This book provides an introduction to various methods developed in string theory to tackle problems in condensed-matter physics. This is the field where string theory has been most largely applied, thanks to the use of the correspondence between anti-de Sitter spaces (AdS) and conformal field theories (CFT). Formulated as a conjecture 20 years ago by Juan Maldacena of the Institute for Advanced Study, the AdS/CFT correspondence relates string theory, usually in its low-energy version of supergravity and in a curved background space–time, to field theory in a flat space–time of fewer dimensions. This correspondence is holographic, which means in some sense that the physics in the higher dimension is projected onto a flat surface without losing information.
The book is articulated in four parts. In the first, the author introduces modern topics in condensed-matter physics from the perspective of a string theorist. Part two gives a basic review of general relativity and string theory, in an attempt to make the book as self-consistent as possible. The other two parts focus on the applications of string theory to condensed-matter problems, with the aim of providing the reader with the tools and methods available in the field. Going into more detail, part three is dedicated to methods already considered as standard – such as the pp-wave correspondence, spin chains and integrability, AdS/CFT phenomenology and the fluid-gravity correspondence – while part four deals with more advanced topics that are still in development, including Fermi and non-Fermi liquids, the quantum Hall effect and non-standard statistics.
Aimed at graduate students, this book assumes a good knowledge of quantum field theory and solid-state physics, as well as familiarity with general relativity.
This book is a collection of articles dedicated to topics within the field of Standard Model physics, authored by some of the main players in both its theory and experimental development. It is edited by Luciano Maiani and Luigi Rolandi, two well-known figures in high-energy physics.
The volume has 21 chapters, most of them devoted to very specific subjects. The first chapters take the reader through a fascinating tour of the history of the field, starting from the earliest days, around the time when CERN was established. I particularly enjoyed reading some recollections of Gerard ’t Hooft, such as: “Asymptotic freedom was discovered three times before 1973 (when Politzer, Gross and Wilczek published their results), but not recognised as a new discovery. This is just one of those cases of miscommunication. The ‘experts’ were so sure that asymptotic freedom was impossible, that signals to the contrary were not heard, let alone believed. In turn, when I did the calculation, I found it difficult to believe that the result was still not known.”
In chapter three, K Ellis reviews the evolution of our understanding of quantum chromodynamics (QCD) and deep-inelastic scattering. Among many things, he shows how the beta function depends on the strong coupling constant, αS, and explains why many perturbative calculations can be made in QCD, when the interactions take place at high-enough energies. At the hadronic scale, however, αS is too large and the perturbative expansion tool no longer works, so alternative methods have to be used. Many non-perturbative effects can be studied with the lattice QCD approach, which is addressed in chapter five. The experimental status regarding αS is reviewed in the following chapter, where G Dissertori shows the remarkable progress in measurement precision (with LHC values reaching per-cent level uncertainties and covering an unprecedented energy range), and how the data is in excellent agreement with the theoretical expectations.
Through the other chapters we can find a large diversity of topics, including a review of global fits of electroweak observables, presently aimed at probing the internal consistency of the Standard Model and constraining its possible extensions given the measured masses of the Higgs boson and of the top quark. Two chapters focus specifically on the W-boson and top-quark masses. Also discussed in detail are flavour physics, rare decays, neutrino masses and oscillations, as is the production of W and Z bosons, in particular in a chapter by M Mangano.
The Higgs boson is featured in many pages: after a chapter by J Ellis, M Gaillard and D Nanopoulos covering its history (and pre-history), its experimental discovery and the measurement of its properties fill two further chapters. An impressive amount of information is condensed in these pages, which are packed with many numbers and (multi-panel) figures. Unfortunately, the figures are printed in black and white (with only two exceptions), which severely affects the clarity of many of them. A book of this importance deserved a more colourful destiny.
The editors make a good point in claiming the time has come to upgrade the Standard Model into the “Standard Theory” of particle physics, and I think this book deserves a place in the bookshelves of a broad community, from the scientists and engineers who contributed to the progress of high-energy physics to younger physicists, eager to learn and enjoy the corresponding inside stories.
This monograph on special relativity (SR) is presented in a form accessible to a broad readership, from pre-university level to undergraduate and graduate students. At the same time, it will also be of great interest to professional physicists.
Relativity Matters has all the hallmarks of becoming a classic with further editions, and appears to have no counterpart in the literature. It is particularly useful because at present SR has become a basic part not only of particle and space physics, but also of many other branches of physics and technology, such as lasers. The book has 29 chapters organised in 11 parts, which cover topics from the basics of four-vectors, space–time, Lorentz transformations, mass, energy and momentum, to particle collisions and decay, the motion of charged particles, covariance and dynamics.
