arXiv subjects

Jian Chai

Publications and source records attributed to Jian Chai.

13 recordsLinked to original sources

arXiv2026

CompanionHarm: A Multi-Turn Benchmark for Detecting Harms in Real-World AI Companion Conversations

As AI companions become increasingly embedded in everyday life, there is an urgent need to detect harms that emerge in social and emotional human-AI interactions. Yet research in this area is constrained by the lack of real-world, multi-turn conversational datasets for operationalizing and evaluating harms that are relational and contextual. In this work, we introduce CompanionHarm, a publicly available benchmark dataset comprising 2,111 real-world, multi-turn conversations (14,051 utterances) between users and the AI companion Replika. 7,016 AI utterances were annotated independently by three annotators across 13 harmful behavior categories grounded in a taxonomy of AI companion harms, and the dataset includes both aggregated labels and annotator-level labels to support model evaluation and systematic disagreement analysis. Evaluations of seven large language models (LLMs) show that harm detection using multi-turn conversational context outperforms detection based on isolated utterances, although current LLMs still struggle to consistently integrate contextual cues, calibrate harm severity, and interpret relational boundaries. We also find substantial annotator disagreement for context-dependent harmful behaviors, with disagreement varying according to annotators' political affiliation, conversation length, and the utterance's position. Together, CompanionHarm provides a foundation for detecting socio-emotional harms in multi-turn human-AI conversations and for rigorously examining how such harms are interpreted by both humans and LLMs. Our dataset is available at https://github.com/HanMeng2004/CompanionHarm.

cs.CY↗

arXiv2026

Polarization fractions and helicity-dependent CP asymmetries in $B_{(s)} \to \rho\rho, \rho K^\ast$ and $K^\ast K^\ast$ decays

In this paper, we present a phenomenological analysis of $B_{(s)} \to \rho\rho, \rho K^\ast$ and $K^\ast K^\ast$ decays using state-of-the-art perturbative QCD (pQCD) calculations. Our study is primarily motivated by recent polarization measurements from the LHCb and Belle II collaborations, which have significantly improved the precision of polarization fractions and enabled the first full determination of polarization-dependent CP asymmetries. This work extends the comprehensive pQCD study of charmless two-body $B$ decays reported in our previous paper [Chin. Phys. C 46 (2022) 123103], with a particular focus on polarization observables, especially the CP asymmetries in each helicity state, which reflect distinct orbital angular momentum configurations between the two vector mesons. Our predictions for the branching ratios and longitudinal polarization fractions in the $B^0 \to K^{\ast 0} {\bar K}^{\ast 0}$ and $B^+ \to \rho^0 K^{\ast +}$ modes are in good agreement with the new experimental data. However, the calculated longitudinal polarization fraction for $B_s \to K^{\ast 0} {\bar K}^{\ast 0}$ is significantly larger than the LHCb measurement. Moreover, the predicted (helicity-dependent) CP asymmetries in $B^+ \to \rho^0 K^{\ast +}$ are about $30 \%$ smaller than the observed values. These discrepancies point to a rich interplay between different topological amplitudes, highlighting the need for further theoretical investigation to resolve the long-standing polarization puzzle in two-body $B$ decays into vector mesons.

hep-ph↗

arXiv2025

The $CP$ violations and branching ratios for $B_c^+\to D_{(s)}^+\pi^+\pi^-(K^{+}K^{-})$ from interference of the vector mesons in Perturbative QCD

Within the framework of the perturbative QCD approach utilizing $K_T$ factorization, we have investigated the CP violations and branching ratios in the decay processes of $B_{c}^{+}\to D_{(s)} ^{+}V(V\rightarrow\pi^{+}\pi^{-})$ and $B_{c}^{+}\to D_{(s)}^{+}V(V\rightarrow K^{+}K^{-})$, where V denotes three vector mesons $\rho^0$, $\omega$, and $\phi$. During the $V\to \pi^+\pi^-$ and $V\to K^+K^-$ decay processes, we incorporated the $\rho^{0}-\omega-\phi$ mixing mechanism to describe the amplitudes of these quasi-two-body decay processes. Within the interference regime of the three vector particles, we observed distinct changes in both CP violations and branching ratios. Furthermore, our study presents evidence for local CP violations and branching ratios that warrants further investigation through experiments.

