Talk:Nucleon Form factors

Reviewer B

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The updated version contains modifications following some of the suggestions and remarks in defining and introducing the subject and in the simplification of the formulas. I particualrly appreciate adding links and further references. I have the following remarks that I would like to see implemented before recommending to accept the paper.

In the Introduction: 1) omega is not defined in the definition of Q^2 - it appears much later, in Fig. 6;

2) The second paragraph should be modified: it seems confusing to define one generic form factor for proton, F(Q^2) and, then F1 and F2. Moreover there is a repetition few lines later, in the History.

3) I would replace

-to the exchange of real (rather than virtual) photons-

by -to the exchange of a low virtuality photon- (the photon,vinternal line that links the electron to the hadron vertex in a t-channel exchange, is always virtual by definition)

4) specify that the mentioned radiative corrections which are effectively used in the analysis programs are first order ones. Higher orders are usually neglected or sometimes taken into account through exponentiation.

5) -Earlier work of Akhiezer and Rekalo (1973, 1974)-

    should be 
  -Earlier work of Akhiezer and Rekalo (1969, 1974)-

6) two lines after Eq. 9:

"or , as illustrated in Fig. 4." should be Fig. 6.

7) as previously mentioned, Eq. 9 is redundant, if considered of pedagogical use, it can be kept, but I would insist on the fact that " In the discussion for the neutron it is worthwhile to note that the corresponding formulas for the neutron in the deuteron – not for a free nucleon) - exist in the literature: (E. Tomasi-Gustafsson, G.I. Gakh, A.P. Rekalo, M.P. Rekalo, Phys.Rev.C70:025202,2004.)"

8) Last sentence before Figs. 7 and 8. Should give reference. All possible double spin correlation as a function of FFs can be found in the literature.

9)Theoretical progress: Indicate which are the selection criteria of the choice of the available models.

10)The verb is missing in the sentence: " The first moment of the GPDs taken in the forward limit yields, according to the Angular Momentum Sum Rule (Ji 1997), a contribution to the nucleon spin from the quarks and gluons, including both the quark spin and orbital angular momentum. "

11)Something is wrong in the ponctuation: " ...experiments. And most recently calculations with the frame work of QCD, solving the Faddeev equation for a quark-diquark system; this approach provides a way to generate the nucleon mass from the kinetic energy of the nucleon constituents."

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The knowledge of nucleon form factors is a fundamental issue in the understanding of nuclear matter. An article at a level of encyclopedia is pertinent and timely: on one side, the experimental and theoretical studies started many decades ago, establishing a solid background of knowledge, on the other side, the technical progress has been so fast during the last decade and the new, precise measurements have brought unexpected results which give rise to many discussions and controversies. More projects and experiments are coming in the near future. In this context, I am in strongly in favor of an article on this subject.

In general

The present version contains formulas and details which could be specified in ‘inserts’, or electronically linked to other articles (for example, Rutherford scattering, which is quite basic, should make an article by itself). In my understanding, an article in an encyclopedia should be ‘self-supporting’: explanations should be simple, short and essential, without need of referring to specialized articles for definitions of symbols. In particular, the illustration of the figures should not refer to other articles. The interested reader must find what is necessary to go deeper in the subject looking in the section: ‘references’ and especially: ‘further reading’. In particular The present article focuses on electromagnetic nucleon form factors in the space-like region (mostly on proton). This should be reflected in the title. Introduction - A simple definition of form factors should be given: (parameterization of charge and magnetic distributions for composite particles etc..). - An ‘operative ‘ definition from experimental as well as theoretical point of view should be already sketched here. - The mention to time like region (line 5): The related processes e++e-p+anti p ( also proton antiproton in e+e- pair should be mentioned.) - Formulas should be simplified; definitions of Sachs/Pauli and Dirac FFs may be given in the history. The history may contain also the theory developments, which were far ahead the experiments. The new technique: - Eq. 9 and 10 are redundant in this context. (Eq 10 should differ from Eq. 6 by the sign only), it should be mentioned that single spin observables are zero (in one photon approximation). Double spin observables using polarized electron beam and either polarized target or measuring the outgoing proton polarization are connected by time invariance (in ep elastic scattering) . - It would be good to develop the discussion on hadron polarimetry (may be it is necessary a specific article, due to the many recent developments), or at least to illustrate the recoil polarization method with a figure. - In the discussion for the neutron it is worthwhile to note that the corresponding formulas for the neutron in the deuteron – not for a free nucleon) - exist in the literature: (E. Tomasi-Gustafsson, G.I. Gakh, A.P. Rekalo, M.P. Rekalo, Phys.Rev.C70:025202,2004.) Theoretical progress: - The theory section should contain some basic definitions and clear properties of form factors from analyticity and perturbative QCD. - The nucleon models should then be classified (either by type, or by chronology) specifying the basic assumptions. - A common discussion and interpretation for neutron and proton should be done (when possible, as the Q^2 range of neutron data is more limited).

Conclusion and/or Perspectives are missing and should afford the questions - which picture of the nucleon is emerging, which hypothesis are clearly ruled out, which of the models rely on parameters, which of the models have real predictive power. - what are the plans (and challenges) for the future –experimentally en theoretically.

References: The format is inhomogeneous, the references sometimes contain the title, sometimes not. Further reading Should contain the ‘historical ‘ papers, and review articles as the one often quoted in the text: Perdrisat et al Prog Part Nucl Phys (2007).

Note for the editors: Fig 3 appears after Fig. 4, in the printed version (not online).

Dear Colleagues,

I would like to start my report by apologizing because I am not an expert in the field, although a researcher in a closely related domain.

I felt that your article would enable me to learn about nucleon form factors. Indeed, I remember from my university studies forty years ago that the nuclear form factors are closely related, at least at low energy, to the charge distributions in nuclei. In the nucleon the situation is probably different because we deal with quark distributions, or partons for people of my generation ...

In the introduction there are many mathematical formulae but the reader does not learn what a form factor is.

I feel that what is missing in the article is a general introduction to explain what a form factor is! From my point of view it is important to strengthen the introduction.

I asked myself: to what public is this article addressed? I presume that it is intended for scholars, PhD students of this and other domains, for scientists and educated journalists, and in this respect it is obvious that the concentration of formulae in the introduction may deter more than one person from reading the article. Furthermore, I don't see any connection between these formulae and the rest of the article; at least this is not obvious to me.

The second part of the article is probably better written and easier to understand, although I feel that it would be nice to have a sketch explaining the double polarization experiments.

I would like to make a remark concerning the figures; they are very difficult to follow. Let me take one example, Figure 3. ...The symbols for [Res. [12,13,14]..... If one wishes to understand the figures one has to read at least four or five associated papers. This is certainly not the aim of an encyclopedia article.

I feel that the topic is very hot. Of importance! Unfortunately the article presently is not well suited for publication in scholarpedia. What is needed, and I really hope that the authors will agree, is to strengthen the introduction, avoid unnecessary formulae, or make a kind of box with the formulae, and to clarify the figures.

I await with interest a new version of the paper.