- Email: [email protected]
Nuclear physics at SLAC
Nuclear Physics A446 (1985) 463~466~ North-HoEand, Amsterdam
NUCLEAR
PHYSICS AT SLAC
Peter BOSTED American University c/o SLAC, P.O. Box 4349, Bin 43, CA 94305, USA
A short overview of past. present, and possible future nuclear physics experiments at SLAC is presented. Until recently. the facilities for doing electron scattering experiments at SLAC consisted of an End Station with three spectrometers of maximum momentum 1.6. 8.0. and 20.0 GeV. and a beam of maximum energy 20 GeV. with a 1.6 psec long pulse with peak current of 40 ma and a repetition rate of less than 180 pps. With this poor duty cycle. only exclusive channel coincidenceexperiments were feasible, but with the high average current at high beam energies. very small cross sections could be measured. One of the first nuclear physics experiments to be done was to extend the measurement of the forward angle elastic form factor of the deuteron A(Q2) to momentum transfers Q2as high as 6 (GeV/cj2
’.
At these small distances the quark degrees of freedom become important, and the
results were examined both from the point of view of perturbative QCD and traditional nuclear physics. The inelastic scattering at forward angles was also measured and was observed to obey scaling laws 2
at sufficiently high Q2
.
The next nuclear experiments measured the forward angle elastic and inelastic cross sections from 3.2 ‘He and ‘He . Again, the results were reviewed with a great deal of interest. While not strictly speaking a nuclear physics experiment. the extraction of the neutron magnetic form factor from quasielastic scattering from deuterium up to Q2--10 (GeV/c)2 also provided results which were interpreted 4.2 in the y scaling picture - Following this was an experiment to measure the magnetic form factor of the proton up to Q2=32 (GeV/c)2 with relatively small error bars5.
This purely high energy physics
experiment was divided into two runs spaced a year apart. with a shorter experiment to measure the ratio of deep inelastic scattering cross sections from various nuclei (the ‘EMC effect’) taking place in 6 the interim - The high beam current at SLAC enabled the experimenters to cover a wide range in x and Q2 and established a solid set of data to which theorists could make detailed comparisons. This vein of experimentation will be continued this fall. when we will measure R.the ratio of longitudinal to transverse scattering in the deep inelastic region. for a variety of nuclei, with particular emphasis on deuterium and hydrogen. Many experiments in nuclear physics require a high current electron beam of energy greater than 800 MeV. At the present time. SLAC is the only place that has such a beam. but to run a 1 GeV beam
0375-9474/85/$03.30 OElsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
464c
P. Bosted /Nuclear physics at SLAC
down a 20 GeV accelerator
is very expensive.
so that in the late 1970’s
it was proposed
to build an
new injector 4/5 of the way down the machine that could provide beams up to 3.5 GeV at a relatively low cost. It would also have the advantage
of providing much higher current below 2 GeV than the full
linac could. After several tries. this proposal was finally approved and the Nuclear Physics Injector was completed
in November
1984. A separate budget was approved for experiments
and the first experiment scattering attempt
took place in January
of electrons
1985.
It consisted
to use the new injector,
of measuring
the threshold
from a variety of nuclei using the 8 GeV spectrometer.
to explain the EMC effect make interesting
predictions
inelastic
Calculations
about the ratio of electron
which
scattering
cross sections from various nuclei in this little explored high x region7. The next experiment of the magnetic
to use the NPI is underway
form factor of the deuteron
at the time of this conference.
uses the NPI to full advantage.
possible beam currents at energies from 800 to 2000 MeV. Since B(Q2) is almost
impossible
headed by American University
University
momentum
spectrometer.
of 700 MeV.
and a 40 cm long liquid target
events by capturing msr acceptance.
Rosenbluth
the recoil deuteron.
The detectors
It is a QQQDD
Therefore
of Massachusetts. system
spectrometer
This spectrometer
is a QQQDDDD
hut consist of a threshold
system
The deuteron
hut has several planes of scintillators
but also to separate deuterons
with about 2.5
gas Cherenkov
define the electron time.
time-of-flight,
is only 2.2
is also needed to tag the elastic
glass shower counter to also separate pions and electrons.
using time-of-flight
and Tel Aviv
with a solid angle of 20
veto pions. a segmented
electron-deuteron
it
the collaboration.
