Spin Physics results at HERMES with Longitudinally Polarized Targets

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Spin Physics results at HERMES with
Longitudinally Polarized Targets
Alessandra Fantoni (on behalf of the HERMES
Collaboration)
1.The spin puzzle the HERMES experiment 2.
Polarised structure function g1 3. Flavor
decomposition polarised quark distribution
functions 4. New 5-flavor extraction 5. New
isoscalar extraction Ds 6. High-pT hadrons, Dg 7.
DVCS for probing GPDs 8. Hint for Excl.
processes with Recoil Detector


International Workshop on Hadron
Structure and Spectroscopy (IWHSS 08)

Torino March 31st April 02nd, 2008
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HERa MEasurement of Spin (DESY)
180 Researchers from 30 Institutions of 12
Countries

• Study of nucleon structure
• double spin asymmetries
• (inclusive, seminclusive)
• single spin asymmetries
• (seminclusive, transversity and exclusive
  processes)

Data taking 1995-2007
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1. The Nucleon Spin Structureand the HERMES
   experiment

QPM
EMC 1988 DS0.1230.0940.138
?SLAC, CERN, DESY DS ?0.2-0.4
Polarised Deep Inelastic Scattering (DIS)
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The Nucleon Structure
HERMES polarised DIS
Unpolarised DIS
v

v



first measurements only recently
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Characteristiques of the HERMES experiment

• resolution
• dp/p2, dqlt1 mrad
• PID leptons with
• e98, contam. lt1
• hadrons?dual RICH p, K,p
• 2ltEhlt15 GeV

Stern-Gerlach separation
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2. Polarised Structure Functions g1
HERMES Collaboration PRD 75 (2007)
HERMES data set precise and complete
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Polarised Structure Functions g1
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Dq and DG from inclusive data
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How to further proceed ?

• Dq and DG from inclusive DIS data via evolution
  equations

• requires wide kinematic
• range in Q2 and x

• only fixed target spin
• experiments so far

OR
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Polarised semi-inclusive DIS
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3. Polarised Quark Distribution Functions
Purity Functions
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Polarised Quark Distribution Functions
HERMES Coll. PRL92(2004), PRD71(2005)
First measurements of Dq by flavour decomposition
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Polarised Quark Distribution Functions
HERMES Coll. PRL 92(2004), PRD 71(2005)
u quarks large positive polarisation
d quarks negative polarisation
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4. Future 5-Flavor Dq(x) Extraction Revisited

• Theres still room for improvement
• Statistical uncertainties
• Not all possible asymmetries were utilized.
  Some may have significant leverage over the sea
  quarks (ks)
• Systematic uncertainties
• The published Monte Carlo related systematic
  uncertainty was hopefully conservative.
• Confirm rigorously
• Potentially reduce!

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Comparison of Uncertainties of Old and New
Asymmetries
2-4 GeV deuterium gives additional pions and kaons
Additional asymmetries available
Proton Target Deuteron Target
HH, HH- P, P
Ks, p0,and L Ks, p0,and L

• Published
• New

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Correlating MC tune and D q(x) systematic
uncertainty
1. Scan c2 surface around best Monte Carlo tune.
Fit with quadratic Polynomial.
68 Contour
Best MC Tune

• 2. Find 68 contour. Two factors
• Height of 68 of d-dimensional Gaussian
  Distribution.
• The height of c2 minimum to accommodate model
  imperfection. PDG does something like this.

c2
c2minC
c2min
parj b
parj a
3. Compute Dq(x) along contour Use Hessian
method to sample along uncorrelated parameter
directions. CTEQ does something like this.
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In reality

• Scan the c2 surface around the best Monte Carlo
  tune.
• Correlations are quite clear between parameters
• Generate and diagonalize the matrix of 2nd
  derivatives to find linear combinations that are
  uncorrelated

Jetset/Lund c2 surface in Fragmentation Parameter
Basis

• Blue ellipses represent 68 contour
• Colored lines represent uncorrelated parameter
  directions

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MC Multiplicities

• Black dots are generated at the best Monte Carlo
  tune.
• Colored dots are generated at the 68 contour in
  the uncorrelated parameter directions
• 9 Hessian vectors
• ? 18 parameter sets to include

So, where are the new Dq(x) uncertainties?! Resul
ts not yet released, but should be greatly
reduced (gt50 in some bins).
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5. New Isoscalar extraction of ?S
HERMES Collaboration arXiv0803.2993 submitted
to Phys. Rev. Lett.
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Isoscalar extraction - formalism
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Isoscalar Formalism contd
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Extraction of ?D(z)dz s
Multiplicity corrected to 4p of charged kaons in
SIDIS from D target
Curves Continuous calculated from xS(x) Dashed
non-strange quark contribution to
multiplicity Dash-dotted strange quark
contribution to multiplicity
Dotted best fit of ? D(z)dz using CTEQ6L PDFs
0.2ltzlt0.8
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Experimental inclusive and kaon spin asymmetries
Lepton-nucleon polarised cross section
asymmetries A,d(x) for inclusive DIS and
A,dK(x) for SIDIS by a D target as a function
of x for identified charged kaons Kaon asymmetry
small and slightly positive
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Non strange quark helicity distributions
0.02ltxbj lt0.6 measured range Q02 2.5 GeV2
a8? Dq80.5860.031 from hyperon b decay
assuming SU(3) deficit violation SU(3) symmetry
or missing octet strength at x below measured
range

