arXiv subjects

Michele Papucci

Publications and source records attributed to Michele Papucci.

At least 19 recordsLinked to original sources

arXiv2026

An introduction to Hammer v2: Helicity Amplitude Module for Matrix Element Reweighting

The Hammer software library provides fast and efficient reweighting of large simulated datasets containing semileptonic $b$-hadron decays to any beyond Standard Model (BSM) theory, or to any form-factor description of the hadronic matrix elements. By enabling reweighting to a different underlying theoretical model after the computationally-expensive detector simulation step has already been completed, Hammer permits experimental analyses to employ forward-folding fitting strategies to recover underlying physical parameters without biases, or to properly characterize theory systematic uncertainties. This publication details upgrades to Hammer functionalities and its application programming interface (API) for version 2.x, and also provides associated documentation of the library's structure, syntactical conventions, and code flow. Substantial optimization of Hammer's internal tensor library now enables computational complexity to generically scale almost linearly with amplitude tensor rank times size rather than as a quartic, enabling reweighting into very high dimension spaces, such as the product of BSM Wilson coefficient and form factor parameter linear spaces, while also retaining Monte Carlo uncertainties. Updated Python bindings are implemented with bijective correspondence to the C++ interface, allowing full access to library functionalities.

hep-ph↗

arXiv2026

What Intermediate Layers Know: Detecting Jailbreaks from Entropy Dynamics

Jailbreak attacks reveal a persistent weakness in aligned Large Language Models: carefully crafted prompts can elicit policy-violating responses despite safety training. While most defenses operate at the prompt or output level, it remains unclear how harmful intent is encoded within the model's internal representations. We investigate this question by analyzing token-level predictive entropy trajectories across layers of a frozen LLM using the logit lens. We find that static aggregate statistics of prompt-level entropy (e.g., mean, variance) carry little discriminative signal, whereas features capturing how entropy evolves across token positions, such as monotonic rank-based trend scores, are substantially more informative. Importantly, this signal is not uniform across model depth: it is concentrated in intermediate layers and degrades at the final layer, indicating that jailbreak-relevant structure is most pronounced in mid-network representations rather than at the output head. Across multiple models (Llama, Qwen, Gemma) and adversarial benchmarks, these entropy dynamics provide architecture-consistent separation without additional training. Together, our findings show that jailbreak behavior is reflected in structured intermediate uncertainty dynamics, clarifying both which entropy-derived features encode harmful intent and where in the network that signal is most pronounced.

cs.CL↗

arXiv2025

Radiative Semileptonic Decays of Beautiful Hadrons

We derive predictions for the hadronic matrix elements of radiative semileptonic decays of beautiful hadrons within Heavy Quark Effective Theory (HQET), relevant for future measurements at Belle II and LHCb. Our study considers Lambda(b) -> Lambda(c), Lambda(*)(c1) and B -> D(*), D** transitions. The symmetries of HQET highly constrain the number of structure-dependent form factors in all cases. In the soft and sub-leading soft regions, all the form factors are fully determined in terms of non-radiative Isgur-Wise functions and the magnetic dipole moments of the heavy hadrons. The structure of higher order corrections is also briefly discussed.

hep-ph↗

arXiv2025

Technical design report for the CODEX-$β$ demonstrator

The CODEX-$β$ apparatus is a demonstrator for the proposed future CODEX-b experiment, a long-lived-particle detector foreseen for operation at IP8 during HL-LHC data-taking. The demonstrator project, intended to collect data in 2025, is described, with a particular focus on the design, construction, and installation of the new apparatus.

physics.ins-det↗

arXiv2025

Stress-testing Machine Generated Text Detection: Shifting Language Models Writing Style to Fool Detectors

Recent advancements in Generative AI and Large Language Models (LLMs) have enabled the creation of highly realistic synthetic content, raising concerns about the potential for malicious use, such as misinformation and manipulation. Moreover, detecting Machine-Generated Text (MGT) remains challenging due to the lack of robust benchmarks that assess generalization to real-world scenarios. In this work, we present a pipeline to test the resilience of state-of-the-art MGT detectors (e.g., Mage, Radar, LLM-DetectAIve) to linguistically informed adversarial attacks. To challenge the detectors, we fine-tune language models using Direct Preference Optimization (DPO) to shift the MGT style toward human-written text (HWT). This exploits the detectors' reliance on stylistic clues, making new generations more challenging to detect. Additionally, we analyze the linguistic shifts induced by the alignment and which features are used by detectors to detect MGT texts. Our results show that detectors can be easily fooled with relatively few examples, resulting in a significant drop in detection performance. This highlights the importance of improving detection methods and making them robust to unseen in-domain texts.

