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Searches with boosted topologies at Run 2

The first LHC run was highlighted by the discovery of the long-awaited Higgs boson at a mass of about 125 GeV, but we have no clue why nature chose this mass. Supersymmetry explains this by postulating a partner particle to each of the Standard Model (SM) fermions and bosons, but these new particles have not yet been found. A complementary approach to address this issue is to widen the net and look for signatures beyond those expected from the SM.

Fig. 1. Expected and observed limits on the cross-section times branching fraction to WZ for a new heavy-vector boson, W´, at √s = 13 TeV. The different limit curves correspond to different decay modes for the W and Z bosons.
Image credit: ATLAS Collaboration.

Searches for new physics in Run 1 found no signals, and from these negative results we know that new particles may be heavy. For this reason, their decay products, such as top quarks, electroweak gauge bosons (W, Z) or Higgs bosons, may be very energetic and could be highly boosted. When such particles are produced with large momentum and decay into quark final states, the decay products often collimate into a small region of the detector. The collimated sprays of hadrons (jets) originating from the nearby quarks are therefore not reliably distinguished. Special techniques have been developed to reconstruct such boosted particles into jets with a wide opening angle, and to identify the cores associated with the quarks using soft-particle-removal procedures (grooming). ATLAS performed an extensive optimisation of top, W, Z and Higgs boson identification, exploiting a wide range of jet clustering and grooming algorithms as well as kinematic properties of jet substructure before the second LHC run. This led to a factor of two improvement in W/Z tagging, compared with the technique used previously in terms of background rejection for the same efficiency for W/Z boson transverse momenta around 300–500 GeV.

ATLAS exploited the optimised boson tagging in the search for heavy resonances decaying into a pair of two electroweak gauge bosons (WW, WZ, ZZ) or of a gauge boson and a Higgs boson (WH, ZH) at 13 TeV collisions. Events are categorised into different numbers of charged/neutral leptons, and all possible combinations are considered except for fully leptonic and fully hadronic WH or ZH decays. For the Higgs boson, only the dominant decay into b quarks is considered. Figure 1 shows the results of WZ searches with a 2015 data set corresponding to 3.2 fb^–1, presented as the lower limits on the production cross-section times the branching fraction for a new massive gauge boson with certain mass. No evidence for new physics has been found with these preliminary searches.

The boosted techniques have evolved into a fundamental tool for beyond SM searches at high energy. ATLAS foresees that the search will be greatly enhanced by the techniques, and seeks opportunities to adapt them in uncharted territory for the upcoming LHC run.

Anisotropic flow in Run 2

Exploiting the data collected during November 2015 with Pb–Pb collisions at the record-breaking energy of √s_NN= 5.02 TeV, ALICE measured for the first time the anisotropic flow of charged particles at this energy.

Fig. 1. Anisotropic flow, v_n, as a function of event centrality.
Image credit: ALICE Collaboration.

Relativistic heavy-ion collisions are the tool of choice to investigate quark–gluon plasma (QGP) – a state of matter where quarks and gluons move freely over distances that are large in comparison to the typical size of a hadron. Anisotropic flow, which measures the momentum anisotropy of final-state particles, is sensitive on the one hand to the initial density and to the initial geometry fluctuations of the overlap region, and on the other hand to the transport properties of the QGP. Flow is quantified by the Fourier coefficients, ν_n, of the azimuthal distribution of the final-state charge particles. The dominant flow coefficient, ν₂, referred to as elliptic flow, is related to the initial geometric anisotropy. Higher coefficients, such as triangular flow (ν₃) and quadrangular flow (ν₄), can be related primarily to the response of the produced QGP to fluctuations of the initial energy density profile of the participating nucleons.

Figure 1 shows the centrality dependence of flow coefficients, both for 2.76 and 5.02 TeV Pb–Pb collisions. Compared with the lower-energy results, the anisotropic flows ν₂, ν₃ and ν₄ increase at the newly measured energy by (3.0±0.6)%, (4.3±1.4)% and (10.2±3.8)%, respectively, in the centrality range 0–50%.

