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Svjetlana Fajfer

Publications and source records attributed to Svjetlana Fajfer.

At least 55 records · Page 3Linked to original sources
arXiv2016

Leptoquark model to explain the $B$-physics anomalies, $R_K$ and $R_D$

We show that a model with a scalar leptoquark of hypercharge $Y=1/6$ which includes the light right-handed neutrinos, can successfully describe both of the $B$-physics anomalies, $R_K^{\rm exp} < R_K^{\rm SM}$ and $R_D^{\rm exp} > R_D^{\rm SM}$. We discuss the corresponding low energy effective theory and, after using the known experimental data as constraints, we show that the model is viable and that it offers several predictions which can be tested experimentally.

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arXiv2016

Lepton flavor non-universality in $b \to s \ell^+ \ell^-$ processes

We explore a scenario of New Physics entering the description of $B\to K^{(\ast )} μμ$ decay through couplings to the operators ${\cal O}_{9,10}^\prime$, satisfying $C_9^\prime = - C_{10}^\prime$. From the current data on ${\cal B}(B_s\to μμ)$ and ${\cal B}(B\to K μμ)_{[15,22] \,\mathrm{GeV}^2}$, we obtain constraints on $\mathrm{Re} C_{10}^\prime$ and $\mathrm{Im} C_{10}^\prime$ which we then assume to be lepton specific, and find $R_K= {\cal B}(B\to K μμ)/ {\cal B}(B\to K ee)_{[1,6]\,\mathrm{GeV}^2}=0.88(8)$, consistent with recent value measured at LHCb. A specific realization of this scenario is the one with a scalar leptoquark state $Δ$, in which $C_{10}^\prime$ is related to the mass of $Δ$ and its Yukawa couplings. We then show that this scenario does not make any significant impact on $B_s-\overline B_s$ mixing amplitude nor to $ {\cal B}(B\to Kν\bar ν)$. Instead, it can modify $R_{K^\ast}= {\cal B}(B\to K^\ast μμ)/ {\cal B}(B\to K^\ast ee)_{[1,6]\,\mathrm{GeV}^2}$, which will soon be experimentally measured and we find it to be $R_{K^\ast}= 1.11(8)$, while $R_{K^\ast}/R_K= 1.27(19)$. A similar ratio of forward-backward asymmetries also becomes lower than in the Standard Model.

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arXiv2016

Is symmetry breaking of SU(5) theory responsible for the diphoton excess?

We advocate the possibility that the observed diphoton excess at 750 GeV at the LHC can be addressed by the scalar field that is a part of the SU(5) symmetry breaking sector. The field in question is the Standard Model singlet that resides in the adjoint representation that breaks SU(5) down to SU(3) x SU(2) x U(1). We also show that the required production and subsequent decay to two photons of this singlet can be induced by individual or combined contribution of two scalar multiplets $S_3$ and $R_2$ that transform as (3,3,-1/3) and (3,2,7/6) under SU(3) x SU(2) x U(1), respectively. The individual dominance of these multiplets is directly related to the issue of the charged fermion mass generation within the SU(5) framework and can be unambiguously tested through the diboson decay signatures of the Standard Model singlet field.

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arXiv2016

Isospin violating decays of positive parity $B_s$ mesons in HM$χ$PT

Recent lattice QCD results suggest that the masses of the first two positive parity $B_s$ mesons lie below the BK threshold, similar to the case of $D^*_{s0}(2317)^+$ and $D_{s1}(2460)^+$ mesons. The mass spectrum of $B_s$ mesons seems to follow pattern of $D_s$ mass spectrum. As in the case of charmed mesons, the structure of positive parity $B_s$ mesons is very intriguing. To shed more light on this issue, we investigate strong isospin violating decays $B_s(0^+) \to B_s^0 π^0$, $B_s(1^+) \to B_s^{*0} π^0$ and $B_s(1^+) \to B_s^0 ππ$ within heavy meson chiral perturbation theory. The two body decay amplitude arises at the tree level and we show that the loop corrections give significant contributions. On the other hand, in the case of three body decay $B_s(1^+) \to B_s^0 ππ$ amplitude occurs only at the loop level. We find that the decay widths for these decays are: $Γ(B_s(1^+) \to B_s^0 ππ)\sim 10^{-3}\,$keV and $Γ(B_s(0^+) \to B_s^0 π^0) \leq 55\,$keV, $Γ(B_s(1^+) \to B_s^{*0} π^0) \leq 50\,$keV. More precise knowledge of the coupling constant describing the interaction of positive and negative parity heavy mesons with light pseudoscalar mesons would help to increase accuracy of our calculation.

