The study of lead–ion collisions at the LHC is a window into the quark–gluon plasma (QGP), a hot and dense phase of deconfined quarks and gluons. An important effect in heavy-ion collisions is jet quenching – the suppression of particle production at large transverse momenta (pT) due to energy loss in the QGP. This suppression is quantified by the nuclear-modification factor RAA, which is the ratio of particle production rate in Pb–Pb collisions to that in proton–proton collisions, scaled for the number of binary nucleon–nucleon collisions. A measured nuclear modification factor of unity would indicate the absence of final-state effects such as jet quenching.
Previous measurements of peripheral collisions revealed less suppression than seen in head-on collisions, but RAA remained significantly below unity. This observation indicates the formation of a dense and strongly interacting system – but it also poses a puzzle. In p–Pb collisions, no suppression has been observed, even though the energy densities are similar to those in peripheral Pb–Pb collisions.
The ALICE collaboration has recently put jet quenching to the test experimentally by performing a rigorous measurement of RAA in narrow centrality bins. The results (figure 1, left) show that the trend of a gradual reduction in the suppression of high-pT particle production as one moves from the most central collisions (corresponding to the 0% centrality percentile) to those with a greater impact parameter does not continue above a centrality of 75%. Instead, the data show a dramatically different behaviour: increasingly strong suppression for the most peripheral collisions. The change at 75% centrality shows that the suppression mechanism for peripheral collisions is fundamentally different from that observed in central collisions, where the suppression can be explained by parton energy loss in the QGP.
In a single Pb–Pb collision several nucleons collide. It has recently been suggested that the alignment of each nucleon collision plays an important role: if the nucleons are aligned, a single collision produces more particles, which results in a correlation between particle production at low pT, which is used to determine the centrality, and at high pT, where RAA is measured. The suppression in the peripheral events can be modelled with a simple PYTHIA- based model that does not implement jet-quenching effects, but incorporates the biases originating from the alignment of the nucleons, yielding qualitative agreement above 75% centrality (figure 1, right).
These results demonstrate that with the correct treatment of biases from the parton–parton interactions the observed suppression in Pb–Pb collisions is consistent with results from p–Pb collisions at similar multiplicities – an important new insight into the nuclear modification factor in small systems.
The LHCb collaboration has discovered a new pentaquark particle, dubbed the Pc(4312)+, decaying to a J/ψ and a proton, with a statistical significance of 7.3 standard deviations. The LHCb data, first presented at Rencontres de Moriond in March, also confirm that the Pc(4450)+ structure previously reported by the collaboration in 2015 has now been resolved into two narrow, overlapping peaks, the Pc(4440)+ and Pc(4457)+, with a statistical significance of 5.4 standard deviations compared to the single-peak hypothesis (figure 1). Together, the results offer rich studies of the strong internal dynamics of exotic hadrons.
In the famous 1964 papers that set out the quark model, Murray Gell-Mann and George Zweig mentioned the possibility of adding a quark–antiquark pair to the minimal meson and baryon states qq̅ and qqq, thereby proposing the new configurations qqq̅q̅ and qqqqq̅. Nearly four decades later, the Belle collaboration discovered the surprisingly narrow X(3872) state with a mass very close to the D0D̅*0 threshold, hinting at a tetraquark structure (cc̅uu̅). A decade after that, Belle discovered narrow Zb0,± states just above the BB̅* and B*B̅* thresholds; this was followed by observations of Zc0,± states just above the equivalent charm thresholds by BES-III and Belle. The existence of charged Zb± and Zc± partners makes the exotic nature of these states clear: they cannot be described as charmonium (cc̅) or bottomonium (bb̅) mesons, which are always neutral, but must instead be a combination such as cc̅ud̅. There is also evidence for broad Zc± states from Belle and LHCb, such as the Zc(4430)±.
A major turning point in exotic baryon spectroscopy was achieved by LHCb in July 2015 when, based on an analysis of Run 1 data, the collaboration reported significant pentaquark structures in the J/ψ−p mass distribution in Λb0→ J/ψpK− decays. A narrow Pc(4450)+ and a broad Pc(4380)+ were reported, both with minimal quark content of cc̅uud (CERN Courier September 2015 p5).
The new results use the data collected at LHCb in Run 1 and Run 2, providing a Λb0 sample nine times larger than that used in the 2015 paper. The new data reproduce the parameters of the Pc(4450)+ and Pc(4380)+ states when analysed the same way as before. However, the much larger dataset makes a more fine-grained analysis possible, revealing additional peaking structures in the J/ψ-p invariant mass spectrum that were not visible before. A new narrow peak, with a width comparable to the mass resolution, is observed near 4312 MeV, right below the Σ+cD̅0 threshold. The structure seen before at 4450 MeV has been resolved into two narrower peaks, at 4440 and 4457 MeV. The latter is right at the Σ+cD̅*0 threshold.
These Pc states join a growing family of narrow exotic hadrons with masses near hadron–hadron thresholds. This is expected in certain models of loosely bound “molecular” states whose structure resembles the way a proton and neutron bind to form a deuteron. Other models, such as of tightly bound pentaquarks, could also explain the Pc resonances. A more complete understanding will require further experimental and theoretical investigation.
Searching for the decay μ+ → e+γ is like looking for a needle in a haystack the size of the Great Pyramid of Giza. This simile-stretching endeavour is the task of the MEG II experiment at the Paul Scherrer Institute (PSI) in Villigen, Switzerland. MEG II is an upgrade of the previous MEG experiment, which operated from 2008 to 2013. All experimental data so far are consistent with muon decays that conserve lepton flavour by the production of two appropriately flavoured neutrinos. Were MEG II to observe the neutrinoless decay of the muon to a positron and a photon, it would be the first evidence of flavour violation with charged leptons, and unambiguous evidence for new physics.
