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% declarations for front matter
\title{Kaon-nucleon/nuclei interaction studies by kaonic atoms measurements: 
the SIDDHARTA experiment at DA$\Phi$NE}

\author{
        M.Bazzi\address[INFN]{INFN, Laboratori Nazionali di Frascati, Frascati
           (Roma), Italy },
       G.Beer\address[Canada]{Dep. of Phys. and Astro., Univ. of Victoria,
          Victoria B.C., Canada},
       L.Bombelli\address[Milano]{Politecnico di Milano, Sez. di Elettronica,
         Milano, Italy},
       A. M. Bragadireanu\addressmark[INFN],\address[IFIN]{IFIN-HH, Magurele,
         Bucharest, Romania },
       M. Cargnelli\address[Vienna]{Stefan-Meyer-Institut f\"{u}r subatomare
         Physik, Vienna, Austria},
       G. Corradi\addressmark[INFN],
       C. Curceanu (Petrascu)\addressmark[INFN],
       A. d'Uffizi\addressmark[INFN],
       C. Fiorini\addressmark[Milano],
       T. Frizzi\addressmark[Milano],
       F. Ghio\address[RomaI]{INFN Sez. di Roma I and Inst. Superiore di
         Sanita, Roma, Italy},
       B. Girolami\addressmark[RomaI],
      C. Guaraldo\addressmark[INFN],
      R.S. Hayano\address[Tokyo]{Univ. of Tokyo, Tokyo, Japan},
      M.~Iliescu\addressmark[INFN], \addressmark[IFIN],
      T. Ishiwatari\addressmark[Vienna],
      M. Iwasaki\address[Riken]{RIKEN, The Inst. of Phys. and Chem. Research, Saitama, Japan },
      P. Kienle\addressmark[Vienna],\address[TUM]{Tech. Univ. M\"{u}nchen, Physik Dep., Garching,
        Germany},
      P. Levi Sandri\addressmark[INFN],
      A. Longoni\addressmark[Milano],
      V. Lucherini\addressmark[INFN],
      J.~Marton\addressmark[Vienna],
      S. Okada\addressmark[INFN],
      D. Pietreanu\addressmark[INFN],
      T. Ponta\addressmark[IFIN],
      A. Rizzo\addressmark[INFN],
      A. Romero Vidal\addressmark[INFN],
      A. Scordo\addressmark[INFN],
      H. Shi\addressmark[Tokyo],
      D.L.~Sirghi\addressmark[INFN],\addressmark[IFIN],\thanks{Corresponding
        author: Tel : 0039 0694032511; fax: 00390694032559; e-mail: sirghi@lnf.infn.it},
      F. Sirghi\addressmark[INFN],\addressmark[IFIN],
      H. Tatsuno\addressmark[Tokyo],
      A. Tudorache\addressmark[IFIN],
      V. Tudorache\addressmark[IFIN],
      O. Vazquez Doce\addressmark[INFN],
      E.~ Wiedmann\addressmark[Vienna],
      J. Zmeskal\addressmark[Vienna]}









    
 




       
\begin{document}

\begin{abstract}



The SIDDHARTA experiment (SIlicon Drift Detector for Hadronic Atom Research by Timing Application)
 had the aim to perform a precise measurement of K-series
kaonic hydrogen x-rays and the first-ever measurement of the kaonic
deuterium x-rays to determine the strong-interaction energy-level shifts
and widths of the lowest lying atomic states.

These measurements offer a unique possibility to precisely determine the
isospin-dependent kaon-nucleon(KN) scattering lengths which are directly
connected with the physics of the KN interaction.

The experiment combined the excellent low-energy kaon beam generated at
DA$\Phi$NE, allowing to use gaseous targets, with excellent fast X-rays
detectors: Silicon Drift Detectors. SIDDHARTA was installed on DA$\Phi$NE in
autumn 2008 and took data till late 2009.

Apart of the kaonic hydrogen and kaonic deuterium measurements, we have
performed the kaonic helium transitions to the 2{\em p} level (L-lines)
measurements: for the first time in a gaseous target for kaonic helium 4 and for the
first time ever for kaonic  helium 3.
In this paper, an overview of this experiment and 
preliminary results are presented, together with future plans.



