[Halld-physics] Ashot, Re: Primakov eta hadronic background

[Alexander Somov]

Dear Ashot,

I think that we can eventually use TOF as well.
I just started with the SC as it is the closest
detector to the target.

Important is to understand/study the kinematics of
p eta pi0 and n eta pi+ (I hope that Sascha Sibirtzev
will help us with that) and tune Pythia accordingly
if needed. Pythia might predict too 'conservative'
angular distributions (similar to what David has
observed for elastic eta p decays).

For the PAC, the estimates we have (+ futher plans
how we are going to tackle the bg) might be enough...
(though more work will be needed in the future).

Cheers,
        Sascha




On Tue, 1 Dec 2009, Ashot Gasparian wrote:

>
>  Dear Sascha,
>
> This is a great progress on the way of background
> simulations for the eta-Primakoff proposal.
> It certainly shows that with the multi-function and
> ~4pi GlueX setup one can effectively separate an
> elastic channel at the end of the spectrum, at list.
> I will take a more careful look on your plots, but from
> the first look they are already very promising. I agree
> that we need more statistics for this background
> estimation.
> I will also double check the cross sections you are using
> for the Primakoff part and contact to you soon.
> In regard to n,eta,pi+ channel: can we use the information
> from the forward time-of-flight detectors and effectively
> suppress this background? There should be an easier ways
> for the charged channels.
>
> Thank you for your efforts and contributions to this project.
>
> Ashot
>
>
> .............................................................
> Ashot Gasparian                    Phone:(336)285-2112 (NC A&T)
> Professor of Physics
> Physics Department                       (757)-269-7914 JLab
> NC A&T State University              Fax:(757)-269-6273 JLab
> Greensboro, NC 27411               email: gasparan at jlab.org
> .............................................................
>
>
> On Tue, 1 Dec 2009, Alexander Somov wrote:
>
>> Hello Ashot,
>> 
>> Finally, I took a quick look on the hadronic bg for Primakov
>> eta. I considered bg from inclusive eta photoproduction
>> predicted by Pythia ( The inclusive eta background should
>> dominate combinatorial bg)
>> 
>> 
>> I put a few plots to
>> 
>> http://www.jlab.org/~somov/primex
>> 
>> 
>> 
>> 
>> ---MC sample----
>> 
>> The MC sample corresponds to 2 10^7 Pythia events generated
>> in the beam energy range of 9 GeV < E_gamma < 12 GeV
>> (I selected events containing at least one eta in the final
>> state  with an angle < 1.6 deg, and processed these events
>> through Geant). The magnetic field was switched OFF.
>> 
>> 
>> This sample corresponds to about 32 reconstructed 'Primakov'
>> eta's in the eta->2gamma decay channel.
>> 
>> N_prim_eta(0 - 0.5 deg)_produced =  2e7 * 7.2e-4 / 122 = 118
>> N_prim_eta_rec = 118 * 0.71 * 0.39 ~ 32,
>> 
>> where 7.2 ub -  is the Primakov production Xsection p(0. - 0.5 deg)
>>      122 ub -  is the total photoproduction Xsection
>>      0.39   -  is the branching fraction for eta -> 2 gamma
>>      0.71   -  is the prim eta acceptance (according to Aram).
>> 
>> 
>> --Eta reconstruction--
>> 
>> - considered events with only 2 clusters in the FCAL
>> - eta p elastic decays were excluded from the sample
>> - Primakov signal region was defined as:
>>                    E_beam - E_2gamma > -0.4 GeV
>>  (more than 3 sigma region  according to the eta mass constrained fit).
>>                    Angle(gamma gamma) < 0.5 deg.
>> 
>> 
>> Below are descriptions to plots
>> -------------------------------
>>
>>     angle_vs_energy.pdf
>>     -------------------
>>
>>     Reconstructed angle of 2 photons versus ( E_beam - E_2gamma )
>>
>>     top right plot: veto events with the bcal total energy (summed
>>                     over all reconstructed photons) larger than
>>                     60 MeV. This cut removes most background from
>>                     p eta pi0 decays. Remaining bg originates mainly from
>>                     n eta pi+
>>
>>  Note: 99% of the p eta (eta->gamma gamma) elastic decays survive this
>>        cut.
>> 
>>
>>     bottom left plot: Exclude events where the reconstructed eta and
>>                       a track candidate having a hit(s) in the SC are
>>                       positioned NOT back-to-back in phi, i.e.,
>>                       delta_phi < 170 deg or delta_phi > 190, where
>>                       delta_phi = Phi(2gamma) - Phi(HIT_SC).
>>                       ( If there are no hits in the SC, we keep this
>>                        event ).
>>
>>                       The delta_phi distribution for the reconstructed
>>                       p eta decays is shown in the file  eta_dphi.ps.
>>                                                           -----------
>>                       The remaining event close to the signal region is
>>                       p eta pi+; I need large statistics to study it in
>>                       detail. We can presumably measure p eta pi+ decays
>>                       and subtract them from the 'Primakov' signal
>>                       region.
>> 
>> 
>> All in all, I don't see large background in the Primakov signal region
>> (If here is no bug in my code, of course ). I hope that you know the ABC 
>> store close  to the lab... :)
>> 
>> We can discuss more details during the meeting. I will need to check evrth. 
>> and generate a large sample...
>> 
>> 
>> Cheers,
>>      Sascha
>> 
>> 
>