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B physics in LHCb

B physics in LHCb. Introduction: LHCb physics The experimental challenge: Vertex reconstruction Particle ID Trigger Flavour tagging Systematic effects Hugo Ruiz – Winter meeting 2007 - Santiago. Introduction. CPV in the SM. SM introduces CP violation through:

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B physics in LHCb

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  1. B physics in LHCb Introduction: LHCb physics The experimental challenge: Vertex reconstruction Particle ID Trigger Flavour tagging Systematic effects Hugo Ruiz – Winter meeting 2007 - Santiago

  2. Introduction

  3. CPV in the SM • SM introduces CP violation through: • the CKM matrix is complex • phases switch sign under CP: VijVij* • After requiring unitarity and removing non-physical phases, the values of Vij are no longer independent • 3 magnitudes + 1 phase fix the matrix: 4 parameters • All measurements related with electroweak quark transitions are coherent with the CKM picture of the SM: • BR, Dm and phases measured fit within a set of values of the 4 CKM parameters • In particular, unitarity triangle closes gently l = 0.2240±0.0036 A = 0.83±0.02 r = 0.168±0.029 h = 0.340±0.017 Wolfenstein parameterization, values from UTFit Hugo Ruiz – Winter Meeting Santiago 2007

  4. New Physics in B decays? • But there is room for NP in B meson decays • And after all, the CKM picture of CPV does not account for the presence of matter in the Universe… • Some quantities very sensitive to NP are yet to be measured or lacking precise measurement • Four examples accessible to LHCb in this talk: • c  arg(Vts)-pvia phase of Bs mixing • CKM fit prediction much more precise than experimental value • g -arg(Vub) at tree level • Tree-level measurements assumed free of NP • Comparison with measurements from loop processes can reveal NP • Branching ratios of rare decays • Expect large contributions from NP models which fit rest of data • Angular distributions • Sensitive to non-SM operators in interactions Hugo Ruiz – Winter Meeting Santiago 2007

  5. 1. Phase of Bs oscillation Vts • Prediction from a global fit to CKM measurements (UT fit): fs= -0.037± 0.002 • Very small, so very sensitive to NP! • Recent D0 measurement () : fs= -0.79±0.56(stat)+0.14-0.01(syst) • Note: no Bs produced in B factories • D0 used golden channel Bs→J/y(m+m-)f(K+K-) • The diagram for Bs oscillation in the SM is • The phase of the oscillation in the SM is given by: • fsSM -2  arg (Vts) -2c = -2l2h up to o(l6) Vts* Hugo Ruiz – Winter Meeting Santiago 2007

  6. 1. Phase of Bs oscillation Bs0 • Bs→J/y(m+m-)f(K+K-) can proceed directly or through mixing Bs0 Bs0 • Measure proper time distribution of events originally tagged as Bs and Bs • Needs flavour tagging! • Build time-dependent CP asymmetry: Tagged Bs Tagged Bs All experimental effects simulated hf = +, - 1 CP eigenstates Strong requirement on vertexing Proper time (ps) Hugo Ruiz – Winter Meeting Santiago 2007

  7. 1. Phase of Bs oscillation From Z. Ligeti et al hep-ph/0604112 Allowed regions CL > 0.90, 0.32, 0.05 • BR=3·10-5 in one nominal LHCb year (2 fb-1): • 33k of events • s(fs)= 0.023 ( UT fit value: -0.037) • Parameterization of NP effects: M12 = (1 + hse 2iss) MSM12 • MSM12= dispersive part of the BS mixing amplitude in the SM • Then Dms and fs can be used to constrain NP in the oscillation: 180o 2006, After first Dms measurement 90o ss 0o 0.5 1.5 2.5 hs 180o fs= 0.04±0.03 90o ss LHCb, L=2fb-1 0o 0.1 0.3 0.5 hs 7 Hugo Ruiz – Winter Meeting Santiago 2007

  8. 2. g from tree processes • garg of (Vub)to o(l4) • Measurement (tree-level only): g = (83 ± 19)o • From global CKM fit (incl. loop processes!): (64.1 ± 4.6)o • Most promising method for tree-level determination: measure BR of B- (K+p-)DK- and the charge-conjugated process • 2 diagrams (via D0 and via D0) contributing with similar amplitudes  large interference effects hence large CPV in the decay • No flavour tagging needed   g 8 Hugo Ruiz – Winter Meeting Santiago 2007

  9. 2. g from tree processes • But several unknowns: relative amplitudes and phases of B, D decays • Need more observables from additional modes to extract g: • CP eigenstates K+K- and p+p-: GLW (Gronau, London, Wiler) • Other decays: ADS (Atwood, Dunietz, Soni) • Current status: B- (K+p-)DK- not yet seen in B factories • Measurement from Dalitz analysis of D0 Ksπ+π– • With LHCb 10fb-1, expected 3.5 K events : • B+: 2500 with B/S ~1.5, B-: 1000 with B/S ~ 4.5 * * Values of rel. amplitudes from Dalitz in B factories Hugo Ruiz – Winter Meeting Santiago 2007

