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.

 

[humanino]

I would be inclined to refer to 0.14% uncertainty as "absolutely incredibly accurate".

Me too, but I tend to be careful with my bias since I work in non-perturbative QCD where 3 = \infty (at the amplitude level) but \pi=1 (although \pi^{2}=10 to compensate)

 

[dkv]

0.14% of W boson is huge uncertainity ...(around 60 Mev)
W Boson is as heavy as atom.
What is the Electron Mass in MeV?
0.511 MeV
The uncertainity is roughly 120 times the mass of an electron..
Electron and Positron can annihilate to yield 1.022 MeV.

Another constant which varies very little is G (the gravitational constant) But when seen in the context of Earth it produces a high Uncertainity in mass.
(percentages just don't add up)

 

[humanino]

The uncertainity is roughly 120 times the mass of an electron..

How many masses of the electron is the uncertainty on the mass of the Sun ?

 

[jal]

Note: for the not so sophisticated readers:
CarlB has a informative page on the process of neutron to proton.
He presents it in the context of neutrino oscillation.
http://carlbrannen.wordpress.com/2008/06/21/neutrino-oscillation-or-interference/

 

[malawi_glenn]

How many masses of the electron is the uncertainty on the mass of the Sun ?

Good point hehe

I can't see what this dkv guy is trying to arguing for by stating that 0.14% accuracy on W-mass is a big problem? What is the uncertainty on tau- and muon mass? Is there an a priori reason why we should measure mass uncertainty in terms of electron masses?

The elekroweak model has done so many predictions, that have been verified to a great extent, so great that it has been rewarded a Nobel prize.

 

[jal]

My search for the understanding of neutrons converting to protons, which is neutron beta decay (n → p + e− + ¯ν) and the reverse, electron capture (e− + p → n + ν), has lead me to Ultra-cold neutrons (UCN), slow neutrons experiments.
The following review paper highlights the cutting edge.
There is a lot that I did not understand in the paper.
Now … I have more questions
I assume that this paper is all “old news” for people like CarlB
http://arxiv.org/abs/nucl-ex/0612022
Experiments in Fundamental Neutron Physics
Authors: J. S. Nico, W. M. Snow
(Submitted on 20 Dec 2006)
Abstract: Experiments using slow neutrons address a growing range of scientific issues spanning nuclear physics, particle physics, astrophysics, and cosmology. The field of fundamental physics using neutrons has experienced a significant increase in activity over the last two decades. This review summarizes some of the recent developments in the field and outlines some of the prospects for future research.
=======
p. 11 The substantial difference between the neutron lifetime of PDG average and that of Serebrov et al. is not understood. It is essential to resolve the disagreement.
ref.
http://arxiv.org/abs/nucl-ex/0702009
Neutron lifetime measurements using gravitationally trapped ultracold neutrons
Authors: A. P. Serebrov, V. E. Varlamov, A. G. Kharitonov, A. K. Fomin, Yu. N. Pokotilovski, P. Geltenbort, I. A. Krasnoschekova, M. S. Lasakov, R. R. Taldaev, A. V. Vassiljev, O. M. Zherebtsov
(Submitted on 6 Feb 2007 (v1), last revised 26 Jul 2007 (this version, v2))

Experiment using gravitationally trapped ultracold neutrons (UCNs) to measure the neutron lifetime is reviewed. The precise value of the neutron lifetime is of fundamental importance to particle physics and cosmology. In our experiment, the UCN storage time is brought closest ever to the neutron lifetime: the probability of the UCN loss from the trap was only 1% of that for neutron beta-decay. The neutron lifetime obtained, 878.5+/-0.7stat+/-0.3sys s, is the most accurate one to date.
=======
How does a lower lifetime affect other process/calculations?
jal

 

[Vanadium 50]

0.14% of W boson is huge uncertainity ...(around 60 Mev)

If you are going to post nonsense, at least be arithmetically correct about it. 60/80,400 = 0.075%, not 0.14%. A bonus would be to use the correct number of 29 MeV, not 60. That gets you to 0.036%.

There have been more W bosons observed (in no fewer than eight experiments) than the number of major league baseball games that have been played. Nobody doubts the existence of baseball.

 

[jal]

I like to phrase my questions so that an amateur can understand?
Is neutron beta decay (n → p + e− + ¯ν), happening faster, ( the W boson doing its job) lowering the life time of the free neutrons, when the neutrons are gathered together as a gas in a container?
How many neutrons do you need to make a sufficient defence (phalanx) against the W boson?
------
http://arxiv.org/abs/0802.4029
The Nuclear Physics of Neutron Stars
Authors: J. Piekarewicz
(Submitted on 27 Feb 2008)
A neutron star is a gold mine for the study of the phase diagram of cold baryonic matter.
========

 

[malawi_glenn]

jal, other conditions such as temperature, presurre etc does not affect the life time of particles, same holds for nuclei. A U-235 nucleus has same lifetime as in gas form as in solid form. And so on.

