PHENIX (Pioneering High Energy Nuclear Interactions eXperiment) is one of the four experiments at the Relativistic Heavy Ion Collider located at the Brookhaven National Laboratory. Its research goals are the discovery and the examination of the quark gluon plasma, a state of matter where the strong interaction is dominant and the quarks and gluons are not bound in hadrons, and the analysis of the spin structure of the proton.

PHENIX consists of several detector types that are designed to detect photonic, leptonic and hadronic signals.

The Science Mission of PHENIX

The PHENIX Collaboration performs basic research with high energy collisions of heavy ions and protons. The primary mission of PHENIX is the following:

* Search for a new state of matter called the quark-gluon plasma, which is believed to be the state of matter existing in the universe shortly after the Big Bang. PHENIX data suggest that a new form of matter has indeed been discovered, and that it behaves like a perfect fluid. PHENIX scientists are now working to study its properties.

* Study matter under extreme conditions of temperature and pressure.

* Learn where the proton gets its spin.

* Study the most basic building blocks of nature and the forces that govern them.

What has PHENIX learned so far?

Here is a summary of published PHENIX results to date, listed in order of publication:

* The particles are flowing like a fluid: PHENIX can measure how much the particles flow around in the collision. PHENIX observes a significant particle flow effect, which is expected when heavy ions collide. However, those high transverse momentum particles surprise again, and show a flow effect that is not yet understood and may be more evidence for the existence of a quark-gluon plasma. The pattern seen is consistent with a fluid that has a very low viscosity.

* Surprising structure is seen in the correlated emission of particles: PHENIX can measure the angle of emission of particles with respect to each other. This analysis shows that there is a surprising structure that appears in heavy ion collisions that is not seen in collisions of elementary particles. This structure appears to be present in the bulk of the created matter and may be due to the generation of shock waves in the medium. PHENIX scientists are working to better characterize this exciting result.

* There appears to be suppression of particles with a high transverse momentum in gold + gold collisions: PHENIX can measure the transverse momentum of charged particles and neutral pions. The transverse momentum is the particle mass times its velocity that goes sideways to the direction that the two ions were travelling before they collided. Particles with a high transverse momentum are generally travelling faster and are more energetic, but they are seen rarely. PHENIX observes that there are fewer particles with a high transverse momentum than what is expected from measurements of simpler proton collisions. This effect was predicted to occur if a quark-gluon plasma is formed, and is referred to as jet suppression, since the majority of these particles are products of a phenomenon called jets.

* There does not appear to be suppression of particles with a high transverse momentum in deuteron + gold collisions: In order to confirm the observation of suppression, a control experiment was run by PHENIX in the Spring of 2003. Here, a collision was studied in which a medium such as the quark-gluon plasma is not expected to be formed. The collisions studied were small deuteron nuclei colliding with gold nuclei. In this case, more, rather than fewer, particles are seen with a high transverse momentum. This observation confirms that the suppression seen in gold + gold collisions is most likely due to the influence of a new state of matter being produced, such as a quark-gluon plasma.

* There are more protons than pions at high transverse momentum: PHENIX can identify different types of particles, including lighter pions and heavier protons and kaons. PHENIX finds that there are more protons than pions at high transverse momentum. This may indicate that the physical processes that produce these particles are occurring differently in heavy ion collisions. Also, there are almost as many anti-protons as protons, which is another indication that conditions are favorable for the production of a quark-gluon plasma.

* A large number of produced particles are observed: PHENIX finds that there are additional particles produced in collisions of gold ions than what would be expected from measurements of simpler collisions of protons. This fact hints that conditions may be favorable for the production of a quark-gluon plasma. Also, more particles are produced when the ions collide head on.

* A large total amount of transverse energy production is observed: PHENIX can measure the amount of energy that comes out sideways, or transverse, to the direction the ions were originally travelling. Like the number of produced particles, the total transverse energy is largest when the ions collide head on. From this measurement, PHENIX estimates that the density of energy in the center of the collision is about 30 times that of a normal nucleus. This fact also hints that conditions may be favorable for quark-gluon plasma production.

* The source of produced particles is large and short-lived: Borrowing a technique from astronomy that has been applied to measure the radius of individuals stars, the size of the source volume where the particles are produced has been measured by PHENIX. The transverse size of the source appears to be much larger than the original size of the gold nuclei, and lives for a very short time. The short life is contrary to what is expected from a quark-gluon plasma and remains a mystery to be solved.

* An electron signal above background is observed: PHENIX is unique at RHIC in that it can identify individual electrons coming from the collision, many of which are the result of decays of heavier particles within the collision. PHENIX measures a number of electrons that is above the expected background. The excess electrons are likely coming from decays of special particles with heavy charm quarks in them. Further study of these charmed particles will help us better understand if a quark-gluon plasma has been formed.

* Non-random fluctuations are observed, but they are likely due to the presence of jets: During a phase transition, it is typical to see fluctuations in some properties of the system. PHENIX has measured fluctuations in the charge and average transverse momentum of each collision. Thus far, PHENIX reports no large charge fluctuations that might be seen if there is a phase transition from a quark-gluon plasma. PHENIX reports that there are excess fluctuations in transverse momentum, but they appear due to the presence of particles from jets. The behavior of the fluctuations is consistent with the jet suppression phenomenon mentioned previously.

* The collisions are strange: PHENIX can identify particles that contain strange quarks, which are interesting since strange quarks are not present in the original nuclei so they all must be produced. It is expected that a quark-gluon plasma will produce a large amount of strange quarks. In particular, PHENIX has measured lambda particles. There are more lambda particles seen than expected.

Has PHENIX found the quark-gluon plasma?

It is too early to say for sure, but the observation of jet suppression, a very large amount of flow, shock wave dynamics, and more high momentum protons than expected are very promising. The collected observations of all of the RHIC experiments are consistent with a state of matter that acts as a perfect fluid. We are continuing to study the properties of the matter produced in RHIC collsions.

PHENIX is designed specifically to look at many possible signatures of the QGP. PHENIX is unique in its ability to look at pure probes of the collision such as electrons and photons. Here are only a few of the questions that the PHENIX Collaboration is currently trying to answer in order to confirm the existence of a QGP.

* Are the jets really disappearing? Do they really look different than what has been seen before in collisions of protons? If the jets are disappearing, where does all of the energy go?

* Is there really a shock wave phenomena in the medium? Many different measurements are being made to better understand this, including correlation measurements of photons with hadrons.

* Are J/ψ particles disappearing? Do they decay differently than expected? Data taken in 2004 should be able to answer this question.

* Can we see photons (particles of light) radiating directly from a quark-gluon plasma? PHENIX has a preliminary measurement that confirms the presence of these direct photons. Data taken in 2004 should improve this measurement.

* Are the masses of the particles moving due to physical effects in a quark-gluon plasma?

* Do the particles decay in the same way as has been measured in simpler particle collisions?

See also

* Relativistic Heavy Ion Collider

External links

* [ PHENIX main page at the Brookhaven National Laboratory]

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