Rare B decays at LHCb Michela Lenzi INFN
Rare B decays at LHCb Michela Lenzi INFN Firenze 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 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: ü Very rare leptonic decays: eg. Bs ü Rare semi-leptonic decays: b sℓℓ (eg. Bd K 0* ) ü Radiative decays: b sg (eg. Bs fg, LB Lg) M. Lenzi SUSY 07 July 31, 2007 2/19
LHCb Detector n=0 LHCb pp interactions/crossing 4 Tm Dipole n=1 Vertex Locator • • RICH counters (p/K/p Id. ) Tracking Calorimeters Muon System LHCb is a single-arm forward spectrometer Forward peaked, correlated bb-pair production bb 500 mb with L 2 x 1032 cm-2 s-1 For 1 nominal year (107 sec) 1012 bb-pairs - M. Lenzi SUSY 07 q( B) , r ad , q(B) July 31, 2007 rad 3/19
LHCb Trigger 10 MHz (visible bunch crossings) L 0 Hardware Trigger Relevant rates: § high p. T + Pile-up veto § On custom boards § Fully synchronized (40 MHz), 4 s fixed latency - LHC: 40 MHz - 2 bunches full: 30 MHz - At least 2 tracks in the 1 MHz (full detector readout) acceptance 10 MHz - bb: 100 k. Hz High Level Trigger (HLT) § Refine p. T 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: 2 ms - Decay of one B in acceptance: 15 k. Hz - Relevant decays BR~10 -4 -10 -9 - cc: 600 k. Hz ≤ 2 k. Hz (storage), ~ 35 k. B/evt M. Lenzi SUSY 07 July 31, 2007 4/19
LHCb performance To exploit the full potential of LHC, the experiment needs: • good system of particle identification (p, K, p, , e) • good tracking and vertexing performances π-K separation: Kaon ID ~ 88% Pion mis-ID ~ 3% Analysis are based on full detailed detector simulation with the realistic reconstruction chain M. Lenzi SUSY 07 July 31, 2007 5/19
LHCb detector in place RICH 2 Magnet RICH 1 Muon Calorimeters Trackers VELO Ø 2007: commissioning phase ü LHCb is confident to be ready for data-taking in spring 2008 Ø 2008: early phase ü Calibration and trigger commissioning at s=14 Te. V ü Start first physics data taking, assume ~ 0. 5 fb– 1 Ø 2009– : stable running ü Full physics data-taking, expected ~ 2 fb– 1/year M. Lenzi SUSY 07 July 31, 2007 6/19
Bs : 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-) tan 6 b/MH 2 - Anomalus magnetic momentum of muon measured at BNL disagrees with SM at 2. 7 : Da = (25. 2 ± 9. 2) x 10 -10 - Within CMSSM for different A 0 at large tanb~50, this indicates that gaugino mass is in the range 400 -650 Ge. V BR(Bs→µ+µ-) in the range 10 -7 – 10 -9 Limit from Tevatron at 90% CL: Current (~2 fb-1) < 7. 5 x 10 -8 Expected final (~8 fb-1) < 2. 0 x 10 -8 ~ 6 times higher than SM! M. Lenzi SUSY 07 July 31, 2007 7/19
Bs : background Extremely low branching ratio main issue is background rejection: Combinatorial – with muons mainly from b decays (b , b ) Mis-identified hadrons – eg. B pp, Kp and KK Bc± → J/y( + -) ±n - Addressed by excellent mass and vertex resolution and particle Identification: For 95% muon efficiency, 0. 6% mis. Id rate for one p from B pp events M. Lenzi SUSY 07 July 31, 2007 8/19
Bs : analysis strategy • Very high trigger efficiency on signal events > 90% • Applying a loose pre-selection, expected: - ~ 35 Bs per fb-1 (SM) - ~ 5 M bb-inclusive decays per fb-1 - ~ 2 M b , b 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 with p and K hypotheses - Invariant mass: [-60, +60] Me. V around Bs peak M. Lenzi SUSY 07 (arbitrary normalization) signal bb inc. . b μ, b μ Bc+ J/Ψμν July 31, 2007 9/19
Limit at 90% C. L. (no signal observed) BR (x 10– 9) LHCb sensitivity LHCb Sensitivity (signal+bkg is observed) Expected final CDF+D 0 Limit 5 Uncertainty in background prediction SM prediction 3 SM prediction Integrated Luminosity (fb-1) L ~ 0. 05 fb-1 (of good quality data) Overtake CDF+DO M. Lenzi Integrated Luminosity (fb-1) L ~ 0. 5 fb-1 1 year@LHCb exclusion @90% CL 3 evidence BR values down to SM of SM signal SUSY 07 L ~ 6 fb-1 5 discovery of SM signal July 31, 2007 10/19
Rare semi-leptonic decays: b sℓℓ In this case the suppression factor is a. EM: 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 + ( -) in forward and backward directions _ in + - 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 B 0 q - + K* AFB(s), theory Solution: use ratios where hadronic uncertainties are significantly reduced: üAFB asymmetry: position of zero crossing of AFB (s 0) is sensitive to New Physics ü Transverse asymmetries s = (m )2 [Ge. V 2] ü Ratio of ee and modes M. Lenzi SUSY 07 July 31, 2007 11/19
