The 17th edition of the International Conference on Strangeness in Quark Matter (SQM 2017) was held from 10 to 15 July at Utrecht University in the Netherlands. The SQM series focuses on new experimental and theoretical developments on the role of strangeness and heavy-flavour production in heavy-ion collisions, and in astrophysical phenomena related to strangeness. This year’s SQM event attracted more than 210 participants from 25 countries, with 20% of attendees made up of female researchers. A two-day-long graduate school on the role of strangeness in heavy-ion collisions with 40 participants preceded the conference.
The scientific programme consisted of 53 invited plenary talks, 70 contributed parallel talks and a poster session. Three discussion sessions provided scope for the necessary debates on crucial observables to characterise strongly interacting matter at extreme conditions of high baryon density and high temperature and to define future possible directions. One of the discussions centred on the production of hadron resonances and their vital interactions in the partonic and hadronic phase, which provide evidence for an extended hadronic lifetime even in small collision systems and might affect other QGP observables. Moreover, future astrophysical consequences for SQM following the recent detection of gravitational waves were outlined: gravitational waves from relativistic neutron-star collisions can serve as cosmic messengers for the phase structure and equation-of-state of dense and strange matter, quite similar to the environment created in relativistic heavy-ion collisions.
Representatives from all major collaborations at CERN’s Large Hadron Collider and Super Proton Synchrotron, Brookhaven’s Relativistic Heavy Ion Collider (RHIC), and the Heavy Ion Synchrotron SIS at the GSI Helmholtz Centre in Germany made special efforts to release new data at this conference. Thanks to the excellent performance of these accelerator facilities, a wealth of new data on the production of strangeness and heavy quarks in nuclear collisions have become available.
Following a successful debut in 2016 that witnessed 4000 people happily packing themselves into a tent over three days in Charlton Park in the UK, CERN returned to the WOMAD festival this year with the Physics Pavilion. Featuring talks on theremins and electric guitars, the physics of the NA62 experiment, and the origins of creativity, the Pavilion was once again packed during a year with record attendance at the festival. Those who craved something more hands-on were not disappointed: The Lab, one of two new additions requested by the festival’s management team, allowed participants to build their own cloud chamber, particle collision event, and more. CERN virtual-reality headsets created a queue at a third location called Outside at The Lab, where people eagerly explored the CMS experiment, and Devoxx4Kids allowed children to understand physics by modelling catapults in Minecraft code – perhaps inspiring the next generation of physics model-builders.
The team behind the Physics Pavilion includes physicists from CERN, the UK Science and Technology Facilities Council, the Institute of Physics, and Lancaster University. The enthusiasm for science was clear to see, with proud parents asking for careers advice for their young Einsteins and Curies. “Can we visit?”, “What if there are things smaller than quarks?”, “What are you looking for now?” and “What can we use the Higgs boson for?” were just some of the tough questions asked by hungry minds at a festival that celebrates diversity and culture. With events like this, CERN and its partners are reaching out to a wide range of people, sharing knowledge and excitement, and showing that science is part of our common cultural heritage.
The Deep Underground Neutrino Experiment (DUNE) in the US, the cavern for which entered construction this summer, will make precision studies of neutrinos produced 1300 km away at Fermilab as part of the international Long-Baseline Neutrino Facility. The DUNE far detector will be the largest liquid-argon (LAr) neutrino detector ever built, comprising four cryostats holding 68,000 tonnes of liquid, and prototype detectors called protoDUNE are being built at CERN.
Each protoDUNE detector comprises a 10 × 10 × 10 m LAr time projection chamber with a single-phase (SP) or dual-phase (DP) configuration, containing about 800 tonnes of LAr. While the two big cryostats housing the detectors are about to be completed, the construction of the protoDUNE detectors themselves has just started. The first of six anode-plane-assembly modules for the protoDUNE-SP detector, which will detect electrons produced by ionising particles passing through the detector (pictured) recently arrived at CERN. The module will be tested, together with its electronics, and then installed in its final position inside the cryostat.
