SAMPLE is a continuing project to study the strange magnetism and the anapole moment of the proton using parity violating electron scattering. The experiment is performed at the MIT/Bates Linear Accelerator Center.
From our previously acquired data, we have determined a value of the strange magnetic moment of the proton
During this funding period, we have developed the capability for an additional data run at a reduced incident beam energy of 120 MeV using our liquid deuterium target. This new experiment will allow further study of the anomalous axial vector coupling that we observed in our previous measurements. The new measurement will improve the statistical precision, add confidence that systematic errors are under control by radically changing the running conditions (such as signal to background), and test that the $Q^2$ dependence of the parity-violating asymmetry is consistent with quasielastic scattering. During May 2001 we performed a test run at the Bates accelerator at 120 MeV. We studied the signal and background levels and found them to be in accord with our previous estimates. Beam quality (such as halo and stability) was excellent, very similar to our previous experience at 200 MeV. The DOE has provided supplemental funds to Bates for this experiment, and a production data run will likely be scheduled for fall 2001.
The SAMPLE experiment continues to provide outstanding opportunities for training and development of young physicists. The proposal for the new low energy version of the experiment was written by a postdoctoral scholar in our group (T. Ito), and this affords him the opportunity (as spokesman for the experiment) to gain invaluable leadership experience and establish his reputation on a very visible project at a very early stage in his career. In addition, a graduate student from Orsay, France will utilize this experiment for his Ph.D. thesis research. We typically run the SAMPLE experiment with the assistance of a substantial number of undergraduate students. These opportunities provide excellent research experience for young scientists and also generate a skilled pool of trained physicists for the extensive programs of parity-violating electron scattering at other facilities (such as the G0 experiment at Jefferson Lab and the E158 project at SLAC).
2.B JLAB 99-118: A-dependence of $R=\sigma_L/\sigma_T$
Jefferson Lab experiment E99-118 was undertaken to confirm the large nuclear effect observed in HERMES experiment for unpolarized inclusive lepton scattering from deuterium and heavy nuclei. Inclusive electron scattering can be described by two virtual photon cross sections representing the longitudinal and transverse polarizations of the virtual photon. The new HERMES data suggests that the ratio of these cross sections - $R=\sigma_L/\sigma_T$ - is much larger for a heavy nucleus ($^{14}$N was measured at HERMES) than for a proton or deuteron at low values of $x\sim 0.01$ and momentum transfer $Q^2 < 1$ (GeV/c)$^2$.
Caltech took part in the data acquisition for the experiment and in the estimation of the key background process whereby electrons are produced from $\pi^0$ production when the decay photons convert to $e^+e^-$ pairs. This background is studied by measuring the cross section for $e^+$ production with reversed magnetic fields in the spectrometers. Publication of the results from this experiment are expected later this year.
2.C HERMES
The HERMES experiment was designed to provide high precision data on the nucleon spin structure using polarized electron and positron beams (from the DESY-HERA accelerator) incident on polarized targets. Kellogg has been involved in HERMES since its inception, but has recently completed their direct involvement in the experiment with the completion of the 2000 running period, however we remain a collaborator on HERMES publications that result from data taken during 2000 and earlier.
2.D Measurement of the Neutron's Beta-Decay Asymmetry using Ultra-Cold Neutrons
Our collaboration to measure the free neutron beta-decay correlation $A$ at the LANSCE facility at Los Alamos was recently given funding approval by DOE and NSF. This measurement is designed to improve the precision in the measurement of $A$ by a factor of five. This correlation relates the neutron spin direction with the momentum of the beta-decay electrons. Combined with the value of the neutron lifetime, the $A$-correlation measurement allows an extraction of the CKM matrix element $V_{ud}$ with considerably reduced statistical and systematic uncertainties compared to existing measurements that rely on studies of $0^+-0^+$ nuclear decays. Measurements with the neutron are free from corrections for nuclear effects and radiative effects due to the large $Z$ of the nucleus. Such a measurement can provide sensitive tests of the standard electroweak model, as well as providing key input in interpreting heavy quark couplings. In fact recent theoretical work in Kellogg has suggested that the unitarity of the CKM matrix may appear to be violated if certain radiative effects due to minimal supersymmetry are present. Our present schedule for this project would allow us to take commissioning beam in 11/02 and to begin full data acquisition in 6/03 after the installation of a new beam kicker magnet by LANSCE. This data will form the thesis of graduate student Junhua Yuan.
