On hadron colliders, dark matter and black holes

[torstan]

Yep. This Higgs is really clear. The problem exists in that the theory breaks at a given energy unless something new happens. The Higgs is the simplest hypothesis that solves it. So we see the Higgs, or something new, but we have to see something.

In the sim, let me say a couple of things. Firstly, we know the physics exactly - all we're doing is taking particles and allow them to interact gravitationally. So no-one's 'just guessing'. Nor are we just saying 'that looks about right, let's leave it at that and call it good'. The consistency checks require that over the volume that's simulated then we get X number of galaxy mass clumps per MPc, and Y number of galaxy cluster sized clumps per 100MPc and so on for every scale. You do detailed statistical analysis to find out where your simulation disagrees with the real universe. And as we can look back in time by looking at light from further away galaxies, we can actually see the development of dark matter clumps over time (to an extent) - so we have a lot of data to compare to. If it doesn't fit, then we can look at why.

It may be that the physics is different - and if so we don't say that it is different. We look at possible explanations of why it might be. And then you are dead right - you use it to constrain predictions for other experiments, like the LHC or looking at colliding galaxy clusters or looking for dark matter annihilation in our own galaxy. So each experiment informs other experiments and all of them taken together start to build a picture. The interesting things occur when two experiments seem to require contradictory theories. Then we need to work to find out if there was an experimental error, or the theory needs adapting.

So we can do a sim of the galaxy and that says that there's about 0.4GeV/c^2 of mass tied up in dark matter per cm^3. So if dark matter is a particle with a mass of 100GeV/c^2 then there's one particle of it per 250 cm^3. Now that tells us how many particles we expect to pass through a direct detection experiment per second. If the detector doesn't see it, then we can constrain the interaction strength of the dark matter with normal matter i.e. if it doesn't interact at all then it doesn't matter how much of it there is, the dark matter detector won't see it. Conversely if it interacts strongly then we should have seen a signal long ago - because the simulation tells us there should be lots of dark matter passing through the detector. So, on the caveat that we've taken a number from simulation, we can use direct detection to place an upper limit on the interaction strength of dark matter with normal matter.

Now the LHC will also place a limit on this interaction strength - because if the interaction is strong then the LHC will produce a load of it. If it's weak then it will produce less. Now if the LHC results require a dark matter particle that interacts more strongly than direct detection limits allow then I can guarrantee that people will go and look very carefully at that number of 0.4GeV/c^2 per cm^2. They'll also be coming up with theories about why it just doesn't interact with direct detectors, but does get produced at the LHC.

I hope that answers the question a bit?

As for the black holes - yes the issue is really one of size. When you have a black hole you have an event horizon, where anything within the horizon gets sucked in, and anything outside can escape. Now say you have an e+ e- pair created out of the vacuum (a particle and an antiparticle can always be created, and quantum mechanics says you can and will do that all the time). Normally they just recombine and annihilate with no discernible effect. However in the case where they turn up on the event horizon, the positron could b created on one side and the electron on the other. Now they can't recombine because nothing crosses the event horizon. So the positron falls in and the electron escapes. To an outside observer that looks like the black hole just emitted an electron! Also, because that electron carries energy, for energy to be conserved, the black hole must have lost energy in the transaction (and complex GR calculations show that this is indeed what happens). So the black hole just emitted an electron and lost energy.

Now this process can happen with any particle. So the black hole can radiate any particle at all. For a large black hole this is a small effect. Basically as volume (which can be taken to be proportional to mass) goes as r^3, and the surface area on which the radiation takes place goes as r^2, for large black holes the mass is much larger than the energy loss from hawking radiation. They do lose energy through it, but it's a small fraction of their mass, and they're accreting new mass anyway. For a small back hole r is very small, so suddenly the rate of energy loss is large compared to your mass. And because the mass is small, the gravitational attraction is really tiny so there's no hope of it accreting new mass to replace the energy it's losing through Hawking radiation, so it just evaporates.

Now because it can evaporate into all particles, it's quite a dramatic signal because you just get a burst of all sorts of different things coming out of a point in space, irrespective of what you threw in (I believe).

Hope that makes some form of sense. I'll go over it more later if you want more details, but this post has already turned out to be a bit of a mammoth.

