A joint Fermilab/SLAC publication

Rare particle decays could indicate presence of new physics

03/30/11

Physicists at the LHCb experiment at the Large Hadron Collider recently reported the first observations of a new way that particles called Bs mesons decay into other particles. Studying this particular decay could provide clues as to why the universe is made up of matter rather than antimatter.

Members of the LHCb collaboration in front of their detector.

Collaboration members stand in front of the LHCb detector.

Equal amounts of matter and antimatter existed in the earliest stages of the universe. But when a matter particle comes in contact with its antimatter counterpart, the two annihilate one another and leave behind pure energy. In principle, all the matter and antimatter in the universe should have annihilated. But matter managed to survive, and scientists are seeking to explain why.

B mesons could hold the answer. Although they don’t exist naturally, they can be created easily in high-energy particle collisions. They contain a bottom antiquark and either an up, down, charm or strange quark. A peculiar trait of some B mesons is that they spontaneously transform into their own antiparticles and back before decaying into new particles. Last year, researchers at Fermilab discovered that certain B mesons decayed into matter particles 1 percent more often than they decayed into antimatter particles, which could account for the imbalance in the universe.

The cause of the imbalance could be the meddling of an unseen, heavier particle, one that physicists have never observed.

“New physics can influence the way B mesons decay,” said LHCb physicist Steve Blusk from Syracuse University. “What we’re trying to do is effectively measure this interference.”

Previous experiments have observed B mesons switching between matter and antimatter with good precision. With higher collision energies and up to 40 times as much data, LHCb could narrow down the uncertainties in existing data and finally explain the mechanism behind the oscillations.

If the culprit is a new particle too massive to be seen at the LHC, it could be observed through its indirect effect on B meson decays. Studying the decays will help scientists understand the forces acting behind the scenes that result in B mesons decaying to matter more than antimatter.

In a paper published this week in Physics Letters B, the LHCb group describes a new decay mode of one particular B meson, Bs (pronounced B-sub-s), which contains a bottom antiquark and a strange quark. After being created in the wake of a proton-proton collision, Bs mesons can decay into a J/psi particle (a charm quark bound to a charm antiquark) and an f0 (a strange quark bound to a strange antiquark). These particles then decay further, leaving trajectories inside the detector for the researchers to observe.

By studying the difference between the way the Bs meson and its antiparticle decay to this
particular final state, the group can measure the interference between new physics and the Standard Model. If these effects are quite large, as earlier data suggests, LHCb researchers could discover new physics in the data recorded this year.

“At the very least, this will establish that the Standard Model is not the end of the road,” Blusk said.

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