High Energy Physics
Physicists Sung-Won Lee (inset left) and Andrew Whitbeck (inset right) are searching for elusive new particles that may hold the key to the nature of the universe. Their work depends on use of CERN's Large Hadron Collider (background), viewed from the inside during construction in this archival photograph. Photo courtesy of CERN.
Physicists Lee & Whitbeck Use CERN
To Search for Exotic New Particles
Written 9.5.19 by Glenys Young
Texas Tech University is well known for its research on topics that hit close to home for us here on the South Plains, like agriculture, water use and climate. But Texas Tech also is making its name known among those who study the farthest reaches of space and the mysteries of matter.
Faculty from the Texas Tech Department of Physics & Astronomy recently presented at the European Physical Society's 2019 Conference on High Energy Physics on the search for dark matter and other new particles that could help unlock the history and nature of the universe.
Texas Tech, led by professor and department chair Sung-Won Lee, has been playing a leading role in new-particle hunt for more than a decade. As part of the Compact Muon Solenoid (CMS) experiment, which investigates a wide range of physics, including the search for extra dimensions and particles that could make up dark matter, Lee has led the new-particle search at the European Organization for Nuclear Research (CERN).
"Basically, we're looking for any experimental evidence of new particles that could open the door to whole new realms of physics that researchers believe could be there," Lee said. "Researchers at Texas Tech are continuing to look for elusive new particles in the CMS experiment at CERN's Large Hadron Collider (LHC), and if found, we could answer some of the most profound questions about the structure of matter and the evolution of the early universe."
The Standard Model
The LHC essentially bounces around tiny particles at incredibly high speeds to see what happens when the particles collide. Lee's search focuses on identifying possible hints of new physics that could add more subatomic particles to the Standard Model of particle physics.
Related article: New ways to approach the most classical search for new particles.
"The Standard Model has been enormously successful, but it leaves many important questions unanswered," Lee said. "It is also widely acknowledged that, from the theoretical standpoint, the Standard Model must be part of a larger theory, 'Beyond the Standard Model' (BSM), which is yet to be experimentally confirmed."
Some BSM theories suggest that the production and decay of new particles could be observed in the LHC by the resulting highly energetic jets that shoot out in opposite directions (dijets) and the resonances they leave. Thus the search for new particles depends on the search for these resonances. In some ways, it's like trying to trace air movements to find a fan you can't see, hear or touch.
In 2018-19, in collaboration with the CMS group, Texas Tech's team performed a search for narrow dijet resonances using a newly available dataset at the LHC. The data were consistent with the Standard Model predictions, and no significant deviations from the pure background hypothesis were observed. But one spectacular collision was recorded in which the masses of the two jets were the same. This evidence allows for the possibility that the jets originated from BSM-hypothesized particle decay.
The illustration above is a three-dimensional display of the event with the second-highest dijet invariant mass at 8 teraelectronvolts. The display shows the energy deposited in the electromagnetic (green) and hadronic (blue) calorimeters and the reconstructed tracks of charged particles (yellow). Illustration courtesy of CERN.
"Since the LHC is the highest energy collider currently in operation, it is crucial to pay special attention to the highest-dijet-mass events where first hints of new physics at higher energies could start to appear," Lee said. "This unusual high-mass event could likely be a collision created by the Standard Model background or possibly the first hint of new physics, but with only one event in hand, it is not possible to say which."
For now, Lee, postdoctoral research fellow Federico De Guio and doctoral student Zhixing (Tyler) Wang are working to update the dijet resonance search using the full LHC dataset and extend the scope of the analysis.
"This extension of the search could help prove space-time-matter theory, which requires the existence of several extra spatial dimensions to the universe," Lee said. "I believe that, with our extensive research experience, Texas Tech's High Energy Physics group can contribute to making such discoveries."
Related article: Enhancing the missing momentum microscope.
Included in the ongoing new-particle search using the LHC is the pursuit of dark matter, an elusive, invisible form of matter that dominates the matter content of the universe.
