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Where and how do you get your<br>electrons for your accelerator?

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How did you make the detectors<br>work in Halls A, B and C?

Why do you need high energy to look at or observe quarks? Since the accelerator at your facility is linear and can't always get as much energy as the circular motion accelerators, what is your particle accelerator used for? What other characteristics would you try to improve at your facility to understand new particle physics? Have you any recent studies to the topic of quark structure? I would also like to know any progress you have made in the field of particle physics and anything related to it.

Why do you need high energy to look at or observe quarks?

Imagine the electrons surrounding a nucleus as a cloud rather than in distinct orbits as they are typically shown. Of course these electrons are negatively charged so they have a negative field that would repel other negative things like other electrons. We need a certain energy to punch through the electron cloud. Beyond that it becomes an issue of resolution. When studying something by shining something on it, like light, the wavelength of that light has to be smaller than what you are studying. You may have heard of electron microscopes. They use electrons rather than light, because the wavelength of any light would be too large to study things as small as you want to study. Likewise with accelerators, increasing energy can be compared to making a shorter wavelength to study smaller things. Our accelerator, which operates at 4 billion electron volts (4 giga electron volt or GeV), has enough energy to penetrate an atom and sometimes tear a proton or neutron off. There are a few accelerators in the world that are much higher energy, but they have to operate at a much lower rate and with what could be described as a fuzzier picture.

Since the accelerator at your facility is linear and can't always get as much energy as the circular motion accelerators, what is your particle accelerator used for?

In a way all of the big accelerators in the world now are in fact linear accelerators. The part of the device where the particles accelerate is expensive to build and operate. They are typically only a small section of the loop. To economize here, as well as at all the other sites, we loop the particles around and send them through the accelerating section many times. The higher the energy of the beam of particles, the larger the radius that is needed to turn the particles around and return them through the accelerator section. Similarly, the higher a car's speed, the radius of its turns must also be larger. The two biggest accelerators in the world, LEP at CERN in Geneva, Switzerland and Fermilab in Batavia, Illinois both have accelerators only a few hundred meters long. The rest of the time the beam is just coasting, being turned by magnets to be sent back through the accelerator section again for another kick. The beam in those accelerators circulates thousands of times before being sent in pulses to the experiments. The Jefferson Lab accelerator has two linear accelerators linked by two arcs of magnets that recirculate the electron beam.

While ours has considerably less energy than those other two, ours has properties that make it special. Ours is a continuous beam. You could compare ours with one continuous string that loops around 4 times before hitting the target and the experiment behind it. You could almost compare it with trying to figure out the shape of an invisible object by spraying it with a hose and noticing where the stream of water gets deflected. It would be a lot harder to figure out that shape if, as in the other machines, the water only came out in short spurts. This continuous beam allows for a "clean" signal and a very high collision rate that allows a very thorough study of some aspect of the structure of the nucleus and the arrangement of the quarks in it. Many of our studies are of the quarks in the nucleus. The big accelerators blast the particles apart and study the shrapnel. It makes for a more difficult study since there are often thousands of particles to follow simultaneously. They need to sort through all that to figure what they are trying to look at. CDF, an experiment at Fermilab, discovered the last quark, the top, a few years ago. It was studied for nearly 10 years (not continuously) to find evidence of 10 top quarks.

What other characteristics would you try to improve at your facility to understand new particle physics?

There is a plan to increase the energy of this accelerator in increments from its current 4 GeV up to 24 GeV. There is always an effort to improve the resolution of the detectors, which are the "eyepiece" of this microscope. The effort includes the ability to better measure the position or the energy of a particle deflected out of the nucleus.

Have you any recent studies to the topic of quark structure?

If you mean structure within the quark itself, there is very little evidence so far. If it does appear that there is structure within the quark, instead of being an infinitely small point, that sort of implies that the quark is made of something else. There have been discussions about that possibility, but at present an accelerator that could look at that is beyond our level of technology.

I would also like to know any progress you have made in the field of particle physics and anything related to it.

The discoveries at all the nuclear and high energy physics labs come in small increments that add up to a bigger picture. Related fields could fill pages. Much of what is developed for particle physics has applications in many areas, especially in medicine. We have developed amongst many other things; a device to scan for breast cancer, a probe to search for cancer sites during surgery, a tube for transmitting UV light for certain chemical agents used to reduce pollution, and a way to illuminate things in areas where it would be dangerous to put lights. We are using our accelerator technology to make a very powerful, inexpensive laser. This laser has many commercial applications for processing many types of material from plastics to very hard and very smooth steel. Check out this cool site: DOE Labs.

Author:

Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)