Experimental Particle Physics Lecture 3 Particle Interactions and

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Experimental Particle Physics Lecture 3 Particle Interactions and Detectors 22 nd February 2007 Fergus

Experimental Particle Physics Lecture 3 Particle Interactions and Detectors 22 nd February 2007 Fergus Wilson, RAL 1

Interactions and Detectors n Last Week q n Ionisation Losses and charged particle detectors

Interactions and Detectors n Last Week q n Ionisation Losses and charged particle detectors This Week q q Photon absorption Electromagnetic Showers Hadronic Showers Multiple Scattering 22 nd February 2007 Fergus Wilson, RAL 2

Radiation Loss by electrons n Bremsstrahlung: electromagnetic radiation produced by the deceleration of a

Radiation Loss by electrons n Bremsstrahlung: electromagnetic radiation produced by the deceleration of a charged particle, such as an electron, when deflected by another charged particle, such as an atomic nucleus. n Photon can be very energetic. Radiation Length E E 0 (gcm-2) E 0/e 22 nd February 2007 Fergus Wilson, RAL X 0 x 3

Radiation Length n Radiation length has 2 definitions; q q mean distance over which

Radiation Length n Radiation length has 2 definitions; q q mean distance over which highenergy electron losses all but 1/e of its energy by bremsstrahlung. 7/9 ths of the mean free path for pair production by a high-energy photon. X 0 (g cm-2) X 0 (cm) Air 37 30, 000 Silicon 22 9. 4 Lead 6. 4 0. 56 22 nd February 2007 Fergus Wilson, RAL 4

Photon Absorption e- n e+ 22 nd February 2007 n Electron-positron pair production Exponential

Photon Absorption e- n e+ 22 nd February 2007 n Electron-positron pair production Exponential absorption Length scale 9/7×X 0 Fergus Wilson, RAL 5

Simple Electromagnetic Shower Ec e+ x 0 X 0 2 X 0 3 X

Simple Electromagnetic Shower Ec e+ x 0 X 0 2 X 0 3 X 0 4 X 0 N 1 2 4 8 16 n 0 n n <E> E 0/2 E 0/4 E 0/8 22 nd February 2007 E 0/16 <Ec Fergus Wilson, RAL Start with electron or photon Depth ~ ln(E 0) Most energy deposited as ionisation. 6

Real EM Shower n Shape dominated by fluctuations As depth of shower increases more

Real EM Shower n Shape dominated by fluctuations As depth of shower increases more energy is carried by photons Tail Maximum close to naïve depth expectation 22 nd February 2007 t = x/X 0 Fergus Wilson, RAL ladd eq 27. 31 here 7

Calorimetry 1 - Homogeneous In homogeneous calorimeters the functions of passive particle absorption and

Calorimetry 1 - Homogeneous In homogeneous calorimeters the functions of passive particle absorption and active signal generation and readout are combined in a single material. Such materials are almost exclusively used for electromagnetic calorimeters, e. g. crystals, composite materials (like lead glass, Pb. WO 4) or liquid noble gases. • • Crystal, glass, liquid Acts as absorber and scintillator Light detected by photodetector E. g. Pb. WO 4 (X 0 ≈ 0. 9 cm) 22 nd February 2007 95% lead Fergus Wilson, RAL 8

Calorimetry 2 – Sampling In sampling calorimeters the functions of particle absorption and active

Calorimetry 2 – Sampling In sampling calorimeters the functions of particle absorption and active signal readout are separated. This allows optimal choice of absorber materials and a certain freedom in signal treatment. n Heterogeneous calorimeters are mostly built as sandwich counters, sheets of heavymaterial absorber (e. g. lead, iron, uranium) alternating with layers of active material (e. g. liquid or solid scintillators, or proportional counters). n Only the fraction of the shower energy absorbed in the active material is measured. n Hadron calorimeters, needing considerable depth and width to create and absorb the shower, are necessarily of the sampling calorimeter type. 9 Fergus Wilson, RAL n 22 nd February 2007

Hadronic Showers n Nuclear interaction length >> radiation length e. g. Lead: X 0

Hadronic Showers n Nuclear interaction length >> radiation length e. g. Lead: X 0 = 0. 56 cm, λ = 17 cm n n Hadron showers wider, deeper, less well understood Need much larger calorimeter to contain hadron shower q Always sampling q Dense metals still good as absorbers q Mechanical/economic considerations often important q Uranium, steel, brass… 22 nd February 2007 Fergus Wilson, RAL 10

