Anomalies in B Decays: A Sign of New Physics?
583
3.2 Simultaneous Explanations of b → sμ + μ − and b → cτ − ¯
ν
Now, (c, s) L is a doublet of SU (2) L . This suggests that, if the NP coupling is purely
LH, b → s and b → c transitions are related. It should therefore be possible to find
NP that can simultaneously explain both the b → sμ + μ − and b → cτ − ¯
ν anomalies
[35].
There are two classes of models that, in principle, can do this:
• A new triplet of vector bosons (W ± , Z ). The W and Z contribute respectively
to b → cτ − ¯
ν and b → sμ + μ − .
• A LQ of charge Q em =
2
3 . It couples to ¯
bμ + and ¯
sμ + (for b → sμ + μ − ) and to
¯
bτ + and c ¯
ν τ (for b → cτ − ¯
ν).
It is found [36–39] that, when all constraints are taken into account, including
those from direct searches at the LHC, the (W ± , Z ) model is excluded. But the
LQ model is viable!
4 Summary
The SM is certainly correct, but it is not complete: there must be physics beyond
the SM. Recently, there have been several measurements of observables that are in
disagreement with the predictions of the SM:
• b → sμ + μ − : These include many observables involving this decay. Some are
clean, while others have important theoretical uncertainties. Global fits allowing
for NP in b → sμ + μ − find improvements over the SM at the level of close to 6σ .
NP Models with an extra Z or with different types of LQs have been proposed
as explanations.
• b → cτ − ¯
ν: here there are several clean observables, with a net deviation from
the SM of ∼ 3σ . These can be explained in models with an extra W or with
different types of LQs.
It is of course possible that these discrepancies with the SM are all statistical
fluctuations, and will go away with more data. This said, their combined statistical
significance is sizeable ( 4σ ), so they will not disappear soon. Hopefully, we are
indeed seeing the first experimental signals of NP.
Acknowledgement This work was financially supported in part by NSERC of Canada.
583
3.2 Simultaneous Explanations of b → sμ + μ − and b → cτ − ¯
ν
Now, (c, s) L is a doublet of SU (2) L . This suggests that, if the NP coupling is purely
LH, b → s and b → c transitions are related. It should therefore be possible to find
NP that can simultaneously explain both the b → sμ + μ − and b → cτ − ¯
ν anomalies
[35].
There are two classes of models that, in principle, can do this:
• A new triplet of vector bosons (W ± , Z ). The W and Z contribute respectively
to b → cτ − ¯
ν and b → sμ + μ − .
• A LQ of charge Q em =
2
3 . It couples to ¯
bμ + and ¯
sμ + (for b → sμ + μ − ) and to
¯
bτ + and c ¯
ν τ (for b → cτ − ¯
ν).
It is found [36–39] that, when all constraints are taken into account, including
those from direct searches at the LHC, the (W ± , Z ) model is excluded. But the
LQ model is viable!
4 Summary
The SM is certainly correct, but it is not complete: there must be physics beyond
the SM. Recently, there have been several measurements of observables that are in
disagreement with the predictions of the SM:
• b → sμ + μ − : These include many observables involving this decay. Some are
clean, while others have important theoretical uncertainties. Global fits allowing
for NP in b → sμ + μ − find improvements over the SM at the level of close to 6σ .
NP Models with an extra Z or with different types of LQs have been proposed
as explanations.
• b → cτ − ¯
ν: here there are several clean observables, with a net deviation from
the SM of ∼ 3σ . These can be explained in models with an extra W or with
different types of LQs.
It is of course possible that these discrepancies with the SM are all statistical
fluctuations, and will go away with more data. This said, their combined statistical
significance is sizeable ( 4σ ), so they will not disappear soon. Hopefully, we are
indeed seeing the first experimental signals of NP.
Acknowledgement This work was financially supported in part by NSERC of Canada.