ATLAS Experiment releases new study of ultra-rare B-meson decay

September 26, 2018, ATLAS Experiment
Measured dimuon mass distributions in the selection channel with highest expected signal purity. Superimposed is the result of a fit to the data. Credit: ATLAS Collaboration/CERN

The study of hadrons—particles that combine quarks to form mesons or baryons—is a vital part of the physics programme by researchers of the ATLAS Experiment at CERN. Their analysis has not only perfected the understanding of the Standard Model, it has also provided excellent opportunities for discovery.

Among the variety of hadrons available in nature and produced in proton-proton collisions at the Large Hadron Collider, B-mesons play a fundamental role. They are bound states of two quarks—one a bottom , the other from among the lighter quarks (up, down, strange or charm)—that decay through the weak interaction to lighter hadrons and/or leptons. In recent decades, physicists have examined rare and precisely predicted phenomena involving neutral B-mesons (i.e. B0or Bs mesons), searching for slight discrepancies from predictions that could be a signal of new physics.

On 20 September 2018, at the International Workshop on the CKM Unitarity Triangle (CKM 2018), ATLAS revealed the most stringent experimental constraint of the very rare decay of the B0meson into two muons (μ). The result is a new milestone that complements analyses previously published by LHC experiments dedicated to the study of B-mesons in a quest spanning almost three decades.

The rareness of this decay is due to the coincidence of two factors: First, the decay requires quantum loops with several vertices, some of which have a low probability to occur; second, angular momentum conservation constrains the decay products of the scalar B0 or Bs into a highly unlikely configuration.

According to the Standard Model, the probability of generating this decay is about 1.1 in 10 billion. The new ATLAS result gets very close, with the tightest available upper limit of 2.1 occurrences in 10 billion at the 95 percent confidence level. The result was obtained using data collected in 2015 and 2016 combined with an analogous analysis of Run 1 data. The result also provided a 4.6 standard deviations evidence for the Bs→ μμ decay, whose branching fraction is measured to be 2.8 +0.8–0.7 x10–9. It confirms previous measurements from the LHCb and CMS collaborations.

This new ATLAS result is the first milestone toward a more precise measurement that will be obtained with the full Run 2 dataset, which is expected to improve the current precision by about 30 percent. further projections toward the high-luminosity LHC (HL-LHC) era predict that ATLAS will be able to further improve the precision of this result by about a factor of three.

Explore further: Long-sought decay of Higgs boson observed

More information: Study of the rare decays of B0s and B0 into muon pairs from data collected during 2015 and 2016 with the ATLAS detector (ATLAS-CONF-2018-046): atlas.web.cern.ch/Atlas/GROUPS … ATLAS-CONF-2018-046/

Related Stories

Long-sought decay of Higgs boson observed

August 28, 2018

Six years after its discovery, the Higgs boson has at last been observed decaying to fundamental particles known as bottom quarks. The finding, presented today at CERN by the ATLAS and CMS collaborations at the Large Hadron ...

The hunt for leptoquarks is on

September 19, 2018

Matter is made of elementary particles, and the Standard Model of particle physics states that these particles occur in two families: leptons (such as electrons and neutrinos) and quarks (which make up protons and neutrons). ...

'New physics' charmingly escapes us

August 3, 2018

In the world of elementary particles, traces of a potential "new physics" may be concealed in processes related to the decay of baryons. Analysis of data from the LHCb experiment at the Large Hadron Collider performed by ...

The potential harbingers of new physics persist in LHC data

August 31, 2018

For some time now, researchers have noted several anomalies in the decays of beauty mesons in the data coming in from the LHCb experiment at the Large Hadron Collider. Are they more than just statistical fluctuations? The ...

Recommended for you

Physicists discover new class of pentaquarks

March 26, 2019

Tomasz Skwarnicki, professor of physics in the College of Arts and Sciences at Syracuse University, has uncovered new information about a class of particles called pentaquarks. His findings could lead to a new understanding ...

Coffee-based colloids for direct solar absorption

March 22, 2019

Solar energy is one of the most promising resources to help reduce fossil fuel consumption and mitigate greenhouse gas emissions to power a sustainable future. Devices presently in use to convert solar energy into thermal ...

Physicists reveal why matter dominates universe

March 21, 2019

Physicists in the College of Arts and Sciences at Syracuse University have confirmed that matter and antimatter decay differently for elementary particles containing charmed quarks.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.