Lepton-flavour conservation is a mainstay of every introductory particle-physics course, yet it is merely a so-called accidental symmetry of the Standard Model (SM). Unlike gauge symmetries, it arises because only massless left-handed neutrinos are included in the model. The corresponding mass and interaction terms of the Lagrangian can therefore be simultaneously diagonalised, which means that interactions always conserve lepton flavour. This is not the case in the quark sector, and as a result quark flavour is not conserved in weak interactions. Since lepton flavour is not considered to be a fundamental symmetry, most extensions of the SM predict its violation at a level that could be observed by state-of-the-art experiments.
Indeed an extension of the SM is already required to include the tiny neutrino masses that we infer from neutrino oscillations. In this extension, neutrino oscillations induce charged lepton-flavour-violating processes but with the branching ratio for μ+ → e+γ emerging to be only 10–54, which cannot be accessed experimentally (see “Charged lepton-flavour violation in the SM” box). A data sample of muons as large as the number of protons in the Earth would not be enough to see such an improbable decay. Charged lepton-flavour violation is therefore a clear signature of new physics with no SM backgrounds.
Finding the needle
The search requires an intense source of muons, and detectors capable of reconstructing the kinematics of the muon’s decay products with high precision. PSI offers the world’s most intense continuous muon beams, delivering up to 108 muons per second. MEG II (previously as MEG) is designed to search for μ+ → e+γ by stopping positive muons on a thin target, and looking for positron–photon pairs from muon decays at rest. This method exploits the two-body kinematics of the decay to discriminate signal events from the backgrounds, which are predominantly the radiative muon decay μ+ → e+ νe ν̅μ γ and the accidental time coincidence of a positron and photon produced by different muon decays.
In the late 1990s, when the first MEG experiment was being designed, theorists argued that the μ+ → e+γ branching ratio could be as high as 10–12 to 10–14, based on supersymmetry arising at the TeV scale. Twenty years later, MEG has excluded branching ratios above 4.2 × 10–13 (figure 1), and supersymmetric particles remain undiscovered at the LHC. Nevertheless, since charged lepton-flavour-violating processes are sensitive to the virtual exchange of new particles, while not requiring their creation as at the LHC, they can probe new physics models (supersymmetry, extra dimensions, leptoquarks, multi-Higgs, etc) up to mass scales of thousands of TeV. Scales such as these are not only unreachable at the LHC, but also at near-future accelerators.
The MEG collaboration therefore decided to upgrade the detectors with the goal of improving the sensitivity of the experiment by a factor of 10. The new experiment, which adopts the same measurement principle, is expected to start taking data at the end of 2019 (figure 2). Photons are reconstructed by a liquid xenon (LXe) detector technology that was pioneered by the MEG collaboration, achieving an unprecedented ~2% calorimetric resolution at energies as low as 52.8 MeV – the energy of the photon in a μ+ → e+γ decay. The LXe detector provides a high-resolution measurement of the position and timing of the photon conversion, precise to a few millimetres and approximately 70 ps. The positrons are reconstructed in a magnetic spectrometer instrumented with drift chambers for tracking, and scintillator bars for timing. A peculiarity of the MEG spectrometer is a non-uniform magnetic field, diminishing from 1.2 T at the centre of the detector to 0.5 T at the extremities. The gradated field prevents positrons from curling too many times. This avoids pileup in the detectors and makes positrons of the same momentum curl with the same radius, independent of their emission angle, thus simplifying the design and operation of the tracking system.
Following a major overhaul that was begun in 2011, all the detectors have now been upgraded. Silicon photomultipliers custom-modified for sensitivity to the ultraviolet LXe scintillation light have replaced conventional photomultipliers on the inner face of the calorimeter. Small scintillating tiles have replaced the scintillating bars of the positron-timing detector to improve timing and reduce pileup. The main challenge when upgrading the drift chambers was dealing with high positron rates. Here, the need for high granularity had to be balanced by keeping the total amount of material low. This reduces both multiple scattering and the rate of positrons annihilating in the material, and contributions to the coincident-photon background in the calorimeter. The solution was the use of extremely thin 40 and 50 μm silver-plated aluminium wires, 20 μm gold-plated tungsten wires, and innovative assembly techniques. All the detectors’ resolutions were improved by a factor of around two with respect to the MEG experiment. The MEG II design also includes a new detector to veto photons coming from radiative muon decays, improved calibration tools and new trigger and data-acquisition electronics to cope with the increased number of readout channels. The improved detector performance will allow the muon beam rate to be more than doubled, from 3.3 × 107 to 7 × 107 muons per second.
The detectors were installed and tested in the muon beam in 2018. In 2019 a test of the whole detector will be completed, with the possibility of collecting the first physics data. The experiment is then expected to run for three years to uncover evidence for the μ+ → e+γ decay if the branching ratio is around 10–13 or set a limit of 6 × 10–14 on its branching ratio.
Charged lepton-flavour violation in the SM – a very small neutrino oscillation experiment
The presence of only massless left-handed neutrinos in the Standard Model (SM) gives rise to the accidental symmetry of lepton-flavour conservation – yet neutrino oscillation experiments have observed neutrinos changing flavour in-transit from sources as far away as the Sun and as near as a nuclear reactor. Such neutral lepton-flavour violation implies that neutrinos have tiny masses and that their flavour eigenstates are distinct from their mass eigenstates. Phases develop between the mass eigenstates as a neutrino travels, and the wavefunction becomes a mixture of the flavour eigenstates, rather than the unique original flavour, as would remain the case for truly massless neutrinos.
The effect on charged lepton-flavour violation is subtle and small. In most neutrino oscillation experiments, a neutrino is created in a charged-current interaction and observed in a later interaction via the creation of a charged lepton of the corresponding flavour in the detector.