• First moments of helicity distributions
• Q 0.359 0.026 0.018
• DS 0.037 0.019 0.027
• Dq8 DQ 2DS 0.285 0.046 0.057

? S(x)dx is 0 within error Earlier HERMES
conclusions of unpolarised sea confirmed
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6. High pT hadrons and sensitivity to Dg First
HERMES Dg/g measurement
HERMES Collaboration, Phys. Rev. Lett. 84 (2000)
2584 Top cite 100
First longitudinal double spin asymmetries for 2
hadrons
Historical plot first HERMES data and future
projections
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?g/g extraction methods I and II
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?G from method I
Assuming ?g(x)/g(x) const over x
h,h- antitagged 4 points between 1.05ltpTlt2.5
GeV h,h- tagged 1 point for pTgt1 GeV Pairs
1 point for GeV2
Only statistical errors are shown

• Results for different data samples (diff.
  mixtures) agree within statistics
• Consistency between the two hadron charges and
  the two targets
• Dominating sample Deuteron antitagged -gt Used
  for Method II and syst. error analysis (charge
  combined)

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?G from method II
(Anti-tagged only)

• Light shaded area range of all data
• Dark shaded area fit center of gravity (span of
  the 4 pt bins)

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?G from method II

• ?2/ndf?5 mainly due to highest pT point
• Model systematic is not included in fit
• 1-2 parameter function is too smooth
• function 1 used as default and function 2 for
  systematics

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Model systematic

• PYTHIA 6.2 has been tuned
• fair agreement in tagged region
• (see plot vs kinematic variables)
• less agreement in anti-tagged region
• some failures in pt dependence
• checks with LO pQCD (collinear)

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Dg/g results vs world data
Black and blue curves pQCD fits to g1 Black
data points CERN exp results Red data
point Prel. HERMES Method I Red curves Prel.
HERMES Method II fit ?g(x)/g(x) with 2 functions
such that
Dg/g0.071 0.034 0.010
HERMES Coll. Paper in progress
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Nucleons 3D structure GPDs
Generalised Parton Distributions
elastic scattering form factors, chargecurrent
DIS quark longitudinal momentumhelicity
distribution
hard exclusive processes complete correlation of
transverse position and longitudinal momentum
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7. GPDs and Exclusive Processes
Q2gtgt, tltlt

• high Q2 ? hard regime
• high luminosity ? s1/Q4, 1/Q6
• high resolution ? exclusivity

? polarisation provides new
observables sensitive to different
(combinations) GPDs
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Exclusive processes at HERMES
e/e- 27.5 GeV
No recoil particle identification
Identification exclusive events by missing
mass Mx
background estimated by MC
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Deeply Virtual Compton Scattering (DVCS)
only at HERA
Beam Spin Asymmetry
? azimuthal asymmetries
HERMES Coll. PRL87(2001) Top cite 100
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HERMES Collaboration arXiv0802.2499 submitted
to JHEP
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Quark total angular momentum
AUT most sensitive observable to access Jq via
GPDs

• HERMES data 2002-04
• U unpolarized beam
• T transv. pol. Target
• 50 of total stat.

GPD model by Goeke et al. (2001)
Ellinghaus et al. (2005)
Projections assume Jd0 (u-quark dominance)
Best fit of Ju between 0.2 0.4 for VGG model
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Exclusive processes at HERMES gt 2006
e/e- 27.5 GeV
Statistics with Recoil larger than pre-Recoil
data taking With Recoil, DVCS events measured
directly, bkgr rejected Pre-Recoil results can be
later refined
Installed Dec05, commissioned and fully
working Dedicated to exclusive processes New
results expected soon
2006 7 M DIS events e- 20 M DIS events
e 2007 20 M DIS events e
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Recoil Detector Overview
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Conclusions

• Inclusive data analysis on g1 completed
• Results from semi-inclusive DIS from purity
  analysis
• Idea of new 5-flavor extraction
• Takes full advantage of the HERMES longitudinal
  target data statistical power by incorporating
  many new asymmetries.
• Addresses (and will reduce considerably!) Monte
  Carlo related systematic uncertainties in a
  rigorous way.
• New Isoscalar Extraction of Ds(x)
• Excellent confirmation of Monte Carlo based
  5-flavor extraction
• Suggests exciting new physics for Ds(x) in the
  xlt0.02 range
• High-pT hadrons with sensitivity to Dg
• Possibility to probe GPDs with Longitudinal and
  Transverse pol. targets
• First hint for future Exclusive Results with
  Recoil Detector