Jobs – Page 107 of 527

ALICE’s dark side

The nature of dark matter (DM) remains one of the most intriguing unsolved questions of modern physics. Astrophysical and cosmological observations suggest that DM accounts for roughly 27% of the mass-energy of the universe, with dark energy comprising 68% and ordinary baryonic matter as described by the Standard Model accounting for a paltry 5%. This massive hole in our understanding of the universe continues to drive multiple experimental searches for DM both in the laboratory and in space. No clear evidence for DM has yet been found, severely constraining the parameter space of the most popular “thermal” DM models.

Assuming DM is a material substance comprised of particles – not an illusion resulting from an imperfect understanding of gravity – there are three independent ways to search for it. One is to directly measure the production of DM particles in a high-energy collider such as the LHC. Another is to infer the presence of DM particles via their scattering off nuclei, as investigated by large underground detectors such as XENON1T and LUX. A third, similarly indirect, strategy is to search for the annihilation or decay of DM particles into ordinary (anti) particles such as positrons or antinuclei – as employed by the AMS experiment on board the International Space Station and in balloon-borne experiments such as GAPS. Low-energy light antinuclei, such as antideuterons and antihelium, are particularly promising signals for such indirect DM searches, since the background stemming from ordinary collisions between cosmic rays and the interstellar medium is expected to be low with respect to the DM signal.

ALICE is the only experiment at the LHC that is able to study the production and annihilation of low-energy antinuclei

The ability to interpret any future observation of antinuclei in our galaxy – especially when trying to identify their creation in exotic processes like DM annihilations – requires a quantitative understanding of light antinuclei production and annihilation mechanisms within the interstellar medium. However, the production of light antinuclei in hadronic collisions between cosmic rays and the interstellar medium is still not fully understood: different models compete to explain how these loosely bound objects can be formed in such high-energy collisions. Furthermore, the inelastic annihilation cross section of light antinuclei with matter is completely unknown in the kinematic region relevant for indirect DM searches, forcing current estimates to rely on extrapolations and modelling.

Fortunately, both the antinuclei production mechanism and the interactions between antinuclei and ordinary matter can be studied on Earth using large accelerators. The main contributions so far have come from the LHC at CERN and from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. Thanks to its unique low-material-budget tracker, which provides excellent tracking and particle-identification performance for low-momentum particles, ALICE is the only experiment at the LHC that is able to study the production and annihilation of low-energy antinuclei.

Antinucleosynthesis in the lab
While antinuclei can also be produced at lower collision energies, only at the LHC are matter and antimatter generated in equal abundances in the region transverse to the beam direction. The most abundant non-trivial antinucleus produced is the antideuteron, which consists of an antineutron and an antiproton. At low momentum, deuterons and antideuterons can be clearly identified thanks to their high energy loss in the ALICE detector’s time-projection chamber. At larger momenta, a clean identification of antideuterons is possible using the ALICE time-of-flight detector. This information, combined with the measured track length and the particle momentum, provide a precise determination of the particle mass. Using these and other techniques, the ALICE collaboration has recently measured the production of (anti)deuterons in proton–proton collisions, as well as in other colliding systems, and set tight constraints on the production models of (anti)nuclei.

Ratio of anti-deuterons to protons — **Competing models** The ratio of (anti)deuterons to protons in p–p, p–Pb and Pb–Pb collisions as a function of the average number of charged particles produced. The purple line shows the expectation of the coalescence model while the solid and dashed black lines represent extreme parameter configurations for the statistical hadronisation model. Credit: *Eur*. Phys. J. C 80 889

There are two main ways to model the production mechanism of (anti)nuclei. Coalescence models assume that primary (anti)neutrons and (anti)protons can bind if they are close enough in phase space. Statistical hadronisation models, on the other hand, assume that hadrons and (anti) nuclei emerge when the collision system reaches thermodynamical equilibrium, making the temperature and the volume of the system the key parameters. Measurements of nuclei-to-proton ratios in various colliding systems have recently enabled the ALICE collaboration to compare the two model approaches in detail (see “Competing models” figure). As can be seen, the two models give different predictions for the evolution of the nuclei-to-proton ratio versus particle multiplicity, with the latest ALICE measurements slightly favouring the coalescence approach.

Similar conclusions about the two models can be drawn using heavier antinuclei, like ³He and ⁴He, which were already measured by ALICE in p–Pb and Pb–Pb collisions. The achievable precision of the measurement is limited by the available data: the antinuclei production rate in pp collisions goes down by a factor of about 1000 for every additional antinucleon in the antinucleus.

