Thursday, June 05, 2014

Stops at \(200\GeV\), a \(W^+W^-\) anomaly may be screaming

While supersymmetry remains the most well motivated candidate for new physics, it seems that all the observations at the LHC up to the \(8\TeV\) run in 2012 show that no new physics is needed and all "excessively bold and obvious" proposals for new physics – those that could explain the unbearable lightness of Higgs' being – have been excluded.

The Standard Model seems to be a more "thrifty" theory than any other bottom-up effective phenomenological model, and because there's no significant contradiction between the Standard Model and the LHC data, one is expected to pick the 40-year-old theory of nearly everything as his preferred effective theory of choice. Empirically speaking, no other theory is doing better.

Well, the first two hep-ph papers on the arXiv today argue that this common wisdom may very well be wrong, in a very exciting way!
Natural SUSY in Plain Sight by Curtin, Maede, Tien [Stony Brook]

'Stop' that ambulance! New physics at the LHC? by Kim, Rolbiecki, Sakurai, Tattersall [Madrid/London/Heidelberg]
The second title probably jokingly refers to the theme of hospitals for theories.

I still think that the seven authors of both papers must be aware of the second group because it would be rather unlikely for two groups to publish a paper with pretty much the same speculative claim on the same day, just minutes after one another. However, I would normally expect some comment of the type "While this paper was being completed, we learned about some damn competing bastards [83] who wanted to scoop us; we're better, faster, correcter, and prettier than them, but we're also generous to cite their future paper because such incomplete future citations aren't counted, anyway."

But I don't see any such comment in either of the two papers so it is in principle conceivable that the papers are independent!

Their detailed argumentation differs and in many technical points, the papers are complementary. However, the basic claims seem to be identical and I will group all these 3+4 authors into one gang.

To prove that they are not Seven Dwarfs, The Magnificent Seven starts with an empirical observation. Neither ATLAS nor CMS has detected a clear 3+ sigma deviation from the Standard Model. But if you combine their results, we do see one 3-sigma deviation in a rather important quantity. It is the \(W^+ W^-\) cross section, i.e. the probability that the proton-proton collisions result in the creation of a particle-antiparticle pair involving the \(W\)-boson.

The ATLAS detector measures this quantity to be about 20% or 2 sigma higher than the Standard Model, and so does the CMS. In combination, it's a tantalizing 3-sigma excess. This may be a fluke, an accident, but it is a rather important quantity and it may mean something.

What is it supposed to mean if this excess is real?

People are tempted to use the one-scale-fits-all method to describe what the LHC has proven and disproven. Because the LHC has placed lower bounds on the masses of many particles in many models that are comparable to \(1\TeV\), and of course that the detailed numbers depend on the model and the detailed particles that are being constrained, people like to sloppily think that "all new particles are probably heavier than \(1\TeV\)".

But as the loophole discussed by these two papers shows (and it is not the only loophole), such a conclusion is too sloppy. In fact, their model is compatible with all the LHC observations, actually produces better predictions than the Standard Model, and it has lots of new particles ligher than \(300\GeV\)!

They present several possibilities and all of them seem to work. The "most extremely light" scenario has both top squarks (stops) around \(200\GeV\), the whole triplet of winos a little bit lighter than that, one bottom squark (sbottom) around \(100\GeV\), and the LSP bino near \(100\GeV\). That's quite a lot of new particles that are almost as light as the Higgs boson – and this scenario is still compatible with all the LHC observations!

In fact, this setup is more accurate in the description of the current LHC data than the Standard Model – who can say that? In particular, the excess in the \(W^+ W^-\) cross section is explained by the additional \(W\)-bosons created from the decay of the stop squarks that are sometimes produced – either direct decays or indirect decays through the winos.

Click the picture if you need to prepare squarks at home. I recommend you to serve it with winos.

I have actually understated how many new particles may be hiding below \(250\GeV\). Most or all the sleptons could be sitting there as well – and still no contradiction with the LHC data would result. In particular, a light smuon could also explain the \((g-2)_\mu\) anomaly – the apparently slightly wrong measured value of the muon's magnetic moment. Several other anomalies and desired properties of dark matter may find a justification in models of this kind although I would say that none of these "additional advantages" is really robustly justifiable if separated from the rest.

Both groups perform various fits and statistical tests and decide that the model with the light stops gives more accurate predictions than the Standard Model – and the statistical significance of this advantage is equivalent to something like a 3-sigma evidence.

To me, the degree of agreement between these two superficially completely independent papers (although both papers are aware of their common pre-history and related papers written by authors from both groups in the past) seems kind of remarkable. I may even imagine that they may have heard about some new top squark rumors a few days ago which made them write these very similar papers.

If one of the scenarios from these papers would be right, naturalness would be completely saved. It's interesting that this possibility has been overlooked. What's "special" and potentially "contrived" about it are some small gaps between the sparticles' masses. There are many pairs or groups of particles that may be assumed to have small gaps and it's plausible that these seven authors are among the first ones who have sufficiently carefully looked into this particular possibility. Well, in this model, it's really the gap between the superpartners and the original Standard Model particles that is "too small to be seen clearly".

