# 2007: Year of the Unparticle

One can get some idea of what progress there might have been in particle theory during 2007 by querying the SPIRES database for 2007 papers that already have lots of citations. Doing

find topcite 50+ and date 2007

turns up 20 papers of which 6 are experimental papers. Remarkably, the 14 other papers are all about one topic: unparticles. These all refer to Howard Georgi’s initial Unparticle Physics paper from March 2007, in which he describes a possible effective field theory that would be scale invariant and correspond to unusual phenomena potentially observable by collider experiments, phenomena he describes in terms of “unparticles”. In less than a year Georgi’s paper has accumulated 118 citations, with the blogger at Resonaances making fun of the phenomenon of “unpapers” with abstracts such as:

We consider unparticles in whatever uncontext. You are encouraged to forget the paper as soon as soon as you add it to your citation list.

but also making the very relevant comment that this does seem to be getting attention because it is a legitimately new idea:

I must give the credit to Howard for drawing our attention to a whole wide class of collider signatures. Besides, I appreciate Howard’s writing style. He is probably the last man on Earth who truly enjoys particle physics.

As far as I can tell, unparticles don’t solve any of the problems of the Standard Model, but they are theoretically possible phenomena of a different kind that experimentalists can look for, and having as many as possible of such phenomena is very worthwhile. The more different things people are looking for, the more likely they’ll find something unexpected that otherwise might not have been noticed.

This week New Scientist has a long and quite good cover story about unparticles, and recent attempts to use them to explain dark matter, which ends with:

Georgi reserves judgement on whether his unparticles really could be the key to solving the dark matter problem until more work is done, but he’s pleased that people are investigating the possibility. “All I knew was that I had found something cool and I wanted other people to take a look and see what kinds of weird things they might be capable of doing – what mysteries they might solve,” he says. “I’m happy because that’s exactly what people are now doing.”

The story has also made it into the Telegraph.

Besides the unparticle phenomenon, there appear to be very few 2007 theory papers that anyone is paying much attention to. I’ve tried to search around and come up with a list of 2007 papers that have so far gotten 25 citations or more, and a list follows. I’m probably missing some [Note added: additions welcome, and will be added to the list]. The main themes shared by most of these papers are AdS/CFT and attempts to construct metastable vacua as part of a study of the landscape [Note added: this characterization is of the hep-th papers, adding in lots of ones I missed from hep-ph shows that the hep-ph ones cover a much wider variety of topics] .

• Metastable vacua and D-branes at the conifold (Argurio, Bertolini, Franco, Kachru) 54 citations.
• Gluon scattering amplitudes at strong coupling, (Alday, Maldacena) 47 citations.
• The Bulk RS KK-gluon at the LHC, (Lillie, Randall, Wang) 38 citations.
• Supersymmetry breaking, R-symmetry breaking and metastable vacua, (Intrilligator, Seiberg, Shih) 35 citations.
• Electroweak constraints on warped models with custodial symmetry, (Carena et al.) 33 citations.
• The Supersymmetric Parameter Space in Light of B-physics Observables and Electroweak Precision Data, (Ellis et al) 31 citations.
• Simple Scheme for Gauge Mediation, (Murayama, Nomura) 31 citations.
• Non-perturbative and Flux superpotentials for Type I strings on the Z(3) orbifold, (Bianchi, Kiritsis) 31citations.
• Phase Structure of a Brane/Anti-Brane System at Large N, (Heckman, Seo, Vafa) 30 citations.
• Thermodynamics of the brane, (Mateos, Myers, Thomson) 30 citations.
• On the Singularities of the Magnon S-matrix, (Dorey, Hofman, Maldacena) 29 citations.
• On the Strong Coupling Scaling Dimension of High Spin Operators, (Alday et al.) 29 citations.
• Charged Lepton Flavour Violation and (g-2)_mu in the Littlest Higgs Model with T-Parity, (Blanke et al.) 29 citations).
• Split states, entropy enigmas, holes and halos, (Denef, Moore) 28 citations.
• Computation of D-brane instanton induced superpotential couplings, (Cvetic, Richter, Weigand) 28 citations.
• Towards an Explicit Model of D-brane Inflation (Bauman et al.) 27 citations.
• MadGraph/MadEvent v4: The New Web Generation, (Alwall et al.) 26 citations.
• Physics of String Flux Compactifications, (Denef, Douglas, Kachru) 25 citations.
• A Measure of de Sitter entropy and eternal inflation, (Arkani-Hamed et al) 25 citations.
• Explaining the Electroweak Scale and Stabilizing Moduli in M Theory, (Acharya et al.) 25 citations.
• SPIRES has yet to compile a 2007 “topcites” list, but it looks like the pattern should be very much the same as the last few years:

