Knocking on Heaven's Door - Lisa Randall [72]
Most of the protons in a bunch won’t find the protons in the other bunch, even when they are directed toward each other so as to collide. Individual protons are only about a millionth of a nanometer in diameter. This means that even though all these protons are kept in bunches of 16 microns, only about 20 protons collide head-on each time the bunches cross.
This is in fact a very good thing. If too many collisions occurred simultaneously, the data would simply be confusing. It would be impossible to tell which particles emerged from which collision. And of course if no collisions occurred, that would be a bad thing as well. By focusing just this number of protons into just this size, the LHC ensures the optimal number of events each time bunches cross.
The individual proton collisions, when they occur, do so almost instantaneously—in a time about 25 orders of magnitude less than a second. This means the time between the sets of proton collisions is set entirely by how frequently the bunches cross, which at full capacity is about every 25 nanoseconds. The beams are crossing more than 10 million times a second. With such frequent collisions, the LHC produces a huge amount of data—about a billion collisions per second. Fortunately, the time between bunch crossing is long enough to let the computers keep track of the interesting individual collisions without confusing collisions that originated in different bunches.
So in the end, the extremes at the LHC are necessary to guarantee both the highest possible energy collisions and the largest number of events that the experiments can handle. Most of the energy just stays in circulation with only the rare proton collision worthy of attention. Despite the massive energy in the beams, the energy of individual bunch collisions involves little more than the kinetic energy of a few mosquitoes in flight. These are protons colliding—not football players or cars. The LHC’s extremes concentrate energy in an extremely tiny region, and in elementary particle collisions that experimenters can follow. We’ll soon consider some of the hidden ingredients that they might find and the insights into the nature of matter and space that physicists hope those discoveries will provide.
CHAPTER NINE
THE RETURN OF THE RING
I entered graduate school for physics in 1983. The LHC was first officially proposed in 1984. So in some sense I’ve been waiting for the LHC for the quarter century of my academic career. Now, at long last, my colleagues and I are finally seeing LHC data and realistically anticipating the insights into mass, energy, and matter that the experiments could soon reveal.
The LHC is currently the most important experimental machine for particle physicists. Understandably, as it commenced operation, my physicist colleagues became increasingly anxious and excited. You couldn’t enter a seminar room without someone inquiring about what was happening. How much energy would collisions achieve? How many protons will beams contain? Theorists wanted to understand minutiae that had previously been almost an abstraction to those of us engaged in calculations and concepts and not machine or experimental design. The flip side was true as well. Experimenters were as eager as I’d ever seen them to hear about our latest conjectures and learn more about what they might look for and possibly discover.
Even at a conference that took place in December 2009, that was purportedly about dark matter, participants were eagerly commenting on the LHC—which had just completed its incredibly successful debut of acceleration and collisions. At the time, after the near despair of a little more than a year before, everyone was ecstatic. Experimenters were relieved they had data they could study to understand their detectors better. Theorists were happy they might get some answers before too long. Everything was working fabulously well.