Z bjet from Tevatron Andrew Mehta Zbjet Motivation

Z/γ + b-jet from Tevatron Andrew Mehta • Z+b-jet Motivation • Z+b-jet Experimental Techniques • Z+b jet Results • γ+b jet Results • Summary 1

Z+b jets Motivation J. Campbell et al. , Phys. Rev. D 69, 074021 (2004). QCD – An important measurement of the b density of the proton. The first two diagrams measure the b density of the proton. Also scale dependences are small. In principle precise test of QCD. MSSM Higgs- Important to understand b density for MSSM Higgs production SM Higgs- Main background to associated production of higgs: so vital to understand this background. Excited quarks- There are other possibilities such as 2

Z+ b-jet Scale Dependence J. Campbell et al. , Phys. Rev. D 69, 074021 (2004). NLO uncertainty estimated to be ~ 15% 3

Z+b-jet Experimental Technique • Reconstruct Z via it’s e and μ decays. • Preliminary measurement based on 2 fb-1 updates publication of 0. 3 fb-1. • Look for b tagged jets in the event with cone size 0. 7, ET>20 Ge. V and |η|<1. 5. • Fit the mass of charged tracks at secondary vertex to find number of reconstructed b jets • Use Monte Carlo to correct for efficiencies, smearing etc. to produce a hadron level cross section. 4

Secondary Vertex Secondary vertex Reconstruct secondary vertex from 2 or more tracks Measure b tagging efficiency directly from data, using events with leptons inside jets. 5

Z Mass for b tagged jets Clear Z signal. Main backgrounds fake e/μ, top, ZZ, but all rather small. 6

ET and η spectra for b tagged jets Jet spectra well described by MC Still a lot of background from charm and light jets 7

Z+b-jet Mass at Secondary Vertex • Mass of charged tracks at Secondary Vertex. • Allows b to be discriminated from c and lights. • Use MC templates and fit to give number of reconstructed b events • b fraction is 0. 45 ± 0. 07 • b efficiency is 32% (in this kinematic range) 8

Z+b-jet Systematic Errors reduced from publication Largest due to uncertainty in ETJet since b tag efficiency is not constant with ETJet 9

Z+b-jet Results Integrated results: • Data agrees with published measurement: • Good agreement with PYTHIA, but ALPGEN, HERWIG, NLO a little low • NLO calculated with MCFM ~2σ lower than data so not very big disagreement • 1 old measurement from D 0: agrees although this assumed ratio of Z+c/Z+b from NLO. 10

Z+b-jet Differential Distributions • PYTHIA in general shows good agreement. • ALPGEN and MCFM show biggest differences at low ET. 11

Z+b-jet Differential Distributions • Also measure jet η and jet multiplicities. 12

Photon+b jet • Similar experimental technique to Z+ b jet • Major difference is very large background from fake photons, but large statistics. • Unfortunately no NLO, but good agreement with PYTHIA 13

Conclusions • New measurements of Z+bjet and γ+bjet. • Good agreement with PYTHIA. • Z+bjet is higher than NLO by ~2σ. • Measurement of differential cross sections in Z bjet for the first time. 14