With New Year’s Eve behind us and the coldest months ahead, most of us probably aren’t thinking of intentionally chilling anything — unless you work at the Relativistic Heavy Ion Collider (RHIC). There, physicists are putting on the big chill, cooling the accelerator’s superconducting magnets to near absolute zero (-273 degrees Celsius) to get ready for the 2012 run. A number of accelerator and detector upgrades — and a swanky new Main Control Room — will help them crank out the data as they strive for deeper understanding of the perfect liquid quark gluon plasma (QGP) and the source of proton spin.
“Due to budget constraints, we’ll be faced with a tighter running schedule this year — about 20 weeks instead of the 26 weeks we operated in 2011,” said Associate Laboratory Director for Nuclear and Particle Physics Steven Vigdor. “But even with reduced running time, I’m confident that the advances we’ve made in accelerator operations will provide high collision rates, and that the STAR and PHENIX experiments will quickly commission their new detectors to gather useful data.”
A cut-away diagram of the EBIS pre-injector.
On the accelerator end, the Electron Beam Ion Source (EBIS) will serve as the main heavy ion injector for a RHIC run for the first time. EBIS enables acceleration of a wider range of ions than the previously used Tandem Van de Graaff accelerators. Though the upcoming RHIC run will be focused in part on colliding polarized proton beams for investigating the mysterious source of proton spin, there will also be a brief (five-or-more-week) heavy-ion run, during which physicists will accelerate and collide copper ions with gold, and if time allows, uranium ions with uranium.
Because EBIS can serve up these different ion beams at practically the same time — or deliver one ion species to RHIC and a different type of ion (think carbon, helium, or iron) to experiments at the NASA Space Radiation Laboratory (NSRL) — multiple research programs can potentially run simultaneously. It also provides heretofore-unavailable ion beams, such as highly non-spherical uranium nuclei. And in contrast to the Tandems, the new beam injector operates practically automatically, with much less need for human intervention.
Of course people still play a vital role in the research at RHIC and the other facilities fed by the Collider-Accelerator Department (CAD), and a brand new, state-of-the-art main control room (MCR) on the second floor of Building 911 is bringing them closer together and streamlining operations.
“We needed to make room for more operators to help manage the multiple research programs that can now run concurrently at RHIC and NSRL,” said CAD Chair Thomas Roser. “During high-demand study periods, we can now integrate all our operators — who run these machines 24 hours a day, seven days a week — in one centralized location for efficient operation.”
Data from accelerator systems now comes to the MCR in fully digital form, making it easier to store and compare — and make adjustments, when needed.
While accelerator physicists make beam adjustments at the macroscopic scale, sophisticated monitoring devices, signaling systems, and electromagnetic “kickers” have been installed to tweak RHIC’s beams in subtle ways that keep the circulating ions tightly bunched. This beam-tweaking system is known as “stochastic cooling” because it monitors the random statistical fluctuations in beam shape and size that occur as the ion bunches naturally spread out, or heat up — and sends corrective signals across the RHIC ring to kick the ions back into place, thus “cooling” the beams.
Run 12 will mark the first time this beam-cooling technique has been fully implemented in both RHIC rings — keeping each heavy ion beam from spreading lengthwise, widthwise, and vertically. These tighter bunches are expected to yield dramatically improved collision rates.
Mike Lenz adjusts part of the forward silicon vertex tracker in the PHENIX detector.
Upgraded and new detector components at both STAR and PHENIX will be ready to capture those collisions and gather the data they need to answer scientific questions.
For instance, PHENIX has installed new silicon tracking detectors, known as the barrel and forward vertex detectors, which allow researchers to identify the production of very short-lived particles that decay mere microns away from the primary collision zone. These will enable the researchers to use so-called heavy quarks as a tool for studying the quark gluon plasma created in RHIC’s high-energy heavy-ion collisions. That’s the free-flowing, superhot (4 trillion-degree!) liquid substance scientists believe filled the early universe some 14 billion years ago, before even protons and neutrons formed. Scientists will use the heavy quarks to see if they, too, are carried along with the flow of the quark-gluon plasma, like heavy rocks in a forcefully flowing stream. A positive result would deepen the mystery about how such strong interactions among the constituents in the QGP arise from a fundamental theory that predicts relatively weak quark-quark and quark-gluon coupling.
For the investigation of proton spin structure, PHENIX has also installed a set of “chambers” that will allow scientists to identify high-momentum muons emerging from proton-proton collisions. These muons are a sign of the production of W bosons, which help elucidate how different “flavors” of quarks contribute to proton spin.
The STAR detector has new systems for tracking W bosons and muons
STAR also has a new detector system to enhance the tracking of W bosons via the energetic particles they decay into to help tease out details of the spin properties of the proton. It is known as a forward GEM tracker, where GEM stands for gaseous electron multiplier — the state of the art for accurate charged particle detectors based on gas. This detector will share components with another precision detector to be installed in the next few years — a technological advancement that has already captured international attention.
For the heavy ion collisions, STAR has added the first set of new detector elements called muon telescope detector (MTD) trays, which will enable measurements of muons that pass through the tons of steel of the STAR magnet. The relative streams of these particles can be translated into precise measurements of the temperature and density of the quark gluon plasma. The system was developed by a collaboration of groups in the U.S., China, and India, and will be continually added to over the next few years.
Both detector groups are excited to explore new kinds of ion collisions at RHIC — for example, interactions between gold ions and smaller copper nuclei, and collisions between football-shaped uranium nuclei.
“The ability to circulate and collide two completely different beams is unique to RHIC, and offers the opportunity to study the asymmetric conditions that result and how those conditions affect the patterns of particles emerging from the collisions,” said Brookhaven physicist David Morrison, deputy spokesperson for the PHENIX collaboration. “It will be interesting to study collisions where, under the right conditions, the smaller copper ion gets completely occluded by, or ‘buried’ in, the larger gold ion, providing new insight into the mechanism by which quarks and gluons lose energy as they traverse the quark-gluon plasma.”
In the case of the uranium collisions, the shape of the ions and their higher density of protons and neutrons (compared with gold) could offer new insight.
“Because of the oblong shape of these nuclei, the shape of the resulting collision zone varies greatly depending on whether the ions collide tip-to-tip, like spiraling footballs — producing energy densities even higher than those produced by colliding spherical gold ions — or slam into one another broadside,” said STAR deputy spokesperson James Dunlop, a physicist at Brookhaven Lab. “By studying the effect of these various geometrical configurations, and their fluctuations, on the patterns seen in the STAR detector, we expect to get a much deeper understanding of the way that the initial shape of the collision zone is transferred through the evolution of the system — and therefore of the properties of the liquid produced at RHIC.”
With all these new avenues to explore, the big challenge will come from the brevity of the run. Getting new detector systems up and running will take some time before the scientists can begin making the measurements that will help them answer their physics questions.
As Morrison summed it up, “We don’t have to hit the ground running, we have to hit it sprinting!”
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