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arXiv2016

Vector leptoquark resolution of $R_K$ and $R_{D^{(*)}}$ puzzles

We propose that three recent anomalies in $B$ meson decays, $R_{D^{(*)}}$, $R_K$, and $P_5'$, might be explained by a single vector leptoquark weak triplet state. The constraints on the parameter space are obtained by considering $t \to b τ^+ ν$ data, lepton flavor universality tests in the kaon sector, bounds on $B \to K^{(*)} \bar νν$, bound on the lepton flavor violating decay $B \to K μτ$, and measurements of $b \to c μ^- \barν$ decays. The presence of such vector leptoquark could be exposed in precise measurements of $t \to b τν$ and $B \to K^{(*)} \bar νν$ decays. The model also predicts approximate equality of lepton flavor universality ratios $R_{K^*}$, $R_{K}$, and suppressed branching fraction of $B_s \to μ^+ μ^-$.

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arXiv2016

Angular distributions of $\bar B \to D^{(\ast)}\ell\bar ν_\ell$ decays and search of New Physics

We derive the expressions for the full angular distributions of $\bar B\to D\ell\bar ν_\ell$ and $\bar B\to D^{\ast }\ell\bar ν_\ell$ decays and discuss the spectra on each angle separately. The coefficient functions, depending on helicity amplitudes, can then be combined in an ensemble of observables which can then be used to check for the presence of New Physics. We examine the sensitivity of each of these observables on the presence of non-Standard Model interaction terms at low energies. The expressions presented here are general and can be used for studying any other semileptonic pseudoscalar to pseudoscalar/vector meson decay. We also examine the problem of pollution of the $\bar B\to D^{\ast}(\to Dπ)_S\ell\bar ν_\ell$ decay sample by the $\bar B\to D_0^{\ast}(\to Dπ)\ell\bar ν_\ell$ events, and point out that a measurement of two particular quantities could clarify whether or not the $(Dπ)_{S-\rm wave}$ in the vicinity of $D^\ast$-peak is (approximately) described by the Breit-Wigner formula.

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arXiv2015

Chiral loops in the isospin violating decays of D_{s1}(2460) and D*_{s0}(2317)

Positive parity meson states D*_{s0}(2317) and D_{s1}(2460) have masses slightly below the DK threshold. Therefore, both states can strongly decay only into isospin violating decays D_{s1}(2460) --> Ds ππ, D_{s1}(2460) --> Ds* π0 and D*_{s0}(2317) --> Ds π0. The pion states are having rather small energies, what makes these decays appropriate to study within heavy meson chiral perturbation theory and calculate loop contributions. The D_{s1}(2460) --> Ds ππdecays occur only at the loop level. Amplitude is a result of chiral loop contributions, which then have to be finite. However, in the case of D_{s1}(2460)--> Ds* π0 and D*_{s0}(2317)--> Ds π0 decays, there is a tree level contribution. We find that chiral loop contributions might be important in both cases. The calculated amplitudes are sensitive on the coupling constant describing the interaction of positive and negative parity heavy meson multiplets with the light pseudoscalars. The counter-terms contributions are also present in the amplitudes D_{s1}(2460) --> Ds* π0 and D*_{s0}(2317) --> Ds π0. We explore experimentally known ratio of the decay widths for these two decay modes to estimate the size of counter-terms contributions. We determine decay widths for both decay modes to be Gamma (D_{s1}(2460) --> Ds π+ π-) = 0.25 keV and Gamma (D_{s1}(2460) --> Ds + π0 π0 ) = 0.15 keV.

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arXiv2015

New Physics Models Facing Lepton Flavor Violating Higgs Decays at the Percent Level

We speculate about the possible interpretations of the recently observed excess in the $h \to τμ$ decay. We derive a robust lower bound on the Higgs boson coupling strength to a tau and a muon, even in presence of the most general new physics affecting other Higgs properties. Then we reevaluate complementary indirect constraints coming from low energy observables as well as from theoretical considerations. In particular, the tentative signal should lead to $τ\to μγ$ at rates which could be observed at Belle II. In turn we show that, barring fine-tuned cancellations, the effect can only be accommodated within models with an extended scalar sector. These general conclusions are demonstrated using a number of explicit new physics models. Finally we show how, given the $h \to τμ$ signal, the current and future searches for $μ\to e γ$ and $μ\to e$ nuclear conversions unambiguously constrain the allowed rates for $h \to τe$.

