gluons in the proton and exclusive hard diffraction
DESCRIPTION
Gluons in the proton and exclusive hard diffraction. Introduction data on exclusive vector meson electroproduction sizes of gluon cloud sizes of photon configurations comparison to theory. Aharon Levy Tel Aviv University. - PowerPoint PPT PresentationTRANSCRIPT
June 13, 2008 Aharon Levy - Torino seminar 1
Gluons in the proton and exclusive hard diffraction
Aharon LevyTel Aviv University
• Introduction
• data on exclusive vector meson electroproduction• sizes of gluon cloud
• sizes of photon configurations
• comparison to theory
June 13, 2008 Aharon Levy - Torino seminar 2
QCD based fits can follow the data accurately, yield parton densities. BUT:• many free parameters (18-30) (only know how parton densities evolve)
• form of parameterisation fixed by hand (not given by theory)
F2 parton densities. * ‘sees’ partons. parton density increases with decreasing x.
June 13, 2008 Aharon Levy - Torino seminar 3
From Pumplin, DIS05
There are signs that DGLAP (Q2
evolution) may be in trouble at small x (negative gluons, high 2 for fits).
Need better data to test whether our parton densities are reasonable. The structure function FL will provide an important test.
all is not well …
Can also get information on gluon density from exclusive hard processes.
June 13, 2008 Aharon Levy - Torino seminar 4
Exclusive VM electroproduction
(V0 = DVCS)
* 0
0 , , , / ,
p V p
V J
June 13, 2008 Aharon Levy - Torino seminar 5
soft to hard transition
IP
‘soft’
WW )(
‘hard’
gg||tbe
dt
d
• Expect to increase from soft (~0.2, from ‘soft Pomeron’ value) to hard (~0.8, from xg(x,Q2)2)
• Expect b to decrease from soft (~10 GeV-2)
to hard (~4-5 GeV-2)
June 13, 2008 Aharon Levy - Torino seminar 6
Below Q2 0.5 GeV2, see same energy dependence as observed in hadron-hadron interactions. Start to resolve the partons.
s0.096
soft hard
2( )2
QF x
June 13, 2008 Aharon Levy - Torino seminar 7
soft to hard transition
IP
‘soft’
WW )(
‘hard’
gg||tbe
dt
d
• Expect to increase from soft (~0.2, from ‘soft Pomeron’ value) to hard (~0.8, from xg(x,Q2)2)
• Expect b to decrease from soft (~10 GeV-2)
to hard (~4-5 GeV-2)
June 13, 2008 Aharon Levy - Torino seminar 8
ingredients
Use QED for photon wave function. Study properties of V-meson wf and the gluon density in the proton.
June 13, 2008 Aharon Levy - Torino seminar 9
Mass distributions KK
/J
June 13, 2008 Aharon Levy - Torino seminar 10
Photoproduction
W process becomes hard as scale (mass) becomes larger.
June 13, 2008 Aharon Levy - Torino seminar 11
(W) – ρ0 Fix mass – change Q2
June 13, 2008 Aharon Levy - Torino seminar 12
Proton dissociationMC: PYTHIA
pdiss: 19 ± 2(st) ± 3(sys) %
June 13, 2008 Aharon Levy - Torino seminar 13
(W) – ρ0,
June 13, 2008 Aharon Levy - Torino seminar 14
(W) - , J/,
June 13, 2008 Aharon Levy - Torino seminar 15
(Q2+M2) - VM
June 13, 2008 Aharon Levy - Torino seminar 16
Kroll + Goloskokov: = 0.4 + 0.24 ln (Q2/4) (close to the CTEQ6M gluon density, if parametrized as xg(x)~x-/4)
June 13, 2008 Aharon Levy - Torino seminar 17
(Q2) nMQ
22
Fit to whole Q2 range gives bad 2/df (~70)
pp 0* VM n comments
ρ 2.44±0.09 Q2>10 GeV2
2.75±0.13 ±0.07
Q2>10 GeV2
J/ 2.486±0.080±0.068
All Q2
1.54±0.09 ±0.04
Q2>3 GeV2
101Q2(GeV2)
June 13, 2008 Aharon Levy - Torino seminar 18
(Q2)
* 2
2
2 2 ( )
1( )
( )p n QQ
Q M
2 2( ) 2.15 0.007n Q Q
(for Q2 > 1 GeV2)
June 13, 2008 Aharon Levy - Torino seminar 19
Proton vertex factorisation
photoproduction
proton vertex factorisation
Similar ratio within errors for and
proton vertex factorisation in DIS
IP IP
Yelastic p-dissociative
June 13, 2008 Aharon Levy - Torino seminar 20
b(Q2) – ρ0,
pp 0*
Fit||tbe
dt
d
:
June 13, 2008 Aharon Levy - Torino seminar 21
b(Q2+M2) - VM
2 2( )r b c
‘hard’
gg
June 13, 2008 Aharon Levy - Torino seminar 22
Information on L and T
Use 0 decay angular distribution to get r0400 density matrix
element 04 04 200 00(cos ) (1 ) (3 1)cosh hf r r
04000400
1
1L
T
rR
r
- ratio of longitudinal- to transverse-photon fluxes ( <> = 0.996)
0400
L L
L T tot
r
using SCHC
June 13, 2008 Aharon Levy - Torino seminar 23
R=L/T (Q2)
pp 0*
When r0004 close to 1, error on R large and asymmetric
advantageous to use r0004 rather than R.
June 13, 2008 Aharon Levy - Torino seminar 24
R=L/T (Q2)
June 13, 2008 Aharon Levy - Torino seminar 25
R=L/T (Mππ)
Why??
