|This plot shows the final data in the exclusive (photon + missing energy) final state, along with the backgrounds as modeled from data. The signal region for this analysis is from 2 to 7 nanoseconds.|
The CDF collaboration has just completed a new search for “delayed” photons, this time in the exclusive photon-plus-missing-energy final state. Delayed photons are a smoking-gun signature of heavy, long-lived particles that are often predicted in gauge-mediated supersymmetry-breaking scenarios. If produced in a collision, a long-lived particle can traverse part of the detector before finally decaying into a photon and a non-interacting particle or particles. Thus the photon will take longer to reach the detector than expected, compared to a photon that originated in the main proton-antiproton interaction—hence the “delayed” term in the moniker.
With the CDF electromagnetic timing system capable of recording the arrival time of photons with a resolution of 0.6 nanoseconds, many timing searches are possible. What is unique about this search is the final state under study: exclusive photon plus missing energy. This has never been done before because of its innate complexity; previous searches of this kind, performed both at CDF and at the LHC, have always relied on jets in the final signature to reduce the backgrounds.
The analysis variable is Δt, defined as the difference between the measured arrival time of the photon and the expected arrival time, assuming the photon travels unimpeded from the main interaction to the detector. New particles would show up as an excess of events in the signal region, which corresponds to Δt values from 2 to 7 nanoseconds.
After analyzing 6.3 inverse femtobarns of data, the analyzers found that the data (322 events) was in agreement with the Standard Model background prediction (286 ± 24 events), showing no evidence of delayed-photon production in the exclusive photon-plus-missing-energy final state. Because of the numerous new-physics models that can produce such a final state, a minimal-supersymmetric-Standard Model Higgs boson decaying into neutralinos being just one example, the findings of this analysis are presented purely as a model-independent signature-based search. With the last third of the Run II Tevatron data at CDF still to be analyzed, this never-before-investigated region still has the opportunity to yield something new and interesting. Stay tuned.
—edited by Andy Beretvas