The transport properties of the created matter are investigated by comparing the experimental results with hydrodynamic model calculations, where the shear-viscosity to entropy density ratio, η/s, is the dominant parameter. Previous studies demonstrated that anisotropic flow measurements are best described by calculations using a value of η/s close to 1/4π, which corresponds to the lowest limits for a quantum fluid. It is observed in figure 1 that the magnitude and the increase of anisotropic flow measured at the higher energy remain compatible with hydrodynamic predictions, favouring a constant value for η/s going from √s_NN= 2.76 to 5.02 TeV Pb–Pb collisions.

It is also observed that the results of the p_T-differential flow are comparable for both energies. This observation indicates that the increase measured in the integrated flow (figure 1) reflects the increase of the mean transverse momentum. Further comparisons of differential-flow measurements and theoretical calculations will provide a unique opportunity to test the validity of the hydrodynamic picture, and the power to further discriminate between various possibilities for the temperature dependence of the shear-viscosity to entropy density ratio of the produced matter in heavy-ion collisions at highest energies.

CMS hunts for supersymmetry in uncharted territory

The CMS collaboration is continuing its hunt for signs of supersymmetry (SUSY), a popular extension to the Standard Model that could provide a weakly interacting massive-particle candidate for dark matter, if the lightest supersymmetric particle (LSP) is stable.

Fig. 1. Exclusion limits as a function of squark and LSP masses, for squark pair production with different flavours and mass-degeneracy scenarios.
Image credit: CMS Collaboration.

With the increase in the LHC centre-of-mass energy from 8 to 13 TeV, the production cross-section for hypothetical SUSY partners rises; the first searches to benefit are those looking for the strongly coupled SUSY partners of the gluon (gluino) and quarks (squarks) that had the most stringent mass limits from Run 1 of the LHC. By decaying to a stable LSP, which does not interact in the detector and instead escapes, SUSY particles can leave a characteristic experimental signature of a large imbalance in transverse momentum.

Searches for new physics based on final states with jets (a bundle of particles) and large transverse-momentum imbalance are sensitive to broad classes of new-physics models, including supersymmetry. CMS has searched for SUSY in this final state using a variable called the “stransverse mass”, MT2, to measure the transverse-momentum imbalance, which strongly suppresses fake contributions due to potential hadronic-jet mismeasurement. This allows us to control the background from copiously produced QCD multi-jet events. The remaining background comes from Standard Model processes such as W, Z and top-quark pair production with decays to neutrinos, which also produce a transverse-momentum imbalance. We estimate our backgrounds from orthogonal control samples in data targeted to each. To cover a wide variety of signatures, we categorise our signal events according to the number of jets, the number of jets arising from bottom quarks, the sum of the transverse momenta of hadronic jets (HT), and MT2. Some SUSY scenarios predict spectacular signatures, such as four top quarks and two LSPs, which would give large values for all of these quantities, while others with small mass splittings produce much softer signatures.

Unfortunately, we did not observe any evidence for SUSY in the 2015 data set. Instead, we are able to significantly extend the constraints on the masses of SUSY partners beyond those from the LHC Run 1. The gluino has the largest production cross-section and many potential decay modes. If the gluino decays to the LSP and a pair of quarks, we exclude gluino masses up to 1550–1750 GeV, depending on the quark flavour, extending our Run 1 limits by more than 300 GeV. We are also sensitive to squarks, with our constraints summarised in figure 1. We set limits on bottom-squark masses up to 880 GeV, top squarks up to 800 GeV, and light-flavour squarks up to 600–1260 GeV, depending on how many states are degenerate in mass.

Even though SUSY was not waiting for us around the corner at 13 TeV, we look forward to the 2016 run, where a large increase in luminosity gives us another chance at discovery.

