The Compact Muon Solenoid Experiment V+jets from CMS

The production of vector bosons (V = W, Z or γ ) in association with jets is a stringent test of perturbative QCD and is a background process in searches for new physics. Total and differential cross-section measurements of vector bosons produced in association with jets (and heavy ﬂavour quarks) are presented. The data have been recorded with the CMS detector at the LHC and are compared to next-to leading order calculations and event simulations that devise matrix element calculations interfaced with parton showers. Abstract The production of vector bosons (V = W, Z, or γ ) in association with jets is a stringent test of perturbative QCD and is a background process in searches for new physics. Total and di ﬀ erential cross-section measurements of vector bosons produced in association with jets (and heavy ﬂavour quarks) are presented. The data have been recorded with the CMS detector at the LHC and are compared to next-to(-next-to)-leading-order calculations and event simulations that devise matrix element calculations interfaced with parton showers.


Introduction
Vector bosons (V = W, Z, or γ) produced in association with jets (V+jets) provide important test of perturbative quantum chromodynamics (QCD) calculation. Precise measurements of these processes can improve the validation and tuning of the models used in Monte Carlo (MC) simulation. Such measurements also provide crucial inputs in the determination of parton distribution functions (PDFs). The V+jets process itself is a background process for both the Higgs boson production and for many searches of physics beyond the standard model (SM). Therefore, a thorough study of these processes is vital at the LHC. Measurements of V+jets from CMS detector [1] using proton-proton collision data collected at √ s = 8 TeV and 13 TeV are reported. The collision data corresponds to integrated luminosities of < 20 fb −1 and 2.5 fb −1 respectively. The measured cross sections are unfolded to particle level in order to correct for detector effects. The measurements are compared to predictions from MC generators * Talk given at 19th International Conference in Quantum Chromodynamics (QCD 16), 4 -8 July 2015, Montpellier -FR * * On behalf of the CMS collaboration Email address: apichart.hortiangtham@cern.ch (A. Hortiangtham) and fixed-order calculations. In the case of fixed-order calculations where parton-level prediction is provided, a nonperturbative correction is applied before comparing to measurements.

Z+jets
Differential cross section measurements of associated production of Z bosons with jets using pp collision data of 2.5 fb −1 at √ s = 13 TeV are presented [2]. The differential cross sections are measured as a function of jet transverse momenta (p T ), jet absolute rapidity (|y|) and scalar sum of p T of jets in an event (H T ) up to the inclusive jet multiplicities of 3. Jets have p T > 30 GeV and |y| < 2.4. Z bosons are identified via their decays into µ + µ − pairs. The measurements are compared to the prediction from MadGraph5 aMC@NLO [3] interfaced with PYTHIA 8 [4] which is an event generator devising next-to-leading-order (NLO) matrix element calculations up to 2 additional partons merged using FxFx scheme. The measurements are also compared to a next-to-next-to-leading-order (NNLO) calculation for Z + 1jet [5]. The jet multiplicity distribution is shown in Figure 1. Figure 2 shows the highest-p T jet p T distribution. Good agreement with multileg NLO generator and NNLO calculation are observed.
A more comprehensive analysis where kinematic distributions of jets are studied up to the inclusive jet multiplicities of 5 has been performed with √ s= 8 TeV data corresponding to integrated luminosity of 19.6 fb −1 [6]. Additionally, the double differential cross section with respect to transverse momentum and rapidity of the highest p T jet, where the rapidity region of the jets is extended to |y| ≤ 4.7, is measured [7] as shown in Figure 3. Theoretical predictions used in comparison are Mad-Graph [8] interfaced with PYTHIA 6 [9] and Sherpa 2 [10].

W+jets
The production of W bosons in association with jets has been studied in pp collision at √ s = 8 TeV for jets with p T > 30 GeV and |η| < 2.4 [11]. The data used corresponds to integrated luminosity of 19.6 fb −1 . The W bosons are identified by their subsequent decay into a muon and a neutrino yielding the final state with one hight p T isolated muon and significant missing transverse energy (E miss T ). Muons are required to have p T > 25 GeV and |η| < 2.   observables are presented. The measurements are compared to several predictions including mulitleg event generators and fixed-order parton-level calculations. The generators used are MadGraph [8] interfaced with PYTHIA 6 [9], which uses a leading-order (LO) matrix element calculation, MadGraph5 aMC@NLO [3] + PYTHIA 8 [4] and Sherpa 2 [10], which use NLO matrix element calculations. The differential cross sections are also compared with the NLO parton-level predictions of BlackHat+SHERPA [12] and with a NNLO calculation for the production of W + 1 jet [5,13]. Good agreement is observed with MadGraph5 aMC@NLO and (NNLO and NLO) fixed order calculations. Figure 4 shows the inclusive jet multiplicity distribution. Figure 5 shows the H T distribution for events with N jet ≥ 1. BlackHat+SHERPA underestimates the data at high H T as expected since it is fixed-order prediction with maximum of 2 jets and contributions from higher jet multiplicities is missing. Fugure 6 shows differential cross section as a function of the rapidity difference between the leading-p T and subleading-p T jets.