However, neutrons bound inside nuclei does not decay in the same way as a free neutron.

 

[jal]

Hi malawi_glenn!
Your points seem to be the concensus.
I would like to read some experiments that could back up that point of view. Free neutrons seem to break that rule. They are theorized to be able to avoid the W boson transformation within 15 minutes and to stay stable. (Neutron stars)
Maybe there are still unknowns ( neutron skin?) in the nuclear data of neutrons that would affect "neutron star models".
-------
http://arxiv.org/abs/0805.1007
Two-fluid models of superfluid neutron star cores
Authors: N. Chamel
(Submitted on 7 May 2008)
A neutron star is mainly composed of three distinct regions: an outer crust, an inner crust characterized by the presence of a neutron ocean and a liquid core which might be solid in the deepest regions (Haensel, Potekhin & Yakovlev 2006). Microscopic calculations of dense nuclear matter suggest that the matter inside neutron stars is superfluid (Dean & Hjorth-Jensen 2003).
We consider a mixture of superfluid neutrons and superconducting protons at zero temperature, taking into account mutual entrainment effects.
With the nuclear models considered in this work, we have found that the neutron relativistic effective mass is even greater than the bare neutron mass in the liquid core of neutron stars.
We have constructed new relativistic mean field models that yield a much better agreement with nuclear data than those considered by Comer & Joynt (2003).

jal

 

[malawi_glenn]

Sorry I seems to have misunderstood your question.

The thing is that you can't bind neutrons toghether using the strong nuclear force (nucleon-nucleon force), since the only bound nucleon nucleon system is the deutron (proton + neutron) - and it has only ONE bound state! (no excitation spectra). So you can't have a neutron ball consisting of 300 neutrons, and so on.

In a neutron star, the force responsible is gravity. In ordinary quantum mechanical systems, we ignore gravity since it is so weak in comparison with the other 3forces (strong(colour), electromagnetic & weak nuclear force).

And since the mass of a bound neutron is less than a free neutron, this affects the lifetime of it. This is also the reason why free protons don't decay, while a proton 'inside' a proton-rich nucleus can 'decay' (Beta plus decay).

I am just a novice in Neutron Star (NS) physics, so i don't know if the neutrons in a NS are stable or undergo decay aswell.

Interresting article, (The Nuclear Physics of Neutron Stars, Authors: J. Piekarewicz) will read it tonight (have not much job to do now) but i prefer reading good ol textbooks in a subject before diving into articles that has not been published.

 

[jal]

So you can't have a neutron ball consisting of 300 neutrons, and so on.

You can have neutron rich nucleons.
I believe that you said that you were on a trip at CERN. Enjoy your stay at CERN.
If you bump into someone from "SIRIUS", they may be more informed and willing/open to a discussion over a beer.
Can I be your waiter?
http://www.sirius.ac.uk/
jal

 

[malawi_glenn]

jal, there exists a thing called "neutron dripline" in nuclear physics. You must have HUGE gravity (and preasure) just as in a NS to make such nuclei bind more neutrons, we can't do it in lab here on earth. (http://en.wikipedia.org/wiki/Neutron_drip_line )

(Yes I am on CERN now, one of the labs where occupied, so we couldn't make more optical fibres =( .. so we are a bit delayed.. I work with beam radiation monitoring group at CMS experiment)

I must check if Sirius is a good place... have never heard of it yet.

 

[jal]

When I start a search I never know where I'll end up.
As a result I found myself searching for the following "terms"
=====
http://arxiv.org/find/all/1/all:+AND+Holographic+QCD/0/1/0/all/0/1
Holographic and QCD
results 1 through 25 (of 160 total)
=======
http://arxiv.org/find/all/1/all:+AND+AdS%2FQCD+models/0/1/0/all/0/1
results 1 through 25 (of 47 total)
AdS/QCD models
=====
http://arxiv.org/abs/0806.3114
Holographic deconfinement temperature at finite densities
Authors: Kyung-il Kim, Youngman Kim, Su Houng Lee
(Submitted on 19 Jun 2008)
Dense matter is one of the most challenging problems of modern physics. Understanding
the properties of such matter is important for the physics of relativistic heavy-ion collisions and dense stellar objects such as neutron stars.
======
Understanding Confinement and Deconfinement seems to be the key to see "new physic".
Does anyone have a favorite paper to recommend?

