arXiv subjects

S. Dey

Publications and source records attributed to S. Dey.

At least 73 records · Page 4Linked to original sources

arXiv2025

New constraints on cosmic ray-boosted dark matter from the LUX-ZEPLIN experiment

While dual-phase xenon time projection chambers (TPCs) have driven the sensitivity towards weakly interacting massive particles (WIMPs) at the GeV/c^2 to TeV/c^2 mass scale, the scope for sub-GeV/c^2 dark matter particles is hindered by a limited nuclear recoil energy detection threshold. One approach to probe for lighter candidates is to consider cases where they have been boosted by collisions with cosmic rays in the Milky Way, such that the additional kinetic energy lifts their induced signatures above the nominal threshold. In this Letter, we report first results of a search for cosmic ray-boosted dark matter (CRDM) with a combined 4.2 tonne-year exposure from the LUX-ZEPLIN (LZ) experiment. We observe no excess above the expected backgrounds and establish world-leading constraints on the spin-independent CRDM-nucleon cross section as small as 3.9 * 10^{-33} cm^2 at 90% confidence level for sub-GeV/c^2 masses.

hep-ex↗

arXiv2025

HydroX, a light dark matter search with hydrogen-doped liquid xenon time projection chambers

Experimental efforts searching for dark matter particles over the last few decades have ruled out many candidates led by the new generation of tonne-scale liquid xenon. For light dark matter, hydrogen could be a better target than xenon as it would offer a better kinematic match to the low mass particles. This article describes the HydroX concept, an idea to expand the dark matter sensitivity reach of large liquid xenon detectors by adding hydrogen to the liquid xenon. We discuss the nature of signal generation in liquid xenon to argue that the signal produced at the interaction site by a dark matter-hydrogen interaction could be significantly enhanced over the same interaction on xenon, increasing the sensitivity to the lightest particles. We discuss the technical implications of adding hydrogen to a xenon detector, as well as some background considerations. Finally, we make projections as to the potential sensitivity of a HydroX implementation and discuss next steps.

hep-ex↗

arXiv2025

Measurement of the branching fraction, polarization, and time-dependent $CP$ asymmetry in $B^0 \to ρ^+ρ^-$ decays and constraint on the CKM angle $ϕ_2$

We present a measurement of the branching fraction and fraction of longitudinal polarization of $B^0 \to ρ^+ ρ^-$ decays, which have two $π^0$'s in the final state. We also measure time-dependent {\it CP} violation parameters for decays into longitudinally polarized $ρ^+ ρ^-$ pairs. This analysis is based on a data sample containing $(387\pm6) \times 10^6$ $Υ(4S)$ mesons collected with the Belle~II detector at the SuperKEKB asymmetric-energy $e^+e^-$ collider in 2019-2022. We obtain $\mathcal{B}(B^0\toρ^+ρ^-) = (2.89 ^{+0.23}_{-0.22} {}^{+0.29}_{-0.27}) \times 10^{-5}, f_{L} = 0.921 ^{+0.024}_{-0.025} {}^{+0.017}_{-0.015}$, \mbox{$S = -0.26\pm0.19\pm0.08$}, and $C = -0.02\pm0.12^{+0.06}_{-0.05}$, where the first uncertainties are statistical and the second are systematic. We use these results to perform an isospin analysis to constrain the CKM angle $ϕ_2$ and obtain two solutions; the result consistent with other Standard Model constraints is $ϕ_2 = (92.6^{+4.5}_{-4.7})^\circ$.

hep-ex↗

arXiv2025

Neutrinoless Double Beta Decay Sensitivity of the XLZD Rare Event Observatory

The XLZD collaboration is developing a two-phase xenon time projection chamber with an active mass of 60 to 80 t capable of probing the remaining WIMP-nucleon interaction parameter space down to the so-called neutrino fog. In this work we show that, based on the performance of currently operating detectors using the same technology and a realistic reduction of radioactivity in detector materials, such an experiment will also be able to competitively search for neutrinoless double beta decay in $^{136}$Xe using a natural-abundance xenon target. XLZD can reach a 3$σ$ discovery potential half-life of 5.7$\times$10$^{27}$ yr (and a 90% CL exclusion of 1.3$\times$10$^{28}$ yr) with 10 years of data taking, corresponding to a Majorana mass range of 7.3-31.3 meV (4.8-20.5 meV). XLZD will thus exclude the inverted neutrino mass ordering parameter space and will start to probe the normal ordering region for most of the nuclear matrix elements commonly considered by the community.

