Neutron to Proton Change: The Role of the W Boson

[jal]

An isolated neutron, changes to a proton within 15 minutes by one of the down quarks becoming an up quark.
Why does the change need to include the intermediate step of the heavy W boson?
Where did all the W boson mass come from and where did it go?
jal

 

[jtbell]

Why does the change need to include the intermediate step of the heavy W boson?

In our current Standard Model of particle physics, all weak interactions involve W or Z bosons, all electromagnetic interactions involve photons, and all strong interactions involve gluons. Have you seen Feynman diagrams? The rules for constructing those diagrams follow from the interaction Lagrangians that we use as the starting point for those theories, and the "vertices" of those diagrams all include one of the "gauge bosons:" W, Z, photon or gluon depending on the nature of the interaction.

Where did all the W boson mass come from and where did it go?

In a process like neutron decay, the W is virtual. Virtual particles don't have to satisfy the relationship E^2 = (pc)^2 + (mc^2)^2. In particle physics jargon, we say that they are "off the mass shell."

Or, you can assume that momentum is conserved, and say that the energy of the W is different from what is given by the formula above, i.e. that energy is briefly not conserved, but it's OK because of the short time involved in the decay, and the Heisenberg Uncertainty Principle.

Either way, virtual particles are different from real ones.

 

[daschaich]

Why does the change need to include the intermediate step of the heavy W boson?

It basically has to do with the mathematical consistency of the theory -- if you don't include the intermediate W boson and just have the neutron decay directly to a proton, electron and antineutrino (or equivalently in terms of quarks), the theory is not renormalizable, it breaks down at high energies.

Historically, Enrico Fermi proposed just such a "four-fermion contact interaction" as a low-energy "effective theory" of neutron decay, recognizing that some new physics would have to appear around energies of tens to hundreds of GeV to keep the math well-behaved. The weak gauge bosons (W and Z) are precisely this effect. You can read a little more about Fermi's theory at http://en.wikipedia.org/wiki/Fermi's_interaction

 

[jal]

I read that.
http://en.wikipedia.org/wiki/Virtual_particles
virtual particles are an artefact of perturbation theory, and do not appear in a nonperturbative treatment. As such, their objective existence as "particles" is questionable;[citation needed] however, the term is useful in informal, casual conversation, or in rendering concepts into layman's terms.[citation needed]
---------

When I start a search I never know where I’ll end up.
Perturbation theory leaves me with more questions than answers.
The nonperturbative treatments might give better answers.
This of course means that we need to figure out how confinement works.
For instance, I read the following interesting approaches.
The W boson is indirectly addressed.
If you can guide me, I’d like to read different approaches.
http://www.phas.ubc.ca/php/directory/research/fac-1p.phtml?entnum=200
Ariel Zhitnitsky
Latest paper
http://arxiv.org/abs/0806.1736v1
Phase Transitions, theta Behavior and Instantons in QCD and its Holographic Model
Authors: Andrei Parnachev, Ariel Zhitnitsky
(Submitted on 10 Jun 2008)
To elucidate the physics of the transition we consider a model where the chiral condensate does not vanish in the deconfining phase. The holographic model of QCD is a good example where this phenomenon occurs. On the field theoretic side this can be achieved by coupling fundamental matter to the hidden gauge group whose dynamically generated energy scale is higher than that of QCD.
========
Stanley J. Brodsky, Robert Shrock also have an interesting paper.
http://arxiv.org/abs/0806.1535
Maximum Wavelength of Confined Quarks and Gluons and Properties of Quantum Chromodynamics
Authors: Stanley J. Brodsky, Robert Shrock
(Submitted on 9 Jun 2008)
=========

 

[humanino]

For starters :
Some Frequently Asked Questions About Virtual Particles

Perturbation theory may or may not be a good approximation. Strictly speaking, Feynman diagrams are (space-time) topological equivalent classes of terms, in an expansion of a scattering amplitude in momentum space. They should certainly not be considered as real processes in general. However, we do speak in those terms on a daily basis, and some might forget the grain of salt they should be taken with.

In another thread, we discussed about how real can a particle be if its mass is comparable to its width for instance.