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arXiv2015

Theoretical perspective on rare and radiative charm decays

Recent experimental bounds on rare charm decays offer a chance to improve our theoretical understanding of physics present in $c \to u γ$ and $c \to u l^+ l^-$ transitions. Standard Model and New Physics contributions are reviewed for inclusive and exclusive $D \to Vγ$, $D^+ \to π^+ l^+ l^-$, $D \to l^+ l^-$ decays. Observables important for search of New Physics are discussed. Possibility to observe CP violation in rare charm decays is questioned.

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arXiv2015

Discerning new physics in charm meson leptonic and semileptonic decays

Current experimental information on the charm meson decay observables in which the $c\to s\ellν_\ell$ transitions occur is well compatible with the Standard Model predictions. Recent precise lattice calculations of the $D_s$ meson decay constant and form factors in $D\to K\ellν$ decays offer a possibility to search for the small deviations from the Standard Model predictions in the next generation of the high intensity flavour experiments. We revisit constraints from these processes on the new physics contributions in the effective theory approach. We investigate new physics effects which might appear in the differential distributions for the longitudinally and transversely polarised $K^\ast$ in $D\to K^\ast\ellν_\ell$ decays. Present constraints from these observables are rather weak, but could be used to constrain new physics effects in the future. In the case of $D\to K\ellν$ we identify observables sensitive on new physics contribution coming from the scalar Wilson coefficient, namely the forward-backward and the transversal muon asymmetries. By allowing that new physics modifies only the second lepton generation but not the first one, we identify allowed region for the differential decay rate for the process $D\to Kμν_μ$ and find that it is allowed to deviate from the Standard Model prediction by only few percent. The lepton flavour universality violation can be tested in the ratio $R_{μ/e}(q^2)\equiv \frac{dΓ^{(μ)}}{dq^2}/\frac{dΓ^{(e)}}{dq^2}$. If the first lepton generation behaves as in the Standard Model, we find, using current constraint on the scalar Wilson coefficient, that the ratio $R_{μ/e}(q^2)$ is currently allowed to be within the range $(0.9, 1.2)$, depending on the value of $q^2$.

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arXiv2014

Cornering Scalar Leptoquarks at LHC

We study implications of large lepton-quark-leptoquark couplings for direct leptoquark searches at Large Hadron Collider. We present all existing flavor constraints on the strength of these couplings assuming that leptoquarks under consideration interact exclusively with charged leptons and quarks of the same generation. We find that these leptoquarks can have sizeable couplings to the Standard Model fermions. This insures a self consistency of our study. We discuss the leptoquark production mechanisms at LHC and demonstrate the importance of inclusion of a t-channel pair production and, in particular, a single leptoquark production through a recast of an existing CMS search at LHC for the second generation leptoquark. Our recast yields the best direct limit on Yukawa coupling of the second generation leptoquark that couples to a muon and a strange quark to date.

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arXiv2014

Light vector-like fermions in a minimal SU(5) setup

The Standard Model fermion sector is enlarged by either one light singlet vector-like down-type quark or one light vector-like lepton doublet, which might be accommodated within a five-dimensional representation of SU(5). At low energies the inclusion of these states affects precisely measured observables in flavor physics, as well as electroweak precision measurements. These experimental results strongly constrain couplings of vector-like states to the Standard Model particles. Having these bounds, we investigate the impact of vector-like fermions on the mass matrices for down-type quarks and charged leptons in an SU(5) setting. We find that unitary transformations relating an arbitrary flavor basis to the mass eigenstate basis depend only on three free parameters. Then we discuss the parameter space constrained by low-energy data assuming vector-like quark and vector-like lepton masses to be 800 GeV and 400 GeV, respectively. We demonstrate that these two scenarios generate unique patterns for relevant proton decay widths. A further improvement of experimental bounds on proton decay modes would thus differentiate the allowed parameter space. We finally present two full-fledged SU(5) models that allow for gauge coupling unification with light vector-like fermions under consideration and discuss their viability.

