Annual Review of Nuclear and Particle Science - Volume 68, 2018

In this article we review the life and work of Guido Altarelli (1941–2015).

Why do we see certain types of strongly interacting elementary particles and not others? This question was posed more than 50 years ago in the context of the quark model. M. Gell-Mann and G. Zweig proposed that the known mesons were and baryons qqq, with the quarks known at the time, u (up), d (down), and s (strange), having charges of 2/3, −1/3, and −1/3, respectively. Mesons and baryons would then have integral charges. Mesons such as and baryons such as would also have integral charges. Why weren't they seen? They have now been seen, but only with additional heavy quarks and under conditions that tell us a lot about the strong interactions and how they manifest themselves. This review describes recent progress in our understanding of such exotic mesons and baryons.

Penning-trap mass spectrometry in atomic and nuclear physics has become a well-established and reliable tool for the determination of atomic masses. In combination with short-lived radioactive nuclides it was first introduced at ISOLTRAP at the Isotope Mass Separator On-Line facility (ISOLDE) at CERN. Penning traps have found new applications in coupling to other production mechanisms, such as in-flight production and separation systems. The applications in atomic and nuclear physics range from nuclear structure studies and related precision tests of theoretical approaches to description of the strong interaction to tests of the electroweak Standard Model, quantum electrodynamics and neutrino physics, and applications in nuclear astrophysics. The success of Penning-trap mass spectrometry is due to its precision and accuracy, even for low ion intensities (i.e., low production yields), as well as its very fast measurement cycle, enabling access to short-lived isotopes. The current reach in relative mass precision goes beyond δm/m=10⁻⁸, the half-life limit is as low as a few milliseconds, and the sensitivity is on the order of one ion per minute in the trap. We provide a comprehensive overview of the techniques and applications of Penning-trap mass spectrometry in nuclear and atomic physics.

We review various applications of dispersion relations (DRs) to the electromagnetic structure of hadrons. We discuss the way DRs allow one to extract information about hadron structure constants by connecting information from complementary scattering processes. We consider the real and virtual Compton scattering processes off the proton, and summarize recent advances in the DR analysis of experimental data to extract the proton polarizabilities, in comparison with alternative studies based on chiral effective field theories. We discuss a multipole analysis of real Compton scattering data, along with a DR fit of the energy-dependent dynamical polarizabilities. Furthermore, we review new sum rules for the double-virtual Compton scattering process off the proton, which allow for model-independent relations between polarizabilities in real and virtual Compton scattering, and moments of nucleon structure functions. Information on double-virtual Compton scattering is used to predict and constrain the polarizability corrections to muonic hydrogen spectroscopy.

Neutrino interactions with nuclei have been the subject of intense interest during the last 15 years. Current and future measurements of neutrino oscillation and exotic physics use order 0.1–10 GeV neutrinos on a range of nuclear targets (¹²C, ¹⁶O, ⁴⁰Ar). As the precision of these experiments has increased, information from their detectors and dedicated experiments has indicated deficiencies in the modeling of neutrino interactions on nuclear targets. Here, we present the current state of knowledge about neutrino–nucleus interactions, the challenge of extracting the cross section of these processes, and current experimental puzzles in the field. We also look forward to new and novel measurements and future efforts that seek to resolve these questions.

We review recent progress in S=−2 hypernuclei such as double-Λ hypernuclei and Ξ hypernuclei, which are composed of a nucleus and one or two hyperons, such as a Λ or a Ξ particle. Through observations of He, such as the Nagara event, we obtain important information about the ΛΛ interaction. Using this information, we perform a four-body calculation of ααΛΛ for Be, which was observed at KEK as the Demachi–Yanagi event. We interpret this event as the excited state. We calculate energy levels of Be within the framework of an ααnΛΛ five-body cluster model, and then interpret the Hida event, which was observed at KEK as the ground state of Be. Motivated by observation of the Kiso event of C, by using the Skyrme–Hartree–Fock and relativistic mean-field frameworks, we calculate the energy spectra of this hypernucleus. We interpret this event as the ¹⁴N(g.s.)+Ξ⁻(0p) state. Finally, we propose an experiment on H and Li to extract information about the spin- and isospin-averaged parts of the ΞN interaction.

Machine learning has played an important role in the analysis of high-energy physics data for decades. The emergence of deep learning in 2012 allowed for machine learning tools which could adeptly handle higher-dimensional and more complex problems than previously feasible. This review is aimed at the reader who is familiar with high-energy physics but not machine learning. The connections between machine learning and high-energy physics data analysis are explored, followed by an introduction to the core concepts of neural networks, examples of the key results demonstrating the power of deep learning for analysis of LHC data, and discussion of future prospects and concerns.

The article describes the early years of the two large general-purpose experiments, ATLAS and CMS, at CERN's Large Hadron Collider (LHC). It covers the early conception of the detector designs to achieve the physics goals, the subsequent building of the worldwide collaborations, the evolution of the designs incorporating advances in technology and other considerations, and the painstaking global construction efforts. A detailed technical description of the detectors is beyond the scope of this review. This article also describes the development and deployment of the software and computing systems, by both the collaborations and the LHC Worldwide Computing Grid, in order to extract the physics results.

The bulk motion of nuclear matter at the ultrahigh temperatures created in heavy ion collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider is well described in terms of nearly inviscid hydrodynamics, thereby establishing this system of quarks and gluons as the most perfect fluid in nature. A revolution in the field is under way, spearheaded by the discovery of similar collective, fluid-like phenomena in much smaller systems including p+p, p+A, d+Au, and ³He+Au collisions. We review these exciting new observations and their profound implications for hydrodynamic descriptions of small and/or out-of-equilibrium systems.

