La physique du quarkonium à l’aube du LHC
Le groupe de travail sur le quarkonium a été constitué en 2002 pour faire avancer la recherche et promouvoir la communication entre théoriciens et expérimentateurs dans ce domaine. La dernière en date des réunions organisées par ce groupe a eu lieu à DESY, à Hambourg, en octobre 2007. Au programme, des nouvelles de la théorie de la production de quarkoniums au Tevatron et dans les usines à B, la production de quarkoniums et leur comportement dans les collisions d’ions lourds, les nouveaux états à résonance étroite découverts à Belle, BaBar et CLEO, les applications aux recherches sur la physique au-delà du modèle standard, et les expériences sur les quarkoniums à l’ère du LHC.
The Quarkonium Working Group (QWG) formed in 2002 to further research in all aspects of quarkonium physics and to bridge communication between theory and experiment in the field. The group has since sponsored a series of workshops on quarkonium physics, starting at CERN in November 2002 (CERN Courier March 2003 p6 and CERN Courier September 2006 p46). The latest meeting took place at DESY, Hamburg, on 17–20 October 2007. Hot topics included recent advances in the theory of quarkonium production at the Tevatron and the B-factories; quarkonium production and in-medium behaviour in heavy-ion collisions; the new narrow-resonance states discovered by the Belle, BaBar and CLEO experiments; applications of quarkonium physics to the search for physics beyond the Standard Model; and quarkonium experiments in the LHC era.
Quarkonium physics has played an important role in establishing QCD as the accepted theory of strong interactions. It has decisively contributed to the development of the quark model of hadrons and to the understanding of the properties of QCD. It also provides a unique window into the interplay between perturbative and nonperturbative QCD. As such, quarkonium physics remains at the forefront of QCD research and is an important testing ground for state-of-the-art computational tools for QCD, such as effective field theories, factorization theorems, higher-order perturbative calculations and lattice QCD. The insights gained from quarkonium studies build greater confidence in predictions for Standard Model processes and, consequently, in predictions of new physics backgrounds at the LHC. The recent discovery of remarkable new resonance states in the charmonium region of the spectrum – exciting in its own right – provides further opportunities to test the theoretical framework of quarkonium physics.
Participants at the DESY meeting learnt of the first complete next-to-leading-order (NLO) QCD corrections to colour-singlet quarkonium production at the Tevatron (figure 1). Surprisingly, these corrections enhance the colour-singlet production rate by an order of magnitude. Such an unprecedented enhancement could potentially lead to a better understanding of the dominant quarkonium-production mechanisms in hadronic collisions and may eventually explain, along with other puzzles of quarkonium production, the absence of the predicted transverse polarization of J = 1 quarkonia at large transverse momenta in the Tevatron measurements. There is also a possible resolution of the apparent order-of-magnitude discrepancy between theory and experiment in exclusive double-quarkonium production at the B-factories – a long-standing puzzle in quarkonium physics. New calculations of corrections at NLO in the strong coupling constant, and at NLO and higher in the nonrelativistic expansion, have brought theory and experiment into agreement, albeit with large uncertainties.
With the advent of the LHC, high-energy physics is entering an exciting and crucial period with great potential for discoveries. The LHC experiments will explore a new energy scale and provide stringent tests of many models, theories and scenarios, both within and beyond the Standard Model. The high-energy frontier, where the increased centre-of-mass energy can lead to the observation of new phenomena, complements high-precision experiments at lower energies. The LHC will provide a laboratory for studying quarkonium production mechanisms in matter, both in the collisions of protons and in the high-density environment that is formed in ultra-relativistic heavy-ion collisions.
The LHC’s heavy-ion programme is not only of great relevance to quarkonium production, but also for finite-temperature studies. Heavy-ion collisions at the LHC will form a hadronic medium with the highest energy density ever produced in a laboratory. Quarkonium studies play a particularly crucial role here since the quarkonium suppression pattern in heavy-ion collisions should serve as a thermo-meter for the hadronic medium. During the four days at DESY, speakers revealed important new insights into the behaviour of the quarkonium states in a hot medium, arising both from finite-temperature lattice QCD approaches and temperature-dependent potential models. For the first time, quarkonium spectral-function calculations from potential models appear to be consistent with lattice calculations of the Euclidean correlators (figure 2). However, the interpretation of the experimental data from RHIC is still incomplete. The next RHIC run, with higher statistics d + Au and p + p data, should pin down the effects of cold nuclear matter more precisely before the LHC starts up.
The recent discoveries of narrow-resonance states at the Belle, BaBar and CLEO experiments at KEK, SLAC and Cornell, respectively, are of special interest to the QWG because some of these resonances have been interpreted as quarkonium states (table 1). These states are currently referred to as X, Y and Z. However, as progress is made in understanding their nature, the assignment of more meaningful names for these states becomes increasingly important. The QWG resolved at the DESY meeting to set aside time at the next workshop for a discussion of appropriate names for these states.
Flavour physics has played a crucial role in the development of the Standard Model and should make important contributions to the understanding of physics beyond the Standard Model, even in the minimal-flavour-violation scenario. Since the flavour sector of the Standard Model is not as well understood as the gauge sector, there remain a number of unresolved questions. How many families exist and why? What is the origin of the quark mass? Are there new sources of CP violation? Is there any relationship between the lepton and quark sectors?
Quarkonium physics plays a role in providing further tests of the Standard Model and the potential for discoveries of new physics at the LHC. In particular, radiative decays of Y resonances into leptons could unveil new physics in connection with the existence of a light Higgs particle. Also, invisible decays of heavy quarkonia might exclude or reveal light dark matter (e.g. very light neutralinos). A recent series of CERN workshops also covered these topics (CERN Courier September 2007 p29).
The workshop noted the changing experimental landscape of quarkonium physics. While the facilities at SLAC and CLEO are reaching the ends of their lifetimes and the future of Fermilab is unclear, the KEK-B facility and the Beijing Spectrometer experiment will continue to perform superbly in the LHC era. However, dedicated quarkonium facilities to follow up on LHC discoveries will be desirable. Other current and future facilities, while not dedicated to quarkonium studies, will add significantly to our understanding of quarkonia.
The proposed future International Linear Collider is a far-reaching project that would provide deeper insights into the laws of nature in many areas of physics, including quarkonium physics. In the meantime, one of the major goals of the planned luminosity upgrade at RHIC is to improve in-medium quarkonium studies. This upgrade will complement quarkonium in-medium studies at the LHC. The quarkonium production rates at the LHC will be similar to those obtained at the upgraded RHIC since the heavy-ion runs at the LHC, while at higher energy and greater luminosity, will be significantly shorter. Quarkonium studies are also a major component of the antiproton and heavy-ion programmes at GSI, Darmstadt.
The workshop concluded with a round-table discussion devoted to a dedicated heavy-flavour facility, the general-purpose Super Flavour Factory project. This high-luminosity machine would make high-precision measurements to search for new physics in the flavour sector and would further contribute to strong-interaction physics.
• The QWG and its workshops provide lively forums where experts in quarkonium physics can assess the most recent advances and set out clear, well defined goals. These goals form a set of action items that are reviewed and updated following each QWG meeting. The action items can be viewed and commented upon from the QWG website at www.qwg.to.infn.it.