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arXiv2014

Colored scalars and the neutron electric dipole moment

We investigate new contributions to the neutron electric dipole moment induced by colored scalars. As an example, we evaluate contributions coming from the color octet, weak doublet scalar, accommodated within a modified Minimal Flavor Violating framework. These flavor non-diagonal couplings of the color octet scalar might account for the $A_{CP} (D^0 \to K^- K^+) - A_{CP} (D^0 \to π^+ π^-)$ asymmetry on the tree level. Using these charm asymmetry constrained couplings, we calculate two-loop contributions to the neutron electric dipole moment and find that they are of the same order as the experimental bound. We comment also on contributions of higher dimensional operators to the neutron electric dipole moment within this framework.

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arXiv2013

Theory of rare charm decays

Recent disagreement between experimental measurments of CP violating asymmetry in D -> K+ K- and D ->pi+ pi- and theoretical Standard model expectation motivated many studies within the Standard model and beyond. Rare charm decays offer new probe of possible signals beyond Standard model. CP conserving and CP violating contributions within the Standard model and beyond are reviewed for inclusive c -> u gamma and c ->u l+ l- and exclusive D ->V gamma, D ->P+ P-, D+ -> pi+ l+ l-, D -> l+ l- decays.

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arXiv2013

Light Higgs and Vector-like Quarks without Prejudice

Light vector-like quarks with non-renormalizable couplings to the Higgs are a common feature of models trying to address the electroweak (EW) hierarchy problem by treating the Higgs as a pseudo-goldstone boson of a global (approximate) symmetry. We systematically investigate the implications of the leading dimension five operators on Higgs phenomenology in presence of dynamical up- and down-type weak singlet as well as weak doublet vector-like quarks. After taking into account constraints from precision EW and flavour observables we show that contrary to the renormalizable models, significant modifications of Higgs properties are still possible and could shed light on the role of vector-like quarks in solutions to the EW hierarchy problem. We also briefly discuss implications of higher dimensional operators for direct vector-like quark searches at the LHC.

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arXiv2013

Minimally flavored colored scalar in $\bar B \to D^{(*)} τ\bar ν$ and the mass matrices constraints

The presence of a colored scalar that is a weak doublet with fractional electric charges of $|Q|=2/3$ and $|Q|=5/3$ with mass below 1,TeV can provide an explanation of the observed branching ratios in $B \to D^{(*)} τ\bar ν$ decays. The required combination of scalar and tensor operators in the effective Hamiltonian for $b \to c τ\bar ν$ is generated through the $t$-channel exchange. We focus on a scenario with a minimal set of Yukawa couplings that can address a semitauonic puzzle and show that its resolution puts a nontrivial bound on the product of the scalar couplings to $\bar τb$ and $\bar c ν$. We also derive additional constraints posed by $Z \to b\bar b$, muon magnetic moment, lepton flavor violating decays $μ\to e γ$, $τ\to μγ$, $τ\to e γ$, and $τ$ electric dipole moment. The minimal set of Yukawa couplings is not only compatible with the mass generation in an SU(5) unification framework, a natural environment for colored scalars, but specifies all matter mixing parameters except for one angle in the up-type quark sector. We accordingly spell out predictions for the proton decay signatures through gauge boson exchange and show that $p \rightarrow π^0 e^+$ is suppressed with respect to $p \rightarrow K^+ \barν$ and even $p \rightarrow K^0 e^+$ in some parts of available parameter space. Impact of the colored scalar embedding in 45-dimensional representation of SU(5) on low-energy phenomenology is also presented. Finally, we make predictions for rare top and charm decays where presence of this scalar can be tested independently.

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arXiv2013

Resonance catalyzed CP asymmetries in D -> P l+ l-

Recently observed increase of direct CP asymmetry in charm meson nonleptonic decays is difficult to explain within the SM. If this effect is induced by new physics, this might be investigated in other charm processes. We propose to investigate new CP violating effects in rare decays $D \to P \ell^+ \ell^-$, which arise due to the interference of resonant part of the long distance contribution and the new physics affected short distance contribution. Performing a model independent analysis, we identify as appropriate observables the differential direct CP asymmetry and partial decay width CP asymmetry. We find that in the most promising decays $D^+ \to π^+ \ell^+ \ell^-$ and $D_s^+ \to K^+ \ell^+ \ell^-$ the "peak-symmetric" and "peak-antisymmetric" CP asymmetries are strong phase dependent and can be of the order 1% and 10%, respectively.

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