Physics Help Forum Majorana Neutrino

 Nuclear and Particle Physics Nuclear and Particle Physics Help Forum

May 13th 2019, 07:26 AM   #11
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 Originally Posted by neila9876 Make clear the very classic concept: charges. Why electron carry the same opposite charge as the proton, why the volume of charge does not change following the movement of paticles...before going deep into nuclei, "standard ***"...Is there a "standard ***" for electron too?
In the standard model, quarks and leptons have charge values and all other particles have their charges derived from those combinations. For example, protons have a +1e charge because they are composed of two up quarks and a down quark (+2/3 + 2/3 - 1/3 = +1) and neutrons have no charge because they are composed of one up quark and two down quarks (+2/3 -1/3 -1/3 = 0).

As for why these particular combinations exist? That's a very difficult question. I think there are hypotheses for why the fundamental particles seem to have the quantum numbers that they do, but I don't know anything about it. Quantum mechanics probably has a lot of involvement here, which is usually the case for anything that seems discretized at particle scales.

 May 13th 2019, 09:49 AM #12 Senior Member     Join Date: Jun 2016 Location: England Posts: 842 One of the reasoning's behind string theory is that the natural "vibration modes" of the "strings" provides a reason why certain characteristics are observed for particles, while others are not. Theoretical Physicists are looking for geometrical/topological mathematical groups which from which the observed particle zoo would become an obvious or even necessary result. They have found interesting hints that such mathematical entities are not impossible, but I don't think anyone has really demonstrated a truly convincing model (yet). topsquark likes this. __________________ ~\o/~
 May 13th 2019, 04:22 PM #13 Senior Member   Join Date: Mar 2019 Location: cosmos Posts: 195 ears vs nose History is ever developing. I' m reluctant to mention politics although it seems to be a very good example to demonstrate what's "convincing". I would rather talk about pig. Two big ears might convince some people that's a pig. But I think that the long nose can convince me that's a pig...
May 13th 2019, 10:08 PM   #14

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 Originally Posted by neila9876 I don't know why people consider so cryptic the neutrino is. I consider it very simple from the physical (structure) angle. Yes, its anti - particle is just itself. "Supersymmetry"...? wo,,,another super - fashion word...
Neutrinos are not their own anti-particles. Neutrinos are "left handed." That means that, no matter what inertial coordinate system we are in, we measure the spin to be +1/2. The anti-neutrino is right handed for all inertial coordinate systems.

This seems like semantics, I mean why wouldn't we just have left and right handed particles and not call them different? The reason is an observed symmetry of nature called lepton (muon, tau) number conservation. The simplest example is beta decay: $\displaystyle n^0 \rightarrow p^+ + e^- + \overline{\nu _e}$ (The $\displaystyle \overline{\nu}_e$ is the electron anti-neutrino.) Notice that the (electron) lepton number ($\displaystyle e^- \text{ and } \overline{\nu}_e$) have a total lepton number 1 - 1 = 0, which is the same as the lepton number of the neutron, which is 0. The electron anti-neutrinos interact with electron neutrinos as a particle anti-particle pair. Verification of this fact is a nightmare of delicacy in experimental proceedure but it has been done.

There are also muon neutrinos and tau neutrinos. (The beta decay version using muons is $\displaystyle n^0 \rightarrow p^+ + \mu ^- + \overline{\nu}_{\mu}$. As the muon has a much larger mass than the electron the reaction is at a much higher energy than beta decay with the electron.) Except for the possible resonance between the leptons there is also muon and tau lepton number conservation and all of them have been measured to be distinct. No one knows why there should be three of these leptons.

There are many particles that are their own anti-particles. The photon is one, the $\displaystyle \pi ^0$, and $\displaystyle \Delta ^0$ are all their own anti-particles. A majorana particle is a fermion (electron, muon, tau, and all the quarks) that is its own anti-particle. None of the photon, $\displaystyle \pi ^0$, or $\displaystyle \Delta ^0$ mentioned above are fermions. The original post refered to an experiment that showed that an electron neutrino is actually its own anti-particle, which is very big news because according to the Standard Model it shouldn't be.

-Dan
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Last edited by topsquark; May 13th 2019 at 10:24 PM.

