sphenixoperamaps

This repository contains magnetic field maps calculated by Wuzheng Meng and Kin Yip for the sPHENIX magnet with OPERA. These maps are intended to be used in the sPHENIX GEANT4 Monte Carlo simulation.

The right-handed coordinate system is show here below. The positions are in centimeters, the field is in Tesla. The symmetry is broken by the magnet's chimney.

The center of the coil is offset from the center of the cryostat by about 28.5 mm as described here: [https://brookhavenlab.sharepoint.com/sites/sPHENIXMagnet/External/SitePages/Parameters.aspx] (formerly [https://collab.external.bnl.gov/sites/sPHENIX-Magnet/SitePages/Parameters.aspx]) and the default maps in the root files reflect that. The phi component of the field is still quite small (< 2 G) and can probably be neglected.

The tracking field map

sphenix3dtrackingmapxyz.root is the field map actually used in sPHENIX track reconstruction. It is not committed to this repository — it lives in the sPHENIX Conditions DB (CDB) on CVMFS. It is the same OPERA field as the maps here, with the same 28.5 mm (2.85 cm) magnetic-center shift, evaluated on a larger grid: 111×111×111 over [−110, 110] cm in x, y, z (2 cm pitch), with bx, by, bz and an extra hz column. Every grid node of sphenix3dmapxyz.root is reproduced byte-for-byte — that map is exactly the central core of this one. The tracking map adds the outer shell (the return-field region, where bz reverses sign) that the tracker acceptance needs.

Map dimensions and sizes

File	Coordinates / grid	Extent	Points	Size
sphenix3dmapxyz.root	Cartesian (x, y, z); bx, by, bz	x, y ∈ [−80, 80]; z ∈ [−100, 100] cm; 2 cm pitch (81×81×101)	662,661	7.2 MB
sphenix3dmaprhophiz.root	cylindrical (ρ, φ, z); brho, bphi, bz	same field as the xyz map re-expressed in cylindrical coords: ρ ∈ [0, 113.1] cm, φ ∈ [−π, π], z ∈ [−100, 100] cm	662,661	7.2 MB
sphenix3dtrackingmapxyz.root	Cartesian (x, y, z); bx, by, bz, hz	x, y, z ∈ [−110, 110] cm; 2 cm pitch (111×111×111)	1,367,631	19.3 MB

All three are TNtuples named fieldmap; positions in cm, field in Tesla.

Locating the tracking map in the CDB

The tracking map is content-addressed by the MD5 of its contents, so it can be identified unambiguously regardless of where it is stored:

• Content (MD5): 8e4d6c3b1660540a658da3a275af2bde — verify any copy with md5sum.
• CDB payload type / domain: FIELDMAP_TRACKING
• CVMFS path (as of 2026-06): /cvmfs/sphenix.sdcc.bnl.gov/calibrations/sphnxpro/cdb/FIELDMAP_TRACKING/8e/4d/8e4d6c3b1660540a658da3a275af2bde_sphenix3dtrackingmapxyz.root (the 8e/4d/ subdirectories are the first nibbles of the MD5)

The CVMFS path may change over time; the MD5 and payload type will not. To resolve the current global tag / interval-of-validity and on-disk path within the sPHENIX offline environment:

recoConsts *rc = recoConsts::instance();
rc->set_StringFlag("CDB_GLOBALTAG", "<global tag>");   // e.g. a production tag
rc->set_uint64Flag("TIMESTAMP",  <run number or timestamp>);

CDBInterface *cdb = CDBInterface::instance();
std::cout << cdb->getUrl("FIELDMAP_TRACKING") << std::endl;

If the printed filename carries the 8e4d… hash, it is this map.