arXiv subjects

Saeed Kamali

Publications and source records attributed to Saeed Kamali.

9 recordsLinked to original sources

arXiv2026

Metallic d-wave altermagnetism in WFeB: a platform for electrically switchable perpendicular spin-splitter response

We report the synthesis and magnetic characterization of WFeB and identify it as a metallic d-wave altermagnet representative of a broader TiNiSi-type family. Neutron diffraction, M\"ossbauer spectroscopy, and magnetometry establish a collinear altermagnetic ordering confirmed by first-principles calculations. The electronic structure shows a nonrelativistic spin splitting of approximately 100 meV, but it also supports a strong spin-splitter transport response. This demonstrates that efficient spin-current generation can occur even with such modest band splitting. Symmetry analysis shows that selected film orientations permit deterministic switching of the N\'eel vector by current-induced staggered torques, enabling electrical control of a perpendicular spin-splitter response. These results establish WFeB and related TiNiSi-type antiferromagnets as a platform for electrically switchable charge-to-spin conversion driven by altermagnetic symmetry.

cond-mat.mtrl-sci↗

arXiv2020

A Measurable Angular Distribution for ${\bar B} \to D^{*} τ^- {\barν}_τ$ Decays

At present, the measurements of $R_{D^{(*)}}$ and $R_{J/ψ}$ hint at new physics (NP) in $b \to c τ^- {\barν}$ decays. The angular distribution of ${\bar B} \to D^* (\to D π) \, τ^{-} {\barν}_τ$ would be useful for getting information about the NP, but it cannot be measured. The reason is that the three-momentum ${\vec p}_τ$ cannot be determined precisely since the decay products of the $τ^-$ include an undetected $ν_τ$. In this paper, we construct a measurable angular distribution by considering the additional decay $τ^- \to π^- ν_τ$. The full process is ${\bar B} \to D^* (\to D π') \, τ^{-} (\to π^- ν_τ) {\barν}_τ$, which includes three final-state particles whose three-momenta can be measured: $D$, $π'$, $π^-$. The magnitudes and relative phases of all the NP parameters can be extracted from a fit to this angular distribution. One can measure CP-violating angular asymmmetries. If one integrates over some of the five kinematic parameters parametrizing the angular distribution, one obtains (i) familiar observables such as the $q^2$ distribution and the $D^*$ polarization, and (ii) new observables associated with the $π^-$ emitted in the $τ$ decay: the forward-backward asymmetry of the $π^-$ and the CP-violating triple-product asymmetry.

hep-ph↗

arXiv2020

Dark sector origin of the KOTO and MiniBooNE anomalies

We present a dark sector model that reproduces the KOTO, MiniBooNE and muon anomalous magnetic moment anomalies. The dark sector is comprised of a light scalar singlet $S$ that has a large coupling to a slightly heavier sterile neutrino that mixes with the active neutrinos. The scalar couples to standard model fermions via Yukawa couplings, and to photons via a higher-dimensional coupling. The KOTO signal is a result of the flavor-changing penguin process $K_L \to π^0 S$ followed by the decay of $S$ to neutrinos. The sterile neutrino produced in neutrino-nucleus scattering at MiniBooNE decays to an active neutrino and $S$, which decays electromagnetically and creates an event excess at low energies.

hep-ph↗

arXiv2020

Resolving the $(g-2)_μ$ and $B$ anomalies with leptoquarks and a dark Higgs boson

At present, there are outstanding discrepancies between standard model predictions and measurements of the muon's $g-2$ and several $B$-meson properties. We resolve these anomalies by considering a two-Higgs-doublet model extended to include leptoquarks and a dark Higgs boson $S$. The leptoquarks modify $B$-meson decays and also induce an $S γγ$ coupling, which contributes to the muon's $g-2$ through a Barr-Zee diagram. We show that, for TeV-scale leptoquarks and dark Higgs boson masses $m_{S} \sim 10-200~\text{MeV}$, a consistent resolution to all of the anomalies exists. The model predicts interesting new decays, such as $B \to K^{(*)} e^+ e^-$, $B \to K^{(*)} γγ$, $K \to πγγ$, and $h \to γγγγ$, with branching fractions not far below current bounds.

