RHIC

The Relativistic Heavy Ion Collider (RHIC ) is the first and one of only two operating heavy-ion colliders, and the only spin-polarized proton collider ever built. Located at Brookhaven National Laboratory (BNL) in Upton, New York, and used by an international team of researchers, it is the only operating particle collider in the US.
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. By colliding spin-polarized protons, the spin structure of the proton is explored.

RHIC is as of 2019 the second-highest-energy heavy-ion collider in the world. As of November 7, 2010, the Large Hadron Collider (LHC) has collided heavy ions of lead at higher energies than RHIC. The LHC operating time for ions (lead–lead and lead–proton collisions) is limited to about one month per year.

In 2010, RHIC physicists published results of temperature measurements from earlier experiments which concluded that temperatures in excess of 345 MeV (4 terakelvins or 7 trillion degrees Fahrenheit) had been achieved in gold ion collisions, and that these collision temperatures resulted in the breakdown of "normal matter" and the creation of a liquid-like quark–gluon plasma.

In January 2020, the US Department of Energy Office of Science selected the eRHIC design for the future electron–ion collider (EIC), building on the existing RHIC facility at BNL.

The accelerator

RHIC is an intersecting storage ring particle accelerator. Two independent rings (arbitrarily denoted as "Blue" and "Yellow") circulate heavy ions and/or polarized protons in opposite directions and allow a virtually free choice of colliding positively charged particles (the eRHIC upgrade will allow collisions between positively and negatively charged particles). The RHIC double storage ring is hexagonally shaped and has a circumference of , with curved edges in which stored particles are deflected and focused by 1,740 superconducting magnets using niobium-titanium conductors. The dipole magnets operate at . The six interaction points (between the particles circulating in the two rings) are in 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 near 6 o'clock. Two large experiments, STAR and PHENIX, are located at 6 and 8 o'clock respectively. The PHENIX experiment is presently undergoing a major upgrade to become sPHENIX.

A particle passes through several stages of boosters before it reaches the RHIC storage ring. The first stage for ions is the electron beam ion source (EBIS), while for protons, the linear accelerator (Linac) is used. As an example, gold nuclei leaving the EBIS have a kinetic energy of per nucleon and have an electric charge ''Q'' = +32 (32 of 79 electrons stripped from the gold atom). The particles are then accelerated by the Booster synchrotron to per nucleon, which injects the projectile now with ''Q'' = +77 into the Alternating Gradient Synchrotron (AGS), before they finally reach 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).

To date the types of particle combinations explored at RHIC are ,
, , , , , , , , and . The projectiles typically travel at a speed of 99.995% of the speed of light. For collisions, the center-of-mass energy is typically per nucleon-pair, and was as low as per nucleon-pair. An average luminosity of was targeted during the planning. The current average luminosity of the collider has reached , 44 times the design value.
The heavy ion luminosity is substantially increased through stochastic cooling.

One unique characteristic of RHIC is its capability to collide polarized protons. RHIC holds the record of highest energy polarized proton beams. Polarized protons are injected into RHIC and preserve this state throughout the energy ramp. This is a difficult task that is accomplished with the aid of corkscrew magnetics called 'Siberian snakes' (in RHIC a chain 4 helical dipole magnets). The corkscrew induces the magnetic field to spiral along the direction of the beam
Run-9 achieved center-of-mass energy of on 12 February 2009. In Run-13 the average luminosity of the collider reached , with a time and intensity averaged polarization of 52%.

AC dipoles have been used in non-linear machine diagnostics for the first time in RHIC.

File:Helium refrigeration system at Relativistic Heavy Ion Collider (RHIC).jpg, The 25 MW Helium refrigeration system that cools the superconducting magnets down to the operating temperature of 4.5 K
File:Arc dipole magnet of Relativistic Heavy Ion Collider (RHIC).jpg, An arc dipole magnet. Electrical bus slots (top and bottom) and beam tube (middle) at the top section of the vacuum shell
File:Curvature of beam tube of Relativistic Heavy Ion Collider arc dipole magnet.jpg, Curvature of beam tube seen through the ends of an arc dipole magnet
File:Two main accelerator rings inside the Relativistic Heavy Ion Collider tunnel.jpg, Two main accelerator rings inside the RHIC tunnel
File:STAR Detector at Relativistic Heavy Ion Collider.jpg, STAR detector
File:A Forward Silicon Vertex Detector (FVTX) sensor on a microscope.jpg, A Forward Silicon Vertex Detector (FVTX) sensor of PHENIX detector on a microscope

The experiments

There is one detector currently operating at RHIC: STAR (6 o'clock, and near the AGS-to-RHIC Transfer Line). PHENIX (8 o'clock) took last data in 2016. PHOBOS (10 o'clock) completed its operation in 2005, and BRAHMS (2 o'clock) in 2006. A new detector sPHENIX is under construction in the old PHENIX hall and is expected to begin collecting data in 2023.

