the bonus experiment at jefferson lab’s clas

The BoNuS Experiment at Jefferson Lab’s CLAS.

Svyatoslav Tkachenko

University of South Carolina

for the CLAS collaboration

Structure functionsand 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

• Moments can be directly compared with OPE (twist expansion), Lattice QCD and Sum Rules– All higher moments are weighted towards large x

Q2=3.15 (GeV/c)2

Ratio to CTEQ6

Q2=3.15 (GeV/c)2

Structure Functions and Resonances

• Precise structure functions in Resonance Region constrain nucleon models[Separate resonant from non-resonant 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 (qp) 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

F2p

14d /u

4 d /u

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.

d

u

4 F2n F2 p 1

4 F2n F2 p

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

pS E S ,p S ; S ES

p S ˆ q

M D /2

pn MD ES ,p S ;

n 2 S

W 2 M 2 2M Q2

W *2 pn q 2 pn pn 2 (MD Es )

p n

q Q 2

M *2 2M(2 S ) Q 2

x Q 2

2 pnq

Q 2

2M (2 S ) *

E = 4.223 GeV

e

p

n <Q2> = 1.19 (GeV/c)2

“Rules” for the spectator.Final state interactions.

The momentum and angular dependence of the ratio of spectral functions with andwithout FSI effects. Blue boxes mark preferred kinematics – regions where FSI havesmaller effect.

Ciofi degli Atti and Kopeliovich, Eur. Phys. J. A17(2003)133

“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]

F2Neff (x 0.6,Q 2 , )

F2Neff (x 0.2,Q 2 , )

Modification of the off-shell scattering amplitude (Thomas, Melnitchouk et al.)

Color delocalizationClose et al.

Suppression of “point-like configurations”Frankfurt, Strikman et al.

pT = 0

939 MeV

905 MeV

823 MeV

694 MeV

“Off-shell” mass of the nucleon M*

Ps = 0 0.09 0.17 0.25 0.32 0.39 GeV/c

… plus 6-quark bags, , MEC…

And of course FSI!

Rules for the spectator.Summary.

Low momentum spectatorsPS < 100 MeV/c

Minimize uncertainty due tothe deuteron wave function and on-shell extrapolation. O (1%) correction.

Backward kinematicsθqp > 110o

Minimize effects from FSI andtarget 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 ?

*BoNuS = Bound Nucleon Scattering

**RTPC = Radial Time Projection Chamber

Radial TPC (view from downstream)

e-backwards p

The Experiment

BoNuS

Region

VIPs

0.07 0.2 GeV/c

D (r p )

2

CLAS

20%

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”)

e- reconstructed in CLAS & RTPC

RTPC Performance

z

=8mm

=4º=1.4º

Out-of-time track suppression

Gain constants for every channel (RTPC-Right on top) – red (blue) indicates “hotter” (“colder”) than average pads

Particle ID (after gain calibration of each channel)

Spectator momentum dependence (preliminary)

Ratio to simulation Effective F2n

Simulation uses PWIA spectator model, radiative effects, full model of RTPC and CLAS.P. Bosted and M.E. Christy F2

n model is used.

80 MeV/c 100 MeV/c

120 MeV/c 140 MeV/c

80 MeV/c 100 MeV/c

120 MeV/c 140 MeV/c

Backwards angles (cos θpq < -0.25) data are shown

Angular dependence(preliminary)

80 MeV/c 100 MeV/c

120 MeV/c 140 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

Extracted F2n (analyses comparison)

(preliminary)▼ - Analysis 1

▲ - Analysis 2

___ Simulation in PWIA spectator picture

- - - CTEQ6X

calculation

Extracted F2n/F2

p (N. Baillie)

(preliminary)

Extracted F2n (N.Baillie)

(preliminary)

“Free” neutron structure function compared with a model by P. Bosted and M.E. Christy

1.6 < Q2 < 1.9 1.9 < Q2 < 2.2 2.7 < Q2 < 3.2

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,epCLAS)pD(e,epRTPC)p

prel

imin

ary

25

Plans for 12 GeV

BoNuS

E12-06-113E12-06-113

• 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 GeVgives max. x* = 0.83

CLAS12CentralDetector

Conclusions

• Preliminary analysis does not contradict spectator model

• Technically different analyses of BoNuS data converge

• Analysis note underway

• BoNuS12 proposal re-submission in preparation