Jobs – Page 165 of 527

The case of the disappearing neutrinos

The neutrino cross-section, divided by the neutrino energy, as measured by IceCube (black line, with shaded regions showing the one-sigma uncertainties), along with previous accelerator data (points in yellow-shaded region). At low energies the cross-section is proportional to the neutrino energy, while above about 3 TeV the increase slows due to the finite W and Z masses.

Neutrinos are popularly thought to penetrate everything owing to their extremely weak interactions with matter. A recent analysis by the IceCube neutrino observatory at the South Pole proves this is not the case, confirming predictions that the neutrino–nucleon interaction cross section rises with energy to the point where even an object as tiny as the Earth can stop high-energy neutrinos in their tracks.

By studying a sample of 10,784 neutrino events, the IceCube team found that neutrinos with energies between 6.3 and 980 TeV were absorbed in the Earth. From this, they concluded that the neutrino–nucleon cross-section was 1.30^+0.21_–0.19 (stat) ^+0.39_–0.43 (syst) times the Standard Model (SM) cross-section in that energy range. IceCube did not observe a large increase in the cross-section as is predicted in some models of physics beyond the SM, including those with leptoquarks or extra dimensions.

The analysis used the 1km ³ volume of IceCube to collect a sample of upward-going muons produced by neutrino interactions in the rock and ice below and around the detector, selecting 10,784 muons with an energy above 1 TeV. Since the zenith angles of these neutrinos are known to about one degree, the absorber thickness can be precisely determined. The data were compared to a simulation containing atmospheric and astrophysical neutrinos, including simulated neutrino interactions in the Earth such as neutral-current interactions. Consequently, IceCube extended previous accelerator measurements upward in energy by several orders of magnitude, with the result in good agreement with the SM prediction (see figure, above).

Neutrinos are key to probing the deep structure of matter and the high-energy universe, yet until recently their interactions had only been measured at laboratory energies up to about 350 GeV. The high-energy neutrinos detected by IceCube, partially of astrophysical origin, provide an opportunity to measure their interactions at higher energies.

In an additional analysis of six years of IceCube data, Amy Connolly and Mauricio Bustamante of Ohio State University employ an alternative approach which uses 58 IceCube-contained events (in which the neutrino interaction took place within the detector) to measure the neutrino cross-section. Although these events mostly have well-measured energies, their neutrino zenith angles are less well known and they are also much less numerous, limiting the statistical precision.

Nevertheless, the team was able to measure the neutrino cross-section in four energy bins from 18 TeV to 2 PeV with factor-of-ten uncertainties, showing for the first time that the energy dependence of the cross section above 18 TeV agrees with the predicted softer-than-linear dependence and reaffirming the absence of new physics at TeV energy scales.

Future analyses from the IceCube Collaboration will use more data to measure the cross-sections in narrower bins of neutrino energy and to reach higher energies, making the measurements considerably more sensitive to beyond-SM physics. Planned larger detectors such as IceCube-Gen2 and the full KM3NeT can push these measurements further upwards in energy, while even larger detectors would be able to search for the coherent radio Cherenkov pulses produced when neutrinos with energies above 10¹⁷ eV interact in ice.

Proposals for future experiments such as ARA and ARIANNA envision the use of relatively-inexpensive detector arrays to instrument volumes above 100 km³, enough to measure “GZK” neutrinos produced when cosmic-rays interact with the cosmic-microwave background radiation. At these energies, the Earth is almost opaque and detectors should be able to extend cross-section measurements above 10¹⁹ eV, thereby probing beyond LHC energies.

These analyses join previous results on neutrino oscillations and exotic particle searches in showing that IceCube can also contribute to nuclear and particle physics, going beyond its original mission of studying astrophysical neutrinos.

Copper reveals nickel’s doubly magic nature

The mass spectrometer setup ISOLTRAP at CERN’s ISOLDE radioactive-beam facility.
Image credit: A Welker.

Teams at CERN’s ISOLDE facility and at RIKEN in Japan have found evidence that an exotic isotope of the metallic element nickel (⁷⁸Ni) is doubly magic, opening a new vista on an important region of the nuclear-stability chart.

