Averages for the PDG 2015 web update

Only results published (or accepted in a refereed journal) by February 28, 2015 have been included in the averages computed by the lifetime and oscillations sub-group of the Heavy Flavour Averaging Group (HFAG) for the 2015 web update of the Particle Data Group. The following material is available publicly: The combination procedures are described in Chapter 3 of the following HFAG writeup: This writeup describes the "Fall 2014" averages, which also include preliminary results; these averages do not include results published after that; hence they are not identical to the ones presented here.

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b-hadron lifetime averages

The lifetimes displayed in the table below have been obtained by combining time-dependent measurements from ALEPH, ATLAS, BABAR, BELLE, CDF, CMS, D0, DELPHI, L3, LHCb, OPAL and SLD.

b-hadron species average lifetime average lifetime relative to B0 average lifetime
B0 1.520 ± 0.004 ps
B+ 1.638 ± 0.004 ps 1.076 ± 0.004
Bs0 1.510 ± 0.005 ps 0.994 ± 0.004
BsL 1.422 ± 0.008 ps
BsH 1.610 ± 0.012 ps
Bc+ 0.507 ± 0.009 ps
Λb 1.466 ± 0.010 ps 0.965 ± 0.007
Ξb− 1.560 ± 0.040 ps
Ξb0 1.464 ± 0.031 ps
Ωb− 1.57 +0.23 −0.20 ps
b-hadron average
(weighted by fractions
in Z decays) 		1.567 ± 0.003 ps

The above B0 lifetime average is obtained assuming there is no decay width difference in the B0 system. The above Bs lifetime is defined as 1/Γs, where Γs = (ΓL + ΓH)/2 is the mean decay width of the Bs system, i.e. the average of the decay widths of the light and heavy states (BsL and BsH). The Λb lifetime average includes a measurement which is 2.5 sigma away from the average re-computed without this measurement; no scale factor was applied on the new combined error, although the Λb lifetime measurements are slightly discrepant (see plot). The b-hadron average ignores the rare species (Bc and b baryons other than Λb).

The tables below give a number of effective Bs lifetime averages, measured from single exponential fits of the proper time distributions of Bs decays to a number of interesting final states. In general each final state may be a different mixture of the two Bs mass eigenstates, and hence the effective lifetime falls somewhere between 1/ΓL and 1/ΓH. The "Bs → flavour specific" lifetime is measured mainly with Bs → Ds lepton X decays; it is used as input to extract the long and short lifetimes of the Bs system (see next section). The "Bs → J/ψ φ" lifetime is an average of the results from single exponential fits. Nowadays, the time dependence and the angular dependence of the Bs → J/ψ φ decays is analysed in a more sophisticated way in order to extract separately the long and short lifetimes (see further below).

mixture of the two
Bs mass eigenstates 		effective lifetime from
single exponential fits
Bs → flavour specific 1.511 ± 0.014 ps
Bs → J/ψ φ 1.479 ± 0.012 ps

The two tables below report effective Bs lifetime averages for final states that are either pure CP-even or pure CP-odd eigenstates. If the corresponding Bs decays are dominated by a single weak phase and if CP violation can be neglected, then the effective lifetime for decays to CP-even (CP-odd) eigenstates corresponds to 1/ΓL (1/ΓH). These averages are used as constraints in the fit to determine Γs and ΔΓs (see further below).

CP-even final state effective lifetime from
single exponential fits
Bs → D+sD−s 1.379 ± 0.031 ps

CP-odd final state effective lifetime from
single exponential fits
Bs → J/ψ f0, J/ψ ππ 1.656 ± 0.033 ps


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Neutral B meson mixing: decay width differences

For both the B0 and Bs systems, the mean decay width and the decay width difference are defined here as ΔΓ = ΓL − ΓH and Γ = (ΓL + ΓH)/2, where ΓL (ΓH) is the decay width of the light (heavy) mass eigenstate. In the Standard Model, one expects ΔΓ > 0, i.e. the light (heavy) mass eigenstate is also the short-lived (long-lived) mass eigenstate. This expectation has been observed to be correct for the Bs system. In the absence of CP violation, the light (heavy) B0 or Bs mass eigenstate is the CP-even (CP-odd) eigenstate. This assumption is made by some analyses included in the combined results given in this section.

