The research in progress in the Kellogg Radiation Laboratory is focussed on the experimental study of fundamental symmetries, testing the standard electroweak theory via precision measurements and the study of ultra-high energy cosmic rays. There is also a broad program of theoretical studies in nuclear physics and low energy QCD. A brief description is provided below; more detailed information is available at our website http://www.krl.caltech.edu and in our proposal  (http://www.krl.caltech.edu/Projects/2000Proposal.html).

Experimental Research

KamLAND - Neutrino Oscillation Search
 

Professor McKeown is a member of a collaboration to perform neutrino oscillation research in Japan. This project, called KamLAND, involves filling the old Kamioka detector with 1000 tons of liquid scintillator to observe anti-neutrinos from power reactors ~200km away. The photo at right shows a view of the 1,922 photomultipliers which were installed in summer 2000. This measurement represents a terrestrial experimental test of the idea that the reduced rate of solar neutrinos is due to oscillation effects. In addition, KamLAND should be capable of detecting very low energy solar neutrinos. One can find additional information at http://citnp.caltech.edu/kamland/docs/pma2.ps or at the KamLAND website http://www.awa.tohoku.ac.jp/html/KamLAND/index.html. The experiment will be built over the next year and begin taking data in early 2002.
 

Ultra-cold Neutron Research
 
 

We have begun a new program to perform fundamental experiments using ultra-cold neutrons. We are collaborating on the development of new techniques to produce high densities of ultra-cold neutrons using the  0.8 GeV proton beam at the Los Alamos Neutron Science Center (LANSCE). Ultra-cold neutrons have energies of a few hundred neV and velocities < 8 m/s, such that they can be contained in bottles of suitable material for times comparable to the neutron lifetime (about 15 minutes). Substantial improvements in precision can be expected in fundamental measurements of neutron decay properties using these confined ultra-cold neutrons.

We are actively involved in a new experiment to measure the parity violating asymmetry in the beta decay of polarized neutrons with unprecedented precision. The goal is to measure the electroweak coupling  between up and down quarks in order to look for new physics beyond the standard model. Such high precision measurements allow testing of the  unitarity of the Kobayashi-Maskawa matrix as well as searching for  new right-handed weak interaction effects. This will require improving upon previous measurements by nearly an order of magnitude. We expect to be able to achieve this precision using the ultra-cold neutrons from LANSCE in a superconducting solenoidal spectrometer coupled with novel detection schemes
for the emitted electrons. Our group is responsible for developing the spectrometer and detectors for the experiment. Development of prototype  detectors (eg. Time-Projection Chambers and Silicon-Strip detectors) is underway using the 150 keV electron gun we have built in the Kellogg Lab and the 1 MeV electron accelerator at JPL. A schematic diagram of the experiment is shown above and more information can be found at http://www.krl.caltech.edu/ucn . It is expected that this experiment will begin taking data in 2003.

A longer term development effort that is just starting is to develop a new experiment to search for the electric dipole moment of the neutron. This T-violating quantity is predicted to be observable in many modern supersymmetric gauge theories.
 
 
 
 

Ultra-high Energy Cosmic Rays

We are presently establishing a new array of detectors located in LA area high schools to study ultra-high energy cosmic rays. The project is called CHICOS, or California HIgh school Cosmic ray ObServatory.

During the last decade, the community of high-energy cosmic ray physicists has constructed a number of very large (up to 100 sq. km area) arrays to study cosmic rays at the very highest energies ever detected (10²⁰ eV). These particles are over one hundred million (10⁸) times more energetic than those that can be produced in modern accelerator laboratories. The astrophysical origin of these particles is still unknown, but it has been generally assumed that they are protons. These particles produce huge ``showers'' of many secondary particles when they collide with atomic nuclei in the upper atmosphere; the most energetic of these showers can simultaneously trigger detectors over a several kilometer radius at the earth's surface. The rate of incidence at the highest energies (10²⁰ eV) is quite low: in a 100 square kilometer area one observes only about one event per year. Therefore, it is essential to sample a large surface area (hundreds of square kilometers) to observe a significant number of these ultra-high energy particles.

In the last several years, the largest previously built array (in Japan) has produced extremely interesting results(see article Science, vol. 281, August 14, 1998, page 893). It appears that these primary particles can be more energetic than previously thought, and so the assumption that they are protons is being seriously questioned. There is even speculation that one could identify ``point'' sources of these particles in the sky. These results have energized this field and there are several proposals to build even larger arrays over the next few years. The primary research goal is to collect more events at these ultra-high energies and characterize their energies and apparent direction of origin.

