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Page 1: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Exclusive Processes at Hermes

Zhenyu Ye, DESY

for the collaboration

HEP 2005, LISBOA, PORTUGAL, JULY 2005

Outline

Motivation

Deeply Virtual Compton Scattering

DVCS Measurements at HERMES

Summary and Outlook

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.1

Page 2: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Motivation — Nucleon Structure

q∆

Lq

∆G

δqLg

Proton Spin

1

2=

1

2(

∼30%︷ ︸︸ ︷∆u + ∆d + ∆s) + Lq

︸ ︷︷ ︸Jq

+ ∆G + Lg︸ ︷︷ ︸Jg

∆q: known from DIS

∆G: first measurments in DIS

Lq, Lg: unknown!

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.2

Page 3: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Motivation — Nucleon Structure

q∆

Lq

∆G

δqLg

Proton Spin

1

2=

1

2(

∼30%︷ ︸︸ ︷∆u + ∆d + ∆s) + Lq

︸ ︷︷ ︸Jq

+ ∆G + Lg︸ ︷︷ ︸Jg

∆q: known from DIS

∆G: first measurments in DIS

Lq, Lg: unknown!

GPDs (x, ξ, t)

tp p′

(x + ξ) · p (x − ξ) · p Generalized Parton Distributions ⇒ Jq, Jg

Ji’s Sum Rule — Ji, PRL 78 (1997) 610

Jq,g =1

2limt→0

∫ 1

−1

dx · x · [Hq,g(x, ξ, t) + Eq,g(x, ξ, t)]

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.2

Page 4: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

How to Access GPDs?

GPD(x, ξ, t)

e

e′

N N′

t

γ∗(q)

γ

x + ξ x − ξ

GPD(x, ξ, t)

e

e′

N N′

t

g

γ∗

L(q)

ρ0, π, . . .

x + ξ x − ξ

GPDs constrained by known quantities (FFs, PDFs, . . . ) and accessible in exclusive processes.

At large Q2 and small t, exclusive electroproduction of real photons or mesons can be factorizedinto a hard, perturbative part and a soft, non-perturbative part (GPDs).

Deeply Virtual Compton Scattering

e + N → e′ + N ′ + γ

described by GPDs H, E, H, E,

simplest process, gluons absent in the leading order.

Exclusive Meson Production

e + N → e′ + N ′ +(ρ0, π, . . .

)

vector mesons (ρ0, ω, φ): H, E,

pseudoscalar mesons (π, η): H, E,

pion pairs (π+π−): H, E,

meson distribution amplitude should be taken care of.

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.3

Page 5: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Deeply Virtual Compton ScatteringDVCS (a) and Bethe-Heithler (b) processes have the same initial and final states:

(a) (b)

p p′ p p′ p p′

e

e′ e e′ e e′

γ

γ γγ∗

γ∗

γ∗

Interference between DVCS and Bethe-Heitler:

dσ(eN → eNγ) ∝ |TBH |2 + |TDV CS |2 + TBHT ∗DV CS + T ∗BHTDV CS︸ ︷︷ ︸I

TBH is parameterized in terms of Dirac and Pauli Form Factors F1, F2, calculable in QED.

TDV CS is parameterized in terms of Compton form factors (convolution of GPDs) H, E, H, E .

At HERMES kinematics, T BH � T DV CS , H, E, H, E are accessed through I.

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.4

Page 6: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Azimuthal Asymmetries in DVCS

x

y

z φ

~pγ

~k

~k′

~qφS

uli

dσ(eN → eNγ) ∝ |TBH |2 + |TDV CS |2 + TBHT ∗DV CS + T ∗BHTDV CS︸ ︷︷ ︸I

I induces azimuthal asymmetries in the cross-section:

Beam-charge asymmetry AC(φ) :dσ(e+, φ) − dσ(e−, φ) ∝ Re[F1H] · cos φ

Beam-spin asymmetry ALU (φ) :dσ(→e , φ) − dσ(

←e , φ) ∝ Im[F1H] · sin φ

Longitudinal target-spin asymmetry AUL(φ) :

dσ(⇐

P , φ) − dσ(⇒

P , φ) ∝ Im[F1H] · sin φ

Transverse target-spin asymmetry AUT (φ, φs) :dσ(φ, φS) − dσ(φ, φS + π)

