HEP2025: 41st Conference on Recent Developments in High Energy Physics and Cosmology [NKUA]

1 Jul 2025, 09:00 → 4 Jul 2025, 17:00 Europe/Athens

“Alkis Argyriadis” Amphitheatre (National and Kapodistrian University of Athens)

posterHEP2025.pdf

timetable.pdf

Tuesday 1 July

09:30 → 11:00 Session 1

09:30 Welcome from the Rector

Speaker: Gerasimos Siasos (NKUA)

10:00 Double Higgs Production via Vector Boson Fusion in SMEFT at the LHC

While gluon fusion dominates Higgs pair production at the LHC, vector boson fusion (VBF) offers a unique window into Beyond the Standard Model (BSM) physics through its distinctive kinematic features and direct sensitivity to Higgs-vector boson interactions. We perform a comprehensive analysis of double Higgs production via VBF in the Standard Model Effective Field Theory (SMEFT), systematically investigating how dimension-6 and dimension-8 bosonic operators. We will present the results from a recent paper, hep-ph/2506.12917.

Speaker: Athanasios Dedes (University of Ioannina)

10:30 Searching for sub-GeV particle dark matter with Spherical Proportional Counters

Spherical proportional counters [1] offer sensitivity to sub-GeV particle dark matter through the combination of single electron threshold, light gaseous targets (H, He, Ne), and highly radio-pure detector construction. Most recently, new constraints on spin-dependent
interactions of dark matter with protons were obtained [2] with the commissioning data of a 140 cm in diameter spherical proportional counter, S140, constructed at LSM using 4N copper with 500 μm electroplated inner layer [3]. The latest physics results will be presented along with the recent developments on the detector instrumentation, namely individual read-out of the multi-anode sensor and electroformation techniques. The path towards DarkSPHERE, a large-scale spherical proportional counter fully electroformed underground at the Boulby Underground Laboratory will be discussed [4].

[1] https://arxiv.org/abs/2502.11870
[2] https://arxiv.org/abs/2407.12769
[3] https://arxiv.org/abs/2008.03153
[4] https://arxiv.org/abs/2301.05183

Speaker: Konstantinos Nikolopoulos (Hamburg University (DE))

11:00 → 11:30 Coffee Break

11:30 → 13:15 Session 2

11:30 Development of Position Sensitive Deep Transient Spectroscopy as a background mitigation tool for CCD-based dark matter searches

The unparalleled sensitivity achieved with skipper CCDs, coupled with ultra-pure high-resistivity substrates (>22 kOhm×cm) and cryogenic operation, has rekindled interest in this technology for low-background experiments (DAMIC@SNOLAB, DAMIC-M, SENSEI, and OSCURA) Such devices offer sub-electron noise resolution, enabling the detection of extremely low-energy interactions critical for rare-event searches, including dark matter detection and neutrino studies. However, exposure to alpha particles and cosmic rays may induce lattice defects with extended annealing times, potentially degrading resolution and increasing background noise. We propose a novel method to mitigate these effects by integrating Laplace Deep-Level Transient Spectroscopy (DLTS) with electrical state pumping through the bias line, combined with the sequential readout of CCDs. Utilizing a lock-in amplifier synchronized to the shift register clock, we achieve pixel-by-pixel readout following charge injection. This approach addresses the inherent loss of timing information in charge-accumulating devices through a frequency scan at the pumping signal level. The method can be applied across various temperature points and injection levels, with an operational range typically spanning 120 K to 200 K. By conducting measurements at multiple injection and thermal conditions, we aim to characterize and mitigate background effects caused by lattice defects.

Speaker: Dr Vagelis Gkougkousis (University of Zurich)

12:00 Constraining Dark Matter with Stellar Objects

Stellar objects provide a unique and powerful laboratory for probing the properties of dark matter. In this talk, I will focus on how the accumulation and annihilation of dark matter can influence stellar evolution, luminosity, and stability. I will present constraints on dark matter particle mass and interactions which offer complementary limits to those derived from cosmological observations and direct detection experiments.

