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LHCP reports from Bologna

The LHCP participants in the San Domenico Centre. Credit: R Serra/FotoBouque

Some 450 researchers from around the world headed to historic Bologna, Italy, on 4–9 June to attend the sixth Large Hadron Collider Physics (LHCP) conference. The many talks demonstrated the breadth of the LHC physics programme, as the collider’s experiments dig deep into the high-energy 13 TeV dataset and look ahead to opportunities following the high-luminosity LHC upgrade.

Both ATLAS and CMS have now detected the Higgs boson’s direct Yukawa coupling to the top quark, following earlier analyses, and the results are in agreement with the prediction from the Standard Model (SM). Further results on Higgs interactions included the determination by ATLAS of the boson’s coupling to the tau lepton with high significance, which agrees well with the previous observation by CMS. Measuring the coupling between the Higgs and the other SM particles is a key element of the LHC physics programme, with bottom quarks now in the collaborations’ sights.

The Bologna event also saw news on the spectroscopy front. CMS reported that it has resolved, for the first time, the J = 1 and J = 2 states of the Xi_b(3P) particle, using 13 TeV data corresponding to an integrated luminosity of 80 fb^–1. The measured mass difference between the two states, 10.60 ± 0.64 (stat) ± 0.17 (syst) MeV, is consistent with most theoretical calculations (see CMS resolves inner structure of bottomonium). Meanwhile, the LHCb collaboration reported the measurement of the lifetime of the doubly charmed baryon Xi_cc⁺⁺ discovered by the collaboration last year, obtaining a value of 0.256 ^+0.024 _–0.022 (stat) ± 0.014 (syst) ps, which is within the predicted SM range (see Charmed baryons strike back).

The Cabibbo–Kobayashi–Maskawa (CKM) matrix, which quantifies the couplings between quarks of different flavours and possible charge-parity (CP) violation in the quark system, was another focus of the conference. LHCb presented a new measurement of the gamma angle, which is the least well measured of the three angles defining the CKM unitary triangle and is associated with the up–bottom quark matrix element. The collaboration obtained a value of 74° with an uncertainty of about 5°, making it the most precise measurement of gamma from a single experiment.

Nuclei–nuclei collisions also shone, with the ALICE collaboration showcasing measurements of the charged-particle multiplicity density, nuclear modification factor and anisotropic flow in Xe–Xe collisons at an energy of 5.44 TeV per nucleon (see Anisotropic flow in Xe–Xe collisions). These and other nuclei–nuclei measurements are providing a deeper insight into extreme states of matter such as the quark–gluon plasma.

Searches for physics beyond the SM by the LHC experiments so far continue to come up empty-handed, slicing into the allowed parameter space of many theoretical models such as those involving dark matter. However, as was also emphasised at this years’ LHCP, there are many possible models and the range of parameters they span is large, requiring researchers to deploy “full ingenuity” in searching for new physics.

These are just a few of the many highlights of this year’s LHCP, which also included updates on the experiments’ planned upgrades for the high-luminosity LHC and perspectives on physics opportunities at future colliders.

Neutrino physics shines bright in Heidelberg

The 28th International Conference on Neutrino Physics and Astrophysics took place in Heidelberg, Germany, on 4–9 June. It was organised by the Max Planck Institute for Nuclear Physics and the Karlsruhe Institute of Technology. With 814 registrations, 400 posters and the presence of Nobel laureates, Art McDonald and Takaaki Kajita, it was the most attended of the series to date – showcasing many new results.

Several experiments presented their results for the first time at Neutrino 2018. T2K in Japan and NOvA in the US updated their results, strengthening their indication of leptonic CP violation and normal-neutrino mass ordering, and improving their precision in measuring the atmospheric oscillation parameters. Taken together with the Super-Kamiokande results of atmospheric neutrino oscillations, these experiments provide a 2σ indication of leptonic CP violation and a 3σ indication of normal mass ordering. In particular, NOvA presented the first 4σ evidence of ν̅_μ– ν̅_e transitions compatible with three-neutrino oscillations.

