Weak Decays Research Articles

QCD Sum Rules Analysis of Weak Decays of Doubly-Heavy Baryons

We calculate the weak decay form factors of doubly-heavy baryons using three-point QCD sum rules. The Cutkosky rules are used to derive the double dispersion relations. We include perturbative contributions and condensation contributions up to dimension five, and point out that the perturbative contributions and condensates with lowest dimensions dominate. An estimate of a part of the gluon–gluon condensates show that it plays a less important role. With these form factors at hand, we present a phenomenological study of semileptonic decays. The future experimental facilities can test these predictions, and deepen our understanding of the dynamics in the decays of doubly-heavy baryons.

Reduction of coherent betatron oscillations in a muon g − 2 storage ring experiment using RF fields

This work demonstrates that two systematic errors, coherent betatron oscillations (CBO) and muon losses, can be reduced through application of radio frequency (RF) electric fields, which ultimately increases the sensitivity of the muon g − 2 experiments. As the ensemble of polarized muons goes around a weak focusing storage ring, their spin precesses, and when they decay through the weak interaction, , the decay positrons are detected by electromagnetic calorimeters. In addition to the expected exponential decay in the positron time spectrum, the weak decay asymmetry causes a modulation in the number of positrons in a selected energy range at the difference frequency between the spin and cyclotron frequencies, ω a. This frequency is directly proportional to the magnetic anomaly a μ = (g − 2)/2, where g is the g-factor of the muon, which is slightly greater than 2. The detector acceptance depends on the radial position of the muon decay, so the CBO of the muon bunch following injection into the storage ring modulate the measured muon signal with the frequency ω CBO. In addition, the muon populations at the edge of the beam hit the walls of the vacuum chamber before decaying, which also affects the signal. Thus, reduction of CBO and unwanted muon loss increases the a μ measurement sensitivity. Numerical and experimental studies with RF electric fields yield more than a magnitude reduction of the CBO, with muon losses comparable to the conventional method.

Hadronic weak decays of the charmed baryon Ωc

Two-body hadronic weak decays of the charmed baryon $\Omega_c$, including Cabibbo-favored (CF), singly Cabibbo-suppressed (SCS) and doubly Cabibbo-suppressed (DCS) modes, are studied systematically in this work. To estimate nonfactorizable contributions, we work in the pole model for the $P$-wave amplitudes and current algebra for the $S$-wave amplitudes. Among all the channels decaying into a baryon octet and a pseudoscalar meson, $\Omega_c\to \Xi^0\overline{K}^0$ is the only allowed CF mode. The predicted branching fraction of order $3.8\%$ and large and positive decay asymmetry of order $0.50$ indicate that a measurement of this mode in the near future is promising. Proceeding through purely nonfactorizable contributions, the SCS mode $\Omega_c\to\Lambda^0\overline{K}^0$ and DCS mode $\Omega_c\to\Lambda^0 \eta$ are predicted to have branching fractions as large as $0.8\%$ and $0.4\%$, respectively. The two DCS modes $\Omega_c\to\Sigma^0\eta$ and $\Omega_c\to\Lambda^0\pi^0$ are suggested to serve as new physics searching channels for their vanishing SM background.

Heavy exotic scalar meson Tbb;u¯s¯−

The spectroscopic parameters and decay channels of the scalar tetraquark ${T}_{bb;\overline{u}\overline{s}}^{\ensuremath{-}}$ (in what follows ${T}_{b:\overline{s}}^{\ensuremath{-}}$) are investigated in the framework of the QCD sum rule method. The mass and coupling of the ${T}_{b:s}^{\ensuremath{-}}$ are calculated using the two-point sum rules by taking into account quark, gluon and mixed vacuum condensates up to dimension 10. Our result for its mass $m=(10250\ifmmode\pm\else\textpm\fi{}270)\text{ }\text{ }\mathrm{MeV}$ demonstrates that ${T}_{b:\overline{s}}^{\ensuremath{-}}$ is stable against the strong and electromagnetic decays. Therefore to find the width and mean lifetime of the ${T}_{b:\overline{s}}^{\ensuremath{-}}$, we explore its dominant weak decays generated by the transition $b\ensuremath{\rightarrow}{W}^{\ensuremath{-}}c$. These channels embrace the semileptonic decay ${T}_{b:\overline{s}}^{\ensuremath{-}}\ensuremath{\rightarrow}{Z}_{bc;\overline{u}\overline{s}}^{0}l{\overline{\ensuremath{\nu}}}_{l}$ and nonleptonic modes ${T}_{b:\overline{s}}^{\ensuremath{-}}\ensuremath{\rightarrow}{Z}_{bc;\overline{u}\overline{s}}^{0}{\ensuremath{\pi}}^{\ensuremath{-}}({K}^{\ensuremath{-}},{D}^{\ensuremath{-}},{D}_{s}^{\ensuremath{-}})$, which at the final state contain the scalar tetraquark ${Z}_{bc;\overline{u}\overline{s}}^{0}$. Key quantities to compute partial widths of the weak decays are the form factors ${G}_{1}({q}^{2})$ and ${G}_{2}({q}^{2})$: they determine differential rate $d\mathrm{\ensuremath{\Gamma}}/d{q}^{2}$ of the semileptonic and partial widths of the nonleptonic processes, respectively. These form factors are extracted from relevant three-point sum rules at momentum transfers ${q}^{2}$ accessible for such analysis. By means of the fit functions ${F}_{1(2)}({q}^{2})$ they are extrapolated to cover the whole integration region ${m}_{l}^{2}\ensuremath{\le}q2\ensuremath{\le}(m\ensuremath{-}\stackrel{\texttildelow{}}{m}{)}^{2}$, where $\stackrel{\texttildelow{}}{m}$ is the mass of ${Z}_{bc;\overline{u}\overline{s}}^{0}$. Predictions for the full width ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{full}}=(15.21\ifmmode\pm\else\textpm\fi{}2.59)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\text{ }\text{ }\mathrm{MeV}$ and mean lifetime ${4.33}_{\ensuremath{-}0.63}^{+0.89}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\text{ }\text{ }\mathrm{s}$ of the ${T}_{b:s}^{\ensuremath{-}}$ are useful for experimental and theoretical investigations of this exotic meson.

