Weighing in at 180 times the mass of the proton, the top quark is the heaviest elementary particle discovered so far. Because of its large mass, it is the only quark that does not form bound states with other quarks but decays immediately after it has been produced. Despite its short lifetime, its existence has far-reaching consequences. It governs the stability of the electroweak vacuum, gives large contributions to the mass of the W boson, and influences many other important observables through quantum-loop corrections. An accurate knowledge of its mass is important for our understanding of fundamental interactions.
The top quark governs the stability of the electroweak vacuum
The LHC’s high centre-of-mass energy makes it an ideal laboratory to study the properties of the top quark with unprecedented precision. Such studies demand that jets originating from light and bottom quarks are measured very accurately, however, subtleties remain even then, as exact calculations are not possible for low-energy quarks and gluons once they start to form bound states. In this regime, our approximations become inaccurate, because the mass of the bound states becomes as large as the energy of the underlying process. An exciting way to overcome these difficulties is to measure top quarks that have been produced with very high transverse momenta and thus large Lorentz boosts. In these topologies, the decay products are highly collimated, and can be clearly assigned to a decaying top quark. Effects from the formation of hadrons play a minor effect in boosted topologies as the top quarks, which were originally produced in quark–antiquark pairs, move apart from each other fast enough that their decays can be considered to happen independently.
Boosted precision
By reconstructing a boosted top quark in a single jet, a measurement of the jet mass can be translated into one of the top-quark mass. The CMS collaboration has carried out such a measurement using the √s = 13 TeV data collected in 2016, reconstructing the top-quark jets with the novel XCone algorithm to obtain a top quark mass of 172.6 ± 2.5 GeV (figure 1). Due to this new way of reconstructing jets, an improvement of more than a factor of three relative to an earlier measurement at √s = 8 TeV has been achieved. Although the uncertainty is larger than for direct measurements, where top quarks are reconstructed from multiple jets or leptons and missing transverse momentum (which currently yield a world average of 172.9 ± 0.4 GeV from a combination of CMS, ATLAS and Tevatron measurements), this new result shows for the first time the potential of using boosted top quarks for precision measurements.
The jet mass can be translated into the top-quark mass
Measuring the properties of the top quark at high momenta enables detailed studies of a theoretically compelling kinematic regime that has not been accessible before. Different effects, such as the collinear radiation of gluons and quarks, govern its dynamics compared to top-quark production at low energies. Exploiting the full Run-2 dataset should allow CMS to extend this measurement to higher boosts, and establish the boosted regime for a number of precision measurements in the top-quark sector in Run 3 and at the high-luminosity LHC.
Almost 90 years since Pauli postulated its existence, much remains to be learnt about the neutrino. The observation in 1998 of neutrino oscillations revealed that the particle’s flavour and mass eigenstates mix and oscillate. At least two must be massive, like the other known fermions, though with far smaller masses. The need for a mechanism to generate such small masses strongly hints at the existence of new physics beyond the Standard Model. Faced with such compelling questions, neutrino experiments are springing up at an unprecedented rate, from a plethora of searches for neutrinoless double-beta decay to gigantic astrophysical–neutrino detectors at the South Pole (IceCube) and soon in the Mediterranean Sea (KM3NeT), and two projects of enormous scope on the horizon in DUNE and Hyper-Kamiokande. Now, then, is a timely moment for the publication of a tutorial for graduate students and young researchers who are entering this fast-moving field.
Access all areas
Edited by former spokesperson of the OPERA experiment Antonio Ereditato, The State of the Art of Neutrino Physics provides an historical account and introduction to basic concepts, reviews of the various subfields where neutrinos play a significant role, and gives a detailed account of the data produced by present experiments in operation. An extremely valuable compilation of topical articles, the book covers essentially all areas of research in experimental neutrino physics, from astrophysical, solar and atmospheric neutrinos to accelerator and reactor neutrinos. The large majority of the articles are written in a didactic style by leading experts in the field, allowing young researchers to acquaint themselves with the diverse research in the field. In particular the chapter describing the formalism of neutrino oscillations should be required reading for all aspiring neutrino physicists. In all cases special attention is given to experimental challenges.
From the theory side, chapters cover measurements at neutrino experiments of the low-energy interactions of neutrinos with nuclei (a key way to reduce systematic uncertainties), the phenomenology and consequences of the yet-to-be-determined neutrino-mass hierarchy, and the possibility of CP violation in the lepton sector. A very detailed account of solar neutrinos and matter effects in the Sun is written by Alexei Smirnov, one of the inventors of the celebrated Mikheyev–Smirnov–Wolfenstein effect, which describes how weak interactions with electrons modify oscillation probabilities for the various neutrino flavours. More speculative scenarios, for example on the possibility of the existence of sterile neutrinos, are discussed as well.
