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Rare B decays at LHCb

Michela Lenzi INFN Firenze. Rare B decays at LHCb. On behalf of LHCb collaboration. 15 th International Conference on Supersymmetry and the Unification of Fundamental Interactions Karlsruhe, Germany July 26 - August 1, 2007. Motivation.

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Rare B decays at LHCb

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  1. Michela Lenzi INFN Firenze Rare B decays at LHCb On behalf of LHCb collaboration 15th International Conference on Supersymmetryand the Unification of Fundamental Interactions Karlsruhe, GermanyJuly 26 - August 1, 2007

  2. Motivation • New Physics is expected to be accessible from box and/or penguin diagrams in which the intermediate particles could be New Physics particles (in addition to SM particles) • This could result in: • unexpected CP violation effects • affected propertiesof rare decays where standard model contributions are suppressed enough to allow potential small New Physics effects to emerge • Regarding the second point, LHCb aims to find New Physics contributions in these processes: • Very rare leptonic decays: eg. Bs mm • Rare semi-leptonic decays: b sℓℓ (eg. Bd  K0*mm) • Radiative decays: b  sg(eg. Bs fg, LB Lg)

  3. pp interactions/crossing LHCb n=0 • (B), rad n=1 • (B), rad LHCb Detector 4Tm Dipole Muon System RICH counters (p/K/p Id.) Vertex Locator Calorimeters Tracking • LHCb is a single-arm forward spectrometer • Forward peaked, correlated bb-pair production • sbb 500mb with L  2 x 1032 cm-2s-1 • For 1 nominal year (107 sec)  1012 bb-pairs

  4. LHCb Trigger 10 MHz (visible bunch crossings) L0 Hardware Trigger • high pT + Pile-up veto • On custom boards • Fully synchronized (40 MHz), 4 s fixed latency • Relevant rates: • LHC: 40MHz • 2 bunches full: 30MHz • At least 2 tracks in the acceptance 10MHz • bb: 100kHz • Decay of one B in acceptance: 15kHz • Relevant decays BR~10-4-10-9 • cc: 600 kHz 1 MHz (full detector readout) High Level Trigger (HLT) • Refine pT measurements + IP cuts • Reconstruct in(ex)clusive decays • In PC farm with ~ 1800 CPUs • Full detector info available, only limit is CPU time • Average latency: 2ms ≤ 2 kHz (storage), ~ 35kB/evt

  5. π-K separation: Kaon ID ~ 88% Pion mis-ID ~ 3% • good tracking and vertexing performances LHCb performance • To exploit the full potential of LHC, the experiment needs: • good system of particle identification (p, K, p, m, e) Analysis are based on full detailed detector simulation with the realistic reconstruction chain

  6. RICH1 VELO RICH2 Magnet Muon Calorimeters Trackers LHCb detector in place • 2007: commissioning phase • LHCb is confident to be ready for data-taking in spring 2008 • 2008: early phase • Calibration and trigger commissioning at s=14 TeV • Start first physics data taking, assume ~ 0.5 fb–1 • 2009– : stable running • Full physics data-taking, expected ~ 2 fb–1/year

  7. Anomalus magnetic momentum of muon measured at BNL disagrees with SM at 2.7s: Dam= (25.2 ± 9.2) x 10-10 • Within CMSSM for different A0 at large tanb~50, this indicates that gaugino mass is in the range 400-650 GeV  BR(Bs→µ+µ-) in the range 10-7 – 10-9 • Limit from Tevatron at 90% CL: • Current (~2fb-1) < 7.5 x 10-8 • Expected final (~8fb-1) < 2.0 x 10-8 ~ 6 times higher than SM! Bs mm: motivation • Very rare decay  very sensitive to NP: • SM prediction (including DMs from CDF): BR(Bs→µ+µ-)= (3.55±0.33) x 10-9 • Could be strongly enhanced by SUSY: BR(Bs→µ+µ-)  tan6b/MH2

