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The nucleon structure, what an Electron-Ion Collider will teach us

The nucleon structure, what an Electron-Ion Collider will teach us. QCD the nearly perfect theory. “Emergent” phenomena not evident from Lagrangian Asymptotic Freedom & Color Confinement Non- perturbative structure of QCD vacuum Gluons: mediator of the strong interactions

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The nucleon structure, what an Electron-Ion Collider will teach us

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  1. The nucleon structure, what an Electron-Ion Collider will teach us DNP, Hawaii, October 2009

  2. QCD the nearly perfect theory • “Emergent” phenomena not evident from Lagrangian • Asymptotic Freedom & Color Confinement • Non-perturbative structure of QCD vacuum • Gluons: mediator of the strong interactions • Determine essential features of strong interactions • Dominate structure of QCD vacuum (fluctuations in gluon fields) • Responsible for > 98% of the visible mass in universe (!) DNP, Hawaii, October 2009 0.2 fm 0.02 fm0.002 fm

  3. Measure Glue through DIS Kinematics: Measure of resolution power Measure of inelasticity Measure of momentum fraction of struck quark Quark splits into gluon splits into quarks … Gluon splits into quarks Increasing resolution higher Q2 DNP, Hawaii, October 2009

  4. Measure Glue through DIS small x Observation of large scaling violations Gluon density dominates large x DNP, Hawaii, October 2009

  5. What do we know till know F2=C(Q2)x-l Q2 = 1GeV  0.2fm EM proton radius: 0.9fm x<0.01 gp Transition, Why, How? Does the rise of F2 set in at the same Q2 for nuclei? DNP, Hawaii, October 2009

  6. Linear BFKL Evolution Density along with c.s. grows as a power of energy: N ~ sΔ Can densities & cross-section rise forever? Black disk limit: σtotal≤ 2πR2 Issues with our Current Understanding • Linear DGLAP evolution scheme • strange behavior of xG from HERA at small x and Q2 • G(x,Q2) < Qsea(x,Q2) ? • Unexpectedly large diffractive cross-section • ~20%; energy independent DNP, Hawaii, October 2009

  7. at small x linear evolution gives strongly rising g(x) BK/JIMWLK non-linear evolution includes recombination effects saturation Dynamically generated scale Saturation Scale: Q2s(x) Increases with energy or decreasing x Scale with Q2/Q2s(x) instead of x and Q2 separately Parton Saturation EIC (eA): • Instead extending x, Q reach •  increase Qs • Pocket formula for nuclei • Remember: Question: What is the relation between saturation and the soft regime? Confinement? as~1 as << 1 HERA (ep): Despite high energy range: • F2, Gp(x, Q2) only outside the saturation regime • Regime where non-linear QCD (saturation phenomena) matter • (Q < Qs) not reached, but close • Only way in ep is to increase √s DNP, Hawaii, October 2009

  8. Beyond collinear pQCD • Suppose we view DIS in rest frame of target • γ* fluctuation into quark, anti-quark (dipole) frozen • w / radial separation r • Dipole interacts with proton/nuclei • Then DIS cross-section • Interesting physics in • What happens @ large r ? • In dipole picture saturates for r>R0 = 1/Qs • assume: • Use BFKL for x dependence of Geometric Scaling  works for proton and nuclei x < 0.01 DNP, Hawaii, October 2009

  9. FL: measures glue directly ⇒G(x,Q2) with great precision FL ~ αs G(x,Q2) requires √sscan Q2/xs = y Plot contains: ∫Ldt = 4/A fb-1(10+100) GeV = 4/A fb-1(10+50) GeV = 2/A fb-1(5+50) GeV statistical errors only • Syst. studies of FL(A,x,Q2) • xG(x,Q2) with great precision • Distinguish between models DNP, Hawaii, October 2009

  10. Diffractive physics: epvseA ? xIP = mom. fraction of pomeronw.r.t. hadron Curves: Kugeratski, Goncalves, Navarra, EPJ C46, 413 Diffraction: DIS: • HERA/ep: ~20% of all events are hard diffractive • Diffractive cross-section σdiff/σtot in e+A ? • Predictions: ~25-40%? • Diffractive structure functions • FLD for nuclei and p extremely sensitive • Exclusive Diffractive vector meson production: dσ/dt ~ [xG(x,Q2)]2 !! • Distinguish between linear evolution and saturation models DNP, Hawaii, October 2009

