Dvcs in an ep collider
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4th EIC Workshop Hampton University 19--23 May 2008. DVCS in an ep Collider. Charles Earl Hyde Universit é Blaise Pascal and Old Dominion University. DVCS. High CM energy less important than high Luminosity.

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Dvcs in an ep collider

4th EIC Workshop

Hampton University

19--23 May 2008

DVCS in an ep Collider

Charles Earl Hyde

Université Blaise Pascal

and

Old Dominion University


Dvcs in an ep collider

DVCS

  • High CM energy less important than high Luminosity.

    • Factor of two range in CM energy can resolve BH•DVCS from |DVCS|2 without positron beams.

  • Precision Cross sections (1% from each independent contribution, 4% total)

    • Spin Observables for H,E,… separations

    • Cannot test factorization (only test models) with relative asymmetries.

  • High Resolution

  • Forward proton tagging essential

    • Exclusivity;

    • Transverse imaging ( resolution).

    • Spectator nucleons in D

    • N final state at threshold ==> Soft Pion Theorems


Dvcs in an ep collider

H(e,e’) Exclusivity Jlab Hall A E00-110

[ H(e,e’)X - H(e,e’)Y ]: Missing Mass2

H(e,e’p

H(e,e’…

H(e,e’p) sample

H(e,e’p) simulation,

Normalized to data

<2% in estimate of

H(e,e)N…

below threshold MX2<(M+m)2


Jlab hall a cross sections c munoz camacho et al prl97 2006

JLab Hall A Cross Sections C. Munoz Camacho et al PRL97 (2006)

  • Isolation of Re and Im parts of Twist-2 DVCS-BH interference.

  • Kinematic weight of DVCS2 term is ≤ 1% in Im[BH*DVCS], 4.5% in Re.

    • DVCS2 term in VGG ≈ 20.

Im[BH*DVCS]

BH2

Re[BH*DVCS] + |DVCS|2


Bh dvcs bilinear dvcs terms

BH-DVCS & Bilinear DVCS terms

  • Variable energy Collider: Scaling with CM energy se = [(k+p)2]1/2

  • Subtract BH2 term by direct calculation. Isolate BH-DVCS from bilinear DVCS terms I with two or more collider CM energies, with variation in se of at least a factor of 2.

  • Complete set of Proton and electron spin observables will separate all DVCS GPDs and bilinear DVCS terms.

  • Test in HallA e07-007 expected 2010


Detector notions design questions

Detector notions / Design Questions

  • Five Detector Segments

    • Ultraforward hadron projectile tagging in beam lattice

    • Forward “hadrons”

    • Central Detector (least important??)

    • Forward “electrons”

    • ≈0˚ tagging of quasi-real photons in electron beam lattice.

  • Where are the angular boundaries between elements?

    • Central  Forward (Electrons, Hadrons)

    • Mininum Forward and Maximum ultra-forward angles

      • (maximum angle obstructed by final focus lattice elements)

      • Maximum Ultra-forward angle acceptance of final focus elements.

P

k

Quasi-real photon tagging

Ultra-peripheral, tagging

« Central »

Forward “Hadrons”

Forward “Electrons”


E e kinematics 5 gev electrons 50 gev c protons

(e,e’) Kinematics5 GeV electrons  50 GeV/c Protons

  • k<k’ for xB<k/P

  • e<30 deg for Q2≤4 GeV2


E e kinematics 10 gev electrons 100 gev c protons

(e,e’) Kinematics10 GeV electrons  100 GeV/c Protons

  • k<k’ for xB<k/P

  • e<30˚ for Q2≤16 GeV2

  • Push “Electron Endcap” to 45˚


Collider vs fixed target kinematics at equal s

Collider vs. Fixed Target Kinematics at equal s

  • sM2 = 2kFM=2kC [E+P]C≈4kCPC

    • kF = kC [E+P]C/M ≈ 4kCP.

  • DVCS: Fixed Target

    • 2 minimized for q’||q

      • q’, q, p’ all co-linear at 2min.

  • Boost by P >>1 along (-)electron beam direction to collider frame.

    • Non-parallel boost to q, q’, p’

      • q2 < 0 = q’2 < p’2

    • Very different rotations of q, q’, p’ into collider frame


Dvcs kinematics where are the photons

DVCS KinematicsWhere are the Photons?

  • Boost =P/E from target rest-frame to Collider frame at same s=(k+P)2.

    • (Electron in z direction)

  • In rest frame at ∆2min, q’||q.

