So this is a blog in conjunction with Teoita's "A Basic Introduction to Astrophysics", only covering particle physics instead. Teoita covered the large stuff, now I'm going to cover the small stuff! Sadly my blog won't have as many pretty pictures, but hopefully it'll still be interesting to at least some of you.
About me: I'm a 2nd year Experimental Particle Physics PhD student, based at CERN, working on the ATLAS detector experiment. My masters' was in theoretical particle physics, looking at properties of micro-black holes that could be produced at the LHC and how they related to extra dimensions in the ADD model.
Contents -- What is Particle Physics? -- The Standard Model -- The Higgs Boson -- What's the problem? -- Beyond the Standard Model -- The Large Hadron Collider -- FAQs / Common Misconceptions
What is Particle Physics?
Whatever you look at can be broken down into smaller things. Skin is made of cells, which are made of molecules, which are made of atoms. Inside an atom are electrons, protons and neutrons. Particle physics is the study of the stuff that makes up the atoms - the fundamental particles of the universe - and in turn, it tries to answer some of the most important questions in the universe, and recreate the fractions of a second after the Big Bang.
The Standard Model
Over the last 100 years of scientific discoveries, some expected and some accidental, physicists have built up a very robust idea of what goes on at the smallest scales in the universe. They call this the Standard Model. The entire thing can be neatly summed up in 1 beautiful equation, shown below for those of you mathematically inclined, but even more succintly can be shown as a table of particles.
The Standard Model Lagrangian, the equation that explains the universe
The particles that make up the Standard Model
The Standard Model is split into two types of particles; fermions, which are sub-divided into leptons (electrons, neutrinos) and quarks (the particles inside protons and neutrons), and bosons. Bosons are 'force-carriers' or intermediary particles, that enable interactions between the fermions. Each boson relates to a fundamental force - electromagnetism, the weak nuclear force, and the strong nuclear force. In addition, every fermion has an anti-matter twin particle, with equal but opposite charge.
Electromagnetism is the most well-known of the forces, and is mediated by the photon - all charged particles can interact through this force, by exchanging or absorbing photons.
The Weak nuclear force is the reason for radioactive atomic beta decay, where a proton becomes a neutron and a positron is emitted. The oddly named W and Z bosons mediate this force, and all fermions can interact in this manner.
The Strong nuclear force binds together quarks, and is strange in that contrary to the other forces, it actually gets stronger the further away the interacting quarks get from each other. The effect of this is that quarks are never found alone - the energy it requires to break a strong force bond is greater than the energy required to create two new quarks! Gluons are the force-carrying boson for this one.
Any eagle-eyed readers out there will notice two things; one, I've not included gravity here, and I'll get back to that later; two, I've also not mentioned the Higgs Boson...
The Higgs boson is a little different, in that it doesn't really relate to a force like the others. To explain the Higgs boson effectively requires a solid understanding of something called Quantum Field Theory. The Wikipedia page for this is actually quite good, if mathematically dense, so for a more detailed look I'd point you to that. But, in a nutshell, QFT states that every particle is actually an energetic state of a field. The most basic example of this would be the photon - an electromagnetic wave, a.k.a. a field, can also be seen as a particle. This rather nebulous concept was demonstrated by the famous Young's Double Slit experiment, and promptly named wave-particle duality.
The two scenarios for the Young's Double Slit experiment; the photon is a wave or a particle, and both interpretations can lead to the same interference pattern on the screen
Similarly, the electron has an electron field, the quarks a quark field. The gluon is a representation of the Strong Nuclear Force Field. And the Higgs is an energetic state of the Higgs Field.
The Higgs Boson
Imagine, if you will, an empty Olympic-sized swimming pool. Walking from one side to the other takes absolutely no work whatsoever - you can go as fast as you like, and you've not really expended much energy. Now, fill the swimming pool with water. Suddenly it takes you a lot more energy to walk across, and you're limited in speed. But a fish can swim across quite happily, much faster than you!
In this analogy, the pool is the universe, you are a fermion, and the fish is a photon. At the start of the universe, there was no water, and all the particles were massless and zipped around quite happily at the speed of light. Then some water was thrown in - the Higgs field. This field permeated throughout space, and some particles were slowed down by it, while some never interacted with it at all and kept zipping around at the speed of light. The amount a particle was 'slowed-down' is related to their mass; the only particles that are massless are the photon and the gluon, and both travel at the speed of light. The Higgs field is the reason why the speed of light is the ultimate speed limit in the universe! The more you push against the field, trying to accelerate a massive particle like an electron, the more the Higgs field resists, and the heavier the particle becomes, requiring more energy to accelerate it...
Interestingly enough, the reason why the W and Z bosons have mass is actually related to the mechanism behind the existence of the Higgs. That same mechanism intrinsically links electromagnetism and the Weak nuclear force, to the extent that at a high enough energy, the two merge into a single force, called the electroweak force. But now we're getting into some pretty hefty maths, so I'll leave it at that; I'm happy to elaborate further in the comments if people want, but I'll move on for now.
