Diffractive production of isolated photons with the ZEUS detector at HERA
The photoproduction of isolated photons has been measured in diffractive events recorded by the ZEUS detector at HERA. Cross sections are evaluated in the photon transverse-energy and pseudorapidity ranges 5 15 GeV and inclusively and also with a jet with transverse energy and pseudorapidity in the ranges 4 35 GeV and -1.5 1.8, using a total integrated electron–proton luminosity of 374 . A number of kinematic variables were studied and compared to predictions from the Rapgap Monte Carlo model. An excess of data is observed above the Rapgap predictions for , where is the fraction of the longitudinal momentum of the colourless “Pomeron” exchange that is transferred to the photon–jet final state, giving evidence for direct Pomeron interactions.
Diffractive production of isolated photons with the ZEUS detector at HERA
Aharon Levy††thanks: Speaker.
On behalf of the ZEUS Collaboration
Tel Aviv University, Tel Aviv, Israel
The HERA collider took data during the period 1992 - 2007. Its rich physics output covered hard processes in the deep inelastic scattering (DIS) regime down to soft interactions in the photoproduction region . Hard processes were studied also in the photoproduction regime when a high scale was present. In the present talk, results are presented of hard scattered isolated photons associated with jets in the photoproduction region. The photoptroduction regime is usually defined as a region where the four-momentum squared between the scatted and the incoming electron, , is less than 1 GeV. In this region no scattered electron is observed.
HERA also showed that diffraction, usually thought to be a soft process, is present also in hard processes  resulting in events with a large rapidity gap. The results presented in this talk are of isolated photons (called hereafter prompt photons) which are diffractively produced. Analyses of prompt photons in the DIS  and the non-diffractive photoproduction  regions have already been presented by the two HERA collaborations.
The reaction studied here can be written as
where LRG is a large rapidity gap and pdis stands for proton dissociation. The LRG is also expressed by a the variable , which is the maximum pseudorapidity for energy-flow objects with energy of 0.4 GeV. Typially for diffraction events 2.5. The brackets on the right-hand side of the reaction describe states that are not observed in the detector. The prompt photon has a high transverse energy, 5 GeV, and represents hadrons or jets. The reaction (1) can be described as a reaction between the virtual photon, at thte electron vertex and the Pomeron, IP, at the proton vertex. The fraction of thte proton energy carried by the Pomeron is given to a good approximation by
where is the energy of the proton beam.
Having a hard scale, the partonic structure of their interaction can be studied. Both the exchanged photon and the Pomeron can act either as a whole (direct) or as a source of partons (resolved). These two classes of process, which are unambiguously defined only at the leading order (LO) of QCD, may be partially distinguished in events containing a high- photon and a jet by means of the quantity
which measures the fraction of the incoming photon energy that is given to the outgoing photon and jet. The quantities and denote the energies of the outgoing photon and the jet, respectively, and denotes the corresponding longitudinal momenta. The suffix “all” refers to all objects that are measured in the detector or, in the case of simulations at the hadron level, all final-state particles except for the scattered beam electron and the outgoing proton. Events with a detected final-state electron are excluded from this analysis.
where the quantities are as before, and corresponds to direct Pomeron events, which are equivalent to the presence of a delta-function in the parton distribution functions at [5, 6]. An event whose observed final state consists only of a prompt photon and a jet has
Examples of diagrams describing the above processes are shown in Fig. 1. A reaction which proceeds through the interaction of a direct photon with a quark from a resolved Pomeron is shown in Fig. 1(a). In (b) a gluon from a resolved photon interacts with a quark from a resolved Pomeron. The interaction of a direct photon with a direct Pomeron is shown in Fig. 1(c).
The data used in this analysis is based on a data samples corresponding to an integrated luminosity of , taken during the years 2004–2007 with the ZEUS detector at HERA. During this period, HERA ran with electron and positron beams111Hereafter, “electron” refers to both electrons and positrons. of energy GeV and a proton beam of energy GeV.
2 The data
The following selections were made:
the forward scattered proton was not measured;
non-diffractive events were removed by the cuts 2.5 and 0.03;
remaining DIS events were removed by excluding events with identified electrons;
Bethe-Heitler () and deeply virtual Compton events were removed;
requirement was made for hard isolated photon candidates (5 GeV) in the barrel calorimeter ();
high-p jets, using a algorithm, were required to be in the region .
3 The outgoing prompt photon
The hard scattered photons can be extracted in a region of the detector which is finely segmented in the direction, namely the electromagnetic section of the ZEUS barrel calorimeter (BEMC), limiting the pseudorapidity of the photon to the region . This fine granularity allows the use of shower-shape distributions to distinguish isolated photons from the product of neutral meson decays such as . To this end, a method was developed  to make use of the energy-weighted width, measured in the direction, of the BEMC energy cluster comprising the photon candidate,
where is the position of the centre of the -th cell, is the energy-weighted centroid of the energy-flow-object cluster, is the width of the cell in the direction, and is the energy recorded in the cell. The sum runs over all BEMC cells in the cluster.
As this analysis studies diffractive prompt photon events having a large rapidity gap, the Rapgap Monte Carlo generator has been use to simulate the expectted distribution of prompt photons coming from a signal event and the distribution of photons coming from the background. In each bin of each physical quantity, the data were fitted to a combination of photon signal and hadronic background. The data together with the fitted results, for events with photon candidastes and at least one jet, are shown in Fig. 2. A good fit to thte data is obtained allowing to extract statistically the photon signal.
After selecting the photon candidate events as described above, the distribution of the is shown in Fig. 3. It peaks toward high and a mixture of 70:30 of direct and resolved photon component events, as obtained from Rapgap gives a reasonable description of the data.
The distribution of is shown in Fig. 4. A sharp peak is observed for 0.9 in contrast to the prediction of the MC generator that described well the . The Rapgap generator does not include the process of a direct Pomeron, as presented in Fig. 1(c). Agreement with the data is obtained by reweighting rapgap (a weight of 7 is applied to the direct photon component). The reweighted Rapgap changes very little in the description of Fig. 3 and still provides an adequate description of the data.
The excess observed for 0.9 is evidence for the presence of a direct Pomeron interaction, predominantly in e direct photon channel.
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