Hall D Internal Note
Photoproduction with Quasi-Real Photons
Richard Jones
July 15, 1998
At the recent CMU workshop we discussed briefly the possibility of doing
Eight+ physics with
low-Q
electron scattering in the place of a real photon beam. In my talk at CMU
I showed a comparison between the virtual and real photon beams in terms of
flux and polarization. There are several advantages of virtual over real
photons. The primary concern that was raised in our discussions was with
the additional backgrounds, particularly Moller scattering, that are present
with electron beams.
I have now done a study of these backgrounds, and have
come to the conclusion that a significant slice of recoil electron kinematics
exists in the far-forward region where backgrounds are modest and a large
fraction of the total hadronic photoproduction cross section can be tagged.
The forward region at high energy can be separated into three regions: very
low pT (electron mass scale) where bremsstrahlung dominates, modest pT
(diffractive scale ~300MeV), and low pT (in between the two at ~30MeV).
The angular distribution of recoil electrons from hadronic photoproduction
processes goes like
1/(pT)
whereas the cross section for background processes (bremsstrahlung, Mott
scattering, Moller scattering) is dropping like
1/(pT) .
What happens at higher CEBAF energies is that this crossover where background
drops below the signal rate gets pushed to small angles of order
1
where it becomes feasible to build a dedicated tagging spectrometer that
does not interfere too much with coverage of the hadronic detector.
CLAS has become interested in this possibility, where they would capture the
electrons which exit through the small hole in the forward region. The
study that I presented at the Hall B session at the recent JLab workshop
showed that they could tag approximately 15% of the total hadronic
photoproduction rate in the endpoint region just covering the region out
to 2
with 6GeV electrons. At 6GeV the backgrounds are modest in this angular
region. At 12GeV they are still better. It occurs to me that this would
be an excellent alternative to bremsstrahlung beams for Eight+. The
advantages for us, as I see them, are as follows.
-
In addition to reading out the energy of the virtual photon, the plane
polarization of the photon is also recorded by the tagger. All of the
photons in this virtual beam are polarized. The polarization is the same as
the theoretical maximum for coherent bremsstrahlung at extreme collimation,
i.e. plane polarization of all photons in the virtual beam is equal to the
polarization of the coherent component from crystal bremsstrahlung at its
maximum right at the edge of the peak. This means that effectively the plane
polarization can be an order of magnitude higher for virtual than for
coherent beams, if you average over the region from 80% to endpoint.
Note that this does NOT diminish the strength of our argument for 12GeV
beams. The polarization is still going to zero at the endpoint. But in
this case the average polarization one can actually achieve in the endpoint
region goes to zero linearly like (k-E) instead of quadradically like
(k-E)
.
-
Only the photons that actually produce hadronic final states in the target
produce counts in the tagger. Compare that with the design with real photons
where only 1/10000 hits in the tagger correspond to actual hadronic events in
the target. This effectively sets aside the present limitations from tagging
which required us to limit fluxes to 3e8/s, roughly comparable to production
rates at a hadronic beam facility.
-
Target can be as small. With a real tagged beam your target has to be
extended in order to get enough hadronic rate, because the photon flux is
limited by the tagging rate. With a virtual beam the scattered electron flux
in the tagger is proportional to the hadronic rate, so you can make the
target thin and compensate with beam current, with no change to the
accidentals situation.
Number 3 is particularly interesting from the point of view of TOF, fast
online track reconstruction, and general compactness of the barrel
components. It also makes building a target much simpler and makes a
polarized target thinkable.
I have included copies of the transparencies from my Hall B talk
(presented at the Workshop) of how this would work with CLAS.
In this study I compared backgrounds from the bremsstrahlung tail,
Mott scattering and Moller scattering in the target, and found that the
backgrounds are substantial at 4GeV, getting better at 6GeV, and really
looking very tame at 12GeV. In talking with others around here (Hall B), I
am beginning to suspect that some years ago people looked into this
possibility for experiments at lower energies and decided that the
backgrounds were too high, and just forgot about it. Things get better at
higher energies. I am suggesting that perhaps we are overlooking something
by assuming that we should carry on at 12GeV the same way that we did at 6GeV
because that was the best way to go at 6GeV. Clearly HERA has proved that
photoproduction physics can be done with high-energy electrons. What I am
finding out is that it seems to work quite well at 12GeV as well. Before we
invest anything in replicating the photon beamline from Hall B, why don't we
look into this possibility?
It also important implications for the larger detector design. In
particular the barrel tracking has to avoid the Moller cone. Things
are much better with a solenoid than with a field like CLAS where the
target sits at a null in the magnetic field, but it has to be looked
at. Beyond
10
the Mollers carry pT of 6MeV or less, so they will not get far from
the axis, but there are lots of them. At lower angles the transverse
momenta are somewhat higher but the intensities are lower. This would
probably be the limiting factor in how high a flux we could run at.
Experience with CLAS shows that they could run with electron beam
currents high enough to gain signficantly in photoproduction rate at
higher energies over bremsstrahlung tagging. With our solenoidal field,
things can only be better. One of the soft spots in our proposal that
has emerged so far has been the plausibility of tagging at 300MHz.
At this point it is too early to draw conclusions, but there are hints
that with electrons this limit can be overcome without stretching things.
Colour copies of these transparencies can be downloaded from the web
site at
http://zeus.phys.uconn.edu/hallB/virtual-6-98.
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