Physics Archives – Page 39 of 147

Quark–gluon plasma narrows jets

**Fig. 1.** Hadrons from a jet (ovals) are reclustered into a tree structure, and the low-energy, wide-angle particles are removed in order to identify the first “hard” splitting, shown in dark purple. Credit: CERN

A new measurement by the ALICE collaboration has demonstrated for the first time that jets become narrower after “quenching” in quark–gluon plasma (QGP). RHIC and LHC data show that the QGP behaves like a strongly-coupled liquid with very low viscosity, but it is an open question how this arises from the asymptotic limit of weakly-coupled quarks and gluons at short lengths. The new results provide quantitative new insights into the hot and dense medium created in heavy-ion collisions and how it modifies the substructure of jets and dissipates part of their energy.

An important property of the QGP is its ability to “resolve” nearby partons as effectively independent colour charges above the medium’s characteristic resolution scale – a parameter that is very poorly predicted by theory, but thought to be in the vicinity of a femtometre or less. In recent years, jet quenching has been proposed to determine this scale. Jets originate from a single quark or gluon that showers into more partons, either by radiating a gluon or splitting into a quark–antiquark pair. When a jet moves through the medium, each individual splitting results in two distinct colour charges that, depending on their angular separation and the medium’s resolution length, can interact as one coherent object or two independent charges. At the LHC, we can put our understanding of this resolution scale to test using special measurements of the angular structure of jets. This allows us to test whether wider jets are more likely to be resolved.

ALICE “groomed” jets using track clustering

The angle between the two prongs of the “hard” splitting — **Fig. 2.** The distribution of the angle between the two prongs of the “hard” splitting measured by ALICE, in both proton–proton and Pb–Pb collisions. The lower panel shows the Pb–Pb/p–p ratio in comparison with theoretical models. Credit: CERN

To identify the relevant two-prong splittings, ALICE “groomed” jets using track clustering. The algorithm reclusters and unwinds the jet shower to find the first parton splitting satisfying a grooming condition (figure 1). The excellent tracking resolution in ALICE allows for very precise measurements of jet substructure even at small angular distance scales. The angular width of the jet was found to be significantly modified in Pb–Pb compared to pp collisions (figure 2). In particular, wider splittings are suppressed in Pb–Pb compared to pp collisions, demonstrating that the interaction of jets with the QGP filters out wide jets.

This measurement is the first of its kind to be fully corrected for large background effects, allowing direct quantitative comparisons with theoretical calculations of jet quenching. Most theoretical models describe the general narrowing trend seen in the data, despite the different implementations of jet-medium interactions. The data is consistent with models implementing an incoherent interaction in which the medium resolves the splittings (Pablos, L_res = 0). Interestingly, however, another calculation demonstrates this narrowing effect with a fully coherent interaction, in which the jet splittings are not resolved, but by modifying the initial quark and gluon fractions (Yuan, quark). While the precision of the data currently precludes a precise extraction of the medium’s resolving power within a given model, the measurement places quantitative constraints on medium properties, and demonstrates for the first time a direct modification to the angular structure of jets in heavy-ion collisions. This opens the door to increasingly precise measurements with the high-precision data anticipated in LHC Run 3.

LHCb studies the intrinsic charm of the proton

**Fig. 1.** gc → Zc scattering is sensitive to valence-like intrinsic charm. Credit: CERN

The possibility that the proton wave function may contain a |uudcc> component in addition to the g → cc splitting arising from perturbative gluon radiation has been debated for decades. In favour of such “intrinsic charm” (IC), light-front QCD (LFQCD) calculations predict that non-perturbative IC manifests as percent-level valence-like charm content in the parton distribution functions (PDFs) of the proton. On the other hand, if the charm–quark content is entirely perturbative in nature, the charm PDF should resemble that of the gluon and decrease sharply at large momentum fractions, x. The proton could also contain intrinsic beauty, but suppressed by a factor of order m²_c/m²_b. The picture for intrinsic strangeness is somewhat murkier due to the lighter mass of the strange quark.

