Nucleon Structure Function, An Introduction of the BoNuS
Program at Hall B Jefferson Laband the G2P Program at Hall A
Jixie Zhang (Jlab)
March. 13, 2013
Outline• Thesis Work:
extract D(e,e`p)p cross section– BoNuS Project– Data Analysis– Result
• Current Work: – G2P Project– Status
• Future Work
2
3
Nucleon Structure Functions
Structure Function Definition (in the quark-parton model)
)(9
1)(
9
4)(
)(9
1)(
9
4))()(()(
12
1
22
xuxdxxF
xdxuxxqxqexxF
x
n
xq
qp
Callan-Gross Relation
4
Strong Model Dependence
SU(6) symmetry uv(x) = 2dv(x)
S = 0 dominant, one-gluon exchangeClose, Phys Lett B43 (1973)Carlitz, Phys Lett B58 (1975)Close and Thomas Phys Lett B212 (1988)Isgur, Phys Rev D59 (1999), Phys Rev Lett 41 (1978)
Sz = 0 dominant, helicity conservationFarrar and Jackson, Phys Rev Lett 35 (1975)Brodsky, Burkardt and Schmidt, Nuc Phys B441 (1995)
SLAC E139 (L. W. Whitlow, et al.), and E140 (J. Gomez, et al.)
Cross Section in the Resonance Region
• Data on the Proton: Clear resonant structure, separation from the non-resonant background is possible
• Data on the deuteron: Kinematically smeared due to binding, off-shell, final state interactions (FSI), etc.
5
Exclusive electro-production D(e,e`p)p
Detect e′, and at least ONE of the two final state protons in D(e,e`p)p to ensure exclusivity and select events where the “spectator” proton has low, backwards momentum. Conservation of energy and momentum allows to determine the initial state of the neutron.
Low momentum “spectator” proton
Novel approach by the BoNuS collaboration: detect the spectator proton directly.
e′
enp
p
ps
d
6
Q2 = -(qµ)2 = 4 sin2(e/2)
W2 = (qµ + nµ )2
= (qµ + dµ - psµ)2
= (µ + pµ )2
Production Kinematics
n - p
* = polar angle of the outgoing in C.M. frame
* = Azimuthal angle of the outgoing in C.M. frame
, q
*
*
n
p
Hadronic plane
E, k
E, k
Leptonic plane
7
Off-Shell and FSI for D(e,e′ps)X
Final State Interactions
Off-shell Effects
VIPs
Select low Ps (<120 MeV/c) and large backward pq (>100o), angle between Ps and virtual photon, to minimize FIS.
Off-shell effects are negligible for small Ps. Choose Ps<120 MeV/c as Very Important Spectator Protons (VIP)
C. Atti et al, Eur. Phys. J. A 19,133-137 (2004).
W. Melnitchouk et al, Phys. Lett. B377, 11 (1996).
8
d
p
ps
n
quasi-free (IA)
primary background
Final State Interations
dp
p
n
p
d
p
p
n
p rescattering
pp rescattering
primary background
9
No theory calculation for * n - p FSI yet. Try to estimate this from FSI ofn - p .......V. Tarasov, A. Kudryavtsev, W. Briscoe, H. Gao, IS, arXiv: 1105.0225
FSI Prediction for D(,p)p, by Laget
R = ratio of the total to the quasi-free cross sectionR = polar angle between photon and spectator protonPR = momentum of the spectator proton
underp rescattering peak region only
Strong momentum and anglular dependence
Cross Section Calculations1. Select exclusive events
2. Apply corrections: background corr., radiative corr., trigger eff., , CLAS proton and RTPC proton detection eff., and acceptance correction.
