Des quarks au menu à Munich

Quelque 400 théoriciens et expérimentateurs du monde entier se sont réunis à Munich du 8 au 12 octobre pour échanger sur les dernières avancées de la théorie des interactions fortes. C’était la dixième édition de la Conférence sur le confinement des quarks et le spectre hadronique (ConfX). Les principaux thèmes de discussion ont été répartis entre sept sessions : structure du vide et confinement ; quarks légers ; quarks lourds ; déconfinement ; QCD et nouvelle physique ; physique nucléaire et physique des astroparticules ; théories à couplage fort. Ont été évoqués également des sujets se situant aux limites du domaine, par exemple les approches tendant à l’application de la chromodynamique quantique (QCD) à la physique nucléaire et à l’astrophysique.

Some 400 theorists and experimentalists from all around the world convened in Munich on 8–12 October to discuss developments in the theory of strong interactions. They were attending the tenth conference on "Quark Confinement and the Hadron Spectrum" (ConfX) at the Garching Research Campus, hosted by the Physics Department of the Technical University of Munich (TUM), with support from the Excellence Cluster "Origin and Structure of the Universe". Topics included areas at the boundaries of the field, such as theories beyond the Standard Model with a strongly coupled sector and QCD approaches to nuclear physics and astrophysics.

Inaugurated in 1994 in Como, Italy, this series of conferences has established itself as an important forum in the field, bringing together people working in strong interactions on approaches that range from lattice QCD to perturbative QCD, models of the QCD vacuum to phenomenology and experiments, the mechanism of confinement to deconfinement and heavy-ion physics, and from effective field theories to physics beyond the Standard Model. Taking place at a particularly important time for particle physics, with the observation of a Higgs-like particle at CERN, the tenth conference provided a valuable opportunity not only to reconsider what was done on past occasions but also to discuss the perspectives for strongly coupled theories.

The scientific focus of ConfX was spread across seven main scientific sessions: vacuum structure and confinement; light quarks; heavy quarks; deconfinement; QCD and new physics; nuclear and astroparticle physics; and strongly coupled theories. These subjects are relevant for the physics of B factories (Belle and BaBar), tau-charm experiments (BESIII), LHC experiments (LHCb, CMS, ATLAS), heavy-ion experiments (RHIC, ALICE), future experiments at FAIR-GSI (Panda, CMB) and in general for many low-energy experiments (such as at Jefferson Lab, COSY, MAMI) and some parts of experimental astrophysics.

It is impossible to summarize here the wealth of results presented at the meeting, the intensity of the discussions and the flow of information. What follows is just a brief selection.

The first plenary session began with recent progress in the theoretical calculations of double parton-scattering at the LHC presented by Aneesh Manohar of the University of California, San Diego. The application of soft collinear effective theory to many collider physics processes was then introduced by Thomas Becher of Bern University and followed by a review of quarkonium production by Kuang-Ta Chao of Peking University. In particular, J/ψ production has now finally been calculated at next-to-leading order in nonrelativistic QCD (NRQCD) and the extraction of colour-octet matrix elements from a combined fit to collider data has become possible for the first time. The current picture hints at the universality of the NRQCD matrix elements and a proof of the NRQCD factorization in the fragmentation approach seems to be close. Predictions for the production of Υ and other quarkonia states at the LHC experiments are now available. The progress in theory together with the new LHC data should soon allow the resolution of the long-standing puzzles about the J/ψ polarization and the production mechanism of quarkonium, both at hadron colliders and at B factories.

Heavy ions and more

The study of quarkonium production and suppression at finite temperature in heavy-ion collisions as a probe of quark–gluon plasma was reviewed in the context of a new effective field-theory approach (potential NRQCD at finite temperature). Here the shift in paradigm from the typical phenomenological description is apparent, the quarkonium dissociation being caused by the emergence of a large imaginary part in the quark–antiquark potential rather than by a Debye screening phenomenon as reported by Jacopo Ghiglieri of McGill University. The effective field-theory approach allows a systematic calculation of the thermal modifications in the energy and width of the Υ(1S) as produced at the LHC in heavy-ion collisions.

There has been great progress in developing the capabilities of the lattice approach to calculate the properties of heavy and light quarks, and also in connection to chiral effective field theories, as Peter Lepage of Cornell University, Laurent Lellouch of the Centre de Physique Théorique, Marseilles, and Zoltan Fodor of the University of Wuppertal reported.

The interest and relevance of light scalars, as well as the long-standing controversy dating back to the 1950s about their existence and nature, has been resolved in recent years by means of better data and more powerful theoretical techniques that include effective Lagrangians and dispersion theory, as José Pelaez of the Complutense University of Madrid argued.

Highlights in strong physics beyond the Standard Model presented at the conference include: composite dynamics as put in context by Francesco Sannino of the Centre for Cosmology and Particle Physics Phenomenology, Odense, at the time of the Higgs discovery; gauge gravity duality; holographic QCD explained by Shigeki Sugimoto from Tokyo University; and applications of anti-deSitter/conformal field theory correspondence to heavy-ion collisions contrasted to proton–proton physics at the LHC now and in the future, including the outstanding LHC results, presented by Günther Dissertori of ETH Zurich. This session culminated in a heated discussion about future strongly coupled scenarios, led by Antonio Pich of Valencia University, in which different views of scenarios beyond the Standard Model were discussed but remained unreconciled among the panel members Estia Eichten of Fermilab, Emanuel Katz of Boston University, Juan Maldacena of the Institute of Advanced Study, Princeton, and Stefan Pokorski of the University of Warsaw.