LHCb awards physics prizes for its Kaggle competition

Machine learning, also known in physics circles as multivariate analysis, is used more and more in high-energy physics, most visibly in data analysis but also in other applications such as trigger and reconstruction. The community of machine-learning data scientists organises “Kaggle” competitions to solve difficult and interesting challenges in different fields.

LHCb Kaggle-competition participants.
Image credit: Marcin Chrzaszcz.

With the aim being to develop interactions with the machine-learning community, LHCb organised such a competition, featuring the search for the lepton-flavour violating decay, τ→ μμμ. This decay is (almost) forbidden in the Standard Model, and therefore its observation would indicate a discovery of “new physics”, which is now the key goal of the LHC. This Kaggle challenge (https://www.kaggle.com/c/flavours-of-physics) was conceived by a group of scientists from CERN, the University of Warwick, the University of Zürich and the Yandex School of Data Analysis. It was financially supported by the Yandex Data Factory, Intel and the University of Zürich. The competition took place over three months between July and October 2015. More than 700 people competed to achieve the best signal-versus-background discrimination and to win the prize awarded to the first three ranked solutions, totalling $15,000.

This particular challenge, using both “real” and simulated LHCb data, has been recognised by the community as more complicated than usual challenges, and therefore a refreshing problem to try and solve. The winners of the competition were awarded their prizes in December at one of the main conferences of the machine-learning community – the Twenty-ninth Annual Conference on Neural Information Processing Systems (NIPS).

In addition to the prizes for the best-ranked solutions, another prize was foreseen for the solution that is the most interesting from a physics point of view. In the event, LHCb decided to award two of these physics prizes of $2000 each to Vincens Gaitan (a former member of the ALEPH collaboration at CERN’s Large Electron–Positron collider) and Alexander Rakhlin. Their solutions are innovative and particularly suitable for cases where the size of the samples used to train the multivariate operator is limited and when the training samples do not perfectly match the real data.

The two awardees collected their prize at a three-day workshop organised at the University of Zürich on 18–21 February, as a follow-up to the Kaggle challenge. This workshop brought together 55 people from the LHC and the machine-learning communities, and interesting ideas have been exchanged. The general conclusion from discussions at this event was that the exercise had been a very positive one, both for LHCb and those that entered the competition.

Another important step for the AWAKE experiment

AWAKE’s 10 m-long plasma cell in the experiment tunnel.
Image credit: CERN.

By harnessing the power of wakefields generated by a proton beam in a plasma cell, the AWAKE experiment at CERN (CERN Courier November 2013 p17) aims to produce accelerator gradients that are hundreds of times higher than those achieved in current machines.

The experiment is being installed in the tunnel that was previously used by the CERN Neutrinos to Gran Sasso facility. In AWAKE, a beam of 400 GeV protons from the CERN Super Protron Synchrotron will travel through a plasma cell and will generate a wakefield that, in turn, will accelerate an externally injected electron beam. A laser will ionise the gas in the cell to become a plasma and seed the self-modulation instability that will trigger the wakefield. The project aims to prove that the plasma wakefield can be driven with protons and that its acceleration will be extremely powerful – hundreds of times more powerful than that achieved today – and eventually to provide a design for a plasma-based linear collider.

The AWAKE tunnel is progressively being filled with its vital components. In its final configuration, the facility will feature a clean room for the laser, a dedicated area for the electron source and two new tunnels for two new beamlines: one small tunnel to hold the laser beam, which ionises the plasma and seeds the wakefields, and a second, larger tunnel that will be home to the electron beamline – the “witness beam” accelerated by the plasma. At the beginning of February, the plasma cell was lowered into the tunnel and moved to its position at the end of the proton line. The cell is a 10 m-long component developed by the Max Planck Institute for Physics in Munich (Germany). A first prototype successfully completed commissioning tests in CERN’s North Area in the autumn of 2015. The prototype allowed the AWAKE collaboration to validate the uniformity of the plasma temperature in the cell.

