arXiv subjects

S. Neshatpour

Publications and source records attributed to S. Neshatpour.

At least 19 recordsLinked to original sources

arXiv2026

Improving lepton flavour universality tests with $K_L$ decays

Rare kaon decays provide sensitive probes of the flavour structure of the Standard Model and of possible new physics. We perform a global analysis incorporating recent experimental results and updated Standard Model predictions, including the latest measurement of $K^+ \to \pi^+ \nu\bar{\nu}$ and lepton flavour universality observables in $K^+ \to \pi^+ \ell^+\ell^-$. The fit favours a best-fit point close to the Standard Model, while a second local minimum remains phenomenologically relevant. We define benchmark scenarios associated with these two regions and investigate the prospective sensitivity of NA62 and KOTO-II to the new physics parameter space. We consider projected measurements of $K^+ \to \pi^+ \nu\bar{\nu}$ and lepton flavour universality observables in $K^+ \to \pi^+ \ell^+\ell^-$ at NA62, and of $K_L \to \pi^0 \nu\bar{\nu}$, $K_L \to \pi^0 e^+e^-$, and $K_L \to \pi^0 \mu^+\mu^-$ at KOTO-II. We find that KOTO-II has significant potential to probe and discriminate between the viable new physics scenarios, with NA62 providing complementary sensitivity.

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arXiv2025

Probing scalar and pseudoscalar new physics using rare kaon decays

Rare kaon decays provide sensitive tests of new physics. In this work, we focus on scalar and pseudoscalar operators, analysing the $K\to \pi \ell^+\ell^-$ and $K\to \ell^+\ell^-$ decays. We highlight the complementary role of different modes: $K^+\to\pi^+\ell^+\ell^-$, in particular the forward-backward asymmetry in the muon channel as a clean probe of scalar effects, the stringent constraints from $K_L\to \mu^+\mu^-$, and the discovery potential of future measurements of $K_S\to \mu^+\mu^-$ and $K_L\to \pi^0 \ell^+\ell^-$. The interplay between charged and neutral modes underscores the complementarity of NA62, the LHCb upgrade, and KOTO-II.

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arXiv2025

Nonlocal form factor of chromomagnetic penguin in $B\to K\ell^+\ell^-$ from QCD light-cone sum rules

The branching fraction of the $B \to K\ell^+\ell^-$ decay has been measured recently by the LHC experiments, showing a deviation from theory predictions based on the Standard Model (SM). A major challenge in achieving a complete SM prediction and interpreting this discrepancy lies in the treatment of nonlocal hadronic effects. In $B \to K\ell^+\ell^-$, these effects are cast in a single nonlocal form factor, a function of squared momentum transfer $q^2$ to the lepton pair. One of the previously used methods provides this form factor in the region of spacelike momentum transfer, $q^2<0$, matching the result to the hadronic dispersion relation, which is then continued to the physical region. The calculation done so far was a combination of QCD factorisation for hard-gluon contributions with light-cone sum rules (LCSRs) for soft-gluon ones. In this work, we calculate for the first time the complete nonlocal form factor at $q^2<0$ for one of the effective operators, the chromomagnetic operator $O_{8g}$, applying the method of LCSRs with $ B$-meson distribution amplitudes. We compute, both analytically and numerically, the operator-product expansion (OPE) diagrams with hard-gluon exchanges, analyse their structure and hierarchy, and obtain their spectral density entering the LCSR together with soft-gluon contributions. This study paves the way for our next task, a complete calculation of nonlocal $B \to K\ell^+\ell^-$ form factor at spacelike $q^2$, including the dominant contributions of current-current operators, known as charm-loops.

