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CERN | Geneva | Switzerland

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All on the Higgs for (nearly) everyone

Like most of my colleagues, the most frequently asked question I get from friends and family these days is: what is this Higgs boson business? Here is what I hope will help not only family members but also struggling physicists. It is not the simplest but it is complete and accurate.

First of all, let’s clarify one point: it has nothing to do with God. This “God’s particle” business has got to go. It was just a bad joke to start with and like any joke, it gets stale fast.

And we need to talk about three separate aspects: the Higgs mechanism, the Higgs field and the Higgs boson.

Currently, there is a theoretical model describing just about everything observed so far in particle physics, called the Standard Model. It rests on two simple principles: 1) all matter is made of particles, namely quarks and leptons and 2) all interactions between particles are mediated by exchange particles associated to the fundamental forces.

The current equations from the Standard Model only produce massless particles when we know these particles all have a mass, as witnessed countless times in our particle detectors.

The “Higgs” mechanism is the mathematical representation of what happens. It is a way to remix the Standard Model equations for the electromagnetic and weak nuclear forces. It brings into the equations three particles called “Goldstone bosons”, that mathematically “absorb” the three massless bosons associated with these forces. Then, out of the new equations pop three massive particles, the W+, the W- and the Z0 bosons, plus the massless photon. These are the four particles we know to be associated with the electromagnetic and weak nuclear forces.

In all fairness, it should be named after all the physicists who contributed to the idea but so many did, it is referred to as “Higgs mechanism” for short.

The mechanism is a mathematical description of a physical entity, the “Higgs field” that permeates all space, just like a gravitational field affects the space around a massive object. Although we cannot see this field, particles feel its effects by acquiring a mass, just like we feel the gravitational attraction du to the Earth’s field.

The Higgs field is what is needed to provide mass to all elementary particles such as the electrons, the quarks, the W and Z bosons. The fact that we have found the W and Z bosons with exactly the mass predicted by the theory is a strong indication that the Higgs mechanism takes place and the Higgs field exists, but there is of course no guarantee until we find the Higgs boson to prove it all.

The Higgs boson is just an excitation of the Higgs field. Ok, I admit, this one is harder to swallow. But think of a hydrogen atom. In its ground state, the hydrogen atom lives eternally. It will never decay into anything more stable. But it becomes “excited” after absorbing energy. Its electron then jumps to a higher level making the atom unstable. In just picoseconds (millionth of millionth of a second), the hydrogen atom will come back to its ground state by emitting a photon, spitting back the excess energy to return to its stable state.

The Higgs field, like a hydrogen atom, can be excited, also only in discrete values of energy corresponding to the Higgs boson mass. The energy released when protons collide in the Large Hadron Collider can excite the Higgs field. The excited state is just the Higgs boson itself. And just like the hydrogen atom in its excited state, it will try to return to its ground state. The Higgs boson is therefore very unstable and will decay into other particles instants after appearing.

What we need to establish now at CERN is exactly if the Higgs field exists and how it operates, how it can be excited, how it all works. This is what all our research around the Higgs boson is about. We want to know the specificities of the Higgs field. The simplest thing is that the first excited state is a single particle, the Higgs boson. That’s what the Standard Model favors. Some other theories bet on many different types of excitations, i.e. many different Higgs bosons or composite objects.

Now, last but not least, how does the Higgs field provide the mass to other particles? Imagine a completely empty pool table, with a perfectly smooth marble surface and perfectly straight marble sides. Toss a billiard ball across the table and it will travel on a straight line. Now glue many rigid posts to form two rows, leaving a narrow path for the ball. This time, the ball will hit the posts, bouncing back and forth along the way. If the table is perfectly smooth and the ball perfectly rigid, it will just bounce back without losing energy. It will eventually make it across the table, taking more time but keeping the same energy. If you measure its speed from how long it took to cross the table, it will appear like it is now travelling slower.

A particle moving in a space filled with the Higgs field would no longer be able to travel in a straight line because of its interaction with the field. It would progress more slowly overall, like a billiard ball interacting with small pegs would take more time to reach the other side of the table.

A physicist would say: there is now dispersion, the tossed ball no longer travels along a straight line but without dissipation, i.e. no energy loss.

The Higgs field acts like the posts glued to the table. It prevents the moving ball from traveling in a straight line but without it losing any energy.

In relativity, the mass is seen as a form of energy. This is expressed by the well-known equation:  E = mc2 where c2 is just like the exchange rate between mass m and energy E. Look at it as having money in your pocket: if you are at CERN, it would be in two currencies, Swiss francs and euro. The sum is the total money you have. For a moving particle, both its movement and its mass contribute to its energy as illustrated below:


For a moving particle, the energy comes from two sources: having speed and having mass. This is a bit like having money in two different currencies. One can convert them into each other.

