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Workshop puts advanced accelerators on track

CERN’s AWAKE experiment. A possible application of this technology would be to use a proton beam from the LHC to accelerate electrons in a few-hundred-metre-long plasma for electron–proton collisions.
Image credit: M Brice/CERN.

Researchers working on advanced and novel accelerator technologies met at CERN on 25–28 April to draw up an international roadmap for future high-energy particle accelerators. Organised by the International Committee for Future Accelerators (ICFA) to trigger a community effort, the Advanced and Novel Accelerator for High Energy Physics Roadmap Workshop saw 80 experts from 11 countries discuss the steps needed to include these new technologies in strategies for future machines in Asia, Europe and the US. It follows recent discussions of national roadmaps in the US and elsewhere, and was timed so that advanced accelerator development can be taken into account in the coming update of the European Strategy for Particle Physics in 2020.

Given the scale of the cost of traditional accelerator technologies, which require large circular or long linear accelerators to reach the highest energies, the past two decades have seen significant progress to find alternative approaches. These include dielectrics and plasmas driven by laser pulses or particle beams, which are able to accelerate particles 1000 times more than the radio-frequency structures used in todayʼs accelerators. Major laboratories including CERN, SLAC, Argonne, DESY and INFN-Frascati are working on various techniques. CERN has recently started the AWAKE experiment, demonstrating that high-energy protons from the SPS can drive large accelerating fields in a plasma.

The next step is to apply these methods to high-energy physics. For example, the acceleration schemes must be tuned to determine their real potential for producing high-energy and high-quality particle bunches; the former has been demonstrated, but the latter remains a challenge. This requires experimental facilities that can only be hosted by international laboratories and a strong, united and co-ordinated community that merges the advanced and traditional accelerator communities.

The April event has now set this process in motion, focusing on the technical milestones that are needed to progress towards an intermediate-size particle accelerator and on the strategies needed to bring communities together. A new working group dedicated to the development of a roadmap will be included in the European Advanced Accelerators Concept Workshop in September 2017 in Elba, Italy.

“This is the first time that the advanced accelerator field is co-ordinated at the international level, and will pull the community together towards the first great challenge ahead, i.e. the achievement of reliable and high-quality particle bunches,” says workshop chair Brigitte Cros. “Further workshops will be organised to strengthen and sustain this co-ordination.”

CAST experiment constrains solar axions

The latest constraints on the two-photon coupling, g_a_γ, of axions and other similar particles as a function of mass, showing the new CAST limits.

In a paper published in Nature Physics, the CERN Axion Solar Telescope (CAST) has reported important new exclusion limits on coupling of axions to photons. Axions are hypothetical particles that interact very weakly with ordinary matter and therefore are candidates to explain dark matter. They were postulated decades ago to solve the “strong CP” problem in the Standard Model (SM), which concerns an unexpected time-reversal symmetry of the nuclear forces. Axion-like particles, unrelated to the strong-CP problem but still viable dark-matter candidates, are also predicted by several theories of physics beyond the SM, notably string theory.

A variety of Earth- and space-based observatories are searching possible locations where axions could be produced, ranging from the inner Earth to the galactic centre and right back to the Big Bang. CAST looks for solar axions using a “helioscope” constructed from a test magnet originally built for the Large Hadron Collider. The 10 m-long superconducting magnet acts like a viewing tube and is pointed directly at the Sun: solar axions entering the tube would be converted by its strong magnetic field into X-ray photons, which would be detected at either end of the magnet. Starting in 2003, the CAST helioscope, mounted on a movable platform and aligned with the Sun with a precision of about 1/100th of a degree, has tracked the movement of the Sun for an hour and a half at dawn and an hour and a half at dusk, over several months each year.

In the latest work, based on data recorded between 2012 and 2015, CAST finds no evidence for solar axions. This has allowed the collaboration to set the best limits to date on the strength of the coupling between axions and photons for all possible axion masses to which CAST is sensitive. The limits concern a part of the axion parameter space that is still favoured by current theoretical predictions and is very difficult to explore experimentally, allowing CAST to encroach on more restrictive constraints set by astrophysical observations. “Even though we have not been able to observe the ubiquitous axion yet, CAST has surpassed even the sensitivity originally expected, thanks to CERN’s support and unrelenting work by CASTers,” says CAST spokesperson Konstantin Zioutas. “CAST’s results are still a point of reference in our field.”

