High-mass Higgs searches from CDF and DZero

The top (CDF) and bottom (DZero) images show the expected and observed 95 percent confidence level upper limits on the production rate of a Higgs boson as a multiple of the Standard Model prediction, assuming standard model decay branching ratios. The solid, horizontal line shows the prediction for the Higgs boson according to the Standard Model. We determine our measurement by how our data relates to this solid line. The figures have two squiggly lines: one dotted and one solid. The dotted line shows what we expected to measure and is surrounded by a bright green band. The band shows how certain we were in our prediction. The best way to interpret this is that the bright green region shows the area where we predicted our measurement should be.

The Standard Model of particle physics needs the Higgs mechanism to explain why all the particles in our universe have mass, but no experiment has yet observed the elusive Higgs boson. Answering the question of whether the Higgs mechanism is correct or whether something else is responsible for the masses of particles is central to our understanding of nature. Many physicists around the world have spent decades searching for the Higgs boson. This week, a crucial step forward in this quest has been made by the CDF and DZero experiments.

All recent Higgs boson mass exclusions have come from combinations of results from more than one experiment. Despite the importance of such combined statements, it is an important milestone when a single experiment reaches the level of sensitivity necessary to rule out or see the Higgs boson. Late last week, the CDF and the DZero experiments crossed this threshold individually. The CDF and DZero experiment collaborations recently updated their Higgs boson searches in the high mass range (130 to 185 Gev). In this range, the Higgs boson mass is high enough to allow it to decay to a pair of W bosons.

Together, the Tevatron experiments put to good use an additional 1.5 inverse femtobarns of data collected since their joint result from last summer, and added several new improvements to their analysis techniques. The new data and improvements have allowed both Tevatron experiments to exclude a portion of the Higgs boson mass range: 158 to 168 GeV for CDF and 163 to 168 GeV for DZero. The CDF and DZero experiments have also combined their results; the region thus excluded is 158 to 173 GeV. A Higgs boson of mass 165 GeV is now excluded at the unprecedented level of more than a 99.5 percent confidence level.

Fermilab currently expects the Tevatron to keep recording data until September 2011. CDF and DZero are also ideally suited to look for the Higgs boson in the low mass range, where the Higgs would decay mainly into bottom quarks. CDF and Dzero expect to present new results in this search region later this year. This large data sample, along with expected analysis improvements, will allow the experiments to either exclude the Higgs boson over the entire mass range of interest if it does not exist or to see hints of it – representing a major breakthrough in our understanding of nature.

— Edited by Andy Beretvas