B What Is an elementary particle?

[microsansfil]

Hi all,

When we read this article http://www.slac.stanford.edu/pubs/beamline/27/1/27-1-weinberg.pdf it seem that there are not single answer to this question.

Does the answer depends on what physical theory we use ? If we use string theory we don't get the same answer as if we use quantum field theory ?

Best regards

 

[alw34]

Physics doesn't answer such a question, but the question is nevertheless a good one.

Does the answer depends on what physical theory we use ?

Our models generally describe observed phenomena and should be useful in making predictions. They rarely tell us "what something is."

The 'particles don't care how we model them, but I 'd agree with your statement: when you ask different scientists, even ask different questions, you'll get different answers.

All known particles are modeled in the Standard Model of particle physics, which is actually a hodge podge of quantum field theories in flat spacetime coupled with observed quantities for which we have no theory, like the mass of an electron. Flat space time means no gravity, no 'graviton' particles.

Rovelli: “…we observe that if the mathematical definition of a particle appears somewhat problematic, its operational definition is clear: particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier” ...

A particle is in some sense the smallest volume/unit in which the field or action of interest can operate….Most discussions regarding particles are contaminated with classical ideas of particles and how to rescue these ideas on the quantum level. "

As presented in Weinberg's "The quantum theory of fields" vol.1: The primary objects are particles described by irreducible unitary representations of the Poincare group. For realistic systems with varying numbers of particles we build the Fock space as a direct sum of products of irreducible representations spaces. Then the sole purpose of quantum fields (=certain linear combinations of particle creation and annihilation operators) is to provide "building blocks" for interacting generators of the Poincare group in the Fock space. In this logic quantum fields are no more than mathematical tools.

And the deeper one goes, the 'crazier' it gets:

It seems that expansion of geometry itself, especially inflation, can produce matter [particles]. Gravitational perturbations [wave inhomogeneaties] in an expanding space produces observable [point] particles. Mathematical transformations between inertial and accelerated frames also seems to produce particles: such different observers see different vacuum energies...and such energy differences result in particle production.

A related phenomena is the "Unruh effect." A vacuum state is observer dependent! The uncertainty principle requires every physical system to have a zero-point energy….http://en.wikipedia.org/wiki/Zero-point_energy “...Vacuum energy is the zero-point energy of all the fields in space...the energy of the vacuum, which in quantum field theory is defined not as empty space but as the ground state of the fields...which leads to virtual particles.

If you are accelerating and I am inertial, we do NOT measure the same vacuum energy! This means if you are accelerating, you detect heat, particles, I do not.

"There is not a definite line differentiating virtual particles from real particles — the equations of physics just describe particles (which includes both equally). ... In the quantum field theory view, "real particles" are viewed as being detectable excitations of underlying quantum fields. As such, virtual particles are also excitations of the underlying fields, but are detectable only as forces but not particles. They are "temporary" in the sense that they appear in calculations, but are not detected as single particles.
http://en.wikipedia.org/wiki/Vacuum_...g_vacuum_state

String theory is unproven and posits particles are actually one dimensional objects, like a string. The Stand Model of particle physics models them as the point like interactions of extended quantum fields.

 

[vanhees71]

Argh! This Wikipedia article about "the vacuum" is very misleading. What's true in it is for sure the first sentence. The rest is too weird to be commented in detail. Particularly the claim that the notion of virtual particles in the form given in this article is accepted is simply ridiculous. What's accepted are the very successful results of perturbation theory in QFT, i.e., a formal expansion of the scattering amplitudes in powers of the coupling or (essentially equivalently) in powers of $\hbar$, leading to the Dyson-Wick series, which is an asymptotic series. These calculations can be tremendously simplified by using Feynman diagrams, which appear to look like scattering processes in a very intuitive manner, but what's behind them really is just a shortcut to write down the expressions for the perturbative S-matrix elements, which can be compared to experimental results concerning scattering processes of real particles.

