Relativistic Heavy Ion Collider

File:Rhictunnel.jpg

The Relativistic Heavy Ion Collider at Brookhaven National Laboratory. Some of the superconducting magnets were manufactured by Northrop Grumman Corp. at Bethpage, New York. Note especially the second, independent ring behind the blue striped one.

The Relativistic Heavy Ion Collider (RHIC, pronounced like "rick", IPA /ˈrɪk/) is a heavy-ion collider located at and operated by Brookhaven National Laboratory in Upton, New York. It is sponsored by the U.S. Department of Energy Office of Science, Office of Nuclear Physics.

The RHIC project had a line-item budget of 616.6 million US dollars. The fiscal year 2005 operational budget is 106 million US dollars. The total investment by 2005 is approximately 1.1 billion US dollars. While suffered under the federal budget cut, a key portion of its operational cost in fiscal year 2006 of 13 million US dollars was contributed privately by a group close to the hedge fund Renaissance Technologies Corp. of East Setauket, New York.

At present, RHIC is the most powerful heavy-ion collider in the world. 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 6 interaction points are at the middle of the 6 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'. 2 interaction points are unused and left for further expansion (refer also to the RHIC Complex diagram).

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, Au 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 Au 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 Synchrontron (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 ${\sqrt {s}}$ is typically 200 GeV (or 100 GeV per nucleon); 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 (Run-4/PHENIX). A center-of-mass energy of 400 GeV was briefly achieved during Run-5, colliding protons.

The experiments

File:First Gold Beam-Beam Collision Events at RHIC at 100 100 GeV c per beam recorded by STAR.jpg

First Gold ion beam-beam collisions at 100 Gev/c per beam on STAR showing explosion of charged particle debris curving in the magnetic field of the instrument.

There are four detectors at RHIC: STAR (6 o'clock, and near the ATR), PHENIX (8 o'clock), PHOBOS (10 o'clock), and BRAHMS (2 o'clock). While the main objective of the two bigger detectors PHENIX and STAR, and also PHOBOS, are the experimental detection and study of the quark-gluon plasma, BRAHMS is mainly interested in the so called "small-x" and saturation physics. There is an additional experiment PP2PP, investigating spin dependence in p + p scattering. The spokespersons for each of the experiments are:

STAR: Timothy Hallman (Brookhaven National Laboratory, Physics Department)
PHENIX: William A. Zajc (Columbia University, Department of Physics and Nevis Laboratories)
PHOBOS: Wit Busza (Massachusetts Institute of Technology Department of Physics and MIT Laboratory for Nuclear Science)
BRAHMS: Flemming Videbaek (Brookhaven National Laboratory, Physics Department)
PP2PP: Włodek Guryn (Brookhaven National Laboratory, Physics Department)

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 $dn/d\phi \propto 1+2v_{2}(p_{\mathrm {T} })\cos 2\phi$ (p_T is the transverse momentum, $\phi$ 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 p_T can serve as a probe for the hot QCD matter, as it loses its energy while traveling through the medium. Experimentally, the quantity R_AA (A is the mass number) being the quotient of observed jet yield in A + A collisions and N_bin × 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 (L. N. Lipatov, Sov. J. Nucl. Phys. 23, 338, 1976) 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 $Q_{s}^{2}\propto \langle N_{\mathrm {part} }\rangle /2$ , with N_part/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 $n_{\mathrm {ch} }/A\propto 1/\alpha _{s}(Q_{s}^{2})$ , supporting the predictions of the color glass condensate model. For a detailed discussion, see e.g. D. Kharzeev, et al. (2002); for an overview of color glass condensates, see e.g. E. Iancu & R. Venugopalan (2003).

Particle ratios. The particle ratios predicted by statistical models allow the calculation of parameters such as the temperature at chemical freeze-out T_ch and hadron chemical potential $\mu _{B}$ . The experimental value T_ch varies a bit with the model used, with most authors giving a value of 160 MeV < T_ch < 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. F. Karsch, 2002).

While theorists are eager to claim RHIC as having discovered the quark-gluon plasma (e.g. Gyulassy & McLarren, 2004), the experimental groups are more careful not to jump to conclusions, citing various variables still in need of further measurement. A recent overview of the physics result is e.g. provided by Adcox, et al. (2004), part of the RHIC Experimental Evaluations 2004, 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 collections 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 will provide significantly higher energies once completed, essentially superseding RHIC.

However, RHIC will likely remain unique in various fields that the LHC in the present state will not be able to cover. Unlike RHIC, LHC is unable to accelerate spin polarized protons, which would leave RHIC remaining 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 (e.g. [1]), also a calorimeter upgrade with partial angular coverage has been proposed for ALICE (e.g. [2], [3]).

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. (2005).

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:

RHIC creates a black hole
RHIC creates a transition into a different quantum mechanical vacuum
RHIC creates strange matter that is more stable than ordinary matter

The (extremely) hypothetical theories are complex, but they predict that at least the Earth and the Solar System would be destroyed within seconds. 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 issue in the controversy was the demand by critics for physicists to show an exactly zero probability for such a catastrophic scenario, which physics cannot provide. However, by following the same argument of the critics, and using the same experimental and astrophysical constraints, physicists are also unable to demonstrate a zero probability, but just a upper limit for the likelihood, that tomorrow Earth will be struck with a "doomsday" cosmic ray, resulting in the same destructive scenarios. According to this argument of upper limits, RHIC would still modify the chance for the Earth's survival by an extremely marginal amount.

The debate started in 1999 with an exchange of letters in Scientific American between W. L. Wagner, World Botanical Gardens, Inc., and F. Wilczek, Institute for Advanced Study, in response to an previous article by M. Mukerjee (1999) in Scientific American. 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, ruling out the catastrophic scenarios depicted (R. Jaffe et al., 2000). W. L. Wagner tried subsequently – as he had attempted with various accelerators before – to stop full energy collision at RHIC by filing Federal lawsuits in San Francisco and New York, but without success (see e.g. [4]).

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 [5] and the New Scientist article [6] 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, similar mathematically. RHIC collisions therefore might be useful to study quantum gravity behavior within AdS/CFT, but the described physical phenomena are not the same.

RHIC in fiction

The novel Cosm (ISBN 0-3807-9052-1) 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 (see [7], page 2).

References

External links