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LHC preparations for precise measurements of muonic very rare B-decays

25. Sept 2006 M.Smizanska, Lancaster University, UK25. Sept 2006 M.Smizanska, Lancaster University, UK

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OutlineOutline

1. Current experimental limits

2. Different strategies of LHC experiments1. Detector layouts and luminositites

2. Detector performance

3. Triggers

3. Challenge of measurements of very rare B-decays to muons1. Signal selections and statistics

2. Background environments – combinatorial and non combinatorial detector dependent backgrounds.

4. Conclusions

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Current Experimental Limits on B

Bs Bd SM 3.5 10-9 0.9 10-10 Ali, Greub, Mannel, DESY-93-

016.

CDF (780 pb-1) 1.0*10-7 95%CL 3.0*10-7 95%CL Note 8176 06-03-16

D0 (700 pb-1) 2.0 *10-7 95%CL 11.1*10-7 95%CL preliminary

Belle 78 fb-1 - 1.6 *10-7 90% CL PRD68, 111101

BaBar 111 fb-1 - 0.61*10-7 90% CL PRL94, 221803

Today experimental limits still factor of 20 above SM –

leave space for NP

Expect improvement by factor of 5-8x by the end

of Tevatron run

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LHC strategies for measurements of B →

LHC pp total = 100 mb

inelastic = 80 mb bb = 500 b

ATLAS/CMS Central detectors:

Muons seen in transverse direction after 11 this limits pT >3-6GeV

LHCb Forward detector

Muon detector in forward direction can be reach by of any pT

pT

one B ‘in’ || < 2.5 pT (B) > 9-10 GeV

~ 100 b

1.9 < <4.9 pT (B) > 2.5 GeV

~ 230 b

Luminosity

for B physics

L = 2 × 1033 cm-2 s-1

rare B 1034 cm-2 s-1

L = 2 × 1032 cm-2 s-1

1 y Statistics

B

1 y @ 1033 cm-2 s-1

~350 in fiducial volume

~7 after trig + signal selections

(<20 backgr.)

1 y @ 2 × 1032 cm-2 s-1

~161 in fiducial volume

~ 17 after trig + signal selections

(<5.7 bckgr.)

Different layouts of LHC detectors - lead to different luminosity, trigger and offline strategies - different strategies in measurements of B →

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Understanding detector performance differences

relevant for B-

Impact parameter resolution LHC b

1/pt distribution for B tracks

Understanding of performance differences for B-- impact parameter resolution

LHCb is precise in R-z so IP precision is determined by large pz lead to 30-50 m resolution for B-tracks even at very low pT>1.3 GeV

ATLAS/CMS are precise in x-y ATLAS B-pT>6GeV 25-70 mCMS B-pT>3-6 GeV 50-90 m

pT- range for muons form B

IP resolution for ATLAS Final detector

CMS ATLAS

< 0.25

Understanding of performance differences for B-pT and mass resolution

CMS =36 MeV,

4 Tesla

ATLAS = 84 MeV

2 TeslaLHCb =18 MeV

Understanding trigger strategies for B-

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ATLAS di-muon triggers for rare decays

LVL1: 2 RoI pT () >

6GeV (~500 Hz @ L=1033cm-2s-

1)

LVL2: Confirm each RoI from LVL1

In precision muon chambers Combine with Inner Detector

track Mass cut 4 GeV < M()< 6

GeVEF: Refit ID tracks in Level-2 RoI

Decay vertex reconstruction

Transverse Decay length cut:

Lxy > 200m

Efficiency estimation L2/EF:

bb+- for both pT>6 GeV– 70% of B +-

– (60% of B K* + -)

Online reconstruction of di- mass, (MeV)

B K* +

-

B+ -

Not normalized

Selected from J.Kirk – this conference

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CMS Triggers for B-

First level trigger:

two muons each with

threshold pT>3GeV.

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LHCb L0 and HLT Trigger - selected features for di-muon case

L0• Pile-up system -reject events with multiple interactions per

bunch crossing • Muon Trigger (high PT muons) -select 2 muons with the highest PT in each

quadrant pT>1.3 GeV for rare decays

HLT (High Level Trigger)• reduce rate from 1MHz to ~2kHz – for di-

muon 600Hz• full detector info available• software trigger

Efficiency of (L0+HLT) for B → signal that passed signal selection cuts (see later) = 79%

Selected from LHCb 2003-165 and Metlica BEACH2006

Offline Selection strategies for B-

and combinatorialbackground

rejection

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LHCb offline signal selections

Later: Bs impact parameter cut was changed to : IP/ < 3

and pointing angle (momentum/decay length) < 5 mrad

17 signal events 2fb-1

<5.7 combinatorial background

More recent (preliminary) study gives 30 signal evts

with no background left of 30M bb sample.

