What Have We Learned from Electron Deep Inelastic Scattering? Xiaochao Zheng Univ. of Virginia October 31, 2011 – Introduction – the four interactions, the Standard Model, and the role of electron scattering How is the nucleon energy shared among quarks? How are quarks polarized inside the nucleon? How do quarks behave in neutral weak interactions? – Summary prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 1/57 What Is the World Made of? Mass of the nucleon: 1 GeV ~ 10-10 Joules ~ (5x10-15m)-1 = (5 fm)-1 ~ (10-24 seconds)-1 ~ 10-24 grams, -35 (10 of Empire State Building) prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 2/57 And How Do They Interact? Electromagnetic 10-2 SU(2)xU(1) Weak 10-5 at low E group theory Strong 10-1~100 SU(3) QCD Gravitational 10-38 General relativity strong SU(3) Color SU(2) EW ? ? U(1) EW Gravitational weak (250 GeV ~ 5 x 1014 GeV ~ 2.4 x 1018 GeV)? prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 3/57 And How Do They Interact? Electromagnetic 10-2 SU(2)xU(1) Weak 10-5 at low E group theory Strong 10-1~100 SU(3) QCD Gravitational 10-38 General relativity Electromagnetic Interaction – Interaction of electric charge (QED) Carried by photons: massless, spin-1, electrically neutral Well understood, higher order processes correctable, tested to 10-9 accuracy vs. prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 4/57 And How Do They Interact? Electromagnetic 10-2 SU(2)xU(1) Weak 10-5 at low E group theory Strong 10-1~100 SU(3) QCD Gravitational 10-38 General relativity Weak Interaction – Interaction of Weak Charges Massive spin-1 bosons Z0, W±; Appears to be weak at low energies; Unified with electromagnetic interaction via the “mixing” of SU(2)xU(1); Violates parity symmetry; Tested to good precision, but is challenged by experiments (neutrino oscillations, etc.) prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 5/57 And How Do They Interact? Electromagnetic 10-2 SU(2)xU(1) Weak 10-5 at low E group theory Strong 10-1~100 SU(3) QCD Gravitational 10-38 General relativity Weak Interaction – Interaction of Weak Charges SU(3) Color (Grand Unification) SU(2) EW ? ? (Theory of Everything) U(1) EW Gravitational (250 GeV ~ 5 x 1014 GeV ~ 2.4 x 1018 GeV)? prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 6/57 And How Do They Interact? Electromagnetic 10-2 SU(2)xU(1) Weak 10-5 at low E group theory Strong 10-1~100 SU(3) QCD Gravitational 10-38 General relativity Weak Interaction – Interaction of Weak Charges Indirect searchSU(3) for Color (signs of) Beyond Direct search the Standard (LHC...) 7 TeV Model Physics, such ? ? as tiny changesSU(2) in EW ? ? interaction strengths, Gravitational eV-GeV U(1) EW Gravitational (250 GeV ~ 5 x 1014 GeV ~ 2.4 x 1018 GeV)? prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 7/57 And How Do They Interact? Electromagnetic 10-2 SU(2)xU(1) Weak 10-5 at low E group theory Strong 10-1~100 SU(3) QCD Gravitational 10-38 General relativity Strong Interaction – Interaction of 3 Color Charges (QCD) Gluons: massless, spin-1, but carries colors Strong force coupling strength Higher order processes gives: 1.5 small couplings at high energies (asymptotic freedom, perturbative QCD) 1.0 large couplings aS>1 at long distances (>0.1fm), calculation very difficult 0.5 Colored objects do NOT exist in Nature – experiments with free quarks and gluons not 0.0 possible; Color confinement cannot be 0 0.05 0.10 0.15 0.20 r(10-15m) computed in QCD (yet). prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 8/57 The Role of Electron-Nucleon Scattering Electromagnetic 10-2 SU(2)xU(1) Weak 10-5 at low E group theory Strong 10-1~100 SU(3) QCD Gravitational 10-38 General relativity Electron beam = a source of photons and Z0's g's: probe structure of the nucleon – how do quarks form the nucleon energy, mass, spin … … via strong interactions? Z's: parity violation electron scattering prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 9/57 What is the Nucleon Made of? The simple quark model of hadrons