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Particle physics meets astrophysics and gravity

The 7th International Conference on New Frontiers in Physics (ICNFP 2018) took place on 4–12 July in Kolymbari, Crete, Greece, bringing together about 250 participants.

The opening talk was given by Slava Mukhanov and was dedicated to Stephen Hawking. To mention some of the five special sessions featured, the memorial session of Lev Lipatov, a leading figure worldwide in the high-energy behaviour of quantum field theory (see CERN Courier January/February 2018 p50), the session on quantum chromodynamics and the round table on the future of fundamental physics chaired by Albert de Roeck, saw a high number of attendees.

Alongside the main conference sessions, there were 10 workshops. Among these, the one on heavy neutral leptons highlighted novel mechanisms for producing sterile-neutrino dark matter and prospects for future searches of such dark matter with the next generation of space-based X-ray telescopes, including Spektr-RG, Hitomi and Athena+.

The workshop on instrumentation and methods in high-energy physics focused on the latest developments and the performance of complex detector systems, including triggering, data acquisition and signal-control systems, with an emphasis on large-scale facilities in nuclear physics, particle physics and astrophysics. This programme attracted many participants and led to the exchange of scientific information between different physics communities.

The workshop on new physics paradigms after the Higgs-boson and gravitational-wave discoveries provided an opportunity both to review results from searches for gravitational waves and to show plans for future precision measurements of Standard Model parameters at the LHC.

The workshop also featured several theory talks covering a wide range of subjects, including the implementation of supersymmetry breaking in string theory, new developments in early-universe cosmology and beyond-Standard Model physics. ICNFP 2018 also saw the first workshop on frontiers in gravitation, astrophysics and cosmology, which strengthened the Asian presence at ICNFP, gathering many participants from the Asia Pacific region.

For the second time in the ICNFP series, a workshop on quantum information and quantum foundations took place, with the aim of promoting discussions and collaborations between theorists and experimentalists working on these topics.

Yakir Aharonov gave a keynote lecture on novel conceptual and practical applications of so-called weak values and weak measurements, showing that they lead to many interesting hitherto-unnoticed phenomena. The latter include, for instance, a “separation” of a particle from its physical variables (such as its spin), emergent correlations between remote parties defying fundamental classical concepts, and a completely top-down hierarchical structure in quantum mechanics, which stands in contrast to the concept of reductionism. As exemplified in the talk of Avshalom Elitzur, the latter could be explained using self-cancelling pairs of positive and negative weak values.

Sandu Popescu, Pawel Horodecki, Marek Czachor and Eliahu Cohen presented many new phenomena involving quantum nonlocality in space and time, which open new avenues for extensive research. Ebrahim Karimi discussed various applications of structured quantum waves carrying orbital angular momentum (either photons or massive particles) and also discussed how to manipulate the topology of optical polarisation knots. Onur Hosten emphasised the importance of cold atoms for quantum metrology.

The workshop also featured many excellent talks discussing the intriguing relations between quantum information and condensed-matter physics or quantum optics. Some connections with quantum gravity, based on entanglement, complexity and quantum thermodynamics, were also discussed. Another topic presented was the comparison between the role of spin and polarisation in high-energy physics and quantum optics. In both of these fields, one should consider the total angular momentum, not the spin alone, and helicity is a very helpful concept in both, too.

Future accelerator facilities such as the low-energy heavy-ion accelerator centres FAIR in Darmstadt, Germany, and NICA at the Joint Institute for Nuclear Research in Dubna, Russia, were also discussed, particularly in the workshop on physics at FAIR-NICA-SPS-BES/RHIC accelerator facilities. Here new ideas as well as overview talks on current and future experiments on the formation and exploration of baryon-rich matter in heavy-ion collisions were presented.

The MoEDAL collaboration at CERN, which searches for highly ionising messengers of new physics such as magnetic monopoles, organised a mini-workshop on highly ionising avatars of new physics. The workshop provided a forum for experimentalists and phenomenologists to meet, discuss and expand this discovery frontier. The latest results from the ATLAS, CMS, MoEDAL and IceCube experiments were presented, and some important developments in theory and phenomenology were introduced for the first time. Highlights of the workshop included monopole production via photon fusion at colliders, searches for heavy neutral leptons and other long-lived particles at the LHC, regularised Kalb–Ramond monopoles with finite energy, and monopole detection techniques using solid-state and Timepix detectors.