μ+ → e+γ may proceed in a similar way, but where the same W boson is involved in both the creation and destruction of the neutrino, and the neutrino oscillates in between (see figure above).
In this process, the neutrino oscillation ν̅μ→ν̅e has to occur at an energy scale E ~ mw, over an extremely short distance of L ~ 1/mw. Considering only two neutrino species with masses m1 and m2, the probability for the oscillation is proportional to sin2 [(m21 – m22) L /4E]. Hence, the μ → eγ branching ratio is suppressed by the tiny factor (m21 – m22)/m2w)2 ≲ 10–49. The exact calculation, including the most recent estimates of the neutrino mixing matrix elements, gives BR(μ → eγ) ~ 10–54.
New directions
In the meantime, PSI researchers are investigating the possibility of building new beamlines with 109 or even 1010 muons per second to allow experimenters to probe even smaller branching ratios. How could a future experiment cope with such high rates? Preliminary studies are investigating a system where photons are converted into pairs of electrons and positrons, and reconstructed in a tracking device. This solution, which has already been exploited previously by the MEGA experiment at Los Alamos National Laboratory, could also improve the photon resolution.
At the same time, other experiments are searching for charged lepton-flavour violation in other channels. Mu3e, also at PSI, will search for μ+ → e+e+e– decays. The Mu2e and COMET experiments, at Fermilab and J-PARC, respectively, will search for muon-to-electron conversion in the field of a nucleus. These processes are complementary to μ+ → e+γ,allowing alternative scenarios to be probed. At the same time, collider experiments such as Belle II and LHCb are working on studies of lepton-flavour violation in tau decays. LHCb researchers are also testing lepton universality, which holds that the weak couplings are the same for each lepton flavour (see The flavour of new physics). As theorists often stress, all these analyses are strongly complementary both with each other and with direct searches for new particles at the LHC.
Ever since the pioneering work of Conversi, Pancini and Piccioni, muons have played a crucial role in the development of particle physics. When I I Rabi exclaimed “who ordered that?”, he surely did not imagine that 80 years later the lightest unstable elementary particle would still be a focus of cutting-edge research.
On 10 April, researchers working on the Event Horizon Telescope – a network of eight radio dishes that creates an Earth-sized interferometer – released the first direct image of a black hole. The landmark result, which shows the radiation emitted by superheated gas orbiting the event horizon of a super massive black hole in a nearby galaxy, opens a brand new window on these incredible objects.
Super massive black holes (SMBHs) are thought to occupy the centre of most galaxies, including our own, with masses up to billions of solar masses and sizes up to 10 times larger than our solar system. Discovered in the 1960s via radio and optical measurements, their origin, as well as their nature and surrounding environments, remain important open issues within astrophysics. Spatially resolved images of an SMBH and the potential accretion disks around them form vital input, but producing such images is extremely challenging.
SMBHs are relatively bright in radio wavelengths. However, since the imaging resolution achievable with a telescope scales with the wavelength (which is long in the radio range) and scales inversely with the telescope diameter, it is difficult to obtain useful images in the radio region. For example, producing an image with the same resolution as the optical Hubble Space Telescope would require a km-wide telescope, while obtaining a resolution that would allow an SMBH to be imaged, would require a telescope diameter of thousands of kilometres. One way around this is to use interferometry to turn many telescopes dishes at different locations into one large telescope. Such an interferometer measures the differences in arrival time of one radio wave at different locations on Earth (induced by the difference in travel path), from which it is possible to reconstruct an image on the sky. This does not only require a large coordination between many telescopes around the world, but also very precise timing, vast amounts of collected data and enormous computing power.
Despite the considerable difficulties, the Event Horizon Telescope project used this technique to produce the first image of an SMBH using an observation time of only tens of minutes. The imaged SMBH lies at the centre of the supergiant elliptical galaxy Messier 87, which is located in the Virgo constellation at a distance of around 50 million light years. Although relatively close in astronomical terms, its very large mass makes its size on the sky comparable to that of the much lighter SMBH in the centre of our galaxy. Furthermore, its accretion rate (brightness) is variable on longer time scales, making it easier to image. The resulting image (above) shows the clear shadow of the black hole in the centre surrounded by an asymmetric ring caused by radio waves that are bent around the SMBH by its strong gravitational field. The asymmetry is likely a result of relativistic beaming of part of the disk of matter which moves towards Earth.
The team compared the image to a range of detailed simulations in which parameters such as the black hole’s mass, spin and orientation were varied. Additionally, the characteristics of the matter around the SMBH, mainly hot electrons and ions, as well as the magnetic field properties were varied. While the image alone does not allow researchers to constrain many of these parameters, combining it with X-ray data taken by the Chandra and NuSTAR telescopes enables a deeper understanding. For example, the combined data constrain the SMBH mass to 6.5 billion solar masses and appears to exclude a non-spinning black hole. Whether the matter orbiting the SMBH rotates in the same direction or opposite to the black hole, as well as details on the environment around it, will require additional studies. Such studies can also potentially exclude alternative interpretations of this object; currently, exotic objects like boson stars, gravastars and wormholes cannot be fully excluded.
The work of the Event Horizon Telescope collaboration, which involves more than 200 researchers worldwide, was published in six consecutive papers in TheAstrophysical Journal Letters. While more images at shorter wavelengths are foreseen in the future, the collaboration also points out that much can be learned by combining the data with that from other wavelengths, such as gamma-rays. Despite this first image being groundbreaking, it is likely only the start of a revolution in our understanding of black holes and, with it, the universe.
