Why are W and Z bosons considered force carriers?

[Eelco]

This question has always somewhat perplexed me; if the W and Z boson are force carriers, then why arnt cannon balls force carriers too?

Ive read before that the line between force carrier and matter is somewhat arbitrary, but what makes the W and Z fall on the force-carrier side? Sure, they transmit forces, but so do cannon balls.

The more obvious interpretation seems to me (not knowing much at all about the subject, I should warn) that they are just massive particles, possibly composite ones such as in the Rishon model. Why isn't such an interpretation preferred? Is it a matter of this making things simpler only on a deep mathematical level, or is there some common-sense argument for this too?

 

[michael879]

W and Z bosons fall on the "force-carrier" side because they are the quantized "particles" of the weak fields. For example, if you were to treat the electroweak lagrangian classically, you wouldn't find any W and Z particles, you would find 3 massive gauge fields with charge/hypercharge/isospin that are produced by a more complicated SU(2) version of Maxwell's equations (or Proca's equations to be more exact, since they're massive). Quantum forces are a lot more complicated that just shooting cannon balls back and forth, and honestly, I think the exchange particle view is pretty misleading (the best example of this is when you try to use it to explain attraction).

While you can also treat the chiral fields in the standard model lagrangian classically, and get a classical theory of all fields, or a classical theory with chiral fields and boson matter, I believe the language is just a matter of convention. Classically we view fermions as matter, and photons as a force-carrier, so therefore quantumly, we treat bosons as the force carrying particles.

 

[Eelco]

Im not quite sure I am following.

When you say they are the quantized particles of the weak field, you mean we can treat them analogous to photons (quantized paritcles of the electric field), for instance. I would agree this gives some intuitive sense to considering them as force carriers.

Are you saying in your last paragraph that we could treat electrons or neutrinos for instance in the same way, or alternatively, treat photons as matter? That would indeed make the choice rather arbitrary.

So the convention is to draw the line based on spin statistics. But this doesn't seem to make much sense to me, absent strong evidence that the massive bosons are not simply composite particles (such as in the Rishon model), deriving their spin statistics from this fact.

The massive / lightspeed distinction seems the altogether more fundamental one to me; that is, from my indeed rather outsiderish perspective.

 

[Kevin_Axion]

All particles are quantizations of fields, fermions and bosons alike. Although fermions obey Fermi-Dirac Statistics (http://en.wikipedia.org/wiki/Fermi–Dirac_statistics) and Bosons obey Bose-Einstein Statistics (http://en.wikipedia.org/wiki/Bose–Einstein_statistics), essentially fermions don't occupy the same state (The Pauli Exclusion Principle (http://en.wikipedia.org/wiki/Pauli_Exclusion_Principle)) and Bosons tend to like coherent lasers.

 

[daschaich]

When you say they are the quantized particles of the weak field, you mean we can treat them analogous to photons (quantized paritcles of the electric field), for instance. I would agree this gives some intuitive sense to considering them as force carriers.

One of the crowning achievements of twentieth century particle physics was the discovery of a unified "electroweak" interaction. Weak and electric interactions are different aspects of this unified electroweak interaction. So the W and Z bosons are not just analogous to the photon, they are (orthogonal components of) the same "thing".

The fact that the W and Z are massive, while the photon is not, means that the full symmetry of this electroweak interaction is somehow "hidden". The generic way this happens was figured out in the 1960s; discovering the specifics of what actually occurs in nature is the primary goal of the LHC.

Are you saying in your last paragraph that we could treat electrons or neutrinos for instance in the same way, or alternatively, treat photons as matter? That would indeed make the choice rather arbitrary.

Think about what matter is: stuff is made of molecules are made of atoms are made of electrons, protons and neutrons. Electrons, protons and neutrons are all fermions! Of course, it was later discovered that protons and neutrons are composite systems, but this simply introduced another level of fermions in the form of quarks.

Meanwhile, photons are not bound in atoms, but are bouncing around and interacting with everything that has electric charge. The W and Z gauge bosons similarly interact with everything that has weak charge.

To summarize: Matter is made out of fermions, so fermions are sometimes called "matter particles". Gauge bosons interact with all particles that feel the corresponding force, so they're sometimes called "force carriers". Maybe this can be useful to try to describe particle physics without doing any math, but I strongly agree with michael879 that taking these nicknames too seriously can be confusing and misleading.

So the convention is to draw the line based on spin statistics. But this doesn't seem to make much sense to me, absent strong evidence that the massive bosons are not simply composite particles (such as in the Rishon model), deriving their spin statistics from this fact.