 

[jal]

I found the following interesting experimental proposals
Experiments on decay of the free neutrons
http://arxiv.org/abs/hep-ex/0602047
An experiment for the measurement of the bound-beta decay of the free neutron
Authors: W. Schott, G.Dollinger, T. Faestermann, J. Friedrich, F.J.Hartmann, R.Hertenberger, N.Kaiser, A. R. Müller, S.Paul, A.Ulrich
(Submitted on 26 Feb 2006)
=====
http://arxiv.org/abs/0704.2365
Neutron Beta Decay: Status and Future of the Asymmetry Measurement
Authors: Takeyasu M. Ito
(Submitted on 18 Apr 2007)
=======
http://arxiv.org/abs/0709.4440
A clean, bright, and versatile source of neutron decay products
Authors: D. Dubbers, H. Abele, S. Baessler, B. Maerkisch, M. Schumann, T. Soldner, O. Zimmer
(Submitted on 27 Sep 2007)
====
Chemical equilibrium among these particles is established by weak interactions such as neutron beta decay (n → p + e− + ¯ν) and electron capture (e− + p → n + ν), and the nuclear symmetry energy plays an important role in determining the relative abundance of neutrons and protons. (The W boson is understood to be involved)

Has there been an experiment with a free proton capturing an electron and observing that the proton changed to a neutron?
--------

 

[dkv]

Is W boson real? If they are not real and do not need to satisfy the energy-momentum conservation then why are they heavy ?

 

[humanino]

Has there been an experiment with a free proton capturing an electron and observing that the proton changed to a neutron?

I have plenty of ep\rightarrow en\pi^{+} on tape if you want
ep\rightarrow n\nu_{e} is much more challenging, because basically your final state is almost invisible ! It has been done, however with little statistics and poor accuracy compared to the hadronic reaction.

 

[daschaich]

Is W boson real? If they are not real and do not need to satisfy the energy-momentum conservation then why are they heavy ?

In order to get a real ("on-mass-shell") W boson, one has to have enough energy -- this was first done experimentally in 1983 at the CERN SPS accelerator. Neutrons, with roughly one-eightieth the W boson's mass, do not have enough energy to emit a real W boson; the W bosons we say are involved in beta decay must be virtual, off-mass-shell.

 

[jtbell]

Is W boson real? If they are not real and do not need to satisfy the energy-momentum conservation then why are they heavy ?

Just like photons can be either real or virtual, so can W (and Z) bosons also be real or virtual. Real ones do satisfy the mass/energy/momentum relationship. Producing real W's and Z's takes a lot of energy because they're so massive. It was first done at CERN in the mid 1980s, and led to a Nobel Prize.

Also because of the large masses, the probability of producing virtual W's or Z's is very small. If you like to think in terms of temporary violation of energy conservation under cover of the uncertainty principle, larger violations are less likely than smaller ones. That's one reason why the weak interaction is so weak.

 

[dkv]

I thought virtual particles are a matter of convenience for the actual physical processes.
If something can be achieved using virtual particles then do we need real particles?

 

[daschaich]

I thought virtual particles are a matter of convenience for the actual physical processes.
If something can be achieved using virtual particles then do we need real particles?

Virtual particles and real particles are the same particles -- but the latter obey relativistic mass-energy relations (are "on mass shell") while the former need not thanks to the time-energy uncertainty relation. Have you read the FAQ humanino posted? It discusses this.

 

[jal]

I have plenty of ep\rightarrow en\pi^{+}
on tape if you want
ep\rightarrow n\nu_{e}
is much more challenging, because basically your final state is almost invisible ! It has been done, however with little statistics and poor accuracy compared to the hadronic reaction.

Can you suggest a paper for me to read?
I assume that it would have something to do with holding a proton in a “trap” and hitting it with an electron.
--------
A little bit of amateur humor.
It takes less than 15 minutes for a “Dr. W boson” to find any free neutron, reach inside, do a sex change, and give birth to an electron.
Where did all the “Heavy Dr.W bosons” come from and where did they go?
--------
http://en.wikipedia.org/wiki/Beta_particle
http://en.wikipedia.org/wiki/Beta_decay
http://en.wikipedia.org/wiki/W_boson

 

[daschaich]

...
I assume that it would have something to do with holding a proton in a “trap” and hitting it with an electron.
...

More likely an electron-proton collider such as HERA (http://en.wikipedia.org/wiki/Hadron_Elektron_Ring_Anlage). I would also expect accelerating electron beams onto liquid hydrogen targets to work as well -- the atomic forces within and between the hydrogen atoms should be negligible. I believe this was a common accelerator-detector setup back in the '50s-'60s, though that's well before my time and well outside my area of expertise.

