The SPD project at NICA

Imagine the universe is built out of tiny, invisible Lego bricks called protons and neutrons. For a long time, scientists thought they knew exactly how these bricks were put together. But in the 1970s, a famous experiment revealed a shocking secret: the tiny pieces inside the proton (quarks) only account for a small fraction of the proton's "spin" (its internal twirling motion). It's like spinning a top and realizing the visible parts only explain 30% of the spin; the rest must be coming from something hidden inside.

This is the mystery the SPD project at the NICA facility in Russia aims to solve. Think of NICA as a massive, high-speed racetrack where scientists crash tiny particles together to see what flies out. The SPD (Spin Physics Detector) is a giant, high-tech camera and sensor suite built right at the crash site to take 3D pictures of these collisions.

Here is a simple breakdown of what they are doing and why it matters, based on the paper:

1. The Goal: Finding the "Ghost" Spin

The main suspect for the missing spin is gluons. If quarks are the bricks, gluons are the super-strong glue holding them together. The SPD wants to map out exactly how these gluons are spinning and moving inside the proton and a heavier cousin called the deuteron (a proton and neutron stuck together).

They aren't just looking at the "forward" spin; they want to see the "sideways" spin and how the particles move in 3D space. It's like trying to understand a spinning basketball not just by watching it rotate, but by seeing how the air swirls around it and how the leather stretches.

2. The Tools: Three Special "Flashlights"

To see these invisible gluons, the SPD uses three specific "probes" (ways of crashing particles) that act like different colored flashlights to reveal hidden details:

Charmonia: Crashing particles to create heavy, short-lived "ghost" particles that reveal the gluon's structure.
Open Charm: Creating particles that contain "charm" quarks to trace the path of the gluons.
Prompt Photons: Catching high-energy flashes of light (photons) that are born directly from the collision, acting as a direct signal of the gluon's behavior.

By comparing the results from these three methods, they can build a complete picture, much like using X-rays, MRIs, and CT scans to get a full view of a human body.

3. The Unique Advantage: The Only Game in Town

The paper highlights a crucial point: NICA is currently the only place on Earth that can smash together polarized (aligned-spin) protons and deuterons at these specific speeds.

The Energy Range: Most other machines are either too slow (only seeing the "soft" physics) or too fast (only seeing the "hard" physics). NICA is special because it can scan the energy range from slow to fast. This allows scientists to see exactly where the rules of physics change, like a camera zooming in and out to find the perfect focus.
The Deuteron Mystery: The SPD plans to smash deuterons together. Since a deuteron is made of two particles, it might have a special "tensor" spin (a complex, multi-directional twist) that single protons don't have. If they find a new type of spin here, it could mean there are entirely new rules or "degrees of freedom" in how matter is built.

4. The Machine: A High-Speed Camera

The detector itself is described as a "universal 4π detector." Imagine a sphere of sensors surrounding the crash point, capturing everything flying out in every direction.

The Silicon Vertex Detector: This is the high-resolution lens. It's so precise it can spot a particle decay happening in a space smaller than a human hair (100 micrometers).
The Magnet: A giant superconducting magnet bends the paths of the particles, allowing the computer to calculate their speed and mass.
The "Triggerless" System: Usually, cameras take a photo only when you press a button. This system is like a security camera that records everything 24/7 without stopping, because the collisions happen so fast (4 million times a second) that they can't afford to miss a single frame.

5. The Timeline: Building the Future

The project is currently in the construction phase.

Phase 1 (Now): They will start with a simpler setup, running at lower speeds and lower intensity. This is like a "soft opening" to test the equipment and study basic collisions.
Phase 2 (The 2030s): Once fully built, the machine will run at its full power, aiming to solve the big mysteries of gluon spin and provide the world with the most detailed 3D map of the proton ever made.

In summary: The SPD project is a massive international effort to build the ultimate microscope for the atomic world. By smashing spinning particles together in a unique way that no other machine can do, they hope to finally answer the decades-old question: "What is the proton made of, and how does it spin?"

Technical Summary of the SPD Project at NICA

Problem and Scientific Context
The fundamental origin of the nucleon spin remains a central question in quantum chromodynamics (QCD). While the EMC experiment at CERN revealed that quark spins contribute only a small fraction to the proton's spin, the contributions from gluon spin (helicity) and orbital angular momentum require further investigation. Previous experiments at SLAC, SMC, COMPASS, HERMES, and RHIC have provided constraints on gluon helicity and transverse momentum distributions (TMDs), yet significant gaps remain, particularly regarding the 3D structure of the proton and the behavior of gluons at intermediate to high Bjorken- $x$ . Furthermore, the gluon transversity distribution, which vanishes for spin-1/2 nucleons, offers a unique probe for new degrees of freedom in the spin-1 deuteron. The NICA (Nuclotron-based Ion Collider fAcility) complex at JINR aims to address these questions by operating as a dedicated polarized hadron collider, a capability unique among current and planned facilities.

