The JLab BoNuS Experiment: measurement of the free neutron structure function at large x and nuclear effects in deuterium via spectator tagging.

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The JLab BoNuS Experiment measurement of the
free neutron structure function at large x and
nuclear effects in deuterium via spectator
tagging.
M. Eric Christy Hampton University
n
p
ECT Workshop - Trento, Italy June 3, 2008
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Why study large-x structure functions?

• Study pQCD DGLAP evolution
  
• Separation of parton distributions, e.g.,
• 
  
  
  
• Precise PDFs needed to constrain limits on new
  physics at
• LHC and Tevatron
  
• Separation and study of perturbative /
  non-perturbative

• d/u for x ? 1,
• singlet / non-singlet separation.

• Higher-twist operators
  (parton-parton correlations)?
• Study quark-hadron duality

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Much can be learned about pQCD from DIS on a
neutron target

• Need proton and neutron targets to pin down u/d
  PDFs from DIS

At Leading order
x4/9u(x) 1/9d(x)
x4/9d(x) 1/9u(x)
At large x proton dominated by u(x) and neutron
by d(x)? due to charge weighting.

• Proton minus neutron allows separation of
  singlet and non-singlet (valence) contributions
  to structure functions at all x

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PDFs Uncertainties
u(x)?
d(x)?
PDFs least well known at large x Gluon
comparable in size to dv at x0.3 not
well known here. Proton neutron data provides
way to separate valence cleanly.
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• Deuterium proton data does not allow clean
  determination of neutron for x ? 1 due to nuclear
  corrections

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The EMC Effect
- Effects such as binding, off-shell, and final
state interactions have largest impact on
classic EMC region - Need to get a handle on
these to study more exotic effects.
Hard to imagine understanding the EMC effect
without a firm grasp of nuclear effects in the
deuteron
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Inclusive Neutron Resonance Electroproduction

• Neutron structure functions in the deuteron have
  contributions from binding, fermi motion, nucleon
  off-shell effects, final state interactions
  (FSI), etc.
• We would like to know the free neutron structure
  to learn about
  - moments of
  the neutron structure function (and
  non-singlet) -neutron
  (transition) form factors -
  quark-hadron duality for the neutron

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Method of Spectator Tagging
nuclear impulse approximation ? the proton is a
pure spectator and recoils with momentum ps -p
With S the nucleon spectral function in the
deuteron and F2n(eff) the effective off-shell
neutron structure function
X
?(q)?
F2n(eff)?
The spectator protons four momentum pµ -(Es
MD, ps)? Light-cone momentum fraction as
(Es psz)/M
n(p)?
d(pd)?
p(ps)?
Spectator
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Experimental Setup I CLAS Spectrometer
Beam

• Detect electrons in CLAS Spectrometer
• 
  
• Detect slow protons in radial time
  projection chamber (RTPC)
• 
  
• Moller electrons bottled up by Solenoid field
  around target
• 
  
• Solenoid field allows momentum determination

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Experimental Setup II BoNuS RTPC
Fit RTPC points to determine helix of proton
trajectory. Momentum determined from track
curvature in solenoid field. dE/dx along track
in RTPC also provides momentum information.
to CLAS
BoNuS RTPC
n
Solenoid Magnet
p
To BoNuS RTPC
Moller Catcher
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RTPC Performance
electrons measured in both RTPC and CLAS after
calibrations.
?p/p
Proton momentum measured in RTPC and
reconstructed from CLAS utilizing e D ?
e' P p- Ps D production
Tagged PID from dE/dx
dE/dx vs Pproton
protons
Nuclei
dE/dx helps provide particle ID
NIM-A report accepted for publication
doi10.1016/j.nima.208.04.047
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Kinematic Coverage
E 5.262 GeV
E 4.223 GeV
W (GeV)?
W (GeV)?
Q2 (GeV2)?
Q2 (GeV2)?
E 2.140 GeV
pspec (MeV)?
W (GeV)?
cos(?pq)?
Q2 (GeV2)?
VIPs
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Towards the free Neutron Structure from BoNuS
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Kinematic tuning cleanly isolates free neutron
structure from nuclear effects
Target fragmentation
Binding effects
Final state interactions
BoNuS VIP 'very important protons'
region 70 lt ps lt 200
MeV/c 120o lt Q
pspec (MeV)?
BoNuS VIP Region
VIPs
cos(?pq)?
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Kinematic reconstruction with tagged protons
E 4.223 GeV
W2 (pn q)2 pnm p nm 2(MD-Esn pn . q)
Q2 M2 2Mn(2- as ) - Q2
Spectator protons four momentum pnµ -(Es
MD, ps)? Light-cone momentum fraction as (Es
psz)/M
W2 M2 2Mn - Q2
Method Works! - Neutron Elastic and D peaks are
very prominent and nicely separated
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First (preliminary)Results
Need to know acceptance and efficiency of tagged
protons (lumped together as 'Tagging
efficiency'). This will vary with kinematics
and position of track in RTPC.
For the moment just normalize tagged/untagged
data to model here. For sn / sp multiply by sD
/ sp utilizing recent fits to world data proton
M.E. Christy and P.E. Bosted, arXiv
0712.3731 deuteron P.E. Bosted and M.E.
Christy, arXiv07110159 (neutron)?
Difference in tagged/untagged acceptance edge
Above normalization is consistent with tagging
efficiency determined from comparisons to
previously measured quasi-elastic region (0.8 lt
W lt 1.07 GeV) Final determination will be done
utilizing Monte Carlo after match to data in
well measured regions.
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First (preliminary) Results F2n/F2p
F2n/F2p

