Is Antimatter a Fundamental Property or a Label?

[Faradave]

"Antimatter": Property Or Label?

I am interested in learning whether a particle’s status as "matter" or "antimatter" is an independent property of that particle, a constellation of other properties or a (somewhat nonspecific) designation. Opinions are welcome if a definitive answer has not been established.

I understand that for elementary particles to annihilate on contact (or emerge from pair creation), they must be of identical mass, spin state and opposite charge [1] but I don’t believe this answers the question, in fact, I think it raises it.

1] http://en.wikipedia.org/wiki/Antiparticle

 

[JDługosz]

To give a specific example, an electron and a positron are both fundamental particles, and are different and distinct species.

They are related via symmetry. For virtual particles, you could say some reaction was due to an electron going one way or a positron going the other way. That is why anti-particles must exist, with the stated properties.

 

[Leperous]

It's a designation based upon a collection of properties that is not just due to its electrical charge (see, for example, antiquarks). I'm not sure if it's anything more special than a name - annihilation is probably just a fancy word for the particular interaction between the two species which result in a tremendous energy release (a particle and something similar to, but not quite, the antiparticle - example - may well have a similar consequence).

 

[niklaus]

If all the matter around us was made out of what we call antiprotons, antineutrons and positrons, we would have called those matter and once we found out about protons, neutrons and electrons would label them antimatter.

Leperous, how are antiquarks different from the corresponding quarks other than charge?

 

[Leperous]

Have a look at the link and the corresponding "flavour quantum number" article. According to that, antiquarks have all opposite flavour numbers (five in total): electric charge is a quantity derived from two of these and a third quantity that is equal for all quarks (I didn't know that before just now

 

[Faradave]

...a particle and something similar to, but not quite, the antiparticle - example - may well have a similar consequence).

Thanks for the link. This was also discussed in my recent thread, AntiSocial Behavior. The point being that even though different matter:antimatter composite particles may annihilate to some degree, it is still the exact twin:antitwin elementary particle pairs within, which are actually annihilating.

 

[Leperous]

When particles interact, certain quantities are conserved, much like momentum in the classical (& quantum...) world. If you react a particle with its antiparticle, these quantities are exactly opposite and so sum to zero: you end up with a charge-less, baryon/lepton-number-less quantity (which I guess looks like a photon).

If you react a particle with some other antiparticle (example) these quantities are conserved but don't sum to zero, so you get something else.

So really it does look like semantics - I know I'm combinging "composite" and "elementary" particles but I hope you see my point. Interesting question though, never really thought about why.

 

[Phrak]

Nothing physically changes with arbitrary label swapping. Protons and antiprotons could be called antiprotons and protons respectively, or the s and anti s label-swapped.

 

[Faradave]

If you react a particle with its antiparticle, these…so sum to zero: you end up with…photon(s)…If you react a particle with some other antiparticle (electron capture)…you get something else…semantics

I’ve altered your quote, but not maliciously. It gets right to the question I’m asking. You see, if any antiparticle will annihilate as much as it can of any normal particle, then it would seem that being one or the other is a property of particles. If annihilation requires a specific combination of other properties, then it seems merely a label for that.

We could get quite complicated (and thus, beyond me), but I don’t think we need to. Everything we see around us in nature is comprised by the four, first-generation fermions (electrons (e), up quarks (u), down quarks (d), electron neutrinos). So let’s stick with them as much as possible. We know a u will annihilate with an up antiquark (u-bar). So, here’s a new question. Will a u annihilate with a d-bar? I don’t think they'd even try because both have negative electric charge. Bottom line, matter doesn't always annihilate with antimatter, it might even behave socially (coexist locally).

The example you cite is not just complicated because the particles are both composite and elementary (thanks for noting this) but under the current model, we have matter (the electron) being consumed by other matter, a proton (uud)!

e +p(uud) \rightarrow n(udd) + \nu_e

There’s not an antimatter particle in sight! Or is there? Right there, in plain view, for everyone to see.*

*with appropriate detectors, of course.

 

[Faradave]

Nothing physically changes with arbitrary label swapping.

Safe bet! But I think a little label swapping might help solve, "…one of the greatest unsolved problems in physics"[1], matter:antimatter asymmetry.

Passive acceptance of the ancient designation, "matter" for all the material around us has some remarkable consequences. First, it makes an unfounded assumption of homogeneity. Because all the material around us is ordinary, it is assumed to be all normal matter. But as I noted (post #9), some matter and some antimatter particles might be able to coexist. Second, it makes an asymmetry guarantee. If all the commonly encountered particles are presumed to be normal matter, then of course, the antiparticles to these will have to be uncommon!

