Monday, September 30, 2013

CMS: \(2.5\)-\(\sigma\) excess in top+Higgs production with dileptons

If real, it's one of the most universal signs of the gluino

The CMS experiment just released a new paper which I mention because it explicitly claims an excess, unlike the overwhelming majority of papers that agree with the Standard Model almost perfectly:
Search for the standard model Higgs boson produced in association with top quarks in multilepton final states (CMS PAS HIG-13-020)
The heaviest known elementary particle is the top quark, \(t\), along with its antiparticle. Their mass is \((173\pm 1)\GeV\) or so. The second heaviest known elementary particle is the Higgs boson, \(h\), whose mass \((125.7\pm 0.4)\GeV\) is the only completely new number that the LHC has taught us so far.

In the search discussed in the paper above, they looked for events in which these three fatties (\(t\bar t h\)) were produced simultaneously and they subsequently decayed into final states with many leptons. And the Standard Model wasn't too perfect.

The most relevant figure is Figure 4 and I am sure that you are already curious what this image shows.

Here is what it looks like:

Click to zoom in.

You may see that the same-sign dilepton final states boast something like a 2.5-sigma excess over the Standard Model prediction. This excess gets inherited by the combination of the channels, too. Moreover, Figure 2 reveals that the excess resides squarely in the \(\mu^\pm\mu^\pm\) channel while the \(\mu^\pm e^\pm\) and \(e^\pm e^\pm\) channels agree with the Standard Model.

I have no immediate interpretation except that the CMS was designed and named to make discoveries using muons. ;-) But make no mistake about it: of course that exactly this final state – top-Higgs associated production with like-sign leptons – could be e.g. a sign of the gluino, the supersymmetric partner of the gluon! The gluino has been one of the most urgently expected superpartners at the LHC for almost two years.

The excess may also be related to the Christmas same-sign dimuon rumor.

If SUSY is too hard for you, let me just tell you that the gluino is a blonde vampire alchemist.


  1. What is the actual number of events found? If these are the expected number of signals from the data set,

    all signals 2.7+/-0.4 1.2 +/-0.2 3.7+/-0.6 4.4+/-0.8 0.52+/- 0.09

  2. So, we can expect some sort of modification to the Standard Model to account for this? Isn't the gluino already in it?

  3. There is no gluino in the Standard Model. Gluino is a particle in supersymmetric models, a partner of the gluon.

  4. How long till it may become a 5-sigma signal?

  5. Well, you normally need to quadruple the number of collisions to double the number of sigmas. So one needs 80/fb for 5 sigma - later in 2015. However, it's likely that if this bump is real, there may be faster way to find the source of the new physics, too.

  6. Doesn't the theory of supersymmetry predict the masses of the superpartners? I.e. we know the masses of electron, of gluon, of quark. Why do the superpartners not have the same mass as the original particles? How do they gain mass? Is it also through Higgs mechanism (through higgs or higgsino field?)

  7. Mephisto, this is a really elementary question for someone who has commented on dozens of texts on SUSY.

    Unbroken SUSY predicts that the superpartners are as heavy as the known particles/partners. In Nature, SUSY is spontaneously broken and broken SUSY allows the masses to differ. SUSY-breaking mechanisms produce soft terms as an approximate effect at long distances. They include masses for the bosons.

    When unbroken SUSY is combined with the Higgs mechanism, then both partners' masses are produced from interactions with the Higgs - not surprising because the masses are and have to be the same.

    The bosons' extra masses that violate the supersymmetric equality of the superpartner masses are generated (mostly) by non-Higgs physics, however.

  8. Thanks for the enlightening answer, but you must be mistaken. I hardly commented on dozens of texts on SUSY. (quantum mechanics yes, but not SUSY)
    So if I understand it correctly: SUSY breaking causes the superpartners to weight more than the regular particles which means more energy at the collider is needed to produce these particles (according to E=mc2). If only one superparticle is found (say gluino) and we know the mass of the gluon and gluino (difference of their masses), will we be able to predict the masses of all other superpartners? (as some function of the difference in masses).
    And why don't the superpartners exist in nature?

  9. Dear Mephisto, there's no general way to find out the masses of other superpartners given the known mass of one of them - but various particular SUSY breaking scenarios "almost" achieve that and correlate the superpartner masses in analogous ways. However, there's always more than 1 independent new parameter for the superpartner masses.

    Most superpartners don't exist in Nature for the same reason why top quarks and Higgs bosons don't - these particles are unstable and decay in a microscopic time.

    However, one SUSY particle is most likely found in Nature and it actually represents most of the matter (excluding dark energy) in the Universe. It's called dark matter. ;-) If SUSY is right, it's extremely likely that the lightest superpartner (LSP, typically a neutralino or gravitino) is the dominant component of the dark matter.

  10. Thank you. Dilletantes like me are interested in yhis stuff, but we often lack basic info and misunderstand.

  11. Yep, TRF is the best place to learn about actual cutting edge nice fundamental physics for dilletantes like me too ... :-)

  12. I was always planning to read this post but repeatedly forgot.

    This is indeed very interesting, but is there any news on the last post in which you said that third generation superpartners may have been found at the CMS (?) .