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Performance Studies for the LHCb Experiment

Direct measurements of angles and sides in particle interactions, sensitivity studies for new physics, and reconstruction strategies in LHCb experiment at CERN.

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Performance Studies for the LHCb Experiment

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  1. Performance Studies for the LHCb Experiment Marcel Merk NIKHEF Representing the LHCb collaboration 19 th International Workshop on Weak Interactions and Neutrinos Oct 6-11, Geneva, Wisconsin, USA

  2. B Physics in 2007 • Direct Measurement of angles: • s(sin(2b)) ≈ 0.03 from J/y Ks in B factories • Other angles not precisely known • Knowledge of the sides of unitary triangle: (Dominated by theoretical uncertainties) • s(|Vcb|) ≈ few % error • s(|Vub|) ≈ 5-10 % error • s(|Vtd|/|Vts|) ≈ 5-10% error (assuming Dms < 40 ps-1) • In case new physics is present in mixing, independent measurement of g can reveal it…

  3. ,K Bs K K Ds  B Physics @ LHC bb production: (forward) qb qb • Large bottom production cross section: 1012 bb/year at 2x1032 cm-2s-1 • Triggering is an issue • All b hadrons are produced: Bu (40%), Bd(40%), Bs(10%), Bc and b-baryons (10%) • Many tracks available for primary vertex • Many particles not associated to b hadrons • b hadrons are not coherent: mixing dilutes tagging B Decay eg.: Bs->Dsh • LHCb: Forward Spectrometer with: • Efficient trigger and selection of many • B decay final states • Good tracking and Particle ID performance • Excellent momentum and vertex resolution • Adequate flavour tagging

  4. Simulation and Reconstruction All trigger, reconstruction and selection studies are based on full Pythia+GEANT simulations including LHC “pile-up” events and full pattern recognition (tracking, RICH, etc…) No true MC info used anywhere ! T3 T2 T1 TT RICH1 VELO Sensitivity studies are based on fast simulations using efficiencies and resolutions and from the full simulation

  5. Evolution since Technical Proposal • Reduced • material • Improved • level-1 trigger

  6. Track finding strategy T track Upstream track VELO seeds Long track (forward) Long track (matched) VELO track T seeds Downstream track Long tracks  highest quality for physics (good IP & p resolution) Downstream tracks  needed for efficient KS finding (good p resolution) Upstream tracks  lower p, worse p resolution, but useful for RICH1 pattern recognition T tracks  useful for RICH2 pattern recognition VELO tracks  useful for primary vertex reconstruction (good IP resolution)

  7. On average: 26 long tracks 11 upstream tracks 4 downstream tracks 5 T tracks 26 VELO tracks T3 Resultof track finding T2 T1 Typical event display: Red = measurements (hits) Blue = all reconstructed tracks TT VELO 2050 hits assigned to a long track: 98.7% correctly assigned Efficiency vs p : Ghost rate vs pT : Ghost rate = 3% (for pT > 0.5 GeV) Eff = 94% (p > 10 GeV) Ghosts: Negligible effect on b decay reconstruction

  8. Experimental Resolution Momentum resolution Impact parameter resolution sIP= 14m + 35 m/pT dp/p = 0.35% – 0.55% 1/pT spectrum B tracks p spectrum B tracks

  9. Particle ID RICH 2 RICH 1 e (K->K) = 88% e (p->K) = 3% Example: B->hh decays:

  10. pile-up L0 Trigger 40 MHz Calorimeter Muon system Pile-up system Level-0: pT of m, e, h, g 1 MHz Vertex Locator Trigger Tracker Level 0 objects Level-1: Impact parameter Rough pT ~ 20% L1 B->pp Bs->DsK 40 kHz ln IP/IP ln IP/IP HLT: Final state reconstruction Full detector information Signal Min. Bias 200 Hz output ln pT ln pT

  11. p+ p- B0 Knowledge of flavour at birth is essential for the majority of CP measurements B0 D K- l b Bs0 s b sources for wrong tags: Bd-Bd mixing (opposite side) b → c →l (lepton tag) conversions… s K+ u u εtag [%] Wtag [%] εeff [%] Combining tags Bdp p 42 35 4 Bs K K 50 33 6 Flavour tag tagging strategy: • opposite side lepton tag ( b →l) • opposite side kaon tag ( b → c → s ) (RICH, hadron trigger) • same side kaon tag (for Bs) • opposite B vertex charge tagging effective efficiency: eff= tag(1-2wtag)2

  12. Efficiencies, event yields and Bbb/S ratios Nominal year = 1012 bb pairs produced (107 s at L=21032 cm2s1 with bb=500 b) Yields include factor 2 from CP-conjugated decays Branching ratios from PDG or SM predictions

  13. CP Sensitivity studies CP asymmetries due to interference of Tree, Mixing, Penguin, New Physics amplitudes: fnew + + + fmix ftree fpen Measurements of Angle g: Mixing phases: 1. Time dependent asymmetries in Bs->DsK decays. Interference between b->u and b->c tree diagrams due to Bs mixing • Sensitive to g + fs (Aleksan et al) 2. Time dependent asymmetries in B->pp and Bs->KK decays. Interference between b->u tree and b->d(s) penguin diagrams • Sensitive to g, fd, fs(Fleischer) 3. Time Integrated asymmetries in B-> DK* decays. Interference between b->u and b->c tree diagrams due to D-D mixing • Sensitive to g(Gronau-Wyler-Dunietz) • Time dependent asymmetry in Bd->J/y Ks decays • Sensitive to fd • Time dependent asymmetry in Bs->J/yf decays • Sensitive to fs

