Tracking efficiency at the LHCb experiment Alexander Shires IC HEP 1 st Year PhD Alexander Shires - IC HEP First Year 02/03/2010 1 PhD Talk
Outline • LHCb Physics – overview • The LHCb detector – Vertex locator – RICH – Trigger • Relative tracking efficiency – LHCb Data 2009 – k-Nearest-Neighbour algorithm – Kolmogorov-Smirnov test • Future work Figures taken from The LHCb Detector at the LHC, Journal of Instrumentation, Vol. 3, No. 08. (2008), pp. S08005-S08005 Alexander Shires - IC HEP First Year 02/03/2010 2 PhD Talk
B-physics at the LHCb experiment • Discover new physics – Measure suppressed final states – New physics in loop diagrams • Measure CP violation in B,D mesons – SM doesn’t account for asymmetry in nature Alexander Shires - IC HEP First Year 02/03/2010 3 PhD Talk
Requirements for B Physics • B mesons produced • σ bb = 0.5% σ Tot at small polar angles – require good signal/background separation – Particle ID / Trigger • B flight distance – Only 7mm – Vertex resolution • Forward spectrometer to take advantage of this Alexander Shires - IC HEP First Year 02/03/2010 4 PhD Talk
LHCb Detector Alexander Shires - IC HEP First Year 02/03/2010 5 PhD Talk
LHCb Detector: VErtex Locator (VELO) • Precise tracker • Gives r and φ coordinate information • Moveable – Position dependant on beam size – Insanely close when closed • 50 fs lifetime resolution – B lifetime ~ 1000 fs Alexander Shires - IC HEP First Year 02/03/2010 6 PhD Talk
LHCb Detector: RICH (1&2) • Particle Identification using Cherenkov LHCb data RICH 1 (preliminary) radiation Kaon ring • ID from size of ring Radiator Momentum Range Aerogel Few GeV C 4 F 10 < 60 GeV/c CF 4 15 GeV/c to beyond 100 GeV/c Alexander Shires - IC HEP First Year 02/03/2010 7 PhD Talk
LHCb Detector: Trigger • Extract possible Up to 40 MHz signal candidates Level 0 – hardware - high from messy hadronic p T objects background 1 MHz HLT1 – software – confirms L0 decision • Requires a farm of 40 kHz 2000 CPU cores for HLT 2 – software - Inclusive the software or exclusive reconstruction processing of each 2 kHz event Alexander Shires - IC HEP First Year 02/03/2010 8 PhD Talk
Pion relative tracking efficiency • Aim: To evaluate quality of the K 0 S π + LHCb Monte Carlo reconstruction θ and compare with the data. CoM frame • Use K 0 S → ππ π - – Know physics π + seen – Don’t know production cross section – k-nearest-neighbour method... Lab frame • Decay angle is sensitive to π - lost efficiency K 0 S Alexander Shires - IC HEP First Year 02/03/2010 9 PhD Talk
LHCb data from 2009 LHCb Preliminary 2009 • √s = 900 GeV • VELO at 15mm • Very tight cuts to ensure pure K 0 S sample • 1000 candidate decays – K 0 S → ππ • (low statistics!) Alexander Shires - IC HEP First Year 02/03/2010 10 PhD Talk
k-Nearest-Neighbour method • Select decays with • Choose a candidate similar kinematics decay • Select 100 events • Choose events near from data with lowest each other in phase distance space • Select MC events within a 4-D • For each event, hypersphere compare data and MC neighbours Alexander Shires - IC HEP First Year 02/03/2010 11 PhD Talk
Comparing nearest neighbours • Compare distribution of decay Plot the confidence angle level for all • Kolmogorov-Smirnov test candidate decays • Example: the nearest Very preliminary! neighbours of a random event • Probability = 0.36 Monte Expect flat DATA Carlo distribution if the data agrees with Monte Carlo Alexander Shires - IC HEP First Year 02/03/2010 12 PhD Talk
Future work • Investigate the events with low Kolmogorov-Smirnov probabilities – as a function of kinematic variables • More statistics smaller region in phase space – Better comparison! • Evaluate the quality of Monte Carlo simulation • Expand the algorithm to other candidate decays – Such as Δ ,D mesons • Help discover new physics! Alexander Shires - IC HEP First Year 02/03/2010 13 PhD Talk
Backup Alexander Shires - IC HEP First Year 02/03/2010 14 PhD Talk
Decay angle Polar angle of the pions in the rest frame of the K 0 S Toy Monte Carlo: K 0 S π + ε at (2 GeV) = 0 θ ε at (10 GeV) = 0.95 π - Spin- 0 → Isotropic distribution If relative efficiency varies with pion momenta → anisotropic distribution. Black = total pions Alexander Shires - IC HEP First Year 02/03/2010 15 PhD Talk Red = seen pions
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