Understanding Antimatter Annihilation: Exploring Particle Pair Requirements

[Faradave]

I’m an interested amateur. In reading what I can about antimatter in Wikipedia, I find articles on annihilation for electron:positron, proton:antiproton and neutron:antineutron pairs. I am given the impression that exact twin:antitwin particle pairs are required for annihilation to occur. Is this correct, or can there be other annihilations such as a positron (mass ≈ 0.000511 GeV/c²) annihilating part of a muon (mass ≈ 0.106 GeV/c²)?

 

[MathematicalPhysicist]

It's all about probabilities of processes to occur.

It's more likely that an electron will get anihliated with a positron than with a plus muon.

Take my words with doubt, cause my knowledge in particle physics is really shaky as it is.

 

[Bob S]

True annihilation into (the equivalent of*) pure photons with zero charge, lepton number or baryon number, can only occur between a particle and its CPT antimatter counterpart. Positive muons and electrons form hydrogen-like muonium atoms, but the muon usually decays into a positron and two neutrinos.

*proton antiproton annihilation normally results in a few pions, but the net charge is still zero..

Bob S

 

[Faradave]

Thanks. Do you believe a hybrid nucleus, composed of protons and antineutrons could be stable? Here, though the baryons are clearly not CPT twin:antitwin, among their constituent quarks (proton=UUD, antineutron=UDD) there would be such pairs. It seems like a recipe for energetic pions (UD).

 

[Bob S]

No..

 

[Faradave]

My fault (sorry). Is that,
a. "No, I don't expect the nucleus would be stable." presumed answer, or
b. "No, it's not a recipe for pions." Implies you would expect the nucleus to be stable.

 

[Bob S]

No, not stable
Yes, lots of pions (multiplicity ≈ 3 to 7), 1 extra pi-plus.

Bob S

 

[Faradave]

I’ve spent most of my life under the naive impression that antimatter always annihilates on contact with matter, not paying attention to material makeup. Of course, that would be true for atoms and antiatoms in contact, their elementary components being symmetrical matches. Thus, the great accomplishment by physicists at CERN to recently isolate and briefly contain antihydrogen. http://cosmiclog.msnbc.msn.com/_news/2010/11/17/5482096-antimatter-atoms-caught-at-last

However, the strong CPT symmetry requirement for annihilation seems actually to engender matter:antimatter compatibility for particle combinations not meeting those requirements. Matter and antimatter particles would seem "social", coexisting together, so long as no symmetrical pairs could be made. - Thanks again.

 

[Faradave]

In poking around, have come across three, recent, mutually consistent statements, among others.

"At first physicists saw no reason why antimatter and matter shouldn't behave symmetrically, that is, obey the laws of physics in the same way. But if so, equal amounts of each would have been made in the big bang -- in which case they should have mutually annihilated, leaving nothing behind. And if somehow that fate were avoided, equal amounts of matter and antimatter should remain today, which is clearly not the case."[1]

"In the beginning, equal amounts of matter and antimatter came into existence. At least that's what scientists believe. Today, antimatter is virtually absent in the natural world. Physicists assume that all that antimatter was annihilated when it came into contact with matter -- and that for some as-yet-unknown reason, the matter we know and love had enough of an advantage for a remnant to survive."[2]

"There is considerable speculation as to why the observable universe is apparently almost entirely matter, whether there exist other places that are almost entirely antimatter instead, and what might be possible if antimatter could be harnessed. At this time, the apparent asymmetry of matter and antimatter in the visible universe is one of the greatest unsolved problems in physics."[3]

It seems perfectly natural for us to assume that the universe of particles ordinarily observed are "normal" and that the rare particles which annihilate them are antimatter. But in view of the asymmetry problem, that might be worth reconsidering.

Granted that two particles capable of annihilation (or emerging from pair creation) comprise a CPT twin:antitwin pair, discerning which is which might be tricky. If we consistently designate the rarer partner "antimatter" then matter:antimatter asymmetry is, in fact guaranteed, and should come as no surprise at all.

As noted in the CPT requirement above, matter and antimatter particles can be quite social (compatible) when that requirement is not met at the fundamental level. This will seem naïve, but I feel I must take a hint from the following:

"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."[4]

Indeed, gravity might have been overlooked completely, except for the fact that electric charge is so well balanced in the material around us. So if positive and negative charge were considered markers for matter and antimatter respectively, the asymmetry problem would become equally small for both. Electrons though now considered antimatter would still obediently orbit protons. Protons and neutrons would be considered hybrid matter:antimatter particles (like mesons) but since Up quarks (still matter) are not CPT counterparts for Down quarks (now antimatter) they remain social, as we observe.

As for baryon asymmetry, the vast majority of the mass we observe is comprised by these Up and Down quarks. There seems to me, unavoidable symmetry in that the positive Up quark has half the mass but twice the charge of the negative Down quark. In this scenario, matter and antimatter balance, though they are not twin:antitwin. Of course, that is no different than what we already accept for electric charge. Electrons are clearly not twins with protons but the charges balance.

It would seem at first, that the conservation baryon number (CBN) law would be sacrificed under matter:antimatter redesignation by charge. Perhaps not. Consider that baryon number (BN) is already highly contrived, ignoring particle mass, charge, and spin among other properties (covered by their own conservation laws). I would suggest that CBN might be salvaged if it continues as an absolute value except in the special cases of pair creation (BN then allowed to increase) and annihilation (BN then allowed to decrease). CBN currently does function as an absolute value with respect to electric charge and would naturally have to do the same if that became the designator of matter:antimatter status.