In parallel with the anode-plane-assembly, other parts of the protoDUNE-SP detector are being assembled at CERN, including the field cage, which keeps the electric field uniform inside the volume of the detector. Around a quarter of the 28 field-cage modules have already been assembled and are stored in CERN’s EHN1 hall, ready to be installed. The assembly and installation of the detector parts is expected to be completed by spring next year, in order for protoDUNE-SP to take data in autumn 2018.
The protoDUNE detectors are among several major activities taking place at the CERN neutrino platform, which was initiated in 2013 to develop detector technology for neutrino experiments in the US and Japan.
The origin of solar flares, powerful bursts of radiation appearing as sudden flashes of light, has puzzled astrophysicists for more than a century. The temperature of the Sun’s corona, measuring several hundred times hotter than its surface, is also a long-standing enigma.
A new study suggests that the solution to these solar mysteries is linked to a local action of dark matter (DM). If true, it would challenge the traditional picture of DM as being made of weakly interacting massive particles (WIMPs) or axions, and suggest that DM is not uniformly distributed in space, as is traditionally thought.
The study is not based on new experimental data. Rather, lead author Sergio Bertolucci, a former CERN research director, and collaborators base their conclusions on freely available data recorded over a period of decades by geosynchronous satellites. The paper presents a statistical analysis of the occurrences of around 6500 solar flares in the period 1976–2015 and of the continuous solar emission in the extreme ultraviolet (EUV) in the period 1999–2015. The temporal distribution of these phenomena, finds the team, is correlated with the positions of the Earth and two of its neighbouring planets: Mercury and Venus. Statistically significant (above 5σ) excesses of the number of flares with respect to randomly distributed occurrences are observed when one or more of the three planets find themselves in a slice of the ecliptic plane with heliocentric longitudes of 230°–300°. Similar excesses are observed in the same range of longitudes when the solar irradiance in the EUV region is plotted as a function of the positions of the planets.
If true, our findings will provide a totally different view about dark matter
Konstantin Zioutas
These results suggest that active-Sun phenomena are not randomly distributed, but instead are modulated by the positions of the Earth, Venus and Mercury. One possible explanation, says the team, is the existence of a stream of massive DM particles with a preferred direction, coplanar to the ecliptic plane, that is gravitationally focused by the planets towards the Sun when one or more of the planets enter the stream. Such particles would need to have a wide velocity spectrum centred around 300 km s–1 and interact with ordinary matter much more strongly than typical DM candidates such as WIMPs. The non-relativistic velocities of such DM candidates make planetary gravitational lensing more efficient and can enhance the flux of the particles by up to a factor of 106, according to the team.
Co-author Konstantin Zioutas, spokesperson for the CAST experiment at CERN, accepts that this interpretation of the solar and planetary data is speculative – particularly regarding the mechanism by which a temporarily increased influx of DM actually triggers solar activity. However, he says, the long persisting failure to detect the ubiquitous DM might be due to the widely assumed small cross-section of its constituents with ordinary matter, or to erroneous DM modelling. “Hence, the so-far-adopted direct-detection concepts can lead us towards a dead end, and we might find that we have overlooked a continuous communication between the dark and the visible sector.”
Models of massive DM streaming particles that interact strongly with normal matter are few and far between, although the authors suggest that “antiquark nuggets” are best suited to explain their results. “In a few words, there is a large ‘hidden’ energy in the form of the nuggets,” says Ariel Zhitnitsky, who first proposed the quark-nugget dark-matter model in 2003. “In my model, this energy can be precisely released in the form of the EUV radiation when the anti-nuggets enter the solar corona and get easily annihilated by the light elements present in such a highly ionised environment.”
The study calls for further investigation, says researchers. “It seems that the statistical analysis of the paper is accurate and the obtained results are rather intriguing,” says Rita Bernabei, spokesperson of the DAMA experiment, which for the first time in 1998 claimed to have detected dark matter in the form of WIMPs on the basis of an observed seasonal modulation of a signal in their scintillation detector. “However, the paper appears to be mostly hypothetical in terms of this new type of dark matter.”
