ptilparticle ph iph ysics 627 - texas a&m...
TRANSCRIPT
About MeG d t k t CLEO• Graduate work at CLEO e+e‐collider at Cornell
• Postdoctoral work with CLEO• Postdoctoral work with CLEO (some) and with CDMS(Cryogenic Dark Matter Search)( y g )
About Me• Majority of my postdoctoral work has been with the• Majority of my postdoctoral work has been with the Cryogenic Dark Matter Search (CDMS) experiment, searching for the Dark Matter in Universesearching for the Dark Matter in Universe
• CDMS leads the world among ~30 experiments
Coincidence or Clue? A Convergence
Current DM AbundanceExplained by MassiveParticle with Weak σ
0χ0
Supersymmetry
Compelling evidence to suspect LSP χ0 as the WIMP
Direct Detection• Di tl D t t WIMP E th• Directly Detect WIMPs as Earth
Ploughs through the DM Halo• Measure Mass and Scattering
cross sectioncross‐section• Huge Challenges of background• Extremely rare and difficult search,
b ll b k d l!billion to 1 background to signal!Out there & may interact on earth!
SuperCDMS CollaborationCollaboration is growing; 4 new groups added!
CDMS InstitutionsDOE L b tDOE LaboratoryFermilabNIST
DOE UniversityyCalTechFloridaMinnesotaMITStanfordStanfordUC Santa Barbara
NSFCase Western ReserveColorado (Denver)Santa ClaraUC BerkeleySyracuseTexas A&MTexas A&M
CanadaQueens
Particle Detection
• To Study Particle Physics phenomena, one must first have a way of studying their behaviorfirst have a way of studying their behavior
• Particles must be first detectedThi i d th h th i i t ti ith di– This is done through their interaction with ordinary matter in detector (s). Directly or indirectly, they transfer energy to the medium they are traversinggy y g
– Particle detectors are designed to take advantage of the energy loss suffered by high energy particles
h ltraversing their activematerial
• Among possible interactions, only electro‐ti i t ti t i ll d f d t timagnetic interaction typically used for detection
Particle Energy Loss Mechanism1. Ionization: A charged particle traversing
material produces ionization, in which electrons are stripped off their atomic shells
2. Scintillation: In some material, the energy loss is also (~3%) through excitation of atomic electrons to higher states and eventual
i i f h h h i d lemission of photon when the excited electron relaxes back. This phenomenon is called scintillationscintillation
3. Cerenkov: The particle may also emit radiation due to its velocity being higher than the speeddue to its velocity being higher than the speed of light in that detector medium
Ionization Loss of Charged Particles
Moderately relativistic charged particles such as protons, α, atomic ions (other than electrons) lose p , , ( )energy in matter primarily by ionization and atomic excitation. Mean rate of energy loss gy(stopping power) given by Bethe –Bloch equation:
In this form, the Bethe Bloch equation describes the energy loss of pions in a material such as copper to about 1% accuracy for energies between about 6MeV and 6GeVaccuracy for energies between about 6MeV and 6GeV (momenta between about 40MeV/c and 6GeV/c).
Ionization Loss of Charged Particles
n = electron density NA.Z.ρ/A
Primarily a function of β only, for all practical purposes
n electron density NA.Z.ρ/AI = mean excitation potential of the target = 10Z eV
1 dE/dx is independent of the mass of the charged particle1. dE/dx is independent of the mass of the charged particle2. Varies as 1/v2 at non‐relativistic velocities3. Minimum at E ≅ 3 Mc2 (called Minimum Ionizing)4. Rises logarithmically with γ = E/ Mc2 = (1‐β2)‐1/2 at high βγ5. Dependence on medium is very week. Z/A ≅ 0.5 for most
material, except hydrogen and heavy materialsmaterial, except hydrogen and heavy materials6. Numerically (dE/dx)min ≅ 1 – 1.5 MeV cm2 g‐1
Details of Ionization Energy Loss• Bulk of the energy loss results in formation of Ion Pairs in the medium
• Two stages in this process:1. Incident particle produces primary ionization in atomic
collisions Knocked out electrons have energy distributioncollisions. Knocked out electrons have energy distribution dE’/(E’2)
2. Higher energy electrons (δ rays) can themselves produce fresh ions (secondary ionisation)fresh ions (secondary ionisation).