Speaker: Dr Ioannis Dalianis (University of Cyprus)

12:30 Precision Gravity: From the LHC to LISA/ET

Speaker: Rafael Porto (DESY)

13:15 → 14:30 Lunch Break

14:30 → 15:20 Session 3

14:30 Search for Exotic Dark Matter using Radioactive Sources

Search for exotic dark matter, we have developed and operated two complementary experiments utilizing radioactive sources and crystal scintillator detectors. The Global Rare Anomalous Nuclear Decay Experiment (GRANDE) experiment employs the Source-as-Detector technique by embedding isotopes such as ¹³⁹Ce and ⁵⁷Co within CeBr₃ crystal scintillators, enabling high-efficiency self-detection of rare nuclear decay processes. The setup, installed at the Yemilab underground laboratory with a 1000-m rock overburden, incorporates a 4π BGO veto system and comprehensive passive shielding to minimize background. Initial data from CeBr₃:¹³⁹Ce have enabled a new branching ratio limit for the 65.86 keV M1 gamma transition, probing axion-like particle signatures.
In parallel, the Kyungpook National University Advanced Positronium Annihilation Experiment (KAPAE) Phase II detector targets invisible decays of positronium, particularly those involving dark photons, mirror world, and other exotic dark matter. The detector consists of a 5 × 5 array of BGO scintillators (30 × 30 × 150 mm³) coupled to silicon photomultipliers for high sensitivity to rare events and precise energy measurement. Operation at -37°C in the Yemi lab, with multi-layer lead and oxygen-free copper shielding, ensures low background conditions. We present the combined results and performance evaluation from both experiments, highlighting new constraints on exotic dark matter scenarios.

Speaker: HongJoo Kim (Kyungpook National University)

15:00 First Operations of the MIGDAL experiment at NILE

Many dark matter experiments are exploiting the Migdal effect, a rare atomic process, to improve sensitivity to low-mass WIMP-like dark matter candidates. However, this process is yet to be directly observed in nuclear scattering. The MIGDAL experiment aims to make the first unambiguous measurement of the Migdal effect in nuclear scattering. A low-pressure optical Time Projection Chamber is used to image in 3-dimensions the characteristic of a Migdal event: an electron and a nuclear recoil track sharing a common vertex.  Nuclear recoils are induced using fast neutrons from a DD source, which scatter in the gaseous volume of the detector. The experiment is operated with 50 Torr of CF4 using two glass GEMs for charge amplification. Both scintillation light and ionisation charge are read-out, and these measurements are combined for full-track reconstruction. 

In this talk I will present the results of the commissioning of the detector with fast neutrons at the Neutron Irradiation Laboratory for Electronics (NILE) at the Rutherford Appleton Laboratory in the UK along with the first science operations of the experiment.

Speaker: Lex Millins (University of Birmingham (GB))

15:20 → 15:50 Young Researchers (part A)

15:20 Topological Defect Lines in the Free Boson RCFT

We explore the enhanced symmetry structure that scale invariance introduces into quantum field theory, with a particular focus on two-dimensional systems. In this setting, scale invariance naturally extends to conformal invariance, enabling the construction of rich Hilbert space structures, including Fock spaces and Verma modules. We then specialize to the compactified free boson, a central example in two-dimensional conformal field theory. Finally, we introduce the concept of defect lines, discussing the criteria under which such defects become topological and outlining how they can be employed to connect or modify conformal field theories while preserving specific symmetries.

Speaker: Anastasios Katsipis (EKPA)

15:35 Equivariant Localization in Supergravity

Constructing explicit solutions to higher dimensional supergravity theories has proven notoriously difficult. Equivariant localization circumvents these problems by offering a novel method for computing BPS observables in the dual field theories of $AdS \times M$ backgrounds arising in the low-energy limits of string theory and M-theory. In general given appropriate symmetries acting on a space, equivariant localization
reduces certain integrals over the whole space, to a finite sum over its
fixed points under the action. We apply the localization formula to
compute the central charge of a $\mathcal{N} = 1$ SCFT dual to a $AdS_5×M6$ solution
of eleven dimensional supergravity, using the holographic dictionary. We
show that this can be done just by using topological data and without
solving the Killing spinor equations. We then compare with known results
obtained from explicit supergravity solutions.

Speaker: Antonis Stefas (National and Kapodistrian University of Athens)

Wednesday 2 July

09:30 → 11:00 Session 4

09:30 New physics implications from the first CEvNS observation using solar $^8$B neutrinos

I will discuss the implications of the first observation of coherent elastic neutrino–nucleus scattering (CEνNS) induced by $^8$B solar neutrinos in dark matter direct detection experiments. By analyzing recent CEvNS data from the XENONnT and PandaX-4T collaborations, I will present new constraints on novel neutrino interactions and non-trivial electromagnetic properties.

Speaker: Dr Dimitrios Papoulias (IFIC (CSIC/Valencia U.))

