Jobs – Page 114 of 521

50 years of the GIM mechanism

GIM originators 50 years on — Hong-Jian He, John Ellis, John Iliopoulos, Sheldon Lee Glashow, Verónica Riquer and Luciano Maiani at a celebration of 50 years of the GIM mechanism in Shanghai. Credit: J Liu

In 1969 many weak amplitudes could be accurately calculated with a model of just three quarks, and Fermi’s constant and the Cabibbo angle to couple them. One exception was the remarkable suppression of strangeness-changing neutral currents. John Iliopoulos, Sheldon Lee Glashow and Luciano Maiani boldly solved the mystery using loop diagrams featuring the recently hypothesised charm quark, making its existence a solid prediction in the process. To celebrate the fiftieth anniversary of their insight, the trio were guests of honour at an international symposium at the T. D. Lee Institute at Shanghai Jiao Tong University on 29 October, 2019.

The UV cutoff needed in the three-quark theory became an estimate of the mass of the fourth quark

The Glashow-Iliopoulos-Maiani (GIM) mechanism was conceived in 1969, submitted to Physical Review D on 5 March 1970, and published on 1 October of that year, after several developments had defined a conceptual framework for electroweak unification. These included Yang-Mills theory, the universal V−A weak interaction, Schwinger’s suggestion of electroweak unification, Glashow’s definition of the electroweak group SU(2)_L×U(1)_Y, Cabibbo’s theory of semileptonic hadron decays and the formulation of the leptonic electroweak gauge theory by Weinberg and Salam, with spontaneous symmetry breaking induced by the vacuum expectation value of new scalar fields. The GIM mechanism then called for a fourth quark, charm, in addition to the three introduced by Gell-Mann, such that the first two blocks of the electroweak theory are made each by one lepton and one quark doublet, [(ν_e, e), (u, d)] and [(ν_µ, µ), (c, s)]. Quarks u and c are coupled by the weak interaction to two superpositions of the quarks d and s: u ↔ d_C , with d_C the Cabibbo combination d_C = cos θ_C d + sin θ_C s, and c ↔ s_C , with s_C the orthogonal combination. In subsequent years, a third generation, [(ν_τ, τ ), (t, b)] was predicted to describe CP violation. No further generations have been observed yet.

Problem solved

The GIM mechanism was the solution to a problem arising in the simplest weak interaction theory with one charged vector boson coupled to the Cabibbo currents. As pointed out in 1968, strangeness-changing neutral-current processes, such as K_L → µ⁺µ⁻ and K⁰ – K⁰ mixing, are generated at one loop with amplitudes of order G sinθ_C cosθ_C (GΛ²), where G is the Fermi constant, Λ is an ultraviolet cutoff, and GΛ² (dimensionless) is the first term in a perturbative expansion which could be continued to take higher order diagrams into account. To comply with the strict limits existing at the time, one had to require a surprisingly small value of the cutoff, Λ, of 2 − 3 GeV, to be compared with the naturally expected value: Λ = G^-1/2 ~ 300 GeV. This problem was taken seriously by the GIM authors, who wrote that “it appears necessary to depart from the original phenomenological model of weak interactions”.

GIM mechanism Feynman diagrams — One-loop quark diagrams for K⁰ – K⁰ mixing in the light of the GIM mechanism. The charm-quark amplitudes have the same magnitude but opposite sign as for up-quark lines, leading to a perfect cancellation, cos θ sin θ + (- sin θ) cos θ = 0, in the case where m_c = m_u and suggesting an explanation for the suppression of processes with strangeness-changing neutral currents.

