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Standard Model stands strong at Moriond

**Mountain encounters** Flavour physics took centre stage at Moriond this year. Credit: L Fayard.

The 66th Rencontres de Moriond, held in La Thuile, Italy, took place from 16 to 30 March, with the first week devoted to electroweak interactions and unified theories, and a second week to QCD and high-energy interactions. More than 200 physicists took part, presenting new results from precision Standard Model (SM) measurements to new exotic quark states, flavour physics and the dark sector.

A major theme of the electroweak session was flavour physics, and the star of the show was LHCb’s observation of CP violation in charm decays (see LHCb observes CP violation in charm decays). The collaboration showed several other new results concerning charm- and B-meson decays. One much anticipated result was an update on R_K, the ratio of rare decays of a B⁺ to electrons and muons, using data taken at energies of 7, 8 and 13 TeV. These decays are predicted to occur at the same rate to within 1%; previous data collected are consistent with this prediction but favour a lower value, and the latest LHCb results continue to support this picture. Together with other measurements, these results paint an intriguing picture of possible new physics (p33) that was explored in several talks by theorists.

Run-2 results

The LHC experiments presented many new results based on data collected during Run 2. ATLAS and CMS have measured most of the Higgs boson’s main production and decay modes with high statistical significance and carried out searches for new, additional Higgs bosons. From a combination of all Higgs-boson measurements, ATLAS obtained new constraints on the important Higgs self-coupling, while CMS presented updated results on the Higgs decay to two Z bosons and its coupling to top quarks.

Precision SM studies continued with first evidence from ATLAS for the simultaneous production of three W or Z bosons, and CMS presented first evidence for the production of two W bosons in two simultaneous interactions between colliding partons. The very large new dataset has also allowed ATLAS and CMS to expand their searches for new physics, setting stronger lower limits on the allowed mass ranges of supersymmetric and other hypothetical particles (see Boosting searches for fourth-generation quarks and Pushing the limits on supersymmetry). These also include new limits from CMS on the parameters describing slowly moving heavy particles, and constraints from both collaborations on the production rate of Z′ bosons. ATLAS, using the results of lead–ion collisions taken in 2018, also reported the observation of light-by-light scattering – a very rare process that is forbidden by classical electrodynamics.

New results and prospects in the neutrino sector were communicated, including Daya Bay and the reactor antineutrino flux anomaly, searches for neutrinoless double-beta decay, and the reach of T2K and NOvA in tackling the neutrino mass hierarchy and leptonic CP violation. Dark matter, axions and cosmology also featured prominently. New results from experiments such as XENON1T, ABRACADABRA, SuperCDMS and ATLAS and CMS illustrate the power of multi-prong dark-matter searches – not just for WIMPs but also very light or exotic candidates. Cosmologist Lisa Randall gave a broad-reaching talk about “post-modern cosmology”, in which she argued that – as in particle physics – the easy times are probably over and that astronomers need to look at more subtle effects to break the impasse.

Moriond electroweak also introduced a new session: “feeble interactions”, which was designed to reflect the growing interest in very weak processes at the LHC and future experiments.

LHCb continued to enjoy the limelight during Moriond’s QCD session, announcing the discovery of a new five-quark hadron, named P_c(4312)⁺, which decays to a proton and a J/ψ and is a lighter companion of the pentaquark structures revealed by LHCb in 2015 (p15). The result is expected to motivate deeper studies of the structure of these and other exotic hadrons. Another powerful way to delve into the depths of QCD, addressed during the second week of the conference, is via the B_c meson family. Following the observation of the B_c(2S) by ATLAS in 2014, CMS reported the existence of a two-peak feature in data corresponding to the B_c(2S) and the B_c^*(2S) – supported by new results from LHCb based on its full 2011–2018 data sample. Independent measurements of CP violation in the B_s system reported by ATLAS and LHCb during the electroweak session were also combined to yield the most precise measurement yet, which is consistent with the small value predicted by the SM.

