Particle Physics by Martin B. R. Shaw G. & G. Shaw

Author:Martin, B. R.,Shaw, G. & G. Shaw
Language: eng
Format: epub
ISBN: 9781118912218
Publisher: John Wiley & Sons, Inc.
Published: 2016-11-05T00:00:00+00:00

7.1.3.(c) Exotic baryons

From time to time, experiments have claimed evidence for the existence of baryons with exotic quantum numbers, but as more experiments have been performed and more data accumulated, the evidence has receded. The present consensus is that there is no convincing evidence for such baryons. However, following the discovery of the tetraquark–charmonium states described above, it was natural to search for the analogous pentaquark–charmonium states , where q = u, d, s, which would not have exotic quantum numbers. The first evidence for such states was published in 2015 by the LHCb Collaboration, who used the detector described in Section 4.5.2. This detector was designed specifically to study the decays of short-lived particles containing heavy quarks, which are produced in large numbers at the LHC. In this case, it was used to measure about 26 000 decays of the form

(7.10a)

Here Λ0b denotes Λ0b(5624) = ubd, the lightest baryon with a non-zero bottom quantum number. The main contribution to the decay (7.10a) is expected to arise from the sequence

(7.10b)

where Λ* is any resonant excitation of the Λ(1116) = uds state discussed in Section 3.4. This is confirmed by the observed invariant mass distribution of the K−p pairs shown in Figure 7.11(a), which shows a prominent peak corresponding to the Λ(1520) = uds resonance and a broader enhancement at higher masses to which several known Λ* states can contribute. In addition, the J/ψp mass distribution, shown in Figure 7.11(b), exhibits a peak at about , which is naturally interpreted as a J/ψp resonance P+c occurring in the decay sequence

(7.10c)

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