Z 0 / γ ∗ +Jet via electron decay mode at s = $\boldsymbol {\sqrt {s}=}$ 7 TeVin CMS@LHC

Abstract

Abstract. The area-normalized angular distributions in events containing a Z 0 boson and a jet, using the electron decay mode are presented. The data samples correspond to 5f b −1 of proton-proton collisions at √ s=7 TeV, collected by the CMS detector. Events in which there is a Z boson and at least one jet, with a jet transverse momentum threshold of 30 GeV /c and absolute jet rapidity less than 2.4, are selected for this analysis. We compare our measurements with a next-to-leading-order perturbative QCD calculation and two generators that combine tree-level matrix element calculations with parton showers. IntroductionThe measurement of the angular distributions has been a tool for understanding the structure and interactions of matter from the time of Rutherford s first scattering experiments. At the Large Hadron Collider (LHC), copious amounts of prompt photons and Z bosons (collectively called V) in association with jets are produced. The measurement of the rapidity distributions of Z+jet events is necessary for the characterization of the Higgs boson properties, and provides an important test of the modeling of these properties in theory calculations. For Z boson decays to electron and muon pairs, the signal is nearly background-free, and triggering is very efficient. From a theoretical point of view, the presence of the electroweak vertex makes the perturbative calculation of dynamical quantities more stable, and next-to-leading order (NLO) perturbative quantum chromodynamics (pQCD) calculations exist for Z+up to 4 jets [1] and photon+jet production [2,3]. The rapidity of a particle is defined as Y = 1/2 log((E + pz)/(E − pz)), where E is the energy and p z is the momentum component along the direction of one of the proton beams. If the V and the jet are the only objects in the event, the polar scattering angle (θ * ) is defined with respect to the incident proton direction in the two-object center of momentum (COM) reference frame, and can be written in terms of the measured quantities Y V and Y jet as cosθ * = tanh(Y dif )/β * ,where β * is the speed of the V, andapproximates the rapidity boost from laboratory to the COM reference frame. In the laboratory frame, Y V and Y jet are highly correlated because there is usually a relatively high momentum quark interacting with a low momentum gluon or anti-quark. The transformed rapidity relations (Y sum , Y dif ) are effectively a rotation in phase space of the system (Y V , Y jet ), which results in the two approximately uncorrelated quantities. At leading-order (LO), matrix element calculations of V + jet events have asymptotic small angle cosθ * distributions which vary in proportion to 1/(1−|cosθ * |), while the di-jet spectrum is proportional to 1/(1−|cosθ * |) 2 . The difference is due to the spin of their respective propagators. Theory predictions of all the four Y distributions: Y V , Y jet , Y sum and Y dif , are compared with data taken by the Compact Muon Solenoid (CMS) detector.