Physicists from the University of Zurich have discovered a previously unknown particle composed of three quarks in the Large Hadron Collider (LHC) particle accelerator. A new baryon could thus be detected for the first time at the LHC. The baryon known as Xi_b^* confirms fundamental assumptions of physics regarding the binding of quarks.
In particle physics, the baryon family refers to particles that are made up of three quarks. Quarks form a group of six particles that differ in their masses and charges. The two lightest quarks, the so-called "up" and "down" quarks, form the two atomic components, protons and neutrons. All baryons that are composed of the three lightest quarks ("up," "down" and "strange" quarks) are known. Only very few baryons with heavy quarks have been observed to date. They can only be generated artificially in particle accelerators as they are heavy and very unstable.
In the course of proton collisions in the LHC at CERN, physicists Claude Amsler, Vincenzo Chiochia and Ernest Aguiló from the University of Zurich's Physics Institute managed to detect a baryon with one light and two heavy quarks. The particle Xi_b^* comprises one "up," one "strange" and one "bottom" quark (usb), is electrically neutral and has a spin of 3/2 (1.5). Its mass is comparable to that of a lithium atom. The new discovery means that two of the three baryons predicted in the usb composition by theory have now been observed.
The discovery was based on data gathered in the CMS detector, which the University of Zurich was involved in developing. The new particle cannot be detected directly as it is too unstable to be registered by the detector. However, Xi_b^* breaks up in a known cascade of decay products. Ernest Aguiló, a postdoctoral student from Professor Amsler's group, identified traces of the respective decay products in the measurement data and was able to reconstruct the decay cascades starting from Xi_b^* decays.
The calculations are based on data from proton-proton collisions at an energy of seven Tera electron volts (TeV) collected by the CMS detector between April and November 2011. A total of 21 Xi_b^* baryon decays were discovered -- statistically sufficient to rule out a statistical fluctuation.
The discovery of the new particle confirms the theory of how quarks bind and therefore helps to understand the strong interaction, one of the four basic forces of physics which determines the structure of matter.
The University of Zurich is involved in the LHC at CERN with three research groups. Professor Amsler's and Professor Chiochia's groups are working on the CMS experiment; Professor Straumann's group is involved in the LHCb experiment.
The CMS detector is designed to measure the energy and momentum of photons, electrons, muons and other charged particles with a high degree of accuracy. Various measuring instruments are arranged in layers in the 12,500-ton detector, with which traces of the particles resulting from the collisions can be recorded. 179 institutions worldwide were involved in developing CMS. In Switzerland, these are the University of Zurich, ETH Zurich and the Paul Scherrer Institute.
- CMS Collaboration. Observation of an excited Xi(b) baryon. Physical Review Letters, Submitted 26 Apr 2012; [link]
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