Dark Matter in Neutron Stars and its Impact on the Trace Anomaly

A recent study has investigated how the presence of dark matter (DM) affects the conformal properties and phase structure of dense matter in neutron stars. Using a self-consistent single-fluid framework, researchers have developed an exact decomposition of the total trace anomaly (Δtot) into its microscopic contributions. This approach allows for the analysis of dark matter's impact not only on the composition but also on the fundamental physics of these stars, which serve as natural laboratories for matter under extreme conditions. For collider-motivated Higgs-portal dark matter benchmarks, direct contributions from Higgs, vector-mediator, and contact interactions are found to be negligible. However, heavy nonrelativistic dark matter possesses an intrinsic trace anomaly close to the nonrelativistic limit (Δχ ≈ 1/3). This implies that the dark matter rest-mass energy fraction is the dominant factor in modifying Δtot, producing a smooth upward shift of up to approximately 0.1 for a dark matter fraction Fχ of about 0.2% of the baryonic mass. In the pressureless, comoving heavy-WIMP regime, a universal dark-sector scaling governed by the mass-loading parameter λ = Fχmχ/mN, where mχ is the dark matter particle mass and mN is the nucleon mass, has been identified. Numerical calculations demonstrate that, for a fixed λ, different combinations of Fχ and mχ exhibit overlapping responses in the trace anomaly, sound speed, and mass-radius relations for a given baryonic equation of state. In hybrid stars, dark matter leaves the coexistence pressure and chemical potential essentially unchanged, whereas first-order hadron-quark deconfinement produces sharp discontinuities in the squared sound speed (c_s²), Δtot, and Δtot - ΔB. This combined softening substantially reduces the maximum stellar mass, placing predictions in tension with observed 2 M☉ neutron stars if Fχ is around 0.2%. These findings suggest that the trace anomaly and its density evolution provide sensitive diagnostics for distinguishing smooth dark matter-induced modifications from genuine first-order deconfinement. This opens new avenues for understanding the internal composition of neutron stars and potentially for detecting the presence of dark matter in these compact objects, which would have profound implications for astrophysics and particle physics.

Particle Physics: Why Are Neutrinos So Light?

An international team of scientists has proposed a new theory that could explain why neutrinos, some of the most abundant elementary particles in the universe, possess such a tiny mass compared to other leptons. The hypothesis suggests that neutrinos acquire their mass through a different mechanism than other particles, involving a type of interaction not yet directly observed. The Standard Model of particle physics predicts that neutrinos should be massless particles. However, experiments like Super-Kamiokande and SNO have shown that they oscillate between different types (flavors), which implies they must have a non-zero mass. This discrepancy has been one of the most persistent problems in particle physics for decades. The new proposal seeks to resolve this anomaly by introducing new particles and symmetries that interact weakly with neutrinos. The theory is based on the existence of an "invisible" dark matter sector that interacts with neutrinos via an as-yet unknown force mediator. This mechanism, dubbed the "inverse seesaw mechanism," would allow neutrinos to obtain their mass indirectly, without needing to interact directly with the Higgs boson in the same way as other particles. This would result in extremely small masses, consistent with current observations. While the theory is purely speculative for now, it offers a promising direction for future research at the Large Hadron Collider (LHC) and other neutrino physics experiments. The detection of mediating particles or new interactions could provide the necessary evidence to validate this hypothesis and shed light on one of the greatest mysteries of the subatomic universe.

Chain Seesaw Mechanism for Neutrinos and Charged Fermions with High-Quality Axion

Researchers have proposed a new chain seesaw mechanism that not only generates the masses of neutral neutrinos but also those of charged fermions. This model extends the Standard Model gauge symmetries (SU(3)c × SU(2)L × U(1)Y) with an additional U(1)Y' gauge symmetry. The inclusion of this extra symmetry allows for a deeper unification in understanding the origin of fundamental particle masses, addressing one of the persistent shortcomings of the Standard Model, which does not explain neutrino mass or the origin of charged fermion masses in a unified way. The mechanism relies on interaction chains for down-type and up-type quarks, which differ in the number of "links." These links are formed by mediator vector-like fermions and new Higgs scalars, each carrying specific U(1)Y' charges. This differentiated architecture is crucial for generating the distinct masses observed in quarks. Furthermore, the Peccei-Quinn (PQ) global symmetry, which is fundamental for solving the strong CP problem in quantum chromodynamics (QCD) by introducing the axion, is automatically embedded in this U(1)Y' gauge symmetry. The automatic embedding of the Peccei-Quinn symmetry within the U(1)Y' gauge symmetry is a key aspect of this proposal. This integration is sufficient to guarantee the "high quality" of the axion, meaning that the resulting axion is robust against quantum perturbations that could reintroduce the strong CP problem. A high-quality axion is essential for its viability as a dark matter candidate and for the consistency of the model with cosmological and particle physics observations. This approach offers a promising avenue to address multiple fundamental problems in particle physics within a unified framework.

