Transcript The BoNuS Experiment at Jefferson Lab’s CLAS. Svyatoslav Tkachenko University of South Carolina for the CLAS collaboration.

The BoNuS Experiment at
Jefferson Lab’s CLAS.
Svyatoslav Tkachenko
University of South Carolina
for the CLAS collaboration
Structure functions
and parton distribution functions
Structure Functions and Moments
qup(x)
qdown(x)
• Precise PDFs at large x
needed as input for LHC
– Large x, medium Q2 evolves
to medium x, large Q2
Q2=3.15 (GeV/c)2
Q2=3.15 (GeV/c)2
• Moments can be directly
compared with OPE (twist
expansion), Lattice QCD
and Sum Rules
– All higher moments are
weighted towards large x
Ratio to CTEQ6
Structure Functions and Resonances
• Precise structure functions in
Resonance Region constrain
nucleon models
[Separate resonant from nonresonant background; isospin
decomposition]

• Needed as input for spin
structure function data, radiative
corrections,…
• Compare with DIS structure
functions to test duality
d(x) and u(x) as x  1
• Valence structure of the nucleon - sea quarks and gluons don’t contribute
• SU(6)-symmetric wave function of the proton in the quark model:
• In this model: d/u = 1/2, u/u*) = 2/3, d/d = -1/3 for all x
• Hyperfine structure effect (1-gluon exchange): S=1 suppressed 
d/u = 0, u/u = 1, d/d = -1/3 for x  1
• pQCD: helicity conservation (qp) 
d/u = 1/5, u/u = 1, d/d = 1 for x  1
• Wave function of the neutron via isospin rotation:
replace u  d and d  u => using experiments with protons and
neutrons one can extract information on u, d, u and d in the valence
quark region.
*)
helicity q = (q - q) for Nucleon N
To extract d/u ratio, we need neutron data.
F2n 1  4d / u


F2 p 4  d / u
d 4 F2n F2 p  1

u 4  F2n F2 p
Extracting structure function
ratio is model dependent and the
results from the same data set
might differ a lot depending on the
model applied for analysis.
Large x - Large Nuclear Effects
• Even simple “Fermi
Smearing” leads to
significant
dependence on D
wave function
• Different models for
off-shell and “EMC”
effects lead to large
additional variations
• Contributions from
MEC, (1232) and
“exotic” degrees of
freedom unknown
• FSI?
Bound neutron… Free neutron…
 How can we study free neutron structure
without free neutrons available?
 Emulate them with nuclear targets:
–
–
In 3He, due to fortuitous cancellation of proton
spins, we can study neutron spin structure.
If we can find observables that are mostly sensitive
to the low-momentum part of the deuteron wave
function, we can treat the nucleons as quasi-free
and thus study neutrons.
Spectator tagging
(aka pinpointing the low-momentum part of
the deuteron wave function)
Spectator Tagging
W 2  M 2  2M  Q2
pn  M D E S , pS  ;
E = 4.223 GeV
n  2 S 

e
<Q2> = 1.19 (GeV/c)2
n
p
p S  E S , p S ;  S 
ES  p S  qˆ
M D /2
W * 2   pn  q  pn pn  2(M D  Es )  pn  q  Q 2
2
 M * 2 2M (2 S ) Q 2
x*
Q2
Q2


2 p n q 2M (2   S )
“Rules” for the spectator.
Final state interactions.
Ciofi degli Atti and Kopeliovich, Eur. Phys. J. A17(2003)133
The momentum and angular dependence of the ratio of spectral functions with and
without FSI effects. Blue boxes mark preferred kinematics – regions where FSI have
smaller effect.
“Rules” for the spectator.
“Off-shellness” depends on the spectator momentum magnitude.
Ratio of the bound to free F2 neutron structure functions vs spectator momentum.
Model by W.Melnitchouk.
Deviations from free structure function:
Off-shell Effects [should depend on (ps), x, Q2]
eff
F2N
(x  0.6,Q 2 , )
pT = 0
eff
F2N
(x  0.2,Q 2 , )
Modification of the off-shell
scattering amplitude (Thomas,

