FATAL ERROR: ISR/EPA handler: event does not contain two radiated particles

Asked by Pablo Martin on 2021-02-25

Hi,

I am trying to simulate the process e+e- -> t tbar, with each top decaying as t -> W b -> l nu b, i.e. dileptonic channel. I am getting the following error:

FATAL ERROR: ISR/EPA handler: event does not contain two radiated particles

I paste the .sin file below. I am using version 2.8.4. Thanks for your help!

# Select model
model = SM

# Define masses
mc = 0
ms = 0
mb = 4.7 GeV

me = 0
mmu = 0

# Define process (step number 1)
process ttbar = e1, E1 => t, tbar

process t_dec = t => Wp, b
process Wp_dec_1 = Wp => E1, n1
process Wp_dec_2 = Wp => E2, n2

process tbar_dec = tbar => Wm, B
process Wm_dec_1 = Wm => e1, N1
process Wm_dec_2 = Wm => e2, N2

compile

# Define energy (step number 2)
sqrts = 500 GeV

# Phase space parametrization (step number 3)
integrate (t_dec, Wp_dec_1, Wp_dec_2, tbar_dec, Wm_dec_1, Wm_dec_2)
integrate (ttbar) {iterations = 10:1000000:"gw", 5:500000:""}

# Define unstable particles and their corresponding decays (step number 4)
unstable t (t_dec)
unstable tbar (tbar_dec)
unstable Wp (Wp_dec_1, Wp_dec_2)
unstable Wm (Wm_dec_1, Wm_dec_2)

# Define beam spectrum and ISR
beams = e1, E1 => circe2 => isr

?isr_handler = true
$isr_handler_mode = "recoil"
isr_alpha = 0.0072993
isr_mass = 0.000510997 GeV

$circe2_file = "ilc500.circe"
$circe2_design = "ILC"
?circe2_polarized = false

# Define cuts
real default_M_cut = 4 GeV
real default_jet_cut = 10 GeV
real default_E_cut = 10 GeV
real default_Q_cut = 4 GeV
alias quarks = u:d:s:c:b:U:D:S:C:B
alias leptons = e1:e2:E1:E2

cuts = all M > default_M_cut [leptons, leptons]
       and all M > default_jet_cut [quarks, quarks]
       and all M < - default_Q_cut [incoming e1, e1]
       and all M < - default_Q_cut [incoming E1, E1]
       and all Pt > 5 GeV [quarks]

# Define parton shower and hadronization
?ps_fsr_active = true
?ps_isr_active = false
?hadronization_active = true
$shower_method = "PYTHIA6"
!?ps_PYTHIA_verbose = true

$ps_PYTHIA_PYGIVE = "MSTJ(28)=0; PMAS(25,1)=120.; PMAS(25,2)=0.3605E-02; MSTJ(41)=2; MSTU(22)=2000; PARJ(21)=0.40000; PARJ(41)=0.11000; PARJ(42)=0.52000; PARJ(81)=0.25000; PARJ(82)=1.90000; MSTJ(11)=3; PARJ(54)=-0.03100; PARJ(55)=-0.00200; PARJ(1)=0.08500; PARJ(3)=0.45000; PARJ(4)=0.02500; PARJ(2)=0.31000; PARJ(11)=0.60000; PARJ(12)=0.40000; PARJ(13)=0.72000; PARJ(14)=0.43000; PARJ(15)=0.08000; PARJ(16)=0.08000; PARJ(17)=0.17000; MSTP(3)=1;MSTP(71)=1"

# Define sample size
n_events = 1000

# Define output format
sample_format = lhef

# Simulate
simulate (ttbar)

And part of the relevant output:

