Nima Arkani-Hamed’s letter on multi-muons – and my reply

I had the pleasure of seeing Nima Arkani-Hamed -an IAS/Harvard University theoretician- visit this blog this morning. Nima, together with some colleagues, published three weeks ago a couple of papers where they discussed the possibility that hadron colliders put in evidence a signature of new physics in the form of lepton jets, produced by particles with long lifetime – a signature strikingly similar to the one CDF published a few days ago.

In the thread developed at Peter Woit’s site Neal Weiner, a colleague of Nima and a co-author of his papers, claimed:

“I can tell you officially we had no word on this. This blog is, in fact, the first I’d heard of it.”

and I replied, in my usual talk-first-think-later style:

“that is pretty hard to digest. Lepton jets with lifetimes. Come on. I think you owe it to the physics community to let us know where the leak came from.”

I later regretted, as I often do, to have been so explicit in saying what I think. But the fact remains that many among my colleagues think the same thing, so the physics community does need some sort of explanation. Nima’s comment, which I paste below, somehow fills that gap, although he also complains with me for what I wrote, implying I said Neal is a liar. I did not call Neal a liar: their paper has many authors, and even just one of them might have heard news about a multi-lepton signal in CDF to come out soon, and this might have been the source of the coincidence. Anyway, here is what Arkani-Hamed wrote:

Hi,

As most of my friends and colleagues know, I have a very dim view of
the physics blogosphere, and avoid interacting with it. However, your
statements about me and my collaborators in connection with the recent CDF anomaly quite clearly crossed a line, and I feel compelled to respond. In doing so I am adopting an attitude Howard Georgi once described when dealing with non-perturbative QCD effects in heavy quark effective theory: once I am finished dealing with the brown muck, I will wash my hands.

It was stimulating to see the CDF paper. However, this is an extremely challenging analysis, and many further cross-checks will have to be done to take it as a serious indication for physics beyond the standard model. We are all well aware that particle physics anomalies have come and gone in the past two decades, in analyses that are less complicated than this one; of course the collaboration made no claim to have discovered new physics. Keeping this in mind, let me first make comments on some physics I have been involved with, and end with some comments on sociology.

I have been working recently with Doug Finkbeiner,Tracy Slatyer and Neal Weiner on a theory for dark matter motivated by a growing number of anomalies in astrophysics, most recently PAMELA/ATIC.
This work is a direct continuation of (in my view) beautiful earlier work pioneered by Neal, Doug and Dave Tucker-Smith, who have collectively pushed it for many years. In fact, Neal, Doug and others previously talked about GeV scalars decaying to leptons in the context of “exciting” dark matter, to explain the INTEGRAL signal, as well as the HEAT excess predecessor to PAMELA. So the idea of GeVish mass particles decaying to leptons, motivated by Dark Matter anomalies, goes back to Feb 2007–Feb 2008. See e.g. astro-ph/0702587 and especially arXiv:0802.2922.

What we did in our four author paper was to show that all these strands fit together into a simple unified picture that also makes very good particle physics sense. The idea can be summarized in one sentence: Dark Matter is charged under a non-Abelian gauge symmetry broken at the GeV scale. We pointed out two additional major motivations for the GeV scale–first, the new vectors with this mass naturally “Sommerfeld enhance” the annihilation cross section as appears necessary to explain the PAMELA/ATIC signals from DM
annihilation; second, the broken gauge symmetry at the GeV scale radiatively induces splittings between the different states in the DM multiplet at the \sim MeV scale, which is precisely what is needed in “exciting” and “inelastic” DM explanations of the INTEGRAL and DAMA signals. All of these phenomena, scattered over energy scales ranging from a TeV to an MeV, are essentially a consequence of the single sentence I used to describe our picture above. We find this compelling.

Since the GeV gauge sector is non-Abelian, there are a number of
states, minimally including vectors and higgses, all GeVish in mass.
One thing that happens when at least some of these GeV particles are vectors is that they can easily talk to the Standard Model, via kinetic mixing with the photon. The mixing is naturally small, so directly producing this particle is challenging–though people have
talked about doing it at low-energy e+ e- machines (B-factories,
DAPHNE, BESS). Neal and I considered the simplest marriage of our DM picture with low-energy SUSY, which further naturally generates the GeV dark gauge symmetry breaking scale. We also
pointed out that in this set-up, one could produce particles in this new GeV sector much more copiously, not directly, but indirectly through SUSY production: every SUSY event will end with MSSM LSP’s which then decay into the true LSP in the dark sector; essentially all of these will also be accompanied by some of the GeVish particles that re-decay back into SM leptons. One would expect a cascade of decays in the GeV sector given the multiplicity of states, and thus the aptly named “lepton jets”. We discussed displaced vertices as a possibility, though they are not guaranteed. So, seeing “lepton jets” as an O(1) fraction of SUSY signals is what we talked about as the smoking gun of our model at the LHC. It would indeed be an amazing signal, which is why we were and continue to be very excited about it!

