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

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 properties of 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)

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

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

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

LHCb detector in place Muon Calorimeters RICH2 Trackers Magnet RICH1 VELO 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

Bs  mm: motivation Very rare decay  very sensitive to NP: 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 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 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: 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 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: 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% signal bb inc.. b μ, b μ Bc+  J/Ψμν (arbitrary normalization)

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

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! m+ K* B0 m- q 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 s = (m)2 [GeV2] AFB(s), theory _ AFB asymmetry: position of zero crossing of AFB (s0) is sensitive to New Physics Transverse asymmetries Ratio of ee and mm modes Solution: use ratios where hadronic uncertainties are significantly reduced:

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! Bd m s b g K* d But NP diagrams could also contribute at the same levels! Background dominated by uncertainties on non-resonant (Bd  Kpmm) Large background fraction from bmX, bmX Expected B/S = 0.5 ± 0.2 Signal events expected for 2 fb-1 (1 year): 7200 ± 180 (stat) ± 2100 (from BR) In LHCb: signal trigger and selection efficiency: (1.11 ± 0.03)%

Bd  K*mm: AFB sensitivity Measure the angular distribution of 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 Determine s0, the M2mm for which AFB=0 s= 0.27 GeV2 L = 10fb-1 = 0.46 GeV2 L = 2fb-1 Mmm2 (GeV2) fast MC: 2fb-1 Mmm2 (GeV2)

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 SUSY 1 SUSY II Longitudinal polarization FL Asymmetry AT(2) SM NLO 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)

RK in B+  K+ℓℓ In SM: RK = 1 ± 0.001 MFV model: Rk-1 ~ BR(Bs→µµ) Hiller & Krüger, PRD69 (2004) 074020) Predicted by MFV model Babar & Belle (Rk) CDF & D0 (BR(Bs→µµ)) Excluded by 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 LHCb projection if SM holds

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 decay amplitudes in 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 _

Bs  fg: yields _ _ _ LHCb: B0s  f ( K+K-) g B0s  f g Direct CP asymmetry that results in a difference of the decay rates for B  Xg and B  Xg: theoretical prediction for inclusive decays is rather clean and may 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 decay amplitudes 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!

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) 2fb-1 10fb-1 L(1115) decays need special reconstruction since L flies (ct ~ 7.9cm)  L vertex doesn’t define the Lb vertex Most L decay after escaping the vertex detector Decay 2 fb-1 yield B/S Lb  L(1115) g 750 < 42 Lb  L (1670) g 2500 < 18 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 distribution is flat  less information)

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!