The experience gained by CAST over the past 15 years will help physicists to define the detection technologies suitable for a proposed, much larger, next-generation axion helioscope called IAXO. Since 2015, CAST has also broadened its research at the low-energy frontier to include searches for dark-matter axions and candidates for dark energy, such as solar chameleons.

Belle II rolls in

The Belle II detector is now in place at the SuperKEKB facility in Japan.
Image credit: KEK.

On 11 April, the Belle II detector at the KEK laboratory in Japan was successfully “rolled-in” to the collision point of the upgraded SuperKEKB accelerator, marking an important milestone for the international B-physics community. The Belle II experiment is an international collaboration hosted by KEK in Tsukuba, Japan, with related physics goals to those of the LHCb experiment at CERN but in the pristine environment of electron–positron collisions. It will analyse copious quantities of B mesons to study CP violation and signs of physics beyond the Standard Model (CERN Courier September 2016 p32).

“Roll-in” involves moving the entire 8 m-tall, 1400 tonne Belle II detector system from its assembly area to the beam-collision point 13 m away. The detector is now integrated with SuperKEKB and all its seven subdetectors, except for the innermost vertex detector, are in place. The next step is to install the complex focusing magnets around the Belle II interaction point. SuperKEKB achieved its first turns in February 2016, with operation of the main rings scheduled for early spring and phase-III “physics” operation by the end of 2018.

Compared to the previous Belle experiment, and thanks to major upgrades made to the former KEKB collider, Belle II will allow much larger data samples to be collected with much improved precision. “After six years of gruelling work with many unexpected twists and turns, it was a moving and gratifying experience for everyone on the team to watch the Belle II detector move to the interaction point,” says Belle II spokesperson Tom Browder. “Flavour physics is now the focus of much attention and interest in the community and Belle II will play a critical role in the years to come.”

CERN on the road

The speed limit will be reduced to 50 km h^–1 at the point where the Esplanade des Particules crosses the pedestrianised area.
Image credit: CERN.

CERN has begun major work to create a new visitor space called Esplanade des Particules, to welcome the ever-growing numbers of visitors to the laboratory each year. The project, undertaken in conjunction with the Etat de Genève, will integrate the laboratory better into the local urban landscape, making it more open and easily accessible, with work to last until summer 2018.

A competition was launched in 2011 to showcase the public entrance to CERN. Landscape-architects Studio Paolo Bürgi won with a design for a large space dedicated to pedestrians that connects CERN’s reception to the Globe of Science and Innovation. The Esplanade des Particules will see the current “Flags Car Park” replaced by a blue pedestrianised area in which the flags of CERN Member States will cross the main road to the laboratory.

LHCb finds new hints of Standard Model discrepancy

The measured value (black) of R_K*⁰ in two regions of the μ⁺μ^– or e⁺e^– invariant mass squared, q², and various independent SM theoretical predictions (colours).

At a seminar at CERN on 18 April, the LHCb collaboration presented new results in flavour physics that show an interesting departure from Standard Model (SM) predictions. The new measurement concerns a parameter called R_K*⁰, which is the ratio of the probabilities that a B⁰ meson decays to K^*0μ⁺μ^– and to K^*0e⁺e^– (where the K^*0 meson was reconstructed through its decay into a charged kaon K⁺ and a pion π^–).

Lepton universality – a cornerstone of the SM – states that leptons have the same couplings to gauge bosons and therefore that R_K*⁰ is expected to be close to unity (apart from well-understood effects related to the different masses of the leptons, which change this value slightly). Any conclusive observation of a violation of this rule would indicate the existence of physics beyond the SM. Based on analysis of data from Run 1, the LHCb measurement differs from the prediction with a significance between 2.1 and 2.5 standard deviations in the two regions of q² (the μ⁺μ^– or e⁺e^– invariant mass squared) in which the measurement is performed.

