New ATLAS result of ultra-rare B-meson decay to muon pair

25 September 2018 | By

Figure 1: Measured dimuon mass distributions in the selection channel with highest expected signal purity. Superimposed is the result of a fit to the data. The total fit result is shown (black continuous line), with the observed signal component (dashed red line), b→μμ X background (dashed blue), and continuum background (dashed green). Signal components are grouped in one single curve, including the B0s → μ+μ- and B0 → μ+μ- components. The peaking B0(s) → hh′ background (brown dashed line) is, for all BDT bins, very close to the x-axis. (Image: ATLAS Collaboration/CERN)

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

Among the variety of hadrons available in nature and produced in LHC proton-proton collisions, B-mesons play a fundamental role. They are bound-states of two quarks – one a bottom quark, the other one of the lighter quarks (up, down, strange or charm) – that decay through the weak interaction to lighter hadrons and/or leptons. Over the past decades, physicists have examined rare and precisely predicted phenomena involving neutral B-mesons (i.e. B0 or Bs mesons), searching for slight discrepancies from theory 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 B0 meson 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.

ATLAS revealed the most stringent experimental constraint of the very rare decay of the B0 meson into two muons.

Figure 2: Likelihood contours for the combination of the Run 1 and 2015-2016 Run 2 results (in black). The solid, dashed and dashed-dotted contours delimit the one, two and three standard deviation regions, respectively. The contours for the Run 2 2015-2016 result alone are overlaid in blue. The Standard Model prediction and its uncertainties are shown by the solid cross. (Image: ATLAS Collaboration/CERN)

The rareness of this decay is due to the coincidence of two factors: first, the decay requires quantum loops with several weak interaction vertices, some of which have a low probability to occur; second, angular momentum conservation constrains the decay products of the scalar B0 or Bs meson 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 ten billion at the 95% 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 towards 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%. Further projections towards 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 3.