Physicists working with the LHCb collaboration at CERN’s Large Hadron Collider have reported evidence for an extremely rare particle decay that researchers have pursued for more than 40 years. The decay is consistent with the Standard Model of particle physics, and the result provides a new measurement of how often a neutral B meson transforms into two muons.
Dr Harry Cliff of the Cavendish Laboratory at the University of Cambridge described the observation as the decay of one of these particles into two muons, the heavier cousins of electrons.
“What we have discovered with these latest results is that on incredibly rare occasions one of these beautiful particles decays, where it transforms into two even smaller particles called muons,” Cliff said.
“This process occurs in only 1 in 10 billion beauty particles, so it’s incredibly difficult to spot.”
The process involves a B0 meson—also known as a Bd0 meson—a short-lived particle containing a beauty quark. The Standard Model predicts that approximately one in every 10 billion B0 mesons will decay into a pair of muons.
Evidence from the LHCb experiment
The search began in 1984, when the CLEO experiment established the first upper limit on the decay rate. In a paper submitted to Physical Review X, Cliff and his colleagues reported an excess of these events over the expected background, with a statistical significance of 3.0 standard deviations.
When combined with earlier LHCb data, the significance rises to 3.2 standard deviations.
“Seeing the results for the first time was a really thrilling moment,” Cliff said. “It was a real honor to see what people have been looking for since before I was born.”
“We’re knocking on the door of the Zeptverse now, and we can’t wait to explore deeper and see what we find.”
Using the new data alone, the LHCb team measured the branching fraction—the proportion of B0 mesons that undergo this decay—at 1.26 in 10 billion, with a statistical uncertainty of about 0.5 in 10 billion. The Standard Model predicts a value of 1.03 in 10 billion. When earlier data are included, the measured value is 1.14 in 10 billion.
“Physicists have been tracking this decay for more than 40 years,” said Dr Marco Santimaria, a physicist at INFN’s Frascati Institute. “Selecting muons from an overwhelming background requires great precision.”
Measurements of a related decay
The researchers also measured a related process: the decay of a neutral Bs0 meson into two muons. This decay is more common than the B0 process, but remains extremely rare.
They measured a branching fraction of 3.31 parts per billion, compared with the Standard Model prediction of 3.66 parts per billion. The collaboration said this was the most precise determination to date.
The team also measured the effective lifetime of the Bs0 meson in this decay, a quantity that can reveal contributions from physics beyond the Standard Model. The new measurement is 1.42 picoseconds, or trillionths of a second, with a statistical uncertainty of 0.18 picoseconds. Combined with previous LHCb data, the result is 1.60 picoseconds.
“LHCb is a great success story for CERN, and we are proud that Oxford has played a leading role since the project began in 1995,” said Professor Guy Wilkinson, a physicist at the University of Oxford.
“Today’s announcement heralds a new era of experimental discovery and vividly demonstrates the increased sensitivity we have achieved in our quest to explore beyond our current understanding of physics at its most fundamental level.”
Professor Tim Gershon, a researcher at the University of Warwick, said: “This impressive result demonstrates the extraordinary ability of LHCb to probe nature at a fundamental level.”
Source: www.sci.news