Twarde procesy ekskluzywne i partonowa struktura nukleonu

1
Twarde procesy ekskluzywne i partonowa struktura
nukleonu
Andrzej Sandacz
Instytut Problemów Jadrowych, Warszawa

• DVCS gleboko wirtualne rozpraszanie Comptona

• Ekskluzywna produkcja mezonów wektorowych na
  akceleratorze HERA

• Spinowa asymetria w produkcji ?0 z poprzecznie
  spolaryzowana tarcza

• Ekskluzywna produkcja pionów

• Projekt pomiarów DVCS i produkcji mezonów w
  COMPASS-ie

Seminarium Fizyki Wielkich Energii, Uniwersytet
Warszawski
11 stycznia 2008
2
Generalized Parton Distributions
g
g
Factorisation Q2 large, -tlt1 GeV2
hard
xx
x-x
soft
GPDs
t
p (P2, s)
p (P1, s)


depending on 3 variables x, x, t
4 Generalised Parton Distributions H, E, H, E
for each quark flavour and for gluons
x xB/(2-xB )
for DVCS gluons contribute only at higher orders
in as
3
GPDs properties and links to standard physics
for P1 P2 recover usual parton densities
no similar relations these GPDs decouple for P1
P2
needs orbital angular momentum between partons
4
Holy Grails or the main goals

? GPD a 3-dimensional picture of the partonic
nucleon structure or spatial parton
distribution in the transverse plane
H(x, ?0, t) ? H(x,, rx,y )
probability interpretation
Burkardt

y
x P
r?
x
z

• Contribution to the nucleon spin puzzle
• E related to the orbital angular momentum
• 2Jq ? x (Hq (x,?,0) Eq (x,?,0) ) dx
• ½ ½ ?S ?G lt Lzq gt lt Lzg gt

E
5
Observables and their relationship to GPDs
T
The imaginary part of amplitude T probes GPD at x
x
The real part of amplitude T depends on the
integral of GPD over x
DGLAP
DGLAP
ERBL
notation
6
DVCS Asymmetries measured up to now
e
e

p
p
BH calculable
DVCS

DsC cosf Re H xH
H

DsLU sinfImH xH kE
H


DsUL sinfImH xH
H
DsUT sin(f-?s)cos?Im(F2H F1E)
H, E
kinematically suppressed
x xB/(2-xB ),k t/4M2
7
CLAS DVCS - BSA
Accurate data in a large kinematical domain
Integrated over t
8
Results from JLAB Hall A E00-100
Q2 2.3 GeV2 xB 0.36
PRL97, 262002 (2006)
Difference of polarized cross sections
Twist-2
Twist-3
Unpolarized cross sections
extracted twist-3 contribution small
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s1int
dominant term at small t
GPD !!!
No Q2 dependence strong indication for scaling
and handbag dominance
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Towards E and Ju, Jd
Hermes DVCS-TTSA
Hall A nDVCS-BSA

• Neutron obtained combining
• deuteron and proton
• F1 small and u d cancel in

x0.36 and Q21.9GeV2
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Present status of the MODEL-DEPENDENT Ju-Jd
extraction
Lattice LHPC hep-lat 0705.4295
With VGG Code
12
2007 results from LHPC Collaboration (Lattice)
µ2 4 GeV2
Lq Jq ?Sq / 2
extrapolations to mp,phys using ChPT (numbers
below for covariant barion ChPT)
Jud 0.213(26) Ju
0.214(16) Jd -0.001(16)
Lud 0.006(38) Lu -0.195(38)
Ld 0.200(38)
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Unpolarised DVCS cross sections from HERA
sDVCS at small xB (lt 0.01) mostly sensitive to

Hg, Hsea
Q² and W dependence NLO predictions
bands reflect experimental error on slope b 5.26
lt b lt 6.40 GeV-2
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t dependence of DVCS cross section
New H1 results on slopes - DIS07
DIS06
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Lessons from DVCS at H1/ZEUS
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Hard exclusive meson production

• factorisation proven only for sL

sT suppressed of by 1/Q2
necessary to extract longitudinal contribution to
observables (sL , )

• allows separation and wrt
  quark flavours

Flavour sensitivity of DVMP on the proton
?0 2ud, 9g/4
? 2u-d, 3g/4
f s, g
? u-d
J/? g
conserve flip nucleon helicity

• quarks and gluons enter at the same order of aS

• wave function of meson (DA F)

additional information/complication
17
Dipole models for exclusive VM production at
small x

• at small x sensitivity mostly to gluons

• at very small x huge NLO corrections, large
  ln(1/x) terms (BFKL type logs)

• pQCD models to describe colour dipole-nucleon
  cross sections and meson WF

dipole transv. size W-dep.
t-dep.
large weak
steep
small strong
shallow

• Frankfurt-Koepf-Strikman (FKS) Phys.Rev. D57
  (1998) 512

sensitivity to different gluon density
distibutions

• Martin-Ryskin-Teubner (MRT) Phys.Rev. D62
  (2000) 014022

• Farshaw-Sandapen-Shaw (FSS) Phys.Rev. D69
  (2004) 094013

• Kowalski-Motyka-Watt (KMW) Phys.Rev. D74
  (2006) 074016

sensitivity to ?0 wave function

• Dosch-Fereira (DF) hep-ph/0610311 (2006)

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Recent ZEUS results on exclusive ?0 production
significant increase of precision extended Q2
range
96-00 data 120 pb-1
arXiv0708.1478hep-ex
W-dependence s 8 Wd

• steeper energy dependence with increasing Q2

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W-dependence for hard exclusive processes at
small x
?
?
J/?

