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Old 13th March 2013, 08:47 PM   #1041
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Farsight has also been vague about what force keeps photons in their orbits. There's nothing in the Standard Model that makes that possible, and any deviations from the Standard Model at the electron's mass scale are very tiny. Check on some of the upper limits for non-SM effects at Particle Data Group some time.

There is a Standard-Model photonlike particle that can confine itself, however: the gluon, which can make glueballs. It can do that because at energies around 1 GeV, its self-interaction is superstrong. However, a glueball state has yet to be unambiguously identified, in part because it acts much like a flavorless meson state. That's a meson with its valence quark and antiquark having the same flavor. In fact, glueballs may mix with flavorless mesons with the same quantum numbers and close masses.
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Old 13th March 2013, 09:49 PM   #1042
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RÉSONAANCES: When shall we call it Higgs?
Quote:
Although in private it's always the Higgs, in official communications the particle discovered at the LHC last year is being called different names: sometimes the 125 GeV particle, sometimes a scalar boson (as opposed to scalar fermions), and most often a Higgs-like boson. This caution was understandable at the early stage, given the fresh memory of faster-than-light neutrinos. However, since it's been walking and quacking like a duck for more than half a year now, there's a discussion among experimenters when they will be allowed to drop the derogatory "-like" suffix.
"Scalar boson" is also distinct from "vector boson", what gauge particles are. The Standard-Model ones are the photon, W, Z, and gluon.

However, the supersymmetry partners of the elementary fermions are sometimes called "scalar fermions", despite the term being an oxymoron.

Also, I've seen the Higgs particle called the Brout-Englert-Higgs or BEH particle, honoring Robert Brout, François Englert, and Peter Higgs. We may also add Gerald Guralnik, C.R. Hagen, and Tom Kibble. The BEHGHK particle?

But sad to say, Robert Brout died on May 3, 2011, a Moses-like death.

-

In the rest of his entry, Jester, the Résonaances blogger, mentioned that the Higgs particle being spin 2 would be even more awkward than for spin 0. It would have to have non-renormalizable interactions, and that requires some new physics around 1 TeV to give it such interactions as a low-energy limit.

Why are there no spin 3/2 or higher fundamental particles in the standard model of particle physics? - Quora (registration required) Physicist David Simmons-Duffin answered, and I'll elaborate on it as appropriate.


A spin-n particle is described by a field that is a tensor with n indices. One has to be careful to project out the lower-spin modes, and that introduces some awkward features into the particle's "propagator". That's a function that says what its field its like at a point after being created at some other point.

One starts getting trouble even for spin 1. In momentum space, the W's propagator is proportional to
1/(p2 - m2)*(gij - pipj/m2)

for space-time indices i,j, metric g, momentum p, and mass m. That makes the W's interactions non-renormalizable with a breakdown energy scale of about 1 TeV. However, electroweak symmetry breaking has a cure for this problem. Its energy scale gives the massive W a maximum energy; above that, the W is effectively massless.

In general, a spin-n particle's propagator has momentum dependence O(p2n-2). Spin-0: O(1/p2), spin-1: O(1), etc. This is true for fermions as well as for bosons. For fermions, one treats the spinor part as being like 1/2 a coordinate index. This a spin-1/2 particle has behavior O(1/p), a spin-3/2 one like O(p), etc.

So if a massive particle has a negative power of p in its propagator for high momenta, it is well-behaved, but not otherwise.

It's true that there are numerous bound states with spins >= 1, but their interactions' non-renormalizability is no problem. That's because they have a maximum energy scale: the energy needed to destroy them.


For massless particles, one gets a different problem. Steven Weinberg, in volume 1 of his big tome, considers soft (low-energy) interactions of particles with different spins. Photons (spin 1) are associated with conservation of electric charge, gravitons (spin 2) with conservation of energy-momentum, but higher spins would be even more restrictive, not allowing interactions to happen. A spin-3/2 particle would be restricted to interacting like a gravitino, etc.

Also, the photon's interactions are renormalizable, and the same is true for nonabelian (self-interacting) gauge fields like the gluon. But the graviton's interactions are not, with the maximum energy scale being the Planck mass.
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Old 14th March 2013, 01:34 AM   #1043
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RÉSONAANCES: Higgs: what have we learned
Some of it repeats what I'd posted on earlier -- rough agreement with the Standard Model's predictions for

(H->top-top) * (H->WW), (H->ZZ), (H->tau-tau)

About measuring the Higgs particle's spin, Jester was sure that it had to be 0, but he noted that the spin fits also constrain additional interactions with the W and Z particles. So far, this particle does not have any big differences from the Standard Model there also.

