Jobs – Page 203 of 527

Chern–Simons (Super) Gravity – 100 Years of General Relativity (vol. 2)

By Mokhtar Hassaine and Jorge Zanelli
World Scientific

Written on the basis of a set of lecture notes, this book provides a concise introduction to Chern–Simons (super) gravity theories accessible to graduate students and researchers in physics and mathematics.

Chern–Simons (CS) theories are gauge-invariant models that could include gravity in a consistent way. As a consequence, they are very interesting to study because they can open up the way to a common description of the four fundamental interactions of nature.

As is well known, three such interactions are described by the Standard Model as Yang–Mills (YM) theories, which are based on the principle of gauge invariance (requiring a correlation between particles at different locations in space–time). The particular form of these YM interactions makes them consistent with quantum mechanics.

On the other hand, gravitation – the fourth fundamental force – is described by general relativity (GR), which is also based on a gauge principle, but cannot be quantised following the same steps that work in the YM case.

Gauge principles suggest that a viable path is the introduction of a peculiar, yet generic, modification of GR, consisting in the addition of a CS term to the action.

Besides being mathematically elegant, CS theories have a set of properties that make them intriguing and promising: they are gauge-invariant, scale-invariant and background-independent; they have no dimensionful coupling constants; and all constants in the Lagrangian equation are fixed rational coefficients that cannot be adjusted without destroying the gauge invariance.

Wisdom of the Martians of Science: In Their Own Words with Commentaries

By Balazs Hargittai and Istvan Hargittai
World Scientific

The “Martians” of science that the titles refers to are five Jewish-Hungarian scientists who distinguished themselves for significant discoveries in fundamental science that contributed to shaping the modern world. These great scientists are John von Neumann, a pioneer of the modern computer; Theodore von Kármán, known as the scientist behind the US Air Force; Loe Szilard, initiator of the development of nuclear weapons; Nobel laurate Eugene P Wigner, who was the world’s first nuclear engineer; and Edward Teller, colloquially known as “the father of the hydrogen bomb”.

Born to upper-middle-class Jewish families and raised in the sophisticated atmosphere of liberal Budapest, they were forced to leave their anti-Semitic homeland to emigrate to Germany, and ultimately to the US, which became their new home country, to the point that they devoted themselves to its defence.

The book comes as a follow-up to a previous title, The Martians of Science, which drew the profiles of these five scientists and presented their contributions to their fields of research. The aim of this second volume is to show the wisdom of the Martians by presenting their thoughts and ideas with their own words and putting them into context. Through direct quotes from the five characters and commentaries from other people who knew them, the authors offer an insight into the thinking of such great minds, which they find instructive and entertaining. They are witty, provocative, intriguing and, as the author says, never boring.

Excitons and Cooper Pairs: Two Composite Bosons in Many-Body Physics

By Monique Combescot and Shine-Yuan Shiau
Oxford University Press

This book deals with two major but different fields of condensed-matter physics, semiconductors and superconductors, starting from the consideration that the key particles of these materials, which are excitons and Cooper pairs, are actually composite bosons. The authors are not interested in describing the physics of these materials, but in better understanding how composite bosons made of two fermions interact and, more specifically, identifying the characteristics of their fermionic components that control many-body effects at a microscopic level.

The many-body physics of elementary fermions and bosons has been largely studied using Green functions and with the help of Feynman diagrams for visualisation. But these tools are not easily applicable to many-body physics of composite bosons made of two fermions. Consequently, a new formalism has been developed and a new type of graphic representation, the “Shiva diagrams” (so named because of the multi-arm structure reminiscent of the Hindu god Shiva) adopted.

After two sections dedicated to the mathematical and physical foundation of Wannier and Frenkel excitons and of Cooper pairs, the book continues with a discussion on composite particles made of excitons. In the fourth and last part, the authors look at some aspects of the condensation of composite bosons, which they call “bosonic condensation”, and which is different from the Bose–Einstein condensation of free elementary bosons. Other important issues are discussed, such as the application of the Pauli exclusion principle on the fermionic components of bosonic particles.

Although suitable for advanced undergraduate and graduate students in physics without a specific background, this text will also appeal to researchers in condensed-matter physics who are willing to obtain insight into the many-body physics of two composite bosons.

Effective Field Theories

By Alexey A Petrov and Andrew E Blechman
World Scientific

The importance of effective field theory (EFT) techniques cannot be over-emphasised. In fact, all theories are, in some sense, effective. A book that discusses these techniques, groups different cases in which EFTs are necessary, and provides numerous examples, is therefore necessary.

