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PHYS 3313 – Section 001 Lecture #22

PHYS 3313 – Section 001 Lecture #22. Wednesday, Nov. 268, 2012 Dr. Jaehoon Yu. Particle Accelerators Particle Physics Detectors Hot topics in Particle Physics What’s coming in the future?. Announcements. Your presentations are in classes on Dec. 3 and Dec. 5

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PHYS 3313 – Section 001 Lecture #22

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  1. PHYS 3313 – Section 001Lecture #22 Wednesday, Nov. 268, 2012 Dr. Jaehoon Yu • Particle Accelerators • Particle Physics Detectors • Hot topics in Particle Physics • What’s coming in the future? PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  2. Announcements • Your presentations are in classes on Dec. 3 and Dec. 5 • All presentation ppt files must be sent to me by 8pm this Sunday, Dec. 2 • Final exam is 11am – 1:30pm, Monday, Dec. 10 • You can prepare a one 8.5x11.5 sheet (front and back) of handwritten formulae and values of constants for the exam • No formulae or values of constants will be provided! • Planetarium extra credit • Tape one side of your ticket stubs on a sheet of paper with your name on it • Submit the sheet on Wednesday, Dec. 5 • Please be sure to fill out the feedback survey. • Colloquium this Wednesdayat 4pm in SH101 PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  3. Introduction • What are elementary particles? • Particles that make up all matters in the universe • What are the requirements for elementary particles? • Cannot be broken into smaller pieces • Cannot have sizes • The notion of “elementary particles” have changed from early 1900’s through present • In the past, people thought protons, neutrons, pions, kaons,ρ-mesons, etc, as elementary particles • Why? • Due to the increasing energies of accelerators that allowed us to probe smaller distance scales • What is the energy needed to probe 0.1–fm? • From de Broglie Wavelength, we obtain PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  4. Forces and Their Relative Strengths • Classical forces: • Gravitational: every particle is subject to this force, including massless ones • How do you know? • Electromagnetic: only those with electrical charges • What are the ranges of these forces? • Infinite!! • What does this tell you? • Their force carriers are massless!! • What are the force carriers of these forces? • Gravity: graviton (not seen but just a concept) • Electromagnetism: Photons PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  5. Forces and Their Relative Strengths • What other forces? • Strong force • Where did we learn this force? • From nuclear phenomena • The interactions are far stronger and extremely short ranged • Weak force • How did we learn about this force? • From nuclear beta decay • What are their ranges? • Very short • What does this tell you? • Their force carriers are massive! • Not really for strong forces • All four forces can act at the same time!!! PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  6. Interaction Time • The ranges of forces also affect interaction time • Typical time for Strong interaction ~10-24sec • What is this time scale? • A time that takes light to traverse the size of a proton (~1 fm) • Typical time for EM force ~10-20 – 10-16 sec • Typical time for Weak force ~10-13 – 10-6 sec • In GeV ranges, the four forces (now three since EM and Weak forces are unified!) are different • These are used to classify elementary particles PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  7. Elementary Particles • Before the quark concepts, all known elementary particles were grouped in four depending on the nature of their interactions PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  8. Elementary Particle Interactions • How do these particles interact?? • All particles, including photons and neutrinos, participate in gravitational interactions • Photons can interact electromagnetically with any particles with electric charge • All charged leptons participate in both EM and weak interactions • Neutral leptons do not have EM couplings • All hadrons (Mesons and baryons) respond to the strong force and appears to participate in all the interactions PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  9. Elementary Particles: Bosons and Fermions • All particles can be classified as bosons or fermions • Bosons follow Bose-Einstein statistics • Quantum mechanical wave function is symmetric under exchange of any pair of bosons • xi: space-time coordinates and internal quantum numbers of particle i • Fermions obey Fermi-Dirac statistics • Quantum mechanical wave function is anti-symmetric under exchange of any pair of Fermions • Pauli exclusion principle is built into the wave function • For xi=xj, PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  10. Bosons, Fermions, Particles and Antiparticles • Bosons • All have integer spin angular momentum • All mesons (consists of two quarks) are bosons • Fermions • All have half integer spin angular momentum • All leptons and baryons (consist of three quarks) are fermions • All particles have anti-particles • What are anti-particles? • Particles that has same mass as particles but with opposite quantum numbers • What is the anti-particle of • A π0? • A neutron? • A K0? • A Neutrino? PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  11. Quantum Numbers • When can an interaction occur? • If it is kinematically allowed • If it does not violate any recognized conservation laws • Eg. A reaction that violates charge conservation will not occur • In order to deduce conservation laws, a full theoretical understanding of forces are necessary • Since we do not have full theory for all the forces • Many of general conservation rules for particles are based on experiments • One of the clearest conservation is the lepton number conservation • While photon and meson numbers are not conserved PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  12. Baryon Numbers • Can the decay occur? • Kinematically?? • Yes, proton mass is a lot larger than the sum of the two masses • Electrical charge? • Yes, it is conserved • But this decay does not occur (<10-40/sec) • Why? • Must be a conservation law that prohibits this decay • What could it be? • An additive and conserved quantum number, Baryon number (B) • All baryons have B=1 • Anti-baryons? (B=-1) • Photons, leptons and mesons have B=0 • Since proton is the lightest baryon, it does not decay. PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  13. The Standard Model of Particle Physics • Prior to 70’s, low mass hadrons (mesons and baryons) are thought to be the fundamental constituents of matter, despite some new particles that seemed to have new flavors • Even lightest hadrons, protons and neutrons, show some indication of substructure • Such as magnetic moment of the neutron • Raised questions whether they really are fundamental particles • In 1964 Gell-Mann and Zweig suggested independently that hadrons can be understood as composite of quark constituents • Recall that the quantum number assignments, such as strangeness, were only theoretical tools rather than real particle properties PHYS 3446, Fall 2006 Jae Yu

