Observation of the strange sea in the proton via inclusive φ-meson production in neutral current deep inelastic scattering at HERA

Inclusive φ(1020)-meson production in neutral current deep inelastic ep scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 45 pb. The φ mesons were studied in the range 10 < Q < 100 GeV, where Q is the virtuality of the exchanged photon, and in restricted kinematic regions in the transverse momentum, pT , pseudorapidity, η, and the scaled momentum in the Breit frame, xp. Monte Carlo models with the strangeness-suppression factor as determined by analyses of ee annihilation events overestimate the cross sections. A smaller value of the strangenesssuppression factor reduces the predicted cross sections, but fails to reproduce the shapes of the measured differential cross sections. High-momentum φ mesons in the current region of the Breit frame give the first direct evidence for the strange sea in the proton at low x. The ZEUS Collaboration S. Chekanov, D. Krakauer, S. Magill, B. Musgrave, J. Repond, R. Yoshida Argonne National Laboratory, Argonne, Illinois 60439-4815 n M.C.K. Mattingly Andrews University, Berrien Springs, Michigan 49104-0380 P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, G. Cara Romeo, L. Cifarelli, F. Cindolo, A. Contin, M. Corradi, S. De Pasquale, P. Giusti, G. Iacobucci, A. Margotti, R. Nania, F. Palmonari, A. Pesci, G. Sartorelli, A. Zichichi University and INFN Bologna, Bologna, Italy e G. Aghuzumtsyan, D. Bartsch, I. Brock, S. Goers, H. Hartmann, E. Hilger, P. Irrgang, H.-P. Jakob, A. Kappes, U.F. Katz, O. Kind, E. Paul, J. Rautenberg, R. Renner, H. Schnurbusch, A. Stifutkin, J. Tandler, K.C. Voss, M. Wang, A. Weber Physikalisches Institut der Universität Bonn, Bonn, Germany b D.S. Bailey, N.H. Brook, J.E. Cole, B. Foster, G.P. Heath, H.F. Heath, S. Robins, E. Rodrigues, J. Scott, R.J. Tapper, M. Wing H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom m M. Capua, A. Mastroberardino, M. Schioppa, G. Susinno Calabria University, Physics Department and INFN, Cosenza, Italy e J.Y. Kim, Y.K. Kim, J.H. Lee, I.T. Lim, M.Y. Pac Chonnam National University, Kwangju, Korea g A. Caldwell, M. Helbich, X. Liu, B. Mellado, Y. Ning, S. Paganis, Z. Ren, W.B. Schmidke, F. Sciulli Nevis Laboratories, Columbia University, Irvington on Hudson, New York 10027 o J. Chwastowski, A. Eskreys, J. Figiel, K. Olkiewicz, P. Stopa, L. Zawiejski Institute of Nuclear Physics, Cracow, Poland i L. Adamczyk, T. Bo ld, I. Grabowska-Bo ld, D. Kisielewska, A.M. Kowal, M. Kowal, T. Kowalski, M. Przybycień, L. Suszycki, D. Szuba, J. Szuba Faculty of Physics and Nuclear Techniques, University of Mining and Metallurgy, Cracow, Poland p A. Kotański, W. S lomiński Department of Physics, Jagellonian University, Cracow, Poland