A charmed life

In the heavy-ion arena, ALICE highlighted its observation that baryons containing charm quarks are produced more often in proton–proton collisions than in electron–positron collisions. Initial measurements in lead–lead collisions suggest an even higher production rate for charmed baryons, similar to what has been observed for strange baryons. These results indicate that the presence of quarks in the colliding beams affects the hadron production rate. The collaboration also presented the first measurement of the triangle-shaped flow of J/ψ particles in lead–lead collisions, showing that even heavy quarks are affected by the quarks and gluons in the quark–gluon plasma and retain some memory of the collisions’ initial geometry.

The SM still stands strong after Moriond 2019, and the observation of CP violation in D mesons represents another victory, concluded Shahram Rahatlou of Sapienza University of Rome in the experimental summary. “But the flavour anomaly is still there to be pursued at low and high mass.”

Pushing the limits on supersymmetry

**Fig. 1.** New constraints (red) on a simplified SUSY model describing the production of a pair of charginos that both subsequently decay to a W boson and the lightest neutralino – a stable and invisible SUSY particle. The new sensitivity is significantly improved over that of the previous analysis (grey).

A report from the ATLAS experiment

Supersymmetry (SUSY) introduces a new fermion–boson symmetry that gives rise to supersymmetric “partners” of the Standard Model (SM) particles, and “naturally” leads to a light Higgs boson with mass close to that of the W and Z. SUSY partners that are particularly relevant in these “natural SUSY” scenarios are the top and bottom squarks, as well as the SUSY partners of the weak SM bosons, the neutralinos and charginos.

Despite the theory’s many appealing features, searches for SUSY at the LHC and elsewhere have so far yielded only exclusion limits. With LHC Run 2 completed as of the end of 2018, the ATLAS experiment has recorded 139 fb^-1 of physics-quality proton–proton collisions at a centre-of-mass energy of 13 TeV. Three recent ATLAS SUSY searches highlight the significant increase in sensitivity offered by this dataset.

The first search took advantage of refinements in b-tagging to search for light bottom squarks decaying into bottom quarks, Higgs bosons and the lightest SUSY partner, which is assumed to be invisible and stable (a candidate for dark matter). The data agree with the SM and lead to significantly improved constraints, with bottom squark masses now excluded up to 1.5 TeV.

If the accessible SUSY particles can only be produced via electroweak processes, the resulting low-production cross sections present a challenge. The second search focuses on such electroweak SUSY signatures with two charged leptons and a significant amount of missing momentum carried away by a pair of the lightest SUSY partners. The current search places strong constraints on SUSY models with light charginos and more than doubles the sensitivity of the previous analysis (figure 1).

A third recent analysis considered less conventional signatures. Top squarks – the bosonic SUSY partner of the top quark – may evade detection if they have a long lifetime and decay at macroscopic distances from the collision point. This search looked for SUSY particles decaying to a quark and a muon, looking primarily for long-lived top squarks that decayed several millimetres into the detector volume. The observed results are consistent with the background-only expectation.

These analyses represent just the beginning of a large programme of SUSY searches using the entirety of the Run-2 dataset. With a rich signature space left to explore, there remains plenty of room for discovery in mining the riches from the LHC.

Boosting searches for fourth-generation quarks

**Fig. 1.** Mass limits for T quarks in GeV. The limits are shown as a function of the probability to decay to a b quark and a W boson, versus the probability to decay to a top quark and a Higgs boson, assuming all other decays are to a top quark and a Z boson.

A report from the CMS experiment

Ever since the 1970s, when the third generation of quarks and leptons began to emerge experimentally, physicists have asked if further generations await discovery. One of the first key results from the Large Electron–Positron Collider 30 years ago provided evidence to the contrary, showing that there are only three generations of neutrinos. The discovery of the Higgs boson in 2012 added a further wrinkle to the story: many theorists believe that the mass of the Higgs boson is unnaturally small if there are additional generations of quarks heavier than the top quark. But a loophole arises if the new heavy quarks do not interact with the Higgs field in the same way as regular quarks. The search for new heavy fourth-generation quarks – denoted T – is therefore the subject of active research at the LHC today.

