Spanning 13 decades in energy and more than 26 decades in intensity, cosmic rays are one of the hottest topics in astroparticle physics today.Spectral features such as a “knee” at a few PeV and an “ankle” at a few EeV give insights into their varying origins, but studies of their arrival direction can also provide valuable information. Though magnetic fields mean we cannot normally trace cosmic rays directly back to their point of origin, angular anisotropies provide important independent evidence towards probable sources at different energies. This week, at the 37th International Cosmic Ray Conference (ICRC), a range of space- and ground-based experiments greatly increased our knowledge of cosmic-ray anisotropies, with new results spanning 10 decades in energy, from GeV to tens of EeV.
At sub-TeV energies, spectral features seen by the AMS-02 and CALET detectors on the International Space Station and the Chinese–European DAMPE satellite could potentially be explained by a local galactic source such as a supernova remnant like Vela (see “Spectral” figure). If a nearby source is indeed responsible for a significant fraction of the cosmic rays observed at such energies, it could show up in the arrival direction of these cosmic rays in the form of a dipole feature, despite bending by galactic magnetic fields; however, results from AMS-02 at ICRC showed no evidence of a dipole in the arrival direction of protons or any other light nucleus. This was confirmed by DAMPE, which excluded dipole features with amplitudes above about 0.1% in the 100s of GeV energy range. The search continues, however, with DAMPE, AMS-02 and CALET all set to take further data over the coming years.
Close to the knee, the dipole has a maximum rather than a minimum close to the galactic centre
Moving to higher energies, clear anisotropic dipoleexcesses have beenobserved over the last decade by ground-based experiments such as the ARGO-YBJ observatory in China, the HAWC observatory in Mexico and the IceCube observatory at the South Pole – though with different “phases” at different energies. The anisotropy in the TeV to the 100s of TeV energy range could point towards a nearby source, though models proposing the structure of the interstellar magnetic field as the true origin for the anisotropy also exist. This feature was further confirmed this year by the LHAASO experimentin China, using a year of data that was taken while constructing the detector. The results from LHAASO also confirm a switch in the phase of the anisotropy when moving from 100s of TeV to PeV energies, as reported by IceCube and other experiments in recent years: at PeV energies, close to the knee, the dipole has a maximum rather than a minimum close to the galactic centre. This could indicate an excess of “pevatron” sources near the galactic centre.
While results up to PeV energies give an insight into sources within our galaxy, it is theorised that the flux starts to be dominated by extragalactic sources somewhere between the knee and the ankle of the cosmic-ray spectrum. Evidence for this was increased by new results from the Pierre Auger Observatory in Argentina and the Telescope Array in the US. These two observatories, which observe different hemispheres, find strong evidence for excesses in the cosmic-ray flux in certain regions of the sky at energies exceeding EeV. At energies as high as these, cosmic rays point more clearly to their origin, and galactic cosmic rays should have very clear point-like sources that are not observed, providing evidence that they originate outside of our galaxy. A prime candidate for such sources are so-called starburst galaxies, wherein star formation happens unusually rapidly, during a short period of the galaxy’s evolution (see “Antennae galaxies” figure).As presented at ICRC 2021, the available data was fitted to models where starburst galaxies are the primary source of EeV cosmic rays. The model fits the anisotropy data with more than 4σ significance relative to the null hypothesis with normal galaxies, indicating starburst galaxies to likely be at least one source of EeV cosmic rays.
While some of the features will likely be fully confirmed within the coming years simply by accumulating statistics, new features are also likely to arise. One example is further constraints on the lack of any observed anisotropy at sub-TeV energies using data from space-based missions, while new data from ground-based experiments will start to bridge the measurement gap between PeV and EeV energies. The latter will be especially important in gaining an understanding of the energy scale at which extragalactic sources start to dominate. To fully exploit the data it will be necessary to compare complex cosmic-ray-propagation simulations with diverse data such as the pevatron sources discovered this year by LHAASO.
From 25 to 28 May, the long-lived particle (LLP) community marked five years of stretching the limits of searches for new physics with its ninth and best-attended workshop yet, with more than 300 registered participants.
LLP9 played host to six new results, three each from ATLAS and CMS. These included a remarkable new ATLAS paper searching for stopped particles – beyond-the-Standard Model (BSM) LLPs that can be produced in a proton–proton collision and then get stuck in the detector before decaying minutes, days or weeks later. Good hypothetical examples are the so-called gluino R-hadrons that occur in supersymmetric models. Also featured was a new CMS search for displaced di-muon resonances using “data scouting” – a unique method of increasing the number of potential signal events kept at the trigger level by reducing the event information that is retained. Both experiments presented new results searching for the Higgs boson decaying to LLPs (see “LLP candidate” figure).
