LHCb Discovers New Resonances

LHCb has uploaded a new preprint presenting the discovery of two new resonances. Various media reports have been breathlessly declaring this to be the discovery of two new particles which, while not technically incorrect, is quite misleading. What they found was what looks like two new resonances (very short lived particles) related to a bound state of quarks. The headlines imply that these are some totally new phenomenon such as a new elementary particle, but in reality these look like excited states of an already-known composite particle.

In the analysis, LHCb looked at the center of mass energy of Ξb0 + π final states in proton-proton collisions. The  Ξb0 (the Ξ family are called xi baryons or cascade particles depending on who you talk to) is a baryon (like a proton or neutron) with an up-strange-bottom valence quark structure and an angular momentum (spin+rotational) of 1/2. Two peaks in the spectrum are seen with an energy that is very slightly different than the total mass of the final state particles. These peaks are generally interpreted as new bound states. These decays should occur via the strong force, and the width (inverse lifetime) of the heavier particle is large enough to be measured to a precision of around 20%.

Since the particles must decay strongly, the final state indicates that they have a down-strange-bottom valence structure, giving a charge of -1. One is identified as having an an angular momentum of 1/2 and the other with 3/2. This interpretation is based on predictions of various models but confirmation with data may come in the future when more data is analyzed. Measurements of the mass splittings and widths can help refine theoretical models of baryons, which are actually much more complex than just three quarks orbiting around each other.

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