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

)