Posted on 18/07/2013
Rob Cantrill (left), an RHUL PhD student working on the ATLAS Higgs discovery, explains the statistics behind the analysis of the data at the Royal Society Summer Science Exhibition (4th July, 2013)
The beginning of this month marked the first anniversary of the discovery of the Higgs boson. On 4 July 2012, the ATLAS and CMS experiments in the Large Hadron Collider at CERN announced the discovery of anew particle found in their data. After sifting through the high energy proton-proton collision data they had accumulated in the previous year, the experiments confirmed they had independently found very strong evidence ("5 sigma") for the discovery of a new particle.
To celebrate this milestone, this year's Summer Science Exhibition at the Royal Society in London included an exhibit on "Understanding the Higgs boson" (see also www.understanding-the-higgs-boson.org). The RHUL Centre for Particle Physics is a member of the ATLAS experimental collaboration, and made several contributions to the Royal Society exhibition:from a "Higgs Hunt" activity for young children, to a demonstration of the probability and statistics behind the discovery, and including written eyewitness accounts of aspects of the journey towards discovery.
Dr Pedro Teixeira-Dias, leader of the ATLAS group at RHUL, said "Those announcements finally set in motion the settling of a major fundamental question in physics, which had been puzzling physicists for the last five decades. The question was 'How is it that elementary particles, such as the electron or the top quark, actually get their mass?' To put the relevance of this question in perspective, the origin of other basic properties of elementary particles - their electric charge, for instance - has been well understood for a while."
The otherwise very successful mathematical theory that summarises our knowledge of elementary particles and how they interact with one another (a.k.a. the Standard Model) did not, in its original form, allow any elementary particles to have nonzero mass. While a few elementary particles, such as the photon, are indeed massless, most are undoubtedly not - hence the big question. (The top quark, the heaviest elementary particle we know about, is hardly massless: it is almost as heavy as a Gold atom, which has nearly 200 protons and neutrons inits nucleus.)
Several physicists worked out a possible theoretical solution to this conundrum in the 1960s. They are sometimes referred to as the "gang of six": Higgs, Englert & Brout, Kibble, Guralnik and Hagen. One unavoidable consequence of the "mass mechanism" that they proposed was that nature must have anew particle with certain specific properties, in addition to all the other elementary particles we have observed so far. Over the years this particle became known as the Higgs boson. Experimentally observing the Higgs particle(or alternatively, establishing once and for all that it didn't exist) therefore became key in the quest to understand the origin of mass. The discovery last year of the new particle at CERN, which we now know behaves consistently with what is expected of a Higgs boson, has therefore been a quantum leap in our understanding of how nature works at its most elementary level. We can now say for the first time that we understand how elementary particles actually can have nonzero mass.
After the discovery, the ATLAS and CMS experimental collaborations have continued to analyse their data in order to measure in detail the properties of the new particle: in particular its spin angular momentum (the Higgs boson is predicted to be the only elementary particle with spin 0) and its production and decaymodes. On July 4th 2013 - the anniversary of the discovery - the ATLAS experiment made public its findings (it walks like a duck and quacks like a duck…). The corresponding scientific papers are available for download*.
*The recent ATLAS publications on the measurement of the Higgs properties are available for download:
1) Evidence for the spin-0 nature of the Higgs boson using ATLAS data (http://arxiv.org/abs/1307.1432)
2) Measurements of Higgs production and couplings using diboson final states with the ATLAS detector at the LHC (http://arxiv.org/abs/1307.1427)
The Large Hadron Collider (LHC) is the highest energy proton-proton collider in the world. It is installed in a 27 km-long underground tunnel at CERN. CERN is the European Laboratory for Particle Physics, in Geneva, Switzerland. The ATLAS and CMS experiments are complex and sophisticated particle detectors installed in the LHC, to record and then study the result of the proton-proton collisions in detail. The RHUL Centre for Particle Physics was a founding member of the ATLAS collaboration, and numbers about 20 members (eight PhD students, five post-doctoral research assistants, one engineer and four academics). The ATLAS group at RHUL works on the Higgs discovery and measurements; the study of the properties of the top-quark (the heaviest known elementary particle); the search for exotic particles such as the graviton, and statistical methods for data analysis in particle physics.