PHENIX and the 500 GeV Run
By John Haggerty
Long before the first 500 GeV proton-proton collisions were seen at PHENIX on February 24, 2009, the detector was being prepared for this milestone in RHIC running. The north muon tracker had been extensively modified to split the signals from the chambers so that fast trigger decisions could be made with it in future runs, but every signal cable and electronic chassis inside the steel magnet return yoke had to be modified, and during the run, it would be essentially impossible to repair or modify any of it. The east carriage, with its seven different kinds of detectors, had been rolled out of the interaction region in order to allow the work on the muon tracker, and had not been fully operated since the end of Run 8, the previous March. The Hadron Blind Detector, an innovative detector that will enable an excellent measurement of low mass electron pairs in the next heavy-ion run, had been extensively reconstructed, and was ready for a long shakedown run with proton-proton collisions. Another new detector, the muon RPC's ("Resistive Plate Chambers"), which will give the muon system an excellent time resolution when it is fully installed, was slated to have its first prototype run this year as a second component of greatly improved triggering on muons. Of course, the rest of the detectors and electronics, many of which have been in use since the very first RHIC run in 2000, had to be tested, repaired if necessary, and brought into operation again, and configured for measuring the higher energy particles that would result from collisions with 500 GeV.
After a period of intense activity in January by physicists, engineers, and technicians from the many institutions that make up PHENIX, the detector was fully reassembled by the end of the month, and gas was flowing through the chambers as it took its first breath of operation. The four foot thick shield wall was assembled from concrete blocks and rolled in, leaving the detector connected by an umbilical cord of cables, fibers, and pipes under control of PHENIX physicists in the control room remotely monitoring the detector. While the RHIC magnets were being cooled down to superconducting temperatures, just above absolute zero, PHENIX physicists began 24-by-7 operation of the detector, making sure that it was operated safely, and testing it by looking at tracks from cosmic rays traversing the detector.
One of the main goals for Run 9 was to produce and detect the particle known as the W boson. The W boson has played a major role in shaping our understanding the interactions between particles in nature that are responsible for the four known forces that govern how chemical reactions work, how stars generate energy, and how atomic nuclei are held together. Enrico Fermi was the first to explain "weak" nuclear decays -- so called because the decays result in relatively long lifetimes compared to "strong" interactions -- by hypothesizing the existence of a heavy particle responsible for the decay, which was called the W (for "weak"). By 1957, two young theorists (C.N Yang and T.-D. Lee) working together at Brookhaven realized that the decays of the W might violate parity (the notion that natural phenomena might not be able to distinguish left and right), and received the Nobel Prize for their insight. In the late 1970's, a model of the weak interaction was awarded another Nobel Prize (and was eventually called "The Standard Model") because of the elegant unification of the weak interaction with the electromagnetic interaction, proving that the W boson is a close relative of the photon. At this point, the W was still not detected directly, but was only inferred by its role in the weak interaction. Finally, in 1983, after innovations in accelerator physics at CERN, two experiments were able to directly observe the decay of the W boson, and Carlo Rubbia and Simon van der Meer received the Nobel Prize for the accelerator and experiment. As a young graduate student in the early 1980's, I took a particle physics course from Rubbia, and so it has been interesting to be looking for the W twenty five years later, at RHIC.
Our reason for looking for the W this year demonstrates the principle in physics that "last year's discovery is this year's calibration." In this case, the interest in the W is not to discover new properties of the W (although there are some things that will be interesting to measure), but to use the W to measure how the three spin 1/2 quarks making up the proton combine with the spin 1 gluons to make a particle which always has spin 1/2. The coupling of the W to quarks (explained by the unified "electro-weak" theory mentioned previously) can now be used to distinguish up and down quarks in the proton.
Still, it's not that easy. The W decay that can be seen in the PHENIX central arms produces a high energy electron and a neutrino. The neutrino is not visible in the detector, and the techniques usually used to infer its presence ("missing energy") are not practical in PHENIX because there are many places where energy can escape due to the limited acceptance of the detector. The best clue to the existence of a W decay is to look for the highest energy electrons from W decay, which simulation says should have higher energy than those come from any other source of electrons. But there are many ways other particles might be confused for electrons, or a lower energy electron might be accompanied by a jet of particles that would confuse our search algorithms, so it is still an experimental challenge. On top of that, the weak interaction is... weak, so even in the best running conditions, we would not produce more than about 20 per day of running, and probably quite a few less. We didn't want to wait months before seeing anything in our data, so we arranged to redirect the small fraction of the 5000 to 6000 events we record every second which we suspect as candidate W events to files that we could analyze more rapidly in what we called our "Fastrak" analysis. That analysis is still going on even after the end of the 500 GeV run, while we try to find the few needles in the haystack.
After seeing the first collisions at 500 GeV in late February, the hard work started to increase the luminosity and then manipulate the spins of the protons to get the highest possible polarization, in which as many of the protons in the 109 bunches are aligned as possible, and finally to rotate those spins so that instead of being aligned vertically, are aligned parallel and anti-parallel to each other when they collide in the middle of the PHENIX detector. Although our shift crews took data whenever there was a good fill in the machine, mostly in the wee hours of the morning, it wasn't until March 17 that the luminosity and polarization were high enough, and the spin "rotators" were in operation, that we began taking data in earnest. During the setup time, we took lots of data which has confirmed the operation of the HBD, added the readout of the new muon trigger electronics, and commissioned the new muon RPC's, in addition to shaking down the older detectors, the data acquisition, and the trigger system. So when "physics running" began on March 17, we were already well prepared to start accumulating data.
Experimental physics is... well, experimental. You can't be
sure what's going to happen, and sometimes what you observe is
confusing. We knew that there would be particles that don't come
from the interaction of the two beams, but from interactions of
the beam particles with the beam pipe and magnets and many other
accelerator components. Back in Run 3 (in 2003), we added
shielding in the RHIC tunnel outside the PHENIX interaction
region to filter these out before they get to our detectors. To
confuse matters further, the high luminosity achieved by RHIC
also meant that almost every bunch crossing had a collision of
some sort, and so detectors that couldn't clearly distinguish
crossings (which are only 0.1 microseconds apart) could confuse
background with collisions. There was also risk that we could
damage some of the detectors with the high rates of particles
traversing them. To make a long story short, we eventually
agreed on a way of operating in which the beam is cleaned up as
much as possible, by moving absorbers close to the beam (a
process called "collimation"). We then took data until about the
very last moment of the the 500 GeV run, on April 13.
That's really the beginning of the story, not the end. In Run 9, we had our first peek at 500 GeV collisions, but it was only a peek. We know we need improved muon triggering in order to see the W decay into muons as well as electrons (that's the reason for the new detectors), and we we have learned more about the response of the detector to high energy electrons, but there is still much work ahead to calibrate all the detectors and analyze the data offline to be able to show unmistakable evidence for W decays in PHENIX. We think we should have accumulated a couple of hundred W decays to electrons, but we are now trying to make sure the detector was working as planned and we are not making any errors in our data analysis. We have accumulated a large number of J/psi decays to muons, which is an important measurement in its own right, as well as providing important information about the muon trigger upgrades.
So far in telling this story, I haven't acknowledged the hundreds of people who have contributed to it. An experiment like PHENIX, at an accelerator like RHIC, takes the combined intelligence and effort of people with expertise in disciplines that range from computer science to rigging, from electronics to beam dynamics. The final result is impossible without all of them working in concert far more harmoniously than any orchestra. There is always more to do, and things that can be done better, but for the moment, all of you should accept my applause for this job well done.