Towards a GPDDVCS program at COMPASS

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Towards a GPD/DVCS program at COMPASS
The proposal - a first draft for the scientific
case - to be completed for
autumn 2008 The detectors and target to be
implemented First tests of 5 days in 2008
Pilot GPD run of 1 month in 2009
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In DVCS and meson production we measure Compton
Form Factor

For example at LO in ?S
DGLAP
t, ?xBj/2 fixed
q(x)
DGLAP
DGLAP
ERBL
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At COMPASS with ? and ?- ? access both Im H and
Re H
with DVCS BH with polarized and charged
leptons and
unpolarized target
Absolute cross section measurements dependence
in ? and Q2
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• Importance to measure both Im H and Re H
• a dispersion relation
• Fitting procedure or GPD quintessence function
• based on dual parametrization (D. Mueller, M.
  Polyakov)
• partial wave expansion with respect to the
  angular momentum
• partial wave amplitude of mesonic exchanged in
  the t-channel
• ? Regge phenomenology is a guide line towards
  realistic GPD ansatze

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3-dim picture of the partonic nucleon structure
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at large distance the gluon density generated
by the pion cloud increase of the N
transverse size for xBj lt mp/mp0.14
(chiral dynamics prediction)
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2 Parametrizations of GPDs
Factorization H(x,?,t) q(x) F(t) or
Regge-motivated t-dependence more realistic
with x-t correlation it considers
that fast partons in the small valence core
and slow partons at larger distance
(wider meson cloud) H(x,0,t)
q(x) e t ltb?2gt ltb2?gt
4 aln 1/x B C(1-x)2
transverse extension of partons
transverse size in hadronic collisions
has to be finite
due to
confinement (aslope of Regge
traject.) a 1 GeV2 for valence quark to
reproduce FF ? meson Regge traj. a 0.02
GeV2 for gluon (J/?) ltlt a 0.25 GeV2 for
soft Pomeron traj.
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µ
?
?
Beam Charge and Spin Asymmetry at E? 100 GeV
COMPASS prediction
µ
p
?
6 month data taking in 2010 250cm H2 target 25
global efficiency
BCSA
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µ
?
?
Beam Charge and Spin Asymmetry at E? 100 GeV
COMPASS prediction
µ
p
?
BCSA
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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2nd ultimate goal with GPD

• Contribution to the nucleon spin knowledge
• ½ ½ ?S ?G lt Lzq gt lt Lzg gt
• the GPDs correlation between the 2 pieces of
  information
• -distribution of longitudinal momentum carried
  by the partons
• -distribution in the transverse plane
• the GPD E is related to the angular
  momentum
• 2Jq ? x (Hq (x,?,0) Eq (x,?,0) ) dx
• ? with a transversely polarized target DVCS
  et MV

E
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How to get the GPD E ?
With a transversely polarized target
DVCS
Vector meson (M) production

• transversely polarized target inside the recoil
  detector
• ? Study for a second stage of the experiment

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Hard exclusive meson production at COMPASS
Collins et al. (PRD56 1997) -factorization
applies only for ?L -probably at high Q2
Filter of GPDs
? production presently studied at COMPASS
Vector Meson (?,?,?) H and E Pseudo-scalar
Meson (?,?,) H and E

Different flavor contents
H?0 1/?2 (2/3 Hu 1/3 Hd 3/8 Hg) H? 1/?2
(2/3 Hu 1/3 Hd 1/8 Hg) H?
-1/3 Hs - 1/8 Hg
measurement of d? /dt and ratio of
meson production
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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
if intensity ? 2, Q2 range up to 11
GeV2 after 2010, which 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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Additional equipment to the COMPASS setup
DVCS µp ? µp?
all COMPASS trackers SciFi, Si, µO, Gem, DC,
Straw, MWPC
? ECal1 ECal2 ?? ?
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2.5m cryogenic target to be designed and built

• - 2011 H2
• later transversely polarized

additional calorimeter ECal0 at larger angle
Recoil proton detector to insure exclusivity to
be designed and built
Nµ2.108/SPS cycle (duration 5.2s, each
16.8s) Possibility for an increase of intensity?
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Requirements for the recoil detector 1) Time
of Flight measurement ? t and ? (sectorization)
?(ToF) lt 300 ps ? ? P/P 3 à 15 t (p-p)²
2m(m-Ep) ? t/t 2 ? P/P ? 10 bins in t from
tmin to 1 GeV2
t is the Fourier conjugate of the impact
parameter r? t is the key of the measurement

