High Energy Physics - Phenomenology (hep-ph)

Abstract: Hadron femtoscopy has turned into a powerful tool for accessing space-time information of heavy-ion collisions as well as for studying final-state interactions of hadrons. Recently, heavy-flavor femtoscopy has become feasible using the ALICE detector at the LHC. We compute the correlation function of $D$ mesons and light mesons using an off-shell $T$-matrix approach to access the two-meson wave function, and predict the correlation functions involving charged $D^+, D^{*+},D_s^+$ and $D_s^{*+}$ with $\pi^\pm$ and $K^\pm$. From the obtained results -- all of them accessible in $p+p$ collision experiments -- we point up the case of $D^+ \pi^-$ which is sensitive to the lower state of the two-pole $D_0^* (2300)$ system. The presence of such poles imprints a depletion on the correlation function, which could potentially be detected in experiments. While preliminary ALICE data do not show evidence of this effect, we suggest to look into the $D_s^+ K^-$ system to explore the higher pole of the $D_0^* (2300)$, as the depletion in the correlation function is more pronounced. Using heavy-quark spin symmetry we also propose exploring the effect of the two poles of the $D_1(2430)$ and predict similar structures in the correlation functions of the $D^{*+} \pi^-$ and $D_s^{*+} K^-$ pairs.

Abstract: We propose a novel method for renormalization group improvement of thermally resummed effective potential. In our method, $\beta$-functions are temperature dependent as a consequence of the divergence structure in resummed perturbation theory. In contrast to the ordinary $\overline{\text{MS}}$ scheme, the renormalization group invariance of the resummed finite-temperature effective potential holds order by order, which significantly mitigates a notorious renormalization scale dependence of phase transition quantities such as a critical temperature even at the one-loop order. We also devise a tractable method that enables one to incorporate temperature-dependent higher-order corrections by fully exploiting the renormalization group invariance.

Abstract: We propose a novel direct search experiment for X17 using the reaction $\gamma D \to e^+ e^- pn$. X17 is a hypothetical particle conjectured by the ATOMKI collaboration to explain anomalous signals around 17 MeV in excited ${}^8$Be, ${}^4$He and ${}^{12}$C nuclear decays via internal pair creation. It has been subject to a global experimental and theoretical research program. The proposed direct search in $\gamma D \to e^+ e^- pn$ can verify the existence of X17 through the production on a quasi-free neutron, and determine its quantum numbers separate from ongoing and planned nuclear-decay experiments. This is especially timely in view of the theoretical tension between results from the ${}^{12}$C and ${}^8$Be measurements. Using the plane-wave impulse approximation, we quantify the expected signal and background for pseudoscalar, vector and axial-vector X17 scenarios. We optimize the kinematics for the quasi-free neutron region with the upcoming MAGIX experiment at MESA in mind and show that for all three scenarios the X17 signal is clearly visible above the QED background.

Abstract: In this study, we present a comprehensive analysis of the electroweak sphaleron formalism and its application to electroweak phase transition (EWPT) patterns in extensions of the Standard Model scalar sector with electroweak multiplets. We offer an equivalence proof for different choices for the form of sphaleron configurations; construct the previously unestablished high-dimensional $\text{SU}(2)$ sphaleron transformation matrix; and revisit the required boundary conditions needed for solving the sphaleron field equations. We then investigate the leading order sphaleron dynamics in the context of a multi-step EWPT. We showcase two distinct analytical approaches for extending the $\text{SU}(2)$ scalar multiplet to the standard model (SM) under differing EWPT scenarios, and perform an explicit calculation of the sphaleron energy using a septuplet example. In the context of a single-step EWPT leading to a mixed phase, we find that the additional multiplet's contribution to the sphaleron energy is negligible, primarily due to the prevailing constraint imposed by the $\rho$ parameter. Conversely, in a two-step EWPT scenario, the sphaleron energy can achieve significantly high values during the initial phase, thereby markedly preserving baryon asymmetry if the universe undergoes a first-order EWPT. In both cases, we delineate the relationship between the sphaleron energy and the parameters relevant to dark matter phenomenology.