The next-generation long-baseline experiments DUNE and Hyper-Kamiokande in the US and Japan, respectively, were discussed in depth. These experiments have the capability to measure CP violation and mass ordering in the neutrino sector with a sensitivity of more than 5σ, with great potential in other searches like proton decay, supernovae, solar and atmospheric neutrinos, and indirect dark-matter searches.

All the reactor experiments – Daya Bay, Double Chooz and Reno – have improved their results, providing precision measurements of the oscillation parameter θ₁₃ and of the reactor antineutrino spectrum. The Daya Bay experiment, integrating 1958 days of data taking, with more than four million antineutrino events on tape, is capable of measuring the reactor mixing angle and the effective mass splitting with a precision of 3.4% and 2.8%, respectively. The next-generation reactor experiment JUNO, aiming at taking data in 2021, was also presented.

The third day of the conference focused on neutrinoless double-beta decay (NDBD) experiments and neutrino telescopes. EXO, KamLAND-Zen, GERDA, Majorana Demonstrator, CUORE and SNO+ presented their latest NDBD search results, which probe whether neutrinos are Majorana particles, and their plans for the short-term future. The new GERDA results pushed their NDBD lifetime limit based on germanium detectors to 0.9 × 10²⁶ years (90% CL), which represents the best real measurement towards a zero-background next-generation NDBD experiment. CUORE also updated its results based on tellurium to 0.15 × 10²⁶ years.

Neutrino telescopes are of great interest for multi-messenger studies of astrophysical objects at high energies. Both IceCube in Antarctica and ANTARES in the Mediterranean were discussed, together with their follow-up IceCube Gen2 and KM3NeT facilities. IceCube has already collected 7.5 years of data, selecting 103 events (60 of which have an energy of more than 60 TeV) and a best-fit power law of E^–2.87. IceCube does not provide any evidence for neutrino point sources and the measured ν_e:ν_μ:ν_τ neutrino-flavour composition is 0.35:0.45:0.2. A recent development in neutrino physics has been the first observation of coherent elastic neutrino–nucleus scattering as discussed by the COHERENT experiment (CERN Courier October 2017 p8), which opens the possibility of searches for new physics.

A very welcome development at Neutrino 2018 was the presentation of preliminary results from the KATRIN collaboration about the tritium beta-decay end-point spectrum measurement, which allows a direct measurement of neutrino masses. The experiment has just been inaugurated at KIT in Germany and aims to start data taking in early 2019 with a sensitivity of about 0.24 eV after five years. The strategic importance of a laboratory measurement of neutrino masses cannot be overestimated.

A particularly lively session at this year’s event was the one devoted to sterile-neutrino searches. Five short-baseline nuclear reactor experiments (DANSS, NEOS, STEREO, PROSPECT and SoLid) presented their latest results and plans regarding the so-called reactor antineutrino anomaly. These are experiments aimed at detecting the oscillation effects of sterile neutrinos at reactors free from any assumption about antineutrino fluxes. There was no reported evidence for sterile oscillations, with the exception of the DANSS experiment reporting a 2.8σ effect, which is not in good agreement with previous measurements of this anomaly. These experiments are only at the beginning of data taking and more refined results are expected in the near future, even though it is unlikely that any of them will be able to provide a final sterile-neutrino measurement with a sensitivity much greater than 3σ.

Further discussion was raised by the results reported by MiniBooNE at Fermilab, which reports a 4.8σ excess of electron-like events by combining their neutrino and antineutrino runs. The result is compatible with the 3.8σ excess reported by the LSND experiment about 20 years ago in an experiment taking data in a neutrino beam created by pion decays at rest at Los Alamos. Concerns are raised by the fact that even sterile-neutrino oscillations do not fit the data very well, while backgrounds potentially do (and the MicroBooNE experiment is taking data at Fermilab with the specific purpose of precisely measuring the MiniBooNE backgrounds). Furthermore, as discussed by Michele Maltoni in his talk about the global picture of sterile neutrinos, no sterile neutrino model can, at the same time, accommodate the presumed evidence of ν_μ–ν_e oscillations by MiniBooNE and the null results reported by several different experiments (among which is MiniBooNE itself) regarding ν_μ disappearance at the same Δm².