Ok, let’s stretch our imagination one bit further and now imagine a massless billiard ball, a ball with absolutely no mass. If we toss that massless billiard ball across the table, all its energy comes from its motion since it has no mass. Its effective speed is lower since it no longer travels on a straight line. A physicist would talk of the group velocity here, the speed at which the ball seems to progress in the right direction.

For a massless particle, i.e a particle without any mass, all the energy comes from its motion. It’s like having only one currency in one’s wallet.

So, here is the million Swiss franc question: If it is slower, the energy share coming from its velocity is smaller, then how comes the ball still has the same energy? Simple: this ball has acquired mass through its interaction with the Higgs field. The contribution to the total energy from its mass is no longer zero and it adds up to the reduced contribution associated with movement to give the same total energy value.

A particle interacting with the Higgs field moves more slowly but its total energy is unchanged. It therefore has less energy in the form of motion and some now coming from the newly acquired mass just as if one had converted some Swiss francs into euro.

To summarize: The Higgs field provides the mass and the Higgs boson is just an excited state of this field. All particles that interact with the Higgs field acquire mass since they travel less rapidly but still have the same energy. The Higgs mechanism expresses all that mathematically.

Sleep tight.

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline or sign-up on this mailing list to receive and e-mail notification.


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30 Responses to “All on the Higgs for (nearly) everyone”

  1. Bruce says:

    Higgs analogies are hard to come up with but am not convinced this analogy works! Seems to me the ball will take the same time to cross the table if the collisions truly don’t change the velocity of the ball and there is perfect reflection at the rods because the distance traveled is the same. Actually I dont understand why you chose rods rather than just walls, so perhaps I have missed some key point…

    • Andrew says:

      Even if speed doesn’t change with collisions (remember velocity is speed and direction so it _does_ change) the path is longer due to the extra bounces. Longer paths take more time.

    • Max says:

      If Energy are equal to mass plus motion, the well know equation E = mc2, should be E = m+c2, or not?

  2. Andy Peters says:

    Thanks! Best layman’s explanation I’ve ever read. Much appreciated.

  3. Steeph says:

    So does that mean that if a particle has zero motion, it would also have zero mass? Because without motion, the higgs field can convert that into mass, or is that a misinterpration of the given example?

  4. Jose Carlos Picarra says:

    Would like to know how the Higgs theory compares with de M theory, since the M theory belives that it is the strings movement throu the eleven dimentions, that gives mass to the matter.

    Thank you in advance.

    Jose Carlos Picarra

    • Pauline Gagnon says:

      Sorry, this is beyond my reach. You would need to talk to a string theorist about that. M theory is even further out than the Higgs mechanism.

      Cheers, Pauline

  5. Adam says:

    How do you explain then, in this beautiful picture, the fact that photons are massless and remain so after the interaction with the Higgs field? Is there something that “protects” them?

    • Pauline Gagnon says:

      Indeed, they simply do not interact with the Higgs field so they remain massless.

      Think of it as someone talking to you in a language you absolutely don’t understand. It won’t affect you, no matter what is said (providing of course the tone or anything else gives it away).

      The photon does not feel the effects of this field, that’s why.

      Pauline

    • Adam says:

      So if we’re going to push things further, why is that they do not share the same language? Is it like if they do not share a common feature that couples them as to provide mass to the photon – a coupling constant for example. This is forbidden from the Electro-Weak Symmetry Breaking…You see, there’s the W3 and B fields, that “eat” some Goldstones – whatever that horror scenario means- and they become Z who gets mass, and gamma, who doesn’t. I get the impression that this is for good! From now on, this prevents real photon having mass and interacting with the Higggs field , although the virtual photon gets mass… how, if he can’t couple to the Higgs field?? So at some point, the photon doesn’t interacts with the Higgs field, and then it gets away from our sight and it does?! He’s kind of playing tricks on us, does he?

  6. Ivan Kirkpatrick says:

    Very good explanation. I have been following this pretty closely for a layman and this helps me understand it all much better. I find this whole study of particle physics quite fascinating. I even created a computer model of my own to help me understand it.

  7. sudhir mittal says:

    Thanks for a layman’s view. Could you pse also touch upon the larger implications of the discovery or otherwise of the field.

    thanks

    sudhir

    • Pauline Gagnon says:

      There is no known application yet. At this point, the goal is simply to better understand the world we live in. That’s the way research work. Applications generally come later, once things are better understood. Nobody would have thought that studying the electron one hundred years ago would lead to electronics, communication, computers, internet etc. It’s the same here. Applications may come or not There is never a guarantee but an increase knowledge is a good goal in itself.

      Pauline

  8. Lorne Wardell says:

    The movement of a particle through spacetime is a process. If the time aspect is stretched i.e. the process is “slowed” then mass is added, if I read this right.
    So the older a process is the more it weighs in a gravitational field?

