The Large Underground Xenon (LUX) experiment located at Sanford Underground Research Facility (SURF) in South Dakota, US, has released its latest results in the search for dark matter. Following the completion of its final 20 month-long run, during which the detector amassed a data set that is four times larger than before, no signal was found and the results were consistent with background expectations.
LUX is based on a 370 kg liquid-xenon dual-phase time-projection chamber that offers a high sensitivity to spin-independent nuclear-recoil interactions. It entered operation in 2013 to search directly for WIMPs in addition to other dark-matter candidates. The experiment’s latest results, which were presented on 21 July at the 2016 International Dark Matter conference in Sheffield, UK, carve out previously un-probed parameter space and exclude spin-independent WIMPS at the level of 0.22 zeptobarns. The collaboration plans further analyses of other dark-matter candidates, including axions.
Researchers are looking ahead to the next-generation LUX-ZEPLIN (LZ) detector, also located at SURF, which is scheduled to start operations by the end of the decade. With an active mass of seven tonnes, LZ will be sensitive to WIMP masses ranging from a few GeV to hundreds of TeV, and will therefore probe even deeper into dark-matter parameter space.
The LHCb collaboration has reported the observation of three new exotic hadrons and confirmed the existence of a fourth by analysing the full data sample from LHC Run 1. Although the theoretical interpretation of the new states is still under study, the particles each appear to be formed by two quarks and two antiquarks. They also do not seem to contain the lightest up and down quarks, which means they could be more tightly bound than other exotic particles discovered so far.
Until recently, all observed hadrons were formed either by a quark–antiquark pair (mesons) or by three quarks only (baryons). The underlying reason has remained a mystery, but during the last decade several experiments have found evidence for particles formed by more than three quarks. For example, in 2009 the CDF collaboration at Fermilab in the US observed evidence for a tetraquark candidate dubbed X(4140), which was later confirmed by the CMS and D0 collaborations (the latest LHCb analysis yields a clear observation of this state, although finds a slightly larger width than the other experiments). Then, in July 2015, LHCb announced the first observation of two pentaquark particles, which are hadrons composed of five quarks.
Each of the four states observed by LHCb – dubbed X(4274), X(4500) and X(4700), in addition to the X(4140) – has a statistical significance above five standard deviations. Sophisticated analysis of the angular distribution of B+ meson decays into J/ψ, φ and K+ mesons also allowed the collaboration to determine the quantum numbers of the exotic states with high precision. Alas, the data could not be described by a model that contains only ordinary mesons and baryons.
The binding mechanism of the new states could involve tightly bound tetraquarks or strange charmed meson pairs bouncing off each other and rearranging their quark content to emerge as a J/ψφ system. The high statistics of the LHCb data set and the sophisticated techniques exploited in the analysis will help to shed further light on the production mechanisms of these particles.
LHCb has made several other important contributions to the investigation of exotic particles. In February 2013, the quantum numbers of the X(3872) particle discovered in 2003 by the Belle experiment in Japan were determined, and in April 2014 the collaboration showed that the Z(4430) particle (also discovered at Belle) is composed of four quarks: ccdu. The latest exotic results from LHCb, which were first presented in June at the Meson 2016 workshop in Cracow, Poland, have been submitted for publication.
When heavy nuclei collide at high energies, a novel state of hot and dense matter – the quark–gluon plasma (QGP) – is expected to form. As a region of QGP expands and cools, it dissociates into a very large number of particles. Earlier findings at Brookhaven’s RHIC and CERN’s LHC revealed that the QGP exhibits strong collective behaviour comparable to a fluid. One of the key experimental pieces of evidence for this state is the observation of long-range anisotropic azimuthal correlations between particles emitted over a wide rapidity range, often referred to as the “ridge phenomenon”. In a typical proton–proton (pp) collision, a ridge correlation is not expected because the system is too dilute to produce a fluid-like state.
