The Large Hadron Collider is not a circle, but instead has eight large arcs with eight shorter straight sections. The straight sections don’t appear to all correspond with detectors. Do they have some purpose, or are they just an artifact of using the existing tunnel from the Large Electron–Positron Collider?
They do correspond to the old LEP tunnel shape, but they serve the same purpose for the LHC. The LHC beam can only curve if its in a beefy magnet, and if you want to do anything to the beam, the magnet would be in the way. So, you need the simpler straight sections to do other things.
As you note, the experiments are housed in straight sections, but you also need beam injection and extraction points, collimation (beam “cleaning”), beam abort systems, radio frequency systems, and sundry other beam monitoring and manipulation items. All of these essential systems are located where the beam isn’t hidden deep inside a giant dipole magnet.
giant liquid helium cooled superconducting dipole magnet. Not just awkward - cold and awkward 
How much new physics was–unsure of verbs here–created, devised, bettered, had to devised, fell out of the engineering of the LHC?
It wasn’t really so much new as improving existing technology, but I read that a major challenge was optimizing the software algorithms to filter the massive amounts of data collected. Essentially the hardware can’t keep up with storing the data (and there isn’t room to store it all), so the software has to be able to determine if each data point is a potentially interesting event almost instantly and either discard it or store it. I understand there were some cool advances in data analysis techniques that came out of this.
The accelerator complex and the detectors required tremendous engineering and material science advancements, but “new physics” isn’t the phrase I would use for most of it. And as TroutMan mentioned, the raw data rate and sheer scope of the physics program requires a lot of frontier-pushing IT technology. Even the “simple” downstream step of having collaborators around the world access the same data files requires new feats in network design.
I don’t want to deprecate the achievements at LHC in any way - by no means. However I’m not sure what you mean.
The “Supercollider” would have been about 3 times the design capacity at 2 x 20 Tev vs the 2 x 7Tev of LHC and the SSC started construction in 1991 whereas didn’t break ground until 1998.
While higher energy does introduce particular challenges, they are a small subset of the total complexity of the projects. The SSC and LHC had/have very different magnet requirements, detector requirements, luminosity requirements, … and by all major measures, the LHC was a significantly tougher engineering challenge, despite its operating at 14 TeV center-of-mass energy. The LHC and its detectors also went through the complex step of actually being built, and issues naturally appear (and get solved) along the way on any major project.
Some thoughts and examples to expand:
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The SSC would have used a brand new 87-km tunnel – 3 times longer than the existing LEP tunnel that now houses the LHC. As a result, the SSC needed dipoles with magnetic field strength of 6.6 Tesla whereas the LHC magnets are 8.4 Telsa. (The higher field is needed to bend the LHC protons around the annoyingly small 27-km tunnel, even though the protons are at lower energy.) This pushes hard on magnet technology.
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You also need two sets of dipoles around the ring for a proton-proton collider since one beam bends clockwise and the other beam bends counterclockwise. The SSC used essentially two independent systems, one mounted above the other, with each superconducting magnet living in its own cryostat. The LHC, in contrast, uses a single cryostat for both dipoles: a tremendous feat, since you’ve got two oppositely directed 8.4 Tesla magnets sitting inches from one another. The LHC cryostats also use superfluid helium (1.9 K) rather than vanilla liquid helium (~4 K). Since the magnets are a major cost driver, the very success of the LHC is in part due to the fresh approach to magnet design.
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The LHC has a factor of ten higher luminosity than the SSC. (Luminosity is proportional to the number of protons in the beam and inversely proportional to high tightly squeezed the beam is, since a narrower beam means more collisions.) Getting more protons in and squeezing them together more tightly is a challenge in beam optics and radiation loss handling.
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As a result of the lower luminosity at the SSC, its detectors would have seen about one collision per “bunch crossing” (i.e., per each time a chunk of protons passed another chunk within view of the detector). The LHC sees an order of magnitude more. This means that each “picture” of a bunch crossing in the LHC has as many as two dozen p-p collisions all piled up on top of one another. The data acquisition system has to be able to handle this, but the detector also needs to be designed from the ground-up such that you have a hope of disentangling the mess. The LHC detectors’ central trackers, thus, have stringent requirements on spatial and temporal resolution.
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For a low-mass Higgs, the Higgs–>(gamma gamma) decay channel is an important one. One’s ability to observe this channel in a detector depends on the design of the so-called “electromagnetic calorimeter”. The CMS detector uses a crystal calorimeter, and major advances in crystal design (light yield, radiation hardness, density, cost [since you need tons of these crystals]) were need to get CMS the energy resolution it needed on EM showers.
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Photon and other channels can also be limited if there is too much material between the interaction point and the calorimeter. This puts additional stresses on the inner components, especially the central trackers and their support systems which are pushing in the opposite direction (more pixels = better!).
On a general note, the decade or two in between the SSC and LHC designs didn’t stop R&D, so the LHC and its detectors are much evolved from the earlier era. In 1990, we could build a Toyota Corolla, but we couldn’t build a 2013 Toyota Corolla in 1990.
