GPDs studies at COMPASS
DVCS studies for a GPD experiment in 2010 Physics impact Experimental issues Recoil detector prototype
Exclusive 0 production in present COMPASS data Principle Preliminary 2002 results
Etienne Burtin, CEA/Saclay, DAPNIA/SPhN on behalf of the COMPASS collaboration
SIR workshop 2005May 19, 2005 – Newport News, VA
Generalized Parton Distributions
H(x,0,0) = q(x) measured in DIS
F(t) measured in elastic scattering
GPDsx+ x-
P P’
hard
soft
* ,Factorisation:Q2 large, -t<1 GeV2
Generalized Parton Distributionsfor quarks : 4 functions H(x,,t)
t
),dxH(x, t
DVCS observables
1
1
),,(
xtxHdxDVCS P
Cross section
High energy beam
Single Spin Asymmetry
),,( txH
Polarised beam
Beam Charge Asymmetry
1
1
),,(
xtxHdxP
+/- charged beam
Deep VCS Bethe-Heitler
Lower energy => use interference - holography
2),,( tHi
COMPASS muon beam can do all !
Requirements for DVCS:-same energy-maximum intensity-opposite polarisation to a few %-change of beam charge frequent
Solution proposed by Lau Gatignon:1) To select Pπ=110GeV and Pμ=100GeV to maximise the muon flux2) To keep constant the collimator settings which define the π and μ momentum spreads
Pol μ+ = -0.8 and Pol μ- = +0.8 I(μ+) ~ 2. I(μ-) switch in ~10 mins (once per shift?)
Muon section 400mHadron decay section 600m
Compasstarget
scrapers
Be absorbersProtons400 GeV
T6 primary Be target
Collimators1 2 3 4H V H V
1.3 1013 protons/spill2.108 muons/spill
Polarized μ+ and μ- beams
Extraction of GPDs in the case of -μ μ-
t)ξ,ξ,H(xξxt)ξ,H(x,dx
iξxt)ξ,H(x,dx π1
111
A
i -P
DVCS)pp(
dσ(μpμp) = dσBH + dσDVCSunpol
+ Pμ dσDVCSpol
+ eμ aBH Re ADVCS + eμ Pμ aBH Im ADVCS cos nφ sin nφ
φθμ’μ *
p Pμ+=-0.8 Pμ-=+0.8
~σσ t)ξ,ξ,(x H μμ -
11
μμξxt)ξ,H(x,dx ~σσ P-
t, ξ~xBj/2 fixed
Extraction of GPDs in the case of μ+ / μ-
Kinematical domain
Collider :H1 & ZEUS 0.0001<x<0.01
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
Limitation due to luminosity
E=19
0, 1
00G
eV
Nx2
new LINAC 4 (SPS injection) in 2010could COMPASS benefit from this ?
BCA
model 1
model 2
COMPASS
Model 2: from Goeke, Polyakov and Vanderhaeghen
sensitivity to the different spatial distribution of partons when xBj
Model 1: H(x,ξ,t) ~ q(x) F(t)
H(x,0,t) = q(x) e t <b2>
= q(x) / xα’ t
Sensitivity of BCA to models
Good sensitivity to models in COMPASS xBj range
Projected errors of a possible DVCS experiment
Ebeam = 100 GeV6 month data taking25 % global efficiency
6/18 (x,Q²) data samples
Good constrain for models
Model 1 : H(x,ξ,t) ~ q(x) F(t)
Model 2 :
Beam Charge Asymmetry L = 1.3 1032 cm-2 s-1
H(x,0,t) = q(x) / xα’t
3 bins in xBj= 0.05, 0.1, 0.26 bins in Q2 from 2 to 7 GeV2
μ’
p’μ
Additionnal equipment to the COMPASS setup
COMPASS equipmentwith additional calorimetry at large angle
Recoil detector to insure exclusivity
to be designed and built
2.5 m Liquid H2 target to be designed and built
ECAL 1 or 2 12°
all COMPASS trackers:SciFi, Si, μΩ, Gem, DC, Straw, MWPC
Physical Background to DVCS
detector requirements: 24° coverage for neutral 50 MeV calorimeter threshold 40° for charged particles
Source : Pythia 6.1 generated DIS eventsApply DVCS-like cuts one ’,,p in DVCS range no other charged & neutral in active volumes
in this caseDVCS is dominant
12°
24°
4m
ECAL0
Possible solution for the recoil detector proposed in the Workshop on the Future Physics at COMPASS 26 Sept2002
Received funding by EU FP6 (Bonn-Mainz-Saclay-Warsaw )Goal : full test of a prototype of a 45 degree sector - scintillating material studies (timing at 200 ps over 4 m) - fast triggering and multi-hit ADC/TDC system
