The charm of high-statistics data samples

The distribution of the difference between the reconstructed masses of the studied particles
c(2595) and Λc(2625)) and the Λc ground state obtained from data, together with the fit function.

There are six types of quarks, and studies of the transitions among them provide important insight to the basic principles of nature. But nature does not provide free quarks. They are always bound by the strong interaction into a pair (a quark and an anti-quark), or into collections of three quarks. The three-quark states are referred to as baryons, the nucleons being the lightest and most prominent representatives of them. Heavier versions of the nucleons contain a bottom or charm quark. They are collectively called heavy-quark baryons.

Many physicists try to identify new physics, and they need a good understanding of how quarks are bound together. However, this is difficult to calculate within the theory of strong interactions. So scientists have to rely on experimental studies to increase their understanding in this field. One way is to analyze the spectra of heavy-quark baryons, to measure properties such as mass and decay width.

Recently, CDF physicists measured the properties of several baryons that contain a charm quark, referred to as Σc(2455), Σc(2520), Λc(2595) and Λc(2625). While all these baryons are known, former experiments observed only a few hundred events for each of them. The new measurement uses several thousands of signal events in each case, which allows more precise measurements of the masses and decay widths of these particles. Furthermore, this statistical gain reveals a new effect concerning the Λc(2595) baryon. It prefers a decay in which the sum of the daughter particle masses is very close to the mass of the baryon itself. Treating this threshold effect properly for the first time results in a measured mass difference (see figure) which is significantly lower and more accurate (305.8 +/- 0.2) then previous measurements (308.9 +/- 0.6 MeV/c2).

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Edited by Andy Beretvas

These physicists were responsible for this analysis. From left: Felix Wick, Thomas Kuhr and Michael Feindt, all from Karlsruhe Institute of Technology in Germany; and Michal Kreps, University of Warwick in the United Kingdom.