The first half of the book derives basic consequences of the SR assumptions with a minimum of mathematical tools. It concentrates on the explanation of apparently paradoxical results, presenting and refuting counterarguments as well as debunking various incorrect statements in elementary textbooks. This is done by cleverly exploiting the Galilean method of a dialogue between a professor, his assistant and a student, to bring out questions and objections.
The importance of correctly analysing the consequences for extended and accelerating bodies is clearly presented. Among the many “paradoxes”, one notes the accelerating rocket problem that the late John Bell used to tease many of the world’s most prominent physicists with. Few of them provided a perfectly satisfactory answer.
The second half of the book, starting from part VII, covers the usual textbook material and techniques at graduate level, illustrated with examples from the research frontier. The introductions to the various chapters and subsections are still enjoyable for a broader readership, requiring little mathematics. The author does not avoid technicalities such as vector and matrix algebra and symmetries, but keeps them to a minimum. However, in the parts dealing with electromagnetism, the reader is assumed to be reasonably familiar with Maxwell’s equations.
There are copious concrete exercises and solutions. Throughout the book, indeed, every chapter is complemented by a rich variety of problems that are fully worked out. These are often used to illustrate quantitatively intriguing topics, from space travel to the laser acceleration of charged particles.
An interesting afterword concluding the book discusses how very strong acceleration becomes a modern limiting frontier, beyond which SR in classical physics becomes invalid. The magnitude of the critical accelerations and critical electric and magnetic fields are qualitatively discussed. It also briefly analyses attempts by well-known physicists to side-step the problems that arise as a consequence.
Relativity Matters is excellent as an undergraduate and graduate textbook, and should be a useful reference for professional physicists and technical engineers. The many non-specialist sections will also be enjoyed by the general, science-interested public.Torleif Ericson, CERN
The Compact Linear Collider (CLIC) workshop is the main annual gathering of the CLIC accelerator and detector communities, and this year attracted more than 220 participants to CERN on 22–26 January. CLIC is a proposed electron-positron linear collider envisaged for the era beyond the high-luminosity LHC (HL-LHC), that would operate a staged programme over a period of 25 years with collision energies at 0.38, 1.5, and 3 TeV. This year the CLIC workshop focused on preparations for the update of the European Strategy for Particle Physics in 2019–2020.
The initial CLIC energy stage is optimised to provide high-precision Higgs boson and top-quark measurements, with the higher-energy stages enhancing sensitivity to effects from beyond-Standard Model (BSM) physics (CERN Courier November 2016 p20). Following a 2017 publication on Higgs physics, the workshop heard reports on recent developments in top-quark physics and the BSM potential at CLIC, both of which are attracting significant interest from the theory community.
Speakers also reported extensive progress in the validation and performance of the new detector model. To ensure that its performance meets the challenging specifications, a new approach to tracking has been commissioned, and the particle flow analysis and flavour-tagging capabilities have been consolidated. Updates were presented on the broad and active R&D programme on the vertex and tracking detectors, which aims to find technologies that simultaneously fulfil all the CLIC requirements. Reports were given on test-beam campaigns with both hybrid and monolithic assemblies, and on ideas for future developments. Many of the tracking and calorimeter technologies under study for the CLIC detector are also of interest to the HL-LHC, where the high granularity and time-resolution needed for CLIC are equally crucial.
For the accelerator, studies with the aim of reducing the cost and power have particular priority, presenting the initial CLIC stage as a project requiring resources comparable to what was needed for LHC. Key activities in this context are high-efficiency RF systems, permanent magnet studies, optimised accelerator structures and overall implementation studies related to civil engineering, infrastructure, schedules and tunnel layout.
A key aspect of the ongoing accelerator development is moving towards industrialisation of the component manufacture, by fostering wider applications of the CLIC 12 GHz X-band technology with external partners. In this respect, the CLIC workshop coincided with the kick-off meeting for the CompactLight project recently funded by the Horizon 2020 programme, which aims to design an optimised X-ray free-electron laser based on X-band technology for more compact and efficient accelerators (CERN Courier December 2017 p8).
Last year also saw the realisation of the CERN Linear Electron Accelerator for Research (CLEAR), a new user facility for accelerator R&D whose programme includes CLIC high-gradient and instrumentation studies (CERN Courier November 2017 p8). Presentations at the workshop addressed the programmes for instrumentation and radiation studies, plasma-lensing, wakefield monitors and high-energy electrons for cancer therapy.
During 2018 the CLIC accelerator and detector and physics collaborations will prepare summary reports focusing on the 380 GeV initial CLIC project implementation as inputs for the update of the European Strategy for Particle Physics, including plans for the project preparation phase in 2020–2025.
