Observing a heavy bottom baryon

This shows part of the mass spectrum for the Λb*0 candidates. The projection of the corresponding unbinned likelihood fit is superimposed.

Physicists from the CDF collaboration have observed the Λb*0 baryon. This state is made of two light quarks (up and down) and a heavy (bottom) quark. It has exactly the same quark content as the ground state Λb0 baryon. The intriguing difference between the newly observed state and the well-known Λb0 is that the pair of light quarks (u, d) in Λb*0 orbits the heavy b quark with an orbital angular momentum that we denote L=1. Physicists call such states orbitally excited resonances. The newly observed state is extremely unstable, as its decay is driven by the strong force. The Λb*0 disintegrates promptly, in less than a billionth of a trillionth of a second (10-22 seconds) to a stable Λb0 and two pions. The CDF detector allows physicists to reconstruct the paths of the two prompt pions and of the ground state Λb0 particle. Unlike the case of the orbitally excited Λb*0 decay, it is the weak force that is responsible for the decay of the Λb0, which goes slowly (it takes about a billionth of a second).

From the decay particles (Λb0 and two pions), CDF physicists were able, using the full Run II data sample, to obtain a mass spectrum. The distribution is shown in the top figure. The narrow peak at approximately 21 MeV represents the orbitally excited Λb*0 resonance state with a total spin (J) of 3/2. We observe 17 ± 5 signal events. This work confirms the previous observation of the Λb*0 made by the LHCb experiment at the LHC. The mass of the Λb*0 is now determined to be 5919.22 ± 0.84 MeV/c2.

The observation of this heavy bottom baryon provides a new benchmark in the spectrum of hadron states to which predictions by theoretical models can be compared. Measurements of the masses of these states are important for our understanding of quantum chromodynamics, the fundamental theory of strongly interacting matter. Handling the enormous amount of collected data was a particularly challenging part of this analysis.

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edited by Andy Beretvas

These CDF physicists contributed to this data analysis. From left: Sally Seidel, Igor Gorelov and Prabhakar Palni, all from the University of New Mexico.