The lively sessions at Neutrino 2018, summarised in the final two beautiful talks by Francesco Vissani (theory) and Takaaki Kajita (experiment), reinforce the vitality of this field at this time (see A golden age for neutrinos).

A golden age for neutrinos

Prototype detector module — Inside a prototype detector module for the international DUNE experiment, which was built at CERN and is about to be filled with liquid argon before undergoing its first tests with beam. Credit: CERN-201710-248-3

On 3 July 1998, researchers working on the Super-Kamiokande experiment in Japan announced the first evidence for atmospheric-neutrino flavour oscillations. Since neutrinos can only oscillate among different flavours if at least some of them have a non-zero mass, the result proved that neutrinos are massive, albeit with very small mass values. This is not expected in the Standard Model.

Neutrino physics was already an active field, but the 1998 observation sent it into overdrive. The rich scientific programme and record attendance of the Neutrino 2018 conference in Heidelberg last month (see Neutrino physics shines bright in Heidelberg) is testament to our continued fascination with neutrinos. Many open questions remain: what generates the tiny masses of the known neutrinos, and what is their mass ordering? Are there more than the three known neutrino flavours, such as additional sterile or right-handed versions? Is there CP violation in the neutrino sector and, if so, how large is it? In addition, there are solar neutrinos, atmospheric neutrinos, cosmic/supernova neutrinos, relic neutrinos, geo-neutrinos, reactor neutrinos and accelerator-produced neutrinos – allowing for a plethora of experimental and theoretical activity.

Many of these questions are expected to be answered in the next decade thanks to vigorous experimental efforts. Concerning neutrino-flavour oscillations, new results are anticipated in the short term from the accelerator-based T2K and NOvA experiments in Japan and the US, respectively. These experiments probe the CP-violating phase in the neutrino-flavour mixing matrix and the ordering of the neutrino mass states; evidence for large CP violation could be established, in particular thanks to the planned ND280 near-detector upgrade of T2K.

Albert De Roeck is a staff physicist at CERN, a professor at the University of Antwerp, Belgium, a member of the CMS collaboration and group leader of the CERN experimental neutrino group.

The next generation of accelerator-based experiments is already under way. The Deep Underground Neutrino Experiment (DUNE) in South Dakota, US, which will use a neutrino beam sent from Fermilab, is taking shape and two large prototypes of the DUNE far detector are soon to be tested at CERN. In Japan, plans are shaping up for Hyper-Kamiokande, a large detector with a fiducial volume around 10 times larger than that of Super-Kamiokande, and this effort is complemented with other sensitivity improvements and a possible second detector in Korea for analysing a neutrino beam sent from J-PARC in Japan. These experiments, which are planned to come online in 2026, will allow precision neutrino-oscillation measurements and provide decisive statements on the neutrino mass hierarchy and CP-violating phase.

Important insights are also expected from reactor sources. In China, the JUNO experiment should start in 2021 and could settle the mass-hierarchy question and determine complementary oscillation parameters. Meanwhile, very-short-baseline reactor experiments – such as PROSPECT, STEREO, SoLid, NEOS and DANSS – are soon to join the hunt for sterile neutrinos. Together with detectors at the short-baseline neutrino beam at Fermilab (SBND, MicroBooNE and ICARUS), the next few years should see conclusive results on the existence of sterile neutrinos. In particular, the recently reported update on the intriguing excess seen by the MiniBooNE experiment will be scrutinised.

Together with the ever-increasing sensitivities achieved by double-beta-decay experiments, which test whether neutrinos have a Majorana mass term, the SHiP experiment is proposed to search for right-handed neutrinos, while KATRIN in Germany has just started its campaign to measure the mass of the electron antineutrino with sub-eV precision. The interplay with astronomy and cosmology, using detectors such as IceCUBE and KM3NeT, which survey atmospheric neutrinos, further underlines the vibrancy and breadth of modern neutrino physics. Also, the European Spallation Source, under construction in Sweden, is investigating the possibility of a precise neutrino-measurement programme.

Neutrino experiments are spread around the globe, but Europe is a strong player. A discussion forum on neutrino physics for the update of the European strategy for particle physics will be hosted by CERN on 22–24 October. Clearly, neutrino science promises many exciting results in the near future.