Just to elaborate on what Kevin_Axion hinted at, the fact that matter is made out of fermions at the level of nucleons and electrons is essential to the existence of life. If these were bosons, then all bosonic-electrons would roll down to the same, lowest-energy atomic orbital, and there would be no chemistry or molecules as we know (and are made out of) them.

The possibility of compositeness at much smaller scales is irrelevant to those considerations. The main question this "Rishon model" (which I hadn't heard of before) raised in my mind was what implications it would have for the unified electroweak interaction. I took a quick look at Harari's 1979 paper (http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-2310.pdf), and saw that he claims the model maintains spontaneously-broken electroweak gauge invariance. He's the expert on his model, so I'll trust him on this for the time being.

Just for the record, I should mention that experimental searches for this sort of elementary-particle substructure have been going on continuously for decades, finding zilch. To demand strong evidence against such models could sound a little churlish to some readers, given that there is (to my knowledge) absolutely no experimental evidence in favor of these proposals.

 

[genneth]

Actually, you certainly can use cannon balls to transmit forces. Unfortunately, because they're very heavy, the range of those forces are very small (something like 1/mass, in appropriate units). People should bear in mind that at the inter-nucleon level, mesons provide the mediating forces.

Anything can be used as a mediating particle, the key thing being the ability to have virtual processes which create and annihilate the particle, with off-shell momenta. The heavier the particle, the more unlikely this becomes, and the smaller the force.

We thus really care about light particles, preferably massless. There are two very good ways to get massless particles --- break a global symmetry (get Goldstone boson) or enforce a "gauge symmetry" (which is a terrible name; I can't think of a better one though...) In crystalline solids, the breaking of translational symmetry gives phonons, which do indeed mediate some very long-ranged interactions in solids, e.g. superconductivity, various photo-electric/polaron effects. In vacuum, the latter is often used, and we get photons, W+Z (with caveat below), gluons.

But, you might say, W+Z aren't massless! Yes, but that's because in electroweak theory, that "gauge symmetry is broken" (another terrible term; gauge symmetry is not a symmetry, and symmetries can't be broken anyway...) which gives them mass (look up Higgs mechanism). However, in high energy field, we still tend to call anything gauge symmetry originated "forces", because we remember that they can be massless, and so truly mediate some long ranged interactions, which are probably the most important (technically, the most important amongst first order interactions).

 

[Eelco]

One of the crowning achievements of twentieth century particle physics was the discovery of a unified "electroweak" interaction. Weak and electric interactions are different aspects of this unified electroweak interaction. So the W and Z bosons are not just analogous to the photon, they are (orthogonal components of) the same "thing".

The fact that the W and Z are massive, while the photon is not, means that the full symmetry of this electroweak interaction is somehow "hidden". The generic way this happens was figured out in the 1960s; discovering the specifics of what actually occurs in nature is the primary goal of the LHC.

Or that this symmetry doesn't quite exist. I am more than a little bothered that no one is willing to put down a number at which energy we will find a higgs boson; or I believe many people have, but they have all been wrong so far. I am more than a little skeptical we will ever find one; I am more inclined towards the view that this whole electroweak thing simply doesn't quite fly 100%. It must scratch the surface of truth somewhere considering it has given us the Z-boson, but we don't necessarily need electroweak theory to explain this.

Think about what matter is: stuff is made of molecules are made of atoms are made of electrons, protons and neutrons. Electrons, protons and neutrons are all fermions! Of course, it was later discovered that protons and neutrons are composite systems, but this simply introduced another level of fermions in the form of quarks.

Meanwhile, photons are not bound in atoms, but are bouncing around and interacting with everything that has electric charge. The W and Z gauge bosons similarly interact with everything that has weak charge.

To summarize: Matter is made out of fermions, so fermions are sometimes called "matter particles". Gauge bosons interact with all particles that feel the corresponding force, so they're sometimes called "force carriers". Maybe this can be useful to try to describe particle physics without doing any math, but I strongly agree with michael879 that taking these nicknames too seriously can be confusing and misleading.

That makes sense.

The possibility of compositeness at much smaller scales is irrelevant to those considerations. The main question this "Rishon model" (which I hadn't heard of before) raised in my mind was what implications it would have for the unified electroweak interaction. I took a quick look at Harari's 1979 paper (http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-2310.pdf), and saw that he claims the model maintains spontaneously-broken electroweak gauge invariance. He's the expert on his model, so I'll trust him on this for the time being.

Just for the record, I should mention that experimental searches for this sort of elementary-particle substructure have been going on continuously for decades, finding zilch. To demand strong evidence against such models could sound a little churlish to some readers, given that there is (to my knowledge) absolutely no experimental evidence in favor of these proposals.

Exactly how these sub-particles would be bound, and with what spatial extent is a completely open question; all scattering experiments can tell us that there is no substructure in terms of the known forces.

