Elementary Particles The particles we will be looking at in this - PowerPoint PPT Presentation

Heidi Schellman Northwestern Elementary Particles The particles we will be looking at in this course are either elementary , like the electron , or composite , like the pion and proton . The elementary particles can be characterized by their ”quantum numbers” . A quantum number is a quantity that is conserved by at least some of the fundamental interactions. June 2010 HUGS 1

Heidi Schellman Northwestern Charge tandard Model of Elementary Particles mass,MeV June 2008 Masses are in MeV 3 Generations of Fermions Force Carriers 2/3 2/3 2/3 0 Strong u c t Interactions Q u 1.5-3 ~1250 ~173,000 0 a r − 1/3 − 1/3 − 1/3 k 0 s Electro- γ d s b magnetism 0 3-7 70-120 ~4200 0 ν 1 ν 3 0 ν 2 L Z e 91,188 >0? >0? >0? p Weak t Interactions o ±1 n ± µ e s τ W 0.511 105.66 1777.2 81,400 June 2010 HUGS 2

Heidi Schellman Northwestern The stable elementary particles that occur in nature at low energies are the electron, electron-neutrino , the u and d quarks, the gluon and the photon . You don’t see u and d quarks, or gluons directly as they are all bound up in protons and neutrons . June 2010 HUGS 3

Heidi Schellman Northwestern Feynman Diagrams Feynman Diagrams are both a way of illustrating what is going on in an interaction, and a way of calculating that interaction. What one does is one draws the particles in 1 space dimension and 1 time dimension. space e- time An electron at rest in space-time. June 2010 HUGS 4

Heidi Schellman Northwestern electron-electron interaction e e γ e e Here two electrons interact by exchanging a photon. Time flows from left to right. This is how the electromagnetic force works - exchange of a force particle. June 2010 HUGS 5

Heidi Schellman Northwestern Compton Scattering e − e − e − e − e � e � γ γ γ γ The scattering of an electron and a photon. You could imagine aiming an X-ray at an atom and knocking out an electron this way. June 2010 HUGS 6

Heidi Schellman Northwestern Look closer e − e − e − e − e � e � γ γ γ γ • You can see the photon and electron moving towards each other in space as they come together and then moving apart later. • The electron in the middle of the left diagram is a bit strange. it does not have the right mass - it has the center of mass energy and not m e . This is called a virtual particle and it can only exist for a time ∆ t such that ∆ m ∆ t ≥ ¯ h For a 500 keV photon hitting an electron at rest out of an atom, the ”virtual” electron has a mass of 878 keV. The thing to remember is that energy and momentum are conserved at each point in the interaction, but the masses of particles can wander off for very short periods of time. June 2010 HUGS 7

Heidi Schellman Northwestern More to think about • The interaction of the particles is at a ”vertex” and that’s the only way they interact. • The lines themselves are called ”propagators”, they are particles propagating through space. Because charge is conserved, electron lines have to continue through the diagram. Once we get into weak interactions, your electron may be able to change into an electron neutrino, but the ”lepton” line can’t break. Same for the quarks - they can change type but you have to conserve quarkness. Photons and other bosons can come and go as needed. • There are two diagrams - there are actually two ways for an electron and photon to scatter. Feynman diagrams illustrate quantum mechanical amplitudes and the probability we observe in the real world is the square of the amplitudes. If two diagrams have the same initial and final states they can interfere. June 2010 HUGS 8

Heidi Schellman Northwestern e − e + scattering e − e − e − e − γ e + γ e + e + e + There are two diagrams as in Compton scattering. June 2010 HUGS 9

Heidi Schellman Northwestern γ m = 0 Q=0 couples to charge force carrier for Electro-Magnetism no strong or weak interaction e- e γ photon-electron scatter e- e γ e- pair-production of e electron/positron e γ in field of a nucleus e + γ∗ Ze June 2010 HUGS 10

Heidi Schellman Northwestern e electron Q = - e m = 0.511 MeV/c 2 stable couples to γ , W , Z e e- γ∗ e- e e e e- γ∗ γ Ze bremsstrahlung June 2010 HUGS 11

Heidi Schellman Northwestern Photon detection photon pair-produces, electrons radiate, products do the same. Length scale for one interaction is X0 ~ 0.3 cm in lead, 9 cm in silicon Shower length ~ X0 log(ZE) June 2010 HUGS 12

Heidi Schellman Northwestern Electron detection electron bremsstrahlungs, photon pair-produces, electrons radiate, products do the same. Length scale for one interaction is X0 ~ 0.3 cm in lead, 9 cm in silicon June 2010 HUGS 13 Shower length ~ X0 log(ZE)

Heidi Schellman Northwestern V Sampling Calorimeter Uranium Liquid Argon Electrons knocked loose in the Argon are detected� Get 15%/sqrt(E) energy resolution June 2010 HUGS 14

