Diapositive 1

1
GPDs studies for COMPASS

• Comments on the calorimetry coverage
• Comments on the scattering muon cut
• Marcin Stolarski update on MC simulation
• -ECAL0 MC status
• -resolution on xBj, Q2
• -comments on the small 'y' region
• High Q2 trigger

Nicole dHose, CERN, 30 May 2007
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Study of DVCS photon acceptance at E?100 GeV
Works of Andrzej and Etienne
Cuts Q2 gt 1 GeV2
Q2 gt 1.5 GeV2
0.1 lt y lt 0.85
0.03 lt xBj lt 0.27 0.06 lt t lt
0.6 GeV2 0.06 lt t lt 0.6 GeV2

• Conclusions
• Acceptance ECAL1ECAL2 85
  Acceptance ECAL1ECAL2 good
• 
  for 0.03 lt xBj lt 0.13
• largest losses of ? in the hole of ECAL2
• 
  
  Necessity of ECAL0
• gain of DVCS events due to ECAL0 lt 1
  for 0.13 lt xBj lt 0.27

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Large difference between 0.1 lt y lt 0.85 and
0.03 lt xBj lt 0.27
y0.9
y0.8
y0.5
y0.2
Traditional cut
y0.1
y0.05
New cut envisageable
? max

• ? when Q2 ?
• or when xBj ?

0.03 0.07 0.13 0.27
xBj
3 xBj bins of study ? for 0.13 lt xBj lt 0.27
necessity of y ? 0.05
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Present size of the calorimeter (as indicated in
the 2006 files)
JURA side
ECAL1_at_11.11m 3.4m
ECAL2_at_33.25m
-143cmltvertlt143cm
-91.92cmltvertlt91.92cm
-198.66cmlthorilt198.66cm
-122.56cmlthorilt122.56cm hole in the
middle -30.64cmltvertlt30.64cm
-19.15cmltvertlt19.15cm
-53.62cmlthorilt53.62cm
0cmlthorilt38.3cm
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Acceptance of the DVCS photons
0
X0.13
X0.27
??lab (deg)
??lab (deg)
??lab (deg)
At small xBj lt 0.03 the DVCS ? are in ECAL2
hole At 0.03 lt xBj lt 0.07 and 0.07
lt xBj lt 0.13 The DVCS ? are
mostly in ECAL12
At
0.13 lt xBj lt 0.27
The DVCS ? are mostly
outside ECAL1
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Calorimeter coverage foreseen for DVCS ? and ?
DVCS ? impact point at ECAL 0 location
DVCS ? kinematics
But ygt0.05?
ECAL 1
ECAL 2
(existing)
(existing)
ECAL 0
To be built
Studied with the Dubna Group
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Calorimeter acceptance
(mainly suppressed by the condition ygt0.05 ?)
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Conclusions for DVCS photons ECAL12
collect most of the DVCS ? for 0.03 lt xBj lt 0.13

(bins around 0.05 and 0.10) ECAL0 is
necessary for DVCS ? for 0.13 lt xBj lt 0.27

(bins around 0.20) and for the
2 decay ? of ?
------- Andrzej and Etienne are comparing their
2 generators (Freund, Frankfurt and
Strikmann, and Vanderhaeghen, Guichon,
Guidal)
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Physical Background to DVCS
Source Pythia 6.1 generated DIS events
Apply DVCS-like cuts one m,g,p in DVCS range
no other charged neutral in active volumes
detector requirements 24 coverage for
neutral 50 MeV?? calorimeter threshold 40
for charged particles
in this case DVCS is dominant
And with y gt 0.05 ?
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Requirement for ECAL0 To complete the DVCS
photon collection at xBj0.2 4 GeV lt E?
lt 25 GeV ?? lt 24 To suppress the ?0
background ?? lt 24 E? as low
as 50 MeV but what is the
effect of the condition ygt0.05??? if E?
as low as 200 MeV life will be better
? an urgent need of simulation of this
point
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possible solution to complete the COMPASS setup
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Exclusivity hermetic detector ECAL0
to reject p contamination Recoil detector for
proton (and also pions, neutron)
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The scattered muon detection for DVCS
middle outer calo(hadro!)
0.03 lt xBj lt 0.07 0.07 lt xBj lt 0.13 0.13 lt xBj
lt 0.27
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SM2
?
The scattered muon detection
?
??
?30 mrad
at fixed Q2 and xBj E? ? ???

• aperture of SM2 (?30 mrad)
• Domain in (Q2, xBj) smaller
• at E ? 100 GeV than E ? 160 GeV
• Necessity of a trigger
• after SM1 / Upstream SM2

large Q2 trigger under development (see
TB on 7 april 2007)
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Remarks on the resolution on muons
(cf COMPASS spectrometer paper) On
the incident muon ?(k)/k lt 10-2 (we had
considered 5.10-3) On the scattered muon
detected after SM2 ?? 0.3 ?B.dl /k ?
?(k)/k2 ?(??)/ 0.3?B.dl and ?(??)40?rad
at 160 GeV ?(k)/k2 2.5 10-5 and ?(k)/k
4 10-3 at 100 GeV ?(k)/k2 4. 10-5
and ?(k)/k 6 10-3 Consequences 1)
resolution on Q2 ?(Q2)/ Q2 10-2
2) resolution on xBj
?(xBj)0.0013 at xBj 0.05
0.030.07 ?(xBj)
0.005 at xBj 0.1 0.07 0.13
?(xBj) 0.02 at xBj 0.2
0.13 0.27 ? small y
3) resolution on phi at small t (cf Laurent
Schoeffels talk
and the azimuthal
angle analysis) And on the scattered muon
detected after SM1???
See Marcins talk
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