The second Future Circular Collider (FCC) physics workshop was held at CERN on 15–19 January, gathering particle physicists from around the world for talks and detailed discussions on the physics capabilities of future electron–positron, electron–proton, and proton–proton colliders.
The FCC study, which emerged following the 2013 European Strategy for Particle Physics, is a five-year project led by CERN to investigate a circular collider built in a new 100 km-circumference tunnel in the Geneva region. Such a tunnel could host an e+e– collider (called FCC-ee), a 100 TeV proton–proton collider (FCC-hh) or an electron–proton collider (FCC-eh). Further opportunities include the collision of heavy ions in FCC-hh and FCC-eh, and fixed-target experiments using the injector complex.
Last year saw a significant evolution in the maturity of the physics studies for these machines, with many detailed results presented. These results include new techniques to determine the properties of the Higgs boson, such as the all-important Higgs potential, and how these relate to fundamental questions at the smallest distance scales. New ideas about how to search for new particles interacting very weakly with normal matter – such as new species of neutrinos, dark photons or other new light scalar particles – were also studied in depth.
The January workshop was preceded by a dedicated meeting to determine whether the unprecedented precision of physics measurements provided by FCC machines could be compared against equally high-precision theoretical predictions.
The results of this study were affirmative, as reported on the first day of the FCC physics workshop.
A major theme that emerged during the workshop was the depth of complementarity between the capacities of the different FCC modes in exploring the questions that will remain open after the completion of the LHC programme. Combining their individual strengths will enable comprehensive exploration in search of answers to the pending questions in particle physics.
Multi-messenger astronomy, neutrino physics and dark matter are among several topics in astroparticle physics set to take priority in Europe in the coming years, according to a report by the Astroparticle Physics European Consortium (APPEC).
The APPEC strategy for 2017–2026, launched at an event in Brussels on 9 January, is the climax of two years of talks with the astroparticle and related communities. 20 agencies in 16 countries are involved and includes representation from the European Committee for Future Accelerators, CERN and the European Southern Observatory (ESO).
Lying at the intersection of astronomy, particle physics and cosmology, astroparticle physics is well placed to search for signs of physics beyond the standard models of particle physics and cosmology. As a relatively new field, however, European astroparticle physics does not have dedicated intergovernmental organisations such as CERN or ESO to help drive it. In 2001, European scientific agencies founded APPEC to promote cooperation and coordination, and specifically to formulate a strategy for the field.
Building on earlier strategies released in 2008 and 2011, APPEC’s latest roadmap presents 21 recommendations spanning scientific issues, organisational aspects and societal factors such as education and industry, helping Europe to exploit tantalising potential for new discoveries in the field.
There are plans to join forces with experiments in the US to build the next generation of NDBD detectors.
The recent detection of gravitational waves from the merger of two neutron stars (CERN Courier December 2017 p16) opens a new line of exploration based on the complementary power of charged cosmic rays, electromagnetic waves, neutrinos and gravitational waves for the study of extreme events such as supernovae, black-hole mergers and the Big Bang itself. “We need to look at cross-fertilisation between these modes to maximise the investment in facilities,” says APPEC chair Antonio Masiero of the INFN and the University of Padova. “This is really going to become big.”
APPEC strongly supports Europe’s next-generation ground-based gravitational interferometer, the Einstein Telescope, and the space-based LISA detector. In the neutrino sector, KM3NeT is being completed for high-energy cosmic neutrinos at its site in Sicily, as well as for precision studies of atmospheric neutrinos at its French site near Toulon. Europe is also heavily involved in the upgrade of the leading cosmic-ray facility the Pierre Auger Observatory in Argentina. Significant R&D work is taking place at CERN’s neutrino platform for the benefit of long- and short-baseline neutrino experiments in Japan and the US (CERN Courier July/August 2016 p21), and Europe is host to several important neutrino experiments. Among them are KATRIN at KIT in Germany, which is about to begin measurements of the neutrino absolute mass scale, and experiments searching for neutrinoless double-beta decay (NDBD) such as GERDA and CUORE at INFN’s Gran Sasso National Laboratory (CERN Courier December 2017 p8).
There are plans to join forces with experiments in the US to build the next generation of NDBD detectors. APPEC has a similar vision for dark matter, aiming to converge next year on plans for an “ultimate” 100-tonne scale detector based on xenon and argon via the DARWIN and Argo projects. APPEC also supports ESA’s Euclid mission, which will establish European leadership in dark-energy research, and encourages continued European participation in the US-led DES and LSST ground-based projects. Following from ESA’s successful Planck mission, APPEC strongly endorses a European-led satellite mission, such as COrE, to map the cosmic-microwave background and the consortium plans to enhance its interactions with its present observers ESO and CERN in areas of mutual interest.
