Unparticles

[ajb]

Unparticles are something I don't know much about. I was hoping some of the people on here are more familiar than I am with them. As such I wanted to start this thread. The purpose is to discuss unparticles and point to useful papers on the subject. If you are going to join in here, then I expect you to know a little about QFT and the standard model.

 

So, what do I know?

 

Well unparticles are a strongly coupled scale-invariant sector which interacts weakly with the standard model. The propagators are non-local and the spectrum of the field operators is continuous and so does not describe particles.

 

Questions.

 

a) What Lagrangians have been proposed and are they renormalisable?

b) How do they couple to gauge fields? (Not sure if minimal couple works here).

c) Do they effect anomaly cancellation in the standard model?

d) What strange possible phenomenology could we expect?

 

Ref

H. Georgi, Phys. Rev. Lett. 98 (2007) 221601

H. Georgi, Phys. Lett. B 650 (2007) 275

 

Plus many more on SPIRES we should look at.

[Farsight]

I think it's good that Georgi has been thinking about particles and gauge, but I also think that this really is the wrong approach:

 

http://www.physorg.com/news100753984.html

[ajb]

As I am quite busy, I have not had much time to look at papers on unparticles. I was hoping this tread would evolve into a "reading group".

 

Anyway, from what I can gather (please correct me if I am wrong or have missed important papers)

 

a)Georgi originally considers unparticles in an effective field theory. He suggest that although the high-energy regime is very complicated, phenomenology can be constructed using a low energy effect theory. As such, this is not renormalisable. I don't know the status of constructing the full theory, anyone?

 

b) The non-local nature of the field means that minimal coupling fails. The

 

coupling is via a Wilson line. So it is something like;

 

[math] S = \int dxdy U^{\dag}(x)\Delta(x-y)F(x,y)[/math],

 

where

 

[math]F(x,y)= P exp\left( -i g \int_{x}^{y}A \right)U(y)[/math].

 

(P is the path ordering).

 

More details, including the Feynman rules can be found in the paper

 

Some Issues in a Gauge Model of Unparticles.

Yi Liao . Apr 2008.

e-Print: arXiv:0804.4033

 

 

As for anomalies and interesting phenomenology I need to continue my reading.

[Graviphoton]

Snap.

 

Don't know anything or much of unparticles. I will read through this. Very interesting.

[BenTheMan]

I would also point out that most of the literature on unparticles likely contains wrong assumptions. In particular, I think it was Terning and some others who showed that the scaling dimension of an operator in a theory with a conformal symmetry has to be greater than 3, I think. It might also have been in the paper by Galloway, Martin, and Stancato.

 

Either way, I think that there is a lot of the literature that should be viewed with suspicion.

[Graviphoton]

You seem wise on this Ben. Can you please tell me what the major implications of unparticles are...?

 

Cheers...

[BenTheMan]

You seem wise on this Ben

 

Heh...yeah...wise.

 

I studied unparticles for a while. Basically, it's a kind of neat idea that doesn't really solve any problems or anything. You have to be famous like Georgi to propose these sorts of things.

 

I don't have nearly as deep an understanding of these things as you would give me credit for. It has been a while since I've read any unparticle papers, so I will have to go back and spend some time going over notes.

[Graviphoton]

Oh i see, its a theory that predicts much, but doesn;t answer for as much?

[iNow]

Oh, for the love of Thor, graviphoton. Can you possibly be serious?

[Graviphoton]

Only serious by these assumptions...

 

Spoken by Ben...

 

''Basically, it's a kind of neat idea that doesn't really solve any problems or anything. ''

 

Therefore,

 

I hold

 

''its a theory that predicts much, but doesn;t answer for as much?''

 

Is the two not the same?

 

Oh for the love of thor, please read what is being said

[ajb]

Please, lets not stray to far from my original intentions of this post.

 

Ben is indeed right. There is no "reason" for unparticles in the sense that they solve any open questions about the standard model. However, it appears that such a conformal sector could be consistent with what we know and have seen of the standard model.

 

Over the weekend, provided I get time I will have a look at anomaly cancellation in the standard model and if unparticle sector has any effect on this. (I don't think is does, but I would not like to commit myself)

[BenTheMan]

Hi ajb. See the paper by Terning. I THINK he works it out there.