Measurements of charm-hadron production in deep-inelastic scattering and in fixed-target experiments, with typical momentum transfers below Q = 10 GeV, have been interpreted as evidence both for and against the IC predicted by LFQCD. Even though such experiments are in principle sensitive to valence-like c-quark content, interpreting low-Q data is challenging since it requires a careful theoretical treatment of hadronic and nuclear effects. Recent global PDF analyses, which also include measurements by ATLAS, CMS and LHCb, are inconclusive and can only exclude a relatively large IC component carrying more than a few percent of the momentum of the proton.

Using its Run-2 data, LHCb recently studied IC by making the first measurement of the fraction of Z+jet events that contain a charm jet in the forward region of proton–proton collisions. Since Zc production is inherently at large Q, above the electroweak scale, hadronic effects are small. A leading-order Zc production mechanism is gc → Zc scattering (figure 1), where in the forward region one of the initial partons must have large x, hence Zc production probes the valence-like region.

**Fig. 2.** The measured production cross-section ratio (grey bands) for three intervals of forward Z rapidity, compared to theory predictions without IC (purple circles), with the charm PDF shape allowed to vary, hence permitting IC (orange squares), and with IC as predicted by LFQCD with a mean momentum fraction of 1% (red triangles). The predictions are offset in each interval to improve visibility. Credit: CERN

The spectrum observed by LHCb exhibits a sizable enhancement at forward Z rapidities (figure 2), consistent with the effect expected if the proton wave function contains the |uudcc> component predicted by LFQCD. Incorporating these results into global PDF analyses should strongly constrain the large-x charm PDF, both in size and shape – and could reveal that the proton contains valence-like intrinsic charm.

These results demonstrate the unique sensitivity of the LHCb experiment to the valence-like content of the proton. Looking forward to Run 3, increased luminosity will lead to a substantial improvement in the precision of this measurement, which should provide an even clearer picture of just how charming the proton is.

Muon detector probes long-lived particles

New ways to detect long-lived particles (LLPs) are opening up avenues for searching for physics beyond the Standard Model (SM). LLPs could provide evidence for a hidden dark sector of particles that includes dark-matter candidates and could be studied via “portal interactions” with the visible universe. By employing the CMS experiment’s muon spectrometer in a novel way, the collaboration has recently deployed a powerful new technique for detecting LLPs that decay between 6 and 10 metres from the primary interaction point.

An LLP decaying in the endcap muon spectrometer volume should produce a particle shower when its decay products interact with the return yoke of the CMS solenoid. The secondary particles produced by the shower would traverse the gaseous regions of the cathode-strip chamber (CSC) detector and produce a large multiplicity of signals on the wire anodes and strip cathodes. Localised hits are reconstructed by combining these signals using a density-based clustering algorithm. This is the first time the CSC detectors have been used as a sampling calorimeter to try to detect and identify LLP decays.

**Fig. 1.** Distributions of the number of hits in the cluster (N_hits) in the search region. Signal models where the Higgs boson plays the role of the dark-portal messenger are shown (dashed lines). The Higgs boson is assumed to decay to dark-sector scalar particles with masses of 7 (red), 15 (orange), 40 (cyan) or 55 GeV (purple), while the dark scalar decays with a macroscopic displacement to quarks. Credit: CERN

Searching for CSC clusters with a sufficiently large number of hits suppresses background processes while maintaining a high efficiency for detecting potential LLP decays. The large amount of steel in the CMS return yoke nearly eliminates “punch-through” hadrons that are not fully stopped by the calorimeter, potentially mimicking the signature of an LLP. The largest remaining source of backgrounds is known LLPs produced by SM processes such as the neutral kaon, K_L. These particles are copiously produced in LHC collisions and, on rare occasions, traverse the material without being stopped. Kaons are predominantly produced with much lower energies than the signal LLPs and therefore result in clusters with a smaller number of hits. Requiring clusters with more than 130 CSC hits suppresses these dominant background events to a negligible level (see figure 1).

This search improves on the previous best results by more than a factor of six

Using the full Run-2 dataset, the CMS collaboration detected no excess of particle-shower events above the expected backgrounds, setting constraints on a benchmark-simplified model of scalar LLP production mediated by the Higgs boson (a so-called Higgs portal model). This search improves on the previous best results by more than a factor of six (two) for an LLP mass of 7 GeV (≥ 15) GeV for a proper decay length (cτ) of the scalar larger than 100 m. It is the first to be sensitive to LLP decays with cτ up to 1000 m and masses between 40 and 55 GeV at branching ratios of the Higgs to a pair of LLPs below 20%.

