Tevatron's Higgs Quest Quickens

Physicists from Fermilab’s Tevatron collider have just reported their most comprehensive summary yet of physics at the highest laboratory energies. At last week’s American Physical Society (APS) meeting in Jacksonville, Florida they delivered dozens of papers on a spectrum of topics, many of which are related in some way to the Higgs boson.

The Higgs is the cornerstone ingredient in the standard model of high energy physics. It is the particle manifestation of the curious mechanism that kicked in at an early moment in the life of the universe: the W and Z bosons (the carriers of the weak force) became endowed with mass while the photon (the carrier of the electromagnetic force) did not. This asymmetry makes the two forces very different in the way they operate in the universe.

Validating this grand hypothesis by actually making Higgs particles in the lab has always been a supreme reason for banging protons and antiprotons together with a combined energy of 2 TeV. Nature is prodigal in its creativity, however, and the search for Higgs is expected to be shadowed by the production of other rare scattering scenarios, some of them nearly as interesting as the Higgs itself.

The Tevatron labors can be compared to work at the Burgess Shale, the fossilbed in the Canadian Rockies where archeologists uncovered impressions of organisms that hadn’t been seen in 600 million years, including some new phyla. No new phyla (no new particles) were reported at the Florida meeting, but much preparatory work-the necessary chipping away of outer layers at the physicists’ equivalent of a high-energy “rockface”-was accomplished. According to Jacobo Konigsberg (Univ Florida), co-spokesperson for the CDF collaboration (one of the two big detector groups operating at the Tevatron, the search for the Higgs is speeding up owing to a number of factors, including the achievement of more intense beams and increasingly sophisticated algorithms for discriminating between meaningful and mundane events, a bread-and-butter issue when sifting through billions of events.

Here is a catalog of some of the freshest results from the Tevatron. Kevin Lannon (Ohio State) reported a new best figure (170.9 GeV, with at uncertainty of 1%) for the mass of the top quark. Lannon also described the class of event in which a proton-antiproton smashup resulted in the production of a single top quark via a weak-force interaction, a much rarer event topology than the one in which a top-antitop pair is made via the strong force.

Moreover, observing these single-top events allows a first rudimentary measurement of Vtb, a parameter (one in a spreadsheet of numbers, called the CKM matrix, that characterize the weak force) proportional to the likelihood of a top quark decaying into a bottom quark. Gerald Blazey (Northern Illinois Univ), former co-spokesperson of the D0 collaboration, reported on the first observations of equally exotic collision scenarios, those that feature the simultaneous production of an observed W and Z boson, and those in which two Z bosons are observed.

Furthermore, he said that when the new top mass is combined with the new mass for the W boson, 80.4 GeV, one calculates a new likely upper limit on the mass of the Higgs. This value, 144 GeV, is a bit lower than before, making it just that much easier to create energetically. Ulrich Heintz (Boston Univ) reported on the search for exotic particles not prescribed by the standard model.

Again, no major new particles were found, but further experience in handling myriad background phenomena will help prepare the way for what Tevatron scientists hope will be their main accomplishment: digging evidence for the Higgs out from a rich seam of other particles. To start with, Heintz broached but then dismissed rumors of pseudo-Higgs “bumps” in the data. The artefacts in question-the presumed exotic particle decaying into a pair of tau leptons-were of too low a statistical stature to take seriously, he said, at least for now.

Other exotic particles not found, but for which there are now new lower mass limits, include such things as excited (extra heavy) electrons or Z and W bosons, extra dimensions, so-called leptoquarks (which turn bosons into leptons and vice-versa), and supersymmetric particles, a whole hypothetical family of particles for which all known bosons would have fermion counterparts and vice versa. Besides the consideration of having enough energy in the collision to create the Higgs and other interesting particles, a vital requirement in producing rare eventualities is possessing a large statistical sample.

All the results above are based on a data-recording sample of one inverse-femtobarn (fb^-1), a unit denoting the integrated amount of scattering events up till now. By the end of the summer, the amount of data analyzed will be two fb^-1. By the end of 2007, the amount will have doubled again, and by 2009 doubled once again (8 fb^-1). For finding the Higgs, energy and statistics will tell.