Experiments at Fermilab have placed new constraints on the mass of the Higgs particle that suggest that it might be within reach of the Tevatron. Tevatron scientists have also detected rare processes and tightened constraints on some exotic particles. These are among the many results from the Tevatron presented at the April Meeting.
The Tevatron smashes together protons and antiprotons with combined energies near 1.96 TeV. Kevin Lannon of Ohio State University said in a press conference that the Tevatron is now generating data at its highest rate ever. The key to success is sophisticated data analysis, he said.
Among the recent results Lannon described is a new measurement of the top quark mass. The Tevatron scientists report a top quark mass of 170.9 GeV, with 1% uncertainty. This measurement gives indirect information on the mass of the Higgs particle, said Lannon.
Lannon also discussed evidence for the extremely rare production of single top quarks via a weak-force process. Top quarks are usually produced in top-antitop pairs by a strong force process. The DZero collaboration at Fermilab identified about 60 single top events out of billions of collisions.
The rate of single top production places constraints on the parameter V_tb, which is related to the probability of a top quark decaying into a bottom quark. The Tevatron single-top data limit V_tb to lie between about .68 and 1. This provides strong evidence that only the six known types of quarks exist, said Lannon. Continued data analysis will constrain V_tb further. “This is not the end of the story for single tops. It’s just the beginning,” said Lannon.
Another April Meeting speaker, Gerald Blazey of Northern Illinois University, said that the latest measurement of the W mass and the top mass favor a light Higgs. The new W mass (80.4 GeV), along with the new top quark mass, constrains the Higgs mass to be less than 144 GeV, with 95 percent confidence, Blazey said. This value, slightly lower than previous limits, puts the Higgs potentially within reach of the Tevatron. The Higgs boson is the only particle predicted by the standard model that has not yet been detected.
Ulrich Heintz of Boston University described Tevatron searches for some exotic particles and new physics beyond the standard model. No such particles have been observed, but the Tevatron research has put some limits on several possibilities. They have excluded squarks and gluinos below about 300 to 400 GeV, and placed limits on neutralinos and non-standard Higgs particles, said Heintz. Tevatron searches have also placed mass limits on other exotic particles, including leptoquarks, excited gravitons, massive non-standard W and Z bosons, and excited electrons, said Heintz. However, new physics might be soon found at energy scales of around a few TeV, he suggested.