MEDEX'25

Jun 23, 2025, 8:00 AM → Jun 27, 2025, 2:45 PM Europe/Prague

Conference room (Institute of Experimental and Applied Physics, CTU in Prague)

The MEDEX (Matrix Elements for the Double beta decay EXperiments) conference is dedicated to presenting different methods of nuclear matrix elements (NMEs) calculations in connection with the nuclear double beta decay processes.

Several double beta decay experiments are collecting data with enriched isotope masses ranging from a few kilograms to over 100 kg, with some reaching tonnes, and there are ongoing plans to further extend these isotope quantities. These efforts involve various isotopes and utilize a broad array of detection techniques (e.g., KamLAND-Zen, SNO+, EXO-200/nEXO, NEXT, LEGEND (GERDA + Majorana), CUORE, CUPID, SuperNEMO, AMoRE, BINGO, COBRA, etc.). Experiments of this scale place enormous demands on the progress and reliability of nuclear matrix element calculations. Additionally, research in special modes of ββ decay, such as β⁺β⁺ or 2νECEC, is becoming increasingly interesting from both experimental and theoretical perspectives (e.g., XENON1T, COBRA, TGV, etc.). Further development of the theory behind these processes is crucial for the continuation of experimental activities in this field.

The extended scope of the MEDEX conference also includes other areas where the same techniques are applied, namely in dark matter detection, rare electroweak decays, and neutrino-nucleus scattering processes.

The MEDEX conference has been organized every second year since 1997 by the Institute of Experimental and Applied Physics, Czech Technical University in Prague (Prague, Czech Republic); the University of La Plata (La Plata, Argentina); and the University of Jyväskylä (Jyväskylä, Finland).

MEDEX25_logo.png

Circulars MEDEX25_1st_Circular.pdf MEDEX25_2nd_Circular.pdf

MEDEX'25 Agenda MEDEX25_Agenda_v4.pdf

PoS PoS_slides.pdf

Mon, June 23

8:45 AM MEDEX'25 Registration & Badges issuing

9:30 AM Opening of the MEDEX’25 conference

Theory

1 New probes of rates of neutrinoless double beta decays

Neutrinoless double beta decay (NDBD) involves virtual transitions through states of an intermediate nucleus. The wave functions of these states can be efficiently probed by the ordinary muon capture (OMC), a process where a nucleus captures a negative muon from the lowest atomic orbital. The OMC on the NDBD daughter populates the states of the intermediate nucleus of the NDBD, like in the case of $^{136}$Xe NDBD the capture on $^{136}$Ba populates the states of $^{136}$Cs. Since both processes have similar momentum exchanges, they can populate intermediate states of high excitation energy and high angular momentum. This is how the OMC probes effectively the wave functions of all the intermediate states relevant for the NDBD. First attempts to correlate the NDBD amplitudes through the intermediate states and the OMC rates to these states have recently been performed in this NDBD system.

Furthermore, the rates of NDBD and OMC depend on the value of the weak axial coupling, $g_{\rm A}$. In particular, the NDBD rate is highly sensitive to the effective value of $g_{\rm A}$. This effective value can be probed in the NDBD-relevant momentum exchanges by OMC, but also recently by charged-current neutrino-nucleus scattering of stopped-pion neutrinos on $^{127}$I.

Speaker: Jouni Suhonen (University of Jyväskylä)

10:15 AM Coffee break

Theory

2 Precission double-beta nuclear matrix elements

The exploration of physics beyond the Standard Model in nuclear physics is closely tied to investigating rare electroweak transitions. The most promising process is neutrinoless double-beta decay ($0\nu\beta\beta$), a nuclear transition where two neutrons simultaneously convert into two protons with the emission of only two electrons. If observed, this second-order decay would prove that neutrinos are Majorana particles, shed light on the existence of massive neutrinos, and help explain the matter–antimatter imbalance in the universe. The half-lives depend on the square of the nuclear matrix elements (NMEs), which must be computed since $0\nu\beta\beta$ has not yet been observed.

In this talk, I will discuss computations of the NMEs at next-to-next-to-leading order (N$^2$LO) within the nuclear shell model (NSM) and QRPA frameworks. These calculations aim to reduce uncertainty in the NMEs. First, I will present the contribution of ultrasoft (low-momentum) neutrinos, which can dominate in scenarios involving light sterile neutrinos, then show the full N$^2$LO NME results, and provide further detailed analysis.

Finally, if time permits, I will briefly address the recent addition of novel next-to-leading order (NLO) terms in two-neutrino $\beta\beta$ decay NMEs within the NSM, focusing on transitions from the $0^+$ ground state to the first $0^+$ excited states.

Speaker: Daniel Castillo Garcia (University of Barcelona/ICCUB)

3 Calculation of kinematic factors for beta and double-beta decays

The kinematic factors in beta and double-beta decays are essential for understanding the decay rates, energy distributions and angular correlations of the emitted leptons, and also represent a testing ground for various BSM effects. Thus, their accurate calculation is very needed as theoretical support for experiments. In my talk I’ll give a review of the present status of these calculations including various new effects that can alter the electron spectra and may be signals for new physics.

Speaker: Sabin Stoica (International Centre for Advanced Training and Research in Physics)

4 Loop Effects in Probing Lepton Number Violation

The discovery of lepton number violation would be a clear sign of physics beyond the Standard Model, with neutrinoless double beta decay (0νββ) as its most sensitive probe. Within the SMEFT framework, we show that one-loop effects can significantly strengthen tree-level bounds on new-physics scales for several dimension-7 operators across flavours. Using UV model examples, we illustrate the interplay between 0νββ contributions from dimension-7 and loop-induced dimension-5 SMEFT operators.