CERN marks beginning of a luminous future

Time capsule — A time capsule is buried at point 5 of the LHC, symbolising the start of civil-engineering work for the HL-LHC upgrade. From left to right: Lucio Rossi (head of the HL-LHC project), Sijbrand de Jong (president of the CERN Council), Pascal Larour (adjoint au Maire de Cessy), Stéphane Bouillon (préfet de la Région Auvergne-Rhône-Alpes), Fabiola Gianotti (CERN Director-General), Pierre Maudet (president of the Conseil d’Etat de la République et Canton de Genève), Pierre-Alain Tschudi (mayor of Meyrin) and Frédérick Bordry (CERN director for accelerators and technology).

A ceremony at CERN on 15 June celebrated the start of civil-engineering works for the high-luminosity upgrade of the Large Hadron Collider (HL-LHC). The upgrade will allow about 10 times more data to be accumulated by the LHC experiments between 2026 and 2036, corresponding to a total integrated luminosity of 3000 fb^–1, thereby enhancing the chances of discovery and bringing increased precision to measurements.

The HL-LHC project began in earnest in November 2011 as an international endeavour today involving 29 institutes from 13 countries. Two years later, the project was identified as one of the main priorities of the European Strategy for Particle Physics. The upgrade, targeting a luminosity of at least 5 × 10³⁴ cm^–2 s^–1, was formally approved by the CERN Council in June 2016.

Although it concerns only about 5% of the current machine, the HL-LHC is a major upgrade requiring a number of innovative technologies, many of which pave the way for future higher-energy colliders. At its heart are powerful new dipole and quadrupole magnets that operate at unprecedented fields of 11 and 11.5 T, respectively, and which employ novel niobium-tin superconducting cables. The quadrupoles, which will be installed on both sides of the collision points, will squeeze the proton beams to increase the probability of a collision (CERN Courier March 2017 p23).

Sixteen brand-new radio-frequency “crab cavities” will also be installed around the ATLAS and CMS experiments to maximise the overlap of the proton bunches at the collision points (CERN Courier May 2018 p18). Their function is to tilt the bunches so that they appear to move sideways, and the first ever tests of this technology in a proton beam were successfully carried out at the Super Proton Synchrotron in May.

To prepare the CERN accelerator complex for the immense challenges of the HL-LHC, the LHC Injectors Upgrade project (LIU) was launched in 2010. In addition to enabling the necessary injector chains to deliver the HL-LHC beams, the LIU project is also tasked with replacing ageing equipment and improving radioprotection measures (CERN Courier October 2017 p32).

Overall, more than 1.2 km of the current LHC will need to be replaced with new components. This requires civil-engineering work at two main sites in Switzerland and in France, involving the construction of new buildings, shafts, caverns and underground galleries (CERN Courier March 2017 p28). The LHC will continue to operate until
early December.

“The High-Luminosity LHC will extend the LHC’s reach beyond its initial mission, bringing new opportunities for discovery, measuring the properties of particles such as the Higgs boson with greater precision, and exploring the fundamental constituents of the universe ever more profoundly,” said CERN Director-General Fabiola Gianotti during the ceremony.

On 25 June, the Canadian government announced a contribution of C$10 million to the HL-LHC, with an additional C$2 million in in-kind contributions. Working with Canadian researchers and industry, the TRIUMF laboratory will lead the production of five cryogenic modules for the HL-LHC crab cavities.

Muons accelerated in Japan

Installation of the RFQ in the experimental area.

Muons have been accelerated by a radio-frequency accelerator for the first time, in an experiment performed at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai, Japan. The work paves the way for a compact muon linac that would enable precision measurements of the muon anomalous magnetic moment and the electric dipole moment.

