arXiv subjects

Ulrich Nierste

Publications and source records attributed to Ulrich Nierste.

At least 37 records · Page 2Linked to original sources

arXiv2023

Impact of $Λ_b\to Λ_cτν$ measurement on New Physics in $b\to c \, l ν$ transitions

Measurements of the branching ratios of $B \to D^{(*)}τ\barν/B \to D^{(*)}\ell\barν$ and $B_c\to J/ψ\, τ\barν/B_c\to J/ψ\, \ell\barν$ by the BaBar, Belle and LHCb collaborations consistently point towards an abundance of taus compared to channels with light leptons. However, the ratio $Λ_b \toΛ_c τ\barν/Λ_b \toΛ_c \ell\barν$ shows a relative deficit in taus. In this paper, we critically address whether data still points towards a coherent pattern of deviations, in particular in light of the sum rule relating these decays in a model-independent way. We find that no common new physics explanation of all ratios is possible (within $2σ$ or $1.5σ$, depending on the ${\cal R}(Λ_c)$ normalization to light lepton channels). While this inconsistency could be a statistical fluctuation, further measurements are required in order to converge to a coherent pattern of experimental results.

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arXiv2022

Addendum: Towards next-to-next-to-leading-log accuracy for the width difference in the $\mathbf{B_s-\bar{B}_s}$ system: fermionic contributions to order $\mathbf{(m_c/m_b)^0}$ and $\mathbf{(m_c/m_b)^1}$

We calculate the three-loop master integrals of Ref. [1] [arXiv:1709.02160] in analytic form. This allows us to present the fermionic contributions to the $ΔB=2$ Wilson coefficients of the $B$-$\bar B$ decay matrix in next-to-next-to-leading order of QCD with full analytic dependence on the mass of the charm quark in the fermionic loops.

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arXiv2022

The width difference in the $B-\bar{B}$ system at next-to-next-to-leading order of QCD

We extend the theoretical prediction for the width difference $ΔΓ_q$ in the mixing of neutral $B$ mesons in the Standard Model to next-to-next-to-leading order in $α_s$. To this aim we calculate three-loop diagrams with two $|ΔB|=1$ current-current operators analytically. In the matching between $|ΔB|=1$ and $|ΔB|=2$ effective theories we regularize the infrared divergences dimensionally and take into account all relevant evanescent operators. Further elements of the calculation are the two-loop renormalization matrix $Z_{ij}$ for the $|ΔB|=2$ operators and the $\mathcal{O}(α_s^2)$ corrections to the finite renormalization that ensures the $1/m_b$ suppression of the operator $R_0$ at two-loop order. Our theoretical prediction reads $ΔΓ_s/ΔM_s = {(4.33\pm 0.93)\cdot 10^{-3}}$ if expressed in terms of the bottom mass in the $\overline{\rm MS}$ scheme and $ΔΓ_s/ΔM_s = {(4.20\pm 0.95)\cdot 10^{-3}}$ for the use of the potential-subtracted mass. While the controversy on $|V_{cb}|$ affects both $ΔΓ_s$ and $ΔM_s$, the ratio $ΔΓ_s/ΔM_s$ is not affected by the uncertainty in $|V_{cb}|$ .

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arXiv2022

Higgs decay into a lepton pair and a photon: a roadmap to $\mathbf{H\to Zγ}$ discovery and probes of new physics

The decay $H\to \ell^+\ell^- γ$, $\ell=e,μ$, receives contributions from $H\to Z[\to \ell^+\ell^-] γ$ and a non-resonant contribution, both of which are loop-induced. We describe how one can separate these sub-processes in a gauge-independent way, define the decay rate $Γ(H\to Zγ)$, and extract the latter from differential $H\to \ell^+\ell^- γ$ branching ratios. For $\ell=μ$ 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 milestones (i) discovery of $H\to Z γ$, (ii) discovery of $H\to μ^+μ^- \left. γ\right|_{\rm tree}$, and (iii) quantification of new physics in both the effective $H$-$Z$-$γ$ and non-resonant $H$-$\ell^+$-$\ell^-$-$γ$ couplings.

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arXiv2022

The width difference in \bbm\ at order $α_s$ and beyond

We complete the calculation of the element $Γ_{12}^q$ of the decay matrix in $B_q-\bar{B}_q$ mixing, $q=d,s$, to order $α_s$ in the leading power of the Heavy Quark Expansion. To this end we compute one- and two-loop contributions involving two four-quark penguin operators. Furthermore, we present two-loop QCD corrections involving a chromomagnetic operator and either a current-current or four-quark penguin operator. Such contributions are of order $α_s^2$, i.e. next-to-next-to-leading-order. We also present one-loop and two-loop results involving two chromomagnetic operators which are formally of next-to-next-to-leading and next-to-next-to-next-to-leading-order, respectively. With our new corrections we obtain the Standard-Model prediction $ΔΓ_s/ΔM_s= (5.20\pm 0.69)\cdot 10^{-3}$ if $Γ_{12}^s$ is expressed in terms of the $\overline{\rm MS}$ b-quark mass, while we find $ΔΓ_s/ΔM_s= (4.70\pm 0.96)\cdot 10^{-3}$ instead for the use of the pole mass.

