Particle Physicists Discover Lowest Energy "Bottomonium" Particle
Written by Ian O'Neill
Particle physicists working with the BaBar detector at Stanford Linear Accelerator Center have discovered a new particle in the bottomonium family of "quarkonium" particles. Technically it isn't a "new particle" it is a previously unobserved state of particle, but when we are talking about subatomic particles, their energy states become a big deal (and their names get very cool). We are in the realms of the vanishingly small and the discovery of the lowest energy bottomonium particle may not seem very significant. But in the world of quantum chromodynamics, this completes the long quest to find experimental evidence for this elusive meson and may help explain why there is more matter than anti-matter in the Universe…
Quarkonia are types of mesons containing two quarks: one quark and its anti-quark (they are therefore "colourless"). They belong to one of two families: "bottomonium" or "charmonium". As the names suggest, bottomonium contains a bottom quark and anti-bottom quark; charmonium contains a charm quark and anti-charm quark. Groups of three quarks (interacting via the strong force) are baryons (i.e. protons and neutrons) whereas groups of two quarks are mesons. Mesons are all thought to be made from a quark-antiquark pair and are therefore of huge importance when studying why there is more matter than anti-matter in the Universe.
This is where the BaBar detector at the Stanford Linear Accelerator Center (SLAC), CA, comes in. The BaBar international collaboration investigates the behaviour of particles and anti-particles during the production of the bottomonium meson (bottom-antibottom quark pairs) in the aim of explaining why there is an absence of anti-particles in everyday life.
For each particle of matter there exists an equivalent particle with opposite quantum characteristics, called an anti-particle. Particle and anti-particle pairs can be created by large accumulations of energy and, conversely, when a particle meets an anti-particle they annihilate with intense blasts of energy. At the time of the big-bang, the large accumulation of energy must have created an equal amount of particles and anti-particles. But in everyday life we do not encounter anti-particles. The question, therefore, is "What has happened to the anti-particles?" – From the BaBar/SLAC collaboration pages.
All matter has a "ground state", or the lowest energy the system is trying to attain. As particles for instance try to reach this ground state, they lose energy, often in the form of electromagnetic radiation. Once reached, the ground state determines the baseline at which measurements can be made for higher energy states of those particles. And this is what the BaBar team has done, they have been able to isolate the lowest possible energy state for the bottomonium particle (which is far from easy). So what have they named the ground state of bottomonium? Quite simply: ηb, pronounced "eta-sub-b".
The bottomonium particle was generated during a collision between an electron and positron. The energy generated by this collision created a bottom quark and an anti-bottom quark bound together. At this point, the bottomonium particle was of too high an energy, but it very quickly decayed, emitting a gamma ray leaving the ηb behind. However, ηb's are highly unstable and will quickly decay into other particles, plus they are very rare and difficult to detect. This particular decay event only occurs once in every two or three thousand higher energy bottomonium decays, so many collisions had to be measured and a huge amount of data had to be gathered by the BaBar detector before a precise measurement of the ηb ground state could be gained.
"This very significant observation was made possible by the tremendous luminosity of the PEP-II accelerator and the great precision of the BaBar detector, which was so well calibrated over the BaBar experiment's 8-plus years of operation. These results were highly sought after for over 30 years and will have an important impact on our understanding of the strong interactions." – Hassan Jawahery, BaBar Spokesperson, University of Maryland.
If you want to find out more, you can check out the BaBar team's publication (with the longest list of co-authors I've ever seen!) or the SLAC press release.
Source: SLAC
Filed under: Physics
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July 10th, 2008 at 9:57 am
I was thinking of why there is less anti-matter, or at least, more hidden or whatever.
Not an expert at this mind you (been out of particle physics for awhile to learn other things), but i will explain as best as possible.
Matter emits radiation as it looses energy, right?
What does anti-matter emit?
If a positron were to fall a level, would it emit something similar to that of an electron falling through the same level? (does anyone even *know* this yet? Didn't some guys create anti-hydrogen awhile back? What happened there?)
Matter can absorb electromagnetic radiation and gain energy, does anti-matter do the same, the exact same way?
I bring this up because there are all these ideas about negative mass, negative energy and so on.
Hopefully LHC will bring more insight into this.
July 10th, 2008 at 10:43 am
So what about "toponium"?
Can't an anti-charm and a bottom come together to form a meson too? Or does it have to be two of the same 'kind'?
July 10th, 2008 at 4:28 pm
Hi Hunnter: I'm no expert either, but I think anti-matter pretty much acts in the same way as matter. So you can expect positrons and electrons to emit radiation as they drop through energy levels.
To Sili: Apparently toponium is not possible as the top quark's mass is too large. There is quite a nice writup on Wikipedia (I know the purists out there don't like Wikipedia, but it is a good source if all the references are stated): http://en.wikipedia.org/wiki/Quarkonium
Hope that helped a little
Ian
July 10th, 2008 at 6:32 pm
There is indeed mixed family mesons. The kaon for example has one strange or anti-strange quark bound to either an up or down (or anti) quark. This is not quarkonium since it is not a bound state of a same quark with it's anti-quark. As to what is emitted when a positron jumps to a lower state in anti-hydrogen it's the same as hydrogen (as far as can be determined)… i.e. a photon with a specific energy. Since a photon can be considered as it's own anti-particle it makes sense that both electrons and positrons emit (or absorb) a photon when transitioning to a different energy level.
July 11th, 2008 at 2:20 am
We have observed quarkonium states for all of the quarks except the top quark.
up, down: Mix together to make the neutral pion
strange: Phi meson
charm: J or Psi meson
bottom: B or Upsilon meson
Source: http://everything2.com/title/meson
July 11th, 2008 at 7:14 am
A wild thought! Can any three dimension particle be split if enough energy is available to do the job? If so, is there a finite limit to this "splitting?"
July 11th, 2008 at 7:23 am
Since energy is motion or the capacity to move, theh how can you have "negative energy"? How do you move slower then standing still?
July 11th, 2008 at 8:48 am
neoguru:
Well, with negative energy, things defy common sense laws of the world we are used to.
The Wikipedia article on it explains it fairly well, my explanation was much much worse… heh.
http://en.wikipedia.org/wiki/Exotic_matter
If matter like this could exist and used, it could potentially be the biggest breakthrough since electricity.
There are a bunch of things that could be created to (ab)use the "negative" properties.
One could be near-perfect frictionless surfaces, lightweight too since the negative mass would repel itself as well, as long as you create a strong-enough bond that is.
July 11th, 2008 at 7:46 pm
Not a new particle per se, but just a higher excited state of a bottom quark and a bottom anti-quark.