Physics Archives – Page 26 of 147

Proton structure consists of three distinct regions

Researchers at Jefferson lab in the US have gained a deeper understanding of the role of gluons in providing mass to visible matter. Based on measurements of the photoproduction of J/ψ particles, the findings suggest that the proton’s structure has three distinct regions, with an inner core driven by gluonic interactions making up most of its mass.

Although the charge and spin of the proton have been extensively studied for decades, relatively little is known about its mass distribution. This is because gluons, which despite being massless provide a sizeable contribution to the proton’s mass, are neutral and thus cannot be studied directly using electromagnetic probes. The Jefferson team instead used the gluonic gravitational form factors (GFFs). Similar to electromagnetic form factors, which provide information about a hadron’s charge and magnetisation distributions, the GFFs (technically the matrix elements of the proton’s energy–momentum tensor) encode mechanical properties of the proton such as its mass, density, pressure and shear distributions.

To access the GFFs, the team measured the threshold cross section of exclusive J/ψ photoproduction at different energies by forcing photons with energies between 9.1 and 10.6 GeV to interact with a liquid hydrogen target. Gluons dominate the production of J/ψ at small momentum transfer since J/ψ mesons share no valence quarks with the proton. Due to the J/ψ’s vector quantum numbers, this process can occur at certain energies by gluons in scalar (dilaton-like) and tensor (graviton-like) states. The researchers fed their cross-section results into QCD models describing the gluonic GFFs and extracted the parameters defining the GFFs, enabling them to deduce one mass radius and one scalar radius.

We need a new generation of high-precision J/ψ experiments to get a better picture
Zein-Eddine Meziani

The analysis revealed a scalar proton radius of 1 fm, which is substantially larger than both the charge radius (around 0.85 fm) and the proton mass radius (0.75 fm). This led the team to propose that the proton structure consists of three distinct regions: an inner core that makes up most of the mass radius and is dominated by the tensor gluonic field structure, followed by the charge radius resulting from the relativistic motion of quarks, all enveloped in a larger confining scalar gluon density.

“Given that the proton’s scalar gluon radius is the largest we need to understand how this converts to our understanding of the gluonic structure of nuclei. For example, what would be the scalar radius of ⁴He compared to its charge radius?” says study leader Zein-Eddine Meziani of Argonne. The team plans to extend its studies to include the J/ψ muon final state decay, doubling the statistics of the current measurement, and to extract the gluon pressure distribution. “It is hard to say much right now, but this is a field in its infancy and the direct role of gluons in nuclei is not well understood,” adds Meziani. “We need a new generation of high-precision J/ψ experiments to get a better picture.”

Cold atoms for new physics

atom_interferometry_workshop_2023 — A planned network of MAGIS and AION very-long-baseline atom interferometers to detect gravitational waves and dark matter. Credit: University of Birmingham

On 13 and 14 March CERN hosted an international workshop on atom interferometry and the prospects for future large-scale experiments employing this quantum-sensing technique. The workshop had some 300 registered participants, of whom about half participated in person. As outlined in a keynote introductory colloquium by Mark Kasevich (Stanford), one of the pioneers of the field, this quantum sensing technology holds great promise for making ultra-sensitive measurements in fundamental physics. Like light interferometry, atom interferometry involves measuring interference patterns, but between atomic wave packets rather than light waves. Interactions between coherent waves of ultralight bosonic dark matter and Standard Model particles could induce an observable shift in the interference phase, as could the passage of gravitational waves.

Atom interferometry is a well-established concept that can provide exceptionally high sensitivity, e.g., to inertial/gravitational effects. Experimental designs take advantage of features used by state-of-the-art atomic clocks in combination with established techniques for building inertial sensors. This makes atom interferometry an ideal candidate to hunt for physics beyond the Standard Model such as waves of ultralight bosonic dark matter, or to measure gravitational waves in a frequency range around 1 Hz that is inaccessible to laser interference experiments on Earth, such as LIGO, Virgo and KAGRA, or the upcoming space-borne experiment LISA. As discussed during the workshop, measurements of gravitational waves in this frequency range could reveal mergers of black holes with masses intermediate between those accessible to laser interferometers, casting light on the formation of the supermassive black holes known to inhabit the centres of galaxies. Atom interferometer experiments can also explore the limits of quantum mechanics and its interface with gravity, for example by measuring a gravitational analogue of the Aharonov-Bohm effect.