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arXiv2021

Modified majoron model for cosmological anomalies

The vacuum expectation value $v_s$ of a Higgs triplet field $Δ$ carrying two units of lepton number $L$ induces neutrino masses $\propto v_s$. The neutral component of $Δ$ gives rise to two Higgs particles, a pseudoscalar $A$ and a scalar $S$. The most general renormalizable Higgs potential $V$ for $Δ$ and the Standard-Model Higgs doublet $Φ$ does not permit the possibility that the mass of either $A$ or $S$ is small, of order $v_s$, while the other mass is heavy enough to forbid the decay $Z\to A S$ to comply with LEP 1 data. We present a model with additional dimension-6 terms in $V$, in which this feature is absent and either $A$ or $S$ can be chosen light. Subsequently we propose the model as a remedy to cosmological anomalies, namely the tension between observed and predicted tensor-to-scalar mode ratios in the cosmic microwave background and the different values of the Hubble constant measured at different cosmological scales. Furthermore, if $Δ$ dominantly couples to the third-generation doublet $L_τ=(ν_τ,τ)$, the deficit of $ν_τ$ events at IceCube can be explained. The singly and doubly charged triplet Higgs bosons are lighter than 280 GeV and 400 GeV respectively, and could be found at the LHC.

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arXiv2021

Two-loop QCD penguin contribution to the width difference in $B_s-\bar{B}_s$ mixing

We consider two-loop QCD corrections to the element $Γ_{12}^q$ of the decay matrix in $B_q-\bar{B}_q$ mixing, $q=d,s$, in the leading power of the Heavy Quark Expansion. The calculated contributions involve one current-current and one penguin operator and constitute the next step towards a theory prediction for the width difference $ΔΓ_s$ matching the precise experimental data. We present compact analytic results for all matching coefficients in an expansion in $m_c/m_b$ up to second order. Our new corrections are comparable in size to the current experimental error and slightly increase $ΔΓ_s$.

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arXiv2020

Higgs decay into a lepton pair and a photon revisited

We present new calculations of the differential decay rates for $H\to \ell^+\ell^- γ$ with $\ell=e$ or $μ$ in the Standard Model. The branching fractions and forward-backward asymmetries, defined in terms of the flight direction of the photon relative to the lepton momenta, depend on the cuts on energies and invariant masses of the final state particles. For typical choices of these cuts we find the branching ratios $B(H\to e \bar e γ)=5.8\cdot 10^{-5}$ and $B(H\to μ\bar μγ)=6.4\cdot 10^{-5}$ and the forward-backward asymmetries $\mathcal{A}^{(e)}_{\text{FB}}=0.343$ and $\mathcal{A}^{(μ)}_{\text{FB}}=0.255$. We provide compact analytic expressions for the differential decay rates for the use in experimental analyses.

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arXiv2020

Cornering Spontaneous CP Violation with Charged-Higgs Searches

Decades of precision measurements have firmly established the Kobayashi-Maskawa phase as the dominant source of the CP violation observed in weak quark decays. However, it is still unclear whether CP violation is explicitly encoded in complex Yukawa matrices or instead stems from spontaneous symmetry breaking with underlying CP-conserving Yukawa and Higgs sectors. Here we study the latter possibility for the case of a generic two-Higgs-doublet model. We find that theoretical constraints limit the ratio $t_β$ of the vacuum expectation values to the range $0.22 \leq t_β\leq 4.5$ and imply the upper bounds $M_{H^\pm}\leq 435$ GeV, $M_{H_{2}^0} \leq 485$ GeV and $M_{H_{3}^0} \leq 545$ GeV for the charged and extra neutral Higgs masses. We derive lower bounds on charged-Higgs couplings to bottom quarks which provide a strong motivation to study the non-standard production and decay signatures $p p \to qb H^\pm(\to q^\prime b)$ with all flavors $q,q^\prime=u,c,t$ in the search for the charged Higgs boson. We further present a few benchmark scenarios with interesting discovery potential in collider analyses.