• increasing dominance of research into AdS/CFT (597 citations of Maldacena’s paper in 2007, versus 551 in 2006)
• particle theory basically died at the end of the 20th century with the only post 1999 paper getting more than 150 citations the KKLT one reflecting the rise of landscape research.
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### 31 Responses to 2007: Year of the Unparticle

1. piscator says:

the positive side of 2007 is the preparation of lots of people for the model bonfire that the folks in geneva are warming up (or rather cooling down) for. frankly, the last thing particle theory needs now is another over-hyped and over-cited model solving all open problems (insert your favourite example)

i think there is a lot of retooling going on in preparation for the LHC, there will soon be a large parting of the ways between those primarily interested in physics as an experimental subject and those interested in more formal topics.

piscator is right. There’s a big divide at the moment between the more formal side of particle physics, which seems to be in the doldrums, and the experimental/phenomenological side which is booming thanks to the forthcoming LHC. Of course, the former should feed off the latter and hopefully this will be the case over the next few years.

3. Bee says:

We had a post on the topic in September, see Unparticles.

4. Domenic says:

Hey, how cool—today is exactly the day that Georgi arrives at Caltech (where I’m currently being an undergrad) to give a talk on unparticles. I put it on my calendar because it looked more interesting/introductory/understandable than most talks, so I’ve actually been looking forward to this for a week. Nice to know that it’s an interesting idea; should be fun!

Two (I think) interesting papers missing from your 2007 list

Mateos, Myers, Thomson

http://arxiv.org/abs/hep-th/0701132 30 citations

Denef and Moore

http://arxiv.org/abs/hep-th/0702146 28 citations

there are probably others…..

6. Peter Woit says:

Thanks a lot, I’ve probably missed quite a few. Just added those two, information about any others that should be added is very welcome.

7. hepph says:

Quite a few papers missing from the hep-ph side that *weren’t* unparticles. Granted these don’t necessarily all resemble theoretical breakthroughs but they represent a sampling of things that the community is working on:

J.~R.~Ellis, S.~Heinemeyer, K.~A.~Olive, A.~M.~Weber and G.~Weiglein arXiv:0706.0652 31 citations

B.~Lillie, L.~Randall and L.~T.~Wang hep-ph/0701166 38 citations

M.~Blanke, A.~J.~Buras, B.~Duling, A.~Poschenrieder and C.~Tarantino hep-ph/0702136 29 citations

M.~S.~Carena, E.~Ponton, J.~Santiago and C.~E.~M.~Wagner hep-ph/0701055 33 citations

H.~Murayama and Y.~Nomura hep-ph/0701231 31 citations

J.~Alwall {\it et al.} arXiv:0706.2334 26 citations

8. alex says:

Acharya, Bobkov, Kane, Kumar, Shao; hep-th/0701034 25 citations

9. David Nataf says:

In terms of particle physics in 2007, I think an interesting result would be the Auger experiment’s confirmation of the GZK effect.

10. stan says:

Initially, “Unparticles“ seemed like a joke to me but now after all these citations, I cant even laugh. Add an Un to every particle physics process and write a paper and cite everyone that wrote before you. I might write an UNString theory paper to see what is going on.

Another interesting one (although it is not exactly “particle physics”)

Elvang and Figueras

http://arxiv.org/abs/hep-th/0701035 25 citations

12. Peter Woit says:

hepph and alex,

Thanks for the additions, it is true that they reflect a wider variety of activity in hep-ph than in hep-th.

Thanks for pointing that out, but I won’t try and keep track of papers like that one that are much more gr-qc than hep-th.

13. Eric says:

Perhaps you should add the string phenomenology papers hep-th/0703028 and arXiv:0707.1871 to your list.

14. Peter Woit says:

Thanks Eric,

I added the first of those, the second only has 15 citations so far, below the cutoff of 25.

To make explicit the obvious, the list compiled here is weighted towards papers from early in the year, which have had more time to accumulate citations. As such, to rank them by how much they’re being cited, you really should weight by when they appeared. My idea in accumulating this list was more to see what papers are getting significant numbers of citations, even if in a crude way. One thing this crude method does is it tends to ignore papers from late in the year, but in any case it is too early to get much of an idea of how often they’ll be cited as time goes on.

To make explicit the even more obvious: citation counts are about sociological, not long-term scientific impact.

15. Jon Lester says:

I find very silly to try to consider importance of papers in a year by their citation number. In about 30 years the only criterion used by particle physics community to decide what is good and what is bad is fashion. This is clearly seen also in the peer-review process and I think the merit of arxiv and its creator will be manifest in a few years when we will realize that the most important papers are those that were systematically overlooked in the year they appeared and was also rejected by the highest ranked journals.

Jon

16. Somdatta Bhattacharya says:

This idea of unparticles, it’s utter nonsense. Does it solve any of the outstanding problems of SM? Is there any motivation for it coming from anything at all?

