Physics Archives – Page 36 of 147

Heavy-ion physics: past, present and future

**Hot and dense** Tracks from a lead–lead collision recorded by the ALICE TPC. Credit: ALICE

Ultra-relativistic collisions between heavy nuclei probe the high-temperature and high-density limit of the phase diagram of nuclear matter. These collisions create a new state of matter, known as the quark–gluon plasma (QGP), in which quarks and gluons are no longer confined in hadrons but instead behave quasi-freely over a relatively large volume. By creating and studying this novel state of matter, which last existed in the microseconds after the Big Bang, we gain a deeper understanding of the strong nuclear force and quantum chromodynamics (QCD).

Nearly 50 years ago, the first relativistic heavy-ion collision experiments were performed at the Bevatron at Berkeley, reaching energies of 1 to 2 GeV. Since then, heavier ions were collided at higher energies at Brookhaven’s AGS, CERN’s SPS and Brookhaven’s RHIC facilities. Since 2010, heavy-ion physics has entered the TeV regime with lead–lead (PbPb) collisions at 2.76 and 5.02 TeV at the LHC. While the ALICE detector is designed specifically to focus on such collisions, all four large LHC experiments have active heavy-ion physics programmes and are contributing to our understanding of extreme QCD matter.

In a heavy-ion collision, the initial energy deposited by the colliding nuclei undergoes a fast equilibration, within roughly 10^–24 s, to form the QGP. The resulting deconfined and thermalised medium expands and cools over the next few 10^–24 s, before the quarks and gluons recombine to form a hadron gas. It is the goal of heavy-ion experiments at the LHC to use the detected final-state hadrons to reconstruct the properties and dynamical behaviour of the system throughout its evolution. So far, the LHC experiments have delivered a series of results that are sensitive to various aspects of the heavy-ion collision system, with Run 3 set to push our understanding much further.

Properties and dynamics

The initial energy-density distribution and subsequent expansion of the heavy-ion collision system is largely determined by the geometrical overlap of the colliding nuclei. Collisions can range from head-on “central” collisions, where the nuclear overlap is large, to glancing “peripheral” collisions where the overlap region is smaller and roughly almond-shaped. Since the interaction region in non-central events is not rotationally symmetric, anisotropic pressure gradients build up. These preferentially boost particles along the minor axis of the ellipsoidal overlap region, resulting in an observable anisotropy in the distribution of final-state hadrons. The distribution of the particles in the azimuthal angle can be described well by a Fourier cosine series, where the largest term is the second harmonic, characterised by the parameter v₂, due to the ellipsoidal shape of the nuclear overlap region. Fluctuations in the positions of the individual constituent nucleons lead to significant higher-order terms. It was discovered that these Fourier coefficients, v_n, are best described by models where the QGP dynamics obeys hydrodynamic equations, and thus behaves as a liquid exhibiting what we call “collective flow”.

A global Bayesian ﬁt to measurements of centrality dependence — **Flow coeﬃcients** A global Bayesian ﬁt to ALICE measurements of the centrality dependence of the ﬂow coeﬃcients v₂, v₃ and v₄ is used to extract the viscosity of the QGP system. Source: *Nat. Phys.* 15 1113

Remarkably, in order to fit hydrodynamic models to experimental data it is necessary for the medium’s viscosity to be very low, corresponding to a shear-viscosity to entropy-density ratio of the order η/s ∼ 0.1. With a shear viscosity that is orders of magnitude smaller than other materials, the QGP is known as the “perfect” liquid. Measurements of the higher order harmonics, as well as their event-by-event fluctuations and correlations, provide even greater sensitivity to medium properties and the initial-state dynamics. Precision measurements of the v_n harmonics, charged-particle density, mean transverse momentum p_T, and mean-p_T fluctuations by ALICE have been used to extract the shear and bulk viscosity of the system as a function of temperature (see “Flow coefficients” figure).

While the QGP created in heavy-ion collisions is too small and short-lived to be examined with conventional probes, its properties can be investigated using the products of hard (high momentum-transfer, q²) scatterings that occur in the early stages of the collision and then propagate through the medium as it evolves. The production rates of these internally generated hard probes can be calculated in perturbative QCD and thus are considered calibrated probes of the QGP medium. The high-momentum quarks and gluons produced in these hard scatterings traverse the medium and fragment into collimated jets of hadrons. While these jets appear as a small signal on top of a large, fluctuating background, advances in re-clustering algorithms as well as the higher production rates of jets at the LHC have made it possible to study jets with high precision across a wide range of energies.

The increase in the LHC luminosity will allow us to perform measurements that were previously inaccessible

Compared to jets in proton–proton (pp) collisions, jets in nucleus–nucleus (AA) collisions appear significantly suppressed or “quenched” due to their interactions with the medium (see “Jet quenching” figure). This is in contrast to electroweak probes, which interact only minimally with the coloured QGP medium. When the presence of hard scattering is identified by a high-p_T jet, photon or Z boson, the recoiling jet measured in the opposite direction is often reconstructed with a significantly lower energy, indicating that some of its energy has been transferred and absorbed by the medium. Recent, detailed jet-structure studies show that jets in heavy-ion collisions are softer (they fragment into lower-p_T hadrons) and broader than their counterparts in pp collisions, due to their interactions with the surrounding coloured QGP medium.

Another class of hard probes are heavy-flavour hadrons, since even heavy quarks (charm and beauty) with low p_T are produced in high-q² processes. Similar to jets, which mainly come from the fragmentation of light quarks and gluons, heavy hadrons are also suppressed in heavy-ion collisions relative to pp collisions. Recent precision measurements at the LHC of the yield of D mesons (containing charm quarks) as well as non-prompt D and J/ψ mesons (from the decays of hadrons containing beauty quarks), compared to the yields in pp collisions, demonstrate a mass-dependent suppression. This observation is consistent with the “dead cone” effect, which predicts that quarks with larger masses will be less significantly suppressed than those with smaller masses. The suppression of quarkonia (quark–antiquark bound states) depends on the binding energy, with loosely bound states such as the Υ(3S) and ψ(2S) more likely to become dissociated in the hot and dense medium than the tightly bound Υ(1S) and J/ψ(1S) states. However, it was discovered at the LHC that final-state J/ψ are actually less suppressed than in lower energy AA collisions at RHIC. This was attributed to the larger number of charm quarks being produced at LHC energies, which enhances the probability that charm and anti-charm quarks can recombine to form J/ψ states within the QGP. These dual effects of suppression and recombination are considered a signature of the production of a deconfined, thermalised medium in heavy-ion collisions.

Freeze out

As the QGP expands and cools, it undergoes a phase transition into a hadron gas in which quarks and gluons become confined into hadrons. At chemical freeze-out, inelastic collisions cease and the thermochemical properties of the system become fixed. Comparing ALICE measurements of the inclusive yields of multiple hadron species with a model of statistical hadronisation shows excellent agreement over nine orders of magnitude in mass, from pions to anti-⁴He nuclei (see “Statistical production” figure). This indicates that the bulk chemistry of the QGP freeze-out can be described by purely statistical particle production from a system in thermal equilibrium with a common temperature (155 MeV) and volume (~5000 fm³).

**Jet quenching** Suppression of the number of reconstructed jets with respect to the expected yields from an equivalent number of independent pp collisions, R_AA, for which ALICE, ATLAS and CMS provide complementary measurements. Source: *Phys. Rev. C* **101** 034911

One of the first surprising results to come from the LHC was the discovery of azimuthal correlations between particles over large distances in pseudorapidity in small collision systems, pp and pPb. These long-range correlations are observed in heavy-ion collisions, where they are traditionally attributed to anisotropic flow (parameterised by v_n coefficients). However, the presence of collective behaviour in small systems, where a QGP was not expected to be formed, raised many questions about our understanding of both large and small nuclear collisions.