1] http://www.sciencedaily.com/releases/2010/11/101117141523.htm
2] http://cosmiclog.msnbc.msn.com/_news/2010/11/17/5482096-antimatter-atoms-caught-at-last
3] http://en.wikipedia.org/wiki/Antimatter
4] http://en.wikipedia.org/wiki/Baryon_asymmetry

 

[Bob S]

Antiparticles do not have to annihilate only with their exact mirror particles, as long as certain key quantities are conserved in the reaction. Antiprotons were observed to annihilate with neutrons in deuterium bubble chambers many many years ago. The recoil spectator proton and electron were the signature. This annihilation reaction conserves baryon number and charge. See

http://www.springerlink.com/index/AKT1U7G337641634.pdf

Also see

http://www.google.com/url?sa=t&source=web&cd=1&sqi=2&ved=0CBMQFjAA&url=http%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2F0550321375906434&ei=n1IKTYKbJZCusAOiuMz9Cg&usg=AFQjCNFV4DnErIxBSvMdF5nt2XsmkSbCSA&sig2=iZxPq3pNDIWcxB3iPQ3h-Q

Bob S

 

[Faradave]

Thanks for the links. They are consistent with what you said above for the proton, antineutron combination. But these interactions of different composite particles don’t seem to rule out the "CPT counterpart" requirement for annihilation that you cited above (post #3) for elementary particles. I’m sure you were not surprised to learn that antihydrogen is devilishly unstable, even when there is no ordinary hydrogen in the environment. All it needs is an encounter with its component counter particles to begin annihilating.

Practically speaking, because of their apparent overwhelming presence, it comes down to the compatibility of these:
Up quark (mass ≈ 0.003 GeV/c², charge = +2/3),
Down quark (mass ≈ 0.006 GeV/c², charge = -1/3)
electron (mass ≈ 0.000511 GeV/c², charge = -1)

Is it sufficient to say with certainty, that a Down quark is not antimatter, simply because it does not annihilate with an Up quark? This is the key question. Electrons, even if they are antimatter (as I suggest), are so isolated by orbital constraints that they would typically be compatible with nucleons, regardless of their status.

 

[Bob S]

Every quark has its antimatter counterpart, such as the positive and negative pions; u d-bar and d u-bar. The antiproton is u-bar u-bar d-bar. The positron is the antimatter counterpart of the electron. Both are non-composite "point" particles and have no quarks. Electrons and positrons are leptons, lepton number, like baryon number and charge, is a conserved quantity.

Bob S

 

[Faradave]

I get that. So when you cited antiproton-neutron annihilation as an example of non mirror image particles annihilating (post #10), my impression was, not really. A u-bar and a d-bar in the antiproton will find mirror image counterparts (u and d quarks) in the neutron. I think it very unlikely that the remaining u-bar from the antiproton will annihilate with the remaining d quark from the neutron. If I’m wrong, please let me know.

Its good you brought up lepton number because redesignation of matter and antimatter by positive and negative charge respectively would necessitate use of absolute values in both conservation of baryon number (CBN) and conservation of lepton number (CLN) except when pairs are created (numbers then rise) or annihilate (numbers then fall).

Those caveats may cause some to hesitate in adopting this solution to the matter:antimatter asymmetry problem. But pair annihilations and always observe conservation of energy (and mass if new particles result). It is in these that nature demonstrates its own absolute value function. Energy resulting from annihilations is always "normal". Electron annihilation with a positron releases two gamma photons, not a gamma photon and a gamma anti-photon. Mass (and presumably gravity) also appear to ignore matter/antimatter status, being positive in either case.

 

[Faradave]

If redesignation of matter:antimatter status by positive:negative charge were to be entertained, a natural question might be, "What is to be done with the chargless particles?"

Of course, this necessarily refers to elementary particles since composite particles like the neutron, though electrically neutral, are not strictly "chargeless". They are composed of charged quarks, verified in part because the "Magnetic moment of a neutron is nonzero, unexpected from an electrically neutral particle."[1]

This now raises questions about neutrinos. When they were thought to be massless, they were understandably considered chargeless. Now, though still considered elementary (electron and muon neutrinos are a fraction of the electron mass) the, "neutrino flavor oscillations implies that neutrinos have mass. The existence of a 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 would seem to imply charge, however minute.

The Z boson is considered chargeless and is its own antiparticle (coincidence?) and therefore does not contribute to matter:antimatter asymmetry. Besides, they exist for such short times, "In practice they can be considered to be virtual particles."[4]

When it comes to the truly massless "particles" such as photons, though I acknowledge the great utility of this concept, I have proposed an alternative to their actual existence[3] and would not like to risk another PF infraction by going further here. Suffice it to say that in any case, massless particles are as easily considered energy which, as noted above (post #13), arises as a natural absolute value with respect to positive:negative charge and matter:antimatter status. Neither photons nor gluons list a specific antiparticle, do not annihilate in a traditional sense and do not contribute to matter:antimatter asymmetry (zero mass and like z bosons, they are said to be their own antiparticles and are thus, self balanced) [5,6].

If there is a truly neutral particle with rest mass greater than zero, it would mean that charge, though a marker for matter:antimatter status, is only a marker and that there is an deeper factor determining such status. So far, though possible, that case cannot, in my view, be made.

1] http://en.wikipedia.org/wiki/Neutron
2] http://en.wikipedia.org/wiki/Neutrino
3] https://www.physicsforums.com/showthread.php?t=408345
4] http://en.wikipedia.org/wiki/W_and_Z_bosons
5] http://en.wikipedia.org/wiki/Photon
6] http://en.wikipedia.org/wiki/Gluon