Speaker: Lukas Graf (Nikhef/Charles University)

12:15 PM Lunch

Experiment

5 Latest results from the CUORE experiment

The Cryogenic Underground Observatory for Rare Events (CUORE) is the first bolometric experiment searching for 0νββ decay that has successfully reached the one-tonne mass scale. The detector, located at the LNGS in Italy, consists of an array of 988 TeO$_2$ crystals arranged in a compact cylindrical structure of 19 towers. CUORE has been collecting data continuously at ~10 mK since 2017, achieving a 90% uptime and amassing over 2.5 tonne-years of TeO$_2$ exposure. In March 2024 the collaboration released the most recent result of the search for 0νββ, corresponding to two tonne-year TeO$_2$ exposure. This is the largest amount of data ever acquired with a solid state cryogenic detector, which allows for further improvement in the CUORE sensitivity. In this talk, we will present the current status of the CUORE search for 0νββ with the updated statistics of two tonne yr exposure. This statistics also allows for one of the most detailed background reconstructions in the field and enables a precision measurement of the $^{130}$Te 2νββ decay half-life. The study of 2νββ has significant implications in nuclear physics, as a precise measurement of the transition half-life and spectral shape.

Speaker: Mattia Beretta (University and INFN of Milano Bicocca)

6 The first results of the LEGEND-200 detector

The LEGEND experiment is designed to search for neutrinoless double beta decay using Ge-76 enriched high purity germanium detectors that are immersed in liquid argon. LEGEND-200 (L200), operating at LNGS in Italy, builds on the successes in background suppression and analysis techniques from the Majorana Demonstrator and GERDA experiments. L200’s first results are based on 61 kgyr of exposure with an estimated background index of $0.5^{+0.3}_{-0.2}$ cts/(keV ton yr). Data from GERDA and the Majorana Demonstrator were combined with L200’s for a joint analysis, yielding a 90% CL sensitivity of $2.8x10^{26}$ yr and setting a new lower limit of > $1.9\times10^{26}$ yr, for the half-life of 0vbb. Assuming the decay mechanism is mediated by the exchange of a light Majorana neutrino, this half-life limit corresponds to an upper limit on the effective Majorana mass of mbb < 75-200 meV.

This work is supported by the U.S. DOE and the NSF, the LANL, ORNL and LBNL LDRD programs; the European ERC and Horizon programs; the German DFG, BMBF, and MPG; the Italian INFN; the Polish NCN and MNiSW; the Czech MEYS; the Slovak RDA; the Swiss SNF; the UK STFC; the Canadian NSERC and CFI; the LNGS and SURF facilities.

Speaker: William Quinn (UCL)

7 New results of the experiment to search for double beta decay of $^{106}$Cd using $^{106}$CdWO$_4$ scintillator

A low-background experiment to study double-beta decay processes in $^{106}$Cd using a $^{106}$CdWO$_4$ crystal scintillator (mass 215.4 g) enriched in $^{106}$Cd to 66$\%$, has been performed at the National Laboratories of Gran Sasso (LNGS), in Italy. Events in the $^{106}$CdWO$_4$ detector are recorded in (anti)coincidence with two large-volume CdWO$_4$ scintillation counters. The setup, designed for high detection efficiency and background suppression, was operated for 1075 days. Energy and timing calibrations, pulse-shape discrimination, and Monte Carlo simulations were used to characterize the detector response and background components. No evidence of double-beta decay was observed. New half-life limits were set for various decay modes and channels, reaching sensitivities in the range T$_{1/2}\sim10^{20}-10^{22}$ yr. In particular, the limit on the $2\nu\varepsilon\beta^+$ decay to the ground state of $^{106}$Pd was established at T$_{1/2}>7.7\times10^{21}$ yr (90% C.L.), approaching the region of theoretical predictions.

Speaker: Alice Leoncini (Università di Roma Tor Vergata)

8 Status and prospect of GRANDE

The Global Rare Anomalous Nuclear Decay Experiment (GRANDE) aims to push the frontiers of nuclear and particle physics by investigating rare nuclear decay processes. A key focus of GRANDE is the measurement of extremely rare nuclear transitions and the search for exotic dark matter particles, including axion-like particles, anapole dark matter, and dark photons in nuclear transitions. Based on the extremely rare electron-capture decay branching of isotopes such as ⁵⁷Co, ¹³⁹Ce, and ⁴⁴Ti.
The Source-as-Detector technique was chosen for radiation detection by embedding a radioactive source within a CeBr₃ crystal scintillator. We present the fabrication process and scintillation performance of CeBr₃:¹³⁹Ce and CeBr₃:⁵⁷Co, 4π BGO veto system and shielding setup at the Yemilab with a 1000-m-rock overburden. The first experimental run was performed with CeBr₃:¹³⁹Ce at a data acquisition rate of 20 kHz. We present the analysis of 1010 recorded events, including a preliminary branching ratio limit for the missing 65.86 keV M1 gamma transition of ¹³⁹Ce, which could be indicative of axion-like particle signatures.

Speaker: Hong Joo Kim (Kyungpook National University)

3:45 PM Coffee break

Round-Table Discussion

Tue, June 24

Theory

9 Second-order processes: a window into 0nbb decay

The detection of neutrinoless double beta decay (0nbb) remains elusive in spite of the intense experimental efforts to observe it. Indeed, the estimated half-life of the decay depends on nuclear matrix elements (NMEs) which are highly uncertain. A promising avenue to gain insights on the 0nbb NMEs is to explore related second-order processes in the weak (two-neutrino double-beta decay) and electromagnetic (double-magnetic dipole transitions) sectors. In this presentation I will argue that confronting theoretical predictions with experimental measurements for these two processes can be a powerful tool to reduce the uncertainty on the values of the 0nbb NMEs.