Since the binding energy of the deuteron
is used, a 0’
in the electron
is much smaller than A(Q2).
separation.
with help from SLAC. The University
decided to build a 180”
msr and a maximum MeV.
to measure with a standard
The measurement
as it requires the highest
counter
to
and scintillators
to
to not only measure the
from the much more copious protons
within the hut. Both huts contain several planes of proportional
wire chambers
to
define the tracks. To date this experiment (GeV/c)2.
and will continue
to find out whether approximation.
has made measurements
of B(Q2)
up to as high a momentum
the form factor has a minimum
at Q2 = 1.2. 1.5. 1.75. 2.0. and 2.5
transfer as possible.
around Q2=2
(GeV/c2)
as predicted
in the impulse
or whether this feature is filled in by meson exchange currents and isobar contributions.
Inelastic spectra extending from threshold to the quasi-elastic to measure Wt(u,Q’)
peak will also be taken at several settings
at the lowest possible values of the scaling variable w’.
I would now like to address the question of what future experiments Due to the high cost involved. so experiments
Of special interest will be
there will probably
of running time available
at SUK.
be very little running with the high energy beam.
will have to use the NPI with a present
current of 80 ma. and a pulse width of 1.6 psec.
could be performed
maximum
energy of 3.5 GeV. a maximum
There will probably be between one and three months
each year at the full repetition
rate of 180 pps. with another
month or so
of checkout running at 10 pps avaibbfe. When the new SK
klystrons are instaRed. the energy will
increase to 5 or 6 GeV and the pulse width will be widened by perhaps a factor of two. One of the most obvious experiments that could be dnne is to measure the backward elastic and inelastic scattering of electrons from 3He. This might
well
use
La
existing 180” spectrometer with
minor modifications. Measuring the form factors of tritium is also quite interesting. but obtaining approval for a target may prove difficult. An experiment that has already been approved to run this January wig measure the width and position of the A(1236)
for various nuclei at higher Q2 than can be
done elsewhere. Other experiments may concentrate on transv~~ion~tud~na~
separations in the A
region. Sing&earm inelastic scatter&g torrid also be used to study the deuteron ~t~djsint~at~on
just
above threshold using the I.6 GeV spectrometer. This was not possible using the 180” spectrometer due to the large contribution from the target endcaps. The photodisintegration of the deuteron is also of great interest, where our present knowledge could be oxtendod to photon energies as high as 2 GaV over a wide angular range using the 1.6 GeV spectrometer. Other photoproduction experiments could include kaon production from a variety of nuclei to study hypemuclear states that are hard to observe with simple kaon scattering. Beyond these relatively straightforward experiments comes the possibility of installing a potarired source at the NPI. This could make possibfe the separation of the charge and quadropoie elastic form factors of the deuteron. A pofarimeter woutd need to be made to measure the~oIa~xati~ recoil dwterons.
of tfte
An even more difficult experiment would be the measurement of the neutron form
factor using quasi-elastic scattering from deuterium and a neutron polarimeter. Detailed studies would be needed to see if such an experiment could be done with the poor duty cycle of the NPI. There is also the possibility nf using gas jet targets in one of the two stnrege rings at SLAC. A wealth nf information about polariration form factors of the proton and deutoron could possibly be obtained using a combination of facilities available nowhere else in the world, but much study is needed to figure out the best approach, ard a fairly large group of physicists would need to be assembfed to perform these very difficult experiments. There is certainty a bright future ahead for nuctear physics and reiated high energy physics experiments at SLAC.
REFERENCES 1) 2) 3) 4) 5) 6) 7)
R.G. Arnold et al., Phys. Rev. Lett. 35 (1976) 776. P. Basted et al., Phys. Rev. Lett. 49 (1982) 1380. R.G. Arnold et al., Phys. Rev. Lett. 40 (19771 429. S. Rock et al., Phys. Rev. Lett. 49 (1980) 1139. American University et al., analysis in progress. R.G. Arnold et al., Phys, Rev. Lett. 52 (1984) 727. University of Virginia et al., analysis in progress.