He also mentioned constraints on Higgs -> Z-photon, mu-mu, and invisible. These could provide constraints on new physics, since from the Standard Model, they are still too small too see.
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Old 14th March 2013, 04:02 AM   #1044
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Physicists say they are now confident they have discovered the long-sought Higgs boson
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Old 14th March 2013, 04:47 AM   #1045
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New results indicate that new particle is a Higgs boson | CERN - the likely source of that AP article.

Rencontres de Moriond - the Moriond conference that the article refers to. Very technical, but I can understand much of it.

ATLAS Experiment - Photos - includes animations of the Higgs-particle bumps emerging with the collection of more data.
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Old 14th March 2013, 05:02 PM   #1046
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Originally Posted by lpetrich View Post
Farsight has also been vague about what force keeps photons in their orbits.
That is basically because the papers that he relies on are equally vague about this. They evoke magic, e.g. In Is the electron a photon with toroidal topology? the photon is in a "self-contained" state that forces photons to exhibit "toroidal topology".
We already know that this speculation is invalid because no matter what topology they come up with, a photon can never be a source of charge.
Of course the other big problem is they have no mechanism to prevent every photon turning into an electron !
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Old 14th March 2013, 09:12 PM   #1047
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Moriond QCD 2013 Agenda has "Measurements of Higgs Boson Properties in ATLAS" with these upper limits:

Ratio is to Standard-Model rate
Higgs -> muon-antimuon: ratio < 9.8
Higgs -> Z-photon: ratio < 18.2

If they get enough events to be able to see these modes, they will get enough to see bottom-antibottom and likely also charm-anticharm. So we'll get the Higgs particle's interactions with the W, Z, top, bottom, tau, charm, muon -- plenty of testing of the mass-proportionality hypothesis.

Assuming Standard-Model values of all the couplings, the branching fraction to invisible particles they find to be less than 0.6.


Also in that presentation was cross sections in picobarns for different production processes, calculated with the Standard Model and a Higgs mass of 125 GeV:

19.5 pb - gluon fusion: 2 gluons - top-antitop - H
1.6 pb - vector boson fusion: 2 quarks -> each one radiates a W or Z -> WW or ZZ -> H
1.1 pb - quark-antiquark -> W,Z -> radiates a H
0.1 pb - 2 gluons -> each one makes top-antitop -> one top-antitop makes a H

So the W and Z rates combined are about 1/8 of the total, the rest being almost entirely top quark. The bottom-quark rate is about 1500 times less, and the other quarks' rates even less.


For spin-2 tests, they are using graviton-like interactions. That's good for positive parity, but they'd have to modify that for mixed or negative parity. BTW, they are also doing mixed-parity tests for spin 0, and so far, the mixture is consistent with being all-positive.

They are doing spin and parity tests mainly with H -> ZZ, and to a lesser extent with H -> WW. But they might eventually extend that to elementary fermions, though they'd have to use mainly H -> top-antitop.
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Old 14th March 2013, 09:44 PM   #1048
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Originally Posted by lpetrich View Post
Farsight has also been vague about what force keeps photons in their orbits.
The force that keeps photons in their orbits is that Farsight drew a picture of the photon in such an orbit. Once Farsight has drawn such a picture, the photon looks at it thinks, "hey, if that's what Einstein said between the lines, that's what I gotta do."
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Old 20th March 2013, 12:27 PM   #1049
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RÉSONAANCES: Higgs: more of the same
reporting on Review of the Higgs-to-2-Photon Data | Of Particular Significance

Rate / (Standard-Model prediction)
ATLAS: 1.65 +- 0.30
CMS: 0.8 +- 0.3
Naive combination: 1.2 +- 0.2

So it looks like that's close to the Standard Model also. It also means that the coupling of the Higgs particle to the elementary fermions, or at least to the top quark, is not opposite in sign from what the Standard Model predicts.
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Old 21st March 2013, 08:24 AM   #1050
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Is it true that the mass of the Higgs boson means that the universe has a half-life?

http://www.escapistmagazine.com/news...Whole-Universe

I actually like this scenario better than heat death. A quick, painless end (probably a long time in the future) seems more merciful than any of the other possible ends of the universe I've heard about.
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Old 21st March 2013, 01:42 PM   #1051
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Originally Posted by Puppycow View Post
Is it true that the mass of the Higgs boson means that the universe has a half-life?

http://www.escapistmagazine.com/news...Whole-Universe
Vacuum instability is entirely speculative to begin with; the statement that a 125 GeV Higgs predicts vacuum instability is, I think, extremely hypothetical and/or premature. Lykken is quoted (after giving a talk; he has not published any papers or preprints on this topic) as saying he "thinks" the idea is "gaining traction", which is a very, very soft claim.
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Old 21st March 2013, 02:33 PM   #1052
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RÉSONAANCES: What's the deal with vacuum stability? has a good discussion of this question, complete with this disclaimer:
Quote:
All this discussion is valid assuming the standard model is the correct theory all the way up to the Planck scale, which is unlikely.
Supersymmetric extensions of the Standard Model don't have this instability problem.

