Introduction to Experimental Particle Physics : Lecture 1

Transcription

1 Introduction to Experimental Particle Physics : Lecture 1 Graz, September 2012 John Swain, Northeastern University, Boston

2 Very, very, very important... We re just starting!...whole field is about 100 years old. See my Santoro festschrift article... Better a little less with more clarity or inspiration...please don t let me lose you!

3 Plan by Material (not necessarily by lecture!) I Introduction, Motivation, Early History II Particles, sources, accelerators III Detectors I : zero mass (trackers) IV Detector II : infinite mass (calorimeters) V Triggers, computing, simulation, reconstruction, analysis, publication VI Outlook for the future

4 The Intellectual Content of Experimental Physics Victor Weisskpof, CERN Yellow Report Question: Why do so many of the brightest students want to do theory? Answer: 1) almost all courses are theoretical 2) almost all textbooks are theoretical

5 Constraints This (not even all of it!) is all you get to work with...

6 Actually, at the start we know much less... Text N.B.: Conceptions of fundamental change -- important to know that they are modes of thought rather than intrinsic realities. (This isn t wrong...these are phases of matter.)

7 Tools get better though...and we start to learn more...

8 Terrestrial Radioactivity Antoine Henri Becquerel (15 December August 1908) Discovers radioactivity in phosphorescent uranium salts Nobel prize (with M. and P. Curie) in 1903 By no means trivial -- Kelvin predicts age of earth to be My

9 Rutherford s alpha, beta, gamma rays ( ) helpful2.htm nowe_teksty/htmla/1_26a.html Now: Helium nuclei, fast electrons (and e+!), photons

10 Rutherford scattering 1911: Discovery of the atomic nucleus in a scattering experiment prototype of most particle physics progress: source of particles collisions detection data collection analysis

11 Really an extension of vision Descartes Ipanema using natural photons of a few ev and photographic film/eye. ( HEP: replace natural sources (often) and replace eye with detectors!

12 In anticipation of later... You only see what you look for. Post-processing (human brain always, perhaps other things too!) influence what s selected Even after selection, you can still miss things! (Basketball plus gorilla experiment)

13 Discovery of the Neutron Chadwick (1932) Essentially same idea as Rutherford experiment 3P13_E/DiscoverNeutron_E.htm

14 Radiation from Space Victor Francis Hess(24 June Dec 1964) Discovers radioactivity increasing with altitude Nobel prize in 1936 Again, by no means trivial Led to a vast array of discoveries in particle physics and astrophysics and is still doing so!

15 free natural (uncontrolled) accelerator, very primitive detector Hess s Great Work discovers radioactivity increasing with altitude above 1km some evidence before, but Hess flies vastly improved electroscopes (!) in balloons up to 5.3 km...and does it himself! (and at great personal risk)

16 Feynman and Einstein If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis that All things are made of atoms-little particles that that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied. 1906: Einstein s annus mirabilis : photoelectric effect and Brownian motion

17 Particles Discovered via Cosmic Rays 1: The Positron Cloud chamber with magnetic field Curvature less at top means comes up Must be positive de/dx means can t be proton C.D. Anderson, Physical Review 43, 491 (1933) First time a predicted (Dirac 1928) particle is discovered!!

18 The mu and pi mesons & more... The muon: 1937 by J. C. Street and E. C. Stevenson: "New Evidence for the Existence of a Particle Intermediate Between the Proton and Electron", Phys. Rev. 52, 1003 (1937) in a cloud chamber The pion: 1947 by Cecil Powell, César Lattes, Giuseppe Occhialini et al. in Bristol, England in emulsion; predicted by Yukawa in 1935 (Nobel in 19490); another Nobel to Powell in 1950 later Lamda, Xi and Sigma all found in cosmic ray interactions

19 What sorts of particles do we know? photons electrons, muons ( who ordered that? ) protons, neutrons, nuclei pions, kaons neutrinos understood from representation of the Poincare group: (mass, spin)

20 What is a particle? The key modern idea - invariances under rotations, boosts, and translations Interesting non-invariance under a scale change... Question: Is special relativity trivial for you? This is not a trick question -- please be honest!

21 Nontrivial Scale Transformations in QFT Obvious absolute scale not present in Maxwell theory provided by particle masses..but it s not as simple as no scale invariance.

22 Particles and Forces The Periodic Table in 2012

23 A feeling for energies Room temperatare kt ~ 1/40 ev Flashlight battery ~1.5 ev Visible light ~ a few ev UV - ionization of hydrogen at 13.6 ev X-rays - kev Nuclear - MeV Inside a proton - GeV

24 Why accelerate? nature gives us radioactive decays: kev to MeV, but we want more, More, MORE!!! high energies as a microscope (colliding Swiss watches to see what s inside!) high energies as a window back in time QFT: create new particles (!) Not just Poincare transformations -- things change under scale transformations too!

25 Virtual Processes Everything happens when you re not looking (2-slit experiment) Uncertainty principle as an alternative to going to higher energies Simple example: muon decay Rare decays, precision g-2, etc.

26 4 Forces we could use for acceleration Need to apply a FORCE through a DISTANCE Electromagnetism -- good candidate! Gravity -- too weak Weak -- too short range and too weak Strong -- too short range

27 What we can accelerate?... Nuclei and protons have structure which even changes with scale collisions are dirty Electrons pointlike, but still have structure due to QFT Bremsstrahlung: Prob ~ square of mass

28 Fixed target vs. collisions All energy available to make particles only in collider Q: Is this easy for you to see? To work out? This is not a trick question -- please be honest!