Long-lived particles can also be produced in a collision inside ATLAS, CMS or LHCb and live long enough to drift entirely outside of the detector volume. To ensure that this discovery avenue is also covered for the future of the LHC’s operation, there is a rich set of dedicated LLP detectors either approved or proposed, and LLP9 featured updates from MoEDAL, FASER, MATHUSLA, CODEX-b, MilliQan, FACET and SND@LHC, as well as a presentation about the proposed forward physics facility for the High-Luminosity LHC (HL-LHC).
Reinterpreting machine learning
The liveliest parts of any LLP community workshop are the brainstorming and hands-on working-group sessions. LLP9 included multiple vibrant discussions and working sessions, including on heavy neutral leptons and the ability of physicists who are not members of experimental collaborations to be able to re-interpret LLP searches – a key issue for the LLP community. At LLP9, participants examined the challenges inherent in re-interpreting LLP results that use machine learning techniques, by now a common feature of particle-physics analyses. For example, boosted decision trees (BDTs) and neural networks (NNs) can be quite powerful for either object identification or event-level discrimination in LLP searches, but it’s not entirely clear how best to give theorists access to the full original BDT or NN used internally by the experiments.
LLP searches at the LHC often must also grapple with background sources that are negligible for the majority of searches for prompt objects. These backgrounds – such as cosmic muons, beam-induced backgrounds, beam-halo effects and cavern backgrounds – are reasonably well-understood for Run 2 and Run 3, but little study has been performed for the upcoming HL-LHC, and LLP9 featured a brainstorming session about what such non-standard backgrounds might look like in the future.
Also looking to the future, two very forward-thinking working-group sessions were held on LLPs at a potential future muon collider and at the proposed Future Circular Collider (FCC). Hadron collisions at ~100 TeV in FCC-hh would open up completely unprecedented discovery potential, including for LLPs, but it’s unclear how to optimise detector designs for both LLPs and the full slate of prompt searches.
Simulating dark showers is a longstanding challenge
Finally, LLP9 hosted an in-depth working-group session dedicated to the simulation of “dark showers”, in collaboration with the organisers of the dark-showers study group connected to the Snowmass process, which is currently shaping the future of US particle physics. Dark showers are a generic and poorly understood feature of a potential BSM dark sector with similarities to QCD, which could have its own “dark hadronisation” rules. Simulating dark showers is a longstanding challenge. More than 50 participants joined for a hands-on demonstration of simulation tools and a discussion of the dark-showers Pythia module, highlighting the growing interest in this subject in the LLP community.
LLP9 was raucous and stimulating, and identified multiple new avenues of research. LLPX, the tenth workshop in the series, will be held in November this year.
The 19th international conference on strangeness in quark matter (SQM) was hosted virtually by Brookhaven National Laboratory from 17 to 22 May, attracting more than 300 participants. The series deals with the role of strange and heavy-flavour quarks in high-energy heavy-ion collisions and astrophysical phenomena.
New results on the production of strangeness in heavy-ion collisions were presented for a variety of collision energies and systems. In an experimental highlight, the ALICE collaboration reported that the number of strange baryons depends more on the final-state multiplicity than the initial-state energy. On the theory side, it was shown that several models can explain the suppression of strange particles at low multiplicities. ALICE also presented new measurements of the charm cross section and fragmentation functions in proton–proton (pp) collisions. When compared to e+e– collisions, these results suggest that the universality of parton-to-hadron fragmentation may be broken.
Moving on to heavy flavours, the ATLAS collaboration presented results for the suppression of heavy-flavour production compared to pp collisions and the angular anisotropy of heavy mesons in heavy-ion collisions. These measurements are crucial for constraining models of in-medium energy loss. Interestingly, while charm seems to follow the flow of the quark–gluon plasma, beauty does not seem to flow. Better statistics are needed to constrain theoretical models. On the theory side, extremely interesting new calculations using open quantum systems coupled with potential non-relativistic QCD calculations were used to compute both the suppression and anisotropic flow of bottomonium states.
Hints of extrema
Another important goal of the field is to determine experimentally whether a critical point exists in the phase diagram of strongly interacting matter, and, if so, where it is located. The STAR experiment at the Relativistic Heavy Ion Collider (RHIC) presented results on higher order cumulants of net-proton fluctuations over a range of collision energies. Extrema as a function of beam energy are expected to indicate critical behaviour. New data from the Beam Energy Scan II programme at RHIC is expected to provide much-needed statistics to confirm hints of extrema in the data. On the theory side, new lattice QCD calculations of second-order net-baryon cumulants were presented, as well as new expansion schemes to extend the lattice-QCD equation of state to larger net baryon chemical potentials that are not computable directly, because of the fermion-sign problem. Another study included the lattice-QCD equation of state and susceptibilities in a hydrodynamic calculation to allow for a more direct comparison to experimental measurements of net-proton fluctuations. Significant differences between net-proton and net-baryon fluctuations were quantified.
