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Measuring the open to closed beauty ratio in heavy-ion collisions at LHC

Periodic Reporting for period 1 - CMSHInBeauty (Measuring the open to closed beauty ratio in heavy-ion collisions at LHC)

Reporting period: 2015-07-01 to 2017-06-30

The goal of this project is to extend the knowledge of J/psi production in the heavy-ion collision. In contrast to ordinary matters in the Universe, a different state is predicted to occur when sufficiently high temperature and density are reached. In this new regime, quarks and gluons are not confined, they form the Quark Gluon Plasma (QGP). Similar extreme conditions can be reproduced by colliding heavy ions at high center-of-mass energy at the Large Hadron Collider (LHC). In the QGP, the production of quarkonia could be suppressed to that of proton-proton collisions due to color charge screening which leads to dissociation of quarkonium state. On the other hand, quarkonia production can be enhanced because of a large number of charm and beauty quarks in the medium. For quantitative measurement of the final production, modifications to the quarkonia production have been estimated with the nuclear modification factor, RAA. The RAA is a ratio of the particle productions in heavy-ion collisions to that in proton-proton collisions normalized by binary collisions. When the RAA is less or larger than 1, the particle production in heavy-ion collisions is suppressed or enhanced than proton-proton collisions, respectively.

A complement to the RAA measurement is the elliptic anisotropy coefficient v2. This is the second Fourier coefficient in the expansion of the azimuthal angle distribution of the J/psi mesons, the azimuthal angle of the participant plane calculated for each event. The participant plane is defined by the beam direction and the direction of the shorter axis of the overlap region of the two colliding nuclei. Together with RAA, v2 can provide more detailed information on the J/psi meson production in heavy-ion collisions and the properties in the QGP.

The main challenge in this project was enhancing precision for the J/psi RAA as well as v2. J/psi has two categories depending on its origin. Prompt J/psi is in a family of quarkonium states which is created at the initial stage of heavy-ion collisions, while nonprompt J/psi is not quarkonium but an open beauty flavored meson. Complicate mathematical models were developed and tested to distinguish prompt and nonprompt J/psi with the highest precision. The new concept of mathematical models is correcting each event with corresponding measurement error. With this technique, the fraction of nonprompt J/psi out of total J/psis mesons was measured within a few percentages of systematical uncertainties.

This project measured the RAA and v2 of J/psi in PbPb collisions at 2.76 and the RAA at 5.02 TeV, recorded by CMS detector. The measurements cover J/psi rapidity less than 2.4 and transverse momentum from 3 to 30-50 GeV/c depending on the collision energy, which has not been measured by other experiments.
With this project, one paper (Eur. Phys. J. C 77 (2017) 252) has been published, and one preliminary result (CMS-HIN-16-025) is opened to the public. Both J/psi measurements share similar analysis methods in general, but the paper is based on collisions at a center of mass energy 2.76 TeV, and the preliminary result is on 5.02 TeV.

All analysis shared similar steps to process dataset and perform the analysis. After events with muons were selected by trigger system and reconstructed, various conditions were studied and applied to reject background events as well as select J/psi events with high purity. The centrality of heavy-ion collisions, the geometrical overlap of the incoming nuclei, was defined as percentiles of the distribution of the energy deposited in the forward calorimetry detectors. Monte Carlo simulations were used to model the shapes of signals and evaluate efficiencies.

The J/psi RAA and v2 paper introduced event-by-event efficiency corrections in rapidity, transverse momentum, centrality and the most probable decay length of b hadron in the laboratory frame.

Both prompt and nonprompt J/psi RAA showed suppression over all measured region. Suppression was more significant in the most central collisions. Prompt J/psi v2 was not zero while nonprompt J/psi v2 was compatible with zero within large uncertainty.
J/psi v2 has been measured for the first time with heavy-ion collisions from this project. The main challenge for the measurement was extracting J/psi signals and correct them with high precision, which required a development of new mathematical models. Also, collecting sufficient amount of J/psi events under high luminosity was done with efficient muon trigger system which was developed and upgraded for this project. This triggers became a baseline for the next PbPb collision data taking, at 2015.

All the new J/psi RAA and v2 measurements are important to heavy-ion communities, theoretical and experimental side. This will give a missing piece information to connect with other measurements at different collision energy as well as different nucleus collisions. Properties of the QGP is not entirely revealed yet, and this project provides a key to understand behaviors of charm and beauty quarks in the QGP.
Prompt J/psi RAA vs. transverse momentum compared to other experimental results
Nonprompt J/psi RAA
Nonprompt J/psi RAA vs. transverse momentum compared to other experimental results
Prompt J/psi v2
Nonprompt J/psi v2 compared to models
Nonprompt J/psi RAA vs. transverse momentum compared to models
Prompt J/psi RAA