University of Ljubljana Eksperimentalna fizika jedra in osnovnih delcev (EFJOD) osnovnih delcev (EFJOD) - Uvod Uvod E Experimental particle and nuclear i t l ti l d l physics – Introduction Peter Križan EFJOD - uvod Peter Križan, Ljubljana
Contents Introduction Experimental methods Experimental methods Accelerators Spectrometers Particle detectors Particle detectors Analysis of data Peter Križan, Ljubljana
Particle physics experiments Accelerate elementary particles, let them collide � energy released in the collision is converted into l d i th lli i i t d i t mass of new particles, some of which are unstable T Two ways how to do it: h t d it Fixed target experiments Collider experiments Peter Križan, Ljubljana
How to accelerate charged particles? • Acceleration with electromagnetic waves (typical frequency is 500 MHz – mobile phones run at 900 frequency is 500 MHz – mobile phones run at 900, 1800, 1900 MHz) • Waves in a radiofrequency cavity: c<c 0 Waves in a radiofrequency cavity: c c 0 elektron ... Similar to surfing the waves Si il fi h Peter Križan, Ljubljana
Electric field positron it Peter Križan, Ljubljana
Stability of acceleration •For a synchronous particles (A): energy loss = energy received from the RF field i d f th RF fi ld •A particle that comes too late (B), gets more energy, the one that is too fast (C), gets less � •OK if particle •OK if particle ~ in phase � stable orbit •Not OK if too far away y Peter Križan, Ljubljana
Synchrotron Acceleration RF cavity Acceleration RF cavity Dipole magnets for beam deflection for beam deflection “Injection kicker” “abort kicker” quadrupole magnets quadrupole magnets For beam focusing LINAC Peter Križan, Ljubljana
Electron position collider: KEK-B Peter Križan, Ljubljana
Large hadron collider CERN CERN LHC Peter Križan, Ljubljana
Interaction region: Belle Collisions at a finite angle +-11mrad Course at University of Tokyo Peter Križan, Ljubljana
Accelerator figure of merit 1: Center of mass energy Center-of-mass energy If there is enough energy available in the collission, il bl i h lli i new, heavier particles can be produced be produced. E CMS > mc 2 e g LHC CERN Tevatron: e.g. LHC, CERN, Tevatron: search for Higgs bosons, Livingston plot m Higgs > 100GeV Higgs Peter Križan, Ljubljana
Accelerator figure of merit 2: Luminosity Luminosity Observed rate of events = Cross section x Luminosity Obse ed ate o e e ts C oss sect o u os ty dN = σ L dt dt Accelerator figures of merit: luminosity L Accelerator figures of merit: luminosity L and integrated luminosity and integrated luminosity ∫ = ( ) L L t dt int Peter Križan, Ljubljana
Luminosity vs time A high luminosity is needed for studies of rare processes. Peter Križan, Ljubljana
How to understand what happened in a collision? •Measure the coordinate of the point (‘vertex’) where the reaction occured, and determine the positions and directions of particles that have been produced of particles that have been produced •Measure momenta of stable charged particles by measuring their radius of curvature in a strong magnetic field (~1T) th i di f t i t ti fi ld ( 1T) •Determine the identity of stable charged particles (e, μ , π , K, p) •Measure the energy of high energy photons γ γ gy g gy p Peter Križan, Ljubljana
How to understand what happened in a collision? Illustration on an Illustration on an example: B � K S J/ ψ S J/ ψ B 0 � K 0 S � π - π + K 0 J/ ψ � μ - μ + J/ ψ � μ μ Peter Križan, Ljubljana
Search for particles which decayed close to the production point close to the production point How do we reconstructing final states which decayed to several stable particles (e.g., 1,2,3)? l t bl ti l ( 1 2 3)? From the measured tracks calculate the invariant mass of the system (i= 1 2 3): of the system (i= 1,2,3): r ∑ ∑ ∑ ∑ = − 2 2 2 2 ( ) ( ) Mc E p c i i The candidates for the X � 123 decay show up as a peak in the distribution on (mostly combinatorial) k i th di t ib ti ( tl bi t i l) background. The name of the game: have as little background under The name of the game: have as little background under the peak as possible without loosing the events in the peak (=reduce background and have a small peak width). Peter Križan, Ljubljana
