Search for new particles in events with energetic jets and large missing transverse momentum in proton-proton collisions at root s=13 TeV

A search is presented for new particles produced at the LHC in proton-proton collisions at root s = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb(-1), collected in 2017-2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb(-1), collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.Peer reviewe

Probing effective field theory operators in the associated production of top quarks with a Z boson in multilepton final states at root s=13 TeV

Peer reviewe

Observation of tW production in the single-lepton channel in pp collisions at root s=13 TeV

A measurement of the cross section of the associated production of a single top quark and a W boson in final states with a muon or electron and jets in proton-proton collisions at root s = 13 TeV is presented. The data correspond to an integrated luminosity of 36 fb(-1) collected with the CMS detector at the CERN LHC in 2016. A boosted decision tree is used to separate the tW signal from the dominant t (t) over bar background, whilst the subleading W+jets and multijet backgrounds are constrained using data-based estimates. This result is the first observation of the tW process in final states containing a muon or electron and jets, with a significance exceeding 5 standard deviations. The cross section is determined to be 89 +/- 4 (stat) +/- 12 (syst) pb, consistent with the standard model.Peer reviewe

Measurement of the top quark mass using events with a single reconstructed top quark in pp collisions at root s=13 TeV

Abstract:A measurement of the top quark mass is performed using a data sample en-riched with single top quark events produced in thetchannel. The study is based on proton-proton collision data, corresponding to an integrated luminosity of 35.9 fb−1, recorded at√s= 13TeV by the CMS experiment at the LHC in 2016. Candidate events are selectedby requiring an isolated high-momentum lepton (muon or electron) and exactly two jets,of which one is identified as originating from a bottom quark. Multivariate discriminantsare designed to separate the signal from the background. Optimized thresholds are placedon the discriminant outputs to obtain an event sample with high signal purity. The topquark mass is found to be172.13+0.76−0.77GeV, where the uncertainty includes both the sta-tistical and systematic components, reaching sub-GeV precision for the first time in thisevent topology. The masses of the top quark and antiquark are also determined separatelyusing the lepton charge in the final state, from which the mass ratio and difference aredetermined to be0.9952+0.0079−0.0104and0.83+1.79−1.35GeV, respectively. The results are consistentwithCPTinvariance

Search for a heavy Higgs boson decaying into two lighter Higgs bosons in the tau tau bb final state at 13 TeV

A search for a heavy Higgs boson H decaying into the observed Higgs boson h with a mass of 125 GeV and another Higgs boson h(S) is presented. The h and h(S) bosons are required to decay into a pair of tau leptons and a pair of b quarks, respectively. The search uses a sample of proton-proton collisions collected with the CMS detector at a center-of-mass energy of 13TeV, corresponding to an integrated luminosity of 137 fb(-1). Mass ranges of 240-3000 GeV for m(H) and 60-2800 GeV for m(hS) are explored in the search. No signal has been observed. Model independent 95% confidence level upper limits on the product of the production cross section and the branching fractions of the signal process are set with a sensitivity ranging from 125 fb (for m(H) = 240 GeV) to 2.7 fb (for m(H) = 1000 GeV). These limits are compared to maximally allowed products of the production cross section and the branching fractions of the signal process in the next-to-minimal supersymmetric extension of the standard model.Peer reviewe

Z boson production in association with jets: measurement and phenomenology

In this thesis, we study two Z boson production mechanisms: the Drell-Yan process andreal electroweak corrections.The transverse momentum spectrum of the Drell-Yan lepton pair is of general interest asit is sensitive to the hadron structure. Moreover, the small transverse momentum region ,qT < mll, is sensitive to the 3D structure of the hadron. To describe the qT < mll region,resummation to all orders is needed. However, the qT ≪ mll region is non-perturbativeand has to be modelled. This region is modelled via a Gaussian distribution of the intrinsictransverse momentum of the incoming partons. Nonetheless, this is insufficient, and verysoftgluon radiation is needed. We investigate the effect of very-soft gluon radiations in theqT ≪ mll region within the Parton Branching framework, and how it relates to the Collins-Soper kernel of the TMD factorisation. The very-soft gluon radiation is removed throughthe soft-gluon resolution scale in the parton showers of Monte-Carlo event generators, suchas Pythia8 and Herwig7. We demonstrate that this leads to a centre-of-mass energydependent intrinsic transverse momentum, by tuning the intrinsic transverse momentum.At high partonic centre-of-mass energies, electroweak corrections become large, and theZ boson might be produced as a real electroweak correction. In QCD processes, at highjet transverse momentum, virtual electroweak corrections become large, and at the TeVscale, their contribution to the cross-section is of the order of 10%. We measure the contributionof real electroweak corrections to QCD events as a function of the jet transversemomentum. We analyse proton-proton collisions at a centre-of-mass energy of 13 TeV atthe LHC, recorded by the CMS experiment during 2016, corresponding to an integratedluminosity of 36.3 fb−1. We measure the contribution of the real electroweak correctionto a 1 TeV jet transverse momentum to be 1-2 %. Moreover, we investigate differentobservables sensitive to real electroweak corrections to test their modelling in differenttheoretical scenarios

