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StevenO
2009-May-24, 01:16 AM
Why do particles and antiparticles with mass annihilate eachother into photons when they are electrons/anti-electrons, protons/anti-protons or neutrons/anti-neutrons, but not when they are quarks/anti-quarks?

robross
2009-May-24, 01:52 AM
Why do particles and antiparticles with mass annihilate eachother into photons when they are electrons/anti-electrons, protons/anti-protons or neutrons/anti-neutrons, but not when they are quarks/anti-quarks?

anti-quarks *do* annihilate with normal quarks, but only of the same kind of quark with all the quantum numbers reversed. So an anti-blue up quark would annihilate with a normal blue up quark, but not say, a red up quark.

We don't see free quarks in nature, so the only time we see quark/anti-quark annihilation is in particle accelerators.

Rob

chornedsnorkack
2009-May-24, 07:55 AM
anti-quarks *do* annihilate with normal quarks, but only of the same kind of quark with all the quantum numbers reversed. So an anti-blue up quark would annihilate with a normal blue up quark, but not say, a red up quark.


Right. If it is annihilation into photons, or gravitons, that have no charge and no colour. Even then, it is a slow/unlikely process because due to fine structure contact, electromagnetic interaction is slower than strong interaction.

Quark and antiquark can also annihilate into gluons, or into weak interaction vector bosons. Or vice versa.

Proton and antiproton annihilation is very similar to the annihilation of neutron and antiproton, or proton and antineutron. And the usual result is 4 or 5 pions - not 3.

Pions are white particles, so they cannot annihilate into gluons. Neutral pions annihilate rapidly into photons. Charged pions cannot, so they annihilate slowly into vector bosons (and onwards, usually into a muon and neutrino).

Fortis
2009-May-24, 04:26 PM
Why do particles and antiparticles with mass annihilate eachother into photons when they are electrons/anti-electrons, protons/anti-protons or neutrons/anti-neutrons, but not when they are quarks/anti-quarks?
If you consider what a proton (and anti-proton) consists of, you may want to reconsider your view that quarks and anti-quarks can't annihilate each other.

StevenO
2009-May-25, 02:29 PM
Quark and antiquark can also annihilate into gluons, or into weak interaction vector bosons. Or vice versa.

How could a proton/antiproton containing only up/down quarks with only a few MeV mass annihilate into a W/Z boson of tens of GeV mass?

I also have trouble understanding how W/Z bosons regulate forces inside a nucleus if a proton/neutron itself has <1GeV mass?

alainprice
2009-May-25, 02:41 PM
You can thank Heisenberg.

High masses can exist as virtual particles for very short times. Luckily it doesn't take long to get from one quark to another. Ergo, uncertainty allows these high mass particles to do their thing.

StevenO
2009-May-25, 02:58 PM
You can thank Heisenberg.

High masses can exist as virtual particles for very short times. Luckily it doesn't take long to get from one quark to another. Ergo, uncertainty allows these high mass particles to do their thing.
How can a force be regulated by uncertainty? And how can the uncertainty in the mass of the proton be a hundred times higher than it's intrinsic mass?

alainprice
2009-May-25, 03:02 PM
Uncertainty applies to pairs of variables. Energy and time is a famous one.

Don't forget that forces(save the gravity debate) are mediated by carrier particles.

It's not the mass of the proton that's uncertain. It's the mass of the particles inside of it mediating the color force(chromodynamics).

I think you should take a good look at virtual particles and uncertainty. It's even weirder than you might think.

Fortis
2009-May-25, 05:48 PM
How could a proton/antiproton containing only up/down quarks with only a few MeV mass annihilate into a W/Z boson of tens of GeV mass?
Are you referring to real W and Z, or virtual W and Z that then decay into other particles?

StevenO
2009-May-25, 08:22 PM
Uncertainty applies to pairs of variables. Energy and time is a famous one.

Don't forget that forces(save the gravity debate) are mediated by carrier particles.

