Jobs – Page 171 of 521

Gerhard Lutz 1939–2017

One of the pioneers of silicon radiation detectors, Gerhard Lutz, passed away in Vienna on 28 April. He will be remembered for numerous inventions that shaped the field of silicon detectors, his deep insight into detector physics and analysis methods, his role as mentor of many young scientists, and his modest and charming personality.

Gerhard Lutz made numerous seminal contributions in the field of silicon detectors. Image credit: PNSensor.

Gerhard Lutz was born in Klagenfurt, Austria, in 1939. He studied physics at the Technical University of Vienna and obtained his PhD from the University of Hamburg under Willibald Jentschke, the founder of DESY and later a Director-General of CERN. His thesis concerned the coherent bremsstrahlung and pair production on diamond crystals using the DESY synchrotron, and demonstrated the production of GeV photons with a polarisation in excess of 70%. In 1967 he moved to Northeastern University in Boston and contributed to a spectrometer experiment at Brookhaven, which had aimed to follow up spectacular results reported earlier by the “CERN missing mass spectrometer”: the splitting of the A₂ resonance and the observation of narrow high-mass resonances. Based on high-quality data and a painstaking analysis, he showed that the CERN results were incorrect.

In 1972, Lutz took a position at MPI-Munich and initiated a precision measurement of the reaction π^– p(↑) → π^– π⁺ n. He organised and ran the experiment, wrote the event-reconstruction software and developed the complex mathematical formalism necessary to interpret the results – marking a milestone in the understanding of exclusive hadronic reactions. In the late 1970s the CERN–Munich Group expanded into the ACCMOR collaboration, which pioneered the use of high-precision silicon tracking detectors. Together with Josef Kemmer and Robert Klanner, he developed silicon microstrip detectors using planar technology and built the vertex telescope for the CERN fixed-target experiments NA11 and NA32. The achieved precision of this device (5 μm), its ability to operate reliably in a high-intensity beam and identify charm particles against a huge background of hadronic events, unleashed the success story of silicon detectors. Today, practically all high-energy physics experiments rely on this technology.

Lutz’s contributions in the field of silicon detectors are numerous: the understanding of detector instabilities due to surface effects; the development of double-sided silicon-strip detectors; the concept of fully depleted pnCCDs based on the principle of sideway depletion; the realisation of novel concepts for silicon sensors with intrinsic gain; and the invention of the DePFET detector-amplifier structure. His developments found their way into many experiments outside particle physics, in particular in astrophysics and X-ray science, and also industry. Lutz co-founded the Max-Planck-Institut Halbleiterlabor (HLL) semiconductor laboratory in 1992, the research company PNSensor in 2002, and the instrumentation company PNDetector in 2007. Until the very end he contributed to the success of both companies with his sharp mind and inventions, while his guidance, inspiration and ideas have been essential for the success of semiconductor developers in the Munich area.

Those who had the opportunity to work with Gerhard Lutz appreciated his gentle and quiet way, his competence and deep insight. His scientific standards were very high and he detested superficial statements. His unconventional and original ideas inspired many colleagues and students, and his book Semiconductor Radiation Detectors has become a classic in the field. Gerhard Lutz’s innovative and influential work was honoured by the 1966 Röntgen Award, the 2011 Radiation Instrumentation Outstanding Achievement Award, and the 2017 High Energy Physics Prize of the European Physical Society (see “EPS awards prizes for high-energy physics”).

Cécile DeWitt-Morette 1922–2017

Cécile DeWitt-Morette, founder of the Les Houches summer school, passed away on 8 May at the age of 94. Born in Caen, she studied in Paris after completing her bachelor degree. In 1944 her mother, sister and grandmother were killed in the Allied bombing of Caen, but in Paris she secured a job at CNRS and was awarded a PhD three years later with a thesis about meson production. She was then invited to the Institute for Advanced Study in Princeton by Robert Oppenheimer, where she met her future husband, the US physicist Bryce DeWitt (they would go on to have four daughters).

Cécile DeWitt-Morette founded the Les Houches summer school.