Summary: the Standard Model gets stability problems at an energy scale of about 1010 GeV. Using the (rather small) experimental limits on the mass of the top quark and the strength of the QCD interaction, the stability problems set in at about 108 GeV to 1014 GeV.


To see why this happens, I'll explain how the Higgs-particle field behaves. I'll simplify it from a complex doublet to a real singlet, though the real-singlet field value will act like the magnitude of the full value.

The Higgs potential is (1/2)*V2*F2 + (1/4)*V4*F4
for Higgs field F and parameters V2 and V4.

The Higgs-field equation of motion is, in this approximation,
D2F + dV/dF = 0

D2 = d2/dt2 - Dspace2

I'll ignore the spatial variation and focus on the time variation, giving us
d2F/dt2 + dV/dF = 0


Let's now see how the field behaves. We can carry over techniques from classical mechanics: look for fixed points and see how the field behaves around those points. Does it oscillate around the point? Does it move away from the point?

Using dV/dF = F*(V2 + V4*F2)
there's an obvious fixed point: F = 0. Oscillations around it behave as dF = dF0*exp(i*w*t) + complex conjugate, where w is the angular frequency of oscillation. It is
w = sqrt(V2)

If V2*V4 < 0, there is another fixed point, F = sqrt(-V2/V4). Its oscillation angular frequency is
w = sqrt(-2V2)

If the angular frequency is real, then the point is stable. Otherwise, it is unstable, with exponential departure.


The Higgs mechanism works by having V2 < 0 and V4 > 0, making a stable nonzero fixed point F = sqrt(-V2/V4). It has the lowest energy that the field can have, thus making it the ground state. It's that nonzero value of F that gives other Standard-Model particles their masses.

But if one extrapolates to energy scales above the electroweak one, V2 becomes positive and the Higgs mechanism no longer works. Of the Standard-Model particles, only the Higgs one is then massive.

The stable fixed point is for F = 0 in this case, what one normally expects of a spin-0 particle.

The interesting thing here is what happens to V4, often written lambda. It's a measure of the Higgs particle's self-interaction, and in the bare Standard Model, it becomes negative at energies of 108 GeV - 1010 GeV - 1014 GeV.

But for the Universe to self-destruct by Higgs instability, the Higgs field must quantum tunnel from near 0 to near sqrt(-V2/V4), and the rate of that tunneling is ~ exp(1/V4). So if V4 is not much less than 0, our Universe is metastable, with a Higgs-decay lifetime longer than its age.
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Old 10th September 2014, 12:31 AM   #1053
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Sorry for necro-ing this thread, but I've been wanting to know how this discovery has affected the theory SuperSymmetry?

(As a layman, my limited understanding of Physics comes soley from documentaries and popular media, so a maths filled post, while pretty, is way over my head).

Does it mean that SuperSymmetry is dead in the water? Do the remaining possible SuperSymmetry variations still help to explain things?

What I'm trying to get at, is where do we go from here? The Standard Model is now supposedly complete. What now? Can the higgs help explain the three mysterious Darks? (Matter/Energy/Fluid)
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Old 10th September 2014, 06:56 AM   #1054
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Supersymmetry is still up in the air. No superpartner of a known partner has been detected, and the easier-to-make ones are out of the energy range of the LHC's first runs.

But the Higgs particle's mass is in the range predicted by some versions of the Minimal Supersymmetric Standard Model. This is significant because it is a free parameter in the Standard Model.
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Old 10th September 2014, 09:28 AM   #1055
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Originally Posted by Octavo View Post
What I'm trying to get at, is where do we go from here? The Standard Model is now supposedly complete. What now?
Now we try to fix it. The Standard Model doesn't predict any more fundamental particles to find, but it's still fundamentally flawed. We only keep it around because we haven't managed to find anything better yet. Technically that's true for all theories, but they don't usually have quite such obvious gaping holes.

Quote:
Can the higgs help explain the three mysterious Darks? (Matter/Energy/Fluid)
Firstly, that would be the two darks - matter and energy. Dark fluid is a pretty fringe theory that attempts to unify the two, but I'm not aware of any particularly widespread support for the idea, and even if there were that would make it one dark rather than three.