Jefferson Lab Experiment E03-012
Barely off-shell Nucleon Structure (BoNuS)
• Electron beam energies: 2.1, 4.2, 5.3 GeV• Spectator protons were detected by the newly built
Radial Time Projection Chamber (RTPC)• Scattered electrons and other final state particles were
detected by CEBAF Large Acceptance Spectrometer (CLAS)
• Target: 7 atm D2 gas, 20 cm long
• Data were taken from Sep. to Dec. in 2005
N.Baillie, S. Tkachenko, J. Zhang,
K. Griffioen, S. Kuhn, Gail Dodge, C. Keppel, M.E. Christy,
W. Melnitchouk, H. Fenker, S. Bültmann
RTPC Sits in the Center of CLAS
FC
CLAS
BoNuS RTPC
stopper
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Radial Time Projection Chamber (RTPC)
proton
Deuteron / Helium
Helium/DME at 80/20 ratio
Sensitive to protons with momenta of 67-250 Mev/c
3 layers of GEM
3200 pads (channels)
5 Tesla B field
Particles ID by dE/dx
100 µm
3-D tracking:time of drift -> rpad position -> ,
z
dE/dX
7Atm. D2 gas target, 20cm in length
Trigger Electron
14
RTPC Resolution
Trigger electrons measured by CLAS are compared to the same electrons measured in BoNuS during High Gain Calibration runs.
H. Fenker, et.al. Nucl.Instrum. Meth. A592:273-286,2008
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Analysis Outline1. Quality Checks
2. Vertex correction and vertex z cuts
3. Particle identifications (electron, and proton )
4. Fiducial cut for trigger electron, and proton
5. Energy loss correction for electron, and proton
6. Exclusive cut (2- Missing Mass cut)
7. Acceptance cut and acceptance correction
8. Background subtraction
9. Trigger efficiency correction, CLAS proton and RTPC proton detection efficiency correction
10.Radiation correction
11.Luminosity analysis
12.Extract the cross section and structure functions
13.Systematic error estimation
Simulation Overview
RTPC(Geant4) CLAS(geant3) Reconstruction Analysis
What have been done with simulation?• Debug/optimize RTPC reconstruction packages• Generate energy loss correction tables, radiation length tables• Study Detector’s acceptance for D(e,e`pCLAS)p and D(e,e`pRTPC)p• Study particle detection efficiency • Model the background… 17
Kinematic coverage and binning, 5 GeV
cos*: 8 bins with boundaries at -1.0, -0.5, -0.1, 0.3, 0.55, 0.7, 0.8, 0.9,1.0
*: 15 bins, 24 degrees each bin, [0.0,360.0)
W: 150 MeV each bin, [1.15,2.95)
Q2 : 6 bins with boundaries at 0.1309, 0.3790, 0.7697, 1.0969, 1.5632, 2.6594, 4.5243
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Missing Mass Cut: from Gaussian Center
5 GeV 5 GeV
background
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Beam Energy MM Cut of D(e,e`pCLAS)p MM Cut of D(e,e`pRTPC)p
2.x 0.9436 +/- 0.0509 0.9374 +/- 0.0400
4.x 0.9401 +/- 0.0605 0.9356 +/- 0.0501
5.x 0.9389 +/- 0.0718 0.9350 +/- 0.0615
Background Subtraction
f1, f2, f3 and f4 are the fitted background fraction functions for real data. f1
sim, f2sim
, f3sim and f4
sim are the fitted background fraction functions for simulated data.
Final background correction factor:
where 0.9545 is the coverage of 2 in a gaussian distribution.
Simulated Data
average
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Background of D(e,e`pCLAS)p, 5G Real
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f1f2
f3f4
Background of D(e,e`pCLAS)p, 5G Sim.
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f1sim f2
sim
f3sim
f4sim
Radiation Correction, E=5.3 GeV
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W`=1.23
*
/
bo
rn
Leading Log approximation Calculation Based on Maid07
Using upraded “Exclurad”. Updated with MAID07 multipole model for both neutron and proton. A dedicate clas-note will be vesy soon.
CLAS trigger efficiency, 5G
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Trigger efficiency is the fraction that the trigger particle (electron) is detected. It is obtained by scaling the simulation data to the real data, then calculating the ratio of real event counts in each p- grid to the simulated event counts in the same grid.
(deg)
P
(GeV
)
Data outside the curve will not be considered!
CLAS proton efficiency
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RTPC proton efficiency
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D(e,ep)p Acceptance Correction
• Binning information:
W: 150 MeV each bin, [1.15,3.10);
Q2 : 6 bins, {0.1309, 0.3790, 0.7697, 1.0969, 1.5632, 2.6594, 4.5243 };
cos*: 8 bins, 0.25 each bin, [-1.0,1.0);
*: 15 bins, 24 degrees each bin, [0.0,360.0).
• Acceptance is the ratio of the number of events detected in a given bin to the number of generated events in the same bin.
