Hard photoproduction

The canonical HERA event proceeds as illustrated in Figure 1(a). The incoming positron is scattered through a large angle exchanging a photon probe of (negative) virtuality as high as $Q^2 \sim 5 \cdot 10^4$ GeV². The structure of the proton may be studied down to values of the Bjorken-x_p variable as low as $x_p \sim 5 \cdot 10^{-3}$.

 

Figure:   Diagrams showing HERA processes. The canonical electroproduction process is shown in (a). A leading order direct photoproduction process is shown in (b) while an example of a leading order resolved photoproduction process is shown in (c).

Of course the electroproduction cross section is strongly peaked to $Q^2 \sim 0$ and the events most copiously produced at HERA are soft photoproduction events. However photoproduction events which lead to the production of high transverse energy jets in the final state are also characterized by a large (negative) squared momentum transfer Q². An example is shown in Figure 1(b). For these hard photoproduction events the negative of the squared invariant mass of the photon is denoted P² and of course, $P^2 \sim 0$. Again, very low values of x_p of the proton may be probed and note that the photoproduction processes (in contrast to the electroproduction processes) are directly sensitive to the gluon content of the proton.

The incoming photon may fluctuate into a hadronic state before interaction with the proton. This situation is illustrated in Figure 1(c). The momentum fraction variable $x_{\gamma}$ has been introduced, where $x_{\gamma}$ represents the fraction of the photon's momentum which participates in the hard interaction. The class of events represented by Figure 1(b) are known as direct photoproduction events and have $x_{\gamma} = 1$. Resolved photoproduction events are represented by Figure 1(c) and have $x_{\gamma} < 1$. The present discussion is clearly limited to leading order processes although a definition of $x_{\gamma}$ may be made which is calculable to all orders and allows for a well defined separation of direct and resolved photoproduction processes [3].

A hard photoproduction event in the ZEUS detector is shown in Figure 2. In the z - R display on the left-hand side the positrons approach from the left and the protons from the right. The e⁺ beam has an energy of 27.5 GeV and the p beam has an energy of 820 GeV. The calorimeter is deeper in the ``forward'' or proton direction, to cope with this asymmetry in the beam energies. This proton direction is the direction of positive pseudorapidity, $\eta = -\ln \tan (\vartheta / 2)$, where $\vartheta$ is the polar angle with respect to the p beam direction.

 

Figure:   A hard photoproduction event in the ZEUS detector. The z -R longitudinal view is shown on the left hand side. In the upper right hand corner the $\eta$ and $\varphi$ coordinates of the hit calorimeter cells are shown, weighted by their transverse energies. In the lower right hand corner the x - y or transverse view is shown.

Two jets of large transverse energy are measured in the tracking chambers and the calorimeter and are clearly apparent in all three views. The jets are both at $\eta \sim 1$ ($\vartheta \sim 40^{\circ}$)and are back to back in $\varphi$. It is the energy deposits and tracks of these jets which we use to select a sample of hard photoproduction events. Notice that there is a large energy deposit in the far-forward region next to the beam pipe. This energy is associated with the proton remnant. There is also a large energy deposit in the rear direction which could be called the photon remnant if this were considered a resolved photon event. (The transverse energy of the rear jet in this particular event is actually sufficiently large that it may be appropriate to consider this a higher order direct photoproduction event.) Notice that there is no energy deposit which could be associated with the scattered e⁺, which is lost down the rear beam pipe in photoproduction processes.