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CMS offline selections

6.1 Evts/10fb-1

Background 13.8 +22.0 -13.8

ATLAS Offline Selections

• M = MBs+140

-70 MeV (asymmetry to distinguish B0s

and B0d)

• isolation: no charged tracks with pT > 0.8 GeV in cone q < 15 degrees

• vertex fit with pointing to primary vertex constraint• transverse decay length Lxy/s(Lxy) > 11

Isolation

Decay Length

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LHC overview rare B-decays: for early data and later luminosity conditions

Integral LHC Luminosity

Experiment Expected Signal

Combinatorial background

Upper limit at 90% CL

100 pb-1 ATLAS or CMS

~ 0 ~ 0.2 6 ×10-8

(each)

10 fb-1 1 year@1033 ATLAS ~ 7 ~ 20 1.2×10-8

CMS ~ 6.1 ~13.8 (+ 22.0 – 13.8) 1.4×10-8

2 fb-1 1 year @2.1032

LHCb ~17 <5.7 Not given yet

10 fb-1 5 years @2.1032

~54 <27

30 fb-1 3 years@1033 ATLAS ~ 21 ~ 60 7 ×10-9

(each)CMS ~18.3 ~41.4

1 year @1034

But can run as long as LHC

ATLAS (2000) ~92 Bs ~660

CMS (2000) ~26 Bs ~6.4

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BR used in the MC

Models used in MC or to confront experimental sensitivities.

3.5 10-9 Bs →

Ali, Greub, Mannel, DESY-93-016.

0.9 10-10 Bd →

1.0 10-10

1.9 10-8

1.9 10-8

Bd→

Bs →

Bd→

Melikhov, Nikitin, PRD70, 2004

WC: SM Buras, Munz, PRD52, 1995.

Other rare decays close to B

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ATLAS: B0d,s →µ+µ-γ as BG to B0

d →µ+µ-

Interesting study (since far limited to “particle-level” = fiducial and trigger cuts) checks B0

d,s →µ+ µ- γ as a possible background to B0d →µ+ µ-. Study concluded

the background is small in comparison with signal and negligible comparing to combinatorial background.

Plan is to study a feasibility of extraction of B0d,s →µ+ µ- γ as a signal. Preliminary

results show potential background from channel B0d,s →µ+ µ- 0

Number of events

pT(γ) < 2 GeV

← φ – resonant

contribution

B0s →µ+ µ- γ

B0d →µ+ µ- γ

Mµµ GeV

B0d →µ+ µ-

pT(γ) < 4 GeV

B0d →µ+ µ-

← φ – resonant

contribution

B0s →µ+ µ- γ

B0d →µ+ µ- γ

Number of events

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Review of non combinatorial BG sources for B-at LHC

BG process Br Effective Br in B- signal region

(ATLAS )

B0 → π - µ+ νµ ~10-4 ~ 5 ∙ 10-8

B+ → µ+µ- ℓ+νℓ < 5 ∙10-6 < 5 ∙10-8

B+ → J/ (µ+µ-) ~ 6 ∙ 10-5 ~ 10-8

Bc → µ+µ- ℓ+νℓ < 10-4 < 10-8

B0d

→ π0 µ+µ- ~ 2 ∙ 10-8 ~ 10-10

B0s →µ+ µ- γ ~ 2 ∙10-8 ~ 10-10

Bd → KBs →KK 2 ∙ 10-5 < 10-9

0.5 10-9 ( LHCb)

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B0s→hh background at LHCb, Kirill Voronchev

Misidentification and Fake Rates in LHCb

Misindetification and fake rates are detector dependent.Two-body hadronic decays in LHCb

B0d,s → +-, B0

d,s → K-+, B0d,s → K+K-

are estimated to have effective branching~ 0.5·10-9

in signal region.Mass resolution Mass resolution is important is important ((LHCbLHCb == 18 MeV) 18 MeV)

estimate of B0s→hh background

at LHCb: convoluted fake probability with K, spectrum

BR(B0s→KK) ~ 2 · 10-6

BR(B0s→K) ~ 5 · 10-6

=> this background under control - results in ~ 2 events / 2 fb-1 (in ± 2· mass window)

log 1

0(e

vent

s)

Fake Rates Spectrum

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Particle level study (ATLAS) ofbackgrounds from B0

d→π -μ+νμ and B+→

Br(B+ → µ+µ- ℓ+νℓ ) ≈ 5*10-6

Number of events

B+ → µ+µ- ℓ+νℓ

pT(ℓ+) < 0.5 GeV

B+ → µ+µ- ℓ+νℓ

pT(ℓ+) < 0.5 GeV

Fake events from

B0d→π -μ+νμ

Fake events from

B0d→π -μ+νμ

B0s →µ+ µ-

B0d →µ+ µ-

B0s →µ+ µ-

B0d →µ+ µ-

Number of events

MµµMµµ

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Br( B0 → π - µ+ ν ) ~ 10-4

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Conclusions

• All LHC experiments confirm to be able to search for B → signature starting from the early LHC run:

Their Lo/L1 triggers are capable to take di-muon signatures with high efficiency

HLT software is written and tested to reconstruct data online

• All three experiments are capable to measure signal of Bs → at luminosity of

1-2 1033

• All experiments are able to continue at luminosity of 1034 and improve measurements of Bs → signal and make sensitivity search for Bd →

• Combinatorial background cannot be well estimated within available CPU capacities before LHC start, but factorization of cuts give prediction roughly at the level of signal ( higher in ATLAS/CMS, and lower in LHCb).

• Specific backgrounds need estimation! LHC will be sensitive to Br where this background is relevant. ( Tevatron did not reach this sensitivity so they may not seen them).

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BackupsBackups

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