Particle Physics LECTURE 7 Georgia Karagiorgi Department of Physics, Columbia University

Experimental Methods in Particle Physics How we study particles in the “lab” Last time!

Today’s agenda Review of particle properties Passage of particles through matter Ionization Scintillation Cherenkov radiation Particle detectors Modern detectors Last time!

Particle detection We exploit primarily the type of interaction that a particle most typically undergo: Particles Type of interaction Signature neutrinos usually none missing energy electrons ionization, electromagnetic track and electromagnetic shower muons ionization penetrating track p, K, p ionization, hadronic track and hadron shower photons electromagnetic electromagnetic shower neutrons, K0 hadronic hadron shower Last time!

Particle detection Last time!

Light detection Last time! A photomultiplier tube (PMT) is a commonly used instrument used to detect visible photons. Basis of operation: photoelectric effect  single photons converted to electrons and multiplied to a measurable electronic signal Last time!

A note about cross-section A measure of interaction probability in particle physics “Effective area of collision” Used to calculate predicted interaction rates N(E) = F(E) × s(E) interaction rate (events/sec) cross-section, or probability of interaction (cm2) flux of incoming particles (particles/cm2/sec) Last time!

Cherenkov detector Last time!

Cherenkov detector Last time!

Ionization detector Detection method and choice of material Induced electrical signal on anode can be measured to estimate number of drift electrons  E lost to ionization

Ionization detector Geiger counter

Time Projection Chamber (TPC) Exploits ionization energy losses of charged particles Electrons are drifted onto a fine grained plane of wires, and the particle trajectories can be mapped out, along with their ionization energy loss, dE/dx.

Time Projection Chamber (TPC) Example: ICARUS liquid argon TPC detector at Gran Sasso Lab, Italy

Scintillation detectors Light output and type of particles/radiation

Scintillation detectors Emitted light depends on detector material. Usually in the visible to UV range. Sometimes requires the use of wavelength-shifting materials to shift UV light to visible, so it can be efficiently measured by commonly used photomultiplier tubes.

Scintillation detectors

Application of scintillation detectors Cosmic ray muon detection

Cosmic ray muon detection incident muon

Cosmic ray muon detection Measurement of the muon lifetime Measure tdecay (difference between muon signal and decay signal in the second scintillator paddle) of a sample N0 of low energy muons Fit the data to an exponential curve of the form: N(t) = N0e -t/T where T = muon lifetime

Cosmic ray muon detection Measurement of the muon lifetime Measure tdecay (difference between muon signal and decay signal in the second scintillator paddle) of a sample N0 of low energy muons Fit the data to an exponential curve of the form: N(t) = N0e -t/T where T = muon lifetime Lifetime T: T = 2.14μs Tth = 2.1970μs

Modern particle detectors: @ LHC

Why LHC? Large proton accelerator complex Long-lived charged particles can be accelerated to high momenta using electromagnetic fields. e+, e−, p, p̅, µ±(?) and Au, Pb & Cu nuclei have been accelerated so far... Why accelerate particles? High beam energies ⇒ high ECM ⇒ more energy to create new particles Higher energies probe physics at shorter distances De-Broglie wavelength: e.g. 20 GeV/c probes a distance of 0.01 fm. An accelerator complex uses a variety of particle acceleration techniques to reach the final energy.

Particle accelerators around the world

Head-on collisions at the LHC

The CMS detector 15 m 22 m

A slice of CMS

Event signatures: the beauty of a magnetic field

Solenoid magnet Solenoid Field direction along beam axis. Homogeneous field inside the coil. Need surrounding iron structure to capture the 'return field'. CMS: I = 20 kA, B = 4T. Superconducting (4K).

Guess what event this is?

Guess what event this is?

What about “dark matter” ? Does not interact through weak, electromagnetic, or strong force But it does interact, very very weakly We need very “quiet” (cryogenic, deep underground) detectors to see dark matter interactions Scattering off of nuclei  transfer of kinetic energy

The XENON dark matter experiment

The XENON dark matter experiment

Modern particle detectors: @ South Pole

Modern particle detectors: @ South Pole

Modern particle detectors: @ South Pole

Modern particle detectors: @ South Pole

What are we trying to measure?

IceCube detector @ South Pole A neutrino detector Neutrino telescope Looks for extra- terrestrial sources of neutrinos; like taking an “X-ray” picture of the Universe

IceCube detector @ South Pole Made up of strings of >5,000 basketball- sized photon detectors called digital optical modules, or DOMs

IceCube detector @ South Pole Made up of strings of >5,000 basketball- sized photon detectors called digital optical modules, or DOMs

IceCube detector @ South Pole How does IceCube “see” neutrinos? A neutrino interacts with an atom of ice, and produces a muon. The muon radiates blue light that is detected by the DOMs. The direction and intensity of the light is used to determine where the neutrino was coming from in the Universe. http://icecube.wisc.edu/about/explained

IceCube detector @ South Pole South pole  maps out neutrinos from the northern sky:

IceCube detector @ South Pole Used to study extra-galactic, extremely high-energy sources. Neutrinos, being nearly massless and without charge, are ideal messengers. They carry directional information. Charged particles are bent by magnetic fields, neutrons decay before reaching the earth and high-energy photons are absorbed. E.g. gamma ray bursts: hadronic or electromagnetic origin?

IceCube detector @ South Pole Can only be done at the South Pole ! Vast, extremely clean ice; it’s there for free! Recall, detector constraints: cost and detector technology