Meson Investigations by the MAMI A2 Collaboration 15th International Workshop on Meson Physics Philippe Martel Krakow, Poland - 12 June 2017 Institute for Nuclear Physics Johannes Gutenberg University of Mainz A 2
Thanks • Thanks to the organizers for the invitation to speak Philippe Martel - Meson A2 1/26
Thanks • Thanks to the organizers for the invitation to speak • Thanks to all of you for still being here (perhaps to the rain for ‘encouraging’ you all to come to the last day) Philippe Martel - Meson A2 1/26
Thanks • Thanks to the organizers for the invitation to speak • Thanks to all of you for still being here (perhaps to the rain for ‘encouraging’ you all to come to the last day) • Since we’re here, let’s talk about some meson physics at MAMI Philippe Martel - Meson A2 1/26
Outline 1. What should we do 2. What can we do 3. What have we done 4. What are we doing Philippe Martel - Meson A2 2/26
What should we do
Should we study mesons? • We’ve had four days of talks regarding this... Philippe Martel - Meson A2 - What should we do 3/26
Should we study mesons? • We’ve had four days of talks regarding this... • If you are not already convinced, I’m not going to change your mind Philippe Martel - Meson A2 - What should we do 3/26
Should we study mesons? • We’ve had four days of talks regarding this... • If you are not already convinced, I’m not going to change your mind • Preferred stance of experimentalists: “Just let me go measure things.” Philippe Martel - Meson A2 - What should we do 3/26
Should we study mesons? • We’ve had four days of talks regarding this... • If you are not already convinced, I’m not going to change your mind • Preferred stance of experimentalists: “Just let me go measure things.” • Of course it’s always nice if your work is beneficial, so what would the theorists like to have... Philippe Martel - Meson A2 - What should we do 3/26
Observables Beam Target Recoil x y z x ′ y ′ z ′ Unpolarized σ T P Linear Σ H P G O x ′ T O z ′ Circular F E C x ′ C z ′ Beam Target/Recoil x y z x ′ y ′ z ′ x ′ y ′ z ′ x ′ y ′ z ′ Unpolarized T x ′ T z ′ Σ L x ′ L z ′ Linear σ L z ′ E L x ′ C z ′ C x ′ T z ′ F T x ′ Circular G O z ′ O x ′ H Philippe Martel - Meson A2 - What should we do 4/26
Observables Beam Target Recoil Both x y z x x ′ z ′ x ′ z ′ Unpolarized σ T T x ′ T z ′ Linear Σ H P G O x ′ O z ′ L z ′ L x ′ Circular F E C x ′ C z ′ As L. Tiator described: • 16 total observables • 8 observables without recoil polarization • 8 observables without target polarization • Do not need all 16 to have complete picture Philippe Martel - Meson A2 - What should we do 4/26
What can we do
Mainz Microtron (MAMI) e − Beam • Injector → 3.5 MeV • RTM1 → 14.9 MeV • RTM2 → 180 MeV • RTM3 → 883 MeV • HDSM → 1.6 GeV Philippe Martel - Meson A2 - What can we do 5/26
Polarized Photon Beam A high energy electron can produce Bremsstrahlung (‘braking radiation’) photons when slowed down by a material. e /P 1 γ P • Longitudinally polarized electron 0.8 beam produces circularly 0.6 polarized photon beam (helicity 0.4 transfer) 0.2 • P e measured with a Mott 0 0 50 100 150 200 250 300 350 400 450 E (MeV) polarimeter before the RTMs. γ • Circular beam helicity flipped by 4 E γ E e − E 2 γ alternating the e − beam P γ = P e 4 E 2 e − 4 E γ E e + 3 E 2 γ polarization ( ≈ 1 Hz). Philippe Martel - Meson A2 - What can we do 6/26
Polarized Photon Beam A high energy electron can produce Bremsstrahlung (‘braking radiation’) photons when slowed down by a material. 3 Normalised Enhancement • Diamond radiator produces 2.5 linearly polarized photon beam (coherent Bremsstrahlung) 2 • Polarization determined by fitting 1.5 the Bremsstrahlung distribution. 1 • Linear beam orientation typically 100 150 200 250 300 350 400 450 500 550 600 γ Incident energy [MeV] flipped every two hours. Philippe Martel - Meson A2 - What can we do 6/26
Photon Tagging • e − beam with energy E 0 , strikes radiator producing Bremsstrahlung photon beam with energy distribution from 0 to E 0 . • Residual e − paths are bent in a spectrometer magnet. • With proper magnetic field, array of detectors determines the e − energy, and ‘tags’ the photon energy by energy conservation. Philippe Martel - Meson A2 - What can we do 7/26
