arXiv subjects

Robert Szafron

Publications and source records attributed to Robert Szafron.

At least 37 records · Page 2Linked to original sources

arXiv2024

Leading order track functions in a hot and dense QGP

We study the modifications to the fragmentation pattern of partons into charged particles in the presence of a hot and dense Quark Gluon Plasma. To this end, we analyze the perturbative renormalization group equations of the track functions, which describe the energy fraction carried by charged hadrons. Focusing on pure Yang-Mills theory, we compute the lowest order moments of the medium-modified track functions, which are found to be sensitive to the reduced phase space for emissions in the medium and to energy loss. We use the extracted moments to calculate the Energy Energy Correlator (EEC) on tracks in the collinear limit. The EEC on medium-evolved tracks does not differ qualitatively from the EEC on vacuum tracks despite being sensitive to the color decoherence transition and suppressing the distribution due to quenching, as seen in other jet observables.

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arXiv2023

Advancing the understanding of energy-energy correlators in heavy-ion collisions

We investigate the collinear limit of the energy-energy correlator (EEC) in a heavy-ion context. First, we revisit the leading-logarithmic (LL) resummation of this observable in vacuum following a diagrammatic approach. We argue that this route allows to naturally incorporate medium-induced effects into the all-orders structure systematically. As an example, we show how the phase-space constraints imposed by the medium on vacuum-like emissions can be incorporated into the LL result by modifying the anomalous dimensions. On the fixed-order side, we calculate the $\mathcal{O}(α_s)$ expansion of the in-medium EEC for a $γ\to q\bar q$ splitting using, for the first time, the exact matrix element. When comparing this result to previously used approximations in the literature, we find up to $\mathcal{O}(1)$ deviations in the regime of interest for jet quenching signatures. Energy loss effects are also quantified and further suppress the EEC at large angles. These semi-analytic studies are complemented with a phenomenological study using the jet quenching Monte Carlo JetMed. Finally, we argue that the imprint of medium-induced effects in energy-energy correlators can be enhanced by using an alternative definition that takes as input Lund primary declusterings instead of particles.

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arXiv2023

Endpoint divergences in inclusive $\bar B \to X_s γ$

The subleading so-called resolved contributions represent the largest uncertainty in the inclusive decay mode $\bar B \to X_s γ$. However there had been no complete proof of factorization of these subleading contributions. This failure of factorisation can be traced back to endpoint divergences and cured by recently proposed refactorisation techniques.

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arXiv2023

Refactorisation in subleading $\bar B \to X_s γ$

We establish refactorisation conditions between the subleading ${O}_8$-${O}_8$ contributions to the inclusive $\bar B \to X_s γ$ decay suffering from endpoint divergences and prove a factorisation theorem for these contributions to all orders in the strong coupling constant. This allows for higher-order calculations of the resolved contributions and consistent summation of large logarithms, consequently reducing the recently found large-scale dependence in these contributions. We implement the concept of refactorisation in a heavy flavour application of SCET, which includes nonperturbative functions as additional subtlety not present in collider applications.

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arXiv2022

Theory Techniques for Precision Physics -- Snowmass 2021 TF06 Topical Group Report

The wealth of experimental data collected at laboratory experiments suggests that there is some scale separation between the Standard Model (SM) and phenomena beyond the SM (BSM). New phenomena can manifest itself as small corrections to SM predictions, or as signals in processes where the SM predictions vanish or are exceedingly small. This makes precise calculations of the SM expectations essential, in order to maximize the sensitivity of current and forthcoming experiments to BSM physics. This topical group report highlights some past and forthcoming theory developments critical for maximizing the sensitivity of the experimental program to understanding Nature at the shortest distances.

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arXiv2022

Power-enhanced leading-logarithmic QED corrections to $B_q \to μ^+μ^-$

We provide a systematic treatment of the previously discovered power-enhanced QED corrections to the leptonic decay $B_q\to μ^+μ^-$ ($q = d, s$) in the framework of soft-collinear effective theory (SCET). Employing two-step matching on SCET$_\text{I}$ and SCET$_\text{II}$, and the respective renormalization group equations, we sum the leading-logarithmic QED corrections and the mixed QED-QCD corrections to all orders in the couplings for the matrix element of the semileptonic weak effective operator $\sim Q_9$. We propose a treatment of the $B$-meson decay constant and light-cone distribution amplitude in the presence of process-specific QED corrections. Finally we include ultrasoft photon radiation and provide updated values of the non-radiative and radiative branching fractions of $B_q \to μ^+μ^-$ decay that include the double-logarithmic QED and QCD corrections.

