semi-inclusive dis experiments using bigbite and super bigbite spectrometers in hall a
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Semi-Inclusive DIS Experiments Using BigBite and Super BigBite Spectrometers in Hall A. Andrew Puckett University of Connecticut and Jefferson Lab SBS Collaboration Meeting July 7, 2014. Outline. Introduction—Semi-Inclusive DIS, TMDs, flavor tagging - PowerPoint PPT PresentationTRANSCRIPT
Semi-Inclusive DIS Experiments Using BigBite and Super BigBite
Spectrometers in Hall AAndrew Puckett
University of Connecticut and Jefferson LabSBS Collaboration Meeting
July 7, 2014
July 2014 SBS Collaboration Meeting 2
Outline• Introduction—Semi-Inclusive DIS, TMDs, flavor tagging• SIDIS studies using BigBite and Super BigBite in Hall A• Approved experiment E12-09-018 (neutron transversity):
Collins/Sivers effects in SIDIS on transversely polarized 3He• New Hall A Collaboration Proposal to PAC42—SIDIS on
longitudinally polarized 3He, high-statistics measurements of A1n
h in n(e,e’h)X• Expected results and impact on nucleon spin-flavor decomposition
• Summary and conclusions
7/7/2014
July 2014 SBS Collaboration Meeting 3
Semi-Inclusive Deep Inelastic Scattering
• Detecting leading (high-energy) hadrons in DIS, N(e,e’h)X reaction provides sensitivity to additional aspects of the nucleon’s partonic structure not accessible in inclusive DIS:• quark flavor• quark transverse motion• quark transverse spin
• Goal of SIDIS studies is (spin-correlated) 3D imaging of nucleon’s quark structure in momentum space.• Transverse Momentum Dependent (TMD) PDF formalism: Bacchetta et al. JHEP 02 (2007) 093, Boer and Mulders, PRD 57, 5780 (1998), etc, etc...
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July 2014 SBS Collaboration Meeting 4
SIDIS Kinematics—Notation and Definitions
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July 2014 SBS Collaboration Meeting 5
General Expression for SIDIS Cross Section: Bacchetta et al. JHEP 02, 093 (2007)
• SIDIS structure functions F depend on x, Q2, z, pT
• U, L, T subscripts indicate unpolarized, longitudinally and transversely polarized beam, target, respectively • S = nucleon spin• λ = lepton helicity• Eight terms survive at leading twistlarge Q2 crucial for “clean” interpretation
• Sivers• Collins• “Pretzelosity”
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July 2014 SBS Collaboration Meeting 6
Quark-parton Model Interpretation of SIDIS: Transverse Momentum Dependent PDFs (TMDs)
7/7/2014
Quark polarization
Unpolarized(U)
Longitudinally Polarized (L)
Transversely Polarized (T)
Nucleon Polarization
U
L
T
July 2014 SBS Collaboration Meeting 7
SIDIS Structure Functions in Terms of TMDs• Only f1, g1, h1 survive integration over quark kT
• All eight leading-twist TMDs are accessible in SIDIS with polarized beams/targets via azimuthal angular dependence of the SIDIS cross section• Physical observables are convolutions over two (unobserved) transverse momenta: • Initial quark kT
• Hadron pT relative to recoiling quark, generated during fragmentation
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July 2014 SBS Collaboration Meeting 8
JLab 11/8.8 GeV DIS Kinematics
7/7/2014
W > 2 GeV
• Optimal orientation of hadron arm is along virtual photon direction—q-direction varies linearly with x for fixed electron scattering angle
• Need forward-angle hadron detection capability!
• To reach high x in the DIS regime, need large scattering angles/high-Q2
July 2014 SBS Collaboration Meeting 97/7/2014
July 2014 SBS Collaboration Meeting 10
SIDIS Using BigBite and SBS in Hall A
7/7/2014
• Above: Schematic of SIDIS experiment(s)
• Independent electron and hadron arms:• Large momentum bite• Moderate solid angle• High-rate capability• Excellent PID
• h+/h- symmetric acceptance
SIDIS w/BB 30 deg, SBS 14 deg.
SIDIS w/BB 30 deg, SBS 10 deg.
