From identical S- and P-wave pT/M spectra to maximally distinct polarizations: probing NRQCD with chi states

A global analysis of ATLAS and CMS measurements reveals that, at mid-rapidity, the directly-produced $\chi_{c1}$, $\chi_{c2}$ and J/$\psi$ mesons have differential cross sections of seemingly identical shapes, when presented as a function of the mass-rescaled transverse momentum, $p_{\rm T}/M$. This identity of kinematic behaviours among S- and P-wave quarkonia is certainly not a natural expectation of non-relativistic QCD (NRQCD), where each quarkonium state is supposed to reflect a specific family of elementary production processes, of significantly different $p_{\rm T}$-differential cross sections. Remarkably, accurate kinematic cancellations among the variegated NRQCD terms (colour singlets and octets) of its factorization expansion can lead to a surprisingly good description of the data. This peculiar tuning of the NRQCD mixtures leads to a clear prediction regarding the $\chi_{c1}$ and $\chi_{c2}$ polarizations, the only observables not yet measured: they should be almost maximally different from one another, and from the J/$\psi$ polarization, a striking exception in the global panorama of quarkonium production. Measurements of the difference between the $\chi_{c1}$, $\chi_{c2}$ and J/$\psi$ polarizations, complementing the observed identity of momentum dependences, represent a decisive probe of NRQCD.Comment: Submitted to European Physical Journal

Measurement of Quarkonium Polarization to Probe QCD at the LHC

With the first proton-proton collisions in the Large Hadron Collider (LHC) at CERNin 2010, a new era in high energy physics has been initiated. The data collected bythe various experiments open up the possibility to study standard model processes withhigh precision, in new areas of phase space. The LHC provides excellent conditions forstudies of quarkonium production, due to the high quarkonium production rates giventhe high center-of-mass energy and high instantaneous luminosity of the colliding protonbeams. Studies of the production of heavy quarkonium mesons – bound states of a heavyquark and its respective antiquark – are very important to improve our understanding ofhadron formation. Until quite recently, experimental and phenomenological efforts havenot resulted in a satisfactory overall picture of quarkonium production cross sections andquarkonium polarizations.The Compact Muon Solenoid (CMS) detector is ideally suited to study quarkoniumproduction in the experimentally very clean dimuon decay channel, up to considerablyhigher values of transverse momentum than accessible in previous experiments. Thescope of this thesis is to describe in detail the measurements of the polarizations ofthe Υ(nS) bottomonium states and (in less detail) of the ψ(nS) charmonium states,based on a dimuon data sample collected with the CMS detector in proton-protoncollisions at a center-of-mass energy of 7 TeV. Surprisingly, no significant polarizationswere found in any of the studied quarkonium states, in none of the studied referenceframes, nor in a frame-independent analysis. From an experimental point of view, theseresults, together with recent results from other experiments, clarify the confusing pictureoriginating from previous measurements, which were plagued by experimental ambiguitiesand inconsistencies.The currently most favored approach to model and understand quarkonium productionis non-relativistic quantum chromodynamics (NRQCD), a QCD-inspired model whichallows color-octet pre-resonant quark-antiquark states to contribute to quarkonium boundstate formation. The measurements obtained as a result of this work, together withother LHC measurements in the field of quarkonium production, are interpreted withan original phenomenological approach within the theoretical framework of NRQCD,guided by the observation of a few general features of the data, and corroborated bya detailed study of the quarkonium production cross section and polarization observables.This phenomenological analysis leads to a coherent picture of quarkonium productioncross sections and polarizations within a simple model, dominated by one single coloroctet production mechanism. These findings provide new insight in the dynamics ofheavy quarkonium production at the LHC, an important step towards a satisfactoryunderstanding of hadron formation within the standard model

Quarkonium production and polarization in pp collisions with the CMS detector

Heavy quarkonium mesons constitute an ideal laboratory to study hadron formation, through the understanding of the properties of quarkonium production and polarization in hadron collisions. The currently most favored approach to model quarkonium production is non-relativistic quantum chromodynamics (NRQCD), taking advantage of the heavy quark masses to simplify QCD calculations. The experimental situation in the pre-LHC era was not satisfactory, given the ambiguous and inconsistent measurements of quarkonium polarization. New experimental input, relying on improved analysis methodologies, is required to advance the understanding of hadron formation. The LHC is a quarkonium factory, providing ideal conditions to study quarkonia in proton-proton collisions. CMS is ideally suited to study the decays of S-wave quarkonia in two muons. Furthermore, given the high granularity silicon tracker, radiative decays of P-wave quarkonia can be reconstructed with excellent momentum resolution through the reconstruction of photon conversions to e$^+$e$^-$ pairs. This allowed CMS to perform measurements of cross-section ratios involving the $\chi$ states, in both the charmonium and bottomonium families. This document discusses results of CMS quarkonium production and polarization data analyses, in proton-proton collisions, at $\sqrt{s}=$7~TeV and 8~TeV, emphasizing the most recent measurements, including the measurement of the polarizations of all five S-wave quarkonium vector states. Results are compared to corresponding results of other experiments, and to state-of-the-art NRQCD model calculations