Mixing with the best of US physics made her realise the poor situation of the field in France, especially particle physics, and drove her to do something about it. Precisely at that time, a summer school was organised at the university of Michigan in Ann Arbor, and Cécile had the idea to create such an event in France. Her beautiful eyes with double-iris rings and considerable powers of persuasion, not to mention a fantastic intuition for selecting the best possible lecturers, were difficult to resist. She had a friend whose father, the architect Albert Laprade, loaned her a piece of land at La Côte des Chavants, just above the village of Les Houches in the Arve valley, among farms and cottages. Financial input soon followed thanks to her skilful negotiating tactics, and in the summer of 1951 I was one of a few candidates to attend the school for a period of three months. She had chosen fantastic professors: Léon Van Hove for quantum mechanics and Viki Weisskopf for nuclear physics, both of whom would be future Director-Generals of CERN; Res Jost for field theory; Walter Kohn (a future Nobel Prize winner) for solid state physics; plus seminars by giants such as Wolfgang Pauli. We worked very hard, except for some excursions in the mountains, and learnt a lot.

The Les Houches school, of which Cécile remained director for 22 years, continued to be a complete success. Many of its students and some teachers received the Nobel prize, the Wolff prize or the Fields Medal. Among them were Pierre Gilles De Gennes and Claude Cohen-Tannoudji. The demand for basic courses dissipated over the years, but the school became a place for high-level specialised topics, and continues to be so.

Cécile also played an important role in founding the Institut des Hautes Scientifiques (IHES) in Bures sur Yvette, and did important work on functional integration, also collaborating with her mathematical-physicist husband. They were professors at the University of North Carolina at Chapel Hill and at the University of Texas at Austin, successively. Bryce died in 2004 just as he was about to receive the Einstein Prize from the American Physical Society.

I met Cécile for the last time at IHES in 2011 where she was made Officier De la Légion dʼHonneur. On 7 May, the day before she died, I understand that she was delighted to learn that the anti-European candidate as president of France, Marine Le Pen, had been defeated.

Lithuania joins CERN as associate member

CERN Director-General, Fabiola Gianotti, president of Lithuania, Dalia Grybauskaitė, and Lithuanian minister of foreign affairs, Linas Antanas Linkevičius, at the signing of the agreement in Vilnius.
Image credit: R Dačkus.

On 27 June, representatives of CERN and the Republic of Lithuania signed an agreement in the capital city of Vilnius admitting Lithuania as an associate Member State. The agreement will enter into force once official approval is received from the Lithuanian government.

“The involvement of Lithuanian scientists at CERN has been growing steadily over the past decade, and associate membership can now serve as a catalyst to further strengthen particle physics and fundamental research in the country,” said CERNʼs Director-General, Fabiola Gianotti. “We warmly welcome Lithuania to the CERN family, and look forward to enhancing our partnership in science, technology development and education and training.”

Lithuania’s relationship with CERN dates back to a co-operation agreement signed in 2004, which paved the way to participation of Lithuanian universities and scientific institutions in high-energy physics experiments at CERN. Lithuania has been a long-time contributor to the CMS experiment and has also played an important role in developing databases for the experiment. The country actively promoted the BalticGrid in 2005, and more generally participates in detector development relevant to the High Luminosity LHC.

Lithuania’s associate membership will strengthen the long-term partnership between CERN and the Lithuanian scientific community. It will allow Lithuania to take part in meetings of CERN Council and its committees, and Lithuanian scientists will be eligible for staff appointments. Finally, once the agreement enters into force, Lithuanian industry will be entitled to bid for CERN contracts.

Physics resumes at LHC

LHC Run 2 has seen a record increase in peak luminosity.