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arXiv2022

Muon g-2 and a Geocentric New Field

Light scalars can in principle couple to both bulk matter and fermion spin, with hierarchically disparate strengths. Storage ring measurements of fermion electromagnetic moments via spin precession can be sensitive to such a force, sourced by the Earth. We discuss how this force could lead to a deviation of the measured muon anomalous magnetic moment, $g-2$, from the Standard Model prediction. Due to its different parameters, the proposed JPARC muon $g-2$ experiment can provide a direct test of our hypothesis. A future search for the proton electric dipole moment can have good sensitivity for the coupling of the assumed scalar to nucleon spin. We also argue that supernova constraints on axion-muon coupling may not be applicable in our framework.

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arXiv2022

Soft-Collinear Gravity and Soft Theorems

This chapter reviews the construction of ``soft-collinear gravity'', the effective field theory which describes the interaction of collinear and soft gravitons with matter (and themselves), to all orders in the soft-collinear power expansion, focusing on the essential concepts. Among them are an emergent soft background gauge symmetry, which lives on the light-like trajectories of energetic particles and allows for a manifestly gauge-invariant representation of the interactions in terms of a soft covariant derivative and the soft Riemann tensor, and a systematic treatment of collinear interactions, which are absent at leading power in gravity. The gravitational soft theorems are derived from soft-collinear gravity at the Lagrangian level. The symmetries of the effective theory provide a transparent explanation of why soft graviton emission is universal to sub-sub-leading power, but gauge boson emission is not and suggest a physical interpretation of the form of the universal soft factors in terms of the charges corresponding to the soft symmetries. The power counting of soft-collinear gravity further provides an understanding of the structure of loop corrections to the soft theorems.

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arXiv2022

Is the $\bar θ$ parameter of QCD constant?

Testing the cosmological variation of fundamental constants of Nature can provide valuable insights into new physics scenarios. While many such constraints have been derived for Standard Model coupling constants and masses, the $\barθ$ parameter of QCD has not been as extensively examined. This letter discusses potentially promising paths to investigate the time dependence of the $\barθ$ parameter. While laboratory searches for CP-violating signals of $\barθ$ yield the most robust bounds on today's value of $\barθ$, we show that CP-conserving effects provide constraints on the variation of $\barθ$ over cosmological timescales. We find no evidence for a variation of $\barθ$ that could have implied an "iron-deficient" Universe at higher redshifts. By converting recent atomic clock constraints on a variation of constants, we infer $ d({\barθ}^2)/dt \leq 6\times 10^{-15}\text{yr}^{-1}$, at 1-$σ$. Finally, we also sketch an axion model that results in a varying $\barθ$ and could lead to excess diffuse gamma ray background, from decays of axions produced in high redshift supernova explosions.

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arXiv2022

Snowmass Theory Frontier: Effective Field Theory

We summarize recent progress in the development, application, and understanding of effective field theories and highlight promising directions for future research. This Report is prepared as the TF02 "Effective Field Theory" topical group summary for the Theory Frontier as part of the Snowmass 2021 process.

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arXiv2022

Inclusive Production Cross Sections at N3LO

We present for the first time the inclusive cross section for associated Higgs boson production with a massive gauge boson at next-to-next-to-next-to-leading order in QCD. Furthermore, we introduce n3loxs, a public, numerical program for the evaluation of inclusive cross sections at the third order in the strong coupling constant. Our tool allows to derive predictions for charged- and neutral-current Drell-Yan production, gluon- and bottom-quark-fusion Higgs boson production and Higgs boson associated production with a heavy gauge boson. We discuss perturbative and parton distribution function (PDF) uncertainties of the aforementioned processes. We perform a comparison of global PDF sets for a variety of process including associated Higgs boson production and observe $1σ$ deviations among predictions for several processes.

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arXiv2022

Endpoint factorization and next-to-leading power resummation of gluon thrust

Endpoint divergences in the convolution integrals appearing in next-to-leading-power factorization theorems prevent a straightforward application of standard methods to resum large logarithmic power-suppressed corrections in collider physics. We study the power-suppressed configuration of the thrust distribution in the two-jet region, where a gluon-initiated jet recoils against a quark-antiquark pair. With the aid of operatorial endpoint factorization conditions, we derive a factorization formula where the individual terms are free from endpoint divergences and can be written in terms of renormalized hard, (anti) collinear, and soft functions in four dimensions. This framework enables us to perform the first resummation of the endpoint-divergent SCET$_{\rm I}$ observables at the leading logarithmic accuracy using exclusively renormalization-group methods.