A second surprising observation was made in the measurement of the ratios of strange and multistrange hadrons (e.g. K⁰_S, Λ, Ξ and Ω) with respect to pions, as a function of the number of particles produced in the collision (multiplicity). The enhancement of strangeness production in AA compared to pp collisions was historically predicted as a signature of the formation of a QGP, although it is now understood as being due to the suppression of strangeness in small systems. However, measurements by ALICE showed a smooth increase in the strangeness enhancement with multiplicity across all collision systems: pp, pPb, XeXe and PbPb – opening further questions about the presence of a thermalised medium in both small and large systems.

In contrast, the suppression of hard probes, which has long been viewed as a complementary effect to anisotropic flow, has not been observed in pp or pPb collisions within current experimental uncertainties. In order to gain a more complete understanding of QCD from the soft to the hard scales, and from small to large systems, we must expand our experimental programmes.

To Run 3 and beyond

All four large experiments at the LHC have undergone significant upgrades during Long Shutdown 2 to extend their reach and allow the collection of heavy-ion data at higher luminosities. The increase in luminosity by a factor of 10 in Runs 3 and 4 at the LHC will allow us to make precision measurements of soft and hard probes of the QGP. Rare probes such as heavy-flavour hadrons will become accessible with high statistical precision, and we will be able to explore the charm and beauty sector at a level commensurate with that of the strangeness studies in Runs 1 and 2. Jet measurements will become significantly more precise as we further explore the medium-induced modification of well-calibrated probes such as γ– and Z-tagged jets.

Our understanding of collective behaviour and the medium evolution will be enhanced by studies of the correlations and fluctuations of flow coefficients, which provide additional and complementary information above and beyond what we learn from v_n alone. Measurements that were severely statistically-limited in Runs 1 and 2, such as those of virtual photons produced as thermal radiation, will be performed with unprecedented precision in Runs 3 and 4. The higher order fluctuations of identified particles, which are expected to be sensitive to critical behaviour around the phase transition, will also come within reach in Runs 3 and 4 and make it possible to map out the phase diagram of QCD matter in great detail.

**Statistical production** Thermal-model ﬁts to the p_T-integrated yields of many hadron species measured in ALICE show excellent agreement with data. Source: *Nucl. Phys. A* **971** 1

Furthermore, studies of small systems will continue to shed light on the development of QGP-like signals from pp to AA collisions. In particular, oxygen nuclei will be collided at the LHC, which will allow us to investigate collective effects in collisions with a geometry similar to PbPb collisions but with multiplicities of the order of those in pp and pPb collisions. High-precision and multi-differential jet measurements in pp, pPb and OO collisions will finally allow us to resolve open questions about the relationship between jet quenching and collective behaviour, and whether such effects are observed across all nuclear collision systems. Through these experimental measurements, we will make major progress in our understanding of nuclear matter from small to large collision systems, towards our ultimate goal of a unified description of QCD phenomenology from the microscopic level to the emergent bulk properties of the QGP.

While the heavy-ion physics programme in Runs 3 and 4 will provide deep insights into the rich field of QCD phenomenology, open questions will remain that can only be addressed with further advancements in detector performance and with the significant increase in heavy-ion luminosity anticipated in Run 5 (expected in 2035–2038). This extension of the LHC heavy-ion programme through the 2030s has been supported by the 2020 update of the European strategy for particle physics, and the LHC-experiment collaborations are exploring the potential for novel measurements in light- and heavy-ion collision systems based on their planned detector upgrades. In particular, ALICE is proposing to build a new dedicated heavy-ion experiment, ALICE 3, based on a large-acceptance ultra-light (low material budget) silicon tracking system surrounded by multiple layers of particle identification technology. The increase in the LHC luminosity coupled with state-of-the-art detector upgrades will allow us to dramatically extend our experimental reach and perform measurements that were previously inaccessible. The goals of the future heavy-ion programme at the LHC – from measuring electromagnetic radiation from the QGP and exotic heavy-flavour hadrons to beyond-the-Standard-Model searches for axions – will provide unprecedented insight into the fundamental constituents and forces of nature.

Accessing the precursor stage of QGP formation

ALICE figure 1 — **Fig. 1.** The correlation of the squared anisotropic flow v²_n and the mean transverse momentum〈p_T〉as a function of collision centrality for PbPb (left) and XeXe (right) collisions. Source: arXiv:2111.06106

The primary goal of the ultrarelativistic heavy-ion collision programme at the LHC is to study the properties of the quark–gluon plasma (QGP), a state of strongly interacting matter in which quarks and gluons are deconfined over large distances compared to the typical size of a hadron. The rapid expansion of the QGP under large pressure gradients is imprinted in the momentum distributions of final-state particles. The azimuthal-anisotropy flow coefficients v_n and the mean transverse momentum 〈p_T〉 of particles, which are described by hydrodynamic models, have been extensively measured by experiments at the LHC and at the RHIC collider. These observables are also used as experimental inputs to global Bayesian analyses that provide information on both the initial stages of the heavy-ion collision, before QGP formation, and on key transport coefficients of the QGP itself, such as the shear and bulk viscosities. However, due to the limited constraints on the initial conditions, uncertainties remain in the QGP’s transport coefficients.

The ALICE collaboration recently reported correlations between v_n and 〈p_T〉 in terms of the modified Pearson coefficient ρ. The measurements were performed in lead–lead (PbPb) and xenon–xenon (XeXe) collisions at centre-of-mass energies per nucleon–nucleon collision of 5.02 and 5.44 TeV, respectively. As the correlations between v_n and 〈p_T〉 are predicted to be mainly driven by the shape and size of the initial profile of the energy distribution in the transverse plane, these studies provide a new approach to characterise the initial state.

The measurements show a positive correlation between v_n and 〈p_T〉 in both PbPb and XeXe collisions (figure 1). These measurements are compared to hydrodynamic calculations using the initial-state models IP-Glasma (based on the colour-glass-condensate effective theory with gluon saturation) and Trento, a parameterised model with nucleons as the relevant degrees of freedom. The centrality dependence of ρ is better described by IP-Glasma than by Trento. In particular, the positive measured values of ρ suggest an effective nucleon width of the order of 0.3–0.5 fm, which is significantly smaller than what has been extracted in all Bayesian analyses using Trento initial conditions. The Pearson correlation measurements can now be included in Bayesian analyses to better constrain the initial state in nuclear collisions, thus impacting the resulting QGP parameters. As a bonus, the measurements in XeXe collisions are sensitive to the quadrupole deformation parameter β₂ of the ¹²⁹Xe nucleus, potentially opening a new window for studying nuclear structure with ultrarelativistic heavy-ion collisions.

Higgs-boson charm coupling weaker than bottom

ATLAS figure 1 — **Fig. 1.** The distribution of the invariant mass of the two jets, after background subtraction. Source: arXiv:2201.11428

Within the Standard Model (SM), the Higgs boson is predicted to interact with (or couple to) quarks with a strength proportional to their mass. By measuring these interaction strengths, physicists can test this prediction and gain insight into possible physics beyond the SM, where such couplings can be modified. In a new analysis exploiting the full Run-2 dataset, the ATLAS collaboration experimentally excludes new-physics scenarios which predict that decays of the Higgs boson to a pair of charm quarks (H → cc) are as frequent as those to bottom quarks (H → bb).

The search for H → cc is hampered by abundant background processes. In order to identify charm–quark signatures, a new multivariate classification method was developed to identify charm hadrons within jets, while simultaneously reducing the probability of misidentifying jets originating from a bottom quark. To maximise the sensitivity to the signal, events with one or two charm-tagged jets were selected. Background processes were further suppressed by selecting Higgs-boson events produced together with a weak boson, VH(cc), where the weak boson (V = W or Z) decays to 0, 1 or 2 electrons or muons. In total, 44 regions were fitted simultaneously to measure the H → cc process.

In the SM, the H → cc process accounts for only 3% of all Higgs-boson decays. The ATLAS analysis found no significant sign for this process in the data, setting an upper limit on the rate of the VH(cc) process 26 times the SM rate at 95% confidence level. This limit constrains the Higgs-to-charm coupling strength to less than 8.5 times the predicted SM value. The analysis strategy is validated by measuring events with two vector bosons that contain the decay of a W boson to one charm quark, VW(cq), or the decay of a Z boson to two charm quarks, VZ(cc), whose rates are found to agree with the predictions. The combined dijet-mass distribution, after subtraction of the backgrounds, is shown in figure 1.

