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Rolf Ent (Jefferson Lab) Baryons 2013 Glasgow, UK June 27

Semi-Inclusive Charged-Pion Electro-production off Protons and Deuterons: Cross Sections, Ratios and Transverse Momentum Dependence. Rolf Ent (Jefferson Lab) Baryons 2013 Glasgow, UK June 27. Pre-Amble.

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Rolf Ent (Jefferson Lab) Baryons 2013 Glasgow, UK June 27

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  1. Semi-Inclusive Charged-Pion Electro-production off Protons and Deuterons: Cross Sections, Ratios and Transverse Momentum Dependence Rolf Ent (Jefferson Lab) Baryons 2013 Glasgow, UK June 27

  2. Pre-Amble Semi-Inclusive Charged-Pion Electro-production off Protons and Deuterons: Cross Sections, Ratios and Transverse Momentum Dependence • HERMES data established the potential for semi-inclusive DIS (SIDIS) • JLab/Hall C’s basic SIDIS cross section data at a 6-GeV JLab showed agreement with partonicexpectations and hints at a flavor dependence in transverse momentum dependence, laying the foundation for a vigorous 12-GeV SIDIS program. • T. Navasardyan et al., Phys. Rev. Lett. 98 (2007) 022001; • H. Mkrtchyan et al., Phys. Lett. B665 (2008) 20; • R. Asaturyan et al., Phys. Rev. C 85 (2012) 015202. • Also M. Osipenko et al. (CLAS), Phys. Rev. D 80 (2009) 032004. • Recently also extensive set of unpolarized SIDIS cross section data from both HERMES and COMPASS: • A. Airapetyan et al., Phys. Rev. D 87 (2013) 074029. • C. Adolph et al., arXiv:1305.7317v1 (2013).

  3. Outline Semi-Inclusive Charged-Pion Electro-production off Protons and Deuterons: Cross Sections, Ratios and Transverse Momentum Dependence • Semi-Inclusive Deep Inelastic Scattering – Introduction • Towards a Partonic Description • Semi-Inclusive Deep Inelastic Scattering – Formalism • Transverse Momentum Dependence – Flavor Dependence • Unpolarized SIDIS Cross Section Measurements @12 GeV Charged Pionsand Neutral Pions

  4. Structure functions, quark longitudinal momentum & helicity distributions Proton form factors, transversecharge & current densities Beyond form factors and quark distributions Generalized Parton and Transverse Momentum Distributions 1990’s Correlated quark momentum and helicity distributions in transverse space - GPDs 2000’s Extend longitudinal quark momentum & helicity distributions to transverse momentum distributions - TMDs

  5. The road to orbital motion Swing to the left, swing to the right: A surprise of transverse-spin experiments The difference between the p+, p–, and K+ asymmetries reveals that quarks and anti-quarks of different flavor are orbiting in different ways within the proton. dsh ~ Seq2q(x) dsfDfh(z) Sivers distribution

  6. 1 DS + Lq + Jg 2 1 2 The Incomplete Nucleon: Spin Puzzle = • DS ~ 0.25 (world DIS) • DG small (RHIC+DIS) • Lq? Longitudinal momentum fraction x and transverse momentum images Longitudinal momentum fraction x and transverse spatial images Up quark Sivers Function 12 GeV projections: valence quarks well mapped

  7. SIDIS – Flavor Decomposition DIS probes only the sum of quarks and anti-quarks  requires assumptions on the role of sea quarks Solution: Detect a final state hadron in addition to scattered electron  Can ‘tag’ the flavor of the struck quark by measuring the hadrons produced: ‘flavor tagging’ SIDIS z= Eh/n Mx2 = W’2 ~ M2 + Q2 (1/x – 1)(1 - z) Measure inclusive (e,e’) at same time as (e,e’h) • Leading-Order (LO) QCD • after integration over pT andf • NLO: gluon radiation mixes • x and z dependences • Target-Mass corrections at large z • ln(1-z) corrections at large z : parton distribution function : fragmentation function

