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Nicola Serra

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hep-ex hep-ph astro-ph.CO physics.data-an Artificial Intelligence Machine Learning physics.ins-det

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preprint2026arXiv

Joint Treatment Effect Estimation from Incomplete Healthcare Data: Temporal Causal Normalizing Flows with LLM-driven Evolutionary MNAR Imputation

Target trial emulation (TTE) enables causal questions to be studied with observational data when randomized controlled trials (RCTs) are infeasible. Yet treatment-effect methods often address causal estimation, missingness, and temporal structure separately, limiting their robustness in electronic health records (EHRs), where time-varying confounding and missing-not-at-random (MNAR) biomarkers can reach 50%--80%. We propose a two-stage pipeline for treatment effect estimation from incomplete longitudinal EHRs. First, CausalFlow-T, a directed acyclic graph (DAG)-constrained normalizing flow with long short-term memory (LSTM)-encoded patient history, performs exact invertible counterfactual inference, avoiding approximation errors from variational inference and separating confounding through explicit causal structure. Ablations on four synthetic and one semi-synthetic benchmark with known counterfactuals show that DAG constraints and exact inference address distinct failure modes: neither compensates for the other. Second, because CausalFlow-T requires completed inputs, we introduce an LLM-driven evolutionary imputer that proposes executable imputation operators rather than individual entries, and evaluate it with three large language model (LLM) backends, including two open-source models. Across 30%--80% MNAR missingness, this imputer achieves the best pooled rank over biomarker and causal metrics, leading in point-wise accuracy and temporal extrapolation while preserving average treatment effect (ATE) recovery as statistical baselines degrade. On Swiss primary-care EHRs from adults with type 2 diabetes initiating a GLP-1 receptor agonist or SGLT-2 inhibitor, the pipeline estimates a per-protocol weight-loss difference of -0.98 kg [95% CI -1.01, -0.96] favoring GLP-1 receptor agonists, consistent with randomized evidence and obtained from realistically incomplete real-world EHRs.

preprint2026arXiv

Large Language Models for Physics Instrument Design

We study the use of large language models (LLMs) for physics instrument design and compare their performance to reinforcement learning (RL). Using only prompting, LLMs are given task constraints and summaries of prior high-scoring designs and propose complete detector configurations, which we evaluate with the same simulators and reward functions used in RL-based optimization. Although RL yields stronger final designs, we find that modern LLMs consistently generate valid, resource-aware, and physically meaningful configurations that draw on broad pretrained knowledge of detector design principles and particle--matter interactions, despite having no task-specific training. Based on this result, as a first step toward hybrid design workflows, we explore pairing the LLMs with a dedicated trust region optimizer, serving as a precursor to future pipelines in which LLMs propose and structure design hypotheses while RL performs reward-driven optimization. Based on these experiments, we argue that LLMs are well suited as meta-planners: they can design and orchestrate RL-based optimization studies, define search strategies, and coordinate multiple interacting components within a unified workflow. In doing so, they point toward automated, closed-loop instrument design in which much of the human effort required to structure and supervise optimization can be reduced.

preprint2026arXiv

Towards replacing detector simulation with heterogeneous GNNs in flavour physics analyses

Driven by the increasing volume of recorded data, the demand for simulation from experiments based at the Large Hadron Collider will rise sharply in the coming years. Addressing this demand solely with existing computationally intensive workflows is not feasible. This paper introduces a new fast simulation tool designed to address this demand at the LHCb experiment. This tool emulates the detector response to arbitrary multibody decay topologies at LHCb. Rather than memorising specific decay channels, the model learns generalisable patterns within the response, allowing it to interpolate to channels not present in the training data. Novel heterogeneous graph neural network architectures are employed that are designed to embed the physical characteristics of the task directly into the network structure. We demonstrate the performance of the tool across a range of decay topologies, showing the networks can correctly model the relationships between complex variables. The architectures and methods presented are generic and could readily be adapted to emulate workflows at other simulation-intensive particle physics experiments.

preprint2021arXiv

A general effective field theory description of $b \to s l^+ l^-$ lepton universality ratios