Finally, on the education and outreach front, Despina Hatzifotiadou gave LHC “masterclasses” in collaboration with EKFE (the laboratory centre for physical sciences) to 30 high-school students and teachers, who had the opportunity to analyse data from the ALICE experiment and “observe” strangeness enhancement in relativistic heavy-ion collisions.

The next ICNFP conference will take place on 21–30 August 2019 in Kolymbari, Crete, Greece.

Hans Paar 1944–2018

Hans Paar, emeritus professor of physics at the University of California, San Diego (UCSD), passed away on 17 June after a short illness. Paar was initially trained at Delft University of Technology in his native country of the Netherlands. This engineering background served him well throughout his career, allowing him to take on important tasks in the design, construction and testing of equipment in all the particle-physics experiments he participated in.

Paar started his particle-physics career at Columbia University in the US, where he worked with Leon Lederman on one of the first experiments at Fermilab (E70). After completing his PhD thesis on this project, he relocated to Europe to work as a CERN fellow with another Nobel Laureate, Jack Steinberger, on WA1, the first experiment with the high-energy neutrino beam of the newly commissioned Super Proton Synchrotron (SPS).

In 1978, Paar joined a team at NIKHEF, the Dutch National Institute for Subatomic Physics, that worked on the TPC/2γ experiment at the SLAC National Accelerator Laboratory in the US, and he quickly became one of the leaders of the collaboration that carried out this experiment. His visibility at SLAC led to an offer from the UCSD, which he then joined as a faculty member in 1986 and where he remained for the rest of his career.

Paar was an internationally recognised physicist. He studied the properties of the bottom quark at electron–positron colliders since the early 1990s, first as a member of the CLEO collaboration (Cornell) and later the BaBar collaboration (SLAC). He also made essential contributions to the design and construction of novel types of calorimeters, in the context of the SPACAL and DREAM projects at CERN.

Later in Paar’s career, his research interests included observational cosmology. Paar and his colleagues set out to detect the B-mode polarisation of the cosmic microwave background radiation to address one of the most fundamental problems in astrophysics – the inflation of the early universe. Paar made crucial contributions to the realisation of this project, named POLARBEAR, which is carried out at high altitude in the Atacama Desert in Chile. Not only did he provide expert leadership, design and analysis skills, he also secured a $600,000 private donation, which helped enable the fabrication of the telescope.

Paar cared deeply about education and creating a nurturing, motivating environment for students. He was instrumental in modernising the UCSD’s curriculum on quantum mechanics at all levels and authored the textbook An Introduction to Advanced Quantum Physics. As part of the UCSD Research Experience for Undergraduates programme, he gave a “Physics of Sailing” course consisting of lectures on the physics of the sport, followed by a full day of sailing on San Diego Bay.

Hans had many interests outside of physics. He was also a devoted husband, (step)father and a very good friend to many. He was a gifted piano player and a serious model-train enthusiast. He helped to create an atmosphere of creative thought and friendliness within every group he was part of. Our thoughts go to his wife Kim, his daughter Suzanne and his stepsons Eric and Alain. We all owe Hans for many happy memories.

Suppression of the Λ(1520) resonance in Pb–Pb collisions

The pT-integrated ratio of Λ(1520)/Λ production as a function of charged-particle multiplicity, also showing predictions from several models.

The ALICE collaboration has recently reported the first measurement of the hadronic resonance Λ(1520) in heavy-ion collisions at the LHC. In such collisions, a deconfined plasma of quarks and gluons called quark–gluon plasma (QGP) is formed, which expands and cools. Eventually, the system undergoes a transition to a dense hadron gas (hadronisation), which further expands until all interactions among hadrons cease. Short-lived hadronic resonances are sensitive probes of the dynamics and properties of the medium formed after hadronisation. Due to their short lifetimes, they decay when the system is still dense and the decay products scatter in the hadron gas, reducing the observed number of decays.