In any case, that's part of why I say the Standard Model doesn't work - whatever dark energy and matter ultimately turn out to be, it's something the Standard Model simply can't account for. As is, perhaps a little more worryingly, gravity. The Standard Model has been a great tool, but we've known pretty much since the beginning that it couldn't be the full answer. Things like supersymmetry, string theory, and so on, have all been attempts to expand the Standard Model to actually address these, and various other, problems.

In fact, probably the biggest surprise of the LHC was that the results matched the Standard Model so well. While no-one was quite sure what to expect, most were pretty sure that there would be something weird seen that would give some indication of which of the various theories had the right idea.
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Old 10th September 2014, 10:45 PM   #1056
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Originally Posted by Cuddles View Post
Now we try to fix it. The Standard Model doesn't predict any more fundamental particles to find, but it's still fundamentally flawed. We only keep it around because we haven't managed to find anything better yet. Technically that's true for all theories, but they don't usually have quite such obvious gaping holes.



Firstly, that would be the two darks - matter and energy. Dark fluid is a pretty fringe theory that attempts to unify the two, but I'm not aware of any particularly widespread support for the idea, and even if there were that would make it one dark rather than three.

In any case, that's part of why I say the Standard Model doesn't work - whatever dark energy and matter ultimately turn out to be, it's something the Standard Model simply can't account for. As is, perhaps a little more worryingly, gravity. The Standard Model has been a great tool, but we've known pretty much since the beginning that it couldn't be the full answer. Things like supersymmetry, string theory, and so on, have all been attempts to expand the Standard Model to actually address these, and various other, problems.

In fact, probably the biggest surprise of the LHC was that the results matched the Standard Model so well. While no-one was quite sure what to expect, most were pretty sure that there would be something weird seen that would give some indication of which of the various theories had the right idea.
Ah, ok I see (I think). The Standard model only predicts the particles we found, therefore it' cannot be complete because it doesn't account for Dark Matter/Energy. And I further assume that the Standard Model can't be extended or reworked sufficiently to predict anything new?

Slightly different question now:
I know that we've managed to discover anti-particles; does it make sense to think there might be an anti-higgs? If not, why not?

Are there any ideas about what sort of technologies a better understanding of the higgs could lead us toward? Like (spitballing here) ways to reduce the mass of a spaceship, allowing more efficient use of accelerate?
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Old 11th September 2014, 12:19 AM   #1057
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Originally Posted by Octavo View Post
...
Are there any ideas about what sort of technologies a better understanding of the higgs could lead us toward?...
The end of the universe according to Stephen Hawking.

Talk about a sore loser. You'd reckon after 2 years he wouldn't still be crying about losing an $100 bet.
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Old 11th September 2014, 07:18 AM   #1058
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Originally Posted by Octavo View Post
Ah, ok I see (I think). The Standard model only predicts the particles we found, therefore it' cannot be complete because it doesn't account for Dark Matter/Energy. And I further assume that the Standard Model can't be extended or reworked sufficiently to predict anything new?
It depends a bit on what exactly you mean by "extended or reworked". Things like supersymmetry are attempts to extend the standard model. It's just that once you've extended it that much, it's no longer the standard model, in the same way that relativity is no longer Newtonian mechanics.

Quote:
Slightly different question now:
I know that we've managed to discover anti-particles; does it make sense to think there might be an anti-higgs? If not, why not?
As with the photon and Z boson, the Higgs is its own anti-particle.

Quote:
Are there any ideas about what sort of technologies a better understanding of the higgs could lead us toward? Like (spitballing here) ways to reduce the mass of a spaceship, allowing more efficient use of accelerate?
When it comes to mass, that's based on a very common misunderstanding. The Higgs boson does not "give particles mass" as is often claimed; it's simply that the Higgs mechanism is part of the explanation for why particles are the mass they are. If the Higgs mechanism didn't exist, some particles would have different masses, but given that the mechanism does exist there's nothing you can do to change those masses. That would be like claiming that understanding electromagnetism should allow us to change the charge of an electron. That understanding allows us to manipulate electrons in all kinds of clever ways, but both its charge and its mass are fundamental properties that we can't do anything about.

As for what technologies it might enable, no-one has the slightest idea. There are much better known and more easily produced and studied particles that haven't led to any useful technology. In fact, the electron and photon are the only fundamental particles that actually have. Knowing about the others is important for our understanding of physics, and that understanding is what all our technology ultimately rests on, but no other particles have led to any technology in themselves.
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Old 11th September 2014, 07:41 AM   #1059
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I don't think nuclear energy would be possible without understanding the behaviour of neutrons and protons.
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Old 11th September 2014, 02:19 PM   #1060
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Originally Posted by Roboramma View Post
I don't think nuclear energy would be possible without understanding the behaviour of neutrons and protons.
So the Sun didn't exist before the Curies were born? The natural reactors at Oklo never happened?

