Treatment of spinor bilinears in FeynCalc

[vsht]

[mescia]

I am using FeynCalc 9.3.1 (stable version).

From DiracSimplify[Spinor[Momentum[ps], ms].Spinor[Momentum[ps], ms]] I get ms, no spinor anymore. Why do the spinors disappear?

Thanks, Federico.

[vsht]

Because it corresponds to ubar(p,m) u(p,m), which is 2m in the relativistic normalization.

[vsht]

Perhaps I misunderstood the question. Was it about whether the spinors can be kept despite being equal to 2m?

[mescia]

I did not formulate my question well. Sorry for this. I am doing an operator matching in the collinear regions. My idea was to keep all the spinor structures for 4 and 6 fermions. But at some point I noticed I had no spinors in my amplitudes (panic!). This was because as you correctly noticed Feyncalc automatically evaluates the Dirac structure of tescalar densities, ub(p).ub(p) as 2m. This is only done for scalar dentisities. Unfortunately in the matching I don't need to directly evaluate amplitudes because quarks are not free. I need only to collect all the billinears and then I will calculate the corresponding matrix elements using form factors. Technically, to keep the spinor structures up to then makes the calculation symmetric with respect to all the other fermion bilinears. and it is usell to build the operator basis in a large calcualtion. One more thing, I used DiracSimplify to apply the equation of motion after divergences in the loop are cancelled out. Then, DiracSimply apply the equation of motion reducing my operator basis. But at this point it also evaluates the scalar density and spinor disappears which is complictaed to classify the operator basis. Is there a Flag in DiracSimply to stop it? thanks very much and sorry for the confusion. Federico.

…

On Mon, 7 Sep 2020 at 10:27, Vladyslav Shtabovenko ***@***.***> wrote: Perhaps I misunderstood the question. Was it about whether the spinors can be kept despite being equal to 2m? — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#60 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AEBUE74WKJRS4X3ASC46UELSESKNBANCNFSM4PLYUA3Q> . Aquest missatge, i els fitxers adjunts que hi pugui haver, pot contenir informació confidencial o protegida legalment i s’adreça exclusivament a la persona o entitat destinatària. Si no consteu com a destinatari final o no teniu l’encàrrec de rebre’l, no esteu autoritzat a llegir-lo, retenir-lo, modificar-lo, distribuir-lo, copiar-lo ni a revelar-ne el contingut. Si l’heu rebut per error, informeu-ne el remitent i elimineu del sistema tant el missatge com els fitxers adjunts que hi pugui haver. Este mensaje, y los ficheros adjuntos que pueda incluir, puede contener información confidencial o legalmente protegida y está exclusivamente dirigido a la persona o entidad destinataria. Si usted no consta como destinatario final ni es la persona encargada de recibirlo, no está autorizado a leerlo, retenerlo, modificarlo, distribuirlo o copiarlo, ni a revelar su contenido. Si lo ha recibido por error, informe de ello al remitente y elimine del sistema tanto el mensaje como los ficheros adjuntos que pueda contener. This email message and any attachments it carries may contain confidential or legally protected material and are intended solely for the individual or organization to whom they are addressed. If you are not the intended recipient of this message or the person responsible for processing it, then you are not authorized to read, save, modify, send, copy or disclose any part of it. If you have received the message by mistake, please inform the sender of this and eliminate the message and any attachments it carries from your account.

-- *Federico Mescia* Catedràtic de Física Teòrica | Universitat de Barcelona Dept Física Quàntica i Astrofísica & Institut de Ciències del Cosmos p: +34 9340 21161 o: 6.07, 6ª planta a: Diagonal 647, Facultat de Física, Barcelona w: icc.ub.edu e: mescia@ub.edu s: federico.mescia

[vsht]

No problem. I see that the question is not a trivial one, which is good.

At the moment it is not really possible to prevent DiracSimplify from doing
this simplification, since it is hard-coded here.

It is also the only place in the whole package where this replacement is applied.
If it is urgent, you could just comment the code between the lines 476 and 486.

In any case I agree that this is something that should be configurable, especially
since I also would like different normalization conventions to be available via an
option.

It's also not really much work, just a bit of code and a bunch of compulsory unit tests.
Then I would also be able to merge #64 to finally have the Gordon identities built-in.

Let me see if I manage to add this to the dev version within this week.

[vsht]

Ok, all done. The new option SpinorChainEvaluate -> False in the dev version should do the trick

DiracSimplify[
 x SpinorUBarD[p].GAD[mu].SpinorUD[p] + y SpinorUBarD[p].SpinorUD[p], 
 SpinorChainEvaluate -> False, FCE -> True]

TID[(x SpinorUBarD[p].GAD[nu].SpinorUD[p] + 
    y SpinorUBarD[p].SpinorUD[p]) FVD[l, mu] FAD[l, l - p], l, 
 SpinorChainEvaluate -> False, FCE -> True]

[mescia]

Thanks very much. Federico

…

On Sat, 12 Sep 2020, 23:47 Vladyslav Shtabovenko, ***@***.***> wrote: Ok, all done. The new option SpinorChainEvaluate -> False in the dev version should do the trick DiracSimplify[ x SpinorUBarD[p].GAD[mu].SpinorUD[p] + y SpinorUBarD[p].SpinorUD[p], SpinorChainEvaluate -> False, FCE -> True] TID[(x SpinorUBarD[p].GAD[nu].SpinorUD[p] + y SpinorUBarD[p].SpinorUD[p]) FVD[l, mu] FAD[l, l - p], l, SpinorChainEvaluate -> False, FCE -> True] — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#60 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AEBUE72OWZRI3DWZY2CTT6LSFPT7BANCNFSM4PLYUA3Q> . Aquest missatge, i els fitxers adjunts que hi pugui haver, pot contenir informació confidencial o protegida legalment i s’adreça exclusivament a la persona o entitat destinatària. Si no consteu com a destinatari final o no teniu l’encàrrec de rebre’l, no esteu autoritzat a llegir-lo, retenir-lo, modificar-lo, distribuir-lo, copiar-lo ni a revelar-ne el contingut. Si l’heu rebut per error, informeu-ne el remitent i elimineu del sistema tant el missatge com els fitxers adjunts que hi pugui haver. Este mensaje, y los ficheros adjuntos que pueda incluir, puede contener información confidencial o legalmente protegida y está exclusivamente dirigido a la persona o entidad destinataria. Si usted no consta como destinatario final ni es la persona encargada de recibirlo, no está autorizado a leerlo, retenerlo, modificarlo, distribuirlo o copiarlo, ni a revelar su contenido. Si lo ha recibido por error, informe de ello al remitente y elimine del sistema tanto el mensaje como los ficheros adjuntos que pueda contener. This email message and any attachments it carries may contain confidential or legally protected material and are intended solely for the individual or organization to whom they are addressed. If you are not the intended recipient of this message or the person responsible for processing it, then you are not authorized to read, save, modify, send, copy or disclose any part of it. If you have received the message by mistake, please inform the sender of this and eliminate the message and any attachments it carries from your account.