B meson
In particle physics, a B meson (also known as a bottom meson) is an unstable meson composed of a bottom quark (or antiquark) bound to a lighter up, down, strange, or charm antiquark (or quark), resulting in pseudoscalar particles with spin-parity quantum numbers J^{PC} = 0^{-+}. These mesons, with masses ranging from approximately 5.28 GeV/c^2 for the lightest (B^\pm, B^0) to 6.27 GeV/c^2 for the doubly heavy B_c^\pm, decay primarily via the weak interaction on timescales of picoseconds and are essential for probing flavor-changing processes, CP violation, and the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements that describe quark mixing in the Standard Model. The B^\pm mesons (quark content u\bar{b} and \bar{u}b) have a mass of $5279.41 \pm 0.07 MeV/c^2 and a mean lifetime of (1.638 \pm 0.004) \times 10^{-12} s, while the neutral B^0 and \bar{\mathrm{B}}^0 ( d\bar{b} and \bar{d}b) have masses of $5279.63 \pm 0.20 MeV/c^2 and mean lifetimes of (1.517 \pm 0.004) \times 10^{-12} s. The B_s^0 and \bar{\mathrm{B}}_s^0 ( s\bar{b} and \bar{s}b), which include a strange quark, exhibit a mass of $5366.91 \pm 0.11 MeV/c^2, a mean lifetime of (1.516 \pm 0.006) \times 10^{-12} s, and notable mixing dynamics with a width difference \Delta \Gamma / \Gamma = 0.124 \pm 0.007. The B_c^\pm ( c\bar{b} and \bar{c}b) stands out with a mass of $6274.47 \pm 0.32 MeV/c^2 and a shorter mean lifetime of (0.510 \pm 0.009) \times 10^{-12} s due to decay channels involving both charm and bottom quarks. These properties, derived from high-precision measurements at electron-positron and hadron colliders, enable detailed studies of rare decays and oscillations that test the Standard Model's predictions.[1] The bottom quark was discovered in 1977 through the observation of the \Upsilon(1S) resonance—a bound state of a bottom quark and antiquark—at the Fermilab proton synchrotron, confirming the existence of a third generation of quarks and motivating the search for B mesons. Direct observation of B mesons occurred in the early 1980s at electron-positron colliders tuned to the \Upsilon(4S) resonance, which decays almost exclusively to B\bar{\mathrm{B}} pairs, with initial evidence from the CLEO experiment at Cornell Electron Storage Ring (CESR) via inclusive lepton spectra from semileptonic decays. Subsequent experiments, including ARGUS, BaBar, Belle, and LHCb, have revolutionized B physics by measuring mixing parameters, branching ratios for hundreds of decay modes, and asymmetries that reveal subtle CP-violating effects, providing stringent tests of the Standard Model and hints of potential new physics in rare processes like b \to s \ell^+ \ell^- transitions.Introduction
Definition and Composition
B mesons are a family of bottom-flavored hadrons classified as mesons, consisting of a bottom antiquark (\bar{b}) bound to a lighter quark through the strong nuclear force described by quantum chromodynamics (QCD). In the quark model, these particles form color-neutral quark-antiquark pairs, where the bottom antiquark pairs with an up (u), down (d), strange (s), or charm (c) quark.[2] The binding arises from the exchange of gluons, which confines the quarks within a potential that dominates at short distances and ensures the overall stability of the meson.[2] The specific members of the B meson family are denoted by their quark content and charge: the B^+ meson contains a u quark and \bar{b} antiquark (u\bar{b}), the neutral B^0 consists of d\bar{b}, the B_s^0 is made of s\bar{b}, and the B_c^+ comprises c\bar{b}. These notations follow the standard convention for open-flavor mesons in particle physics, distinguishing them from hidden-bottom states like bottomonium (b\bar{b}).[2] As ground-state mesons in the quark model, B mesons are pseudoscalars with total angular momentum and parity quantum numbers J^P = 0^-, corresponding to zero orbital angular momentum (L=0) between the quark and antiquark, combined with their intrinsic spins.[2] The mass of the B meson is largely determined by the heavy bottom antiquark, which has a mass of approximately 4.18 GeV/c^2 in the \overline{\rm MS} scheme at the scale of its own mass.[3] This heavy quark dominance simplifies the description of their dynamics compared to lighter mesons, allowing effective field theory approaches to model their behavior under QCD.[2]Historical Discovery
The B mesons were predicted within the framework of the Standard Model's quark model in the 1970s, as bound states involving the bottom quark, which was theorized as part of a third generation of quarks to accommodate CP violation in weak interactions. This prediction stemmed from the work of Kobayashi and Maskawa, who proposed the existence of top and bottom quarks to extend the Cabibbo model and explain observed CP asymmetries in kaon decays. The bottom quark was experimentally discovered in 1977 by the E288 collaboration at Fermilab, through the observation of a narrow resonance at 9.4 GeV in proton-nucleus collisions, interpreted as the Υ particle—a bottom-antibottom quarkonium state—confirming the quark's mass around 5 GeV/c² and its role in completing the generational structure. The first direct observation of charged B