Of Particular Significance

Our Survey of Exotic Decays of the Higgs is Done

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 12/19/2013

After many months gestation and a difficult labor, a behemoth is born!  Yes, it’s done, finally: our 200 page tome entitled “Exotic Decays of the 125 GeV Higgs Boson“.  Written by thirteen hard-working theoretical particle physicists, this is a paper that examines a wide class of possible decays that our newly found Higgs particle might exhibit, but that would not occur if the Standard Model of particle physics (the equations we use to describe the known elementary particles and forces plus the simplest possible type of Higgs particle) were all there was to see at the Large Hadron Collider [LHC], the giant proton-proton collider outside of Geneva, Switzerland.  

[Non-experts; sorry, but this paper was written for experts, and probably has a minimum of two words of jargon per sentence. I promise you a summary soon.]

Why is looking for unusual and unexpected decays of the Higgs particle so important?  [I’ve written about the possibility of these “exotic” decays before on this website (see herehere,  hereherehereherehere and here).]  Because Higgs particles are sensitive creatures, easily altered, possibly in subtle ways, by interactions with new types of particles that we wouldn’t yet know about from the LHC or our other experiments. (This sensitivity of the Higgs was noted as far back to the early 1980s, though its generality was perhaps only emphasized in the last decade.)  The Higgs particle is very interesting not only on its own, for what it might reveal about the Higgs field (on which our very existence depends), but also as a potential opportunity for the discovery of currently unknown, lightweight particles, to which it might decay.  Such particles might be the keys to unlocking secrets of nature, such as what dark matter is, or maybe even (extreme speculation alert) the naturalness puzzle — very roughly, the puzzle of why the mass of the Higgs particle can be so small compared to the masses of the smallest possible black holes.

The goal of our paper, which is extensive in its coverage (though still not comprehensive — this is a very big subject) is to help our experimental colleagues at ATLAS and CMS, the general purpose experiments at the LHC, decide what to search for in their current (2011-2012) and future (2015-) data, and perhaps assist in their decisions on triggering strategies for the data collecting run that will begin in 2015.  (Sorry, LHCb folks, we haven’t yet looked at decays where you’d have an advantage.) And we hope it will guide theorists too, by highlighting important unanswered questions about how to look for certain types of exotic decays.  Of course the paper has to go through peer review before it is published, but we hope it will be useful to our colleagues immediately. Time is short; 2015 is not very far away.

Although our paper contains some review of the literature, a number of its results are entirely new.  I’ll tell you more about them after I’ve recovered, and probably after most people are back from break in January.  (Maybe for now, as a teaser, I’ll just say that one of the strongest limits we obtained, as an estimate based on reinterpreting published ATLAS and CMS data, is that no more than a few × 10-4 of Higgs particles decay to a pair of neutral spin-one particles with mass in the 20 – 62 GeV/c2 range… and the experimentalists themselves, by re-analyzing their data, could surely do better than we did!)  But for the moment, I’d simply like to encourage my fellow experts, both from the theory side and the experimental side, to take a look… comments are welcome.

Finally, I’d like to congratulate and thank my young colleagues, all of whom are pre-tenure and several of whom are still not professors yet, on their excellent work… it has been a pleasure to collaborate with them.  They led the way, not me.  They are (in alphabetical order): David Curtin, Rouven Essig, Stefania Gori, Prerit Jaiswal, Andrey Katz, Tao Liu, Zhen Liu, David McKeen, Jessie Shelton, Ze’ev Surujon, Brock Tweedie, and Yi-Ming Zhong. They hail from around the world, but they’ve worked together like family… a great example of how our international effort to understand nature’s deep mysteries brings unity of purpose from a diversity of origins.

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20 Responses

  1. Hmm it appears like your website ate my first ccomment (it
    was super long) so I guess I’ll jist sum it up what Isubmitted
    and say, I’m thoroughly enjoying your blog. I too am an aspiring
    blog writer but I’m still new to the whole thing. Do youu have any tips and hints for novice
    blog writers? I’d definitely appreciate it.

  2. I seldom comment here since, while my business was quantum chemical physics, and am very well read in relativistic high energy physics, seriously studying the math, I’m absolutely not an expert.

    The comment is: it is a remarkably accessible paper. I actually understand it, congratulations on writing clarity!

  3. Content aside, it is certainly worth noting the very unusual presentation decision made with this paper to include a url for a summary website that is intended to be updated long after the original pre-print was issued in the abstract. It is the first time that I have seen this done and I read probably 30%-40% of the pre-print abstracts on arXiv that come out each year. If this is not the first time that this has ever been done, it is certainly one of the very first few times that it has been done.

