[Clascomment] OPT-IN:Measurement of Exclusive pi0 Electroproduction Structure Functions and their Relationship to Transversity GPDs

[Jean-Marc Laget]

Dear Lead Authors and Colleagues,

The paper gives the false impression that the inclusion of transversity in the GPD approach is the only way to interpret the data.

Although it represents a interesting novel aspect in the GPD game, the strong differences (in shape for the unpolarized cross section, in magnitude for the interference cross sections) in the results of the two models demonstrate the contrary, show that there is a model dependency and kill the argument.

Also you are well aware that other hadronic approaches (that you "put under the carpet" in the introduction) reproduces your data.

One of those was the first to reproduce HallA and HallB data (you already have in hand a comparison between this model and your data), as well as the data at the real photon point. It relies on the coupling to the charged rho production channel (of which the cross section has been, by the way, determined in CLAS). It is basically parameter free in that sense that it relies upon on shell amplitudes of know channels. One cannot escape this unitary contribution.

Now, the interesting question is why this (Regge) coupled channel approach and the GPD approach including transversity lead to similar results. On may argue that duality allows to sum up over all the possible intermediate states and replace the complete hadronic basis  by a quark basis. Unfortunately, at JLab6 the cm energy is quite limited (about 1 to 1.5 GeV above the nucleon mass) and simply does not allow to sum up over all those hadronic intermediate states. Furthermore, at large Q2, it turns out that only one channel dominates the neutral pion production: the charged rho production channel.

Since this coupled channel approach gives a fair description of the data, it is highly likely that any parametrization of GPDs, that enter the quark approach, mokes up  the corresponding unitary hadronic contribution!. The extraction of transversity from neutral pion production is simply not possible under the kinematics conditions of JLab6.

I strongly urge you to soften your conclusion and give more credit to the hadronic approaches.

All the best,

JM