Either way, I don't care so much about the rishon model in particular or its implications for leptons; i was just using it as an example: in general, has the W or Z been tested for substructure? It seems like that would be well out of the range of our experimental fu, no?

 

[daschaich]

Or that this symmetry doesn't quite exist. I am more than a little bothered that no one is willing to put down a number at which energy we will find a higgs boson; or I believe many people have, but they have all been wrong so far. I am more than a little skeptical we will ever find one; I am more inclined towards the view that this whole electroweak thing simply doesn't quite fly 100%. It must scratch the surface of truth somewhere considering it has given us the Z-boson, but we don't necessarily need electroweak theory to explain this.

The Higgs boson is completely irrelevant! It is but one of many possibilities that could occur in nature. The Higgs boson could exist, or not, without consequence for the generic "spontaneously-broken electroweak gauge symmetry". (As genneth points out, this is terrible jargon on a number of levels. Chris Quigg has convinced me to try to talk about "hiding" rather than "breaking" gauge invariance, but the latter is far more common.) Discovering the specifics of how the full symmetry of the electroweak interaction is actually hidden in nature is the primary goal of the LHC.

The electroweak theory itself has been precisely tested to about one part in 10000, by a wide array of experiments over the course of decades. It is a law of nature in the same sense as Newton's law of universal gravitation: not necessarily perfectly correct at all length scales and at all energies, but a fully accurate description of nature within its region of validity (which so far encompasses all we are able to study experimentally).

Exactly how these sub-particles would be bound, and with what spatial extent is a completely open question; all scattering experiments can tell us that there is no substructure in terms of the known forces.

Either way, I don't care so much about the rishon model in particular or its implications for leptons; i was just using it as an example: in general, has the W or Z been tested for substructure? It seems like that would be well out of the range of our experimental fu, no?

I'm not quite sure what you mean by "substructure in terms of the known forces". Sub-particle constituents of (currently-)elementary particles can be bound together by any means necessary, but because the particles themselves possesses given quantum numbers, at least some of their constituents must carry the corresponding charges.

Scattering experiments, precision measurements, and searches for a "fifth force" can thus probe substructure regardless of the physics responsible for the substructure itself. And they've done so without finding anything down to a level of about 10^-17 m. The LHC should move this down to about 10^-18 m -- the "nanonanoscale".

A quick Google search for Z boson composite gave me
http://arxiv.org/abs/hep-ph/9904263
http://arxiv.org/abs/0907.0906
http://arxiv.org/abs/1006.3322
and many other articles, mostly irrelevant. The second one especially looks like it has a good discussion of current constraints on W and Z substructure. (And I just realized the third has been sitting in one of my "to-read" piles for months!)

Most research of this sort has focused on the possibility that the W and Z bosons "eat" composite degrees of freedom (as opposed to components of an elementary Higgs doublet) to become massive in the process of electroweak symmetry breaking. These scenarios run the gamut from models with composite Higgs bosons, to "techincolor" models in which there's (typically) no Higgs boson at all; to models that play all sorts of tricks with extra dimensions (and also tend not to need any Higgs bosons).

 

[Eelco]

The Higgs boson is completely irrelevant! It is but one of many possibilities that could occur in nature. The Higgs boson could exist, or not, without consequence for the generic "spontaneously-broken electroweak gauge symmetry". (As genneth points out, this is terrible jargon on a number of levels. Chris Quigg has convinced me to try to talk about "hiding" rather than "breaking" gauge invariance, but the latter is far more common.) Discovering the specifics of how the full symmetry of the electroweak interaction is actually hidden in nature is the primary goal of the LHC.

Let me get this straight: we havnt actually ever seen electroweak symmetry, the unification energy being far out of our experimental reach; yet somehow I am supposed to be convinced that it exists, yet is hidden? Would you excuse me if i said that sounds fishy?

The electroweak theory itself has been precisely tested to about one part in 10000, by a wide array of experiments over the course of decades. It is a law of nature in the same sense as Newton's law of universal gravitation: not necessarily perfectly correct at all length scales and at all energies, but a fully accurate description of nature within its region of validity (which so far encompasses all we are able to study experimentally).

I don't doubt the electrical and weak force exist and can be accurately described using the photon and W and Z; its the part where describing them as aspects of one unifying force supposedly adds any explanatory power where I get lost.

I'm not quite sure what you mean by "substructure in terms of the known forces". Sub-particle constituents of (currently-)elementary particles can be bound together by any means necessary, but because the particles themselves possesses given quantum numbers, at least some of their constituents must carry the corresponding charges.

I mean that a substructure in terms of the known forces would have a theoretically predictable spatial arrangement/seperation, which would be liable to falsification. Given that we have as little evidence that Rishons exists as we have about the mechanism that binds them together, we cannot predict how they would be held together, and it is not impossible to presume they are bound at smaller scales than our current experiments can probe.

