Hello, all,
The endgame is being played now on several fronts...
The LHC operators have said that they've achieved their primary goal for 2010, and that is to reach the magical 1032 instantaneous luminosity number (that's 1 with 32 zeroes following, which is the equivalent number of protons passing through an area of 1 square centimeter every second).
Caution: tedious didactic aside:
For those of you who've ever shot a shotgun, think of trying to increase the number of pellets in each casing, and then increasing how fast you can reload and shoot, and then also narrowing the spread of the pellets to a pencil width or better. Now think of someone 100 yards away from you trying to do the same thing, and you're both shooting in each other's directions trying to get the pellets to collide in the middle, without anyone getting hurt. That's what the LHC is trying to do with protons! -except that the LHC is a bit more economical about it, more "green" if you will. Here's a freebie t-shirt logo: "LHC: We recycle protons". By bending the protons around in a circle those 1032 protons are really the same 1013 protons going around the ring 11,000 times every second.
Anyway, that's what the LHC has been working on all this year. We've reached a point now where we can collect more data in less than a day than it took all of March to August to collect. Just to give you some perspective, we started somewhere in the high 20's (e.g., 10somewhere in the high 20's protons per sq. cm. per second) back in March, and the design goal for the LHC ultimately is 1034. So we've increased by a factor of 10,000, and we have another factor of 100 or so to go.
The shooting analogy is an apt one also because particle physicists measure how likely it is for two protons to hit each other in "barns". As in, "that uranium nucleus is as big as a barn". "Which uranium nucleus?" "Right there! Can't you see it? It's as big as a barn!" or "That mor-on couldn't hit the broadside of a nucleus!" - doesn't have the same ring, does it? So the barn is called a unit of "cross-section", or area, like a square foot.
Uranium nuclei are pretty large, for particle physicists anyway, so we have to employ the standard set of prefixes, like milli-, micro-, nano-, pico-, femto-, chico-, zeppo-, harpo-, groucho-, gummo-, (what, you've never heard of grouchometers?) ... to describe smaller targets, like single protons. In fact, let's give it a shot: I read from the wikipedia entry that I linked above that a barn is equivalent to 10-24 square centimeters (try hitting that with your shotgun). And I said that they've achieved 1032 protons per square centimeter per second. What is that amount using barns instead? No cheating now...
Time. Here's the answer: 108 protons per barn per second ("108/barn⋅sec"), because 32-24=8. But as I say, with protons and smaller particles a barn is not as useful a quantity. Why? Because not only do physicists use the barn to to measure how easy it is to hit a stationary target, or how likely it is for two particles to collide, but they also use it for more esoteric processes like particle production and particle decay. How likely is it, for instance, that given two protons colliding, they will produce a black hole that swallows the earth? This would be measured also in barns, and this is where it gets confusing. Why would a unit of area also be used to measure probability?
Hey, look at that! What's that over there? (CLOP CLOp CLop Clop, clop...) sorry, but I'm not sure how best to answer that question, it's always bothered me too.
How about a little archery practice? What's the likelihood that you'll make a bullseye? How about if the bullseye was twice as large? Ten times as large? How about if you had a team of archers all shooting arrows whose trajectories were exactly parallel, all at the same time? One might see that the probability of hitting the bullseye is proportional to the area of the bullseye.
Now let's say it's pitch-dark, and there are multiple targets with different size bullseyes. You don't know how large the bullseyes are, but you know they're there, because when you hit one, you see a flash of light with a distinct color that tells you which target you hit. The best way to determine the size of each bullseye is to keeping shooting as many arrows as you can, keeping track of how many you shoot, and count the number of flashes of each color you see. The more flashes of a particular color you see, the larger the bullseye on the associated target.
The more arrows you shoot, the better your estimate becomes. This is true even if you don't see any flashes of a particular color at all. If you shoot just one arrow and see nothing, you can't conclude much, but if you've shot a million, you might reasonably conclude that this one bullseye is very small indeed, or non-existent. The more arrows you need to get a good estimate the longer it takes, unless you increase the rate of arrow shooting.