  14. Bs oscillation frequency: ms • Needed for the observation of CP asymmetries with Bs decays • Use BsDs • If ms= 20 ps1 • Can observe >5 oscillation signal ifwell beyond SM prediction Expected unmixed BsDs sample in one year of data taking. (ms) = 0.011 ps1 Full MC ms < 68 ps1 Proper-time resolution plays a crucial role

  15. Mixing Phases • Bs mixing phase using Bs->J/yf • Bd mixing phase using B->J/y Ks Angular analysis to separate CP even and CP odd Background-subtracted BJ/()KS CP asymmetry after one year Time resolution is important: st = 38 fs Proper time resolution (ps) If ms= 20 ps1: s(DGs/Gs) = 0.018 (sin(d)) = 0.022 NB: In the SM, s = 2 ~ 0.04 (sin(fs)) = 0.058

  16. After one year, if ms= 20 ps1, s/s = 0.1, 55 <  < 105 deg, 20 < T1/T2 < 20 deg: () = 1415 deg No theoretical uncertainty; insensitive to new physics in B mixing (after 5 years of data) 1. Angle  from BsDsK (2 Tree diagrams due to Bs mixing) Simultaneous fit of Bs->Dsp and Bs->DsK: • Determination of mistag fraction • Time dependence of background Time dependent asymmetries: Bs(Bs)->Ds-K+: →DT1/T2 + (g+fs) Bs(Bs)->Ds+K-: →DT1/T2 – (g+fs) ADs-K+ ADs+K-

  17. 2. Angle  from B and BsKK (b->u processes, with large b->d(s) penguin contributions) • Measure time-dependent CP asymmetries in B and BsKK decays: ACP(t)=Adir cos(m t) + Amix sin(m t) • Method proposed by R. Fleischer: • SM predictions: Adir (B0 ) = f1(d, , ) Amix(B0 ) = f2(d, , , d) Adir (BsKK ) = f3(d’, ’, ) Amix(BsKK ) = f4(d’, ’, , s) • Assuming U-spin flavour symmetry(interchange of d and s quarks): d = d’and = ’ • 4 measurements (CP asymmetries) and 3 unknown (, d and )  can solve for  d exp(i) = function of tree and penguin amplitudes in B0  d’ exp(i’) = function of tree and penguin amplitudes in Bs KK

  18. “fake” solution blue bands from BsKK (95%CL) red bands from B (95%CL) ellipses are 68% and 95% CL regions(for input = 65 deg) If ms= 20 ps1, s/s=0.1, d =0.3, = 160 deg, 55 <  < 105 deg: () = 46 deg U-spin symmetry assumed; sensitive to new physics in penguins d vs  2. Angle  from B and BsKK (cont.) • Extract mistags from BK and BsK • Use expected LHCb precision on d and s pdf for  pdf for d

  19. √2 A2 √2 A2 A3 2 A3  A1 = A1 55 <  < 105 deg 20 <  < 20 deg () = 78 deg No theoretical uncertainty; sensitive to new physics in D mixing 3. Angle  from B DK* and B DK* (Interference between 2 tree diagrams due to D0 mixing) • Application of Gronau-Wyler method to DK* (Dunietz): • Measure six rates (following three + CP-conjugates): • 1) B D(K)K*,2) B DCP(KK)K*, 3) B D (K)K* • No proper time measurement or tagging required • Rates = 3.4k, 0.6k, 0.5k respectively (CP-conj. included), with B/S = 0.3, 1.4, 1.8, for =65 degrees and =0

  20. Measurement of angle g: New Physics? 2. B->pp, Bs->KK 3. B->DK* 1. Bs->DsK g not affected by new physics in loop diagrams g affected by possible new physics in penguin g affected by possible new physics in D-D mixing • Extract the contribution of new physics to the oscillations and penguins • Determine the CKM parameters A,r,h independent of new physics

  21. Systematic Effects Possible sources of systematic uncertainty in CP measurement: • Asymmetry in b-b production rate • Charge dependent detector efficiencies… • can bias tagging efficiencies • can fake CP asymmetries • CP asymmetries in background process Experimental handles: • Use of control samples: • Calibrate b-b production rate • Determine tagging dilution from the data: e.g. Bs->Dsp for Bs->DsK, B->Kp for B->pp, B->J/yK* for B->J/yKs, etc • Reversible B field in alternate runs • Charge dependent efficiencies cancel in most B/B asymmetries • Study CP asymmetry of backgrounds in B mass “sidebands” • Perform simultaneous fits for specific background signals: e.g. Bs->Dsp inBs->DsK , Bs->Kp& Bs->KK, …

  22. Conclusions • LHC offers great potential for B physics from “day 1” LHC luminosity • LHCb experiment has been reoptimized: • Less material in tracking volume • Improved Level1 trigger • Realistic trigger simulation and full pattern recognition in place • Promising potential for studying new physics

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