Baseline Jet Energy Corrections for the CMS Experiment at the CERN LHC

Ο Μεγάλος Επιταχυντής Αδρονίων LHC (Large Hadron Collider) στο Εργαστήριο CERN παρέχει συγκρούσεις πρωτονίου-πρωτονίου στα 13 TeV, η υψηλότερη ενέργεια μέχρι σήμερα, καθιστώντας τα πειράματα ικανά να ελέγξουν και να διερευνήσουν το Καθιερωμένο Πρότυπο (ΚΜ) με μεγάλη ακρίβεια. Οι αδρονικοί πίδακες είναι οι πειραματικές υπογραφές των κουάρκ και γκλουονίων που παράγονται στην τελική κατάσταση υψηλοενεργειακών σκεδάσεων, όπως αυτές που πραγματοποιούνται στο πείραμα CMS (Compact Muon Solenoid) στον LHC. Ένα μεγάλο πλήθος αναλύσεων φυσικής που στοχεύουν όχι μόνο στην μελέτη υπογραφών του Καθιερωμένου Προτύπου, αλλά και στην εύρεση φυσικής πέραν αυτού, χρησιμοποιούν τους αδρονικούς πίδακες, κάνοντας επιτακτική την ανάγκη για μεγάλης ακρίβειας μετρήσεις της ενέργειας τους και των συστηματικών αβεβαιοτήτων. Στην διπλωματική αυτή εργασία θα παρουσιασθούν οι βασικές διορθώσεις ενέργειας που παράγονται χρησιμοποιώντας προσομοιώσεις (Monte Carlo) και αποτελούν το πρώτο και κύριο μέρος της αλυσίδας των διορθώσεων ενέργειας. Οι διορθώσεις αυτές χωρίζονται σε δύο επίπεδα, αυτές που αφαιρούν την επιπρόσθετη ενέργεια που οφείλεται σε δευτερεύουσες αλληλεπιδράσεις πέραν της κύριας, και αυτές που στοχεύουν στην εξίσωση της ενέργειας σε επίπεδο ανακατασκευής με την αντίστοιχη στο επίπεδο του γεννήτορα που δεν λαμβάνει υπ' όψιν τις ατέλειες του ανιχνευτή και της ανακατασκευής του γεγονότος. Πριν συζητηθούν οι διορθώσεις της ενέργειας, θα παρουσιασθούν τα βασικά χαρακτηριστικά της Κβαντικής Χρωμοδυναμικής και του σχηματισμού των αδρονικών πιδάκων. Επιπλέον, θα περιγραφεί η ανιχνευτική διάταξη του πειράματος CMS και θα συζητηθούν οι τρόποι με τους οποίους ανακατασκευάζονται και ομαδοποιούνται οι αδρονικοί πίδακες στο CMS.Jets are the experimental signature of quarks and gluons produced in the final state of high energy scatterings, as the ones that take place in the CMS experiment at the Large Hadron Collider. Numerous physics analyses that not only aim to study Standard Model (SM) Processes, but also to search for physics beyond the SM, use jets, thus creating the need for high accuracy in the estimation of their energy and the associated uncertainties. The factorized Jet Energy Corrections (JEC) that are utilized by the CMS experiment aim to provide exactly that. In this thesis the baseline jet energy corrections that are derived from simulation and constitute the first and main step of the JEC chain, will be presented. These corrections are divided into two levels; the pileup offset corrections (L1) whose goal is to remove the excess jet energy that is caused by additional proton - proton collisions different from the main hard scatter, and the relative and absolute corrections (L2L3) that aim to refer the jet energy at reconstruction level to the respective one at generator level , thus proving an estimation of the true jet energy without the detector resolution and reconstruction effects being considered. Before discussing the jet energy correction estimation and showing the final results, the theoretical aspects of Quantum Chromodynamics and jet formation will be presented. Furthermore, a description of the CMS detector layout will be provided, along with the methods used in order to reconstruct and cluster jets in the CMS experiment

Measurement of the double-differential inclusive jet cross section in proton-proton collisions at s\sqrt{s} = 5.02 TeV

International audienceThe inclusive jet cross section is measured as a function of jet transverse momentum $p_\mathrm{T}$ and rapidity $y$. The measurement is performed using proton-proton collision data at $\sqrt{s}$ = 5.02 TeV, recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 27.4 pb$^{-1}$. The jets are reconstructed with the anti-$k_\mathrm{T}$ algorithm using a distance parameter of $R$ = 0.4, within the rapidity interval $\lvert y\rvert$$\lt$ 2, and across the kinematic range 0.06 $\lt$$p_\mathrm{T}$$\lt$ 1 TeV. The jet cross section is unfolded from detector to particle level using the determined jet response and resolution. The results are compared to predictions of perturbative quantum chromodynamics, calculated at both next-to-leading order and next-to-next-to-leading order. The predictions are corrected for nonperturbative effects, and presented for a variety of parton distribution functions and choices of the renormalization/factorization scales and the strong coupling $\alpha_\mathrm{S}$