So perhaps you see that a unit of area is as good as any to measure probability, particularly where projectiles and targets are concerned.
Anyway, black hole production is quite unlikely, even if the theory that postulates its possibility really is true to nature. Almost all of the really interesting processes we mean to look for are like needles in a haystack, one in a million. We measure their probabilities with picobarns, or pb. Now here it gets even more confusing: we measure how much data we've collected in inverse picobarns, or 1/pb. "Hah? Come again?" Look back at that instantaneous luminosity: if we can run now with a luminosity of 108/barn⋅sec, and we run for one day = 24*60*60 ~ 105 seconds, that's a total of 1013/barn, or 10/pb, or 10 pb-1 (1 picobarn=10-12 barn). So while a picobarn is 12 orders of magnitude smaller than (one trillionth of) a barn, an inverse picobarn is 12 orders of magnitude (a trillion times) larger than an inverse barn! I can feel my mind bending... Meaning if we run long enough, we'll have to start using the Marx Brother prefixes to talk about how much data we've collected: zeppobarns, gummobarns...
Perhaps now you can understand this plot a little better.
Now the really useful thing about using picobarns this way is that we can then figure out, given how much data we've collected, how many events of a given process we expect to see in that data. All you have to do is multiply cross section times integrated luminosity, the "pb's" cancel and you're left with a unitless number, the number of events. So if some theory predicts a cross-section for black-hole production at the LHC of 1 pb, we expect by that prediction to see 10/pb * 1pb = 10 black holes in one day of running at 1032. If we don't see it...the theory is wrong and we can breathe a sigh of relief. Dodged that bullet, anyway...
Normal blogging resumed.
So what's all this got to do with the endgame? Well. Very soon now the proton-proton program will be over for the year. This will be the dataset I write my thesis on. If we collect enough data in these last few days the thesis will actually be able to contain new scientific information.
When proton collisions are done, then there's maybe a one or two day "technical stop", while they gear up for...nuclei. Colliding lead nuclei. Lead's chemical symbol is also Pb. But these nuclei are as big as a barn! Never mind, these puns are going over like a...whatever. So one month of the so called "Heavy Ion" campaign, where they leave behind the black holes and instead attempt to recreate the Big Bang, and then the LHC is done for the year.
So I've been at this now for over seven years, "the whole PhD thing", and I'm finally into the endgame. I'm attached to a "search" for new physics (as opposed to a "measurement" of known physics), which is the possible existence of heavy neutrinos (a contradiction in terms) and "right-handed W's". We hope to use this year's data in order to set a limit on the masses and/or production cross-sections of these particles that exceeds the current limit set by the Tevatron collider outside Chicago. Then it's finish the thesis, defend in the spring, and look for my next job at the same time.
This means we might be in our endgame with respect to living here. Chief among our concerns is getting B sufficient schooling, with an aide to accompany him. We applied for such an aide in May, with the intent that he begin school in September. The French government said they would take four months to decide, which ended a couple weeks ago. At the same time, we heard rumors that in conjunction with all the other budget cutting measures the French government is taking, the aide program for special needs kids was also being cut. We got the impression that there was no chance B would receive any assistance, even though the principal and teachers of the school across the street were eager to have B join them. This virtually sealed our return to the U.S. sooner rather than later...except then we got a letter from the French government, saying they approved an aide for B!! We're in the processing of finalizing that now, and should have more news quite soon.
Still the acid test is what kind of job I will be able to get that can satisfy the many constraints we have on our living situation. We shall see. Our endgame plan for this year is to stay in Europe over the holidays and use the lull to work on the thesis, then take our family vacation back to the U.S. when I fly back to defend sometime in the spring.
Speaking of endgames, I can't wait for Tuesday. My prediction is that regardless of what happens, the proverbial fecal matter is going to hit the rotary oscillator; i.e., it's not going to be pretty.
So take care, "Kyrie eleison" on the road that you must travel,
-PKBGD
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