Laura Gilbert How We Study Particles

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

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

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

Why do we want to study particles? The “Standard Model”: particles we have detected IForces: IIIIIGeneration: QUARKS LEPTONS u d c s t b e μτ υeυe υμυμ υτυτ g γ Z W We think we know how these interact with each other. +2/3 -1/3 0 charge EM Strong Weak

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?

How do we study the world around us? SourceTargetDetectorAnalysis SunlightCatEyeBrain

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

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

How do we study the world around us? SourceTargetDetectorAnalysis SunlightCatEyeBrain Particle beam ParticlesParticle Detectors Computers “fixed target” Source of high energy particles Particles target detectors

How do we study the world around us? SourceTargetDetectorAnalysis SunlightCatEyeBrain Particle accelerator ParticlesParticle Detectors Computers “colliding beam” accelerator detectors

How do we get high energies? 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 (=1.6x J) Charged plates -V+V Potential difference Constant velocity Acceleration Acceleration!

Particle Accelerators “Bunch” of +ve protons 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.

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. 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: Particle Accelerators Magnetic fields curve particle paths Electric fields accelerate

Particle Accelerators Magnetic fields curve particle paths Electric fields accelerate 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: (Downside: synchrotron radiation from circular acceleration of charged particles.)

CERN (birthplace of the World Wide Web!) 8.5km The path of the LHC… 100m below ground ATLAS – proton beams collide here The SPS 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 ( eV). The SPS will accelerate protons to around 7 TeV ( eV).

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

Detecting Particles We can see particles when they interact: ForceRelative Strength Range Strong1≤ m Electromagnetic1/100 ∞ Weak1/100000~ m

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

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

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

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: Charged particle decays into charged + neutral Particles appear from nowhere! “Kink” in track Charged particle – detect ionisation

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

F F v v -ve charged particle +ve charged particle B into picture 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”). How do we measure… Charge? Use a magnetic field. Mass? Indirectly…

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. Particles slow down gradually shower

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

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

Colliding beam experiment Higher energies than ever recorded before Looking for Higgs boson, new particles, new physics Massive project: 4000 physicists at 150 universities in 34 countries. Expected cost £200 million. ATLAS physicist! Electromagnetic calorimeters hadron calorimeters magnets - to curve charged particle tracks muon detectors 44m 22m

The “Particle Zoo” μ-μ- K 0 / K ± π 0 / π ± γ n p e-e- Symbol 207Muon 974/9660 / ±1Kaons 264/2730 / ±1Pions 00Photon 18390Neutron Proton 1Electron Mass (9.1x kg) Charge (1.6x C) Name Atomic matter etc… Identify particles by their charge and mass!

Over to you… You will be asked to identify some particles from an e + e - annihilation. Electron + positron of known energy annihilate producing a photon Which turns into a particle/antiparticle pair: e+e+ e-e- μ + π + K + or p μ - π - K - or p From their radius of curvature in a B field, find momentum (p =rQB) and then mass (E 2 =p 2 +m 2 ) γ