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 detectorbetween 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+ candidate
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:
 |
and red curve is the Λb → &Lambdac+ π- signal). The invariant mass distribution is described by several components: |
Analysis Cuts
From the previous Σ b measurement we know that the main background source is due toreal Λb combined with random tracks from the hadronization and the underline event.
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
to be one of the high pT tracks from the trigger.
Fitter Description:
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:
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 following function:
| ([Q+m(π)]2 -thr2)0.5 ·(C + b1 · Q + b2 (2Q2 − 1)) |
Systematic Errors
There are three different sources of systematic uncertainties:- 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 fitting
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.
Results
we measure the mass differences (Q-values,
Q = M(Λb0 π±)-M(Λb0)-mπ±) :
- Q(Σb+) = 52.0+0.9-0.8(stat.) ± +0.09-0.4(syst.) MeV/c2
- Q(Σb¯) = 56.2 +0.6-0.5(stat.) +0.07-0.4(syst.) MeV/c2
- Q(Σb*+) = 72.7 ± 0.7 (stat.) +0.12-0.6(syst.) MeV/c2
- Q(Σb*¯) = 75.7 ± 0.6(stat.) +0.08-0.6(syst.) MeV/c2
From these masses we obtain the isospin mass splitting to be:
- m(Σb+) - m(Σb-) = -4.2+1.1-0.9(stat.) +0.07-0.09(syst.)
- m(Σb*+) - m(Σb*-) = -3.0 ± 0.9 (stat.)+0.12-0.13(syst.) MeV/c2
We also measure the widths:
- Γ(Σb+) = 9.2+3.8-2.9(stat.) +1.0-1.1(syst.) MeV/c2
- Γ(Σb-) = 4.3+3.1-2.1(stat.) +1.0-1.1(syst.) MeV/c2
- Γ(Σb+*) = 10.4+2.7-2.2(stat.) +0.8-1.2(syst.) MeV/c2
- Γ(Σb*-) = 6.4 +2.2-1.8(stat.) +0.7-1.1(syst.) MeV/c2
We measure the yields:
- N(Σb+) = 468+110-95(stat.) +18-15(syst.) MeV/c2
- N(Σb-) = 333+93-73(stat.) ± 35 (syst.) MeV/c2
- N(Σb+*) = 782+114-103(stat.) +25-27(syst.) MeV/c2
- N(Σb*-) = 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:
mass measurement we extract the following absolute masses:
- m(Σb+) = 5811.2+0.9-0.8(stat.) ± 1.7(syst.) MeV/c2
- m(Σb¯) = 5815.5 +0.6-0.5(stat.) ± 1.7(syst.) MeV/c2
- m(Σb*+) = 5832.0 ± 0.7 (stat.) ± +1.8(syst.) MeV/c2
- m(Σb*¯) = 5835.0 ± 0.6(stat.) ± +1.8(syst.) MeV/c2
The following are the fits to the Σb Q-value distributions:
 |
 |
Σb(*)- |
Σb(*)+ |