ATLAS figure 2 — **Fig. 2.** Likelihood scan (red) of the ratio of the Higgs-charm and Higgs-bottom coupling modifiers. The vertical green lines indicate the scenario of equal couplings. Source: arXiv:2201.11428

Since H → cc and H → bb decays lead to very similar signatures in the ATLAS detector, a combined analysis of both processes is key to a common interpretation. The multivariate classification method is used to identify jets as originating from a bottom quark, a charm quark or lighter quarks. Since a fraction of the H → bb events passes the selection criteria of the H → cc analysis and vice versa, the individual analyses are designed to ensure that no collision events are counted twice. This orthogonality between the analyses enabled a simultaneous measurement of the two processes for the first time.

Within the SM, the ratio of the couplings of bottom and charm quarks to the Higgs boson is given by their mass ratio: m_b/m_c = 4.578 ± 0.008, obtained from lattice-QCD calculations. With its novel combination of H → cc and H → bb decays, the ATLAS analysis excludes the hypothesis that the Higgs-boson interaction with charm quarks is stronger than or equal to the interaction with bottom quarks at 95% confidence level (figure 2). For the first time, this measurement establishes that the Higgs-boson coupling is smaller for charm quarks than for bottom quarks.

LHCb constrains cosmic antimatter production

LHCb figure 1 — **Fig. 1.** Measurement of the displacement of candidate antiproton tracks from the collision vertex. “Prompt” antiprotons peak at 0, whereas if the value is above 3, they most likely originate from “detached” antihyperon decays or in secondary collisions with the detector material. Source: LHCb-PAPER-2022-006

During their 10 million-year-long journey through the Milky Way, high-energy cosmic rays can collide with particles in the interstellar medium, the ultra-rarefied gas filling our galaxy and mostly composed of hydrogen and helium. Such rare encounters are believed to produce most of the small number of antiprotons, about one per 10,000 protons, that are observed in high-energy cosmic rays. But this cosmic antimatter could also originate from unconventional sources, such as dark-matter annihilation, motivating detailed investigations of antiparticles in space. This effort is currently led by the AMS-02 experiment on the International Space Station, which has reported results with unprecedented accuracy.

The interpretation of these precise cosmic antiproton data calls for a better understanding of the antiproton production mechanism in proton-gas collisions. Here, experiments at accelerators come to the rescue. The LHCb experiment has the unique capability of injecting gas into the vacuum of the LHC accelerator. By injecting helium, cosmic collisions are replicated in the detector and their products can be studied in detail. LHCb already provided a first key input into the understanding of cosmic antimatter by measuring the amount of antiprotons produced at the proton–helium collision vertex itself. In a new study, this measurement has been extended by including the significant fraction (about one third) of antiprotons resulting from the decays of antihyperons such as Λ, which contain a strange antiquark also produced in the collisions.

These antiprotons are displaced from the collision point in the detector, as the antihyperons can fly several metres through the detector before decaying. Different antihyperon states and decay chains are possible, all contributing to the cosmic antiproton flux. To count them, the LHCb team exploited two key features of its detector: the ability to distinguish antiprotons from other charged particles via two ring-imaging Cherenkov (RICH) detectors, and the outstanding resolution of the LHCb vertex locator. Thanks to the latter, when checking the compatibility of the identified antiproton tracks with the collision vertex, three classes of antiprotons can be clearly resolved (figure 1): “prompt” particles originating from the proton–helium collision vertex; detached particles from Λ decays; and more separated particles produced in secondary collisions with the detector material.

The majority of the detached antiprotons are expected to originate from Λ particles produced at the collision point decaying to an antiproton and a positive pion. A second study was thus performed to fully reconstruct these decays by identifying the decay vertex. The results of this complementary approach show that about 75% of the observed detached antiprotons originate from Λ decays, in good agreement with theoretical predictions.

These new results provide an important input for modelling the expected antiproton flux from cosmic collisions. No smoking gun for an exotic source of cosmic antimatter has emerged yet, while the accuracy of this quest would profit from more accelerator inputs. Thus, the LHCb collaboration plans to expand its “space mission” with the new gas target SMOG2. This facility/device could also enable collisions between protons and hydrogen or deuterium targets, further strengthening the ties between the particle and astroparticle physics communities.

Science diversity at the intensity and precision frontiers

**North Area** The EHN1 experimental hall in the North Area, showing the Morpurgo magnet (left) and the NA61 experiment (right). Credit: CERN-PHOTO-202001-006-9

While all eyes focus on the LHC restart, a diverse landscape of fixed-target experiments at CERN have already begun data-taking. Driven by beams from smaller accelerators in the LHC chain, they span a large range of research programmes at the precision and intensity frontiers, complementary to the LHC experiments. Several new experiments join existing ones in the new run period, in addition to a suite of test-beam and R&D facilities.

At the North Area, which is served by proton and ion beams from the Super Proton Synchrotron (SPS), new physics programmes have been underway since the return of beams last year. Experiments in the North Area, which celebrated its 40th anniversary in 2019, are located at different secondary beamlines and span QCD, electroweak physics and QED, as well as dark-matter searches. “During Long Shutdown 2, a major overhaul of the North Area started and will continue during the next 10 years to provide the best possible beam and infrastructure for our users,” says Yacine Kadi, leader of the North Area consolidation project. “The most critical part of the project is to prepare for the future physics programme.”

The first phase of the AMBER facility at the M2 beamline is an evolution of COMPASS, which has operated since 2002 and focuses on the study of the gluon contribution to the nucleon spin structure. By measuring the proton charge radius via muon–proton elastic scattering, AMBER aims to clarify the long-standing proton–radius puzzle, offering a complementary approach to previous electron–proton scattering and spectroscopy measurements. A new data-acquisition system will enable the collaboration to measure the antiproton production cross-section to improve the sensitivity of searches for cosmic antiparticles from possible dark-matter annihilation. A third AMBER programme will concentrate on measurements of the kaon, pion and proton charge radii via Drell-Yan processes using heavy targets.

A second North Area experiment specialising in hadron physics is NA61/SHINE, which underwent a major overhaul during Long Shutdown 2 (LS2), including the re-use of the vertex detector from the ALICE experiment. Building on its predecessor NA49, the 17 m-long NA61/SHINE facility, situated at the H2 beamline, focuses on three main areas: strong interactions, cosmic rays and cross-section measurements for neutrino physics. The collaboration continues its study of the energy dependence of hadron production in heavy-ion collisions, in which NA49 found irregularities. It also aims to observe the critical point at which the phase transition from a quark–gluon plasma to a hadron gas takes place, the threshold energy for which is only measurable at the SPS rather than at the higher energy LHC or RHIC experiments. By measuring hadron production from pion–carbon interactions, meanwhile, the team will study the properties of high-energy cosmic rays from cascades of charged particles. Finally, using kaons and pions produced from a target replicating that of the T2K experiment in Japan, NA61/SHINE will help to determine the neutrino flux composition at the future DUNE and Hyper-Kamiokande experiments for precise measurements of neutrino mixing angles and the CP-violating phase.

New physics

Situated at the same H2 beamline, the new NA65 “DsTau” experiment will study the production of D_s mesons. This is important because D_s decays are the main source of ν_τ’s in a neutrino beam, and are therefore relevant for neutrino-oscillation studies. After a successful pilot run in 2018, a measurement campaign began in 2021 to determine the ν_τ-production flux.

**East Area** The newly renovated East Area hosts the CLOUD experiment (cylinder on the left), and the CHARM irradiation facility (concrete block on the right). Credit: CERN-PHOTO-202101-009-6

At the K12 secondary beamline, NA62 continues its measurement of the ultra-rare charged kaon decay to a charged pion, a neutrino and an antineutrino, which is very sensitive to possible physics beyond the Standard Model. The collaboration aims to increase its sensitivity to a level (10%) approaching theoretical uncertainties, thanks to further data and experimental improvements to the more than 200 m-long facility. One is the installation during LS2 of a muon veto hodoscope that helps to determine whether a muon is coming from a kaon decay or from other interactions. Since 2021, NA62 also operates as a beam-dump experiment, where its primary focus is to search for feebly-interacting particles. Here, the ability to determine whether muons come from the target absorber is even more important since they make up most of the background.