The Los Angeles basin is quite unique in that there is a very large area ( > 5000 km²) of uniformly dense population with available high school infrastructure. We have obtained 164 scintillation detectors from a decommissioned cosmic ray experiment in New Mexico, and are presently working to instrument these detectors in an array with area of more than 400 km². We presently plan to deploy this array in the next 3 years (2001-2003), and then it will be the largest of its type in the world.

Parity Violation in Electron Scattering

This program of experiments has two main thrusts. In SLAC E158, we will perform a precision measurement of parity violation in electron-electron scattering as a stringent test of the electroweak theory. In addition, we will use parity-violating electron scattering as a new probe of the the quark flavor structure of protons and neutrons.

SLAC Experiment E158

We are active collaborators in a new recently approved SLAC experiment ( E158 , see (http://www.slac.stanford.edu/exp/e158/) that will perform a precision measurement of the electroweak mixing angle, sin² _w. The experiment will search for parity violation in the scattering of high energy (50 GeV) polarized electrons off unpolarized electrons in a liquid hydrogen target. This pure e-e scattering process is extremely clean theoretically. The goal of the experiment is to measure with high precision sin² _w at an energy scale far from the Z-boson mass. The experiment is complementary to tests of the electroweak theory coming from LEP II at CERN and is sensitive to new physics such as would be given by new Z bosons or new contact interactions. The experiment is part of the SLAC fixed target program and is expected to run in the year 2002 and 2003. The Caltech group is responsible for the liquid hydrogen target and has a major role in the experiment.

Strangeness in the Nucleon

The magnetic moment of the proton, first measured in 1933 by Frisch and Stern, was the earliest experimental evidence for the internal structure of the nucleon. Although the theory of strong interactions, Quantum Chromodynamics (QCD), is over 20 years old, a quantitative description of the magnetic moments of the nucleons based on QCD remains an elusive goal. The phenomenal quantitative success of the standard electroweak theory now allows one to use the weak interaction to obtain additional information on the magnetic properties of the nucleon. In particular, the measurement of the strength of the magnetic interaction with the neutral weak boson Z₀ enables a decomposition of the nucleon magnetism into the contributions arising from the three relevant quark flavors.

The photo above shows Caltech graduate student Bryon Mueller working on the Cerenkov detector used in the SAMPLE experiment at the Bates Linear Accelerator Center, which measures for the first time the neutral weak magnetism of the proton and its contribution from the strange quark sea (http://www.npl.uiuc.edu/exp/sample/sampleMain.html). The experimental method involves the detection of the parity violation in the elastic scattering of longitudinally polarized electrons. The interference of weak (Z₀ exchange) and electromagnetic (photon exchange) amplitudes causes the cross section to depend on the helicity of the incident electron. This helicity dependence corresponds to a breakdown of parity symmetry and thus is a signal of the presence of the neutral weak interaction. The effect is quite small (~10^-6) due to the feeble strength of the weak interaction at low energies, and its measurement represents a formidable experimental challenge. We have determined the strange quark contribution to the magnetic moment of the proton, and have obtained evidence for an anomalously large anapole moment (parity violating electromagnetic interaction).

    Additional future experiments to further explore features of neutral weak currents and strange form factors of the nucleon are planned for Jefferson Lab (CEBAF). We are collaborators on what should be the definitive measurement of these effects, the G0 experiment (see http://www.npl.uiuc.edu/exp/G0/G0Main.html). This experiment will study the charge and magnetic properties of the strange quarks in the nucleon. We are building the liquid hydrogen target apparatus for the G0 experiment, and expect to begin taking data in 2002.

These experiments will open a promising new window on the quark structure of the nucleon, and hopefully will provide important information towards a more complete understanding of nucleon structure in the context of QCD.
 

Theoretical Research

Selected Publications

1. "Shell model Monte Carlo methods", S.E. Koonin, D.J. Dean, and K. Langanke Physics Reports 278, 1 (1997)
2. "Strangeness in the Nucleon", R.D. McKeown, ftp://ftp.krl.caltech.edu/preprints/OAP/OAP-745.ps
3. "Strange Magnetism and the Anapole Structure of the Proton", R. Hasty et al., Science 290, 2117 (2000).
4.  "Nuclear beta-decay, atomic parity violation, and new physics",  M.J. Ramsey-Musolf, Phys. Rev. D62, 056009 (2000).
5. "Nuclear anapole moment and parity-violating ep scattering", S.-L. Zhu et al, Phys. Rev. D62, 033008 (2000).

The Contents contains links to the other Physics departments.

More information may be found at the following WWW addresses:
PMA Home Page: http://www.pma.caltech.edu
Caltech Home Page: http://www.caltech.edu