Rare B decays at LHCb Michela LenziINFN Firenze

On behalf of LHCb collaboration

15th International Conference on Supersymmetryand the Unification of Fundamental Interactions

Karlsruhe, GermanyJuly 26 - August 1, 2007

M. Lenzi July 31, 2007SUSY07 2/19

MotivationNew 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

Rare semi-leptonic decays: b sℓℓ (eg. Bd K0*)

Radiative decays: b s (eg. Bs B )

M. Lenzi July 31, 2007SUSY07 3/19

LHCb Detector

• LHCb is a single-arm forward spectrometer• Forward peaked, correlated bb-pair production

- bb 500mb with L 2 x 1032 cm-2 s-1

- For 1 nominal year (107 sec) 1012 bb-pairs

(B), rad (B), rad

MuonSystem

VertexLocator

RICH counters/K/p Id.) Tracking Calorimeters

4TmDipole

pp interactions/crossing

LHC

b

n=0

n=1

M. Lenzi July 31, 2007SUSY07 4/19

LHCb Trigger

L0 Hardware Trigger high pT + Pile-up veto On custom boards Fully synchronized (40 MHz), 4 s fixed latency

10 MHz (visible bunch crossings)

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

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

M. Lenzi July 31, 2007SUSY07 5/19

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, , e)

Analysis are based on full detailed detector simulation with the realistic reconstruction chain

• good tracking and vertexing performances

M. Lenzi July 31, 2007SUSY07 6/19

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

Muon Calorimeters RICH2Trackers

Magnet RICH1VELO

M. Lenzi July 31, 2007SUSY07 7/19

Bs motivationVery rare decay very sensitive to NP:

- SM prediction (including Ms from CDF): BR(Bs→µ+µ-) = (3.55±0.33) x 10-9

- Could be strongly enhanced by SUSY:BR(Bs→µ+µ-) tan6/MH

2

- Within CMSSM for different A0 at large tan~50, this indicates that gaugino mass is in the range 400-650 GeV BR(Bs→µ+µ-) in the range 10-7 – 10-9

- Anomalus magnetic momentum of muon measured at BNL disagrees with SM at 2.7a= (25.2 ± 9.2) x 10-10

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!

M. Lenzi July 31, 2007SUSY07 8/19

Bs : backgroundExtremely low branching ratio main issue is background rejection:- Combinatorial – with muons mainly from b decays (b , b )- Mis-identified hadrons – eg. B , K and KK

- Bc± → J/

Addressed by excellent mass and vertex resolution and particle Identification:

For 95% muon efficiency, 0.6% misId rate for one from B events

M. Lenzi July 31, 2007SUSY07 9/19

• Very high trigger efficiency on signal events > 90%

• Applying a loose pre-selection, expected: ~ 35 Bs per fb-1 (SM) ~ 5M bb-inclusive decays per fb-1

~ 2M 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 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 per fb-1 (SM) ~ 5M bb-inclusive decays per fb-1

~ 2M b , b per fb-1

• Very high trigger efficiency on signal events > 90%

Bs : analysis strategy

signalbb inc..b μ, b μBc+ J/Ψμν

(arbitrary normalization)

M. Lenzi July 31, 2007SUSY07 10/19

LHCb sensitivity

Integrated Luminosity (fb-1)

LHCb Sensitivity(signal+bkg is observed)

5

3

BR

(x10

–9)

SM prediction

Integrated Luminosity (fb-1)

Limit at 90% C.L. (no signal observed)

Uncertainty in background prediction

Expected final CDF+D0 Limit

BR

(x10

–9)

SM prediction

1 year@LHCb3 evidence of SM signal

L ~ 0.05 fb-1

(of good quality data)Overtake CDF+DO

L ~ 0.5 fb-1

exclusion @90% CLBR values down to SM

L ~ 6 fb-1

5 discovery of SM signal

M. Lenzi July 31, 2007SUSY07 11/19

s = (m)2 [GeV2]

AFB(s), theory

Rare semi-leptonic decays: b sℓℓ

AFB asymmetry: position of zero crossing of AFB (s0) is sensitive to New Physics Transverse asymmetries Ratio of ee and modes

Solution: use ratios where hadronic uncertainties are significantly reduced:

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

_

In this case the suppression factor is 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!