  8. E00-108 Experiment in Hall C/JLab x ~ 0.3, Q2 ~ 2.3 (GeV/c)2 0.2 < x < 0.6, 2 < Q2 < 4, 0.3 < z < 1 1) Probe p+ and p- final states 2) Use both proton and neutron (deuteron) targets 3) Combination of precise cross sections and ratios allows confirmation of interpretation in terms of convolution of quark distribution and fragmentation function 4) Combination allows, naively, a separation of quark kt-widths from fragmentation pt-widths (if sea quark contributions small) D region Mx2 = W’2 ~ M2 + Q2 (1/x – 1)(1 - z) Convolution of CTEQ5 quark distribution and BKK fragmentation function Mx2 z = Eh/n

  9. How Can We Verify Factorization? • Neglect sea quarks and assume no ktdependence to parton distribution functions • Fragmentation function dependence drops out in Leading Order [sp(p+) + sp(p-)]/[sd(p+) + sd(p-)] = [4u(x) + d(x)]/[5(u(x) + d(x))] ~ sp/sdindependent of z and kt [sp(p+) - sp(p-)]/[sd(p+) - sd(p-)] = [4u(x) - d(x)]/[3(u(x) + d(x))] independent of z and kt, but more sensitive to assumptions

  10. E00-108: Onset of the Parton Model GRV & CTEQ, @ LO or NLO Good description for p and d targets for 0.4 < z < 0.65 (Note: z = 0.65 ~ Mx2 = 2.5 GeV2) Closed (open) symbols reflect data after (before) events from coherent r production are subtracted

  11. p quark E00-108: Onset of the Parton Model Collinear Fragmentation Seq2q(x) Dqp(z) (Deuterium data) factorization (Resonances cancel (in SU(6)) in D-/D+ ratio extracted from deuterium data)

  12. Resonances cancel in D-/D+ ratio extracted from deuterium! From deuterium data: D-/D+ = (4 – Np+/Np-)/(4Np+/Np- - 1) F. Close et al : SU(6) Quark Model How many resonances does one need to average over to obtain a complete set of states to mimic a parton model? 56 and 70 states o.k. for closure Destructive interference leads to factorization and duality

  13. E00-108: Onset of the Parton Model in SIDIS Solid (open) symbols are after (before) subtraction of diffractive revents x = 0.32 N-D region CTEQ5M x = 0.4 Phys. Rev. C85: 015202 (2012) Curves are partonmodel calculations using CTEQ5M parton distributions at NLO and BKK fragmentation functions. Agreement with the partonmodel expectation is always far better for ratios, also for D/H, Al/D, or for ratios versus z or Q2. Bodes well for SIDIS at 12 GeV dv/uv extracted from differences and ratios of p+ and p- cross sections off H and D targets

  14. New Observable Reveals Interesting Behavior of Quarks 1stmeasurement of 3He (neutron) single-spin asymmetries (SSA) Measurement of Sivers & Collins SSA’s in X. Qianet al., PRL 107, (2011) 072003 1st measurement of ALT beam-target double-spin asymmetry J. Huang et al., PRL 108, (2012) 052001 Target: (transversely) polarized 3He ~ polarized neutron • Indications: • A non-vanishing quark “transversal helicity” distribution, revealsalignment of quark spin transverse to neutron spin direction • Quark orbital motions

  15. m p x TMD SIDIS – kT Dependence Final transverse momentum of the detected pion Pt arises from convolution of the struck quark transverse momentum kt with the transverse momentum generated during the fragmentation pt. TMDu(x,kT) f1,g1,f1T ,g1T h1, h1T ,h1L ,h1 Pt= pt +zkt+ O(kt2/Q2) Linked to framework of Transverse Momentum Dependent Parton Distributions

  16. m p X TMD Transverse momentum dependence of SIDIS Linked to framework of Transverse Momentum Dependent Parton Distributions s Unpolarized target Longitudinally pol. target TMDq(x,kT) Transversely pol. target UnpolarizedkT-dependent SIDIS: in framework of Anselmino et al. described in terms of convolution of quark distributions f and (one or more) fragmentation functions D, each with own characteristic (Gaussian) width  Emerging new area of study • Basic precision cross section measurements: • Crucial information to validate theoretical understanding • Convolution framework requires validation for most • future SIDIS experiments and their interpretation • Can constrain TMD evolution • Questions on target-mass corrections and ln(1-z) re-summations require precision large-z data f

