Neutron structure with spectator tagging at MEIC

Transcription

1 Neutron structure with spectator tagging at MEIC C. Weiss (JLab), Users Group Workshop 2014, JLab, 03 Jun 14 Light ion physics with EIC e D pol. e p, n High energy process Forward spectators detected Physics objectives Polarized deuterium Spectator nucleon tagging Forward p/n tagging with deuterium Free neutron through on-shell extrapolation Kinematic reach in x,q 2 Ion polarization L, T, tensor Forward detection of p,n,d Precision measurements using ed with forward tagging Bound nucleon structure Polarization Shadowing and coherence at x 0.1 R&D status and prospects Forward detection with JLab MEIC JLab 2014 LDRD project

2 Light ions: Energy, luminosity, polarization 2 Luminosity [cm -2 s -1 ] Mainz Bates JLab12 Bonn CEIC 1 SLAC HERMES CEIC 2 MEIC 1 COMPASS µp E665 EMC/NMC EIC designs MEIC 2 HL-RHIC erhic HERA ep e CM energy [GeV] 2 Q, x ep / µp facilities LHeC CM energy GeV/nucleon MEIC 1. Higher energy upgrade Q 2 few 10 GeV 2 for DIS x for sea quarks, gluons Luminosity cm 2 s 1 Exceptional configurations in target Multi-variable final states Polarization effects Polarized light ions erhic: unpol D, pol 3 He MEIC: polarized D and 3 He Figure-8 design A. ep physics program 2012 White Paper, reviews. Talk A. Deshpande

3 Light ions: Physics objectives 3 n Neutron structure Flavor decomposition of quark spin, sea quarks ū, d, gluon polarization g How to account for binding, polarization, final state interactions? Bound nucleon in QCD Modification of basic quark/gluon structure by nuclear medium, QCD origin of nuclear forces How to control nuclear environment? Coherence and saturation Interaction of high energy probe with coherent quark/gluon fields How to quantify onset of coherence? Signatures of saturation? [Nucleus rest frame view] Challenges to be addressed by theory and new experimental techniques!

4 Light ions: Deuterium and spectator tagging 4 Polarized deuterium p S = 1 Wave function simple, known incl. Light-cone wave function for high energy processes Neutron spin polarized n + D wave Limited possibilities for nuclear final state interaction Coherent effects at N = 2 Complementary to saturation in large nuclei Spectator nucleon tagging e e Detection of forward proton or neutron High energy process Identifies active nucleon, controls quantum state D pol. p, n Forward spectators detected Unique for collider: No target material, forward detection of charged/neutral p s, polarized ion beams Tagging with fixed target: CLAS BONUS, limited to recoil momenta p R > 100 MeV

5 Spectator tagging: Extracting neutron structure 5 Light-cone momentum of recoil proton ( t MN) 2 D n t ( p p ) = R D 2 X p α R, on shell point F2n ( x, Q 2 ) prt α R 2 = E R + p z R E D + p z D and p RT Cross section in impulse approximation Frankfurt, Strikman 81 dσ dx dq 2 (dα R /α R ) d 2 p RT = flux factor ( ) x S D (α R,p RT ) F 2n,Q 2 2 α R Deuteron LF spectral fn Neutron structure fn +... d σ/ d [..] 0.1 GeV 2 On-shell extrapolation t M 2 N Cf. Chew Low extrapolation in πn, N N scattering Free neutron structure at pole m N 2 t = function(α R,p RT ) t Pole value not affected by FSI Sargsian, Strikman 05: No-loop theorem Model independent method!

6 Spectator tagging: EIC projections JLab LDRD project 6 dσ / dx dq 2 (d 3 p R / E R ) [nb/gev 4 ] t min = Tagged cross section ed e +p+x α R = x = 0.01, Q 2 = 50 x = 0.001, Q 2 = 5 α R = 0.01, t = Neutron active, proton tagged Integrated luminosity 10 6 nb -1 t t m N 2 F 2D (x, Q 2 ; α R, t ) (-t / Res) Tagged structure function with pole factor removed -t min = α R = x = 0.01, Q 2 = 50 x = 0.001, Q 2 = 5 α R = 0.01, t = Neutron active, proton tagged Integrated luminosity 10 6 nb t [GeV 2 ] -t [GeV 2 ] Stat errors based on integrated lumi 10 6 nb 1 2 weeks at cm 2 s 1 Overall error systematics-dominated, full MC simulation in progress On-shell extrapolation appears feasible Extrapolation smooth after taking out nucleon pole factor Test universality: t dependence at different α R Excellent rates for spin/flavor asymmetries

7 Spectator tagging: Applications 7 Unpolarized neutron structure F n 2,F n L Isovector p n at x < 10 1 constrains sea quark flavor asymmetry d ū Bound proton through neutron spectator tagging Compare tagged SF at t = m 2 N with free proton measurement to validate method Quantify nuclear binding effect on quark/gluon distributions through t dependence: Connection with short-range N N correlations? Neutron spin structure functions g n 1,g n 2 in progress Isoscalar p + n for G, especially at large x Isovector p n for u d Bjorken sum rule: Fundamental quantity, tests high-order pqcd calculations Cleanest possible extraction of neutron spin structure! Other DIS final states: Semi-inclusive, exclusive, DVCS