Understanding the origin of the elements has been a decades-long pursuit, with many open questions remaining. Old stars found in the Milky Way and its dwarf satellite galaxies can provide answers because they preserve clean element abundance patterns of the nucleosynthesis processes that operated some 13 billion years ago, enabling reconstruction of the chemical evolution of the elements. This review focuses on the astrophysical signatures of heavy neutron-capture elements made in the s-, i-, and r-processes found in old stars. A highlight is the recently discovered r-process galaxy Reticulum II, which was enriched by a neutron star merger. These results show that old stars in dwarf galaxies provide a novel means to constrain the astrophysical site of the r-process, ushering in much-needed progress on this major outstanding question. This nuclear astrophysics research complements the many experimental and theoretical nuclear physics efforts into heavy-element formation, and also aligns with results on the gravitational-wave signature of neutron star mergers.

We review the development of silicon-based calorimeters from the very first applications of small calorimeters used in collider experiments to the large-scale systems that are being designed today. We discuss silicon-based electromagnetic calorimeters for future e⁻e⁺ colliders and for the upgrade of the CMS experiment's endcap calorimeter to be used in the high-luminosity phase of the LHC. We present the intrinsic advantages of silicon as an active detector material and highlight the enabling technologies that have made calorimeters with very high channel densities feasible. We end by discussing the outlook for further extensions to the silicon calorimeter concept, such as calorimeters with fine-pitched pixel detectors.

The automation of one-loop amplitudes plays a key role in addressing several computational challenges for hadron collider phenomenology: They are needed for simulations including next-to-leading-order corrections, which can be large at hadron colliders. They also allow the exact computation of loop-induced processes. A high degree of automation has now been achieved in public codes that do not require expert knowledge and can be widely used in the high-energy physics community. In this article, we review many of the methods and tools used for the different steps of automated one-loop amplitude calculations: renormalization of the Lagrangian, derivation and evaluation of the amplitude, its decomposition onto a basis of scalar integrals and their subsequent evaluation, as well as computation of the rational terms.

This article reviews our present understanding of neutrino properties with a particular emphasis on observable differences between Majorana and Dirac neutrinos. We summarize current and future experimental efforts toward measuring neutrino properties and describe consequences of the Majorana versus Dirac nature of neutrinos on neutrino masses, neutrino decays, and neutrino electromagnetic properties.

Heavy ion collisions quickly form a droplet of quark–gluon plasma (QGP) with a remarkably small viscosity. We give an accessible introduction to how to study this smallest and hottest droplet of liquid made on Earth and why it is so interesting. The physics of heavy ion collisions ranges from highly energetic quarks and gluons described by perturbative QCD to a bath of strongly interacting gluons at lower energy scales. These gluons quickly thermalize and form QGP, while the energetic partons traverse this plasma and end in a shower of particles called jets. Analyzing the final particles in various ways allows us to study the properties of QGP and the complex dynamics of multiscale processes in QCD that govern its formation and evolution, providing what is perhaps the simplest form of complex quantum matter that we know of. Much remains to be understood, and throughout the review big open questions are encountered.

In this review, we first reassess the supernova remnant paradigm for the origin of Galactic cosmic rays in the light of recent cosmic-ray data acquired by the Voyager 1 spacecraft. We then describe the theory of light-element nucleosynthesis by nuclear interaction of cosmic rays with the interstellar medium and outline the problem of explaining the measured beryllium abundances in old halo stars of low metallicity with the standard model of the Galactic cosmic-ray origin. We then discuss the various cosmic-ray models proposed in the literature to account for the measured evolution of the light elements in the Milky Way, and point out the difficulties that they all encounter. It seems to us that, among all possibilities, the superbubble model provides the most satisfactory explanation for these observations.

Jefferson Lab's upgrade of its Continuous Electron Beam Accelerator Facility (CEBAF) has recently been completed. The project involved an upgrade of the accelerator to achieve a maximum beam energy of 12 GeV and the construction of a fourth end station, Experimental Hall D, as well as new detector equipment for two of the three existing halls (A, B, and C). A broad experimental program has been developed to map the nucleon's intrinsic quark distributions in transverse space and in longitudinal momentum through measurements of deeply exclusive and semi-inclusive processes, and to probe color confinement by studying the spectrum of hadrons with active gluon degrees of freedom in the wave function. Other programs include the forward parton distribution function at large quark momentum fraction x, the quark and gluon polarized distribution functions, measurements of electromagnetic form factors of the nucleon ground state and of nucleon resonance transitions at short distances, and the exploration of physics beyond the Standard Model in high-precision parity-violating processes and in the search for signals of dark matter. The higher beam energy is also suitable for exploration of quark hadronization properties using the nucleus as a laboratory. This review highlights major areas of hadron and nuclear science that will be the focus of the first 5 years of operation.

Colliders, among the most successful tools of particle physics, have revealed much about matter. This review describes how colliders contribute to the search for particle dark matter, focusing on the highest-energy collider currently in operation, the Large Hadron Collider (LHC) at CERN. In the absence of hints about the character of interactions between dark matter and standard matter, this review emphasizes what could be observed in the near future, presents the main experimental challenges, and discusses how collider searches fit into the broader field of dark matter searches. Finally, it highlights a few areas to watch for the future LHC program.