• 60 cm polarized 3He• 10.5 atm• Ibeam ≥ 40 μA
BigBite (SBS) as electron (hadron) arm
July 2014 SBS Collaboration Meeting 11
<Q2> of SBS+BB SIDIS: > HERMES, < COMPASS
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July 2014 SBS Collaboration Meeting 12
SIDIS Kinematic Coverage
7/7/2014
• Distributions of SIDIS kinematic variables—normalized to 10 (5) days at each SBS angle setting for E = 11 (8.8) GeV
• θSBS = 10 deg; θSBS = 14 deg
July 2014 SBS Collaboration Meeting 13
SIDIS Phase Space Coverage
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July 2014 SBS Collaboration Meeting 14
Charged Hadron PID—SBS RICH Detector
7/7/2014
• Re-use HERMES dual-radiator RICH detector• Aerogel n=1.0304• C4F10 gas n=1.00137
• NIM A 479, 511 (2002)• Above: RICH schematic• Top right: HERMES RICH
implemented in SBS GEANT4• Bottom right: actual and
reconstructed θC from GEANT4
“True”
Reconstructed
July 2014 SBS Collaboration Meeting 15
Expected PID performance (IRT algorithm)
7/7/2014
5 GeV pion 5 GeV kaon
• MC PID results include acceptance effects—showing RICH geometry is well-matched to SBS magnet/tracker acceptance
July 2014 SBS Collaboration Meeting 16
SBS+BB Resolution—Charged Hadrons
7/7/2014
• SBS+BB resolution more than adequate for SIDIS on 3He—kinematic bin migration/resolution dominated by Fermi-smearing.
• Bin migration due to kinematic smearing becomes significant for ΔxBj < ~0.1
July 2014 SBS Collaboration Meeting 17
Neutral pion detection—Acceptance comparison
7/7/2014
• π0 detected in HCAL via two high-energy hits separated by at least one pixel
• Apertures of GEM/RICH limit useful area of HCAL for π0 detection
• Pixel size 15 x 15 cm2 limits coord. resolution for EM showers to 15 cm/sqrt(12) = 4.3 cm
• Estimated HCAL resolution for EM showers is dE/E ~ 14%/sqrt(E in GeV)
July 2014 SBS Collaboration Meeting 18
SBS+BB Resolution—Neutral Pions
7/7/2014
• π0 kinematic resolution dominated by HCAL coordinate/energy resolution—two-photon invariant mass resolution ~21 MeV
• To-do—full MC study of accidental/combinatorial background for π0 reconstruction.
July 2014 SBS Collaboration Meeting 19
Unique Advantages of SBS+BB for SIDIS Physics• The combination of moderate solid angle, large momentum acceptance and
high-rate capability at forward angles is ideal for high-luminosity experiments (e.g., polarized 3He), SIDIS at high Q2 • For polarized proton SIDIS at lower luminosity, competitiveness of SBS+BB less
clear; large acceptance detectors have a bigger advantage• Independent electron and hadron arms—straight-line tracking in “field-free”
regions behind dipole magnets• Simple, reliable reconstruction and data analysis• Change magnet polarity of e(h) arm without changing h(e) acceptance:
• Most accurate possible measurement of pair-production background in BigBite (important background for 3He targets w/thick glass walls, especially at low x)
• SBS polarity reversals to increase ϕh coverage and make h+/h- acceptances identical• Ability to measure K and π0 simultaneously in addition to charged pions• Excellent systematics control for charge-sum and difference asymmetries used to
separate valence/sea quark polarizations• Complementarity with CLAS12/SOLID/Hall C experiments—precise, timely
neutron data w/unique kinematic coverage; can run within first five years of 12 GeV
7/7/2014
July 2014 SBS Collaboration Meeting 20
Transverse target spin effects in SIDIS
Transverse target spin-dependent cross section for SIDIS
• Collins effect—chiral-odd quark transversity DF; chiral-odd Collins FF• Sivers effect—access to quark OAM and QCD FSI mechanism• “Transversal helicity” g1T—real part of S wave-P wave interference (Sivers = imaginary part) (requires polarized beam)• “Pretzelosity” or Mulders-Tangerman function—interference of wavefunction components differing by 2 units of OAM
7/7/2014
P-25 Seminar, LANL 21
The Sivers Effect: a Probe of Quark OAM
10/24/2011
x = 0.2
A. Prokudin• Sivers effect: a left-right asymmetry in the transverse momentum distribution of unpolarized quarks in a transversely polarized nucleon
• Proton spin is along +y axis (up)• Proton momentum into screen• Regions of higher/lower quark density in transverse momentum space
