On hadron colliders, dark matter and black holes

[torstan]

Absolutely, red dwarf was a stroke of genius.

I have to say that I've greatly enjoyed writing the posts in this thread (did the length of the posts give it away ?) and I'm glad that people are enjoying the read.

[helium3]

Interesting.

I didn't know that failing to produce/find the Higgs Boson would result in disproving the entire Standard Model.

What are the competing theories that would then move up to replace it, if that happens? Or would theoretical physicists just stand around looking confused for a while?

Also, there's a pretty decent hard sci-fi book called "Einstein's Bridge" by John Cramer. It's about a mist-like event that occurs at the (never built in our timeline) SSC in Texas.

Oh, and I think the physics shop here at the UW in Seattle built some of the components of one of the detector modules. I've actually seen what they look like on the inside. Lot's and lot's of tubes. Not sure if the tubes were for coolant or "detector fluid."

[torstan]

Failing to produce a Higgs boson will disprove the Standard Model. That doesn't mean it's not useful. The major elements of the structure will be part of any future theory. Think of it this way. We know Newton's theory of gravity is fundamentally wrong. It claims that gravity is a force that acts instantaneously at a distance. So if the sun vanished now, the earth would immediately stop going in a circle and start going in a straight line. Now that's not the case. From relativity we know that gravity is communicated by the shape of space and time, and that changes to that shape propagate at the speed of light. Therefore if the sun vanishes in an instant, the earth will keep going in a circle for around 8 minutes, until the effects reach us. However that doesn't mean that all the results of Newton's theory of gravity are null and void. We can still use it for most calculations about the movements of planets, the course of satellites and space missions with complete faith. However the vision of reality that underlies those equations is wrong. In the case of the Standard Model, if we don't find a Higgs boson then the structure that the Standard Model is based on is wrong. However, the equations that describe it do an excellent job of describing reality in all the situations we have investigated so far - and they will continue to be a good description for calculational purposes, whether we find a Higgs or not. It's just that if the Higgs ain't there, then we know the basic principles are wrong.

There are a few theories that would predict no Higgs boson. There's three reasons we might not see a Higgs boson, and each have theories associated with them.

1. The Higgs boson interacts very weakly to normal matter and could thus avoid detection. So it's there, and possibly even produced, but we don't see it.
2. The Higgs boson is very heavy, so it is outside the reach of this accelerator.
3. There is no Higgs boson at all.

Now 1 is very tightly constrained already. This is because a Higgs boson must couple to matter to give it a mass. So there is only so far you can go in making such a particle 'invisible' before it just stops doing its job. The only models I know of with such an invisible Higgs also have heavier Higgs bosons to help finish the job. An example of a model with a light invisible Higgs is the Next to Minimal Supersymmetric Standard Model (NMSSM). Yes, we are very bad at making up names. It's a failing.

In the case of 2 this is also pretty constrained. Now the standard model can't have a heavy Higgs. This is because the Higgs boson is tightly linked to the W and Z bosons, that have been found already. Now the Z boson has a mass of 91.14 (in units of giga-electron volts or GeV - it's a convenient unit of mass, because if we weigh things in grams the numbers are so small they get very cumbersome very fast). Now for a first calculation of the Higgs mass in the Standard Model you find that it must have a mass of equal to or less that the Z mass. However experiment tells you it can't be lighter than 114GeV. So you see the problem. A more detailed calculation in the Standard Model allows you to get up to about 135GeV before you run out of options. Now the LHC will run at an energy of 14TeV (=14000GeV) so we should definitely be able to produce a Higgs if it weighs less than 135GeV. So a heavy Higgs (outside the reach of the LHC) is well beyond what could be done in the Standard Model. Furthermore, because of the nature of the W and Z bosons, a heavy Higgs is always going to be very problematic as, by its nature, it must be directly involved with the W and Z bosons. If it is separated from them in mass by a lot, then this connection is very hard to maintain. To achieve this you need to alter the nature of the W and Z bosons from the form they take in the Standard Model.

