What Is an elementary particle?

[LaserMind]

It goes back early atomism - matter is made of particles that are indivisible.

The key idea is *indivisibility* which persists through the ages to today.

But today we term a particle as something that interacts and makes tracks in bubble chambers
and forget about indivisibility.

 

[A. Neumaier]

forget about indivisibility.

Even today, an elementary particle is indivisible in the sense of being described by an irreducible representation of the Poincare group, while composite particles are divisible, represented by reducible representations (tensor products).

 

[bob012345]

Even today, an elementary particle is indivisible in the sense of being described by an irreducible representation of the Poincare group, while composite particles are divisible, represented by reducible representations (tensor products).

You've said nothing about what a particle is physically.

 

[A. Neumaier]

You've said nothing about what a particle is physically.

Physically, we cannot know what it is, as we can only observe its traces or decay products in interactions with macroscopic matter (in a bubble chamber, a wire detector, a Geiger counter, etc.)

Observed is the macroscopic response, the properties of the particle itself are only inferred. That's why particles are defined as a mathematical entity in the context of a (quantum field) theory that predicts how this entity becomes observable in scattering events.

 

[bob012345]

Physically, we cannot know what it is, as we can only observe its traces or decay products in interactions with macroscopic matter (in a bubble chamber, a wire detector, a Geiger counter, etc.)

Observed is the macroscopic response, the properties of the particle itself are only inferred. That's why particles are defined as a mathematical entity in the context of a (quantum field) theory that predicts how this entity becomes observable in scattering events.

Well, you could say that about everything, that you cannot say what it is physically but that is a rather poor attitude to take for a physicist in my opinion. Alternatively, you can define the particle in terms of its properties interactions, not in terms of a particular mathematical theory.

 

[vanhees71]

Of course, "particles" are not some mathematical abstractions but real things that can be observed. E.g., take electrons. They were discovered at the end of the 19th century by J. J. Thomson in cathode ray tubes by studying their reflection in electric and magnetic fields. They were "visible" for him due to the excitation of the rest gas in the tube.

Of course, this and other discoveries at the time (like radioactivity) started a whole new world for the physicists, namely the world of atomism in a broad sense, i.e., the fact that matter is composed of particles. As it turned out, however, the meaning of the concept of "particle" had to be drastically revised, which lead to the discovery of quantum mechanics in the 1925 and very quickly thereafter also of quantum field theory. These theories are pretty abstract but necessary to fully describe the behavior of "particles" and to make sense out of this picture of matter at all. It is thus not so easy to answer the question, what a particle is. Strictly speaking, physics doesn't give an answer at all, and that's also not the purpose of physics. The natural sciences try to describe as good as one can and as accurately as possible all quantitative observations in Nature, but it doesn't tell you what it is what we observe.

From a theoretical point of view, you thus get sometimes answers like "an elementary particle is what can be described as the asymtotically free Fock states of elementary quantum fields". This is the most accurate answer you can get from the point of view of a theoretician using the most recent scientific knowledge about how to get an accurate description of what an elementary particle is.

This is, however a pretty incomplete picture, because physics is after all an empirical science about observable facts, and thus this very abstract "definition" of the theorist must be always seen as complemented by how experimentalists observe the "particles" and how they make sure that the theoretical description as an "elementary particle" is really right. So the theorists make a model, based on knowledge about observations concerning "particles" and provide the experimentalists with all kinds of observable properties of what they call "elementary particle". Then the experimentalist can build devices to test these hypotheses.

This endeavor went on from the early days of Thomson, the Curies, Rutherford et al until today, and with more and more technological progress (among them the ability to build accelerators that brought the "particles" to higher and higher energies, and very fancy and accurate dection methods) and more and more progress in describing the particles. From this endeavor the "Standard Model of Elementary Particle Physics" emerged as the so far most comprehensive model. It was finished in the early 1970ies with the discovery of asymptotic freedom of QCD, I'd say. Since then it was more and more confirmed, and this success is on the other hand also a kind of problem, because there are some reasons to hope for it to fail, i.e., to make some observation that contradicts the predictions made by it. That's why at the LHC, after having its discovery of the last building block of the Standard Model, the Higgs boson, one looks for "physics beyond the standard model" with the hope to find a hint, how to get to an even better description, hopefully also closer to an understanding what the famous "dark matter" or even "dark energy" might be. From the point of view of cosmology we know only about 5% of the energy content of the universe, which is the amount made up by the known "elementary particles" of the Standard Model. About 20% or so is "dark matter", which should exist because of the discrepancy between the motion of stars in our galaxy and the expectation of this motion given the amount of visible "Standard Model matter" and the theory of gravitation (general relativity). The rest of about 75% is "dark energy", which is the most mysterious piece of contemporary physics. It's described by the cosmological constant, but it's totally unknown, why it has the value observed by accurate measurements of the fluctuations of the cosmic microwave radiation and the redshift-distance relationship (Hubble law) measuring supernovae, now used as "standard candles" to the largest distances possible.

 

[bob012345]

That is a nice answer Vanhees71. I appreciate the experimental aspects. It's interesting to note that the best theoretical answers changed throughout the twentieth century and likely will continue to change into the future.