The precision of the measurements from proton–proton collisions places strong constraints on the production models, which can then be used to predict the antinuclei fluxes in space. Indeed, the ALICE measurements combined with different coalescence models have already been employed to estimate the antideuteron and antihelium flux from cosmic-ray interactions measurable by the AMS and GAPS experiments. These predictions will allow correct interpretations of the eventual antinuclei signal that might be observed in the future by the two collaborations.

Further helping clarify the results of indirect DM searches, ALICE has recently performed the first measurement of the antideuteron inelastic cross section in the momentum range 0.3 < p < 4 GeV/c – significantly extending our knowledge about this cross section from previous measurements at momenta of 13 and 25 GeV/c at the Serpukhov accelerator complex in Russia in the early 1970s. The collaboration took advantage of the ability of antideuterons produced at the LHC to interact inelastically with the detector material. To quantify this process, ALICE has employed a novel approach based on the antideuteron- to-deuteron ratio reconstructed in collisions of protons and heavy ions at a centre-of-mass energy per nucleon–nucleon pair of 5.02 TeV. Such a ratio depends on both of the inelastic cross sections of deuterons and antideuterons. The former has been measured in various previous experiments at different momenta; the latter can be constrained from the ALICE data by comparing the measured ratio with detailed Monte Carlo simulations.

**Interaction probability** Antideuteron inelastic interaction cross section measured in different ALICE subdetectors. The dashed red line represents the parameterisations used for antideuterons in the Geant4 toolkit, and the full grey line shows the parameterisations for deuterons. The full points correspond to the experimental measurements, whereas the purple and orange bands are the one and two sigma uncertainty intervals, respectively. Credit: arXiv:2005.11122

The resulting antideuteron inelastic cross section is shown (see “Interaction probability” figure), where the two panels correspond to the different sub-detectors employed in the analysis and therefore to different average material crossed by (anti)deuterons – corresponding to a difference of about a factor two in average mass number. The inelastic cross sections include all possible inelastic antideuteron processes such as break-up, annihilation or charge exchange, and the analysis procedure was validated by demonstrating consistency with existing antiproton results from traditional scattering experiments.

The momentum range covered is of particular importance to evaluate the signal predictions for indirect dark-matter searches

The momentum range covered in this latest analysis is of particular importance to evaluate the signal predictions for indirect dark-matter searches. Additionally, these measurements can help researchers to understand the low-energy antideuteron inelastic processes and to model better the inelastic antideuteron cross sections in widely-used toolkits such as Geant4. Together with the proper modelling of antinuclei formation, the obtained results will impact the antideuteron flux expectations at low momentum for ongoing and future satellite- and balloon-borne experiments.

The heavier, the better
ALICE is studying the full range of antinuclei physics with unprecedented precision. These results, which have started to emerge only since 2015, are contributing significantly to our understanding of antinuclei formation and annihilation processes, with important ramifications for DM searches. Both the statistical hadronisation and coalescence models can describe antideuteron production at the LHC, while the detector material can be used as an absorber to study the antinuclei inelastic cross section at low energies relevant for the astrophysical applications.

For the foreseeable future, ALICE will continue to provide an essential reference for the interpretation of astrophysics measurements of antinuclei in space. With the increased integrated luminosity that will be acquired by ALICE during LHC Run 3 from early 2022, it will be possible to extend the current analyses to heavier (anti)nuclei, such as ³He and ⁴He, with even better precision than the currently available measurements for (anti)deuterons. This will allow the collaboration to perform fundamental tests of the production and annihilation mechanisms with heavier, doubly-charged antinuclei, which are more easily identified by satellite-borne experiments and thus expected to provide an even clearer DM signature.

Pulsars hint at low-frequency gravitational waves

NANOGrav uses pulsate to detect potential distortions in space time — NANOGrav uses variations in the arrival time of the signals from an array of very stable, quickly rotating pulsars to detect potential distortions in space time caused by low-frequency gravitational waves. Credit: D Champion

The direct detection of a gravitational wave (GW) in 2015 by the LIGO and Virgo collaborations confirmed the existence of these long sought after events. However, these and other GW events detected so far constitute only a small fraction – in the kHz regime — of the vast GW spectrum. As a result, they only probe certain phenomena such as stellar mass black-hole and neutron-star mergers. On the opposite side of the spectrum to LIGO and Virgo are Pulsar Timing Array (PTA) experiments, which search for nHz frequency GWs. Such low-frequency signals can originate from supermassive black-hole binaries (SMBHBs), while in more exotic models they can be proof of cosmic strings, phase transitions or a primordial GW background. The NANOGrav (North American Nanohertz Observatory for Gravitational Waves) collaboration has now found possible first hints of low-frequency GWs.

