1. 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

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 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)

3. 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

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~
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. 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

6. 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

7. 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 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!

8. 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

9. 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)

10. 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
3  evidence
of SM signal
L ~ 6 fb-1
5  discovery
of SM signal 3
SM prediction
L ~ 0.5 fb-1 CL
BR values down to SM
Integrated Luminosity (fb-1)
Integrated Luminosity (fb-1)

11. 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:

12. Bd  K*mm: yields In SM the decay is a b  s penguin decay:
In SM: BR = ( ) 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)%

13. 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)

14. BdK*mm transverse asymmetries
Recent theoretical work has highlighted other asymmetries to study (Phys Rev D71: , 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 = 2fb-1

15. RK in B+  K+ℓℓ In SM: RK = 1 ± 0.001
MFV model: Rk-1 ~ BR(Bs→µµ)
Hiller & Krüger, PRD69 (2004) )
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 ± 227
Gives RK = 1 (fixed) ± 0.043
LHCb projection
if SM holds

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 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 _

17. 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 < CL
B0s  f g
Sensitivity under study!

18. 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)

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) ± fb-1
Good potential for study of radiative B-decays:
Bs  fg : Yield per 2 fb-1 of 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!