Higgs Hunting at the LHC
Since 2009, the Large Hadron Collider (LHC) at CERN (Geneva, Switzerland) collides protons at a center-of-mass energy of 7 TeV, the highest energy ever reached by a particle accelerator. One of the main goals of the LHC is the search for the Higgs boson, the last piece of the Standard Model that remains undiscovered. The so-called Higgs mechanism was introduced in 1964 to explain the breaking of the electroweak symmetry, leading to a massless photon, mediator of the electromagnetic force, and very heavy W and Z bosons, mediators of the weak interaction. The Higgs mechanism would also explain the mass of fermions as the Higgs field permeates the Universe and interacts with all particles endowing them with their mass. Finally, the existence of a Higgs boson associated with the Higgs field is postulated, although its mass is not predicted and must be determined experimentally.
During the last four decades, particle physicists have searched for the Higgs boson. In the 90's, the LEP electron-positron collider at CERN concluded that the Higgs boson, if it exists, should have a mass larger than 114.4 GeV at 95% confidence level (C.L). Since 2002, the quest for the Higgs boson was mainly undertaken by the CDF and D0 experiments at the 1.96 TeV proton-antiproton Tevatron collider at Fermilab (near Chicago, USA). The experiments were able to extend the excluded mass range at 95% C.L. to 156-177 GeV, still leaving much room for the Higgs boson to hide.
The analysis of the data delivered in 2011 by the LHC to the ATLAS and CMS experiments has translated into a huge step forward in the quest for the Higgs boson. In particular, the combination of searches at the ATLAS experiment has excluded at 95% C.L. the presence of a Higgs boson with mass in the ranges: 112.7-115.5 GeV, 131-237 GeV, and 251-453 GeV [1-4]. Most importantly, the experiment observes a suggestive excess of events around 126 GeV (see Fig. 1) which would be consistent with the potential signal of a Higgs boson, although the observation is not yet statistically significant. It corresponds to a 3.6 standard deviation from the background-only hypothesis (see Fig. 2) but, after including look-elsewhere effects, it translates into a 1% probability to be a simple background fluctuation.
In 2012 the LHC experiments will collect much more data, which should allow them to either discover the Higgs boson or completely exclude its existence. A. Juste and M. Martínez lead the analysis effort of the ATLAS data at IFAE, playing a central role in those channels whe