physics.ins-det↗

arXiv2025

Search for lepton-flavor-violating $τ^- \to \ell^- K_s^0$ decays at Belle and Belle II

We present the results of a search for charged-lepton-flavor violating decays $τ^{-} \rightarrow \ell^{-}K_{S}^{0}$, where $\ell^{-}$ is either an electron or a muon. We combine $e^+e^-$ data samples recorded by the Belle II experiment at the SuperKEKB collider (428 fb$^{-1}$) with samples recorded by the Belle experiment at the KEKB collider (980 fb$^{-1}$) to obtain a sample of 1.3 billion $e^+e^-\toτ^+τ^-$ events. We observe 0 and 1 events and set $90\%$ confidence level upper limits of $0.8 \times 10^{-8}$ and $1.2 \times 10^{-8}$ on the branching fractions of the decay modes $τ^{-} \rightarrow e^{-}K_{S}^{0}$ and $τ^{-} \rightarrow μ^{-}K_{S}^{0}$, respectively. These are the most stringent upper limits to date.

hep-ex↗

arXiv2025

Search for $B^0 \to K^{\ast 0} τ^+ τ^-$ decays at the Belle II experiment

We present a search for the rare flavor-changing neutral-current decay $B^0 \to K^{\ast 0} τ^+ τ^-$ with data collected by the Belle II experiment at the SuperKEKB electron-positron collider. The analysis uses a 365 fb$^{-1}$ data sample recorded at the center-of-mass energy of the $Υ(4S)$ resonance. One of the $B$ mesons produced in the $Υ(4S)\to B^0 \bar{B}^0$ process is fully reconstructed in a hadronic decay mode, while its companion $B$ meson is required to decay into a $K^{\ast 0}$ and two $τ$ leptons of opposite charge. The $τ$ leptons are reconstructed in final states with a single electron, muon, charged pion or charged $ρ$ meson, and additional neutrinos. We set an upper limit on the branching ratio of $BR(B^0 \to K^{\ast 0} τ^+ τ^-) < 1.8 \times 10^{-3}$ at the 90% confidence level, which is the most stringent constraint reported to date.

hep-ex↗

arXiv2025

Measurement of $B \to K{}^{*}(892)γ$ decays at Belle II

We present measurements of $B \to K{}^{*}(892)γ$ decays using $365\,{\rm fb}^{-1}$ of data collected from 2019 to 2022 by the Belle~II experiment at the SuperKEKB asymmetric-energy $e^+e^-$ collider. The data sample contains $(387 \pm 6) \times 10^6$ $Υ(4S)$ events. We measure branching fractions ($\mathcal{B}$) and $C\!P$ asymmetries ($\mathcal{A}_{C\!P}$) for both $B^{0}\to K{}^{*0}γ$ and $B^{+}\to K{}^{*+}γ$ decays. The difference in $C\!P$ asymmetries ($Δ\mathcal{A}_{C\!P}$) and the isospin asymmetry ($Δ_{0+}$) between these neutral and charged channels are also measured. We obtain the following branching fractions and $C\!P$ asymmetries: $\mathcal{B} (B^{0} \to K{}^{*0}γ) = (4.14 \pm 0.10 \pm 0.11 ) \times 10^{-5}$, $\mathcal{B} (B^{+} \to K{}^{*+}γ) = (4.04 \pm 0.13 {}^{+0.13}_{-0.15} )\times 10^{-5}$, $\mathcal{A}_{C\!P} (B^{0} \to K{}^{*0}γ) = (-3.3 \pm 2.3 \pm 0.4 )\%$, and $\mathcal{A}_{C\!P} (B^{+} \to K{}^{*+}γ) = (-0.7 \pm 2.9 \pm 0.5 )\%$. The measured difference in $C\!P$ asymmetries is $Δ\mathcal{A}_{C\!P} = (+2.6 \pm 3.8 \pm 0.6 )\%$, and the measured isospin asymmetry is $Δ_{0+} = (+4.8 \pm 2.0 \pm 1.8 )\%$. The first uncertainties listed are statistical and the second are systematic. These results are consistent with world-average values and theory predictions.

