18.11.2011

The Higgs particle is running out of places to hide

Largest hunt so far with the Large Hadron Collider LHC

Scientists working at the world’s largest particle accelerator LHC near Geneva have presented the so far most extensive search for the Higgs particle, the last missing piece of the Standard Model of particle physics. It is the first combined analysis of data from the large detectors ATLAS and CMS. The LHC is well underway to solve the Higgs enigma by the end of next year, the physicists told the Hadron Collider Physics Symposium in Paris on Friday. The new analysis of up to 160000 billion proton collisions rules out any Standard Model Higgs particle in the energy range between 145 and 470 Giga electron volts (GeV).

Peter Higgs visits the CMS detector. Photo: CERN

The search for the Higgs particle is one of the most important tasks of the Large Hadron Collider (LHC) at the European centre for nuclear research CERN. The elusive particle is the last piece in the puzzle of the established Standard Model of the structure of matter, as it is not possible to explain the particles’ mass in this model without the Higgs mechanism. The Deutsche Elektronen-Synchrotron DESY in Hamburg and Zeuthen participates in the operation and data evaluation of the LHC detectors, with own control rooms for the CMS and ATLAS detectors at the LHC.

„The window closes for the Higgs,“ explains Professor Joachim Mnich, DESY director in charge of particle physics  and member of the CMS team. “This is no surprise. The space remains open where we always assumed it to be.” The still open energy range for the Higgs boson is the most challenging one for the experiments at the LHC – the physicists need more data for reliable conclusions. Until the end of 2012, physicists expect to get evidence of the existence or non-existence of the Higgs particle. Compared to the current analysis, the detectors already have a multiple amount of data. By the end of the year, this amount will double again.

Ground-breaking results are almost guaranteed: „Finding the Higgs particle would be a discovery, ruling it out would be a revolution,” Mnich emphasises. Should the physicists not find the Higgs, this would be the clue for so far undetected physics giving particles their mass. “The Higgs particle is the so-called headstone of the Standard Model,”  DESY physicist Thomas Naumann from the ATLAS team explains. “If you pull it out, the whole building collapses.”

For the Higgs particle search, hydrogen nuclei (protons) collide in the LHC at almost the speed of light, with unprecedented energy; a shower of secondary particles emerging from the concentrated collision energy. According to the Standard Model, there will only be one Higgs particle emerging from about a billion collisions or so. To make it even more difficult, it cannot be detected directly but only via its decay products. As it may decay into various different particles, according to its mass, the search is very complex.

Physicists often specify the mass of particles as an energy equivalent with thestandard unit electron volt (eV), according to Einstein’s famous formula E=mc².  In the range between 145 and 470 Giga electron volts (GeV), the scientists ruled out the existence of the Higgs particle. Former accelerators already searched within this range up to 114.4 GeV. A window between 114 and 145 GeV would still be a place for the Higgs to hide. Thus, the search has already covered a substantial area. The existence of the Standard Model Higgs is very unlikely above the currently specified area of exclusion.

The new analysis also demonstrates the excellent performance of the machines, as Mnich stresses: “The detectors proved to be as sensitive as expected”. Therefore, the scientists are confident to present definite search results in the coming year.

Background information:

The Higgs particle is named after the emeritus Scottish physicist Peter Higgs. In 1964, he and other physicists conceived a trick to overcome fundamental inconsistencies of the otherwise outstandingly successful Standard Model of the structure of matter: the theory established in a period of decades only allows massless interaction mediating particles like the photon. In order to also explain massive interaction mediators as the W and Z bosons of the weak interaction, Peter Higgs introduced the field later named after him. According to this theory, the Higgs field permeates the universe like syrup and interacts with exchange particles in different degrees, slows them down and gives them mass. Just as the photon is an electromagnetic field excitation, the Higgs particles are a Higgs field excitation – like clots in the syrup.