E. Cisbani / SBS for GEp5 @ JLab12 1

Large acceptance magnetic spectrometer for the 12 GeV2 GEp experiment

(at Jefferson Lab)

E. CisbaniINFN Rome – Sanità Group and

Italian National Institute of Healthfor the SBS collaboration

26/Oct/2013 (DNP2013)

Outlook

• Electromagnetic form factors at high Q2

• Experimental requirements

• Spectrometer details

http://hallaweb.jlab.org/12GeV/SuperBigBite

DNP 2013 – Newport News – 26/Oct/2013

E. Cisbani / SBS for GEp5 @ JLab12 2

Form Factors: discovery and formalism• R.W. McAllister, R. Hofstadter Phys. Rev. 102 (1956) 851 “First measurement of

the proton electromagnetic radius”: RMS E/M radius of =(0.74 ± 0.24) 10-13 cm

26/Oct/2013 (DNP2013)

Nucleon electromagnetic current operator has two “unknown” functions (Dirac and Pauli FFs) that describe the internal structure of the nucleon (one photon exchange approx.):

In terms of Sachs FFs:

Elastic Cross section (Rosenbluth):

Sachs FFs are FT of the charge and magnetization distributions in the nucleon (in Breit frame)

E. Cisbani / SBS for GEp5 @ JLab12 3

Proton GE/GM – an «unexpected» discrepancy

26/Oct/2013 (DNP2013)

22MpEp GG

d

d

Rosenbluth Separation: assume single photon approximation

2tan

2

)( e

p

ebeam

l

t

Mp

Ep

M

EE

P

P

G

G

Polarization transfer from the incident electron to the scattered proton

Prior to JLab/2000, expectations were that proton GE/GM fairly constant with Q2

At JLab, new class of experiments show proton GE/GM decreasing linearly with Q2

Two Photon Exchange – favorite candidateDA3: T. AverettHA2: M. Kohl

E. Cisbani / SBS for GEp5 @ JLab12 4

Proton GE/GM - Theoretical models

• Many theoretical models– VMD (Iachello, Lomon, Bijker),

generally good description of all FF– Relativistic CQM (Miller, Gross, ...)

spin dependent quark density– Lattice QCD, start to give prediction – Dyson-Schwinger, dressed quarks,

diquark correlation, ...

– pQCD-based: GE/GMconst Q2

– GPD-based: direct connection to quark OAM, FF’s constraint GPD’s

Most of them agree with current data but diverge at higher, unexplored, Q2

26/Oct/2013 (DNP2013)

E. Cisbani / SBS for GEp5 @ JLab12 5

«Modern» Form Factor measurements at high Q2

Method: Polarization Transfer: Target Perp. Polarization:

Measure(one photon approx.) Pt, Pl : trans. and long. polarization of the recoil

protonN+ and N- : events with opposite transverse target polarization

Many systematics effects (theory and exp.) cancel in ratio

Figure of Merit (stat.): acceptaceL: Luminositys: elastic xsec Pb: beam polarization

Ay: polarimeter analyzing power: polarimeter efficiency

PT : Target polarization

At Q2~10 GeV2 expected: FoMpol_trans ~ 10 FoMtarg_pol (target polarization cannot tolerate large L)

26/Oct/2013 (DNP2013)

Challenges at high Q2:Maximize (coincidence) acceptanceMaximize luminosityMazimize polarization efficiencyMaximize beam polarization

(... having the needed beam energy)

... keeping costs at «affordable» level

26/Oct/2013 (DNP2013) E. Cisbani / SBS for GEp5 @ JLab126

Jefferson Lab - CEBAF after 2013

CHL-2

Upgrade magnets and power supplies

add Hall D (and beam line)6 GeV CEBAF (< 2013)

Max Current: 200 mAMax Energy: 0.8 - 5.7 GeVLong. Polarization: 75-85%

12 GeV CEBAF(>2013)

Max Current: 90 mAMax Energy Hall A,B,C: 10.9 GeVMax Energy Hall D: 12 GeVLong. Polarization: 75-85%

Doubling Beam Energy

E. Cisbani / SBS for GEp5 @ JLab12 7

Proton GE/GM at large Q2 by polarization transferBeam:

Current= 75 mA,Polarization= 85% long.Energy= 6, 8 and 11 GeV

Target:H2 Liquid

Length= 40 cm

Luminosity = 8 · 1038 Detectors:

P-arm: SBS + PolarimeterE-arm: BigCal + Coordinate

26/Oct/2013 (DNP2013)

GOAL: Extend the measurement of the proton form factor ratio GE/GM to the maximum Q2 that is possible with 11 GeV beam with constraints:

Absolute error < 0.1 Beam time = 60 days

GEp5 experiment in HallA

(SBS)

E. Cisbani / SBS for GEp5 @ JLab12 8

New SuperBigbite Spectrometer (SBS) in Hall A

26/Oct/2013 (DNP2013)

Large luminosity“Large” acceptanceForward angles Reconfigurable detectors

Large luminosity“Large” acceptanceForward angles Reconfigurable detectors

• Support event rate 10x higher than with standard small acceptance spectrometer

• GEM chambers to handle the high rate of the background

High photon up to 250 MHz/cm2 and electron 160 kHz/cm2

background

E. Cisbani / SBS for GEp5 @ JLab12 9

Large Luminosity Large Background• Must be supported by the

detectors GEM technology• Must be handled by the trigger:

