diffraction at hera
DESCRIPTION
Diffraction at HERA. Anna Mastroberardino Calabria University On behalf of the H1 & ZEUS Collaborations. HSQCD 2004 St. Petersburg, Russia 18 – 22 May 2004. Outline. Introduction to diffraction Diffractive structure function of the proton QCD fits of diffractive data - PowerPoint PPT PresentationTRANSCRIPT
1
Anna MastroberardinoCalabria University
On behalf of the H1 & ZEUS Collaborations
Diffraction at HERA
HSQCD 2004St. Petersburg, Russia
18 – 22 May 2004
2
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
3
What is diffraction?
XW
Q2e (k)e’(k’)
P (p) P’(p’)t
IPxIP
*(q)
Diffraction: exchange of colour singlet (IP) producing a rapidity GAP in the particle flow
Q2
WThe struck quark carries fraction x Q2/W2 of the proton momentum
W = photon-proton centre of mass energy
The pomeron carries fraction xIP of the initial proton momentum The struck parton carries fraction β of the Pomeron momentum
Standard DIS in a frame in which the proton is very fast (Breit frame):
4
Why diffraction ?
),()],(1[2
14 2
22
2
4
2
2
2
QxFQxR
yy
xQdxdQ
d
i
ii x,QxfeF 222
),,,()1(2
14 2)4(
2)4(
2
4
2
2
4
txQFR
yy
QdtdxdQd
dIP
DD
IP
Diffractive DIS probes the partonic structure of colour singlet exchange
DIS probes the partonic structure of the proton
Diffractive structure function Diffractive cross section
920 GeV proton 27.5 GeV electron
5
252
10
GeV10
GeV 300
x
Q
W
~ 10% of low-x DIS diffractive at HERA What role does it play?
HERA has opened up the small x domain
5
Selection methods
Large Rapidity Gap MX – Method Proton Tagging
ln MX2
-2 0 2 4 6 8
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 peak
no forward energy deposition
Diffractive events are characterized by:
flat vs ln MX2 distribution
scattered proton almost intact
6
Factorization in Diffractive DIS
QCD factorization for diffractive DIS holds (Collins, Bereira & Soper, Trentadue & Veneziano)
),(),( 22/
)4(2 QFtxfF IP
IPpIPD
iD
i/pD fF ̂ ~ 2 universal partonic cross section
(same as in inclusive DIS)
diffractive parton distribution function – evolve according to DGLAPuniversal for diffractive ep DIS (inclusive, dijets, charm)
If in addition postulate Regge factorization (Ingelman & Schlein)
equations DGLAPfollowing evolves ),( 22 QF IP
7
New results from ZEUS
Transition from very low Q2 to DIS (0.03 <Q2<100 GeV2)
MN < 2.3 GeV
Proton tagging method MX method
8
Recent results from H1
)4(
2
2)4(
2)4(
)2
1(2
DL
DDr F
yy
yF
0 if
low yat
2
2
D
LDD
r
DDr
FF
F
Integrate over t )3(Dr
22 GeV 16005.1 Q
high precision measurement of and Q2 dependences
QCD fit (DGLAP evolution of diffractive pdfs) (coming later)
9
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
10
H1 NLO QCD fit – diffractive PDFs
Parametrize Flavour Singlet (quarks + antiquarks) and gluons at Q2 = 3 GeV2
Evolve according to NLO DGLAP and fit Determine quark sea and gluon distribution
Integrated fraction of exchanged momentum carried by gluons
(75 15)%
Diffractive interactions gluon dominated
Diffractive data fitted in similar way to proton F2 data
11
QCD fit describes data
fractional gluon momentum
shape of pdfs not well constrained
))%(9)(882( sysstat
)36/9.37/( 2 ndf
[F2D(3)cc from DESY-03-094]
(LPS)
ZEUS NLO QCD fit to F2D and charm
• xIP <0.01
• 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-flavour- number-scheme
12
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?
Factorization in Diffractive DIS – experimental test
iD
i/pD fF ̂ ~ 2 universal partonic cross section
(same as in inclusive DIS)
diffractive parton distribution function – evolve according to DGLAPuniversal for diffractive ep DIS (inclusive, dijets, charm)
13
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
14
diffractive DIS ?
factorization breaking
Diffractive structure function of antiproton
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 ?
Factorization in Diffractive DIS – experimental test
It holds
Factorization not expected to work - Indeed it does not:
hadron-hadron scattering ?
diffractive dijets at the Tevatron: suppression by a factor of 10
15
Diffractive dijets in photoproductionReal photon (Q2~ 0) can develop hadronic structure
photoproduction similar to
hadron-hadron interaction
LO comparison: no evidence for a suppression of resolved with respect to direct
NLO comparison ?
X= partonic momentum for dijet production
photon remnant energy 1 - x
16
Diffractive dijets in photoproduction
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
NLO calculations compared to preliminary H1 data (Klasen and Kramer, DESY 04-011)
17
Vector Meson production
(JPC=1--): JIP
p p
V
pp
V
W
cross section:
226
2
),( ~ effS QxxG
Q
2
22
~W
MQx V
8.0~ ~
~),( /for 2.02
W
xQxxGJ eff
rise with W: increasing with hard scale
probability of finding 2 gluons in the proton
2-gluon exchange Exclusive VM production calculable in pQCD
NLO calculation available for J/photoproduction
Sensitivity to gluons in proton
18
Vector Meson production
(p Vp), Q2=0
W x
MV
p centre-of-mass energy
Small MV (MV2 1 GeV2):
Incoming dipole behaves like a normal-size hadron. Flat vs W reflects flat gluon distribution for Q2 0
Large MV :
Fast growth of with W reflects growth of gluon distribution with decreasing x
Soft regime
Hard regime
19
Exclusive J/ Meson production
- pQCD models describe data
- strong sensitivity to (generalized) gluon
- need NLO to constrain gluon density
L/T vs Q2
Pomeron trajectory
0<Q2<100 GeV2
IP(t) not consistent with
soft diffractive measurement
20
Summary
Non perturbative phenomenology
HERA–II: a lot of more data coming
Need to discriminate models
pQCD
New high precision HERA data have improved our understanding of diffraction:
Diffractive processes are dominated by gluons
Regge factorization works to a good approximation
Diffractive pdfs are universal within HERA - QCD factorization holds in diffractive DIS
Vector mesons: steep W dependence - Pure pQCD approach successful
- On the way to understanding the large breaking of factorization at Tevatron – soft re-scattering