Three years ago, LHCb found a similar discrepancy for the quantity R_K – in which the B⁰ meson is replaced by a B⁺ and the K^*0 meson by a K⁺. In addition, another class of measurements concerning different ratios of B-meson decay rates involving τ and muon leptons also exhibit some tensions with predictions. While intriguing, none of the differences are yet at the level where they can be claimed to exhibit evidence for physics beyond the SM.

The LHCb collaboration has a wide programme of lepton-universality tests based on different R measurements in which other particles replace the K^*0 or K⁺ mesons in the ratios. The R_K*⁰ and R_K measurements so far were obtained using the entire Run 1 data sample, corresponding to an integrated luminosity of 3 fb^–1 at an energy of 7 and 8 TeV. Data collected in Run 2 already provide a sample more than twice as large, and it is therefore of great importance to see whether updates of the present analysis will confirm or rule out the discrepancies.

Proton–proton collisions become stranger

Particle-to-pion ratios versus the number of charged particles produced in an event, normalised to the average values measured in proton–proton collisions.

Recreating the intense fireball of quarks and gluons that existed immediately after the Big Bang, the quark–gluon plasma (QGP), traditionally requires high-energy collisions between heavy ions such as lead-on-lead. Recently, however, the ALICE experiment has seen tentative evidence that the extreme QGP state is created in much smaller systems generated by selected proton–proton collisions.

In a paper published in Nature Physics, the collaboration reports an enhanced production of strange and multi-strange hadrons in high-multiplicity proton–proton (pp) interactions at a centre-of-mass energy of 7 TeV. This phenomenon was one of the earliest proposed indicators for the formation of a QGP, and is very similar to that found in lead–lead (Pb–Pb) collisions and proton–lead (p–Pb) collisions. Measured at mid-rapidity, the production rate of strange particles increases with the event “activity” (quantified by the charged-particle multiplicity density) faster than that of non-strange ones, leading to an enhancement relative to pions.

The enhancement in strangeness is expected to be more pronounced for multi-strange baryons, and this was confirmed in collisions of heavy nuclei at the SPS, RHIC and the LHC. The remarkable similarity between strange particle production in pp, p–Pb and Pb–Pb collisions is complemented by other pp and p–Pb measurements. All exhibit characteristic features from high-energy heavy-ion collisions that are understood to be connected to the formation of a deconfined QCD phase at high temperature and energy density.

The observed multiplicity-dependent enhancement (see figure) follows a hierarchy connected to the strangeness in the hadron. No enhancement is observed for protons (which have no valence strange quarks), demonstrating that the observed increase is strangeness rather than mass related. The results have been compared with Monte Carlo models commonly used at the LHC, of which none can reproduce satisfactorily the observations.

It is not yet clear if the ALICE data truly signal the progressive onset of a QGP medium in small systems. On the other hand, these measurements unveil another remarkable similarity with phenomena known from high-energy nuclear reactions, opening up new possibilities to investigate the underlying dynamical mechanisms of the QGP. Either way, the ability to isolate QGP-like phenomena in a smaller and simpler system opens up an entirely new dimension for the study of the properties of the fundamental state that our universe emerged from.

ATLAS explores the energy frontier

The highest mass dijet event selected by ATLAS in its dijet resonance search, corresponding to a mass of 8.1 TeV.

The start of LHC Run 2 in 2015 saw the centre-of-mass energy of proton–proton collisions increase from 8 to 13 TeV, dramatically increasing the possibility to create heavy particles predicted by many models of new physics. The ATLAS collaboration has recently released the first search results from its analysis of the full 2015 and 2016 data sets, providing the largest combined LHC data set analysed so far.

New heavy particles are likely to decay immediately inside the detector into known objects such as pairs of jets, leptons or bosons. These decay products will typically have large transverse momentum, due to the high mass of the parent particle, and this raises challenges both for the detector and the algorithms used to identify the decay products.

Utilising pairs of jets (dijets), a recent ATLAS search was able to probe the highest invariant mass of any of its searches, measuring events with energies as high as 8.1 TeV and thereby pushing up the experimentʼs sensitivity to hypothetical new resonances. Additionally, ATLAS has released the results of searches in events containing pairs of muons or electrons or single muons/electrons plus a neutrino, which extend the sensitivity to new resonance masses up to 4.5 and 5.1 TeV, respectively. Heavy particles with an affinity for coupling to the Higgs boson were also examined up to a mass of 3.7 TeV.