...
........
n_events = 1000
| Starting simulation for process 'ttbar'
| Simulate: using integration grids from file 'ttbar.m1.vg'
| Simulate: activating ISR handler
| Simulate: ISR/EPA handler mode: pair recoil
| Simulate: activating decays
| Simulate: activating parton shower
     MSTU(12) changed from 0 to 12345
| Shower: Using PYTHIA6 shower
| Simulate: activating hadronization
| Hadronization: Using PYTHIA6 interface for hadronization and decays
| Simulate: using integration grids from file 'Wp_dec_1.m1.vg'
| Simulate: using integration grids from file 'Wp_dec_2.m1.vg'
| Simulate: using integration grids from file 't_dec.m1.vg'
| Simulate: using integration grids from file 'Wm_dec_1.m1.vg'
| Simulate: using integration grids from file 'Wm_dec_2.m1.vg'
| Simulate: using integration grids from file 'tbar_dec.m1.vg'
| RNG: Initializing TAO random-number generator
| RNG: Setting seed for random-number generator to 12520
| Simulation: requested number of events = 1000
| corr. to luminosity [fb-1] = 1.8235E+00
| Events: writing to LHEF file 'ttbar.lhe'
| Events: writing to raw file 'ttbar.evx'
| Events: generating 1000 unweighted, unpolarized events ...
| Events: event normalization mode '1'
 Particle set:
------------------------------------------------------------------------
 Particle 1 [i] f(11)
 E = 2.500000000000E+02
 P = 0.000000000000E+00 0.000000000000E+00 2.500000000000E+02
 T = 0.000000000000E+00
 Children: 3 4
 Particle 2 [i] f(-11)
 E = 2.500000000000E+02
 P = 0.000000000000E+00 0.000000000000E+00 -2.500000000000E+02
 T = 0.000000000000E+00
 Children: 3 4
 Particle 3 [o] f(6)c(1 )
 E = 2.500000000000E+02
 P = 8.917823768270E+00 -3.746694311532E+01 -1.762188712733E+02
 T = 2.996361000000E+04
 Parents: 1 2
 Particle 4 [o] f(-6)c(-1 )
 E = 2.500000000000E+02
 P = -8.917823768270E+00 3.746694311532E+01 1.762188712733E+02
 T = 2.996361000000E+04
 Parents: 1 2
******************************************************************************
******************************************************************************
*** FATAL ERROR: ISR/EPA handler: event does not contain two radiated particles
******************************************************************************
******************************************************************************
| There were no errors and 7 warning(s).
WHIZARD run aborted.

Question information

Language:
English Edit question
Status:
Solved
For:
WHIZARD Edit question
Assignee:
Juergen Reuter Edit question
Solved by:
Pablo Martin
Solved:
2021-02-25
Last query:
2021-02-25
Last reply:
2021-02-25
Juergen Reuter (j.r.reuter) said : #1

Dear Pablo,
thanks for your question. The problem can be solved by putting the beams statement _after_ the integrate statements of the decays but _before_ the integrate statements of the production process. What happens otherwise is that the integration is done for structureless beams (no beamstrahlung, no ISR), and then also for the event generation, and hence the ISR handler for generating the photon pT rightfully complains about missing ISR photons in the events. So just do

....
integrate (t_dec, Wp_dec_1, Wp_dec_2, tbar_dec, Wm_dec_1, Wm_dec_2)
# Define energy (step number 2)
sqrts = 500 GeV

# Define beam spectrum and ISR
beams = e1, E1 => circe2 => isr

# Phase space parametrization (step number 3)
integrate (ttbar) { ... }
----

Cheers,
   JRR (J├╝rgen Reuter)

Pascal Stienemeier (pstienem) said : #2

Hi Pablo,

Sindarin is a script language, so to be on the safe side, you might want to also put all other statements that affect the integration, i.e. the cuts and also the settings for the beamstrahlung and ISR, before the integrate statement.

Cheers,
Pascal Stienemeier

Pascal Stienemeier (pstienem) said : #3

One minor addition: in this case, this is not critical as the cuts are just acting on non-top quarks and leptons which are not present in the integrated process before the top decays so the cuts will have no effect on the integration anyways. I just wanted to emphasize that it's in general better to get used to putting the cuts before the integrate statement.

Cheers,
Pascal

Pablo Martin (pmartin7) said : #4

Hi Juergen and Pascal,

thank you so much for such a quick and illustrative reply. That definitely solved the issue, and it has been great to understand a bit better how Whizard works!

Best,

Pablo