On 4 July, the Republic of Slovenia became an associate member of CERN in the pre-stage to membership. It follows official notification to CERN that Slovenia has completed internal approval procedures, entering into force an agreement signed in December 2016. “It is a great pleasure to welcome Slovenia into our ever-growing CERN family as an associate Member State in the pre-stage to membership,” said CERN Director-General Fabiola Gianotti. “This now moves CERN’s relationship with Slovenia to a higher level.”
Slovenian physicists contributed to CERN’s programme long before Slovenia became an independent state in 1991, participating in an experiment at LEAR (the Low Energy Antiproton Ring) and on the DELPHI experiment at CERN’s previous large accelerator, the Large Electron–Positron collider (LEP). In 1991, CERN and Slovenia concluded a co-operation agreement concerning the further development of scientific and technical co-operation in the research projects of CERN. In 2009, Slovenia applied to become a Member State of CERN. For the past 20 years, Slovenian physicists have participated in the ATLAS experiment at the Large Hadron Collider. Their focus has been on silicon tracking, protection devices and computing at the Slovenian TIER-2 data centre, and on the tracker upgrade, making use of the research reactor in Ljubljana for neutron irradiation studies.
“Sloveniaʼs membership in CERN will on the one hand facilitate, strengthen and broaden the participation and activities of Slovenian scientists (especially in the field of experimental physics), on the other it will bring full access of Slovenian industry to CERN orders, which will help to break through in demanding markets with products with a high degree of embedded knowledge,” said Maja Makovec Brenčič, Slovenian minister of education, science and sport.
Slovenia joins Cyprus and Serbia as an associate Member State in the pre-stage to membership. After a period of five years, the CERN Council will decide on the admission of Slovenia to full membership.
CERN’s long-running radioactive-ion-beam facility ISOLDE, which produces beams for a wide range of scientific communities, has recently been upgraded to allow higher-energy beams.
In July, the second phase of the High-Intensity and Energy upgrade (HIE-ISOLDE) saw its first user experiments get under way using the high-resolution Miniball germanium detector, which is specially designed for studying nuclear reactions with low-intensity radioactive ion beams. One of the first experiments looked at electromagnetic interactions between selenium-70 and a platinum target, which allow researchers to determine the shape of this radioactive nucleus. It was carried out by a team from the University of the Western Cape in South Africa, marking the first African-led experiment to be carried out at CERN.
Although HIE-ISOLDE’s first physics experiments began in late 2016, earlier this year the facility added a further cryomodule that had to be calibrated, aligned and tested. Each cryomodule contains five superconducting radio-frequency cavities to accelerate the beam to higher energies, and the facility is now able to accelerate nuclei up to an average energy of 7.5 MeV per nucleon, compared with 5.5 MeV last year. The higher energy allows physicists to study the properties of heavier isotopes, and in 2018 a fourth cryomodule will be added to the HIE-ISOLDE linac to reach the final design energy of 10 MeV per nucleon.
The HIE-ISOLDE beams will be available until the end of November, with 13 experiments hoping to use the facility during that time – more than double the number that took data last year.
A team in Germany has made the most precise measurement to date of the mass of a single proton, achieving a precision of 32 parts-per-trillion (ppt). The result not only improves on the precision of the accepted CODATA value by a factor of three but also disagrees with its central value at a level of 3.3 standard deviations, potentially shedding light on other mysteries surrounding the proton.
The proton mass is a fundamental parameter in atomic and particle physics, influencing atomic spectra and allowing tests of ultra-precise QED calculations. In particular, a detailed comparison between the masses of the proton and the antiproton offers a stringent test of the fundamental CPT invariance of the Standard Model.
The team at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg and collaborators from RIKEN in Japan used a bespoke electromagnetic Penning trap cooled to 4 K to store individual protons and highly charged carbon ions. By measuring the characteristic cyclotron frequencies of the trapped particles using ultra-sensitive image-current detectors, the mass of the proton in natural units follows directly.