  10. 2. g from tree processes • Most powerful methods for g measurement in LHCb: s(g) = 2.4o Free from NP Strong requirement on PId (*): with a weak assumption on U-spin symmetry, could be affected by NP

  11. 2. g from tree processes  • Status of g from tree processes now and in ~ 2013: Current from tree processes only g From BDK, Tree process, LHCb10 fb-1  g g From UT fit, quantities affected by loops |Vub/Vcb| from semileptonic BRs NP! 11 11 11 31/05/2007 31/05/2007 This is what we know about CKM if we suspect NP in loop processes! Hugo Ruiz – Winter Meeting Santiago 2007

  12. 3. BR of Bsm+m- 5 BR (x10-9) SM prediction 3 Integrated Luminosity (fb-1) SM • Occurs via loops: • Small BR in SM: (3.4 ± 0.4) x 10-9 • Sensitive to NP! • Strongly enhanced by some SUSY models • Ex: up to x100 by CMSSM with parameters ‘preferred’ by anomalous m mag. moment in BNL. • Limit from Tevatron at 90% CL: • Current (1 fb-1)< 7·10-8 • Expected final (8 fb-1): < 2·10-8 • ~ x10 higher than SM! MSSM LHCb Sensitivity (signal+bkg is observed) LHCb: with L=2fb-1, 3s observation if SM value Yesterday’s talk by Diego Martinez

  13. 3. BR of Bsm+m- gμ- 2 Bs → μ+μ- SM prediction • An example of complementarity of high-pT & flavour physics: • tan b: ratio of Higgs vacuum expectation values • m½ : gaugino mass • A0: trilinear soft SUSY-breaking parameter • CMSSM:soft SUSY-breaking scalar and gaugino masses are each equal at GUT input scale. Only 4 independent parameters in MSSM, see hep-ph/0411216 Hugo Ruiz – Winter Meeting Santiago 2007

  14. 4. B0 K*0m+m- AFB(s), theory + B0 q K* – s = (m)2 [GeV2] b s • Forward-backward asymm AFB(s) in  rest-frame sensitive to chiral structure of the process • SM: BR = (1.22+0.38-0.32) 10-6 • 2 fb-1: 7200±2100 evts, B/S < 0.5: d d Bd K* m FBA g m 2 fb-1 s = (m)2 [GeV2] • Other variables (eg. difference  -polarizations) promising (hep-ph/0612166) Hugo Ruiz – Winter Meeting Santiago 2007

  15. LHCb • The LHCb collaboration: • 619 Scientists • 46 Institutes • 14 Countries • Spain: • Universidade de Santiago de Compostela • Universitat de Barcelona - Universitat Ramon Llull Hugo Ruiz – Winter Meeting Santiago 2007

  16. Detector overview Muon System RICHES: PID: K, separation VELO: primary vertex impact parameter displaced vertex 0.4 rad PileUp System x mrad Interaction region Calorimeters: PID: e,, 0 Trigger Tracker: p for trigger and Ksreco Tracking Stations: p of charged particles Hugo Ruiz – Winter Meeting Santiago 2007

  17. A single-arm spectrometer? • (B), rad • (B), rad Direction of bb pairs: Pythia bb production cross-section: • Within LHCb acceptance: ~ 1012 b hadrons per year • ~104 more than in B factories pT (GeV) h b and b very close in direction, important for flavour tagging! Hugo Ruiz – Winter Meeting Santiago 2007

  18. Experimental issues: A typical bb event: • Vertex reconstruction • Particle identification: distinguish Bdpp, Bs KK • Trigger, especially for hadronic final states • Mass resolution: distinguish Bs & Bd, reduce comb. bckgr. • Flavour tagging Dh~0.7 1 fm <L> ~ 8 mm Primary Vertex (PV): pp interaction Dh~1.4 B meson 1 IP B meson 2 Hugo Ruiz – Winter Meeting Santiago 2007

  19. Vertex reconstruction

  20. Luminosity • Luminous region(within 1 sigma): • With nominal LHC lumi(1034 cm-2s-1): 23 interactions per bunch crossing  23 PVs • Difficult to find secondary vertices! • LHCb needs a lower luminosity: • Chosen to maximize the probability of a single interaction: 2 – 5 · 1032cm-2s-1 • 50 times lower than LHC design lumi • LHCb will probably reach its ‘design luminosity’ before ATLAS and CMS ~ 1 mm 5 cm Num. of pp collisions LHCb ~ Maximum for detector radiation Hugo Ruiz – Winter Meeting Santiago 2007

