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Measurement of b-jet Shapes at CDF

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1 Measurement of b-jet Shapes at CDF
Alison Lister UC Davis Hadronic interactions and QCD Jet Shape definition b-quark Jet Shapes Motivation Analysis Methodology Systematics Results Conclusions - What are jet shapes? - Why are b jet shapes special? - How do we define the shape? - What do the results show? - Conclusions DPF2006, 31st October 2006 Alison Lister, UC Davis

2 Outline Motivation Definition Unfolding Results Conclusions
DPF2006, 31st October 2006 Alison Lister, UC Davis

3 Hadronic interactions
Hadron colliders Only small fraction of initial particles participate in hard interaction Rest goes into underlying event Total centre of mass energy of each interaction not known Theoretical cross-section Fragmentation + hadronisation processes Initial parton → Set of hadrons Jet algorithms Kinematics as close as possible to initial parton kinematics Can only know that the total transverse momentum of each interaction must be zero. Hadrons (or tracks or energy deposits in calorimeters) Mostly “close” to direction of initial parton Many different algorithms to define the jet objects Want jet energy as close as possible to initial parton energy Jet defined by direction and energy Cone algorithms also by cone size DPF2006, 31st October 2006 Alison Lister, UC Davis

4 Jet Shape Definition Integrated shape
Fractional pT of jet inside cone of size r around jet axis 1 R r Can also define differential quantity Rapid = 0.5 ln(E+pt/E-pt) DPF2006, 31st October 2006 Alison Lister, UC Davis

5 Motivation Jet shapes Probe internal structure of jets Dependence on initial parton Testing Monte Carlo simulation S. Frixione et al. hep-ph/ b-jet shapes First measurement of shapes of b-jets at hadron colliders b-quarks from gluon splitting mostly inside the same jet 1b 2b Say: tool for top physcis and H->bb search Thorough understanding of b-jets crucial for many potential discovery channels at the Tevatron and LHC! DPF2006, 31st October 2006 Alison Lister, UC Davis

6 Single b-quark Fraction
Parameters of unfolding depend on relative amount of gluon splitting to flavour creation b-jets Use comparison between Pythia Tune A with NLO calculation at hadron level (using scale with largest difference) Max f1b difference 0.2 M. D’Onofrio Re-weight the MC samples to account for a decrease in f1b by 0.2 DPF2006, 31st October 2006 Alison Lister, UC Davis

7 Datasets + MC Samples Jet datasets MC samples Jets
~300 pb-1 of CDF Run II data Trigger relying only on calorimetry Require > 99% trigger efficiency MC samples Pythia di-jet Herwig di-jet Jets MidPoint cone 0.7, merge fraction 75% Jet pT corrected back to hadron level Central jets with |Y| < 0.7 One well reconstructed primary vertex Pythia Tune A: Tuned to CDF Run I underlying event Used for unfolding and comparison to data Herwig Systematic studies Comparison to data pT limits [Gev/c] Njets Ntagged jets 52-80 ~160’000 ~4’700 80-104 ~354’000 ~13’400 ~135’000 ~5’900 378’000 ~18’700 DPF2006, 31st October 2006 Alison Lister, UC Davis

8 b-jet Shapes Fraction of b-quark jets after tagging not 100%
We want Fraction of b-quark jets after tagging not 100% Need to separate statistically b-jets from background Tagging requirement biases measured shapes Makes b-jet shapes narrower, nonb-jet shapes wider Detector independent measurement Unfolding equation can be re-written as Measured after tagging Purity Biases Purity Biases Measured ~inclusive jet shape DPF2006, 31st October 2006 Alison Lister, UC Davis

9 Unfolding parameters The different parameters needed for the unfolding are Detector level jet shapes for tagged jets Measured in data Detector level jet shapes for inclusive jets Measured in data (incl ~ nonb) Purity of the samples Secondary vertex mass fit Tagging biases (from MC) b-jets and nonb-jets Hadron level corrections to b-jets (from MC) DPF2006, 31st October 2006 Alison Lister, UC Davis

10 Purity Extraction Look at the secondary vertex mass distribution of tagged jets Different for b-jets and nonb-jets Define set of templates Fit data distribution to templates Extract fraction of tagged jets which have a b-quark inside jet cone (pb) DPF2006, 31st October 2006 Alison Lister, UC Davis

11 Total Systematics Dominant sources of systematic errors
b-quark jet shapes reconstructed from raw track shapes Effect of use of particular fragmentation, hadronisation and underlying event models Jet energy scale Variation of fraction of c-jets from gluon splitting Add GeV/C DPF2006, 31st October 2006 Alison Lister, UC Davis

12 Hadron Level b-jet Shapes
Herwig predicts slightly broader b-jet shapes Agreement between data and MC significantly better for jets with a larger fraction of gluon splitting DPF2006, 31st October 2006 Alison Lister, UC Davis

13 Hadron Level b-jet Shapes
MC / Data Pythia Tune A Herwig Yellow band: total errors on measurement Black hashed bands: statistical error on measurement DPF2006, 31st October 2006 Alison Lister, UC Davis

14 Hadron Level b-jet Shapes vs pT
Fractional pT outside sub-cone of size 0.2 Larger fraction means wider jets Difference observed between inclusive and b-quark jet shapes Evolution with pT flatter for b-jets than for inclusive jets DPF2006, 31st October 2006 Alison Lister, UC Davis

15 Conclusions b-jet shapes measured for the first time at a hadron collider Important for the understanding of b-jets Needed for many discovery channels at Tevatron and LHC Fraction of b-jets with two b-quarks inside the same jet cone seems to be significantly underestimated in LO MC Most likely the rate of gluon splitting to b bbar pairs is underestimated Important for modeling top, higgs and new physics with heavy flavour DPF2006, 31st October 2006 Alison Lister, UC Davis

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