If you have followed this blog for long enough, dear readers, the words "multi-muons", "anomalous muons", or even "lepton jets" are not foreign to you. They all refer to a paper appeared on the ArXiv on the evening of Halloween last year. In the paper the CDF collaboration published the results of a detailed analysis which described how a component of collider data containing two or more muons could not be explained by known Standard Model processes.

Described as above, "a component not explained" is less than exciting; however, the peculiarities of the sample of events unearthed by CDF did lend itself to speculate that the Standard Model would soon crumble under its own weight, yielding in a way that was unexpected -that is, in the modeling of the lifetime of observed muons in hadron collisions. A detailed explanation in four parts of the results of the CDF study is available at the following links: part I, part II, part III, part IV.

Having followed closely the internal review process of the surprising new CDF analysis, and having observed with some amusement the brutally failed attempts of the experiment at keeping the highest level of secrecy on an analysis it had ever attempted to install since the years of the first top quark evidence (1993-94), I knew that the whole story had leaked outside the collaboration. I had even been shown by a friend how some of my collaborators had placed copies of the draft, and even slides of internal meetings where the analysis was being reviewed, in their own public web areas, maybe to make space in other disks, or maybe -just maybe- to distribute the reserved information to outside observers. Friends, fellow theorists, or -God forbid- competitors.

Two weeks before the publication, I had thus not been too surprised to see a paper discussing signatures of dark matter appearing in the Arxiv, where some discussion was made of "lepton jets", with leptons coming from long-lifetime particle decays, and having a small combined invariant mass. The authors foresaw the occurrence of that phenomenon at hadron colliders as a possible signal of new physics, a signal of a hidden sector of particles extraneous to the Standard Model ones.

To me, the matter was clear: the authors had heard news of the forthcoming CDF analysis, and they had worked hard to put together in a timely fashion a model that could predict exactly the signature seen by CDF. In my mind, they had certainly played a tricky hand, but in earnest I could not blame them for their attempt at a quick Nobel Prize, which would have then been easy to claim, had the CDF signal been proven a true emergence of physics beyond the Standard Model. However, I could not really understand how they would be going to handle the obvious objection: the burden of proving that they had had no news of the CDF results) was, in my mind, entirely theirs.

When the news of the surprising CDF preprint finally hit the blogosphere, the matter indeed caused some ripples. The high-traffic site of Peter Woit was the first to report on the matter, and here is what Peter wrote on that occasion:

This is not a signature characteristic of supersymmetry or any of the other known heavily-studied classes of models. If real, as far as I'm aware it's something genuinely unexpected. Perhaps phenomenology experts can point to some less well-known models with this kind of signature. The only such thing I'm aware of is a very recent paper from three weeks ago by Arkani-Hamed and Weiner, entitled LHC Signals for a SuperUnified Theory of Dark Matter. They discuss a theory of dark matter involving a new hidden gauge symmetry, broken near the GeV scale, saying that this is "motivated directly by striking Data from the PAMELA and ATIC collaborations". In these models there can be Gev-scale Higgs and gauge particles decaying to an anomalously large number of leptons. They discuss the question of whether the parameters of such models can be adjusted to give large decay lengths, and predict the observation of events that "contain at least two lepton jets": collections of n > 2 leptons, with small angular separations and GeV scale invariant masses, pretty much just what CDF sees . Since the CDF paper undoubtedly has been the topic of intense discussion among the 450 or so physicists in the collaboration for many months now, the most likely explanation for the appearance of a new theory paper a few weeks ago discussing exactly the signatures in question is that news of what's in the paper got out to some theorists early.

Peter's post received immediate attention in the blogosphere. The fourth note in the comments thread of Peter's post was by Prof. Neal Weiner (right), one of the authors of the paper mentioned in the blog post. Just a few hours after the posting, here is what Neal wrote:

I can tell you officially we had no word on this. This blog is, in fact, the first I'd heard of it. (But have now looked at the paper.) What we predicted was just what was needed to explain PAMELA we thought.

I read Neal's comment shortly after he issued it. I honestly did not believe it, and replied rather rudely in the thread:

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.

The matter, of course, was not settled by a firearms duel, but neither it died out too quickly. Indeed, I had probably gone a bit too far, accusing openly the authors of the phenomenological paper -all distinguished theorists- of little short of scientific plagiarism. I must acknowledge Neal Weiner's good dose of fair play, since he reacted politely to my accusation, and after an e-mail exchange we settled the matter at the "agree to disagree" level.