Like electrons in an atom, protons and neutrons in a nucleus have a penchant for configurations that offer extra stability, called magic numbers. Nuclei that have magic numbers of both protons and neutrons are of particular interest for understanding how nucleons bind together. Examples are ¹⁶O, containing eight protons and eight neutrons, and ⁴⁰Ca (20 protons and 20 neutrons), both of which are stable nuclides.

One of the main efforts in modern nuclear physics is to create systems at the extremes of nuclear stability to test whether these magic numbers, and the nuclear shell model from which they derive, are still valid. Two usual suspects are ¹³²Sn (with a half-life of 40 s) and ⁷⁸Ni (0.12 s). Sn (tin) is the element with the highest number of stable isotopes (10), attesting to the magic nature of its 50 protons.

The next magic number is 82, corresponding to the number of neutrons in ¹³²Sn. Nickel has a magic number of 28 protons but the recipe for adding the magic 50 neutrons to make ⁷⁸Ni has proven challenging for today’s radioactive beam factories. CERN’s ISOLDE facility has now got very close, taking researchers to the precipice via nickel’s nuclear neighbour ⁷⁹Cu containing 50 neutrons and 29 protons.

Andree Welker of TU Dresden and collaborators used ISOLDE’s precision mass spectrometer ISOLTRAP to determine the masses and thus binding energies of the neutron-rich copper isotope ⁷⁹Cu, revealing that this next-door neighbour of ⁷⁸Ni also exhibits a binding-energy enhancement. To probe the enhancement, Ruben de Groote of KU Leuven and collaborators used another setup at ISOLDE called CRIS to measure the electromagnetic moments of the odd-N neighbour ⁷⁸Cu, providing detailed information about the underlying wave functions. Both the ISOLTRAP masses and the CRIS moments were compared with large-scale shell-model calculations involving the many relevant orbitals. Both are in excellent agreement with the ISOLDE results, suggesting that the predictions for the neighbouring ⁷⁸Ni can be taken with great confidence.

An independent study of ⁷⁹Cu carried out by Louis Olivier at the IN2P3–CNRS in France and colleagues based on a totally different technique has reached the same conclusion. Using in-beam gamma-ray spectroscopy of ⁷⁹Cu at the Radioactive Isotope Beam Factory at RIKEN in Japan, the team produced ⁷⁹Cu via proton “knockout” reactions in a 270 MeV beam of ⁸⁰Zn. No significant knockout was observed in the relevant energy region, showing that the ⁷⁹Cu nucleus can be described in terms of a valence proton outside a ⁷⁸Ni core and affirming nickel’s doubly magic character.

Novartis acquires CERN spin-off

AAA’s headquarters in Saint-Genis-Pouilly, France, just across the border from CERN.
Image credit: AAA.

Global healthcare company Novartis has announced plans to acquire Advanced Accelerator Applications (AAA), a spin-off radiopharmaceutical firm established by former CERN physicist Stefano Buono in 2002. With an expected price of $3.9B, said the firm in a statement, the acquisition will strengthen Novartis’ oncology portfolio by introducing a new therapy platform for tackling neuroendocrine tumours. Trademarked Lutathera, and based on the isotope lutetium-177, the technology was approved in Europe in September 2017 for the treatment of certain neuroendocrine tumours and is under review in the US.

With its roots in nuclear-physics expertise acquired at CERN, AAA started its commercial activity with the production of radiotracers for medical imaging. The successful model made it possible for AAA to invest in nuclear research to produce innovative radiopharmaceuticals. “We believe that the combination of our expertise in radiopharmaceuticals and theragnostic strategy together with the global oncology experience and infrastructure of Novartis, provide the best prospects for our patients, physicians and employees, as well as the broader nuclear medicine community,” said Buono, who is CEO of AAA.

Fermilab joins CERN openlab on data reduction

CERN’s computing centre, photographed in 2017.
Image credit: R Hradil.

In November, Fermilab became a research member of CERN openlab – a public-private partnership between CERN and major ICT companies established in 2001 to meet the demands of particle-physics research. Fermilab researchers will now collaborate with members of the LHC’s CMS experiment and the CERN IT department to improve technologies related to physics data reduction, which is vital for gaining insights from the vast amounts of data produced by high-energy physics experiments.

