Physicists with the LHCb collaboration at CERN’s Large Hadron Collider have reported evidence for the extremely rare decay of a neutral beauty meson into two muons. The result, which follows more than 40 years of searching, is consistent with the Standard Model of particle physics and reaches a statistical significance of 3.0 standard deviations in the new data, or 3.2 standard deviations when combined with earlier LHCb results.
A decay tracked for more than four decades
The process involves a B0 meson—also called a B0d or Bd meson—a short-lived particle containing a beauty quark. In the decay, the meson transforms into two muons, the heavier cousins of electrons.
According to the Standard Model, this decay should occur only once in approximately 10 billion B0 mesons. “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,” said Dr Harry Cliff of the Cavendish Laboratory at the University of Cambridge.
“This process occurs in only 1 in 10 billion beauty particles, so it’s incredibly difficult to spot.”
The search began in 1984, when the CLEO experiment established the first upper limit on the decay rate. Detecting the muons among the overwhelming background requires highly precise measurements.
In a paper submitted to Physical Review X, Dr Cliff and his colleagues reported an excess of B0 → μ+μ− events over the expected background. The excess has a statistical significance of 3.0 standard deviations. Including previous LHCb data raises the significance to 3.2 standard deviations.
“Seeing the results for the first time was a really thrilling moment,” Dr 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.”
Measured decay rate
The LHCb team measured the branching fraction—the proportion of B0 mesons that decay through this channel—as 1.26 per 10 billion using the new data alone, with a statistical uncertainty of approximately 0.5 per 10 billion.
The Standard Model prediction is 1.03 per 10 billion. When older data are included, the measured value is 1.14 per 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.”
A related decay and the meson’s lifetime
The researchers also measured the related decay of a B0s meson into two muons. This process is more common than the B0 decay, but it remains extremely rare.
The measured branching fraction was 3.31 parts per billion, compared with the Standard Model prediction of 3.66 parts per billion. The collaboration described this as the most accurate determination to date.
LHCb also measured the effective lifetime of the B0s meson in this decay. The lifetime can provide information about possible 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. When combined with previous LHCb data, the result is 1.60 picoseconds.
Reactions from the physics community
“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 the LHCb to probe nature at a fundamental level.”
Source: www.sci.news
