arXiv subjects

Abner Soffer

Publications and source records attributed to Abner Soffer.

At least 19 recordsLinked to original sources

arXiv2026

Probing $\nu$SMEFT at Belle II with displaced dilepton vertices

The existence of right-handed (RH) neutrinos is strongly motivated by the observation of small neutrino masses and as a means to shed light on the origin of parity violation in the Standard Model (SM). Experimental searches for the corresponding mass eigenstates, referred to as heavy neutral leptons (HNLs), have been carried out in a variety of experiments and within different HNL models. In the current work we use the SM effective field theory with added RH neutrinos, known as $\nu$SMEFT, to study GeV-scale HNL production in $e^+e^-$ collisions at the Belle~II experiment. We focus on leptonic decays of long-lived HNLs that produce a displaced vertex in the Belle~II detector. Accounting for detection efficiency and backgrounds, we estimate the sensitivity of Belle~II to the new-physics scale of four-fermion, Higgs-current, and dipole operators in $\nu$SMEFT. We find that for most operators, the search we propose probes large regions of parameter space that are not excluded by other searches.

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arXiv2025

Searching for long-lived light neutralinos from $B$-meson decays with baryonic R-parity violation at Belle II

In a supersymmetry scenario with R-parity violation (RPV), neutralinos with GeV-scale mass, which are necessarily bino-like, are allowed by all constraints and can be produced in association with a baryon in $B$-meson decays via certain $\bar U \bar D \bar D$ operators. In this work, we investigate this scenario with two non-vanishing RPV couplings at the low-energy scale. With one RPV coupling governing the neutralino production rate and another determining its lifetime, this scenario can lead to observable signals with displaced-vertex signatures in the tracking volume of $B$-factories. To maximize the sensitivity to such signals, we develop a new partial-reconstruction technique that yields high efficiency and utilizes most of the decays of relatively heavy, long-lived particles, achieving much better sensitivity than standard full reconstruction. We consider potential background sources and devise selection criteria to suppress their event yields to very low levels. Using a parameterized model of the detector, we estimate in detail the displaced-vertex reconstruction efficiency as a function of neutralino lifetime and mass. For squark masses beyond the LHC limits, we calculate the signal sensitivity of Belle~II, showing that the experiment can probe the RPV couplings well beyond the present bounds, obtained from searches for dinucleon decays, baryon-antibaryon oscillations, and $B^+\to p +\text{missing}$.

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arXiv2025

Estimating the track-reconstruction efficiency in phenomenological proposals of long-lived-particle searches

Phenomenological proposals of searches for new, long-lived particles face a challenge when estimating the reconstruction efficiency of displaced charged-particle tracks. The efficiency depends not only on the charged-particle's momentum vector, but also on its production point in relation to the detector boundary and substructure elements. Phenomenological studies generally do not have access to GEANT4-based simulation, which is used for calculating the efficiency in experimental analyses. To address this need, we have developed TrackEff, a python software package for estimating the track-reconstruction efficiency. The default detector geometry within TrackEff is that of the Belle~II drift chamber. However, tunable parameters enable modification of the geometry to correspond to other tracker configurations. TrackEff uses a simplified model of the tracker to determine the number of detector hits associated with each track. The user can then decide whether the track would be detected based on the number of hits, potentially in association with any other information.

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arXiv2024

Entanglement and Bell nonlocality with bottom-quark pairs at hadron colliders

It has been shown that entanglement and Bell nonlocality, which are key concepts in Quantum Mechanics, can be probed in high-energy colliders via processes of fundamental particle scattering. In fact, the ATLAS and CMS collaborations have measured entanglement using top-quark pairs produced in proton-proton collisions at the LHC. Recently, it was shown that spin correlations can be measured in pairs of bottom quarks at the LHC, despite the fact that bottom quarks, unlike top quarks, hadronize before decaying. Here, we demonstrate that quantum correlations can also be studied using bottom-quark pairs, and analyze the feasibility of the observation of entanglement and Bell nonlocality in several collider experiments. Given the low mass of the bottom quark relative to typical energies accessible at the LHC, many of the bottom-quark pairs are in the ultrarelativistic regime, where they can exhibit strong spin entanglement. We find that entanglement of bottom-quark pairs may be measurable even with the LHC Run 2 data, especially with the CMS $B$ parking dataset, while observation of Bell nonlocality may become feasible at the high-luminosity phase of the LHC.

