Puzzles, achievements and perspectives in quarkonium production studies
by
Carlos Lourenço, Pietro Faccioli
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UTC
Description
Introduction by Carlos Lourenço:
Quarkonium production and polarization in pp collisions with the CMS detector
Thanks to a dedicated dimuon trigger strategy, combined with the record-level energy and luminosity provided by the LHC, the CMS experiment collected large samples of pp collisions at 7 and 8 TeV, including quarkonium states decaying in the dimuon channel. This allowed the CMS collaboration to perform a series of systematic measurements in quarkonium production physics, including double-differential cross sections and polarizations, as a function of rapidity and pT, for five S-wave quarkonia: J/psi, psi(2S), Y(1S), Y(2S), and Y(3S). Some of these measurements extend well above pT~50 GeV, probing regions of very high pT/mass. Thanks to its high-granularity silicon tracker, CMS can reconstruct low-energy photons through their conversions to e+e- pairs, thereby accessing the radiative decays of the P-wave quarkonium states, with a very good mass resolution, so that the J=1 and J=2 1P states can be resolved, in both the charmonium and bottomonium families, allowing the measurement of their cross sections, ratios and feed-down decay fractions. This talk presents the CMS quarkonium production results in pp collisions, placing emphasis on the polarizations of all five S-wave states, the most comprehensive measurement of quarkonium polarization made so far. We will also present brand-new results on P-wave quarkonium production in the bottomonium family.
Abstract (by Pietro Faccioli):
Puzzles, achievements and perspectives in quarkonium production studies
Heavy quarkonia are elementary manifestations of the strong binding force and allow us to address in the most direct way the fundamental question of how quarks combine to form hadrons.
Nonrelativistic QCD (NRQCD), a rigorous and consistent effective theory based on QCD, should provide an accurate description of quarkonium production. The validation of NRQCD as a working framework would have important applications in the study of the Higgs coupling to charm (H -> J/psi gamma decay) and open the path to the general understanding of the formation of heavy QCD bound states, crucial for predictions of new resonances like toponium and gluino-onium.
After solving, in the mid 90's, the so-called "psi(2S) anomaly" through the introduction of colour-octet processes, for almost two decades NRQCD is being challenged by measurements of quarkonium decay angular distributions. While the disagreement shown by early J/psi and Upsilon(1S) measurements could not considered to be conclusive, because of experimental inconsistencies, insufficient high-pT coverage and neglected effects of indirect production, recent CMS measurements of the polarizations of (directly produced) psi(2S) and Upsilon(3S) have seemingly removed any residual ambiguity in the evidence for what is generally considered one of the most serious and persistent mismatches between data and predictions in the present particle physics landscape.
After proposing improved techniques for unambiguous polarization analyses, now adopted by all LHC experiments, and while leading the CMS quarkonium polarization measurements, we are now addressing the quarkonium production puzzle through a significant reconsideration of the strategy for theory-data comparison. While the polarization data are traditionally excluded from global NRQCD analyses of quarkonium production (and confined to the role of a posteriori verifications of the predictions), we realized that they are actually the most stringent and straightforward constraints in discriminating the underlying fundamental processes and we moved them from the periphery to the centre of the study. With this crucial premise, instead of asking if NRQCD is the correct theory, we systematically search for its domain of validity through a scan of the kinematic phase space, including a rigorous treatment of theoretical and experimental correlated uncertainties.
According to the first, promising results, this "Copernican revolution" seems to provide a straightforward solution to the puzzle, at the same time highlighting definite hierarchies in the nonperturbative parameters of NRQCD, to be interpreted as strong indications for the understanding of the mechanisms of bound-state formation (with implications for the study of quarkonium absorption in nuclear collisions).
After decades of unsolved puzzles and erratic explorations of possible alternative models of empirical more than fundamental interest, the newly established success of NRQCD will have soon the potential to ultimately turn quarkonium production measurements into precision studies of the most intriguing and challenging aspect, the long-distance one, of the theory of strong interactions.
