The Exotic η π Wave in 190 GeV π p η π p at COMPASS

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1 The Exotic η π Wave in 19 GeV π p η π p at COMPASS Tobias Schlüter for the COMPASS collaboration Ludwig-Maximilians-Universität München June 16, 11

2 πη in diffractive scattering Possible quantum numbers for the πη system: L S-wave P-wave D-wave F-wave G-wave J PC Hence: P-wave resonant exotic meson. This system has been studied by the following experiments: experiment beam momentum reaction year published VES 37 GeV/c π N η π N 1993, 5 E85 18 GeV/c π p η π p 1 They all see a very strong P-wave.

3 πη in diffractive scattering Possible quantum numbers for the πη system: L S-wave P-wave D-wave F-wave G-wave J PC Hence: P-wave resonant exotic meson. This system has been studied by the following experiments: experiment beam momentum reaction year published VES 37 GeV/c π N η π N 1993, 5 E85 18 GeV/c π p η π p 1 COMPASS 19 GeV/c π p η π p 1 (?) They all see a very strong P-wave.

4 Previous πη results VES Results from VES (Be target, 37 GeV): VES sees the a (13) (peak in D + -wave) VES says: there may be an a (17) explaining the broad structure in the D + -wave VES says: there may be an exotic π 1 (16) Note the jump in the relative P + D + phase near GeV

5 Previous πη results BNL E85 Results from BNL E85 (proton target, 18 GeV): they see the a (13) (peak in D + -wave) they add a G + -wave to the fit, gives: a 4 (4) they explain the broad D + -wave with an a (17) and the P + -wave with an exotic resonance π 1 (16) they find an unusual t-slope Note the various jumps at GeV

6 Remarks on PWA formalism For a given mass, two-body states in the reflectivity basis (ǫ, l, m) have the form (θ, φ Gottfried-Jackson angles) { A ǫlm Yl m sin(mφ) ǫ = +1 (m > ) (θ, ) cos(mφ) ǫ = 1 (m ) The observed intensity is then proportional to I(θ, φ) = T +1,lm A +1,lm (θ, φ) lm + T 1,lm A 1,lm (θ, φ) where the production amplitudes T ǫlm were introduced. Important observations: only negative reflectivity (= unnatural exchange) contributes to intensity at φ = ; all positive reflectivity waves with m = 1 have the same φ dependency. Negative reflectivity compatible with zero in VES, BNL, COMPASS analyses. lm

7 The COMPASS spectrometer fixed target experiment at CERN s SPS accelerator variety of beams available (pos/neg muon, pos/neg hadron) variety of targets (polarized targets, LH, solid state) diverse physics program 8, 9 : hadron beam runs with various targets In this talk: 8 data, negative pion beam at 191 GeV, LH target

8 Data selection entries /.5 MeV Final state selected: exclusive 3 tracks, photons COMPASS 8 - π p π π π + γγp m(γγ) entries [MeV] ( entries per event) COMPASS 8 - π p π π π + ηp w/o acceptance correction m(π π + η) [GeV] select exclusive events with 3 tracks + recoil proton, good ECAL clusters select η γ (left) select η π π + η (right)

9 Data selection entries /.5 MeV entries / MeV Final state selected: exclusive 3 tracks, photons COMPASS 8 - π p π π π + γγp m(γγ) COMPASS 8 - π p π η (π π + γγ)p w/o acceptance correction m(πη ) [GeV] entries [MeV] ( entries per event) Result: COMPASS 8 - π p π π π + ηp w/o acceptance correction m(π π + η) [GeV] 18 events with m(η π) < GeV/c, 35 total mass reach beyond GeV/c additionally, about 3 events in πη, η 3π channel

10 First look at the data: t slopes We find indication for a continuous transition between different production mechanisms, fitting the t distribution in several areas mass bin fit with Aexp( B t ) fit with A t exp( B t ) m < < m < < m < < m < (BNL fitted with a simple exponential between.5 < t < 1. GeV/c, they found B =.93/GeV ) We find: higher mass broader slope and: clear contradicition with BNL

11 Input to the PWA Like previous analyses, we used all waves with l, m 1 and additionally the ǫ = +1, l = 4, m = 1. I.e.: ǫ = +1 P + D + G + ǫ = 1 S P P D D As expected from Pomeron (i.e., natural) exchange, the negative reflectivity waves turn out compatible with zero (below GeV). Ambiguities are bounded by the size of the negative waves, i.e. they are not a problem.

12 PWA results P + and G + waves + events / 4 MeV/c COMPASS 8 PC -+ P + (J = 1 ) intensity Intensity P + -1 Phase D + P / P + ) [deg] arg(d COMPASS 8 phase D - P events / 4 MeV/c COMPASS 8 PC ++ D + (J = ) intensity Intensity D Clear phase-motion from a (13), jump in phase near GeV, slow phase-motion in range of P + -wave intensity peak.

13 PWA results can the G + -wave clarify the picture? + events / 4 MeV/c COMPASS 8 COMPASS 8 8 COMPASS 8 PC -+ P + (J = 1 ) intensity - phase D - P phase G - P Int. P Φ(D P + ) Φ(G 3 + P + ) / P + ) [deg] + arg(d / P + ) [deg] + arg(g events / 4 MeV/c COMPASS 8 4 COMPASS 8 PC ++ D + (J = ) intensity 35 phase G - D Int. D Φ(G 5 + D + ) / D + ) [deg] arg(g events / 4 MeV/c Int. G + COMPASS 8 PC ++ G + (J = 4 ) intensity Clear phase-motion in G + -wave relative to D + wave, compatible with a 4 (4). Again: jump at GeV in phase relative to P + wave. But: unlike between P + and G + no rapid phase jump between D + and G + waves at GeV

14 Transition between different production processes? cos θ GJ COMPASS 8 w/o acceptance correction Depicted: cos θ GJ of the η in the π η GJ restframe vs. m(πη ). Low masses show P and D wave interference, a 4 near GeV/c, above that strong forward/backward peaking indicative of central production. Question: How does the forward/backward peaking at high masses affect the interpretation at low masses?

15 Comparison to ηπ cos θgj We also selected the ηπ final state along the same lines. No PWA yet, for comparison, here s the same plot as on the previous slide, but for the πη: COMPASS 8 π-p π-ηp w/o acceptance correction Depicted: cos θgj of the η in the π η GJ restframe vs. m(πη). Dominated by a (13), structures due to a4 (4) visible, again forward/backward peaking at high masses.

16 Conclusions COMPASS can confirm previous observations of a strong P-wave in η π and in addition finds confirming evidence for the a 4 (4) π η the t distribution shows a decreasing slope with increasing m(η π ) and the slope disagrees with the findings of E85 resonant interpretation of the P-wave cannot be confirmed (at this point) The road ahead: Primary Objective: clarify what happens in the transition between the regimes below and above GeV/c Secondary Objective: use this to gain clearer understanding of the nature of π η P-wave Thank you!