Gell-Mann (Nishijima) 1961-1964/1969 prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 10/57 Exploring Nucleon Structure Using Electron Scattering k '=E ' ,k ' W 2 =P' 2 t q= ,q Q 2=−q 2 k= E ,k P= M, 0 x The inclusive scattering “cross section” describes the probability of electrons being scattered to E' and Ω. d 2 σ = σ αF Q 2 ,ν +βF Q 2 ,ν d dE' Mott[ 1 2 ] For point-like target prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 11/57 Exploring Nucleon Structure Using Electron Scattering k '=E ' ,k ' W 2 =P' 2 q= ,q Q 2=−q 2 k= E ,k P= M, 0 prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 12/57 Exploring Nucleon Structure Using Electron Scattering Elastic, quasi-elastic, resonances, deep inelastic (Quasi-) elastic – the nucleus (nucleon) appears as a rigid body Resonance region – quarks inside the nucleon excited to resonance modes Deep Inelastic Scattering (DIS): start to see deep inside the nucleon (individual constituents) (highly non-pertubative, phenomenology models) prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 13/57 Exploring Nucleon Structure Using Electron Scattering Elastic, quasi-elastic, resonances, deep inelastic (Quasi-) elastic – the nucleus (nucleon) appears as a rigid body Resonance region – quarks inside the nucleon excited to resonance modes Deep Inelastic Scattering (DIS): start to see deep inside the nucleon (individual constituents) (highly non-pertubative, phenomenology models) prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 14/57 Exploring Nucleon Structure Using Electron Scattering Elastic, quasi-elastic, resonances, deep inelastic (Quasi-) elastic – the nucleus (nucleon) appears as a rigid body Resonance region – quarks inside the nucleon excited to resonance modes Deep Inelastic Scattering (DIS): Start to see deep inside the nucleon (individual constituents) prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 15/57 Virtual photon compared to (real, visible) photon k '=E ' ,k ' W 2 =P' 2 t q= ,q Q 2=−q 2 k= E ,k P= M, 0 Focus (spatial resolution): 1/Q2 x Time exposure 1/n prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 16/57 “Resonance” “Elastic” “Deep Inelastic” prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 17/57 Phys. Rev. D 66, 010001 (2002) ) /57 2 18 2 (GeV 2 Q X. Zheng, October 2011, Colloqium at UVa at Colloqium 2011, October Zheng, X. 0.1 1.0 10 10 10 1.0 0.1 0 3 2 1 2.5 1.5 0.5 3.5 F 2 ( x,Q2 )+c(x) ) 6 2 10 5 (GeV 2 n / Q 2 10 4 10 3 prepared as ppt file on daisy with openoffice.org3 with daisy on file ppt as prepared 10 2 From ElectronFrom DeepScattering Inelastic shifted according to Q to according shifted 0.1 1.0 10 10 10 1.0 0.1 1 0 3 2 5 4 8 7 6 9 10 F 2 ( x,Q2 )+c(x) From Electron Deep Inelastic Scattering F 2 ( 10 “scaling”: x , Q 2 9 ) What we observe is + c independent of the resolution ( 8 x ) of the photon probe (1/Q2) 7 6 5 the constituents must be 4 point-like particles, spaced far apart. 3 2 1 0 0.1 1.0 10 102 103 104 105 106 Q2(GeV2) Friedman, Kendall, Taylor et al. 1968/1990 prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 19/57 From Electron Deep Inelastic Scattering F 2 ( 10 x , “scaling violation” Q 2 9 ) 2 + Log(Q ) dependence c ( 8 x ) 7 6 5 the quarks are weakly 4 interacting inside the nucleon “Asymptotic Freedom” 3 2 4π 2 α Q = S 2 2 11−2n f /3 ln Q / Λ 1 0 t Hooft 1972/1999 0.1 1.0 10 102 103 104 105 106 Gross, Wilczek, Politzer 1972/2004 Q2(GeV2) Agree very well with perturbative QCD calculations prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 20/57 How Does the (inside of) Nucleon Look Like? The simple quark model of hadrons Gell-Mann (Nishijima) 1961/1964/1969 prepared as ppt file on daisy with openoffice.org3 X. Zheng, October 2011, Colloqium at UVa 21/57 Feynman's Quark Parton Model In DIS, x=Q2/(2Mn) P(nucleon) -> ∞ 0 1/3 1 x each quark carries x-fraction of P; 0 1/3 1 x Parton distribution function q(x): Probability to find the u, d, s... quarks carrying x-fractionprepared as ppt file on ofdaisy withthe openoffice.org3 nucleon'sX.