A deep shaft at Point 4 of the LHC is a promising location for an atom interferometer with a vertical baseline of over 100 m

Although the potential of atom interferometers for fundamental scientific measurements was the principal focus of the meeting, it was also emphasised that technologies based on the same principles also have wide-ranging practical applications. These include gravimetry, geodesy, navigation, time-keeping and Earth observation from space, providing, for example, a novel and sensitive technique for monitoring the effects of climate change through measurements of the Earth’s gravitational field.

Several large atom interferometers with a length of 10m already exist, for example at Stanford University, or are planned, for example in Hanover (VLBAI), Wuhan and at Oxford University (AION). However, many of the proposed physics measurements require next-generation setups with a length of 100m, and such experiments are under construction at Fermilab (MAGIS), in France (MIGA) and in China (ZAIGA). The Atomic Interferometric Observatory and Network (AION) collaboration is evaluating possible sites in the UK and at CERN. In this context, a recent conceptual feasibility study supported by the CERN Physics Beyond Colliders study group concluded that a deep shaft at Point 4 of the LHC is a promising location for an atom interferometer with a vertical baseline of over 100 m. The March workshop provided a forum for discussing such projects, their current status, future plans and prospective sensitivities.

Looking further ahead, participants discussed the prospects for one or more km-scale atom interferometers, which would provide the maximal sensitivity possible with a terrestrial experiment to search for ultralight dark matter and gravitational waves. It was agreed that the global community interested in such experiments would work together towards establishing an informal proto-collaboration that could develop the science case for such facilities, provide a forum for exchanging ideas how to develop the necessary technological advances and develop a roadmap for their realisation.

A highlight of the workshop was a poster session that provided an opportunity for 30 early-career researchers to present their ideas and current work on projects exploiting the quantum properties of cold atoms and related topics. The liveliness of this session showed how this interdisciplinary field at the boundaries between atomic physics, particle physics, astrophysics and cosmology is inspiring the next generation of researchers. These researchers may form the core of the team that will lead atom interferometers to their full potential.

ATLAS increases precision on W mass

Latest ATLAS measurement — **On point** The latest ATLAS measurement compared to other published results, with the vertical band showing the Standard Model prediction and light- and dark-blue horizontal bands showing the statistical and total uncertainties of the results. Credit: CERN

Since the discovery of the W boson at the SppS 40 years ago, collider experiments at CERN and elsewhere have measured its mass ever more precisely. Such measurements provide a vital test of the Standard Model’s consistency, since the W mass is closely related to the strength of the electroweak interaction and to the masses of the Z boson, top quark and Higgs boson; higher experimental precision is needed to keep up with the most recent electroweak calculations.

The latest experiment to weigh in on the W mass is ATLAS. Reanalysing a sample of 14 million W candidates produced in proton–proton collisions at 7 TeV, the collaboration finds M_w = 80.360 ± 0.005(stat) ± 0.015(syst) = 80.360 ± 0.016 GeV. The value, which was presented on 23 March at the Rencontres de Moriond, is in agreement with all previous measurements except one – the latest measurement from the CDF experiment at the former Tevatron collider at Fermilab.

In 2017 ATLAS released its first measurement of the W-boson mass, which was determined using data recorded in 2011 when the LHC was running at a collision energy of 7 TeV (CERN Courier January/February 2017 p10). The precise result (80.370 ± 0.019 GeV) agreed with the Standard Model prediction (80.354 ± 0.007 GeV) and all previous experimental results, including those from the LEP experiments. But last year, the CDF collaboration announced an even more precise measurement, based on an analysis of its full dataset (CERN Courier May/June 2022 p9). The result (80.434 ± 0.009 GeV) differed significantly from the Standard Model prediction and from the other experimental results (see figure), calling for more measurements to try to identify the source of the discrepancy.

In its new study, ATLAS reanalysed its 2011 data sample using a more advanced fitting technique as well as improved knowledge of the parton distribution functions that describe how the proton’s momentum is shared amongst its constituent quarks and gluons. In addition, the collaboration verified the theoretical description of the W-production process using dedicated LHC proton–proton runs. The new result is 10 MeV lower than the previous ATLAS result and 15% more precise.

