Jet Reconstruction in CMS using Charged Tracks only

Transcription

1 Jet Reconstruction in CMS using Charged Tracks only Andreas Hinzmann for the CMS Collaboration JET Aug 2010

2 Jet Reconstruction in CMS Calorimeter Jets clustered from calorimeter towers independent detectors Track Jets clustered from tracks Jet plus Tracks correct Calorimeter Jets using momentum of tracks Particle Flow Jets clustered from identified particles reconstructed using all detector components make use of full CMS detector Talk by Sevil Salur 2 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

3 Introduction to TrackJets TrackJets measure the charged component of a jet which typically corresponds to ~60% of the actual quark/gluon momentum Charged jet component is the one measured with the best precision in the tracker TrackJets are a very reliable way to find jets of any energy, down to very low p T (very low fake rate) Jet direction is accurately measured with TrackJets TrackJets are transparent to pile-up, thanks to clean vertex association This talk: Performance of TrackJets, Commissioning using 7 TeV data, First data analyses using TrackJets Documented in CMS PAS JME Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

4 The CMS detector 4 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

5 The CMS tracking system Covers η < 2.4 (η = ln[tan(θ/2)]) Pixel detector: 3 barrel layers, 2 forward disks Strip detector: barrel and forward layers, ~40% with stereo views Iterative tracking using combinatorial track finder CMS PAS TRK Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

6 TrackJet reconstruction Select tracks with good momentum error p T > 300 MeV distance to primary vertex (d xy <2mm, d z <5mm) tight cuts on χ 2 /ndof and impact parameter significance TrackJets built by clustering selected tracks using anti-k T algorithm with R = 0.5 separately for each primary vertex 6 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

7 Energy response MC s=14 TeV CMS PAS JME Average fraction of charged particles in jets: ~60% 7 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

8 Position resolution CMS PAS JME Position resolutions of ~0.04 at p T =10 GeV generator level jets 8 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

9 Reconstruction efficiency Reconstruction efficiency and fake rate studied on MC More than 99% of true GenJets with p T > 15 GeV/c are reconstructed as TrackJets Less than 1% for TrackJets with p T > 10 GeV/c do not correspond to a true GenJet 9 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

10 TrackJet commissioning March 30, 2010 CMS started collecting data from LHC protonproton collisions at 7 TeV Present results based on minimum bias events, corresponding to 100 μb 1 of integrated luminosity Minimum bias events triggered by requiring activity in the beam scintillation detectors on each side of the CMS detector and the reconstruction of one good primary vertex Data is compared to a reference minimum-bias sample produced with the PYTHIA 8.1 event generator and processed with a simulation of the CMS detector response based on GEANT Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

11 Inclusive TrackJets p T > 10 GeV Green error-band corresponds to 10% uncertainty on the luminosity Default tune of PYTHIA 8.1 reproduces multiplicity and transverse momentum distribution 11 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

12 Inclusive TrackJets p T > 10 GeV p T > 10 GeV MC and detector simulation reproduces angular distribution of inclusive TrackJets 12 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

13 TrackJet constituents p T > 10 GeV p T > 10 GeV Median number of tracks per TrackJet: ~6 Median fraction of transverse momentum carried by leading track: ~35% 13 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

14 Di-jet events p T1 > 10 GeV p T2 > 10 GeV p T1 > 10 GeV p T2 > 10 GeV Di-jet asymmetry sensitive to TrackJet energy resolution nicely described in MC and detector simulation Di-jet event back-to-back in ΔФ 14 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

15 Matching with other jet types Matching efficiency (using ΔR<0.5) 95% for p T > 40 GeV for TrackJets and CaloJets lower for p T < 40 GeV due to angular resolution of CaloJets 95% for p T > 15 GeV for TrackJets and ParticleFlowJets 15 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

16 TrackJets in pile-up events Example: t t µ + µ - + jets candidate event TrackJets clustered separately for each primary vertex 16 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

17 Effect of pile-up on multi-jet events H T = Σ p T for all jets with p T >3 GeV/c in an event Vertex association eliminates pile-up effects for TrackJets Systematic effects Other systematic effects: beam background, electronic noise, mis-alignment, material budget, track quality selection: < 1% effect on TrackJets reconstruction efficiency + fake rate 17 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

18 Analyses with TrackJets Measurement of the Underlying Event Activity (CMS PAS QCD , CMS PAS QCD ) Study number and momentum of charged particles in region transverse to leading TrackJet 18 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

19 Analyses with TrackJets Open beauty production cross section with muons (CMS PAS BPH ) Transverse momentum of the muon with respect to the closest TrackJet discriminates b events from background 19 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

20 Conclusions TrackJets are an orthogonal approach to CaloJets in CMS used for cross-validation of jets used in analyses focusing on jet finding rather than full energy measurement TrackJets have been commissioned using 7 TeV collision data good agreement with simulation TrackJets are expected to be transparent to pile-up effects in high LHC luminosity scenarios 20 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only

21 References CMS PAS TRK : Tracking and Primary Vertex Results in First Collisions CMS PAS TRK : Tracking and Primary Vertex Results in First 7 TeV Collisions CMS PAS JME : Performance of Jet Reconstruction with Charged Tracks only CMS PAS JME : Commissioning of TrackJets in pp Collisions at s=7 TeV CMS PAS JME : CMS Jet Performance in pp Collisions at s=7 TeV CMS PAS QCD : First Measurement of the Underlying Event Activity in Proton-Proton Collisions at 900 GeV at the LHC CMS PAS QCD : Measurement of the Underlying Event Activity at the LHC with s = 7 TeV and Comparison with s = 0.9 TeV CMS PAS BPH : Open beauty production cross section with muons in pp collisions at s = 7 TeV 21 Andreas Hinzmann Jet Reconstruction in CMS using Charged Tracks only