3 August 2020
New results have been announced by particle physicists at CERN that will move them closer to understanding the basic forces that shape our universe.
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.
Event in which a candidate Higgs boson produced by vector boson fusion (VBF) decays into two muons. The forward jets from the VBF are depicted by the orange cones and the muons are drawn as long red lines.
Credit: CMS Collaboration
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.
UK CMS Spokesperson Professor Claire Shepherd-Themistocleous from STFC’s Rutherford Appleton Laboratory said:
“The extraordinary capabilities of the CMS experiment and the ingenuity of the members of the collaboration from around the world, including, of course, the UK, have once again produced a beautiful result.
The observation of Higgs boson decays to the second generation of fundamental particles enables tests of the standard model and opens up a new avenue of exploration for evidence of physics beyond our current understanding.”
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.
Leader of the Higgs CMS group Dr Nicholas Wardle, from the Department of Physics at Imperial, said:
"Neither of these results would have been possible without the excellent calorimetry, trigger or tracking sub-systems at CMS. The UK plays a huge role in the design and construction of these systems."
In physics a 3-sigma detection event has a 0.3% probability of occurring by chance, and a 5-sigma event has just a 0.00006% probability of occurring by chance. Physicists traditionally call a 3-sigma detection "evidence", while a 5-sigma detection is considered a "discovery".
A candidate ATLAS event display of a Higgs boson decay to two muons
Credit: ATLAS Collaboration/CERN
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.
“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,” said Professor Sinead Farrington, University of Edinburgh, Spokesperson for the UK institutes on ATLAS.
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.
Last updated: 04 September 2020