bonus (barely off-shell nucleon structure) experiment update thia keppel cteq meeting november 2007
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BONUS (Barely Off-Shell Nucleon Structure) Experiment Update
Thia Keppel
CTEQ Meeting
November 2007
Large x Parton Distribution Functions - Large Large x Parton Distribution Functions - Large UncertaintiesUncertainties
Where can we turn for help….
u(x) d(x)
Textbook Physics, Major ProblemTextbook Physics, Major Problem
Quark-Parton Model
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Without a free neutron we are forced to extract F2
n from a bound system. However, a large dependence on correct modeling of the deuteron comes into play for x >~0.5
⎟⎠
⎞⎜⎝
⎛ +≈
⎟⎠
⎞⎜⎝
⎛ +≈+=
→
→∑
)(9
1)(
9
4)(
)(9
1)(
9
4))()(()(
12
1
22
xuxdxxF
xdxuxxqxqexxF
x
n
xq
qp
F2n/F2
p
Many More Sophisticated Many More Sophisticated PredictionsPredictions
F2n/F2
p d/u
SU(6) 2/3 1/2Scalar Diquark 1/4 0H-P Quark Model 1/4 0 pQCD 3/7 1/5Counting Rules 3/7 1/5
Review Articles : Isgur, Phys. Rev. D59, 34013 (1999) Brodsky et al., Nucl. Phys. B441, 197 (1995)Melnitchouk and Thomas, Phys. Lett. B377, 11 (1996)
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Bound Neutron Structure FunctionsBound Neutron Structure Functions
• Simple subtraction (deuteron-proton) yields nonsense
• Kinematic shift of the effective Bjorken variable x or of W
0.70 0.690.80 0.780.90 0.851.00 0.90
F2n = F2d - F2p ?
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3
W [GeV]
F2
F2d
F2p
F2d-F2p
€
xmeasured =Q 2
2Mνxrelevant =
Q 2
2 Enν −r p n ⋅
r q ( )
+ Binding (off-shell) effects, coherent scattering, final state interactions, non-nucleonic degrees of freedom in the ground state, nucleon structure modification (“EMC”-effect)
Can we reduce model dependence?Can we reduce model dependence?Yes! Within the nuclear impulse approximation. The virtual photon interacts with the neutron on a short enough time scale that the proton doesn’t know what happened. The spectator continues on unperturbed w/ momentum ps = -p
2sin'4
/)(
'
22 θα
ν
EEQ
MpE
EEz
ss
=
−=
−=
γ*(q)
d(pd)
n(p)
p(ps)
X
F2n(eff)
S
Can we reduce model dependence?Can we reduce model dependence?
2sin'4
/)(
'
22 θα
ν
EEQ
MpE
EEz
ss
=
−=
−=
γ*(q)
d(pd)
n(p)
p(ps)
X
F2n(eff)
S
Yes! Within the nuclear impulse approximation. The virtual photon interacts with the neutron on a short enough time scale that the proton doesn’t know what happened. The spectator continues on unperturbed w/ momentum ps = -p
BoNuS
Region
Can we reduce model dependence?Can we reduce model dependence?
2sin'4
/)(
'
22 θα
ν
EEQ
MpE
EEz
ss
=
−=
−=
γ*(q)
d(pd)
n(p)
p(ps)
X
F2n(eff)
S
Yes! Within the nuclear impulse approximation. The virtual photon interacts with the neutron on a short enough time scale that the proton doesn’t know what happened. The spectator continues on unperturbed w/ momentum ps = -p
BoNuS
Region
We focus on VIPs (Very Important Protons) where Rn > 99%
n
e
p
e
p
n
Detect very important Detect very important low momentum low momentum protons. If the proton is protons. If the proton is also going also going backwardsbackwards in in the lab frame it is the lab frame it is almost guaranteed to almost guaranteed to be only a spectator. be only a spectator.
n
e
p
<-- ambiguous -->proton target neutron target
neutron target !