 May 14th 2019, 12:09 AM #15 Senior Member   Join Date: Mar 2019 Location: cosmos Posts: 195 pig vs pig @ dragon: You are telling the story "God is left - handed" which was the brilliant story of two (exactly should be three) dragons? (God is mentioned by scientists at that time, not me). That original experiment is one of the subjects I still want to take a look but I can get no detailed material. I wonder if guys can really see that a neutrino spins just alike a coin on the table? If you watch from the top, a coin is spining left handed then if you watch from the bottom, it will spin right handed. But from the angle of physical structure, the coin does not change... I mention a pig above, it means "different methods of desription".
May 14th 2019, 06:40 AM   #16

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 Originally Posted by neila9876 @ dragon: You are telling the story "God is left - handed" which was the brilliant story of two (exactly should be three) dragons? (God is mentioned by scientists at that time, not me). That original experiment is one of the subjects I still want to take a look but I can get no detailed material. I wonder if guys can really see that a neutrino spins just alike a coin on the table? If you watch from the top, a coin is spining left handed then if you watch from the bottom, it will spin right handed. But from the angle of physical structure, the coin does not change... I mention a pig above, it means "different methods of desription".
When we say that an electron has a quantity called "spin" we aren't talking about something rotating about an axis. Spin is a result of Quantum Mechanics merged with SR. (For non-Relativistic systems we can add a factor to the solution of the Schrodinger equation. You can see this most vividly by the solution of an electron orbiting a nucleus, hydrogen specifically. The solution simply does not work without spin included.) The spin is not in "real space", but in an "internal space." In fact, the electron has spin 1/2 and thus has to go around twice in order to get back to its original orientation!

The reason the term spin is used is because it does share similar Mathematical properties with the spin of an object in real space. It's unfortunate that rotation and spin have similar uses. The reason it has survived is because, while spin is not the same thing as a rotation, it still contributes to the angular momentum of the electron. So the notation will live on.

Ever since QM was born Physicists have tried to explain what's happening by analogy and many of those analogies don't do a good job of explaining it. Spin and orbital are terms that should probably be revamped. But we've been doing it for over 100 years so I doubt anything is going to change.

-Dan
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 May 14th 2019, 08:06 AM #17 Senior Member   Join Date: Mar 2019 Location: cosmos Posts: 195 @dragon What's "left - hand", "right - hand"?
May 14th 2019, 09:59 AM   #18

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 Originally Posted by neila9876 @dragon What's "left - hand", "right - hand"?
Unfortunately after I just went on a rant about making analogies I'm now going to make an analogy! Sorry.

Let's use a rotation in real space for simplicity. Say that we have a particle that is moving along the, say, +z axis and that the particle has a spin. Use your right hand "grab" the velocity vector (pointing in the +z direction) with your thumb in the direction of the velocity. Now, if the particle is rotating in the direction your fingers curl then the spin is "right handed." If the spin is in the opposite direction then it's "left handed." Most particles are either right handed or left handed depending on the reference frame of the observer, just like you would expect. But a neutrino is only left handed and an anti-neutrino is always right handed no matter what the reference frame is. Unfortunately there is no simple picture I can give you for the neutrino.

There is a way to do this just using Mathematics but unless you've worked with the Dirac equation it would likely confuse you. Let's leave it at the level of there is a way to do it. (By the way, the Dirac equation is a spin 1/2 relativistic equation, much like the Schrodinger equation is a spin 0 non-Relativistic equation.)

-Dan
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Last edited by topsquark; May 14th 2019 at 10:03 AM.

 May 14th 2019, 04:54 PM #19 Senior Member   Join Date: Mar 2019 Location: cosmos Posts: 195 pig vs pig @dragon: Good Kongfu (好功夫)... Thank you. I remember a sentence of one QM founder (but I forgot his name): "We don't know why we should calculate it in that way, also we don't know what's the actual physical meaning, we only know that if we calculate in that way, we will get interesting result." The terms "real space" as well as "internal space" of your post above make me feel that people have wandered on the brink of another spacial dimension for many decades..Nothing should be hypothetical in real cosmos. My thread in the Quantum column "wavicle: an old topic" actually is trying to find out some physical meaning, even a bit, at the very beginning of QM, it will be a good thing... The method of description is not sole. 横看成岭侧成峰，远近高低各不同；不知庐山真面，只缘身在此山中。
May 15th 2019, 04:02 AM   #20

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 Originally Posted by neila9876 The terms "real space" as well as "internal space" of your post above make me feel that people have wandered on the brink of another spacial dimension for many decades..Nothing should be hypothetical in real cosmos.
I think we have a bit of a language problem here. I'll try to fix it but I'm not good at this sort of thing.

When I use the term "real" in "real space" I am refering to the fact that we are working with a physical 4D space-time. The internal spaces are real in the sense that they exist in some fashion, just not in the physical space-time. Whether they are real in the sense of being physical is a matter for the Philosophers.

-Dan
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