The LHCb collaboration has discovered two new particles, the Ξ´–b and Ξ*–b. Predicted to exist by the quark model, they are both baryons containing three quarks, in this case, b, s and d. The new particles – which thanks to the heavyweight b quarks are more than six times as massive as the proton – join the Ξ–b, found several years ago by the D0 and CDF experiments at Fermilab.
The three particles are differentiated by the spin, j, of the sd diquark, and the overall spin-parity, JP, of the baryon, and in turn the relative spins of the quarks affect the masses of the particles. With j = 0 and JP = ½+, the Ξ–b is the lightest, and so decays relatively slowly through the weak interaction, leading to its discovery at Fermilab’s Tevatron. The Ξ´–b and Ξ*–b have j = 1, and JP = ½+ and JP = 3/2+, respectively, and should decay either strongly or electromagnetically, depending on their masses.
LHCb analysed proton–proton collision data from the LHC corresponding to an integrated luminosity of 3.0 fb–1, to observe the new particles through their decay to Ξ0b π–. A third of the data were collected at a centre-of-mass energy of 7 TeV, the remainder at 8 TeV. Signal candidates were reconstructed in the final state Ξ0b π–, where the Ξ0b was identified through its decay Ξ0b → Ξ+c π–, Ξ+c → p K– π+.
The figure shows the distribution of δm, defined as the invariant mass of the Ξ0 bπ– pair minus the sum of the π– mass and the measured Ξ0b mass. This definition means that the lightest possible mass for the Ξ0b π– pair – the threshold for the decay – is at δm = 0. The two peaks are clear observation of the Ξ´–b(left) and Ξ*–b (right) baryons above the hatched-red histogram representing the expected background. The Ξ*–b is clearly the more unstable of the two, because its peak is wider. This is consistent with the pattern of masses: the Ξ´–bmass is just slightly above the energy threshold, so it can decay to Ξ0b π–, but only just – its width is consistent with zero, with an upper limit of Γ(Ξ´–b) < 0.08 MeV at 95% confidence level.
The results show the extraordinary precision of which LHCb is capable: the mass difference between the Ξ´–b and the Ξ0b is measured with an uncertainty of about 0.02 MeV/c2, less than four-millionths of the Ξ0b mass. By observing these particles and measuring their properties with such accuracy, LHCb is making a stringent test of models of nonperturbative QCD. Theorists will be able to use these measurements as an anchor point for future predictions.
Further reading
LHCb Collaboration 2014 arXiv:1411.4849 [hep-ex].