I'd like to ask about parity of baryon. When I search a parity section of textbook, it only explain about parity of meson, not baryon. And I can't find experimental method for parity determination of baryons. Am I missing something? Or is there special reason for baryon?
3
-
$\begingroup$ Maybe you have not looked at en.wikipedia.org/wiki/List_of_baryons . slide 2 here www.physics.ohio-state.edu/~kass/P780_L6_sp03.ppt . baryons by convention have parity + $\endgroup$– anna vCommented Sep 24, 2014 at 12:57
-
$\begingroup$ @annav While it's true that protons and neutrons have positive parity, that's not a general statement true for all baryons. Counterexamples here, here, here. $\endgroup$– rob ♦Commented Oct 4, 2014 at 15:05
-
$\begingroup$ @Rob you are correct. The convention is for protons and neutrons $\endgroup$– anna vCommented Oct 4, 2014 at 18:31
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
The proton and neutron are defined by convention to have wavefunctions that don't change sign under parity transformations, or positive parity. An antifermion has the opposite intrinsic parity of its matter counterpart, so the antiproton and antineutron have negative parity. Other baryon states have measured parities which are tabulated by the Particle Data Group; there are both positive- and negative-parity baryons.
To measure the parity of an unstable particle, you have look at what its decay products are and measure their angular distributions. For example, the N(1440) with $J^P=\frac12^+$ may decay to a $\Delta$ plus a pion with one unit of orbital angular momentum ("$p$-wave"), or to a nucleon and two pions with zero orbital angular momentum ("$s$-wave"). Since the pion has negative intrinsic parity, and the rule that the orbital wavefunctions with parity $(-1)^\ell$ are positive for $s$-wave and negative for $p$-wave, you can see that both these examples are positive-parity final states, and predict that the dominant decay mode $N\pi$ must have one unit of orbital angular momentum.
