First physics for Belle II

10 April 2020
Belle II

The Belle II collaboration at the SuperKEKB collider in Japan has published its first physics analysis: a search for Z′ bosons, which are hypothesised to couple the Standard Model (SM) with the dark sector. The team scoured four months of data from a pilot run in 2018 for evidence of invisibly decaying Z′ bosons in the process e+e→μ+μZ′, and for  lepton-flavour violating Z′ bosons in e+e→e±μZ′, by looking for missing energy recoiling against two clean lepton tracks. “This is the first ever search for the process e+e→μ+μZ′ where the Z′ decays invisibly,” says Belle II spokesperson Toru Iijima of Nagoya University.

The team did not find any excess of events, yielding preliminary sensitivity to the coupling g′ in the so-called Lμ−Lτ extension of the SM, wherein the Z′ couples only to muon and tau-lepton flavoured SM particles and the dark sector. This model also has the potential to explain anomalies in b → sμ+μ decays reported by LHCb and the longstanding muon g-2 anomaly, claims the team.

Belle II Z

The results come a little over a year since the first collisions were recorded in the fully instrumented Belle II detector on 25 March 2019. Following in the footsteps of Belle at the KEKB facility, the new SuperKEKB b-factory plans to achieve a 40-fold increase on the luminosity of its predecessor, which ran from 1999 to 2010. First turns were achieved in February 2016, and first collisions between its asymmetric-energy electron and positron beams were achieved in April 2018. The machine has now reached a luminosity of 1.4 × 1034 cm-2 s-1 and is currently integrating around 0.7 fb-1 each day, exceeding the peak luminosity of the former PEP-II/BaBar facility at SLAC, notes Iijima.

By summer the team aims to exceed the Belle/KEKB record of 2.1 × 1034 cm-2 s-1 by implementing a nonlinear “crab waist” focusing scheme. First used at the electron-positron collider DAΦNE at INFN Frascati, and not to be confused with the crab-crossing technology used to boost the luminosity at KEKB and planned for the high-luminosity LHC, the scheme stabilises e+e beam-beam blowup using carefully tuned sextupole magnets located symmetrically on either side of the interaction point. “The 100 fb-1 sample which we plan to integrate by summer will allow us to provide our first interesting results in B physics,” says Tom Browder of the University of Hawaii, who was Belle II spokesperson until last year.

Flavour debut

Belle II will make its debut in flavour physics at a vibrant moment, complementing  efforts to resolve hints of anomalies seen at the LHC, such as the recent test of lepton-flavour universality in beauty-baryon decays by the LHCb collaboration.

We will then look for the star attraction of the dark sector, the dark photon

Tom Browder

As well as updating searches for  invisible decays of the Z′ with one to two orders of magnitude more data, Belle II will now conduct further dark-sector studies including a search for axion-like particles decaying to two photons, the Z′ decaying to visible final states and dark-Higgstrahlung with a μ+μ pair and missing energy, explains Browder. “We will then look for the star attraction of the dark sector, the dark photon, with the difficult signature of e+e to a photon and nothing else.”


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