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Marco Stratmann

INT workshop, Seattle, 11/19/10 The science case for an EIC. Spin and Flavor Structure of the Nucleon compelling bread & butter physics at an EIC. Marco Stratmann. marco@bnl.gov. wiki–page : https://wiki.bnl.gov/eic/index.php/Nucleon_Spin_and_Imaging.

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Marco Stratmann

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  1. INT workshop, Seattle, 11/19/10 The science case for an EIC Spin and Flavor Structure of the Nucleon compelling bread & butter physics at an EIC Marco Stratmann marco@bnl.gov wiki–page : https://wiki.bnl.gov/eic/index.php/Nucleon_Spin_and_Imaging

  2. based on presentations on PDF studies by & discussions with S. Alekhin, E. Aschenauer, J. Blümlein, A. Cooper-Sarkar, M. Diehl, A. Guffanti, K. Kumar, S. Moch, P. Nadolsky, F. Olness, M. Pfeuffer, R. Sassot, M. Savage, H. Spiesberger, W. Vogelsang THANK YOU !

  3. main theme: HERA an unfinished business 16yrs of data taking leave a rich legacy of knowledge & by now textbook results (steep rise of F2; small-x gluons, diffraction, e-w effects, photoproduction, spin structure, … ) so, what did we miss which is still of interest in 2020+ ? spin structure “only” studied in fixed-target regime (HERMES) only proton beams – neutron structure ? – nuclei ? L = 500 pb-1 and variation of Ep not sufficient to really study FL JLab12? LHC? completely unfold flavor & spin structure: strangeness & s – s asymmetry ? - d/u and the gluon @ large-x ? concepts/processes introduced but neither fully explored nor understood: GPDs, unintegrated PDFs, diffraction, role of heavy flavors, photoproduction, electroweak physics in ep, semi-inclusive processes, … . . . considerable overlap with physics agenda of the LHeC

  4. path to address & answer open PDF issues at an EIC uniqueness feasibility relevance

  5. key: large & variable kinematic coverage E. Aschenauer, T. Burton

  6. kinematics – work in progress • find out how low in y we can go • increase x,Q2 coverage • more overlap between different • energy settings • more lever-arm for Q2 evolution • at fixed x • upper y cut has much less impact • QED radiative corrections needs to be studied in detail but expected to be under good control Monte Carlo tools at hand scatt. electron tracking @ EIC is a big advantage compared to HERA • known to be significant at HERA • devise strategies to control them • i.e., reconstruct true x, Q2 reliably • explore different methods to reconstruct • x,Q2 (“electron”, “Jacquet-Blondel”, “combined”) • more relevant for eA (?) Aschenauer, Spiesberger

  7. Opportunities in inclusive (un)polarized DIS

  8. polarized DIS and impact on Δg(x,Q2) strategy to quantify impact: global QCD fit with realistic toy data W2 > 10GeV2 W2 > 10GeV2 • DIS data sets produced for stage-1 [5x50, 5x100, 5x250, 5x325] and 20x250, 30x325, … • DIS statistics “insane” after 1 month of running (errors MUCH smaller than points in plots)

  9. what can be achieved for Δg? x recall: • low x behavior unconstrained • no reliable error estimate • for 1st moment • (entering spin sum rule) • find DIS & pp DSSV global fit de Florian, Sassot, MS, Vogelsang RHIC pp pQCD scaling violations positive Dg

  10. what can be achieved for Δg? – cont’d how effective are scaling violations at the EIC… Sassot, MS χ2profile slims down significantly already for EIC stage-1 (one month of running) DSSV+ includes also latest COMPASS (SI)DIS data (no impact on DSSV Δg) • with 30x325 one can reach down to x ≈ 3×10-5(impact needs to be studied)

  11. what can be achieved for Δg? – cont’d what about the uncertainties on the x-shape … Sassot, MS … wow – cool! • even with flexible DSSV x-shape we can now determine up to ± 0.07 • work in progress: try weird x-shapes below x = 10-4 to improve/check error estimate

