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Quantum C h r o m o dynamics (QCD)

Quantum C h r o m o dynamics (QCD). Andrew Brandt UT-Arlington/D Ø Experiment. Physics 3313 November17, 2003. Structure of Matter. Matter. Molecule. Atom. Nucleus. Baryon. Quark. (Hadron). u. 10 -14 m. cm. 10 -9 m. 10 -10 m. 10 -15 m. <10 -19 m. Chemistry. protons, neutrons,

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Quantum C h r o m o dynamics (QCD)

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  1. Quantum Chromodynamics (QCD) Andrew Brandt UT-Arlington/DØ Experiment Physics 3313 November17, 2003

  2. Structure of Matter Matter Molecule Atom Nucleus Baryon Quark (Hadron) u 10-14m cm 10-9m 10-10m 10-15m <10-19m Chemistry protons, neutrons, mesons, etc. p,W,L... top, bottom, charm, strange, up, down Atomic Physics Nuclear Physics Electron Mass (Lepton) proton ~ 1 GeV/c2 <10-18m High Energy Physics

  3. Forces work by the exchange of Boson’s Electromagnetic:Photon Exchange Weak Nuclear Force:Causes Nuclear Decays Forces neutron proton W- boson e- p+ photon  electron n

  4. Forces: Strong Nuclear or Color • Strong Nuclear Force:Quantum ChromodynamicsGluon Exchange, also holds the nucleus together.All quarks carry a color chargeGluons carry two color charges • Different from other Forces:Gluons can interact with other gluons. Quarks and gluons are free at small distances (asymptotic freedom), but not at large distances (confinement)  cannot observe bare color Always observe quarks in multiplets: Baryons qqq (Proton neutron) and Mesons (quark antiquark pair ) Proton: uudAlso contains gluons and quark-antiquark pairs in a sea. Neutron:udd Pion: ud

  5. A word about units:HEP uses “natural units” Collide protons and antiprotons each with 900 GeV of kinetic energy. Proton Antiproton Collisions 900 GeV Antiprotons 900 GeV Protons • The mass of a proton is then given by

  6. Life at Fermilab

  7. Particle Colliders as Microscopes QM: large momenta = small distances How we see different-sized objects:

  8. The actual result was very different.“It was almost as incredible as if you fired a 15 inch shell at a piece of tissue paper and it came back at you” Implied the existence of the nucleus. We perform a similar experiment at Fermilab to look for fundamental structure Rutherford Scattering

  9. Proton contains three valance quarks: uud Also contains sea of virtual quark anti-quark pairs. All held together by gluons Quarks and gluons are called partons. Proton with momentum P. Individual parton carries momentum xP Proton Structure u u d u s uv uv s u d dv

  10. Described by QCD. Parton-Parton Scattering Scattered Parton Anti-Proton 900 GeV Proton 900 GeV Scattered Parton

  11. Perturbative QCD and Jet Production ^ ^ s ~ a2s (LO) s ~ a3s (NLO) Observable jet of particles in detector q jet q (x1) Parton distribution (PDF) g q (x2) q jet Fragmentation into hadrons p Hard scatter (pQCD) Includes radiative corrections and gluon emission - much of current QCD is a study of this additional radiation p

  12. Jets are formed by the scattered partons. QCD requires that colourless objects are produced (hadrons) e.g..:, K, , etc. At DØ a jet is defined to be the energy deposited in a cone of radius: Jets

  13. Measured Event Variables • In a Two Jet event the following is measured: Jet 1: ET1, h1, 1   ET = Energy x sin  Jet 2: ET2, h2, 2 h = 0

  14. y   x z The DØ Detector

  15. Detection Muon Tracks Charged Particle Tracks Energy Scintillating Fiber Silicon Tracking Calorimeter (dense) Interaction Point Absorber Material Ä B EM hadronic electron photon Wire Chambers jet muon neutrino -- or any non-interacting particle missing transverse momentum 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

  16. Single Inclusive Jets: Inclusive Jet Cross Section as a Test of the Standard Model (pQCD)

  17. CH “calorimeter jet” hadrons FH  EM “particle jet” Time “parton jet” Jet Production and Reconstruction Highest ET dijet event at DØ • Fixed cone-size jets • Add up towers • Iterative algorithm • Jet quantities:

  18. “Typical DØ Dijet Event” ET,1 = 475 GeV, h1 = -0.69, x1=0.66 ET,2 = 472 GeV, h2 = 0.69, x2=0.66 MJJ = 1.18 TeV Q2 = ET,1×ET,2=2.2x105 GeV2

