[Clascomment] OPT-IN:Coherent photoproduction of pi+ from 3He

[Charles Hyde]

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\\title{Comments on CLAS paper ``Coherent Photoproduction of $\\pi^+$ from $^3$He}


\\author{Charles Hyde}


\\date{11 October 2010}

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I think this is an excellent paper with fascinating results.
We must include a table of cross sections, these results are too important to  publish only as figures.

I think we could add a few qualitative sentences at the end about the striking high $Q^2$ behavior of the data.
For example \\\\ ``At high energy, the data exhibit an onset of a striking energy independence at each angle (Fig. 11) and
angle independence at each energy (Fig. 12).\'\'  \\\\
If you want to be bolder, you could say\\\\
``This is suggestive of scattering from a point-like configuration of the three body system\'\'.




%\\setcounter{section}{-1}
\\section*{Latex and Typographic comments}
I have some small cosmetic suggestions

\\section{Introduction}
Second paragraph, suppress a ``the\'\'\\\\
The contribution of mesonic ... three-nucleon systems for which {\\color{red} \\sout{the}} accurate...

\\section{}
\\section{Experiment and Data Analysis}
\\subsection{}
\\subsection{Event Selection}
Table I, the missing mass squared rows for the $\\pi^+$ and triton are interchanged.  Also, there is a latex error for
the $\\pi^+$ (the $+$ is ill placed) the label should be 
\\begin{verbatim} $MM_{\\pi^+}^2$\\end{verbatim}

In Fig 6, lower left panel I think it is unnecessary and incorrect to suppress pion missing mass squared less than zero.
Experimental resolution effects make it perfectly physical for the value of missing energy squared minus missing momentum
squared to be negative.  You correctly understand this in Table II, where your pion identification cuts extend to $-0.06$ GeV$^2$.
The plot should also extend to negative missing mass squared. Also, as in Table I, the pion and triton figures are mis-identified on
the bottom row.

There are some additional (root, latex?) ackwardness with the axis captions of the fig 6.  I think root does not process
pure latex in the formattinging of sub and super script.  The axis labels have $t$ or $\\pi$ as a subscript of the $^2$ superscript,
rather than a subscript of the `MM\'.  The main problem is that this makes the $t$ or 
$\\pi$ unreadably small.

The last paragraph/sentance could be deleted.  It is unnecessary and redundant with the first sentence  of the next
sub section.


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\\subsection{Systematic Uncertainties}
I find Table III a little bit disingenuous.  If the error on the target density is $2\\%$, then quote $2\\%$.
If the uncertainty stemming from the kinematic cuts is up to $10\\%$, then we must either quote $10\\%$,
or specify the error point-by-point.  We cannot say the total error is smaller than $20\\%$ but then fail to give the
reader an actual number.

\\section{Results and Discussion}
We must include a table of cross sections, these results are too important to just publish them as figures.

The sixth paragraph starts with ``Drechsel {\\em et al.} [23] and Strueve...\'\'  The second sentance says
``Both models described the experimental data...\'\'  Which data?  Elastic form factors?  Can you be more specific?

Minor latex point on page 9, you have $^3$He and $^4He$...  I think $^4$He given by
\\begin{verbatim}
$^4$He
\\end{verbatim}
is preferred.  The reference to CLAS data in the concluding sentence of this paragraph is a little odd:\\\\
``Perhaps data are available ... CLAS ...\'\'  Wouldn\'t any $^3$He$(\\gamma,\\pi^0)^3$He data from CLAS be in this
same data set?

In the final paragraph, I do not think the sentence ``It is important to emphasize that the interpretation of
these data is model dependent.\'\' is necessary.  It suffices to start the following sentence with ``We have compared the
data with the results...\'\'



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