We construct an expression for a general lepton flavour universality (LFU) ratio, $R_{X}$, in $b\to s l^+ l^-$ decays in terms of a series of hadronic quantities which can be treated as nuisance parameters. This expression allows to include any LFU ratio in global fits of $b\to s l^+ l^-$ short-distance parameters, even in the absence of a precise knowledge of the corresponding hadronic structure. The absence of sizeable LFU violation and the approximate left-handed structure of the Standard Model amplitude imply that only a very limited set of hadronic parameters hamper the sensitivity of $R_X$ to a possible LFU violation of short-distance origin. A global $b\to s l^+ l^-$ combination is performed including the measurement of $R_{pK}$ for the first time, resulting in a significance of new physics of $4.2\,σ$. In light of this, we evaluate the impact on the global significance of new physics using a set of experimentally promising non-exclusive $R_X$ measurements that LHCb can perform, and find that they can significantly increase the discovery potential of the experiment.

preprint2020arXiv

Probing effects of new physics in $Λ^0_{b}\toΛ^+_{c}μ^{-}\barν_μ$ decays

We present, for the first time, the six-fold differential decay density expression for $Λ^0_b\toΛ^+_{c} l^- \barν_{l}$, taking into account the polarisation of the $Λ^0_b$ baryon and a complete basis of new physics operators. Using the expected yield in the current dataset collected at the LHCb experiment, we present sensitivity studies to determine the experimental precision on the Wilson coefficients of the new physics operators with $Λ^0_{b}\toΛ^+_{c}μ^{-}\barν_μ$ decays in two scenarios. In the first case, unpolarised $Λ^0_{b}\toΛ^+_{c}μ^{-}\barν_μ$ decays with $Λ^+_c\to p K^+ π^-$ are considered, whereas polarised $Λ^0_{b}\toΛ^+_{c}μ^{-}\barν_μ$ decays with $Λ^+_c \to p K^0_S$ are studied in the second. For the latter scenario, the experimental precision that can be achieved on the determination of $Λ^0_b$ polarisation and $Λ^+_c$ weak decay asymmetry parameter is also presented.

preprint2020arXiv

zfit: scalable pythonic fitting

Statistical modeling is a key element in many scientific fields and especially in High-Energy Physics (HEP) analysis. The standard framework to perform this task in HEP is the C++ ROOT/RooFit toolkit; with Python bindings that are only loosely integrated into the scientific Python ecosystem. In this paper, zfit, a new alternative to RooFit written in pure Python, is presented. Most of all, zfit provides a well defined high-level API and workflow for advanced model building and fitting, together with an implementation on top of TensorFlow, allowing a transparent usage of CPUs and GPUs. It is designed to be extendable in a very simple fashion, allowing the usage of cutting-edge developments from the scientific Python ecosystem in a transparent way. The main features of zfit are introduced, and its extension to data analysis, especially in the context of HEP experiments, is discussed.

preprint2018arXiv

A global fit of the MSSM with GAMBIT

We study the seven-dimensional Minimal Supersymmetric Standard Model (MSSM7) with the new GAMBIT software framework, with all parameters defined at the weak scale. Our analysis significantly extends previous weak-scale, phenomenological MSSM fits, by adding more and newer experimental analyses, improving the accuracy and detail of theoretical predictions, including dominant uncertainties from the Standard Model, the Galactic dark matter halo and the quark content of the nucleon, and employing novel and highly-efficient statistical sampling methods to scan the parameter space. We find regions of the MSSM7 that exhibit co-annihilation of neutralinos with charginos, stops and sbottoms, as well as models that undergo resonant annihilation via both light and heavy Higgs funnels. We find high-likelihood models with light charginos, stops and sbottoms that have the potential to be within the future reach of the LHC. Large parts of our preferred parameter regions will also be accessible to the next generation of direct and indirect dark matter searches, making prospects for discovery in the near future rather good.

preprint2018arXiv

Global fits of GUT-scale SUSY models with GAMBIT

We present the most comprehensive global fits to date of three supersymmetric models motivated by grand unification: the Constrained Minimal Supersymmetric Standard Model (CMSSM), and its Non-Universal Higgs Mass generalisations NUHM1 and NUHM2. We include likelihoods from a number of direct and indirect dark matter searches, a large collection of electroweak precision and flavour observables, direct searches for supersymmetry at LEP and Runs I and II of the LHC, and constraints from Higgs observables. Our analysis improves on existing results not only in terms of the number of included observables, but also in the level of detail with which we treat them, our sampling techniques for scanning the parameter space, and our treatment of nuisance parameters. We show that stau co-annihilation is now ruled out in the CMSSM at more than 95\% confidence. Stop co-annihilation turns out to be one of the most promising mechanisms for achieving an appropriate relic density of dark matter in all three models, whilst avoiding all other constraints. We find high-likelihood regions of parameter space featuring light stops and charginos, making them potentially detectable in the near future at the LHC. We also show that tonne-scale direct detection will play a largely complementary role, probing large parts of the remaining viable parameter space, including essentially all models with multi-TeV neutralinos.