• Utilize sn/sD
• 
  and
• 
  
  
• Good agreement with previous data in smaller x
  region.
• 
  
• Full acceptance correction method forthcoming.

BoNuS Preliminary
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Nuclear effects Studieswith BoNuS
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Complementary measurements D(e,e',ps)X in CLAS
(DeePs)?
Q pq gt 108o
72.5O lt Q pq lt 107.5O
W lt 1.1 GeV
W lt 1.1 GeV
1.1 lt W lt 2
1.1 lt W lt 2
1.1 lt W lt 2
Tag proton exclusively in CLAS Ps gt
250 MeV/c
2 lt W
2 lt W
Complementary to BoNuS for studying nuclear
effects Ps lt 200 MeV/c
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Off-Shell Effects
(Effective off-shell to on-shell structure
function ratio)
Covariant spectator model W. Melnitchouk, A.W.
Schreiber, and A.W. Thomas
Relativistic quark spectral function model
F. Gross and S. Liuti
BoNuS Region
CLAS Region

• Significantly different x and ps dependence
• between models

• Tagging more forward angle protons
• in CLAS at higher ps provides significantly
• increased lever arm for discriminating
• different models.

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Final State Interactions
Maximize sensitivity to FSIs by looking at high
spectator momenta around Qpq 90o
Ciofi degli Atti and Kopeliovich, Eur. Phys. J.
A17(2003)133
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BoNuS kinematics
pT 0
Thomas, Melnitchouk et al. 'off-shell
modifications'
Close et al. 'Color
delocalization'
Frankfurt, Strikman et al.
'PLC suppression'
Nucleon Off-shell mass
939 MeV
905 MeV
823 MeV
694 MeV
Ps 0 0.09 0.17 0.25 0.32
0.39 GeV/c
BoNuS will provide - True 'on-shell' free
neutron structure. - Additional kinematic
constraints on models by looking at Ps, a, and
qpq variation of effective structure function.
especially when combined with spectator tagging
data from other experiments at higher Ps
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The Future of BoNuS

• Other ongoing analyses
• Semi-Inclusive DIS
• Exclusive meson production
• Neutron form factors?

• F2n to higher x and Q2
• Semi-Inclusive DIS higher W'
• Exclusive meson production higher Q2
• Neutron form factors higher Q2

BoNuS will 11 GeV CEBAF
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BoNuS _at_ 11 GeV
Proposed Kinematics
Neutron structure function
d/u
? transition form factor from neutron
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Backup Slides
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PDFs Uncertainties
u(x)?
d(x)?
PDFs least well known at large x Gluon
comparable in size to dv at x0.3 not
well known here. Proton neutron data provides
way to separate valence cleanly.
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Resonance Proton fit (M.E. Christy)

• Kinematic range of fit
• 0 lt Q2 lt 8
• and W??thresh lt W lt 3
• reproduces cross section data to 3

Photoproduction (Q2 0)?
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Fit compared to deuteron data
Hall C Jan05 prelim Hall C I. Niculescu
(published)? Hall C Spring'03 prelim. Hall B
published 2006
SLAC E133 published Hall C I. Niculescu
published Hall C Spring'03 (S. Malace
Thesis)? Hall B published 2006
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L/T Separated Structure Functions on Nuclei (JLab
E02-109, E04-001 and E06-009)?

• L/T Separation Data Targets D, C, Al, Fe -
  Final uncertainties 1.6 pt-pt in ? (2
  normalization) - essentially, duplicate proton
  data from E94-110.

Data from Jan '05
Spring 2007 Kinematics
L/T separations where multiple energies.

• Low Q2 modeling data
• Targets H,D, C, Al
• Uncertainties in preliminary data
• estimated at 3 - 8
• (Larger RCs and rates)?