“Due to an undetected neutrino in the particle’s decay, the W-mass measurement is among the most challenging precision measurements performed at hadron colliders. It requires extremely accurate calibration of the measured particle energies and momenta, and a careful assessment and excellent control of modelling uncertainties,” says ATLAS spokesperson Andreas Hoecker. “This updated result from ATLAS provides a stringent test and confirms the consistency of our theoretical understanding of electroweak interactions.”

The LHCb collaboration reported a measurement of the W mass in 2021, while the results from CMS are keenly anticipated. In the meantime, physicists from the Tevatron+LHC W-mass combination working group are calculating a combined mass value using the latest measurements from the LHC, Tevatron and LEP. This involves a detailed investigation of higher-order theoretical effects affecting hadron-collider measurements, explains CDF representative Chris Hays from the University of Oxford: “The aim is to give a comprehensive and quantitative overview of W-boson mass measurements and their compatibilities. While no significant issues have been identified that significantly change the measurement results, the studies will shed light on their details and differences.”

Searching for dark photons in beam-dump mode

**Intense** Part of the NA62 detector in the ECN3 experimental hall in Prevessin, where beam travels from right to left. On the right-hand side is the STRAW spectrometer, with the analysing magnet in blue. Four large-angle vetoes serving to clean the samples from non-forward events are visible in white, while the green region houses the RICH detector. Credit: CERN-PHOTO-202104-059-6

Faced with the no-show of phenomena beyond the Standard Model at the high mass and energy scales explored so far by the LHC, it has recently become a much considered possibility that new physics hides “in plain sight”, namely at mass scales that can be very easily accessed but at very small coupling strengths. If this were the case, then high-intensity experiments have an advantage: thanks to the large number of events that can be generated, even the most feeble couplings corresponding to the rarest processes can be accessible.

Such a high-intensity experiment is NA62 at CERN’s North Area. Designed to measure the ultra-rare kaon decay K → πνν, it has also released several results probing the existence of weakly coupled processes that could become visible in its apparatus, a prominent example being the decay of a kaon into a pion and an axion. But there is also an unusual way in which NA62 can probe this kind of physics using a configuration that was not foreseen when the experiment was planned, for which the first result was recently reported.

During normal NA62 operations, bunches of 400 GeV protons from the SPS are fired onto a beryllium target to generate secondary mesons from which, using an achromat, only particles with a fixed momentum and charge are selected. These particles (among them kaons) are then propagated along a series of magnets and finally arrive at the detector 100 m downstream. In a series of studies starting in 2015, however, NA62 collaborators with the help of phenomenologists began to explore physics models that could be tested if the target was removed and protons were fired directly into a “dump” that can be arranged by moving the achromat collimators. They concluded that various processes exist in which new MeV-scale particles such as dark photons could be produced and detected from their decays into di-lepton final states. The challenge is to keep the muon-induced background under control, which cannot be easily understood from simulations alone.

A breakthrough came in 2018 when beam physicists in the North Area understood how the beamline magnets could be operated in such a way as to vastly reduce the background of both muons and hadrons. Instead of using the two pairs of dipoles as a beam achromat for momentum selection, the currents in the second pair are set to induce additional muon sweeping. The scheme was verified during a 2021 run lasting 10 days, during which 1.4 × 10¹⁷ protons were collected on the beam dump. The first analysis of this rapidly collected dataset – a search for dark photons decaying to a di-muon final state – has now been performed.

Hypothesised to mediate a new gauge force, dark photons, A′, could couple to the Standard Model via mixing with ordinary photons. In the modified NA62 set-up, dark photons could be produced either via bremsstrahlung or decays of secondary mesons, the mechanisms differing in their cross-sections and distributions of the momenta and angles of the A′. No sign of A′ → μ⁺μ^– was found, excluding a region of parameter space for dark-photon masses between 215 and 550 MeV at 90% confidence. A preliminary result for a search for A′ → e⁺e^– was also presented at the Rencontres de Moriond in March.

“This result is a milestone,” explains analysis leader Tommaso Spadaro of LNF Frascati. “It proves the capability of NA62 for studying physics in the beam-dump configuration and paves the way for upcoming analyses checking other final states.”

