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BaBar Particle Identification and Measuring Direct CP Asymmetry in b→sγ Piti Ongmongkolkul 1.

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Presentation on theme: "BaBar Particle Identification and Measuring Direct CP Asymmetry in b→sγ Piti Ongmongkolkul 1."— Presentation transcript:

1 BaBar Particle Identification and Measuring Direct CP Asymmetry in b→sγ Piti Ongmongkolkul 1

2 Outline BaBar Particle Identification(PID) Is this track Kaon, Pion, Electron or Proton? BaBar Detector Decision Tree Error Correcting Output Code(ECOC) Results b→sγ Direct CP Asymmetry Measurement Motivation Current Progress Outlook 2

3 BaBar Experiment Located at SLAC PEPII Asymmetric e+e- machine 3 GeV and 9 GeV Main Goal is to measure CP violation in B meson Generic enough to do many more interesting physics 3

4 BaBar Experiment 2000-2008 ~470 million B Bbar pairs +Others Off resonance (udsc pairs) Υ(3S) Υ(2S) ~1$ per B Bbar pairs 4

5 SVT Silicon Vertex Tracker 5 Layer Silicon Strip Close to interaction region tracking dE/dx SVT 5

6 DCH Drift CHamber 40 layer wire chamber filled with He-based gas provides tracking dE/dx 6

7 DIRC Detection of Internally Reflected Cherenkov Quartz bars and Photo Multiplier Tube array the end Measure Cherenkov radiation angle θ c (function of β) Mainly for π-K separation DIRC PMT 7

8 EMC ElectroMagnetic Calorimeter 6.5k CsI(Tl) Photo diode at the end measure energy deposited in the crystals Electron -- bremsstrahlung + ionizing photon -- pair production pKπμ -- ionizing through E/p is a very good variable for identifying electron. Electrons leave ~all of its energy in EMC. EMC 8

9 IFR Instrumented Flux Return Many Resistive Plates Identifying K-Long and muon Just count how many plate it pass through IFR magnet 9

10 BaBar PID Identify track whether it’s kaon pion electron or proton Get B meson’s flavor needed for CP violation study sin(2β) golden mode Cleaning up combinatoric background Used in almost all analysis 10

11 BaBar PID - How? Each subsystem provides some information about which type of particle pass through. SVT DCH provides dE/dx, p, charge DIRC provides Cherenkov angle θ c EMC provides energy deposited in CsI and various quantity associated with energy cluster (eg. how wide spread it is ) + combinations of above Combine all the information Decision Tree ECOC (Error Correcting Output Code) 11

12 Decision Tree Binary question with many input -- given all these information, is this track an e or a π? Cuts scan for each variable. Pick a split that maximize the separation (ex gini- index) Repeat the process until the node considering has event less than some number 12

13 Combining Decision Trees Many trees are trained with slight difference. Use different set of input variables, different set of weight or different subset of sample. Average the result X 100 = Better Classifier 13

14 Added Bonus Intuitive to see importance of input variables Sum of change in “score” for each variable splitting deltaFOM Reduce number of input variables 14

15 ECOC* Decision Tree can answer binary question. “Is this track a p, π, K or e?” is a multiple choice question. Error Correcting Output Code -- combining binary classifiers to make a multiclass classifier. This is a more pedagogical example actual implementation is slightly different but the idea is the same *Dietterich, T., Bakiri, G. (1995). "Solving multiclass problem via error-correcting output code" 15

16 ECOC -- indicator matrix 1 vs All is an obvious choice Is it (e) or (p π K)--I (p) or (e π K)--II (K) or (e p π)--III (π) or (e p K)--IV Indicator Matrix Template for answers Ask all the question to an unknown track Pick the closest one IIIIIIIV e 1000 p 0100 K 0010 π 0001 (?) 1000 1100 16

