Particle Physics Status and Perspectives
Part 2
142.095
Claudia-Elisabeth Wulz
Institute of High Energy PhysicsAustrian Academy of Sciences
c/o CERN/PH, CH-1211 Geneva 23
Tel. 0041 22 767 6592, GSM: 0041 75 411 0919E-mail: [email protected]
http: //home.cern.ch/~wulz
3 May 2017
Particle detectorsNo single detector is optimal to measure simultaneously time, position, momentum and energy of particles and to identify them.
Photons
Electrons
Muons
Pions, protons
Neutrons
inside ... outside ...
DetectorTracker Electrom. Hadron Muon system
calorimeter calorimeter
1
Time measurement
Charged particles suffer energy losses through exitation and ioniziation of atoms in the detector medium. Part of the excitation energy emerges in suitable media as visible light, which can be transported to a photodetecting device through light guides by total reflection -> scintillation counter. Photomultipliers (“PM”) are common photodetection devices. The duration of the electric pulses may be a few ns (organic scintillators, wavelength shifters are necessary)! Time resolution down to 200 ps -> use as triggering devices!Other uses are as coincidence counters and for beam definition.Problems: Adaptation of the scintillator geometry to the PM as well as operation in magnetic fields.
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Photomultiplier
]
Photocathode
Light fromscintillator 1st Dynode
…………. Anode
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Photomultiplier
Photocathode: electron are emitted through the photoelectric effect. Dynodes: Intermediate electrodes for secondary emission, with successively increasing potential difference.PM’s have in general 10 to 14 stages. Gains up to 108 can be reached. The different voltages of the dynodes are generated by voltage dividers. The efficiency for photoelectron conversion at the cathode depends strongly on the frequency of the incident light and of the material.
For most metals η < 0.1%! Semiconductors have η between 10 and 30%. GaP (doted with zinc and caesium) has η ≈ 80%! η is maximal for wavelengths of about 400 nm.
Number of emitted photoelectrons
Number of incident photons on the cathode (λ)Quantum efficiency η(λ) =
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Exaples of scintillator propertiesNaI(Tl) BGO CsI(Tl) Polystyrene
+p-terphenyle
Decay time / ns 250 300 1000 3λmax (nm) 410 480 565 355Relative light yield 1.0 0.15 0.40 0.13
Scintillators generate large output pulses with short risetimes. Spatial resolution is not good, however, since there is no clear correlation between particle trajectory and pulse. If spatial information is needed, one can arrange several small scintillation counters in series. For exact beam definition, several counters are operated in coincidence (“beam telescope”). This is for example useful in test beams.
Beam definition
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Pion decay in photographic
emulsionPrinciple: ionization.Ionization products are collected at electrodes, or the ionization trail is made visible.
Historic examples: photographic emulsions, cloud chamber, bubble chamber
Emulsions have a resolution < 1 µm, but are continuously sensitive, and the events must be scanned with a microscope (or digitally)!
C. Lattes et al., Nature 159 (1947) 694
Position measurement
π
600
µm
µ
e
π+ -> µ+ + νµ
µ+ -> e+ + νe + νµ-
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K+
µ+
3 cm lead}
Charged V event: K+ -> µ + + νµ
Rochester & Butler, Nature 160 (1947) 855
Wilson’s Cloud Chamber
Condensation of water vapour is enhanced if ions are present. Filled with air almost saturated with water vapour. On expansion: air cools, formation of droplets along trajectories of ions caused by traversing charged particles. Sensitive only during the expansion time, long deadtime afterwards. 1952 replaced by bubble chamber. The latter is filled with liquid instead of gas, which allows to use it also as a target.
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Big European Bubble Chamber (BEBC)
Photo: CERN13
Bubble chamber event (neutral current)
Photo: CERN14
Proportional chambers
104 to 105 V/cm -> Number of secondary electrons is proportional to the number of primary ion pairs (≈ 105 / primary ion pair).
r… radial distance, a … radius of cylinder, b … wire radiusFilled with gas, e.g. Argon. “Quenching” component (e.g. methane) required for higher voltages, to stop propagation of electrons and ions.
