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In a , the neutrino scatters from a d quark, remaining unchanged. This is an example of the weak neutral current. In b , the neutrino changes to an electron, and the d quark changes to a u quark. This is an example of the more common weak charged current. These two types of scattering are illustrated in Figure 4. Experimentally, observing an elastic scattering of a neutrino is extremely difficult because the neutrino leaves no trace. The scattering can be observed only by seeing that a proton sitting in a neutrino beam is given a kick, without any charged lepton emerging from the event.

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In , an experiment at CERN produced the first evidence of neutral current scattering, which was soon confirmed by experiments at Fermilab. The number of events observed was consistent with the prediction of the electroweak theory, and this discovery provided the first real evidence that theorists were on the right path.

Theorists had successfully predicted the existence of new particles, but this was the first time that a fundamental particle physics interaction had first been predicted by theory and then discovered experimentally. From that time on, a wide range of many different types of experiments studied neutral currents, and electroweak theory was able to accurately predict the results of all of them.

Some experiments used neutrinos, and others were. A precise measurement of electron scattering at SLAC saw a tiny difference of about one part in one hundred thousand in the scattering rate of left- and righthanded electrons, exactly as predicted by electroweak theory. Evidence that the electromagnetic and weak forces were unified was overwhelming; the next task was to actually discover the predicted gauge bosons of the weak interaction. The second major prediction of electroweak theory was the existence of W and Z bosons.

However, a new breakthrough in accelerator technology was necessary. The experiments of the s, which used proton beams hitting stationary targets, could not produce a particle with a mass greater than about 10 GeV.

The Standard Model - with Harry Cliff

To reach higher energies, it was necessary to use a proton-antiproton collider, at which much higher energies were accessible. Starting in the mids, the Super Proton Synchrotron SPS accelerator at CERN was converted to a proton-antiproton collider capable of reaching an energy of GeV in the center of mass of the collision, about 20 times the energy possible in fixed-target experiments. For the first time since the Bevatron was built in the s to produce the antiproton, a new accelerator was built with the express purpose of discovering a new particle predicted by theory.

Two experiments were constructed to observe the rare events in which bosons were produced and then decayed to leptons. Decades after the discovery that the photon had no mass, its massive siblings the gauge bosons of the weak force—were observed in the laboratory. During the past decade, two new electron-positron colliders dramatically improved our understanding of weak interactions. A much smaller linear electron-positron collider at SLAC called the Stanford Linear Collider [SLC] produced fewer Z bosons, but its innovative design allowed experiments with polarized beams, where the spins of beam particles were aligned to a common orientation SLC was also important as a prototype for a possible future linear collider, discussed in Chapter 6.

For the first time, precise measurements of the fundamental parameters of electroweak theory could be made. These measurements could then be used to probe its validity, in much the same way that precise tests of electromagnetic theory have been made for 50 years. One outcome of these. If the electroweak theory was assumed to be correct, then the relationships between the electroweak parameters measured at LEP and SLC depended on the t quark mass. The excellent agreement between this indirect measurement of t quark mass and the direct measurement made once it was discovered, provided a stringent test of physicists' understanding of the electroweak force.

The current status is that in dozens of measurements to precisions of fractions of a percent, electroweak theory and experimental measurements are in spectacular agreement everywhere. The second innovation in the description of forces in the Standard Model is the theory of the strong force, known as quantum chromodynamics QCD. In , physicists could explain hadrons, such as the proton, as composites of quarks. From detailed experiments at SLAC, it was known that if high-energy electrons were fired into a proton, they would scatter off its quarks, which acted like hard objects much smaller than the proton itself.

There was no theory to explain how three quarks would bind together to make a proton or neutron or to explain why isolated quarks were never observed. Around , however, QCD was developed. It accounted for the observation that quarks are effectively confined inside the proton. The massless force carriers, analogous to the photon, were called gluons, because they provided the ''glue" that held the proton together gluons, like quarks, could not be observed in isolation.

Experimental study of QCD as the theory of the strong force has been much more difficult than studying electromagnetic or weak forces. Nevertheless, QCD has been verified experimentally, and there is continuing progress in developing the calculational tools necessary for precision tests of the theory. QCD predicted that the strength of the strong force would decrease slightly with increasing energy. In the mids, this was beautifully confirmed by a series of precision experiments at Fermilab, SLAC, and CERN, which scattered electrons, muons, and neutrinos from protons.

