Detecting Particles Martin Gallacher – University of Birmingham

Contents  The Spark chamber  Early particle detectors  The ATLAS detector  The Atlantis Event Display  How to recognise particle events at ATLAS

The Spark Chamber The spark chamber detects cosmic ray muons The spark chamber detects cosmic ray muons The spark shows the track of the muon through the chamber The spark shows the track of the muon through the chamber

The Origin of Cosmic Rays StarsSupernovae Black HolesOther galaxies

Cosmic ‘Rays’ reach Earth

Muons and Neutrinos MUON (µ) – has charge and mass (like a heavy electron). MUON (µ) – has charge and mass (like a heavy electron). Easy to detect! Arrive at a rate of a few per cm 2 per min NEUTRINO (ν) – no charge, negligible mass NEUTRINO (ν) – no charge, negligible mass Difficult to detect! Arrive at a rate of hundreds of millions per cm 2 per min

How the Spark Chamber Works

Early Particle Detectors Cloud Chamber Bubble Chamber A discarded bubble chamber at Fermilab The cloud chamber at the Cavendish museum

Identifying Particles Cloud chamber photo Evidence for the positron Bubble chamber photo Electron-positron production

Modern Particle Detectors

The ATLAS Detector 7000 tonnes of detector sits 100m underground

The ATLAS Detector 44m in length and 22m In diameter (about the size of a 5 storey building – bigger than the Poynting building) The ATLAS experiment is a collaboration of about 2100 scientists from 167 institutions in 37 different countries.

A Typical Detector Inner detector (Tracker) Measures charge and momentum of charged particles in magnetic field Electro-magnetic calorimeter Measures energy of electrons, positrons and photons Hadronic calorimeter Measures energy of hadrons (particles containing quarks), such as protons, neutrons, pions, etc. Muon detector Measures charge and momentum of muons Neutrinos are only detected indirectly via ‘missing energy’ not recorded in the calorimeters

Inner Detector (Tracker) Comprises of pixel detectors in the centre than semiconductor tracker (SCT) and finally transition radiation tracker (TRT). 1.15m in radius and 7m long

Solenoid Magnet Surrounds the inner detector and is contained in a cryostat Produces a 2T magnetic field

EM Calorimeter Barrel in cryostat Endcap

Hadronic Calorimeter The calorimeters fill the gap between the outside of the inner solenoid and the muon system

Toroid Magnets The largest toroid magnet ever built ! Outer diameter 20.1m and 25.3m long

Muon Detector Muons are the only charged particles which can pass through the calorimeters. The muon system therefore acts like the inner tracker but outside the calorimeters to measure the muon properties alone.

Proton Collisions in ATLAS

End-on view Side view Rolled out energy plot Control Panel

Inner detector (Tracker) Electromagnetic calorimeter Hadronic Calorimeter Muon detector

Example: W  e Spotting an electron! Look for a track in the tracker and a deposit of energy in the electromagnetic calorimeter

To find out more about an object in the event click on ‘Pick’ Then click on the object you want to look at eg the track of the electron.

The selected object then becomes grey The transverse momentum (p T ) then appears in the bottom left corner

Example: W  e So what about the neutrino? Dashed red line means no track – something is missing Neutrino is indicated by missing E T of more than 10GeV

Next event Click on ‘Next’

Example: W  Spotting a muon Track in the tracker with high p T Track in the muon detector (orange) Also have some other fragments not associated with the muon

Example: Z  ee Characteristics: 2 electrons in the event

Example: Z  Characteristics: 2 muons in the event Here: one in central region Example: Z  2 muons in the event Here: one in central region one in forward region Particles in forward region are not seen in “end-on” projection! Only in “side” projection Example: Z  Characteristics: 2 muons in the event

Example: background Characteristics: Does not contain W  e, W , Z  ee, Z  Example: background Characteristics: Does not contain W  e, W , Z  ee, Z  Typically bundles of particles (jets) are produced NB: Jets have hits in both the Electromagnetic and Hadronic calorimeters

Recap Analyse each event and classify into 1 of 5 categories (W  e, W , Z  ee, Z , background) by ticking the correct box Analyse each event and classify into 1 of 5 categories (W  e, W , Z  ee, Z , background) by ticking the correct box Click ‘Next’ to go to the next event Click ‘Next’ to go to the next event WARNING: There is one event that won’t fit into one of the above categories – the elusive Higgs boson event (H , H  eeee or H  ee  ) – find this and win a prize! WARNING: There is one event that won’t fit into one of the above categories – the elusive Higgs boson event (H , H  eeee or H  ee  ) – find this and win a prize!