M. Lenzi July 31, 2007SUSY07 12/19

Bd K*: yieldsIn 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 SM the decay is a b s penguin decay:

Bd

sb

K*

d d

But NP diagrams could also contribute at the same levels!

- Background dominated by uncertainties on non-resonant (Bd K)

- Large background fraction from bX, bX- 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)%

M. Lenzi July 31, 2007SUSY07 13/19

Bd K*: AFB sensitivity

fast MC: 2fb-1

M2

(GeV2)

= 0.27 GeV2

L = 10fb-1

= 0.46 GeV2

L = 2fb-1

M2 (GeV2)

M2 (GeV2)

-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

-Determine s0, the M2 for which AFB=0

M. Lenzi July 31, 2007SUSY07 14/19

- Recent theoretical work has highlighted other asymmetries to study (Phys Rev D71: 094009, 2500)

- Describe the decay in terms of 4 parameters:– s = mass squared l = FBA angle (between and B in rest-frame) K* = equivalent K* angle (between K and B in K* rest-frame) = angle between K* and decay planes

SUSY 1

SUSY II

Longitudinal polarization FL Asymmetry AT(2)

SMNLO

BdK*transverse asymmetries

2 fb-12

0

2

||2

202 )(

AAA

AqFL

2

||2

2

||2

2)2( )(AA

AAqAT

2 fb-1

- FL measurement looks plausible with 2fb-1 ( = 0.016) – but theory errors inhibit discrimination between models

- AT2 looks more difficult: = 0.42 @ 2fb-1 (0.16 @10fb-1)

M. Lenzi July 31, 2007SUSY07 15/19

RK in B+ K+ℓℓ

ds

dsR

q

m dseKeBd

q

m dsKBd

K

2max

2

2max

2

4

)(

4

)(

MFV model: Rk-1 ~ BR(Bs→µµ)Hiller & Krüger, PRD69 (2004) 074020)

Predicted by MFV model

Excluded by

Babar & Belle (R k)

CDF & D0 (BR(Bs→µµ))

LHCb projectionif SM holds

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 K 18774 ± 227

Gives RK = 1 (fixed) ± 0.043

M. Lenzi July 31, 2007SUSY07 16/19

Radiative Decays: motivation• b s proceeds only via loop diagram

- SM: BR(b→s) = (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

_

M. Lenzi July 31, 2007SUSY07 17/19

Bs : yields

= 71 MeV

s

Sensitivity under study!

- Direct CP asymmetry that results in a difference of the decay rates for B Xand B X 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 decay amplitudes when B0

s and B0s

have transitions to the same final state X

LHCbs

_

- signal trigger and selection efficiency: 0.28%

- signal events expected for 2 fb-1 = 11500

- expected B/S < 0.55 @90% CL

__

M. Lenzi July 31, 2007SUSY07 18/19

2fb-1

10fb-1

b → polarization

Decay 2 fb-1 yield B/S

b 750 < 42

b 2500 < 18

- decays need special reconstruction since flies (c ~ 7.9cm) vertex doesn’t define the b vertex- Most decay after escaping the vertex detector

Sensitivity:

b decay most promising: LHCb can measure the right-handed component of photon polarization down to 15% at 3at L = 10 fb-1

~5% worse using only (1520), (1670), (1690) (p,1/2

= 0 proton angular distribution is flat less information)

Photon polarization can be probed in polarized b-baryons decays: b ((1115) p), b ((X) p) expect b to be polarized (assume 20% for now)

M. Lenzi July 31, 2007SUSY07 19/19

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 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 : Yield per 2 fb-1 of 11500 with B/S < 0.55 b (1115) : 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!