For a book like this, which has the ambition to address a broad palette of neutrino questions, it is always difficult to be totally complete, but it comes close. Some topics have evolved in the details since 2016, when the material upon which the book is based was written, but that doesn’t take away from the book’s value as a tutorial. I recommend it very highly to young and not-so-young aspiring
neutrino aficionados alike.
After the discovery of the long‑sought Higgs boson at a mass of 125 GeV, a major question in particle physics is whether the electroweak symmetry breaking sector is indeed as simple as the one implemented in the Standard Model (SM), or whether there are additional Higgs bosons. Additional Higgs bosons would occur, for example, in the presence of a second Higgs field, as realised in two‑Higgs doublet models, among which is the well‑known minimal supersymmetric extension of the SM (MSSM). The discovery of additional Higgs bosons could therefore be a gateway to new symmetries in nature.
ATLAS has recently released results of a search for heavy Higgs bosons decaying into a pair of tau leptons using the complete LHC Run 2 dataset (139 fb–1 of 13 TeV proton–proton data). The new analysis provides a considerable increase in sensitivity to MSSM scenarios compared to previous results.
The MSSM features five Higgs bosons
The MSSM features five Higgs bosons, among which, the observed Higgs boson can be the lightest one. The couplings of the heavy Higgs bosons to down‑type leptons and quarks, such as the tau lepton and bottom quark, are enhanced for large values of tan β – the ratio of the vacuum expectation values of the two Higgs doublets, and one of the key parameters of the model. The heavy neutral Higgs bosons A (CP odd) and H (CP even) are produced mainly via gluon–gluon interactions or in association with bottom quarks. Their branching fractions to tau leptons can reach sizeable values across a large part of the model‑parameter space, making this channel particularly sensitive to a wide range of MSSM scenarios.
The new ATLAS search requires the presence of two oppositely charged tau‑lepton candidates, one of which is identified as a hadronic tau decay, and the other as either a hadronic or a leptonic decay. To profit from the enhancement of the production of signal events in association with bottom quarks at large tan β values (for example when the heavy Higgs boson is radiated by a b‑quark produced in the collision of two gluons), the data are further categorised based on the presence or absence of additional b‑jets. One of the challenges of the analysis is the misidentification of backgrounds with hadronic jets as tau candidates. These backgrounds are estimated from data by measuring the misidentification probabilities and applying them to events in control regions representative of the event selection. The final discriminant is on the quantity mTtot, which is built from the combination of the transverse masses of the two tau‑lepton decay products (figure 1).
The data agree with the prediction assuming no additional Higgs bosons, despite a small, non‑significant excess around a putative signal mass value of 400 GeV. The measurement places limits on the production cross section that can be translated into constraints on MSSM parameters. One realisation of the MSSM is the hMSSM scenario, in which the knowledge of the observed Higgs‑boson mass is used to reduce the number of parameters. The A/H → ττ exclusion limit dominates over large parts of the parameter space (figure 2), but still leaves room for possible discoveries at masses above the top‑anti‑top quark production threshold. ATLAS continues to refine this and conduct further searches for heavy Higgs bosons in various final states.
The coupling between quarks and gluons depends strongly on the energy scale of the process. The same is true for the masses of the quarks. This effect – the so‑called “running” of the strong coupling constant and the quark masses – is described by the renormalisation group equations (RGEs) of quantum chromodynamics (QCD). The experimental verification of the RGEs is both an important test of the validity of QCD and an indirect search for unknown physics, as physics beyond the Standard Model could modify the RGEs at scales probed by the Large Hadron Collider. The running of the strong coupling constant has been established at many experiments in the past, and, over the past 20 years, evidence for the running of the masses of the charm and bottom quarks was demonstrated using data from LEP, SLC and HERA, though the running of the top‑quark mass has hitherto proven elusive.
CMS has probed the running of the mass of the top quark for the first time
The CMS collaboration has now, for the first time, probed the running of the mass of the top quark. The measurement was performed using proton–proton collision data at a centre‑of‑mass energy of 13 TeV, recorded by the CMS detector in 2016. The top quark’s mass was determined as a function of the invariant mass of the top quark–antiquark system (the energy scale of the process), by comparing differential measurements of the system’s production cross section with theoretical predictions. In the vast majority of the cases, top quarks decay into a W boson and a bottom quark. In this analysis, candidate events are selected in the final state where one W boson decays into an electron and a neutrino, and the other decays into a muon and a neutrino.
