Since restarting on 13th of March and until last week, the LHC had produced very little collision data for the thousands of physicists like me eager to prepare new results for the summer conferences. Why? Mostly because the whole Large Hadron Collider (LHC) team was busy setting-up the machine to recover the luminosity obtained at the end of last year and more recently, scrubbing the beam pipe.
Since the LHC is a brand new machine, you may wonder why did it need such a giant spring clean? Being new is precisely the point. It is just like buying a new car: just as they smell of new vinyl and whatever materials are used nowadays, the LHC needed to get rid of that brand-new smell.
Anybody who has ever worked with an extremely high vacuum knows that when pumping down on a new system, it’s like getting the smell out of a new car. The new materials are “outgassing”, that is, releasing molecules trapped in their surfaces. This is why to achieve an ultra pure vacuum, you use non-porous materials, like glass or stainless steel. Nevertheless, molecules of all sorts, and grease in particular, always seem to manage to permeate any surface, especially when you are pumping down to less than a millionth of a millionth of the atmospheric pressure like at the LHC.
Throughout the first full year of operation in 2010, as the LHC operators and accelerator physicists gained expertise, they kept raising the LHC beam intensity nearly every week, overcoming hitches as they found them. Just think of a garden hose. If the water flow in the hose increases under more pressure, any loose material or dirt attached to the hose wall will eventually come loose. As you double, decuple or even increase the pressure by a hundred, more junk will start coming out, to the point where your water could be contaminated. And that’s precisely what we saw at the end of last year when the LHC had reached “luminosities” (something akin to the pressure in a hose) one hundred thousand times higher than at the beginning.
For the LHC, loose molecules released from the beam pipe ended up getting in the beam’s way. Because we don’t deal with electrically neutral material like water but with a flow of charged protons, the outgassed molecules got ionized, i.e. lost electrons, and electron clouds formed around the beam, making it increasingly difficult to operate at higher luminosities. The vacuum in the beam pipe was no longer good enough. This prevented the injection of more and more protons to increase the luminosity, that is, the chances of having more collisions needed for discoveries.
What was done for ten days was to inject high intensity but low energy beams inside the machine to get a squeaky-clean beam pipe while vacuum pumps evacuated all molecules released from the pipe in the process. In fact, the same electrons that were in the way were put to use to clean the beam pipe surface. As bunches of protons passed by, they electrically repelled the electrons, sending them forcefully against the pipe wall. And that’s how the scrubbing occurred. By doing this repeatedly, the vacuum improved, and the LHC team could inject higher intensity beams for the next step. As with the water hose analogy, they increased the pressure until the water coming out at the other end was crystal clear again.
And the payoffs were immediate: as soon as they resumed operation, they could reach higher luminosities than last year. Within a week, we have already collected more data than in all of 2010! So yes, that means the LHC is on a roll.
This means us experimentalists are getting what we wanted: lots of collisions to get a chance to see extremely rare events. To get there, the strategy is to keep adding more protons per beam. But that’s easier said than done! This is like asking a juggler to go from keeping three or four balls up in the air to several hundreds! The protons in the beams are regrouped in bunches, with as many as one hundred thousand million protons per bunch. As of now, the operators can manage 480 bunches in the machine at once, with 36 bunches clustered in trains, and each bunch kept 50 nanoseconds apart (yes, that’s 50 billionth of a second!). And this occurs simultaneously for the two beams, with each beam circulating in opposite direction. All this to maximize the number of protons in the two circulating beams and increase the number of collisions produced in each detector. The goal is to get as close as possible to the theoretical limit with 50ns bunch spacing of 1318 bunches per beam sometime this year.
Pauline Gagnon
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