– spatial and time correlation between electron and proton elastically scattered

– «high» energy threshold in segmented CALO’s

26/Oct/2013 (DNP2013)

Good tracking resolution needed - momentum resolution: 1 % - angular resolution: 1 mrad - vertex reconstruction: 5 mm

Hit

Red: p0 photoproductionBlack: Elastics

Blue: Sum

For Emiss<0.35 GeV, remaining p0 background: 10%

Adequate proton polarization precession reconstruction (next slide)

E. Cisbani / SBS for GEp5 @ JLab12 10

GEp5: Proton Polarimeter (PP) Number of scattered protons:

Require: Dipole magnet to precess Pl at target to Pypp

Polarimeter only measures components of proton spin that are transverse to the proton’s momentum direction

Track in

Track out

Track in

Track out

Pypp Px

pp

N=number of scattered proton, Pe beam polarization

where refers to electron beam helicity

26/Oct/2013 (DNP2013)

Maximize Pe

A (a.u.)

Use azimuthal asymmetry of the proton scattering off matter induced by spin-orbit coupling

E. Cisbani / SBS for GEp5 @ JLab12 11

Beam

SBS Dipole Magnet / 48D48 from BNL

26/Oct/2013 (DNP2013)

Magnet Parameters Integral field strength 1.82 T-m

2.28 T-m with pole shims Yoke length 1.22 m Gap: 47 cm 121.9 cm Yoke Weight 85 tons 6 1008 steel sectors, largest is 18.3 tons

Adapted from Robin Wines / JLab

J

(deg) (mrs)

5 12

15 72

30 76

Magnetic field needed for:• Momentum measurement• Polarimetry• Sweep off low energy charged particles

Yoke modifications to allow beam pipe passage at forward angle kinematics

E. Cisbani / SBS for GEp5 @ JLab12 12

GEM Working principle

26/Oct/2013 (DNP2013)

Ionization

Multiplication

Readout

Multiplication

Multiplication

Recent Technology:F. Sauli, Nucl. Instrum. Methods A386(1997)531

GEM foil: 50 mm Kapton + few mm copper on both sides

with 70 mm holes, 140 mm pitch

Strong electrostatic field in GEM holes

SBS

Gain vs Particle Flux

Support high particle flux ( MHz/cm2)

Intrinsic resolution at 50 mm level

Relatively unexpensive

Robust / Slow aging

E. Cisbani / SBS for GEp5 @ JLab12 13

SBS - GEM Front Tracker• Six 150x40 cm2 chambers with small dead area (~10%)• Each chamber consists of 3 50x40 cm2 lightweight 3xGEM

modules with x/y strip readout (0.4 mm pitch)• Readout electronics based on high channel density APV25 ASIC

driven by VME64x modules

26/Oct/2013 (DNP2013)

Large SNR

Use x/y charge correlation for false hit suppression

E. Cisbani / SBS for GEp5 @ JLab12 14

GEM Front Tracker MonteCarloRealistic MC and digitization• Tracking efficiency 99%-85%

depending on background• Track parameter resolutions at

acceptable values even at largest background

26/Oct/2013 (DNP2013)

E. Cisbani / SBS for GEp5 @ JLab12 15

CH2 Polarimeters with GEM tracking• Two Polarimeters in series to increase statistics by ~50%• Each polarimeters consists of CH2 analyzer (50 cm) and four

50x2000 cm2 GEM chambers• Each chamber is made of five 50x50 cm2 GEM modules• Similar design of GEM front tracker, optimized for focal

polarimetry (less demanding particle rate respect to main tracker)

26/Oct/2013 (DNP2013)

N. Liyanage et al. / UVa qpp (deg)

Num

ber o

f sca

tter

ed p

roto

ns

E. Cisbani / SBS for GEp5 @ JLab12 1626/Oct/2013 (DNP2013)

PJ6: B. Quinn

G. Franklin et al. / Carnegie Mellon

E. Cisbani / SBS for GEp5 @ JLab12 17

High Luminosity, impact on Trigger / DAQ

26/Oct/2013 (DNP2013)

• Must efficienty select electron elastic scattering by angular correlation

• First level (L1) from electron arm– Energy information (with cuts to reduce inelastic)– Rate (from SLAC high energy data and RCS experiments):

• Hadron Arm:– Energy information (with cuts to reduce inelastic)– Rate: 1.5 MHz

• Second level (L2) from two-arm coincidence: – in 30 ns gate: 9 kHz– AND geometrical correlation: 2 kHz

Ethr/Emax % 50 75 85 90Rate [kHz] 1400 203 60 38

DJ4: A. Camsonne

E. Cisbani / SBS for GEp5 @ JLab12 18

SBS, is a cost effective, new magnetic spectrometer; will use the recent GEM technology to operate at high luminosity, providing “large” acceptance and high reconstruction accuracy

SBS will permit unprecedented measurements of the proton and neutron Form Factors at high Q2 as well on SIDIS physics

Conclusions

26/Oct/2013 (DNP2013)

Likely from 2016

NC9: A. Puckett

http://hallaweb.jlab.org/12GeV/SuperBigBite

Expected results on proton GE/GM Kinematics and expected accuracy

E(GeV)

Q2 (GeV2)

Days DmGE/GM

6.6 5.0 1 0.023

8.8 8.0 10 0.032

11.0 12.0 30 0.074