(I know what you meant, but your wording was fairly unfortunate. Physics doesn't depend on knowledge or belief, which is one of the things that distinguishes it from, say, religion.)
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Old 11th September 2014, 04:32 PM   #1061
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Originally Posted by xtifr View Post
So the Sun didn't exist before the Curies were born? The natural reactors at Oklo never happened?

(I know what you meant, but your wording was fairly unfortunate. Physics doesn't depend on knowledge or belief, which is one of the things that distinguishes it from, say, religion.)
Haha, nice. Yeah, by "nuclear energy" I was talking about nuclear power plants and nuclear bombs. Specifically the technology of nuclear energy. I should have worded that better.
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Old 11th September 2014, 11:23 PM   #1062
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Originally Posted by Cuddles View Post
It depends a bit on what exactly you mean by "extended or reworked". Things like supersymmetry are attempts to extend the standard model. It's just that once you've extended it that much, it's no longer the standard model, in the same way that relativity is no longer Newtonian mechanics.
I understand that, however I imagine that the mathematical algorithms and terms used to describe Newtonian mechanics are completely different to the algorithms that describe relativity. Is it not the case that the algorithms that describe "the standard model" can only be tweaked or modified so far before you need to throw them away and start with new algorithms and different terms? Otherwise isn't supersymmetry just a heavily tweaked version of the standard model?

Please excuse my ignorance here I just want to learn!

Originally Posted by Cuddles View Post
As with the photon and Z boson, the Higgs is its own anti-particle.
Ok. You're going to have to explain that one. My understanding is that a particle and it's anti-particle would annihilate each other and photons obviously don't annihilate each other. The concept of something being it's own anti-particle doesn't make sense given how I understand anti-matter to work. (Again, please excuse my scientific illiteracy - I'm sure this must seem an obvious question to the initiated).

Originally Posted by Cuddles View Post
When it comes to mass, that's based on a very common misunderstanding. The Higgs boson does not "give particles mass" as is often claimed; it's simply that the Higgs mechanism is part of the explanation for why particles are the mass they are. If the Higgs mechanism didn't exist, some particles would have different masses, but given that the mechanism does exist there's nothing you can do to change those masses. That would be like claiming that understanding electromagnetism should allow us to change the charge of an electron. That understanding allows us to manipulate electrons in all kinds of clever ways, but both its charge and its mass are fundamental properties that we can't do anything about.
There are other mechanisms/explanations that also contribute to the mass of particles? (I mean I understand that relativity implies that mass is dependent on velocity, but I suspect there's more to it than that?)

Perhaps I'm reading you wrong, but the paragraph above implies that even if we understood the Higgs perfectly, we still wouldn't necessarily understand why matter has mass? We would only understand why the masses they have are the values they are - is that correct?

Originally Posted by Cuddles View Post
As for what technologies it might enable, no-one has the slightest idea. There are much better known and more easily produced and studied particles that haven't led to any useful technology. In fact, the electron and photon are the only fundamental particles that actually have. Knowing about the others is important for our understanding of physics, and that understanding is what all our technology ultimately rests on, but no other particles have led to any technology in themselves.
I present you with:
http://en.wikipedia.org/wiki/Proton_therapy
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Old 12th September 2014, 12:28 AM   #1063
Roboramma
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Please note: I'm not an expert, but the below should be correct in general even if some details are off. :P

Originally Posted by Octavo View Post
There are other mechanisms/explanations that also contribute to the mass of particles? (I mean I understand that relativity implies that mass is dependent on velocity, but I suspect there's more to it than that?)
I think it depends on what you want to call a "particle". For instance, I think you know that the mass of a Uranium atom is more than the sum total of the masses of the protons and neutrons in the nucleus, yes? Split the atom and you get two atoms of smaller mass whose sums are less than that of the uranium atom by a small amount, that deficit happens to be equal to the amount of energy released.

But what form does that energy take in the Uranium atom? It's the energy of interaction between the particles. Or perhaps put better, between the particles and the fields they produce.

The same is true in a Hydrogen atom. A certain amount of the mass of the hydrogen atom is the energy of interaction between the electron and the proton.

But you wouldn't call those particles.

How about a proton? Well, a proton is made up of quarks. The mass of a quark is determined by the Higgs mechanism, but you can't just sum up the masses of the quarks in a proton to give you the proton mass. You'll be way under. There's also the contribution from the interaction of the quarks. That part of the energy is actually much greater than the part that comes about through coupling to the Higgs field.