Dark interactions

Searching for new physics is the focus of NA64 at the H4 beamline, which studies the interaction between an electron beam and an active target to look for a hypothetical dark-photon mediator connecting the SM with a possible dark sector. With at least five times more data expected this year, and up to 10 times more data during the period of LHC Run 3, it could be possible to determine whether the dark mediator, should it exist, is either an elastic scalar or a Majorana particle. Adding further impetus to this programme is an unexpected 17 MeV peak reported in e⁺e^–internal pair production by the ATOMKI experiment and, more significantly, the tension between the measured and predicted values of the anomalous magnetic moment of the muon (g-2)_μ, for which possible explanations include models that invoke a dark mediator. During a planned muon run at the M2 beamline, the collaboration aims to cover the relevant parameter space for the (g-2)_μ anomaly.

NA63 also receives electrons from the H4 beamline and uses a high-energy electron beam to study the behaviour of scattered electrons in a strong electromagnetic field. In particular, the experiment tests QED at higher orders, which have a gravitational analogue in extreme astroparticle physics phenomena such as black-hole inspirals and magnetars. The NA63 team will continue its measurements in June.

Besides driving the broad North Area physics programme, the SPS serves protons to AWAKE – a proof-of-principle experiment investigating the use of plasma wakefields driven by a proton bunch to accelerate charged particles. Following successful results from its first run, the collaboration aims to further develop methods to modulate the proton bunches to demonstrate scalable plasma-wakefield technology, and to prepare for the installation of a second plasma cell and an electron-beam system using the whole CNGS tunnel at the beginning of LS3 in 2026.

Located on the main CERN site, receiving beams from the Proton Synchrotron (PS), the East Area underwent a complete refurbishment during LS2, leading to a 90% reduction in its energy consumption. Its main experiment is CLOUD, which simulates the impact of particulates on cloud formation. This year, the collaboration will test a new detector component called FLOTUS, a 70 litre quartz chamber extending the simulation from a period of minutes to a maximum of 10 days. The PS also feeds the n_TOF facility, which last year marked 20 years of service to neutron science and its applications. A new third-generation spallation target installed and commissioned in 2021 will enable new n_TOF measurements relevant for nuclear astrophysics.

Different dimensions

Taking CERN science into an altogether different dimension, the PS also links to the Antimatter Factory via the Antiproton Decelarator (AD) and ELENA rings, where several experiments are poised to test CPT invariance and antimatter gravitational interactions at increased levels of precision (see “Antimatter galore at ELENA” panel). Even closer to the proton beam source is the PS Booster, which serves the ISOLDE facility. ISOLDE covers a diverse programme across the physics of exotic nuclei and includes MEDICIS (devoted to the production of novel radioisotopes for medical research), ISOLTRAP (comprising four ion traps to measure ions) and COLLAPS and CRIS, which focus on laser spectroscopy. Its post-accelerators REX/HIE-ISOLDE increase the beam energy up to 10 MeV/u, making ISOLDE the only facility in the world that provides radioactive ion-beam acceleration in this energy range.

Antimatter galore at ELENA

**Slow down** Experiments in the AD hall. Credit: F Butin

Served directly by the Antiproton Decelerator (AD) for the past two decades, experiments at the CERN Antimatter Factory are now connected to the new ELENA ring, which decelerates 5.3 MeV antiprotons from the AD to 100 keV to allow a 100-fold increase in the number of trapped antiprotons. Six experiments involving around 350 researchers use ELENA’s antiprotons for a range of unique measurements, from precise tests of CPT invariance to novel studies of antimatter’s gravitational interactions.

The ALPHA experiment focuses on antihydrogen-spectroscopy measurements, recently reaching an accuracy of two parts per trillion in the transition from the ground state to the first excited state. By clocking the free-fall of antiatoms released from a trap, it is also planning to measure the gravitational mass of antihydrogen. ALPHA’s recent demonstration of laser-cooled antihydrogen has opened a new realm of precision on anti-hydrogen’s internal structure and gravitational interactions to be explored in upcoming runs.

ASACUSA specialises in spectroscopic measurements of antiprotonic helium, recently finding surprising behaviour. The experiment is also gearing up to perform hyperfine-splitting spectroscopy in antihydrogen using atomic-beam methods complementary to ALPHA’s trapping techniques.

GBAR and AEgIS target direct measurements of the Earth’s gravitational acceleration on antihydrogen. GBAR is developing a method to measure the free-fall of antihydrogen atoms, using sympathetic laser cooling to cool antihydrogen atoms and release them, after neutralisation, from a trap directly injected with antiprotons from ELENA, maximising antihydrogen production. AEgIS, having established pulsed formation of antihydrogen in 2018, is following a different approach based on measuring the vertical drop of a pulsed cold beam of antihydrogen atoms travelling horizontally through a device called a Moiré deflectometer.

BASE uses advanced Penning traps to compare matter and antimatter with extreme precision, recently finding the charge-to-mass ratios of protons and antiprotons to be identical within 16 parts per trillion. The data also allowed the collaboration to perform the first differential test of the weak equivalence principle using antiprotons, reaching the 3% level, with experiment improvements soon expected to increase the sensitivities of both measurements. The BASE team is also working on an improved measurement of the antiproton magnetic moment, the implementation of a transportable antiproton trap called BASE-STEP and improved searches for millicharged particles.

The newest AD experiment, PUMA, which is preparing for first commissioning later this year, aims to transport trapped antiprotons collected at ELENA to ISOLDE where, from next year, they will be annihilated on exotic nuclei to study neutron densities at the surface of nuclei.

“Thanks to the beam provided by ELENA and the major upgrades of the experiments, we hope to see big progress in ultra-precise tests of CPT invariance, first and long-awaited antihydrogen-based studies of gravity, as well as the development of new technologies such as transportable antimatter traps,” says Stefan Ulmer, head of the AD user committee.

Stable and highly customisable beams at the North and East areas also facilitate important detector R&D and test-beam activities. These include the recently approved Water-Cherenkov Test Experiment, which will help to develop detector techniques for long-baseline neutrino experiments, and new detector components for the LHC experiments and proposed future colliders. The CERN Neutrino Platform is dedicated to the development of detector technologies for neutrino experiments across the world. Upcoming activities including ongoing contributions to the future DUNE experiment in the US, in particular the two huge DUNE cryostats and R&D for “vertical drift” liquid-argon detection technology. In the East Area, the mixed-field irradiation (CHARM) and proton-irradiation (IRRAD) facilities provide key input to detector R&D and electronics tests, similar to the services provided by the SPS-driven GIF irradiation facility and HiRadMat.

With the many physics opportunities mapped out by Physics Beyond Colliders and the consolidation of our facilities, we are looking into a bright future
Johannes Bernhard

Fixed-target experiments in the North and East areas, along with experiments at ISOLDE and the AD, demonstrate the importance of diverse physics studies at CERN, when the best path to discover new physics is unclear. Some of these experiments emerged within the Physics Beyond Colliders initiative and there are many more on the horizon, such as KLEVER and the SPS Beam Dump Facility. “With the many physics opportunities mapped out by Physics Beyond Colliders and the consolidation of our facilities, we are looking into a bright future,” says Johannes Bernhard, head of the liaison to experiments section in the beams department. “We are always aiming to serve our users with the highest beam quality and performance possible.”

