Overview of particle physics

Transcription

1 Overview of particle physics The big qestions of particle physics are 1. What is the niverse mae of? 2. How is it hel together? We can start at orinary istances an work or way own. Macroscopic stff is mae of molecles. Molecles are mae of atoms. An atom has a clo of electrons (e) srroning a ncles. Nclei are mae of protons (p) an netrons (n), which are calle ncleons. The electrons are hel to the ncles by electromagnetic (EM) forces mainly jst the Colomb force. The EM force can be escribe by saying that the charge particles exchange the qanta of the EM fiel photons (γ ). The ncleons are hel together in the ncles by strong interactions. The associate fiel is calle the color fiel. It is a generalization of the EM fiel. The qanta of the color fiel are calle glons (g). (Yo mst be wary of the jargon in high energy physics. For example, the wor color here is jst a name; it has nothing to o with real colors like re an ble that we perceive irectly. Also some of the jargon is imaginative, an some of it is jst plain mb. We will see why glons have their name shortly.) At this point, it might seem that a satisfactory niverse col be bilt from e, p, n, γ, an g. Only the strong force wol nee more iscssion. Things i not work ot that way at all! For reasons that are not known, the niverse is far richer an more interesting than that. (Perhaps it s to keep s oing physics an away from mischief that col be mch worse.) Here are some things that exist bt are not obviosly necessary for a nice niverse: 1. Antiparticles: For example, the antiparticle of the electron is the positron e +. It is jst like the electron except for having the opposite charge. Similarly, the antiproton p an the antinetron n exist. Since the netron has no charge, how o the netron an the antinetron iffer? The netron (as well as the proton) has another kin of charge calle baryon nmber B which is not zero. The n has B=1 an the n has =-1. For the electron, B=0. Some particles sch as the photon are trly netral an are their own antiparticle. Antiparticles seem to be reqire by relativity an locality/casality. There is also a nice particle-antiparticle symmetry calle TCP for time reversal, charge conjgation, an parity. Since these things are pleasing to s, we can accept antiparticles withot too mch worry. Bt then we mst ask why there are so many particles aron an so few antiparticles. There is a efinite asymmetry in the qantity of matter vs. antimatter in the niverse. There is mch more hyrogen with ep than antihyrogen with e + p. There are some interesting ieas on this bt no efinite answer yet. This general sitation where there is a symmetric nerlying law bt asymmetric phenomena is a recrring theme that is calle spontaneos symmetry breaking. 2. β-ecay of the netron: A netron that is not bon in a ncles is nstable an ecays with a lifetime of 887 sec. via n peν e. Associate with this, there is a new particle, the netrino (In this case, it s the electron s antinetrino.), an a new force, the weak interaction. 3. Qarks: The strong interaction is not a simple force between ncleons. The ncleons are not fnamental. Each ncleon is mae of three qarks. So far as we know toay, the qarks are fnamental spin-1/2 fermions. Two kins of qarks are neee to make orinary matter. The label for the ifferent kins is calle flavor. Of corse, this has nothing to o with orinary flavor. The two flavors are p an own. (I tol yo that some of the jargon is mb.) A proton is, an a netron is. It is the qarks that have the color charge of the color fiel. Each flavor of qark comes in three colors: re, ble, an green. The theory of the colore qarks an the color fiel is calle