The work will take place within an existing CERN openlab project with Intel on big-data analytics. The goal is to use industry-standard big-data tools to create a new tool for filtering many petabytes of heterogeneous collision data to create manageable, but still rich, datasets of a few terabytes for analysis. Using current systems, this kind of targeted data reduction can often take weeks, but the Intel-CERN project aims to reduce it to a matter of hours.

The team plans to first create a prototype capable of processing 1 PB of data with about 1000 computer cores. Based on current projections, this is about one twentieth of the scale of the final system that would be needed to handle the data produced when the High-Luminosity LHC comes online in 2026. “This kind of work, investigating big-data analytics techniques is vital for high-energy physics — both in terms of physics data and data from industrial control systems on the LHC,” says Maria Girone, CERN openlab CTO

SESAME sees first light

SESAME beamline scientist Messaoud Harfouche points out SESAME’s first monochromatic light.
Image credit: SESAME.

At 10.50 a.m. on 22 November 2017, the third-generation light source SESAME in Jordan produced its first X-ray photons, signalling the start of the regional laboratory’s experimental program. Researchers sent a beam of monochromatic light through the XAFS/XRF (X-ray absorption fine structure/X-ray fluorescence) spectroscopy beamline, the first to come on stream at SESAME and targeted at research ranging from solid state physics to environmental science and archaeology.

Obtaining first light is an important step in the commissioning of a new synchrotron light source, and the milestone comes 10 months after SESAME circulated its first electrons (CERN Courier March 2017 p8). Nevertheless, it is just one step on the way to full operation. The SESAME synchrotron is currently operating with a beam current of just over 80 milliamps while the design value is 400 milliamps. Over the coming weeks and months as experiments get underway, the current will be gradually increased.

SESAME’s initial research program will be carried out at two beamlines, the XAFS/XRF beamline and an infrared spectro-microscopy beamline that is scheduled to join the XAFS/XRF beamline this year. A third beamline devoted to materials science will come on stream in 2018. “After years of preparation, it’s great to see light on target,” said XAFS/XRF beamline scientist Messaoud Harfouche. “We have a fantastic experimental programme ahead of us, starting with an experiment to investigate heavy metals contaminating soils in the region.”

SwissFEL carries out first experiment

SwissFEL project leaders Hans Braun (left) and Luc Patthey in front of the end-station where the first experiment took place.
Image credit: M Dzambegovic/PSI.

The free-electron X-ray laser SwissFEL at the Paul Scherrer Institute (PSI) in Switzerland has hosted its inaugural experiment, marking the facility’s first science result and demonstrating that its many complex components are working as expected. Construction of 740m-long SwissFEL began in April 2013, with the aim of producing extremely short X-ray laser pulses for the study of ultrafast reactions and processes.

Between 27 November and 4 December 2017, PSI researchers and a research group from the University of Rennes in France conducted the first in a series of pilot experiments.

The high-energy X-ray light pulses enabled the team to investigate the electrical and magnetic properties of titanium pentoxide nanocrystals, which have potential applications in high-density data storage. This and further pilot experiments will help hone SwissFEL operations before regular user operations begin in January 2019.

Searches for dark photons at LHCb

Comparison of the new LHCb results to existing constraints from previous experiments. Red and green curves show the predictions from LHC Run 3, while the dashed cyan curve is the prediction rescaled to the 2016 data sample.

The possibility that dark-matter particles may interact via an unknown force, felt only feebly by Standard Model (SM) particles, has motivated an effort to search for so-called dark forces.

The force-carrying particle for such hypothesised interactions is referred to as a dark photon, A’, in analogy with the ordinary photon that mediates the electromagnetic interaction. While the dark photon does not couple directly to SM particles, quantum-mechanical mixing between the photon and dark-photon fields can generate a small interaction. This provides a portal through which dark photons may be produced and through which they might decay into visible final states.

The minimal A’ model has two unknown parameters: the dark photon mass, m(A’), and the strength of its quantum-mechanical mixing with the photon field. Constraints have been placed on visible A’ decays by previous beam-dump, fixed-target, collider, and rare-meson-decay experiments.