Leptoquarks and Lepton Anomalous Magnetic Moments

A new study explores the implications of scalar leptoquarks, both SU(2)L singlet and triplet, regarding the observed anomalies in lepton anomalous magnetic moments (g-2) and charged lepton flavor-violating decays. The research reveals significant correlations between the values of Δaₑ and Δaᵤ, as well as with decay rates such as e_b → e_aγ and the decays of Higgs and Z bosons into lepton pairs e_b⁺e_a⁻. The proposed extended Standard Model, which incorporates these leptoquarks, cannot simultaneously accommodate sizable values for both the electron's magnetic anomaly (|Δaₑ| ~ 10⁻¹³) and the muon's (|Δaᵤ| ~ 10⁻¹⁰). If a significant value for the muon anomaly is assumed, for instance |Δaᵤ| ~ 10⁻¹⁰ and a decay rate Br(μ → eγ) > 10⁻¹⁵, the model predicts that the decay rates of tau into an electron and a photon, Br(τ → eγ), would be very small, below 10⁻¹², and Higgs decays into tau and electron, Br(h → τe), would also be suppressed, below 10⁻⁶. However, other decays could be within the reach of future experimental sensitivities. Conversely, if significant values for the electron anomaly are considered, |Δaₑ| ~ 10⁻¹³, the model predicts a similar suppression for Br(τ → μγ) decays below 10⁻¹¹ and Br(h → τμ) below 10⁻⁷. These results suggest that while leptoquarks could offer an explanation for some observed anomalies, their ability to resolve all of them simultaneously is limited, and future measurements of lepton flavor-violating decay rates will be crucial for validating or ruling out this type of Standard Model extension.

New Machine Learning Model Improves Collider Event Reconstruction

Scientists have developed VyPER, a novel geometric learning framework for event reconstruction in particle collider experiments. This system addresses the challenge of inferring the kinematics of short-lived particles from the stable final states recorded by detectors. Reconstruction is decomposed into two primary tasks: assigning measured jets and charged leptons to their parent particles, and predicting the kinematics of unmeasured neutrinos. VyPER represents collider events as hypergraphs with a physics-inspired topology. It combines supervised classification of hyperedges for particle assignment with a diffusion model for predicting neutrino kinematics. Both tasks are unified through a joint loss function, optimizing reconstruction within a single framework. This approach enables more precise inference of the properties of ephemeral particles. VyPER's performance has been demonstrated across various proton-proton collision processes, outperforming existing analytical and machine-learning-based reconstruction techniques. This advancement facilitates accurate event reconstruction across a diverse range of Standard Model physics processes, opening new avenues for precision measurements in the Higgs boson, electroweak, and top-quark sectors.

Argon Could Resolve Nature of 248 keV Recoil in LZ

A recent analysis suggests that argon could be key to discerning the nature of a 248 keV recoil event observed in the LZ experiment, designed for dark matter searches. This event, which could be interpreted as the de-excitation of a pseudo-Dirac state or as the interaction of a Higgsino with halo particles, yields distinct predictions for detection in argon-based detectors. The ability to discriminate between these two interpretations lies in the interaction kinematics and nuclear form factors. If the 248 keV signal in LZ is due to the de-excitation of a pseudo-Dirac state, a nuclear recoil line in argon is predicted at an energy of |δ|mχ/(mχ+mAr), which for a 200 GeV mass would be 336 keV. This signal would be expected at a rate ten times higher than that observed in xenon. Detecting this line and its energy, combined with the xenon data, would allow for the determination of the dark matter particle's mass. Conversely, if the event is the result of a ground-state Higgsino interacting with the fastest halo particles, no argon events would be expected, as the argon detection threshold would exceed the maximum halo particle energy. The ratio between detection rates and recoil energies in xenon and argon depends solely on interaction kinematics and nuclear form factors, making it a robust test. Data already recorded by the DEAP-3600 experiment, which uses liquid argon, could contain tens of events if the pseudo-Dirac state hypothesis is correct. Analyzing these data, or conducting future argon experiments, would offer a direct path to confirm or refute these interpretations, shedding light on the nature of dark matter.