July 2014 SBS Collaboration Meeting 22
JLab Experiment E12-09-018
7/7/2014
• Primary physics goal: measure transverse target SSA in 3He(e, e’h)X in SIDIS kinematics in the valence region• Extract neutron SSAs from Helium-3 using
effective polarization approximation—relatively small theoretical uncertainty
• Wide, multi-dimensional kinematic coverage• Detect π±/0 and K± simultaneously in identical
acceptance/kinematics (for charged hadrons)• First precision SSA data in a multi-
dimensional phase space• Main experiment parameters:
• Electron-polarized neutron luminosity: • Helium-3 target polarization: 60%• Electron beam polarization: 80-85%
• Approved by JLab PAC38 for 64 beam-days, including:• 40 beam-days production at Ebeam = 11 GeV• 20 beam-days production at Ebeam = 8.8 GeV• 4 beam-days for calibrations, configuration
changes
E12-09-018, E=11 GeV
E12-09-018, E=8.8 GeV
E06-010, E=5.9 GeV
July 2014 SBS Collaboration Meeting 23
E12-09-018: Vast Improvement over Current Knowledge
7/7/2014
1D binned neutron precision ~0.2%
π± , K± Sivers compared to HERMES, COMPASS, theory fit
FOM: Improvement on existing data by 2+ orders of magnitude
• E12-09-018 will achieve statistical FOM for the neutron ~100X better than HERMES proton data and ~1000X better than E06-010 neutron data.• Kaon and neutral pion data will aid flavor decomposition, and understanding of reaction-mechanism effects.• Provide precise data in the unexplored region x > 0.3 valence-dominated
July 2014 SBS Collaboration Meeting 24
E12-09-018: First Precision Multi-Dimensional Analysis
7/7/2014
• 2D Extraction: Sivers AUT in n(e,e’π+)X, 6 x bins 0.1<x<0.7, 5 z bins 0.2<z<0.7• Curves are theory predictions (Anselmino et al.) with central value and error band
Uncertainty in this x, z bin ~ 0.6%
Large neutron π+ asymmetry expectation at high z, large
uncertainty
July 2014 SBS Collaboration Meeting 25
E12-09-018: Fully Differential Analysis
7/7/2014
Increasing z
In
crea
sing
pT
Sivers AUT, n(e,e’π+)X vs. x, 40 days @ 11 GeV
• 6 (0.1 < x < 0.7) × 5 (0.2 < z < 0.7) × 6 (0 < pT (GeV) < 1.2) 3D binning• Q2 dependence with E = 11 and 8.8 GeV data gives fully-differential analysis• Typically 120 bins with good stats per beam energy • Statistical precision:• 83% of 3D bins have separated Collins/Sivers neutron asymmetry error of less than 5% (absolute)• Average stat. err ~4%• Most probable stat. err ~1.5%
July 2014 SBS Collaboration Meeting 26
New Proposal to PAC42—PR12-14-008
7/7/2014
Measurements of Semi-Inclusive DIS Double-Spin Asymmetrieson a Longitudinally Polarized 3He Target
A Hall A Collaboration Proposal
July 2014 SBS Collaboration Meeting 27
PR12-14-008 Collaboration—Author list as of 6-1-2014
7/7/2014
July 2014 SBS Collaboration Meeting 28
Proton Spin Crisis/Puzzle
“Crisis”: EMC collaboration, NPB 328, 1 (1989)
• 1989: Fraction of proton spin carried by quarks is “small”—“crisis” for the parton model• Modern (DSSV2008) value of ΔΣ ≈ 0.24-0.37, depending on (controversial) behavior of strange sea polarization Δs • Remaining ~70% of nucleon spin distributed among gluon spin and orbital motion of quarks/gluons; poorly known but much recent progress both theoretically and experimentally
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July 2014 SBS Collaboration Meeting 29
Polarized DIS and Nucleon Spin Structure
PDG2010 compilation of g1 data DSSV NLO global fit: PRD 80, 034030 (2009)
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July 2014 SBS Collaboration Meeting 307/7/2014
July 2014 SBS Collaboration Meeting 31
PR12-14-008: Projected Precision vs x for all hadrons
7/7/2014
• Projected asymmetry precisions (stat. only) in A1n
h vs x, integrated over z, pT, compared to prediction of “DSSV+” NLO global fit: http://arxiv.org/abs/1108.3955
• Fit includes COMPASS 2010 p and d data: http://arxiv.org/abs/1007.4061
• <Q2> between HERMES and COMPASS
• More details, numerical tables available at: https://userweb.jlab.org/~puckett/PAC42_deltad/projections/
July 2014 SBS Collaboration Meeting 32
PR12-14-008: Projected Precision vs (x,z), E = 11 GeV
7/7/2014
• Left-right, top-bottom: π+, π-, π0, K+, K-.