Speaker: Javier Menendez (University of Barcelona)

10 Pinning Down Two Right-Handed Neutrinos with Neutrinoless Double Beta Decay

The addition of two gauge singlet right-handed neutrinos to the Standard Model conveniently explains neutrino oscillations, while also potentially explaining the matter-antimatter asymmetry. The possible Majorana nature of neutrinos resulting from this modification can lead to observable signals in the form of neutrinoless double beta decay. Recent calculations show that the neutrinoless double beta decay rates may be underestimated in the standard parametrisation, calling for a better computation from an EFT perspective. The computations reveal significant differences in the amplitude, especially for light neutrinos where the ultrasoft mode becomes relevant. We show that, in the inverted mass ordering of neutrinos, with future limits on neutrinoless double beta decay, it is possible to precisely pin down the regions in the mass-coupling parameter space where a model with two right-handed neutrinos could exist, and that these regions are subject to cosmological constraints while also being a target for future collider and beam dump searches.

Speaker: Vaisakh Plakkot (University of Amsterdam & Nikhef)

10:15 AM Coffee break

Theory

11 QRPA and PQRPA studies of ordinary muon capture processes

The proton-neutron quasiparticle random-phase approximation (QRPA) and the particle number projected QRPA(PQRPA) are used to study the ordinary muon capture (OMC) reaction. In the present work, we have applied both methods to calculate the OMC on the 0$^+$ ground state of $^{76}$Se and $^{136}$Ba and included a comparison with the results obtained in shell model calculations. The nuclei $^{76}$Se and $^{136}$Ba are the daughter nucleus in the neutrinoless double beta (0$\nu\beta\beta$) decays of $^{76}$Ge and $^{136}$Xe, respectively. The analysis of both processes, the OMC reaction and the (0$\nu\beta\beta$) decay, shows the complementarity existing between a direct reaction mechanism and a decay. Own to this complementarity we have explored the range of possible values of nuclear matrix elements which are relevant for the determination of the Majorana neutrino mass.

Speaker: Osvaldo Civitarese (Department of Physics, University of La Plata, Argentina)

12 Relativistic chiral effective field theory for neutrinoless double beta decay

The nuclear matrix elements for neutrinoless double-beta decay play an important role in interpreting the experimental half-life limits, yet are hampered by large theoretical uncertainties. One crucial aspect of the uncertainties is the leading-order short-range decay operator identified by the nonrelativistic chiral effective field theory. This short-range operator is required to achieve renormalizability of the $nn\rightarrow pp ee$ amplitude, but its size is highly uncertain due to the absence of lepton-number-violating data.

In this presentation, I will demonstrate that such a leading-order short-range decay operator is not needed in the relativistic chiral effective field theory, by performing a renormalization group analysis of the $nn\rightarrow pp ee$ amplitude. To validate the relativistic approach, I will present the predictions of $nn\rightarrow pp ee$ amplitude from relativistic chiral effective field theory and compare the results with the recent lattice QCD simulations. Finally, I will show how the relativistic chiral decay operator can be applied in nuclear-structure calculations of nuclear matrix elements.

Speaker: Yilong Yang (Peking University)

13 A new possibility for experimental tests of nuclear matrix elements in double beta decay

Nuclear matrix elements in double beta decay are crucial for probing the nature of neutrinos. These quantities can, in principle, be inferred from experimental observables through various nuclear reactions. Examples include double charge exchange reactions connecting an initial and a final state, as well as single charge exchange and two-nucleon transfer reactions, which involve multiple intermediate spectator states. Despite extensive experimental efforts, however, no data with sufficient precision are currently available.

In this work, we present a new approach by introducing two complete sets of (A-1)-nucleon states and a complete set of A-nucleon states. This method enables the extraction of matrix elements from one-nucleon transfer amplitudes, which are generally easier to measure than those from other reactions. We demonstrate that, although many virtual states are involved and most are not experimentally accessible, only a few low-energy states make significant contributions. Furthermore, while limited, the available experimental data provide meaningful validation of the shell-model valence spaces. This approach offers a promising avenue for improving theoretical descriptions of double beta decay and guiding future experiments.

Speaker: Latsamy Xayavong (Yonsei University)

12:15 PM Lunch

Experiment

14 NuDoubt++: Search for Double Beta Plus Decays with a Hybrid Opaque Scintillation Detector

Positron-emitting double beta decay modes are rare nuclear processes. Their detection is challenging due to extremely low decay probabilities, complex experimental signatures, and the low natural abundance of suitable isotopes. Studying these decays can offer valuable insights into nuclear structure and fundamental symmetries. The decay rate is influenced by nuclear matrix elements (NMEs) and phase space factors (PSFs) - both essential for interpreting results and refining theoretical models. In this presentation, we introduce NuDoubt++, a new detector concept designed to search for double beta plus decays with unprecedented sensitivity. It combines hybrid and opaque scintillation technologies for the first time, along with advanced light readout techniques, to enhance detection of positron-based signatures. With this approach, we expect to observe two-neutrino double beta plus decay (2νβ⁺β⁺) within a 1-tonne-week exposure. Beyond that, NuDoubt++ will significantly improve sensitivity to neutrinoless double beta plus decay (0νβ⁺β⁺), pushing the current experimental limits by several orders of magnitude.

