S. Levonian, DESY

New Results on Diffraction at HERA

) GeVπ) - m(Ksππm(K0.14 0.145 0.15 0.155 0.16 0.165

N(D*

)

0

50

100

150

200

250

300

350

400

D* in diffractive DIS

H1 Preliminarydatasignalbackground

Wγπ [GeV]

σ(γπ

+--- >

ρ0 π+)

[µb]

10 20 30 400

1

2

3

4

5

Wγπ [GeV]

σ(γπ

+--- >

ρ0 π+)

[µb]

10 20 30 400

1

2

3

4

5

Wγπ [GeV]

σ(γπ

+--- >

ρ0 π+)

[µb]

10 20 30 400

1

2

3

4

5H1 data H1

)2 (GeV2Q0 10 20 30 40 50 60 70 80

R

0

0.2

0.4

0.6

0.8(1S)ψJ/σ

(2S)ψσR =

-1ZEUS 468 pb

-1H1 27 pb

-1p: 6.3 pbγH1

HIKT: GBW-BT

HIKT: GBW-LOGAR: b-CGC

AR: IP-Sat

= 0δKMW: FFJS

KNNPZZLM

= 2δKMW:

ZEUS

2 HERA: The World’s Only ep Collider

Days of running

H1

Inte

grat

ed L

umin

osity

/ p

b-1

Status: 1-July-2007

0 500 1000 15000

100

200

300

400electronspositronslow E

HERA-1

HERA-2

• 1998 Ep upgrade: 820 ⇒ 920 GeV(√s : 301 ⇒ 319 GeV)

• 2001 HERA-2 upgrade: L × 3, Polarised e+/e−

(〈P 〉 = 40%)

HERA-1 (1993-2000) ≃ 120 pb−1

HERA-2 (2003-2007) ≃ 380 pb−1

Final Data samples

H1+ZEUS: 2 × 0.5 fb−1

3 Diffraction at HERA. Factorisation properties

pp

X (M )X

e

QCD collinearfactorisation at

fixed x , t

(b)

(x )IP

(t)

e

(Q )2g*

}

}

IP

γ

ppIP

IPq

qγ

ppIP= x

QCD versus Regge factorisation

QCD factorisation(rigorously proven for

DDIS by Collins et al.):

Regge factorisation(conjecture, e.g. RPM

by Ingelman,Schlein):

σD(4)r ∝ ∑

i σ̂γ∗i(x,Q2) ⊗ fDi (x,Q2;xIP , t)

• σ̂γ∗i – hard scattering part, same as in inclusive DIS

• fDi

– diffractive PDF’s, valid at fixed xIP , t which obey (NLO) DGLAP

FD(4)2 (xIP , t, β,Q

2) = Φ(xIP , t) · F IP2 (β,Q2)

• In this case shape of diffractive PDF’s is independent of xIP , t

while normalization is controlled by Regge flux Φ(xIP , t)

4 Selection of Diffractive Events

5 Diffraction at HERA: Some old Results

Inclusive Diffraction and DPDFs: gluon dominated IP

]2 [GeV2Q10 210

D(3

)rσ

IP .

xl 3

-210

-110

1

10

210

310

410=0.005 (l=11)β

=0.008 (l=10)β

=0.013 (l=9)β

=0.02 (l=8)β

=0.03 (l=7)β

=0.05 (l=6)β

=0.08 (l=5)β

=0.13 (l=4)β

= 0.2 (l=3)β

=0.32 (l=2)β

= 0.5 (l=1)β

= 0.8 (l=0)β

=0.01IPx < 1.6 GeV)

YH1 LRG (M

H1 FPS HERA-II x 1.2

H1 2006 DPDF Fit B

(extrapol. fit)

H1 0

0.2

0.4

0.6

0.8

0.2 0.4 0.6 0.8z

z⋅gl

uon(

z)

gluonµf

2=25 GeV2

H1

0

0.05

0.1

0.15

0.2

0.2 0.4 0.6 0.8z

z⋅si

ngle

t(z)

singletµf

2=25 GeV2

H1

5 Diffraction at HERA: Some old Results

Inclusive Diffraction and DPDFs: gluon dominated IP

]2 [GeV2Q10 210

D(3

)rσ

IP .