How do we know it was precisely this reaction? precisely this reaction? π − π + S J/ ψ ψ B 0 � K 0 S π − π + K 0 S � detect J/ ψ � μ − μ + For π − π + in μ − μ + pairs we calculate the invariant mass: the invariant mass: M 2 c 4 =(E 1 + E 2 ) 2 - (p 1 + p 2 ) 2 μ − μ + + Mc 2 must be for K 0 S close to 0.5 GeV, for J/ ψ close to 3 1 GeV for J/ ψ close to 3.1 GeV. e - e + e e 2.5 GeV 3.0 3.5 2.5 GeV 3.0 3.5 Rest in the histrogram: random Rest in the histrogram: random coincidences (‘combinatorial 10. oktober 2006 background’) EFJOD - uvod Peter Križan, Ljubljana
Experimental aparatus Detector form: symmetric for colliders with symmetric energy beams; extended Detector form: symmetric for colliders with symmetric energy beams; extended in the boost direction for an asymmetric collider; very forward oriented in fixed target experiments. cms lab p* βγ p* BELLE BELLE CLEO Peter Križan, Ljubljana
Example of a fixed target experiment: HERA-B Peter Križan, Ljubljana
Belle spectrometer at KEK B at KEK-B μ and K L detection system Aerogel Cherenkov Counter 3.5 GeV e + Silicon Vertex Detector Silicon Vertex Detector Electromagnetic. Cal. g 8 GeV e - (CsI crystals) Central Drift Chamber 1.5T SC solenoid ToF counter Peter Križan, Ljubljana
ATLAS at LHC A physicist... Peter Križan, Ljubljana
Components of an experimental apparatus (‘spectrometer’) apparatus ( spectrometer ) • Tracking and vertexing systems Tracking and vertexing systems • Particle identification devices • Calorimeters (measurement of energy) C l i t ( t f ) Peter Križan, Ljubljana
Components of an experimental apparatus (‘spectrometer’) apparatus ( spectrometer ) • Tracking and vertexing systems Tracking and vertexing systems • Particle identification devices • Calorimeters (measurement of energy) C l i t ( t f ) Peter Križan, Ljubljana
Silicon vertex detector (SVD) pitch 20 cm 20 cm 50 cm Two coordinates measured at the same time time Typical strip pitch ~50 μ m, resolution about ~15 μ m June 5-8, 2006 Course at University of Tokyo Peter Križan, Ljubljana
Interaction of charged particles with matter g p Energy loss due to ionisation: depends on βγ, typically about 2 MeV/cm ρ /(g cm -3 ). b t 2 M V/ /( 3 ) Minimum ionizing Liquids, solids: few MeV/cm particles (MIP) Gases: few keV/cm Gases: few keV/cm Primary ionisation: charged particle y g p kicks electrons from atoms. In addition: excitation of atoms (no free electron), on average need W i (>ionisation energy) to create e ion pair e-ion pair. W i typically 30eV � per cm of gas about 2000eV/30eV=60 e ion about 2000eV/30eV=60 e-ion pairs Peter Križan, Ljubljana
Ionisation n prim is typically 20-50 /cm (average value, Poisson like distribution – used in measurements of n prim ) The primary electron ionizes further: secondary e-ion pairs, typically about 2-3x more. t i ll b t 2 3 Finally: 60-120 electrons /cm Can this be detected? 120 e-ion pairs make a pulse of Can this be detected? 120 e ion pairs make a pulse of V=ne/C=2mV (at typical C=10pF) � NO -> Need multiplication -> Need multiplication Peter Križan, Ljubljana
Multiplication in gas p g Simplest example: cylindrical counter, radial field electrons drift to the anode in the center field, electrons drift to the anode in the center E = E(r) α 1/r If the energy eEd gained over several mean free paths (d around 10mm) exceeds the ionisation energy � new electron around 10mm) exceeds the ionisation energy � new electron Electric field needed � E = I/ed = 10V/mm = 10kV/cm Peter Križan, Ljubljana
Multiplication in gas Electron travels (drifts) towards the anode (wire); close to the wire the electric field becomes high enough (several kV/cm), the the electric field becomes high enough (several kV/cm), the electron gains sufficient energy between two subsequent collisions with the gas molecules to ionize -> start of an avalanche. Peter Križan, Ljubljana
Signal development 3 Time evolution of the signal Q Q t t = − + ( ) ln( 1 ) u t πε 4 l t 0 0 with no RC filtering ( τ = inf.) and with time constants 10 μ s and 100 μ s. If faster signals are If faster signals are needed � smaller time constants � smaller signals smaller signals e.g. τ =40ns: max u(t) is about ¼ of ( ) the τ = inf. case Peter Križan, Ljubljana
Multiwire proportional chamber (MWPC) Typical parameters: Typical parameters: L=5mm, d=1-2mm, wire radius =20 mm P. Križan, Ionisation counters Peter Križan, Ljubljana
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