Υ(3S)

Υ(3S)
Name	Υ(3S)
Other names	Upsilon(3S)
Type	Meson
Composition	b anti-b
Mass	10.3552 GeV/c2
Width	20.32 keV
Discovered	1982
Discoverers	SLAC, Cornell
Decay modes	mu+ mu-, hadrons, radiative transitions

Υ(3S) is the third radial excitation of the bottomonium vector meson family, a bound state of a bottom quark and bottom antiquark. It occupies a key position in the spectroscopy of heavy quarkonia, bridging lower states such as the Υ(1S) and Υ(2S) with open-bottom thresholds and higher excitations including the Υ(4S) and exotic candidates. Measurements of its mass, leptonic widths, radiative transitions, and hadronic decays have provided constraints for quantum chromodynamics and potential models developed at institutions like CERN, Fermilab, KEK, and DESY.

Discovery and Naming

The state was first observed in e+e− annihilation experiments at storage rings operated by SLAC and Cornell following earlier discoveries of the Υ family by collaborations including E598 and groups at the DORIS facility. The naming follows the historical convention established for the bottomonium sequence after the initial Υ discovery: Greek letter Υ with parenthetical radial quantum number designator "(3S)" to indicate the third S-wave vector excitation, a convention shared with charmonium states such as J/ψ and ψ(2S). Announcements of the Υ(3S) appearance were reported in conference proceedings alongside results from collaborations like CLEO, ARGUS, and later precision measurements by BaBar and Belle.

Properties and Quantum Numbers

Υ(3S) is characterized by quantum numbers JPC = 1−−, isospin I = 0, and flavor quantum numbers reflecting a b anti-b composition. Its measured mass near 10.355 GeV/c2 and narrow total width on the order of tens of keV place it below the BB̄ production threshold associated with the onset of open-bottom production that characterizes the Υ(4S) region; this positioning affects its available decay channels and transition rates observed by detectors at KEKB, PEP-II, and storage rings at Frascati. The state’s leptonic branching fractions, particularly to μ+μ− and e+e−, permit extraction of the radial wavefunction at the origin, a quantity compared against predictions from potential models by theorists at Princeton University, MIT, and University of Cambridge.

Production and Decay Modes

Υ(3S) production has been achieved primarily via e+e− annihilation at center-of-mass energies tuned to the resonance peak in facilities such as CESR, KEKB, and PEP-II, as well as in hadron colliders like LHC experiments that can produce bottomonia in proton–proton collisions recorded by ATLAS, CMS, and LHCb. Decay modes include electromagnetic transitions (E1 and M1) to lower bottomonium states—e.g., radiative decays to χbJ(1P) and χbJ(2P) multiplets studied by the CLEO and Belle II collaborations—leptonic channels to μ+μ− and e+e− used by BaBar for normalization, and hadronic annihilation modes into light hadrons. The proximity to open-beauty thresholds suppresses some annihilation rates while enhancing multiphoton and cascade transitions, phenomena investigated by groups at IHEP and SLAC National Accelerator Laboratory.

Spectroscopy and Relation to Other Bottomonium States

Within the bottomonium spectrum, Υ(3S) connects radially to Υ(1S) and Υ(2S) through measured E1 transitions and isospin-neutral hadronic cascades, and it sits below the open-bottom Υ(4S) which predominantly decays to B meson pairs observed by Belle and BaBar. The χbJ(1P) and χbJ(2P) triplets couple strongly via photons to the Υ(3S) and provide spectroscopic fingerprints exploited to map fine and hyperfine splittings; these splittings are compared to potentials formulated by researchers at University of Chicago and Caltech. Investigations of nearby states such as ηb(1S) and ηb(2S) have used Υ(3S) radiative transitions as tagging channels in analyses by BABAR and Belle II, illuminating spin-dependent forces in quantum chromodynamics as modeled by groups at IHEP Beijing and INFN.

Experimental Measurements and Observations

Precision determinations of the Υ(3S) mass and total width have emerged from energy scans and resonance fits conducted by collaborations including CLEO, BABAR, and Belle, with complementary production studies at LHCb and earlier results from ARGUS. Measurements of branching fractions to μ+μ− and radiative decays to χbJ states provide inputs for global fits of bottomonium parameters performed by theorists at SLAC and DESY. Unexpected observations—such as anomalous dipion transitions to Υ(1S) and Υ(2S) and searches for charged bottomonium-like structures—have prompted joint analyses by Belle and BaBar that reference exotic candidates reported by Belle II and theoretical interpretations considered at Perimeter Institute.

Theoretical Interpretations and Models

Υ(3S) has been a testing ground for nonrelativistic potential approaches, lattice quantum chromodynamics computations, and effective field theories like nonrelativistic QCD (NRQCD) developed by researchers at CERN, BNL, and Johns Hopkins University. Potential models—Cornell, Buchmüller-Tye, and screened potentials—are benchmarked against Υ(3S) level spacings and leptonic widths in studies by groups at University of Tokyo, Ohio State University, and Rutgers University. Lattice QCD collaborations, including HPQCD and RBC-UKQCD, have computed mass splittings and decay matrix elements to confront experimental results. Discrepancies between model predictions and precision data have motivated refinements invoking coupled-channel effects, spin-dependent interactions, and mixing with hybrid or molecular configurations examined at theoretical centers such as MIT and YITP.

Category:Bottomonium Category:Mesons