\vspace{1pc}
\end{abstract}

% typeset front matter (including abstract)
\maketitle

\section{The SIDDHARTA scientific case}

The precision measurement of kaonic atoms at the  DA$\Phi$NE accelerator of the
LNF-INFN Laboratories was performed in the framework of the SIDDHARTA
international Collaboration. The SIDDHARTA experiment continued and deepen
the successful line, initiated by the DEAR experiment \cite{Beer05}, in performing precision
measurements of X-ray transitions in exotic (kaonic) atoms at DA$\Phi$NE .

The scientific aim of the experiment is a precision determination of the
isospin-dependent antikaon-nucleon scattering lengths, through few eV
precision measurement of $K_{\alpha}$ line shift ($\epsilon$) and width
($\Gamma$) in kaonic hydrogen, and a similar, first
time measurement of kaonic deuterium. SIDDHARTA measures the X-ray transitions
occurring in the cascade processes of kaonic atoms.



% A kaonic atoms is formed when a negative kaon enters in the target, it lose its kinetic energy through
%(by) the ionization and excitation of the atoms
% and molecules of the medium and is eventually captured into an excited 
%atomic orbit around the nucleus  by replacing the electron. 
%
%The kaon then cascades down through a series of atomic states to the lower states, via different cascade process (Auger effect, Coulomb deexcitation, scattering).

%When a low-n state with a small angular momentum is reached, the strong interaction with the nucleus comes into play and the kaon it is absorbed by the proton through the strong interaction with the nucleus. At same final n-value, nuclear absorption will dominate and the x-ray series will terminate.
The strong interaction between the kaon (hadron) and the nucleus causes a
shift ($\epsilon$) in the energy of
 the last energy levels from their purely electromagnetic values and a
 finite lifetime of the state - corresponding to an increase in the observed
 level width ($\Gamma$).


For kaonic hydrogen and kaonic deuterium, the K-series transitions are 
of main experimental interest, since they are the only ones affected by the
strong interaction; for higher n-states the effect is very small.
%Ka line is more important than the other K-series lines in determine the level
%shift and width of the 1s state because K are clearly separated from the
%higher K transitions.

The shift($\epsilon$) and the width($\Gamma$) of the 1{\em s} state for kaonic hydrogen  and kaonic
deuterium are related in a
fairly model-independent way to the real and imaginary part of the complex
{\em s}-wave scattering length, $a_{K^{-}p}$ and, respectively,  $a_{K^{-}d}$
\cite{Deser54}.

%This expression is known as the Desser-Trueman formula \cite{Deser54}.A similar relation
%applies to the case of kaonic deuterium and its corresponding length, ak-d.
%The measured scattering lengths are then related to the isospin-dependent
%scattering lengths, a0  and a1:


%The extraction of Ak-n from ak-d requires a more complicated analysis than the
%impulse approximation: higher-order contributions associated with the K-d
%three=body Faddeev equations by the use of potentials, taking into account the
%coupling among the multichanel interactions.

An accurate determination of the K-N isospin-dependent scattering lengths will
place strong constraints on the low-energy K-N dynamics, which in turn
constraints in SU(3) description of chiral symmetry breaking.





\section{The SIDDHARTA experimental setup} 

The precision obtained by the DEAR experiment for the kaonic-hydrogen X-ray
transitions to the 1{\em s} level \cite{Beer05}, was clearly limited by the poor signal to
background ratio (1/70), so another experimental technique had to be developed
to reduce the huge background: large area soft X-ray detectors with good energy and timing resolution, high
 counting rate capability and enough robustness to be operated in the radiation
environment of an accelerator, namely the Silicon Drift Detector(SDD).

The SIDDHARTA experimental setup \cite{Zmeskal08} consists of main three parts: a kaon detector, an X-ray
detection system and a cryogenic target system. 

An overview of the experimental setup is presented in Fig. \ref{setup}.


\begin{figure}[htb]
%\vspace{9pt}
%\framebox[55mm]{\rule[-21mm]{0mm}{43mm}}
\includegraphics[scale=0.7]{setup1.eps}
\caption{The SIDDHARTA setup.}
\label{setup}
\end{figure}







