[Primexd] Beamline background MC simulation & meeting tomorrow

[Alexander Somov]

Hi Igal,

My point was that with such a small MC sample it's hard to
conclude anything (and exclude the em background).

Your yield may actually be not far away. For 414M beam photons
(with E > 8 GeV) we would expect to produce 9 elastic etas (below 5 deg)
on LH2 (assuming the total cross section of 60 nb) and about 10 times less
Primakoff events, i.e. 1 event.

For the He target, the number of expected coherent etas is 1.

Even if your signal to background is 1/1, you will need way more MC events.

(we can discuss it on Friday, or offline).

Cheers,
             Sasha


________________________________
From: Igal Jaegle <ijaegle at jlab.org>
Sent: Tuesday, March 16, 2021 8:09 PM
To: Alexander Somov <somov at jlab.org>; primexd at jlab.org <primexd at jlab.org>
Subject: Re: Beamline background MC simulation & meeting tomorrow

Sasha, you are correct.

The full sample corresponds to 414M photons launched with an energy above 8GeV i.e. I did expect to see at least one hit for an eta-candidate below 1 degree. What is seen is 10's of etas above 1 degree (see fig. below after all selection criteria is applied).  So either the beamline implemented in Geant3/4 is not realistic and/or the Geant4 physics list is not appropriate for what we want to simulate.

[cid:6c39a37d-2674-40a6-b49a-d147efcb3767]



Without the elasticity cut, the diphoton invariant mass looks like this: it is cleaner than the data ...

[cid:bc7627fc-f681-4e65-96d4-75a13098ac29]
But for this sample I place the photon beam 2 m upstream the target. Now, I am producing 4B photons with an energy above 8GeV ~22 m upstream from the target i.e. at the GlueX cave collimator.

tks ig.
________________________________
From: Alexander Somov <somov at jlab.org>
Sent: Tuesday, March 16, 2021 7:09 PM
To: Igal Jaegle <ijaegle at jlab.org>; primexd at jlab.org <primexd at jlab.org>
Subject: Re: Beamline background MC simulation & meeting tomorrow

Thanks Igal,

If I understand you correctly, this sample is statistically
insignificant. Your generated samples is worth of a second
compared with the typical run time of hours.

Just on the example of the empty target run:

The photon flux equivalent to all empty target runs is
~2.4 x 10^12 (with E_gamma > 8 GeV). For all production runs
it's several times larger.

The total number of runs in PrimEx is ~420; you processed
10^6 events for each run, i.e., 4.2 x 10^8 events in total (?)

If you reconstruct 1000 empty-target BG events below 1 degree
(as an example), the average production rate is 1 event per 2.4 x 10^9 photons.
You need ~5 more samples like this to see at least one bg event
in the empty target run (on average).

(I guess that your bg is less than 1000 events below one degree).

The background might also be a combination of the em and hadronic events.

You need many more samples like this to exclude the em bg (if I understood
you correctly).


Cheers,
       Sasha
________________________________
From: Primexd <primexd-bounces at jlab.org> on behalf of Igal Jaegle <ijaegle at jlab.org>
Sent: Monday, March 15, 2021 8:38 AM
To: primexd at jlab.org <primexd at jlab.org>
Subject: [Primexd] Beamline background MC simulation & meeting tomorrow

All,

The first batch of background produced along the beamline can be found here:
/cache/halld/gluex_simulations/PRIMEX/primex-beam-line-bkg-batch01-15032021

In this batch, 1M photons with an energy above 8 GeV and following the DB flux distributions are shot upstream the target for each run with an entry for the flux in the DB. There are no entries below 1 degree for all runs combined (Be, He, empty targets). All the entries above 1 degree correspond to hadronic processes (following the SAID DB) producing pi0/eta/etaprime. It is not clear if the physics list is adapted for what we want to simulate.

Also, the meeting will be short as we have an 11 am GlueX Physics Meeting. Note, Sasha and I might be a little bit late as we have a mandatory meeting between 9:30 and 10:30 am.

tks ig.
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