• Need to generate tables for D(e,epRTPC)p and D(e,epCLAS)p reaction separately.
• Applied event by event
• Need to have acceptance cut to ensure the reliabilities
For CLAS channel: 0.008 and less than 10% uncertainty (> 100 detected events)
For RTPC channel: 0.003 and less than 15% uncertainty (> 40 detected events) 27
Cross Section Calculations1. Select exclusive events
2. Apply corrections: background corr., radiative corr., trigger eff., , CLAS proton and RTPC proton detection eff., and acceptance correction.
Cross Section: BoNuS Vs MAID and SAID
4 GeV, W` = 1.23 GeV
0.85
Cos
0.75
0.625
0.95
MAID 07 SAID 08 D(e,epCLAS)pD(e,epRTPC)p
prel
imin
ary
GeV2
Minimizing Final State Interactions2 GeV, W` = 1.23 GeV
VIP Cut= pq>100o and 70<ps<120 MeV
prel
imin
ary
With VIP Cut
Without VIP Cut
Prediction: FSI + Binding < 20% under VIP cutData: 1)Better agreement between CLAS and RTPC channel for low Q2; 2) No obvious change other than removing data points to the results with larger Q2
Cos=0.85
Cos=0.85
Cos=0.95
Cos=0.95
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Systematic Error 4 GeV, W` = 1.23 GeV
15% in average, mainly come from the following (values are all in average):
Cos=0.85
Cos=0.95
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D(e,epCLAS)pD(e,epRTPC)p
Rad. corr. ~5% Background corr. 5%
Acceptance cut 10% Missing mass cut 5%
Detection eff. 8% Luminosity 2%
GeV2
FSI is estimated to be 15%, not included in the figure yet
Fit for the Structure Functions
A0 A1 Cos* A2 Cos2*= + +32
Structure Functions: BoNuS Vs MAID 5 GeV, W` = 1.525 GeV
VIP = 70<ps<120 MeV/c, and pq>100o
D(e,epRTPC)p + VIPD(e,epCLAS)p + VIP
prel
imin
ary
33FSI + Binding < 20% under VIP cut
A0: BoNuS Vs MAID, 4 GeV with VIP cut
prel
imin
ary
34
BoNuS Kinematic Correction
BoNuS
Region
VIPs
ps distribution
70, 100, 200 MeV/c
• Very Important Protons 70<ps<100 MeV/c
• Corrections make resonances stand out
• F2n/F2p can be measured at high x*
Accidental Backgrounds
• Fits to the triangular background allows us to measure backgrounds underneath the peak
• Rbg = Blue area / Pink area, Rbg is independent of kinematics
Two Analysis Methods
• The Ratio Method★ measure tagged counts divided by inclusive counts★ correct this ratio for backgrounds★ one scale factor gives F2n/F2d
• The Monte Carlo Method★ measure tagged counts★ divide by spectator model Monte Carlo results★ multiply by F2n used in the model
• The two methods have different systematic errors, but give very similar results.
Comparison of These 2 Methods
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1, by S. Tkachenko, MC method2, by N. Baillie, Ratio method
BoNuS F2n
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BoNuS F2n/F2
p • F2
n/F2n vs. x
• Curves are CETQ error bands
• CETQ cuts off at low x because Q2 is too low
• Lower cuts in W* imply higher x but the inclusion of resonance contributions.
• Results are consistent with CETQ trends at high x.
E12-06-113E12-06-113
BoNuS Plans for 12 GeV
Data taking:
– 35 days on D2
– 5 days on H2
– L = 2 x 1034 cm-2 sec-1
DIS region: – Q2 > 1 GeV2
– W* > 2 GeV
– ps < 100 MeV/c
– θpq > 110°
– x*max = 0.80W* > 1.8 GeV: x*max = 0.83
Summary (I) • Measured absolute cross sections for D(e,e′p)p reaction over a
wide kinematic range, 1.1<W`<2.8 GeV, 0.13<Q2<4.5 GeV2, full range of cos* and *.
• We see qualitative consistency in most bins between our results and model predictions. In most bins, the pRTPC and pCLAS channels are consistent.
• Huge increase in available data points (about 5000) for n p. Currently only ~900 points available in the world database. BoNuS data will be used to improve our understanding of neutron structure, as part of fits to world data (SAID, MAID…).