The LHCb collaboration has released a much anticipated update on its measurement of RK – a ratio that describes how often a B+ meson decays to a charged kaon and either a μ+μ– or an e+e– pair, and therefore provides a powerful test of lepton universality. The more precise measurement, officially revealed at Rencontres de Moriond on 22 March, suggests that the intriguing current picture of flavour anomalies persists.
Since 2013, several results involving the decay of b quarks have hinted at deviations from lepton universality, a tenet of the Standard Model (SM), though none is individually significant enough to constitute evidence of new physics. LHCb has studied a number of ratios comparing b-decays to different leptons and also sees signs that something is amiss in angular distributions of B→K*μ+μ− decays. Data from BaBar and Belle add further intrigue, though with lower statistical significances.
The latest measurement from LHCb is the first lepton-universality test performed using part of the 13 TeV Run 2 data set (2015–2016) together with the full Run 1 data sample, representing in total an integrated luminosity of 5fb-1. The blinded analysis was performed in the range 1.1<q2<6.0 GeV2, where q2 is the invariant mass of the μ+μ– or e+e– pair. It found RK = 0.846+0.060–0.054 (stat) +0.016–0.014 (syst), the most precise measurement to date. However, having shifted closer to the Standard Model prediction, the value leaves the overall significance unchanged at about 2.5 standard deviations.
“I cannot tell you if lepton-flavour universality is broken or not, so sorry for this!” said Thibaud Humair of Imperial College London, who presented the result on behalf of the LHCb collaboration. “All LHCb results for RK are below SM expectations. Together with b → sμ+μ− results, RK and RK* constitute an interesting pattern of anomalies, but the significance is still low,” he said.
Humair’s talk generated much discussion, with physicists pressing LHCb on potential sources of uncertainties and other possible explanations such as the dependence of RK on q2. Other experiments also showed new measurements of lepton universality and other related tests of the Standard Model, such as ATLAS on the branching ration of Bs→μ+μ− and an update from Belle on both RD(*) and RK*. The current experimental activity in flavour physics was reflected by several talks at Moriond from theorists.
“It’s not a discovery, but something is going on,” says David Straub of TUM Munich, who had spent the previous 24 hours working solid to update a global likelihood fit of all parameters relevant to the b anomalies with the updated LHCb and Belle results. The fit, which involves 265 observables showed that b → sl+l– observables such as RK continue to show a “large pull” towards new-physics. “The popular ‘U1 leptoquark’ is still giving excellent fit to the data”, says Straub.
Further reduction in the uncertainty on RK can be expected when the data collected by LHCb in 2017 and 2018 are included in a future analysis. Meanwhile, in Japan, the Belle II physics programme has now begun in earnest and the collaboration is expected to bring further statistical power to the b-anomaly question in the near future.
On the morning of 21 March, at the 2019 Rencontres de Moriond in La Thuile, Italy, the LHCb collaboration announced the discovery of charge-parity (CP) violation in the charm system. Met with an impromptu champagne celebration, the result represents a milestone in particle physics and opens a new area of investigation in the charm sector.
CP violation, which results in differences in the properties of matter and antimatter, was first observed in the decays of K mesons (which contain strange quarks) in 1964 by James Cronin and Val Fitch. Even though parity (P) violation had been seen eight years earlier, the discovery that the combined C and P symmetries are not conserved was unexpected. The story deepened in the early 1970s, when, building on the foundations laid by Nicola Cabibbo and others, Makoto Kobayashi and Toshihide Maskawa showed that CP violation could be included naturally in the Standard Model (SM) if at least six different quarks existed in nature. Their fundamental idea – whereby direct CP violation arises if a complex phase appears in the CKM matrix describing quark mixing – was confirmed 30 years later by the discovery of CP violation in B-meson decays by the BaBar and Belle collaborations. Despite decades of searches, CP violation in the decays of charmed particles escaped detection.
LHCb physicists used the unprecedented dataset accumulated in 2011–2018 to study the difference in decay rates between D0 and D̅0 (which contain a c quark or antiquark) decaying into K+K– or π+π– pairs. To differentiate between the identical D0 and D̅0 decays, the collaboration exploited two different classes of decays: those of D*+/- mesons decaying into a D0 and a charged pion, where the presence of a π+(π–) indicates the presence of a D0(D̅0) meson; and those of B mesons decaying into a D0, a muon and a neutrino, in which the presence of a μ+(μ–) identifies a D0(D̅0). Counting the number of decays present in the data sample, the final result is ΔACP= -0.154±0.029%. At 5.3 standard deviations from zero, it represents the first observation of CP violation in the charm system.
“This is a major result that could be obtained thanks to the very high charm- production cross section at LHC, and to the superb performance of both the LHC machine and the LHCb detector, which provided the largest sample of charm particles ever collected,” says LHCb spokesperson Giovanni Passaleva. “Analysing the tens of millions of D0 mesons needed for such a precise measurement was a remarkable collective effort by the collaboration. The result opens up a new field in particle physics, involving the study of CP-violating effects in the sector of up-type quarks and searches for new-physics effects in a completely new domain.”
CP violation is a thought to be an essential ingredient to explain the observed cosmological matter-antimatter asymmetry, but the level of CP violation observed in the SM is only able to explain a fraction of the imbalance. In addition to hunting for novel sources of CP violation, physicists are making precise measurements of known sources to look for deviations that could indicate physics beyond the SM. The SM prediction for the amount of CP violation in charm decays is estimated to be in the range of 10-4 – 10-3 in the decay modes of interest. The new LHCb measurement is consistent with the SM expectation but falls at the upper end of the range, generating much discussion at Moriond 2019. Unusually for particle physics, the experimental measurement is much more precise than the SM prediction. This is due to the lightness of charm quarks, which means that reliable perturbative QCD and other approximate calculation techniques are not possible. Future theoretical improvements, and data, will establish whether the seminal LHCb result is consistent with the SM.