  • q’z≈gq’Rest(cosqRest+)

    • q’z>0 For >cos(Rest)

    • Photon is boosted into “hadronic hemisphere”


5x50 gev 2 collider q 2 5 gev 2

5x50 GeV2 Collider, Q2=5 GeV2

Angle of Final Real Photon, Relative to Proton Axis

Central

Hadronic

Forward Hadronic

Forward Electron Side

Calorimeters in shadow of FF Quads

Central

Electron


10x100 gev 2 collider

10x100 GeV2 Collider,

Central

Hadronic

Forward Hadronic

Forward Electron

Calorimeters in shadow of Final Focus Quads

Central

Electron


Where does the dvcs proton go

Where does the DVCS Proton Go?

  • DVCS variables

    • =(q-q’) = (P’-P)

      •  = Component of  perp. to (P’+P)

      • C = Component of  perp. to P.

    • xB=Q2/(2q•P)

    •  = (q+q’)2/[4(q+q’)•(P+P’)]  xB / (2- xB)

    • 2 = (2•P)/(2k•P) ≈ 2

  • All variables are equivalent, to order 2/Q2

    • .  xB / (2- xB)

    • C 

  • 2=[4M2+ C]/(1-2)(exact)

  • For design study, assume C ≤ 1 GeV2

    • tan(P’)] =[ |C | / P(1-2) ]

    • Constraint of Final Focus Aperture is independent of Q2.


Final focus aperture r at distance l dvcs recoil protons

Final Focus Aperture R at Distance L & DVCS “Recoil” Protons

  • Constraint [R/L] > [ |C | / P(1-2) ]

    • 5mr limits |C | < 500 MeV/c at P=100 GeV/c

    • 5mr limits |C | < 250 MeV/c at P=50 GeV/c

    • Pion pfrom N final state p< 60 MeV/c ≈ [ 5 mr] [mP/(M+m)]


Dvcs recoil proton tagging

DVCS Recoil Proton Tagging

  • Large aperture Lattice for tagging P’ is more important to DVCS than small angle detection in the forward hadron side (final photon).

  • Large Aperture Final Focus Quadrupoles (≥10mr)

    • Compact Calorimetry in front of Quadrupole “shadow” to catch the narrow range of xB≈2z for which final DVCS photon is occluded by Final Focus Quadrupoles


Exclusivity for 1b we better deliver

Exclusivity,for $1B, we better deliver

  • Exclusivity means different things to different people.

  • “Photon Exclusivity”

    • Resolve P(e,e’)P’ from P(e,e’)P’

      • Distance to calorimeter and granularity is sufficient to separate  opening angle of m/E≈ m/q’

      • Detector is sufficiently hermetic to detect  decays with good efficiency (including asymmetric decays over ≈ 50% of CM)

  • “Baryon Exclusivity”

    • Resolve P(e,e’)P’ from P(e,e’)N etc.

    • Missing Mass Squared (k+P-k’-q’)2 < (M+ m)2=1.15 GeV2

    • Tag final proton P’ in lattice.


Exclusivity resolution study

Exclusivity Resolution Study

  • Incident Beam Spread

    • p||/p = 0.001

    • p/p = [p*]1/2

      • (k,P) = (70,0.7)•10-6m

      • * =5mm (ELIC, maximum luminosity

        • P=160MeV/c @ 100 GeV/c

  • Scattered Electron

    • k’/k’ = 1 mrad

    • k’||/k’ = 1%  1%[1GeV/k’]1/2

  • Internal pre- and post-bremsstrahlung

  • Final Photon

    • q’/q’ = 1 mrad

    • q’/q’ = 1%  4%[1GeV/q’]1/2

  • Final Proton (for  Study)

    • p/p = 200 m / 10 m = 20 rad

    • p||/p = 200 m / 2m = 0.01%


5 gev 50 gev c e p

5 GeV  50 GeV/c (e P)

  • Q2=4 GeV2

  • 2= 0.2

  • P’ tagging required

    • Exclusivity

    •  Resolution

      • () ≈ 0.3GeV2 without tagging

      • Transverse Imaging


10 gev 100 gev c e p

10 GeV  100 GeV/c (e P)

(Turn off beam spread)

  • Q2=8 GeV2

  • 2= 0.05

    • q’ ≈ 5 GeV

  • P’ tagging required

  • [Missing Mass]2 resolution terrible at high energy

  •  Resolution unchanged from 5  50

    • Transverse Imaging


Conclusions eic dvcs

Conclusions: EIC-DVCS

  • Forward (|q|<45˚) region more important than central

    • High energy final photon in “forward hadron” side of detector

  • Full Ultra-forward tagging of final proton essential:

    • Exclusivity

    • Transverse Imaging

      • Transverse emittance of beams cannot be neglected at ultimate luminosity

    • Tagged proton and neutron DVCS in Deuteron

      • e+d --> e’++NDVCS+Nspectator

    • Large aperture Final Focus Quads required.


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