Polarized positive + and negative - polarized positive + and negative - muon beams @ to perform DVCS...
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Transcript of Polarized positive + and negative - polarized positive + and negative - muon beams @ to perform DVCS...
polarized positive polarized positive ++ and negative and negative --
muon beams muon beams
@@
to perform DVCS measurements to perform DVCS measurements for GPD study for GPD study
Nicole d’Hose, CEA-SaclayOn behalf of the COMPASS collaboration
workshop on positronsJLab 25 March 2009
exclusive single-photon productionexclusive single-photon production
μμ p p μμ p p
dd |T |TDVCSDVCS||22 + |T + |TBHBH||22 + + Interference TermInterference Term
DVCSDVCS
small t
p
p
BHBH
GPD
slow pslow p small t
At COMPASS or BH or DVCS can dominate the cross section
COMPASS : domain 10COMPASS : domain 10-2-2 < x < 10 < x < 10-1-1
JLab
CO
MP
AS
S
HE
RM
ES
ZEUSH1
Limit due to luminosity ?
Limit due to acceptance
BHdominance
|t |<1 GeV2
xB=0.05
xB=0.1xB=0.2
Q2=2Q2=2
Q2=2
Q2=4
Q2=4
Q2=4
DVCS dominatesDVCS dominatesBH important
|t |<1 GeV2
|t |<1 GeV2
|t |<1 GeV2
|t |<1 GeV2|t |<1 GeV2
DVCS /2BH/10
DVCS /2BH/10
Comparison BH and DVCS at 160 GeVComparison BH and DVCS at 160 GeV
dσ(μpμp) = dσBH + dσDVCSunpol
+ Pμ dσDVCSpol
+ eμ aBH Re ADVCS + eμ Pμ aBH Im ADVCS
Twist-3 M01
DVCS + BH with DVCS + BH with ++ andand -- beambeam and unpolarized and unpolarized targettarget
Twist-2 M11 Twist-2 gluon M-11>>
θμ’μ
*
p
μ
pμ
pBH calculableDVCS
+
Belitsky,Müller,Kirchner
DVCSBH Aa e )coscoscos()()(
323210
213
6IntIntIntInt cccc
PtPxye
)sin( DVCSDVCSpol
sQy
ed
122
6
= 2Pμ
+ 2eμ
Bea
m
Char
ge
& s
pin
diff
eren
ce
DU,CS
dσ(μpμp) = dσBH + dσDVCSunpol
+ Pμ dσDVCSpol
+ eμ aBH Re ADVCS + eμ Pμ aBH Im ADVCS
Twist-3 M01
DVCS + BH with DVCS + BH with ++ andand -- beambeam and unpolarized and unpolarized targettarget
Twist-2 M11 Twist-2 gluon M-11>>
φφ
θμ’μ
*
p
μ
pμ
pBH calculableDVCS
+
Belitsky,Müller,Kirchner
DVCSBH Aa e )coscoscos()()(
323210
213
6IntIntIntInt cccc
PtPxye
)sin( DVCSDVCSpol
sQy
ed
122
6
= 2Pμ
+ 2eμ
This is the good way to get rid of the large BH contribution if we are able to control the + and - fluxes and efficiencies we can extract c0
Int and c1Int
Bea
m
Char
ge
& s
pin
diff
eren
ce
DU,CS
dσ(μpμp) = dσBH + dσDVCSunpol
+ Pμ dσDVCSpol
+ eμ aBH Re ADVCS + eμ Pμ aBH Im ADVCS
Twist-3 M01Twist-2 M11 Twist-2 gluon M-11>>
φ
θμ’μ
*
p
μ
pμ
pBH calculableDVCS
+
Belitsky,Müller,Kirchner
)coscos()()(
),,(
2
21021
2BHBHBHBBH cCc
PP
tQxd
)sinsin()()(
221
213
6IntInt ss
PtPxye
)coscos( 221022
6DVCSDVCSDVCSDVCS
unpolcCc
Qye
d
= 2
+ 2
+ 2 eμ Pμ
Bea
m
Char
ge
& s
pin
s
um
SU,CS
DVCS + BH with DVCS + BH with ++ andand -- beambeam and unpolarized and unpolarized targettarget
dσ(μpμp) = dσBH + dσDVCSunpol
+ Pμ dσDVCSpol
+ eμ aBH Re ADVCS + eμ Pμ aBH Im ADVCS
Twist-3 M01Twist-2 M11 Twist-2 gluon M-11>>
φφ
θμ’μ
*
p
μ
pμ
pBH calculableDVCS
+
Belitsky,Müller,Kirchner
)coscos()()(
),,(
2
21021
2BHBHBHBBH cCc
PP
tQxd
)sinsin()()(
221
213
6IntInt ss
PtPxye
)coscos( 221022
6DVCSDVCSDVCSDVCS
unpolcCc
Qye
d
= 2
+ 2
+ 2 eμ Pμ
After integration over φ in all the acceptance and subtraction of BH contrib. contribution we can get DVCS contribution only, C0
DVCS
Bea
m
Char
ge
& s
pin
s
um
SU,CS
DVCS + BH with DVCS + BH with ++ andand -- beambeam and unpolarized and unpolarized targettarget
Using DU,CS / SU,CS : Beam Charge and Spin Asymmetry
