Mass in Beta - Decay
Asked by: puzzledoldman-ga
List Price: $25.00
04 Aug 2003 05:52 PDT
Expires: 03 Sep 2003 05:52 PDT
Question ID: 238805
Using the word mass in its invariant form, rather than its relativistic form, what are the best current estimates of the masses of the neutron and of the proton, electron, and anti-neutron into which the neutron is believed sometimes to disintegrate?
Re: Mass in Beta - Decay
Answered By: mathtalk-ga on 04 Aug 2003 19:51 PDT
Hi, puzzledoldman-ga: By "invariant form" of mass, I suspect you mean the "rest mass" of a particle, which is independent of its velocity ("relativistic form" ?). On the other hand perhaps you are asking for the mass of the particles to be given in "grams" rather than in millions of electron volts, which might be another meaning of "relativistic form". I'll try to accomodate both points. The beta-decay of a free neutron produces: - a proton - an electron (or "beta" particle) - an (electron) anti-neutrino through a "weak interaction". See here: [Decay of the Neutron] http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton.html#c4 The neutron is a baryon, like the proton, and an electron is a lepton with lepton number 1. The presence of all three particles in the "yield" provides for conservation of both baryon number (1 in, 1 out) and lepton number (because the lepton number of the electron anti-neutrino is -1). [Physical Constants and Conversion Factors] http://newton.ex.ac.uk/research/semiconductors/theory/collabs/constants.html The rest mass of the neutron is experimentally determined to be: mass of neutron = 1.6749286(10)e-24 gram The rest mass of the proton is experimentally determined to be: mass of proton = 1.6726231(10)e-24 gram The rest mass of the electron is experimentally determined to be: mass of electron = 9.1093897(54)e-28 gram Experimental determination of the rest mass of the electron anti-neutrino has proven to be a difficult and perhaps profound issue. All known experimental evidence is consistent with a zero rest mass for the electron neutrino (equiv. for the electron anti-neutrino), but a tiny mass on the order of 1eV or 1.78e-33 gram has not yet been excluded. For links to papers on measurements of neutrino masses, see here: [Neutrino Mass - Direct Measurement] http://www.to.infn.it/~giunti/NU/Neutrino_Mass/ On theoretical grounds it is believed that observations of "oscillation" between flavors of neutrinos, which are thought to be three (electron, muon, tau), show that some (presumably all) of the flavors have mass: [Neutrinos Have Mass] http://focus.aps.org/story/v2/st10 Please advise me if further clarification would be useful. regards, mathtalk-ga
Re: Mass in Beta - Decay
From: hedgie-ga on 09 Aug 2003 06:56 PDT
What does that means in physics? Modern physics equations are formulated so that they are valid in all frames of references. We can transform the quantities from one frame of reference into another, e.g. into one which is moving with respect to the former. The laws of transformation are different in Newtonian and Relativistic physics. (Both sets of equations are invariant these days. It was not so when people were still arguing about heliocentric vs.geocentric models). In Newton's universe the masses before and after an event add up to the same number. In this sense mass is conserved, and is also truly invariant, i.e. same in all frames. Better way to say it, mass is a scalar. So is Energy . Both are conserved. Event may be collision, disintegration, etc. In Einstein's universe it is more complex. There are quantities which are conserved, and some of them are scalars, e.g. the lepton and barion numbers mathtalk is talking about. Some are vectors, and they have a complex law of transformation. Since classical physics approximates relativity in same cases, we can point out a quantity which has some properties of classical mass and energy. This, this correspondence, is source of a lot of semantic confusion you see on the Internet. Mass, or more exactly mass-energy is not a scalar in this aproximation or analogy. In the context of your question it means: there is no single-number-quantity, which you can add (for all particles in the event) and get same value, before and after the event, in any frame. In other words, there is no exact equivalent of scalar mass in relativity. There are many quotes about Internet. One cartoon says 'On Internet no one knows you are a dog' (or a hedgehog :-) I would add to the collection, the following: "On Internet, everybody understands relativity. But many understand it differently than most textbooks." If you are seriously interested, you may ask for recommended reading, to cover the area of interest at an appropriate level of complexity. This is just a free comment. It recommends a critical, skeptical approach to what you read. Particularly when it is on the Internet, and for free.
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