Abstract: The current framework for dark matter searches at beam dump and fixed target experiments relies on four benchmark models, the complex scalar, inelastic scalar, pseudo-Dirac and finally, Majorana DM models. While this approach has so far been successful in the interpretation of the available data, it a priori excludes the possibility that DM is made of spin-1 particles -- a restriction which is neither theoretically nor experimentally justified. In this work we extend the current landscape of sub-GeV DM models to a set of models for spin-1 DM, including a family of simplified models (involving one DM candidate and one mediator -- the dark photon) and an ultraviolet complete model based on a non-abelian gauge group where DM is a spin-1 Strongly Interacting Massive Particle. For each of these models, we calculate the DM relic density, the expected number of signal events at beam dump experiments, the rate of energy injection in the early universe thermal bath and in the Intergalactic Medium, as well as the helicity amplitudes for forward processes subject to the unitary bound. We then compare these predictions with experimental results from Planck, CMB surveys, IGM temperature observations, LSND, MiniBooNE, NA64, and BaBar and with available projections from LDMX and Belle II. Through this comparison, we identify the regions in the parameter space of the models considered in this work where DM is simultaneously thermally produced, compatible with present observations, and within reach at Belle II and LDMX. We find that the simplified models are strongly constrained by current beam dump experiments and the unitarity bound, and will thus be conclusively probed in the first stage of LDMX data taking. We also find that the SIMP model explored in this work predicts the observed DM abundance, is compatible with current observations and within reach at LDMX in a wide region of the parameter space.

Abstract: In cross sections with angular cuts, an intricate pattern of enhanced higher-order corrections known as non-global logarithms arises. The leading logarithmic terms were computed numerically two decades ago, but the resummation of subleading non-global logarithms remained a challenge that we solve in this Letter using renormalization group methods in effective field theory. To achieve next-to-leading logarithmic accuracy, we implement the two-loop anomalous dimension governing the resummation of non-global logarithms into a large-$N_c$ parton shower framework, together with one-loop matching corrections. As a first application, we study the interjet energy flow in $e^+e^-$ annihilation into two jets. We then present, for the first time, resummed predictions at next-to-leading logarithmic accuracy for a gap-between-jets observable at hadron colliders.

Abstract: We elucidate the structure of the next-to-leading-power soft-gluon expansion of arbitrary one-loop massless-QCD amplitudes. The expansion is given in terms of universal colour-, spin- and flavour-dependent operators acting on process-dependent gauge-invariant amplitudes. The result is proven using the method of expansion-by-regions and tested numerically on non-trivial processes with up to six partons. In principle, collinear-region contributions are expressed in terms of convolutions of universal jet operators and process-dependent amplitudes with two collinear partons. However, we evaluate these convolutions exactly for arbitrary processes. This is achieved by deriving an expression for the next-to-leading power expansion of tree-level amplitudes in the double-collinear limit, which is a novel result as well. Compared to previous studies, our analysis, besides being more general, yields simpler formulae that avoid derivatives of process-dependent amplitudes in the collinear limit.

Abstract: In this paper we revisit $D_s \to f_0$ form factors from the light-cone sum rules with the light meson light-cone distribution amplitudes. The main motivation of this study is the differential decay width of $D_s \to \left[\pi\pi \right]_{\rm S} e \nu_e$ measured recently by BESIII collaboration and the $D_s \to f_0$ form factor extracted under the intermediate resonant model. Our result of the differential width of $D_s^+ \to f_0 (\to \left[ \pi\pi \right]_{\rm S}) e^+ \nu_e$ decay obtained under the narrow width approximation is a litter bit lower than the data, the result obtained under the resonant Flatt\'e model is in consistent with the data while shows a litter bit larger, indicating a sizable mixing $\sim 20\degree$ between ${\bar s}s$ and ${\bar u}u+{\bar d}d$ of $f_0$. In order to obtain a model independent prediction, we suggest to calculate $D_s \to \left[ \pi\pi \right]_{\rm S}$ form factors with the isoscalar scalar dipion light-cone distribution amplitudes. Our calculation of $D_s \to \left[ \pi\pi \right]_{\rm S}$ form factors is carried out at the leading twist level due to the finite knowledge of dipion system, the result of differential width shows a moderate evolution in contrast to that obtained from the narrow width approximation and the Flatt\'e model, revealing a bright prospect to study the four-body leptonic decays of heavy mesons with the dimeson light-cone distribution amplitudes.

