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<title>Sartre - Generating Ultra-Peripheral Collisions</title>
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<h1 class="title">Generating Ultra-Peripheral Collisions</h1>
<p>
There are a few things to consider while writing a user program
for simulation of ultra peripheral hadron-hadron collisions (UPC):
</p>
<ul>
<li>
In Sar<em>t</em>re, the beam particle emitting the photon is
always the beam particle coming from the right, <em>i.e.</em> with
negative momentum in the <var>z</var> direction. For UPC events
with symmetric beams one then needs to reflect the entire event
in <var>the xy</var>-plane
in half of the events, preferably chosen at random.
</li>
<li>
When running UPC for symmetric beams, the cross-section given
by calling <code><a href="refSartre.html#totalCrossSection">Sartre::totalCrossSection()</a></code> has to
be multiplied by a factor 2.
</li>
<li>
When running with asymmetric beams, <em>e.g. </em>proton-gold, one has
to run events with A=1 and UPCA=197, as well as events with
A=197 and UPCA=1. These events then has to be weighted by
respective cross-section. This can be avoided if one can
make experimental cuts such that one of the processes dominates
the events. </li>
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