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arXiv2025

Data-driven analyses and model-independent fits for present $b\to s \ell \ell$ results

We present a critical assessment of the present $B$ anomalies in the exclusive $b \to s \ell\ell$ mode based on the QCD factorisation (QCDf) approach. In particular, we analyse the impact of different local form factor calculations and of the largest bin in the low-$q^2$ region. We also present a model-independent analysis of the new results of the LHCb and CMS experiments on the $B \to K^* \mu^+\mu^-$ angular observables. In addition, we update the global fit by including all $b \to s$ observables incorporating the new data from CMS and LHCb. In these analyses, we use 10% or higher guesstimates of the non-factorisable power corrections as additional uncertainties, serving as a placeholder for robust estimates of these contributions. Updating earlier results, we also analyse the combined LHCb and CMS data on the $B \to K^* \mu^+\mu^-$ angular observables using data-driven approaches to find indications whether these tensions between the QCDf predictions and the present data are due to underestimated subleading hadronic contributions or due to new physics effects.

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arXiv2025

Experimental Study of Rare Kaon Decays at J-PARC with KOTO and KOTO II

The rare kaon decay $K_L\to\pi^0\nu\bar{\nu}$ is extremely sensitive to new physics, because the contribution to this decay in the Standard Model (SM) is highly suppressed and known very accurately; the branching ratio is $3\times 10^{-11}$ in the SM with a theoretical uncertainty of just 2%. The measurement of this branching ratio could provide essential new information about the flavor structure of the quark sector from the $s\to d$ transition. The decay is being searched for in the KOTO experiment at J-PARC, which has obtained the current best upper limit on the branching ratio of $2.2\times 10^{-9}$; a sensitivity to branching ratios below $10^{-10}$ is achievable by the end of the decade. A next-generation experiment at J-PARC, KOTO II, was proposed in 2024 with 82 members worldwide, including significant contributions from European members. The goal of KOTO II is to measure the $K_L\to\pi^0\nu\bar{\nu}$ branching ratio with sensitivity below $10^{-12}$ in the 2030s. Discovery of the decay with $5\sigma$ significance is achievable at the SM value of the branching ratio. An indication of new physics with a significance of 90% is possible if the observed branching ratio differs by 40% from the SM value. Another important goal of KOTO II is to measure the branching ratio of the unobserved $K_L\to \pi^0e^+e^-$ decay, which can give an input to flavor structures of new physics. Other rare $K_L$ decays and hidden-sector particles are also in the scope of the study. After 2026, KOTO will be the only dedicated rare kaon decay experiment in the world, and KOTO II is the only future rare kaon decay project currently proposed. We would like to lead a global initiative for the experimental study of rare kaon decays, with significant contributions and support from the European community.

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arXiv2024

Theoretical implications for a new measurement of $K_L\to \pi^0 \ell\ell$

Kaon physics is at an important experimental juncture with respect to the ongoing measurements of several observables. This work will build on the existing status by formulating different phenomenological analyses corresponding to different paths that may lie ahead. Beginning with the golden channels, $K^+\rightarrow\pi^+\nu\bar\nu$ and $K_L\rightarrow\pi^0\nu\bar\nu$, the paper will eventually cast the spotlight on the importance of a precise measurement of BR($K_L\rightarrow\pi^0\ell\bar\ell$). The phenomenological analyses involve sequentially adding kaon physics observables at the projected final precision of their respective measurements to the global fit. More specifically, we consider three different scenarios with different sets of observables assumed at their final precisions. Beginning with BR($K^+\rightarrow\pi^+\nu\bar\nu$) and BR($K_L\rightarrow\pi^0\nu\bar\nu$), we sequentially add BR($K_L\rightarrow\pi^0 e\bar e$) and BR($K_L\rightarrow\pi^0 \mu\bar\mu$) to the global fit. The evolution of the result from one scenario to the next makes a strong case for the consideration of future measurement of BR($K_L\rightarrow\pi^0\ell\bar\ell$).

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arXiv2024

Workshop summary -- Kaons@CERN 2023

Kaon physics is at a turning point -- while the rare-kaon experiments NA62 and KOTO are in full swing, the end of their lifetime is approaching and the future experimental landscape needs to be defined. With HIKE, KOTO-II and LHCb-Phase-II on the table and under scrutiny, it is a very good moment in time to take stock and contemplate about the opportunities these experiments and theoretical developments provide for particle physics in the coming decade and beyond. This paper provides a compact summary of talks and discussions from the Kaons@CERN 2023 workshop.

