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22 responses to “Articles”

1. Christopher F Van Meier

I have been following the construction of the LHC with great interest over the years. I am a high school history and English teacher, but have a huge personal interest in forefront of modern physics.
A perplexing question has arisen for me. When I look at the tracks of the colloisions, there are noticible curved tracks. Are these related to spin? And if not, what is the reqason behind the curved treacks.
Thanks
Van Meier

• “The second is the degree of bending of the particle’s path in the presence of a magnet, which we install in some of the sub-detectors. ”

About halfway through the detectors is a giant solenoid, which generates a multi-Tesla magnetic field running parallel to the beampipe inside the tracker. This causes the tracks of charged particles to curve (specifically, to form a part of a helix, whose axis is parallel to the beampipe). The direction of curvature tells us the particle’s charge; the degree of curvature tells us its momentum. In fact this technique has been used for over a century; it is the principle behind mass spectrometers and many other devices.

Your question reminds me that I need to do some more work on the above article!

• Alice Roberts

Put even more simply: the curvature is not “natural”, but caused by huge magnets built into the detectors specifically to cause it. This lets the scientsts figure out the charge of the particle (by the direction of curvature), and the charge/momentum ratio. The charge interacting with the magentic field causes a curving force; the ratio of that force and the particle’s momentum determines the amount of curvature. Since charges other than ±1 are very rare, for practical purposes this gives you the momentum of the particle.

Although the detector magnets do not have as high a field strength (measured in Tesla) as the main dipole magnets in the accelerator, they are still very strong, and vastly larger, making them the highest-powered magnets in the whole complex by many measures.

All 1232 dipoles around the full LHC circumference have a combined stored energy of 11 GJ, when operating at 14 TeV, which won’t actually happen until after the 2013 long shutdown. The CMS solenoid magnet alone has a stored energy of 2.3 GJ, at the strength they’re running it at today. For details, see Analysis and design of the CMS magnet quench protection.

The magnets are oriented in such a way that they have minimal effects on particles travelling straight down the middle of the detector, notably the colliding protons themselves. But anything that sprays outward will cross field lines and be bent.

2. Christopher F Van Meier

The article was very clear. It is more that I am a) a layman and b) right in the midst of a new book on the LHC.
Some of the things brought up in the book have generated a lot of interest for me. You can read that as a lack of knowledge. Spin, in particular, has me a bit perplexed. I am of two thoughts here. The first is that left and right spin is litteral, hence the question about the curved tracks (i was thinking torque), and the sesecond is that spin is simply a method of differentiation between two sub atomic particles.

• Spin *is* confusing. It is literal, but very counter-intuitive. I have avoided trying to explain it because I haven’t thought of a great way to do it yet. It is possible for certain elementary particles to spin, somewhat the way the earth does, around an axis. That’s the picture you should have in mind. But what is counter-intuitive is that there is nothing you can do that could eliminate or increase that spin; it is a fundamental property of the particle. An electron will always spin; the only thing you can do, potentially, is change the axis of spin, or flip the spin from clockwise from counterclockwise. [Nobody would have guessed particles spin in this strange way if it were not for (a) an experiment on electrons that was hard to explain, and (b) math that showed that spinning particles with this strange behavior explained the otherwise-difficult-to-explain experiment. And since the same math also eventually explained the entire periodic table, and many other experiments before and since, we accept this odd feature of particles as true, despite its challenge to our daily conception of what it means for something to rotate on its axis. It is neither the first time nor the last that nature has forced us to give up an intuitive concept in favor of a richer one.]

3. Christopher F Van Meier

A follow up…your response makes clear that it is the magnets that influence the path of the particles. Got that. However, at 6 a.m. it has raised another odd question. A regular curved path makes sense to me. However, there are the occaisional paths with a seeming ever increasing curve (the curve sharpens and begins to close ever tighter in on itself)

• Right — if a particle hits something inside the detector, it loses energy and momentum; that makes its path curve more sharply. The reverse never happens!

• Alice Roberts

Again, at the risk of stating the obvious: any time you can see the curve, it’s because the particle is actually hitting parts of the detector, which means losing energy to it.

The most interesting particles are usually the high-energy ones, whose path barely curves. Low-energy particles start out curving strongly, and then lose even more energy, so they end up spiralling down to zero (and getting absorbed by the detector material). You see lots of those.

The stronger the magnet, the easier it is to measure the momentum of high-energy particles accurately, which is good. But some low-energy particles can be curved so sharply that they never make it out of the beam pipe to the detector! This leads to a demand for a small and thin-walled beam pipe, surrounded by a very small “inner detector”. But so close to the beam, the inner detector gets hammered by radiation.

