After two years of repairs and upgrades, the world’s largest and most powerful particle accelerator is back in action. The Large Hadron Collider (LHC), located at the European Organization for Nuclear Research (CERN) near Geneva, Switzerland, restarted operations in April and will soon provide data to scientists from all over the world to help them uncover secrets in the universe.
The University of California, Santa Barbara Experimental High Energy Physics Group plays a strong role in the Compact Muon Solenoid (CMS) experiment. The team consists of about 30 graduate students, postdoctoral researchers, engineering staff, and undergraduate students, led by four faculty members of the Physics department—Claudio Campagnari, Jeffrey Richman, Joe Incandela, and David Stuart.
The UCSB group is involved in the detector’s design, construction and operations, software development, data analysis, and the management of several other parts of the experiment. The team is supported by a grant from the Department of Energy.
In 2012, scientists from the CMS and the similar ATLAS experiment at the LHC reported the discovery of the Higgs boson, a subatomic particle that is a telltale sign of the mechanism that gives masses to elementary particles. At the time, CMS was led by UCSB’s Incandela. CMS and ATLAS are both major international projects, in which thousands of physicists from around the world work as a team to build and operate the giant experiments.
The Higgs mechanism was proposed in the 1960s by Peter Higgs, Francois Englert, and several other theorists. “It is a truly astonishing idea,” said Professor Richman. “We know now that elementary particles like the electron acquire their masses by interacting with an invisible field, the Higgs field, which extends throughout all of space.”
According to Richman, what we think of as the vacuum—empty space—actually has complex properties. “The Higgs boson is a particle that is an excitation of this field and its discovery, with all the right properties, gives us experimental verification of the Higgs mechanism,” says Richman, “It was a great moment when Peter Higgs saw the results from CMS and ATLAS and realized that his decades-old prediction had been confirmed. He was in tears.”
The CMS experiment is located in an underground cavern about the size of a four-story building. It lies at one of several interaction regions dotted around the giant, 27-km circumference LHC circular accelerator ring.
Beams of protons circulate at nearly the speed of light around this ring in opposite directions, guided by magnetic fields created by powerful superconducting magnets. The beams collide at a point in the middle of the CMS experiment, where their tremendous kinetic energy is converted into matter.
“We need high energies to produce heavy particles,” said Richman. “The higher the energy, the greater the mass of the particle that can be formed, in accordance with Einstein’s famous equation relating mass to energy: E=mc2.”
Upgrades and repairs performed on the LHC and its detectors will enable scientists to study collisions at even higher energies than in the past.
Some other unanswered questions that CMS wants to look into relate to another theoretical idea called supersymmetry. Supersymmetry connects two classes of elementary particles: bosons and fermions. Fermions have an intrinsic angular momentum, or “spin,” equal to ½, whereas bosons have integer spins. Electrons are fermions, while photons are bosons. Supersymmetry posits that, for every type of particle that is a fermion, there is a corresponding particle that is a boson, and vice versa.
Some theorists believe that the invisible “dark matter” that makes up most of the matter in the universe could consist of a particular type of supersymmetric particle.
“In the dream scenario,” says Richman, “we would discover supersymmetric particles at the LHC, while our colleagues in the Physics department, Professors Harry Nelson and Michael Witherell, would detect astrophysical dark matter in a separate experiment called LUX. Then we would work to find out whether the two particles are the same thing.”