This novel approach to identifying showers in muon detectors opens up an exciting new programme of searches for LLPs in a wide variety of theoretical models. Potential frameworks range from Higgs-portal models to other portals to a dark sector, including neutrinos, axions and dark photons. The on-going development of a dedicated Level-1 and High-Level Trigger focusing on particle showers detected in the CMS muon spectrometer promises an order of magnitude improvement in the discovery sensitivity for LLPs in the forthcoming run of the LHC.

Star-forming galaxies rule gamma-ray sky

Five-year sky map — **Diffuse sky** Fermi–LAT’s five-year sky map, indicating the various diffuse components that make up 80% of all photons detected. Credit: NASA

The diffuse photon background that fills the universe does not limit itself to the attention-hogging cosmic microwave background, but spans a wide spectrum extending up to TeV energies. The origin of the photon emission at X-ray and gamma-ray wavelengths, first discovered in the 1970s, remains poorly understood. Many possible sources have been proposed, ranging from active galactic nuclei to dark-matter annihilation. Thanks to many years of gamma-ray data from the Fermi Large Area Telescope (Fermi-LAT), a group from Australia and Italy has now produced a model that links part of the diffuse emission to star-forming galaxies (SFGs).

As their name implies, SFGs are galaxies in which stars are formed, and therefore also die through supernova events. Such sources, which include our own Milky Way, have gained interest from gamma-ray astronomers during the past decade because several resolvable SFGs have been shown to emit in the 100 MeV to 1 TeV energy range. Given their preponderance, SFGs are thus a prime-suspect source of the diffuse gamma-ray background.

Clear correlation

The source of gamma rays within SFGs is very likely the interaction between cosmic rays and the interstellar medium (ISM). The cosmic rays, in turn, are thought to be accelerated within the shockwaves of supernova remnants, after which they interact with the ISM to produce a hadronic cascade. The cascade includes neutral pions, which decay into gamma rays. This connection between supernova remnants and gamma rays is strengthened by a clear correlation between the star-formation rate in a galaxy and the gamma-ray flux they emit. Additionally, such sources are theorised to be responsible for the neutrino emission detected by the IceCube observatory over the past few years, which also appears to be highly isotropic.

Gamma-ray background model — **Good fit** The observed gamma-ray background (black points) compared to the new model prediction (thick red line) and its various components (DIGB, diffuse isotropic γ-ray background; EBL, extragalactic background light). Source: arXiv.org:2109.07598

Based on additional SFG gamma-ray sources found by Fermi–LAT, which could be used for validation, the Australian/Italian group developed a physical model to study the contribution of SFGs to the cosmic diffuse gamma-ray background. The model used to predict the gamma-ray emission from galaxies starts with the spectra of charged cosmic-rays produced in the numerous supernovae remnants within a galaxy, and greatly benefits from data collected from several such remnants present in the Milky Way. Subsequently the production and energies of gamma rays through their interaction of cosmic rays with the ISM is modelled, followed by the gamma-ray transport to Earth, which includes losses due to interactions with low-energy photons leading to pair production.

The main uncertainty in previous models was the efficiency of a galaxy to transform the energy from cosmic rays into gamma rays, since it is not possible to use our own galaxy to measure it. The big breakthrough in the new work is a more thorough theoretical modelling of this efficiency, which was first tested extensively using data from resolved SFG sources. After such tests proved successful, the model could be applied to predict the gamma-ray emission properties of galaxies spanning the history of the universe. These predictions indicate that the low-energy part of the spectrum can be largely attributed to galaxies from the so-called cosmic noon: the period when star formation in large galaxies was at its peak, about 10 billion years ago. Nearby galaxies, on the other hand, explain the high-energy part of the spectrum, which, for old and distant sources, is absorbed in the intergalactic medium by low-energy photons undergoing pair production with TeV emission. Overall, the model predicts not only the spectral shape but also the overall flux (see “Good fit” figure), negating the need for other possible sources such as active galactic nuclei or dark matter.

