The Way I See It:
Particle physics enters new territory
BY KARL ECKLUND
Special to the Rice News
I just returned from a trip to Geneva where I am participating in the largest science experiment ever undertaken. It’s called the Large Hadron Collider (LHC), the massive particle accelerator that set a new record this week at the European physics laboratory called CERN. LHC set a new mark on Tuesday for the highest energy ever achieved in a particle accelerator. The record heralds the start of a new era of discovery in particle physics, an era that Rice faculty and students are helping to bring about.
I was in the control room of the Compact Muon Solenoid experiment, or CMS, on Tuesday when the LHC’s proton beams were accelerated to an energy level of 3.5 trillion electron volts. That’s approximately the amount of energy it takes for a mosquito to buzz around your head, but in the LHC, physicists concentrate that energy into a single proton — instead of the billion-trillion protons in the atoms of a mosquito. With that kind of energy, the protons in the LHC buzz around 27-kilometer tracks at nearly the speed of light!
Thousands of physicists from across the world have spent more than a decade designing, building and assembling the LHC. Rice’s contingent is working on CMS, one of LHC’s four large experiments. The CMS weighs 13,000 tons, and most of it is located deep underground in tunnels where the protons — those particles that are going nearly the speed of light — are smashed together in head-on collisions.
In his famous equation, E=mc2, Albert Einstein illustrated the relationship between energy and matter in our universe. Because of that relationship, physicists can use machines like the LHC to convert huge quantities of energy into massive, exotic subatomic particles that last existed less than one second after the Big Bang.
CMS is a collection of instruments that are designed to measure these exotic particles. The CMS instruments are located in the tunnels where the proton beams collide. The CMS particle detectors are nested, like Russian dolls, and each one is specialized to capture the traces of particular kinds of particles.
My work in the past five years has concentrated on the innermost detector, the silicon pixel detector, which is just four centimeters from the beam collisions. The pixel detector is not very different from a 66-megapixel camera, except that it takes 40 million pictures per second, and instead of recording colors and intensity of light, the pixel detector records the passage of charged particles.
As the detector closest to the head-on proton collisions in the CMS, the pixel detector gives a precise view of the particles that are emerging from the collisions. Work on the detector brought me into collaboration with hundreds of scientists in Switzerland, Austria, Italy, Germany, Belgium, Hungary, France and across the United States. In total, about 2,000 people contributed to the detector and other parts of CMS.
Coming to Rice two years ago, I joined three other Rice faculty who were also working on the CMS project. Paul Padley, Jabus Roberts and Frank Geurts — all from the Department of Physics and Astronomy — contributed to the End Cap Muon system, or EMU, which surrounds the ends of the CMS detector. The EMU will capture the traces of muons, heavier cousins of the electron. These traces can provide telltale clues about the nature of matter, the rules that govern how matter interacts and the structure of space-time itself.
Collecting data from those first record-setting collisions Tuesday was very gratifying for me and all the physicists involved in the project. The work of many years reached a new level as the detectors, so carefully prepared, began to record what happens when two protons collide with a combined energy of 7 trillion electron volts (TeV). All parts of the detector worked in concert, just as the teams who built them had. We saw clearly on the multiple computer monitors the evidence that the beams were actually colliding.
It was the start of the scientific adventure of a lifetime for me and the other physicists on the project, but it is just the beginning. The LHC will run at its current energy level for two years. Then plans call for a much longer run at double the current energy level. There’s still lots of work to bring all the complex of instruments at LHC online. But we have already moved the frontier of particle physics from 2 TeV up to 7 TeV, and it’s time to scout out the new territory.
A good place to start is hunting for the top quark. Physicists discovered this particle a decade ago in collisions at lower energies, and we expect the LHC to produce as many top quarks in the next few months as have been produced in the past 10 years in other detectors. I am working with Vesna Cuplov, a Rice postdoctoral research associate, and a team of international collaborators on the CMS to find evidence of top quarks in LHC data. Over the past year, Rice undergraduate Diego Caballero has also been working with us as part of his senior thesis.
Searching for the top quark is interesting work. It is one of the things we expect to find. Another is the Higgs boson, which has sometimes been referred to as the “God particle.” But the biggest draw for physicists like Paul, Jay, Frank, Vesna, Diego and myself is the prospect of finding something completely new, something no theorist has predicted. What does nature hold for us? We don’t yet know, but this week’s milestone has brought us one step closer to finding out.
–Karl Ecklund is an assistant professor of physics and astronomy at Rice.