17 ECOC -- Exhaustive Matrix All the binary questions one could ask In general there are 2 n-1 -1 Those 4 and (e π) or (p K)--V (e p) or (π K)--VI (e K) or (p π)--VII Recovery power IIIIIIIVVV1VII e 1000111 p 0100010 K 0010001 π 0001100 17

18 ECOC -- Recovery Power IIIIIIIVVV1VII e 10001110 p 01000104 K 00100014 π 00011004 (e) 1000111 win 4 outputs. 1 for each hypothesis Hamming Distance 18

19 ECOC -- Recovery Power IIIIIIIVVV1VII e 10001111 p 01000103 K 00100015 π 00011005 (e) 1100111 still win 19

20 ECOC -- Recovery Power IIIIIIIVVV1VII e 10001112 p 01000102 K 00100014 π 00011006 (e) 1100011 A draw Our implementation uses real numbers instead of 1 and 0 and use sum of square ~2 mistake to change the answer if we are unlucky 20

21 ECOC--recovery power In 1 VS All allow ~1 mistake Used in the the previous PID IIIIIIIV e 1000 p 0100 K 0010 π 0001 (e) 1100 21

22 IIIIIIIVVV1VII e 1000111 p 0100010 K 0010001 π 0001100 What I did Adding 3 columns IIIIIIIV e 1000 p 0100 K 0010 π 0001 22

23 e vs pion Color Legend 1 vs All Matrix Exhaustive Old likelihood based 23

24 Color Legend 1 vs All Matrix Exhaustive Old likelihood based 24

25 Conclusion I New method for PID system ECOC with exhaustive matrix Adding 3 columns makes a huge difference Currently the recommended one at BaBar 25

26 Measuring Direct CP Violation in Using Sum of Exclusive Modes 26

27 Introduction and has slightly different branching ratio SM* predicts *Tobias Hurth, et al. arXiv:hep-ph/0312260v2 25 Nov 2005 27

28 Feature Highly suppressed Flavor changing neutral current CKM suppressed* GIM suppressed* Require interference of Wilson coefficients New physics could lift CKM or GIM suppression or change in C’s up to 15% Kagan Neubert PHYSICAL REVIEW D, VOLUME 58, 094012 *Glashow, Iliopoulos, and MaianiGlashowlos, and M *Cabibbo Kobayashi Maskawa 28

29 More Small long distance contribution (~1%)* Depend very weakly on photon energy cutoff* low energy photon will bring large background Good probe for new physics. Happy middle ground for both theorist and experimentalist. Kagan Neubert PHYSICAL REVIEW D, VOLUME 58, 094012 29

30 Current State Acp Standard Model (note %) BaBar(2008) −0.011 ± 0.030 ± 0.014 Belle(2004) 0.002 ± 0.050 ± 0.030 Belle arXiv:hep-ex/0308038v4 22 Jul 2004 ~140e6 events (full data set 700e6) BaBar arXiv:0805.4796v3 [hep-ex] 7 Dec 2008 ~380e6 events (full data set 471e6) Our analysis will be done using full babar data set ~ 22% increased in data 30

31 Measurement Goal is to get the Acp Select Event as clean as possible Reconstruct B from 16 final states For charged B total charge tells us the flavor Kaon identification tell us neutral B flavor Get the yield for each flavor and done Blind Analysis 31

32 But.... For 1 event, there are many ways to reconstruct B All we have is a list of tracks with some PID associate with it. Remember 1 event has 2 B’s although we only use one. Need to match which tracks belong B of our interest. Even with mass, energy, vertexing among other things. There are still ~10-100 B candidates/event. Select the best candidate B doesn’t always decay to X s gamma but could be mis-reconstructed as B->X s γ branching fraction ~ 3x10^-4 Photons from pi0 e+e- collision doesn’t always go to B pairs light quark pairs Best Candidate Selection Peaking BBbar Fake high energy photon from pi0 continuum background 32