For the new measurement, the team stored one proton and one highly charged carbon ion in separate compartments of the apparatus and then transported them alternately into the central measurement compartment. Purpose-built electronics allowed the proton to be interrogated under identical conditions as the carbon ion, despite its 12-fold lower mass and six-fold smaller charge, and the ratio of the two measured values results directly in the proton mass in atomic units: 1.007276466583±15 (stat)±29 (syst).
The sensitive single-particle detectors were partly developed by the RIKEN group, drawing on experience gained with similar traps for antimatter research at CERN’s Antiproton Decelerator (AD) – specifically the BASE experiment. “The group around Sven Sturm and Klaus Blaum from MPIK Heidelberg, which did the measurement, has great expertise with carbon, whereas the BASE group contributed proton expertise based on 12 years dealing with protons and antiprotons,” explains RIKEN group leader and BASE spokesperson Stefan Ulmer. “We shared knowledge such as know-how on ultra-sensitive proton detectors and the ‘fast-shuttling’ method developed by BASE to perform the proton–antiproton charge-to-mass ratio measurement.”
Interestingly, the new value of the proton mass is significantly smaller than the accepted one and could therefore be linked to well-known discrepancies in the mass of the heaviest hydrogen isotope, tritium. “Our result contributes to solving this puzzle, since it corrects the proton’s mass in the proper direction,” says Blaum. The result also improves the proton–electron mass ratio by a factor two, achieving a relative precision of 43 ppt, where the uncertainty arises nearly equally from the proton and the electron mass.
Although carefully conducted cross-check measurements confirmed a series of previously published values of the proton mass and showed that no unexpected systematic effects were imposed by the new method, such a striking departure from the accepted value will likely challenge other teams to revisit the proton mass. The discrepancy has already inspired the MPIK-RIKEN team to further improve the precision of its measurement, for instance by storing a third ion in the trap and measuring it simultaneously to eliminate uncertainties originating from magnetic-field fluctuations, which are the main source of the systematic error using the new technique.
“It is also planned to tune the magnetic field to even higher homogeneity, which will reduce additional sources of systematic error,” explains BASE member Andreas Mooser. “The methods that will be pioneered in the next step of this experiment will have immediate positive feedback to future BASE measurements, for example to improve the precision in the antiproton-to-proton charge-to-mass ratio.”
The KEDR collaboration has used the VEPP-4M electron–positron collider at the Budker Institute in Russia to make the most precise measurement of the quantity “R” in the low-energy range. R is defined as the ratio of the radiatively corrected total hadronic cross-section in electron–positron annihilation to the Born cross-section of muon pair production. The dependence of R on the centre-of-mass energy is critical for determining the running strong coupling constant and heavy-quark masses, the anomalous magnetic moment of the muon and the value of the electromagnetic fine structure constant at the Z peak. A substantial contribution to the uncertainties on these quantities comes from the energy region below charm threshold, where KEDR measurements were made.
The KEDR team performed a precise measurement of R at 20 points: in the energy ranges 1.84–3.05 and 3.12–3.72 GeV the weighted averages of R are 2.225±0.051 and 2.189±0.047, respectively, in good agreement with perturbative QCD. At present, it is the most accurate measurement of R in this energy range, to which more than 10 experiments have contributed. It involved a challenging analysis in which the hadronisation of light quarks at low energies was modelled by tuning distributions of parameters essential for the event selection in the various generator codes.
The collaboration now plans to measure R in the range 5–7 GeV, where the last similar experiment was carried out more than a quarter of a century ago.
On 14 July, the Square Kilometre Array (SKA) organisation signed an agreement with CERN to formalize their collaboration in the area of extreme-scale computing. The agreement will address the challenges of “exascale” computing and data storage, with the SKA and the Large Hadron Collider (LHC) to generate an overwhelming volume of data in the coming years.
When completed, SKA will be the world’s largest radio telescope with a total collecting area of more than 1 km2 using thousands of high-frequency dishes and many more low- and mid-frequency aperture array telescopes distributed across Africa, Australia and the UK. Phase 1 of the project, representing approximately 10% of the final array, will generate around 300 PB of data every year – 50% more than has been collected by the LHC experiments in the last seven years. As is the case at CERN, SKA data will be analysed by scientific collaborations distributed across the planet. The acquisition, storage, management, distribution and analysis of such volumes of scientific data is a major technological challenge.