Abstract: We present a phenomenological study on the role of charm contribution and $SU(2)$ isospin symmetry in the extraction of the $\Lambda$ polarizing fragmentation functions from $e^+e^- \to \Lambda^\uparrow (\bar\Lambda^\uparrow) \,h + X$ annihilation processes. We adopt the well-established transverse-momentum-dependent factorization formalism, within the Collins-Soper-Sterman evolution scheme at next-to-leading logarithm accuracy, carefully exploiting the role of the nonperturbative component of the polarizing fragmentation function. We then discuss the impact of these results on the predictions for transverse $\Lambda$, $\bar{\Lambda}$ polarization in semi-inclusive deep inelastic scattering processes at typical energies of the future Electron-Ion Collider.

Abstract: The active galaxy NGC 1068 was recently identified by the IceCube neutrino observatory as the first known steady-state, extragalactic neutrino point source, associated with about 79 events over ten years. We use the IceCube data to place limits on possible neutrino self-interactions mediated by scalar particles with mass between 1 - 10 MeV. We find that a flavor-specific $\nu_{\tau}$ self-interaction is constrained beyond existing published bounds, while a flavor-universal self-interaction is not.

Abstract: We study the molecular probability of the $X(3872)$ in the $D^0 \bar D^{*0}$ and $D^+ D^{*-}$ channels in several scenarios. One of them assumes that the state is purely due to a genuine nonmolecular component. However, it gets unavoidably dressed by the meson components to the point that in the limit of zero binding of the $D^0 \bar D^{*0}$ component becomes purely molecular. Yet, the small but finite binding allows for a nonmolecular state when the bare mass of the genuine state approaches the $D^0 \bar D^{*0}$ threshold, but, in this case the system develops a small scattering length and a huge effective range for this channel in flagrant disagreement with present values of these magnitudes. Next we discuss the possibility to have hybrid states stemming from the combined effect of a genuine state and a reasonable direct interaction between the meson components, where we find cases in which the scattering length and effective range are still compatible with data, but even then the molecular probability is as big as $95 \%$. Finally, we perform the calculations when the binding stems purely from the direct interaction between the meson-meson components. In summary we conclude, that while present data definitely rule out the possibility of a dominant nonmolecular component, the precise value of the molecular probability requires a more precise determination of the scattering length and effective range of the $D^0 \bar D^{*0}$ channel, as well as the measurement of these magnitudes for the $D^+ D^{*-}$ channel which have not been determined experimentally so far.

Abstract: In this work we introduce $\nu^2$-Flows, an extension of the $\nu$-Flows method to final states containing multiple neutrinos. The architecture can natively scale for all combinations of object types and multiplicities in the final state for any desired neutrino multiplicities. In $t\bar{t}$ dilepton events, the momenta of both neutrinos and correlations between them are reconstructed more accurately than when using the most popular standard analytical techniques, and solutions are found for all events. Inference time is significantly faster than competing methods, and can be reduced further by evaluating in parallel on graphics processing units. We apply $\nu^2$-Flows to $t\bar{t}$ dilepton events and show that the per-bin uncertainties in unfolded distributions is much closer to the limit of performance set by perfect neutrino reconstruction than standard techniques. For the chosen double differential observables $\nu^2$-Flows results in improved statistical precision for each bin by a factor of 1.5 to 2 in comparison to the Neutrino Weighting method and up to a factor of four in comparison to the Ellipse approach.

