Transcript Slides

Particles, Colliders, and the Higgs
Boson
Tim Wiser
Splash P2506
3 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 3rd 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: 2.0023228 (1 loop)
– Measured: 2.0023193
• 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, gp=2 and gn=0
• But…
• Experimentally, gp=5.6 and gn=-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 (quarkantiquark 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: 3rd generation of matter
Practice with Feynman Diagrams
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•
•
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Beta decay
e+ e- -> μ+ μπ+ -> μ+ νμ
K0 -> K0 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+ecolliders
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•
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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 lowestenergy 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-GuralnikHagen-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
– http://www.particleadventure.org/
• 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 http://www.interactions.org/blogs/
More questions?
• Contact me:
– [email protected]