The possibility suggested by these papers should be rather safely confirmed or eliminated by the 2015 LHC run at \(13\) or \(14\TeV\). But it's somewhat plausible that some stronger evidence for or against this possibility could emerge from more careful analyses of the existing 2012 data, too.


  1. Nice :-)

    Maybe a new ATLAS/CMS contest should be opend to analyse the data with respect to this signal (and the other light particles) ?

    I suspect Lumo would find all of them, ha ha :-D

  2. LOL, I am sure they wouldn't offer this yummy toast to the public so that someone else could scoop their otherwise inevitable Nobel prize. ;-)

  3. Guys, experience sadomaso with BDSM girls, who are live now on -> .

  4. Haha yes, you are probably right, but neglicting any prizes and from an exlusively scientific point of view, the should open such a contest to find (or exclude) new physics as fast as possible ...

    Yes I have seen your nice questions, thanks for it :-)

    And many thanks for alerting Lisa Randall about PhysicsOverflow, this is cool :-D. Of course I agree that she is most probably too busy ...

    Urs Schreiber has asked some great questions too, and it is a pitty that the relevant people who were active on TP.SE have partly reclaimed their accounts but are not posting, such that he did not get answers so far.

    Maybe it just needs some more time for people to note that we are not a uncivil barbarian wilde horde, as some moderators on P.SE loudly and repeatedly claim ... ;-)

  5. Yes! I don't see why it could very well be so that these sparticles interact so weakly with the clearly known energy-matter menagerie that many more runs of the LHC at the level of energy already tried could cause the sparticles to be disCERNed. %-)

  6. Dear Peter, at some fundamental level, it's correct to say that the superpartners can't interact "more weakly" than the known particles. They interact exactly equally strongly - that's a part of what the "supersymmetry" means. So for example, each superpartner has the same electric charge as the original particle, so when it comes to the strength of the "interaction vertex", both particles interact exactly equally strongly with the electromagnetic field (photons). For other types of interactions, similar constraints also exist.

  7. Thanks Lumo! It looks like what you told me has been one of my unknown unkowns.:<
    Am on my way'back to the lavatory/bathroom for a thoroughly cleansing brain-wash of my actention selection serving system! :)

  8. That would be very impressive - who's up for the prize if/when SUSY is confirmed? - Like your squarky - but personally I prefer them with pivo - I suppose that would be a pair of squark with antisquark

  9. “Supersymmetry remains the most well motivated candidate for new physics!”
    OK but it is not super symmetric enough,
    we still need symmetry of ourselves , our mirror consciousness coming from
    symmetric worlds outside our own, just like what the rigveda suggests by the
    multi-headed Perushi God representing the raspberry multiverse..

    Just like my former reaction to

    Kashyap Vasavada: The veda says:

    “The answer in Vedas is that the source of consciousness is outside. We
    are merely reflecting it as multiple mirrors would reflect a single object!”

    Could you imagine that the reflecting mirrors are entangled
    (anti-material) copies of me, living in entangled universes far away? The
    creation of the multiverse, which makes the big bang a symmetrical ( Charge
    Parity) process?

  10. Rising the energy in the next run will not help too much if the particles are sub-TeV. Moreover, the idea of pushing on the luminosity is not great, as many experts say now...
    Anyway, in those papers they have more parameters than the SM, so no wonder that the fit can be better. You should know the overfitting phaenomenon since you participate to the Higgs challenge. More parameters can better accomodate experimental errors (especially systematic ones).
    All this is not against SUSY or similar theories. I would be very happy if all this will be actually true. It is also not a big news the idea that light (so to say) particles will remain undiscovered by LHC: this problem is discussed over and over in the collaborations.

  11. see perhaps also: Democratic Free Will in the instant entangled Multiverse:

  12. I guess the bounds from neutron and electron EDM experiments would disfavor new physics at 100's of GeV as suggested by these papers..... unless for some reason CP violating phases vanish or are highly suppressed for the sparticles.

  13. How many articles have you published on this stupid site that included the words, "hints" and "supersymmetry" followed by an exclamation point? Looks like another nonevent that you have gotten way too excited about.

  14. Lubos
    Congratulations you are back to 10th place with a 3.68376. Very good. I hope you get another break through to hit 3.7.

    Cheers from waterloo. Its 12 am.

  15. Thanks, Svik, your advise may have been a good one. Why didn't I think about it before?

    Now I have breached the 3.7 barrier and I am at the fourth place right now. ;-)

  16. There exist limits coming from LEP2, 183-208 GeV cm energy. I hope they have checked that their hypothesis does not go below those mass limits ( summary ) over 90 GeV. I searched for "LEP" and found no quote. in the papers.

  17. ATLAS and CMS should finally publish their 8 TeV data on the WW cross section, which they are holding back for quite some time now (most likely because they see something non-SM-like...)

  18. Your speculation sounds both reasonable and intriguing, Sven. ;-)