  14. The Standard Model of Particle Physics • In late 60’s, Jerome Friedman, Henry Kendall and Rich Taylor designed an experiment with electron beam scattering off of hadrons and deuterium at SLAC (Stanford Linear Accelerator Center) • Data could be easily understood if protons and neutrons are composed of point-like objects with charges -1/3e and +2/3e. • A point-like electrons scattering off of point-like quark partons inside the nucleons and hadrons • Corresponds to modern day Rutherford scattering • Higher energies of the incident electrons could break apart the target particles, revealing the internal structure PHYS 3446, Fall 2006 Jae Yu

  15. The Standard Model of Particle Physics • Elastic scatterings at high energies can be described well with the elastic form factors measured at low energies, why? • Since the interaction is elastic, particles behave as if they are point-like objects without a substructure • Inelastic scatterings cannot be described well w/ elastic form factors since the target is broken apart • Inelastic scatterings of electrons with large momentum transfer (q2) provides opportunities to probe shorter distances, breaking apart nucleons • The fact that the form factor for inelastic scattering at large q2 is independent of q2 shows that there are point-like object in a nucleon • Bjorken scaling • Nucleons contain both quarks and glue particles (gluons) both described by individual characteristic momentum distributions (Parton Distribution Functions) PHYS 3446, Fall 2006 Jae Yu

  16. The Standard Model of Particle Physics • By early 70’s, it was clear that hadrons (baryons and mesons) are not fundamental point-like objects • But leptons did not show any evidence of internal structure • Even at high energies they still do not show any structure • Can be regarded as elementary particles • The phenomenological understanding along with observation from electron scattering (Deep Inelastic Scattering, DIS) and the quark model • Resulted in the Standard Model that can describe three of the four known forces along with quarks, leptons and gauge bosons as the fundamental particles PHYS 3446, Fall 2006 Jae Yu

  17. Quarks and Leptons Q 0 -1 • In SM, there are three families of leptons •  Increasing order of lepton masses • Convention used in strong isospin symmetry, higher member of multiplet carries higher electrical charge • And three families of quark constituents • All these fundamental particles are fermions w/ spin Q +2/3 -1/3 PHYS 3446, Fall 2006 Jae Yu

  18. PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  19. The Standard Model Discovered in 1995, ~175mp Make up most ordinary matters ~0.1mp • Total of 16 particles make up the matter in the universe! Simple and elegant!!! • Tested to a precision of 1 part per million! PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  20. Quark Content of Mesons • Meson spins are measured to be integer. • They must consist of an even number of quarks • They can be described as bound states of quarks • Quark compositions of some mesons • Pions Strange mesons PHYS 3446, Fall 2006 Jae Yu

  21. Quark Content of Baryons • Baryon spins are measured to be ½ integer. • They must consist of an odd number of quarks • They can be described as bound states of three quarks based on the studies of their properties • Quark compositions of some baryons • Nucleons Strange baryons Other Baryons • s=1 s=2 • Since baryons have B=1, the quarks must have baryon number 1/3 PHYS 3446, Fall 2006 Jae Yu

