Major Accelerators Closing in on Elusive Higgs Particle

The game’s afoot! Particle physicists at Fermilab’s Tevatron and CERN’s Large Hadron Collider (LHC) are closing on the last remaining undiscovered particle in the Standard Model: the Higgs boson, thought to pervade the vacuum of space, interacting with particles to give them mass. According to various speakers at the APS April Meeting in St. Louis, physicists are fast approaching the energies and luminosities required to detect the Higgs particle.

Fermilab’s Tevatron is reaching its performance peak, with energies quite sufficient to create a particle in the expected energy range for the Higgs: between 114 GeV and 190 GeV, according to current theoretical calculations. The primary issue is luminosity, or the density of the beam particles that collide per second, and the Tevatron recently set a record high luminosity of 3.1 x 10³²/cm², raising hopes that the accelerator might beat the long-awaited LHC to the punch.

Brian Winer of Ohio State University said that the “most Higgs-like Higgs event” observed to date at the Tevatron involved a proton-antiproton collision in April 2005 that produced a fireball which then decayed into a W boson and a Higgs particle. The Higgs in turn quickly decayed into a bottom-antibottom quark pair with a combined mass of 120 GeV.

However, this does not constitute “discovery” of the Higgs, since it is just one event. The Tevatron would have to find a substantially larger number of candidate events than would be expected from the usual noise of background events that could mimic the Higgs signature.  According to Winer, only time and further luminosity improvements will tell whether enough Higgs events have been collected to constitute a statistically significant “discovery.” Fermilab physicist Dmitri Denisov estimated that when the CDF and D0 collaborations begin to wrap up in 2010, luminosity would probably be twice what it is now, and as much as 4 to 8 times more data would have been analyzed.

Should Fermilab fail to uncover the Higgs, the LHC’s higher collision energy is expected to produce an abundance of the elusive particle. Official estimates from CERN’s leadership indicate the cool-down process for the LHC’s magnets should be complete by mid-June, with the first beam injection occurring two months later. Although the accelerator is designed to produce proton beams at 7 TeV, initially the LHC will produce beams at a much lower 5 TeV.

Abraham Seiden of the University of Santa Cruz presented a timeline plotting the data to be collected at the LHC as a function of time, pointing out where key expected discoveries are most likely to be made. Potential milestones include discovery of the Higgs particle around 2009, assuming it is around 200 GeV in mass. Should the Higgs be closer to 120 GeV in mass, the chart indicates discovery around 2011, since it is harder to detect at that lower energy because it decays into a key signature involving photons that is very similar to other decay signatures.

LHC data should also provide evidence for supersymmetry in 2009 if the energy scale for supersymmetry breaking turns out to be 1 TeV. Should the appropriate energy scale be 3 TeV, that discovery would more likely show up much later, around 2017. If there are extra dimensions of space, scientists might be able to detect them when energy scales reach 9 TeV in 2012. Evidence for a new type of Z' force, assuming it exists, is unlikely to be observed until at least 2019.