Jobs – Page 164 of 521

Alper Garren 1925–2017

Alper Abdy Garren was born on 30 April 1925 in Oakland, California, and died peacefully on 25 June in the same place.

Alper Garren designed the lattice for the SSC. Image credit: Berkeley Lab.

Al attended the US Naval Reserve Midshipmenʼs School at the University of Notre Dame in 1945 and served as a commissioned lieutenant in the US Naval Reserve through 1947. By 1950 he had received undergraduate and masters degrees from the University of California, Berkeley, and in 1955 he completed his PhD at the Carnegie Institute of Technology.

A career particle physicist at Berkeley Lab, located on the hill above the UC Berkeley campus, Al wrote his first paper for what was then the Radiation Laboratory in 1949. He wrote his final paper in 1991 at what had become the Lawrence Berkeley National Laboratory (LBL).

Al was a brilliant scientist who designed the accelerator lattice for the Superconducting Super Collider (SSC), in particular inventing the “diamond bypass” to allow two beams to be injected and aborted from just one straight section. His career also included work on the Tevatron, the asymmetric B-Factory at SLAC’s PEP-II accelerator and SYNCH – a computational tool used extensively at particle-physics labs around the world and for which he held a patent. He contributed to the design and orbit theory of the following machines: the Bevatron, Magnetic Mirror Fusion Reactors, 88-inch Cyclotron, Advanced Light Source (ALS), Fermilab Proton Synchrotron, the Large Proton–Proton Storage Rings LSR (CERN), ISABELLE (BNL), and the High Energy Heavy Ion Facility SUMATRAN (Japan).

Al was a sweet, kind, generous man who made friends easily and kept them for life. He loved to travel and was especially drawn to the culture and people of Asia. He loved the performing arts and was a patron of the San Francisco Opera, the San Francisco Symphony and the Philharmonia Baroque Orchestra. He was a dedicated philanthropist, supporting some 200 environmental, human-rights and performing-arts organisations in his later years.

Physicist, teacher, mentor, world traveller, sailor, philanthropist and above all a dear friend, Al enriched many lives during his 92 years.

Xenon beams light path to gamma factory

The SPS, pictured during a recent technical stop, was loaded with beams of partially ionised xenon atoms in September.
Image credit: M Brice/CERN.

On 14 September, CERN injected a beam of partially ionised xenon atoms into the Super Proton Synchrotron (SPS) and kept it circulating for a short period. The successful demonstration, carried out by the SPS operations and radio-frequency teams, is the first of a series of experimental steps to explore the feasibility of a gamma-ray source with an intensity several orders of magnitude higher that those currently in operation.

Earlier this year, CERN’s accelerator complex demonstrated its flexibility by producing a beam of fully ionised xenon atoms for the fixed-target experiment NA61, which studies the physics of strong interactions. Profiting from this achievement, the gamma-factory study group – which is part of CERN’s Physics Beyond Colliders study – requested dedicated beam tests with partially ionised xenon atoms in the SPS. The beam was composed of xenon nuclei carrying 15 out of the 54 electrons present in the neutral atom, the missing 39 electrons having been stripped off before reaching the SPS.

The xenon beams injected into the SPS are the most fragile of any beam so far accelerated to, and stored at, ultra-relativistic energies at CERN. A loss of even a single electron changes the magnetic rigidity of the stored particles and leads to beam loss. The losses for the xenon-39 beam due to interactions of the beam with the residual gas in the SPS vacuum pipe were expected to be severe, and the tests confirmed that the beam lifetime is indeed short (of the order of one second). However, the lifetime is expected to be significantly higher for lead beams with only one or two attached electrons, which are the principal candidates to drive the high-energy gamma factory. Tests with lead atoms will be carried out next year in parasitic mode during the LHC’s heavy-ion programme, when the CERN accelerator teams aim not only to inject partially ionised lead atoms into the SPS but also into the LHC.

