
\documentclass[nocopyright]{cimento}
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\title{Monitored neutrino beams: ENUBET design and prototype performance}
\author{L.~Halić\from{ins:x} on behalf of nuSCOPE collaboration
        %\atque
%T.~Moore\from{ins:y}\thanks{Any footnote to author.}
}
\instlist{\inst{ins:x} Center of Excellence for Advanced Materials and Sensing Devices, Ruder Bo\v{s}kovi\'c Institute, 10000 Zagreb, Croatia
}
%% When only one author is present, please do not use the command \from{} near the author name.





\begin{document}

\maketitle

\begin{abstract}
Fundamental neutrino physics experiments at GeV scale struggle with large uncertainties in neutrino interaction cross sections, especially when statistical uncertainty for next generation experiments will no longer be the dominant uncertainty. The nuSCOPE collaboration, born from merger of ENUBET and NuTag collaborations, is trying to tackle this issue by designing a beamline which leverages two novel technologies, those of monitored (ENUBET) and tagged (NuTag) neutrino beams. This beamline enables flux monitoring at the percent level and provides neutrino energy measurement independent of final state particle reconstruction at the detector. nuSCOPE collaboration is exploring the potential implementation of such a beamline at CERN. This poster focuses on the work of the ENUBET collaboration and the monitoring of the charged leptons produced in the decay tunnel with the goal of estimating the neutrino flux. An instrumented decay tunnel prototype of ENUBET design was built and its performance tested multiple times using a charged-particle beam at the CERN PS accelerator.
\end{abstract}

\section{Introduction}
With the recent advancements in beam power and detector masses, future neutrino oscillation experiments (DUNE \cite{ref:dune}, Hyper-Kamiokande \cite{ref:hyperk}, ESSnuSB \cite{ref:essnusb}) are no longer limited by the statistical, but by the systematic uncertainty. The main sources of systematic uncertainties for next-generation oscillation experiments are related to the modeling of neutrino interactions with matter \cite{ref:nustec}. Measuring neutrino interaction cross sections using conventional neutrino beams is extremely difficult, mainly due to the poor knowledge of the initial neutrino flux.

In an attempt to solve these issue, nuSCOPE collaboration \cite{ref:nuscope} was formed, with the aim to construct a neutrino interaction cross section measurement experiment with a beamline that takes advantage of two different novel techniques. First technique comes from the NuTag project \cite{ref:nutag}. It is a tagged neutrino beam, where a neutrino detected in the detector is uniquely associated with a meson decay in the decay tunnel. In a two body decay production of muon neutrinos, this technique allows for an unprecedented energy resolution (better than $1$\%) for the incident neutrino, as opposed to the standard energy reconstruction using final state particles, which are highly dependent on poorly constrained nuclear models. The other technique exploited by nuSCOPE is a monitored neutrino beam, whose design follows the ENUBET \cite{ref:enubet} design of a monitored beam. ENUBET's design uses a momentum selected meson beam (using a series of dipoles and quadrupoles) with an instrumented decay tunnel to observe large-angle electrons and positrons created in pion and kaon decays. Additionally, it is able to monitor the outgoing muons as well by instrumenting the hadron dump. By counting the number and measuring the energies of electrons, positrons and (anti)muons it is able to reduce the uncertainty on the initial neutrino flux down to 1\% for both electron and muon neutrinos. This proceeding will focus on the ENUBET's monitored beam design and performance.

\section{nuSCOPE's beamline design}
The key component of the nuSCOPE beamline is a fully static focusing system of the secondary mesons. The advantage of such a system compared to the standard horn-based design is that it allows for a much longer proton extraction time which is a key ingredient for a working tagged and monitored neutrino beam. This lowers the intensity of secondary mesons in the decay tunnel which in turn reduces the possibility of the pile-up at the instrumented calorimeter and meson trackers. This design selects a quasi-monoenergetic beam of mesons (mainly pions and kaons), which produces a neutrino beam with two well separated populations coming from the two types of mesons. The incident protons of $400$ GeV/c hit a graphite target, while $8.5$ GeV/c charged mesons are then focused with a $5-10$\%  momentum bite. The target design is optimized to maximize the kaon and pion yield. Similarly, the transfer line length is optimized to maximize the kaon survival rate before the decay tunnel. As it can be seen from Fig. \ref{beamline}, the trackers used for tagging (marked in red) are placed along the beamline. More precisely, one set is placed between last two dipoles, the second and third set are placed before and after the last three quadrupoles. These measure the position and momentum of secondary mesons which is later used to uniquely asocciate a meson decay to the neutrino interaction in the detector.

