Martin Pohl, Uni Genève
On July 4th 2012, shortly before a detailed presentation at the biannual International Conference on High Energy Physics, ICHEP 2012, the two LHC experiments ATLAS 1 and CMS 2 jointly announced the discovery of a new particle, compatible with being the long-sought Higgs boson 3. By interaction with this ubiquitous particle, quarks and leptons as well as the weak force carriers (the W and Z vector bosons) are prevented to move with the speed of light, as they otherwise would. Mass is thus not an intrinsic property, like electric charge, but a dynamical property of particles. This mechanism, proposed by Peter Higgs (Figure 1) and others a long time ago, reconciles gauge symmetry, the basis of the Standard Model of electroweak and strong interactions, and massive particles. It thus explains, why the Standard Model is indeed renormalizable 4 and able to make the high precision predictions confirmed by equally high precision experiments for decades.
Prior to this discovery, it had been know from experiments at the Large Electron Positron collider LEP that the Higgs mass would be beyond, but not much beyond the experimental limit of 114.4 GeV at 95% confidence level 5. Precision measurements of electroweak parameters limited its mass from above at 152 GeV 6. Experiments at the Tevatron proton-antiproton collider of the Fermi National Laboratory had in vain tried to find a signal 7, excluding only a narrow mass range between 162 and 166 GeV. This left the LHC experiments to search in a mass range, where many decay channels can contribute, including at the per mill level the decay into two photons.
The two experiments conducted a dedicated search for the characteristic decay processes of the Higgs boson, mostly using the data collected in 2011 and 2012. "The search is more advanced today than we imagined possible," said ATLAS spokesperson Fabiola Gianotti. "We observe in our data clear signs of a new particle, at the level of 5 sigma, in the mass region around 126 GeV. The outstanding performance of the LHC and ATLAS and the huge effort of many people have brought us to this exciting stage." Swiss groups from ETHZ and PSI participate in CMS, researchers from Bern and Geneva collaborate on the ATLAS project.
Since this announcement, both collaborations have refined their results and published them, in two detailed papers 8 simultaneously submitted to Physics Letters B on July 31, 2012. The results are based on a total of about 10 fb-1 integrated luminosity, collected by each experiment at 7 and 8 TeV center-of-mass energy during the last two years. Each experiment observes a signal for the new particle with a local significance between 5 and 5.9 standard deviations, corresponding to a probability for a background fluctuation of a few parts per billion. As expected, the most significant signals are observed among candidates for the Higgs decay into two photons (Figure 2) or two Z bosons (Figure 3), which present the best mass resolution. A clear bump is seen in the invariant mass distribution of two photons between 120 and 130 GeV (Figure 4). The mass of the new particle is found to be 126.0±0.4±0.4 GeV by ATLAS, 125.3±0.4±0.5GeV by CMS, where the first error is statistical, the second systematic. The observation of the decay into two photons proves that the new particle is indeed a boson with a spin different from 1. The rates observed in the different decay channels are compatible with those predicted by the Standard Model Higgs mechanism, within the rather limited statics available to date.
In the future, vast amounts of new data will be required to firmly establish the properties of the new boson and confirm that it is indeed responsible for dynamically generating all masses, including its own. For that, its couplings to various more rare or difficult to identify decay products like tau leptons and b quarks need to follow the predicted mass-coupling relation. One must also see if the new boson is the only one of its species or if several exist, as e.g. foreseen in supersymmetry. While supersymmetric Higgs bosons may be found at LHC if they exist, the precision study of Higgs couplings may be beyond its capabilities. This latter aspect of Higgs physics may then well require the construction of a new accelerator to firmly conclude.
1 See http://www.atlas.ch
2 See http://cms.web.cern.ch/
3 F. Englert and R. Brout, Broken symmetry and the mass of gauge vector mesons, Phys. Rev. Lett. 13 (1964) 321; P. W. Higgs, Broken symmetries, massless particles and gauge fields, Phys. Lett. 12 (1964) 132; P. W. Higgs, Broken symmetries and the masses of gauge bosons, Phys. Rev. Lett. 13 (1964) 508; G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble, Global conservation laws and massless particles, Phys. Rev. Lett. 13 (1964) 585; P. W. Higgs, Spontaneous symmetry breakdown without massless bosons, Phys. Rev. 145 (1966) 1156.; T. W. B. Kibble, Symmetry breaking in non-Abelian gauge theories, Phys. Rev. 155 (1967) 1554.
4 G. ’t Hooft and M. Veltman, Regularization and Renormalization of Gauge Fields, Nucl. Phys. B 44 (1972) 189.
5 ALEPH, DELPHI, L3, OPAL Collaborations, and LEP Working Group for Standard Model Higgs Boson Searches, Search for the standard model Higgs boson at LEP, Phys. Lett B 565 (2003) 61
6 ALEPH, CDF, D0, DELPHI, L3, OPAL, SLD Collaborations, the LEP Electroweak Working Group, the Tevatron Electroweak Working Group, and the SLD Electroweak and Heavy Flavor Groups, Precision Electroweak Measurements and Constraints on the Standard Model, CERN PH-EP-2010-095 (2010). For most up-to-date constraints, see http://lepewwg.web.cern.ch/LEPEWWG/plots/winter2012/.
7 CDF and D0 Collaborations, Combination of Tevatron Searches for the Standard Model Higgs Boson in the W+W- Decay Mode, Phys. Rev. Lett. 104 (2010) 061802, arXiv:1207.1707, arXiv:1207.6436, and FERMILAB-PUB-12-406-E.
8 The CMS Collaboration, Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC, arXiv:1207.7235v1. The ATLAS Collaboration, Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC, arXiv:1207.7214v1.
[Released: September 2012]