Huizhou hadron spectrometer: a high-rate experiment poised to transform light hadron physics

State-of-the-Art Detector Technologies

HHaS is built around a suite of cutting-edge detector systems housed inside a solenoidal magnet. A five-dimensional silicon pixel tracker based on Monolithic Active Pixel Sensor (MAPS) technology achieves a hit position resolution of ~30 μm, enabling precise track reconstruction with ~1% momentum resolution. Low-Gain Avalanche Detector (LGAD) time-of-flight panels deliver ~30 ps timing resolution, providing clean separation of various charged particles. The Cherenkov-scintillation dual-readout electromagnetic calorimeter achieves ~3% energy resolution at 1 GeV by simultaneously reading Cherenkov and scintillation light, matching the performance of far more expensive crystal calorimeters. Together, these systems give HHaS good momentum and energy resolutions as well as the ability to identify e±, γ, π±, K±, p, p̅, d, t, ³He, and ⁴He—all at event rates of 1–100 MHz.

Unprecedented Statistical Power

The standout feature of HHaS is its ultra-high event rate capability of 1–100 MHz—several orders of magnitude faster than existing high-energy nuclear physics experiments. Unlike experiments that rely on selective triggers, HHaS records all collision signals with time information in all three detector systems, and reconstructs them offline—ensuring no rare physics is missed. This continuous, unbiased data acquisition underpins HHaS's unprecedented statistical reach.

Probing Dark Sector Portals and Fundamental Symmetries

The η and η′ mesons are uniquely powerful probes of physics beyond the Standard Model. Their decays are strongly suppressed by C, CP, and other discrete symmetries, making their anomalous branching ratio a clean signal of new physics. HHaS will accumulate η meson samples three orders of magnitude larger than the world total, enabling searches for dark photons and dark Higgs bosons with sensitivity two orders of magnitude beyond current limits. Simultaneously, precision measurements of η decays will provide stringent tests of C, CP symmetries and lepton flavor conservation—complementing searches at high-energy colliders with a qualitatively different experimental approach.

Probing Exotic States of Matter

HHaS opens new windows into exotic hadronic structures, like di-baryons and pentaquark states. Pentaquark states with heavy quarks have been a hot research topic in recent years, but their partner states composed purely with light quarks have not been confirmed. The multi-body decay signatures of these exotic hadron searches demand large acceptance, excellent momentum and energy resolutions, as well as clean identification of a wide range of final state particles—precisely what HHaS provides. Heavy-ion collisions at HIAF can also produce hyperon-rich environments where hypernuclei form through coalescence, offering unique insights into hyperon-nucleon interactions, which are tightly related to the hyperon puzzle in neutron star physics.

Mapping the QCD Phase Diagram

One of the most profound goals of modern nuclear physics is to map the phase diagram of quantum chromodynamics (QCD)—the theory describing how quarks and gluons transition between hadronic matter and the quark-gluon plasma. HHaS’s large angular acceptance and excellent charged-particle identification capabilities make it uniquely suited to search for the predicted first-order phase transition and critical point in the high baryon density region of the QCD phase diagram. By measuring higher-order moments of net-proton multiplicity and light nucleus production ratios, HHaS will contribute to the global effort alongside RHIC, NICA, and FAIR.

Setting New Standards

With its comprehensive multi-disciplinary reach—from dark sector searches to QCD phase structure—HHaS is positioned to become one of the most versatile and competitive spectrometers worldwide, addressing some of the deepest questions in nuclear physics from a single experimental setup.

The full conceptual design report of HHaS has been submitted for publication. The HHaS collaboration invites interested researchers and institutions to join the project. Detailed simulation studies for additional physics measurement channels are underway.

The complete study is via by DOI: https://doi.org/10.1007/s41365-026-01996-1