Quark structure of the Pentaquark(?) Jo Dudek, Jefferson Lab

2 Pentaquark ? baryon seen in must have strangeness = +1 isospin = 0,1 - unless decay is I-violating minimal quark structure then is naïve application of constituent quark model ? ? fall-apart decay mechanism

Jo Dudek, Jefferson Lab3 in expt ? experimental candidate is doubly mysterious why so light? why so narrow? in addition we dont know the low spins usually lightest S -wave decay is unlikely small width makes production mechanism mysterious if is small then why produced by kaon exchange? would require an internal P -wave in quark picture

Jo Dudek, Jefferson Lab4 multitude of states is problem common to all multiquark models

Jo Dudek, Jefferson Lab5

6 achieved prominance through prediction of Diakonov, Polyakov & Petrov in Chiral Soliton Model degrees of freedom are pion & kaon fields baryons are solitons in the pion & kaon fields predicts a with why so light? chiral properties of the model make it reasonable though, since rather than adding constituent quarks to the nucleon mass, we add a pseudo-Goldstone not obvious – scale set by assuming is in the, splitting depends upon (unsettled) nucleon sigma term value why so narrow? relies upon a cancellation between two couplings in the model. accidental? – no obvious physical origin

Jo Dudek, Jefferson Lab7 flavour sym colour antisym spin sym diquarks are identical bosons space Jaffe & Wilczek degree-of-freedom SCALAR DIQUARK

Jo Dudek, Jefferson Lab8 no axials/tensors ad-hoc?ad-hoc? scalar diquarks solve(?) another hadronic puzzle - LIGHT SCALAR MESONS

Jo Dudek, Jefferson Lab9 Light Scalar Mesons a la Jaffe diquarks are light

Jo Dudek, Jefferson Lab10 Width in Fall-Apart Dynamics 1 fm square well nK 1540 MeV CG(F) x CG(C) x CG(LS) fall-apart decay mechanism

Jo Dudek, Jefferson Lab11 Light Scalar Mesons a la Jaffe a multitude of multiquarks the width problem

Jo Dudek, Jefferson Lab12 Lattice QCD lattice field theory on a computer has been used to look at a theory similar to QCD with light quarks - quenched lattice QCD with fairly light quarks no quark loopspion masses down to 500 MeV at these heavier quark masses, our familiar lightest hadron resonances become bound states

Jo Dudek, Jefferson Lab13 Lattice QCD what is the lattice method extracting? hopefully – the lightest asymptotic state with the quantum numbers of a chosen interpolating operator not seen

Jo Dudek, Jefferson Lab14 Lattice QCD for the Lasscock et al

Jo Dudek, Jefferson Lab15 Lattice QCD for the Lasscock et al no bound state observed

Jo Dudek, Jefferson Lab16 SummarySummary existence of the light and narrow pentaquark presents a challenge to many theoretical ideas existence is an issue to be decided by experiment, not theories or models