Rare Meson Decay Studied by NSU and BINP Researchers

One of the collaborators, a researcher from BINP SB RAS and an instructor at the Chair of Theoretical Physics at NSU Leonid Kardapoltsev speaks on the search for the η′→e+e− decay and the results of the collective research.

Particle interaction sheds light on the processes in nature going on at very short distances. For instance, the η′-meson is an unstable particle with a short lifetime and fast decay. The meson has different decay modes, with some of them being rare and the choice of the mode being random. Every mode has its own probability, which can be measured, says Leonid Kardapoltsev.

Mesons are made of a quark and its antiquark and have a baryon number B = 0. They also have a whole spin number (possibly 0) and are essentially bosons. The eta (η) and eta prime meson (η′) are neutral particles with 0 isospin and hidden strangeness.

The team of scientists decided to measure the η′→e+e- decay by conducting an experiment at the VEPP-2000 e+e- collider, which had been created at BINP, using the SND detector. Leonid says that the advantage of studying such decays makes it possible to study the interaction between η′ and photons.

While studying the decay, one part of the process is described through electromagnetic interaction and can be measured precisely, whereas the other, strong part, when the η′ interacts with a pair of virtual photons, is described via a form-factor. Having measured the probability of η′→e+e- decay, we can measure this form-factor. Such experiments are important for both understanding the composition of η′ and for the hadronic light-by-light scattering contribution to the muon's anomalous magnetic moment.

There are a number of ways to measure the tiny values of η′→e+e- decay probability, explains Leonid. The most obvious one is through the reaction generating the η; then we gather statistic data and look for e+e- decays. Another way is to use the reverse reaction, when the electron and positron dematerialize into the η′. During the latter experiment we can dramatically decrease the influence of other processes and thus improve the measurement accuracy.

The researcher points out that although the results obtained are not a sensation, the experiment is quite important for further investigations. We have managed to obtain an almost vanishing background level for the process under study and get a high sensitivity. Working with the VEPP-2000 collider for only two weeks and using the energy equal to the mass of the η′, we were able to downgrade the upper limit of η′→e+e- decay about 50-fold. I’d like to emphasize the importance of the system which contributed to such high sensitivity. It is the system for precision measurement of beam energy in the collider, which was developed in collaboration with NSU.

The article Search for the η′ →e+e- decay with the SND detector has been published in Physical Review D — Particles, Fields, Gravitation and Cosmology. A report about the study was delivered at the international conference Phi to Psi 2015 in China.

Prepared by Anastasiya Anikina