Targets Polarized frozen spin butanol target • Dynamic Nuclear Polarization (DNP) • Butanol (C 4 H 9 OH) for polarized protons or D-Butanol (C 4 D 9 OD) for polarized deuterons • P max > 90%, τ > 1000 h T Unpolarized targets • LH2/LD2 • 4 He • Solid targets (C, Al, Pb, etc.) Philippe Martel - Meson A2 - What can we do 8/26
Detectors Crystal Ball (CB) TAPS • 672 NaI Crystals BaF2 PbWO4 • 24 Particle Identification Detector (PID) Paddles Target • 2 Multiwire Proportional Chambers (MWPCs) Two Arms Photon CB Spectrometer (TAPS) NaI • 366 BaF 2 and 72 PbWO 4 PID MWPC Crystals • 384 Veto Paddles Philippe Martel - Meson A2 - What can we do 9/26
What have we done
Busy Two Years • Since Meson2016, we’ve been quite productive... Philippe Martel - Meson A2 - What have we done 10/26
Busy Two Years • Since Meson2016, we’ve been quite productive... • Taken • 3 weeks polarized target data • 6 weeks recoil polarimeter data • 6 weeks 4 He target data • 3 weeks LD2 data • 16 weeks LH2 data • 2 weeks of tests • Total = 36 weeks (feels like more) Philippe Martel - Meson A2 - What have we done 10/26
Busy Two Years • Since Meson2016, we’ve been quite productive... • Taken • 3 weeks polarized target data • 6 weeks recoil polarimeter data • 6 weeks 4 He target data • 3 weeks LD2 data • 16 weeks LH2 data • 2 weeks of tests • Total = 36 weeks (feels like more) • 1 EPJA, 1 PRL, 1 PLB, and 5 PRCs published • 1 PRC and 1 PRD accepted • 1 PLB submitted Philippe Martel - Meson A2 - What have we done 10/26
Σ - γ p → π 0 p [S. Gardner, EPJA 52, 333 (2016)] Well that’s a lot of data. Philippe Martel - Meson A2 - What have we done 11/26
Σ - γ p → π 0 p [S. Gardner, EPJA 52, 333 (2016)] That’s a little bit better. Philippe Martel - Meson A2 - What have we done 11/26
Σ - γ p → π 0 p [S. Gardner, EPJA 52, 333 (2016)] 2 l max Σ( W , θ ) = σ 0 ( W , θ )Σ( W , θ ) = q ˇ a Σ n ( W ) P 2 � n ( cos θ ) k n =2 2 Philippe Martel - Meson A2 - What have we done 11/26
γ p → η p / γ p → η ′ p [V. Kashevarov, PRL 118, 212001 (2017)] 0.08 (a) (b) 0.06 0.2 0.04 0.1 d σ /d Ω [μ b/sr ] d σ /d Ω [μ b/sr ] 0.02 W=1888 MeV W=1888 MeV W=1888 MeV W=1888 MeV W=1925 MeV W=1925 MeV W=1938 MeV W=1938 MeV 0 0 0.3 (c) (d) 0.08 0.2 0.06 0.04 0.1 0.02 W=1938 MeV W=1938 MeV W=1956 MeV W=1956 MeV W=1944 MeV W=1944 MeV W=1956 MeV W=1956 MeV 0 0 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 * cos Θ η cos θ η′ Present A2 data in magenta, previous in blue, CLAS [M. Williams et al., PRC 80, 045213 (2009)] in black crosses, CBELSA/TAPS [V. Crede et al., PRC 80 055202 (2009)] in open circles Philippe Martel - Meson A2 - What have we done 12/26
γ p → η p / γ p → η ′ p [V. Kashevarov, PRL 118, 212001 (2017)] Run I Run II Run III 1.5 CBELSA/TAPS-09 Run III CBELSA/TAPS-09 SAPHIR-98 3 ABBHHM-68 σ [μ b ] σ [µ b ] AHHM-76 1 2 0.5 1 0 1.65 1.7 1.75 1.8 1.85 1.9 1.95 2 2.05 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 W [ GeV ] W [ GeV ] Present A2 data in magenta, CBELSA/TAPS [V. Crede et al., PRC 80 055202 (2009)] in open circles, with η MAID-2003 [Nucl. Phys. A700, 429 (2002)] (black dotted), SAID-GE09 [Phys. Rev. C 82, 035208 (2010)] (blue), BG2014- 2 [EPJA 47, 153 (2011); EPJA 48, 15 (2012)] (magenta) Philippe Martel - Meson A2 - What have we done 12/26
π 0 → e + e − γ [S. Prakov, PRC 95, 025202 (2017)] Transition Form Factors (see talk by L. Heijkenskjoeld in Parallel Session B4) • Pion-exchange term a π 0 µ in HLbL scattering • Decay width of π 0 → e + e − 1.3 1.3 1.3 2 | p0 p0 p0 p0 0.03113 0.03113 0.03113 0.03113 0.01709 0.01709 0.01709 0.01709 p0 p0 p0 p0 0.02924 0.02924 0.02924 0.02924 0.0115 0.0115 0.0115 0.0115 p0 p0 p0 p0 0.02983 0.02983 0.02983 0.02983 0.009552 0.009552 0.009552 0.009552 ± ± ± ± ± ± ± ± ± ± ± ± γ 0 π |F 1.25 1.25 1.25 This Work: Run I This Work: Run II This Work: Data 1.2 1.2 1.2 This Work: Fit I This Work: Fit II This Work: Fit Pad e approxim. Pad e approxim. Pad e approxim. 1.15 1.15 1.15 DA DA DA 1.1 1.1 1.1 1.05 1.05 1.05 1 1 1 0.95 (a) 0.95 (b) 0.95 (c) 0.9 0.9 0.9 0.02 0.04 0.06 0.08 0.1 0.12 0.02 0.04 0.06 0.08 0.1 0.12 0.02 0.04 0.06 0.08 0.1 0.12 2 2 2 m [GeV/c ] m [GeV/c ] m [GeV/c ] + - + - + - e e e e e e Philippe Martel - Meson A2 - What have we done 13/26
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