Thursday, July 10, 2014

CMS "sees" \(650\GeV\) leptoquarks

It seems pretty obvious to me by now that the LHC experiments were publishing lots of papers where things look "clear" and nothing new is observed and they were increasingly scrutinizing and postponing the searches and surveys that are potentially "seeing something new". Just to be sure, one may feel uncomfortable about this bias because the message extracted from the papers published before a moment \(t\) is skewed. On the other hand, this bias is understandable because extraordinary claims deserve extraordinary care (and require extraordinary evidence).

So one should arguably accept that it takes a longer time to complete papers with "positive claims" – and at the same moment, everyone should be aware of this bias and realize that the body of papers published up to an early enough moment looks more conservative than the actual evidence that the experimenters are probably seeing at that moment. You simply have to accept that the literature can't be mindlessly trusted as a faithful picture of "what science actually knows right now", and this bias is just one minor aspect of this disclaimer.

The avalanche of anomalies in new papers published since the early July 2014 seems too strong to me to consider this clustering a set of coincidences. Note that ATLAS has published a paper on the anomalously high \(W^+W^-\) cross section and CMS saw that an estimated 1% of Higgs decays go to \(\mu^\pm \tau^\mp\), a flavor-violating combination that should be nearly absent and where the excess is 2.5 sigma.

Today, the CMS released another paper with an anomaly, a paper on their search for leptoquarks:
Search for Pair-production of First Generation Scalar Leptoquarks in \(pp\) Collisions at \(\sqrt{s} = 8\TeV\)
Note that leptoquarks are (or would-be) speculative new particles that combine the charges of leptons as well as quarks. So a leptoquark carries (or, quite possibly, doesn't carry because it doesn't exist) a nonzero lepton number (like a lepton) and a nonzero baryon number (like a quark) and transforms as a color triplet (as a quark).

The particular leptoquarks searched for in this paper were spin-0 scalar particles.

And this is the money graph.

As the picture at the top shows, the leptoquark (a single particle) is supposed to decay to a combination of lepton+quark. The decay necessarily exists because we don't observe any light and therefore stable leptoquarks; leptons and quarks have to be among the final products because of the lepton and baryon number conservation laws. The lepton may be either charged or neutral (neutrino). The percentage of the decays involving a (visible) charged lepton is known as \(\beta\) (which stands for "branching ratio").

On the \(x\)-axis, you see the assumed mass of the leptoquark, on the \(y\)-axis, you see the maximum possible \(\beta\). For a \(650\GeV\) leptoquark, one predicts \(20\pm 2\pm 2\) events (and the ability to prove something like \(\beta\lt 0.075\)) but they observe \(36\) events (and can only say that \(\beta\lt 0.2\)). Quite a disagreement but the significance is actually just 2.4 sigma.

Also, the shape is diluted – the excess exists for a wide range of leptoquark masses above \(300\GeV\), while a "real" leptoquark would be expected to produce a narrower excess as a function of the mass. In fact, you could be bothered even by the other 2-sigma excess for masses around \(850\GeV\).

Leptoquarks are in principle possible additions to the particle zoo, even according to some of the string vacua, but many people's (and my) knee-jerk reaction would be to say that they're bizarre and unmotivated. And they could cause lots of trouble. Of course that if this fluke turned out to be a signal, people would jump on this bizarre possibility rather quickly.

I would bet on a fluke but I am not really sure.


  1. "The avalanche of anomalies in new papers published since the early July
    2014 seems too strong to me to consider this clustering a set of

    Of course it is not a coincidence, you have to think of the logistics of experimental groups, graduate students , thesis subjects. I would bet that a large percentage of those 3000+ names on the papers of the two large LHC experiments are graduate students who have been working for years on their thesis. How many students do you think could get a thesis by studying Higgs production?

    Afaik a thesis subject still has to have an original contribution. student advisers have their pet analysis honed on monte carlo events maybe for ten years, and do not be surprised to see limits from even more far out phenomenological models. There have been for years working groups on leptoquarks trying to bolster the claims for ILC. .

  2. Would not Cumrun Vafa at least be happy about certain (chiral) leptoquarks ... :-)?

    Sorry for not being able to link to the Google search resalts I have found using certain search terms from my smart phone...

    When I have my laptop again tomorrow evening, I will maybe ask about it on a certain physics Q&A site.

  3. Less than 4 sigma is just desperation from theorists.
    DAMA is 6-9 sigma BTW. But nobody cares as it does not fit
    the intellectual orthodoxy...

  4. A rational theorist never goes to any "desperation"; instead, he is refining a (most) likely picture how the Universe works according to the available data. Whatever the data are, he sees a solution.

    The fact that the expected evidence *does* depend on the agreement with the "official orthodoxy" - you mean previous scientific evidence and principles extracted from it - is known as rationality. Different magnitudes of excesses are needed in different contexts because the prior probabilities are different, and so are the probabilities of a completely different (more mundane) explanation.

    If you think that the expected confidence level should be independent of the character of the new claim, you are only proving that you are a hopeless crank who has no clue what science is and how it works when it works.

  5. You may be right, Dilaton, he could be happy. Well, I know some other F-theorists whose happiness in that case seems more guaranteed. ;-)

  6. This kind of trollong comments I have seen at other dark places in the internet, do not belong here.

    Go home !

  7. As designer I did research on sub-quantum form possibilities for quarks and leptons based on well known decay products. As surprising automatic result the Muon and Tau become members of the Down respectivily Charm quarks. See figure : Geometrical proposal for the sub-quantum structure of 6 Leptons and Quarks interpreted as compound Photon/Electron Positron propeller combinations.
    Muon- and Tau particles seem to show up as family members of the Down respectivily Charm Quarks.

  8. How could the existence of leptoquarks 'cause lots of trouble'?

  9. Looks like he has in mind some sort of new Israel in Ukraine.