Particle Detectors for Colliders

1. Particle Detectors for Colliders Robert S. Orr University of Toronto

2. Plan of Lectures • I’ve interpreted this title to mean: • Physical principles of detectors • How these principles are applied to representative devices • Physical principles are not particular to colliders • Realization as devices probably is, a bit • An enormous field – I can only scratch the surface • At Toronto I give this material as about 15 lectures • More comprehensive (?) notes from UofT lectures • These pages also have some notes on accelerators http://hep.physics.utoronto.ca/~orr/wwwroot/phy2405/Lect.htm

3. High Energy Physics experiments? • Collide Particles • Detect Final State • Understand Connection of 1 + 2 Accelerators & Beams Detectors Analysis

4. Generic Detector • Layers of Detector Systems around Collision Point R.S. Orr 2009 TRIUMF Summer Institute

5. Generic Detector • Different Particles detected by different techniques. • Tracks of Ionization – Tracking Detectors • Showers of Secondary particles – Calorimeters R.S. Orr 2009 TRIUMF Summer Institute

6. Generic Detector • Different Particles detected by different techniques. • Tracks of Ionization – Tracking Detectors • Showers of Secondary particles – Calorimeters R.S. Orr 2009 TRIUMF Summer Institute

7. ATLAS Detector R.S. Orr 2009 TRIUMF Summer Institute

8. ATLAS Detector • Different Particles detected by different techniques. • Tracks of Ionization – Tracking Detectors • Showers of Secondary particles – Calorimeters R.S. Orr 2009 TRIUMF Summer Institute

9. Interaction of Charged Particles with Matter • All particle detectors ultimately use interaction of electric charge with matter • Track Chambers • Calorimeters • Even Neutral particle detectors • Ionization • Average energy loss • Landau tail • Multiple Scattering • Cerenkov • Transition Radiation • Electron’s small mass - radiation

10. Heavy charged particle interacting with atomic electrons • All electrons with shell at impact parameter b • Energy loss - symmetry • Gauss • Density of electrons Energy Loss to Ionization R.S. Orr 2009 TRIUMF Summer Institute

11. Maximum minimum b • In a classical head-on collision • Relativistically Physical limits of integration R.S. Orr 2009 TRIUMF Summer Institute

12. Time for EM interaction Physical limits of integration • Electrons bound in atoms • Time of interaction must be small, compared to orbital period, else energy transfer averages to zero • Orbital period • Collision time • Put in integration limits R.S. Orr 2009 TRIUMF Summer Institute

13. Ionization Loss • This works for heavy particles like α • Breaks down for • Correct QED treatment gives Bethe – Bloch equation Maximum energy transfer in single collision Mean excitation potential of material Density correction Shell correction R.S. Orr 2009 TRIUMF Summer Institute

14. Mean excitation potential • This is main parameter in B –B • Hard to calculate • measure • infer I Bethe – Bloch Equation • Empirically R.S. Orr 2009 TRIUMF Summer Institute

15. Electric field polarizes material along path • Far off electrons shielded from field and contribute less • Polarization greater in condensed materials, hence density correction Relativistic rise & Density Correction R.S. Orr 2009 TRIUMF Summer Institute

16. Particle Identification depends on velocity Usually measure determines mass R.S. Orr 2009 TRIUMF Summer Institute

17. Particle Identification R.S. Orr 2009 TRIUMF Summer Institute

18. expressed as (mass)x(thickness) is relatively constant over a wide range of materials density ln variation Roughly constant over periodic table Independent of material Fe, Al, …. Cu or Mass Stopping Power Mixtures of Materials Bragg’s Rule fraction by weight No of atoms of i element molecule 10 MeV proton loses same energy in R.S. Orr 2009 TRIUMF Summer Institute

19. Electron Energy Loss • More complicated than heavy particles discussed so far • Small mass radiation (bremsstrahlung) dominates • Above critical energy, radiation dominates • Below critical energy, ionization dominates • What constitutes a heavy particle, depends on energy scale R.S. Orr 2009 TRIUMF Summer Institute

