Particles, Colliders, and the Higgs Boson Tim Wiser Splash P Nov 2012

Plan Standard Model of Particle Physics – Particles, interactions, and the Higgs field Drawing Feynman diagrams – These simple pictures are actually calculational tools for physicists! Particle colliders – How they tell us what stuff is made of What’s left? – Incompleteness of the Standard Model Q&A

What is matter made of? Atoms (~100 elements) Protons (p), neutrons (n), and electrons (e-) p, n made of quarks (up & down type) All the matter around us is made of u, d, e- But there’s more! 6 quarks and 6 leptons, plus antiparticles

What holds it together? Four fundamental forces: – Gravity – Electromagnetism – Strong force – Weak force Gravity is by far the weakest, and it’s different than all of the others. So we will ignore it today!

Forces in Particle Physics In the Standard Model, all forces work the same way: by exchanging particles. – E&M: photon – Strong force: gluon – Weak force: W, Z bosons Two electrons can “toss” a photon back and forth between them, and repel each other as a result.

e-/e- repulsion

What about attractive forces…? You might think that exchanging particles can only result in repulsive forces. But the exchanged particles are not “real”… Virtual particles can move left but carry rightward momentum! Kind of like throwing a boomerang…

Feynman Diagrams There are three rules in particle physics: – Conserve energy – Conserve momentum – Conserve charges (electric, and more…) As long as those rules are satisfied, everything that is allowed WILL happen with some probability! Feynman diagrams automatically obey the 3 rd rule. “Calculating” the diagram tells us the probability. There are usually lots of diagrams for the same process, so we will need to add them all together.

QED (Quantum Electrodynamics) QED is the simplest part of the Standard Model. There is only one possible Feynman vertex:

Electron Scattering Let’s say we want to see how two electrons “scatter” off of each other. We need to draw all Feynman diagrams with two electrons in and two electrons out. We already saw one:

How can we possibly deal with an infinite number of Feynman diagrams??

Order of importance Fortunately for us, the more complicated the diagram, the smaller its value! Each vertex multiplies the probability by a small number (in QED, 1/137) Every loop divides the probability by about 25,000! So, we only need to think about the simplest possible diagrams.

“Bending” diagrams It’s not against the rules to have electron lines go “backwards in time” Such electrons would act exactly like oppositely-charged particles moving forward in time—antimatter! (This doesn’t make time travel possible. Sorry!)

Pair production If a photon has enough energy (rule #1!) it can produce an electron and its antiparticle, the positron. (It turns out that this can only happen if the photon hits something first, due to rule #2.)

Annihilation If we read the diagram the other way, we see that an electron and positron can “annihilate” and produce a photon. (Well, actually two photons—we need to conserve momentum!)

Evidence for QED Besides the fact that we have detected electrons, positrons, and photons and they work just like QED says… QED predicts the “g-factor” of an electron to be almost, but not quite, 2. – Prediction: (1 loop) – Measured: If you add in the 2-loop correction, they agree to 10 decimal places!

Protons & Neutrons For a while, scientists thought that these were elementary particles like the electron and photon. If that were true, g p =2 and g n =0 But… Experimentally, g p =5.6 and g n =-3.8 This can only happen if the proton and neutron are made of smaller particles!

So, what’s inside? We only have one good way of finding out what’s inside of particles… Smash them together!

A plenitude of particles When we started smashing protons and neutrons together, we started discovering all sorts of new particles: – 8 mesons: 3 pions, 4 kaons, and the eta – 8 baryons: p, n, 3 sigmas, 2 xis, and the lambda But as we built bigger, better colliders we found even more: there are now hundreds of mesons and baryons known.

Simplifying We wanted to find what protons and neutrons were made of… But we found a bunch of composite particles like them instead! We can explain the structure of hadrons (mesons and baryons) if we guess that there are three “quarks”—up, down, and strange. But we’ve never seen quarks by themselves, so the force that holds them together must be really strong!

Hadron Structure Mesons: Baryons:

Quantum Chromodynamics (QCD) In fact, there is a way for this all to work… Three quarks: up, down, and strange In addition to electric charge, “color charge” – Call them red, green, and blue Force carrier particle: gluon

QCD Feynman Diagrams

Confinement Because gluons themselves have color charge, the force between two quarks doesn’t get weaker as they get further apart! If you pull hard enough, you will just create new particles until everything is color neutral. This explains why we see mesons (quark- antiquark pairs) and baryons (three quarks or three antiquarks) but never quarks or gluons by themselves.

Jets If we never see quarks or gluons in nature, why are they useful? It turns out that QCD gets weaker at high energies! So we can describe collider physics with quarks and gluons… which “hadronize” as they leave the collision point. The resulting bunches of hadrons are called jets.