BONUS: An Effective Free Neutron Target from Deuterium….(e- + d e- + p + x)
Spectator proton
e-
e-
e-
BoNuS Radial TPC DesignBoNuS Radial TPC Design
3 cm He
φ, z from pads
r from time
dE/dx from charge along track (particle
ID)
3 cm Helium/DME at 80/20 ratio
Stagger pads in z to improve theta angle reconstruction
Big PictureBig Picture
• rTPC inside Solenoid inside CEBAF Large Acceptance Spectrometer (CLAS)
• Track scattered e- in CLAS• Locate e- interaction point
in target.
• Electron tracked in CLAS provides trigger to BoNuS radial TPC
• Link pspectator with electron vertex.CLAS (taking data since 1997) is
made of Drift Chambers, Time of Flight Scintillators, Cerenkov counters and Electromagnetic Calorimeters for tracking, momentum determination, and PID
Solenoid Magnet
BoNuSBoNuS in CLASBoNuS in CLAS
RTPC viewed during insertioninto CLAS (above), andfrom the solenoid downstream end (left)
Proton Tracks in rTPCProton Tracks in rTPC
e- beam coming out of pageFit to tracke- beam traversing
target
RTPC entrance window
RTPC readout plane
Proton track identifiable by consecutive charge deposit
BoNuS RTPC in actionBoNuS RTPC in action
The z coordinate of the trigger electron’s vertex is compared with the z vertex of the track in the RTPC. A clear peak around Δz = 0 is seen riding on top of accidental coincidences.
∆p/p
Compare proton momentum measured by the RTPC to CLAS
prediction σ ≈ 20%
BoNuS RTPC in actionBoNuS RTPC in action
The z coordinate of the trigger electron’s vertex is compared with the z vertex of the track in the RTPC. A clear peak around Δz = 0 is seen riding on top of accidental coincidences.
∆p/p
Compare proton momentum measured by the RTPC to CLAS
prediction σ ≈ 20%
BoNuS RTPC in actionBoNuS RTPC in action
The z coordinate of the trigger electron’s vertex is compared with the z vertex of the track in the RTPC. A clear peak around Δz = 0 is seen riding on top of accidental coincidences.
⎥⎦
⎤⎢⎣
⎡
calcdxdQ
dxdQ
/
/log exp
10
The measured dQ/dx is compared w/ Bethe-Bloch prediction for a proton having the measured momentum.
protons
heavy nuclei background
Status of BoNuS AnalysisStatus of BoNuS Analysis
The spectator proton’s four momentum: pnμ = -(Es – MD, pps)
Light-cone momentum fraction: αs = (Es – psz)/M
€
W 2 = M 2 + 2Mν − Q2
€
W 2 = pn + q( )2 = pn
μ pnμ + 2 (M D − Es )ν −r p n ⋅
r q ( )−Q 2
≈ M *2 +2Mν (2−α S )−Q 2
E = 4.223 GeV
Status of BoNuS AnalysisStatus of BoNuS Analysis
n
np
CLAS
RTPC
N
N
σσσ
ε)( +
=
model for σn/σD by P. Bosted
Choose the QUASI-ELASTIC region (0.8 < W < 1.07 GeV) to make a first approximation of the tagging efficiency
Tag
gin
g E
ffici
en
cy (
%)
Status of BoNuS AnalysisStatus of BoNuS Analysis
So far…So far…
•RTPC effectively reconstructed and identified slow protons
•spectator tagging technique works
•VERY PRELIMINARY results for n/d and n/p
What the future holds…What the future holds…•compare experimentally determined tagging efficiency w/ MC
•Complete final CLAS and RTPC corrections and calibrations, including radiative corrections
•Finish acceptance and charge measurement studies in order to extract absolute cross-sections
preliminary
preliminary
preliminary
Kinematic CoverageKinematic Coverage
E = 5.262 GeV E = 4.223 GeV
E = 2.140 GeV
W (
GeV
)
W (
GeV
)
W (
GeV
)
Q2 (GeV2) Q2 (GeV2)
Q2 (GeV2) cos(θpq)
psp
ec
(MeV
)