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\title{Intriguing Aspects of Nucleus-Nucleus Collisions at the AGS}

\author{Huan Z. Huang\address{Department of Physics and Astronomy\\
        University of California\\
        Los Angeles, CA 90095-1547}}

\begin{document}

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\section{Introduction}

The Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory
(BNL) has accelerated gold ions to a beam momentum up to 11.6 GeV/c per
nucleon in fixed target experiments. The major thrust of heavy ion
physics program at the AGS is to study nucleus-nucleus collisions in the 
regime of high baryon density and high temperature, and to search for
signatures of the Quark Gluon Plasma (QGP) and any other exotic phenomenon. 
For over a decade the first generation major
experiments at the AGS (E802/E859/E866/E917~\cite{E802spec}, 
E814/E877~\cite{E814spec} and E810/E891~\cite{E810spec}) have measured
features of nucleus-nucleus collisions in an extensive physics
program. The second generation major 
experiments (E864~\cite{E864spec}
and E896~\cite{E896spec}) 
have searched for novel phenomena in nucleus-nucleus collisions: E864 for
strange quark matter and rare composite objects; E896 for H dibaryons
and Lambda polarization. The LBL EOS TPC was moved to BNL to study nuclear
collisions as a function of excitation energy(E895)~\cite{E895}. I shall review
particle production in nucleus-nucleus collisions at the AGS, mostly using
data from E802, E864 and E896 because I am more familiar with these
experiments.

\section{High Baryon Density and Large Strangeness Production at the AGS}

The baryon rapidity loss in p+A collisions has been studied extensively
prior to the heavy ion physics program. It was found that an incident
proton, on average, losses over two units of rapidity in a central p+A
collision with a heavy target~\cite{Busza}. Thus the prediction was 
that there would be a
large stopping of baryons at central rapidity in nucleus-nucleus
collisions at the AGS where the beam rapidity is about 3.2 for Au and 3.6
for Si ions. Early measurements of the transverse energy~\cite{E802Neu}~\cite{E814Neu}, 
and subsequent measurements of the proton rapidity density distribution 
in Si+A collisions at the AGS indeed 
showed a large stopping of the incident baryons at central 
rapidity~\cite{Stopping}~\cite{Stop1}.

\begin{figure}[htb]
   \centering
   \epsfxsize=8cm
   \epsfysize=8cm
   \leavevmode
  \epsffile{k_yield.eps}
\caption[fig:yield]{Integrated $K^{+}$ and $K^{-}$ yield as a function of number of 
participants. Data are from the E802/E859/E866 collaboration.}
\label{fig:yield}
\end{figure}

Determination of the energy and baryon number densities in nuclear
collisions has been problematic. Bjorken scaling has provided a hydrodynamic 
interaction picture which can be used to estimate
the energy density of the initial conditions for nuclear collisions~\cite{BJ84}.
However, nucleus-nucleus collisions at the AGS and the SPS have exhibited no
constant rapidity density plateau for an extended region at mid-rapidity, 
i.e., Bjorken scaling is not valid at
these energies. In a full stopping picture two Lorentz-contracted nuclei
collide with each other in the Center-of-Mass (CM) frame and overlap completely.
The resulting baryon density is then $2\gamma n_{o}$,
where $\gamma $ is the Lorentz contraction factor and $n_{o}$ is the normal
nuclear matter density. At the AGS energy this expression gives a baryon
density of about 5 times the normal nuclear density in central
nucleus-nucleus collisions. Dynamical models which include temporal
evolution have predicted a baryon density 
higher than what we have obtained from the simple expression. RQMD~\cite{RQMD} and 
ARC~\cite{ARC} models have predicted a
baryon density as high as 10 times of the normal nuclear matter density in
central Au+Au collisions at the AGS. It has been a dilemma that it would be
very unlikely for ordinary baryons to keep their identity in such a high
density, whereas cascade models mostly using free-space baryon properties could
describe many features of nucleus-nucleus collisions.

A large $K^{+}/\pi ^{+}$ ratio of $\sim 20\%$ from central Si+Au collisions,
in contrast to a ratio of $5\%$ in p+p collisions, was measured by E802~\cite{E802Kaon}.
Large Kaon production was considered a signature of the
Quark Gluon Plasma (QGP) formation. However, dynamical models without the QGP formation seem
to reproduce the observed Kaon yield reasonably well~\cite{RQMDKaon}. Rescattering
processes are believed to play an important role for strangeness production in 
nucleus-nucleus collisions at the AGS.
Figure~\ref{fig:yield} shows the $K^{+}$ and $K^{-}$
yield as a function of number of participants, 
where the yield is obtained by integrating over 
whole phase space. The $K^{+}$ to $\pi^{+}$
ratio is about $20\%$ at midrapidity in central Si+Au and Au+Au collisions. 
As shown in Figure~\ref{fig:kratio} the ratio of $K^{+}$ to $K^{-}$ is large with 
little dependence on centrality of the collisions. 
Note that a similarly large value for the $K^{+}/K^{-}$ ratio
is also observed in proton+nucleus collisions at the AGS~\cite{Stopping}. This feature is
probably characteristic for strangeness production in the stopping regime
where baryon collision dynamics play an significant role.

