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.!).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).
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.
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\[
m(\mu^\pm\mu^\pm)=105\GeV.
\] 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.
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