hep-ex↗

arXiv2025

Measurement of the Branching Fraction of $Λ_c^+ \to p K_S^0 π^0$ at Belle

We report a precise measurement of the ratio of branching fractions $\mathcal{B}(Λ_c^+\to p K_S^0 π^0)/\mathcal{B}(Λ_c^+\to p K^- π^+)$ using 980 fb$^{-1}$ of $e^+e^-$ data from the Belle experiment. We obtain a value of $\mathcal{B}(Λ_c^+\to p K_S^0 π^0)/\mathcal{B}(Λ_c^+\to p K^- π^+)=0.339\pm 0.002\pm 0.009$, where the first and second uncertainties are statistical and systematic, respectively. This Belle result is consistent with the previous measurement from the CLEO experiment but has a fivefold improvement in precision. By combining our result with the world average $\mathcal{B}(Λ_c^+\to p K^- π^+)$, we obtain the absolute branching fraction $\mathcal{B}(Λ_c^+\to p K_S^0 π^0)=(2.12\pm 0.01\pm 0.05 \pm 0.10)\%$, where the uncertainties are statistical, systematic, and the uncertainty in the absolute branching fraction scale $\mathcal{B}(Λ_c^+\to p K^- π^+)$, respectively. This measurement can shed light on hadronic decay mechanisms in charmed baryon decays.

hep-ex↗

arXiv2025

Observation of the decay $B^0 \to J/ψω$ at Belle II

We measure the branching fraction of the decay $B^0 \to J/ψω$ using data collected with the Belle II detector at the SuperKEKB collider. The data contain $(387 \pm 6) \times 10^6$ $B\overline{B}$ meson pairs produced in energy-asymmetric $e^+e^-$ collisions at the $Υ(4S)$ resonance. The measured branching fraction $\mathcal{B}(B^0 \to J/ψω) = \left( 2.16 \pm 0.30 \pm 0.14 \right) \times 10^{-5}$, where the first uncertainty is statistical and the second is systematic, is more precise than previous results and constitutes the first observation of the decay with a significance of $6.5$ standard deviations.

hep-ex↗

arXiv2025

Measurement of the inclusive branching fractions for $B_s^0$ decays into $D$ mesons via hadronic tagging

We report measurements of the absolute branching fractions $\mathcal{B}(B_s^0 \to D_s^{\pm} X)$, $\mathcal{B}(B_s^0 \to D^0/\bar{D}^0 X)$, and $\mathcal{B}(B_s^0 \to D^{\pm} X)$, where the latter is measured for the first time. The results are based on a 121.4\,fb$^{-1}$ data sample collected at the $Υ(10860)$ resonance by the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We reconstruct one $B_s^0$ meson in $e^+e^- \to Υ(10860) \to B_s^{*} \bar{B}_s^{*}$ events and measure yields of $D_s^+$, $D^0$, and $D^+$ mesons in the rest of the event. We obtain $\mathcal{B}(B_s^0 \to D_s^{\pm} X) = (68.6 \pm 7.2 \pm 4.0)\%$, $\mathcal{B}(B_s^0 \to D^0/\bar{D}^0 X) = (21.5 \pm 6.1 \pm 1.8)\%$, and $\mathcal{B}(B_s^0 \to D^{\pm} X) = (12.6 \pm 4.6 \pm 1.3)\%$, where the first uncertainty is statistical and the second is systematic. Averaging with previous Belle measurements gives $\mathcal{B}(B_s^0 \to D_s^{\pm} X) = (63.4 \pm 4.5 \pm 2.2)\%$ and $\mathcal{B}(B_s^0 \to D^0/\bar{D}^0 X) = (23.9 \pm 4.1 \pm 1.8)\%$. For the $B_s^0$ production fraction at the $Υ(10860)$, we find $f_s = (21.4^{+1.5}_{-1.7})\%$.