To detect such rumblings of space—time, which also have minute amplitudes, researchers need to track subtle movements of measurement points spread out over the size of a galaxy. For this purpose, the NANOGrav collaboration uses millisecond pulsars, several tens of which have been detected in our galaxy. Pulsars are quickly rotating neutron stars which emit cones of electromagnetic emission from their poles. When a pole points towards Earth it is detected as a short pulse of electromagnetic radiation. Not only is the frequency of millisecond pulsars high, making it easier to detect small variations in arrival time, but it is very stable over periods of many years. Combined with their great distances from Earth, this makes millisecond-pulsar emissions sensitive to any small alterations in their travel path — for example, those introduced by distortions of space–time by low-frequency gravitational waves. Such waves would cause the pulses to arrive a few nanoseconds early during January and a few nanoseconds late in June, for instance. By observing the radio emission of these objects once a week throughout many years, researchers can search for such effects.

The new results show a clear sign of a common spectrum between the studied pulsars

The problem is that GWs are not the only things which can cause a change in the arrival time of the pulses. Changes in the Earth’s atmosphere already alter the arrival time, as do changes in the position of the pulsar itself (which is usually part of a quickly rotating binary system), and the movement of Earth with respect to the source. The complexity of the measurements lies mostly in correcting for all of these effects. The latest results from NANOGrav, for example, reduce systematics by incorporating unprecedented precision (of the order of tens of km) in the orbital parameters of Jupiter.

Whereas previous results by NANOGrav and other PTA collaborations only allowed upper limits to be set on the amplitude of the GW background travelling through our galaxy, the new results show a clear sign of a common spectrum between the studied pulsars. Based on 12.5 years of data and a total of 47 pulsars studied using the ultra-sensitive Arecibo Observatory and Green Bank Telescope, the spectrum of variations in the pulsar signal arrival time was found to agree with theoretical predictions of the GW background produced by SMBHBs. The uncertainties remain large, however, which admits alternative interpretations such as cosmic strings which predict only a slightly different spectral shape. Furthermore, a key ingredient is still missing: a spatial correlation between the pulsar variations, which would confirm the quadrupole nature of GWs and provide clear proof of the nature of the signal. Finding this “smoking gun” will require longer observation times, more pulsars and smaller systematic errors — something the NANOGrav team is now working towards.

While the NANOGrav collaboration remains cautious, several exotic interpretations have already been proposed. The final sentences of their preprint summarise the status of this exciting field well: “The LIGO–Virgo discovery of high-frequency, transient GWs from stellar black-hole binaries appeared meteorically, with incontrovertible statistical significance. By contrast, the PTA discovery of very-low-frequency GWs from SMBHBs will emerge from the gradual and not always monotonic accumulation of evidence and arguments. Still, our GW vista on the unseen universe continues to get brighter”.

Willem de Boer 1948–2020

Willem (“Wim”) de Boer passed away on 13 October, aged 72. Wim studied physics at the University of Delft and graduated in 1974 with a thesis on the dynamic orientation of nuclei at low temperatures, which laid the foundation of polarised targets in high-energy physics. Following a CERN fellowship, he joined the University of Michigan, Ann Arbor and worked on polarised proton–proton scattering at the ANL synchrotron, where he found an unexplained difference in the cross sections for parallel and antiparallel spins.

In 1975 Wim took up a position at the Max Planck Institute for Physics in Munich where he stayed, interrupted by a sabbatical at SLAC in 1987, for 14 years. In Munich he joined the team working on the CELLO experiment at DESY, where he took responsibility for the data-acquisition system. The CELLO years were instrumental for precision studies of QCD, out of which the triple-gluon coupling and the running of the strong coupling constant emerged – a subject Wim pursued ever after.

Following his appointment to a professorship at the University of Karlsruhe in 1989, Wim created research groups at LEP’s DELPHI experiment, the AMS-02 experiment on the International Space Station, and he coordinated a group at the LHC’s CMS experiment. Having studied the running of the coupling constants of the weak, electromagnetic and strong interactions, Wim found, together with Ugo Amaldi and Hermann Fürstenau, that these could only meet in a unified way at high energies if phenomena beyond the Standard Model, such as supersymmetry, existed. This was published in their seminal 1991 paper “Comparison of grand unified theories with electroweak and strong coupling constants measured at LEP”, which led to the expectation that a new energy domain would open up at the TeV scale with the lightest supersymmetric particle constituting dark matter. The paper has been cited almost 2000 times.