The study of the quark–gluon plasma’s vorticity via the measurement of the polarisation of hyperons was also a major topic. Theoretical calculations obtain the opposite sign to the data for the angular differential measurement. Attempts to solve this discrepancy presented at SQM 2021 featured shear-dependent terms and a stronger “memory” of the strange-quark spin.
Various new applications of machine learning and artificial intelligence were also discussed, for example, for determining the order of the phase transition and constraining the neutron-star equation of state.
Overall, there were 41 plenary and 96 parallel talks at SQM 2021, poignantly including presentations in memory of Jean Cleymans, Jean Letessier, Dick Majka and Jack Sandweiss, who all made exceptional impacts on the field.
The next SQM conference will be held from 13 to 18 June 2022 in Busan, South Korea.
More than 1000 physicists took part in the ninth Large Hadron Collider Physics (LHCP) conference from 7 to 12 June. The in-person conference was to have been held in Paris: for the second year in a row, however, the organisers efficiently moved the meeting online, without a registration fee, thanks to the support of CERN and IUPAP. While the conference experience cannot be the same over a video link, the increased accessibility for people from all parts of the international community was evident, with LHCP21 participants hailing from institutes across 54 countries.
The LHCP format traditionally has plenary sessions in the mornings and late afternoons, with parallel sessions in the middle of the day. This “shape” was kept for the online meeting, with a shorter day to improve the practicality of joining from distant time zones. This resulted in a dense format with seven-fold parallel sessions, allowing all parts of the LHC programme, both experimental and theoretical, to be explored in detail. The overall vitality of the programme is illustrated by the raw statistics: a grand total of 238 talks and 122 posters were presented.
Last year saw a strong focus on the couplings to the second generation
Nine years on from the discovery of the 125 GeV Higgs boson, measurements have progressed to a new level of precision with the full Run-2 data. Both ATLAS and CMS presented new results on Higgs production, helping constrain the dynamics of the production mechanisms via differential and “simplified template” cross-section measurements. While the couplings of the Higgs to third-generation fermions are now established, last year saw a strong focus on the couplings to the second generation. After first evidence for Higgs decays to muons was reported from CMS and ATLAS results earlier in the year, ATLAS presented a new search with the full Run-2 data for Higgs decays to charm quarks using powerful new charm-tagging techniques. Both CMS and ATLAS showed updated searches for Higgs-pair production, with ATLAS being able to exclude a production rate more than 4.1 times the Standard Model (SM) prediction at 95% confidence. This is a process that should be observable with High-Luminosity LHC statistics, if it is as predicted in the SM. A host of searches were also reported, some using the Higgs as a tool to probe for new physics.
Puzzling hints
The most puzzling hints from the LHC Run 1 seem to strengthen in Run 2. LHCb presented analyses relating to the “flavour anomalies” found most notably in b→sµ+µ− decays, updated to the full data statistics, in multiple channels. While no result yet passes a 5σ difference from SM expectations, the significances continue to creep upwards. Searches by ATLAS and CMS for potential new particles or effects at high masses that could indicate an associated new-physics mechanism continue to draw a blank, however. This remains a dilemma to be studied with more precision and data in Run 3. Other results in the flavour sector from LHCb included a new measurement of the lifetime of the Ωc, four times longer than previous measurements (CERN Courier July/August 2021 p17) and the first observation of a mass difference between the mixed D0–D0 meson mass eigenstates (CERN Courier July/August 2021 p8).
A wealth of results was presented from heavy-ion collisions. Measurements with heavy quarks were prominent here as well. ALICE reported various studies of the differences in heavy-flavour hadron production in proton–proton and heavy-ion collisions, for example using D mesons. CMS reported the first observation of Bc meson production in heavy-ion collisions, and also first evidence for top-quark pair production in lead–lead collisions. ATLAS used heavy-flavour decays to muons to compare suppression of b- and c-hadron production in lead–lead and proton–proton collisions. Beyond the ions, ALICE also showed intriguing new results demonstrating that the relative rates of different types of c-hadron production differ in proton–proton collisions compared to earlier experiments using e+e− and ep collisions at LEP and HERA.
Looking forward, the experiments reported on their preparations for the coming LHC Run 3, including substantial upgrades. While some work has been slowed by the pandemic, recommissioning of the detectors has begun in preparation for physics data taking in spring 2022, with the brighter beams expected from the upgraded CERN accelerator chain. One constant to rely on, however, is that LHCP will continue to showcase the fantastic panoply of physics at the LHC.
