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
The prime directive is for sissies, like Pickard and his feckless crew. Kirk was a real captain. He boffed a new pretty girl every week. Those were the glory days of the Federation.
I saw him do it to Majel Barrett in the Bloopers reel. So there, dweller of the basement of the science building.
"Stuck various organs into every green-skinned alien chick he could find?" - HEY! There weren't -any- green-skinned alien chicks on Star Trek, were there?
Oh there weren't? Explain this Orion Slave Girl, coke-bottle-bottom glasses-boy. (and click this if you dare)
You LAME-O!!!! I can't believe you didn't know that the Orion Slave Girl was in Episode 16, "The Menagerie"!!! Boy, people like YOU shouldn't be ALLOWED TO REPRODUCE, if you ever could even find a woman!!!! I bet you even think that an Imperial Star Destroyer could take on a Galaxy-Class Star Crusier!!! Why don't you just go to Germany and GOOSESTEP you NAZI! (spittle flying)
You were not rude at all. I just popped on for a sec. I will be back on this evening. :-)
I probably have just enough knowledge to lead you astray.
Many of the behaviors of semiconductors were mysterious until a refined understanding of electrons was developed.
First imagine a single neutral hydrogen atom in space. The electron can be in the "ground state" which is the lowest energy level. External energy can be absorbed by the atom causing the electron to occupy a higher energy level. Later, the atom may emit that energy by emitting a photon whose frequency is proportional to that energy and the electron can return to the ground state.
There are many energy levels possible for the electron and many different frequencies of photons which might be emitted when the electron moves from the higher energy level to the lower level. These various frequencies make up the spectrum of hydrogen. These same frequencies can be observed in the spectum or our sun, providing the proof that there is hydrogen in the sun.
The spectrum of the sun shows not only the spectrum of hydrogen but of other elements as well. The spectral content due to helium was the first observation of that element ( the name comes from the Greek helios, meaning "sun" ). Helium was found on earth much later.
The spectrum of helium is similar to that of hydrogen but the energy levels are different because the two protons in the helium atom exert a stronger force on an electron than the single proton of a hydrogen nucleus.
Imagine now, that there are two neutral hydrogen atoms located far apart in space. Each will behave independently of the other and each will exhibit the spectrum of hydrogen when disturbed by outside forces.
Now gradually decrease the distance between the two hydrogen atoms. Obviously, when the two protons making up the two hydrogen atoms come close enough together, they will constitute a helium nucleus. The two electrons, which initially were orbiting their own hydrogen atom, will begin to interact as the protons are brought together, until finally the two electrons will behave as the two electrons behave in helium.
In the transition between having two isolated hydrogen atoms and one helium atom, there are intermediate situations in which each electron is affected by both protons. The possible energy levels and the detailed shape of the orbits are a function of the spacing between the two protons. Some of the electron orbits of the original hydrogen atom were spherical and some were not. In this intermediate state, even more exotic shapes might occur to describe the expected positions of the electrons.
Quantum physics deals with the fact that entities like electrons sometimes behave like particles and sometimes behave like waves. The "orbit" of an electron is described by solving a "wave equation" whose values are related to the likelihood of finding the electron in any given position during an experiment. The Pauli "exclusion principle" dictates that only one electron can be present for any one suitably specific solution of the wave equation. It is like having a unique address for each electron ( the quantum numbers describing its state ) and the electon is either at home or not at home. The exclusion principle states that there can never be two electrons in the same home. Think of it like a bucket of water that can be either full or empty. You can't have a bucket which contains two buckets worth of water. The water that is there "excludes" adding any more.
Semiconductors are a more complicated case of the two atoms described above.
A single crystal of silicon consists of a regularly repeating interlocking pattern of silicon atoms, with each atom sharing covalent bonds with four neighboring atoms.
In isolation, a neutral silicon atom has four electrons in its outer shell. This shell would be full if it contained eight electrons. Electrons making up a full shell tend to be tightly bound. An electron which is the only electron in a shell tends to be loosely bound.
The regular arrangement of silicon atoms in a crystal and the covalent bonding tends to make the electons in the crystal tightly bound. A piece of pure silicon crystal at absolute zero temperature is non-conductive. Applying a voltage to it will not cause electrons to move.
Now heat the crystal slightly. The heat will cause the atoms to be agitated and the resulting motion can cause an electron to be bumped loose from its covalent bond. Under the influence of an external voltage, this electron can move through the crystal. Surprisingly, the "hole" left behind can also move under the influence of the external voltage. Simplistically, the hole can be thought to move to the left by having an electron shift to the right by filling in the hole. Holes behave differently from electrons because the amount of movement which is created by an external voltage is different. The amount of movement caused is a measure of the "mobility" of the hole or electron.
The number of conduction electrons ( and the identical number of holes ) which is created in a pure silicon crystal is an exponential function of temperature. One can exploit this behavior to make a temperature measuring device called a "thermistor".
Let's imagine now that we can somehow remove all of the electrons from a piece of pure silicon crystal without changing the regular location of the silicon nuclei. Let's then add back the electrons one at a time.
The crystal acts like a giant nucleus and the electron will occupy one of many possible energy levels, just like the electron in a single hydrogen atom. If we cool the crystal to near absolute zero and wait a while, we can expect the electron to find the lowest level. As before, we expect that the electron "orbit" will be the solution to a quantum-physical wave equation with a particular set of quantum numbers. If we add in additional electrons, they will settle into their own "orbits" within the crystal. Later electrons will tend to occupy "orbits" with higher energy levels. Because the crystal is so large, the energy levels of succeeding electrons can differ by extremely small amounts from the energy levels of prior electrons.