The particle interpretation in relativistic QT is pretty narrow in the sense that it is possible only for socalled "asymptotic free states". The picture is the following: If you have a short-ranged interaction you can describe particles that are much farther away from each other than the range of the interaction as non-interacting ("free") particles. Mathematically they are free when the distance between becomes "asymptotically large", that's why one talks about asymptotic free states. What's happening when the particles come close to each other and the interaction becomes relevant, is solely describable by quantum-field theoretical abstract quantities, and it is not generally clear, how to interpret this "transient states" in terms of particles. Usually it is save to say that you better don't attempt to interpret them as particles, and usually you cannot resolve the time evolution of a transition. So it is save to say not to talk about "virtual particles" but only about observable facts.

Indeed there are a lot of manifestation of socalled "radiative corrections" (which is a terminus from the time where relativistic QFT was merely restricted to QED, i.e., the description of the electromagnetic interaction, but it applies as well to the entire standard model of elementary particle physics). The paradigmatic once in QED are the Lambshift of the hydrogen-atom levels and the anomalous magnetic moment of the electron. They are due to the corrections of perturbation theory beyond the leading order, where loops occur in the Feynman diagrams. They describe the fluctuations of the quantum fields that can indeed be qualitatively understood by the energy-time uncertainty relation, but the measurable consequences have little to do with particle-like notions. In the case of the Lamb shift you measure a feeble splitting between hyperfine transition lines which are degenerate in the leading-order (tree-level) approximation. For the anomalous magnetic moment of the electron it's the deviation of the socalled Lande-gyro factor relating the electron spin with its intrinsic magnetic moment from its tree-level value 2. In both cases the calculations have been driven to high orders in perturbation theory and the agreement between the QED prediction (including also corrections from the strong interaction, which introduces the largest uncertainty in the theoretical calculation) and measurement is overwhelming, but it has nothing to do with "virtual particles", which you cannot sensibly define at all.

 

[alw34]

To clarify a bit,Micro, your original post is on target. Explaining particles, really the models we use to describe their behavior, gets complicated very quickly. Are quantum fields 'real' or just tools? Different people may emphasize one or the other.

[Vanhees knows way more about all this than I. I have to keep it basic for myself as well as for you.]

Here is another source, but it too has some statements experts here will find extremely objectionable.
http://www.physics.ucdavis.edu/Text/Carlip.html#Hawkrad Parts I like, which may be somewhat less than perfectly technically correct, follow:

So not only do different models, different physical theories yield different answers, people often don't agree on the explanations, that is, the interpretation of the math.

A classical configuration of a field typically does not have a single frequency, but it can be Fourier decomposed into modes [broken down into components] with fixed frequencies. In quantum field theory, modes with positive frequencies correspond to particles, and those with negative frequencies correspond to antiparticles. {and complex numbers correspond to virtual particles}…..

So note that people look at math and conclude: That seems to look like this, or that, experimental observation, a measurement. Is it 'real' or just a fantasy? It's 'real' if that interpretation turns out to make valid predictions and matches observational measurements; if not, the interpretation has to be tossed out for,say, different math or a different interpretation.

[If you have had trig already, you might think of relationships like Sin2X = 2SinXCosXas a type of 'decomposition']. So a sine wave of one frequency can be considered identical to the product of two other trig functions as shown of different frequency...and you can do this endlessly for the myriad of trig relationships.

But those frequencies may not be so simple to interpret: We know from special relativity that frequency depends on time, and in particular on the choice of a time coordinate; two observers in relative motion will see different frequencies [!] for the same source. In special relativity, though, while Lorentz transformations [used in Einstein's special relativity] can change the magnitude of frequency, they can't change the sign, so observers moving relative to each other with constant velocities will at least agree on the difference between particles and antiparticles.

For accelerated motion this is no longer true[!] , even in a flat spacetime. A state that looks like a vacuum to an unaccelerated observer will be seen by an accelerated observer as a thermal bath of particle-antiparticle pairs. This predicted effect [is the] Unruh effect….

The above is a decent intuitive description of how difficult it is to define a 'particle', even to determine how we objectively detect such a particle, in a manner that all theorists would agree is 'accurate'.

So the lesson for me is that physical models describe interactions, not the object particles themselves.

If all this is not enough, search these forums for "What is a particle." and settle in for a lot of reading!