hep-ph↗

arXiv2019

CP Violation in ${\bar B}^0\to D^{*+}μ^-{\barν}_μ$

In order to explain the observed anomalies in the measurements of $R_{D^{(*)}}$ and $R_{J/ψ}$, a variety of new-physics (NP) models that contribute to $b\to cτ^-{\barν}$ have been proposed. In this paper, we show how CP-violating observables can be used to distinguish these NP models. Because ${\vec p}_τ$ cannot be measured (the decay products of the $τ$ include the undetected $ν_τ$), obtaining the angular distribution of ${\bar B}^0\to D^{*+}τ^{-}{\barν}_τ$ is problematic. Instead, we focus here on ${\bar B}^0\to D^{*+}(\to D^0 π^+)μ^- {\barν}_μ$. This process may also receive contributions from the same NP, and LHCb intends to measure the CP-violating angular asymmetries in this decay. There are two classes of NP models that contribute to $b\to cμ^-{\barν}_μ$. These involve (i) a $W'$ (two types) or (ii) a leptoquark (LQ) (six types). The most popular NP models predict no CP-violating effects, so the measurement of nonzero CP-violating symmetries would rule them out. Furthermore these measurements allow one to distinguish the $W'$ and LQ models, and to differentiate among several LQ models.

hep-ph↗

arXiv2019

New physics in inclusive semileptonic $B$ decays including nonperturbative corrections

In this work we study the effects of New Physics (NP) operators on the inclusive $\bar{B} \to X_c τ^- \barν_τ$ decay including power $(\mathcal{O}(1/m_b^2))$ corrections in the NP operators. In analogy with $R(D^{(*)})$ observables, we study the observable $R(X_c)=\frac{\mathcal{B}(\bar{B} \to X_c τ^- \barν_τ)}{\mathcal{B}(\bar{B} \to X_c \ell^- \barν_\ell)}$. We present some numerical results for $R(X_c)$ and compare the results for this observable with and without power corrections in the NP contributions.

hep-ph↗

arXiv2018

New physics in inclusive $B \to X_c\ell \barν$ decay in light of $R(D^{(*)})$ measurements

In this work we study the effects of new-physics (NP) operators with different Lorentz structures on the inclusive $B \to X_cτ\barν$ decay and make predictions for the ratio of total decay rates $R(X_c)=\frac{Γ(B \to X_cτ\barν_τ)}{Γ(B \to X_c \ell \barν_\ell)}$ with $\ell=e, μ$, the differential decay rates, $\frac{dΓ}{dq^2}$ and $\frac{dΓ}{dE_τ}$, forward-backward asymmetry $A_{FB}$ and the ratio of the differential decay rates $B(q^2)=\frac{dΓ(B \to X_cτ\barν_τ)/dq^2}{dΓ(B \to X_c \ell \barν_\ell)/dq^2}$. In addition, we introduce some leptoquark models as explicit models of the NP operators and study their effects on the inclusive decay. We consider $\mathcal{O}(α_s)$ radiative and $1/m_b$ nonperturbative corrections to the Standard Model (SM) decay rate and ignore their small effects in the NP contributions.

hep-ph↗

arXiv2017

Phenomenology of $Λ_b \to Λ_c τ\barν_τ$ using lattice QCD calculations

In a recent paper we studied the effect of new-physics operators with different Lorentz structures on the semileptonic $Λ_b \to Λ_c τ\barν_τ$ decay. This decay is of interest in light of the $R({D^{(*)}})$ puzzle in the semileptonic $\bar{B} \to D^{(*)} τ{\barν}_τ$ decays. In this work we add tensor operators to extend our previous results and consider both model-independent new physics (NP) and specific classes of models proposed to address the $R({D^{(*)}})$ puzzle. We show that a measurement of $R(Λ_c) = {\cal B}[Λ_b \to Λ_c τ\barν_τ] / {\cal B}[Λ_b \to Λ_c \ell \barν_{\ell}]$ can strongly constrain the NP parameters of models discussed for the $R({D^{(*)}})$ puzzle. We use form factors from lattice QCD to calculate all $Λ_b \to Λ_c τ\barν_τ$ observables. The $Λ_b \to Λ_c$ tensor form factors had not previously been determined in lattice QCD, and we present new lattice results for these form factors here.

hep-ph↗

arXiv2015

Testing General Relativity with Present and Future Astrophysical Observations

One century after its formulation, Einstein's general relativity has made remarkable predictions and turned out to be compatible with all experimental tests. Most of these tests probe the theory in the weak-field regime, and there are theoretical and experimental reasons to believe that general relativity should be modified when gravitational fields are strong and spacetime curvature is large. The best astrophysical laboratories to probe strong-field gravity are black holes and neutron stars, whether isolated or in binary systems. We review the motivations to consider extensions of general relativity. We present a (necessarily incomplete) catalog of modified theories of gravity for which strong-field predictions have been computed and contrasted to Einstein's theory, and we summarize our current understanding of the structure and dynamics of compact objects in these theories. We discuss current bounds on modified gravity from binary pulsar and cosmological observations, and we highlight the potential of future gravitational wave measurements to inform us on the behavior of gravity in the strong-field regime.

gr-qc↗