CMS researchers have recently completed a search for such “vector-like” quarks using a new machine-learning method that exploits special relativity in a novel way. If the new T quarks exist, they are expected to decay to a quark and a W, Z or Higgs boson. As top quarks and W/Z/H bosons decay themselves, production of a T quark–antiquark pair could lead to dozens of different final states. While most previous searches focused on a handful of channels at most, this new analysis is able to search for 126 different possibilities at once.

The key to classifying all the various final states is the ability to identify high-energy top quarks, Higgs bosons, and W and Z bosons that decay into jets of particles recorded by the detector. In the reference frame of the CMS detector, these particles produce wide jets that all look alike, but things look very different in a frame of reference in which the initial particle (a W, Z or H boson, or a top quark) is at rest. For example, in the centre-of-mass frame of a Higgs boson, it would appear as two well-collimated back-to-back jets of particles, whereas in the reference frame of the CMS detector the jets are no longer back-to-back and may indeed be difficult to identify as separate at all. This feature, based on special relativity, tells us how to distinguish “fat” jets originating from different initial particles.

Modern machine-learning techniques were used to train a deep neural-network classification algorithm using simulations of the expected particle decays. Several dozen properties of the jets were calculated in different hypothetical reference frames, and fed to the network, which classifies the original fat jets as coming from either top quarks, H, W or Z bosons, b quarks, light quarks, or gluons. Each event is then classified according to how many of each jet type there are in the event. The number of observed events in each category was then compared to the predicted background yield: an excess could indicate T-quark pair production.

CMS found no evidence for T-quark pair production in the 2016 data, and has excluded T-quark masses up to 1.4 TeV (figure 1). The collaboration is working on new ideas to improve the classification method and extend the search to higher masses using the four-times larger 2017 to 2018 dataset.

ALICE sheds new light on high-p_T suppression

**Fig. 1.** Left: nuclear-modification factor versus p_T for 5% wide centrality classes, showing the suppression of high-p_T particle production in Pb–Pb collisions. The most peripheral 25% of ion collisions (coloured) exhibit completely different behaviour from the most central 75% (grey). Coloured bars show uncertainty on the normalisation. Right: mean R_AA (red) compared with a new model of suppression in peripheral collisions (black).

A report from the ALICE experiment

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 (p_T) due to energy loss in the QGP. This suppression is quantified by the nuclear-modification factor R_AA, 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 R_AA 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 R_AA in narrow centrality bins. The results (figure 1, left) show that the trend of a gradual reduction in the suppression of high-p_T 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 p_T, which is used to determine the centrality, and at high p_T, where R_AA 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.

New pentaquarks resolved by LHCb

**Fig. 1.** The LHCb collaboration has discovered a new pentaquark, and resolved a previously discovered peak into two. The Λ_b⁰ → J/ψpK⁻ data sample was fitted with three narrow resonances (blue, purple and cyan) plus a polynomial background.

The LHCb collaboration has discovered a new pentaquark particle, dubbed the P_c(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 P_c(4450)⁺ structure previously reported by the collaboration in 2015 has now been resolved into two narrow, overlapping peaks, the P_c(4440)⁺ and P_c(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 Z_b^0,±states just above the BB̅^* and B^*B̅^* thresholds; this was followed by observations of Z_c^0,± states just above the equivalent charm thresholds by BES-III and Belle. The existence of charged Z_b^± and Z_c^± 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 Z_c^± states from Belle and LHCb, such as the Z_c(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 Λ_b⁰ → J/ψpK⁻ decays. A narrow P_c(4450)⁺ and a broad P_c(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 Λ_b⁰ sample nine times larger than that used in the 2015 paper. The new data reproduce the parameters of the P_c(4450)⁺ and P_c(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 P_c 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 P_c resonances. A more complete understanding will require further experimental and theoretical investigation.

Hunting the muon’s forbidden decay

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.

