measuring the size of proton-proton collisions

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Measuring the Size of Proton-Proton Collisions. Thomas D. Gutierrez University of California, Davis March 14, 2002 Department of Physics Sonoma State University. Quarks knocked loose during a collision quickly form bound states through a process called hadronization . - PowerPoint PPT Presentation

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  • Measuring the Size of Proton-Proton CollisionsThomas D. GutierrezUniversity of California, Davis

    March 14, 2002Department of PhysicsSonoma State University

  • http://particleadventure.orgParticle Physics at a Glance

  • Perspectives on Temperature~10-6 K~3 K~300 K~6000 K~106 K~1012 K~ 120 MeVTrapped IonsCosmic Microwave BackgroundRoom Temperature ~ 1/40 eVSolar SurfaceSolar Interior~109 KNeutron Star Thermonuclear Explosion~10-10 KRhodium metal spin cooling (2000)(Low-T World Record!)(Terrestrial Nuclear explosions)~107 KGraphic courtesy JLKNucleus-Nucleus collisions

  • Nuclear Collisions in Action

  • Proton-proton (pp) collisions are the simplest case of nucleus-nucleus (AA) collisions...pp collisions form the baselinefor AA collisionsWhy study proton-proton and nucleus-nucleus collisions at all?

  • Why collide protons at all?The Relativistic Heavy Ion Collider (RHIC)on Long Island, NY slams gold nuclei head-on at 0.99995c,creating little Big Bangs!

  • 1. Space-Time Evolution of High Energy Nucleus-Nucleus CollisionQuark Formation & creation ~ 1fm/cQGPPTMixed PhaseHadron GasNKThermal Freeze-outztProjectile Fragmentation Region

  • 2. Space-Time Evolution of proton-proton CollisionQuark scattering and creationPTztThats why pp collisions area cleaner probe of what is going on during hadronization

  • Why measure the size of pp collisions?Source sizes are measured using a technique called Hanbury-Brown TwissIntensity Interferometry (or just HBT for short)

  • What is HBT?The technique was originally developed by two English astronomers Robert Hanbury-Brown and Richard Twiss (circa 1952) (Sadly, RHB passed away just this January)Its form of Intensity Interferometry-- as opposed to regular amplitude-level(Young or Michelson) interferometry -- and was used to measure the angular sizes of starsA quantum treatment of HBT generated much controversy and led to a revolution in quantum optics (photons can act strangely!)Later it was used by high energy physicists to measure source sizes of elementary particle or heavy ion collisionsBut how does HBT work? And why use it instead of regular interferometry?

  • Monochromatic SourcePlane wavedTwo slit interference (between coherent sources at A and B)ABrA1rB1P1source geometry is related to interference pattern

  • Two monochromatic but incoherent sources (i.e.with random, time dependent phase)produce no interference patternat the screen -- assuming we time-averageour measurement over manyfluctuationsrA1rB1P1Two slit interference (between incoherent sources at A and B)

  • What does mean?Position on the screen in radians (for small angles)

  • As before...HBT Example (incoherent sources)But if we take the product before time averaging...whereABImportant: The random phase terms completely dropped outand left us with a non-constant expression!(will be related to source and detector geometry)Difference of the path length differences

  • This quantity is known as a correlation functionIt is important to note that for coherent sources (remembering in that case =I)Time average of the productProduct of the time averagessoC=1

  • What does C mean?Its not exactly the usual statistical correlation functionbut it is relatedI1I2If we independently monitor the intensity as a function of time at two points on the screen...

  • For two incoherent point sources.The momentum difference is called:Recall

  • Particle physicsAstronomyFor fixed kIll drop

  • Bosons and FermionsBosons are integer spin particles. Identical Bosons have a symmetric two particle wave function -- any number may occupy a given quantum state...Fermions are half-integer spin particles. Identical Fermions have an antisymmetric wave function -- only one particle may occupy a quantum statePhotons and pions are examples of BosonsProtons and electrons are examples of Fermions

  • More about Correlation functionsAt the quantum levela non-constant C(Q) arisesbecause ofI) the symmetry of the two-particle wave functionfor identical bosons or fermions andII) the kind of statistics particles of a particular type obeyA series of independent events should give C=1 (same as a coherent source)

  • HBT Summary and ObservationsThe correlation function contains information about the source geometry

    The width of the correlation function goes like 1/(source width)

    The HBT correlation function is insensitive to random phases that would normally destroy regular interference patterns

  • Back to pp Collisions

    Pions (also bosons) are used in the HBT rather than photons

    Basic idea is the same: Correlation function contains information about pion emission source size in the collision and may give clues about the nature of hadronization

  • Real Data!500k pp events from Experiment NA49 at CERNGeVGeVGeV1. signal2. random3. correlationQQQ1. Generate a cumulative signal histogram by taking the momentum difference Q between all combinations of pion pairs in one pp event; repeat this for all pp events2. Generate a random background histogram by taking the momentumdifference Q between pions pairs in different events3. Generate a correlation function by taking the ratio of signal/random

  • C(Q)Q (MeV/c)NA44 at CERNNPA610 240 (96)

    From Craig Ogilvie(2 Dec 1998)Typical AA DataThis isnt my analysisC is narrower so R is biggerJust for comparison...R really increases with system size!

  • My current research related to this workAnalysis: HBT for pp collisions at NA49 (at CERN) and STAR (at RHIC)Evaluate phase space density of the pp system, extract temperature!

    Current pure theory project (mostly unrelated to particle physics): What are theoretical correlation functions for parastatistical particles and anyons?

    Lots of room for student involvement at various levels!

  • What have we learned?pp collisions are smaller than AA collisions!HBT can be subtle and funQuark hadronization is complicated but studying the size of proton-proton collisionsusing HBT may be able to tell us something about it

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