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My Physics Research Activities in LHCb experiment

My Physics Research Activities in LHCb experiment. Yuehong Xie 16-Jan-04. Some words before talking about physics. I Was @CERN in LHCb for two years: know how the experiment is going on and how people collaborate

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My Physics Research Activities in LHCb experiment

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  1. My Physics Research Activities in LHCb experiment Yuehong Xie 16-Jan-04

  2. Some words before talking about physics • I Was @CERN in LHCb for two years: know how the experiment is going on and how people collaborate • Wrote XML geometry database for inner tracker: learned something about the (inner ) tracker detectors • Developed the offline VeloTT tracking program: got the basic knowledge how tracks are reconstructed within Brunel framework • Developed the DaVinci software package for Ks reconstruction and got to know some technical details how to get information for analyses with DaVinci framework: MC association, PID information, vertexing, various tools, especially Ks stuff • Need TECHNICAL help? Don’t hesitate to talk to me.

  3. Work on Physics Analysis • Ongoing: Bf Ks for sin(2b) • To understand current performance numbers in this channel and identify the difficulties • To improve yield, B/S • To improve robustness against worse background situation • Prospective: BDcpKs for g • New clean strategy to determine g

  4. - q p 2Iml 1+ |l|2 |l|2 -1 |l|2 +1 A A S = A = l = xf SM: bs Penguin phase = (cc) K0 - CP Violation in b sqq ACP = S sin(DmDt) + A cos (DmDt) C(Babar)=-A(Belle) Mixing induced CPV Direct CPV f + + New Physics with New Phase Sbs¹ Sbc , A can¹ 0 Sbs=sin2f1, A=0 “b  ccs: sin2f1” (SM reference)deviation

  5. Bf Ks: New physics or not? b→s penguin LP2003: Belle B0fKs “sin2f1” = -0.96  0.50 3.5 σ from SM +0.09 -0.11 LP2003: Babar B0fKs “sin2b” = +0.45  0.43  0.07 2.1 σ discrepancy WA Average B0fKs “sin2b” = -0.14  0.33 2.6 σ from SM b→ccs WA NEW 2003 WA: b  ccs “sin2b” = +0.736  0.049

  6. “sin2f1” 2003 Status “SM” Av = -0.14±0.33 (2.6s away)

  7. Precision requirement Acpmix (B0f Ks) - Acpmix (B0J/y Ks) = o(1) + o(l) + o(l2) NPI=1 NPI=0 SM (R. Fleischer, hep-ph/0103121 ) An experimental precision of O(l2) for measurement of mixing induced CP asymmetry is needed to distinguish between SM and new physics contribution in B0f Ks ! We need to understand how well LHCb will be able to do the work.

  8. Direct CP violation in Bφ K Acpdir(B±f K±) =+0.04  0.09  0.01 (hep-ex/0309025) Compared with Acpdir(B0f Ks) = +0.38  0.37  0.12 Model independent parameterization (R. Fleischer, hep-ph/0103121 ) [Acpdir(B±f K±) + Acpdir(B0f Ks)]/2 = o(1) + o(l2) [Acpdir(B±f K±) - Acpdir(B0f Ks)]/2= o(l) + o(l2) NPI=0 SM NPI=1 SM Higher precision of Acpdir measurement is needed in B0f Ks to distinguish between new physics from SM contribution

  9. Personal feeling • Statistics for b →s penguin modes are very limited • Purity for b →s penguin is ~ 60%; it is on average >90% for ccs channels. • There is indeed a tendency to have a smaller effective sin(2b) in B decays with dominating or big penguin contribution. • So far it is still an experimental problem! Discrepancy between Babar, Belle and SM can still be explained as fluctuation. • Need a huge increase of statistics and more careful study of background, to establish a discovery of new physics or converge to SM!