10:00 Cosmic Microwave Background measurements with microwave telescopes

One of the major challenges of modern cosmology is the detection of primordial B‐mode polarization anisotropies in the CMB, a smoking gun for inflation. The B-modes is the most promising direct observational signature of the inflationary phase that is thought to have taken place in the early Universe, generating primordial perturbations, producing Standard Model elementary particles and giving its generic features to our Universe (flatness, homogeneity. . .). Science aims at testing models of inflation by measuring or putting upper limits on r, the ratio of tensor fluctuations to scalar fluctuations. A detection of tensor modes would yield the first evidence of quantum gravity and point to inflationary physics near the energy scale associated with grand unified theories, probing energy scales far beyond the reach of the LHC or any conceivable collider experiment, and providing additional evidence in favour of the idea of the unification of forces.

Results from Planck, BICEP and other previous and current experiments have shown how challenging is the search for primordial B-mode polarization because of many difficulties: weakness of the expected signal, instrumental systematics that could possibly induce polarization leakage from the large E signal into B, polarized foregrounds (dust) larger than anticipated…

A review of results and projects will be given, with emphasis on two experiments:

• CMB-Stage 4 is the next generation project, aiming to reach a precision on r of 5x10-4, (5σ discovery of r>3x10-3 or an upper limit of 10-3 at 95% C.L.), allowing the detection of r for the favoured models of inflation. CMB-S4 is being reconfigured to be installed on the Chilean Andes. It will target additional rich science goals including the measurement of the number of light relic species with an error of 0.03, corresponding to a freeze-out temperature TF>1GeV, of the sum of the masses of neutrinos, the study of galaxy clusters and of the millimetre-wave transient sky.

• QUBIC, now with the technical demonstrator installed in Argentina and starting commissioning observations of the sky, is designed to address the foreground subtraction with a novel kind of instrument, a Bolometric Interferometer, with spectral imaging, allowing to better disentangle primordial B-polarization from complex, decorrelated foregrounds.

On this aspect, the contribution of the Pasiphae projects, in Crete and Sutherland, will be outlined.

Speaker: Sotiris Loucatos (Université Paris-Saclay (FR) and APC)

10:30 Muon Performance in ATLAS with Run-3 Data

Muons play a central role in many physics analyses at the LHC, offering clean signatures and reliable performance. With the start of Run 3, significant updates to the ATLAS muon system -- including the installation of the New Small Wheel and a new reconstruction framework -- make it important to review and optimize combined muon performance. This talk presents the latest results based on 2022-2023 data, focusing on efficiency, momentum scale and resolution, and isolation performance.

Speaker: Stylianos Angelidakis (National and Kapodistrian University of Athens (GR))

11:00 → 11:30 Coffee Break

11:30 → 13:00 Session 5

11:30 ATLAS highlights

Speaker: Anna Sfyrla (Universite de Geneve (CH))

12:15 CMS highlights

Speaker: Hafeez Hoorani (Imperial College (GB))

13:00 → 14:30 Lunch Break

14:30 → 16:00 Session 6

14:30 CMS HGCAL Status

The CMS Collaboration is getting ready to replace its current endcap calorimeters with a high-granularity calorimeter (HGCAL) for the HL-LHC era, featuring a previously unrealised transverse and longitudinal segmentation, for both the electromagnetic and hadronic compartments. The current design uses silicon sensors for the electromagnetic section and high-irradiation regions of the hadronic section, while in the low-irradiation regions of the hadronic section plastic scintillator tiles equipped with on-tile silicon photomultipliers (SiPMs) are used. In this talk we present the ideas behind the HGCAL, the current status of the project, the evolution of the detector systems with what is learned from the beam tests as well as the challenges that lie ahead. Insights to future systems will be also discussed.

Speaker: Dimitra Tsionou (National Taiwan University (TW))

15:00 Measurements of the Higgs boson differential and production mode cross sections in the H->ZZ*->4l channel at √s = 13.6 TeV with the ATLAS detector

A measurement of Higgs boson properties is pp collisions and the 4l decay channel using the data collected by the ATLAS detector in 2022 and 2023 at √s = 13.6 TeV is presented. Inclusive fiducial and total cross-sections are measured in addition to the differential cross-sections for the observables: pT4l, |y4l|, njets and the invariant mass of the sub-leading lepton pair in the event, m34. Furthermore, production cross-sections for individual Higgs boson production modes are probed for the first time at this centre-of-mass energy. All measurements are in agreement with the Standard Model predictions.

