Of Particular Significance

Higgs Discovery: Is it a Higgs?

Matt Strassler [July 10, 2012]

A new particle has been discovered — but is this a Higgs particle or not? And if so, is it an example of the simplest possible Higgs — the Standard Model Higgs particle — or is it not?  

In this article I’ll address the first question; the answer to the second will appear in a sequel.  [You may want to read my articles on how the Standard Model Higgs is produced, decays, and can be studied before you try to make sense of this article.]

Unlike many other articles that I put on this site, this one reflects my own personal view more than the consensus view among my colleagues — mainly because I don’t know what the consensus view is yet. Probably the consensus is still evolving; my own view may as well. I think everyone agrees about what the questions are, but they probably still disagree as to how plausible the different possible answers are.

Also, although I’m thinking for myself here, I believe that all of the points that I’m making here have appeared in various people’s papers (some of them very recent and still in the form of unpublished preprints.)   I’m not making any intentionally original remarks.

Simply put, I am largely convinced this new particle is a Higgs particle of some type. This is a theory bias, not based firmly on evidence from data but on indirect evidence from many years of experience with the theoretical options. And this bias, since it doesn’t come directly from data, is subject to change; if one of my colleagues gives me a good argument as to why I should be less convinced of my viewpoint, I may well change my mind. So no promises are given here.

Things we know or almost know

Before introducing any bias, what do we know from the data? The fact that we observe this particle decaying to two photons as a bump in a plot already tells us, just from basic conservation laws, that the particle

  • has a mass of about 125 GeV/c2
  • has a lifetime which is long compared to its “heartbeat” (my term), i.e. the period of its natural vibrational frequency [I’ll explain this in a moment]
  • is electrically neutral (i.e. has no electric charge)
  • is not directly affected by the strong nuclear force (i.e., in technical language, it is chromodynamically neutral, or “color”-neutral for short)
  • is a boson (it must have spin that is an integer [a brief discussion of spin follows])
  • cannot have spin 1 (a spin 1 particle cannot decay to two photons due to a slightly tricky argument)

Moreover, from the facts that

  1. we also observe it decaying to two lepton/anti-lepton pairs at about a rate 20 to 40 times smaller than its decay to two photons, and
  2. other production and decay modes, expected to be more difficult to observe, have indeed not yet been solidly confirmed in the data,

we may infer that

  • the particle appears to interact much more strongly with Z particles than it does with photons, as would be expected for almost any type of Higgs particle, including the simplest one.

Finally, if we take the claim of the Tevatron experiments CDF and DZero at face value that they also see evidence of the Higgs particle, and we note that certain forms of evidence have not yet been seen at ATLAS and CMS, it must be that

  • the particle cannot interact much more weakly with W and Z particles than is expected of a simplest Higgs;
  • the particle cannot interact much more strongly with W and Z particles than is expected of a simplest Higgs, unless it decays most of the time in a way that the simplest Higgs is not expected to do and that is hard to observe.

There are a few other statements one can make, but they all come with more complex caveats, so I’ll save them for later.

Explanatory asides

Let me explain two of the remarks made above.

“Heartbeat”, or natural vibrational frequency: a particle is a ripple in a field; specifically, it is the wave of smallest possible intensity. Like any ripple, it is vibrating; at any one location in space, the field becomes larger, then smaller, then larger, then, smaller as time goes by. (Think of sitting on a moored boat as a set of wave troughs and crests go by, you’ll be oscillating up and down in time the wave’s own vibrations.) The time between back-and-forth vibrations of a particle at rest is h / m c2 , where m is the mass of the particle, c is the speed of light, and h is Planck’s constant , h = 4.1356675 × 10−15 electron-volt·seconds. Informally, I call this a particle’s “heartbeat”. Notice that a heavy particle has a quicker heartbeat! And we can see from the data that the new particle “lives” (i.e., exists for) many heartbeats (the narrowness of the bump implies at least 50 or so, and possibly many more) before it falls apart.

“Spin”, or intrinsic angular momentum: any spinning object, such as the earth, is said to have angular momentum. Angular momentum is interesting because total angular momentum is conserved, like energy and ordinary momentum. Due to very non-intuitive quantum mechanical effects, it turns out that every type of particle (whether elementary or not) is intrinsically rotating — or at least, it has intrinsic angular momentum, which we call spin. [Whether or not we should picture this as rotation like the Earth’s is a bit in question — I’ve never found having such a picture in my head terribly useful, and most of the time I just treat this little feature of the world using math rather than pictures.][Thanks to quantum mechanics,

  • this intrinsic rotation may be oriented in any direction, but it has a magnitude which is fixed for each type of particle
  • the spin S is defined to be the angular momentum J divided by h / 2π where h is Planck’s constant (see above).
  • it is a theorem that the combination of quantum mechanics and Einstein’s relativity implies that S must either be an integer (for a particle that is a boson) or 1/2 plus an integer (for a particle that is a fermion.) [Read about fermions and bosons here.]