315 ? 12 ps have been achieved during the
2006 test intrinsic limit due to the thin
layer A
2) Hermiticity huge background high counting
rates

• Rejection of extra ? at large angle
• ? study of rejection with angular and coplanarity
  constrains
• if not, detection of extra ? at a reasonable
  cost

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Calorimeter acceptance
y gt 0.05
E. Burtin
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Planning Proposal - a first draft for
the physics case is available - to be
completed for autumn - Lau Gatignon
discussion - target / RPD /
calorimetry - simulations and
predictions Presentation to SPSC ready for the
end of 2008 Good timing for detector improvement
and realization 2008 tests during 5
days 2009 one month for a pilot GPD/DVCS
run after 2010 the complete experiment _at_
COMPASS with H2 target RPD
ECALs still later with a transversely polarized
target
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2008 DVCS feasibility study in the
COMPASS environment
µ p ? µ ? p
in the RPD
in ECAL12
2008 Request to the collaboration for
switch-over from pion beam to muon beam
for 5 days Goal 1 a better knowledge of the ?
and ?- beams beam size (Si),momentum
(BMS), beam halo (forward spectro, RPD) Goal 2
a better knowledge of the RPD and its
extrapolation to a larger size comparison
hadrons (5?107 ? / 10s (5 MHz)) muons (2?5 108
? / 5s (50 MHz)) - rates in PMT, multipl.,
waveforms using Rand., Incl., RPD
triggers with the exclusive rho production
(1000 evts in 5 days) with Incl. trigger
- efficiency, purity of the RPD -
kinematical fit, azimuthal correlation -
comparison t from recoil proton detector
or from COMPASS spectro t
(p - p)2 (q? - q?)2
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• Goal 3 a better knowledge of the Calorimetry
• ? detection
• as a function of E? and the E? threshold
  energy
• in ECAL2 (x_Bj 0.05 and E? DVCS gt 10 GeV)
• in ECAL1 (x_Bj 0.1 and E? DVCS gt 5 GeV)
• thresholds of 1 GeV in ECAL1 and 2 GeV in ECAL2
• should be necessary for a good ?/? separation
• low noise level and pile up rejection (with the
  SADC)
• study as a function of the beam
  intensit
• necessity to measure with random
  trigger without/with beam
• effect of the charged tracks given by the muons
• selection of ? exclusif with exclusivity
  constrains

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• General requirement for these tests
• (compared to the hadron beam)
• ? or ?- beam
• BMS stations to be reinstalled
• ? evaluated time for the reinstallation 5
  days
• SciFi 01 02 04 06 to be reinstalled
• SciFi 05 and 08 are always in place
• Muon trigger (Random Trigger
• Outer Middle Calo triggers)
  Q2gt 1 GeV2
• ? one part of the Outer Trigger (HO03Y1) has to
  be reinstalled
• ? impact of the small hole in ECal2, HCal2
• ? strategy to set up the trigger with the beam
  to save time

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The scattered muon detection
middleoutercalo
0.03 lt xBj lt 0.07 0.07 lt xBj lt 0.13 0.13 lt xBj
lt 0.27
For the tests Q2gt1 GeV2 and 0.01ltxBj

XBj0.27
Strategy to set up the trigger with the beam to
save time 4 hours to fix the Calo trigger GPD
tests can really start 12 hours to fix the
Middle trigger 12 hours to fix the Outer
trigger to have the most complete set of
trigger
XBj0.03
XBj0.01
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in 2008 in 5 days goal 1 the ? and ?-
beam goal 2 the RPD in the muon beam
environment goal 3 the detection of low energy
photons in ECAL12
Tentative for a proposed planning
5 days at the end of the run
? ? ? ?- ?-
At least 5 days for instal- lation
2.108
gt 2.108
gt 2.108
2.108
2.108
Hadron run
Trigger setting
? detection
? and ? detection
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in 2009 ? with 30 days of dedicated
beam a  GPD  pilot run
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µ
?
?
?
at E? 100 GeV
6 month data taking in 2010 250cm H2 target 25
global efficiency
µ
p
?
Beam Charge and Spin Asymmetry
14000 evts
780 evts
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µ p?µ p ?
global efficiency of 25 ?
Data taking SPS availability 0.80
Spectro availability 0.90
DAQ livetime 0.93
Inter run 0.99 Spill
gate 0.96 Quality
checks 0.92 Trigger Veto
livetime 0.80 Trigger
efficiency 0.80 Reconstruction Incident
muon 0.93 Scattered
muon 0.90 ? 31 DVCS photon 0.90 would be
a guess! Recoil
proton 0.90
Control of absolute cross section within 5-10
- determination of F2 (already done in
NMC within 2) - determination of
Bethe-Heitler cross section (in 2009)
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DVCS BH with polarized and charged leptons
and unpolarized target
ds(µp?µp?) dsBH dsDVCSunpol Pµ
dsDVCSpol eµ aBH Re ADVCS
eµ Pµ aBH Im ADVCS
? Known expression
Pµ ?
eµ ?
eµ Pµ ?
Twist-3 M01
Twist-2 M11
Twist-2 gluon M-11
gtgt
Belitsky,Müller,Kirchner
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Naturally Polarized µ and µ- beams