  12. other opportunities in polarized DIS • in 10+ years the NNLO corrections will be available • (needed to match precision of data) Moch, Vogt, … Bartels, Ermolaev, Ryskin; Greco, Troyan; … • watch out for surprises at small-x = deviations from DGLAP • (expected to set in earlier than in unpol. DIS; showing up as tension in global fit (?)) • strong coupling from scaling violations (needs to be worked out / quantified) • Bjorken sum rule: • CBj known to O(αs4) Kodaira; Gorishny, Larin; Larin, Vermaseren; Baikov, Chetyrkin, Kühn, ... • but not a tool to determine αs(1% change in αs translates in 0.08% change of Bj sum ) • experimental challenge: effective neutron beam (3He), very precise polarimetry, … • theor. motivation for precision measurement: Crewther relation non-trivial relation of two seemingly unrelated quantities Adler function D(Q2) in e+e- Bj sum CBj(Q2) in DIS deviation from exact conformal symmetry

  13. unpolarized DIS at an EIC • precision data for F2 may help to resolve some issues with old fixed target data • (nice to have, but only “incremental” with little impact; cannot beat HERA at small x) • longitudinal structure function FL- basically missed at HERA (fixed Ee, Ep) interesting for several reasons: • hard to get; recall •  contributes mainly at large y (= low x for a given Q2) strategies: • indirect measurement from deviation of σr from “F2 only fit” • slope of y2/Y+for different S at fixed x and Q2strength of an EIC • FL starts only at O(αs) (due to helicity conservation) this is the LO expression

  14. longitudinal structure function FL - cont’d best motivation for a precise measurement at the EIC in 10+ years is not to determine the gluon density but to understand pQCD series • known up to three loops (NNLO) Moch, Vermaseren, Vogt • leading small x term • appears first at NNLO • sensitivity to small x term • at lowish Q2 values (few GeV2)

  15. 1st feasibility study for E. Aschenauer 5x50 - 5x325 running FL “slopes” (examples) TO DO: refine & test how well we can extract FL with high precision

  16. Semi-inclusive DIS &Flavor Separation

  17. selected open issues in flavor structure strangeness was identified to be one of the least known quantities – both unpolarized and polarized – where significant progress is unlikely w/o the EIC DSSV (incl. all latest COMPASS data) NNPDF collaboration data • surprise: Δs small & positive from SIDIS data • but 1st moment is negative and sizable due • to “constraint” from hyperon decays (F,D) • (assumed SU(3) symmetry debatable M. Savage) • drives uncertainties on ΔΣ (spin sum) • substantial uncertainties • known issues with HERMES data at large x • hot topic: we really need to determine it ! (as well as their u,d quark colleagues)

  18. flavor separation with semi-inclusive DIS at LO: extra weight for each quark allows for full flavor separation if enough hadrons are studied complications/additional opportunities: actual analysis of data requires NLO QCD where x, z dependence is non-trivial • PDF information entangled with fragmentation functions • should be not a problem: already known pretty well (DSS), more data (Belle, LHC, …) • EIC: if needed, can play with x & z integration/binning to reduce uncertainties • (needs to be studied in more detail) relevant quantities/measurements: • (un)polarized SIDIS cross sections (we don’t want to study asymmetries anymore at an EIC) • for u, ubar, d, dbar, s, sbar separation need H = π+, π-, K+, K- (nice to have more)

  19. 1st studies done for charged kaons Aschenauer, MS compute K+ yields at NLO with 100 NNPDF replicas z integrated to minimize FF uncertainties (work in progress) 5×250 GeV actual uncertainties much smaller than points one month of running PYTHIA agrees very well (despite different hadronization) --> confidence that we can use MC to estimate yields & generate toy data

  20. kaon studies – cont’d how about K- (relevant for separation) next step: assess impact of data on PDFs with “reweighting method” (using full set of stage-1 energies: 5×50 – 5×325) Giele, Keller; NNPDF to do: include also π±; polarized SIDIS and impact on global fit