Proportional tube+V0
Signal
Anode wire
Cathode
1 V0E =
r ln(b/a) _ _____
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Gas amplification regions
16Ions per primary pair for a typical detector with 1 wire for highly (α particles) and weakly (electron) ionizing particles
Charpak (1968, Nobel prize 1993): many anode wires between two cathode plates. Position resolution: ≈ 300-500 µm, Time resolution ≈ 30 ns.
Multiwire Proportional Chamber (MWPC)
L ≈ 5-8 mm, d ≈ 1-2 mm, Wire diameter 20-40 µm
Lines of equipotential and field lines in MWPC
Only 1 coordinate given by addresses of the hit wires!
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Meseasurement of the second coordinate
x-y (u-v) configuration
Multiwire Proportional Chamber (MWPC)
Crossed wire planes
“Ghost” hits, therefore only suitable for lower multiplicities
“Charge Division” y
L
Particle trackAnode wire
ADC(Analog/digital
converter)
QB QAADC
y QA___ = ___________L QA+ QB
y___) ≈ 0.4 % σ ( L 18
Multiwire Proportional Chamber (MWPC)
Photo: CERN19
Drift chamber
Replacement of MWPC. Resolution 100-200 µm.
Scintillation counter starts a timer (TDC) and defines t0 .t1 is the arrival time of the electrons at the anode wire.vD must be constant as far as possible. Typical values about 5 cm/µs.A drift cell is typically a few cm long or wide.
to
t1
Drift cell
t0
t1
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Measurement of the arrival time of the electrons at the wire relative to a start time t0
Drift chamber
Drift chambers exist in planar (e.g. CMS experiment at CERN) and cylindric arrangements (“Jet chambers”, e.g. OPAL experiment at CERN).
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Drift chamber
Straw tracker of the ATLAS experiment
Photo: CERN22
Streamer chamber
Gas amplification 108 electrons per primary ion pair -> “Streamer mode” (local plasma) -> by recombination of ions visible light emerges from streamers -> electric pulse. Electrodes are parallel plates, HV 10-50 kV/cm with pulse length 3 - 50 ns results in streamer with a few mm length. Resolution ~ 200 µm. Electronic analogue of the bubble chamber.
Photo: CERN23
Resistive Plate Chamber (RPC)
Derived from proportional chambers. Working point near streamer mode (strong emission of photons).
Electrodes e.g. from bakelite (ρ ≈ 109-1010 Ωcm) with graphite coating. Time dispersion: ≈ 1-2 ns -> suitable for triggering!RPC’s exist also in arrangements with several gas gaps. Better efficiencies and time resolution can be achieved. 24
Spacers Bakelite Readout strips
Time Projection Chamber (TPC)
3-dimensional tracking detector, based on ideas of the MWPC and the drift chamber. Mainly in use at e+e- colliders and ion experiments. A TPC consists of a large, gas-filled cylinder with a thin HV- electrode plate in the center -> uniform electric field. In addition, a parallel magnetic field is applied. At the sides of the cylinder sectors of planes of anode wires are placed (endcaps). Parallel to each wire ther are cathode pads. The electrons produced by the passage of a particle drift to the endcaps. 1 coordinate is given by the position of the hit anode, the 2nd by the signal induced on the cathode pads. The 3rd coordinate along the cylinder axis is given by the drift time of the ionization electrons. One obtains many space points along a track. The magnetic field prevents diffusion. Signal amplitudes in the endcaps are proportional to the energy loss dE/dx. The momentum can be derived from the curvature -> particle identification.
