Measuring the b quark: Beauty is in the eye of the beholder

The top plot shows the difference in masses for Σb, while the bottom plot shows the same for Σb+ candidates.

The Σb particle was discovered by CDF in 2006. Physicists from the CDF collaboration have measured various properties of the particle since then, including the negative and positive versions of the sigma sub b particle. Both versions include a b quark, which is called the bottom quark or the beauty quark. The scientists focused their efforts on precise measurements of the masses and widths of the components of the sigma sub b particle. The mass measurements provide new benchmarks to compare to predictions of theoretical models. Measurements of the widths give insight into the dynamics between the heavy b quark and its pair of light quark partners. Both types of measurements are important for our understanding of the strong force, the fundamental theory of matter that interacts strongly.

Once produced in the CDF detector, the Σb particles decay within 10-22 second, releasing a tiny amount of energy. The CDF online selection system identifies data containing possible Σb candidates whose properties match patterns developed by the physicists. Handling the enormous amount of collected data was a particularly challenging part of the project. The CDF team sorted these into two spectra, corresponding to the types of quarks in the particle.

Each of the four identified widths has a statistical significance well above six standard deviations, meaning that the chance the measurements are mistaken is two parts in a billion. CDF scientists measured four precise masses and widths of the Σb particle. This is the first measurement of the natural widths of these evanescent particles. The CDF scientists found some of nature’s beauty in the b quark.

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—Edited by Andy Beretvas 

These physicists were responsible for this analysis. Clockwise from top left: Igor Gorelov, University of New Mexico; Constantino Calancha, CIEMAT, Spain; Luis Labarga, CIEMAT, Spain; Sally Seidel, University of New Mexico; Juan Pablo Fernandez, CIEMAT, Spain.