Smashing Protons in the Cathedrals of Science
The search for the Higgs boson at the Large Hadron Collider
The picturesque Franco-Swiss borderland between the Alps and the Jura Mountains has long beckoned skiers and hikers from around the world. But on March 30, physicists and journalists converged on this valley, near Geneva, eager to witness history being made 100 meters beneath the surfac
The visitors found their way to the control room of the Compact Muon Solenoid (CMS), a device housed in one of the Large Hadron Collider’s four enormous underground caverns (physicists like to call them “cathedrals of science”) that are connected by 27 kilometers of tunnels. Measuring 15 meters in diameter by 22 meters long and weighing 12,500 tons–more than the Eiffel Tower–the CMS stood ready to send hundreds of billions of protons hurtling toward each other through small vacuum pipes at velocities approaching the speed of light. When they crashed, the device would for the first time measure the precise position and energy of the particles produced by such collisions, giving physicists their first glimpse at the underlying physics.
By lunchtime, the CMS control center was packed. At 12:58 p.m., the rate monitors that track activity inside the CMS detector spiked, revealing the presence of the first proton-proton collisions. Seconds later, the first images of proton interactions and the new particles they produced lit up the screens. Reconstructed charged particles appeared as bright yellow dots with golden tentacles; energy depositions were indicated by red and blue rectangles of different sizes. Wild applause and cheers erupted and went on for minutes. A palpable sense of relief flooded the room. Twenty-five years after the idea of a Large Hadron Collider at CERN was first conceived, and 12 years after construction began, the LHC had achieved the highest-energy proton-proton collisions ever, beginning a new era in particle physics.
At CERN headquarters that evening, the LHC host lab’s restaurants buzzed with discussions of the day’s events. Like the usual lively conversations about snow quality in the Alps, the World Cup in soccer, and the latest developments in physics, computing, and engineering, these exchanges were conducted in English at various levels of proficiency, though a dozen other languages could also be heard. Reaching the milestone of the first collisions at seven teraelectronvolts was cause for celebration. A failure on that day would have been really bad for our field.
Since March, the LHC has performed well, producing numerous collisions. The first observations of W and Z boson events in LHC experiments are being discussed; because we had already measured the bosons’ properties with very high precision in smaller detectors, they serve as “standard candles” for alignment and calibration of the LHC. The first top quarks, the heaviest known elementary particles, are expected to appear soon. Scientists have known about these particles for many years, but the physics community is greatly excited at the prospect of seeing evidence of them firsthand at the LHC. And that excitement will only intensify if we see truly new phenomena.
It may take years before we find answers to such big questions as where the mass of elementary particles comes from, what the dark matter observed in galaxies consists of, and whether supersymmetry and the extra dimensions proposed by string theory really exist. But we are getting closer, now that we’re able to create new massive particles and probe matter at very small scales. The results of the LHC experiments will set the agenda for future generations of particle physicists.
The goal of my team at MIT is to find the long-sought-after Higgs boson, whose discovery would yield information about how particles acquire mass. Though the Higgs boson entered the theoretical landscape 40 years ago, it remains elusive because it is rarely produced and decays instantaneously. The experimental challenge is to identify the signature of its decay products–a task complicated by the existence of other processes with similar signatures. The challenge can be compared to finding not a needle in a haystack but, rather, one distinct piece of hay. I began working on this as an undergraduate almost 12 years ago, and now the scientific community finally has the tools in hand to make the discovery possible. It’s an exciting time to be a particle physicist–and a once-in-a-lifetime opportunity to be working at the center of this excitement.
Markus Klute is an assistant professor of physics and leads the MIT team searching for the Higgs boson at the LHC. To see images from the March 30 CMS experiment, go to technologyreview.com/LHC.