[Redrobes]

No thats great. I knew that the radiation emitted was from tunneling across the event horizon because the instantaneous position is statistical from Heisenbergs. I didn't know that it emitted a particle tho. I guess I didn't think how the thing started but I remember that you can have these spontaneous evolutions of stuff that normally recombine. Ok so bursts of all sorts then.

On the sim you say that the physics is all known or exact but then say that it might need to be modified. I think I know what your saying but it seems as tho there is some educated and experimental backed guesswork going on and some of the parameter twiddling sounds like the interaction factor for dark matter and it sounds like the LHC is definitely going to constrain the twiddle range. I guess if some theories require that to be in a range that is outside of the LHC results then its struck off the cards or modified. This sounds a lot like the hubble constant where the telescope nailed it down after a few years and left us all with one theory (expanding wasn't it) instead of three (constant or contracting).

So I have a better idea of what and why its happening now thanks. Ill prob ask more in time.

btw: Is it just me but does anyone else think that the big detector in the photo looks like the MCP from Tron...

[torstan]

Ah, okay. So on large scales the dominant interaction is gravity - by many orders of magnitude. The self interaction between dark matter particles will have to be very very small so it's a small perturbation on the dominant interaction - so we know the most important numbers already. That's not to say that a small non-zero interaction strength wouldn't do something - and indeed simulations are looking at the variation from that. That's one twiddle. Another is the interaction between the dark matter and the normal matter in galaxies - that will be small, but could be non-zero and might have an effect on how galaxies form. However the large scale bulk properties should be well handled by just dealing with gravity.

If we see some big conflicts between different areas then we can look at what might be screwing up, but we generally start with the simplest cases.

[RPMiller]

btw: Is it just me but does anyone else think that the big detector in the photo looks like the MCP from Tron...

Don't be sad...

http://tron-2-trailer.blogspot.com/

[Redrobes]

Ok so it didn't take me long to think of another question - sorry...

If a black hole is emitting particles then is it true that it emits about as many particles as anti particles. I mean do electrons come off a black hole and its the positrons that always stay inside in an e- e+ pair.

If as I expect its about the same then the hole is emitting streams of mixed particles then do these recombine and if so does that generate the photons or do they annihilate perfectly with nothing emitted at all just as it would if the e+ e- had recombined back within the event horizon.

In which case wouldn't some or a lot of the particles from a black hole out of the event horizon just pop out of existence and we wouldn't see or be able to detect most of them anyway.

Oh and Tron link - cool though I don't think its going to grab people like the first movie. I collect Syd Mead stuff - not the real stuff I should add, not being a millionaire n'all. But I have some fairly rare stuff. I am missing Kronovector or however you spell it but then I don't have the laser disk player for it anyways. I think there's a book I am still missing as well. Anyhoo thats all a bit off topic. Syd's art is well cool. Its great that many of original cast are back in and lissenberger (sorry spelling again) is part of it. I didnt know about the sequel.

[torstan]

Not quite, but I really need to get on the subway and head home.

If the particles have travelled away from the event horizon then they must have a non-zero momentum. Therefore when they recombine they must annihilate to something with non-zero momentum - which is a real thing, not just the generic vacuum that they could annihilate to if they collide head on. So they produce a real photon that travels out (if it's an e+ e- collision) and we might be able to see that. So that's why you might hear of small black holes 'shining'.

But yes, your intuition is right and you should get as many particles as anti-particles. It's just that once you have a stream of particles going out, anything they annihilate into must also go out, due to conservation of momentum. So you'll see stuff traveling out from a black hole, even if the stuff you see isn't quite the same stuff that was created around the event horizon.

[Redrobes]

Cool and thanks - you had better get home Appreciate the explanations

[torstan]

No problem. Happy to answer them.

[torstan]

First beams have been injected into 2 of the eight sectors successfully. Also, they've had beam through LHCb - one of the detectors that's looking into the nature of bottom quarks (or investigating beauty - whichever naming convention you decide to follow).

[Gidde]

I've been reading a book about this stuff lately (specifically The Elegant Universe by Brian Greene) and even though it's right at the edge of my capacity to understand, I'm finding it fascinating. My question is, are we to the point yet where we have a hope of detecting things like gravitons at the LHC, or is that in the future once it gets ramped up to its full capacity?

Also, I was thinking about picking up Lisa Randall's book next; are there any others out there you'd recommend for a layperson interested in string theory/m-theory?