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arXiv2024

Probing dark photons from a light scalar at Belle II

In the minimal $U(1)$ extension of the Standard Model (SM), a new gauge boson referred to as "dark photon" is predicted. The dark-photon mass can be generated from an additional Higgs mechanism associated with a dark scalar boson. At $B$-factories such as Belle II, large numbers of $B$-mesons are produced and can decay to a kaon plus the dark scalar via the latter's mixing with the SM Higgs boson. We evaluate the sensitivity of Belle II for the case in which the dark scalar decays exclusively into a pair of dark photons via the new $U(1)$ gauge coupling, and the dark photons are long lived owing to a small kinetic mixing $\epsilon$. We study the experimental signature in which each dark photon decays into a pair of charged leptons, pions, or kaons, resulting in a pair of displaced vertices, and argue that the search is essentially background-free. We perform detailed Monte-Carlo simulations to determine the expected number of signal events at Belle II with an integrated luminosity of 50 ab$^{-1}$, taking into account the efficiencies for both final-state-particle identification and displaced tracking. We find that for experimentally allowed values of the scalar mixing angle and kinematically allowed dark-photon and dark-scalar masses, the proposed search is uniquely sensitive to the medium-$\epsilon$ regime, which is currently mostly unexcluded by experiments.

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arXiv2022

Probing $R$-parity violation in $B$-meson decays to a baryon and a light neutralino

We propose a search for $B$ meson decays to a baryon plus missing energy at the Belle II experiment to probe supersymmetry with a GeV-scale lightest neutralino ${\tilde{\chi}_1^0}$ and $R$-parity violation (RPV). We perform analytic computations of the signal branching fractions in the framework of effective field theory, with a single nonzero RPV operator $\lambda''_{ij3}\bar{U}_{i}^c\bar{D}_{j}^c\bar{D}_3^c$, where $i,j=1,2$. The hadronic form factors are calculated using an SU(3) phenomenological Lagrangian approach for the proton, as well as several hyperons and charmed baryons. Since the decay of the neutralino is kinematically and CKM suppressed in this theoretical scenario, it decays outside the detector and appears experimentally only as missing energy. We detail the analysis techniques at the experimental level and estimate the background in the ${B^+ \to p {\tilde{\chi}_1^0}}$ search using published results for $B^+\to K^+ \nu\bar\nu$. Our final sensitivity plots are shown for both $\lambda''_{113}$ versus the squark mass $m_{\tilde{q}}$ and $\lambda''_{113}/m^2_{\tilde{q}}$ versus the neutralino mass $m_{{\tilde{\chi}_1^0}}$. We find that the search at Belle II could probe $\lambda''_{113}/m^2_{\tilde{q}}$ down to the order of $10^{-8}$ GeV$^{-2}$ in the kinematically allowed $m_{{\tilde{\chi}_1^0}}$ range. We also obtain current limits on $\lambda''_{123}$ by recasting an existing search interpreted as ${B^0 \to \Lambda^0 {\tilde{\chi}_1^0}}$, and comment about searches for ${B^+ \to \Sigma^+ {\tilde{\chi}_1^0}}$, ${B^0 \to \Sigma^0 {\tilde{\chi}_1^0}}$, ${B^+ \to \Lambda_c^+ {\tilde{\chi}_1^0}}$, and ${B^+ \to \Xi_c^+ {\tilde{\chi}_1^0}}$. In closing, we briefly discuss potential searches at the LHCb and BESIII experiments.

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arXiv2022

Heavy QCD Axion at Belle II: Displaced and Prompt Signals

The QCD axion is a well-motivated addition to the standard model to solve the strong $C\!P$ problem. If the axion acquires mass dominantly from a hidden sector, it can be as heavy as $O(1)$ GeV, and the decay constant can be as low as $O(100)$ GeV without running into the axion quality problem. We propose new search strategies for such heavy QCD axions at the Belle II experiment, where the axions are expected to be produced via $B\to K a$. We find that a subsequent decay $a\to 3π$ with a displaced vertex leads to a unique signal with essentially no background, and that a dedicated search can explore the range $O(1$-$10)$ TeV of decay-constant values. We also show that $a\to γγ$ can cover a significant portion of currently unexplored region of $150 \lesssim m_a \lesssim 500$ MeV.