  22. Z and W Boson Decays • The weak vector bosons (discovered in early 1980’s) couple quarks and leptons • Thus they decay to a pair of leptons or a pair of quarks • Since they are heavy, they decay instantly to the following channels and their branching ratios • Z bosons: MZ=91GeV/c2 • W bosons: MW=80GeV/c2 PHYS 3446, Fall 2006 Jae Yu

  23. Z and W Boson Search Strategy • The weak vector bosons have masses of 91 GeV/c2 for Z and 80 GeV/c2 for W • While the most abundant decay final state is qqbar (2 jets of particles), the multi-jet final states are also the most abundant in collisions • Background is too large to be able to carry out a meaningful search • The best channels are using leptonic decay channels of the bosons • Especially the final states containing electrons and muons are the cleanest • So what do we look for as signature of the bosons? • For Z-bosons: Two isolated electrons or muons with large transverse momenta (PT) • For W bosons: One isolated electron or muon with a large transverse momentum along with a signature of high PT neutrino (Large missing ET). PHYS 3446, Fall 2006 Jae Yu

  24. What do we need for the experiment to search for vector bosons? • We need to be able to identify isolated leptons • Good electron and muon identification • Charged particle tracking • We need to be able to measure transverse momentum well • Good momentum and energy measurement • We need to be able to measure missing transverse energy well • Good coverage of the energy measurement (hermeticity) to measure transverse momentum imbalance well PHYS 3446, Fall 2006 Jae Yu