Light source

An eventual gamma factory would use beams of highly ionised atoms to drive a novel type of light source. The idea is to insert the ion beams into a storage ring and illuminate them with a laser that excites the electrons to a higher energy state, leading to spontaneous emission of secondary photons. In this scheme, the initial laser-photon frequency is boosted by a factor of up to 4γ²_L, where γ_L is the Lorentz factor of the ion beam. With the LHC as a storage ring, photons in the energy range 1–400 MeV would therefore be possible. Such a source of gamma rays would open many scientific opportunities, such as precision atomic electroweak physics with high-Z hydrogen-like atoms, dark-matter searches using photon beams, and neutron dipole moment and neutron–antineutron oscillations. It would also act as a test bed for a future neutrino factory or a TeV-scale muon collider, says the team.

Meanwhile, independent activities during machine-development periods this year will see xenon atoms injected and brought into collision in the LHC. “The beauty of the operation mode of the CERN accelerator complex is not only that the xenon-39 beam tests in the SPS could be done with no influence on the LHC pp operation, but that they could be done concurrently to injecting and accelerating other types of beam in the SPS – e.g. two cycles for the fixed-target programme and one parasitic cycle for xenon-39,” says Witold Krasny of the gamma-factory study group.

CERN’s Physics Beyond Colliders initiative was launched in 2016 to explore the opportunities offered by the CERN accelerator complex and infrastructure “to get new insights into some of today’s outstanding questions in particle physics through projects complementary to high-energy colliders and other initiatives in the world” (CERN Courier November 2016 p28).

CLEAR prospects for accelerator research

CLEAR’s plasma-lens experiment will test ways to drive strong currents through a plasma for particle-beam transverse focusing.
Image credit: M Volpi.

A new user facility for accelerator R&D, the CERN Linear Electron Accelerator for Research (CLEAR), started operation in August and is ready to provide beam for experiments. CLEAR evolved from the former CTF3 test facility for the Compact Linear Collider (CLIC), which ended a successful programme in December 2016. Following approval of the CLEAR proposal, the necessary hardware modifications started in January and the facility is now able to host and test a broad range of ideas in the accelerator field.

CLEAR’s primary goal is to enhance and complement the existing accelerator R&D programme at CERN, as well as offering a training infrastructure for future accelerator physicists and engineers. The focus is on general accelerator R&D and component studies for existing and possible future accelerator applications. This includes studies of high-gradient acceleration methods, such as CLIC X-band and plasma technologies, as well as prototyping and validation of accelerator components for the high-luminosity LHC upgrade.

The scientific programme for 2017 includes: a combined test of critical CLIC technologies, continuing previous tests performed at CTF3; measurements of radiation effects on electronic components to be installed on space missions in a Jovian environment and for dosimetry tests aimed at medical applications; beam instrumentation R&D; and the use of plasma for beam focusing. Further experiments, such as those exploring THz radiation for accelerator applications and direct impedance measurements of equipment to be installed in CERN accelerators, are also planned.

The experimental programme for 2018 and beyond is still open to new and challenging proposals. An international scientific committee is currently being formed to prioritise proposals, and a user request form is available at the CLEAR website: cern.ch/clear.

China neutron source sees first beam

An aerial view of the CSNS facility in August this year.
Image credit: IHEP.

In late August, the China Spallation Neutron Source (CSNS) produced its first neutron beam, representing an important milestone for the $280 million project. The world’s fourth pulsed spallation neutron source, following ISIS in the UK, SNS in the US and J-PARC in Japan, CSNS is located in the city of Dongguan in Guangdong province and is expected to become an important base for research and innovation in China and the surrounding region. CSNS entered construction in 2011 and is being built and operated by the Institute of High Energy Physics in collaboration with the Institute of Physics, both part of the Chinese Academy of Sciences.

A spallation neutron source uses intense pulses of protons to strike a target, producing a beam of neutrons that have been knocked out of the target nuclei. CSNS is driven by a 80 MeV H^– linac and a 1.6 GeV rapid cycling synchrotron, providing a 100 kW proton beam. The protons strike a solid tungsten target and the emerging neutrons are slowed using three moderators, before being delivered to the instrumentation facilities. A second phase of the project, upgrading the linac to 250 MeV and the proton beam power to 500 kW, is planned for the near future.

