How do physicists running experiments with the particle accelerators detect that they’ve found a massless particle? Also, are there special quantum mechanics formulas to describe the dynamics of a massless particle? I WAG they only have an energy balance (both in the lab and on the chalkboard) as their guide since all the clues a mass would manifest are simply not present.
Why would detection of massless particles be any different than massive ones? The easiest particle in the world to detect, the photon, is massless.
You can detect massless particles with your eyes (photons).
Massless particles are highly relativistic and so you can’t use vanilla quantum mechanics to describe them, you need to invoke special relativity (in some way or other). This in itself brings a few of the problems associated with relativistic quantum mechanics, but the problems aren’t unique to massless particles.
To discuss some other day: Aren’t “photons” really just an outdated and simplistic term for the particle behavior of the now better understood wavicle property of light?
I’m talking about detecting a massless subatomic particle from the output of a collision that exist for a blip and are gone. Are you going to see it as light? How do you know you’ve discovered a massless particle? You pull out your massless particle detector, right?
I thought particle physics totally depended on studying collisions applying conservation of mass, momentum, and energy. Well, let’s say you collide Mass-a with Mass-b and get Mass-c and energy out. Perhaps the only clue you also have a massless particle in the output mix is if one finds they got out more energy than one’s theoretical calculations predict, I WAG?
No. It would be a mistake to consider them as tiny billiard balls, but ‘particle’ is a perfectly fine word or concept; they just behave quantum-mechanically rather than classically.
Mass isn’t the part that makes detection complicated; photomultipliers, for example, are great at detecting photons. Neutrinos, for example, are reasonably hard to detect not because they’re so light, but because they don’t participate in any interactions besides the weak force (they do experience gravity, but its effect is completely undetectable). Besides, the masses involved in high-energy physics are so small (it’s not uncommon to do calculations, for example, in which protons and neutrons are taken to have infinite mass) that trying to sort out or detect particles by gravitational effects is a nonstarter.
Mass isn’t conserved. Conservation of 4-momentum, though, does give evidence for the existence of specific particles (if I remember correctly, the neutrino was originally proposed to carry off momentum in beta decay). Mass isn’t really the main issue, though; gamma rays are very easy to detect, and in fact are used to find more complicated decays.
Right now there are only two known massless particles, the photon and the gluon.
The gluon normally is confined to the inside of a hadron but can be produced by particle accelerators. It was detected because of the particles it broke down to, in accordance with QCD. It is a three-jet event.
That’s the way almost all the new particles are found. Even if they can’t be seen directly they have known interactions and breakdowns, whether they have mass or not. It’s not a straightforward process - look at the 40 years needed to find the Higgs - but positive results are well recognized because of the physics involved.
The one theoretically remaining massless particle is the graviton. From what I’ve read, that will not be detectable by any known means. That’s why there are so many experiments running trying to detect gravitational waves, which if found would imply their existence. The physics is always there: the effects of a particle is enough to prove that a particle exists.
We will probably never detect individual gravitons, and yes, I do mean the word “never” literally there: It’s theoretically possible, and “just an engineering problem”, but an engineering problem so immense that there is no plausible model of technological advance that would ever get us there. A gravitational wave, however, can be described as being a stream of a great many gravitons, in the same way that an electromagnetic wave can be described as a stream of photons, and we should detect gravitational waves in the near future.
That said, though, the fact that gravitons are massless is essentially unrelated to the difficulty in detecting them.
I assume the difficulty detecting gravitons is due to the extraordinary weakness of the gravitational force, correct?
Right. In fact, from skimming over a paper on the arxiv, it looks the total cross-section should be something like ~G[SUP]2[/SUP] (by analogy with classical electromagnetic scattering) or ~G (through more sophisticated arguments). The implied constants are very small, and the setup is just not feasible given the necessary time constraints and detector size (the paper uses a proton as a lower bound). It’s not exactly clear how the interaction should look, given how difficult it is to come up with a coherent QFT theory of gravity, but the force is so small that it’s not likely to be possible regardless.
There’s that, and you also have the difficulty that gravitons produced by any plausible process would be exceedingly low-energy. We expect gravitational waves in the Universe to top out at around a kilohertz, and gravitons would have the same frequency-energy relation as photons.