  25. Particle Accelerators • How can one obtain high energy particles? • Cosmic ray  Sometimes we observe 1000TeV cosmic rays • Low flux and cannot control energies too well • Need to look into small distances to probe the fundamental constituents with full control of particle energies and fluxes • Particle accelerators • Accelerators need not only to accelerate particles but also to • Track them • Maneuver them • Constrain their motions to the order of 1μm or better • Why? • Must correct particle paths and momenta to increase fluxes and control momenta PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  26. Particle Accelerators • Depending on what the main goals of physics are, one needs different kinds of accelerator experiments • Fixed target experiments: Probe the nature of the nucleons  Structure functions • Results also can be used for producing secondary particles for further accelerations  Tevatron anti-proton production • Colliders: Probes the interactions between fundamental constituents • Hadron colliders: Wide kinematic ranges and high discovery potential • Proton-anti-proton: TeVatron at Fermilab, at CERN • Proton-Proton: Large Hadron Collider at CERN (turned on early 2010) • Lepton colliders: Very narrow kinematic reach, so it is used for precision measurements • Electron-positron: LEP at CERN, Petra at DESY, PEP at SLAC, Tristan at KEK, ILC in the med-range future • Muon-anti-muon: Conceptual accelerator in the far future • Lepton-hadron colliders: HERA at DESY PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  27. Electrostatic Accelerators: Cockcroft-Walton • Cockcroft-Walton Accelerator • Pass ions through sets of aligned DC electrodes at successively increasing fixed potentials • Consists of ion source (hydrogen gas) and a target with the electrodes arranged in between • Acceleration Procedure • Electrons are either added or striped off of an atom • Ions of charge q then get accelerated through series of electrodes, gaining kinetic energy of T=qV through every set of electrodes • Limited to about 1MeV acceleration due to voltage breakdown and discharge beyond voltage of 1MV. • Available commercially and also used as the first step high current injector (to ~1mA). PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  28. Electrostatic Accelerators: Van de Graaff • Energies of particles through DC accelerators are proportional to the applied voltage • Robert Van de Graaff developed a clever mechanism to increase HV • The charge on any conductor resides on its outermost surface • If a conductor carrying additional charge touches another conductor that surrounds it, all of its charges will transfer to the outer conductor increasing the charge on the outer conductor, thereby increasing voltage higher PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  29. Electrostatic Accelerators: Van de Graaff • Sprayer adds positive charge to the conveyor belt at corona points • Charge is carried on an insulating conveyor belt • The charges get transferred to the dome via the collector • The ions in the source then gets accelerated to about 12MeV • Tandem Van de Graff can accelerate particles up to 25 MeV • This acceleration normally occurs in high pressure gas that has very high breakdown voltage PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  30. Resonance Accelerators: Cyclotron • Fixed voltage machines have intrinsic limitations in their energy due to breakdown • Machines using resonance principles can accelerate particles to even higher energies • Cyclotron developed by E. Lawrence is the simplest and first of these • The accelerator consists of • Two hallow D shaped metal chambers connected to alternating HV source • The entire system is placed under strong magnetic field PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  31. Resonance Accelerators: Cyclotron • While the D’s are connected to HV sources, there is no electric field inside the chamber due to Faraday effect • Strong electric field exists only in the gap between the D’s • An ion source is placed in the gap • The path is circular due to the perpendicular magnetic field • Ion does not feel any acceleration inside a D but gets bent due to magnetic field • When the particle exits a D, the direction of voltage can be changed and the ion gets accelerated before entering into the D on the other side • If the frequency of the alternating voltage is just right, the charged particle gets accelerated continuously until it is extracted PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  32. Resonance Accelerators: Cyclotron • For non-relativistic motion, the frequency appropriate for alternating voltage can be calculated from the fact that the magnetic force provides centripetal acceleration for a circular orbit • In a constant angular speed, ω=v/r. The frequency of the motion is • Thus, to continue accelerating the particle, the electric field should alternate in this frequency, cyclotron resonance frequency • The maximum kinetic energy achievable for an cyclotron with radius R is PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  33. Resonance Accelerators: Linear Accelerator • Accelerates particles along a linear path using resonance principle • A series of metal tubes are located in a vacuum vessel and connected successively to alternating terminals of radio frequency oscillator • The directions of the electric fields changes before the particles exits the given tube • The tube length needs to get longer as the particle gets accelerated to keep up with the phase • These accelerators are used for accelerating light particles to very high energies PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  34. Synchroton Accelerators • For very energetic particles, the relativistic effect must be taken into account • For relativistic energies, the equation of motion of a charge q under magnetic field B is • For v ~ c, the resonance frequency becomes • Thus for high energies, either B or ν should increase • Machines with constant B but variable ν are called synchro-cyclotrons • Machines with variable B independent of the change of νis called synchrotrons PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  35. Synchroton Accelerators • Electron synchrotrons, B varies while νis held constant • Proton synchrotrons, both B and νvaries • For v ~ c, the frequency of motion can be expressed • For an electron • For magnetic field strength of 2Tesla, one needs radius of 50m to accelerate an electron to 30GeV/c. PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  36. Synchroton Accelerators • Synchrotons use magnets arranged in a ring-like fashion. • Multiple stages of accelerations are needed before reaching over GeV ranges of energies • RF power stations are located through the ring to pump electric energies into the particles PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  37. Comparisons between Tevatron and LHC • Tevatron: A proton-anti proton collider at 2TeV • Need to produce anti-protons using accelerated protons at 150GeV • Takes time to store sufficient number of anti-protons • Need a storage accelerator for anti-protons • Can use the same magnet and acceleration ring to circulate and accelerator particles • LHC: A proton-proton collier at 14TeV design energy • Protons are easy to harvest • Takes virtually no time to between a fresh fill of particles into the accelerator • Must use two separate magnet and acceleration rings PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  38. Chicago CDF p DØ Tevatron p • World’s Highest Energy p-p collider • 27km circumference, 100m underground • Design Ecm=14 TeV (=44x10-7J/p 362M Joules on the area smaller than 10-4m2) • Equivalent to the kinetic energy of a B727 (80tons) at the speed 193mi/hr 312km/hr • ~3M times the energy density at the ground 0 of the Hiroshima atom bomb • First 7TeV collisions on 3/30/10  The highest energy humans ever achieved!! • First 8TeV collisions in 2012 on April 5, 2012 FermilabTevatron and LHC at CERN • World’s Highest Energy proton-anti-proton collider • 4km circumference • Ecm=1.96 TeV (=6.3x10-7J/p 13M Joules on the area smaller than 10-4m2) • Equivalent to the kinetic energy of a 20t truck at the speed 81mi/hr 130km/hr • ~100,000 times the energy density at the ground 0 of the Hiroshima atom bomb • Was shut down at 2pm CDT, Sept. 30, 2011 • Vibrant other programs running!! PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  39. LHC @ CERN Aerial View CMS France Geneva Airport Swizerland ATLAS PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  40. Particle Detectors • Subatomic particles cannot be seen by naked eyes but can be detected through their interactions within matter • What do you think we need to know first to construct a detector? • What kind of particles do we want to detect? • Charged particles and neutral particles • What do we want to measure? • Their momenta • Trajectories • Energies • Origin of interaction (interaction vertex) • Etc • To what precision do we want to measure? • Depending on the answers to the above questions we use different detection techniques PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  41. Calorimeter (dense) Muon Tracks Charged Particle Tracks Energy Scintillating Fiber Silicon Tracking Interaction Point B EM hadronic Magnet Wire Chambers Particle Detection Techniques electron photon jet muon We know x,y starting momenta is zero, but along the z axis it is not, so many of our measurements are in the xy plane, or transverse neutrino -- or any non-interacting particle missing transverse momentum PHYS 3446, Fall 2006 Jae Yu