The team now plans to produce a full simulation of planetary lensing taking into account the simultaneous effect of all the planets in the solar system, and to extend the analysis to include sunspots, nano-flares and other solar observables. CAST, the axion solar telescope at CERN, will also dedicate a special data-taking period to the search for streaming DM axions.
“If true, our findings will provide a totally different view about dark matter, with far-reaching implications in particle and astroparticle physics,” says Zioutas. “Perhaps the demystification of the Sun could lead to a dark-matter solution also.”
The COHERENT collaboration at Oak Ridge National Laboratory (ORNL) in the US has detected coherent elastic scattering of neutrinos off nuclei for the first time. The ability to harness this process, predicted 43 years ago, offers new ways to study neutrino properties and could drastically reduce the scale of neutrino detectors.
Neutrinos famously interact very weakly, requiring very large volumes of active material to detect their presence. Typically, neutrinos interact with individual protons or neutrons inside a nucleus, but coherent elastic neutrino–nucleus scattering (CEνNS) occurs when a neutrino interacts with an entire nucleus. For this to occur, the momentum exchanged must remain significantly small compared to the nuclear size. This restricts the process to neutrino energies below a few tens of MeV, in contrast to the charged-current interactions by which neutrinos are usually detected. The signature of CEνNS is a low-energy nuclear recoil with all nucleon wavefunctions remaining in phase, but until now the difficulty in detecting these low-energy nuclear recoils has prevented observations of CEνNS – despite the predicted cross-section for this process being the largest of all low-energy neutrino couplings.
The COHERENT team, comprising 80 researchers from 19 institutions, used ORNL’s Spallation Neutron Source (SNS), which generates the most intense pulsed neutron beams in the world while simultaneously creating a significant yield of low-energy neutrinos. Approximately 5 × 1020 protons are delivered per day, each returning roughly 0.08 isotropically emitted neutrinos per flavour. The researchers placed a detector, a caesium-iodide scintillator crystal doped with sodium, 20 m from the neutrino source with shielding to reduce background events associated with the neutron-induced nuclear recoils produced from the SNS. The results favour the presence of CEνNS over its absence at the 6.7σ level, with 134±22 events observed versus 173±48 predicted.
Crucially, the result was achieved using the world’s smallest neutrino detector, with a mass of 14.5 kg. This is a consequence of the large nuclear mass of caesium and iodine, which results in a large CEνNS cross-section.
The intense scintillation of this material for low-energy nuclear recoils, combined with the large neutrino flux of the SNS, also contributed to the success of the measurement. In effect, CEνNS allows the same detection rates as conventional neutrino detectors that are 100 times more massive.
“It is a nearly ideal detector choice for coherent neutrino scattering,” says lead designer Juan Collar of the University of Chicago. “However, other new coherent neutrino-detector designs are appearing over the horizon that look extraordinarily promising in order to further reduce detector mass, truly realising technological applications such as reactor monitoring.”
Yoshi Uchida of Imperial College London, who was not involved in the study, says that detecting neutrinos via the neutral-current process as opposed to the usual charged-current process is a great advantage because it is “blind” to the type of neutrino being produced and is sensitive at low energies. “So in combination with other types of detection, it could tell us a lot about a particular neutrino source of interest.” However, he adds that the SNS set-up is very specific and that, outside such ideal conditions, it might be difficult to scale a similar detector in a way that would be of practical use. “The fact that the COHERENT collaboration already has several other target nuclei (and detection methods) being used in their set-up means there will be more to come on this subject in the near future.”
The Particle Zoo, a California-based company that produces soft-toy versions of elementary particles and other outreach materials, is searching for a buyer to take the reins.
Julie Peasley, a graphic designer with a keen interest in physics, started the company in 2008 after having attended a public lecture about cosmology at the University of California at Los Angeles and a craft fair in the same weekend. Setting up an online store, she began selling her particles, slowly expanding the range after requests from physicists. Beginning with little sewing experience, Peasley can now produce around four particle toys per hour and has created more than 50,000 units to date.