\begin{figure}[htb]
   \centering
   \epsfxsize=8cm
   \epsfysize=8cm
   \leavevmode
  \epsffile{k_ratio.eps}
\caption[fig:kratio]{$K^{+}$ to $K^{-}$ ratio as a function of number of participants. 
Note that a similarly large ($\sim$4-5) ratio for p+A collisions has also been observed 
at the AGS.}
 \label{fig:kratio}
\end{figure}


\begin{figure}[htb]
   \centering
   \epsfxsize=8cm
   \epsfysize=8cm
   \leavevmode
   \epsffile{e896yield.eps} 
\caption[fig:e896yield]{Invariant yield of the $\Lambda$ hyperon per central Au+Au collisions 
from E896 in comparison with RQMD calculation.}
\label{fig:e896yield}
\end{figure}


The large $K^{+}/K^{-}$ ratio is indicative of a large hyperon production
where strangeness is mostly balanced between $s$ quarks in hyperons and 
$\overline{s}$ quarks in $K^{+}$. Both E891~\cite{E891Lambda} and E877~\cite{E877Lambda} 
have reported measurements of the
$\Lambda $ rapidity density distribution in the forward region. The E877
data are slightly below the E891 data, and match more closely the RQMD prediction.
Recently E896 has measured the $\Lambda $ spectrum in a similar rapidity region with
very high statistics. Figure~\ref{fig:e896yield} shows preliminary 
E896 data in comparison with 
RQMD calculation for central Au+Au collisions. RQMD can satisfactorily
reproduce the E896 measurement. E896 has also attempted to measure the
transverse polarization of $\Lambda ^{\prime }s$ from central Au+Au
collisions~\cite{Kelly}.

\section{Searches for Exotic Particles}

Two salient features--high baryon density and large strangeness 
production--characterize nucleus-nucleus collisions at the AGS. 
A blob of dense nuclear
matter with large strangeness content formed early in the collision could
lead to exotic phenomenon in the subsequent evolution. One possible scenario
proposed by C. Greiner et al.~\cite{Greiner} is the strangeness distillation, in which
Kaons and pions are emitted by the dense blob of matter 
(presumably in the QGP state initially) carrying away
entropy and anti-strange quarks, leaving behind a baryonic dense matter with
a net strange quark content. Strange quark matter~\cite{Witten} might be
formed during the evolution if strange quark matter is stable or metastable.

E864 is an open geometry spectrometer experiment with a high rate
capability. A special trigger for heavy objects was used to enhance the
sensitivity of the experiment for rare objects. Details of the experiment
can be found in reference~\cite{E864spec}. The experiment sampled approximately 
$50$x$10^{9}$
central Au+Pb collisions; a sensitivity of a few times $10^{-9}$ per central
Au+Pb collision for stable ($\tau >50$ ns) strange quark matter production
has been achieved with a $90\%$ confidence level~\cite{E864SQM}. E864 has also measured
light ion production up to elements $^{7}Li$ and $^{7}Be$~\cite{E864Ion}. Existing
coalescence models cannot explain the yield of these elements in a satisfactory framework.

The large $\Lambda $ yield in central nucleus-nucleus collisions provides a
unique channel for H dibaryon (uuddss)~\cite{Jaffe} formation via coalescence of two 
$\Lambda ^{\prime }s.$ The coalescence yield of H dibaryon depends
sensitively on many factors which cannot be reliably estimated. A production
rate as high as $10^{-2}$ has been predicted for central nucleus-nucleus
collisions at the AGS~\cite{Dover}. E896 was designed to search for H dibaryon through
its weak decay channels, particularly H$\rightarrow \Sigma ^{-}p$ where 
$\Sigma ^{-}\rightarrow n\pi ^{-}$. E896 collected about 80 million events of
central Au+Au collision during a four-week running in the spring of 1998.
Data analysis for the H decay topology is in progress~\cite{E896spec}.

