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Oral Candidacy Presentation

Oral Candidacy Presentation. David Doll. Outline. Thesis topic: b →s γ Motivation Previous Analysis Babar overview Subdetector introduction and impact on thesis Previous work: Introduction to Random Forest method Applications to thesis topic and plans. b →s γ Motivation. γ.

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Oral Candidacy Presentation

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  1. Oral Candidacy Presentation David Doll

  2. Outline • Thesis topic: b→sγ • Motivation • Previous Analysis • Babar overview • Subdetector introduction and impact on thesis • Previous work: • Introduction to Random Forest method • Applications to thesis topic and plans

  3. b→sγ Motivation γ • Flavor changing neutral current decay (absent at tree level in SM) • Precision test of SM • Different models may enhance or suppress the BF Source: U. Haisch, FPCP Conference Taipei, 2008

  4. b→sγ Motivation • For photon energy cut Eγ> 1.6 GeV in B meson rest frame • NNLO theoretical calculation • HFAG experimental results (as of March 15, 2007) Source U. Haisch FPCP Violation 2008 Error associated with subtraction of events Combined statistical and systematic error Systematic uncertainty associate with Ecut = [1.8,2.0]

  5. b→sγ Motivation • Photon energy spectrum • In b quark rest frame (impossible to boost to), this would be a delta function (≈mb/2) • b quark motion within meson smears this spectrum • Spectral shape dependent on modeling of spectator quark • In the framework of HQET, the parameters λ1 and (or equivalently mb) may be determined from the first two moments of the spectrum • Beyond SM theories are not predicted to influence this much

  6. Different Photon Models • Shape function of b quark motion is universal • Applicable to all decays involving tranistions to massless states (B→Xsγ, B→ Xdγ, etc.) • Different shape function models exist Source Eur. Phys. Jour. C7, 5-27 (1999) Gaussian Ansatz A.L. Kagan and M. Neubert propose an exponential shape function (KN model): where:

  7. Dependence on Ecut • D. Benson, I.I. Bigi, and N. Uraltsev also investigate an exponential and a Gaussian ansatz (with minimal difference between the two) • Use purely perturbative spectrum calculated by Z. Ligeti, M. Luke, A.V. Manohar, and M. Wise as a starting point • Add nonperturbative pieces to the energy moments • Finally, they investigate the effects of the minimum photon energy cut to evaluate a bias in mb and μπ2 Without perturbative corrections Complete bias difference Source D. Benson, I.I. Bigi and N. Uraltsev FPCP Violation 2004

  8. Other Photon Models • B. Lange, M. Neubert, and G. Paz also present shape functions based on exponential, gaussian, and hyperbolic functions (hep-ph/0504071) • They detail how to fit the parameters: First and second moments are directly relatable to and μπ2 and: Is a good model for Exponential, gaussian, or hyperbolic

  9. Search Strategy for b→sγ • Performing a sum of exclusive states • 38 states total • Update of former analysis published in Phys. Rev. D (2005) 052004 • Based on 89.1 fb-1 of data collected at the Y(4S) Source Babar doc

  10. On CP Asymmetry Measurement • Because of the final state reconstruction, a direct CP asymmetry measurement is possible with this strategy in modes of definite flavor (yellow) • Recently investigated with ~80% of the total data • Possible source of other new physics • Search to be performed in another analysis Source Babar doc

  11. Former Analysis Procedure • Reconstruct event candidates into the 38 different decay modes • Use a Neural Network to reject continuum events based on event shape variables • Set the lower cutoff energy at Eγ> 1.9 GeV (or equivalently at MXs between 0.6-2.8 GeV/c2) to limit peaking B background • Choose the ‘best’ candidate as the one that minimizes ΔE Source Babar doc

  12. Former Analysis Procedure • Subtract off the continuum, generic BB, and cross-feed backgrounds by fitting the beam substituted mass, mES and fit to signal on bin-by-bin basis in MXs • The peaking background contribution (cross-feed and generic BB) is fit with a Novosibirsk function: • The continuum contribution is fit with an ARGUS function • As a default signal model, they use the KN exponential model with mb= 4.65 GeV/c2 and λ1=-0.30 GeV2/c4 • Kagan and Neubert recommend treating only the range between MXs={1.1 GeV/c2, 2.8 GeV/c2} as a non-resonant spectrum • Below MXs= 1.1 GeV/c2, they recommend using K*γ MC below 1.1 GeV/c2

