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Annual Users Meeting

June 1-5, 2009
Brookhaven National Laboratory


Three's a Charm: The Story Behind the First 'Renowned' Result From RHIC

Garbor DavidBy Gabor David, BNL

500 citations - a threshold defined by SPIRES to be where a paper becomes "renowned" - is no small feat in physics, particularly in a field as cutting edge as relativistic heavy ion physics.  The first two RHIC publications to reach this distinguished status were the PHENIX and STAR "White Papers", 100-plus page review articles summarizing dozens of important results from the first three years of RHIC operations.

Physical Review LettersIt is even more remarkable for a paper on a single measurement to be cited more than 500 times, and thus become "renowned" - but this is exactly what happened recently to "Suppression of Hadrons with Large Transverse Momentum in Central Au+Au Collisions at sqrt(s)=130GeV", Phys. Rev. Lett. 88 (2002) 022301 (also the subject of an APS Focus story in December 2001). The paper is affectionately known within PHENIX as "PPG003", since its "Paper Preparation Group" (the cast of characters actually writing the paper) was only the third ever formed in PHENIX history.  The paper was based on data from the very first RHIC run (Summer 2000), when gold ions were collided at 130 GeV per nucleon-nucleon pair.  The circumstances were less than auspicious for a scientific breakthrough. Not only was this the first time when the incredibly complex accelerator came online and the Collider-Accelerator Division was just learning how to operate it, but the major detectors themselves were not complete and fully instrumented yet.  Idiosyncrasies of the brand new detectors had to be understood and tamed, as is normal with any complex apparatus.  At that point, PHENIX could only collect about  2 million clean collisions in a ten week period (the data-taking rate is a thousand times higher today!) - while large momentum particles are typically quite rare and require enormous event samples.

What was the discovery?  The probability of producing high transverse momentum hadrons in proton-proton collisions can be understood using established theories, since they are generally typically the "leading", or most energetic, particles in jets.  Then in the early 1990s theorists predicted that if a hot, dense partonic matter (e.g. a quark gluon plasma) is really formed in relativistic heavy ion collisions, the production rate of those leading particles will be much smaller than expected from a straightforward extrapolation of proton-proton rates.  The parent quark or gluon will lose energy by interacting with the hot, dense matter created and the resulting jet will be "quenched". This turned out to be one of the first predictions, made before RHIC startup, that experiments actually confirmed.  Despite the small amount of data available (and thus the very limited transverse momentum range -- just 4 GeV/c), PPG003 established that neutral pions are indeed significantly suppressed at higher transverse momenta.  This suggested that jet quenching really occurs, and that it is quite strong in Au+Au collisions.  Along with a similar result from STAR, this result turned out to be a major sensation at the Quark Matter 2001 conference held at Stony Brook, January 2001 and spawned literally hundreds of theoretical publications.

Not that it was easy to make that claim.  Data analysis is always tricky, particularly so when your detector sees real data for the first time.  Also, if instead of neutral pions (the six solid circles on the bottom of the main data figure, which made the cover of Physics Review Letters) you look for charged hadrons (solid squares higher up), they appear to be much less suppressed - a difference no one really understood back in 2001.  Later it turned out to be the first hint of another important discovery, but that's for another time.  While the large systematic errors made it difficult to argue that the measurements were inconsistent with each other, everyone on PHENIX had the sense that the two results were somehow fundamentally different.  Thus, both analyses were crossed-checked several times, and no error was found in either of them.  So after several months of scrutiny, we decided to trust our own work and say that this is what Nature tells us: the result should be published as-is, even if we don't yet fully understand the underlying message.  It took quite some intellectual courage when the paper was submitted on September 5, 2001, but it turned out to be the right decision.  A combination of higher statistics and better-quality measurements in later runs fully confirmed (and explained) both of the initial results presented in PPG003, which was ultimately published in Physical Review Letters on December 21, 2001.

A combination of many factors preceeded these great results and their publication: years of preparation, endless simulations, meticulous understanding and calibrating the detectors, cleaning up the data, refining the analysis techniques, and so on.  But the dedication alone probably would not necessarily have resulted in a paper with over 500 citations.  One more component was needed: being at the right place at the right time.  Heavy ion collisions at RHIC provided an effect so strong that it turned out to be unambiguous despite the experimental limitations, and it heralded the creation of a new state of matter.  Clearly, without the highest-quality work put into building the detector and analyzing the data, no amount of luck would have sufficed - but in order to produce a "renowned" paper, a rare constellation of circumstances was also necessary.  While we are duly proud of PPG003 and are celebrating it within PHENIX, we shouldn't forget the dozens of other outstanding RHIC papers which were essential to solidify the case for the discovery of the strongly interacting quark gluon plasma, and contributed to the exploration of the detailed properties of this new form of matter.