Raynette van Tonder appointed convenor of the Semileptonic Physics Group of Belle II
ETP PostDoc Raynette van Tonder has been appointed co-convener of the “Semileptonic and Missing Energy” (or short “S(L)”) physics group of Belle II. Belle II, located at the SuperKEKB accelerator in Japan, has a diverse Physics program and is organized into eight subgroups covering different areas, from charm and rare B meson decays to measurements of time-dependent CP violation.
Analyses in the S(L) working group include precision measurements of semileptonic or leptonic B-meson decays, involving a final state with a lepton-neutrino pair. The neutrino remains unobserved as it fully traverses through the Belle II Detector and is thus classified as “missing energy”. Belle II is the perfect laboratory to study decays with neutral particles, since the unique experimental setup provides a clean environment with lower backgrounds than generally encountered at hadron colliders like LHCb.
“I’m thrilled to co-convene this physics working group because we have almost 70 motivated physicists worldwide working on more than 20 different analyses.”
Semileptonic B decays, exhibiting clean experimental signatures and controllable theoretical uncertainties, provide an ideal toolkit to test lepton flavor universality (LFU). Due to the cancellation of various systematic uncertainties, measurements of ratios of decay rates can be achieved to a high precision - providing stringent tests of LFU. The main driver of the current anomalies in the field of flavor physics involve ratios with semitauonic decays, namely R(D(*)). The tension between the average R(D(*)) measurements of different experiments and the corresponding Standard Model predictions currently stands at 3.6σ. Additional measurements of various b → cτν decays and other observables can be studied to probe LFU, including the D* longitudinal polarization and angular asymmetries in differential decays rates. Another topic of the S(L) working group is precision measurements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements |Vub| and |Vcb|. Determinations of |Vub| and |Vcb| are extracted by employing two complementary approaches: the exclusive approach focuses on the reconstruction of a specific decay mode, while the inclusive approach aims to measure the sum of all possible final states entailing the same quark-level transition. Current world averages of |Vub| and |Vcb| from exclusive and inclusive determinations exhibit disagreements of approximately 3σ between both techniques, posing a longstanding, unsolved puzzle.
Dr. van Tonder takes over convenership as many analysts are starting to include both Run 1 and Run 2 collision data in their studies, allowing previously statistically limited measurements to show exciting results with world leading precision at upcoming major conferences.
“My personal goal for my convenership apart from thoroughly reviewing analyses and ensuring timely delivery of results, is to encourage innovative analyses performing groundbreaking measurements to confirm or deny the existing anomalies in flavor physics.”