Relativistic Heavy Ion Collider

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. 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.[1]
Hadron Colliders
Intersecting Storage Rings CERN, 1971–1984
Super Proton Synchrotron CERN, 1981–1984
ISABELLE BNL, cancelled in 1983
Tevatron Fermilab, 1987–2009
Relativistic Heavy Ion Collider BNL, operational since 2000
Superconducting Super Collider Cancelled in 1993
Large Hadron Collider CERN, 2008–
Very Large Hadron Collider Theoretical

The Relativistic Heavy Ion Collider (RHIC, pronounced like "rick", IPA: /ˈrɪk/) is a heavy-ion collider located at and operated by Brookhaven National Laboratory (BNL) in Upton, New York.[2] 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,[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.[4] It is also distinctive in its capability to collide spin-polarized protons.

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) 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 the RHIC Complex diagram).

Sitting on the 8 o'clock interaction point is the PHENIX detector. Visible is the green painted Central magnet in the interaction region (IR), with the beam pipe in its center. To the right is in an extracted position, the East Carriage with the ring imaging Cherenkov detector (RICH).

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{s_{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 (Run-4). 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[5]) and AC dipoles. The AC dipoles have been also used in non-linear machine diagnostics for the first time in RHIC.[6]

The experiments

First gold ion beam-beam collisions at a momentum of 100 + 100 GeV/c per nucleon on STAR showing hadronized 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, pronounced like "phoenix", IPA /ˈfiːnɪks/), PHOBOS (10 o'clock), and BRAHMS (2 o'clock).[1] 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 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 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:[7]

While in the first years, theorists were eager to claim that RHIC has discovered the quark-gluon plasma (e.g. Gyulassy & McLarren[12]), though the experimental groups were more careful not to jump to conclusions, citing various variables still in need of further measurement.[13] 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 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.[14] 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,[15] also a calorimeter upgrade with partial angular coverage has been proposed for ALICE.[16]

Two planned upgrades should enhance the future scientific output of RHIC in these areas:

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.[18]

Fears among the public

Further information: Safety of particle collisions at the Large Hadron Collider

Before RHIC started operation, there were fears among the public that the extremely high energy could produce one of the following catastrophic scenarios:[19]

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.[20]

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[21][22] and in the scientific community.[23] 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.[24] 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."[25] After detailed studies, scientists reached such conclusions as "beyond reasonable doubt, heavy-ion experiments at RHIC will not endanger our planet"[26] and that there is "powerful empirical evidence against the possibility of dangerous strangelet production."[27]

The debate started in 1999 with an exchange of letters in Scientific American between Walter L. Wagner,[28] and F. Wilczek,[29] Institute for Advanced Study, in response to a previous article by M. Mukerjee.[30] The media attention unfolded with an article in U.K. Sunday Times of July 18, 1999 by J. Leake,[31] closely followed by articles in the U.S. media.[32] 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.[20] 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.[33] 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.[34]

On March 17, 2005, the BBC published an article[35] implying that researcher Horaţiu Năstase believes black holes have been created at RHIC. However, the original papers of H. Năstase[36] and the New Scientist article[37] 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.[38] It had a line-item budget of 616.6 million U.S. dollars.[39] The annual operational budgets were:[40]

The total investment by 2005 is approximately 1.1 billion U.S. dollars. Though operation under the fiscal year 2006 federal budget cut[41] 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.[42]

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.[43]