CDF sets W mass against the Standard Model

CDF_detector — The CDF detector operated at the Tevatron from 1984-2011. Credit: Fermilab

Ever since the W boson was discovered at CERN’s SppS four decades ago, successive collider experiments have pinned down its mass at increasing levels of precision. Unlike the fermion masses, the W mass is a clear prediction of the Standard Model (SM). At lowest order in electroweak theory, it depends solely on the mass of the Z boson and the value of the weak mixing angle. But higher-order corrections introduce an additional dependence on the gauge-boson couplings and the masses of other SM particles, in particular the heavy top quark and Higgs boson. With the precision of electroweak calculations now exceeding that of direct measurements, better knowledge of the measured W mass provides a vital test of the SM’s consistency.

The immediate reaction was silence
Chris Hays

A new measurement by the CDF collaboration based on data from the former Tevatron collider at Fermilab throws a curveball into this picture. Published today in Science, the CDF W-mass measurement – the most precise to date – stands 7σ from the SM prediction, upsetting decades of steady convergence between experiment and theory.

“I would say the immediate reaction was silence,” says Chris Hays, one of the CDF analysis leads, of the moment the measurement was unblinded on 19 November 2020. “Then there was some discussion to ensure the unblinding worked, i.e. that the value was correct, and to decide what would be the next steps.”

Long slog
CDF physicists have been measuring the mass of the W boson for more than 30 years via its decays to a lepton and a neutrino. In 2012, shortly after the Tevatron shut down, CDF published a W mass of 80,387 ± 12 (stat) ± 15 (syst) MeV based on 2.2 fb^-1 of data, which significantly exceeded the precision of all previous measurements at that time combined. After 10 years of careful analysis and scrutiny of the full Tevatron dataset (8.8 fb^-1, corresponding to about 4.2 million W-boson candidates), and taking into account an improved understanding of the detector and advances in the theoretical and experimental understanding of the W’s interactions with other particles, the new CDF result is twice as precise: 80,433.5 ± 6.4 (stat) ± 6.9 (syst) MeV.

In addition to the four-fold increase in statistics, the measurement benefits from a better understanding of systematic uncertainties. One significant change concerns the proton/antiproton parton distribution functions (PDFs), where the addition of LHC data to the PDF fits has reduced the uncertainty from 10 MeV to 3.9 MeV while also slightly raising the central value of the 2012 result.

LHCb-FIGURE-2022-003 — The new CDF measurement of the W mass along with other Tevatron results, the LEP experiments, ATLAS and LHCb, showing the total (dark blue) and statistical (light-blue band) uncertainties. The Standard Model prediction is shown in green. Credit: LHCb

“The 2012 and 2022 CDF values are in agreement at the level of two sigma accounting for the fact that approximately 25% of the events are in common, so the internal tension is not so significant,” explains CDF collaborator Mark Lancaster, who was an internal reviewer for the result. “But the tension with other results — particularly ATLAS at 80,370 ± 19 MeV and the SM at 80,357 ± 6 MeV — is significant. Many people from the LHC, Tevatron and theory community are presently working together to combine the results from the Tevatron, LHC and LEP and understand the correlations between them, e.g. in the PDFs and some of the higher order QCD and QED effects.”

It’s now up to theorists and other experiments to follow up on the CDF result, comments CDF co-spokesperson David Toback. “If the difference between the experimental and expected value is due to some kind of new particle or subatomic interaction, which is one of the possibilities, there’s a good chance it’s something that could be discovered in future experiments,” he says.

Cross checks
Results from the LHC experiments are crucial to enable a deeper understanding. One of the challenges in measuring the W mass in high-rate proton-proton collisions at the LHC is event “pile-up”, which makes it hard to reconstruct the missing transverse energy from neutrinos. The higher collision energy at the LHC also means W bosons are produced with larger transverse momenta with respect to the beam axis, which needs to be properly modeled in order to measure the W boson mass precisely.

It takes years to build up the knowledge of the detector necessary to be able to address all the issues satisfactorily
Florencia Canelli

The ATLAS collaboration published the first high-precision measurement of the W mass at the LHC in 2018 based on data collected at a centre-of-mass energy of 7 TeV, and is currently working on new measurements. In September, based on 2016 data, LHCb published its first measurement of the W mass: 80,354 ± 32 MeV, and estimates that an uncertainty of 20 MeV or less is achievable with existing data. CMS is also proceeding with analyses that should soon see its first public result. “It’s an important measurement of our physics programme,” says CMS physics co-cordinator Florencia Canelli. “As the CDF result shows, precision physics can be a challenging and lengthy process: it takes a very long time to understand all aspects of the data to the level of precision required for a competitive W-mass measurement, and it takes years to build up the knowledge of the detector necessary to be able to address all the issues satisfactorily.”

The CDF result reiterates the central importance of precision measurements in the search for new physics, describe Claudio Campagnari (UC Santa Barbara) and Martijn Mulders (CERN) in a Perspective article accompanying the CDF paper. They point to the increased precision that will be available at the High-Luminosity LHC and the capabilities of future facilities such as the proposed Future Circular Collider, the e⁺e^– mode of which “would offer the best prospects for an improved W-boson mass measurement, with a projected sensitivity of 7 ppm”. Such a measurement would also demand the SM electroweak calculations be performed at higher orders, a challenge firmly in the sights of the theory community.

Following the 2012 discovery of the Higgs boson, it is not easy to tweak the SM parameters without ruining the excellent agreement with numerous measurements. Furthermore, unlike calculations such as that of the muon anomalous magnetic moment, which relies on significant input from QCD, the prediction of the W mass relies mostly on “cleaner” electroweak computations. Surveying possible new physics that could push the W mass to higher values than expected, the CDF paper points to hypotheses that offer a deeper understanding of the Higgs field, from which the SM particles get their masses. These include supersymmetry and Higgs-boson compositeness, both of which include a potential source of dark matter.

“Supersymmetry could make a significant change to the SM prediction of the W mass, although it seems difficult to explain as big an effect as seen experimentally,” says theorist John Ellis. “But one prediction I can make with confidence is a tsunami of arXiv papers in the weeks ahead.”

Dijet excess intrigues at CMS

The Standard Model (SM) has been extremely successful in describing the behaviour of elementary particles. Nevertheless, conundrums such as the nature of dark matter and the cosmological matter-antimatter asymmetry strongly suggest that the theory is incomplete. Hence, the SM is widely viewed as an effective low-energy limit of a more fundamental underlying theory which must be modified to describe particles and their interactions at higher energies.

A powerful way to discover new particles expected from physics beyond the SM is to search for high-mass dijet or multi-jet resonances, as these are expected to have large production cross-sections at hadron colliders. These searches look for a pair of jets originating from a pair of quarks or gluons, coming from the decay of a new particle “X” and appearing as a narrow bump in the invariant dijet-mass distribution. Since the energy scale of new physics is most likely high, it is natural to expect these new particles to be massive.

CMS_Figure1 — **Fig. 1.** Display of the highest mass event with a four-jet mass of 8 TeV, in which each pair of jets has a dijet mass of 1.9 TeV. Source: CMS-PAS-EXO-21-010.

CMS and ATLAS have performed a suite of single-dijet-resonance searches. The next step is to look for new identical-mass particles “X” that are produced in pairs, with (resonant mode) or without (non-resonant mode) a new intermediate heavier particle “Y” being produced and decaying to pairs of X. Such processes would yield two dijet resonances and four jets in the ﬁnal state: the dijet mass would correspond to particle X and the four-jet mass to particle Y.

The CMS experiment was also motivated to search for Y→ XX → 4-jets by a candidate event recorded in 2017, which was presented by a previous CMS search for dijet resonances (figure 1). This spectacular event has four high transverse-momentum jets, forming two dijet pairs each with an invariant mass of 1.9 TeV and a four-jet invariant mass of 8 TeV.

CMS_Figure2 — **Fig. 2.** Number of events observed (colour scale) within bins of the four-jet mass and the average mass of the two dijets. Magenta ellipses show the 1 and 2σ resolution contours, respectively, from a signal simulation of a four-jet resonance (Y), with a mass of 8.4 TeV, decaying to a pair of dijet resonances (XX), each with a mass of 2.1 TeV. Source: CMS-PAS-EXO-21-010.