hep-ph↗

arXiv2025

Form factors of light pseudoscalar mesons from the perturbative QCD approach

We study the electromagnetic and meson-photon transition form factors (TFF) of light pseudoscalar mesons from the perturbative QCD (pQCD) approach. To comprehensively account for both the longitudinal and transverse nonperturbative dynamics of hadronic constituents, we incorpoarate intrinsic transverse momentum distributions (iTMDs) alongside the conventional light-cone distribution amplitudes (LCDAs). The main motivations of this work are the disjointedness of electromagnetic form factors between the theoretical predictions and the experimental measurements, and the BaBar-Belle tension of pion-photon transition form factor in the large momentum transfers. Our calculation is carried out at the next-to-leading-order for the contributions from leading and subleading twist LCDAs, and leading order for the twist four contributions. Notably, this work presents the first systematic evaluation of higher-twist contributions to meson-photon TFFs. The key findings are: (a) iTMDs play a crucial role in describing form factor data, particularly in the small-to-intermediate momentum transfer region where they induce significant modifications to pQCD predictions. (b) The extracted transverse size parameters for valence quark states are found to be $\beta_\pi^2 = 0.51 \pm 0.04$ GeV$^{-2}$ and $\beta_K^2 = 0.30 \pm 0.05$ GeV$^{-2}$, the chiral mass of pion meson $m_0^\pi$ at $1$ GeV is determined to be $1.84 \pm 0.07$ GeV. (c) The meson-photon TFFs are predominantly governed by leading-twist LCDAs. The iTMDs-enhanced pQCD results show better agreement with Belle's pion TFF data across intermediate and large momentum transfers and favor a small $\eta-\eta^\prime$ mixing angle. (d) Remarkably, the inclusion of iTMDs extends the applicability of pQCD calculations down to a few GeV$^2$ for all considered form factors, significantly improving the theory-data consistency.

hep-ph↗

arXiv2024

Shedding light on the intrinsic transversal momentum distributions of pion and kaon

We propose to introduce the intrinsic transversal momentum distribution functions (iTMDs), in conjunction with the light-cone distribution amplitudes (LCDAs), to elucidate the probability amplitude of encountering a meson state wherein the partons swiftly traverse along the longitudinal axis while gently oscillating in the transversal plane. The primary motivation stems from the oversight of soft transverse dynamics within the $k_T$ factorization formalism of an exclusive QCD process, which confines perturbative QCD (pQCD) predictions to scenarios involving large momentum transfers. We meticulously investigate the $\pi$ and $K$ electromagnetic form factors using the iTMDs-improved pQCD calculation at next-to-leading order. By analyzing data in the timelike physical regions, we obtain the transversal-size parameters $\beta_\pi^2 = 0.51 \pm 0.04$ GeV$^{-2}$ and $\beta_K^2 = 0.30 \pm 0.05$ GeV$^2$. We then extract the chiral mass of pion to be $m_0^\pi(1 \, {\rm GeV}) = 1.84 \pm 0.07$ GeV and explain the precise measurements of kaon form factor in the perturbative timelike region. As a remarkable byproduct, we found that the incorporation of iTMDs improves the pQCD predictions for electromagnetic form factors, extending the applicable range to a few GeV$^2$. This improvement allows for direct comparison with existing measurements and lattice QCD evaluations.

hep-ph↗

arXiv2024

$B\to K\bar K(\pi\eta)h$ decays in the presence of isovector scalar resonances $a_0(980,1450)$

Different from the previous treatment in a two-body framework, we introduce the dimeson distribution amplitudes (DAs) to describe the strong dynamics between the S-wave resonances $a_0(980, 1450)$ and the $K\bar K (\pi\eta)$ pair, where the Gegenbauer coefficient required is determined from the experimental data on the time-like form factors involved. The branching ratios and direct CP asymmetries of the decays $B \to a^{(\prime)}_0 h \to K\bar K(\pi\eta) h$, with $a_0=a_0(980)$, $a^{\prime}_0=a_0(1450)$ and $h$ referring to a pion or a kaon, are then calculated in the perturbative QCD (PQCD) approach. We find that the branching ratios of the corresponding quasi-two-body decays $B\to a^{(\prime)}_0 K$ obtained with the narrow width approximation are closer to those predicted in the QCD factorization (QCDF) approach compared to the previous PQCD calculations, no matter a three-body or a two-body framework is assumed. Furthermore, all our predictions for these $B\to a^{(\prime)}_0 K$ decays are below the current experimental upper limits except for those of decays $B^0\to a^{(\prime)-}_0K^+$, which are (slightly) larger than the upper limits. Under the narrow width approximation, the branching ratios of the decays $B^+\to a^{(\prime)+}_0\pi^0$, $B^0\to a^{(\prime)+}_0\pi^-$ and $B^0\to a^{(\prime)0}_0\pi^0$ are comparable to or agree well with the previous PQCD and the QCDF calculations. While for the decays $B^+\to a^{(\prime)0}_0\pi^+$ and $B^0\to a^{(\prime)-}_0\pi^+$, their branching ratios are predicted to be unexpectedly large, for example, the obtained branching ratio of decay $B^+\to a^0_0\pi^+$ is even higher than the current experimental upper limit.

hep-ph↗

arXiv2023

Enhanced photovoltaic effect in graphene-silicon Schottky junction under mechanical manipulation