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Time Projection Chamber (TPC)
Cathode pads
Anoden wires
Endkappen
Driftingelectrons
High voltage
Electronsdrift
Electric fieldparallel magnetic field
Particles
HV plane
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Time Projection Chamber (TPC)
About 1000 tracks 27
Semiconductor detectors
Without reverse-bias voltage initially a diffusion of holes to the n-region and of electrons to the p-region takes place. The drifting electrons fill holes in the p-region, the holes capture electrons in the n-region. Since the n- and p-regions were electrically neutral originally, charging on both sides of the p-n junction occurs. The p-region becomes negative, the n-region positive. Thereby a field gradient emerges, which eventually stops the diffusion. A zone free of mobile charge carriers is produced.
n p+ -
Depletion zonewithout voltage
Depletion zonewith voltage
p-n junction reverse-bias
Doping:n: As, P, Sb (5 valence el.)p: Ga, B, In (3 valence el.)
Electron-hole pairs play the role of ion pairs in gaseous detectors.
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By applying a reverse-bias voltage (order of 100V) the thin depletion zone is extended across the whole area of the junction.
Through energy deposit in the depleted zone from crossing charged particles) free electron-hole-pairs are formed. Electrons are lifted from the valence band into the conduction band, a hole in the valence band results results from this. In the electric field the electrons and holes drift to the electrodes - a measurable current arises.
The measured signal is proportional to the ionization.Electron-hole pairs thus play the role of electron-ion pairs in gaseous detectors.
The required energy for ionization is however about 10 times smaller than for gas ionization. Therefore one achieves better position resolutions than in gaseous detectors.
29
Semiconductor detectors
Silicon microstrip detectors
They are used as precision tracking devices. Very good resolution, up to 5 µm (through “charge division”).
Spatial information by segmentation of the p-layer -> single-sided microstrip detector. Double-sided detectors by additional segmentation of the n-layer.
Readout strips
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Readout capacitors
Two 15x15 cm2 silicon microstrip detectors with readout chip(CMS experiment)
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Silicon microstrip detectors
Tracker of the DELPHI experiments at LEP
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Silicon microstrip detectors
Event in the DELPHI vertex detector
1.2 million cells
Hit resolution10 µm in the barrel 0.0 7.5 cm
33
Silicon microstrip detectors as vertex detectors
Silicon pixel detectors
• Diode matrix made of silicon• Readout electronics with same geometry• Connections through bump bonding• Used as precision vertex detectors
16x24 pixel matrix (BELLE)
50 µm
100 µm
34
Momenta are determined by measuring curved tracks of charged particles in a magnetic field -> spectrometer. at colliders they surround the interaction point.
Momentum measurement
35
Part of a helicoidal track
Magnetic field configurations
Dipole
Solenoid
• DipoleField lines perpendicular to the beam direction. Best momentum resolution for particles in forward directions. Frequent at fixed target experiments.
• SolenoidField lines parallel to the beam direction. Best momentum resolution for particles perpendicular to the beam direction.
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Magnetic field configurations of ATLAS and CMS
ATLASToroids + central solenoid
CMSlong solenoid
37
ATLAS- Detector
A Toroidal LHC Apparatus 38
CMS Detector
Compact Muon Solenoid39
Calorimeters measure energy and position.Principle: total absorption. Measurement of charged and neutral particles is possible. During absorption the particle interacts with the absorber material and produces secondary particles, which in turn produce more particles -> cascade (shower). Therefore calorimeters are sometimes also called shower counters.The shower develops mainly in longitudinal direction. A smaller transverse component arises through multiple scattering and transverse momentum components of the produced particles.
Energy measurement
L
rΘ
θ0 = θRMS = <θ2>1/2 , rRMS = Lθ0
θ0 = _____________ q √ L/X0 {1+0.038 ln(L/X0)} 13.6 MeVβcp
X0 … Radiation length q …. Charge
40
Calorimeters are particularly suited to measure highly energetic particles. The absorption process is a statistical process, therefore at high energies:
There are two basic types of calorimeters:homogeneous calorimeters and sandwich calorimetersHomogeneous calorimetersAbsorber and detector are the same, e.g. lead glass. Only for electromagnetic calorimeters.Sandwich calorimeterAbsorber (Pb, Fe, Cu, …) and detector (scintillator, …) in alternating layers (“sampling calorimeter”).