New resonances would reveal themselves as excesses of events (open circles and squares) in the dijet mass spectrum. ATLAS searches for them by comparing the data (black) to an estimate of the spectrum produced if no new particles are present (red). The most significant excess (vertical blue lines) is compatible with a statistical fluctuation.

ATLAS has also searched for vector-like top-quark partners, which are strongly interacting particles invoked by models with new high-scale symmetries and which may be produced at the LHC. The final states sought in these analyses are a single high-transverse-momentum electron or muon, plus either several jets and a large component of missing transverse momentum or a large-radius jet consistent with a W or Z boson plus some missing transverse momentum and one b-tagged jet. The presence of vector-like top quarks is excluded for particle masses of up to 1.35 TeV, depending on the physics model chosen.

Finally, ATLAS has performed direct searches for dark matter by looking for single energetic photons plus missing transverse momentum and for a Higgs boson plus missing transverse momentum. These are potential signatures of the production and decay of a pair of weakly interacting massive particles (with the photon arising from initial-state radiation and the Higgs boson being produced in the decay of a Z’ dark-matter mediator).

The data are found to be consistent with Standard Model predictions for all of the searches conducted thus far. The second phase of Run 2 is about to begin and is scheduled to continue until the end of 2018, roughly tripling the integrated luminosity collected so far. This huge amount of data yet to be recorded will further extend the reach of these searches for new physics.

SUSY searches in the electroweak sector

SUSY exclusion limits for the production of chargino–neutralino pairs for decays with intermediate sleptons (left) and decays involving W, Z or H bosons (right).

The sensitivity of searches for supersymmetry (SUSY) has been boosted by the increased centre-of-mass energy of LHC Run 2. Analyses of the first Run 2 data recorded in 2015 and early 2016 focused on the production of strongly interacting SUSY particles – the partners of Standard Model (SM) gluons (“gluinos”) and quarks (“squarks”).

With the large data set accumulated during the rest of 2016, attention now turns to a more challenging but equally important part of the SUSY particle spectrum: the supersymmetric partners of SM electroweak gauge (“winos”, “binos”) and Higgs (“higgsinos”) bosons. The spectrum of the minimal supersymmetric extension of the SM contains six of these particles: two charged (“charginos”) and four neutral (“neutralinos”) ones. The cross-sections for the direct production of pairs of these particles are typically three to five orders of magnitude lower than that for gluino pair production, but such events might be the only indication of supersymmetry at the LHC if the partners of gluons, quarks and leptons are heavy.

CMS has recently reported searches for electroweak production of neutralinos and charginos in different final states. Decays of these particles to the lightest SUSY particle (LSP) – which are candidates for dark matter – are expected to produce Z, W and H bosons, or photons. If the SUSY partners of leptons (sleptons) are sufficiently light they can also be part of the decay chain. In all of these cases, since final states with two or more leptons constitute a large fraction of the signal events, CMS has searched for supersymmetry in final states with multiple leptons. These searches are complemented by analyses targeting hadronic decays of Higgs bosons in these events.

None of the searches performed by CMS show any significant deviation of the observed event counts from the estimated yields for SM processes. In benchmark models with reduced SUSY particle content, the strongest constraints on the electroweak production of pairs of the lightest chargino and the second-lightest neutralino are obtained by assuming their decay chains involve sleptons, with mass limits reaching up to 1.15 TeV, depending on the slepton’s mass and flavour. For direct decays of the chargino (neutralino) to a W (Z) boson and the lightest neutralino, the excluded regions reach up to 0.61 TeV.

A particularly interesting case, favoured by “natural” supersymmetry, are models with small mass differences between the lightest chargino and neutralino states. In these models, the transverse momenta of the leptons can be significantly lower than the typical thresholds of 10–20 GeV used in most analyses. CMS has designed a specific search to enhance the sensitivity to final states with two low-momentum leptons of opposite charge that includes a dedicated online selection for muons with transverse momenta as low as 3 GeV. The search reaches an unprecedented sensitivity: for a mass difference of 20 GeV, the exclusion reaches a mass of 230 GeV.