One-loop agreement
The cross section was determined using a maximum‑likelihood fit to multi‑differential distributions of final‑state observables, allowing the precision of the measurement to be significantly improved by comparison to standard methods (figure 1). The measured cross section was then used to extract the value of the top‑quark mass as a function of the energy scale. The running was determined with respect to an arbitrary reference scale. The measured points are in good agreement with the one‑loop solution of the RGE, within 1.1 standard deviations, and a hypothetical no‑running scenario is excluded at above 95% confidence level.
This novel result supports the validity of the RGEs up to a scale of the order of 1 TeV. Its precision is limited by systematic uncertainties related to experimental calibrations and the modelling of the top‑quark production in the simulation. Further progress will not only require a significant effort in improving the calibrations of the final‑state objects, but also substantial theoretical developments.
Jets are the most abundant high‑energy objects produced in collisions at the LHC, and often contaminate searches for new physics. In heavy‑ion collisions, however, these collimated showers of hadrons are not a background but one of the main tools to probe the deconfined state of strongly interacting matter known as the quark‑gluon plasma.
There are many open questions about the structure of the quark‑gluon plasma: What are the relevant degrees of freedom? How do high‑energy quarks and gluons interact with the hot QCD medium? Do factorisation and universality hold in this extreme environment? To answer these questions, experiments study how jets are modified in heavy‑ion collisions, where, unlike in proton‑proton collisions, they may interact with the constituents of the quark‑gluon plasma. Since jet production and interactions can be computed in perturbative QCD, comparing theoretical calculations to measurements can provide insight to the properties of the quark‑gluon plasma.
Soft power
In this spirit, the ALICE collaboration has measured the inclusive jet production yield in both Pb‑Pb and proton–proton (pp) collisions at a centre‑of‑mass energy of 5.02 TeV. Jets were reconstructed from a combination of information from the ALICE tracking detectors and electromagnetic calorimeter for a variety of jet radii R. The detectors’ excellent performance with soft tracks was exploited to allow the measurements to cover the lowest jet transverse momentum (pT,jet) region measured at the LHC, where jet modification effects are predicted to be strongest. The measured jet yields in Pb‑Pb collisions exhibit strong suppression compared to pp collisions, consistent with theoretical expectations that jets lose energy as they propagate through the quark‑gluon plasma (figure 1). For relatively narrow R = 0.2 jets, the data show stronger suppression at lower pT, jet than at higher pT,jet, suggesting that lower pT,jet jets lose a larger fraction of their energy. Additionally, the data show no significant R dependence of the suppression within the uncertainties of the measurement, which places constraints on the angular distribution of the “lost” energy.
Several theoretical models, spanning a range of physics approximations from jet‑medium weak‑coupling to strong‑coupling, were compared to the data. The models are able to generally describe the trends of the data, but several models exhibit hints of disagreement with the measurements. These data complement existing jet measurements from ATLAS and CMS, and take advantage of ALICE’s high‑precision tracking system to provide additional constraints on jet‑quenching models in heavy‑ion collisions at low pT. Moreover, these measurements can be used in combination with other jet observables to extract properties of the medium such as the transverse momentum diffusion parameter, which describes the angular broadening of jets as they traverse the quark–gluon plasma, as a function of the medium temperature and the jet pT.
The “reference” measurements in pp collisions contain important QCD physics themselves. This new set of measurements was performed systematically from R= 0.1 to R= 0.6, in order to span from small R, where hadronisation effects are large, to large R, where underlying event effects are large. These data can be used to constrain the perturbative structure of the inclusive jet cross section, as well as hadronisation and underlying event effects, which are of broad interest to the high‑energy physics community.
Going forward, ALICE is actively working to further constrain theoretical predictions in both pp and Pb‑Pb collisions by exploring complementary jet measurements, including jet substructure, heavy‑flavour jets, and more. With a nearly 10 times larger Pb‑Pb data sample collected in 2018, upcoming analyses of the data will be important for connecting observed jet modifications to properties of the quark‑gluon plasma.
One of the many doors to new physics that have been opened by the discovery of the Higgs boson concerns the possibility of finding charge-parity violation (CPV) in Higgs-boson interactions. Were CPV to be observed in the Higgs sector, it would be an unambiguous indication of physics beyond the Standard Model (SM), and could have important ramifications for understanding the baryon asymmetry of the universe. Recently, the ATLAS and CMS collaborations reported their first forays into this area by measuring the CP-structure of interactions between the Higgs boson and top quarks.