June 13, 2008 Aharon Levy - Torino seminar 26
R=L/T (Mππ)
Possible explanation:
2
1R
M
example for =1.5
June 13, 2008 Aharon Levy - Torino seminar 27
Photon configuration - sizessmall kT large
kTlarge config.
small config.
T: large size small size
strong color forces color screening
large cross section small cross section
*: *T, *L
*T – both sizes, *L – small size
Light VM: transverse size of ~ size of proton
Heavy VM: size small cross section much smaller (color transparency) but due to small size (scale given by mass of VM) ‘see’ gluons in the proton ~ (xg)2 large
June 13, 2008 Aharon Levy - Torino seminar 28
L/tot(W)
L and T same W dependence
L in small configuration
T in small and large configurations
small configuration steep W dep
large configuration slow W dep
large configuration seems to be suppressed
June 13, 2008 Aharon Levy - Torino seminar 29
L/tot(t)
TL bb
size of *L *T
large configuration seems to be ssuppressed
June 13, 2008 Aharon Levy - Torino seminar 30
(W) - DVCS* p p
Final state is real T
using SCHC initial * is *T
but W dep of steep
large *T configurations seem to be suppressed
June 13, 2008 Aharon Levy - Torino seminar 31
Effective Pomeron trajectory
ρ0
photoproduction
Get effective Pomeron trajectory from d/dt(W) at fixed t
2[2 ( ) 2]( ) ( ) IP td
W F t Wdt
Regge:
June 13, 2008 Aharon Levy - Torino seminar 32
Effective Pomeron trajectoryρ0 electroproduction
' 1
Tk
‘hard’
gg
June 13, 2008 Aharon Levy - Torino seminar 33
Comparison to theory
• All theories use dipole picture
• Use QED for photon wave function
• Use models for VM wave function – usually take a Gaussian shape
• Use gluon density in the proton
• Some use saturation model, others take sum of nonperturbative + pQCD calculation, and some just start at higher Q2
• Most work in configuration space, MRT works in momentum space. Configuration space – puts emphasis on VM wave function. Momentum space – on the gluon distribution.
• W dependence – information on the gluon
• Q2 and R – properties of the wave function
June 13, 2008 Aharon Levy - Torino seminar 34
ρ0 data (ZEUS) - Comparison to theory
• Martin-Ryskin-Teubner (MRT) – work in momentum space, use parton-hadron duality, put emphasis on gluon density determination. Phys. Rev. D 62, 014022 (2000).
• Forshaw-Sandapen-Shaw (FSS) – improved understanding of VM wf. Try Gaussian and DGKP (2-dim Gaussian with light-cone variables). Phys. Rev. D 69, 094013 (2004).
• Kowalski-Motyka-Watt (KMW) – add impact parameter dependence, Q2 evolution – DGLAP. Phys. Rev. D 74, 074016 (2006).
• Dosch-Ferreira (DF) – focusing on the dipole cross section using Wilson loops. Use soft+hard Pomeron for an effective evolution. Eur. Phys. J. C 51, 83 (2007).
June 13, 2008 Aharon Levy - Torino seminar 35
ρ0 data (H1) - Comparison to theory
• Marquet-Peschanski-Soyez (MPS): Dipole cross section from fit to previous , and J/ data. Geometric scaling extended to non-forward amplitude. Saturation scale is t-dependent.
• Ivanov-Nikolaev-Savin (INS): Dipole cross section obtained from BFKL Pomeron. Use kt-unintegrated PDF and off-forward factor.
• Goloskokov-Kroll (GK): Factorisation of hard process and proton GPD. GPD constructed from standard PDF with skewing profile function.
June 13, 2008 Aharon Levy - Torino seminar 36
Q2
KMW – good for Q2>2GeV2 miss Q2=0
DF – miss most Q2
FSS – Gauss better than DGKP
June 13, 2008 Aharon Levy - Torino seminar 37
Q2
Data seem to prefer
MRST99 and CTEQ6.5M
June 13, 2008 Aharon Levy - Torino seminar 38
W dependence
KMW - close
FSS:
Sat-Gauss – right W-dep.
wrong norm.
MRT:
CTEQ6.5M – slightly better in W-dep.
June 13, 2008 Aharon Levy - Torino seminar 39
L/tot(Q2)
June 13, 2008 Aharon Levy - Torino seminar 40
L/tot(W)
All models have mild W dependence. None describes all kinematic regions.
June 13, 2008 Aharon Levy - Torino seminar 41
L,T (Q2+M2)
• Different Q2+M2 dependence of L and T (L0 at Q2=0)
• Best description of L by GK; T not described.
June 13, 2008 Aharon Levy - Torino seminar 42
Density matrix elements - ,
• Fair description by GK
• r500 violates SCHC
June 13, 2008 Aharon Levy - Torino seminar 43
VM/tot - ???
June 13, 2008 Aharon Levy - Torino seminar 44
Summary and conclusions• HERA data shows transition from soft to hard interactions.• The cross section is rising with W and its logarithmic derivative in W,
, increases with Q2.• The exponential slope of the t distribution decreases with Q2 and
levels off at about b = 5 GeV-2. Transverse size of gluon density (0.6 fm) inside the charge radius of the proton (0.8 fm).
• Proton vertex factorisation observed also in DIS.• The ratio of cross sections induced by longitudinally and
transversely polarised virtual photons increases with Q2, but is independent of W and t. The large configurations of the transversely polarised photon seem to be suppressed.
• The effective Pomeron trajectory has a larger intercept and smaller slope than those extracted from soft interactions.
• All these features are compatible with expectations of perturbative QCD.
• None of the models which have been compared to the measurements are able to reproduce all the features of the data.
• Precision measurements of exclusive vector meson electroproduction can help determine the gluon density in the proton.