[Clascomment] OPT-IN:Measurements of ep -> e'pi+pi-p' Cross Sections with CLAS at 1.40 GeV < W < 2.0 GeV and 2.0 GeV2 < Q2 < 5.0 GeV2

[Viktor Mokeev]

 Dear Michel,

 Thank you for your comments, in particular on Fig. 8!

With the particle momenta shown in Fig 8a we have to add half circle of pi, as you note, in order to be consistent with Eqs(6-9) which are our definition of $\phy_{j}$. THe symbol j stands for either \pi^+, or \pi^-, or p'. Fig.8 shows the case when j=\pi^-.

In my view, Eqs (4-9) do define the angle \phy_{\pi-} unambiguously and in the graph-independent way, if n_{z} unit vector is collinear with the virtual photon three momentum and n_{z}=[n_{x} x n_{y}]. Are you agree or I missed something in you comments?

The initial state in the exclusive electron scattering ep--->e'p'pi^+pi^- consists of two particles e and p-target. Your equation gives us (3*2-4)=2 independent variables.
The \phy_{e_scatt} dependence, you mentioned, emerges not for the initial state, but for the final scattered electron. When we bin our events over W and Q^2, we integrate over  \phy_{e_scatt} eliminating this dependence. Furthermore, as you mentioned, the  \phy_{e_scatt} dependence can emerge only for the scattering of transversely polarized electron. I am not aware about such experiments with the CLAS detector at all. For the above reasons, I think that in our measurement the initial state kinematics does fully described by W and Q^2. Do I miss anything?

  Thank you!

  Best Regards,
                  Victor

 

----- Original Message -----
From: "Garcon Michel" <michel.garcon at cea.fr>
To: "clasmbr" <clasmbr at jlab.org>, "clascomment" <clascomment at jlab.org>, "burkert" <burkert at jlab.org>, "Kenneth Hicks" <hicks at ohio.edu>, "Viktor Mokeev" <mokeev at jlab.org>, ioana at jlab.org, "Ivan Bedlinskiy" <bedlinsk at jlab.org>, "bsi" <BSI at depni.sinp.msu.ru>, "Evgeny Isupov" <isupov at jlab.org>, mkunkel at jlab.org
Sent: Tuesday, March 28, 2017 9:43:38 AM
Subject: OPT-IN:Measurements of ep -> e'pi+pi-p' Cross Sections with CLAS at 1.40 GeV < W < 2.0 GeV and 2.0 GeV2 < Q2 < 5.0 GeV2

Hello,

I only have very detailed comments, avoiding repetition with Dan's.

- line 25: define r,v
- line 175: threshold in mV is meaningless for the reader. If you want to indicate a threshold, translate in energy. In EPJA 24, 445 (the first paper on e1-6 data), it was about (since it is not a sharp threshold in energy) 575 MeV.
- lines 300+: the same reasoning (which is right) would lead to (3n-4)-differential cross sections for n particles in the final state, hence 8-fold, and not 7 for eppipi. I would add in line 309: "... variables W, Q2 that, in the absence of any dependence on the scattered electron azimuthal angle, fully define...." or something to that effect (the 8th variable is the lab phi_e which does not enter in the absence of transverse polarization in the initial state)
- line 283: "with its direction given as shown in Fig. 8" is an ambiguous definition. One needs a definition which is independent of the figure. In general one uses "along the direction of k x k'" (or e x e', or Pe x Pe'), although your Fig. 8 seems opposite to that.
- lines 330+: the definitions of angles are cumbersome. I would a minima merge Eqs 4 and 5 in a single one, and Eqs 6 and 7 in a single one. Preferably, I would remove all eqs 3-9 and write: "In Fig. 8, we denote by theta_pi- the angle between the initial virtual photon and the final pi- in the CM frame. The phi_pi- angle is defined as the angle between the electron scattering plane (C) and the gammapi- plane (A), with the convention that it is 0 when e x e' and gamma x pi- point in the same direction (or reverse ??) and between O and pi when the pi- is emitted in the same hemisphere as e x e' ( or Pypi- >0) (or reverse ??). " I am not sure what your convention is since Fig. 8a shows 0 < phi_pi- < pi/2 whereas Pxpi- and Pypi- look negative. If I am not mistaken, to make consistent Fig. 8a with your eqs (provided you keep them), you would need to add a half circle to the phi_pi- angle.
- Likewise, the definition of alpha can be simplified in the same spirit. " We define  alpha as the angle between the pi+p' plane (B) and the gammapi- plane (A), with the convention that ...."
- lines 350 and 353: is not there a (language) contradiction between "impossible to evaluate 5-fold sigmas" and "integrating over these 5-fold..." (the second statement implies that you calculated these cross sections) ?
- line 384: "After applying the fiducial cuts,...
- Fig. 12 caption: "The horizontal lines represent +/- 10% deviations from unity."
- line 582: contributions
- line 663: by more than a factor 2
- line 706: more slowly

Best regards,
Michel Garçon.

PS: I agree with Reinhard's comment on the title.