• Systematic error is estimated to be ~15% (not include FSI).
• Plan to repeat this analysis using other deuteron data, i.e. CLAS E6 data. Will also use BoNuS 12GeV data to increase statistics.
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Summary (II)
43
• We have measured F2n on a “free” neutron target
• No effects from Fermi motion and final-state interactions
• No evidence for off-shell structure for ps<100 MeV/c• F2
n/F2p behaves at high x much like CETQ high-x fits
• F2n resonance data will significantly improve the
world data set, which up to now came from d with nuclear corrections
Backup Slices
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Motivation• Purpose: In order to understand the structure of the
nucleon (neutron and proton) we need to study the excited states (resonances).
• We have a lot of data on the proton but almost nothing on the neutron – both are needed for a complete understanding.
• Strategy is to use pion production: * + n p
• We have some real photon data, almost no electroproduction (virtual photon)
• Difficulty: No free neutron target, need to use deuteron instead.
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46
Deep Inelastic Electron Scattering
Need to know this ratio to extract structure function information from inclusive processes (Rosenbluth separation)
•Provided first experimental evidence for the “partonic” structure of the nucleon
•As we increase the momentum transferred to the target, the resolving power increases
Deep Inelastic ScatteringDeep Inelastic Scattering
MQ
x2
2
e- e-
*
Nq
q
Q2 > 1 GeV2, W > 2 GeV
2cos
'4 24
22 Q
EMott
Resonances predicted by Quark Model vs. experimental measurements
From http://pdg.lbl.gov/2009/reviews/rpp2009-rev-quark-model.pdf
Full lines: tentative assignment to
observed states
Dashed lines: so far no observed
counterparts.
47
Resonances6 families, depending on quark content (Isospin):
N*(1/2), (3/2),(0), (1), (1/2), (0)
N*, Resonance, combination of u and d only
Notation: L2I2J
L: orbital angular momentum
I: Isospin
J: total angular momentum J = L+S
S,P,D,F,G,H L= 0,1,2,3,4,5
+N + N
Possible for N not
Heisenberg Uncertainty Principle:
short life time wide energy
Measured from the decay products
I=1 I=1/2
+N + NI=0 I=1/2
Possible for both N and
48
Resonances examples
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Exclusive Cross Section
Unpolarized virtual photon cross-section of *+n +p
Degree of transverse polarization:
,
50
d
p
ps
n
quasi-free (IA)
primary background
Final State Interations
d
p
0
p
p
dp
p
n
p
d
p
p
n
p rescattering
pp rescattering ep scattering
primary background
51
52
Continuous Electron Beam Accelerator Facility (CEBAF)
53
Jefferson Lab
A B C
Continuous Electron Beam Accelerator Facility
E: 0.75 –6 GeV Imax: 200A RF: 1499 MHz Duty Cycle: 100% (E)/E: 10-4
Polarization: 85% Simultaneous
distribution to 3 experimental Halls
Injector
LIN
AC L
INA
CExperimental Halls
54
CLAS in Jefferson Lab, Hall B
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Radial Time Projection Chamber (RTPC)
56
Analysis Procedure1. Quality Checks
2. Vertex correction and cuts
3. Particle identification (electron, and proton )
4. Fiducial cut for trigger electron, and protons
5. Energy loss correction
6. Exclusive cut (Missing Mass cut)
7. Acceptance correction
8. Background subtraction
9. Radiation correction
10.Particle detection efficiency for e, proton and RTPC proton
11.Extract cross section and structure functions
Run Selection: Quality Checks
4 GeV 5 GeV
Without Stopper
Bad Runs
58
Zel-Z2
(mm)
Sector 2
Sector 6
Sector 4
Sector 1
Sector 3
Sector 5
Zel-Z2
(mm)
Before and After Vertex Correction
Using real data, find the best beam position to minimize the vertex difference among coincident particles
59
Vertex Cut
60
Electron Identification
• rtr
1. Negative charge.
2. Number of photo electrons (Nphe) > 1.5
3. E_inner > 0.06 and 0.016*P+0.15 < E_total/P < 0.34
4. Pass Osipenko cut (geometry matching between SC and CC)61
CLAS Particle IdentificationsNegative non-trigger particles:
(1) above purple curve
(2) Among the rest, between purple and light blue curve K
(3) Among the rest, below light blue curve anti-proton
Positive particles:
(1) between light blue and green curve proton
() below green curve Deuteron
(3) Among the rest, above purple curve
(4) Among the rest, between purple and light blue curve K
62
RTPC Proton Identification
proton
Deuteron Helium
dQ/dx
calcdxdQ
dxdQ
/
/log exp
10
protons
NucleiReal data
63
The Drift Path of An Ionized Electron
•The red lines show the drift path of each ionization electron that would appear on a given channel.