TeV Higgsino Interpretation of LUX-ZEPLIN High-Recoil Event

A recent high-recoil event reported by the LUX-ZEPLIN (LZ) experiment has motivated a new theoretical interpretation within the framework of supersymmetry. This event, featuring a significant recoil, could be explained by the interaction of a Higgsino with a mass on the order of a TeV, characterized by a sub-MeV energy splitting between its neutral states. In the Minimal Supersymmetric Standard Model (MSSM), such a small gap in the Higgsino's neutral states, absent specific cancellations, typically implies electroweak gaugino masses on the order of $10^7$ GeV, a value much higher than the Higgsino's own mass. Researchers have proposed that a non-universal boundary condition for gaugino masses ($M_1^G/M_2^G = -3/5$) at the SU(5) Grand Unified Theory (GUT) scale could resolve this discrepancy. This condition would allow for the cancellation of leading bino and wino contributions and would be preserved under homogeneous one-loop evolution down to the Higgsino scale. This opens the possibility for a controlled tree-level solution with a wino mass near 120 TeV for a 350 keV gap, a 1.091 TeV Higgsino, and $\tan\beta = 10$. Mixed SU(5) representations that realize the required gaugino ratio have been identified. It is crucial to note that, despite this new interpretation, a full-density thermal Higgsino remains subject to published solar-capture bounds. This is because the inelastic Z coupling remains essentially unsuppressed, implying that the astrophysical and transport assumptions used in solar-capture limits are still valid for this scenario. The radiative sensitivity and spectrum consistency of this theoretical construction have been examined to ensure its viability.

Inelastic Electroweak Dark Matter Proposed for LZ Events

A new study proposes an electroweak dark matter model that could explain the high-energy inelastic nuclear recoil events recently reported by the LUX-ZEPLIN (LZ) Collaboration. This model extends minimal electroweak dark matter with a Majorana and a Dirac multiplet, coupled through Higgs interactions, providing a predictive framework for high-energy inelastic nuclear recoils. In this scheme, electroweak symmetry breaking induces a neutral-state splitting, δ, and an off-diagonal Z interaction. For a series of coupled multiplets (such as 3M2D, 5M4D, etc.), both the splitting and the leading inelastic interaction are universal for a fixed (mχ, y), independent of the electroweak representation. The high recoil energy observed by LZ points to splittings of a few hundred keV. The fixed Z-mediated rate allows for the determination of a mass-dependent δLZ(mχ) by requiring one expected inelastic event in the LZ exposure, with a two-sided 90% Poisson band. This defines a universal region in the (mχ, y) plane. Intersecting this region with the representation-dependent thermal relic trajectories selects a benchmark for each multiplet. The corresponding electroweak representation then predicts a correlated loop-induced elastic spin-independent signal at lower recoil energies, which could be sought in future experiments.

1.1 TeV Higgsino Explains Dark Matter Anomalies in LZ and Fermi-LAT

A new study proposes that the Higgsino, a supersymmetric particle, could be responsible for anomalies observed in the LZ and Fermi-LAT experiments, which search for dark matter signals. This interpretation suggests that the Higgsino, a WIMP (Weakly Interacting Massive Particle) candidate from the Minimal Supersymmetric Standard Model, would have a mass of approximately 1.1 TeV. This mass value is derived from the relic abundance of dark matter in the universe, while the nuclear recoil spectrum observed by LZ points to a mass splitting of a few hundred keV. The LZ experiment, an underground liquid xenon detector, has reported an excess of nuclear recoil events that cannot be explained by known backgrounds. Simultaneously, 14 years of data from the Fermi-LAT space telescope show a mild preference for a gamma-ray signal in the Galactic center, consistent with the annihilation of dark matter particles. The 1.1 TeV Higgsino hypothesis successfully reconciles both results, providing a unified explanation for these seemingly disparate observations. In addition to explaining the LZ and Fermi-LAT anomalies, this Higgsino interpretation is also consistent with solar neutrino constraints imposed by the IceCube experiment over 10 years. The model predicts the existence of a gamma-ray line and an endpoint signal in the Galactic center, which are near the current sensitivity of the H.E.S.S. observatory and within the projected reach of the future Cherenkov Telescope Array Observatory (CTAO). This means that this classic dark matter hypothesis could be experimentally tested in the near future, opening a window for the direct detection of these elusive particles.

