Pion-Photon Reactions and Chiral Dynamics inPrimakoff Processes at COMPASS

Markus Krämer on behalf of the COMPASS collaboration

supported by: Maier-Leibnitz-Labor der TU und LMU München,Cluster of Excellence: Origin and Structure of the Universe, BMBF

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 1

The COMPASS Experiment at CERN

• Located at the CERN SPS (Super Proton Synchrotron)• Beam (Rates):- (tertiary) Muon - (secondary) Hadron (,,…)

• Two stage magnetic spectrometer with large angular acceptance

COmmon Muon and Proton Apparatus for Structure and Spectroscopy

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 2

Motivation

• COMPASS measure pion-nucleon reaction at relativistic energies• The electric field of the nucleus act like quasi free photons on the pion• The effect is described by the equivalent-photon approximation

• Primakoff reaction allows to access reaction• beam contribution allows normalization, i.e. absolute measurements • Data record at COMPASS

- 2004 : - 2009 : - 2012 : (work in progress)

• Several topics of interest• Pion polarizability• Chiral dynamics near threshold• Radiative width of resonances

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Pion Polarizability

• Pion polarizability shows up as a modification to the Born cross-section of process

• The Compton cross-section at high photon energies is modified as compared to a point like particle (assuming )

• Using Monte-Carlo (point like particle) the ratio can be determined• beam used to study systematics

(COMPASS 2009)

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 4

*C. Adolph et al. , Phys. Rev. Lett. 114, 062002 (2015)

Pion Polarizability

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(COMPASS 2009)

The measured polarizability of the pion is

*C. Adolph et al. , Phys. Rev. Lett. 114, 062002 (2015)

Chiral Dynamics Near Threshold

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1N. Kaiser, Nucl. Phys. A848, 198-217 (2010)

Predictions from chiral perturbation theory (ChPT) for and

Chiral Dynamics Near Threshold (COMPASS 2009)Kinematic distribution – invariant mass of the 3-pion final state

All Events Beam Particle: Pion

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 7

Chiral Dynamics Near Threshold (COMPASS 2009)of the 2-pion subsytems

Beam Particle: Pion

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 8

Beam Particle: Pion

Charged Neutral

Chiral Dynamics Near Threshold (COMPASS 2009)Partial-Wave Analysis• Extended isobaric model

• Isobaric model- Decay channels for various : ,,,- Additional quantum number : Projection of on production z-axis

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 9

• Extended isobaric model• Isobaric model

- Decay channels for various : ,,,- Additional quantum number : Projection of on production z-axis

• Chiral Wave- Including LO+NLO+diagrams including single or double 1

- Up to 780 MeV/ () used exclusively for - Conservative model: Chiral wave stop at 780 MeV/- Alternative model: Chiral up to 1580 MeV/ (presented here)

Chiral Dynamics Near Threshold (COMPASS 2009)Partial-Wave Analysis

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 10

1 Calculation provided by N. Kaiser (TUM)

• Extended isobaric model• Isobaric model

- Decay channels for various : ,,,- Additional quantum number : Projection of on production z-axis

• Chiral Wave- Including LO+NLO+diagrams including single or double 1

- Up to 780 MeV/ () used exclusively for - Conservative model: Chiral wave stop at 780 MeV/- Alternative model: Chiral wave up to 1620 MeV/ (presented here)

• Partial coherence model Accounting for experimental resolution• Allows to distinguish Diffractive () and Primakoff () production

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 11

1 Calculation provided by N. Kaiser (TUM)

Chiral Dynamics Near Threshold (COMPASS 2009)Partial-Wave Analysis

Partial-Wave Analysis

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Results for “Spin Totals” (Sum of amplitudes)

: Diffractive production : Primakoff production

: Diffractive production

Partial-Wave AnalysisImportant Waves

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 13

: Diffractive production

: Primakoff production : Diffractive production

Partial-Wave AnalysisImportant Waves

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 14

: Primakoff production : Diffractive production

Partial Wave AnalysisImportant Waves

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 15

Partial-Wave Analysis

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Chiral Intensity

Partial Wave Analysis

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Chiral Intensity – Fit in t’-bins : get Diff. BG

Cross Section

• Account for the Nuclear Target• Weizsäcker-Williams Approximation

• Use free decay for normalization

Primakoff Photon Production

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 18

Chiral Dynamics Near Threshold – (COMPASS 2009)

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Work in progress – to be finalized soon

?

Chiral Dynamics Near Threshold

• Data recorded in 2004 @ COMPASS• PB target with pion beam• Reaction:

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(COMPASS 2004, )

1C. Adolph et al., Phys. Rev. Lett. 108, 192001 (2012)

Chiral Dynamics Near Threshold

• Data recorded in 2004 @ COMPASS• PB target with pion beam• Reaction: • Partial-wave analysis of the data

- M=0: Diffractive production- M=1: Primakoff production- ChPT amplitude included

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(COMPASS 2004, )

1C. Adolph et al., Phys. Rev. Lett. 108, 192001 (2012)

Chiral Dynamics Near Threshold

• Measurement of the cross-section applying Kaon normalization

(COMPASS 2004, )

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1C. Adolph et al., Phys. Rev. Lett. 108, 192001 (2012)

Radiative Width (COMPASS 2004)

Markus Krämer, Technische Universität München, mkraemer@tum.de17/11/2015 23

• The cross section of a resonance X decaying to is given by

• The decay into the observed final state is given by

• With the Primakoff cross-section

• One gets

1C. Adolph et al., EPJA 50 (2014) 79

Radiative Width (COMPASS 2009)

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Γ 0 (𝑎2 (1320 )→𝜋𝛾 )=(358±6stat± 42syst ) keV

)

1C. Adolph et al., EPJA 50 (2014) 79

Conclusion/Outlook

• Published results of• The Polarizability of the pion• Chiral dynamics near threshold (, COMPASS 2004)• Radiative width of the and (, COMPASS 2004)

• Progress of the analysis of COMPASS 2009• Chiral dynamics coming soon• Possible measurement of radiative width

• Analysis of the Primakoff data taken in 2012• Work in progress• Increased statistics for neutral channels• Spectrometer and trigger improvements

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Thank you for your attention

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Backups

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Pion Polarizability

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Systematic Uncertainty

Kaon Normalization

• Recorded data at COMPASS (2009)• Produced by either Photon or Pomeron (strong interaction) exchange

- (low t’ process)

• decay for determine the integrated luminosity Calculate cross section of

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Partial Wave Analysis

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Wave Set – 37 Waves

Cross SectionLuminosity

• Luminosty determined using events (cancel systematic uncertainties)

• Calculation of cross section

• : Diffractive BG, bin migration, beam PID efficiency

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