Precision Magnetic Field Direction Measurements for Spin Structure Studies n n Murchhana Roy and Wolfgang Korsch (for the A1 /d2 Collaboration) University of Kentucky NUCLEON SPIN STRUCTURE COMPASS DESIGN RESULTS • The experiments carried out at CERN and • The compass comprising of a • Holding magnetic field directions were SLAC in the 80s on the integral of the neodymium magnetic rod (0.25” determined along the target length in p polarized structure function g1 showed that diameter, 2” long) mounted on a the absolute Hall C coordinate system by the three valence account for only a floating disk was conceptualized analyzing the survey data. fraction of the proton's total spin. and constructed at the • All angles were computed w.r.t. the beam • This discovery has resulted in multiple prolific experimental and University of Kentucky. The air-compass direction (+z direction). theoretical research endeavors. Current understanding of the nucleon • A cylindrical magnet was placed inside a V-shaped groove to ensure • All the systematic uncertainties were spin is that the total spin is distributed among valence quarks, sea its position repeatability. propagated or added in quadrature to get quarks, their orbital angular momenta and . • The compass magnet had two mirrors and circular scales mounted on the total uncertainties associated with each • Due to non-perturbative nature of strong interactions, it is extremely both sides. The circular scale had markings every 30˚. data point. difficult to make absolute predictions from QCD on how the nucleon • Field direction was measured by reflecting a laser beam off the spin is decomposed in all the components. compass mirrors, aligned perpendicular to the magnetic axis of the SYSTEMATIC UNCERTAINTIES • Measurements of nucleon spin structure functions g1 and g2 in magnet as precisely as possible. inclusive deep inelastic scattering are used to investigate the nucleon 1. Error in determining the angle (θ) the magnetic field makes with spin structure. COMPASS MIRROR ALIGNMENT beam line: ~ ±(0.04°-0.08°) • g1 can be understood in terms of naïve -Parton model but θ → function of all surveyed coordinate g is one of the cleanest higher twist observables that contains • The geometric and magnetic axes of the = variables (x ) 2 2 i information on quark- correlations. cylindrical magnet do not coincide. 𝜕𝜕θ 2 → error associated with that coordinate θ 𝑖𝑖 𝑥𝑥𝑖𝑖 • d , the third moment of linear combination of g and g , is the clean • Reflected laser beam from the compass 2. Errorsσ from∑ the𝜕𝜕𝑥𝑥𝑖𝑖 compassσ mirror alignment (θ ): ~ ±(0.04°-0.06°) 2 1 2 𝑥𝑥𝑖𝑖 M probe to the quark-gluon correlations. mirror inscribed an ellipse on a screen as a The misalignment betweenσmagnetic axis of the cylindrical magnet and result of 360° scan of the magnet. compass mirror generated additional errors: THE EXPERIMENT: NEUTRON g2 AND d2 • The compass mirrors were aligned parallel to the A. Projection of the fitted straight line on the horizontal axis​ magnetic axis of the compass magnet B. Fit parameter errors • The experiment E12-06-121 (Neutron g2 and d2) aims to do a 2 to minimize the horizontal error by reducing 3. Laser beam spot size: ~ ±0.006° precision measurement of neutron g2 and d2 at high Q . the ellipse to a straight line. Lenses were used to make the laser beam spot diameter ~2 mm. • The compass mirror alignment data were fitted 4. Position of incident laser beam on the compass mirror: ~ ±0.01° with a straight line and the horizontal error was The laser beam always reflected off the center of the compass mirror determined. Mirror 1 data within 0.5 mm uncertainty. MEASUREMENTS IN HALL C SUMMARY • The magnetic field direction was scanned Hall C Layout in three different locations along the • The field direction measurement was successfully completed in • SHMS and HMS will collect data to cover the kinematic range 0.20 < x target length for all four polarization March, 2020. < 0.95, 2.5 < Q2 < 6.0 (GeV/c)2 . directions (+X, -X, +Z, -Z) and two • The uncertainty in the field direction measurement was limited to which satisfies requirement for the d n experiment. • Polarized 3He cell (40 cm long) will be used as an effective polarized kinematic settings of the experiment. ±0.1° 2 neutron target with in both transverse and • The incident and reflected laser beam • The Hall C is currently getting ready for data taking. ~50% polarization n 2 longitudinal directions. spots for each measurement were • The value of d2 will be measured at four truly constant Q values marked on a transparent screen. Compass installed in Hall C for the very first time. THE NEED FOR PRECISION • The results will provide insight on the neutron spin structure and quark-gluon correlations. • If the target polarization direction is a little different from 90°/270° , • Physics will be explored to test different theoretical predictions the longitudinal asymmetry (A ) contributes to the total asymmetry including Lattice QCD. in same order as the transverse asymmetry (A ). ∥ • A precision of ±0.1° in the measurement of polarization direction is ⊥ required in the Hall C coordinate system. Longitudinal Transverse ACKNOWLEDGEMENT • A novel air-compass was developed and built as the commercially • The alignment group at the Jefferson Lab surveyed the points in the This work is partially supported by the U.S. Department of Energy Office available compasses cannot achieve the desired level of precision. absolute Hall C coordinate system. of Nuclear Physics under Contract No. DEFG02-99ER41101.