# What prevent reaction changing leptons to hadrons?

1. Oct 10, 2011

### ndung200790

What prevent the reaction changing leptons to hadrons and vice versa?.Because both leptons and hadrons are ruled by electro-weak interaction,so why the reaction can not happen?
Thank you very much in advance.

2. Oct 10, 2011

Staff Emeritus
Try and write down a reaction that does this.

3. Oct 10, 2011

### humanino

$$\pi^\pm\rightarrow\mu^\pm\nu_\mu$$

4. Oct 10, 2011

### Polyrhythmic

Where did you read that it wasn't possible?

5. Oct 10, 2011

### Bill_K

The Wikipedia page on "Proton decay". It's never been observed, and current experimental limits give a lifetime greater than 1033 years.

6. Oct 10, 2011

### ndung200790

So,considering electro-weak interaction,the lepton and baryon number conservation is violated?

7. Oct 10, 2011

### ndung200790

And I do not understand why proton can not decay under electro-weak interaction?

8. Oct 10, 2011

### Polyrhythmic

9. Oct 10, 2011

### ndung200790

I hear that the Standard Model can deduce the lepton and baryon number conservation laws.Is that correct?

10. Oct 10, 2011

### ndung200790

Sorry,now I realize in pion decay process both lepton and baryon number are conservable.

11. Oct 10, 2011

### humanino

12. Oct 10, 2011

### humanino

Mesons have zero hadron number (quark-anti quark pair). The lepton number on the left hand side of the reaction above is also zero (lepton-antilepton).

13. Oct 10, 2011

### humanino

It can, but within the standard model it is a non-perturbative anomaly with exponentially suppressed probability, at least at the current temperature of the universe. Did you read the wikipage
http://en.wikipedia.org/wiki/Proton_decay
?

14. Oct 10, 2011

### ndung200790

Please teach me(sketch) how to demontration the lepton and baryon number conservation laws with Standard Model?

15. Oct 10, 2011

### arivero

Guys, instead of addressing the original poster, what about addressing the original post? I mean, the question is interesting by itself, to write down where Barion and Lepton numbers do appear in the standard model, and how the ungauged quantity upgrades to a gauged one in SU(5), in SO(10) and, differently perhaps, in Pati-Salam.

16. Oct 12, 2011

### ndung200790

So,at Standard Model level of knownledge,the lepton and baryon number conservation laws are based on experiment but not theoretical theory?

17. Oct 12, 2011

### arivero

Well, the SM preserves B and L obviously, in fact Weinberg discuss L explicitly in his fundational paper. When you go up to unified theories you find couplings violation B and L, but amazingly B+L (or B-L) is still preserved. So B violation (aka proton decay) become a warhorse against GUT theories: experiment does not allow for a gauge boson corresponding to B, L related charges.

More intriguingly, Salam and others proved that the putative SU(3)xSU(2)xU(1) symmetry of some 7 differential manifolds do not allow for charges as the standard model, but then Bailin and others hinted that using still an extra dimension solved the problem. And you get GUT groups then. But you are not supposed to have more than 7 extra dimensions, so this addition should not produce a gauge boson.

In any case, if you play some days (or weeks) with the hypercharges of the standard model, eventually you will find that a right isospin plus a B+L number do a better, more aesthetic, play that the SM hypercharge. Try.

18. Oct 12, 2011

### kloptok

As far as I know, yes - conservation of baryon and lepton number is primarily an experimental fact. (Although I would say that basically B and L are introduced just because they are a conserved quantity.)

More precisely, B and/or L conservation is not a gauged symmetry of the Standard Model, and therefore not as profound as for example conservation of electric charge. Therefore future unified theories could allow for B/L violation, as mentioned in previous posts.

19. Oct 13, 2011

### ndung200790

Thank very much all of you.Now I think that by combination color and flavor we can deduce the lepton and baryon number conservation laws, because the weak gauge bosons(that are (1,3) and (1,1)) can not change quark(that are (3,2) and (3,1)) into lepton(that are (1,2) and (1,1)).But in SU(5) theory,the conservation laws are violated,because in this theory there are superheavy gauge bosons(that are (3,2) and (bar3,2)) that permit quark change to lepton.For example (u,d)--->e+(X,Y)(that is (3,2)--->(1,1)x(3,2)).Is that correct?

20. Oct 13, 2011

### ndung200790

I mean in Standard Model,by combination of color and flavor we can deduce the lepton and baryon number conservation laws.