Getting back to basics, whenever there is a twin:antitwin annihilation (or pair creation) of elementary particles, one has a positive electric charge and one negative. I think nature might be giving us a hint that positive:negative electric charge might be seen potentially as a marker for matter:antimatter status. Sticking with first generation fermions, the electron (now antimatter) is able to coexist with matter nicely isolated by orbital constraints.

As for Leperous’s example of electron consumption, this is only consistent in that, occasionally an electron (now antimatter) reacts with a proton’s up quark (still matter). Meanwhile, the up quark - down antiquark (u, d-bar) combination I mentioned in post #9 will be compatible because both are now matter.

Regarding the great matter:antimatter asymmetry mystery. Well, from my admittedly amateur view, it looks like the bulk of observable mass is about half up quarks and half down quarks. The up quarks (still matter) have half the mass and twice the charge magnitude of the down quarks (now antimatter). What asymmetry? The mass-charge products are balanced.

There's more, but no sense going on if this is all wrong.

1] http://en.wikipedia.org/wiki/Antimatter

 

[Phrak]

Safe bet!

More than that, it is true.

 

[Faradave]

Nothing physically changes with arbitrary label swapping…it’s true.

It’s also true that the sky is blue. No one is suggesting arbitrary label swapping. I am suggesting that we consider systematic redesignation of particles as "matter" when associated with positive electric charge and "antimatter" when negative. The CPT symmetry requirement for annihilation[1] seems to allow stable coexistence of matter and antimatter particles so long as that requirement is not met.

You're still right, nothing physically changes. But this could save us a, so far futile, search for antimatter universes because baryons would then be as balanced as charge. "The baryon asymmetry problem in physics refers to the apparent fact that there is an imbalance in baryonic matter and antibaryonic matter in the universe. Neither the standard model of particle physics, nor the theory of general relativity provide an obvious explanation for why this should be so; and it is a natural assumption that the universe be neutral with all conserved charges."[2] Charge is so well balanced that we experience much-weaker gravity as a dominant influence.

Consistent association of charge, mass and spin might also open the door to a deeper understanding of each of these properties.

1] http://en.wikipedia.org/wiki/Antiparticle
2] http://en.wikipedia.org/wiki/Baryon_asymmetry

 

[Phrak]

"You're still right, nothing physically changes. But this could save us a, so far, futile search for antimatter universes because baryons would then be as balanced as charge. "The baryon asymmetry problem in physics refers to the apparent fact that there is an imbalance in baryonic matter and antibaryonic matter in the universe. Neither the standard model of particle physics, nor the theory of general relativity provide an obvious explanation for why this should be so; and it is a natural assumption that the universe be neutral with all conserved charges."[2] Charge is so well balanced that we experience much weaker gravity as a dominant influence.

Consistent association of charge, mass and spin might also open the door to a deeper understanding of each of these properties.

OK. See if you can use your ideas to explain why there is an apparent dominance of hydrogen over antihydrogen.



1] http://en.wikipedia.org/wiki/Antiparticle
2] http://en.wikipedia.org/wiki/Baryon_asymmetry[/QUOTE]

 

[JDługosz]

I am suggesting that we consider systematic redesignation of particles as "matter" when associated with positive electric charge and "antimatter" when negative.

No, the families are layed out due to how they interact, forming various patterns of relationships. As mentioned, the "first generation" has neutrino, electron, up, and down. They are all "matter", not anti. The Weak interaction treats the first two in the same way as the second two. The glue force sees the u and d as being the same kind of color charges. The u has a positive charge, but the negative anti-u has different color charges as well.

 

[Phrak]

No, ...

"No" what? It is a proposition for a convention, not of requirements upon physical substance. If what you state below is what your prefer rather than a physical requirement, you should state why.

...the families are layed out due to how they interact, forming various patterns of relationships. As mentioned, the "first generation" has neutrino, electron, up, and down. They are all "matter", not anti. The Weak interaction treats the first two in the same way as the second two. The glue force sees the u and d as being the same kind of color charges. The u has a positive charge, but the negative anti-u has different color charges as well.

 

[Faradave]

...explain why there is an apparent dominance of hydrogen over antihydrogen.