  17. SIDIS Formalism General formalism for (e,e’h) coincidence reaction with polarized beam: [A. Bacchetta et al., JHEP 0702 (2007) 093] (y = azimuthal angle of e’ around the electron beam axis w.r.t. an arbitrary fixed direction) Use of polarized beams will provide useful azimuthal beam asymmetry measurements (FLU) at low PT If beam is unpolarized, and the (e,e’h) measurements are fully integrated over f, only the FUU,T and FUU,L responses, or the usual transverse (sT) and longitudinal (sL) cross section pieces, survive. Unpolarized kT-dependent SIDIS: FUUcos(f) and FUUcos(2f), in framework of Anselmino et al. described in terms of convolution of quark distributions f and (one or more) fragmentation functions D, each with own characteristic (Gaussian) width.

  18. Transverse momentum dependence of SIDIS General formalism for (e,e’h) coincidence reaction with polarized beam: [A. Bacchetta et al., JHEP 0702 (2007) 093] (y = azimuthal angle of e’ around the electron beam axis w.r.t. an arbitrary fixed direction) Azimuthal fh dependence crucial to separate out kinematic effects (Cahn effect) from twist-2 correlations and higher twist effects. data fit on EMC (1987) and Fermilab (1993) data assuming Cahn effect→ <m02> = 0.25 GeV2 (assuming m0,u =m0,d)

  19. Hall C: Transverse momentum dependence E00-108 Pt dependence very similar for proton and deuterium targets, but deuterium slopes systematically smaller? Pt dependence very similar for proton and deuterium targets

  20. Unpolarized SIDIS – Simple Analysis Constrain kT dependence of up and down quarks separately 1) Probe p+ and p- final states 2) Use both proton and neutron (d) targets 4) Combination allows, in principle, separation of quark width from fragmentation widths (if sea quark contributions small) Example 1st example: Hall C, Phys. Lett. B665 (2008) 20 Numbers are close to expectations! But, simple model only with many assumptions (factorization valid, fragmentation functions do not depend on quark flavor, transverse momentum widths of quark and fragmentation functions are gaussian and can be added in quadrature, sea quarks are negligible, assume Cahn effect, etc.), incomplete cos(f) coverage, uncertainties in exclusive event & diffractive r contributions. x = 0.32 z = 0.55 Example <pt2> (favored) <kt2> (up)

  21. Unpolarized SIDIS – Transverse Momentum Warning: we used here an overly simplistic model analysis in an early effort to show the perspective of Pt-dependent SIDIS experiments. For instance, the assumption of Cahn dominance may not be justified. But, the Pt dependence of D seems shallower than H, with an intriguing explanation in terms of flavor/ktdeconvolution. • An alternate analysis was performed in • Schweitzer, Teckentrup and Metz, PRD 81 (2010) 094019 • Gauss model for Pt distributions • - Do not assume kinematic dominance of Cahn effect • Showing consistency of CLAS, Hall C, HERMES data • Gaussian approach also describes Drell-Yan data, giving • credence to the factorization approach used Warning again: a gaussian approach can formally not be correct

  22. Transverse momentum dependence of SIDIS Gaussian approach of Schweitzer, Teckentrup and Metz, PRD 81 (2010) 094019 E00-108 CLAS x = 0.32 Gauss: <Ph (z)>2 = p/4 <Ph2(z)> HERMES Curves are from the Gauss model with the Gauss width fixed from CLAS data (also consistent with CLAS)

  23. Transverse momentum dependence of SIDIS Intrinsic value of SIDIS to establish transverse momentum widths of quarks with different flavor and polarization now well established (and they can be different). Steps towards QCD evolution taken. Need precision at large z to validate fragmentation process, verify target-mass correction and ln(1-z) re-summation, etc. CLAS COMPASS Avakian et al., PRL 105 (2010) 262002 Adolph et al., arXiv:1305.7317v1 (2013) Double Spin Asymmetry