Each of the Zoo’s 36 felt particles, plus a range of related products from stickers to pillows, is designed to reflect the properties of its real counterpart: the W boson is double-sided to represent its positive and negative aspects; neutrinos are masked to reflect their elusive nature; and each toy is stuffed with a different material to reflect the mass hierarchy of the real particles.
Looking for a new career path, Peasley says the business has tremendous potential for growth with the right owner and a little capital. “To this day, there are no competitors – I am still the only person selling plush particles! I just never had the capital to take it to the next level,” she told CERN Courier.
Peasley piloted the mass-production of her two most popular particles – the Higgs boson and the electron – in China for three years. However, she struggled to secure the investment required to continue with the mass-production of all the particles. “Iʼd really love to see the business continue rather than die because it brings smiles to anyone who sees it.” Interested potential particle zookeepers can get in touch at particlezoo.net.
Launched in 2014, the Distributed Electronic Cosmic-ray Observatory (DECO) enables Android smartphone cameras to detect cosmic rays. In response to the increasing number of events being recorded, however, the DECO team has developed a new machine-learning analysis that classifies 96% of events correctly.
Similar to detectors used in high-energy physics experiments, the semiconductors in smartphone camera sensors detect ionising radiation when charged particles traverse the depleted region of their sensor. The DECO app can spot three distinct types of charged-particle events: tracks, worms and spots (see image). Each event can be caused by a variety of particle interactions, from cosmic rays to alpha particles, and a handful of events can be expected every 24 hours or so.
These events have so far been classified by the users themselves, but the increasing number of images being collected meant there was a need for a more reliable computerised classification system. Due to the technological variations of smartphones and the orientation of the sensor when a cosmic ray strikes, traditional algorithms would have struggled to classify events.
The DECO team used advances in machine learning similar to those widely used in high-energy physics to design several deep neural-network architectures to classify the images. The best performing design, which contained over 14 million learnable parameters and was trained with 3.6 million images, correctly sorted 96% of 100 independent images. An iOS version of DECO is currently in the beta stage and is expected to be released within the next year.
In addition to the direct production of new particles in high-energy collisions, evidence for new physics beyond the Standard Model (SM) could arise through precision measurements. Two particularly important parameters in this regard are the mass of the W boson, Mw, and the electroweak mixing angle, sin2θW. The CMS collaboration has recently reported a precise measurement of the effective electroweak mixing angle based on Drell–Yan production of leptons.
The electroweak mixing angle is a key parameter defining how the SM unifies the electromagnetic and weak forces, and at first order it is related to the W and Z bosons masses by the simple expression: sin2θW = 1 − M2w / M2Z. An uncertainty in sin2θW of ±0.0005 is equivalent to an indirect measurement of Mw to a precision of 25 MeV, which corresponds to the precision of the direct measurements at hadron colliders.
The most precise measurements of sin2θW were performed at the LEP and SLD electron–positron collider at CERN and SLAC. While the uncertainties are small (0.00026 and 0.00029), the central values differ by more than three standard deviations, motivating further precise measurements. CMS aims to measure sin2θW with a precision matching LEP and SLD based on the forward–backward asymmetry of lepton pairs with an invariant mass near MZ. The asymmetry of the production of a negative lepton with respect to the direction of the quark is small in this region, as a result of the axial-vector Z boson self-interference, and is sensitive to the effective mixing angle, sin2θlepteff. In contrast, the asymmetry at higher and lower mass is much larger, originating from the interference of the weak and electromagnetic interactions, and this asymmetry is not sensitive to sin2θlepteff. The leptons are produced via quark–antiquark annihilation in the LHC’s proton–proton collisions, and limited knowledge about the quark and antiquark parton distribution functions (PDFs) lead to systematic uncertainties that currently dominate the measurement uncertainty.
Using a novel technique with lepton pairs at high and low invariant mass to reduce the PDF uncertainties, CMS has analysed the full electron and muon data samples recorded at an energy of 8 TeV and finds that the effective mixing angle, 0.23101±0.00052, is consistent with the SM prediction 0.23152±00005. With analysis of the 13 TeV LHC data, we can expect that the uncertainties will be significantly smaller and test the electroweak sector further.