First stable beams in the LHC were declared on 23 May, just 25 days after the first beam was injected and almost three weeks ahead of schedule. Since then, interleaved with physics operation and remaining commissioning activities, the LHC teams have been busy ramping up the intensity of the beams. During this procedure, the number of proton bunches circulating the machine is increased in a stepwise manner: beginning with three bunches per beam and going up to 12, 72, 300, 600, 900, 1200, 1800, 2400 and finally 2556 bunches per beam. To ensure that all systems work as they should, each step requires a minimum of 20 hours of stable-beam operation and that the machine is filled three times. As the Courier went to press on 28 June, 2556 bunches were circulating in the machine and already the experiments had clocked an integrated luminosity of around 5 fb^–1.

Another important procedure during the LHC restart is the so-called scrubbing run to condition the vacuum chamber, which took place in early June. Despite the ultra-high vacuum of the LHC beam pipe, residual gas molecules and electrons remain trapped on the walls of the chamber and can be liberated by the circulating beam, eventually heating the walls and destabilising the beam. Such “electron-cloud” effects can be reduced by repeatedly filling the LHC with closely spaced bunches, provoking intense electron clouds that gradually become less prone to produce further electrons.

The rapid and smooth restart of the LHC this year, which marks the continuation of Run 2 at a centre-of-mass energy of 13 TeV, is due to the excellent availability of the machine and its injector chain, and also the dedication of many specialists. The LHC is now ready to continue the intensity ramp for physics-data collection, with the ambitious goal of reaching an integrated luminosity of 45 fb^–1 for 2017.

LHCb discovers new baryon

(Left) Mass spectrum recorded in 2016 showing the right-sign (RS) signal sample Ξ++cc → Λc+ K– π+ π+ along with two control samples: sideband (SB) Λ+c K– π+ π+ candidates, and wrong-sign (WS) Λ+c K– π+ π– candidates, each normalised to have the same area as the RS sample. The inset shows the Λ+c mass distributions with the signal (cross-hatch) and sideband (vertical lines) regions. (Right) An illustration of the spin-1/2 baryon multiplet including light quarks and the charm quark, with the Ξ++cc located on the top floor.

The LHCb collaboration has discovered a new weakly decaying particle: a baryon called the Ξ⁺⁺_cc, which contains two charm quarks and an up quark. The discovery of the new particle, which was observed decaying to the final-state Λ⁺_c K^– π⁺ π⁺ and is predicted by the Standard Model, was presented at the European Physical Society conference in Venice on 6 July.

Although the quark model of hadrons predicts the existence of doubly heavy baryons – three-quark states that contain two heavy (c or b) quarks – this is the first time that such states have been observed unambiguously with overwhelming statistical significance (well in excess of 5σ with respect to background expectations). The properties of the newly discovered Ξ++cc baryon shed light on a long-standing puzzle surrounding the experimental status of doubly charmed baryons, opening an exciting new branch of investigation for LHCb.

The team scrutinised large high-purity samples of Λ⁺_c → p K^– π⁺ decays in LHC data recorded at 8 and 13 TeV in 2012 and 2016, respectively, and discovered an isolated narrow structure in the Λ⁺_c K^– π⁺ π⁺ mass spectrum (associating the Λ⁺_c baryon with further particles) at a mass of around 3620 MeV/c². After eliminating all known potential artificial sources, the collaboration concluded that the highly significant peak is a previously unobserved state. Corroboration that it is the weakly decaying Ξ⁺⁺_cc came from examining a subset of data in which the reconstructed baryons lived for a measurable period before decaying. Such a requirement eliminates all promptly decaying particles, leaving only long-lived ones that are the hallmark of weak transitions.

Although the existence of baryons with valence-quark content ccu and ccd (corresponding to the Ξ⁺⁺_cc and its isospin partner Ξ⁺_cc) is expected, the experimental status of these states has been controversial. In 2002, the SELEX collaboration at Fermilab in the US claimed the first observation of this class of particle by observing a significant peak of about 16 events at a mass of 3519±1 MeV/c² in the Λ⁺_c K^– π⁺ mass spectrum, which they identified as the closely related state Ξ⁺_cc. Puzzlingly, the short lifetime (which was too small to be measured at SELEX) and the very large production rate of the state seemed not to match theoretical expectations for the Ξ⁺_cc. Despite SELEXʼs confirmation of the observation in a second decay mode, all subsequent searches – including efforts at the FOCUS, BaBar and Belle experiments – failed to find evidence for doubly charmed baryons. That left both theorists and experimentalists awaiting a firm observation by a more powerful heavy-flavour detector such as LHCb. Although the new result from LHCb does not fully resolve the puzzle (with a mass difference of 103±2 MeV/c², LHCbʼs Ξ⁺⁺_cc and SELEXʼs Ξ⁺_cc seem irreconcilable as isospin partners), the discovery is a crucial step to an empirical understanding of the nature of doubly heavy baryons.