X-ray source could reveal new class of supernovae

**Stellar nursery** A possible precursor to a type1a supernova has been spotted in the Large Magellanic Cloud, a satellite galaxy of the Milky Way almost 200,000 light-years from Earth. Credit: NASA/ESA Hubble Space Telescope

Type1A supernovae play an important role in the universe, both as the main source of iron and as one of the principal tools for astronomers to measure cosmic-distance scales. They are also important for astroparticle physics, for example allowing the properties of the neutrino to be probed in an extreme environment.

Type1A supernovae make ideal cosmic rulers because they all look very similar, with roughly equal luminosity and emission characteristics. Therefore, when a cosmic explosion that matches the properties of a type1A supernova is detected, its luminosity can be directly used to measure the distance to its host galaxy. Despite this importance, the details surrounding the progenitors of these events are still not fully understood. Furthermore, a group of outliers, now known as type1ax events, has recently been identified that indicate there might be more than one path towards a type1A explosion.

The reason that typical type1A events all have a roughly equal luminosity is because of their progenitors. The general explanation for these events includes a binary system with at least one white dwarf: a very dense old star consisting mostly of oxygen and carbon that is not undergoing fusion. The system is only prevented from collapsing into a neutron star or black hole due to electron-degeneracy pressure. As the white dwarf accumulates matter from a nearby companion, its mass increases to a precise critical limit at which an uncontrolled thermonuclear explosion starts, resulting in the star being unbounded and seen as the supernova.

This peculiar binary system provides strong hints of a new type of progenitor that can explain up to 30% of all supernovae 1a events

As several X-ray sources were identified in the 1990s by the ROSAT mission as being white dwarfs with hydrogen burning on their surface, the source of matter that is accumulated by the white dwarf was long thought to be hydrogen from a companion star. The flaw with this model, however, is that type1A supernovae show no signs of any hydrogen. On the other hand, helium has been seen, particularly in the outlier type1ax supernovae events. These 1ax events, which are predicted to make up 30% of all type1A events, can be explained by a white dwarf accumulating helium from a companion star that has already shed all of its hydrogen. If the helium was able to accumulate on the surface in a stable way, without intermediate explosions due to violent ignition of the helium, it reaches a mass where it violently ignites on the surface. This in turn triggers the ignition of the core and could explain the type1ax events. Evidence of helium accumulating white dwarfs has, however, not been found.

Now, a group led by researchers from the Max Planck Institute for Extraterrestrial Physics (MPE) has used both optical data and X-ray data from the eROSITA and XMM Newton missions to find the first clear evidence of such a progenitor system. The group found an object, known as [HP99] 159, located in the Large Magellanic Cloud, which shows all the characteristics of a white dwarf surrounded by an accretion disk of helium. Using historical X-ray data from as far back as 50 years, the team also showed that the brightness of the source is relatively stable, thereby indicating that it is accumulating the helium at a stable rate, despite the accumulation rate being lower than theoretically predicted for stable burning. This indicates that the system is working its way towards ignition in the future.

The discovery of this new X-ray source therefore proves the existence of white dwarfs that accumulate helium from a companion star at a steady rate, thereby allowing them to reach the conditions to produce a supernova. This peculiar binary system already provides strong hints of a new type of progenitor that can explain up to 30% of all supernovae 1a events. Follow-up measurements will provide further insight into the complex physics at play in the thermonuclear explosions that produce these events, while [HP99] 159’s characteristics can be used to find similar sources.

First collider neutrinos detected

Electron neutrino charged-current interaction — **New source** A candidate high-energy electron neutrino charged-current interaction recorded by FASERν, with the electron shower (left of the image) balanced by several charged particle tracks (right). Credit: FASER

Since their discovery 67 years ago, neutrinos from a range of sources – solar, atmospheric, reactor, geological, accelerator and astrophysical – have provided ever more powerful probes of nature. Although neutrinos are also produced abundantly in colliders, until now no neutrinos produced in such a way had been detected, their presence inferred instead via missing energy and momentum.