8 Spectator tagging: Coherent effects 8 01 X Shadowing in inclusive DIS x n p X n p interference Diffractive scattering on single nucleon Leading-twist effect! Seen at HERA Interference between scattering on p and n Calculable: Gribov 70 s. Frankfurt, Guzey, Strikman 02+ Shadowing in tagged DIS Clean coherent effect with N = 2 Essential for systematics in p n Also polarized. Needs to be controlled! R(x,Q 2,p) p 0.9 z =0 p t = p t =50 MeV/c p t =100 MeV/c Frankfurt, Guzey, Strikman 11. Guzey LDRD 2014 x Strong low-energy FSI between p and n distorts p T spectrum, spin/isospin dep. Guzey, Strikman, CW; in progress 01 X S p T

9 MEIC: Full acceptance detector [Slide P. Nadel-Turonski] 9 Design goals Detection/identification of complete final state IR Optics Acceptance and resolution for forward protons, fragments, neutrons Low Q 2 electron tagger for photoproduction More information at

10 MEIC: Far-forward detection [Slide P. Nadel-Turonski] 10 Good acceptance for all ion fragments rigidity different from beam Large magnet apertures (small gradients at a fixed maximum peak field) Roman pots not needed for spectators and high-p T fragments Good acceptance for low p T recoils rigidity similar to beam Small beam size at detection point (downstream focus, efficient cooling) Large dispersion (generated after the IP, D = D = 0 at the IP) With 10σ beam size cut, the low p T recoil proton acceptance is Energy up to 99.5% of the beam for all angles Angular down to 2 mrad for all energies Good momentum and angular resolution Should be limited only by initial state (beam) Longitudinal dp/p: Angular in θ, for all φ: 0.2 mrad p RT 15MeV/c resolution for tagged 50 GeV/A deuterium beam Long, instrumented drift space (no apertures, magnets, etc.) Sufficient beam line separation ( 1 m)

11 MEIC: Momentum spread in beam 11 nominal crossing angle Intrinsic momentum spread in ion beam smears recoil momentum electron p R (measured) p R (vertex) ion δ p D, δ θ Dominant uncertainty for MEIC Larger than detector resolution. Different for erhic! Actual distribution 2 of t = t m N at vertex x = Q = 15 20GeV 2 PRELIMINARY At nominal MEIC emittance δp D /p D = δθ = Effect on t = t M 2 N Dominant effect from ion δθ Smearing width bin size Ch. Hyde, K. Park et al.: JLab LDRD project On-shell extrapolation appears feasible!

12 LDRD: Polarized light ions with 12 JLab FY14 LDRD Project D. Higinbotham, W. Melnitchouk, P. Nadel Turonski, K. Park, C. Weiss (JLab), Ch. Hyde (ODU), M. Sargsian (FIU), V. Guzey (PNPI), with collaborators W. Cosyn (Ghent), S. Kuhn (ODU), M. Strikman (PSU), Zh. Zhao (JLab) Objectives Develop physics models for DIS processes on polarized light ions (D, 3He) with spectator tagging Develop event generators for MC simulations of inclusive, diffractive and exclusive final states Simulate processes with schematic modeling of EIC beam/detector/ir characteristics Analyze pseudodata and quantify physics output of spectator tagging Resources 50% FTE experimental physics postdoc, shared with ODU: Kijun Park Theory collaborators as long-term visitors in Summer 2014: Sargsian, Guzey, Cosyn 10% FTE of JLab Staff: Weiss, Higinbotham Collaboration Open for collaboration with Users! Physics models and generators to be made available Extension to other processes of interest possible More information on Wiki

13 LDRD: Status and next steps Physics models Unpolarized ed e + N + X with nuclear binding, final-state interactions theory+codes ready, testing/documentation in progress 13 Unpolarized ed e + pn + X with diffraction/shadowing theory+code ready, low-energy final-state interaction in progress Polarized ed e + N + X theory+code developing, Polarized e 3 He N + X scheduled (summer 14) MC generators FSGEN-based generator (nucleus rest frame) adapted from fixed-target code New generator developed for collider kinematics (detector frame), including intrinsic momentum spread of beam particles Codes available on github, testing/documentation in progress Polarized beams, diffractive final state ed e + pn + X scheduled (summer 14) Process simulations On-shell extrapolation in ed e + N + X Effect of intrinsic momentum spread Hookup to GEMC detector MC to study tracking, acceptance Physics extraction from pseudodata, extension to polarized ed (summer 14)

14 Summary 14 Next-generation nuclear DIS measurements enabled by Polarized deuterium beam Forward p, n detection Unique combination! EIC kinematic reach R&D to establish forward tagging at MEIC as standard method Theory: Polarization, final state interactions,... Simulations: Acceptance, tracking, systematic errors,... Users welcome! Natural continuation of JLab 6/12 GeV nuclear program: BONUS, SRCs, EMC effect, DVCS, exclusive processes Tools made available: Physics models, event generators Increasing interest