We are proceeding with construction of the UCN source based upon a solid deuterium moderator that our collaboration has pioneered. Our prototype source holds the world record for UCN density ($\sim 100$ UCN/cm$^3$). Our final source is expected to have a stored density $> 300$ UCN/cm$^3$.
Caltech is responsible for the superconducting magnetic spectrometer and the design, construction and calibration of the electron detectors. We have recently been awarded a Major Research Instrumentation (MRI) award from NSF for this project (525k\$ from the NSF and 225k\$ in matching funds from Caltech).
In collaboration with American Magnetics Inc. we have designed the superconducting spectrometer that provides a field uniformity of $< 5 \times 10^{-4}$ in the neutron decay region with a 4 cm port through the side of the solenoid. We are presently preparing a purchase order to begin construction of the spectrometer.
We are also developing several detector systems for the experiment. We have built a Multi-Wire Proportional Chamber to measure the fiducial volume of the neutron decay region as well as several options for a scintillator detection system for measuring the total energies of the decay electrons. In addition we are developing a Multi-strip Si detector that will provide much improved energy resolution but with larger effects due to backscattering of incident electrons.
2.E G0
The G0 project will provide a thorough and definitive study of the contribution of strange quarks to the charge and magnetization distributions of the proton. It involves a large international collaboration and will take place at Jefferson Laboratory. Construction of the experimental apparatus is nearly complete, and installation in the experimental hall is scheduled to begin in February 2002.
The primary responsibility of our research group is to construct the cryogenic liquid hydrogen target. Substantial progress has been made in this grant period towards the completion of the target system. Fabrication of the cryoloop and cells was completed and the system has been assembled and tested. The LH pump was tested extensively at $77^\circ$K and ran reliably for more than 200 hours. The gas panel that supplies the hydrogen and helium gas to the cryoloop has been assembled and fully leak tested. The target support and service module system was constructed under sub-contract by Thermionics Northwest and delivered to Jefferson Lab in January 2001. The complete target system is assembled, leak tested, and ready for cooldown at the test lab at Jlab. Cold tests with He gas in the cryoloop will take place during the summer of 2001.
We have conducted a thorough study of pionic backgrounds in the experiment, and although it appears that these backgrounds are manageable in backward angle running of the experiment with a hydrogen target, running with deuterium will certainly require implementation of a pion identification system. After examination of several alternatives, we have proposed addition of an aerogel Cerenkov counter to facilitate particle identification for backward angle running. Such a system would have sufficient pion rejection capability ($> 100:1$) to enable acceptable running of both hydrogen and deuterium at backward angles. The G0 collaboration has adopted this proposal and and we have built a small prototype in our lab for tests. These tests include studies of reflecting material for the interior walls, magnetic shielding for the photomultipliers, and measurements of light yield and time response. We are presently constructing a full scale octant prototype for furhter tests in our lab this summer. Our G0 collaborators from TRIUMF have secured funding for 4 octants based on our design, and our French collaborators are considering providing the remaining 4 octants.
We are presently studying the possibility of measuring the parity violating asymmetry in $\Delta$ photoproduction during the G0 backward angle running. This asymmetry is related to the parity violating asymmetries measured in strangeness changing hyperon decays, which are anomalously large.
The G0 experiment is the Ph.D. research project of one of our graduate students (S. Covrig) and also provides valuable research experience for several postdoctoral scholars in our group. In particular, J. Martin (postdoctoral scholar) and L. Hannelius (graduate student) have benefited from the valuable opportunity to develop the aerogel Cerenkov detector concept.
2.F SLAC E158
The goal of the E158 experiment at SLAC is a precision measurement of the weak mixing angle via parity-violating M\o ller scattering. This will provide an stringent and important test of the standard electroweak theory. Our group had the responsibility to construct the liquid hydrogen target for this experiment. Following successful off-line tests, the target was installed in End Station A in November/December 2000. The target was tested with liquid hydrogen in January 2001. These tests indicated that the pump motor was not powerful enough to circulate the LH at the desired flow rate, and the pump exhibited occasional instability that led to target boiloff. A larger (2 hp) motor was installed, and some changes to the valve configuration were implemented (to avoid triggering the rupture disk during boiloff) after these tests. The target performed successfully with beam during the commissioning run in April-May 2001.