hep-ex↗

arXiv2025

Measurement of $B^+\toτ^+ν_τ$ branching fraction with a hadronic tagging method at Belle II

We present a measurement of the branching fraction of $B^+\toτ^+ν_τ$ decays using $(387\pm6)\times 10^6$ $Υ(4S)$ collected between 2019 and 2022 with the Belle II detector at the SuperKEKB $e^+e^-$ collider. We reconstruct the accompanying $B^-$ meson using the hadronic tagging method, while $B^+\toτ^+ν_τ$ candidates are identified in the recoil. We find evidence for $B^+\toτ^+ν_τ$ decays at 3.0 standard deviations, including systematic uncertainties. The measured branching fraction is $\mathcal{B}(B^+\toτ^+ν_τ) = [1.24 \pm 0.41 (\text{stat.}) \pm 0.19 (\text{syst.})] \times 10^{-4}$.

hep-ex↗

arXiv2024

Search for lepton flavor-violating decay modes $B^0\to K_S^0τ^\pm\ell^\mp~(\ell=μ, e)$ with hadronic $B$-tagging at Belle and Belle II

We present the first search for the lepton flavor-violating decay modes $B^0 \rightarrow K_S^0 τ^\pm \ell^\mp~(\ell=μ, e)$ using the 711 fb$^{-1}$ and 365 fb$^{-1}$ data samples recorded by the Belle and Belle II detectors, respectively. We use a hadronic $B$-tagging technique, and search for the signal decay in the system recoiling against the fully reconstructed $B$ meson. We find no evidence for $B^0 \rightarrow K_S^0 τ^\pm \ell^\mp$ decays and set 90\% confidence level upper limits on the branching fractions in the range of $[0.8,\,3.6]\times10^{-5}$.

hep-ex↗

arXiv2024

Measurement of the branching fraction and $\it CP$-violating asymmetry of the decay $B^{0} \rightarrow π^{0} π^{0}$ using $387$ million bottom-antibottom meson pairs in Belle II data

We measure the branching fraction and $\it CP$-violating flavor-dependent rate asymmetry of $B^{0} \to π^{0} π^{0}$ decays reconstructed using the Belle II detector in an electron-positron collision sample containing $387 \times 10^{6}$ $B\overline{B}$ pairs. Using an optimized event selection, we find $126\pm 20$ signal decays in a fit to background-discriminating and flavor-sensitive distributions. The resulting branching fraction is $(1.25 \pm 0.23)\times 10^{-6}$ and the $\it CP$-violating asymmetry is $0.03 \pm 0.30$.

hep-ex↗

arXiv2024

Observations of the singly Cabibbo-suppressed decays $Ξ_c^{+} \to pK_{S}^{0}$, $Ξ_c^+ \to Λπ^+$, and $Ξ_c^+ \to Σ^{0} π^+$ at Belle and Belle II

