EPTA Basic Introduction to Particle Physics - Page 2
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y0su
Finland7871 Posts
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helpman169
28 Posts
Do you have to constantly modify computer code to test your predictions? Do you use some sort of supercomputer for analysis? Does it also mean that you have long waiting times for analysis to complete? | ||
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opisska
Poland8852 Posts
On April 28 2015 17:40 eonrulz wrote: Ah yes, the infamous "The lagrangian is wrong!" argument :D There's an extra hbar.c term added in there for no apparent reason - but you're right, I hadn't noticed the extra bar on the psi. That'll teach me to google and use the first image that appears. Can't be bothered to try to change it now, though, haha. There is nothing infamous on requiring that if you show a Lagrangian, it has at least kinetic terms for all the fields! And the +h.c. terms stand for "+ hermitian conjugate", hbar and c are both equal to one anyway, right?  . And you can't "fix" it by just removing the bar, because that would just be the mass term and not only would the mass of all particles (assuming you use the shorthand that psi is a matrix of all fermionic fields) would have the same and unit mass but also the inability to have such a term (because of different SU(2) symmetry of left- and right-handed parts) is the very reason for the Higgs mechanism in the first place. But since you are not using that Lagrangian for anything, it's just nitpicking anyway.On April 28 2015 18:04 Cascade wrote: Haha, no, not Pythia, I'm sorry. Torbjorn Sjostrand (the Pythia author/god) was like 30m down the corridor. My event generator was called DIPSY, and focused on minimum bias of p-p and also deep inelastic scattering (p-gamma) and (inelastic) diffraction, so not really going after those super-rare high-PT events. We were mainly using the minimum-bias data taken at the very start when they powered up LHC at lower energies, with much lower luminosities, so almost no pileup, which made minimum bias a whole lot easier. Actually minimum-bias LHC physics is very important for us, because that's the interface bewteen accelerator and cosmic ray hardonic physics, high-PT stuff is all but irrelevant for us. You people are true warriors of science, among a thousand of people in each detector collaboration who look for Higgses, SUSY and whatnot flashy cool stuff, minimum bias physics is actually immensly usefull for modelling of air-showers from high-energy cosmic rays. | ||
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eonrulz
United Kingdom225 Posts
On April 28 2015 18:24 helpman169 wrote: I was wondering what you guys are actually doing on a day to day basis when you say you are analysing LHC results? Do you have to constantly modify computer code to test your predictions? Do you use some sort of supercomputer for analysis? Does it also mean that you have long waiting times for analysis to complete? All of the above. I wrote this blog late last night as I was waiting for code to finish running - took me about 4 hours to write, and that was only about half of how long the code took. There is the 'Grid', which is a worldwide distributed network of servers and supercomputers designed to speed up code processing, but there's currently a bug in the script I use to submit to the Grid, so I can't submit my code there at the minute  Basically its just, we have these huge datasets of events, and we apply various selections to the datasets that are designed to separate the background and our signal. Then we do whatever it is we need to do, and this varies from analysis to analysis. Mostly it involves comparing our recorded data to our simulated events, to see a) how good our simulations are, and b) to see whether or not the data agrees with a model. Its... actually really dull, and mostly computing rather than physics. But then we get (hopefully) nice, pretty plots out at the end that make everyone go "oooh, ahhh!" and clap politely. | ||
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eonrulz
United Kingdom225 Posts
On April 28 2015 18:44 opisska wrote: There is nothing infamous on requiring that if you show a Lagrangian, it has at least kinetic terms for all the fields! And the +h.c. terms stand for "+ hermitian conjugate", hbar and c are both equal to one anyway, right? . And you can't "fix" it by just removing the bar, because that would just be the mass term and not only would the mass of all particles (assuming you use the shorthand that psi is a matrix of all fermionic fields) would have the same and unit mass but also the inability to have such a term (because of different SU(2) symmetry of left- and right-handed parts) is the very reason for the Higgs mechanism in the first place. But since you are not using that Lagrangian for anything, it's just nitpicking anyway.Right, right, but in the Lagrangian that's generally shown, there's two "+ hbar.c" terms, and I'm pretty sure if you follow it through, it turns out that you're off by a factor of two because of it? Or something like that, its been a while since I've actually worked it through personally. Doesn't really matter anyway :D | ||
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Cascade
Australia5405 Posts