Hadronic Calorimeter L 3 Alternating layers of steel and streamer chambers SLD 22 nd

Hadronic Calorimeter L 3 Alternating layers of steel and streamer chambers SLD 22 nd February 2007 Fergus Wilson, RAL 11

Energy Resolution Limitations n EM Calorimeter q q q n the intrinsic limitation in

Energy Resolution Limitations n EM Calorimeter q q q n the intrinsic limitation in resolution results from variations in the net track length of charged particles in the cascade. Sampling Fluctuations Landau Distribution Hadronic Calorimeter q q q 22 nd February 2007 A fluctuating pi 0 component among the secondaries which interacts electromagnetically without any further nuclear interaction (pi 0 ->gg). Showers may develop with a dominant electromagnetic component. A sizeable amount of the available energy is converted into excitation and breakup of nuclei. Only a small fraction of this energy will eventually appear as a detectable signal and with large event-to-event fluctuations. A considerable fraction of the energy of the incident particle is spent on reactions which do not result in an observable signal. Such processes may be energy leakage of various forms, like: n Backscattering n Nuclear excitation n slow neutrons, neutrinos Fergus Wilson, RAL 12

Multiple Scattering n Elastic scattering from nuclei causes angular deviations: θ n n Approximately

Multiple Scattering n Elastic scattering from nuclei causes angular deviations: θ n n Approximately Gaussian Can disrupt measurements in subsequent detectors 22 nd February 2007 Fergus Wilson, RAL 13

Creating a detector 22 nd February 2007 Fergus Wilson, RAL 14

Creating a detector 22 nd February 2007 Fergus Wilson, RAL 14

1) Vertex Detectors Purpose: Ultra-high precision trackers close to interaction point to measure vertices

1) Vertex Detectors Purpose: Ultra-high precision trackers close to interaction point to measure vertices of charged tracks n n n 22 nd February 2007 Fergus Wilson, RAL Spatial resolution a few microns Low mass A few layers of silicon 15

2) Tracking Detectors Purpose: Measure trajectories of charged particles n n n Low mass

2) Tracking Detectors Purpose: Measure trajectories of charged particles n n n Low mass q Reduce multiple scattering q Reduce shower formation High precision Multiple 2 D or 3 D points Drift chamber, TPC, silicon. . . Can measure momentum in magnetic field (p = 0. 3 q. BR) 22 nd February 2007 Fergus Wilson, RAL 16

3) Particle ID Purpose: Distinguish different charged “stable” particles n n n Muon, pion,

3) Particle ID Purpose: Distinguish different charged “stable” particles n n n Muon, pion, kaon, proton Measured momentum and energy: m 2 = E 2 – p 2 q Difficult at high energy E ~ p Different d. E/dx in tracking detectors q Only for low energy 1/ 2 region, no good for MIPs Measure time-of-flight q Fast scintillator Measure directly q Cerenkov radiation Measure directly q Transition radiation 22 nd February 2007 Fergus Wilson, RAL 17

4) EM Calorimeter Purpose: Identify and measure energy of electrons and photons n n

4) EM Calorimeter Purpose: Identify and measure energy of electrons and photons n n Need ~ 10 X 0 q 10 cm of lead Will see some energy from muons and hadrons Homogenous q Crystal q Doped glass Sampling q Absorber + scintillator/MWPC/… 22 nd February 2007 Fergus Wilson, RAL 18

5) Hadron Calorimeter Purpose: Identify and measure energy of all hadrons n n Need

5) Hadron Calorimeter Purpose: Identify and measure energy of all hadrons n n Need ~ 10 λ q 2 m of lead Both charged and neutral Will see some energy from muons Sampling q Heavy, structural metal absorber q Scintillator, MWPC detector 22 nd February 2007 Fergus Wilson, RAL 19

6) Muon Detectors Purpose: Identify muons Muons go where other particles cannot reach: q

6) Muon Detectors Purpose: Identify muons Muons go where other particles cannot reach: q No nuclear interactions q Critical energies >> 100 Ge. V Ø Always a MIP q Stable (τ = 2. 2 μs) n n A shielded detector can identify muons q “shielding” often calorimeters q Scintillator, MWPC, drift chambers… 22 nd February 2007 Fergus Wilson, RAL 20

Next Time. . . Putting it all together - building a particle physics experiment

Next Time. . . Putting it all together - building a particle physics experiment 22 nd February 2007 Fergus Wilson, RAL 21