A new LHC experiment called FASER, which entered operations at the start of Run 3 last year, has changed this picture with the first observation of collider neutrinos. Announcing the result on 19 March at the Rencontres de Moriond, and in a paper submitted to Physical Review Letters on 24 March, the FASER collaboration reconstructed 153 candidate muon neutrino and antineutrino interactions in its spectrometer with a significance of 16 standard deviations above the background-only hypothesis. Being consistent with the characteristics expected from neutrino interactions in terms of secondary-particle production and spatial distribution, the results imply the observation of both neutrinos and antineutrinos with an incident neutrino energy significantly above 200 GeV. In addition, an ongoing analysis of data from an emulsion/tungsten subdetector called FASERν revealed a first electron–neutrino interaction candidate (see image).

“FASER has directly observed the interactions of neutrinos produced at a collider for the first time,” explains co-spokesperson Jamie Boyd of CERN. “This result shows the detector worked perfectly in 2022 and opens the door for many important future studies with high-energy neutrinos at the LHC.”

The extreme luminosity of proton–proton collisions at the LHC produces a large neutrino flux in the forward direction, with energies leading to cross-sections high enough for neutrinos to be detected using a compact apparatus. FASER is one of two new forward experiments situated at either side of LHC Point 1 to detect neutrinos produced in proton–proton collisions in ATLAS. The other, SND@LHC, also reported its first results at Moriond. The team found eight muon–neutrino candidate events against an expected background of 0.2, with an evaluation of systematic uncertainties ongoing.

Covering energies between a few hundred GeV and several TeV, FASER and SND@LHC narrow the gap between fixed-target and astrophysical neutrinos. One of the unexplored physics topics to which they will contribute is the study of high-energy neutrinos from astrophysical sources. Since the production mechanism and energy of neutrinos at the LHC is similar to that of very-high-energy neutrinos from cosmic-ray collisions with the atmosphere, FASER and SND@LHC can be used to precisely estimate this background. Another application is to measure and compare the production rate of all three types of neutrinos, providing an important test of the Standard Model.

Beyond neutrinos, the two experiments open new searches for feebly interacting particles and other new physics. In a separate analysis, FASER presented first results from a search for dark photons decaying to an electron-positron pair. No events were seen in an almost background-free analysis, yielding new constraints on dark photons with couplings of 10^–5 to 10^–4 and masses of between 10 and 100 MeV, in a region of parameter space motivated by dark matter.

New insights into CP violation via penguin decays

LHCb figure 1 — **Fig. 1.** Observed rate asymmetry between decays of flavour-tagged B⁰_s or B⁰_s mesons to a ϕϕ pair as a function of decay time, t, folded in a single oscillation period (black points) and overlaid with the projection of the best fit (blue solid line). The red dashed line corresponds to the hypothesis of CP symmetry. Source: LHCB-PAPER-2023–001

At the recent Moriond Electroweak conference, the LHCb collaboration presented a new, high-precision measurement of charge–parity (CP) violation using a large sample of B⁰_s → ϕϕ decays, where the ϕ mesons are reconstructed in the K⁺K^– final state. Proceeding via a loop transition (b → sss, such “penguin” decays are highly sensitive to possible contributions from unknown particles and therefore provide excellent probes for new sources of CP violation. To date, the only known source of CP violation, which is governed by the Cabibbo–Kobayashi–Maskawa matrix in the quark sector, is insufficient to account for the huge excess of matter over antimatter in the universe; extra sources of CP violation are required.

A B⁰_s or B⁰_s meson can change its flavour and oscillate into its antiparticle at a frequency Δm_s/2π, which has been precisely determined by the LHCb experiment. Thus a B⁰_s meson can decay either directly to the ϕϕ state or via changing its flavour to the B–⁰_s state. The phase difference between the two interfering amplitudes changes sign under CP transformations, denoted ϕ_s for B⁰_s or –ϕ_s for B⁰_s decays. A time-dependent CP asymmetry can arise if the phase difference ϕ_s is nonzero. The asymmetry between the decay rates of initial B⁰_s and B⁰_s mesons to the ϕϕ state as a function of the decay time follows a sine wave with amplitude sin(ϕ_s) and frequency Δm_s/2π. In the Standard Model (SM) the phase difference is predicted to be consistent with zero, ϕ^SM_s = 0.00 ± 0.02 rad.