preprint2018arXiv

Prospects for disentangling long- and short-distance effects in the decays $B\to K^* μ^+μ^-$

Theory uncertainties on non-local hadronic effects limit the New Physics discovery potential of the rare decays $B\to K^*μ^+μ^-$. We investigate prospects to disentangle New Physics effects in the short-distance coefficients from these effects. Our approach makes use of an event-by-event amplitude analysis, and relies on the state of the art parametrisation of the non-local contributions. We find that non-standard effects in the short-distance coefficients can be successfully disentangled from non-local hadronic effects. The impact of the truncation on the parametrisation of non-local contributions to the Wilson coefficients are for the first time systematically examined and prospects for its precise determination are discussed. We find that physical observables are unaffected by these uncertainties. Compared to other methods, our approach provides for a more precise extraction of the angular observables from data.

preprint2018arXiv

Status of the scalar singlet dark matter model

One of the simplest viable models for dark matter is an additional neutral scalar, stabilised by a $\mathbb{Z}_2$ symmetry. Using the GAMBIT package and combining results from four independent samplers, we present Bayesian and frequentist global fits of this model. We vary the singlet mass and coupling along with 13 nuisance parameters, including nuclear uncertainties relevant for direct detection, the local dark matter density, and selected quark masses and couplings. We include the dark matter relic density measured by Planck, direct searches with LUX, PandaX, SuperCDMS and XENON100, limits on invisible Higgs decays from the Large Hadron Collider, searches for high-energy neutrinos from dark matter annihilation in the Sun with IceCube, and searches for gamma rays from annihilation in dwarf galaxies with the Fermi-LAT. Viable solutions remain at couplings of order unity, for singlet masses between the Higgs mass and about 300 GeV, and at masses above $\sim$1 TeV. Only in the latter case can the scalar singlet constitute all of dark matter. Frequentist analysis shows that the low-mass resonance region, where the singlet is about half the mass of the Higgs, can also account for all of dark matter, and remains viable. However, Bayesian considerations show this region to be rather fine-tuned.

preprint2017arXiv

FlavBit: A GAMBIT module for computing flavour observables and likelihoods

Flavour physics observables are excellent probes of new physics up to very high energy scales. Here we present FlavBit, the dedicated flavour physics module of the global-fitting package GAMBIT. FlavBit includes custom implementations of various likelihood routines for a wide range of flavour observables, including detailed uncertainties and correlations associated with LHCb measurements of rare, leptonic and semileptonic decays of B and D mesons, kaons and pions. It provides a generalised interface to external theory codes such as SuperIso, allowing users to calculate flavour observables in and beyond the Standard Model, and then test them in detail against all relevant experimental data. We describe FlavBit and its constituent physics in some detail, then give examples from supersymmetry and effective field theory illustrating how it can be used both as a standalone library for flavour physics, and within GAMBIT.

preprint2017arXiv

GAMBIT: The Global and Modular Beyond-the-Standard-Model Inference Tool

We describe the open-source global fitting package GAMBIT: the Global And Modular Beyond-the-Standard-Model Inference Tool. GAMBIT combines extensive calculations of observables and likelihoods in particle and astroparticle physics with a hierarchical model database, advanced tools for automatically building analyses of essentially any model, a flexible and powerful system for interfacing to external codes, a suite of different statistical methods and parameter scanning algorithms, and a host of other utilities designed to make scans faster, safer and more easily-extendible than in the past. Here we give a detailed description of the framework, its design and motivation, and the current models and other specific components presently implemented in GAMBIT. Accompanying papers deal with individual modules and present first GAMBIT results. GAMBIT can be downloaded from gambit.hepforge.org.

preprint2015arXiv

Extracting Angular Observables without a Likelihood and Applications to Rare Decays