∝ Im[F2H− F1E] · sin (φ − φS) cos φ + Im[F2H − F1ξE] · cos (φ − φS) sin φ

=⇒ the only place E enters in the leading order =⇒ Asin (φ−φS) cos φ

UT sensitive to Jq

Jq,g =1

2limt→0

∫ 1

−1dx · x · [Hq,g(x, ξ, t) + Eq,g(x, ξ, t)]

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.5

Page 7: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Azimuthal Asymmetries in DVCS

x

y

z φ

~pγ

~k

~k′

~qφS

uli

dσ(eN → eNγ) ∝ |TBH |2 + |TDV CS |2 + TBHT ∗DV CS + T ∗BHTDV CS︸ ︷︷ ︸I

I induces azimuthal asymmetries in the cross-section:

Beam-charge asymmetry AC(φ) :dσ(e+, φ) − dσ(e−, φ) ∝ Re[F1H] · cos φ

Beam-spin asymmetry ALU (φ) :dσ(→e , φ) − dσ(

←e , φ) ∝ Im[F1H] · sin φ

Longitudinal target-spin asymmetry AUL(φ) :

dσ(⇐

P , φ) − dσ(⇒

P , φ) ∝ Im[F1H] · sin φ

Transverse target-spin asymmetry AUT (φ, φs) :dσ(φ, φS) − dσ(φ, φS + π)

∝ Im[F2H− F1E] · sin (φ − φS) cos φ + Im[F2H − F1ξE] · cos (φ − φS) sin φ

=⇒ the only place E enters in the leading order =⇒ Asin (φ−φS) cos φ

UT sensitive to Jq

Jq,g =1

2limt→0

∫ 1

−1dx · x · [Hq,g(x, ξ, t) + Eq,g(x, ξ, t)]

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.5

Page 8: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Azimuthal Asymmetries in DVCS

x

y

z φ

~pγ

~k

~k′

~qφS

uli

dσ(eN → eNγ) ∝ |TBH |2 + |TDV CS |2 + TBHT ∗DV CS + T ∗BHTDV CS︸ ︷︷ ︸I

I induces azimuthal asymmetries in the cross-section:

Beam-charge asymmetry AC(φ) :dσ(e+, φ) − dσ(e−, φ) ∝ Re[F1H] · cos φ

Beam-spin asymmetry ALU (φ) :dσ(→e , φ) − dσ(

←e , φ) ∝ Im[F1H] · sin φ

Longitudinal target-spin asymmetry AUL(φ) :

dσ(⇐

P , φ) − dσ(⇒

P , φ) ∝ Im[F1H] · sin φ

Transverse target-spin asymmetry AUT (φ, φs) :dσ(φ, φS) − dσ(φ, φS + π)

∝ Im[F2H− F1E] · sin (φ − φS) cos φ + Im[F2H − F1ξE] · cos (φ − φS) sin φ

=⇒ the only place E enters in the leading order =⇒ Asin (φ−φS) cos φ

UT sensitive to Jq

Jq,g =1

2limt→0

∫ 1

−1dx · x · [Hq,g(x, ξ, t) + Eq,g(x, ξ, t)]

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.5

Page 9: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

DVCS Measurements at HERMESFixed target experiment, Forward spectrometer

Tracking: δP/P < 2%, δθ < 1 mrad

Particle Identification: εe > 99%, hadron contamination < 1%

Photons: calorimeter δEγ/Eγ ∼ 5%

Recoiling protons not detected ⇒ missing mass technique (ep → e′pγ)

M2x = (Pe + Pp − Pe′ − Pγ)2

Background contribution ∼ 5% is determined from MC and corrected.

γ

1

0

2

-1

-2

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� � � � �

m

LUMINOSITY

CHAMBERSDRIFT

FC 1/2

DVC

MC 1-3MONITOR

BC 1/2

BC 3/4 TRD

PROP.CHAMBERS

FIELD CLAMPS

PRESHOWER (H2)

CALORIMETER

DRIFT CHAMBERS

TRIGGER HODOSCOPE H1

0 1 2 3 4 5 6 7 8 9 10

RICH

MUON HODOSCOPEWIDE ANGLE

FRONTMUONHODO

MAGNET

m

IRON WALL

e

27.6 GeV

MUON HODOSCOPES

STEEL PLATE

TARGETCELL

HODOSCOPE H0

LAMBDAWHEELS

e’

HadronsPROTONRECOIL

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.6

Page 10: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

DVCS Measurements at HERMESFixed target experiment, Forward spectrometer

Tracking: δP/P < 2%, δθ < 1 mrad

Particle Identification: εe > 99%, hadron contamination < 1%

Photons: calorimeter δEγ/Eγ ∼ 5%

Recoiling protons not detected ⇒ missing mass technique (ep → e′pγ)

M2x = (Pe + Pp − Pe′ − Pγ)2

Background contribution ∼ 5% is determined from MC and corrected.