After illustrating the ubiquitousness of EFTs with a discussion of Newtonian gravity, superconductivity, and the Euler–Heisemberg theory of photon–photon scattering below the electron mass, the book splits into different directions to examine qualitatively diverse situations where EFTs are used. Fermi theory, chiral perturbation theory, heavy-quark effective theory, non-relativistic quantum electrodynamics (chromodynamics), and even the EFT for physics beyond the Standard Model, are all discussed with a common language that allows the reader to find analogies and appreciate the different physics of these fundamentally different systems.

Soft collinear effective theory (SCET) and non-relativistic general relativity provide a different context in which EFTs are useful as a computational tool. The text exploits the intuition developed in the previous examples to identify the relevant expansion parameters and to organise hierarchically the different contributions to the scattering amplitudes.

Admittedly, the book focuses on high-energy physics topics, neglecting many applications in soft and condensed matter.

The volume is very well written, it is continuous, and includes a rich introduction on the main topics necessary to understand and use EFTs, such as symmetries, renormalization-group methods and anomalies. As an advanced quantum field theory (QFT) book, it exploits the possibility of relying on the previous knowledge of the reader and concentrates on the relevant issues; the introduction is written in a practical way, providing EFT jargon and highlighting the differences between renormalisable and non-renormalisable theories.

The tone of the book makes it suitable not only for practitioners in the field, but also for students looking for a broad perspective on different QFT topics – the common EFT language providing the thread – and for teachers searching for analogies and similarities between advanced and classical topics.

Introduction to Soft-Collinear Effective Theory

By Thomas Becher, Alessandro Broggio and Andrea Ferroglia
Springer
Also available at the CERN bookshop

The volume provides an essential and pedagogical introduction to soft-collinear effective field theory (SCET), one of the low-energy effective field theories (EFTs) of the Standard Model developed in the last two decades. EFTs are used when the problem that is tackled requires a separation of the low-energy contributions from the high-energy part, to be solved.

SCET has already been applied to a large variety of processes, from B-meson decays to jet production at the LHC. As a consequence, the need was felt for a self-contained text that could make this subject easily accessible to students, as well as to researchers who are not experts in the subject. Nevertheless, a background in quantum field theories and perturbative QCD is a prerequisite for the book.

The basics of the construction of effective theory are presented in detail. The expansion of Feynman diagrams describing the production of energetic particles is described, followed by the construction of an effective Lagrangian, which produces the different terms that contribute to the expanded diagrams. The case of a scalar theory is considered first, then the construction is extended to the more complex case of QCD.

To show the method at work, the authors have included some collider-physics example applications (the field where, in the last few years, SCET has been applied the most). In particular, the soft-gluon resummation for the inclusive Drell–Yan cross-section in proton–proton collisions is discussed, and SCET formalism is used to perform transverse-momentum resummation. In addition, the application of SCET methods to a process with high energetic particles in many directions is analysed, and the structure of infrared singularities in n-point gauge-theory amplitudes derived.

Quark–Gluon Plasma 5

By Xin-Nian Wang (ed.)
World Scientific

As the fifth volume in a series on quark–gluon plasma (QGS), this text provides an update on the recent advances in theoretical and phenomenological studies of QGS. Quark–gluon plasma (also informally called “quark soup”) is a state of matter in quantum chromodynamics (QCD) hypothesised to exist at extremely high temperatures and densities, in which the constituents of hadrons, i.e quarks and gluons, are in a special condition of high freedom.

The book is a collection of articles written by major international experts in the field, with the aim to meet the needs of both novices – thanks to its pedagogical and comprehensive approach – and experienced researchers.

A significant amount of space is given – of course – to the impressive progress in experimental and theoretical studies of new forms of matter in high-energy heavy-ion collisions at RHIC, as well as at the LHC. The strong coupled quark–gluon plasma (sQGP) discovered at RHIC has attracted the attention of many researchers and defined the path for future studies in the field. At the same time, the heavy-ion collisions at unprecedented high energies at the LHC have opened up new lines of research.

This updated and detailed overview of QGS joins the previous four volumes in the series, which altogether present a comprehensive and essential review of the subject, both for beginners and experts.

Images of Time: Mind, Science, Reality

By George Jaroszkiewicz
Oxford University Press

For ages, sundials have been used to measure time, with typical accuracies in the order of a few minutes. After Galileo discovered that the small oscillations of a pendulum are isochronous, Huygens built the first prototype of a pendulum clock reaching the remarkable accuracy of a few seconds. Today, improved measurements of time and frequency are at the heart of quantum electrodynamics (QED) precision tests. The anomalous magnetic moment of the muon is measured with an accuracy of more than one part in a billion. The global-positioning system (GPS) and satellite communications, as well as other technological applications, are based (directly or indirectly) on accurate measurements of time.

