Experimental Methods of Particle Physics

Transcription

1 Experimental Methods of Particle Physics (PHY461) Fall 015 Olaf Steinkamp 36-J

2 Overview 1) Introduction / motivation measurement of particle momenta: magnetic field early detectors: cloud chamber, bubble chamber, spark chamber ) Gaseous tracking detectors Multi Wire Proportional Chamber Drift Chambers Micro-Pattern Gaseous Detectors 3) Silicon detectors micro-strip detectors (pixel detectors) 4) Track reconstruction pattern recognition track fitting Introduction ()

3 Literature Main sources used in preparing this lecture: C. Niebuhr, Detektoren für die Teilchenphysik, Vorlesungen Uni Hamburg D. Bortoletto, An Introduction to Semiconductor Detectors, lecture at 004 Vienna Conference on Instrumentation R. Frühwirth et al., Data Analysis Techniques for High Energy Physics Cambridge Monographs on Particle Physics, Nuclear Physics and Cosmology, 000 Other useful resources: K. Kleinknecht, Detectors for Particle Radiation, J. Ferbel, Experimental Techniques in High Energy Physics Particle Data Group web page, also: R. K. Bock and A. Vasilescu, Particle Detector Briefbook, Cambridge University Press Introduction (3)

4 Particle Physics Experiments for Dummies Accelerate a beam of (stable & charged) particles to high energies electrons/positrons, protons/antiprotons, heavy ions Bring them into collision with another beam of particles ( collider experiment ) a target at rest ( fixed-target experiment ) Measure properties of the particles created in the collision production & decay vertices position-sensitive detectors (in magnetic field) flight paths momenta speed Cherenkov detectors penetration power muon detectors energy calorimeters charged particles only charged and neutral analyse and interpret data from many collisions (at LHCb: 109 / year) Introduction (4)

5 Particle Physics Detector for Dummies Detection based on interaction of particles in detector material energy deposition mostly due to excitation / ionisation ( Bethe-Bloch) creation of free electric charge carriers or of scintillation light Electronic readout of detector signals: (not discussed in this lecture) apply electric field across detector volume, collect charges on electrodes electronically integrate & amplify signal pulse digitize the signal: discriminator binary information (hit / no hit) analog-to-digital converter (ADC) encode pulse height time-to-digital converter (TDC) encode signal arrival time transfer digital data to a huge computer farm for processing and storage need a trigger signal to decide when to read out the detector ( Lea) Introduction (5)

6 Tracking Detectors Obtain position information from finely segmented readout electrodes granularity determined by particle density and required spatial resolution close to interaction point: small tracking volume but high particle density fine granularity and excellent position resolution further away: lower particle density but large tracking volume coarser granularity, lower position resolution Example: ATLAS experiment at the LHC radius technology cell size area 5-1 cm silicon pixels 50 x 400 μm 1.8 m² cm silicon strips 80 μm x 13 cm 60 m² cm drift tubes (TRT) 4 mm x 75 cm ( 680 m²) cm drift tubes (MDT) 3 cm x 6 m 5500 m² Introduction (6)

7 ATLAS Detector Introduction (7)

8 ATLAS Inner Tracker silicon pixels silicon strips (SCT) straw drift tubes (TRT) all: barrel + endcaps Introduction (8)

9 Additional Requirements rate capability limited by charge collection time and dead-time of read-out electronics must match the expected rate of charged particles in the experiment material budget multiple scattering in detector material limits spatial resolution especially important if particle momenta are low radiation hardness degradation of detector material due to radiation damage detector must survive several (typically 10) years in the experiment cost!!! often dominated by number of electronic readout channels detector granularity as fine as needed but not much finer than that different detector technologies to match different conditions Introduction (9)

10 Momentum Measurement Measure curvature ρ of the particle trajectory in a magnetic field B gives momentum component transverse to magnetic field: p T = q B ρ also: direction of curvature gives sign of the particle charge Typical collider experiment: solenoid magnet p T [GeV ] = 0.3 B [T ] ρ [m] Typical fixed-target experiment: field lines parallel to beam axis barrel and endcap detectors inside magnetic field dipole magnet field lines orthogonal to beam axis planar detection layers before and after the magnet Δθ B-field B-field tracking detectors Introduction (10)

11 Momentum Resolution (I) Determine sagitta of trajectory from three position measurements from geometry: L φ φ L/ = sin ρ ( s = ρ 1 cos φ ) [ ( ρ 1 1 ( ) )] 1 φ = ρ φ 8 x1 x x3 s ρ from deflection in magnetic field: L 0.3 B L φ = ρ = pt B-field (for φ not too large) 0.3 L B s = 8 pt for position measurements with resolution σx: x +x s = x σ = σ x s Introduction (11) σ (p T ) σs = = pt s 8 pt 3 σ x 0.3 B L

12 Momentum Resolution (II) For N equidistant measurements (N 10): σ (p T ) σ = κκ = pt pt 70 σ x N B L with κ = 1/ρ = curvature [Gluckstern, NIM 4 (1963) 381] Relative momentum resolution deteriorates linearly with transverse momentum improves linearly with the strength of B-field improves quadratically with the length of the measured track segment large size of high-energy particle physics experiments e.g. ATLAS magnetic field: overall diameter: 5 m overall length: T 46 m Introduction (1)

13 Momentum Resolution (III) Additional uncertainty from multiple scattering average deflection angle in bending plane ϑ rms ( L' L' = z ln β p [GeV ] X0 X0 ( )) X0 = radiation length of detector material L' = L / sin θ = passlength through detector material ( Katharina ) B L' θ L Total uncertainty on momentum measurement σp p ( ) = ( ( σ x p sinθ 70 N B L β B L sinθ X 0 + ( σ θ cot θ ) ) ) spatial resolution multiple scattering θ resolution Introduction (13)

14 Early Tracking Detectors (I) Cloud Chamber (Wilson, 191) vessel filled with supersaturated water vapour (created by rapid adiabatic expansion) charged particle creates ionisation clusters ionisation clusters act as condensation nuclei trail of water droplets along particle trajectory photograph trails through windows in the vessel spatial resolution ~ 100 μm estimate particle energy from density of droplets most important experimental tool until 1950s main disadvantages: large dead time of the order of seconds need to compress/expand vessel to remove droplets after each exposure photographs need to be analysed manually Introduction (14) discovery of positron (Anderson, 193)

15 Early Tracking Detectors (II) Bubble Chamber (Glaser, 195) vessel filled with superheated transparent liquid (created by rapid adiabatic expansion) energy deposition brings liquid to boil trail of bubbles along particle trajectory photograph trails through windows in the vessel spatial resolution ~ 100 μm estimate particle energy from density of bubbles advantage compared to Cloud Chamber: higher density of detection medium detection medium can serve as target material for fixed-target experiments at particle beams gives higher sensitivity to rare processes disadvantages: same as for Cloud Chamber discovery of neutral currents (Gargamelle, CERN 1973) Introduction (15)

16 Early Tracking Detectors (III) Spark Chamber (Fukui/Myamoto, 1959) stack of thin metal plates filled with He/Ne gas apply high voltage between alternate layers just below the break-down voltage charged particle ionizes gas molecules in gap causes discharge in between adjacent plates creates trail of sparks along particle trajectory reduce high voltage to stop discharges readout mostly optical (also: acoustic/electronic) advantage: detector dead-time only ~ ms factor 100 faster than Bubble chambers disadvantage: spatial resolution only ~ mm factor 10 worse than Cloud and Bubble chambers Introduction (16) discovery of muon neutrino (BNL, 196)