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Status and prospects for GPDs studies at COMPASS

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June 7, 2006 - Trento, Italia. Generalized Parton Distributions. H(x,0,0) = q(x) measured in DIS ... Blue is background. upstream. PMT. downstream. PMT. 1 2 3 4 ... – PowerPoint PPT presentation

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Title: Status and prospects for GPDs studies at COMPASS


1
Status and prospects forGPDs studies at COMPASS
Etienne Burtin, CEA/Saclay, DAPNIA/SPhN on
behalf of the COMPASS collaboration
1- Physics impact 2- Experimental issues
3- Recoil detector prototype
GPD2006 June 7, 2006 - Trento, Italia
2
Generalized Parton Distributions
g
g,p,r
Factorisation Q2 large, -tlt1 GeV2
hard
xx
x-x
soft
GPDs
P
P
t
Generalized Parton Distributions
H(x,0,0) q(x) measured in DIS
for quarks 4 functions H(x,x,t)
F(t) measured in elastic
scattering
3
DVCS observables
Deep VCS
Bethe-Heitler
High energy beam
Lower energy gt use interference - holography
Cross section
COMPASS muon beam can do all !
4
µ
µ
DVCS Bethe Heitler
p
p
BH calculable
The high energy muon beam at COMPASS allows to
play with the relative contributions
DVCS-BH which depend on 1/y 2 mp El xBj
/Q2
Higher energy DVCSgtgtBH ? DVCS Cross section
  • Smaller energy DVCSBH
  • Interference term will provide
  • the DVCS amplitude

5
Polarized µ and µ- beams
  • Polarized beam Ep110 GeV ? Em100 GeV
  • P(m) -0.8 2.108 m/spill (5s)
  • P(m-) 0.8 2.108 m/spill (5s)

Same collimator settings beam profile
unchanged Switch in 10 mins could be done
every 8h
2.108 m/spill
1.3 1013 protons/spill
6
Extraction of GPDs in the case of µ / µ-
t, ?xBj/2 fixed
Pµ-0.8 Pµ-0.8
7
Kinematical domain
Ix2
Collider H1 ZEUS 0.0001ltxlt0.01
Fixed target JLAB 6-11GeV SSA,BCA? HERMES
27 GeV SSA,BCA
COMPASS could provide data on Cross section
(190 GeV) BCA (100 GeV) Wide Q2 and xbj
ranges Limitations due to luminosity
new LINAC 4 (SPS injection) in 2010
Radioprotection limits needs better
shielding
8
Sensitivity of BCA to models
Model 1 H(x,?,t) q(x) F(t)
Model 2 from Goeke, Polyakov
and Vanderhaeghen
H(x,0,t) q(x) e t ltb?2gt q(x) /
xa t
COMPASS
sensitivity to the different spatial
distribution of partons ? when xBj ?
Good sensitivity to models in COMPASS xBj
range
9
Projected errors of a possible DVCS experiment
Beam Charge Asymmetry
L 1.3 1032 cm-2 s-1
Ebeam 100 GeV 6 month data taking 25 global
efficiency
6/18 (x,Q²) data samples
3 bins in xBj 0.05, 0.1, 0.2 6 bins in Q2 from 2
to 7 GeV2
Model 1 H(x,?,t) q(x) F(t) Model 2
H(x,0,t) q(x) / xat
Good constrains for models
10
Hard Exclusive Meson Production (?,?,?,p,? )
Scaling predictions
1/Q6
1/Q4
Collins et al. (PRD56 1997) 1.
factorization applies only for gL 2. sT ltlt
sL
vector mesons pseudo-scalar mesons
?0 largest production present study ?0
? p p-
with COMPASS
11
Hard Exclusive Meson Production
It comes for free with the recoil detector and
existing COMPASS trackers
Cross section
Vector meson production (?,?,?) ? H
E Pseudo-scalar production (p,? ) ? H
E


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
Can be investigated with present COMPASS data
Single spin asymmetry E/H
for a transverse polarized target
12
Compass Set-up 2002-2003
at CERN 250 physicists 26 institutes
magnets
muon filter
Calorimeters
160 GeV pol. m beam
200 detection planes Silicon, SciFi,
Micromegas, Drift chambers, GEM, Straw chambers,
MWPC
RICH
polarized target
13
Incoherent exclusive r0 production in COMPASS DATA
Preview of A. Sandaczs talk
Impact on GPD sL is dominant at high
Q² (factorisation only valid for sL)
14
Additionnal equipment to the COMPASS setup
required for DVCS
all COMPASS trackers SciFi, Si, µO,
Gem, DC, Straw, MWPC
µ
2.5 m Liquid H2 target to be designed and built
ECAL 1 or 2 ?? ? 12
?
COMPASS equipment with additional calorimetry
at large angle
p
µ
Recoil detector to insure exclusivity to be
designed and built
15
Recoil detector design
Goals Detect protons of 250-750 MeV/c
t resolution gt sTOF 200 ps
exclusivity gt Hermetic detector
Design 2 concentric barrels of 24
scintillators counters read at both sides
European funding (127 k) through a JRA for
studies and construction of a prototype ( Bonn,
Mainz, Saclay, Warsaw)
16
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
17
Timing and triggering issues
Slow protons TOF60ns
Fast protons TOF3ns
PMT
PMT
B 0.7 MHz/counter
PMT
PMT
A 2 MHz/counter
  • Time window
  • kinematics (60ns)
  • light propagation (up to 40 ns)
  • safety margin (30 ns)
  • gt all signal in a 128 ns window
  • Light attenuation
  • - ADC(PMT) Edep e-x/l

