Measurement of the diffractive longitudinal structure function

First measurements are presented of the diffractive cross section σep→eXY at centre-of-mass energies √ s of 225 and 252 GeV, together with a precise new measurement at √ s of 319 GeV, using data taken with the H1 detector in the years 2006 and 2007. Together with previous H1 data at √ s of 301 GeV, the measurements are used to extract the diffractive longitudinal structure function F L in the range of photon virtualities 4.0 ≤Q2 ≤ 44.0 GeV2 and fractional proton longitudinal momentum loss 5× 10−4 ≤ xP ≤ a e-mail: daum@mail.desy.de bAlso at Physics Department, National Technical University, Zografou Campus, 15773 Athens, Greece cAlso at Rechenzentrum, Universitat Wuppertal, Wuppertal, Germany dAlso at University of P.J. Safarik, Kosice, Slovak Republic eAlso at CERN, Geneva, Switzerland fAlso at Max-Planck-Institut fur Physik, Munchen, Germany gAlso at Comenius University, Bratislava, Slovak Republic hAlso at Faculty of Physics, University of Bucharest, Bucharest, Romania iAlso at Ulaanbaatar University, Ulaanbaatar, Mongolia jSupported by the Initiative and Networking Fund of the Helmholtz Association (HGF) under the contract VH-NG-401 kAbsent on leave from NIPNE-HH, Bucharest, Romania lOn leave of absence at CERN, Geneva, Switzerland mSupported by the Bundesministerium fur Bildung und Forschung, FRG, under contract numbers 05H09GUF, 05H09VHC, 05H09VHF, 05H16PEA nSupported by the UK Science and Technology Facilities Council, and formerly by the UK Particle Physics and Astronomy Research Council 3× 10−3. The measured F L is compared with leading twist predictions based on diffractive parton densities extracted in NLO QCD fits to previous measurements of diffractive Deep-Inelastic Scattering and with a model which additionally includes a higher twist contribution derived from a colour dipole approach. The ratio of the diffractive cross section induced by longitudinally polarised photons to that for transversely polarised photons is extracted and compared with the analogous quantity for inclusive Deep-Inelastic Scattering. oSupported by FNRS-FWO-Vlaanderen, IISN-IIKW and IWT and by Interuniversity Attraction Poles Programme, Belgian Science Policy pPartially Supported by Polish Ministry of Science and Higher Education, grant DPN/N168/DESY/2009 qSupported by the Deutsche Forschungsgemeinschaft rSupported by VEGA SR grant No. 2/7062/27 sSupported by the Swedish Natural Science Research Council tSupported by the Ministry of Education of the Czech Republic under the projects LC527, INGO-LA09042 and MSM0021620859 uSupported by the Swiss National Science Foundation vSupported by CONACYT, Mexico, grant 48778-F wRussian Foundation for Basic Research (RFBR), grant no 1329.2008.2 xThis project is co-funded by the European Social Fund (75%) and National Resources (25%)–(EPEAEK II)–PYTHAGORAS II ySupported by the Romanian National Authority for Scientific Research under the contract PN 09370101 zPartially Supported by Ministry of Science of Montenegro, No. 051/3-3352