• Curves: “DSSV+”: http://arxiv.org/abs/1108.3955
• More details including numerical tables at: https://userweb.jlab.org/~puckett/PAC42_deltad/projections/
• Relatively weak z dependence of DSSV+ curves is a NLO QCD effect.
• Strong hadron dependence of A1nh clear indication of flavor
sensitivity of SIDIS
July 2014 SBS Collaboration Meeting 33
Impacts on Nucleon Spin-Flavor Decomposition
7/7/2014
• Left: Projected precision of five-flavor Δq/q extraction using LO “purity” method• Excellent precision/sensitivity to d and dbar, as
expected.• Below: existing data, from DSSV2008 analysis: http://
journals.aps.org/prd/abstract/10.1103/PhysRevD.80.034030
July 2014 SBS Collaboration Meeting 34
Impacts on Spin-Flavor decomposition, II
7/7/2014
• Left: valence d polarization from Helium-3 charge-difference asymmetries using LO Christova-Leader method.
• Below: polarized sea asymmetry assuming proton data of comparable precision to this proposal
July 2014 SBS Collaboration Meeting 35
Impact on Spin-Flavor Decomposition, III
7/7/2014
Preliminary results of DSSV impact study
indicate dramatic impact of PR12-14-008 to dbar polarization (and very significant impacts to
ubar, sbar)Flavor-separated SIDIS data
important cross-check to RHIC W asymmetry data
July 2014 SBS Collaboration Meeting 36
SBS+BB SIDIS: Challenges/Status/Future Plans• “Dependencies”:
• High-luminosity polarized 3He target w/flexible spin orientation (transversity)• Design shielding of SBS stray field
• RICH detector:• Require TDC readout to reduce effective
occupancy• Gas handling system in Hall A: availability of
C4F10 or its equivalent (e.g., C4F8O)?
• GEM occupancies and tracking:• SIDIS luminosity ~= GEP luminosity/40, but:• No exclusivity constraint on track search area• HCAL coordinate info can help• Need realistic MC
• Background rates: • RICH—new estimates with fully detailed
MC for approved and proposed SIDIS expt’s.
• GEMs—define BB+SBS tracker geometry for SIDIS—demonstrate tracking feasibility for SIDIS
• Beamline backgrounds shielding design• 3He target in vacuum?
• Trigger & DAQ:• Desired trigger thresholds for SIDIS:
• BigBite: Electron p > 1 GeV• SBS: Hadron p > 2 GeV
• Online trigger rate for SBS 14 deg. SIDIS configuration estimated at ~18 kHz in PR12-09-018 @PAC38 (dominated by accidentals)• ECAL-only trigger for BB assumed—includes
~90% photon-induced triggers• Need estimate for 10 deg. setting—may
require higher threshold, higher minimum z (z >? 0.3)
7/7/2014
SBS SIDIS experiments (approved and proposed), are very challenging—SIDIS
requirements need more attention/consideration in SBS design efforts, especially if PAC42 proposal
approved!
July 2014 SBS Collaboration Meeting 37
Summary and Conclusions• The BigBite-SBS spectrometer pair in Hall A is ideally suited for high-
luminosity polarized (and unpolarized) SIDIS experiments:• BigBite as electron arm as in several other 12 GeV expt’s. • SBS as hadron arm, equipped with existing RICH for high-performance PID
• Experiment E12-09-018 (transversity) already approved for 64 beam-days, A- rating by PAC38• New proposal PR12-14-008 (A1n
h SIDIS) submitted to PAC42 for 33 beam-days, high-impact data for nucleon spin-flavor decomposition, relevant to future EIC program• SBS+BB SIDIS, with unique kinematic coverage and excellent
systematics control, is complementary to other approved polarized SIDIS experiments such as CLAS12, SOLID, etc. • Impact studies of proposed measurements to NLO global QCD analysis
are underway by Dr. R. Sassot of DSSV group.
