hps and beam polarization
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
HPS and beam polarization
Michel GuidalIPN Orsay
HPS collaboration meeting17/06/2014
Any perspective for HPS ?
The QED (trident) background amplitude is purely real=> No sensitivity to beam spin
The JLab beam comes polarized to a high degree for (almost) free
The A’ amplitude has an imaginary part (width)ÞThrough interference with the trident amplitude,
s ++ s -
s +- s -
there should be a beam spin asymmetry (BSA)
Spin observables are well-known to be sensitive to small effects
An asymmetry (ratio) doesn’t need the perfect knowledge of the normalization and shape of the QED background
In principle, any non-zero asymmetry would sign an A’
Motivation
Tree-level diagrams for ep->epe+e-
V = g or A’ (only timelike)
from T. Beranek and M. Vanderhaeghen arXiv:1303.2540 [hep-ph](Relation to M. Guidal and M. Vanderhaeghen (Double DVCS) Phys.Rev.Lett. 90 (2003) 012001)
(with antisymmetrization)
Kinematics
At fixed beam energy, there are 8 independent variables:
Ee’, qe’ , fe’ , qA’ , fA’ , Me+e-, qcm , qcm
fA’
fe’
Theoretical beam spin asymmetries
qcm=0 deg, fcm=0 degqcm=20 deg, fcm=70 degqcm=70 deg, fcm=170 degqcm=170 deg, fcm=250 deg
NOANTISYMMETRIZATION
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.)
qcm=0 deg, fcm=0 degqcm=20 deg, fcm=70 degqcm=70 deg, fcm=170 degqcm=170 deg, fcm=250 deg
WITHANTISYMMETRIZATION
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)
Theoretical beam spin asymmetries
fA’ (deg.)
What happens to the BSA when one integratesover the 8 kinematic variables ?
Complicated numerical problem:8 variables, structures and peaks,…
Monte-Carlo integration, ~stable results with 1011 events (around mA’ ; less if only background),24 hours with use of IN2P3 grid
qv (« vertical » angle)
fh (« horizontal » angle)
y
x
qe’ , fe’ , Me+e- , Ee+ , qv (e+), fh (e+), qv (e-), fh (e-),
In the following, we will use the 8 independent variables:
-.015 (rad)
-.05 (rad)
-.06 (rad)
.015 (rad)
.06 (rad)
.05 (rad)
MA’=50 MeV, e=10-2 G ~ 10 eV(a’/a=10-4)
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)
qv (« vertical » angle)
fh (« horizontal » angle)
y
x
qv (e+) [.015,.06]
A «bit more» (~3/1000) e+ on the left side than on the right side
Count rates/Statistics
• N(A’)~4.1011 (arb. units proportional to cross section)
• N(backgr)~1.5 1011 (arb. units proportional to cross section)
• DN(A’)~ 8.108 BSA ~ 8.108/4.1011 ~ 2/1000
If one takes the fh (e+) bin where BSA peaks (i.e. fh (e+) ~2 deg.):
backgr
A’
Count rates/Statistics
• N(A’)~4.1011 (arb. units proportional to cross section)
• N(backgr)~1.5 1011 (arb. units proportional 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 takes the fh (e+) bin where BSA peaks (i.e. fh (e+) ~2 deg.):
Count rates/Statistics
• N(A’)~4.1011 (arb. units proportional to cross section)
• N(backgr)~1.5 1011 (arb. units proportional 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 takes the fh (e+) bin where BSA peaks (i.e. fh (e+) ~2 deg.):
If one scales over a DMe+e=1 MeV bin: • N(A’)~4.1011 (arb. units proportional to cross section)
• N(backgr)~1.5 1011 X 103 (arb. units proportional to cross section)• DN(A’)~ 8.108 BSA ~ 8.108/1.5 1014 ~ 5/106
Count rates/Statistics
• N(A’)~4.1011 (arb. units proportional to cross section)
• N(backgr)~1.5 1011 (arb. units proportional 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 takes the fh (e+) bin where BSA peaks (i.e. fh (e+) ~2 deg.):
If one scales over a DMe+e=1 MeV bin: • N(A’)~4.1011 (arb. units proportional to cross section)
• N(backgr)~1.5 1011 X 103 (arb. units proportional to cross section)• DN(A’)~ 8.108 BSA ~ 8.108/1.5 1014 ~ 5/106
And we expect 107 events in a 1 MeV bin for HPS…
Doesn’t look too good…
But maybe still some hope:
Instead of integrating over all 8 variables andall HPS acceptance, identify a particular cornerof the phase space where BSA is of the orderof the percent and, if doing so, one goes from107 to 104 events… (one can always dream )
Stay tuned !