the super bigbite project: physics and optimization

The Super BigBite project:physics and optimization

• The Super Bigbite form factor experiments.

• The physics of the nucleon form factors, and what is most important.

• How the Super Bigbite project is optimized to access the highest-impact physics.

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• E12-09-016: measurement of GEn/GMn to Q2=10 GeV2.

• E12-07-109: measurement of GEp/GMp to Q2=15 GeV2.

• E12-09-019: measurement of GMn/GMp to Q2=18 GeV2.

The Super Bigbite ProjectWill provide the necessary (additional) apparatus to perform well

optimized measurements of three out of four ground state electromagnetic nucleon form factors

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Figure-of-merit X50 better than any experiment on the books.



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The Super Bigbite Project truly represents a “3rd generation” series of FF measurements.

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Layout for measuring GEp

On the proton arm: “The Super Bigbite Spectrometer” (SBS), comprised of: the new Super Bigbite magnet, a GEM based double recoil polarimeter and a hadron calorimeter.

On the electron arm, the (existing) BigCal Lead-Glass Calorimeter instrumented with a GEM based tracker.

Electron Arm

Beam

.

.

Target

Proton form factors ratio, GEp(5) (E12-07-109)

Proton Arm

BigCalLead-GlassCalorimeter

BNLINFN

HCalo

Al filter

48D48

GEM

GEM

GEM

BigBenGEM

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Expt. to measure GEp/GMp

p(!e, e!!p)

Layout for measuring GEn and GMn

Electron arm: Existing “BigBite” spectrometer instrumented with GEM trackers.

Hadron arm: Hadron calorimeter to detect the neutron (or proton).

Super Bigbite magnet, used to deflect protons to aid in distinguishing protons versus neutrons.

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3!"He(!e, e!n)pp

Expt. to measure GEn/GMn

d(e, e! p)nd(e, e! n)p

Expt. to measure GMn/GMp

The physics of form factors

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Nucleon form factors and nucleon structure

It was noted in the 2007 NSAC Long Range Plan that measurements of the ground-state form factors at CEBAF

after the 12 GeV upgrade, “... remain the only source of information

about quark distributions at small transverse distance scales."

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Nucleon form factors and nucleon structure

It was noted in the 2007 NSAC Long Range Plan that measurements of the ground-state form factors at CEBAF

after the 12 GeV upgrade, “... remain the only source of information

about quark distributions at small transverse distance scales."

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And to access short distance scales, one must go to high Q2.

The ground-state electromagnetic form factors are an essential part of the

description of the nucleon

The hadronic current:

J µhadronic = eN(p!)

!!µF1(Q2) +

i"µ!q!

2MF2(Q2)

"N(p)

Dirac FF Pauli FF

GE = F1 ! !F2 and GM = F1 + F2

! = Q2/4M2nucleon

The Sachs FFs:

where

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The expectation was that the ratio GEp/GMp would remain constant.

Data on GEp/GMp prior to JLab

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Precise data on GEp/GMp at high Q2 from JLab

Totally unexpected, these data forced a reconsideration of nucleon structure.

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Precise data on GEp/GMp at high Q2 from JLab

]2 [GeV2Q

0 5 10 15

p M/G

p E G

p!

0

1

GEP-I

GEP-II

VMD - Bijker and Iachello (2004)

CQM - Miller (2002)

VMD - Lomon (2002)

Roberts - DSE

2)/Q2!/2(Q2 ln"

1/F

2F

Stat. Error Only

The theoretical studies that have been performed in response to the new data on GEp/GMp represent some of the most sophisticated efforts to

date to understand the nucleon in terms of QCD degrees of freedom

Over 500 citations!

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The evolution of the FFs using pQCD

The corrections result from including in the light-cone quark wave function components with L≠0, indicating the dynamic

importance of quark orbital angular momentum.

Hall A results From Belitski, Ji and Yuan

From simple pQCD counting rules, Q2F2p/F1p should become constant.

With logarithmic corrections, scaling appears to be restored.

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The DSE/Faddeev calculation from ArgonneA sophisticated QCD-based nucleon model

The three constituent quarks then serve as the degrees of freedom for a calculation involving a Faddeev equation.

Diquark-coupling is included.

While still a model, the calculation has features that move toward a true analytical approach.

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Dyson-Schwinger equations are used to “dress” the quark operators, thus dynamically

generating their mass.

Relativistic constituent quark models

Explains the data well, in part by including substantial violation of

hadron helicity conservation, i.e. an important dynamic role for quark

orbital angular momentum.12

Super Bigbite offers precision just where it is needed, at high Q2

2 in GeV

2Q

0 5 10

n M/G

n EG

nµ

0.0

0.5

1.0

VMD - E. Lomon (2002)

RCQM - G. Miller (2002)

DSE - C. Roberts (2009)

- Schiavilla & Sick20

d(e,e'd) T

= 300 MeV!, 2)/Q2!/2(Q2 ln" 1

/F2

F

Galster fit (1971)

Madey

E02-013

E12-09-006, SHMS

E12-09-016, SBS (This Experiment)

Data between 10-15 GeV2 will cleanly distinguish between

some of the models.

Proton Neutron

At 7 GeV2 some models are still

close to one another.

By 10 GeV2, it is possible to

discriminate between all the

best models. 13

Hall A measurement of GMn/GMp

Will represent the most precise measurement to date of GMn at high Q2.

Tests both model predictions (such as discussed earlier) as well as pQCD scaling.

Needed to extract GEn from measurements of GEn/GMn .

Needed to perform flavor separations of all FFs.

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Generalized Parton Distributions (GPDs)and the nucleon form factors

Jq =12

! 1

!1x dx [Hq(x, !, 0) + Eq(x, !, 0)]

Ji’s Sum Rule yields the total angular momentum assoicated with the quarks

The elastic form factors provide critical constraints on the GPDs.