Each type of particle — whether or not it is elementary — has a definite spin. Among the apparently-elementary known and suspected particles, the spins are

  • S=0: Higgs particle
  • S=1/2: all matter particles known so far — electrons, muons, taus, all neutrinos, all quarks, (and all of their anti-particles)
  • S=1: the carriers of the three known non-gravitational forces: photons, gluons, W and Z particles (which are their own anti-particles, except the W+ and W- which are anti-particles of each other)
  • S=2: the graviton (the carrier of gravity)

There are various theoretical reasons to expect lightweight elementary particles to have spin no greater than 2, and for the only particle of spin 2 to be the graviton.

But the limitations of elementary particles do not apply to more complicated composite objects like hadrons (particles made from quarks, anti-quarks and gluons) or atomic nuclei (made from protons and neutrons). Among hadrons there are many, many examples

  • S=0: pions, kaons (the lightest mesons)
  • S=1/2: protons, neutrons (the lightest baryons)
  • S=1: rho mesons, omega mesons, phi mesons
  • S=3/2: Delta baryons
  • S=2: f2 mesons
  • S>2: nobody remembers the names of the higher-spin hadrons, all of which are very short-lived, but there are lots of them going up to S=6 and beyond.

Atomic nuclei also have many spins.

The interactions with photons and with Z particles

The new particle interacts with Z particles much more strongly than with photons or gluons: How can we see this is true? A simple way is to note that

  1. this fact is true of the simplest possible Higgs, and
  2. the theory with the simplest Higgs in it predicts the ratio for the production of Higgs particles decaying to two photons to the production of Higgs particles decaying two lepton/anti-lepton pairs (henceforth “four-leptons” for short), and
  3. the data about the new particle agrees, to within about a factor of two, with the prediction of the theory of the simplest Higgs; so the new particle too, like the simplest Higgs, must interact more strongly with Z’s than with photons.

The reason why four-lepton events are rare, despite the stronger coupling of the new particle to Z’s, is that

This fact is very important, because it makes it implausible (and in many cases impossible) that this new particle could interact with photons in a way that is similar to how it interacts with W and Z particles. Instead, as is the case for the simplest Higgs, the interaction of the new particle with W and Z particles must be direct, while the interaction of the new particle with photons must be indirect (via virtual particles). This constraint immediately eliminates many types of possible alternative explanations for this particle. [Essentially, since photons and Z particles are mixtures of weak-isospin and hypercharge particles, getting the new particle to interact relatively strongly with Z’s and relatively weakly with photons requires a very delicate balancing act, one which is automatic for a Higgs particle but is in general not going to happen for any particle that isn’t similarly quite tied up with the generation of the W and Z particle masses.]

So we are almost guaranteed that the new particle has something to do with how the W and Z got their masses — maybe in a more complicated way than for a Higgs, but it can’t be completely disconnected from it. If it were, it would treat photons and Z particles too similarly to agree with data.

On top of this, there is a weak but interesting argument that the strength of the new particle’s interaction with W and Z particles tentatively looks as though it is rather close to the right size to be a Higgs particle.  If the interaction were much weaker, then (no matter how you twiddle things) the Tevatron experiments CDF and DZero should see no evidence at all of the Higgs particle in the ways they are looking for it.  [You might choose to disregard their evidence as unconvincing; that’s up to you.] If the interaction were much stronger, then you’d expect much stronger evidence of the new particle in at least one LHC or Tevatron search where little or no evidence is yet seen [except in the one case where all of the new particle’s interactions are uniformly stronger than for a simplest Higgs and yet it has a new exotic way of decaying that is hard to observe and dominates all the others.]

Some Theory Bias — Reasonable but Not Without Holes

Now comes the part that is a little more theoretical and harder to explain. If you take a theory of a new particle of spin 0 or 2, along with massive W and Z particles and massless photons and gluons, and you try to make equations in which this particle interacts directly with the W and Z particles and interacts indirectly with the photons and gluons, the equations, depending on their details, can give a wide range of answers for the production of Higgs particles that decay to photons and leptons, and can give a wide range of possible ratios for the decays to two photons and to four leptons. The equations can also give you whatever you want for the strength of the new particle’s interaction with the W and Z. Uniquely (I think) in the case that this particle is a Higgs particle of some type, and thus is directly associated with the Higgs field that gives the W and Z particles its mass (or one of several such particles and fields) you will automatically get the interactions of the particle with photons, W and Z particles to be of the right size so that you are roughly consistent with the data; and you can easily get the interaction with the gluons with roughly the right size. In all other cases, you can get things to come out right, but not automatically: it’s only as an accident.