The production yield of the Λ(1520) baryon resonance was measured at mid-rapidity in lead–lead (Pb–Pb) collisions at a centre of mass energy per nucleon–nucleon pair of 2.76 TeV. The resonance is reconstructed in the Λ(1520) → pK⁻ (and its charge-conjugate) hadronic decay channel and its production is measured as a function of the collision centrality. The ratio of the number of
measured Λ(1520) baryons to that of its stable counterpart, Λ, highlights the characteristics of resonance production directly related to the particle lifetime, since possible effects due to valence-quark composition (e.g. strangeness enhancement) cancel in the ratio. A gradual decrease of the Λ(1520)/Λ yield ratio with increasing charged-particle multiplicity is observed from peripheral to central Pb–Pb collisions (see figure).

The result provides the first evidence for Λ(1520) suppression in central heavy-ion collisions compared to peripheral collisions, achieving a 3.1σ confidence level once cancellations of correlated systematics are taken into account. An earlier measurement at lower collision energy by the STAR experiment at Brookhaven’s Relativistic Heavy-Ion Collider showed a similar suppression, but with much larger uncertainties. The ratio of the Λ(1520) resonance yield with respect to non-resonant Λ baryons reduces by about 45% in central collisions compared to peripheral collisions.

The EPOS3 model, which describes the full evolution of a heavy-ion collision and includes re-scattering in the hadronic phase, describes this suppression well, although it systematically overestimates the data. The relative decrease of the Λ(1520) resonance yield is also slightly smaller in the EPOS3 model than observed in the data, suggesting a longer lifetime of the hadronic phase (about 8.5 fm/c in EPOS3), or that the description of the relevant hadronic cross-sections in the transport phase is imprecise. The mean transverse momentum is also shown to increase with increasing charged-particle multiplicity, hence with increasing collision centrality. The EPOS3 model can quantitatively describe this feature. It is noteworthy that the model does not describe the data when the microscopic transport stage responsible for the re-scattering effect inside the hadronic medium (as described by the UrQMD model), is disabled.

In summary, these measurements add further support to the formation of a dense hadronic phase in Pb–Pb collisions, highlighting its relevance and the importance of a microscopic description of the latest stages of the evolution of heavy-ion collisions.

Search for new quarks addresses unnaturalness

Left: the lower limit on the mass of a vector-like top quark as a function of its branching ratio to Wb and Ht. Right: the upper limit on the coupling strength of the vector-like top quark as a function of the particle’s mass.

The Standard Model (SM) is a triumph of modern physics, with unprecedented success in explaining the subatomic world. The Higgs boson, discovered in 2012, was the capstone of this amazing theory, yet this newly known particle raises many questions. For example, interactions between the Higgs boson and the top quark should lead to huge quantum corrections to the Higgs boson mass, possibly as large as the Planck mass (>10¹⁸ GeV). Why, then, is the observed mass only 125 GeV? Finding a solution to this “hierarchy problem” is one of the top motivations of many new theories of particle physics.

A common feature in several of these theories is the existence of vector-like quarks – in particular, a vector-like top quark (T) that could naturally cancel the large quantum corrections caused by the SM top quark. Like other quarks, vector-like quarks are spin-½ particles that interact via the strong force and, like all spin-½ particles, they have left-handed and right-handed versions. The unique feature of vector-like quarks is their ambidexterity: while the weak force only interacts with left-handed SM particles, it would interact the same way with both the right- and left-handed versions of vector-like quarks. This also gives vector-like quarks more options in how they can decay. Unlike the Standard Model top quark, which almost always decays to a bottom quark and W boson (t→Wb), a vector-like top quark could decay three different ways: T→Wb, T→Zt, or T→Ht.

The search for vector-like quarks in ATLAS spans a wide range of dedicated analyses, each focusing on a particular experimental signature (possibly involving leptons, boosted objects or large missing transverse energy). The breadth of the programme allows ATLAS to be sensitive to most relevant decays of vector-like top quarks, and also those of vector-like bottom quarks, thus increasing the chances of discovery. The creation of particle–antiparticle pairs is the most probable production mechanism for vector-like quarks with mass around or below 1 TeV. For higher masses, single production of vector-like quarks may have a larger rate.