While CPV is well established in the weak interactions of quarks (most recently in the charm system by the LHCb collaboration), and is explained in the SM by the existence of a phase in the CKM matrix, the amount of CPV observed is many orders of magnitude too small to account for the observed cosmological matter-antimatter imbalance. Searching for additional sources of CPV is a major programme in particle physics, with a moderate-significance suggestion of CPV in lepton interactions recently announced by the T2K collaboration. It is likely that sources of CPV from phenomena beyond the scope of the SM are needed, and the detailed properties of the Higgs sector are one of several possible hiding places.
Based on the full LHC Run 2 dataset, ATLAS and CMS studied events where the Higgs boson is produced in association with one or two top quarks before decaying into two photons. The latter (ttH) process, which accounts for around 1% of the Higgs bosons produced at the LHC, was observed by both collaborations in 2018. But the tH production channel is predicted to be about six times rarer. This is due to destructive interference between higher order diagrams involving W bosons, and makes the tH process particularly sensitive to new-physics processes.
Exploring the CP properties of these interactions is non-trivial
According to the SM, the Higgs boson is “CP-even” – that is, it is possible to rotate-away any CP-odd phase from the scalar mass term. Previous probes of the interaction between the Higgs and vector bosons by CMS and ATLAS support the CP-even nature of the Higgs boson, determining its quantum numbers to be most consistent with JPC = 0++, though small CP-odd contributions from a more complex coupling structure are not excluded. The presence of a CP-odd component, together with the dominant CP-even one, would imply CPV, altering the kinematic properties of the ttH process and modifying tH production. Exploring the CP properties of these interactions is non-trivial, and requires the full capacities of the detectors and analysis techniques.
The collaborations employed machine-learning (Boosted Decision Tree) algorithms to disentangle the relative fractions of the CP-even and CP-odd components of top-Higgs interactions. The CMS collaboration observed ttH production at significance of 6.6σ, and excluded a pure CP-odd structure of the top-Higgs Yukawa coupling at 3.2σ. The ratio of the measured ttH production rate to the predicted production rate was found by CMS to be 1.38 with an uncertainty of about 25%. ATLAS data also show agreement with the SM. Assuming a CP-even coupling, ATLAS observed ttH with a significance of 5.2σ. Comparing the strength of the CP-even and CP-odd components, the collaboration favours a CP-mixing angle very close to 0 (indicating no CPV) and excludes a pure CP-odd coupling at 3.9σ. ATLAS did not observe tH production, setting an upper limit on its rate of 12 times the SM expectation.
In addition to further probing the CP properties of the top–Higgs interaction with larger data samples, ATLAS and CMS are searching in other Higgs-boson interactions for signs of CPV.
Recent years have seen the dawn of multi-messenger astrophysics. Perhaps the most significant contributor to this new era was the 2017 detection of gravitational waves (GWs) in coincidence with a bright electromagnetic phenomenon, a gamma-ray burst (GRB). GRBs consist of intense bursts of gamma rays which, for periods ranging from hundreds of milliseconds to hundreds of seconds, outshine any other source in the universe. Although the first such event was spotted back in 1967, and typically one GRB is detected every day, the underlying astrophysical processes responsible remain a mystery. The joint GW–electromagnetic detection answered several questions about the nature of GRBs, but many others remain.
Recently, researchers made the first attempts to add gamma-ray polarisation into the mix. If successful, this could enable the next step forward within the multi-messenger field.
So far, three photon parameters – arrival time, direction and energy – have been measured extensively for a range of different objects within astrophysics. Yet, despite the wealth of information it contains, the photon polarisation has been neglected. X-ray or gamma-ray fluxes emitted by charged particles within strong magnetic fields are highly polarised, while those emitted by thermal processes are typically unpolarised. Polarisation therefore allows researchers to easily identify the dominant emission mechanism for a particular source. GRBs are one such source, since a consensus on where the gamma rays actually originate from is still missing.
Difficult measurements
The reason that polarisation has not been measured in great detail is related to the difficulty of performing the measurements. To measure the polarisation of an incoming photon, details of the secondary products produced as it interacts in a detector need to be measured. With gamma rays, for example, the angle at which the gamma ray scatters in the detector is related to its polarisation vector. This means that, in addition to detecting the photon, researchers need to study its subsequent path. Such measurements are further complicated by the need to perform them above the atmosphere on satellites, which complicates the detector design significantly.