To sidestep this problem, Glashow, Iliopoulos and Maiani brought in the fourth “charm” quark, already introduced by Bjorken, Glashow and others, with its typical coupling to the quark combination left alone in the Cabibbo theory: c ↔ s_C = − sinθ_C d + cosθ_C s. Amplitudes for s → d with u or c on the same fermion line would cancel exactly for m_c= m_u, suggesting a more natural means to suppress strangeness-changing neutral-current processes to measured levels. For m_c>> m_u, a residual neutral-current effect would remain, which, by inspection, and for dimensional reasons, is of order G sinθ_C cos θ_C (Gm_c²). This was a real surprise: the “small” UV cutoff needed in the simple three-quark theory became an estimate of the mass of the fourth quark, which was indeed sufficiently large to have escaped detection in the unsuccessful searches for charmed mesons that had been conducted in 1960s. With the two quark doublets included, a detailed study of strangeness changing neutral current processes gave m_c ∼ 1.5 GeV, a value consistent with more recent data on the masses of charmed mesons and baryons. Another aspect of the GIM cancellation is that the weak charged currents make an SU(2) algebra together with a neutral component that has no strangeness changing terms. Thus, there is no difficulty to include the two quark doublets in the unified electroweak group SU(2)_L×U(1)_Y of Glashow, Weinberg and Salam. The 1970 GIM paper noted that “in contradistinction to the conventional (three-quark) model, the couplings of the neutral intermediary – now hypercharge conserving – cause no embarrassment.”

The GIM mechanism has become a cornerstone of the Standard Model and it gives a precise description of the observed flavour changing neutral current processes for s and b quarks. For this reason, flavour-changing neutral currents are still an important benchmark and give strong constraints on theories that go beyond the Standard Model in the TeV region.

Astroparticle physicists head down under

Yvonne Wong at TeVPA 2019 — Yvonne Wong (UNSW) tells TeVPA delegates how cosmological data can be used as a test of neutrino physics – and vice versa. Credit: S Hickey

Despite the thick haze of bushfire smoke hanging over the skyline, 200 delegates gathered in Sydney from 2 to 6 December for the 14th edition of the TeV Particle-Astrophysics conference (TeVPA), to discuss the status and future of astroparticle physics.

The week began with a varied series of talks on dark matter. Luca Grandi (Chicago) and Tom Thorpe (LNGS) updated delegates on progress towards the next generation of xenon and argon-based experiments: these massive underground detectors are now approaching total masses in the multiton-scale. Experiments like XENON, LZ and DarkSide are poised to be so sensitive to rare signals that they will even able to detect coherent elastic neutrino-nucleus scattering – the ultimate background to direct dark-matter searches. Meanwhile, Greg Lane (Australian National University) brought us news of exciting developments in Australian dark-matter research. The Stawell Underground Laboratory—the first deep underground site in the southern hemisphere—will host part of the SABRE experiment, which aims to test the annually modulating event rate seen by the DAMA experiment. This highly controversial, dark-matter-like signal has been observed for two decades by DAMA, but remains in irreconcilable tension with null results from many other experiments. Excavation at Stawell is underway as of October last year. The site will form a central component of the Centre of Excellence for Dark Matter Particle Physics, recently awarded by the Australian Research Council.

Galaxies can be used as laboratories for particle physics

Eminent astrophysicist Joe Silk (IAP) reviewed the many ways in which galaxies can be used as laboratories for particle physics. One of the most persistent hints of dark-matter particle interactions in astrophysical data is the notorious excess of GeV gamma rays coming from the galactic centre. Recent analyses of the excess using improved statistical techniques and better models for the Milky Way’s central bulge were detailed by Shunsaku Horiuchi (Virginia Tech). While dark-matter-related explanations remain tempting, there is growing evidence in support of millisecond pulsars being responsible, given the spatial morphology of the excess. Francesca Calore (LAPTh) told us that multi-wavelength probes of the excess will be possible in the near-future, and may finally allow us to conclusively determine the origin of the signal.

Probing the cosmos

Delegates enjoyed a stirring series of talks on the ever-increasing number of probes of cosmology. Following a review of the post-Planck status of cosmology by Jan Hamaan (UNSW), Xuelei Chen (CAS) explained how the unique 21 cm radio line can be used to map neutral hydrogen throughout the universe and across cosmic time. A host of upcoming ground and space-based experiments attempting to observe the sky-averaged 21 cm line will hopefully allow us to peer back to the birth of the first stars at “cosmic dawn”. We also heard from Yvonne Wong (UNSW) about how cosmological data can be used as a test of neutrino physics and how neutrino physics may in turn be a means to alleviate tensions between cosmological datasets. For example, strong self-interactions between neutrinos could bring the two increasingly divergent measurements of the Hubble constant, from the cosmic microwave background and type-1a supernovae respectively, into agreement.

