%
% ****** maiksamp.tex 29.11.2001 (ENGLISH VERSION) ******
%
\RequirePackage{lineno}
\documentclass[
aps,%
12pt,%
final,%
notitlepage,%
oneside,%
onecolumn,%
nobibnotes,%
nofootinbib,% 
superscriptaddress,%
noshowpacs,%
centertags]%
{revtex4}

\usepackage[caption=false]{subfig}

\begin{document}
\linenumbers
\selectlanguage{english}

\title{Status of the MPD experiment at NICA}%

\author{\firstname{P.}~\surname{Parfenov}}
\email{pparfenov@jinr.int}
\affiliation{%
JINR, NRNU MEPhI
}%
\collaboration{for the MPD Collaboration}

\begin{abstract}
\textbf{Abstract} --- The Multi-Purpose Detector (MPD) at the NICA facility in Dubna is undergoing its final phase of assembly and commissioning, with the first beams expected at the end of 2026. Covering the collision energy range $\sqrt{s_{NN}}=2.4$--$3.5$~GeV in fixed-target mode and $4$--$11$~GeV in collider mode, MPD will probe the region of the QCD phase diagram where a first-order phase transition and critical point may occur.
Following the first NICA run in 2026, preparations for MPD data taking are ongoing. This report summarizes the current project status and recent feasibility studies of MPD physics performance.
\end{abstract}

\maketitle

\section{Introduction}
Relativistic heavy-ion collisions provide a unique tool for recreating, under controlled
conditions, strongly interacting matter under extreme temperature and density. The conditions
similar to those that existed in the early Universe microseconds after the Big Bang are reached
in the collision-energy range available at the Large Hadron Collider (LHC) and at the top
energies of the Relativistic Heavy Ion Collider (RHIC). At sufficiently high temperature and
energy density, hadronic matter is expected to undergo a cross-over phase transition into a deconfined state of quarks and
gluons, commonly referred to as quark--gluon matter (QGM). Experiments at RHIC and the LHC
have established key properties of this strongly coupled matter, primarily in the region of the
QCD phase diagram characterized by low net-baryon density.
The NICA accelerator complex is designed to explore a distinct sector of the QCD phase diagram,
corresponding to high baryon density and moderate temperatures~\cite{Kekelidze:2018fvh,Cleymans:2010pw}.
Such baryon density conditions are relevant to the matter in neutron star mergers.
Some theoretical models predict that this region may host a first-order phase transition and a critical end-point (CEP).
The detailed study of heavy ion collisions at NICA energies will provide an insight into the properties of nuclear matter under extreme conditions, the transition from hadronic matter to QGM, the collective dynamics of strongly interacting matter, and the restoration of chiral symmetry, a fundamental property of QCD.





\section{Nuclotron-based Ion Collider facility (NICA)}

The NICA accelerator complex, located at the Veksler-Baldin Laboratory
for High Energy Physics of JINR, includes a heavy-ion source, linear
accelerator, booster synchrotron, Nuclotron superconducting synchrotron,
and two collider rings~\cite{Kekelidze:2018fvh}. The Nuclotron provides
heavy-ion beams up to 6\textit{A}~GeV for isospin-symmetric nuclei. The
first stage of the heavy-ion program uses xenon beams, with Xe+W fixed-target
and Xe+Xe collider operation~\cite{MPD:2022qhn,MPD:2025jzd,Parfenov:2025ulm}.
In 2026, the first NICA run achieved two stable xenon beams in the collider
rings.

Extracted Nuclotron beams are used by the BM@N fixed-target experiment
~\cite{Afanasiev:2023opv}, while the NICA collider is designed for protons,
light nuclei, and heavy ions up to xenon and bismuth. 
The main experimental areas are MPD for heavy-ion studies~\cite{MPD:2022qhn,MPD:2025jzd,Golovatyuk:2019rkb} 
and SPD for polarized proton and deuteron spin physics~\cite{Abramov:2021vtu}.
While the NICA collider operates in the energy range $\sqrt{s_{NN}}$~=~4--11~GeV for heavy ions, the MPD experiment
can also reach $\sqrt{s_{NN}}$~=~2.4--3.5~GeV in fixed-target mode.






\section{Multi-Purpose Detector (MPD) experiment at NICA}

The MPD is a 4$\pi$ spectrometer for heavy-ion studies at NICA in the
high baryon-density region. Its first-stage configuration includes the TPC,
TOF, ECal, FHCal, FFD, and a superconducting solenoid with an iron return yoke,
providing tracking, particle identification, calorimetry, triggering, and
event-plane measurements for the MPD physics program~\cite{MPD:2022qhn,MPD:2025jzd,Parfenov:2025ulm}.