Around 15 years ago, the E821 storage-ring experiment at Brookhaven National Laboratory (BNL) reported the most precise measurement of the muon anomalous magnetic moment (g-2). Achieving an impressive precision of 0.54 parts per million (ppm), the measured value differs from the Standard Model prediction by more than three standard deviations. Following a major effort over the past few years, the BNL storage ring has been transported to and upgraded at Fermilab and recently started taking data to improve on the precision of E821. In the BNL/Fermilab setup, a beam of protons enters a fixed target to create pions, which decay into muons with aligned spins. The muons are then transferred to the 14 m-diameter storage ring, which uses electrostatic focusing to provide vertical confinement, and their magnetic moments are measured as they precess in a magnetic field.

The new J-PARC experiment, E34, proposes to measure muon g-2 with an eventual precision of 0.1 ppm by storing ultra-cold muons in a mere 0.66 m-diameter magnet, aiming to reach the BNL precision in a first phase. The muons are produced by laser-ionising muonium atoms (bound states of a positive muon and an electron), which, since they are created at rest, results in a muon beam with very little spread in the transverse direction – thus eliminating the need for electrostatic focusing.

Turning on the RFQ — When turning on the RFQ (red), a peak is observed at the expected timing (green). The peak disappears when turning off the RFQ (blue), suggesting acceleration has taken place.

The ultracold muon beam is stored in a high-precision magnet where the spin-precession of muons is measured by detecting muon decays. This low-emittance technique, which allows a smaller magnet and lower muon energies, enables researchers to circumvent some of the dominant systematic uncertainties in the previous g-2 measurement. To avoid decay losses, the J-PARC approach requires muons to be accelerated via a conventional radio-frequency accelerator.

In October 2017, a team comprising physicists from Japan, Korea and Russia successfully demonstrated the first acceleration of negative muonium ions, reaching an energy of 90 keV. The experiment was conducted using a radio-frequency quadrupole linac (RFQ) installed at a muon beamline at J-PARC, which is driven by a high-intensity pulsed proton beam. Negative muonium atoms were first accelerated electrostatically and then injected into the RFQ, after which they were guided to a detector through a transport beamline. The accelerated negative muonium atoms were identified from their time of flight: because a particle’s velocity at a given energy is uniquely determined from its mass, its type is identified by measuring the velocity (see figure).

The researchers are now planning to further accelerate the beam from the RFQ. In addition to precise measurements in particle physics, the J-PARC result offers new muon-accelerator applications including the construction of a transmission muon microscope for use in materials and life-sciences research, says team member Masashi Otani of KEK laboratory. “Part of the construction of the experiment has started with partial funding, which includes the frontend muon beamline and detector. The experiment can start properly three years after full funding is provided.”

Muon acceleration is also key to a potential muon collider and neutrino factory, for which it is proposed that the large, transverse emittance of the muon beam can be reduced using ionisation cooling (see Muons cooled for action).

Jefferson Lab inaugurates upgraded CEBAF

Experimental hall D — Left to right: US Department of Energy undersecretary for science Paul Dabbar, Jefferson Lab director Stuart Henderson and congressman Bobby Scott in the new experimental hall D. Credit: Jefferson Lab

On 2 May, the Thomas Jefferson National Accelerator Facility in Virginia, US, celebrated the completion of the 12 GeV CEBAF upgrade project. The $338 million upgrade has tripled CEBAF’s original operating energy and will allow, among other studies, more in-depth investigations of nuclear confinement.

CEBAF (Continuous Electron Beam Accelerator Facility) provides high-quality beams of polarised electrons that allow physicists to extract information on the quark and gluon structure of nucleons.
The CEBAF accelerator started up in 1994 and originally delivered 4 GeV beams, which were later pushed to 6 GeV thanks to efficiencies in the machine’s design and extensive experience gained during operation. Previously, CEBAF operated as a pair of superconducting radio-frequency linear accelerators in a “racetrack” configuration, capable of delivering 6 GeV electron beams simultaneously to three experimental halls. In 2008 work began on a major upgrade project to double the maximum energy and add new
experimental setups.

The 12 GeV CEBAF upgrade project required 10 high-performance, superconducting radio-frequency cryomodules, doubling the capacity of the existing cryogenics plant, and the addition of eight superconducting magnets and other system upgrades. The upgrade also includes the construction of a new experimental area (“hall D”) for dedicated research on exotic mesons produced by energetic photons incident on a target. CEBAF’s newly energetic and precise beams will enable the first 3D views of the structure of protons and neutrons, the study of the origin of confinement in QCD, and the investigation of physics beyond the Standard Model by testing the theory’s completeness at low energies.

