Quantum Chromodynamics QCD Andrew Brandt UTArlingtonD Experiment Physics
Quantum Chromodynamics (QCD) Andrew Brandt UT-Arlington/DØ Experiment Physics 3313 November 17, 2003
Structure of Matter Molecule Atom Nucleus Baryon Quark (Hadron) u cm Mass proton ~ 1 Ge. V/c 2 10 -14 m 10 -9 m 10 -10 m Chemistry Atomic Physics Nuclear Physics 10 -15 m <10 -19 m protons, neutrons, top, bottom, charm, strange, mesons, etc. up, down , W, L. . . Electron (Lepton) <10 -18 m High Energy Physics
Forces l Forces work by the exchange of Boson’s l Electromagnetic: Photon Exchange e- photon l Weak Nuclear Force: Causes Nuclear Decays neutron proton W- boson p+ electron n
Forces: Strong Nuclear or Color l Strong Nuclear Force: Quantum Chromodynamics Gluon Exchange, also holds the nucleus together. All quarks carry a color charge Gluons carry two color charges l 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: uud Also contains gluons and quarkantiquark pairs in a sea. Neutron: udd Pion: ud
Proton Antiproton Collisions l A word about units: HEP uses “natural units” l The mass of a proton is then given by 900 Ge. V Protons 900 Ge. V Antiprotons l Collide protons and antiprotons each with 900 Ge. V of kinetic energy.
Life at Fermilab
Particle Colliders as Microscopes QM: large momenta = small distances How we see different-sized objects:
Rutherford Scattering l 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” l Implied the existence of the nucleus. l We perform a similar experiment at Fermilab to look for fundamental structure
Proton Structure l Proton contains three valance quarks: uud l Also contains sea of virtual quark anti-quark pairs. l All held together by gluons d u s uv uv l Quarks and gluons are called partons. l Proton with momentum P. Individual parton carries momentum x. P u s u dv
Parton-Parton Scattering l Described by QCD. Scattered Parton Anti-Proton 900 Ge. V Scattered Parton
Perturbative QCD and Jet Production ^ ~ a 2 (LO) s s Parton distribution (PDF) q (x 1) q (x 2) p g q jet Observable jet of particles in detector Fragmentation into hadrons Hard scatter (p. QCD) ^ ~ a 3 (NLO) s s p Includes radiative corrections and gluon emission - much of current QCD is a study of this additional radiation
Jets l Jets are formed by the scattered partons. l QCD requires that colourless objects are produced (hadrons) e. g. . : , K, , etc. l At DØ a jet is defined to be the energy deposited in a cone of radius:
Measured Event Variables l In a Two Jet event the following is measured: Jet 2: ET 2, 2 Jet 1: ET 1, 1 ET = =0 Energy x sin
The DØ Detector y z x
Detection Charged Particle Tracks ÄB Calorimeter (dense) EM Absorber Material Interaction Point Scintillating Fiber Silicon Tracking Muon Tracks Energy 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
Inclusive Jet Cross Section as a Test of the Standard Model (p. QCD) Single Inclusive Jets:
Highest ET dijet event at DØ CH FH EM hadrons Time “parton jet” “particle jet” “calorimeter jet” Jet Production and Reconstruction Fixed cone-size jets l Add up towers l Iterative algorithm l Jet quantities: l
“Typical DØ Dijet Event” ET, 1 = 475 Ge. V, 1 = -0. 69, x 1=0. 66 ET, 2 = 472 Ge. V, 2 = 0. 69, x 2=0. 66 MJJ = 1. 18 Te. V Q 2 = ET, 1×ET, 2=2. 2 x 105 Ge. V 2
High Energy Art
The DØ Central Inclusive Jet Cross Section DØ Run 1 B l 0. 0 0. 5 S JETRAD l Is NLO ( l Are quarks composite? d 2 /d. ET d m How well do we know proton structure (PDF)? Phys. Rev. Lett. 82, 2451 (1999) ) QCD “sufficient”? PDF, substructure, … ? ET
Data Selection and Corrections Unfold effects of finite jet energy resolutions from very steeply falling inclusive jet cross sections “smearing” “true” “observed” DØ “unsmearing” or “unfolding” E 0 0. 98 Smearing 0. 94 Correction 0. 90 0. 86 50 100 150 200 250 300 350 400 450 500 ET(Ge. V)
Data Selection and Corrections E = (EObs-Offset)*Det. Uniformity RH * Out of Cone Showering “parton jet” “particle jet” Jet energy scale correction: “calorimeter” “particle” jet “calorimeter jet” Cut on central p-pbar vertex position Eliminate events with large missing ET Apply jet quality cuts CH FH EM hadrons
Jets in PDFs x-Q region spanned by experimental data in modern fits Tevatron jets in blue CTEQ 5 Q (Ge. V) 101 100 101 102 103 104 1/ x Tevatron jet data serves as stronger constraint in medium x region for CTEQ. MRST uses does not use these data.
