Mar 20 2013

The Higgs Story-Part 1: The three faces of Higgs

Around the time of reports last year about the discovery of the Higgs particle at the LHC (Large Hadron Collider), reader Anthony in a private email to me asked a good question. The Higgs particle is repeatedly referred to as the means by which all other particles get their mass. If not for the Higgs, elementary particles like the electron, muon, and the like would be massless and like all massless particles would be zipping around at the speed of light. At present, there is no explanation for why these particles have mass at all let alone the actual values that they do have. According to current theory, it is the Higgs phenomenon that gives all the other particles their mass. So how does that happen?

In many popular explanations, a metaphor is used. It asserted that these Higgs particles permeate all space and act as a kind of viscous fluid that ‘slows’ the other particles down, which is equivalent to giving them mass. But this raises the question of how the Higgs particle gets its own mass. Surely it cannot be slowing itself down? This is the good question that Anthony raised, and rather than give a short and glib answer, I thought that the scientifically-minded readers of this blog might like more solid information about the much-ballyhooed discovery of this particle last July. The catch is that this will turn out to be one of my dreaded multi-part series, so buckle up, it’s going to be a bumpy ride*. To make it easy to refer back to the other posts in this series, I will present the material in bite-sized chunks every weekday so that people have time to digest it before moving on to the next part. I have created a new category so that the entire series will be in one place for easy reference.

Although the discovery of the Higgs is by now somewhat old news, I hope to provide more solid information than the hyperventilating popular media accounts that tend to go along the lines of how ‘the god particle’ will unlock the secrets of the universe. At the same time I will avoid the difficult mathematics. This series of posts will try to explain why the search was pursued with such intensity, some of the basic physics underlying it, and why the entire process was so difficult and costly. This will involve plenty of hand-waving arguments using metaphors and analogies at various points because the theory is pretty mathematical and complicated. Also, this is not my area so I am not going to claim 100% accuracy for everything that follows but it should be pretty close.

It would be helpful to rectify three popular misconceptions right at the beginning.

The first is that there are three related phenomena associated with the name Higgs: two of them refer to specific entities and are the Higgs field and the Higgs particle (also called a boson for technical reasons that I won’t be going into), while the third is the Higgs mechanism, which represents a process involving those two. Although it was the discovery of the Higgs particle that made the headlines, the other two are as important. The Higgs particle derives its significance from the fact that it provides concrete evidence that we are on the right track in postulating the existence of the Higgs field and the Higgs mechanism that follows from it. If the Higgs particle had not been found, it would have cast serious doubt on what has been called the Standard Model of particle physics that has been so successful so far in explaining the properties of fundamental particles. The discovery of the Higgs does not mean that there are no more unanswered questions. But it is an important part of the structure.

The second thing to note is that although the name Higgs is the one that has stuck to all three phenomena, there were actually six people in three groups working independently who in 1964 published papers contributing to the eventual theory: Robert Brout and Francois Englert; Peter Higgs (working alone); and Carl Richard Hagen, Gerald Guralnik, and Tom Kibble. So why does only Higgs get his name attached to this? The way things get named in physics is often an accident and the first easily remembered and plausible name that comes along tends to get fixed in people’s minds. In the spirit of sharing credit more appropriately, some have tried to have the label BEHHGK (pronounced ‘beck’) replace Higgs but it hasn’t caught on.

The third popular misconception is that although it is often asserted that the Higgs particle gives mass to everything, this statement has to be modified in two ways: one is that it is the Higgs field, via the Higgs mechanism, that gives rise to mass; and the second is that it gives mass only to the elementary particles, not to all the objects in the universe that have mass.

Next: How ordinary matter is made.

(* I know that this allusion is to a common misquote of the original line from the classic 1950 film All About Eve but the misquote is more generally serviceable than the correct “Fasten your seat belts. It’s going to be a bumpy night“, which may be why it has caught on. Too bad that line wasn’t from that other classic 1957 film The Three Faces of Eve to better fit with the title of this post.)


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  1. 1

    ” and the second is that it gives mass only to the elementary particles, not to all the objects in the universe that have mass.”

    Are the other “objects” that have non-Higgs Mechanism mass things like dark matter or black holes?

  2. 2
    Mano Singham

    That is a good question. What makes up an ‘elementary’ particle and its relationship other forms of matter is something that I will address in the next two posts.

  3. 3

    Even less exotic than that actually. All of the bulk matter that we run into on the macroscopic scale, anything made out of protons and neutrons, get the overwhelming majority of their mass from the binding energies of the strong nuclear force.

  4. 4

    The third popular misconception is that although it is often asserted that the Higgs particle gives mass to everything, this statement has to be modified in two ways: one is that it is the Higgs field, via the Higgs mechanism, that gives rise to mass; and the second is that it gives mass only to the elementary particles, not to all the objects in the universe that have mass.

    As a physicist, or rather, physics grad student, I’ve been trying to take up this torch and correct this misconception where I see it. It would be good to have something from you to be able to pop out a quick link to, rather than type out something lengthy of my own :)

  5. 5
    Doug Little

    Sean Carroll’s Blog has a good discussion going on about where the mass of particles comes from in the comments section here.