There are some who argue that, while time is measured accurately, its nature is debatable in so far as it appears ubiquitously in physics (from the second law of thermodynamics to the early universe) but often with slightly different meanings. There are even some who claim that time is a mystery whose foundations are sociological, biological and psychological. This recent work by George Jaroszkiewicz suggests that different disciplines (or even different areas of physics) elaborated diverse images of time through the years. The ambitious and erudite purpose of the book is to collect all of the imageries related to the conceptualisation of time, with particular attention to the physical sciences.

The book is neither a treatise on the philosophy of science nor is it a monograph of physics. The author tries to find a balance between physical concepts and philosophical digressions, but this goal is not always achieved: various physical concepts are introduced by insisting on a mathematical apparatus that seems, at once, too detailed for the layman and too sketchy for the scholar. Through the book’s 27 chapters (supplemented by assorted mathematical appendices), the reader is led to reflect on the subjective, cultural, literary, objective, and even illusionary, images of time. Each chapter consists of various short subsections, but the guiding logic of the chapter is sometimes lost in the midst of many interesting details. The overall impression is that different branches of physics deal with multiple images of time. Because these conceptualisations are not always consistent, time is perceived by the reader (and partly presented by the author) as an enigmatic theme of speculation. A malicious reader might even infer that after nearly five centuries of Galilean method, the physicists are dealing daily with something they do not quite understand.

This knowledgeable review of the different images of time is certainly valuable, but it fails to explain why improved measurements of time and frequency are correlated with the steady development of modern science in general and of physics in particular. The truth is that physical sciences thrive from a blend of experiments, theories and enigmas: without mysteries driving our curiosity, we would not know why we should accurately measure, for instance, the anomalous magnetic moment of the muon. However, by only contemplating time as an enigma, we would probably still be stuck with sundials.

The LHC: Run 2 has restarted

Applause in the LHC control room as the first particles began circulating in the LHC.
Image credit: CERN.

At the end of March, the LHC opened its doors to allow particles to travel around the ring for the first time since the year-end technical stop began in December 2015. Progress was good, and the phase of recommissioning with beam could rapidly start. The LHC team worked with low-intensity beam for a few weeks to re-commission all systems and to check all aspects of beam-based operation, to ensure that the LHC was fully safe before declaring “stable beams” – the signal that the experiments could start taking data.

“Beam-splash” events recorded by CMS during the run restart.
Image credit: CMS.

Before the protons could circulate again, the machine underwent the final phase of preparation – known as the machine checkout. During this phase, all of the LHC’s systems are put through their paces without beam. A key part of the process is driving the magnetic circuits, radiofrequency accelerating cavities, collimators, transverse dampers, etc, repeatedly through the nominal LHC cycle.

ATLAS “beam-splash” events.
Image credit: ATLAS.

A full programme of beam instrumentation checks took place, to ensure sure that active elements were working and that the complex acquisition chain was functioning properly. Detailed checks were performed on the collimation systems.

The radiofrequency system was re-commissioned and the LHC beam-dump system was subject to stringent operational checks. In parallel, a pilot beam extracted from the Super Proton Synchrotron (SPS) was sent down the two SPS-LHC transfer lines to the beam dumps just before the start of the LHC.

While the machine checkout was ongoing, the experiments were finishing their own last interventions before the closure of the caverns.

2015 saw the start of Run 2 for the LHC, during which the proton–proton collision energy reached 13 TeV. Beam intensity has increased, and by the end of the 2015 run, 2240 proton bunches per beam were being collided. This year, the aim is to increase the number of bunches even further, to the target of 2748. The goal is to reach an integrated luminosity of around 25 inverse femtobarns (fb^–1), up from the 4 fb^–1 reached by the end of last year. One fb^–1 corresponds to around 80 million million collisions.

At the heart of every LHC collision

Fig. 1. The figure shows some of the cross-section measurements made by ATLAS at √s = 7.8 and 13 TeV, including data collected in 2015.
Image credit: ATLAS Collaboration.

At the heart of every LHC collision are the constituents of protons: the quarks and gluons, collectively known as partons. These partons can undergo hard-scattering processes, producing a plethora of final states ranging from the massless to the very massive, such as W and Z bosons or top-quark pairs. Understanding these production cross-sections and their evolution as a function of the centre-of-mass energy, √s, of the LHC are important components to understanding all of the measurements performed by ATLAS, including searches for new physics beyond the Standard Model.