Which charmed baryon decays first? The LHCb collaboration recently challenged the received wisdom offixed-target experiments by almost quadrupling the measured lifetime of the doubly strange Ωc0. Now, a follow-up measurement by the collaboration confirms the revised hierarchy, offering valuable input to theoretical models of the decays.
The situation changed dramatically in 2018
Ground-state baryons containing a charm quark (c), such as Λc+ (udc), Ξc+ (usc), Ξc0 (dsc) and Ωc0 (ssc), decay via the weak interaction. The ordering of their lifetimes has long been thought to be τ(Ξc+) > τ(Λc+) > τ(Ξc0) > τ(Ωc0), based on measurements from fixed-target experiments nearly 20 years ago. However, the situation changed dramatically in 2018 when LHCb joined the game using a sample of Ωc0 baryons obtained from bottom- baryon semileptonic decays. That LHCb study measured the Ωc0 lifetime to be nearly four times larger than previously measured, transforming the hierarchy into τ(Ξc+) > τ(Ωc0) > τ(Λc+) > τ(Ξc0). One year later, LHCb significantly improved the precisions of the lifetimes of the other three charmed baryons using the same method, also finding the lifetime of the Ξc0 baryon to be larger than the world-average value by about 3σ (figure 1).
Theoretically challenging
The corresponding theoretical calculations are challenging. In the charm sector, an effective theory of heavy-quark expansion is taken to calculate lifetimes of charmed baryons through an expansion in powers of 1/mc, where mc is the constituent charm–quark mass. Calculations up to order 1/mc3 imply a lifetime hierarchy consistent with the original fixed-target measurements, though only qualitatively. Attempts at higher-order calculations up to order 1/mc4, however, cannot accommodate the old hierarchy, but can explain the new one if a suppression factor to the constructive Pauli-interference and semileptonic terms is written in. The origin of the suppression factor is still unknown, but probably due to even higher order effects. An independent measurement was therefore needed to confirm the experimental situation.
The charmed-baryon lifetime puzzle has now been resolved by a new measurement from LHCb using a much larger sample of Ωc0 and Ξc0 baryons produced directly in proton–proton collisions. Both particles are detected in the final state pK–K–π+. The measurement is made relative to the lifetime distribution of the charmed meson D0 via D0→ K+K–π+π– decays, in order to control systematic uncertainties. Taking advantage of the performance and detailed understanding of the LHCb detector, the lifetimes of the Ωc0 and Ξc0 baryons are found to be τ(Ωc0) = 276.5 ± 13.4 (stat) ± 4.5 (syst) fs and τ(Ξc0) = 148.0 ± 2.3 (stat) ± 2.2 (syst) fs, respectively, where the precision of the Ωc0 lifetime is improved by a factor of two compared to the semileptonic measurement. The new results are consistent with the previous LHCb measurements, and hence establish the new lifetime hierarchy. Combining this measurement with the previous LHCb results gives τ(Ωc0) = 274.5 ± 12.4 fs and τ(Ξc0) = 152.0 ± 2.0 fs, the most precise charm-baryon lifetimes to date. The newly confirmed lifetime hierarchy will help improve our knowledge of QCD dynamics in charm hadrons, and provides a crucial input to calibrate theoretical calculations.
The production of four top quarks is an extremely rare event at the LHC, with an expected cross section five orders of magnitude below the production of a top-quark pair. With the heaviest elementary particle in the Standard Model produced four times in the final state, it is also one of the most spectacular processes accessible at the LHC. By combining two analyses, the ATLAS collaboration has uncovered the first strong evidence to support the existence of this unique event topology with sensitivity to theories beyond the Standard Model (BSM).
This is the only process that could probe potentially anomalous effective four-heavy-fermion operators
As a result of its large mass, the top quark plays a special role in numerous BSM theories, and many of these theories predict an increase in the four-top-quark production cross section. In particular, four-top-quark production is the only process that could probe potentially anomalous effective four-heavy-fermion operators. The cross section is also sensitive to the value of the top-quark Yukawa coupling, as a result of contributions mediated by Higgs bosons. However, until now, four-top-quark production has not been observed, in part because of its tiny production rate, and in part because the experimental signature of this process is very complex, requiring up to 12 particles to be reconstructed from the top-quark decays. The search is also affected by background sources in kinematic regions that are at the limit of the domain of validity of the simulations.