“Both CERN and SKA are and will be pushing the limits of what is possible technologically, and by working together and with industry, we are ensuring that we are ready to make the most of this upcoming data and computing surge,”says SKA director-general Philip Diamond.
CERN and SKA have agreed to hold regular meetings to discuss the strategic direction of their collaborations, and develop demonstrator projects or prototypes to investigate concepts for managing and analysing exascale data sets in a globally distributed environment. “The LHC computing demands are tackled by the Worldwide LHC computing grid, which employs more than half a million computing cores around the globe interconnected by a powerful network,” says CERN’s director of research and computing Eckhard Elsen. “As our demands increase with the planned intensity upgrade of the LHC, we want to expand this concept by using common ideas and infrastructure into a scientific cloud. SKA will be an ideal partner in this endeavour.”
On 19 June, the Nuclear Physics European Collaboration Committee (NuPECC) released its long-range plan for nuclear research in Europe, following 20 months of work involving extensive discussions with the scientific community. The previous long-range plan was issued in 2010.
Today, nuclear physics is a broad field covering nuclear matter in all its forms and exploring their possible applications. It encompasses the origin and evolution of the universe, such as the quark deconfinement at the Big Bang, the physics of neutron stars and nucleosynthesis, and addresses open questions in nuclear structure, among other topics.
A number of research programmes are at the interface between nuclear and particle physics, where CERN plays an important role. This can be seen clearly in the six chapters of the NuPECC report devoted to: hadron physics; properties of strongly interacting matter; nuclear structure and dynamics; nuclear astrophysics; symmetries and fundamental interactions; and applications. For symmetries and fundamental interactions, particular emphasis is given to experiments (such as those with antihydrogen) where nuclei are sensitive to physics beyond the Standard Model. Concerning broader societal benefits, CERN’s MEDICIS facility, based on expertise from ISOLDE, is of particular interest (CERN Courier October 2016 p28).
The Facility for Antiproton and Ion Research (FAIR), a major investment that just entered construction in Germany (CERN Courier July/August 2017 p41), also has high prominence. Three other prominent recommendations have particular relevance to CERN: support for world-leading isotope-separation facilities (ISOLDE at CERN together with SPIRAL2 in France and SPES in Italy); support for existing and emerging facilities (including the new ELENA synchrotron at CERN’s Antiproton Decelerator); and support for the LHC’s heavy-ion programme, in particular ALICE. Emerging facilities – the extreme-light source ELI-NP in Bucharest, and NICA and a superheavy element factory in Dubna – are also highlighted.
Research in nuclear physics involves several facilities of different sizes that produce complementary scientific results, and in Europe they are well co-ordinated. International collaborations beyond Europe, mainly in the US (JLAB in particular) and in Asia (Japan in particular), add much value to this field.
NuPECC’s latest report is expected to help co-ordinate and guide this rich field of physics for the next 6–7 years. Its recommendations were extensively discussed and can be read in full at nupecc.org/pub/lrp2017.pdf.
Five years ago, the ATLAS and CMS collaborations at the LHC announced the discovery of a new particle with properties consistent with those of a Standard Model Higgs boson. Since then, based on proton–proton collision data collected at energies of 7 and 8 TeV during LHC Run 1 and at 13 TeV during Run 2, many measurements have confirmed this hypothesis. Several decay modes of the Higgs boson have been observed, but the dominant decay into pairs of b quarks, which is expected to contribute at a level of 58%, had up to now escaped detection – largely due to the difficulty in observing this decay mode at a hadron collider.
On 6 July, at the European Physical Society conference in Venice, the ATLAS collaboration announced that they had found evidence for H → bb, representing an immense analysis achievement. By far the largest source of Higgs bosons is their production via gluon fusion, gg → H → bb, but this is overwhelmed by the huge background of bb events, which are produced at a rate 10 million times higher. The associated production of a Higgs with a W or Z vector boson (jointly denoted V) offers the most sensitive alternative, despite having a production rate roughly 20 times lower than H→ bb, because the vector bosons are detected via their decay to leptons and therefore allow efficient triggering and background rejection. Nevertheless, the signal remains orders of magnitude smaller than the backgrounds, which arise from the associated production of vector bosons with jets and from top-quark production.