The 21 cm radio line can be used to map neutral hydrogen throughout the universe and across cosmic time

Much of the week’s schedule was devoted to cosmic-ray research, gamma rays and indirect searches for dark matter. The antimatter cosmic-ray detector AMS, mounted on the International Space Station, is making measurements of cosmic-ray spectra to within 1% accuracy. Weiwei Xu (Shandong) summarised an impressive array of physics results made over almost a decade by AMS, including the most recent measurement of the positron flux, which has a clear high-energy component with a well-defined cutoff at 810 GeV – just as expected for galactic dark-matter annihilations. As with the GeV gamma-ray excess, however, pulsars represent a possible natural astrophysical explanation. The mystery could be resolved by the fact that, unlike pulsars, dark-matter annihilations are expected to produce antiprotons. While current antiproton data show a tantalisingly similar trend to the positron spectrum, more data is needed to identify the origin of the high-energy positrons. Many ongoing and upcoming observatories in the fields of cosmic-ray and gamma-ray research were also introduced to us, such as DAMPE (Jingjing Zang, CAS), the Cherenkov Telescope Array (Roberta Zanin, CTAO), the Pierre Auger Observatory (Bruce Dawson, U. Adelaide) and LHAASO (Zhen Cao, CAS). We are entering an exciting time when many of the enticing but ambiguous anomalies in cosmic-ray spectra will be definitively tested, potentially identifying a signal of dark matter in the process.

Gamma ray bursts (GRBs) generated much enthusiasm this year, with Edna Ruiz-Velasco (MPIK) and Elena Moretti (IFAF) talking about brand new observations of GRBs from the H.E.S.S. and MAGIC collaborations, including the first detection of a GRB afterglow at very high energies (>100 GeV), by H.E.S.S. These observations have helped resolve long-standing mysteries surrounding the complex array of processes that are needed to produce the phenomenal energies of GRB emission. An important contribution is now known to be “synchrotron self-Compton” – emission in which a synchrotron photon generated from an electron spiralling around a magnetic field line is Compton up-scattered by the same electron that produced it.

Many well-motivated theories of modified gravity are now finding little room to hide

Finally, the subject of gravitational waves continues to surge in popularity within this community. We were first given a summary by Susan Scott (Australian National University) of over 50 confirmed gravitational-wave discoveries made by Advanced LIGO and Advanced Virgo to date, and from Tara Murphy (Sydney), about the intense work involved in rapidly following-up luminous gravitational-wave events with radio observations. LIGO’s discoveries of neutron-star and black-hole mergers are a window into the one of the strongest regimes of gravity we have ever been able to see. With general relativity still holding up as robustly as ever, many well-motivated theories of modified gravity are now finding little room to hide.

The next TeVPA will take place in late October 2020 in Chengdu, China.

Linacs pushed to the limit in Chamonix

This past June in Chamonix, CERN hosted the 12^th edition of an international workshop dedicated to the development and application of high-gradient and high-frequency linac technology. These technologies are making accelerators more compact, less expensive and more efficient, and broadening their range of applications. The workshop brought together over seventy 70 and engineers involved in a wide range of accelerator applications, with common interest in the use and development of normal-conducting radio-frequency cavities with very high accelerating gradients ranging from around 50 MV/m to above 100 MV/m.

Applications for high-performance linacs such as these include the Compact Linear Collider (CLIC), compact XFELs and inverse-Compton-scattering photon sources, medical accelerators, and specialised devices such as radio-frequency quadrupoles, transverse deflectors and energy-spread linearisers. In recent years the latter two devices have become essential to achieving low emittances and short bunch lengths in high-performance electron linacs of many types, including superconducting linacs. In the coming years, developments from the high-gradient community will be increasing the energy of beams in existing facilities through retrofit programs, for example in an energy upgrade of the FERMI free-electron laser. In the medium term, a number of new high-gradient linacs are being proposed, such as the room-scale X-ray-source SMART*LIGHT, the linac for the advanced accelerator concept research accelerator EUPRAXIA, and a linac to inject electrons into CERN’s Super Proton Synchrotron for a dark-matter search. The workshop also covered fundamental studies of the very complex physical effects that limit the achievable high gradients, such as vacuum arcing, which is one of the main limitations for future technological advances.