In 2010, however, CMS reported an unexpected observation of a ridge phenomenon in pp events with high particle multiplicities. This finding was regarded as a hint for possible collective effects that might occur in rare pp interactions with similar high particle densities as those in heavy-ion collisions. Due to statistical limitations of the LHC Run 1 data set, detailed studies of the ridge phenomenon in pp collisions, especially concerning its connection to collectivity, remained inconclusive. With the increased collision energy of the LHC Run 2, high-multiplicity pp events are produced at significantly higher rates. This makes it possible to perform a detailed examination of ridge and collective phenomena in pp collisions.
The CMS silicon tracking system is ideally suited for measurements of multi-particle correlations and therefore for studies of the collective nature of the ridge. Recently, the CMS collaboration measured the “elliptic-flow” harmonic v2 in pp events from LHC Run 2 as a function of particle multiplicity (see figure above). This is the first extraction of v2 – an observable that can be used to quantify the ridge signal – in pp collisions using a multi-particle correlation technique.
Strikingly, the observed v2 signals show nearly no dependence on the number of correlated particles, providing direct evidence for a collective origin for the long-range ridge observations. Similar observations were previously made in proton–lead and lead–lead collisions (see figure, middle and right, respectively), providing a consistent picture for the collective nature of the medium in all three collision systems. Furthermore, CMS studied long-range correlations using identified K0s and Λ hadrons, observing a clear dependence of the v2 signal on the particle species in high-multiplicity events. Such “mass ordering” provides further evidence for collective behaviour.
These intriguing observations, revealed through the study of small hadronic systems, therefore open up unexpected territory in the understanding and characterisation of dense quark–gluon matter. With the larger data samples of pp and pPb collisions to be collected in the future, CMS is poised for additional exciting physics discoveries in the coming years.
Precise measurements of final states containing multiple electroweak bosons (W, Z or γ) offer a powerful probe of the gauge structure of the Standard Model (SM), and are therefore a promising avenue to search for new physics. Using proton–proton collision data from the LHC collected at centre-of-mass energies of 7, 8 and 13 TeV, the ATLAS collaboration has recently measured the cross-section for boson-pair final states with unprecedented precision, challenging calculations from quantum chromodynamics (QCD).
For example, the uncertainty on the total production cross-section measurement of the WZ final state at 7 TeV is approximately 10%, which is large enough to cover the differences between the next-leading-order (NLO) and next-to-next-to-leading-order (NNLO) QCD predictions. At 8 and 13 TeV, the uncertainties on the ATLAS measurement reveal a tension with the NLO prediction, although this has been mitigated by recent NNLO QCD calculations.
The large size of the 8 TeV data setallowed ATLAS, for the first time, to become sensitive to tri-boson production and also to boson pairs produced through vector-boson scattering. The production cross-section of such processes in the fiducial volume where the measurement is performed is of the order of 1 fb or less, which is very difficult to measure. ATLAS performed significant cross-section measurements of the Zγγ, Wγγ, and W±W±+2j final states, while limits were set on the production cross-section of the WWW and WZ+2j final states.
These measurements probe the gauge-boson self-couplings, which are sensitive to contributions from new physics – especially at high energies. To parameterise possible anomalous gauge couplings, an effective-field-theory (EFT) approach was used. The ATLAS collaboration has searched for deviations in the coupling of three gauge bosons in the WZ final state using data collected at collision energies of 8 TeV and 13 TeV. By including the 13 TeV data, the previous sensitivity on the EFT coefficients (CWWW/Λ2, CW/Λ2, and CB/Λ2) has improved by 40%. No evidence for new physics has been found and new limits have been derived.
The large data set from LHC Run 2 will provide sensitivity to rare processes that have not been observed so far, and the expected high accuracy on multi-boson cross-sections will allow higher-order QCD and electroweak corrections to be probed.