  19. High Energy Art

  20. How well do we know proton structure (PDF)? • Is NLO ( ) QCD “sufficient”? • Are quarks composite? PDF, substructure, … ? DØ Run 1B d2/dET d ET The DØ Central Inclusive Jet Cross Section • 0.0    0.5 • JETRAD Phys. Rev. Lett. 82, 2451 (1999)

  21. E0 “smearing” 0.98 “true” “observed” Smearing Correction DØ 0.94 0.90 0.86 “unsmearing” or “unfolding” 50 100 150 200 250 300 350 400 450 500 ET (GeV) Data Selection and Corrections Unfold effects of finite jet energy resolutions from very steeply falling inclusive jet cross sections

  22. CH “calorimeter jet” hadrons FH  EM “particle jet” E = (EObs-Offset)*Det.Uniformity RH * Out of Cone Showering “parton jet” Data Selection and Corrections • Cut on central p-pbar vertex position • Eliminate events with large missing ET • Apply jet quality cuts Jet energy scale correction: “calorimeter”  “particle” jet

  23. CTEQ5 Jets in PDFs 101 x-Q region spanned by experimental data in modern fits Tevatron jets in blue Q (GeV) 101 100 100 101 102 103 104 1/ x Tevatron jet data serves as stronger constraint in medium x region for CTEQ.MRST uses does not use these data.

  24. Inclusive Jets- CDF

  25. Inclusive Jet Cross Section at 1.8TeV Preliminary PRL82, 2451 (1999) D0 and CDF data in good agreement. NLO QCD describes the data well.

  26. DØ Preliminary Run 1B • 0.0    0.5 • 0.5    1.0 • 1.0    1.5 • 1.5    2.0 • 2.0    3.0 d2 dET d (fb/GeV) Nominal cross sections & statistical errors only ET (GeV) Rapidity Dependence of the Inclusive Jet Cross Section

  27. Continuing Search for fundamental building blockAtom  Nucleus  Nucleons  Quarks Three quark and lepton generations suggests that quark and leptons are composites. Question Are Quarks composite particles? Search for compositeness in Proton Anti-proton collisions Compositeness Atom Nucleus Nucleon Quark

  28. The presence of three quark and lepton generations suggests that they could be composite particles Composed of “preons” Define the preons interaction scale as  Existence of substructure at energies below  indicated by presence of four-fermion contact interactions. Strength of interactions related to Search for Compositeness Proton Quark Preons? M cos

  29. If quarks are made up of smaller particles then expect more events at high mass, and at smaller scattering angles Predictions Prediction for composite quarks Number of Events Number of Events Prediction for fundamental quarks cos * M

  30. Dijet Production • To search for compositeness we need a good prediction for Standard Model dijet production  NLO QCD. • NLO event generator JETRAD (Giele, Glover, Kosower Nucl. Phys. B403, 633) • Need to choose pdf • Choose Renormalization and Factorization scales (set equal) • Rsep: maximum separation allowed between two partons to form a jet (mimic exp. algorithm)Rsep=1.3R(Snowmass: Rsep=2.0R) 1.3R 2R

  31. Dijet Cross Section Phys. Rev. Lett. 82, 2457 (1999)

  32. Cross Section Ratio • Calculate Ratio of Cross Sections. • Two different angular regions Submitted to PRL: hep-ex/9807014 Model with LL coupling

  33. Quark-Quark Compositeness Limits Limit on size of preons is fempto-meters

  34. Conclusions • No evidence for Compositeness found at the Tevatron • Standard Model (QCD) in excellent agreement with the data • Quark-Quark Compositeness •  > 2 to 3 TeV depending on models

  35. Numerous other QCD studies to probe scattering dynamics W, Z + +... q(x) W/Z PT,W/Z+Jets  Jets in High E Limit Photons Color Flow Diffraction etc...

  36. Measurement of aS from Inclusive Jet Production NLO x-section can be parametrized as Measured by CDF Obtained from JETRAD • Fitting the NLO prediction to the data determines aS(ET) • aS(ET) is evolved to aS(MZ) using 2-loop renormalization group equation • Systematic uncertainties (~8%) from understanding of calorimeter response • Measured value consistent with world average ofaS(MZ)=0.119±0.004 New measurement of aS by a single experiment & from a single observable over a wide range of Q2.

  37. Conclusions • Standard Model (QCD) in excellent agreement with the data • No evidence for Compositeness of quarks found at the Tevatron • Studies continue improving theory, detectors, and using better microscopes

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