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Large Hadron Collider captures incredibly rare particle decay for the first time

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Experts say findings will help guide the quest to explain what physics cannot

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For the first time, scientists at CERN have observed extremely rare leftovers from subatomic particles. The observations support a fundamental theory of particle physics, according to a paper published today in the journal Nature. It's hoped that the research will help guide future investigations into dark matter and other phenomena that the Standard Model of particle physics cannot explain. The findings are based on data collected between 2011 and 2012 at CERN's Large Hadron Collider (LHC) in Switzerland.

Particle decay occurs when elementary particles spontaneously transform into other elementary particles. In the LHC experiments, protons collided at high energy to create 1 trillion particles known as neutral B mesons, some of which then decayed into pairs of oppositely charged muons — heavier "cousins" of electrons. The decay of one type of B mesons, known as "strange" mesons, occurred at the same frequency predicted by the Standard Model (about four in 1 billion), with a confidence level high enough to qualify as a discovery. The decay of non-strange B mesons also aligned with Standard Model predictions (about one in 10 billion), albeit at a lower confidence level (99.7 percent).

"We are constraining these theories, we are not saying they are wrong."

The Standard Model of particle physics explains how the universe's fundamental particles interact through strong, electromagnetic, and weak forces — but it doesn't explain everything. Although previous experiments have supported the model with increased precision, the Standard Model still fails to account for gravity, and cannot explain the dark matter that holds galaxies together. Physicists have proposed many theories to account for these shortcomings, including supersymmetry, which posits the existence of high-mass "superparticles" that could account for dark matter. Neutral B meson decays, which are rare, may help account for where superparticles and other unobserved "new physics" phenomena come into play. CERN's researchers hope their latest findings will provide more precise limitations to help refine non-Standard Model theories.

"It can help people doing these theories to better understand which parts are correct and which parts are not," says Marc-Olivier Bettler, a CERN research fellow and one of the paper's authors. "We are constraining these theories, we are not saying they are wrong. We are just telling them that they cannot cover all the space that they were covering before."

LHC's reboot holds promise for future experiments

The findings announced today are based on combined results from two previous CERN experiments: one carried out by the LHCb team, and another by the Compact Muon Solenoid (CMS) team. Each experiment found evidence for the decay of neutral B mesons, but the researchers were only able to observe it at a high statistical significance when combining their databases.

"Both experiments on their own have published their measurements before, so in that sense it's not a surprise," says Philip Burrows, a professor of physics at the University of Oxford who was not involved in the CERN research. "But it's the combined precision of the measurements which is the important thing."

The good news for CERN and other physicists is that more data is on the way. Last month, the LHC restarted after a two-year hiatus for maintenance and repairs, and its proton collisions are expected to carry twice as much energy as they did at its debut. That will result in more precise observations, experts say, and, perhaps, a blueprint to explain the vast parts of the universe that the Standard Model cannot.

"They've established the technique for measuring these rare processes, and they're going to have a whole lot more data over the next 10 to 20 years," Burrows says. "And that means their measurements will get a whole lot more precise as the size of the data sample increases."

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