Higgs centre opens for business

A new facility called the Higgs Centre for Innovation opened at the Royal Observatory in Edinburgh on 25 May as part of the UK government’s efforts to boost productivity and innovation. The centre, named after Peter Higgs of the University of Edinburgh, who shared the 2013 Nobel Prize in physics for his theoretical work on the Higgs mechanism, will offer start-up companies direct access to academics and industry experts. Space-related technology and big-data analytics are the intended focus, and up to 12 companies will be based there at any one time. According to a press release from the UK Science and Technology Facilities Council (STFC), the facility incorporates laboratories and working spaces for researchers, and includes a business incubation centre based on the successful European Space Agency model already in operation in the UK.

“Professor Higgs’ theoretical work could only be proven by collaboration in different scientific fields, using technology built through joint international ventures,” said principal and vice-chancellor of the University of Edinburgh Peter Mathieson. “This reflects the aims and values of the Higgs Centre for Innovation, which bring scientists, engineers and students together under one roof to work together for the purpose of bettering our understanding of space-related science and driving technological advancement forward.”

The Higgs Centre for Innovation was funded through a £10.7 million investment from the UK government via STFC, which is also investing £2 million over the next five years to operate the centre.

Largest WIMP survey sets new limits

The latest limits on WIMP interactions, derived from one year of XENON1T data. The inset compares the new XENON1T limit and sensitivity with those of previous experiments.

On 28 May, the world’s largest and most sensitive detector for direct searches of dark matter in the form of weakly interacting massive particles (WIMPs) released its latest results. XENON1T, a 3D-imaging liquid-xenon time projection chamber located at Gran Sasso National Laboratory in Italy, reported its first results last year (CERN Courier July/August 2017 p10). Now, the 165-strong international collaboration has presented the results from an unprecedentedly large exposure of approximately one tonne × year.

The results are based on 1300 kg out of the total 2000 kg active xenon target and 279 days of data-taking, improving the sensitivity by almost four orders of magnitude compared to XENON10 (the first detector of the XENON dark-matter project, which has been hosted at Gran Sasso since 2005). The data are consistent with background expectations, and place the most stringent limit yet on spin-independent interactions of WIMPs with ordinary matter for a WIMP mass higher than 6 GeV/c².

XENON1T spokesperson Elena Aprile of Columbia University in the US describes the result as a milestone in dark-matter exploration. “Showing the result after a one tonne × year exposure was important in a field that moves fast,” she explains. “It is also clear from the new result that we will win faster with a yet-larger mass and lower radon background, which is why we are now pushing the XENONnT upgrade.”

ISOLDE mints chromium for structure studies

Resonant ionisation laser-ion source — ISOLDE’s resonant ionisation laser-ion source (RILIS), which provided the first beams of neutron-rich chromium isotopes. Credit:CERN-201602-031-7

CERN’s radioactive ion-beam facility ISOLDE has stamped a new coin in its impressive collection. Long considered the domain of high-energy, in-flight rare-isotope facilities, chromium has now been produced at ISOLDE in prodigious quantities, thanks to a new resonant ionisation laser-ion source (RILIS) scheme. Together with the latest calculations based on chiral effective field theory, the result provides important guidance for improving theoretical approaches that bridge the gap between nuclear matter and the low-energy extension of quantum chromodynamics (QCD).

Certain configurations of protons and neutrons are more bound than others, revealing so-called magic numbers. Chromium has 24 protons, situating it squarely between magic calcium (with 20 protons) and nickel (with 28). Of particular interest to nuclear physics are isotopes with a large excess of neutrons.

The RILIS is a chemically selective ion source which relies on resonant excitation of atomic transitions using a tunable laser. In the new ISOLDE experiment, Maxime Mougeot of CSNSM/Université Paris-Saclay and collaborators used RILIS to venture 10 neutrons further on the nuclear chart to ⁶³Cr. With a total of 39 neutrons, ⁶³Cr lies exactly between the magic neutron numbers 28 and 50 and has a half-life of just 130 ms.