  10. LHCb status of BfKs • Several groups work on the channel (Russian, French) • Recent report by Olivier Leroy from Marseille on LHCb week November 2003 • “Status and future of BdKS” • LHCb can select ~800 untagged BdKS /yearMost dangerous backgroundareb X events • Studying 10M bb and 1M b X gives: • Btot/S < 1.3 • sin2 [0.27; 0.41](extrapolated from BdJ/KS) • can be compared with BaBar+Belle 2007: sin(2) = 0.165 • Improvements are possible, especially in trigger and tagging (also useful for other channels !) • Example: • Increase tagging power from 3.9% (today’s TDR) to 6.4% (TP’s value) is equivalent to reduce sin2 from 0.27 to 0.165(i.e BaBar+ Belle 2007 value) ! • This means, for example, tag = 40% and  = 30% (are 40.9% and 34.6% in TDR)

  11. Experimental limits • Low trigger efficiency because of Ks • Low effective tagging efficiency: a general problem with LHCb • Can we do something on trigger and tagging or leave it to experts? • Hard to estimate background level due to limited simulated bb events • 2.6GeV window used for bb events • B/S result is biased due to over-tuning of cuts to get rid of bb background events • Statistics will be increased in next data challenge • Need unbiased way to estimate B/S

  12. Our aims • Get unbiased yield and B/S estimate • More statistics • Unbiased method, no over-tuning • Mainly long term: prepare selection codes and software tools for final analysis with real data • Event selection tools: Ks, f, vertexing, lifetime… • Analysis tools: ntuple, CP fitting … • Systematics • Provide robustness against worse background level • Understand background sources • Get rid of them as much as possible

  13. Reconstructability & reconstruction performance Reported on LHCb week September 2003 A Ks package for general use New bb stripping done with UoE pre-selection code Private code to fill nutple with necessary information Some background investigation Ks reconstruction improvement Upgrade downstream tracking Remove prompt Ks Solve technical problems, like Ks vertex fitting with mass constraint to improve B mass resolution f selection improvement (reported in LHCb light meeting Dec 16,2003) Include downstream tracks Isolate f What I have done/am doing for Bf Ks done ongoing A complete result to be worked out

  14. Difficult-to-remove background • True Ks from primary vertices • In DD case, IP is not helpful • True Ks from K* decays • True f from PV (13%) • True f from decays of • D0→fp+p-,fp0, fr0 (20%) • Ds+ → fp+,fp+p+p- , fp+p0 , fK+( 58%) • D +→fp+,fp+p0 (7%)

  15. Removing f from D decays • All f from Ds+ and D + and 20% of f from D0 decays have partner charged tracks which originate from the decay vertex of f • No such track in B→f Ks • Strategy: if find a partner track then drop the f • Challenge: too many tracks in high multiplicity events which may lead to loss of B→f Ks

  16. An effort to Isolate f • Find partner long tracks • IP significance >3 wrt all PV • IP significance <1.5 wrt f vertex • Polar angle > 65 mrad • Angle between track direction and the line connecting PV and f vertex <80 mrad • Similar way to find velo and VTT partner tracks • If a partner track is found, throw away the f

  17. Rejection power • If use IP significance <2 wrt f vertex, loss increases to 15%, and rejection factor for Ds is about 60% • Rejection factor for Ds can be close to 100% with sizable loss of signal. • This is preliminary result. Improvement is possible • Ks tracks were not excluded in the search of partners Signal 7.6% Ds± 39% D0 30% D±32%

  18. Removing promt Ks • For LL and LU Ks, IP significance can be used to get rid of Ks from PV • In DD Ks case, IP error is on the order of 1mm, much bigger than the true IP value. Then IP is useless • Pt is not so powerful as pt/p, the polar angle • For DD case, pt/p removes half of bb true Ks with a signal loss of 18%

  19. Pt vs pt/p

  20. Using upstream tracks for f→KK • 20% of LU f normalized to LL f • Much worse mass resolution • Worse B mass resolution • A trick to play: constrain f mass to nominal massbecause f massresolution >> f width, comparable B mass can be achieved (to be proved)