Combined result on the relative decay width difference in the B0 system:

s×ΔΓd/Γd = 0.001 ± 0.010 from Belle, BABAR, D0, DELPHI and LHCb

The quantity s = sign(Re(λCP)), where λCP = (q/p)×ACP/ACP refers to a CP-even final state (e.g. J/ψKL), is predicted to be equal to s= +1 to a high degree of confidence from the Standard Model fits to all available constraints on the unitarity triangle.

The ATLAS, CDF and D0 analyses of the Bs → J/ψ φ decay, as well as the LHCb analyses of the Bs → J/ψKK and Bs → J/ψππ decays provide information on Γs, ΔΓs and the weak phase φsccs, defined as the phase difference between the mixing amplitude and the b→ccs decay amplitude of the Bs meson. Combined values of the average decay width Γs and the decay width difference ΔΓs are obtained from of a two-dimensional fit of the experimental results. In this fit, the CP-violating phase φsccs is fixed to zero, which is consistent with the φsccsaverage described below. The correlation between Γs and ΔΓs in each analysis is taken into account (but correlations with other parameters are ignored). The following additional constraints are applied, using effective lifetime measurements:

  1. Bs → J/ψf0(980) lifetime measurement from CDF and Bs → J/ψπ+π− lifetime measurement from LHCb (pure CP-odd final states), which average to τ(Bs→ CP-odd) = 1.656 ± 0.033 ps, taken to be equal to (1/ΓH)×[1−(φsccs)2×ΔΓs/4];
  2. Bs → Ds+Ds− lifetime measurement from LHCb (pure CP-even final states), which average to τ(Bs→ CP-even) = 1.379 ± 0.031 ps, taken to be equal to (1/ΓL)×[1+(φsccs)2×ΔΓs/4];
  3. flavour-specific Bs lifetime average τ(Bs → flavour specific) = 1.511 ± 0.014 ps, taken to be equal to (1/Γs) × (1 + (ΔΓs/Γs)2/4) / (1 − (ΔΓs/Γs)2/4).
The implementation of contraints I and II, described in full in the literature [R. Fleischer and R. Knegjens, Eur. Phys. J. C (2011) 1789], neglects here possible sub-leading Penguin contributions and possible direct CP violation. The table below shows the results with and without these additional constraints. The default (i.e. recommended) set of results is the one with all the constraints applied.

Fit results from
ATLAS, CDF, D0
and LHCb data 		without constraint
from effective
lifetime measurements 		with constraints
I and II 		with constraints
I, II and III
Γs 0.6622 ± 0.0027 ps−1 0.6614 ± 0.0026 ps−1 0.6623 ± 0.0023 ps−1
1/Γs 1.510 ± 0.006 ps    1.512 ± 0.006 ps    1.510 ± 0.005 ps   
τShort = 1/ΓL 1.427 ± 0.008 ps    1.422 ± 0.008 ps    1.422 ± 0.008 ps   
τLong = 1/ΓH 1.603 ± 0.015 ps    1.614 ± 0.013 ps    1.610 ± 0.012 ps   
ΔΓs +0.077 ± 0.008 ps−1 +0.084 ± 0.008 ps−1 +0.082 ± 0.007 ps−1
ΔΓs/Γs +0.116 ± 0.013         +0.127 ± 0.012         +0.124 ± 0.011        
correlation ρ(Γs, ΔΓs) −0.335 −0.286 −0.204

The two plots below show contours of Δ(ln(L)) = 0.5 (39% CL for the enclosed 2D regions, 68% CL for the bands), in the plane (Γs, ΔΓs) on the left and in the plane (1/ΓL, 1/ΓH) on the right. The average of all Bs → J/ψ φ measurements is shown as the red contour, and the constraints given by the effective lifetime measurements of Bs → J/ψf0(980), Bs → K+K− and Bs to flavour-specific final states ( 1.511 ± 0.014 ps) are shown as the green, purple and blue bands, respectively. The average taking all constraints into account is shown as the gray filled contour. The yellow band is a theory prediction ΔΓs = 0.087 ±0.021 ps−1 which assumes no new physics in Bs mixing [A. Lenz and U. Nierste, arXiv:1102.4274 [hep-ph]].