Bd K* : yields In SM the decay is a b s penguin decay: d d Bd b s g But NP diagrams could also contribute at the same levels! K* 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! 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) - Background dominated by uncertainties on non-resonant (Bd Kp ) - Large background fraction from b X, b X - Expected B/S = 0. 5 ± 0. 2 M. Lenzi SUSY 07 July 31, 2007 12/19
Bd K* : AFB sensitivity -Measure the angular distribution of the + in the rest frame relative to the B direction -Measure the forward-backward Asimmetry (AFB) of the distribution as a function of the invariant mass = 0. 46 Ge. V 2 L = 2 fb-1 -Determine s 0, the M 2 mm for which AFB=0 Mmm 2 (Ge. V 2) fast MC: 2 fb-1 = 0. 27 Ge. V 2 L = 10 fb-1 Mmm 2 (Ge. V 2) M. Lenzi Mmm 2 (Ge. V 2) SUSY 07 July 31, 2007 13/19
Bd K* transverse asymmetries - Recent theoretical work has highlighted other asymmetries to study (Phys Rev D 71: 094009, 2500) Describe the decay in terms of 4 parameters: – s = mass squared – ql = FBA angle (between and B in rest-frame) – q. K* = equivalent K* angle (between K and B in K* rest-frame) – f = angle between K* and decay planes Longitudinal polarization FL Asymmetry AT(2) 2 fb-1 SUSY 1 2 fb-1 SM NLO SUSY II - FL measurement looks plausible with 2 fb-1 ( = 0. 016) – but theory errors inhibit discrimination between models - AT 2 looks more difficult: = 0. 42 @ 2 fb-1 (0. 16 @10 fb-1) M. Lenzi SUSY 07 July 31, 2007 14/19
RK in B+ K+ℓℓ MFV model: Rk-1 ~ BR(Bs→µµ) Hiller & Krüger, PRD 69 (2004) 074020) In SM: RK = 1 ± 0. 001 But neutral Higgs corrections could be O(10%) Measure RK 1 New Physics ) µµ)) k R y ( b → ed Belle R(Bs d clu ar & 0 (B x E ab B F&D CD Predicted by MFV model LHCb 10 fb-1 yields: § Bd ee. K 9240 ± 379 § Bd K 18774 ± 227 Gives RK = 1 (fixed) ± 0. 043 LHCb projection if SM holds M. Lenzi SUSY 07 July 31, 2007 15/19
Radiative Decays: motivation • b sg proceeds only via loop diagram - SM: BR(b→sg) = (3. 7± 0. 3) x 10 - Sensitive to New Physics, eg charged Higgs, gluino, -4 • neutralino loops The emitted photon is predicted to be mainly lefthanded in SM - right-handed components arise in several new physics models - Several methods proposed, e. g. : • M. Lenzi CP asimmetries in the interference between mixing and decay amplitudes in radiative B neutral _ 0 0 decays require both B and B 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 SUSY 07 July 31, 2007 16/19
Bs fg: yields - Direct _ asymmetry that results in a difference of the decay rates for B Xg _ CP 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 _ 0 0 mixing and decay amplitudes when B s and B have transitions to the same final state Xg: s LHCb: B 0 s f ( K+K-) g - signal trigger and selection efficiency: 0. 28% - signal events expected for 2 fb-1 = 11500 - expected B/S < 0. 55 @90% CL = 71 Me. V B 0 s f g Sensitivity under study! M. Lenzi SUSY 07 July 31, 2007 17/19
Lb → Lg polarization Photon polarization can be probed in polarized b-baryons decays: Lb (L(1115) pp)g, Lb (L(X) p. K)g expect Lb to be polarized (assume 20% for now) - 2 fb-1 L(1115) decays need special reconstruction since L flies (ct ~ 7. 9 cm) 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 10 fb-1 Sensitivity: üLb L(1115) g decay most promising: LHCb can measure the right-handed component of photon polarization down to 15% at 3 at L = 10 fb-1 ü~5% worse using only L(1520), L(1670), L(1690) (ap, 1/2 = 0 proton angular distribution is flat less information) M. Lenzi SUSY 07 July 31, 2007 18/19
Conclusions • LHCb has good sensitivity for new physics discovery: Bs § Potential to exclude BR between 10 -8 and SM with 0. 5 fb-1 § Potential for 3 (5 ) observation with ~ 2 fb-1 (~ 6 fb-1) Bd K* § Yield per 2 fb-1 of 7200 ± 180(stat) ± 2100(BR) with B/S = 0. 5 § AFB zero-crossing point s 0 = 0. 46 Ge. V 2 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 3 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! M. Lenzi SUSY 07 July 31, 2007 19/19
- Slides: 19