I found this, "It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees. A few minutes afterward, starting with only protons and neutrons, nuclei up to lithium and beryllium (both with mass number 7) were formed, but only in relatively small amounts."[1]

and "For reasons that remain uncertain, during the process of leptogenesis there was an excess in the number of electrons over positrons. Hence, about one electron in every billion survived the annihilation process. This excess matched the excess of protons over anti-protons, in a condition known as baryon asymmetry, resulting in a net charge of zero for the universe."[2, 3] which, I gather you are well aware of. You can certainly envision H formation without H-bar from there.

So, I gather what you are asking really boils down to, "Why is it just u quarks (u) and d quarks (d) without u-bar and d-bar (from which H-bar would arise)?"

I don’t know. One could imagine the equivalent of a neutral hexaquark (3u,3d) in the "plasma" freezing out, then decaying into a proton, neutron and electron (plus neutrinos) with no asymmetry unless we blindly insist on designating them all "matter". It bears repeating, though the term "antimatter" is relatively new, it cannot escape the fact that it reflects directly from just such a designation "matter", handed down through the millennia.

Redesignation would have, u as matter, d as antimatter. Since u has half the mass but twice the charge magnitude as d, there is already balance. There is no abundance of u-bar or d-bar because none is called for. Under redesignation, neither conservation of mass, charge, spin nor baryon number is violated. Granted the pile of u’s is not identical to the pile of d’s but neither tips the scales (adding the leptons as needed). It remains typical today that particle decay results in non-twin products, but still follows the conservation laws (i.e. they balance). There is some asymmetry internally but none externally.

You might still ask, "Why is u not a positive twin of d?", which would have both internal and external symmetry. The answer has to be, if u and d were twin:antitwin, they would annihilate. That universe would self destruct. So, it might be argued that a universe with internal asymmetry is the only type which could arise.

Think about this. How often does an overstretched guitar string break dead center? One side curls clockwise, the other spins oppositely. If you measure closely enough, they never come out to be exactly equal in length. Yet they still add up to a single, uncurled guitar string. There is no need to suppose a separate guitar string broken in exactly the opposite way.

We live in a guitar string universe. Rock on!

1] http://en.wikipedia.org/wiki/Nucleosynthesis
2] http://en.wikipedia.org/wiki/Electron
3] http://en.wikipedia.org/wiki/Leptogenesis_(physics )

 

[Faradave]

…first generation…are all "matter", not anti…The glue force sees the u and d as being the same kind...

Honestly, as an amateur, I find the color scheme currently beyond me. Remember, I suggest no physical change (thanks Phrak), just redesignation of matter:antimatter status by positive:negative electric charge. The reason I can sleep without understanding color lies in the classic annihilation of electron and positron. This pair seems to contain whatever it is that constitutes matter:antimatter status, without involving the strong force (glue). Essentially, all I see in that pair is mass, spin, charge and momentum.

 

[Phrak]

I found this, ""For reasons that remain uncertain, during the process of leptogenesis there was an excess in the number of electrons over positrons. Hence, about one electron in every billion survived the annihilation process. This excess matched the excess of protons over anti-protons, in a condition known as baryon asymmetry, resulting in a net charge of zero for the universe."[2, 3] which, I gather you are well aware of. You can certainly envision H formation without H-bar from there.

It's not necessary to start talking about quarks. I've put in bold the critical part.

I think the proper way to formulate the paradox is this: "Why was the excess one way rather than the other?"

One possibility I've considered is that it is a result of positive feedback from CPT violation. Who knows; There may even by an article on this very idea. In fact, anything I have ever conceived has already been conceived by someone else, better equipped and already published, so there is a very good change.

The idea is this: Vacuum fluctuations, first favoring one orientation (handedness, or parity) will effect other matter (see "CPT violation"), also causing it to favor the same handedness. Now, this puts things in a bit of a bind. One region of space in the early universe will not communicate it's preference, by feedback to another distant regions where influence must propagate at sub-light speed. I think this demands that regions of the universe differ: Some are composed of matter, others are composed of antimatter. Once having formed, these would maintain separation for the most part, due to radiation pressure from matter and antimatter annihilation at the boundary. Maybe there is a good reason to reject this idea that I havn't heard of , like the light signature of annihilation happening at boundaries.

The other idea goes beyond the constraints imposed by this forum regarding personal theories, so I cannot comment, but it suffers the same fate but in stronger form, demanding rather than equivocating that the universe is composed of regions of both matter and antimatter.

 

[Faradave]

...There may even by an article on this very idea... I think this demands that regions of the universe differ: Some are composed of matter, others are composed of antimatter. Once having formed, these would maintain separation for the most part, due to radiation pressure from matter and antimatter annihilation at the boundary. Maybe there is a good reason to reject this idea that I havn't heard of , like the light signature of annihilation happening at boundaries...