  21. VErtexLOcator: VELO 21 stations 1 m Silicon sensors Interaction region R sensors R sensor: pitch: 38 μm - 103μm thickness: 300μm φ sensor: pitch: 39 μm - 98μm thickness: 300 μm f sensors 8mm Hugo Ruiz – Winter Meeting Santiago 2007

  22. VELO VELO sensors • The closest to the beam, the less extrapolation distances, and the better IP and vertex resolution • But if too close: unstable beams at fill-up could damage detector • Solution: retractable detector • At 3 cm at beginning of fill • Moved to 8 mm when stable beams declared • Even though lower lumi, higher dose than ATLAS and CMS pixel detectors • Will have to replace in a few years RF Foil Hugo Ruiz – Winter Meeting Santiago 2007

  23. VELO The 2 VELO modules are assembled: View of the foil separating the secondary vacuum of VELO sensors: Hugo Ruiz – Winter Meeting Santiago 2007

  24. IP resolution • IP resolution is  1/pT, because of: • Multiple scattering produces dI 1/p for a given amount of material • Material traversed  1/cosq PV dIP≃ 14mm ± 35 mm/pT Signal B IP 1/pt distribution for B tracks Hugo Ruiz – Winter Meeting Santiago 2007

  25. Secondary vertex resolution Relevant resolution PV resol (~X VELO tracks): σz=47 μm, σx=8 μm z=168 m Bs→DsK B lifetime: z coordinate: • Typical s: 37 fs(2p·Dms-1~350 fs) • ATLAS: 83 fs, CMS: 77 fs • CDF ~ 87 fsfor fully reconstructed decays PRL 242003 (2006) Bs→J/yf Hugo Ruiz – Winter Meeting Santiago 2007

  26. Particle identification

  27. K-p separation • B physics require separation between final states with p and K • Best example: extraction of g from ACP in BsK+K- and Bd p+p-. What happens if we are p-K blind? • If all tracks considered to be pions: m(B0) = 5279 MeV m(Bs) = 5367 MeV

  28. RICH detectors • Ring Imaging Cherenkov detectors measure angle of Cherenkov emission, a function of velocity of particles • Different radiating materials separate p-K in two different ranges of momentum: Hugo Ruiz – Winter Meeting Santiago 2007

  29. RICHes 80mm 120mm RICH 1 Hybrid Photo Diodes Typical event (RICH2): HPD arrays out of acceptance. Each containing a 1024 Si pixel Granularity: 2.5 x 2.5 mm2 Hugo Ruiz – Winter Meeting Santiago 2007

  30. Performance of RICHes Kaon identification: • Effect on Bdp+p-: Hugo Ruiz – Winter Meeting Santiago 2007

  31. And some RICH pictures… RICH2 HPD Column: RICH2 in the pit: Hugo Ruiz – Winter Meeting Santiago 2007

  32. MASS RESOLUTION

  33. Momentum measurement • Momentum is measured from curvature of tracks between VELO and main tracking stations • The LHCb (hot) magnet: • ∫B dL = 4 Tm • Field reversal to reduce syst. effects on CP asymmetries

  34. Tracking stations Outer Tracker : 450 cm 595 cm • Occupancy: Inner tracker Silicon detector, 198m pitch Outer tracker 4 layers of straws (0o,-5o,5o,0o) each Track 5mm straws e- e- e- 3 stations (T1 –T3) pitch 5.25 mm e- e- Hugo Ruiz – Winter Meeting Santiago 2007

  35. Tracking performance • Typical bb event: • 20-50 hits assigned to each long track • 98.7% correctly • For tracks with p>12GeV: • Efficiency >95% • Ghost rate <7% • Robust: if track multiplicity x2… • efficiency 91% • ghost rate 14% Note 1-D missing! Hugo Ruiz – Winter Meeting Santiago 2007

  36. Mass resolution Momentum resolution: • Mass resolution: Bs m+m- sm=18 MeV dp/p ≃0.35%–0.55% p distribution for B tracks • CMS: 36 MeV (ms have large p) • ATLAS: 77 MeV(lower ∫BdL) • N combinatorial background  sm! Resolution dominated by multiple scattering (over detector resolution) up to 80 GeV Hugo Ruiz – Winter Meeting Santiago 2007

  37. Trigger

  38. The LHC environment Particles reconstructed • Relevant rates: • LHC: 40 MHz, 2 bunches full: 30 MHz • At least 2 tracks in acceptance 10 MHz • bb:100 KHz • Decay of one B in acceptance:15 KHz • relevant decays BR ~10-4 – 10-9 • cc: 600 KHz Hugo Ruiz – Winter Meeting Santiago 2007