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arXiv2021

Probing charged lepton flavor violation with axion-like particles at Belle II

We study charged lepton flavor violation associated with a light leptophilic axion-like particle (ALP), $X$, at the $B$-factory experiment Belle II. We focus on production of the ALP in the tau decays $τ\to X l$ with $l=e,μ$, followed by its decay via $X\to l^- l^+$. The ALP can be either promptly decaying or long-lived. We perform Monte-Carlo simulations, recasting a prompt search at Belle for lepton-flavor-violating $τ$ decays, and propose a displaced-vertex (DV) search. For both types of searches, we derive the Belle~II sensitivity reaches in both the product of branching fractions and the ALP coupling constants, as functions of the ALP mass and lifetime. The results show that the DV search exceeds the sensitivity reach of the prompt search to the relevant branching fractions by up to about a factor of 40 in the long decay length regime.

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arXiv2021

Probing the $ΔU=0$ Rule in Three Body Charm Decays

CP violation in charm decay was observed in the decays $D^0\rightarrow P^{\pm}P^{\mp}$ of a $D^0$ meson to two pseudoscalars. When interpreted within the SM, the results imply that the ratio of the relevant rescattering amplitudes has a magnitude and phase that are both of $O(1)$. We discuss ways to probe similar ratios in $D^0\rightarrow V^{\pm}P^{\mp}$ decays, where $V$ is a vector that decays to two pseudoscalars, from the Dalitz-plot analysis of time-integrated three-body decays. Compared to two-body decays, three-body decays have the advantage that the complete system can be solved without the need for time-dependent CP violation measurements or use of correlated $D^0$--$\overline{D}^0$ production. We discuss the decays $D^0\rightarrow π^+π^-π^0$ and $D^0\rightarrow K^+K^-π^0$ as examples by considering a toy model of only two overlapping charged resonances, treating the underlying pseudo two-body decays in full generality.

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arXiv2021

Long-lived light neutralinos at Belle II

We consider light neutralinos of mass about 1 GeV, produced from $τ$ lepton rare decays at Belle II, in the context of R-parity-violating (RPV) supersymmetry. With large and clean samples of $τ$ leptons produced at the Belle II experiment, excellent sensitivity to such light neutralinos with the exotic signatures of displaced vertices is expected. We focus on two benchmark scenarios of single RPV operators, $λ'_{311} L_3 Q_1 \bar{D}_1$ and $λ'_{312} L_3 Q_1 \bar{D}_2$, which induce both the production and decay of the lightest neutralino. For the reconstruction of a displaced vertex, we require at least two charged pions in the final states. We perform Monte-Carlo simulations for both signal and background events, and find that Belle II can explore regions in the parameter space competitive with other probes. In particular, for the $λ'_{311}$ scenario, it can put limits up to two orders of magnitude stronger than the current bounds.

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arXiv2019

Measuring CP violation in $b\to cτ^-\barν_τ$ using excited charm mesons

There is growing evidence for deviation from the standard model predictions in the ratios between semi-tauonic and semi-leptonic $B$ decays, known as the $R(D^{(*)})$ puzzle. If the source of this non-universality is new physics, it is natural to assume that it also breaks CP symmetry. In this paper we study the possibility of measuring CP violation in semi-tauonic $B$ decays, exploiting interference between excited charm mesons. Given the current values of $R(D^{(*)})$, we find that our proposed CP-violation observable could be as large as about 10%. We discuss the experimental advantages of our method and propose carrying it out at Belle II and LHCb.

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arXiv2018

Collider Searches for Long-Lived Particles Beyond the Standard Model

Experimental tests of the Standard Model of particle physics (SM) find excellent agreement with its predictions. Since the original formation of the SM, experiments have provided little guidance regarding the explanations of phenomena outside the SM, such as the baryon asymmetry and dark matter. Nor have we understood the aesthetic and theoretical problems of the SM, despite years of searching for physics beyond the Standard Model (BSM) at particle colliders. Some BSM particles can be produced at colliders yet evade being discovered, if the reconstruction and analysis procedures not matched to characteristics of the particle. An example is particles with large lifetimes. As interest in searches for such long-lived particles (LLPs) grows rapidly, a review of the topic is presented in this article. The broad range of theoretical motivations for LLPs and the experimental strategies and methods employed to search for them are described. Results from decades of LLP searches are reviewed, as are opportunities for the next generation of searches at both existing and future experiments.

