Neutrons in a Spin: Nucleon Structure at Jefferson Lab

Transcription

1 Neutrons in a Spin: Nucleon Structure at Jefferson Lab Daria Sokhan University of Glasgow, UK on behalf of the CLAS Collaboration IoP Nuclear Physics Group Conference, York 8 th April 2013

2 Nucleon structure Lepton (eg: electron, neutrino) scattering off a nucleon reveals different aspects of nucleon structure. Elastic Scattering e γ e' N N ' Cross-section parameterised in terms of Form Factors Transverse quark distributions: charge, magnetisation.

3 Charge density inside a nucleon Proton Neutron G. Miller, PRL 99, (2007) negative inner core positive outer surface

4 Deep Inelastic Scattering e e' γ N First experimental evidence of partons inside a nucleon Cross-section parameterised in terms of Structure Functions Longitudinal momentum distributions of partons

5 Parton Distribution Functions x f(x) Momentum distributions of quarks and gluons within a nucleon x: longitudinal momentum of parton as a fraction of nucleon s momentum x

6 Deep Exclusive Reactions Generalised Parton Distributions (GPDs) relate transverse position of partons, b, to their longitudinal momentum. Tomography: 3D image of the nucleon.

7 The Nucleon Spin Puzzle What contributes to nucleon spin? 1980 s: European Muon Collaboration (EMC) measures contribution of valence quarks to proton spin to be ~ 30 %. Subsequent deep inelastic scattering (DIS) experiments confirm. Where is the rest? Proton spin crisis! Quark spin: extracted from helicity distributions measured in polarised DIS. 1 1 J N= = Σ+ Lq+ G+ 2 2 Quark orbital angular momentum: can be accessed via GPDs, which contain information on total angular momentum, J q. L g Gluon spin: measurements of DIS and polarised proton collisions indicate G is very small. Caveat: The three terms not yet accessible within a single formalism.

8 Deeply Virtual Compton Scattering One of the cleanest experimental processes in which GPDs can be accessed is Deeply Virtual Compton Scattering (DVCS). e p γ N ( Q 2 ) x+ξ e' q ~ E, E ~, H, H ( x, ξ, t) γ x ξ t N' p' Q 2 2 = q = ( e e' ) Bjorken variable x B x±ξ : longitudinal momentum 2 fractions of struck quark xb ξ 2 x B t 2 2 Q = 2 p q = ( p p') 2 At high exchanged Q 2, access to four GPDs: E q ~ ~, E, H, H ( x, ξ, t) q q q

9 Experimental extraction of GPDs DVCS and Bethe-Heitler (BH) experimentally indistinguishable. Process measured in experiment: e e ' γ* γ + e e ' γ* γ + e γ γ* e ' N N ' N N ' N N ' DVCS Bethe - Heitler dσ T DVCS 2 + T BH 2 + T BH T * DVCS + T DVCS T * BH Amplitude parameterised in terms of Compton Form Factors Amplitude calculable from elastic Form Factors and QED! Interference term T 2 << DVCS T BH 2

10 Experimentally, access Compton Form Factors (CFF), which are combinations of GPDs integrated over x and functions of GPDs at x=ξ. Only ξ and t are thus experimentally directly accessible! T DVCS GPDs x ( x, ξ ± ξ, t ) dx ± iπ GPDs ( ± ξ, ξ, t ) + K Cross-sections σ Cross-section differences (asymmetries) σ

11 Extracting Compton Form Factors For example, from DVCS spin asymmetries: γ φ Beam, target polarisation e - p/n A LU = r dσ r dσ + s dσ s dσ = σ LU r s dσ+ dσ ~ σ LU ~ sinφ Im{F 1 H + ξ(f 1 +F 2 )H - kf 2 E}dφ ξ = x B /(2-x B ) k = t/4m 2 e e γ leptonic plane Proton hadronic plane p Neutron ~ Im{H p, H p, E p } ~ Im{H n, H n, E n } e - e - σ UT ~ cosφ Im{k(F 2 H F 1 E) +.. }dφ ~ σ LL ~ (A+Bcosφ) Re{F 1 H+ξ(F 1 +F 2 ) (H + x B /2E) }dφ Im{H p, E p } Im{H n } ~ Re{H p, H p } Re{H n, E n, E ~ n }

12 Neutron DVCS GPDs from proton and neutron: flavour separation Neutron DVCS extremely sensitive to E, least-known and least-constrained GPD. e - n Polarized beam, unpolarized neutron target: ~ σ LU ~ sinφ Im{F 1 H + ξ(f 1 +F 2 )H - kf 2 E}dφ e e γ γ leptonic plane hadronic plane n ~ H n, H n, E n φ Suppressed because F 1 (t) is small Suppressed because of cancellation between PDF s of u and d quarks Ji s Sum Rule : J 1 = 1 J 1 g = 2 2 { H ( x, ξ,0) E ( x, ξ,0) } q xdx 1 q + q 1 1 J N= = Σq+ Lq+ 2 2 J g Important missing link in the nucleon spin puzzle!