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arXiv2020

Penguin contribution to width difference and CP asymmetry in $B_q$-$\bar B_q$ mixing at order $α_s^2 N_f$

We present new contributions to the decay matrix element $Γ_{12}^q$ of the $B_q$-$\bar B_q$ mixing complex, where $q=d$ or $s$. Our new results constitute the order $α_s^2 N_f$ corrections to the penguin contributions to the Wilson coefficients entering $Γ_{12}^q$ with full dependence on the charm quark mass. This is the first step towards the prediction of the CP asymmetry $a_{\rm fs}^q$ quantifying CP violation in mixing at next-to-next-to-leading logarithmic order (NNLO) in quantum chromodynamics (QCD) and further improves the prediction of the width difference $ΔΓ_q$ between the two neutral-meson eigenstates. We find a sizable effect from the non-zero charm mass and our partial NNLO result decreases the NLO penguin corrections to $a_{\rm fs}^q$ by 37\% and those to $ΔΓ_q$ by 16\%. We further update the Standard-Model NLO predictions for $a_{\rm fs}^q$ and the ratio of the width and mass differences of the $B_q$ eigenstates: If we express the results in terms of the pole mass of the bottom quark we find $a_{\rm fs}^s=(2.07 \pm 0.10)\cdot 10^{-5}$, $a_{\rm fs}^d=(-4.71 \pm 0.24)\cdot 10^{-4}$, $ΔΓ_s/Δ{M}_s = (4.33 \pm 1.26)\cdot 10^{-3}$, and $ΔΓ_d/Δ{M}_d = (4.48 \pm 1.19)\cdot 10^{-3}$. In the $\overline{\rm MS}$ scheme these numbers read $a^s_{\rm fs} =(2.04 \pm 0.11)\cdot 10^{-5}$, $a^d_{\rm fs} = (-4.64 \pm 0.25)\cdot 10^{-4}$, $ΔΓ_s/Δ{M}_s = (4.97 \pm 1.02)\cdot 10^{-3}$, and $ΔΓ_d/Δ{M}_d = (5.07 \pm 0.96)\cdot 10^{-3}$.

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arXiv2020

Higgs portal to dark matter and $B\to K^{(*)}$ 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 $R_ξ$ 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 $b\to s h_2$ 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.

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arXiv2020

Charm decays

I discuss hadronic decays of $D$ mesons with emphasis on the recent discovery of charm CP violation in $D^0\to K^+K^-,π^+π^-$ decays. The measured difference $Δa_{CP} \, \equiv \; a_{CP}^{\mathrm{dir}}(D^0\rightarrow K^+K^-) - a_{CP}^{\mathrm{dir}}(D^0\rightarrowπ^+π^-) = \; (-15.4\pm 2.9)\cdot 10^{-4}$ of two direct CP asymmetries exceeds the SM prediction by a factor of 7. A possible explanation is an enhancement of the penguin amplitude entering $ a_{CP}^{\mathrm{dir}}$ by QCD effects which are not understood yet. Alternatively, $Δa_{CP}$ could be dominated by contributions from new physics. In order to distinguish these two hypotheses further CP asymmetries should be measured. To this end CP asymmetries resulting from the interference of two tree-level amplitudes auch as $a_{CP}^{\mathrm{dir}}(D^0\rightarrow K_SK_S)$ or $a_{CP}^{\mathrm{dir}}(D^0 \to K^{*0} K_S)$ are especially interesting.

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arXiv2019

Addendum to "Impact of polarization observables and $B_c\to τν$ on new physics explanations of the $b\to c τν$ 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 $3.1\,σ$ 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 ${\cal R}(Λ_c)$ from our sum rule reads ${\cal R}(Λ_c)= \mathcal{R}_{\rm SM}(Λ_c) \left( 1.15 \pm 0.04 \right) = 0.38 \pm 0.01 \pm 0.01$. We also comment on theoretical predictions for the fragmentation function $f_c$ of $b\to B_c$ and their implication on the constraint from $B_{u/c}\toτν$ data.

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arXiv2019

Little hierarchies solve the little fine-tuning problem: a case study in supersymmetry with heavy guinos

Radiative corrections with new heavy particles coupling to Higgs doublets destabilize the electroweak scale and require an ad-hoc counterterm cancelling the large loop contribution. If the mass scale m1 of these new particles in in the TeV range, this feature constitutes the "little fine-tuning problem". We consider the case that the new-physics spectrum has a little hierarchy with two particle mass scales m1, m2 and m2 = O(10 m1) and no tree-level couplings of the heavier particles to Higgs doublets. As a concrete example we study the (next-to-)minimal supersymmetric standard model ((N)MSSM) for the case that the gluino mass M3 is significantly larger than the stop mass parameters m_{L,R} and show that the usual one-loop fine-tuning analysis breaks down. If m_{L,R} is defined in the dimensional-reduction (DR-bar) or any other fundamental scheme, corrections enhanced by powers of M3^2/m_{L,R}^2 occur in all higher loop orders. After resumming these terms we find the fine-tuning measure substantially improved compared to the usual analyses with M3 <~ m_{L,R}. In our hierarchical scenario the stop self-energies grow like M3^2, so that the stop masses m_{L,R}^{OS} in the on-shell (OS) scheme are naturally much larger than their DR-bar counterparts m_{L,R}^{DR-bar}. This feature permits a novel solution to the little fine-tuning problem: DR-bar stop masses are close to the electroweak scale, but radiative corrections involving the heavy gluino push the OS masses, which are probed in collider searches, above their experimental lower limits. As a byproduct, we clarify which renormalization scheme must be used for squark masses in loop corrections to low-energy quantities such as the B-B-bar mixing amplitude.