This is similar to what you wrote but no mention of "feedback".
"Another possible explanation of the apparent baryon asymmetry is that there are regions of the universe in which matter is dominant, and other regions of the universe in which antimatter is dominant, and these are widely separated. The problem therefore becomes a matter/antimatter separation problem, rather than a creation imbalance problem. Antimatter atoms would appear from a distance indistinguishable from matter atoms, as both matter and antimatter atoms would produce light (photons) in the same way. Only in the border between a matter dominated region and an antimatter dominated region would the antimatter's presence be detectable, as only there would matter/antimatter annihilation (and the subsequent production of gamma radiation) occur. How easy such a boundary would be to detect would depend on its distance and what the density of matter and antimatter is along it. Presumably such a boundary would lie (almost by necessity) in deep intergalactic space, and the density of matter in intergalactic space is reasonably well established at about one atom per cubic meter. Assuming this is the typical density of both matter and antimatter near a boundary, the gamma ray luminosity of the boundary interaction zone is easily calculated. Approximately 30 years of scientific research have placed boundaries on how far away, at a minimum, any such boundary interaction zone would have to be, as no such zones have been detected. Hence, it is now considered unlikely that any region within the observable universe is antimatter dominated."[1]

They don’t sound optimistic. I also don’t anticipate (as they clearly do) that light from positron transitions will act the same as light from electrons. You may recall this from my previous thread[2], for which I was awarded a three month vacation (from PF, if you know what I mean). So for now, let me put the original question back on the floor, in the simplest case.

Since we all agree that electron:positron annihilation is representative of matter:antimatter annihilation (though I suggest reversing their designations), is there some property of these particles, separate from mass, spin and electric charge which makes one matter and the other antimatter, or does the designation derive from those three? From what do we get the signs of lepton numbers?

1] http://en.wikipedia.org/wiki/Baryon_asymmetry
2] https://www.physicsforums.com/showthread.php?t=408345

 

[Phrak]

You are very hell-bent on finding a physically motivated division, rather than a man made division for the classification of particles into particle/antiparticle sets. You should really call this hypothetical division something different, like 'munglik' and 'antimunglik', or whatever, to distinguish the two. You might find something for your quest in researching CP symmetry.

 

[Faradave]

To answer my own question, "From what do we get the signs of lepton numbers"? I see the designation as archaic, arbitrary, incorrect and potentially obstructive of deeper truths such as, what is electric charge, what is force, what is spin and what is mass? When animal species are misclassified, we get the same kind of obstruction in discerning branches of the evolutionary tree. Can we ever hope to unify the "fundamental" properties if they aren't attributed correctly?

…finding a physically motivated division…into particle/antiparticle sets. You should really call this hypothetical division something different…to distinguish the two.

Before adding a new term, I think we might get rid of an obsolete one, "antimatter". There are still fundamental particles and antiparticles, but at any higher level from baryons to matter in its common usage (material made from atoms), "antimatter" won’t do under redesignation. It’s all heterogeneous as both positive and negative (anti) particles are present. I would still refer to that composed of common particles as matter and the stuff made of their rare counter particles (e.g. H-bar) would be "comp matter". (charge-opposite, mixed particle). The term quite intentionally also connotes "complementary" and "composite".

 

[Faradave]

To its credit, the PF, "Similar Threads" auto-search engine (below) recommended a discussion from 2006.[1] I have copied three quotes from it here for recognition and comment.

It's the weak interactions (flavordynamics) not the strong (chromodynamics) that violate CP, thereby allowing us to distinguish anti-matter from matter. Griffiths chapter on CP violation (4.8) gives a convention-free definition of positive charge. It is the charge carried by the lepton preferentially produced in the decay of the long-lived neutral K meson.

Once we have a set definition of positive, we can choose how we want to define anti-matter and matter (and how we define right-handed and left-handed). We choose that the +2/3e charged quark is the up quark and the negative one is the antiquark. Once we make that choice, the other quarks fall into their place. Baryons are comprised of three quarks or of three anti-quarks. Mesons have one quark and one anti-quark. You can't have an up and a down quark in a meson, only the up and the anti-down (pion +) or the anti-up and the down (pion -).

I'm not sure if defining the quarks that way absolutely forces us to define the leptons as we do. But the universe certainly gives us a hint. Everything around us is pretty much made up of protons and neutrons (with quarks we defined as matter) and electrons. Defining the electrons as matter makes sense. Then the other leptons (including neutrinos) have to fall in on the electron into their proper matter/anti-matter formation.