In a collision between two nuclei that exhibits a large impact parameter, the initial spatial anisotropy of the overlap region was conjectured to be smooth and almond shaped. In the past few years, however, experimental measurements and hydrodynamical calculations have changed this paradigm. We now know that the overlap region has an irregular shape originating from the initial random distribution of the gluons and nucleons in the nuclei, which fluctuates from one event to the next. These fluctuations appear as azimuthal correlations between final-state particles relative to the system’s symmetry plane. These correlations are quantified by a Fourier series of the azimuthal distribution of particle production relative to the system’s symmetry plane.
The second harmonic – v2, which is also known as the elliptic-flow coefficient – has recently been the main focus of the heavy-ion community. Indeed, elliptic-flow measurements and hydrodynamical calculations led to the revelation that the quark–gluon plasma (QGP) generated in heavy-ion collisions behaves as an almost perfect liquid. The ratio of shear viscosity to entropy density (η/s) in the QGP, which is a measure of its fluidity, is very close to the lower bound of ħ/4π kB – as conjectured by the anti-de-Sitter/conformal field theory (AdS/CFT) correspondence.
However, there are also higher-order contributions to the particle correlations and these are more sensitive probes of the QGP state than elliptic flow. Indeed, by carrying out such studies for identified particles rather than unidentified ones, it is also possible to probe the effect of the dissipative, late-stage hadronic re-scattering on the flow coefficients.
Profiting from its excellent particle-identification capabilities, the ALICE collaboration has recently used lead–lead collisions recorded in 2011 at a collision energy of 2.76 TeV to measure the elliptic (v2), triangular (v3), quadrangular (v4) and pentagonal (v5) flow coefficients of charged π and K mesons, protons and antiprotons for different centrality intervals (see figure). For ultra-central collisions, in which the evolution of the system is predominantly driven by the initial-state fluctuations, one observes significant nonzero values for all harmonics and particle species. In addition, v3 and v4 become progressively dominant with increasing transverse momentum, while even v5 for pT > 4 GeV/c is comparable to v2. For mid-central collisions, v2 is the dominant flow harmonic and has a significantly larger value. Higher harmonics also have significant nonzero values but do not seem to change significantly with centrality.
These observations confirm that elliptic flow is driven mainly by the anisotropy in the collision geometry, whereas the initial-state fluctuations are the main driving force behind higher harmonics. A mass ordering expected in hydrodynamical calculations is seen that describes the QGP as a nearly perfect liquid. In addition, in the intermediate-pT region, the flow harmonics of different hadrons show a baryon–meson grouping. The analysis of lead–lead collisions collected in 2015 will allow for a higher-precision measurement of such effects and therefore place more stringent limits on η/s and the initial conditions of a heavy-ion collision.
The European Union project AIDA-2020 (Advanced European Infrastructure for Detectors and Accelerators), in which CERN is a partner, has launched a proof-of-concept fund for breakthrough projects in the field of detector development and testing.
The fund will provide up to €200k in total to support innovative projects with a focus on industry-orientated applications and those beyond high-energy physics. Up to four projects will be funded based on a competitive selection process, and the deadline is 20 October 2016. More information can be found at aida2020.web.cern.ch/content/poc.
Physicists in China have passed an important milestone towards an accelerator-driven sub-critical (ADS) system, a novel accelerator design for advanced energy and other technologies. On 2 July, teams working on “Injector I” at the Chinese Academy of Sciences’ Institute of High Energy Physics (IHEP) in Beijing succeeded in accelerating a proton beam to an energy of 10.11 MeV with a peak beam current of 10.5 mA in pulse mode. “This is a major breakthrough for the ADS Injector I after five years of hard work by scientists from the Institute of High Energy Physics, and marks a new step for high-current proton-linear-accelerator technology worldwide,” explains IHEP deputy-director Weimin Pan.
ADS technology directs high-energy protons towards a heavy target, whereupon spallation reactions produce dozens of neutrons for every proton. A portion of these neutrons may then be used to drive a sub-critical nuclear reactor, with the remaining neutrons used for nuclear-waste transmutation or other applications. Indeed, the past 10 years has seen the development of various combined ADS systems aiming at different applications – including proposals to generate nuclear power from thorium instead of uranium fuel.
