Fisica del b in CMS
N. Magini
Università & INFN Firenze
Parma, 19 Gennaio 2006
The CMS detector
4 Tesla Solenoid
Muon system in return yoke
Tracker (Pixel + Silicon Microstrip)
|h| < 2.4
ECAL & HCAL
inside solenoid
22 m long, 15 m diameter,
14.000 ton Detector
Parma, 19/01/06
Staged Scenario for start-up
1. 3rd forward pixel disks missing
2. RPC up to |h| < 1.6
3. Single muon trigger up to |h| < 2.1,
Fisica del B in CMS
dimuon up to |h| < 2.4
N. Magini
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B Physics at CMS
B-decays program
Inclusive b production
CP violation
B0s Mixing
Rare decays
Bc
b production at LHC
Starting luminosity  1033 cm-2s-1
s = 0.5 mb  about 0.5 x 106 b pairs/s
only 100 ev/s on tape for ALL interesting
physics channels
The trigger strategy is a great challenge
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Inclusive b production
bb differential cross section
(from b m X)
Expected CMS reach – up to pT ~ 1 TeV/c
b m X rate
extracted from
total m rate
fitting the
distributions of
m pT w.r.t. the
closest B jet
b
c
udsg
120 < pT < 170 GeV/c
Valery Andreev, David B. Cline, Stan Otwinowski
Parma, 19/01/06
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Benchmark channels
Decays into muons (di-muon Lvl-1 Trigger) :
• BS  J/   mm KK (Bs mixing, CPV)
• BS(d)  mm (rare decays)
Fully hadronic channel
(Single muon Lvl-1 Trigger from other B  m + X ) :
• BS  DS  (KK)  (Bs mixing)
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B0s oscillations
s
B0s
B0s
ss
Bs are too heavy to be produced at B factories
 studied at hadron colliders
Standard Model predictions
DmS < 22.2 x 1012 s-1 @ 95% CL , DGs/Gs ~ 0.12
PDG Experimental limits @ 95% CL
DmS > 14.4 x 1012 s-1, DGs/Gs < 0.59
Latest result by CDF : DGs/Gs = 0.65
D0 : DGs/Gs = (25+14-15)%
Parma, 19/01/06
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Bs  J/
ℓ+
BR(BsJ/ )=(9.3±3.3)x10-4
J/ℓ+ℓ- (BR≈6%)
K+K- (BR≈49%)
K+
ℓK-
p
CP violation weak phase
s= 2dg = 22h
SM predicts s ~ O(0.03)
p
Main ongoing activity: benchmark channel for B
Physics studies in CMS Physics TDR – Vol. 2
Angular distributions of decay products depend on
Gs, DGs, DMs (B0s mixing) and s (CP Violation)
N. Magini, V. Ciulli, T. Speer, K. Prokofiev, L. Wilke, S. Shulga, T. Ilicheva
Parma, 19/01/06
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Main backgrounds
s at LHC
–
Signal B0s  J/   m+m- K+K-
167 fb
–
Exclusive bkg B0d  J/ K*  m+m- K
900 fb
–
–
Inclusive backgrounds :
b  J/ X
–
Prompt pp  J/ X
–
–
In all samples : pT m > 2 GeV/c
In signal + Bd bkg : pT m > 0.5 GeV/c
Parma, 19/01/06
51.4 nb
310 nb
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B0s  J/   m+m- K+KSelection strategy
Optimization performed vs. the
following backgrounds:
Trigger
L1 : double muon trigger
L2 : regional J/ reco with fast tracking
Prompt pp  J/ X
L3 : regional  + B0s reco with fast tracking
• Inclusive b  J/ X
•B0d  J/ K*
Offline
Kinematic fitting
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CMS Trigger & DAQ
Two level trigger: Lvl-1 and HLT
40 MHZ
HW
50 kHz
SW
100 Hz
4 DAQ slices at start-up  50 kHZ
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Level 1 Muon Trigger rates
Low Lumi
Lvl-1 thresholds
optimized for
discovery physics
h < 2.1
High - pT
processes are
selected
b-jets are
selected mainly by
1m and 2m trigger
Parma, 19/01/06