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arXiv2018

Identifying a $Z'$ behind $b \to s \ell \ell$ anomalies at the LHC

Recent $b\to s\ell\ell$ anomalies may imply the existence of a new $Z'$ boson with left-handed $Z'bs$ and $Z'μμ$ couplings. Such a $Z'$ may be directly observed at LHC via $b \bar s \to Z' \to μ^+μ^-$, and its relevance to $b\to s\ell\ell$ may be studied by searching for the process $gs \to Z'b \to μ^+μ^- b$. In this paper, we analyze the capability of the 14 TeV LHC to observe the $Z'$ in the $μ^+ μ^-$ and $μ^+μ^- b$ modes based on an effective model with major phenomenological constraints imposed. We find that both modes can be discovered with 3000 fb$^{-1}$ data if the $Z'bs$ coupling saturates the latest $B_s-\bar B_s$ mixing limit from UTfit at around $2σ$. Besides, a tiny right-handed $Z'bs$ coupling, if it exists, opens up the possibility of a relatively large left-handed counterpart, due to cancellation in the $B_s-\bar B_s$ mixing amplitude. In this case, we show that even a data sample of $\mathcal{O}(100)$ fb$^{-1}$ would enable discovery of both modes. We further study the impact of a $Z'bb$ coupling as large as the $Z'bs$ coupling. This scenario enables discovery of the $Z'$ in both modes with milder effects on the $B_s-\bar B_s$ mixing, but obscures the relevance of the $Z'$ to $b \to s\ell\ell$. Discrimination between the $Z'bs$ and $Z'bb$ couplings may come from the production cross section for the $Z'b\bar{b}$ final state. However, we do not find the prospect for this to be promising.

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arXiv2015

Searches for Light Scalars, Pseudoscalars, and Gauge Bosons

In the past few years there has been a great deal of theoretical and experimental activity related to the search for low-mass scalars, pseudoscalars, and vectors in various scenarios of physics beyond the standard model. I review the current status of this topic, focusing on results obtained since FPCP 2014.

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arXiv2014

Constraints on dark forces from the B factories and low-energy experiments

The idea that dark-matter interactions with Standard-Model particles may be mediated by new bosons with masses in the MeV-to-GeV range took off several years ago. Constraints on such models were soon calculated based on older measurements. Subsequently, active collaborations conducted dedicated searches for these bosons, and new experiments were planned to improve the search sensitivity. In this note I briefly cover the basic models that predict dark vectors and scalars in this mass range, review the constraints from electron-positron colliders, fixed-target experiments, and hadron colliders, and comment on the sensitivities of future experiments.

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arXiv2014

B-Meson Decays into Final States with a tau Lepton

Decays of B mesons into final states containing a tau lepton are sensitive to new charged-current interactions that break lepton-flavor universality. These decays have been studied only at e+e- colliders, where the low-background environment and well-known initial state make it possible to observe small signals with undetectable neutrinos. In particular, the large data samples of the B factories and recent advances in techniques for full-event reconstruction have led to evidence for the decay B --> tau nu and unambiguous observation of the decays B --> D(*)tau nu. These results have led to exclusion of large regions of the parameter space for a variety of new-physics models. Furthermore, the branching fraction for B --> D(*)tau nu has been measured to be higher than the standard-model expectation by at least 3.4 standard deviations, making this an interesting topic for further research. This letter reviews the theoterical and experimental status of this topic, summarizing the results at this time and outlining the path for further improvements.

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arXiv2010

Measuring the Magnitude of the Fourth-Generation CKM4 Matrix Element V_{t'b'}

We study the decays of heavy chiral (t', b') quarks in a four-generation extension of the Standard Model. If the difference between the t' and b' masses is smaller than the W mass, as favored by precision electroweak measurements, then the Cabibbo-Kobayashi-Maskawa favored, on-shell decay t'-->b'W is forbidden. As a result, other t' decays have substantial branching fractions, which are highly sensitive to the magnitude of the diagonal CKM matrix element V_{t'b'}. We show that |V_{t'b'}| can be determined from the ratio of the two-body and three-body t'-decay branching fractions and estimate the precision of such a measurement at the LHC. We discuss the hadronization of a t' for large enough values of |V_{t'b'}|.

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arXiv2009

Estimate of the Branching Fraction of tau --> pi eta' nu_tau

We calculate the expected branching fraction of the second-class-current decay tau --> pi eta' nu, motivated by a a recent experimental upper-limit determination of this quantity. The largest contribution to the branching fraction is due to the intermediate a0(980) scalar meson, assuming it is a $\bar ud$ state. Smaller contributions arise from a0(1450), rho(770), and rho(1450). Our calculated values are substantially below the experimental upper limit, and are smaller still if the a0(980) is a four-quark state, as often suggested. Thus, a precise measurement or tight upper limit has the potential to determine the nature of the a0(980), as well as search for new scalar interactions.

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