  11. VM production @ small x W &t dependences: probe transition from softhard regime r f J/Y U s ~ Wd s ~ e-b|t| steep energy dependence of s in presence of the hard scale universality of b-slope parameter: point-like configurations dominate DNP, Hawaii, October 2009

  12. Measure the Gluon Form Factor RA = 1.2A1/3fm Elastic scattering on full nuceus  long wavelength gluons (small t) Expectation for 1M J/y Requirement: Momentum resolution < 10MeV great t resolution Need to detect nuclei break-up products DNP, Hawaii, October 2009

  13. From DIS at HERA: At small-medium Q2, σ(NC) >> σ(CC) For Q2 > MZ2 and MW2, σ(NC) ~ σ(CC) EW Unification Already a textbook figure ... Unification • What about on the parton scale? • Small-x running-coupling BFKL • QCD evolution predicts: • QSapproaches universal behaviour • for all hadrons and nuclei • No dependence on A!! • Not only functional form f(QS) • universal, but even QSitself • becomes universal A.H. Mueller, hep-ph/0301109 • Radical View: • Nuclei and all hadrons have a component of their wave function with the same behaviour • This is a conjecture! Needs to be tested EW Unification DNP, Hawaii, October 2009

  14. Important to understand hadron structure: Spin DG SqLq Lg SqDq SqDq Lg SqLq dq DG dq Is the proton spinning like this? N. Bohr W. Pauli gluon spin “Helicity sum rule” Where do we go with solving the “spin puzzle” ? angular momentum total u+d+s quark spin DNP, Hawaii, October 2009

  15. Polarized Quark Distributions eRHIC: 10GeV@250GeV at 9 fb-1 0.8 0. 0.2 -0.8 DSSV: arXiv:0904.3821 DNP, Hawaii, October 2009 X

  16. The Gluon Polarization x RHIC range 0.05·x·0.2 can be extended a bit by changing √s to 64GeV & 500GeV small-x 0.001<x<0.05 large-x x>0.2 Dg(x) very small at medium x best fit has a node at x~0.1 huge uncertainties at small x Need to enlarge x-range Dg(x) small !? g*p D0 + X DNP, Hawaii, October 2009

  17. How to measure DS and DG • DG: Indirect from scaling violation g1@eRHIC Integrated Lumi: 5fb-1 DNP, Hawaii, October 2009

  18. Beyond form factors and quark distributions X. Ji, D. Mueller, A. Radyushkin (1994-1997) Proton form factors, transversecharge & current densities Structure functions, quark longitudinal momentum & helicity distributions Generalized Parton Distributions Correlated quark momentum and helicity distributions in transverse space - GPDs DNP, Hawaii, October 2009

  19. How to access GPDs? quantum number of final state selects different GPDs: • theoretically very clean • DVCS(g):H, E, H, E • VM(r, w, f):H E • info on quark flavors • PS mesons(p, h):H E ~ ~ ~ ~ DNP, Hawaii, October 2009

  20. Proton Tomography Allows for Transverse Imaging Fourier transform in momentum transfer x < 0.1 x ~ 0.3 x ~ 0.8 gives transverse spatial distribution of quark (parton) with momentum fraction x DNP, Hawaii, October 2009

  21. Proton Tomography probing partons with specified long. momentum @transverse position b T [M. Burkardt, M. Diehl 2002] FT (GPD) : momentum space  impact parameter space: polarized nucleon: u-quark d-quark [x=0] from lattice DNP, Hawaii, October 2009

  22. Results from Theory Lattice: K. Kumericki & D. Mueller arXiv: 0904.0458 contribution to nucleon spin CLAS BSA Hermes BCA First hints for a small JqLq What about the Gluons ? Hall A Hall A mp2 GeV2 different GPD parametrisations LHPC Collab. hep-lat/0705.4295 t=0 t=-0.3 DNP, Hawaii, October 2009

  23. DVCS @ eRHIC • dominated by gluon contributions • Need wide x and Q2 range to extract GPDs • Need sufficient luminosity to bin in multi-dimensions DNP, Hawaii, October 2009

  24. More insights to the proton - TMDs Explore spin orbit correlations Single Spin Asymmetries Unpolarized distribution function q(x), G(x) Transversity distribution function dq(x) First data on proton and deuterium targets from Hermes, Compass still kinematics and statistics limited Bell: results on Collins and IFF FF RHIC AN: no separation in underlying subprocesses dominated by gluon Siversdistribution function Boer-Muldersdistribution function Correlation between and Helicity distribution function Dq(x), DG(x) peculiarities of f^1T chiral even naïve T-odd DF related to parton orbital angular momentum violates naïve universality of PDFs QCD-prediction: f^1T,DY = -f^1T,DIS Correlation between and Correlation between and DNP, Hawaii, October 2009

