- The Lambda (uds) - 1115 MeV
- The Sigma (uds) - 1192 MeV
Other than the 77 MeV, why are these 2 different particles? Is it the spin of 1 quark? If so, is it possible to determine which one? If not spin, what could it be?
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Looking at the Particle Data Group summaries, the $\Lambda $ bayrons have strong isospin $I=0$, which means that there is only one charge state. The $\Sigma$ baryons, on the other hand, have $I=1$.
This means that all $\Lambda$ baryons are electrically neutral, while the $\Sigma$ (like the $\pi$ meson, which also has $I=1$) may have charge zero or $\pm1$.
For what it's worth, the names attached to these particles categorize them by their quantum numbers; the links above list dozens of particles classified as $\Lambda$ or $\Sigma$. The lowest-mass $\Sigma^0$, which I assume you're referring to as "the Sigma," isn't heavy enough for the isospin-conserving decay $\Sigma^0\to\Lambda\pi$, and therefore decays via the isospin-changing channel $\Sigma^0\to\Lambda\gamma$. However, the more massive $Σ(1385)$ may undergo the isospin-conserving decay, and therefore has a much briefer lifetime.
The lightest $\Lambda$ baryons, however, can't decay without undergoing a strangeness-changing weak interaction, so the decay of the lightest $\Lambda^0$ is $10^{10}$ times slower than the decay of the lightest $\Sigma^0$.
rob
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"[Sigma baryons] are closely related to the Lambda baryons, which differ only in the wavefunction's behaviour upon flavour exchange."
my2cts
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