The shaded distributions show the subset of those poins which enter the NA44 acceptance. These distributions are shown for two different angular settings of the detector relative to the incoming beam.
These figures show that pions from the back of the fireball do not reach the detector, because they are streaming away from the detector, and that the apparent size, as measured with HBT (the width of the shaded distribution) is much smaller than the width of the true pion source.
Therefore, HBT measurements also contain implicit information about collective flow.
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