What's the Problem?
It sounds like the universe is pretty well understood then, right? We've got an understanding of what particles are, from QFT, we know how charged particles interact through electromagnetism and photons. We have some knowledge of how quarks interact through the Strong force, but the equations are pretty nasty and aren't entirely solveable analytically. The Weak force has a solid theoretical background, and its interactions can be well modelled. All of these have decades now of strong experimental evidence backing them up, and with the recent discovery of the Higgs boson, a big hole about where mass comes from has been filled. So why are physicists still worried?
Unfortunately, the Standard Model still has some fairly fatal flaws in its construction. I left out gravity earlier, and that's for a good reason - the Standard Model has no explanation for it. Like, at all. Seriously, its not even mentioned. "But eon!", I hear you cry, "Gravity is obviously there, everything relies on gravity!". Well, physicists know this, but as of yet no theory linking the quantum mechanical world of the subatomic and the macroscopic world we see every day has ever been proven. The greatest minds in the last hundred years have tried, but to no avail - Einstein himself called it his greatest failure. So there's obviously something we're missing.
There's other issues as well. Given that electromagnetism and the weak force unify into the electroweak force at high energies, one would expect the strong force to join them - however using theoretical predictions from the SM, this doesn't happen - in fact, they miss by several orders of magnitude. And again, gravity doesn't even appear on the scale. In fact, gravity is 10^37 times weaker than the other forces; that's a 1 followed by 37 zeroes. Its damn weak. This discrepancy between gravity and the other forces is known as the Heirarchy Problem.
A graph showing energy scale on the x-axis and field strength on the y-axis. In the Standard Model, the three forces don't unify at high energy scales - it requires a modification like Supersymmetry in order for them to unify.
Then there's the problem of, well... the universe itself. When we look around us, everything we see is matter - Teoita's blog is a shining example of that. But, at the very start of the universe, matter and antimatter should have been created in equal amounts - and when they interact and annihilate, they only do it in pairs. So there should either be no matter in the universe, or we should still be able to see antimatter galaxies! There has to be some sort of process by which matter dominates over antimatter, but this violates something called Charge-Parity conservation, a core tenet of the Standard Model which says that anything matter can do, antimatter can also do. Again, there's something we're missing.
The last big issue, amongst many other more technical points, is that cosmological measurements say that only 4% of the universe is the matter we can see around us. 25% of the universe is some form of matter that doesn't (or, incredibly rarely) interact with normal matter - so-called dark matter. We know its there; it can be seen through gravitational lensing effects, and other indirect observations. But we're yet to observe it directly, in a lab. We don't even know what kind of particle it is - and regardless, its yet another thing the Standard Model doesn't have any explanation for. Let alone the other 73% of the universe! Dark energy dominates that, and that's even more obscure than dark matter.
Without gravitational lensing from dark matter, this image would look like a normal set of stars and galaxies! Its images like this that indicate to us that dark matter is real, but we need to observe it directly before we can say what it is.
Beyond the Standard Model
So with everything that's wrong with the SM, why do physicists cling to it? Quite frankly, its because there's no direct evidence of anything else. Every measurement made in the last 20 years agrees almost perfectly with SM predictions. Any new theory, then, has to build upon the SM, filling in the holes that are there whilst agreeing with all previous measurements. Quite a difficult task indeed, but there are some options.
Supersymmetry: the crowd favourite. An extension of the SM, this theory predicts that for every particle in the SM, there exists a supersymmetric partner particle. There's about a billion different interpretations of this, but all of them handily solve most of the issues plaguing the SM. Dark matter is the lightest supersymmetric particle (LSP), which doesn't interact with anything and sits around inert. The Hierarchy Problem is solved by the possible extra interactions with the SUSY particles. Gravity... still kinda an issue, but some SUSY models predict the existence of the graviton which would provide the missing link between particle physics and cosmology. Basically, its the All-Star all-rounder of the theoretical line-up, and its no wonder people (including me!) are most looking forward to this model.
Leptoquarks: vaguely plausible. This model predicts the existence of a third type of fermion, that allows gluons and the Strong force to interact with leptons like electrons. Its been around for just about forever, but nobody's been able to prove or disprove it conclusively.
Technicolor: like a bad rash, it keeps coming back in waves of popularity. It postulates additional Strong force interactions that haven't been observed yet. Similar to leptoquarks in that nobody's been able to conclusively disprove it, but there's no evidence for it either, and it regularly falls out of favour.
Extra dimensions: a personal favourite. There's lots of different models of these, from the ADD model of large but compacted dimensions, to naturally small dimensions, and every possibility in between. The general idea is that if a dimension is somehow folded or warped to be smaller than an atom, we can't see it, but particles might be able to - particles like the graviton, which could then escape into these other spatial dimensions and weaken gravity in our dimensions. Again, like SUSY, these theories are pretty handy at explaining a lot of things, and have the added bonus of dealing with gravity. Plus, some have the hilarious side-effect of the possibility of creating black holes at the LHC... dun dun duuuuun!