At 10.56 a.m. on 28 August, a proton beam pulse from the accelerator collided with the tungsten target for the first time. Neutron detectors located at two of the facility’s 20 beamlines measured the neutron spectrum, showing that the neutron beam had been successfully produced. The spectrum was consistent with the prediction from Monte Carlo simulations, with a higher neutron yield than expected.

Construction of the first three neutron spectrometers is also complete. A general-purpose powder diffractometer will be used to study crystal and magnetic structures of materials, while a small-angle neutron-scattering instrument will probe structures such as polymers at the level of 1–100 nm. A third instrument, a multipurpose reflectometer, will analyse neutrons reflected from a sample to study the surface and interface structure of materials.

These and other instruments will soon be available to users from around the world for research in materials science and technology, life sciences, physics, the chemical industry, environment, energy and other fields. Commissioning of the spectrometers is under way, on track for the facility to open to users in the spring of 2018.

Servers for SESAME

On 12 September, 56 servers left CERN bound for the SESAME light-source facility in Jordan. “These servers are a very valuable addition to the SESAME data centre,” said Salman Matalgah, head of IT at SESAME. “They will help ensure that we’re able to provide first-class computing support to our users.” Speaking for CERN, Charlotte Warakaulle, director for international relations, said: “After many other successful donations, it’s great that we can extend the list of beneficiaries to include SESAME: a truly inspiring project showcasing and building on scientific capacity in the Middle East and neighbouring regions.” Pictured are CERN’s head of IT Frédéric Hemmer (left), Charlotte Warakaulle and president of SESAME Council Rolf Heuer, with the servers packed and ready to go.

UK neutrino investment steps up

Prototype liquid-argon detectors for the international DUNE project are currently being built at CERN.
Image credit: M Brice and J Ordan/CERN.

The UK is to invest £65 million (€74 million) in the Long-Baseline Neutrino Facility (LBNF) and Deep Underground Neutrino Experiment (DUNE) in the US, announced UK science minister Jo Johnson on 20 September. Currently under construction at Fermilab and Sanford Underground Research Laboratory, LBNF/DUNE will investigate crucial questions about neutrinos such as their mass ordering and CP-violating properties.

The latest investment makes the UK, already a major scientific contributor with 14 universities and two Science and Technology Facilities Council laboratories providing expertise and components, the largest country in the LBNF/DUNE project outside of the US.

“We have been working towards this for a long time and it is important both for the UK and for DUNE overall,” says co-spokesperson of the DUNE collaboration Mark Thomson of the University of Cambridge. “Specifically, the investment will allow the UK to play a major role in the construction of the DUNE far detector (read-out TPC wire planes and DAQ system) and in the neutrino beamline (super-conducting RF for the PIP-II LINAC and the LBNF neutrino target).”

LBNF/DUNE is the first major project to be addressed by a broader UK–US science and technology agreement, the first between the two countries, signed in January to strengthen UK–US co-operation.

CERN is an important partner in LBNF/DUNE and is developing prototype liquid-argon detectors for the project as part of its dedicated neutrino platform.

Fastest spinning fluid clocked by RHIC

Charged particles from a gold–gold nuclei collision in the STAR detector. Measurements of the alignment between the global angular momentum of non-central collisions and the spin of emitted particles reveal that the QGP is the most vortical system so far observed.

Experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory have found that droplets of quark–gluon plasma (QGP) can spin faster than any other fluid. The immensely hot and fast-expanding QGP is already known to behave as a near “perfect” liquid, exhibiting a viscosity lower than any other. Now, researchers on RHIC’s STAR experiment report that the vorticity (or curl) of the fluid produced in RHIC’s relativistic heavy-ion collisions is about 9 × 10²¹ s^–1. That exceeds the rotation of a super-cell tornado by a factor 10²⁰ and is 14 orders of magnitude higher than any fluid ever observed, beating the previous spin record held by nano-droplets of superfluid helium. The results will aid descriptions of quark–gluon plasma and, with more data, offer a way to measure the strength of the plasmaʼs magnetic field.

