Posts Tagged ‘particle physics’

So Just HOW Do You Measure the Shape of the Electron?

May 26, 2011 3 comments

A paper recently published in Nature is generating quite a bit of media buzz. [PhysOrg, BBC, Fox, PhysicsWorld]

The paper is entitled ‘Improved measurement of the shape of the electron’ and describes, well, a new method for measuring the electron’s shape.

I love when physics paper titles are easy to understand :)

Anywho, the main thrust of the paper is that the electron appears to be spherical to a very high degree of accuracy. In fact, the press release from the Imperial College of London states that,

the electron differs from being perfectly round by less than 0.000000000000000000000000001 cm. This means that if the electron was magnified to the size of the solar system, it would still appear spherical to within the width of a human hair.

Wow! Now that is pretty darn spherical.

But now you’re thinking “Hey wait, I thought the electron was a wave? Or a string of energy? Or a cloud of virtual particles? How can it actually be spherical?”

These are excellent questions. Indeed, when we first learn about atomic structure in science class the electron, protons and neutrons are all depicted as perfect spheres. As we learn more, we know that this is only an approximation, an easier way of visualizing the complicated subatomic structure.

The truth is, the electron isn’t (well, probably not) spherical. We don’t really know for sure. Current theories point to the most accurate picture being that the electron is a cloud of particles, blinking in and out of existence, which contribute to its mass and size.

So what this paper and these news outlets are actually saying is that this experiment has shown that the electron behaves as though it is a sphere.

Even more accurately, the electric dipole moment of the electron is approximately zero, which is what we would expect from a perfect sphere with uniform charge distribution.

Ok, I know I just said a mouthful. So let’s go through exactly what I mean about the electric dipole moment, and then we’ll go through what this paper actually measured.

Let’s begin with what an electric dipole moment actually is. Imagine you had two particles, one with a negative charge (-q), and one with a positive charge (+q). If you put these two charges close together, you will create special electric field pattern. This type of arrangement creates what is called an electric dipole moment (EDM). The EDM vector (p) is defined as the of the charge on the two particles (q) times the displacement vector between them (d).

The electric dipole moment vector (blue arrow) points, by definition, from the negative charge to the positive charge.

However, you can also create an EDM if you were to have a particle with an uneven charge distribution.

For example, imagine you had a sphere with a total charge +q. In this case, the charge is evenly distributed and you don’t get an EDM.

A perfect sphere with a uniform charge distribution does not have an electric dipole moment.

But now imagine you had an oddly shaped particle that was “squished” at one end.

In this case, there is more charge at one end of the particle than there is at the other. This uneven charge distribution gives the particle its own EDM.

A not-so-perfect sphere has a non-uniform charge distribution. The higher concentration of positive charge at one end creates an electric dipole moment (red arrow).

So if the electron is not perfectly spherical, it should have an EDM. If it has an EDM, we should be able to measure it to infer the electrons shape. Simple, right?

Now, the Standard Model of Physics predicts that the EDM of the electron is too small for us to currently measure; our equipment is just not sensitive enough. But there are variations on the theory that say the electron’s EDM may actually be larger enough to measure using our current technology.

So finding the electron’s EDM will help narrow down our current theories on the subatomic universe.

The existence of an EDM may also help explain why there is so much matter in the Universe and so little antimatter. If the reason for this apparent imbalance in matter and antimatter is the result of an as-of-yet-undiscovered particle interaction, then the current theories of particle physics predict that there should be a measurable EDM for the electron.

So this explains why this experiment is so important. Now lets explain the experiment.

In a simplified picture, the electron EDM in an applied electric field will either point in the same direction as the field, or in the opposite direction. The energy of an EDM in an electric field depends on the direction of the EDM in relation to the electric field.

This means that the EDMs that align with the electric field will have a different energy than those that align against the field. This difference in energy is proportional to the magnitude of the EDM.

So how does one measure this energy difference? One way is to align the spins perpendicular to the field, which will cause them to precess and you can then measure the precession rate, which is proportional to the energy difference.

This effect can also be described in terms of how the two energy states interfere with one another. This interference between the two states can be measured using an interferometer. If there is an EDM present, then a phase shift should be seen in the interferometer signal. If the applied electric field is reversed, then the phase shift should change sign.

So the authors of the paper went looking for this phase shift. They used molecules of Yttrium Fluoride and fired them at a speed of 590 m/s into an apparatus which has a constant electric and magnetic field.

