Transcript Diffraction in ep collisions at HERA

Diffraction in ep Collisions at HERA
Introduction
Vector Mesons
DVCS
Diffractive DIS
Final States:
K.Hiller
DESY Zeuthen
Charm & Jets
Collaborations
on behalf of the
H1 & ZEUS
Summary
K.Hiller ISMD 2003 Cracow
1
Kinematics of Diffraction
Standard DIS variables:
x
Q2
y
W
s
-
fractional parton/proton momentum
neg. virtual photon momentum2
fractional electron energy loss
g-p center-of-mass energy
e-p center-of-mass energy
g*
Additional for diffraction:
xP = Q2 + MX2 / ( Q2 + W2 )
fractional Pomeron momentum
b = Q 2 / ( Q 2 + MX 2 )
fractional parton/Pomeron momentum
t = (p – p’)2 , proton momentum transfer2
K.Hiller ISMD 2003 Cracow
t
2
Signatures of Diffraction
diffractive event
non-diffractive event
no visible
forward
activity
Two systems X and Y well separated in phase
space with low masses MX ,MY << W
System Y : proton or p-dissociation carries
most of the hadronic energy
Pomeron
System X : vector meson, photon
or photon-dissociation
Exchange of colourless object, Pomeron, with low momentum fraction xP
K.Hiller ISMD 2003 Cracow
3
Soft Diffraction Models
Notation: soft = non-perturbative process, hadron level
Regge model : diffraction described by exchange of Pomeron trajectory
 P t  =  P 0   P'  t , P 0 = 1  
 and ’ result from fit of energy
wit h  = 0.08,  P' = 0.25GeV  2
dependence of hadronic cross sections
 slow increasing total cross section
s(W) = Wd with d = 4(  ’/B)
 steep t-dependence with shrinkage
s(t) = exp(-Bt) with B = B0 + 2’ ln(W2/W02)
 low MX - pure Pomeron exchange
s(MX) = MX-2(1+2
 large MX - Reggeon & Pion exchange
Reggeon R(t) = 0.55 + 0.86 GeV-2 t
Pion
p(t) = 0
+ 1 GeV-2 t
 photon dissociation: triple Regge
K.Hiller ISMD 2003 Cracow
4
Hard Diffraction Models
Notation: hard = perturbative process, parton level
Starting from alternative frames  two classes of models :
Proton rest frame
formation time
~ 1 / Mp x
long at small x
Breit frame
Standard DIS
scheme
LO: 2 gluons , … gluon ladders
←
Exchange
→
fluctuates
in colour dipoles
_
_
← Virtual photon →
qq, qq+g, …
combine soft & hard processes by
←
different parton transverse momentum
Colour Dipole Models
Dynamics
→
object with partonic structure
point-like couplings to partons,
standard partonic cross sections
evolve diffractive PDFs in x / Q2
by DGLAP / BFKL schemes
Resolved Pomeron Models
K.Hiller ISMD 2003 Cracow
5
Selection Methods
1) Large Rapidity Gap / H1
2) MX– Method / ZEUS
-2
Typical cut: 0max < ~ 1.5
*)
* h = -ln tan (Q / 2)
0
2
4
6
8
ln MX2
3) Proton Tagging / H1, ZEUS
FPS / LPS & beam line optics
Fit excess above
exponential fall-off
K.Hiller ISMD 2003 Cracow
6
HERA Domain
….or why diffraction at HERA ?