FCC-ee to explore flavor-violating interactions beyond Standard Model

A new study explores the capability of the future electron-positron Future Circular Collider (FCC-ee) to detect flavor-changing neutral currents (FCNCs) in the third generation of fermions. These interactions, mediated by Z bosons and photons, are a fertile ground for the search for new physics, as the properties of the third generation, such as their significantly larger masses, are less precisely measured and could reveal deviations from the Standard Model (SM). Researchers employed the Standard Model Effective Field Theory (SMEFT) formalism, using dipole and Higgs-current operators, to analyze transitions between the third and the first two generations of fermions. They used the optimal observable technique (OOT) to determine the optimal sensitivity of the FCC-ee at different center-of-mass energies. Additionally, they complemented this analysis with existing constraints from low-energy flavor-violating observables and heavy fermion decay channels. The study also revealed characteristic interference patterns among the dipole contributions, which vary depending on the underlying flavor transition and exhibit distinct behavior between the Z pole and higher-energy FCC-ee stages. Projections indicate improved sensitivity for several of these interactions compared to current limits, and comparable sensitivity for others. This highlights the importance of a systematic assessment across the different FCC-ee energy stages to obtain a comprehensive picture of its potential in exploring flavor-violating phenomena and its complementarity with low-energy flavor experiments.

Composite Higgs Models Predict Dark Matter with Magnetic Moment

Researchers have explored the interaction of dark matter (DM) with nucleons and nuclei within the framework of Composite Higgs Models (CHMs). In these models, the lightest Dirac composite particle (LDCP) can be stable and constitute a significant fraction of the observed relic dark matter abundance. The study focuses on the elastic scattering of the LDCP on nucleons, as well as on xenon (Xe) and argon (Ar) nuclei, considering a non-zero magnetic dipole moment for the LDCP. The peculiarity of this scenario is that the LDCP's magnetic moment, its mass, and its coupling to the Higgs doublet are suppressed by an approximate U(1) symmetry. A non-zero magnetic dipole moment for the LDCP can lead to a substantial enhancement in the differential event rate in direct detection (DD) experiments, especially at low nuclear recoil energies. This effect could be key to its detection. Assuming the LDCP constitutes at least 10% (ξ ≥ 0.1) of the relic dark matter, the authors have identified a region of the parameter space where this enhancement in the event rate could be potentially observable. Furthermore, they have specified some observables that could be useful in discriminating between dark matter fermions with a magnetic moment and other types of dark matter particles that do not possess similar electromagnetic properties. This opens a path to identify the nature of dark matter if a signal with these characteristics is detected.

Vector-Like Fermions: New Sensitivity Limits at FCC-ee Higgs Factory

Researchers have calculated the one-loop effects of vector-like fermions (VLFs) on the Higgs-strahlung cross-section. This observable is a flagship precision measurement for future $e^+e^-$ Higgs factories like the FCC-ee. The study considers four benchmark extensions of the Standard Model (SM), including VLF quark and lepton doublets and singlets that couple to the Higgs boson, or that mix directly with third-generation SM fermions. The calculations, performed using the on-shell renormalization scheme and imposing perturbativity bounds on couplings, include both universal self-energy and non-universal vertex contributions to the Higgs-strahlung amplitude. Oblique parameters S and T, and the $H\to\gamma\gamma$ decay rate were also computed as complementary cross-checks. The results indicate that a sizable fraction of the VLF parameter space, still allowed by current LHC searches, could produce shifts in $\sigma(ZH)$ detectable by the FCC-ee, with a per-mille sensitivity, offering a radiative probe of these couplings. For scenarios where interactions are driven by Yukawa couplings, the FCC-ee could provide a new window to explore VLFs. However, in mixing-driven scenarios, existing electroweak precision bounds on the relevant mixing angles already preclude an observable effect. This work highlights that VLF Yukawa couplings are the more promising target for Higgs-strahlung precision measurements at a future $e^+e^-$ collider, rather than any residual mixing with SM fermions.

Dimension-8 Operators in Baryogenesis via Sphaleron Decoupling

A recent study investigates whether the observed baryon asymmetry of the Universe, the imbalance between matter and antimatter, can be explained by a baryogenesis mechanism known as sphalerogenesis within the Standard Model Effective Field Theory (SMEFT) framework. This mechanism posits that the baryon asymmetry is generated through a CP-asymmetric decoupling of electroweak (EW) sphaleron-like transitions. Researchers introduced seven CP-violating dimension-8 operators constructed from the Higgs doublet and the SU(2)L gauge fields. They demonstrated that five of these operators can individually account for the observed baryon asymmetry while satisfying experimental constraints from particle colliders or electron electric dipole moment measurements. This finding suggests new avenues for understanding the origin of matter in the universe. The study also examined the impact of these dimension-8 operators in the presence of a CP-violating dimension-6 operator. It was found that the dimension-8 contributions can be comparable to those from the dimension-6 operator when the former are generated at one loop. This result highlights that loop-order counting can be as crucial as canonical mass-dimension counting in sphalerogenesis models, offering a more nuanced perspective on the relevance of different operators in physics beyond the Standard Model.

