Higgs portal to dark matter and B→K(∗)B\rightarrow K^{(*)} decays

We consider a Higgs portal model in which the 125-GeV Higgs boson mixes with a light singlet mediator h$_{2}$ coupling to particles of a Dark Sector and study potential b→sh$_{2}$ decays in the Belle II experiment. Multiplying the gauge-dependent off-shell Standard-Model b-s-Higgs vertex with the sine of the Higgs mixing angle does not give the correct b-s-h$_{2}$ vertex. We clarify this issue by calculating the b-s-h$_{Zahl}$ vertex in an arbitrary R$_{ξ}$ gauge and demonstrate how the ξ dependence cancels from physical decay rates involving an on-shell or off-shell h$_{2}$. Then we revisit the b→sh$_{2}$ phenomenology and point out that a simultaneous study of B→K∗h$_{2}$ and B→Kh$_{2}$ helps to discriminate between the Higgs portal and alternative models of the Dark Sector. We further advocate for the use of the h$_{2}$ lifetime information contained in displaced-vertex data with h$_{2}$ decaying back to Standard-Model particles to better constrain the h$_{2}$ mass or to reveal additional h$_{2}$ decay modes into long-lived particles

Higgs portal to dark matter and B→K(∗)B\to K^{(*)} decays

We consider a Higgs portal model in which the 125-GeV Higgs boson mixes with a light singlet mediator $h_2$ coupling to particles of a Dark Sector and study potential $b\to s h_2$ decays in the Belle II experiment. Multiplying the gauge-dependent off-shell Standard-Model $b$-$s$-Higgs vertex with the sine of the Higgs mixing angle does not give the correct $b$-$s$-$h_2$ vertex. We clarify this issue by calculating the $b$-$s$-$h_2$ vertex in an arbitrary $R_\xi$ gauge and demonstrate how the $\xi$ dependence cancels from physical decay rates involving an on-shell or off-shell $h_2$. Then we revisit the $b\to s h_2$ phenomenology and point out that a simultaneous study of $B\to K^* h_2$ and $B\to K h_2$ helps to discriminate between the Higgs portal and alternative models of the Dark Sector. We further advocate for the use of the $h_2$ lifetime information contained in displaced-vertex data with $h_2$ decaying back to Standard-Model particles to better constrain the $h_2$ mass or to reveal additional $h_2$ decay modes into long-lived particles.Comment: 9 pages, 4 figures. v.2: minor clarifications, added Table 1, added references, acknowledgements extende

Flavorful leptoquarks at the LHC and beyond: Spin 1

Evidence for electron-muon universality violation that has been revealed in $b\to s \ell\ell$ transitions in the observables $R_{K,K^*}$ by the LHCb Collaboration can be explained with spin-1 leptoquarks in $SU(2)_L$ singlet $V_1$ or triplet $V_3$ representations in the ${\cal{O}}(1-10)$ TeV range. We explore the sensitivity of the high luminosity LHC (HL-LHC) and future proton-proton colliders to $V_1$ and $V_3$ in the parameter space connected to $R_{K,K^*}$-data. We consider pair production and single production in association with muons in different flavor benchmarks. Reinterpreting a recent ATLAS search for scalar leptoquarks decaying to $b \mu$ and $j \mu$, we extract improved limits for the leptoquark masses: For gauge boson-type leptoquarks ($\kappa=1$) we obtain $M_{V_1}> 1.9$ TeV, $M_{V_1}> 1.9$ TeV, and $M_{V_1}> 1.7$ TeV for leptoquarks decaying predominantly according to hierarchical, flipped and democratic quark flavor structure, respectively. Future sensitivity projections based on extrapolations of existing ATLAS and CMS searches are worked out. We find that for $\kappa=1$ the mass reach for pair (single) production of $V_1$ can be up to 3 TeV (2.1 TeV) at the HL-LHC and up to 15 TeV (19.9 TeV) at the FCC-hh with $\sqrt{s}=100$ TeV and 20 \, \mbox{ab}^{-1}. The mass limits and reach for the triplet $V_3$ are similar or higher, depending on flavor. While there is the exciting possibility that leptoquarks addressing the $R_{K,K^*}$-anomalies are observed at the LHC, to fully cover the parameter space $pp$-collisions beyond the LHC-energies are needed.Comment: 16 pages, 8 figures; v2: Clarifying comments, four plots and few references added. Few typos corrected. Conclusions unchange

Higgs boson decay into a lepton pair and a photon: A roadmap to the discovery of H →zγ and probes of new physics

The decay H→ℓ$^{+}$ℓ$^{–}$γ, ℓ=e, μ receives contributions from H→Z[→ℓ$^{+}$ℓ$^{–}$]γ and a nonresonant contribution, both of which are loop induced. We describe how one can separate these subprocesses in a gauge-independent way, define the decay rate Γ(H→Zγ), and extract the latter from differential H→ℓ$^{+}$ℓ$^{–}$γ branching ratios. For ℓ=μ, also the tree decay rate, which is driven by the muon Yukawa coupling, is important. We propose kinematic cuts optimized to separate the three contributions, paving the way to the milestone (i) discovery of H→Zγ, (ii) discovery of H→μ$^{+}$μ$^{–}$γ| $_{tree}$, and (iii) quantification of new physics in both the effective H−Z−γ and nonresonant H−ℓ$^{+}$ℓ$^{–}$−γ couplings

