0 25. sept 2006 m.smizanska, lancaster university, uk lhc preparations for precise measurements of...
TRANSCRIPT
1
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
2
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
3
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
4
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 →
5
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-
9
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
10
CMS Triggers for B-
First level trigger:
two muons each with
threshold pT>3GeV.
11
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
13
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.
14
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
16
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
6
17
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
18
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
19
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)
13
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
21
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
12
Br( B0 → π - µ+ ν ) ~ 10-4
22
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).
23
BackupsBackups
24
25