Probing Higgs self-coupling at future lepton colliders

Researchers have investigated the sensitivity to the Higgs self-coupling through W boson fusion di-Higgs production at the Compact Linear Collider (CLIC), operating at a center-of-mass energy of 3 TeV. This study focuses on the interplay between the Higgs self-coupling modifier (κ_λ) and the Higgs-gauge coupling modifiers (κ_V and κ_2V) within the κ framework. The results demonstrate that future high-energy lepton colliders, combined with advanced machine learning techniques, offer exceptional sensitivity to the Higgs self-coupling, significantly exceeding the projections of the High-Luminosity Large Hadron Collider (HL-LHC). To enhance the separation between signal and background, the team developed a classifier based on graph neural networks (GNNs). This approach allowed them to achieve a signal significance of approximately 20σ with an integrated luminosity of 5 ab⁻¹. This level of precision is crucial for disentangling the nature of electroweak symmetry breaking, distinguishing between linearly and non-linearly realized scenarios, and provides a powerful tool for the search for new physics in the electroweak sector. The Higgs boson self-coupling is a fundamental parameter that governs the shape of the Higgs potential and, therefore, the mechanism of electroweak symmetry breaking. Its precise measurement is essential for confirming the Standard Model and for searching for deviations that could indicate the presence of particles or interactions beyond it. The capability of CLIC, coupled with GNNs, to probe this coupling with such precision opens new avenues for exploring fundamental phenomena in particle physics.

Revised Exclusion Limits for Doubly Charged Higgs Bosons

A new analysis has revised the exclusion limits for doubly charged Higgs bosons (H±±), hypothetical particles that would extend the Standard Model. This work re-examines a search conducted by the ATLAS collaboration at CERN's Large Hadron Collider (LHC), utilizing the full Run 2 dataset at a collision energy of 13 TeV. The original ATLAS search, published in Eur. Phys. J. C 83 (2023) 605, set the strongest limits to date on the mass of these doubly charged scalars, largely driven by an essentially background-free four-lepton channel. The current study identifies a discrepancy in the four-lepton signal efficiency reported by ATLAS. The authors demonstrate that the implied efficiency from the original analysis exceeds a strict, mass-independent upper bound derived from key assumptions such as equal branching ratios, leptonic tau (τ) branching fractions, and ATLAS lepton reconstruction efficiencies. This discrepancy cannot be explained by hadronic τ or jet misidentification without invoking unrealistically high fake rates. To correct this inconsistency, the researchers independently regenerated the signal and recomputed the exclusion limits. They used the corrected signal yields, ATLAS background predictions and uncertainties, and the same CLs procedure (modified frequentist confidence limit) implemented in pyhf. The results indicate that the expected lower mass bound shifts from 1065 GeV to approximately 950 GeV for the left-right symmetric type-II seesaw model, and from 880 GeV to about 770 GeV for the Zee-Babu model. These new limits are systematically higher than the ATLAS expected limits by a factor of two or more, implying that the excluded mass region is smaller than initially thought.

Spatial Curvature and Dynamical Dark Energy Reconcile Inflation with Observations

A new analysis of cosmological data suggests that spatial curvature and dynamical dark energy could resolve the tension between inflationary model predictions and current observations of the spectral index $n_s$. Traditionally, the standard cosmological model $\Lambda$CDM shows a slight discrepancy with inflationary models such as Starobinsky, Higgs, and the simplest $α$-attractors. This study proposes that by considering a universe with a small negative spatial curvature or time-evolving dark energy, this tension is significantly alleviated.

New technique for strong primordial phase transitions

Researchers have proposed a new resummation technique to calculate the effective potential in very strong first-order phase transitions in the early universe. These transitions are crucial for predicting stochastic gravitational wave backgrounds, which could be detected by next-generation observatories like LISA. Established techniques, such as dimensional reduction, rely on high-temperature expansions that are expected to break down for the most intense transitions, precisely those that could generate observable signals. The study argues that 2PI (Two-Particle Irreducible) effective action techniques enable consistent resummation for these extreme transitions. The authors applied these techniques to calculate the next-to-leading order (NLO) effective potential in the Abelian Higgs model, considering a strong transition in a general covariant gauge. This approach is fundamental for obtaining reliable predictions for the bubble nucleation rate, a key parameter in the early evolution of the universe. The results show that the potential obtained by this method can be recovered from a Daisy-resummed potential by modifying the power counting. Furthermore, it was explicitly verified that the solution satisfies the leading-order Nielsen identity, which ensures the gauge independence of the predictions. This is vital for the robustness of calculations, as physics must be independent of the arbitrary choice of gauge. Consistency with previous results for small Higgs condensates was also confirmed, validating the applicability of the new technique in different regimes.