 

[A. Neumaier]

thus get sometimes answers like "an elementary particle is what can be described as the asymtotically free Fock states of elementary quantum fields".

As I had remarked before, this confirms

why particles are defined as a mathematical entity in the context of a (quantum field) theory that predicts how this entity becomes observable in scattering events.

... and of course how they can be prepared, manipulated, etc. - all the stuff that is of experimental significance. But this doesn't give more insight into what a particle ''is', only how it can be created and how it behaves. The ''is'' must be a theoretical definition: ''an electron is what behaves like what QED predicts for it''.

Well, you could say that about everything, that you cannot say what it is physically but that is a rather poor attitude to take for a physicist in my opinion.

Once something is properly understood, the best one can say about what it is is that it is ''something conforming to the theory within a specified accuracy''. Indeed, any deviation from the theory would point to it being a different kind of object. For example Herbert B. Callen, the author of a very famous book on (mostly phenomenological) thermodynamics, writes on p.15: ''Operationally, a system is in an equilibrium state if its properties are consistently described by thermodynamic theory.'' This over 50 years old definition is the unsurpassed and most practical definition of equilibrium systems I have ever seen.

 

[vanhees71]

Of course, giving a name to something is a theoretical construct, be it a "naive" qalitative one or a quantitative one involving some mathematics, but just a mathematical definition doesn't explain what it is. That's of course also true for the example cited from Callen's marvelous book on (not only phenomenological) thermodynamics [*]. There appears the word "properties", and properties are something observable, and phenomenological thermodynamics describes a many-body system using observable quantities like temperature, pressure, particle number (or moles), etc. Then you have rather abstract quantities like entropy that describes equilibrium as that state for which it takes its maximum value, given the constraints due to the fundamental conservation laws (energy, momentum, angular momentum). Of course, the theory also gives you the properties of the system in equilibrium for the measurable quantities and then you can determine by measurements if it is in this state, but just a mathematical definition without empirical foundation is not physics. That's why I tried to bring in the empirical foundations in addition to your theoretical answer.

I strongly believe that physics won't make much progress, if one looses the empirical foundations for theory building. Of course, if there are open mathematical issues as in the 1960ies about the weak interactions, then the ball is in the theory park to find a description of the known facts, and it was found with the Glashow-Salam-Weinberg chiral Higgsed gauge theory and 't Hooft and Veltman's renormalizability proof of non-Abelian gauge theories. If, however, there's no such open question concerning empirical facts in comparison to theory, it's very unlikely to find the right idea to find a new theory. The only concrete hint that something is wrong with the standard model is the very plausible existence of "dark matter", and that's why theorists where thinking for some decades now about an extension of the standard model, employing ideas like super symmetry, and that's why at the LHC one is sweeping the parameter space of some minimal SUSY extensions of the standard model. So far to no success. Maybe SUSY isn't the right way to make progress, but that you cannot know without making these measurements (to go beyond the minimal SUSY extensions in the search seems to be very tough, because there are so many parameters, and you have to rule out a huge parameter space without such a constraint to special cases).

 

[mfb]

The only concrete hint that something is wrong with the standard model is the very plausible existence of "dark matter"

Neutrino masses, gravity, baryogenesis, Landau poles, (fine-tuning) - there has to be something at higher energies, as the standard model does not work up to infinity.

The LHC won't be able to exclude SUSY, but if it finds nothing, the theoretical arguments for SUSY get much weaker. And it has a chance to find SUSY or something else new, of course.

 

[vanhees71]

I should have said empirical hints, and you are right, the CP violation of the standard model is not sufficient to explain the matter-antimatter asymmetry of the universe, and also neutrino masses are physics beyond the standard model.

But why shouldn't the LHC be able to exclude or confirm some SUSY model? As far as I know, a lot of SUSY models are already excluded (not only from LHC but also from astrophysical observations). It's of course very tough, if you remember that for finding the Higgs boson the only still not too much constrained parameter was the Higgs mass, but for SUSY you have a multidimensional parameter space to cover!

 

[mfb]

Exclude some SUSY models, or some parameter space: sure. The LHC did that already and will continue to exclude more.
Exclude all SUSY models everywhere: no way.

 

[vanhees71]

I see. So the parameter space for all SUSY models is simply too large.

 

[mfb]

Right.
MSSM ("minimal supersymmetric model") is in serious trouble, NMSSM ("next to MSSM", the "backup" if MSSM is excluded) gets its phase space reduced already, but overall SUSY has more than 100 free parameters. You can always tune the coupling to standard model particles to be small enough, or the particles to be heavy enough, to escape detection at every reasonable current or future accelerator. But if SUSY is too hard to detect, it can also not help solving the theoretical issues with the standard model, and the dark matter argument would get weaker as well.

 

[vanhees71]

That's an example for what I meant with saying that without empirical foundations we are lost in finding new better models. So we can only hope that the LHC finds something new to give more hints for that model building.

BTW is there some recent review on the status of SUSY search at the LHC?

 

[mfb]

Probably, but I don't know where.
Also, I think we are deviating from the original topic.