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Hi Ilya,</div>
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the cross section is in nb, and is the result of the real gamma gamma to pi0 pi0
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<span> </span>mesured<span> </span></span>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.</div>
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Jose</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Halld-npp <halld-npp-bounces@jlab.org> on behalf of Elton Smith <elton@jlab.org><br>
<b>Sent:</b> Monday, March 2, 2020 7:48 AM<br>
<b>To:</b> Ilya Larin <ilarin@jlab.org><br>
<b>Cc:</b> Halld-npp@jlab.org <Halld-npp@jlab.org><br>
<b>Subject:</b> Re: [Halld-npp] Primakoff cross section</font>
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<div class="PlainText">Hi Ilya,<br>
<br>
The cross section sigma(gg -> pipi) is a two body reaction, so the <br>
energy dependence can be expressed as a function of the cm energy <br>
(Wpipi). It is the cross section as a function of energy but not <br>
differential in that variable. It reproduces the Crystal Ball Fig. 8 <br>
data (see attachment). Note the dimensions of nb. From Eq. 4 of the <br>
proposal (see attached) you can check the dimensions, which is nb.<br>
<br>
Hope this helps. We need to agree on this cross section. Thanks, Elton.<br>
<br>
Elton Smith<br>
Jefferson Lab MS 12H3<br>
12000 Jefferson Ave STE 4<br>
Newport News, VA 23606<br>
(757)269-7625<br>
(757)269-6331 fax<br>
<br>
On 3/1/20 6:06 AM, Ilya Larin wrote:<br>
><br>
> Hi Elton,<br>
><br>
> I can just comment that the 200 nb value I got has already a factor of <br>
> 1.25 taken into account.<br>
> I also do not quite understand (maybe because it's 6 am already) why <br>
> in Fig. 3 the units<br>
> are nb, not nb/GeV, maybe it is nb per histogram bin size, which is <br>
> 50MeV I assume, or something else. Having Fig. 3 understood is <br>
> essential for the cross-section results comparison,<br>
> because I used values from Jose and can/should put Fig. 3 values <br>
> instead to compare the results directly.<br>
><br>
> Thank you,<br>
> Ilya<br>
><br>
><br>
> ------------------------------------------------------------------------<br>
> *От:* Elton Smith <elton@jlab.org><br>
> *Отправлено:* 28 февраля 2020 г. 15:15<br>
> *Кому:* Elton Smith <elton@jlab.org><br>
> *Копия:* Halld-npp@jlab.org <Halld-npp@jlab.org>; Ilya Larin <br>
> <ilarin@jlab.org><br>
> *Тема:* Primakoff cross section<br>
> Hi Ilya,<br>
><br>
> I compared your calculation of the cross section in the LOI. See figs.<br>
> 3-5. I get a cross section/nucleus of 345 nb. If you took Jose's cross<br>
> section directly, you need to divide your value 200 nb by 0.8 since it<br>
> is for |costhe|<0.8, which gives 250 nb, or 70% of my value.<br>
><br>
> You can check my parameterization of sigma(gg->pi0pi0) (Fig. 3) with<br>
> Jose's. I assumed it is essentially flat above the threshold instead of<br>
> having a shape. This may explain most of the remaining discrepancy. We<br>
> may wish to update the document with your value since Jose's<br>
> parameterization is more constrained, but we can discuss this.<br>
><br>
> Cheers, Elton.<br>
><br>
> Elton Smith<br>
> Jefferson Lab MS 12H3<br>
> 12000 Jefferson Ave STE 4<br>
> Newport News, VA 23606<br>
> (757)269-7625<br>
> (757)269-6331 fax<br>
><br>
> On 2/28/20 2:36 PM, Elton Smith wrote:<br>
> > Dear collaborators,<br>
> ><br>
> > I have posted the minutes from the NPP meeting this morning on the<br>
> > wiki at<br>
> > <br>
> <a href="https://halldweb.jlab.org/wiki/index.php/Pi0_Polarizability_Meeting_Feb_28,_2020#Minutes">
https://halldweb.jlab.org/wiki/index.php/Pi0_Polarizability_Meeting_Feb_28,_2020#Minutes</a>.
<br>
><br>
> ><br>
> ><br>
> > Ilya: You can go ahead an link your presentation directly to the<br>
> > meeting page.<br>
> ><br>
> > Thanks, Elton.<br>
> ><br>
><br>
<br>
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