Our goal is to obtain a complete set of angular observables arising in a generic multi-body process. We show how this can be achieved without the need to carry out a likelihood fit of the angular distribution to the measured events. Instead, we apply the method of moments that relies both on the orthogonality of angular functions and the estimation of integrals by Monte Carlo techniques. The big advantage of this method is that the joint distribution of all observables can be easily extracted, even for very few events. The method of moments is shown to be robust against mismodeling of the angular distribution. Our main result is an explicit algorithm that accounts for systematic uncertainties from detector-resolution and acceptance effects. Finally, we present the necessary process-dependent formulae needed for direct application of the method to several rare decays of interest.

preprint2015arXiv

Search for New Physics in SHiP and at future colliders

SHiP is a newly proposed fixed-target experiment at the CERN SPS with the aim of searching for hidden particles that interact very weakly with SM particles. The work presented in this document investigates SHiP's physics reach in the parameter space of the Neutrino Minimal Standard Model ($ν$MSM), which is a theory that could solve most problems left open by the Standard Model with sterile neutrinos. A model introducing an extra $U(1)$ symmetry in the hidden sector, providing a natural candidate for dark matter, is also explored. This work shows that the SHiP experiment can improve by several orders of magnitude the sensitivity to Heavy Neutral Leptons below 2 GeV, scanning a large part of the parameter space below the $B$ meson mass. The remainder of the $ν$MSM parameter space, dominated by right-handed neutrinos with masses above 2 GeV, can be explored at a future $e^+e^-$ collider. Similarly, SHiP can greatly improve present constraints on $U(1)$ dark photons.

preprint2014arXiv

A new relation between the zero of $A_{FB}$ in $B^0 \to K^* μ^+μ^-$ and the anomaly in $P_5^\prime$

We present two exact relations, valid for any dilepton invariant mass region (large and low-recoil) and independent of any effective Hamiltonian computation, between the observables $P_i$ and $P_i^{CP}$ of the angular distribution of the 4-body decay $B \to K^*(\to Kπ) l^+l^-$. These relations emerge out of the symmetries of the angular distribution. We discuss the implications of these relations under the (testable) hypotheses of no scalar or tensor contributions and no New Physics weak phases in the Wilson coefficients. Under these hypotheses there is a direct relation among the observables $P_{1}$,$P_2$ and $P_{4,5}^\prime$. This can be used as an independent consistency test of the measurements of the angular observables. Alternatively, these relations can be applied directly in the fit to data, reducing the number of free parameters in the fit. This opens up the possibility to perform a full angular fit of the observables with existing datasets. An important consequence of the found relations is that a priori two different measurements, namely the measured position of the zero ($q_0^2$) of the forward-backward asymmetry $A_{FB}$ and the value of $P_5^\prime$ evaluated at this same point, are related by $P_4^2(q_0^{2})+P_5^2(q_0^{2})=1$. Under the hypotheses of real Wilson coefficients and $P_4^\prime$ being SM-like, we show that the higher the position of $q_0^{2}$ the smaller should be the value of $P_5^\prime$ evaluated at the same point. A precise determination of the position of the zero of $A_{FB}$ together with a measurement of $P_4^\prime$ (and $P_1$) at this position can be used as an independent experimental test of the anomaly in $P_5^\prime$. We also point out the existence of upper and lower bounds for $P_1$, namely $P_5^{\prime 2}-1 \leq P_1 \leq 1-P_4^{\prime 2}$, which constraints the physical region of the observables.

preprint2012arXiv

Experimental constraints from flavour changing processes and physics beyond the Standard Model

Flavour physics has a long tradition of paving the way for direct discoveries of new particles and interactions. Results over the last decade have placed stringent bounds on the parameter space of physics beyond the Standard Model. Early results from the LHC, and its dedicated flavour factory LHCb, have further tightened these constraints and reiterate the ongoing relevance of flavour studies. The experimental status of flavour observables in the charm and beauty sectors is reviewed in measurements of CP violation, neutral meson mixing, and measurements of rare decays.

preprint2012arXiv

Search for new physics in $B_s \rightarrow μ^+ μ^-$ and $B \rightarrow K^{(*)} μ^+ μ^-$

Transitions of the type $b \to s l^+ l^-$ are flavour changing neutral current processes where new physics can enter in competing loop diagrams with respect to the Standard Model contributions. In these decays several observables sensitive to new physics, and where theoretical uncertainties are under control, can be constructed. Particularly interesting are the angular asymmetries in the decay $B_d \to K^* μ^+ μ^-$ and the measurement of the branching fraction of the decays $B_{s,d} \to μ^+ μ^-$. Recent measurements of these observables and the measurement of the isospin asymmetry in the decays $B \to K^{(*)} μ^+ μ^-$ are presented.