To find evidence for the H → bb decay in the VH production channel, it is necessary to use detailed information on the properties of the decay products. The jets arising from b quarks contain b hadrons, whose long lifetime can be used in sophisticated b-tagging algorithms to discriminate them from jets originating from the fragmentation of gluons or other quark species. These algorithms have benefitted significantly from the new innermost pixel layer installed in ATLAS before Run 2. The kinematic properties of the decay products can also be used to enhance the signal-over-background ratio. The property with the most discriminatory power is the invariant mass of the two-b-jet system, which for the signal accumulates at the mass of the Higgs boson (see figure). To increase the sensitivity of the analysis, this mass is used together with several other kinematic variables as input to a multivariate analysis.
Based on data collected during the first two years of LHC Run 2 in 2015 and 2016, evidence for the H → bb decay is obtained at the level of 3.5σ, slightly increased to 3.6σ after combination with the Run 1 results (compared to an expected significance of 4σ). The measured signal yield is in agreement with the Standard Model expectation, within an uncertainty of 30%. The associated VZ production, with Z → bb, allows for a powerful cross-check of the analysis, as the final states are very similar except for the location of the two-b-jet mass peak (see figure); VZ production is observed with a significance of 5.8σ in the Run 2 data, in agreement with the Standard Model prediction.
This analysis opens a way to study about 90% of the Higgs boson decays expected in the Standard Model, which is a sharp increase from the approximately 30% observed previously. With much more data expected by the end of Run 2 in 2018, a definitive 5σ observation of the H → bb decay may be in sight, with the increased precision providing new opportunities to challenge the Standard Model.
The quest to find dark matter (DM) has inspired new searches at CMS, specifically looking for interactions between DM and quarks mediated by particles of previously unexplored mass and width. If the DM mediator is a leptophobic vector resonance coupling only to quarks with a universal coupling g′q, for instance, its decay also produces a dijet resonance (see bottom figure, left) and the value of g′q determines the width of the mediator.
CMS has traditionally searched for peaks from narrow resonances on the steeply falling dijet invariant mass spectrum predicted by QCD. This search has been updated with the full 2016 data set and limits set on a DM mediator, constraining g′q for resonances with a mass between 0.6 and 3.7 TeV and width less than 10% of the resonance mass. Two additional dijet searches have now been released: a boosted-dijet search sensitive to lower mediator masses, and an angular-distribution search sensitive to larger couplings and widths.
The first search gets round the limitations of the narrow-resonance search, which only applies above a minimum mass that satisfies the dijet trigger requirements, by requiring resonance production in association with a jet (bottom figure, middle). In such events the resonance is highly boosted and by analysing the jet substructure the QCD background can be highly suppressed, making the search sensitive in a lower mass range. The mass spectrum of the single jet was used to search for resonances over a mass range of 50–300 GeV, and the corresponding constraints on g′q and the mediator width from boosted dijets explore the lowest mediator masses.
For large couplings and widths, the sensitivity of searches for dijet resonance peaks is strongly reduced. However, a search for a very wide resonance can be performed by studying dijet angular distributions such as the scattering angle between the incoming and outgoing partons. These distributions differ significantly, depending on whether a new particle is produced in the s-channel or from the QCD dijet background, which is dominated by t-channel production (bottom figure, right). Being sensitive to both large-width resonances and non-resonant signatures, this search also sets lower limits on the scale of contact interactions that may arise from quark compositeness in the range 6–22 TeV, as well as signatures of large extra dimensions and quantum black holes. The same search, when interpreted in the context of a vector mediator coupling to DM, excludes values of g′q greater than 0.6, corresponding to widths higher than 20% of the resonance mass, and extending to mediator masses as high as 5 TeV.