Wim contributed a multitude of ideas, studies and publications to each of the experiments he worked on, driven by the single question: where is supersymmetry? He looked for dark-matter signals at the lowest energies in our galaxy using earth-bound observatories, balloon experiments and satellites, at signals from direct production at LEP and the LHC, and in anomalous decay modes of bottom mesons using data from the Belle and BaBar experiments, among others.

It is our belief that Wim was most fascinated by AMS-02. Not only did he and his group contribute an electronic readout system to the detector, he also saw it take off from Cape Canaveral with the penultimate Space Shuttle flight in 2011, celebrated by the visit of the whole crew of astronauts to Karlsruhe later that year.

Wim’s career saw him work across detectors using gases, liquids, silicon and diamonds, and study their performance in magnetic fields and high-radiation backgrounds. He also investigated the use of detectors for medical and technical applications. His last R&D effort began only a few weeks before his death: the development of a novel cooling system for high-density batteries.

Our field has lost a great all-round physicist with unparalleled creativity and diligence, a warm collegiality and a very characteristic dry humour. Well aware of his rapid illness, his last words to his family were: “Hij gaat nog niet, want hij heeft nog zoveel ideeën!” (roughly “He’s not going yet, because he still has so many ideas!). He will be missed deeply.

American Physical Society announces 2021 awards

Henry Sobel (Top) and Edward Kearns (Bottom). Credit: UCI/Boston University

W.K.H. Panofsky Prize

The W K H Panofsky Prize in experimental particle physics has been awarded to Henry Sobel, professor emeritus of the University of California, Irvine and Edward Kearns of Boston University for pioneering and leadership contributions to large underground experiments for the discovery of neutrino oscillations and sensitive searches for baryon-number violation. As the US co-spokesperson, Sobel is heavily involved with Japan’s Super-Kamiokande experiment (Super-K), and is also involved in the next-generation neutrino experiments – DUNE, in the US and Hyper-K in Japan. Kearns is also involved in Super-K and DUNE, along with being a member of the Tokai-To-Kamioka (T2K) experiment and active in the search for dark matter using techniques based on cryogenic noble liquids.

J.J. Sakurai Prize

The J J Sakurai Prize for theoretical physics has been given to Vernon Barger of the University of Wisconsin-Madison for pioneering work in collider physics contributing to the discovery and characterisation of the W boson, top quark and Higgs boson, and for the development of incisive strategies to test theoretical ideas with experiments.

Robert R. Wilson Prize

In the field of accelerators, Yuri Fyodorovich Orlov, formerly of Cornell University, was awarded the Robert R Wilson Prize for his pioneering innovation in accelerator theory and practice. Orlov received the news shortly before his passing on 27 September.

Maria Goeppert Mayer Award

Phiala E Shanahan of the Massachusetts Institute of Technology has been granted the Mario Goeppert Mayer Award, which recognises an outstanding contribution to physics research by a women, “for key insights into the structure and interactions of hadrons and nuclei using numerical and analytical methods”.

Chanda Prescod-Weinstein Credit: University of New Hampshire

Edward A.Bouchet Award

The Edward A Bouchet Award, which promotes the participation of underrepresented minorities in physics, has been awarded to Chanda Prescod-Weinstein of the University of New Hampshire for her contributions to theoretical cosmology and particle physics and for co-creating the Particles for Justice movement.

Herman Feshbach Prize

The Herman Feshbach Prize in theoretical nuclear physics has been awarded to Berndt Mueller of Brookhaven National Laboratory for his contributions to the identification of quark-gluon plasma signatures.

Henry Primakoff Award

The 2021 Henry Primakoff Award for early-career particle physics has gone to Jaroslav Trnka of the University of California, Davis for seminal work on the computation of particle scattering amplitudes.

Dwight Nicholson Medal

The 2020 Dwight Nicholson Medal for Outreach has been given to Michael Barnett of Lawrence Berkeley National Laboratory “for a lifetime of innovations in outreach bringing the discoveries and searches of particle physicists and cosmologist to multitudes of students and lay people around the world.”

Yuri Orlov 1924–2020

Yuri Orlov, a world-renowned accelerator physicist and a leading figure in the worldwide campaign for human rights in Soviet Russia, passed away at the end of September at the age of 96.