Calorimeters usually only serve for one kind of particles (e/γ, hadrons). Properties of electromagnetic and hadronic showers are not the same.
_____ ~ ___ΔE 1E √ E
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Energy measurement
Electromagnetic showers
Highly energetic e+/e-: Energy loss mainly from bremsstrahlung. Highly energetic photons: Energy loss mainly from pair production. A eine cascade of e+/e– pairs and photons emerges, until the energies of the secondary electrons fall below the critical energy Ec, at which ionization losses are equal to the bremsstrahlung losses (Ec ≈ 600 MeV/Z).
Transverse size of an electromagnetic shower (95% the shower cone is contained in a cylinder with radius 2 RM) (“Molière radius”):
Es … mec2 √ 4 π/α = 21.2 MeVe.g. lead glass: RM = 1.8 cm, X0 = 3.6 cm
RM = X0 ___ Es Ec
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Longitudinal development of an electromagnetic shower
Simple model: Each e with E > Ec (initial energy E0 , E0 >> Ec) loses after 1 X0 half of its energy to a bremsstrahlung photon, each photon with Eγ > Ec loses after 1 X0 its energy by producing a e+/e- pair. Electrons with E < Ec do not radiate any longer and lose the rest of their energy by collisions.
e+
γ
e- e-
e- e-
e-
e+e-
e-
γ
γ
γ
γ
t = 0 1 2 4Radiation lengths
e+
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Longitudinal develoment of an electromagnetic showerAfter t radiation lengths there are about 2t particles in the shower.
Mean energy of the e/γ:
The shower development stops when E(t) = Ec:
Electromagnetic shower in a cloud chamber
E0
2tEt (t) = ____
tmax = t (Ec) = ____________ln (E0/Ec)ln 2
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Electromagnetic calorimeters
Typicaldepth: for particles with 30 GeV energy --> more than20 X0 .
Energy resolution:
a … Stochastic term; a ≈ (2 … 15)%
b … Constant term (Inhomogenities, intercell calibration, nonlinearities) -> dominates at high energies; b ≈ (0.5 … 5) %
c … Noise term (electronic noise, radioactivity, pile-up)
The spatial and angular resolution also show a 1/√E dependence.45
Hadronic showersQualitatively similar to electromagnetic showers, but more complex (inelastisch) processse occure. More fluctuations -> worse energy resolution than for electromagnetic calorimeters. Typically: a ≈ (50 … 100)%, b ≈ (4 … 10)%.
The shower size is defined by the absorption length λa. It is always larger than X0 -> hadron calorimeters are always thicker than electromagnetic ones. Typical depths: 10 λa and more. Losses through nuclear excitation, “leakage” vof decay muons and neutrinos from the calorimeter -> visible energy is 20 to 30% smaller than for electrons -> nonlinearity! Compensation can be achieved by clever arrangement of the samples and other methods.
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Particle identification
Distinction of π/K, K/p, e/π, γ/π0, ...Calorimeters, muon detectors, vertex detectors …Methods depend very much on the energy range one is interested in.. It is possible to simultaneously measure dE/dx and p, time of flight, and to use Cerenkov light and transition radiation.
Measurement of dE/dx
Simultaneous measurement of p and dE/dx defines the mass and thus the identity of a particle.
p = mβγ1β2dE/dx ~ ___ ln (β2γ2)
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Measurement of dE/dx
Mean energy loss for e, µ, π, K, p in 80/20 Ar/CH4
} π/K – separation requires dE/dx resolution < 5%!
e
µ
Kp
π
48
Monte-Carlo
dE/dx in the DELPHI detector
pKπ
Data
e
49
Time of Flight Counters
Limited to particles with momenta smaller than a few GeV.