The LHCb collaboration has observed the rare baryonic decay B0→ pp, as first presented at the European Physical Society conference in Venice in early July. The branching fraction was measured at the level of about 1.3 per 100 million decays, which makes this decay mode the rarest decay of a B0 meson ever observed. It is also the rarest observed hadronic decay of all beauty mesons.
Our knowledge of baryonic B decays has increased considerably in the last few years. The LHCb experiment, which is primarily designed to search for new physics in CP-violation and rare decays of particles with heavy flavour, has been pursuing a programme to study the decays of B mesons to final states containing baryons. Among the recent achievements it is worth emphasising the first observation of a baryonic B0s decay, B0s→ p Λ K–, and that of B0 and B0s decays to pp plus a pair of light charged mesons. The B0s is the last of the B meson species for which a baryonic decay mode had yet to be observed. The large data samples available at LHCb have made it possible to study the two-body baryonic final-state decays, which are suppressed with respect to higher-multiplicity decay modes.
A search for the rare decays B0→ pp and B0s→ pp had previously been performed by LHCb with 2011 data only, obtaining evidence for B0→ pp. The collaboration now used the full 3 fb–1 data sample collected during the first run of the LHC, approximately three times more data than in the previous search, to confirm the evidence for this decay. An excess of B0→ pp candidates with respect to the background-only hypothesis is now observed with a statistical significance of 5.3 standard deviations. The hint of a B0s→ pp signal reported in 2013 is, however, not confirmed, and an upper limit for the corresponding branching fraction has been set.
The measured B0→ pp and B0s→ pp branching fractions are compatible with the latest theoretical calculations. The observation of the latter will allow a quantitative comparison of various QCD-inspired models describing baryonic B decays.
The ALICE collaboration has reported a measurement of the azimuthal anisotropy in the production of D mesons, which contain a charm quark, when measured relative to the estimated reaction plane of lead–lead collisions at 5.02 TeV at LHC Run 2. The new measurement is a factor two more precise than the Run 1 measurement and clearly indicates that charm takes part in the collective flow of the colour-deconfined quark–gluon plasma (QGP).
Since heavy quarks are produced in hard-scattering processes on a timescale shorter than the QGP formation time, they experience all stages of the system’s evolution. Evidence of interactions between charm quarks and the medium is provided by the observed strong modification of the transverse momentum (pT) distribution of D mesons in heavy-ion collisions with respect to pp collisions. This is understood in terms of elastic scatterings and gluon-radiation in the medium. Measurements of anisotropies in the azimuthal distribution of heavy-flavour hadrons provide further information to determine the transport properties of the medium. The QGP expansion converts the initial spatial anisotropy, which originates from the geometry of the collision, into a particle-momentum anisotropy. The elliptic anisotropy is characterised by the second Fourier coefficient (v2) of the particle’s azimuthal angle distribution relative to the estimated reaction plane.
The measurement of v2 for the D meson addresses the issue of whether the flow that is generated by the expansion of the collision system and is known to affect light quarks is also imparted to the much heavier charm quarks. Additionally, hadronisation of charm quarks via recombination with light quarks could contribute to the D-meson flow. This motivates a comparison of the v2 of D mesons with and without strange-quark content.
ALICE measured the v2 of D0, D+, D*+ and, for the first time at the LHC, of D+s mesons in mid-central lead–lead collisions at an energy of 5.02 TeV per nucleon. The average v2 of non-strange D mesons is larger than zero and similar to that of pions, which is compatible with a similar recent measurement by CMS. The D+s v2 was found to be positive and compatible with that of non-strange D mesons (top panel of the figure). The improved precision of the new measurement compared to the Run 1 measurement at an energy of 2.76 TeV per nucleon will significantly improve the determination of the heavy-quark diffusion coefficient in the QGP at LHC, which is one of the fundamental properties of this exotic form of matter.
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