[ajb]

Thank you Ben, I will have a look for his paper(s) over the weekend.

 

(suggesting papers and giving pointers is exactly what I wanted from this post, not a free for all slagging match nor a short exchange of opinions and random thoughts.)

[ajb]

In Anomalies, Unparticles and Seiberg Duality Galloway, McRaven, and Terning ( arXiv:0805.0799v1 [hep-ph]) show that for fermionic unparticles (unfermions) the Adler-Bell-Jackiw (ABJ or V-V-A triangle) anomaly does not depend on the scaling dimension of the unfermions. They claim (I think correctly so) that this is obvious as the anomalies only have one loop contributions. However, they show this to be the case by applying the Feynman rules of unfermions to the V-V-A diagrams (now not only a triangle!).

 

So, I imagine this means that we can have V-V-A anomaly cancellation form different sectors. I want to read this paper more carefully.

 

Some questions arise

a) Can Fujikawas's method be applied to the V-V-A anomaly?

b) Can you have chiral unfermions? If so what about the non-abelian anomaly?

c) Can we use BRST cohomology here?

[KALSTER]

Found a pdf for you, hope it helps.

[ajb]

Found a pdf for you, hope it helps.

 

Thank you.

 

That presentation is useful. I recommend anyone interested in following this thread have a look at it. It covers most of the basics of unparticles. It does not however discuss "chiral unfermions", which I imagine can exist.

 

As it is still a new area of investigation, I expect some of my questions could be open at this time.

[BenTheMan]

Can Fujikawas's method be applied to the V-V-A anomaly?

 

This question has been turning over in my mind for a few days. I guess unparticle fermions would enter the path integral in the same manner, but I don't know---I've never seen someone write an un-path integral.

[Reaper]

Wait, hold on! Please excuse me for asking this question, but are unparticles supposed to, you know, be real (physically exist)? Or are they just a mathematical tool, because that is what it seems to me so far....

 

I don't really know that much about this branch of physics, sorry, but any clarification will be appreciated.

[ajb]

This question has been turning over in my mind for a few days. I guess unparticle fermions would enter the path integral in the same manner, but I don't know---I've never seen someone write an un-path integral.

 

I too am not very clear on this. The papers I have looked at don't really explain or explicitly present the "un-path integral". Maybe it really is still an open question right now? If you see any references that even hint at this let us know!

 

 

Wait, hold on! Please excuse me for asking this question, but are unparticles supposed to, you know, be real (physically exist)? Or are they just a mathematical tool, because that is what it seems to me so far....

 

I don't really know that much about this branch of physics, sorry, but any clarification will be appreciated.

 

Unparticles are a proposed so far hidden scale invariant sector of the standard model. The state-space looks more like a collections on n non-integer particles. So, they do not represent particles in the usual sense. They would be detected by carrying away energy and quantum numbers in high energy colliders. The dynamical signature makes them distinguishable from other possible processes. Please read the papers I have already referenced if you want to contribute to this thread.

Edited May 31, 2008 by ajb

[ajb]

The issue of deriving the correct Feynman rules for (scalar) unparticles coupled to a gauge field is more complicated than one might at first think.

 

The point here is how does one define the derivative of the Wilson Line [math] W(x,y)= P exp(-i g \int_{x}^{y}A)[/math]?

 

Galloway, Martin and Stancato arXiv:0802.0313 follow earlier work of Mandelstam and suggest that

 

[math]\frac{\partial}{\partial y^{\mu}}W(x,y) \approx A_{\mu}(y)W(x,y)[/math],

 

as this reproduces minimal coupling. But doesn't it depend on the path? Is this a problem?

 

Licht arXiv:0801.0892 shows that for a straight line path you get more complicated Feynman rules that first suggested.

 

Licht then (in arXiv:0801.1148 ) shows using operator methods how to couple gauge fields and unparticles in an effective action which reproduces the Feynman rules as derived earlier by Terring et al arXiv:0708.0005.

 

So at this stage I would say it is not fully understood or really agreed on how to derive the Feynman rules here. The issue must be how to deal with the Wilson path.

Edited June 27, 2008 by ajb