"Currently, the LHC is producing the highest-energy collisions from an accelerator in the world, and my primary research interest is in understanding whether or not new states of matter are being produced in these collisions," said Andrew Whitbeck, an assistant professor in the Department of Physics & Astronomy.
"Specifically, we are looking for dark matter produced in association with quarks, the constituents of the proton and neutron. These signatures are important for both understanding the nature of dark matter, but also the nature of the Higgs boson, a cornerstone of our theory for how elementary particles interact."
Beyond Higgs Boson
The discovery of the Higgs boson at the LHC in 2012 was a widely celebrated accomplishment of the LHC and the detector collaborations involved. However, the mere existence of the Higgs boson has provoked a lot of questions about whether there are new particles that could help us better understand the Higgs boson and other questions, like why gravity is so weak compared to other forces.
As an offshoot of that finding, Whitbeck has been working to better understand a type of particle called neutrinos.
"Neutrinos are a unique particle in the catalog of known particles in that they are the lightest matter particles, and they only can interact with particles via the Weak force, which, as its name suggests, only produces a feeble force between neutrinos and other matter," Whitbeck said. "Neutrinos are so weakly interacting at the energies produced by the LHC that it is very likely a neutrino travels through the entire earth without deviating from its initial trajectory.
The illustration above is a graphical display of measurements made by the CMS detector. Looking in a two-dimensional projection along the cylinder of the detector, the trajectory of charged particles can be seen in the yellow lines emerging from the center. Streams of these particles are combined into jets grouped in the orange cones, and the size of their combined energy is shown as the sum of blue and green bars. The imbalance of their total momentum is seen in the purple arrow. Could this be a new particle exiting the detector without leaving a measurement, or is it just invisible Standard Model particles that were not detected, like neutrinos?
"Dark matter is expected to behave similarly given that, despite being all around us, we don't directly see it. This means that in looking for dark matter produced in proton-proton collisions, we often find lots of neutrinos. Understanding how many events with neutrinos there are is an important first step to understanding if there are events with dark matter."
Since the discovery of the Higgs boson, many of the most obvious signatures have come up empty for any signs of dark matter, and the latest results are some of the most sensitive measurements done to date. However, Whitbeck and his fellow scientists will continue to look for many more subtle signatures as well as a very powerful signature in which dark matter hypothetically is produced almost by itself, with only one lonely proton fragment visible in the event. The strategy provides powerful constraints for the most difficult-to-see models of dark matter.
"With all of the traditional ways of searching for dark matter in proton-proton collisions turning up empty, I have also been working to design a new experiment, the Light Dark Matter eXperiment (LDMX), that will employ detector technology and techniques similar to what is used at CMS to look for dark matter," Whitbeck said. "One significant difference is that LDMX will look at electrons bombarding a target. If the mass of dark matter is somewhere between the mass of the electron and the mass of the proton, this experiment will likely be able to see it."
Texas Tech also is working to upgrade the CMS detector so it can handle much higher rates of collisions after the LHC undergoes some upgrades of its own. The hope is that with higher rates, they'll be able to see not only new massive particles but also the rarest of processes, such as the production of two Higgs bosons. This detector construction is ramping up now at Texas Tech's new Advanced Physics Detector Laboratory at Reese Technology Center.
Besides being a background for dark matter searches, neutrinos also are a growing focus of research in particle physics. Even now, the Fermi National Accelerator Laboratory is able to produce intense beams of neutrinos that can be used to study their idiosyncrasies, but there are plans to upgrade the facility to produce the most intense beams of neutrinos ever and to place the most sensitive neutrino detectors nearby, making the U.S. the center of neutrino physics. Measurements done with these neutrinos could unlock whether these particles play a big role in the creation of a matter-dominated universe.
Texas Tech's High Energy Physics group hopes that, in the near future, it can help tackle some of the challenges this endeavor presents.