# Higgs boson for the uninitiated

1. ### jctyler

8
I usually visit this forum as a guest. I am not a physicist but these visits always teach me something and keep me somewhat informed of what goes on in the world of physics. But now I have a question which will probably seem stupid to you all. It is apparent that the Higgs boson plays a most prominent role in modern physics/research. Everybody I know has heard of it, yet nobody I know has a basic idea of what is. My question therefore:

I usually read that the Higgs boson is a massive particle but I have also read that it generates mass in other particles. Does this mean that the particle is massive and passes this mass onto other particles or does this mean that it is not massive itself but does something that gives other particles mass?

I am aware this may sound like a stupid question to all of you but this is truly something that confuses me and would appreciate a however simplistic reply.

2. ### chrispb

100
The mechanism by which the higgs gives other particles mass is related to the reason it has a mass itself. However, the higgs having a mass alone is insufficient to generate masses for other particles; the more crucial feature is that it develops a vacuum expectation value (a vev), meaning that the lowest energy configuration for the field is a nonzero constant throughout all of time and space. It's this feature that's responsible for giving other particles masses.

3. ### jctyler

8
Do I understand correctly from this that the Higgs boson has mass itself and that this mass is required for the function of the Higgs which is to generate mass for other particles? In other words, the Higgs is something of a mass generator mechanism?

4. ### epenguin

2,211
I started a thread on the question that was open to being serious and by about post 41 had begun to get some answers. https://www.physicsforums.com/showpost.php?p=3676434&postcount=41 We'll get more complete ones we can understand eventually.

I think we can understand that it has a mass itself because that would seem to be the point of the experiments - the mass corresponds to an energy which was more than could be created in collisions in any machine till now. The indications seem to change fast but I see they are giving 115-130 GeV for it which if I am not mistaken is of the order of the mass of an iodine or a barium atom. But don't take my statement as official.

5. ### blechman

779
Your "stupid" question is much more subtle and difficult than you realize!

Particle physicists believe (and with good reason!) that particles get mass via the HIGGS MECHANISM, a mathematical algorithm that allows particles to acquire nonzero masses. This is a somewhat complicated procedure involving something called "Spontaneous Symmetry Breaking". Let's hold that thought for a moment...

Now even though we believe that the "Higgs Mechanism" is at work, it is not clear how this mechanism is realized by nature. The "Higgs Boson" is ONE such way the Higgs Mechanism might be accomplished. It is not the only one (there are several competing models out there) and until it is found, we won't know. That's what we're trying to do at the LHC.

Now the Higgs boson can (does, if it exists!) also have mass. This is actually a very nontrivial point! We have **NO IDEA** where the Higgs boson mass comes from. In fact, the problem is even worse than it seems, since we particle physicists try to calculate the Higgs mass and we get that it is infinite! This problem is so immense it has been given a name, "The Hierarchy Problem".

Much of theoretical particle physics research in the last four decades has been dedicated to trying to find an explanation for WHY the Higgs mass is what it is. Up to this time, nobody has an answer. Hopefully, if the Higgs boson exists, the LHC will find it AND give us some hints as to what causes its mass to be what it is. If the Higgs boson does not exist, the LHC should be able to find out what causes the Higgs MECHANISM to work. But we have to run the experiment to find out...

Hope that helps!

6. ### SuperNova1

10

I noticed you mentioned there are other models which are competing, what other models are these? i would like to know of the other ideas and theories surrounding the higgs mechanism and would appreciate it if you explained more :D (simplified please)

7. ### jctyler

8
Thanks for the reassurance, I'm really out of my depth.

If I understand this correctly the Higgs mechanism is believed to produce the mass without which the particles that need mass could not be, that this mechanism relies on a mathematical algorithm and that the LHC tests the correctness of this algorithm? In other words, until that algorithm is proven correct, nobody knows for sure how mass is generated? And if the LHC proved the algorithm incorrect, this would destabilize the main theory?

This iI don't understand. Wherever I try to find answers to what bosons are it always says that in essence they are massless and have a certain spin which makes them different from those particles that have mass and have another spin. So this boson would be a freak?

Infinite mass? That is quite interesting. Does that not also imply that no mass is ever lost? First thing that comes to my mind is absolute recycling. This I could imagine to an extent based on real life situations. But in your field? Maybe if I could see mass in this context as energy in disguise I could imagine it as infinite or never lost but I am tripping over myself. Do physicists think of mass as infinite or is that part of what bugs them about the Higgs mechanism?

If the Higgs boson did not exist, wouldn't that disprove the concept of the Higgs mechanism?

Also, this makes me wonder: if the Higgs boson did not exist, one would not know what produces mass, wouldn't that then make it impossible to understand how mass gets to where it is supposed to get? Surely understanding the one must come before the other?

