# Relativistic Heavy Ion Collider

Relativistic Heavy Ion Collider

Caption=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. Barely visible and between the white and red pipes on the left wall, is the orange Crash Cord, which should be used to stop the beam in the case a person is still left in the tunnel.see also [http://www.sciencedirect.com/science/journal/01689002 Nucl. Instr. Meth. Phys. Res. A] 499:2&ndash;3, p. 428ff; preprints are available at [http://www4.rcf.bnl.gov/brahms/WWW/publications.html BRAHMS,] [http://www.phenix.bnl.gov/techpapers.html PHENIX,] [http://www.phobos.bnl.gov/Publications/Technical/phobos_technical_publications.htm PHOBOS,] and [http://www.star.bnl.gov/STAR/publications/technical_publications STAR.] ]
The Relativistic Heavy Ion Collider (RHIC, pronounced like "rick", IPAEng|ˈrɪk) is a heavy-ion collider located at and operated by Brookhaven National Laboratory (BNL) in Upton, New York. [ [http://dx.doi.org/10.1016/S0168-9002(02)01937-X M. Harrison, T. Ludlam, & S. Ozaki, Nucl. Instr. Meth. Phys. Res. A 499:2&ndash;3, 235 (2003);] [http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.nucl.52.050102.090650 M. Harrison, S. Peggs, and T. Roser, Ann. Rev. Nucl. Part. Phys. 52, 425 (2002);] [http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.nucl.53.041002.110450 E. D. Courant, Ann. Rev. Nucl. Part. Phys. 53, 1 (2003).] ] 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, [e.g. [http://www.sciam.com/article.cfm?articleID=0009A312-037F-1448-837F83414B7F014D M. Riordan and W. A. Zajc, Scientific American 294:5, 34 (2006);] [http://podcast.sciam.com/sa_podcast_060426.mp3 Scientific American Podcast, April 26 2006] (MPEG-1 Audio Layer 3).] 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 [cite web |url=http://lhc-commissioning.web.cern.ch/lhc-commissioning/commissioning-ions.htm |title=LHC Lead Ion Beam Commissioning|accessdate=2008-09-15 |work= |publisher= |date=August 2008 ] . 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 [http://www.bnl.gov/RHIC/RHIC_complex.htm 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, 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 $sqrt\left\{s_\left\{NN$ 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 ( [http://www.agsrhichome.bnl.gov/AP/RHIC2004/ Run-4] / [http://www.phenix.bnl.gov/ PHENIX] ). A center-of-mass energy of 400 GeV was briefly achieved during Run-5, colliding protons.

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 [ [http://www.cerncourier.com/main/article/42/3/2 Description of Siberian Snakes in the CERN Courier] ] ) and AC dipoles. The AC dipoleshave been also used in non-linear machine diagnostics for the first time in RHIC. [ [http://prst-ab.aps.org/abstract/PRSTAB/v8/i2/e024001?qid=0c6964f5219d9555&qseq=3&show=10 AC dipole as a non-linear diagnostic tool] ]

The experiments

There are four detectors at RHIC: STAR (6 o'clock, and near the ATR), PHENIX (8 o'clock, pronounced like "phoenix", IPA IPA|/ˈfiːnɪks/), 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 [http://www.rhic.bnl.gov/pp2pp/ PP2PP] , investigating spin dependence in p + p scattering.

The spokespersons for each of the experiments are:
*STAR: Nu Xu (Lawrence Berkeley Laboratory, [http://www.lbl.gov/nsd Nuclear Science Division] )
*PHENIX: Barbara Jacak (Stony Brook University, [http://www.physics.sunysb.edu/Physics/ Department of Physics and Astronomy] )
*PHOBOS: Wit Busza (Massachusetts Institute of Technology [http://web.mit.edu/physics/ Department of Physics] and [http://pierre.mit.edu/ MIT Laboratory for Nuclear Science] )
*BRAHMS: Flemming Videbaek (Brookhaven National Laboratory, [http://www.bnl.gov/physics/ Physics Department] )
*PP2PP: Włodek Guryn (Brookhaven National Laboratory, [http://www.bnl.gov/physics/ Physics Department] )

Current results

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: [ [http://www.physicstoday.org/vol-56/iss-10/p48.html T. Ludlam & L. McLerran, Phys. Today October 2003, 48 (2003).] ]

* Collective anisotropy, or elliptic flow. The multiplicity of the particles' bulk with lower momenta exhibits a dependency as $dn/dphi propto 1 + 2 v_2\left(p_mathrm\left\{T\right\}\right) cos 2 phi$ ("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 "RAA" ("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 "Q"² for deep inelastic scattering with small Bjorken-"x", saturate at a unitarity limit $Q_s^2 propto langle N_mathrm\left\{part\right\} angle/2$, 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 $n_mathrm\left\{ch\right\}/A propto 1/alpha_s\left(Q_s^2\right)$, supporting the predictions of the color glass condensate model. For a detailed discussion, see e.g. Kharzeev "et al."; [ [http://arxiv.org/abs/hep-ph/0210332 D. Kharzeev "et al.", Phys. Lett. B 561, 93 (2002).] ] for an overview of color glass condensates, see e.g. Iancu & Venugopalan. [ [http://arxiv.org/abs/hep-ph/0303204 E. Iancu & R. Venugopalan, in "Quark Gluon Plasma 3", edited by R. C. Hwa & X.-N. Wang, (World Scientific, Singapore, 2003), p. 249.] ]