String theory: I don't really have much to say on this. Its a nice idea, that happens to explain everything, but there's no way to detect it. And in general, string-based theories rely on other theories being correct as well - all string theories are dead in the water without extra dimensions, for which we've seen no evidence yet, and most are boned without SUSY as well. Personally, I think its a load of bollocks, but it keeps theorists busy and off our backs, so hey
The great thing about all of these theories - except string theory - is that with the restart of the LHC, currently scheduled for late June/early July, the potential for a discovery is hitting its peak.
The Large Hadron Collider
100m underground, passing under the French/Swiss border by Geneva, is a 27km circumference tunnel. Originally built to house the Large Electron-Positron collider (LEP), the tunnel at CERN is now home to the LHC, the most powerful particle accelerator ever made.
The LHC takes bunches of protons and accelerates them up to 13TeV - 13 x 10^12 electron-volts, or 2.1 x 10^-6 joules. This might not sound a lot, but given how small a proton is, its the equivalent energy of a freight train travelling at 70mph! These bunches of protons are made to collide at 4 points around the ring, and at each point there's a detector to see the explosive results. 40,000,000 collisions happen a second, and each collision takes a few gigabytes of hard drive space. Storing every event is impossible - so an insanely complicated triggering process running at speeds ~25ns per event decide which events are 'physics-worthy' and are saved. Even still, a full dataset is roughly 10 Petabytes on disk!
A single event from the ATLAS event display software. Particles are determined by their trajectory and the energy they deposit in the detector - between the two, we can determine their energy, momentum and charge.
Four main experiments do the brunt of the physics analysis on the LHC; ATLAS, CMS, LHCb and ALICE.
The ATLAS detector
The CMS detector
ATLAS and CMS are general purpose detectors - designed using different methods and properties, they're designed as counterparts to validate any discovery the other makes. After all, science should be repeatable, and the LHC is one-of-a-kind! These detectors do a huge range of physics, from Standard Model measurements to CP-violation studies, to searches for new physics like SUSY and extra dimensions. They're also massive; ATLAS is 45m long and 23m high - the cavern its housed in is the size of the Notre Dame cathedral!
A diagram of LHCb - the beampipe is the thin red line on the left. Unfortunately there's not much to see of the detector in photos
LHCb is a very-forward angle detector. While ATLAS and CMS form a (nearly) completely sealed environment around the collisions, LHCb drops all but a few degrees around the beampipe. What it lacks in 360 degree coverage, it more than makes up for in its ability to measure momentum, and even directly identify particles with its RICH (Ring-Imaging CHerenkov) detector system. Its objective is to study b-quarks, to find more about CP violation and the difference between matter and antimatter.
The ALICE detector - probably the coolest looking of the bunch!
Unlike the other detectors, ALICE doesn't look at the main proton-proton collisions. Instead, it has its own special run of lead-ion collisions. A much lower rate of collisions, but what it allows for is the study of the quark-gluon plasma. This unique state of matter is representative of what the universe was like fractions of a second after the Big Bang, and it leads to some very interesting results. Though, admittedly, due to a severe lack of UK involvement in this project, I sadly know very little about what ALICE actually does
The LHC is now poised in a fantastic position for scientific revolution, with the restart at the full 13TeV about to begin. Either we find evidence of new physics, and change our understanding of the universe - or we don't, and we're forced to throw away not only the more recent theories, but the Standard Model itself. Without any way of fixing its flaws, the Standard Model would be thrown into doubt, and a radical new approach must be found.
Either way - now is an incredibly exciting time to be a particle physics PhD student :D
FAQs/Common misconceptions
-- But... black holes! You'll kill us all!
Yeeeeeeah, not really. If a very, very small subset of theories is correct, we'll create micro black-holes at the LHC. But, I must emphasise the micro - not only are these black holes too tiny to see, they also decay (or disappear) through Hawking radiation instantaneously. Like, actually instantly. Not enough time to stabilise, even if they were big enough to start pulling in other types of energy. So no, no world-ending black holes, sorry. Besides, please do bear in mind - the LHC is the most powerful man-made particle collider ever, but our own atmosphere is subject to far more powerful collisions with cosmic rays on a daily basis; and if millions of years of insanely high-energy cosmic rays isn't enough to create a black hole big enough to destroy the world, its unlikely we'll be able to manage it at the LHC.
-- What industrial applications does this have?
*Ahem* well, uh... directly? Little to nothing. But! Remember that its impossible to say where technology will end up in the future. And in the meantime, CERN revolutionised the design and production of superconductors in the process of building the LHC, and we're starting to see the effects of that in MRIs and other industrial and medical applications. In fact, nearly all of the most recent diagnostic breakthroughs in medical physics have come from particle physicists, and I know for a fact there's a team in Bristol who gave up working on the LHC to focus on a new proton radiotherapy treatment to cure cancer.
