[Halld-npp] Primakoff cross section

[Jose Goity]

Hi Ilya,
the cross section is in nb, and is the result of the   real gamma gamma to pi0 pi0  mesured  cross section for a given value of the pi0 pi0 invariant mass. If you check the units of the formula you use for the Primakoff differential cross section, the gamma gamma ->pi0 pi0 cross section mist be in nb.

Jose
________________________________
From: Halld-npp <halld-npp-bounces at jlab.org> on behalf of Elton Smith <elton at jlab.org>
Sent: Monday, March 2, 2020 7:48 AM
To: Ilya Larin <ilarin at jlab.org>
Cc: Halld-npp at jlab.org <Halld-npp at jlab.org>
Subject: Re: [Halld-npp] Primakoff cross section

Hi Ilya,

The cross section sigma(gg -> pipi) is a two body reaction, so the
energy dependence can be expressed as a function of the cm energy
(Wpipi). It is the cross section as a function of energy but not
differential in that variable.  It reproduces the Crystal Ball Fig. 8
data (see attachment). Note the dimensions of nb. From Eq. 4 of the
proposal (see attached) you can check the dimensions, which is nb.

Hope this helps. We need to agree on this cross section.  Thanks, Elton.

Elton Smith
Jefferson Lab MS 12H3
12000 Jefferson Ave STE 4
Newport News, VA 23606
(757)269-7625
(757)269-6331 fax

On 3/1/20 6:06 AM, Ilya Larin wrote:
>
> Hi Elton,
>
> I can just comment that the 200 nb value I got has already a factor of
> 1.25 taken into account.
> I also do not quite understand (maybe because it's 6 am already) why
> in Fig. 3 the units
> are nb, not nb/GeV, maybe it is nb per histogram bin size, which is
> 50MeV I assume, or something else. Having Fig. 3 understood is
> essential for the cross-section results comparison,
> because I used values from Jose and can/should put Fig. 3 values
> instead to compare the results directly.
>
>  Thank you,
>  Ilya
>
>
> ------------------------------------------------------------------------
> *От:* Elton Smith <elton at jlab.org>
> *Отправлено:* 28 февраля 2020 г. 15:15
> *Кому:* Elton Smith <elton at jlab.org>
> *Копия:* Halld-npp at jlab.org <Halld-npp at jlab.org>; Ilya Larin
> <ilarin at jlab.org>
> *Тема:* Primakoff cross section
> Hi Ilya,
>
> I compared your calculation of the cross section in the LOI. See figs.
> 3-5. I get a cross section/nucleus of 345 nb. If you took Jose's cross
> section directly, you need to divide your value 200 nb by 0.8 since it
> is for |costhe|<0.8, which gives 250 nb, or 70% of my value.
>
> You can check my parameterization of sigma(gg->pi0pi0) (Fig. 3) with
> Jose's. I assumed it is essentially flat above the threshold instead of
> having a shape. This may explain most of the remaining discrepancy. We
> may wish to update the document with your value since Jose's
> parameterization is more constrained, but we can discuss this.
>
> Cheers, Elton.
>
> Elton Smith
> Jefferson Lab MS 12H3
> 12000 Jefferson Ave STE 4
> Newport News, VA 23606
> (757)269-7625
> (757)269-6331 fax
>
> On 2/28/20 2:36 PM, Elton Smith wrote:
> > Dear collaborators,
> >
> > I have posted the minutes from the NPP meeting this morning on the
> > wiki at
> >
> https://halldweb.jlab.org/wiki/index.php/Pi0_Polarizability_Meeting_Feb_28,_2020#Minutes.
>
> >
> >
> > Ilya: You can go ahead an link your presentation directly to the
> > meeting page.
> >
> > Thanks, Elton.
> >
>

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