Lambda_b particle lifetime measurement

Λb lifetime measurement
with Λb → J/ψ Λ

Régis Lefèvre, Ahmed Rebai
Laboratoire de Physique Corpusculaire – IN2P3/ CNRS
Université Blaise Pascal – Clermont-Ferrand II

Outline
Motivations
MC samples
Pre-selection
Selection
Lifetime Fit
Outlook

Motivations
HQE calculations up to O(1/mb4)
τ(Λb) / τ(B0) = 0.88 ± 0.05
Calculations up to O(1/mb3)
were giving around 0.94
Assuming τ(B0) = 1.530 ± 0.009 ps
(PDG2008), this leads to the
prediction: τ(Λb) = 1.346 ± 0.077 ps
CDF Run II J/ψ Λ Prelim. (March 2007)
2.0σ higher than prediction
… and 3.3σ higher than PDG2006
CDF Run II Λc π Prelim. (July 2008)
seems to resolve the puzzle
τ(Λb) = 1.410 ±0.046 ±0.029 ps
– Compatible with prediction
… and 2.0σ higher than PDG2006
… 1.8σ lower than J/ψ Λ measurement
Worth doing another precise measurement in LHCb
First study done on DC04 by Eric Conte and Stéphane Monteil
Within Eric’s PhD: final state also used for T-violation studies with Ziad Ajaltouni

Régis Lefèvre - Λb Lifetime - Flavour Physics Working Group - May 20th 2009 2

MC Samples: DC06
Signal: Λb → J/ψ (→ μμ) Λ (→ pπ)
239,621 events - Leq = 592 pb-1 assuming BR(Λb → J/ψ Λ) = 4.7 ⋅ 10-4
Prompt J/ψ background: J/ψ (→ μμ) X
3,991,942 events - Leq = 1.20 pb-1
J/ψ from B background
Inclusive b di-muon: at least one B hadron in 400 mrad and two opposite signs
muons between 10 mrad and 400 mrad
25,865,679 events – Leq = 3.84 pb-1
B0 → J/ψ (→ μμ) X
5,415,440 events - Leq = 51.2 pb-1 with BR(B0 → J/ψ X) = 1.55%
B+ → J/ψ (→ μμ) X Note: in PDG2008
6,034,777 events - Leq = 57.1 pb-1 with BR(B+ → J/ψ X) = 1.76% BR(B±/B0 → J/ψ X)
Bs → J/ψ (→ μμ) X = (1.092 ± 0.032)%
640,611 events - Leq = 24.3 pb-1 with BR(Bs → J/ψ X) = 1.56%
Fake J/ψ background
Inclusive b di-muon
An underestimate checked with inclusive b (at least one B hadron in 400 mrad)
– 32,109,506 events - Leq = 0.105 pb-1


Towards pre-selection
DaVinci v22r0p2
Loose pre-selection
J/ψ from Standard Loose Muons
ADMass(J/ψ) < 100 MeV
Λ from
– Standard Loose Protons, Standard Loose Downstream Protons
– Standard Loose Pions, Standard Loose Downstream Pions
ADMass(Λ) < 30 MeV
Λb from J/ψ and Λ
ADMass(Λb) < 500 MeV
Running over 30,000 signal events
Retains 76,210 combinations in 9,364 events
2,341 matched combinations
– Acceptance × reconstruction efficiency of 7.8%
with respect to DecProdCut
» 1.62% with respect to 4π


Pre-selection (1/4)

Cut Matched Not matched
Track χ2 Probability (μ+) > 10-6 96% 91%
Track χ2 Probability (μ-) > 10-6 97% 92%
Track χ2 Probability (p) > 10-6 94% 82%
Track χ2 Probability (π) > 10-6 84% 55%
All Track χ2 Probability Cuts 74% 37%
Vertex χ2 Probability (J/ψ) > 10-3 98% 93%
Vertex χ2 Probability (Λ) > 10-6 95% 68%
Vertex χ2 Probability (Λb) > 10-6 96% 41%
All Vertex χ2 Probability Cuts 89% 26%
IP χ2 (Λb) < 25 98% 58%

Efficiency on matched and not matched combinations
in signal sample (applying cuts sequentially)


Pre-selection (2/4)


Pre-selection (3/4)
Cut Matched Not matched
ADMass(J/ψ) < 50 MeV 97% 96%
ADMass(Λ) < 10 MeV 98% 35%
All ADMass Cuts 95% 34%
pT (μ+) > 200 MeV 100% 99.7%
pT (μ-) > 200 MeV 100% 99.6%
pT (p) > 500 MeV 99.8% 56%
pT (π) > 100 MeV 98.9% 79%
All pT Cuts 99% 44%
All Pre-selection Cuts 61% 0.9%

