Exclusive processes in leptoproduction at COMPASS

1
Exclusive processes in leptoproduction at COMPASS
Andrzej Sandacz
Soltan Institute for Nuclear Studies, Warsaw
on behalf of the COMPASS collaboration
Photon 2009
Int. Conference on the Structure and the
Interactions of the Photon
11-15 May 2009, DESY, Hamburg
2
Present analyses of exclusive channels in
leptoproduction at COMPASS

• Physics analyses for ?0 and f channels

• transverse target spin asymmetry
  ?0 on p, d

• double spin asymmetry
  ?0, f on d

• SDMEs
  ?0, f on p, d

• cross sections, R(sL/sT), t-slopes
  ?0, f on p, d

• Searches for signals of exclusive J/?(?µµ-),
  ?0, p0 production

3
COMPASS setup
as in ? runNIM A 577(2007) 455
two stages spectrometer Large Angle Spectrometer
(SM1) Small Angle Spectrometer (SM2)

• high energy beam
• large angular acceptance
• broad kinematical range

variety of tracking detectors to cope with
different particle flux from ? 0 to ? 200 mrad
Luminosity 51032 cm-2s-1
SciFi Silicon Micromegas GEMs
Straws SDC MWPC W45
MuonWall
SM2
E/HCAL
E/HCAL
50 m
SM1
MuonWall
Polarised Target
calorimetry, PID
RICH
RICH detector
PKU-RBRC Workshop on Transverse Spin Physics,
June 30, 2008
F. Bradamante
4
COMPASS polarised ammonia target (2007)
polarised material - protons in NH3 molecule
transverse/longitudinal polarisation 90
dilution factor f 0.14
Microwave reversal every week
5
Q2 gt 1 GeV2
Selections of exclusive ?0 events
W gt 5 GeV
0.005 lt xBj lt 0.1
Transversely polarised proton target (NH3), PT
90, 2007 data
0.05 lt pt2 lt 0.5 GeV2

• recoil proton (recoiling system) not detected

• charged pion mass assumed for h(h-)

-0.3 lt Mpp M?(PDG) lt 0.3 GeV/c 2

• cuts on pt2 to remove coherent production from N

and further suppress non-exclusive background
6
Observables in hard exclusive meson production
relevant for GPDs
for vector mesons
unpolarised cross section
transverse target spin asymmetry
access to GPD E
related to orbital momentum
of the GPDs Hq,g , Eq,g
HM , EM
are weighted sums of integrals
weights depend on contributions of various quark
flavours and of gluons to the production of meson
M
Give access to the orbital angular momentum of
quarks
So far GPD E poorly constrained by data (mostly
by Pauli form factors)
7
Transverse target spin asymmetry for exclusive
?0 production
definitions
to disentangle contributions from ?L and ?T the
distribution of ?0 decay polar angle needed in
addition
Diehl and Sapeta (2005)

• Spin-dependent photoabsorption cross sections
  and interference terms sijmn

Aim amplitudes for subprocess ? p ? ?0 p
with proton polarisation i and photon
polarisation m

• Virtual photon polarisation parameter

(if ml can be neglected)
8
Cross sections in terms of target polarisation
wrt virtual photon
(in deep inelastic kinematics)
9
Extraction of transverse target spin asymmetry
Flux Acceptance Dilution factor Mean
target polarisation
Numer of target nucleons
Asymmetry extraction from double ratio method
using 3 targets with two polarisations each
in the double ratio Flux and s0 cancel
also Acceptance cancels provided no changes
between spin reversals
AUT from a fit to DR (?-?s)
10
Transverse target spin asymmetry polarised
protons (2007)
NH3 target
new
lt Q2 gt 2.2 (GeV/c)2 lt xBj
gt 0.04 lt pt2 gt 0.18 (GeV/c)2
In progress L/T ? separation (using ?0 decay
angular distribution)
11
Transverse target spin asymmetry polarised
deuterons (2002-2004)
6LiD target
lt Q2 gt 2.0 (GeV/c)2 lt
xBj gt 0.03 lt pt gt 0.11
GeV/c
In progress L/T ? separation (using ?0 decay
angular distribution)
and coherent / incoherent separation for deuteron
12
Comparison to a GPD model and to HERMES
COMPASS p?
?
GPD model
Goloskokov, Kroll
2008
W 10 GeV
t integrated
K0
?0
?
predictions for protons
AUT(?) -0.02
AUT(?) -0.10
similar for both experiments
compatible with 0
HERMES extracted also ?0 TTSA separately for ?L
and ?T !
13
Longitudinal double-spin asymmetry for exclusive
?0 production
EPJ C 52 (2007)
lt PB gt -0.76 longitudinally polarised
deuteron target (6LiD) PT 50 f 0.37