Speaker: Veronika Palusova (JGU Mainz)

15 Validating LEGEND-200 detector model with calibration data using Monte Carlo simulations

One of the most puzzling open questions in physics is whether neutrinos are their own antiparticles - are they Majorana particles? Demonstrating this property would impact our understanding of the neutrino mass ordering and the matter-antimatter asymmetry in the Universe. The Large Enriched Germanium Experiment for Neutrinoless $\beta\beta$ Decay (LEGEND) aims to shed light on this puzzle by searching for the neutrinoless double beta ($0\nu\beta\beta$) decay of $^{76}$Ge. Since 2023, LEGEND-200 has been operating 142 kg of $^{76}$Ge-enriched detectors placed in a cryostat filled with active liquid argon, which is itself housed in a large water tank for additional shielding. LEGEND-200 targets a half-life sensitivity beyond $10^{27}$ years after an exposure of 1 tonne-year.

Calibration of the $^{76}$Ge detectors is essential to ensure accurate energy reconstruction, long-term detector stability and effective discrimination between potential $0\nu\beta\beta$ signals and background events. My contribution focuses on comparing the calibration data with Monte Carlo simulated data to validate the LEGEND-200 detector model. This work also supports the optimisation of calibration source placement to achieve uniform irradiation of all detectors in the array. Such uniformity is crucial for improving the precision of the energy calibration and enhancing the overall reliability of the physics analysis.

Speaker: Aravind Remesan Sreekala (University of Zurich)

16 Sensitivity of the CUPID experiment to $0\nu\beta\beta$ decay of $^{100}$Mo

Neutrinoless double-beta decay (0νββ) is a key process addressing some of the most significant open questions in particle physics, the conservation of lepton number and the Majorana nature of the neutrino. Over the past decades, extensive efforts have been dedicated to improving the sensitivity of 0νββ half-life measurements across multiple isotopes. The next generation of experiments aims to probe half-lives greater than 10$^{27}$ years, reaching the sensitivity required to explore the Inverted-Ordering region of the neutrino mass spectrum. Among the various techniques employed, low-temperature calorimetry has proven exceptionally promising and is expected to maintain a leading role in future searches through the CUPID experiment. CUPID will search for the 0νββ decay of $^{100}$Mo and will exploit the existing cryogenic infrastructure and expertise gained from CUORE, currently operating at the Laboratori Nazionali del Gran Sasso in Italy. We developed a statistical analysis in a Frequentist and a Bayesian framework to evaluate the discovery and the exclusion sensitivity. In this talk, we will describe this analysis and we will show the impact of background levels and energy resolutions differing from those of the baseline scenario. We will provide and overview of the current status of the experiment and outline the upcoming steps.

Speaker: Léonard Imbert (INFN Milano-Bicocca)

17 Development of 3d printable scintillating plastic for bolometric 0νββ decay experiments

The 3dSPARK (3D-Printed Scintillating Polymer Assembly for Rare Events at milliKelvin Temperature) project aims to develop a novel type of assembly for next-generation bolometric neutrinoless double beta decay experiments. A significant part of the background in bolometric experiments originates from contamination of the copper frames traditionally used in the assembly. By using a 3D-printed polymer-based mechanical structure, whose design can be highly optimised thanks to the flexibility of additive manufacturing, the mass is reduced, and the gamma interaction probability is lowered due to the material's low atomic number. Additionally, this approach enables the structure to function as an active veto by incorporating a scintillating compound, which can ultimately help to further reduce the background contribution from the detector structure. In this talk, we will present an overview of the project and show the potential of this technology to decrease the background level, as demonstrated by Geant4 simulation studies. We will also present the first results on the optical characterisation of the 3d scintillating plastic.

Speaker: Léonard Imbert (INFN Milano-Bicocca)

3:30 PM Coffee break

Round-Table Discussion

7:00 PM Social dinner - Banquet

Wed, June 25

Theory

18 Theoretical uncertainties and beyond-Standard-Model signatures in double-beta decay spectra

Double-beta decay provides a promising probe sensitive to physics beyond the Standard Model, especially due to its potential to uncover the Majorana nature of neutrinos. For interpreting data from current and next-generation double-beta decay experiments, including SuperNEMO – which will be capable of measuring not only energy spectra but also angular correlations of emitted electrons – it is of direct relevance to focus on very precise theoretical predictions. In this work, we perform a detailed analysis of the shapes of single and summed energy distributions, as well as angular correlations of the electrons emitted in double-beta decay, with particular emphasis on theoretical uncertainties arising from phase-space factor calculations and nuclear modelling. In particular, we focus on nuclear matrix element calculations, approximations for Dirac wave functions, and weak magnetism correction, and we investigate their impact on distinguishing the standard two-neutrino double-beta decay from new physics signals, such as neutrinoless double-beta decay accompanied by massive scalar emission.

Speaker: Dominik Starý

19 Effects of perturbation for transition operators of double-β decay on nuclear matrix element

We calculate the nuclear matrix elements of the neutrinoless and the two-neutrino double-β decays for 136Xe to 136Ba with the higher-order corrections for the transition operators in terms of perturbation interaction. This is a study of the effects that cannot be included in the initial and the final nuclear wave functions. The new terms are derived by an extension of the usual leading-order calculation using the perturbation theory. We use the lowest-order corrections of the vertex correction and the two-body current. It turned out that the absolute values of these correction terms are comparable with that of the leading term. The effective axial-vector current coupling for the two-neutrino double-β decay with the new nuclear matrix element to reproduce the experimental half-life is significantly larger than that of the leading-order calculation and closer to the bare value for nucleons in the vacuum. We obtain the effective axial-vector current couplings using the leading-order components to reproduce the perturbed nuclear matrix elements with the bare coupling. Those effective couplings for the neutrinoless and the two-neutrino decays are not significantly different. This is the new important finding.