On April 28 2015 18:44 opisska wrote: Actually minimum-bias LHC physics is very important for us, because that's the interface bewteen accelerator and cosmic ray hardonic physics, high-PT stuff is all but irrelevant for us. You people are true warriors of science, among a thousand of people in each detector collaboration who look for Higgses, SUSY and whatnot flashy cool stuff, minimum bias physics is actually immensly usefull for modelling of air-showers from high-energy cosmic rays. And it turned out that the heavy ion community was very interested in the event generator as well, as we included initial state saturation effects... So towards the end of my PhD (+ short postdoc) I got drawn into the heavy ion community, almost against my will. Cosmic rays would be like proton - small nucleus? I guess it can be approximated well by proton-proton? Well, if not, DIPSY would be a tool to handle it.  What kind of CoM energies are we talking? We do low-x approximation, so better be high CoM, like at least 200GeV, preferably 1 TeV and above. | ||
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opisska
Poland8852 Posts
On April 28 2015 19:14 Cascade wrote: And it turned out that the heavy ion community was very interested in the event generator as well, as we included initial state saturation effects... So towards the end of my PhD (+ short postdoc) I got drawn into the heavy ion community, almost against my will. Cosmic rays would be like proton - small nucleus? I guess it can be approximated well by proton-proton? Well, if not, DIPSY would be a tool to handle it. What kind of CoM energies are we talking? We do low-x approximation, so better be high CoM, like at least 200GeV, preferably 1 TeV and above. The problem with cosmic rays is that is almost "whatever - whatever" collisions. The target is always N or O (there is 1 percent Ar in the air, but that's not really important), but the projectile is anything from p to Fe as we now are pretty sure that the primary beam is not pure p. Also not only the primary interaction is important, but the subsequent cascade, where if the primary is Fe, then almost any fragment may happen. The CMS energy of primary interaction that we have is easily in the 100 TeV range. There are no data for that of course, but as close as you can get is interesting for us - not only because then you extrapolate less but also because all these many secondary interactions. At the moment, we use basically two generators - EPOS and QGSJET which are being actively maintained and updated to be compatible with as many LHC data as possible while providing truly minimum bias events (incl. diffraction) - ironically, QGSJET is so cosmic-ray oriented that it does nor for example even produce charmed particles and as far as I know no EW process are there whatsoever, these things are just too rare to even matter. EPOS you maybe have heard of, that's more complete (developed largely off RHIC data lately) and can be with some success used to compare with LHC data even at mid-rapidity. Anyway, if you have a generator that produces truly minbias events (that is, can reproduce what happens when you smash two things together even if the result is largely invisible to the LHC), is tuned to at least some LHC data and can handle up to Fe-O interaction, it could be of interest to see what it has to say on CR data. The issue would be integration in our simulation frameworks as we are running essentially a monolithic F77 code into which the models are kinda hacked to work and I am not sure if I have the manpower to play with porting a model at the moment, but I may find a bored student. | ||
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SoSexy
Italy3725 Posts
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Cascade
Australia5405 Posts
That said, I haven't worked on it for more than 2 years (switched to computational biology), so I can't personally support you. If you want to know more, you can just check me up on spires: Christoffer Flensburg, and you will find the relevant papers. It's called DIPSY. Im not sure what they are doing with it now, but I think they have a student working on it. The person to contact if you are interested is my ex supervisor Leif Lönnblad. Say Hi from me if you contact him. I think it can give good results, but it may be messy to get running as you say. | ||
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opisska
Poland8852 Posts
On April 28 2015 21:13 Cascade wrote: OK, that seems to fit actually. It is indeed zero bias, to the extent that the total cross section comes out of it naturally, and does from p-p up to Pb-Pb (although that takes hours per event). That said, I haven't worked on it for more than 2 years (switched to computational biology), so I can't personally support you. If you want to know more, you can just check me up on spires: Christoffer Flensburg, and you will find the relevant papers. It's called DIPSY. Im not sure what they are doing with it now, but I think they have a student working on it. The person to contact if you are interested is my ex supervisor Leif Lönnblad. Say Hi from me if you contact him. I think it can give good results, but it may be messy to get running as you say.Thanks, if I can get myself to work instead mucking around on TL, I may have a look on that. | ||
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Paljas
Germany6926 Posts
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kingjames01
Canada1603 Posts
