Strangelet Search at RHIC
By J. Sandweiss
Strange
Quark Matter (SQM) are hypothesized to be nuclei which consist
of a single "bag" of up (u), down (d) and strange (s) quarks.
This is in contrast with ordinary nuclei which are composed of
nucleons i.e. groups of three quarks (two up, one down for
protons and two down, one up for neutrons). The existence of
stable SQM was predicted by Witten1 in
1984. For a variety of reasons, it has not been possible to
prove or to disprove this hypothesis. Recently an experiment
with STAR at RHIC2 has added to this situation by
searching for SQM produced in 200 GeV Au-Au collisions. None
were found but limits were set using an ingenious experimental
approach.
That SQM might be stable can be understood by considering the Pauli exclusion principle for fermions (particles with half integer spin like quarks). According to this principle no two fermions can exist in the same quantum state. With two types of quarks (e.g. up and down) it becomes more energy efficient to start a new system once three quarks are in the nucleon. However, with three different quarks (u,d,s) it may be possible to have an unlimited number of quarks in the system. Whether or not this is the case cannot be determined from the theory (Quantum Chromodynamics or QCD) even though we believe the theory to be correct and applicable. The reason is that the calculation involves the nonperturbative region of QCD which is too difficult, at least at present, to calculate.
An important feature of SQM is due to the strange quark content with its negative charge (-1/3). As a results SQM have a very small charge to mass ratio.
Calculations have been done with models, the favorite being the MIT "bag" model. These indicate that SQM might be stable but are not definitive. The situation is then left to experiment. A number of experiments have been done looking for SQM particles, (called "strangelets"). These include searches for relic strangelets on earth, and experiments aiming to produce them in accelerators (especially high energy heavy ion collisions. So far all results have been negative but do not prove that strangelets do not exist.
A heavy ion collision does produce the ingredients for a strangelet, namely a large number of up, down, and strange quarks. The problem is that these quarks are produced at high energy so the chance that they will combine into a "cold" nuclear system as a strangelet is very small.
Recently, the STAR experiment was inspired by a new calculation that indicated that strangelets could be produced by a different mechanism which could be observed at RHIC energies.3 The experiment utilizes the "zero degree calorimeters" (ZDCs) which detect particles which are either neutral or have such small charge to mass ratios that they are not appreciably deflected by the strong magnetic fields of the DX magnets which couple the Au beams into and out of the interaction region.
Figure 1. The shower profile of neutron clusters (left) and strangelets (right) from simulations.2
For this experiment the ZDCs were augmented by a counter system that provided detailed information on the impact point of the particle in the ZDC. The figure (Figure 2 of the paper) shows the pattern which a strangelet would make and the pattern which a background spray of neutrons would make. Because of the high energy of the collision, the strangelets that would be produced would travel fast enough so that the relativistic time dilation would make the transit time from the collision to the ZDC appear to be between one nanosecond and one microsecond, depending on the strangelet mass.
Thus the experiment was sensitive not only to stable strangelets but also to those with proper lifetimes in this range or longer.
The experimental limits essentially showed that such strangelets of mass greater than about 40 GeV to 100 GeV were produced less often that about once per million collisions.
Like the previous searches, this experiment does not prove that SQM does not exist. It is difficult to do so since reliable the estimates of production in accelerator experiments are not really possible with the current state of the theory.
As a final comment, we note that there is a way to obtain a definitive answer to the question of SQM. This is a space experiment searching with a broad mass range for strangelets in the cosmic rays. This relies on the fact that if SQM is indeed stable then all compact stars are not neutron stars but strange stars. Binary collisions of such stars would put a measurable flux of strangelets into the cosmic radiation. But that is another story!
References
1) E. Witten, Phys. Rev. D30, 272 (1984).
2) B.I. Abelev et al., Phys. Rev. C76, 011901(R) (2007) (arXiv:nucl-ex/0511047v2).
3) M. Bleicher et al., Phys. Rev. Lett. 92, 072301 (2004) (arXiv:hep-ph/0205182).
The Relativistic Heavy Ion Collider at Brookhaven National
Laboratory is a world-class scientific research facility primarily
funded by the U.S. Department of Energy Office of Science. Hundreds
of physicists from around the world use RHIC to study what the
universe may have looked like in the first few moments after its
creation. What physicists learn from these collisions may help us
understand more about why the physical world works the way it does,
from the smallest subatomic particles, to the largest stars.