The E158 commissioning run was quite successful, with major progress in the operation and understanding all subsystems. The level of background is quite sensitive to beam tune, and it is suspected that the collimation system for the spectrometer needs some improvements. The data from the commissioning run are under study and discussion; modifications to the apparatus are being considered before the next run in fall 2002.
This project has provided excellent training for a Caltech undergraduate (A. Scott, who wrote a senior thesis on the target performance) as well as postdoctoral scholars J. Gao and K. Gustafsson. In particular, J. Gao developed extensive and valuable experience in the implementation of the control system using the Labview software environment. K. Gustafsson gained important experience in commissioning the LH target system, which will be very beneficial for the G0 project at Jlab.
2.G CHICOS
The California HIgh school Cosmic ray ObServatory is a collaborative project with California State University at Northridge (CSUN), University of California at Irvine, and Los Angeles area high schools to field a large array of cosmic ray detectors in the high schools. These detectors will enable study of ultra-high energy ($> 10^{18}$ eV) cosmic ray air showers and offer high school students and teachers a unique opportunity to collaborate in research with world-class universities.
We have developed the basic initial hardware and software configuration for the detector stations. The data is to be acquired at each high school using a PC workstation running a Labview executable. Pulses from the detector are processed using a NI-6602 timer counter, and time stamping of the events is facilitated by a GPS receiver. Pulse heights are derived using a time-over-threshold circuit. Data from the various high school sites will be transferred via internet to a central computer at Caltech where analysis will be done to locate and reconstruct the air showers. Outstanding progress has been made on all these components and we have demonstrated that a complete system works in our laboratory. The timing of the events has been verified to be accurate to about 12 nsec, which is sufficient for very good pointing accuracy ($<1^\circ$).
In addition, we have taken possession of over 150 1 m${}^2$ scintillation counters (including weatherproof enclosures) from the decommissioned CYGNUS experiment at Los Alamos National Lab. These detectors are ideal for our purposes. We have also received about 200 photomultiplier tubes with bases from the decommissioned Palo Verde Neutrino Oscillation Experiment and have begun installing some in the CYGNUS detectors.
We anticipate fielding a few (8-10) sites in high schools this summer. A course in the relevant cosmic ray physics and instrumentation is to be offered at Cal. State Northridge by a Kellogg Visiting Associate, R. Seki as part of the training program for the high school teachers. In addition, we have established an electronic logbook for use in communication, archiving results, and database management for the whole collaboration.
This project has afforded an excellent research training opportunity for a group of students from Caltech, CSUN, and Hong Kong University. They participated in all the software/hardware development and brought the project to its present state of readiness. We also successfully engaged about a dozen high school teachers and some high school students. The high school teachers have been developing relevant curriculum materials for use in the high school classroom during the coming year, and some are participating in equipment construction in our lab this summer. This project offers great potential as an outreach program to engage a large number of high schools in a real research project with significant technical infrastructure. CHICOS provides a novel and unique mechanism to identify promising scientists from diverse backgrounds in Los Angeles, to motivate these students to study science, and to provide pipelines to enable them to pursue higher education in science at world-class universities such as Caltech.
2.H Theory
2.H.1 Parity-Violation in electron scattering: A collaboration involving visiting associates M. Ramsey-Musolf and S. Zhu, along with visiting graduate student S. Puglia and B. Holstein (Umass) has completed an analysis of parity-violating (PV) electropion production near threshold using chiral perturbation theory (CHPT). This project has resulted in one publication, now in press at Phys. Rev. C. The results indicate that subleading chiral contributions could be as important as the leading-order effect, which is dominated by the PV pion-nucleon coupling. The process is sensitive to a new low-energy effective interaction which may contribute to other low-energy hadronic PV observables.
2.H.2 Resonant PV electro- and photoproduction of pions: A collaboration involving M. Ramsey-Musolf, S. Zhu, B. Holstein, post-doctoral fellow C. Maekawa, and visiting student G. Sacco have completed two studies of resonant PV pion production using real and virtual photons. The results indicate that the resonant PV eletroproduction asymmetry does not vanish at the photon point due to a new PV electric dipole nucleon to delta transition interaction not previously measured. Its determination could shed new insight into long standing, unsolved puzzles in strangeness changing nonleptonic decays, where the analogous E1 transitions are a factor of five times larger than expected on general grounds. For both real and virtual photons, the PV resonant pion production asymmetry could be of the order of a few ppm, making them large enough to be measured in an experiment at Jefferson Lab. Two publications have been submitted based on this work.
