Exclusive r 0 Production at COMPASS Etienne Burtin
Exclusive r 0 Production at COMPASS Etienne Burtin, CEA-Saclay on behalf of the COMPASS Collaboration - Physics motivations Experimental setup Data analysis Preliminary results Baryons 2004, Palaiseau October 28, 2004
Physics motivations m’ m g* N r 0 pp+ N’ • Diffractive processes : Pomerons or Reggeons • r 0 Production : test of s-Channel Helicity Conservation (SCHC) • Hard regime : Generalized Parton Distributions
COMPASS experimental set-up ~ 250 physicists from 28 institutes use 160 Ge. V/c polarised m beam from the CERN SPS m filter Trackers: Si, Sci. Fi, Micromegas, GEM, DC, Straw, MWPC Magnets Trigger hodoscopes Polarised target am e b m H&E Cals RICH COMPASS @ Baryons 2004 K. Kurek : DG from high p. T Y. Bedfer : Spin physics results R. Webb : Collins and Sivers asym.
Selection of the sample m’ m • Topology N – 1 incident and 1 scattered m – only 2 hadrons of opposite charge – vertex in the 6 Li. D polarized target g* r 0 pp+ N’ • Kinematical conditions: – n > 30 Ge. V – Em’ > 20 Ge. V – Q² > 0. 01 Ge. V² Focus on Incoherent Exclusive r 0 production. . .
Incoherent exclusive r 0 production Mpp Emiss p t² Assuming both hadrons are p: 0. 5 < Mpp< 1 Ge. V Background processes rejected : Exclusivity of the reaction Emiss=(M²X-M²N)/2 MN -2. 5 < Emiss < 2. 5 Ge. V Incoherent production: 0. 15 < pt²< 0. 5 Ge. V² scattering off a quasi-free nucleon 6 Li. D
Kinematical domain of final sample after event selection 10 -4 10 -3 10 -2 10 -1 Q² [Ge. V²] 2002 : this analysis ~ 700, 000 evts (whole range) ~ 20, 000 evts (Q²>1 Ge. V²) 1 10 High statistics from g-production to hard regime 2003 and 2004: - trigger coverage improved - enlarged acceptance at high Q² (up to 25 Ge. V) - ~ 4 times more beam time
r 0 angular distributions Definitions : in g*-p CM frame p’ m’ g* m F p f r° p- q p+ in r° rest frame The angular distribution W(cosq, f, F) depends on the r 0 spin density matrix elements 23 (15) measurable with polarised (unpol. ) beam This analysis : one-dimension distributions test of s-Channel Helicity Conservation SCHC studied by NMC, Zeus, H 1, Hermes, E 665
Determination of the Acceptance • Use DIPSI event generator of exclusive meson production developed for ZEUS. • Fed through the GEANT-based simulation and reconstruction programs of COMPASS. • Same cuts as real data applied. • Calculation of the acceptance • Estimation of the effect of smearing and binning – reject Q²<0. 01 Ge. V² data due to high smearing
Angular distributions 0. 01 < Q² < 0. 05 < Q² < 0. 3 < Q² < 0. 6 < Q² < 2. 0 < Q² Preliminary : - Corrected for Acceptance - Background not subtracted Statistical error only, limited by MC (not last Q² bin)
Measurement of r Distribution : Spin density matrix elements: Tlg are helicity amplitudes meson photon 04 00
Determination of Rr° =s. L/s. T If SCHC holds : only T 00≠ 0 T 11≠ 0 Then : 2002 2003 Impact on GPD : s. L is dominant at high Q² factorisation only valid for s. L
Measurement of r Distribution : n beam polarisation if SCHC holds: ak e w vio io t a l 04 1 -1 and Im r 3 1 -1
Measurement of r 11 -1 Distribution, if SCHC holds: with if in addition, natural parity exchange in the t-channel holds:
Angular distributions (2) 0. 01 < Q² < 0. 05 < Q² < 0. 3 < Q² < 0. 6 < Q² < 2. 0 < Q²
Conclusions and perspectives • Incoherent Exclusive r 0 production channel under investigation: – high statistics from photo-production to the hard regime – larger acceptance at high Q² in 2003 and 2004 (up to 25 Ge. V²) • Preliminary results on Spin Density Matrix Elements: – – only statistical errors were shown (dominated by MC) in agreement with other experiments weak violation of SCHC observed systematics to be evaluated especially on background • Spin asymmetries for longitudinally and transversally polarised target will also be determined. • Exclusive production of f and J/Y also observed and will be studied
r 0 mass distribution Söding parametrization: No acceptance corrections yet ! (subtracted)
- Slides: 16