γ

1

0

2

-1

-2

� � � � �

� � � � �

� � � � �

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� � � � �

� � � � �

� � � � �

� � � � �

m

LUMINOSITY

CHAMBERSDRIFT

FC 1/2

DVC

MC 1-3MONITOR

BC 1/2

BC 3/4 TRD

PROP.CHAMBERS

FIELD CLAMPS

PRESHOWER (H2)

CALORIMETER

DRIFT CHAMBERS

TRIGGER HODOSCOPE H1

0 1 2 3 4 5 6 7 8 9 10

RICH

MUON HODOSCOPEWIDE ANGLE

FRONTMUONHODO

MAGNET

m

IRON WALL

e

27.6 GeV

MUON HODOSCOPES

STEEL PLATE

TARGETCELL

HODOSCOPE H0

LAMBDAWHEELS

e’

HadronsPROTONRECOIL

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.6

Page 11: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

DVCS Measurements at HERMESFixed target experiment, Forward spectrometer

Tracking: δP/P < 2%, δθ < 1 mrad

Particle Identification: εe > 99%, hadron contamination < 1%

Photons: calorimeter δEγ/Eγ ∼ 5%

Recoiling protons not detected ⇒ missing mass technique (ep → e′pγ)

M2x = (Pe + Pp − Pe′ − Pγ)2

Background contribution ∼ 5% is determined from MC and corrected.

exclusive peak ep → e′pγ

semi-incl.bkd. ep → e′X(π0 . . . )↪→ γ

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.6

Page 12: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

DVCS Measurements at HERMESFixed target experiment, Forward spectrometer

Tracking: δP/P < 2%, δθ < 1 mrad

Particle Identification: εe > 99%, hadron contamination < 1%

Photons: calorimeter δEγ/Eγ ∼ 5%

Recoiling protons not detected ⇒ missing mass technique (ep → e′pγ)

M2x = (Pe + Pp − Pe′ − Pγ)2

Background contribution ∼ 5% is determined from MC and corrected.

exclusive peak ep → e′pγ

semi-incl.bkd. ep → e′X(π0 . . . )↪→ γ

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.6

Page 13: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Beam-Charge Asymmetry in DVCS

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

-3 -2 -1 0 1 2 3

φ (rad)

AC

e± p → e±’γ X (Mx< 1.7 GeV)HERMES PRELIMINARY (<-tc> = 0.12 GeV2)

c0 + c1 cos φ + s1 sin φ

χ2/ ndf : 11.47/ 8c0 = 0.009 ± 0.020 (stat)c1 = 0.059 ± 0.028 (stat)s1 = 0.094 ± 0.028 (stat)

-0.2

0

0.2

0.4

-2 0 2

φ (rad)

Ac

HERMES PRELIMINARY

e± d → e±' γ X (all d)A=c0 + c1 cosφ + s1 sinφ (Mx<1.7 GeV)

c0= 0.003 +/- 0.013 (stat.)c1= 0.061 +/- 0.018 (stat.)s1= 0.010 +/- 0.018 (stat.)

χ2/ndf = 2.26

Beam-Charge Asymmetry:

AC(φ) =dσ(e+, φ) − dσ(e−, φ)

dσ(e+, φ) + dσ(e−, φ)∝ Re [F1H] · cos φ

proton: Acos φC

= 0.059 ± 0.028(stat)

deuteron: Acos φC

= 0.061 ± 0.018(stat)

proton deuteron

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.7

Page 14: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Beam-Charge Asymmetry in DVCS

GPD Model: M.Vanderhaeghen et al.PRD 60 (1999) 094017

Contributions to ed → eXγ

ed → edγ coherent production ∼ 20%

ed → epnγ incoherent production ∼ 60%

ed → e∆γ associated processes ∼ 15%

coherent contribution enhanced at small −t

first −t-bin ∼ 40%

neutron contribution ONLY at large −t

last −t-bin ∼ 25%

t-dependence of BCA can be used to constrain GPD models

limited by e−p sample (L∼ 10 pb−1), HERMES is running with e− beam in 2005.