A radiofrequency pulse is applied which excites the molecules into their respective energy states. They are then allowed to interact for a certain amount of time (a few milliseconds) and it is during this time that the molecules in the different energy states develop a phase difference.

A second radiofrequency pulse is applied and the number of molecules which end up in the lower energy state is measured and is proportional to the phase difference they developed during their interaction time in the electric field.

This phase difference is measured via the applied magnetic field and creates an interference curve.

An example of an interference curve from measuring the phase difference via the magnetic field. (Figure 3 from this paper)

If the electric field is reversed, then a small phase shift in the interference curve is seen. Remember that the phase shift is proportional to the electron EDM.

So by varying certain parameters like the magnetic field and the frequency of the radiofrequency pulses, the authors were able to extract the numerical of the electron EDM from the data.

Over 25 million pulses of YbF were used to collect this data. Not only that, but many experiments had to be done to determine systematic sources of error in the experimental setup.

Things like fluctuations in the applied magnetic field, electric field plate potentials not being completely symmetric, magnetic fields generated in the magnetic shielding during switching of the electric field are all sources of error which had to be considered.

So after all this work they finally arrived at their calculated value of the EDM for the electron. The value turned out to be de = (-2.4 ± 5.7stat ± 1.5syst) × 10-28 e · cm, where the first error term is from statistical uncertainty and the second is from systematic uncertainty.

Notice that the error on this measurement makes it consistent with zero and consistent with previous work.

However, this measurement is 54 times more precise than the previous one the author’s previous measurement and puts an upper limit on the EDM of the electron which must be less than 10.5 × 10-28 e · cm.

The next step in these types of experiments is to reduce the uncertainty of the measurements. The authors believe that they should be able to do this using cold molecule techniques and get their measurement down into the 10-29 e · cm range.

Be sure to check out another blog post about this paper by Chad Orzel, author of the blog “Uncertain Principles” and the book “How to Teach Physics to Your Dog”.

Hudson, J., Kara, D., Smallman, I., Sauer, B., Tarbutt, M., & Hinds, E. (2011). Improved measurement of the shape of the electron Nature, 473 (7348), 493-496 DOI: 10.1038/nature10104

One Step Closer to Finding the God Particle

July 28, 2010 1 comment

The God Particle, or the Higgs Boson as its known in the Physics world, is coming closer and closer to being found. (I recently wrote an article  about the Higgs, you can check it out here for a bit of background info)

Experimenters at Fermilab, which is a particle accelerator laboratory in the United States, are competing with Europeans at the new-fangled Large Hadron Collider (LHC) in Geneva, Switzerland.

They are competing for the ultimate prize: finding the Higgs Boson, and experimenters at Fermilab just narrowed the search a bit.

I see this competition like a nerdy version of Rocky IV. Fermilab is Rocky, the hard-nosed American underdog (Fermilab is much less powerful than the LHC) and the LHC is the engineered Russian super-athlete.

Fermilab vs. LHC

One of the biggest problems with finding the Higgs is that no one knows exactly what its mass is (i.e. how heavy it is). But we do know that the mass should be between 114 and 185 GeV/c2

Oh, and  GeV/c2 is a unit of mass that particle physicists use. I’m not gonna go into a whole lot of detail, but for comparisons sake the proton is roughly 1 GeV/c2

So the Higgs boson is supposed be roughly between 114 and 185 times larger than the proton.

But Fermilab just released some results which showed that the Higgs is NOT between the masses of 158 and 175 GeV/c2

So this narrows the search parameters a little bit, and hopefully it results in finding the Higgs a bit sooner.

Of course, NOT finding the Higgs boson would just as huge a result. It would mean the Universe is a whole lot weirder than we already thought, and there are those who think we won’t find it.

So exciting times in physics world. But of course its ALWAYS exciting in the physics world! You can try and keep up with all the excitement by following me on Twitter

The (Simple) Physics of the ‘God Particle’

July 21, 2010 1 comment

The ‘God Particle’.

Pretty catchy name. Its been in the news quite a bit lately. But what is it exactly? And why would they call it the ‘God Particle’? Especially since science and religion get along about as well as Frank and Estelle Costanza!

Well in this blog post, I’m going to give you a basic and (hopefully!) entertaining explanation of what the God Particle is, and why we should care. So let’s start at the beginning.

The ‘God Particle’ is also (and more accurately) known as the Higgs Boson. Described in a single sentence, it is believed to be the particle that gives mass to all other particles in the Universe.