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Only left plot: png / eps / pdf /
Only right plot: png / eps / pdf /

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B0 mixing: oscillations and mass difference

Combined result on B0 mixing, obtained separately from time-dependent measurements of the oscillation frequency Δmd (at high energy colliders and asymmetric B factories) and from time-integrated measurements of the mixing probability χd at symmetric Υ(4S) machines:

Δmd = 0.510 ± 0.003 ps−1 from time-dependent measurements at ALEPH, DELPHI, L3, OPAL, CDF, D0, BABAR, BELLE, LHCb
χd = 0.182 ± 0.015 from time-integrated measurements at ARGUS and CLEO

Assuming no CP violation in the mixing and no width difference in the B0 system, and using the B0 lifetime average of 1.520 ± 0.004 ps (the experimental average listed above), all above measurements can be combined to yield the following world averages:

Δmd = 0.510 ± 0.003 ps−1
   xd = 0.775 ± 0.006
χd = 0.1875 ± 0.0017 		from all ALEPH, DELPHI, L3, OPAL, CDF, D0, BABAR, BELLE, LHCb, ARGUS and CLEO measurements

In the plot below, all individual measurements are listed as quoted by the experiments; they might assume different physics inputs. The averages (which take into account all known correlations) are quoted after adjusting the individual measurements to the common set of physics inputs. The χd average from ARGUS and CLEO is converted to a Δmd measurement assuming no CP violation, no width difference in the B0 system and a B0 lifetime of 1.520 ± 0.004 ps.


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Same without average including time-integrated (χd) measurements:
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Only measurements and average at LEP and CDF1:
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Only measurements and average at LEP:
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Only measurements and average at asymmetric B factories:
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In the plot below, the individual experiment averages are listed as quoted by the experiments (or computed by the working group without performing any adjustments); they might assume different physics inputs. The global averages are quoted after adjusting the individual measurements to the common set of physics inputs. The χd average from ARGUS and CLEO is converted to a Δmd measurement assuming no CP violation, no width difference in the B0 system and a B0 lifetime of 1.520 ± 0.004 ps.


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2D average of Δmd and τ(B0)

BABAR and Belle have performed simultaneous measurements of Δmd and τ(B0). The Belle analysis is actually a simultaneous measurement of Δmd, τ(B0) and τ(B+), and has been converted, for the purpose of averaging with the BABAR results, into a 2D measurement of Δmd and τ(B0). The plot below displays these measurements (after adjustments to a common B+ lifetime of 1.638 ± 0.004 ps) together with their 2D average. The result of this 2D combination is Δmd = 0.509 ± 0.006 ps−1 and τ(B0) = 1.527 ± 0.010 ps, with a total (stat+syst) correlation coefficient of −0.23 (note that this result on Δmd is already included in the Δmd world average quoted above).


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Bs mixing: oscillations and mass difference

Combined result on B0s mixing, obtained from time-dependent measurements of the oscillation frequency Δms at high-energy hadron colliders:

Δms = 17.757 ± 0.021 ps−1 CDF, LHCb

With a mean B0s lifetime of 1/Γs = 1.510 ± 0.005 ps, a decay width difference of ΔΓs = +0.082 ± 0.007 ps−1 and the assumption of no CP violation in B0s mixing, this leads to

xs = 26.81 ± 0.10
χs = 0.499308 ± 0.000005

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Neutral B meson mixing: CP violation

Several different parameters can be used to describe CP violation in B mixing: |q/p|, the so-called dilepton asymmetry ASL, and the real part of εB. The relations between these parameters are as follows (all are exact except the last one which is an approximation valid for small CP violation):
   ASL = (|p/q|2−|q/p|2)/(|p/q|2+|q/p|2) = (1 − |q/p|4)/(1+|q/p|4)
   |q/p| = [(1−ASL)/(1+ASL)]**0.25
   εB = (p−q)/(p+q)
   q/p = (1−εB)/(1+εB)
   ASL ~ 4 Re(εB)/(1+|εB|2)

The parameters |q/p|, ASL and Re(εB)/(1+|εB|2) are thus equivalent. There is CP violation in the mixing if |q/p| is different from 1, i.e. ASL is different from 0.