Yuri was born in Moscow in 1924. He studied and worked there until 1956, when a critical pro-democracy speech he gave at the Institute for Theoretical and Experimental Physics resulted in him being fired and banned from scientific work. He then moved to the Yerevan Physics Institute in Armenia where he earned his first doctorate (“Nonlinear theory of betatron oscillations in the strong-focusing synchrotron”) in 1958, followed by the award of a second doctorate in 1963. While in Yerevan, he designed the 6 GeV electron synchrotron, became head of the electromagnetic interaction laboratory, and was elected to the Armenian Academy of Sciences.

In 1972 Yuri returned to Moscow and joined the influential dissident movement that included Andrei Sakharov and Aleksandr Solzhenitsyn. When the final documents of the Helsinki Conference on Security and Co-operation in Europe were signed in 1975, Yuri founded the Moscow Helsinki Group with the aim of having all human rights guaranteed in the Helsinki documents accorded to all citizens of the Soviet Union. As was to be expected, Yuri was arrested in 1977, tried in a political mock trial in 1978 and convicted to seven years in a labour camp in Perm.

As soon as Yuri Orlov’s ordeal became known in Europe and North America, physicists began to protest against the treatment of their colleague. At CERN, where several physicists had had personal contacts with Yuri, the Yuri Orlov Committee was founded with Georges Charpak as one of its founding members. The long-standing fruitful scientific collaboration with the Soviet Union was challenged and the support of eminent political leaders of the CERN member states was solicited.

Surviving a total of seven years of labour camp under extreme conditions, Yuri was deported to Siberia for a period of five years. Because of continuing international pressure, he was then deported to the US in 1986, where he was offered a position at Cornell University. Soon after his forced emigration, Yuri visited CERN and he spent a sabbatical there in 1988/1989 working in the accelerator division to develop the idea of ion “shaking”. He joined the muon g-2 experiment at Brookhaven National Laboratory and worked on Brookhaven proposals to measure the electric dipole moments of protons, electrons and deuterons. At Cornell he pursued this work as well as an alternative design for the proposed B-factory, and wrote on the foundations of quantum mechanics. In 2008 he was named a professor of physics and professor of government, and taught physics and human rights until his retirement in 2015.

Yuri authored or co-authored more than 240 scientific papers and technical reports, and wrote a memoir, Dangerous Thoughts: Memoirs of a Russian Life (William Morrow & Co, 1991). Among the many honours Yuri received are the American Physical Society’s 2006 Sakharov prize “For his distinction as a creative physicist and as a life-long, ardent leader in the defence and development of international human rights, justice and the freedom of expression for scientists”, and the APS 2021 Wilson Prize for outstanding achievements in the physics of particle accelerators, of which he was notified shortly before his death.

Yuri’s example as a scientist committed to the freedom of science, its cultural dimension in world affairs and his defence of the human right of expression of one’s convictions is an example and inspiration to all of us.

Glen Lambertson 1926–2020

Glen Lambertson, one of the early giants of US accelerator physics, passed away on 30 August aged 94. Glen is best known for the injection/extraction magnet that bears his name. His greatest achievements, however, were, to quote from the American Physical Society (APS) 2006 Wilson Prize citation, “… fundamental contributions … in the area of beam electrodynamics including the development of beam instrumentation for the feedback systems that are essential for the operation of high luminosity electron and hadron colliders”.

Glen’s studies at the University of Colorado were interrupted by World War II, during which he saw action serving in the legendary 10th Mountain Division. Severely wounded in Northern Italy, his life was saved by the newly discovered wonder drug “penicillin”. (Incidentally, he remained an avid skier well into his 80s.) After the war he completed his degree at Colorado in engineering physics and did graduate work at the University of California, quickly becoming involved with accelerators. His first contact was as an operator of the 184-Inch Synchrocyclotron, where he commented that Ernest Lawrence would often reach over his shoulder to “tweak a knob”.

Glen played a large part in the design of the magnet system for the Bevatron at Lawrence Berkeley National Laboratory, and in 1960 was instrumental in the retrofitting of a resonant extraction system for this machine, vastly improving its performance and effectiveness as a discovery tool for the newly established field of particle physics. His patent for the “Lambertson magnet” is dated 1965, and this concept is still widely used for the injection and extraction of beams in synchrotrons and storage rings.