Meanwhile, another author of the paper in question, Professor Nima Arkani-Hamed (left), also commented on my own blog on the matter. Here is his incipit:

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.

Nima's comment is very long and if you want more details you should read it directly at the source, or in another post where it is further discussed. In the thread, I just answered like this:

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.

The bubble inflated further -it even popped up on the Russian edition of Newsweek, which rather than on the surprising new CDF result, decided to write two full pages on the controversy between Nima, Neal, and I- and then, as happens with bubbles, blew up. I could meet Nima at the CERN cafeteria a couple of months later, and I found him totally at ease and not bearing a grudge. We chatted amiably about the matter, and I mentioned to him that I was still unsure of what to think, but I admitted I should not have reacted so openly in my blog and in Peter Woit's.

Time passed, and I got to meet other people who had an informed view of the issue -the main author of the CDF analysis, other theorists, other experimentalists. I also took the time to study in some more detail than I had so far the theory of a "Hidden Valley", of which the CDF signal might appear to be a signature. And my opinion slowly changed. However, I felt I had not really insulted anybody with my outing on October 31st, 2008, so I let the story die out.

One month ago, however, I had the pleasure of meeting in person, and then having dinner with -at a table in the wonderful setting of the Palazzo della Ragione- none other than Neal Weiner. Neal had come to Padova to give a talk at the Planck 2009 conference: a quite interesting talk, by the way.

Of course, Neal and I discussed face to face the incident of last October, and I came clean with him, apologizing and explaining that I had, in fact, changed my mind on the issue. It was an amusing conversation, which confirmed my view that intelligent people are hard to offend: Neal had not felt offended by my open disbelief of their "no prior knowledge". He also explained to me his side of the story: he was caught by surprise by the CDF paper just before leaving on a trip to China.

I asked Neal to tell that story for you, dear readers, and for the record. Here it is, in his own words:

Hi Tommaso,

Given that everyone has no hard feelings, there's not a great reason to dwell on the past, except to convey the humorous events of the day when the CDF result appeared. Indeed, I would think that anyone who had seen us that day would never have thought we knew about the result in advance.

I received an email forward from Nima (who had been forwarded the link from a postdoc) pointing me to the blog post noting the multi-muon result. I immediately wrote back "wow! they discovered lepton jets?" and responded to the blog comment that we must have known something in advance with my brief remark that we had known nothing, thinking that this would help settle
speculation. Not a good move, perhaps, but I was very, very tired when I read the post.

The next morning, a number of us gathered in an experimental colleague's office, hiding from anyone who might distract us. I was about to leave for China for a month (to attend a workshop at the KITPC), and so it was necessary to avoid anyone who might want to talk about anything other than the new result. We spent the whole morning going over the details of the paper, and calling our experimental friends asking for their opinion and insight. (We were occasionally distracted by an ongoing discussion in the blogosphere.) I was so excited about the CDF result that I asked our fantastic admin at the CCPP to look into changing my flight plans to China(!), as if this was real and consistent with our predictions, it was certainly the most important thing around, and I was ready to change all of my plans to focus on it! It was only when the large cross section was pointed out that we realized that this was certainly _not_ the signal we had
predicted, and, although we continued to speculate on how it might be related, a great deal of wind was taken from our sails. I told our admin that I was still going to China.

So, truly, we were caught as off guard as anyone, and if only you could have seen us that Friday, I think it would have been comically obvious to anyone how ignorant we were!

But, still, a fun morning. You don't get a lot of days that are that exciting, even if fleetingly so.

As for the whole discussion, I happily accept your retractatio! These things should never be taken too seriously.

Neal Weiner

So this settles the matter: the present posting represents a formal Retractatio of my previous accusation. I now believe that Neal Weiner, Nima Arkani-Hamed, and their collaborators had indeed no prior knowledge of the CDF result when they published their preprint.

I am also happy to take this occasion to say that I sometimes do change my mind: it is a very healthy habit, which aging minds should exercise at least once a week. As some wise man once said, only the fool never changes his mind.

As for the CDF anomaly, the issue is still open. True, DZERO did publish a conference proceedings where they claim to not see any excess of muons with large impact parameter in a dataset similar to the one where CDF sees a large one. However, there are a few things that make me doubt that the DZERO analysis is a conclusive rebuttal of the CDF signal, primum inter pares the fact that DZERO has a tracking efficiency that dies out quickly for tracks not pointing toward the primary vertex. But that is another story, which I am sure you will only like to hear on another day.