xl 3

-210

-110

1

10

210

310

410=0.005 (l=11)β

=0.008 (l=10)β

=0.013 (l=9)β

=0.02 (l=8)β

=0.03 (l=7)β

=0.05 (l=6)β

=0.08 (l=5)β

=0.13 (l=4)β

= 0.2 (l=3)β

=0.32 (l=2)β

= 0.5 (l=1)β

= 0.8 (l=0)β

=0.01IPx < 1.6 GeV)

YH1 LRG (M

H1 FPS HERA-II x 1.2

H1 2006 DPDF Fit B

(extrapol. fit)

H1 0

0.2

0.4

0.6

0.8

0.2 0.4 0.6 0.8z

z⋅gl

uon(

z)

gluonµf

2=25 GeV2

H1

0

0.05

0.1

0.15

0.2

0.2 0.4 0.6 0.8z

z⋅si

ngle

t(z)

singletµf

2=25 GeV2

H1

αIP(t)=α(0)+α′t

VM: soft vs hard IP

soft

hard

transition from soft to hard regime at µ2 ≃ 4÷5 GeV2

6 Selected new Results

x = E /EL pn

. .

γ π+

π−

π+

0

t’

t

γ Wγ

+πW

p

PI

ρ

p

π

n

� Diffractive Photoproduction of Isolated Photons [ZEUS-prel-2015]

� D∗ Meson Production in Diffractive DIS at HERA [H1-prel-2016]

� Cross-section Ratioσψ(2S)σJ/ψ(1S)

in Exclusive DIS [ZEUS-pub-2016]

� Exclusive ρ0 Meson Photoproduction with a Leading Neutron [H1-pub-2016]

Isolated Photons in Diffractive Photoproduction

8 Isolated Photons in Diffractive PHP

4 < Eγt < 15 GeV

−0.7 < ηγ < 0.9

• Use energy-weighted e.m. cluster width 〈δZ〉to distinguish γ from π0, η background

• Diffraction: LRG signature, and xIP < 0.03

9 Isolated Photon+ Jet: Data vs MC model

Photon Jet All well described,except highest zIPComparison with NLO QCD to follow

D* in Diffractive DIS at HERA

11 D∗ Production in Diffractive DIS: Data sample

) GeVπ) - m(Ksππm(K0.14 0.145 0.15 0.155 0.16 0.165

N(D

*)

0

50

100

150

200

250

300

350

400

D* in diffractive DIS

H1 Preliminarydatasignalbackground

[nb]

eY

X(D

*)→

ep

σ0

0.2

0.4

D* in diffractive DIS

H1 Preliminarydata

scale variation⊕ cmNLO QCD, H1 2006 Fit B

scale variation⊕ cmNLO QCD, H1 2006 Fit B

scale variation⊕ cmNLO QCD, H1 2006 Fit B

2 < 100 GeV25 < Q < 0.03IPx

0.02 < y < 0.65 < 1.6 GeVYM

> 1.5 GeVT,D*

p 2|t| < 1 GeV

< 1.5D*

η-1.5 <

Based on 280 pb−1 HERA-2 data

Open charm tagged with D∗

D∗+ →D0π+slow→(K−π+)π+

slow + C.C.

LRG selection of diffraction (∼ 1100D∗)

12 D∗ Production in Diffractive DIS: Data vs NLO

• NLO QCD by HQVDIS in FFNS

(H1 DPDF-2006, mc = 1.5GeV,

µ2r = µ2

f = m2c + 4Q2)

in good agreement with data

• Charm fragm.func. as determined

in H1 non-diffractive D∗ analysis

works here ⇒ supports

universality of charm fragmentation

• Data could be used as

additional input to the global

DPDF fit

Cross−section Ratio in DISσψ(2S)σJ/ψ(1S)