This is the most precise single measurement to date

The observed asymmetry as a function of the B⁰_s → ϕϕ decay time and the projection of the best fit are shown in figure 1 for the Run 2 data sample. The measured asymmetry is diluted by the finite decay-time resolution and the nonzero flavour mis-identification rate of the initial B⁰_s or B⁰_s state, and averaged over two types of linear polarisation states of the ϕϕ system that have CP asymmetries with opposite signs. Taking these effects into account, LHCb measured the CP-violating phase using the full Run 2 data sample. The result, when combined with the Run 1 measurement, is ϕ_s = –0.074 ± 0.069 rad, which agrees with the SM prediction and improves significantly upon the previous LHCb measurement. In addition to the increased data sample size, the new analysis benefits from improvements in the algorithms for vertex reconstruction and determination of the initial flavour of the B⁰_s or B⁰_s mesons.

This is the most precise single measurement to date of time-dependent CP asymmetry in any b → s transition. With no evidence for CP violation, the result can be used to derive stringent constraints on the parameter space of physics beyond the SM. Looking to the future, the upgraded LHCb experiment and a planned future phase II upgrade will offer unique opportunities to further explore new-physics effects in b → s decays, which could potentially provide insights into the fundamental origin of the puzzling matter–antimatter asymmetry.

Beauty quark production versus particle multiplicity

ALICE figure 1 — **Fig. 1.** The ratio between the fraction of non-prompt D⁰ and D⁺ mesons measured in three different multiplicity intervals and the multiplicity-integrated fraction in pp collisions at a centre-of-mass energy of 13 TeV. The PYTHIA 8 curves obtained with different tunes describe the data satisfactorily, except for the CR-BLC 3 (cyan), which underestimates the ratio between non-prompt fractions at low p_T for the high-multiplicity class of events. Source: arXiv:2302.07783

Measurements of the production of hadrons containing heavy quarks (i.e. charm or beauty) in proton–proton (pp) collisions provide an important test of the accuracy of perturbative quantum chromodynamics (pQCD) calculations. The production of heavy quarks occurs in initial hard scatterings of quarks and gluons, whereas the production of light quarks in the underlying event is dominated by soft processes. Thus, measuring heavy-quark hadron production as a function of the charged-particle multiplicity provides insights into the interplay between soft and hard mechanisms of particle production.

Measurements in high-multiplicity pp collisions have shown features that resemble those associated with the formation of quark–gluon plasma in heavy-ion collisions, such as the enhancement of the production of particles with strangeness content and the modification of the baryon-to-meson production ratio as a function of transverse momentum (p_T). These effects can be explained by two different types of models: statistical hadronisation models, which evaluate the population of hadron states according to statistical weights governed by the masses of the hadrons and a universal temperature, or models that include hadronisation via coalescence (or recombination) of quarks and gluons which are close in phase space. Both predict an enhancement of the baryon-to-meson and strange-to-non-strange hadron ratios as a function of charged-particle multiplicity.

In the charm sector, the ALICE collaboration has recently observed a multiplicity dependence of the p_T-differential Λ_c⁺/D⁰ ratio, smoothly evolving from pp to lead–lead collisions, while no dependence was observed for the D_s⁺-meson production yield compared to the one of the D⁰ meson. Measurements of these phenomena in the beauty sector are needed to shed further light on the hadronisation mechanism.

To investigate beauty-quark production as a function of multiplicity and to put it in relation with that of charm quarks, ALICE measured for the first time the fraction of D⁰ and D⁺ originating from beauty-hadron decays (denoted as non-prompt) as a function of transverse momentum and charged-particle multiplicity in pp collisions at 13 TeV, using the Run 2 dataset. The measurement exploits different decay-vertex topologies of prompt and non-prompt D mesons with machine-learning classification techniques. The fractions of non-prompt D mesons were observed to somewhat increase with p_T from about 5 to 10%, as expected by pQCD calculations (figure 1). Similar fractions were measured in different charged-particle multiplicity intervals, suggesting either no or only mild multiplicity dependence. This suggests a similar production mechanism of charm and beauty quarks as a function of multiplicity.

The possible influence of the hadronisation mechanism was investigated by comparing the measured D-meson non-prompt fractions with predictions based on Monte Carlo generators such as PYTHIA 8. A good agreement was observed with different PYTHIA tunes, with and without the inclusion of the colour-reconnection mechanism beyond the leading colour approximation (CR-BLC), which was introduced to describe the production of charm baryons in pp collisions. Only the CR-BLC “Mode 3” tune that predicts an increase (decrease) of hadronisation in baryons for beauty (charm) quarks at high multiplicity is disfavoured by the current data.