  13. Former Analysis Procedure • Bin-by-bin fit to signal gives Partial Branching Fractions, PBF(MXs) • Need to correct PBF(MXs) for fractional coverage of inclusive b→sγ decays to get Total Branching Fractions, TBF(MXs) • Convert TBF(MXs) to TBF(Eγ) • Fit TBF(Eγ) to different expected models, allowing extraction of inclusive Branching Fraction measurement to lower Eγ • Also extract shape function paramters, mb and λ1 from model fit

  14. Impact of competing models • Ideally, photon spectrum measurement is model independent • Analysis strategy forbids this • Need idea of total decay coverage in each MXs bin • Not able to extrapolate to a total BF without introducing some model dependencies • Use a sample with a flat Eγ distribution to reweight to any model chosen • Measure parameters in all models considered (everyone’s happy)

  15. Results of Former Analysis • Quote results of KN fit, ‘kinetic’ model (BBU from above), and ‘shape function’ models (average of 3 BNP shapes from above) Source Babar doc

  16. PEP-II at SLAC Source Babar Doc • e- (at 9.0 GeV) on e+ (at 3.1 GeV) • CM energy = 10.58, the mass of the Υ (3S) • Lorentz boost of βγ = 0.56 • B meson lifetime 1.5-1.6 ps → Δz ≈ 250-270 μm • Turned off in April with a total of ~485 fb-1 at or just below the Υ(4S).

  17. Babar Detector Solenoid Magnet (1.5 T) EMC e + DIRC SVT e - DCH IFR

  18. Subsystems Overview

  19. SVT and DCH SVT DCH • SVT • 5 layers of double-sided silicon strip sensors • φ measuring strips parallel to the beam, z measuring strips perpendicular to the beam • 20-40 μm resolution in all 5 layers. • DCH • 7,104 small drift cells arranged in 40 cylindrical layers • dE/dx measured by total charge deposited in each cell Source Babar doc

  20. DIRC • Particle ID for particles with momentum above 750 MeV/c • 144 fused, synthetica silica bars arranged in a 12-sided polygon • Readout by 11,000 PMTs Source Babar doc

  21. IFR Muon efficiency Pionmis-id rate • Segmented steel flux return (later also brass), instrumented in gaps • Originally used resistive plate chambers (RPC) to detect streamers from ionizing particles • Upgraded to limited streamer tubes (LST) starting in 2004 RPC data LST data Source Babar doc.

  22. EMC • Designed to operate over the energy range of 20MeV to 9GeV • 6,580 CsI(Tl) crystals separated into 5,760 in the barrel, and 820 in the endcap • 16.1 X0 in the backward half of the barrel, to 17.6 X0 in endcap • Each crystal read out by two 1cm2 Si photodiodes • Calibration at low energy using a 6.13MeV photon source and at high energies using Bhabha events • Studies of the low energy calibrations have shown light yield falloff to total around 8% or less after the run of the experiment (depending on crystal manufacturer). Angular resolution vs photon energy Source Babar doc.

  23. B+→K+νν (or the benefits of a multivariate classifier) • Performed search with D. Hitlin, I. Narsky, and B. Bhuyan • Also a FCNC, and therefore highly suppressed in the SM arXiv:0708.4089v2 [hep-ex]

  24. Analysis Procedure, Tagging • Perform a ‘semileptonic’ tagged analysis • Fully reconstruct the ‘tag B’ in the decay • Look at the rest of the event for our signal Tag B Signal B

  25. Analysis Procedure, Cuts • Separately pursued two different techniques to suppress background • Standard Rectangular Cut method • More sophisticated Multivariate technique with a Random Forest • For Rectangular Cuts, separated the Monte Carlo (MC) into 3 sets: train, valid, test; in a 2:1:1 ratio • Optimized the ‘Punzi’ Figure of Merit: Where S is the number of signal, Nσ is the sigma level of discovery, and B is the number of background

  26. Rectangular Cut Results

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