Tuesday, December 25, 2012

Christmas rumor: \(105\GeV\) dimuon excess at 5 sigma

Update: The best physicist on the territory of Argentina right now, Paul Frampton, wrote me that the signal could perhaps be a sign of a dilepton from the 331 models which enhance the electroweak \(SU(2)_L\) to an \(SU(3)_L\). The lower limit on the mass of such states also seems to be \(1\TeV\) but weaker coupling constants could perhaps work. Check e.g. this 2000 paper for a quick review of the particle content of the 331 models or, even more relevantly, this 1992 paper discussing dileptons in 331 models and highlighting Paul Frampton's own pioneering contributions (thanks, Joseph S.!).

Also, the 331 gauge group may be embedded into \(E_6\) GUT which offers additional possible explanations of like-sign dileptons, for example a leptophobic \(Z'\) boson.
I hope that the TRF readers are enjoying their Christmas, their Saturnalia, their Hanukkah, or at least their first days after the winter solstice (except for TRF readers in Islamic countries where all such pagan holidays are banned: those readers are wished to survive instead).

We have gotten used to the news from the LHC that meticulously and precisely confirms every small feature of every graph of every final state that may possibly occur when a proton pair collides. Because of this "habit" of ours, the following rumor may sound shocking, stunning, unbelievable.

Well, it's a rumor – and one posted at a highly unreliable place – so it should remain unbelievable for some time and at least to some extent. But it's interesting enough so that I can't miss it because if the rumor is true, it's an amazing Christmas gift from the LHC.

Phil Gibbs claims that he has been browsing through some really stinky garbage at a notorious crackpots' discussion forum led by an immoral sourball when...

...when he saw a rumor by the user named "crossing symmetry" who wants to remain anonymous, who claims to have an ATLAS friend, and who says that he was told that the ATLAS detector has seen a very strong, 5.02-sigma signal (probably a local significance) in the graph of the number of like-sign dimuon events localized near the invariant mass\[


\] It is supposed to be based on 14 events.

Note that the total electric charge of these pairs of muons or antimuons is equal to \(Q=+2\) or \(Q=-2\) which makes the suggested intermediate particle rather unusual. Moreover, it shouldn't be an "ordinary" doubly charged Higgs boson that would belong into an electroweak triplet (well, I wouldn't call any doubly charged Higgs boson ordinary so the quotes were needed) because the lower limit on those beasts' mass has been already set to \(300\)-\(400\GeV\).

The combination of the strange charge with the strange apparent lepton number of the new particle, also \(L=\pm 2\), strengthens my belief that this rumor is a Christmas chimera but I would be lying if I told you that I am not trying to search for models that would incorporate such an animal. ;-)

The rumor passes the basic test. Those 14 events are supposed to arise from \(13/{\rm fb}\) of the \(8\TeV\) 2012 data. One may look at \(4.7/{\rm fb}\) of the \(7\TeV\) 2011 data which was evaluated in a recent October 2012 preprint and one sees some highly suggestive excesses near \(70\GeV\) and \(100\GeV\) on Figure 1b copied above. It seems totally plausible to me that the excesses above have grown to the 5-sigma excess grown in the rumor. Note that these excesses would be independent so one could vaguely combine the 2-sigma excess on the picture above (2011) with the 5-sigma excess from the rumor (2012) to get a 6-sigma excess (assuming the masses agree well enough) which would arguably be enough for 5 sigma even after the look-elsewhere taxation.

In fact, you may see significant excesses in a similar region reported already on Figure 1 of this January 2012 paper based on \(1.6/{\rm fb}\) of the 2011 data only, too.

OK, now, let me mention that the like-sign dimuon events have been searched for as evidence for supersymmetric models. See, for example, the 2004 Fermilab thesis by Yurkewicz. It was not written by Katie Yurkewicz if you happen to know her but it was dedicated to her by an Adam with the same last name. ;-)

In this thesis, you may learn about something that Phil Gibbs has already mentioned. Like-sign dimuon events are usually looked at in situations when this lepton pair may be a subset of a three (or more) final leptons – multileptons – and the condition that two leptons have the same charge is a convenient constraint that eliminates the "way too ordinary" events that boil down to opposite-sign leptons.

However, I must tell you that in the most conventional SUSY models, one doesn't expect a resonance – a sharply determined value of "the mass" – because the new particles are being created in pairs and the invariant mass of the pair is inevitably continuous (but bounded by an inequality to lie above a threshold).

If you want to see a cute nostalgic paper on this issue, look at this 1990 paper on the production of gluinos at the Superconducting Supercollider (murdered 3-4 years later). We learn from the abstract that "The like-sign dimuon signature displays the Majorana property of gluinos." You surely want more than just an abstract. Here you have a 2002 thesis by Chadd Smith. You may learn about a similar dilepton search in which gluino pairs are produced. The (like-sign) correlation between the charges of the resulting muons arising from the two gluinos boils down to the Majorana spinor properties of the gluinos (if they're Majorana).

