p. marage université libre de bruxelles
DESCRIPTION
Exclusive (hard) diffraction at HERA (DVCS and Vector Mesons). P. Marage Université Libre de Bruxelles. The HERA harvest. initial state : real and virtual photons final state VM : g, r, f, J/ y, y (2s), U final state baryonic syst.: p, p. diss. M V 0 – 10 GeV - PowerPoint PPT PresentationTRANSCRIPT
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P. MarageUniversité Libre de Bruxelles
Exclusive (hard) diffraction at HERA
(DVCS and Vector Mesons)
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The HERA harvest
initial state : real and virtual photons
final state VM : J/(2s),
final state baryonic syst.: p, p. diss.
MV 0 – 10 GeV
Q2 0 – 80 GeV2 photoproduction & DIS
W ≈ √(Q2/x) 30 – 300 GeV
|t| = |pt,miss|2 0 – 30 GeV2 small (< 0.5 – 3 GeV2) & large |t| (2 < |t| < 10-30 GeV2)
> 30 H1 + ZEUS published exp. papers most recent results (2008-2009)
- DVCS ZEUS (LPS) + H1 DESY-08-132 + H1-prelim-09-014 - photoprod. H1 H1-prelim-09-016- and electroprod. H1 DESY-09-093 (to appear soon)- photoprod. ZEUS DESY-09-036
- real at large |t| H1 DESY-08-077
P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’09 – CERN 29/06-03/07/2009
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ContextQCD frameworks
collinear factorisation (GPD) proton rest frame factorisation (dipole approach)
scale - long. amplitudes
- transv. amplitudes
Content DVCSd / dQ2, dW, dt (small t) VM helicity amplitudes
elast. / proton diss.; Regge factorisationd / dW, dt (large t); helicity amplitudes
2 2 2(1 ) ( )z z Q M2 2 21 0.5 ( ) / 4z z Q M 2 2 2( ) / 4Q M
P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’09 – CERN 29/06-03/07/2009
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Event selection and backgroundsPhotoprod. (Q2 ~ 0) / electroprod. (Q2 > 1 GeV2) : absence / presence of electron cand.
VM selection : 2 tracks + inv. mass selection
elastic events : LPS (ZEUS) or no tag in forward detectors (H1, Zeus) |t| < 0.5 GeV2
proton dissociation : tag events, |t| <3 GeV2
backgrounds:
elastic <-> p. diss. events
other VM’s (’)
continuum
P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’09 – CERN 29/06-03/07/2009
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DVCS
P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’09 – CERN 29/06-03/07/2009
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Q2 dependenceGPD and dipole
models
GPD take into account
skewing
but kin. skewing is
not sufficient
LPS
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interference with Bethe-Heitler
→ access to Re contributions
Beam charge asymmetry (e+ vs. e-)
in agreement with dispersion relation analysis
W, t dependences; effective Regge trajectory : see below
Re / Im (BCA)
Re / Im 0.20 0.05 0.08DVCS DVCSA A
2tan ( ( ) / 8) 0.25 0.03 0.05Q
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Q2 dependences (VM)
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Excellent ZEUS-H1 agreement on total cross sections
(also for with ZEUS p. diss. bg. subtraction as for )
and (el. and p. diss.)
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NB fixed W, varying x
and – polarised cross sections
2 2 2 4
2 2
6 8
2 2
| ( ) | / ( )
/ /
formally 1/ , 1/
but dependence of | ( ) | (and of )
L V
L T V
L T
xG x Q M
R Q M
Q Q
Q xG x R
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(approximate) universalityCross sections scaled by
quark charge content
Very strong Q2 dependence
approximate universality
as a function of Q2+M2
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W dependences
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Photoproduction : mass dependence
soft
tot
hard
J/
(skewing,
Re GPD)
W
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DVCS and electroprod.: Q2 dependence
see below – effective Regge trajectory
DVCS
J/
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t dependences
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t dependences
b td
edt
DVCS (LPS)
elast. p. diss.
DVCS
(p. diss.)
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b slopes
optical model
- b decrease with dipole size (Q2, mq)
- universal scale dependence for > 5 GeV2 = (Q2 + M2)/4 for VM= Q2 for DVCS
- b(, at large Q2) -> b (J/) -> small room for VM form factor bVM
( )dipole exch Y VMb b b b b
P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’09 – CERN 29/06-03/07/2009
elastic p. dissoc.
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bel – bpd and Regge factorisationVertex (“Regge”) factorisation
bel. – bp. diss.
bel. – bp. diss. ~ cte (?) ~ 5.5 for
= 4 for DVCS (2 = Q2)
= 3.5 for J/
-> factorisation for > 5 GeV2 ?
2/ (p.diss.)( 0) indep. of Q
/ (elast.)
d dtt
d dt
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Effective Regge trajectory
P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’09 – CERN 29/06-03/07/2009
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W dependence
~ W (t) – 1) (t) = (0) + ’ t
shrinkage of diffractive peak
W – t correlation
t dependence as a function of W W dependence as a function of t
Effective trajectory
0 4( (0) ' 1)
0 0
( )
4 ' ln( / )
b tb t tdW e e W
dtb b W W
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Shrinkage Xx
Put rho photoprod.photoprod.
electroprod.