 

[vanhees71]

Well, there are some links to excellent websites about the standard model on the popular-science level, e.g.,

http://www.particleadventure.org/

 

[microsansfil]

Hi alw34,
Thank for your answer.

As presented in Weinberg's "The quantum theory of fields" vol.1: The primary objects are particles described by irreducible unitary representations of the Poincare group. For realistic systems with varying numbers of particles we build the Fock space as a direct sum of products of irreducible representations spaces. Then the sole purpose of quantum fields (=certain linear combinations of particle creation and annihilation operators) is to provide "building blocks" for interacting generators of the Poincare group in the Fock space. In this logic quantum fields are no more than mathematical tools.

Is it the same mathematical representation for elementary and composed particle ? In fact my question is about elementary relate to particle. Thank,
Patrick

 

[microsansfil]

Well, there are some links to excellent websites about the standard model on the popular-science level, e.g.,

http://www.particleadventure.org/

Thank i am going to read it

Patrick

 

[mfb]

Is it the same mathematical representation for elementary and composed particle ? In fact my question is about elementary relate to particle.

You can use the same representation if the internal details do not matter, e.g. for a proton in a hydrogen atom. That gives a very good approximation.

 

[microsansfil]

Hi all

Does we can say that only elementary particle acquire mass with Higgs mechanism and therefore this is not the case for proton in a hydrogen atom ?

Is there any fundamental physical properties that characterize unambiguously the elementary particles ?

Patrick

 

[mfb]

Does we can say that only elementary particle acquire mass with Higgs mechanism and therefore this is not the case for proton in a hydrogen atom ?

The quark masses contribute to the proton mass (although that contribution is small for a proton). Most of the mass is independent of the Higgs mechanism, however.

Is there any fundamental physical properties that characterize unambiguously the elementary particles ?

They are not made out of other particles?

 

[microsansfil]

They are not made out of other particles?

Composite particles being made of elementary particles, thus all physical properties that characterised elementary particles also characterised composite particles and reciprocally ?

For example, there are different types of quarks described with properties as flavor, generation, color. Each type of quark has properties that allows it to bind together with other quarks. Does this properties make physical sense also for composite particles ?

Thanks
Patrick

 

[mfb]

Some properties of composite particles don't make sense for elementary particles and vice versa. Some properties are relevant for both.

As an example, composite particles have a non-zero size. Elementary particles do not (according to current knowledge).
Quarks are sorted by generations, sorting hadrons in generations in the same way doesn't work. As a classical analogy: people have an age, but groups of people do not have a single value "age".

 

[Garlic]

I want to add something.
Why are electrons considered to be elementary particles despite the existence of quasiparticles?

 

[mfb]

Quasiparticles are quantum states in many-body systems.

 

[bob012345]

Physics does not answer such a question, in fact physicists stare at you funny for even asking such a question which is telling.

It suggests the state of the theory is woefully incomplete and they get defensive. It's shocking that after a century of particle physics they have no real idea of what a particle is.

 

[vanhees71]

I don't understand this statement.

By definition an elementary particle is one that can be described by a quantum field in the formalism of relativistic quantum field theory. Empirically it depends on the energy scale you look at it, whether this holds true to a good accuracy or not. E.g., at low scattering energies you can treat protons as elementary particles described by a Dirac field with good accuracy. This holds no longer true for higher scattering energies. In fact with high-energy electron scattering on protons it was revealed that the proton seems to be a bound state of three partons. That lead to the discovery that Gell-Mann and Zweig's "mathematical constructs" they called "quarks" are in fact reall things rather than mathematical constructs. Nowadays we consider the quarks as elementary particles described by Dirac fields and the proton as a complicated bound state of quarks and gluons within the theory of the strong interaction, called Quantum Chromodynamics (QCD), which has the funny property (indeed not yet fully understood) that the elementary quanta described by them (quarks and gluons) never occur as free particles, which is called confinement.

 

[bob012345]

I don't understand this statement.

By definition an elementary particle is one that can be described by a quantum field in the formalism of relativistic quantum field theory.