• Solution proposed by Lau Gatignon
• To select Pp110GeV and Pµ100GeV
• to maximise the muon flux
• 2) To keep constant the collimator
• settings which define
• the p and µ momentum spreads
• ? Pol µ -0.8 and Pol µ- 0.8
• Nµ 1.6 ? Nµ-
• 2.108 µ- per SPS cycle (5.2s, 16.8s)

Requirements for DVCS -same energy -maximum
intensity -opposite polarisation to a few
-switch one per day
2.108 muons/spill
1.3 1013 protons/spill
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?
Criteria for a good proton

• geometrical configuration
• Correlation energy loss and ?measured
• Good timing and good vertex with the muon

? ? ?
online (2001)
offline (2001)
protons
Zproton-Zmuon cm

• Efficiency 90
• Purity 80

• Control with the present 1m long RPD

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Requirements for calorimetry
Competing reactions to DVCS DVCS µp ? µp?
? HEpP µp ? µpp ?
? ?? Dissociation of the proton
µp ? µN? ?/10
? Np DIS µp ?µpX dominant
with 1?, 1p, 2p,? Beam pile-up
DVCS photon kinematics
ECAL1
ECAL0
Q21 GeV2
?? ? E? DVCS? and E? threshold
for ? detection ?
xBj0.05 min E?DVCS10 GeV E? thr.
2 GeV in ECAL2 xBj0.1 min
E?DVCS5 GeV E? thr. 1 GeV in ECAL1

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Goal to detect DVCS photons of E?
5 GeV in ECAL1 ? We have to reconstruct the
2 decay photons

E?1 Lab
2.5 GeV
3.75 GeV
E? 5 GeV
4.27 GeV
?CM46
? ? ?1 ?2
Prob cos?CM
0.73 GeV
1.25 GeV
2.5 GeV
E?2 Lab
If E? threshold 1.25 GeV
Prob 50 Prob 70
If E? threshold 0.73 GeV
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• Goal to detect DVCS photons of
• E?10 GeV in ECAL2
  and E? 5 GeV in ECAL1
• ? We have to reconstruct the 2 decay photons
• for ? of 10 GeV in ECAL2 and 5 GeV in
  ECAL1
• in ECAL2
  in ECAL1
• If E? threshold 2.5 GeV
  1.25 GeV
• ? prob50
  ? prob50
• If E? threshold 1.5 GeV
  0.73 GeV
• ? prob70
  ? prob70

? the ? energy threshold has to be of the order
of 1 GeV in ECAL 1 2
GeV in ECAL 2 for DVCS study
and good ?/? separation ? 1 GeV
threshold is also the result of simulation
(Etienne)
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Physical background to DVCS
exclusive ?0 production in COMPASS data
Source Pythia 6.4 generated DIS events
One scattered muon One photon with E?1 gt 3
GeV -2.5 lt Emiss lt 2.5 GeV 0.03ltxBjlt0.3 and
0.1ltylt0.9 No other photon in calorimeters
1GeV lt E?gt1 lt 3 GeV One proton in
recoil detector No charged tracks in spectrometer
-2.5 2.5 GeV
Emiss(MX2-MP2)/2MP
in this case DVCS is dominant

• But ideal case
• Only acceptance
• No resolution
• No pile up
• angular correlation
• And coplanarity

Etienne Burtin
Q2
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Recoil detector extra calorimetry