Scattering and Neutrino Detector (SND)

The "Scattering and Neutrino Detector" is not part of the Hidden Sector Decay Spectrometer, but a separate detector located upstream of it. While the main goal of the SHiP Experiment is the search for long-lived hidden-sector particles, the SND provides an additional program for studying high-energy neutrino interactions, especially for \(\nu_\tau\) produced by the beam-dump target.

From the \(400\,\text{GeV}\) proton beam a large number of Charm mesons, in particular D-Mesons, gets produced:

\[ p + A \to D + X \]

These subsequently decay into a large flux of tau-neutrinos:

\[ D_S^\pm \to \tau^\pm \nu_\tau \]

and

\[ \tau^\pm \to \nu_\tau + X \]

making the SHiP Beam dump an intense neutrino source. The main motivation for the SND is the analysis of \(\nu_\tau\) interactions. Since the \(\nu_\tau\) remains the least experimentally studied neutrino flavour, physical measurement for the following quantities are still unexplored:

neutrino-nucleon cross-sections
tau production mechanisms
Charm production in neutrino interactions
nucleon structure functions
strange-quark content of the nucleon

The SND aims to measure the processes

\[ \nu_\tau + N \to \tau^- + X \]

and

\[ \bar{\nu}_\tau + N \to \tau^+ + X \]

Furthermore, the SND is also designed to reconstruct electromagnetic showers produced in \(\nu_e\) interactions.

Detector Concept

The detector follows a concept that was already used at the OPERA and DONuT experiment. The following diagram summarizes the structure of the detector:

flowchart TD

    Beam(["Beam"]) --> A
    A["Emulsion Target"] --> B["Silicon Target"]
    B --> C["Magnetic Tracking Calorimeter"]

A CAD model of the SND is shown in Fig. 1.

**Fig. 1:** A CAD Drawing of the Scattering and Neutrino Detector (SND) within the SHiP experiment.¹

Emulsion Target

The "Emulsion Cloud Chamber" (ECC) is a repeated combination of Emulsion film and Tungsten plates. The high density tungsten plates are used to increase the interaction probability of \(\nu_\tau\):

\[ \nu + W \to X \]

Within the emulsion film, which is made out of silver iodide crystals dissolved in gelatin, the charged particles leave a trace of silver grains. The emulsion is used to reconstruct the tracks of the charged particles with a spatial resolution of \(\approx 1\,\mu\text{m}\). This was desired to reconstruct the short-lived \(\tau\) leptons with a decay length of \(L_\tau \approx 87\,\mu{}\text{m}\)². The ECC part of the SND reconstructs the primary interaction vertex and the displaced decay vertex of the produced \(\tau\) lepton:

\[ \nu_\tau + N \to \tau^- + X\,. \]

Additional "Scintillating Fibre trackers" (SciFi) are used for a coarse position reconstruction³. In order to detect the traces within the emulsion films, the films are developed and scanned with an automated optical microscope and relies on the adjustment of the focal plane of the objective lens through the whole thickness of the emulsion plate³. Even though this process can be automated, it would still require a single microscope one to two months to fully scan a single emulsion brick. Therefore, ten microscopes should be run in parallel. The emulsions are replaced every \(\approx 10\,\text{fb}^{-1}\) to \(\approx 25\,\text{fb}^{-1}\). A fully exposed plate will conain \(\approx 10^5\) tracks per \(\text{cm}^2\)[1 3]. The ECC is making up the first \(50\,\text{cm}\) of the SND.

Silicon Target

The "Silicon Target" is a \(1.5\,\text{m}\) long section made out of alternating layers of \(300\,\mu\text{m}\) Silicon and Tungsten. It is used to reconstruct \(\nu_\tau\) events on an event-by-event basis for muons and hadrons with the momenta and energy information from the MTC. Furthermore it links the ECC tracks to the MTC tracks and provides timing informations. Since the ECC relies on the emulsion films, it has a low time resolution and cannot provide timing information for the reconstructed tracks. It would be hard to separate events from different spills and reconstruct longer vertices only with the ECC. Therefore, tracking information from the Silicon Target is used to link the ECC tracks to the MTC tracks and provide timing information for the reconstructed events.

Magnetic Tracking Calorimeter (MTC)

The "Magnetic Tracking Calorimeter" is the last section of the SND with a length of \(3.5\,\text{m}\). It is made out of a repeated combination of thick iron plates with Scintillating Fibers (SciFi) and Scintillator tiles in between them. The iron plates are used as magnetic absorbers, using the return field of the Muon Shield yoke rather than a dedicated magnet. The SciFi planes reconstruct the neutrino interaction vertices and measure the momentum of a charged track by measuring its deflection radius \(r\):

\[ \begin{equation} p_T = q \cdot B \cdot r\,. \end{equation} \]

Furthermore, they can measure the missing transverse momentum of \(\tau\) decays, carried away by neutrinos. It can also detect the long traces of muons by their long trace through the detector without large energy deposits. The scintillating tiles are used to measure the energy of hadronic shower events¹.

https://arxiv.org/pdf/2504.06692, Section 4 ↩↩
https://link.springer.com/article/10.1140/epjc/s10052-022-10346-5, Section 4.2 ↩
https://openscience.ub.uni-mainz.de/items/901302bf-320c-4ebb-8c77-0136b5770b88, Section 2.2.3.2 ↩↩