wide range of Q2 and x , W gt 7.5 GeV , 0.15
lt pt2 lt 0.5 GeV2
curve
where A1 inclusive asymmetry (d)
A1? on polarised deuterons consistent with 0
14
Longitudinal double-spin asymmetry for exclusive
?0 production (cont.d)
estimate of contribution of unnatural exchanges
(p, a1)








if SCHC
The asymmetry is a sensitive probe of unnatural
parity exchanges

• at small Q2 and x data provides precise limits
  on their contribution

Goloskokov, Kroll (2007)
HERMES
W 5 GeV
COMPASS
W 10 GeV
small value of A1? at W 10 GeV due to
approximate cancellation of gluon and sea
contributions
15
Spin Density Matrix Elements
VM angular distributions W(cos?, fp, ?) depend
on the spin density matrix elements (SDME)
? 23 (15) observables with polarized
(unpolarized) beam
T?m ?? (?N ? mN)
SDMEs are bilinear combinations of the helicity
amplitudes
?? 1, 0 ?m 1, 0
(averaged over nucleon spins)

• describe helicity transfer from ? to VM

s-channel helicity conservation (SCHC)

• describe parity of t-channel exchange

(NPE vs. UPE)

• impact on GPD studies determination of sL

SDME
16
Measurement of r and determination of R
sL/sT
04 00
0.01 lt Q² lt 0.05 lt Q² lt 0.3 lt Q² lt 0.6 lt
Q² lt 2.0 lt Q² lt 10 GeV2
W(cos ?)
2002

• High statistics from quasi-photoproduction to
  hard production

• Impact on GPD studies determination of sL

17
0.01 lt Q² lt 0.05 lt Q² lt 0.3 lt Q²
lt 0.6 lt Q² lt 2.0 lt Q² lt10 GeV2
W(f)
(ffp)
2002
beam polarisation
if SCHC holds
weak violation of SCHC
18
Future GPD program _at_ COMPASS

• The GPDs program is part of the COMPASS Phase II
  (2010-2015) proposal to be submitted to CERN in
  2009.

• The first stage of this program requires a 4 m
  long recoil proton detector (RPD) together with a
  2.5 m long LH2 target. Upgrades of
  electromagnetic calorimeters to enlarge coverage
  at large xB and reduce bkg.

• The second stage requires either a transversely
  polarized NH3 target inserted in the RPD or a new
  SciFi (?) RPD inserted in the existing NH3 target
  system.

primary physics goal is DVCS
meson production also planned
cf. Laurent Schoeffel talk
Stage 1 (2011) to constrain H ds/dt ? t-slope
parameter b d?(??, ?) d?(?-?, ?) ? Im(F1 H)
sin ? d?(??, ?) - d?(?-?, ?) ? Re(F1 H) cos
? Stage 2 (2013) to constrain E d?(?, ?S) -
d?(?, ?Sp) ? Im(F2 H F1 E) sin (?- ?S) cos ?
19
2008 DVCS test run
Goal evaluate feasibility to detect DVCS/BH
in the COMPASS setup
µ p ? µ p ?
Use COMPASS hadron set-up
in EMCals
LH2 40 cm
in the small RPD
2 days of 160 GeV muon beam (µ and µ-)
20
Kinematic constraints in the transverse plane
?p? P?µ?-P?RPD ?pperplt 0.2 GeV
???miss- ?RPD ??lt 36 deg
21
Azimuthal distribution for exclusive single
photon events
Monte-Carlo simulation of BH (dominant)
and DVCS
Bethe-Heitler
DVCS
After all cuts, Q2gt1GeV2
Clear signature of dominant BH events
22
Conclusions and outlook

• New results on transverse target spin
  asymmetries for ?0 production

compatible with 0 both for the proton and the
deuteron targets
ongoing work on L/T separation, and coh./incoh.
separation for d

• Published results on double spin asymmetry for
  ?0 production on d

compatible with 0 in a wide x and Q2 range
precise upper limits on contribution of unnatural
parity exchanges

• Expected high precision results on ?0 and f
  SDMEs and cross sections

• In preparation proposal aiming at the GPD
  physics

equipment needed 4m long RPD, 2.5m LH2 target,
extented calorimetry
RPD with polarised target

• DVCS test runs 2 days in 2008, 2 weeks in
  2009

muon beam and hadron setup including 40cm LH2
and the small RPD