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Relativistic Heavy Ion Collider
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The Relativistic Heavy Ion Collider (RHIC, pronounced like "rick", ) is a heavy-ion collider located at and operated by Brookhaven National Laboratory (BNL) in Upton, New York. By using RHIC to collide ions traveling at relativistic speeds, physicists study the primordial form of matter that existed in the universe shortly after the Big Bang, and also the structure of protons.
At present, RHIC is the most powerful heavy-ion collider in the world, although the LHC is expected to collide ions at higher energies in late 2009.
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Encyclopedia
The Relativistic Heavy Ion Collider (RHIC, pronounced like "rick", ) is a heavy-ion collider located at and operated by Brookhaven National Laboratory (BNL) in Upton, New York. By using RHIC to collide ions traveling at relativistic speeds, physicists study the primordial form of matter that existed in the universe shortly after the Big Bang, and also the structure of protons.
At present, RHIC is the most powerful heavy-ion collider in the world, although the LHC is expected to collide ions at higher energies in late 2009. It is also distinctive in its capability to collide spin-polarized protons.
The accelerator
RHIC is an intersecting storage ring (ISR) particle accelerator. Two independent rings (arbitrarily denoted as "blue" and "yellow" rings, see also the photograph) allow a virtually free choice of colliding projectiles. The RHIC double storage ring is itself hexagonally shaped and 3834 m long in circumference, with curved edges in which stored particles are deflected by 1,740 superconducting niobium-titanium magnets. The six interaction points are at the middle of the six relatively straight sections, where the two rings cross, allowing the particles to collide. The interaction points are enumerated by clock positions, with the injection point at 6 o'clock. Two interaction points are unused and left for further expansion (refer also to ).
A particle passes through several stages of boosters before it reaches the RHIC storage ring. The first stage for ions is the Tandem Van de Graaff accelerator, while for protons, the 200 MeV linear accelerator (Linac) is used. As an example, gold nuclei leaving the Tandem Van de Graaff have an energy of about 1 MeV per nucleon and have an electric charge Q = +32 (32 electrons stripped from the gold atom). The particles are then accelerated by the Booster Synchrotron to 95 MeV per nucleon, which injects the projectile now with Q = +77 into the Alternating Gradient Synchrotron (AGS), before they finally reach 8.86 GeV per nucleon and are injected in a Q = +79 state (no electrons left) into the RHIC storage ring over the AGS-To-RHIC Transfer Line (ATR), sitting at the 6 o'clock position.
The main types of particle combinations used at RHIC are p + p, d + Au,
Cu + Cu and Au + Au. The projectiles typically travel at a speed of 99.995% of the speed of light in vacuum. For Au + Au collision, the center-of-mass energy is typically 200 GeV (or 100 GeV per nucleus); a luminosity of 2 × 1026 cm-2 s-1 was targeted during the planning. The current luminosity performance of the collider is 2.96 × 1026 cm-2 s-1 (). For p + p collision, Run-9 achieved center-of-mass energy of 500 GeV at February 12, 2009.
One unique characteristic of RHIC is its capability to produce polarized protons. RHIC holds the record of highest energy polarized protons. Polarized protons are injected into RHIC and preserving this state throughout the energy ramp. This is a difficult task that can only be accomplished with the aid of Siberian Snakes (a chain of solenoids and quadrupoles for aligning particles) and AC dipoles. The AC dipoles
have been also used in non-linear machine diagnostics for the first time in RHIC.
The experiments
There are four detectors at RHIC: STAR (6 o'clock, and near the ATR), PHENIX (8 o'clock, pronounced like "phoenix", IPA ), PHOBOS (10 o'clock), and BRAHMS (2 o'clock). Two of them are still active, with PHOBOS having completed its operation after 2005 and Run-05, and BRAHMS after 2006 and Run-06.
Among the two larger detectors, STAR is aimed at the detection of hadrons with its system of time projection chambers covering a large solid angle and in a conventionally generated solenoidal magnetic field, while PHENIX is further specialized in detecting rare and electromagnetic particles, using a partial coverage detector system in a superconductively generated axial magnetic field. The smaller detectors have larger pseudorapidity coverage, PHOBOS has the largest pseudorapidity coverage of all detectors, and tailored for bulk particle multiplicity measurement, while BRAHMS is designed for momentum spectroscopy, in order to study the so called "small-x" and saturation physics. There is an additional experiment , investigating spin dependence in p + p scattering.