Methodology and Experimental Setup
The Spin Physics Detector (SPD) is designed as a universal, $4\pi$ detector located at one of the two interaction points of the NICA collider. The facility utilizes high-intensity polarized proton and deuteron beams with collision energies up to $\sqrt{s} = 27$ GeV and a design luminosity of $10^{32} \text{ cm}^{-2} \text{ s}^{-1}$ .

The experimental methodology relies on measuring specific single and double spin asymmetries using three complementary hard probes to access the polarized gluon content:

Inclusive production of charmonia (e.g., $J/\psi$ , $\psi(2S)$ ).
Open charm production.
High- $p_T$ prompt photon production.

These processes are sensitive to the gluon helicity function $\Delta g(x)$ and gluon TMDs. Additionally, the experiment will utilize inclusive production of light mesons (pions) and Drell-Yan processes to probe quark distributions. The detector design incorporates:

Silicon Vertex Detector: For secondary vertex reconstruction of $D$ -meson decays with resolution $<100 \, \mu\text{m}$ .
Straw Tube Tracking System: Located within a 1 T superconducting solenoid, providing transverse momentum resolution $\sigma_{p_T}/p_T \approx 2\%$ at 1 GeV/ $c$ .
Time-of-Flight (ToF) System: With $\sim 60$ ps resolution for particle identification ( $\pi/K/p$ separation).
FARICH: A Focusing Aerogel Ring-Imaging Cherenkov detector for extended momentum range identification.
Electromagnetic Calorimeter: Sampling type with energy resolution $\sim 5\%/\sqrt{E} \oplus 1\%$ .
Range System: For muon identification and rough hadron calorimetry.
Luminosity and Polarimetry: Beam-Beam Counters and Zero Degree Calorimeters.

The data acquisition system is designed to be free-running (triggerless) to handle collision rates up to 4 MHz. The construction of the first stage, including the magnet, tracker, and calorimeters, is included in the JINR seven-year development plan (2024–2030).

Key Contributions and Physics Program
The paper outlines a physics program divided into two phases:

First Stage (Low Energy/Reduced Luminosity): Operating at $\sqrt{s} < 9.4$ GeV for $p-p$ and $\sqrt{s} < 4.5$ GeV/nucleon for $d-d$ collisions. This phase focuses on:
- Polarized elastic $p-p$ and $d-d$ scattering.
- Spin effects in hyperon production and multiquark resonances.
- Near-threshold charmonia production.
- Studies of tensor polarization in deuterons.
Main Physics Program (High Energy): Operating at $\sqrt{s} \leq 27$ GeV. The primary goals include:
- Gluon Helicity: Determining $\Delta g(x)$ via double spin asymmetries ( $A_{LL}$ ) in charmonium, open charm, and prompt photon production. Prompt photons are specifically noted for their ability to determine the sign of the gluon helicity.
- Gluon TMDs: Investigating the gluon Sivers function and other TMD PDFs via transverse single-spin asymmetries (SSAs).
- Gluon Transversity: Searching for non-zero gluon transversity in the deuteron, which would indicate new degrees of freedom beyond the simple sum of nucleon contributions.
- Unpolarized Gluon PDFs: Constraining the unpolarized gluon density at high $x$ ( $x > 0.5$ ) in protons and deuterons.
- Factorization Limits: By scanning energies from the non-perturbative regime (few GeV) to the perturbative regime (27 GeV), SPD aims to experimentally determine the limits of applicability for various factorization approaches.

Results and Claims
The paper does not present experimental data, as the facility is under construction. Instead, it presents the design specifications and the expected physics reach. The authors claim that the SPD will:

Provide unique access to gluon TMDs and the gluon transversity distribution in the deuteron, which are inaccessible at other facilities.
Fill the energy gap between low-energy fixed-target experiments and RHIC, allowing for a continuous study of spin-dependent partonic structures.
Offer a unique laboratory for studying the tensor structure of the deuteron through tensor-polarized beams.
Complement the physics programs of the Electron-Ion Collider (EIC) and other facilities by focusing on the intermediate energy range and specific gluon probes in hadronic collisions.

Significance
The significance of the SPD project lies in its potential to advance the 3D tomography of the nucleon, specifically focusing on the gluonic sector. As the world's only planned polarized hadron collider for the foreseeable future, NICA/SPD offers a unique opportunity to study spin-dependent effects in nuclear collisions across a wide energy range. The project aims to resolve open questions regarding the magnitude and sign of the gluon helicity contribution to the proton spin and to explore the internal structure of the deuteron through tensor polarization. The paper emphasizes that the implementation of the full gluon physics program is planned for the 2030s, positioning SPD as a critical, complementary component of the global effort to understand the strong interaction.