16 kHz DAQ
3.6 kHz for m, mm
14 ; 3,3
2.7+0.9=3.6 kHz
eW
=90%
eZ
=99%
eBsmm =15%
Bs  J/  efficiency 33.3%
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Using the Tracker at HLT
Muon HLT stream optimized for high pT isolated muons  use
the Silicon Tracker instead to reconstruct the decay chain at
HLT
HLT track reconstruction has to be fast but does not need to be
global as the offline one, therefore it can be:
– Regional
pT
• Restricted to some phase-space region defined
from external Lvl-1 information (e.g. a cone
around muon/jet direction or the set of tracks
coming from a vertex or above a PT threshold )
– Partial
• Stopped after a number of hits have been
assigned to the track
– Conditional
• Stopped when enough resolution is reached or
on other condition
Full tracker
performance
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Bs  J/
HLT L2 strategy
Signal x 103
b  J/ X
prompt J/
 150 MeV/c2 around J/ mass
Transverse decay length resolution ~ 100
mm
– c2 < 10 and LT/s(LT) > 3
Tracks reconstructed with 5 hits around
L1 muons - |Dh| < 0.15 , |D| < 0.5
Parma, 19/01/06
 Rate = 10 Hz, <t> ~ 190 ms
~ 80% of L2 J/ are from b
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Bs  J/
HLT L3 strategy
Regional tracking around J/ direction |Dh| < 0.9, |D| < 0.9
 mass  20 MeV/c2
B0s mass  190 MeV/c2
HLT
e
10.1%
Parma, 19/01/06
HLT Rate
Events/10fb-1
0.05 Hz
169000
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Bs  J/ 
Offline selection strategy
• Combinatorial decay chain reconstruction
• Kinematic fitting – application of constraints from decay
kinematics
– J/ mass constraint
DM()<12 MeV/c2,
DM(B0s) < 67 MeV/c2
Parma, 19/01/06
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Bs  J/ 
Proper time resolution
Parma, 19/01/06
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Event selection
Vertex pointing (cos a > 0.9)
Dm  < 12 MeV/c2
Dm B0s < 67 MeV/c2
Events selected per 10 fb-1
Estimated S/B ~ 6, but very limited statistics for
backgrounds  larger MC sample produced
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Bs J/ Analysis
Two methods may be used to extract parameters of interest from angular
distributions
– Unbinned maximum Likelihood fit
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Angular moments method
W( t ,  ) =
d 3G
d cos1d cos2 d
=
6
9
64
b
g
i ( t ) i (1 ,  2 ,  )
i =1
Time evolution is a function of the transversity
amplitudes A0(t), A||(t), A(t) which contain the
parameters of interest
 A set of 6 weighting
functions wi(1,2, ) is
defined to separate the 6 bi(t)
components
Parma, 19/01/06
Known functions of the
three helicity angles
1,2, describing decay
product kinematics
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Angular moments method
Statistical error with 64k generator level events
Sources of systematics:
• Background contamination
• Detector resolution
• effect shown to be negligible
• Selection efficiency bias
Taking into account these effects, after 3 years at
low luminosity (60 fb-1): error on ΔΓs /Γs < 0.018
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Bs(d)mm
FCNC bs or bd only at loop-level in SM
 BR(Bsmm) = (3.5±1.0)x10-9
MSSM enhancement (high tanβ)  probe for new physics
BR(Bsμμ)=3x10-6(tanβ/50)6(200 GeV/c2 / mA)4
• Lvl-1: 2m pT> 3 GeV/c, e=15.2%
• HLT strategy:
– Select pixel seeds with pT > 4 GeV/c in h- region around trigger m’s