Speaker: Jun Terasaki (Czech Technical University in Prague)

10:15 AM Coffee break

Theory

20 Can Pions and Weak Magnetism in 2νββ Tell Us Something About BSM Physics?

Corrections to neutrinofull double beta decay observables typically focus on QED interactions or on refining the treatment of nuclear matrix elements. We introduce a new kind: chiral. These corrections involve "Yukawa-like" pion exchanges between the two decaying nuclei, as well as weak force magnetism. We explore how these effects alter decay rates and spectra, and whether they can mimic or suppress beyond the Standard Model observables, such as sterile neutrinos. Furthermore, we relate the neutrinofull double beta decay pion correction to neutrinoless double beta decay nuclear matrix elements, to assess whether constraining the former results in constraining the latter. We conclude by outlining the energy resolution necessary to measure these corrections.

Speaker: Saad el Morabit (University of Amsterdam, Nikhef)

21 Theoretical Analysis of 97Tc Electron Capture with Applications to Neutrino Mass Studies

In this presentation, we investigate the electron capture decay of 9797Tc to the 320 keV excited state of 9797Mo, exploring its potential application in neutrino mass determination. We calculate the decay half-life and the energy distribution released following the capture. Due to uncertainties in the angular momentum of the final state, we consider multiple possible transition types. By comparing our results with the experimentally measured total half-life, we are able to point out unlikely angular momentum assignments currently listed in nuclear databases.

Our calculations employ a self-consistent Dirac–Hartree–Fock–Slater approach to determine electron wave functions and binding energies. We implement a refined energy conservation scheme that accounts for changes in the energy of spectator electrons, enhancing the accuracy of predicted energy peaks observed in detectors. Additionally, we incorporate corrections for exchange and overlap effects, as well as shake-up and shake-down processes.

Speaker: Vasile-Alin Sevestrean (FF-UB, CIFRA, IFIN-HH)

22 Beta decay studies for Physics beyond the Standard Model

The study of weak interaction nuclear processes in general, and nuclear β-decay in particular, plays a key role in multiple avenues of searches for physics beyond the standard model. The search for the rare neutrinoless double β-decay(0νββ) and exotic dark matter in nuclear laboratory-scale experiments are among such searches that aim to answer foundational questions in physics. In the searches for exotic dark matter, unknown forbidden electron-capture decay can appear as an irreducible internal background. Therefore, giving theoretical estimates for branching ratios of such unknown decays is of utmost importance in experimental confirmation of the detection of exotic dark matter. On the 0νββ decay front, understanding the phenomenology of effective axial vector coupling (𝑔𝐴𝑒𝑓𝑓) is key for determining the sensitivity of underground experiments designed to detect this rare decay. In particular, the understanding of 𝑔𝐴𝑒𝑓𝑓 is terra incognita in the case of forbidden non-unique β+/electron capture decays, it being key for 0νββ decays searches on the β+/electron capture side. The novel Branching Ratio Method (BRM) is introduced to explore this uncharted territory. The talk aims to walk through these novel facets of physics that lie at the forefront of weak interaction physics.

Speaker: Aagrah Agnihotri (University of Jyväskylä)

12:15 PM Lunch

Experiment

23 LEGEND-1000 - a next generation detector for searches of neutrino-less double beta decay

The LEGEND experiment has been designed to search for neutrinoless double-beta decay in Ge-76. Its discovery would have profound implications for neutrino physics and cosmology providing unambiguous evidence for the Majorana nature of neutrinos, lepton number non-conservation and the absolute neutrino mass scale. The LEGEND-1000 detector represents the ton-scale phase of the LEGEND program, following
the current intermediate stage (LEGEND-200) carried out at LNGS in Italy. The LEGEND-1000 will be based on germanium detectors enriched in Ge-76 up to about 90 %. The detectors will be operated in an active shield based on underground argon This approach proved to guarantee the lowest background levels and the best energy resolution at the decay Q value as established by the GERDA and MAJORANA DEMONSTRATOR experiments. The anticipated quasi background-free operation will allow for search of neutrinoless double-beta decay in Ge-76 at a half-life beyond 10^28 yr and a discovery sensitivity spanning the inverted-ordering neutrino mass scale. The LEGEND Collaboration is successfully seeking funding from US and European agencies. The construction of the detector in Hall C of the underground laboratory of LNGS in Italy should start in 2026 and will take about 8 years. Start of data taking is foreseen for 2029.

Speaker: Grzegorz Zuzel (Jagiellonian University)

24 Double beta decay of $^{150}$Nd to the excited levels of $^{150}$Sm

The search for double beta decay of $^{150}$Nd to the excited levels of $^{150}$Sm was performed at the Gran Sasso underground laboratory of INFN (Italy) with a four-crystal HPGe gamma spectrometer over 5.845 yr by using a highly purified neodymium-containing sample. The two-neutrino double beta transition of $^{150}$Nd to the first 740.5 keV $0^+$ level of $^{150}$Sm was detected in both one-dimensional and coincidence spectra taken, with a half-life $T_{1/2}=[0.83^{+0.18}_{-0.13}(\rm{stat})^{+0.16}_{-0.19}(\rm{syst})] \times 10^{20}$ yr, in agreement with the results of the previous experiments. Some excess of the 334.0-keV peak is observed that is an indication of the $2\nu2\beta$ decay of $^{150}$Nd to the 334.0 keV $2^+$ excited level of $^{150}$Sm with the half-life $T_{1/2} = [1.5^{+2.3}_{-0.6}(\rm{stat}) \pm0.4(\rm{syst})] \times 10^{20}$ yr, that leads also to slightly higher half-life for the $0^+$ to $0^+_1$ transition: of $T_{1/2}=[1.03^{+0.35}_{-0.22}(\rm{stat})^{+0.16}_{-0.19}(\rm{syst})] \times 10^{20}$ yr. The half-life value for the $0^+$ to $2^+_1$ decay does not contradict the existing limits and agrees with the half-life range calculated in the framework of proton-neutron QRPA with isospin restoration combined with like-nucleon QRPA for a description of excited states in the final nuclei. A possible advancement in experimental sensitivity will be briefly discussed.