[One interesting accident and resulting loophole occurs in the case where the theory above the TeV energy scale becomes scale-invariant. In this case the interaction of the new particle with gluons becomes larger and the interactions with photons and with W and Z particles becomes smaller in just such a way as to remain consistent with present data — as long as you completely discard the evidence of the Higgs from the Tevatron experiments and any of the limited signs of the p p –> q q H process, called “vector boson fusion”.]

[I don’t know of any other interesting accidents like this, but would be interested to learn of other reasonable examples.]

[I am grateful to several colleagues, especially Ben Gripaios, for discussions on this point.]

Summing Up

In the end, however, I must remind you this is all talk.  The question of whether this particle has spin larger than 0 [or, for experts, is spin 0 but is CP-odd instead of CP-even], is something that can be determined experimentally, and this will gradually happen over the next few years. So you won’t have to accept this theory bias in the long run; we’ll know, from data.  For a spin-0 and CP-even particle, it will be trickier to settle the issue, but at some point we’ll gradually be convinced that the alternatives don’t make sense. All I’m telling you today is that I’d personally be very surprised if this were not a Higgs particle of some type. If you accept this point of view (and you shouldn’t do so without thinking it over and remembering the loopholes), then the next question is: is this an example of a simplest Higgs, or is it an example of a more complicated Higgs? That’s the subject of the next article.

25 Responses

  1. Thank you very much.

    I’m 17 and I live in the Netherlands. If you finish high school here, you have to write an essay about a subject. You are free to choose any subject you want, and we (me and a friend) chose to write an essay about the higgs discovery. After writing alot about CERN, the standard model and a lot(!) more, we still had no clue how they found the higgs particle. We understood what it was and why it was hard to detect. We still had no clue how it was detected…

    Then we found your site, you are a legend.

    We understand now how they found the higgs particle, This is the clearest explanation I’ve found on the internet.

    (sorry for my English, it isn’t my primairy language)

  2. Very interesting and clear as usual!

    Just a question. When you say “There are various theoretical reasons to expect lightweight elementary particles to have spin no greater than 2, and for the only particle of spin 2 to be the graviton.” … what reasons do you have in mind? Thanks in advance.

    1. This is complicated and sophisticated; it involves subtle theorems in quantum field theory. It certainly isn’t obvious. That’s why I didn’t say much about it. Sorry I don’t have a better explanation for you.

  3. Two questions.

    1. Will the discovery of the Higgs-Boson at 125 GeV/c2, and all of the other discoveries that you have outlined above, help to “tune” LIGO when it switches back on?

    2. If there is a Higgs-Boson…can there be an anti-Higgs-Boson?

    1. 1. There is no connection between the Higgs field and gravity, so there will be no impact on LIGO. [Many people think “Higgs — mass — gravity”, but this is an error: First, gravity is always universal (affects all particles) while the Higgs field is not universal: it only provides mass for the known particles, while other hypothetical particles, including perhaps dark matter particles, likely get their masses in other ways. Even the Higgs particle gets only part of its mass from the Higgs field; the other part of its mass is not predicted by the Standard Model’s equations and its source is therefore unknown. Second, gravity in Einstein’s theory is not about mass, as Newton’s is; it is about energy and momentum. For slow particles, energy and mass are proportional (E is approximately m c-squared if a particle moves with speed slow compared to light-speed) so that’s why the difference between Newton’s theory and Einstein’s are hard to notice in ordinary life. But it’s crucial to understand this distinction. So you should think: Higgs — non- universal giver of mass to the known particles, and partly itself; gravity — universal force that arises from the energy and momentum of all types of particles.

      2. Like the photon and the Z particle, the Higgs boson is its own antiparticle.

  4. ‘the particle … has a lifetime which is short compared to its “heartbeat”’. Did you mean “long”? Otherwise I can’t reconcile this with your later statement “the new particle “lives” (i.e., exists for) many heartbeats (the narrowness of the bump implies at least 50 or so, and possibly many more) before it falls apart”.

