On Thursday, David Tucker-Smith, associate professor of physics, delivered the third talk in this year’s faculty lecture series, titled “Searching for the Higgs at the LHC,” in Wege Auditorium.
In his lecture, Tucker-Smith, a particle physicist, focused on the recent progress scientists have made in their attempts to prove the existence of the the Higgs boson, a hypothetical elementary particle, using the Large Hadron Collider (LHC), the world’s largest and highest-energy particle accelerator. The LHC is located near Geneva, Switzerland.
Physicists use the LHC to collide protons at extremely high speeds to try to produce the Higgs boson, which is also known as the “God particle,” Tucker-Smith explained. In theory its existence accounts for why fundamental particles have mass. “It is crucial in making our world have some necessary properties that we take for granted,” Tucker-Smith said.
He went on to describe the Standard Model of particle physics, in which equations “exhibit a high degree of symmetry” and can therefore be manipulated. To demonstrate this concept called “gauge symmetry,” Tucker-Smith showed the audience an overhead slide depicting a circle. He explained that no matter how much or which way the circle is rotated, it stays the same.
“This is not an accident,” Tucker-Smith said of the Standard Model’s gauge-symmetric equations. “[These equations] don’t have many free parameters, and because of that [they] are remarkably predictive.”
However, he continued, if the equations incorporate the particle’s mass, the symmetry is spoiled. Tucker-Smith demonstrated this problem by depicting a circle with a splotch protruding from it that clearly did not belong. He explained that when mass is accounted for, as when the splotch is accounted for, the equation’s symmetry is spoiled just as is that of the circle, which now could not be rotated in a way that preserves its symmetry.
“We have to have mass, it’s there,” Tucker-Smith said. The solution to the problem with gauge symmetry is the Higgs mechanism, which allows physicists to write down a gauge-symmetric equation and produce outcomes that do not look symmetric.
To illustrate this idea, Tucker-Smith balanced a pencil on its tip, explaining that no matter how still he attempts to keep his hand, there will always be some motion. “The pencil will randomly choose a direction and the equation is not symmetric at all,” he said. The Higgs mechanism is what gives mass to objects and “chooses” a direction, Tucker-Smith explained, illustrating how necessary the Higgs is to explaining the physical world. With the LHC, physicists continue to search for the Higgs particle by condensing “a lot of energy in a small space,” Tucker-Smith said. “It’s energy associated with motion being converted to energy associated with mass.”
Researchers at the European Organization for Nuclear Research (CERN), where the LHC is housed, use the particle accelerator’s 27 kilometer-long collision tunnel to collide hundreds of millions of protons per second in an attempt to produce the Higgs particle. Tucker-Smith explained that while scientists cannot analyze every collision, the large body of data is crucial because “the events that they are looking for are so rare.”
Tucker-Smith explained that because the Higgs is produced once per 10 billion collisions, the Higgs particle should have been produced approximately 100,000 times between the completion of the LHC in 2008 and now. “The bottom line is, we should be able to make the Higgs,” Tucker-Smith said. But he noted that “if you assume the Standard Model, mass is the only thing we don’t know about the Higgs.” He added that, when the Higgs is produced, “it often decays right away into lighter [particles].”
Thus, scientists look for the Higgs’ presence indirectly. The presence of either two photons or four leptons may indicate a decayed Higgs particle, because “it will do that maybe two or three times out of a thousand,” Tucker-Smith said. “This is one of the most promising ways to search for the Higgs particle.”
Scientists then calculate the mass of these lighter particles, which, in theory, “should come out to be the same thing over and over again,” Tucker-Smith said.
While this is not always the case, scientists have pinpointed the range of the Higgs particle’s mass as between 115 and 130 gigaelectronvolts. Tucker-Smith explained that physicists have arrived at this figure by simulating collisions in which no Higgs are present and analyzing the statistical fluctuation between the computer simulations and the actual experiments.
Tucker-Smith explained that the Higgs “has almost been discovered this past year,” and scientists working with the LHC expect three times as much data this year. Next week’s faculty lecture by Associate Professor of Music Marjorie Hirsch is titled “Gothic Tales of Horror and the Music of Franz Schubert.”