Presented on 14 March at Rencontres de Moriond, the CMS collaboration has found another very similar event in a new search optimised for this specific Y→ XX → 4-jets topology. These events could originate from quantum-chromodynamics processes, but those are expected to be extremely rare (figure 2). The two candidate events are clearly visible at high masses, distinct from all the rest. Also shown (magenta) is a simulation of a possible new-physics signal – a diquark decaying to vector-like quarks – with a four-jet mass of 8.4 TeV and a dijet mass of 2.1 TeV, which very nicely describes these two candidates.

The hypothesis that these events originate from the SM at the observed X and Y masses is disfavoured with a local significance of 3.9σ. Taking into account the full range of possible X and Y mass values, the compatibility of the observation with the SM expectation leads to a global significance of 1.6σ.

The upcoming LHC Run 3 and future High-Luminosity LHC runs will be crucial in telling us whether these events are statistical fluctuations of the SM expectation, or the first signs of yet another groundbreaking discovery at LHC.

Graph neural networks boost di-Higgs search

**Fig. 1.** Observed (solid) and expected (dashed) exclusion limit on the HH production cross section times the branching fraction into four bottom quarks as a function of κ_2V with other couplings fixed to the SM values. The crossings of the observed limit and the theoretical cross section (red line) indicate the allowed range of the coupling values: 0.6 < κ_2V < 1.4. Source: CMS-PAS-B2G-22-003

Two fundamental characteristics of the Higgs boson (H) that have yet to be measured precisely are its self-coupling λ, which indicates how strongly it interacts with itself, and its quartic coupling to the vector bosons, which mediate the weak force. These couplings can be directly accessed at the LHC by studying the production of Higgs-boson pairs, which is an extremely rare process occurring about 1000 times less frequently than single-H production. However, several new-physics models predict a significant enhancement in the HH production rate compared to the Standard Model (SM) prediction, especially when the H pairs are very energetic, or boosted. Recently, the CMS collaboration developed a new strategy employing graph neural networks to search for boosted HH production in the four-bottom-quark final state, which is one of the most sensitive modes currently under examination.

H pairs are produced primarily via gluon and vector-boson fusion. The former production mode is sensitive to the self-coupling, while the latter probes the quartic coupling involving a pair of weak vector bosons and two Higgs bosons. The extracted modifiers of the coupling-strength parameters, κ_λ and κ_2V, quantify their strengths relative to the SM expectation.

This latest CMS search targets both production modes and selects two Higgs bosons with a high Lorentz boost. When each Higgs boson decays to a pair of bottom quarks, the two quarks are reconstructed as a single large-radius jet. The main challenge is thus to identify the specific H jet while rejecting the background from light-flavour quarks and gluons. Graph neural networks, such as the ParticleNet algorithm, have been shown to distinguish successfully between real H jets and background jets. Using measured properties of the particles and secondary vertices within the jet cone, this algorithm treats each jet as an unordered set of its constituents, considers potential correlations between them, and assigns each jet a probability to originate from a Higgs-boson decay. At an H-jet selection efficiency of 60%, ParticleNet rejects background jets twice as efficiently as the previous best algorithm (known as DeepAK8). A modified version of this algorithm is also used to improve the H-jet mass resolution by nearly 40%.

Using the full LHC Run-2 dataset, the new result excludes an HH production rate larger than 9 times the SM cross-section at 95% confidence level, versus an expected limit of 5. This represents an improvement by a factor of 30 compared to the previous best result for boosted HH production. The analysis yields a strong constraint on the HH production rate and κ_λ, and the most stringent constraint on κ_2V to date, assuming all other H couplings to be at their SM values (see figure 1). For the first time, and with the assumption that the other couplings are consistent with the SM, the result excludes the κ_2V = 0 scenario at over five standard deviations, confirming the existence of a quartic coupling between two vector bosons and two Higgs bosons. This search paves the way for a more extensive use of advanced machine-learning techniques, the exploration of the boosted HH production regime, and further investigation into the potentially anomalous character of the Higgs boson in Run 3 and beyond.

Extending the reach on Higgs’ self-coupling

**Fig. 1.** Observed (solid) and expected (dashed) upper limits at 95% CL on the non-resonant HH production cross section as a function of the coupling strength of the Higgs boson trilinear-self-coupling κ_λ. The lower and upper limits on κ_λ are determined by the crossing points of the upper limits on the HH cross section with the theory prediction. Source: ATLAS-CONF-2021-052

The discovery of the Higgs boson and the comprehensive measurements of its properties provide a strong indication that the mechanism of electroweak symmetry breaking (EWSB) is compatible with the one predicted by Brout, Englert and Higgs (BEH) in 1964. But there remain unprobed features of EWSB, chiefly whether the form of the BEH potential follows the predicted “Mexican hat” shape. One of the parameters that determines the form of the BEH potential is the Higgs boson’s trilinear self-coupling, λ. Experimentally, this fundamental parameter can be measured via Higgs-boson pair (HH) production, where a single virtual Higgs boson splits into two Higgs bosons. However, such a measurement is very challenging as the Standard Model (SM) HH production cross-section is more than 1000 times lower than that of single Higgs-boson production.

Beyond the SM (BSM) physics with modified or new Higgs-boson couplings could lead to significantly enhanced HH production. Some BSM scenarios predict new heavy particles that may lead to resonant HH production, contrary to the non-resonant production featured by the triple-Higgs-boson coupling. New ATLAS results set tight constraints on both the non-resonant and resonant scenarios, showing that the boundaries of what can be achieved with the current and future LHC datasets can be significantly pushed.

The ATLAS collaboration recently released results of searches for HH production in three final states – bbγγ, bbττ and 4b (where one Higgs boson decays into two b-quarks and the other into two photons, two tau-leptons or two b-quarks) and their combination, exploiting the full LHC Run-2 dataset. The first two analyses target both resonant and non-resonant HH production, while the 4b analysis targets only resonant HH production. These three channels are the most sensitive final states in each scenario. The three decay modes of the second Higgs boson provide good sensitivity in different kinematic regions, so that the analyses are highly complementary. The HH → bbγγ process has the lowest branching ratio but high efficiency to trigger and reconstruct photons, as well as an excellent diphoton mass resolution, leading to the best sensitivity at low HH invariant masses. The HH → 4b final state has the highest branching ratio but suffers from the requirement to impose high transverse momentum b-jet trigger thresholds, the ambiguity in the Higgs boson reconstruction and the large multijet background. However, it provides the best sensitivity at high HH invariant masses. Finally, the HH → bbττ decay has a moderate branching ratio as well as a moderate background contamination, giving the best sensitivity in the intermediate HH mass range.

BSM physics with new Higgs-boson couplings could lead to significantly enhanced HH production

With the latest analyses, a remarkably stringent observed (expected) upper limit of 3.1 (3.1) times the SM prediction on non-resonant HH production was obtained at 95% confidence level (CL). The coupling strength of the Higgs boson trilinear self-coupling in units of the SM value κ_λ is observed (expected) to be constrained between –1.0 and 6.6 (–1.2 and 7.2) at 95% CL (see figure 1). These are the world’s tightest constraints obtained on this process. The observed (expected) exclusion limits at 95% CL on the resonant HH production cross-section range between 1.1 and 595 fb (1.2 and 392 fb) for resonance masses between 250 and 5000 GeV.

The sensitivity of the current analyses is still limited by statistical uncertainties and is expected to improve significantly with the future luminosity increase during LHC Run 3 and the HL-LHC programme. A comparison between the current results and previous partial Run-2 dataset results has shown that an improvement by more than a factor of three on the limits is achieved. A factor of two was expected from the larger dataset, and the remaining improvements arise from better object reconstruction and identification techniques, and new analysis methods.

These latest results inspire confidence that the observation of the SM HH production and a precise measurement of the Higgs-boson trilinear self-coupling may be possible at the HL-LHC.

Exploring the CMB like never before

To address the major questions in cosmology, the cosmic microwave background (CMB) remains the single most important phenomenon that can be observed. Not this author’s words, but those of the recent US National Academies of Sciences, Engineering, and Medicine report Pathways to Discovery in Astronomy and Astrophysics for the 2020s (Astro2020), which recommended that the US pursue a next-generation ground-based CMB experiment, CMB-S4, to enter operation in around 2030.