These new results once again indicate the importance of star-forming regions for astrophysics, after also recently being proposed as a possible source of PeV cosmic rays by LHAASO (CERN Courier July/August 2021 p11). Furthermore, it shows the potential for an expansion to other astrophysical messengers, with the authors stating their ambition to apply the same model to radio-emission and high-energy neutrinos.

MicroBooNE sees no hint of a sterile neutrino

The existence of an eV-scale sterile neutrino looks less likely today than at any time in the past 20 years. Such a particle has long been considered to be the simplest explanation for several related anomalies in neutrino physics, but results released yesterday by Fermilab’s MicroBooNE collaboration disfavour its existence relative to the Standard Model.

“MicroBooNE has made a very comprehensive exploration through multiple types of interactions, and multiple analysis and reconstruction techniques,” says co-spokesperson Bonnie Fleming of Yale. “They all tell us the same thing, and that gives us very high confidence in our results that we are not seeing a hint of a sterile neutrino.”

The collaboration says that the analyses favour the Standard Model over the anomalous signal seen by sibling-experiment MiniBooNE at more than 99% confidence, should its true origin be electrons from a neutrino oscillation via a hitherto-undetected sterile neutrino. “But that earlier data from MiniBooNE doesn’t lie,” says former co-spokesperson Sam Zeller of Fermilab. “There’s something really interesting happening that we still need to explain.”

There’s something really interesting happening that we still need to explain
Sam Zeller

Neutrinos suffer from an identity crisis regarding their mass. As a result, the three known flavours morph into each other as phase differences develop between three mass eigenstates. However, well before this model solidified around the turn of the millennium, a measurement by the LSND collaboration at Los Alamos in the US suggested the existence of an additional neutrino which had to be “sterile” with respect to the weak, electromagnetic and strong interactions, and much more massive, given how rapidly the oscillation developed. Since this first hint, the tale of the sterile neutrino has taken multiple twists and turns.

Twists and turns

In the mid-1990s, LSND reported seeing a 3.8σ excess of electron antineutrinos in a beam of accelerator-generated muon antineutrinos, but the KARMEN experiment at the Rutherford Appleton Laboratory in the UK failed to reproduce the effect. Evidence for an eV-scale sterile neutrino mounted with the observation of a deficit of electron neutrinos from ³⁷Ar and ⁵¹Cr electron-capture decays at Gran Sasso in Italy and at the Baksan Neutrino Observatory in Russia (the gallium anomaly), and a reported deficit of electron antineutrinos from nuclear reactors (the reactor anomaly). Troublingly, however, long-baseline accelerator neutrino experiments such as MINOS+ do not observe the requisite “disappearance” of muon neutrinos required by the principle of unitarity, and the existence of such a sterile neutrino is also starkly incompatible with current models of cosmology. While the gallium anomaly should soon be probed definitively by the BEST experiment at Baksan (Phys. Rev. D 2018 97 073001), recent calculations of reactor fluxes may now be dissolving the reactor anomaly (see, for example, arXiv:2110.06820). But the most compelling single piece of evidence in favour of sterile neutrinos came when the MiniBooNE experiment at Fermilab tried to reproduce the LSND effect. In November 2018, the collaboration reported a 4.5σ excess of electron neutrinos and antineutrinos compared to Standard-Model expectations.

Few neutrino physicists foresaw that MicroBooNE would disfavour both hypotheses

Sibling experiment MicroBooNE has now released its first round of tests of the MiniBooNE anomaly. Equipped with a cutting-edge liquid-argon time-projection chamber, the collaboration observed neutrino interactions at the level of individual particle tracks – a key advantage compared to a Cherenkov detector such as MiniBooNE, which could not distinguish electrons from photons. The collaboration has now used half of its available data to probe which particle is the true origin of the anomaly. Earlier this month, MicroBooNE tested the hypothesis that MiniBooNE’s excess was actually due to an underestimated single-photon background, perhaps caused by a difficult-to-model rare decay of a Δ resonance. Now, MicroBooNE has tested the hypothesis that the MiniBooNE excess was caused by single electrons, most likely the result of neutrino oscillations via an eV-scale sterile neutrino. Few neutrino physicists foresaw that MicroBooNE would disfavour both hypotheses.

