Inclusive diffraction at HERA
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Transcript Inclusive diffraction at HERA
Diffraction at HERA
Anna Mastroberardino
Calabria University
On behalf of the
H1 & ZEUS Collaborations
HSQCD 2004
St. Petersburg, Russia
18 – 22 May 2004
1
Outline
Introduction to diffraction
Diffractive structure function of the proton
QCD fits of diffractive data
Test of QCD factorization with jets and charm
Exclusive vector mesons
Summary
2
What is diffraction?
Standard DIS in a frame in which the proton
is very fast (Breit frame):
Q2
The struck quark carries fraction x Q2/W2
of the proton momentum
W
W = photon-proton centre of mass energy
Diffraction: exchange of colour singlet (IP)
producing a rapidity GAP in the particle flow
Q2
*(q)
W
X
xIP
IP
t
The pomeron carries fraction xIP of the
initial proton momentum
The struck parton carries fraction β of
the Pomeron momentum
3
Why diffraction ?
d 2
4 2
y2
2
1
y
F2 ( x, Q )
2
4
2
dxdQ
xQ
2[1 R( x, Q )]
F2 ei2 xfi x,Q2
DIS probes the partonic
structure of the proton
i
Diffractive structure function
Diffractive cross section
d 4
4 2
2
ddQ dxIP dt
Q 4
D ( 4)
y2
2
1
y
F
(
,
Q
, xIP , t )
2
D ( 4)
2(1 R
)
Diffractive DIS probes the partonic
structure of colour singlet exchange
HERA has opened up the small x domain
920 GeV proton
27.5 GeV electron
~ 10% of low-x DIS diffractive at HERA
W 300 GeV
Q 2 105 GeV 2
x 105
What role does it play?
4
Selection methods
Two systems X and Y well separated in phase
space with low masses MX ,MY << W
Y : proton or p-dissociation
carries most of the hadronic energy
X : vector meson, photon or photon-dissociation
Diffractive events are characterized by:
scattered proton almost intact
no forward energy deposition
flat vs ln MX2 distribution
-2
Large Rapidity Gap
0
2
4
6
8
MX – Method
ln MX2
Diffractive peak
Proton Tagging 5
Factorization in Diffractive DIS
QCD factorization for diffractive DIS holds
(Collins, Bereira & Soper, Trentadue & Veneziano)
F2D ~ fi/pD ˆi
universal partonic cross section
(same as in inclusive DIS)
diffractive parton distribution function – evolve according to DGLAP
universal for diffractive ep DIS (inclusive, dijets, charm)
If in addition postulate Regge factorization (Ingelman & Schlein)
F2D( 4) f IP / p ( xIP , t )F2IP (Q2 , )
F2IP (Q2 , ) evolves followingDGLAP equations
6
New results from ZEUS
Proton tagging method
MX method
MN < 2.3 GeV
Transition from very low Q2 to DIS (0.03 <Q2<100 GeV2)
7
Recent results from H1
r
D(4)
F2
D(4)
y2
2(1 y
2
y
)
2
FL
D(4)
r D F2 D at low y
r D F2 D if FL D 0
Integrate over t
rD ( 3)
1.5 Q2 1600GeV2
high precision measurement of
and Q2 dependences
QCD fit
(DGLAP evolution of diffractive pdfs)
(coming later)
8
Measurement of & Q2 dependences
Regge factorization holds for xIP< 0.01
Weak dependence: looks like a photon more than a proton
Scaling violations positive up to large : large gluon contribution
DGLAP evolution based fit describes the data
9
H1 NLO QCD fit – diffractive PDFs
Integrated fraction of exchanged
momentum carried by gluons
(75 15)%
Diffractive data fitted in similar way to proton F2 data
Parametrize Flavour Singlet (quarks + antiquarks)
and gluons at Q2 = 3 GeV2
Evolve according to NLO DGLAP and fit
Determine quark sea and gluon distribution
Diffractive interactions
gluon dominated
10
ZEUS NLO QCD fit to F2D and charm
• xIP <0.01
(LPS)
• QCDNUM
• Regge factorisation assumption
possible for this small data set
• DL flux
• initial scale Q2=2 GeV2
• zf(z)=(a1+a2z+a3z2)(1-x)a4
• other PDFs parametrisation tried
• Thorne-Robert variable-flavournumber-scheme
QCD fit describes data
( 2 / ndf 37.9 / 36)
fractional gluon momentum
(82 8( stat ) 9( sys ))%
shape of pdfs not well constrained
[F2D(3)cc from DESY-03-094]
11
Factorization in Diffractive DIS – experimental test
F2D ~ fi/pD ˆi
universal partonic cross section
(same as in inclusive DIS)
diffractive parton distribution function – evolve according to DGLAP
universal for diffractive ep DIS (inclusive, dijets, charm)
If QCD factorization holds diffractive parton densities are universal
- Test: use diffractive pdfs obtained so far from inclusive data to
predict other final state cross sections
diffractive DIS ?