In the mid-1970s Glen was a major contributor to the ESCAR project – a first attempt to build a small (4 GeV) superconducting accelerator to provide data and experience for future large superconducting machines. While funds were not available to complete the project, two quadrants of dipoles were built and successfully tested, along with the necessary cryogenic and control-system infrastructures. Later in the 1970s, following the developments in stochastic cooling by Simon Van der Meer, Glen led the successful experiment to demonstrate stochastic cooling at the Fermilab 200 MeV cooling test ring. His techniques were transferred to rings at Fermilab and Brookhaven.

His most productive studies were in beam instabilities, in particular the instrumentation to detect and control electron-cloud instabilities. He was a key figure in the successful commissioning of both the PEP-II B-factory at SLAC, and the Advanced Light Source at Berkeley. He also had close contacts with CERN, serving as a visiting scientist in 1993 and later playing an important role in calculating the impedance of injection-line components for the LHC.

Glen’s work was widely recognised. In addition to the APS Wilson prize, he was an APS fellow and also won the US Particle Accelerator School Prize for Achievement in Accelerator Science and Technology.

His always relaxed demeanour and sage advice were a constant inspiration to us, and we forgave him his incredibly awful puns. Rest in peace, Glen!

Electron makeover proposed for the SPS

CERN’s Super Proton Synchrotron (SPS) could be upgraded so that not only protons have the possibility to be accelerated, but also electrons. A 173-page conceptual design report posted on arXiv on 15 September describes the installation of a high-energy electron accelerator that could have the potential to be used for accelerator R&D, dark-sector physics, and for electro-nuclear measurements crucial for future neutrino experiments. The “eSPS”, proposed in 2018 by Torsten Åkesson of Lund University and colleagues at CERN, would marry technology developed for the Compact Linear Collider (CLIC) and the Future Circular Collider (FCC), and could also provide a step towards a potential electron-positron Higgs factory. The facility could be made operational in about five years and would operate in parallel and without interference with the next run of the LHC, Run 4, write the authors.

The SPS is one of CERN’s longest running accelerators, commissioned in June 1976 at an energy of 400 GeV and serving numerous fixed-target experiments ever since. It was later converted into a proton-antiproton collider which was used to discover the W and Z bosons in 1983. Then, in addition to its fixed-target programme, the SPS became part of the injection chain for LEP, and most recently, has been used to accelerate protons for the LHC.

The changeover time for using the SPS as a proton accelerator to an electron accelerator is estimated to be around ten minutes

Electrons would be injected into the SPS at an energy of 3.5 GeV by a new compact high-gradient linac based on CLIC’s X-band radio-frequency (RF) cavity technology, which would fill the circular machine with 200 ns-duration pulses at a rate of 100 Hz. An additional 800 MHz superconducting RF system, similar to what is needed for FCC-ee, would then accelerate the electron beam from 3.5 GeV to an extraction energy up to 18 GeV. The changeover time for using the SPS as a proton accelerator to an electron accelerator is estimated to be around ten minutes.

Serving experiments

The requirements of the primary electron beam to be delivered by the eSPS were determined by the needs of the proposed Light Dark Matter eXperiment (LDMX), which would use missing-momentum techniques to explore potential couplings between hidden-sector particles and electrons in uncharted regions. The experiment could be housed in a new experimental area (see figure). The beam directly from the linac could also serve two experimental areas for a broad range of accelerator R&D; for example, it could provide multi-GeV drive beam bunches and electron witness bunches for plasma wakefield acceleration.

In a second phase, the facility could be geared to deliver positron witness bunches, which would make it a “complete facility” for plasma wakefield collider studies. Such a programme would naturally build on the work done by the AWAKE collaboration, which uses protons as a drive beam, and significantly broaden plasma wakefield R&D at CERN in line with priorities set out by the recent update of the European strategy for particle physics. Positron production would be a crucial element for any future Higgs-factory, while it would also allow studies of the Low EMittance Muon Accelerator (LEMMA) – a novel scheme for obtaining a low-emittance muon beam for a muon collider, by colliding a high-energy positron beam with electrons in a fixed target configuration at the centre of mass energy required to create muon pairs.

The eSPS proposal came about as a result of work in CERN’s Physics Beyond Colliders study group, and an Expression of Interest that was submitted to the SPS Committee in September 2018.