Speaker: Fedir Danevych (INFN sez. Tor Vergata, Rome, Italy)

25 Launch and operation of the PIKACHU experiment to search for double beta decay in 160Gd

Gadolinium-160 ($^{160}$Gd) is a candidate for double beta decay with relatively high natural abundance (21.9%). However, its low Q-value (1.73 MeV) makes the observation of even the two-neutrino double beta decay (2$\nu$2$\beta$) extremely challenging. Previous experiments using a 2-inch Gd$_2$SiO$_5$ (GSO) scintillator couldn’t detect 2$\nu$2$\beta$ due to significant background from intrinsic uranium and thorium series in GSO. As a result, the search established a lower limit of 1.9$\times$10$^{19}$ years on the 2$\nu$2$\beta$ half-life. Meanwhile, a theoretical prediction suggests a 2$\nu$2$\beta$ half-life of approximately 7.4$\times$10$^{20}$ years.
The PIKACHU experiment is designed to overcome the limitations by employing large Gd$_2$Ga$_3$Al$_2$O$_{12}$ (GAGG) single crystals and to observe 2$\nu$2$\beta$. GAGG offers several advantages over GSO: higher light yield, possibility of pulse shape discrimination, and a higher $^{160}$Gd content by increasing in size. We planned two phases: Phase 1 aims to update the current lower limit on the 2$\nu$2$\beta$ half-life, and Phase 2 is intended to achieve a sensitivity approximately an order of magnitude better than previous study, with the goal of observing the decay.
In this presentation, I will introduce the PIKACHU experiment, present on the development of high-purity GAGG crystals, and report the current status of data acquisition and analysis for Phase 1.

Speaker: Takumi Omori (University of Tsukuba)

26 Total capture rate analysis of 136Ba target through Ordinary Muon Capture in the MONUMENT experiment

MONUMENT (Muon Ordinary capture for NUclear Matrix element) measures Ordinary Muon Capture (OMC) on isotopes relevant for the neutrinoless double beta (0nbb) decay searches. OMC is a powerful tool to study the $0\nu\beta\beta$-decay NMEs as it involves similar momentum transfer and allows to experimentally probe the intermediate virtual transitions involved in the decay. OMC on $^A_{Z+2}X$ nucleus populates several excited states of a daughter nucleus $^A_{Z+1}Y^*$. The main goal of MONUMENT is to extract the total and partial muon capture rates. The total capture rate determines the lifetime of the muonic atom and can be obtained by studying the time evolution of the de-excitation of the γ rays after OMC. In this talk, I present a general overview of the MONUMENT experiment with details of the experimental setup designed to accurately extract the capture rates. I also discuss the current status of the analysis of the total capture rate of $^{136}$Ba from the data taken at Paul Scherrer Institute in Switzerland during the 2021 campaign.

Speaker: Dhanurdhar Bajpai (The University of Alabama, Tuscaloosa)

3:45 PM Coffee break

Round-Table Discussion

Thu, June 26

Theory

27 EFT Approach to Radiative Neutrino Masses: Beyond Cut-off Regularisation

Lepton number violating (LNV) interactions occur in the Standard Model Effective Field Theory (SMEFT) at odd dimensions starting from the dimension-5 Weinberg operator. The operators higher than dimension-5 are suppressed by additional powers of the heavy new scale. However, they can be crucial when traditional seesaw mechanisms leading to tree-level dimension-5 contributions are absent. Considering the example of minimal tree-level UV-completions for dimension-7 SMEFT LNV operators, I will discuss how a dimensional-regularisation-based approach can provide a more accurate estimate for the radiative neutrino masses when the new physics fields are hierarchical in mass, as compared to the cut-off-regularisation-based approach often employed in the literature. This opens up new viable regions of parameter space close to the current limits set by searches for neutrinoless double beta decay and the LHC that would previously have been thought to be excluded by neutrino-mass constraints.

Speaker: Chandan Hati (Instituto de Física Corpuscular (IFIC), CSIC-UV, Spain)

28 A classification of UV models for higher-dimensional SMEFT operators for neutrinoless double beta decay

The observation of lepton number violation (LNV) would be clear evidence for physics beyond the Standard Model. Famous examples for processes that violate lepton number by two units are neutrino mass mechanisms and neutrinoless double beta decay.
In the Standard Model Effective Field Theory (SMEFT), a ubiquitous framework used for indirect new physics searches, $\Delta L =2$ operators appear at dimension-5 and higher odd dimension.
The dimension-5 Weinberg operator, that can explain neutrino masses, does not have to be realised at tree-level but could arise at higher loop order in the UV models. These models, however, could produce higher dimensional operators at tree-level, leaving the question which contribution dominates the neutrinoless double beta decay.
We use a diagrammatic approach to systematically list all possible tree-level models for dimension-9 operators and perform a scan over which lower dimensional operators they produce. Finally, we address the question raised above for an explicit example.

Speaker: Fabian Esser (Charles University Prague)

10:15 AM Coffee break

Theory

29 Two-neutrino 0+ → 0+ double beta decay of 48Ca in the DFT-NCCI framework

Two-neutrino double beta decay (2νββ) is a second-order weak-interaction process. Consequently,it is among the rarest radioactive processes observed in nature.

The 2νββ decay has recently attracted significant attention due to substantial investments in the search for the yet unobserved neutrinoless double beta decay (0νββ), a process considered a potential gateway to new physics beyond the Standard Model. Current efforts focus on high-precision half-life measurements and, on the theoretical side, on high-precision calculations of the nuclear matrix elements using various theoretical models.