“Every time we look at neutrinos, we seem to find something new or unexpected,” says MicroBooNE co-spokesperson Justin Evans of the University of Manchester. “MicroBooNE’s results are taking us in a new direction, and our neutrino programme is going to get to the bottom of some of these mysteries.” The collaboration will now investigate whether more exotic topologies such as electron-positron pairs could be the source of the MiniBooNE anomaly. Such a final state might suggest the existence of heavier sterile neutrinos, say theorists.

“eV-scale sterile neutrinos no longer appear to be experimentally motivated, and never solved any outstanding problems in the Standard Model,” says theorist Mikhail Shaposhnikov of EPFL. “But GeV-to-keV-scale sterile neutrinos – so-called Majorana fermions – are well motivated theoretically and do not contradict any existing experiment. They can explain neutrino masses and oscillations, give a dark-matter candidate, and produce a baryon asymmetry in the universe: all the problems that the Standard Model is incapable of addressing. Experimental efforts at the intensity frontier should now be concentrated in this direction.”

Fermilab: a future built on international engagement

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Future scientific breakthroughs in high-energy physics will require unprecedented levels of international engagement, building on the successful model of the Large Hadron Collider at CERN. Joe Lykken, Fermilab deputy director for research, will describe how Fermilab is moving forward rapidly with CERN and other international partners to realise this vision.

The questions under scrutiny range from the nature of the Higgs field to the question of whether neutrinos play a role in the matter-antimatter asymmetry observed in the universe. PIP-II, an upgrade to the Fermilab accelerator complex that includes a leading-edge superconducting linear accelerator, is already under construction, with major “in-kind” contributions and expertise from partners in India, Italy, the UK, France and Poland. PIP-II will enable the world’s most intense beam of neutrinos for the Deep Underground Neutrino Experiment (DUNE), which will deploy 70,000 tonnes of liquid argon detectors in a deep underground site 1300 km from Fermilab. DUNE was formulated as an international project from the start, and now includes more than a thousand collaborators from 30 countries. Two large prototype detectors for DUNE have been successfully constructed and tested at the CERN Neutrino Platform. DUNE will have remarkable capabilities to determine how the properties of neutrinos have shaped our universe. At the same time, Fermilab has been developing and building next-generation superconducting magnets that will be deployed in the HL-LHC accelerator at CERN, and is the US lead for ambitious upgrades to the CMS experiment for the HL-LHC era. These technological capabilities will also make Fermilab an important partner for the proposed Future Circular Collider.

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Joseph Lykken is Fermilab’s deputy director of research and leads the Fermilab Quantum Institute. A distinguished scientist at the laboratory, Lykken was a former member of the Theory Department, researching string theory and phenomenology, and is a member of the CMS experiment on the Large Hadron Collider at CERN. He received his PhD from the Massachusetts Institute of Technology and has previously worked for the Santa Cruz Institute for Particle Physics and the University of Chicago. Lykken began his tenure at Fermilab in 1989. He is a former member of the High Energy Physics Advisory Panel, which advises both the Department of Energy and the National Science Foundation, and served on the Particle Physics Project Prioritization Panel, developing a road map for the next 20 years of US particle physics. Lykken is a fellow of the American Physical Society and of the American Association for the Advancement of Science.

LHCb tests lepton universality in new channels

Measurements of the ratios of muon to electron decays by LHCb (purple) and previous measurements from Belle (red). Source: *Phys. Rev. Lett.* **126** 161801 and arXiv:2110.09501

At a seminar at CERN today, the LHCb collaboration presented new tests of lepton universality in rare B-meson decays. While limited in statistical sensitivity, the results fit an intriguing pattern of recent results in the flavour sector, says the collaboration.

Since 2013, several measurements have hinted at deviations from lepton-flavour universality (LFU), a tenet of the Standard Model (SM) which treats charged leptons, ℓ, as identical apart from their masses. The measurements concern decay processes involving the transition between a bottom and a strange quark b→sℓ⁺ℓ^–, which are strongly suppressed by the SM because they involve quantum corrections at the one-loop level (leading to branching fractions of one part in 10⁶ or less). A powerful way to probe LFU is therefore to measure the ratio of B-meson decays to muons and electrons, for which the SM prediction, close-to-unity, is theoretically very clean.

