LHC News

The LHC has been in the news a lot recently, here here* and here**, and the press has been less than positive. Not inaccurate per se, but twisted and seen in the most negative light possible. In this post I'll try to lay the current status and future plans out for you, explain what they mean, and give my perspective on them. You should know that I'm certainly not an optimist when it comes to the LHC. As they say, once bitten, twice shy.


The facts are these: September 19th, 2008 the LHC experienced a major accident while trying to commission the magnets to reach their design current capacity. While ramping up the current in a set of superconducting magnets a defect in the connections between magnets caused a part of the assembly go resistive, resulting in an electrical arc which punctured the cryostat walls, bringing the superfluid helium into contact with room temperature air and, most generally, releasing a lot of energy in a short period of time (i.e. explosion, although no combustion occurred). That was a mouthful. In simple terms, the magnets had an enormous amount of energy flowing through them unhindered and then all of a sudden the energy encountered a blockage and dumped itself into the surrounding material. This great release of energy melted the interface between the magnets, their cooling systems, and the tunnel which meant that their cooling medium, superfuild helium (1.7K, colder than outer space!), hit the warm air of the tunnel. The helium went from a superfluid state to gas, resulting in a 400x increase in volume and pressure waves which rocked the 1o ton magnets out of their anchors, moving some by almost half a meter and blowing the doors of the tunnel off their hinges. Pretty spectacular, but also pretty devastating to a machine which had been decades in the making.

It wasn't until about six months after the accident that the LHC team truly understood the cause of the problem, a faulty design for the interconnects between the magnets, called bus bar splices. They used a design that was less expensive than the standard one used in other superconducting machines but which ultimately could not withstand the currents required by the LHC. There are thousands of these splices which will need to be replaced before the LHC can go to its design energy, 7 TeV.

These faulty splices do not prevent the LHC from running at a lower energy, however. Each splice has been measured to find out how much current it can withstand and the LHC believes it can safely run at up to 5 TeV. The current run plan, announced last week, is to start at 3.5 TeV beams, take a sizable amount of data at that energy then slowly ramp up to 5 TeV by the end of 2010. This procedure will nominally begin in late November, from that one can project first collisions by late December/early January at injection energy (450 GeV) and then 3.5 TeV collisions a month or so later. Now, I don't believe that schedule. I fully expect it to slip by a couple of months because things never run as smoothly as expected. However, I do think we will get an analyzable amount of data in the first half of 2010.



So, what does that all mean? What is a TeV and why does it matter if the machine runs at 3.5,5, or 7? A TeV stands for a trillion electron volts, or a trillion times the energy an electron accumulates while traveling through a one volt potential. It's an arcane unit, but us physicists are fond of them, so they pop up all over the place. The important thing to remember is that it is a unit of energy. The important parameters of the LHC are what energy it will collide protons at and how many collisions it will produce (termed its luminosity).

Now on to the why does it matter part, and it all comes down to

E = mc^2

Einstein's famous equation, embedded in the public consciousness yet not always understood, means that energy can be converted to mass and vice versa. For particle physicists this equation means that we can collide particles (protons) to create new particles and the energy of those collisions determines how massive a particle we can create. As we increase the energy of the collisions we can produce more massive particles and probe the laws of physics at a deeper level.

The other important parameter is the number of collisions because the outcome of any single collision is a priori unknown. There is a probability of producing particle X, Y or Z in a collision but we don't know that collision 1 will produce X, collision 2 will produce Y and so on. We just know that there is a (very small) probability that X will be produced so we have to collide enough protons to know that 10 or 100 or 1000 Xs should've been produced. We then look for X in our data and we can verify whether or not the probability we expected was correct.

Collision II by Dawn Mason

These probabilities change with energy: it's easier to produce heavy particles when there is more energy in a collision. So the LHC is trying to maximize the energy and number of collisions as soon as possible. 3.5 TeV will be a good start. It's already over 3 times the current highest energy particle accelerator at Fermi Lab.

The 2010 data will not be a big leap forward for particle physics. More likely it will be like the little hop and step the long jumper takes before hurtling herself through the air. (edit: long jumpers don't actually take that hop and skip. I was thinking of a cross between a long jump and a triple jump) Unless we are extremely lucky and Nature has been extremely gracious, the real discoveries won't come until 2011 or 2012. But 2010 will be good enough for me to graduate. And that's about all I care about right now.


*This article, the most positive of the three is nearly impossible to find on the NYTimes website, whereas the others are quite easy to find.
**This article is the worst thing I've read in a long time. Absolutely terrible and I can't believe the NYTimes published it as 'journalism'.