Problem with color sextets

[thoron]

Hi,

I implemented in SARAH a simple extension of the SM with color-sextet VL fermions:
FermionFields[[6]] = {X, 1, xc, 1/3, 1, 6};
FermionFields[[7]] = {XP, 1, xpc, -1/3, 1, -6};

I managed to create UFO files with no error (I put them in the attachment). I was then trying to use them in MadGraph to generate the procces "p p > new color sextets" and it seemed to be working, but when I tried to calculate the cross-section I got the error message:

Command "output VL_sextet_1" interrupted with error:
ValueError : String -3/16 T6(-1000,-1007) T6(-1000,-1007) cannot be simplified to a number!

I contacted the MadGraph team first and received the following information from Olivier Mattelaer:

"I know that some FR model have that issue but it is the first time that I hear that from SARAH. The origin of the problem should be the same some particle should be in the wrong representation (in this case 6 instead of -6). Since that error is SO common for triplet, MG5aMC has an automatic way to flip triplet representation but we do not have the equivalent for sextet so you have to have a really valid UFO model in that case. For the help on doing that, I can only suggest to contact SARAH authors."

I was then wondering if this problem can be solved somehow from the SARAH side.

Thanks.
Kamila

[FStaub]

Hi Kamila,

I worry that the support of sextets has been implemented years ago but were never used in a serious application (at least the UFO output).
Did you try, if I might work if you just switch the sign of the colour representation of your sextet?

I.e., changing in particles.py

Code: Select all

fc = Particle(pdg_code =5000003, 
	 name = 'fc' ,
	 antiname = 'fcbar' ,
	 spin = 2 ,
	 color = -6 ,
	 mass = Param.Mfc ,
	 width = Param.Wfc ,
	 GhostNumber = 0, 
	 line = 'swavy' ,
	 charge = 1/3 ,
	 texname = 'fc' ,
	 antitexname = 'fcbar' ) 

and putting a +6 there. If the -6 is correct, but Matlab needs everything in terms of positive colour representations, it might be possible to redefine your Dirac spinor from {XP, conj[XPC]} to {XPC, conj[XP]}. Sorry, all of that is just guesswork, because I'm officially out of business, but maybe it helps.

Cheers,
Florian