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First hints of the Higgs boson?

The CMS detector under construction in its underground cavern at CERN

The CMS detector under construction in its underground cavern at CERN

16 December 2011

Physicists working at the Large Hadron Collider have received an early Christmas present. Using apparatus partly designed in Bristol, the CMS (Compact Muon Solenoid) collaboration has presented the first tentative evidence of the Higgs boson

The Higgs boson is a key component of the Standard Model of matter and forces.  This theory describes how physics works ‘behind the scenes’, and describes how the Universe looked around one picosecond after the Big Bang.  The Higgs is often described as giving the other particles mass, but in fact plays a more fundamental role: it allows three of the four known forces of nature to be described by a single equation.  This paves the way for ‘grand unified theory’ at yet higher energies, combining all known fundamental laws into a simple mathematical structure.

The Large Hadron Collider is the world’s largest scientific instrument, and is sited at CERN, Geneva.  It collides bunches of protons 40 million times per second, and with the highest energies yet achieved.  CMS is one of two giant experiments which identify new particles produced in the collisions, and seeks to answer basic questions about matter, space and time.

The University of Bristol particle physics group has been working since 1993 to construct and operate the CMS experiment.  A key subcomponent for the Higgs search, the electromagnetic calorimeter, was developed and tested at Bristol in collaboration with other universities in the UK and around the world.  Bristol also helped to design the worldwide computing system which moves millions of gigabytes of data from CERN to universities, where it is analysed.

The results from CMS, and its competing experiment, ATLAS, indicate that the Higgs will be found with a mass of around 125GeV, roughly the mass of a caesium atom.  The Higgs is expected to be produced only very rarely, and searching for the new particle is akin to finding a needle in 10,000 haystacks.

After careful filtering of the data, CMS has observed an excess of events which could be due to the decay of Higgs into two photons, or into two W bosons.  However, more data will be needed before a decisive statement can be made. The LHC will operate from March 2012 with higher energy and intensity, which will allow progress to be made very quickly.

Bristol is also using CMS data to search for other new phenomena.  Researchers based in the UK and at CERN are searching for evidence of supersymmetry, a theory which will help explain the presence of dark matter in the universe, and improve our understanding of space and time themselves.  A new study in this area, led by a Bristol academic, has been featured in a Viewpoint article in the journal Physics.  The results indicate that if supersymmetry does exist, it is in a more complex form than originally foreseen.

The LHC physics programme will continue for at least fifteen years, and the Bristol team are already involved in the design upgraded detectors to be used from 2017.  The field of elementary particle physics is entering one of the most exciting phases in its history, with many long-standing questions due to be answered.

Dave Newbold, Head of High Energy Particle Physics Group, CMS Collaborator, School of Physics

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