Results from the ATLAS and CMS collaborations, presented at the biannual Lepton-Photon conference in Mumbai, India today, show that the elusive Higgs particle, if it exists, is running out of places to hide.
Higgs Boson Particle Simulated
The Higgs boson is a hypothetical massive elementary particle predicted to exist by the Standard Model of particle physics. The existence of the particle is postulated to resolve inconsistencies in theoretical physics and attempts are being made to find the particle by experiment, using the Large Hadron Collider (LHC) at CERN and the Tevatron at Fermilab.
The Higgs boson is the only Standard Model particle that has not been observed in particle physics experiments. It is a consequence of the so-called Higgs mechanism which is the part of the Standard Model that explains how most of the known elementary particles become massive. For example, the Higgs boson would explain the difference between the massless photon, which mediates electromagnetism, and the massive W and Z bosons, which mediate the weak force. If the Higgs boson exists, it is an integral and pervasive component of the material world and would be of a class of particles known as scalar bosons.
Bosons have integer spin and scalar bosons have spin 0. The photon is a kind of boson and so is the less-familiar gluon, along with the W and Z particles mentioned above. These particles are all vector bosons, with spin 1. At present there are no known elementary scalar bosons in nature, although many composite spin-0 particles are known.
Proving or disproving the existence the Higgs boson, which was postulated in the 1960s as part of a mechanism that would confer mass on fundamental particles, is among the main goals of the LHC scientific programme. ATLAS and CMS have excluded the existence of a Higgs over most of the mass region 145 to 466 GeV with 95 percent certainty.
As well as the Higgs search results, the LHC experiments will be presenting new results across a wide range of physics. Thanks to the outstanding performance of the LHC, the experiments and the Worldwide LHC Computing Grid, some of the current analyses are based on roughly twice the data sample presented at the last major particle physics conference in July.
"These are exciting times for particle physics," said CERN's research director, Sergio Bertolucci. "Discoveries are almost assured within the next twelve months. If the Higgs exists, the LHC experiments will soon find it. If it does not, its absence will point the way to new physics."
The Standard Model Higgs mechanism is one of a range of ways that fundamental particles could acquire their masses. According to the Higgs mechanism, space is filled with a so-called Higgs field with which particles interact. Those that interact strongly with the field have more mass than those that interact weakly, rather like a streamlined racing car cuts through air more easily than a bus.
At the first major particle physics conference of 2011, the European Physical Society's High Energy Physics conference held in Grenoble, France, in July, both ATLAS and CMS were careful to stress that possible hints of a Higgs signal in their data could be explained by statistical fluctuations. Now, with additional data analysed, the significance of those fluctuations has slightly decreased.
"Thanks to the superb performance of the LHC, we have recorded a huge amount of new data over the last month," said ATLAS spokesperson Fabiola Gianotti. "This has allowed us to make great strides in our understanding of the Standard Model and in the search for the Higgs boson and new physics."
CMS Spokesperson Guido Tonelli concurred, saying: "It's great that the LHC's fantastic performance this year has brought us this close to a region of possible discovery. Whatever the final verdict on Higgs, we're now living in very exciting times for all involved in the quest for new physics."
The Lepton-Photon conference runs until 27 August. There will be a press conference on 25 August at which CERN Director General, Rolf Heuer, will be one of the speakers. CERN's LHCb experiment will present its latest measurements on the Standard Model on Saturday 27 August. Following the Lepton Photon conference, the results from the LHC experiments will be available through the CERN website.
The LHC is on track to at least double the amount of data delivered so far to the experiments by the end of the year.
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