Analog trigger - hard to implement - background
(next slides )
Digital trigger - could work at 50 MHz -
tgttrigger latency
One solution use the inclusive trigger and
sample the signals DAQ rate 100 kHz
18
Part I Simulation of the Recoil Detector
Goals - evaluate front-end requirements -
tune reconstruction algorithms
  • Dedicated Géant 3.21 program
  • LH2 target with envelope
  • inner scint. 4 mm thick
  • outer scint. 5 cm thick
  • light attenuation
  • all processes turned on
  • custom beam halo generation
  • PMT waveform generation

19
Effect of d-rays
20
Digitization Waveform generation
  • For each Geant step in active volume
  • record DE,z,t
  • propagate step to photocathode exit by applying
  • decay constant of the scintillator
  • speed of light in the medium
  • attenuation length
  • light yield quantum efficiency
  • fill the histogram of time of production of
    gelectrons (0.1 ns bins)
  • Add contribution from additional muons according
    to intensity
  • (2 108 m in a 5s long spill)
  • Smear with a gaussian
  • of s3ns to mimic the
  • time response of the PMT
  • now 1 ns bins (1Gs/s)

up
down
-30ns 120ns
21
PMT signals only 1m in the set-up
22
PMT signals 2 108 m/spill (5s)
recording the waveform of all signals and
segmentation are mandatory
  • Hints for analysis
  • facing counters
  • m m vertex (z,t)
  • DEINNER vs DEOUTER
  • DEOUTER vs b

23
Simple waveform analysis
  • For each PMT signal (up to 5 hits)
  • perform leading edge discrimination
  • correct for time-walk effect
  • extract pulse height and integrate charge (35ns
    window)
  • For each up/down pair (up to 5525 points)
  • calculate time position of crossing point using
    time information
  • unfold attenuation length to extract energy loss
  • request that position is within scintillator
    volume /- 10cm

24
Criteria for proton candidates
  • Have points in corresponding A and B counters
  • For each pair of points
  • Energy loss correlation
  • Energy loss vs bmeas correlation

( no additional beam muons in this plot
just for pedagogy )
25
Coincidence with the scattered muon
Use reconstructed muon vertex time to constraint
proton candidates
Use vertex position to evaluate the effective
signal
26
Performances
trigger one event with at least one good
combination of A and B with hits identified
proton f of proton candidate matches f of
generated proton
27
Recoil Detector Prototype
Mechanical design for the prototype 30 sector ,
Scale 1 of what could be the DVCS recoil detector
28
tests on the m beam in fall-2006
  • Goals for this 2-month data taking period
  • Validate detection with sTOF 250 ps
  • Scintillators Light guides PMT
    electronics
  • Measure background with equivalent CH2 target (
    1MHz/detector)
  • use 1GHz sampler of the signals with a
    stand-alone DAQ
  • and standard multihit TDCs and ADCs

29
Conclusions
This initiative is now an Express of Interest
SPSC-EOI-005 http//doc.cern.ch//archive/electroni
c/cern/preprints/spsc/public/spsc-2005-007.pdf To
wards a GPD experiment using COMPASS -
COMPASS is complementary to other experiments
- has good sensitivity to GPD models through BCA
- has good Q² range for 0.03ltxbjlt0.2 Experiment
al challenges - measure TOF with good
resolution in high background conditions - 1
GHz sampling and recording of the PMT signals
- increase beam intensity - recoil detector
prototype test this fall
next step write the proposal !
30
Spare
31
Roadmap for GPDs at COMPASS
  • 2005 Expression of interest SPSC-EOI-005
  • 2006 Test of recoil detector prototype
  • Proposal
  • 2007-2009 construction of
  • recoil detector
  • LH2 target
  • ECAL0
  • 2010 Study of GPDs at COMPASS
  • In parallel present COMPASS studies with
    polarised target
  • Complete analysis of ? production
  • Other channels ?, 2p
  • GPD E/H investigation with the transverse
    polarized target

32
Proton luminosity upgrade at CERN
Gatignon
compass
33
Front-End Electronics
  • COMPASS DAQ 100 kHz no dead time
  • Data flow from recoil detector 2.5 GByte/s
  • 100 kHz, 100 channels, 128 samples, 10 bits
  • gt 25 increase in event
    size
  • discussion about the DC282 module
  • 10 bits resolution (16 bits used for coding)
  • 350ns dead time
  • memory depth 256-1024MBytes
  • 400 MBytes/s PCI transfer
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