Drinking from the fire hose: searching for new physics at the world's largest machine

August 13, 2015

As an experimental high-energy physicist, Assistant Professor Mike Hance's research studies particle interactions by colliding them at the highest possible energies. To do this, he works on the ATLAS experiment, one of two multi-purpose experiments collecting data from the CERN Large Hadron Collider, or LHC, located near Geneva, Switzerland. Joining ATLAS colleagues at UCSC and the Santa Cruz Institute for Particle Physics (SCIPP), he's working to understand whether our existing model for particle interactions, the Standard Model, is still valid at LHC energies, and whether there are any new particles hiding in the huge number of events collected every day by ATLAS. One recent LHC discovery was the existence of the Higgs boson, a particle predicted by the Standard Model that is intimately connected with why other particles have mass. To make this discovery, the experiments sifted through over a quadrillion events to find a few thousand that look like they might contain a Higgs boson, a signal to background ratio of one over a trillion! By continuing to sift through this mountain of data, Hance and the UCSC ATLAS group hope to make more new discoveries in the years ahead.

As a graduate student at the University of Pennsylvania, Hance worked on the construction and installation of the Transition Radiation Tracker (TRT), a part of the inner tracking detector of ATLAS. The TRT consists of almost 300,000 gas-filled straws used to map the passage of charged particles emerging from the collision region. Hance and the Penn group had primary responsibility for the electronics used to process the signals coming from the straws, and for the software used to format the TRT data for analysis. The data from the TRT is useful in identifying electrons and other charged particles, as well as for reconstructing the trajectory of energetic photons that interact with detector material and produce electron-positron pairs. Almost half of all photons "convert" this way in a large detector like ATLAS, and understanding that process was important in the search for Higgs bosons decaying to pairs of photons, one of the ways the Higgs was eventually discovered. For his thesis, Hance worked on the first measurement of energetic photon production in ATLAS, an important stepping stone to the Higgs to diphoton search.

As a Chamberlain Fellow at Lawrence Berkeley National Laboratory, Hance continued his research on the ATLAS experiment, first focusing on the search for the Higgs boson, which was discovered in 2012. As part of the discovery, we also learned that its mass is relatively small, around one hundred times heavier than a proton, but still less than three-quarters the mass of the top quark. This makes the Higgs boson heavy enough that it evaded detection in earlier experiments, but light enough that many theories predict that new physics will be hiding at energies that the LHC can now access.

So, after the Higgs discovery, Hance shifted his attention to searches for physics beyond the Standard Model. Focusing on events with multiple leptons and/or bosons, Hance worked on several searches for new physics models that might explain the mass of the Higgs, the non-zero masses of neutrinos, and the particle nature of dark matter. He served as the leader of the ATLAS working group on searches for exotic new physics with multiple leptons and bosons, and oversaw their preparations for the second data-taking period of the LHC, which began this summer. With the LHC now colliding protons at a new world-record energy of 13 TeV, the possibilities for discovery are greater than ever!

In joining the ATLAS group at UCSC/SCIPP, Hance will continue his research on LHC physics. One ongoing focus will be the search for new physics in the data that is currently being collected. The increased energy and rate of collisions from the accelerator will make the next few years an extremely exciting time in particle physics, and Santa Cruz will be leading several searches for supersymmetry, dark matter, and more exotic models of physics beyond the Standard Model.

Another important project will be preparing for an upgrade of the ATLAS experiment, which will be needed to cope with the increased collision rate expected from the accelerator in the middle of the next decade. The instrumentation group at SCIPP is a world-recognized authority in designing and building silicon tracking detectors, and SCIPP has a leading role in designing the upgraded ATLAS silicon tracker. When the R&D phase ends and construction begins, Santa Cruz will be an assembly site for one of the largest and most sophisticated tracking detectors ever built.

Finally, even as the LHC moves into the heart of its physics program, plans for the next generation of particle colliders are already being discussed. One option for a future facility would be a machine like the LHC, but seven times as powerful, that would come online in the middle of the century. While this may be a long ways off, the planning for such large projects can take decades. Hance and others in SCIPP are already thinking about how to use some of the novel detector technologies developed in Santa Cruz to build future experiments, to ensure that any discoveries made at the LHC can be studied with an even more powerful machine by the next generation of physicists.