CMU physicists explore the foundations of the universe
In recent decades, significant advances in particle physics have been made through experiments conducted by particle accelerators. While the most well-known and talked about accelerator is the Large Hadron Collider (LHC), there are many other accelerators around the world, including the Beijing Electron-Positron Collider (BEPC) and the Thomas Jefferson National Accelerator Facility in Virginia — all of which Carnegie Mellon professors have worked with.
The LHC is a gigantic particle accelerator that lies 100 meters underground between the borders of Switzerland and France. It accelerates protons or lead ions inside its circular track, which then collide head-on at high energy levels. The particles created by these collisions are then analyzed in detectors.
“Carnegie Mellon has been involved in the design and construction of the CMS,” said physics professor Helmut Vogel, one of several faculty members working with the LHC, in reference to the Compact Muon Solenoid detector. “We also helped write the software and build part of the hardware. We are now involved in data analysis.”
One of the main goals of the LHC is to detect the elusive Higgs boson, a particle theorized to give other particles mass. However, many of the particles that the LHC seeks to discover, including the Higgs, have lifespans that are much too short to leave tracks in the detectors. Instead, scientists use simulations to predict the decay products of such particles, and look for these in the detectors.
“We have found something that in some ways behaves like the predictions for the Higgs, although we don’t have the amount of data yet to tell with high precision,” Vogel said. Further research supporting this newly discovered Higgs-like particle would prove that the current model of how the world works is correct.
Meanwhile, a different type of particle physics experiment is going on at the BEPC in China. While the work being done at the LHC is referred to as the “energy frontier,” the BEPC focuses more on the “intensity frontier.” Instead of searching for the existence of undetected particles, precision measurements of particular known particles are studied to test how well their behavior agrees with theoretical models.
“Electron-positron collisions are used to produce various bound states of charm quarks,” wrote Carnegie Mellon physics professor Roy Briere, who collaborates with the BEPC, in his research statement. Data from this collider will answer important questions, such as the dominance of matter over antimatter in the universe.
In Virginia, the Jefferson Lab is probing the behavior of quarks. Michael Darcy, a junior physics major who interned with the lab over the summer, described the Continuous Electron Beam Accelerator Facility, the lab’s particle accelerator: “It accelerates electrons with superconducting radio-frequency cavities around a racetrack-shaped tunnel.”
“Jefferson Labs currently has three running experimental stations called Hall A, B, and C,” he said, describing the lab. “Electrons from the beamline can be split into these three different rooms and each room has a different kind of particle detector in it.” A fourth hall is currently under construction and will be used to perform the GlueX experiment, for which physics professor Curtis Meyer is the spokesperson. GlueX aims to discover exotic mesons — a postulated but yet-undiscovered type of meson made of two quarks and two anti-quarks.
So why is it important to spend time and money on these immense projects? Vogel gave two reasons.
The first is cultural: “What is the benefit of knowing the Earth revolves around the sun and not the other way around? It is knowledge that contributes to the culture of society. Even if it has no direct impact on building a better microwave, or better cars.... It is valuable knowledge.”
His second reason is more practical for those seeking a career: “Particle accelerators are immensely complicated apparatuses that take people with very sophisticated skills to build. These experiments are training a new generation of people with enormous skills that are very marketable, and very sought after, in many fields.”