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Asked by anon-345100 on 8 Dec 2022.
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Joel Goldstein answered on 8 Dec 2022:
In general, to “discover” a particle you have to either isolate it or produce it, then detect it or its decay products (if it is unstable). Measuring the spin, mass and other properties then follows. Some examples of discovery techniques and associated measurements are:
Electron – isolated by applying an electric field to metal in a vacuum (cathode ray tube), detected on a fluorescent screen, mass measured by observing deflection in a magnetic field. Spin was measured by passing beam through an inhomogeneous magnetic field
Electron neutrino – produced in nuclear reactors, detected by observing interactions of neutrinos with protons in water. Mass and spin still not measured directly
Muon – produced in cosmic radiation, observed in a cloud chamber, mass measured by deflection in magnetic field
Up and down quarks – observed by firing high-energy electrons at protons and seeing how they scatter.
Top quark – produced in high-energy proton-antiproton colliisions, decay products detected in sophisticated electronic detectors. Spin and mass measured by looking at the decay products.
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Patrick MacGregor answered on 9 Dec 2022:
Every particle is different! Some are abundant all over Earth (e.g. electrons), but measuring properties of the heavier particles is very difficult without a particle accelerator. A great example of discovering other particles is the discovery of the muon. Physicists in the early 20th century were looking at cloud chambers to identify particles — this is where particles leave trails in a special vapour so that you can see how they travel. It’s a bit like working out how a plane travels by the trail it leaves behind it, though in this case you wouldn’t be able to see the plane at all!
When the cloud chamber is placed in a magnetic field, any particles with an electric charge start moving in circles rather than straight lines. The radius of the circle depends on the mass and charge of the particle, which is how the muon was identified. When cosmic rays were passing through the cloud chamber in the magnetic field, the muons curved differently than the electrons, which indicated that there was a different type of particle that existed.
Other particles like neutrinos are not measured directly, but we can infer their existence from beta decay – a beta decay in a nucleus always emits an electron, and should always release the same amount of energy. But when physicists measure the energy of the electron and the daughter nucleus from the decay, they get a wide range of energies instead of a single value. Therefore, some of this energy is being carried away by something else that is very hard to detect, and this helped them discover the neutrino.
For heavier particles, you never detect them directly because they decay too fast for us to make them go into a detector. Instead, we have to look at what they decay into, and work out what the original particle is like. It’s a bit like having a crime scene, and looking for clues to work out the identity of the criminal from what they left behind. The hope is that the LHC, or an even bigger accelerator, will discover new and different particles that aren’t in the standard model. The hope is by understanding these, we will learn more about our universe and why it behaves in the way that it does!
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Vichayanun Wachirapusitanand answered on 12 Dec 2022: last edited 13 Dec 2022 11:53 am
There is no limit on how many particles can exist in the fundamental model, but so far we do not find any more fundamental particles other than what we have in the Standard Model. Currently we have three generations of quarks, three generations of leptons, and five bosons.
What I mean by “generations” of quarks has to do with the history of when particles are discovered. The first generation of quarks are up and down quarks, which are “discovered”, or more like proposed by a quark model by Gell-Mann and Zweig back in 1961. The model also proposes another type of quark called strange quarks. The second generation of quarks are strange and charm quarks, with the charm quark proposed by Glashow, Iliopoulos, and Maiani in 1970. The third generation of quarks are bottom and top quarks, proposed by Kobayashi and Maskawa in 1973, and later discovered in 1976 and 1995.
The same idea of generations of particles are also applied to leptons as well. Electrons are discovered first by Thompson in 1897 (!), muons are first observed in 1936, and tau leptons are discovered in 1975. On the other hand, neutrinos are discovered in 1956, 26 years after a proposal by Pauli.
As of the current Standard Model, we do not have any upper limit of the number of quarks, leptons, and bosons. There is currently an ongoing effort to find the next generation of quarks and leptons, but so far what we as physicists can do is to set the lower limit of masses of these particles. We have not discovered the fourth generation of quarks and leptons yet. We also have not discovered a new type of boson yet.
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