You also need to do this if there is no Higgs boson at all - option 3.

As the Higgs boson is intimately related to the W and Z boson, the lack of a Higgs boson at the LHC will have clear implications for our understanding of those particles (that we can produce, and in large numbers). Models that have no Higgs, or a heavy Higgs, generally fall into the category of composite Higgs models, or extra-dimensional models. One clear problem is that if you take the Standard Model as is, and take out the Higgs, funny things start to happen. One such funny thing is that if you collide two W bosons off each other, then at high energy the chance of getting two W bosons out of the collision is greater than 1. Now that is definitely not allowed. The inclusion of the Higgs boson sorts this out.

So you can measure the odds of getting two Ws out of such a collision over a range of energies in your experiment and see whether the result agrees with the Standard Model or deviates from it. If it deviates, and you've found no Higgs boson, then you know you've found evidence of non-standard model physics. This would be an obvious place to look if we don't find a Higgs boson. This could tell us that the W and Z bosons are not fundamental particles, but that they are made up of smaller components - like an atomic nucleus is not solid but rather a collection of protons and neutrons. It could also tell us that W and Z bosons can move in extra dimensions of space that we haven't been able to access before. This can result in alterations to their collision behaviour which avoids the embarrassing problem of getting more out than you put in that I mentioned above.

So there are a few avenues to consider if we don't find a Higgs boson, and some obvious areas to study that should break if the Higgs boson isn't where we expect it to be. However, the expectation is that we'll probably find a number of different Higgs bosons as well as a slew of other stuff.

As for whether the scientists will be scratching their heads? Well, yes. It's not the expected result - and it will disprove a lot more than just the Standard Model if we don't find a Higgs - most of the extensions have one or more Higgs bosons in them already. However it will only be a few weeks before people start coming up with weird and wonderful solutions that fit the data and can themselves be tested. Hence the progress of science

Hmmmm, that got a bit long and technical. Sorry about that. Hope it wasn't too heavy. If you want me to clarify any of it, please say so.

Yes, the SSC (Superconducting Super Collider) was never built - though they did dig the tunnel. It was supposed to do what the LHC is doing now. They screwed up the budgeting for it and congress threw it out. It's now one of the most expensive mushroom farms ever built. I'll have to look out that sci-fi book you mention. Sounds like an interesting read.

[NeonKnight]

I theorize that the reason we see no Higgs boson is because they exist outside the 4 dimensions we know.

[Redrobes]

Heres a set of wooly questions. So the engineers finish the building tweaking and tuning and hand the keys to the big red button to the physicists. Whats the order of tests that they would perform - i.e. is looking for Higgs the first test and is that going to be done with the two W boson probability test thing. Its like building the Hubble and asking so what do you point it at first ?

Also, I am not clear about how you try to generate a Higgs Boson. Is the idea that you accelerate matter in the ring and then crash it into each other ? What matter do you accelerate ? Does it have to be a stream of protons or something like that or does the accelerator just gain more energy in massless 'particles' and then these new particles just take some of this energy and create themselves. If not then whats special about the matter used that is thought to create the Higgs. Doesn't all the matter go around the ring at the same speed or is it switched into a stationary 'block' of something and thats where all the action takes place.

And...(as if that wasn't enough)... why doesn't the system create more lighter particles instead of one heavy one - is that just down to probability and you get a slew of everything.

[torstan]

That is indeed quite a series of questions. I'll try to get through them all, but not necessarily in the order they were asked:

1. Is the idea that you accelerate matter in the ring and then crash it into each other?
Yes.


2. What matter do you accelerate?
Protons. This is because they are charged - necessary if you are going to accelerate things with an electric field. Also they are relatively heavy in the world of subatomic particles. This means they don't lose as much energy when spun round a ring. Electrons lose a lot, which is why they aren't used any more.


