Meson
Ten to the tenth of them. They hit a piece of platinum, at ninety-nine
hundredths the speed of light, every seven seconds.
But they
were so small! Over half sneaked between
the platinum atoms to the other side. Those
that hit weren’t even enough to move the thing!
The
student sat in a prefab shed on the other side of a ten-foot concrete wall. Every time the bell rang, to announce that the
machine had completed another cycle, he glanced at a row of registers to see their
count augment. He was waiting
impatiently for the one marked T0 to reach 200. Then the electronics would automatically stop
while they went out to change the film in the automatic camera looking at the
spark chamber. After they did that he
could go off to the canteen for a coke and candy bar.
He
thought: if Pavlov turned off the bell, would I still look at the registers at
the right time? T0 did not advance with every
bell. It recorded the number of times
the electronics concluded there was an event worth photographing in the spark
chamber, so that a picture was taken and the film wound to another frame. Sure, after the machine’s protons hit the
platinum on the other side of the wall, there were a
lot of ð+ coming through the narrow channel cut in the concrete. There were a lot with just the right velocity
for their paths to be bent through 20 degrees by the big magnet and hit the
hydrogen target outside the shed, used as a source of protons for the ð+
to hit. But most of the mesons just went
through the target, and of those that didn’t only a small fraction made the
kind of reaction chain they were interested in:
ð+ + p à Ó+ + K+, and then Ó+ à
p + ð0. If this
happened and if the proton from the second reaction went to the left rather
than the right it would head into the spark chamber. There was a special device he didn’t understand
too well to detect the K+, and it would cause the camera shutter to
open and 15,000 volts to activate the spark chamber, so as to photograph what
the proton did and test some theories, and also T0 would advance,
every minute or so.
The
student yawned, and the physicist looked up from working on a circuit with a soldering
iron. “Tired?” It was
“No, I
slept all day yesterday, like a log. I’m
just sick of waiting for T0.”
“Take a
break. I can come get you when it’s time
to change the film.” The physicists in
this group, unlike most at the Lab, treated grad students like people.
“That’s
OK. Only 20 more to
go. Besides, we’ll probably have
to go get some more hydrogen at the next shutdown. The level was getting low the last I looked.” This would involve them putting metal
sheathes on their shoes to avoid collecting static electricity near the
flammable hydrogen; turning various valves on or off; picking up a heavy
container between the two of them and carrying it to an area outside the
building; exchanging it for a full one; and retracing all the steps. Twenty minutes, maybe more.
“I guess
you’re right, and I want to finish this.”
The physicist turned back to his circuit. After an hour checking wire connections he had
figured out which component was defective and was replacing it. He didn’t bother to turn the power off for
that, but just held rubber-handled forceps with one hand and a soldering iron
with the other. (He’d get a hefty shock
if his hand slipped.) One of the “shoe-string
and sealing wax” men who’d been around even before the machine was built. He’d told the student a night or two before
that they had built all their own circuits when there was only an ordinary
cyclotron, and he couldn’t get used to the Lab’s new production shop where you
just went in and showed the foreman a schematic and he got you the circuit two
days later if there were no snags.
Not much
on theory, though. He asked the student,
“did you go to that seminar last week where they
covered the sigma decay parameters?”
“Yes,
very interesting.”
A cough from the corner of the shed. A technician was scanning yesterday’s film
with a microfilm reader. (He’d been told
to weed out the events where the proton just went through the chamber in a
straight line and didn’t interact, so that later they could look carefully at the events where
it did something, without wasting time.) They glanced at him but said nothing.
“What’d
they say? Leave out the math.”
“How can
I leave it out? It’s a mathematical
theory, expressed in terms of Lie groups.”
“Lie
groups, schmee groups! I guess you haven’t been around long enough
to learn to distill what’s useful in the theoretician’s talk out of all the folderol.
OK, I suppose they’re saying it’s a
question of which Lie group governs elementary particles. So what does the asymmetry parameter in sigma
decay turn out to be if the group is what they want?”
“They
didn’t get down to the level of calculating empirical quantities.”
“Empirical, schmerical! Those guys almost never talk to
experimenters, and when they do it’s always with the bubble chamber.” The group that operated the bubble chamber,
the student had been learning, seemed to have the ear of everyone important,
and its physicists always seemed to be the ones who got mentioned in popular
accounts of what went on at the Lab. But
he supposed this was because their device had been invented five years earlier,
whereas spark chambers were new. But the
physicists in his group never tired of telling him how worthless the bubble
chamber physicists were because they had a special technical crew to operate all
their equipment and never got into the nitty-gritty themselves. “What is empirical is how many times the decay
proton scatters to the right when it hits a carbon atom in one of the plates of
the spark chamber compared with how many to the left. The asymmetry parameter is the difference of
these divided by their sum, and a theoretical quantity that the theoreticians
are supposed to have models of.”
“I think
they see it as our job to calculate things like asymmetry parameters from the
basic group representations themselves. I know that means we have to learn some group
theory, but don’t you agree they have a point? After all, we’re testing their theory, and it’s
the theory that is interesting, not the asymmetry parameter that derives from
it.”
The
physicist grew angry. “I’ll learn group
theory when those bastards come down here from cloud nine and learn how to
replace a bad capacitor. Let them take a
chance on getting electrocuted before they tell me I have to spend my spare
time, which I don’t even have, in learning another crazy new mathematical
scheme. Anyway, we’re not doing this
just to test their theory. It was before
your time, but the history is that we put in the proposal for machine time
because the overthrown of parity conservation made it possible that there is a
non-zero asymmetry parameter, and because we’d perfected the K+
detector, and also of course because we’d developed the spark chamber to make
the observations possible. Let the
theoreticians tell us what they want us to look for or not, I don’t care.”
“Sorry, I
didn’t understand.” The student decided
against pointing out that the physicist could have turned the power to the
circuit off while doing the soldering. “I
hope it doesn’t upset you, but the students from the bubble chamber group told
me all the physicists there are learning group theory.”
The
physicist threw up his hands. “That’s
because they never get their hands dirty. I suppose they have time for it.”
The
student was looking in his seminar notes. “Wait a minute. There is a formula here on how to calculate
the ratio of the asymmetry parameter for Ó+ decay to that for
Ë0 decay if SU(3) is the right group. That’ll give us something to compare with
experiment once we put numbers into the formula. Didn’t you say were going to switch over to ð++
n à Ë0 + K+
next week?”
The
physicist’s voice lowered. “If the lithium deuteride for the neutron
target gets here on time.”
“Lithium deuteride? Oh, I get it, that’s lithium hydride but with
heavy hydrogen. But why don’t we just
use liquid deuterium instead of hydrogen?”
“It’s too
expensive.”
“I
see. Well, where’s the stuff coming from
that there might be a problem?”
The
physicist looked around to make sure the technician was absorbed in his film
scanning, and lowered his voice further.
“
A buzzer
sounded. T0 had reached its
quota and all the registers stopped counting. They got up to go change the film and check
the liquid level in the target.
The
student said, “I don’t know if I like that. I was told that we only do unclassified
research here, and I believe in academic freedom.”
“We do
do only unclassified research,
and you have academic freedom. For
instance, another student is writing his thesis on this run if it goes OK, and
you’re next in line to write on the next experiment if you like the subject. Do you want to get your degree so you can stop
going through the registration line hassle every semester?”
“You have
a point.”
The
technician coughed. They glanced at him
as they went out, but said nothing.
(from the early
1980s)
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