“This is an important milestone in the study of CP violation,” Kobayashi, now professor emeritus at KEK in Japan, tells CERN Courier. “I hope that analysis of the results will provide a clue to new physics.”
Neutrinos, discovered in 1956, play an exceptional role in particle and nuclear physics, as well as astrophysics, and their study has led to the award of several Nobel prizes. In recognition of their importance, the first International Conference on the History of the Neutrino took place at the Université Paris Diderot in Paris on 5–7 September 2018.
The purpose of the conference, which drew 120 participants, was to cover the main steps in the history of the neutrino since 1930, when Wolfgang Pauli postulated its existence to explain the continuous energy spectrum of the electrons emitted in beta decay. Specifically, for each topic in neutrino physics, the aim was to pursue an historical approach and follow as closely as possible the discovery or pioneering papers. Speakers were chosen as much as possible for their roles as authors or direct witnesses, or as players in the main events.
The first session, “Invention of a new particle”, started with the prehistory of the neutrino – that is, the establishment of the continuous energy spectrum in beta decay – before moving into the discoveries of the three flavour neutrinos. The second session, “Neutrinos in nature”, was devoted to solar and atmospheric neutrinos, as well as neutrinos from supernovae and Earth. The third session covered neutrinos from reactors and beams including the discovery of neutral-current neutrino interactions, in which the neutrino is not transformed into another particle like a muon or an electron. This discovery was made in 1973 by the Gargamelle bubble chamber team at CERN after a race with the HPWF experiment team at Fermilab.
The major theme of neutrino oscillations from the first theoretical ideas of Bruno Pontecorvo (1957) to the Mikheyev–Smirnov–Wolfenstein effect (1985), which can modify the oscillations when neutrinos travel through matter, was complemented by talks on the discovery of neutrino oscillations by Nobel laureates Takaaki Kajita and Art McDonald. In 1998, the Super-Kamiokande experiment, led by Kajita, observed the oscillation of atmospheric neutrinos, and in 2001 the Sudbury Neutrino Observatory experiment, led by McDonald, observed the oscillation of solar neutrinos.
The role of the neutrino in the Standard Model was discussed, as was its intrinsic nature. Although physicists have observed the rare process of double beta decay with neutrinos in the final state, neutrinoless double beta decay with no neutrinos produced has been searchedfor for more than 30 years because its observation would prove that the neutrino is Majorana-type (its own antiparticle) and not Dirac-type.
To complete the panorama, the conference discussed neutrinos as messengers from the wider universe, from the Big Bang to violent phenomena such as gamma-ray bursts or active galactic nuclei. Delegates also discussed wrong hints and tracks, which play a positive role in the development of science, and the peculiar sociological aspects that are common to particle physics and astrophysics.
Following the conference, a website dedicated to the history of this fascinating particle was created: https://neutrino-history.in2p3.fr.
In a workshop held at CERN on 16–17 January, researchers presented the findings of the Physics Beyond Colliders (PBC) initiative, which was launched in 2016 to explore the opportunities at CERN via projects complementary to the LHC and future colliders (CERN Courier November 2016 p28). PBC members have weighed up the potential for such experiments to explore open questions in QCD and the existence of physics beyond the Standard Model (BSM), in particular including searches for signatures of hidden-sector models in which the conjectured dark matter does not couple directly to Standard Model particles.
The BSM and QCD groups of the PBC initiative have developed detailed studies of CERN’s options and compared them to other worldwide possibilities. The results show the international competitiveness of the PBC options.
The Super Proton Synchrotron (SPS) remains a clear attraction, offering the world’s highest-energy beams to fixed-target experiments in the North Area (see Fixed target, striking physics). The SPS high-intensity muon beam could allow a better understanding of the theoretical prediction of the muon anomalous magnetic moment (MUonE project), and a significant contribution to the resolution of the proton radius puzzle by COMPASS(Rp). The NA61 experiment could explore QCD in the interesting region of “criticality”, while upgrades of NA64 and a few months of NA62 operation in beam-dump mode (whereby a target absorbs most of the incident protons and contains most of the particles generated by the primary beam interactions) would explore the hidden-sector parameter space. In the longer term, the KLEVER experiment could probe rare decays of neutral kaons, and NA60 and DIRAC could enhance our understanding of QCD.
A novel North Area proposal is the SPS Beam Dump Facility (BDF). Such a facility could, in the first instance, serve the SHiP experiment, which would perform a comprehensive investigation of the hidden sector with discovery potential in the MeV–GeV mass range, and the TauFV experiment, which would search for forbidden τ decays. The BDF team has made excellent progress with the facility design and is preparing a comprehensive design study report. Options for more novel exploitation of the SPS have also been considered: proton-driven plasma- wakefield acceleration of electrons for a dark-matter experiment (AWAKE++); the acceleration and slow extraction of electrons to light–dark-matter experiments (eSPS); and the production of well-calibrated neutrinos via a muon decay ring (nuSTORM).
Fixed-target studies at the LHC are also considered within PBC, and these could improve our understanding of QCD in regions where it is relevant for new-physics searches at the high-luminosity LHC upgrade. The LHC could also be supplemented with new experiments to search for long-lived particles, and PBC support for a small experiment called FASER has helped pave the way for its installation in the ongoing long shutdown of CERN’s accelerator complex.
2018 was a notable year for the gamma factory, a novel concept that would use the LHC to produce intense gamma-ray beams for precision measurements and searches (CERN Courier November 2017 p7). The team has already demonstrated the acceleration of partially stripped ions in the LHC, and is now working towards a proof-of-principle experiment in the SPS. Meanwhile, the Electric Dipole Moment (CPEDM) collaboration has continued studies, supported by experiments at the COSY synchrotron in Germany (CERN Courier September 2016 p27), towards a prototype storage ring to measure the proton EDM.