ATLAS recently performed a statistical combination of all the individual searches that looked for pair-production of vector-like quarks. While the individual analyses were designed to be sensitive to particular sets of decays, the combined results provide increased sensitivity to all considered decays of vector-like top quarks with masses up to 1.3 TeV. No vector-like top or bottom quarks were found. The combination allowed ATLAS to set the most stringent exclusion bounds on the mass of a vector-like top quark for arbitrary sets of branching ratios to the three decay modes (figure, left).

As the limits on vector-like quarks reach higher masses, the importance of searching for their single production rises. Such searches are also interesting from a theoretical perspective, since they allow one to constrain parameters of the production model (figure, right).

Given these new strong limits on vector-like quarks and the lack of evidence for supersymmetry, the theoretical case for a naturally light Higgs boson is not looking good! But nature probably still has a few tricks up her sleeve to get out of this conundrum.

CMS detects first production of top quark and photon

Left: the reconstructed top-quark mass from a b-tagged jet, muon and missing energy. Right: the multivariate estimator for data and SM predictions after performing the fit. The inset presents a zoom of the last three bins plotted on log scale, while the hatched band shows the statistical and systematic uncertainties in the estimated signal and background yields, and the vertical bars on the points represent the statistical uncertainties of the data. The ratio of the data to the SM prediction is shown in the bottom panels.

It is well known that the top quark, the heaviest known elementary particle, plays an important role in electroweak-symmetry breaking, and is also one of the most promising particles to be investigated in the search for new physics. Numerous measurements of top-quark interactions have been performed at the Tevatron and LHC since the discovery of this particle at the Tevatron in 1995. The associated production of a top quark with a photon (tγj, where j indicates a jet) via electroweak interactions provides a powerful tool to probe the couplings of the top quark with the photon and the couplings of the W boson with the photon. The small production rate of the tγj process at the LHC makes its observation very challenging. However, any excess observed above the Standard Model (SM) rate would indicate new physics.

The CMS collaboration has released evidence for the tγj process using events with one isolated muon, a photon and jets in the final state. The results are based on proton–proton collision data recorded in 2016 at a centre-of-mass-energy of 13 TeV. The tγj process results in an interesting final state, which requires information from all sub-detectors of the CMS experiment, from the innermost tracker layers to the outermost muon systems.

The predicted cross section for tγj, including the branching fraction, is 81 fb, which corresponds to a few hundred events in the whole dataset. Therefore, a sophisticated method is needed to separate the signal events from the huge number of background events originating from several other SM processes. In addition, to achieve the highest signal-to-background ratio, a robust multivariate technique is used to estimate the contribution of the background in which a jet is misidentified as a photon. After these methods are employed, the largest background contribution comes from events that contain a top-quark pair associated with a photon.

CMS observed an excess of tγj events over the background-only hypothesis with a significance of 4.4 standard deviations, which corresponds to a p-value of 4.3 × 10^–6. The measured value of the signal cross section in the considered phase space is 115 ± 34 fb. The measurement is in agreement with the SM prediction within one standard deviation. This result is the first experimental evidence of the direct production of a top quark and a photon. Upcoming results, exploiting the full 13 TeV dataset, will further improve the precision of the measurement.

Burton Richter 1931–2018

Burton Richter, a major figure in particle physics who shared the Nobel Prize for the co-discovery of the J/ψ meson, passed away on 18 July in Palo Alto, California, at the age of 87.

Born in Brooklyn, New York, in 1931, Richter’s love of science began with the nightly blackouts during World War II, which revealed an unparalleled view of the night sky.

He studied physics at the Massachusetts Institute of Technology (MIT), where he was introduced to the electron–positron system by Martin Deutsch, who was conducting classical positronium experiments. He wrote his thesis on the quadratic Zeeman effect in hydrogen and completed his PhD in 1956 on the photoproduction of pi-mesons from hydrogen.

That year, Richter moved to Stanford University’s high-energy physics laboratory as a research associate. In 1960, he became an assistant professor of physics, then associate professor in 1963 and professor in 1967. During this time, Richter married his wife, Laurose, and had two children, Elizabeth and Matthew. By 1970, Richter’s talents in experimental particle physics and accelerator physics led to the Stanford Positron-Electron Asymmetric Ring (SPEAR) at the Stanford Linear Accelerator Center (SLAC). It included a groundbreaking type of general-purpose detector that has been used in particle colliders ever since, and it would eventually produce his biggest discovery.

