Hall B staff recently installed Jefferson Lab's newest detector: a hybrid calorimeter nicknamed HYCAL. It was designed and built in just over three years by the PrimEx Collaboration. The detector system weighs in at about six tons and required nearly a million dollars to construct. According to PrimEx spokesperson Ashot Gasparian, an associate professor at North Carolina A&T State University, the calorimeter is designed to make high precision measurements of the lifetime of the chargeless pion, or pi-0, particle. Physicists hope this information will tell them more about symmetry in nature.
The Chargeless Pion
Pions are the lightest particles made-up of quarks, and they are commonly produced in experiments with CEBAF. There are three types of pions, referred to by their electric charge: a positively charged, a negatively charged, and a neutrally charged, or chargeless, pion. Positive and negative pions each contain two quarks. But the chargeless pion is different in that it can't be described quite so easily; physicists can't pin down its constituents at any one moment in time. The chargeless pion is best described by a mathematical formula physicists have derived for it from statistical probabilities. According to this formula, it contains some combination of four different quarks.
"The pion is the lightest hadron [subatomic particle composed of quarks] in nature, and its properties are the easiest to calculate from theory," Gasparian explains. Early particle theory suggested that the lifetime of a pion should be around 0.0000000000001 second, or one femtosecond. But experimental observations of pi-0 showed that the lifetime is actually much smaller, 0.0000000000000001 second, or one attosecond. This may not look like a huge difference, but it's the equivalent of expecting a turtle to live about 1,000 years and finding that it only lives a single year. Theorists found that calculations incorporating symmetry violation resulted in a figure that was much closer to the actual pion lifetime. It's thought that a more precise experimental measurement will provide information on the pion and on symmetry violation.
A Hybrid Calorimeter
The chargeless pion leads such a short life that it's difficult to measure directly. However, practically all pions decay into two photons, and in certain circumstances, a pion can also be created with two photons. The process is called the Primakoff Effect, in honor of Henry Primakoff, who, in 1951, postulated that physicists could measure the lifetime of chargeless pions indirectly by way of this two-photon creation and decay process.
According to Dan Dale, PrimEx spokesperson and associate professor at the University of Kentucky, PrimEx scientists will produce a pion by bombarding a nucleus with photons. The nucleus will interact with a photon by spitting out one of its own. "The nucleus always has a cloud of photons around it. We call this cloud the Coulomb field," Dale explains. When the two photons collide, a chargeless pion is formed. The pion cruises along for about one attosecond before it decays back into two photons. The lifetime of a pion created by this simple process will be measured indirectly by measuring the energy and position of the two photons.
"This experiment detects the two photons. And for that, we needed a big calorimeter with high resolution," Dale adds, "The best option available for measuring these photons was newly developed detectors containing fast scintillator crystals made from lead and tungsten." But at $600 for a detection area of two by two centimeters, the scintillator crystal detector assemblies were expensive.
"Our initial R&D showed that if we were to make a big calorimeter with all crystal detectors, it would be very costly. So we needed a compromise of cost and performance," Gasparian says. To balance the high cost of the scintillator crystal detectors with the need for a large detection area, the scientists decided to make a hybrid calorimeter. Part of the package would use the crystals, but the other part would make use of a less expensive detector: lead glass Cerenkov counters costing $500 per four by four centimeters of detection area.
How It Works
HYCAL's final design called for a matrix of detectors: the inner section employs 480 lead tungstate scintillator crystal assemblies, and the outer section contains 663 lead glass Cerenkov counters. "The high resolution part of HYCAL is twice the size it was originally designed to be, and we hope that will make the experiment more successful," Gasparian says.
"The most energetic photons should enter the detector closest to the beamline. These particles will enter the scintillator crystal detectors, which are capable of providing the best quality information," Dale adds. Once a photon enters a scintillator crystal, it's converted into a shower of particles that emit photons of visible light. These light photons are trapped in the crystal by a layer of reflecting material wrapped around each crystal and sealed with an ultrathin layer of KevlarŪ. The light photons travel through the 18-centimeter long crystals into photomultiplier tubes. These photomultiplier tubes detect, amplify and transform the light photons into electrical pulses, which are then digitized and sent to the data acquisition system designed by the PrimEx collaboration.
Photons entering the 45-centimeter long lead glass cylinders of the Cerenkov detectors go through a similar process, though the principles of detecting particles with lead glass are slightly different than those for lead tungstate. Photons entering the Cerenkov detector modules generate showers of particles as they do in lead tungstate. The speed of light within the lead glass is slower than that in a vacuum, and many of these shower particles are able to travel faster than light can travel in the lead glass. As a result, a flash of light is emitted, somewhat analogous to the shock wave of a supersonic boom emitted by a plane traveling faster than the speed of sound. This so-called Cerenkov light is detected in the photomultiplier tube at the end of the glass.
"From these measurements, we are gauging the energy and position of each particle that enters the system. We can reconstruct the event, which gives us so-called invariant pion mass -- how heavy was that particle. And that way, we are actually detecting pi-0s through the photons they emit," Gasparian explains.
HYCAL began taking experimental data September 27. It will take data on pions that emerge from three different targets: carbon, lead and tin. Measuring the differences in the pion lifetime that each target gives will serve as a kind of quality control check for the experiment. Scientists hope to get enough high-quality information to fine-tune the lifetime of the chargeless pion and to test the theoretical predictions of this lifetime.
"The next experiment will measure the lifetime of the next particle up, the eta meson. For experimental physicists this is very interesting and exciting work," says Gasparian. In addition, Dale is looking forward to further experiments on the chargeless pion, which may yield a more detailed picture of the particle's structure.
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