 

[dkv]

There are two types of W bosons .. real and virtual. Real ones can be identified. Virtuals can't be identified. However as you say both W are the same..
Virtual W bosons do not exist physically where real ones exist physically. How can we say that there are virtual w bosons??
I am all confused.. I read the faq but it is not clear that whether W virtual bosons are real or not ?

 

[daschaich]

There are two types of W bosons .. real and virtual. Real ones can be identified. Virtuals can't be identified. However as you say both W are the same..
Virtual W bosons do not exist physically where real ones exist physically. How can we say that there are virtual w bosons??
I am all confused.. I read the faq but it is not clear that whether W virtual bosons are real or not ?

Real W bosons are those W bosons with energy and momentum obeying the relativistic formula E^2 = m^2 + p^2 (with c = 1). We say these W bosons are on-mass-shell or just on-shell. Virtual bosons are those W bosons that are off-shell, with energy and momentum E^2 -p^2 \ne m^2.

At first glance this looks like it violates relativity and therefore can't physically exist. The catch is that E can only be known up to an irreducible uncertainty \Delta E given by the uncertainty relation \Delta E \Delta t \gtrsim \hbar, where \Delta t is the amount of time we spend measuring E. If virtual particles are only around for a short time, their energy becomes indeterminate.

Perhaps instead of existence or reality it would be better to talk about observability -- only W bosons obeying relativity (that is, on-shell) can be observed. But the effects of virtual W bosons can still be predicted using the actual math that we're trying to describe in terms of "virtual particles" and other hand-wavy phrases, and tested in experiments. And as you're probably aware, agreement between predictions and experiments are spectacular.

 

[dkv]

Ok. There are mathematical evidences for fractional charges (see Quantum Hall effect) but these pseudo charges... They do not exist for real but they help to solve the equations more efficiently...
Based on such "indirect " evidences we can not conclude existence of "Virtual" particles.
The underlying physics is different... Virtual particles offer mathematical convinience..
I think it is better to call the virtual particles as pseudo particles.

 

[daschaich]

We know that W bosons exist. They can be and have been directly observed experimentally when on shell. Quantum field theory (the underlying physics) involves both on-shell and off-shell states in scattering and decay processes.

You can use the term "pseudo particle" instead of "virtual particle" for off-shell particles if you like. That is a matter of language which doesn't affect the physics. However, it's usually more convenient to use the most common language, whether or not you think it is the most appropriate. That way others will have an easier time understanding what you're talking about.

 

[humanino]

I think it is better to call the virtual particles as pseudo particles.

If I may repeat what daschaich said, it would be a terrible idea to choose to use one's own preferred language. I could give you a list of all the scientific terms which upset me, and some of them are really confusing, but we must use the same terms as in the books for otherwise students will be lost. Change to the official terms for well-grounded reasons, be them philosophical, must be submitted, reviewed and discussed by some sort of authority. In France, I guess that would be the Academy. I do not know who that would be in the U.S., maybe the N.S.F. ?

 

[jal]

I've looked over the "Similar Threads for: Neutron to Proton" and I do not find any repetitition of info
"The quest continues ..."
jal

 

[dkv]

That makes it much easier to understand.
Generally it is said that Virtual W bosons are Virtual because their Mass is Less Than Actual W Boson when infact this pseudo-Boson never existed...
When the Virtual Boson decays into electron and anti-electron neutrino... in a way we can say that the underlying process generates electron and anti-electron neutrino... simply because Neutron decays into Proton...
The same decay of proton into neutron can be understood in terms of real Bosons...
But the question is why do we need Real Bosons if Pseudo Boson can do the job?
As far nomenclature is concerned Biology is far superior at classifying its characteristics.
May be it is too early to need a revision but some day we may need to revise.

 

[daschaich]

...Change to the official terms for well-grounded reasons, be them philosophical, must be submitted, reviewed and discussed by some sort of authority. In France, I guess that would be the Academy. I do not know who that would be in the U.S., maybe the N.S.F. ?

Well, that might be nice in an ideal world, but what we really have are not official terms so much as common or accepted terms, without any authority behind them besides popular usage/acceptance. So everything's rather anarchic... an example that comes to mind is the debate over "rest mass" vs. "relativistic mass", which was mainly carried out in articles like "The Concept of Mass" by Lev Okun (Physics Today, June 1989, http://www.physicstoday.org/vol-42/iss-6/vol42no6p31_36.pdf ). Groups like the NSF, APS, or AAPT might be convinced to take a side in such a debate, though I wouldn't expect that to happen until the question had been largely resolved in practice.