7/7/2014
July 2014 SBS Collaboration Meeting 38
Acknowledgements• Thanks to all who contributed to successful
development/submission of new proposal• PR12-14-008 Spokespeople: • X. Jiang (contact), N. Liyanage
• Hall A Collaborators/reviewers • SBS Collaborators• R. Sassot for grid of DSSV+ predictions and
forthcoming impact studies.• S. Riordan for development of GEANT4 framework
for SBS/BB MC simulations
7/7/2014
July 2014 SBS Collaboration Meeting 39
Backup Slides
7/7/2014
July 2014 SBS Collaboration Meeting 40
Major Systematic Uncertainties
7/7/2014
• See proposal for additional details: https://userweb.jlab.org/~puckett/PAC42_deltad/submitted_Deltaq.pdf
Identical phase space for π+, π- leads to excellent systematic control of ratio r:
July 2014 SBS Collaboration Meeting 41
Comparison With Other Approved Experiments
7/7/2014
• Naive FOM comparison from basic experiment parameters• Generally: High-luminosity 3He in Hall A roughly 10-100X higher FOM for
neutron than CLAS12 ND3 (kinematics-dependent)• At same kinematics, SBS+BB and SOLID FOM are of the same order-of-magnitude
• SBS+BB higher luminosity partially offsets SOLID advantage in solid-angle
July 2014 SBS Collaboration Meeting 42
SBS+BB vs. SOLID: Complementarity of kinematic coverages
7/7/2014
• Left: Q2-x of SBS+BB vs. SOLID: SBS+BB reaches higher Q2 at similar x due to larger electron scattering angles.
• Right: theta-vs.-phi coverage between SBS+BB and SOLID
July 2014 SBS Collaboration Meeting 43
Detailed FOM comparison—SBS+BB vs. SOLID
7/7/2014
July 2014 SBS Collaboration Meeting 44
Experiment design considerations
7/7/2014
Azimuthal coverage: full coverage of Sivers and Collins angles Charged and neutral pions and kaonsFlavor decomposition of PDF and FF As large as possible Q2: DIS regime, factorization Low-to-moderate pT: ΛQCD ~< pT << Q Applicability of TMD formalism Wide, independent coverage of xBj, z = p/ : n factorization Reach high x ~ 0.5-0.7, where observable asymmetries are expected to be large
Challenges:• A high performance polarized target• Low event rates at high Q2 and high xBj—high luminosity.• High-performance particle ID—separate different hadron species• Proton and neutron targets—flavor decomposition• Non-SIDIS backgrounds: • Radiative tails of exclusive and
resonant electroproduction• Charge-symmetric (e+/e- pair-
production) background
Below: Zhongbo Kang seminar, LANL, 4/2011
July 2014 SBS Collaboration Meeting 45
Electron Arm—BigBite Spectrometer
7/7/2014
BigBite @ 6 GeV (E06-010 transversity expt):• Three MWDCs for tracking (18 wire planes)• Pre-shower/shower calorimeter for trigger and PID• Scintillator hodoscope for timing• Dipole magnet:
BigBite @ 12 GeV:• Detector upgrades including:
• GEM chambers for high-rate, high-resolution tracking (resolve higher electron momenta at same field integral)
• Gas Cherenkov for higher-fidelity e/π separation• New detector support frame
• BigBite parameters in E12-09-018:• Central angle = 30 deg.• Target to magnet yoke distance = 1.5 m
July 2014 SBS Collaboration Meeting 46
Hadron Arm—Super BigBite Spectrometer
7/7/2014
Super BigBite Spectrometer• Originally designed for nucleon elastic form
factor measurements at large Q2. • 48D48 magnet: acquired from BNL by
JLab, Bdl ~ 2 Tm.• Flexible, modular design w/ basic detector
package consisting of:• GEMs• HCAL
• Suitable for SIDIS with modest addition: • Re-use RICH detector from HERMES
for hadron PID
SBS main parameters for E12-09-018:• Central angle = 14 deg.• Target to magnet yoke distance ~ 2.5 m• Solid angle ~40 msr• Momentum acceptance: p > 1 GeV• More info: http://
hallaweb.jlab.org/12GeV/SuperBigBite
HERMES RICH performance characteristics• π/K/p separation from 2-15 GeV using dual-
radiator (aerogel + heavy gas) design• RICH details: NIM A 479 (2002) 511
July 2014 SBS Collaboration Meeting 47
High-luminosity polarized 3He target
7/7/2014
Basic Target Parameters in E12-09-018• Polarization: 60-65% based on alkali-hybrid spin-exchange
optical pumping technology• Beam current: 40 μA• Target cell length along beam-line: 60 cm• Electron-polarized neutron luminosity: • Luminosity * Pol.2 capability upgraded (relative to previous
targets) by using convection-driven circulation of gas between “pumping chamber” and “target chamber” (already demonstrated in bench tests) and metal end-windows to prevent cell rupture (under development)
• Spin orientation in “any” direction; holding field ~25 G• Fast spin rotation: Change spin orientation every ~120 s.