Despite these challenges, the ATLAS collaboration has recently released two studies of four-top-quark production using its full Run-2 data sample. The first study searches for events with two leptons (electrons or muons) with the same electric charge or with three leptons. This selection corresponds to only 13% of all possible four-top-quark final states, but is contaminated by only a small background, mainly from the production of a top-quark pair with a W, Z or Higgs boson and additional jets, or from events with one lepton with misidentified electric charge or a “fake” lepton that doesn’t correspond to a W or Z boson decay. Background processes were primarily simulated using the best available theoretical predictions; the rates of the most difficult ones were measured using control samples with similar properties to the signal events. The second study searches for events with one lepton or two oppositely-charged leptons. This selection retains 57% of the possible four-top-quark final states, but suffers from a large background from top-quark pairs produced in association with many jets, some of which are consistent with originating from b-quarks (b-jets). This background is difficult to model and was determined using data control samples. To better isolate the signal from the background, multivariate discriminants were trained in both analyses using distinct features of the signal, such as the number of b-jets and the kinematic properties of the reconstructed particles (see figure 1).
Results from the two studies were combined, leading to a four-top-quark cross-section measurement at 13 TeV of 25+7–6 fb, which is consistent with the Standard Model prediction of 12.0 ± 2.4 fb within 2.0σ (see figure 2). The statistical significance of the signal corresponds to 4.7σ, providing strong evidence for this process, close to the observation threshold of 5σ. LHC Run-3 data, possibly at a higher centre-of-mass energy, will allow ATLAS to verify whether the larger measured cross section relative to the prediction is confirmed or not.
The European Consortium for Astroparticle theory (EuCAPT) held its first annual symposium from 5 to 7 May. Hundreds of theoretical physicists from Europe and beyond met online to discuss the present and future of astroparticle physics and cosmology, in a dense and exciting meeting that featured 29 invited presentations, 42 lightning talks by young researchers, and two community-wide brainstorming sessions.
Participants discussed a wide array of topics at the interface between particle physics, astrophysics and cosmology, with particular emphasis on the challenges and opportunities for these fields in the next decade. Rather than focusing on experimental activities and the discoveries they might enable, the sessions were structured around thematic areas and explored the interdisciplinary multi-messenger aspects of each.
Two sessions were dedicated to cosmology, exploring the early and late universe. As stressed by Geraldine Servant (Hamburg), several unresolved puzzles of particle physics – such as the origin of dark matter, the baryon asymmetry, and inflation – are directly linked to the early universe, and new observational probes may soon shed new light on them.
Julien Lesgourgues (Aachen) showed how the very same puzzles are also linked to the late universe, and cautiously elaborated on a series of possible inconsistencies between physical quantities inferred from early- and late-universe probes, for example the Hubble constant. Those inconsistencies represent both a challenge and an extraordinary opportunity for cosmology, as they might “break” the standard Lambda–cold-dark-matter model of cosmology, and allow us to gain insights into the physics of dark matter, dark energy and gravity.
We are witnessing a proliferation of theoretically well-motivated models
New strategies to go beyond the standard models of particle physics and cosmology were also discussed by Marco Cirelli (LPTHE) and Manfred Lindner (Heidelberg), in the framework of dark-matter searches and neutrino physics, respectively. Progress in both fields is currently not limited by a lack of ideas – we are actually witnessing a proliferation of theoretically well-motivated models – but by the difficulty of identifying experimental strategies to conclusively validate or rule them out. Much of the discussion here concerned prospects for detecting new physics with dedicated experiments and multi-messenger observations.
Gravitational waves have added a new observational probe in astroparticle physics and cosmology. Alessandra Buonanno (Max Planck Institute for Gravitational Physics) illustrated the exciting prospects for this new field of research, whose potential for discovering new physics is attracting enormous interest from particle and astroparticle theorists. The connection between cosmic rays, gamma rays and high-energy neutrinos was explored in the final outlook by Elena Amato (Arcetri Astrophysical Observatory), who highlighted how progress in theory and observations is leading the community to reconsider some long-held beliefs – such as the idea that supernova remnants are the acceleration sites of cosmic rays up to the so-called “knee” – and stimulating new ideas.
In line with EuCAPT’s mission, the local organisers and the consortium’s steering committee organised a series of community-building activities. Participants stressed the importance of supporting diversity and inclusivity, a continuing high priority for EuCAPT, while a second brainstorming session was devoted to the discussion of the EuCAPT white paper currently being written, which should be published by September. Last but not least, Hannah Banks (Cambridge), Francesca Capel (TU Munich) and Charles Dalang (University of Geneva) received prizes for the best lightning talks, and Niko Sarcevic (Newcastle) was awarded an “outstanding contributor” prize for the help and support she provides for the analysis of the EuCAPT census (pictured).
The next symposium will take place in 2022, hopefully in person, at CERN.
Recent years have seen rapid growth in high-energy gamma-ray astronomy, with the first measurement of TeV photons from gamma-ray bursts by the MAGIC telescope and the first detection of gamma rays with energies above 100 TeV by the HAWC observatory.