AWAKE is a collaborative endeavour with institutes and organisations participating around the world. The synchronised proton, electron and laser beams provided by CERN are an integral part of the experiment. After installation of the plasma cell, the next step will be installation of the laser, the vacuum equipment and the diagnostic system for both laser and proton beams.

Beam commissioning for the proton beamline is scheduled to start this summer. The programme will continue with installation of the electron line, with the aim of starting acceleration tests at the end of 2017.

BESIII makes first direct measurement of the Λ_c at threshold

The figure shows the beam-constrained mass M_BC (M_BCc²= √(E²_beam − p²c²)) distribution for the simple tag samples. The mass peak is at (2286.46±0.14) MeV/c².
Image credit: BESIII Collaboration.

The charmed baryon, Λ_c, was first observed at Fermilab in 1976. Now, 40 years later, the Beijing Spectrometer (BESIII) experiment at the Beijing Electron–Positron Collider II (BEPCII) has measured the absolute branching fraction of Λ⁺_c→ pΚ^–π⁺ at threshold for the first time.

Because the decays of the Λ⁺_c to hadrons proceed only through the weak interaction, their branching fractions are key probes for understanding weak interactions inside of a baryon. In particular, precise measurements of the decays of the Λ⁺_c will provide important information on the final-state strong interaction in the charm sector, thereby improving the understanding of quantum chromodynamics in the non-perturbative energy region. In addition, because most of the excited baryons of the Λ_c and Σ_c types, as well as the b-flavoured baryons, eventually decay into a Λ⁺_c, studies of these baryons are directly connected to understanding the ground state, Λ⁺_c.

Most decay rates of the Λ⁺_c are measured relative to the decay mode, Λ⁺_c→ pΚ^–π⁺, but there are no completely model-independent measurements of the absolute branching fraction for this decay mode. Moreover, most measurements of the ground-state Λ⁺_c were made more than 20 years ago.

In 2014, BESIII accumulated a data sample of e⁺e^– annihilations with an integrated luminosity of 567 pb^–1 at a centre-of-mass energy of 4.599 GeV. This is about 26 MeV above the mass threshold for a Λ⁺_cΛ^–_c, so no additional hadrons accompanying the Λ⁺_cΛ^–_c are produced.

The BESIII collaboration measures hadronic branching fractions at the Λ⁺_cΛ^–_c threshold using a double-tagging technique that relies on fully reconstructed Λ⁺_cΛ^–_c decays. This technique obviates the need for knowledge of the luminosity or the Λ⁺_cΛ^–_c production cross-section. To improve precision, BESIII combines 12 Cabibbo-favoured decay channels and implements a global least-squares fit by considering their correlations. This leads to a result for the branching fraction for Λ⁺_c → pΚ^–π⁺ of B(Λ⁺_c → pΚ^–π⁺) = (5.84±0.27±0.23)%.

This is the first measurement of the absolute branching fraction of the decay Λ⁺_c→ pΚ^–π⁺ at threshold, and it has the advantage of incorporating an optimal understanding of model uncertainty. In addition, BESIII has made significantly improved measurements of the other 11 Cabibbo-favoured hadronic-decay modes.

In 2015, based on the same data set, BESIII also measured the absolute branching fraction of the semi-leptonic decay Λ_c⁺→ Λe⁺ν_e, using a missing-neutrino technique. In future, a larger Λ_c⁺ threshold sample will help to improve further understanding of the properties of the Λ_c⁺.

‘First turns’ for SuperKEKB

The three middle “spikes” are signals from CLAWS, a subsystem of the BEAST II detector triggered by the SuperKEKB injection signal.
Image credit: SuperKEKB.

On 10 February, the SuperKEKB electron–positron collider in Tsukuba, Japan, succeeded in circulating and storing a positron beam moving close to the speed of light through 1000 magnets in a narrow tube around the 3 km circumference of its main ring. And on 26 February, it succeeded in circulating and storing an electron beam around its ring of magnets in the opposite direction.

