By comparing standard theory and experiment, they show a discrepancy which can indicate new physics

The search for physics beyond the Standard Model which until now has explained all interactions between the elementary particles, is an important area of research. Now, there is evidence that the Standard Model is not a complete description of the microscopic world of elementary particles. Scientists at Institute of Mathematical Sciences (IMSc), Chennai, have come up with a calculation based on the observations made at the LHCb (Large Hadron Collider beauty) experiment at CERN, which shows definitively that in a specific interaction, the standard model predictions are violated and there is indirect evidence of new physics. The research is published in the journal Physical Review D.

Suppressed reaction

The interaction that has been studied involves the decay of the B meson into a lighter boson called the vector-Kaon meson and a pair of muon (a heavier cousin of the electron) and its antiparticle. The process is interesting as it involves a change in a quantum number known as “flavour” without the change of charge. The process can happen only because of the quantum nature of elementary particles and involves virtual creation of heavier particles for a very short time — a process known as the loop diagram.

It is known that loop processes themselves are very tiny in relation to louder signals from more dominant processes. Flavour-changing loop processes like this one are further suppressed because of the nature of flavour-changing neutral current involved. Because of this weakness of the signal, the process is sensitive to violation of the Standard Model. Rusa Mandal and Rahul Sinha of IMSc, Chennai, have pointed out the precise discrepancy that shows up when the theoretical calculation based on the Standard Model is compared with the precise experimental observations from the LHCb at CERN.

First mooted

“It was at IMSc in 1996 and 1999 that we first showed how this mode would allow a plethora of related observables to be measured allowing for a very sensitive search for new physics,” says Dr. Sinha. “Today, the study of this mode attracts the most attention among physicists studying flavour-changing loop processes and is a focus at all major high energy physics conferences in the world.”

When asked to comment on this work, Prof. S Umasankar from IIT Bombay, who is not part of this group, said: “This is an important result resting on the premise that the present consensus on theoretical understanding is correct. The only previous result looked at a different variable and found a discrepancy at 4-sigma. In this paper, the authors have obtained a difference that is more than 5 sigma in a large number of cases. It is a result that both theoreticians and experimentalists should scrutinise with utmost seriousness.”

Identification of this mode of decay as the one that would show up new physics has resulted in many physicists around the world studying it. While other groups have also been pursuing this line, the approach of the IMSc group has nearly eliminated all hadronic uncertainties.

The researchers are now working on a paper that outlines more clearly the exact nature of the new physics involved.