• Resultant total number of ion pairs is 3‐4 times the number of primary ionisations and is proportionalp y p pto the energy loss of incident particle
• A stochastic process; Hard to predict for single int.l b d d b f l• Fluctuations about mean, dominated by few close primary collisions with large E’: Landau distribution
Radiation Loss by Electronsl lElectrons lose energy in 2 ways1. Ionization energy loss (already discussed)2. Bremsstrahlung – braking radiation
Strong nuclear electric field decelerates electron and the energy change appears as a photon
The energy spectrum is ~ dE’/E’Integrated over the photon spectrum, total loss of an electron
with energy E in traversing dx is gy g(dE/dx)rad = ‐E/X0, where X0 is radiation length
– Contrast this with the (dE/dx)ion, which is constant with energy Average energy lost after traveling distance xAverage energy lost after traveling distance x<E> = E0 exp(‐x / X0), Radiation length may be defined as the thickness of the medium
th t d th b bthat reduces the beam energy by e
For electron: Radiation loss dominates ionization loss at high energies
Energy Loss by γ‐rays3 ways γ rays lose energy in matter:3 ways γ‐rays lose energy in matter:1. Photoelectric absorption (1/E3)2 C i (1/E)2. Compton scattering (1/E)3. Pair production
Pair Production, where a high energy photon converts to a e+ gy pand e‐ in the field of a nucleus is similar to electron bremsstrahlung
I = I0 exp (‐7X/9X0)
Conversion length for γ defined similar t th di ti l th f l t
Photon Energy
to the radiation length for electrons
Detectors
• Detectors are designed to take advantage of the charged particle interaction with matterthe charged particle interaction with matter which releases various forms of energy:
Ionization– Ionization
– Scintillation
C k li ht– Cerenkov light
• For charged particles, they detect– Position
– Arrival time
– Identity
Detectors
• Precise position information helps reconstruct particle trajectory in the medium
• Also helps find the momentum from the deflection in a magnetic field
• Precise timing helps correlate more than one particle from same reaction (e.g. Time of flight)
• Identity of a particle may be established from simultaneous measurement of velocity (by time‐of‐flight or Cerenkov radiation) and momentum and hence rest mass
• Neutral particles detected through interaction of their decay products (e.g. K0→π+π‐ or interaction with matter (π0 → 2γ, γ → e+e‐) leading to secondary charged particles(π → γ, γ → e e ) ead g to seco da y c a ged pa t c es
Detectors
• No single detector can satisfy all these needsNo single detector can satisfy all these needs• Combination of detectors of different types neededneeded
• Typically each detector component works for a very specific purposevery specific purpose – Tracking Chamber (to measure momentum, charge)– Scintillation Chamber (to measure timing, velocity)( g, y)– Calorimeter (to measure energy)– Cerenkov counter (to measure velocity)( y)
QuizA 1GeV γ enters a crystal of thickness 30cm (>5 conversion lengths)
List what would typically happentypically happen
Proportional CountersCounters
• One of the oldest devices for recording Ionization• Gaseous Ionization Detector
– An incoming ionizing particle liberates orbital electrons of the g g pgas atoms, leaving an electron and positively charged atom, commonly known as an ion pair.
– As the charged particle travels through the chamber it leaves a trail of ion pairs along its trajectorytrail of ion pairs along its trajectory.
– The electrons created in this process drift toward a readout electrode (anode) under the influence of an applied electric field. At the same time, the positive ions drift towards thefield. At the same time, the positive ions drift towards the cathode, at much lower speed
• Proportional Counters operate at higher electric field soProportional Counters operate at higher electric field so that the primary ions themselves create secondary ions– Total number of ions is proportional to the energy deposited
Drift Chamber
Drift in low field and amplify in high field near the anode regionDrift time provides precise measurement of the position of interaction
Geiger Counter: Spark Chamber
• If applied voltage is very high (> 5 kV/cm)If applied voltage is very high (> 5 kV/cm), electric breakdown happens. A charged particle propagates through the electrodesparticle propagates through the electrodes and creates an avalanche of ion column linking anode and cathodeanode and cathode
• Geiger Counter to detect radiation level, acts as a counter Clicks every time there is breakas a counter. Clicks every time there is break down
Scintillation Detector: First use 1903!• A charged particle traversing matter leaves behind it a wake of excited g p g
molecules. Certain types of molecules, however, will release a small fraction (~3%) of this energy as optical photons
l d h f h f ll l h• Fluors are used as wave shifters to shift scintillation light to a more convenient wavelength. Nano to micro second time scale (contrast with phosporescence which is ms to secs)
• Use photomultiplier or photodiode to detect these optical photons
Scintillation Counter
• Types of scintillators:I i (N I C I)– Inorganic (NaI, CsI)
• Scintillation mechanism requires crystal band structure
• Water is bad for these crystals!• Water is bad for these crystals!
• Most are impurity activated for providing luminescence
– Organic plastics– Organic plastics
– Organic liquids
Inert liquids (Xenon Argon)– Inert liquids (Xenon, Argon)
Noble Gas and Liquids
• Scintillation mechanism different.Scintillation mechanism different.• Noble gases monoatomic, but excited atoms can produce dimers Ar2*can produce dimers Ar2
• It decays by photon emission with photon energy less than what is needed to exciteenergy less than what is needed to excite monomer.
