Search for Higgs Boson (4)
 Figure 1 : Higgs boson and the Standard Model particles. The Higgs boson is a particle predicted as the origin of mass for all elementary particles. (Courtesy of Fermilab Visual Media Service) |
 Figure 2 : The peak luminosity of each store in the proton-antiproton collisions at the Tevatron accelerator. |
 Figure 3 : The integrated luminosity obtained at the CDF experiment. The black line shows the luminosity for the proton-antiproton beam intersections in the CDF detector, and the purple line shows the recorded luminosity as data. |
This is a follow-up report on the search for the Higgs boson in the CDF experiment, in which the High Energy Physics Laboratory of Osaka City University is participating. The Higgs boson is a particle predicted by the Standard Model of particle physics as the origin of mass for all elementary particles (Figure 1). For our previous efforts in this research, please refer to our earlier articles (High Energy Physics Laboratory News, dated August 7, 2008, May 15, 2008, and September 4, 2007).
More than two years have passed since the last report, and during that time, the amount of data collected in the CDF experiment has steadily increased. As of October 2010, it has roughly doubled compared to August 2008. In experiments like the CDF experiment, where particle beams collide head-on from opposite directions, the statistical amount of the data is expressed as “integrated luminosity.” Luminosity, also known as “brightness” in Japanese, is defined as L = N/σ, where N is the number of reactions occurring per unit time in the beam intersection, and σ is the reaction cross-section. Although there could be lack precision, simply put, luminosity is the number of particle encounters per unit time and area. The integrated luminosity is the amount of luminosity accumulated over the entire experiment time. When proton and antiproton beams are loaded into the Tevatron accelerator to conduct collision experiments, it is called a “store”. Figure 2 shows the peak luminosity at the start of collisions for each store, along with the store numbers. Despite short-term fluctuations due to accelerator troubles and adjustments, the overall luminosity is steadily increasing. This indicates continuous improvement in accelerator performance, thanks to the efforts of the accelerator researchers at Fermilab. On April 17th this year, the highest value of 409.6×1030 cm−2s−1 was recorded. Figure 3 shows the integrated luminosity obtained from the CDF experiment along with store numbers. In this figure, the black line represents the luminosity delivered by the beam intersections in the CDF detector (Delivered luminosity), and the purple line represents the portion of luminosity actually recorded as data after subtracting detector downtime (Acquired luminosity). In August 2008, the acquired luminosity (purple line) was 4 fb−1, but by October 2010, it had reached 8 fb−1.
Figure 4 : Plot of the ratio between an upper limit of the Higgs boson production cross section obtained from the CDF/DØ experiments and the predicted cross section of the Standard Model as a function of the Higgs mass. It can be said that the possibility that there exists the Higgs boson having the mass from 158 to 175 GeV/c2 is extremely small since the observed upper limit is lower than the theoretical prediction.
This indicates significant progress in the search for the Higgs boson. Data analysis takes time, so as of October 2010, results using up to 5.9 fb−1 of data have been published. Unfortunately, the Higgs boson has not yet been discovered, but from the experimental fact of “not being discovered,” we statistically derive the upper limit of the production cross-section with 95% confidence level and express this upper limit as a ratio to the Standard Model’s predicted value (Figure 4). This result is an analysis of combined data from the two experiments conducted at the Tevatron, the CDF experiment and the DØ experiment. Since both detectors have nearly identical performance, combining the two datasets approximately doubles the statistical power. This figure shows that the experimental upper limit falls below the theoretical prediction in the mass range of 158 to 175 GeV/c2. This means that a Higgs boson, as predicted by the Standard Model, does not exist in this range with 95% statistical confidence level. Compared to the previous article (High Energy Physics Laboratory News, dated August 7, 2008), significant improvement is evident.
The Tevatron accelerator will continue to operate next year in 2011, and data collection for the CDF and DØ experiments will also continue. This is expected to increase the integrated luminosity by about another 30%, improving the sensitivity for detecting the Higgs boson. Whether the Higgs boson exists as theorized, and if so, what its mass is, remains a topic of keen interest for future research results.