It's not the mass of the proton that's uncertain. It's the mass of the particles inside of it mediating the color force(chromodynamics).

I think you should take a good look at virtual particles and uncertainty. It's even weirder than you might think.
I have trouble understanding how a proton of 1 GeV mass can contain particles of 80GeV mass...

According to this author, cited on Wikipedia, virtual particles are no more than a mathematical tool:




Virtual particles?

The calculational tool represented by Feynman diagrams suggests an often abused picture
according to which “real particles interact by exchanging virtual particles”. Many
physicists, especially nonexperts, take this picture literally, as something that really and
objectively happens in nature. In fact, I have never seen a popular text on particle physics
in which this picture was not presented as something that really happens. Therefore, this
picture of quantum interactions as processes in which virtual particles exchange is one of
the most abused myths, not only in quantum physics, but in physics in general. Indeed,
there is a consensus among experts for foundations of QFT that such a picture should
not be taken literally. The fundamental principles of quantum theory do not even contain
a notion of a “virtual” state. The notion of a “virtual particle” originates only from a
specific mathematical method of calculation, called perturbative expansion. In fact, perturbative
expansion represented by Feynman diagrams can be introduced even in classical
physics, but nobody attempts to verbalize these classical Feynman diagrams in terms of classical
“virtual” processes. So why such a verbalization is tolerated in quantum
physics? The main reason is the fact that the standard interpretation of quantum theory
does not offer a clear “canonical” ontological picture of the actual processes in
nature, but only provides the probabilities for the final results of measurement outcomes.
In the absence of such a “canonical” picture, physicists take the liberty to introduce various
auxiliary intuitive pictures that sometimes help them think about otherwise abstract
quantum formalism. Such auxiliary pictures, by themselves, are not a sin. However, a
potential problem occurs when one forgets why such a picture has been introduced in the
first place and starts to think on it too literally.

Fortis
2009-May-25, 08:55 PM
I have trouble understanding how a proton of 1 GeV mass can contain particles of 80GeV mass...

According to this author, cited on Wikipedia, virtual particles are no more than a mathematical tool:
That's not a bad description. Feynman diagrams appear when you carry out a perturbative expansion of the S-matrix. What is the S-matrix? It is the mathematical object that, in essence, provides the likelihood of a transition from one state to another. When you carry out this expansion you find that you have a sum of terms, each of which contains expressions that look like combinations of particle like objects (if you recall, in QM, particles with a well defined momentum look like complex plane waves.) This complex set of mathematical expressions can be written using a graphical description, where each of these particle-like objects is associated with a line in the Feynman diagram. These diagrams have really nice properties, for example at the vertices (i.e. where the "particles" meet) momentum and energy is conserved. The internal lines (i.e. those between the "in" state and the "out" state) do have some strange properties, however. The energies and momenta are typically not "on-shell". ("On-shell" means that the momentum and energies of the "particles" are not on the "mass-shell", i.e. they do not satisfy m0^2.c^4 = E^2-p^2.c^2.) These clearly cannot be "real" particles, and hence are described as "virtual particles".

Are virtual particles real? They clearly are not, in a classical sense. Is the concept of virtual particles useful? Absolutely. They provide an intuitive way to work with quantum field theory. Without the Feynman approach, we would probably not have made the great progress that we currently have.

Hope that helps. (As you can see, it isn't straightforward, but then again, the universe doesn't have to comply with our desire for it to behave the way we would like it.) :)

stutefish
2009-May-25, 08:58 PM
I have trouble understanding how a proton of 1 GeV mass can contain particles of 80GeV mass...
I wonder if maybe "contain" is the wrong concept to apply in this context.

Perhaps it might be more "accurate" to say that the actuality of a 1 GeV mass is by its nature accompanied by the possibility of one or more 80 GeV masses, and this possibility may be realized under certain conditions.


According to this author, cited on Wikipedia, virtual particles are no more than a mathematical tool:
I am guessing that you mean to imply by the use of "no more than" that this mathematical tool does not do anything useful.

If this is the case, you might want to consider what is implied by the word "tool" in this context...

