Only five of the about 100 PhD students who graduated in 2011 received the ATLAS Thesis Award 2011, and we are very proud to know that Christian Ohm, PhD student at the Oskar Klein Centre is one of them. The other winners are Michael Hance, David Lopez Mateos, David Miller and Verena Martinez Outschoorn.
“It is very exciting!” tells me a radiant Christian when he joins me in my office “if you think that the other four are all Americans, and I am the only European! And they come from places like Harvard, Caltech and Berkeley!”
The five winners were selected by the CB Chair Advisory Committee, which acted as the Thesis Awards Committee, from a selection of 21 nominations received.
My thesis centers around two searches for beyond Standard Model (SM) physics but contains published work of technical nature as well. The two physics analyses I contributed to are both a signature-driven searches for new physics giving rise to so-far unobserved massive long-lived particles. The ATLAS experiment was built primarily to directly detect SM particles, and most searches for new physics rely on the assumption that new states produced in the pp collisions will decay promptly and leave SM particles detectable in the final state. Many extensions of the SM feature new particles with decay lengths on the scale of modern collider experiments, and these two analyses instead search for new physics based on direct detection. Christian explains.
The first search exploits that these particles are typically expected to be produced with momentum comparable to their mass, meaning they would be slow-moving. The sought signature was based on penetrating particles using time-of-flight measurements from the hadronic Tile calorimeter (developed at Stockholm University) and ionization energy loss measurements in the innermost tracker, the Pixel detector. No excess above the expected background yields were observed in the 2010 data set, corresponding to 34 pb-1, and cross-section limits were calculated as a function of mass. The results were interpreted as mass limits for long-lived gluinos (e.g. Split Supersymmetry) and long-lived squarks (R-parity violating SUSY, etc). A stable gluino with mass up to 586 GeV was excluded, as were stop and sbottom squarks with masses below 309 and 294 GeV, respectively.
The second search is less driven by theory and looks for possibly signals of new physics using more of a “blue-sky” approach. The target of this search were heavily ionizing long-lived massive particles, i.e. new states
possessing either high electric charge or magnetic charge (motivated by e.g. Dirac’s explanation of electric charge quantization based on the existence of a magnetic monopole). Such particles are expected to undergo dramatic energy losses when passing through the detector material, and frequently come to a stop before escaping ATLAS. Using ionization energy loss measurements from the outermost tracker, the Transition Radiation Tracker, and shower shape variables in the electromagnetic calorimeter based on liquid argon technology, SM background processes were rejected efficiently while maintaining sensitivity to the sought signal. Also here the observations in experimental data (3 pb-1) matched those expected for a background-only hypothesis, and generic cross-section limits were calculated in windows of geometric and kinematic acceptance.
During his PhD, Christian continued some technical work which he had initiated earlier, on a system based on simple electrostatic beam pick-ups used to monitor the proton beams and timing signals of the Large Hadron Collider (LHC). The system proved particularly important during the start-up of the LHC and was used in ATLAS to trigger and record the very first pp collision events.
The thesis was defended in November, but since he spent most of his phd at CERN, Christian is now finishing his last courses while thinking about the next step. He has received offers for a postdoc at New York University and a CERN fellowship, and recently interviewed with LBL (Berkeley). “I’ve been very lucky and most likely I’ll return to CERN in May, though it’s still not clear who I will work for. But first thing’s first, March and April are reserved for snowboarding, an attempt to finally take a much overdue driver’s license, and some traveling”
The award ceremony will take place during the ATLAS Week, on Thursday 2 February 2012 at 14:00 in the Main Auditorium. Congratulations Christian!