Comparison to different models from VGG
statistical errorsstill under debate
H(x,0,t) = q(x) e t
<b
2>
= q(x) / x α’ t
<b2> = α’ ln 1/x
(α’~0.1)
H(x,ξ,t) ~ q(x) F(t)
New predictions from Dieter MuellerNew predictions from Dieter Mueller
BC&SADieter Mueller uses a Global Fita Global Fit
on H1/ZEUS, HERMES, CLAS
and JLab Hall A
and no JLab Hall A
Using SU,CS : dDVCS / dt ~ exp(Bt)
Projections for the t-slope measurement at COMPASS
αα’’ =0. =0.125 GeV125 GeV-2-2
b0 =5.83 0.5 GeV-2
x0=0.0012 (Q2=8,W=82)1<Q2<8
B(x) = b0 + 2 αα’’ ln(x0/x)
for valence quark αα’’ ~ 1 GeV-2 to reproduce FF meson Regge traj.
for gluon αα’’ ~ 0.164 GeV-2 (J/ at Q2=0)
αα’’ ~ 0.02 GeV-2 (J/ at Q2=2-80 GeV2)
<< αα’’ ~ 0.25 GeV-2
for soft Pomeron
FFS modelAdapted by Sandacz
Still many open questions on FoM and Still many open questions on FoM and Systematic Systematic effects:effects:
Efficiency to reject background that simulates exclusive μp μ’p events
Final accuracy of the + and - fluxes and detection efficiencies
to combine both + and - measurements for DU,CS
Final accuracy on the procedure to subtract the known BH contribution
correctly renormalized to the measured yield for SU,CS
Goal of a DVCS pilot run in 2009:Goal of a DVCS pilot run in 2009:
- do an absolute measurement of BH (dominant at small x) FoM + - record and identify a few DVCS events very important for - collect μp μ’X or μ’ + (-) (°) X
μp μ p (°) ()
In 2008In 2008 with 1.5 days of beam of low intensity done in a rush before the shut down (LHC accident)
we have observed ~100 Single Eclusive photons
identified to BH due to their distribution in
analysis to be released tomorrow
bins for BH with minimal DVCS contaminationbins for BH with minimal DVCS contamination to study the phi dependence to study the phi dependence to determine the FoMto determine the FoM
1<Q2<2 0.005<x<0.01 300 BH (5 DVCS + int.) 6 % 6 % 2<Q2<4 0.01<x<0.02 121 BH (3 DVCS + int.) 9 % 9 %
in 7 days accuracy on BHin 7 days accuracy on BH
in 2009in 2009 absolute measurement of BH at small xabsolute measurement of BH at small x
* μ’μ
*
p
Characteristic feature of the -dependenceof BH and DVCS
first insight for DVCSfirst insight for DVCS
Experimental determination of the DVCS yield to control the credibility of estimation
in 2009in 2009
15
Perform accurate control of: - the + and - fluxes - the halos flux - the detection efficiencies
collect with collect with μμ+ and + and μμ- - inclusive, semi-inclusive, semi-inclusive inclusive and exclusive events:and exclusive events:
μμp p μμ’X ’X or or μμ’ ’ + (+ (-) (-) (°) X °) X or or μμ’’ p p ( (°) (°) ())
roadmaproadmap
- ’’Letter of Intent’’ CERN-SPSC-2009-003 a complete proposal for june 2009
- Demonstrate the feasibility of such an experiement
- 1rst phase in ~2011/12 DVCS with unpolarised proton target
- 2nd phase in ~2013/14 DVCS with polarised proton target
new equipments needed
d /dt impact parameter b
Proposal to study ‘’GPDs @ COMPASS’’ in 2 phases
Phase 1: DVCS experiment in ~2011/12 to constrain GPD H
with +, - beam + unpolarized long LH2 (proton) target + recoil detector + ECAL1,2 (possibly 0) + all COMPASS equipment
Phase 2: DVCS experiment in ~2013/14 to constrain GPD E
with + and transversely polarized NH3 (proton) target + recoil detector + ECAL1,2 (possibly 0) + all COMPASS equipment
d( +, ) + d( -, ) Im(F1 H) sin d( +, ) - d( -, ) Re(F1 H) cos
d(, S) - d(, S+π) Im(F2 H – F1 E) sin (- S) cos