Session W08: QCD

The charm of the proton: using forward Z+c production to study intrinsic charm at LHCb

While the proton is typically considered to be composed of three quarks at the valance level, current experimental limits also allow for a percent-level contribution from valance-like charm quarks (intrinsic charm). The production of Z bosons in association with charm jets in the forward region provides a direct probe of the charm content of the proton. We will present results from a recent analysis of forward Z+c production at LHCb, which show strong hints of an intrinsic charm component.   

Measurement of four-jet production in the ATLAS experiment

Measurements of multi-jet cross section ratios are presented.

The measured quantities are the ratio of the four-jet and three-jet

cross sections and the ratio of the four-jet and two-jet cross sections.

The data were collected with the ATLAS detector in proton-proton

collisions at a center of mass energy of 13 TeV during 2015-2018.

The results are presented as a function of the transverse jet momentum.

Predictions from different Monte Carlo event generators are compared

to the data.   

Multijet correlations in CMS

Collinear (small-angle) and large-angle, as well as soft and hard radiations, are investigated in three-jet and Z + two-jet events collected in proton-proton collisions at the LHC. The normalized production cross sections are measured as a function of the ratio of transverse momenta of two jets and their angular separation. The measurements in the three-jet and Z + two-jet events are based on data collected at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.8 fb^−1. The Z + two-jet events are reconstructed in the dimuon decay channel of the Z boson. The three-jet measurement is extended to include sqrt(s) = 13 TeV data corresponding to an integrated luminosity of 2.3 fb^−1. The results are compared to predictions from event generators that include parton showers, multiple parton interactions, and hadronization. The collinear and soft regions are in general well described by parton showers, whereas the regions of large angular separation are often best described by calculations using higher-order matrix elements.   

The latest results in the search for four top production at CMS in the opposite-sign dilepton channel

The production of four top quarks is predicted by the Standard Model with a cross-section of 12fb, 69,000 times rarer than top quark-anti-quark production, and has not yet been observed. With the high invariant mass of the system and multiple decay channels, the process produces distinct signatures including 4 b-jets and multiple lighter-quark jets (up to 8 in the all-hadronic decay) and leptons (up to 4 in the fully-leptonic decay). The latest results from CMS for 2017 and 2018 data taking at the LHC are presented for the opposite-sign dilepton channel and are expected to bring additional sensitivity in combination with other final states.   

Measurement of the Z boson production in association with at least two b jets in pp collisions at √s = 13 TeV with CMS

The cross sections of the Z boson production in association with at least two b jets as a function of various kinematic variables are measured in pp collisions at √s = 13 TeV using 137 1/fb of data collected by the CMS experiment at LHC. The Z boson decays to electrons or muons are considered with leading (sub-leading) lepton transverse momentum pT > 35 (25) GeV and pseudorapidity |eta|< 2.4, and the invariant mass within 71 and 111 GeV. Jets are selected with pT > 30 GeV and |eta| < 2.4. The results are compared to various QCD calculations.   

Identification of additional b-jets in the tt- plus heavy flavor production with two leptons of the opposite sign in the final state using BDT for differential cross- section measurements

Top quark pair-production in association with one or two additional b-jets is one of the main backgrounds in the search for tt^-+H production, and in BSM physics searches involving top and bottom quarks in the final states. Precise measurement of tt^-+b and tt^-+bb differential cross sections will help to significantly reduce systematic and modeling uncertainties in future BSM searches and measurements of the top-Higgs coupling. A BDT that discriminates between b-jets which do or do not originate from top quarks decays has been developed in the opposite-sign dilepton channel. The method has higher purity compared to the existing method based on the angular distributions between b-jets and leptons. Preliminary results show that with the new method we can accurately measure the p_T spectra of b-jets from top quark decays and of the leading additional b-jet up to 400 GeV, and the sub-leading additional b-jets p_T up to 300 GeV. The analysis uses LHC proton-proton collision data at the center-of-mass energy √s=13 TeV with an integrated luminosity of 139 fb^-1 collected with the ATLAS detector in 2015-2018.   

Studies of C-jet spectra in various Flavor Schemes for $\bar{t}t+c$ cross section measurement by the ATLAS Detector

The analysis uses LHC proton-proton collision data at the center-of-mass energy $\sqrt{s}=13\ TeV$ with an integrated luminosity of $139\ fb^{-1}$ collected with the ATLAS detector in 2015-2018. A comparison of three flavor scheme, four flavor scheme and five flavor scheme has been performed using top quark pair-production with additional charm jet ( $\bar{t}t+c$) events simulated by MadGraph and showered by Pythia8. Spectra of additional c-jets that do not originate from the top quark decay are compared in the three schemes. These studies are motivated by the upcoming  $\bar{t}t+c$ jets cross section measurement. No significant differences in spectra of additional c-jets are observed in the different flavor schemes.   

Measurement of Z plus Heavy-Flavor Jets Differential Cross Section with Run-2 ATLAS data

Measurements of the production rate of Z bosons in association with heavy quarks provide sensitive tests of perturbative quantum chromodynamics (pQCD) predictions, which are made at next-to-leading-order (NLO) accuracy using either a 4-flavor number scheme (4FNS) or 5-flavor number scheme (5FNS). In the 4FNS, b-quarks are not present in the parton distribution functions (PDFs) and only appear as a product of gluon splitting (g→bb). In the 5FNS, on the other hand, a (massless) b-quark PDF is included. A previous analysis studying Z→ee/μμ + b-jet events using 2015 & 2016 data showed that the 5FNS predictions match the data well, while the 4FNS predictions underestimate the data. The uncertainties are substantial, however. In our analysis we are attempting to further investigate these results by also including Z + c-jet events and looking at the combined "heavy-flavor" (b+c) region to reduce uncertainties. We are also updating the 2015-2016 results with 140 fb−1 (up from 35.6 fb−1) of ATLAS Run-2 data at (√s)=13 TeV. This is still a work in progress, but important milestones will be presented.   

Cross section measurement of associated J/ψ+W± production with the ATLAS detector.

The cross-sections of prompt and non-prompt production of J/ψ+W^± are being measured by the ATLAS Collaboration using the 139 fb^-1 Run II 13 TeV pp collision data set. The prompt production of J/ψ associated with a W^±, where both particles are produced in a single parton-parton interaction, provides constraints on models of non-relativistic QCD, especially in the realm of heavy quarkonia production. The measurement is an all-leptonic final state with J/ψ→μ⁺μ^- and W^±→l^±ν, and it is performed differentially in J/ψ transverse momentum and rapidity. Finally, the non-prompt measurement allows for the study of top production and b-quark fragmentation.