14 Motivation

pQCD predictions: R(Q2 = 0) ≃ 0.17 and rises withQ2

15 Data samples and Decay channels

(GeV)µµM2 3 4 5

Ent

ries

10

210

310

410

-1ZEUS 468 pb

Background fit

ZEUS

(1S)ψJ/

(2S)ψ

5<Q2<80GeV2 L = 468pb−1

16 Results: σψ(2S)/σJ/ψ(1S) vs Q2, W and |t|

)2 (GeV2Q10 20 30 40 50 60 70 80

com

bR

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-1ZEUS 468 pb

ZEUS

W (GeV)40 60 80 100 120 140 160 180 200

com

bR

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

-1ZEUS 468 pb

ZEUS

)2|t| (GeV0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

com

bR

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

-1ZEUS 468 pb

ZEUS

)2 (GeV2Q0 10 20 30 40 50 60 70 80

R

0

0.2

0.4

0.6

0.8(1S)ψJ/σ

(2S)ψσR =

-1ZEUS 468 pb

-1H1 27 pb

-1p: 6.3 pbγH1

HIKT: GBW-BT

HIKT: GBW-LOGAR: b-CGC

AR: IP-Sat

= 0δKMW: FFJS

KNNPZZLM

= 2δKMW:

ZEUS

• Ratio rises with Q2 and

is constant in W and |t|

• HERA data in qualitative agreement

with pQCD models

• Some discriminating power

(albeit statistically limited)

Rho−0 with a Leading Neutron at HERA

18 HERA as a ‘4P ’ facility

HERA enables to study structure of

Proton – F2, FL, ...Photon – g/γ

Pomeron – FD2 , F

DL

Pion – F π2

0.2

0.4

0.6

0.8

1

­410 ­310 ­210 ­110 1

HERAPDF2.0 NLO uncertainties: experimental model parameterisation HERAPDF2.0AG NLO

x

xf 2 = 10 GeV2f

µ

vxu

vxd

0.05)×xS (

0.05)×xg (

H1 and ZEUS

0

1

2

3

4

5

-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0

log10(xγ)

x γ g(x

γ)/α

H1 1993 (dijet events)

H1 1994 (high pT track events)

GRV-LO

LAC1-LO

SAS1D-LO

‹pT2› = 38 (GeV/c)2ˆ

0

0.2

0.4

0.6

0.8

0.2 0.4 0.6 0.8z

z⋅gl

uon(

z)

gluonµf

2=25 GeV2

H1

18 HERA as a ‘4P ’ facility

HERA enables to study structure of

Proton – F2, FL, ...Photon – g/γ

Pomeron – FD2 , F

DL

Pion – F π2

0.2

0.4

0.6

0.8

1

­410 ­310 ­210 ­110 1

HERAPDF2.0 NLO uncertainties: experimental model parameterisation HERAPDF2.0AG NLO

x

xf 2 = 10 GeV2f

µ

vxu

vxd

0.05)×xS (

0.05)×xg (

H1 and ZEUS

0

1

2

3

4

5

-1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0

log10(xγ)

x γ g(x

γ)/α

H1 1993 (dijet events)

H1 1994 (high pT track events)

GRV-LO

LAC1-LO

SAS1D-LO

‹pT2› = 38 (GeV/c)2ˆ

0

0.2

0.4

0.6

0.8

0.2 0.4 0.6 0.8z

z⋅gl

uon(

z)

gluonµf

2=25 GeV2

H1

ρ 0 + p

+πγ n

W

W

0

π+γ

γπ

p

t

x = E /En

t’

pL

γ

π+

π

π+

−

.

ρ

p n

IP

Here for the first time we investigate the reaction involving all these objects simultaneously:

No hard scale present ⇒ Regge framework is most appropriate

19 ρ0 with Leading Neutron: S/B decomposition

x = E /EL pn

.

γ π+

π−0

t’

π+*

γ π+

−0

t’

π+

π

W. .

γ π+

π−

π+

0

t’

t

γ Wγ

+π

γ+

−0

t’

ππ

Y

pN

ρ

PI

np

PI

ρ

n n

p

PI

ρ

p

π

n

ρ

PI

p n

M

(a) (b) (c) (d)

20 ρ-meson shape

M(π+π-) [GeV]

dN/d

M [

GeV

-1]

ρ0 with Forward Neutron

0.5 1 1.5

0

400

800

1200

M(π+π-) [GeV]

dN/d

M [

GeV

-1]

ρ0 with Forward Neutron

0.5 1 1.5

0

400

800

1200H1 dataρ0: BWρ0: BW RSx

interference termω reflection

M(π+π-) [GeV]

dN/d

M [

GeV

-1]

ρ0 with Forward Neutron

0.5 1 1.5

0

400

800

1200

full fit

(a)