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arXiv2024

Exploring scalar contributions with $K^+ \to \pi^+ \ell^+ \ell^-$

The rare kaon decay $K^+ \to \pi^+\ell^+\ell^-$ offers insights into Standard Model (SM) physics and beyond. Driven by vector form factor in the SM, it can also probe non-standard contributions. In this letter we study the scalar contribution, $f_S$. Using differential decay width and Forward-Backward Asymmetry we propose a simultaneous fit to vector and scalar contributions which is necessary for a consistent analysis. Novel bounds on $|f_S|$ are presented for the first time through a reinterpretation of the E865, NA48/2, and NA62 experimental results. The analysis results in the most precise bound $f_S < 7.9\times 10^{-6}$ at 90$\%$ confidence level.

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arXiv2023

Beyond the Standard Model prospects for kaon physics at future experiments

Rare kaon decays offer a powerful tool for investigating new physics in $s\to d$ transitions. Currently, many of the interesting decay modes are either measured with rather large uncertainties compared to their theoretical predictions or have not yet been observed. The future HIKE programme at CERN will provide unprecedented sensitivity to rare kaon decays, allowing for strong constraints on new physics scenarios with lepton flavour universality violation. We present the overall picture that emerges from a study of the different decay modes with a global analysis considering projections based on the HIKE programme, both with and without KOTO-II future measurements. We also highlight the most relevant decays and identify that in addition to the "golden channel", $K^+\to\pi^+\nu\bar\nu$, the rare $K_L\to\pi^0 \ell\bar\ell$ decay, especially in the electron sector offers strong constraints on short-distance physics.

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arXiv2023

Post-LS3 Experimental Options in ECN3

The Experimental Cavern North 3 (ECN3) is an underground experimental cavern on the CERN Prévessin site. ECN3 currently hosts the NA62 experiment, with a physics programme devoted to rare kaon decays and searches of hidden particles approved until Long Shutdown 3 (LS3). Several options are proposed on the longer term in order to make best use of the worldwide unique potential of the high-intensity/high-energy proton beam extracted from the Super Proton Synchrotron (SPS) in ECN3. The current status of their study by the CERN Physics Beyond Colliders (PBC) Study Group is presented, including considerations on beam requirements and upgrades, detector R&D and construction, schedules and cost, as well as physics potential within the CERN and worldwide landscape.

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arXiv2023

$B$ anomalies in the post $R_{K^{(*)}}$ era

We discuss the status of $b \to s \ell^+\ell^-$ decays in the post-$R_{K^{(*)}}$ era. The recent LHCb update of $R_K$ and $R_{K^*}$ measurements which are now compatible with the Standard Model, constrain new physics contributions to be lepton flavor universal, allowing only small deviations from this limit. Besides the latest LHCb measurements of $R_K$ and $R_{K^{*}}$, we also include the recent CMS measurements of $R_K$ and of the branching ratio of $B^+ \to K^+ μ^+μ^-$. We present a model-independent analysis of the $b\to s \ell^+ \ell^-$ data and investigate the implications of the different sets of observables. In addition, we consider multi-dimensional fits and discuss the significance of more complex new physics scenarios compared to one- and two-dimensional scenarios.

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arXiv2022

HIKE, High Intensity Kaon Experiments at the CERN SPS

A timely and long-term programme of kaon decay measurements at a new level of precision is presented, leveraging the capabilities of the CERN Super Proton Synchrotron (SPS). The proposed programme is firmly anchored on the experience built up studying kaon decays at the SPS over the past four decades, and includes rare processes, CP violation, dark sectors, symmetry tests and other tests of the Standard Model. The experimental programme is based on a staged approach involving experiments with charged and neutral kaon beams, as well as operation in beam-dump mode. The various phases will rely on a common infrastructure and set of detectors.