  21. Charged & Neutral Current Probes

  22. main objective / why interesting at high enough Q2 electroweak probes become relevant • neutral currents (γ, Z exchange, γZ interference) • charged currents (W exchange) parameterized by new structure functions which probe combinations of PDFs different from photon exchange --> flavor decomposition without SIDIS, e-w couplings hadron-spin averaged case: studied to some extent at HERA (limited statistics) Wray; Derman; Weber, MS, Vogelsang; Anselmino, Gambino, Kalinowski; Blumlein, Kochelev; Forte, Mangano, Ridolfi; … hadron-spin difference: contains e-w propagators and couplings unexplored so far – unique opportunity for an EIC

  23. what can be learned in the parton model (for simplicity) NC: CC: requires a positron beam • NLO QCD corrections all available • can be easily put into global QCD analysis • enough combinations for a flavor separation (no fragmentation) de Florian, Sassot; MS, Vogelsang, Weber; van Neerven, Zijlstra; Moch, Vermaseren, Vogt

  24. feasibility – 1st exploratory studies Q2 > 1 GeV2 Ringer, Vogelsang no y cut y > 0.1 20×250 HERA 30×325 2nd indep. study: Kumar, Riordan, Deshpande, Taneja, Paschke

  25. feasibility – cont’d Ringer, Vogelsang 20 × 250 GeV Q2 > 1 GeV2 0.1 < y < 0.9 10 fb-1 DSSVPDFs very promising! even doable with 5x250 GeV

  26. feasibility – cont’d Ringer, Vogelsang 20 × 250 GeV Q2 > 1 GeV2 0.1 < y < 0.9 10 fb-1 DSSVPDFs NC electron beam TO DO: refine studies & quantify impact on global PDF fits compare with independent studies by Kumar, Riordan, Deshpande, Taneja, Paschke

  27. other avenues to be explored further • accessing fundamental electroweak parameters aq mainly constrained by xF3γZ vq mainly constrained by F2Z Can we do better than HERA ? What does it take (energy, luminosity)? needs to be investigated

  28. other avenues to be explored further – cont’d • SIDIS through e-w boson exchange π, K some studies available from “Future Physics at HERA” workshops: Maul, Contreras, Ihssen, Schafer; Contreras, De Roeck, Maul (based on PEPSI Monte Carlo) TO DO: re-do for EIC kinematics • CC charm production as a probe of strangeness idea: at O(αs0) at O(αs1) can potentially spoil sensitivity to strangeness also, need to keep full dependence on charm mass in EIC kinematics • NLO available (pol + unpol) Kretzer, MS • again, studies performed for HERA • gluon channel suppressed for z > 0.2 • in D meson production Δs < 0 Δs ≈ 0 errors assume 500pb-1 TO DO: exhume codes & re-do for the EIC

  29. Heavy Flavors

  30. treatment of heavy quarks ( = getting used to acronyms) heavy quarks: mQ >> ΛQCD(i.e., charm, bottom, top) • no mass singularities -> no evolving, genuine heavy quark PDFs • asymptotically large logarithms in DIS different ways to treat heavy quarks in calculations: (use charm as an example) • fixed flavor-number scheme FFNS • only u, d, s, g are active partons; charm produced though • NLO parton-level MC (HVQDIS) Harris, Smith • zero mass variable flavor-number scheme ZM-VFNS • standard evolution with massless partons above “threshold” Q = mc • general mass variable flavor-number scheme GM-VFNS • attempt to match two distinct theories (nf=3+mc vs. nf=4) • needs some matching & “interpolating” coefficient fcts. • details matter in global fits ! not a priori clear if / where logs matter

  31. treatment of heavy quarks – cont’d each PDF group has its own favorite scheme: CTEQ: ACOT, ACOT-χ, S-ACOT, S-ACOT-χ; MSTW: TR, TR’; NNPDF: FONLL; ABKM: BMSN but VFNS must be derived from FFNS: relations between nf and nf+1 partons Buza, Matiounine, Smith, van Neerven; Bierenbaum, Blümlein, Klein; …. BMSN construction for F2charm : (used by Alekhin, Blümlein, Klein, Moch) exact massive part mc ≠ 0 zero mass part mc = 0 ln Q/mc resummed asymptotic part ln Q/m another issue: quark masses in PDF fits • choice of mc part of uncertainty • all fits use pole mass • consistently lower than PDG value • future: use running mass in DIS fits • (work in progress Alekhin, Moch)