H1

pT2 [GeV2]

n RS

Exclusive ρ0 photoproduction

0.01 0.1 10

2

4

6

8

fit n 0(pT2+M2)-β

pT2 [GeV2]

n RS

Exclusive ρ0 photoproduction

0.01 0.1 10

2

4

6

8

H1 dataZEUS-1994 (γp ----> ρ0p)

(b)

H1

dN(Mππ)dMππ

∝ BWρ(Mππ)(

Mρ

Mππ

)nRS

M = 764 ± 3 MeV

Γ = 155 ± 5 MeV

Analysis region: 0.6<Mπ+π−<1.1 GeV extrapolated using BW to the full range: 0.28<Mρ0<1.5 GeV

21 Cross sections definitions

0

π+

π+

π

π−

+γ

γ

π

ρ

PI

np

e

e’

f

f /p

/eσγp =σepΦγ

Φγ =∫

fγ/e(y,Q2)dydQ2

σγπ =σγpΓπ

Γπ =∫

fπ/p(xL, t)dxLdt

VMD:

OPE:

. fγ/e(y,Q2) =

α

2πQ2y

[

1 + (1 − y)2 − 2(1 − y)

(

Q2min

Q2− Q2

M2ρ

)]

1(

1 + Q2

M2ρ

)2

. fπ/p(xL, t) =1

2π

g2pπN

4π(1 − xL)

−t(m2

π − t)2exp[−R2

πn

m2π − t

1 − xL]

21 Cross sections definitions

Wγp [GeV]

σ γp [n

b]

20 40 60 80 1000

100

200

300

400

500

Wγp [GeV]

σ γp [n

b]

20 40 60 80 1000

100

200

300

400

500

H1 data

Wγp [GeV]

σ γp [n

b]

20 40 60 80 1000

100

200

300

400

500

POMPYT

Wγp [GeV]

σ γp [n

b]

20 40 60 80 1000

100

200

300

400

500

fit W δ (δ=--0.26+--- 0.06stat+--- 0.07sys)

H1ρ0 with Forward Neutron

Wγπ [GeV]

σ(γπ

+--- >

ρ0 π+)

[µb]

10 20 30 400

1

2

3

4

5

Wγπ [GeV]

σ(γπ

+--- >

ρ0 π+)

[µb]

10 20 30 400

1

2

3

4

5

Wγπ [GeV]

σ(γπ

+--- >

ρ0 π+)

[µb]

10 20 30 400

1

2

3

4

5H1 data H10

π+

π+

π

π−

+γ

γ

π

ρ

PI

np

e

e’

f

f /p

/e

VMD:

OPE:

. fγ/e(y,Q2) =

α

2πQ2y

[

1 + (1 − y)2 − 2(1 − y)

(

Q2min

Q2− Q2

M2ρ

)]

1(

1 + Q2

M2ρ

)2

. fπ/p(xL, t) =1

2π

g2pπN

4π(1 − xL)

−t(m2

π − t)2exp[−R2

πn

m2π − t

1 − xL]

22 Constraining pion flux

xL

dσγp

/dx L

[µb

]

0.4 0.5 0.6 0.7 0.8 0.90

0.2

0.4

0.6

0.8

1H1 data (ϑn<0.75 mrad)

Normalised to data:

xL

dσγp

/dx L

[µb

]

0.4 0.5 0.6 0.7 0.8 0.90

0.2

0.4

0.6

0.8

1

FMSNSSS

xL

dσγp

/dx L

[µb

]

0.4 0.5 0.6 0.7 0.8 0.90

0.2

0.4

0.6

0.8

1

H1

xL

dσγp

/dx L

[µb

]

0.4 0.5 0.6 0.7 0.8 0.90

0.2

0.4

0.6

0.8

1H1 data (ϑn<0.75 mrad)

Normalised to data:

xL

dσγp

/dx L

[µb

]

0.4 0.5 0.6 0.7 0.8 0.90

0.2

0.4

0.6

0.8

1

Bishari-0HoltmannKPPMST

xL

dσγp

/dx L

[µb

]

0.4 0.5 0.6 0.7 0.8 0.90

0.2

0.4

0.6

0.8

1

H1

pT,n2 [GeV2]

d2 σ γp/(

dxLdp

T,n

2) x

3j [

µb/G

eV2]

0 0.1 0.2 0.3

0.1

1

10

100 0.35<xL<0.50 (j=3)

0.50<xL<0.65 (j=2)

