Sarah Eno1 Particles and Matter Sarah Eno MD Quarknet 9 July 2003 Selection and Comments Jim Linnemann MSU

Sarah Eno2 Detectors Goal: produce some sort of detectable signal that depends on the things we want to measure (energy, position, particle type) current in a wire charge on a capacitor light (detectable with photomultiplier, for example) The physics processes we care about will be the ones that lead to this kind of signal. References: The Physics of Particle Detectors, Dan Green,Cambridge University Press (2000) Techniques for Nuclear and Particle Physics Experiments, William Leo, Springer-Verlag (1987) Radiation Detection and Measurement, Glenn Knoll, John Wiley and Sons (1979)

Sarah Eno3 A minor Miracle Detectors must find 1 particle among How??? –The particle is very energetic So it behaves differently than the others –The detector is in a special meta-stable state So the particle disturbs it and causes a physical change This amplifies the effect Then an electronic device can amplify it further

Sarah Eno4 Particles What particles do we detect? But those that are stable on the time scale of a few microseconds when moving fast Not the fundamental particles Mostly… electrons, muons, neutrinos, photons (light), charged pions (bound state of ud quarks) Also occasionally proton, neutron, alpha particle (ionized hydrogen), charged kaon (bound state of su quark)

Sarah Eno5 Bulk Matter What happens when a high speed particle passes through bulk matter? Non-destructive elastic scattering (+ionization) scintillation Cherenkov radiation transition radiation destructive bremsstrahlung pair-production Nuclear interactions

Sarah Eno6 Units is the energy an electron gains when it is accelerated through 1 Volt. Light from the sun keV → atomic energies X rays MeV → nuclear energies Gamma Rays GeV → high energy physics 1eV = 1.6x J eV

Sarah Eno7 Cross section Measure of the probability for an interaction to occur. Units of area. 1 barn = m 2 L is the luminosity, which is a measure of the flux of the incident particles (number per area per time)

Sarah Eno8 Elastic energy When a charged particle passes through a bulk material (say gas, Silicon, etc), it will scatter elastically off the nuclei in the material (change direction) and inelastically off the electrons (lose energy) (we’ll discuss inelastic scattering of nucleii later) transferred energy can do 2 things: atomic excitation or ionization. Atomic excitation can lead to the production of photons (talk about later), ionization of the bulk material -> freed electrons can be collected to give a current or charge tell us if the particle is charged or neutral energy loss/length depends on the particles velocity

Sarah Eno9 Bethe-Bloch Formula Units MeV-cm 2 /g Because, for the same thickness, you’ll put more energy into a dense material than a non-dense one. Multiply by the density to get MeV/length. Lead: p=18 g/cm3 MeV/cm=36 Argon: p= g/cm3 MeV/cm= Tells you the total energy lost to both elastic and inelastic collisions

Sarah Eno10 Bethe-Bloch Why that shape? At low energy-> slower. Spends more time near each nucleus -> more interactions. Higher energy… can’t go faster than c, so doesn’t keep getting smaller once near c At high energy->relativity Imagine a heavy particle with mass mp and momentum P incident on an electron at rest. Maximum kinetic energy that the electron receives is: If ignore g, KE approaches a fixed value as v->c This g 2 factor accounts for the relativistic rise

Sarah Eno11 To keep in mind The constancy of energy loss at high speed That means the total loss proportional to the length of the path traversed

Sarah Eno12 Electron-Ion Pairs When energy is transferred from the high energy particle to the bulk material, some of it causes excitations of the atom, and some causes ionization (delta rays). Typically 1 ion-electron pair per 30 eV of energy lost. Leo

Sarah Eno13 Uses Most commonly used in “tracking” chambers. High energy particle going through gas or silicon. Freed electrons are collected to make a signal. Tells you where the particle is. Can tell you its velocity as well. And maybe radio signals?