2.H.3 Supersymmetry in neutron beta-decay: M. Ramsey-Musolf and visiting student A. Kurylov have a completed a computation of the one-loop electroweak radiative corrections to charged current observables in the context of the minimal supersymmetric standard model (MSSM). Preliminary results indicate these corrections could modify the value of the asymmetry parameter measured in neutron beta-decay at an observable level. Consequently, a measurement of this parameter could add new constraints to the MSSM parameter space, complementing direct limits from collider experiments. A publication with these results is in preparation.
2.H.4 Hadronic PV: M. Ramsey-Musolf, S. Zhu, C. Maekawa, S. Puglia, in collaboration with B. Holstein (Umass) and U. van Kolck (Arizona) have undertaken a reanalysis of nuclear and hadronic PV in the context of effective field theory (EFT). Preliminary results indicate the EFT framework introduces several qualitatively new features not present in the conventional meson-exchange framework. A publication with these results is in preparation.
2.H.5 Three-pion interferometry in relativistic heavy-ion collisions: R. Seki and H. Nakamura have completed an anlysis of the recent two- and three-pion interferometry data from CERN, combined with the existing pion multiplicity information. Coherent pions are found to be generated mostly in the central events rather than in the peripheral events, about one coherent source being created in each central event and emitting about half of the total pions. More data, such as those from the RHIC, could verify the finding. A publication has been submitted based on this work.
2.A SAMPLE
Based on our recent test run, we have determined that it is feasible and very attractive to pursue measurements with the SAMPLE apparatus at lower energy (120 MeV). Promising developments in using strained GaAs for higher polarization electron beams at Bates offer an option to further improve the quality of this experiment.
2.B JLAB 99-118: A-dependence of $R=\sigma_L/\sigma_T$
The E99-118 experiment at JLAB was performed in summer 2000 and data are presently being analyzed. A preliminary result for the ratio of cross sections for $^{12}$C:D was presented at the spring 2000 meeting of APS and is shown in Fig. 2.B.1. While naively the effect appears to be reduced at JLAB, final corrections for kinematic effects and radiative corrections are not yet completed.
\midinsert \centerline{\psfig{figure=nucr_plot.eps,height=2.75in,angle=0}} \medskip \vbox{\narrower\narrower\baselineskip=12pt \noindent{\bf Fig. 2.B.1. Ratio of cross sections for nuclei compared to deuterium as a function of the Bjorken $x$ variable.}} \endinsert
2.C HERMES
A very interesting result from HERMES was recently submitted for publication, indicating clear evidence for deeply virtual compton scattering (DVCS). This process has been shown to be sensitive to a newly identified set of generalized parton distributions. The process is observed via an interference with a QED radiative effect where the incident or scattered lepton emits a photon. The interference is measured via a beam-spin asymmetry vs the azimuthal angle $\phi$. This asymmetry is shown in Fig. 2.C.1 where a missing mass cut has been applied to select events dominated by virtual Compton scattering from the proton.
\midinsert \centerline{\psfig{figure=hermes_dvcs.eps,height=3in,angle=0}} \vbox{\narrower\narrower\baselineskip=12pt \noindent{\bf Fig. 2.C.1. Azimuthal asymmetry for photon production from polarized positrons incident on a polarized hydrogen target. The dashed curve is a simple $\sin \phi$ fit, while the solid curve is a prediction based on generalized parton distributions.}} \endinsert
2.D Measurement of the Neutron's Beta-Decay Asymmetry using Ultra-Cold Neutrons
As part of our calibration system for the detectors we have completed construction of a 150 keV electron accelerator in the Pelletron Lab of Kellogg. This device allows us to measure the response of various detectors to monoenergetic electron beams and to measure other detector properties such as backscattering probability. The backscattering process is not well characterized at the energies of neutron decay (0 - 800 keV) so we have begun a series of measurements to allow us to calibrate a full Monte Carlo simulation of the detectors. An example of these measurements is shown in Fig. 2.D.1 where the energy and angular distribution of backscattered electrons is measured with a Si surface barrier detector. This work formed the senior thesis of a Caltech Undergraduate who is presently performing nuclear physics experiments at the 88 inch cyclotron at Berkeley as a graduate student at Vanderbuilt University. This data is being prepared for publication.
