Researchers have discovered a previously unknown particle composed of three quarks in the Large Hadron Collider (LHC) particle accelerator. The baryon known as Xi_b^* confirms fundamental assumptions of physics regarding the binding of quarks.

 The baryon family refers to particles that are made up of three quarks and quarks form a group of six particles that differ in their masses and charges. The two lightest quarks, “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. 

During proton collisions in the LHC, physicists Claude Amsler, Vincenzo Chiochia and Ernest Aguiló from the University of Zurich’s Physics Institute detected 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 mass difference of the peak is 14.84 +/- 0.74 (stat.) +/- 0.28 (syst.) MeV. The new state most likely corresponds to the Xi(b)^{*0} baryon, the J^P=3/2^+ excitation of the Xi(b)^0.


Observation of an excited Xi_b baryon. Credit: CMS collaboration

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. 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 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 – 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.

CMS Collaboration, Observation of an excited Xi(b) baryon, Submitted to Physical Review Letters, http://arxiv.org/abs/1204.5955v1