TOP 1 - Large kinematic range
920 GeV proton ↔ 27.5 GeV electron, W  300 GeV
Q2  105 GeV2 photo- & electroproduction
x = Q2 / y s  10-5
TOP 2 - Large acceptance
H1/ZEUS ~ 4p to measure final state particles, important for g* dissociative system
TOP 3 - Large cross sections
~ 40 % of stot , ~10 % of DIS is diffractive
TOP 4 - Point-like couplings
to probe the Pomeron structure, not possible in hadron-hadron processes
TOP 5 – Different varying scales
MV2, Q2, t to access the transition region from soft to hard processes
HERA opened a new window for diffraction
K.Hiller ISMD 2003 Cracow
7
Total gp Cross Section
Typical soft process: quasi-real photon Q2  0 , tag e+ at low angles
H1:
stot(gp) = 165 ± 2 ± 11 mb
W = 200 GeV
ZEUS : stot(gp) = 174 ±1 ± 13 mb
W = 209 GeV
Fit: Pomeron + Reggeon contributions
s tot = A  W 2  B  W 2h
 = 0.093 0.002
Energy dependence of gp
resembles soft hadronic
processes  try to understand
diffraction in frame of QCD
K.Hiller ISMD 2003 Cracow
8
Vector Mesons : Overview
Exclusive processes in photo- and electroproduction : r, w, f, J/Y, Y(2S), U
Photoproduction r, f, J/Y high t
Hadron level:
Vector Meson
Dominance
& Regge model
QCD level:
with 2 gluon
exchange
Large variety of processes to study dynamics versus scales: MV2, Q2, t
K.Hiller ISMD 2003 Cracow
9
Vector Mesons: MV2- Dependence
H1 and ZEUS
photoproduction
Fit: s ~ Wd with d = 4P(0) -1)
W-dependence steeper with MV2:
dr ~ 0.2 > d Y2S) ~ 1.0
Large MV supplies a scale for hard
processes  apply pQCD models
K.Hiller ISMD 2003 Cracow
10
Vector Mesons: Q2- Dependence
 Photoproduction of light VM well described by
Regge Model
 pQCD predicts (Q2 + M2)–n dependence for
hard processes
ρ
W-dependence steeper with increasing Q2
n = 2.60
J/ψ
n = 2.70
Increasing Q2  hard processes dominate,
pQCD models in good agreement with data
K.Hiller ISMD 2003 Cracow
11
Vector Mesons : t - Dependence
Low t – region:
well-described by exp(-bt), with b(W)
High t – region:
pQCD predicts non-exponential dependenc
ρ
φ
fit t-n with:
J/ψ
Universal t-dependence in scale Q2 or M2
n(r = 3.2,
n(f) = 2.7,
n(J/ Y) = 1.7
pQCD model works fine at t > 1 GeV2
K.Hiller ISMD 2003 Cracow
12
Vector Mesons: Soft & Hard Processes
Indicator: s ~
Wd
_
with d = 4P(0) – 1) related to the exchanged object
Light VM: smooth transition from soft to hard regime
Heavy VM: flat W-dependence, hard regime already at low Q2
K.Hiller ISMD 2003 Cracow
13
Vector Mesons : SU(4), Universality
SU(4) prediction : r : w : f : J/Y = 1 : 1/9 : 2/9 : 8/9
assume SCHC, neglecting masses, meson-WF
SU(4) restoration at t ~ 5
GeV2,
Q2
~ 10
GeV2
All VM cross sections scaled by SU(4) factors:
Universal Q2 + M2 dependence for all VM
reflects common underlying dynamics
K.Hiller ISMD 2003 Cracow
14
Deeply Virtual Compton Scattering
 measure electron and photon
 topology similar to VM production:
replace the VM by a photon
 clean QCD process with point-like
couplings, no wave function
x1
x2
x1
x2
 skewed / generalized PDFs G(x1,x2,Q2)
Measurement problem:
 elastic BH process has same signature,
but much larger cross section
Bethe-Heitler QED process
K.Hiller ISMD 2003 Cracow
15
DVCS: W and Q2-Dependences
Fit Wd with
δ ~ 1 indicates hard process
W / GeV
Fit σ ~ Q-3 ↔ pQCD ~ Q-4
 soft processes essential
 NLO QCD Freund & with 2 sets of GPDFs
 Colour dipole models Donnachie &, Favart &
Both theoretical approaches
consistent with measurements
Q2 / GeV2
K.Hiller ISMD 2003 Cracow
16
Diffractive Deep Inelastic Scattering : σrD
Complete set of variables:
Q2, xP, t, MX, MY
System Y not measured
 integrate over MY < 1.6 / 2.3 GeV, t < 1GeV2
and measure reduced cross section σr :
FL unknown, FL = 0 or FL = F2  few % error
K.Hiller ISMD 2003 Cracow
17
DDIS: xP-Dependence & αP(0)
Use Ingelman&Schlein resolved Pomeron ansatz:
σdiff = flux(xP) · object (β,Q2)
For large xP > 0.01 add Reggeon exchange :
with flux in Regge limit:
P(0) indicates
hard Pomeron
at high Q2
Reggeon essential at large xP > ~ 0.01
Resoved Pomeron ansatz works for xP-dependence fine
K.Hiller ISMD 2003 Cracow
18
DDIS: QCD Analysis
QCD Fit Model:
1) Use QCD hard scattering factorization:
σg*p → p’X = σg*i  fiD
σg*i = universal partonic cross section
same as in inclusive DIS
D
fi = diffractive PDFs, xP& t = const.
