13.12.2011

The search for the Higgs makes a significant step forward

Physicists at the world’s largest particle accelerator LHC made a significant step forward on the hunt for the sought-after Higgs particle. The teams of both large detectors – ATLAS and CMS – narrowed down further the last possible hideout of the hypothetical elementary particle. This is the result of a first evaluation of all observational data obtained in 2011, which both groups presented on Tuesday at a seminar at the European particle research centre CERN near Geneva, home of the LHC. To date, the scientists analysed about 400 billion particle collisions – over a hundred times more data than in the year before.

Simulated signature of a Higgs boson decay. Image: CERN

In this search, both detectors revealed exciting hints of the Higgs particle, although not enough to make any conclusive statements on its existence. “The observations are still compatible with statistical background fluctuations, but also with what is to be expected of the Higgs on the basis of the current amount of data,” explains Professor Joachim Mnich, director of research at Deutsches Elektronen-Synchrotron DESY and member of the CMS team.

Taken individually, none of the observed excesses is statistically any more significant as rolling a dice and coming up with two sixes in a row. Nevertheless, multiple independent measurements point to excesses in the same measuring range of 125 Giga electronvolts (GeV: billion electronvolts). Physicists often specify elementary particles’ mass as an energy equivalent, according to Einstein’s famous formula E=mc², the common unit being an electronvolt (eV).

The search for the Higgs particle at the LHC and at previous accelerators browsed the mass region, except for a narrow range between 115 and 130 GeV, which is extremely difficult to explore. “We know that the Higgs particle – if it exists – would hide in the farthest and less accessible corner,” Mnich clarifies. “However, this is the place where we always expected it to be, due to indirect hints,” the particle physicist adds.

For the discovery of a new particle, physicists must overcome enormous obstacles, since statistical outliers may lead to see such signals. Only when it is possible to exclude likely random fluctuations with a probability of less than one in a million, an observation is regarded as a discovery in particle physics. Mathematically, this means aa excess of five so-called standard deviations, and it is about as probable as rolling a dice and coming up with eight sixes in a row.

The search for the Higgs particle is one of the most important tasks of the LHC. The Higgs is seen as the last missing piece of the well established Standard Model of the structure of matter, because – without the Higgs mechanism – it is not possible to describe the elementary particles’ mass in this model. DESY in Hamburg and in Zeuthen is participating in the operation and data evaluation of the LHC detectors, with on-site control rooms for the CMS and ATLAS detectors.

For the Higgs particle hunt, the LHC collides protons at almost the speed of light and with unprecedented energies. The intense collision energy produces a shower of secondary particles. According to the Standard Model, there is also a Higgs particle among them in less than a billionth of cases. Observations of the Higgs cannot be made directly but only through the decay products. Additionally, the Higgs boson has different modes of decay, which also depend on its mass. “We are searching for the needle not only in one but in 100 000 hay stacks,” DESY physicist Professor Thomas Naumann from the ATLAS team emphasises.

By the end of 2012, the physicists expect a definitive statement on the existence or non-existence of the Higgs particle, as anticipated by the Standard Model.