The Omega Baryon

The Field Guide to Particle Physics : Season 2
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The Omega Baryon

Introduction

The Omega Baryon is the strangest particle we have encountered so far. It may also be the strangest particle known to Science, literally.

With a mass of 1672.4 MeV, the Omega Baryon is heavy. As well it should be. It is comprised of three, strange quarks. The three strange quarks gives the Omega an electric charge of three times minus one third, or minus one.

Those strange quarks also gives it the unusually long lifetime of about 8% of a nanosecond.

While short by our standards - even a bit shorter than some other strange particles - a solid fraction of a nanosecond is an enormous lifetime for a particle with such an enormous mass.

The Decay of Strange Quarks

As if on brand, this strangest of the strange particles lives for so long precisely because its made from only strange quarks. The strange quarks, you might recall, struggle to decay. They wouldn’t decay- actually - if not for a mild identity crisis.

The strange quarks talk to other particles both by photon, gluon and by W boson. That is, in addition to the electromagnetic force, strange quarks communicate via both the strong and weak nuclear forces. From the strong force’s perspective, strange quarks are distinct. Just like the up and down quarks. Nobody is confused, all that that subnuclear gu respects their identity as strange quarks.

The weak force hedges a bit. The W-boson in particular is a little confused on who is who, and from its perspective down and strange quarks are a little mixed. Just like North and West mean slightly different things to a compass or a cartographer, down and strange quarks appear slightly different to the strong and weak forces. They’re almost aligned, but not quite.

As a result, the strange quark decays by W boson as if it were a down quark. That decay is amplified by the strange quark’s heavy mass, but its still a small effect. The weak nuclear force is… well… weak.

Being made of three strange quarks, the Omega baryon decays once one of its constituents does.

Omega Baryon Decay Channels

The Omega minus decays when one of its strange quarks throws out a W boson, changing its identity to an up quark.  Typically the W-boson then decays to a pair of quarks itself, an antiup and a down quark. This all happens quickly inside the baryon itself, which subsequently explodes into a pair or triplet of particles. There there are a number of possible results.

Two-thirds of the time, that anti-up quark SCORES and runs away with one of the Omega’s other strange quarks, creating one of those tricky K minus mesons that we’ve discussed previously. What’s left over? An up, a down and a strange quark, which manifests as a Lambda zero baryon.

Twenty-three percent of the time, that anti-up quark isn’t so lucky. It remains stuck so the down quark that came from W boson, which together run away as a negatively charged pion.  The quarks that remain - two strange and an up - comprise the neutral cascade or Xi baryon, which of course leads to its own cascade of particle decays.

Almost all the rest of the time - that’s about 8% for you bean counters out there - the Omega baryon spits out a neutral pion, decaying to a Xi minus instead.  For this to happen, that down quark has to hold on tight to that pair of strange quarks that didn’t decay.

On extremely rare occasions, instead of a neutral pion, the Omega decays to Xi minus by spitting out a pi+ pi- pair. This could happen, for instance, the resulting up and antiup quarks happened to find a down-anti down pair inside the subnuclear goo.

Spin and the Decuplet

In addition to having three strange quarks, the Omega baryon also has three times the angular momentum of most baryons we’ve encountered so far. Inside the baryon, those three strange quarks are all zooming around each other, extra fast.

We’ve seen this behavior before, when we looked into the Delta baryons. The Delta Plus Plus baryon, you might remember had three up quarks. And the Delta Minus baryons, which had three down quarks.

In a sense, the Omega baryon is the strange version of those beasts. Because of that simple, three strange quark arrangement, the Omega baryon was predicted to exist well before it was found. 

Well, that’s… not exactly right. In fact, it’s exactly backwards. Back in the 50s and early 60s nobody knew what a quark was, or how baryons were even organized. They just had all those wild names: Delta. Sigma. Xi. This zoo of strange particles was something of a mystery. 

The physicist Gell-Mann (and, independently Ne'eman) chased the patterns of all the particles and their decays and divsed the quark model to fit those data. There was only one problem: one particle was missing.  

The Omega baryon was discovered as a short stub of a line which appeared on a photographic plate at Brookhaven national Lab. It had essentially the same mass, spin and charge that Gell-Mann predicted, ushering in the first of many experimental verifications of the quark model of subnuclear physics.

And that concludes our second - STRANGE - season of the Field Guide to Particle Physics! We’ve got a few bonus episodes, stories and other extras in store, including a bonus series on Gell-Mann’s Eightfold Way and more details on how particles like the Down and Strange quarks mix. So please stay tuned and subscribe for more!