An electron on the other hand is, as far as we can tell, "fundamental", and as such the mass it has is all explained by the Higgs mechanism.
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Old 12th September 2014, 01:50 AM   #1064
Octavo
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Originally Posted by Roboramma View Post
Please note: I'm not an expert, but the below should be correct in general even if some details are off. :P


I think it depends on what you want to call a "particle". For instance, I think you know that the mass of a Uranium atom is more than the sum total of the masses of the protons and neutrons in the nucleus, yes? Split the atom and you get two atoms of smaller mass whose sums are less than that of the uranium atom by a small amount, that deficit happens to be equal to the amount of energy released.
Yes, I think I have a fair understanding of nuclear decay and fission, but I had thought that the energy release/mass loss was due to matter to energy conversion? E=mc2 and all that? It made sense to me given the huge amounts of energy released by nuclear fission - even a tiny amount of matter converted to energy would be massive. If I understand the rest of your post correctly, you seem to be saying that there isn't a matter-energy conversion, it's just that the energy created by the interactions between the quarks/particles/whatever itself confers mass, and the break-up of the atom results in the energy previously locked up in interactions being released. Since the interactions are no longer happening, the atom loses mass at the same time.... is that correct?

Originally Posted by Roboramma View Post
But what form does that energy take in the Uranium atom? It's the energy of interaction between the particles. Or perhaps put better, between the particles and the fields they produce.

The same is true in a Hydrogen atom. A certain amount of the mass of the hydrogen atom is the energy of interaction between the electron and the proton.

But you wouldn't call those particles.
I think this must be above my pay-grade. I have a difficult enough time understanding exactly what "energy" actually is. Now you're saying that the energy of interaction between these "particles" has mass?

Originally Posted by Roboramma View Post
How about a proton? Well, a proton is made up of quarks. The mass of a quark is determined by the Higgs mechanism, but you can't just sum up the masses of the quarks in a proton to give you the proton mass. You'll be way under. There's also the contribution from the interaction of the quarks. That part of the energy is actually much greater than the part that comes about through coupling to the Higgs field.

An electron on the other hand is, as far as we can tell, "fundamental", and as such the mass it has is all explained by the Higgs mechanism.
OK, so quarks have mass and the interactions between the quarks that form a proton confer additional mass? Is that right? How the hell does that work?
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Old 12th September 2014, 04:08 AM   #1065
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I'd thought of describing the Standard Model's problems in more detail, but I then remembered that I'd created a thread on that subject some time ago: The Standard Model: Where are we now? - JREF Forum

Empirical problems:
  • Gravity
  • Neutrino masses
  • Matter-antimatter antisymmetry
  • Dark matter
  • Dark energy
  • Cosmic inflation
Theoretical problems:
  • Number of parameters: 19
  • Hierarchy problem: SM masses << Planck mass
  • Sources of neutrino masses: tiny Higgs couplings or very massive right-handed neutrinos?
  • Strong CP problem: how to keep QCD from violating CP without fine tuning)
  • Higgs-particle instability at high energy scales: prevented by SUSY partners (higgsinos)
  • Elementary-fermion unification: each generation fits into 2 SU(5) multiplets or 1 SO(10) multiplet without needing a lot of extra particles
  • Gauge unification: in the Minimal Supersymmetric Standard Model, the QCD, weak-isospin, and weak-hypercharge interaction strengths converge on one value at about 2*10^(16) GeV, not much less than the Planck mass
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Old 12th September 2014, 06:18 AM   #1066
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Originally Posted by Octavo View Post
I understand that, however I imagine that the mathematical algorithms and terms used to describe Newtonian mechanics are completely different to the algorithms that describe relativity. Is it not the case that the algorithms that describe "the standard model" can only be tweaked or modified so far before you need to throw them away and start with new algorithms and different terms? Otherwise isn't supersymmetry just a heavily tweaked version of the standard model?
Newtonian mechanics isn't as different from relativity as you think - if you set velocity to zero, the equations governing special relativity are identical to Newtonian mechanics. Isn't supersymmetry just a heavily tweaked version of the Standard Model? Sure. But that tweaking makes it different enough that it no longer makes sense to refer to it by the same name (at least in most cases; the Minimal Supersymmetric Standard Model is close enough that it does keep the name). Some approaches, such as string theory, do pretty much throw everything out and start from scratch, but that's not the only way to come up with something new.