13 Jefferson Lab (Virginia, USA) CEBAF: Continuous Electron Beam Accelerator Facility: Duty cycle: ~ 100% Energy up to ~6 GeV Electron polarisation up to ~85% CLAS in Hall B: Drift chambers Toroidal magnetic field Cerenkov Counters Scintillator Time of Flight Electromagnetic Calorimeters Extremely large angular coverage

14 Neutron DVCS: Eg1-dvcs experiment Data taken: Feb Sept 2009 Beam: polarised electrons E e = 4.7 to 6 GeV polarisation ~ 85% Longitudinally polarised targets: NH3 (95 days) ND3 (33 days) Proton / neutron pol. ~ 80 / 40 % r r e + d e' + γ + n + ( ps ) CLAS plus Inner Calorimeter (IC) Exclusive reconstruction of e, N, and γ. Spectator proton identified via missing mass. high-energy forward photon detection

15 Particle ID in CLAS q and p from track-curvature through drift chambers in magnetic field Separation from π - : on basis of energy deposit in electromagnetic calorimeter (EC) and number of photoelectrons produced in Cerenkov counters (CC). β from neutral particles time of # of photoelectrons (x10) in CC flight to EC π - e - β > 0.95: photons β < 0.95: neutrons Forward, low-angle photons in additional Inner Calorimeter

16 Reaction Identification Select kinematic region where GPD formalism holds: Energetic photon and recoil nucleon: E γ > 1 GeV Q 2 > 1 GeV 2 p n > 0.4 GeV/c W > 2 GeV/c 2 where W is the missing mass of ( en e' X ), remove resonance region of remaining γn Additionally, require: Coplanarity between γ and N Missing momentum from ed e' N'γ Should be low for spectator nucleon in quasi-free reaction Missing mass from the above reaction should correspond to mass of nucleon. X γ cone angle Difference between calculated and measured γ direction should be low.

17 DVCS on different targets Free proton F.-X. Girod et al, PRL. 100 (2008) H2 n Calculate DVCS on a free neutron NH3 Free proton in nuclear medium ND3 Quasi-free proton in deuterium and in heavier nuclear medium ND3 Quasi-free neutron in deuterium and in heavier nuclear medium

18 A LU check on proton DVCS in NH 3 and ND 3 Fit: NH3 PRELIMINARY!!! p0 sinϕ A= 1+ p cosϕ 1 ND3 PRELIMINARY!!! Previously measured result onh 2 is in range F.-X. Girod et al, PRL. 100 (2008) N P( N + + N + N 0.23± 0.02 ) Uncorrected for π 0 contamination N P( N + + N + N ) actual A LU larger! Deuterium target smearing due to Fermi motion requires wider data cuts. 0.16± 0.02 π 0 contamination more significant measured A LU lower than on NH3.

19 A LU and A UL in neutron DVCS on ND 3 Beam-spin asymmetry: One previous measurement from Hall JLab, A LU ~ 0. Big statistical and systematic uncertainties, slightly different kinematic region. (M. Mazouz et al, PRL 99 (2007) ) Fit: P RE L I M I NA RY!!! A LU = p 0 sinϕ Uncorrected for π 0 contamination, includes neutrons from N! p ± 0.06 p1 0.11± 0.06 N P( N + + N + N 0.06± 0.02 ) Target-spin asymmetry: First measurement! PR EL I MI NAR Y!!! Fit: A UL = p 0 sinϕ+ p0 sin 2ϕ

20 Jefferson 12 GeV CEBAF: Continuous Electron Beam Accelerator Facility, upgrade from current 6 GeV to 12 GeV underway. Add new hall Open up much larger phase space in Q 2 and x B Hall B 11 GeV to the upgraded detector system CLAS12 Scheduled completion ~ 2014

21 A LU in Neutron 11 GeV E e = 11 GeV J u = 0.3, J d = -0.1 J u = 0.3, J d = 0.1 J u = 0.1, J d = 0.1 J u = 0.3, J d = 0.3 A LU At 11 GeV, beam spin asymmetry (A LU ) in neutron DVCS is very sensitive to J u, J d Wide coverage needed! VGG Model (calculations by M. Guidal) φ( o ) Fixed kinematics: x B = Q 2 = 2 GeV t 2 = 0.4 GeV