33 Best Candidate One event has many B candidates ee→Υ(4S)→BB We expect each B to have half of the beam energy in CM frame. If we pick the right set of tracks. Minimize Used in all previous analysis but.... Best Candidate Selection Fake high energy photon from pi0 continuum background Peaking BBbar 33

34 Better Best Candidate Selection Peaking BBbar Fake high energy photon from pi0 continuum background Binary question if looking candidate individually Is this B candidate correctly reconstructed? Classifier(Decision Tree) Separate correctly reconstruct B and mis- reconstructed one Exploit more information* XsMass Minimum pi0 momentum Thrust of B ΔE (normalized by resolution) Fox Wolfram Moment 0 and 5 Multiple candidates each with classifier score. Select the one with the best classifier score *selected from deltaFOM 34

35 More Information 35

36 Improvement Best Candidate Selection Peaking BBbar Fake high energy photon from pi0 continuum background Better 36

37 Peaking BBbar Bonus from SSC We trained the classifier to separate correctly reconstructed B and mis-reconstructed one BBbar background is mis-reconstructed by definition Cutting on the output gives us handle on peaking BBbar Background Best Candidate Selection Peaking BBbar Fake high energy photon from pi0 continuum background 37

38 Pi0 Veto Fake high energy photon from pi0 pi0 decays primarily to 2 photon Is this photon from pi0? Best Candidate Selection Peaking BBbar Fake high energy photon from pi0 continuum background 38

39 Pi0 Veto Pair up given high energy photon with all other photons in the event many pi0 candidates per photon Train a classifier separate true pi0 candidate and fake pi0 candidate* Invariant Mass Energy of the other photon Take the maximum output The higher the score the more likely it comes from pi0 Since it’s linked with continuum background we used this as a variable for another classifier Best Candidate Selection Peaking BBbar Fake high energy photon from pi0 continuum background *selected from deltaFOM 39

40 Continuum* light quark pairs udsc. Very jetty event. mass of Υ(4S) ~ 2*mass of B. Isotropic. Build classifier to separate continuum and BBbar Best Candidate Selection Peaking BBbar Fake high energy photon from pi0 continuum background *Done by Dr. David Doll Legendre Moments along photon axis and ratio cosine angle of B and beam axis in CM frame cosine angle thrust of B candidate and thrust of rest of event in CM frame cosine angle of photon and thrust of rest of event various momentum flow(momentum around B axis in various cone size) pi0 Classifier 40

41 Combining them Optimizing 41

42 SSC ΔE 42

43 Getting Acp Work in progress Simultaneous Fitting of should peak around mass of B 43

44 Resolution Generate 2000 set of sample based on pdf and refit to get Acp Compared to previous analysis of 0.030 ~ 1.5-2 times better* with only 20% more data The improvement comes from better candidate selection and handle on BBbar background. *There is some precision problem with the fitting program though but the residual should be correct 44

45 More to be done Finalize Fitting procedure Subtract off Inherent Detector CP Asymmetry (~1%) Our detector is made of matter Sideband/Offpeak Acp in peaking BBbar component if any Dilution from mis-PID (expected to be negligibly small) Fitting Systematic 45

46 Conclusion BaBar PID ECOC with exhaustive matrix Current recommended one Acp Event selection is finalized Need to Extract Acp Do systematic and etc. 46

47 Backup 47

48 Different level of tightness IIIIIIIVVV1VII e 10001110 p 01000104 K 00100014 π 00011004 Each analysis has different requirement. Some need sample to be really clean and can take a hit from efficiency. Some need efficiency and just need pid to clean up a bit. 4 output 1 for each hypothesis Picking the best one means e

a and e/K>b and e/π>c 48

49 Bethe-Bloc Mostly ionization for pi p K Ionization and bremsstrahlung (~β) for electron which lose almost all of its energy in CsI 49

50 Acp formula helper 50

51 Cutoff Dependence a27 weak dependence a87 none a28 yes but small compared to other two 51

52 Standard Model 52


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