After Richter secured funding for SPEAR in 1970, it took him just 27 months to build the accelerator, at a cost of $6 million. Experiments commenced in 1973 and, famously, in November 1974, SPEAR flushed out what the SLAC team dubbed the “psi” meson – a bound state of two charm quarks. Simultaneously, at Brookhaven National Laboratory on the other side of the continent, Sam Ting and his group had spotted the same resonance, which they christened the “J”. Just two years later, Richter and Ting shared the 1976 Nobel Prize in Physics for their pioneering discovery of the J/ψ, which proved the existence of a fourth type of quark (charm). It was a major step towards the establishment of the Standard Model of particle physics.

Before he received the Nobel Prize, in 1975 Richter began a sabbatical year at CERN, during which he pursued an experiment at CERN’s Intersecting Storage Rings (ISR) – the world’s first hadron collider. He was hosted by Pierre Darriulat and worked on adding a muon spectrometer arm to the R702 experiment. Richter also worked out the general energy-scaling laws for high-energy electron–positron colliding-beam storage rings, looking specifically at the parameters of a collider with a centre-of-mass energy in the range 100–200 GeV, arguing that such a machine would be required to better understand the relationship between the weak and electromagnetic interactions: “That study turned into the first-order design of the 27 km-circumference LEP project at CERN that was so brilliantly brought into being by the CERN staff in the 1980s,” he wrote in his Nobel biography.

His influential paper “Very High Energy Electron–Positron Colliding Beams for the Study of the Weak Interactions” (Nucl. Instrum. Methods 136 47) was followed by two detailed studies: one concerning the physics, published in November 1976 as CERN Yellow Report 76-18, of which Burt was a co-author, and an accelerator study headed by Kjell Johnsen. “Burt’s paper and his personal advocacy of high-energy electron–positron collision triggered interest at CERN, and had a powerful impact on the development of the Laboratory, also paving the way for the LHC and the discovery of the Higgs boson,” says CERN’s John Ellis.

In 1978, along with others at SLAC, Richter began to investigate the possibility of turning the 3.2 km linear accelerator at SLAC into a linear electron–positron collider. Construction of the SLAC Linear Collider (SLC) began in 1983, and Richter became director of SLAC the following year, until stepping down in 1999. During that time, he oversaw the construction of the SLC, the only linear electron–positron collider yet to be built, and led the way to other machines for photon science. While SLAC director, Richter also initiated interregional collaborations with DESY in Germany and KEK in Japan, and was a proponent of bringing into existence a high-energy linear collider as a global collaboration.

“Perhaps his greatest contribution as director was, in the 1990s, designing a future for SLAC that would look very different from the past,” said Stanford Provost Persis Drell, who served as SLAC director from 2007 to 2012. “He recognised that pursuing an X-ray free-electron laser at SLAC could be used to provide a revolutionary science opportunity to the photon science community, who use X-rays as their tool for discovery. This vision became the Linac Coherent Light Source. Burt recognised that outstanding science needed to drive the future of the institution, and he did not flinch from designing that future.”

When he stepped down as SLAC director, Richter focused on public policy issues in science and energy, for which he received the prestigious 2007 Philip Hauge Abelson Prize from the American Association for the Advancement of Science. In 2010, he published Beyond Smoke and Mirrors: Climate Change and Energy in the 21st Century, an apolitical layperson’s exploration of the facts of climate and energy. Among his many accolades, Richter received the US National Medal of Science, the nation’s highest scientific honour, in 2014; the Enrico Fermi Award in 2012; and the Ernest Orlando Lawrence Award in 1976.

“In my career I have met no one who has made more fundamental contributions in electron–positron and electron–electron colliders, in the precision instrumentation used in colliders and in experimental physics,” says Ting. “After we received the Nobel Prize together in 1976, I met him many times and we became good friends. My wife, Susan, and I are going to miss him deeply.”

Nobel work shines a light on particle physics

This year’s Nobel Prize in Physics was shared between three researchers for groundbreaking inventions in laser physics. Half the prize went to Arthur Ashkin of Bell Laboratories in the US for his work on optical tweezers, while the other half was awarded jointly to Gérard Mourou of the École Polytechnique in Palaiseau, France, and Donna Strickland of the University of Waterloo in Canada “for their method of generating high-intensity, ultra-short optical pulses”.

