Measurement of the Coulomb quadrupole amplitude in the γ*pΔ(1232)in the low momentum transfer region

Nikos Sparveris

Massachusetts Institute of Technology

Hall A Proposal PR-08-010

u d

u

u d

u

γ* Μ1 , Ε2 , C2

Μ1+ , Ε1+ , S1+ πo

p(qqq)

I = J =

938 MeV

2

1

2

1

Δ(qqq)

I = J =

1232 MeV

2

3

2

3Ν Δ(1232)

Spherical M1

Deformed M1 , E2 , C2Deformation signal

Experimental confirmation of the deviation of the proton structure from spherical symmetry is fundamental and has been the subject of intense experimental and theoretical interest

Studied through the measurement of the electric and Coulomb quadrupole amplitudes (E2,C2) in the predominantly M1 (magnetic dipole-quark spin flip) NΔ(1232) transition

The issue

Experimental activity: MAMI, Bates (low-Q2), JLab (Hall A, B & C) mapping from Q2=0.06 (GeV/c)2 up to 6 (GeV/c)2

Theoretical activity: dynamical calculations, phenomenology, ChEFT, Lattice (Sato-Lee, DMT, MAID, SAID, Pascalutsa-Vanderhaeghen, Alexandrou et al)

Quark model predictions are 30% too low for M1 and an order of magnitude lower for the quadrupole amplitudes

This issue of the quark core and pion cloud contributions has been addressed in a meson exchange model by Sato & Lee – the model quantitatively makes up for the deficiencies of the quark model

The dynamic Sato-Lee calculations are in excellent agreement with the CLAS data

But:

Sato-Lee not in agreement with the low Q2 data taken at Bates and MAMI near the predicted peak of the pion cloud contribution at 0.1 (GeV/c)2.

No data available lower than Q2 = 0.06 (GeV/c)2

There are some discrepancies between the MAMI and the Bates data at Q2 = 0.126 (GeV/c)2 that make the picture unclear at a crucial point.

The status

effect of quark core + pion cloud

Sato-Lee calculation

effect of quark core

Resonant amplitudes in the low Q2 region

Extracting the signal

Separation of the partial cross sections with measurements at various azimouthal angles

p

oΗ(e,e’p)πo

RLT

RL+RT

RTT

RLT’

MultipoleTruncationMultipoleTruncation

Modelinterpretation

Modelinterpretation

Multipoledecomposition

Multipoledecomposition

CMR , EMR

Data Background Signal

Capability to place the spectrometers in small angles and high resolution spectrometers

The lowest Q2 measurements taken at MAMI (Q2=0.06 (GeV/c)2) were constrained by space limitations (lower limits for the 2 MAMI spectrometers are 23o and 15.1o)

The 2 HRS spectrometers in Hall A can go down to 12.5o thus providing access to lower Q2 values.

Experiment requirements:

Hall A standard equipment only

The 2 HRS spectrometers for e and p detection respectively (with their standard detector packages: VDCs, scintillators, Cherenkov, lead-glass)

A 6 cm LH2 target

Beam: Eο=1115 MeV and I=75 μΑ (beam energy will stay constant during the experiment)

Beam energy can be easily adjusted around the above value to accommodate beam energies of other experiments

Why Hall A ?

Φpq= 0o

Φpq= 180o

σLT = ( σ(Φpq=180o) - σ(Φpq=0o) ) / 2 vLT

p

o

The Experiment

H(e,e’p)πo

Kinematical Settings

20% dead-time & 99% detection efficiency have been assumed

47 hrs production + 8 hrs calibrations + 17 hrs config. changes = 72 hrs

6 cm LH2 target , Eo = 1115 MeV , I=75 μA

8.5 hrs

9 hrs

29.5 hrs

Trues / Accidentals

2 ns timing window & 60 MeV Missing-mass cut around pion mass

• Phase space (W,θpq,Q2) will be matched for Φpq = 0o , 180o measurements

• analysis bin size: ΔW = ± 4 MeV , Δθpq = ± 2.5o , ΔQ2 = ± 3*10-3 – 4.5*10-3 (GeV/c)2