Also studying a scintillating fibers solution (Monte-Carlo) with good timing provided by incoming or scattered muons
Vector meson production (ρ,ω,…) H & E
Pseudo-scalar production (π,η… ) H & E~ ~
Hρ0 = 1/2 (2/3 Hu + 1/3 Hd + 3/8 Hg)
Hω = 1/2 (2/3 Hu – 1/3 Hd + 1/8 Hg)
H = -1/3 Hs - 1/8 Hg
Single spin asymmetry ~ E/H
for a transverse polarized target
Cross section:
Hard Exclusive Meson Production
Can be investigated
with present COMPASS data
It comes for free with the recoil detector and existing COMPASS trackers
Compass Set-up 2002-2003
polarized target
magnets
RICH
Calorimeters
muon filter
~ 200 detection planesSilicon, SciFi, Micromegas,Drift chambers, GEM, Strawchambers, MWPC
at CERN 250 physicists 26 institutes
160 GeVpol. beam
Incoherent exclusive 0 sample selection
Exclusivity of the reactionEmiss=(M²X-M²N)/2MN-2.5 < Emiss < 2.5 GeV
Incoherent production:0.15 < pt²< 0.5 GeV² scattering off a quasi-free nucleon
Assuming both hadrons are 0.5 < M< 1 GeV
M
Emiss
pt²
Event topology:
Kinematics :• > 30 GeV• E’ > 20 GeV• Q² > 0.01 GeV²
’
N N’
*0
0 angular distributionsDefinitions :in *-p CM frame
’
pp’
in rest frame
The angular distribution W(cosdepends on the 0 spin density matrix elements 23 (15) measurable with polarised (unpol.) beamThis analysis : one-dimension distributions test of s-Channel Helicity Conservation SCHC
Distributions are corrected for acceptance. Background not subtracted. Statistical error limited by MC sample (except last Q² bin).
Measurement of r0400
θ1)cos(3r)r(143) W(cos 204
000400
201
211
211TTL
T
200
20104
00
TTTN/σσR
δ)R)(ε(1NTδ)(εT
r
Distribution :
Spin density matrix elements:
T are helicity amplitudesmeson photon
Measurement of r and Im r
2cos21 [21)( 04
11 rW
if SCHC holds:0r0r 3
1104
11 Im
041-1
31-1
weak violationbeam polarisation
]2sin12 2311 PrIm
R can be extracted
2002
Determination of R° =L/T
2003+
as SCHC holds …:only T00≠0 T11≠0Then :
0400
0400
T
L
r1r
δ)(ε1
σσR
High statistics from -production to hard regime
better coverage at high Q²more data to come …
Impact on GPD :L is dominant at high Q²
(factorisation only valid for L)
ConclusionsExclusive vector meson production in COMPASS data - preliminary results from 2002 data - allow test of SCHC - shows dominance of L at Q²>2GeV² - more data on tape with better coverage at high Q² to come - insight on GPDs : transverse asymmetry and cross section measurementsTowards a GPD experiment using COMPASS… - COMPASS is complementary to other experiments - has good sensitivity to GPD models through BCA - has good Q² range for 0.03<xbj<0.2 - will be able to measure Hard Exclusive Meson Production - seeking collaborators…This initiative is now an “Express of Interest” : SPSC-EOI-005http://doc.cern.ch//archive/electronic/cern/preprints/spsc/public/spsc-2005-007.pdf
COMPASS should also be a good place to study GPD in 2010 !
Proton luminosity upgrade at CERN
Gatignoncompass
Task NameLINAC4
Design ref inment
Construction
Commissioning
Start operation with PSB
SPLDesign ref inment
Construction
Linac4 displacement + commissioning
Start operation as PS Injector
12/31
12/30
2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016
DVCS+ Bethe Heitler
The high energy muon beam at COMPASSallows to play with the relative contributions DVCS-BHwhich depend on
1/y = 2 mp Eℓ xBj /Q2
Higher energy: DVCS>>BH DVCS Cross section
Smaller energy: DVCS~BHInterference term will provide the DVCS amplitude
φθμ’μ *
p
μp
μp BH calculable
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