pdf file - EPFL

Measurement of mixing parameters
of the B meson s meson at LHCb
A.Hicheur, A.Hicheur,
EPFL
Swiss Physical Society meeting
February 20-21, 20 21, 2007
1

Outline
Physics motivation
LHCb experiment
Main steps of the study
Results of sensitivity studies
Conclusion
2

Motivation
LHCb Physics program: dedicated to b physics
• All B s mixing parameters: Δms s , φs , ΔΓ
ΔΓs s
Bs→Dsπ, , b→c cbar s modes: J/ΨΦ J/ ΨΦ, , J/Ψη J/ Ψη, , η
Sensitivity to new cΦ
physics:
• CKM angles α β and γ
• Rare B decays
Radiative, Radiative,
electroweak, gluonic penguins
Bs→μμ →μμ (rare box diagram)
New particles in loop diagrams
• Bc , b-baryon b baryon physics, charm physics, etc… etc
B mixing and decays:
Combined decays to CP eigen states and
non CP eigen states to extract the mixing
parameters:
1. Precision measurement for Δms for CP eigenstates
2. φs measurement
(e.g e.g: : J/ΨΦ J/ ΨΦ, , J/Ψη J/ Ψη, , ηcΦ)
For Bs→D -
s π + , f ≠ f
Small (φ ( s ) SM (-0.04): 0.04): possible
early detection of New Physics
3

Mixing and decay asymmetry for b → cc
s modes
Mixing: mass eigenstates ≠ CP eigenstates
B = p B ± q B , ΔM
= M − M , ΔΓ = Γ − Γ
L / H
q
q
Mixing phase φs= = 2Arg[V *
Mixing ts Vtb
Mixing phase φ tb] ] for B s +φNP s
NP with new physics
q
H
L
q
L
H
Decay
Final phase for asymmetry ratio ≈ φs s (+φ (+ NP )
Time dependent asymmetry: extraction of
φs and ΔΓ s (ΔMs obtained from control
sample B 4
s→D -
s π + )

pp collisions @ 14 TeV
Correlated forward
production σb ≈ 500 μb
Forward spectrometer
Primary
vertex
LHCb experiment
bb
Need prim. & b vertex reco for
time measurements +
subsequent decays separation
B signal
Vertexing
D
≈ 1 cm btag Flavour tagging
K
π
RICH system for PID
Tracking Muon detector
PID to identify K/ π
5

Channels for mixing parameters determination
Admixture of CP eigenstates (ηη f = -1,+1) 1,+1)
• Bs→J/ J/Ψ(μ + μ- )Φ(K (K + K- )
Large yield but angular analysis to disentangle final states
CP Even eigenstates (ηη f = +1)
• Bs→J/ J/Ψ(μ + μ- )η(γγ γγ,π + π-π0 ), J/Ψ(μ J/ + μ- )η’(π + π- η,ρ0γ), ),
ηc(h (h + h-h + h- )Φ(K (K + K- )
Low yield, high background
• Bs→ D +
s (K + K-π + )Ds )D - (K + K-π- )
Low yield, degradation of proper time resolution
Extraction of ΔMs using flavor-specific flavor specific control
channel
• Bs→D -
s (K + K-π- )π +
6

Basic ingredients for the analyses
Candidate selection
• Bs mass resolution
Proper time
• Vertex/time resolution and time-dependent time dependent efficiency ε(t) (t)
Tagging
• Mistag rate and tagging efficiency
Full Monte Carlo simulation results (based on 2 fb -1 )
J/Ψ J/ η’(π + π- η(γγ γγ)) ))
2.2
1.0
19
33.0
19
31
64
7

Ideal rate
B -
s→Ds π + decay rates and ΔMs e
2
ΔΓst
(cosh ± cos ΔM
t)
2
( ( ) )
) ( −
−ΔΓst
R B s t → Dsπ
∝
s
+ dilution from flavour tagging
e
2
ΔΓst
(cosh ± D cos ΔM
t)
2
( ( ) )
) ( −
−ΔΓst
R B s t → Dsπ
∝
s
+ Proper time resolution ~40 fs
R
( ) ( −
true
true rec
B s ( t ) → Dsπ
) ⊗G(
t − t , Στσ
τ )
+ Background S/B ~ 3
+ Acceptance
( −)
rec
true
true rec
ε ( t ). R(
B s ( t ) → Dsπ
) ⊗G(
t − t , Στσ
τ )
1000
1000
Events 800
800
600
600
400
400
200
200
Perfect Perfect Perfect reconstruction
reconstruction
reconstruction
+ + flavour flavour tagging
tagging
+ proper time resolution
resolution
+ background
+ acceptance
0
0
0 0 1 1 2 2 2 3 3 4 4 5
5
Proper Proper Proper time time time (ps)
(ps) (ps)
8