Good comment but I fail to see how, "the other quarks fall into place". The notion that baryons must be purely matter or purely antimatter seems arbitrary. Neutral mesons clearly demonstrate our acceptance that hybrid (matter:antimatter) particles are possible within the standard model or under redesignation. This suggests the same can be true for baryons.

"Defining the electrons as matter" does not make sense unless you want to have a great asymmetry mystery.

If I understand it, there is nothing that forces us (in the Std. Model, at least), to name the electron the same way as we do the up and down quark (right?), but what I was really wondering was if we had to name the up and down the same way, and it seems that the answer is "yes", due to their coupling via the W.

I started wondering about this because of the matter/anitimatter asymmetry problem, and I wondered if the ratio was really an absolute thing or if it was in any way a by-product of convention, i.e. if we could decide electrons are actually antimatter, and then the asymmetry would be somewhat reduced, but obviously nowhere near enough to resolve the problem.

The W (and Z) bosons exist for such short times they are for all intents, "virtual particles" [2] so we should use caution giving them the deciding role in matter:antimatter status for real particles. Redesignation would switch boson status so W^- would be the antiparticle to W⁺. This eliminates discrepancy in beta decay of a neutron.

d \rightarrow u + W^- \rightarrow u + e^- + \nu_e

Yes, it's quite arbitrary. What we commonly see in our chunk of spacetime is called "matter", rare stuff we call "antimatter". And there are things that are on the line, such as the pions. These are composites, and they are made up of partly what we call quarks and partly what we call anti quarks. And the pi+ and pi- are equally common, so we can't decide whether to call them matter or antimatter. On the other hand, protons are very common in our neck of the woods, antiprotons are not, so that's what we call them.

In Feynman's interpretation, antiparticles are particles traveling backwards in time. If we could distinguish between particles that travel forwards in time from particles that travel backwards, we could distinguish between particles and antiparticles.

One of the mysteries of the Dirac equation was that it is natural to put together eigenstates of velocity. This was cause for much debate back when it was a new idea. Most of the debate has died away. However, the fact that the Dirac equation solutions can be put into eigenstates of velocity provides a clue for how to distinguish between particles and antiparticles in a non arbitrary (well, less arbitrary) way.

If you can expand the Dirac equation so that it contains solutions for more particles, you can look at the velocity eigenstates to divide the elementary particles into two sets, and then arbitrarily call one set particles, and the other set anti particles. You then have a 50% chance of being right, which is better than if you assign particle / anti particle labels to all the elementary particles at random.

Well said.

1] https://www.physicsforums.com/showthread.php?t=145435
2] http://en.wikipedia.org/wiki/W_and_Z_bosons

 

[geophysics10]

Think about this. How often does an overstretched guitar string break dead center? One side curls clockwise, the other spins oppositely. If you measure closely enough, they never come out to be exactly equal in length. Yet they still add up to a single, uncurled guitar string. There is no need to suppose a separate guitar string broken in exactly the opposite way.

I'm not sure I understand your analogy. Failure of a string under tension at a specific place is due to defects in the material. Are you suggesting an inherant asymmetry? Or considering a perfect string which deforms uniformly and a probabilistic asymmetry?

 

[Faradave]

Good question! I don’t think we can know until we understand something more about the composition of elementary particles.

The guitar string analogy (post #16) assumes redesignation of elementary particles by electric charge so negative particles are the antiparticles their positive counterparts. This would make baryons a hybrid of particles (up quarks, u) and antiparticles (down quarks, d). The fact that u has half the mass and twice the charge magnitude of d would not be viewed as coincidental, rather an indication that both "broke" from a common ancestor. If that object was under a sort of tension (like a guitar string), one might imagine an asymmetric break where the smaller piece flies away in one direction (say to the right) spinning (say clockwise) faster because of its lower mass. The other, more massive piece recoils left, spinning slower and counterclockwise. In this imagined scenario, while angular momentum is conserved, electric charge derives from angular velocity (positive = clockwise, negative = counterclockwise).

Why an asymmetric break? It seems forced by the assumption that a symmetric breaks would lead to particle extinction through annihilation. In that sense, the scenario seems probabilistically derived from a "perfect" guitar string. On the other hand, imperfection is suggested by the existence of leptons, appearing as important bits of shrapnel. Even, "neutrino mass strongly suggests the existence of a tiny neutrino magnetic moment of the order of 10^-19 μB, allowing the possibility that neutrinos may interact electromagnetically as well."[2] This seems to imply charge, however minute.

1] http://en.wikipedia.org/wiki/Neutrino