Weak Interactions In nature, we observe “flavor-changing” interactions Nuclear beta decay (n->p+e+?) – d quark -> u quark How can we explain this? QED and QCD are “flavor-blind”

Neutrinos It looks like beta decay doesn’t conserve momentum! That’s ridiculous, there must just be an invisible particle as well. Call it a “neutrino” (quasi-Italian for little neutral particle)

Changing Flavors To change from a d quark to a u quark, we must emit a charge -1 particle That particle must then emit an electron and an anti-neutrino. W - boson (there is also a W + boson, of course.) To explain the “weakness” of the weak force, the W bosons must be heavy. (This will be important later!)

Constructing the Standard Model A series of surprises, predictions, and experiments. Prediction: pion as nuclear force mediator Surprise: muon (a heavier electron!) Experiment: quarks are real Prediction: charm quark (confirmed!) Prediction: W and Z bosons (confirmed!) Surprise: 3 rd generation of matter

Practice with Feynman Diagrams Beta decay e + e - -> μ + μ - π + -> μ + ν μ K 0 -> K 0 bar

Testing the Standard Model High-energy tests – Particle colliders – Cosmic rays Precision tests – g-2 experiments – Rare particle decays

Colliders 2 things come in, n things go out Higher energy means we can make heavier particles in the collision Two main types: linear (like SLC) and circular (like LHC)

Electron Colliders The easiest particles to accelerate Since they’re elementary particles, easy to calculate and to measure the results Hard to make circular colliders (LEP was one) Lots of linear colliders, including one at SLAC! Link

A few discoveries made by e+e- colliders Countless hadrons Charm quark (in the form of the J/ψ meson) Tau lepton Precision measurements of W and Z bosons

Hadron Colliders Protons and/or antiprotons Tevatron (p-pbar) and LHC (p-p) are the major HCs Pros: high energy, can be circular (cheaper), strong interactions Cons: hadrons are composite, strong interactions

Discoveries at Hadron Colliders Bottom and top quarks (Tevatron) W and Z bosons (SPS) Countless MORE hadrons

Collider Physics I: Acceleration Powerful electric fields speed up charged particles In practice, “RF cavities” are used – Kind of like a tuned microwave oven… In a linear collider, we get one shot to accelerate In a circular collider, we can accelerate it over and over again

Collider Physics II: Bending and Focusing Electric fields speed up the particles, but we use magnetic fields to focus and aim the beam Magnets have to be kept very cold so that the wires superconduct and produce very strong magnetic fields

Collider Physics III: Collision Finally, two beams of particles will collide with each other How do we see what is produced? Massive detectors around the collision point can track the paths of particles and measure their energies

The LHC Large Hadron Collider At CERN, near Geneva, Switzerland 17 mile circumference, >150 ft underground – Passes under both Switzerland and France 2 primary detectors, ATLAS and CMS 2 special-purpose detectors, LHCb and ALICE Several minor detectors

ATLAS

CMS

The Higgs Boson The Standard Model as we have talked about so far makes a prediction: All elementary particles are massless! – (Composite particles like hadrons can still have mass, though.) This is obviously not true…but the Standard Model works so well, we have to try and save it.

Symmetry The SM has a property called “gauge symmetry” which describes the properties of the three forces Mass is incompatible with gauge symmetry! But removing gauge symmetry gets rid of all of the predictive power.

Broken Symmetry In quantum field theory, particles are actually ripples of fields Most fields have the value of 0 in the lowest- energy state. If a field’s lowest energy state is not zero, then it is said to “break” a symmetry. – The symmetry still exists, but it is ‘hidden’ at low energies.

Higgs Field Peter Higgs* discovered the Higgs mechanism, where a field breaks a gauge symmetry. Then, the gauge boson (force-carrying particle) will become massive. This could be how the W and Z bosons get mass! *(It should really be called the Anderson-Higgs-Brout-Englert-Guralnik- Hagen-Kibble mechanism.)

Prediction Higgs realized that the presence of this field meant there would be a new boson that interacts with all massive particles. If we find the Higgs boson, we will finally complete the Standard Model! But it won’t be easy: the Higgs interacts proportionally with mass of particles, so electrons, ups, and downs barely interact with it at all. Then, the Higgs will decay long before it reaches one of our detectors. We will only be able to see it indirectly.

Higgs at the LHC Production – Gluon fusion – Vector boson fusion Decay – WW, ZZ – bb – 2 photons?

Discovery Announced July 4, 2012

What’s Next? There are a few ways the SM is incomplete: – Gravity – Dark Matter & Dark Energy – Lots of free parameters (unsatisfying) – Fine-tuned (maybe not a problem?) So, we keep looking for new physics: – Supersymmetry – Extra dimensions – Or something unexpected…

Questions?

Learn More The Particle Adventure – Popular Science Books: – Brian Greene (quantum physics, string theory) – Lisa Randall (beyond SM physics) – Sean Carroll (search for the Higgs, coming out soon) Take a physics class! Particle physics blogs – Partial list at

More questions? Contact me: –