Nucleon Structure Functions:An Introduction of the BoNuS

Program at Hall B Jefferson Laband the G2P Project at Hall A

Jixie Zhang (Jlab)

March. 13, 2013

Outline• Thesis Work

– BoNuS Project– Data Analysis:

Exclusive: Extract D(e,e`p)p cross section

Inclusive: F2n structure function

– Results

• Current Work– G2P Project– Status

• Future Work2

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

Dominated by the valence quarks’ contribution at large x

4

Very 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 (J. Gomez et al.) and E140 (L. W. Whitlow et al.)

Cross Section in the Resonance Region

• Goal: to understand neutron as well as proton, understand u and d quark structure function ...

• 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

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).

7

d

p

ps

n

quasi-free (IA)

primary background

FSI for D(e,e`p)p

dp

p

n

p

d

p

p

n

p rescattering

pp rescattering

primary background

8

No theory calculation for * n - p FSI yet. Try to estimate this from FSI of n - 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

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, P. Bosted and ......

RTPC Sits in the Center of CLAS

FC

CLAS

BoNuS RTPC

stopper

11

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

12

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

13

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… 14

Q2 = -(qµ)2 = 4 sin2(e/2)

W2 = (qµ + nµ )2

= (qµ + dµ - psµ)2

= (µ + pµ )2

Exclusive Electro-Production Analysis

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

15

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

16

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.

Missing Mass Cut: from Gaussian Center

5 GeV 5 GeV

background

18

Beam Energy MM Cut of D(e,e`pCLAS)p MM Cut of D(e,e`pRTPC)p

2.1 0.9436 +/- 0.0509 0.9374 +/- 0.0400

4.2 0.9401 +/- 0.0605 0.9356 +/- 0.0501

5.2 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

19

Radiation Correction, E=5.3 GeV

20

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.

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

)CLAS trigger efficiency, 5G

21

Data outside the curve will not be considered!

CLAS proton efficiency

22

RTPC proton efficiency

23

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) 24

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

26

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

27

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*= + +28

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

29FSI + Binding < 20% under VIP cut

Summary of Exclusive Analysis • 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.

30

Inclusive Analysis

• The Ratio Method★ measure tagged counts divided by inclusive counts★ correct this ratio for backgrounds★ one scale factor gives F2

n/F2d

• The Monte Carlo Method★ measure tagged counts★ divide by spectator model Monte Carlo results★ multiply by F2

n used in the model

• The two methods have different systematic errors, but give very similar results.

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/F2

p 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

Comparison of These 2 Methods

34

1, by S. Tkachenko, MC method2, by N. Baillie, Ratio method

BoNuS F2n

35

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 of Inclusive Analysis

38

• 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

The G2P Project at Hall A

39

My Contributions:•Simulation

• Helped to optimize run plan • Optimized the local dump and its shielding• Estimate the radiation damage• Concept design for the 3rd arm

•Field mapping •Made the optics run plan•Managed the shift schedule, helped to organize meetings•Measured the beam polarization with Moller•Event reconstruction (Optics)

40

41

42

43

44

45

46

47

The Geant4 Simulation Program, HRSMC

48

•Designed to support all Hall HRS experiment•BitBite geometry and detector included •Use SANKE transport packages. HRS QQDQ field not include•Used by G2P, GEP, CREX

Adjust Simulation to Match Data

Working on a new SNAKE model. Courtesy to Chao Gu and Min Huang. 49

484816 no raster

Isolate Target Field from SNAKE Model

• Optics models could be simplied and well known for no target field ......• Can we isolate the target filed behavious in g2p|gep optics?

• 5.0 Tesla target field, the integrated BdL_y from 800 mm to beyond is only 0.01 Tesla.m, about 1.46% of the total. 50

Optics: No Target Field

• Vertex to Sieve: propagated by Geant4 with a physics model (i.e. MSC, Decay, Ionization, EM physics...)

• Sieve to Focal: project sieve to target then using SNAKE forward model, 8-10 software collimator cuts are applied along the trajectory to make sure particle is really hitting the focal plane.

• Focal to target: using SNAKE backward model or real optics matrix.• Target to Vertex: linear projection.

Jixie Zhang HRS Optics 51

Target plane

Vertex planeSieve planeVirtual boundary

Focal plane

Snake forward

Snake

backward

Propagated by Geant4

Optics: With Target Field

• Vertex to Sieve: propagated by Geant4 with a physics model• Sieve to Focal: project sieve to target then using SNAKE forward model, 8-10

software collimator cuts are applied along the trajectory to make sure particle is really hitting the focal plane.

• Focal to Sieve: using SNAKE backward model or real optics matrix from focal to target, then project from target to sieve.

• Sieve to Target: using Drift-In-Field model.• Target to Vertex: using Drift-In-Field model.Jixie Zhang HRS Optics 52

Target plane

Vertex planeSieve planeVirtual boundary

Focal plane

Snake forward

Snake

backward

Propagated by Geant4

Drift-In-Field

Resolution of HRSMC Using SNAKE Model 484816+shim, No Target Field

53

Raster=2.4cm, BPM resolution: 1 mm in vertical, No target field

Resolution of HRSMC Using SNAKE Model 484816+shim, Stop at Exact Z0,5T

54

Raster=2.4cm, BPM resolution: 1 mm in vertical, 5.0T target fieldReconstruct to thrown vertex z (perfect Ytg_tr resolution)

Resolution of HRSMC Using SNAKE Model 484816+shim, Stop at Target Plane,5T

55

Raster=2.4cm, BPM resolution: 1 mm in vertical, 5.0 T target fieldReconstructed to the target center

How well can we determine the central value?

56

Summary for the G2P Project

• We managed to accompolish most of the proposed physics goal.• Detector calibration is almost done. Only BPM calibration is left over.• The strategy of the reconstruction with target field is:

No-Target-Filed-HRS + Drift-in-Taget-Field .

This strategy works well.• Geant4 simulation program for HRS is ready to use. This program can

also be used for any other HRS experiements with minimum modification (just add the target geometry).

• There is huge difficulty to find the vertex plane (interaction plane). We can only reconstruct events back onto the target plane. That said we will not have good event by event resolution for the scattering angle. But we will be able to determine the XS central value up to 1%.

57

Thanks

58

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.

59

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

60

Resonances examples

61

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

62

CLAS in Jefferson Lab, Hall B

63

Radial Time Projection Chamber (RTPC)

64

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

Vertex Cut

66

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.

67

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

68

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)69

Energy Loss Correction for RTPC Proton

70

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

71

Background of D(e,e`pCLAS)p, 5G Real

72

f1f2

f3f4

Background of D(e,e`pCLAS)p, 5G Sim.

73

f1sim f2

sim

f3sim

f4sim

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) 74

Acceptance Correction, E=5.3 GeV

75

Acceptance Correction, E=5.3 GeV

76

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.

77

Time integrated Luminosity

78

Sim Vs Real

Real 5G

Simula

tion 5

G

79

A0: BoNuS Vs MAID, 4 GeV with VIP cut

prel

imin

ary

80

81

82

83

84

85

86