Search for Higgs Boson
Figure 1 : An image of Higgs particles tangling in a particle trying to move with a speed of light.
“How did the mass of matter come to be?” This simple question has intrigued many people for a long time. In reality, the “mass” of matter is acquired in stages at various hierarchical scales. So, where does the mass of the “elementary particles,” the smallest units of matter, come from? Physics is striving to answer this question. Through years of research in particle physics, it has been understood that the acquisition of mass is significantly related to the properties of the “vacuum.” In 1964, British physicist P. W. Higgs proposed a theory that particles acquire mass due to a phase transition in the vacuum caused by spontaneous symmetry breaking. This mechanism is known as the Higgs mechanism. Although the vacuum might seem to be empty, it is not so when delved into the world of elementary particles. According to the Higgs mechanism, the current vacuum is filled with a field called the “Higgs field,” which has a finite vacuum expectation value and constantly interacts with elementary particles. The particles causing this interaction are called “Higgs particles,” denoted by the symbols H or h. In the early universe, elementary particles moved freely at the speed of light, but due to the phase transition in the vacuum, Higgs particles began to interact with them, causing the elementary particles to slow down and thus acquire mass (Figure 1).
 Figure 2 : Production processes of a Higgs particle. (a): Gluon fusion, (b): Vector boson associated production. |
 Figure 3 : Production cross-sections of the Higgs particle. (Red lines: Vector boson associated production, Blue line: Gluon fusion) |
 Figure 4 : Decay modes of the Higgs particle. |
 Figure 5 : Invariant mass distribution of H → bb candidates. |
 Figure 6 : Upper limits of the Higgs particle production cross-section obtained at CDF+DØ. |
To verify this idea, it is essential to detect Higgs particles. Although attempts have been made with large accelerators, Higgs particles have yet to be discovered. Exploration experiments with the Large Electron-Positron Collider (LEP) at CERN, which started operating in 1989, indicated that if Higgs particles exist, their mass MH must be heavier than 114.4 GeV/c2. However, LEP was shut down in 2000 to make way for the construction of the Large Hadron Collider (LHC), scheduled to begin operations next year. As of 2007, Tevatron at Fermilab is the only facility worldwide capable of searching for Higgs particles.
Tevatron is a proton-antiproton collider with a center-of-mass energy of 1.96 TeV. The primary production processes for Higgs particles are “vector boson associated production” (Figure 2(a)), where Higgs particles are emitted through intermediate states of vector bosons from quark-antiquark collisions:
qq′ → WH, qq → ZH
and “gluon fusion” (Figure 2(b)) where Higgs particles are formed by gluon-gluon collisions:
gg → H
The cross-sections of these production processes are theoretically calculated to be between 0.1 and 1 pb, as shown in Figure 3. With Tevatron’s current luminosity of approximately 35 pb−1/week, it is estimated that about 40 Higgs particles are produced weekly. However, the Higgs particles, being unstable, decay immediately after their production, and we can only observe the final state where they have decayed into stable particles. The decay modes are shown in Figure 4, where MH < 130 GeV/c2 mainly decays into bb, and MH > 130 GeV/c2 primarily decays into WW. A b quark further decays into lighter quarks, and a W boson also decays into a pair of leptons or quarks.To observe Higgs particles, it is necessary to filter out the decay events of Higgs particles from a large number of background events using the distribution characteristics of kinematic variables, resulting in an observation efficiency of around 0.1%. Therefore, even if 40 Higgs particles are produced weekly, several years of data would be required to observe them.
The CDF experiment at Tevatron (with participation from Osaka City University) and the DØ experiment have continuously collected data since 2001, and approximately 2.5 fb−1 of data has been accumulated so far. This time, the data analysis of 1 to 2 fb−1 of proton-antiproton collision events was completed, and the search results for Higgs particles were announced. The analysis of each mode concerning the generation and decay:
qq′ → WH → ℓνbb
qq → ZH → ℓ+ℓ−bb / ννbb
gg → H → WW → ℓ+ℓ−νν
Figure 5 shows the result for qq′→ WH → ℓνbb. This figure represents the invariant mass distribution of bb candidates, and if Higgs particles exist in sufficient statistics, a peak like the red line in the figure will appear. Although no signals indicating the production of Higgs particles were obtained in this analysis, including other modes, the upper limit of the Higgs particle production cross-section was calculated from this fact, as shown in Figure 6. The figure represents the ratio of the theoretical prediction. While the current sensitivity has not yet reached the theoretical prediction value (the line marked SM at 1 on the vertical axis), it is less than twice that for some Higgs masses. As Tevatron continues to collect and analyze data, the sensitivity will increase, and Higgs particles might be discovered soon.