Ok, that SOUNDS important, but its still a bit hard to understand, so here’s a bit more thorough explanation.

Everything in the universe is made up of particles. And there are several different kinds of particles.

All the matter in the universe is made up of atoms. Atoms are made of a nucleus, which is found at the center of the atom and has neutrons and protons in it. Surrounding the nucleus are electrons, which are much smaller and fly around the nucleus in a circle, or an ‘orbit’.

Groups of atoms can get together and form molecules, and big groups can get together to form rocks, trees, and Maria Sharapova.

So thats 3 particles we have already described (protons, neutrons, and electrons). These 3 particles have mass; this essentially means that they weigh something.

But there are other types of particles out there too. For example, there is the photon.

Photons are are basically light. They are tiny packages of energy that make up a beam of light. They also make up radio waves, x-rays, and gamma rays (the stuff that gave the Fantastic Four their powers).

But photons are different from, say, protons, because they don’t have any mass. They carry light energy from the sun, for example, to the Earth. Or they can carry radio messages from the radio station to your house. So photons are like messengers; as such, they are sometimes called “messenger particles”.

A “messenger particle” is also called a boson. Bosons are really cool because they actually DO something. What do I mean by that?

Well, if you remember high school physics or chemistry class, you know there are 4 forces in nature. Gravity is one of them, and it is the most familiar too us. Its what keeps us firmly planted to the ground. Electromagnetism is what makes electricity, light beams, radio waves, and magnets work.

You Can Visualize a Magnetic Field with Iron filings and a Bar Magnet

The other two forces are a little less familar. They are called the Strong nuclear force, and the Weak nuclear force. These two are basically what holds the nucleus of the atom together, and make it behave the way it does.

So what does this have to do with anything? Well remember bosons are messenger particles. The photon carries the electromagnetic force “message”. The other forces in nature have bosons as well that carry their “messages”. Gravity has the “graviton” (which hasn’t been observed yet but we think its out there). The strong nuclear force has the “Gluon” (because it ‘glues’ the nucleus together). And the weak nuclear force has the “W” boson (it doesn”t get a cool name because its not cool).

Ok, now we get to the Higgs boson. So, since it is a boson, it must be the “messenger” of something right? So what is it the messenger of?

Well, remember I said that protons, neutrons and electrons have mass? But the photon does not have any mass. Why is that? What is it that makes one particle have mass, and the other not have mass? Even a couple of the bosons have mass! Thats just freakin’ weird.

So particle physicists (one of the named Peter Higgs, oddly enough) came up with a theory. They think there is some kind of a field in the universe called the “Higgs Field”. Its kind of like a gravity field, or a magnetic field. Just like a magnetic field will interact with some iron to pull it in one direction, the Higgs Field will also interact with particles. But instead of pulling at them, the Higgs Field gives these particles mass! It makes them heavy!

The theory also says there should be something called the Higgs boson: an actual particle that carries the Higgs field “message”. And thats what we are trying to find. The Higgs boson is the messenger particle of the Higgs Field, which is (theoretically) what gives particles their mass. If we do find it, then we know our theories about how the universe is made are on the right track. It would be HUGE breakthrough for physics!

One problem: the Higgs boson is supposed to be heavy! Well, for a particle its pretty heavy.

In fact, the only way to actually “make” one is by slamming together stuff like protons at close to the speed of light in what we call a “particle collider”. And we need to slam them together at a really BIG energy, so we need a BIG collider. Thats why we have the Large Hadron Collider.

The Large Hadron Collider in Geneva, Switzerland

Ok, so thats the explanation of what the Higgs boson is and why we should care. So why is it called the ‘God Particle’.

Well, a guy named Leon Lederman wrote a book called “The God Particle: If the Universe Is the Answer, What Is the Question?which was actually about the Higgs boson. Calling it the “God Particle” was a kind of grandiose name because it suggested we knew what it was that gave particles mass, what made they heavy or “real”. Very “god-like” I suppose”.

The term “God Particle” also showed up in Dan Brown’s novel “Angels and Demons”. In the book some claimed that the discovery of the particle would prove the existence of God.

Finding the Higgs would be great, but would hardly prove the existence of God. The use of these terms is largely to increase media interest.

Phew, well there you have it. If you want to hear more about Physics news as it happens you can follow me on Twitter! Or you can follow the Large Hadron Collider on Twitter!