Professor John Butler of Boston University briefed the Dartmouth’s Physics Department on the state of the Large Hadron Collider (LHC) at last Friday’s colloquium. Butler is an experimental high-energy physicist, which places him at the forefront of efforts to test the validity of particle physics’ “theory of almost everything,” the Standard Model. Prior to his current involvement at the ATLAS detector of the LHC, Butler aided in the discovery of the top quark at the Fermilab TEVATRON accelerator. The LHC, which has been in operation for over three years, is currently smashing proton streams together at unprecedented energies to hunt for a missing linking particle in the tried-and-true Standard Model, the Higgs Boson.
Dryly noting “you know that you’ve entered the pop culture zeitgeist when you become an Internet meme,” Butler acknowledged public interest in the frontiers of particle physics, but emphasized the significance of the LHC’s work in his introduction. The Standard Model categorizes various subatomic particles into the broad categories of fermions, which form the basis of matter as spatially separated particles, and bosons, which provide the electromagnetic, weak and strong fundamental forces (gravity’s source remains hypothetical). Theorists predict that the W and Z bosons, which unify the electromagnetic and weak forces, must receive their mass from phenomena associated with the undetected Higgs Boson. Detecting the particle, however, requires enormous amounts of energy – the Higgs Boson itself is predicted to have energy of up to 145 gigaelectron volts (GeV), generating physics that violate the standard model on the energy/mass scale of 1 TeV.
Fortunately, a small window exists at lower proton collision energies to detect decay products of the Higgs Boson. Butler presented results from ATLAS experimentation that suggests the presence of a Higgs Boson signature in a channel corresponding to its decay into gamma rays at 126 GeV. He admitted that attempting to locate a trace of the Higgs Boson is akin to “finding a needle in a needle stack,” and attributed much of the difficulty to the excess of background interactions caused bymore than one billion proton collisions per second in the LHC. While pre-filtering of data to remove extraneous events has proven useful to increase signal-noise ratio in favor of the Higgs Boson, Butler believes that significant particle interactions can be missed in the process. Regardless, results from 2011 suggested that the probability of background noise in the 110-150 GeV range fluctuating to the level observed in the 126 GeV channel was roughly seven percent, a first clue that the Higgs Boson may actually exist.
Butler concluded by predicting a banner year for the LHC in 2012. The LHC is in the process of ramping up the energy of its proton beams to its final collision energy of 14 TeV – enough energy to melt 15 tons of copper – and the results presented by Butler may well be outdated by the summer conference circuit. Regardless of future results regarding the Higgs Boson, however, particle physicist interest in the LHC seems primed to grow. In fact, Butler and other scientists are ready to discard the Higgs Boson and the Standard Model entirely if a new physics presents itself. Many eyes will be trained on data from the LHC in 2012. Over 3,000 collaborators, including roughly 1,000 graduate students, are currently working on the ATLAS project, which serves as only one of six detectors on the LHC circuit. Butler, who called the collaboration as a “United Nations of Science,” expects the LHC to continue to both pose and answer fundamental questions in physics in the near future.
Note: A recording of Professor John Butler’s presentation will be made available on the Dartmouth Physics website https://physics.dartmouth.edu/news/john-butler-boston-university (not uploaded at time of press).