The Chinese ADS Injector I is the world’s first proton accelerator to use low-β superconducting “spoke cavities”. It consists of an electron cyclotron resonance (ECR) ion source, a 325 MHz radio-frequency quadrupole accelerator, a superconducting linac containing 14 spoke cavities in two cryomodules, beam transport lines and a beam dump. During beam commissioning, the proton beam reached a final energy of 10.5 MeV with a beam current of 10.11 mA. The cavities achieved an accelerating gradient of 7 MV m–1 and beam transmission through the superconducting linac was 100%.
“This is an important focus of development for ADS accelerators, which lays the foundations for the future Chinese ADS project,” says Pan.
SESAME, the pioneering synchrotron facility for the Middle East and neighbouring countries, located in Jordan, has announced its first call for proposals for experiments. A third-generation light source with a broad research capacity, SESAME’s first beams are due to circulate in the autumn and its experimental programme is scheduled to start in 2017. SESAME is already host to a growing user community of some 300 scientists from across the region and is open to proposals for the best science, wherever they may come from.
SESAME will start up with two beamlines, one delivering infrared light and the other X-rays. The laboratory’s full scientific programme will span fields ranging from medicine and biology, through materials science, physics and chemistry to healthcare, the environment, agriculture and archaeology. Proposals can be submitted through the SESAME website (www.sesame.org.jo) and will be examined by a proposal-review committee.
“This is a very big moment for SESAME,” says SESAME director-general Khaled Toukan. “It signals the start of the research programme at the first international synchrotron research facility in our region.”
The largest 3D map of distant galaxies ever made has allowed one of the most precise measurements yet of dark energy, which is currently driving the accelerating expansion of the universe. The new measurements, which were carried out by the Baryon Oscillation Spectroscopic Survey (BOSS) programme of the Sloan Digital Sky Survey-III, took five years to make and include 1.2 million galaxies over one quarter of the sky – equating to a volume of 650 cubic billion light-years.
BOSS measures the expansion rate by determining the size of baryonic acoustic oscillations, which are remnants of primordial acoustic waves. “We see a dramatic connection between the sound-wave imprints seen in the cosmic microwave background to the clustering of galaxies 7–12 billion years later,” says co-leader of the BOSS galaxy-clustering working group Rita Tojeiro. “The ability to observe a single well-modelled physical effect from recombination until today is a great boon for cosmology.”
The map shows galaxies being pulled towards each other by dark matter, while on much larger scales it reveals the effect of dark energy ripping the universe apart. It also reveals the coherent movement of galaxies toward regions of the universe with more matter, with the observed amount of in-fall explained well by general relativity. The results have been submitted to the Monthly Notices of the Royal Astronomical Society.
The International Union of Pure and Applied Chemistry (IUPAC) has announced the provisional names of four new super-heavy elements that complete the seventh row of the periodic table. The researchers responsible for the discoveries, which were made in Japan, Russia and the US during the past decade, proposed the following names for peer review: nihonium (Nh) for element 113; moscovium (Mc) for 115; tennessine (Ts) for 117; and oganesson (Og) for 118.
Having reviewed the proposals and recommended them for acceptance, the IUPAC Inorganic Chemistry Division set in motion a five-month public review that will come to an end on 8 November, prior to formal approval by the IUPAC Council. Keeping with tradition, newly discovered elements can be named after a mythological concept or character (including an astronomical object); a mineral or similar substance; a place or geographical region; a property of the element; or a scientist.
In conjunction with the International Union of Pure and Applied Physics, IUPAC has also attached priority to the discovery claims. Element 113 was discovered by a collaboration at RIKEN in Japan, while elements 115 and 117 were synthesised at the U-400 accelerator complex at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, via a collaboration with the Lawrence Livermore National Laboratory (LLNL) and Oak Ridge National Laboratory in the US. The discovery of element 118 was attributed to a JINR–LLNL collaboration, which in 2011 was also acknowledged by IUPAC for the discovery of elements 114 (flerovium) and 116 (livermorium).
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