Measurement of the double-differential inclusive jet cross section in proton-proton collisions at s\sqrt{s} = 5.02 TeV

International audienceThe inclusive jet cross section is measured as a function of jet transverse momentum $p_\mathrm{T}$ and rapidity $y$. The measurement is performed using proton-proton collision data at $\sqrt{s}$ = 5.02 TeV, recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 27.4 pb$^{-1}$. The jets are reconstructed with the anti-$k_\mathrm{T}$ algorithm using a distance parameter of $R$ = 0.4, within the rapidity interval $\lvert y\rvert$$\lt$ 2, and across the kinematic range 0.06 $\lt$$p_\mathrm{T}$$\lt$ 1 TeV. The jet cross section is unfolded from detector to particle level using the determined jet response and resolution. The results are compared to predictions of perturbative quantum chromodynamics, calculated at both next-to-leading order and next-to-next-to-leading order. The predictions are corrected for nonperturbative effects, and presented for a variety of parton distribution functions and choices of the renormalization/factorization scales and the strong coupling $\alpha_\mathrm{S}$

Two-particle Bose-Einstein correlations and their Lévy parameters in PbPb collisions at sNN\sqrt{s_\mathrm{NN}} = 5.02 TeV

Two-particle Bose-Einstein momentum correlation functions are studied for charged-hadron pairs in lead-lead collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV. The data sample, containing 4.27$\times10^{9}$ minimum bias events corresponding to an integrated luminosity of 0.607 nb$^{-1}$, was collected by the CMS experiment in 2018. The experimental results are discussed in terms of a L\'evy-type source distribution. The parameters of this distribution are extracted as functions of particle pair average transverse mass and collision centrality. These parameters include the L\'evy index or shape parameter ($\alpha$), the L\'evy scale parameter ($R$), and the correlation strength parameter ($\lambda$). The source shape, characterized by $\alpha$, is found to be neither Cauchy nor Gaussian, implying the need for a full L\'evy analysis. Similarly to what was previously found for systems characterized by Gaussian source radii, a hydrodynamical scaling is observed for the L\'evy $R$ parameter. The $\lambda$ parameter is studied in terms of the core-halo model.Comment: Submitted to Physical Review C. All figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-21-011 (CMS Public Pages

Measurement of the double-differential inclusive jet cross section in proton-proton collisions at s\sqrt{s} = 5.02 TeV

International audienceThe inclusive jet cross section is measured as a function of jet transverse momentum $p_\mathrm{T}$ and rapidity $y$. The measurement is performed using proton-proton collision data at $\sqrt{s}$ = 5.02 TeV, recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 27.4 pb$^{-1}$. The jets are reconstructed with the anti-$k_\mathrm{T}$ algorithm using a distance parameter of $R$ = 0.4, within the rapidity interval $\lvert y\rvert$$\lt$ 2, and across the kinematic range 0.06 $\lt$$p_\mathrm{T}$$\lt$ 1 TeV. The jet cross section is unfolded from detector to particle level using the determined jet response and resolution. The results are compared to predictions of perturbative quantum chromodynamics, calculated at both next-to-leading order and next-to-next-to-leading order. The predictions are corrected for nonperturbative effects, and presented for a variety of parton distribution functions and choices of the renormalization/factorization scales and the strong coupling $\alpha_\mathrm{S}$

Measurement of the double-differential inclusive jet cross section in proton-proton collisions at s\sqrt{s} = 5.02 TeV

International audienceThe inclusive jet cross section is measured as a function of jet transverse momentum $p_\mathrm{T}$ and rapidity $y$. The measurement is performed using proton-proton collision data at $\sqrt{s}$ = 5.02 TeV, recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 27.4 pb$^{-1}$. The jets are reconstructed with the anti-$k_\mathrm{T}$ algorithm using a distance parameter of $R$ = 0.4, within the rapidity interval $\lvert y\rvert$$\lt$ 2, and across the kinematic range 0.06 $\lt$$p_\mathrm{T}$$\lt$ 1 TeV. The jet cross section is unfolded from detector to particle level using the determined jet response and resolution. The results are compared to predictions of perturbative quantum chromodynamics, calculated at both next-to-leading order and next-to-next-to-leading order. The predictions are corrected for nonperturbative effects, and presented for a variety of parton distribution functions and choices of the renormalization/factorization scales and the strong coupling $\alpha_\mathrm{S}$

