EXPERIMENTAL AND THEORETICAL
NUCLEAR PHYSICS
Professorial Faculty: | Research Fellows and Associates: | |
The research in progress in the Kellogg Radiation Laboratory is focussed on the experimental study of fundamental symmetries and testing the standard electroweak theory via precision measurements. 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 in our progress report (http://www.krl.caltech.edu/Projects/98progress.html).
The present experimental research of Professors Filippone, Hughes, and McKeown includes studies of parity violation in electron scattering, precision measurements of neutron beta decay, and a search for neutrino oscillations. These projects are all being developed over the next 2-3 years and represent excellent opportunities for graduate students entering this year.
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 sin2w 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 2000 and 2001. The Caltech group is responsible for the liquid hydrogen target and has one of the leadership roles on 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.
A schematic diagram indicating the configuration of the experimental equipment is shown in the accompanying figure. The experiment employs a 200 MeV polarized electron beam incident on a 40 cm long cryogenic liquid hydrogen target. The scattered electrons were detected in a large solid angle (approximately 1.5 steradian) air Cerenkov detector system consisting of 10 mirrors and 10 large photomultipier tubes. The reliable determination of the small parity-violating asymmetry in the scattering cross section required great care in eliminating sources of systematic error associated with reversal of the beam helicity. The precision of the measurement will improve with additional planned running this summer. 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 2001.
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.
Ultra-cold Neutron Research
We have begun a new program to perform fundamental experiments using ultra-cold neutrons. New techniques are being developed to produce an intense flux 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.
Professors Filippone and McKeown are involved in a new experiment to study the parity violating asymmetry in the beta decay of polarized neutrons with unprecedented precision. The goal is to test the unitarity of the Kobayashi-Maskawa matrix and search for new right-handed weak interaction effects. This will require improving upon present measurements by nearly an order of magnitude. We expect to be able to achieve this precision using the ultra-cold neutrons from LANSE in a solenoidal spectrometer coupled with novel detection schemes for the emitted electrons that are being developing in the Kellogg Lab. A schematic diagram of the experiment is shown to the right. It is expected that this experiment will take data beginning in the year 2000.
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.
Neutrino Oscillation Search
Professor McKeown is a member of a new collaboration to perform neutrino oscillation research in Japan. This project, called KamLAND, involves filling the old Kamioka detector with scintillator to observe neutrinos from power reactors ~200km away. This measurement represents a terrestrial experimental test of the idea that the reduced rate of solar neutrinos is due to oscillation effects. One can find additional information 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 2001.
The theoretical program is under the guidance of Professor S. Koonin and Senior Research Fellow B. van Kolck; a number of graduate students and postdoctoral researchers are involved. Some theoretical work is related to the experimental effort, while other studies deal with problems in nuclear physics, astrophysics, and many-body physics. The overall theme is the fundamental and phenemenological treatment of diverse quantum many-body systems in the laboratory and in astrophysical situations.
How the dynamics of QCD gives rise to strong-coupled, many-body bound states (nuclei) is the least understood aspect of the standard electroweak/strong theory. Drs. Seki and van Kolck are involved in efforts to relate the symmetry properties of the theory to the basic structure of, and forces between, protons and neutrons. This is done in the framework of Effective Field Theories, which explore ideas of the renormalization group to build a systematic treatment of systems that lack a conventional perturbative expansion. While this framework is widely used in other areas of physics, it is for the first time being developed in a non-perturbative context. One recovers not only features of non-relativistic quantum mechanics, but also novel dynamics that hint at a consistent theory of nuclear matter. This theory has had its first successes when confronted with experiment, but its full formulation remains a task for the next few years.
Drs. Seki and van Kolck are involved in efforts to relate the symmetry properties of QCD to the basic forces between protons and neutrons.
The theoretical work is an integral part of the research in nuclear physics and nuclear astrophysics in the W.K. Kellogg Radiation Laboratory and enjoys a mutually beneficial relationship with the group's experimental activities.
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