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.7

Page 15: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Beam-Spin Asymmetry in DVCS

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

-3 -2 -1 0 1 2 3

φ (rad)

ALU

HERMES 2000 (refined)PREL.e→ + p → e+ γ X (Mx< 1.7 GeV)

P1 + P2 sin φ + P3 sin 2φ

<-t > = 0.18 GeV2, <xB> = 0.12, <Q2> = 2.5 GeV2

P1 = -0.04 ± 0.02 (stat)P2 = -0.18 ± 0.03 (stat)P3 = 0.00 ± 0.03 (stat)

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

-3 -2 -1 0 1 2 3

φ (rad)

ALU

HERMES PRELIMINARYe→ + d → e+ γ X (Mx< 1.7 GeV)

P1 + P2 sin φ + P3 sin 2φ

<-t > = 0.20 GeV2, <xB> = 0.10, <Q2> = 2.5 GeV2

P1 = -0.04 ± 0.02 (stat)P2 = -0.15 ± 0.03 (stat)P3 = 0.03 ± 0.03 (stat)

Beam-Spin Asymmetry: — [HERMES, PRL 87 (2001) 182001]

ALU (φ) =1

|PB |· dσ(

→e , φ) − dσ(

←e , φ)

dσ(→e , φ) + dσ(

←e , φ)

∝ Im [F1H] · sin φ

proton: Asin φLU

= −0.18 ± 0.03(stat)

deuteron: Asin φLU

= −0.15 ± 0.03(stat)

proton deuteron

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.8

Page 16: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Longitudinal Target-Spin Asymmetry in DVCS

-0.4

-0.2

0

0.2

0.4

0.6

-3 -2 -1 0 1 2 3

φ [rad]

AU

L

HERMES PRELIMINARYe+ p

→ → e+γ X (Mx<1.7 GeV)(in HERMES acceptance)

<-t>=0.12 GeV2, <xB>=0.10, <Q2>=2.5 GeV2

χ2/ ndf: 8.5/ 7s0: -0.009 ± 0.024 (stat.)s1: -0.071 ± 0.034 (stat.)s2: -0.113 ± 0.034 (stat.)

A = s0 + s1 sin φ + s2 sin 2φ

-0.4

-0.2

0

0.2

0.4

0.6

-3 -2 -1 0 1 2 3

φ [rad]

AU

L

HERMES PRELIMINARY 99,00e+ d

→ → e+γ X (Mx<1.7 GeV)(in HERMES acceptance)

<-t>=0.13 GeV2, <xB>=0.10, <Q2>=2.5 GeV2

χ2/ ndf: 6.2/ 7s0: 0.030 ± 0.017 (stat.)s1: -0.036 ± 0.024 (stat.)s2: -0.039 ± 0.023 (stat.)

A = s0 + s1 sin φ + s2 sin 2φ

Longitudinal Target-Spin Asymmetry:

AUL(φ) =1

|PT | ·dσ(⇐

P , φ) − dσ(⇒

P , φ)

dσ(⇐

P , φ) + dσ(⇒

P , φ)∝ Im[F1H] · sin φ

proton: Asin φUL

= −0.071 ± 0.034(stat)

deuteron: Asin φUL

= −0.036 ± 0.024(stat)

proton deuteron

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.9

Page 17: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Transverse Target-Spin Asymmetry in DVCS

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 0.5

-0.2

-0.4

-0.6

0

0.2

AU

Tsi

n(φ-

φ s)cos

(φ) HERMES

PRELIMINARY(in HERMES acceptance)

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 0.2 0.4

e+ p⇑ → e+ γ X(Mx < 1.7 GeV)

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 5 10

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 0.5

-0.2

-0.4

-0.6

0

0.2

0 0.25 0.5

-t (GeV2)

AU

Tco

s(φ-

φ s)sin

(φ)

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 0.2 0.4

0 0.1 0.2 0.3

xB

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 5 10

0 2.5 5 7.5 10

Q2 (GeV2)

Transverse Target-Spin Asymmetry:

AUT (φ, φs) =1

|PT | ·dσ(P⇑, φ, φs) − dσ(P⇓, φ, φ′s)

dσ(P⇑, φ, φs) + dσ(P⇓, φ, φ′s)

∝ Im[F2H− F1E] · sin (φ − φS) cos φ + Im[F2H − F1ξE ] · cos (φ − φS) sin φ

∝ Im[F2H− F1E]

∝ Im[F2H − F1ξE ]

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.10

Page 18: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Transverse Target-Spin Asymmetry in DVCS

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 0.5

-0.2

-0.4

-0.6

0

0.2

AU

Tsi

n(φ-

φ s)cos

(φ) HERMES

PRELIMINARY(in HERMES acceptance)