The PBC technology team has also been working to leverage CERN’s skills base to novel experiments, for example by exploring synergies across experiments and collaboration in technologies – in particular, concerning light-shining-through-walls experiments and QED vacuum-birefringence measurements.
Finally, some PBC projects are likely to flourish outside CERN: the IAXO axion helioscope, now under consideration at DESY; the proton EDM ring, which could be prototyped at the Jülich laboratory, also in Germany; and the REDTOP experiment devoted to η meson rare decays, for which Fermilab in the US seems better suited.
The PBC groups have submitted their full findings to the European Particle Physics Strategy Update (http://pbc.web.cern.ch/).
On 11 December 2018, 25 years after its inaugural meeting, the BaBar collaboration came together at the SLAC National Accelerator Laboratory in California to celebrate its many successes. David Hitlin, BaBar’s first spokesperson, described the inaugural meeting of what was then called the Detector Collaboration for the PEP-II “asymmetric” electron–positron collider, which took place at SLAC at the end of 1993. By May 1994 the collaboration had chosen the name BaBar in recognition of its primary goal to study CP violation in the neutral B-B̅ meson system. Jonathan Dorfan, PEP-II project director, recounted how PEP-II was constructed by SLAC, LBL and LLNL. Less than six years later, PEP-II and the BaBar detector were built and the first collision events were collected on 26 May 1999. Twenty-five years on, and BaBar has now chalked up more than 580 papers on CP violation and many other topics.
BaBar has now chalked up more than 580 papers on CP violation and many other topics.
The “asymmetric” descriptor of the collider refers to Pier Oddone’s concept of using unequal electron and positron beam energies – tuned to 10.58 GeV, the mass of the ϒ(4S) meson and just above the threshold for producing a pair of B mesons. This relativistic boost enabled measurements of the distance between the points where the mesons decay, which is critical for the study of CP violation. Equally critical was the entanglement of the B meson and anti-B meson produced in the ϒ(4S) decay, as it marked whether it was the B0 or B̅0 that decayed to the same CP final state by tagging the flavour of the other meson.
By October 2000 PEP-II had achieved its design luminosity of 3 × 1033 cm–2 s–1 and less than a year later BaBar published its observation of CP violation in the B0 meson system based on a sample of 32 × 106 pairs of B0-B̅0 mesons – on the same day that Belle, its competitor at Japan’s KEK laboratory, published the same observation. These results led to Makoto Kobayashi and Toshihide Maskawa sharing the 2008 Nobel Prize in Physics. The ultimate luminosity achieved by PEP-II, in 2006, was 1.2 × 1034 cm–2s–1. BaBar continued to collect data on or near the ϒ(4S) meson until 2007 and in 2008 collected large samples of ϒ(2S) and ϒ(3S) mesons before PEP-II was shut down. In total, PEP-II produced 471 × 106 B-B̅ pairs for BaBar studies – as well as a myriad of other for other investigations.
The anniversary event also celebrated technical innovations, including “trickle injection” of beam particles intoPEP-II, which provided a nearly 40% increase in integrated luminosity; BaBar’s impressive particle identification, made possible by the DIRC detector; and the implementation of a computing model – spurred by PEP-II delivering significantly more than design luminosity – whereby countries provided in-kind computing support via large “Tier-A” centres. This innovation paved the way for CERN’s Worldwide LHC Computing Grid.
Notable physics results from BaBar include the first observation in 2007 of D–D̅ mixing, while in 2008 the collaboration discovered the long-sought ηb, the lowest energy particle of the bottomonium family. The team also searched for lepton-flavour violation in tau–lepton decays, publishing in 2010 what remain the most stringent limits onτ → μγand τ → eγ branching fractions. In 2012, making it onto Physics World’s top-ten physics results of the year, the BaBar collaboration made the first direct observation of time-reversal violation by measuring the rates at which the B0 meson changes quantum states. Also published in 2012 was evidence for an excess of B̅→ D(*)τ– ν̅τ decays, which challenges lepton universality and is an important part of the current Belle II and LHCb physics programmes. Several years after data-taking ended, it was recognised that BaBar’s data could also be mined for evidence of dark-sector objects such as dark photons, leading to the publication of two significant papers in 2014 and 2017. Another highlight, published last year, is a joint BaBar–Belle paper that resolved an ambiguity concerning the quark-mixing unitarity triangle.
Although BaBar stopped collecting data in 2008, this highly collegial team of researchers continues to publish impactful results. Moreover, BaBar alumni continue to bring their experience and expertise to subsequent experiments, ranging from ATLAS, CMS and LHCb at the LHC, Belle II at SuperKEKB, and long-baseline neutrino experiments (T2K, DUNE, HyperK) to dark-matter (LZ, SCDMS) and dark-energy (LSST) experiments in particle astrophysics.
There has never been a better time to be a physicist. The questions on the table today are not about this-or-that detail, but profound ones about the very structure of the laws of nature. The ancients could (and did) wonder about the nature of space and time and the vastness of the cosmos, but the job of a professional scientist isn’t to gape in awe at grand, vague questions – it is to work on the next question. Having ploughed through all the “easier” questions for four centuries, these very deep questions finally confront us: what are space and time? What is the origin and fate of our enormous universe? We are extremely fortunate to live in the era when human beings first get to meaningfully attack these questions. I just wish I could adjust when I was born so that I could be starting as a grad student today! But not everybody shares my enthusiasm. There is cognitive dissonance. Some people are walking around with their heads hanging low, complaining about being disappointed or even depressed that we’ve “only discovered the Higgs and nothing else”.
So who is right?