  21. 0.487GeV<ksmass<0.51GeV kschi2<20 ksipsig>3. ksvz>0 0.99GeV<phimass<1.05GeV k1DLLk>0.and.k2DLLk>0 phichi2<7 phipt/phip>0.05 phipt>1.0GeV phiipsig>3 0.<bchi2<10 bipsig<3 B open Angle < 20mrad 0<Bdecaytimefit<15ps bDecayTimeFitchi2<15 No f partner tracks 202 signals before trigger 0 events in 7M bb and 1M phiX events Trigger efficiency is 17% on selected events LU f LL Ks

  22. 0.475GeV<ksmass<0.520GeV kschi2<15 kspt/ksp>0.05 ksvz>200mm 0.99GeV<phimass<1.05GeV k1DLLk>0.and.k2DLLk>0 phichi2<7 phipt>1.0GeV phipt/phip>0.05 phiipsig>3 0.<bchi2<10 bipsig<3 B open Angle < 60mrad 0.5ps<Bdecaytimefit<15ps bDecayTimeFitchi2<5 No f partner tracks 422 signals before trigger 0 events in 7M bb and 1M phiX events Trigger efficiency is 10% on selected events LU f DD Ks

  23. LU f vs LL f • From 311K signal events, 624 LU f untagged signal events selected before trigger, compared with 3596 LL f signals selected in Olivier’s talk: 13% increase of annual yield taking into account trigger. But • Only 70% bb sample used: the other events are not available due to crashed stripping jobs • New cuts not used for LL f, tuning needed • And some bugs in common software fixed

  24. Plan on Bf Ks • Continue current basic work stated on previous pages • Request for more signal events in next data challenge • Work out a complete (unbiased) result with new data • Think if we can contribute to the improvement of trigger (concerning channels with Ks) and B flavor tagging? • At least we should understand the trigger and tagging performance numbers and express our hope! • Understand systematical errors • Develop necessary tools for CP fitting • Do we still need a sensitivity study in post-TDR days

  25. Determine g in BDcpKs • R.Fleishcer hep-ph/0301255 • Efficient, theoretically clean determination of angle r using BD Ks where D denotes CP even and odd eigenstates of neutral D meson-system. • B and anti-B may both decay into D0Ks, leading to interference effects between B-B mixing and decay process, which involve the weak phase fd+g c u b b D0 D0 B u B c s s Ks Ks d d d d Vub•Vcs Vcb•Vus

  26. Untagged observables • Measure asymmetry • Which should satisfy and • New bound on g

  27. Tagged observables • Measure the usual time dependant rate asymmetry and obtain the direct and mixing-induced asymmetries: • Define • Determine g using

  28. Tagged observables (cont.) • Probe strong phase ds • Single solution for ds using plausible assumption • The strong phase ds offers valuable insights into hadronic physics • Special treatment needed if

  29. Similar channels: Bs D  h ,h`, f • Either fix fs with other channels and obtain g • Or fix sign or tang with BD Ks and obtain sign of cos fs using the right side of the equation

  30. Feasibility • Only untagged asymmetry and mixing-induced CP violation observables have to be measured • Decay of neutral D into CP eigenstates is experimentally challenging • Belle: • Untagged asymmetry can be measured to give bound on g • With about only 1thousand untagged events/year for D+Ks and a few thousand untagged events/year for D-Ks

  31. Feasibility (cont.) • Measurement of time dependant asymmetry very difficult due to • Small annual yields • Bad lifetime resolution when there is no charged track from B vertex • Bs D f is not clear: no estimate of branching ratio • Concluding the channel BDcpKs • It is possible to measure untagged asymmetry to give new bound on g, which will help to resolve ambiguities in other methods to determine g in decays with only tree contribution (BsDsK). We will do it! • The feasibility to use tagged events to determine g needs to be investigated.

  32. Questions regarding computing resources • How to make use of the local resources? • Can we access common LHCb data using Scott Grid? • Can we run jobs locally? How ? • Can we copy (stripped) data to local disk? • Can we do some production locally? • Can we save huge ntuples locally?

  33. Summary • Work on Bf Ks is in progress • Feasibility study on BDcpKs is to be started • Need to set up a local LHCb software environment to do physics analysis work • Possible more • Other channels • Trigger and tagging if new ideas come

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