  25. Sivers function and OAM Model dependent statement: Siversfct. from fit to M. Burkardt et al. anomalous magnetic moment: ku = 1.67 kd = -2.03 Lattice: P. Haegler et al. lowest moment of distribution of unpol. q in transverse pol. proton and of transverse pol. quarks in unpol. proton How are TMDs related to the multi parton correlations in CGC or ones based on an eikonalized DGLAP x Anselmino et al. arXiv:0809.2677 DNP, Hawaii, October 2009

  26. Summary DNP, Hawaii, October 2009 • EIC: a machine, which will allow to develop a unified picture from the nucleus to nuclei • only a small fraction of the physics program presented • beam energy variability and luminosity > Hera crucial for the physics program • International Interest in an EIC growing • 2 “proposals” in Europe: • very low energy: ENC @ FAIR (e: 3.5 GeV, p: 15GeV) • very high energy: LHeC @ CERN (e: 70 – 140GeV, p/A:LHC) • Upcoming events: • INT workshop October 19-23 on physics for MeRHIC • 2nd meeting with EIC-IAC @ JLab 1st & 2nd of Nov. • Collaboration meeting 10th – 1212th of January 2010 @ SBU

  27. EIC: one solution eRHIC @ BNL 5 mm 5 mm 5 mm 5 mm 20 GeV e-beam 16 GeV e-beam Common vacuum chamber 12 GeV e-beam 8 GeV e-beam 2 x 200 m SRF linac 4 (5) GeV per pass 5 (4) passes eRHIC detector Gap 5 mm total 0.3 T for 30 GeV Polarized e-gun 10-20 GeV ex 325 GeV p 130 GeV/u Au possibility of 30 GeV @ low current operation Beam dump MeRHIC detector Coherent e-cooler PHENIX STAR 4 to 5 vertically separated recirculating passes DNP, Hawaii, October 2009

  28. Detector Requirements from Physics DNP, Hawaii, October 2009 • ep-physics • the detector needs to cover inclusive (ep -> e’X)  semi-inclusive (ep -> e’hadron(s)X)  exclusive reactions (ep -> e’pp) • large acceptance absolutely crucial • particle identification (p,K,p,n) over wide momentum range • excellent vertex resolution (charm) • particle detection for very low scattering angle • around 1o in e and p/A direction  in big contradiction to high focusing quads close to IP • small systematic uncertainty for e/p polarization measurements • very small systematic uncertainty for luminosity measurement • eA-physics • requirements very similar to ep • most challenging get information on recoiling heavy ion from exclusive and diffractive reactions.

  29. First ideas for a detector concept Solenoid (4T) Dipol 3Tm Dipol 3Tm FPD FED // // ZDC / TRD • Dipols needed to have good forward momentum resolution • Solenoid no magnetic field @ r ~ 0 • DIRC, RICH hadron identification p, K, p • high-threshold Cerenkov  fast trigger for scattered lepton ~15m DNP, Hawaii, October 2009

  30. MeRHIC Detector in Geant-3 Thank you for your attention DNP, Hawaii, October 2009

  31. BACKUP DNP, Hawaii, October 2009

  32. The √s vs. luminosity landscape Diffraction exclusive DIS (PS & VM) electro-weak exclusive DIS (DVCS) semi-inclusive DIS inclusive DIS 20x100 20x250 10x100 4x100 DNP, Hawaii, October 2009

  33. The Nuclear Enhancement Factor • Enhancing Saturation effects: • Probes interact over distances L ~ (2mnx)-1 • For probes where L > 2RA (~ A1/3), cannot distinguish between nucleons in the front or back of of of the nucleus. • Probe interacts coherently with all nucleons. • Probes with transverse resolution 1/Q2 (<< Λ2QCD) ~ 1 fm2 will see large colour charge fluctuations. • This kick experienced in a random walk is the resolution scale. Simple geometric considerations lead to: Nuclear Enhancement Factor: Enhancement of QS with A:⇒ non-linear QCD regime reached at significantly lower energy in e+A than in e+p DNP, Hawaii, October 2009