cs.CL↗

arXiv2025

CODEX-b: Opening New Windows to the Long-Lived Particle Frontier at the LHC

This document is written as a contribution to the European Strategy of Particle Physics (ESPP) update. We offer a detailed overview of current developments and future directions for the CODEX-b detector, which aims to detect long-lived particles beyond the Standard Model. We summarize the scientific motivation for this detector, advances in our suite of simulation and detector optimization frameworks, and examine expected challenges, costs, and timelines in realizing the full detector. Additionally, we describe the technical specifications for the smaller-scale demonstrator detector (CODEX-$β$) we have installed in the LHCb experimental cavern.

hep-ex↗

arXiv2024

Off-shell vertices in heavy particle effective theories and $B\rightarrow Dπ\ell ν$

We study the modifications to decay amplitudes in heavy to heavy semileptonic decays with multiple hadrons in the final state due to intermediate heavy hadrons being off-shell or having a finite width. Combining Heavy Hadron Chiral Perturbation Theory (HH$χ$PT) with a BCFW on-shell factorization formula, we show that these effects induce $O(1/M)$ corrections to the standard results computed in the narrow-width approximation and therefore are important in extracting form factors from data. A combination of perturbative unitarity, analyticity, and reparameterization invariance fully determine these corrections in terms of known Isgur-Wise functions without the need to introduce new form factors. In doing so, we develop a novel technique to compute the boundary term at complex infinity in the BCFW formula for theories with derivatively coupled scalars. While we have used the $\bar B\rightarrow Dπ\ellν$ decay as an example, these techniques can generally be applied to effective field theories with (multiple) distinct reference vectors.

hep-ph↗

arXiv2023

Appraising constrained second-order power corrections in HQET with $Λ_b \to Λ_c l ν$

We derive the $Λ_b \to Λ_c$ form factors for the Standard Model and beyond at second order in Heavy Quark Effective Theory (HQET), applying the recently-proposed Residual Chiral Expansion (RCE) to reduce the set of unknown subsubleading hadronic functions to a single, highly-constrained function, that is fully determined by hadron mass parameters at zero recoil. We fit a form factor parametrization based on these results to all available Lattice QCD (LQCD) predictions and experimental data. We find that the constrained and predictive structure of the form factors under the RCE is in excellent agreement with LQCD predictions and experimental data, as well as prior HQET-based fits.

hep-ph↗

arXiv2023

Energetic long-lived particles in the CMS muon chambers

We present a recast in different benchmark models of the recent CMS search that uses the endcap muon detector system to identify displaced showers produced by decays of long-lived particles (LLPs). The exceptional shielding provided by the steel between the stations of the muon system drastically reduces the Standard Model background that limits other existing ATLAS and CMS searches. At the same time, by using the muon system as a sampling calorimeter, the search is sensitive to LLPs energies rather than masses. We show that, thanks to these characteristics, this new search approach is sensitive to LLPs masses even lighter than a GeV, and can be complementary to proposed and existing dedicated LLP experiments.

hep-ph↗

arXiv2023

Exploring the $τ$ polarization in $B\to Xτ\barν$ along different axes

The $τ$ polarization in semileptonic $B$ decays provides probes of new physics complementary to decay rate distributions of the three-body final state. Prior calculations for inclusive decays used a definition for the polarization axis that is different from the choice used in calculations (and the only measurement) for exclusive channels. To compare inclusive and exclusive predictions, we calculate the $τ$ polarization in inclusive $B\to Xτ\barν$ using the same choice as in the exclusive decays, and construct a sum rule relating the inclusive $τ$ polarization to a weighted sum of exclusive decay polarizations. We use this relation, experimental data, and theoretical predictions for the decays to the lightest charm or up-type hadrons to make predictions for excited channels.

