Knocking on Heaven's Door - Lisa Randall [118]
Not only is the LHC seven times higher in energy than the Tevatron—the highest-energy collider before—but it also generates events at a rate 50 times faster. The LHC needs to handle what are essentially extremely high resolution pictures of events that are happening at a rate of up to about a billion collisions per second. The “picture” of each event contains about a megabyte of information.
This would be way too much data for any computing system to deal with. So trigger systems make decisions on the fly about which data to keep and which to throw away. By far the most frequent collisions are just ordinary proton interactions that occur via the strong force. No one cares about most of these collisions, which represent known physical processes but nothing new.
The collisions of protons are analogous in some respects to two beanbags colliding. Because beanbags are soft, most of the time they wilt and hang and don’t do anything interesting during the collision. But occasionally when beanbags bang together, individual beans hit each other with great force—maybe even so much so that individual beans collide and the bags themselves break. In that case, individual colliding beans will fly off dramatically since they are hard and collide with more localized energy, while the rest of the beans will fly along in the direction in which they started.
Similarly, when protons in the beam hit each other, the individual subunits collide and create the interesting event, whereas the rest of the ingredients of the proton just continue in the same direction down the beampipe.
However unlike bean collisions, in which the beans simply collide and change directions, when protons bang into each other, the ingredients inside—quarks, antiquarks, and gluons—collide together—and when they do the original particles can convert into energy or other types of matter. And, whereas at lower energies, collisions involve primarily the three quarks that carry the proton charge, at higher energies virtual effects due to quantum mechanics create significant gluon and antiquark content, as we saw earlier in Chapter 6. The interesting collisions are those in which any of these subcomponents of the protons hit each other.
When the protons have high energy, so do the quarks, antiquarks, and gluons inside them. Nonetheless, that energy is never the entire energy of the proton. In general, it is a mere fraction of the total. So more often than not, quarks and gluons collide with too small a fraction of the proton’s energy to make heavy particles. Due possibly to a smaller interaction strength or to the heavier mass expected for new particles, interesting collisions involving as-yet-unseen particles or forces occur at a much lower rate than “boring” Standard Model collisions.
As with the beanbags, most of the collisions therefore are uninteresting. They involve either protons just glancing off each other or protons colliding to produce Standard Model events that we already know should be there and that won’t teach us much. On the other hand, predictions tell us that roughly one-billionth as often as that the LHC might produce a new exciting particle such as the Higgs boson.
The upshot is that only in a small but lucky fraction of the time does the good stuff get made. That’s why we need so many collisions in the first place. Most of the events are nothing new. But a few rare events could be very special and informative.
It’s up to the triggers—the hardware and software designed to identify potentially interesting events—to ferret these out. One way to understand the enormity of this task (once you account for different possible channels) is as if you had a 150-megapixel (the amount of data from each bunch crossing) camera that can snap pictures at a rate of 40 million per second (the bunch crossing rate). This amounts to about a billion physics