# EPA dependence on epa_mass

Asked by Roberto Franceschini on 2019-10-01

I am trying to use the equivalent photon calculation for a cross-section photon photon > b bbar

What is the minimal sindarin to compute the total cross-section of this process?

process proc = "A", "A" => "b","bbar"
beams = "e-", "e+" => epa, epa
epa_x_min = 1e-10 !mandatory
epa_mass = 0.105 GeV ! 511 keV !mandatory
!epa_q_min = 0.105 GeV ! suggested in "minimal viable scenario" of the Manual

Changing epa_mass in this setting gives very little variation compared to the expectation. e.g. at 3 TeV e+e- I get 9 pb for epa_mass 511 keV and 3 pb for epa_mass 0.105 GeV. As ln(200)~5 the two cross-section cannot be just a factor 3 different, unless I am badly misunderstanding the dependence of this cross-section on the emitter mass.

Thanks for the clarification.
Roberto

## Question information

Language:
English Edit question
Status:
Open
For:
WHIZARD Edit question
Assignee:
Wolfgang Kilian Edit question
Last query:
2019-11-04
 Juergen Reuter (j.r.reuter) said on 2019-11-01: #1

Roberto, sorry we didn't have the chance to sit down in Sendai. Did you try to run the same setting with an epa_x_min of the order of 0.01 or 0.001?

 Roberto Franceschini (franceschini-roberto) said on 2019-11-04: #2

Hi there, sorry but I thought this was a dropped case, maybe misunderstood.
Anyhow, yes I have tried larger x cuts and there is a dependence on that x _min value.

\gamma\gamma\to b\bar{b} 3 TeV inclusive
3 = lpp1 photon from electron 6 pb
beams = "e-", "e+" => epa, x_{min}=0.001 epa_mass=501 keV 3.76(2) pb
beams = "e-", "e+" => epa, x_{min}=0.001 epa_mass=105 MeV 1.518(7) pb
x_{min}=0.01 epa_mass=501 keV 0.237(1)
x_{min}=1e-4 epa_mass=501 keV 7.77(6) pb
x_{min}=1e-4 epa_mass=105 MeV 3.06(2) pb
x_{min}=1e-5 epa_mass=501 keV 9.06(10) pb
x_{min}=1e-6 epa_mass=501 keV 9.20(10) pb
x_{min}=1e-10 epa_mass=501 keV 9.13(12) pb
x_{min}=1e-10 epa_mass=105 MeV 3.59(4) pb
x_{min}=1e-10 epa_mass=105 MeV epa_q_min=105 MeV isr_mass=105 MeV 0.602(4) pb

It clearly saturates when x_min becomes small enough that all the phase space for bottom pair creation is included in the range. The issue that I see is that the scaling with the epa_mass is too weak compared to the log I was expecting. Am I expecting the wrong thing?

 Wolfgang Kilian (whkilian) said on 2019-11-05: #3

Thanks for providing those details! There may be in issue with the EPA formulas or their implementation that got unnoticed. There are inconsistent results also elsewhere. I'll try to investigate this more closely.