Observation of four top quark production in proton-proton collisions at s\sqrt{s} = 13 TeV

International audienceThe observation of the production of four top quarks in proton-proton collisions is reported, based on a data sample collected by the CMS experiment at a center-of-mass energy of 13 TeV in 2016-2018 at the CERN LHC and corresponding to an integrated luminosity of 138 fb$^{-1}$. Events with two same-sign, three, or four charged leptons (electrons and muons) and additional jets are analyzed. Compared to previous results in these channels, updated identification techniques for charged leptons and jets originating from the hadronization of b quarks, as well as a revised multivariate analysis strategy to distinguish the signal process from the main backgrounds, lead to an improved expected signal significance of 4.9 standard deviations above the background-only hypothesis. Four top quark production is observed with a significance of 5.6 standard deviations, and its cross section is measured to be 17.7$^{+3.7}_{-3.5}$ (stat) $^{+2.3}_{-1.9}$ (syst) fb, in agreement with the available standard model predictions

Search for new heavy resonances decaying to WW, WZ, ZZ, WH, or ZH boson pairs in the all-jets final state in proton-proton collisions at s\sqrt{s} = 13 TeV

International audienceA search for new heavy resonances decaying to WW, WZ, ZZ, WH, or ZH boson pairs in the all-jets final state is presented. The analysis is based on proton-proton collision data recorded by the CMS detector in 2016-2018 at a centre-of-mass energy of 13 TeV at the CERN LHC, corresponding to an integrated luminosity of 138 fb$^{-1}$. The search is sensitive to resonances with masses between 1.3 and 6 TeV, decaying to bosons that are highly Lorentz-boosted such that each of the bosons forms a single large-radius jet. Machine learning techniques are employed to identify such jets. No significant excess over the estimated standard model background is observed. A maximum local significance of 3.6 standard deviations, corresponding to a global significance of 2.3 standard deviations, is observed at masses of 2.1 and 2.9 TeV. In a heavy vector triplet model, spin-1 Z' and W' resonances with masses below 4.8 TeV are excluded at the 95% confidence level (CL). These limits are the most stringent to date. In a bulk graviton model, spin-2 gravitons and spin-0 radions with masses below 1.4 and 2.7 TeV, respectively, are excluded at 95% CL. Production of heavy resonances through vector boson fusion is constrained with upper cross section limits at 95% CL as low as 0.1 fb

Measurement of the dependence of the hadron production fraction ratio fs/fuf_\mathrm{s} / f_\mathrm{u} and fd/fuf_\mathrm{d} / f_ \mathrm{u} on B meson kinematic variables in proton-proton collisions at s\sqrt{s} = 13 TeV

International audienceThe dependence of the ratio between the B$_\mathrm{s}^0$ and B$^+$ hadron production fractions, $f_\mathrm{s} / f_\mathrm{u}$, on the transverse momentum ($p_\mathrm{T}$) and rapidity of the B mesons is studied using the decay channels B$_\mathrm{s}^0$$\to$ J$/\psi\,\phi$ and B$^+$$\to$ J$/\psi$ K$^+$. The analysis uses a data sample of proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the CMS experiment in 2018 and corresponding to an integrated luminosity of 61.6 fb$^{-1}$. The $f_\mathrm{s} / f_\mathrm{u}$ ratio is observed to depend on the B $p_\mathrm{T}$ and to be consistent with becoming asymptotically constant at large $p_\mathrm{T}$. No rapidity dependence is observed. The ratio of the B$^0$ to B$^+$ hadron production fractions, $f_\mathrm{d} / f_\mathrm{u}$, measured using the B$^0$$\to$ J$/\psi$ K$^{*0}$ decay channel, is found to be consistent with unity and independent of $p_\mathrm{T}$ and rapidity, as expected from isospin invariance

Observation of the rare decay of the η\eta meson to four muons

A search for the rare $\eta$$\to$$\mu^+\mu^-\mu^+\mu^-$ double-Dalitz decay is performed using a sample of proton-proton collisions, collected by the CMS experiment at the CERN LHC with high-rate muon triggers in 2017-2018 and corresponding to an integrated luminosity of 101 fb$^{-1}$. A signal having a statistical significance well in excess of 5 standard deviations is observed. Using the \emm decay as normalization, the branching fraction $\mathcal{B}($η$\to \mu^+\mu^-\mu^+\mu^-)$ = ( 5.0 $\pm$ 0.8 (stat) $\pm$ 0.7 (syst) $\pm$ 0.7 $\mathcal{B}_{2\mu}$ ) $\times$ 10$^{-9}$ is measured, where the last term is the uncertainty in the normalization channel branching fraction. This is the first measurement of this branching fraction and is found to be in agreement with theoretical predictions