Conceptual design of SIDIS target w/metal end
windows
July 2014 SBS Collaboration Meeting 48
Experiment status, future plans, conclusion• Experiment E12-09-018 represents an exciting near-term
opportunity to elucidate neutron transverse spin structure.• Super BigBite Spectrometer (SBS) recently funded by DOE,
construction underway, with contributions from INFN (GEM), UVA (GEM), CMU (HCAL), JLab (Magnet, infrastructure, program management, etc.) and others. Exciting program of high-impact approved experiments:• Nucleon elastic Form Factors at large Q2—GMn, GEn, GEp • Neutron transversity in SIDIS on Helium-3
• Custody of half of HERMES RICH detector (and all aerogel) transferred to JLab, currently in controlled storage at UVA, plan to start refurbishment at UConn soon.• Many exciting physics opportunities beyond initial approved
program (if beam time in Hall A is available)7/7/2014
July 2014 SBS Collaboration Meeting 49
Acknowledgements
• E12-09-018 co-spokespeople:• Gordon Cates (UVA), Evaristo Cisbani (INFN), Gregg
Franklin (CMU), Bodgan Wojtsekhowski (JLab)• E12-09-018 collaboration• SBS collaboration• A. Prokudin (for phenomenological model fit
results and “theoretical” uncertainty projections)• US DOE
7/7/2014
July 2014 SBS Collaboration Meeting 50
Electron-Nucleon Scattering: Kinematics
7/7/2014
Incident electron four-momentumScattered electron four-momentum
Initial nucleon four-momentum
Squared Momentum Transfer
Energy Transfer (nucleon rest frame)
Bjorken “x” variable
Fractional electron energy loss (nucleon rest frame)
Invariant mass of virtual-photon + initial nucleon system
July 2014 SBS Collaboration Meeting 51
Kinematic coverage from simulation
7/7/2014
BB+SBS solid angle coverage
Direction of q vs Bjorken x @ 11 GeV
Above: azimuthal coverage vs pT, Below: Kinematic coverage
Transversity 2014 52
Accessible phase space in fixed-target at 11 GeV
Above: phase space with SIDIS cuts (before considering any detector acceptances), E=11 GeV
6/12/2014
July 2014 SBS Collaboration Meeting 53
Transverse spin dynamics in eqeq
7/7/2014
• Magnitude of quark normal and in-plane transverse polarization components is reduced by a factor of • Dnn = (1-y)/(1-y+y2/2), where y = (1 - cosθCM)/2 is invariant (y=(ν/E)LAB).
• Direction of normal polarization is unchanged• In-plane transverse polarization component in the cms rotates with quark momentum vector—corresponds
to a spin flip in target rest frame (P, q collinear)• Simplified view—ang. mom. conservation requires spin flip for quark to absorb transverse virtual
photon• DNN, an inherent feature of the hard partonic subprocess, suppresses the observable SSA corresponding
to Collins effect, esp. at large y!
July 2014 SBS Collaboration Meeting 54
Where do the azimuthal dependences come from?• Sivers effect is due to the correlation between unpolarized quark kT and nucleon transverse polarization:
• Collins effect is due to the left-right asymmetry in the fragmentation of a transversely polarized quark.• The observable asymmetry results from the convolution of the transversity distribution and the Collins fragmentation function.
• The modified azimuthal dependence of the Collins SSA relative to Sivers is due to a spin-flip of the in-plane component of the quark’s transverse polarization component by the virtual photon (ang. mom. conservation)
7/7/2014