Marie Sklodowska-Curie fellowship
Fellow: Jean Wicht
Scientist in charge: Michal Kreps
Summary of the project:
Over the last fifteen years spectroscopy, the study of the hadronic states, has received a lot of attention. This has been driven by the observation of many new 'exotic' states that do not fit into the traditional picture of mesons (composed of a quark-antiquark pair) and baryons (three quarks) in the constituent quark model. One prominent example is the Z(4430)+ particle, which was discovered in 2008 by the Belle experiment and was recently confirmed by LHCb. Its nature sparked a lot of interest in the theory community because it is charged and decays to charmonium. The LHCb analysis establishes for the first time that the Z(4430)+ is a four quark state. The next bit of puzzle came in 2015, when LHCb collaboration announced observation of two exotic baryons (called pentaquarks later), which are analogues of the Z(4430)+ state and which cannot be accommodated in the traditional picture. It was long standing question, why all hadrons discovered could fit to very simple picture despite not having reason why it should be case and with recent observations this question is settled. But it opens much bigger question, because any sensible explanation of these exotic hadrons predict many others. In fact we do not even have clear understanding of internal structure of those which are already known. In order to understand exotic hadrons, much more experimental information is needed.
The pentaquark states observed would be in terms of quark described as states having charm quark and antiquark along with two up quarks and one down quark. But if such hadrons exists, there should be also hadrons where one of the up quark is exchange for strange quark. With view of this, in this project we embarked on search for decays of Lambda_b baryon to J/psi meson along with Lambda baryon and pair of other particles (pions or kaons). These would be possible places where to search for pentaquarks decaying to J/psi and Lambda, but nobody ever searched for these decays of Lambda_b baryon. To make first step, this project just exactly that search. Outcome of the measurement is promising and will show interesting results once it is finalised.