\midinsert \centerline{\psfig{figure=be.120eta.eps,height=3in,angle=0}} \medskip \vbox{\narrower\narrower\baselineskip=12pt \noindent{\bf Fig. 2.D.1. Energy distribution (scaled by the beam energy) of 126 keV electrons scattered from a thick Be target. The different curves correspond to different final electron scattering angles (the angle $\theta$ is measured with respect to the normal to the target).}} \endinsert
We also have access to an electron Dynamitron at JPL that provides electrons from 200 - 1000 keV energy. We have used our iron-free Helmholtz spectrometer to calibrate this accelerator at low energies (we have also calibrated our electron gun in Kellogg with this device). We will continue to perform backscattering measurements at JPL to characterize this process to higher energies.
2.E G0
Further studies of the capabilities of the G0 experiment are leading to proposals to pursue additional physics topics. We have considered the possibility of studying parity violation in threshold pion photo- and electroproduction and determined that, although there is considerable interest in these measurements, they are not really feasible at Jlab. More recently, we have been motivated by our local theorists to consider the possibility of measuring parity-violating photoproduction of the $\Delta$ resonance. This measurement appears quite feasible and looks promising as a parasitic measurement with the large angle elastic scattering measurements planned for G0. Further work is in progress to assess the potential of this type of measurement.
2.G CHICOS
We have studied the potential of the proposed CHICOS array to detect and characterize ultra-high energy cosmic rays. We have determined the efficiency for detection for the San Gabriel Valley sites as a function of the incident cosmic ray energy. The threshold for detection of air showers with this array is about $10^{18}$eV. We would detect over 100 events per year, and about 2 per year above $10^{20}$eV with the proposed array (80 sites using the existing CYGNUS detectors). This rate of detection is of great interest in this field.
2.H Theory
2.H.1 Parity-Violation in electron scattering: A collaboration involving visiting associates M. Ramsey-Musolf and S. Zhu, along with visiting graduate student S. Puglia and B. Holstein (Umass) has completed an analysis of parity-violating (PV) electropion production near threshold using chiral perturbation theory (CHPT). This project has resulted in one publication, now in press at Phys. Rev. C. The results indicate that subleading chiral contributions could be as important as the leading-order effect, which is dominated by the PV pion-nucleon coupling. The process is sensitive to a new low-energy effective interaction which may contribute to other low-energy hadronic PV observables.
2.H.2 Resonant PV electro- and photoproduction of pions: A collaboration involving M. Ramsey-Musolf, S. Zhu, B. Holstein, post-doctoral fellow C. Maekawa, and visiting student G. Sacco have completed two studies of resonant PV pion production using real and virtual photons. The results indicate that the resonant PV eletroproduction asymmetry does not vanish at the photon point due to a new PV electric dipole nucleon to delta transition interaction not previously measured. Its determination could shed new insight into long standing, unsolved puzzles in strangeness changing nonleptonic decays, where the analogous E1 transitions are a factor of five times larger than expected on general grounds. For both real and virtual photons, the PV resonant pion production asymmetry could be of the order of a few ppm, making them large enough to be measured in an experiment at Jefferson Lab. Two publications have been submitted based on this work.
2.H.3 Supersymmetry in neutron beta-decay: M. Ramsey-Musolf and visiting student A. Kurylov have a completed a computation of the one-loop electroweak radiative corrections to charged current observables in the context of the minimal supersymmetric standard model (MSSM). Preliminary results indicate these corrections could modify the value of the asymmetry parameter measured in neutron beta-decay at an observable level. Consequently, a measurement of this parameter could add new constraints to the MSSM parameter space, complementing direct limits from collider experiments. A publicaton with these results is in preparation.
2.H.4 Hadronic PV: M. Ramsey-Musolf, S. Zhu, C. Maekawa, S. Puglia, in collaboration with B. Holstein (Umass) and U. van Kolck (Arizona) have undertaken a reanalysis of nuclear and hadronic PV in the context of effective field theory (EFT). Preliminary results indicate the EFT framework introduces several qualitatively new features not present in the conventional meson-exchange framework. A publication with these results is in preparation.
2.H.5 Three-pion interferometry in relativistic heavy-ion collisions: R. Seki and H. Nakamura have completed an anlysis of the recent two- and three-pion interferometry data from CERN, combined with the existing pion multiplicity information. Coherent pions are found to be generated mostly in the central events rather than in the peripheral events, about one coherent source being created in each central event and emitting about half of the total pions. More data, such as those from the RHIC, could verify the finding. A publication has been submitted based on this work.