The plenary session on Wednesday morning was dedicated to the impact of QCD on nuclear and astroparticle physics. Opening the session, Ulrich Wiedner of Ruhr University Bochum presented a comprehensive review of the highlights and future of low-energy experiments in hadron physics. An effective field theory and lattice description of a variety of nuclear bound states and reactions, as well as a review of the low-energy interaction of strange and charm hadrons with nucleons and nuclei, were presented by Evgeny Epelbaum, also of Bochum, and William Detmold at Massachusetts Institute of Technology. Charles Horowitz of Indiana University spoke about multimessenger observations of neutron-rich matter, describing the Lead Radius Experiment (PREX) at Jefferson Lab, which measures the neutron density of 208Pb using parity-violating electron scattering. This has important implications for neutron-rich matter and neutron stars. He also described X-ray observations of radii of neutron stars, which are possibly model dependent, and their implications for the equation of state. Gravitational-wave observations of merging neutron stars and r-mode oscillations were discussed in terms of the equation of state, mechanical properties and bulk and shear viscosities of neutron-rich matter. This prepared the ground for the roundtable discussion on "What can compact stars really tell us about dense QCD matter", chaired by Andreas Schmitt of the Vienna University of Technology.

On Thursday morning, Pich gave an overview of the perturbative determination of αs in which he presented the final value of 0.1187 ± 0.0007 and discussed the impact of the different type of αs extractions on the final result.

A number of low-energy precision measurements are sensitive to new physics either because the Standard Model prediction for the measured quantity is precisely known – for example, the anomalous magnetic moment of the muon (g-2) – or because the Standard Model "background" is small, as in the case of electric dipole moments (EDMs). Timothy Chupp of the University of Michigan presented several studies that are under way to probe physics beyond the Standard Model, including g-2 and EDMs. He also described the prospects for the precision measurement of the Cabibbo-Kobayashi-Maskawa matrix element, Vud, from neutron decay, i.e. the neutron lifetime and measurement of the axial-vector coupling constant (gA), as well as couplings beyond the Standard Model accessible from neutron decay. The discussion culminated in the roundtable "Resolving physics beyond the Standard Model at low energy" led by Susan Gardner of the University of Kentucky.

The final plenary session on Friday afternoon started with a talk by Mikko Laine of the University of Bern, in which he drew analogies and relationships between hot QCD and cosmology. John Harris of Yale University went on to review the latest heavy-ion data from Brookhaven’s Relativistic Heavy-Ion Collider (RHIC) and the LHC. In particular, the data show how the "soup" of quark–gluon plasma flows easily, with extremely low viscosity – suggesting a near-perfect liquid of quarks and gluons. However, it appears opaque to energetic partons at RHIC and less so to the extremely energetic parton probes available in collisions at the LHC. This review was followed by presentations on the theoretical challenges and perspectives in the exploration of the hot QCD matter, including recent highlights in lattice calculations at finite temperature and finite density as presented by Peter Petreczky of Brookhaven National Laboratory. The session culminated with a roundtable about "Quark Gluon Plasma: what is it and how do we find it out?" chaired by Berndt Mueller of Duke University.

Yiota Foka of GSI and CERN reported on the International Particle Physics Outreach Group, which has developed an educational activity that brings LHC data into the classroom. Each year since 2005, thousands of high-school students in many countries go to nearby universities or research centres for one day to unravel the mysteries of particle physics and to be "scientists for a day". In 2012, 10,000 students from 130 institutions in 31 countries took part in the popular event over a four-week period.

The conference featured a plenary session and seven sessions running in parallel on the subjects of the seven topical sections, with a total of 250 parallel talks. The sections on vacuum structure and confinement and on deconfinement constituted almost two conferences in themselves, with a total of 54 talks in 17.5 hours and 57 talks in 24 hours, respectively. The conference as a whole ended with a visionary talk by Chris Quigg of Fermilab on "Beyond Confinement". The extraordinary scientific discussion and exchange that characterized the conference has served as a trigger for a document "Strongly Coupled Physics: challenges, scenarios and perspectives" that is currently in preparation in collaboration with the section conveners.

During the poster session, participants could also enjoy tasting cheese and a variety of wine from all of the countries represented. A ride down the gigantic slide belonging to the Mathematics Department complemented the lively scientific discussions. An evening session on the "Colourful world of quark and gluons" given by Gerhard Ecker, "The shaping of QCD", and Thomas Mannel, "The many facets of QCD", attracted the public from Garching city and from the many campus research institutes, as well as conference participants. Tours of Munich, glimpses of Bavarian culture at the famous Hofbräuhaus and a social dinner at the Hofbräukeller complemented the opportunity to discover the local campus facilities (the TUM Institute of Advanced studies and the TUM engineering, mathematics and physics departments).