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 (now part of STAR), investigating spin dependence in p + p scattering.

The spokespersons for each of the experiments are:
*STAR: Helen Caines (Yale University) and Lijuan Ruan (Brookhaven National Laboratory)
*PHENIX: Yasuyuki Akiba ( Riken)
*sPHENIX: Gunter Roland (MIT) and David Morrison (Brookhaven National Laboratory)

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 both the 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 major part of the particles with lower momenta is emitted following an angular distribution $dn/d\phi \propto 1 + 2 v_2(p_\mathrm) \cos 2 \phi$ (''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 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 $Q_s^2 \propto \langle N_\mathrm \rangle/2$, 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 $n_\mathrm/A \propto 1/\alpha_s(Q_s^2)$, supporting the predictions of the color glass condensate model. For a detailed discussion, see e.g. Dmitri 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 ''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).

While in the first years, theorists were eager to claim that RHIC has discovered the quark–gluon plasma (e.g. Gyulassy & McLarren), 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 provided by th
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 collection at RHIC.

New results were published in ''Physical Review Letters'' on February 16, 2010, stating the discovery of the first hints of symmetry transformations, and that the observations may suggest that bubbles formed in the aftermath of the collisions created in the RHIC may break parity symmetry, which normally characterizes interactions between quarks and gluons.

The RHIC physicists announced new temperature measurements for these experiments of up to 4 trillion kelvins, the highest temperature ever achieved in a laboratory. It is described as a recreation of the conditions that existed during the birth of the Universe.

Possible closure under flat nuclear science budget scenarios

In late 2012, the Nuclear Science Advisory Committee (NSAC) was asked to advise the Department of Energy's Office of Science and the National Science Foundation how to implement the nuclear science long range plan written in 2007, if future nuclear science budgets continue to provide no growth over the next four years. In a narrowly decided vote, the NSAC committee showed a slight preference, based on non-science related considerations, for shutting down RHIC rather than canceling the construction of the Facility for Rare Isotope Beams (FRIB).

By October 2015, the budget situation had improved, and RHIC can continue operations into the next decade.

The future

RHIC began operation in 2000 and until November 2010 was the most powerful heavy-ion collider in the world. The Large Hadron Collider (LHC) of CERN, while used mainly for colliding protons, operates with heavy ions for about one month per year. The LHC has operated with 25 times higher energies per nucleon. As of 2018, RHIC and the LHC are the only operating hadron colliders in the world.

Due to the longer operating time per year, a greater number of colliding ion species and collision energies can be studied at RHIC. In addition and unlike the LHC, RHIC is also able to accelerate spin polarized protons, which would leave RHIC as the world's highest energy accelerator for studying spin-polarized proton structure.

A major upgrade is the Electron–Ion Collider (EIC), the addition of a 18 GeV high intensity electron beam facility, allowing electron–ion collisions. At least one new detector will have to be built to study the collisions. A review is given by A. Deshpande ''et al.'' A more recent description is at:

On January 9th, 2020, It was announced by Paul Dabbar, undersecretary of the US Department of Energy Office of Science, that the BNL eRHIC design has been selected for the future electron–ion collider (EIC) in the United States. In addition to the site selection, it was announced that the BNL EIC had acquired CD-0 (mission need) from the Department of Energy.

Critics of high-energy experiments

Before RHIC started operation, critics postulated that the extremely high energy could produce catastrophic scenarios,
such as creating a black hole, a transition into a different quantum mechanical vacuum (see false vacuum), or the creation of strange matter that is more stable than ordinary matter. These hypotheses are complex, but many predict that the Earth would be destroyed in a time frame from seconds to millennia, depending on the theory considered. 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 and other man-made colliders 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 popular science media. 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, in response to a previous article by M. Mukerjee. The media attention unfolded with an article in UK '' 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, 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
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