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\large{\bf Anomalous single top photoproduction at the LHC}

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by K. Piotrzkowski (on behalf of J. de Favereau, UCLouvain)
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{\bf Summary}
FCNC processes are expected in many extensions of the Standard Model. Measurements at HERA show that 
anomalous couplings $\kappa_{\gamma q t}$ can be strongly constrained by studying the single top 
photoproduction. Of course, in the Standard Model the single top cross-section is strongly suppressed, 
as it is zero at the tree level. The recent limits from the ZEUS and H1 collaborations \cite{top-hera} 
significantly reduce the allowed range for anomalous couplings, in particular for $\kappa_{\gamma u t}$.
Rate of the photon-induced electroweak processes is significant at the LHC \cite{kp}, and the energy 
reach is about 10 times higher than at HERA. Apart from the low-mass muon pairs, the highest 
cross-section of about 40 pb is expected for the single W boson photoproduction, which is still at about 
1 pb for photon-parton cms energies above 1 TeV. In case of the direct photoproduction the W boson is 
accompanied by at least one jet, making such W+jets events the major background in searches for the
anomalous single top photoproduction at the LHC, in case when the Wj invariant mass is close to the top 
mass and jet is misidentified as a b-quark jet. It should be noted that the NLO calculations has been
done for the HERA kinematics \cite{spira}, showing complete dominance of the direct photoproduction 
for the W boson transverse momenta of about 20 GeV, and above.

Initial studies of such measurements at the LHC have been made using the adopted for photon 
interactions MADGRAPH package \cite{fabio}, interfaced to Pythia for the final state hadronization, and
PGS - a generic, fast MC program to simulate detector and experimental effects \cite{pgs}.
Leptonic decays of the W boson, from the top quark decay, were assumed and b-jet tagging was requested.
The signal selection was done by requesting an electron or muon at large transverse momenta, significant
missing energy, the reconstructed invariant mass of the Wj system compatible with the top mass, and by a
cut on the direction of the b-jet in the c.m. frame of the Wj system. Photoproduction was identified or by
lack of significant energy flow into one of very forward calorimeters, or by requesting a hit in one of 
the roman pot proton detectors. Very preliminary results suggest that even at very low, initial LHC 
luminosity the present limits on $\kappa_{\gamma u t}$ and $\kappa_{\gamma c t}$ could be significantly 
improved. In the near future, the analysis will include full detector simulation and more complete 
background considerations.
 
\bibitem{top-hera} ZEUS Collab., Phys. Lett. B559 (2003) 153-170; H1 Collab., 
Eur. Phys. J. C33 (2004) 9-22.

\bibitem{kp} K. Piotrzkowski, these proceedings.

\bibitem{spira} K.-P.O. Diener, Ch. Schwanenberger and M. Spira, Eur.Phys.J.C25 (2002) 405-411.

\bibitem{fabio} F. Maltoni and T. Stelzer, JHEP 0302 (2003) 027.

\bibitem{pgs} http://www.physics.ucdavis.edu/~conway/research/software/pgs/pgs4-olympics.htm

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