3. Doesn't all the matter go around the ring at the same speed or is it switched into a stationary 'block' of something and that's where all the action takes place?
Half the protons go round the ring one way, half the other. They collide head on at two points on the ring. That's where the energy is produced, from the annihilating matter in the protons. It's this energy - that only gets created here - that allows for the production of new particles.

4. Does it have to be a stream of protons or something like that or does the accelerator just gain more energy in massless 'particles' and then these new particles just take some of this energy and create themselves?
It can be any type of particle that has mass - because massive particles must couple to the Higgs. It must also be charged - hence the proton. Actually the proton is a collection of quarks - which are the things that really collide. The quarks do couple to the Higgs and so can be used to create one.

5. Why doesn't the system create more lighter particles instead of one heavy one - is that just down to probability and you get a slew of everything.
It will create more light particles than heavy ones. Most of the interactions will spit out a massive array of Standard Model particles and experimentalists will have to hunt for the interesting new signals amongst that morass. That's really why we need the vast computing power of a world wide grid. We're hunting for one MB sized needle in a petabyte of haystack.

The exception is when you are precisely on the mass of the particle you want to produce. Then there are effects that kick in that greatly enhance the likelihood of getting that state over other states of different mass. But yes, fundamentally it's quantum mechanical, so it is always about probabilities.

6. What's the order of tests that they would perform?
Well sadly this isn't like the Hubble. This machine just does one thing. You turn it on at one energy and then watch what comes out. It will precisely measure every interaction for 10 years. If some of those interactions produce interesting stuff then we are in luck.

6b. So the question really is, when we get the first years data set, what's the first thing the experimentalists will look for in it?
The first thing to look for is the Standard Model. Not a Higgs, but all of the W and Z bosons, the quarks, electrons and so on. The Standard Model has never been studied at these energies before, so they need to understand that before they go any further. Then they will look for a heavy copy of the Z boson. This is predicted by a few theories and is really easy to spot. Then they'll go to work on the Higgs, as that is the most widely sought particle. They'll also look for weird and wonderful new signals such as dark matter.

If they find nothing, then they start looking at W-W scattering. The reason to wait is that this is a subtle effect - not the production of a new particle, but rather the modification of the scattering of two particles. Now rmember we're colliding protons, not Ws, at the LHC. That means the Ws are going to have to have been created from a proton-proton collision and then have been created in such a way that they collide with each other - and then we still have to be paying enough attention to have seen it. So it's a phenomenally tricky measurement to make - and probably won't be done unless we really need to figure out what's up with our broken models.

So the order is:
1. Find the Standard Model again
2. Look for easy signals - like a heavy Z (imaginatively titled the Z')
3. Look for the Higgs (well even experimentalists can't resist the lure of a Nobel Prize)
4. Look for clear signals of the big contenders for theories of physics - supersymmetry and extra dimensions. This will be a matter of looking for dark matter and fancy new particles
5. Look for the tricky signals - such as WW scattering. These tend not to involve the creation of new particles, but are instead a modification of existing processes.

[Redrobes]

Thanks, thats really lit up some dark corners about all of this in my head. I studied electronics so I did bits of Maxwells and Schrodingers but there are only a handful of everyday electronic components that work at the quantum level so I never did any subatomic physics, not counting the electron of course. Its really interesting and moving very fast.

[torstan]

Roughly the speed of light normally

Sorry, couldn't resist.

[Ascension]

Don't know if any of you knew this or not, but this weekend there were 2 separate programs on the LHC on the Science Channel...one called "The 6 billion dollar experiment" and another called "The big bang machine". I watched em both cuz, well, that's what I do while I map, unless there's a Cardinals game on.

[torstan]

Yep, there was a big article on the BBC as well talking about how the LHC was set to become the coldest place in the universe which isn't an unreasonable claim.

2 weeks to start up if nothing goes wrong.

Were the programs any good?