Now, the Large High Altitude Air Shower Observatory (LHAASO) in China has increased the energy scale at which the universe has been observed by a further order of magnitude. The recent LHAASO detection provides the first clear evidence of the presence of galactic “pevatrons”: sources in the Milky Way capable of accelerating protons and electrons to PeV energies. Although PeV cosmic rays are known to exist, magnetic fields pervading the universe perturb their direction and therefore do not allow their origin to be traced. The gamma rays produced by such cosmic-rays, on the other hand, point directly to their source.
Wide field of view
LHAASO is located in the mountains of the Sichuan province of China and offers a wide field of view to study both high-energy cosmic and gamma rays. Once completed, the observatory will contain a water Cherenkov detector with a total area of about 78,000 m2, 18 widefield- of-view Cherenkov telescopes and a 1 km2 array of more than 5000 scintillator- based electromagnetic detectors (EDs). Finally, more than 1000 underground water Cherenkov tanks (the MDs) are placed over the grid to detect muons.
The latter two detectors, of which only half were finished during data-taking for this study, are used to directly detect the showers produced when high-energy particles interact with the Earth’s atmosphere. The EDs detect the shower profile and incoming angle, using charge and timing information of the detector array, while the MDs are used to distinguish hadronic showers from the electromagnetic showers produced by high-energy gamma rays. Thanks to both its large size and the MDs, LHAASO will ultimately be two orders of magnitude more sensitive at 100 TeV than the HAWC facility in Mexico, the previous most sensitive detector of this type.
The measurements reported by the Chinese-led international LHAASO collaboration reveal a total of 12 sources Astrowatch Mountain observatory nets PeV gamma rays located across the galactic plane (see image above). This distribution is expected, since gamma rays at such energies have a high cross-section for pair production with the cosmic microwave background and therefore the universe starts to become opaque at energies exceeding tens to hundreds of TeV, leaving only sources within our galaxy visible. Of the 12 presented sources, only the Crab nebula can be directly confirmed. This substantiates the pulsar-wind nebulae as a source in which electrons are accelerated beyond PeV energies, which in turn are responsible for the gamma rays through inverse Compton scattering.
Of specific interest is the source responsible for the photon with the highest energy, 1.4 PeV
The origin of the other photons remains unknown as the observed emission regions contain several possible sources within them. The sizes of the emission regions exceed the angular resolution of LHAASO, however, indicating that emission takes place over large scales. Of specific interest is the source responsible for the photon with the highest energy, 1.4 PeV. This came from a region containing both a supernova remnant as well as a star-forming cluster, both of which are prime theoretical candidates for hadronic pevatrons.
Tip of the iceberg
More detailed spectrometry as well as morphological measurements, in which the differences in emission intensity throughout the sources are measured, could allow the sources of > 100 TeV gamma rays to be identified in the next one or two years, say the authors. Furthermore, as the current 12 sources were visible using only one year of data from half the detector, it is clear that LHAASO is only seeing the tip of iceberg when it comes to high-energy gamma rays.
More than a century after its discovery, the proton remains a source of intrigue, its charge-radius and spin posing puzzles that are the focus of intense study. But what of its mortal sibling, the neutron? In recent years, discrepancies between measurements of the neutron lifetime using different methods constitute a puzzle with potential implications for cosmology and particle physics. The neutron lifetime determines the ratio of protons to neutrons at the beginning of big-bang nucleosynthesis and thus affects the yields of light elements, and it is also used to determine the CKM matrix-element Vud in the Standard Model.
The neutron-lifetime puzzle stems from measurements using two techniques. The “bottle” method counts the number of surviving ultra-cold neutrons contained in a trap after a certain period, while the “beam” method uses the decay probability of the neutron obtained from the ratio of the decay rate to an incident neutron flux. Back in the 1990s, the methods were too imprecise to worry about differences between the results. Today, however, the average neutron lifetime measured using the bottle and beam methods, 879.4 ± 0.4 s and 888.0 ± 2.0 s, respectively, stand 8.6 s (or 4σ) apart.
We think it will take two years to obtain a competitive result from our experiment
Kenji Mishima
In an attempt to shed light on the issue, a team at Japan’s KEK laboratory in collaboration with Japanese universities has developed a new experimental setup. Similar to the beam method, it compares the decay rate to the reaction rate of neutrons in a pulsed beam from the Japan Proton Accelerator Research Complex (J-PARC). The decay rate and the reaction rate are determined by simultaneously detecting electrons from the neutron decay and protons from the reaction 3He →3H in a 1 m-long time-projection chamber containing diluted 3He, removing some of the systematic uncertainties that affect previous beam methods. The experiment is still in its early stages, and while the first results have been released – τn = 898 ± 10(stat)+15–18 (sys) s – the uncertainty is currently too large to draw conclusions.
