Study of Cold Nuclear Matter Effects on Heavy Quarkonia in Proton-Lead Collisions at LHCb

Proton-nucleus ($p\rm{A}$) collisions play an important role in high energy nuclear physics as they allow to study nuclear matter effects and the parton distribution functions in the nuclear environment (nPDF). The quantum chromodynamics (QCD) phase transition from hadron gas to the the quark-gluon plasma (QGP) is not expected to occur in a $p\rm{A}$ collision due to its limited space-time size. Therefore, the $p\rm{A}$ collisions provide an ideal platform to study cold nuclear matter (CNM) effects, which are also known as normal nuclear matter effects. The measurements of the productions and correlations of the final-state particles in $p\rm{A}$ collisions serve the purpose to test various theoretical models for CNM effects, to constrain the benchmarking nPDFs, and thus provide a baseline to understand and interpret the QGP created in ultra-relativistic heavy-ion collisions. Heavy quarkonia (including charmonia and bottomonia), which are produced at the early stage of heavy-ion collisions, are considered good probes for nuclear matter effects. Heavy quarkonia are easy to dissociate due to color screening effects in the QGP, leading to the suppression of their production relative to proton-proton ($pp$) collisions. This suppression effects are proposed to be one of the signatures of the formation of a QGP. However, heavy quarkonium production can also be suppressed by the competitive CNM effects in heavy-ion collisions. Therefore, the investigation of the suppression effects for the heavy quarkonia in $p\rm{A}$ collisions is a prerequisite to understand and interpret the QGP. In early 2013, the LHCb detector at the Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN) collected proton-lead ($p\rm{Pb}$) data of an integrated luminosity of about $2\rm{nb^{-1}}$ at a nucleon-nucleon center-of-mass energy of $\sqrt{s_{\tiny{\it NN}}}= 5\rm{TeV}$. In this thesis, with the $p\rm{Pb}$ data at LHCb, the differential production cross-sections are measured for prompt $J/\psi$ mesons and $J/\psi$ from $b$-hadron decays, and the total production cross-sections are measured for $\Upsilon(1S) $, $\Upsilon(2S) $, and $\Upsilon(3S) $ mesons, in the rapidity coverage of $1.5 < y < 4.0$ (forward region) and $-5.0 < y < -2.5$ (backward region) and the transverse momentum range of $p_{\rm T}(J/\psi) < 14$ GeV/$c$ and $p_{\rm T}(\Upsilon)<15$ GeV/$c$. Based on the cross-section measurements, the nuclear modification factor, $R_{p\rm{Pb}}$, and the forward-backward production ratio, $R_{\rm FB}$, which are sensitive to CNM effects, are determined for these heavy quarkonium states in $p\rm{Pb}$ collisions at $\sqrt{s_{\tiny{\it NN}}}= 5\rm{TeV}$ for the first time. The production ratios of the $\Upsilon(2S) $ and $\Upsilon(3S) $ states to the $\Upsilon(1S) $ ground state are also studied, reflecting the sequential suppression of different quarkonium states due to different binding energies. The measurements show a clear evidence for CNM effects in $p\rm{Pb}$ collisions. They agree well with theoretical predictions within uncertainties. In the forward region, clear suppression of prompt $J/\psi$ production relative to $pp$ collisions is observed; the measurements suggest a modest suppression of the production of $J/\psi$ from $b$-hadron decays, reflecting the CNM effects on $b$ hadrons; there is a week indication of the production suppression for $\Upsilon(1S)$ mesons, however with a rather large uncertainty. In the backward region the CNM effects on all the measured heavy quarkonia are found to be insignificant; the slightly enhanced yield of $\Upsilon(1S)$ mesons is compatible with the nuclear antishadowing effects. The measurements show that CNM effects are very important in the $p\rm{Pb}$ collisions and it is necessary to consider CNM effects in the interpretation of QGP signatures in lead-lead collisions. CNM shows different effects on different heavy quarkonia states. The results will shed light on the constraints of the nPDFs.