It boils down to what you think particle physics is really about, and what motivates you to get into this business. One view is that particle physics is the study of the building blocks of matter, in which “new physics” means “new particles”. This is certainly the picture of the 1960s leading to the development of the Standard Model, but it’s not what drew me to the subject. To me, “particle physics” is the study of the fundamental laws of nature, governed by the still mysterious union of space–time and quantum mechanics. Indeed, from the deepest theoretical perspective, the very definition of what a particle is invokes both quantum mechanics and relativity in a crucial way. So if the biggest excitement for you is a cross-section plot with a huge bump in it, possibly with a ticket to Stockholm attached, then, after the discovery of the Higgs, it makes perfect sense to take your ball and go home, since we can make no guarantees of this sort whatsoever. We’re in this business for the long haul of decades and centuries, and if you don’t have the stomach for it, you’d better do something else with your life!
Isn’t the Standard Model a perfect example of the scientific method?
Sure, but part of the reason for the rapid progress in the 1960s is that the intellectual structure of relativity and quantum mechanics was already sitting there to be explored and filled in. But these more revolutionary discoveries took much longer, involving a wide range of theoretical and experimental results far beyond “bump plots”. So “new physics” is much more deeply about “new phenomena” and “new principles”. The discovery of the Higgs particle – especially with nothing else accompanying it so far – is unlike anything we have seen in any state of nature, and is profoundly “new physics” in this sense. The same is true of the other dramatic experimental discovery in the past few decades: that of the accelerating universe. Both discoveries are easily accommodated in our equations, but theoretical attempts to compute the vacuum energy and the scale of the Higgs mass pose gigantic, and perhaps interrelated, theoretical challenges. While we continue to scratch our heads as theorists, the most important path forward for experimentalists is completely clear: measure the hell out of these crazy phenomena! From many points of view, the Higgs is the most important actor in this story amenable to experimental study, so I just can’t stand all the talk of being disappointed by seeing nothing but the Higgs; it’s completely backwards. I find that the physicists who worry about not being able to convince politicians are (more or less secretly) not able to convince themselves that it is worth building the next collider. Fortunately, we do have a critical mass of fantastic young experimentalists who believe it is worth studying the Higgs to death, while also exploring whatever might be at the energy frontier, with no preconceptions about what they might find.
What makes the Higgs boson such a rich target for a future collider?
It is the first example we’ve seen of the simplest possible type of elementary particle. It has no spin, no charge, only mass, and this extreme simplicity makes it theoretically perplexing. There is a striking difference between massive and massless particles that have spin. For instance, a photon is a massless particle of spin one; because it moves at the speed of light, we can’t “catch up” with it, and so we only see it have two “polarisations”, or ways it can spin. By contrast the Z boson, which also has spin one, is massive; since you can catch up with it, you can see it spinning in any of three directions. This “two not equal to three” business is quite profound. As we collide particles at ever increasing energies, we might think that their masses are irrelevant tiny perturbations to their energies, but this is wrong, since something must account for the extra degrees of freedom.
The whole story of the Higgs is about accounting for this “two not equal to three” issue, to explain the extra spin states needed for massive W and Z particles mediating the weak interactions. And this also gives us a good understanding of why the masses of the elementary particles should be pegged to that of the Higgs. But the huge irony is that we don’t have any good understanding for what can explain the mass of the Higgs itself. That’s because there is no difference in the number of degrees of freedom between massive and massless spin-zero particles, and related to this, simple estimates for the Higgs mass from its interactions with virtual particles in the vacuum are wildly wrong. There are also good theoretical arguments, amply confirmed in analogous condensed-matter systems and elsewhere in particle physics, for why we shouldn’t have expected to see such a beast lonely, unaccompanied by other particles. And yet here we are. Nature clearly has other ideas for what the Higgs is about than theorists do.
Is supersymmetry still a motivation for a new collider?
Nobody who is making the case for future colliders is invoking, as a driving motivation, supersymmetry, extra dimensions or any of the other ideas that have been developed over the past 40 years for physics beyond the Standard Model. Certainly many of the versions of these ideas, which were popular in the 1980s and 1990s, are either dead or on life support given the LHC data, but others proposed in the early 2000s are alive and well. The fact that the LHC has ruled out some of the most popular pictures is a fantastic gift to us as theorists. It shows that understanding the origin of the Higgs mass must involve an even larger paradigm change than many had previously imagined. Ironically, had the LHC discovered supersymmetric particles, the case for the next circular collider would be somewhat weaker than it is now, because that would (indirectly) support a picture of a desert between the electroweak and Planck scales. In this picture of the world, most people wanted a linear electron–positron collider to measure the superpartner couplings in detail. It’s a picture people very much loved in the 1990s, and a picture that appears to be wrong. Fine. But when theorists are more confused, it’s the time for more, not less experiments.
What definitive answers will a future high-energy collider give us?
First and foremost, we go to high energies because it’s the frontier, and we look around for new things. While there is absolutely no guarantee we will produce new particles, we will definitely stress test our existing laws in the most extreme environments we have ever probed. Measuring the properties of the Higgs, however, is guaranteed to answer some burning questions. All the drama revolving around the existence of the Higgs would go away if we saw that it had substructure of any sort. But from the LHC, we have only a fuzzy picture of how point-like the Higgs is. A Higgs factory will decisively answer this question via precision measurements of the coupling of the Higgs to a slew of other particles in a very clean experimental environment. After that the ultimate question is whether or not the Higgs looks point-like even when interacting with itself. The simplest possible interaction between elementary particles is when three particles meet at a space–time point. But we have actually never seen any single elementary particle enjoy this simplest possible interaction. For good reasons going back to the basics of relativity and quantum mechanics, there is always some quantum number that must change in this interaction – either spin or charge quantum numbers change. The Higgs is the only known elementary particle allowed to have this most basic process as its dominant self-interaction. A 100 TeV collider producing billions of Higgs particles will not only detect the self-interaction, but will be able to measure it to an accuracy of a few per cent. Just thinking about the first-ever probe of this simplest possible interaction in nature gives me goosebumps.