"I don't understand this statement" typifies the modern physics mind well. "By definition..." Fundamental particles are not defined, they are measured. Your math to describe them is defined and thus is only a model, not the thing itself. Reality is forced to conform to the theory rather than the other way around. What if field theory is not the best way to describe particles? But you have defined particles as only that which can fit into field theory.

 

[vanhees71]

Well, of course, it's to be measured whether the particle behaves like an elementary one. You need, however, to define what an elementary particle is, before you can compare the definition with your measurements. There's no measurement possible without theory. Even as apparently simple a quantity as the length of my table needs an assumption about geometry before you can measure it!

 

[mfb]

What if field theory is not the best way to describe particles?

It is the best way we found so far. And it works incredibly well. It is a very good model, and physics is all about models.

 

[alw34]

It suggests the state of the theory is woefully incomplete and they get defensive.

There is some merit to this, to be sure. People who have spent careers studying such things are apt to defend the vast wealth of knowledge that has been accumulated. Someone said something to the effect "We know so much, we understand so little." And that can thinking can infuriate people in almost any field. So better to focus on open questions and future opportunities, perhaps?

For example, we don't know how to economically develop fusion power,anti gravity flight vehicles or transport between dimensions, maybe because we don't even know if such dimensions [parallel universes, for example] exist.

It is the best way we found so far. And it works incredibly well. It is a very good model, and physics is all about models.

I like "...is the best way we have found so far", but I don't like to say our physics 'works incredibly well' so much.
For me, 'incredibly well' would answer what particles are, where they come from, and the Standard model would include gravity and be based on fundamental principles. We don't have a physical 'theory of everything" yet.

Physics is great within its confines and limitations, but vast new new horizons remain. On the other hand, compared to our understanding of the human body and, say, the brain, or human behavior, I'd be willing to argue no branch of science surpasses that of the achievements of physics.

Hmmm, do the vaccines of medicine and perhaps genetics give physics a 'run for its money'? Maybe, but I'd say viruses are still smarter and faster than we.

 

[mfb]

"what particles are" is philosophy. Where the current particles were created is well-known.
What does "be based on fundamental principles" mean? Is gauge invariance a "fundamental principle"?

Combining nonlinear gravity and the Standard Model: well, working on it. There are just two known cases where both at the same time are relevant, and both of them are very far away (in time or space) which makes them hard to study.

 

[vanhees71]

Generally one can say that bob012345 has somehow misunderstood how science works. No serious scientist thinks that the contemporary theories or models are the final answer to all questions about nature. Science aims at an as accurate description (NOT "explanation") of empirical facts about nature. To do any quantitative measurement you need a theory, defining the quantities. E.g., for measuring a length and angles you need geometry. To measure kinematical quantities like position, velocity, acceleration of a point particle you even need a space-time model. To understand dynamics you need a model of interactions etc.

The best model concerning "elementary particles" we have today is the Standard Model of elementary particles, based on relativistic quantum field theory and the symmetries of special-relativistic space-time. It is very accurate to an astonishing degree. In fact, it is hard to find deviations from the standard model. The physicists, particularly at the LHC at CERN, are very eager to find "physics beyond the Standard Model", because it's quite clear that the Standard Model cannot be the final answer (hierarchy problem, nature of dark matter). The only solid violation of the Standard Model are the neutrino oscillations, and here the situation is not fully understood. Only recently there was an interesting finding by the Daya Bay collaboration on antineutrino data, indicating that there may be even more (sterile?) neutrinos than known so far. There are also other indications (decay of B mesons) that there might be a deviation from the Standard Model.

So the aim of physics is, to test models to their extreme and find deviations to learn something new. It's not as you claim, that physicists think their models are correct at any cost, but to the contrary, they are vigorously checked in more and more refined measurements and at higher and higher collisions energies. Physics finally is an empirical science with a strong theoretical component to order the empirical facts and subsume them under as simple as possible basic concepts. In contemporary physics these are symmetry principles, the symmetries closely related to conservation laws (Noether's Theorems). They are also guidelines for model building, i.e., they constrain the possible form of the natural laws, but they are themselves of course subject to experimental tests.

 

[microsansfil]

Hi mfb

"what particles are" is philosophy.

