LHCb May Have Just Discovered Pentaquarks

In what may be the biggest non-Higgs news yet to come out of the LHC, the LHCb experiment has a brand new preprint reporting on a measurement of what looks like a couple new resonances consistent with pentaquarks – a theoretical particle that appears as a bound state of four quarks and an antiquark (or four antiquarks and a quark). The paper has been submitted to PRL, which in high energy physics is the most prestigious journal. Hopefully everything checks out and this really is a new discovery.

The analysis looks at decays of the neutral Λb baryon, which has a quark content of udb, a mass of 5.6 GeV and a lifetime of 1.4 ps. One of its more prominent decay modes is to a J/ψ (charmonium meson) and a regular Λ baryon, which generally decays to a pion and a nucleon. This paper looks at a similar final state of  Λb→J/ψ K p. The presence of the kaon means that the Kp system could not be from a Λ, since a Λ isn’t heavy enough. They also only consider the case where the J/ψ decays to muons only, allowing its invariant mass to be reconstructed very accurately.

LHCb shows that they can reconstruct the invariant mass peak of the Λb using the three hadron final state. Where things get interesting is the two-particle invariant masses. Instead of looking at all three particles, they look at the invariant mass of the Kp system and the J/ψ p system. The Kp system shows many different peaks and significant deviation from the expectation just from random throws in phase space. This is expected since many excited Λ resonances exist that can decay into this mode. In the J/ψ p no such peaks are expected. However, there is a prominent bump in the spectrum around 4.4-4.5 GeV that looks very statistically significant. LHCb finds that the two spectra are modeled pretty well if they add two new resonances, one at 4450 MeV and a wider one at 4380. Looking in more detail at these peaks, they also find that things are best modeled by particles with total spins of J= 3/2 and 5/2 (apparently they can’t really tell which particle has which spin) and with opposite parities. Since everything here appears to point to new particle, they have been labeled Pc(4450)+ and Pc(4380)+. The widths indicate that these particles have lifetimes of order 10-23 s or so, which is indicative of strong decays. Strong decays preserve the quark content, so it seems likely that these particles would need to be pentaquarks with a quark content of uudc̅c.

Physicists thought that they found pentaquarks in the 2000s, but further studies suggested that that discovery was really just a statistical fluctuation. I think that this past experience means that we should be careful before trumpeting a definitive discovery, but this paper looks pretty convincing to me (I’m not an expert on precision b physics though). If this really is a new discovery, then we should continue to see better and better precision as more data is taken. If Λb particles can be created in large numbers in a lower energy collider such as one of the b factories, that might be the best way to really pin down the properties of this possible new class of particles.