Negative Crosssection is the output and no events are getting generated.

Asked by Atanu Guha on 2020-07-20

Dear Sir,

 I am getting a strange problem while incorporating polarization by the following code (only relevant parts I am giving). Without the polarization option the code s working perfectly fine. But for the given polarization choice (+80,+30) it yields negative crossection and no events are being generated.

Please suggest me if I am doing anything wrong.

##################################
Model: SM
Model changed: False
Gauge: Feynman
Process: e%,E%->n,N,A

Composite: n=ne,nm
Composite: N=Ne,Nm

Kinematics : 12 -> 34 , 5
Kinematics : 34 -> 3 , 4

pdf1: ISR & Beamstrahlung
pdf2: ISR & Beamstrahlung

Bunch x+y sizes (nm) : 480
Bunch length (mm) : 0.3
Number of particles : 2E+10

Polarization1:0.8
Polarization2:0.3

p1: 500
p2: 500
##################################

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Status:
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Solved by:
Alexander Belyaev
Solved:
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This question was reopened

Revision history for this message
Alexander Belyaev (alexander.belyaev) said :
#1

Hello,
without kinematical cut on the electron, this process is divergent. This
is the reason of your problem. You should apply, for example pt >20 cut
on the photon.
Alexander

On Mon, 20 Jul 2020, 06:35 Atanu Guha, <email address hidden>
wrote:

> New question #691965 on CalcHEP:
> https://answers.launchpad.net/calchep/+question/691965
>
> Dear Sir,
>
> I am getting a strange problem while incorporating polarization by the
> following code (only relevant parts I am giving). Without the polarization
> option the code s working perfectly fine. But for the given polarization
> choice (+80,+30) it yields negative crossection and no events are being
> generated.
>
> Please suggest me if I am doing anything wrong.
>
> ##################################
> Model: SM
> Model changed: False
> Gauge: Feynman
> Process: e%,E%->n,N,A
>
> Composite: n=ne,nm
> Composite: N=Ne,Nm
>
> Kinematics : 12 -> 34 , 5
> Kinematics : 34 -> 3 , 4
>
> pdf1: ISR & Beamstrahlung
> pdf2: ISR & Beamstrahlung
>
> Bunch x+y sizes (nm) : 480
> Bunch length (mm) : 0.3
> Number of particles : 2E+10
>
> Polarization1:0.8
> Polarization2:0.3
>
> p1: 500
> p2: 500
> ##################################
>
> --
> You received this question notification because your team CalcHEP is an
> answer contact for CalcHEP.
>

Revision history for this message
Alexander Belyaev (alexander.belyaev) said :
#2

P.S.

I meant
"without kinematical cut on the PHOTON this process is divergent. "
PT>20GeV cut on the poton
i.e.
T(A)>20
will solve the problem just put this instruction (see example in the batch template which comes with CalcHEP)

Alexander

Revision history for this message
Atanu Guha (atanu-guha) said :
#3

Ok. I put

########
Cut parameter: T(A)
Cut invert: False
Cut min: 10
Cut max: 500

Cut parameter: E(A)
Cut invert: False
Cut min: 10
Cut max: 500

Cut parameter: Y(A)
Cut invert: False
Cut min: -2.5
Cut max: +2.5
#########

Ok, I am checking the modification.

Revision history for this message
Alexander Belyaev (alexander.belyaev) said :
#4

T(A) is enough

On Mon, 20 Jul 2020, 11:11 Atanu Guha, <email address hidden>
wrote:

> Question #691965 on CalcHEP changed:
> https://answers.launchpad.net/calchep/+question/691965
>
> Status: Answered => Solved
>
> Atanu Guha confirmed that the question is solved:
> Ok. I put
>
> ########
> Cut parameter: T(A)
> Cut invert: False
> Cut min: 10
> Cut max: 500
>
> Cut parameter: E(A)
> Cut invert: False
> Cut min: 10
> Cut max: 500
>
> Cut parameter: Y(A)
> Cut invert: False
> Cut min: -2.5
> Cut max: +2.5
> #########
>
>
> Ok, I am checking the modification.
>
> --
> You received this question notification because your team CalcHEP is an
> answer contact for CalcHEP.
>

Revision history for this message
Atanu Guha (atanu-guha) said :
#5

Ok. But including these also did not solve the problem. If I remove the polarization, then only the problem is solved.

Revision history for this message
Atanu Guha (atanu-guha) said :
#6

Hii.. I modified my code... and this is the modified version now. Still I am facing the same issue.

##############################################################
Model: SM
Model changed: False
Gauge: Feynman
Process: e%,E%->n,N,A

Composite: n=ne,nm
Composite: N=Ne,Nm

Kinematics : 12 -> 34 , 5
Kinematics : 34 -> 3 , 4

pdf1: ISR & Beamstrahlung
pdf2: ISR & Beamstrahlung

Bunch x+y sizes (nm) : 480
Bunch length (mm) : 0.3
Number of particles : 2E+10

Polarization1:0.8
Polarization2:0.3

p1: 500
p2: 500

Cut parameter: T(A)
Cut invert: False
Cut min: 20
Cut max: 500

Number of events (per run step): 10000
Filename: ee_neNeA

NTuple: False
Cleanup: False
Max number of cpus: 8

nSess_1: 5
nCalls_1: 100000
nSess_2: 5
nCalls_2: 100000
##########################################

Revision history for this message
Atanu Guha (atanu-guha) said :
#7

Hi.... Sorry.....

Revision history for this message
Atanu Guha (atanu-guha) said :
#8

Hi...

I found my problem...

Polarization1:0.8
Polarization2:0.3

This choice is making the problem. Maximum polarization is 0.5, so if I need to use 80% porlaized beam, I should set the value 0.5*0.8... right?

Revision history for this message
Best Alexander Belyaev (alexander.belyaev) said :
#9

This is correct
80% -> 0.5*0.8

Regards
Alexander

Revision history for this message
Alexander Belyaev (alexander.belyaev) said :
#10

P.S.
You can cross check your results with zero polarization

for
Process: e%,E%->n,N,A
and
Polarization1:0.0
Polarization2:0.0

you should get the same cross section as for
Process: e,E->n,N,A

Alexander

Revision history for this message
Atanu Guha (atanu-guha) said :
#11

Thanks Alexander Belyaev, that solved my question.