The Heart of the Matter (Physics)
The Heart of the Matter
By Robert Matthews, Physics
All you see at first are some sparkling white buildings scattered almond trees and fields on the edge of Newport News, Virginia, on the Atlantic coast of America. But looking around, you may catch a glimpse of strange-looking grassy mounds rising out of the ground. Clearly manmade, they are the first indication of the dramatic events that are taking place beneath your feet.
You have arrived at the Thomas Jefferson National Accelerator Facility, and 10 metres below where you stand, an army of scientists is using the world's largest electron microscope to probe deep into the very heart of atoms. Their mission is to study the ultimate building blocks of matter and the bizarre force that binds them together: the 'strong nuclear force'; the strongest force in the universe.
Not so long ago, scientists had a very simple picture of matter. They thought it was made of building-blocks called atoms, and that it was affected by just two fundamental forces: gravity and electromagnetism. Then, in the early part of this century, scientists discovered that the atom was not fundamental at all, but contained a central nucleus made up or two types of particles: a central nucleus made up of two types of particles: uncharged neutrons and positively charged protons.
This discovery created a puzzle: as similarly charged objects repel each other, what was preventing the protons from being blown apart? There had to be another force at work in the nucleus, thousands of times stronger than the electromagnetic force trying to force the protons apart. It was duly called the strong nuclear force.
This, in turn, raised another puzzle: what was the source of the strong force? Not until the late 1960s did the answer finally emerge. Using very high-energy accelerators capable of firing electrons deep into the nucleus, scientists showed that neither protons nor neutrons are fundamental. They too contain particles, known as quarks.
Clustered in groups of three inside neutrons and protons, quarks are now thought to be the truly ultimate building blocks of matter. They are bound together by particles aptly named gluons, whose grip on the nucleus makes them the ultimate source of the strong nuclear force.
That, at least, is what scientists believe is happening within all matter, from the simplest bacterium to the largest stars. But many mysteries remain - mysteries that are now set to be solved by the scientists at 'Jefferson Lab', named after the eponymous third President of the United States.
Solving the Mystery
The giant underground machine is what they will use: the Continuous Electron Beam Accelerator Facility (CEBAF); the most powerful machine ever built to delve inside the hearts of atoms. Delivering a constant stream of millions of electrons travelling at virtually the speed of light, CEBAF will give scientists their first detailed picture of quarks and gluons in their natural state inside the nucleus.
The results are eagerly awaited by scientists who have spent decades trying to understand the ultimate structure of matter - with little more than mathematics to light their path to enlightenment. For as Jefferson Lab's theory group leader Nathan Isgur points out, everyday intuition and experience could count for little in the realm of the quarks.
'The quark structure of matter is a radical departure,' he says. 'Until quarks, each sub-unit of matter - atom, nucleus and proton and neutron - has been separable from the objects it makes. But at the quark level, we encounter a new phenomenon: a force so strong that it prevents quarks ever being seen on their own.'
Understanding the strong force
Understanding this bizarre feature of the strong force is a major aim of Jefferson Lab research. So far, the best theory to account for it is quantum chromodynamics (QCD), a mathematical description of the strong force in terms of different types of quarks and the gluons that flit between them.
According to QCD, the force between quarks has a ppeculiar property quite unlike familiar forces such as gravity: it doesn't grow weaker with increasing distance. As a result, attempts to extract a single quark from inside the proton or neutron are doomed to failure. The harder one pulls, the stronger the force becomes.'
'Precisely how QCD leads to such a force is currently a glaring gap in our misunderstanding of the world,' admits Isgur. Mystery also surrounds the way quarks give neutrons and protons their observed properties. Both contain two types of quark, known whimsically as 'up' and 'down': the proton contains two 'up' quarks and a 'down' quark, while the neutron contains two 'downs' and an 'up'.
This seems simple enough. But according to QCD, there is far more inside protons and neutrons than just quarks. There are the gluons, binding the quarks together, plus a seething cloud of quarks and antiquarks, constantly popping in and out of existence. Somehow, this complex melee of activity leads to the constant and familiar properties of neutrons and protons.
'Being a theorist, I believe I know how Nature accomplishes this feat, but we need to do the experiments to be sure,' says Isgur. 'The experimental program at Jefferson Lab looks inside the nuclear particles to see what is in fact going on: it really is like a giant electron microscope.'
