Posted on 11/15/2003 8:43:52 PM PST by Diddley
The Belle collaboration at the KEK laboratory in Japan has discovered a new sub-atomic particle which it is calling the "X(3872)". The particle does not fit into any known particle scheme and theorists are speculating that it might be a hitherto unseen type of meson that contains four quarks (arxiv.org/abs/hep-ex/0309032; Phys. Rev. Lett. to be published).
The discovery has been confirmed by the CDF collaboration at Fermilab in the US, where the new particle is being called the "mystery meson". Mesons are particles that contain a quark and an antiquark that are held together by the strong nuclear force.
Since there are six different "flavours" of quark - up, down, strange, charm, bottom and top - it is possible to form a large number of different mesons.
The Belle team measured the decay of B-mesons - mesons that contain a bottom quark - produced in electron-positron collisions at the KEK B-factory in Japan. The team plotted the number of candidate events for B mesons against mass and observed a significant spike in the distribution at 0.775 GeV. This corresponds to a mass of nearly 3872 MeV. The particle decayed almost immediately into other, longer lived particles.
The KEK team says that the mass of this new meson is higher than theoretical predictions. Moreover, the way in which it decays also differs from theory. One possibility is that current models of the strong force need to be modified. Alternatively it could be that X(3872) is the first example of a "molecular state" meson that contains two quarks and two antiquarks.
Until recently particle physicists had only ever detected particles that contain two or three quarks. However, in the past year evidence has emerged for another four-quark particle known as the Ds(2317) and a five-quark particle known as the pentaquark.
Author Belle Dumé is Science Writer at PhysicsWeb
Ping. Hard experimental evidence for the existence of the graviton.
Can anybody offer an "explanation" for what the distance law is for the strong force?
Simply, the geometrical argument behind the inverse square law for gravity and electromagnetism is predicated upon two facts: the carrier particles are massless, and they don't interact. In the case of QCD (the modern theory of the strong force), the carrier particles are massless, but they interact via the strong force. In other words, the force is self-coupled. The result is that the force is proportional to distance.
[Geek alert: In the old Yukawa theory of the strong force, the carrier particles are massive, which leads to a force that drops off exponentially. This is the inter-hadron force that binds nuclei together. The carrier particle for this force is the pion, which can be envisioned as a quark-antiquark pair. The reason that this very real force isn't typically mentioned in discussions of the Standard Model is because it isn't a fundamental force. It is analogous to the Van der Waals force in electromagnetism.]
You can envision the force this way. As you pull two quarks apart, they exchange gluons, which are coupled to the color charges of the quarks. Each gluon also carries a color charge and an anticolor charge, so the exchanged gluons will also exchange gluons between them, and thus will be drawn together. The farther this "color flux tube" between the quarks is drawn out, the more gluons will be exchanged by the gluons mediating the quark-quark force, and the more tightly they will be bound. The flux tube thus acts like a spring. If you pull the quarks far enough apart, you will put enough energy into the flux tube to create a new quark-antiquark pair, which will "screen" the separating quarks from each other (i.e., snap the long flux tube into two short ones). It's rather like trying to separate a north pole from a south pole by cutting a magnet in half; all you get is two short magnets. The color charges are thus "confined" to the hadrons, which have a net color charge of zero.
Thanks. I get it. I think.
Physicist said: In other words, the force is self-coupled. The result is that the force is proportional to distance."
I don't get it. Do you have a simpler way to explain what the term "self-coupled" means in this context. What are the alternatives to being "self-coupled"? Did you mean to say that the force is constant with distance or proportional to distance. The former was the case I thought applied. Is the strong force proportional to distance?
Doesn't string theory offer a strong gravitational force - weakness being the illusion?
Thomas Bowles of Los Alamos National Laboratory in New Mexico. "In the quantum realm, the gravitational force is so weak that it is difficult to observe quantum effects."
The strong force is proportional to distance, not constant with distance.
By "self-coupled", I mean that the strong force interacts via the strong force. Imagine what optics would be like, if every photon carried an electromagnetic charge.
One of the serious problems with classical physics is the presumed possibility of action at a distance. Mathematically, it is possible to treat the net force on a particle as if it is the result of a "field" where the field strength at any point in space is the vector sum of all the individal sources of force.
For example, the electric field in a space containing four protons can be calculated at any point in space by assuming that a sample charge exists at that point and is being acted upon by the four protons. Once you know the value of the field, you can ignore the protons.
The problem with this is that the field then becomes the cause of motion of a particle being acted upon. There is no delay associated with the distances to the protons. If one of the protons moves, the field would instantly reflect that motion and any sample charge would feel this effect immediately.
In the real universe, there is a delay between the motion of a charged particle and the effect on another particle at a distance. For electric charges in free space, that delay is dictated by the speed of electromagnetic radiation; photons.
The analogy I think of for exchanging particles, is that of two platforms sitting ten feet apart on a frozen lake. Imagine a man standing on each platform and taking turns throwing a ball back and forth. When the man on the right throws to the left, he imparts leftward momentum to the ball and a equal rightward momentum to the righthand platform.
When the man on the left catches the ball, the ball transfers its leftward momentum to the platform on the left. When the man on the left throws, he transfers rightward momentum to the ball and leftward to his platform.
The result is that the platforms drift apart as if a force were causing them to be repelled. If something should cause a shift in the position of one of the platforms, the effect of that shift could not be detected until sufficient time had passed for a ball to travel between the platforms.
It's not really an approximation.
Yes, I agree. The calculus yields an exact answer.
I was open to the possibility that some problems in General Relativity might yield a slightly different answer, since GR predicts some things that Newton would not have predicted. But Newton's assumptions were exactly accounted for in his calculations.
Do you know whether GR invalidates Newton's conclusion or can a spherical mass always be represented by a point source?
Richard Feynman. IIRC, he once suggested that there was actually only one electron, but that it was able to travel in time ;)
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