Problem with beam polarisation in equivalent photon approximation (EPA)
Dear authors,
I tried to generate a process:
alias quark = u:d:s:c:b
alias QUARK = U:D:S:C:B
process aeqqn = A, E1 => ( quark, QUARK, N1 )
using EPA in e+e- collisions at 1.5 TeV CLIC:
beams = e1, E1 => circe2 => epa, isr
epa_x_min = 0.013333333 # 10 GeV from 750 GeV beam
epa_mass = me
epa_q_max = 4 GeV
Without beam polarisation the calculated cross section is about 4790 fb.
When I set electron beam polarisation the cross section is exactly twice as large (9580 fb) for both -80% and +80% polarisation. Something has to be wrong in my opinion, as the unpolarised cross section should be the average of -80% and +80% cross sections (the positron beam is assumed not to be polarised). The cross section is the same for any polarisation setting. Even if I set it to 0%:
beams_pol_density = @(-1), @(+1)
beams_pol_fraction = 0%, 0%
it is 9580 fb. But when I remove these lines (comment them out) it is back to 4790 fb...
I use same circe2 file in all cases (standard CLIC file). I tested it with version 2.7.0 and 2.8.2 (same results).
I will be grateful for your reply. Which cross section number is the correct one?
Regards
Filip
Question information
- Language:
- English Edit question
- Status:
- Solved
- For:
- WHIZARD Edit question
- Assignee:
- Wolfgang Kilian Edit question
- Last query:
- 2020-01-24
- Last reply:
- 2020-03-13
Launchpad Janitor (janitor) said : | #1 |
This question was expired because it remained in the 'Open' state without activity for the last 15 days.
Juergen Reuter (j.r.reuter) said : | #2 |
Was opened on Christmas. Strange. Nobody reacted^^
Additional hint:
It looks like this behaviour is only observed when circe2 is switched on. If there is no circe2 beam
spectrum, the cross section between the case of beams_pol_fraction = 0%, 0% and the one without
any polarisation setting are identical.
Launchpad Janitor (janitor) said : | #4 |
This question was expired because it remained in the 'Open' state without activity for the last 15 days.
Juergen Reuter (j.r.reuter) said : | #5 |
We still need some more time.
Juergen Reuter (j.r.reuter) said : | #6 |
Filip, I can confirm your findings, using the SINDARIN below for the simple process
gamma e+ -> u dbar nuebar, the unpolarized cross section using 1.4 TeV is 2.5 pb, the
cross section for the polarization combinations are
e-, e+ -+/ ++ 10 pb, while -- and +- are 10^-12 pb. So the average would be the sum of the
four divided by 4, which 20/4 = 5 pb. This is what I get when I do
beams_pol_density = +1,+1
beams_pol_fraction = 0,0
then this cross section corresponds to 5 pb, so the one expected from the average of the
polarized components. The same is true for starting with a forbidden combination of
beams_pol_density = -1,-1
beams_pol_fraction = 0,0
Dear Juergen,
Thank you for checking. All the numbers you give agree with what I get from my simulation.
BUT! The point is that, if you remove the polarisation definition from your script, i.e. remove the lines:
beams_pol_density = -1,-1
beams_
you will get only 2.5 pb (and not 5pb, as one would expect). Please check!
I would assume that without any polarisation definition the unpolarised cross section should be calculated. But then it should be equal to the cross section for 0%x0% polarisation...
When not using CIRCE2 spectra (monochromatic beam) the cross section for "unpolarised" beams (no polarisation settings) and 0%polarisation are both around 3pb. So it looks like CIRCE2 is doubling the polarised EPA cross section...
Regards
Filip
Juergen Reuter (j.r.reuter) said : | #8 |
Yes, I also see that not using a beam polariztaion setting, the cross section gets diminuished by a factor of 2. I haven't switched of CIRCE2 yet, but I checked that switching off ISR on the positron hard process leg does not change the picture.
Wolfgang Kilian (whkilian) said : | #9 |
After a more careful inspection (US & WK):
1.
Without CIRCE, one should set
beams_
to inspect the zero-polarizati
2.
With the "tiny polarization" setting, the problem occurs both with and without CIRCE.
3.
There is no problem for processes that do not involve EPA.
My conclusion: The chaining of the polarization density matrix from the beam with the polarization transfer matrix for EPA is wrong. Most likely this is an actual bug. I'll create a gitlab issue for that and will try to find a solution.
Juergen Reuter (j.r.reuter) said : | #10 |
This is fixed in svn, public git, and nightly builds, and will be released in v2.8.3 and v3.0.0beta, cf. the corresponding linked bug.
Juergen Reuter (j.r.reuter) said : | #11 |
Fix committed.