Using data samples of 983.0~$\rm fb^{-1}$ and 427.9~$\rm fb^{-1}$ accumulated with the Belle and Belle~II detectors operating at the KEKB and SuperKEKB asymmetric-energy $e^+e^-$ colliders, singly Cabibbo-suppressed decays $Ξ_c^{+} \to pK_{S}^{0}$, $Ξ_c^+ \to Λπ^+$, and $Ξ_c^+ \to Σ^{0} π^+$ are observed for the first time. The ratios of branching fractions of $Ξ_{c}^{+}\to p K_{S}^{0}$, $Ξ_{c}^{+}\to Λπ^{+}$, and $Ξ_{c}^{+}\to Σ^{0} π^{+}$ relative to that of $Ξ_c^+ \to Ξ^- π^{+} π^{+}$ are measured to be \begin{equation} \frac{{\cal B}(Ξ_c^+ \to pK_S^0)}{{\cal B}(Ξ_c^{+} \to Ξ^{-} π^+ π^+)} = (2.47 \pm 0.16 \pm 0.07)\% \notag, \end{equation} \begin{equation} \frac{{\cal B}(Ξ_c^+ \to Λπ^+)}{{\cal B}(Ξ_c^{+} \to Ξ^{-} π^+ π^+)} = (1.56 \pm 0.14 \pm 0.09)\% \notag, \end{equation} \begin{equation} \frac{{\cal B}(Ξ_c^+ \to Σ^0 π^+)}{{\cal B}(Ξ_c^{+} \to Ξ^{-} π^+ π^+)} = (4.13 \pm 0.26 \pm 0.22)\% \notag. \end{equation} Multiplying these values by the branching fraction of the normalization channel, ${\cal B}(Ξ_c^{+} \to Ξ^{-} π^+π^+) = (2.9 \pm 1.3)\%$, the absolute branching fractions are determined to be \begin{equation} {\cal B}(Ξ_c^{+} \to p K_{S}^{0}) = (7.16 \pm 0.46 \pm 0.20 \pm 3.21) \times 10^{-4} \notag, \end{equation} \begin{equation} {\cal B}(Ξ_c^{+} \to Λπ^+) = (4.52 \pm 0.41 \pm 0.26 \pm 2.03) \times 10^{-4} \notag, \end{equation} \begin{equation} {\cal B}(Ξ_c^{+} \to Σ^0 π^+) = (1.20 \pm 0.08 \pm 0.07 \pm 0.54) \times 10^{-3} \notag. \end{equation} The first and second uncertainties above are statistical and systematic, respectively, while the third ones arise from the uncertainty in ${\cal B}(Ξ_c^{+} \to Ξ^{-} π^{+} π^{+})$.

hep-ex↗

arXiv2024

Measurement of the $e^+e^- \to π^+π^-π^0$ cross section in the energy range 0.62-3.50 GeV at Belle II

We report a measurement of the $e^+e^- \to π^+π^-π^0$ cross section in the energy range from 0.62 to 3.50 GeV using an initial-state radiation technique. We use an $e^+e^-$ data sample corresponding to 191 $\text{fb}^{-1}$ of integrated luminosity, collected at a center-of-mass energy at or near the $Υ{(4S)}$ resonance with the Belle II detector at the SuperKEKB collider. Signal yields are extracted by fitting the two-photon mass distribution in $e^+e^- \to π^+π^-π^0γ$ events, which involve a $π^0 \to γγ$ decay and an energetic photon radiated from the initial state. Signal efficiency corrections with an accuracy of 1.6% are obtained from several control data samples. The uncertainty on the cross section at the $ω$ and $ϕ$ resonances is dominated by the systematic uncertainty of 2.2%. The resulting cross sections in the 0.62-1.80 GeV energy range yield $ a_μ^{3π} = [48.91 \pm 0.23~(\mathrm{stat}) \pm 1.07~(\mathrm{syst})] \times 10^{-10} $ for the leading-order hadronic vacuum polarization contribution to the muon anomalous magnetic moment. This result differs by $2.5$ standard deviations from the most precise current determination.

hep-ex↗

arXiv2024

Two-neutrino double electron capture of $^{124}$Xe in the first LUX-ZEPLIN exposure

The broad physics reach of the LUX-ZEPLIN (LZ) experiment covers rare phenomena beyond the direct detection of dark matter. We report precise measurements of the extremely rare decay of $^{124}$Xe through the process of two-neutrino double electron capture (2$ν$2EC), utilizing a $1.39\,\mathrm{kg} \times \mathrm{yr}$ isotopic exposure from the first LZ science run. A half-life of $T_{1/2}^{2\nu2\mathrm{EC}} = (1.09 \pm 0.14_{\text{stat}} \pm 0.05_{\text{sys}}) \times 10^{22}\,\mathrm{yr}$ is observed with a statistical significance of $8.3\,σ$, in agreement with literature. First empirical measurements of the KK capture fraction relative to other K-shell modes were conducted, and demonstrate consistency with respect to recent signal models at the $1.4\,σ$ level.