Vacuum arcing is one of the main limitations for future technological advances

Originated by the CLIC study, the focus of the workshop series has grown to encompass high-gradient radio-frequency design, precision manufacture, assembly, power sources, high-power operation and prototype testing. It is also notable for having a strong industrial participation, and plays an important role in broadening the applications of linac technology by highlighting upcoming hardware to companies. The next workshop in the series will be hosted jointly by SLAC and Los Alamos and take place on the shore of Lake Tahoe from 8 to 12 June.

Space–time symmetries scrutinised in Indiana

**Back to Bloomington** The eighth CPT and Lorentz Symmetry meeting took place at Indiana University. Credit: Y Ding

The space–time symmetries of physics demand that experiments yield identical results under continuous Lorentz transformations – rotations and boosts – and under the discrete CPT transformation (the combination of charge conjugation, parity inversion and time reversal). The Standard-Model Extension (SME) provides a framework for testing these symmetries by including all operators that break them in an effective field theory. The first CPT and Lorentz Symmetry meeting, in Bloomington, Indiana, in 1998, featured the first limits on SME coefficients. Last year’s event, the 8th in the triennial series, brought 100 researchers together from 12 to 16 May 2019 at the Indiana University Center for Spacetime Symmetries, to sample a smorgasbord of ongoing SME studies.

Most physics is described by operators of mass dimension three or four that are quadratic in the conventional fields – for example the Dirac lagrangian contains an operator ψ ∂̸ ψ (mass dimension 3/2 + 1 + 3/2 = 4) and an operator ψψ (mass dimension 3/2 + 3/2 = 3), with the latter controlled by an additional mass coefficient – however, the search for fundamental symmetry violations may need to employ operators of higher mass dimensions and higher order in the fields. One example is the Lorentz-breaking lagrangian-density term (k_VV)^μν(ψγ_μψ) (ψγ_νψ), which is quartic in the fermion field ψ. The coefficient k_VV carries units of GeV^–2 and controls the operator, which has mass dimension six. Searches for Lorentz-symmetry breaking seek nonzero values for coefficients like k_VV. In the 21 years since the first CPT meeting, theoretical studies have uncovered how to write down the myriad operators that describe hypothetical Lorentz violations in both flat and curved space–times. Meanwhile, experiments in particle physics, atomic physics, astrophysics and gravitational physics continue to place exquisitely tight bounds on the SME coefficients, motivated by the intriguing prospect of finding a crack in the Lorentz symmetry of nature.

The SME has revealed uncharted territory that requires theoretical and experimental expertise to navigate

Comparisons between matter and antimatter offer rich prospects for testing Lorentz symmetry, because individual SME coefficients can be isolated. The AEgIS, ALPHA, ASACUSA, ATRAP, BASE and gBAR collaborations at CERN, as well as ones at other institutions, are working to develop the challenging technology for such tests. Several presenters discussed Penning traps – devices that confine charged particles in a static electromagnetic field – for storing and mixing the ingredients for antihydrogen, the production of antihydrogen, spectroscopy for the hyperfine and 1S–2S transitions, and the prospects for interferometric measurements of antimatter acceleration. The commissioning of ELENA, CERN’s 30 m-circumference antiproton deceleration ring, promises larger quantities of relatively slow-moving antiprotons in support of this work.

Lorentz violation can occur independently in each sector of the particle world, and participants discussed existing and future limits on SME coefficients based on the muon g-2 experiment at Fermilab, neutrino oscillations at Daya Bay in China, kaon oscillations in Frascati, and on positronium decay using the Jagellonian PET detector, to name a few. Dozens of Lorentz-symmetry tests have probed the photon sector of the SME with table-top devices such as atomic clocks and resonant cavities, and with astrophysical polarisation measurements of sources such as active galactic nuclei, which leverage vast distances to limit cumulative effects such as the rotation of a polarisation angle. In the gravity sector, SME coefficient bounds were presented from the 2015 gravitational-wave detection by the LIGO collaboration, as well as from observations of pulsars, cosmic rays and other phenomena with signals that are proportional to the travel distance. Symmetry-breaking signals are also sought in matter-gravity interactions with test masses, and here CPT’19 included discussions of short-range spin-dependent gravity and neutron-interferometry physics.