Speaker: Konstantinos Sioulas (National and Kapodistrian University of Athens (GR))

15:20 Data-Driven Background Estimation for tt̄H(H→bb̄) in the Dilepton Channel with CMS

Prospects for the measurement of top quark–antiquark associated Higgs boson production ($t\bar{t}H$) in Run 3 and the HL-LHC era are presented. The study is performed in the opposite-sign dilepton channel, focusing on the Higgs boson decay to bottom quarks ($H \rightarrow b\bar{b}$). A novel approach is introduced, and projection studies for the HL-LHC are explored. The analysis strategy relies on a data-driven method, the Tag Rate Function, to estimate the dominant background contribution from $t\bar{t}$ events. Improvements in the analysis are also investigated by applying the Run 3 tagging algorithm to simulated events.

Speaker: Haris Painesis (National and Kapodistrian University of Athens (GR))

15:40 Muon Reconstruction in the ATLAS Experiment for Phase-II

Muon Reconstruction in the ATLAS Experiment for Phase-II

Towards the High Luminosity LHC era which is going to bring major improvements to the performance of the accelerator, the ATLAS detector is expected to be operational in a demanding environment. To maintain the performance of the track reconstruction under these conditions, the ATLAS software is undergoing a significant Phase II Upgrade. The ATLAS Collaboration has chosen to adopt ACTS (A Common Tracking Software), an experiment-independent toolkit for charged-particle track
reconstruction.
This contribution presents the latest developments and results of the muon reconstruction software as it is being upgraded for Run 4. Specifically, the strategy for integrating ACTS into offline muon reconstruction includes the development of a new tracking geometry schema, updated navigation models, and new reconstruction algorithms.

Speaker: Dimitra Amperiadou (Aristotle University of Thessaloniki (GR))

16:00 → 16:30 Coffee Break

Thursday 3 July

09:30 → 11:00 Session 7

09:30 On Final Results From the Muon $g\textrm{-}2$ Experiment at Fermilab

The Muon $g\textrm{-}2$ Experiment at Fermilab has measured the muon anomalous magnetic moment, $a_\mu$, with the unprecedented precision of $127$ ppb, leveraging data from Runs $1$–$6$ that is $21$ times larger than its Brookhaven predecessor. We will present the experiment’s final result, which will serve as a lasting benchmark for testing the Standard Model with high precision. The measurement was performed using a storage ring with a highly uniform magnetic field, where a precise understanding of beam dynamics was vital to determine the muon anomalous precession frequency. Key to this effort were advanced simulation tools including COSY INFINITY, precise fringe field modelling, and calculations of beam dynamics characteristics like tunes and chromaticity. We will conclude by discussing the implications of the Muon $g\textrm{-}2$ Theory Initiative's $2025$ white paper and the Muon $g\textrm{-}2$ Experiment's result for the Standard Model.

Speaker: Dr Eremey Valetov (Michigan State Univeristy)

10:00 Advancements in Resistive Silicon for 4D Tracking: Sensor Design, Environmental Testing, and Machine Learning-Based Reconstruction for Next-Generation HEP Detectors

Speaker: Gaetano Barone (Brown University)

10:30 Massive spin-2 particles: Bimetric gravity and its applications to cosmology

GR is understood as the unique theory of a single massless spin-2 particle, the graviton. From a particle physics perspective, the most natural and minimal extension to gravity entails the introduction of a massive spin-2 field that does not spoil the successful predictions of GR.
Bimetric gravity is a theory of gravity that posits the existence of two interacting and dynamical metric tensors. The spectrum of bimetric gravity consists of a massless and a massive spin-2 particle. The form of the interactions between the two metrics is constrained by requiring absence of the so called Boulware-Deser ghost. In this talk, I will discuss extended bimetric gravity theories, primarily focusing on their applications in cosmology.

Speaker: Ioannis Gialamas (National Institute of Chemical Physics and Biophysics)

11:00 → 11:30 Coffee Break

11:30 → 13:15 Session 8

11:30 TBA

Speaker: Johannes Henn (Humboldt University Berlin)

12:15 Fibre Inflation

Speaker: George Leontaris (University of Ioannina (GR))

12:45 Towards numerical two-loop integrand reduction

We present a method for the integrand-level reduction of two-loop helicity amplitudes in both d=4−2ϵ and d=4 dimensions. The amplitude is expressed in terms of a set of Feynman integrals and their coefficients that depend on the external kinematics. The method presented in this talk, in conjunction with the ongoing development of the computational framework HELAC-2LOOP, paves the road for the construction of an automated program for numerical two-loop amplitude calculations.

