# Neutron to Proton

1. Jun 21, 2008

### 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

2. Jun 21, 2008

### Staff: Mentor

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.

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.

3. Jun 21, 2008

### daschaich

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

4. Jun 22, 2008

### jal

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]
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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)
=========

5. Jun 22, 2008

### humanino

For starters :

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.

6. Jun 23, 2008

### 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?
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7. Jun 23, 2008

### 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 ?

8. Jun 23, 2008

### humanino

I have plenty of $$ep\rightarrow en\pi^{+}$$ on tape if you want :tongue2:
$$ep\rightarrow n\nu_{e}$$ 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.

9. Jun 23, 2008

### daschaich

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.

10. Jun 23, 2008

### Staff: Mentor

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.

11. Jun 23, 2008

### 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?

12. Jun 23, 2008

### daschaich

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.

13. Jun 23, 2008

### jal

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.
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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?
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http://en.wikipedia.org/wiki/Beta_particle
http://en.wikipedia.org/wiki/Beta_decay
http://en.wikipedia.org/wiki/W_boson

14. Jun 23, 2008

### daschaich

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.

15. Jun 24, 2008

### dkv

There are two types of W bosons .. real and virtual. Real ones can be identified. Virtuals cant 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 ?

16. Jun 24, 2008

### daschaich

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.

17. Jun 24, 2008

### 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.

18. Jun 24, 2008

### 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.

19. Jun 24, 2008

### humanino

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. ?

20. Jun 24, 2008

### 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