**Mirror mirror** MEG II’s liquid xenon photon detector. Credit: MEG II collaboration.

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⁺ν_e ν̅_μ γ and the accidental time coincidence of a positron and photon produced by different muon decays.

**Fig. 1.** Limits on the branching ratios of flavour-violating muon decays. In the mid 1950s, experiments using stopped pion beams improved on previous cosmic-ray limits, before experiments with stopped muons allowed further improvements in the mid 1970s. The current best limits in each channel were set by the MEG, SINDRUM-II and SINDRUM collaborations, respectively, each at PSI. Credit: *Riv. Nuovo Cimento* 41 71.

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.

**Fig. 2.** Illustration of a μ⁺ → e⁺γ event in MEG II. Credit: MEG II Collaboration.

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 ν̅_μ → ν̅_ehas to occur at an energy scale E ~ m_w, over an extremely short distance of L ~ 1/m_w. 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)/m²_w)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.

Serbia becomes CERN Member State

**Expanding family** Ana Brnabić, Prime Minister of the Republic of Serbia, talking with CERN director-general Fabiola Gianotti during a visit of the Serbian delegation to CERN in September 2018. Credit: CERN-PHOTO-201809-216-26.

Serbia became the 23rd Member State of CERN, on 24 March, following receipt of formal notification from UNESCO. Ever since the early days of CERN (former Yugoslavia was one of the 12 founding Member States of CERN in 1954, until its departure in 1961), the Serbian scientific community has made strong contributions to CERN’s projects. This includes at the Synchrocyclotron, Proton Synchrotron and Super Proton Synchrotron facilities. In the 1980s and 1990s, physicists from Serbia worked on the DELPHI experiment at CERN’s LEP collider. In 2001, CERN and Serbia concluded an International Cooperation Agreement, leading to Serbia’s participation in the ATLAS and CMS experiments at the LHC, in the Worldwide LHC Computing Grid, as well as in the ACE and NA61 experiments. Serbia’s main involvement with CERN today is in the ATLAS and CMS experiments, in the ISOLDE facility, and on design studies for future particle colliders – FCC and CLIC – both of which are potentially new flagship projects at CERN.

Serbia was an Associate Member in the pre-stage to membership from March 2012. As a Member State, Serbia will have voting rights in the CERN Council, while the new status will also enhance the recruitment opportunities for Serbian nationals at CERN and for Serbian industry to bid for CERN contracts. “Investing in scientific research is important for the development of our economy and CERN is one of the most important scientific institutions today,” says Ana Brnabić, Prime Minister of Serbia. “I am immensely proud that Serbia has become a fully-fledged CERN Member State. This will bring new possibilities for our scientists and industry to work in cooperation with CERN and fellow CERN Member States.”

Welcome to the Science Gateway

**Connecting** An impression of the Science Gateway looking down Route de Meyrin towards Geneva. Credit: CERN-PHOTO-201904-083-1.

On 8 April, CERN unveiled plans for a major new facility for scientific education and outreach. Aimed at audiences of all ages, the Science Gateway will include exhibition spaces, hands-on scientific experiments for schoolchildren and students, and a large amphitheatre to host science events for experts and non-experts alike. It is intended to satisfy the curiosity of hundreds of thousands visitors every year and is core to CERN’s mission to educate and engage the public in science.

“We will be able to share with everybody the fascination of exploring and learning how matter and the universe work, the advanced technologies we need to develop in order to build our ambitious instruments and their impact on society, and how science can influence our daily life,” says CERN director-general, Fabiola Gianotti. “I am deeply grateful to the donors for their crucial support in the fulfilment of this beautiful project.”

**Iconic** A scale model of the facility. Credit: CERN-PHOTO-201904-083-18.

The overall cost of the Science Gateway, estimated at 79 m Swiss Francs, is entirely funded through donations. Almost three quarters of the cost has already been secured, thanks in particular to a contribution of 45 m Swiss Francs from Fiat Chrysler Automobiles. Other donors include a private foundation in Geneva and Loterie Romande, which distributes its profits to public utility projects. CERN is looking for additional donations to cover the full cost of the project.