As long as the meaning is clear, I try not to get too excited about the terminology, but it is true that poor choice of terminology can obscure meaning. I remember reading in Crease and Mann's Second Creation that Julian Schwinger made a number of significant discoveries that went unnoticed because he used his own personal notations and terminologies that few others were familiar with.

In the case at hand, I feel "on-shell" and "off-shell" are superior to "real" or "virtual" or "pseudo", or even "observable", both because they contain more information, and because they're what physicists (in my experience) actually use in practice -- when not trying to explain things in "layman's terms".

 

[daschaich]

...
Generally it is said that Virtual W bosons are Virtual because their Mass is Less Than Actual W Boson when infact this pseudo-Boson never existed...

Terminology again: what do you mean by "existed"? It has real, physical, predicable and observable effects, which satisfies me.

...
When the Virtual Boson decays into electron and anti-electron neutrino... in a way we can say that the underlying process generates electron and anti-electron neutrino... simply because Neutron decays into Proton...

If you go back to the first few posts in this thread, you'll see that this is backwards -- W boson decay to an election and antineutrino is the underlying process in n \to p + e + \overline\nu.

...
But the question is why do we need Real Bosons if Pseudo Boson can do the job?

Again, they're the same particle, "pseudo" or "virtual" or whatever just means it's off-shell, E^2 - p^2 \ne m^2. You could say we "need" them to regularize Fermi's four-point interaction, which blows up in the absence of such intermediate particles. You could also say we "need" them because they've been physically proven to exist -- the same reason we "need" muons, pions, photons and neutrinos, etc.

One point jtbell made that may be worth raising again is that all particles can be on-shell or off-shell, photons, electrons, you-name-it. This is how quantum field theory works (and work it does).

 

[dkv]

There has been no direct evidences of W boson.
Its mass is calculated by applying conservation of energy and momentum.
When the Energy is not conserved it is called Virtual .. When Energy is conserved it is called real ...
Where is proof that real W Boson is actually real and not a special case of Virtual Boson?

 

[daschaich]

There has been no direct evidences of W boson.

There most certainly has! Won the 1984 Nobel prize in physics -- see
http://en.wikipedia.org/wiki/W_boson#Discovery
http://nobelprize.org/nobel_prizes/physics/laureates/1984/

 

[daschaich]

But continuing on beyond my initial outburst...

...
Where is proof that real W Boson is actually real and not a special case of Virtual Boson?

This makes it sound like the identity of a particle depends on its momentum -- why should that be the case?

 

[dkv]

The article says unambiguous "signs" of were seen.The actual W particle is said to decay in 10^-35 seconds.. I repeat the mass was inferred from the dynamics. It was not seen directly.
Actually we know that Virtual Bosons violate the mass energy conservation because the same physical interaction doesn't satisfy the conservation principle..
(Note Neutrino can not be observed directly and they do not leave any trace... )

Moreover there is a huge uncertainity in the calculated mass (around 0.14%)...
Isnt that strange when QED claims to right upto 12th decimal place?

 

[dkv]

We tend to infer that if Energy and Momentum is not conserved then the particle must be virtual.Apparently it is assumed that the identity of particle depends on the conservation of energy and momentum... (not just on momentum)

 

[daschaich]

...
Moreover there is a huge uncertainity in the calculated mass (around 0.14%)...
Isnt that strange when QED claims to right upto 12th decimal place?

1) This isn't QED, so, no, that's not strange.

2) This is the first time in my life I have seen 0.14% (0.0014) referred to as "huge". I would be inclined to refer to 0.14% uncertainty as "absolutely incredibly accurate".

PS. The key word in the part of your post I replaced with ellipses is "unambiguous", as in "no ambiguity" that W and Z bosons exist.

 

[humanino]

Moreover there is a huge uncertainity in the calculated mass (around 0.14%)...

Look for you credibility : 0.14% is a respectable precision. It is actually about the borderline of what we would call "precision physics" in general, as opposed to exploratory physics where 1 to 10% accuracy is enough compared to orders of magnitudes.

Anyway, basically you are critisizing the standard electroweak model because energy-momentum conservation can be broken according to Heisenberg relations. However, those relations and the concept of a virtual particle encompasses much more than the W boson on one hand. On the other had, the standard electroweak model has more to it that the single W boson, and it passes all the tests we can figure out so far.