Quote:
Ok. You're going to have to explain that one. My understanding is that a particle and it's anti-particle would annihilate each other and photons obviously don't annihilate each other. The concept of something being it's own anti-particle doesn't make sense given how I understand anti-matter to work. (Again, please excuse my scientific illiteracy - I'm sure this must seem an obvious question to the initiated).
There's a decent explanation here. In short, photons do annihilate with each other, it's just not a common event due to small collision cross-sections and other effects. The important thing with respect to the general question is that antimatter is not defined by annihilation, it's a consequence of quantum physics - the equations that predict regular matter have symmetric solutions that correspond to almost identical particles with opposite charge. For neutral particles, that means that both solutions are actually identical - the particle and antiparticle are the same thing.

It's similar to quadratic equations always having two solutions. For example, x2 = 1 has solutions of 1 and -1. x2 = 0 also has two solutions, but they're both 0. That may just seem to be a bit of meaningless nitpicking when you learn about it in maths classes, but it's important when it comes to particle physics because having photons and antiphotons both acknowledged as solutions has physical meaning even though they're the same thing.

Quote:
There are other mechanisms/explanations that also contribute to the mass of particles? (I mean I understand that relativity implies that mass is dependent on velocity, but I suspect there's more to it than that?)
The Higgs mechanism was originally developed to answer the problem that without it, electroweak theory (it hadn't developed to the standard model by this point) predicted that the W and Z bosons had zero mass. But this was a problem specifically with getting the weak force involved - we could already account for the masses of things like electrons via gauge theories that didn't try to unify electromagnetism with the weak force. It's not as simple as:
Standard model without Higgs - no masses
Standard model with Higgs - everything has mass
The Higgs mechanism and its consequences are just one part of the whole puzzle. Things wouldn't work properly without it, but it's really no more fundamental or important than any other part, it just happens to be the most recently confirmed.

Quote:
Perhaps I'm reading you wrong, but the paragraph above implies that even if we understood the Higgs perfectly, we still wouldn't necessarily understand why matter has mass? We would only understand why the masses they have are the values they are - is that correct?
Well yes, but that's not really a point I was trying to make. If anything, that's more philosophy than anything else. No theory can really answer why things are the way they are. We can describe how the universe works and what the properties of things in it are, but we can't say why they are that way rather than something else.

Quote:
Note that I used the term fundamental particles. We have lots of uses for all kinds of composite particles, starting with protons and neutrons and then combining them with other things to produce pretty much everything we do. But other than electrons and photons, I'm not aware of any technology resulting from the discovery of a fundamental particle, and even in those two cases many of the uses ultimately stem from empirical testing prior to the discovery of the actual particles - we had electric lights well before we knew what was going on at the particle level, or even that there was a particle level at all.

Obviously I'm not saying particle physics is useless; that would be rather hypocritical given my job. It's just that new discoveries in particle physics do not tend to lend themselves to immediate technological use. It's been nearly a century since we last discovered a particle that has actually been used to do something practical. We've discovered a whole lot more since then, but while things like computers rely very much on the associated advances in physics, the particles themselves haven't led directly to anything.
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Old 12th September 2014, 06:45 AM   #1067
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Originally Posted by Cuddles View Post
There's a decent explanation here. In short, photons do annihilate with each other, it's just not a common event due to small collision cross-sections and other effects. The important thing with respect to the general question is that antimatter is not defined by annihilation, it's a consequence of quantum physics - the equations that predict regular matter have symmetric solutions that correspond to almost identical particles with opposite charge. For neutral particles, that means that both solutions are actually identical - the particle and antiparticle are the same thing.

It's similar to quadratic equations always having two solutions. For example, x2 = 1 has solutions of 1 and -1. x2 = 0 also has two solutions, but they're both 0. That may just seem to be a bit of meaningless nitpicking when you learn about it in maths classes, but it's important when it comes to particle physics because having photons and antiphotons both acknowledged as solutions has physical meaning even though they're the same thing.
Ok, that makes sense (if anything in physics does). Cool article as well; thanks!

Originally Posted by Cuddles View Post
<Respectful snip>
Thanks Cuddles - I'm not sure I fully grok the rest of your post, so I will spend some more time digesting and reading up, but I really appreciate the effort to educate!
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Old 12th September 2014, 07:13 AM   #1068
DeiRenDopa
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Originally Posted by Cuddles View Post
...

Obviously I'm not saying particle physics is useless; that would be rather hypocritical given my job. It's just that new discoveries in particle physics do not tend to lend themselves to immediate technological use. It's been nearly a century since we last discovered a particle that has actually been used to do something practical.
That would be the positron, right?

As in PET (positron emission tomography), for example.

Quote:
We've discovered a whole lot more since then, but while things like computers rely very much on the associated advances in physics, the particles themselves haven't led directly to anything.
Isn't it more shades of grey?