Abstract: We compute the two-loop BSM contributions to the $h\longrightarrow \gamma\gamma$ decay width in the aligned THDM. We adopt the simplifying assumptions of vanishing EW gauge couplings and vanishing mass of the SM-like Higgs boson, which allow us to exploit a low-energy theorem connecting the $h\gamma\gamma$ amplitude to the derivative of the photon self-energy w.r.t. the Higgs field. We briefly discuss the numerical impact of the newly-computed contributions, showing that they may be required for a precise determination of $\Gamma[h\rightarrow \gamma \gamma]$ in scenarios where the quartic Higgs couplings are large.

Abstract: Factorization underlies all predictions at the Large Hadron Collider (LHC), but has only been rigorously proven in a few cases. One of these cases is the Drell-Yan process, $pp \to Z/\gamma + X$, in the limit of small boson transverse momentum. We introduce a one-parameter family of observables, that we call vector angularities, of which the transverse momentum is a special case. This enables the study of factorization violation, with a smooth transition to the limit for which factorization has been established. Like the angularity event shapes, vector angularities are a sum of transverse momenta weighted by rapidity, but crucially this is a vector sum rather than a sum of the magnitude of transverse momenta. We study these observables in Pythia, using the effect of multi-parton interactions (MPI) as a proxy factorization violation, finding a negligible effect in the case where factorization is established but sizable effects away from it. We also present a factorization formula for the cross section, that does not include factorization violating contributions from Glauber gluons, and thus offers a baseline for studying factorization violation experimentally using vector angularities. Our predictions at next-to-leading logarithmic accuracy (NLL$'$) are in good in agreement with Pythia (not including MPI), and can be extended to higher order.

Abstract: Based on recent progress in the systematic analysis of $C$ and $CP$ violation in the light-meson sector, we calculate the $C$-odd transition amplitudes $\eta\to\pi^0\ell^+\ell^-$ and $\eta'\to\eta\ell^+\ell^-$. Focusing on long-distance contributions driven by the lowest-lying hadronic intermediate states, we work out the correlations between these beyond-the-Standard-Model signals and the Dalitz-plot asymmetries in $\eta \rightarrow \pi^0 \pi^+ \pi^-$ and $\eta' \rightarrow \eta \pi^+ \pi^- $, using dispersion theory.

Abstract: Puzzles in the determination of the hadronic-vacuum-polarization contribution currently impede a conclusive interpretation of the precision measurement of the anomalous magnetic moment of the muon at the Fermilab experiment. One such puzzle concerns tensions between evaluations in lattice QCD and using $e^+e^-\to\text{hadrons}$ cross-section data. In lattice QCD, the dominant isospin-symmetric part and isospin-breaking (IB) corrections are calculated separately, with very different systematic effects. Identifying these two pieces in a data-driven approach provides an opportunity to compare them individually and trace back the source of the discrepancy. Here, we estimate the IB component of the lattice-QCD calculations from phenomenology, based on a comprehensive study of exclusive contributions that can be enhanced via infrared singularities, threshold effects, or hadronic resonances, including, for the first time, in the $e^+e^-\to3\pi$ channel. We observe sizable cancellations among different channels, with a sum that even suggests a slightly larger result for the QED correction than obtained in lattice QCD. We conclude that the tensions between lattice QCD and $e^+e^-$ data therefore cannot be explained by the IB contributions in the lattice-QCD calculations.