Sarah Eno14 Scintillation As we discussed, when a charged particle goes through bulk matter, it can excite the atoms of this matter. Some materials, when they deexcite, emit photons with visible light wavelengths (usually blue, around 400 nm), and these (rather than electrons from ionization) can be collected as the signal, using a photomultiplier tube for example (which converts a photon to a current). Fluorescent materials: emission of photons with a decay constant of s Leo

Sarah Eno15 Scintillators Common Plastics: Polyvinyltoluene, polyphenylbenzene, polystyrene + fast, cheap, flexible, easy to machine - suffer radiation damanage Inorganic Crystals: NaI, CsI + when need precision measurements - expensive. Other Organic scintillator (aromatic hydrocarbon compounds containing linked or condensed benzene-ring structures. ) Organic Crystals (C 14 H 10 (anthracene), C 14 H 12 (trans-stilbene), C 10 H 8 (naphthalene)) Organic Liquids (P-Terphenyl, PBD,PPO, and POPO, xylene, toluene, benzene, …)

Sarah Eno16 Total Internal Reflection Particle Quartz bar Cherenkov light Active Detector Surface

Sarah Eno17 Total Internal Reflection Note: can only happen when n 1 >n 2 !

Sarah Eno18 Scintillator On display at MoMA in NY until Aug 31 (then on world world tour, “Signatures of the Invisible”, Scintillator from D0.

Sarah Eno19 Scintillator

Sarah Eno20 Uses Time of flight counters (measure particle speed) calorimeters (measure particle energy) D0 tracking system

Sarah Eno21 Cherenkov Radiation When a particle moves through bulk matter with a speed faster than the speed of light in that medium, it emits radiation (well, not really. Particles moving at constant speed don’t radiate. But, its field interacts with the medium, which emits photons) in an electro-magnetic analog of the “sonic boom” that happens when a jet moves faster than the speed of sound or around the bow of a boat moving faster than the speed of sound in water. (happens for any kind of wave, not just sound!)

Sarah Eno22 Cherenkov Radiation Radiation is emitted at a fixed angle to the particle. Cone shape. When photons hit flat surface, make circle. Size of circle depends on particle speed. However, can’t do a good measurement as  ->1 Hygen’s Principal Green

Sarah Eno23 Cherenkov Minimum speed for emission n-=1.33 (water)  >0.752 q max=41 degrees

Sarah Eno24 Cherenkov Energy loss for a solid around MeV cm 2 g -1. Photons typically in the visible light frequencies. About 1 part per thousand of the ionization energy loss

Sarah Eno25 Cherenkov Radiation BaBar

Sarah Eno26 Uses Mostly used to get particle type (tell kaons from pions) Cosmic Rays: look at extensive air showers directly

Sarah Eno27 Destructive Measurements Typically happen with the aid of a heavy nucleus Only way to measure neutral particles.

Sarah Eno28 Bremsstrahlung  e Energy lost to this goes as 1/M 4, so basically only happens to electrons (happens also for very very high energy muons (100 GeV)) When in the presence of a heavy nuclei, a particle can “bremsstrahlung” off a high energy photon (gamma ray). Because the electron undergoes a large acceleration due to nuclei’s field. Mass of muon is 105 MeV, electron is 0.51 MeV. How does the radiation loss for muons compare to electrons? 40,000

Sarah Eno29 Bremsstrahlung The photon is not low energy! It is (almost) equally probable for the photon to have any fraction of the electrons energy, from 0% to 100%. On average, thus, will get about ½ the electron’s energy.

Sarah Eno30 Bremsstrahlung (“Brem”) Happens more often in materials with heavy nucleii Material distance an electron loses 63% of its energy to brem Be 35 cm C 19 cm Al 9 cm Pb 6 cm U 0.3 cm Air 30 m (at sea level)

Sarah Eno31 Pair Production When in the presense of a heavy nucleii, a photon with energy above MeV can turn into an electron- antielectron (positron) pair. On average, each will have ½ the photons energy. Why MeV?

Sarah Eno32 Destructive Calorimeter: use these two processses to measure the energies of electrons and photons. More from Greg later this week.

Sarah Eno33 Nuclear Interactions Hard to predict the cross section, properties of these interactions from first principles, because they involve the strong force.

Sarah Eno34 Nuclear Interactions Rarer than, for example, brem. For example, a high energy electron will lose 64% of its energy to brem every 6.4 g/cm 2 in Pb, while a high energy pion will do the same via nuclear interactions only every 194 g/cm 2. A puzzle: nuclear interactions are due to the “strong” interaction. So why are they rarer than the electromagnetic interactions?

Sarah Eno35 Nuclear Interactions Messier than Brem. Green

Sarah Eno36 Nuclear Interactions Sometimes the interactions can produce neutral pions, which decay to two photons. These photons will then pair- produce, brem, pair-produce just like photons that came from the interaction point.

Sarah Eno37 Nuclear interactions Study empirically, using accelerator data Green