From: Bruce 20/06/99 18:02:18 Subject: Those wacky particles post id: 18679 I've been reading earlier posts and heard a lot about all those different types of particles- bosons, leptons, baryon, mesons, quarks. Could someone please lay down the law for me? Which is which, what do they do, and why the funky names? Thank you Bruce From: Dr. Ed G (Avatar) 21/06/99 2:29:33 Subject: re: Those wacky particles post id: 18694 Okay, some definitions. Leptons: the most common lepton is the electron. These are fundamental particles in that there is nothing to suggest that they are made of anything smaller. Leptons can be group into three pairs - there's the electron (negative charge) paired with an associated electron neutrino, there's the heavier muon (with the same negative sharge as the electron) paired with an associated muon neutrino, then there's the heaviest lepton, the tau (again, same charge as the electron) paired with its associated tau neutrino. Then there's associated anti-particles of these the anti-electron or positron, the anti-electron neutrino, the anti-muon, the anti-muon neutrino, and the anti-tau, and the anti-tau neutrino, respectively. Baryons: the most common of these are the proton and the neturon (both also known as nucleons 'cause they appear in the nucleus of atoms). Baryons are not considered fundamental as they are each made up of three quarks. Mesons: not very common at all, similar to baryons, but they are made up of only two quarks. Quarks: These are particles which are NEVER seen on their own, but which make up the majority of all matter (except for leptons). Like the leptons, quarks are divided into three groups of two, being the up and down quarks, the strange and the charm quarks, and the bottom and top quarks (also known as beauty and truth depending on which side of the Atlantic you're from). And then there are their corresponding anti-particles, the anti-up, anti-down, anti-strange, anti-charm, anti-bottom, and anti-top. Quarks have fractional charges of either 1/3 or 2/3 that of the electron, so that when you combine them to form a baryon you always get +1, -1, or 0 times the charge of the electron - so the proton has a positive charge exactly equal to that of the electron, and the neutron has no charge. Bosons: technically, bosons are just particles which have what is known as integral spin and obey Bose-Einstein statistics, as opposed to fermions which have 1/2 integral spin which obey Fermi-Dirac statistics (leptons, and baryons are all fermions). When it comes to particle physics, they more often refer to exchange bosons which are particles that particles like leptons, baryons, quarks, and mesons exchange in order to interact with each other. The most important boson is the photon which is the exchange particle by which charged particles can be said to interact - the photon is the exchange particle of the electromagnetic force. For example, in order to accurately describe the orbit of a negatively charged electron around a positively charged proton as in a Hydrogen atom, quantum mechanics considers the exchange of an infinite number of photons between the electron and the proton as they interact. This may seem freaky, but it works with an astonishing degree of accuracy. Another important example of an exchange boson that we're looking for, but have not found, is the graviton, which is the particle expected to dictate how massive particles (literally particles which possess mass) interact by gravity. In this case the graviton is postulated as the exchange particle of the gravitational force. For completeness the exchange particles for the weak nuclear interaction are known as the W+, W-, and Z0 particles (if I recall correctly), and for the strong nuclear interaction are gluons So, basically, theoretical particle physics is the study of how leptons, baryons, quarks and mesons interact by the exchange of photons, W+, W-, and Z0 particles, and gluons. I don't know if this helps, but hey, you asked! :-) Soupie twist, Ed G. From: Todd Collins 21/06/99 3:44:22 Subject: re: Those wacky particles post id: 18695 If quarks never exist on their own, how come we know that they exist. Couldn't the larger multi-quark complexes we have seen be single entities, where some different multi-quarks share some common features, but also have some different features to other multi-quarks? Why can't you separate Quarks? (I know I've probably asked a question that'll take months to answer, but I'm curious about Quantum Physics (without being curious enough to actually bother reading the necessary physics textbooks)) From: Chris (Avatar) 21/06/99 11:55:10 Subject: re: Those wacky particles post id: 18721 Really? Which side is which? Papers I have seen which deal with the 3rd generation quarks tend to refer to them as the b quark and t quark, probably half for brevity and half because there isn't really any agreement on the whole bottom and top or beauty and truth thing (although I think top and bottom is now more common). I'd also suggest that it isn't so strictly defined as to who uses what. The majority of papers I've seen using beauty and truth have been US, but then I've seen some from CERN and other Swiss authors using beauty and truth as well. How do we know quarks exist if we can't see them singly? The theory which incorporates quarks has been sufficiently successful for it to be generally accepted. It predicts properties for the different quarks which turn out to be testable. All six quark flavours have been detected at this time without observing them singly. Why can't quarks appear singly? The force which holds quarks together in baryons and mesons actually gets stronger the further the separation between the quarks - ie the opposite to the e/m and gravity fields' falling off as r2. Also quarks have a special property called colour charge* in addition to the usual spin, electric charge (which is fractional in quarks), flavour, etc. This colour charge comes in three varieties known as red, blue and green (there is also antired, antiblue and antigreen) and forms the basis of the theory of quantum chromodynamics. This theory holds that the total colour charge on any non-quark particle must be neutral ("white"), since colour is not observed in any single particles. In other words a red quark and anitred quark may combine to make a meson or a blue, a red and a green quark may combine to make a baryon. Hope this helps! Chris *Colour doesn't actually refer to a colour (quarks are to small to scatter or reflect visible light), it's just a name for the phenomenon. This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.