Graphene-silicon Schottky junction (GSJ) which has the potential for large-scale manufacturing and integration can bring new opportunities to Schottky solar cells for photovoltaic (PV) power conversion. However, the essential power conversion limitation for these devices lies in the small open-circuit voltage ($V_{oc}$), which depends on the Schottky barrier height (SBH). In this study, we introduce an electromechanical method based on the flexoelectric effect to enhance the PV efficiency in GSJ. By atomic force microscope (AFM) tip-based indentation and in situ current measurement, the current-voltage (I-V) responses under flexoelectric strain gradient are obtained. The $V_{oc}$ is observed to increase for up to 20$\%$, leading to an evident improvement of the power conversion efficiency. Our studies suggest that strain gradient may offer unprecedented opportunities for the development of GSJ based flexo-photovoltaic applications.

cond-mat.mtrl-sci↗

arXiv2022

Charmless two-body $B$ meson decays in perturbative QCD factorization approach

The perturbative QCD (PQCD) approach based on $k_T$ factorization has made a great achievement for the QCD calculation of the hadronic B decays. Regulating the endpoint divergence by the transverse momentum of quarks in the propagators, one can do the perturbation calculation for kinds of diagrams including the annihilation type diagrams. In this paper, we review the current status of PQCD factorization calculation of two-body charmless $B\to PP, PV, VV$ decays up to the next-to-leading order (NLO) QCD corrections. two new power suppressed terms in decaying amplitudes are also taken into account. By using the universal input (non-perturbative) parameters, we collected the branching ratios and ${\bf CP}$ asymmetry parameters for all the charmless two body $B$ decays, calculated in the PQCD approach up to the NLO, including some power suppressed terms in decaying amplitudes. The results are compared with the ones from QCD factorization approach, soft-collinear effective theory approach and the current experimental measurements. For most considered B meson decays, the PQCD results for branching ratios agree well with other approaches and the experimental data. The PQCD predictions for the ${\bf CP}$ asymmetry parameters for many of the decay channels do not agree with other approaches, but have a better agreement with the experimental data. The longstanding $K π$ puzzle about the pattern of the direct CP asymmetries of the penguin-dominated $B \to K π$ decays can be understood after the inclusion of the NLO contributions in PQCD. The NLO corrections and power suppressed terms play an important role in the color suppressed and pure annihilation type $B$ decay modes. These rare decays are more sensitive to different types of corrections, providing opportunity to examine the factorization approach with the more precise experimental measurements.

hep-ph↗

arXiv2022

Determination of pion distribution amplitudes from the electromagnetic form factor with the data-driven dispersion relation

We study the pion electromagnetic form factor in the modulus squared dispersion relation, and do the model independent extraction of the most important nonperturbative parameters in pion light-cone distribution amplitude. The motivation of this work is the recent measurement of timelike form factor in the resonant regions, which makes up the piece lacking solid QCD-based calculation. With the perturbative QCD calculation up to next-to-leading-order QCD corrections and twist four level of meson distribution amplitudes, we obtain the chiral mass of pion meson as $m_0^π(1 \, {\rm GeV}) = 1.31^{+0.27}_{-0.30} \, {\rm GeV}$. More accuracy measurement of the timelike form factor are desirable to help us to determine the lowest gegenbauer moments in pion distribution amplitudes with the data-driven dispersion relation method developed in this work.

hep-ph↗

arXiv2022

Probing isovector scalar mesons in the charmless three-body $B$ decays

We propose to study the multiparticle configurations of isovector scalar mesons, saying $a_0(980)$ and $a_0(1450)$, in the charmless three-body $B$ decays by considering the width effects. Two scenarios of $a_0$ configurations are assumed, in which the first one take $a_0(980)$ as the lowest-lying $q{\bar q}$ state and $a_0(1450)$ as the first radial excited state, the second one take $a_0(1450)$ as the lowest-lying $q{\bar q}$ state and $a_0(1950)$ as the first radial excited state while $a_0(980)$ is not a $q{\bar q}$ state. Within these two scenarios, we do the PQCD calculation for the quasi-two-body $B \to a_0 \left[\to K{\bar K}/πη\right] h$ decays and extract the corresponding branching fractions of two-body $B \to a_0 h$ decays under the narrow width approximation. Our predictions show that the first scenario of $a_0(980)$ configuration can not be excluded by the available measurements in $B$ decays, the contributions from $a_0(1450)$ to the branching fractions in most channels are comparable in the first and second scenarios. Several channels are suggested for the forthcoming experimental measurements to reveal the multiparticle configurations of $a_0$, such as the channel $B^0 \to a_0^-(980) \left[\to π^-η\right] π^+$ with the largest predicted branching fraction, the channels $B^0 \to a_0^{\pm}(1450) \left[\to K^\pm{\bar K}^0, π^\pmη\right] π^\mp$ whose branching fractions obtained in the second scenario is about three times larger in magnitude than that obtained in the first scenario, and also the channels $B^+ \to a_0^+(1950) \left[ K^+{\bar K}^0/π^+η\right] K^0$ whose branching fractions are linear dependent on the partial width $Γ_{a_0(1950) \to KK/πη}$.