Using these three complementary techniques, CMS has now explored a large range in mass, coupling and width, extending the scope of searches for DM mediators. The expected volume of data from the LHC in upcoming years will allow CMS to extend this reach even further, with the study of three-jet topologies allowing the uncovered mass range of 300–600 GeV to be explored.
In 2016 the LHC collided protons and lead nuclei for the first time at a centre-of-mass energy of 8.16 TeV per nucleon–nucleon pair. In lead–lead collisions, the formation of the quark–gluon plasma (QGP), a deconfined system where quarks and gluons can move freely, is a subject of intense studies at the LHC. By contrast, proton–lead collisions represent the best available environment to quantify nuclear effects that are not related to the QGP.
Our knowledge of the partonic content of nuclei suffers from large uncertainties, particularly at low momentum where large modifications of the partonic flux with respect to the free nucleon are expected. The particular design of the LHCb experiment, with its fully instrumented forward acceptance, offers a unique opportunity to access production processes in which one parton carries a momentum fraction of the incoming nucleon inside the lead nucleus of approximately 10–5–10–4 (covering the proton fragmentation region) and 10–3–10–1 for the lead fragmentation region.
The LHCb collaboration recently submitted the first paper at the LHC based on results obtained with the 2016 proton–lead data sample. This measurement of J/ψ production profits from an integrated luminosity about 20 times larger than the proton–lead sample collected by LHCb during the 2013 run. The nuclear modification factor RpPb as a function of transverse momentum is shown in the figure: J/ψ mesons produced in the interaction point (prompt) are found to be suppressed by about a factor two at low transverse momentum, while RpPb approaches unity at higher transverse momenta. Those arising from the decays of long-lived beauty hadrons (non-prompt) follow a similar pattern. This is the most precise measurement to date of inclusive beauty production in nuclear collisions.
The results can be compared with perturbative QCD calculations based on collinear nuclear parton distribution functions (nPDFs) or with calculations within the colour-glass condensate (CGC) framework, which takes into account gluon saturation. The large uncertainties on the nPDFs compared to the data show the importance of new experimental data to better constrain them, while the CGC-based calculation reproduces the observed dependence accurately.
The large 2016 data set will allow for a precise study of heavy-flavour production with different hadron species, and also of cleaner electromagnetic/electroweak probes. These measurements will test which frameworks adequately describe the modification of the partonic flux in nuclear collisions. Additionally, other mechanisms such as partonic energy loss due to gluon radiation, which is very relevant for nuclear modifications.
Quarkonium states, such as the J/ψ meson, are prominent probes of the quark–gluon plasma (QGP) formed in high-energy nucleus–nucleus (AA) collisions. That bulk J/ψ production is suppressed in AA collisions with respect to proton–proton collisions had been reported by ALICE five years ago. However, measurements of J/ψ production in proton–lead collisions, where the formation of the QGP is not expected, are essential to quantify effects that are present in AA collisions but not associated with the QGP. In a recent study, ALICE has shown that the production of J/ψ mesons in proton–lead collisions is strongly correlated with the total number of produced particles in the event (event multiplicity), and that this correlation varies as a function of rapidity.
In ALICE, the J/ψ measurements are performed at forward (proton direction), mid- and at backward-rapidity (lead direction). An increase of the J/ψ yield relative to the event-averaged value with the relative charged-particle multiplicity is observed for all rapidity domains, with a similar slope at low multiplicities (see figure). At multiplicities a factor two above the event average, the trend at forward rapidity is very different from those at mid- and backward-rapidity. In the forward rapidity window, a saturation of the relative yield sets in at high multiplicities, which is interesting because the forward region with low parton fractional momentum is in the domain of gluon shadowing/saturation.
Models incorporating nuclear parton distribution functions with significant shadowing have previously been shown to describe J/ψ measurements performed in event classes selected according to the centrality of the collision. The present measurement, exploring significantly more “violent” events (below 1% of the total hadronic interaction cross-section), suggests that effective gluon depletion in the colliding lead nucleus is larger in high-multiplicity events. However, there are additional concepts to describe this regime of QCD, and it remains to be seen whether such models can also describe the saturation of the yields at forward rapidities.
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