The SME has revealed uncharted territory that requires theoretical and experimental expertise to navigate. CPT’19 showed that there is no shortage of physicists with the adventurous spirit to explore this frontier further.

Hyper-active neutrino physicists visit London

The sixth edition of Prospects in Neutrino Physics (NuPhys19) attracted almost 100 participants to the Cavendish Conference Centre in London from 16 to 18 December. Jointly organised by King’s College London and the Institute for Particle Physics Phenomenology at Durham University, the conference provides a much-needed snapshot of the fast-moving field of neutrino physics.

The neutrino community’s current challenge is to understand the origin of neutrino masses and lepton mixing. This means establishing whether neutrinos are Dirac or Majorana fermions, their absolute mass scale, the order of the measured mass splittings (the neutrino mass ordering), whether there is leptonic CP violation, the precise value of other parameters in the neutrino mixing matrix, and, finally, whether there is an indication of physics beyond the standard three-neutrino paradigm, for example through the detection of sterile neutrinos.

Construction of the Hyper-Kamiokande experiment will begin in 2020

2015 Nobel laureate Takaaki Kajita (University of Tokyo) opened the conference by confirming that construction of the Hyper-Kamiokande experiment will begin in 2020, following the allocation by the Japanese government of a supplementary budget on 13 December. Hyper-Kamiokande will be a water-Cherenkov detector with a total mass of 260 kton — almost an order of magnitude larger than its famous predecessor Super-Kamiokande, where atmospheric neutrino oscillations were discovered, and far larger than KamiokaNDE, which observed solar neutrinos and supernova SN1987A. Hyper-Kamiokande will eventually replace Super-Kamiokande as the far detector for the upgraded J-PARC neutrino beam, which is situated on the far side of Japan (essentially a comprehensive upgrade of the T2K experiment), with the aim of measuring CP violation in the leptonic sector. It will also provide high statistics for proton-decay searches, supernova neutrino bursts, atmospheric and solar neutrinos, and indirect searches for dark matter. Hyper-Kamiokande will therefore soon join DUNE in the US as a next-generation long-baseline neutrino-oscillation experiment under construction. Together the detectors will provide a far wider coverage of physics signals than either could manage alone.

Critical mass

News of KATRIN’s record-breaking new upper limit on the electron-antineutrino mass was complemented by a report by Joseph Formaggio (MIT) on the successful “Project 8” demonstration in the US of a new approach to directly measuring neutrino masses wherein the energies of beta-decay electrons are determined from the frequency of cyclotron radiation as the electrons spiral in a magnetic field. This work will be complemented by the JUNO experiment in China which will in 2021 begin to constrain the ordering of the neutrino-mass eigenvalues.

The search for neutrinoless double-beta decay also has the potential to provide information on neutrino masses. A potentially unambiguous indication of lepton-number violation and the postulated Majorana nature of neutrinos, it is being pursued aggressively as experiments compete to reduce backgrounds and increase detector masses to the ton-scale. Several talks emphasised the complementary progress by the theory community to better estimate nuclear effects, and reduce the errors arising from the differences between different nuclear models and different isotopes. These calculations are equally important for NOvA and T2K, which is now beginning to probe leptonic CP conservation at the 3? level.

The cosmological upper limit on the sum of neutrino masses could be relaxed upwards

Current and future cosmological constraints of neutrino properties were reviewed by Eleonora Di Valentino (Manchester), whose recent work with Alessandro Melchiorri and Joe Silk reinterprets Planck-satellite data to favour a closed universe at more than 99% significance – an inference which could lead to the current cosmological upper limit on the sum of neutrino masses being relaxed upwards if it is accepted by the community. Conversely, astrophysical neutrinos are also powerful tools for studying astrophysical objects. One key development in this field is the doping of Super-Kamiokande with gadolinium, currently underway in Japan. This will soon give the detector sensitivity to the diffuse supernova-neutrino background.

