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Experimental Results on Nucleon Structure Lecture II

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Experimental Results on Nucleon Structure Lecture II Experimental results on nucleon structure Lecture II Barbara Badelek University of Warsaw National Nuclear Physics Summer School 2013 Stony Brook University, July 15 – 26, 2013 Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 1 / 86 Course literature Outline 1 Course literature 2 Introduction 3 Nucleon elastic form factors Basic formulae Form factor measurements Radiative corrections 4 Parton structure of the nucleon Feynman parton model Partons vs quarks Introducing gluons Parton distribution functions EMC effect Fragmentation functions Sum rules 5 “Forward” physics Phenomena at low x Diffraction Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 2 / 86 Introduction Outline 1 Course literature 2 Introduction 3 Nucleon elastic form factors Basic formulae Form factor measurements Radiative corrections 4 Parton structure of the nucleon Feynman parton model Partons vs quarks Introducing gluons Parton distribution functions EMC effect Fragmentation functions Sum rules 5 “Forward” physics Phenomena at low x Diffraction Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 3 / 86 Nucleon elastic form factors Outline 1 Course literature 2 Introduction 3 Nucleon elastic form factors Basic formulae Form factor measurements Radiative corrections 4 Parton structure of the nucleon Feynman parton model Partons vs quarks Introducing gluons Parton distribution functions EMC effect Fragmentation functions Sum rules 5 “Forward” physics Phenomena at low x Diffraction Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 4 / 86 Nucleon elastic form factors Basic formulae Outline 1 Course literature 2 Introduction 3 Nucleon elastic form factors Basic formulae Form factor measurements Radiative corrections 4 Parton structure of the nucleon Feynman parton model Partons vs quarks Introducing gluons Parton distribution functions EMC effect Fragmentation functions Sum rules 5 “Forward” physics Phenomena at low x Diffraction Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 5 / 86 Nucleon elastic form factors Basic formulae Rutherford scattering In 1910 – 1911, Ernest Rutherford + students: H. Geiger and E. Marsden First exploration of atomic structure: small, massive, positive nucleus and negative charge around it Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 6 / 86 Nucleon elastic form factors Basic formulae Probing the structure of the proton From: M.A. Thomson, Michaelmas Term 2011 Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 7 / 86 Nucleon elastic form factors Basic formulae Elastic electron – nucleon scattering Rutherford scattering: a particle ze of E scatters off Ze at rest and changes its momentum vector by #: dσ (zeZe)2 = dΩ 2 2 4 # R (4π0) (4E) sin 2 This formula is nonrelativistic; target recoils is neglected (= target is very heavy), i.e. E = E0; j~pj = j~p0j; j~qj = j~kj = 2j~pj sin #=2 A relativistic formula( E ≈ j~pjc) and z = 1: dσ Z2α2( c)2 = ~ (4) dΩ 2 4 # R 4E sin 2 At relativistic energies, Rutherford formula is modified by spin effects. If electron relativistic and its spin included =) Mott cross-section (still no recoil): dσ ∗ dσ # 4Z2α2( c)2E02 # = · 1 − β2 sin2 = ~ cos2 (5) 4 dΩ Mott dΩ R 2 j~qcj 2 (an asterisk means the recoil of the nucleus is neglected) For β !1, # = π supressed on spinless target (a consequence of a helicity, h, conservation, h = ~s~p=j~sjj~pj). Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 8 / 86 Nucleon elastic form factors Basic formulae Elastic electron – nucleon scattering...cont’d Mott expression agrees with the data for j~qj ! 0(# ! 0) but at higher j~qj experimental cross-sections are smaller: a form factor! dσ dσ ∗ = · jF (~q 2)j2 (6) dΩ exp dΩ Mott (for spherically symmetric systems, the form factor depends on ~q only! Determination of a form factor: measure of dσ=dΩ at fixed E and diferent # (= various j~qj) and divide by the Mott cross- section. Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 9 / 86 Nucleon elastic form factors Basic formulae Elastic electron – nucleon scattering...cont’d First measurements at SLAC in early 50-ties, Ee = 0.5 GeV. Z Define a charge distribution function f by ρ(~r) = Zef(~r) so that f(~r)d3r = 1. Then the form factor: Z F (~q 2) = ei~q~r=~f(~r)d3r (7) but only under conditions: no recoil, Ze small (or Zα 1)! For spherically symmetric cases f depends only on r = j~rj. Then Z sin(j~qjr= ) Z 1 F (~q 2) = 4π f(r) ~ r2dr 1 = 4π f(r)r2dr j~qjr=~ 0 The radial charge distribution, f(r) cannot be determined from the inverse Fourier transform of F (~q2) due to limited interval of measured values of j~qj. Thus a procedure of finding F (~q 2): choose parameterisation of f(r), calculate F (~q 2) and vary its parameters to get best fit to data. Observe: Z 1 dF (~q2) hr2i = 4π r2f(r)dr = −6 2 (8) ~ 2 0 d~q ~q2=0 Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 10 / 86 Nucleon elastic form factors Basic formulae Examples of form factors and charge densities From: M.A. Thomson, Michaelmas Term 2011 Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 11 / 86 Nucleon elastic form factors Basic formulae Form factors of the nucleons Studies of nucleon structure (r ∼ 0.8 fm) demand E & 1 GeV; thus nucleon recoil can no longer be neglected. With recoil: dσ dσ ∗ E0 = · (9) dΩ Mott dΩ Mott E Also: a Q2 = −q2 = 4EE0 sin2 #=2 needed instead of ~q 2 in the Mott cross-section Apart of e-N Coulomb interaction, now also electron current - nucleon’s magnetic moment. e Reminder: a pointlike, charged particle of spin 1/2 has a magnetic moment: µ = g ~ 2M 2 (g=2 from relativistic Q.M.). Magnetic interaction associated with a flip of the nucleon spin. Scattering at # = 0 not consistent with helicity (and angular momentum) conservation; scattering at # = π is preferred. Thus: dσ dσ # Q2 = 1 + 2τ tan2 ; τ = (10) 2 dΩ point;spin1=2 dΩ Mott 2 4M New, magnetic term (above) is large at large Q2 and #. Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 12 / 86 Nucleon elastic form factors Basic formulae Form factors of the nucleons...cont’d Anomalous magnetic moments for nucleons: e~ e~ µp = +2:79::: · = +2:79::: · µN µn = −1:91::: · = −1:91::: · µN (11) 2M 2M −14 where µN = 3:1525 · 10 MeV/T = nuclear magneton. Charge and currrent distributions described by two (Sachs) form factors (Rosenbluth): " 2 2 2 2 # dσ dσ GE (Q ) + τGM (Q ) 2 2 2 # = · + 2τGM (Q ) tan (12) dΩ dΩ Mott 1 + τ 2 2 2 Here: GE (Q ) and GM (Q ) are the electric and magnetic form factors. 2 2 2 At very low Q , GE (Q ) and GM (Q ) are Fourier transforms of the charge and magnetization current densities inside the nucleon. At Q2 ! 0: p n GE = 1;GE = 0 (13) p n GM = 2:79;GM = −1:91 2 2 How do we measure (separate) GE (Q ) and GM (Q ) ? Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 13 / 86 Nucleon elastic form factors Form factor measurements Outline 1 Course literature 2 Introduction 3 Nucleon elastic form factors Basic formulae Form factor measurements Radiative corrections 4 Parton structure of the nucleon Feynman parton model Partons vs quarks Introducing gluons Parton distribution functions EMC effect Fragmentation functions Sum rules 5 “Forward” physics Phenomena at low x Diffraction Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 14 / 86 Nucleon elastic form factors Form factor measurements Measurement of form factors – Rosenbluth method Independent measurement 2 2 of GE (Q );GM (Q ) needed. This is done at fixed Q2 and different # (or energies E). The measured cross-section is then divided by σMott. Example of results: 2 2 Slope of the line yields GM (Q ), intercept at # = 0 =) GE (Q ) For a long time it seemed that: Gp (Q2) Gn (Q2) Gp (Q2) = M = M = E 2:79 −1:91 Q2 −2 = G (Q2) = 1 + D 0:71(GeV=c)2 Fig. from the book of B.Povh et al. Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 15 / 86 Nucleon elastic form factors Form factor measurements Measurement of form factors – Rosenbluth method...cont’d All published Rosenbluth separation data for the proton: 2 2 p At Q &1 GeV ;GE inaccurately determined... p 2 2 ...but GM errors small up to Q ∼ 30 GeV . Neutron data (from elastic and break-up ed ecattering) of poorer quality. Old paradigm (until 1998 ! JLAB): proton from factors similar, and close to GD. www.scholarpedia.org/article/Nuclear_Form_factors Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 16 / 86 Nucleon elastic form factors Form factor measurements Measurement of form factors – polarisation transfer method 1998: a new method of form factors determination (coincided with opening of the JLAB): polarisation observables instead of cross-sections. 1) ~ep ! e~p or 2) ~e~p ! ep www.scholarpedia.org/article/Nuclear_Form_factors Barbara Badelek (Univ. of Warsaw ) Experimental results on nucleon structure, II NNPSS 2013 17 / 86 Nucleon elastic form factors Form factor measurements Measurement of form factors – polarisation transfer...cont’d 1998: a new method of form factors determination (coincided with opening of the JLAB): polarisation observables instead of cross-sections. 1) ~ep ! e~p or 2) ~e~p ! ep Polarisation of the recoil proton contains terms proportional to Gp Gp p E M so that GE may be determined even when it is small.
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