64% on matched combinations
with ADMass(Λb) < 50 MeV

Efficiency on matched and not matched combinations
in signal sample (applying cuts sequentially)


Pre-selection (4/4)

Fraction of events pre-selected
Λb → J/ψ Λ : 6.04%
J/ψ X : 0.660%
Inclusive b di-muon : 0.0537%
Bd → J/ψ X : 1.10%
Bu → J/ψ X : 1.11%
Bs → J/ψ X : 1.08%
Inclusive b : 0.00279%


Selection (1/4)
Cut Signal Prompt J/ψ J/ψ from B Fake J/ψ
t(Λ)>0 99.3% 53% 47% 51%
t(Λ)/σt(Λ)>5 92% 21% 17% 12%
p track type = π track type 91% 72% 69% 73%
pT(μ+)+pT(μ-) > 2 GeV 99.9% 99.6% 100% 96%
DMass(J/Ψ) ∈ ]-30; 28[ MeV 97% 95% 95% 56%
ADMass(Λ) < 6 97% 86% 79% 76%
IP χ2 (Λb) < 16 98% 98% 84% 83%
pT(Λ) > 1.6 GeV 83% 27% 34% 39%
pT(Λb) > 3 GeV 81% 36% 65% 43%
All Selection Cuts 67% 9.9% 22% 17%

On selected signal events one has about 1/4 of
Long-Long and 3/4 of Downstream-Downstream

Efficiency on signal [matched & ADMass(Λb) < 50 MeV]
and background combinations (applying cuts sequentially)

Selection (2/4)

Signal
Prompt J/ψ
J/ψ from B
Fake J/ψ


Selection (3/4)

Signal
Prompt J/ψ
J/ψ from B
Fake J/ψ


Selection (4/4)
Lifetime fit cuts
-1ps < t(Λb) < 10ps
σt(Λb) < 70fs
8MeV < σm(Λb) < 20MeV
Efficiency: 98.5% for signal, 91% to 94% for backgrounds
Select only one combination per event
Min [ Vertex χ2 (Λb) + IP χ2 (Λb) ]
2.9% of selected signal events have more than one selected combination, on that:
83% have only matched combinations, 3% have only not matched combinations,
14% have matched and not matched combinations
– Select a matched combination in 88% of those 14% (of 2.9%)
Expected S/B in a Λb mass window of ±50MeV
∀ τ(Λb) : 0.604 ± 0.041
τ(Λb) > 0.2ps : 8.61 ± 0.39
Lifetime fit exercise will be done using:
Signal events passing selection and lifetime fit cuts
Background events passing pre-selection and lifetime fit cuts
Statistics limited by number of pre-selected J/ψ prompts in the inclusive J/ψ sample
Leq = 147.7 pb-1

Lifetime fit
RooFit
Maximum likelihood

L=Π
N

i =1
[ f P (t σ )P (σ )P (m σ )P (σ ) + (1 − f )P (t σ )P (σ )P (m σ )P (σ ) ]
S S
t
i i
t σt
S i
t
S
m
i i
m σm
S i
m
S
t
B i i
t σt
B i
t
B
m
i i
m σm
B i
m

Proper time pdfs
Signal: convolution with a Gaussian (μ = 0 ; σ = σti × Fσt) of a positive exponential
Background: convolution with a Gaussian (μ = 0 ; σ = σti × Fσt) of
– a negative exponential
– two positive exponentials
– and a prompt contribution
Proper time error pdfs: convolutions of a Gaussian and a positive exponential
Mass pdfs
Signal: Gaussian (μ ; σ = σmi × Fσm)
Background: 2nd order polynomial
Mass error pdfs: convolutions of a Gaussian and a positive exponential
→ 25 parameters in total
Fit results (Leq = 147.7pb-1)
τ(Λb) = (1.237 ± 0.035) ps


Fit projections (1/3)


Toy MC Study: goodness of fit, pull…


Outlook
Fake J/ψ background
Inclusive b sample leads to a fake J/ψ background estimation
13.3 (+9.9/-7.7) times higher (4 fake J/ψ events passing the selection)
than the estimation from the inclusive b di-muon sample
Fake J/ψ background would then contribute to as much as about 1/4 of
the prompt J/ψ background
Re-making the study increasing the fake J/ψ background by 7 [21] leads to
a degradation on the average expected statistical error on τ(Λb) of only
(0.7±0.5)% [(1.1±0.7)%]
To do
Control channel: B0 → J/ψ (→ μμ) KS (→ ππ)
Estimation of systematic uncertainties
Redo the study on DC09
Asked for signal sample equivalent to DC06 one
– This time would prefer to have Λb decayed in phase space
– Channel also used for the PhD of Marwa Jahjah
under the supervision of Ziad Ajaltouni
» BR(Λb → J/ψ Λ), Λb polarization …
… Redo on data …


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