Gauge Coupling Beta Functions Calculated to Four Loops in Standard Model

Researchers have calculated the beta functions of the gauge couplings and the anomalous dimensions of the gauge fields in the unbroken phase of the Standard Model, achieving four-loop precision. This advancement represents the most precise calculation to date for these fundamental quantities, which describe how the strength of fundamental interactions (strong, weak, and electromagnetic forces) varies with the energy scale. Beta functions are crucial in particle physics because they dictate the behavior of force couplings at different energies, a phenomenon known as "running" of the couplings. Understanding this running is essential for predicting how particles behave at very high energies, such as those reached in colliders like the LHC, or under the extreme conditions of the early universe. Anomalous dimensions, in turn, describe how the properties of quantum fields are modified due to interactions with other particles. The unbroken phase of the Standard Model refers to the regime where electroweak symmetry is not yet broken, which is relevant for understanding physics at very high energy scales, long before the Higgs boson acquires its expected value. These high-precision calculations are fundamental for refining theoretical predictions of the Standard Model and for searching for possible deviations that could indicate the existence of new physics beyond this model.

Supersymmetric μνSSM Model Impacts B Meson Decay

A theoretical study has investigated how new physics, specifically within the Supersymmetric Standard Model with neutrinos (μνSSM), influences the rare inclusive decay $B \to X_{\mathrm{s}} l^{+} l^{-}$. This decay is of particular interest because its rate and characteristics can be sensitive to particles and forces not accounted for in the Standard Model of particle physics. Researchers have identified the main contributions to the relevant Wilson coefficients, which are parameters describing the strength of interactions in the decay, and the particles associated with them within the μνSSM. The analysis focused on a systematic scan of the μνSSM parameter space, which allowed for the elucidation of the underlying physical mechanisms governing these dominant contributions. The obtained results are consistent with experimentally allowed regions, suggesting that the μνSSM could offer an explanation for potential future deviations observed in these decays. Experimental constraints from other relevant decays, such as $\bar{B} \to X_{\mathrm{s}}\gamma$, $B_{\mathrm{s}}^{0} \to \mu^{+} \mu^{-}$, and the 125 GeV Higgs boson mass, were also incorporated. A key part of the study was the systematic interference decomposition of the Wilson coefficient contributions to the forward-backward asymmetry (AFB). It was identified that the $C_7C_{10}$ and $C_9C_{10}$ interference terms are the dominant contributions governing the behavior of the AFB in both low- and high-$q^2$ (momentum transferred to the lepton pair) regions. Understanding these contributions is crucial for interpreting future measurements of the AFB, which is an observable sensitive to new physics and could reveal the existence of supersymmetric particles or other extensions of the Standard Model.

Re-evaluating Supersymmetry Limits with LHC Data

A new study has re-examined the constraints imposed by the Large Hadron Collider (LHC) on supersymmetry (SUSY), focusing on the electroweak-ino sector. Using public data from ATLAS Run 2 searches, researchers employed the SModelS v3.0 tool to reproduce and extend the original analysis, also incorporating results from the CMS experiment. This work is crucial for understanding which regions of the vast SUSY parameter space remain unexplored or are compatible with current observations. The ATLAS collaboration recently published an extensive scan of the phenomenological Minimal Supersymmetric Standard Model (pMSSM), with a particular focus on electroweak-inos, which are the supersymmetric partners of gauge and Higgs bosons. This scan aimed to determine how searches for electroweak production of SUSY particles in LHC Run 2 constrained this model. The simulation data (SLHA files) and the constraints from eight individual searches were made publicly available, allowing other research groups to validate and expand upon the results. The research team used this data to evaluate the capability of SModelS v3.0 to replicate the ATLAS constraints. Furthermore, they explored how the inclusion of CMS results alters the picture and what benefits are gained from the statistical combination of analyses from both experiments. The results underscore the need for a broad, multifaceted approach to maximize sensitivity and close loopholes in the extensive SUSY parameter space. The study also discusses the part of the parameter space with light electroweak-inos that remains valid despite the stringent LHC limits, indicating that supersymmetry is not ruled out, but its manifestations might be more subtle than anticipated.