The next edition of NuPhys will take place in London from 16 to 18 December 2020.

Crisis for cosmology?

Planck data on the cosmic microwave background (CMB) have been reinterpreted to favour a closed universe at more than 99% confidence, in contradiction with the flat universe favoured by the established ΛCDM model of cosmology. In their new fit to Planck’s 2018 data release, Eleonora Di Valentino (Manchester), Alessandro Melchiorri (La Sapienza) and Joe Silk (Oxford) exchanged an anomalously large lensing amplitude (a phenomenological parameter that rescales the gravitational-lensing potential in the CMB power spectrum) for a higher energy density.

In addition to the lensing anomaly, which leads to inconsistencies between large and small scales, the flat interpretation is already plagued by a 4.4σ tension with the latest determination of the Hubble constant using observations of the recession of Cepheid stars – a tension that grows to 5.4σ in a closed universe.

The inconsistencies between data sets signal “a possible crisis for cosmology”, argue the authors.

CLIC most flexible option for Europe, study leaders contend

Simulated production of a top-quark pair at a collision energy of 3 TeV at the proposed Compact Linear Collider. Credit: CLIC.

The proposed Compact Linear Collider (CLIC) offers the most flexible option for European particle physics in the post-LHC era, write the leaders of the CLIC study in a preprint posted on arXiv on 15 January. Responding to a preprint by 53 authors in late December which backed a Future Circular Collider (FCC) over CLIC, the CLIC team argues that moving forward quickly with a linear collider “would allow a vibrant high-energy frontier programme to be maintained over the coming decades, while pursuing in parallel the accelerator R&D required to open future options”.

Acknowledging the widespread consensus that the next major collider should be an electron–positron collider to explore the Higgs sector in detail, the authors argue that the discussion of what will be the most appropriate high-energy frontier machine afterwards “must be kept open” such that it can be guided by new physics results and new technology. The three-page long note states that an initial CLIC programme undertaken in parallel with strong accelerator R&D and HL-LHC, followed by the best possible high-energy frontier machine when technologies are mature, “thus provides the most flexible and appealing strategic option” for collider physics in Europe.

Although FCC-ee is unique in offering a very high-statistics Z physics programme, state the CLIC authors, the potential for Higgs-boson studies with a first-stage 380 GeV CLIC or 365 GeV FCC-ee is similar when assuming equivalent running times. They also say that both machines have a similar performance at the top-quark energy and the same accelerator performance risk. “Previous limits of both circular and linear electron–positron colliders have been understood and overcome thanks to vast efforts in hardware developments and large-scale system tests across the planet,” says coauthor Daniel Schulte of CERN. “Both colliders have ambitious parameters, but we are confident that they can be achieved, as confirmed in detailed reviews of both projects.”

Ultimately we all want what is best for our science
Aidan Robson

CLIC studies during the past few years have focused on energy consumption and construction costs, which CLIC project leader Steinar Stapnes of CERN says are now “very favourable” compared with FCC-ee. “Owing to CLIC’s compactness the construction is relatively fast, and we have also deliberately kept the 380 GeV baseline operation time relatively short at eight years,” he says. “We feel strongly that the possibilities for the subsequent step – whether a linear collider energy extension, or a proton or muon collider option – need to be kept on timescales that are not too far away.”

Priorities for European particle physics are under discussion this week at a meeting in Bad Honnef, Germany, as the update of the European strategy for particle physics enters its final stages.

“The European strategy is being developed in a complex environment where particle physics projects continue to become larger and longer-scale,” says Aidan Robson of the University of Glasgow, who is spokesperson of the CLIC detector & physics collaboration. “Ultimately we all want what is best for our science. CLIC at 380 GeV offers a rapid and exciting e⁺e^– programme, and opens doors for R&D for several possible future colliders going much higher in energy. This provides the key elements that offer attractive and challenging opportunities for the young people who will drive the future of our field.”