The measurements of non-prompt D⁰and D⁺ mesons represent an important test of production and hadronisation models in the charm and beauty sectors, and pave the way for future measurements of exclusive reconstructed beauty hadrons in pp collisions as a function of charged-particle multiplicity.

Searching for electroweak SUSY: a combined effort

CMS figure 1. — **Fig. 1.** Exclusion limits at 95% confidence level for the production of (left) “wino-like” chargino–neutralino pairs decaying via a W and a Z boson, and of (right) “higgsino-like” neutralino–neutralino, chargino–chargino and chargino–neutralino pairs, where the charginos (neutralinos) decay via a W (Higgs) boson, and the lightest chargino and the second- and third-lightest neutralinos are mass-degenerate. On the right, in addition to exclusion limit contours on the mass of the involved SUSY particles, upper limits on the production cross section are also shown. Source: CMS-PAS-SUS-21-008

The CMS collaboration has been relentlessly searching for physics beyond the Standard Model (SM) since the start of the LHC. One of the most appealing new theories is supersymmetry or SUSY – a novel fermion-boson symmetry that gives rise to new particles, “naturally” leads to a Higgs boson almost as light as the W and Z bosons, and provides candidate particles for dark matter (DM).

By the end of LHC Run 2, in 2018, CMS had accumulated a high-quality data sample of proton–proton (pp) collisions at an energy of 13 TeV, corresponding to an integrated luminosity of 137 fb^–1. With such a large data set, it was possible to search for the production of strongly interacting SUSY particles, i.e. the partners of gluons (gluinos) and quarks (squarks), as well as for SUSY partners of the W and Z bosons (electroweakinos: winos and binos), of the Higgs boson (higgsinos), and of the leptons (sleptons). The cross sections for the direct production of SUSY electroweak particles are several orders of magnitude lower than those for gluino and squark pair production. However, if the partners of gluons and quarks are heavier than a few TeV, it could be that the SUSY electroweak sector is the only one accessible at the LHC. In the minimal SUSY extension of the SM, electroweakinos and higgsinos mix to form six mass eigenstates: two charged (charginos) and four neutral (neutralinos). The lightest neutralino is often considered to be the lightest SUSY particle (LSP) and a DM candidate.

CMS has recently reported results, based on the full Run 2 dataset, from searches for the electroweak production of sleptons, charginos and neutralinos. Decays of these particles to the LSP are expected to produce leptons, or Z, W and Higgs bosons. The Z and W bosons subsequently decay to leptons or quarks, while the Higgs boson primarily decays to b quarks. All final states have been explored with complementary channels to enhance the sensitivity to a wide range of electroweak SUSY mass hypotheses. These cover very compressed mass spectra, where the mass difference between the LSP and its parent particles is small (leading to low-momentum particles in the final state) as well as uncompressed scenarios that would instead produce highly boosted Z, W and Higgs bosons. None of the searches showed event counts that significantly deviate from the SM predictions.

CMS maximised the output of the Run 2 dataset, providing its legacy reference on electroweak SUSY searches

The next step was to statistically combine the results of mutually exclusive search channels to set the strongest possible constraints with the Run 2 dataset and interpret the results of searches in different final states under unique SUSY-model hypotheses. For the first time, fully leptonic, semi-leptonic and fully hadronic final states from six different CMS searches were combined to explore models that differ depending on whether the next-to-lightest supersymmetric partner (NLSP) is “wino-like” or “higgsino-like”, as shown in the left and right panels of figure 1, respectively. The former are now excluded up to NLSP masses of 875 GeV, extending the constraints obtained from individual searches by up to 100 GeV, while the latter are excluded up to NLSP masses of 810 GeV.

With this effort, CMS maximised the output of the Run 2 dataset, providing its legacy reference on electroweak SUSY searches. While the same data are still being used to search for new physics in yet uncovered corners of the accessible phase-space, CMS is planning to extend its reach in the upcoming years, profiting from the extension of the data set collected during LHC Run 3 at an unprecedented centre-of-mass energy of 13.6 TeV.