Now, even if the CDF anomalies are an indication of new physics–which I think in all of our views is _very_ far from obvious– it can not be due to the signal Neal and I talked about, arising from SUSY cascade decays. The rate of the CDF anomaly is absolutely
enormous–you are talking about 70,000 “ghost” events! If there is a connection to our model at all, it would likely have to be through direct production of the GeV sector particles, that still cascade decay in the dark sector and produce the lepton jets. As I mentioned there are limits one can put on this from e+ e- data, and a number of us had been wondering what could be done at hadron colliders, but at least my instinct was that the rates wouldn’t be high enough and it would be far too messy and difficult to extract a signal. We were planning on thinking about this in more detail soon, but the exploration of the consequences of our model is very new, and for now most of us have been focusing on getting out the important predictions on the DM side for GLAST/Fermi, as well as fleshing out the big O(1) LHC predictions. Obviously, stimulated by the CDF result, studying the question of direct production at the Tevatron is much higher on our list of priorities, and we are looking at it now to see if it can even be in the right ballpark. But to re-iterate: even if the CDF anomaly is new physics, and even if it is connected to our model, (and needless to say these are two very big “ifs”), it would be a wonderful surprise to me since I had expected probing direct production of the GeV sector to be incredibly difficult at a hadron collider.

So much for the physics. Turning to the sociology: you publicly suggested that we had gotten wind of the CDF experimental result ahead of time, and casually wrote this paper pointing out the signal before the experimental result was published. Your only evidence is that we made a surprising prediction of a signal experimentalists hadn’t thought of before and put it out before the experimental
results were made public–Gasp! Shocking! Scandalous! Never happened in the history of physics! Not contained in the definition of the word “pre”diction! This was a hilariously preposterous accusation;
however it stopped being funny when you went further and all but
called Neal a liar when he stated unequivocally that we had no advance knowledge of the CDF result–this is outrageous. As Neal said, we had no knowledge about the experimental result ahead of time _at all_. We didn’t even talk about the Tevatron in our paper! And as I said above, even if this anomaly is real and even if it is related to our model, it can not be literally the signal we talked about. Also, the thought that we could somehow cook up a model motivated by explaining dark matter anomalies as a cover to explain rumored events at CDF is absurd–ask any theorist friends you may have whether this is feasible and you will get a good chuckle out of them. You think this signal “came out of the blue”, but if you have been following any of the developments in BSM collider physics in the past couple of years you will realize that signals involving high particle multiplicities with displaced vertices have been discussed for quite a while–check out the repeated use of the phrase “hidden valley” in my paper with Neal for references to work by Strassler, Zurek et. al. Our contribution is that a rather specific version of such a picture–with the GeV mass scale and coupling to the SM through kinetic mixing with the photon–is naturally singled out by the new picture of dark matter we put forward with Doug and Tracy, giving a potentially exciting connection between what we are now seeing in the sky and what we might see at colliders. As it happens this type of hidden valley model had not been discussed in the literature, so we happily pointed out some of their possibly dramatic LHC consequences. Perhaps if you had bothered to even superficially read our papers and think about the physics, it wouldn’t seem so “out of the blue” to you, and you would understand that these light GeV particles decaying to leptons are a necessary feature of our model of Dark Matter, whether CDF saw any hints for them or not.

I find your cynicism remarkable. We are entering what promises to be a golden era of amazing experiments in high-energy physics, astrophysics and cosmology, which may very well lead to profound advances in our understanding of Nature at a fundamental level. All of us–experimentalists and theorists alike–are fantastically excited about this and are doing everything we can to give it the best chance of happening. And at least most of us don’t think of physics as a soap opera rife with rumor and innuendo, or spend the precious time we have cynically tossing around completely baseless and deeply offensive accusations.

Speaking of precious time, I’m sure you’ll agree that there is more critical physics to do than there are hours in the day to do it,
and I for one would like to get back to work.

Nima Arkani-Hamed

And here is my reply:

Dear Nima,

thank you for taking the time to explain more in detail what was the creative process behind the two papers you recently published. If you read my comment on Peter’s blog, you know I did ask for something like what you wrote above: an explanation of how you came to consider the striking signature we are discussing about.

If I insulted you or your colleagues with my remark, then please accept my apology, and forward these to them. I am a sceptic not only with respect to SUSY (which may be excused, since I am in good company), but also with respect of the CDF result itself. And I found it really a remarkable coincidence, to avoid putting new words out which may be found aggressive, that no more than three weeks before the CDF result is aired, you come up with lepton jets, with long lifetime, and with small invariant mass. Of course, that is not a copyrighted signature, so I am the one at fault – I am speculating. But indeed, I was not the only one who found this coincidence fishy. Many of my colleagues in CDF did, and so did others outside.

So, to summarize: I am pleased that you chose to come down to this blog to explain what caused you to discuss that signature. I will need time to digest what you wrote, because I am basically an ignorant. In earnest, I have to say I am still sceptical that there were no influences in your creative process. But I guess that is ok. The material is there for anybody to read and make their own opinion, and your text above will help creating those opinions.

Keep up with the good work,
T.

So, I should like to open a poll for those heroic readers who came to the bottom of this post: did you read Nima’s papers ? Do you think there was a leak or do you rather think it was their own cooking ? I would be happy to hear I am the only one who still believes there was some internal information which was passed to the group who wrote those papers…