The Science Gateway will be hosted in iconic buildings with a 7000 m²footprint, linking CERN’s Meyrin site and the Globe of Science and Innovation. It is being designed by renowned architects Renzo Piano Building Workshop and intends to “celebrate the inventiveness and creativity that characterise the world of research and engineering”. Construction is planned to start in 2020 and be completed in 2022.

First images of the centre of a galaxy

**Dark secrets** The black hole’s event horizon is around 2.5 times smaller than the shadow (orange) it casts and measures just under 40 billion kilometres across – large enough to contain the entire solar system. Credit: EHT collaboration.

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 The Astrophysical 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.

BELLA sets new record for plasma acceleration

**Light work** The plasma channel’s electron density profile (blue) formed inside a sapphire tube (grey) with the combination of an electrical discharge and ultrashort laser pulse (red, orange, and yellow). Credit: G Bagdasarov/A Gonsalves/J-L Vay.

A world record for laser-driven wakefield acceleration has been set by a team at the Berkeley Lab Laser Accelerator (BELLA) Center in the US. Physicists used a novel scheme to channel 850 TW laser pulses through a 20 cm-long plasma, allowing electron beams to be accelerated to an energy of 7.8 GeV – almost double the previous record set by the same group in 2014.

Proposed 40 years ago, plasma-wakefield acceleration can produce gradients hundreds of times higher than those achievable with conventional techniques based on radio-frequency cavities. It is often likened to surfing a wave. Relativistic laser pulses with a duration of the order of the plasma period generate large-amplitude electron plasma waves that displace electrons with respect to the background ions, allowing the plasma waves to accelerate charged particles to relativistic energies. Initial work showed that TeV energies could be reached in just a few hundred metres using multiple laser-plasma accelerator stages, each driven by petawatt laser pulses propagating through a plasma with a density of about 10¹⁷ cm^–3. However, this requires the focused laser pulses to be guided over distances of tens of centimetres. While a capillary discharge is commonly used to create the necessary plasma channel, achieving a sufficiently deep channel at a plasma density of 10¹⁷ cm^–3 is challenging.

In the latest BELLA demonstration, the plasma channel produced by the capillary discharge was modified by a nanosecond-long “heater” pulse that confined the focused laser pulses over the 20 cm distance. This allowed for the acceleration of electron beams with quasi-monoenergetic peaks up to 7.8 GeV. “This experiment demonstrates that lasers can be used to accelerate electrons to energies relevant to X-ray free-electron lasers, positron generation, and high-energy collider stages,” says lead author Tony Gonsalves. “However, the beam quality currently available from laser-wakefield accelerators is far from that required by future colliders.”

The quality of the accelerated electron beam is determined by how background plasma electrons are trapped in the accelerating and focusing “bucket” of the plasma wave. Several different methods of initiating electron trapping have been proposed to improve the beam emittance and brightness significantly beyond state-of-the-art particle sources, representing an important area of research. Another challenge, says Gonsalves, is to improve the stability and reproducibility of the accelerated electron beams, which are currently limited by fluctuations in the laser systems caused by air and ground motion.

In addition to laser-driven schemes, particle-driven plasma acceleration holds promise for high-energy physics applications. Experiments using electron-beam drivers are ongoing and planned at various facilities including FLASHForward at DESY and FACET-II at SLAC (CERN Courier January/February 2019 p10). The need for staging multiple plasma accelerators may even be circumvented by using energetic proton beams as drivers. Recent experiments at CERN’s Advanced Wakefield Experiment demonstrated electron acceleration gradients of around 200 MV/m using proton-beam-driven plasma wakefields (CERN Courier October 2018 p7).

Experiments at Berkeley in the next few years will focus on demonstrating the staging of laser-plasma accelerators with multi-GeV energy gains. “The field of plasma wakefield acceleration is picking up speed,” writes Florian Grüner of the University of Hamburg in an accompanying APS Viewpoint article. “If plasma wakefields can have gradients of 1 TV/m, one might imagine that a ‘table-top version of CERN’ is possible.”

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