Strategy drafting under way in Bad Honnef

Today, senior figures in European particle physics have gathered in the small town of Bad Honnef, Germany, for a week of intense discussions that will guide the future of fundamental exploration. The “strategy drafting session” marks the final stage of the update of the European strategy for particle physics. Convened by the European Strategy Group (ESG) — which includes a scientific delegate from each of CERN’s member and associate-member states, directors and representatives of major European laboratories and organisations and invitees from outside Europe – the 60 or so attendees are tasked with identifying a set of priorities and recommendations to the CERN Council.

The ESG, a special body set up by the CERN Council approximately every five years, was invited to formulate an update of the European strategy for particle physics in September 2017. A call for input in 2018 attracted 160 submissions, which were discussed at an open symposium in Granada, Spain, in May 2019. The ESG then published a 200-page briefing book which distilled the input into an objective scientific summary and will form the basis for discussions in Germany this week.

The start of a new project in the early 2040s is crucial to keep the community motivated and engaged
Fabiola Gianotti

The focus of the latest strategy update, the third since 2005, is which major project should follow the LHC once its high-luminosity phase comes to an end in the late 2030s. There is broad support for an electron—positron collider that will explore the Higgs sector in detail, as well as for a high-energy proton–proton collider at CERN. In Europe, the possible options are the Compact Linear Collider and the Future Circular Collider, while an International Linear Collider (ILC) in Japan and a large Circular Electron-Positron Collider in China are also contenders. The strategy update will also consider non-collider experiments, computing, instrumentation and other key aspects of growing importance to the field such as energy efficiency and communication.

The previous strategy update, which concluded in 2013, made several high-priority recommendations: the full exploitation of the LHC, including the high-luminosity upgrade of the machine and detectors; R&D and design studies for a future energy-frontier machine at CERN; establishing a neutrino programme at CERN for physicists to develop detectors for experiments at accelerator-based neutrino facilities around the world; and the welcoming of a proposal from Japan to discuss the possible participation of Europe in the ILC. The first three are well under way, while a decision on the ILC still rests with the Japanese government. Other conclusions of the 2013 update included the need for closer collaboration with the astroparticle and nuclear physics communities, which has been met for example via the recently launched centre for astroparticle physics theory (EuCAPT) and the new Joint ECFA-NuPECC-APPEC Seminar series, JENAS. There was also a call for greater scientific diversity, leading to the CERN-led Physics Beyond Colliders initiative, which will also form a central part of this week’s discussions.

The recommendations from the ESG are due to formally be approved by the CERN Council on 25 May at an event in Budapest, Hungary.

During her annual address to personnel on 14 January, CERN Director-General Fabiola Gianotti acknowledged the enormous efforts that have gone into the strategy update, and said that she hoped that a recommendation on CERN’s next major collider would be among the ESG’s priorities.

“The start of a new project in the early 2040s is crucial to keep the community motivated and engaged,” said Gianotti, noting that CERN and Europe should also be open to participate in projects at the forefront of particle physics elsewhere in the world. “The Higgs boson is a guaranteed deliverable. It is related to the most obscure and problematic sector of the Standard Model and carries special quantum numbers and a new type of interaction. It is therefore a unique door into new physics, and one that can only be studied at colliders.”

Brookhaven to host Electron-Ion Collider

RHIC tunnel — The US Department of Energy has selected a plan whereby accelerator infrastructure at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory (pictured) will be reconfigured to boast a new electron storage ring in addition to existing infrastructure for the acceleration of heavy ions. Credit: BNL

Brookhaven National Laboratory (BNL) on Long Island, New York, has been selected as the site for the planned Electron-Ion Collider (EIC). The decision, announced by the US Department of Energy (DOE) on 9 January, will see the laboratory’s Relativistic Heavy-Ion Collider (RHIC) reconfigured to include a new electron storage ring to facilitate electron-ion collisions. Scheduled to enter operation at the end of the decade, the new electron-ion collider will pivot BNL’s physics focus from the study of the quark-gluon plasma to nuclear femtography.