When we add the last electron back to the crystal, it will tend to occupy the highest energy level orbit in the now electrically neutral crystal. The electrons are now occupying fully what is called the "valence" band. At zero degrees absolute temperature, this is where all the electrons will be and the crystal will be a non-conductor. At slightly higher temperatures, some electrons will gain enough energy to move up into what is called the "conduction" band. Such electrons will move under the application of an external voltage, but if there are not many electrons, then there won't be much current.
Now comes the neat part. Imagine that one silicon atom out of each one-hundred thousand is replaced by a phosphorus atom. Phosphorus comes next after silicon in the periodic table. It has an additional proton and an additional electron in the neutral atom. When it finds itself in the middle of a regular arrangement of silicon atoms, all covalently bonded to each other, the phosphorus atom will take up a similar position. Four of the five electrons in the atoms outer shell will form covalent bonds with the neighboring silicon atoms.
The arrangement of the four covalent bonds of the phosphorus atom leaves no lower energy place for the fifth electron. It is forced to shift away from the phosporus nucleus and to inhabit an energy level which is such that the electron is much less tightly bound to the phosphorus atom.
The situation just described is what would occur at absolute zero temperature. Elevating the temperature slightly will cause electrons to become unbound from the phosphorus atoms and the crystal will become a conductor. In the case of the pure silicon, the number of electrons moving into the conduction band was small. In the case of this "doped" semiconductor, the amount of energy needed to free the fifth electron from each phosphorus atom is so low that they are virtually all in the conduction band at even very low temperatures. The amount of doping can be chosen so that the amount of conduction is nearly independent of temperature.
Back now to the original question. Does an electron which enters one end of a conducting crystal eventually make its way to the other end? A similar question might asked about buckets of water.
Imagine a line of one hundred buckets of water, each completely full. There is no convenient way to move water from one bucket to the next because they are full.
Now remove a teaspoon of water from each bucket. It will now be possible to move water. Pour one teaspoon of water from the first bucket to the second, then from the second to the third, and so on until a teaspoon of additional water arrives at the one-hundredth bucket.
Did any particular molecule of water move from the first bucket to the second? From the second to the third? Is there even any way to tell? In the case of water, we could make one molecule from a rare isotope of oxygen, and then follow its progress. Will it move from one end to the other or not? The answer is sometimes but not often and in a very unpredictable fashion.
Electrons cannot be labelled by choosing a special constituent for a particular electron. They are all indistinguishable from each other.
Note that other materials that behave as conductors, metals, consist of atoms which have excess electrons in their outer shells. These electrons are readily freed from the constituent atoms at very low temperatures and will occupy energy levels which are analogous to those in a semiconductor crystal.
I would say that your suggestion number two is close to a description consistent with what I know. An electron moving from its own quantum state to a nearby state under the influence of an applied field is a quantum physical event which can be approximated as the drift of an electron acted on by a distant charge and slowed by periodic interaction with other "particles" in the crystal.
D'oh !!! I've referred to my periodic table three times today. Should've been four.
Good point about how our predecessors deserve a lot of credit for what they were able to discover.
One of the things that I admire Newton for was his single minded pursuit of a way to prove that the gravitational attraction of a spherical mass can be approximated very closely by a point source at its center. I've never been so fascinated by a problem that I have invented calculus to solve it.
Some of the applications of bucky balls and bucky tubes are getting interesting. The regular repeating structure is similar to the underlying structures of semiconductors and applications are being worked on to exploit the possibilities. It's fascinating that C60 was discovered just because there was an unexplained peak on a spectrum.
Has anyone made a bucky ball out of silicon yet? Perhaps it is unable to support its own weight under 1 gravity. Sounds like a mission for the space station. Has anybody done any work on this? If not, I wan't it named "WilliamTell-ine". It's only fair.
Thinking about it, I'd be surprised if the current flow doesn't start everywhere in the wire at once, although I'm not sure why I think so or how in detail it works. One feature of a good conductor material is that it only has a loose hold on its outer electrons.
If gravitons exist, then you don't need an objective space to be bent to distort light, gravitons could act on the photons of light and make light bend.
Standing fields make a certain sense, though. I doubt the concept is going away.
I don't want to say that the "objective space" and "graviton" models are mutually exclusive, they're not, but at the same time, if you have one you don't need the other.
If you don't have vector bosons, you have the ever-unpopular "action at a distance."
Am I making any sense?
Not a lot overall, but I'm just another amateur sci-article junkie.
Are you sure about that? Is it inevitable?
There's a heck of a lot more than 6. There are 6 quarks, 6 leptons (electron, muon, tau plus their respective neutrinos), photon, gluon, graviton, the W boson, and last but not least, the Z boson. I count 17, plus most if not all have a corresponding anti-particle, for upwards of 34. Did I miss any?
Thanks. That was interesting.
I don't see anything there which suggests why the force is constant rather than inverse square.
Square law forces such as that which is exhibited by the spring in a spring-mass system result in solutions which are sinusoidal.
Inverse-square law behavior results in conic sections such as circular or elliptical orbits, or, I believe, even hyperbola shaped paths for objects with escape velocity.
Is there some theoretical basis for believing that the strong force is constant with distance? How is this known? Is it constant within some precision?
The linked site points out that one consequence of a constant force with distance is that there is no escape velocity. That is, no matter what initial velocity a particle has, eventually a constant force will be able to bring it to a halt and return it to the source of the force.
Here's another question which arises from the information on the linked site. It states that neutron decay takes a billionth of a billionth of a second. Is this number just an upper bound on a process which could be occuring much faster or is there reason to believe that it is taking this long? Is this the observed half-life of the W- particle? Light would travel in vacuum about 3 angstroms or three hydrogen atom diameters in that much time, I think.
A Thirty-Fourth degree Meson?
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.