2nd-Meeting Ionization energy loss Multiple Coulomb scattering (plural and single scattering, too) Tracking chambers
#2 -Particle Physics Experiments at High Energy Colliders John Hauptman, Kyungpook National University, Fall 2016 Today: particles traveling through atoms: electrons, e - : ionization energy loss scintillation light Cerenkov light nuclei, Ze - : multiple Coulomb scattering single/plural scattering bremsstrahlung pair production radiation by muons tracking systems: sagitta measurement impact parameter measurement tracking history Particle Physics Experiments at High Energy Colliders (v. 2.0) Instructor ka-talk: John Hauptman, email: hauptman@iastate.edu Text Particle Physics Experiments at High Energy Colliders, Wiley-VCH, 2011. Class time: One 150-minute class per week, on a day and time best suited to students Class # Date Day Chapter Topics 1 7Sept W 1-2 Introduction to particles and particle physics 2 12 Sep M 3.1-3.2 Particle detectors - tracking chambers 3 19 Sep M How to test detectors in beams 4 21 Sep W 3.3-3.6 particles detectors - calorimeters 5 28 Sep W How to test detectors in beams - 5Oct W no class - CERN test beam - 12 Oct W no class - CERN beam test - 19 Oct W no class - CERN beam test 6 26 Oct W 4 Particle identification e, µ, 0 7 2 Nov W 5 Particle beams, accelerators, and colliders 4 Nov proton ramp for test beams 8 9 Nov W 6 General principles: magnetic field configuration and tracking systems 9 16 Nov W 6 General principles: calorimeter and muon systems 10 23 Nov W App. B Detector design strategies: Steinberger, creativity; atlas, cms; test beams 11 30 Nov W 7 4th Concept detector 2Dec 12 7Dec W 8 Sociology and psychology in experimental physics 13 14 Dec W Reports and reviews of assignment
Calorimeters & particle interactions: Compact Muon Solenoid (CMS) experiment ( standard detector geometry) EM hadronic hadronic scales as interaction length λint ~ 20cm EM scales in depth as radiation length X0 ~ 1cm
The CMS figure shows simulated events. On the left is an EM (e-) event recorded in a drift chamber, and the bottom left is an event in a liquid Argon TPC. Below is a simulation of a pion interaction.
CMS (Compact Muon Solenoid) is a conventional collider experiment Vertex chamber (silicon) Main tracking chamber (silicon strips) EM (electromagnetic calorimeter) PWO crystals HadCal (hadronic calorimeter) brass-scintillator SC solenoid (~4T) Iron-based hadron-filter muon system It is like an onion with layer after layer of detectors. There are additional pieces, e.g., more calorimeter after the hadronic calorimeter. It takes 20 years to design and build, at a cost of $1 billion. It will run at the LHC for 20-30 years; easily 1/2 the lifetime of a physicist. Students and postdocs may spend their entire early lives on one experiment, maybe switch to one of the other experiments (ATLAS, ALICE, LHCb) for a permanent position. This is both good and bad, and very personality dependent.
How and why did CMS turn out the way it did? That might be an unanswerable question. CMS follows, basically, the design of the Magnetic Detector or Mark I at SPEAR/SLAC in the early 1970s. However, ATLAS is much different.
ATLAS end view during assembly.
We will not try to understand how or why these specific detectors were designed and built the the way they were, or to contrast different detectors. This was an original goal of mine, but I quickly gave it up as nearly impossible. Any look at a TDR ( Technical Design Report ) for an LHC (or an SSC) experiment will convince you that this is too large a task. Instead, we will develop the simplest possible understandings of particles and their measurements, including particle identification methods, then consider general ideas for formulating the main parts of any big detector: tracking system for measuring momenta, a calorimeter system for measuring energies, and a magnetic field configuration.
Particle detectors are made of atoms: There are many excellent texts of particle detectors and particle interactions with matter. The point here will be simplicity and clarity of thought. There are interactions with the electrons and with the nucleus. Particle interactions with atomic electrons (e - ): Ionization energy loss de/dx e - b atomic electron, ev high energy >> ev passing charged particle +e 2b v
+ - + - F = 1 e 2 4 0 b 2 p = F t = 1 e 2 4 0 b 2 E = ( p)2 2m e t = 2b v 2b v = 2~c bv ( = e2 4 0 ~c 1 137 ) = ( 2~c)2 2m e c 2 1 b 2 1 2 / 1/ 2 ( = v/c) Integrated over cylinders of electrons of depth dx in material of density (g/cm 3 ), the minimum ionization rate is de/dx 1.6 MeV/g cm 2 (g/cm 3 ) E de/dx 1.6 MeV/cminwater + - E smaller + - + - + - E + - + + v + v - + - + E partially screened
Altogether, exact, in an infinite medium, the ionization energy loss (in MeV/g-cm -2 ): Lorentz contraction of E field ionization potential maximum energy transfer to electron polarization of the medium where K =4 N A r 2 em e c 2 =0.307075 MeV / g cm 2 This elegant formula ( Bethe-Bloch ) is seldom useful: finite media, B-field, δ-rays, UV, low-energy particles, Energy loss to nuclei is negligible ~ (m/m)
Nygren TPC, ~1980
Particle interactions with atomic electrons (e - ): Scintillation light emission Some atomic and molecular media have energy level structures that can be excited, filling upper levels, which quickly de-excite, yielding scintillation light. Since is a quantum process, there is a exponential lifetime to these excited states. For fast scintillators, this time is typically 1-2 ns. Inorganic scintillators, PWO, BGO, BSO, NaI, etc., have typically longer quantum lifetimes and, often, have multiple exponential lifetimes. Organic scintillators can be plastic or liquid.