Based on data recorded in 2016, CMS has covered models of electroweak production of “wino”-like charginos and neutralinos with searches in different final states. More results are expected soon, and the sensitivity of the searches will largely profit from the extension of the data set in the remaining two years of LHC Run 2.

CERN and APS sign open access agreement

On 27 April, CERN and the American Physical Society (APS) signed an agreement for the Sponsoring Consortium for Open Access Publishing in Particle Physics (SCOAP³). Under the agreement, high-energy physics articles published in three leading journals of the APS will be open access from January 2018 onwards. All authors worldwide will be able to publish their articles in Physical Review C, Physical Review D and Physical Review Letters at no direct cost. The aim is to allow free and unrestricted exchange of scientific information within the global scientific community and beyond. As a result of the agreement, SCOAP³ will cover about 90 per cent of all the journal literature in high-energy physics.

Convened and managed by CERN and launched in 2014, SCOAP³ is the largest-scale global open access initiative ever built, involving 3000 libraries and research institutes from 44 countries. The initiative is possible through funds made available from the redirection of former subscription monies: publishers reduce subscription prices for journals participating in the initiative, and those savings are pooled by SCOAP³ partners to pay for the open access costs.

“Open access reflects values and goals that have been enshrined in CERN’s Convention for more than 60 years, such as the widest dissemination of scientific results. We are very pleased that the APS is joining SCOAP³ and we look forward to welcoming more partners for the long-term success of this initiative,” says CERN Director-General Fabiola Gianotti.

Survey reveals edge of dark-matter halos

Simulation showing the density of a galaxy cluster, with a clear edge in the density (dashed line) induced by the process of “splash back”.
Image credit: ApJ/DOI:10.1088/0004-637X/810/1/36.

Gravitational-lensing measurements indicate that clusters of galaxies are surrounded by large halos of dark matter. By studying the distribution and colour of galaxies inside galaxy clusters using data from the Sloan Digital Sky Survey (SDSS), researchers have now measured a new feature of the shape of these halos. The results show that the density of dark matter in a halo does not gradually fall off with distance, as might be expected, but instead exhibits a sharp edge.

According to the standard cosmological model, dark-matter halos are the result of small perturbations in the density of the early universe. Over time, and under the influence of gravity, these perturbations grew into large dense clumps that affect surrounding matter: galaxies in the vicinity of a halo will initially all move away due to the expansion of the universe, but gravity eventually causes the matter to fall towards and then orbit the halo. Studying the movements of the matter inside halos therefore provides an indirect measurement of the interaction between normal and dark matter, allowing researchers to probe new physics such as dark-matter interactions, dark energy and modifications to gravity.

Using the SDSS galaxy survey, Bhuvnesh Jain and Eric Baxter from the University of Pennsylvania and colleagues at other institutes report new evidence for an edge-like feature in the density profile of galaxies within a halo. The large amount of SDSS data available allowed a joint analysis of thousands of galaxy clusters each containing thousands of galaxies, revealing an edge inside clusters in agreement with simulations based on “splash-back” models. The edge is associated with newly accreted matter which, after falling into the halo, slows down as it reaches the extremity of its elliptical orbit before falling back towards the halo centre. As the matter “splashes back” it slows down, which leads to a build-up of matter at the edge of the halo and a steep fall-off in the amount of matter right outside this radius.

The authors found additional evidence for the edge by studying the colour of the galaxies. Since new stars that formed in hydrogen-rich regions are more bright in the blue part of the spectrum, galaxies with large amounts of new-star formation are more blue than those with little star formation. As a galaxy travels through a cluster, different mechanisms can strip it of the gasses required to form new blue stars, reducing star formation and making the galaxy appear more red. Models therefore predict galaxies still in the process of falling into the halo to be more blue, while those which already passed the edge and are in orbit have started to become red – exactly as data from the SDSS galaxy survey showed.

A range of ongoing and new galaxy surveys – such as Hyper Suprime-Cam, Dark Energy Survey, Kilo-Degree Survey and the Large Synoptic Survey Telescope – will measure the galaxy clusters in more detail. Using additional information on the shape of the clusters, says the team, it is possible to study both the standard physics of how galaxies interact with the cluster and the possible unknown physics of what the nature of dark matter and gravity is.

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