Scientists at CERN one step closer to understanding the Higgs boson due to new observations

The ATLAS and CMS experiments at CERN’s Large Hadron Collider (LHC) part-funded by the Science and Technology Facilities Council (STFC), have announced their observation of a phenomenon that has never been seen before, where the Higgs boson decays into two elementary particles, called muons.

The ATLAS and CMS collaborations both have members from UK universities and the STFC Rutherford Appleton Laboratory. UK institutes have been members of these collaborations since their very inception and part of the design, build and current operation of these huge detectors.

The Higgs boson discovery in 2012 at the LHC famously provided evidence for a missing piece in the Standard Model of Particle Physics that describes how elementary particles acquire mass. In terms of our understanding of matter and the basic forces shaping the Universe, this is a critical issue: without mass, there would be no matter as we know it.

Since its discovery, the Higgs boson has been scrutinized by physicists eager to discover whether this particle behaves exactly as expected within the Standard Model or whether its properties will reveal yet more insight into the many mysteries that remain to be explained in fundamental physics. The decay of a Higgs boson to muons provides a new chapter to the exploration of its properties.

Muons, like electrons, are elementary particles that make up matter. While electrons are classified as a first-generation particle, muons belong to the second generation. The physics process of the Higgs boson decaying into muons is a rare phenomenon, as only about one Higgs boson in 5000 decays into muons. These new results have pivotal importance for CERN’s physics programme because they indicate for the first time that the Higgs boson interacts with second generation elementary particles.

CMS achieved evidence of this decay with 3 sigma, which means that the chance of the observed result arising from a statistical fluctuation is less than one in 700. The two sigma result from ATLAS means the chances are one in 40. The combination of both results would increase the significance well above 3 sigma and demonstrates even stronger evidence for the Higgs boson decay to two muons.

Professor Sinead Farrington, School of Physics and Astronomy, spokesperson for the UK institutes commented:

ATLAS is a hugely complex machine that enables tremendous precision measurements and searches for new phenomena.  It was built thanks to the efforts of thousands of physicists worldwide including many key contributions from UK physicists. Ingenuity - drawn worldwide including from the UK - is required to record, process and analyse this data, in which to search for fantastically rare processes such as this Higgs decay to a second-generation fermion. Seeing this type of decay for the first time is an exciting landmark that opens a completely unexplored domain for characterising the Higgs sector.

Professor Victoria Martin, ATLAS collaborator, School of Physics and Astronomy said:

Finding out more about the Higgs boson is always an exciting occasion for the ATLAS experiment collaboration, especially so for the over 500 collaboration members from the UK. After 25 years of effort to plan, build, operate and collect and analyse data from the ATLAS experiment, we now have a better answer to one of the questions that the LHC and ATLAS was designed to explore: how and why matter (stuff) gets mass. Taken together, the new observations announced this week by ATLAS provide a lot of extra weight to the theory that fundamental subatomic particles obtain their mass through interactions with the Higgs boson.

With more data, the collaborations will improve the precision of these and other measurements and probe the decay of the Higgs boson, always watching for deviations in the data that could point to physics beyond the Standard Model.