May 2000 Measurement of the Proton Structure Function F 2 at Very Low Q 2 at HERA ZEUS

A measurement of the proton structure function F2(x,Q ) is presented in the kinematic range 0.045GeV < Q < 0.65GeV and 6 ·10 < x < 1 ·10. The results were obtained using a data sample corresponding to an integrated luminosity of 3.9 pb in ep reactions recorded with the ZEUS detector at HERA. Information from a silicon-strip tracking detector, installed in front of the small electromagnetic calorimeter used to measure the energy of the final-state positron at small scattering angles, together with an enhanced simulation of the hadronic final state, has permitted the extension of the kinematic range beyond that of previous measurements. The uncertainties in F2 are typically less than 4%. At the low Q values of the present measurement, the rise of F2 at low x is slower than observed in HERA data at higher Q and can be described by Regge theory with a constant logarithmic slope. The dependence of F2 on Q 2 is stronger than at higher Q values, approaching, at the lowest Q values of this measurement, a region where F2 becomes nearly proportional to Q. The ZEUS Collaboration J. Breitweg, S. Chekanov, M. Derrick, D. Krakauer, S. Magill, B. Musgrave, A. Pellegrino, J. Repond, R. Stanek, R. Yoshida Argonne National Laboratory, Argonne, IL, USA p M.C.K. Mattingly Andrews University, Berrien Springs, MI, USA G. Abbiendi, F. Anselmo, P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, G. Cara Romeo, G. Castellini, L. Cifarelli, F. Cindolo, A. Contin, N. Coppola, M. Corradi, S. De Pasquale, P. Giusti, G. Iacobucci, G. Laurenti, G. Levi, A. Margotti, T. Massam, R. Nania, F. Palmonari, A. Pesci, A. Polini, G. Sartorelli, Y. Zamora Garcia, A. Zichichi University and INFN Bologna, Bologna, Italy f C. Amelung, A. Bornheim, I. Brock, K. Coböken, J. Crittenden, R. Deffner, H. Hartmann, K. Heinloth, E. Hilger, P. Irrgang, H.-P. Jakob, A. Kappes, U.F. Katz, R. Kerger, E. Paul, H. Schnurbusch, A. Stifutkin, J. Tandler, K.C. Voss, A. Weber, H. Wieber Physikalisches Institut der Universität Bonn, Bonn, Germany c D.S. Bailey, O. Barret, N.H. Brook, B. Foster, G.P. Heath, H.F. Heath, J.D. McFall, D. Piccioni, E. Rodrigues, J. Scott, R.J. Tapper H.H. Wills Physics Laboratory, University of Bristol, Bristol, U.K. o M. Capua, A. Mastroberardino, M. Schioppa, G. Susinno Calabria University, Physics Dept.and INFN, Cosenza, Italy f H.Y. Jeoung, J.Y. Kim, J.H. Lee, I.T. Lim, K.J. Ma, M.Y. Pac Chonnam National University, Kwangju, Korea h A. Caldwell, W. Liu, X. Liu, B. Mellado, S. Paganis, S. Sampson, W.B. Schmidke, F. Sciulli Columbia University, Nevis Labs., Irvington on Hudson, N.Y., USA q J. Chwastowski, A. Eskreys, J. Figiel, K. Klimek, K. Olkiewicz, K. Piotrzkowski, M.B. Przybycień, P. Stopa, L. Zawiejski Inst. of Nuclear Physics, Cracow, Poland j B. Bednarek, K. Jeleń, D. Kisielewska, A.M. Kowal, T. Kowalski, M. Przybycień, E. RulikowskaZarȩbska, L. Suszycki, D. Szuba Faculty of Physics and Nuclear Techniques, Academy of Mining and Metallurgy, Cracow, Poland j A. Kotański Jagellonian Univ., Dept. of Physics, Cracow, Poland k L.A.T. Bauerdick, U. Behrens, J.K. Bienlein, C. Burgard, D. Dannheim, K. Desler, G. Drews, A. Fox-Murphy, U. Fricke, F. Goebel, P. Göttlicher, R. Graciani, T. Haas, W. Hain, G.F. Hartner, D. Hasell, K. Hebbel, K.F. Johnson, M. Kasemann, W. Koch, U. Kötz, H. Kowalski, L. Lindemann, B. Löhr, M. Mart́ınez, M. Milite, T. Monteiro, M. Moritz, D. Notz, F. Pelucchi, M.C. Petrucci, M. Rohde, P.R.B. Saull, A.A. Savin, U. Schneekloth, F. Selonke, M. Sievers, S. Stonjek, E. Tassi, G. Wolf, U. Wollmer, C. Youngman, W. Zeuner Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany C. Coldewey, A. Lopez-Duran Viani, A. Meyer, S. Schlenstedt, P.B. Straub DESY Zeuthen, Zeuthen, Germany