-- What if we had quantum control over our minds and blah blah blah pseudoscience blah
Humans will never be able to control quantum mechanical processes with our minds. We can't even physically imagine what they would be like, which is why we have to use computer simulations, complicated equations and random number generators (Monte Carlo, anyone?). And if we somehow, beyond all logic and scienctific reasoning, actually manage to get to that point... would we really be human anymore?
-- Will there be a larger Large Hadron Collider?
Hopefully, yes! There's several potential colliders in the works, including two linear colliders (rather than circular like the LHC, both styles have their merits and drawbacks) and anything from a 60km ring to a 100km ring! Plus smaller colliders designed to study e.g. B-Physics (the upcoming Belle II detector in Japan), or the Higgs in more detail (China have proposed a 'Higgs factory' accelerator to do just that). Ultimately, it depends on funding, and that in itself depends on the LHC. As interesting as finding no new physics would be from a scientific point of view, to the tax-payer and to funding agencies, a positive result is far more likely to lead to enough funding to build a new collider. Regardless, nothing is expected to be ready for at least another 15 years.
-- Hur hur, hadron sounds like hardon
Yes, very good.
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Well, that's been a very brief overview of particle physics - hopefully its been interesting! Please, do feel free to ask questions in the comments, I'm always happy to answer to the best of my abilities. And thanks to Teoita for inspiring me to write this :D
That Lagrangian looks fucking terrifying holy shit.
So regarding supersymmetry, what are the actual characteristics of dark matter candidates like wimps? All i know is pretty much "yeah dark matter could be super symmetrical particles that only interact gravitationally" and that's really it.
Also, do exentions of the Standard Model compute vacuum energy density in any reasonable way (as in, not 104 or whatever it was orders of magnitude off what the standard cosmological model comes up with)?
second edit: also, can you tell me anything more specific about the last AMS-02 results that were announced a couple of weeks ago regarding potentially detecting signals from the annihilation of dark matter particles?
Thank you, I enjoyed the read. One thing that has been bothering me...
You said earlier that gravity is many times weak than other fundamental forces. How do you perform an apples to apples comparison? I mean, the gravitational force of the Earth on the moon is much stronger than the electromagnetic force. Therefore, the electromagnetic force of the Earth on the moon is much weaker. In contrast, if I put 1 Coulomb of charge on to a 1 kg metal sphere, the electromagnetic force on an identical sphere 10 meters away will be much stronger than the gravitational force. How do you choose the mass and charge of each object for a fair comparison?
5/5, great blog. This should absolutely be featured!
Can you tell me why you chose to go into particle physics and whether or not you are seriously interested in it as a career? I'm asking because right now I'm at the point in my life where I need to make some decisions about what I want to do. I'm finishing high school in 3 weeks, going to college in the fall.
Last summer, I worked for a physics professor at Bellarmine University in Kentucky analyzing collision data from ATLAS about the Z prime boson. It was really exciting work, particularly because the data we got was indicative of the existence of the Z prime! Supersymmetry yay! I also finished AP Physics C as a junior and am doing a independent study of basic QM with the Physics C teacher at my high school, who is a really smart guy. So I really love this stuff, but I don't know how far I want to pursue it.
On April 28 2015 08:19 micronesia wrote: You said earlier that gravity is many times weak than other fundamental forces. How do you perform an apples to apples comparison? I mean, the gravitational force of the Earth on the moon is much stronger than the electromagnetic force. Therefore, the electromagnetic force of the Earth on the moon is much weaker. In contrast, if I put 1 Coulomb of charge on to a 1 kg metal sphere, the electromagnetic force on an identical sphere 10 meters away will be much stronger than the gravitational force. How do you choose the mass and charge of each object for a fair comparison?
Just take 2 identical charged objects and work out the gravitational force between them and the electric force between them.
For example, take 2 protons 1 metre apart.
F_E = k q_1 * q_2 / r^2, remember q_1 = q_2 F_E = (9.0 x 10^9 N * m^2 / C^2) (+1 e)^2 / (1 m)^2 F_E = (9.0 x 10^9 N * m^2 / C^2) (1.6 * 10^(-19) C)^2 / (1 m)^2 F_E = 2.3 x 10^(-28) N
F_G = G m_1 * m_2 / r^2, remember m_1 = m_2 F_G = (6.7 x 10^(-11) N * m^2 / kg^2) (1.7 x 10^(-27) kg)^2 / (1 m)^2 F_G = 1.9 x 10^(-64) N
That's a REALLY large difference! The electric force is about 1.2 x 10^36 times larger. Let's turn that around and see how much mass we would need to get roughly the same scale.
F_E / F_G = 1 [2.3 x 10^(-28) N] / [(6.7 x 10^(-11) N * m^2 / kg^2) (m_1)^2 / (1 m)^2] = 1
Solving for m_1, gives m_1 = 1.9 x 10^(-9) kg.