Averages are given below separately for the B0 and the Bs systems. Two sets of averages are given for the B0 system in the first table: a first set using only measurements performed at Υ(4S) machines, and a second set using all measurements (excluding those that assume no CP violation in Bs mixing). The second table presents an average for the Bs system. Measurements performed at high energy that do not separate the B0 and Bs contributions are no longer used to obtain the final averages (at this time, the only measurements at high energy used in the averages are from D0 and LHCb).

CP violation parameter in B0 mixing
|q/p| = 1.0009 ± 0.0013
ASL = −0.0019 ± 0.0027
Re(εB)/(1+|εB|2) = −0.0005 ± 0.0007
from measurements at the Υ(4S)
|q/p| = 1.0007 ± 0.0009
ASL = −0.0015 ± 0.0017
Re(εB)/(1+|εB|2) = −0.0004 ± 0.0004
world average

CP violation parameter in Bs mixing
|q/p| = 1.0038 ± 0.0021
ASL = −0.0075 ± 0.0041
world average

The above world averages ASL(B0) = −0.0015 ± 0.0017 and ASL(Bs) = −0.0075 ± 0.0041 are obtained from a two-dimensional fit of the CLEO, BABAR, Belle, D0 and LHCb results: the correlation coefficient between them is found to be −0.158 . This is illustrated in the plot below, where the vertical band is the average of the pure B0 measurements performed at CLEO, BABAR, Belle, D0 and LHCb, the horizontal band is the average of the pure Bs measurements performed at D0 and LHCb with semileptonic Bs decays, the green ellipse is the D0 measurement with same-sign dileptons, and the read ellipse is the result of the two-dimensional averaging. The red point close to (0,0) is the Standard Model prediction [A. Lenz and U. Nierste, arXiv:1102.4274 [hep-ph]] with errors bars multiplied by 10. The prediction and experimental average deviate from each other by 1.5 σ.


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CP violation in Bs mixing is caused by the weak phase difference φ12=arg[−M12/Γ12], where M12 and Γ12 are the off-diagonal elements of the mass and decay matrices. The tangent of this phase difference can be estimated (approximately) as ASL(Bs) Δms/ΔΓs= −1.6 ± 0.9 using the above averages of ASL(Bs), Δms and ΔΓs.

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Phase difference between Bs-mixing and b→ccs amplitudes

The weak phase difference φsccs between the Bs mixing amplitude and the b→ccs decay amplitude of the Bs meson (for example in Bs → J/ψφ) is predicted by the Standard Model to be approximately equal to −2βs, where βs = arg(−(Vts Vtb*)/(Vcs Vcb*)) ~ 1 degree. The phase −2βs is the equivalent of 2β for the B0 meson. The phase φsccs has been measured in six analyses: three Bs → J/ψφ analyses from CDF, D0 and ATLAS, one Bs → J/ψK+K− analysis from LHCb, one Bs → J/ψπ+π− analysis from LHCb, and one Bs → DsDs analysis from LHCb. A combined 2D fit of φsccs and ΔΓs, without external assumption, yields two symmetric solutions related through φsccs ↔ π− φsccs and ΔΓs ↔ −ΔΓs. Only the solution with positive ΔΓs is shown in the table below, as detailed analysis of the strong phases in Bs → J/ψK+K− by LHCb (arXiv:1202.4717 [hep-ex]) has shown that this is the physical solution. Only the correlations between the measurements of φsccs and ΔΓs are taken into account (possible correlations with other parameters are ignored).

Combined result from
CDF, D0, ATLAS and LHCb data
φsccs −0.013 ± 0.037

The recommended final average of ΔΓs is the one given in the section on decay width differences, where all the available information on ΔΓs is used.