Inclusive Jets- CDF
Inclusive Jet Cross Section at 1. 8 Te. V Preliminary PRL 82, 2451 (1999) D 0 and CDF data in good agreement. NLO QCD describes the data well.
Rapidity Dependence of the Inclusive Jet Cross Section DØ Preliminary Run 1 B d 2 d. ET d (fb/Ge. V) l H s n t 0. 0 0. 5 1. 0 1. 5 2. 0 3. 0 Nominal cross sections & statistical errors only ET (Ge. V)
Compositeness l Continuing Search for fundamental building block Atom Nucleus Nucleons Quarks l Three quark and lepton generations Nucleus suggests that quark and leptons are composites. l Question Nucleon Are Quarks composite particles? l Search for compositeness in Proton Anti-proton collisions Quark Atom
Search for Compositeness Proton Quark l Define the preons interaction scale as l Existence of substructure at energies below indicated by presence of four-fermion contact interactions. l Strength of interactions related to Preons? l The presence of three quark and lepton generations suggests that they could be composite particles l Composed of “preons” M cos
Predictions Number of Events Prediction for composite quarks M Prediction for fundamental quarks Number of Events l If quarks are made up of smaller particles then expect more events at high mass, and at smaller scattering angles cos *
Dijet Production l To search for compositeness we need a good prediction for Standard Model dijet production NLO QCD. l NLO event generator JETRAD (Giele, Glover, Kosower Nucl. Phys. B 403, 633) l Need to choose pdf l Choose Renormalization and Factorization scales (set equal) l Rsep: maximum separation allowed between two partons to form a jet (mimic exp. algorithm) Rsep=1. 3 R (Snowmass: Rsep=2. 0 R) 1. 3 R 2 R
Dijet Cross Section Phys. Rev. Lett. 82, 2457 (1999)
Cross Section Ratio Submitted to PRL: hep-ex/9807014 l Calculate Ratio of Cross Sectio l Two different angular regions Model with LL coupling
Quark-Quark Compositeness Limit on size of preons is fempto-meters
Conclusions l No evidence for Compositeness found at the Tevatron l Standard Model (QCD) in excellent agreement with the data l Quark-Quark Compositeness l > 2 to 3 Te. V depending on models
Numerous other QCD studies to probe scattering dynamics W, Z + +. . . q(x) W/Z PT, W/Z+Jets in High E Limit Color Flow Photons Diffraction etc. . .
Measurement of a. S 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 a. S(ET) • a. S(ET) is evolved to a. S(MZ) using 2 -loop renormalization group equation • Systematic uncertainties (~8%) from understanding of calorimeter response • Measured value consistent with world average of a. S(MZ)=0. 119± 0. 004 New measurement of a. S by a single experiment & from a single observable over a wide range of Q 2.
Conclusions l Standard Model (QCD) in excellent agreement with the data l No evidence for Compositeness of quarks found at the Tevatron l Studies continue improving theory, detectors, and using better microscopes
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