In March this year, an LHCb measurement of R_K = BR(B⁺→K⁺μ⁺μ^–)/BR(B⁺→K⁺e⁺e^–) based on the full LHC Run 1 and 2 dataset showed a 3.1σ difference from the SM prediction. This followed departures at the level of 2.2—2.5σ in the ratio R_K*⁰ (which probes B⁰→K*⁰ℓ⁺ℓ^– decays) reported by LHCb in 2017. The collaboration has also seen slight deficits in the ratio R_pK, and departures from theory in measurements of the angular distribution of final-state particles and of branching fractions in neutral B-meson decays. None of the results is individually significant enough to constitute evidence of new physics. But taken together, say theorists, they point to a coherent pattern.

We are seeing a similar deficit of rare muon decays to rare electron decays that we have seen in other LFU tests
Harry Cliff

The latest LHCb analysis clocked the ratio of muons to electrons in the isospin-partner B-decays: B⁰→ K_S⁰ℓ⁺ℓ^– and B⁺→K*⁺ℓ⁺ℓ^–. As well as being a first at the LHC, it’s the first single-experiment observation of these decays, and the most precise measurement yet of their branching ratios. Being difficult to reconstruct due to the presence of a long-lived K_S⁰in the final state, however, the sensitivity of the results is lower than for previous “R_K” analyses. LHCb found R(K_S⁰) = 0.66+0.2/-0.15 (stat) +0.02/-0.04 (syst) and R(K*⁺) = 0.70+0.18/-0.13 (stat) +0.03/-0.04 (syst), which are consistent with the SM at the level of 1.5 and 1.4σ, respectively.

“What is interesting is that we are seeing a similar deficit of rare muon decays to rare electron decays that we have seen in other LFU tests,” said Harry Cliff of the University of Cambridge, who presented the result on behalf of LHCb (in parallel with a presentation at Rencontres de Blois by Cambridge PhD student John Smeaton). “With many other LFU tests in progress using Run 1 and 2 data, there will be more to come on this puzzle soon. Then we have Run 3, where we expect to really zoom in on the measurements and obtain a detailed understanding.”

The experimental and theoretical status of the flavour anomalies in b→sℓ⁺ℓ and semi-leptonic B-decays will be the focus of the Flavour Anomaly Workshop at CERN on Wednesday 20 October, at which ATLAS and CMS activities will also be discussed, along with perspectives from theorists.

Gauge–boson polarisation observed in WZ production

**Fig. 1.** Left: the measured WZ production cross section in various final states compared to predictions at different orders in perturbative QCD and electroweak theory (EWK). Right: measured polarisation fractions for the W boson in WZ production, compared with the Standard Model expectation (◊). Credit: CERN

At the collision energies of the LHC, diboson processes have relatively high production cross sections and often produce relatively clean final states with two or more charged leptons. Consequently, multilepton final states resulting from diboson processes are powerful signatures to study the properties of the electroweak sector of the Standard Model. In particular, WZ production is sensitive to the strength of the triple gauge coupling that characterises the WWZ vertex, which derives from the non-Abelian nature of the electroweak sector. Additionally, as the Higgs mechanism is responsible for the appearance of longitudinally polarised gauge bosons, studying W and Z boson polarisation indirectly probes the validity of the Higgs mechanism.

The results include the first observation at any experiment of longitudinally polarised W bosons in diboson production

A recent result from the CMS collaboration uses the full power of the data taken during Run 2 of the LHC to learn as much as possible from WZ production in the decay channels involving three charged leptons (electrons or muons). The results include the first observation at any experiment of longitudinally polarised W bosons in diboson production.

Reconstruction and event selection were optimised to reduce contributions from processes with “non-isolated” electrons and muons produced in hadron decays – traditionally one of the primary sources of experimental uncertainty in such measurements. The total production cross section for WZ production was measured with a simultaneous fit to the signal-enriched region and three different control regions. This elaborate fitting scheme paid off, as the final result has a relative uncertainty of 4%, down from the 6% obtained in past iterations of the measurement. The results are all consistent with state-of-the-art theoretical predictions (figure 1, left).