Speaker: Giuseppe Bevilacqua (NCSR Demokritos)

13:15 → 14:30 Lunch Break

14:30 → 16:00 Session 9

14:30 Probing X→YH signatures in the bbγγ final state with parametric neural networks

Searches for resonant Higgs boson pair production (HH) have been gaining increasing attention at the Large Hadron Collider (LHC) experiments. In particular, the CMS experiment has reported an excess in the X$\rightarrow$Y(bb)H(γγ) final state, with a local (global) significance of 3.8σ (2.6σ) for $m_\text{X}\approx$ 650 GeV and $m_\text{Y}\approx$ 90 GeV. This observation aligns with other CMS excesses, such as one found around $m_\text{H}$ = 95 GeV in the search for a low-mass H$\rightarrow$γγ. To shed light on these results, we present a complementary analysis targeting the X$\rightarrow$Y(γγ)H(bb) final state. The strategy used for this analysis revolves around the usage of a parametric neural network, designed to probe a wide range of mass hypotheses efficiently and deliver competitive exclusion limits. Extensions of the analysis strategy to searches in similar final states are also discussed, laying the groundwork for a framework that can be used to cover all of the resonant HH and YH$\rightarrow$bbγγ efforts.

Speaker: Manos Vourliotis (Univ. of California San Diego (US))

15:00 Study of Higgs boson pair production in the $b\bar{b}\gamma\gamma$ final state with $308~\mathrm{fb}^{-1}$ of data collected at $\sqrt{s} = 13~\mathrm{TeV}$ and $13.6~\mathrm{TeV}$ by the ATLAS experiment

One of the biggest open questions in particle physics today is how the Higgs boson interacts with itself. This “self-coupling” plays a pivotal role in determining the shape of the energy potential of the Higgs field, influencing both the early Universe evolution and the mechanism that gives mass to elementary particles. We present a search for Higgs boson pair production in the $b\bar{b}\gamma\gamma$ final state, combining the full Run2 dataset ($140~\mathrm{fb}^{-1}$ at $13~\mathrm{TeV}$) with early Run 3 data ($168~\mathrm{fb}^{-1}$ at $13.6~\mathrm{TeV}$) collected by the ATLAS detector. This is the first ATLAS result based on over $300~\mathrm{fb}^{-1}$ of $pp$ collision data. The measured Higgs pair production cross section, normalised to the Standard Model prediction, is $\mu_{HH} = 0.9^{+1.4}_{-1.1}$ ($1.0^{+1.3}_{-1.0}$ expected), leading to a 95% CL upper limit of $\mu_{HH} < 3.8$. At the same confidence level, the Higgs self-coupling modifier is constrained to $-1.7 < \kappa_\lambda < 6.6$ ($-1.8 < \kappa_\lambda < 6.9$ expected).

Speaker: Spyros Merianos (Aristotle University of Thessaloniki (GR))

15:20 Study of tt̄H(H→bb̄) Events in the Opposite-Sign Dilepton Channel with the CMS Experiment Focusing on the Mass Reconstruction Method

The study of tt̄H production in the opposite-sign dilepton channel, where the Higgs boson decays to a pair of b quarks (H → bb), is presented in the context of Run 3 and HL-LHC projections. A dedicated Mass Reconstruction Method is used to identify the Higgs jet pair by evaluating all jet combinations, scanning over top and W masses, and ranking them with parton distribution function weights. Its performance has been validated on Run 2 data and further evaluated on simulated Run 3 sample, aiming to improve the overall signal significance and Higgs reconstruction efficiency in this challenging final state.

Speaker: Polytimi Iosifidou (National and Kapodistrian University of Athens (GR))

15:40 $t\bar{t}t\bar{t}$: NLO QCD corrections in production and decays for the $4\ell$ and $3\ell$ channels

We present results for four-top quark production at the LHC at next-to-leading order (NLO) accuracy in perturbative QCD, focusing on the $4\ell$ and $3\ell$ decay channels. The QCD corrections are included in both the production and decay stages using the narrow-width approximation, which preserves spin correlations at NLO accuracy in QCD without any approximation. We briefly summarize the impact of higher-order QCD effects and compare the various narrow-width approximation treatments applied for both decay channels.

