A University of Melbourne researcher analysed data from the Belle experiment, searching for an invisible new particle produced in B meson decays.
Published in Physical Review Letters, the study by Dr. Daniel Marcantonio examined data from the Belle particle physics experiment at the KEK laboratory in Japan, which collided electrons with positrons, their antimatter counterparts.
The collision energy was tuned to produce B mesons, which are heavy, unstable particles that contain a bottom quark. B mesons are valuable to study because their decay can be sensitive to new, yet-undiscovered particles.
Searching for potential dark matter particles
Dr Marcantonio searched the Belle data for evidence of “feebly interacting particles” (FIPs), a hypothetical particle that interacts extremely rarely with ordinary matter.
Many theories about particle physics predict FIPs, with some theories suggesting that the particles could be candidates for dark matter or as messengers between ordinary matter and a hypothetical “dark sector”.
Marcantonio’s analysis studied five different decay channels in B mesons- three of which had never been searched for before- seeking an invisible new particle accompanied by a known particle such as a pion, kaon, proton or heavier meson.
The Belle dataset contains 711 inverse femtobarns of electron-positron collision data, containing more than 770 million pairs of B mesons. Analysis used the B-tagging technique, where one B meson in each event is fully reconstructed, allowing the properties of the other B meson’s decay to be tightly constrained.
Marcantonio commented: “Because this is the first search for several of these decay channels, I hope it will motivate further exploration of even more variations of these decays, both at Belle II and at other experiments.”
The null result helps narrow down lines of investigation
As no new particles were identified from the data, the analysis suggests there is an upper limit to how often these decays can occur. This does not rule out the existence of the particles entirely, but will help future studies narrow down the parameters and conditions under which they can exist.
These are the tightest such constraints to date for all five channels, which can be translated into constraints on the interaction strength between hypothetical new particles -such as axion-like particles and dark scalars- and documented ones.
Understanding cosmic imbalance
One of the decay channels studied by Marcantonio involved a proton in the final state, which can constrain a theoretical mechanism called “B-mesogenesis”, where it is posited that B meson decays in the early universe could have channelled antimatter into a dark sector, which could explain the matter-dominated universe observed today.
The analysis rules out B-mesogenesis for a range of masses of the hypothetical dark matter particles and Marcantonio hopes that this will help focus future studies on physics beyond the Standard Model.
“These results are applicable to more theoretical models than we were able to cover in the paper, and I hope the broader community will use them to constrain even more scenarios than we have considered.”