Tevatron experiments close in on Higgs particle
The combined Tevatron results exclude the existence of a Higgs particle with a mass between 100-108 and 156-177 GeV/c2. For the range 110-155 GeV/c2, the experiments are now extremely close to the sensitivity needed (dotted line below 1) either to see a substantial excess of Higgs-like events or to rule out the existence of the particle. The small excess of Higgs-like events observed by the Tevatron experiments in the range from 120 to 155 (see solid curve) is not yet statistically significant.
Scientists of the CDF and DZero collaborations at Fermilab continue to increase the sensitivity of their Tevatron experiments to the Higgs particle and narrow the range in which the particle seems to be hiding. At the European Physical Society conference in Grenoble, Fermilab physicist Eric James reported today that together the CDF and DZero experiments now can exclude the existence of a Higgs particle in the 100-108 and the 156-177 GeV/c2 mass ranges, expanding exclusion ranges that the two experiments had reported in March 2011.
Last Friday, the ATLAS and CMS experiments at the European center for particle physics, CERN, reported their first exclusion regions. The two experiments exclude a Higgs particle with a mass of about 150 to 450 GeV/c2, confirming the Tevatron exclusion range and extending it to higher masses that are beyond the reach of the Tevatron. Even larger Higgs masses are excluded on theoretical grounds.
This leaves a narrow window for the Higgs particle, and the Tevatron experiments are on track to collect enough data by the end of September 2011 to close this window if the Higgs particle does not exist.
James reported that the Tevatron experiments are steadily becoming more sensitive to Higgs processes that the LHC experiments will not be able to measure for some time. In particular, the Tevatron experiments can look for the decay of a Higgs particle into a pair of bottom and anti-bottom quark which are the dominant, hard-to-detect decay mode of the Higgs particle. In contrast, the ATLAS and CMS experiments currently focus on the search for the decay of a Higgs particle into a pair of W bosons, which then decay into lighter particles.
This graph shows the improvement in the combined sensitivity of the CDF and DZero experiments to a Higgs signal over the last couple of years. When the sensitivity for a particular value of the Higgs mass, mH, drops below one, scientists expect the Tevatron experiments to be able to rule out a Higgs particle with that particular mass. By early 2012, the Tevatron experiments should be able to corroborate or rule out a Higgs particle with a mass between 100 to about 190 GeV/c2.
The LHC experiments reported at the EPS conference an excess of Higgs-like events in the 120-150 GeV/c2 mass region at about the 2-sigma level. The Tevatron experiments have seen a small, 1-sigma excess of Higgs-like events in this region for a couple of years. A 3-sigma level is considered evidence for a new result, but particle physicists prefer a 5-sigma level to claim a discovery. More data and better analyses are necessary to determine whether these excesses are due to a Higgs particle, some new phenomena or random data fluctuations.
In early July, before the announcement of the latest Tevatron and LHC results, a global analysis of particle physics data by the GFitter group indicated that, in the simplest Higgs model, the Higgs particle should have a mass between approximately 115 and 137 GeV/c2.
"To have confidence in having found the Higgs particle that theory predicts, you need to analyze the various ways it interacts with other particles," said Giovanni Punzi, co-spokesperson of the CDF experiment. "If there really is a Higgs boson hiding in this region, you should be able to find its decay into a bottom-anti-bottom pair. Otherwise, the result could be a statistical fluctuation, or some different particle lurking in your data."
The CDF and DZero experiments will continue to take data until the Tevatron shuts down at the end of September.
"The search for the Higgs particle in its bottom and anti-bottom quark decay mode really has been the strength of the Tevatron," said Dmitri Denisov, DZero co-spokesperson
"With the additional data and further improvements in our analysis tools, we expect to be sensitive to the Higgs particle for the entire mass range that has not yet been excluded. We should be able to exclude the Higgs particle or see first hints of its existence in early 2012."
The details of the CDF and DZero analysis are described in this note, which will be posted later today, as well as submitted to the arXiv.