The CMB comprises the photons created in the Big Bang. These photons have therefore experienced the entire history of the universe. Everything that has happened has left an imprint on them in the form of anisotropies in their temperature and polarisation with characteristic amplitudes and angular scales. The early universe was hot enough to be completely ionised, which meant that the CMB photons constantly scattered off free electrons. During this period the primary CMB anisotropies were imprinted, tracing the overall geometry of the universe, the fraction of the energy density in baryons, the number of light-relic particles and the nature of inflation. After about 375,000 years of expansion the universe cooled enough for neutral hydrogen atoms to be stable. With the free electrons rapidly swept up by protons, the CMB photons simply free-streamed in whatever direction they were last moving in. When we observe the CMB today we therefore see a snapshot of this so-called last-scattering surface.

The continued evolution of the universe had two main effects on the CMB photons. First, its ongoing expansion stretched their wavelengths to peak at microwave frequencies today. Second, the growth of structure eventually formed galaxy clusters that changed the direction, energy and polarisation of the CMB photons that pass through them, both from gravitational lensing by their mass and from inverse Compton scattering by the hot gas that makes up the inter-cluster medium. These secondary anisotropies therefore constrain all of the parameters that this history depends on, from the moment the first stars formed to the number of light-relic particles and the masses of neutrinos.

The temperature anisotropies of the CMB — **Relic radiation** The temperature anisotropies of the cosmic microwave background imprinted on the sky, as observed by Planck, show fluctuations that correspond to regions of slightly different densities, representing the seeds of all cosmological structure formation. Credit: ESA and the Planck Collaboration

As noted by the Astro2020 report, the history of CMB research is that of continuously improving ground and balloon experiments, punctuated by comprehensive measurements from the major satellite missions COBE, WMAP and Planck. The increasing temperature and polarisation sensitivity and angular resolution of these satellites is evidenced in the depth and resolution of the maps they produced (see “Relic radiation” image”). However, such maps are just our view of the CMB – one particular realisation of a random process. To derive the underlying cosmology that gave rise to them, we need to measure the amplitude of the anisotropies on various angular scales (see “Power spectra” figure). Following the serendipitous discovery of the CMB in 1965, the first measurements of the temperature anisotropy were made by COBE in 1992. The first peak in the temperature power spectrum was measured by the BOOMERanG and MAXIMA balloons in 2000, followed by the E-mode polarisation of the CMB by the DASI experiment in 2002, and the B-mode polarisation by the South Pole Telescope and POLARBEAR experiments in 2015.

CMB-S4, a joint effort supported by the US Department of Energy (DOE) and the National Science Foundation (NSF), will help write the next chapter in this fascinating adventure. Planned to comprise 21 telescopes at the South Pole and in the Chilean Atacama Desert instrumented with more than 500,000 cryogenically-cooled superconducting detectors, it will exceed the capabilities of earlier generations of experiments by more than an order of magnitude and deliver transformative discoveries in fundamental physics, cosmology, astrophysics and astronomy.

The CMB-S4 challenge

Three major challenges must be addressed to study the CMB at such levels of precision. Firstly, the signals are extraordinarily faint, requiring massive datasets to reduce the statistical uncertainties. Secondly, we have to contend with systematic effects both from imperfect instruments and from the environment, which must be controlled to exquisite precision if they are not to swamp the signals. Finally, the signals are obscured by other sources of microwave emission, especially galactic synchrotron and dust emission. Unlike the CMB, these sources do not have a black-body spectrum, so it is possible to distinguish between CMB and non-CMB sources if observations are made at enough microwave frequencies to break the degeneracy.

Power spectra of the CMB — **Power spectra** The temperature and polarisation power spectra of the CMB, illustrating features that can answer key questions in cosmology and fundamental physics. The CMB polarisation is decomposed into a curl-free E-mode and divergence-free B-mode by analogy with electromagnetism, with r quantifying the scalar-to-tensor ratio (the size of the B-modes relative to that of the temperature power spectrum). Credit: J Borrill

This third challenge actually proves to be an astrophysical blessing as well as a cosmological curse: CMB observations are also excellent legacy surveys of the millimetre-wave sky, which can be used for a host of other science goals. These range from cataloguing galaxy clusters, to studying the Milky Way, to detecting spatial and temporal transients such as gamma-ray bursts via their afterglows.

Coming together

In 2013 the US CMB community came together in the Snowmass planning process, which informs the deliberations of the decadal Particle Physics Project Prioritization Panel (P5). We realised that achieving the sensitivity needed to make the next leap in CMB science would require an experiment of such magnitude (and therefore cost) that it could only be accomplished as a community-wide endeavour, and that we would therefore need to transition from multiple competing experiments to a single collaborative one. By analogy with the US dark-energy programme, this was designated a “Stage 4” experiment, and hence became known as CMB-S4.

In 2014 a P5 report made the critical recommendation that the DOE should support CMB science as a core piece of its programme. The following year a National Academies report identified CMB science as one of three strategic priorities for the NSF Office of Polar Programs. In 2017 the DOE, NSF and NASA established a task force to develop a conceptual design for CMB-S4, and in 2019 the DOE took “Critical Decision 0”, identifying the mission need and initiating the CMB-S4 construction project. In 2020 Berkeley Lab was appointed the lead laboratory for the project, with Argonne, Fermilab and SLAC all playing key roles. Finally, late last year, the long-awaited Astro2020 report unconditionally recommended CMB-S4 as a joint NSF and DOE project with an estimated cost of $650 million. With these recommendations in place, the CMB-S4 construction project could begin.

**Constraining inflation** Current (light blue and purple) and anticipated (red) CMB-S4 constraints on the scalar-to-tensor ratio r compared with the predictions of various inflationary models that naturally explain the observed value of the spectral “tilt” of the power spectrum, n_s = 0.965. The popular Starobinsky and Higgs inflation models are shown as grey and black circles. The lines show models with different masses of the inflaton in units of the Planck mass M_P and N_* is the number of e-folds. The corresponding inflation potentials φ_P all either polynomially or exponentially approach a plateau. Source: arXiv:1907.04473

From the outset, CMB-S4 was intended to be the first sub-orbital CMB experiment designed to reach specific critical scientific thresholds, rather than simply to maximise the science return under a particular cost cap. Furthermore, as a community-wide collaboration, CMB-S4 will be able to adopt and adapt the best of all previous experiments’ technologies and methodologies – including operating at the site best suited to each science goal. One third of the major questions and discovery areas identified across the six Astro2020 science panels depend on CMB observations.

The critical degrees of freedom in the design of any observation are the sky area, frequency coverage, frequency-dependent depth and angular resolution, and observing cadence. Having reviewed the requirements across the gamut of CMB science, four driving science goals have been identified for CMB-S4.

For the first time, the entire community is coming together to build an experiment defined by achieving critical science thresholds

The first is to test models of inflation via the primordial gravitational waves they naturally generate. Such gravitational waves are the only known source of a primordial B-mode polarisation signal. The size of these primordial B-modes is quantified by the ratio of their power to that of the temperature power spectrum – the scalar-to-tensor ratio, designated r. For the largest and most popular classes of inflationary models, CMB-S4 will make a 5σ detection of r, while failure to make such a measurement will put an upper limit of r ≤ 0.001 at 95% confidence, setting a rigorous constraint on alternative models (see “Constraining inflation” figure). The large-scale B-mode polarisation signal encoding r is the faintest of all the CMB signals, requiring both the deepest measurement and the widest low-resolution frequency coverage of any CMB-S4 science case.

