Session R10: QCD, EWK, and Jet Physics

Measurement of the Rapidity Dependence of Dijet Azimuthal Decorrelations and Determination of the Strong Coupling Constant

The first measurement of the rapidity dependence of dijet azimuthal decorrelations is presented. The measurement is based on data taken with the DO detector in Run II of the Fermilab Tevatron Collider. Dijet azimuthal decorrelations are measured as a function of the scalar sum of the transverse jet momenta in different regions of the dijet scattering angle. The results are compared to pQCD predictions in NLO, and used to test the running of the strong coupling constant in a novel energy regime.   

Measurement of $Z$ boson production in association with heavy flavor jets at D0

$Z$ boson production processes in association with $b$ and $c$ quarks provide important tests of Quantum Chromodynamic calculations. Several measurements based on data sample corresponding to an integrated luminosity of 8 fb$^{-1}$ collected by D0 in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV are discussed. They include measurements of $Z+b$-jet differential cross-sections and $Z+b$- and $Z+c$-jet rates in $Z$ events with at least one jet. $b$- and $c$-jet fractions are determined by means of a dedicated algorithm that employes kinematic properties of tracks associated to the jet. Measurements are compared to the next-to-leading order theoretical predictions.   

Measurement of the forward-backward charge asymmetry in $Z/\gamma^*\to e^+e^-$ events with the D0 detector at the Tevatron

We present a measurement of the forward-backward charge asymmetry for dielectrons produced via an intermediate $Z/\gamma^*$ boson with mass between 50 and 1000 GeV, using about 10 fb$^{-1}$ of data collected by the D{\O} detector in $p\bar{p}$ collisons at $\sqrt{s}=1.96$ TeV. The extraction of the weak mixing angle $\sin^2\theta^W_{eff}$ and of the couplings of light quarks to the $Z$ boson are also presented.   

Measurement of the Underlying Event Activity for p-p collisions with $\sqrt{s} = 7$ TeV and 0.9 TeV at CMS

A measurement of the underlying activity in scattering processes with a $p_{T}$ scale of several GeV/c is performed on proton-proton collisions at $\sqrt{s} = 7$ TeV and 0.9 TeV using corrected data collected by the CMS experiment at the LHC. The production of charged particles is studied with reference to the azimuthal direction of the leading track with $|\eta| < 0.8$ and $p_{T} > 0.5$ GeV/c. A significant increase of the average number of charged particles and of the average scalar $\Sigma p_{T}$ is observed followed by a saturation at large values of the $p_{T}$ scale. A significant growth of the activity in the transverse region is observed when increasing the energy from $\sqrt{s} = 0.9$ TeV to $\sqrt{s} = 7$ TeV. Various PYTHIA based QCD-inspired models are compared with the data after full detector simulation.   

Measurement of the Triple Differential Cross Section of Photon plus Jet in pp Collisions at 7 TeV

A measurement of the triple differential cross section for the process $pp \rightarrow \gamma + jet + X$ in proton-proton collisions at a centre-of-mass energy of 7 TeV is presented. The data sample corresponds to an integrated luminosity of 2.1 inverse femtobarns recorded by the CMS detector at the LHC. The measurement covers the photons with pseudorapidity range $|eta|<2.5$ and the transverse energy range $40 < ET < 300$ GeV and a leading jet with pseudorapidity range $|eta|<2.5$ and the transverse energy $ET > 20$ GeV. The measured cross section is presented as a function of ET of the photon for four pseudorapidity regions of the photon two pseudorapidity regions of the jet. The result is compared with the next-to-leading-order perturbative QCD calculations.   

Measurement of the Muon Charge Asymmetry from W Bosons Produced in pp Collisions at a center-of-mass energy of 7 TeV

We present a measurement of the muon charge asymmetry from W Boson produced in proton-proton collisions at a center-of-mass energy of 7 TeV with the CMS detector at the LHC. The asymmetry is measured in the W Boson to muon decay channel as a function of the muon pseudorapidity using data collected in 2010 and 2011 from the CMS detector. The result is compared against predictions from different parton distribution functions.   

New Jet Physics with ATLAS

Measurements of jet shape and substructure variables for high $p_{T}$ jets with $35.0 \pm 1.1$ pb$^{-1}$ of integrated luminosity collected in $2010$ using the ATLAS detector are presented. Jet mass, width, eccentricity, planar flow and angularity are reconstructed using the anti-$k_{T}$ jet algorithm with resolution parameters of $0.6$ and $1.0$. The measurements are compared to those predicted by the {{\sc PYTHIA}} and {{\sc HERWIG++}} Monte Carlo generators assuming the data are dominated by QCD dijet production, and in the case of jet masses, to analytic NLO QCD calculations for quark and gluon jets. Monte Carlo generators reproduce the gross features of jet masses and shapes seen in data, but do not reproduce some of the finer details of substructure distributions. The applicability of these variables to boosted particle searches is discussed.   

Tagging $q/g$ and $g\rightarrow b\bar{b}$ Jets at ATLAS

We present first results on jet tagging in ATLAS. First, we present the results of an implementation of a quark/gluon tagger built with tracking variables, and show that strong gluon rejection is possible even in high luminosity environments. We use template methods to validate the quark/gluon tagger in data. Second, we present results on a new technique developed to identify jets originating from a gluon that has split to $b\bar{b}$. Jet substructure variables enable high levels of double $b\bar{b}$-jet rejection. The ability to separate quark from gluon jets and single $b$-jets from gluons splitting to $b\bar{b}$-jets can enhance the LHC discovery potential by rejecting gluon-dominated backgrounds.