“In the current situation, it is important to verify the puzzle by experiments in which different systematic errors dominate,” says Kenji Mishima of KEK, adding that further improvements in the statistical and systematic uncertainties are underway. “We think it will take two years to obtain a competitive result from our experiment.”
Several new-physics scenarios have been proposed as solutions of the neutron lifetime puzzle. These include exotic decay modes involving undetectable particles with a branching ratio of about 1%, such as “mirror neutrons” or dark-sector particles.
One day, around the time I started properly reading, somebody gave me a book about the sky, and I found it fascinating to think about what’s beyond the clouds and beyond where the planes and the birds fly. I didn’t know that you could actually make a living doing this kind of thing. At that age, you don’t know what a cosmologist is, unless you happen to meet one and ask what they do. You are just fascinated by questions like “how does it work?” and “how do you know?”.
Was there a point at which you decided to focus on theory?
Not really, and I still think I’m somewhat in-between, in the sense that I like to interpret data and am plugged-in to observational collaborations. I try to make connections to what the data mean in light of theory. You could say that I am a theoretical experimentalist. I made a point to actually go and serve at a telescope a couple of times, but you wouldn’t want to trust me in handling all of the nitty-gritty detail, or to move the instrument around.
What are your research interests?
I have several different research projects, spanning large-scale structure, dark energy, inflation and the cosmic microwave background. But there is a common philosophy: I like to ask how much can we learn about the universe in a way that is as robust as possible, where robust means as close as possible to the truth, even if we have to accept large error bars. In cosmology, everything we interpret is always in light of a theory, and theories are always at some level “spherical cows” – they are approximations. So, imagine we are missing something: how do I know I am missing it? It sounds vague, but I think the field of cosmology is ready to ask these questions because we are swimming in data, drowning in data, or soon will be, and the statistical error bars are shrinking.
This explains your current interest in the Hubble constant. What do you define as the Hubble tension?
Yes, indeed. When I was a PhD student, knowing the Hubble constant at the 40–50% level was great. Now, we are declaring a crisis in cosmology because there is a discrepancy at the very-few-percent level. The Hubble tension is certainly one of the most intriguing problems in cosmology today. Local measurements of the current expansion rate of the universe, for example based on supernovae as standard candles, which do not rely heavily on assumptions about cosmological models, give values that cluster around 73 km s–1 Mpc–1. Then there is another, indirect route to measuring what we believe is the same quantity but only within a model, the lambda-cold-dark-matter (ΛCDM) model, which is looking at the baby universe via the cosmic microwave background (CMB). When we look at the CMB, we don’t measure recession velocities, but we interpret a parameter within the model as the expansion rate of the universe. The ΛCDM model is extremely successful, but the value of the Hubble constant using this method comes out at around 67 km s–1 Mpc–1, and the discrepancy with local measurements is now 4σ or more.
What are the implications if this tension cannot be explained by systematic errors or some other misunderstanding of the data?
The Hubble constant is the only cosmological parameter in the ΛCDM universe that can be measured both directly locally and from classical cosmological observations such as the CMB, baryon acoustic oscillations, supernovae and big-bang nucleosynthesis. It’s also easy to understand what it is, and the error bars are becoming small enough that it is really becoming make-or-break for the ΛCDM model. The Hubble tension made everybody wake up. But before we throw the model out of the window, we need something more.
How much faith do you put in the ΛCDM model compared to, say, the Standard Model of particle physics?
It is a model that has only six parameters, most constrained at the percent level, which explains most of the observations that we have of the universe. In the case of Λ, which quantifies what we call dark energy, we have many orders of magnitude between theory and experiment to understand, and for dark matter we are yet to find a candidate particle. Otherwise, it does connect to fundamental physics and has been extremely successful. For 20 years we have been riding a wave of confirmation of the ΛCDM model, so we need to ask ourselves: if we are going to throw it out, what do we substitute it with? The first thing is to take small steps away from the model, say by adding one parameter. For a while, you could say that maybe there is something like an effective neutrino species that might fix it, but a solution like this doesn’t quite fit the CMB data any more. I think the community may be split 50/50 between being almost ready to throw the model out and keeping working with it, because we have nothing better to use.
It is really becoming make-or-break for the ΛCDM model
Could it be that general relativity (GR) needs to be modified?
Perhaps, but where do we modify it? People have tried to tweak GR at early times, but it messes around with the observations and creates a bigger problem than we already have. So, let’s say we modify in middle times – we still need it to describe the shape of the expansion history of the universe, which is close to ΛCDM. Or we could modify it locally. We’ve tested GR at the solar-system scale, and the accuracy of GPS is a vivid illustration of its effectiveness at a planetary scale. So, we’d need to modify it very close to where we are, and I don’t know if there are modifications on the market that pass all of the observational tests. It could also be that the cosmological constant changes value as the universe evolves, in which case the form of the expansion history would not be the one of ΛCDM. There is some wiggle room here, but changing Λ within the error bars is not enough to fix the mismatch. Basically, there is such a good agreement between the ΛCDM model and the observations that you can only tinker so much. We’ve tried to put “epicycles” everywhere we could, and so far we haven’t found anything that actually fixes it.