That's about 1.1 x 10^18 times larger!
So, to recap, let's take an imaginary object comprised of 1 x 10^18 protons and electrons, ie. 1 000 000 000 000 000 000 protons and give it one more proton than electron so it's charge is +1 e. The gravitational force between these objects is now roughly the same as the electric force between them.
Edit: I'll clean this up and LaTeX it if I have time later.
On April 28 2015 11:05 TheLordofAwesome wrote: 5/5, great blog. This should absolutely be featured!
Can you tell me why you chose to go into particle physics and whether or not you are seriously interested in it as a career? I'm asking because right now I'm at the point in my life where I need to make some decisions about what I want to do. I'm finishing high school in 3 weeks, going to college in the fall.
Last summer, I worked for a physics professor at Bellarmine University in Kentucky analyzing collision data from ATLAS about the Z prime boson. It was really exciting work, particularly because the data we got was indicative of the existence of the Z prime! Supersymmetry yay! I also finished AP Physics C as a junior and am doing a independent study of basic QM with the Physics C teacher at my high school, who is a really smart guy. So I really love this stuff, but I don't know how far I want to pursue it.
So why did you choose particle physics?
EDIT: Just noticed the FAQs, those are hilarious.
Unless you want to be a professor, at least here in the US pursuing math and physics is pretty grim career-wise. I have an undergrad degree in math/physics and a MS in applied mathematics and I work in a call center for tech support. It's horribly depressing and I hope you have better luck if you go in the field.
On April 28 2015 08:19 micronesia wrote: You said earlier that gravity is many times weak than other fundamental forces. How do you perform an apples to apples comparison? I mean, the gravitational force of the Earth on the moon is much stronger than the electromagnetic force. Therefore, the electromagnetic force of the Earth on the moon is much weaker. In contrast, if I put 1 Coulomb of charge on to a 1 kg metal sphere, the electromagnetic force on an identical sphere 10 meters away will be much stronger than the gravitational force. How do you choose the mass and charge of each object for a fair comparison?
Just take 2 identical charged objects and work out the gravitational force between them and the electric force between them.
For example, take 2 protons 1 metre apart.
F_E = k q_1 * q_2 / r^2, remember q_1 = q_2 F_E = (9.0 x 10^9 N * m^2 / C^2) (+1 e)^2 / (1 m)^2 F_E = (9.0 x 10^9 N * m^2 / C^2) (1.6 * 10^(-19) C)^2 / (1 m)^2 F_E = 2.3 x 10^(-28) N
F_G = G m_1 * m_2 / r^2, remember m_1 = m_2 F_G = (6.7 x 10^(-11) N * m^2 / kg^2) (1.7 x 10^(-27) kg)^2 / (1 m)^2 F_G = 1.9 x 10^(-64) N
That's a REALLY large difference! The electric force is about 1.2 x 10^36 times larger. Let's turn that around and see how much mass we would need to get roughly the same scale.
F_E / F_G = 1 [2.3 x 10^(-28) N] / [(6.7 x 10^(-11) N * m^2 / kg^2) (m_1)^2 / (1 m)^2] = 1
Solving for m_1, gives m_1 = 1.9 x 10^(-9) kg.
That's about 1.1 x 10^18 times larger!
So, to recap, let's take an imaginary object comprised of 1 x 10^18 protons and electrons, ie. 1 000 000 000 000 000 000 protons and give it one more proton than electron so it's charge is +1 e. The gravitational force between these objects is now roughly the same as the electric force between them.
Edit: I'll clean this up and LaTeX it if I have time later.
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.
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.
You are right, but when we talk about a force being stronger than another in the context of particle physics, then we mean this at the scales of particle physics, using the particles that exists at that scale. The table on this link shows that gravity is slightly stronger at the proton/neutron scale than at the fundamental quark scale.
On April 28 2015 16:00 helpman169 wrote: Why is it confirmed that there only 4 fundamental forces? Is it possible that additional forces exist?
As far as I know it is not. However, I think we can safely say that all forces we know now have a massive impact on the way the universe functions, so we'd expect to have seen some evidence of an additional force. It'd have to be really really weak.
Is it possible that there are other particles that carry the electromagnetic force similar to the photon?
Not within our current theory (which we are quite certain of, since it has pretty much provided the most accurate predictions in the history of science). The particle that mediates the fundamental forces of the SM are introduced as a consequence of their associated symmetry. In case of EM, this symmetry requires a quantum field that corresponds with a single particle.
How big would a particle accelerator have to be in order to be able to create the energies necessary for testing quantum gravity?
Hard to say, but to give you an idea, in the 70s we were building accelerators that reached energies of 1 GeV. We now have the LHC which operates at around 1000 GeV. Gravity is expected to kick in at the Planck scale, which is around 10^19 GeV. So yeah...
On April 28 2015 08:18 Teoita wrote: That Lagrangian looks fucking terrifying holy shit.