The spokespersons for each of the experiments are:
Current results
For a complementary discussion, see also quark-gluon plasma.
For the experimental objective of creating and studying the quark-gluon plasma, RHIC has the unique ability to provide baseline measurements for itself. This consists of the both lower energy and also lower mass number projectile combinations that do not result in the density of 200 GeV Au + Au collisions, like the p + p and d + Au collisions of the earlier runs, and also Cu + Cu collisions in Run-5.
Using this approach, important results of the measurement of the hot QCD matter created at RHIC are:
- Collective anisotropy, or elliptic flow. The multiplicity of the particles' bulk with lower momenta exhibits a dependency as (pT is the transverse momentum, angle with the reaction plane). This is a direct result of the elliptic shape of the nucleus overlap region during the collision and hydrodynamical property of the matter created.
- Jet quenching. In the heavy ion collision event, scattering with a high transverse pT can serve as a probe for the hot QCD matter, as it loses its energy while traveling through the medium. Experimentally, the quantity RAA (A is the mass number) being the quotient of observed jet yield in A + A collisions and Nbin × yield in p + p collisions shows a strong damping with increasing A, which is an indication of the new properties of the hot QCD matter created.
- Color glass condensate saturation. The Balitsky–Fadin–Kuraev–Lipatov (BFKL) dynamics which are the result of a resummation of large logarithmic terms in Q² for deep inelastic scattering with small Bjorken-x, saturate at a unitarity limit , with Npart/2 being the number of participant nucleons in a collision (as opposed to the number of binary collisions). The observed charged multiplicity follows the expected dependency of , supporting the predictions of the color glass condensate model. For a detailed discussion, see e.g. Kharzeev et al.; for an overview of color glass condensates, see e.g. Iancu & Venugopalan.
- Particle ratios. The particle ratios predicted by statistical models allow the calculation of parameters such as the temperature at chemical freeze-out Tch and hadron chemical potential . The experimental value Tch varies a bit with the model used, with most authors giving a value of 160 MeV < Tch < 180 MeV, which is very close to the expected QCD phase transition value of approximately 170 MeV obtained by lattice QCD calculations (see e.g. Karsch).
While in the first years, theorists were eager to claim that RHIC has discovered the quark-gluon plasma (e.g. Gyulassy & McLarren), though the experimental groups were more careful not to jump to conclusions, citing various variables still in need of further measurement. The present results shows that the matter created is a fluid with a viscosity near the quantum limit, but is unlike a weakly interacting plasma (a widespread yet not quantitatively unfounded belief on how quark gluon plasma looks).
A recent overview of the physics result is e.g. provided by the , a community-wide effort of RHIC experiments to evaluate the current data in the context of implication for formation of a new state of matter. These results are from the first three years of data collection at RHIC.
The future
RHIC began operation in 2000 and is currently the most powerful heavy-ion collider in the world. It is expected, however, that the Large Hadron Collider (LHC) of CERN, completed in 2008, will provide significantly higher energies once fully operational in spring 2009.
However, RHIC will likely remain unique in various fields that the LHC in the present state will not be able to cover. Unlike LHC, RHIC is able to accelerate spin polarized protons, which would leave RHIC as the world's highest energy accelerator for studying spin-polarized proton structure. And ALICE, the dedicated heavy ion detector at LHC, unlike STAR and PHENIX, lacks a calorimeter for jet tomographic studies. As a result, heavy ion studies with the hadronic detectors of LHC has been proposed, also a calorimeter upgrade with partial angular coverage has been proposed for ALICE.
Two planned upgrades should enhance the future scientific output of RHIC in these areas:
- RHIC-II: An upgrade that will increase the luminosity by a further factor of 10, together with upgrades to the detectors STAR and PHENIX.
- eRHIC: Construction of a 10 GeV high intensity electron/positron beam facility, allowing electron-ion collisions. At least one new detector will have to be built to study the collisions. A recent review is given by A. Deshpande et al..
In October 2006, then Interim Director of BNL, Sam Aronson, has contested the claim in a Physics Today report that "Tevatron is unlikely to outlive the decade. Neither is ... the Relativistic Heavy Ion Collider", referring to a report of the National Research Council.