• steep energy dependence for all vector mesons
  in presence of hard scale

Q2 and/or M2

• universality of energy dependence at small x?
  

at Q2M2 10 GeV2 still significant difference
between ? and J/?
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ds / dt - ?0
example (1 out of 6)
ZEUS
Fit

• shallower t-dependence with increasing Q2

21
t-dependence for hard exclusive processes at
small x
recent data suggestive of possible 15
difference between ? and J/?
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Selected results on R sL/sT for ?0 production
the same W- and t-dependence for sL and sT
dL dT
bL bT
the same size of the longitudinal and transverse
? involved in hard ?0 production
i.e. contribution of large qqbar fluctuations of
transverse ? suppressed
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Comparison to dipole models
extensive comparison of the models to recent ZEUS
?0 data in
arXiv0708.1478hep-ex
below just selected examples

• considered models describe qualitatively all
  features of the data reasonably well

• recent ZEUS data are a challenge none of the
  models gives at the moment

satisfactory quantitative description of all
features of the data
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Comparison to GPD model
Hand-bag model GPDs from DD using CTEQ6

• Goloskokov-Kroll

power corrections due to kt of quarks included
arXiv0708,3569hep-ph
both contributions of ?L and ?T calculated
W90 GeV
H1
? ZEUS
W90 GeV
ZEUS (recent)
s/10
W75 GeV
complete calculation
leading twist only (in collinear approx.)

• leading twist prediction above full
  calculation, even at Q2 100 GeV2

20

• contribution of sT decreases with Q2, but does
  not vanish even at Q2 100 GeV2

10

• sea quark contribution, including interference
  with gluons, non-negligible

25 at Q2 4 GeV2
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Transverse target spin asymmetry for exclusive
?0 production
Give access to GPD E related to the orbital
angular momentum of quarks
Jis sum rule
q
q
So far GPD E poorly constrained by data mostly
by Pauli form factors
The asymmetry defined as
to disentangle contributions from ?L and ?T the
distribution of ?0 decay polar angle needed in
addition
Diehl and Sapeta
Eur. Phys. J.C 41, 515 (2005)
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Method for L/T separation used by HERMES
A. Rostomyan and J. Dreschler arXiv0707.2486
Angular distribution W(cos ?, f, fs) and
Unbinned Maximum Likelihood fit
a prerequisite for the method determination of
acceptance correction
as a function of cos ?, f and fs
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?0 transverse target spin asymmetry from HERMES
Transversely polarised proton target, PT 75
2002-2005 data, 171.6 pb-1

• for the first time ?0 TTSA extracted separately
  for ?L and ?T

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?0 transverse target spin asymmetry from HERMES

• in a model dependent analysis data favours
  positive Ju

in agreement with DVCS results from HERMES
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?0 transverse target spin asymmetry from COMPASS
Transversely polarised deuteron target (6LiD),
PT 50 2002-2004 data
obtained using Double Ratio Method
in bins of ? f fS
u (d) are for upstream (downstream) cell of
polarised target
arrows indicate transverse polarisation of
corresponding cells
raw asymmetry e from the fit to DR(?)
dilution factor f 0.38
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?0 transverse target spin asymmetry from COMPASS
new

• asymmetry for deuteron target consistent with
  zero

ongoing work on

• longitudinal/transverse separation

• separation of incoherent/coherent

in 2007 data taken with transversely polarised
proton target (NH3)
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Exclusive p production from HERMES
e p ? e n p

• at leading twist sL sensitive to GPDs H and E

• L/T separation not possible at HERMES, sT
  expected to be supressed as 1/Q2

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Beam spin asymmetry in exclusive p0 production
from CLAS

• at leading twist sL sensitive to GPD H

e p ? e p p0

• no t-channel pion-pole (in contrast to
  exclusive p production)

• any non-zero BSA would indicate L-T
  interference, i.e. contribution not described

in terms of GPDs
preliminary
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Cross sections for exclusive p0 production from
JLAB HALL A DVCS Collab.
E00-110
h 1 is the beam helicity
from P. Bertin, Baryon-07
t-slope close to 0, maybe even small negative

34
dashed prediction for sL from VGG model using
GPDs
Vanderhaeghen, Guichon, Guidal
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Summary for existing measurements

• New precise data on cross sections and
  asymmetries ? significantly

more stringent constraints on the models for GPDs

• Results on DVCS promissing

indication of scaling and handbag dominance in
valence region
good agreement with NLO for HERA data