2 qantm chromoynamics (QCD). It is a generalization of the Maxwell eqations, which are the basic eqations of electroynamics. This color force acts to gle the three qarks together to make the ncleon. That is why the qanta of the color fiel are calle glons. Now for a little bit on nits. Every specialty has its own convenient nits. In high energy physics, we ten to measre many things in energy nits. The convenient energy nits are MeV or GeV millions or billions of electron volts. One electron volt is the energy that an electron gains by falling throgh an electric potential rop of one volt = 1.60x10-19 J. Now introce the spee of light c. Notice that MeV/c has momentm nits an MeV/c 2 has mass nits. The next step is to choose length an time nits so that the spee of light c=1. Then energy, momentm, an mass are all measre in energy nits. (This brings to min the famos Einstein relation E=mc 2.) To get lengths into the pictre, recall another famos relation (from e Broglie this time) p=h/λ= k. In this, p is the momentm, h is Planck s constant, k=2π/λ is the wave nmber, an =h/2π. The nits of are momentm length. Now choose length nits so that =1. That leaves length with nits of inverse momentm or eqivalently inverse energy. This is all very weir bt very convenient once yo get se to it. Ths, energy, momentm, an mass are all in MeV an length is in 1/MeV. To get back to stanar nits, yo nee to know two nmbers c=3x10 8 m/s an c=197mev f. (f=fermi=fm=10-15 m.) For example, for mass, 1 GeV=1 GeV/c 2 =1.78x10-27 kg, an for length, 1/GeV= c/gev=0.197x10-15 m. The proton an netron masses are both close to 1 GeV. The proton mass is 938.3MeV, an the netron is 1.3MeV heavier at 939.6MeV. Let s retrn to the big qestions. As we nerstan it toay, the orinary stff of the niverse is mae of for matter particles. There are the electron e an its netrino ν e, which are calle leptons, an the two qarks an. For each qark, there are the three colors, an for each particle, there is also its antiparticle so we are really talking abot ( )x2=16 particles. Particle Charge Mass electron netrino ν e 0 < 17eV electron e MeV p qark 2/3 4.3MeV own qark -1/3 7.5MeV This is calle the first family.

3 The niverse is hel together by for forces. For each of these forces, there is an associate fiel an an associate qantm for that fiel. The fiel qantm appears as a particle. Force Qantm Charge Spin Mass Gravity graviton EM photon γ Weak W +, W -, Z 0 +1, -1, , 80.2, 91.2GeV Strong glon g I will explain later why some of the masses are in qotes. An now for the eep mystery: In some extravagant, implse shopping, Natre boght two more families of qarks an leptons. These have the same strctre as the first family. The ifference is that the masses are larger. Particle Charge Mass mon netrino ν µ 0 < 0.27MeV mon µ MeV charm qark c 2/3 1800MeV strange qark s -1/3 150MeV Particle Charge Mass ta netrino ν τ 0 < 31MeV ta τ MeV top qark t 2/3 175GeV bottom qark b -1/3 4.5GeV No one nerstans the reason for more than one family or the vales of the masses. These are active research areas. Since the six qarks an their antiparticles can be combine into bon states in many ways, many particles in aition to the p an n are possible. Hnres of these have been observe. One of the great accomplishments of the qark moel has been to give a nifie escription of these many states. Here is some more jargon: These are names for classes of particles. 1. Fermions are particles for which yo are allowe to pt only one of them in a given state. This is calle Fermi-Dirac statistics. All the leptons an qarks are fermions. 2. Bosons are particles for which yo are allowe to pt any nmber in the same state. In fact, ptting more in the same state is favore. This is calle Bose-Einstein statistics. The photon, the glon, an the other qanta of the force fiels are bosons. 3. Leptons are the fermions that o not have strong interactions. These are the e, µ, τ, an their netrinos. 4. Harons are the strongly interacting particles. a) Mesons are the harons that are also bosons, e.g. π, ρ, ϖ, an K. b) Baryons are the harons that are also fermions, e.g. p, n,, Λ, an Σ.

