Inclusive diffraction at HERA

Report
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
ddQ 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  105
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

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