22 CLAS12 Design luminosity L ~ cm -2 s -1 Acceptance for charged particles: Central (CD) 40 o < θ < 135 o Forward (FD) 5 o < θ < 40 o Central Detector Acceptance for photons: IC 2 o < θ < 5 o EC 5 o < θ < 40 o High luminosity & large acceptance: Concurrent measurement of deeply virtual exclusive, semi-inclusive, and inclusive processes Forward Detector PCAL

23 Recoil DVCS neutrons in CLAS12 Beam-spin asymmetry in neutron DVCS at 11 GeV extremely sensitive to J q en e ' n ' γ Exclusive reconstruction of the DVCS process require detection and measurement of all three final state particles. P N (GeV) P N Vs θ N θ N (deg) Simulation at E e = 11 GeV Over 80% of neutrons recoil at θ lab > 40 with peak momentum at ~ 0.4 GeV/c. Requires central neutron detector sensitive to 0.2 < p n < 1.2 GeV/c.

24 Neutron Detector for CLAS12 y Available: 10 cm of radial space in a high magnetic field (~ 5T) Detector proposal approved: Plastic scintillator barrel: 3 layers, 48 paddles in each Length ~ 70 cm, inner radius 28.5 cm Long (~ 1.5 m) light-guides PMT read-out upstream, out of high B field z x Light guides U-turn light guide PMT 1 PMT 2 Scintillators

25 CND Simulation (Geant 4) Neutron efficiency ~ 8-9 % Good separation of neutrons and γ up to ~ 1 GeV/c σ p p 5 12 % σ θ 2 o 3 1 3% contamination from misreconstructed hits β = v/ c θ = 60º p (GeV/c) β Detector under construction at IPN Orsay, France, for 2014.

26 In Conclusion: GPDs provide a 3D image of the internal dynamics of the nucleon and are experimentally accessible in exclusive reactions such as DVCS. More on testing GPD predictions in talk by J. Sjögren: Mon 14.45, Session 2 Beam-spin asymmetry in DVCS on the neutron, particularly in the kinematic range opening up with CLAS12, will offer vital information on the composition of nucleon spin. The Central Neutron Detector is under construction to allow exclusive reconstruction of neutron DVCS with CLAS12 at Jefferson Lab. A preliminary extraction of DVCS on 6GeV is underway indications of a low measurable beam-spin and target-spin asymmetry on the neutron.

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28 Back-up slides

29 Particle ID Photons and Neutrons βfrom neutral particles time of flight to EC Forward, low-angle photons in additional Inner Calorimeter β < 0.95: neutrons β > 0.95: photons Hits in IC with E deposit > 1 GeV Neutrons: p n β m = n 1 β 2 n 2 n E = m + p 2 n Photons: p γ = E deposited in calorimeter

30 Neutron DVCS in ND 3 data cuts II p n > 0.4 GeV/c Recoiling nucleon should not have a low p φ < 10 Coplanarity between γ andn γ cone angle < 5 Difference between calculated and measured γ direction φ γ cone angle p x Missing momentum from ed e' N'γ Should be low for spectator nucleon in quasi-free reaction X

31 First measurement of ndvcs: Hall A M. Mazouz et al., PRL 99 (2007) E e = 5.75 GeV/c P e = 75 % L = cm -2 s -1 /nucleon e HRS e LH 2 / LD 2 target γ Electromagnetic Calorimeter (PbF 2 ) Active nucleon identified via missing mass Analysis done in the impulse approximation: D ( e, e γ ) X = p( e, e γ ) p + n( e, e γ ) n + d ( e, e γ ) d +L Q 2 = 1.9 GeV 2 x B = GeV 2 < -t < 0.5 GeV 2 Subtraction of quasi-elastic proton contribution deduced from H 2 data convoluted with initial motion of the nucleon Twist-2

32 ndvcs in Hall A: results M. Mazouz et al., PRL 99 (2007) F. Cano, B. Pire, Eur. Phys. J. A19 (2004) 423 Q 2 = 1.9 GeV 2 - x B = 0.36 Model dependent extraction of J u and J d S. Ahmad et al., PR D75 (2007) VGG, PR D60 (1999) Im(C I n ) compatible with zero ( too high x B?) Strong correlation between Im[C I d ] and Im[CI n ] Big statistical and systematic uncertainties (mostly coming from H 2 and π 0 subtraction)

33 A LU from neutron DVCS with CLAS12 ~ r e + d e' + n + γ + 80 days of data taking L = cm 2 s 1 /nucleon CLAS12 + Forward Calorimeter + Neutron Detector Model predictions (VGG) for different values of quarks angular momentum: ( ps ) -0.2 BSA 0.2 σ LU ~ sinφ Im{F 1 H + ξ(f 1 +F 2 )H -kf 2 E}dφ The most sensitive observable to the GPD E -t J u =.3, J d =.1 J u =.1, J d =.1 J u =.3, J d =.3 J u =.3, J d =-.1 φ