Fears among the public
Before RHIC started operation, there were fears among the public that the extremely high energy could produce one of the following catastrophic scenarios:
These hypothetical theories are complex, but they predict that at least the Earth would be destroyed within seconds, to years, to millennia, depending on the theories. However, the fact that objects of the Solar System (e.g., the Moon) have been bombarded with cosmic particles of significantly higher energies than that of RHIC for billions of years, without any harm to the Solar System, were among the most striking arguments that these hypotheses were unfounded.
The other main controversial issue was a demand by critics for physicists to reasonably exclude the probability for such a catastrophic scenario. Physicists are unable to demonstrate experimental and astrophysical constraints of zero probability of catastrophic events, nor that tomorrow Earth will be struck with a "doomsday" cosmic ray (they can only calculate an upper limit for the likelihood). The result would be the same destructive scenarios described above, although obviously not caused by humans. According to this argument of upper limits, RHIC would still modify the chance for the Earth's survival by an infinitesimal amount.
Concerns were raised in connection with the RHIC particle accelerator, both in the media and in the scientific community. The risk of a doomsday scenario was indicated by Martin Rees, with respect to the RHIC, as being at least a 1 in 50,000,000 chance. With regards to the production of strangelets, Frank Close, professor of physics at the University of Oxford, indicates that "the chance of this happening is like you winning the major prize on the lottery 3 weeks in succession; the problem is that people believe it is possible to win the lottery 3 weeks in succession." After detailed studies, scientists reached such conclusions as "beyond reasonable doubt, heavy-ion experiments at RHIC will not endanger our planet" and that there is "powerful empirical evidence against the possibility of dangerous strangelet production."
The debate started in 1999 with an exchange of letters in Scientific American between Walter L. Wagner, and F. Wilczek, Institute for Advanced Study, in response to a previous article by M. Mukerjee. The media attention unfolded with an article in U.K. Sunday Times of July 18, 1999 by J. Leake, closely followed by articles in the U.S. media. The controversy mostly ended with the report of a committee convened by the director of Brookhaven National Laboratory, J. H. Marburger Megatron, ostensibly ruling out the catastrophic scenarios depicted. However, the report left open the possibility that relativistic cosmic ray impact products might behave differently while transiting earth compared to "at rest" RHIC products; and the possibility that the qualitative difference between high-E proton collisions with earth or the moon might be different than Gold on Gold collisions at the RHIC. Wagner tried subsequently to stop full energy collision at RHIC by filing Federal lawsuits in San Francisco and New York, but without success. The New York suit was dismissed on the technicality that the San Francisco suit was the preferred forum. The San Francisco suit was dismissed, but with leave to refile if additional information was developed and presented to the court.
On March 17, 2005, the BBC published an article implying that researcher Horatiu Nastase believes black holes have been created at RHIC. However, the original papers of H. Nastase and the New Scientist article cited by the BBC state that the correspondence of the hot dense QCD matter created in RHIC to a black hole is only in the sense of a correspondence of QCD scattering in Minkowski space and scattering in the AdS5 × X5 space in AdS/CFT; in other words, it is similar mathematically. Therefore, RHIC collisions might be useful to study quantum gravity behavior within AdS/CFT, but the described physical phenomena are not the same.
Financial information
The RHIC project is sponsored by the United States Department of Energy, Office of Science, Office of Nuclear Physics. It had a line-item budget of 616.6 million U.S. dollars. The annual operational budgets were:
- fiscal year 2005: 131.6 million U.S. dollars
- fiscal year 2006: 115.5 million U.S. dollars
- fiscal year 2007, requested: 143.3 million U.S. dollars
The total investment by 2005 is approximately 1.1 billion U.S. dollars. Though operation under the fiscal year 2006 federal budget cut was uncertain, a key portion of the operational cost (13 million U.S. dollars) was contributed privately by a group close to Renaissance Technologies of East Setauket, New York.
RHIC in fiction
- The novel Cosm (ISBN 0-380-79052-1) by the American author Gregory Benford takes place at RHIC. The science fiction setting describes the main character Alicia Butterworth, a physicist at the BRAHMS experiment, and a new universe being created in RHIC by accident, while running with uranium ions.
- The zombie apocalypse novel The Rising by the American author Brian Keene referenced the media concerns of activating the RHIC raised by the article in The Sunday Times of July 18, 1999 by J. Leake,. As revealed very early in the story, side effects of the collider experiments of the RHIC (located at "Havenbrook National Laboratories") were the cause of the zombie uprising in the novel and its sequel City of the Dead.
See also
External links
- , Physics News, March 15, 2005
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