17. piscator says:

For the clearest evidence of why citation counts do not directly measure scientific impact, do the following:

1. Look up the two Randall-Sundrum warping papers.

2. Take the ph paper, convert the ‘p’ to a ‘t’, and look up the hep-th paper with the same number.

3. Look at the topic of this paper and the papers that cite it, and decide for yourself where these citations were intended.

Conclusion: citation counts sometimes measure neither scientific merit, nor sociological trends, but rather the touch-typing proficiency of particle theorists 🙂

18. string-dude says:

Hi Somdatta, why should it solve the “outstanding” problems of the standard model for it to be interesting?

Unparticles are talking about a possible low energy phenomenon which might have gone un-noticed because we never thought of looking for it. This in itself is a valid thing to consider, whatever ones prejudices against high energy physics might be.

But the fact that you claim that unparticles are “utter nonsense”, an THEN ask questions about it, makes me think that you form your opinions a tad quicker than I do…

19. Somdatta Bhattacharya says:

Dear String dude,
Just because I wrote the questions after the statement doesn’t necessarily mean that the latter is not a consequence of the former.
One could come up with numerous theories or scenarios that supplement the SM but there must have to be a “motivation” for them. For me investigating them just because they are a possible scenario with possible experimental signatures is like putting the cart before the horse. Since experiments at the next energy scale are near, it makes no sense to just go on increasing upon the number of possible scenarios that might be there, especially when there is no motivation to do such things. If it solved or aspired to solve some problem like the hierarchy problem or that of the abundance of parameters in the SM or the unification one, I would see a point to it. Otherwise its just a load of c***.

20. gambler says:

Somdatta Bhattacharya, if you want to win the lottery, you have to buy the tickets before the drawing.

21. Thomas D says:

Neubert had a nice paper 0708.0036 showing that ‘unparticles’ are actually rather similar to strongly interacting particles, in terms of the supposedly distinctive experimental signature.

Also, as soon as you couple them to the SM their conformal invariance will be explicitly broken (whether to a small or large extent), which somewhat, or completely, spoils their original neat rationale.

So between experiment and theory, I don’t know what is supposed to be attracting us towards them. As I said some time ago: there is a continuous infinity of field theory models of new physics, we would like to have some coherent rationale to choose some directions in this infinite space of theories. Experimental considerations do play a role: if a model has clear collider signatures that otherwise would be overlooked, for example. It’s not clear to me whether there really is one clear signature of unparticles.

Of course, had the LHC turned on in 2005-6, as was hoped some time ago, there would very probably be no-one saying ‘unparticle’ today at all. Theorists need something to keep their minds active.

22. Somdatta Bhattacharya says:

Thanks Thomas D,
However, yes, theorists might need to keep their minds active, but it would do them helluva lot of good if they, especially people like Georgi, came up with things that are worthwhile, otherwise its a waste of time not only for them, but for the whole community. Because had I published such a paper, no one would have payed any attention, but since it came from Georgi, everyone decided to latch on.

23. string-dude says:

>Because had I published such a paper, no one would have payed
>any attention, but since it came from Georgi, everyone decided to
>latch on.

I agree that there is truth in this statement, but I find it hardly worth getting so upset about. Science is an enterprise that involves primates working in groups. Mavericks and rebels have to work their way up the hierarchy by paying their dues. I promise you that once you produce a paper of sufficient import, your next paper, even if it is mediocre, will have a fan-following.

Mind you that I am not endorsing this phenomenon. But I somehow think that this meta-level anger, righteous indignation, etc., is completely useless. The only real way in which to make sure that people work on good ideas is to OFFER them a good idea to work on. What is the point in bitching about other ideas? The oldest rule of progress in science is to work on *something* rather than nothing, and I can’t really blame the troops for latching on to a moderately new idea.

That said, I looked at the paper that Thomas D talked about and agree that unparticles suddenly seem a lot less interesting than what I had thought after reading the original paper by Georgi (which I bothered to do only after seeing Peter’s blog).

Sleep tight and don’t let the bedbugs bite,
Chethan Krishnan (aka string dude)

24. Somdatta Bhattacharya says:

>I promise you that once you produce a paper of sufficient import, >your next paper, even if it is mediocre, will have a fan-following.

I hope not everybody thinks like this. In the enterprise of physics, it’s not fan-followings that one is after, but the truth. Mediocre papers ought to get the treatment they deserve, which is complete disregard.

>What is the point in bitching about other ideas?

In case you haven’t noticed, this IS the place to bitch about other ideas, to thrash things out with people like you.

25. Professor R says:

hi Peter,
I was delighted to see one of the top-cite papers you list (Supersymmetry Breaking, R-Symmetry Breaking and Metastable Vacua) gives the proper reference for the O’ Raifeartaigh Model for a change – and so it should, as the model is heavily referenced and mentioned on every page!