Electroweak corrections to high transverse momentum jets

The production of electroweak (EW) bosons in association with jets has been extensively studied at particle colliders. The EW boson is considered the outcome of the hard process and the jets a product of parton evolution. These events are a great test of quantum chro- modynamics and allow to study parton density functions and parton evolution equations. So far, light quarks and gluons are considered in the parton evolution. However, with increasing centre-of-mass energies the probability of radiating heavier particles increases. In this analysis, the production of EW bosons in association with jets is studied specifically with the aim to investigate EW boson emitted in the parton shower. To this end, events with high transverse momentum jets are studied. Then, the contribution of EW boson emissions is measured. Preliminary results are presented with data collected in 2016, corre- sponding to an integrated luminosituy of 36.3 fb−1. The contribution of the Z boson is studied in the leptonic decay channel

On Broad Kaluza-Klein Gluons

In theories with a warped extra dimension, composite fermions, as e.g. the right-handed top quark, can be very strongly coupled to Kaluza-Klein (KK) fields. In particular, the KK gluons in the presence of such composite fields become very broad resonances, thus remarkably modifying their experimental signatures. We have computed the pole mass and the pole width of the KK gluon, triggered by its interaction with quarks, as well as the prediction for proton-proton cross-sections using the full propagator and compared it with that obtained from the usual Breit-Wigner approximation. We compare both approaches, along with the existing experimental data from ATLAS and CMS, for the $t\overline{t}$, $t\overline{t}W$, $t\overline{t}Z$, $t\overline{t}H$, and $tt\overline{tt}$ channels. We have found differences between the two approaches of up to about 100%, highlighting that the effect of broad resonances can be dramatic on present, and mainly future, experimental searches. The channel $tt\overline{tt}$ is particularly promising because the size of the cross-section signal is of the same order of magnitude as the Standard Model prediction, and future experimental analyses in this channel, especially for broad resonances, can shed light on the nature of possible physics beyond the Standard Model

Deep Inelastic Scattering events for photon and heavy boson TMDs

Deep Inelastic Scattering (DIS) events at HERA are the most precise DIS measurement to date, which constitutes an important tool to study transverse momentum dependent (TMD) parton distribution functions. In this study, we introduce standard DIS Monte Carlo events recreating HERA conditions, and apply their cross section as a constraint for both photon and heavy boson TMDs in the Parton Branching Method

Back-to-back azimuthal correlations in Z+\mathrm {Z} +jet events at high transverse momentum in the TMD parton branching method at next-to-leading order

Azimuthal correlations in $\mathrm {Z} +$jet production at large transverse momenta are computed by matching Parton-Branching (PB) TMD parton distributions and showers with NLO calculations via MCatNLO. The predictions are compared with those for dijet production in the same kinematic range. The azimuthal correlations $\Delta \phi$ between the Z boson and the leading jet are steeper compared to those in dijet production at transverse momenta $\mathcal{O}(100)$ GeV , while they become similar for very high transverse momenta ${{\mathcal {O}}}(1000)$ GeV . The different patterns of $\mathrm {Z} +$jet and dijet azimuthal correlations can be used to search for potential factorization-breaking effects in the back-to-back region, which depend on the different color and spin structure of the final states and their interferences with the initial states. In order to investigate these effects experimentally, we propose to measure the ratio of the distributions in $\Delta \phi$ for $\mathrm {Z} +$jet- and multijet production at low and at high transverse momenta, and compare the results to predictions obtained assuming factorization. We examine the role of theoretical uncertainties by performing variations of the factorization scale, renormalization scale and matching scale. In particular, we present a comparative study of matching scale uncertainties in the cases of PB-TMD and collinear parton showers