\section{Antihyperon and Antideuteron Yield}

E864 has measured the yield of antiprotons including those originating as 
decay products from
antihyperon decays at $p_{t}$ near zero in Au+Pb collisions with various
centralities. By comparing with an antiproton measurement from E878~\cite{E878} 
which does not
include a significant contribution from antihyperon decays, 
E864 inferred the antihyperon to
antiproton ratios as a function of centrality presented in 
Figure~\ref{fig:hratio}~\cite{E864Ratio}. In central
Au+Pb collisions the inferred antihyperon to antiproton ratio is about 3.5.
A recent measurement with very limited statistics from E917 is consistent with
a large ratio~\cite{E917Ratio}. Previously E859 reported a ratio of about 2 in central Si+Au
collisions~\cite{E859Ratio}. Existing models (RQMD and ARC, e.g.) cannot account for the large
antihyperon to antiproton ratio. A large antihyperon to antiproton ratio has been
considered a signature of the QGP formation~\cite{Strange}. 
However, we do not know enough details of the
antihyperon dynamics during the evolution of the collision to draw a firm
conclusion.

\begin{figure}[htb]
   \centering
   \epsfxsize=8cm
   \epsfysize=8cm
   \leavevmode
   \epsffile{antih_p_ratio.eps} 
\caption[fig:hratio]{Ratio of antihyperon to antiproton as a function of collision 
centralities for Au+Pb collisions. Arrows indicate the lower limit of the ratio 
at a $98\%$ confidence level.}
\label{fig:hratio}
\end{figure}

\begin{figure}[htb]
   \centering
   \epsfxsize=8cm
   \epsfysize=8cm
   \leavevmode
   \epsffile{dbar_yield.eps} 
\caption[fig:e864dbar]{Invariant yield of antiproton and antideuteron per central 
Au+Pb collisions from E864. The arrow indicates a previously published upper limit on
antideuteron production.}
\label{fig:e864dbar}
\end{figure}


Antideuteron production is a unique phenomenon at the AGS because an individual
nucleon-nucleon collision at the AGS is below the threshold for
deuteron-antideuteron pair production. Thus antideuterons can only be formed
through coalescence of antinucleons which must be produced from different
nucleon-nucleon collisions in a nucleus-nucleus collision.  The yield of
antideuterons is very sensitive to collision dynamics. E864 has observed
about 20 antideuterons from approximately $10$x$10^{9}$ central Au+Pb
collisions~\cite{E864dbar}. Figure~\ref{fig:e864dbar} 
shows the antideuteron invariant multiplicity in
comparison with that of antiprotons. There is a difference of 5 orders of
magnitude in the antiproton and antideuteron production. A coalescence parameter 
$B_{2}=y_{inv}^{\overline{d}}/(y_{inv}^{\overline{p}}y_{inv}^{\overline{n}})$
has been calculated for antideuterons, where $y_{inv}$ measures invariant
yield ($y_{inv}=d^{2}n/(2\pi p_{t}dydp_{t})$). Assuming the same yield for
antiprotons and antineutrons and correcting for antiprotons from decays of antihyperons
which cannot participate in the coalescence process, an antideuteron
coalescence parameter approximately equal to that for deuteron has been
obtained. Thus there is no significant difference in the formation process
once nucleons or antinucleons are assembled together.

\section{Reflections on the AGS Heavy Ion Physics Program}

We have had a vigorous heavy ion research program at the AGS. The new
national facility at BNL, Relativistic Heavy Ion Collider (RHIC), 
represents a major scientific leap from current SPS and AGS programs: a
factor of ten increase in CM energy from the SPS, and forty from the AGS.
The commencement of nucleus-nucleus collisions at RHIC will usher in an
exciting new era for relativistic heavy ion physics. However, heavy ion
physics at the AGS represents a unique high baryon density regime. Many
intriguing topics have not yet been fully explored. Some of my favorite topics
are: 1) search for $\Omega $ production -- the AGS is well below the 
$\Omega $ pair production threshold and a measurement of $\Omega $ particle
would signal quark-level clustering from the QGP phase; 2) search for
metastable strange quark matter with a hyperon decay lifetime on the order of 
$10^{-10}$ second; 3) sensitive search for the H and other dibaryons; 4) systematic
measurements of antihyperons and antiprotons over a large phase space; and 5)
vector meson production through dilepton channels.  

\section{Acknowledgment}

I gratefully acknowledge enlightening discussions with Jack Sandweiss, Richard Majka, 
Charles Whitten and Sean Kelly over the years on physics topics
that I have reported here. Fuqiang Wang and Chellis Chasman provided E866 data for my
presentation.
I thank the organizers of this CRIS2000 conference
for their financial support and for providing a stimulating environment
during the conference at Cantania.  This work is supported by the US
Department of Energy.

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\end{thebibliography}

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