It strikes me once more that one of the drawbacks in the way citations are counted is that there is no distinction between a passing citation, and a model that is the central to the entire paper…part of the reason I still having difficulty persuading Dad’s own countrymen that his work was important….Cormac

26. Coin says:

Neubert had a nice paper 0708.0036 showing that ‘unparticles’ are actually rather similar to strongly interacting particles, in terms of the supposedly distinctive experimental signature.

Also, as soon as you couple them to the SM their conformal invariance will be explicitly broken (whether to a small or large extent), which somewhat, or completely, spoils their original neat rationale… It’s not clear to me whether there really is one clear signature of unparticles.

Hm, so this is very interesting to me. So is there *ANY* detectable difference between a SM+unparticles unparticle and a strongly-interacting particle? That is to say, if we detected a particle signature for some new strongly-interacting particle, would there be grounds to distinguish whether it was an actual particle or a disguised unparticle?

I guess I’m still trying to understand exactly what the unparticle stuff is. I’d be curious to know whether I have this right: It appears that an unparticle field is a just a QFT field (I.E., you can’t ever have say an electron unparticle– qft fields consist of either all particles or all unparticles) except that field has a particular property (conformal invariance) which is mathematically well-defined, but which we don’t ever consider in normal physical theories. Is that about it?

Given (let’s assume that we go looking for unparticles and don’t find any) that there don’t seem to be any unparticles in nature, is this something that needs to be explained? Is there some natural or obvious reason why only fields without the unparticle property would be selected to exist in nature, besides “that’s what we observe”?

27. David B. says:

Coin:

Unparticle events would just be like hadronic jets in a hidden sector. Jets usually involve many particles in the final state and give different energy curves for how much energy was dumped into them than single particle production. If you can not see the particles, your only signature is missing energy. So long as the perturbations from conformal invariance are small, the unparticle’ calculation is for the most part fine, unless you are below treshold for production of unparticles (which could have a tiny mass).

Unparticles is in the end just another nickname for conformal field theory (in practice all you need is an approximate scale invariant sector). It is the fact that conformal field theories have scaling properties that lets you do a calculation without a specific model of high energies. This is why it is looked at. It covers many possible models of ultrahigh energy physics with very few parameters ( a scaling dimension, and the strength of some coupling constants).

Regarding the use of Conformal Field Theories in the rest of physics, I suggest you look at the theory of second order phase transitions in condensed matter systems: those are conformal field theories that can be realized in laboratories, and the scaling exponents can be measured quite precisely.

Unparticles do not have to be there. They are also not forbidden from being there. There doesn’t necessarily need to be an explanation for why nature is the way it is. That is a theoretical prejudice. You have to be open to the idea that there might be surprises in physics and some might just have to be due to no particular reason whatsoever. If you care about phenomenology, you want to figure out which of all the possibilities are realized in nature. You might have to do that by excluding all theories on a case by case basis, in which case you need to have a proper accounting of all the possibilities out there.

28. Professor R says:

“This week New Scientist has a long and quite good cover story about unparticles, and recent attempts to use them to explain dark matter”…
I agree it is a nice article and thanks for pointing it out.
However: did you notice the clanger in the introduction?
“Georgi…pioneered suspersymmetry, a theory he proposed in 1981 with Stavros Dimopoulos at Stanford University….”
Surely this is quite a howler!

Most particle physicists agree that the theory of supersymmetry was originally proposed in the late 1960s by Soviet theorists Gol’fand and Likhtman, and developed by Julius Wess and Bruno Zumino in 1973. Indeed, simple models of supersymmetry breaking (the O’Raifeartaigh Model) had already emerged by 1975.

Presumably the NS meant that the first realistic supersymmetric version of the Standard Model was proposed in 1981 by Georgi and Dimopoulos….the trouble with errors like this is that one is left wondering how accurate the rest of the piece is…..Cormac

29. Peter Woit says:

Cormac,

I did notice that howler. Georgi certainly isn’t responsible for the idea of supersymmetry, or even for supersymmetric gauge theories, which were well known by 1981. From what I remember, he and Dimopoulos were working on supersymmetric versions of GUTs, and I guess as part of that may have been the first to note that one should think about the minimal supersymmetric extension of the Standard Model, together with all possible soft breaking terms (the so-called MSSM).

30. Martin Bauer says:

I don’t really get whats the new idea here. There are a lot of papers out there, assuming hidden sector physics. Is it the conformal invariance? If so, I don’t even get the point why it should be (only) conformal invariant. Why not just take some arbitrary symmetry group?

(I should probably add that all my knowledge in the field comes from a talk I just attended.)

31. Hrvoje Nikolic says:

As I argue in http://xxx.lanl.gov/abs/0801.4471
unparticle can be viewed a particle with an arbitrary mass.