4 Even thogh there are jst for interactions, particle physicists ream of an work on a more nifie escription. There has been no great progress so far. However, some important work has been one. The Glashow-Weinberg-Salam theory of the electroweak interaction sort of nifies the EM an weak interactions in a way that is not particlarly pretty (except that it acconts for a hge boy of experimental reslts). The electroweak theory combine with QCD is referre to as the Stanar Moel. It contains one more fnamental particle that was not liste above. It is calle the Higgs boson. It is closely relate to the W an Z an to the large masses that they have. It has not yet been observe. One of the biggest activities in high energy physics these ays is looking for the Higgs. There is no experimental reslt that forces s to look beyon this theory. Nevertheless, consierable effort is evote to looking for more nifie theories. Those that nify QCD an the electroweak theories are calle gran nifie theories (GUTs). The simplest of these is calle SU(5). (The name comes from the fact that SU(5) is the symmetry grop se for this theory.) The SU(5) moel has many nice featres. It is niqe in making preictions that are ifficlt to wiggle ot of. Unfortnately, one of these is wrong. Most GUTs, incling SU(5), preict an nstable proton. SU(5) gives an actal nmber for the ecay rate, which wol have been seen by now in the crrent ambitios experiments. It has not been seen. However, there is new hope for the SU(5) GUT in a spersymmetric (see below) version in which the proton lifetime is large enogh to be ot of range of crrent experiments. Even more ambitios theories try to get gravity into the pictre. Recent attempts along this line are characterize by a new symmetry calle spersymmetry. This symmetry transforms fermions into bosons an visa-versa. Early versions were calle spergravity moels. Later versions are in the context of string theory an are calle sperstring moels. In string theory, the fnamental objects are not particles or fiels; they are strings with extent in one imension. Decays an lifetimes The trly stable (not yet observe to ecay) particles are γ, ν s, e, an p. All others ecay by weak, EM, or strong interactions. Those that are trly stable an those that ecay by weak or EM interactions have mch longer lifetimes than they wol if they col ecay strongly. They are calle stable particles. Weak ecays The classic weak ecay is netron beta ecay n peν e. W e τ = 887sec. ν e

5 Another typical weak ecay is µ ν µ eν e with a mean life τ sec. The pictre for this is µ e ν µ W ν e The π meson is a bon state of a qark an an anti-p qark. It is a haron that ecays only weakly an only into leptons. π µν µ τ = sec. π W µ ν µ The W an Z bosons of the weak interaction can be proce, an their properties can be observe. Since the W an Z are so heavy, there are many ways that they can ecay, an the lifetimes are very short. W eν e, µν µ,, Z e + e, µ + µ,, There is also Z νν, an this affects the Z for each netrino with m ν < m Z / 2. In the three known families, even thogh the qarks an charge leptons get heavier an heavier, the netrinos are all light. A precise measrement of the Z lifetime tells the nmber of light netrinos. If there were more families with qarks an charge leptons too heavy to have been iscovere, bt with light netrinos, like the known families, of the Z lifetime wol reveal them. The experiments on the Z lifetime at SLAC/SLC an CERN/LEP observe the process e + e Z an show that the nmber of light netrinos is three. Ths if there is a forth family, it mst have heavy netrinos.

6 EM ecays Here the rates are generally faster (other things being eqal). For example: γ γ π 0 γγ τ = sec. Strong ecays These are faster still. For example, the ρ meson can ecay into two π mesons. π 0 ρ g π ρ ππ The lifetime is of the orer of sec. Fast inee! If a particle can ecay strongly, it will an will be gone before any possible EM or weak ecays have a significant chance to work. If no strong ecays are possible, then an EM ecay may happen. If that is not possible, then finally the weak ecay may be seen. Strong ecays conserve everything. EM ecays o not conserve something calle I-spin. Weak ecays o not conserve many other qantities. Some things like EM charge an energy an momentm seem to be conserve for goo reason. Some others like baryon nmber an lepton nmber seem to be conserve bt for no apparent reason. Cross sections Cross sections are another measre of interaction. They are se to escribe the scattering of a beam of one kin of particle on a target of another kin. The cross section is basically the area for scattering. It is the size in area that one particle appears to be to the other. Cross sections follow a pattern that is analogos to lifetimes. Strong interaction cross sections are large. For π + p, σ T 30mb 1b =1barn = cm 2 ( ). EM cross sections are smaller. For γp, σ T 100µb. Weak cross sections are very small. For νp, σ T mb at E ν = 1GeV.

7 Probing short istances reqires high energy an large, expensive accelerators. The ltimate high energy experiment has alreay been one. It was the big bang. Or niverse is the final state from that event. Cosmology an general relativity tell s that the temperatre T of the niverse is relate to the time t after the big bang by T t 1/2 at early times. So at sfficiently early times, particle energies were arbitrarily large. For example, at t = sec, kt 1TeV. There is now a lot of interesting work combining particle physics an astrophysics.