The 2020s should see the start of a new type of astrophysics
Recent progress has shown that, although challenging, polarisation measurements are possible. The most recent example came from the POLAR mission, a Swiss, Polish and Chinese experiment fully dedicated to measuring the polarisation of GRBs, which took data from September 2016 to April 2017. The team behind POLAR, which was launched to space in 2016 attached to a module for the China Space Station, recently published its first results. Though they indicate that the emission from GRBs is likely unpolarised, the story appears to be more complex. For example, the polarisation is found to be low when looking at the full GRB emission, but when studying it over short time intervals, a strong hint of high polarisation is found with a rapidly changing polarisation angle during the GRB event. This rapid evolution of the polarisation angle, which is yet to be explained by the theoretical community, smears out the polarisation when looking at the full GRB. In order to fully understand the evolution, which could give hints of an evolution of a magnetic field, finer time-binning and more precise measurements are needed, which require more statistics.
POLAR-2
Two future instruments capable of providing such detailed measurements are currently being developed. The first, POLAR-2, is the follow-up of the POLAR mission and was recently recommended to become a CERN-recognised experiment. P OLAR-2 w ill b e a n order of magnitude more sensitive (due to larger statistics and lower systematics) than its predecessor and therefore should be able to answer most of the questions raised by the recent POLAR results. The experiment will also play an important role in detecting extremely weak GRBs, such as those expected from GW events. POLAR-2, which will be launched in 2024 to the under-construction China Space Station, could well be followed by a similar but slightly smaller instrument called LEAP, which recently progressed to the final stage of a NASA selection process. If successful, LEAP would join POLAR-2 in 2025 in orbit on the International Space Station.
Apart from dedicated GRB polarimeters, progress is also being made at other upcoming instruments such as NASA’s Imaging X-ray Polarimetry Explorer and China-ESA’s enhanced X-ray Timing and Polarimetry mission, which aim to perform the first detailed polarisation measurements of a range of astrophysical objects in the X-ray region. While the first measurements from POLAR have been published recently, and more are expected soon, the 2020s should see the start of a new type of astrophysics, which adds yet another parameter to multi-messenger exploration.
Physicists working on the T2K experiment in Japan have reported the strongest hint so far that charge-conjugation × parity (CP) symmetry is violated by the weak interactions of leptons. Based on an analysis of nine years of neutrino-oscillation data, the T2K results indicate discrepancies between the way muon-neutrinos transform into electron-neutrinos and the way muon-antineutrinos transform into electron-antineutrinos, at 3σ confidence. While further data are required to confirm the findings, the result strengthens previous observations and offers hope for a future discovery of leptonic CP violation at T2K or at next-generation long-baseline neutrino-oscillation experiments due to come online this decade.
These exciting results are thanks to the hard work of hundreds of T2K collaborators
Federico Sanchez
“These exciting results are thanks to the hard work of hundreds of T2K collaborators involved in the construction, data collection and data analysis for T2K over the past two decades,” says T2K international co-spokesperson Federico Sanchez of the University of Geneva.
Discovered in 1964, CP violation has so far only been observed in the weak interactions of quarks, mostly recently in the charm system by the LHCb collaboration. Since the size of the effect in quarks is too small to explain the observed matter-antimatter disparity in the universe, finding additional sources of CP violation is one of the outstanding mysteries in particle physics. The quantum mixing of neutrino flavours as neutrinos travel over large distances, the discovery of which was marked by the 2015 Nobel Prize in Physics, provides a way to probe another potential source of CP violation: a complex phase, δCP, in the neutrino mixing matrix. Though models indicate that no value of δCP could explain the cosmological matter-antimatter asymmetry without new physics, the observation of leptonic CP violation would make models such as leptogenesis, which feature heavy Majorana partners for the Standard Model neutrinos, more plausible.
The T2K (Tokai-to-Kamioka) experiment uses the Super Kamiokande detector to observe neutrinos and antineutrinos generated by a proton beam at the J-PARC accelerator facility 295 km away. As the beams travel through Earth, a fraction of muon neutrinos and antineutrinos in the beam oscillate into electron neutrinos that are recorded via nuclear-recoil interactions in Super Kamiokande’s 50,000-tonne tank of ultrapure water, where the charged lepton generated by the weak interaction creates a Cherenkov ring which can be distinguished as being created by an electron or muon (see image above). Since the beam-line and detector components are made out of matter and not antimatter, the observation of neutrinos is already enhanced. The T2K analysis therefore includes corrections based on data from magnetised near-detectors (ND280, which uses the magnet originally built for the UA1 detector at CERN’s Spp̄S collider) placed 280m from the target.