BNL has edged out competition to host the EIC from the Thomas Jefferson National Accelerator Facility (JLab) in Virginia, which boasts the recently upgraded Continuous Electron-Beam Accelerator Facility (CEBAF). Under the JLab proposal, CEBAF would have been augmented with a new heavy-ion accelerator. JLab is now expected to be a major partner in the project and take the lead in aspects of accelerator R&D. The project is foreseen to cost between $1.6bn and $2.6bn, with first physics planned in 2029 or 2030, following the completion of RHIC’s science programme at the STAR and newly upgraded sPHENIX experiments. “Our plan, working with the DOE Office of Nuclear Physics, remains unchanged,” says BNL’s associate laboratory director for nuclear and particle physics Berndt Mueller: “to complete the RHIC science mission by bringing sPHENIX into operation for three years of data taking.”

Nuclear femtography

EIC will perform precision “nuclear femtography” by zeroing in on the substructure of quarks and gluons in heavy ions using collisions with high-energy electrons, in a comparable manner to the seminal studies of the proton using electron-proton collisions at DESY’s HERA accelerator between 1992 and 2007. While HERA ran at a centre-of-mass energy of 318 GeV, the EIC will operate from 20 to 140 GeV. “The upper centre-of-mass energy limit is chosen to be sufficient for access to the predicted gluon saturation regime in electron-heavy nucleus collisions,” says Mueller, referring to the state known as a colour-glass condensate, a nonlinear regime of quantum chromodynamics where the rate of gluon recombination rivals that at which gluons are radiated. “The lower centre-of-mass energy limit is optimised for the three-dimensional imaging of quark and gluon distributions in the proton and other nuclei, which will utilise the much higher luminosity projected for the EIC compared to HERA,” continues Mueller. “If required by the evolving physics programme, the energy range of the BNL EIC could be raised in the future, for example by increasing the strength of the magnets in the hadron ring.”

The lower energy limit is optimised for the three-dimensional imaging of quark and gluon distributions
Berndt Mueller

The selection of BNL allows work to begin on EIC’s conceptual design, but is not a final approval, with the project still required to clear several hurdles relating to its design, cost and construction schedule. Meanwhile, a complementary project, the Electron-Ion Collider of China (EicC), which primarily targets sea quarks rather than gluons, is also moving forward, though on a longer timescale. The EicC garnered publicity in December with news that design work will proceed with a view to beginning construction at a new campus in Huizhou, in Guangdong province in southern China. First physics is foreseen towards the end of the next decade.

“This brings to conclusion the hard work over the last 20 years to make the case for an EIC, and gives for the community the signal to start finalising the design and construct the EIC over the coming years,” says BNL’s Elke-Caroline Aschenauer. “To finally have the opportunity to image quarks and gluons, and their interactions, and to explore the new QCD frontier of strong colour fields in nuclei – to understand how matter at its most fundamental level is made – is the best new year present one can imagine.”

Croatia becomes an associate member of CERN

Vesna Batistic Kos and Fabiola Gianotti — **Strong partners** Vesna Batistic Kos, ambassador of Croatia to the United Nations in Geneva (left), and CERN Director-General Fabiola Gianotti exchanged signed agreements on 10 October. Credit: J Ordan/CERN

On 10 October CERN welcomed the Republic of Croatia as an Associate Member State, following receipt of official notification that Croatia has completed its internal approval procedures in respect of an agreement signed on 28 February.

“It is a great pleasure to welcome Croatia into the CERN family as an associate member. Croatian scientists have made important contributions to a large variety of experiments at CERN for almost four decades, and as an associate member, new opportunities open up for Croatia in scientific collaboration, technological development, education and training,” said CERN Director-General Fabiola Gianotti.

Researchers from Croatia have contributed to many experiments at CERN, and a cooperation agreement concluded in 2001 increased the country’s participation in CERN’s research and educational programmes. As an Associate Member State, Croatia will be represented at the CERN Council and be entitled to attend meetings of the finance committee and the scientific policy committee. Nationals of Croatia will be eligible to apply for limited-duration positions as staff members and fellows, while firms offering goods and services originating from Croatia will be entitled to bid for CERN contracts, creating opportunities for industrial collaboration in advanced technologies.

Croatia joins India, Lithuania, Pakistan, Turkey and Ukraine as Associate Member States, while Cyprus and Slovenia are Associate Member States in the pre-stage to membership.

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