In this talk, we present the results of our seminal calculation of the nuclear matrix element for the 2νββ decay 48Ca → 48Ti, performed using a post-Hartree-Fock (HF) Density Functional Theory-based No-Core Configuration-Interaction (DFT-NCCI) framework developed by our group. The preliminary value we have obtained for the nuclear matrix element describing this process, |M2νββ | = 0.063(6) MeV−1, is in excellent agreement with the results of the shell-model study by Horoi et al., which yielded 0.054 (0.064) MeV−1 for the GXPF1A (GXPF1) interactions, respectively.

The consistency of our prediction with the shell-model results strengthens our confidence in the nuclear modeling of this extremely rare process, which is of paramount importance for the further modeling of the 0νββ decay.

Speaker: Jan Miśkiewicz (Physics Faculty, University of Warsaw)

30 Description of Double Beta Decay within EMPM and STDA

Double beta decay (DBD) is a phenomenon which provides us unique window to physics beyond Standard Model and which lies at the intersection of particle, nuclear and atomic physics. It is of crucial importance to distuinguish whether DBD occurs solely in two-neutrino or also neutrino-less variant. Possible discovery of neutrino-less or other exotic mode of DBD would have big consequences in next development of particle physics as well as cosmology. One of challenges in theoretical description of DBD is to provide precise nuclear matrix elements (NME) which enter to calculations of DBD's half-life.

In our contribution we discuss Equation of Motion Phonon Method (EMPM), and additionally also Second Tamm Dancoff Approximation (STDA). Comparison of both methods, EMPM and STDA, was recently studied in more detail [1]. While in the paper [1] we described electromagnetic nuclear excitations, in this conference contribution we show that the generalization of STDA and EMPM, in which we apply the particle-hole configurations which change type of nucleon, is suitable for calculations of DBD in Calcium-48.

References
[1] F. Knapp, P. Papakonstantinou, P. Veselý, G. De Gregorio, J. Herko, and N. Lo Iudice, Phys. Rev. C 107 (2023) 014305.

Speaker: Petr Vesely (Nuclear Physics Institute, Czech Academy of Sciences)

31 Neutrinoless double beta decay NME for LRSM

Left-right symmetric model (LRSM) offers rich phenomenon of particle physics. One of which is the neutrinoless double beta decay, besides the light neutrino mass mechanism, it also provide possibilities of other mechanisms. In my talk, I will focus on the mechanism mediated by the light neutrino, and give the corresponding NMEs from two nuclear many-body approaches: Large Scale Shell Model and proton-neutron quasi-particle random phase approximation. We also compare the current results with the popular EFT master formula.

Speaker: DongLiang Fang (Institute of Modern Physics, Chinese Academy of sciences)

12:15 PM Lunch

Experiment

32 Search for inelastic Dark Matter induced deexcitations of 178m2Hf isomer in the Gran Sasso underground laboratory

The 2nd isomeric state of 178Hf has high energy of 2.446 MeV and big half-life of 31 yr. Normally it decays spontaneously to the ground state of 178Hf by isomeric transition with emission of cascade of gamma quanta with energies up to ~600 keV. Possible interactions of 178m2Hf with some given inelastic Dark Matter (iDM) candidates could lead to emission of gammas with energies >1 MeV from the excited levels of 178Hf not populated usually. The search for such gammas was performed deep underground at the Gran Sasso National Laboratory of the INFN in measurements of 178m2Hf source with activity of ~70 Bq with two ultra-low-background HPGe spectrometers over 1170 h. This is the first 178m2Hf measurements in the low-background conditions which allowed the enhancement of the sensitivity – in the framework of the models considered here – to hypothesized iDM induced decay at energies >600 keV thanks to reduced natural radioactive background. Improved T1/2 limits for several hypothesized iDM induced transitions were set, at the level of lim T1/2 ~ 10^6 - 10^7 yr, 1 – 2 orders of magnitude higher than those known previously. Constraints on the parameters' space of iDM are derived under the considered assumptions.

Speaker: Volodymyr Tretyak (INR NASU, Ukraine & LNGS INFN, Italy)

33 The RES-NOVA observatory

The RES-NOVA project detects cosmic neutrinos via coherent elastic neutrino-nucleus scattering (CEνNS) using archaeological Pb-containing PbWO4 cryogenic detectors. RES-NOVA plans to conduct a direct detection campaign while waiting for neutrinos of astrophysical origin. The natural abundance of Pb-207 offers sensitivity to spin-dependent dark matter interactions. Additionally, Effective Field Operators offer a comprehensive way to interpret experimental data for the search of the elusive dark matter particles. Most of these operators feature spin proportionality and, hence, knowledge of the nuclear spin structure is key for accurate model evaluation. Thanks to one existing calculation of the Pb-207 spin structure allowed RES-NOVA to accurately compute sensitivity predictions to spin dependent DM particles interactions in the light of the SUSY neutralino dark matter candidate.

The target material contains tungsten, an interesting candidate to search for double electron capture in W-180 and W-186, a key process to search for exotic properties of neutrinos. A precise evaluation of the nuclear matrix elements is crucial to translate experimental data to the life time of these isotopes.

In this contribution the experimental reach of RES-NOVA will be outlined with emphasis on how the knowledge of key nuclear structures impacts the precision of the planned results.

