# spinmode onshell problem (continued)

Asked by Sihyun Jeon on 2018-01-08

Hi Olivier, the problem never ends sadly.

We expected the ratio of SS and OS to be 1 since we use following commands (with spinmode onshell)

define mu = mu+ mu-
generate p p > mu n1 [QCD]
decay n1 > mu j j

which would generate mu+mu+, mu-mu-, mu+mu-, mu-mu+ combinations.

However we found that (mu+mu+)&(mu-mu-) / (mu+mu-)&(mu-mu+) ratio is not 1, about 0.7.

Is there a possible fix for this?

Or should we have to generate the hard way?
(divide the samples into 4 categories mu+mu+, mu-mu-, mu+mu- and mu-mu+ separately)

Thanks, Sihyun Jeon.

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2018-01-15
2018-01-14
 Olivier Mattelaer (olivier-mattelaer) said on 2018-01-10: #1

Hi,

I guess that this is actually the continuation of the same problem that you break quantum mechanics.

When receiving an events with not same sign muon.
you have two possibility to assign the muon (i.e. which one in production and which one in decay)
But according to this choice, the result that you expect is different.
The code allows only one matrix-element per set of pdg in the final state.
Therefore the code always assigns the positive one in the production (or the opposite).

Consequently, you are indeed force to split your production in (at least) two. (At least with this strategy)
Only two production should be enough to my understanding since you can do this:

> define mu = mu+ mu-
> generate p p > mu+ n1 [QCD]
> decay n1 > mu j j

and

> define mu = mu+ mu-
> generate p p > mu- n1 [QCD]
> decay n1 > mu j j

Cheers,

Olivier

> On 8 Jan 2018, at 16:32, Sihyun Jeon <email address hidden> wrote:
>
> New question #662694 on MadGraph5_aMC@NLO:
>
>
> Hi Olivier, the problem never ends sadly.
>
> We expected the ratio of SS and OS to be 1 since we use following commands (with spinmode onshell)
>
> define mu = mu+ mu-
> generate p p > mu n1 [QCD]
> decay n1 > mu j j
>
> which would generate mu+mu+, mu-mu-, mu+mu-, mu-mu+ combinations.
>
> However we found that (mu+mu+)&(mu-mu-) / (mu+mu-)&(mu-mu+) ratio is not 1, about 0.7.
>
> Is there a possible fix for this?
>
> Or should we have to generate the hard way?
> (divide the samples into 4 categories mu+mu+, mu-mu-, mu+mu- and mu-mu+ separately)
>
> Thanks, Sihyun Jeon.
>
> --

 Sihyun Jeon (shjeon) said on 2018-01-12: #2

Hi Olivier,

we have one more question regarding this charge ratio.

It seems that for mass of n1 bigger than 80GeV, (here we use default spinmode) we see the ratio was normal (SS:OS = 1:1).

One of the questions that were asked to us was "Why is charge ratio 1:1 for m(n1) > 80 and not 1:1 for m(n1) < 80".

Can you tell us why it is different?

Is it purely due to madspin spinmode or something else?

 Olivier Mattelaer (olivier-mattelaer) said on 2018-01-14: #3

Hi,

Did you check with the spinmode=None for n1 bigger than 80GeV?

Cheers,

Olivier
> On 12 Jan 2018, at 19:57, Sihyun Jeon <email address hidden> wrote:
>
> Question #662694 on MadGraph5_aMC@NLO changed:
>
>
> Sihyun Jeon is still having a problem:
> Hi Olivier,
>
> we have one more question regarding this charge ratio.
>
> It seems that for mass of n1 bigger than 80GeV, (here we use default
> spinmode) we see the ratio was normal (SS:OS = 1:1).
>
> One of the questions that were asked to us was "Why is charge ratio 1:1
> for m(n1) > 80 and not 1:1 for m(n1) < 80".
>
> Can you tell us why it is different?
>
> Is it purely due to madspin spinmode or something else?
>
> --

 Sihyun Jeon (shjeon) said on 2018-01-14: #4

Yes, they are 1:1 within stat errors.

With this result, then we can assume that this is due to onshell spinmode?

Aside from this I have one more question to ask

1) I found that in madspin
decay n1 > mu+ w-, w- > j j
AND
decay n1 > mu+ w-
decay w- > j j

these two command lines give a bit big difference in cross sections.

This would be due to nwa? Anyways, could you tell me which cross section would be more accurate or recommended to trust?

 Olivier Mattelaer (olivier-mattelaer) said on 2018-01-14: #5

Sorry I did not ask the correct question.
Did you test madspin=onshell for the case where the normal madspin can be used?
If those are in 1:1 then something weird happens.

Did you test by splitting in two sample like i was suggesting?

> 1) I found that in madspin
> decay n1 > mu+ w-, w- > j j
> AND
> decay n1 > mu+ w-
> decay w- > j j

In the second case, you do not ask the W- coming from the n1 decay to decay.
Therefore the two cross-section will differ by BR(w- > j j )
In the second case, the last line is irrelevant if you do not have any W in the production matrix-element.

Cheers,

Olivier

> On 14 Jan 2018, at 15:27, Sihyun Jeon <email address hidden> wrote:
>
> Question #662694 on MadGraph5_aMC@NLO changed:
>
>
> Sihyun Jeon is still having a problem:
> Yes, they are 1:1 within stat errors.
>
> With this result, then we can assume that this is due to onshell
> spinmode?
>
>
> Aside from this I have one more question to ask
>
> 1) I found that in madspin
> decay n1 > mu+ w-, w- > j j
> AND
> decay n1 > mu+ w-
> decay w- > j j
>
> these two command lines give a bit big difference in cross sections.
>
> This would be due to nwa? Anyways, could you tell me which cross section
> would be more accurate or recommended to trust?
>
> --

 Sihyun Jeon (shjeon) said on 2018-01-14: #6

I tested two cases with m(n1) = 500 GeV (where default madspin mode should be used)

[A]
generate p p > mu+ n1 [QCD]
set spinmode onshell
define mu = mu+ mu-
decay n1 > mu j j

[B]
define mu = mu+ mu-
generate p p > mu n1 [QCD]
set spinmode onshell
decay n1 > mu+ j j

For case[A] the ratio is 1:1 and [B] is 0.33:0.67

 Sihyun Jeon (shjeon) said on 2018-01-14: #7

Ah actually the [B] didn't have to be tested since it's w+/w- ...

 Sihyun Jeon (shjeon) said on 2018-01-14: #8

[A]
generate p p > mu+ n1 [QCD]
set spinmode onshell
define mu = mu+ mu-
decay n1 > mu j j

Sorry it's not 1:1.
I could only check this with high stats.

For case[A] with "spinmode none" it's 50:50 with #events 49.95 : 50.05 with #events 80000
However, with "spinmode onshell" it's 49.32 : 50.68 even with #events 160000

I think this does make some difference when using onshell, no?

 Sihyun Jeon (shjeon) said on 2018-01-15: #9

Sorry for multiple notes.

I am testing with more stats and there was some error in the previous comment.

[A]
generate p p > mu+ n1 [QCD]
define mu = mu+ mu-
decay n1 > mu j j

For case[A] with "spinmode none" it's just 50.01:49.99 with #events 160000
However, with "spinmode onshell" it's 49.30 : 50.70 even with #events 240000

I think this does make some difference when using onshell, no?