The δCP parameter is a cyclic phase: if δCP=0, neutrinos and antineutrinos will change from muon- to electron-types in the same way during oscillation; any other value would enhance the oscillations of either neutrinos or antineutrinos, violating CP symmetry. Analysing data with 1.49×1021 and 1.64×1021 protons produced in neutrino- and antineutrino-beam mode respectively, T2K observed 90 electron-neutrino candidates and 15 electron-antineutrino candidates. This may be compared with the 56 and 22 events expected for maximal antineutrino enhancement (δCP=+π/2), and the 82 and 17 events expected for maximal neutrino enhancement (δCP=−π/2). Being most compatible with the latter scenario, the T2K data disfavour almost half of the possible values of δCP at 3σ confidence. For the “normal” neutrino-mass ordering favoured by T2K and other experiments, and averaged over all other oscillation parameters, the measured 3σ confidence-level interval for δCP is [−3.41, −0.03], while for the “inverted” mass ordering (in which the first mass splitting is greater than the second) it is [−2.54, −0.32]. Averaged over all oscillation parameters, δCP=0 is now disfavoured at 3σ confidence, though it is still within the 3σ bound for some allowed values of the mixing angle θ23 (see figure, above).
“Our results show the strongest constraint yet on the parameter governing CP violation in neutrino oscillations, one of the few parameters governing fundamental particle interactions that has not yet been precisely measured,” continues Sanchez. “These results indicate that CP violation in neutrino mixing may be large, and T2K looks forward to continued operation with the prospect of establishing evidence for CP violation in neutrino oscillations.”
Next steps
To further improve the experimental sensitivity to a potential CP-violating effect, the collaboration plans to upgrade the near detector to reduce systematic uncertainties and to accumulate more data, while J-PARC will increase the beam intensity by upgrading its accelerator and beam line.
“This is the first time ever CP-violation is glimpsed in the lepton sector and it has the potential of being a very large effect,” says Albert De Roeck, group leader of the CERN neutrino group, which has participated in the T2K experiment since last year. “Future neutrino CP violation measurements will be further performed by currently running neutrino experiments, and then the torch will be passed to the planned high precision neutrino experiments DUNE and Hyper-Kamiokande that will provide measurements of the exact degree of CP violation in the neutrino system.”
The Belle II collaboration at the SuperKEKB collider in Japan has published its first physics analysis: a search for Z′ bosons, which are hypothesised to couple the Standard Model (SM) with the dark sector. The team scoured four months of data from a pilot run in 2018 for evidence of invisibly decaying Z′ bosons in the process e+e−→μ+μ−Z′, and for lepton-flavour violating Z′ bosons in e+e−→e±μ∓Z′, by looking for missing energy recoiling against two clean lepton tracks. “This is the first ever search for the process e+e−→μ+μ−Z′ where the Z′ decays invisibly,” says Belle II spokesperson Toru Iijima of Nagoya University.
The team did not find any excess of events, yielding preliminary sensitivity to the coupling g′ in the so-called Lμ−Lτ extension of the SM, wherein the Z′ couples only to muon and tau-lepton flavoured SM particles and the dark sector. This model also has the potential to explain anomalies in b → sμ+μ− decays reported by LHCb and the longstanding muon g-2 anomaly, claims the team.
The results come a little over a year since the first collisions were recorded in the fully instrumented Belle II detector on 25 March 2019. Following in the footsteps of Belle at the KEKB facility, the new SuperKEKB b-factory plans to achieve a 40-fold increase on the luminosity of its predecessor, which ran from 1999 to 2010. First turns were achieved in February 2016, and first collisions between its asymmetric-energy electron and positron beams were achieved in April 2018. The machine has now reached a luminosity of 1.4 × 1034 cm-2 s-1 and is currently integrating around 0.7 fb-1 each day, exceeding the peak luminosity of the former PEP-II/BaBar facility at SLAC, notes Iijima.
By summer the team aims to exceed the Belle/KEKB record of 2.1 × 1034 cm-2 s-1by implementing a nonlinear “crab waist” focusing scheme. First used at the electron-positron collider DAΦNE at INFN Frascati, and not to be confused with the crab-crossing technology used to boost the luminosity at KEKB and planned for the high-luminosity LHC, the scheme stabilises e+e– beam-beam blowup using carefully tuned sextupole magnets located symmetrically on either side of the interaction point. “The 100 fb-1 sample which we plan to integrate by summer will allow us to provide our first interesting results in B physics,” says Tom Browder of the University of Hawaii, who was Belle II spokesperson until last year.