MPD can also operate in the fixed-target mode using a thin wire target of about
100~$\mu$m diameter, shifted by about $1.5$~cm from the central beam line near
the MPD barrel edge~\cite{Parfenov:2025ojv}. This mode covers
$\sqrt{s_{NN}}=2.4$--$3.5$~GeV and can compensate for early collider luminosity
limitations, with rates mainly constrained by the data-acquisition bandwidth.

The first-stage MPD subsystems are now in final integration, commissioning, and
calibration for initial physics running.





\section{Highlights from the feasibility studies at the MPD experiment}

Recent feasibility studies demonstrate the strong potential of MPD for early
heavy-ion measurements at NICA. Collider-mode simulations of Bi+Bi collisions at
$\sqrt{s_{NN}}=9.2$~GeV show that the first-stage detector can reconstruct and
identify the main physics probes with the required performance~\cite{MPD:2025jzd}.
The studies use modern event generators, GEANT4 transport through the full MPD
geometry, and standard reconstruction~\cite{GEANT4:2002zbu,Allison:2006ve}.
Detailed collider mode results are given in~\cite{MPD:2022qhn,MPD:2025jzd}. The
following highlights focus on fixed-target feasibility studies.

Fig.~\ref{fig:ChHadrons} shows $p_\mathrm{T}$ spectra of identified $\pi^+$,
$K^+$, and protons in Xe~+~W collisions at $\sqrt{s_\mathrm{NN}}=2.87$~GeV.
Using TPC and TOF information, the reconstructed spectra agree well with UrQMD
predictions~\cite{Bass:1998ca,Bleicher:1999xi}, confirming the charged-particle
tracking and identification performance.

Figure~\ref{fig:Flow} presents the directed ($v_1$) and elliptic ($v_2$) flow of $\pi^+$ mesons and
protons in Xe~+~W and Xe~+~Xe collisions at $T=2.5A$~GeV
($\sqrt{s_\mathrm{NN}}=2.87$~GeV). The symmetric Xe~+~Xe system provides a
reference for detector effects. Reconstructed $v_1$ and $v_2$ values agree with
the UrQMD input, demonstrating the feasibility of flow measurements in
fixed-target mode.

Figure~\ref{fig:WeakDecays} shows $p_\mathrm{T}$ spectra of weakly decaying
$\Lambda$ hyperons and $K^0_S$ mesons in different centrality intervals. The
collider-mode reconstruction procedure~\cite{MPD:2025jzd}, adapted to the
fixed-target geometry, preserves both spectral shapes and normalizations.

Figure~\ref{fig:PolLambda} shows the invariant-mass fit and centrality dependence
of global $\Lambda$ polarization in Xe~+~W collisions at
$\sqrt{s_\mathrm{NN}}=2.87$~GeV. A custom afterburner was used to introduce the
polarization signal, and the generalized invariant-mass fit method was applied
as described in~\cite{Troshin:2025fko}. The reconstructed values reproduce the
generated $P_\Lambda$ signal.

Figure~\ref{fig:Resonances} shows $p_\mathrm{T}$ spectra of the $K^*(892)^0$ and
$\phi(1020)$ resonances. The agreement between reconstructed spectra and UrQMD
predictions indicates that the yield of the resonance particles can be measured at low
$p_\mathrm{T}$ in different centrality intervals.


\section{Summary}

The MPD experiment at NICA is close to the first data taking with
the first-stage detector subsystems in final integration, commissioning, and
calibration. The NICA energy range will provide access to the high-baryon-density
region of the QCD phase diagram, where the properties of strongly interacting
matter, possible signs of a first-order phase transition, and the critical
end-point can be investigated.

Recent feasibility studies confirm that MPD can perform key measurements in
both collider and fixed-target modes. In fixed-target Xe~+~W and Xe~+~Xe
collisions, realistic simulations demonstrate reliable reconstruction of
identified charged hadrons, collective flow, weakly decaying particles, global
$\Lambda$ polarization, and hadronic resonances. The good agreement between
reconstructed observables and model input indicates that MPD is well prepared
for the initial heavy-ion physics program at NICA.




\begin{acknowledgments}
The work was funded by the Ministry of Science and Higher Education of the Russian Federation, Project "Studying physical phenomena in the micro- and macro-world to develop future technologies" FSWU-2026-0010.
\end{acknowledgments}