However, much of the A’ parameter space that is of greatest interest (based on quantum field theory arguments) is currently unexplored. Using data collected in 2016, LHCb recently performed a search for the decay A’→μ⁺μ^– in a mass range from the dimuon threshold up to 70 GeV. While no evidence for a signal was found, strong limits were placed on the A’–photon mixing strength. These constraints are the most stringent to date for the mass range 10.6 < m(A’) < 70 GeV and are comparable to the best existing limits on this parameter.

Furthermore, the search was the first to achieve sensitivity to long-lived dark photons using a displaced-vertex signature, providing the first constraints in an otherwise unexplored region of A’ parameter space. These results demonstrate the unique sensitivity of the LHCb experiment to dark photons, even using a data sample collected with a trigger that is inefficient for low-mass A’ decays. Looking forward to Run 3, the number of expected A’→μ⁺μ⁻ decays in the low-mass region should increase by a factor of 100 to 1000 compared to the 2016 data sample. LHCb is now developing searches for A’→e⁺e⁻ decays which are sensitive to lower-mass dark photons, both in LHC Run 2 and in particular Run 3 when the luminosity will be higher. This will further expand LHCb’s dark-photon programme.

ATLAS extends searches for natural supersymmetry

(Left) Summary of ATLAS exclusion limits (95% C.L.) on top squark pair production considering various decay possibilities. The x-axis represents the mass of the top squark while the y-axis is the mass of the lightest SUSY particle. (Right) Exclusion limits on higgsino pair production for scenarios where the lightest higgsino is the lightest SUSY particle, with grey representing the LEP exclusion.

Despite many negative searches during the last decade and more, supersymmetry (SUSY) remains a popular extension of the Standard Model (SM). Not only can SUSY accommodate dark matter and gauge–force unification at high energy, it offers a natural explanation for why the Higgs boson is so light compared to the Planck scale. In the SM, the Higgs boson mass can be decomposed into a “bare” mass and a modification due to quantum corrections. Without SUSY, but in the presence of a high-energy new physics scale, these two numbers are extremely large and thus must almost exactly oppose one another – a peculiar coincidence called the hierarchy problem. SUSY introduces a set of new particles that each balances the mass correction of its SM partner, providing a “natural” explanation for the Higgs boson mass.

Thanks to searches at the LHC and previous colliders, we know that SUSY particles must be heavier than their SM counterparts. But if this difference in mass becomes too large, particularly for the particles that produce the largest corrections to the Higgs boson mass, SUSY would not provide a natural solution of the hierarchy problem.

New SUSY searches from ATLAS using data recorded at an energy of 13 TeV in 2015 and 2016 (some of which were shown for the first time at SUSY 2017 in Mumbai from 11–15 December) have extended existing bounds on the masses of the top squark and higgsinos, the SUSY partners of the top quark and Higgs bosons, respectively, that are critical for natural SUSY. For SUSY to remain natural, the mass of the top squark should be below around 1 TeV and that of the higgsinos below a few hundred GeV.

ATLAS has now completed a set of searches for the top squark that push the mass limits up to 1 TeV. With no sign of SUSY yet, these searches have begun to focus on more difficult to detect scenarios in which SUSY could hide amongst the SM background. Sophisticated techniques including machine learning are employed to ensure no signal is missed.

First ATLAS results have also been released for higgsino searches. If the lightest SUSY particles are higgsino-like, their masses will often be close together and such “compressed” scenarios lead

to the production of low-momentum particles. One new search at ATLAS targets scenarios with leptons reconstructed at the lowest momenta still detectable. If the SUSY mass spectrum is extremely compressed, the lightest charged SUSY particle will have an extended lifetime, decay invisibly, and leave an unusual detector signature known as a “disappearing track”.

Such a scenario is targeted by another new ATLAS analysis. These searches extend for the first time the limits on the lightest higgsino set by the Large Electron Positron (LEP) collider 15 years ago. The search for higgsinos remains among the most challenging and important for natural SUSY. With more data and new ideas, it may well be possible to discover, or exclude, natural SUSY in the coming years.

CMS studies rare top-quark processes

The tZq fit, displaying the Boosted Decision Tree output for one analysis region.

Now that all the particles predicted in the Standard Model (SM) have been discovered, most recently the Higgs boson in 2012, experiments at the LHC are active on two fronts: a deeper scrutiny of the SM and the search for new particles produced by beyond-SM (BSM) physics. Recent studies of rare processes involving the top quark serve both purposes. On one hand, they probe SM predictions and parameters in regions not accessed so far. On the other hand, if BSM couplings to the massive particles of the third generation of the SM are substantial, rare processes involving the top quark are golden candidates to reveal signs of BSM physics.