The two plots below show different 68% confidence-level contours in the (φsccs, ΔΓs) plane. The left plot shows the individual contours of ATLAS, CDF, D0 and LHCb, their combined contour (solid line and shaded area), as well as the Standard Model predictions (thin black rectangle). The prediction for φsccs is taken as the indirect determination of −2βs via a global fit to experimental data within the Standard Model, −2βs = −0.0363 +0.0012 −0.0014 [CKMfitter, Phys. Rev. D84, 033005 (2011), updated with Moriond14 results], while the Standard Model prediction for ΔΓs is 0.087 ±0.021 ps−1 [A. Lenz and U. Nierste, arXiv:1102.4274 [hep-ph]]. The combined result is consistent with these predictions. The right plot shows the same combined contour and SM predictions together with the regions allowed at 68% and 95% CL by the average measurements ASL(Bs) = −0.0075 ± 0.0041 and Δms = 17.757 ± 0.021 ps−1, through the relation tanφ12 = ASL(Bs) × Δms / ΔΓs, where φ12 = arg(−M12/Γ12) is the phase mismatch between the off-diagonal elements of the mass and decay matrices of the Bs−Bs system. This region is drawn under the assumption that possible new physics will not affect the phase difference φsccs−φ12, i.e. that this phase difference is equal to its Standard Model prediction [A. Lenz and U. Nierste, arXiv:1102.4274 [hep-ph]].


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b-hadron fractions in Υ(4S) decays

The B+ and B0 fractions below are for an unbiased sample of B mesons produced in Υ(4S) decays. Most analyses measure the ratio f+−/f00 assuming isospin invariance in charged and neutral B decays, and relying on our knowledge of the B+/B0 lifetime ratio. Combining all these analyses from BABAR, BELLE and CLEO leads to the average f+−/f00 = 1.059 ± 0.027 after adjusting to a common B+/B0 lifetime ratio of 1.076 ± 0.004 (the current average given above). On the other hand, BABAR measured directly f00 = 0.487 ± 0.013 without assuming isospin invariance nor relying on the B+/B0 lifetime ratio.

f+−/f00 = 1.059 ± 0.027 from ratios of reconstructed B+ and B0 mesons at BABAR, BELLE and CLEO
(assumptions made, see text above)
f00 = 0.487 ± 0.013 from absolute measurement of B0 mesons at BABAR
(no assumptions)

Assuming f+− + f00 = 1, the above two independent results (which are consistent with each other) can be combined to yield:

b-hadron species fraction in Υ(4S) decay ratio
B+ B− f+− = 0.514 ± 0.006 f+−/f00 = 1.058 ± 0.024
B0 B0 f00 = 0.486 ± 0.006
Note that the ratio f+-/f00 differs from unity by 2.4 σ.

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b-hadron fractions in Υ(5S) decays

The table below show the fraction of events containing non-strange B mesons (fu,d), strange B mesons (fs), or no B mesons at all (fnoB) in a sample of Υ(5S) decays, or more precisely in a sample of bb events produced in e+e− collisions at a centre-of-mass energy equal to the Υ(5S) mass. The sum of the three fractions is constrained to unity: fu,d+fs+fnoB=1. Their combined values have been obtained by combining model-dependent estimates of CLEO3 and Belle based on the measurements of several inclusive Υ(5S) branching fractions, after performing adjustments to common external inputs. A one-sided constraint on fnoB from the direct measurements of Υ(5S) decays to final states without bottom mesons has been used, caused the strongly asymmetric uncertainty on the final value of fnoB.

final states fraction in Υ(5S) decays ratio of fractions
Bu,d(*) Bu,d(*)(π(π)) fu,d  = 0.761 +0.027 −0.042
Bs(*) Bs(*) fs     = 0.200 +0.030 −0.031 fs / fu,d = 0.263 +0.052 −0.044
no open bottomness fnoB = 0.039 +0.050 −0.004


The plot below shows the published measurements of fs. All values have been obtained assuming fnoB=0. They are quoted as in the original publication, except for the most recent measurement of Belle which is quoted as fs = 1−fud. The average value of all these measurements is quoted with or without the assumption that fnoB=0, after performing adjustments to common external inputs.


