Measurements of prompt $J/\psi$ and $\psi(2S)$ production and polarization at CMS

Abstract

Quarkonia, bound states of a heavy quark and its antiquark, are theideal probe to study how quarks form bound states via strong interactions. Non Relativistic QuantumChromoDynamics (NRQCD) is aQCD inspired model that factorizes the production of a bound stateinto two steps: the creation of the initial quark-antiquark pair andthe transformation of the initial pair to a bound state. The NRQCDfactorization approach relies on experimental data to describe thenon-perturbative evolution of the initial pair to a bound state.This thesis describes the measurements of the production cross sections and polarizations of the prompt J/ψ and ψ(2S) mesons, whichare c¯ states that are produced either directly or via the decay ofca short-lived intermediate state. The measurements are based on adimuon data sample collected by the CMS experiment at the LHC√in proton-proton collisions at s = 7 TeV, corresponding to a total integrated luminosity of 4.9 fb−1 . The prompt J/ψ and ψ(2S)production cross sections are determined in four equidistant rapidityranges as well as over a wider integrated rapidity interval of |y| < 1.2.Four different polarization (preferred spin alignment) scenarios areconsidered. The cross sections have been measured up to transversemomenta, pT , of 120 GeV and 100 GeV for the J/ψ and ψ(2S), respectively, thereby tremendously extending the pT reach of previousCMS measurements. Moreover, the ratio of ψ(2S) to J/ψ cross sections was explicitly determined.Polarization is always measured with respect to a reference frame.The polarizations of the prompt J/ψ and ψ(2S) mesons are determined in three reference frames (HX, CS, PX) as functions of pTand |y|. In addition, a frame-invariant approach is applied. Theresults are obtained in two (three) rapidity bins and extend up topT = 70 (50) GeV in case of the J/ψ (ψ(2S)). The measured polarization values are close to zero, showing no evidence of any strongpolarization. This is in disagreement with existing NRQCD calculations that predict strong transverse polarization, especially at highpT values.The quarkonium results provided by CMS have significantly contributed to improve the understanding of hadron formation, especially because they probe the theoretical calculations in a kinematicregion where NRQCD factorization is believed to be most reliable