nucl-ex↗

arXiv2024

Search for lepton-flavor-violating $τ^- \to μ^-μ^+μ^-$ decays at Belle II

We present the result of a search for the charged-lepton-flavor violating decay $τ^- \to μ^-μ^+μ^-$ using a $424fb^{-1}$ sample of data recorded by the Belle II experiment at the SuperKEKB $e^{-}e^{+}$ collider. The selection of $e^{-}e^{+}\toτ^+τ^-$ events is based on an inclusive reconstruction of the non-signal tau decay, and on a boosted decision tree to suppress background. We observe one signal candidate, which is compatible with the expectation from background processes. We set a $90\%$ confidence level upper limit of $1.9 \times 10^{-8}$ on the branching fraction of the \taumu decay, which is the most stringent bound to date.

hep-ex↗

arXiv2024

Measurements of the branching fractions of $Ξ_{c}^{0}\toΞ^{0}π^{0}$, $Ξ_{c}^{0}\toΞ^{0}η$, and $Ξ_{c}^{0}\toΞ^{0}η^{\prime}$ and asymmetry parameter of $Ξ_{c}^{0}\toΞ^{0}π^{0}$

We present a study of $Ξ_{c}^{0}\toΞ^{0}π^{0}$, $Ξ_{c}^{0}\toΞ^{0}η$, and $Ξ_{c}^{0}\toΞ^{0}η^{\prime}$ decays using the Belle and Belle~II data samples, which have integrated luminosities of 980~$\mathrm{fb}^{-1}$ and 426~$\mathrm{fb}^{-1}$, respectively. We measure the following relative branching fractions $${\cal B}(Ξ_{c}^{0}\toΞ^{0}π^{0})/{\cal B}(Ξ_{c}^{0}\toΞ^{-}π^{+}) = 0.48 \pm 0.02 ({\rm stat}) \pm 0.03 ({\rm syst}) ,$$ $${\cal B}(Ξ_{c}^{0}\toΞ^{0}η)/{\cal B}(Ξ_{c}^{0}\toΞ^{-}π^{+}) = 0.11 \pm 0.01 ({\rm stat}) \pm 0.01 ({\rm syst}) ,$$ $${\cal B}(Ξ_{c}^{0}\toΞ^{0}η^{\prime})/{\cal B}(Ξ_{c}^{0}\toΞ^{-}π^{+}) = 0.08 \pm 0.02 ({\rm stat}) \pm 0.01 ({\rm syst}) $$ for the first time, where the uncertainties are statistical ($\rm stat$) and systematic ($\rm syst$). By multiplying by the branching fraction of the normalization mode, ${\mathcal B}(Ξ_{c}^{0}\toΞ^{-}π^{+})$, we obtain the following absolute branching fraction results $(6.9 \pm 0.3 ({\rm stat}) \pm 0.5 ({\rm syst}) \pm 1.3 ({\rm norm})) \times 10^{-3}$, $(1.6 \pm 0.2 ({\rm stat}) \pm 0.2 ({\rm syst}) \pm 0.3 ({\rm norm})) \times 10^{-3}$, and $(1.2 \pm 0.3 ({\rm stat}) \pm 0.1 ({\rm syst}) \pm 0.2 ({\rm norm})) \times 10^{-3}$, for $Ξ_{c}^{0}$ decays to $Ξ^{0}π^{0}$, $Ξ^{0}η$, and $Ξ^{0}η^{\prime}$ final states, respectively. The third errors are from the uncertainty on ${\mathcal B}(Ξ_{c}^{0}\toΞ^{-}π^{+})$. The asymmetry parameter for $Ξ_{c}^{0}\toΞ^{0}π^{0}$ is measured to be $α(Ξ_{c}^{0}\toΞ^{0}π^{0}) = -0.90\pm0.15({\rm stat})\pm0.23({\rm syst})$.

hep-ex↗