Observation ofJ/ψpResonances Consistent with Pentaquark States inΛb0

Abstract

Observations of exotic structures in the J=ψp channel, which we refer to as charmonium-pentaquark states, in Λ 0 b → J=ψK − p decays are presented. The data sample corresponds to an integrated luminosity of 3 fb −1 acquired with the LHCb detector from 7 and 8 TeV pp collisions. An amplitude analysis of the three-body final state reproduces the two-body mass and angular distributions. To obtain a satisfactory fit of the structures seen in the J=ψp mass spectrum, it is necessary to include two Breit-Wigner amplitudes that each describe a resonant state. The significance of each of these resonances is more than 9 standard deviations. One has a mass of 4380 AE 8 AE 29 MeV and a width of 205 AE 18 AE 86 MeV, while the second is narrower, with a mass of 4449.8 AE 1.7 AE 2.5 MeV and a width of 39 AE 5 AE 19 MeV. The preferred J P assignments are of opposite parity, with one state having spin 3=2 and the other 5=2.

Baryons as relativistic three-quark bound states

“…One could construct antisymmetric spin-flavour combinations, although this would yield (2,8) and (4, 1) and thus no decuplet baryons in orbital ground states (the ∆ ++ is a particularly apposite example). The situation can be remedied by introducing a new quantum number, namely the SU (3) of colour [99][100][101], and hence an antisymmetric component Ψ colour to the baryon wave function:…”

“…[282,283] for recent reviews. In the baryon sector, five-quark states are of considerable interest especially since the experimental signature of a potential pentaquark has been reported at LHCb [2]. However, besides this new state (and its potential cousins detected in the future) there are also well-established states such as the Λ(1405) that are heavily debated since their location in the baryon spectrum does not agree with (potentially naive) expectations from the quark model.…”

“…K (2) and K (3) are the irreducible two-and three-body kernels. The Bethe-Salpeter wave functions are the bound-state amplitudes with dressed quark propagators attached, so the inverse propagators that appear in…”

“…Observe that because P f,i are the onshell momenta one has 17) where τ = Q 2 /(4M 2 ), and therefore only Q 2 remains as an independent variable. Hence, for spacelike photon virtualities Q 2 > 0 the momenta Q and q are real (the loop momenta are always real) whereas the average 13 By comparison, the analogous current matrix element for mesons has only two topologies: the impulse approximation and a diagram with K [µ] (2) . In that case a rainbow-ladder truncation does indeed induce the impulse approximation.…”

“…states with three valence quarks and an additional valence quark-antiquark pair. Especially the latter possibility has generated a tremendous amount of interest over the years with many renewed activities after the recent report of an experimental signature of a potential pentaquark at LHCb [2]. In addition, well-established states such as the Λ(1405) have been discussed to contain a strong 'penta'KN -component that may be responsible for its unexpectedly low mass.…”