•In green is the spatial reconstruction of where the ionization took place.
•In reconstruction, hits which are close to each other in space are linked together and fit to a helical trajectory.
•This resulting helix tells us the vertex position and the initial three momentum of the particle.
A MAGBOLTZ simulation of the crossed E and B fields, together with the drift gas mixture, determines the drift path and the drift velocity of the electrons.
64
RTPC Gain CalibrationChannel by Channel gain multipliers can be determined for each run by comparing the track’s expected energy loss to the measured value.
After applying the gain corrections, a clear separation of protons and heavier particles through dE/dx has been achieved.
Gain constants (vs phi and z) determined independently for two different runs.Before and After Gain Calibration
65
Energy Loss Correction for CLAS Particles
Binned by measured , , z, p;
Based on simulation, fit ‘P0-P vs P’ with an integral Bethe-Bloch function for all particles
Pr
e
P0-P
me
asu
red (
Ge
v/c
)
Pmeasured (Gev/c)66
Energy Loss Correction for RTPC Proton
67
Fiducial Cut Fitting Function
Fiducial cut is the geometry cut which is used to remove inefficiency part of the detector.
Momentum dependent
Electron: = 0.35,
Pion: = 1.0
Ptoton : = 3.0
Fitting function:
New Fiducial cut for e
69
Fiducial cut for
70
Fiducial cut for proton
71
Exclusive Events Selection1. Select good trigger (scattered electron)
q<0 , CC + Osipenko (CC geometry match cut)EC (Ein>0.06, without Etot/p Cut)Theta_Z_Cut
2. Vertex Z correlation cut|z_el –Z_i| < 2.71 cm, i could be , fast proton or RTPC proton.
3. Identify the , if not found, use a negative non-trigger particle as
4. Identify protons, if not found, use a positive particle as proton
5. Apply vertex and energy loss corrections6. Apply 2- missing mass cut
72
D(e,ep)p Acceptance Correction
• Binning information:
W: 150 MeV each bin, [1.15,3.10);
Q2 : 6 bins, {0.1309, 0.3790, 0.7697, 1.0969, 1.5632, 2.6594, 4.5243 };
cos*: 8 bins, 0.25 each bin, [-1.0,1.0);
*: 15 bins, 24 degrees each bin, [0.0,360.0).
• Acceptance is the ratio of the number of events detected in a given bin to the number of generated events in the same bin.
• Need to generate tables for D(e,epRTPC)p and D(e,epCLAS)p reaction separately.
• Applied event by event
• Need to have acceptance cut to ensure the reliabilities
For CLAS channel: 0.008 and less than 10% uncertainty (> 100 detected events)
For RTPC channel: 0.003 and less than 15% uncertainty (> 40 detected events) 73
Acceptance Correction, E=5.3 GeV
74
Acceptance Correction, E=5.3 GeV
75
Target Thickness
Calculate the effective target pressure, Peff , for a data set by weighting the pressure of each run with the number of good electrons in that run.
76
Time integrated Luminosity
77
Sim Vs Real
Real 5G
Simula
tion 5
G
78
A0: BoNuS VIP Vs MAID, 2 GeV
prel
imin
ary
79
Cross Section: BoNuS Vs MAID and SAID
2 GeV, W` = 1.23
0.85
Cos
0.75
0.625
0.95
MAID 07 SAID 08 D(e,epCLAS)pD(e,epRTPC)p
prel
imin
ary
80
Cross Section: BoNuS Vs MAID and SAID
5 GeV, W` = 1.23
0.85
Cos
0.75
0.625
0.95
MAID 07 SAID 08 D(e,epCLAS)pD(e,epRTPC)p
prel
imin
ary
81
A0: BoNuS VIP Vs MAID, 4 GeV
prel
imin
ary
82
A0: BoNuS VIP Vs MAID, 5 GeV
prel
imin
ary
83