  32. heavy quarks – do they ever become “light” long-standing question … (example from ‘94 Glück, Reya, MS) mc= 0 mc= 0 mc≠ 0 mc≠ 0 • even at high Q2 or W2, mc = 0 approx. not effective • no smooth transition/matching • existing HERA data described well with mc ≠ 0 • differences more dramatic for FLc • (never measured) target for an EIC

  33. expectations for F2c and FLc ABKM (S. Alekhin) • shown: • F2C BMSN • (close to mc ≠ 0) • FLC is not small • (mc ≠ 0) TO DO: collect predictions for F2c, FLc from all PDF groups

  34. 1st feasibility study – reduced cross section (charm) E. Aschenauer 5x50 - 5x325 running for charm (via D mesons) bin needs 30x325 “FL slopes” for fixed x,Q2 TO DO: refine & test how well we can extract FLc

  35. intrinsic charm ? Brodsky, Hoyer, Peterson, Sakai can we finally settle this? M. Guzzi, P. Nadolsky, F. Olness (work in progress)

  36. charm contribution to g1 • so far safely ignored: << 1% to existing g1 fixed-target data • relevance at an EIC depends strongly on size of Δg • need massive Wilson coefficients (charm not massless for most of EIC kinematics) • so far only known to LO (NLO is work in progress Kang, MS) some expectations: (need to be studied in detail) very small (1-2% of g1uds) ≈ 2x10-5 ≈ 2x10-3 10-15% of g1uds

  37. Photoproduction

  38. main objective / why interesting • make use of bulk of events sitting at low Q2 • access to non-perturbative structure of photons why should I bother about yet another non perturbative function ? • needed for consistent factorization in all processes with quasi-real photons • ILC has a program for γγ physics perhaps even with polarization • unpolarized photon structure not well known: LEP γ*γ DIS, some HERA data • (a global analysis was never performed; no error estimates) • polarized photon structure is completely unknown • non-trivial inhomogeneous Q2 evolution • (due to pointlike coupling of photons to quarks) • pQCD framework more involved than for DIS-type processes

  39. photoproduction basics cross sections consist of two contributions, e.g. at parametrically of “direct photon” contribution “resolved photon” contribution need to be added for physical cross sections linked through factorization • most processes of interest (charm, hadrons, jets, photons) are known to NLO (pol+unp) • strategies to enhance sensitivity to resolved part known from HERA: • single-inclusive: need to look into rapidity dependence • di-jets: can define resolved sample (LO only)

  40. example I for EIC kinematics TO DO: work out in detail estimate uncertainties • polarized photon structure from 1-jet production • (very similar: 1-hadron production Jäger, MS, Vogelsang) pTjet > 4 GeV 10x250 GeV 1-jet different assumptions about Jäger arXiv:0807.0066 lepton proton xγ << 1 probes unknown photon PDFs xγ ≈ 1 probes proton PDFs

  41. example II for EIC kinematics • unpolarized photoproduction of charm H. Spiesberger (ongoing work) sum direct resolved 5×325 GeV 3 < pT < 5 GeV

  42. photon content of protons photons are also part of protons: • source for isospin violation • only one PDF analysis available Martin, Roberts, Stirling, Thorne • extraction in ep depends on how QED radiative corrections are treated • (defines the factorization scheme) • needed for consistent factorization of electroweak higher • order corrections (collinear photon radiation) • --> relevant for precision calculations for the LHC Diener, Dittmaier, Hollik; … interest twofold at an EIC (work in progress) C. Pisano; M. Pfeuffer, A. Schafer, W. Vogelsang • step 1: can we extract them at the EIC? • step 2: important background to Drell-Yan process in ep • (can we control it ?)

  43. Summary “Golden PDF Measurements”

  44. looks like we can deliver a pile of new results (but we still need to brush up our appearance)

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