Thursday, October 20, 2011

CMS sees SUSY-like trilepton excesses

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

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

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).


  1. Hello Lubos,

    Would it be accurate to say they have identified "a gap" so as to identify the missing energy from the total energy used to seek the decay products and the loss of energy accountable

    Missing Energy Kicks New Physics Models Off The Board

    The signature of large missing energy and jets is arguably one of the most important avenues for the study of potential new physics signatures at today's hadron colliders.

    The above concept marks an interesting turn of events: the years of the glorification of charged leptons as the single most important tools for the discovery of rare production processes appears behind us. The W and Z discovery in 1983 by UA1 at CERN, or the top quark discovery by CDF and DZERO in 1995 at Fermilab, would have been impossible without the precise and clean detection of electrons and muons. However, with time we have understood that missing energy may be a more powerful tool for new discoveries.


  2. Dear Plato, I don't understand your "gap" question. Missing energy is simply the part of energy carried away in invisible stable (or almost totally stable) particles such as the LSP, the lightest superpartner (probably a neutralino). Such an invisible escaped particle, much like in the case of neutrinos, simply displays itself as an apparent violation of the energy (and momentum) conservation law.

    The quote by Dorigo you copied shows that Dorigo is not exactly a seer (or a guy witth a good intuition) and is pretty much obsolete today. it was indeed fashionable to rely on MET but as the first year of the serious LHC run showed, they overrelied on it and good old leptons and multileptons could be a better path to discovery.

  3. Hi Lubos,

    Yes I appreciate perspectives about the truth of what is happening. The truth, of what you and Matt are writing.

    So yes, I see Dorigo at the time may be writing about science and what one is seeing as it unfolds, so their may be corrections after time, as to what Matt is talking about the missing energy.


  4. Lubos,

    "As energy in the accelerator increases to its peak of 3.5 TeV and the beams get closer to light speed, however, relativity makes the 15 seconds stretch out little by little. At some point the difference between the beams becomes small enough that it can be overcome with a minor adjustment in their orbits – essentially the proton ends up going just a bit farther than the lead does, so they both take the same time to make one revolution.Accelerator soup: Scientists to mix elements in LHC to study recipe for heavy-ion collisions

    You may be interested in article above?