  6. 6

    Can you explain what you mean by getting mass from binding energy?

  7. 7

    Thanks for your contribution to explain this “phenomenon” of the nature. I hope you will succeed to explain the relationship between particle and field. It seems to me that to do with category theory in mathematics.

  8. 8
    Doug Little

    Here is another good discussion about where the mass of protons and neutrons comes from and also what percentage the Higgs mechanism is responsible for. Article Here

  9. 9
    moroplogo mrplgo

    Very thanks for your contribution to explain this “phenomenon” of the nature. I hope that you succeed to clarify the relationship between particle and field. It seems to me that to do with the category theory in mathematics.
    Another important thing to talk is dark matter and dark energy , as suggested by curcuminoid.

  10. 10
    moroplogo mrplgo

    I am sorry for the previous comment which is only repetition of the comment below.

    I wanted to point out the diffusion of the news of the space by ESA tomorrow live from their website : http://spaceinvideos.esa.int/esalive
    It’s about Planck Cosmology Findings.

  11. 11

    Something about 99% of the mass of baryonic matter has a different source: color confinement. From Quantum Chromodynamics, if quarks and gluons start getting more than about 10^(-15) m away from each other, their interaction becomes superstrong. This forces protons and neutrons’ quarks to have kinetic energies of a few hundred MeV, about 100 times their rest masses (they are up and down quarks).

    The Higgs mechanism makes masses for every other Standard-Model particle, except for the photon and the gluon, which stay massless.

    Dark matter is something-or-other outside of the Standard Model, so it would get its mass from a different source.

    Though the Standard Model does not explain the origin of the Higgs particle’s mass, supersymmetric extensions of the Standard Model typically explain the Higgs mass as a result of supersymmetry breaking. That is also the origin of the masses of some dark-matter candidate particles.

  12. 12
    Doug Little

    From what I’ve read 60% of the mass of the proton and neutron is the result of the Higgs field as the field also influences QCD which would be different if the Higgs field did not exist.

    This is from a comment on Matt Strassler’s blog that he agreed with,

    The Higgs field is actually responsible for about 60% of the mass of the proton and the neutron.

    It is true that the proton and the neutron get their mass from non-perturbative QCD effects, and this mass is set by Lambda_QCD. *However* Lambda_QCD is itself set by the Higgs field. The fact that the Higgs field is `on’ means the top, bottom and charm all decouple from the running of the strong coupling constant at their mass scales, and this happens only because the Higgs field give them mass.

  13. 13

    “one of my dreaded multi-part series”
    You are kidding. Those multi-part series contained the best articles on your old site. I have been missing them since you moved to ftb, even though I discovered your blog via ftb!
    Ed Brayton should contractually oblige you to do at least one each year, I say.

  14. 14

    It’s only an indirect involvement, unlike its direct involvement in the masses of the up and down quarks.

    As an elementary particle travels, it can turn into some other particles, which then turn back into the original particle. A photon can turn into a charged particle and its antiparticle, and these particles can turn back into a photon. A charged particle can radiate a photon, then re-absorb it. Etc for all the others. One has to integrate over all possible energies of a particle in these loops, and the resulting integrals typically diverge. But with a trick called “renormalization”, these divergences can often be subtracted out by redefining various quantities to include them.

    But these loops continue to have an effect. These loops make effective masses, coupling parameters, etc. change as a result of interaction energy. One can observe such effects for the effective electric charge, and especially for the effective QCD “charge”. But some loop particles will make a significant contribution only if their combined masses are above the interaction energy.

    For particles orbiting each other, their interaction energy, or more precisely, momentum, is about (hbar*c)/(separation), or 1/(separation) in units that particle physicists like to use: hbar = c = 1. So probing higher and higher energies is equivalent to probing shorter and shorter distances.

    Loop particles will not make a significant contribution to a parameter’s effective value if the interaction energy is below the energy needed to create them (remember E = mc^2). This corresponds to a size scale greater than the loop particles’ smallest Compton wavelength.

    For the QCD “charge”, gluons will always contribute, because they are massive, and because gluons interact with each other. However, quarks will only contribute if the interaction energy is above their masses. Let’s see how it goes from LHC energies of about a TeV to light-hadron energies of a few hundred MeV. At a TeV, the QCD version of the fine structure constant is about 0.09, small enough to allow quarks and gluons to act like almost independent particles. With lower energy, it increases, and it becomes near 1 near a GeV, causing “color confinement”. Gluon-gluon interactions are the cause of this effect, with gluon-quark interactions acting in the opposite direction, but only partially canceling out the gluon-gluon interactions.

    From 1 TeV to 173 GeV, all six quarks contribute.
    The top quark stops contributing.
    From 173 to 4.5 GeV, the remaining five quarks contribute.
    The bottom quark stops contributing.
    From 4.5 GeV to 1.3 GeV, the remaining four quarks contribute.
    The charm quark stops contributing.
    From 1.3 GeV to a few hundred MeV, the remaining three quarks contribute — up, down, strange.
    We run into color confinement.

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