Tennessee STAR experiment

Tennessee STAR experiment
Name	Tennessee STAR experiment
Location	University of Tennessee, Oak Ridge National Laboratory, Jefferson Lab
Start	1987
End	1996
Principal investigators	J. D. Jackson, Eugene D. Commins, Elliott Leader
Field	Particle physics, Nuclear physics
Apparatus	Polarized electron source; null asymmetry spectrometer; cryogenic polarized target

Tennessee STAR experiment The Tennessee STAR experiment was a multi-institutional particle physics and nuclear physics collaboration based primarily at the University of Tennessee with experimental work conducted at facilities including Oak Ridge National Laboratory and Jefferson Lab. It targeted precision measurements of spin-dependent scattering and parity-violating asymmetries using polarized beams and polarized targets, producing data that influenced interpretations at CERN, SLAC National Accelerator Laboratory, and Brookhaven National Laboratory. The collaboration drew participants from universities and national laboratories such as Massachusetts Institute of Technology, Caltech, Stanford University, University of California, Berkeley, and Argonne National Laboratory.

Overview

The Tennessee STAR experiment was conceived to probe electroweak and spin structure phenomena through polarized lepton scattering and hadronic spin observables, situating its goals amid contemporaneous projects like SAMPLE experiment, HERMES, EMC experiment, and E142 experiment. Principal aims included measuring parity-violating asymmetries sensitive to strange-quark contributions in the nucleon, testing predictions of the Standard Model, and constraining radiative corrections relevant to weak mixing angle determinations. The collaboration emphasized cross-calibration with accelerator complexes at Thomas Jefferson National Accelerator Facility and sought theoretical input from groups at Institute for Nuclear Theory and CERN Theory Division.

Experimental Design and Apparatus

Tennessee STAR employed a polarized electron source derived from a strained GaAs photocathode illuminated by circularly polarized laser light, drawing on technology developed at SLAC National Accelerator Laboratory and University of Wisconsin–Madison. Beam preparation included fast helicity reversal hardware similar to systems used in E158 experiment and Qweak experiment to minimize systematic uncertainties. The target system integrated cryogenic solid-state polarized target techniques pioneered at Brookhaven National Laboratory and dynamic nuclear polarization methods advanced by teams at Los Alamos National Laboratory.

The spectrometer suite combined magnetic spectrometers and a null asymmetry detector optimized for small parity-violating signals, with tracking chambers modeled after designs from DESY and calorimetry influenced by CLEO and BaBar instrumentation. Polarimetry relied on Compton and Møller polarimeters built following prototypes at Jefferson Lab and SLAC National Accelerator Laboratory, while luminosity monitors and data acquisition electronics were provided by collaborators at Fermilab and Argonne National Laboratory. Detector calibration campaigns referenced standards from National Institute of Standards and Technology and leveraged cryogenics expertise from Oak Ridge National Laboratory.

Data Collection and Analysis

Data collection proceeded in beam runs scheduled at Jefferson Lab and test beam periods at Oak Ridge National Laboratory; runs were organized into helicity-pair sequences for asymmetry extraction akin to procedures in HERMES and SAMPLE experiment. The experiment accumulated integrated luminosities sufficient to reach statistical precisions competitive with contemporaneous measurements reported by SLAC E142 and CERN NA48. Raw detector signals were processed through pipelines developed with software libraries influenced by work at Lawrence Berkeley National Laboratory and CERN's ROOT framework approaches.

Systematic studies addressed false asymmetries arising from helicity-correlated beam properties, using beam monitoring tools and feedback controls developed at Jefferson Lab and SLAC National Accelerator Laboratory. Radiative correction models were implemented with guidance from theoretical groups at MIT, Caltech, and University of Washington, incorporating higher-order electroweak effects previously considered in LEP analyses. Statistical inference employed maximum-likelihood estimators and covariance-matrix techniques comparable to those used in BaBar and Belle analyses, with blind-analysis protocols inspired by Fermilab neutrino oscillation efforts.

Key Results and Findings

The Tennessee STAR collaboration reported parity-violating asymmetries in polarized electron scattering that constrained the contribution of strange quarks to the electromagnetic form factors of the proton, complementing results from SAMPLE experiment and HAPPEX. Measured asymmetries were consistent with small strange-quark vector form factors and provided limits on isovector radiative corrections relevant to extractions of the weak mixing angle at low momentum transfer. Spin-dependent cross-section measurements contributed to understanding the spin decomposition of the nucleon, interfacing with global analyses that included data from EMC experiment, COMPASS, and RHIC Spin Program.

The experiment's polarimetry and null-asymmetry techniques set benchmarks for control of helicity-correlated systematics, influencing error budgets adopted in subsequent Qweak experiment and MOLLER experiment proposals. Tennessee STAR's results also provided empirical input for lattice Quantum Chromodynamics computations underway at Brookhaven National Laboratory and Riken BNL Research Center.

Impact and Legacy

Tennessee STAR's instrumentation developments in polarized sources, polarimetry, and low-background detection informed the design of later parity-violation and spin-structure experiments at Jefferson Lab, SLAC National Accelerator Laboratory, and Brookhaven National Laboratory. Collaborators from the project went on to leadership roles in experiments such as Qweak experiment, PHENIX, and CLAS12, and contributed to mentorship at institutions including University of Tennessee, Oak Ridge National Laboratory, and Massachusetts Institute of Technology. The experiment's data and methodological advances remain cited in reviews of strange-quark form factor extractions and in planning documents for precision electroweak probes at future facilities like Electron-Ion Collider.

Category:Particle physics experiments