  42. The ATLAS and CMS Detectors • Fully multi-purpose detector with emphasis on lepton ID & precision E & P • Weighs 7000 tons and 10 story tall • Records 200 – 400 collisions/second • Records approximately 350 MB/second • Record over 2 PB per year 200*Printed material of the US Lib. of Congress PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  43. Scintillation Counters • Two types of scintillators • Organic or plastic • Tend to emit ultra-violate • Wavelength shifters are needed to reduce attenuation • Faster decay time (10-8s) • More appropriate for high flux environment • Inorganic or crystalline (NaI or CsI) • Doped with activators that can be excited by electron-hole pairs produced by charged particles in the crystal lattice • These dopants can then be de-excited through photon emission • Decay time of order 10-6sec • Used in low energy detection PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  44. Scintillation Detectors & Photo-multiplier Tube • Scintillation detectors consist of a doped plastic that emits lights when a particle loses its energy via atomic excitation and transition back to lower energy states • The light produced by scintillators are usually too weak to see • Photon signal needs amplification through photomultiplier tubes • Gets the light from scintillator directly or through light guide • Photocathode: Made of material in which valence electrons are loosely bound and are easy to cause photo-electric effect (2 – 12 cm diameter) • Series of multiple dynodes that are made of material with relatively low work-function • Operating at an increasing potential difference (100 – 200 V) difference between dynodes PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  45. Scintillation Detector Structure HV PS Scintillation Counter Light Guide/ Wavelength Shifter PMT Readout Electronics Scope PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  46. Some PMT’s Super-Kamiokande detector PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  47. Time of Flight • Scintillator + PMT can provide time resolution of 0.1 ns. • What position resolution does this corresponds to? • 3cm • Array of scintillation counters can be used to measure the time of flight (TOF) of particles and obtain their velocities • What can this be used for? • Can use this to distinguish particles with about the same momentum but with different mass • How? • Measure • the momentum (p) of a particle in the magnetic field • its time of flight (t) for reaching some scintillation counter at a distance L from the point of origin of the particle • Determine the velocity of the particle and its mass PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  48. Cerenkov Detectors • What is the Cerenkov radiation (covered in CH2)? • Emission of coherent radiation from the excitation of atoms and molecules • When does this occur? • If a charged particle enters a dielectric medium with a speed faster than light in the medium • How is this possible? • Since the speed of light is c/n in a medium with index of refraction n, if the particle’s β>1/n, its speed is larger than the speed of light • Cerenkov light has various frequencies but blue and ultraviolet band are most interesting • Blue can be directly detected w/ standard PMTs • Ultraviolet can be converted to electrons using photosensitive molecules mixed in with some gas in an ionization chamber PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  49. Cerenkov Detectors • Threshold counters • Particles with the same momentum but with different mass will start emitting Cerenkov light when the index of refraction is above a certain threshold • These counters have one type of gas but could vary the pressure in the chamber to change the index of refraction to distinguish particles • Large proton decay experiments use Cerenkov detector to detect the final state particles, such as p  e+π0 • Differential counters • Measure the angle of emission for the given index of refraction since the emission angle for lighter particles will be larger than heavier ones • Ring-imaging Cerenkov Counters (RICH) • An energetic charged particle can produce multiple UV distributed about the direction of the particle • The now stopped BaBar experiment at Stanford Linear Accelerator Center (SLAC) used RICH as the primary detector system PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

  50. Super Kamiokande A Differential Water Cerenkov Detector • Kamioka zinc mine, Japan • 1000m underground • 40 m (d) x 40m(h) SS • 50,000 tons of ultra pure H2O • 11200(inner)+1800(outer) 50cm PMT’s • Originally for proton decay experiment • Accident in Nov. 2001, destroyed 7000 PMT’s • Dec. 2002 resumed data taking • This experiment was the first to show the neutrinos oscillate PHYS 3313-001, Fall 2012 Dr. Jaehoon Yu

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