hep-ph↗

arXiv2022

Constrained second-order power corrections in HQET: $R(D^{(*)})$, $|V_{cb}|$, and new physics

We postulate a supplemental power counting within the heavy quark effective theory, that results in a small, highly-constrained set of second-order power corrections, compared to the standard approach. We determine all $\bar{B} \to D^{(*)}$ form factors, both within and beyond the standard model to $\mathcal{O}(α_s/m_{c,b}, 1/m_{c,b}^2)$, under truncation by this power counting. We show that the second-order power corrections to the zero-recoil normalization of the $\bar{B} \to D^{(*)} l ν$ matrix elements ($l = e$, $μ$, $τ$) are fully determined by hadron mass parameters, and are in good agreement with lattice QCD (LQCD) predictions. We develop a parametrization of these form factors under the postulated truncation, that achieves excellent fits to the available LQCD predictions and experimental data, and we provide precise updated predictions for the $\bar{B} \to D^{(*)} τ\barν$ decay rates, lepton flavor universality violation ratios $R(D^{(*)})$, and the CKM matrix element $|V_{cb}|$. We point out some apparent errors in prior literature concerning the $\mathcal{O}(1/m_cm_b)$ corrections, and note a tension between commonly-used simplified dispersive bounds and current data.

hep-ph↗

arXiv2022

Interpreting LHCb's $Λ_b\to Λ_cτ\barν$ measurement and puzzles in semileptonic $Λ_b$ decays

Normalizing the recent LHCb measurement of $Λ_b \to Λ_c τ\barν$ to the standard model (SM) prediction for the $Λ_b \to Λ_c μ\barν$ rate, instead of a LEP measurement, provides a more consistent comparison with the SM prediction for the lepton flavor universality ratio $R(Λ_c)$. This modestly increases $R(Λ_c)$ compared to the quoted LHCb result, such that it no longer hints at a suppression compared to the SM, which would be hard to accommodate in new physics scenarios that enhance $R(D^{(*)})$. We point out that the fraction of excited states in inclusive semileptonic $Λ_b$ decay may be significantly greater than the corresponding fraction in $B$ decays. Possible implications are speculated upon.

hep-ph↗

arXiv2022

The Road Ahead for CODEX-b

In this Snowmass contribution we present a comprehensive status update on the progress and plans for the proposed CODEX-b detector, intended to search for long-lived particles beyond the Standard Model. We review the physics case for the proposal and present recent progress on optimization strategies for the detector and shielding design, as well as the development of new fast and full simulation frameworks. A summary of the technical design for a smaller demonstrator detector (CODEX-$β$) for the upcoming Run~3 of the LHC is also discussed, alongside the road towards realization of the full experiment at the High-Luminosity LHC.

hep-ex↗

arXiv2022

Form Factor Counting and HQET Matching for New Physics in $Λ_b \to Λ_c^*lν$

We calculate the $Λ_b \to Λ_c^*(2595) l ν$ and $Λ_b \to Λ_c^*(2625) l ν$ form factors and decay rates for all possible $b \to c l \barν$ four-Fermi interactions in and beyond the Standard Model (SM), including nonzero charged lepton masses and terms up to order $\mathcal{O}(α_s, 1/m_{c,b})$ in the heavy quark effective theory (HQET). We point out a subtlety involving the overcompleteness of the representation of the spin-parity $1/2^+ \to 3/2^-$ antisymmetric tensor form factors, relevant also to other higher excited-state transitions, and present a general method for the counting of the physical form factors for any hadronic transition matrix element and their matching onto HQET. We perform a preliminary fit of a simple HQET-based parametrization of the $Λ_b \to Λ_c^*$ form factors at $\mathcal{O}(α_s, 1/m_{c,b})$ to an existing quark model, providing preliminary predictions for the lepton universality ratios $R(Λ_c^*)$ beyond the SM. Finally, we examine the possible incompatibility of recent lattice QCD results with expectations from the heavy-quark expansion and available experimental data.

