Two thousand twelve will be remembered as the year of the Higgs boson discovery. Hypothesized more than 40 years ago, the Higgs boson is the key to the question of how fundamental particles acquire their mass and how the weak force is broken. Its observation by the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland, completes the Standard Model (SM) of particle physics. Named breakthrough of the year by Science magazine, the Higgs discovery is the experimental confirmation of a theoretical prediction and an incredible triumph of science.
Only 3 years ago, on March 30, 2010, the first collisions took place at CERN between two 3.5 TeV proton beams, setting the world record for the highest-energy particle collisions, about a factor 4 larger than previously achieved. Thus, the LHC research program and the hunt for the Higgs boson began. Based on previous experimental data and theoretical input, the LHC experiments were bound to make a major discovery.
“There’s two possible outcomes: if the result confirms the hypothesis, then you’ve made a discovery. If the result is contrary to the hypothesis, then you've made a discovery." -Enrico Fermi
The observation of a new particle was first presented on July 4, 2012, by ATLAS and CMS teams using final states with photons and Z bosons. The complete dataset collected by ATLAS and CMS during the first 3 years of LHC operation is now 2.5 times larger than what was available when the observation was made in July 2012. Further analysis of this data now also shows evidence of decays to a pair of W bosons and strong indication for couplings to taus.
With the discovery of the Higgs boson, the next big question is whether or not it marks the beginning of a new age of discovery in particle physics. We can shed light on this question by measuring the properties of the Higgs boson with high precision. The accumulated data allows the measurement of the Higgs mass to about 125 GeV with a precision of better than 1% (Figure i). Thus, with the knowledge of its mass, all other properties of the Higgs boson can be predicted by the SM. The precise measurements of couplings, spin, and parity challenge the theory and may well lead to the next discovery.
Through analysis of the kinematic, angles, and masses of Higgs decay products, we have sensitivity to the spin and parity of the new particle. We observe signatures consistent with the hypothesis of a Standard-Model Higgs boson, a spin-0 particle with even parity, and can exclude pure alternative models with a pseudoscalar (odd parity) or spin- 2 particle (Figure ii). The final distinguishing characteristic of the Higgs field is the form of its interactions with the other particles and the coupling to itself. So far we have confirmed, with still limited precision, that elementary particles couple as expected with a strength proportional to their mass (Figure iii).
Combining all available information on the Higgs boson, the data shows no significant sign of physics beyond the SM, and the newly-discovered Higgs boson is truly Standard Model-like.
"If it looks like a Higgs, swims like a Higgs, and quacks like a Higgs, then it's probably a Higgs." -Markus Klute
However, our understanding of all this is not yet complete. Although the SM requires only one Higgs field, we are not sure how many kinds of Higgs fields there are. In fact, we suspect that the SM will be superseded by a more complete theory. For example, we already know that the SM is incomplete through the observation of dark matter (the SM does not provide a candidate for 85% of the matter content of the universe, a severe shortcoming). Leading contenders for a more complete theory are extensions of the SM known as supersymmetric models, which would require a minimum of two Higgs fields.
Even more data will be required to explore these exciting questions. To upgrade the LHC to higher collision energies and intensities, it was shut down in February 2013. When the LHC comes back online in Spring 2015, the center-of-mass energy will be increased from 8 to 13 TeV, and intensities will be doubled. The LHC upgrade will again open a new window in the search for new physics beyond the Standard Model.