The achievement of “first turns”, which means storing the beam in the ring through many revolutions, is a major milestone for any particle accelerator.

SuperKEKB, along with the Belle II detector, is designed to search for new physics beyond the Standard Model by measuring rare decays of elementary particles such as beauty quarks, charm quarks and τ leptons.

Unlike the LHC, which is the world’s highest-energy machine, SuperKEKB/Belle II is designed to have the world’s highest luminosity – a factor of 40 higher than the earlier KEKB machine, which holds many records for accelerator performance. SuperKEKB will therefore be the leading accelerator on the “luminosity frontier”.

The Belle II detector at SuperKEKB was designed and built by an international collaboration of more than 600 physicists from 23 countries. This collaboration is working closely with SuperKEKB accelerator experts to optimise the machine performance and backgrounds.

At the same time as first turns were achieved, the BEAST in its cave at Tsukuba Hall awakened from its slumber. The BEAST II detector is a system of detectors designed to measure the beam backgrounds of the SuperKEKB accelerator. The parasitic radiation produced by electromagnetic showers when the beam collides with the walls of the vacuum pipe not only obscure the signals that we wish to observe, but can also damage the detector. Therefore, when operating the new accelerator, these beam backgrounds must be well understood.

The BEAST II detector will collect data in the unique environment produced by SuperKEKB’s first beams, allowing Belle II to safely roll into the beam in 2017.

TPS exceeds design goal of 500 mA stored current

In December last year, the 3 GeV Taiwan Photon Source (TPS) of the National Synchrotron Radiation Research Center (NSRRC) stored 520 mA of electron current in its storage ring, and gave the world a bright synchrotron light as the International Year of Light 2015 came to an end. This is the second phase of commissioning conducted after the five-month preparation work set to bring the electron current of TPS to its design value of 500 mA (CERN Courier June 2010 p16 and April 2015 p22).

After the first light of TPS shone on 31 December 2014, the beam injection stored an electron current greater than 100 mA with the efficiency of the booster to storage ring exceeding 75% using Petra cavities. To overcome the instability of the electron beam, high chromaticity and a vertical feedback system were applied to damp the vertical instability at a high current, in this case close to 100 mA, whereas the longitudinal instability appeared when the beam current reached around 85 mA. Subsequently, the dynamic pressure of the vacuum conditioning reached 10^–7 Pa at 100 mA after feeding 35 amps-per-hour beam dose. At this stage, the TPS was ready for the upgrade implementation scheduled for the remainder of 2015.

Several new components were installed during this phase, including new undulators and superconducting cavities, while the cryogenic and control systems were completed.

The upgrade activities also involved the injection system and the transfer line between booster and storage, to improve the injection efficiency and the stability of the system. In addition, 96 fast-feedback corrector magnets were placed at both ends of the straight sections, as well as upstream of the dipole magnets.

After several test runs in the fourth quarter of 2015, an unusual and unfamiliar phenomenon began to emerge, preventing the electron current from progressing beyond 230 mA. The pressure of the vacuum chamber located in the first dipole of the second arc section in the storage ring repeatedly surged to more than 300 times the normal value of 10 × 10^–9 Pa when the beam current increased to 190 mA. A small metal–plastic pellet that contaminated the vacuum environment was removed and the staff performed flange welding on the spot.

After the vacuum problem had been solved, commissioning of TPS went smoothly, ramping from 0 to 520 mA in 11 minutes on 12 December.

While the TPS was ramping up to its stored-current target value, two beamlines – the protein microcrystallography beamline (TPS-05) and the temporally coherent X-ray diffraction beamline (TPS-09) – were in the commissioning phase. The TPS beamlines will be open for use in 2016.

DZero discovers new four-flavour particle

The m(B⁰_sπ^±) distribution together with the background distribution and the fit results after applying the cone cut.
Image credit: DZero Collaboration.

