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Charmonium production in hadron collisions at the COMPASS experiment
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Resumo(s)
COMPASS (Common Muon and Proton Apparatus for Structure and Spectroscopy) is a fixed-target experiment with a beam of negative pions at 190 GeV. The COMPASS setup includes 3 target materials placed sequentially along the beamline: two cells of solid ammonia (NH3) crystals embedded in a helium bath (3He and 4He); a aluminium cell (27Al) and a tungsten plug with its first 20 cm used as a target(184W). In the COMPASS dimuon mass spectra, we observe two peaks on top of a decreasing continuum: J/ψ at 3.097 GeV/c 2 and ψ(2S) at 3.686 GeV/c 2 . J/ψ and ψ(2S) are charmonium resonances made of a quark charm and an antiquark charm. The goal of this thesis is to obtain the nuclear modification factors of J/ψ production as a function of transverse momentum (pT ) and Feynman-x (xF), using COMPASS 2018 data to access Cold Nuclear Matter (CNM) effects: Parton Energy Loss; Nuclear Shadowing and Anti-shadowing and EMC effect. The purity of J/ψ signal in the COMPASS dimuon mass spectra is evaluated in a cocktail fit, considering all the physics processes in J/ψ mass region. To calculate the J/ψ differential cross-section, the acceptance is considered, given as the ratio of the number of J/ψ events reconstructed over the number of events generated by Pythia, using a model based in Non-Relativistic QCD. The acceptance is calculated in bins of transverse momentum, pT , Feynman-x, xF, and vertex position along the beamline, zvertex. The J/ψ Reconstructed Monte Carlo is compared with the real data in the range of J/ψ highest purity, obtaining discrepancies of 50 %, demonstrating that NRQCD model doesn’t describe well charmonium production at the COMPASS energies. The discrepancies are corrected, applying event reweighting in the muon laboratory variables. Finally, the differential cross-section ratios between the three target materials are calculated.
Descrição
Tese de Mestrado, Engenharia Física, 2025, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
J/ψ ψ(2S) PDF Drell-Yan NRQCD