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Tags cern , higgs boson , physics

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Old 13th March 2013, 08:47 PM   #1041
lpetrich
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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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