Search for Higgs Boson (5)
Figure 1 : Tevatron accelerator at Fermilab.
We are pleased to report another advancement in the search for the Higgs boson in the CDF experiment, in which the High Energy Physics Laboratory at Osaka City University is participating. The Higgs boson is a particle that is considered the origin of the mass of all elementary particles in the Standard Model of particle physics. For information on our previous efforts in this research, please refer to earlier articles (High Energy Physics Laboratory News dated October 14, 2010; August 7, 2008; May 15, 2008; and September 4, 2007).
At the end of last September, the Tevatron accelerator (Figure 1), where the CDF detector is installed, ceased operations. This update presents the results of analyzing nearly all the data collected up to that point. The Tevatron had two independent detectors, CDF and DØ, both of which ultimately collected data amounting to a statistical amount of 10 fb−1 and proceeded with their analyses.
 Figure 2 : Higgs boson production process through gluon fusion, which is the most important channel at LHC. |
 Figure 4 : Higgs boson production process through vector boson associated production, which is the most important channel at the Tevatron. |
 Figure 4 : Appearance of a b quark decay. It decays at a location (Secondary Vertex) slightly away from the collision point of a proton and an antiproton (Primary Vertex). |
 Figure 5 : Plot of the ratio between an upper limit of the Higgs boson production cross section obtained from CDF/DØ experiments and the prediction of the Standard Model of particle physics as a function of the Higgs boson. A signal excess is seen in the Higgs boson mass range of 115 to 135 GeV/c2 with the statistical significance more than 2σ compared with the assumption that there is no Higgs boson. |
In December of last year, the results of the search conducted by the ATLAS and CMS detectors installed at the LHC (Large Hadron Collider) of the European Organization for Nuclear Research (CERN) were announced, revealing signs of the Higgs boson in the mass region of 124 to 126 GeV/c². At the LHC, protons are accelerated to 3.5 TeV and their head-on collisions create a center-of-mass energy state of 7 TeV, aiming to generate the Higgs boson from these collisions. According to theoretical calculations, in the very high-energy collisions of 3.5 TeV protons, many Higgs bosons are expected to be produced through the fusion of two gluons. After being produced, the Higgs boson quickly decays into various modes, but if it decays into a quark pair, it becomes difficult to distinguish from QCD jet events, which are also produced at a very high rate from proton-proton collisions. Therefore, the ATLAS and CMS experiments focused on the channel where the Higgs boson decays into two photons (H → γγ, Figure 2). The results showed an excess of events in the relatively light mass region of 124 to 126 GeV/c², with a local statistical significance of 3.6 to 2.6 sigma, and an overall significance of 2.3 to 1.9 sigma compared to the case where there is no Higgs boson.
In contrast, the Tevatron is a proton-antiproton collider, so a significant portion of the Higgs bosons are produced via quark-antiquark collisions through W or Z bosons (vector boson associated production, Figure 3). When W or Z bosons decay, they emit high-momentum electrons, muons, or neutrinos with a certain probability, which can be used as clues for the search. Even if the Higgs boson itself decays into quarks, if the quarks are b quarks, they can be identified. Since b quarks have relatively long lifetimes, they often decay at a location slightly away from the collision point, and by reconstructing the tracks of the particles produced from the decay, it can be determined whether they were b quarks (Figure 4). Therefore, the Tevatron explored the Higgs boson in complementary modes different from the LHC:
qq′ → WH → ℓνbb
qq → ZH → ℓ+ℓ−bb, ννbb .
The results are shown in Figure 5. Compared to the assumption that there is no Higgs boson, an excess of events with a statistical significance of more than 2σ was observed in the mass range of 115 to 135 GeV/c². This result does not contradict the results from the ATLAS and CMS experiments at the LHC, and it might indicate that the Tevatron also saw signs of the Higgs boson. Unfortunately, since the Tevatron has been shut down, no more data can be collected. We now await the LHC experiments to definitively confirm the existence of the Higgs boson. If the Higgs boson is indeed discovered, it would be a result achieved through the collaboration of researchers around the world across multiple experiments.