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’ slope
hadron-hadron inter. : ‘ = 0.25 GeV-2
H1 DVCS : ‘ = 0.03 ± 0.09 ± 0.11 GeV-2
NB J/ 2 < |t| < 30 GeV2 photoprod.,
‘ = - 0.02 ± 0.01 ± 0.01 GeV-2 (H1 + ZEUS)
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P(0)
= (Q2 + M2)/4 for VM = Q2 for DVCS
hardening with Q2 and mass (dipole size)
light VM : getting similar to J/ for larger than about 5 GeV2
4( ( ) 1) ( ) (0) 'W t t t
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Remark on scales
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Helicity amplitudes
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spin density matrix elements3 angles describe VM production and decay
→ 15 spin density matrix elements
bilinear combinations of
helicity amplitudes T
(NPE is assumed)
SCHC T00 T11
single flip T01 T10
double flip T-1 1
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spin density matrix elements (Q2) ()5 SCHC elements
compared to GPD calculations
Other elements (dashed lines) compatible with 0 or small)
except ~ T01 T00*
several models
(GPD, unintegrated kt,
dipole + saturation)
can reproduce
general features of
SCHC amplitudes + hierarchy
but not details
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R(Q2) ; Q2 / M2
pQCD : formally R ~ Q2 / M2
scaling plot (Q2 / M2V)
- scaling OK
- lower than 1
- damping at large Q2
2 2 2 200 10 00 00
2 2 2 2 2 211 01 11 11 01 11
2L
T
T T T TR
T T T T T T
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R(W)
No W dependence of L / T
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R (t) ; bL - bT
( ) exp ( )LL T
TR t b b t
H1 Q2 > 5 GeV2 : bL - bT <0 1.5 (stat. + syst.)
also t dependence of T11 / T00 – see below
indication that transverse size of dipoles from transverse photons is larger than from longitudinal photons
Q2 > 5 GeV2
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R (m) ()
Decrease of R(m) is qualitatively consistent with
formal pQCD calculations R ~ Q2 / M2
if M is diquark / dipion mass rather than resonance mass
Support to models (MRT) with small relevance of the VM WF
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Global fit of 15 SDME -> 4 amplitude ratios (supposed to be purely imaginary)
(Ivanov-Kirschner)
Q2 dependences (higher twist) of all amplitude ratios
t dependences of helicity flip amplitudes T01/T00 T10/T00 T-11/T00
NB double flip amplitude -> gluon polarisation
t dependence of T11 / T00 - cf. R(t) : different slopes for L and T – here 3 effect
amplitudes ratios (t, Q2) ()
Q2 < 5
Q2 > 5
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SCHC amplitudes phases (Q2) ()
phase between T00 and T11 ≠ 0
phase difference decreases with Q2
NB phase difference related to different values of = Re / Im,
which are related through dispersion relations to the W evolutions
-> indication of different W evolution of transverse and longitudinal ampltudes ?
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Large |t|
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Power like t dependences for real , , J
Quote measured powers for photon, rho, jpsi
J BFKL running s excluded BFKL favoured
DGLAP OK (t < M
see also helicity below
t dependences
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J/
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W dependences
rise of with W described
by BFKL,
not by DGLAP at large |t|
BFKL pred.
J/
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SDME ()
“naïve” pQCD predicts large
helicity flip, with long. dominating at large |t| (spin flip ~ t)
But SCHC T → T dominates
+ double flip T → T
Reason :
chiral odd contribution in (due to constituent quark mass)
→ no orbital momentum needed for T
→ SCHC
BFKL model describes data,
except for sign of
cf. also t and W dependences
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SDME (J/)
SDME compatible with SCHC,
as expected from models
and compact J/ WF
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Summary and conclusions
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summaryMeasurements of kinematic and angular distributions of real , and , J/, in photo- and electroprod.,
small and large |t|
DVCS
Q2, W, t dependences; beam charge asymmetry -> Re/Im amplitudes
VM at small |t|
Q2, W, t dependences of DVCS, and , J/
hard features for Q2 + M2 10-20 GeV2
helicity amplitudes
SCHC + helicity violations
Q2 and t dependences understood in QCD
indication of bL – bT 0
cos 1
effective Regge trajectory
pom(0) increases with scale. ‘ < 0.25 GeV-2
GPD and dipole models describe main features, but differences in details
VM at large |t|
W, t dependences; helicity amplitudes
BFKL describes main features, but differences in details P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’09 – CERN 29/06-03/07/2009
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conclusions
Very rich and varied landscape,
explored by ZEUS and H1, complementing lower energy experiments
A semi-quantitative understanding is achieved in a QCD framework,
in the two complementary approaches of GPD’s and dipole models
and, at large |t|, in the BFKL framework
Many thanks to the organisers,
and to all those to whom I borrowed data – plots – ideas
P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’09 – CERN 29/06-03/07/2009