Charm Baryon Spectroscopy

Primary Authors: Felix Wick, Michal Kreps, Thomas Kuhr, Michael Feindt


Analysis Overview

The excited charm baryons &Lambdac(2595), &Lambdac(2625), &Sigmac(2455) and &Sigmac(2520) are examined in their strong decays to the &Lambdac ground state, &Lambdac*+ &rarr &Lambdac+ &pi+ &pi- respective &Sigmac0,++ &rarr &Lambdac+ &pi-,+. Measurements of the mass differences of these resonances to the &Lambdac mass and the corresponding decay widths are performed. It turns out that the &Lambdac(2595) mass shape is affected by kinematical threshold effects in the resonant subdecays &Lambdac(2595) &rarr &Sigmac(2455) &pi . This leads to a &Lambdac(2595) mass which is approximately 3 MeV/c2 lower than the previously measured values.

The results were blessed on July 1, 2010.

1. Motivation

Because of its rich mass spectrum and the relatively narrow widths of the resonances the charmed baryon system makes a good testing ground for the heavy quark symmetry. With the excellent tracking and mass resolution of the CDF detector and the large amount of available data it is possible to improve previous mass difference and decay width measurements of the states &Lambdac(2595)+, &Lambdac(2625)+, &Sigmac(2455)0,++ and &Sigmac(2520)0,++.

References to previous experimental results:

&Lambdac(2595)+ &Lambdac(2625)+ &Sigmac(2455)0,++ &Sigmac(2520)0,++
Phys. Lett. B402, 207 Phys. Lett. B402, 207 Phys. Rev. D65, 071101 Phys. Rev. D71, 051101
Phys. Lett. B365, 461 Phys. Rev. Lett. 74, 3331 Phys. Lett. B525, 205 Phys. Rev. Lett. 78, 2304
Phys. Rev. Lett. 74, 3331 Phys. Rev. Lett. 72, 961 Phys. Lett. B488, 218

By means of a proper inclusion of kinematical threshold effects in the resonant decays &Lambdac(2595)+ &rarr &Sigmac(2455)0,++ &pi+,-, a direct experimental determination of the pion coupling constant h2 in the chiral Lagrangian is feasible (Phys. Rev. D67, 074033). The knowledge of h2 provides information about other excited charm and bottom baryons (Phys. Rev. D56, 5483; Phys. Rev. D56, 6738).

2. Trigger and Selection

We take advantage of the hadronic trigger on displaced tracks for the selection of secondary vertex decays (Two Track Trigger). The &Lambdac+ is reconstructed in its decay to p K- &pi+ and then combined with one respective two additional tracks with &pi+ mass hypothesis to build the &Sigmac and &Lambdac* candidates. Our data sample corresponds to an integrated luminosity of 5.2 fb-1.

After some slight precuts, neural networks (NeuroBayes program package) are applied in two successive steps to distinguish between signal and background. First, a pure &Lambdac network is employed which is then used as input for &Sigmac and &Lambdac* networks. Thereby, the trainings are solely based upon real data by means of sPlot weights, what has the advantage of being independent of simulated events.

3. Fit Procedure

A binned maximum likelihood method is employed in order to fit the distributions of the mass differences m(&Lambdac+ &pi-)-m(&Lambdac+), m(&Lambdac+ &pi+)-m(&Lambdac+) and m(&Lambdac+ &pi+ &pi-)-m(&Lambdac+) of the selected candidates. Thereby, the signals are convolutions of nonrelativistic Breit-Wigner functions with the corresponding detector resolutions which are determined from Monte Carlo simulations.

For the &Lambdac(2595)+, the consideration of kinematical threshold effects in the resonant decays to &Sigmac(2455)0,++ &pi+,- is necessary. This is done by using a mass-dependent width in the Breit-Wigner function. Then, instead of &Gamma(&Lambdac(2595)+), the second parameter of the Breit-Wigner function is h2 which can therefore be determined directly.

The backgrounds consist of three different constituents:

4. Systematic Uncertainties

Main sources of systematic uncertainties:

5. Validation of Detector Resolutions

In order to estimate the reliability of the detector resolutions determined from Monte Carlo simulations, the reference decays D*(2010)+ &rarr D0 &pi+ and &psi(2S) &rarr J/&psi &pi+ &pi- are considered because of the similarities of their decay topologies to &Sigmac0,++ &rarr &Lambdac+ &pi-,+ and &Lambdac*+ &rarr &Lambdac+ &pi+ &pi-, respectively. In particular, the dependencies of the detector resolutions in data and Monte Carlo on the transverse momenta of the slow pion(s) are examined.

Final Results

m - m(&Lambdac+) [MeV/c2] &Gamma [MeV/c2] h22
&Sigmac(2455)0 167.28 ± 0.03 (stat.) ± 0.12 (syst.) 1.65 ± 0.11 (stat.) ± 0.49 (syst.)
&Sigmac(2455)++ 167.44 ± 0.04 (stat.) ± 0.12 (syst.) 2.34 ± 0.13 (stat.) ± 0.45 (syst.)
&Sigmac(2520)0 232.88 ± 0.43 (stat.) ± 0.16 (syst.) 12.51 ± 1.82 (stat.) ± 1.37 (syst.)
&Sigmac(2520)++ 230.73 ± 0.56 (stat.) ± 0.16 (syst.) 15.03 ± 2.12 (stat.) ± 1.36 (syst.)
&Lambdac(2595)+ 305.79 ± 0.14 (stat.) ± 0.20 (syst.) 2.59 ± 0.30 (stat.) ± 0.47 (syst.) 0.36 ± 0.04 (stat.) ± 0.07 (syst.)
&Lambdac(2625)+ 341.65 ± 0.04 (stat.) ± 0.12 (syst.) < 0.97 (90% CL)

Comparison of our measurements (statistical and systematic uncertainties added in quadrature) with PDG values (in parentheses):

m - m(&Lambdac+) [MeV/c2] &Gamma [MeV/c2]
&Sigmac(2455)0 167.28 ± 0.12 (167.30 ± 0.11) 1.65 ± 0.50 (2.2 ± 0.4)
&Sigmac(2455)++ 167.44 ± 0.13 (167.56 ± 0.11) 2.34 ± 0.47 (2.23 ± 0.30)
&Sigmac(2520)0 232.88 ± 0.46 (231.6 ± 0.5) 12.51 ± 2.28 (16.1 ± 2.1)
&Sigmac(2520)++ 230.73 ± 0.58 (231.9 ± 0.6) 15.03 ± 2.52 (14.9 ± 1.9)
&Lambdac(2595)+ 305.79 ± 0.24 (308.9 ± 0.6) 2.59 ± 0.56 (3.6+2.0-1.3)
&Lambdac(2625)+ 341.65 ± 0.13 (341.7 ± 0.6) < 0.97 (90% CL) (1.9 (90% CL))

The analysis at hand is the one with the highest number of signal events for all the reviewed resonances, what leads to the most accurate values for the &Lambdac* properties. The significant difference in m(&Lambdac(2595)+)-m(&Lambdac+) is due to our proper treatment of the kinematical threshold effects. Furthermore, some of the previously measured values of the &Sigmac properties show tensions between the different experiments, so that our measurements can have important impact.

List of main Tables

List of main Figures

List of supporting Tables

List of supporting Figures