We will then look for the star attraction of the dark sector, the dark photon
Tom Browder
As well as updating searches for invisible decays of the Z′ with one to two orders of magnitude more data, Belle II will now conduct further dark-sector studies including a search for axion-like particles decaying to two photons, the Z′ decaying to visible final states and dark-Higgstrahlung with a μ+μ– pair and missing energy, explains Browder. “We will then look for the star attraction of the dark sector, the dark photon, with the difficult signature of e+e– to a photon and nothing else.”
On 25 January, European Space Agency astronaut Luca Parmitano stepped outside a half-million-kilogramme structure travelling at tens of thousands of kilometres per hour, hundreds of kilometres above Earth, and, tethered by a thin cord, ventured into the vacuum of space to check for a leak.
It was the fourth such extravehicular activity (EVA) he’d been on in two months. All things considered, the task ahead was relatively straightforward: to make sure that a newly installed cooling system for the Alpha Magnetic Spectrometer (AMS), the cosmic-ray detector that has been attached to the International Space Station (ISS) since 2011, had been properly plumbed in.
Heart-stopping spacewalks
The first EVA on 15 November saw Parmitano and fellow astronaut, NASA’s Drew Morgan, remove and jettison the AMS debris shield, which is currently still spiralling its way to Earth, to allow access to the experiment’s cooling system. The CO2 pumps, needed to keep the 200,000-channel tracker electronics at a temperature of 10 ± 3 °C, had started to fail in 2014 – which was no surprise, as AMS was initially only supposed to operate for three years. During the second EVA on 22 November, the astronauts cut through the cooling system’s eight stainless-steel lines to isolate and prepare it for removal, and a critical EVA3 on 2 December saw Morgan and Parmitano successfully connect the new pump system, which had been transported to the ISS by an Antares rocket the previous month. Then came a long wait until January to find out if the repair had been successful.
“EVA4 was the heart-stopping EVA because that’s where we did the leak tests on all those tubes,” says Ken Bollweg, NASA’s AMS project manager. The success of the previous EVAs suggested that the connections were going to be fine. But Parmitano arrived at the first tube, attached one of 29 bespoke tools developed specially for the AMS repair, and saw that the instrument had issued a warning signal. “I see red,” he reported to anxious teams at NASA’s Johnson Space Center’s Mission Control Center and the AMS Payload Operations Control Centre (POCC) at CERN’s Prévessin site, from where spokesperson Sam Ting and his colleagues were monitoring proceedings closely. Though not huge, the leak was serious enough not to guarantee that the system would work, jeopardising four years of preparation involving hundreds of astronauts, engineers and scientists. Following procedures put in place to deal with such a situation, Parmitano tightened the connection and waited for about an hour before checking the tube again. A leak was still present. Then, after re-tightening the troublesome connection again, while the team was preparing a risky “jumper” manoeuvre to bypass the leak and make a new connection, he checked a third time: “No red!” Happy faces lit up the POCC.
NASA has learned a lot of new things from this
Ken Bollweg
AMS was never designed to be serviceable, and the repair, unprecedented in complexity for a space intervention, required the avoidance of sharp edges and other hazards in order to bring it back to full operational capacity. The chances of something going wrong were high, says Bollweg. “NASA has learned a lot of new things from this. We really pushed the envelope. It showed that we have the capabilities to do even more than we have done in the past.” EVA4 lasted almost six hours. Five hours and two minutes into it, Parmitano, who returned safely to Earth on 6 February, broke the European record for the most time spent spacewalking (33 hours and nine minutes). It’s not a job for the fainthearted. During a spacewalk in 2013, while wedged into a confined space outside the ISS, a malfunction in Parmitano’s spacesuit caused his helmet to start filling with water and he almost drowned.
“Building and operating AMS in space has been an incredible journey through engineering and physics, but today it is thanks to the NASA group that in AMS we can continue this journey and this is amazing. An enormous thanks to the EVA crew,” said AMS integration engineer Corrado Gargiulo of CERN. The day after EVA4, the POCC team spent about 10 hours refilling the new AMS cooling system with 1.3 kg of CO2 and started to power up the detector. At noon on 27 January, all the detector’s subsystems were sending data back, marking a new chapter for AMS that will see it operate for the lifetime of the ISS.