“It is important at this time to put together the human forces,” says Masiero. “APPEC will exercise influence in the European Strategy for Particle Physics, and has a significant role to play in the next European Commission Framework Project, FP9.”
A substantial investment is needed to build the next generation of astroparticle-physics research, the report concedes. According to Masiero, European agencies within APPEC currently invest around €80 million per year in astroparticle-related activities, in addition to funding large research infrastructures. A major effort in Europe is necessary for it to keep its leading position. “Many young people are drawn into science by challenges like dark matter and, together with Europe’s existing research infrastructures in the field, we have a high technological level and are pushing industries to develop new technologies,” continues Masiero. “There are great opportunities ahead in European astroparticle physics.”
Researchers at TRIUMF in Canada have reported the first production of ultracold neutrons (UCN), marking an important step towards a future neutron electric dipole moment (nEDM) experiment at the Vancouver laboratory. Precision measurements of the nEDM are a sensitive probe of physics beyond the Standard Model: if a nonzero value were to be measured, it would suggest a new source of CP violation, possibly related to the baryon asymmetry of the universe.
The TUCAN collaboration (TRIUMF UltraCold Advanced Neutron source) aims to measure nEDM a factor 30 better than the present best measurement, which has a precision of 3 × 10–26ecm and is consistent with zero. For this to be possible, physicists need to provide the world’s highest density of ultracold neutrons. In 2010 a collaboration between Canada and Japan was established to realise such a facility and a prototype UCN source was shipped to Canada and installed at TRIUMF in early 2017.
The setup uses a unique combination of proton-induced spallation and a superfluid helium UCN source that was pioneered in Japan. A tungsten block stops a beam of protons, producing a stream of fast neutrons that are then slowed in moderators and converted to ultracold speeds (less than around 7 ms–1) by phonon scattering in superfluid helium. The source is based on a non-thermal down-scattering process in superfluid helium below 1 K, which gives the neutrons an effective temperature of a few mK. The ultracold temperature is below the neutron optical potential for many materials, which means the neutrons are totally reflected for all angles of incidence and can be stored in bottles for periods of up to hundreds of seconds.
Tests late last year demonstrated the highest current operation of this particular source, resulting in the most UCNs it has ever produced (> 300,000) in a single 60-second-long irradiation at a 10 µA proton beam current. This is a record for TRIUMF, but the UCN source intensity is still two orders of magnitude below what is needed for the nEDM experiment.
Funding of C$15.7 million to upgrade the UCN facility, a large proportion of which was granted by the Canada Foundation for Innovation in October 2017, will enable the TUCAN team to increase the production of neutrons at higher beam current to levels competitive with other planned nEDM experiments worldwide. These include proposals at the Paul Scherrer Institute in Switzerland, Los Alamos National Laboratory in the US, the Institut Laue–Langevin in France and others in Germany and Russia. The neutron EDM is experiencing intense competition, with most projects differing principally in the way they propose to produce the ultracold neutrons (CERN Courier September 2016 p27).
The nEDM experimental campaign at TRIUMF is scheduled to start in 2021. “The TRIUMF UCN source is the only one combining a spallation source of neutrons with a superfluid helium production volume, providing the project its uniqueness and competitive edge,” says team member Beatrice Franke.
On 8 January the Republic of Lithuania formally became an associate Member State of CERN, following the completion of its internal approval procedures (CERN Courier July/August 2017 p7). Lithuania’s relationship with CERN dates back to an International Cooperation Agreement signed in 2004, with Lithuanian researchers contributing to the CMS experiment since 2007. Its new status strengthens the long-term partnership between CERN and the Lithuanian scientific community, makes Lithuanian scientists eligible for staff appointments, and entitles Lithuanian industry to bid for CERN contracts.
Since 4 December, around 500 technicians and engineers have been working flat-out to maintain and upgrade the Large Hadron Collider (LHC) and other parts of the CERN accelerator complex. The current year-end technical stop will last until 9 March, and preparations for the machine and its infrastructure for the High Luminosity LHC (HL-LHC) have been a focus of activities.
Collimators are key to operating the HL-LHC, which will have roughly twice the stored energy (700 MJ) as the present machine. These devices control losses from the circulating proton beams so that they can be constrained to a small section of the machine’s circumference. Continuing work undertaken during last year’s extended year-end stop (CERN Courier March 2017 p9), two new collimators are being installed at point 1 containing a wire that generates an electromagnetic field to compensate for long-range beam–beam effects.