Figure 1 illustrates some of the cross-section measurements made by ATLAS at √s = 7, 8 and 13 TeV. The new 13 TeV data collected in 2015 greatly extend the lever arm of the investigation of the √s evolution, with increased cross-sections for W and Z bosons and top-quark pairs by factors of approximately two and three, respectively, from their values at 8 TeV.

The final states observed from hard scattering tell a story of which partons participated in the collisions: e.g. top-quark production is related to the gluon composition of the proton, whereas Z-boson production provides insight into the quark sea, and W-boson production on the relationship between the valence quarks. These measurements are pieces of the proton puzzle, and because the √s evolution changes the range of the parton momentum fractions probed by the collisions, the 13 TeV data open up a new kinematic region of investigation.

Via hard scattering, one can also test the predictions of perturbative QCD – a key component of the Standard Model. Single and dibosons are currently predicted at next-to-next-to-leading order (NNLO), and top-quark pair production at NNLO plus next-to-next-to-leading log (NNLL). As √s increases, the mix of the hard-scattering processes changes, and the precision measurements become increasingly dependent on the knowledge of growing electroweak corrections currently available at NLO. With higher √s, rarer processes like Z-boson pair production (ZZ) become more accessible and open an enticing window onto potential new physics.

As is evident from figure 1, results match well with Standard Model expectations. Apart from a common beam-luminosity uncertainty, the measurements at 13 TeV have an experimental precision ranging from under 1% for Z bosons, to 3% for W bosons and top-quark pairs, to 14% for ZZ – the latter still being dominated by statistical uncertainties. However, measuring ratios of cross-sections can benefit from the cancellation of many experimental uncertainties. This is evident from the W⁺/W^– cross-section ratio at 13 TeV, which has a total systematic uncertainty of less than 1%, rivalling the precision of the current predictions of parton-distribution functions but whose central value is consistently lower than predictions. Results such as those presented here will contribute significantly to the understanding of the large 13 TeV data set expected in the coming years.

ALICE finds a new source of charmonium

The ALICE collaboration has studied the production of charmonium – bound states of charm and anti-charm quarks – in hadronic as well as in ultra-peripheral collisions of lead nuclei at √s_NN= 2.76 TeV. In the latter case, the nuclei do not overlap, and the charmonium is produced through a photonuclear interaction (CERN Courier November 2012 p9). Recently, however, ALICE has found a clear signal for what appears to be photoproduction of J/ψ mesons, the lowest vector state of charmonia, also in collisions with significant nuclear overlap.

Fig. 1. Lower part of the transverse momentum (p_T) distribution for J/ψ candidates, decaying into two muons with opposite electric charges (denoted OS in the figure), in peripheral collisions of lead nuclei, as measured by ALICE. The red line represents the expectation of coherent photoproduced J/ψ, according to the STARLIGHT Monte Carlo.
Image credit: ALICE Collaboration.

The nuclear overlap of heavy-ion collisions can be classified based on centrality, which is expressed as a percentile between 0 and 100%, corresponding to head-on and grazing collisions, respectively, or expressed in terms of the impact parameter, which is the distance between the centres of the two colliding nuclei in a plane that is transverse to the beam axis (CERN Courier May 2013 p31). The signal is most clearly seen in the transverse-momentum (p_T) distribution shown in figure 1. Hadronically produced J/ψ mesons have a mean p_T around 2 GeV/c, and the spectrum shown in figure 1 is consistent with hadronic production down to a p_T of about 0.3 GeV/c. Below this value, there is a very strong excess, which cannot be reproduced by any model assuming hadronic production, but which is consistent with the sum of the expected hadronic production plus a contribution from coherently photoproduced J/ψ. This last contribution is shown with the Monte Carlo template in the figure. The yield in this region of phase space is about a factor of seven above what is expected from a scaling of the hadronic yield with the number of binary nucleon–nucleon collisions. This unexpectedly large value implies that there is a physics process at play that has not been taken into account in currently available models. Assuming that the underlying process is photoproduction, ALICE obtained the corresponding cross-section.

While in hadronic collisions the nuclei break, they each act as one entity in coherent photoproduction, where the smallness of the p_T is related, via the Heisenberg uncertainty principle, to the size of the lead nucleus. Interestingly, current models of coherent photoproduction integrated over the impact-parameter range corresponding to peripheral collisions predict cross-sections with the right magnitude.

New Pb–Pb collision data at √s_NN= 5.02 TeV recorded by ALICE in 2015 should allow us to quantify this excess with higher precision and to evaluate its strength in more central collisions. Whether this new source of very-low-p_T J/ψ will provide an additional probe of the properties of the QGP remains an open question.

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