Strong Evidence for Existence of Higgs Boson
 Figure 1 : Fermi National Accelerator Laboratory. A ring at the back is the Tevatron and a ring at the front is the Main Injector. |
 Figure 2 : CDF detector (side picture). |
 Figure 3 : Higgs boson production process through gluon fusion, which is the most important channel in the Higgs search at LHC. |
 Figure 4 : Higgs boson production process through vector boson associated production, which is the most important channel at the Tevatron. |
 Figure 5 : Plot of the ratio between an upper limit of the Higgs production cross section obtained from the CDF/D0 experiments to the prediction of the Standard Model as a function the Higgs mass. A signal excess over a statistical significance of 2σ is seen in the Higgs mass range of 115 – 135 GeV/c2 compared with a hypothesis that the Higgs boson does not exist. |
 Figure 6 : The p-value distribution of background. (Probability in which data and samples can be explained by a statistical fluctuation of background) The dashed line shows the background p-value of an assumption in which the Higgs signal is added to the expected background. |
 Figure 7 : Background p-value limited to the bb sample. The dashed line shows the background p-value for an assumption in which the Higgs signal is added to the expected background. |
 Figure 8 : Invariant mass distribution reconstructed from two-jet events with the b-quark tag. Upper left : Plot requiring that one of jets is a b (or b) quark. Upper right : Plot requiring that two of jets are b (or b) quarks. Lower : Combination of above two. |
On July 4th, it was announced that a new particle, believed to be the Higgs boson, was discovered at CERN’s Large Hadron Collider (LHC). Concurrently, significant results suggesting the existence of the Higgs boson were also obtained from high-energy proton-antiproton collision experiments at the Fermi National Accelerator Laboratory (Fermilab) in the United States (Figure 1), in which the High Energy Physics Laboratory of Osaka City University is participating.For more details on the ongoing efforts of this research, please refer to previous articles (dated April 26, 2012, October 14, 2010, News, August 7, 2008, May 15, 2008, News, and September 4, 2007, in OCU HEP Lab. News).
The Higgs boson is a particle believed to be the origin of the mass of all elementary particles. In the widely accepted Standard Model of elementary particles, it is explained as follows. After the universe was born from the Big Bang, a phase transition occurred in the vacuum as the universe cooled, resulting in the creation of the Higgs field. Consequently, the present vacuum is thought to be filled with the Higgs field. Particles that would otherwise move at the speed of light interact with this field, causing them to move slower and thereby acquire mass. The particles of this field are the Higgs bosons, and particles that couple more strongly with the Higgs boson are heavier. This mechanism, named after its proposer P. W. Higgs, is called the “Higgs mechanism.” It also serves as an important example of “spontaneous symmetry breaking” in nature, a concept discovered by Dr. Yoichiro Nambu, who is also the Special Emeritus Professor at Osaka City University.
At the end of September last year, the Tevatron accelerator, a proton-antiproton collider at Fermilab, ceased operations, and the recent announcement is based on the analysis of all data obtained to date. The Tevatron had two independent detectors, CDF and DØ, both of which accumulated data with an amount of statistics of 10fb-1. Osaka City University is participating in the CDF experiment (Figure 2 shows the CDF detector).
Recently, results from experiments conducted with the ATLAS and CMS detectors installed at CERN’s LHC were announced. Both reported the discovery of a new particle, believed to be the Higgs boson, in the mass range of 125-126 GeV/c². The LHC creates a center-of-mass energy of 8 TeV by colliding protons, aiming to generate Higgs bosons. According to theoretical calculations, high-energy proton collisions produce many Higgs bosons primarily through gluon fusion. After its creation, the Higgs boson quickly decays into various modes. If it decays into quark pairs, it becomes challenging to distinguish from the high rate of QCD jet events produced by proton collisions. Therefore, the ATLAS and CMS experiments focused on the decay channel where the Higgs boson decays into two photons (H → γγ, Figure 3). As a result, they reported a statistical significance of 4.9-5.0σ for an excess of events in the mass range of 125-126 GeV/c², compared to the null hypothesis.
In contrast, the Tevatron, a proton-antiproton collider with a center-of-mass energy of 2TeV, has a significant production mode for Higgs bosons through quark-antiquark collisions mediated by W or Z bosons (vector boson associated production). Additionally, the Tevatron focused on the decay channel where the Higgs boson decays into bb pairs, which has the largest branching ratio for a light Higgs boson (Figure 4). When W or Z decays, high-momentum electrons or muons, or neutrinos, are emitted with a certain probability, and b-quarks, which have a relatively long lifetime, typically decay slightly away from the collision point. By leveraging these characteristics, the following modes were investigated for Higgs boson production:
qq′ → WH → ℓνbb
qq → ZH → ℓ+ℓ−bb, ννbb
The results are shown in Figures 5-7. It was possible to exclude the Higgs boson mass range of 147-180 GeV/c² and below 103 GeV/c² at a 95% confidence level (Figure 5). Moreover, compared to the null hypothesis, an excess of events was observed with a local significance of 3.0σ and a global significance of 2.5σ in the mass range of 115-135 GeV/c² (Figure 6). For the H→bb mode alone, an excess was observed with a local significance of 3.2σ and a global significance of 2.9σ (Figure 7). Figure 8 shows the invariant mass distribution reconstructed from two-jet events with the b-quark tag. The data is consistent with the background-only distribution within errors, but when the expected H→bb distribution assuming a 120 GeV/c² Higgs boson is overlaid (green histogram in Figure 8), a better match with the obtained data is seen. The results obtained from the Tevatron strongly support the findings from the ATLAS and CMS experiments at the LHC, suggesting a very high probability that evidence for the Higgs boson was observed at the Tevatron as well.
However, particles, including the Higgs boson, have intrinsic properties such as spin and parity in addition to mass and charge (it decays into two neutral particles or one particle and one antiparticle, so the charge is zero). Determining whether the newly discovered particle has the same properties as the theoretically predicted Higgs boson will be a future task. Through these verifications, research in particle physics and cosmology will progress beyond the Standard Model into a new era.