Speaker: Nahuel Ferreiro Iachellini (University of Milano-Bicocca)

34 Search for neutrinoless double beta decay of Sn-124 at Yemilab

The KIMS collaboration had reported that a lower limit on the Sn-124 half-life using an organotin-loaded liquid scintillation detector remained the best result. And recent results from other experiments calls that the next generation of neutrinoless double beta decay experiment requires a substantial quantity of target isotope in order to attain the enhanced sensitivity of 1E28 year. Consequently, the loading of organotin in the liquid scintillation detector has the potential application in the large-scale experiment to obtain the sensitivity.
The preliminary R&D stage has been initiated for the neutrinoless double beta decay of Sn-124 with tetrabutyltin, which is well soluble in aromatic solvents and its commercial availability. Following the optimisation of the detection system, the test experiment will be conducted in the Yemilab, which is located at a depth of 1 km underground in Jeongseon, Korea.
This presentation will comprise a discussion of two key elements.
The results of the properties of the detector will be discussed, and the scheme of the experiment will be presented.

Speaker: Jooyoung Lee (Kyungpook National University)

35 Double beta decay with PandaX liquid xenon detectors

PandaX-4T is a liquid xenon time projection chamber (TPC) that searches for dark matter particles and neutrinoless double beta decay of xenon isotopes. In this talk, I will present our latest work on the double beta decay half-life limit established by PadnaX-4T for Xe-136(-134), as well as the effort to utilize the spectral information of Xe-136 decay for NME and new physics analysis.

Speaker: Ke Han (Shanghai Jiao Tong University)

3:45 PM Coffee break

Round-Table Discussion

Fri, June 27

Experiment

36 Measurement of the electron capture of $^{76}$As

Large-scale experiments like LEGEND are searching for the neutrinoless double beta decay of $^{76}$Ge. The measurement of the half-life of this process would give access to the neutrino mass using the nuclear matrix element. This can be expressed as a sum over all transitions along states of the intermediate nucleus $^{76}$As. Experimentally the contribution of the $^{76}$As ground state can be investigated via the branching ratios of its $\beta^-$ and electron capture decay branches. Energetically, the electron capture of $^{76}$As into the $^{76}$Ge ground state and first excited state are possible.

In an experimental study at TU Dresden the branching of $^{76}$As is investigated. $^{76}$As is produced via $^{75}$As(n,$\gamma$) on a thin As$_2$O$_3$ sample. A Silicon drift detector measures characteristic X-rays emitted by the Germanium atoms left with an inner vacancy after the electron capture. A HPGe detector with excellent energy resolution is sensitive to the 563 keV deexcitation gammas emitted after electron capture into the excited state. Investigation of coincident signals in both detectors allows to distinguish between both electron capture channels. The experimental procedure, analysis and preliminary results will be presented.

Speaker: Hans Hoffmann (TU Dresden)

37 Measurement of New Ac-228 Isomers Using a Ra-228 Doped CeBr3 Scintillation Crystal

Despite significant research efforts, the precise measurements of certain radioactive decays remain elusive. In this study, we have measured the energy and decay time of Ac-228 isomers produced by the beta decay of Ra-228. This was achieved using a novel method where a Ra-228 radioactive source is deposited in a scintillator. Given the low energy (45.8 keV) Q value of Ra-228 beta decay and the corresponding low energy emission of the resulting Ac-228 isomers, these measurements are challenging. To overcome this, we coated a Ra-228 radioactive source inside a quartz ampoule and grew CeBr3 using the Bridgman method, thereby embedding the Ra-228 source within the CeBr3 scintillator. The CeBr3 scintillator, chosen for its high light yield, good resolution, and fast decay time, was critical in measuring the low energy and fast decay time of the Ac-228 isomers. Since Ac-228 is produced via the beta decay of Ra-228, the isomer’s gamma emission coincides with the beta particle. We utilized the delayed coincidence method to measure the energy and decay time of the isomers. In this presentation, we will discuss the development of the detector using this novel method and the measured energy and decay time of Ac-228 isomers.

Speaker: Doohyeok Lee (Kyungpook National University)

10:15 AM Coffee break

Experiment

38 Status of the SuperNEMO Experiment and First Simulation-Based Sensitivity Estimates

The SuperNEMO Experiment has entered its physics data-taking phase as of April 2025, becoming the only operational double beta decay detector capable of full topological event reconstruction via the tracker-calorimeter design. This topology-driven approach provides powerful discrimination of signal and background, and is uniquely suited to explore a wide range of BSM scenarios. The detector, located at the Laboratoire Souterrain de Modane (LSM) in France,uses 6.11 kg of enriched $^{82}$Se as its double beta decay source.

We present the first simulation-based sensitivity estimates using the newly developed tracking algorithm and an updated analysis framework, targeting both the neutrinoless double beta decay mode and other exotic decays such as Majoron-emitting decays and the right-handed currents. The extended analysis takes advantage of SuperNEMO’s capability to measure not only the total electron energy but also single-electron energies and angular correlations. These results represent an important step toward quantifying SuperNEMO’s sensitivity to a broad range of double beta decay processes.

Speaker: Maros Petro (IEAP CTU in Prague)

39 Dark matter search with PICO bubble chambers

Modern bubble chambers offer a unique opportunity to probe the dark matter parameter space. These detectors use superheated fluids such as C₃F₈ to detect elastic scatters on target nuclei. Nuclear recoils that deposit energy above the detector’s thermodynamic threshold—set by its operating temperature and pressure—nucleate visible bubbles, which are recorded by high-speed cameras.

The PICO collaboration is leading the effort with bubble chambers at the SNOLAB underground laboratory in Sudbury, Canada. PICO-40L is a fully assembled and operational detector that uses a new “Right Side Up” design, placing the compression and expansion system below the target fluid to reduce background signals observed in earlier detectors. PICO-500 is the next-generation, ton-scale bubble chamber currently under construction. Building on experience from previous detectors, it incorporates major improvements in size, design, and background control.

This talk will present an overview of both detectors, their design improvements, background mitigation strategies, and early commissioning results.

Speaker: Babar Ali (IEAP CTU in Prague)

Round-Table Discussion

12:15 PM Lunch

1:45 PM End of the conference!