 The whole system
was installed at the  $e^{+}e^{-}$ interaction point of the DA$\Phi$NE collider.
The target cell is  cylindrical, with a radius of 6 cm and a height of 12
cm. It was filled with gaseous hydrogen, at an equivalent of 0.01 of the
liquid hydrogen density.
% The bottom of the vacuum  chamber has a circular window made of Kapton
%foil, with a radius of 6.5 cm, trough which kaons enter in the target cell. An
%energy degrader, installed on the bottom of the vacuum chamber, was adjusted
%in thickness to optimize the maximum number of kaonic hydrogen X-rays.
%It had a step-like shape to compensate 
%the phi boost effect caused by a finite crossing angle of the electron and positron 
 
The kaon detector \cite{Lucherini03} consists of two scintillators installed on the top and
bottom of the beam pipe in the IR, which detects the $K^{+}K^{-}$ signal in
coincidence, using TOF technique. The slow kaon pairs are clearly separated
from the fast minimum ionizing particles (MIPs), due to excellent time
resolution ( $\leq$ 100ps FWHM) and the stability of the clock pulses (380 MHz)
delivered by DA$\Phi$NE.
% A correlation of the time difference on the two
%scintillators is shown in Fig. 2

The kaonic hydrogen  X-rays were detected using recently developed large are
Silicon Drift Detectors (SDD), having an active area of 1 $cm^{2}$ and a thickness
of 450 $\mu$m  \cite{Lechner01}.
After detailed performance tests, 144 SDDs, chips were installed surrounding
the target cell. 
%The SDDs were cooled to a temperature of 170K with a
%stability of +- 0.5 K. 

The SIDDHARTA experimental setup was installed in DA$\Phi$NE accelerator in middle
2008 and started to take data from end 2008 until December 2009. The big part
of the data taking was  dedicated to the hydrogen and deuterium data, the main
goal of the experiment,
 but we have, as well,  performed measurements  of kaonic helium4 and, for
 the first time ever, of kaonic helium3 atoms.
The analysis of these data is ongoing and, in this paper, only preliminary
results are shown. 
%For all these measurement we used the method of the triple coincidence between
%the X-rays which are 
%hitting the SDDs and the K-K+ pairs, emitted back-to-back, form the phi decay, 
%which allows us to obtained a background suppression more than 104 , as is
%clearly seen in Fig. 3.


\section{Kaonic Helium4 measurement }

To test  and optimize our experimental technique, we measured the
L-transitions for kaonic helium4 atoms, in particular the transition 3{\em d}$\rightarrow$
2{\em p}, $L_{\alpha}$ transition. The use of this transition is motivated by
its yield, a factor at least 10 times higher than the kaonic hydrogen
$K_{\alpha}$ one.


Part of the data for kaonic helium4 atoms, was taken  for
about two weeks in January 2009, while a Fe55 source was
installed inside the apparatus. In this way, detailed studies of systematic
of calibration was done.
The result obtained from these  data for the 2{\em p} level  strong interaction shift of the
kaonic helium4 is \cite{Bazzi09}:

\begin{equation}
\label{shift}
 \Delta E = E_{exp}- E_{e.m.}=0 \pm 6(stat) \pm 2(syst) eV
\end{equation}

More kaonic helium4 data were taken during 2009 without the Fe55 source.  
The preliminary X-ray energy spectrum for the new  kaonic helium4 data is shown in  Fig. \ref{khe4}.

\begin{figure}[htb]
%\vspace{9pt}
%\framebox[55mm]{\rule[-21mm]{0mm}{43mm}}
\includegraphics[scale=0.4]{khe4.eps}
\caption{Kaonic helium 4 X-ray spectrum}
\label{khe4}
\end{figure}






The kaonic helium4  $L_{\alpha}$ line at 6.4 keV is clearly seen, together with the
other kaonic helium4  L-lines and background lines.


The kaonic helium has physics-interest in itself: before the KEK E570 \cite{Okada07}
experiment, 
the experimental data were in contradiction with theory (so called ``kaonic helium puzzle''). 
The resultant shift obtained by the SIDDHARTA experiment
\cite{Bazzi09} confirms the result obtained by the E570 group, definitely
solving the ``kaonic helium puzzle''.
Presently, the data analysis of the overall kaonic helium4 data is ongoing, in
order to get a better precision and information about yields of L-lines.

 



\section{Kaonic Helium3 measurements}

During the SIDDHARTA data taking, we
measured also  the L-transitions for kaonic helium3, for which no experimental data existed until now. 