A highlight of the analysis is the study of the polarisation of both the W and the Z bosons in the helicity frame, using missing transverse energy as a proxy for the transverse momentum of the neutrino in the W decay. This choice, coupled with the precisely measured four-momenta of the three leptons and the requirement that W boson be on-shell, allows both the W and Z momenta to be fully reconstructed. The angle between the W (Z) boson and the (negatively) charged lepton originating from its decay is then computed. The resulting distributions are fitted to extract the polarisation fractions f_R, f_L, and f_o, which correspond to the proportion of bosons in the left, right and longitudinally polarised states in WZ production.

The measured polarisation fractions are consistent within 1σ with the Standard Model predictions (figure 1, right), in accordance with our knowledge of the electroweak spontaneous symmetry breaking mechanism. The significance for the presence of longitudinally polarised vector bosons is measured to be 5.6σ for the W boson and well beyond 5σ for Z-boson production. These new studies pave the way for future measurements of doubly polarised diboson cross sections, including the challenging doubly longitudinal polarisation mode in WW, WZ or ZZ production.

BICEP crunches primordial gravitational waves

The BICEP/Keck collaboration has published the strongest constraints to date on primordial gravitational waves, ruling out parameter space for models of inflation in the early universe (Phys. Rev. Lett. 2021 127 151301). A conjectured rapid expansion of the universe during the first fraction of a second of its existence, inflation was first proposed in the early 1980s to explain the surprising uniformity of the universe over scales which should not otherwise have been connected, and may have left an imprint in the polarisation of the cosmic-microwave background (CMB). Despite a high-profile false detection of gravitational-wave-induced “B-modes” by BICEP in 2014, which was soon explained as a mis-modelling of the galactic-dust foreground, the search for primordial gravitational waves remains one of the most promising avenues to study particle physics at extremely high energies, as inflation is thought to require a particle-physics explanation such as the scalar “inflaton” field proposed by Alan Guth.

Certain ‘standard’ types of inflation are now clearly disfavoured
Kai Schmitz

In its latest publication, the BICEP/Keck collaboration has managed to significantly improve the upper bound on the strength of gravitational waves produced during the epoch of inflation. “This is important for theorists because it further constrains the allowed range of viable models of inflation, and certain ‘standard’ types of models are now clearly disfavoured,” explains CERN theorist Kai Schmitz. “It’s also a great experimental achievement because it demonstrates that the sources of systematic uncertainties such as dust emission in our Milky Way are under good control. That’s a good sign for future observations.”

The BICEP/Keck collaboration searches for the imprint of gravitational waves in the polarisation pattern of the CMB, emitted 380,000 years after the Big Bang. Telescopes at the South Pole receive incoming CMB photons and focus them through plastic lenses onto detectors in the focal plane which are cooled to 300 mK, explains principal investigator Clem Pryke of the University of Minnesota. As the telescopes scan the sky they record the tiny changes in temperature due to the intensity of the incoming microwaves. The detectors are arranged in pairs with each half sensitive to one of two orthogonal linear polarisation components. The telescopes take their best data during the six-month long Antarctic night, during which intrepid “winter-overs” maintain the detectors and upload data via satellite to the US for further analysis.

“The big change since 2014 was to make measurements in multiple frequency bands to allow the removal of the galactic foreground,” says Pryke. “Back then we had data only at 150 GHz and were relying on models and projections of the galactic foreground – models which turned out to be optimistic as far as the dust is concerned. Now we have super-deep maps at 95, 150 and 220 GHz allowing us to accurately remove the dust component.”

The current analysis uses data recorded by BICEP2, the Keck Array and BICEP3 up to 2018. Since then, the collaboration has installed a new more capable telescope platform called the BICEP Array designed to increase sensitivity to primordial gravitational waves by a factor of three, in collaboration with a large-aperture telescope at the South Pole called SPT3G. With 21 telescopes at the South Pole and in the Chilean Atacama desert, the proposed CMB Stage-4 project plans to improve sensitivity by a further factor of six in the 2030s.