Based on data taken during 2016, the CMS Collaboration has recently published two such studies of rare top quark processes: the production of a single top quark associated with a Z boson and one or more jets (tZq) and the production of four top quarks (tttt). Detecting these processes is very difficult due to their tiny cross sections (about 0.8 pb for tZq and 0.01 pb for tttt in proton–proton collisions at 13 TeV), which means that no more than a few hundred tZq events and a dozen tttt events were expected after selection. If this was not challenging enough, these events have to be separated from an overwhelming amount of background from several other SM processes. To achieve a sufficient control of the background, the analyses are restricted to final states containing multiple electrons and muons. Furthermore, the tZq analysis uses multivariate techniques to classify event candidates according to their topologies.

Probing the Yukawa coupling between the top and the Higgs via the ̄tt̄tt analysis.

In both analyses, the signal is extracted with maximum-likelihood fits performed simultaneously in the control regions with different selections. As a result, CMS was able to report evidence of the tZq process with a significance of 3.1 standard deviations (3.7 expected) against the background-only hypothesis, and a cross section of 0.123^+0.033_–0.031 (stat) ^+0.029_–0.023 (syst) pb, in agreement with the SM. CMS also reported a small excess of tttt events over the background-only hypothesis, with a significance of 1.6 standard deviations (1.0 expected), and derived an upper limit of 0.0208^+0.0112_–0.0069 pb on the tttt production cross section. The high energy and the large integrated luminosities provided by the LHC have opened a new window on precision physics, in which measurements of rare processes involving top quarks play a central role.

As more LHC data become available, these studies will provide more stringent tests of the SM while increasing the chances of revealing BSM processes.

Longitudinal asymmetry tracked in heavy-ion collisions

Measured values of coefficient c₁ as a function of estimated values of mean rapidity-shift (<y₀>) for different collision centralities. The lines show linear fits passing through the origin and the differences in slopes are due to changes in the width of the rapidity distributions for different centralities.

In a heavy-ion collision, a longitudinal asymmetry arises due to unequal numbers of participating nucleons from the two colliding nuclei, causing a shift in the centre-of-mass (CM) of the overlapping “participant zone” with respect to the nucleon–nucleon CM.

The asymmetry may be expressed as α = (A-B)/(A+B), where A and B are the number of nucleons participating from the two colliding nuclei. This shifts the rapidity (y₀) of the participant zone with respect to the nucleon-nucleon CM rapidity, where y₀ ≅ ½ ln (A/B).

First results on the asymmetry in the longitudinal direction and its effect on the pseudorapidity distributions in lead-lead collisions at a nucleon-nucleon CM energy of 2.76 TeV have been obtained with the ALICE detector, allowing investigations of the effect of variations in the initial conditions on other measurable quantities.

Since the number of participants cannot be measured directly, the asymmetry in an event was estimated by measuring the energy in the forward neutron-zero-degree-calorimeters (ZNs) in the ALICE detector. The observed distribution of asymmetry in ZNs, α_zn, is used to classify events into symmetric and asymmetric by a choice of α_zn. A Monte Carlo simulation using a Glauber model for the colliding nuclei is tuned to reproduce the spectrum in the ZNs and provides a relation between the measurable longitudinal asymmetry and the shift in the rapidity of the participant zone formed by the unequal number of participating nucleons.

The effect of the longitudinal asymmetry was measured on the pseudorapidity distributions of charged particles in the mid and forward regions by taking the ratio of the pseudorapidity distributions from events corresponding to different regions of asymmetry (see figure). The coefficients of a polynomial fit to the ratio characterise the effect of the asymmetry, with the coefficient of the linear term in the polynomial expansion, c₁, showing a linear dependence on the mean value of y₀.

This analysis confirms that the longitudinal distributions are affected by the rapidity-shift of the participant zone with respect to the nucleon-nucleon CM frame, highlighting the relevance of nucleon numbers in the production of charged particles, even at high energies. The method is potentially a new event classifier for the study of initial state fluctuations and different particle production mechanisms.

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