You may also look for the 2010 diploma thesis by Jason Mansour who tried to use the same signature to search for the associated production of charginos and neutralinos at the D0 experiment.

It would be too brutal to think that \(105\GeV\) could be the combined mass of the gluino pair. But for some other gauginos, this super-low mass could be at least conceivable. The resonant character of the excess could be due to some fast enough decrease of the signal above the threshold but let me admit that yes, I am just offering you some wishful thinking not supported by solid calculations at this point. Phenomenologists should be able to clarify these issues quickly.


  1. I saw this at Phil's site too, and regarding the source of the rumor, I thought some sourballs are again making fun of people interested in BSM physics over there by confusing Christmas with April 1st etc ... :-/

    But or course it would be nice to get such a Christmas present.

    I am curious about the models Lumo will bring up which could allow for such a signal !

    Merry Christmas to you Lumo and everybody else :-)

  2. Merry Christmas to you too and to all TRF folks!
    Off-topic. Have any of you read this paper, and if so, what's your opinion on it?
    "The presheaf of Quantum Realities and constructions of the space-time from the Space of Ultimation" http://arxiv.org/abs/1212.5585

  3. Franca Jones ClericiDec 25, 2012, 5:03:00 PM

    "Nazi roots" ? Is your deadly disease gone now Lubos?

  4. Lubos, how come the Tevatron didn't see anything like this?

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  6. Hi, well, whatever it is, the LHC has already accumulated many more expected events - and stronger signal - than the Tevatron. It's the signal from 13/fb at 8 TeV. Tevatron had less then 10/fb at 2 TeV and because the muons have the same signs, they could actually be more frequent in proton-proton collisions than proton-antiproton collisions.

    With the usual expected cross sections, the Tevatron has eliminated particles whose mass would be 105/2 GeV, but at some lower cross sections, more unusual models etc., many things are possible. At any rate, the LHC has already entered the realm of hegemony so it may discover something new at any moment and the previous colliders can't prohibit it because they're equivalent to the LHC in the past.

  7. Merry Xmas Lubos

    I am curious if it is really 14 events. How could one get 5 sigma out of 14 events?

  8. Hi Anna, well, if the background were small, you can get 5 sigma out of 1 event, too. If it can't happen without new physics and it happens, you are sure that there's new physics. This situation isn't too different because the number of SM background events is predicted to be less than one.

  9. When we talk of events and sigmas, we talk of statistics. True, even one event is significant if none is expected, but not in a statistical manner. The plots you show have a lot of backgrounds other than the standard model. I suspect that the person who leaked this does not really know what he/she is talking about.

  10. No, you're wrong, Anna, there is nothing wrong about 14 events' producing 5 sigma at these backgrounds.

  11. It could have grown to 5 sigma, but not with 14 events over background. Just from statistics one needs 25 events over background to call it five sigma, at

  12. Anna,

    If the background is zero, then I think a single event counts as infinite-sigma.

    The chisquare statistic is the difference between the data and the model divided by the square root predicted by the model, not the square root of the data.

    For 14 events to be 5 sigma, that means that the background is expected to be 7.84 events, and that ~22 events were in fact detected. If you expect 8 events, and you find 22, then you have a 5-sigma excess equivalent to 14 eevents.

  13. “We got so caught up in the little things of Christmas, like love and
    family that we almost forgot it's buying things that makes our economy
    thrive.” - South Park's Ms. Choksondik

    Original South Park short from back in 1992:


  14. Well, in my books as an experimentalist , the sigmas are counted from the square root of the measured events. I believe the reason is that "models may change, data is data". I think the argument of 1 event can have high significance, holds in the case where it is strict physics laws, like conservation of energy, or some other basic model assumptions, like phase space, as happened with the discovery of the W+/-. There is plenty of Monte Carlo phase space under 105 GeV ( detector effects and all) .

  15. Dear Anna, David is right. To count the sigmas properly, you need to calculate the p-value of the signal for each number of events. Five sigma is at whatever number of events you need where the p-value is 1:3,000,000 or whatever is the exact number. There's no room for personal experimenters' "books". One must either calculate the things from the right values, or he must say that it is prohibited to talk about sigmas when the dependence on the number of events is so nonlinear, and it surely is when the background is very weak.

  16. Dividing by the expectation of the models is the way to get Brazil plots, not discoveries. I will be really surprised if the experiment comes out to declare a five sigma observation with 14 events over background. Lets wait and see if this materializes.

  17. Abit out of topic, have you seen this http://cms.web.cern.ch/news/cms-observes-unexplained-particle-states ? two new low mass states confirmed, not in the SM, according to them.

  18. Cool what and that Prof. Frampton has written to you Lumo :-)