StevenO
2009-May-25, 10:08 PM
I wonder if maybe "contain" is the wrong concept to apply in this context.

Perhaps it might be more "accurate" to say that the actuality of a 1 GeV mass is by its nature accompanied by the possibility of one or more 80 GeV masses, and this possibility may be realized under certain conditions.

I am guessing that you mean to imply by the use of "no more than" that this mathematical tool does not do anything useful.

If this is the case, you might want to consider what is implied by the word "tool" in this context...
I don't want to discredit the mathematical tool as useless. I understand that the expectation value of all possible realizations leads to the prediction of actual final states. The question is: are these "possible" 80 GeV masses as "real" as e.g. an electrical or gravitational field?

tusenfem
2009-May-26, 09:27 AM
StevenO, in the case of putting stuff together in particle physics, you have to take into account that the binding energy of the particle is negative, e.g. combine a hydrogen atom with a neutron to get a deuteron atom. You will find that the mass of the deuteron is smaller that the added mass of the proton and the neutron.

chornedsnorkack
2009-May-26, 09:52 AM
Are virtual photons real enough?

Because they are a way to express electromagnetic fields other than freely propagating electromagnetic waves.

cfgauss
2009-May-26, 11:41 AM
Are virtual particles real? They clearly are not, in a classical sense. Is the concept of virtual particles useful? Absolutely. They provide an intuitive way to work with quantum field theory. Without the Feynman approach, we would probably not have made the great progress that we currently have.


This is a little misleading. Elementary particles are not real in the classical sense either! But thinking of an interaction as the sum of all possible Feynman diagrams contributing to an interaction is a real "physical" way to think of what's going on--as much as thinking of the trajectory of a quantum particle as the sum over all allowed classical trajectories.


I have trouble understanding how a proton of 1 GeV mass can contain particles of 80GeV mass...

According to this author, cited on Wikipedia, virtual particles are no more than a mathematical tool:

Err, wikipedia is not the greatest resource in this area. The problem is that a real understanding of these ideas involves a great deal of math, and is very difficult to explain with words (which, you know, is the whole point at using all that math). And that's if the author is actually competent to talk about the subject, which, even with many textbooks is sadly, often not the case*. Also, as someone who's read enough of these types of things, when you start to see words like "ontological" they've jumped off the deep end and you need to pick up another book ;).

Anyway, this particular description is not good because it's like saying that thinking of e^x as 1 + x + x^2/2 + ... is bad because "originates only from a
specific mathematical method of calculation." In many cases, it's critically useful to view e^x like this! It's an easy way, for example, to define matrix exponents.

It's the same thing with feynman diagrams. They really should be taken literally in the same sense that you can take the terms of the series expansion for e^x "literally" as being e^x.

It is actually an interesting thing to note that every one of the founders of quantum mechanics would disagree with the statement about not taking the math "too literally." Much of the progress in quantum field theory was made by taking the math more literally!

Although it's certainly the case that you need to know what you're doing.

* In fact, frequently when I'm in our physics library here, I see at least one obviously crackpot textbook on the shelves next to real texts. You'd think we'd have someone review what books come into our library, but apparently not.

Edit:
That quote also mentions the same expansion can be done in classical mechanics, but isn't. The reason it isn't is because the kinds of questions we ask in classical mechanics are better suited to be answered in other ways. And, in fact, much technically simpler ways. If you want, though, you can get perfectly good answers out of this. And they have good, physical interpretations if you know what you're doing, too! There are even papers about it. I recall reading one as an undergrad, and, IIRC, they corresponded to an order-by-order expansion in something like energy or momentum transfer. So it still gives you a very useful thing, but one that can AFAIK, always be obtained in a much better way.

