PROVIDENCE, R.I. [Brown University] — The world’s most powerful particle accelerator has ended its two-year hiatus. The Large Hadron Collider restarted over the weekend and over the next year will smash particles at nearly twice the energy as its previous run. Brown University physicists working at the collider are excited to see what the machine turns up.
Four Brown physicists, along with several postdoctoral researchers and graduate students, will continue to work at the LHC, which is headquartered in Geneva, Switzerland. Other group members will be connected over the net from Brown or from Fermilab, near Chicago. The Brown group is a member of CMS, one of the two large-scale experiments going on at the LHC. The first test beams started circling the collider’s 17-mile underground ring last Sunday. Data collection from particle collisions will start soon after testing is complete.
“It’s a very exciting time because we’re crossing a new energy threshold,” said Greg Landsberg, a Brown physicist who recently completed a term as physics coordinator for CMS. “We’re hopeful that at these higher energies we’ll see something spectacular in this next run.”
The collider’s first run from 2010 to 2013 certainly didn’t lack spectacle. In 2012, the machine detected the elusive Higgs boson, the manifestation of an invisible field that gives some elementary particles their mass. The Higgs was the last missing piece of the standard model of particle physics, but its discovery also suggests that the standard model likely isn’t the end of the story.
“Even though it completes the standard model, the Higgs also tells us that something else is out there,” said Meenakshi Narain, professor of physics.
In particular, the observed mass of the Higgs creates a sticky theoretical problem. According to theory, the Higgs should gain mass from its interactions with other particles — particularly the top quark, heaviest of the known quarks. At high energies, the Higgs’ interactions with the top quark should make its mass practically infinite. Yet when detected at the LHC, the Higgs tipped the scales at a rather svelte 125 gigaelectron volts (GeV). Explaining the lightweight Higgs in terms of the standard model requires some fancy mathematical footwork — too fancy for many physicists. That has fueled the search for explanations outside the standard model.
Narain and her students will be looking for signs of one such explanation — a heavy partner to the top quark. The heavy partner, if it exists, would counteract the top quark’s contribution to the Higgs’ mass and explain why the particle is so light. There were no signs of the heavy partner in the collider’s first run, but higher energy increases the chance of making heavier particles. So this next run may detect the heavy partner or rule it out.
Narain, who is leading the LHC Physics Center at Fermilab, played a key role in the discovery of the top quark at the Tevatron collider in 1995. “It would be great if we were to discover its partner almost exactly 20 years later,” she said.
But the top quark partner isn’t the only possible explanation for the lightweight Higgs. Another is supersymmetry — the idea that all standard model particles have shadowy cousins with slightly different properties. Among the predictions made by supersymmetry is that there should be additional Higgs bosons hiding out there. Ulrich Heintz, along with a postdoctoral researcher and a student, will be looking for those.
“If there were supersymmetry, there would have to be more Higgs bosons,” Heintz said. “In supersymmetric theories, you end up with five different kinds of Higgs bosons, in the least.”
It’s possible, Heintz says, that there’s a heavier Higgs with a mass of 300 GeV that decays into two standard 125 GeV Higgs particles. The higher-energy run should eventually create of few of these heavier Higgs bosons if they exist.
“The higher energy gives us an increase in the number of heavy things we can make if they exist,” Heintz said. “That’s the real boost that we’re getting here. We may be able to access particles that were just out of range with the last run.”
In addition to the heavy Higgs search, Heintz is also managing an upgrade to the readout electronics of the CMS detector. The upgrade will help the detector deal better with the higher frequency of collisions in the next run.
David Cutts and his graduate student have been looking for another potential manifestation of supersymmetry in the form of so-called long-lived particles. Most particles formed as a result of high-energy collision decay within microseconds. But some theories suggest that there should be heavy particles that survive much longer and travel farther from the site of the collision.
“To find some of these long-lived particles would be an unambiguous sign of something new,” Cutts said. “For example, there are varieties of supersymmetry that would give rise to production of a heavy particle that would have a significant lifetime.”
Cutts and his student are in the process of recalibrating their search for the next run. Collisions will happen more frequently in the higher-energy regime. Since their search is focused on the space between collisions, they must change their search parameters to account for the fact that collisions happen more frequently.
Landsberg will be looking for yet another particle outside the standard model — a dark matter particle. In particular, Landsberg’s search for dark matter will build on a potential discovery made by Savvas Koushiappas, another Brown physicist. Last month, Koushiappas and his colleagues detected a potential gamma ray signal coming from a dwarf galaxy, which could be due to dark matter particles at the galaxy’s center slamming into each other and annihilating into gamma rays and quarks. It might be possible, Landsberg said, to produce the same reaction at the LHC, only in reverse.
“Instead of two dark matter particles annihilating to produce quarks, we might be able to take a pair of quarks and annihilate them to produce dark matter particles.”
The collider is scheduled to run nonstop throughout this year until a brief pause in December. The Brown physicists expect many of the questions they and other physicists are looking into will be answered this year.
“The next run could be one in which we find all these new particles that send theorists back to the chalkboard to make sense of all of it,” Narain said. “It’s really a great time to be in particle physics.”