PHYS 210Spring 2006High-Energy Physics Experimental Techniques1. One-page review of high energy physicsRecall that atoms are made of electrons, which are elementary particles, and nuclei, whichconsist of roughly equal numbers of protons and neutrons. Protons and neutrons, in turn, aremade from three quarks, which are also believed to be elementary particles. In addition to thesecommon stable particles there is a number of unstable elementary particles and many morecomposite particles. Some of them are listed with a few of their properties in the table below.See attached chart of fundamental particles and interactions for more information.Name Composition Mass q (e) Lifetime Decays NotesElectron (e) Lepton (elem.) 0.5 MeV -1 Stable NoneMuon (µ)Lepton (elem.) 105 MeV -12.2 µsec eνeνµSecondary cosmic raysTau (τ)Lepton (elem.) 1777 MeV -12.9×10-13 secµντνµMany other decay modesNeutrinos(νe, νµ, ντ)Leptons(elementary)0<mν<2 eV0 Stable ChangeflavorsNon-zero mass onlyconfirmed in the last yearUp (u) Quark (elem.) ~ 3 MeV 2/3 Stable NoneDown (d) Quark (elem.) ~ 6 MeV -1/3 10-8 sec to ∞1ueνeStrange (s) Quark (elem.) ~100 MeV -1/3 ~10-8 secueνeCharm (c) Quark (elem.) 1200 MeV 2/3 ~10-12 secseνeBottom (b) Quark (elem.) 4200 MeV -1/3 ~10-12 secceνeExist only bound insidebaryons or mesons. Haveone of 3 different“colors” – charges of thestrong forceTop (t) Quark (elem.) 174 GeV 2/3 ~10-24 secbeνeDiscovered in 1995Proton (p) Baryon (uud) 938 MeV21 Stable None Primary cosmic raysNeutron (n) Baryon (udd) 939 MeV20 887 secpeνeStable inside nucleiPion (π+)Meson (u d )139 MeV212.8×10-8 secµ+νµKaon (K+)Meson (u s )493 MeV211.2×10-8 secµ+νµNotes to Table: All particles have corresponding anti-particles with the same mass and opposite charge.1 MeV = 1.8×10-30 kg. (1) Lifetime depends on binding inside mesons or baryons. All decays must satisfyconservation of energy, charge, baryon and lepton nmmber. (2) Masses of quarks do not add up to masses ofbaryons and mesons because of extra binding energy (mass) of the gluons holding them together.The goal of modern particle physics experiments is to search for new, heavier particles and tomake precision measurements of properties of the known ones in a quest to arrive at a completepicture of fundamental interactions. For example, it is believed that there is an additionalelementary particle called Higgs boson with a mass in excess of 115 GeV whose interactions areresponsible for giving mass to all other particles. Additional elementary particles, calledsupersymmetric partners (one for every known elementary particle) are believed to exist in themass range of 200 to 1000 GeV. Another active area of research is violation of CP (charge-parity) symmetry that is related to the origin of asymmetry between matter and anti-matter in theUniverse.Most high-energy experiments are performed at large accelerators (construction cost >b$) incollaborations of ~1000 physicists, although smaller teams and even table-top experimentsaddressing specific questions of particle physics also exist.2. Interactions of high-energy particles with matter and their detection.The purpose of high-energy particle detectors is to track particles produced in a high-energycollision and measure their properties, such as energy, momentum, charge and mass.a) Ionization energy lossAny charged high-energy particle traveling through matter looses energy byelectromagnetic interactions with electrons in the material. Most of this energy goes intokicking off electrons bound in atoms and producing ions and free electrons. The amountof energy loss is proportional to the density of the material and is only weakly dependenton the particle energy or type of the material. Numerically dE/dx > 1 MeV/cm ρ (g/cm3),so in a typical solid material particles loose several MeV per cm of travel, while in air(ρ=0.0012g/cm3) they loose 0.1 MeV/m.Several types of detectors are based on measuring the ionization energy deposited in thematerial: Ionization counters and drift chambersThese detectors consist of a tank filled with argon or othersuitable gas and many thin wires with a high voltage (about 5kV) applied between them. Electrons from the ionizationtrack left by the high-energy particle drift to the closest wire,get amplified by electrical breakdown in the high electric fieldnear the wire, and generate an electric current in the wire. The time of signal arrivaldepends on the distance from the track to the wire.Scintillation countersThese detectors typically consist of a transparent plastic material with a specialchemical called “wavelength shifter”. In a solid material the electrons and ionsproduced by the high-energy particle recombine and emit photons in the ultra-violentwavelength range. These photons cannot travel very far, but the wavelength shifterabsorbs the photons and reemits them in the visible range. The visible photons travelthrough the plastic to a photomultiplier tube which converts them to an electricalsignal.Photomultiplier tubes are basedon the photoelectric effect. Eachphoton ejects a single electronfrom the photocathode. Thiselectron is accelerated by anelectric field to secondaryelectrodes called dynodes whereit ejects several additional electrons. With many dynodes a large amplification factoris achieved, resulting in a strong electrical pulse at the output. Silicon strip detectorsElectrons+−TrackScintillatorplasticLightguideOutputPhotocathodeHigh VoltagePhotomultiplierTubeElectrontrajectoryThese detectors consist of long silicon p-n junctions, similar to solar cells we willdiscuss later in the class. The ionization track generates an electric current that isamplified by electronics located on the same piece of silicon.b) Cherenkov RadiationIf a charged particle travels at a speed greater than the speed of light in a given medium itemits Cherenkov radiation. Recall that the speed of light in a transparent medium is v = c/nwhere the index of refraction n = 1.3 - 1.5 for glass or water. Highenergy particles with total energy much greater than their rest masstravel nearly at the speed of light. The Cherenkov radiation is similar tothe sound shock wave created by an object traveling faster than thespeed of sound. The Cherenkov radiation is emitted at an anglecos θ =1/βn to the path of the particle. This direction and opening angleof the Cherenkov radiation cone can be used to determine the speed anddirection of the particle. The Cherenkov