Irwin T. Lee

The Search for Stable, Massive, Elementary Particles

In this paper we review the experimental and observational searches for stable, massive, elementary particles other than the electron and proton. The particles may be neutral, may have unit charge or may have fractional charge. They may interact through the strong, electromagnetic, weak or gravitational forces or through some unknown force. The purpose of this review is to provide a guide for future searches — what is known, what is not known, and what appear to be the most fruitful areas for new searches. A variety of experimental and observational methods such as accelerator experiments, cosmic ray studies, searches for exotic particles in bulk matter and searches using astrophysical observations is included in this review.

Search for Free Fractional Electric Charge Elementary Particles Using an Automated Millikan Oil Drop Technique

A direct search was carried out in bulk matter for free fractional electric charge elementary particles using the largest mass single sample ever studied| about 17.4 mg of silicone oil. The search used an improved and highly automated Millikan oil drop technique. No evidence for fractional charge particles was found. The concentration of particles with fractional charge more than 0.16 e (e being the magnitude of the electron charge) from the nearest integer charge is less than 4.71 x 10{sup -22} particles per nucleon with 95% confidence.

Production of dry powder clots using a piezoelectric drop generator

We have demonstrated that piezoelectrically driven, squeeze mode, tubular reservoir liquid drop generation, originally developed as a “drop-on-demand” method for ejection of microdrops of liquids or suspensions, can successfully operate with dry powder. Spherical silverpowder with maximum particle diameter of 20 μm was loaded into and ejected from a 100 μm orifice glass dropper with a flat piezoelectric disk driver. Time of flight experiments were performed to optimize the dropper operation and to determine the size and velocity of the ejected particles. It was found that at certain values of the amplitude, duration, and repetition rate of the voltage pulses applied to the piezoelectric disk, one can eject powder clots of a stable size, comparable with the dropper orifice diameter. In contrast to the operation with a liquid, a clot is not ejected at each pulse, but quasiperiodically with an interval corresponding to thousands of pulses. The application for injection of atoms into helium buffer gas at cryogenic temperatures is discussed.

Automated electric charge measurements of fluid microdrops using the Millikan method

Automated measurements of the electric charge of fluid microdrops precise to up to 0.016 of an electron charge have been made using machine-vision systems to observe the motion of fluid microdrops in air under the influence of an oscillating electric field. The fluid drop diameters have ranged from 7 to 25 mu m with smaller diameter drops being measured to higher precision. The experimental runs performed for the purpose of attempting to find isolated fractionally electrically charged particles have measured the charges of tens of millions of fluid microdrops using piezoelectrically driven drop-on-demand inkjet-like droplet ejectors as fluid drop sources.

A new method for searching for free fractional charge particles in bulk matter

We present a new experimental method for searching for free fractional charge in bulk matter; this new method derives from the traditional Millikan liquid drop method but allows the use of much larger drops, 20–100 μm in diameter, compared to the traditional method that uses drops less than 15 μm in diameter. These larger drops provide the substantial advantage that it is then much easier to consistently generate drops containing liquid suspensions of powdered meteorites and other special minerals. These materials are of great importance in bulk searches for fractional charge particles that may have been produced in the early universe.

Sizing of Microdrops

Several techniques for determining the size of small fluid microdrops with diameters ranging from 5 to 30 μm have been developed and evaluated using an automated variation on the Millikan oil drop experiment. The average diameter of a large sample of monodisperse fluid drops was determined by measuring their terminal velocity in air, or if charged, their motion under the influence of an electric field, as well as by measurement of the magnitude of their Brownian motion. The diameter of individual drops was determined optically, by direct observation using an imaging system based on a charge-coupled-device camera. The technique used to analyze the image data is based on a best-fit technique taking the point spread function of the lens into account, and yields results accurate to 1% (based on a single image) without the need for any calibration. By combining this technique with terminal velocity measurements, the density of the fluid can be determined to similar accuracy.

The Search for fractional charge elementary particles and very massive particles in bulk matter

The authors describe their ongoing work on, and future plans for, searches in bulk matter for fractional charge elementary particles and very massive elementary particles. Their primary interest is in searching for such particles that may have been produced in the early universe and may be found in the more primeval matter available in the solar system: meteorites, material from the moons surface, and certain types of ancient terrestrial rocks. In the future the authors are interested in examining material brought back by sample return probes from asteroids. The authors will describe their experimental methods that are based on new modifications of the Millikan liquid drop technique and modern technology: micromachining, CCD cameras, and desktop computers. Extensions of the experimental methods and technology allow searches for very massive charged particles in primeval matter; particles with masses greater than 1,013 GeV. In the first such searches carried out on earth there will be uncertainties in the mass search range. Therefore the authors will also discuss the advantages of eventually carrying out such searches directly on an asteroid.

A device for precision neutralization of electric charge of small drops using ionized air

For use in our Millikan type liquid drop searches for fractional charge elementary particles we have developed a simple ionized air device for neutralizing a narrow stream of small drops. The neutralizer has been used for drops ranging in diameter from 10 to 25 μm. The width of the produced charge distribution is given by the Boltzmann equilibrium charge distribution and the mean of the distribution is set by a bias voltage. Using the bias voltage, the mean can be set with a precision of better than e, the electron charge. The use of the neutralizer is illustrated in an application to mineral oil drops produced with charges of the order of 1000e. We also show the interesting case of silicone oil drops that are produced in our drop generator with a charge distribution narrower than the Boltzmann equilibrium charge distribution, the charge distribution being broadened by the neutralizer.