University of Minnesota
School of Physics & Astronomy


Trigger Man

Bryan Dahmes
Bryan Dahmes
Alex Schumann

Sifting through data is a monumental task on physics experiments and the larger the experiment, the more the data piles up. At the Compact Muon Solenoid experiment at the Large Hadron Collider in CERN, Switzerland, the problem is so great that physicists can’t even record all of the events created by the collider. “We get data at a rate of more than a million events per second," says Bryan Dahmes a Research Associate at the University of Minnesota, who lives and works at CERN.

"If we had infinite resources and disc space and could save everything we have, then we could sit down and crunch the data forever." Since the experiment doesn’t have infinite resources, the selection from billions of events to an interesting few has to happen very early in the process. To help make that selection, physicists have invented a "trigger," a computer program which allows them to select which events will be of interest to the various searches at CMS, one of which was the hunt for the Higgs Boson.

"In the context of the Higgs discovery, everyone is obviously very excited. We’ve run for two years exceeding our expectations, and at the end of the day, you have maybe a couple hundred Higgs events total," Dahmes says. But the Higgs is just the beginning, and Dahmes, as the person in charge of the operation of the triggering system for CMS, needs to have a broad knowledge of the various priorities across the whole experiment. As such Dahmes and his team of students have designed a broad program to study in detail collect things both expected and hoped for, that that expect to see, or things they hope to see, using theory as a guide. "It’s the job of the trigger to make sure these events can be selected. This is where that you want to make sure your software and hardware is very reliable so that you can very quickly find the interesting signatures and you don’t spend a lot of time processing garbage." Currently, Dahmes and his team sifting through the are looking at data of collisions spaced 50 nanoseconds apart. This is accomplished by looking at the data with using a farm of 5,000 very high speed computers, 5,000 of the best PCs they could build. All of these computers Working together in synch, these computers decide which events to save.

Right Handed W Boson

"The Higgs is a big deal. People have been looking for it for decades," Dahmes says. Experts in particle theory predicted it and a particle matching the characteristics of the Higgs boson has now been observed. Theorists expected to see and it was found. But Dahmes points out that there are mountains of untested particle theory waiting to be explored. One such direction is the search for the Right-Handed W Boson at CMS, which is an entirely Minnesota-led effort.

Particle theory tends to be symmetrical and when a symmetry is broken, it usually an indication that something strange is happening. The weak interaction, surprisingly, appears to use only is entirely left-handed spin. "There must be some point where the weak interaction behaves like everything else and has some right handed spin at higher energies." Theorists have postulated, that a right-handed counterpart at higher energy which would make everything would be symmetric again. As Dahmes explains, neutrinos twenty years ago, Dahmes explains were thought to be completely waveless and massless. As a result, One consequence of this was that you could only ever find the left-handed neutrino in nature – as a massless, weakly-interacting particle you can only make the left-handed particles. A few years ago physicists measured neutrino oscillation. The easiest way for the neutrino to oscillate, is for it to must have mass and a right-handed form which has never been observed. But then there would have to be some right-handed interactions. Except that no detector has observed what they call the right-handed sector. "So you have a dead particle that only exists to make the math work." Dahmes is involved in a search for right-handed W Bosons that decay to right-handed neutrinos. "Now it turns out that at very high mass, the normal theory doesn’t have a lot of events that could show up. With the data we’ve accumulated this year, we are already cutting into the space where theory says this could exist." If the right-handed W exists, we know from other measurements that it must be heavier than 2.5 TeV and the latest results from Dahmes and his team exclude the right-handed W up to 2.8 TeV, providing new powerful constraints on the ambidexterity of the universe