  39. Trigger overview 10 MHz Calo+ Muon system L0: hightpT+ not too busy • On custom boards • Fully synchr. (40 MHz), 4ms latency Pileup system 1 MHz High Level Trigger (HLT) In PC farm with ~1800 CPUs Refine pT measurement + IP cuts Reconstruct in(ex)clusive decays Whole detector Full detector available (full flexibility) (but no time to process everything for every event). Average latency: 2 ms (ATLAS, CMS: ~ 100 Hz, 1Mb/evt) ~2KHz, ~35Kb/evt Hugo Ruiz – Winter Meeting Santiago 2007

  40. L0 ET triggers • Fast search for ‘high’ pT particles • Calorimeter: look for high ET candidates in three categories: e±, g and p0 • In regions of 2x2 cells • Particle identification from • ECAL / HCAL energy • PS and SPD information • Muons: • Straight line search in M2-M5 • Look for compatible hits in M1 • Momentum measurement 20% Scintillator Pad Detector (SPD) ECAL HCAL Pre-Shower Detector Hugo Ruiz – Winter Meeting Santiago 2007

  41. L0: cuts on global variables Interaction region • Require minimum total ET in HCAL • Reduces background from halo-muons • Rejection of multi-PV and busy events: • They tend to fake B signatures (IP, high combinatorics) • Busy events spend trigger resources without being more signal-like • Better throw them early and use bandwidth to relax other cuts • Two ways: • SPD multiplicity • Pileup system: 2 dedicated VELO layers upstream of interaction region that allow a fast search of PVs Hugo Ruiz – Winter Meeting Santiago 2007

  42. L0 performance • Bandwidth share: • Effinciency (off-line selected evts): e ~ 50 % The only B trigger in ATLAS & CMS, but with pT cut ~ 6 GeV L0 is the bottle-neck of hadronic channels L0 increases B purity 1%  3% Hugo Ruiz – Winter Meeting Santiago 2007

  43. Trigger: HLT • Disk Hadr. alley ECAL alley Muon alley … Exclusive selections HLT • Robust scheme: • Understand and calibrate trigger • Reduction of systematics, ex: flavour tagging Hugo Ruiz – Winter Meeting Santiago 2007

  44. Example: di-hadron alley • L0 hadron: 700 KHz • Reconstruct Velo2D, IP cut (~75mm): 350kHz (~3 candidates) • Reco Velo3D, match to L0 object, IP cut (~75mm): 250 kHz (~2 cands.) • Match to T stations, pT>2GeV: 40 kHz (~1.2 cands.) • RecoVelo3D tracks with IP forming good vertex with 1st candidate • Match them to T stations and cut at pT>1 GeV:8-5 kHz (~1 cand. vertex)

  45. Bandwidth share Calibration and systematic studies Hugo Ruiz – Winter Meeting Santiago 2007

  46. Trigger performance • Overall efficiencies (on offline reconstructed evts): • Hadronic (egBh+h- ): 25 – 35 % • Radiative: 30 – 40% • With dimuons (egBs→J/y(m+m-)f(K+K-)): 60 – 70 % Unbiased B • The unbiased B sample: • 900 Hz of inclusive B  mX, 550 Hz true • ~ 1.5109 fully contained, m-tagged and decay-unbiased B mesons / 2fb-1 • Tagging enhanced: eeff ~ 0.15 • This trigger only: factor of ~10higher yield in BB than B-factories for data mining! PV Trigger m (BR xx) Hugo Ruiz – Winter Meeting Santiago 2007

  47. Flavour tagging

  48. Flavour tagging • Flavour tagging: determination of the flavour of the signal B at production • At a hadroncollider, information can be obtained from: hadron from fragmentation or B** decay (K±, p±) Same side (SS) Signal B PV vertex charge Dx Tagging B Opposite side (OS) kaon (K±) lepton (m±, e±) Hugo Ruiz – Winter Meeting Santiago 2007

  49. Wrong tags… • Flavour tagging algorithms are not perfect! • Backgrounds in tagger selections • The tagging B can itself oscillate (unlike in B-factories): • 40% B±,10% baryons: no oscillation  • 40% Bd:Dmd ~ Gd oscillated 17.5% • 10% Bs:Dms >> Gs  oscillated 50%  • Characterization of tagging algorithms: • etag: fraction of events in which the algorithm gives a tag • w  NW/(NW+NR): wrong tag fraction • eeff  etag(1-2w)2: effective tagging efficiency. • Indicates the reduction in number of events that would account for the same statistical degradation as the fraction of wrong tags Average mixing probability: 13% Hugo Ruiz – Winter Meeting Santiago 2007

  50. Opposite-side tagging (OS) Dx - Tagging B(b) K+ - n m+, e+ • Tagging objects : • Selection tuned to optimize eeff: vertex charge Momentum and IPS cuts: Particle Id: Hugo Ruiz – Winter Meeting Santiago 2007

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