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GPDs provide the most complete description to date of nucleons, providing a unified framework that encompasses both structure functions from DIS as well as the elastic form factors.

GPDs are used to provide what is sometimes called 3D tomography of the nucleon.

They can be used to extract all kinds of important physics, one example coming from what is often called Ji’s Sum Rule

The Super Bigbite design parameters have been optimized for maximum

physics impact.

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Optimizing Form Factor Experiments

For a double-polarization experiment with a recoil polarimeter:

(An extra factor of Q4 comes from AY2. Also, we assume that the solid angles for the electron and hadron arms are appropriately matched.)

Things become challenging at high Q2 !

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Optimizing Form Factor Experiments

From all this, we gather the following:

For a double-polarization experiment with a polarized target:

• All other things being equal, a polarized target experiment (F.o.M. ~ 1/Q12) is preferable to a recoil polarimeter experiment ( F.o.M.~ 1/Q16).

• Want to maximize Ω while maintaining ΔQ2/Q2 ~ 0.1

• Want to maximize L as much as possible.

Figure of Merit ! ! · ! · P 2 · L

! E2

Q12· ! · P 2 · L

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Optimizing acceptance and rates

The Super Bigbite magnet is optimized for FF measurements.

• Single dipole provides adequate momentum resolution (~1%) and large solid angle (~70 msr) acceptance. (Also true of original BigBite magnet).

• Vertical aperture well matched to electron arm while still appropriate for ΔQ2/Q2 ~ 0.1.

• Cut in yoke permits operating at small angles where the recoil is going.

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(More info in talk by John LeRose.)

Optimizing detectors for luminosityThe use of Gas Electron Multiplier (GEM) technology for

tracking ensures the ability to handle very high luminosity.

• Expected hit rate in front tracker is less than 600kHz/cm2, well underneath the capability of GEMs (>60MHz/cm2).

• Up to 14x2 planes provide enormous redundancy for track reconstruction.

• Hadron calorimeter permits relatively high threshold, again increasing rate-handling capability.

(See next talk by Evaristo Cisbani.)

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Measuring GEn/GMn

Here, the measured asymmetry has the form:

A = A! sin !" cos "" + A# cos !" =a · (GE /GM ) sin !" cos ""

(GE /GM )2 + c+

b · cos !"

(GE /GM )2 + c

Where θ* is the angle between the neutron polarization and the q vector.

From the form of the asymmetry given above, the neutron detector needs to be at roughly 90o to the polarization.

This limits the region over which detection is optimal. It precludes, for instance, trying to achieve 2π azimuthal coverage.

In this case, it is feasible to use a polarized 3He target (gaining a factor of Q4 over a recoil polarimeter). The measured asymmetry has the form:

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Why Super Bigbite is optimal for measuring GEn/GMn

• It is feasible to use a polarized 3He target (extra factor of Q4).

• You get close to the largest solid angle (~70 msr) that you can actually use, even if money were no object.

• The luminosity can be quite high (~ 6x1036/cm2s).

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Why Super Bigbite is optimal for measuring GEp/GMp

• For measuring the proton, polarized targets cannot tolerate the high luminosities required, thus a recoil polarimeter is the way to go.

• For the acceptance, Super Bigbite can provide up to 70 msr (we use less, around 40 msr).

• In luminosity, Super Bigbite can tolerate more than 8x1038cm-2s-1

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2, GeV2Q0 2 4 6 8 10 12 14 16

M/G

E G

pµ

-0.5

0

0.5

1

1.5

GEp-I

GEp-II

Qattan et al, 2005

Christy et al, 2004

Andivahis et al, 1994

Hall C PRELIMINARY

Iachello+Bijker, 2004

GPD, Guidal et al, 2005

Miller LFCBM 2002

= 300 MeV!Belitsky et al pQCD 2003,

Lomon VMD 2002

The challenge of measuring GEp/GMp at high Q2 with existing equipment

Scaling from the highest GEp(3) point using existing equipment and TRIPLE the beam time.

F.o.M. ! E2

Q16· ! · L

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2, GeV2Q0 2 4 6 8 10 12 14 16

M/G

E G

pµ

-0.5

0

0.5

1

1.5

GEp-I

GEp-II

Qattan et al, 2005

Christy et al, 2004

Andivahis et al, 1994

Hall C PRELIMINARY

Iachello+Bijker, 2004

GPD, Guidal et al, 2005

Miller LFCBM 2002

= 300 MeV!Belitsky et al pQCD 2003,

Lomon VMD 2002

Super BigBite provides the flexibility to achieve a higher figure of merit.

Larger Figure-of-Meritis absolutely crucial!!

Scaling from the highest GEp(3) point using Super Bigbite and the SAME the beam time.

F.o.M. ! E2

Q16· ! · L

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Hall A Super Bigbite GMn/GMp provides essentially x20 F.o.M. over competing experiments

• Provides much better statistics at high Q2 during 30 day run than CLAS 12 during 56 day run.

• Excellent example of how larger angular coverage (with CLAS 12) does not necessarily help.

• Super Bigbite/BigBite provides 70 msr solid angle with little or no constraint on luminosity.F.o.M. ! E2

Q12· ! · L

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• The Super Bigbite Project represents a highly optimized approach for studying the nucleon elastic form factors.

• It provides excellent precision at high Q2.

• The Super Bigbite program is well optimized to extract the highest-impact physics.

• Even with unlimited funding, it is unclear that one would want to follow a significantly different approach. Super Bigbite is even well optimized from the perspective of cost.

Summary

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the super bigbite project: physics and optimization

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