Christos Leonidopoulos

Christos Leonidopoulos

Particle Physics: Electroweak Symmetry-Breaking mechanism, Exotica searches, hadron-collider triggers, future colliders.

Location University of Edinburgh

Activity

  • Oh that's a tricky one. It turns out that the first W boson in this chain (let's call it W*) is slightly heavier than the "normal" W. How's that possible, you may ask. It is, because particles do not have a fixed mass, but something like a distribution around a nominal mass (if you are curious, check out "Breit Wigner distribution"), so that W* as it turns out...

  • @francisgan When I say "light is light", I meant that all photons travel at the same speed (c) in vacuum. Having said that, of course energy & intensity have a role to play in how harmful a light beam can be. But this does not change how fast the photons travel, if that makes sense?

  • You do have radioactive decays inside the Earth, see e.g. https://physicsworld.com/a/radioactive-decay-accounts-for-half-of-earths-heat/ . The Thorium decay that the article mentions is effectively a beta decay that requires the exchange of a W boson.

  • If by "single" you mean completely isolated/on their own, the answer is no. But they do appear either in pairs or in groups of 3 (sometimes more). The reason for that is QCD (Week-4) and colour-confinement.

  • @DaphneJones Not at all, this is a legit blog post, written by physicists. SSB is the mathematical recipe which (sort of) explains how elementary particles acquire mass. But as I said earlier, it is really just a prescription through which we go from a (non-existing) zero-mass world (W1, W2, B) to the real world in which elementary particles have masses (W+,...

  • We may be talking about different things. You may have several particles contributing to the production of a Higgs boson (e.g. W, Z, electrons, muons, etc) and you can depict all these with Feynman diagrams. In order to get the theory predictions to agree with the experimental data, it turns out that you need to take into account all these interactions (aka:...

  • @DaphneJones I assume you mean the non-expert version? I went on youtube and typed "Feynman diagrams" and I found several good-quality outreach attempts. Want to have a look yourself?

  • The presence-or-lack of vacuum is relevant only when it comes to massless particles (e.g. photons). Massless particles travel at the "speed of light in vacuum", well, when in vacuum. When inside a medium their speed is always smaller than that. Massive (ie. with non-zero mass) particles can never reach that upper-limit of speed (whether in vacuum or not). They...

  • Hi @DaphneJones . All known interactions between particles (including decays) in Quantum Field Theory are described/represented with Feynman diagrams in which a "messenger" particle is being exchanged between the two interacting particles. We don't really know any other way of describing particle interactions.

  • Electric discharge is the transmission of electric charges through a medium across an electric field. It is charged particles (think: electrons), not photons/light.

  • You use the ring at the LHC where protons can go around again & again and pick up energy, and increase their speed.

  • Light is light! what matters is the medium in which photons travel, not the wavelength/frequency.

  • Not quite, but pretty close. Protons at the LHC travel at 99.9999991% the speed of light.

  • @francisgan you can decrease the speed of photons by surrounding them with some material. You cannot increased it though, the speed of light at vacuum is the upper limit according to Special Relativity.

  • All three happen from time to time! I just came back from CERN, teaching at Edinburgh on Monday morning!

  • Compared to a massless particle (e.g. a photon), obviously the massive electron (one of the lightest known particles) has a mass that is gigantic & immense! :)

  • @francisgan Classical Mechanics describes everyday physics, ie what we refer to as typically "large" & "slow" objects (e.g. think of a person or a car moving - yes even cars are slow in this prescription). Relativity enters the picture when we consider very fast objects (e.g. think of a spaceship travelling at 10% of the speed of light). Quantum Mechanics...

  • Are you referring to the fact that we are using natural units here? (ie. some constants like ħ and c are missing) That's ok, this is just a convention to make equations a bit shorter/easier to parse.

  • Not sure I understood the question. Are you asking if the mass at each vertex is conserved? Charge, energy, momentum have to be conserved at each vertex, but mass can be created & destroyed (assuming no rules are violated in the process). Matter & anti-matter enter in other conservation laws (e.g. charge is just one aspect of it), but not really in the mass,...

  • Hi @CarstenDierks
    Q1: can you be more specific about the Feynman diagram you are talking about? Since you are bringing up 3 particles, I am tempted to think that you may have forgotten a photon somewhere, can you please clarify?
    Q2: This is true in general across energies (ie. as long the energy is high enough to create electron-positron pairs)
    Q3: You can...