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arXiv2019

Impact of polarization observables and $ B_c\to τν$ on new physics explanations of the $b\to c τν$ anomaly

The combined analysis of the BaBar, Belle, and LHCb data on $B\to Dτν$, $B\to D^*τν$ and $B_c\to J/Ψτν$ 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 $τ$ polarizations, and especially on the constraint from the branching fraction ${\rm BR}(B_c\toτν)$. We critically review this constraint and do not support the aggressive limit of ${\rm BR}(B_c\toτν)<10\%$ used in some analyses. While the impact of $F_L(D^*)$ is currently still limited, the ${\rm BR}(B_c\toτν)$ 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 $B\to Dτν$, $B\to D^*τν$ and $Λ_b\to Λ_cτν$ which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of ${\rm BR}(Λ_b\to Λ_cτν)$ over its SM prediction by $(24\pm 6)\%$ for the current $\mathcal{R}(D^{(*)})$ data.

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arXiv2019

The width difference in the $B_s$-$\bar B_s$ system: towards NNLO

The width difference $ΔΓ$ among the two mass eigenstates of the $B_s$-$\bar B_s$ system is measured with a precision of 7%. The theory prediction has a larger uncertainty which mainly stems from unknown perturbative higher-order QCD corrections. I discuss the subset of next-to-next-to-leading order diagrams proportional to $α_s^2\, N_f$, where $N_f=5$ is the number of quark flavours. The results are published in [1].

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arXiv2018

Lepton flavour violation in the MSSM: exact diagonalization vs mass expansion

The forthcoming precision data on lepton flavour violating decays require precise and efficient calculations in New Physics models. In this article lepton flavour violating processes within the Minimal Supersymmetric Standard Model (MSSM) are calculated using the method based on the Flavour Expansion Theorem, a recently developed technique performing a purely algebraic mass-insertion expansion of the amplitudes. The expansion in both flavour-violating and flavour-conserving off-diagonal terms of sfermion and supersymmetric fermion mass matrices is considered. In this way the relevant processes are expressed directly in terms of the parameters of the MSSM Lagrangian. We also study the decoupling properties of the amplitudes. The results are compared to the corresponding calculations in the mass eigenbasis (i.e. using the exact diagonalization of the mass matrices). Using these methods, we consider the following processes: $\ell \to \ell' γ$, $\ell \to 3 \ell'$, $\ell \to 2\ell'\ell"$, $h \to \ell\ell'$ as well as $μ\to e$ conversion in nuclei. In the numerical analysis we update the bounds on the flavour changing parameters of the MSSM and examine the sensitivity to the forthcoming experimental results. We find that flavour violating muon decays provide the most stringent bounds on supersymmetric effects and will continue to do so in the future. Radiative $\ell\to\ell^\primeγ$ decays and leptonic three-body decays $\ell\to 3\ell^\prime$ show an interesting complementarity in eliminating "blind spots" in the parameter space. In our analysis we also include the effects of non-holomorphic $A$-terms which are important for the study of LFV Higgs decays.

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arXiv2017

Neutral $D\rightarrow K K^*$ decays as discovery channels for charm CP violation

We point out that the CP asymmetries in the decays $D^0\rightarrow K_{S} K^{*0}$ and $D^0\rightarrow K_{S} \overline{K}{}^{*0}$ are potential discovery channels for charm CP violation in the Standard Model. We stress that no flavor tagging is necessary, the untagged CP asymmetry $a_{CP}^{\mathrm{dir}}(D \to K_S K^{*0})$ is essentially equal to the tagged one, so that the untagged measurement comes with a significant statistical gain. Depending on the relevant strong phase, $\vert a_{CP}^{\text{dir, untag}}\vert$ can be as large as $0.003$. The CP asymmetry is dominantly generated by exchange diagrams and does not require non-vanishing penguin amplitudes. While the CP asymmetry is smaller than in the case of $D^0\rightarrow K_SK_S$, the experimental analysis is more efficient due to the prompt decay $K^{*0}\rightarrow K^+π^-$. One may further search for favourable strong phases in the Dalitz plot in the vicinity of the $K^{*0}$ peak.

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