%
% Bibliography
%
\bibliographystyle{maik}
\bibliography{references}

\newpage

\textbf{Captions to the figures}
\begin{enumerate}
  \item Transverse-momentum spectra of identified charged hadrons, $\pi^+$ (a), $K^+$ (b), and protons (c), reconstructed in Xe~+~W fixed-target collisions at $\sqrt{s_\mathrm{NN}}=2.87$~GeV (markers) and compared with UrQMD model predictions (lines).
  \item Directed flow $v_1$ (a--b) and elliptic flow $v_2$ (c--d) of $\pi^+$ mesons (a, c) and protons (b, d) in Xe~+~W (blue) and Xe~+~Xe (red) collisions at $T=2.5A$~GeV ($\sqrt{s_\mathrm{NN}}=2.87$~GeV). Results from realistic reconstruction (markers) are compared with UrQMD predictions (lines).
  \item Transverse-momentum spectra of weakly decaying particles, $\Lambda$ hyperons and $K^0_S$ mesons, reconstructed in different centrality intervals (markers) and compared with generated distributions (lines).
  \item Global $\Lambda$ polarization in Xe~+~W collisions at $\sqrt{s_\mathrm{NN}}=2.87$~GeV: invariant-mass fit example (a) and centrality dependence of $P_\Lambda$ (b). The measured polarization from realistically reconstructed data (markers) is compared with generated data (lines).
  \item Transverse-momentum spectra of $K^*(892)^0$ and $\phi(1020)$ resonances reconstructed in different centrality intervals and compared with UrQMD model predictions. Results from reconstructed data are shown with markers, while the generated signal is shown with lines.
\end{enumerate}

%
\newpage
%
% Figures
%
\begin{figure}[!tbp]
  \centering
  \subfloat[]{
    \centering
	  \includegraphics[width=0.3\textwidth]{./Figures/ch_hadron_prod_Piplus_pt_centrality0.pdf}
  }
  \hfill
  \subfloat[]{
    \centering
    \includegraphics[width=0.3\textwidth]{./Figures/ch_hadron_prod_Kplus_pt_centrality0.pdf}
  }
  \hfill
  \subfloat[]{
    \centering
    \includegraphics[width=0.3\textwidth]{./Figures/ch_hadron_prod_protons_pt_centrality0.pdf}
  }
  \caption{}
  \label{fig:ChHadrons}
\end{figure}

\begin{figure}[!tbp]
\vspace{-5mm}
  \centering
  \subfloat[]{
    \centering
	  \includegraphics[width=0.4\textwidth]{./Figures/Flow_v1_pi_pos_y.png}
  }
  \hfill
  \subfloat[]{
    \centering
    \includegraphics[width=0.4\textwidth]{./Figures/Flow_v1_proton_y.png}
  }
  \hfill
  \subfloat[]{
    \centering
    \includegraphics[width=0.4\textwidth]{./Figures/Flow_v2_pi_pos_pT.png}
  }
  \hfill
  \subfloat[]{
    \centering
    \includegraphics[width=0.4\textwidth]{./Figures/Flow_v2_proton_pT.png}
  }
  \caption{}
  \label{fig:Flow}
\end{figure}

\begin{figure}[!tbp]
  \centering
  \subfloat[]{
    \centering
	  \includegraphics[width=0.4\textwidth]{./Figures/WeakDecays_L0_pTspectr_paper_centr.pdf}
  }
  \hfill
  \subfloat[]{
    \centering
    \includegraphics[width=0.4\textwidth]{./Figures/WeakDecays_K0_pTspectr_paper_centr.pdf}
  }
  \caption{}
  \label{fig:WeakDecays}
\end{figure}

\begin{figure}[!tbp]
  \centering
  \subfloat[]{
    \centering
	  \includegraphics[width=0.4\textwidth]{./Figures/Polarization_lambda_Minv.pdf}
  }
  \hfill
  \subfloat[]{
    \centering
    \includegraphics[width=0.4\textwidth]{./Figures/Polarization_lambda_cent.pdf}
  }
  \caption{}
  \label{fig:PolLambda}
\end{figure}

\begin{figure}[!tbp]
  \centering
  \subfloat[]{
    \centering
	  \includegraphics[width=0.4\textwidth]{./Figures/Resonances_Kstar_Closure_Test.pdf}
  }
  \hfill
  \subfloat[]{
    \centering
    \includegraphics[width=0.4\textwidth]{./Figures/Resonances_Phi_Closure_Test.pdf}
  }
  \caption{}
  \label{fig:Resonances}
\end{figure}


\end{document}
%
% ****** End of file maiksamp.tex (English version) ******