Melnitchouk et al.)
Color delocalization
Close et al.
Suppression of “point-like
configurations”
Frankfurt, Strikman et al.
“Off-shell” mass of the nucleon M*
939
905
823
694
MeV
MeV
MeV
MeV
… plus 6-quark bags, , MEC…
And of course FSI!
Ps =
0
0.09
0.17
0.25 0.32 0.39 GeV/c
Rules for the spectator.
Summary.
Low momentum spectators
PS < 100 MeV/c
Minimize uncertainty due to
the deuteron wave function and
on-shell extrapolation.
O (1%) correction.
Backward kinematics
θqp > 110o
Minimize effects from FSI and
target fragmentation.
O (5%) correction.
Validation of the spectator tagging method
(BoNuS experiment)
• Check angular dependence of effective (bound)
structure functions in comparison with PWIA
spectator model
• Check spectator momentum dependence of
effective (bound) structure functions in
comparison with PWIA spectator model
Low Spectator Momenta - Nearly Free Neutrons ?
r
D ( p )
2
20%
The
Experiment
BoNuS
Region
VIPs
0.07
0.2 GeV/c
CLAS
backwards p
e-
Radial TPC (view from downstream)
*BoNuS
= Bound Nucleon Scattering
**RTPC
= Radial Time Projection Chamber
Bonus Radial Time Projection Chamber.
(Detector system for slow protons)
• Thin-walled gas target
(7 atm., room temperature)
• Radial Time Projection
Chamber (RTPC) with
Gaseous Electron
Multipliers (GEMs)
• 4 - 5 Tesla longitudinal
magnetic field
(to suppress Möller
electrons and to measure
momentum)
• 3-dimensional readout of
position and energy loss
(“pads”)
RTPC Performance
Particle ID (after gain calibration of each channel)
e- reconstructed in CLAS & RTPC
=8mm
Gain constants for
every channel
(RTPC-Right on
top) – red (blue)
indicates “hotter”
(“colder”) than
average pads
Out-of-time track suppression
z
=1.4º

=4º

Spectator momentum dependence
(preliminary)
Ratio to simulation
Effective F2n
Backwards angles (cos θpq < -0.25) data are shown
80 MeV/c
100 MeV/c
80 MeV/c
100 MeV/c
120 MeV/c
140 MeV/c
120 MeV/c
140 MeV/c
Simulation uses PWIA spectator model, radiative effects, full model of RTPC and CLAS.
P. Bosted and M.E. Christy F2n model is used.
Angular dependence
(preliminary)
80 MeV/c
100 MeV/c
Q2 = 1.66 (GeV/c)2
W* = 1.73 GeV
• No significant deviations
from PWIA (ps<100 MeV/c)
• Possible θ dependence
at higher momenta
120 MeV/c
140 MeV/c
Extracted
F2n (analyses comparison)
(preliminary)
▼ - Analysis 1
▲ - Analysis 2
___ Simulation in
PWIA spectator
picture
- - - CTEQ6X
calculation
Extracted F2n/F2p (N. Baillie)
(preliminary)
Extracted F2n (N.Baillie)
(preliminary)
1.6 < Q2 < 1.9
1.9 < Q2 < 2.2
2.7 < Q2 < 3.2
“Free” neutron structure function compared with a model by P. Bosted and M.E. Christy
Cross Section Fitting (J.Zhang I)
=
A0
+
A1 Cos*
+
A2 Cos2*
24
BoNuS Vs Models, 5 GeV, W = 1.525 (J.Zhang II)
MAID 07
SAID 08
D(e,eppRTPC)p
25
D(e,eppCLAS)p
Plans for 12 GeV
BoNuS
E12-06-113
•
•
•
Data taking of 35 days on D2
and 5 days on H2
with L = 2 · 10 34 cm-2 sec-1
Planned BoNuS detector
DAQ and trigger upgrade
DIS region with
–
–
–
–
•
•
Q 2 > 1 GeV 2/c 2
W *> 2 GeV
ps < 100 MeV/c
pq > 110°
Largest value for x* = 0.80
(bin centered x* = 0.76)
Relaxed cut of W *> 1.8 GeV
gives max. x* = 0.83
CLAS12
Central
Detector
Conclusions
• Preliminary analysis does not contradict
spectator model
• Technically different analyses of BoNuS
data converge
• Analysis note underway
• BoNuS12 proposal re-submission in
preparation