Abstract: Isospin-breaking (IB) effects are required for an evaluation of hadronic vacuum polarization at subpercent precision. While the dominant contributions arise from the $e^+e^-\to\pi^+\pi^-$ channel, also IB in the subleading channels can become relevant for a detailed understanding, e.g., of the comparison to lattice QCD. Here, we provide such an analysis for $e^+e^-\to 3\pi$ by extending our dispersive description of the process, including estimates of final-state radiation (FSR) and $\rho$-$\omega$ mixing. In particular, we develop a formalism to capture the leading infrared-enhanced effects in terms of a correction factor $\eta_{3\pi}$ that generalizes the analog treatment of virtual and final-state photons in the $2\pi$ case. The global fit to the $e^+e^-\to 3\pi$ data base, subject to constraints from analyticity, unitarity, and the chiral anomaly, gives $a_\mu^{3\pi}|_{\leq 1.8\,\text{GeV}}=45.91(53)\times 10^{-10}$ for the total $3\pi$ contribution to the anomalous magnetic moment of the muon, of which $a_\mu^\text{FSR}[3\pi]=0.51(1)\times 10^{-10}$ and $a_\mu^{\rho\text{-}\omega}[3\pi]=-2.68(70)\times 10^{-10}$ can be ascribed to IB. We argue that the resulting cancellation with $\rho$-$\omega$ mixing in $e^+e^-\to 2\pi$ can be understood from a narrow-resonance picture, and provide updated values for the vacuum-polarization-subtracted vector-meson parameters $M_\omega=782.70(3)\,\text{MeV}$, $M_\phi=1019.21(2)\,\text{MeV}$, $\Gamma_\omega=8.71(3)\,\text{MeV}$, and $\Gamma_\phi=4.27(1)\,\text{MeV}$.

Abstract: We demonstrate that existing gravitational wave data from LIGO already places constraints on well motivated Pati-Salam models that allow the Standard Model to be embedded within grand unified theories. For the first time in these models we also constrain the parameter space by requiring that the phase transition completes, with the resulting constraint being competitive with the limits from LIGO data. Both constraints are complementary to the LHC constraints and can exclude scenarios that are much heavier than can be probed in colliders. Finally we show that results from future LIGO runs, and the planned Einstein telescope, will substantially increase the limits we place on the parameter space.

Abstract: We study lepton flavour universality violation in SM boson decays in low-scale seesaw models of neutrino mass generation, also addressing other electroweak precision observables. We compute the electroweak next-to-leading order corrections, which turn out to be important - notably in the case of the invisible decay width of the $Z$ boson, for which the corrections can be as large as the current experimental uncertainty. As a well-motivated illustrative study case, we choose a realisation of the Inverse Seesaw mechanism, and discuss the complementary role of lepton flavour conserving, lepton flavour violating and precision observables, both in constraining and in probing such models of neutrino mass generation. Our findings suggest that invisible $Z$ decays are especially important, potentially at the origin of the most stringent constraints for certain regimes of the Inverse Seesaw (while complying with charge lepton flavour violation and other electroweak precision tests). We also discuss the probing power of the considered observables in view of the expected improvement in experimental precision at FCC-ee.

Abstract: There is currently no evidence for a baryon asymmetry in our Universe. Instead, cosmological observations have only demonstrated the existence of a quark-antiquark asymmetry, which does not necessarily imply a baryon asymmetric Universe, since the baryon number of the dark sector particles is unknown. In this paper we discuss a framework where the total baryon number of the Universe is equal to zero, and where the observed quark-antiquark asymmetry arises from neutron portal interactions with a dark sector fermion $N$ that carries baryon number. In order to render a baryon symmetric universe throughout the whole cosmological history, we introduce a complex scalar $\chi$, with opposite baryon number and with the same initial abundance as $N$. Notably, due to the baryon number conservation, $\chi$ is absolutely stable and could have an abundance today equal to the observed dark matter abundance. Therefore, in this simple framework, the existence of a quark-antiquark asymmetry is intimately related to the existence (and the stability) of dark matter.

Abstract: We present the results of a survey meant to assess the opinion of the high-energy physics theory (HET) community on the January 7th postdoc acceptance deadline - specifically, whether there is a preference to shift the deadline to later in January or February. This survey, which served for information-gathering purpose only, is part of a community conversation on the optimal timing of an acceptance deadline and whether the community would be better served by a later date. In addition, we present an analysis of data from the postdoc Rumor Mill, which gives a picture of the current hiring landscape in the field. We emphasize the importance of preserving a universal deadline, and the current results of our survey show broad support for a shift to a later date. A link to the survey, frequently asked questions, a running list of supporters, and next steps can be found on our companion web page.