• To describe present data on DVMP, both at large
  and small x,

including power corrections (or higher order
pQCD terms) is essential

• First experimental efforts to constrain GPD E
  and quark orbital momentum

36
Generalized Parton Distributions _at_
 Expression of Interest  SPSC-EOI-005 and
presentation to SPSC ? writing of the proposal,
preparation of the future GPD program 2010
Roadmap Now with 6LiD or NH3 polarized target
and no recoil
detector After 2010 with H2 or D2 target with a
recoil detector and an
additional calorimetry
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Competition in the world and COMPASS role
HERA
Ix2
COMPASS at CERN-SPS High energy muon
beam 100/190 GeV µ or µ- change once per
day polar(µ)-0.80 polar(µ-)0.80 2.108 µ per
SPS cycle in 2010 ? new Linac4 (high intensity
H- source) as injector for the PSB
improvements on the muon line
Gluons valence quarks valence quarks
and sea quarks and gluons
COMPASS JLab 12 GeV, FAIR, 2010
2014
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DVCS BH with polarized and charged leptons
and unpolarized target
µ
µ

p
p
BH calculable
DVCS
ds(µp?µp?) dsBH dsDVCSunpol Pµ
dsDVCSpol eµ aBH Re ADVCS
eµ Pµ aBH Im ADVCS
? known
Pµ ?
eµ ?
eµ Pµ ?
Twist-3 M01
Twist-2 M11
Twist-2 gluon M-11
gtgt
Belitsky,Müller,Kirchner
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Both c1Int and s1Int accessible at COMPASS with
? and ?-
with
F1H dominance with a proton target F2E
dominance with a neutron target (F1ltlt) very
attractive for Jis sum rule study
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µ
?
?
Muon Beam Asymmetry at E? 100 GeV COMPASS
prediction With a 2.5m H2 target
µ
p
?
6 month data taking in 2010 25 global efficiency
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µ
?
?
Muon Beam Asymmetry at E? 100 GeV COMPASS
prediction
µ
p
?
VGG double-distribution in x,?
model 1 H(x,?,t) q(x) F(t)
model 2 and 2 correl x and t
ltb2?gt a ln 1/x
H(x,0,t) q(x) e t ltb?2gt q(x) /
xat a slope of Regge traject.
a0.8 a1.1
Guzey Dual parametrization
model 3 also Regge-motivated
t-dependence with a1.1
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Recoil detector design
Goals Detect protons of 250-750 MeV/c
t resolution gt sTOF lt 300 ps
exclusivity gt Hermetic detector
Design 2 concentric barrels of 24
scintillators counters read at both sides
European funding through a JRA for studies
and construction of a prototype ( Bonn, Mainz,
Saclay, Warsaw )
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Recoil Detector Prototype Tests (2006)
All scintillators are BC 408 A 284cm x 6.5cm x
0.4cm Equiped with XP20H0 (screening grid) B
400cm x 29cm x 5cm Equiped with XP4512 Use
1GHz sampler (300ns window) Design by
CEA-Saclay/LAL-Orsay
Installed downstream of COMPASS
Trigger AB coincidences finger pairs
Resolution on TOF Center 340ps HV
low Center 310ps HV high Expected resolution
280 ps
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Studies for a new ECAL0 (Dubna,)
Light brought by LS fibers to Avalanche
Micro-Pixel Photodiode Very Challenging
development for new and cheap AMPDs -
magnetic field, low threshold detection, high
rate environment
ECAL0 modules dimesions 50x50 mm2
(inner), 70x70 mm2 (middle and outer) Inner
part - 1200 modules (7200 AMPDs) Middle
part - 980 modules (3920 AMPDs) Outer part
- 1372 modules (4116 AMPDs) The first scint
3552 AMPDs Total 3552 modules
(15236355218788 AMPDs)
requiremens on ECAL0
geometry
large transverse size
400 x 400 cm2
small depth 25 cm
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The AMPD (1)

• High Gain (105106)
• Good Photon Detection
• Efficiency (1565)
• Compact
• (package size a few mm)
• Relatively Low Cost
• Magnetic-field tolerant
• High dark noise
• (order of 100-1000 kHz)
• Response against input
• light yield is non-linear

46
Conclusions prospects for future COMPASS
measurements

• Possible physics ouput
• Sensitivity to total spin of partons Ju Jd
• Sensitivity to spatial distribution of partons
• Testing a variety of GPD models (VGG, Müller,
  Guzey, FFS-Sch)
• to quantify the Physics potential of DVCS and
  HEMP
• Experimental challenges
• Recoil Detection for proton (and neutron)
• High performance and extension of the EM
  calorimetry
• Roadmap
• Now with the transversely polarized targets
• 6LiD (? 2006) and NH3 (2007)
• 2008-9 A small RPD and a liquid H2 target will
  be available
• for the hadron program (ask for 2
  shifts ? and ?-)
• gt 2010 A complete GPD program at COMPASS
• with a long RPD large liquid
  H2 target
• before the availability of JLab 12
  GeV, FAIR, EIC