The past decade has seen major advances in our understanding of the quark–gluon plasma, with RHIC experiments also reporting recently that the extreme state might even form in collisions involving very light nuclei such as deuterium – in line with recent observations by the LHC experiments of proton-collision systems.

LHCb digs deeper into lepton-flavour universality

1D projections of the 3D fit to the data in two of the fit variables: the invariant-mass squared of the neutrino system (left) and the decay time of the selected Bc candidates (right).

The LHCb collaboration has released yet another result in its campaign to test lepton-flavour universality. Following anomalies already detected in the rate that B mesons decay into muons compared to electrons or tau leptons, the latest result concerns the charmed B meson, B⁺_c.

Using data recorded at collision energies of 7 and 8 TeV during LHC Run 1, LHCb reports evidence for the semi-tauonic decay B⁺_c→J/ψ τ⁺ ν_τ and has performed a measurement of the ratio R(J/ψ) = Br(B⁺_c→J/ψ τ⁺ ν_τ)/Br(B⁺_c→J/ψ μ⁺ν_μ). The ratio is found to be 0.71±0.17±0.18, which is within 2σ of the expected Standard Model (SM) range of 0.25–0.28. The SM prediction of R(J/ψ) deviates from unity only due to the large mass difference between the tau and the muon.

Both the semi-muonic decay, B⁺_c→J/ψ μ⁺ν_μ, and the semi-tauonic decay, B⁺_c→J/ψ τ⁺ ν_τ, with J/ψ → μ⁺μ^– and τ⁺→ μ⁺ ν_μν_τ, lead to a three-muon final state with the two channels distinguished by their decay kinematics. Despite the distinct signature, however, the analysis must overcome several major challenges. For example, since B_c mesons account for less than 0.1% of the b hadrons produced at the LHC energies, light b hadrons are a major source of background when one or more particles in their final states are misidentified in the detector. Fortunately, the presence of two heavy quarks in the B_c means it decays nearly three times faster than its lighter cousins, providing a powerful handle for statistically separating their respective contributions.

The latest LHCb result adds to the intriguing picture emerging from the measurements of semi-tauonic decays of b-flavoured hadrons. Previous studies of the ratios of branching fractions between B → D^(*)τ⁺ν_τ and B → D^(*)μ⁺ν_μ at LHCb, BaBar and Belle have shown hints of departure from lepton-flavour universality. The combined effect is now almost at the level of 4σ with respect to the SM prediction. In addition, previous LHCb analyses of B → K^(*)μ⁺μ^– and B → K^(*)e⁺e^– decays also deviate from the SM by about 2.5σ.

There is much more to come from LHCb on tests of lepton-flavour universality, which remains one of the most enduring hints of deviation from the SM. This includes updates of the results with Run-2 data and measurements from other b-hadron species.

• This article was corrected on 10 November 2017.

CMS observes top quarks in proton–nucleus collisions

Top-quark pair-production cross-section in pp and pPb collisions as a function of the centre-of-mass energy per nucleon pair.

The top quark, the heaviest elementary particle in the Standard Model, has been the subject of numerous detailed studies in proton–antiproton and proton–proton collisions at the Tevatron and LHC since its discovery at Fermilab in 1995. Until recently, however, studies of top-quark production in nuclear collisions remained out of reach due to the small integrated luminosities of the first heavy-ion runs at the LHC and the low nucleon–nucleon (NN) centre-of-mass energies (√s_NN) available at other colliders such as RHIC in the US.

Proton–lead runs at √s_NN = 8.16 TeV performed in 2016 at the LHC have allowed the CMS collaboration to perform the first-ever study of top-quark production in nuclear collisions.