Scientists from the DZero collaboration at the US Department of Energy’s Fermilab have discovered a new particle – the latest member to be added to the exotic species of particles known as tetraquarks.

In 2003, scientists from the Belle experiment in Japan reported the first evidence of quarks hanging out as a foursome, forming a tetraquark. Since then, physicists have glimpsed a handful of different tetraquark candidates, including now the recent discovery by DZero – the first observed to contain four different quark flavours.

DZero scientists first saw hints of the new particle, called X(5568), in July 2015. After performing multiple cross-checks, the collaboration confirmed that the signal could not be explained by backgrounds or known processes, but was evidence of a new particle.

And the X(5568) is not just any new tetraquark. While all other observed tetraquarks contain at least two of the same flavour, X(5568) has four different flavours: up, down, strange and bottom.

Four-quark states are rare, and although there is nothing in nature that forbids the formation of a tetraquark, scientists do not understand them nearly as well as they do two- and three-quark states. This latest discovery comes on the heels of the first observation of a pentaquark – a five-quark particle – announced last year by the LHCb experiment at the LHC.

The next step will be for DZero scientists to understand how the four quarks are put together. Indeed, the quarks could be scrunched together in a tight ball, or they might be a pair of tightly bound quarks revolving at some distance from the other pair. Scientists will sharpen the picture of the quark quartet by making measurements of properties such as the way that X(5568) decays or how much it spins on its axis. As with previous investigations of the tetraquarks, studies of the X(5568) will provide another window into the workings of the strong force that holds these particles together.

Seventy-five institutions from 18 countries collaborated on this result from DZero.

Testing of DUNE tech begins

The DUNE 35 tonne prototype, a test version of the future DUNE detector.
Image credit: Reidar Hahn.

The planned Deep Underground Neutrino Experiment (DUNE) (CERN Courier December 2015 p19) will require 70,000 tonnes of liquid argon, making it the largest experiment of its kind – 100 times larger than the liquid-argon particle detectors that came before. Scientists recently began taking data using a 35 tonne test version of their detector – a significant step towards building the four massive detectors at the Sanford Underground Research Facility (SURF), which will hold the 70,000 tonnes of liquid argon.

Built at the Department of Energy’s Fermi National Accelerator Laboratory, the 35 tonne prototype allows researchers to check that the various detector elements are working properly and to start formal studies. Scientists also use the prototype to assess detector components that have not been tried before. The new parts include redesigned photodetectors – long rectangular prisms with a special coating that changes invisible light to a visible wavelength and bounces the collected light to the detector’s electronic components.

DUNE scientists are also paying special attention to the prototype’s wire planes – pieces that hold the thin wires strung across the detector to pick up electrons. To ensure the frames will fit down the narrow mineshaft at SURF and avoid having to stretch the wires across the long DUNE detectors, risking sagging, scientists plan to use a series of independent 6 m-long and 2.3 m-wide frames. These wire planes should measure tracks in the liquid argon, both in front of and behind them, unlike other detectors.

Engineers have also moved some of the detector’s electronic parts inside the cryostat, which holds liquid argon at –184 °C.

Much like the full detectors, development of the components of the 35 tonne prototype depends on teamwork. For the prototype, Brookhaven and SLAC national laboratories in the US provided much of the electronic equipment; Indiana University, Colorado State University, Louisiana State University and Massachusetts Institute of Technology worked on the light detectors; and the universities of Oxford, Sussex and Sheffield helped to make special digital cameras that can survive in liquid argon, and wrote the software to make sense of the data. Fermilab was responsible for the cryostat and cryogenic support systems.

Scientists will use what they learn from this small prototype version to build one of the full-scale modules for a larger, 400 tonne prototype currently under construction at the CERN Neutrino Platform. A second 400 tonne module using dual-phase technology will also be built at CERN. These will be the final tests before installation of the four huge detectors at SURF for the actual experiment, which is scheduled to start in 2021/2022.

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