CMS observes production of same-sign W-boson pairs

(Left) Distribution of the dijet mass in the signal region, with hatched bars including statistical and systematic uncertainties. Predictions for two scenarios of doubly charged Higgs boson production (red and green) are also shown. (Right) Expected and observed 95% confidence-level upper limits on the product of cross-section and branching fraction for doubly charged Higgs boson decays to a same-sign W-boson pair.

The LHC was built with a guaranteed discovery: the ATLAS and CMS experiments would either find a Higgs boson, or it would discover new physics in vector boson scattering (VBS) at high energies. The discovery of a Higgs-like boson in July 2012 confirmed that the W and Z bosons acquire mass through the Higgs mechanism, but to determine whether the observed particle corresponds to the single Higgs boson expected in the Standard Model (SM), it is now paramount to precisely measure the Higgs boson’s contributions to VBS. Since the behaviour of VBS amplitudes is sensitive to the way Higgs and vector bosons couple to one another and to the Higgs boson’s mass, models of physics beyond the SM predict enhancements to VBS via modifications to the Higgs sector or from the presence of additional resonances.

A recent analysis by CMS aimed to identify events in which a W-boson pair is produced purely via the electroweak interaction. Requiring events to have a same-sign W-boson pair reduces the probability of production via the strong interaction, making it an ideal signature for VBS studies. The first experimental results on this final state were reported by ATLAS and CMS based on 20 fb^–1 of LHC data collected in 2012 at an energy of 8 TeV, but were insufficient to claim an observation. The new study is based on 36 fb^–1 of data collected in 2016 at 13 TeV. Events were selected by requiring they contain two leptons (electrons or muons) with the same electric charge, moderate missing transverse momentum, and two jets with a large rapidity separation and a large dijet mass. About 67 signal events were expected, with the dominant sources of background events coming from top quark–antiquark pairs and WZ boson pairs. The event yield of the signal process is then extracted using a 2D fit of the dijet and dilepton mass distributions (figure, left).

The new CMS study provides the first observation of the electroweak production of same-sign W-boson pairs in proton–proton collisions, with an observed significance of 5.5 standard deviations. The result does not point to physics beyond the SM: a cross-section of 3.8±0.7 fb is measured within the defined fiducial signal region, corresponding to 90±22% of the result expected. An excess of events could have been caused by the presence of a doubly charged Higgs boson that couples to W bosons, and the analysis sets upper bounds on the product of the cross-section and branching fraction for such particles (figure, right). Bounds on the structure of quartic vector-boson interactions are also obtained in the framework of dimension-eight effective field theory operators, and the measurements set 95% confidence-level limits that are up to six times more stringent than previous results.

This first observation of the purely electroweak production of same-sign W-boson pairs is an important milestone towards precision tests of VBS at the LHC, and there is much more to be learned from the rapidly growing data sets. Studies demonstrate that the High Luminosity LHC, due to enter operation in the early 2020s, should even allow a direct investigation of longitudinal W-boson scattering.

ATLAS probes Higgs boson at 13 TeV

(Left) The invariant mass spectrum measured in the four-lepton final state with the expected SM background and signal superimposed. (Right) The measured differential cross-section for the transverse momentum (p_T,4l) of the Higgs boson, compared to theoretical predictions.

The ATLAS collaboration has released new results on measurements of the properties of the Higgs boson using the full LHC proton–proton collision data set collected at a centre-of-mass energy of 13 TeV in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb^–1.

