This web page summarizes the results of the mass and width measurements for
the heavy baryons Σb and Σb* in
the decay channel:
These Σ b(*) states were discovered by CDF
on 2006 using 1.1 fb-1 of data.
This new analysis is based upon 6.0 fb-1 of data.
The results were initally blessed in the B group meeting on July 21, 2010.
Dataset / Trigger
This analysis is based on an integrated luminosity of 6.0 fb-1
collected with the CDF II detector
between March 2002 and February 2010.
The data are collected using the Two Track Trigger (TTT),
which selects events
that contain track pairs with transverse momentum larger that 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+
with a π+ to build the Λb0 candidate.
Finally, the Λb0 is combined with a π-
to build the Σb± candidate.
The analysis is performed on the Q-value,
Q = m(Λb0 π±) - m(Λb0) - m(π±)
because the Λb0 resolution
is canceled by taking the difference.
Λb mass plot:
The result of a binned fit is imposed over the histogram (blue curve
is the total background
and red curve is the
Λb → &Lambdac+
The invariant mass distribution is described by several components:
The discrepancies between the fit and data below the Λb
0 signal region are due to incomplete knowledge
of the branching ratios of the decays in this region.
the Λb → &Lambdac+
a combinatorial background
partially and fully reconstructed B mesons which pass the Λ
c+π- selection criteria
- partially reconstructed Λb0 decays
and fully reconstructed &Lambdab0 other
(e.g. Λb0 → Λc
- Here the .eps file for Λb0 mass plot.
previous Σ b measurement we know that the main background source is
real Λb combined with random tracks from the hadronization and the
Thus, we perform the optimization of our final cuts looking to get as
much Λb as possible.
The optimization is performed based on the
experimental data only. The following are the
final cuts with more power of background rejection:
cτ(Λb)/σc&tau > 12.0
cτ(&Lambdab) > 200 μm
cτ(Λc → Λb) > -150 μm
cτ(Λc → Λb) < 250 μm
|d0(Λb)| < 80 μm
pT(πb) > 1.5 GeV/c
The first two plots reflect that the Λb decays via the weak force.
The following cuts on
cτ(Λc) are related with the weak decay of the Λc.
The cut on d0(Λb) ensures that
the Λb was
produced in the primary interaction. The last cut confirm that the πb uses
one of the high pT tracks from the trigger.
The signal peaks are described by
modifies Breit-Wigner distributions convoluted with two
gaussians to account
for the resolution detector.
The width of the Breit-Wigners are modified with
the following angular momentum correction:
Γ → Γ ⋅ (P/P0)3
where Γ is the corrected natural with, P is the momentum of the softpion
in the Σb rest frame,
and P0 is the same evaluated in
the pole of the Σb mass distribution.
The background is described by the
·(C + b1 · Q + b2 (2Q2 − 1))
where thr is fixed to the mπ (PDG), and b2
is also fixed.
- C and b1 are floating in the fit
There are three different sources of systematic uncertainties:
- Fit procedure.
- Uncertainties on the momentum scale.
- Assumptions made about the fitter. These include the following:
- Fixed parameters describing detector resolution (dominant
contribution for the widths).
- The model describing the background.
The uncertainty on the momentunm scale is estimated extrapolating the Q-value differences
between CDF II and PDG for several decays releasing little amount of energy.
The uncertainty comming from the chosen model describing the background is estimated
the spectra with alternative models describing the background.
The uncertainty comming from the resolution is estimated comparing the
resolution in the
D*±-D0 mass difference distribution for
D*± →D0 π± decays using data and Monte Carlo.
This table summarizes all the systematic uncertainties.
we measure the mass differences (Q-values,
Q = M(Λb0 π±)-M(Λb0)-mπ±) :
= 52.0+0.9-0.8(stat.) ±
= 56.2 +0.6-0.5(stat.)
= 72.7 ± 0.7 (stat.)
= 75.7 ± 0.6(stat.)
From these masses we obtain the isospin mass splitting to be:
- m(Σb+) - m(Σb-)
- m(Σb*+) - m(Σb*-)
= -3.0 ± 0.9 (stat.)+0.12-0.13(syst.)
We also measure the widths:
= 6.4 +2.2-1.8(stat.)
We measure the yields:
± 35 (syst.) MeV/c2
= 522 +85-76(stat.)
± 29 (syst.) MeV/c2
From the above Q-values and using the
best CDF II Λb0
mass measurement we extract the following absolute masses:
= 5811.2+0.9-0.8(stat.) ±
= 5815.5 +0.6-0.5(stat.)
± 1.7(syst.) MeV/c2
= 5832.0 ± 0.7 (stat.)
± +1.8(syst.) MeV/c2
= 5835.0 ± 0.6(stat.)
± +1.8(syst.) MeV/c2
The following are the fits to the Σb Q-value distributions:
- A more detailed summary of the results can be found here: .pdf
- Here the .eps file for Σb−
- Here the .eps file for Σb+
- Table significance "Same Sign" .pdf
- Table significance "Opposite Sign" .pdf
D*± Resolution Studies:
- Comparison Data-MC and Data-Data: .eps
- Resolution Ratio Data-MC and Data-Data: .eps
- Table summary systematics .pdf