  24. Transverse momentum dependence of SIDIS Intrinsic value of SIDIS to establish transverse momentum widths of quarks with different flavor and polarization now well established (and they can be different). Steps towards QCD evolution taken. Need precision at large z to validate fragmentation process, verify target-mass correction and ln(1-z) re-summation, etc. HERMES Hall C Airapetian et al., PRD 107 (2013) 074029 Asaturyan et al., PRC 105 (2012) 015202 Solid (open) triangles: Cornell data @ x = 0.24 & x = 0,50

  25. Hall C SIDIS Program – basic (e,e’p) cross sections (At a 12-GeV JLab, Hall C’s role will be again to provide basis SIDIS cross sections.) HERMES PRD87 (2013) 074029 Low-energy (x,z) factorization, or possible convolution in terms of quark distribution and fragmentation functions, at JLab-12 GeVmust be well validated to substantiate the SIDIS science output. Many questions at intermediate-large z (~0.2-1) and low-intermediate Q2 (~2-10 GeV2) remain. Solid (open) symbols are after (before) subtraction of exclusive revents Why need for (e,e’p0) beyond (e,e’p+/-)? (e,e’p0):  no diffractive r contributions  no exclusive pole contributions  reduced resonance contributions  proportional to average D Ratio of after (before) subtraction of exclusive revents

  26. JLabUnpolarizedSIDIS Program Kinematics Accessible Phase Space for SIDIS (and Deep Exclusive Scattering) at 12-GeV JLab 11 GeV phase space E12-13-007 Neutral pions: Scan in (x,z,PT) Overlap with E12-09-017 & E12-09-002 Charged pions: 6 GeV phase space E12-06-104 L/T scan in (z,PT) No scan in Q2 at fixed x: RDIS(Q2) known Parasitic with E12-13-010 E12-09-017 Scan in (x,z,PT) E00-108 + scan in Q2 at fixed x E12-09-002 + scans in z Typical z range: 0.2 to 0.7 (up to 1.0 for smaller Mx2)

  27. R = sL/sT in SIDIS (ep e’p+/-X) Only existing data: Cornell 70’s data RDIS Conclusion: “data consistent with both R = 0 and R = RDIS” Some hint of large R at large z in Cornell data? RDIS (Q2 = 2 GeV2) Example projections given for E12-06-104 assuming RSIDIS = RDIS

  28. E12-09-017 Projected Results - Kaons III IV II VI I V

  29. Semi-Inclusive Charged-Pion Electro-production off Protons and Deuterons: Cross Sections, Ratios and Transverse Momentum Dependence • Hall C/E00-108 1,2H(e,e’p+/-) cross section data provided the foundation of the SIDIS framework in terms of convolution at lower energies. • Agreement with parton model expectations is always far better for ratios. • Transverse momentum dependence of cross section (and asymmetry data) led to consideration of flavor dependence. • Now the stage of precision data enters, to provide answers to questions of 1) experimental issues such as r contributions, L/T ratios, etc. • 2) flavor dependence of transverse momentum widths • (and fragmentation functions) • 3) QCD evolution and ln(1-z) re-summation • At a 12-GeV JLab precision unpolarized SIDIS experiments approved for: • Measurement of ratio R = sL/sT in SIDIS (E12-06-104) • Measurement of Transverse Momentum Dependence of • Charged-Pion and Kaon Production (E12-09-017) • Precise Measurement of Charged-Pion Ratios to High Q2 (E12-09-002) • Measurement of Semi-Inclusive Neutral-Pion Production (E12-13-007)

  30. E12-09-017 Projected Results - Pions III IV II VI I V

  31. p quark R = sL/sTin (e,e’p) SIDIS Knowledge on R = sL/sT in SIDIS is essentially non-existing! • If integrated over z (and pT, f, hadrons), RSIDIS = RDIS • RSIDIS = RDIS test of dominance of quark fragmentation • RSIDIS may vary with z • At large z, there are known contributions from exclusive • and diffractive channels: e.g., pions from D and r p+p- • RSIDIS may vary with transverse momentum pT • Is RSIDISp+ = RSIDISp- ? Is RSIDISH = RSIDISD ? • Is RSIDISK+ = RSIDISp+ ? Is RSIDISK+ = RSIDISK- ? E2-06-104 measure kaons too! (with about 10% of pion statistics) “A skeleton in our closet” Seq2q(x) Dqp(z)

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