One of the most sensitive measurement channels involves Higgs boson decays via two Z bosons to four leptons (two pairs of oppositely charged electrons or muons). Although only occurring in about one in every 8000 Higgs decays, it gives the cleanest signature of all the Higgs decay modes.

Using this channel, ATLAS measured both the inclusive and differential cross-sections for Higgs boson production. Although these have been measured before at lower LHC collision energy, the increased integrated luminosity and larger cross-section compared to LHC Run 1 allows their magnitudes to be determined with increased precision. In total, around 70 Higgs boson to four-lepton events were measured with a fit to the invariant mass distribution, allowing the inclusive cross-section to be measured with an accuracy of about 16%.

Candidate Higgs boson events were corrected for detector measurement effects and classified according to their kinematic properties to measure differential production cross-sections. Among these, the measurement of the momentum of the Higgs boson transverse to the beam axis probes different Higgs boson production mechanisms. By measuring the number and properties of jets produced in these events, Higgs boson production via the fusion of two gluons was studied. The measured inclusive and differential cross-sections were found to be in agreement with the Standard Model (SM) predictions. The results were used to constrain possible anomalous Higgs boson interactions with SM particles.

ALICE zooms in on evolution of the quark–gluon plasma

Ratio of K^*0 and φ yields to charged-kaon yields at midrapidity for Pb–Pb and pp collisions as a function of charged-particle pseudorapidity density at midrapidity (dN_ch/dη)^1/3, which is related to the final-state freeze-out geometry of the system. Also shown are corresponding results from models with and without re-scattering.

The precise particle-identification and momentum-measurement capabilities of the ALICE experiment allow researchers to reconstruct a variety of short-lived particles or resonances in heavy-ion collisions. These serve as a probe for in-medium effects during the last stages of evolution of the quark–gluon plasma (QGP). Recently, the ALICE collaboration has made a precise measurement of the yields (number of particles per event) of two such resonances: K^*(892)⁰ and φ(1020). Both have similar masses and the same spin, and both are neutral strange mesons, yet their lifetimes differ by a factor of 10 (4.16±0.05 fm/c for K^*0, and 46.3±0.4 fm/c for φ).

The shorter lifetime of the K^*0 means that it decays within the medium, enabling its decay products (π and K) to re-scatter with other hadrons. This would be expected to inhibit the reconstruction of the parent K^∗⁰, but the π and K in the medium may also scatter into a K^∗⁰ resonance state, and the interplay of these two competing re-scattering and regeneration processes becomes relevant for determining the K^*0 yield. The processes depend on the time interval between chemical freeze-out (vanishing inelastic collisions) and kinetic freeze-out (vanishing elastic collisions), in addition to the source size and the interaction cross-sections of the daughter hadrons. In contrast, due to the longer lifetime of the φ meson, both the re-scattering and regeneration effects are expected to be negligible.

Using lead–lead collision data recorded at an energy of 2.76 TeV, ALICE observed that the ratio K^*0/K^– decreases as a function of system size (see figure). In small impact-parameter collisions, the ratio is significantly less than in proton–proton collisions and models without re-scattering effects. In contrast, no such suppression was observed in the φ /K^– ratio. This measurement thus suggests the existence of re-scattering effects on resonances in the last stages of heavy-ion collisions at LHC energies. Furthermore, the suppression of K^*0 yields can be used to obtain the time difference between the chemical and the kinetic freeze-out of the system.

On the other hand, at higher momenta (p_T > 8 GeV/c), these resonances were suppressed with respect to proton–proton collisions by similar amounts. The magnitude of this suppression for K^*0 and φ mesons was also found to be similar to the suppression for pions, kaons, protons and D mesons. The striking independence of this suppression on particle mass, baryon number and the quark-flavour content of the hadron puts a stringent constraint on models dealing with particle-production mechanisms, fragmentation processes and parton energy loss in the QGP medium.

In future, it will be important to perform such measurements for high-multiplicity events in pp collisions at the LHC.

