Last week a team at CERN says they might have seen some pentaquarks! Physicists have been looking for them. Back in 2005 Japanese researchers claimed they saw some, but this was later discredited. I hope this new claim holds up.
What's a pentaquark
? It's not really 5 quarks. It's actually 4 quarks and an antiquark, all held together by exchanging other particles called gluons.
Let's start with something easier: a neutron, as shown here. A neutron consists of 3 quarks: one up quark and two down quarks. They're actually zipping around like mad in a blurry quantum way, but this movie simplifies things.
Besides coming in various kinds, like up and down, quarks have an easily changeable property called color
. This is nothing like ordinary color - but color serves as a convenient metaphor, and physicists occasionally have a sense of humor, so that's what they called it.
There's a lot of math underlying this story, but let's sweep that under the carpet and talk about color in simple terms, so you can explain pentaquarks to your children and parents.
Quarks can be in 3 different colors, called red
. But they can only stick together and form a somewhat stable particle if all three colors add up and cancel out to give something white
. So, protons and neutrons are made of 3 quarks.
The quarks stick together by exchanging gluons
, which have subtler colors like red-antigreen
If you watch this movie of a neutron, you'll see a red quark emit a red-antigreen gluon and turn green. This red-antigreen gluon is then absorbed by a green quark, turning it red. Color is conserved like this! The total color of the neutron remains white.
You can't build something white out of just a single quark, so we never see lone quarks in nature. The closest you can come is at insanely
high temperatures when everything is shaking around like mad and you get a quark-gluon plasma. I'm talking temperatures of several trillion degrees Celsius! People have gotten this to happen at places like the Relativistic Heavy Ion Collider on Long Island, New York.
You also never see a particle built of just 2 quarks. Again, the reason is that it can't be white.
But you can get particles built of a quark and an antiquark - their colors can cancel.
You can't build a particle out of 4 quarks, because the colors can't cancel.
But you can do 3 quarks together with an extra quark and antiquark! And that's called - somewhat misleadingly - a pentaquark.
Here's the paper:
• LHCb collaboration: R. Aaij, B. Adeva, M. Adinolfi, A. Affolder, Z. Ajaltouni, S. Akar, J. Albrecht, F. Alessio, M. Alexander, S. Ali, G. Alkhazov, P. Alvarez Cartelle, A.A. Alves Jr, S. Amato, S. Amerio, Y. Amhis, L. An, L. Anderlini, J. Anderson, G. Andreassi, M. Andreotti, J.E. Andrews, R.B. Appleby, O. Aquines Gutierrez, F. Archilli, P. d'Argent, A. Artamonov, M. Artuso, E. Aslanides, G. Auriemma, M. Baalouch, S. Bachmann, J.J. Back, A. Badalov, C. Baesso, W. Baldini, R.J. Barlow, C. Barschel, S. Barsuk, W. Barter, V. Batozskaya, V. Battista, A. Bay, L. Beaucourt, J. Beddow, F. Bedeschi, I. Bediaga, L.J. Bel, V. Bellee, N. Belloli, I. Belyaev, E. Ben-Haim, G. Bencivenni, S. Benson, J. Benton, A. Berezhnoy, R. Bernet, A. Bertolin, M.-O. Bettler, M. van Beuzekom, A. Bien, S. Bifani and 662 other authors, Observation of J/ψp resonances consistent with pentaquark states in Λ0b→J/ψK−p decays, http://arxiv.org/abs/1507.03414
It's not unusual to have lots of authors on these papers, but it's rather unusual to list them in alphabetical order. I like that system, especially since it usually puts me near the front.
In case you're wondering, the theory behind all this is quantum chromodynamics
, which is based on quantum field theory, in particular Yang-Mills theory, and on the representation theory of the group SU(3).
It is conjectured but not yet proved that quantum chromodynamics is mathematically consistent and that stable particles must all be 'white', that is, transform in the trivial representation of SU(3). We describe quarks using the fundamental representation of SU(3) on C^3, which has 3 basis vectors whimsically called red, blue and green. We describe antiquarks using the dual representation, which has 3 basis vectors called anti-red, anti-blue and anti-green. We describe gluons using the adjoint representation, which has basis vectors like red-antiblue.
If you want to carry the color analogy even further, you can call anti-red, anti-blue and anti-green cyan
. However, you need to be careful. Cyan, yellow and magenta do not combine to form 'black'. They form 'antiwhite', but antiwhite is white - that's what the math says, and the math is more fundamental than the cute analogy to colors.
Also, gluons only come in 8 colors, not 9.Puzzle 1:
Why? If you know some math you may know SU(3) is 8-dimensional so we can't get 9, but try to explain the story in terms of colors.Puzzle 2:
If you build particles using only quarks, not antiquarks, could you build something white with 4 quarks? How about 5? How about 6? What's the rule?
For more, read:https://en.wikipedia.org/wiki/Color_charge #spnetworks