An important verification of the theory came from indirect observation of the gluon. The gluon, like quarks, manifests itself in high-energy collisions as a collimated jet of particles. The perfect environment for observing jets is in. Most of the events were of the type shown in Figure 4.

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Two quarks are produced in the collision that form two jets of particles with equal energy and opposite directions. QCD predicted that at high-energy colliders, three-jet events from the process illustrated in Figure 4. Figure 4. This was dramatic confirmation of a prediction of QCD. In a , an electron-positron pair is annihilated to a quark-antiquark pair. In b , one of these quarks also emits a gluon. Courtesy of Sau Lan Wu. A fundamental property of all forces is their strength. The strength of the strong force has been very difficult to measure, however, in part because quarks and gluons are confined.

Nevertheless, a series of measurements of the strength of the strong interaction g 3 was made, starting in , using very different techniques. It is a severe test of the theory that all of these measurements agree and determine this fundamental constant of nature rather well. Also, it has been shown that accurate measurements of the strength of the forces can be used to test ideas of "grand unification," the possibility that all of the forces derive from a single one. As calculational tools mature, these measurements are continuing. High-energy physicists use the masses of hadrons to probe the strong interaction that binds quarks to form particles in the same way that atomic physicists early in this century used the spectra of atoms to study the electromagnetic interaction that binds electrons in the atom.

As a result of a concerted experimental effort over the past 25 years, approximately 50 new quark bound states have been discovered at a variety of high-energy machines around the world. QCD has the potential to calculate the particle spectrum in terms of the quark constituents and fundamental equations describing the behavior of the strong interaction. This is a difficult and challenging task. One continuing mystery is that QCD predicts an even richer spectrum of states than has so far been observed.

Quantitatively explaining the wealth of experimental data from QCD will continue to be a challenge for theorists of the next decade. Progress over the past 25 years in understanding the constituent particles has been just as dramatic as for the fundamental forces. In , only the first two generations of leptons had been observed: the electron, muon, electron neutrino, and muon neutrino.

Three types of quarks were known: up, down, and strange. Experiments since that time have discovered the tau lepton and its neutrino, as well as three more quarks: charm, bottom, and top. In addition, experimental studies of Z-boson decay have determined that there are exactly three neutrinos, and therefore the familiar pattern of generations of quarks and leptons shown in Table 2. In , knowledge about quarks was still rudimentary. The breathtaking discovery of the charm quark in was hailed as the beginning of "the new physics. At the same time, a group at Brookhaven using protons on a fixed target in a completely different type of experiment also detected the new quark.

The discovery established that the structure of repeating generations seen in the leptons also applied to quarks.

The Standard Model of particle physics: The absolutely amazing theory of almost everything

When the muon was discovered in , physicists wondered why nature created this heavier copy of the electron. It was the first indication of the repeat-. Many groundbreaking discoveries were made using this instrument, including discovery of the tau lepton and codiscovery of the charm quark. It served as the prototype for the next generation of experiments. Courtesy of the Stanford Linear Accelerator Center. In , still flush from the discovery of the charm quark, physicists on the Mark I detector operating at SPEAR discovered a third-generation lepton, the tau.

After years of subsequent research, the properties of the tau lepton have been measured to be precisely as predicted for a heavier repetition of the electron, leading to the conclusion that the three generations of charged leptons are distinguished only by the large differences in mass. The charm quark completed the second quark generation. As soon as evidence for a third generation of leptons was found with the discovery of the tau lepton, physicists intensified their search for more quarks. However, even if such a third generation of quarks did exist, there was no guidance from theory as to what the mass of third-generation quarks might be.

The only palpable evidence of physics beyond the Standard Model

The bottom quark was discovered in in an experiment using the stillnew proton accelerator at Fermilab. Particles of matter transfer discrete amounts of energy by exchanging bosons with each other. The Standard Model includes the electromagnetic, strong and weak forces and all their carrier particles, and explains well how these forces act on all of the matter particles.

However, the most familiar force in our everyday lives, gravity, is not part of the Standard Model, as fitting gravity comfortably into this framework has proved to be a difficult challenge. The quantum theory used to describe the micro world, and the general theory of relativity used to describe the macro world, are difficult to fit into a single framework.