The masses of the newly forged chromium isotopes, as measured by ISOLDE’s precision Penning-trap mass spectrometer ISOLTRAP, offer insights into its shape and structure. Magic-number nuclides have filled orbitals that favour spherical shapes, but not so the chromium nuclides weighed by ISOLTRAP, which are deformed.Whereas in some areas of the nuclear chart deformation sets in very suddenly with the addition of a further neutron, the remarkably smooth neutron binding energies of chromium show that deformation sets in very gradually – contrary to previous conclusions.

The ISOLDE measurements were compared with different theoretical results, including a very first attempt by a new ab-initio approach called valence-space in-medium similarity renormalization group (VS-IMSRG). While several ab-initio approaches exist, until now they have been restricted to the near-spherical cases that have very few valence protons and neutrons. The latest VS-IMSRG results are the first for such open-shell nuclides.

“It turns out that the ab-initio VS-IMSRG, an interaction derived from chiral effective field theory which reduces QCD to its relevant degrees of freedom at the nuclear scale, failed to predict these results,” explains Mougeot. “So the recent chromium measurements are constructive and important for advancing this promising technique, which bridges the gap between first-principle calculations and the structure of nuclei at the extremes of the nuclear landscape.”

CMS resolves inner structure of bottomonium

Fig. 1. The χb(mP) states, seen in the Υ(nS) γ invariant-mass distributions. The inset shows the 10 million Υ(1S), 3.9 million Υ(2S) and 2.6 million Υ(3S) that were reconstructed by CMS using the full data sample at 13 TeV.

A report from the CMS experiment

Bottomonium mesons, composed of beauty quark–antiquark pairs bound to each other through the strong force, play a special role in our understanding of hadron formation because the large quark mass allows important simplifications in the relevant theoretical calculations.

The spectroscopy of the bottomonium family has now been significantly upgraded, thanks to the first observation of the individual χ_b1(3P) and χ_b2(3P) states by the CMS collaboration. Identified via the decay χ_b(3P) → Υ(3S) γ, and adding for the first time all the LHC data collected at an energy of 13 TeV (corresponding to a staggering 80 fb^–1 of integrated luminosity), CMS detected 16.5 million Υ mesons in the dimuon decay channel. The corresponding invariant mass distribution shows well-resolved Υ(1S), Υ(2S) and Υ(3S) resonances (figure 1, inset), which constitute the starting point for the reconstruction of the p-wave bottomonia through the radiative decay χ_b(mP) → Υ(nS) γ.

The main challenge in this study is the low energy of the photons. The CMS analysis uses photons that convert into e⁺e^– pairs and are reconstructed in the silicon tracker with very high precision, leading to clear χ_b(mP) peaks in the resulting Υ(nS) γ invariant mass distributions. The resolution of the χ_b mass measurement scales with the photon energy, or the difference between the masses of the P- and S-wave mesons. The Υ(3S) γ invariant mass is measured with a remarkable resolution, enabling the first observation of a double-peak structure in the χ_b(3P) resonance, which corresponds to the states of total angular momentum J = 1 and J = 2 (figure 2).

Fig. 2. The double-peak structure reflecting the J = 1 and J = 2 states is observed for the first time thanks to the high statistics of the data sample and the excellent mass resolution.

The existence of two peaks is established with a significance exceeding nine standard deviations and the two masses are measured to be 10,513.42 ± 0.41 (stat) ± 0.18 (syst) MeV and 10,524.02 ± 0.57 (stat) ± 0.18 (syst) MeV. The measured mass splitting, 10.60 ± 0.64 (stat) ± 0.17 (syst) MeV, can be used to improve the theoretical calculations, which currently predict values between 8 and 18 MeV depending on the potentials describing the quark–antiquark non-perturbative interaction. The only exception predicts a value of –2 MeV, the negative sign meaning that the χ_b2(3P) has a mass smaller than the χ_b1(3P).

The new measurement is a step forward in completing the spin-dependent bottomonium spectroscopy diagram, and should significantly contribute to an improved understanding of the non-perturbative QCD processes that lead to the binding of quarks and gluons into hadrons.

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