 

[jal]

http://arxiv.org/abs/0805.3491v1
S-pairing in neutron matter. I. Correlated Basis Function Theory
Authors: Adelchi Fabrocini, Stefano Fantoni, Alexey Yu. Illarionov, Kevin E. Schmidt
(Submitted on 22 May 2008)
Superfluidity in neutron matter has been a fascinating topic in many–body physics and astrophysics ever since Migdal [2] proposed the possibility of superfluid matter in neutron stars. In the inner crust of the star, 1S0 pairing in the low density neutron gas permeating the lattice of neutron rich nuclei may occur and peak at densities much lower than the empirical nuclear matter saturation density, ρ0 = 0.16 fm−3. A similar pairing may take place for the low concentration proton component in the highly asymmetrical nuclear matter in the star’s interior. At higher interior densities, neutrons may also pair in the anisotropic 3P2–3F2 partial wave. A realistic evaluation of the density regimes where superfluidity takes place and of the strength of the connected energy gaps is needed for a quantitative understanding of important features of neutron stars, such as the cooling rate[3, 4] and the post–glitch relaxation times [5, 6].
=======
http://arxiv.org/abs/0805.2513
Equation of state of superfluid neutron matter and the calculation of $^1S_0$ pairing gap
Authors: S. Gandolfi, A. Yu. Illarionov, S. Fantoni, F. Pederiva, K. E. Schmidt
(Submitted on 16 May 2008)
========
I assume that the next step, after pairing of neutron, would be a phase change to Quark-gluon liquid.

 

[Atakor]

Hello !

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.

Humanino, what do you mean by the bolded sentence ??
do you mean that feynman diagrams are just a mathemagical artifact ?

Can you give me the link for that thread too ?

Thanks !

 

[humanino]

Humanino, what do you mean by the bolded sentence ??

It was a while ago
I meant that Feynman diagrams are individual terms in an infinite series, and as such should be considered with caution. At first there is nothing more to my statement : if you are to calculate any observable, you'd better calculate it up to the next relevant order, that is up to when your calculation does not improve the result anymore.

do you mean that feynman diagrams are just a mathemagical artifact ?

No, that would be oversimplifying. Particle detected, I know what that is, and scattering matrix. Anything beyond is calculation.

Let me give you an example.

In the business of nucleon structure with an electromagnetic probe, people have assumed 1-photon exchange was a good enough approximation for a long time. And they extract particular form factors. Lately, people found a discrepancy in two ways to extract the ratio of the electric and magnetic form factors. Several possibilities exist to explain this, and it is not yet clear whether those possibilities are equivalent to one another. Anyway, the first thing people came up with is two-photon exchange, which people have written to be negligible (for such and such reason) for years (decades).

This is merely one example.

Another example, people use the optical theorem all the time, where we calculate a probability for a real process as an amplitude for an impossible process. Here again, you'd better not take the Feynman diagram as a real process !

My first example was rather extraordinary event, my second example is rather trivial. My remark applies possibly only at an elementary level. Let me go back to the tree level amplitude for electron-positron scattering. As explained by John Baez
Some frequently asked questions about virtual particles
attraction in the one photon exchange stems from the interference term with nothing happening ! So if you ask very basic questions about interpretations of Feynman diagrams, you sometimes should expect complicated answers.

 

[jal]

If I understood the following paper correctly, they are searching for the decay of a free neutron that does not involve the W boson.
Is that correct?
http://arxiv.org/abs/0710.1389
Comparison of two experiments on radiative neutron decay
Authors: R. U. Khafizov, S. V. Tolokonnikov, V. A. Solovei, M. R. Kolhidashvili
(Submitted on 6 Oct 2007)

First, the results from the first experiment aiming to observe the as yet undiscovered radiative decay mode of the free neutron are reported. Although the experiment could not be performed under ideal conditions, the data collected still allowed one to deduce the B.R. = (3.2±1.6) · 10-3 (99.7 % C.L.) for the branching ratio of radiative neutron decay in the gamma energy region greater than 35 keV. This value is in agreement with the theoretical prediction based on the standard model of weak interactions.
Secondly, the average B.R. value we obtained deviates from the standard model, but because of the presence of a significant error (50%) we cannot make any definite conclusions. Taking into account the fact that the experimental conditions can still be significantly optimized, an e-p coincidence count rate of 5-10 events per second is within reach. Together with the standard model prediction for the branching ratio of this decay mode, this would correspond to a triple e-p-γ coincidence rate of several events per 100 seconds. This can easily be observed with the current experimental set-up, which is now being optimized with a view to performing such an experiment. The aim of that experiment will then not only be to establish the existence of radiative neutron beta decay, but also to study B.R. in more detail. This, in turn, would allow to discover the deviation from standard electroweak theory. According to our estimates, we will be able to make more definite conclusions about deviation from the standard electroweak theory at the precision level of less than 10%.

 

[daschaich]

If I understood the following paper correctly, they are searching for the decay of a free neutron that does not involve the W boson.
Is that correct?
...

Doesn't look like it to me. They are searching for neutron \betadecays in which a photon is radiated,
<br /> n \to p + e + \overline{\nu} + \gamma.<br />
They compare their results with "the theoretical prediction based on the standard model of weak interactions", which of course involves the W boson.

 

[Atakor]

Thanks Humanino for the answer.

Still, I have a problem relative to the interpretation.
I know the use of feynman diagrams to calculate cross-sections and it is of course a matter of precision to stop at a certain level in the development.
But, why can't we say that the real process involves all the different possibilities ? or .. paths to use another term ?

thanks.