Description of the thesis topic

Title: Search for the b → s nu anti-nu transitions with semi-leptonic tag at Belle II

The B → K(*) nu anti-nu decays are a formidable place to look for the new physics needed to complete the standard model of particle physics. On the one hand in the standard model their decay rate is small and well known so that interference with new dynamics could modify its decay rate by a sizeable amount. On the other hand the two final state neutrinos are not observed directly, thus these processes are also sensitive to new invisible particles like those that could compose dark matter.

Belle II will remain in the foreseeable future the only experiment allowing to observe B → K(*) nu anti-nu decays . In this experiment pairs of B mesons are produced in electron-positron collisions. As neutrinos from the signal are undetected information from the other B mesons of the event (referred to as tag B) must be used to constrain their kinematic and correctly identify the signal decay. There are currently two strategies to reconstruct the tag B: the tagged approach where the tag B is reconstructed via a hadronic or a semi-leptonic decay and the untagged approach where the signal is reconstructed first and the rest of the event is assumed to be composed of only the other B. The untagged approach allows the higher efficiency among the three approaches at the price of a lower purity. The hadronic tag allows the best purity and the best resolution on the di-neutrino invariant mass but has a low efficiency. The semi-leptonic tag has intermediate performance in terms of efficiency and purity. The Belle II experiment recently provided the first evidence for the B+ → K+ nu anti-nu decay using the untagged approach and the tagged approach using hadronic decays [1].

The PhD student will search for B → K(*) nu anti-nu decays using the semi-leptonic tag. They will also develop new kinematic variables inspired to supersymmetric searches that optimally exploit the topology of pair-produced heavy particles decaying semi-invisibly [2].

The candidate should be proficient in particle physics They should have analysis and software skills (ROOT, Python, C++) and should be able to communicate in English (at level B2/C1 according to the Common European Framework of Reference for Languages). Experience in B physics would be an advantage but is not mandatory.

[1] https://arxiv.org/abs/2311.14647
[2] https://arxiv.org/abs/2209.03387

Work Context

The Institut Pluridisciplinaire Hubert CURIEN (IPHC), a research facility in conjunction of CNRS and the Université de Strasbourg (UMR7178), is a multidisciplinary laboratory where research teams of different scientific background (ecology, physiology and ethnology, chemistry and sub-atomic physics) develop high level research programs with strong links to scientific instrumentation. The IPHC is organised in three departments and hosts 393 employees in total, amongst them 257 with permanent contacts (119 staff scientists and associated professors, 138 engineers and technicians), 46 fixed term contracts and 102 PhDs.

The PhD student will be welcomed into the Belle II team which includes 4 researchers, two post-doctoral researchers and two doctoral students. The PhD student will become a member of the Belle II international collaboration. Travels to the experiment site in Japan and to national and international conferences and workshops will be planned.

The thesis will be in co-supervision experiment/theory. The PhD student will be supervised by Giulio Dujany and Jérôme Baudot (IPHC Strasbourg) for the experimental side and by Diego Guadagnoli (LAPTh Annecy) for the theory side. A long stay at Annecy is foreseen.

Additional Information

The candidate must hold a French master 2 degree or a comparable international master degree. Applications must include a CV, a motivation letter and the grades of the master studies.
Skills :
- Advanced level Python, C++.
- Advanced level of English at level B2/C1 according to the Common European Framework of Reference for Languages