What's up with the pentaquark?

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Joe - 27 Oct 2004 16:55 GMT

Hi,

I attended a lecture here where a researcher gave a talk on the
pentaquark. He said that in 1997, the pentaquark was theorized, and
then they went around trying to get experimenters interested in
looking to see for its possible existence. And, eventually, some
people got interested and looked for it, and found it! Exactly at the
right mass and width according to the theory. Then other people
started finding it too. And it made some good news. (Just google
pentaquark.) What a success. Right? However, there is a small
problem that the researcher pointed out. The theory behind the
pentaquark is flawed! And that anybody who knows what they're talking
about and looks into it, can see this. He himself went through why he
thinks it is wrong. He dumbed down the problem for us, and I still
couldn't quite follow it. But it involved using a trick in coming up
with the pentaquark, a trick that doesn't quite work in this
particular regime. Even the person who came up with this particular
trick many years ago, has spoken up and said it is being used wrongly.
And yet, the experimenters say they've found it. So it is very
puzzling. Have any of you heard of this? I'm cross posting this to a
moderated newsgroup, unfortunately. But I'd like to read what others
have to say on this matter. So I ask, are any of you aware of this
funny thing in particle physics right now? Is this funny thing
happening? (I can't find any 'controversy' over the net.)

Thanks,
Joe

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Sam Wormley - 03 Nov 2004 16:43 GMT

A Five-Quark State was Discovered in 2003
http://www.aip.org/enews/physnews/2003/split/644-1.html

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Hendrik van Hees - 04 Nov 2004 10:42 GMT

> A Five-Quark State was Discovered in 2003
> http://www.aip.org/enews/physnews/2003/split/644-1.html

Well, that's a hot issue. There are some labs finding the pentaquark
(called Theta^+), some don't see it. I was a bit surprised when I found
out that in the newest particle data booklet this is sufficient to make
it a three-star resonance. The accompanying text in the review of
particle physics from the particle data group is much more careful:

http://pdg.lbl.gov/2004/listings/b152.pdf

Indeed there are some very impressive experimental evidences (if not
even a discovery) of 5-7 \sigma confidence level (JLAB, COSY etc.), but
as I heard last week during the DNP Fall meeting in Chicago the signals
are weak (in the JLAB peak only fourty events are contained). Now we
have to wait for better statistics.

Another issue is the explanation, why many labs (all high-energy
experiments!) do not see the pentaquark, i.e., one has to find out the
reaction mechanism.

It would also be highly interesting to get the precise spin and parity
of the resonance, if there is really one.

So before we claim "discovery" a lot of issues, both experimental and
theoretical, have to be clarified. For these reasons, I'd say there is
some strong evidence, but not a discovery yet.

Signature

Hendrik van Hees Cyclotron Institute
Phone: +1 979/845-1411 Texas A&M University
Fax: +1 979/845-1899 Cyclotron Institute, MS-3366
http://theory.gsi.de/~vanhees/ College Station, TX 77843-3366

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alistair - 03 Nov 2004 17:03 GMT

> Hi,
>
> The theory behind the
> pentaquark is flawed!

A pentaquark is predicted to contain a diquark in which the colour
spins are antiparallel to one another and the electromagnetic spins
are also antiparallel to one another.This forms a very low energy
combination which has been
found in protons and neutrons: electrons will bounce back in the
direction they came from - when they strike a neutron - one quarter of
the number of times that they bounce back in the same direction for a
proton.This has been interpreted (by Wilzcek who recently won the
nobel prize) as happening because there is an isolated negatively
charged down quark in a neutron (the other down quark forms a diquark
with an up quark),which repels the electron more than a corresponding
positively charged, isolated up quark in a proton - the down quark
reducing the chances of a direct hit.

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Bjoern Feuerbacher - 04 Nov 2004 10:38 GMT

>>Hi,
>>
[quoted text clipped - 4 lines]
> spins are antiparallel to one another and the electromagnetic spins
> are also antiparallel to one another.

Sorry, but what do you mean by "colour spins"?

> This forms a very low energy combination which has been
> found in protons and neutrons: electrons will bounce back in the
> direction they came from - when they strike a neutron - one quarter of
> the number of times that they bounce back in the same direction for a
> proton.

This is news to me. Reference, please.

[snip]

Bye,
Bjoern

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drnobes@gmail.com - 03 Nov 2004 17:06 GMT

> I attended a lecture here where a researcher gave a talk on the
> pentaquark. He said that in 1997, the pentaquark was theorized, and
[quoted text clipped - 6 lines]
> problem that the researcher pointed out. The theory behind the
> pentaquark is flawed!

I'm not entirely sure what this means. What "theory" are you talking
about? In strong interaction physics the *theory* is QCD. I don't
think anybody is proposing that that is flawed.

However, QCD is really hard to calculate with, so people make models.
Typically these models include some aspects of QCD, and leave others
out. My knowledge of the zoo of models isn't great, but IIRC the
chiral soliton model was the one that "predicted" the pentaquark. Now
there may well be something "wrong" with the chrial soliton model, in
that it doesn't capture some other aspects of QCD, but that's okay,
since it's only a model.

> And that anybody who knows what they're talking about and looks into

it,

> can see this. He himself went through why he
> thinks it is wrong. He dumbed down the problem for us, and I still
> couldn't quite follow it.

Without more details (which model, what's wrong with it) I can't
comment.

> But it involved using a trick in coming up
> with the pentaquark, a trick that doesn't quite work in this
> particular regime. Even the person who came up with this particular
> trick many years ago, has spoken up and said it is being used wrongly.
> And yet, the experimenters say they've found it. So it is very
> puzzling. Have any of you heard of this?

I've certainly heard of the pentaquark, and the chiral soliton model.
What's needed is full QCD calculations, using lattice qcd. But these
are very hard to do.

This paper, http://www.arxiv.org/abs/hep-lat/0410016, is probably a
good place to start if you want to learn the status of lattice QCD
calculations of pentaquark properties.

> I'm cross posting this to a
> moderated newsgroup, unfortunately. But I'd like to read what others
> have to say on this matter. So I ask, are any of you aware of this
> funny thing in particle physics right now? Is this funny thing
> happening? (I can't find any 'controversy' over the net.)

It's "happening" though I personally don't see a controversy. There's
a bunch of models, which you can do calculations with, and then there's
the actual theory, where it's hard to do calculations.
Matt

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alistair - 06 Nov 2004 18:03 GMT

Bjoern Feuerbacher wrote in message:
alistair wrote:

> A pentaquark is predicted to contain a diquark in which the colour
> spins are antiparallel to one another and the electromagnetic spins
> are also antiparallel to one another.

>Sorry, but what do you mean by "colour spins"?

The orientation of the colour charge.

>This is news to me. Reference, please.

The reference is The New Scientist, July 3rd 2004
Article "The power of five."

JR Minkel spoke to Frank Wilczek and others.

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Natural Science Forum / Physics / Particle Physics / November 2004

What's up with the pentaquark?