W + 2b quark jets
Measurement of the W boson production cross section in association with two b jets in pp collisions at √ s = 8 TeV is studied with data corresponding to integrated luminosity of 19.8 fb −1 [14]. The W bosons are reconstructed via their leptonic decays to muons and electrons channel. The resulting cross section for the combined lepton channels is 0.69 ± 0.02(stat) ± 0.11(syst) ± 0.07(theo) ± 0.02(lumi) pb. The measurement is compared to theoretical predictions including MCFM [15,16] (corrected for hadronization), Mad-Graph 5 [8]+PYTHIA 6 in the four-and five-flavour schemes (FS), and MadGraph 5+PYTHIA 8 in the fourflavour scheme. The resulting cross section predictions in the fiducial volume at the hadron level and including the estimated hadronization and DPS corrections when needed are compared in Figure 7 with the measured value. The predictions agree with the measured cross section within one standard deviation.

Z + b quark jets
The measurement of the production of a Z boson in association with at least one jet originating from a b quark in proton-proton collisions at √ s = 8 TeV is presented [17]. Differential cross sections are measured on data for a total integrated luminosity of 19.8 fb −1 using Z boson decays into electrons or muons, and identified b quark jets. Cross sections are measured in combined  lepton channels as a function of observables characterizing the b jet and Z boson kinematics, and ratios of differential cross sections for the associated production with at least a b jet and with any jet are presented. The measured fiducial cross sections for events with at least one b jet, noted as Z(1b), and with at least two b jets noted as Z(2b) are 3.55 ± 0.12(stat.) ± 0.21(syst.) pb and 0.331 ± 0.011(stat.) ± 0.035(syst.) pb respectively. The ratio of the cross sections in the fiducial phase space for the production of at least two and at least one b jet is (9.3 ± 0.4(stat.) ± 0.7(syst.)) ×10 −2 . Results are compared to theoretical predictions including MadGraph 5+PYTHIA 6 in the four-and five-flavour schemes and NLO POWHEG [18] in the five-flavour scheme. An overall good agreement is observed between unfolded data and the 5FS-based theoretical calculations for the Z(1b) final state in most of the ranges of the measured observables. Figure 8 shows the differential Z(1b) cross section as a function of the leading b jet p T .

Photon (γ) + jets
The differential cross sections for the process Z/γ * + jets and photon (γ) + jets is presented. The measure- ments are based on data at √ s = 8 TeV corresponding to an integrated luminosity of 19.7 fb −1 [19]. The differential cross sections and their ratios are presented as functions of p V T of the vector bosons (V = Z or γ) for inclusive jet multiplicities of 1 and 2. The analysis also presents the ratio p Z T /H T , log 10 (p Z T /p j1 T ), and the inclusive 1-jet over inclusive 2-jet p V T cross section ratio. For γ + jets, the γ has p T > 20 GeV and |y| < 1.4. Jets have p T > 30 GeV and |η| < 2.4. The measurements are compared to theoretical predictions including Mad-Graph+PYTHIA 6 and BlackHat. Figure 9 shows the measured p γ T spectrum for N jets ≥ 1 compared to theoretical predictions. There are deviations for low p T values, and NLO BlackHat does better than LO MadGraph in describing the data.

Conclusion
CMS has an active physics program to measure vector boson production in association with jets including heavy flavour jets providing precision tests for perturbative QCD calculation. The data delivered by LHC at 8 TeV allows possibility to probe extreme kinematics    Figure 7: Comparison between the measured W+bb cross section and various QCD predictions. The blue error bars on the predictions represent the uncertainty in the given sample associated with PDF choice and the black bars represent the total uncertainty. In the case of the MadGraph+PYTHIA 6 (5F) sample, the effects of double parton interaction (DPS) are already included in the generated sample so the extra DPS factor was not needed and the blue and black error bars overlap perfectly.   19.7 fb Figure 9: Differential cross section for photon production as a function of p γ T for an inclusive γ+jets, N jets ≥ 1 selection for central rapidities |y| < 1.4, compared with predictions from MadGraph+PYTHIA 6 and BlackHat. The bottom plots give the ratio of the various theoretical predictions to the data. that were not previously accessible. The early results of Z+jets at 13 TeV is also presented. Measured V+jets differential cross sections in various observables have been compared to several predictions. In general, better agreement is observed with higher order (NLO or NNLO) predictions than LO predictions.