Top-quark cross-sections at the LHC can be computed with great accuracy via perturbative quantum chromodynamics (pQCD) methods, thus making this quark a “standard candle” and a tool for further investigations. In proton–nucleus collisions, in particular, the top quark is a novel probe of the nuclear gluon density at high virtualities in the unexplored high Bjorken-x region. In addition, a good understanding of top-quark production in proton–nucleus collisions is crucial for studies of the space–time structure of the quark–gluon plasma formed in nucleus–nucleus collisions.

Invariant mass distribution of the hadronic top-quark candidates in selected events with two b-tagged jets.

Once produced, each top quark decays promptly into a W boson plus a bottom quark, with the W boson further decaying into either a charged lepton and a neutrino or a pair of light quarks. To identify pair-produced top quarks, CMS therefore selected events containing one isolated electron or muon, two “b-tagged” jets, and two jets that fail b tagging. The amount of signal in the selected sample is inferred by a fit to the invariant mass of the two untagged jets, interpreted as W boson decay products (W → qq′). The amount of non-top background is constrained by two complementary event samples, with zero or one b-tagged jets, also included in the fit. In this way, the background behaviour in this (so far unexplored) phase-space region and the b-tagging efficiency are evaluated in situ with only minimal assumptions, independent of prior inputs. As a validation, the outcome of the fit is used to model the signal and background invariant mass distributions of the top-quark candidate in the hadronic decay channel (t → Wb → qq′b), which are in agreement with the data (see figure).

The excess of events with respect to the background-only hypothesis corresponds to a significance of more than 5σ, even under the most conservative assumptions. The measured top-pair cross-section is consistent with the expectations from scaled proton–proton collision data as well as pQCD predictions at next-to-next-to-leading order with next-to-next-to-leading-log accuracy (figure, right). This result paves the way towards future studies of top-quark production in the hot and dense matter created in nucleus–nucleus collisions.

ATLAS experiment makes precision measurement of top-quark mass

(Left) The reconstructed top-quark mass recorded at 8 TeV together with the best-fit template. (Right) The new ATLAS combined result compared to those from other experiments.

The top quark is copiously produced at the LHC, allowing for very precise measurements of its properties. The mass of the top quark, m_top, plays a special role in the Standard Model (SM) of particle physics. It is a key part of the mechanism of electroweak symmetry breaking and one of the parameters governing the stability of the universe in the SM.

Following years of meticulous work, ATLAS presented a new measurement of m_top at the 10th International Workshop on Top Quark Physics held in Braga, Portugal, in late September. The measurement was performed using around 100,000 proton–proton collision events at an energy of 8 TeV, each containing a top-quark pair reconstructed in the single-lepton final state. In this channel, each top quark immediately decays to a W boson and a bottom quark, and one W boson decays to an electron or muon and a neutrino, while the second W boson decays to two light quarks.

A simultaneous measurement of m_top together with a global jet-energy scale factor and a relative bottom-to-light jet-energy scale factor was performed. The inclusion of these scale factors strongly reduces systematic uncertainties. The precision of the measurement is further improved by differentiating between correctly reconstructed top-quark events and events where the final-state objects are incorrectly assigned to the two top quarks. While only retaining 40% of the events, the total uncertainty is improved by 19%, leading to a top-quark mass of 172.08±0.91 GeV.

The power of this measurement, which is the second most precise individual top-quark mass measurement made by ATLAS to date, is revealed when combined with previous ATLAS measurements in the single-lepton channel at 7 TeV and the dilepton channel at 8 TeV. This combination relies on a careful evaluation of the correlation between measurements for all sources of systematic uncertainty. In both channels at 8 TeV, the analysis optimisation trades reduced systematic against increased statistical uncertainty, thereby reducing the correlation among the measurements. The combined result thus has a 41% smaller uncertainty than the single most precise measurement. The current combined value is 172.51±0.50 GeV with a relative precision of 0.29%, which is mainly limited by the calibration of the jet-energy scales and is similar to that of the leading single-experiment combined measurements.

The current precision on m_top represents a significant achievement that demonstrates the precise understanding of all the relevant aspects of the ATLAS detector. The measurement will allow further and deeper tests of the consistency of the SM.

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