  8. Addressed by excellent mass and vertex resolution and particle Identification: For 95% muon efficiency, 0.6% misId rate for one p from B pp events Bs mm: background Extremely low branching ratio  main issue is background rejection: • Combinatorial – with muons mainly from b decays (b  m, b  m) • Mis-identified hadrons – eg. B  pp, Kp and KK • Bc± → J/y(m+m-)m±n

  9. (arbitrary normalization) signal bb inc.. b μ, b μ Bc+  J/Ψμν Bs mm: analysis strategy • Very high trigger efficiency on signal events > 90% • Applying a loose pre-selection, expected: • ~ 35 Bs mm per fb-1 (SM) • ~ 5M bb-inclusive decays per fb-1 • ~ 2M b  m, b  m per fb-1 • The pre-selected events are weighted with the likelihoods for these 3 distributions: • Combined geometry variable [0,1]: impact parameters, distance of closest approach, lifetime, vertex isolation • Particle-ID [0,1]: difference in likelihood of m with p and K hypotheses • Invariant mass: [-60, +60] MeV around Bs peak • Very high trigger efficiency on signal events > 90% • Applying a loose pre-selection, expected: • ~ 35 Bs mm per fb-1 (SM) • ~ 5M bb-inclusive decays per fb-1 • ~ 2M b  m, b  m per fb-1 • Very high trigger efficiency on signal events > 90%

  10. L ~ 0.05 fb-1 (of good quality data) Overtake CDF+DO L ~ 0.5 fb-1 exclusion @90% CL BR values down to SM L ~ 6 fb-1 5  discovery of SM signal 1 year@LHCb 3  evidence of SM signal LHCb sensitivity Limit at 90% C.L. (no signal observed) LHCb Sensitivity (signal+bkg is observed) BR (x10–9) BR (x10–9) Expected final CDF+D0 Limit 5 Uncertainty in background prediction SM prediction 3 SM prediction Integrated Luminosity (fb-1) Integrated Luminosity (fb-1)

  11. AFB(s), theory m+ B0 q K* m- Solution: use ratios where hadronic uncertainties are significantly reduced: • AFB asymmetry: position of zero crossing of AFB (s0) is sensitive to New Physics • Transverse asymmetries • Ratio of ee and mm modes s = (m)2 [GeV2] Rare semi-leptonic decays: b  sℓℓ In this case the suppression factor is aEM: BR(b→sℓℓ)= (4.5±0.1) x 10-6 BR(B+→sℓℓ)= (0.5±0.1) x 10-6 Currently the rarest observed B decay! Branching ratio and forward-backward asymmetry AFB (defined as asymmetry between m+ (m-) in forward and backward directions in m+m- pair rest frame, with respect to the B (B) direction) are sensitive to New Physics: • Inclusive decay well described theoretically but difficult to access experimentally • Exclusive decays affected by hadronic uncertainties _

  12. But NP diagrams could also contribute at the same levels! d d Bd K* m g m In LHCb: signal trigger and selection efficiency: (1.11 ± 0.03)% Signal events expected for 2 fb-1 (1 year): 7200 ± 180 (stat) ± 2100 (from BR) b • Background dominated by uncertainties on non-resonant (Bd Kpmm) • Large background fraction from bmX, bmX • Expected B/S = 0.5 ± 0.2 s Bd K*mm: yields In SM the decay is a b s penguin decay: • In SM: BR = (1.22+0.38-0.32) x 10-6 • The measured BR agrees within 30% with the SM prediction. • However New Physics could modify the angular distributions much more than this!

  13. = 0.46 GeV2 L = 2fb-1 Mmm2 (GeV2) • s= 0.27 GeV2 • L = 10fb-1 Mmm2 (GeV2) Bd K*mm: AFB sensitivity • Measure theangular distributionof the m+ in the mm rest frame relative to the B direction • Measure the forward-backward Asimmetry (AFB) of the distribution as a function of the mm invariant mass • Determines0, the M2mm for which AFB=0 fast MC: 2fb-1 Mmm2 (GeV2)

