I finally got to a very, very large screen that I could blow up the image of so I have corrected many of the typos and bad edits that I can't see on my usual computer screen - yeesh, if my eyes get much worse I'll have to do this by sound instead of sight.
As to the meaning of 1 Ge V and 1 Te V, that refers to the size of the particles being looked for. I'll give you this by way of explanation from the articles about some of those experiments from the CERN Accelerator site. It gives you an idea of the difficulties and enormous physical apparatus required to find these things, one of the very real limits in the ability of theoretical physicists in getting the experimental evidence they need to go on more than speculation (ever open to wishful thinking) or, really, just making stuff up, what I think of as writing sci-fi in equations instead of words.
One of the fundamental characteristics of a particle is its mass, which determines not only how heavy it is (its weight under gravity), but how hard it is to accelerate. For example, a car is much harder to push by hand than a bicycle. Einstein’s famous equation E=mc2 tells us that mass, m, and energy, E, are proportional (related by the speed of light, c, squared). This means that a heavy particle requires much more energy to create than a light one. Particle physicists use this relationship to measure particle masses in terms of ‘electron volts’, where 1 electron volt is the energy acquired by an electron when it is accelerated by an electric field of 1 volt. This is an extremely small unit, and the proton has a mass of about 1 giga electron volts, i.e. 1000 million electron volts or 1 GeV for short, equivalent to 1.8x10-27 kg.
In these units, the up, down and strange quarks have masses of less than 0.1 GeV; the charm quark, 1.3 GeV; and the bottom quark, 4.2 GeV. So, it was natural to assume that the top quark fit this sequence – with a mass of perhaps 10 to 20 GeV. Surely, after the discovery of the bottom quark, the top quark would be ‘just around the corner’.
As each new and more powerful particle accelerator or collider began its work, physicists hoped it would have enough energy to discover the top quark. But no convincing hints were seen, and the first data from the CDF and D0 experiments at Fermilab’s Tevatron proton–antiproton collider in the early 1990s showed that if the top quark exists, its mass must be more than about 100 GeV. On the other side of the Atlantic Ocean, experimentalists at CERN’s Large Electron–Positron Collider (LEP) in Geneva, Switzerland, were probing the top quark indirectly through precise measurements of the decays of the Z boson (a fundamental particle connected to the electroweak interaction) into different types of quarks and antiquarks. Due to conservation of energy, the Z boson, with a mass of about 90 GeV, would not be heavy enough to decay into a top quark–antiquark pair if the top quark (and top antiquark) mass is greater than 45 GeV. Nevertheless, the relative proportions of Z boson decays into other types of quarks could be subtly influenced by even the possibility of decays into top quarks, and measurements at LEP suggested the top-quark mass should be somewhere between 150 and 200 GeV. But did it really exist?
In a particle collider, collisions between high-energy protons and antiprotons can be understood as collisions between two opposing ‘bags’ of quarks or antiquarks, the constituents of the (anti)protons. The total energy of the accelerated proton is shared among the three quarks, with a fraction also going to gluons, other particles in the proton that represent the force binding the three quarks together. Physicists expected that the most likely way to produce top quarks in Tevatron’s 1.8 TeV (1800 GeV) collisions was through a head-on collision of a quark from the proton and an antiquark from the antiproton, producing a top quark and corresponding top antiquark (a 'top-pair'). Again, due to conservation of energy, this process would require the initial quark and antiquark to have at least twice the energy equivalent of the top-quark mass – that’s more than their fair share of their parent proton’s energy. This is rather unlikely, making top-pair production a rare process that becomes even rarer if the top quark is very heavy.
In the early 1990s, the CDF and D0 experiments began to accumulate evidence for the production of top–antitop pairs in their data sample. They finally announced their joint discovery of the top quark in 1995, measuring its mass to be about 180 GeV. This was around 10 times larger than the original expectation, but in agreement with the indications from LEP. Over the next 16 years, tens of thousands of top-quark events were recorded and studied by the two experiments, allowing physicists to build a first portrait of this new particle. As far as they could see, it behaved just as a partner of the bottom quark would be expected to – but why was it so heavy?
You can read more by yourself, they have articles about finding the latest and grooviest of them, the one the filled in the last known piece of the Standard Model jig saw puzzle, the Higgs boson that was hyped so much a few years back. Though a lot of their articles would seem to be more hype to keep the project funded and going, or at least that's what some of it seems to me. What practical use any of it is, I have no idea, nor does it look to me as if they're anywhere near having a complete picture of things, notice how many questions they include in their articles. Frankly, I'd rather they be spending resources and money on saving us from global warming or feeding, housing, clothing and fueling our species on an equitable basis without destroying the biosphere.
No comments:
Post a Comment