  4. Fascinating stuff, that would be insane if we discovered lightweight new particles. This is the reason I love science because it adds new dimensions to everything we already understand about nature. It’s addictive. Question Matt, I read an article in Quanta Magazine about Amplituhedrons and was wondering if they have potential for anything. Do you have any familiarity with these things?


    Thanks for your great work. Happy holidays.

    1. The amplituhedron is a mathematical structure that may prove useful for doing calculations in quantum field theory. Right now it works for imaginary worlds with maximal supersymmetry, see http://profmattstrassler.com/2013/11/20/quantum-field-theory-string-theory-and-predictions-part-7/ . Whether it is of practical use for doing calculations for real-world particles isn’t clear yet. The folks just downstairs from me at Harvard are sitting around trying to see if they can make it work in settings somewhat closer to the real world — but this make take a few years.

  5. Matt, impressive paper. May be the answer is in your 200 page paper. But being lazy, let me ask anyway! What is the total width of Higgs? How much of it is already accounted for by the known decay modes? Thanks.

    1. Let me rephrase my question little better. On second thoughts, I realized that since Higgs is observed channel by channel we do not know the total width. Also taking the figure 40000 literally for the ratio of mass to width I get only about 3 Mev ! is this right?

      1. 4 MeV, but yes, basically right. That’s why the Higgs is so sensitive to new phenomena, and is one of the main motivations for our efforts. See section 1.1 of our paper, where this is laid out.

    2. The width cannot be directly measured at the LHC, so your second question is not answerable.

      The first question can only be answered by theory at the moment. The **Standard Model** Higgs (the simplest possible type) would have a width [i.e. Planck’s quantum constant divided by its lifetime] of 0.004 GeV, which is tiny by comparison with the mass of the Higgs, 125 GeV/c^2. The width of the Higgs we’ve discovered might be somewhat larger than this.

      1. Thanks. I quickly glanced at 1.1 and found (to my great surprise!) the sentence
        “We know the mass of (at least one) Higgs boson, and we also know that its branching fraction into exotic states cannot exceed  60%. ” Do I understand it right that at least 40% of decay is into known particles and at the most 60% into exotic particles?

          1. In a sense, yes; it stems from having an upper bound on how strongly the Higgs can interact with the W and Z particle, which is associated with the fact that the W and Z, in a unitary theory, are getting all their mass from the Higgs field (or fields). If you want to have the known Higgs particle decay 99% of the time to exotic things, yet have its observed decays fit the Standard Model to 20% or so, that means you have to produce it 100 times as often as we expect. That requires the coupling to gluons be very large. That would in turn increase the relative rate at which the Higgs should be decaying to gluons (which we can’t observe) and reduce the relative rate at which the Higgs is observed to decay to W and Z particles (which we can observe), and you can’t increase the coupling W and Z particles to compensate. So at some point you just can’t fit the data anymore.

        1. This is obtained by fitting the strengths of the various types of interactions to the data. You’d have to play very, very fast and loose to get anything more than about 60%, and even that would be pushing very hard. A more reasonable upper bound on the fraction of decays that are to exotic particles is probably 20%.

  6. A most impressive accomplishment!! Just reading the authors’ names buoys my spirit. Another example of how science can transcend the petty boundaries of world politics. Congrats to this truly diverse team of international experts!!

  7. Prof. Strassler: At the bottom of page 10 of your 200 page tome, I noticed a mention of the “vector portal” and the “neutrino portal”. According to Mathew Graham, in connection with the vector portal it seems that there might be “a new U1 gauge boson in the hidden sector (“a dark photon”).”
    http://indico.cern.ch/conferenceDisplay.py?confId=285642 “Searching for hidden sectors via the vector portal”, Dec. 2013, by Mathew Graham (SLAC)
    What are the possibilities for the properties of a hypothetical “dark photon”? Are there any other “portals” in addition to the Higgs, vector, neutrino, and axion portals?

    1. This field of research goes back to 2006. http://arxiv.org/abs/hep-ph/0604261 . As we emphasized in our first papers on this subject, there are many portals [we called them something else, the word “portal” was introduced later by Wilczek], and many possibilities for discoveries of new particles in hidden valleys (hidden sectors with lightweight particles, some of which can decay back to known particles.)

  8. according to some CERN people, spherical shapes are more stable than non-spherical. https: // www. youtube. com/ watch?v=x8Jdu9O2RhU
    a one unit diameter sphere’s volume=4/3*pi*r^3=1/6pi units cubed
    a 2 unit diameter sphere will hold 2^3 (8) one unit diameter spheres.
    a 3 unit diameter sphere will hold 3^3 (27) one diameter spheres.
    a X unit diameter sphere will hold X^3 one diameter spheres.

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