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Last edited: Dec 23, 2011
8. ### blechman

779
There are many ideas out there, mostly under the (rather unimaginative) name of "Higgsless Theories". The famous example is "Technicolor", which is an attempt to apply the confinement ideas from QCD to electroweak physics. All of these models have problems. Then again, so does the "Higgs Boson model"! So we'll just have to see what the LHC has to say.

That's what this forum is here for! Ask away!

Short answer: Yes. Long answer: as I hinted at, nobody really argues anymore that the Higgs MECHANISM is at work - this knowledge comes from many indirect measurements. If we are wrong, we would have seen evidence of failure in the last 30 years of measurements. But even though we know the MECHANISM is correct, we don't know if there is a Higgs BOSON, or if there is something else.

I don't think I'm following your question... Maybe if you rephrase it....

I think you are taking this too seriously. "Infinite mass" is nonsense! We calculate the Higgs mass using Quantum Field Theory (the mathematical language of particle physics) and we get an infinite answer. This means that we are missing something! This is different than any other particle (quarks, electrons, photons, etc): when we calculate the mass of other particles we get finite (or zero!) answers. So something goes wrong with the Higgs boson.

No, and no! Remember what I said: Higgs MECHANISM vs Higgs BOSON!! We are pretty sure the MECHANISM is correct. But if it's not the BOSON then it must be something else. What else? Only further study will tell us.

9. ### bapowell

1,835
We really shouldn't be calling it an algorithm -- it's not. It's a physical mechanism described as part of the theory. The presence of the Higgs field in the standard model is necessitated by the apparent need for gauge symmetry in the Standard Model (SM). (This is technical so I won't go into details on a first pass, but I'm happy to discuss more if you have questions.) Essentially, if the particles have masses at the level of their equations of motion (e.g., the Dirac equation for a massive particle is $-i\hbar \gamma^\mu \partial_\mu \psi + mc\psi = 0$, the 2nd term on the LHS is the mass term) then the theory does not exhibit the gauge symmetry. Why is the gauge symmetry important? Imposing gauge invariance on a free field (such as the free fermion described by the Dirac equation) generates interactions. One finds that in order for the Dirac equation to exhibit gauge invariance it must interact with forces. So, the simplest gauge symmetry (called U(1)), results in the coupling of charged fermions to the electromagnetic potential -- one ends up with quantum electrodynamics. Other gauge symmetries (SU(2), SU(3)) result in additional forces. But all this only works if there are no explicit mass terms in the equations! The Higgs mechanism is an elegant way of retaining the vitally important gauge symmetry while allowing for masses to be introduced into the theory. The Higgs mechanism makes use of a process called spontaneous symmetry breaking (which, admittedly, sounds confusing since it's supposed to help retain the symmetry, not break anything. In a way, it does both.)
Yes, bosons have integer spin (this is what makes them bosons.) They differ from fermions that have spin 1/2. The only bosons that have been seen in nature are the ones that mediate the fundamental forces (so-called gauge bosons for reasons that I briefly sketched above). The photon is one such boson that is in fact massless; the W and Zs of the weak force are actually massive (as a result of the Higgs mechanism). So having a mass does not make a boson a freak. The mass of the Higgs boson itself comes from the fact that it self-interacts, something that is possible because it is a spin zero particle. Additional mass comes from its interactions with other particles, more on this below...
It bugs them. The Higgs interacts with all massive particles in the SM. Quantum mechanically, these interactions occur through the exchange of gauge bosons. However, quantum mechanics tells us that we should sum over all possible such exchanges (think perturbation expansion if you are familiar). Each exchange contributes to the mass of the Higgs. If you naively count up all these contributions, it goes off to infinity. This is a problem. (The technical term for this process would be to say that the Higgs field receives radiative corrections from all the other particles in the theory.) So, something must stop this counting process, i.e. there needs to be a cutoff where we say, "OK, we're done counting." Of course, we should stop counting at the Planck scale, but this is still way too high. So there needs to be a much lower energy cutoff that protects the Higgs mass and keeps it relatively light. Nobody knows yet how this happens; supersymmetry is probably the best guess right now.
Excellent question. No! Spontaneous symmetry breaking, and the Higgs mechanism in particular (as concepts), are seen in condensed matter systems. For example, the Misner effect and ferromagnetism, below the Curie temperature are examples and consequences of spontaneous symmetry breaking. Although I have a feeling you're referring to the Higgs mechanism in particle physics. If there is no Higgs boson discovered at the LHC, we of course can't rule out the possible existence of the Higgs mechanism operating elsewhere at higher energies in the SM. However, it would complicate our understanding of electroweak processes.

10. ### jctyler

8
I guess in my world it would be said that the machine which produces mass has been found but that the engineers have not yet found the engine that drives the machine.

