Precise measurement of the absolute fluorescence yield of the 337 nm   band in atmospheric gases
  A measurement of the absolute fluorescence yield of the 337 nm nitrogen band, relevant to ultra-high energy cosmic ray (UHECR) detectors, is reported. Two independent calibrations of the fluorescence emission induced by a 120 GeV proton beam were employed: Cherenkov light from the beam particle and calibrated light from a nitrogen laser. The fluorescence yield in air at a pressure of 1013 hPa and temperature of 293 K was found to be $Y_{337} = 5.61\pm 0.06_{stat} \pm 0.21_{syst}$ photons/MeV. When compared to the fluorescence yield currently used by UHECR experiments, this measurement improves the uncertainty by a factor of three, and has a significant impact on the determination of the energy scale of the cosmic ray spectrum. 
AIRFLY Collaboration: M. Ave, M. Bohacova, E. Curry, P. Di Carlo, C.   Di Giulio, P. Facal San Luis, D. Gonzales, C. Hojvat,...
astro-ph.IM hep-ex physics.ins-det