The second goal concerns the dark universe. Dark matter and dark energy make up 95% of the universe’s mass-energy content, and their particular form and composition impact the growth of structure and thus the small-scale CMB anisotropies. The collective influence of the three known light-relic particles (the Standard Model neutrinos) has already been observed in CMB data, but many new light species, such as axion-like particles and sterile neutrinos, are predicted by extensions of the Standard Model. CMB-S4’s goal, and the most challenging measurement in this arena, is to detect any additional light-relic species with freeze-out temperatures up to the QCD phase-transition scale. This corresponds to constraining the uncertainty on the number of light-relic species N_eff to ≤ 0.06 at 95% confidence (see “Light relics” figure). Precise measurements of the small-scale temperature and E-mode polarisation signals that encode this signal require the largest sky area of any CMB-S4 science case. In addition, since the sum of the masses of the neutrinos impacts the degree of lensing of the E-mode polarisation into small-scale B-modes, CMB-S4 will be able to constrain this sum around a fiducial value of 58 meV with a 1σ uncertainty ≤ 24 meV (in conjunction with baryon acoustic oscillation measurements) and ≤ 14 meV with better measurements of the optical depth to reionisation.

Current and anticipated CMB-S4 constraints — **Light relics** Current (black) and anticipated (magenta) CMB-S4 constraints on the effective number of light-relic species N_eff = N_SMeff + ∆N_eff, with N_SMeff = 3.045 from neutrinos. The plot shows the contributions of a single massless particle (which decoupled from the SM at freeze-out temperature T_F) to N_eff, with the displayed values on the right indicating observational thresholds for particles with different spins. Source: arXiv:1907.04473

The third science goal is to understand the formation and evolution of galaxy clusters, and in particular to probe the early period of galaxy formation at redshifts z > 2. This is enabled by the Sunyaev–Zel’dovitch (SZ) effect, whereby CMB photons are up-scattered by the hot, moving gas in the intra-cluster medium. This shifts the CMB photons’ frequency spectrum, resulting in a decrement at frequencies below 217 GHz and an increment at frequencies above, therefore allowing clusters to be identified by matching up the corresponding cold and hot spots. A key feature of the SZ effect is its red-shift independence, allowing us to generate complete, flux-limited catalogues of clusters to the survey sensitivity. The small-scale temperature signals needed for such a catalogue require the highest angular resolution and the widest high-resolution frequency coverage of all the CMB-S4 science cases.

Finally, CMB-S4 aims to explore the mm-wave transient sky, in particular the rate of gamma-ray bursts to help constrain their mechanisms (a few hours to days after the initial event, gamma-ray bursts are observable at longer wavelengths). CMB-S4 will be so sensitive that even its daily maps will be deep enough to detect mm-wave transient phenomena – either spatial from nearby objects moving across our field, or temporal from distant objects exploding in our field. This is the only science goal that places constraints on the survey cadence, specifically on the lag between repeated observations of the same point on the sky. Given its large field of view, CMB-S4 will be an excellent tool for serendipitous discovery of transients but less useful for follow-up observations. The plan is therefore to issue daily alerts for other teams to follow up with targeted observations.

Survey design

While it would be possible to meet all of the CMB-S4 science goals with a single survey, the result – requiring the sensitivity of the inflation survey across the area of the light-relic survey – would be prohibitively expensive. Instead, the requirements have been decoupled into an ultra-deep, small-area survey to meet the inflation goal and a deep, wide-area survey to meet the light-relic goal, the union of these providing a two-tier “wedding cake” survey for the cluster and gamma-ray-burst goals.

Having set the survey requirements, the task was to identify sites at which these observations can most efficiently be made, taking into account the associated cost, schedule and risk. Water vapour is a significant source of noise at microwave frequencies, so the first requirement on any site is that it be high and dry. A handful of locations meet this requirement, and two of them – the South Pole and the high Chilean Atacama Desert – have both exceptional atmospheric conditions and long-standing US CMB programmes. Their positions on Earth also make them ideally suited to CMB-S4’s two-survey strategy: the polar location enables us to observe a small patch of sky continuously, minimising the time needed to reach the required observation depth, and the more equatorial Chilean location enables observations over a large sky area.

CMB-S4 observatory telescopes — **Looking up** The full facility will employ 18 0.5 m small-aperture telescopes (top left), three per mount, fielding 150,000 detectors; one 5 m large-aperture telescope (right) fielding 130,000 detectors; and two 6 m large-aperture telescopes fielding 275,000 detectors (bottom left). Credit: CMB-S4

Finally, we know that instrumental systematics will be the limiting factor in resolving the extraordinarily faint large-scale B-mode signal. To date, the experiments that have shown the best control of such systematics have used relatively small-aperture (~0.5 m) telescopes. However, the secondary lensing of the much brighter E-mode signal to B-modes, while enabling us to measure the neutrino-mass sum, also obscures the primordial B-mode signal coming from inflation. We therefore need a detailed measurement of this medium- to small-scale lensing signal in order to be able to remove it at the necessary precision. This requires larger, higher-resolution telescopes. The ultra-deep field is therefore itself composed of coincident low- and high-resolution surveys.

A key feature of CMB-S4 is that all of the technologies are already well-proven by the ongoing Stage 3 experiments. These include CMB-S4’s “founding four” experiments, the Atacama Cosmology Telescope (ACT) and POLARBEAR/Simons Array (PB/SA) in Chile, and BICEP/Keck (BK) and the South Pole Telescope (SPT) at the South Pole, which have pairwise-merged into the Simons and South Pole Observatories (SO and SPO). The ACT, PB/SA, BK and SPT are all single-aperture, single-site experiments, while SO and SPO are dual-aperture, single sites. CMB-S4 is therefore the first experiment able to take advantage of both apertures and both sites.

The key difference with CMB-S4 is that it will deploy these technologies on an unprecedented scale. As a result, the primary challenges for CMB-S4 are engineering ones, both in fabricating detector and readout modules in huge numbers and in deploying them in cryostats on telescopes with unprecedented systematics control. The observatory will comprise: 18 small-aperture refractors collectively fielding about 150,000 detectors across eight frequencies for measuring large angular scales; one large-aperture reflector with about 130,000 detectors across seven frequencies for measuring medium-to-small angular scales in the ultra-deep survey from the South Pole; and two large-aperture reflectors collectively fielding about 275,000 detectors across six frequencies for measuring medium-to-small angular scales in the wide-deep survey from Chile (see “Looking up” image). The final configuration maximises the use of available atmospheric windows to control for microwave foregrounds (particularly synchrotron and dust emission at low and high frequencies, respectively), and to meet the frequency-dependent depth and angular-resolution requirements of the surveys.

CMB-S4 will be able to adopt and adapt the best of all previous experiments’ technologies and methodologies

Covering the frequency range 20–280 GHz, the detectors employ dichroic pixels at all but one frequency (to maximise the use of the available focal plane) using superconducting transition-edge sensors, which have become the standard in the field. A major effort is already underway to scale up the production and reduce the fabrication variance of the detectors, taking advantage of the DOE national laboratories and industrial partners. Reading out such large numbers of detectors with limited power is a significant challenge, leading CMB-S4 to adopt the conservative but well-proven time-domain multiplexing approach. The detector and readout systems will be assembled into modules that will be cryogenically cooled to 100 mK to reduce instrument noise. Each large-aperture telescope will carry an 85-tube cryostat with a single wafer per optics tube; and each small-aperture telescope will carry a single optics tube with 12 wafers per tube, with three telescopes sharing a common mount.

Prototyping of detector and readout fabrication lines, and building up module assembly and testing capabilities, is expected to begin in earnest this year. At the same time, the telescope designs will be refined and the data acquisition and management subsystems developed. The current schedule sees a staggered commissioning of the telescopes in 2028–2030, and operations running for seven years thereafter.

Shifting paradigms

CMB-S4 represents a paradigm shift for sub-orbital CMB experiments. For the first time, the entire community is coming together to build an experiment defined by achieving critical science thresholds in fundamental physics, cosmology, astrophysics and astronomy, rather than by its cost cap. CMB-S4 will span the entire range of CMB science in a single experiment, take advantage of the best of all worlds in the design of its observation and instrumentation, and make the results available to the entire CMB community. As an extremely sensitive, two-tiered, multi-wavelength, mm-wave survey, it will also play a key role in multi-messenger astrophysics and transient science. Taken together, these measurements will constitute a giant leap in our study of the history of the universe.

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