NUCLEAR
PHYSICS AT SLAC
Peter BOSTED American University c/o SLAC, P.O. Box 4349, Bin 43, CA 94305, USA
A short overview of past. present, and possible future nuclear physics experiments at SLAC is presented. Until recently. the facilities for doing electron scattering experiments at SLAC consisted of an End Station with three spectrometers of maximum momentum 1.6. 8.0. and 20.0 GeV. and a beam of maximum energy 20 GeV. with a 1.6 psec long pulse with peak current of 40 ma and a repetition rate of less than 180 pps. With this poor duty cycle. only exclusive channel coincidenceexperiments were feasible, but with the high average current at high beam energies. very small cross sections could be measured. One of the first nuclear physics experiments to be done was to extend the measurement of the forward angle elastic form factor of the deuteron A(Q2) to momentum transfers Q2as high as 6 (GeV/cj2
’.
At these small distances the quark degrees of freedom become important, and the
results were examined both from the point of view of perturbative QCD and traditional nuclear physics. The inelastic scattering at forward angles was also measured and was observed to obey scaling laws 2
at sufficiently high Q2
.
The next nuclear experiments measured the forward angle elastic and inelastic cross sections from 3.2 ‘He and ‘He . Again, the results were reviewed with a great deal of interest. While not strictly speaking a nuclear physics experiment. the extraction of the neutron magnetic form factor from quasielastic scattering from deuterium up to Q2--10 (GeV/c)2 also provided results which were interpreted 4.2 in the y scaling picture - Following this was an experiment to measure the magnetic form factor of the proton up to Q2=32 (GeV/c)2 with relatively small error bars5.
This purely high energy physics
experiment was divided into two runs spaced a year apart. with a shorter experiment to measure the ratio of deep inelastic scattering cross sections from various nuclei (the ‘EMC effect’) taking place in 6 the interim - The high beam current at SLAC enabled the experimenters to cover a wide range in x and Q2 and established a solid set of data to which theorists could make detailed comparisons. This vein of experimentation will be continued this fall. when we will measure R.the ratio of longitudinal to transverse scattering in the deep inelastic region. for a variety of nuclei, with particular emphasis on deuterium and hydrogen. Many experiments in nuclear physics require a high current electron beam of energy greater than 800 MeV. At the present time. SLAC is the only place that has such a beam. but to run a 1 GeV beam
0375-9474/85/$03.30 OElsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
464c
P. Bosted /Nuclear physics at SLAC
down a 20 GeV accelerator
is very expensive.
so that in the late 1970’s
it was proposed
to build an
new injector 4/5 of the way down the machine that could provide beams up to 3.5 GeV at a relatively low cost. It would also have the advantage
of providing much higher current below 2 GeV than the full
linac could. After several tries. this proposal was finally approved and the Nuclear Physics Injector was completed
in November
1984. A separate budget was approved for experiments
and the first experiment scattering attempt
took place in January
of electrons
1985.
It consisted
to use the new injector,
of measuring
the threshold
from a variety of nuclei using the 8 GeV spectrometer.
to explain the EMC effect make interesting
predictions
inelastic
Calculations
about the ratio of electron
which
scattering
cross sections from various nuclei in this little explored high x region7. The next experiment of the magnetic
to use the NPI is underway
form factor of the deuteron
at the time of this conference.
uses the NPI to full advantage.
possible beam currents at energies from 800 to 2000 MeV. Since B(Q2) is almost
impossible
headed by American University
University
momentum
spectrometer.
of 700 MeV.
and a 40 cm long liquid target
events by capturing msr acceptance.
Rosenbluth
the recoil deuteron.
The detectors
It is a QQQDD
Therefore
of Massachusetts. system
spectrometer
This spectrometer
is a QQQDDDD
hut consist of a threshold
system
The deuteron
hut has several planes of scintillators
but also to separate deuterons
with about 2.5
gas Cherenkov
define the electron time.
time-of-flight,
is only 2.2
is also needed to tag the elastic
glass shower counter to also separate pions and electrons.
using time-of-flight
and Tel Aviv
with a solid angle of 20
veto pions. a segmented
electron-deuteron
it
the collaboration.