mesons occurred in 1983 by the CLEO experiment at the Cornell Electron Storage Ring (CESR), where electron-positron collisions at the Υ(4S) resonance—decaying almost exclusively to B \overline{B} pairs—yielded reconstructed exclusive hadronic decays such as B^+ \to D^0 \pi^+ and B^+ \to \overline{D}^0 \pi^+, establishing the charged B mass at approximately 5.28 GeV/c² with a statistical significance exceeding 5σ. Similarly, the neutral B^0 meson was observed in 1983 at the PEP storage ring at SLAC via high-energy e^+ e^- annihilation into continuum events, with the MARK II detector identifying semileptonic decays consistent with neutral B production and measuring its mass close to that of the charged counterpart, supporting the quark model's expectations for light-quark partners to the bottom quark. A pivotal advancement came in 1987 with the ARGUS and CLEO collaborations' confirmation of B^0-\overline{B}^0 mixing, where neutral B mesons were observed to oscillate into their antiparticles via second-order weak processes, with the mixing frequency measured at about 0.5 ps^{-1} from dilepton events at the Υ(4S). This discovery validated theoretical predictions of flavor-changing neutral currents mediated by the top quark and opened avenues for probing CP violation in the B system. The subsequent shift to high-precision measurements was enabled by the asymmetric-energy B factories: BaBar at SLAC, operating from 1999 to 2008, and Belle at KEK, running from 1999 to 2010, which amassed billions of B mesons to refine properties and search for new physics beyond the Standard Model.Fundamental Properties
Masses and Lifetimes
The ground-state B mesons, consisting of a bottom antiquark paired with a light up, down, strange, or charm quark, exhibit masses determined through precision measurements in high-energy collider experiments. The charged B⁺ meson, composed of u \bar{b}, has a mass of $5279.41 \pm 0.07 MeV/c^2, while the neutral B⁰ meson (d \bar{b}) has $5279.63 \pm 0.20 MeV/c^2. The Bₛ⁰ meson (s \bar{b}) is heavier at $5366.91 \pm 0.11 MeV/c^2, reflecting the increased mass of the strange quark, and the B_c⁺ meson (c \bar{b}) reaches $6274.47 \pm 0.32 MeV/c^2 due to the heavy charm quark component.[4] These masses arise primarily from the constituent quark masses within the meson, with binding effects contributing smaller corrections; the progression from lighter to heavier partner quarks establishes the scale of bottom-flavored pseudoscalar mesons. The near-degeneracy between B⁺ and B⁰ masses exemplifies isospin symmetry, where the up and down quarks are treated as nearly identical under the strong interaction, with small deviations (~0.2 MeV) attributable to electromagnetic interactions and quark mass differences.[4]| Meson | Mass (MeV/c^2) |
|---|---|
| B⁺ | 5279.41 ± 0.07 |
| B⁰ | 5279.63 ± 0.20 |
| Bₛ⁰ | 5366.91 ± 0.11 |
| B_c⁺ | 6274.47 ± 0.32 |
| Meson | Lifetime (ps) |
|---|---|
| B⁺ | 1.638 ± 0.004 |
| B⁰ | 1.517 ± 0.004 |
| Bₛ⁰ | 1.516 ± 0.006 |
| B_c⁺ | 0.510 ± 0.009 |
Quantum Numbers
B mesons in their ground state are pseudoscalar particles, with total spin J = 0 and negative parity P = -1. For the neutral members, such as B^0 and B_s^0, the charge conjugation quantum number is C = +1, yielding J^{PC} = 0^{-+}. Charged B mesons, including B^+ and B_c^+, lack a well-defined C due to their non-neutrality, so their assignment is J^P = 0^-. These quantum numbers arise from the quark model description of B mesons as q \bar{b} bound states in an S-wave (L = 0), with the light quark q and antiquark \bar{b} in a spin-singlet configuration (S = 0).[4][2] All B mesons carry baryon number B = 0 and lepton number L = 0, consistent with their classification as quark-antiquark pairs rather than three-quark states or leptons. The electric charge Q is +1/e for B^+ and B_c^+, and $0 for B^0 and B_s^0, determined by the charges of their constituent quarks via the Gell-Mann–Nishijima formula.[4] Flavor quantum numbers further distinguish B meson species. Every ground-state B meson has bottomness b = +1, reflecting the presence of the \bar{b} antiquark (which carries b = +1, opposite to the b quark's b = -1). Strangeness S = 0 for B^+, B^0, and B_c^+, but S = -1 for B_s^0 due to its strange quark content. Charm quantum number C = 0 except for B_c^+, where C = +1 from the charm quark. Topness t = 0 for all.[4][2] In terms of isospin, the B^+ = u\bar{b} and B^0 = d\bar{b} form an I = 1/2 doublet, with I_3 = +1/2 for B^+ and I_3 = -1/2 for B^0, analogous to the light-quark isodoublet structure. The B_s^0 = s\bar{b} and B_c^+ = c\bar{b} each have I = 0, as the strange and charm quarks are isospin singlets. These assignments stem from the SU(3) flavor symmetry breaking in the quark model.[2][4] The following table summarizes the key quantum numbers for the ground-state B mesons:| Particle | Quark Content | J^{PC} or J^P | Charge Q | Isospin I (I_3) | Bottomness b | Strangeness S | Charm C |
|---|---|---|---|---|---|---|---|
| B^+ | u\bar{b} | $0^- | +1 | 1/2 (+1/2) | +1 | 0 | 0 |
| B^0 | d\bar{b} | $0^{-+} | 0 | 1/2 (-1/2) | +1 | 0 | 0 |
| B_s^0 | s\bar{b} | $0^{-+} | 0 | 0 (0) | +1 | -1 | 0 |
| B_c^+ | c\bar{b} | $0^- | +1 | 0 (0) | +1 | 0 | +1 |