* 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. Karsch [ [http://arxiv.org/abs/hep-lat/0106019 F. Karsch, in "Lectures on Quark Matter", Lect. Notes Phys. 583 (Springer, Berlin, 2002), p. 209.] ] ).

While in the first years, theorists were eager to claim that RHIC has discovered the quark-gluon plasma (e.g. Gyulassy & McLarren [ [http://arxiv.org/abs/nucl-th/0405013 M. Gyulassy & L. McLarren, Nucl. Phys. A 750, 30 (2005).] ] ), though the experimental groups were more careful not to jump to conclusions, citing various variables still in need of further measurement. [ [http://www.bnl.gov/discover/Spring_04/RHIC_1.asp K. McNulty Walsh, "Latest RHIC Results Make News Headlines at Quark Matter 2004", "Discover Brookhaven" 2:1, 14&ndash;17 (2004).] ] 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 [http://www.phenix.bnl.gov/WWW/info/comment/ 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. [ [http://arxiv.org/abs/nucl-ex/0410020 I. Arsene "et al." (BRAHMS collaboration), Nucl. Phys. A 757 1, (2005);] [http://arxiv.org/abs/nucl-ex/0410003 K. Adcox "et al." (PHENIX Collaboration), Nucl. Phys. A 757, 184 (2005);] [http://arxiv.org/abs/nucl-ex/0410022 B. B. Back "et al." (PHOBOS Collaboration), Nucl. Phys. A 757, 28 (2005);] [http://arxiv.org/abs/nucl-ex/0501009 J. Adams "et al." (STAR Collaboration), Nucl. Phys. A 757, 102 (2005).] ] 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.Fact|Tevatron article makes highest-energy claim (1.96 TeV in CM). If both claims are true, please make distinction.| ()|date=August 2008 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 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, [ [http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/SM/ions/ ATLAS Experiment Heavy Ion Physics Group] ] also a calorimeter upgrade with partial angular coverage has been proposed for ALICE. [ [http://nuclear.ucdavis.edu/~jklay/ALICE/EMCAL/ALICE_EMCAL.html The Case for a Large EMCalorimeter in ALICE;] [http://aliceinfo.cern.ch/Collaboration/ALICE_Project/EMCAL/index.html DOE Review 2005] ]

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.". [ [http://arxiv.org/hep-ph/0506148 A. Deshpande "et al.", Ann. Rev. Nucl. Part. Sci. 55, 165 (2005).] ]

In October 2006, the 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. [ [http://www.physicstoday.org/vol-59/iss-10/p15a.html S. Aronson, Phys. Today, October 2006, 15.] ]

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 [ [http://www.findarticles.com/p/articles/mi_m2843/is_3_24/ai_62102225 T. D. Gutierrez, "Doomsday Fears at RHIC," Skeptical Inquirer 24, 29 (May 2000)] ] :
*RHIC creates a black hole
*RHIC creates a transition into a different quantum mechanical vacuum (see false vacuum)
*RHIC creates strange matter that is more stable than ordinary matter

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 Fact|date=January 2008 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 [cite journal |journal=New Scientist |date=28 August 1999 |title=A Black Hole Ate My Planet |url=http://www.newscientist.com/article/mg16322014.700-a-black-hole-ate-my-planet.html |last=Matthews |first=Robert |authorlink=Robert Matthews] [citation |title=Horizon: End Day |publisher=BBC |year=2005] and in the scientific community. [W. Wagner, "Black holes at Brookhaven?" and reply by F. Wilzcek, Letters to the Editor, Scientific American July 1999] 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. [Cf. Brookhaven Report mentioned by Rees, Martin (Lord), "Our Final Century: Will the Human Race Survive the Twenty-first Century?", U.K., 2003, ISBN 0-465-06862-6; note that the mentioned "1 in 50 million" chance is disputed as being a misleading and played down probability of the serious risks (Aspden, U.K., 2006)] 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." [BBC "End Days" (Documentary)] After detailed studies, scientists reached such conclusions as "beyond reasonable doubt, heavy-ion experiments at RHIC will not endanger our planet" [A. Dar, A. De Rujula, U. Heinz, "Will relativistic heavy ion colliders destroy our planet?", Phys. Lett. B470:142-148 (1999) [http://www.arxiv.org/abs/hep-ph/9910471 arXiv:hep-ph/9910471] ] and that there is "powerful empirical evidence against the possibility of dangerous strangelet production." [W. Busza, R. Jaffe, J. Sandweiss, F. Wilczek, "Review of speculative 'disaster scenarios' at RHIC", Rev. Mod. Phys.72:1125-1140 (2000) [http://www.arxiv.org/abs/hep-ph/9910333 arXiv:hep-ph/9910333] ]