hadron – hadron scattering?
12
A test of QCD factorization: jets and charm (H1)
Use results of NLO QCD fit to predict
the rate of diffractive production of
dijets and charm in DIS
NLO calculations based on H1 pdfs describe data well
QCD factorization in DDIS holds
13
Factorization in Diffractive DIS – experimental test
diffractive DIS ?
It holds
Diffractive structure function of antiproton
hadron-hadron scattering ?
Factorization not expected to work Indeed it does not:
diffractive dijets at the Tevatron:
suppression by a factor of 10
factorization breaking
understood in terms of (soft) rescattering
corrections of the spectator partons
(Kaidalov, Khoze, Martin, Ryskin)
But several other approaches …
also a suppression of resolved
processes, supposed to be similar to pp ?
14
Diffractive dijets in photoproduction
Real photon (Q2~ 0) can develop
hadronic structure
photoproduction similar to
hadron-hadron interaction
X= partonic momentum for dijet production
photon remnant energy 1 - x
LO comparison: no evidence for a suppression
of resolved with respect to direct
NLO comparison ?
15
Diffractive dijets in photoproduction
NLO calculations compared to
preliminary H1 data
(Klasen and Kramer, DESY 04-011)
NLO comparison: agreement between
data and MC found if resolved
contribution suppressed by a factor
of 0.34
rate of suppression expected
from theoretical models
16
Vector Meson production
V
V
(JPC=1--): r, f, J/y,U,...
W
p
IP
p
p
Exclusive VM production calculable in pQCD
p
2-gluon exchange
NLO calculation available for J/ photoproduction
probability of finding
2 gluons in the proton
Sensitivity to gluons in proton
cross section:
~
S2
Q6
2
xG( x, Qeff
)
2
Q 2 M V2
x~
W2
rise with W: increasing with hard scale
2
for J / xG( x, Qeff
) ~ x 0.2
~ W ~ 0.8
17
Vector Meson production
(p
Vp), Q2=0
Soft regime
Small MV (MV2 1 GeV2):
Incoming dipole behaves like
a normal-size hadron.
Flat vs W reflects flat gluon
distribution for Q2 0
MV
Hard regime
Large MV :
Fast growth of with W
reflects growth of gluon
distribution with decreasing x
W 1/ x
p centre-of-mass energy
18
Exclusive J/ Meson production
0<Q2<100 GeV2
L/T vs Q2
Pomeron trajectory
- pQCD models describe data
- strong sensitivity to (generalized) gluon
- need NLO to constrain gluon density
IP(t) not consistent with
soft diffractive measurement
19
Summary
New high precision HERA data have improved our understanding of diffraction:
Diffractive processes are dominated by gluons
pQCD
Regge factorization works to a good approximation
Diffractive pdfs are universal within HERA
- QCD factorization holds in diffractive DIS
Non perturbative
phenomenology
- On the way to understanding the large breaking
of factorization at Tevatron – soft re-scattering
Vector mesons: steep W dependence
- Pure pQCD approach successful
Need to discriminate models
HERA–II: a lot of more data coming
20