What are the prospects for future dark-matter searches?
Beyond the measurements of the Higgs properties, there are all sorts of exciting signals of new particles that can be looked for at both Higgs factories and 100 TeV colliders. One I find especially important is WIMP dark matter. There is a funny perception, somewhat paralleling the absence of supersymmetry at the LHC, that the simple paradigm of WIMP dark matter has been ruled out by direct-detection experiments. Nope! In fact, the very simplest models of WIMP dark matter are perfectly alive and well. Once the electroweak quantum numbers of the dark-matter particles are specified, you can unambiguously compute what mass an electroweak charged dark-matter particle should have so that its thermal relic abundance is correct. You get a number between 1–3 TeV, far too heavy to be produced in any sizeable numbers at the LHC. Furthermore, they happen to have miniscule interaction cross sections for direct detection. So these very simplest theories of WIMP dark matter are inaccessible to the LHC and direct-detection experiments. But a 100 TeV collider has just enough juice to either see these particles, or rule out this simplest WIMP picture.
What is the cultural value of a 100 km supercollider?
Both the depth and visceral joy of experiments in particle physics is revealed in how simple it is to explain: we smash things together with the largest machines that have ever been built, to probe the fundamental laws of nature at the tiniest distances we’ve ever seen. But it goes beyond that to something more important about our self-conception as people capable of doing great things. The world has all kinds of long-term problems, some of which might seem impossible to solve. So it’s important to have a group of people who, over centuries, give a concrete template for how to go about grappling with and ultimately conquering seemingly impossible problems, driven by a calling far larger than themselves. Furthermore, suppose it’s 200 years from now, and there are no big colliders on the planet. How can humans be sure that the Higgs or top particles exist? Because it says so in dusty old books? There is an argument to be made that as we advance we should be able to do the things we did in the past. After all, the last time that fundamental knowledge was shoved in old dusty books was in the dark ages, and that didn’t go very well for the West.
What about justifying the cost of the next collider?
There are a number of projects and costs we could be talking about, but let’s call it $5–25 billion. Sounds like a lot, right? But the global economy is growing, not shrinking, and the cost of accelerators as a fraction of GDP has barely changed over the past 40 years – even a 100 TeV collider is in this same ballpark. Meanwhile the scientific issues at stake are more profound than they have been for many decades, so we certainly have an honest science case to make that we need to keep going.
People sometimes say that if we don’t spend billions of dollars on colliders, then we can do all sorts of other experiments instead. I am a huge fan of small-scale experiments, but this argument is silly because science funding is infamously not a zero-sum game. So, it’s not a question of, “do we want to spend tens of billions on collider physics or something else instead”, it is rather “do we want to spend tens of billions on fundamental physics experiments at all”.
Another argument is that we should wait until some breakthrough in accelerator technology, rather than just building bigger machines. This is naïve. Of course miracles can always happen, but we can’t plan doing science around miracles. Similar arguments were made around the time of the cancellation of the Superconducting Super Collider (SSC) 30 years ago, with prominent condensed-matter physicists saying that the SSC should wait for the development of high-temperature superconductors that would dramatically lower the cost. Of course those dreamed-of practical superconductors never materialised, while particle physics continued from strength to strength with the best technology available.
What do you make of claims that colliders are no longer productive?
It would be only to the good to have a no-holds barred, public discussion about the pros and cons of future colliders, led by people with a deep understanding of the relevant technical and scientific issues. It’s funny that non-experts don’t even make the best arguments for not building colliders; I could do a much better job than they do! I can point you to an awesomely fierce debate about future colliders that already took place in China two years ago: (Int. J. Mod. Phys. A31 1630053 and 1630054). C N Yang, who is one of the greatest physicists of the 20th century and enormously influential in China, came out with a strong attack on colliders, not only in China but more broadly. I was delighted. Having a serious attack meant there could be a serious response, masterfully provided by David Gross. It was the King Kong vs Godzilla of fundamental physics, played out on the pages of major newspapers in China, fantastic!
What are you working on now?
About a decade ago, after a few years of thinking about the cosmology of “eternal inflation” in connection with solutions to the cosmological constant and hierarchy problems, I concluded that these mysteries can’t be understood without reconceptualising what space–time and quantum mechanics are really about. I decided to warm up by trying to understand the dynamics of particle scattering, like collisions at the LHC, from a new starting point, seeing space-time and quantum mechanics as being derived from more primitive notions. This has turned out to be a fascinating adventure, and we are seeing more and more examples of rather magical new mathematical structures, which surprisingly appear to underlie the physics of particle scattering in a wide variety of theories, some close to the real world. I am also turning my attention back to the goal that motivated the warm-up, trying to understand cosmology, as well as possible theories for the origin of the Higgs mass and cosmological constant, from this new point of view. In all my endeavours I continue to be driven, first and foremost, by the desire to connect deep theoretical ideas to experiments and the real world.
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The presence of only massless left-handed neutrinos in the Standard Model (SM) gives rise to the accidental symmetry of lepton-flavour conservation – yet neutrino oscillation experiments have observed neutrinos changing flavour in-transit from sources as far away as the Sun and as near as a nuclear reactor. Such neutral lepton-flavour violation implies that neutrinos have tiny masses and that their flavour eigenstates are distinct from their mass eigenstates. Phases develop between the mass eigenstates as a neutrino travels, and the wavefunction becomes a mixture of the flavour eigenstates, rather than the unique original flavour, as would remain the case for truly massless neutrinos.