No one has managed to make the two mathematically compatible in the context of the Standard Model. But luckily for particle physics, when it comes to the minuscule scale of particles, the effect of gravity is so weak as to be negligible. Only when matter is in bulk, at the scale of the human body or of the planets for example, does the effect of gravity dominate.

So the Standard Model still works well despite its reluctant exclusion of one of the fundamental forces. Even though the Standard Model is currently the best description there is of the subatomic world, it does not explain the complete picture. The theory incorporates only three out of the four fundamental forces, omitting gravity. Last but not least is a particle called the Higgs boson , an essential component of the Standard Model.

This particle is consistent with the Higgs boson but it will take further work to determine whether or not it is the Higgs boson predicted by the Standard Model. The Higgs boson, as proposed within the Standard Model, is the simplest manifestation of the Brout-Englert-Higgs mechanism. Provided by Northwestern University. This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. Jellyfish thrive in the man-made disruption of the oceans 1 hour ago.

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User comments. Oct 17, I would like to see a 'video' of an electron emitting photons and assuming lower energy orbital. THAT is the degree of precision and information gathering that they need to get to before they find some of these answers. Until they all they can say is, "Well, it does it. Report Block. Video or it didn't happen?


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You need lots of photons to capture video of an event. How can you take a video of an event involving the collapse of an electron orbit and the emission of a single photon?

Study supports Standard Model of particle physics, excludes alternative models

The investigation of the dipole moment of an electron has to be undertaken indirectly. This experiment measures the properties of photons emitted from collapsing electron shells and infers from there. Your demand for a video is And there's no need to waste the internet's bandwidth with opinions about science that aren't grounded in science. LOLOL, Considering they are announcing they have 'confirmed' the spherical nature of the electron to a higher degree of certainty, but they still don't know exactly what it is, or how it does it's photon stripping trick. I am not saying video or it did not happen, I am merely looking forward to us being ABLE to do it, maybe within my lifetime as fast as things have been going.

No need to Auto-Bash anyone. Infer is just an 'educated guesstimate'. Oct 18, What an extraordinary admission! That ontological problem is a real zinger, huh. Steelwolf amazingly wrote, "I am merely looking forward to us being ABLE to do it make videos of one-photon events , maybe within my lifetime as fast as things have been going. The Standard Model isn't complete. That doesn't mean it's wrong, dude. It just means there's a lot more to the story than the model can tell us.

Gotta love smack-talking geeks! Even as an inference, it's interesting. If you think it's wrong, get your PhD and get in the game! When they are doing work on This scale, which would have been laughed at by You and Your gang of 1 voters just days ago of someone said it could be done. Since THIS is out, I am looking forward to being able for them to make virtual vids of such things as a photon being released from an electron and the elecron's relaxing to lower energy state. They ARE using things other than photons to do this kind of imagery.

I remember when writing IBM in singe atoms was a huge thing. Now they just filmed an ultrafast laser photon packet in real time, I do not see where there should be difficulties in making further advances along that line. Surely it is on their Wish List of Things to do. Fast camera capture of light is a different thing entirely. The camera samples photons in a stream. You can't use that technique to capture video of a single photon emerging as an electron orbit collapses.

With one photon, you get one interaction. Run it into a sensor and it's absorbed.


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You don't see it until you see it; and you only see it once. You can't make a video from one frame. But then you said 'virtual' video, and that makes me think that what you're really asking for is CGI. CGI is great. But you do know that CGI isn't showing an actual thing, right? It's a cartoon. Do you have understanding of such things as the study of X-ray diffraction patterns, electron beam microscopy etc?

1. Introduction

And how the scale has been getting finer and finer, and yet they show their schematics etc of Exactly What they have found in their studies, but yes, it is done with computer generated representations, but anymore, working at atomic level or near it, you HAVE to use different forms of inferring what is going on. For them to have enough actual information to be able to create a pure, fully realized model of EXACTLY how photons interact with electrons, both capture and release.

The rest of your tutelage does fine for a beginner, but is useless verbiage otherwise, all you are trying to do is the Capt Stumpd trick of making fun of someone and maligning them by attributing their work to, say a cartoon Would you tell major physicists that their models were wrong since they look like cartoons? How about the chalkboard work of decades past. Will get there yet. Don't be a Luddite.