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 0.2 0.4

e+ p⇑ → e+ γ X(Mx < 1.7 GeV)

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 5 10

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 0.5

-0.2

-0.4

-0.6

0

0.2

0 0.25 0.5

-t (GeV2)

AU

Tco

s(φ-

φ s)sin

(φ)

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 0.2 0.4

0 0.1 0.2 0.3

xB

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0 5 10

Ju=0.2Ju=0.4

Ju=0

(hep-ph/0506264)

0 2.5 5 7.5 10

Q2 (GeV2)

AUT (φ, φs) ∝ Im[F2H− F1E] · sin (φ − φS) cos φ + Im[F2H − F1ξE] · cos (φ − φS) sin φ

Asin (φ−φS) cos φ

UTsensitive to Ju and not to GPD model parameters (hep-ph/0506264)

=⇒ allows extraction of Ju within these GPD models

More data is coming (HERMES 2005 e−p⇑, about the same statistics as here)

∝ Im[F2H− F1E]

∝ Im[F2H − F1ξE ]

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.10

Page 19: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Summary and OutlookMeasurements of exclusive processes can increase our knowledge on thenucleon structure by determining the Generalized Parton Distributions.

Measurement of the Transverse Target-Spin Asymmetry in DVCS will allowthe first determination of Ju through certain GPD models.

Dedicated study on exclusive processes at HERMES with the newrecoil detector starts at the end of 2005 (BSA, BCA in DVCS):

Allows to detect the recoiling protonBackground ’free’ DVCS: Background ∼ 5% ⇒< 1%

⇒ Talk by Nils Pickert (404)

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.11

Page 20: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Summary and OutlookMeasurements of exclusive processes can increase our knowledge on thenucleon structure by determining the Generalized Parton Distributions.

Measurement of the Transverse Target-Spin Asymmetry in DVCS will allowthe first determination of Ju through certain GPD models.

Dedicated study on exclusive processes at HERMES with the newrecoil detector starts at the end of 2005 (BSA, BCA in DVCS) :

Allows to detect the recoiling protonBackground ’free’ DVCS: Background ∼ 5% ⇒< 1%

⇒ Talk by Nils Pickert (404)

,

γ

1

0

2

-1

-2

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

m

LUMINOSITY

CHAMBERSDRIFT

FC 1/2

DVC

MC 1-3MONITOR

BC 1/2

BC 3/4 TRD

PROP.CHAMBERS

FIELD CLAMPS

PRESHOWER (H2)

CALORIMETER

DRIFT CHAMBERS

TRIGGER HODOSCOPE H1

0 1 2 3 4 5 6 7 8 9 10

RICH

MUON HODOSCOPEWIDE ANGLE

FRONTMUONHODO

MAGNET

m

IRON WALL

e

27.6 GeV

MUON HODOSCOPES

STEEL PLATE

TARGETCELL

HODOSCOPE H0

LAMBDAWHEELS

e’

HadronsPROTONRECOIL

Photon Detector

Water Coolingfor Si Det.

SciFi/LightguideConnector Ring

C3 Collimator(Tungsten)

Connectors

(3 layers tungsten/scintillator)

SciFi 2

Si Det. Hybrid

Thin−walledScatteringChamber

SciFi 1

Si Det.

TIGRE Sensors

Si Det. Frame(Aln)

CellTarget

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.11

Page 21: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Backup Slides!

Backup Slides!

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.12

Page 22: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Nucleon Structure

FF PDF GPD

Proton Spin1

2=

1

2(

∼30%︷ ︸︸ ︷∆u + ∆d + ∆s) + Lq

︸ ︷︷ ︸Jq

+∆G + Lg︸ ︷︷ ︸Jg

Ji’s Sum Rule

Jq =1

2

∫ 1

−1dx · x[Hq(x, ξ, 0) + Eq(x, ξ, 0)]

Nucleon Form Factors

well known, measured by e.g. elastic scattering:

e + N → e′ + N

provide the 1st direct knowledge about the internalstructure of the nucleon [Hofstadter1955−61]

Parton Distribution Functions

measured through e.g. deep inelastic scattering:

e + N → e′ + h + X

unpolarized quark density q(x)

longitudinal spin density ∆q(x) = q→

⇒(x) − q→

⇐(x)

transverse spin density δq(x) = q↑⇑(x) − q↑⇓(x)

gluon polarization ∆G/G

Generalized Parton Distributions

accessible in exclusive processes, e.g.:

e + N → e′ + γ + N

provide a 3D picture of the nucleon structure

access the total angular momentum of quark Jq

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.13

Page 23: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Generalized Parton Distributions

GPDs (x, ξ, t)

tp p′

(x + ξ) · p (x − ξ) · ptwist-2 GPDs H, E, H, E (x, ξ, t) for spin 1/2 hadron

x ± ξ: longitudinal momentum fractions of the partons,

ξ: fraction of the momentum transfer, ξ ' xB2−xB

,

t: invariant momentum transfer, t ≡ (p − p′)2.