Addendum to "Impact of polarization observables and Bc→τνB_c\to \tau \nu on new physics explanations of the b→cτνb\to c \tau \nu anomaly"

In this addendum to arXiv:1811.09603 we update our results including the recent measurement of ${\cal R}(D)$ and ${\cal R}(D^*)$ by the Belle collaboration: ${\cal R}(D)_{\rm Belle} = 0.307\pm0.037\pm0.016$ and ${\cal R}(D^*)_{\rm Belle}=0.283\pm0.018\pm0.014$, resulting in the new HFLAV fit result ${\cal R}(D) = {0.340\pm0.027 \pm 0.013}$, ${\cal R}(D^*) = {0.295\pm0.011 \pm 0.008 }$, exhibiting a $3.1\,\sigma$ tension with the Standard Model. We present the new fit results and update all figures, including the relevant new collider constraints. The updated prediction for ${\cal R}(\Lambda_c)$ from our sum rule reads ${\cal R}(\Lambda_c)= \mathcal{R}_{\rm SM}(\Lambda_c) \left( 1.15 \pm 0.04 \right) = 0.38 \pm 0.01 \pm 0.01$. We also comment on theoretical predictions for the fragmentation function $f_c$ of $b\to B_c$ and their implication on the constraint from $B_{u/c}\to\tau\nu$ data.Comment: 9 pages, 7 figures. v2: references added, matches PRD versio

Revisiting lifetimes of doubly charmed baryons

We present updated predictions for lifetimes of doubly charmed baryons, within the heavy quark expansion, including available NLO $\alpha_s$ contributions and newly-computed terms in the $1/m_c$ series. Our improved results confirm the expected hierarchy $$\tau(\Xi_{cc}^{+}) < \tau(\Omega_{cc}^{+}) < \tau(\Xi_{cc}^{++}) \,,$$ while the predicted lifetime $\tau(\Xi_{cc}^{++}) = 0.32 \pm 0.5 ^{+0.8}_{-0.7} \,\textrm{ps}$ is consistent with the recent LHCb determination. We provide predictions for the lifetime ratios of the $\Xi_{cc}^{+}$ and $\Omega_{cc}^+$ baryons relative to the $\Xi_{cc}^{++}$ baryon, namely $\tau(\Xi_{cc}^{+})/\tau(\Xi_{cc}^{++})=0.22\pm 0.05\pm 0.04$ and $\tau(\Omega_{cc}^{+})/\tau(\Xi_{cc}^{++})=0.52\pm 0.13^{+0.03}_{-0.02}$.Comment: 25 pages, 2 figures; v2: few clarifications added, one reference added, matches version accepted for publicatio

Impact of polarization observables and Bc→τν B_c\to \tau \nu on new physics explanations of the b→cτνb\to c \tau \nu anomaly

The combined analysis of the BaBar, Belle, and LHCb data on $B\to D\tau\nu$, $B\to D^*\tau\nu$ and $B_c\to J/\Psi\tau\nu$ decay observables shows evidence of physics beyond the Standard Model (SM). In this article, we study all the one- and two-dimensional scenarios which can be generated by adding a single new particle to the SM. We put special emphasis on the model-discriminating power of $F_L(D^*)$ and of the $\tau$ polarizations, and especially on the constraint from the branching fraction ${\rm BR}(B_c\to\tau\nu)$. We critically review this constraint and do not support the aggressive limit of ${\rm BR}(B_c\to\tau\nu)<10\%$ used in some analyses. While the impact of $F_L(D^*)$ is currently still limited, the ${\rm BR}(B_c\to\tau\nu)$ constraint has a significant impact: depending on whether one uses a limit of $60\%$, $30\%$ or $10\%$, the pull for new physics (NP) in scalar operators changes drastically. More specifically, for a conservative $60\%$ limit a scenario with scalar operators gives the best fit to data, while for an aggressive $10\%$ limit this scenario is strongly disfavored and the best fit is obtained in a scenario in which only a left-handed vector operator is generated. We find a sum rule for the branching ratios of $B\to D\tau\nu$, $B\to D^*\tau\nu$ and $\Lambda_b\to \Lambda_c\tau\nu$ which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of ${\rm BR}(\Lambda_b\to \Lambda_c\tau\nu)$ over its SM prediction by $(24\pm 6)\%$ for the current $\mathcal{R}(D^{(*)})$ data.Comment: 16 pages, 6 figures; v2: clarifying comments and few references added, matches version published in PRD; v3: typo corrected in eq. (28). Updated results based on the HFLAV average for spring 2019 are available in arXiv:1905.0825