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Relativistic Heavy Ion Collider

From Academic Kids

Missing image
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) is a heavy-ion collider located at and operated by the Brookhaven National Laboratory in Upton, New York. It is sponsored by the U.S. Department of Energy Office of Science (http://www.er.doe.gov/), Office of Nuclear Physics. The RHIC project had a line-item budget of 616.6 million US dollars.

Contents

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) allows 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 projectiles 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 crosses, allowing the projectiles 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 (also refer the RHIC Complex diagram (http://www.bnl.gov/RHIC/RHIC_complex.htm)).

Main types of projectile 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 <math>\sqrt{s}<math> is currently up to 200 GeV per nucleon (or 100 GeV per nucleon for each projectile), and 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 (http://www.agsrhichome.bnl.gov/AP/RHIC2004/)/PHENIX (http://www.phenix.bnl.gov/)).

A projectile passes 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 nucleuses leaving the Tandem Van de Graaff have a energy of about 1 MeV per nucleon and have Q = +32 (32 electrons stripped from the Au atom). The projectiles then are continued to be 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 experiments

Missing image
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.

RHIC consists of four detectors: STAR (http://www.star.bnl.gov/) (6 o'clock and near the ATR), PHENIX (http://www.phenix.bnl.gov/) (8 o'clock), PHOBOS (http://www.phobos.bnl.gov/) (10 o'clock), and BRAHMS (http://www4.rcf.bnl.gov/brahms/WWW/brahms.html) (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 quark-gluon plasma, BRAHMS is mainly interested in the so called "small-x" and saturation physics. There is a further experiment PP2PP (http://www.rhic.bnl.gov/pp2pp/), 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 quark-gluon plasma, RHIC has the unique ability to provide baseline measurement for itself. This consists of the both lower energy and also lower mass number projectile combinations that does not result in the density of 200 GeV per nucleon 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 <math>dn/d\phi \propto 1 + 2 v_2(p_\mathrm{T}) \cos 2 \phi<math> (pT is the transverse momentum, <math>\phi<math> 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 property 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 is the result of a resummation of large logarithmic terms in Q2 for deep inelastic scattering with small Bjorken-x, saturates at a unitarity limit <math>Q_s^2 \propto \langle N_\mathrm{part} \rangle/2<math>, with Npart/2 being the number of participants nucleons in a collision (as opposed to number of binary collisions). The observed charged multiplicity follows the expected dependency of <math>n_\mathrm{ch}/A \propto 1/\alpha_s(Q_s^2)<math>, supporting the predictions of the color glass condensate model. For a detailed discussion, see e.g. D. Kharzeev, et al. (2002) (http://arxiv.org/abs/hep-ph/0210332), for an overview of color glass condensate, see e.g. E. Iancu & R. Venugopalan (2003) (http://arxiv.org/abs/hep-ph/0303204).
  • 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 <math>\mu_B<math>. 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 value of QCD phase transition of approximately 170 MeV obtained by lattice QCD calculations (see e.g. F. Karsch, 2002 (http://arxiv.org/abs/hep-lat/0106019)).

While theorists are more eager to claim RHIC having discovered the quark-gluon plasma (e.g. Gyulassy & McLarren, 2004 (http://arxiv.org/abs/nucl-th/0405013)), the experimental groups are more careful to jump to a conclusion, 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) (http://www.arxiv.org/abs/nucl-ex/0410003), part of the RHIC Experimental Evaluations 2004 (http://www.phenix.bnl.gov/WWW/info/comment/), 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 the RHIC.

The future

RHIC began its operation in 2000 and is currently the most powerful heavy-ion collider in the world. It is expected, however, that the Large Hadron Collider of the Conseil Européenne pour la Recherche Nucléaire (CERN) will provide significantly higher energy once completed, essentially superseding RHIC. It is expected, however, that RHIC will remain unique in various fields that Large Hadron Collider will not be able to cover, like tomographic study of quark-gluon plasma, spin physics and saturation physics. Two planned upgrades should enhance the future scientific output of RHIC in these fields:

  • RHIC-II: An upgrade that increase the luminosity by a further factor 10, together with upgrades to the detectors STAR and PHENIX.
  • eRHIC: Construction of a 20 GeV electron storage ring together with a specialized eRHIC detector, allowing electron-ion collisions.

Fears among the public

Before RHIC started its operation, there have been fears among the public that the extremely high energy could produce one of the following catastrophic scenarios:

The hypothetical theories are complex, but they predict that at least the Earth and the Solar System would be destroyed within few seconds. However, the fact that objects of the Solar System (e.g. the Moon) are 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 strking arguments that these hypotheses were unfounded.

The other main issue in the controversy is the demand by the critics of RHIC to physicists to show an exactly zero probability for such a catastrophic scenario, which the physics cannot provide. However, by following the same argument of the critics, and using the same experimental and astrophysical constraints, the physics is also not able 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. By choosing the standpoint to argue with upper limits, RHIC would still modify the chance for the Earth's survival by a extremely marginal amount.

The debate started in 1999 with an exchange of letters in Scientific American between W. L. Wagner, World Botanical Gardens, Inc. (http://www.wbgi.com/) and F. Wilczek, Institute for Advanced Study to an previous article by M. Mukerjee (1999) in Scientific American. The media attention unfolded with the article in U.K. Sunday Times of July 18, 1999 by J. Leake [1] (http://www.wisdomofsolomon.com/bigbang.html), closely followed by 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 depited (R. Jaffe et al., 2000). W. L. Wagner tried subsequently – as he 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. [2] (http://www.msnbc.com/news/314049.asp)).

On March 17, 2005, the BBC reported that researcher Horatiu Nastase believes this has indeed occurred.[3] (http://news.bbc.co.uk/1/hi/sci/tech/4357613.stm) However, the report is very likely to be the product of the journalist's misconception. Both the original papers of H. Nastase [4] (http://arxiv.org/abs/hep-th/0501068) and the New Scientist article [5] (http://www.newscientist.com/channel/fundamentals/mg18524915.400) cited by the BBC states 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. RHIC collisions therefore might be useful to study various quantum gravity behaviors within the AdS/CFT, but the described physical phenomena are not the same.

RHIC in fictional literature

The novel "Cosm" (http://www.amazon.com/exec/obidos/ASIN/0380790521/) 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 [6] (http://www.bnl.gov/bnlweb/pubaf/bulletin/1998/bb022098.pdf), page 2).

References

External links

pt:Colisor Relativístico de Íons Pesados

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