Scattering experiments, precision measurements, and searches for a "fifth force" can thus probe substructure regardless of the physics responsible for the substructure itself. And they've done so without finding anything down to a level of about 10^-17 m. The LHC should move this down to about 10^-18 m -- the "nanonanoscale".

I know.

 

[Vanadium 50]

Let me get this straight: we havnt actually ever seen electroweak symmetry, the unification energy being far out of our experimental reach;

Where did you get that?

Here is a "picture of unification" - i.e. that the curves become parallel around 10⁴.

[PLAIN]http://www.sps.ch/typo3temp/pics/3c6f69d170.jpg

 

[daschaich]

Let me get this straight: we havnt actually ever seen electroweak symmetry, the unification energy being far out of our experimental reach; yet somehow I am supposed to be convinced that it exists, yet is hidden? Would you excuse me if i said that sounds fishy?

No, I wouldn't. When I said, "The electroweak theory itself has been precisely tested to about one part in 10000, by a wide array of experiments over the course of decades", well, that is what it means to "see" something in physics. Vanadium 50 has kindly posted just one of the many pieces of experimental evidence I referred to. You yourself mentioned some others: the existence of the weak neutral current and its manifestation as a "force" "carried" by the Z boson (with precisely predictable couplings to weakly-charged particles).

I don't doubt the electrical and weak force exist and can be accurately described using the photon and W and Z; its the part where describing them as aspects of one unifying force supposedly adds any explanatory power where I get lost.

This is where the W and Z bosons come from, and how their properties and interactions are predicted (up to a small number of parameters that need to be measured experimentally). They aren't just randomly tossed into the mix, they're predictions of the electroweak theory.

I mean that a substructure in terms of the known forces would have a theoretically predictable spatial arrangement/seperation, which would be liable to falsification. Given that we have as little evidence that Rishons exists as we have about the mechanism that binds them together, we cannot predict how they would be held together, and it is not impossible to presume they are bound at smaller scales than our current experiments can probe.

Of course you can always claim that anything so-far-undetected occurs only at so-far-undetectable energy levels or length scales. However, the only confining "known force" is QCD, and it has an intrinsic length scale of about a femtometer, which has been thoroughly probed experimentally. Any substructure of elementary particles must be due to "new physics", but this doesn't weaken the power of experiments to search for and constrain it.

 

[michael879]

Just to clarify what I was saying about the labeling convention being arbitrary, it really is. The reason we classify fermions as "matter" and bosons as "forces" is because all of the stable, massive particles are fermions (up quarks, down quarks, electrons, and neutrinos), and all of the stable massless particles that appear to mediate forces between these fermions are bosons (photons and gluons). Since all of the remaining fermions are just later generations of the stable ones, its natural to assign the label "matter" to them too. Likewise, since the weak force is so closely related to the E&M force (and really the same thing), the W and Z bosons are labeled force carriers.

However, boson's can exchange virtual fermions and you could treat these interactions as bosons interacting through some fermion force. Also in supersymmetric theories (this is the extent of my current knowledge on them), there is a corresponding boson field for each fermion field in the SM, and a fermion field for each boson field in the SM. In these theories there is actually a deep symmetry between bosons and fermions, and they are just part of the same thing (I believe the symmetry is spontaneously broken similarly to in electroweak theory). The problem is really in classical physics, where a distinction is made. The labeling of quantum fields is just to provide a better analogue.

 

[bcrowell]

I think the exchange particle view is pretty misleading (the best example of this is when you try to use it to explain attraction).

I've seen it used to explain attraction as well. A fermion just has to absorb a boson and reemit it in the forward direction with a higher energy, causing itself to recoil in the direction from which the boson came.

 

[bcrowell]

I would say that to some extent calling bosons "force carriers" is an arbitrary and possibly oversimplified convention, probably arising simply from popular science writers' need for a term that would be more understandable to laypeople than "bosons." "Bosons" also sounds funny to most laypeople, like "bozos" or something. I don't think it's a coincidence that you hear the Higgs referred to in press releases these days as the "Higgs particle" rather than the "Higgs boson." Taxpayers paying for something as expensive as the LHC don't want to hear that the main payoff will be something with a silly sounding name.

However, I think there is at least one reasonable fundamental justification for the "force carrier" tag. If you start off with a theory of noninteracting fermions and try to add gauge symmetry, the theory loses its mathematical self-consistency. The only way to fix it is by adding a boson field that describes interactions between the fermions. There is a nontechnical explanation here: http://en.wikipedia.org/wiki/Introduction_to_gauge_theory#Gauge_bosons