Stop decay widths in UDD RPV

Questions concerning the different calculations performed by SARAH and the output of the information into other formats
jmontejo
Posts: 3
Joined: 12. Jul 2017, 11:30

Stop decay widths in UDD RPV

Postby jmontejo » 11. Jan 2018, 14:36

Dear Florian,
I'm using SPheno+SARAH with the MSSMBpV model to study stop pair production with subsequent stop->bs or stop->tN1 decays.
When comparing to other results (1601.03737) I noticed I get a different BR for stop->bs. The partial width for stop->tN1 agrees but the width for stop->bs differs by a factor 2. I find this extremely surprising since I would expect a difference in convention leading to a factor 4 difference.

As an example, I use a purely right-handed stop, a bino-like LSP and m(stop)=620 GeV, m(N1)=200 GeV, lambda’’323=0.1
I get back Width(stop->bs)=0.49 GeV and Width(stop->tN1)=0.91. From the formula A.1 in 1601.03737 I get half the width for bs.

Would you have some insight on this? The convention seems to be the same so I would expect the results to match but maybe I'm overseeing some detail.

Cheers
Javier

FStaub
Site Admin
Posts: 546
Joined: 13. Apr 2016, 14:05

Re: Stop decay widths in UDD RPV

Postby FStaub » 22. Jan 2018, 12:48

Hi,

did you try to compare with another reference? For instance, using the UFO output and calculate the width with MG. If this doens't agree with SARAH/SPheno, I need to check. However, I have hardly time right now. Sorry.

Florian

jmontejo
Posts: 3
Joined: 12. Jul 2017, 11:30

Re: Stop decay widths in UDD RPV

Postby jmontejo » 22. Jan 2018, 12:57

Hi Florian,
running the same scenario through Madgraph I reproduce the numbers in 1601.03737, so factor of 2 disagreement with SARAH.

Cheers
Javier

FStaub
Site Admin
Posts: 546
Joined: 13. Apr 2016, 14:05

Re: Stop decay widths in UDD RPV

Postby FStaub » 22. Jan 2018, 15:02

Hi,

I was quickly looking at the code which reads

Code: Select all

Do gt1= 1, 3
  Do gt2= gt1, 3
m1out = MFd(gt1)
m2out = MFd(gt2)
coupL = cplcFdcFdcSuL(gt1,gt2,i1)
coupR = cplcFdcFdcSuR(gt1,gt2,i1)
Call ScalarToTwoFermions(m_in,m1out,m2out,coupL,coupR,gam)
If (gt1.ne.gt2) gam = 2._dp*gam
gPartial(i1,i_count) = 2*gam
gT(i1) = gT(i1) + gPartial(i1,i_count)
i_count = i_count +1
  End Do
End Do


Does the width into two identical states (i.e. bb or ss) agree with your expectations? In that case, the relevant line is

Code: Select all

If (gt1.ne.gt2) gam = 2._dp*gam


i.e. SARAH multiplies the width into q_i q_j with a factor of 2 for i =! j. This factor is there, because it doesn't calculate q_i q_j and q_j q_i individually as you can so on the do-loops (gt1,gt2 are generation indices of the external states). Does this make sense...?

Cheers,
Florian

jmontejo
Posts: 3
Joined: 12. Jul 2017, 11:30

Re: Stop decay widths in UDD RPV

Postby jmontejo » 22. Jan 2018, 17:02

Hi Florian,
I'm not familiar with the code so there might be some context I'm missing here, apologies if I misunderstood the code.
This is a UDD decay so the case where gt1==gt2 should always be zero, how would I get stop->bb?
When gt1!=gt2 it seems that you are multiplying twice by 2, there is a second one just after the If, is this intended? maybe the color factor?

Reading through the appendices in 1601.03737 and 0406039 what I understand is that the convention lambda''/2 UDD is chosen so that the antisymmetry in the coupling is already taken into account and there would be no need to for

Code: Select all

If (gt1.ne.gt2) gam = 2._dp*gam


Cheers
Javier

FStaub
Site Admin
Posts: 546
Joined: 13. Apr 2016, 14:05

Re: Stop decay widths in UDD RPV

Postby FStaub » 23. Jan 2018, 13:05

Hi,

of course, the decay into identical states is forbidden. Yes, the other factor of 2 is just the colour as you say.
However, the factor of 2 for non-identical generation indices has nothing do to with the coupling. It would also be there for decays H->h_i h_j for multi-Higgs models for i =! j.

Cheers,
Florian


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