Measuring the charged pion polarizability in the γγ π + π reac5on

Transcription

1 Measuring the charged pion polarizability in the γγ π + π reac5on David Lawrence, JLab Rory Miskimen, UMass, Amherst Alexander Mushkarenkov, UMass, Amherst Elton Smith, JLab

2 Mo5va5on Electro (α π ) and Magne5c (β π ) Polarizabili5es represent fundamental proper5es of the charged pion in the low- energy sector of QCD α π and β π are related to the charged pion weak form factors F V and F A : where the low- energy constants L r 10 and Lr 9 are part of the Gasser- Leutwyler effec5ve Lagrangian Measuring the polarizabili5es of the charged pion can be used to test the even- parity part of the Chiral Lagrangian (as opposed to the odd- parity sector which is tested via anomalous processes such as π o - >γγ) Improved measurement of α π -β π would reduce uncertainty contribu5on of hadronic light- by- light scauering to SM predic5on of anomalous magne5c moment of the µ: (g µ - 2)/2 (see K. Engel, H. Patel, M. Ramsey- Musolf, arxiv: v2 [hep- ph]) 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

3 LO O(p 4 ) ChPT calcula5ons give: α π - β π = 5.6 ± 0.2 x 10-4 fm 3 with Donoghue and Holstein, 1989 α π + β π = 0.0 fm 3 NLO O(p 6 ) correc5ons are rela5vely small α π - β π = 5.7 ± 1.0 x 10-4 fm 3 with α π + β π = 0.16 ± 0.1 x 10-4 fm 3 Bürgi 1996, Gasser et al Dispersion Rela5ons have been used to as well, but do not agree: +2.6 α π - β π = 13.0 x 10-4 fm Fil kov et al. 2006* α π - β π = 5.7 x 10-4 fm 3 Pasquini et al /6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

4 Experimental Access This experiment Primakoff effect SIGMA RadiaSve pion photo- producson MAMI PACHRA Light by light scaxering (by crossing symmetry) PLUTO DM1 DM2 MARK- II 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

5 ) 3 fm -4 ( 10 π - β DM2 (DCI) experiment theory α π 40 PLUTO (DESY) DM1 (DCI) PACHRA (LEBEDEV) MAMI (MAINZ) SIGMA (Serpukov) Fil'kov (DR) ChPT MARK II (SLAC) Gasser (ChPT) Pasquini (DR) 0 - γγ π + π γ p nπ + γ π A π' γ A Theory Predictions 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

6 Backgrounds Experiment will measure reac5on: γ Pb - > Pb π + π - Primary background will be coherent ρ o produc5on followed by ρ- >ππ decay Will use angular distribu5ons to separate Primakoff from coherent ρ o produc5on (see later slides) Currently gathering list of other poten5ally relevant backgrounds including: σ meson produc5on (angular distribusons same as Primakoff) incoherent π + π - produc5on 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

7 Kinema5cs of Experiment π ± Generated values for threshold W ππ polar angle distribution in lab frame MeV/c August 3, 2012 DL Generated values for threshold W ππ π ± momentum distribution in lab frame MeV/c August 3, 2012 DL Acceptance at nominal GlueX target center π + - π π - π θ (degrees) φ ππ is angle between ππ system and incident photon polariza5on vector in CM frame ψ ππ is angle between ππ scauering plane and polariza5on vector in helicity frame γ φ π π p' X φ Momentum (GeV/c) θ π + θ cm Z ψ π π - π p 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

8 Linear Polariza5on of incident photon beam helps dis5nguish Primakoff from coherent ρ o produc5on Primakoff + ρ o Primakoff only W ππ (GeV/c 2 ) (invariant mass of π + π - system) (invariant mass of π + π - system) φ ππ ψ ππ 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

9 Rela5ng cross- sec5on to α π -β π Figure 5. from Pasquini et al. Phys. Rev. C 77, (2008) γγ - > π + π - Cross- sec5on for γγ - > π + π - calculated based on two values of α π -β π : α π -β π = 13.0 α π -β π = 13.0 x 10-4 fm 3 (top, doued line) α π -β π = 5.7 x 10-4 fm 3 (solid and dashed lines) α π -β π = 5.7 (Data points from MARK- II) Cross- sec5on varies by ~10% for factor of 2 varia5on in α π -β π Need measurement of σ(γγ - > π + π - ) at few percent level Invariant mass of π + π - do5ed: subtracted DR calcula5on with α π -β π = 13.0 dashed: subtracted DR calcula5on with α π -β π = 5.7 solid: unsubtracted DR calcula5on with α π -β π = 5.7 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

10 The GlueX Detector in Hall- D New Proposal will use GlueX detector in Hall- D: Linearly polarized photon source (~9GeV) 2T solenoidal magne5c field (δp/p = few %) Drir chambers High resolu5on Time- of- flight detector Modifica5ons to standard GlueX setup: Replace LH2 target with thin Pb target GlueX diamond wafer electron beam tagger magnet photon beam tagger to detector distance is not to scale Move target upstream to improve low- angle acceptance Alternate start- counter? barrel calorimeter target electron beam time-of -flight superconducting magnet forward calorimeter central drift chamber forward drift chambers 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

11 Detector Rates/Acceptance 10 7 tagged photons/second on 5% radia5on length Pb target 500 hours of running W ππ acceptance down to ~320 MeV/c 2 (working to improve acceptance to even lower W ππ ) Es5mated ~36k* Primakoff events (contrast this with the ~400 events in the acceptance of the MARK- II measurement) * before detector acceptance 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

12 Summary Next to leading order ChPT predic5on of α π - β π is 5.7 ± 1.0 x 10-4 fm 3 Previous measurements of α π - β π range from x 10-4 fm 3 A new proposal to measure the charge pion polarizability α π - β π via the γγ*- >π + π - reac5on is being developed that will use the GlueX detector at Jefferson Lab LeUer of Intent submiued to PAC in June PAC has encouraged development of full proposal will be submiued in next PAC, spring/summer 2013 Work is ongoing to iden5fy relevant backgrounds and determine detector acceptance An improved measurement of α π - β π would improve the SM predic5on of the anomalous magne5c moment of the µ: (g µ - 2)/2 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

13 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics

14 Anomalous magnet moment of the µ: (g µ - 2)/2 Experimental uncertainty of ~ 63 x SM calcula5on has uncertainty of ~ 49 x Hadronic light- by- light (HLBL) scauering is one of two major contributors to SM uncertainty (other is hadronic vacuum polariza5on) π polarizability is poten5ally significant contribu5on to HLBL that is currently omiued from current SM calcula5on g- 2 collabora5on at Fermilab is preparing a measurement that will reduce experimental uncertainty by a factor of 4 A measurement of the π polarizability could help reduce the SM uncertainty significantly For detailed info on planned Fermi- lab experiment, see hxp://gm2.fnal.gov/public_docs/proposals/proposal- APR5- Final.pdf 8/6/12 Measuring the charged pion polarizability in the γγ π+π reac5on D. Lawrence Chiral Dynamics