Friday, March 01, 2013

Is the Higgs boson just Higgs-like?

Jester wrote a satirical essay,
When shall we call it Higgs?
that I fully subscribe to. The ATLAS and CMS papers – and some media and theoretical papers inspired by them – keep on referring to the particle officially discovered on July 4th, 2012 as "the 125 GeV particle", "the scalar boson", or – most often – as the "Higgs-like boson".

But it's been quacking like a duck for quite some time. Moreover, from a theoretical perspective, it looks pretty much inevitable that a Higgs boson has to be there because the probabilities of the longitudinal W-bosons' scattering could otherwise exceed 100 percent at energies near a TeV. While we have 5-sigma known that there was a new particle around 126 GeV for half a year if not more than a year, we weren't sure whether its properties exactly agreed with what is required from a Higgs boson. Well, we will never be sure. In fact, we are sure that the properties won't exactly agree with those envisioned by any particular known theory.

It's still a Higgs boson. We're not saying it's exactly the Standard Model Higgs boson – note that the Standard Model is absent in the previous sentence – but it is a Higgs boson and as long as it is the only known one, we may also call it "the Higgs boson" (don't forget that as a linguistic Slav, I am no expert in "the" and "an", whether they are particles or articles, I don't even know how to call these bastards). Papers have shown that it must be a scalar, not a pseudoscalar, at a rather high confidence level. It is a boson i.e. a particle with integer spin. And the spin one is prohibited by the Landau theorem.

In practice, it can't have a positive even spin, either. It does interact with the gauge bosons by interactions that pretty much agree with the Higgs boson expectations, and so on. Assuming that you want Nima Arkani-Hamed to stay alive, it can't be a dilaton of a sort, and so on: he would otherwise have to commit suicide, as promised during a talk. But even if you don't care about his life, it's silly to say that it is not a Higgs boson.

If we used the caution consistently, we could also continue to call W-bosons "the W-like bosons", the carbon nuclei would be the "carbon-like nuclei with six protons and six neutrons", while the Sun could become the "Sun-like star we seem to observe behind Venus", assuming that Venus wouldn't be just a "Venus-like lemon on the sky". But lemon could be a "lemon-like citrus" and the sky could be the "sky-like photographic plate above our heads".

And so on. More seriously, I want to say that our knowledge of the 126 GeV Higgs boson is already comparable to our knowledge of the top quark and other particles.

To be sure, there exist papers in which I would have some understanding for the "Higgs-like" expletive. Today, a new paper by Carena et al. appeared
MSSM Higgs Boson Searches at the LHC: Benchmark Scenarios after the Discovery of a Higgs-like Particle
Here, the "Higgs-like" construct is justified because the title needs to emphasize the contrast between the properties of the 126 GeV particle that have been established and those that remain unclear. Because the very point of the paper is to compare scenarios – especially MSSM scenarios – that try to interpret the Higgs boson in one of the particles in a broader theory with additional features and parameters, it's good to use a more vague term for the new particle than the "Higgs boson" because this term could be misinterpreted as the "Standard Model Higgs boson" and some (or all) theories beyond the Standard Model could be implicitly banned by the wording.

But in many other cases, there is no similar excuse. One may talk about the Higgs boson and it doesn't really imply that he needs the Standard Model to be the exactly right theory. Everyone who is not insane knows that it's ultimately not the exact theory. But we know that it must be a big portion of the truth. The LHC phenomena still seem to be compatible with the Standard Model. It would be more reasonable if such general papers just started to use the term "Higgs boson" for the new particle of mass 126 GeV or so.

Incidentally, the paper by Carena et al. is pretty interesting. It shows that the newly discovered Higgs boson is compatible both with the Standard Model and the Minimal Supersymmetric Standard Model. But they say much more – and more interesting things. If the stau slepton is light, the decay rate of the known Higgs boson to two photons gets enhanced at large tan(beta), something that could possibly explain a minor observed anomaly. And if the stop squark is light, the lightest neutral CP-even Higgs boson may have suppressed probabilities to be created by the vector boson fusion which could correspond to the reality if the lightest CP-even neutral Higgs boson is actually lighter than the known one and unobserved so far while the 126 GeV Higgs boson is the heavier among the two CP-even neutral Higgs bosons of the MSSM.

