Number 861 #1, April 16, 2008 by Phil Schewe
Finding The Higgs Boson.
Is the imperative of the two most powerful particle accelerators ever built---the Tevatron at Fermilab, now reaching the peak of its decades-long performance, and the Large Hadron Collider (LHC) at CERN, where beams will circulate for the first time around a 27-km track within the next few months. The Higgs has not yet been discovered, but at this week’s meeting of the American Physical Society (APS) in St. Louis dozens of talks referred to the status of the Higgs search. Why is the Higgs so important? Because it is thought to pervade the universal vacuum; not, as with the old aether, to provide a material substrate for the propagation of electromagnetic waves, but rather to interact with particles and confer mass upon them.
The Higgs’ ministrations are usually hidden away in the vacuum, but if enough energy is brought to bear in a tiny volume of space---at the point where two energetic particles collide---then the Higgs can be turned into an actual particle whose existence can be detected. Theoretical calculations made using the standard model of particle physics combined with previous experiments serve to limit the possible range of masses for the Higgs particle. Right now that mass is thought to be larger than 114 GeV but smaller than about 190 GeV.
The Tevatron delivers more than enough energy to create a particle in that energy range. The main issue, then, is luminosity, or the density of beam particles crashed together per second. The Tevatron recently established a record high luminosity: 3.1 x 10^32 per cm^2 per second. That would a Higgs event look like? One speaker at the meeting, Brian Winer (Ohio State), said that the “most Higgs-like Higgs event” seen so far was on in which (it is surmised) the proton-antiproton collision at the Tevatron had created a fireball which then decayed into a W boson (one of the carriers of the weak nuclear force) and a Higgs particle.
The Higgs in turn quickly decayed into a bottom-antibottom quark pair whose combined mass amounted to 120 GeV. By itself such an event does not constitute a discovery. Successfully observing the Higgs involves finding an inventory of candidate events substantially larger than the number of expected background events from collisions which to not produce a Higgs particle but which mimic some features of the Higgs. Time (and luminosity) will tell whether the Tevatron accumulates enough Higgs candidate events to establish a statistically-satisfactory “discovery.”
One Tevatron physicist, Dmitri Denisov (firstname.lastname@example.org) summarized the likely status of things when the experiments (the CDF and D0 detector groups) start to wrap up in the year 2010. The luminosity, he said, would probably be twice what it is now and that 4 to 8 times more data would be analyzed than is available today. The Higgs, if it exists, is expected to show up in abundance at the LHC, where the collision energy is much higher than at the Tevatron. Abraham Seiden (email@example.com) of UC Santa Cruz summarized the current status of the LHC.
In the CERN lab scientists and engineers are now chilling down the magnets which steer protons around their proper trajectory to the near-absolute-zero temperatures needed for operating in a superconducting mode. Although designed to produce proton beams at 7 TeV, the accelerator will at first hold to a more conservative 5 TeV. As for the present schedule, Seiden quoted a recent CERN report specifying mid June as the time when the machine would be cooled and ready to circulate beams around the ring and August as the time when actual particle collisions will commence. However, several scientists at the meeting, when asked, were somewhat skeptical that this timeline would be met.
As for the prospective scenario for discoveries at LHC in coming years, Seiden said that finding evidence for a supersymmetric particle (one of a large family of hypothetical particles) might be possible as early as the year 2009, while finding the Higgs might be possible by 2010.
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