pdgLive

MASS LIMITS for ${{\mathit b}^{\,'}}$ (4${}^{\mathit th}$ Generation) Quark or Hadron in ${{\mathit e}^{+}}{{\mathit e}^{-}}$ Collisions

INSPIRE JSON (beta) PDGID:

Search for hadrons containing a fourth-generation $−$1/3 quark denoted ${{\mathit b}^{\,'}}$.

The last column specifies the assumption for the decay mode (${{\mathit C}}{{\mathit C}}$ denotes the conventional charged-current decay) and the event signature which is looked for.

VALUE (GeV)

CL%

DOCUMENT ID

TECN

COMMENT

$\bf{> 46.0}$

95

ALEP

any decay

• • We do not use the following data for averages, fits, limits, etc. • •

$\text{none 96 - 103}$

95

DLPH

${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit b}}{{\mathit Z}}$, ${{\mathit c}}{{\mathit W}}$

L3

Quarkonium

$>44.7$

95

L3

$\Gamma\mathrm {({{\mathit Z}})}$

$>45$

95

DLPH

$\Gamma\mathrm {({{\mathit Z}})}$

$\text{none 19.4 - 28.2}$

95

VNS

Any decay; event shape

$>45.0$

95

DLPH

B(${{\mathit C}}{{\mathit C}}$) = 1; event shape

$>44.5$

95

DLPH

${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit c}}{{\mathit H}^{-}}$, ${{\mathit H}^{-}}$ $\rightarrow$ ${{\overline{\mathit c}}}{{\mathit s}}$, ${{\mathit \tau}^{-}}{{\mathit \nu}}$

$>40.5$

95

DLPH

$\Gamma\mathrm {({{\mathit Z}} \rightarrow hadrons)}$

$>28.3$

95

TOPZ

B(${\mathrm {FCNC}})=100\%$; isol. ${{\mathit \gamma}}$ or 4 jets

$>41.4$

95

OPAL

Any decay; acoplanarity

$>45.2$

95

OPAL

B(${{\mathit C}}{{\mathit C}}$) = 1; acoplanarity

$>46$

95

OPAL

${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit \gamma}}{+}$ any

$>27.5$

95

VNS

B(${{\mathit C}}{{\mathit C}}$) =1; ${{\mathit \mu}}$, ${{\mathit e}}$

$\text{none 11.4 - 27.3}$

95

VNS

B(${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit b}}{{\mathit \gamma}}$) $>$ 10$\%$; isolated ${{\mathit \gamma}}$

$>44.7$

95

MRK2

B(${{\mathit C}}{{\mathit C}}$)= 100$\%$; isol. track

$>42.7$

95

MRK2

B(${{\mathit b}}{{\mathit g}}$)= 100$\%$; event shape

$>42.0$

95

MRK2

Any decay; event shape

$>28.4$

95

TOPZ

B(${{\mathit C}}{{\mathit C}}$) =1; ${{\mathit \mu}}$

$>28.8$

95

AMY

B(${{\mathit C}}{{\mathit C}}$) ${ {}\gtrsim{} }90\%$; ${{\mathit \mu}}$, ${{\mathit e}}$

$>27.2$

95

AMY

any decay; event shape

$>29.0$

95

AMY

B(${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit b}}{{\mathit g}}$) ${ {}\gtrsim{} }$ 85$\%$; event shape

$>24.4$

95

AMY

${{\mathit \mu}},{{\mathit e}}$

$>23.8$

95

AMY

event shape

$>22.7$

95

MRKJ

${{\mathit \mu}}$

$>21$

TASS

$\mathit R$, event shape

$>19$

TASS

Aplanarity

¹

DECAMP 1990F looked for isolated charged particles, for isolated photons, and for four-jet final states. The modes ${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit b}}{{\mathit g}}$ for B(${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit b}}{{\mathit g}}$) $>$ 65$\%$ ${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit b}}{{\mathit \gamma}}$ for B(${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit b}}{{\mathit \gamma}}$) $>$ 5$\%$ are excluded. Charged Higgs decay were not discussed.

²

ABDALLAH 2007 searched for ${{\mathit b}^{\,'}}$ pair production at $\mathit E_{{\mathrm {cm}}}=196 - 209$ GeV, with 420$~$pb${}^{-1}$. No signal leads to the 95$\%$ CL upper limits on B(${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit b}}{{\mathit Z}}$) and B(${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit c}}{{\mathit W}}$) for ${\mathit m}_{{{\mathit b}^{\,'}}}$ = 96 to 103 GeV.

³

ADRIANI 1993G search for vector quarkonium states near ${{\mathit Z}}$ and give limit on quarkonium-${{\mathit Z}}$ mixing parameter $\delta {{\mathit m}^{2}}_{}<(10 - 30$) GeV${}^{2}$ (95$\%$CL) for the mass $88 - 94.5$ GeV. Using Richardson potential, a 1S (${{\mathit b}^{\,'}}{{\overline{\mathit b}}^{\,'}}$) state is excluded for the mass range $87.7 - 94.7$ GeV. This range depends on the potential choice.