If we add "in the context of physics" for me is a physical question unrelated with ontology. I should have written "What is an elementary particle in the context of physics"

Thanks
Patrick

 

[alw34]

"what particles are" is philosophy. Where the current particles were created is well-known.
What does "be based on fundamental principles" mean? Is gauge invariance a "fundamental principle"?

I recognize that as a widespread, perhaps even almost universal perspective. But such a view of "What particles are", I think, and its a personal opinion, short changes physics. I happen to think we can do better.

It's as if medical researchers were satisfied that a certain drug 'cures' certain symptoms, but didn't really know why, because the 'cure' arose simply from trial and error. That seems like a horrible perspective. I just saw, for example, some medical researchers are now proposing Alzheimers might arise from virus or bacteria. If true, that would seem to require entire new areas of research.

(mentor note: removed strikeout text as it didn't seem to match the discussion.)

 

[Drakkith]

Several off-topic posts have been removed. I remind all PF members to please stay on topic, which is about what fundamental particles are as described by mainstream science.

 

[alw34]

I think I picked up this link from a discussion here in physics forums.

http://www.nature.com/news/not-quite-so-elementary-my-dear-electron-1.10471

Not-quite-so elementary, my dear electron 4/2012
Fundamental particle ‘splits’ into quasiparticles, including the new ‘orbiton’.

"...Isolated electrons cannot be split into smaller components, earning them the designation of a fundamental particle. But in the 1980s, physicists predicted that electrons in a one-dimensional chain of atoms could be split into three quasiparticles: a ‘holon’ carrying the electron’s charge, a ‘spinon’ carrying its spin (an intrinsic quantum property related to magnetism) and an ‘orbiton’ carrying its orbital location1.

“These quasiparticles can move with different speeds and even in different directions in the material,” says Jeroen van den Brink, a condensed-matter physicist at the Institute for Theoretical Solid State Physics in Dresden, Germany. Atomic electrons have this ability because they behave like waves when confined within a material. “When excited, that wave splits into multiple waves, each carrying different characteristics of the electron; but they cannot exist independently outside the material,” he explains.

 

[mfb]

I recognize that as a widespread, perhaps even almost universal perspective. But such a view of "What particles are", I think, and its a personal opinion, short changes physics. I happen to think we can do better.

It's as if medical researchers were satisfied that a certain drug 'cures' certain symptoms, but didn't really know why, because the 'cure' arose simply from trial and error. That seems like a horrible perspective. I just saw, for example, some medical researchers are now proposing Alzheimers might arise from virus or bacteria. If true, that would seem to require entire new areas of research.

I don't think that analogy works that way.
A medical researcher is satisfied if they understand which molecule of the drug reacts where in which way. They do not discuss "do molecules actually exist?" - the existence of molecules is a really good model.
Particle physicists are not satisfied if they just know "okay, we get a Higgs once in a while" - but they are satisfied if they can predict how often they get one, what its energy distribution looks like, what its decay modes and the angular distributions of the decay products are, which other particles are produced in the collisions, and so on. We do not need to discuss "do quantum fields and particle-like excitations of those actually exist?" - their existence is a really good model.

 

[alw34]

We do not need to discuss "do quantum fields and particle-like excitations of those actually exist?" - their existence is a really good model.

maybe, but isn't it still the case that we don't know the origin, the cause, of the Big Bang?

 

[Drakkith]

maybe, but isn't it still the case that we don't know the origin, the cause, of the Big Bang?

Yes, but that is beyond the scope of this thread and is something that we may never be able to answer.

 

[alw34]

A medical researcher is satisfied if they understand which molecule of the drug reacts where in which way. They do not discuss "do molecules actually exist?" - the existence of molecules is a really good model.

Glad you posted that.

made me realize my post was incomplete, inadequate: Instead of just understanding the action of the drug molecule, I'd sure like them to know the answer to this question: "What went wrong in Alw34's DNA to make such a drug therapy necessary?" In other words, root cause.

And that is not the end of the investigation either, but a good start. I think an even better question would be: "How do we prevent such aberrations in DNA?" [Which leads to : "How do we make better DNA sequences!"

Edit: oops, I see we are, I am at least, straying from the original question...enough...
good thread.

 

[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.