trinitree88
2009-May-26, 12:10 PM
While the Z-pole at ~ 80 Gev exists as a resonance, in particle/antiparticle annihilations, it is not the only form of the Z. Since the Z is neutral, it can be any particle/antiparticle pair,...electron/positron, neutrino/antineutrino, pion/antipion, kaon/antikaon...etc, so it can rear it's ugly head in surprising places.
A few of the putative "new physics" rumors from CERN, Fermilab, SLAC....have in all probability been transient Z's, leading to several authors removing their names from papers. pete see:http://www.bautforum.com/universe-today-story-comments/80836-forget-lhc-aging-tevatron-may-have-uncovered-some-new-physics.html

stutefish
2009-May-26, 05:34 PM
I don't want to discredit the mathematical tool as useless. I understand that the expectation value of all possible realizations leads to the prediction of actual final states. The question is: are these "possible" 80 GeV masses as "real" as e.g. an electrical or gravitational field?
Well, obviously they're not "real" in the same way as an electrical or gravitational field... But why does it matter what kind of "real" they are?

A Jenson Button (http://www.formula1.com/results/driver/2009/6.html) had the potential to win the Formula 1 Grand Prix de Monaco (http://www.formula1.com/results/season/2009/810/6633/) last week. And in fact he realized that potential, crossing the finish line ahead of all other racers on May 24th, 2009.

The potential was real; and the win, when it happened, was real. But of course we don't talk about the two realities in the same way; nor do we understand the reality of the potential in terms of the reality of the win--nobody gets a trophy for being the potential winner, after all.

Fortis
2009-May-26, 06:44 PM
This is a little misleading. Elementary particles are not real in the classical sense either! But thinking of an interaction as the sum of all possible Feynman diagrams contributing to an interaction is a real "physical" way to think of what's going on--as much as thinking of the trajectory of a quantum particle as the sum over all allowed classical trajectories.
A sloppy use of the word "classical". :( (This is what happens when you post late at night on an empty stomach, but that's another story...) It is the off-shell nature of virtual particles that I was using to differentiate them from what people think of as "real" particles.

cfgauss
2009-May-26, 08:39 PM
A sloppy use of the word "classical". :( (This is what happens when you post late at night on an empty stomach, but that's another story...) It is the off-shell nature of virtual particles that I was using to differentiate them from what people think of as "real" particles.

Well, yeah. There's certainly something funny about virtual particles :). But I suppose thinking about that in detail is a bit beyond the scope of a mathless discussion ;).

StevenO
2009-May-27, 12:48 PM
Well, obviously they're not "real" in the same way as an electrical or gravitational field... But why does it matter what kind of "real" they are?
In physics we have gotten used to see the an electrical field as "real" (as the chair you sit on) since it obeys causality even though it only exchanges "virtual" photons. I just have no idea how to compare that to a virtual Z boson that does not obey causality but can exist by probability also.
I mean: what is real, except hadronization?


A Jenson Button (http://www.formula1.com/results/driver/2009/6.html) had the potential to win the Formula 1 Grand Prix de Monaco (http://www.formula1.com/results/season/2009/810/6633/) last week. And in fact he realized that potential, crossing the finish line ahead of all other racers on May 24th, 2009.

The potential was real; and the win, when it happened, was real. But of course we don't talk about the two realities in the same way; nor do we understand the reality of the potential in terms of the reality of the win--nobody gets a trophy for being the potential winner, after all.
Yes. Personally I did'nt watch the race, but some people want to make me believe that he won the race because for some instants of time he turned into Zenson Button flying a spaceship twice around the earth through improbability drive :)

robross
2009-May-29, 08:48 AM
A sloppy use of the word "classical". :( (This is what happens when you post late at night on an empty stomach, but that's another story...) It is the off-shell nature of virtual particles that I was using to differentiate them from what people think of as "real" particles.

"Off shell?" Does this imply that virtual electrons don't have quantized energy states?

Rob

Fortis
2009-May-29, 05:45 PM
"Off shell?" Does this imply that virtual electrons don't have quantized energy states?

Rob
This isn't the shell that people refer to in the context of electron shells, or orbitals. This is more of a mathematical statement that the relationship between energy, momentum, and rest mass, doesn't hold as an equality for virtual particles.