  • It was noticed at some point that one could picture (for example) a negative-charge particle moving forward in time as a positive-charge particle moving backward in time. And that's about all there is to it. It is a mathematical manipulation that is used to try to make sense of something (negative energies) that looks very unintuitive. Of course, now we know...

  • The negative-energy solutions in these QM equations appear when we try to switch from the classical description of the equations of motion to the relativistic one. It turns out that this is not good enough, and we need to switch to the full description of Quantum Field Theory with continuous fields that allows for particles to be created and annihilate. In...

  • tell your friend that it's you who is right! :)

  • @francisgan "The main facet of modern science is that we can repeat an experiment easily." Are you referring to reproducibility of the results here? For the specific case of the Higgs boson discovery we accomplish this by having two independent collaborations that report on their findings separately/independently and without advance notice of what the other...

  • "most promising" means what we consider is most interesting from a physics point of view. For example, it could be because the fragments of the collisions look consistent with the decay of a Higgs boson. In this case, we record the collision so we can study it more carefully offline. Less interesting can be less rare or exotic physics processes that we...

  • @francisgan "if everything goes well" means if we have done a good job of having the two beams aligned and we manage to achieve a good collision at the centre of the experimental apparatus so we can record it. If not, we are missing out.

  • There are people with a very wild/wide range of backgrounds here. But we are trying to make sure we all learn while having fun.

  • we all do!

  • (a) Einstein, approaching this from a theoretical point of view, wanted to make Maxwell's electromagnetic equations uniform across all reference frames. For this to work, the speed of light would have to be constant.

    (b) The Michelson-Morley result provided the experimental confirmation that the speed of light is, indeed, the same in different reference...

  • Special Relativity is based on the observation (if you an experimentalist) or postulate (if you are a theorist) that the speed of light in vacuum is the same in all reference frames (in other words: all observers agree that the light travels at the same speed, no matter what the relative velocity among the various observers).

    This does not make sense from...

  • @francisgan yes, every Higgs boson decays practically instantaneously. What the plot is showing is the process of data-collection where we accumulate a large number of Higgs bosons over a long period of data-taking.

  • @JohnLateano The photons are mass-less, we are fairly certain about this. What you describe as "massive photons" at the time of collision/interaction is in reality the effect of momentum. If a mosquito hits the windshield of a moving car, it is not its mass that creates the "thump", it is its momentum. Closely related, but not the same. Funny thing, the...

  • @francisgan yes, we get about one Higgs per billion of collisions. The tricky part is that we do not get to store every single of these collisions but a very small fraction, the "most promising" ones. We do this with a highly selective filtering mechanism, called the Trigger. We will be discussing all this in Week-6.

  • @JohnLateano The "mass x velocity" formula is indeed from the pre-Relativity era. The new/adjusted formula is modified to be "gamma x mass x velocity", where gamma is the relativistic Lorentz factor. For a slow massive particle, gamma ⁓ 1, and we recover the classic formula. For a photon with mass = 0, gamma is not defined (or you could say that gamma = ∞, if...

  • Good to have you back, @SaraLeyshon !

  • Good to have you onboard, @DanielJohnson

  • I think the way to think about this is whether you need to "pay" to break free (e.g. think of a piece of metal in a magnetic field), or not (e.g think of a bullet moving in a zero-gravitational field: nothing can stop it). The +/- helps us picture whether we have an (energy) debt or a positive balance in our energy-bank account :)

  • I am not sure I understand your question, but my guess is you are referring to the filtering mechanism of the experiments. We have about a billion collisions per sec, and we only select a few thousands of them (so: a few kHz). We accumulate data over a long period of time, and then we look for fragments/evidence of the Higgs in the recorded datasets. Does this...

  • Good to see this attitude, @JosephineLord , and please remember: we are here to help.

  • No, it is actually the force and the *change* in the direction of movement that have to be parallel to each other (or in the same direction). This can also be seen in more complex systems, e.g. 2D & 3D. Imagine an object moving on a circular trajectory, e.g. a planet moving around a heavier sun. To force the planet to be on a circular motion you need a...

  • Hi @JohnLateano . The answer to your question is: both! We will discuss some of the technical details (not to mention, experimental challenges) in Week 6.

  • Anyway, as it happens with these things, in 2012 Toichiro Kinoshita and collaborators calculated 12,672 such Feynman diagrams, which gave a theoretical prediction on alpha/2pi with a precision of 10 significant digits, or 0.25 parts per billion. This may sound crazy (it is) and unnecessary (it isn't). It turns out that experimentalists from Brookhaven about...

  • This result was so important for physics that it got engraved on Schwinger's tombstone, see:
    https://ibb.co/Wg34rkX

    This theoretical development led to an arms race: experimentalists started getting more precise results, which forced theorists to include even more corrections in their calculations, and so on.

    Mind you: These "corrections" are not...

  • If you are a physicist, there is a very good chance you know that tomorrow a new result will be announced from the Muon g-2 experiment at Fermilab on the (so-called) anomalous muon magnetic moment. This is related to the spin of fundamental particles, and how fast they would wobble if they enter a magnetic field.

    Paul Dirac (about a century ago or so)...

  • It is that a collision is understood in today's terms as an interaction between two particles that exchange energy/momentum via a so-called mediator particle. So, every collision can be mathematically described via the exchange of a "messenger" between two interacting particles.

  • Does this answer your question?

  • @ThomasPruefling "collision" is another word for "interaction". QFT is teaching us that when two particles get "close enough" to each other (which is really determined by the range of the force/interaction) they interact. Sometimes they annihilate each other, create new particles, etc. Or you could have an inelastic collision, where the two particles push each...

  • @ChamodSamarasinghe the size of the accelerator site has to do with the design energy the beam particles are accelerated to: the higher the energy, the larger the collider. If you think the (27-km long) LHC is large, you should read about the new 100-km long Future Circular Collider, which is currently one of the options for future colliders under...

  • It is a subtle point, maybe easier to explain if I use the example given on pg. 11 of the lecture notes linked above. In that diagram we see a (so-called) off-shell Z (sometimes also called "virtual" Z or Z*) decaying into a "regular" Z and a Higgs boson (H). We could have replaced the Z/Z* by W/W*. The point is that Z & Z* (or W & W*) are different particles,...

  • Hi @GuusLöhlefink . Yes, this indeed the case, but of course you can only check for a limited number of problems and biases with the much smaller dataset obtained with this method.

  • @GarethWilliams x-axis: time elapsed in ps (or: travel time of B meson), y-axis: event-counts at given timestamp. Without the oscillations, one would expect an exponential decay: you start with a given number of B-mesons, you expect an ever-decreasing number following an exponential decay law. With the oscillations, you see this flickering pattern convoluted...

  • Is that all?
    No, there is something else. To understand this oscillation, we also need to add in the mix Einstein's Special Relativity that is telling us that very fast-moving particles tend to experience life quite differently than slow ones, and that the extra time they seem to gain (called time dilation) allows them to travel just far enough for us to be...

  • But what I wanted to point out is something slightly different: In this plot you see two distributions, one created with particles (blue) and a second one created with anti-particles (red):
    https://ibb.co/SQLfVH5

    Do you see that strange oscillating pattern? Do you know what is causing this?
    This is the experimental observation of matter being converted...

  • [Posting this a bit early in the Week discussing the experimental searches, as more closely related]

    There is a good chance you may have already seen today's announcement from the LHCb collaboration at CERN about a reported difference between the way electrons and muons seem to interact with (so-called) B mesons.

    Why do we care?
    Oh, we care a lot....

  • Good to have you on the course, Jackie.

  • So, we fixed the problem of negative energies by noticing that a particle traveling in one direction is equivalent to an anti-particle traveling in the opposite direction, very much like a negative particle traveling from left to right is equivalent to a positive particle traveling from right to left. You may have seen this trick before in condensed matter...

  • Ok, let's take a step back and give a bit of context on the notion of the negative time (ie. particles traveling backwards in time). You are a theorist and are writing down equations (NB: this is happening at a time before anti-particles have been discovered or hypothesised). You are solving the QM equations and you find that one of the solutions to these...

  • @GuusLöhlefink Yes, lots of data already publicly open for people to explore and search for hints of their favourite theoretical model: http://opendata.cern.ch/

  • "why does matter and antimatter, annihilate?"

    I am not sure if I can answer this in a satisfactory way. Quantum Mechanics comes with a bunch of rules, as in: "process A can happen", "process B is forbidden", etc. So, matter & anti-matter annihilation is one of those things that are allowed (because it is not explicitly forbidden by a long series of QM...

  • Hi @SeanBottrill.

    I assume you are talking about the diagram on the right at the 6:25 timestamp. You can look at this in one of the following ways: You can either have a photon decaying into an electron and a positron (equivalently, an electron and a positron annihilating into a photon), OR, look at the whole thing as an electron traveling (from left to...