MicroBooNE homes in on the sterile neutrino

Excitement is building in the search for sterile neutrinos – long-predicted particles which would constitute physics beyond the Standard Model. Although impervious to the electromagnetic, weak and strong interactions, such a fourth “right-handed” neutrino flavour could reveal itself by altering the rate of standard-neutrino oscillations – tantalising hints of which were reported by Fermilab’s MiniBooNE experiment in 2007. In a preprint published last week, sibling experiment MicroBooNE strongly disfavours a mundane explanation for such hints, with further scrutiny by the collaboration expected to be announced later this month.

“If the MiniBooNE effect is indeed a sterile neutrino, this of course would be a major discovery which would revolutionise particle physics, opening up a whole new sector to explore,” says MicroBooNE co-spokesperson Justin Evans of the University of Manchester.

The story of the sterile neutrino began in the 1990s, when the LSND experiment at Los Alamos reported seeing 88±23 (3.8σ) more electron antineutrinos than expected in a beam of accelerator-generated muon antineutrinos. This apparent short-baseline oscillation from muon to electron antineutrinos was incompatible with the oscillation rates established by Super-Kamiokande in 1998 and SNO in 2002, and would have to occur via an unknown intermediate neutrino flavour with a mass of about an electron-Volt. This hypothesised neutrino was dubbed sterile, as it would have to be insensitive to all interactions but gravity for it to have remained undiscovered this long.

The photon hypothesis

The plot thickened in 2007 when the MiniBooNE experiment at Fermilab tried to reproduce the LSND anomaly. The team also saw an excess of electron-like signals, though not quite at the energy corresponding to the LSND effect. The significance of the MiniBooNE anomaly grew to 4.5σ by the time the experiment finished running in November 2018. But a mundane possible explanation poured cold water on hopes for new physics: as a mineral-oil Cherenkov detector, MiniBooNE could not differentiate electrons from photons, and one particularly tricky-to-model background process might be contributing more photons than expected.

Many of us suspected that there could be something wrong with predictions for this background
Joachim Kopp

“High-energy single photons can be produced when a neutrino scatters on a nucleon via a neutral-current interaction and excites the nucleon to a Δ(1232) resonance,” explains CERN theorist Joachim Kopp. “Most of the time, the resonance decays to a pion and a nucleon, but there is a rare decay mode to a nucleon and a photon. The rate for this mode is very hard to predict, and many of us suspected that there could be something wrong with predictions for this background.”

Enter MicroBooNE, a liquid-argon time-projection-chamber sibling experiment to MiniBooNE which is capable of studying neutrino interactions in photographic detail, and differentiating the two signals. Having detected its first neutrino interactions in 2015, the MicroBooNE team has now set a limit on the neutral-current Δ→Nγ process is more than a factor of 50 better than existing constraints, explains Evans. “With this MicroBooNE result, we reject a Δ→Nγ model of the low-energy excess at 94.8% confidence, a strong indication that we must look elsewhere for the source of the excess.”

The electron hypothesis

Now that MicroBooNE has strongly disfavoured a leading-photon model for the MiniBooNE anomaly, attention shifts to the electron hypothesis – which would hint at the existence of a sterile neutrino, or something more exotic, if proven. And we don’t have long to wait. The MicroBooNE collaboration plans to release its search for an electron-like low-energy excess on 27 October, with results from three independent analyses looking at a range of inclusive and exclusive channels.

Beyond that, there is more to come, says Evans. “Our current round of results use only the first half of the total MicroBooNE data-set, and this is a programme that is only just beginning, with ICARUS and SBND within Fermilab’s short-baseline programme now coming online to turn this into a multi-baseline exploration of the richness of neutrino physics with unparalleled detail.”

The global picture is complex. In 2019, for example, the MINOS+ experiment failed to confirm the MiniBooNE signal (CERN Courier March/April 2019 p7). Were the sterile neutrino to exist, it should also have significant cosmological consequences which remain unobserved. But the anomalies are accumulating, says Kopp.

“LSND and MiniBooNE are quite consistent, and the short-baseline reactor experiments require parameters in the same region of parameter space, though these results are very much in flux and it’s not clear which ones are trustworthy, so it’s hard to make precise statements. The good news is that there’s realistic hope of resolving these puzzles over the next few years. ”

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Clear correlation

Twists and turns

The photon hypothesis

The electron hypothesis