preprint2011arXiv

Tests of Factorization and SU(3) Relations in B Decays into Heavy-Light Final States

Using data from the B factories and the Tevatron, we perform tests of how well non-leptonic B decays of the kind B -> D^{(*)}_{(s)} P, where P is a pion or kaon, are described within the factorization framework. We find that factorization works well - as is theoretically expected - for color-allowed, tree-diagram-like topologies. Moreover, also exchange topologies, which have a non-factorizable character, do not show any anomalous behavior. We discuss also isospin triangles between the B -> D^{(*)} pi decay amplitudes, and determine the corresponding amplitudes in the complex plane, which show a significant enhancement of the color-suppressed tree contribution with respect to the factorization picture. Using data for B -> D^{(*)} K decays, we determine SU(3)-breaking effects and cannot resolve any non-factorizable SU(3)-breaking corrections larger than \sim 5%. In view of these results, we point out that a comparison between the \bar B^0_d -> D^+π^- and \bar B^0_s -> D_s^+π^- decays offers an interesting new determination of f_d/f_s. Using CDF data, we obtain the most precise value of this ratio at CDF, and discuss the prospects for a corresponding measurement at LHCb.

preprint2010arXiv

New Strategy for B_s Branching Ratio Measurements and the Search for New Physics in B^0_s -> mu^+ mu^-

The LHCb experiment at CERN's Large Hadron Collider will soon allow us to enter a new era in the exploration of B_s decays. A particularly promising channel for the search of "new physics" is B^0_s -> mu^+ mu^-. The systematic key uncertainty affecting the measurement of this -- and in fact all B_s-decay branching ratios -- is the ratio of fragmentation functions f_d/f_s. As the presently available methods for determining f_d/f_s are not sufficient to meet the high precision at LHCb, we propose a new strategy using \bar B^0_s -> D_s^+ pi^- and \bar B^0_d -> D^+ K^-. It allows us to obtain a lower experimental bound on BR(B^0_s -> mu^+ mu^-) which offers a powerful probe for new physics. In order to go beyond this bound and to determine f_d/f_s with a theoretical precision matching the experimental one it is sufficient to know the SU(3)-breaking correction to a form-factor ratio from non-perturbative methods at the level of 20%. Thanks to our strategy, we can detect new physics in B^0_s -> mu^+ mu^- at LHCb with 5 sigma for a branching ratio as small as twice the Standard-Model value, which represents an improvement of the new-physics reach by about a factor of two with respect to the current LHCb expectation.

Nicola Serra

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19 published item(s)

Joint Treatment Effect Estimation from Incomplete Healthcare Data: Temporal Causal Normalizing Flows with LLM-driven Evolutionary MNAR Imputation

Large Language Models for Physics Instrument Design

Towards replacing detector simulation with heterogeneous GNNs in flavour physics analyses

A general effective field theory description of $b \to s l^+ l^-$ lepton universality ratios

Probing effects of new physics in $Λ^0_{b}\toΛ^+_{c}μ^{-}\barν_μ$ decays

zfit: scalable pythonic fitting

A global fit of the MSSM with GAMBIT

Global fits of GUT-scale SUSY models with GAMBIT

Prospects for disentangling long- and short-distance effects in the decays $B\to K^* μ^+μ^-$

Status of the scalar singlet dark matter model

FlavBit: A GAMBIT module for computing flavour observables and likelihoods

GAMBIT: The Global and Modular Beyond-the-Standard-Model Inference Tool

Extracting Angular Observables without a Likelihood and Applications to Rare Decays

Search for New Physics in SHiP and at future colliders

A new relation between the zero of $A_{FB}$ in $B^0 \to K^* μ^+μ^-$ and the anomaly in $P_5^\prime$

Experimental constraints from flavour changing processes and physics beyond the Standard Model

Search for new physics in $B_s \rightarrow μ^+ μ^-$ and $B \rightarrow K^{(*)} μ^+ μ^-$

Tests of Factorization and SU(3) Relations in B Decays into Heavy-Light Final States

New Strategy for B_s Branching Ratio Measurements and the Search for New Physics in B^0_s -> mu^+ mu^-