For all the fanfare last summer surrounding the Higgs boson, it might have seemed like the search for new elementary particles had come to a triumphant end. The Higgs was, after all, the last missing piece in the Standard Model of particle physics. But even with the big prize bagged, they won’t be boarding up the Large Hadron Collider (LHC) anytime soon.
Despite the Higgs discovery — and partly because of the Higgs — the search for new physics beyond the Standard Model is just getting started. Michael Luk, a Ph.D. candidate in physics and one of four 2013 winners of the Joukowsky Outstanding Dissertation Prize, spent the last two years as part of that search. His award-winning dissertation outlines his work at the LHC in search of a hypothetical particle called a T-quark. If such a particle exists, it could answer some puzzling questions about the Higgs boson and the universe itself.
“The reason we’re looking for these particles is because the Higgs was discovered last year, and its mass hints that our theory about the particles we observe is not the complete picture,” Luk said. “So the idea is to try to find the missing parts of this theory.”
Luk’s dissertation work is the most comprehensive search yet undertaken for T-quarks. “People had looked in a little corner here or a little corner there,” said Ulrich Heintz, professor of physics and Luk’s adviser. “But looking at the whole parameter space up to the energies we can currently reach at the LHC, no one had done that before.”
While the work didn’t turn up any new particles, it did set new constraints on the possible existence of such particles that will help guide new theories. Luk’s methods will also serve as a template, Heintz said, for continuing the T-quark search when the LHC restarts at higher energies in 2015.
The Higgs: A blessing and a curse
The Higgs boson solves a lot of problems in physics. The theory behind the particle predicts a mechanism that breaks the symmetry between electromagnetic force and the weak nuclear force. That broken symmetry explains why quarks and leptons — the building blocks of matter — have mass, while other particles such as photons (particles of light) are massless. The Higgs boson is the physical manifestation of that symmetry-breaking, mass-giving mechanism. Gerald Guralnik, the Chancellor’s Professor of Physics and one of Luk’s dissertation committee members, is one of the originators of the theory.
But the Higgs’ own mass creates a theoretical problem. The Higgs should gain mass through its interactions with other particles, particularly a particle known as the top quark. According to theory, those interactions should cause the Higgs’ mass to be practically infinite at high energies. But when detected at the LHC, the Higgs came in at a relatively svelte 125 GeV (giga-electron volts). That’s 125 times heavier than a proton, but still far from infinity.
That raises a question: Why is the observed mass of the Higgs so small? There are several theories, most of which suggest the existence of undiscovered particles beyond the Standard Model. One of those theories suggests that the top quark, the one that contributes so much mass to the Higgs, has a hidden partner: the T-quark. The T-quark would cancel out the top quark’s contributions to the Higgs’ mass and explain the seemingly unnatural lightness of the Higgs.
While working with the Compact Muon Solenoid (CMS) experiment at the LHC, Luk devised an analytical method to isolate any possible sign of the T-quark amid the unimaginable piles of data gathered by the CMS detector.
Mounds of data
The concept behind the work at the LHC is pretty simple: Smash particles together at high energies and use a detector to see what falls out. But catching a glimpse of a new particle in these collisions is an incredibly rare event. In order to find new particles, or to rule out their existence, physicists need to look at a mind-boggling number of collisions.
The search for the T-quark involved over 19 inverse femtobarns worth of data. One inverse femtobarn is equivalent to around 70 trillion particle collisions. Luk’s task was devising ways to distinguish the possible T-quark signatures from all the unimportant background data gathered by the detector.
“The T-quark decays into particles that look like those produced by expected Standard Model background processes,” Luk explained. So distinguishing signal from background required incredibly sensitive statistical tools. One of the tools Luk used is called a “boosted decision tree,” a technique often used in finance and economic forecasting.
The process is a kind of divide-and-conquer strategy. After plugging in a variable of interest, the algorithm divides the data into two branches, with events that look more like signal on one side and events that look more like background on the other. Each of those branches can then be divided over and over by looking at other variables. By dividing the data recursively, the algorithm helps to get the best possible separation between signal and background.
The technique enabled the researchers to look at multiple variables at the same time. That was important because the T-quark could show up in the detector as any of three different signatures. Luk’s analysis looked simultaneously for all of those signals. “That’s what’s novel about this work,” Heintz said. “No one had looked at the T-quark across these three different final states.”
Unfortunately, there were no convincing signs of the T-quark. The thoroughness of the analysis allowed the researchers to rule out the existence of the particle at masses currently attainable at the LHC. When the collider starts up again at twice the energy in two years, physicists can use the techniques Luk has developed to search higher mass ranges. If it doesn’t turn up then, the T-quark can be ruled out as the source of the Higgs’ mass cancellation.
“Michael ended up determining the best lower limits for the mass of vector-like quarks to date,” Heintz wrote in a letter recommending Luk for the Joukowsky award. “This analysis Michael carried out all on his own and he demonstrated that he really mastered the techniques.”
While Luk was somewhat disappointed that he didn’t find the T-quark, he says that such negative results are an occupational hazard of particle physics.
“The probability of finding a new fundamental particle, such as the one we searched for, is miniscule,” he said. “Sure, it’s slightly disappointing that we didn’t find evidence for this particular particle, but we’re pushing back the shroud of the unknown, gaining more knowledge and understanding of the universe every time we do so, and that’s really what’s interesting when doing research in physics. By not finding evidence for the T-quark, we are setting restrictions on an entire class of new theories and influencing this entire field of research.”
From quarks to chips
Luk also honed skills that will serve him well in the next stage of his career. After graduating, he’ll take a job at Intel helping to develop the next generation of computer chips. The jump from looking for new theoretical particles to making computer chips may seem like a big one, but the necessary skills are similar.
“I’ll be analyzing the mountains of data that are being produced in the development stages of these new technologies, looking for patterns in the data and optimizations in the process,” Luk said. “This is pretty much the same thing that I’ve been doing throughout my Ph.D. — analyzing data and looking for patterns. Many of the statistical tools will be the same, and techniques will be similar.”
Luk says he’s excited about going in a new direction, despite the fact that he thought since age 12 that he would eventually be a physicist.
“It’s still on the leading edge of research but it provides the opportunity to learn about a completely new field and also it gives access to a whole new set of information that I would never otherwise see. It’s a chance I couldn’t pass up.”