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arXiv2022

Neutral current B-decay anomalies

We discuss the implications of $b \to s \ell^+\ell^-$ measurements and their deviations with respect to the Standard Model predictions in a model-independent framework. We highlight in particular the impact of the recent updated measurements including the updated $B_s \to ϕμ^+μ^-$ branching ratios and angular observables, the recent CMS measurement of the branching ratio of $B_s\toμ^+ μ^-$, and the LHCb measured lepton flavour universality violating ratios $R_{K_S^0}$ and $R_{K^{*+}}$. In addition, we check the compatibility of the new physics effect for the theoretically clean observables with the rest of the neutral $B$ decays observables.

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arXiv2022

Anatomy of kaon decays and prospects for lepton flavour universality violation

The kaon sector is characterised by several processes which are under active investigation across different experiments. In this work, we present the global picture that emerges from a study of the different decay modes. We begin by revisiting the theoretical component of these decays and providing up-to-date predictions of the Standard Model as well as the corresponding uncertainties. Several new features emerge, in particular for $K_{S,L}\to μ\barμ$, and are presented in considerable detail. This offers an ideal platform for extracting the parameter space supported by the existing data. Motivated by possible lepton flavour universality violation in $B$ decays, we investigate such Beyond the Standard Model effects also in rare kaon decays. Without loss of generality, our primary analyses correspond to the paradigm where the Wilson coefficients for operators involving tau leptons are chosen to be equal to that involving the muon, i.e. $δC^τ=δC^μ$. We conclude by presenting the possible picture that can be achieved towards the end of the run of data accumulation in the planned experiments. This includes assumptions on possible sensitivity goals that the experiments can aim to achieve, in order to extract the kind of physics highlighted in this paper.

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arXiv2022

Determination of the structure of the $K \to πππ$ amplitudes from recent data

This Letter provides a new determination of the ten real coefficients that describe the structure of the $K \to πππ$ amplitudes in the limit where isospin is conserved and complex phases, due to either CP violation or final-state rescattering, are neglected. This determination is obtained through a fit to the data on the Dalitz-plot structures and partial-decay rates collected during the last twenty years by several high-precision experiments. The fitting procedure and the way the experimental data have been handled in the fit are discussed in detail. Our fit leads to a more precise determination of the coefficients describing the linear and quadratic slopes of the $K \to πππ$ amplitudes.

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arXiv2022

Flavour Physics Phenomenology with SuperIso

We present an overview of SuperIso v4.1 which is a public program for the calculation of flavour physics observables. We give examples of using SuperIso to constrain new physics scenarios with kaon physics and present the implications of rare B-decays.

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arXiv2021

Implications of LHCb Data for Lepton Flavour Universality Violation

We analyse the new physics implications of theoretically clean $b \to s$ observables in a model-independent approach and compare their coherence with the implications of other rare $B$-decays. A statistical comparison is done between the New Physics explanation and hadronic contributions as the source of the anomalies in angular observables of the $B\to K^*μμ$ decay. We make projections for future measurements that indicate that LHCb will be in the position to discover lepton non-universality via a single observable using the Run 3 data. The global fit of rare $B$-decays is given within a multidimensional fit involving all the 20 relevant Wilson coefficients.

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arXiv2021

More Indications for Lepton Nonuniversality in $b \to s \ell^+ \ell^-$

Recently the LHCb collaboration has confirmed the evidence for lepton flavour nonuniversality at the $3.1σ$ level via an updated measurement of $R_K$. In this work we analyse this evidence within a model-independent approach. We make projections for future measurements which indicate that LHCb will be in the position to discover lepton nonuniversality with the Run 3 data in a single observable. We analyse other ratios based on our analysis of the present measurements of the ratios $R_{K^{(*)}}$ and analyse if they are able to differentiate between various new physics options within the effective field theory at present or in the near future. We also compare the present deviations in the ratios with NP indications in the angular observables of exclusive $b \to s \ell\ell$ transitions. Finally, we update our global analysis considering all $b \to s \ell\ell$ observables altogether, including a 20-parameter fit in connection of a Wilks' test.

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