From what I just looked up to quote properly, bosons are usually considered massless and all 'observed bosons have integer spin'. By contrast the Higgs boson, although having integer spin, HAS mass and is NOT observed. Now, I would think that something that has no mass is harder to detect than something that has mass. But it is the contrary. The massless bosons which by definition would seem harder to see (to me) are observed whereas the massive Higgs boson which I would suppose is easier to see because it has mass has not been observed. To non-physicists that sounds like someone saying that he can see the sound but not the mountain. You understand why I am baffled?

Yes, that is an ugly problem. In my work, when I checked all my input and the result is still wrong, I usually start again by dissecting my initial assumption but that seems a bit difficult in your case.

Knowing that something happens but not knowing why it happens? Ever heard of John Bennet Lawes? He revolutionized farming by discovering that acid would turn mineral phosphates into highly efficient fertilizer but he didn't understand why. Only far later was it discovered that the acid is needed to activate the calcium phosphate. In this context: if the Higgs mechanism was physics' calcium phospate, you haven't found out yet what turns it on? Maybe the Higgs mechanism is triggered into mass production by its equivalent of Lawes' acid? And that might then not be a boson at all? Looks like you have your work all cut out.

11. ### blechman

779

"The massless bosons which by definition would seem harder to see..." -- Not at all! Why would you say that? And why "by definition"?! Particle physics doesn't really work like that. Your eyeball sees photons all the time, and photons are massless! I don't really see the contradiction here.

But you correctly put your finger on the problem that has troubled our field for decades!

That might not be a bad analogy either. Particle physicists know that the Higgs Mechanism is responsible for giving mass to the particles; but we don't know what the "acid" component is. That's why we have built the LHC - to try and answer that question!

12. ### bapowell

1,835
A photon has no mass, but that doesn't stop it from being readily detected, by your eye for example. It's all about interaction strength, not mass per se. The more massive the particle, the more difficult it is to produce, and also the more rapidly it decays. If the Higgs has a mass in the neighborhood of 125 GeV, it's certainly being produced every day at the LHC. But particle physicists have to filter out the Higgs decays from all the background decays. This is tough work and it takes lots and lots of decays before a statistically definitive signature begins to emerge. So there's really not a clear cut dependence on mass here. Also, all bosons, by definition, have integer spin -- it's not a matter of observation.

13. ### jctyler

8
Sorry, I misinterpreted the term. An algorithm in this context is then not so much a function as a description.

If I understand this halfways, the mechanism changes one thing into another by breaking up the original symmetry, then reassembling it so that there is a new symmetry? That's cubism. Makes sense to me.

I see how that would be a problem. But what keeps it from indiscriminate promiscuity? Is there a selection process ordering the exchange?

That is what I mean by the above.

Couldn't that be seen more philosophically? The mechanism is said to exist I'm told so that's half the job done. If the boson turns out not to exist, it only means that physicists will need a new approach. Maybe it's only a matter of being temporarily stuck in a dead end? Definitely one of the ultimate mind games.

14. ### jctyler

8
Yes, I became vaguely aware of my non-scientific interpretations a minute after I posted it. I did a similar error in chemistry when I initially believed that the metals must be the solid stuff and the non-metals the, well, non-solids. "Common sense" vs science.

15. ### bapowell

1,835
The interactions determine the exchanges. The problem is that the Higgs is, well, fairly promiscuous -- it couples to everything with mass.
Indeed. Some have said that a null result would indeed be an exciting time for particle physics.

16. ### jctyler

8
It was something along those lines that made me rethink. Unfortunately, I had posted. Thanks for the correction. Never too late to learn.

17. ### jctyler

8
Thank you all, especially blechman and bapowell, for having replied and explained so well something that I wanted to understand that essential bit better because it is so characteristic of our times.

Have a merry, festive season all!

18. ### bapowell

1,835
No problem! Happy to help. Happy Holidays to you as well!

19. ### blechman

779
And to you! Hope I helped to give you a flavor of the puzzle that faces us with the Higgs!

20. ### nikkkom

866
This particle has to be massive and unstable, otherwise we'd see it in experiments.

And it "does something" that gives other particles mass.

The subtle part is "how it can give other particles mass when it is not usually present?".

I think the simple (simplified?) answer is that Higgs field is special in the following way: all currently known fields go to zero when we move sufficiently far away from the corresponding charge carriers. In empty space very far away from anything, magnetic field is zero, electric field is zero, gravitational field is zero, and of course strong and weak nuclear fields are zero.

But Higgs field is not! It's lowest possible level is nonzero! That's why this field acts on particles even when there is no real (non-virtual) Higgs boson nearby: since field is non-zero, then any particle which is affected by Higgs field will interact with it via virtual Higgs bosons.