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HPS and beam polarization

HPS and beam polarization. HPS collaboration meeting 17/06/2014. Michel Guidal IPN Orsay. Motivation. The JLab beam comes polarized to a high degree for ( almost ) free. The QED (trident) background amplitude is purely real => No sensitivity to beam spin.

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HPS and beam polarization

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  1. HPS and beampolarization HPS collaboration meeting 17/06/2014 Michel Guidal IPN Orsay

  2. Motivation The JLabbeamcomespolarized to a highdegree for (almost) free The QED (trident) background amplitudeispurelyreal => No sensitivity to beam spin The A’ amplitude has an imaginary part (width) • Throughinterferencewith the trident amplitude, Any perspective for HPS ? • thereshouldbe a beam spin asymmetry (BSA) • In principle, any non-zeroasymmetrywouldsign an A’ • Spin observables are well-known to be sensitive to smalleffects • An asymmetry (ratio) doesn’tneed the perfectknowledge • of the normalization and shape of the QED background • s +- s - s ++s -

  3. Tree-leveldiagrams for ep->epe+e- fromT. Beranekand M. VanderhaeghenarXiv:1303.2540 [hep-ph] (Relation to M. Guidaland M. Vanderhaeghen(Double DVCS) Phys.Rev.Lett. 90 (2003) 012001) (withantisymmetrization) V = g or A’(onlytimelike)

  4. Kinematics fe’ fA’ Atfixedbeamenergy, there are 8 independent variables: Ee’, qe’ , fe’ , qA’ , fA’ , Me+e-, qcm , qcm

  5. Theoreticalbeam spin asymmetries qcm=0 deg, fcm=0 deg NO ANTISYMMETRIZATION qcm=20 deg, fcm=70 deg qcm=70 deg, fcm=170 deg qcm=170 deg, fcm=250 deg Ebeam=2.2 GeV, qe=0.5 deg, Ee’=1 GeV, qA’=2 deg, Me+e-=50 MeV MA’=50 MeV, e=10-2 (a’/a=10-4) fA’ (deg.)

  6. Theoreticalbeam spin asymmetries qcm=0 deg, fcm=0 deg WITH ANTISYMMETRIZATION qcm=20 deg, fcm=70 deg qcm=70 deg, fcm=170 deg qcm=170 deg, fcm=250 deg Ebeam=2.2, GeVqe=0.5 deg, Ee’=1 GeV, qA’=2 deg, Me+e-=50 MeV MA’=50 MeV, e=10-2 (a’/a=10-4) fA’ (deg.)

  7. Whathappens to the BSA when one integrates over the 8 kinematic variables ? Complicatednumericalproblem: 8 variables, structures and peaks,… Monte-Carlo integration, ~stable resultswith1011events (around mA’ ; lessif onlybackground), 24 hourswith use of IN2P3 grid

  8. y .06 (rad) qv (« vertical » angle) .015 (rad) .05(rad) x -.05(rad) fh (« horizontal » angle) -.015 (rad) -.06 (rad) In the following, wewill use the 8 independent variables: qe’ , fe’ , Me+e- , Ee+ , qv(e+), fh(e+), qv(e-), fh(e-),

  9. MA’=50 MeV, e=10-2 G ~ 10 eV (a’/a=10-4)

  10. Integrating over: qe’ [0,p](rad) fe’[0,2p](rad) Me+e- 1 keV around MA’ Ee+[300,2000] (MeV) qv (e-) [-.06,-.015]+ [.015,.06](rad) fh (e-) [-.05,.05] (rad) qv (e+) [.015,.06] (rad) fh (e+) [-.05,.05](rad)

  11. y qv (« vertical » angle) qv (e+) [.015,.06] x fh (« horizontal » angle) A «bit more» (~3/1000) e+ on the leftsidethan on the right side

  12. Count rates/Statistics • If one takes the fh (e+) binwhere BSA peaks(i.e. fh (e+) ~2 deg.): • N(A’)~4.1011 (arb. unitsproportional to cross section) • N(backgr)~1.5 1011 (arb. unitsproportional to cross section) • DN(A’)~ 8.108 BSA ~ 8.108/4.1011 ~ 2/1000

  13. A’ backgr

  14. Count rates/Statistics • If one takes the fh (e+) binwhere BSA peaks(i.e. fh (e+) ~2 deg.): • N(A’)~4.1011 (arb. unitsproportional to cross section) • N(backgr)~1.5 1011 (arb. unitsproportional to cross section) • DN(A’)~ 8.108 BSA ~ 8.108/4.1011 ~ 2/1000 This all over a DMe+e=1 keV bin !

  15. Count rates/Statistics • If one takes the fh (e+) binwhere BSA peaks(i.e. fh (e+) ~2 deg.): • N(A’)~4.1011 (arb. unitsproportional to cross section) • N(backgr)~1.5 1011 (arb. unitsproportional to cross section) • DN(A’)~ 8.108 BSA ~ 8.108/4.1011 ~ 2/1000 This all over a DMe+e=1 keV bin ! • If one scales over a DMe+e=1 MeV bin: • N(A’)~4.1011 (arb. unitsproportional to cross section) • N(backgr)~1.5 1011 X 103(arb. unitsproportional to cross section) • DN(A’)~ 8.108 BSA ~ 8.108/1.5 1014 ~ 5/106

  16. Count rates/Statistics • If one takes the fh (e+) binwhere BSA peaks(i.e. fh (e+) ~2 deg.): • N(A’)~4.1011 (arb. unitsproportional to cross section) • N(backgr)~1.5 1011 (arb. unitsproportional to cross section) • DN(A’)~ 8.108 BSA ~ 8.108/4.1011 ~ 2/1000 This all over a DMe+e=1 keV bin ! • If one scales over a DMe+e=1 MeV bin: • N(A’)~4.1011 (arb. unitsproportional to cross section) • N(backgr)~1.5 1011 X 103(arb. unitsproportional to cross section) • DN(A’)~ 8.108 BSA ~ 8.108/1.5 1014 ~ 5/106 • And weexpect 107events in a 1 MeV bin for HPS…

  17. Doesn’t look too good… But maybestillsomehope: Instead of integrating over all 8 variables and all HPS acceptance, identify a particular corner of the phase spacewhere BSA is of the order of the percent and, if doingso, one goesfrom 107 to 104events… (one canalwaysdream) Staytuned !

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