On April 28 2015 22:36 Paljas wrote: can anyone explain neutrino oscillation to me? So, I'll take this in stages: Basic: At one point we thought we understood the Sun and how it produces energy quite well. When we set out to confirm our hypothesis, it was revealed that we were missing a lot of the neutrinos that we expected. So, we didn't understand the basic nuclear reactions which drive the Sun or we didn't understand neutrinos or perhaps some combination. It turns out that there are 3 kinds of neutrinos (flavours) and they have a small probability to change forms. This ability is what we call neutrino oscillation. Anyway, because the neutrinos have a very far distance to travel from the Sun to the Earth, many of them had changed forms by the time they arrived. Advanced: In Physics and Mathematics, you can use a specific representation to describe a quantity or state. For example, if I want to describe an electron, I might use the position representation to describe its location. From this representation, I could work out other important values but it would take some manipulation. Alternatively, I could simply change the representation and we could skip the step where we have to work out relationships. For example, I could instead describe the electron in the momentum representation. Then we could work with the momentum directly rather than having to work it out from the position representation. You might know this as "change of bases". Next, I'll point out that you can describe neutrinos in their flavour representation or their mass representation. Anyway, what drives neutrino oscillation is that the mass representations are slightly rotated from the flavour representations, where the weak interaction applies to. So, when the Sun creates a neutrino in one flavour, it's actually a superposition of 3 different mass states. As it travels, they move through the various flavours. This is periodic so it's called an oscillation. | ||
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kingjames01
Canada1603 Posts
On April 28 2015 12:48 micronesia wrote: I don't feel like this addresses my concern. Why is comparing the electrostatic force between two protons and the gravitational force between two protons a valid way of showing which force is 'stronger?' Maybe protons just have really huge charges considering their mass. The proton has a charge of +1 e. That's the smallest possible charge for a free particle. So, I purposely fabricated an example with the smallest possible charge and made my conclusion from there. The example is not meant to be a proof, rather, it is a demonstration of scale. When you want to actually compare the strengths between two different interactions, the usual practice is to first calculate the coupling constants of the interaction and then compare those values. To discuss the underlying reason between the difference in strengths we would have to look into the models for the unification of the fundamental forces. | ||
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helpman169
28 Posts
How can you detect particles? Don't they have to interact with other particles such as photons so that you can 'see' them? Or is all detection indirect? How does the detection influence the particle itself? Does accelerating particles to high speeds and letting them collide with other particles give the same picture of the particles as you would find in real world atoms? | ||
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kingjames01
Canada1603 Posts
On April 29 2015 03:47 helpman169 wrote: How can you detect particles? Don't they have to interact with other particles such as photons so that you can 'see' them? Or is all detection indirect? There are many methods for particle detection. I'll just discuss a few common methods: Charged particles: - Accelerating charged particles radiate energy in the form of electromagnetic radiation. If you can detect the radiation, then you know that there was a charged particle moving around. - When a charged particle moves past other charged particles, the latter move around. Since they pick up extra energy from the electromagnetic attraction/repulsion, they accelerate. This brings us back to the point above. Neutral particles: - You can't actually see these guys moving around directly since they have no charge. Usually you wait for them to interact with matter, which causes charged particles to accelerate. For example, to detect fast neutrons, you wait for them to collide with a nucleus, which obviously is positively charged. In the case of photons, which are neutral, they will interact with the electrons, which are charged. The most popular material for gamma detection (photons produced from nuclear reactions) are Germanium and, in very recent years, Lanthanum Bromide. - You can wait for neutral particles to undergo a nuclear reaction, and then detect the "signatures" that are unique to that particular reaction. This is a common tool for slow neutron detection and the basis for neutrino detection. There are other methods of course, but a full discussion would fill a textbook. On April 29 2015 03:47 helpman169 wrote: How does the detection influence the particle itself? In general: For the methods that rely on nuclear reactions, the particle is destroyed and you measure the product. If the basis of your detection is scatter, then the incoming particle rebounds with a change in energy. On April 29 2015 03:47 helpman169 wrote: Does accelerating particles to high speeds and letting them collide with other particles give the same picture of the particles as you would find in real world atoms? I'm not sure what you mean as the same picture. Think of in-beam experiments as follows. Suppose you bought a car and you brought it over to my house. I, being a very curious person, want to understand how it works. I can start by walking around it