HERA Anomaly and Hard Charm in the Nucleon.pdf

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HERA Anomaly and Hard Charm in the Nucleon

a r X i v : h e p - p h / 9 7 0 7 3 8 7 v 2 1 5 S e p 1 9 9 7 August 1997 U.MD. PP# 98-003 DOE/ER/40762-125 ADP-97-25/T260 HERA Anomaly and Hard Charm in the Nucleon W. Melnitchouk1 and A. W. Thomas2 1 Department of Physics, University of Maryland, College Park, Maryland 20742, USA 2 Department of Physics and Mathematical Physics, and Special Research Center for the Subatomic Structure of Matter, University of Adelaide, Adelaide 5005, Australia Abstract We explore the possibility that the excess neutral and charged current events seen by the H1 and ZEUS Collaborations at HERA at large x and Q2 could arise from a hard charm component of the nucleon. While the symmetric intrinsic charm hypothesis is unable to account for the HERA anomaly, a non-symmetric charm distribution generated non-perturbatively, for which c is much harder than c, can produce significant enhancement of cross sections at the HERA kinematics. PACS numbers: 13.60.Hb, 14.65.Dw, 12.38.Qk Typeset using REVTEX 1 The recent observation by the H1 [1] and ZEUS [2] Collaborations at HERA of an excess of events at large x and Q2 in e+p neutral current (NC) and charged current (CC) deep- inelastic scattering (DIS) has prompted numerous speculations about whether one has seen evidence for physics beyond the standard model. Most explanations have concentrated on leptoquarks or contact interactions as the source for the anomalously large cross sections — for a recent overview of the theoretical discussions see Ref. [3]. Using a simple, non- perturbative model for the intrinsic charm component of the nucleon wave function, we find a hitherto unnoticed asymmetry, with the c component significantly harder than c, which has important consequences for the HERA NC and CC events. Confirmation of such an asymmetry would challenge our understanding of the non-perturbative structure of hadrons within QCD. The ZEUS experiment collected only NC data, while H1 observed both NC and CC DIS events. The evidence for the ex

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