What about possible sources of experimental error?
Systematics are always unknowns that may be there, but the level of sophistication of the analyses suggests that if there was something major then it would have come up. People do a lot of internal consistency checks; therefore, it is becoming increasingly unlikely that it is only due to dumb systematics. The big change over the past two years or so is that you typically now have different data sets that give you the same answer. It doesn’t mean that both can’t be wrong, but it becomes increasingly unlikely. For a while people were saying maybe there is a problem with the CMB data, but now we have removed those data out of the equation completely and there are different lines of evidence that give a local value hovering around 73 km s–1 Mpc–1, although it’s true that the truly independent ones are in the range 70–73 km s–1 Mpc–1. A lot of the data for local measurements have been made public, and although it’s not a very glamorous job to take someone else’s data and re-do the analysis, it’s very important.
Is there a way to categorise the very large number of models vying to explain the Hubble tension?
Until very recently, there was an interpretation of early versus late models. But if this is really the case, then the tension should show up in other observables, specifically the matter density and age of the universe, because it’s a very constrained system. Perhaps there is some global solution, so a little change here and a little in the middle, and a little there … and everything would come together. But that would be rather unsatisfactory because you can’t point your finger at what the problem was. Or maybe it’s something very, very local – then it is not a question of cosmology, but whether the value of the Hubble constant we measure here is not a global value. I don’t know how to choose between these possibilities, but the way the observations are going makes me wonder if I should start thinking in that direction. I am trying to be as model agnostic as possible. Firstly, there are many other people that are thinking in terms of models and they are doing a wonderful job. Secondly, I don’t want to be biased. Instead I am trying to see if I can think one-step removed, which is very difficult, from a particular model or parameterisation.
What are the prospects for more precise measurements?
For the CMB, we have the CMB-S4 proposal and the Simons Array. These experiments won’t make a huge difference to the precision of the primary temperature-fluctuation measurements, but will be useful to disentangle possible solutions that have been proposed because they will focus on the polarisation of the CMB photons. As for the local measurements, the Dark Energy Spectroscopic Instrument, which started observations in May, will measure baryon acoustic oscillations at the level of galaxies to further nail down the expansion history of the low-redshift universe. However, it will not help at the level of local measurements, which are being pursued instead by the SH0ES collaboration. There is also another programme in Chicago focusing on the so-called tip of the red-giant-branch technique, with more results to come out. Observations of multiple images from strong gravitational lensing is another promising avenue that is very actively pursued, and, if we are lucky, gravitational waves with optical counterparts will bring in another important piece of the puzzle.
If we are lucky, gravitational waves with optical counterparts will bring in another important piece of
the puzzle
How do we measure the Hubble constant from gravitational waves?
It’s a beautiful measurement, as you can get a distance measurement without having to build a cosmic distance ladder, which is the case with the other local measurements that build distances via Cepheids, supernovae, etc. The recession velocity of the GW source comes from the optical counterpart and its redshift. The detection of the GW170817 event enabled researchers to estimate the Hubble constant to be 70 km s–1 Mpc–1, for example, but the uncertainties using this novel method are still very large, in the region of 10%. A particular source of uncertainty comes from the orientation of the gravitational-wave source with respect to Earth, but this will come down as the number of events increases. So this route provides a completely different window on the Hubble tension. Gravitational waves have been dubbed, rather poetically, “standard sirens”. When these determinations of the Hubble constant become competitive with existing measurements really depends on how many events are out there. Upgrades to LIGO, VIRGO, plus next-generation gravitational-wave observatories will help in this regard, but what if the measurements end up clustering between or beyond the late- and early-time measurements? Then we really have to scratch our heads!
How can results from particle physics help?
Principally, if we learn something about dark matter it could force us to reconsider our entire way to fit the observations, perhaps in a way that we haven’t thought of because dark matter may be hot rather than cold, or something else that interacts in completely different ways. Neutrinos are another possibility. There are models where neutrinos don’t behave like the Standard Model yet still fit the CMB observations. Before the Hubble tension came along, the hope was to say that we have this wonderful model of cosmology that fits really well and implies that we live in a maximally boring universe. Then we could have used that to eventually make the connection to particle physics, say, by constraining neutrino masses or the temperature of dark matter. But if we don’t live in a maximally boring universe, we have to be careful about playing this game because the universe could be much, much more interesting than we assumed.
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