So regarding supersymmetry, what are the actual characteristics of dark matter candidates like wimps? All i know is pretty much "yeah dark matter could be super symmetrical particles that only interact gravitationally" and that's really it.
Also, do exentions of the Standard Model compute vacuum energy density in any reasonable way (as in, not 104 or whatever it was orders of magnitude off what the standard cosmological model comes up with)?
second edit: also, can you tell me anything more specific about the last AMS-02 results that were announced a couple of weeks ago regarding potentially detecting signals from the annihilation of dark matter particles?
Haha - its actually not that bad. Each part is an interaction term with one of the forces, and you can break it up and solve each part separately, so its bark is definitely worse than its bite. Well, at leading order anyway...
So the thing is that we really don't know much about the properties of dark matter. As far as SUSY goes for a dark matter candidate, in order for the lightest SUSY particle to be a WIMP, it has to be 'R-parity conserving', meaning it can only decay to other SUSY particles and not anything from the SM (which is why it sits around doing nothing as dark matter), and it has to be <400GeV mass. Any heavier than that and it starts to dominate the universe, which we don't see. It also has to be electrically neutral. Past that, we haven't a clue.
I'm not too sure about the vacuum energy density, actually. Not something that's discussed much. I think there are models that try to explain the suppression, and I have a feeling that the extra dimension-style models provide a neat explanation for this, but as far as I'm aware, SUSY doesn't really deal with it.
Yeah, uh, I've not read much into it. CERN have been burned before by realising results too soon (see, the OPERA experiment), so I'm sure they're confident, but I'll wait for confirmation and reinterpretation by a second experiment first. Don't get me wrong, its a significant result and I'm very excited, but I'd love to see a second experiment repeat the observation. If it is right though, its still not a direct observation of dark matter, but it is a huge step forward as it would place some very stringent limits on the LSP mass.
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.
You are right, but when we talk about a force being stronger than another in the context of particle physics, then we mean this at the scales of particle physics, using the particles that exists at that scale. The table on this link shows that gravity is slightly stronger at the proton/neutron scale than at the fundamental quark scale.
On April 28 2015 16:00 helpman169 wrote: Why is it confirmed that there only 4 fundamental forces? Is it possible that additional forces exist?
As far as I know it is not. However, I think we can safely say that all forces we know now have a massive impact on the way the universe functions, so we'd expect to have seen some evidence of an additional force. It'd have to be really really weak.
Is it possible that there are other particles that carry the electromagnetic force similar to the photon?
Not within our current theory (which we are quite certain of, since it has pretty much provided the most accurate predictions in the history of science). The particle that mediates the fundamental forces of the SM are introduced as a consequence of their associated symmetry. In case of EM, this symmetry requires a quantum field that corresponds with a single particle.
How big would a particle accelerator have to be in order to be able to create the energies necessary for testing quantum gravity?
Hard to say, but to give you an idea, in the 70s we were building accelerators that reached energies of 1 GeV. We now have the LHC which operates at around 1000 GeV. Gravity is expected to kick in at the Planck scale, which is around 10^19 GeV. So yeah...
The standard quoted size is that you need an accelarator the size of earths orbit around the sun to reach the planck scale, where most theories of everything (TOE, a theory that includes SM and gravity consistently) would be measurable. haven't done the calculations myself though. It usually comes with a scientist-type sour joke along the lines of such an accelerator being unrealistic with current fundings.
The idea with gravity being weaker is that the gravity charge (ie mass) of the particles that exists are much smaller than the charges of the other forces. So it comes down to some of the constants in the standard model (controling the force between unit charges) being much larger than the corresponding constants in general relativity.
Also, how can you write this blog without:
:D (note: this was pre-Higgs discovery)
What's your PhD on btw? I did a PhD in particle physics phenomenology in Lund, writing a minimum bias event generator. glgl!
I`ve always felt that scientist need to change something about the way they present particle physics. Even at the simplest level its pretty hard for the "normal" person to understand what is going on.
On April 28 2015 08:19 micronesia wrote: Thank you, I enjoyed the read. One thing that has been bothering me...
You said earlier that gravity is many times weak than other fundamental forces. How do you perform an apples to apples comparison? I mean, the gravitational force of the Earth on the moon is much stronger than the electromagnetic force. Therefore, the electromagnetic force of the Earth on the moon is much weaker. In contrast, if I put 1 Coulomb of charge on to a 1 kg metal sphere, the electromagnetic force on an identical sphere 10 meters away will be much stronger than the gravitational force. How do you choose the mass and charge of each object for a fair comparison?
Well, I mean, just using Newton's Gravitational Equation, F = (GMm)/r^2, you could calculate the gravitational pull between two atoms at some distance (say, nanometres) away from each other. Repeat this for the electroweak and strong forces (not recommended except for the brave or the foolish PhD student), and boom, you've got your apples to apples comparison. I think there's a more mathematically rigorous of doing it, but that would give you an idea of the scale of the problem, certainly.