Mourou and Strickland’s technique, called chirped-pulse amplification (CPA), opens new perspectives in particle physics. Proposed in 1985, and forming the foundation of Strickland’s doctoral thesis, CPA uses a strongly dispersive medium to temporally stretch (“chirp”) laser pulses to reduce their peak power, then amplifies and, finally, compresses them – boosting the intensity of the output pulse dramatically without damaging the optical medium. The technique underpins today’s high-power lasers and is used worldwide for applications such as eye surgery and micro-machining.

Surfing the waves

But CPA’s potential for particle physics was clear from the beginning. In particular, high-power ultra-short laser pulses can drive advanced plasma-wakefield accelerators in which charged particles are brought to high energies over very short distances by surfing longitudinal plasma waves.

“After we invented laser-wakefield acceleration back in 1979, I was acutely aware that the laser community at that time did not have the specification that we needed to drive wakefields, which needed ultrafast and ultra-intense pulses,” explains Toshi Tajima of the University of California at Irvine, a long-time collaborator of Mourou. Tajima became aware of CPA in 1989 and first met Mourou in 1993 at a workshop at the University of Texas at Austin devoted to the future of accelerator physics upon the demise of the Superconducting Super Collider. “Ever since then, Gérard and I have formed a strong scientific and personal bond to promote ultra-intense lasers and their applications to accelerators and other important societal applications such as medical accelerators, transmutation and intense X-rays,” he says.

Today, acceleration gradients two-to-three orders of magnitude higher than existing radio-frequency (RF) techniques are possible at state-of-the-art laser-driven plasma-wakefield experiments, promising more compact and potentially cheaper particle accelerators. Though not yet able to match the quality and reliability of conventional acceleration techniques, plasma accelerators might one day be able to overcome the limitations of today’s RF technology, thinks Constantin Haefner, program director for advanced photon technologies at Lawrence Livermore National Laboratory in the US. “The race has started,” he says. “The ability to amplify lasers to extreme powers enabled the discovery of new physics, and even more exciting, some of the early envisioned applications such as laser plasma accelerators are on the verge of moving from proof-of-principle to real machines.”

Electrons can also be used to drive plasma accelerators, as is being explored at SLAC and in European labs such as LNF in Italy and DESY in Germany. Meanwhile, the AWAKE experiment at CERN has recently demonstrated the first proton-driven plasma-wakefield acceleration (CERN Courier October 2018 p7). Although AWAKE does not use a laser to drive the plasma, it employs a high-power laser to generate the plasma from a gas, at the same time seeding the proton self-modulation process that allows charged particles to be accelerated. CERN is also a partner in a recent project called the International Coherent Amplification Network, led by Mourou and funded by the European Union, to explore advanced wakefield drivers based on the coherent combination of multiple high-intensity fibre lasers that can run at high repetition rates and efficiencies.

“We have a long way to go, but plasma accelerators have game-changing potential for high-energy physics,” says Wim Leemans, director of the accelerator technology and applied physics division and Berkeley Lab Laser Accelerator Center (BELLA) at Lawrence Berkeley National Laboratory. “Other applications already being explored include free-electron lasers, a quasi-monoenergetic gamma-ray source for nonproliferation and nuclear security purposes, and a miniaturised method for brachytherapy, a cancer-treatment modality in which radiation is delivered directly to the site of a tumour.”

Beyond accelerators, the enormous intensity of single-shot pulses enabled by CPA offer new types of experiments in high-energy physics. In 2005, Mourou initiated the Extreme Light Infrastructure (ELI), nearing completion in the Czech Republic, Hungary and Romania, to explore the use of high-power PW lasers such as Livermore Lab’s HAPLS facility (see image on previous page). Going beyond ELI is the International Center for Zetta- and Exawatt Science and Technology (IZEST), established in France in 2011 to develop and build a community around the emerging field of laser-based particle physics. Under Mourou and Tajima’s direction, IZEST will extend existing laser facilities (such as PETAL at the Megajoule Laser facility in France) to the exa- and zettawatt scale, opening studies including “searches for dark matter and energy and probes of the nonlinearity of the vacuum via zeptosecond dynamical spectroscopy.”