  14. Longitudinal polarization FL Asymmetry AT(2) SUSY 1 SM NLO SUSY II BdK*mm transverse asymmetries • Recent theoretical work has highlighted other asymmetries to study (Phys Rev D71: 094009, 2500) • Describe the decay in terms of 4 parameters: • s = mm mass squared • ql = FBA angle (between m and B in mm rest-frame) • qK* = equivalent K* angle (between K and B in K* rest-frame) • f = angle between K* and mm decay planes 2 fb-1 2 fb-1 • FL measurement looks plausible with 2fb-1 (s = 0.016) – but theory errors inhibit discrimination between models • AT2 looks more difficult: s = 0.42 @ 2fb-1(0.16 @10fb-1)

  15. MFV model: Rk-1 ~ BR(Bs→µµ) Hiller & Krüger, PRD69 (2004) 074020) Excluded by Babar & Belle (Rk) CDF & D0 (BR(Bs→µµ)) Predicted by MFV model LHCb projection if SM holds RK in B+ K+ℓℓ In SM: RK = 1 ± 0.001 But neutral Higgs corrections could be O(10%)  Measure RK  1  New Physics LHCb 10 fb-1 yields: • Bd  eeK 9240 ± 379 • Bd  mmK 18774 ± 227 Gives RK = 1 (fixed) ± 0.043

  16. Radiative Decays: motivation • b  sg proceeds only via loop diagram • SM: BR(b→sg) = (3.7±0.3) x 10-4 • Sensitive to New Physics, eg charged Higgs, gluino, neutralino loops • The emitted photon is predicted to be mainly left-handed in SM • right-handed components arise in several new physics models • Several methods proposed, e.g.: • CP asimmetries in the interference between mixing and decayamplitudesin radiative B neutral decays require both B0 and B0 decay to a common state, i.e. with the same photon helicity  if photon is polarized (SM) the CP asymmetry should vanish • Polarized b-baryons decays, where the photon helicity could be probed exploiting the angular correlations between the initial and final states • No clarifying results up to now due to limited statistics _

  17. Bs fg: yields • Direct CP asymmetry that results in a difference of the decay rates for BXgandBXg: theoretical prediction for inclusive decays is rather clean andmay increase up to 10%-40% for contribution of new particles but the experimentally accessible exclusive cases are theoretically much more difficult to calculate • CP violation in the interference between mixing and decayamplitudes when B0s and B0s have transitions to the same final state Xg: • LHCb: B0s f ( K+K-) g _ _ _ s = 71 MeV • signal trigger and selection efficiency: 0.28% • signal events expected for 2 fb-1 = 11500 • expected B/S < 0.55 @90% CL B0s f g Sensitivity under study!

  18. 2fb-1 10fb-1 Lb→Lg polarization Photon polarization can be probed in polarized b-baryons decays:Lb (L(1115)  pp)g, Lb (L(X)  pK)g expect Lb to be polarized (assume 20% for now) • L(1115)decays need special reconstruction since L flies (ct~ 7.9cm)  Lvertex doesn’t define the Lb vertex • Most L decay after escaping the vertex detector • Sensitivity: • LbL(1115) g decay most promising: LHCb can measure the right-handed component of photon polarization down to 15% at 3s at L = 10 fb-1 • ~5% worse using only L(1520), L(1670), L(1690) (ap,1/2 = 0  proton angular distributionis flat  less information)

  19. Conclusions • LHCb has good sensitivity for new physics discovery: • Bs mm • Potential to exclude BR between 10-8 and SM with 0.5 fb-1 • Potential for 3s (5s) observation with ~ 2 fb-1 (~ 6 fb-1) • Bd K*mm • Yield per 2 fb-1 of 7200 ± 180(stat) ± 2100(BR) with B/S = 0.5 • AFB zero-crossing point s0 = 0.46 GeV2 for 2 fb-1 (± 0.27 for 10 fb-1) • RK = 1 (fixed) ± 0.043 @10 fb-1 • Good potential for study of radiative B-decays: • Bs fg : Yield per 2 fb-1 of 11500 with B/S < 0.55 • LbL(1115)g: LHCb can measure the right-handed component of photon polarization down to 15% at 3s at 10 fb-1 • LHCb detector is on a good track to take first physics data in 2008 • The challenge is to achieve that performance with real data!

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