Thursday, April 16, 2015

LHC: chance to find SUSY quickly

This linker-not-thinker blog post will largely show materials of ATLAS. To be balanced, let me begin with a recommendation for an UCSB article Once More Unto the Breach about the CMS' excitement before the 13 TeV run. Note that the CMS (former?) boss Incandela is from UCSB. They consider the top squark to be their main target.

ATLAS is more into gluinos and sbottoms, it may seem. On March 25th, ATLAS released interesting graphs
Expected sensitivity studies for gluino and squark searches using the early LHC 13 TeV Run-2 dataset with the ATLAS experiment (see also PDF paper)
There are various graphs but let's repost six graphs using the same template.

These six graphs show the expected confidence level \(p_0\) (the probability of a false positive; see the left vertical axis) or \(X\)-sigma (see the dashed red lines with explanations on the right vertical line) that a new superpartner will have been discovered after 1, 2, 5, and 10 inverse femtobarns of collisions.

First, the bottom squark production. The sbottom decays to the neutralino and the bottom quark. If the uncertainty of the Standard Model backgrounds is at 40 percent, the graph looks like this:

You see that if the sbottom is at least 700 GeV heavy, even 10/fb will only get you to 3 sigma. Things improve if the uncertainty in the Standard Model backgrounds is only 20%. Then you get to 4.5 sigma

Now, the production of gluino pairs. Each gluino decays to a neutralino and two quarks. With the background uncertainty 40%, we get this:

With the background uncertainty 20%, things improve:

You see that even a 1350 GeV gluino may be discovered at 5 sigma after 10 inverse femtobarns. I do think that I should win a bet against Adam Falkowski after 10/fb of new data because only 20/fb of the old data has been used in the searches and the "total deadline" of the bet is 30/fb.

Things look similar if there is an extra W-boson among the decay products of each gluino. With the 50% uncertainty of the Standard Model backgrounds, the chances are captured by this graph:

If the uncertainty of the Standard Model backgrounds is reduced to 25%, the discovery could be faster:

If you're happy with 3-sigma hints, they may appear after 10/fb even if the gluino is slightly above 1500 GeV.

The probability is small but nonzero that the gluino or especially the sbottom may be discovered even with 5/fb (if not 2/fb and perhaps 1/fb) of the data.

After 300/fb of collisions, one may see a wider and safer region of the parameter space, see e.g. this CMS study.


  1. Jury-rigged theory breaks bad when it says something measurable. Physics Today 684 46 (2015), Figure 6. If the Higgs really masses 125 GeV/c^2, then the universe should not exist versus jiggles - by a rather convincing margin. As supernovae, black holes, etc. are pretty good jiggles, theory is defective. Add a few more parameters to fit the curve. OK, more than MSSM.

  2. alejandro riveroApr 16, 2015, 5:35:00 PM

    Is it possible for two sbottoms plus a gluino to make a bound state and just mimic a up-type antiquark, avoiding detection?

  3. Dear Alejandro, why such a complicated bound state? You used 3 superpartners...

    Superpartners can make bound states with the original particles, too.

    Your particular bound state has 3 (odd number of) superpartners, so it is R-parity-odd, which is why it is very different from an antiquark, although it is also a fermion (as the gluino).

    All these bound states with very heavy particles are pretty much useless because they're so unstable - because the superpartners in them are highly unstable, much like the top quark. This instability also makes the bound states hard to produce.

    To make it stable, you would need to use the stable LSP, the lightest superpartner. But if it is dark, to be a dark matter particle, then it doesn't interact non-gravitationally (except for extremely weak interactions) which is why it basically doesn't form bound states.

    So I would summarize that there are probably no interesting bound states of superpartners that would affect some real-world phenomena.

  4. If SUSY really is nature's way then it must be compatible with MOND because there is too much empirical evidence in favor of MOND — SUSY researchers should take note of MOND.

  5. alejandro riveroApr 16, 2015, 5:52:00 PM

    Well, in this context I was thinking if there was some way for the particles to escape detection by being wrongly tagged as quarks.

    But of course you could remember that ten years ago I was interested in this state because the number of pairs of scalar-(u,d,s,c,b) reproduces the number of quark species... and also the number of lepton species, if you use the anti-squarks. It is so nice match that I would like to find some usage for it.

  6. Dear Lubos,

    If the metric g must be endowed with thermodynamic properties as suggested by black hole entropy, shouldn't actions like

    S = ∫ g(df, df)

    for the free massless scalar also be endowed with thermodynamic properties? Does this have something to do with the fact that Euclidean action essentially is an entropy? Is the Lagrangian/Action an incomplete description?

  7. It sounds like a great system of ideas. So far I don't know how to complete it and answer it but it will make me think. ;-)

  8. Ha ha - coincidentally I'd been reading about the Euclidean Action approach to BH entropy recently, and casting my eye over a couple of short papers:

    though they're somewhat difficult for me to follow to be honest.

    It seems that the definitional subtleties that turn up with "naive" ways of counting microstates in relativisitc QFTs, (different relative observed vacuum excitation due to Unruh effect, infinite Von Neumann Entropy of vacuum etc), are pointing to some new insights as to the right way to formulate the full thermodynamics of Quantum Gravity.

    As far as I can tell from my layman's perspective, this looks like a good approach and an interesting research direction!

    I've seen Bill Unruh make semi-serious provocative joke remarks in some talks on BH information that maybe counting microstates is not going to work, and Entropy/Heat would just have to be reintroduced into physics as a fundamental properties like charge and mass, as in the old pre-Boltzmann days! :D :D

    (I'm pretty sure he never really believed this of course).

    One of the encouraging things I like about this approach, where entropy is related to Wick-rotated path-integrals, is that we seem to arrive at a low-level definition of entropy that can handle fully fledged field degrees of freedom, that is still in a sense giving a measure or a counting of "possible configurations" or "contributing histories", so that the statistical way of thinking about thermodynamics is preserved and deepened...

    As long as we don't have to go back to a caloric theory of intrinsic heat, I don't mind ;-)

  9. I was about to ask Dejan: you are calculating spectrum of scalar field around collapsing star. Why does this time varying metric produce Planckian spectrum? It is fully thermal when horizon forms but looks Planckian already at the beginning of the collapse. Does gravity like temperature and/or Planckian spectrum for some reason?

  10. Really ? I must try thhis one . Thank you for the information:)

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