Speaker: Nikolaos Dimitrakopoulos (RWTH Aachen University)

16:00 → 16:30 Coffee Break

16:30 → 19:00 Hellenic Society for the Study of High Energy Physics

16:30 Memorial for A. Filippas and T. Papadopoulou

16:50 Report from General Secretary of Research & Innovation representative to CERN

17:10 News from Open Symposium on the European Strategy for Particle Physics

17:40 Report from EESFYE’s liaison to Industry

17:50 Report from EESFYE’s representative to ACCU

Speaker: Ioannis Kopsalis (National Technical Univ. of Athens (GR))

18:00 Report from EESFYE's outreach coordinator

Speaker: Christine Kourkoumelis (National and Kapodistrian University of Athens (GR))

18:10 CERN Alumni Network Programme and the Athens Group

The report will make an update about the Network,which celebrated recently its 8th Anniversary and is counting more than eleven thousand members. The activity of Athens group will be mentioned.

Speaker: Michele Barone (Nat. Cent. for Sci. Res. Demokritos (GR))

18:20 Report from the EESFYE Executive Committee

Speaker: Katerina Zachariadou (University of West Attica (GR))

18:40 Round table for EESFYE matters

20:30 → 23:30 Dinner

Friday 4 July

09:30 → 11:00 Session 10

09:30 Axion-Mediated Photonic Transitions in High-Q Dielectric Resonators

Axions, originally proposed to solve the strong CP problem in quantum chromodynamics, are also among the leading candidates for dark matter. A key challenge in their detection lies in their extremely weak coupling to electromagnetic fields. In this work, we present a theoretical framework for substantially enhancing axion-photon interactions using high-finesse spherical dielectric resonators. These structures support long-lived Mie resonances with mode spacing tunable to match the expected axion mass range. We analytically investigate axion-mediated photon-to-photon transitions between resonant modes using first-order perturbation theory and derive the corresponding selection rules via group-theoretical arguments. Our results predict an enhancement of transition rates by more than ten orders of magnitude compared to interactions in homogeneous space, offering a resonantly amplified pathway for axion detection. This approach may also inspire analogous strategies in engineered axion-like quasiparticle systems. A detailed analysis is available at https://arxiv.org/abs/2506.04666

Speaker: Evangelos Almpanis

10:00 Probing the Universe's Topology through a Quantum System?

The global topology of the Universe could, in principle, affect quantum systems through boundary condition constraints. We investigate this connection by analyzing how compact, flat, cosmologically inspired topologies, specifically the $3-$Torus ($E_1$) and half turn space ($E_2$), influence the energy eigenvalues of a quantum particle in the bound state of a 3D Dirac delta potential. Using rigorous renormalization techniques, we derive the equations satisfied by the energy eigenvalues in each topology and develop a systematic method to compute spectral shifts. Our results reveal that each topology induces characteristic deviations in the energy spectrum. In the large$-L$ limit ($L \gg g_R$), to leading order, the energy eigenvalues for both the $E_1$ and $E_2$ spaces can be written in the unified form
$E\simeq -\frac{\hbar^2}{2mg_R^2}(1 + C_\Gamma\,\frac{2g_R}{L}\,e^{-L/g_R})$,
where the topology-dependent coefficient is
$C_\Gamma = 6$ for the $E_1$ space and $C_\Gamma = 4$ for the $E_2$ space, $g_R$ is the characteristic length scale of the quantum system, and $L$ is the side physical length of the fundamental cubic region.
Using the three‐dimensional Dirac potential as a toy model, we show that at the current cosmic epoch ($a=1$), these topological effects are exponentially suppressed, rendering direct observation infeasible. However, such effects may become measurable in the early Universe, when the physical size of the particle horizon is comparable to the characteristic scale of the quantum system. While immediate experimental verification remains impractical, our work offers theoretical insight into how global cosmic topology might manifest in quantum bound states and may inform future studies of early‐Universe quantum phenomena.

Speaker: Evangelos Achilleas Paraskevas (University of Ioannina)

10:20 Entanglement Entropy in Field Theory and Cosmology

We compute the entanglement entropy of a massless scalar field in various curved backgrounds. We discuss applications to cosmology and the calculation of c- and a-functions in field theory.

Speaker: Kostas Boutivas (University of Athens)

10:40 Resolving the $H_0$ ​ tension in Horndeski Gravity

We show that the $H_0$ tension can be alleviated in the framework of Horndeski gravity. In particular, since the terms depending on $G_5$ control the friction in the Friedmann equation, we construct specific sub-classes in which it depends only on the field's kinetic energy. Since the latter is small at high redshifts, namely at redshifts which affected the CMB structure, the deviations from $\Lambda$CDM cosmology are negligible, however as time passes it increases and thus at low redshifts the Hubble function acquires increased values in a controlled way. We consider two Models, one with quadratic and one with quartic dependence on the field's kinetic energy. In both cases we show the alleviation of the tension, resulting to $H_0 \approx 74$ km/s/Mpc for particular parameter choices. Finally, we examine the behavior of scalar metric perturbations, showing that the conditions for absence of ghost and Laplacian instabilities are fulfilled throughout the evolution, and we confront the models with Supernovae type Ia and Cosmic Chronometer data.

