Π polarizability Meeting Mar 22, 2019

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Meeting Info.

Meeting Time And Location

1:00 pm EST (JLab time)

L207

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  1. Previous meeting: Mar 8, 2019
  2. Announcements
  3. TOF Trigger Beam Test
  4. Murray M. Request (see e-mail below)
  5. Other business
  6. Next meeting

Murray's e-mail received 3/22/2019

Hi David,

How goes? 
I am preparing a pion polarizability review article with Stefan Scherer. 
It should include a description of your Jab plans.
I prepared a short one paragraph draft description below for this purpose. 
I'd appreciate it if you could look at this and let me know if it is a good description,
or if you suggest any changes.
I'm not sure how you will actually do PID.
Will it use DIRC or otherwise? 

Do you have predictions from theorists for asymmetry in   γγ → π+π− and 
γγ → mu+mu− γγ → e+e− reactions? 
Are  asymmetries sensitive to polarizabilities? 
Do you have estimates for the magnitudes of these cross sections? 
 
Any progress with plans for a pizero measurement? 

When might JLab E12-13-008 get beam? 

Thanks, Murray
The Jlab pion polarizability experiment E12-13-008 [H] plan to measure γγ → π+π−  cross sections and asymmetries 
via a Primakoff  γγ → π+π- reaction. They plan to use a 9 GeV tagged linearly polarized photon beam produced via 
coherent bremsstrahlung, a Pb “photon target”, the GlueX detector in Hall-D, and auxiliary detectors. Gluex is based on 
a solenoidal hermetic detector optimized for tracking of charged particles and detection of gamma ray, and will achieve 
kaon/pion identification via a quartz-based differential Cherenkov light detector (DIRC). The most important problem in 
studying the  γγ → π+π− reaction is the elimination of the dominant two-prong QED reactions  γγ → e+e− and γγ → mu+mu−
These leptonic backgrounds below Wππ = 0.5 GeV are expected each to be more than 10 times larger than the expected signal.
Mark II eliminated these backgrounds by identifying pion pairs using time of flight, by requiring both tracks to hit an active region 
of the liquid-argon calorimeter, and by requiring both tracks to have a summed transverse momentum with respect to the e+e- axis of
less than 150 MeV/c. TOF will not be useful for the JLab measurement because of the extreme relativistic velocities of the pions, 
and they rather plan is to use a system of hadronic absorbers with MWPC's to distinguish pions from
muons. The JLab experiment plans to use linearly polarized incident photons, which will allow them to 
use the [1 - cos 2phi] azimuthal dependence of the π+π- and mu+mu- systems to help distinguish between 
signal and background. Dispersion relations calculations [10, 45] show in Fig. 5 that the total cross section for |cos(theta)] < 0.6 for γγ → π+π− 
at Wππ = 0.4 GeV equals ~170nb and 210 nb for απ−βπ  equal 5.7 and 13.0, respectively. 
These calculations provide important guidance in planning the JLab experiment that if  απ−βπ ~ 5.7, 
the cross section must be measured to ~3 nb uncertainty to achieve �an accuracy of ~10%.

Minutes

Attending: