The SPD project at NICA

The Spin Physics Detector (SPD) is a universal detector in the one of two interaction points of the NICA collider under construction at JINR, Dubna. SPD plans to study the spin structure of the proton and deuteron and other spin-related phenomena using a unique possibility to operate with polarized proton and deuteron beams at a collision energy up to 27 GeV and a luminosity up to $10^{32}$ cm$^{-2}$ s$^{-1}$. As the main goal, the experiment aims to provide access to the gluon TMD PDFs in the proton and deuteron, as well as the gluon transversity distribution and tensor PDFs in the deuteron, via the measurements of specific single and double spin asymmetries using different complementary probes such as charmonia, open charm, and prompt photon production processes. Other polarized and unpolarized physics is possible, especially at the first stage of NICA operation with reduced luminosity and collision energy of the proton and ion beams. Construction of the first stage of the SPD facility is included in the JINR seven-year development plan for 2024-2030. The physics program of the SPD project and the design of the SPD setup are presented.

💡 Research Summary

The paper presents a comprehensive overview of the Spin Physics Detector (SPD) project, which will be installed at one of the two interaction points of the Nuclotron‑based Ion Collider Facility (NICA) under construction at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia. The primary scientific goal of SPD is to explore the spin structure of the proton and the deuteron, with a particular focus on the gluon sector. By colliding longitudinally and transversely polarized proton and deuteron beams at center‑of‑mass energies up to √s = 27 GeV and luminosities up to 10³² cm⁻² s⁻¹, SPD aims to provide unprecedented access to gluon transverse‑momentum‑dependent parton distribution functions (TMD PDFs), the gluon helicity distribution Δg(x), the gluon transversity (which is absent in spin‑½ nucleons but may appear in the spin‑1 deuteron), and the tensor‑polarized parton distributions unique to the deuteron.

Three complementary hard probes are identified: inclusive production of charmonia (J/ψ, ψ(2S)), open‑charm (D‑mesons), and high‑p_T prompt photons. Charmonium production is dominated by gluon‑gluon fusion, making it highly sensitive to the gluon density and its polarization. Open‑charm production probes gluon–quark scattering and provides a clean channel for reconstructing secondary vertices with a silicon vertex detector. Prompt photons arise mainly from quark‑gluon Compton scattering, offering a direct handle on the sign of Δg(x). Together, these processes cover the kinematic region 0.1 ≲ x ≲ 0.7 and moderate Q², filling the gap between low‑energy fixed‑target experiments and high‑energy colliders such as RHIC.

A distinctive feature of SPD is its capability to collide tensor‑polarized deuterons. This opens the possibility to measure deuteron‑specific structure functions (b₁, h₁ᵗ, etc.) and to search for a non‑zero gluon transversity, which would signal novel degrees of freedom in the deuteron beyond the simple sum of proton and neutron contributions. The paper emphasizes that only the combination b₁ has been measured so far; SPD would be the first facility to explore the full set of tensor PDFs at high energies.

The technical design of SPD is described in detail. It is conceived as a 4π detector with modern tracking, particle‑identification, and calorimetry subsystems:

• A silicon vertex detector (SVD) with <100 µm spatial resolution for secondary‑vertex reconstruction.
• A straw‑tube tracker inside a 1 T solenoidal magnet, delivering σ(p_T)/p_T ≈ 2 % at 1 GeV/c.
• A time‑of‑flight system with ~60 ps resolution, providing 3 σ π/K separation up to 1.2 GeV/c and K/p up to 2.2 GeV/c; an optional F‑ARICH in the end‑caps extends PID to higher momenta.
• An electromagnetic calorimeter with energy resolution ≈ 5 %/√E ⊕ 1 % and a muon range system that also serves as a coarse hadron calorimeter.
• Beam‑beam counters and zero‑degree calorimeters for local polarimetry and luminosity monitoring.
• A trigger‑less (free‑running) data‑acquisition system capable of handling up to 4 MHz collision rates and several hundred thousand readout channels, requiring sophisticated online filtering and distributed offline analysis.

The implementation plan is staged. Phase 1 (2024‑2030) will operate at reduced luminosity and √s < 9 GeV, using a simplified configuration (solenoid, straw tracker, range system, ZDCs, and beam‑beam counters). A Micromegas central tracker will replace the full SVD to keep costs manageable, and a partial electromagnetic calorimeter will be installed. Phase 2, targeted for the 2030s, will complete the full detector suite, achieve the design luminosity, and enable the full physics program.

In the broader context, SPD fills a unique niche. While RHIC provides polarized proton–proton collisions at √s ≈ 200–500 GeV, and future facilities such as the US Electron‑Ion Collider (EIC) and China’s EicC will explore low‑x physics, SPD operates at intermediate energies where the transition from non‑perturbative to perturbative QCD can be studied directly. Its high‑luminosity polarized beams, combined with the ability to collide tensor‑polarized deuterons, make it the only facility worldwide capable of systematically mapping gluon spin and transverse‑momentum structure at large x and of probing deuteron tensor PDFs.

The authors conclude that SPD will deliver critical data to reduce the large uncertainties in Δg(x) at high x, test the existence of gluon transversity, and provide the first comprehensive measurements of deuteron tensor parton distributions. The project is integrated into JINR’s seven‑year development plan (2024‑2030) and invites international collaboration. Its successful realization will significantly advance our understanding of the spin composition of nucleons and light nuclei, complementing the global spin‑physics program.