There are lots of difficult tradeoffs in detector design.

• Just to clarify the way the first sentence in the above comment was worded:

The curvature of the track is caused by the magnetic field, which exerts a force on the moving charged particle, changing its motion and causing it to spiral.

The spiral is very tight or very loose depending on whether the particle has a small or large amount of momentum.

When that particle hits something in the detector, it causes an electronic signal which can be detected. Those detected signals are used to figure out where the particle went. But hitting something also causes the particle to lose just a little bit of momentum (and motion-energy).

As the particle gradually loses momentum through these collisions, its spiral becomes increasingly tight.

4. Christopher, Hi,

The higher the energy of the collision, the higher the chance that it will produce heavier particles to “appear” (some of them, even though are created after the collision, do know really make themselves evident at the detectors). Many of those heavier particles are very unstable: they last as themselves just a small instance of time, and then decay into some other more stable particles.

Many of these decay conversions involve pairs of particle-antiparticle. Most of these pairs (but not all) are charged particles, which means that on each pair, you will have a negative particle and its antiparticle, which will be a positive particle.

The easiest way to “see” these charged pairs in the aftermath of the collision is by applying a very strong magnetic field that will clearly make the pairs evident, as two opposite curls that show up somewhere.

Kind regards, Gastón

5. Regarding spin, we should consider a few things. Spin is more significant for fermions, than for bosons: that is to say, only fermions are affected by Pauli’s exclusion principle.

Another thing regarding spin on fermions was clearly exposed by Dirac’s theory, which he discovered while trying to convert quantum mechanics equations to a Lorentz invariant form: spin turned up naturally in this Lorentz invariant form.

So, while Pauli had to “force” spin into the picture when he deviced his principle, for Dirac spin appeared as a natural consequence of applying special relativity to QM equations.

From this perspective, we could argue that spin appears to be a relativistic effect.

Another interesting thing about spin appears in degenerate matter, like what happens to electrons in the atoms of a star that has collapsed into a white darf, or into a neutron star.

Even though electrons of the same energy level are constrained into very closed quarters, they stay away from each other by increasing its energy and moving wildly. This behaviour is important because it helps to conserve Pauli’s principle. What is the main parameter for this behaviour in the wave function? spin.

Spin is one of the main things that makes fermions such weird particles, besides the fact that fermions are fermions just for having a fractional value of its spin.

Kind regards, Gastón

6. One correction to my last comment: degenerate matter includes white darfs and neutron stars, but my comment on spin should only apply to white darfs, as in a neutron star, matter is so affected by degeneracy (atoms are so collapsed by gravity), that it forces electrons into the nucleus which such pressure that electrons and protons turn into neutrons, so, the resulting degenerate atoms are only made up of neutrons.

Kind regards, Gastón

7. emeka

Firstly, thank you for answering my first post. Secondly, I apologize if the questions below are posted on the wrong page. Internet access is limited in my country. I’m not even sure if this post went through.

My two questions:

A documentary I watched asserted that since photons are responsible for all electron and proton interaction, everything we do in our everyday life relies on the exchange of photons. Can you elaborate on this claim?

Secondly, if gravity has to do with the curvature of spacetime, why do we need the graviton?

Thanks

• It is true that all of ordinary matter is held together by electromagnetic forces. In this sense, the electromagnetic field is essential to everything we do. To say this force is due to the “exchange of photons” is not exactly right. It is due to the exchange of “virtual photons” — which really aren’t photons at all. https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/

What is the relation between a real photon and the electromagnetic field? The electromagnetic field can wave back and forth, and those waves, in quantum mechanics, are made from quanta, called “photons”. In other words, the photon is a ripple in the electromagnetic field. It is not essential for holding atoms together. Only “virtual photons” are needed to do that.

However, photons (real ones, not the virtual not-really-photons) are also essential for almost everything we do — sunlight! Without it our lives would be dark and cold. Our eyes are photon detectors, absorbing one photon at a time.

Similarly, the curvature of space-time gives us gravitational fields, and gravitational forces. “Virtual gravitons” (which really aren’t gravitons at all, but a general effect of the gravitational field) hold the earth together.

Just as for the electromagnetic field, waves in gravitational fields are also possible, and are also, in our quantum world, believed to be made from quanta, which we call gravitons. We don’t “need” them in our daily lives. But they almost certainly exist.

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10. Flox

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