  5. Hi Matt,
    When you talk about “Heartbeat” it called to mind for me the quality factor or “Q” of structures like the RF accelerating cavities for particle accelerators. ( http://en.wikipedia.org/wiki/Q_factor ). Just as you describe the bumps or resonances get narrower in the plots revealing the presence of particles in the ATLAS or CMS data for particles surviving for more heartbeats; the higher the Q of an RF cavity the narrower its bandwidth and the longer it stays oscillating. In fact an RF cavity Q is proportional to the energy it contains divided by the energy it loses per cycle of its natural resonant frequency. I wonder if the similarity is perhaps more than just a coincidence, particularly as particle physicists tend to call the bumps in their data “resonances” too?

    1. It’s not a coincidence — the math and the physics is exactly the same as for any resonance. A particle at rest is a resonance with a natural frequency proportional to its mass and its mass-energy (i.e. rest energy); that’s what I call the “heartbeat”. And like any resonance, it has a damping time which is nothing other than the particle’s “lifetime”.

  6. Is there a missing word or two in “…the interaction of the new particle with W and Z particles must be direct, while the interaction of the new particle [with photons] must be indirect…?” (I filled in my guess for missing words. Virtual words? Perhaps they’ll disappear soon.)

  7. ” the interaction of the new particle with W and Z particles must be direct, while the interaction of the new particle must be indirect”

    Is this sense missing a prepositional phrase somewhere? What interaction is indirect?

  8. M.E.: “… an electron is a point particle, so it can’t be rotating!”

    Very interesting comment. The following is only my personal view.

    The spin of “elementary” particles is a bit different from the rotation of a macro-object. For particle of spin 1/2, it sees two copies of space-time. For particle of spin 1, the two copies of space-time look identical, that is, it sees only one copy of space-time.

    This whole Higgs field issue is about this space-time issue. With the introduction of Higgs field, the two copies of space-time are viewed as one. Without the introduction of Higgs field, there are two copies of space-time. That is, the Higgs field is only a shadow of the reality, a shadow of the two copies of space-time. The Higgs field is not wrong per se. It is simply a bad idea.

  9. Your aside about spin makes me wonder. I never realized the meaning of spin was “in question.” I always saw it explained as “spin is just a number associated with elementary particles; an electron is a point particle, so it can’t be rotating!” But I’ve always wanted some reason for why particles should have a “spin” that behaves mostly like “real” angular momentum but isn’t. So is there some thought about this among physicists, or do they mostly treat spin as “just a number”?

    Then I found out that electrons have a tiny dipole moment which suggests that the electron has a certain distribution in space. Does the electron’s electric dipole axis rotate?

    1. I’m just saying that I don’t have an intuitive way of visualizing it; the equations are very clear. I never said “the meaning of spin is in question”. It’s just that putting that meaning into words and pictures, rather than the technical language of math, seems difficult to do. One has to get used to the idea that a brain’s ability to visualize the possibilities is often less powerful than math’s ability to convey the range of possibilities.

      The electron has a magnetic dipole moment but does not have any distribution in space. Again, I have no intuitive way of visualizing this; but the equations are very clear. Point particles with electric charge and non-zero spin will generally have magnetic dipole moments.

      1. Thanks for your response. By the way I have enjoyed your last series of articles and I hope you keep writing. This is the best blog for laymen about the frontline of physics that I’ve seen.

        Anyway, what you said is, “Whether or not we should picture this as rotation like the Earth’s is a bit in question”.

        All I’m saying is that I’ve been taught that this is not in question, that spin of elementary particles is under no circumstances to be seen as a form of rotation. But you’re implying that maybe some people do think it’s ok to see it that way. So I just wanted a clarification of that.

        Regarding the dipole moment, the electron also has a small electric dipole moment (http://en.wikipedia.org/wiki/Electron_electric_dipole_moment) distinct from the magnetic dipole moment. I don’t know enough quantum physics to understand what the article is actually saying.

        1. I see; you’re pointing out that my language is a bit problematic. [Sometimes it is hard for me to tell who’s a layperson and who’s more educated; I do get it backwards quite often.] Maybe I should reword that.

          About the electron’s electric dipole moment — its existence is a consequence of the violation of time-reversal symmetry in nature. But again, this has nothing to do with the electron having a size, any more than the magnetic dipole moment does. It’s something a point-particle (which interacts with other particles in a way that violates time-reversal symmetry) can have.

          I should add that the notion of what it means for an object to be a “point-particle” is subtle too in quantum field theory. A particle that can interact iwth other fields and particles in some sense always has a cloud of disturbances (virtual “particles”, which aren’t really particles at all) around it. But this cloud dies off gradually, it’s not like you should think of it as a ball with an edge.

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