Higher performing injectors that can produce more intense particle beams are another demand of the HL-LHC, and this aspect is being managed by the LHC Injector Upgrade (LIU) project (CERN Courier October 2017 p32). An upgraded kicker magnet, one of eight fast-pulsed magnets that inject particle beams coming from the Super Proton Synchrotron (SPS) into the LHC, will be installed at point 8. A special coating applied to the inner wall of the ceramic pipe of the magnet is one of several techniques developed to reduce the heating of components in the harsher HL-LHC environment.
While work steps up on the LHC, which has been temporarily emptied of its 120 tonnes of helium coolant, brand-new accelerator technology that will help the HL-LHC achieve its unprecedented luminosities is being prepared for tests in the SPS. Two prototype radiofrequency crab cavities – designed to tilt particle bunches before they collide to maximise the overlapping of the beams and increase the probability of collisions – have been installed for testing during 2018. In around five years from now, during Long Shutdown 3 (LS3), the full system will be installed in the LHC.
Further down the accelerator chain, a major de-cabling campaign is taking place in the Proton Synchrotron (PS) to create space for the deployment of the LIU project during Long Shutdown 2 (LS2) beginning next year. The transfer line linking the PS to the SPS is also having all of its 43 quadrupole magnets replaced, among numerous other works. The whole CERN injector chain is undergoing an annual check-up, in particular concerning the cooling, ventilation, cryogenics and electrical supply systems. Other important activities are taking place to consolidate the infrastructure, such as the installation of a new lift at LHC point 8, and to update the beam control systems.
During the 2018 LHC performance workshop, held in Chamonix from 29 January to 2 February, the performance of the LHC during 2017 was reviewed and operational scenarios for 2018 were discussed. A particular focus of the workshop was on the status of the LIU and HL-LHC projects, which will be rolled-out in LS2 and LS3, respectively. There was lively discussion about the organisation and planning of activities for LS2, and the final session of the workshop covered the full energy exploitation of the LHC. Until LS2 the machine will run at a centre-of-mass energy of 13 TeV, but prospects for running at 14 TeV after LS2 and eventuallly even 15 TeV were also discussed.
The LHCb collaboration has shed light on a long-standing anomaly in the very rare hyperon decay Σ+→ pµ+µ– first observed in 2005 by Fermilab’s HyperCP experiment. The HyperCP team found that the branching fraction for this process is consistent with Standard Model (SM) predictions, but that the three signal events observed exhibited an interesting feature: all muon pairs had invariant masses very close to each other, instead of following a scattered distribution.
This suggested the existence of a new light particle, X0, with a mass of about 214 MeV/c2, which would be produced in the Σ+ decay along with the proton and would decay subsequently to two muons. Although this particle has been long sought in various other decays and at several experiments, no experiment other than HyperCP has so far been able to perform searches using the same Σ+ decay mode.
The large rate of hyperon production in proton–proton collisions at the LHC has recently allowed the LHCb collaboration to search for the Σ+→ pµ+µ– decay. Given the modest transverse momentum of the final-state particles, the probability that such a decay is able to pass the LHCb trigger requirements is very small. Consequently, events where the trigger is activated by particles produced in the collisions other than those in the decay under study are also employed.
This search was performed using the full Run 1 dataset, corresponding to an integrated luminosity of 3 fb–1 and about 1014 Σ+ hyperons. An excess of about 13 signal events is found with respect to the background-only expectation, with a significance of four standard deviations. The dimuon invariant- mass distribution of these events was examined and found to be consistent with the SM expectation, with no evidence of a cluster around 214 eV/c2. The signal yield was converted to a branching fraction of (2.1+1.6–1.2) × 10–8 using the known Σ+→ pπ0 decay as a normalisation channel, in excellent agreement with the SM prediction. When restricting the sample explicitly to the case of a decay with the putative X0 particle as an intermediate state, no excess was found. This sets an upper limit on the branching fraction at 9.5 × 10–9 at 90% CL, to be compared with the HyperCP result (3.1+2.4–1.9± 1.5) × 10–8.
This result, together with the recent search for the rare decay KS→ μ+μ– shows the potential of LHCb in performing challenging measurements with strange hadrons. As with a number of results in other areas reported recently, LHCb is demonstrating its power not only as a b-physics experiment but as a general-purpose one in the forward region. With current data, and in particular with the upgraded detector thanks to the software trigger from Run 3 onwards, LHCb will be the dominant experiment for the study of both hyperons and KS mesons, exploiting their rare decays to provide a new perspective in the quest for physics beyond the SM.
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