0.65<xL<0.80 (j=1)

0.80<xL<0.95 (j=0)

pT,n2 [GeV2]

d2 σ γp/(

dxLdp

T,n

2) x

3j [

µb/G

eV2]

0 0.1 0.2 0.3

0.1

1

10

100

H1 data

Fit: e--bn(x L) p2T,n

H1

ρ0 with Forward Neutron

xL

b n [G

eV-2]

0.4 0.6 0.80

10

20H1 data

xL

b n [G

eV-2]

0.4 0.6 0.80

10

20 Bishari-0HoltmannKPPMSTFMS

H1

ρ0 with Forward Neutron

Failure to describe bn(xL) suggests strong absorptive effects (n rescattering) ⇒ try to quantify

23 Estimate of absorption corrections

Wγπ [GeV]

σ(γπ

+--- >

ρ0 π+)

[µb]

10 20 30 400

1

2

3

4

5

Wγπ [GeV]

σ(γπ

+--- >

ρ0 π+)

[µb]

10 20 30 400

1

2

3

4

5

Wγπ [GeV]

σ(γπ

+--- >

ρ0 π+)

[µb]

10 20 30 400

1

2

3

4

5H1 data H1

γπ γp

(2.33 ± 0.56)µb (9.5 ± 0.5)µb

rel =σγπ→ρ0π

σγp→ρ0p

=

{

0.25 ± 0.06 (exp.extracted)

0.57 ± 0.03 (theo.expected)Kabs = 0.44 ± 0.11

Optical Theorem: dσel

dt|t=0

= belσel ∝ σ2tot

rel = (bγp

bγπ) · (σγπtot/σ

γptot)

2

Eikonal approach: b = 〈R2〉; b12 = b1 + b2

World data: (bpp≃11.7, bπ+p≃9.6, bγp≃9.75) GeV−2

Differential cross section inp2T,ρ

-t' [GeV2]

dσγp

/dt'

[µb/

GeV

2 ]

0 0.2 0.4 0.6 0.8 1

0.01

0.1

1

10

-t' [GeV2]

dσγp

/dt'

[µb/

GeV

2 ]

0 0.2 0.4 0.6 0.8 1

0.01

0.1

1

10

H1 data

-t' [GeV2]

dσγp

/dt'

[µb/

GeV

2 ]

0 0.2 0.4 0.6 0.8 1

0.01

0.1

1

10

Fit: a1eb1t' + a2e

b2t'

b1 = 25.7 +– 3.2 GeV-2, b2 = 3.62 +– 0.32 GeV-2

H1ρ0 with Forward Neutron

q

_q

p������������������������������������������������������������������������������������������������������������������������������������������������������������

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R

��������������������

��������������������

����������������

����������������

π

γ

ISR

M(nπ)/GeV

Geometric interpretation: 〈r2〉 = 2b1·(~c)2 ≃ 2 fm2 ⇒ (1.6Rp)2 ⇒ ultra-peripheral process

DPP explanation: low mass π+n state → large slope, high masses → less steep slope

25 Summary

� Diffraction is an important part of HERA physics landscape.Despite overall consistent picture, the field is challenging, as it representsa complicated interplay of soft and hard phenomena.

� Statistically limited channels have been studied with full HERA data sample.Whenever a hard scale is present, pQCD calculations are successful.

� The data show sensitivity to some QCD models parameters.They can also be used to further constrain DPDF, especially at high zIP .

� Photon-pion elastic cross section is extracted experimentally (in OPE ap-proximation) for the first time.

� Strong absorptive effects are confirmed in Leading Neutron production.Since the nature of these is non-perturbative, exp. results are essential fortuning models of ‘Survival Gap Probability’.

Backup Slides

27 Open questions

� FD(4)2 from HERA-II VFPS data and final DPDF determination without as-

sumption on Regge factorisation.

� Explain factorisation breaking mechanizm in PHP, in particular indepen-dence of Gap Survival Probability on xγ.

� Multiscale problem: (Q2, ET ,MV , t).

� Where is an Odderon ?

� Can one observe Glueball in a double Pomeron reaction in PHP?γp → (IPIP ) → MX (MX =

√xIP1xIP2Wγp = 2 ÷ 4 GeV)

HERA has finished, but not DIS physics.What’s next? eRHIC ? LHeC ?