and looking at everything and anything. However, that only gives me a large-scale picture. I still don't really understand how it's made. In order for me to further my understanding, I would want to take it apart to look at the components. Well, if you want to break apart a molecule, an atom, a nucleus or even a composite particle, you're going to need a lot of energy. In general, the smaller the system, the more energy you will need to open it up. Once you smash it open, you need to be able to identify everything that's produced. Some things will be charged, some neutral. Some will only move an extremely short distance before disappearing. It's very technically challenging. On top of that, you have to be careful what you use to break open your system, because you don't want to mix up where the components you see came from. There are other major factors but that's a very long discussion. | ||
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eonrulz
United Kingdom225 Posts
On April 28 2015 22:36 Paljas wrote: can anyone explain neutrino oscillation to me? I think an easier way to visualise it is to use wave-particle duality. Rather than thinking of a neutrino as a solid particle like a snooker ball, think of it as a wave. There are three types of neutrinos, so the neutrino 'wave' is actually three interacting waves. If it starts as an electron neutrino, the e wave is initially dominant. But, waves go up and down, right? So after some distance, the e wave is actually smaller than the muon (u) wave, so the e neutrino becomes a u neutrino! Then after some further distance, the u wave starts to wane and the neutrino becomes a e again. And sometimes, quite rarely, it becomes a tau (t) neutrino! Though the probability for this is quite low, as you can see from the graph below. ![[image loading]](http://upload.wikimedia.org/wikipedia/en/thumb/1/1f/Oscillations_muon_short.svg/320px-Oscillations_muon_short.svg.png) The actual maths is not really well understood - you can prove that in order for neutrino oscillation to occur, neutrinos have to have mass; but their mass is so small, we've not been able to measure it yet! And we don't even know which neutrino has the lowest or highest mass! But we do know for definite that neutrino oscillation occurs, we've been able to measure it directly in laboratory experiments. There's a lot of weird stuff that happens with neutrinos that we've not wrapped our heads around properly yet. | ||
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kingjames01
Canada1603 Posts
On April 29 2015 04:48 eonrulz wrote: The actual maths is not really well understood - you can prove that in order for neutrino oscillation to occur, neutrinos have to have mass. The requirement is that only one of the neutrinos need to have mass, but excellent explanation! Edit: Wait, what do you mean by this statement? On April 29 2015 04:48 eonrulz wrote: The actual maths is not really well understood We actually understand the underlying math quite well. The mass eigenstates and the leptonic eigenstates are related through the CKM rotation matrix. | ||
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eonrulz
United Kingdom225 Posts
but thanks for appreciating my attempt anyway! | ||
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kingjames01
Canada1603 Posts
On April 29 2015 05:25 eonrulz wrote: Sorry, I meant that the maths behind the neutrino mass isn't very well understood, which I think is accurate. Its late and I've had a couple of g+t's, my wording might not be quite on-point but thanks for appreciating my attempt anyway!Ah! I knew it was just a matter of me misunderstanding your point! :D | ||
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Cascade
Australia5405 Posts
On April 29 2015 03:47 helpman169 wrote: Does accelerating particles to high speeds and letting them collide with other particles give the same picture of the particles as you would find in real world atoms? I think I can help you with this one: Yes, the understanding we have about particles from atomic physics is very much in agreement with particle physics. In fact, the standard model Lagrangian (the equation in the OP) describes not only particle physics, but also atomic physics. My favourite example of this is the Lamb shift. A photon can (for a short while) turn into an electron and a positron, which shortly after re-merge again into a photon (unless something bumps into them before that). This is described by the lagrangian (in what is called a "loop correction" or "next to leading order calculation") and has an important effect in some collisions. It is pretty well studied in particle physics. In atomic physics, it is a very small effect. However, the photons that carry the electromagnetic force between the electron and the nucleus in an atom can form this electron-positron pair while they travel, which affects the forces within the atom ever so slightly. I think the energy level of the atom is shifted in the sixth digit or something due to this effect, which is called the "Lamb shift". However, they do crazy precision experiments in atomic physics, measuring energy levels to like 8 digits, making the Lamb shift very measurable, and it is indeed observed and in perfect agreement with the particle physics calculations. So that's just a little neat example of how it's the same underlying physics that guide atoms or high-energy particle collisions. Also: particle collisions are part of the real world, just as much as atoms.  | ||
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