Heidi Schellman Northwestern muon Q = - e m = 105.66 MeV/c 2 decays to e e couples to γ , W , Z June 2010 HUGS 15

Heidi Schellman Northwestern Charged particles with m >> m e deposit energy at a reasonably constant rate. dE/dx ~ Z 2 / β 2 ρ (g/cm 2 ) Typical energy loss for a fast muon is 2 MeV/gr/cm 2 June 2010 HUGS 16

Heidi Schellman Northwestern Particle ID: TP data 28 Neutral Current Quasi-Elastic Candidate Tammy Walton APS talk ν + p --> ν + p DIS2010 June 2010 HUGS 17

Heidi Schellman Northwestern If you can detect charged particles, you can measure their momentum by applying a magnetic field CFT Trigger XY View 60 y (cm) 8 0.6 Event 8 0.7 . 0 0.6 0.5 5 0.5 0.7 . 1 3 40 . 0.8 1 0.5 0.7 0.8 20 0.5 0 . 7 0.6 5 . 2 3 0 0.5 0.8 0 . 8 0 . 6 3 2 . 0 0 8 . 0.5 0 6 . 2 . 1 -20 0.9 1.2 -40 0 . 7 0.5 0.6 0.9 0.6 0.8 0 0 . . 9 6 -60 -60 -40 -20 0 20 40 60 x (cm) Particles bending in a 2 T magnetic field. The red particles are electrons from a Z decay June 2010 HUGS 18

Heidi Schellman Northwestern u up-quark Q = 2/3 e m ~ 2-3 MeV/c 2 stable? couples to γ , W , Z, gluon d down-quark Q = -1/3 e m ~ 5-8 MeV/c 2 decays to u couples to γ , W , Z, gluon June 2010 HUGS 19

Heidi Schellman Northwestern Quark Confinement Strong force is very strong Try to rip one quark out of a proton u d u u p d d u pair creation is easier than stretching u d u u s s d d d d u u s K + s d d u u d d K 0 get a ’jet’ of particles d u n June 2010 HUGS 20

Heidi Schellman Northwestern Parton View u dbar p Particle View Partons become ’jets’ Jets momentum similar to parton momentum June 2010 HUGS 21 n

Heidi Schellman Northwestern neutrinos Q = - 0 m = 0? stable couples to W , Z June 2010 HUGS 22

Heidi Schellman Northwestern W W boson Q = e m = 80.4 GeV decays to q , q’, l , couples to q, q’, l, v, , Z The W particle is the carrier of the Weak Force e+ v W u d June 2010 HUGS 23

Heidi Schellman Northwestern Z Z boson Q = 0 m = 91.19 GeV/c 2 decays to q , q, l , couples to q, q, l, v, , Z, W The Z particle is also a carrier of the Weak Force v v Z 0 u u June 2010 HUGS 24

Heidi Schellman Northwestern Quantum Numbers - Spin The elementary particles we have seen to date have either spin 1 2 (fermions) or spin 1 (bosons) . We hope to see the Higgs boson soon - which would be our first spin-0 particle. The graviton , the carrier of the gravitational field, is believed to be spin-2 but has not been detected. Spin is conserved in all interactions. June 2010 HUGS 25

Heidi Schellman Northwestern Quantum Numbers - Electromagnetic Charge This is the conserved charge that results from gauge invariance in electricity and magnetism. As far as we know, it is absolutely conserved. The electron is assumed to have charge − 1 in units of e , the electron charge. The Fine Structure Constant α = e 2 ¯ hc is often used in formulas instead of an explicit e 2 . June 2010 HUGS 26

Heidi Schellman Northwestern Quantum Numbers - Color Charge Color charge is the strong interaction analog of electric charge. The strong interaction equivalent of the photon is the gluon . There are 3 color charges. A quark can be red, green or blue or anti-red, anti-green or anti-blue. Gluons can have 8 color combinations like red-anti-blue or green-anti-red. June 2010 HUGS 27

Heidi Schellman Northwestern Strong Force The strong force comes about by the exchange of massless gluons . u u u u g u g u u g u g g g g Not all particles feel the strong force. The ones that don’t, electrons and neutrinos, are called leptons while the ones that do are called quarks . The word hadrons is also used for strongly interacting particles and generally means the bound quark state we see in the lab, like protons and pi-mesons. June 2010 HUGS 28

Heidi Schellman Northwestern Chromodynamics Meson like the π 0 , K + and φ which are made up of quark-anti-quark pairs have color-anti-color charges . Baryons like the proton are made up 3 colored quarks , with 3 different colors so the net color is 0. The strong color force is so strong that you can’t separate the charges and never see a colored object in the lab, only the combination ’red’-’green’-’blue’ or ’color-anti-color’. June 2010 HUGS 29