Edit: just saw your second post, and the reply to it as well. I mean, its a good question, but its definitely a case of you work with what you've got, right? Proton/quark/electron electric charges are just what they are, similarly with their mass, so we either use that and compare them as they are, or we spend our time complaining the universe doesn't do what we want it to do . Electrons have the same charge as a proton, but their mass is many magnitudes smaller; quarks are heavier than electrons but have 1/3 the electric charge. Take your pick for comparing; so long as you're consistent, you should always get the same kind of result.
On April 28 2015 11:05 TheLordofAwesome wrote: 5/5, great blog. This should absolutely be featured!
Can you tell me why you chose to go into particle physics and whether or not you are seriously interested in it as a career? I'm asking because right now I'm at the point in my life where I need to make some decisions about what I want to do. I'm finishing high school in 3 weeks, going to college in the fall.
Last summer, I worked for a physics professor at Bellarmine University in Kentucky analyzing collision data from ATLAS about the Z prime boson. It was really exciting work, particularly because the data we got was indicative of the existence of the Z prime! Supersymmetry yay! I also finished AP Physics C as a junior and am doing a independent study of basic QM with the Physics C teacher at my high school, who is a really smart guy. So I really love this stuff, but I don't know how far I want to pursue it.
So why did you choose particle physics?
EDIT: Just noticed the FAQs, those are hilarious.
Thanks :D
I worked for two summers at the Rutherford Appleton Lab in the UK, on the ISIS neutron + muon source, and that made me want to do a PhD and go into research. Optics and solid state physics bored me, and I never liked the order-of-magnitude hand-waving that astrophysicists tend to do. I loved doing quantum mechanics, and the subatomic world fascinated me, so I chose particle physics - then worked damn hard to do well enough in my Masters and the particle physics courses available that I got accepted for a PhD. Now I'm here... not so sure I want to do it as a career. Not only is funding hard to come by, but its an awful lot of stress. The academics work harder and longer than just about anyone I've ever known, and I'm not sure I'm dedicated enough to want to do that for the rest of my life; not when I can get a 9-5 job that I enjoy that would almost certainly pay better .
That sounds cool, I know a couple of guys working on the Z' analysis, its a pretty good setup they've got! Good luck with finishing high school, and starting college - don't work too hard
eonrulz, I really like you part on BSM, it's nice to see you don't pose you favourite BSM flavour as a fact (many people sadly do that).
On April 28 2015 08:18 Teoita wrote: That Lagrangian looks fucking terrifying holy shit.
Not only it is terrifying, it i also wrong. The second line is messed up, it should be psi-bar D_mu psi to make a kinetic term with the covariant derivative, psi-bar psi-bar breaks like 4 conservation laws at once.
Actually, it can be both made even more terrifying and yet even more simple i a single step - if you write it in terms of physical particle fields, it explodes into a lot more of terms and can easily fill a line - particularly if you are mean or thorough and include counterterms and all the annoying ghosts that pop up at least from QCD (you can have them in QED part as well if you reallly wish). But then if you know how to read it, every term is just a small drawing - it is a vertex in a graph and the fields give you the kind of lines that meet in the vertex and then it is just drawing preety pictures and assinging a value to them (which is however dauntingly difficult to convert from symbols to a number).
On April 28 2015 17:19 Cascade wrote: What's your PhD on btw? I did a PhD in particle physics phenomenology in Lund, writing a minimum bias event generator. glgl!
On April 28 2015 17:32 opisska wrote: eonrulz, I really like you part on BSM, it's nice to see you don't pose you favourite BSM flavour as a fact (many people sadly do that).
Not only it is terrifying, it i also wrong. The second line is messed up, it should be psi-bar D_mu psi to make a kinetic term with the covariant derivative, psi-bar psi-bar breaks like 4 conservation laws at once.
Actually, it can be both made even more terrifying and yet even more simple i a single step - if you write it in terms of physical particle fields, it explodes into a lot more of terms and can easily fill a line - particularly if you are mean or thorough and include counterterms and all the annoying ghosts that pop up at least from QCD (you can have them in QED part as well if you reallly wish). But then if you know how to read it, every term is just a small drawing - it is a vertex in a graph and the fields give you the kind of lines that meet in the vertex and then it is just drawing preety pictures and assinging a value to them (which is however dauntingly difficult to convert from symbols to a number).
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.
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.
You are right, but when we talk about a force being stronger than another in the context of particle physics, then we mean this at the scales of particle physics, using the particles that exists at that scale. The table on this link shows that gravity is slightly stronger at the proton/neutron scale than at the fundamental quark scale.
On April 28 2015 16:00 helpman169 wrote: Why is it confirmed that there only 4 fundamental forces? Is it possible that additional forces exist?
As far as I know it is not. However, I think we can safely say that all forces we know now have a massive impact on the way the universe functions, so we'd expect to have seen some evidence of an additional force. It'd have to be really really weak.