    • Pauline Gagnon says:

      Being slower and being older are two very different things. So I think you are mixing them and getting to wrong conclusions. That is also always the problems with analogies and metaphors… they have their limits.

      Pauline

  9. F.Le Guen says:

    clearest explanation i’ve ever read so far (I am a scientist but never studied particules physic). Thanks. does it mean that if the univers keep cooling photons will acquire a mass and the world we know will desapear due to a short distance EM interaction ?

  10. plusminus3 says:

    I wondered about the same as Steeph. Thank you, Pauline, for these great examples!

  11. Ralph Lausa says:

    At least Pauline is trying to simplyfy the concept for us all. Many thanks Pauline!

    • Pauline Gagnon says:

      You’re most welcome. But it is a complex process and this explanation has its limits. But glad to hear it help you visualize it.

      Pauline

  12. Mike says:

    in reply to Steeph, you can never – in real life – define a free particle, a particle is always in interaction, hence there is no particle with zero motion, unless you reach a temperature of zero kelvin where some believe that motion will stop:s

    to the auther, I would like to always explain it as a result of spontaneous symmetry breaking:)

    I am truely amazed by the idea, but I always thought that there is more to physics than that, physicists always suppose that the initial physical state is symmetric rather than thinking of it as asymmetric, I know that our universe is symmetric(somehow) but what if our universe was originated from a vacuum state that is degenerate( resulted after some spontaneous symmetry breaking of an intial vacuum that was unstable)
    there are some assumptions that are always questionable
    cheers

  13. Softspoken_One says:

    This explains things very well, I think. Many Thanks!

    Mike

  14. Sosh says:

    But the ball is not ‘moving slower’, just in a convoluted path. Are you suggesting it is moving in some hidden dimension? Maybe I’m taking the analogy too literally?

  15. Łukasz says:

    Thank you for this explanation , I hope that Higgs bozon will be observed in data produced by LHC.
    have a nice day Pauline

  16. Dave Hanley says:

    As a chemist with only basic maths, particle physics on the level of billiard balls with cool names and amazing properties is very interesting, and I appreciate Pauline’s post. However…
    I believe that when you look closely at anything really tiny, all you can “see” is…maths…great statistical maths that gives amazing predictive power. Imagining subatomic particles flying around interacting is fun but I suspect completely irrelevant, although a helpful way to label and sort some of the maths. Perhaps protons at low energies are big enough and permanent enough to sometimes behave as little marbles, so we can think of them safely as “particles”. I suspect for anything smaller or more fleeting than a proton, you might just as well imagine a bunch of maths equations banging into each other.

  17. Ed says:

    I’m not an idiot. I was tutoring calculus and auditing upper division (and one grad) classes when I was 15.

    However, I was almost completely unable to follow this description.

  18. Max says:

    Photon is a massless particle when it is quiet, but you really think existing in nature a quiet photon?

    I think not!

    To say about “a quiet massless photon” is equal to say about “a moving photon with infinite mass”.
    You have only to choose your preferred one definition.

    But we have to admit with this method, we can find all particles we want. Higg’s boson also….;)

  19. Paul says:

    In this analogy, are the pegs intended to represent instances of the Higgs boson itself, or do they represent dynamic variations in the Higgs field generally?

  20. Why is it that an electrical field and magnetic field are called physical entity?
    A much better name is mathematical entity.
    I would also call Energy a mathematical entity.
    IMO baryonic matter and non baryonic consists of physical entities
    i.e protons, neutrons, elektrons and photons.
    If you agree is it then not better to call the Higgs field a mathematical
    entity and the Higgs boson a physical entity.
    Of course you can also select different names to distinquish between the two.
    The most important part is not to call everything a physical entity because
    when you do the word physical entity has no meaning.

    In the article the word massless particle is used. What are massless particles
    and how do you know that they are massless.

    The article: http://en.wikipedia.org/wiki/Standard_Model#Higgs_boson claims:
    In particular, the Higgs boson would explain why the photon has no mass,
    while the W and Z bosons are very heavy.

    Sorry to say but after reading the article it is not clear to me how the
    Higgs boson explains the difference in masses of the different phyisical entities.

    • CERN says:

      Hello Nikolaas,

      this might be a matter of taste. The Higgs field **IS** there and so are electric and magnetic fields. The proof of it: take a chaged object or a magnet and you will feel their presence. So they are physical entities, and so is the Higgs field.

      One massless particle is the photon. And this is why it is the only known article that can travel at the speed of light; neutrinos are very light but not massless and travel slower.

      As for why the Higgs gives mass to the W and Z bosons but not to the photon, this is just the way it is It comes out of the equations of the Brout-Engler-Higgs mechanism that way. One starts with all massless particles, but by rearranging everything after adding a few Goldstone boosns, W and Z get a mass, and the photon remains massless. This is just a mathematical facts that reflects the situation that the photon does not interact with the Higgs field.

      Cheers, Pauline

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