Vector Dark Matter: Parametric Resonance Requires New Inflationary Conditions

A new study explores the viability of vector dark matter (VDM) production via parametric resonance in a Higgsed Abelian sector. This mechanism, which involves the amplification of a dark-Higgs field to generate VDM particles, critically depends on the initial displacement conditions of the dark-Higgs field. Researchers analyzed this problem using a calibrated nonlinear broad-resonance relic map and a stochastic inflationary analysis of the dark-Higgs condensate. The results show that a minimal light-spectator realization fails under standard inflationary duration. For broad resonance and isocurvature constraints to hold, an initial dark-Higgs field displacement of \( φ_0/H_I \gtrsim 3.3 imes10^4 \) is required, where \( H_I \) is the Hubble scale during inflation. However, stochastic equilibrium and finite-duration random walk only produce \( φ/H_I=\mathcal O(1) \). This large mismatch in initial displacement represents a robust, model-independent obstruction to the stochastic branch of VDM production. The study identifies a distinct, classically sourced branch where the condensate tracks a time-dependent minimum, \( φ_0=κH_*/\sqrt{λ_4} \), induced by a negative Hubble-induced mass. In this scenario, the radial fluctuation remains heavy during inflation. This sourced branch modifies the scaling relation of the dark matter particle mass, \( m_X \), from \( m_X\propto λ_4^{5/8}H_I^{-3/2} \) to \( m_X\propto κ^{-3/2}λ_4 H_*^{-3/2} \). The authors derived the simultaneous consistency conditions for this branch, including broad resonance, adiabatic tracking, perturbativity, sub-Planckian displacement, thermal non-erasure, spectator backreaction, and control of inflationary vector fluctuations. These findings suggest that Higgsed-vector resonance is not merely a dark matter production mechanism, but also a sensitive probe of the inflationary and reheating dynamics that determine its initial conditions. The work opens new avenues for exploring the connection between dark matter physics and early cosmological processes, potentially leading to tighter constraints on inflation models and the nature of dark matter.

Low-scale cosmological phase transitions and gravitational waves

A new study has investigated low-scale cosmological phase transitions within a dark Abelian Higgs sector, a gauge theory model describing spontaneous symmetry breaking. The work was motivated by recent evidence for a stochastic gravitational-wave (GW) background reported by pulsar timing array (PTA) collaborations. Researchers quantified the impact of thermal resummation, higher-order matching corrections, and higher-dimensional operators on the phase-transition thermodynamics and the resulting gravitational wave signal. Their analysis reveals that the parameter region favored by current PTA observations lies close to the boundary of validity of the effective field theory. In this regime, higher-dimensional operators become increasingly important. Despite substantial shifts induced by higher-order thermal corrections, the predicted signal from the model remains disfavored by PTA data, even within the controlled region of the theory. Furthermore, the study delineated parameter regions where the dark and visible sectors are thermally and hydrodynamically coupled or decoupled. It also revisited dark matter phenomenology, identifying asymmetric freeze-out as naturally compatible with both the observed relic abundance and the gauge couplings favored by strong phase transitions. These results underscore the importance of systematically controlled finite-temperature calculations for reliable GW predictions from low-scale cosmological phase transitions.

Chebyshev Approximations Improve Feynman Integral Calculations

A new method based on Chebyshev approximations promises to accelerate and simplify the calculation of Feynman integrals, essential mathematical tools for particle physics. This advance addresses one of the most significant computational challenges in collider physics, where prediction precision is crucial for interpreting experimental results from accelerators like the LHC. The technique exploits the analytic properties of these integrals to construct rapidly converging polynomial approximations along a path. The method introduces an adaptive approximation that dynamically samples the parameter space to optimize convergence. Implemented with double-precision arithmetic, it has demonstrated stability across the physical phase space, even in complex two-loop, five-point cases, which are representative of advanced quantum field theory calculations. One of its key advantages is the ability to handle spurious singularities with little to no manual intervention, a recurring problem in existing methods. This Chebyshev approximation proves competitive with state-of-the-art one-fold integral methods. By reducing computational complexity and time, this development could enable more precise and faster theoretical predictions for high-energy processes, facilitating the search for new physics beyond the Standard Model and the detailed characterization of known particles such as the Higgs boson.