⁴	ABREU 1990D assumed ${\mathit m}_{{{\mathit H}^{-}}}$ $<$ ${\mathit m}_{{{\mathit b}^{\,'}}}$ $−$ 3 GeV.

⁵	Superseded by ABREU 1991F.

⁶

AKRAWY 1990B search was restricted to data near the ${{\mathit Z}}$ peak at $\mathit E_{{\mathrm {cm}}}$ = $91.26$ GeV at LEP. The excluded region is between $23.6$ and $41.4$ GeV if no ${{\mathit H}^{+}}$ decays exist. For charged Higgs decays the excluded regions are between (${\mathit m}_{{{\mathit H}^{+}}}$ $+1.5$ GeV) and $45.5$ GeV.

⁷

AKRAWY 1990J search for isolated photons in hadronic ${{\mathit Z}}$ decay and derive B(${{\mathit Z}}$ $\rightarrow$ ${{\mathit b}^{\,'}}{{\overline{\mathit b}}^{\,'}})\cdot{}B({{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit \gamma}}$ X)/B(${{\mathit Z}}$ $\rightarrow$ hadrons) $<$ $2.2 \times 10^{-3}$. Mass limit assumes B(${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit \gamma}}$ X) $>$ 10$\%$.

⁸	ABE 1989E search at $\mathit E_{{\mathrm {cm}}}$ = 56$-$57 GeV at TRISTAN for multihadron events with a spherical shape (using thrust and acoplanarity) or containing isolated leptons.

⁹	ABE 1989G search was at $\mathit E_{{\mathrm {cm}}}$ = 55$-60.8$ GeV at TRISTAN.

¹⁰

If the photonic decay mode is large (B(${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit b}}{{\mathit \gamma}}$) $>$ 25$\%$), the ABRAMS 1989C limit is $45.4$ GeV. The limit for for Higgs decay (${{\mathit b}^{\,'}}$ $\rightarrow$ ${{\mathit c}}{{\mathit H}^{-}}$, ${{\mathit H}^{-}}$ $\rightarrow$ ${{\overline{\mathit c}}}{{\mathit s}}$) is $45.2$ GeV.

¹¹	ADACHI 1989C search was at $\mathit E_{{\mathrm {cm}}}$ = $56.5-60.8$ GeV at TRISTAN using multi-hadron events accompanying muons.

¹²	ADACHI 1989C also gives limits for any mixture of ${{\mathit C}}{{\mathit C}}$ and ${{\mathit b}}{{\mathit g}}$ decays.

¹³	ENO 1989 search at $\mathit E_{{\mathrm {cm}}}$ = 50$-60.8$ at TRISTAN.

¹⁴	ENO 1989 considers arbitrary mixture of the charged current, ${{\mathit b}}{{\mathit g}}$, and ${{\mathit b}}{{\mathit \gamma}}$ decays.

¹⁵	IGARASHI 1988 searches for leptons in low-thrust events and gives $\Delta \mathit R({{\mathit b}^{\,'}}$) $<$ $0.26$ (95$\%$ CL) assuming charged current decay, which translates to ${\mathit m}_{{{\mathit b}^{\,'}}}$ $>$ $24.4$ GeV.

¹⁶

SAGAWA 1988 set limit ${\mathit \sigma (}$top${)}$ $<$ 6.1 pb at CL=95$\%$ for top-flavored hadron production from event shape analyses at $\mathit E_{{\mathrm {cm}}}$ = 52 GeV. By using the quark parton model cross-section formula near threshold, the above limit leads to lower mass bounds of 23.8 GeV for charge $−$1/3 quarks.

¹⁷	ADEVA 1986 give 95$\%$CL upper bound on an excess of the normalized cross section, $\Delta \mathit R$, as a function of the minimum c.m. energy (see their figure 3). Production of a pair of 1/3 charge quarks is excluded up to $\mathit E_{{\mathrm {cm}}}$ = 45.4 GeV.

¹⁸	ALTHOFF 1984C narrow state search sets limit $\Gamma\mathrm {({{\mathit e}^{+}} {{\mathit e}^{-}})}{}$B(hadrons) $<$2.4 keV CL = 95$\%$ and heavy charge 1/3 quark pair production $\mathit m$ $>$21 GeV, CL = 95$\%$.

¹⁹	ALTHOFF 1984I exclude heavy quark pair production for 7 $<\mathit m$ $<$19 GeV (1/3 charge) using aplanarity distributions (CL = 95$\%$).

Except where otherwise noted, content of the 2025 Review of Particle Physics is licensed under a Creative Commons Attribution 4.0 International (CC BY 4.0) license. The publication of the Review of Particle Physics is supported by US DOE, MEXT (Japan), INFN (Italy) and CERN. Individual collaborators receive support for their PDG activities from their respective institutes or funding agencies. © 2025. See LBNL disclaimers.