B →J/ J/Ψ(μ s + μ- )Φ(K (K + K- ) case
Transversity amplitudes:
A0, , A // (CP even) A ⊥ (odd)
One angle distribution:
μ -
z
θ
J/ψ J/
Γ(
Bs ( t)
→ f ) ∝ ( 1−
RT
) Γ(
Bs
( t)
→ feven)
+ RT
Γ(
Bs
( t)
→ fodd
)
B s
φ s
μ +
K +
K -
9

Toy Monte Carlo sensitivity studies
From full MC, extract PDFs to model signal and
background
• Mass distributions
• Proper time distributions (including acceptance, tagging,
resolution model)
• Transversity angle θ for Bs→J/ J/ΨΦ ΨΦ
Generate ~ 200 toy experiments, each of them corresponds
to 2 fb -1 of data
• Likelihood maximized for b→ b c⎯c s modes simultaneously with
control sample Bs→Dsπ Standard Model (or measured) values used for the input
parameters:
• M(B s) ) = 5369.6 MeV/c 2 , ΔMs = 17.5 ps -1
• φs s = -0.04 0.04 rad, rad,
ΔΓ s /Γs s = 0.15, τs = 1/Γ 1/ s = 1.45 ps
• RT = 0.2 for Bs→J/ J/ΨΦ ΨΦ
Fitted parameters: φs, , ΔMs, s , ΔΓs
(+ R tag)
, Γ s
ΔΓs (+ RT, ωtag 10

PDF modeling, examples
Bs (→Dsπ) ) mass
Transversity angle for Bs→J/ J/ΨΦ ΨΦ
total
CP even
CP odd
bkg
Proper time model for Bs→J/ J/ΨΦ ΨΦ
total
bkg
Proper time PDF: convolution
(ε(t rec ).R(t true ,ωtag CP-even CP even
component
tag ) (acceptance
function×decay
function decay rate)
⊗ G(t rec – t true , Στστ) ) (resolution function)
- στ (per event) proper time error
- Στ scaling factor to account for bad error
11
estimation

Results for the sensitivities (2 fb -1 )
φs sensitivity
Other mixing parameters
Parameters Sensitivity Decay
ΔΓ s /Γs R T
ΔM s
ωtag tag
σ(φs) ) vs φs B s →η c Φ
Bs→J/ J/ΨΦ ΨΦ
0.0092 Bs→J/ J/ΨΦ ΨΦ
0.0004 Bs→J/ J/ΨΦ ΨΦ
0.007 ps -1 Bs→Dsπ 0.0036 Bs→Dsπ 12

Conclusions
Precision measurement for ΔMs φs s is unmeasured (yet)
LHCb combined statistical sensitivity to φs s is 0.02
for 2 fb -1 1 (a 2σ 2 measurement for a SM value)
• Dominated by Bs→J/ J/ΨΦ ΨΦ
• With the same statistics, ability to detect deviation due to
new physics
• First results expected for 2008. More precision afterwards
More improvement to come
• Adding J/Ψ→e J/ + e- mode increases yield by ~ 20%
• Full angular analysis for Bs→J/ J/ΨΦ ΨΦ
• Optimize use of Bs→Dsπ to determine the tagging
performance of the b→ c⎯c s modes
• Systematics studies
13

Back up
14

∝
∝
∝
∝
Decay rates
b→ c⎯c s modes
B s →D s - π +
15

Decay diagrams
Dominant tree Penguin contribution
∝ λ 2 ∝ λ 4
u
d −
π
16

Tagging performance
e, , μ from semi-leptonic
semi leptonic decays
K ± from the b→c c →s
jet/vertex charge
tagging B
same side π/K /K signal B
Tagging power εD2 = ε(1 (1−2ωω tag)
tag
2 (in %)
Tag εD2 Muon 1.0
Electron 0.4
Kaon 2.4
Jet/vertex
charge
1.0
Same side 2.1
Combined tagging power
for B S in LHCb is ~ 6%
Note ~2% at the Tevatron
~30% at B-Factories Factories
Tagging power for B 0 ~ 4%
(reduced reduced same side tagging) tagging
Recent Neural Network based study achieved 9% for B S tagging!
tagging
17

More sensitivity plots
σ(φs) ) vs ΔΓ s /Γs σ(φs) ) vs RT 18