The preliminary X-ray energy spectrum for kaonic helium3 data,  is presented
in Fig. \ref{khe3}

\begin{figure}[htb]
%\vspace{9pt}
%\framebox[55mm]{\rule[-21mm]{0mm}{43mm}}
\includegraphics[scale=0.4]{khe3.eps}
\caption{Kaonic helium3 X-ray spectrum}
\label{khe3}
\end{figure}

The kaonic helium3  $L_{\alpha}$ line at 6.2 keV is clearly seen, together with the
other kaonic helium3  L-lines and background lines.

The analysis is undergoing and will be soon published.

\section{Kaonic Hydrogen  measurements}

The main part of the data taking campaign was dedicated to the Hydrogen target. 

Figure \ref{kh} shows the K-H preliminary X-ray energy spectrum. The two peaks at 
4.5 and 5.5 keV correspond to: the Ti fluorescence lines, coming from the Ti foil, 
which is activated by the kaons and background, and to the Kaonic-Carbon 6
$\rightarrow$ 5 (at 5.5 keV ) X-ray transitions, produced at the target walls. 

 \begin{figure}[htb]
%\vspace{9pt}
%\framebox[55mm]{\rule[-21mm]{0mm}{43mm}}
\includegraphics[scale=0.4]{kh_prel.eps}
\caption{Kaonic hydrogen X-ray spectrum.}
\label{kh}
\end{figure}


The other two broad peaks are the kaonic hydrogen $K_{\alpha}$line (5.8-6.4keV region) 
and the $K_{complex}$ ($K_{\beta}$,$K_{\gamma}$ ... ) lines (7.0 - 9.0 keV region). 
For fitting the kaonic hydrogen signal a Voigt function is used for each X-ray 
transition ($K_{\alpha}$,$K_{\beta}$,$K_{\gamma}$... ) where the energy level shift with respect to the electromagnetic 
value is common to all lines, and the same for the width.

The analysis for kaonic hydrogen is undergoing, with the aim to 
arrive at a more precise measurement than the DEAR experiment (or similar).
 %The very 
%preliminary results show a shift about 280 eV and a width about 500 eV with a 
%statistical precision better than 30 eV for the shift, and 80 eV for the width, which 
%is an improvement of the measurement with respect to DEAR and KEK. The systematic
%errors are under evaluation.

In the kaonic deuterium case, the accumulated statistics is not enough to distinguish 
a clear signal. The data analysis is ongoing. 


\section{Conclusions}
The SIDDHARTA experiment performed in 2009 precision measurements of kaonic 
helium (3 and 4)and kaonic hydrogen. It performed as well the first ever exploratory 
measurement of the kaonic deuterium. 
DA$\Phi$NE proved to be an ideal �kaonic atoms� factory, representing an unique opportunity to study in a complete way
the kaon-nucleon/nuclei physics at low energy.
Presently, data analyses are ongoing. In parallel plans for an upgrade of the 
experimental setup for an enriched scientific case for the near future are being 
considered. 

\section{Acknowledgements}
We acknowledge the support of the European Community-Research Infrastructure 
Integrating Activity �Study of Strongly Interacting Matter� (acronym HadronPhysics2, 
Grant Agreement n. 227431) under the Seventh Framework Programme 
of EU. 
We thank the  DA$\Phi$NE accelerator division for the excellent working conditions 
provided to SIDDHARTA and for their friendly collaboration. 



\begin{thebibliography}{9}

\bibitem{Beer05} G. Beer et al., Phys. rev. Lett. 94,  (2005)212302.
\bibitem{Deser54} S.Deser et al., Phys. Rev. 98(1954) 774 ; T. L. Truemann, Nucl. Phys. 26
 (1961) 57; A. Deloff, phys. Rev. C 13 (1976) 730.
\bibitem{Zmeskal08} J. Zmeskal, Progr. Part. Nucl. Phys. 61 (2008) 512.
\bibitem{Lucherini03} V. Lucherini, et al., Nucl. Instrum. Methods A 496 (2003) 315.
\bibitem{Lechner01} P. Lechner, et al., Nucl. Instrum. Methods A 458 (2001) 281.
%\bibitem{Fiorini06} C. Fiorini, et al., Nucl. Instrum. Methods A 568 (2006) 322.
\bibitem{Bazzi09} M. Bazzi, et al., Physics Letters B 681 (2009) 310-314.
\bibitem{Okada07} S. Okada, et al., Physics Letters B 653 (2007) 387.


\end{thebibliography}


\end{document}