For example, while there's no equivalent practical use of muons (say) in medicine or heavy industry1, their use as 'heavy electrons' in fields of science beyond fundamental particle physics is quite practical.

Another example might be practical uses of "geo-neutrinos"; a mere ~decade hence, perhaps?

True, practical uses of gluons and Z bosons (for example) do not seem to be even a gleam in the wildest of inventor's eyes yet ...

1 Or not: aren't there scanners - looking for contraband or radioactive materials - which use muons?
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Old 12th September 2014, 05:34 PM   #1069
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Originally Posted by Cuddles View Post
Newtonian mechanics isn't as different from relativity as you think - if you set velocity to zero, the equations governing special relativity are identical to Newtonian mechanics.
Strictly speaking, one has to take the limit, and one has to be a bit careful about how one does so:
t = O(1)
x = O(ε)
v = O(ε)
kinetic energy = O(ε2)
gravitational potential = O(ε2)
etc.
Quote:
Isn't supersymmetry just a heavily tweaked version of the Standard Model? Sure.
The Minimal Supersymmetric Standard Model is the Standard Model with N = 1 supersymmetry added on, the simplest level of SUSY. Most of the MSSM parallels the SM, with the exception of the Higgs particle. In the Standard Model, it is one weak-isospin doublet, but in the MSSM, it must be two WIS doublets, an "up Higgs" and a "down Higgs".

SUSY is, of course, a broken symmetry, and SUSY breaking is necessary for the MSSM to have electroweak symmetry breaking. The breaking of SUSY apparently cannot happen in the MSSM itself, so it is broken in some "hidden sector" and its breaking is conveyed to the MSSM particles by some mechanism.

In the Standard Model, the Higgs particle is a complex WIS doublet, giving 4 particle modes. EWSB turns 3 of the modes into longitudinal modes of the W and Z, with the remaining one being the recently-observed Higgs particle. In the MSSM, it is two complex WIS doublets, giving 8 modes. Three of them go into the W and Z, and the remaining five split up into a charged one with charges +1 and -1 and three neutral ones. One of the neutral ones is an approximation of the Standard-Model after-EWSB Higgs particle.

EWSB also mixes the electroweak-gauge superpartners and the Higgs ones to make 4 "neutralinos" and 2 "charginos" with charges +1 and -1. The lightest neutralino is often speculated to be the dark-matter particle.
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Old 12th September 2014, 05:44 PM   #1070
Craig B
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Originally Posted by lpetrich View Post
EWSB also mixes the electroweak-gauge superpartners and the Higgs ones to make 4 "neutralinos" and 2 "charginos" with charges +1 and -1. The lightest neutralino is often speculated to be the dark-matter particle.
What about Dark Energy? That is much more abundant even than Dark Matter. Is its "particle" likely to be some weird kind of photon?
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Old 12th September 2014, 10:42 PM   #1071
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Originally Posted by Craig B View Post
What about Dark Energy? That is much more abundant even than Dark Matter. Is its "particle" likely to be some weird kind of photon?
That's even more of a mystery than dark matter. But I've seen some theoretical speculations about it called "quintessence". These speculations usually involve some scalar field because such a field can have some nonzero value without the complications that one gets for nonzero spin.

The name is from this whimsical identification:
Baryonic matter -- earth
Dark matter -- water
Neutrinos -- air
Photons -- fire
Dark energy -- aether or quintessence
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Old 13th September 2014, 05:24 AM   #1072
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Originally Posted by DeiRenDopa View Post
That would be the positron, right?

As in PET (positron emission tomography), for example.


Isn't it more shades of grey?

For example, while there's no equivalent practical use of muons (say) in medicine or heavy industry1, their use as 'heavy electrons' in fields of science beyond fundamental particle physics is quite practical.

Another example might be practical uses of "geo-neutrinos"; a mere ~decade hence, perhaps?

True, practical uses of gluons and Z bosons (for example) do not seem to be even a gleam in the wildest of inventor's eyes yet ...

1 Or not: aren't there scanners - looking for contraband or radioactive materials - which use muons?
The use of many of the more unstable particles just isn't as obvious. For example, W and Z bosons are both involved in the processes that are used to produce the positrons and muons used in the applications you mentioned for those particles. And particles like gluons and pions are directly involved in all forms of nuclear fusion and fission.

Obviously extremely short-lived particles that can only produced by extremely high-energy interactions, such as 2nd and 3rd generation quarks, tau particles, and the Higgs boson, aren't likely to have any direct practical applications for quite some time. But even there, stuff we have learned about the fundamental interactions from observing those exotic particles has been invaluable in a large number of practical applications that don't directly make use of those particles.
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