Anadi Canepa, Spokesperson for the CMS Experiment at CERN

Italian physicist Anadi Canepa, a senior scientist at Fermilab, has been appointed spokesperson for the CMS (Compact Muon Solenoid) experiment at CERN. This appointment places her at the helm of one of the world's largest scientific collaborations, marking a significant milestone in high-energy particle physics. The CMS experiment is one of the two major detectors operating at the Large Hadron Collider (LHC) in Geneva, Switzerland. The CMS experiment is crucial for particle physics research, having played a fundamental role in the discovery of the Higgs boson in 2012, alongside the ATLAS experiment. Its primary objective is to explore the physics of the Standard Model and search for new physics beyond it, including the quest for supersymmetric particles, dark matter, and other fundamental interactions. The scale of the collaboration is reflected in the thousands of scientists and engineers from around the world who contribute to its design, construction, operation, and data analysis. The role of spokesperson involves coordinating the scientific and technical activities of the collaboration, as well as representing the experiment to the global scientific community and the public. This leadership is essential for maintaining the coherence and direction of such a large-scale project, ensuring that research objectives are met and that the results obtained are effectively communicated.

High-Scale Mirror Dark Matter and Primordial Gravitational Waves

A recent study explores a dark matter model based on a "mirror sector" of the Standard Model, interacting with ordinary matter solely through gravity. This mirror sector would possess the same symmetries as the conventional Standard Model, but with significantly higher energy scales and different couplings. The research focuses on how these differences, including distinct Yukawa couplings for the mirror Higgs boson and mirror fermions, would influence the early evolution of the universe. The authors predict that in this model, a dark phase transition would occur at high temperatures of the baryonic universe, much earlier than the ordinary Standard Model's electroweak phase transition. This transition could be first or second order, depending on the universe's scale and the Yukawa couplings. The case of a second-order phase transition is particularly interesting, as it could leave a detectable imprint on the spectrum of stochastic gravitational waves, a phenomenon potentially observable by future gravitational wave detectors. The work also examines the ability of this high-scale mirror dark matter to form atoms, showing that in certain scenarios it could have both atomic and subatomic components. An approximation of the total equation of state for this mirror dark matter is provided, and the study discusses how this model could reconcile seemingly contradictory observations from galaxy clusters like the Bullet Cluster and Abell 520, which suggest different dark matter properties at various scales.

The Number of Elementary Particles: 17 or Nearly a Thousand?

The question of how many elementary particles truly exist in the universe is more complex than it appears, with answers varying drastically from the 17 particles of the Standard Model to nearly a thousand. This disparity arises from different definitions of "elementary" and the inclusion of hypothetical particles predicted by theories beyond the Standard Model, such as supersymmetry or string theories. The search for a definitive answer is central to particle physics and the fundamental understanding of matter and forces. The Standard Model of particle physics describes 17 fundamental particles: six quarks (up, down, charm, strange, top, bottom), six leptons (electron, muon, tau, and their respective neutrinos), four force bosons (photon, gluon, W and Z bosons), and the Higgs boson. These particles have been experimentally observed and form the basis of our current understanding of matter and fundamental interactions. However, this model does not explain phenomena such as dark matter, dark energy, or gravity, suggesting the existence of additional, yet undiscovered particles. Theories such as supersymmetry (SUSY) propose that each Standard Model particle has a more massive "superpartner," effectively doubling the number of elementary particles. Other extensions, like string theories or those postulating extra dimensions, could introduce an even larger number of particles, including gravitons, axions, or dark matter particles. The existence of these hypothetical particles is the subject of intense research at accelerators like the LHC and in dark matter detection experiments, with the hope of resolving the inconsistencies of the Standard Model and unifying the fundamental forces.

New Constraints on R²-Higgs Inflation from ACT and SPT Data

Researchers have explored the R²-Higgs inflation model by incorporating nonminimal couplings of the Higgs field to the Ricci scalar. This analysis was conducted in light of recent observations from the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT). The study focuses on how dimension-four and dimension-six operators can influence the scalar spectral index (n_s), a key parameter describing primordial fluctuations in the early universe. The results indicate that the inclusion of dimension-six operators, specifically $|Φ|^2 R^2$ and $|Φ|^4 R$, can accommodate an enhanced n_s value, consistent with preferences from combined cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) analyses. Using a doubly covariant formalism, the team found that the same parameter space region explaining the observed n_s value can also induce rapid preheating through the production of Goldstone modes. This preheating mechanism could be crucial for efficient thermalization of the post-inflationary universe. The implication of this finding is significant, as efficient thermalization via this preheating mechanism could help match the inflationary energy scale with the CMB reference scale. This provides a vital link between theoretical inflation models and cosmological observations, offering a way to better understand the processes that occurred in the very first moments of the universe.