Rare subatomic process observed for the first time by LHC scientists

Data analyzed by CMS and LHCb collaborations show B meson decay that has never been seen before. Three Brazilian research groups participated in the research, published by Nature.

By Elton Alisson

Agência FAPESP – In an article published by the journal Nature researchers from the CMS and LHCb collaborations at CERN’s Large Hadron Collider (LHC) in Switzerland announced the first observation of a very rare subatomic process. CERN is the European Organization for Nuclear Research, the world's largest particle physics laboratory.

Analyzing the combined results of two experiments, they found that the B_s⁰ meson decays (that is, spontaneously transforms) into two muons. The Standard Model predicts that this rare process happens a few times in a billion decays, but it has never been seen before.

A muon is an ultraenergetic atomic particle. The B_s⁰ meson and its cousin the B⁰ are elementary unstable subatomic particles that are only produced in high-energy collisions such as those that occur in particle accelerators like the LHC or that occur in nature, for example, in cosmic-ray interactions.

Three groups of Brazilian researchers contributed to the study. They are affiliated with the São Paulo Research and Analysis Center (SPRACe) at São Paulo State University (UNESP) and the Federal University of the ABC (UFABC), supported by FAPESP; the Brazilian Physics Research Center (CBPF); and the Rio de Janeiro State University (UERJ). The SPRACe group participates in the CMS collaboration.

“Precise observation of rare decays, such as those of B mesons, is a complementary strategy to test the possibility of new physics beyond the Standard Model,” said Sérgio Novaes, a full professor at UNESP and principal investigator at SPRACe.

Novaes told Agência FAPESP that the Standard Model of particle physics, which explains how the fundamental particles that make up all known matter in the universe interact through strong, weak, and electromagnetic forces, predicts a very low probability of B mesons decaying into muons: approximately four times for every 1 billion B_s⁰ mesons produced and once for every 10 billion B⁰ mesons.

The existence of a difference between the predicted probabilities of decay and the experimental observations of these mesons could open a window to theories beyond the Standard Model, including supersymmetry, which posits that for each fermion (quark, electron, or neutrino) there is a corresponding boson, such as the Higgs boson that was found by researchers at the LHC in 2012.

The experiments performed by the CMS and LHCb collaborations, in which protons collided at high energy to create 1 trillion B mesons, confirmed the Standard Model’s predictions with a high level of precision.

“The combined results of the observations performed by the CMS and LHCb collaborations match the predictions of the Standard Model and help to eliminate or constrain a number of theories that predict higher rates of B meson decay than those observed,” Novaes said.

Joint analysis

The two collaborations produced their data in 2011-12 and first released results for B meson decay individually in July 2013.

According to Novaes, while the individual results were in agreement, both were just below the 5 sigma statistical precision historically needed to claim an observation in particle physics.

“Five standard deviations, or 5 sigma, means 99.9994% confidence that the measurement is correct and a 1 in 1.75 million chance that it’s a random fluctuation,” he said.

The combined analysis of the data obtained by both collaborations, taking correlations and uncertainties into account, easily exceeded this statistical precision requirement, reaching 6.2 sigma.

According to the article published in Nature, “both measurements are statistically compatible with Standard Model predictions and allow stringent constraints to be placed on theories beyond the Standard Model”.

“Precision measurements of electroweak effects are an indirect and complementary way to obtain limits that predict new heavy particles,” Novaes said.

The CMS and LHCb collaborations collected data at between 7 and 8 teraelectronvolts (TeV) center-of-mass energy.

The LHC experiments will resume collecting data in the coming weeks, recording collisions at 13 TeV with more intense proton beams, doubling the production of B_s⁰ and B⁰ mesons and consequently making the measurement of their decay rates even more precise.

According to researchers in the field, CERN will also resume a direct search for new heavy particles that may be produced in the LHC and for any signs of new phenomena that extend the Standard Model.

The article “Observation of the rare B_s⁰ →µ+µ− decay from the combined analysis of CMS and LHCb data” (doi: 10.1038/nature14474) can be read at www.nature.com/nature/journal/vaop/ncurrent/full/nature14474.html.