LHCb experiment discovers a new pentaquark | Hacker News Reader

LHCb experiment discovers a new pentaquark

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95 pointsljdk19 comments

So, what is a pentaquark exactly? The article doesn't say.

https://en.wikipedia.org/wiki/Pentaquark

if you click through on the 'read more' to: http://lhcb-public.web.cern.ch/lhcb-public/Welcome.html#Pent...

including the detail that the quark state content is: "duucc: four quarks and one antiquark. "

Theres a further 'read more' link to a powerpoint, http://moriond.in2p3.fr/QCD/2019/TuesdayMorning/Skwarnicki.p... which is way out of my understanding

Penta = five. There's a picture of a ball with five balls in it. It decays to a proton (which as mentioned has three quarks) and another pair of a quark and an anti-quark. I'd say it's there.

Usually quarks are confined in composite particles of two (mesons) or three (baryons) quarks. However they can also in certain conditions form particles of 5 quarks (the pentaquarks).

Oh, so quarks can only be composites of two, three, and five? Any reason that could be stated in layman's terms?

> Oh, so quarks can only be composites of two, three, and five?

They can also be in fours. Possibly other configurations; the four and five quark configurations were theorized in 1964 but only confirmed in 2014 & 2015.

> Any reason that could be stated in layman's terms?

None at all, in fact, the source article does so: “In the conventional quark model, composite particles can be either mesons formed of quark–antiquark pairs or baryons formed of three quarks. Particles not classified within this scheme are known as exotic hadrons. When Murray Gell-Mann proposed the quark model in his fundamental 1964 paper, he mentioned the possibility of exotic hadrons such as pentaquarks, but it took 50 years to demonstrate their existence experimentally.”

(There is slightly more detail on the read more link, too.)

> > Any reason that could be stated in layman's terms?

> None at all, in fact, the source article does so ….

Did you really mean "None at all"? The rest of your post seems to say the opposite.

Quantum chromodynamics (the theory which describes the strong interaction) has a feature called colour confinement, which says that quarks will favour being in colourless configuration (where colourless = zero colour charge). The two easiest ways for this to happen are in mesons (two quarks: one quark and one antiquark of the same colour, like red + antired = colourless), or in baryons (three quarks: 1 red + 1 green + 1 blue = colourless) like the familiar proton and neutron. However other configurations are possible, these are just the simplest ones. In certain incredibly difficult to attain conditions, we can avoid producing either a two- or three-quark composite particle but produce instead a five-quark particle (e.g. 1 red + 1 green + 1 blue + 1 red + 1 antired = colourless).

This "colour" means colour charge, it doesn't have any relation to the regular meaning of "colour of light".

> This "colour" means colour charge.

Color charge is essentially as charge. I.e. instead of having +/- you have A/B/C. Except you also have Anti A/B/C.

Basically, take the numbers 1/3, -1/3, and count how many terms you need to produce something that sums to a whole number. You can get four with (1/3 + 1/3 - 1/3 - 1/3), so it's not an impossible configuration.

> Basically, take the numbers 1/3, -1/3, and count how many terms you need to produce something that sums to a whole number.

Any whole number of terms greater than 1 will allow this, by the way.

> Basically, take the numbers 1/3, -1/3, and count how many terms you need to produce something that sums to a whole number.

Shouldn't there be +2/3 and -2/3 charges as well? Otherwise the only way to do this is with an equal number of +1/3 and -1/3 charges (so not 5 total, for example).

You can get five with (1/3 + 1/3 + 1/3 + 1/3 - 1/3). Note that I'm not really talking about charges directly so they don't need to be equal, but rather I'm using these numbers as a proxy for charges. So this configuration might be something like (red, green, blue, red, antired).

It's just a quick rule for showing how many quarks can fit together, not what kinds of quarks they are.

> You can get five with (1/3 + 1/3 + 1/3 + 1/3 - 1/3).

I'm sorry; although you very clearly referred to winding up with a whole number, I somehow read it as winding up with 0.

If you're willing to dive in a bit, Matt Strassler has a nice blog aimed at the lay reader. His mini-series on the structure of the proton is quite excellent:

https://profmattstrassler.com/articles-and-posts/largehadron...

This is both entertainment and information, so to justify it I'll ask a question about this: https://www.youtube.com/watch?v=j50ZssEojtM (Large Hadron Rap)

Are these goals still accurate or has new knowledge and engineering changed the goals of the detectors?

The rap is pretty bad. It lists four questions: dark matter, matter-antimatter asymmetry (aka baryogenesis, related to CP violation), Higgs boson, and hierarchy problem (why gravity is so weak; rap alludes extra dimension).

We found Higgs boson, which is great. We are no closer to answer other three questions, and frankly, there was no reason to expect LHC to help there, it was all wishful thinking. Sure, it was possible, LHC would do what has never been done, but not very probable.

As I see it, we are basically putting more and more energy into these particle collisions and hoping something unexpected happens.

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