Photons Interacting with Pions at COMPASS Jan M. Friedrich - PowerPoint PPT Presentation

Photons Interacting with Pions at COMPASS Jan M. Friedrich Physik-Department, TU M¨ unchen COMPASS collaboration June 18, 2015

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook CO mmon M uon and P roton A pparatus for S tructure and S pectroscopy J. M. Friedrich — Photon-Pion at COMPASS 2/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook CO mmon M uon and P roton A pparatus for S tructure and S pectroscopy CERN SPS: protons ∼ 400 GeV (5 – 10 sec spills) ( − ) p : up to 2 · 10 7 /s (typ. 5 · 10 6 /s) secondary π, K , Nov. 2004, 2008-09, 2012: hadron spec. & Primakoff reactions tertiary muons: 4 · 10 7 / s 2002-04, 2006-07, 2010-11: spin structure of the nucleon J. M. Friedrich — Photon-Pion at COMPASS 2/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook Physics fields at COMPASS Sept. 2013 (Q) τ decays (N 3 LO) α s Lattice QCD (NNLO) lepton scattering DIS jets (NLO) Heavy Quarkonia (NLO) at high momentum transfer 0.3 e + e – jets & shapes (res. NNLO) Z pole fit (N 3 LO) → partonic structure of the nucleons ( – ) pp –> jets (NLO) 0.2 × 6 π − → π − π + π − 10 (COMPASS 2008) p p 0.1 ) 2 c Preliminary Number of Events / (5 MeV/ QCD α s (M z ) = 0.1185 ± 0.0006 0.35 1 10 100 1000 Q [GeV] 0.3 0.25 0.2 diffractive dissociation of pions and kaons 0.15 → meson spectrometry 0.1 0.05 0 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 − − π π + π Mass of System (GeV/ 2 ) c scattering of pions (and kaons) in nuclear Coulomb field → low-energetic meson-photon reactions πγ → πγ (pion polarisability), πγ → 3 π (chiral dynamics, radiative couplings) J. M. Friedrich — Photon-Pion at COMPASS 3/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook Physics fields at COMPASS Sept. 2013 (Q) τ decays (N 3 LO) α s Lattice QCD (NNLO) lepton scattering DIS jets (NLO) Heavy Quarkonia (NLO) at high momentum transfer 0.3 e + e – jets & shapes (res. NNLO) Z pole fit (N 3 LO) → partonic structure of the nucleons ( – ) pp –> jets (NLO) s. talk tomorrow by A. Ferrero 0.2 × 6 π − → π − π + π − 10 (COMPASS 2008) p p 0.1 ) 2 c Preliminary Number of Events / (5 MeV/ QCD α s (M z ) = 0.1185 ± 0.0006 0.35 1 10 100 1000 Q [GeV] 0.3 0.25 0.2 diffractive dissociation of pions and kaons 0.15 → meson spectrometry 0.1 s. following talk by B. Grube 0.05 0 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 − − π π + π Mass of System (GeV/ 2 ) c scattering of pions (and kaons) in nuclear Coulomb field → low-energetic meson-photon reactions (this talk) πγ → πγ (pion polarisability), πγ → 3 π (chiral dynamics, radiative couplings) J. M. Friedrich — Photon-Pion at COMPASS 3/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook COMPASS Experimental Setup Fixed-target experiment two-stage magnetic spectrometer high-precision, high-rate tracking, PID, calorimetry J. M. Friedrich — Photon-Pion at COMPASS 4/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook COMPASS Experimental Setup Fixed-target experiment two-stage magnetic spectrometer high-precision, high-rate tracking, PID, calorimetry Runs with Hadron Beams 2004, 2008/09, 2012 190 GeV π − beam on p and nuclear targets (C, Ni, W, Pb) Silicon microstrip detectors for “vertexing” recoil and (digital) ECAL triggers J. M. Friedrich — Photon-Pion at COMPASS 4/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook Pion polarisability and ChPT B E + + α π + β π − − − pion polarisabilities α π , β π in units of 10 − 4 fm 3 size of the pion ∼ 1 fm 3 [cf. atoms: polarisability ≈ size ≈ 1 A ◦ 3 ] α π − β π = 5 . 7 ± 1 . 0 Theory: ChPT (2-loop) prediction: = 0 . 16 ± 0 . 