Since the binding energy of the deuteron
is used, a 0’
in the electron
is much smaller than A(Q2).
separation.
with help from SLAC. The University
decided to build a 180”
msr and a maximum MeV.
to measure with a standard
The measurement
as it requires the highest
counter
to
and scintillators
to
to not only measure the
from the much more copious protons
within the hut. Both huts contain several planes of proportional
wire chambers
to
define the tracks. To date this experiment (GeV/c)2.
and will continue
to find out whether approximation.
has made measurements
of B(Q2)
up to as high a momentum
the form factor has a minimum
at Q2 = 1.2. 1.5. 1.75. 2.0. and 2.5
transfer as possible.
around Q2=2
(GeV/c2)
as predicted
in the impulse
or whether this feature is filled in by meson exchange currents and isobar contributions.
Inelastic spectra extending from threshold to the quasi-elastic to measure Wt(u,Q’)
peak will also be taken at several settings
at the lowest possible values of the scaling variable w’.
I would now like to address the question of what future experiments Due to the high cost involved. so experiments
Of special interest will be
there will probably
of running time available
at SUK.
be very little running with the high energy beam.
will have to use the NPI with a present
current of 80 ma. and a pulse width of 1.6 psec.
could be performed
maximum
energy of 3.5 GeV. a maximum
There will probably be between one and three months
each year at the full repetition
rate of 180 pps. with another
month or so
of checkout running at 10 pps avaibbfe. When the new SK
klystrons are instaRed. the energy will
increase to 5 or 6 GeV and the pulse width will be widened by perhaps a factor of two. One of the most obvious experiments that could be dnne is to measure the backward elastic and inelastic scattering of electrons from 3He. This might
well
use
La
existing 180” spectrometer with
minor modifications. Measuring the form factors of tritium is also quite interesting. but obtaining approval for a target may prove difficult. An experiment that has already been approved to run this January wig measure the width and position of the A(1236)
for various nuclei at higher Q2 than can be
done elsewhere. Other experiments may concentrate on transv~~ion~tud~na~
separations in the A
region. Sing&earm inelastic scatter&g torrid also be used to study the deuteron ~t~djsint~at~on
just
above threshold using the I.6 GeV spectrometer. This was not possible using the 180” spectrometer due to the large contribution from the target endcaps. The photodisintegration of the deuteron is also of great interest, where our present knowledge could be oxtendod to photon energies as high as 2 GaV over a wide angular range using the 1.6 GeV spectrometer. Other photoproduction experiments could include kaon production from a variety of nuclei to study hypemuclear states that are hard to observe with simple kaon scattering. Beyond these relatively straightforward experiments comes the possibility of installing a potarired source at the NPI. This could make possibfe the separation of the charge and quadropoie elastic form factors of the deuteron. A pofarimeter woutd need to be made to measure the~oIa~xati~ recoil dwterons.
of tfte
An even more difficult experiment would be the measurement of the neutron form
factor using quasi-elastic scattering from deuterium and a neutron polarimeter. Detailed studies would be needed to see if such an experiment could be done with the poor duty cycle of the NPI. There is also the possibility nf using gas jet targets in one of the two stnrege rings at SLAC. A wealth nf information about polariration form factors of the proton and deutoron could possibly be obtained using a combination of facilities available nowhere else in the world, but much study is needed to figure out the best approach, ard a fairly large group of physicists would need to be assembfed to perform these very difficult experiments. There is certainty a bright future ahead for nuctear physics and reiated high energy physics experiments at SLAC.
REFERENCES 1) 2) 3) 4) 5) 6) 7)
R.G. Arnold et al., Phys. Rev. Lett. 35 (1976) 776. P. Basted et al., Phys. Rev. Lett. 49 (1982) 1380. R.G. Arnold et al., Phys. Rev. Lett. 40 (19771 429. S. Rock et al., Phys. Rev. Lett. 49 (1980) 1139. American University et al., analysis in progress. R.G. Arnold et al., Phys, Rev. Lett. 52 (1984) 727. University of Virginia et al., analysis in progress.