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arXiv2022

Isotope dependence of muon-to-electron conversion

The lepton-flavor-violating conversion of a muon into an electron in the field of a nucleus is one of the most sensitive probes of physics beyond the Standard Model and the experiments Mu2e, COMET, and DeeMe will explore uncharted terrain in the near future. An observation of this $μ^-\to e^-$ conversion process opens up the possibility to distinguish the underlying operator or new-physics model by exploiting the target-nucleus dependence of the conversion rate. To facilitate the choice of optimal targets we provide $μ^-\to e^-$ conversion rates for all stable isotopes and estimate nuclear-structure uncertainties. Our results enable studies of mixed or enriched target materials that are particularly promising for distinguishing scenarios in which the muon converts either on protons or neutrons.

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arXiv2022

Snowmass 2021 White Paper: Resummation for future colliders

Resummation techniques are essential for high-precision phenomenology at current and future high-energy collider experiments. Perturbative computations of cross sections often suffer from large logarithmic corrections, which must be resummed to all orders to restore the reliability of predictions from first principles. The precise understanding of the all-order structure of field theories allows for fundamental tests of the Standard Model and new physics searches. In this white paper, we review recent progress in modern resummation techniques and outline future directions. In particular, we focus on the resummation beyond leading power, the joint resummation of different classes of logarithms relevant for jets and their substructure, small-$x$ resummation in the high-energy regime and the QCD fragmentation process in the small-$z_h$ limit.

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arXiv2022

Soft-collinear gravity beyond the leading power

We construct "soft-collinear gravity", the effective field theory which describes the interaction of collinear and soft gravitons with matter (and themselves), to all orders in the soft-collinear power expansion. Despite the absence of collinear divergences in gravity at leading power, the construction exhibits remarkable similarities with soft-collinear effective theory of QCD (gauge fields). It reveals an emergent soft background gauge symmetry, which allows for a manifestly gauge-invariant representation of the interactions in terms of a soft covariant derivative, the soft Riemann tensor, and a covariant generalisation of the collinear light-cone gauge metric field. The gauge symmetries control both the unsuppressed collinear field components and the inherent inhomogeneity in $λ$ of the invariant objects to all orders, resulting in a consistent expansion.

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arXiv2022

Gravitational soft theorem from emergent soft gauge symmetries

We consider and derive the gravitational soft theorem up to the sub-subleading power from the perspective of effective Lagrangians. The emergent soft gauge symmetries of the effective Lagrangian provide a transparent explanation of why soft graviton emission is universal to sub-subleading power, but gauge boson emission is not. They also suggest a physical interpretation of the form of the soft factors in terms of the charges related to the soft transformations and the kinematics of the multipole expansion. The derivation is done directly at Lagrangian level, resulting in an operatorial form of the soft theorems. In order to highlight the differences and similarities of the gauge-theory and gravitational soft theorems, we include an extensive discussion of soft gauge-boson emission from scalar, fermionic and vector matter at subleading power.

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arXiv2022

The analytic two-loop soft function for leading-jet $p_T$

We present the calculation of the two-loop soft function for the transverse momentum distribution of the leading jet produced in association with any colour-singlet system (e.g.~a Higgs or a $Z$ boson). This constitutes a central ingredient for the resummation of the above distribution as well as the jet-vetoed cross section at the next-to-next-to-next-to-leading logarithmic order, both of which play an important role in the precision physics programme at the Large Hadron Collider. The calculation is performed in soft-collinear effective theory with an appropriate regularisation of the rapidity divergences that occur in the phase-space integrals. We obtain analytic results by employing an exponential regulator and by taking a Laurent expansion in the jet radius $R$. All expressions are presented as ancillary files with this article.

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arXiv2022

Isotope dependence of muon decay in orbit

The decay $μ^-\to e^- \barν_eν_μ$ of a muon that is bound to a nucleus poses an unavoidable background for experiments such as Mu2e, COMET, and DeeMe searching for lepton-flavor-violating $μ^-\to e^-$ conversion and thus requires a precise understanding. We calculate the electron-energy spectra of muon decays in orbit near the electron energy endpoint for all stable isotopes and estimate uncertainties due to the nuclear charge distribution. Our results enable background studies of mixed or enriched target materials that are necessary for the next generation of $μ^-\to e^-$ conversion experiments.

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