GPDs ⇒ Form Factors:∫ 1−1 dx · Hq (x, ξ, t) = F q

1 (t),∫ 1−1 dx · Eq (x, ξ, t) = F q

2 (t),∫ 1−1 dx · Hq (x, ξ, t) = Gq

A(t),

∫ 1−1 dx · Eq (x, ξ, t) = Gq

P(t).

GPDs ⇒ PDFs :

Hq (x, 0, 0) = q (x), Hq (x, 0, 0) = ∆q (x).

Hg (x, 0, 0) = g (x), Hg (x, 0, 0) = ∆g (x).

GPDs ⇒ impact parameter dependent PDFs :

Hq

(x, 0,−∆2

)→ q (x, b⊥),

Hq

(x, 0,−∆2

)→ ∆q (x, b⊥).

GPDs ⇒ Total Angular Momentum of Partons

Jq,g = 12

∫ 1−1 dx · x[Hq,g(x, ξ, 0) + Eq,g(x, ξ, 0)]

PDFs (x)

p p

x · p x · p

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.14

Page 24: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

Exclusive Meson Production

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.15

Page 25: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

TTSA in ρ0 Production

-0.5

0

0.5

0.02 0.08 0.14 0.2

x

AU

T

sin (φ

-φs )

HERMES PRELIMINARYe p Þ e′ ρ0 p

l

n

s

0.0 < -t′ < 0.10.1 < -t′ < 0.20.2 < -t′ < 0.4

Transverse Target-Spin Asymmetry:

AUT (φ, φs) =1

|PT |· dσ(P⇑, φ, φs) − dσ(P⇓, φ, φs)

dσ(P⇑, φ, φs) + dσ(P⇓, φ, φs)

∝ E · H sin (φ − φS)

x

y

z

φS

φ

~Ph

~S⊥

~k

~k′

~q

uli

√Large negative asymmetry at low x and large t

√A

sin (φ−φS)UT

sensitive to Ju (hep-ph/0506264), no direct theoretical comparison yet

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.16

Page 26: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

π+ Cross-Section Measurement

)2 (GeV2Missing Mass-2 0 2 4 6 8

#eve

nts

0

500

1000

1500

2000 +π

normalized-π

=1.77-π/+π

/ ndf 2χ 21.88 / 17Constant 18.9± 341.9 Mean 0.05± 0.87 Sigma 0.036± 0.677

)2 (GeV2Missing Mass-2 0 2 4 6 8

- π-N + π

N

0

200

400

/ ndf 2χ 21.88 / 17Constant 18.9± 341.9 Mean 0.05± 0.87 Sigma 0.036± 0.677

DataFit to dataExclusive MC(arbitrary norm.)

ep → enπ+ cross-section (H, E):

Background estimated from π− yield.

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.17

Page 27: Exclusive Processes at Hermes · Exclusive Processes at Hermes Zhenyu Ye, DESY for the collaboration HEP 2005, LISBOA, PORTUGAL, JULY 2005 Outline Motivation Deeply Virtual Compton

π+ Cross-Section Measurement

1

10

10 2

0 2 4 6 8 10

Q2 ( GeV2 )

σ tot

(n

b)

HERMES PRELIMINARYγ* p → π+ n

uncorrected for radiative effects

0.02 < x < 0.18

0.18 < x < 0.26

0.26 < x < 0.80

VGG: LO

VGG: LO+power corrections

σLσL

GPD Model: M.Vanderhaeghen et al.

σtot = σT + εσL, L/T separation notpossible, but:

σT suppressed by 1/Q2

At HERMES kinematics, 0.8 < ε < 0.96

At large Q2, σL dominates

Q2 dependence is in general agreement with the theoretical expectation

Power corrections (k⊥ and soft overlap) calculations overestimate the data

Zhenyu Ye, HEP 2005, Lisboa, Portugal, July 2005 – p.17