Δt for wavelengths of L = 1mscintillator with σt = 300 psπ/K separation up to 1 GeV
Δt = __ ( __ - __ ) ≈ ____ (m12 - m2
2)Lc
1β1 β2
1 Lc2p2
50
Cerenkov counters
At the passage of a charged particle with velocity v through a medium with refractive index n excited atoms near a particle become polarized. If v > c/n, part of the excitation energy appears as coherent radiation, which is emitted at a typical angle θ with respect to the direction of motion. A determination of θ leads to a direct determination of the velocity. Compared to a typical scintillator (104/cm) only few photons are emitted. Therefore Cerenkov counters are several meters long.
v > c/nβn > 1
cos θ = ____1βn
ct/n θβct
θ
51
Cerenkov countersCerenkov counters are used in two modes of operation:
1) Threshold modeTo detect particles with speeds that exceed a certain value. θ is not measured explicitly.Assumption: 2 particles with β1 and β2 need to be distinguished at a given momentum p. In a suitable medium, where β1n > 1 ≥ β2n, particle 1 will emit Cerenkov radiation, but not particle 2. γ, at which the particle starts to produce Cerenkov light: γthreshold = E/mc2
Medium n-1 Photons/cm γthreshold___________________________________________________________________________________________
He 3.5 . 10-5 0.03 120CO2 4.1 . 10-4 0.4 35Silica gel 0.025-0.075 24-66 4.6-2.7Water 0.33 213 1.52Glass 0.46-0.75 261-331 1.37-1.22
Particle distinction works up to about 30 GeV/c. 52
Cerenkov counters
2) Differential mode (focussing mode)Here the angle theta θ is measured with a mirror system. If all particles travel in the same direction, the cone of the Cerenkov light can be focussed onto a slit aperture and read out by a PM. Once can select the wanted velocity range either by adjusting the aperture or by changing the refractive index through changing the pressure or the composition of the gas.
θ
PrismAperture
Medium
sphericalmirrorto PM’s
53
Cerenkov countersWhen particles do not fly parallel to a fixed axis, one has to use a RICH (Ring Imaging Cerenkov Counter). Used at some collider experiments. θ is determined by intersecting the Cerenkov cone with a photosensitive plane. The radii of the rings depend on the emission angle of the Cerenkov radiation.
Mirror
Cerenkov-medium
54
θ
RICH-Detektoren
Ein RICH mit 2 Medien erlaubt π/K/p-Trennung von 0.7 bis 45 GeV/cz.B. in DELPHI und SLD.DELPHI: Das flüssige Medium kann Teilchen im Impulsbereich 0.7 bis 9 GeV/c identifizieren. Das gasförmige Medium dient zur Teilchenidentifikation von 2.5 bis 25 GeV/c.
DELPHI
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DELPHI RICH
2 particles in a hadronic jet from a Z decay in gaseous and liquid Cerenkov-medium.
π/K hypothesis
56
Transition radiationFor very high energies (γ ≥ 1000). Transition radiation occurs then charged particles traverse layers of different dielectric properties. The intensity of the emitted radiation (in the visible and X-ray regions) reflects the particle energy E = mγc2, not the velocity. Probability for transition radiation grows with γ. Mainly used for electron identification (e.g. at H1 at DESY, D0 at Fermilab or ATLAS at CERN). Distinction of π possible from p > 1 GeV.An X-ray quantum is only emitted with probability 1% per transition -> several 100 transitions in practice, e.g. Li or plastic foils in gas.
ATLAS TRT prototype(Transition Radiation Tracker)
57
Example LHC:Cross sections for different processes vary over many orders of magnitude. inelastic: 109 Hz• W -> ℓν: 100 Hz• tt: 10 Hz• Higgs (100 GeV): 0,1 Hz• Higgs (600 GeV): 0,01 Hz
Required selectivity 1 : 10 10 - 11
Trigger
-
Trigger – Cross sections and rates
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Type of eventProperties of the measured trigger objects
Choice of trigger conditions
Event accepted?T ( ) YES
NO
depends
Triggerobjekte (Kandidaten): e/γ, µ, Hadronjets,τ-Jets, fehlende Energie, Gesamtenergie
Triggerbedingungen: gemäß physikalischen und technischen Prioritäten
successive steps
Trigger
59