A giant microscope
Some microscope! First conceived 20 years ago, and now completed at a cost of US$600 million, CEBAF is a 1.4 kilometre-long machine that accelerates electrons from zero to 300,000 kilometres per second in 30 millionths of a second flat, before smashing in to their target nuclei.
The electrons begin their journey by being blasted from a crystal of gallium arsenide by a powerful laser beam. Once out of the crystal's surface, the electrons are grabbed by electromagnetic fields and rapidly transported to the main part of the machine: the accelerator tunnel or 'racetrack' buried 10 metres underground.
Here they encounter over 300 accelerator cavities, devices containing intense electric fields which progressively kick the electrons to ever higher speeds. It is these accelerator cavities which give CEBAF its name and makes it so special.
Conventional accelerators have to switch their beams of electrons on and off repeatedly to present the accelerator cavities from overheating, but CEBAF's cavities, contained within can-shaped 'cryomodules', degrees above absolute zero. This makes the cavities 'superconducting', virtually eliminating the electrical resistance that causes the overheating. As a result, CEBAF does not have to be switched on and off: it can live up to its name of being a 'continuous beam' accelerator, giving scientists a beautifully rich and consistent stream of electrons with which to probe the nucleus.
Despite the awesome power of the accelerator units (equivalent to the eletricity demands of a small town), it still takes up to five circuits of the racetrack to give the electrons sufficient energy to enable us to see their sub-atomic targets. So, to herd the electrons around the racetrack for each repeat journey, CEBAF uses over 2200 precisely shaped and placed magnets, measuring anything from a few centimetres to 2 metres in length and up to 5 tonnes in weight.
The sheer size of the magnets reflects a bizarre phenomenon at work within CEBAF: At speeds close to the speed of light, Einstein's special theory of relativity predicts that objects appear to gain mass. The electrons in CEBAF move so fast that they act as it they have gained almost 8000 times their normal mass, making them far more reluctant to change course - hence the need for powerful magnets.
Just 30 millionths of a second after leaving the gallium arsenide crystal, the electrons have completed their 7-kilometre ride within CEBAF, and are switched into three experimental halls where they smash into the nuclei of target atoms of lead, carbon and other elements.
It is inside these experimental halls that the secrets of the nucleus are finally revealed. Each contains massive detectors capable of analysing the hundreds of thousands of collisions produced by the electron beam every second.
The detector halls
Hall A's detectors look for evidence of so-called hadrons - particles, such as protons, that are affected by the strong force - produced by the collisions, while in Hall B the behaviour of the particles is analysed. Banana-shaped 'drift chambers', filled with argon and ethane gas, allow researchers to track and identify the nuclear fragments by studying the avalanche of new charged particles created as the fragments fly through the gas.
Hall C is home to the giant high momentum and short orbit spectrometers, which examine the paths of the fastest-moving particles as they scatter out of the collision. Using computers, scientists can examine the paths to work out precisely what CEBAF's electrons have found inside the traget nuclei.
Despite the apparent violence of the events, the detectors used to analyse them are extremely delicate. Hall B's drift chambers, for example, had to be constucted in a 'clean room' where the air contains no more than 350 dust particles per litre - hundreds of times cleaner than normal air.
World centre for study
With its huge array of detectors and its powerfull, clean and continuous source of electrons, the promise of CEBAF is attracting hundreds of scientists from 20 countries, including large contingents from France, Germany, Italy and Holland.
'The instruments here are unique, and international access is essential,' says Dr. Hermann Grunder, the charismatic director of Jefferson Lab, who is widely credited with bringing the huge project in on time and within budget. 'We hope that, working together, we will achieve a deep understanding of quarks and gluons, and precisely how they form all nuclear matter.'
Dr. Grunder adds that the work at Jefferson Lab will benefit more than just the world's particle physicists, however. 'Some of the technologies that need to be developed for our primary mission may well have commercial applications,' he says. 'Collaborations with industry and universities are important to take advantage of these.'
In the past, particle physics has led to many key technologies, from body scanners to television. The first spin-offs are already emerging from the work at Jefferson Lab. Industrial scientists working on the project pointed out that the superconducting niobium cavities used to accelerate the electrons in CEBAF could form the heart of a free electron laser, a device that can cut steel with ease and precision and create revolutionary new man-made textiles.
'Ultimately, however, we all profit by the intellectual stimulation,' says Dr. Grunder. 'The big pay-off is the fundamental knowledge we gain: it goes into our textbooks, and into the technologies of tomorrow.'