hep-ph↗

arXiv2021

Radiative Semileptonic $\bar{B}$ Decays

We consider the form factors for the radiative semileptonic decays $\bar{B}(v) \rightarrow D^{(*)}(v') \ell {\bar ν}_\ell γ$ in the kinematic region where the photon momentum, $k$, is small enough that heavy quark symmetry (HQS) can be applied without the radiated photon changing the heavy quark velocity (i.e., $v^{(\prime)} \cdot k < m_{(b,c)}$). We find that HQS is remarkably powerful, leaving only four new undetermined form factors at leading order in $1/m_{(b,c)}$. In addition, one of them is fixed in terms of the leading order Isgur-Wise function in the kinematic region, $v^{(\prime)}\cdot k < Λ_\text{QCD}$.

hep-ph↗

arXiv2021

Expression of Interest for the CODEX-b Detector

This document presents the physics case and ancillary studies for the proposed CODEX-b long-lived particle (LLP) detector, as well as for a smaller proof-of-concept demonstrator detector, CODEX-$β$, to be operated during Run 3 of the LHC. Our development of the CODEX-b physics case synthesizes `top-down' and `bottom-up' theoretical approaches, providing a detailed survey of both minimal and complete models featuring LLPs. Several of these models have not been studied previously, and for some others we amend studies from previous literature: In particular, for gluon and fermion-coupled axion-like particles. We moreover present updated simulations of expected backgrounds in CODEX-b's actively shielded environment, including the effects of shielding propagation uncertainties, high-energy tails and variation in the shielding design. Initial results are also included from a background measurement and calibration campaign. A design overview is presented for the CODEX-$β$ demonstrator detector, which will enable background calibration and detector design studies. Finally, we lay out brief studies of various design drivers of the CODEX-b experiment and potential extensions of the baseline design, including the physics case for a calorimeter element, precision timing, event tagging within LHCb, and precision low-momentum tracking.

hep-ex↗

arXiv2020

Das ist der HAMMER: Consistent new physics interpretations of semileptonic decays

Precise measurements of $b\to cτ\barν$ decays require large resource-intensive Monte Carlo (MC) samples, which incorporate detailed simulations of detector responses and physics backgrounds. Extracted parameters may be highly sensitive to the underlying theoretical models used in the MC generation. Because new physics (NP) can alter decay distributions and acceptances, the standard practice of fitting NP Wilson coefficients to SM-based measurements of the $R(D^{(*)})$ ratios can be biased. The newly developed HAMMER software tool enables efficient reweighting of MC samples to arbitrary NP scenarios or to any hadronic matrix elements. We demonstrate how HAMMER allows avoidance of biases through self-consistent fits directly to the NP Wilson coefficients. We also present example analyses that demonstrate the sizeable biases that can otherwise occur from naive NP interpretations of SM-based measurements. The HAMMER library is presently interfaced with several existing experimental analysis frameworks and we provide an overview of its structure.

hep-ph↗

arXiv2019

Combined analysis of semileptonic $B$ decays to $D$ and $D^$: $R(D^{()})$, $|V_{cb}|$, and new physics

The measured $\bar{B}\to D^{(*)} l \barν$ decay rates for light leptons ($l=e,μ$) constrain all $\bar{B}\to D^{(*)}$ semileptonic form factors, by including both the leading and ${\cal O}(Λ_{\text{QCD}}/m_{c,b})$ subleading Isgur-Wise functions in the heavy quark effective theory. We perform a novel combined fit to the $\bar{B}\to D^{(*)} l \barν$ decay distributions to predict the $\bar{B} \to D^{(*)} τ\barν$ rates and determine the CKM matrix element $|V_{cb}|$. Most theoretical and experimental papers have neglected uncertainties in the predictions for form factor ratios at order $Λ_{\text{QCD}}/m_{c,b}$, which we include. We also calculate ${\cal O}(Λ_{\text{QCD}}/m_{c,b})$ and ${\cal O}(α_s)$ contributions to semileptonic $\bar{B}\to D^{(*)} \ell \barν$ decays for all possible $b \to c$ currents. This result has not been available for the tensor current form factors, and for two of those, which are ${\cal O}(Λ_{\text{QCD}}/m_{c,b})$, the corrections are of the same order as approximations used in the literature. These results allow us to determine with improved precision how new physics may affect the $\bar{B}\to D^{(*)} τ\barν$ rates. Our predictions can be systematically improved with more data; they need not rely on lattice QCD results, although these can be incorporated.

hep-ph↗