This web page summarizes the result on the observation of the orbitally excited
bottom baryon resonance state Λb*0 in the decay mode:
Λb*0 → Λb0 π− π+ where Λb0 → Λc+ π−, Λc+ → p K− π+
The analysis is based on a complete dataset of 10.0 fb-1 collected with CDF II detector.
The results have been blessed at the B Physics group meeting on June 28, 2012.
Dataset / Trigger
This analysis is based on a complete total integrated luminosity of 10.0 fb-1 collected bya Two Displaced Track Trigger, which selects events that contain at least two tracks
with a transverse momentum above 2 GeV/c and impact parameter 120 μm < d0 < 1mm.
The analysis begins with the reconstruction of the Λc+ → p K− π+ by fitting three tracks
to a common vertex, then we combine our Λc+ candidate with a pion track π-b to build
the Λb0 candidate.
Finally, the Λb0 is combined with two soft pion tracks, π-sπ+s to build the Λb*0 candidate.
The search of a narrow structiure is performed in a Q-value spectrum, where
Q = M(Λb0π-sπ+s) -M(Λb0) - 2⋅M(π±)
The Λb0 resolution and much of the systematic uncertainties are canceled in a mass difference
Q-value spectrum.
Λb mass plot:
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and red curve is the Λb → &Lambdac+ π- signal). The invariant mass distribution is described by several components: |
Analysis Cuts
The following are the analysis cuts used to reconstruct the candidates:-
M(Λc+→p K−π+) ∈ ( m(Λc+)PDG -3⋅&sigma, m(Λc+)PDG +3⋅&sigma )
pT(p) > 2.0 GeV/c
pT(K−) > 400 MeV/c
pT(π+) > 400 MeV/c
M(Λb→&Lambdac+π-b) ∈ ( 5.561, 5.677 ) GeV/c2
cτ(Λb)/σc&tau > 6.0
cτ(&Lambdab) > 200 μm
cτ(Λc → Λb) > -100 μm
|d0(Λb)| < 80 μm
pT(Λb) > 9.0 GeV/c
pT(π-b) > 1.0 GeV/c
pT(π±s) > 200 MeV/c
The Λc+ and Λb candidates are selected in their 3⋅&sigma mass windows around central values taken
from PDG or from the experimental data fit correspondingly.
The momentum cut on a proton confirms the Two Track Displaced Trigger.
The cuts on a proper lifetime of the Λb are motivated by its weak decay mode.
The followig cut on cτ(Λc) is related with the weak decay of the Λc.
The cut on d0(Λb) ensures that the Λb was produced in the primary interaction.
The soft pion tracks are taken with the transverse momenta as low as 200 MeV/c.
The cut on a total transverse momentum of a Λb candidate is used to contain both soft pions
within the acceptance of a CDF tracker system.
Fitter Description:
The narrow signal structure is described by two Gaussians with their widths σn, σw and weightsgn, (1 - gn) fixed according to Monte-Carlo simulation studies. The background is described by
the second order Chebyshev polynomial with a floating normalization factor and coefficients a1, a2.
Soft Pions Momentum Scale
Λb*0 Experimental Q value Spectrum with Fit
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Significance of the observed signal
Significance Estimate Based on the Experimental Data Fits:
- D = -2⋅log(L0/L1) = -2⋅Δ(log L)
- -2⋅Δ(log L) = -2⋅(-12.99), Δ(NDF)=2
- Prob(χ2) = 2.28⋅10-6 or 4.6 σ
Significance Estimate with Statistical Trials:
- Generate samples with H0 but fit with H1
- Search window Q ∈(0., 50.0) MeV/c2
- Parameter of interest: Ncands
- Signal position Q: floating
- Signal shape: fixed
- Background shape: floating
- p = 2.3⋅10-4 or 3.5 σ
Systematic Uncertainties
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Results
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