Session S13: QCD and Colliders II

Performance of Jet Algorithms at CMS

The Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC) at CERN is designed to study a wide range of high-energy processes involving diverse signatures of final states. Almost every process of interest at LHC contains quarks and gluons in the final state which, through the fragmentation process, evolve into collimated spray of particles called jets. Jets are identified by clustering localized energy depositions in the CMS calorimeter detectors. Studies on the performance of the jet algorithms used at CMS will be presented.   

Measuring the Jet Energy Resolution at CMS: A Data-Driven Approach

The Compact Muon Solenoid (CMS) detector at the Large Hadron Collider at CERN is a general-purpose detector designed to study proton-proton collisions at a center-of-mass energy of 14 TeV. Many interesting physics signatures include jets in the final state. Jets are identified by clustering localized energy depositions in the CMS calorimeter detectors. Using a sample of dijet events, we have investigated methods for determining the jet energy resolutions in data as a function of jet's transverse energy and pseudorapidity. Studies demonstrating the validity of these methods using Monte Carlo simulations will be presented.   

A Combined Tracking \& Calorimetry Jet Energy Scale at D0

Currently, D0 jets from p-p(bar) collisions are reconstructed using the signals from a liquid Argon - Uranium sampling calorimeter. Previous work has shown an algorithm combining the precision of the D0 sampling calorimeter at high momenta and the precision of the magnetic tracker at low momenta can significantly improve the raw jet energy resolution. Using the framework of the current calorimeter-only jet energy scale at D0 a new track and calorimeter jet energy scale can be calculated so a direct comparison between the two methods can be made. The modifications of the calorimeter- only jet energy scale are reported, and the resultant improvement in jet energy resolution is demonstrated.   

Photon + Heavy Flavor Jet Production at D0

A measurement of photon + b jet production is presented using about 1 fb~$^{-1}$ of data collected by the D0 detector at the Tevatron p pbar collider at a center-of-mass energy of 1.96~TeV. Isolated photons are selected in the rapidity range of $0 < |y| < 1.0$ and the jets are selected using the D0 Midpoint Cone Algorithm in the rapidity ranges $0 < |y| < 0.8$ or $1.5 < |y| < 2.5$. The measurement is expected to help constrain the understanding of the b quark content of the proton.   

Study of Inclusive Prompt Photon Production at CDF

We present preliminary results on inclusive prompt photon production based on 2 fb$^{-1}$ of CDF Run II data. The measurements are performed as a function of the transverse momentum and the pseudorapidity of the photon and are compared to pQCD predictions.   

Studying Hard Elastic NN Scattering in Isosinglet State

We analyze world data on high momentum transfer pp and pn scattering in order to extract the hard nucleon-nucleon elastic cross sections in isosinglet channel. These data provide us with new constraints in checking the QCD dynamics of elastic nucleon-nucleon scattering. Several aspects of this dynamics are studied, such as the evidence of the oscillations in the energy dependence of $s^{10}$ weighted differential cross section of NN scattering at $90^{\circ}$ center of mass scattering angles and extent of SU6 symmetry breaking in the quark wave function of the nucleon. Studying, both, angular and energy dependences of isosinglet NN cross section we attempted to constrain the contribution due to resonating scattering as well as independent gluon exchanges that could give rise the energy oscillations observed in $pp$ scattering.   

CMS Pixel Hit Reconstruction Using Cluster Templates and its Impact on Tracking and b-Tagging

We present a novel technique for reconstructing hits in the CMS pixel detector based on fits o the observed spatial charge distributions to the templates obtained from a detailed simulation. This technique suppresses resolution tails and provides cluster shape information that is useful in track seeding. We present the improvement this approach brings track reconstruction and the b-tagging using the baseline algorithms currently in use in CMS.   

Level 1 b-tagging Trigger Proposal for CMS

A study of $b$-tagging in CMS Level-1 triggers by simulated data is presented. Two muon momentum cuts, loose and tight, are proposed, which give a Level-1 $b$-tagging efficiency of 7.9\% for jet $|\eta|<2.5$, and 7.3\% for jet $|\eta|<2.0$ respectively. For an instantaneous luminosity of $L=2 \times 10^{33}$ cm$^{-2}$s$^{-1}$and the jet $p_T$ cut of 20 GeV jet pT cut 20GeV, we obtain an integrated background rate 3.8kHz for tight cut, and 5.7kHz for loose, which is a hundred times lower than 210kHz rate of a non-b-tag L1 trigger with the same jet threshold. The proposed trigger is now under consideration by the CMS collaboration.   

Jet and E$_T^{miss}$ Improvement Using Track Information in Atlas

We present a new technique that uses tracks to improve the energy resolution of calorimeter jets. The method relies on the fact that jet energy response depends on the jet particle composition, which can be determined, on a jet-by-jet bases, using track information. The new algorithm corrects the jet energy response as a function of the fraction of charged particle energy, defined as the ratio of track-jet over calorimeter-jet transverse momentum ($f_{trk}$). Preliminary studies with ATLAS full simulation show a 20\% improvement in the fractional jet energy resolution with the new track-based energy response and an improvement in missing transverse energy scale. Future improvements include the use of additional track-based variables, like the fraction of transverse momentum carried by the leading track.