Into the unknown
The 7.5 tonne AMS apparatus has so far recorded almost 150 billion charged cosmic rays with energies up to the multi-TeV range, and its percent-level results show clear and unexpected behaviour of cosmic-ray events at high energies. A further 10 years of operation will allow AMS to make conclusive statements on the origin of these unexpected observations, says Ting. “NASA is to be congratulated on seeing this difficult project through over a period of many years. AMS has observed unique features in cosmic-ray spectra that defy traditional explanations. We’re entering into a region where nobody has been before.”
AMS has observed unique features in cosmic-ray spectra that defy traditional explanations
Sam Ting
The first major result from AMS came in 2013, when measurements of the cosmic positron fraction (the ratio of the positron flux to the flux of electrons and positrons) up to an energy of 350 GeV showed that the spectrum fits well to dark-matter models. The following year, AMS published the positron and electron fluxes, which showed that neither can be fitted with the single-power-law assumption underpinning the traditional understanding of cosmic rays. The collaboration has continued to find previously unobserved features in the measured fluxes and flux ratio of electrons and positrons, publishing the results in several high-profile papers during the past couple of years.
Last year, AMS reaffirmed the complex energy dependence exhibited by the positron flux: a significant excess starting from 25 GeV, a sharp drop-off above 284 GeV and a finite energy cutoff at 810 GeV (figure 1). “In the entire energy range the positron flux is well described by the sum of a term associated with the positrons produced in the collision of cosmic rays, which dominates at low energies, and a new source term of positrons, which dominates at high energies,” says Ting. “These experimental data on cosmic-ray positrons show that, at high energies, they predominantly originate either from dark-matter annihilation or from other astrophysical sources.” Although dark-matter models predict such a cut off, the AMS data cannot yet rule out astrophysical sources, in particular pulsars. Further intrigue comes from the latest, to-be-published, AMS result on antiprotons, which, although rare at high energies, exhibit similar functional behaviour as the positron spectrum (figure 2). “This indicates that the excess of positrons may not come from pulsars due to the similarity of the two spectra and the high mass of antiprotons,” says Ting.
Thanks to the successful installation of the new AMS cooling system, the expected positron spectrum by 2028, in particular the high-energy data points, should enable an accurate comparison with dark-matter models (figure 3). High-energy (>TeV) events are also expected to provide insights into the origins of cosmic electrons, the latest results on which show that the electron flux exhibits a significant excess starting from 42 GeV.
Unlike the positron flux, which has an exponential energy cutoff at 810 GeV, the electron flux does not have a cutoff (at least not below 1.9 TeV). Also: in the entire energy range the electron flux is well described by the sum of two power law components (figure 4), providing “clear evidence”, says Ting, that most high energy electrons originate from different sources than high energy positrons.
Novelties in nuclei
Unexpected results continue to appear in data from cosmic nuclei, which make up the bulk of cosmic rays travelling through space. Helium, carbon and oxygen nuclei are thought to be mainly produced and accelerated in astrophysical sources and are known as primary cosmic rays, while lithium, beryllium and boron nuclei are produced by the collision of heavier nuclei with nuclei of the interstellar matter and are known as secondary cosmic rays.
New properties of primary cosmic rays – helium, carbon and oxygen – have been observed in the rigidity range 2 GV to 3 TV; at high energies these three spectra also have identical rigidity dependence, all deviating from a single power law above 200 GV. Similar oddities have appeared in measurements of secondary cosmic rays – lithium, beryllium and boron – in the range 1.9 GV to 3.3 TV (figure 5). The lithium and boron fluxes have an identical rigidity dependence above 7 GV, all three fluxes have an identical rigidity dependence above 30 GV, and, unexpectedly, above 30 GV the Li/Be flux ratio is approximately equal to two.
The ratio of secondary fluxes to primary fluxes is particularly interesting because it directly measures the amount and properties of the interstellar medium. Before AMS, only the B/C ratio was measured and was assumed to be proportional to RΔ with Δ a constant for R > 60 GV. The latest AMS results on secondary (Li, Be, B) to primary (C, O) flux ratios show that Δ is not a constant, but changes by more than 5σ between the two rigidity ranges, 60 < R < 200 GV and 200 < R < 3300 GV. As with electron and positron fluxes, none of the current AMS results can be explained by existing theoretical models. By 2028, says Ting, AMS will extend its measurements of cosmic nuclei up to Z=30 (zinc) with sufficient statistics to get to the bottom of these and other mysteries. “We have measured many particles, electrons, positrons, antiprotons and many nuclei, and they all have distributions and none agree with current theoretical models. So we will begin to create a new field.”
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