The F₂^$\gamma$ Low-x Problem

The measurement of the low-x behaviour of F₂^$\gamma$ is not trivial for the following reason. In the singly-tagged regime, the determination of x requires both the Q² of the probe photon (measured from the tag) and the invariant mass, W, of the hadronic final state (measured from the particles other than the tag). However, the visible invariant mass is less than the true invariant mass mainly due to losses in the beam pipe and poor hadronic acceptance in the forward regions. This results in increasing the reconstructed x ( x = Q²/(Q² + W²)) and therefore the x distribution has to be corrected by an unfolding procedure to obtain F₂^$\gamma$. This unfolding heavily relies upon information, both before and after detector simulation, from the Monte Carlo that is used to model the tagged two-photon process. The critical point is that this Monte Carlo must correctly model the final state, so that the particle losses are properly accounted for. If an unfolding Monte Carlo has final state particles that are more forward-going than the ones in the data events, the unfolding procedure can falsely increase the result at small x (and correspondingly decrease it at high x) and even introduce a false low-x rise into a result. Clearly, the analysis of the hadronic final state is vital to the low-x analysis.

The energy flow of the final state relative to the tagged electron (or positron) has been introduced [2] and was used to demonstrate that the presently available tagged $\gamma$$\gamma$ Monte Carlos do not model the LEP data very well [3], even in the central acceptance. This results in uncertainties at low-x that are too large to be conclusive about the existence of a low-x rise.