– Conditional tracking:
- stop if pT<4 GeV/c @ 5σ or Nhit= 6 or σ(pT)/pT<0.02
- Bs reconstruction if only 2 track candidates with opposite charge in
150 MeV/c2 window
- Vertex: c2 < 20 and dr > 150 mm
U. Langenegger, A. Starodumov, P. Trüb
Parma, 19/01/06
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Bsmm
Mass resolution
Full Tracker
HLT
s = 50 MeV/c2
s = 74 MeV/c2
Lvl-1
e
e
Global
33.5%
5.1%
HLT
15.2%
e
Events / 10fb-1
Trigger Rate
47
<1.7Hz
Offline analysis (hep-ph/9907256) based on SV cuts (decay length and
direction) + Tk/Calo Isolation: with SM BR=3.5x10-9
10 fb-1  7 signal events with < 1 background
5s observation with 30 fb-1
and analysis could be perfomed at high lumi too
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Bs Ds  (KK) 
BS-BS mixing: DmS  14.4 ps-1 @ 95% CL
P(b  Bs) x Br(Bs DS    K K  ) ~ 5 x 10-5
•
•
L1 1m trigger: pT > 14GeV/c , R=3.2 kHz
HLT strategy:
-
-
pixel seeds in full acceptance and Primary Vertex
Partial tracking: 2 pixel and 1 strip hits
Topological cuts: DR(KK)<0.3, DR()<1.2, DR(DS)<2.0, D(BS,m)>0.6
Kinematical cuts: pT() > 2GeV/c, pT(DS) > 4 GeV/c, pT(BS) > 5 GeV/c
Mass reconstruction: DM<15 MeV/c2, DMDs<75 MeV /c2,
DMBs<270 MeV/c2
HLT efficiency 9%
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Bs Ds  (KK) 
Mass resolutions (only 3 hits are used)

s = 5 MeV/c2
Ds
s = 25 MeV/c2
s = 95 MeV /c2
Bs
1 year low luminosity (20 fb-1):
Lvl-1 1 kHz
HLT 5 Hz
300-400 signal events  DmS up to 20 ps-1
1000 events are needed to test allowed SM range: DmS  26 ps-1 @ 99%CL
Can not be run on full Lvl-1 2.7 kHz because of
large output rate: still room to improve
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Bc Physics
Bc→J/ψµ (J/ψ→µµ)
M = 6287.0 ± 4.8 ± 1.1 MeV/c2
t = 0.46+0.18−0.16 ± 0.03 ps
258 evts after 1 fb-1
(+156 for e chan)
Bc→J/ψπ(J/ψ → µµ)
70 evts after 1 fb-1,
Mass RMS 76 MeV
But still need to study bkg rejection
Decay length residual RMS 25 mm
Parma, 19/01/06
Fisica del B in CMS
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High Level Trigger table for low lumi
Trigger
Threshold
(e=90-95%)
(GeV)
Indiv.
Rate
(Hz)
Cumul
rate
(Hz)
1e, 2e
29, 17
34
34
1g, 2g
80, (40*25)
9
43
1m, 2m
19, 7
29
72
1t, 2t
86, 59
4
76
Jet * Miss-ET
180 * 123
5
81
1-jet, 3-jet,
4-jet
657, 247,
113
9
89
19 * 52
1
90
237
5
95
10
105
e * jet
Inclusive bjets
Calibration/
other
Where is B – physics ?
Table optimized in DAQTDR for discovery physics
Allocation of additional 50 Hz to
“Standard” Physics ? In this
case we would like to trigger on
the full 10 Hz bJ/X rate
Bandwith for B-physics at LHC
start-up will also depend on
actual luminosity
– Lower luminosity at startup
 higher B-bandwith
From DAQ-TDR (CERN/LHCC 2002-26)
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Conclusions
 CMS is a competitive detector for B-physics, even if it is
not designed specifically for it
 Low-pT dimuon L1 Trigger and use of Fast Tracking at
HLT are fundamental for B-decay selection
 We should profit from LHC starting conditions:
Low luminosity for a while  lots of B physics
 In only a few months: the Physics TDR Vol. 2 will
address in deeper detail the CMS B-physics potential,
with a chapter dedicated to the full analysis for the
B0s  J/  benchmark channel and sections on the
other topics
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