lambda_b particle lifetime measurement

Λb lifetime measurement with Λb → J/ψ Λ

Régis Lefèvre, Ahmed RebaiLaboratoire de Physique Corpusculaire – IN2P3/ CNRS

Université Blaise Pascal – Clermont-Ferrand II

OutlineMotivationsMC samplesPre-selectionSelectionLifetime FitOutlook

MotivationsHQE calculations up to O(1/mb

4) 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 theprediction: τ(Λb) = 1.346 ± 0.077 psCDF Run II J/ψ Λ Prelim. (March 2007)

2.0σ higher than prediction… and 3.3σ higher than PDG2006CDF 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 LHCbFirst 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: DC06Signal: 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) X3,991,942 events - Leq = 1.20 pb-1

J/ψ from B backgroundInclusive 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) X5,415,440 events - Leq = 51.2 pb-1 with BR(B0 → J/ψ X) = 1.55%

B+ → J/ψ (→ μμ) X6,034,777 events - Leq = 57.1 pb-1 with BR(B+ → J/ψ X) = 1.76%

Bs → J/ψ (→ μμ) X640,611 events - Leq = 24.3 pb-1 with BR(Bs → J/ψ X) = 1.56%

Fake J/ψ backgroundInclusive 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


Note: in PDG2008 BR(B±/B0 → J/ψ X) = (1.092 ± 0.032)%

Towards pre-selectionDaVinci v22r0p2Loose pre-selection

J/ψ from Standard Loose MuonsADMass(J/ψ) < 100 MeV

Λ from– Standard Loose Protons, Standard Loose Downstream Protons– Standard Loose Pions, Standard Loose Downstream PionsADMass(Λ) < 30 MeV

Λb from J/ψ and ΛADMass(Λb) < 500 MeV

Running over 30,000 signal eventsRetains 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)

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


Cut Matched Not matchedTrack 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%

Pre-selection (2/4)


Pre-selection (3/4)

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


Cut Matched Not matchedADMass(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

Pre-selection (4/4)

Fraction of events pre-selectedΛb → 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)

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


Cut Signal Prompt J/ψ J/ψ from B Fake J/ψt(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

Selection (2/4)


SignalPrompt J/ψJ/ψ from BFake J/ψ

Selection (3/4)


SignalPrompt J/ψJ/ψ from BFake J/ψ

Selection (4/4)Lifetime fit cuts

-1ps < t(Λb) < 10psσt(Λb) < 70fs8MeV < σm(Λb) < 20MeV

Efficiency: 98.5% for signal, 91% to 94% for backgroundsSelect 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 ± 0.041τ(Λb) > 0.2ps : 8.61 ± 0.39

Lifetime fit exercise will be done using:Signal events passing selection and lifetime fit cutsBackground events passing pre-selection and lifetime fit cuts

Statistics limited by number of pre-selected J/ψ prompts in the inclusive J/ψ sampleLeq = 147.7 pb-1


Lifetime fitRooFitMaximum likelihood

Proper time pdfsSignal: convolution with a Gaussian (μ = 0 ; σ = σt

i × Fσt) of a positive exponentialBackground: convolution with a Gaussian (μ = 0 ; σ = σt

i × Fσt) of – a negative exponential– two positive exponentials – and a prompt contribution

Proper time error pdfs: convolutions of a Gaussian and a positive exponentialMass pdfs

Signal: Gaussian (μ ; σ = σmi × Fσm)

Background: 2nd order polynomialMass error pdfs: convolutions of a Gaussian and a positive exponential

→ 25 parameters in totalFit results (Leq = 147.7pb-1)τ(Λb) = (1.237 ± 0.035) ps


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Fit projections (1/3)


Toy MC Study: goodness of fit, pull…


OutlookFake 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 …


lambda_b particle lifetime measurement

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