CMS sees SUSY-like trilepton excesses

Update: See also trijet and nonajet events at CMS...

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Update: See also: Multileptons are only "mostly consistent" with the SM: CMS

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Update: See also Searches for R-parity-violating multileptons at CMS...

While we're patiently waiting for the official announcement of the discovery of the Higgs boson(s) by the LHC, and your humble correspondent is determined not to leak the information about the boson(s) and the mass(es) in advance, we shouldn't forget that the LHC may discover something completely different and much more original at every moment.



CMS detector inside the LHC at CERN

Matt Strassler has just pointed out that such a thing could be occurring right now:

Something Curious at the Large Hadron Collider

According to a very fresh Fedor Ratnikov's talk at the LHC SUSY conference at Berkeley, their trilepton group within the CMS collaboration may have seen more-than-three-sigma excesses in several channels involving three leptons. Ratnikov works in Karlsruhe, Germany and their trilepton CMS colleagues also include experimenters from my graduate Alma Mater, Rutgers University.




The final states of the LHC collisions are divided according to the number of leptons in the final state (the number of collisions with more than three leptons is too low so far, so we talk about 0, 1, 2, 3 leptons and are interested in 3-lepton states here); according to the missing transverse energy's being greater or smaller than 50 GeV; according to some other effective transverse energy index's being below or above 200 GeV; according to the number of tau-leptons among the leptons; according to the presence or absence of oppositely charged leptons of the same flavor; and according to a similar condition, namely a possible/likely appearance of a decaying Z-boson.

So there are dozens of channels or bins and many of them see an excess in the CMS data, especially some of the channels without Z-bosons and/or without oppositely charged leptons (in the latter case, especially if the general transverse energy index is demanded to be high). It may sometimes happen that you see a 3-sigma or stronger excess by chance but seeing 5 of them in the 25 candidate slots is surely unusual. The chance that this occurs by chance is something like 1 in 50 million; of course, we're assuming that there are no "big errors" in their analysis or my calculation. ;-)

What the interpretation could be? Why is it interesting?

Well, it's interesting because the trilepton events are some of the most promising supersymmetry signatures. In fact, if you look at this paper by John Strologas (2009, Fermilab) or Howard Baer (1994, LHC) or one of many other papers on trilepton signatures of supersymmetry, you will learn that the trileptons have been considered to be the "golden channel" for the discovery of supersymmetry at the recently retired Tevatron collider. Such trilepton events may be created from a relatively frequent chargino-neutralino production followed by the decay of these two fermionic superpartners of the well-known gauge bosons or the Higgs bosons:

\( pp \to \tilde W_1 \tilde Z_2 \to 3 {\mathcal l} +\, /\!\!\!\!\!\! E_T \)

Of course, the information about the potential new physics is too fuzzy, uncertain, and ambiguous at this point and we will have to wait whether newer data, more careful analyses, and the ATLAS competitors confirm the excess and/or find a more direct manifestation of the new particles. But if they do, it could be damn interesting! So far, the conservative speaker said that "everything is essentially consistent with the background", just like you would expect in a hard science.

Even if the signals were real and got stronger, supersymmetry is not necessarily the only way how to explain such trilepton events; one could deduce similar signatures from some extra-dimensional or little Higgs models, among others. Also, it's conceivable that detailed observations of the trilepton excesses will eliminate many if not all possible explanations (maybe even all known SUSY explanations). More generally, if you find the trilepton events too abstract rather than object-like, I share your sentiment. While supersymmetry offers the LHC to ultimately find whole fireworks of new physics, it has one property that makes the new physics easy to hide: in the most natural versions, the new particles have to be produced in pairs – because of the R-parity conservation – which means that one doesn't find sharp resonances. That's why we have to look at relatively complicated and obscure final states such as those with 3 leptons and missing transverse energy (coming from the lightest neutralino or something like that).