• No absorption and hence it is transparent toNo absorption and hence it is transparent to its own scintillation light
• High light yield: ~40photons per keV for argonHigh light yield: 40photons per keV for argon
• PMT is the most ubiquitous detector element in most high energy experiments
Principle of a PMT
• High vacuum glass and photocathode material to produce electrons by photo electric effect when photon enters PMTelectric effect when photon enters PMT
• The electron is accelerated from dynode to dynode at successively higher voltages thusdynode at successively higher voltages, thus each pass producing more secondaries
• high gain (108), low noise, high bandwidth,high gain (10 ), low noise, high bandwidth, large collection area
Cerenkov Detectors• If the high energy charged particles velocity exceeds c/n, part of the light emitted by excited atoms appear in form of coherent wave front at a fixedcoherent wave front at a fixed angle with respect to the trajectory (like shockwave)trajectory (like shockwave)
• Wavefront forms the surface of a cone with Cos θ = (ct/n)/(βct)a cone with Cos θ = (ct/n)/(βct) = 1/βn
• Detect this light to measureDetect this light to measure velocity!
A Cerenkov Detector•This two step device converts light•This two step device converts light emitted by a particle in the first boxinto a ring of charge in the second box•Different speeds of particlesidentified by the different sizes of ring
A Brilliant Application: Air Cerenkov Detector
• To detect cosmic gamma rays, one typically flies satellite or balloon abovesatellite or balloon above the earth’s atmosphere to avoid absorption of the
h tgamma ray photons• Air Cerenkov detector coverts the atmosphere incoverts the atmosphere in to an entire detector!
• Detects Cerenkov light from h d f dthe cascade of secondary charged particles on the journey down, which arejourney down, which are highly energetic and travel faster than light in atmos.
Solid State Detectors
• Use semiconductors to detect energy loss
• Used in same manner as scintillation detectors• Used in same manner as scintillation detectors
• Examples are Ge/Si
• As with scintillators, these detectors mainly rely on a photoelectric ionization of the material by the gamma‐ray, but in this case electron/hole pairs are created in the semiconductor material rather than electron/ion pairs as in a scintillator
• Low Threshold and High energy resolution (~500)
ZIP Detectors
Phonon side: 4 quadrants of
(Z-sensitive Ionization and Phonon)
3” (7.6 cm)
qathermal phonon sensorsEnergy & Position (Timing)
1 cm Ge: 250 g Charge side: 2 concentric1 cm gSi: 100 g
Charge side: 2 concentric electrodes (Inner & Outer)Energy (& Veto)
Operated at ~40 mK for good phonon signal-to-noise
Anatomy of an eventQ innerQ outer
-3V Hot charge carriers (3eV/pair)
h+
e-
0V Quasi-diffusive THz phononsBallistic Neganov-Luke phonons
Sensor ASensor BSensor CS D
Ballistic low-frequency phononsSensor D
Phonon Sensors
60
μm
380 μm
6 μ
Al C ll
ΔR
Al
Al Collector W Transition Edge Sensor
(TES)Cooper
Pair
R0 ΔT
T0
Ge or Siphonons
Sensors held in equilibrium between Normal and Super Conducting. Highly sensitive to small energy deposit. Fast signal. SQUID Readout
Electromagnetic Calorimeterh f h h l l b d b l• The energy of a high energy particle can also be measured by total absorption method, known as calorimetry, in which the particle loses all (or most) of its energy in a calorimetric detector
• In the absorption process the incident particle interacts in a large• In the absorption process, the incident particle interacts in a large detector mass, generating secondary particles, that in turn produces tertiary particles and so on
• The detector path length is usually more than a few radiation lengthsp g y g• These devices also essential for measuring energies of neutral hadrons (as
well as charged hadrons)
Cloud Chamber
• Cloud chamber is a well sealed environment with super cooled water or alcohol vapor
• When a charged particle enters this environment, it ionizes itit ionizes it
• The resulting ions act as nucleus for condensation, around which mist will formcondensation, around which mist will form
• Thus a track shows up along the path of the original charged particle
• A magnetic field can also be applied to distinguish positive from negative charged particle
Bubble Chamber
• Same concept as cloud chamber, except have p , psuperheated liquid
• Ionization of the liquid causes bubbles to appear where ions are producedwhere ions are produced
• Can provide very detailed information on complex interactions with many secondarycomplex interactions with many secondary particles, just like cloud chamber
• Making a come back as a Dark Matter detection h l d i f h itechnology, due to its nature of having a
threshold for bubble formation (thus rejecting background below threshold)g )
COUPP Bubble Chamber•Detection of bubble(s) induced by high dE/dx nuclear•Detection of bubble(s) induced by high dE/dx nuclear recoils
•Set threshold to be insensitive to ER
•Easy changing of target mass
•Low cost room temperature
•It is a threshold detector and hence no direct energyIt is a threshold detector and hence no direct energy measurement
~mm Spatial Resolution
A Real Detector EnsembleA Real Detector Ensemble
• A single element is incapable to capturing theA single element is incapable to capturing the entire complex physics phenomena
• Need multiple types of detectors to measure• Need multiple types of detectors to measure various parameters of collision and resulting particle productionparticle production