At room temperature, molecules are at 0.025 ev, and in the S00 state. An ionizing particle transfers KE to the molecules and electrons are excited up into the S1, S2, and S3 but below ionization. The higher states S1, S2, de-excite quickly (picoseconds) down to the S1 states. The S10, S11, S12, S13 states are vibrational and decay down to S10. The scintillation light, or prompt fluorescence, is from the S10 to S0 transition. I(t) =I 0 e t/ =lifetimeofs 10 state 1-2 ns.
Since this excitation is driven by the same process as ionization, the scintillation light generated is proportional to the ionization energy loss, except for recombination of the ionization electrons with the dense column of ions in heavily ionized media, described by Birks law: dl/dx = L 0 de/dx (1+k B de/dx) k B 0.01 g MeV/cm 2 There are many important factors involving scintillators: light yield (photons/mev) emission spectrum (red, yellow, blue) temperature dependence of light yield sensitivity to moisture cost
Detector this scintillation light (in the visible, mostly) with a photo-multiplier tube, PMT, or a Silicon Photo Multiplier, SiPM or photo-diode, photo-triode, etc.
Einstein s photoelectric effect at work
Particle interactions with atomic electrons (e - ): Cerenkov light emission Cerenkov light generation is a collective effect of the cylindrical polarization of the atomic medium, and when the polarization relaxes, a photon is emitted. The light yield is ~1% of typical scintillation yields.
Cerenkov light emission: on a cone of angle cos C = 1 n n =indexofrefractionofopticalmedium, =velocityofparticle Light intensity per ev band and per cm of particle path: d 2 N dedx = ~c sin2 C 370 sin 2 C (photons / ev cm) Spectral (wavelength) distribution is dn C = 1 (since dn C d 2 d =constant)
Particle interactions with nuclei (Ze): multiple Coulomb scattering (+singular and plural) +Ze +e θ1 θ2 +Ze θ3 θ4 +Ze Ze+ Τhe scattering θi angles are just from ordinary Coulomb scattering of a charge +e from a nucleus of charge Ze. Central Limit Theorem of statistics: if you add together, or take an average of, many numbers sampled from finite (i.e., non-infinite) distributions, the distribution of the sum will be Gaussian.
dn/d / e 1 2 ( / rms) 2 rms = 0.0136GeV p c q ` X 0 [1 + 0.038 ln(`/x 0 )] p is momentum, is velocity, ` is pathlegth of particle in material X 0 is radiation length Very basic: rms 14MeV/c p p`/x0
plane rms = p 1 3 0 y rms plane = 1 p 3 x 0 s rms plane = 1 4 p 3 x 0
Radiation length was originally defined as the material needed to reduce the energy of an electron by 1/e. It is also 7/9 of the material needed to convert a photon to e+e-, i.e., the photon mean free path. Thirdly, is defines the spatial scale, the doubling distance, for showers in electromagnetic calorimeters. 1 Xo The Review of Particle Properties displays a calculation of Xo for all materials. Element X 0 (g/cm 2 ) X 0 (cm) He 94.3 568,000 C 42.7 19.3 Al 24.0 8.90 Si 21.8 9.36 Fe 13.8 1.76 Cu 12.9 1.44 W 6.76 0.35 Pb 6.37 0.56 U 6.00 0.32 e - E0 e - (Electron energy lost to bremsstrahlung radiation inside material) Radiation lengths of some common media. E=E0/e
Particle interactions with nuclei (Ze): bremsstrahlung ( braking radiation ) An electron radiates photons continuously, mostly low energy. On the average, the summed energy radiated in one radiation length is all but 1/e of the original electron energy.
Particle interactions with nuclei (Ze): pair production The photon penetrates 9/7 of a radiation length, on the average, before undergoing pair-production.
Particle interactions with nuclei (Ze): bremsstrahlung by muons