experimental physics - What is a Pseudoscalar particle?

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   Can someone explain to me what is a pseudoscalar particle?

   
   


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   And how do experiments figure out that what they're dealing with is a scalar or pseudoscalar?

   
   

Answer

As the Wikipedia page you linked to, pseudoscalar particles are just like scalar particles, but their associated field (or wave function) is assigned an additional sign flip if we decide to study how objects behave under the parity, i.e. under $$ (x,y,z) \to (-x,-y,-z) $$ In general, we may study whether some equations of physics are invariant under this mirror reflection (which changes the left hand to the right hand). They may fail to be symmetric altogether. But if they happen to be symmetric, we are still free to transform scalar fields either as $$ s(x,y,z)\to s'(x',y',z') =+s(-x,-y,-z) $$ which are (true) scalars or $$ p(x,y,z)\to p'(x',y',z') =-p(-x,-y,-z) $$ which are pseudoscalars. Scalars and pseudoscalars have $P=+1$ and $P=-1$ where $P$ is the parity and the sign is correlated with the sign in the transformation law above.

If parity is conserved and we may determine the parity of the decay products, the parity of the original state is simply equal to the parity of the products – which is the product of individual parities in the simplest cases (parity is a multiplicative quantum number). So particles that are quanta of scalar fields or pseudoscalar fields behave in the same way "separately" but they may have different decays. If you want me to mention objects that are scalars or pseudoscalars, let me say that $\vec E\cdot \vec E$ is a scalar (the sign, an energy density, doesn't depend on the mirroring) but $\vec \cdot \vec B$ is a pseudoscalar (because the magnetic field is a pseudovector, so its sign is a matter of conventions and one must revert it if we switch to a mirror-symmetric configuration).

In the case of the Higgs-like particle discovered by the LHC, you may figure out what the parity is e.g. by methods described in today's paper by John Ellis et al.

http://arxiv.org/abs/1208.6002

In general, the relative motion of the final products can compensate the parity and allow the decay of the initial particle regardless of the parity. However, the parity will still influence the angular distributions of the final particles as well as the distribution of their relative energies.

Is a single photon also a Maxwellian wave?

A photon is associated with the equations $h\nu$ and $\frac{hc}{\lambda}$.

My book (Serway Modern Physics) says that Einstein explained the photoelectric effect by assuming that the classical wavefront had its energy distributed over bundles, with energy $h\nu$. (I've been told this is wrong elsewhere, but its not crucial to my question anyways).

With this picture I imagine that the Maxwellian wave is replete with photons, and its not too hard to digest that the energy of a photon could be given by $h\nu$ where $\nu$ is the frequency of the Maxwellian wave.

But what about a single photon produced, e.g., in an electronic transition in an atom? What does the frequency or wavelength variable in its energy a wavelength or frequency of? Is it also of a classical Maxwellian wave? Meaning that a single photon has a complete plane wave associated with it?

How is this Maxwellian wave distributed over space? Is the common picture of the photon as a little sperm correct then?

quantum mechanics - Is there a symmetry associated to the conservation of information?

Conservation of information seems to be a deep physical principle. For instance, Unitarity is a key concept in Quantum Mechanics and Quantum Field Theory.

We may wonder if there is an underlying symmetry, in some space, which may explain this conservation of information.

electromagnetism - How is a magnetic field translated into physical force?

Related to this question Where do magnets get the energy to repel?

If I have a magnet repelling another, eg one in my hand, the other being pushed along the desk, how do the each of the magnet's fields actually "push" against each other? What translates the magnetic field to kinetic energy?

geometry - Painting with a Pendulum: Would it be possible to graph the pattern?

I intend to try and replicate an experiment that I found online:

The idea seems to be:

1. Attach a string to a fixed, overhead object


2. Attach a can of paint to the string


3. Put a hole in the bottom of the can and plug it


4. Pull the can to one side


5. Unplug the hole and release


6. The paint can will apparently paint some sort of reducing, Fibonacci spiral

The experiment seems relatively straightforward.

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However, I'd like to take it one step further:

If possible, I would like to:

1. Ascertain the formula for this line


2. Plug it in to some sort of open-source graphing software


3. Plot on a large-format plotter (mine can print up to 42" wide).


4. Compare the theoretical line to what actually happens in practice (with paint)

Since I would be doing this experiment myself, I think I would be able to determine some of the variables, such as:

1. Volume of the paint can



2. Rate of paint release over time


3. Distance of initial travel from the release point to the natural resting point (would it be an arc?)


4. Other factors?

Question:

What would the formula be for this line?

The catch: I know nothing about math or physics. This would be a learning experience for me, to say the least.

gauge theory - How many fundamental forces could there be?

We’re told that ‘all forces are gauge forces’. The process seems to start with the Lagrangian corresponding to a particle-type, then the application of a local gauge symmetry leading to the emergence of the force bosons via the associated symmetry group.

But where did exactly four forces come from? Could new, perhaps supersymmetric particles hint at new fundamental forces? Is there a deeper theory which predicts what final set of forces we’ll eventually end up with?

Finally, is the concept of force in unified physics really that fundamental at all?

Answer

The very claim that there are "four fources" is an approximation. We know that the electromagnetic and the weak force have to be unified to an electroweak theory. So counting the electroweak theory as one force, there are just three known elementary forces.

The electroweak theory is based on the $SU(2)\times U(1)$ group which has two factors, but these two factors are not in one-to-one correspondence with the electromagnetism and the weak force, respectively.

The strong force with its $SU(3)$ group is another seemingly independent factors, except that there is evidence that all three non-gravitational forces get unified into a grand unified force of a GUT theory at high energies.

String theory unifies the non-gravitational forces with gravity, too.

Every vacuum of string theory predicts gravity described by GR plus extra non-gravitational forces. The number of factors and their Higgs-like breaking patterns are essentially random properties of the string vacua. According to the anthropic picture of the world, the number of low-energy forces is an accidental property of our world that could be different in different parts of the multiverse.

According to non-anthropic reasoning, the precise selection of our vacuum - including the fact that it has 4 low-energy forces - could be derivable from some more unique theoretical principles. However, this research program remains a wishful thinking as of 2011.

condensed matter - References on the physics of anyons

Anyone know some good introductory references on the physics of anyons?

Answer

One of the best recent references is the 2008 RMP article by Nayak et al. Non-Abelian Anyons and Topological Quantum Computation

A somewhat less technical reference is An Anyon Primer by Sumathi Rao.

There are many others but these two are good for someone starting out.