a
2) Parton ansatz for exchange:
Pomeron = ∑q(z)+q(z) + g(z)
3) Use NLO DGLAP to evolve diffractive
PDFs to Q2 > Q02 = 3 GeV2
Gluon momentum fraction 75 ±15 % at Q2 = 10 GeV2
and remains large up to high Q2
K.Hiller ISMD 2003 Cracow
19
DDIS: b and Q2-Dependences (1)
Fit region: 6 < Q2 < 120 GeV2
Flat up to high β, no xP dependence
 Regge factorization works
strong positive scaling violations up to
high $  large gluon component
K.Hiller ISMD 2003 Cracow
20
DDIS: Extrapolation of NLO QCD fit
1.5 < Q2 < 12 GeV2, xP < 0.01
200 < Q2 < 1600 GeV2, xP < 0.03
General in good agreement, confirm diffractive PDFs with gluon dominance
K.Hiller ISMD 2003 Cracow
21
DDIS : Forward Proton Tagging
1) free of badly known p-dissociation corrections, H1/ZEUS MY < 1.6 / 2.3 GeV
2) measure momentum transfer t  F2D4, at least t – slope
3) Cross over to non-diffractive region at xP > 0.05, Reggeon & Pion exchange
B = 7.8±0.5±0.9/0.6 GeV-2
t
Leading proton/neutron xP > 0.10
z
K.Hiller ISMD 2003 Cracow
22
DDIS: Ratio sdiff / stot
ZEUS forward plug: 2 < Q2 < 80 GeV2
Q2-dependence: MX < 35 GeV
decreases with Q2
from ~ 20% at Q2 = 2.7 GeV2
to ~ 10% at Q2 = 27 GeV2
no Q2-dependence for MX > 8 GeV
W-dependence:
MX < 2 GeV ratio falling
MX > 2 GeV ratio constant
W-dependence of ratio surprising,
since Regge model predicts :
W 4((t) - 1) / W 2((t) – 1)
and QCD 2-gluon models:
x g(x) 2 / x g(x)
K.Hiller ISMD 2003 Cracow
23
Final States : Open Charm in DDIS
Open charm production very sensitive_ to
the gluon/Pomeron component: g*  c c
1) Resolved Pomeron - Boson-gluon fusion
2) Colour dipole 2 gluon exchange
~ 260 D* , 1.5 < Q2 < 200 GeV2
Resolved Pomeron :
xP < 0.01
- NLO fit Alvero &
_
2-gluon exchange qq+g:
- Golec-Biernat &
- Bartels &
All models agree with
data for xP < 0.01
K.Hiller ISMD 2003 Cracow
24
Final states: Jets in Photoproduction
 Jet production sensitive to gluon component
due to boson-gluon fusion
 Implement diffractive PDFs into Monte Carlo
RAPGAP and compare with data
 Photon: direct and resolved processes
with LO GRV PDFs
zP, xg :
partonic momentum
for dijet production
Resolved Pomeron
in fine agreement
with data – impoved
to LO PDFs
 improved to LO fit
K.Hiller ISMD 2003 Cracow
25
Summary
 Vector Meson
- large MV or Q2 or t provide a hard scale for application of pQCD models
- in the soft  hard transition region the energy dependence becomes steeper
 DVCS
- tiny cross section measured, but needs more/HERA-2 data
- clean process to measure parton correlation by generalized PDFs G(x1,x2,Q2)
 Diffractive DIS
- positive scaling violations up to β ~ 0.5  gluons dominate 75 ±15 % diffraction
- ratio to inclusive DIS remarkable flat over W
 Charm & Jets
- Models with diffractive PDFs describe different processes well  confirm gluon dominance
 pQCD Models
- Resolved Pomeron model
_ & Regge / QCD factorization very promising
- Colour dipole models: qq+g dominates at high Q2
K.Hiller ISMD 2003 Cracow
26
DDIS : MX- Dependence
ZEUS forward plug :
MX < 35 GeV
σdiff
~W
adiff
_
, adiff = 4(αP-1)
MX < 2 GeV: vector mesons range
little W-dependence  soft
MX > 2 GeV: steeper W-dependece with Q2,
compatible with xP-spectra
K.Hiller ISMD 2003 Cracow
27
Application: Diffractive Jet Production (2)
ZEUS: 3-jets in electroproduction
5 < Q2 < 100 GeV , MX > 23 GeV
 3-Jet fraction ~ 30 %
 at high MX dominant process
photon  qq + g
 gluon jet in Pomeron direction
and broader
 RAPGAP (resoved Pomeron)
SATRAP (colour dipole)
generators within 20 % range
K.Hiller ISMD 2003 Cracow
28