XENON1T releases first data

The XENON1T detector and the three-storey building next door containing auxiliary equipment.
Image credit: R Corrieri and P De Perio.

Researchers from the XENON1T dark-matter experiment at Gran Sasso National Laboratory in Italy reported their first results at the 13th Patras Workshop on Axions, WIMPs and WISPs, held in Thessaloniki from 15–19 May (see “Exploring axions and WIMPs in Greece” in Faces & Places). XENON1T is the first tonne-scale detector of its kind and is designed to search for WIMP dark matter by measuring nuclear recoils from WIMP–nucleus scattering. Continuing the programme of the previous XENON10 and XENON100 detectors, the new apparatus contains 3200 kg of ultra-pure liquid xenon (LXe) – 20 times more than its predecessor – in a dual-phase xenon time projection chamber (TPC) to detect nuclear recoils. The TPC encloses about 2000 kg of LXe, while another 1200 kg provides additional shielding.

The experiment started collecting data in November 2016. A blind search based on 34.2 live days of data acquired until January 2017, when earthquakes in the region temporarily suspended the run, revealed the data to be consistent with the background-only hypothesis. This allowed the collaboration to derive the most stringent exclusion limits on the spin-independent WIMP–nucleon interaction cross-section for WIMP masses above 10 GeV/c², with a minimum of 7.7 × 10^–47 cm² for 35 GeV/c² WIMPs at 90% confidence level.

These first results demonstrate that XENON1T has the lowest low-energy background level ever achieved by a dark-matter experiment, with the intrinsic background from krypton and radon reduced to unprecedented low levels. The sensitivity of XENON1T will continue to improve as the experiment records data until the end of 2018, when the collaboration plans to upgrade to a larger TPC due to come online by 2019. Several other experiments, such as PANDA-X and LUX-ZEPLIN, are also competing for the first WIMP detection.

“With our experiment working so beautifully, even exceeding our expectations, it is really exciting to have data in hand to further explore one of the most exciting secrets we have in physics: the nature of dark matter,” says XENON spokesperson Elena Aprile of Columbia University in the US.

First beam at Muon g-2

The electromagnet for Fermilab’s Muon g-2 experiment, which involves more than 150 scientists and engineers from nine countries.
Image credit: Fermilab.

The Muon g-2 experiment at Fermilab has begun its three-year-long campaign to measure the magnetic moment of the muon with unprecedented precision. On 31 May, a beam of muons was fired into the experiment’s 14 m-diameter storage ring, where powerful electromagnetic fields cause the magnetic moment, or spin, of individual muons to precess. The last time this experiment was performed, using the same electromagnet at Brookhaven National Laboratory in the late 1990s and early 2000s, the result disagreed with predictions by more than three standard deviations. This hinted at the presence of previously unknown particles or forces affecting the muon’s properties, and motivated further measurements to check the result.

Sixteen years later, the reincarnated Muon g-2 experiment will make use of Fermilab’s intense muon beams to definitively answer the questions raised by the Brookhaven experiment. It turned out to be 10 times cheaper to move the apparatus to Fermilab than it would have cost to build a new machine at Brookhaven, and the large, fragile superconducting magnet was transported in one piece from Long Island to the suburbs of Chicago in the summer of 2013.

Since it arrived, the Fermilab team reassembled the magnet and spent a year adjusting or “shimming” the uniformity of its field. The field created by the g-2 magnet is now three times more uniform than the one it created at Brookhaven. In the past year, the team has worked around the clock to install detectors, build a control room and prepare for first beam. The work has included: the creation of a new beamline to deliver a pure beam of muons; instrumentation to measure the magnetic field; and entirely new instrumentation to measure the muonʼs spin-precession signal.

Over the next few weeks the Muon g-2 team will test the equipment, with science-quality data expected later in the year. The experiment aims to achieve a precision on the anomalous magnetic moment of the muon of 0.14 parts per million, compared to around 0.54 parts per million previously. If the inconsistency with theory remains, it could indicate that the Standard Model of particle physics is in need of revision.

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