[Clascomment] OPT-IN: Hard Two-body Photodisintegration of 3He

[Jean-Marc Laget]

Dear collaborators,

I read the draft of the paper "Hard two body photodisintegration of 3He".

While the data are beautiful and worth to be published, I have strong reservations and concerns on the physics interpretation. I already share them with Yordanka, Steffen and Doug. 

Dimensional scaling is well founded and expected at asymptotic energies. Namely the available energy, in the c.m., should be much higher than the mass of the system. Under this circumstance, the only scale available is the energy and the s behavior comes from the norm of the active fields.

The same result is obtained in pQCD, also at high energies. Namely, when the quark-quark interaction becomes constant, the s behavior of the cross section comes from the s dependency of the far off shell active constituent propagators (besides a trivial s dependency of the phase space factor).

The range in s goes from 10 to 15 Gev2, and is not significantly different from the squared mass of the system, about 9 GeV2. It is hard to believe that the observation of approximate scaling in this region is a proof of dimensional scaling and suggests that the quark and gluon description of nuclei (which by the way does not predict the magnitude of the cross section) is  viable even at energies below 1 GeV.

Also it is a little bit misleading to restrict the analysis to a single angle (90 deg). Plotting a broader angular range would for instance reduce the dramatic difference between between CLAS and DAPHNE data at 90 deg, but also it would be a better test of the models.  

On the other hand, it turns out that the baryonic-mesonic description does a fair job in reproducing not only the s behavior of the 90 deg. differential cross section (I do not know how to attach the plot to this comment, but Yordanka has it already) but also the angular distributions (Yordanka has already this plot too). More important it provides a good accounting of the absolute cross section, that dimensional scaling or pQCD are unable to provide.

Furthermore, the same model, after trivial account of wave functions, spin, isospin, etc.., reproduces very well the angular distributions of the two body photodisintegration of 4He (PRC80, 044603 (2009)), measured at CLAS in the same kinematics domain. 

Also, the same model (same code!), explains and reproduces very well the deuteron momentum distribution recorded in Hall A at X=1 (see PRC72, 024001 (2005)) in the study of the two body disintegration of 3He. The probed range of momentum of the deuteron is the same, and three body mechanisms are responsible for the large cross section at large momentum. It is remarkable that photo and electroproduction probe different three body mechanisms: contact and pion exchange in the real photon sector; s-channel nucleon exchange in the virtual photon sector at X=1.

This set of experiments and their interpretation provide us with the answer to a question, and was one of the goals, which motivated the building of CEBAF: understanding the mechanisms of the disintegration of the simplest nuclear systems. We should not be shy and should not put this achievement under the carpet!

In sum, the conclusion of the paper is not scientifically founded and I strongly suggest to reorient the discussion of the meaning of the data within a broader perspective.

For instance, the observation of approximate scaling could be a part of a longer paper where the angular distributions would be presented and which would include a complete discussion of more reasonable alternative approaches, which explain the data too.

All the best,

JM