[Clascomment] OPT-IN: Shrunken particles pass freely through nuclear matter

[Elton Smith]

Color transparency is a subject of high interest and data with a clean interpretation are meager. 

Some of these comments will likely overlap with previous feedback, but it may be easier for the authors to sort out overlapping issues. Also, the Nature format is somewhat unfamiliar, so the comments need to be moderated based on new expectations. In particular, I understand that it is very difficult to write a technical paper for a broad audience. However, I am not sure the right balance has been achieved, especially since quite a bit of jargon is used, and at the same time some concepts are exposed at very basic levels.

page 1. Bold intro. Third sentence. "It is the same force that binds protons and neutrons together to form the nucleus of an atom." This is analogous to talking about energy levels of the hydrogen atom and saying that it is the same as molecular forces. Since the paragraph is intended to emphasize the large color forces, which are much tighter bound than nucleons in a nucleus, this might not be the best analogy.

page 2. Introductory sentence, starting with "Color transparency is one of ...." this is a poor leading sentence, with "well-hidden" color degrees of freedom, color-neutral hadrons, etc. followed by "High momentum-transfer exclusive reactions... If the intent is to bring in a broad audience, you should start slower. Take a look at the introduction to ref [21] for some guidance.

There are several qualitative statements that are important to the paper, which are stated without proof or poorly substantiated. For example, p. 3 end of second paragraph: "Intuitively, one would expect to reach the onset to color transparency at lower Q2 for rho production than in the case of pion production. In the former case, the SSC is directly produced from the virtual photon since both the photon and the rho meson have the same spin." I do not understand why the same spin leads to earlier onset of color transparency. Also on p.4  bottom of third (long) paragraph: "The onset of color transparency in rho electroproduction seems to start much lower (Q2~1 GeV/c2) than for both the pion and the proton measurements [12-21]. If I look at ref [21], I would say that their data and the data in the current paper have a very similar shape vs Q2 and, modulo interpretation, one would conclude they indicate the onset at about the same Q2 (i.e. 1 GeV).

p.5 middle first paragraph: "the next step is to extract quantitatively the SSC expansion time and its interaction in the nuclear medium, which ultimately depend on its size." I'm confused about the emphasis. Do you mean ...that the data could be used to estimate the size of the propagating in-medium SSC? 

p.6 First paragraph under methods and list of corrections: It is mentioned that the data are corrected for Fermi motion. How was this done and how important was this effect?

Related to corrections: I do not see any mention of a correction for the rho decay [See Ref. 29, Section Glauber treatment of rho decay, p. 015208-5]. At Q2<2, this is the most important Q2 dependent effect (larger than CT), which increases with Q2 [see Figs. 8, 13]. Is it assumed that the ratio with deuterium takes care of this effect? Is this sufficient? How does the decay affect the use of the rho in the CT test?

p. 5 Bottom. The CLAS vertex resolution is quoted as being 2 mm. The resolution depends very strongly on the angle of the tracks. It is only 2 mm for very large angle tracks. For very forward tracks it can be 10 times worse and the angle may be correlated with  momentum. Has the separation of targets been studied for very forward tracks?

p. 7. Fig. 2. Are the fits to a linear function (y=a+bx) as suggested by the text or just to a constant? The fits have an imperceptible slope.

p. 9 Ref. 22 and 23 are out of order in the text.