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Strange baryons and exotic resonances in beauty baryon decays

Periodic Reporting for period 1 - LHCbBaryonAmplitudes (Strange baryons and exotic resonances in beauty baryon decays)

Reporting period: 2015-10-07 to 2017-10-06

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 established 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 of hadrons. 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, as any sensible explanation of observed exotic hadrons predict many others, not yet seen. 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 as nobody ever searched for these decays of Lambda_b baryon it is not yet clear how large statistics these will provide. To make first step, this project just exactly that search.

Overall, the project was partially successful. Significant amount of work was done towards achieving scientific objectives, but the analysis is not fully finished and cleared by the LHCb collaboration for public presentation. This is down to unforeseen delay in producing simulated samples needed to estimate efficiencies to detect various decays. The delay was affecting large fraction of the collaboration. The project would need another 3-4 months of work to finalise results to the point when they could be presented outside the LHCb collaboration. The researcher was restarting his scientific career and this restart was successful. The work on the project also resulted in researcher gaining new transferable skills, which are widely applicable.
During the project search for decays Lambda_b -> J/psi Lambda pi+ pi-, Lambda_b -> J/psi Lambda K+/- pi-/+ and Lambda_b -> J/psi Lambda K+ K- is performed. Such measurement is rather challenging, because kaons and pions can be misidentified in experiment and if that happens one decay appears in sample for the other decay. In this way, all of these decays cross-feed between each other, which makes extraction of the number of decays challenging task. This is further complicated by specific way how Lambda baryons are detected in the LHCb detector and the fact that another particle, KS0 can be misidentified as Lambda baryon. In order to reliably extract yields, simultaneous analysis of many samples is necessary. This issue was solved and analysis is close to completion. Once finished, we will be in position to answer question, whether search for pentaquark state with strange quark is possible in near future, or whether much larger samples would be needed, in which case, this can guide ideas for future experiments design.

The researcher significantly contributed to one scientific publication (Phys. Lett. B772 (2017) 265) on discovery of Xi_b- --> J/psi Lambda K- decay, a topic which was closely related to the objectives of this project. As the analysis performed as main part of the project was delayed, it does not had any scientific output yet. Besides the scientific output, researcher had active role in explaining research done by the LHCb collaboration to a wider audience and specially to secondary school students. Researched actively participated in organising yearly International Masterclass event (http://www.physicsmasterclasses.org/index.php) during the project and the ``Antimatter matters'' exhibition at The Big Bang Fair (https://www.thebigbangfair.co.uk) major event which aims at bringing STEM subjects to attention of secondary school students.
The understanding of hadrons is long standing question in particle physics. Despite many years of work spent both on experiment and theory side, there are many unanswered questions. It is also clear that in order to answer at least some of those questions, much more experimental information is needed and it is not even always clear what experiments to do in order to obtain such information. This project tries to address question, whether after observation of first pentaquark baryons LHCb experiment can make further advancement to search for additional pentaquark baryons. Without being able to find more of them and obtain more systematic information, their understanding is going to be very limited.