Satellite premieres in CERN irradiation facility

The CELESTA micro satellite, carrying a space version of the radiation-monitoring system RadMon. — CELESTA contains a space version of CERN’s radiation-monitoring system RadMon, and is the first full satellite tested in the CHARM facility. Image credit: CERN-PHOTO-201807-181-3

CHARM, a unique facility at CERN to test electronics in complex radiation environments, has been used to test its first full space system: a micro-satellite called CELESTA, developed by CERN in collaboration with the University of Montpellier and the European Space Agency. Built to monitor radiation levels in low-Earth orbit, CELESTA was successfully tested and qualified during July under a range of radiation conditions that it can be expected to encounter in space. It serves as an important validation of CHARM’s potential value for aerospace applications.

CELESTA’s main goal is to enable a space version of an existing CERN technology called RadMon, which was developed to monitor radiation levels in the Large Hadron Collider (LHC). RadMon also has potential applications in space missions that are sensitive to the radiation environment, ranging from telecom satellites to navigation and Earth-observation systems.

The CELESTA cubesat, a technological demonstrator and educational project made possible with funding from the CERN Knowledge Transfer fund, will play a key role in validating potential space applications by using RadMon sensors to measure radiation levels in low-Earth orbit. An additional goal of CELESTA is to demonstrate that the CHARM facility is capable of reproducing the low-Earth orbit radiation environment. “CHARM benefits from CERN’s unique accelerator facilities and was originally created to answer a specific need for radiation testing of CERN’s electronic equipment,” explains Markus Brugger, deputy head of the engineering department and initiator of both the CHARM and CELESTA projects in the frame of the R2E (Radiation to Electronics) initiative. The radiation field at CHARM is generated through the interaction of a 24 GeV/c proton beam extracted from the Proton Synchrotron with a cylindrical copper or aluminium target. Different shielding configurations and testing positions allow for controlled tests to account for desired particle types, energies and fluences.

It is the use of mixed fields that makes CHARM unique compared to other test facilities, which typically use mono-energetic particle beams or sources. For the latter, only one or a few discrete energies can be tested, which is usually not representative of the authentic and complex radiation environments encountered in aerospace missions. Most testing facilities also use focused beams, limiting tests to individual components, whereas CHARM has a homogenous field extending over an area of least one square metre, which allows complete and complex satellites and other systems to be tested.

CELESTA is now fully calibrated and will be launched as soon as a launch window is provided. When in orbit, in-flight data from CELESTA will be used to validate the CHARM test results for authentic space conditions. “This is a very important milestone for the CELESTA project, as well as an historical validation of the CHARM test facility for satellites,” says Enrico Chesta, CERN’s aerospace applications coordinator.

First beam at IOTA for accelerator research

Fermilab’s 40 m-circumference Integrable Optics Test Accelerator. Image cregdit: G Stancari

In late August, a beam of electrons successfully circulated for the first time through a new particle accelerator at Fermilab in the US. The Integrable Optics Test Accelerator (IOTA), a 40 m-circumference storage ring, is one of only a handful of facilities worldwide dedicated to beam-physics studies. It forms the centrepiece of the Fermilab Accelerator Science and Technology (FAST) facility, and is the first research accelerator that will be able to switch between beams of electrons and protons.

Researchers will use IOTA to explore multiple accelerator technologies, including several that have been proposed but never tested, in particular targeting ultrahigh-intensity beams. More fundamentally it will allow precise control of a single electron – also opening the door to unique experiments in fundamental physics, such as understanding how the electron’s quantum-mechanical nature blurs its position in space.

For accelerator physicists, IOTA’s key focus is to test the concept of a nonlinear integrable focusing lattice in a realistic storage ring. Whereas contemporary accelerators are designed with linear focusing lattices, in reality machines always have nonlinearities, e.g. resulting from magnet imperfections, which lead to resonant behaviour and particle losses. A nonlinear integrable focusing lattice, proposed in 2010, is predicted to significantly suppress collective instabilities via Landau damping and thus could improve the performance of accelerators such as a Future Circular Collider. IOTA scientists will also capitalise on Fermilab’s existing strengths in accelerator technologies, such as cooling, to make more orderly beams that are easier to manipulate and accelerate.

