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\begin{document}
\title{Experimental constraints on fragmentation functions for strange hadrons in \pp collisions
at $\sqrt{s}=200$ GeV at RHIC}

%***********************************************************************
% AUTHORS INFORMATION AREA
%***********************************************************************
\author{Mark Heinz
\vspace{.2cm}\\
Yale University, Physics Department, WNSL,\\
272 Whitney Ave, New Haven, CT 06520, USA }

\maketitle

\section{Summary of Talk}
Perturbative QCD has proven successful in describing inclusive
hadron production in elementary collisions. Within the theory's
range of applicability, calculations at next-to-leading order (NLO)
have produced accurate calculations for the transverse momentum
spectra of charged hadrons in different collision systems and energy
scales, which has lead to claims of their universality
\cite{universality}. With the new mid-rapidity \pp data at \sqsRhic
GeV collected at RHIC, comparisons to pQCD can now be extended to
identified strange baryons and mesons \cite{STAR:strange}.

In the last 5 years significant improvements have been made in the
field of NLO fragmentation function (FF) parameterizations. Several
groups have updated their parameterizations to include not just \ee
but also (SI)DIS and now \pp data, therefore improving the
constraints on the parameters in the FF. In particular the
gluon-to-hadron FF was never well constrained by \ee and DIS data
since the probability of gluon-jets was low. Conversely, at RHIC we
are probing a very gluon-jet rich parton stage and the majority of
final states are produced by gluons. In particular baryons are
dominated by gluon fragmentation. In fact, according to AKK-FF
protons below \pt 10 GeV/c are produced to 90\% from gluons and only
10\% from quarks. On the other hand pions are produced in equal
parts by gluons and quarks.

Experimentally this fact can be exploited by computing the
baryon-to-meson ratios and fitting them to LO/NLO calculations.
Results by the STAR collaboration confirm that LO models are not
able to accurately describe this ratio
\cite{STAR:protons,STAR:strange} and underpredict the amount of
baryons at low \pt. For strange baryon production (\lam) first NLO
predictions equally fail to describe the data \cite{DSV}. In an
attempt to solve this shortcoming one group (AKK) \cite{AKK}
adjusted the gluon FF until it fit the data. They found the best
result for $D_{g}^{\lam}= D_{g}^{p}/3$. Another group (DSS) has for
the first time performed seperate fits to protons and anti-protons
based on STAR data \cite{DSS2}. They also publish FF for pions,
kaons and charged hadrons from a global analysis \cite{DSS1}.

Another subject which did not receive sufficient attention up until
recently are error estimations on the FF parameterizations. One
group (HKNS) \cite{HKNS} has now published a global (hessian) error
analysis for charged hadrons and finds that the 4 most ubiqitious FF
agree within errors. However this analysis does not include \pp
~data.

The STAR experiment has also measured identified particle spectra in
terms of \mt (=$\sqrt{\pt^{2}+m^{2}}$) which is motivated by a some
earlier papers \cite{Mtscaling}. After arbitrary normalization of
the different particle species, this representation clearly shows
different shapes of the spectra for the baryons and mesons for $\mt
> 2$ GeV/c. Interestingly, a simulation of the same observable with
PYTHIA (LO pQCD) shows a similar feature when selecting gluon-jet
enriched events \cite{Pythia}.

Finally, efforts at RHIC are now being focused in order to extract
medium modified FF. Several approaches are being pursued in
parallel, such as Di-hadron correlations, 3-particle correlations
and also full jet reconstruction.





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\begin{thebibliography}{99}

%universality paper
\bibitem{universality}B.~A.~Kniehl, G.~Kramer and B.~Potter, {\it Nucl. Phys.} B {\bf 597}(2001) 337


%----STAR references
\bibitem{STAR:protons}J.~Adams et al. (STAR collaboration), {\it  Phys. Lett.} B {\bf 637},
(2006) 161
\bibitem{STAR:strange}B.~Abelev et al. (STAR collaboration), {\it  Phys. Rev.} C {\bf 75},
(2007) 064901

%----section NLO
\bibitem{DSV}D.~de Florian, M.~Stratmann, W.~Vogelsang, {\it Phys. Rev.} D
{\bf 57}, (1998) 5811

\bibitem{AKK}S.~Albino et al., {\it Nucl. Phys.} B{\bf 734}, (2006) 50

\bibitem{DSS2}D.~de Florian, M.~Stratmann, M.~Sassot, {\it Phys. Rev.} D
{\bf 76}, (2007) 074033
\bibitem{DSS1}D.~de Florian, M.~Stratmann, M.~Sassot, {\it Phys. Rev.} D
{\bf 75}, (2007) 114010

\bibitem{HKNS}S.~Hirai et al., {\it Phys. Rev.} D{\bf 75}, (2007) 094009

%-----Mt scaling
\bibitem{Mtscaling}
  G.~Gatof and C.Y.~Wong {\it Phys. Rev.} D {\bf 46}, (1992) 997

\bibitem{Pythia}T.~Sjostrand and P.~Z.~Skands, {\it Eur. Phys. J.} C {\bf 39},
(2005) 129

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