• theoretical calculations folded over the acceptance for the extraction of point cross sections

• cross section uncertainties : statistical < ± 1% , systematic < ± 3% , avg.-to-point < ± 0.4%

• σLT uncertainty < ± 8% (depending on kinematics)

• resonant amplitudes will be fitted to the cross sections

• CMR (statistical+systematic) uncertainty < ± 0.20% to < ± 0.28% (from Q2=0.125 to 0.04 (GeV/c)2)

• contributions from background amplitudes from all available models will be introduced to the fits

• Model uncertainty introduced to the CMR < ± 0.30% in all cases

• σLT will be extracted down to Q2= 0.038 (GeV/c)2 and unmatched cross sections in (W,θpq,Q2) will be

extracted down to Q2= 0.036 (GeV/c)2

Data analysis

Phase space : Q2 = 0.125 (GeV/c)2

ΔQ2 = ± 0.0045 (GeV/c)2

ΔW = ± 4 MeVΔθpq = ± 2.5o

analysis bin widths

ΔW cut = ± 5 MeV ΔW cut = ± 5 MeV

Q2 = 0.04 (GeV/c)2 Q2 = 0.09 (GeV/c)2

ΔQ2 = ± 0.003 (GeV/c)2

ΔW = ± 4 MeVΔθpq = ± 2.5o

ΔQ2 = ± 0.004 (GeV/c)2

ΔW = ± 4 MeVΔθpq = ± 2.5o

Phase space

ΔW cut = ± 5 MeV ΔW cut = ± 5 MeV

Projected Results: Q2 = 0.125 (GeV/c)2

Bates results seem to overestimate the MAMI ones at Q2=0.125 (GeV/c)2

Disagreement in the description of the parallel cross section as a function of W

Q2 = 0.06 (GeV/c)2 Q2 = 0.20 (GeV/c)2

Mainz data Mainz data

Q2 = 0.125 (GeV/c)2

proposed measurements

W-dependence at low Q2

Projected Results

Q2 = 0.04 (GeV/c)2

Q2 = 0.09 (GeV/c)2

Projected Results: CMR

going even lower ? Q2 = 0.025 (GeV/c)2 … working on it

900 MeV

Q2 = 0.025 (GeV/c)2

pion cloud ahead

will take an extra 12 hrs (beam on target+config. changes) + Eo = 900 MeV

• CMR will be precisely mapped from Q2=0.125 (GeV/c)2 down to 0.04 (GeV/c)2

• this experiment will provide the lowest Q2 CMR measurements and the most precise ones in the low momentum transfer region

• cross sections will be also be extracted down to 0.036 (GeV/c)2

• discrepancies of other labs (Bates/MAMI) will be resolved

• strong constrains to the most recent theoretical calculations will be provided

• valuable insight to the mechanisms that contribute to the nucleon deformation

Summary

Request:

The 2 HRS spectrometers (with their standard detector packages)

A 6 cm LH2 target

Beam: Eο=1115 MeV and I=75 μΑ (central Eo value adjustable if needed)

3 days of running (including production, calibrations and configuration changes)

Measurement of the Coulomb quadrupole amplitude in the γ*pΔ(1232)in the low momentum transfer region

BACK – UP SLIDES

Results: Q2 = 0.20 (GeV/c)2

Results: Q2 = 0.127 (GeV/c)2

Latest compilation of Bates data and comparison with Mainz data

The Nucleon is DeformedDeformed

Deformed

Spherical

Deformed

Spherical

Ποιοτική διερεύνηση Ποιοτική διερεύνηση MAID MAID στα αποτελέσματαστα αποτελέσματα

CMR = RCMR = RCM (MAID)CM (MAID) •• 1 1

CMR = RCMR = RCM (MAID)CM (MAID) •• 0.5 0.5

CMR = RCMR = RCM (MAID)CM (MAID) •• 0 ( … spherical ) 0 ( … spherical )

RRCM (MAID)CM (MAID) ~ - ~ - 66..55%%