Over the next year, the Fermilab team will install the proton injector. Once it is in place, it will complete the trio of particle accelerators that make up Fermilab’s FAST facility: the proton injector, the electron injector (completed in 2017) and the IOTA ring. FAST has already attracted 29 institutional partners, including European institutions, US universities, national laboratories and members from industry.

“IOTA is one of a kind – a particle storage ring designed and built specifically to host novel experiments with both electrons and protons, and to develop innovative concepts in accelerator science,” says Fermilab physicist Alexander Valishev, head of the team that developed and constructed IOTA. “This facility offers a flexibility that can be useful to a wider community – above and beyond the needs of high-energy physics.”

Beam tests bring ProtoDUNE to life

Cosmic-muon tracks — One of the first cosmic-muon tracks recorded by the ProtoDUNE detector at CERN during September. Three wire planes, each made up of thousands of individual wires, recorded the signal of a muon as it traveled approximately 3.8 m through the detector’s liquid-argon volume. Image credit: DUNE collaboration

The world’s largest liquid-argon neutrino detector has recorded its first particle tracks in tests at CERN, marking an important step towards the international Deep Underground Neutrino Experiment (DUNE) under preparation in the US. The enormous ProtoDUNE detector, designed and built at CERN’s neutrino platform, is the first of two prototypes for what will be a much larger DUNE detector. Situated deep beneath the Sanford Underground Research Facility in South Dakota, four final DUNE detector modules (each 20 times larger than the current prototypes and containing a total of 70,000 tonnes of liquid argon) will record neutrinos sent from Fermilab’s Long Baseline Neutrino Facility some 1300 km away.

DUNE’s scientific targets include CP violation in the neutrino sector, studies of astrophysical neutrino sources, and searches for proton decay. When neutrinos enter the detector and strike argon nuclei they produce charged particles, which leave ionisation traces in the liquid from which a 3D event can be reconstructed. The first ProtoDUNE detector took two years to build and eight weeks to fill with 800 tonnes of liquid argon, which needs to be cooled to a temperature below –184 degrees. It adopts a single-phase architecture, which is an evolution from the 170 tonne MicroBooNE detector at Fermilab’s short-baseline neutrino facility. The second ProtoDUNE module adopts a different, dual-phase, scheme with a second detection chamber.

The construction and operation of ProtoDUNE will allow researchers to validate the membrane cryostat technology and associated cryogenics for the final detector, in addition to the networking and computing infrastructure. Now that the first tracks have been seen, from beam tests involving cosmic rays and charged-particle beams from CERN’s SPS, ProtoDUNE’s operation will be studied in greater depth. The charged-particle beam test enables critical calibration measurements necessary for precise calorimetry, and will also produce valuable data for optimising event-reconstruction algorithms. These and other measurements will help quantify and reduce systematic uncertainties for the DUNE far detector and significantly improve the physics reach of the experiment. “Seeing the first particle tracks is a major success for the entire DUNE collaboration,” said DUNE co-spokesperson Stefan Soldner-Rembold of the University of Manchester, UK.

More than 1000 scientists and engineers from 32 countries in five continents are working on the development, design and construction of the DUNE detectors. For CERN, it is the first time the European lab has invested in infrastructure and detector development for a particle-physics project in the US. “Only two years ago we completed the new building at CERN to house two large-scale prototype detectors that form the building blocks for DUNE,” said Marzio Nessi, head of the neutrino platform at CERN. “Now we have the first detector taking beautiful data, and the second detector, which uses a different approach to liquid-argon technology, will be online in a few months.”

In July, the US Department of Energy also formally approved PIP-II, an accelerator upgrade project at Fermilab required to deliver the high-power neutrino beam required for DUNE. First data at DUNE is expected in 2026. Meanwhile, in Japan, an experiment with similar scientific goals and also with scientific links to the CERN neutrino platform – Hyper-Kamiokande – has recently been granted seed funding for construction to begin in 2020 (CERN Courier October 2018 p11). Together with several other experiments such as KATRIN in Germany, physicists are closing in on the neutrino’s mysteries two decades after the discovery of neutrino oscillations (CERN Courier July/August 2018 p5).

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