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Search for New Particles

The High Energy Physics group at the University of Tennessee has been part of the hunt for the Higgs boson, working with the international collaboration that built and maintains the LHC's Compact Muon Solenoid detector, or CMS.

In 2012 scientists announced that experiments conducted at the Large Hadron Collider (LHC), the most powerful particle accelerator in the world located at CERN, the European Organization for Nuclear Research, have discovered the Higgs boson, the elusive subatomic particle that brings the so-called "theory of almost everything" closer to completion.

The Higgs boson plays a critical role in what physicists call The Standard Model: the arrangement of fundamental particles—and the way they influence one another—that make up everything in the universe. It has proven greatly successful in explaining experimental results and predicting new phenomena since it came of age in the 1970s. The model's architecture includes the 12 building blocks of matter, divided into quarks (which constitute, for example, protons and neutrons) and leptons, the lightest one being the electron. Additional particles carry three of the four fundamental forces that influence quark and lepton behavior (the strong force, weak force, and electromagnetic force). The Higgs particle is a cornerstone of this theory because it's predicted to give all particles their mass by interacting with them. It is named for Peter Higgs, one of the scientists who in 1964 proposed its role in fundamental physics. It is the last experimentally observed particle in the Standard. There are some shortcomings though such that the theory doesn't take into account gravity or the dark energy that comprises most of the universe.

Higgs Event

(Image credit: CERN/CMS) A typical candidate event including two high-energy photons whose energy (depicted by red towers) is measured in the CMS electromagnetic calorimeter. The yellow lines are the measured tracks of other particles produced in the collision. The pale blue volume shows the CMS crystal calorimeter barrel.

The Large Hadron Collider is an underground, 17-mile ring that straddles the French-Swiss border and accelerates protons to enormous energies in opposite directions. Every second, protons collide head-on more than 40 million times at particular locations surrounded by layers of particle detectors. The results of these collisions can be new particles or other phenomena. With multiple layers, the CMS detector can observe these remnants and track their signatures, providing scientists with data to piece together what happened at the heart of a collision.


Graduate student Grant Riley installing the particle tracker called Pixel Luminosity Telescope (PLT) close to the LHC beam pipe. The PLT is the innermost sub-detector of CMS and measures the flux of particles from proton-proton collisions.

Far from Over

There is still much work to be done. After all, the Standard Model does not yet incorporate the physics of dark energy or the full theory of gravitation as described by general relativity. But even the mass of the Higgs, though well constrained by theory, appears a bit like an accident unless there are other force particles involved that still need to be discovered. This means the hunt is far from over. With a signal for the Higgs particle, scientists can now conduct further analysis to determine how compliant its properties really are with the Standard Model predictions. Discrepancies can be first hints that there is more to the model or a new world beyond the Standard Model opens up. As there are many different ways for the Higgs particle to decay for example and the number of Higgs particles produced in proton-proton collisions is small, many more collisions with higher and higher intensities of the proton beams have to be continued. Even more exciting is the prospect to uncover signals for new particles, maybe in unexpected places. This requires to develop smart search strategies and not leave any stone unturned.

Strong computing support is required to analyze the huge amount of data recorded with the CMS detector, and the UT group has a 10-gigabit network connected directly to Fermilab (which receives data from CERN), as well as a computer cluster based on the shared computing concept that has been developed for high energy physics, called GRID. This setup has translated into broader benefits for UT as well: the Newton cluster uses these concepts to provide shared computing resources for researchers across the university.

New detection technologies are needed to keep pace with the increasing beam intensities at the LHC. In this vein, he UT group is studying pixelated artificial diamond detectors with particle beams. The group is exploring whether this technology is radiation hard and can replace particle detectors inside the CMS detector to prepare for even higher beam intensities. Meanwhile, a new particle tracker based on silicon has been installed inside CMS in 2015 to measure inclusively the rate of all particles produced in proton-proton collisions. It is crucial input to turn any particle counting of new signals into absolute measurements. Undergraduate, graduate students, and postdocs from UT were part of constructing and implementing the new instrument and support its operations.


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