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Experimental considerations about gg physics at DA F NE2

Experimental considerations about gg physics at DA F NE2. Physics window at DA F NE2 and yield estimate for the main processes (using W.W. approximation) A feasibility study for gg physics at DA F NE was performed ~12 years ago * : Review of the main results obtained

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Experimental considerations about gg physics at DA F NE2

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  1. Experimental considerations about gg physics at DAFNE2 • Physics window at DAFNE2 and yield estimate for the main processes (using W.W. approximation) • A feasibility study for gg physics at DAFNE was performed ~12 years ago* : • Review of the main results obtained • The tagging system proposed for DAFNE • Very preliminary studies on higher energy option (Eb=1.2 GeV) • Conclusions F. Anulli, D. Babusci, G. Pancheri Laboratori Nazionali di Frascati • * For reference see: • F. Anulli et al., “Two Photon Interaction Measurements with the KLOE Small Angle Tagging System”, LNF-95/007, “The Second DAFNE Handbook” (1995) • G. Alexander et al., “Two-Photon Capabilities of KLOE at DAFNE”, Il Nuovo Cimento, 107 A, 837-862 (1994)

  2. gg luminosity function • Eb=510 MeV : very limited phase-space available for Wgg500 MeV/c2 • Only the low mass region can be effectively exploited: - G(p0,h) → gg - pp at threshold (polarizabilities) • s → p0p0 (challenging, especially if tagging is needed) • Eb=1200 MeV : much more favorable conditions • h, f0(980), a0(980) • s(400÷600) → p0p0 can be effectively studied F. Anulli - INFN road map

  3. Main characteristics • Q2 small  “quasi-real” gg interactions • gg system axis close to that of e+e- • Relatively wide angular distribution: • ~50% of e±scattered at q > 10 mrad • ~15% of e±scattered at q > 100 mrad F. Anulli - INFN road map

  4. Example: e+ e- e+ e- p0p0 pp invariant mass (MeV/c2) Electron Energy (MeV) Electron scattering angle (rad) X = Eg /Ebeam 510 MeV 510 MeV 1200 MeV 1200 MeV F. Anulli - INFN road map

  5. A closer look at Eb=510 MeV operation point Background from f decays Estimated yields • Additional sizable backgrounds from non f decays, like ISR and continuum processes • Kinematics cut would bring a rejection factor <100 (mainly from PTof the hadronic system) • hopeless w/o tagging at the f peak • above the peak, tagging would unambiguously select gg events PT(pp) W(pp) scale factor 104 to be applied! F. Anulli - INFN road map

  6. Electron tagging • Electrons emitted preferentially at small angles • Most of them depart from the main beam orbit after several meters • However, they behave differently w.r.t. the nominal beam when going through magnets, because of lower energy • A tagging system can be conceived only taking into account the constraints imposed by the design of the machine lattice • Interaction with accelerator group to optimize acceptance in specific regions • Careful evaluation of machine backgrounds (mainly radiative Bhabha events) Small angle Tagging (SAT) system proposed for Dafne (1994) • - Collect scattered electrons bent by the Split Field Magnet • SAT located at ~8.5m from the IP • Beam pipe shaped to allow electrons to escape • Quads and sextupoles with large horizontal aperture Ebeam = 510 MeV 0 < qe < ~20 mrad 250 < Ee < 450 MeV IP ~5m BZ = 0.17 T SAT F. Anulli - INFN road map

  7. SAT x beam scattered e- What events would be collected at the SAT - Simulation uses DAFNE old machine layout. - Consider free space after SFM. - SAT location 1.5m downstream the SFM exiting edge Ebeam = 510 MeV Energy of electrons collected at the SAT Radial displacement w.r.t. beam pipe axis E(MeV) beam pipe beam pipe x(m) E(MeV) vs x(m) F. Anulli - INFN road map

  8. What events would be collected at the SAT • Bz of SFM increased to account for higher energy. • - SAT location 1.5m downstream the SFM exiting edge Ebeam = 1200 MeV Energy of electrons collected at the SAT Radial displacement w.r.t. beam pipe axis E(MeV) beam pipe x(m) E(MeV) vs x(m) F. Anulli - INFN road map

  9. Wppvs Tagging Scheme • Tagging at very small angles introduce a cut on the minimun energy for tagged electrons • cut on the maximum photon energy  cut on the high side of the invariant mass spectrum, especially for double small angle tag • problems if working at f peak, need to add a relatively large-angle tagging F. Anulli - INFN road map

  10. Conclusions • gg physics can be successfully exploited at the energy range covered by DAFNE2 (1 < s < 2.4 GeV), with some advice: • It is not clear that the much higher hadronic background can be fully suppressed with kinematic cuts • A tagging system would clean up the selected samples • Eb = 510 MeV • Very few channels available • Precise measurements of radiative p0 and h widths, and pp cross section at threshold • If working at f peak, tagging is necessary, reducing the available phase space  study of the s(400÷600) over the full width looks problematic. • Eb = 1200 MeV • Very interesting physics program. Mgg up to 1 GeV/c2 can be reached, giving access to h, f0, a0, • It should also allow a precise study of the s →p0p0 channel • Single tagging would not cut the invariant mass spectrum • Small yield reduction, but possible full background suppression F. Anulli - INFN road map

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