J/ψ polarisation differs in lead collisions

**Fig. 1.** Inclusive J/ψ polarisation parameters as a function of transverse momentum for Pb–Pb collisions at √(s_NN) = 5.02 TeV. The results are compared to proton–proton collisions observed by LHCb for prompt J/ψ at √s = 7 TeV (*Eur. Phys. J. C* 2013 73 2631). Credit: ALICE

Quarkonia, the bound states of charm and anti-charm or bottom and anti-bottom quarks, are an important tool to test our knowledge of quantum chromodynamics (QCD). At the LHC, the study of quarkonia polarisations offers a valuable new window onto how heavy quarks bind together in such states. Understanding quarkonium polarisation has already proven to be difficult at lower energies, however, and measurements at the LHC pose significant further challenges.

ALICE measures quarkonia spin orientations with respect to a chosen axis via a measurement of the anisotropy in the angular distribution of the decay products. The angular distribution is parametrised in terms of the polarisation parameters, λ_θ, λ_φ and λ_θ_φ, where θ and φ are the polar and azimuthal emission angles. If all of them are null, no polarisation is present, whereas (λ_θ = 1, λ_φ = 0, λ_θ_φ = 0) and (λ_θ = –1, λ_φ = 0, λ_θ_φ = 0) indicate a polarisation of the spin in the transverse and longitudinal directions, respectively.

Polarisation studies represent a valuable tool for the study of the properties of quark–gluon plasma

In pp collisions, polarisation has been mainly used to investigate the J/ψ production mechanism. Reproducing the small values of polarisation parameter λ_θ observed at the LHC is a challenge for many theoretical models. Until recently, no corresponding results were available for nucleus–nucleus collisions, and in this domain polarisation studies represent a valuable tool for the study of the properties of quark–gluon plasma (QGP). The formation of this deconfined, strongly interacting medium impacts differently on the various quarkonium resonances, inducing a larger suppression on the less bound excited states ψ(2S) and χ_c, and modifying their feed-down fractions into the ground state, J/ψ. This effect may lead to a variation of the overall polarisation values since different charmonium states are expected to be produced with different polarisations. In addition, the recombination of uncorrelated heavy-quark pairs inside the QGP gives rise to an extra source of J/ψ, which can further modify the overall polarisation with respect to pp collisions.

The ALICE experiment has recently made the first measurements of the J/ψ and ϒ(1S) polarisation in Pb–Pb collisions. The data correspond to a centre-of-mass energy √(s_NN) = 5.02 TeV, and the rapidity range 2.5 < y < 4. The measurements were carried out in the dimuon decay channel, and results were obtained in two different reference frames, helicity and Collins–Soper, each of them with its own definition of the quantisation axis. In the helicity frame, the quarkonium momentum direction in the laboratory is chosen, while the bisector of the angle formed by the two colliding beams boosted in the quarkonium rest frame is used in the Collins–Soper frame. The J/ψ polarisation parameters, evaluated in three p_T bins covering the range between 2 and 10 GeV, are close to zero, but with a maximum positive deviation for λ_θ (corresponding to a transverse polarisation) of 2σ for 2 < p_T < 4 GeV in the helicity reference frame. Interestingly, the corresponding LHCb pp result for prompt J/ψ at √(s_NN) = 7 TeV instead shows a small but significant longitudinal polarisation.

The observation of a significant difference between J/ψ polarisation results in pp and Pb–Pb collisions motivates further experimental and theoretical studies, with the main goal of connecting this observable with the known suppression and regeneration mechanisms in heavy-ion collisions. For the rarer ϒ(1S), a bound state of a bottom and an antibottom quark, the inclusive polarisation parameters were found to be compatible with zero within sizeable uncertainties. A higher precision and momentum-differential measurement will be enabled by the ten-fold larger Pb–Pb luminosity expected in Run 3 of the LHC.

LHCb explores dark-sector confinement

The possibility that dark-matter particles may interact via an unknown force, felt only feebly by Standard Model (SM) particles, has motivated substantial efforts to search for dark forces. The force-carrying particle for such hypothesised interactions is often referred to as a dark photon, in analogy with the ordinary photon that mediates the electromagnetic interaction.

In the minimal dark-photon scenario, the dark photon does not couple directly to SM particles; however, quantum-mechanical mixing between the photon and dark-photon fields can generate a small interaction, providing a portal through which dark photons may be produced and through which they might decay into visible final states.