Yes, but you made it sound like the LHC couldn’t have been built but for those advances and that’s a bit misleading. It could have been built but it’s specifications and operational parameters would have been different in various ways.
However as with the ring in Batavia, I’m sure you’re well aware that it was upgraded dozens of times over it’s life and the same would have happened with both the SSC and LHC.
Finally, as energy increases in a synchrotron, doesn’t the energy dissapated increase exponentially due to synchrotron radiation? So I’m a little confused by those numbers you gave.
That was my intention. The LHC couldn’t have been build but for those advances. The LHC, despite it’s general-sounding name, is a very specific machine coupled with very specific detectors. Leo Bloom asked what was required for “the engineering of the LHC”, and the answer is unequivocally “a lot”.
The general point you make that we have built accelerators in the past and could re-build them in the future is correct, but that doesn’t get you the LHC. In large part because of the energy limitations due to the LEP tunnel, the LHC and its detectors are way more complex than anything before in order to achieve many of the same physics goals.
Which numbers in particular? I’m happy to expand on them.
In the meantime, I’ll note that radiated power goes not exponentially with the beam energy but still rather painfully as a quartic. It also scales inversely with the radius of curvature, and the overall power loss scales linearly with the number of particles present. All together, the SSC and LHC have similar power loss due to synchrotron radiation, about 8 kW each, and this amount is easy for the RF systems (i.e., the accelerating cavities) to keep up with, as they put way more power than this into the beam when actively ramping up the energy.
Synchrotron radiation is a much bigger deal – a thirteen-order of magnitude bigger deal – in electron rings since the power loss scales as the fourth power of the particle mass.
I think then you should specify what those goals were. But maybe I can save you some time.
In 1998, what was the range of massed the Higgs was thought to occupy? I’ll get to the other issues in due course.
I don’t understand what you mean by this point. Is there a particular claim that you take issue with that your cite refutes? If you are just pointing out the physics goals as a general piece of information, then, yes, that wiki article summarizes many of them.
Well, you said that “The LHC, despite it’s general-sounding name, is a very specific machine coupled with very specific detectors.” I read that imply that there was a reason it should be so. If not, then why all of the “specificity”?
Ah, I see. Thanks.
The specificity comes about because the name “LHC” means “A 14 TeV machine, housed in the old LEP tunnel at CERN, using superconducting magnet technology, serving two general purpose detectors with certain specs, having the ability to accelerate heavy ions in addition to protons, with particular funding sources and particular collaborative organizational schemes.” Less verbosely, the LHC was a specific proposal to build this exact facility, with all its peculiar difficulties.
The specificity comes about naturally, as one would get laughed out of the room if one were to say: “We want to access these physics topics. Give us some money and we’ll build something.” Instead, it has to look like: “We want to build the LHC. Here’s exactly what it will look like. We don’t know how to do all of it yet, but here’s the R&D timeline and cost we think it will take. Overall, the project will require this much money. Here’s where we think funding could come from. Here is the suite of physics we can access with it and why it is interesting. Here’s how the physics reach compares with the SSC (another very specific proposal). Here’s how the design is driven by the physics needs.”
Thus, if you look at an article from 1990 that says “LHC” in it, it’s referring to this exact machine and not a generic machine with unspecified parameters that can get at physics topics A, B, and C.
Oh, I see. Because, you know, I thought that it was like buying a car. I could just go into any dealership and say I need a car and that would be the end of it. :rolleyes:
The specifications had a reason behind them though I presume right? Just like I have certain reasons for buying a 6 speed 2 door coupe and not a soccer mom van.
There are economic and infrastructural reasons to build at CERN, and there are cost savings in using an existing tunnel. This is why the LEP tunnel was so attractive. Once you decide you want to use the LEP tunnel, your energy is constrained by the difficulty in bending the beam, but you can push against that constraint a bit through reach-for-the-sky magnet technology. The optimization there is one of physics reach (higher energy) and engineering cost/risk. For any chosen energy, you need a certain rate of p-p collisions to answer each particular physics questions, so now your luminosity is constrained. Given the required luminosity, the physics detectors pick up some design constraints in order to handle it. And so on.
So basically, you in fact COULD have built a different accelerator there, it just wouldn’t have been as powerful but then that really should have been an issue, right?
By “not as powerful”, do you mean operating at a lower beam energy? I don’t quite understand what you’re asking. The goal with the LHC design was to maximize the capabilities for certain physics questions, and that drove the design. But, I’m not saying anything profound with that statement. If you built a different machine, it would have different physics capabilities. Indeed, the LEP collider lived in the same tunnel, but had a very different physic scope.
Power would be electron volts - do you need a link?
If you could be a bit more descriptive / verbose, it would help me understand what you are driving at. Power isn’t measured in electron volts, energy is, which is why I was asking if you meant “beam energy”. The other thing you might have meant is “powerful” is the vernacular sense, as in “more capable in some generic way” (like, “this computer is more powerful than that one”).
If you actually mean “energy”, then as mentioned above, the energy is only one piece of the puzzle, and reducing the energy means you need to increase capabilities in other ways.