How We Study Particles

1. How We Study Particles Laura Gilbert

2. Fundamental particles: FORCE (“gauge bosons”) The basics of particle physics! Matter is all made up of particles… Fundamental particle: LEPTON

3. Fundamental particles: QUARKS The basics of particle physics! Matter is all made up of particles…

4. Fundamental particles: FORCE Take a closer look at a proton: • Three Quarks (two “up”, one “down”) • Held together by “gluons”: Strong force

5. Generation: I II III Forces: u c t γ QUARKS g d s b e μ τ Z LEPTONS υe υμ υτ W Why do we want to study particles? The “Standard Model”: particles we have detected charge +2/3 EM -1/3 Strong -1 Weak 0 Weak We think we know how these interact with each other.

6. Why do we want to study particles? We are looking for a “Theory of everything”. So what’s missing? • Are these particles “fundamental”? • Are there more? • What is gravity? (force particle? “superstring”?) • How do we get mass? • Why is there more matter than antimatter in the universe? • How did the universe begin?

7. How do we study the world around us?

8. Detecting Small Things • To see small things, we need short wavelengths (<~size of object): target

9. Particles behave like waves with short wavelengths: λ 1/energy. • To see things we need high energies. Detecting Small Things • To see small things, we need short wavelengths (<~size of object): target

10. How do we study the world around us? “fixed target” detectors target Particles Source of high energy particles

11. How do we study the world around us? detectors “colliding beam” accelerator

12. How do we get high energies? Acceleration! We give particles kinetic energy (and mass) by accelerating them. It is simple to accelerate charged particles using electric fields – electrons gain 1eV of energy per volt (=1.6x10-19J) Charged plates Acceleration Constant velocity Constant velocity Potential difference -V +V

13. - - - + + - - + + + Particle Accelerators • Linear array of plates with holes: alternating high energy field applied. • As particles approach a plate they are accelerated towards it by an opposite charge on the plate. • As they pass through the plate, polarity is switched: plate now repels them. They are accelerated towards the next plate. “Bunch” of +ve protons

14. - - + + + Magnetic fields curve particle paths Electric fields accelerate Particle Accelerators • To allow greater acceleration the accelerator is circular. • The path of a charged particle is curved in the presence of a magnetic field. The tracks of the particles are curved to fit using dipole magnets: • Linear array of plates with holes: alternating high energy field applied. • As particles approach a plate they are accelerated towards it by an opposite charge on the plate. • As they pass through the plate, polarity is switched: plate now repels them. They are accelerated towards the next plate.

15. CERN (birthplace of the World Wide Web!) Super Proton Synchrotron (SPS) accelerates protons. Large Hadron Collider (LHC) accelerates further and collides them. A television is an accelerator in which electrons gain around 10 keV (10 000 eV). The SPS will accelerate protons to around 7 TeV (7 000 000 000 000 eV). The SPS 8.5km ATLAS – proton beams collide here The path of the LHC… 100m below ground

16. The ATLAS experiment at CERN Two protons collide at very high energy, producing new particles for us to trap and study. ?

17. Detecting Particles We can see particles when they interact:

18. Electromagnetic Ionisation: We need to make ionisation "visible“.

19. Electromagnetic Ionisation: We need to make ionisation "visible“.

20. Electromagnetic Ionisation: The addition of highly charged wires turns it into a “Drift Chamber”. The electrons form a detectable current.

21. Strong and Weak Uncharged (neutral) particles are unaffected by electromagnetic force. They only interact via strong and weak interactions. We can tell where neutral particles are indirectly as missing tracks: “Kink” in track Charged particle – detect ionisation Particles appear from nowhere! Charged particle decays into charged + neutral

22. Identifying particles Particles can be identified (almost) UNIQUELY by their mass and charge. These are what we need to measure.

23. v -ve charged particle F B into picture F +ve charged particle v How do we measure… Charge? Use a magnetic field. For a charged particle in a magnetic field, the force is perpendicular to velocity → particle moves in circular path. The direction of curvature tells us the sign of the charge (“Flemming’s left hand rule”). Mass? Indirectly…

24. How do we measure… Momentum? Magnetic Field again. From the radius of curvature of the tracks. Energy? Calorimeters. Dense transparent materials. Energetic particles interact, producing “showers” of thousands of secondary particles – particles are stopped dead and energy is absorbed. “Scintillator” material puts out light that can be measured. shower Particles slow down gradually

25. -ve particles: anticlockwise +ve particles: clockwise Example – Bubble chamber B Charged particles B field causes paths of charged particles to curve!

26. Neutral particle decay -ve particles π +ve particles Let’s identify some particles… B electron electrons from ionisation

27. Name Symbol Charge (1.6x10-19C) Mass (9.1x10-31kg) Electron e- -1 1 Atomic matter Proton p +1 1836 Neutron n 0 1839 Photon γ 0 0 Pions π0 / π± 0 / ±1 264/273 Kaons K0 / K± 0 / ±1 974/966 Muon μ- -1 207 The “Particle Zoo” Identify particles by their charge and mass! etc…

28. Over to you… You will be asked to identify some particles from an e+e- annihilation. Which turns into a particle/antiparticle pair: e+ γ μ+π+ K+ or p μ-π- K- or p e- From their radius of curvature in a B field, find momentum (p =rQB) and then mass (E2=p2+m2) Electron + positron of known energy annihilate producing a photon