hep-ph↗

arXiv2021

The role of $D_{(s)}^\ast$ and their contributions in $B_{(s)}\to D_{(s)} h h^\prime$ decays

We demonstrate the roles of $D_{(s)}^\ast$ and their contributions in the quasi-two-body decays $B_{(s)} \to D_{(s)} h h^\prime$ ($h, h^\prime = \{π, K\}$) in the perturbative QCD approach, stemming from the quark flavour changing $\bar{b} \to \bar{c} \, q_2 \, \bar{q}_1$ and $\bar{b} \to c \, \bar{q_1} \, \bar{q_2}$ with $q_1, q_2 = \{s/d, u\}$. The main motivation of this study is the measurements of significant derivations from the simple phase-space model in the channels $B_{(s)} \to D_{(s)} h h^\prime$ at $B$ factories and LHC, which is now clarified as the Breit-Wigner-tail effects from the corresponding intermediate resonant states $D_{(s)}^\ast$. We confirm that these effect from $D^\ast$ is small ($\sim 5\%$) in the quasi-two-body $B_{(s)} \to D ππ(K)$ decaying channels, and predict the tiny ($< 1 \%$) contributions from $D^\ast$ in the $B_{(s)} \to D_s K π(K)$ decaying channels, our result for the $B_s \to D K π(K)$ decaying channels contributed only from the Breit-Wigner-tail effect of $D_s^\ast$ is in agreement with the current LHCb measurement. We recommend the Belle-II and the LHCb collaborations to restudy the processes $B^+\to \bar D^{\ast 0}π^+(K^+) \to D^-π^+π^+(K^+)$ to reveal the structure of $D^{\ast 0}$ and the strong decay $D^{\ast 0} \to D^+ π^-$.

hep-ph↗

arXiv2020

Contributions of $K^_0(1430)$ and $K^_0(1950)$ in the three-body decays $B\to Kπh$

We study the contributions of the resonant states $K_0^{*}(1430)$ and $K_0^{*}(1950)$ in the three-body decays $B\to Kπh$ (with $h=π, K$) in the perturbative QCD approach. The crucial nonperturbative input $F_{Kπ}(s)$ in the distribution amplitudes of the $S$-wave $Kπ$ system is derived from the matrix element of vacuum to $Kπ$ pair. The $CP$ averaged branching fraction of the quasi-two-body decay process $B\to K^*_0(1950)h\to Kπh$ is about one order smaller than that of the corresponding decay $B\to K^*_0(1430)h\to Kπh$. In view of the important contribution from the $S$-wave $Kπ$ system for the $B\to Kπh$ decays, it is not appropriate to neglect the $K_0^{*}(1950)$ in the theoretical or experimental studies for the relevant three-body $B$ meson decays. The predictions in this work for the relevant decays are consistent with the existing experimental data.

hep-ph↗

arXiv2019

Virtual contributions from $D^{\ast}(2007)^0$ and $D^{\ast}(2010)^{\pm}$ in the $B\to Dπh$ decays

We study the quasi-two-body decays $B\to D^*h \to Dπh$ with $h=(π, K)$ in the perturbative QCD approach and focus on the virtual contributions from the off-shell $D^{\ast}(2007)^0$ and $D^{\ast}(2010)^{\pm}$ in the four measured decays $\bar B^0 \to D^0π^+π^-$, $\bar B^0\to D^0π^+K^-$, $B^- \to D^+π^-π^-$ and $B^- \to D^+π^-K^-$. For the $\bar B^0 \to D^{*+}π^-\to D^0π^+π^-$ and $\bar B^0\to D^{*+}K^-\to D^0π^+K^-$ decays, their branching fractions concentrate in a very small region of $m_{D^0π^+}$ near $D^{*+}$ pole mass, and the virtual contributions from $D^{*+}$, in the region $m_{D^0π^+}>2.1$ GeV, are about $5\%$ of the corresponding quasi-two-body results. We define two ratios $R_{D^{*+}}$ and $R_{D^{*0}}$, from which we conclude that the flavor-$SU(3)$ symmetry will be maintained for the $B\to D^* h\to Dπh$ decays with very small breaking at any physical value of the $m_{Dπ}$. The $B^-\to D^{*0}π^-\to D^0π^0π^-$ and $B^-\to D^{*0}K^-\to D^0π^0K^-$ decays can be employed as a constraint for the $D^{*0}$ decay width, with preferred values consistent with previous theoretical predictions for this quantity.

hep-ph↗