Euclid to link the largest and smallest scales

**Counting down** The Euclid payload module at Thales Alenia Space in Cannes, where assembly and testing is taking place ahead of a scheduled launch in July. Credit: ESA/S Corvaja

Untangling the evolution of the universe, in particular the nature of dark energy and dark matter, is a central challenge of modern physics. An ambitious new mission from the European Space Agency (ESA) called Euclid is preparing to investigate the expansion history of the universe and the growth of cosmic structures over the last 10 billion years, covering the entire period over which dark energy is thought to have played a significant role in the accelerating expansion. The 2 tonne, 4.5 m tall and 3.1 m diameter probe is undergoing final tests in Cannes, France, after which it will be shipped to Cape Canaveral in Florida and inserted into the faring of a SpaceX Falcon 9 rocket, with launch scheduled for July.

Let there be light

Euclid, which was selected by ESA for implementation in 2012 with a budget of about €600 million, has four main objectives. The first is to investigate whether dark energy is real, or whether the apparent acceleration of the universe is caused by a breakdown of general relativity on the largest scales. Second, if dark energy is real, Euclid will investigate whether it is a constant energy spread across space or a new force of nature that evolves with the expansion of the universe. A third objective is to investigate the nature of dark matter, the mass of neutrinos and whether there exist other, so-far undetected fast-moving particle species, and a fourth is to investigate statistics and properties of the early universe that seeded large-scale structures. To meet these goals, the six-year Euclid mission will use a three-mirror system to direct light from up to a billion galaxies across more than a third of the sky towards a visual imager for photometry and a near-infrared spectrophotometer.

So far, the best constraints on the geometry and expansion history of the universe come from cosmic-microwave background (CMB) surveys. Yet these missions are not the best tracers of the curvature, neutrino masses and expansion history, nor for identifying possible exotic subcomponents of dark matter. For this, large surveys on galaxy clustering are required. Euclid will use three methods to achieve this. The first is redshift-space distortions, which combines how fast galaxies move away from us due to the expansion of the universe and how fast galaxies move towards a region of strong gravitational pull in our line-of-sight; measuring these deformations in galactic positions enables the growth rate of structures as well as gravity to be investigated. The second is baryonic acoustic oscillations (BAOs), which arose when the universe was a plasma made from baryons and photons and set a characteristic scale that is related to the sound horizon at recombination. After recombination, photons decoupled from visible matter while baryons were pulled in by gravity and started to form bigger structures, with the BAO scale imprinted in galaxy distributions. BAOs thus serve as a ruler to trace the expansion rate of the universe. The third method, weak gravitational lensing, occurs when light from a background source is bent around a massive foreground object such as a galaxy cluster, from which the distribution of dark matter can be inferred.

As the breadth and precision of cosmological measurements increase, so do the links with particle physics. CERN and the Euclid Consortium (which consists of more than 2000 scientists from 300 institutes in 13 European countries, the US, Canada and Japan) signed a memorandum of understanding in 2016 after Euclid gained CERN recognised-experiment status in 2015. The collaboration was motivated by technical synergies for the mission’s Science Ground Segment (SGS), which will process about 850 Gbit of compressed data per day – the largest of any ESA mission to date. CERN is contributing with the provision of critical software tools and related support activities, explains CERN aerospace and environmental applications coordinator Enrico Chesta: “CernVM–FS, developed by the EP-SFT team to assist high-energy physics collaborations to deploy software on the distributed computing infrastructure used to run data-processing applications, has been integrated into Euclid SGS and will be used for software continuous deployment among the nine Euclid science data centres.”

Competitive survey

Euclid’s main scientific objectives also align closely with CERN’s physics challenges. A 2019 CERN-TH/Euclid workshop identified overlapping areas of interest and options for scientific visitor programmes, with topics of potential interest including N-body CMB simulations, redshift space distortions with relativistic effects, model selection of modified gravity, and dark-energy and neutrino-mass estimation from cosmic voids. Over the coming years, Euclid will provide researchers with data against which they can test different cosmological models. “Galaxy surveys have been happening for decades and have grown in scale, but we didn’t hear much about it because the CMB was, until now, more accurate,” says theorist Marko Simonović of CERN. “With Euclid there will be a competitive survey that is big enough to be comparable to CMB data. It is exciting to see what Euclid, and other new missions such as DESI, will tell us about cosmology. And maybe we will even discover something new.”

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Let there be light

Competitive survey