Advanced hadron identification methods for Belle II

After more than ten years of planning, research and development, design, and construction, the Belle II detector at the SuperKEKB electron-positron accelerator is nearly ready for operation. The accelerator will operate at the Upsilon(4S) resonance where pairs of quantum-mechanically entangled B mesons are produced with no additional particles. With its unprecedented performance, Belle II/SuperKEKB will allow for searches for new phenomena beyond the Standard Model of elementary particles and their interactions, complementary to the efforts at the Large Hadron Collider (LHC).

The main purpose of the research with the Belle II detector is to search for new physics phenomena by investigating some selected very rare processes in decays of B and D mesons. Recently, three experiments (LHCb, BaBar, and Belle) have found consistent discrepancies between their results and predictions of SM in certain classes of decays of B mesons that involve leptons in the final state. These include the rates of B → D(*) tau nu decays compared to B → D(*) mu nu and B → D(*) e nu decays, as well as the ratio of rates of B -) K mu mu and B -) K e+e− transitions. These surprising results imply that lepton flavour universality (LFU), the equality of how leptons interact, is violated; the results are extremely interesting, as the LFU is one of the cornerstones of the SM. If experimentally proven, the LFU violation would be a ground-breaking change in the understanding of elementary particles interactions, not only because of the SM breakdown but also because it would point to the presence of NP in nature. At present, these findings are not yet conclusive because of the relatively large experimental uncertainties. Further studies with much larger data samples and methods enabling a reduction of systematic uncertainties should, therefore, allow an investigation these very promising effects and look for others in related processes, and for such a research programme, Belle II is an ideal tool.

For the success of this research programme, novel, highly advanced methods are needed for the identification of charged particles; they are crucial in measurements of extremely rare processes where the sensitivity to a possible contribution of NP is largest, in order to suppress backgrounds arising from other decays, which are sometimes many orders of magnitude more abundant. A typical example of the impact of excellent particle identification in studies of very rare decays is shown in Fig. 1: the final state of a very rare decay B -) rho gamma can only be separated from the much more copious background reaction B -) K* gamma when very advanced identification methods are employed. We also note that in some cases particle identification constitutes the largest contribution to the systematic error of the measurement. The advanced particle identification system of Belle II has detector systems for separation of pions from kaons over the full kinematic range of the experiment (from a few hundred MeV/c up to 4 GeV/c), as well as components to identify electrons and muons.

In order to cover such a wide kinematic are region, our project team, in cooperation with physicists from Japan, USA and Italy, developed two types of advanced ring imaging Cherenkov detectors.

The subject of the proposed research is the development of very advanced methods for the identification of charged pions and kaons that will exploit these new types of detectors and will be adapted to the operation in high-level background conditions as expected in the Belle II detector at the SuperKEKB accelerator.