1 α π + β π experiments for α π − β π lie in the range 4 · · · 14 ( α π + β π = 0 assumed) J. M. Friedrich — Photon-Pion at COMPASS 5/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook Pion polarisability and ChPT B E + + α π + β π − − − pion polarisabilities α π , β π in units of 10 − 4 fm 3 size of the pion ∼ 1 fm 3 [cf. atoms: polarisability ≈ size ≈ 1 A ◦ 3 ] = 2 . 93 ± 0 . 5 α π Theory: ChPT (2-loop) prediction: = − 2 . 77 ± 0 . 5 β π experiments for α π lie in the range 2 · · · 7 ( α π + β π = 0 assumed) J. M. Friedrich — Photon-Pion at COMPASS 5/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook latest publication on the pion polarisability J. M. Friedrich — Photon-Pion at COMPASS 6/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook Principle of the COMPASS measurement high-energetic pion beam on 4mm nickel disk observe scattered pions in coincidence with produced hard photons study of cross-section shape J. M. Friedrich — Photon-Pion at COMPASS 7/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook Polarisability effect in Primakoff technique Charged pions traverse the nuclear electric field typical field strength at d = 5 R Ni : E ≈ 300 kV/fm Bremsstrahlung process: particles scatter off equivalent photons tiny momentum transfer Q 2 ≈ 10 − 5 GeV 2 / c 2 pion/muon (quasi-)real Compton scattering Polarisability contribution Compton cross-section typically diminished corresponding charge separation ≈ 10 − 5 fm · e J. M. Friedrich — Photon-Pion at COMPASS 8/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook Polarisability effect in Primakoff technique Charged pions traverse the nuclear electric field typical field strength at d = 5 R Ni : E ≈ 300 kV/fm details: see later Bremsstrahlung process: 3 particles scatter off 2.5 photon exchange equivalent photons 2 tiny momentum transfer Q 2 ≈ 10 − 5 GeV 2 / c 2 1.5 pion/muon (quasi-)real 1 Compton scattering n g i n t e r a c t i o s t r o n 0.5 Polarisability contribution 0 0 0.05 0.1 0.15 0.2 0.25 Compton cross-section q [GeV/c] T typically diminished corresponding charge separation ≈ 10 − 5 fm · e J. M. Friedrich — Photon-Pion at COMPASS 8/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook Pion Compton scattering: embedding the process − − − π π π γ γ π + Q 2 < 2 m 2 < π (A,Z) (A,Z) Primakoff processes π π + π p n Radiative pion photoproduction Photon-Photon fusion J. M. Friedrich — Photon-Pion at COMPASS 9/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook Pion polarisability: world data before COMPASS π + − π − π − π π γ γ π + Q 2 < 2 m 2 π < π p n Radiative pion (A,Z) (A,Z) Photon-Photon fusion Primakoff processes photoproduction ± world avg.: 12.7 2.5 3 fm χ 50 2 -4 Serpukhov 0.06 / 10 PACHRA Babusci Lebedev 40 PACHRA 0.65 PLUTO, DM1 γ → γ π + π p n β DM2, Mark II MAMI 0.10 - γ γ → π π + - π 0.81 α 30 Sigma (CL=0.67) Serpukhov Fil'kov π → π γ Z Z 20 γ γ → π π + - MAMI γ → γ π p + n Kaloshin 10 γ γ → π π - + GIS '06 0 1980 1985 1990 1995 2000 2005 2010 2015 0 5 10 15 20 25 30 35 α β 3 -4 Donoghue year of publication - / 10 fm GIS (2006) π π Mark II GIS’06: ChPT prediction, Gasser, Ivanov, Sainio, NPB745 (2006), plots: T. Nagel, PhD Fil’kov analysis objected by Pasquini, Drechsel, Scherer PRC81, 029802 (2010) J. M. Friedrich — Photon-Pion at COMPASS 10/32

ChPT & Resonances in π − π − π + COMPASS Intro: Pions & ChPT COMPASS Pion polarisability Summary and Outlook Pion Compton Scattering → π γ π γ Two kinematic variables, in CM: total energy √ s , scattering angle θ cm d Ω cm = α 2 ( s 2 z 2 + + m 4 π z 2 α m 3 π ( s − m 2 π ) 2 d σ πγ − ) π z − ) 2 − π z − ) · P s ( sz + + m 2 4 s 2 ( sz + + m 2 − ( α π − β π )+ s 2 + ( α π + β π ) − ( s − m 2 π ) 2 P = z 2 z 2 z 3 − ( α 2 − β 2 ) m 4 24 s π z ± = 1 ± cos θ cm J. M. Friedrich — Photon-Pion at COMPASS 11/32