Speaker: Maria Petronikolou (National Technical University of Athens, School of Applied Mathematical and Physical Sciences)

11:00 → 11:30 Coffee Break

11:30 → 13:00 Session 11

11:30 The Giant Radio Array for Neutrino Detection (GRAND) - Prototypes and Path Forward

The Giant Radio Array for Neutrino Detection (GRAND) is an ambitious international effort to detect ultra-high-energy cosmic particles through their radio signatures in the atmosphere. Three prototype arrays are currently operational across China, Argentina, and France, validating the detection concept with autonomous radio antennas. These testbeds have delivered promising results in calibration, noise characterization, and cosmic-ray event reconstruction. Building on this, the next phase—GRAND10k—aims to deploy large-scale arrays in both hemispheres. This talk will summarize the current status, key findings from the prototypes, and the roadmap toward a full-scale observatory.

Speaker: Dr Stavros Nonis (Hellenic Open University)

12:00 GRAND experiment: Antenna Modeling, Galactic Background and Trigger Studies

This talk will present recent technical contributions to the GRAND experiment from our group, focusing on key aspects of the detector development and data analysis chain. These include detailed antenna simulations using NEC and MATLAB, the construction of a sky-noise model based on the Galactic radio background, and contributions to the development of the GRANDlib software. Current work involves studies on trigger performance and calibration methods using Galactic signals, in view of optimizing the detection of air-shower events in the prototype arrays.

Speaker: George Vittakis (University of the Aegean)

12:20 Probing Standard Model and Beyond via Elastic Neutrino Electron Scattering at the DUNE Near Detector

The Deep Underground Neutrino Experiment (DUNE) stands at the frontier of neutrino physics, offering unprecedented opportunities to explore both established and new physics. In this talk, I will present a study on the potential of the DUNE Near Detector (ND) to constrain Standard Model parameters and probe new physics through elastic neutrino–electron scattering. The study uses truth-level simulations and applies advanced parametric reconstruction techniques to analyze realistic background conditions across both on-axis and off-axis neutrino fluxes. I will discuss how this approach enables a precise determination of the weak mixing angle and provides sensitivity to potential signatures due to the violation of lepton flavor unitarity, illustrating the broader physics reach of DUNE-ND.

Speaker: Panteleimon Melas

12:40 Measurement of the top quark mass from boosted top antitop processes in proton proton collisions.

This analysis targets the measurement of top quark mass from boosted top antitop pair production in proton proton collisions at $\sqrt{s} = 13.6~\mathrm{TeV}$. The top quark is the heaviest fermion and its properties are important parameters in the Standard Model. It decays mainly via weak interaction, so we can have direct measurements of its mass from the decay products. In the boosted topology, products are highly collimated and reconstructed in one large cone jet. The jet mass distribution and especially the peak position is directly associated to top mass. Studying the fully hadronic decay channel gives the advantage of having no missing transverse momentum, hence all decay products are fully reconstructed. Additionally, backgrounds with leptons in final state are small or negligible. The dominant background in this phase space comes from QCD multijet events. To suppress QCD we use an MVA method, which is a study in a previous analysis in our group. In order to estimate the shape of multijet distribution, a data driven technique is used. Finally, to extract the top quark mass, we perform a parametric simultaneous fit on signal region and QCD enriched control region. The workflow for the measurement is so far tested on RUN 2 datasets and will be applied to RUN 3.

Speaker: Eirini Siamarkou (National Technical Univ. of Athens (GR))

13:00 → 14:30 Lunch Break

14:30 → 15:45 Young Researchers (part B)

Young Researchers (part B)

14:30 Study of pp→HH→bbγγ production at \sqrt{s}=13TeV using ATLAS simulations

Study of the $HH\rightarrow b\bar{b}γγ$ decay channel in pp collisions at $\sqrt{s} =13TeV$ using simulated events from the ATLAS experiment. A neural network classifier is trained on kinematic features to enhance signal-to-background discrimination. The expected sensitivity to the signal is quantified using maximum likelihood techniques.

