Peter Križan (FMF UL and JSI)

My research at Belle and Belle II – the most recent ten years

In the last ten years, my research focused on experimental precision flavour physics with the Belle experiment, preparation of the Belle II experiment, novel identification methods based on Cherenkov radiation and their applications in detectors for medical imaging. In autumn 2020 this research got a further boost by a five-year ERC Advanced Grant FAIME.


Experimental precision flavour physics with the Belle and Belle II detectors

With my research team, we have carried out several very important studies that helped to shape flavour physics in the last decade. My main research interest were studies of particle-antiparticle asymmetries (CP violation) in the B and D meson systems and investigations of rare transitions of B and D mesons, all by using data collected with the Belle detector.

A selected list of the most important papers in this research area (last ten years):

-        M. Starič et al., Phys. Lett. B753 (2016) 412-418: measurement of particle-antiparticle mixing in the neutral D meson system and search for CP violation in the KK and pi pi final states, a follow-up of our pioneering first observation of D mixing by M. Starič in 2007.


-        A. Zupanc et al., Phys. Rev. D80 (2009) 052006: the first measurement of D mixing in the neutral D meson system in the CP=-1 final state with D -> phi KS (PhD Thesis of A. Zupanc)


-        A. Zupanc et al., JHEP 1309 (2013) 139: measurement of the rare leptonic Ds meson decay Ds -> mu nu and extraction of the Ds decay constant (using a novel method, based on the associated D meson production)


-        L. Šantelj et al., JHEP 1410 (2014) 165 and S. Fratina et al., Phys. Rev. Lett. 98 (2007) 221802: measurements of CP violation in B-meson decays, B -> DD and B -> eta KS, in a search for New Physics effects (PhD Theses of my students).


-        M. Starič et al., Phys. Rev. Lett. 108 (2012) 071801: search for CP violation in D -> phi pi  decays.


Members of my team have served as conveners of the most active Belle analysis working group, and have in this capacity overseen and coordinated a large number of Belle publications. We made an essential contribution to the monumental 900-page review of the results of a decade of research of the Belle and BaBar experiments (Eur. Phys. J. C74 (2014) 3026) with B. Golob as one of the five general editors, and A. Zupanc and myself as section editors.


Preparation of the Belle II experiment

My research team has acted as one of the core teams and initiators of the project:

-        I have acted as the first – foundingspokesperson (scientific leader) of the international research team for two terms 2009-2013 after having chaired a steering committee to set up the collaboration (2008-2009). In this capacity, I contributed to the concept and the design of the Belle II spectrometer as well as to the research program of the experiment (invited review in Int. J. Mod. Phys. A30 (2015) 1530029).


-        In the period of 2015-2020 I have been coordinating the construction of the Belle II detector as the Technical Coordinator.


-        I have co-chaired a very intense 4-week workshop at MIAPP Munich (November 2016) to define the physics program of Belle II; I am a co-author of a comprehensive report on the Belle II physics reach and program (E. Kou et al., arXiv:1808.10567 [hep-ex], published as PTEP 2019 (2019) 12, 123C01, PTEP 2020 (2020) 2, 029201 (erratum)


-        My team has significant responsibilities in two novel particle identification devices (see below)


-        The team members have/had several key responsibilities within the collaboration: B. Golob acted as the Physics Coordinator; S. Korpar is leading the RICH detector team in the forward region; M. Bračko, M. Starič, A. Zupanc, and L. Šantelj have been coordinating analysis and software working groups.



Development of novel identification methods, mainly based on Cherenkov radiation


-           I have led several studies of novel particle identification methods that are summarized in my invited comprehensive review, JINST 4 (2009) P11017


-           With my team, I have developed a novel proximity focusing Ring Imaging Cherenkov (RICH) counter with a focusing multilayer radiator (Nucl. Instr. and Meth. A 548 (2005) 383)


-           With my team members we have contributed decisively to the design, preparation, construction, and commissioning of the RICH counter in the forward region of the Belle II experiment where this focusing radiator concept is essential; the finalized apparatus has contributed in an essential way to the success of the first data taking period with the fully equipped Belle II spectrometer in spring 2019.


-           We have led the development of simulation and reconstruction methods for the Time-Of-Propagation counter, a Cherenkov imaging  identification device in the barrel region of the Belle II experiment, and have contributed in an essential way to the conceptual design (M. Starič et al, Nucl. Instrum. Meth. A595 (2008) 252-255); the apparatus has been successfully commissioned in the 2018 campaign.


-           With my team members, we contributed substantially to the advancement in the understanding of fast light sensors, able to detect single photons. Starting from wire chambers with CsI photocathode that we developed in the nineties, we developed, designed, constructed and operated the first full-scale RICH detector with multi-anode photomultiplier tubes for the HERA-B experiment. We then pioneered fast single-photon timing with microchannel plate PMTs, the development of novel hybrid avalanche photodetectors for the Belle II RICH detector, and developed and tested the first RICH prototype with SiPMs as photo-sensors  (invited review in Annu. Rev. Nucl. Part. Sci. 2013. 63:329–49)


-           Based on our experience with fast Cherenkov detectors, we found a novel way to considerably improve the time resolution in time-of-flight positron emission tomography (PET) medical imaging by detecting gamma rays through Cherenkov effect rather than through scintillations.