Hidden-valley scenarios exhibit confinement in the dark sector, similarly to how the strong nuclear force confines quarks

While the minimal dark-photon model is both compelling and simple, it is not the only viable dark-sector scenario. Many other well-motivated dark-sector models exist, and some of these would have avoided detection in all previous experimental searches. Fully exploring the space of dark sectors is vital given the lack of signals observed thus far in the simplest scenarios. For example, so-called hidden-valley (HV) scenarios exhibit confinement in the dark sector, similarly to how the strong nuclear force confines SM quarks, would produce a high multiplicity of light hidden hadrons from showering processes in a similar way to jet production in the SM. These hidden hadrons would typically decay displaced from the proton–proton collision, thus failing the criteria employed in previous dark-photon searches to suppress backgrounds due to heavy-flavour quarks. Therefore, it is desirable to perform experimental searches for dark sectors that are less model dependent, by not focusing solely on the minimal dark-photon scenario.

Using its Run-2 data sample, LHCb recently performed searches for both short-lived and long-lived exotic bosons that decay into the dimuon final state. These searches explored the invariant mass range from near the dimuon threshold up to 60 GeV. None of the searches found evidence for a signal and exclusion limits were placed on the X → μ⁺μ^– cross sections, each with minimal model dependence.

For many types of dark-sector models, these limits are the most stringent to date. This is especially true for the HV scenario (see figure), for which LHCb has placed the first such constraints on physically relevant HV mixing strengths in this mass range.

These results demonstrate the unique sensitivity of the LHCb experiment to dark sectors. Looking forward to Run 3, the trigger will be upgraded, greatly increasing the efficiency to low-mass dark sectors, and the luminosity will be higher. Taken together, these improvements will further expand LHCb’s world-leading dark-sector programme.

CMS reaffirms exotic nature of the X(3872)

Exotic charmonium-like states are a very active field of study at the LHC. These states have atypical properties such as non-zero electric charges and strong decays that violate isospin symmetry. The exotic X(3872) charmonium state discovered by the Belle collaboration in 2003 displays such isospin-violating strong decays and has a natural width of about 1 MeV, which is unexpectedly narrow for a state with mass very close to the D^*⁰ D⁰ threshold.

Luciano Maiani and collaborators pointed out that the new CMS measurement can naturally be explained by a tetraquark model of X(3872)

Several theoretical interpretations of the internal structure of these charmonium-like states have been proposed to explain their peculiar properties. To choose the most adequate model for each state, we must continue studying their properties and improving the determination of their parameters. As for the X(3872), although it is inconsistent with the predicted conventional charmonium states and does not have a definite isospin, its production partially resembles that of ordinary charmonium states such as ψ(2S) or χ_c1(1P). One of the ways to evaluate the degree of similarity between X(3872) and ψ(2S) is to compare their production rates in exclusive b-hadron decays. In the case of ψ(2S), which is a conventional charmonium state, the branching fractions of the decays B⁰_s → ψ(2S)φ, B⁺ → ψ(2S)K⁺, and B⁰ → ψ(2S)K⁰, are approximately equal to each other. Recent CMS measurements of the corresponding rates for decays to X(3872) show differences, however, which may provide a clue to the nature of this exotic charmonium-like state.

Recently the CMS collaboration observed the decay B⁰_s → X(3872)φ for the first time, with a significance exceeding five standard deviations. The X(3872) is reconstructed via its decay to J/ψπ⁺π^–, followed by a decay of the J/ψ meson into a pair of muons, and of the φ meson to a pair of charged kaons (figure 1).

Diquark hypothesis

At a simple Feynman-diagram level, this decay is a close analogue to the B⁺ → X(3872)K⁺ and B⁰ → X(3872)K⁰ decays that have previously been observed. The ratio of the branching fractions of this new B⁰_s decay to that of the B⁺ decay is significantly below unity at 0.48 ± 0.10, while a similar ratio for the decays involving ψ(2S) is consistent with unity. This is not expected from naive “spectator-quark” considerations, based on a simple tree-level diagram, and assuming X(3872) is a pure charmonium state. The measured ratio also happens to be consistent with the analogous ratio for the B⁰ → X(3872)K⁰ to B⁺ → X(3872)K⁺ decays, though the latter ratio has not yet been measured with high accuracy. The results suggest that spectator quarks behave differently in the B⁺ and B⁰₍_s) two-body decays into X(3872) and a light meson. In a recent theoretical paper, former CERN Director-General Luciano Maiani and collaborators pointed out that the new CMS measurement can naturally be explained by a tetraquark model of X(3872), which describes this exotic particle as a bound state of a diquark (charm and up quarks) and its anti-diquark.

Further studies of X(3872) are now important in order to gain a deeper understanding of its exotic properties and uncover its mysterious nature. The results may have interesting consequences for our understanding of quantum chromodynamics.

Topics

Reset your password

Registration complete

Diquark hypothesis