New particles and surprises may await us even below 100 GeV! People who try to suggest that all hypothetical new particles have been expelled from the realm beneath 1 TeV are just excessively sloppy. Many particles may still be much lighter than that. The list includes possible new (neutral as well as chared) Higgs bosons as well as numerous superpartners: neutralinos, charginos – photinos, zinos, higgsinos, winos, binos (these concepts aren't independent from each other: each of them is a superposition of some other ones) – but even gluinos, stop squarks, sbottom squarks, stau sleptons, perhaps even stau sneutrinos and gravitinos and other particles that aren't even superpartners of the known ones. Even when all the completed experiments (and searches) are taken into account, these particles may be lighter or much lighter than 1 TeV. And the LHC is and will be getting capable to discover new particles above or well above 1 TeV, too.

But let me return to the main topic. It's good to be careful but at some moment, one should realize that excessive caution – a diluted version of the denial of evidence – is also a bias and a contrived vocabulary makes the research less efficient and kind of strange. Even if the 126 GeV particle turned out to be something else than a Higgs boson – which is really, really unlikely – it wouldn't be the end of the world even if there were many papers that use the term "Higgs boson" for the new particle. Everyone would understand that all the papers written about this particle should have called it the dilaton or whatever it would be in this hypothetical world. No big deal.


  1. I want to remind people of the history of the muon, called a "meson" because Yukawa had predicted the existence of a meson to mediate strong interactions , back in 1936 and the confusion resolve in 1947, 11 years later.

    We live in accelerated times. I am told that the final higgs paper will be ready combining both collaborations data by summer .

  2. Dear Anna, if you meant the story of the meson as an argument in favor of the Higgs boson caution, I would raise several objections:

    1) the vocabulary actually *was* distorted by the possible misidentification of the particle. For example, the "unified" particle meson/muon was called mesotron at 1938 conferences:

    This awkward notation - that looks like a crazy theoretical curiosity - seems more awkward with the hindsight than it looked before and it didn't help the physicists to identify the things properly, quite on the contrary

    2) the Yukawa theory was a rough guess, a nonrenormalizable effective theory that people didn't quite understand. We understand the electroweak theory very well, at least to all orders of perturbation theory

    3) as you said, the mesotron confusion was already clarified 11 years after the prediction by Yukawa. Today, we are 49 years after the theoretical discovery of the Higgs mechanism which is more than 11, so by your timing, we should have been using the proper final Higgs vocabulary for 38 years anyway

    4) of course that the spin is zero. All these things are already partly evaluated. For example, this CMS paper

    disfavors the pure pseudoscalar hypothesis for the new particle. But people aren't familiar about these papers because only competent people follow these details and they know that the spin parity is 0-plus, anyway.

  3. Well, I was not counting time from Yukawa's expression of his theory, but from the first muon measurement , which was in 1936, that is why I am counting 11 years ( biased towards experiment).

    I think in the summer when they combine the two experiments spin and parity will be nailed with good sigmas.

  4. OK, I think that you are mixing formalities with the actual uncertainties. In the muon case, no one could actually be certain about anything because almost nothing was known about the observed particle.

    In this case, we know its interactions with W-bosons, strongly interacting particles, photons etc. because otherwise it wouldn't have been measured.

    You know that the summer papers won't really be game-changers, don't you? If you think that it will demonstrate that it has some property such as spin/parity that qualitatively differs from the Higgs, I am ready to bet 5-to-1 in your favor.

  5. Of course I expect that the presently limited-sigma confirmations of spin and parity will become maybe 5 sigma and nail it as the Higgs, a cooked duck so to speak.

  6. Right, perhaps, but I don't think that the 5-sigma point is so crucial here. 5-sigma is normally required to abandon a simpler null hypothesis. But now, the spin-parity equal to zero-plus is by far the most likely and simplest theory a priori. A 2-sigma preference for 0-plus just confirms a strong theoretical prior; it strengthens it 20 times even though it was strong to start with. I think that to insist on a 5-sigma evidence for 0-plus before 0-plus is being discussed as a likely fact is a bit too cautious - denial of evidence and almost self-evident facts, I would say.

  7. Nancy B. HultquistMar 1, 2013, 11:09:00 PM

    by John F., not Nancy
    “ . . . officially discovered on July 4th, 2013 . . .”

    Interesting timeline as it is only March!

  8. Another interesting article on the Higgs that sums it up... can't wait for 2014.