\begin{figure}[h!]
\includegraphics[width = \linewidth]{figs/nuscope_beamline.eps}     % includes figure foo.eps
\caption{Schematic of a proposed beamline implementation for nuSCOPE. Parts marked in red are responsible for the neutrino tagging, while parts marked in green are used for the monitoring.}
\label{beamline}
\end{figure}

\section{Instrumented decay tunnel}

The instrumentation of the decay tunnel is envisioned as a calorimeter embedded in the decay tunnel wall with a small veto region that is placed right before the tunnel wall. Calorimeter is segmented into radial, longitudinal and azimuthal directions (Fig. \ref{instrumentation_illu}). Each segment is a longitudinal stack of $0.7$ cm thick plastic scintillator tiles ($3\times3$ cm$^2$) with a $1.5$ cm thick iron plate between each longitudinal segment to induce showering. Calorimeter has 3 radial layers, while the 4th, innermost, radial layer is a veto layer which lies outside of the tunnel walls and is not interleaved by iron plates. This veto region is used to identify showers created by neutral particles like $\gamma$ and $\pi^0$. Following this design, a Demonstrator was built to test its performance. 

\begin{figure}[h!]
\centering
\includegraphics[width = 0.8\linewidth]{figs/instrumentaion_illustration.eps}     % includes figure foo.eps
\caption{Illustration of the instrumented decay tunnel and its radial layers. Marked green are the layers of the calorimeter, while marked orange is the veto layer.}
\label{instrumentation_illu}
\end{figure}

The Demonstrator, $1.5$ m in length and a mass of $3.5$ t, is shown in Fig. \ref{demo}. It was exposed to a charged particle beam at CERN-PS accelerator in 2022, 2023, and 2024. The demonstrator was only partially instrumented and currently consists of 1275 channels. Preliminary analysis from these exposures shows that both the energy linearity and energy resolution (Fig. \ref{energy_reso}) of the calorimeter is enough for $e$/$\pi$ separation in $1-3$ GeV range.

\begin{figure}[h!]
\centering
\includegraphics[width = 0.8\linewidth]{figs/demoatCERN_all.eps}     % includes figure foo.eps
\caption{Demonstrator and the test setup at CERN in 2023.}
\label{demo}
\end{figure}

\begin{figure}[h!]
\centering
\includegraphics[width = 0.7\linewidth]{figs/energy_reso.eps}     % includes figure foo.eps
\caption{Energy resolution of electrons in the calorimeter as tested in 2023 and 2024 test beams at CERN-PS accelerator.}
\label{energy_reso}
\end{figure}

\section{Conclusions}
To better understand neutrino interactions with matter and the current modeling of those interactions for a GeV region, a new approach in neutrino cross section measurements is needed. nuSCOPE provides this by designing a monitored and tagged neutrino beam, which gives us an unprecedented knowledge not only on the initial neutrino flux (around $1$\%), but also on the energy resolution of the interacting neutrino (below $1$\%). 

Tests preformed at CERN by the ENUBET collaboration on the monitored neutrino beam design show that the current design is capable of delivering such performance ($1$\% uncertainty on the electron and muon neutrino flux).

\newpage
\acknowledgments
This project has received funding from the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement no. 681647 and the Italian Ministry for Education and Research (MIUR, bando FARE, progetto NUTECH). It is also supported by the Agence Nationale de la Recherche (ANR, France) through the PIMENT project (ANR-21-CE31-0027) and by the Ministry of Science and Education of Republic of Croatia grant No. KK.01.1.1.01.0001


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\end{thebibliography}

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