Thursday, July 12, 2012

Have we observed the Higgs imposter?

Dozens of outlets including The Daily Mail were attracted by an interesting paper with a catchy title,
Have We Observed the Higgs (Imposter)?
by Ian Low, Joseph Lykken, and Gabe Shaughnessy. Having had the office next to Ian for quite some time, I know him rather well. ;-)

A decade ago, Ian Low Imposter (right) was a component in a triplet (with Matt Headrick and their secretary) that drinks Pilsner Urquell of my hometown (yes, I had bought it) and a Pilsner Urquell Imposter called Czechvar (because crazy U.S. judges don't allow the intermediate, softcore imposter beer to be sold in the U.S. under the original name Budvar in Czech or Budweiser in German that has been locally hijacked by a hardcore Budweiser Imposter called Anheuser-Busch).

In fact, the title and main message was so attractive that days after I had seen the preprint, I was alerted to this paper even by Fred Singer, one of the world's most famous climate skeptics.

I don't plan to discuss the technical content of this paper in too much detail but it's rather original – but much less "contrarian" than the journalists and probably even Fred Singer think.

They eliminate some possibilities what the new particle could be but they still defend the viability of a very unusual culprit. The normal Higgs boson in the Standard Model and its "straightforward enough" extensions is a physical polarization that is left from a doublet \({\bf 2}\) of the electroweak \(SU(2)\) symmetry. The remaining 3 real components of this complex doublet are eaten by the W-boson fields and the Z-boson field in their quest to become massive.

Ian Low Imposter is dancing. The bottom part of the imposter on the right belongs to Marcos Mariño; the imposter behind Ian Low is Andrew Strominger; also seen are Michael Gutperle and Ruth Britto. To protect the privacy of Ian Low and his imposters, I will stop posting extra photos. You could also learn that even Ian Low was high which would harm the logical consistency of this blog. :-)

However, using constructions involving the custodial \(SU(2)\) symmetry and similar decadent pseudo-symmetries that phenomenologists such as these three like, they argue that the new particle could very well be a part of the Higgs triplet (which is also a quintuplet of the custodial symmetry) rather than the usual Higgs doublet. That's extremely unusual for most trained particle physicists. On the other hand, it's still a "Higgs triplet", i.e. a Higgs boson coming from a Higgs field that breaks the symmetry via the Higgs mechanism.

So from the most general qualitative perspective, it's still the same thing and Higgs and Englert and perhaps others would deserve a Nobel prize for this particle even if it were a part of the triplet. (To make their construction look more radical than it is, Low and others try to call the imposter differently even though it is a Higgs triplet and it still has the same name "Higgs" in it.) The construction would be qualitatively different from the usual Standard Model but as far as I can say, it's rather plausible. It's one of the possible avenues how the Higgs sector could be extended relatively to the Standard Model. All of these avenues predict that new Higgs-like particles should be discovered. We're not guaranteed that they will. I still think that the two doublets, i.e. five faces of the God particle, is the most motivated conceivable extension of the Higgs sector. That's because supersymmetry isn't an ad hoc extension. It's a natural symmetry, one justified by string theory, coupling unification, dark matter, and the hierarchy problem, among other things.

I am not sure whether the triplet is really compatible or incompatible with string theory. It's surely unusual in the phenomenological literature if it exists at all. But it may be absent only because people were trained to think about electroweak doublets only; absence due to group think and unjustified habits. I am not able to prove a no-go theorem for Higgs triplets like that. But maybe someone can...

By the way, by a complete coincidence, "Cabibbo What" at a physics blog asked its readers today what is the strange triplet "L-N-L" on the picture below:

Click to zoom in.

No reader over there knew so let me tell you: it was a lepton triplet – charged lepton, neutrino, oppositely charged lepton. These days, we no longer arrange leptons or Higgses into electroweak triplets as we prefer doublets for both. Homework exercise for you: Would there be anything wrong with a model with a single neutrino and two lepton species that is organized as a triplet like on the picture from the 1970s rather than a doublet?


  1. how long should it take to know the properties of the particle and if it can be considered as a "Higgs". i won't be surprised if they find 20 of them. first they said it is one Higgs, then 3, 4, 5 that are a bit different then they will say it is more than those 5 and they are even more different. what i don't understand is which criteria the particle has to pass to be considered a Higgs? if its spin is zero, it is automatically a Higgs?

    the way i see it they crammed many things they did not know about in one particle. the more experiments they do with higher energies, they will discover the actual mechanisms behind the different things they could not explain.

  2. Hi Lubos,

    I'm confused by your statement about whether or not the triplet is really compatible or incompatible with string theory. Can't it just be a symmetric tensor representation of SU(2)_L? This is very easy to accomodate . . .

    Also, I must be missing something obvious - triplet Higgs can easily give the W and Z masses, but what about quarks and leptons!?

    Thanks for the nice posts of late.


  3. Ha ha the pictures are fun, I like them :-D

    In contrast, I`m not yet sure if I like the new phenomenological model ...?

  4. Hi physicist, thanks for your feedback. I may be as wrong as you ;-) but I also think that a non-doublet of the gauge symmetry SU(2) Higgs "h" cannot be contracted with the quark/lepton doublets in a gauge-invariant way so one can't add the Yukawa-like couplings connecting left-handed doublets with right-handed singlets, so one can't get fermion masses.

    Yup, they otherwise propose a simple enough but different representation for the Higgs field. Still, representations one may get in various string scenarios are restricted, aren't they? In heterotic things, one gets reps that must arise from an E8 at the fundamental level, in braneworlds one has to start with bifundamentals of U(N)'s and their products, and there are fancier but very stringent restrictions in F-theory, too.

  5. a neutrino and two leptons can't have spin 1 so they can't be a triplet

  6. A triplet Higgs also gives the wrong value for the rho parameter (i.e. the wrong relationship between the masses of the W and Z).

  7. No, sorry, that's not the right answer. They are proposed to be an electroweak triplet, under the internal SU(2)_W symmetry which is something else than the rotational SU(2) = SO(3) symmetry of the regular space.

  8. Hi, if I understand it, you are saying that the imposter higgs cannot couple to fermions. This seems to rule it out, because (a) it is supposed to get produced by gluon-gluon fusion into top quark loop into higgs, and (b) it is supposed to decay via top quark loop into di-photons. How can they explain the higgs production and (enhanced, not suppressed) di-photon decay?

  9. Hi Lubos,

    I think we're in agreement on most of these things - in this wonderful data filled era good physicists like Joe Lykken can conservatively ask what might be explaning the signals we see without immediately having to "explain properties of all other particles", as you say.

    Regarding fermiophobic issue, I think I missed this. So one of the things they point out is that the triplet requires throwing out the Tevatron data? This seems a bit dubious to me. What do you think?

    Also agreed that representations in the various string scenarios are restricted, which is one of the great virtues of strings in my opinion. I don't immediately see how to get them in the heterotic string, but in orientifold compactifications they are automatically possible, due to the fact that brane-image brane intersections can give symmetric and antisymmetric tensor reps. This sort of behavior should also lift to F-theory.


  10. Thanks for your interesting comments, P! It seems dubious to me, too. As Ghandi surprisingly aptly notices elsewhere in this or nearby thread, one can't really create a Higgs particle by gluon fusion without couplings to the top - because the top quark loop is used in the gluon fusion.

    So I can imagine that the right explanation could be that Lykken et al. have simply neglected these details. ;-)

    Good to know the reps are there in braneworlds and F-theory. I still feel that there could be some extra constraints beyond the "getting of right reps of SU(2)".

    Cheers, LM