  34. nDIS: Clean measurement in ‘cold’ nuclear matter Suppression of high-pT hadrons analogous to, but weaker than at RHIC Interaction of fast probes with gluonic medium Zh = Eh/ν RHIC Au+Au @ 200 GeV/n • Fundamental question: • When do partons get colour neutralized? Parton energy loss vs. (pre)hadron absorption Energy transfer in lab rest frame: EIC: 10 < ν < 1600 GeV HERMES: 2-25 GeV DNP, Hawaii, October 2009

  35. EIC: allows multi-differential measurements of heavy flavour Covers and extends energy range of SLAC, EMC, HERA, and JLab allowing for the study of wide range of formation lengths Charm measurements at an EIC Charm also suppressed at RHIC - above and beyond model predictions DNP, Hawaii, October 2009

  36. How to measure coherent diffraction in e+A? • Can measure the nucleus if it is separated from the beam in Si (Roman Pot) “beamline” detectors • pTmin ~ pAθmin • For beam energies = 100 GeV/n and θmin = 0.08 mrad: • These are large momentum kicks, much greater than the binding energy (~ 8 MeV) • Therefore, for large A, coherently diffractive nucleus cannot be separated from beamline without breaking up DNP, Hawaii, October 2009

  37. Large rapidity gaps at aneRHIC Diffractive events • Method: • Use RAPGAP in diffractive and DIS modes to simulate e+p collisions ateRHICenergies • Clear difference between DIS and Diffractive modes in “most forward particle in event” distributions • Little change in distributions with increasing energy Diffractive DIS DNP, Hawaii, October 2009

  38. Large rapidity gaps at aneRHIC • Efficiency vs Purity: • Efficiency = fraction of diffractive events out of all diffractive events in sample • Purity = fraction of diffractive events out of all events in sample • Possible to place a cut to have both high efficiency and high purity • However, reduce the acceptance by 1 or 2 units of rapidity and these values drop significantly • Need hermetic detector coverage!! Purity Efficiency DNP, Hawaii, October 2009

  39. Machine design options electron storage ring RHIC electron linear accelerator RHIC ✔ L x 10 DNP, Hawaii, October 2009 • two main design options for eRHIC • Ring-Ring: • Linac-Ring:

  40. Parton Propagation and Fragmentation eRHIC HERMES π • nDIS: • Suppression of high-pT hadrons analogous but weaker than at RHIC • eRHIC: Clean measurement in ‘cold’ nuclear matter • Energy transfer in lab rest frame • eRHIC: 10 < ν < 1600 GeV HERMES: 2-25 GeV • eRHIC: can measure heavy flavor energy loss • Work in Progress: • Simulation with PYTHIA 6.4.19 • 10 weeks of beam at eRHIC • 10+100 GeV • Large reach in Q2 and pT • small ν - hadronization inside A • large ν - precision tests of QCD • parton energy loss • DGLAP evolution and showers Nuclear Modification Measure: DNP, Hawaii, October 2009

  41. What is the nature of glue at high density? How do strong fields appear in hadronic or nuclear wave functions at high energies? Do gluon densities saturate? What drives saturation, what’s the underlying dynamics What are the appropriate degrees of freedom (Pomerons?) Does the Color Glass Condensate describe matter at low-x? Questions to Address with the eRHIC • Universality of gluon dynamics & energy dependence • Is there a “fixed” point where all hadronic matter have a component of their wave function with the same behavior • Could a better knowledge of glue help solve the longstanding problem of confinement in QCD? • What’s the role of gluons in the nuclear structure? DNP, Hawaii, October 2009

  42. 4 Key Measurements in e+A Physics • Momentum distribution of gluons in nuclei? • Extract via scaling violation in F2: ∂F2/∂lnQ2 • Direct Measurement: FL ~ xG(x,Q2) - requires √s scan • Inelastic vector meson production (e.g. J/Ψ,ρ) • Diffractive vector meson production (~ [xG(x,Q2)]2) • Space-time distribution of gluons in nuclei? • Exclusive final states (e.g. ρ, J,Ψ) • Deep Virtual Compton Scattering (DVCS) - σ ~ A4/3 • F2, FL for various impact parameters • Role of colour-neutral (Pomeron) excitations? • Diffractive cross-section: σdiff/σtot (~ 10%: HERA e+p; 30%? eRHICe+A?) • Diffractive structure functions and vector meson productions • Abundance and distribution of rapidity gaps • Interaction of fast probes with gluonic medium? • Hadronization, Fragmentation, Energy loss (charm!!) DNP, Hawaii, October 2009

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