The debate started in 1999 with an exchange of letters in Scientific American between Walter L. Wagner [Wagner is a lawyer and former physics lab technician. In 1975, he worked on a project that claimed to discover a magnetic monopole in cosmic ray data ("Evidence for the Detection of a Moving Magnetic Monopole", Physical Review Letters, Vol. 35, (1975)). That claim was later withdrawn in 1978 ("Further Measurements and Reassessment of the Magnetic Monopole Candidate", Physical Review D18: 1382-1421 (1978))] , and F. Wilczek [Wilczek is noted for his work on quarks, for which he subsequently was awarded the Nobel Prize] , Institute for Advanced Study, in response to a previous article by M. Mukerjee. [M. Mukerjee, [http://www.sciam.com Scientific American] 280:March, 60 (1999).] The media attention unfolded with an article in U.K. Sunday Times of July 18, 1999 by J. Leake, [ [http://www.wisdomofsolomon.com/bigbang.html Sunday Times, 18 July 1999.] ] closely followed by articles in the U.S. media. [e.g. [http://web.archive.org/web/20031005104321/abcnews.go.com/sections/tech/FredMoody/moody990914.html ABCNEWS.com,] from the Internet Archive.] The controversy mostly ended with the report of a committee convened by the director of Brookhaven National Laboratory, J. H. Marburger, ostensibly ruling out the catastrophic scenarios depicted [http://arxiv.org/abs/hep-ph/9910333 R. Jaffe "et al.", Rev. Mod. Phys. 72, 1125&ndash;1140 (2000).] ] . 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. [e.g. [http://www.msnbc.msn.com/id/3077374/ MSNBC, June 14 2000.] ] . 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. [United States District Court, Eastern District of New York, Case No. 00CV1672, Walter L. Wagner vs. Brookhaven Science Associates, L.L.C. (2000); United States District Court, Northern District of California, Case No. C99-2226, Walter L. Wagner vs. U.S. Department of Energy, et al. (1999)]

On March 17, 2005, the BBC published an article [ [http://news.bbc.co.uk/1/hi/sci/tech/4357613.stm BBC, 17 March 2005.] ] implying that researcher Horaţiu Năstase believes black holes have been created at RHIC. However, the original papers of H. Năstase [ [http://arxiv.org/abs/hep-th/0501068 H. Nastase, hep-th/0501068 (2005).] ] and the New Scientist article [ [http://www.newscientist.com/channel/fundamentals/mg18524915.400 E. S. Reich, New Scientist 185:2491, 16 (2005).] ] 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 "AdS"5 × "X"5 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. [ [http://www.er.doe.gov/np/ U.S. Department of Energy, Office of Science, Office of Nuclear Physics] ] It had a line-item budget of 616.6 million U.S. dollars. [ [http://dx.doi.org/10.1016/S0168-9002(02)01937-X M. Harrison, T. Ludlam, & S. Ozaki, Nucl. Instr. Meth. Phys. Res. A 499:2&ndash;3, 235 (2003).] ] The annual operational budgets were: [ [http://www.cfo.doe.gov/budget/ U.S. Department of Energy, Office of Budget] ]
* 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. dollarsThe total investment by 2005 is approximately 1.1 billion U.S. dollars. Though operation under the fiscal year 2006 federal budget cut [e.g. [http://www.aip.org/fyi/2005/168.html FYI, November 22 2005;] [http://select.nytimes.com/gst/abstract.html?res=F20713F73C550C748EDDA80994DD404482 New York Times, November 27 2005.] ] 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. [e.g. [http://www.aps.org/apsnews/0306/030604.cfm APS News Online, March 2006;] [http://www.aip.org/fyi/2006/006.html FYI, November 22 2005.] ]

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. [ [http://www.bnl.gov/bnlweb/pubaf/bulletin/1998/bb022098.pdf Brookhaven Bulletin 52, 8 (1998),] p. 2.]

ee also

* The ISABELLE Project

References

* [http://www.bnl.gov/rhic/ Relativistic Heavy Ion Collider]
* [http://www.bnl.gov/cad/ Brookhaven National Laboratory Collider-Accelerator Department]
* [http://www.agsrhichome.bnl.gov/RHIC/Runs/ RHIC Run Overview]
* [http://www.phobos.bnl.gov/images/SiliconModules/some_recent_snap_shorts_of_the_p.htm Various photos of the RHIC tunnel near the PHOBOS experiment]
* [http://www.aip.org/pnu/2005/split/723-2.html A Puzzling Signal in RHIC Experiments] , Physics News, March 15, 2005

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