A joint Fermilab/SLAC publication

Keep it simple, SUSY

05/03/11
This CMS event display from October 2010 captured a collision that produced jets - showers of particles that leave energy deposits in the detectors - and missing energy, represented by the blue line at the bottom left. Experimentalists concluded that this amount of missing energy, 693 GeV, was too small to be a SUSY particle. (Image courtesy CMS/CERN.)

This CMS event display from October 2010 captured a collision that produced very energetic jets - showers of particles that leave energy deposits in the detectors - and an exceptional amount of missing energy, represented by the blue line at the bottom left. Experimentalists and theorists are continuing to analyze collision events such as this one in search of new physics.(Image courtesy CMS/CERN.)

Experimentalists at the Large Hadron Collider recently proved effective a simple, new method of looking for evidence of supersymmetry. The method addresses a challenge particle physicists often face in looking for new particles and processes: They can manifest themselves in a multitude of ways.

How new physics will reveal itself is anybody’s guess. It’s a bit like the game Plinko on “The Price is Right” – contestants drop a token at the top of a pegboard and watch hopefully as it zigzags toward a slot at the bottom that may or may not contain a prize. If the game is played enough times, the token is likely to retrace some of the same routes, but occasionally it will take an unexpected path to the prize.

And so it goes at the LHC. Energy from proton collisions transforms into a handful of massive particles, which can decay in seemingly endless ways to any number of final particles. Collide enough protons and eventually the rarest particles will appear among the commonly produced ones.

“The set of possibilities for finding new physics is so large that it can be overwhelming,” said theoretical physicist Jay Wacker from SLAC National Accelerator Laboratory in California. “We’re trying to go about systematically exploring all of these possibilities. We want to make sure no stone is left unturned.”

Using the first round of data taken in 2010, LHC experimentalists sifted through several billion collisions looking for rare signatures that could represent supersymmetry. They looked where theory stated that these signatures were most likely to appear – the tokens that took a particular path down the pegboard and ended up in a certain slot on the bottom – and came up empty-handed.

The experimental groups are still looking for supersymmetry under different conditions, but it would take far too long to analyze every possible combination of initial particles, decay paths and final particles. To solve this problem, a group of theorists came up with a way to look for supersymmetry in the widest area possible.

Wacker and his colleagues designed a search that is sensitive to a number of different particle signatures that appear in the aftermath of a high-energy proton collision. The goal, he said, was to come up with the easiest way to cover the most possible areas where new physics might pop up.

The search looks at a class of events called jets plus missing energy – proton collisions that result in a shower of hadronic particles plus a stable, neutral particle that escapes detection – and ignores events that show signs of electrons or muons.

Both the theorists and the experimentalists looked only at the pile of tokens that landed in a particular slot at the bottom of the Plinko board. While the experimentalists had a set of guidelines about how the tokens should have gotten there and excluded any tokens that didn’t follow the rules, the theorists didn’t care as much about that. They were primarily concerned with the mass of the initial particles, the mass of the final particles and the ratio between them.

When the initial massive particles decay into lighter ones, the total energy must be conserved. Sometimes this energy goes missing; if the missing energy adds up to a certain amount, it could mean that a supersymmetric particle carried it away without being detected.

“These models are really nice because they allow us to think in terms of particles, not abstract parameters,” said CMS deputy spokesperson Joseph Incandela, from the University of California, Santa Barbara. “As particle physicists, we like that.”

The theorists’ approach also does not consider each individual possibility, but rather a few combinations of particle masses, decay paths and missing energy ranges that are a well-rounded representation of all the possibilities.

From the set, Wacker’s group proposed two dozen model signatures, each of which describes the masses and behaviors of a set of particles corresponding to a spot within the search region. The ATLAS group applied these models to their SUSY search.

“The good news is that they work,” said ATLAS physicist Zach Marshall, who helped test the search strategy against the 2010 LHC data. “We were basically asking, ‘If the signature really did look like that, would we have seen it last year?’ And the answer is that we can exclude many of those points.”

The models not only verify last year’s searches, they also help to optimize them. The groups now have a better understanding of potentially missed supersymmetry signatures and can state much more clearly the mass limits on particles, Marshall explains. What’s more, the models extend even beyond the constraints set by the experiments and are sensitive to many different supersymmetry theories.

“They essentially broke down a very important class of supersymmetry models and found the common denominators,” Incandela said. After certain limits are set with these simple models, scientists will gradually add more variables back into the equation. This will narrow down the number of potential new physics events and will help scientists to get a better handle on what they see in the detectors.

Both the CMS and ATLAS experiments will use the search again as they collect and analyze data from the 2011 run. Meanwhile, the theorists are working to expand these models and apply them to other slots at the bottom of the Plinko board.

“These models created a search more extensive than what has been used, and with reasonable efficacy,” Wacker said. “But this is just the first step in showing what kinds of new physics the LHC is sensitive to.”

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