20. Projectile deflected • Projectile and atomic electrons have equal masses • Also identical particles – statistics • Equal masses non-identical identical Bethe Bloch for electrons R.S. Orr 2009 TRIUMF Summer Institute

21. Below ~100 GeV/c only important for electrons • > 100 GeV/c becomes important for muons • in the GeV range e  Bremsstrahlung atoms /cc independent of  function of material can emit all energy in a few photons -> large fluctuations R.S. Orr 2009 TRIUMF Summer Institute

22. distance over which the electron energy is reduced by1/e on average • Radiation Length Radiation Length assume indep of E • for x expressed in units of R.S. Orr 2009 TRIUMF Summer Institute

23. Electron Energy Loss approx valid for any material electrons in Cu R.S. Orr 2009 TRIUMF Summer Institute

24. good approximation (3%) except for He R.S. Orr 2009 TRIUMF Summer Institute

25. High Energy Muons R.S. Orr 2009 TRIUMF Summer Institute

26. Muons in Cu R.S. Orr 2009 TRIUMF Summer Institute

27. 5 4 3 2 eV Cerenkov Radiation measure known 350 nm to 550 nm R.S. Orr 2009 TRIUMF Summer Institute

28. Can be a very important limitation on detector angle/momentum resolution For Charged particles traversing a material (ignore radiation) Inelastic collisions with electrons - ionization elastic scattering from atomic nuclei Multiple Coulomb Scattering Rutherford scattering •  is polar angle • number of scatters > 20 • negligible energy loss • Gaussian statistical treatment is usually ok vast majority of scatters – small angle

29. probability of scattering through  RMS scattering angle Gaussian Multiple Scattering 15.7 MeV electrons Gaussian cf. experiment more material R.S. Orr 2009 TRIUMF Summer Institute

30. Gaussian Multiple Scattering For detectors usually interested in RMS scattering angle – projected on a plane most detectors measure in a plane R.S. Orr 2009 TRIUMF Summer Institute

31. So far have discussed In general energy loss for a given particle For a mono-energetic beam distribution of energy losses Thick Absorber – Gaussian Energy Loss Thin Absorber – Possibility of low probability, high fractional energy transfers Energy Loss Distribution

32. Typical Energy Loss in Thin Absorber • Scintillator • Wire Chamber Cell • Si tracker wafer • Practical Implications • Use of dE/dx for particle ident • Landau tails cause limitation in separation long tail • Position in tracking chamber • Landau tails smear resolution • Various Calculations • Landau – most commonly used • Vavilov - “improved” Landau • Separation of 1 from 2 particles in an ionization/scintillator counter • Landau tails smear ionization R.S. Orr 2009 TRIUMF Summer Institute

34. Energy Loss of Photons in Matter Important for Electromagnetic Showers • Photoelectric Effect • Compton Scattering • Pair Production – completely dominant above a few MeV • For a beam of  or survival probability of a single  absorption coefficient R.S. Orr 2009 TRIUMF Summer Institute

35. Photoelectric Effect Pb K absorption edge – inner, most tightly bound electrons • Atomic electron absorbs photon and is ejected • Cross section for absorption increases with decreasing energy until • Then drops because not enough energy to eject K-shell electrons • Dependence on material R.S. Orr 2009 TRIUMF Summer Institute

36. Compton Scattering Compton Edge – Detector Calibration R.S. Orr 2009 TRIUMF Summer Institute

37. Pair Production • Central to electromagnetic showers • Can only occur in field of nucleus • Rises with energy cf. Compton and PE • Same Feynman diagram as Brems Mean Free Path Closely related to Radiation Length R.S. Orr 2009 TRIUMF Summer Institute

38. Photon Absorption as Function of Energy Pb MeV R.S. Orr 2009 TRIUMF Summer Institute