Speaker: Eliana Kapsali (Aristotle University of Thessaloniki (GR))

14:45 Implementation of the Artificial Retina Method for Offline Analysis of Test-Beam Data for the ATLAS New Small Wheel Micromegas

The relatively new technology of resistive-strip micromegas [1], which has been implemented in the ATLAS New Small Wheel (NSW) upgrade [2], has been tested in a series of performance studies. NSW modules have been irradiated at the GIF++ facility at CERN, in order to simulate charge accumulation of the high $\gamma$-Ray rate of the HL-LHC [3] environment. In this study, the SM1-M40 spare module of the NSW was subjected to a pion beam at the SPS-H8B line.

Micromegas performance studies, which emphasise on spatial resolution and detector efficiency, require the generation of an external particle track to probe the detector under testing. The Artificial Retina technique [4] provides a set of weights for the cluster centroids, which reproduce the particle track, thus supporting the reconstruction method. A series of Monte-Carlo simulations have been conducted, which provide insight on the sub-methods and limitations of the Artificial Retina. Additionally, the method has been implemented on the analysis of test-beam data and demonstrates its effectiveness.
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References
[1] T. Alexopoulos, et al. (2011) A spark-resistant bulk-micromegas chamber for high-rate applications. Nuclear Instruments and Methods in Physics Research A 640, pp. 110 – 118.
[2] Bernd Stelzer, for the ATLAS Muon Collaboration (2016) The New Small Wheel Upgrade Project of the ATLAS Experiment. Nuclear and Particle Physics Proceedings 273-275, pp. 1160 – 1165
[3] I. Béjar Alonso (Eds), et al. (2020) High-Luminosity Large Hadron Collider (HL-LHC): Technical design report. CERN Yellow Reports: Monographs, CERN-2020-010, Genève, Switzerland.
[4] L. Ristori (2000) An artificial retina for fast track finding. Nuclear Instruments and Methods in Physics Research A 453, pp. 425 – 429.

Speaker: Athanasios Koutsostathis (National Technical Univ. of Athens (GR))

15:00 Studies on high-η muons with the Run3 data of the ATLAS experiment.

High-η muons (2.5 <|η| < 2.7) are important in multi-lepton final states, where geometrical acceptance is crucial. Due to the absence of ID coverage at |η| > 2.5 and the increased background rates, muon reconstruction and identification present certain challenges in that region.

This presentation will address the validation of the criteria for muon identification in this geometrical region for Run3, after the installation of the New Small Wheel in the ATLAS’ Muon Spectrometer.

Speaker: Nikolaos Kamaras (National and Kapodistrian University of Athens (GR))

15:15 Study of anomalous Quartic Gauge Couplings in pp -> ZZ -> 2l2v production at √s = 13 TeV in ATLAS

The standard model effective filed theory provides a theorical framework to explore new physics beyond the electroweak scale. Within this framework, dimension-eight operators can be introduced to the effective Lagrangian, modifying vector boson scattering (VBS) cross sections via anomalous Quartic Gauge Couplings (aQGCs). In this study, focusing on the $pp\rightarrow ZZ \rightarrow 2\ell2\nu$ process in the ATLAS experiment at $\sqrt s $=13 TeV, limits of the aQGCs were derived using a binned likelihood fit to the transverse mass ($m_{TZZ}$). In the analysis full run 2 data as well as Monte Carlo simulated events are used.

Speaker: Eleni Papadimitriou (Aristotle University of Thessaloniki (GR))

15:30 Identification of Non-Collision Background Muons in the ATLAS Detector During Run-3

The purpose of this study is to identify Non-Collision Background (NCB) muons in the ATLAS detector. These muons are produced from interactions with the beam pipe during Run-3 operations and enter the detector from outside—unlike collision muons, which emerge from the interaction point. NCB muons usually travel nearly parallel to the beam line and can potentially contaminate physics analyses as well as detector calibration and monitoring studies. It is therefore essential to distinguish them from collision-induced muons. This analysis uses segment-level information from the New Small Wheel (NSW)—a precision tracking and trigger system recently installed in the forward region of ATLAS—based on both simulated and real data samples to study and compare NCB and collision segments.

The analysis includes a detailed comparison between horizontal and collision-induced muons, highlighting differences in geometrical and timing properties across entrance and exit sides of the detector. In addition, segment matching techniques are applied within and across detector sides, and correlations with MDT information are explored. Based on these studies, selection criteria are defined to identify NCB segments and veto events with significant background activity. The selections derived from monte carlo samples are then validated on real data to evaluate its effectiveness and robustness.

Speaker: Anna Vasileiadou (National and Kapodistrian University of Athens (GR))