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| E791 | |
|---|---|
| Name | Fermilab Fixed-Target Charm Experiment |
| Other names | Fermilab E-791 |
| Facility | Fermilab |
| Location | Batavia, Illinois |
| Collaboration | Fermilab experiment 791 collaboration |
| Beam | 500 GeV/c negative pion beam |
| Target | platinum and carbon foils |
| Detector type | fixed-target spectrometer |
| Run period | 1991 |
| Events recorded | ~20 billion triggers |
| Spokesperson | Eugene T. Worcester |
E791
The project was a high-statistics charm hadroproduction experiment at Fermilab that collected an unprecedented sample of charm and charmed-strange hadrons using a 500 GeV/c negative pion beam and a thin-foil target array. It operated in the early 1990s and sought to study weak decays, lifetimes, form factors, and rare processes of particles such as D mesons, D_s mesons, and charmed baryons including the Lambda_c^+; the dataset enabled precision measurements relevant to tests of the Cabibbo–Kobayashi–Maskawa matrix and searches for physics beyond the Standard Model. Key collaborators included institutions such as University of Chicago, Texas A&M University, University of Mississippi, Fermilab and numerous international universities and laboratories.
The apparatus used a high-energy secondary beam produced in the Tevatron complex and transported to a fixed-target station instrumented with segmented targets of platinum and carbon to optimize charm production and reduce multiple scattering. Downstream of the target, a vertex silicon microstrip detector array provided precise tracking akin to systems used later at SLAC National Accelerator Laboratory and CERN, while large-aperture dipole magnets and multiwire proportional chambers delivered momentum analysis comparable to spectrometers at Brookhaven National Laboratory. Particle identification relied on a multicell threshold Cherenkov detector with performance similar to devices used at DESY and time-of-flight elements influenced by designs from KEK experiments.
Primary objectives included measurement of lifetimes for D^0, D^+, D_s^+, and charmed baryons; determination of branching fractions for semileptonic and hadronic decay modes; studies of mixing and indirect CP violation analogous to searches carried out later by BaBar and Belle; and searches for rare and forbidden decays that could indicate contributions from supersymmetry, heavy Z' boson, or other extensions explored at CERN. Results produced precision lifetime ratios, improved branching fraction determinations for modes like D → Kππ, and limits on charm mixing parameters that constrained theoretical frameworks including heavy-quark effective theory calculations used by groups at University of Oxford and Princeton University.
Analysis exploited large-scale offline computing farms and pattern-recognition algorithms developed contemporaneously with software from FNAL Computing Division efforts and influenced by techniques used at CERN experiments. Event reconstruction combined vertex fitting with kinematic constraints and multivariate selection methods such as likelihood ratio cuts, precursors to machine-learning classifiers later used by ATLAS and CMS. Background suppression used sideband subtraction and invariant-mass fits employing fitting tools similar to those from ROOT workflows developed at CERN. Systematic studies compared Monte Carlo simulations based on generators and detector-response models adapted from GEANT packages and tuning informed by datasets from E691 and other predecessor fixed-target experiments.
The detector featured a high-resolution silicon microstrip vertex detector, large-area drift chambers for tracking, a dipole magnet for momentum analysis, an electromagnetic calorimeter for neutral-particle reconstruction, a hadronic calorimeter, and a muon identification system. The multicell threshold Cherenkov counter provided charged-hadron separation between pions, kaons, and protons over the momentum range of interest, analogous to PID systems used by collaborations at CERN and KEK. Trigger electronics implemented a fast hardware-level selection for high-transverse-momentum tracks and vertex topologies, with a multilevel architecture influenced by designs from SLAC fixed-target programs. Readout and data acquisition systems used custom-built modules and VME-based electronics common to several contemporary experiments at Fermilab and Brookhaven National Laboratory.
The collaboration comprised numerous universities and laboratories across the United States, Europe, and Asia, including groups from University of California, Berkeley, University of Pittsburgh, University of Tokyo, University of Pisa, University of Coimbra, and Texas A&M University, coordinated with beam and infrastructure support from Fermilab. The experiment was proposed and approved in the late 1980s, recorded data during a high-intensity run in 1991, and performed analysis throughout the 1990s with publications appearing in major journals and conference presentations at venues like the International Conference on High Energy Physics and meetings of the American Physical Society Division of Particles and Fields.
The high-statistics dataset influenced subsequent charm-physics programs at collider and fixed-target facilities, contributing to techniques later adopted by CLEO, BaBar, Belle, LHCb, and fixed-target charm initiatives. Detector innovations in vertexing and particle identification informed designs for silicon systems and Cherenkov detectors at CERN and KEK, while data-analysis approaches anticipated the widespread use of multivariate methods and large-scale computing farms at Stanford Linear Accelerator Center and elsewhere. Limits and measurements produced by the collaboration remain part of global fits to charm-sector parameters used by theory groups at CERN and Brookhaven National Laboratory, and alumni of the collaboration went on to leadership roles in experiments including ATLAS, CMS, and LHCb.