See also

References

  1. ↑ 1.0 1.1 see also Nucl. Instr. Meth. Phys. Res. A 499:2–3, p. 428ff; preprints are available at BRAHMS, PHENIX, PHOBOS, and STAR.
  2. ↑ M. Harrison, T. Ludlam, & S. Ozaki, Nucl. Instr. Meth. Phys. Res. A 499:2–3, 235 (2003); M. Harrison, S. Peggs, and T. Roser, Ann. Rev. Nucl. Part. Phys. 52, 425 (2002); E. D. Courant, Ann. Rev. Nucl. Part. Phys. 53, 1 (2003).
  3. ↑ e.g. M. Riordan and W. A. Zajc, Scientific American 294:5, 34 (2006); Scientific American Podcast, April 26, 2006 (MPEG-1 Audio Layer 3).
  4. ↑ "LHC Lead Ion Beam Commissioning" (August 2008). Retrieved on 2008-09-15.
  5. ↑ Description of Siberian Snakes in the CERN Courier
  6. ↑ AC dipole as a non-linear diagnostic tool
  7. ↑ T. Ludlam & L. McLerran, Phys. Today October 2003, 48 (2003).
  8. ↑ L. N. Lipatov, Sov. J. Nucl. Phys. 23, 338 (1976).
  9. ↑ D. Kharzeev et al., Phys. Lett. B 561, 93 (2002).
  10. ↑ E. Iancu & R. Venugopalan, in Quark Gluon Plasma 3, edited by R. C. Hwa & X.-N. Wang, (World Scientific, Singapore, 2003), p. 249.
  11. ↑ F. Karsch, in Lectures on Quark Matter, Lect. Notes Phys. 583 (Springer, Berlin, 2002), p. 209.
  12. ↑ M. Gyulassy & L. McLarren, Nucl. Phys. A 750, 30 (2005).
  13. ↑ K. McNulty Walsh, "Latest RHIC Results Make News Headlines at Quark Matter 2004", Discover Brookhaven 2:1, 14–17 (2004).
  14. ↑ I. Arsene et al. (BRAHMS collaboration), Nucl. Phys. A 757 1, (2005); K. Adcox et al. (PHENIX Collaboration), Nucl. Phys. A 757, 184 (2005); B. B. Back et al. (PHOBOS Collaboration), Nucl. Phys. A 757, 28 (2005); J. Adams et al. (STAR Collaboration), Nucl. Phys. A 757, 102 (2005).
  15. ↑ ATLAS Experiment Heavy Ion Physics Group
  16. ↑ The Case for a Large EMCalorimeter in ALICE; DOE Review 2005
  17. ↑ A. Deshpande et al., Ann. Rev. Nucl. Part. Sci. 55, 165 (2005).
  18. ↑ S. Aronson, Phys. Today, October 2006, 15.
  19. ↑ T. D. Gutierrez, "Doomsday Fears at RHIC," Skeptical Inquirer 24, 29 (May 2000)
  20. ↑ 20.0 20.1 R. Jaffe et al., Rev. Mod. Phys. 72, 1125–1140 (2000).
  21. ↑ Matthews, Robert (28 August 1999). "A Black Hole Ate My Planet". New Scientist. http://www.newscientist.com/article/mg16322014.700-a-black-hole-ate-my-planet.html. 
  22. ↑ Horizon: End Day, BBC, 2005 
  23. ↑ W. Wagner, "Black holes at Brookhaven?" and reply by F. Wilzcek, Letters to the Editor, Scientific American July 1999
  24. ↑ 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)
  25. ↑ BBC End Days (Documentary)
  26. ↑ A. Dar, A. De Rujula, U. Heinz, "Will relativistic heavy ion colliders destroy our planet?", Phys. Lett. B470:142–148 (1999) arXiv:hep-ph/9910471
  27. ↑ W. Busza, R. Jaffe, J. Sandweiss, F. Wilczek, "Review of speculative 'disaster scenarios' at RHIC", Rev. Mod. Phys.72:1125–1140 (2000) arXiv:hep-ph/9910333
  28. ↑ 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))
  29. ↑ Wilczek is noted for his work on quarks, for which he subsequently was awarded the Nobel Prize
  30. ↑ M. Mukerjee, Scientific American 280:March, 60 (1999). The Wagner and Wilczek letters follow in the July issue (vol. 281 no. 1), p. 8.
  31. ↑ Sunday Times, 18 July 1999.
  32. ↑ e.g. ABCNEWS.com, from the Internet Archive.
  33. ↑ e.g. MSNBC, June 14, 2000.
  34. ↑ 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)
  35. ↑ BBC, 17 March 2005.
  36. ↑ H. Nastase, hep-th/0501068 (2005).
  37. ↑ E. S. Reich, New Scientist 185:2491, 16 (2005).
  38. ↑ U.S. Department of Energy, Office of Science, Office of Nuclear Physics
  39. ↑ M. Harrison, T. Ludlam, & S. Ozaki, Nucl. Instr. Meth. Phys. Res. A 499:2–3, 235 (2003).
  40. ↑ U.S. Department of Energy, Office of Budget
  41. ↑ e.g. FYI, November 22, 2005; New York Times, November 27, 2005.
  42. ↑ e.g. APS News Online, March 2006; FYI, November 22, 2005.
  43. ↑ Brookhaven Bulletin 52, 8 (1998), p. 2.

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