Is it possible that there are other particles that carry the electromagnetic force similar to the photon?
Not within our current theory (which we are quite certain of, since it has pretty much provided the most accurate predictions in the history of science). The particle that mediates the fundamental forces of the SM are introduced as a consequence of their associated symmetry. In case of EM, this symmetry requires a quantum field that corresponds with a single particle.
How big would a particle accelerator have to be in order to be able to create the energies necessary for testing quantum gravity?
Hard to say, but to give you an idea, in the 70s we were building accelerators that reached energies of 1 GeV. We now have the LHC which operates at around 1000 GeV. Gravity is expected to kick in at the Planck scale, which is around 10^19 GeV. So yeah...
The standard quoted size is that you need an accelarator the size of earths orbit around the sun to reach the planck scale, where most theories of everything (TOE, a theory that includes SM and gravity consistently) would be measurable. haven't done the calculations myself though. It usually comes with a scientist-type sour joke along the lines of such an accelerator being unrealistic with current fundings.
The idea with gravity being weaker is that the gravity charge (ie mass) of the particles that exists are much smaller than the charges of the other forces. So it comes down to some of the constants in the standard model (controling the force between unit charges) being much larger than the corresponding constants in general relativity.
What's your PhD on btw? I did a PhD in particle physics phenomenology in Lund, writing a minimum bias event generator. glgl!
Oh hey, I remember us talking in that thread about the Winners advantage. :D
Its on indirect and direct searches for new physics at the LHC - so I've just finished with the Bs->mumu analysis, which is interesting because a) its a SM flavour-changing neutral current that's never been observed before, and b) if there's SUSY, that can change the branching ratio away from SM predictions. Can't say anything about ATLAS results yet, but LHCb and CMS have combined 6sigma observation compatible with the SM, but with a 2.8sigma deviation from the SM in the ratio of Bs/Bd decays... And now I'm working on SUSY, searching for the stop .
And of course, how could I forget that amazing (terrible) rap! Thank you for bringing that to everyone's attention xD
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.
You are right, but when we talk about a force being stronger than another in the context of particle physics, then we mean this at the scales of particle physics, using the particles that exists at that scale. The table on this link shows that gravity is slightly stronger at the proton/neutron scale than at the fundamental quark scale.
On April 28 2015 16:00 helpman169 wrote: Why is it confirmed that there only 4 fundamental forces? Is it possible that additional forces exist?
As far as I know it is not. However, I think we can safely say that all forces we know now have a massive impact on the way the universe functions, so we'd expect to have seen some evidence of an additional force. It'd have to be really really weak.
Is it possible that there are other particles that carry the electromagnetic force similar to the photon?
Not within our current theory (which we are quite certain of, since it has pretty much provided the most accurate predictions in the history of science). The particle that mediates the fundamental forces of the SM are introduced as a consequence of their associated symmetry. In case of EM, this symmetry requires a quantum field that corresponds with a single particle.
How big would a particle accelerator have to be in order to be able to create the energies necessary for testing quantum gravity?
Hard to say, but to give you an idea, in the 70s we were building accelerators that reached energies of 1 GeV. We now have the LHC which operates at around 1000 GeV. Gravity is expected to kick in at the Planck scale, which is around 10^19 GeV. So yeah...
The standard quoted size is that you need an accelarator the size of earths orbit around the sun to reach the planck scale, where most theories of everything (TOE, a theory that includes SM and gravity consistently) would be measurable. haven't done the calculations myself though. It usually comes with a scientist-type sour joke along the lines of such an accelerator being unrealistic with current fundings.
The idea with gravity being weaker is that the gravity charge (ie mass) of the particles that exists are much smaller than the charges of the other forces. So it comes down to some of the constants in the standard model (controling the force between unit charges) being much larger than the corresponding constants in general relativity.
What's your PhD on btw? I did a PhD in particle physics phenomenology in Lund, writing a minimum bias event generator. glgl!
Oh hey, I remember us talking in that thread about the Winners advantage. :D
Its on indirect and direct searches for new physics at the LHC - so I've just finished with the Bs->mumu analysis, which is interesting because a) its a SM flavour-changing neutral current that's never been observed before, and b) if there's SUSY, that can change the branching ratio away from SM predictions. Can't say anything about ATLAS results yet, but LHCb and CMS have combined 6sigma observation compatible with the SM, but with a 2.8sigma deviation from the SM in the ratio of Bs/Bd decays... And now I'm working on SUSY, searching for the stop .
And of course, how could I forget that amazing (terrible) rap! Thank you for bringing that to everyone's attention xD
Right, yes! Winners advantage. Well done on the blog! I even got a t-shirt with the SM Lagrangain! :D
On April 28 2015 17:32 opisska wrote: Wow, which code? You might be quite famous
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.
EPT![[image loading]](http://d29qn7q9z0j1p6.cloudfront.net/sites/default/files/highwire/roypta/370/1961/818/embed/graphic-1.gif)
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