'Interference' used to study inner structure of protons, neutrons

Click here for a high resolution photograph.

When you throw two rocks into a pond of water, side-by-side, the ripples created by the rocks will collide and annihilate each other in some areas and combine to make bigger ripples in others. Physicists call this phenomenon interference. In a large Hall A experiment that completed taking data in early December, physicists are using this same phenomenon to get them one step closer to a three-dimensional picture of the inner structure of protons and neutrons in the nucleus of the atom.

The experiment, called DVCS@JLab, ran in two, back-to-back parts from Sept. 18 through Dec. 5.

DVCS, or "deeply virtual Compton scattering," is the process these experiments used to learn about protons and neutrons (nucleons).

In this process, physicists use the CEBAF accelerator to propel a beam of electrons into an experimental target. When these electrons strike the target, many collide with the nucleons in the nuclei of atoms. In the first part of the experimental run, the researchers were interested in those collisions where an electron has struck a quark inside the proton, giving the quark extra energy and knocking it out of the proton.

This quark then gets rid of the excess energy by emitting a photon, or a unit of light, and is reabsorbed by the proton. In the second part of the experiment, the researchers looked at the same process in the neutron.

For the experiment to work, the scientists needed to be able to measure the speed, position and energy of the electron that had bounced off the quark, the photon given off by the quark, and the re-assembled proton.

The researchers used Hall A's High Resolution Electron Spectrometer to measure the electron and a scintillator array they built to measure the proton.

According to Pierre Bertin, Director of Research at the Laboratoire de Physique Corpusculaire de Clermont-Ferrand and lead spokesperson for DVCS@JLab, the most important piece of equipment in the experiment is the new calorimeter the researchers constructed to measure the photon.

"We have built a calorimeter, a very nice one. And it is the heart of the experiment, because we need to know perfectly the energy of the photon. And at this time, we can measure that to within two or three percent.

That is the accuracy of our detector, and it is very necessary because that will allow us to go a factor of 10 better than experiments have done before," he says.

A Calorimeter with No Name

The total cost of the calorimeter is about $750,000. The calorimeter and the scintillator both spent four months in the Test Lab undergoing final checkout procedures before installation in Hall A began in July 2004. Franck Sabatie, a researcher from CEA-Saclay and a spokesperson on the experiment, says the calorimeter is composed of an array of leadfluorite crystals. "The lead-fluorite crystals look like glass, but they're very dense -- as heavy as iron, basically."

"We have 132 block crystals, so it's not too big," Bertin says. Each rectangular crystal rod measures 3 by 3 centimeters on end and nearly 20 centimeters in length. Attached to the end of each crystal is a photomultiplier tube.

"And what happens is -- the photon -- as soon as it enters a very heavy material, creates an electron and positron, and these then radiate more photons," Sabatie explains, "The new photons then create more electrons and positrons... so there's a shower of particles inside the crystal." The result is a flash of light inside the crystal, which is measured by the photomultiplier tube and recorded. "That allows us to reconstruct both the energy and the position of where the original photon hit the calorimeters," Sabatie adds.

As for what the scientists call the calorimeter, Bertin notes, "It doesn't have a name." According to Sabatie, some of the collaborators have taken to calling the calorimeter Calo, because it's a small one as far as calorimeters are concerned.

Installing the equipment into Hall A for the experiment took about two months. "It's one of those experiments where we added several detectors to the main hall instrumentation. And that meant a lot of installation time. So we started in July, and we finished basically when beam arrived on Sept. 18," Sabatie says.

Back to Interference

Bertin says that even with the new dedicated calorimeter, proton scintillator and other equipment, the analysis of the data from the experiment is going to be challenging. That's because the detectors will record information from more than one process that will result in the telltale photon recorded in the calorimeter. "We have two processes in competition. We have the Bethe-Heitler process, where the photon we detect in the calorimeter is simply emitted by an electron. That is one process, then," Bertin explains, "in DVCS, the photon going in the spectrometer is emitted by the proton. But there will be fewer photons produced in this process, and this is the one we're interested in."

The signal the physicists will record in the experiment will contain events from both processes: DVCS as well as the Bethe-Heitler process. It turns out that theory projects these processes will overlap, or interfere, in a predictable way, inflating the photon signal that the detector records in some areas and reducing the signal in others.

The physicists aim to separate out these processes by using what they know about how these processes interfere. The Bethe-Heitler is a well-known process that can be calculated from theory. "We get DVCS just by taking out what we know from the Bethe- Heitler theory," Sabatie explains.

Bertin adds, "And that is the new thing in this physics. This trick will allow us to get the result we're looking for." The researchers have begun analyzing the data and expect to obtain results sometime in the next year.

Generalized Parton Distributions

The goal of the experiment is to test the theory of generalized parton distributions (GPDs). GPDs are a set of mathematical functions that may allow physicists to produce a threedimensional snapshot of the inner structure and dynamics of protons and neutrons in the nucleus. Physicists can use these functions to map out the location and momentum of the quarks and gluons inside protons and neutrons.

"This is the first experiment dedicated to GPDs," Bertin notes.

This experiment won't provide enough information to put together a map of the internal structure of protons and neutrons just yet. Rather, it will provide enough information for a calculation of one aspect of the structure of the nucleon. "We are measuring one point. So the experiment won't give us the whole picture of the nucleon, but it will give us one accurate measurement that relates to the structure of the nucleon. It will allow us to check the theory," Sabatie explains.

He says if the experiment is successful, it will pave the way for broader experiments. "We will need a lot of pictures to reconstruct a 3-D image of the nucleon. So the next step is to plan a much bigger experiment, and this is basically what Jefferson Lab is doing with the 12 GeV Upgrade," he says.