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b-hadron fractions in Z decays

The table below shows the b-hadron fractions in an unbiased sample of weakly decaying b-hadrons produced in Z decays. These fractions have been calculated by combining direct rate measurements performed at LEP with the LEP combined measurement of the time-integrated mixing probability averaged over an unbiased sample of semi-leptonic b-hadron decays, χ = f'(Bd)χd+f'(Bs)χs = 0.1259 ± 0.0042 . This combination relies on the world average of χd, on the assumption χs = 1/2, as well as on the world averages of the lifetimes of the individual b-hadrons species. The B+ and B0 mesons are assumed to be produced in equal amount, the Bc production is neglected and the sum of the fractions is constrained to unity.

b-hadron species fraction in Z decays correlation with f(Bs) correlation with f(b-baryon)
Bs f(Bs) = 0.100 ± 0.008
b baryons f(b-baryon) = 0.082 ± 0.011 +0.075
B0 or B+ f(Bd) = f(Bu) = 0.409 ± 0.007 −0.639 −0.815
Bs / (B0 or B+) ratio f(Bs)/f(Bd) = 0.244 ± 0.023

This is based on the following average of χ in Z decays:

  χ(LEP) = 0.1259 ± 0.0042     LEP average from LEP EW WG


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b-hadron fractions in pp collisions at 1.8−2 TeV

The table below shows the b-hadron fractions in an unbiased sample of weakly decaying b-hadrons produced in pp collisions at √s = 1.8−2 TeV. These fractions have been calculated by combining direct rate measurements performed at Tevatron with the Tevatron combined measurement of the time-integrated mixing probability averaged over an unbiased sample of semi-leptonic b-hadron decays, χ = 0.147 ± 0.011 . This combination relies on the world average of χd, on the assumption χs = 1/2, as well as on the world averages of the lifetimes of the individual b-hadrons species. The B+ and B0 mesons are assumed to be produced in equal amount, the Bc production is neglected and the sum of the fractions is constrained to unity.

b-hadron species fraction in pp collisions
at 1.8−2 TeV		 correlation with f(Bs) correlation with f(b-baryon)
Bs f(Bs) = 0.115 ± 0.013
b baryons f(b-baryon) = 0.187 ± 0.044 −0.414
B0 or B+ f(Bd) = f(Bu) = 0.349 ± 0.020 +0.130 −0.956
Bs / (B0 or B+) ratio f(Bs)/f(Bd) = 0.330 ± 0.040

This is based on the following average of χ in pp collisions at 1.8−2 TeV:

  χ(Tevatron) = 0.147 ± 0.011     Average of CDF and D0 measurements


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b-hadron fractions at high energy

The table below shows the b-hadron fractions in an unbiased sample of weakly decaying b-hadrons produced at high energy, computed under the assumption that they are the same in Z decays, in pp collisions at the Tevatron (√s=1.8−2 TeV) or in proton-proton collisions at the LHC (√s=1.8−7 TeV). They have been calculated by combining direct rate measurements performed by the LEP, CDF and LHCb experiments with the world average of the time-integrated mixing probability averaged over an unbiased sample of semi-leptonic b-hadron decays, χ = 0.1284 ± 0.0069 . This combination relies on the world average of χd, on the assumption χs = 1/2, as well as on the world averages of the lifetimes of the individual b-hadrons species. The B+ and B0 mesons are assumed to be produced in equal amount, the Bc production is neglected and the sum of the fractions is constrained to unity.

b-hadron species fraction at high energy correlation with f(Bs) correlation with f(b-baryon)
Bs f(Bs) = 0.105 ± 0.005
b baryons f(b-baryon) = 0.085 ± 0.011 −0.204
B0 or B+ f(Bd) = f(Bu) = 0.405 ± 0.006 −0.273 −0.886
Bs / (B0 or B+) ratio f(Bs)/f(Bd) = 0.260 ± 0.015

This is based on the following average of χ at high energy:

  χ = 0.1259 ± 0.0042     LEP average from LEP EW WG
  χ = 0.147   ± 0.011     Tevatron average
  χ = 0.1284 ± 0.0069     Weighted average of above two, with error rescaled by factor 1.8 according to PDG prescription

WARNING:




Author: OS 12-Apr-2015, rev. 22-Apr-2016
Latest mod. mer avr 29 01:02:56 CEST 2015