Unification of flavor SU(3) analyses of heavy Hadron weak decays

Analyses of heavy mesons and baryons hadronic charmless decays using the flavor SU(3) symemtry can be formulated in two different forms. One is to construct the SU(3) irreducible representation amplitude by decomposing effective Hamiltonian, and the other is to draw the topological diagrams. In the flavor SU(3) limit, we study various B/Drightarrow PP,VP,VV, B_crightarrow DP/DV decays, and two-body nonleptonic decays of beauty/charm baryons, and demonstrate that when all terms are included these two ways of analyzing the decay amplitudes are completely equivalent. Furthermore we clarify some confusions in drawing topological diagrams using different ways of describing beauty/charm baryons.

Towards a heavy diquark effective theory for weak decays of doubly heavy baryons

We construct a leading-order effective field theory for both scalar and axial-vector heavy diquarks, and consider its power expansion in the heavy diquark limit. By assuming the transition from QCD to diquark effective theory, we derive the most general form for the effective diquark transition currents based on the heavy diquark symmetry. The short-distance coefficients between QCD and heavy diquark effective field theory are also obtained by a tree level matching. With the effective currents in the heavy diquark limit, we perform a reduction of the form factors for semi-leptonic decays of doubly heavy baryons, and find that only one nonperturbative function is remaining. It is shown that this soft function can be related to the Isgur–Wise function in heavy meson transitions. As a phenomenological application, we take a single pole structure for the reduced form factor, and use it to calculate the semi-leptonic decay widths of doubly heavy baryons. The obtained results are consistent with others given in the literature, and can be tested in the future.

The prospect of charm quark magnetic moment determination

In this paper, we discuss the theoretical framework and the experimental measurements of the magnetic moment of the charmed baryons. We first review the theoretical predictions of the varLambda _c^{+} magnetic moment and show that the measurements of the magnetic moments of other charmed baryons, such as varXi _{c}, allow to perform detailed spectroscopy studies. The magnetic moment of the charm quark can be determined using radiative charmonium decay, which can be compared to the varLambda _c^{+} magnetic moment within theoretical models. The present results show a tension with majority of theoretical predictions. The magnetic moment of the charmed baryons could potentially be measured directly, using bent-crystal experiments at LHC. The possibility to measure precisely the magnetic moments of charmed baryons needs precise measurement of their polarisation and weak decay parameters. In this paper, we revisit the formalism of the angular analysis needed for these measurements and make a detailed evaluation of initial polarisation of deflected varLambda _c baryons as a function of crystal orientation. We found a special orientation of the crystal that gives the opportunity to measure the varLambda _c dimensionless electric dipole moment almost with the same precision as its g-factor, which is more than an order of magnitude more efficient than suggested before.

Strangeness production with ALICE at the LHC

The main goal of the ALICE experiment is to study the physics of strongly interacting matter under extremely high temperature and energy density conditions to investigate the properties of the Quark-Gluon plasma (QGP). The enhanced production of strange hadrons with respect to non-strange particles was historically considered as one of the signatures of QGP formation during the evolution of the system created in heavy-ion collisions. The excellent tracking and particle identification capabilities of the ALICE experiment allow the reconstruction of multi-strange baryons via their weak decay channels over a large range of transverse momentum. The yields and the relative production rates of strange and multi-strange particles normalized to pions measured in pp, p–Pb, Pb–Pb and Xe–Xe collisions are presented as a function of particle multiplicity. The strangeness production dependence on the collision energy is also shown. Results are compared to QCD-inspired and statistical hadronization model predictions.

Progress on the Electromagnetic Calorimeter Trigger Simulation at the Belle II Experiment

The Belle II experiment at KEK in Japan has started real data taking from April 2018 to probe a New Physics beyond the Standard Model by measuring CP violation precisely and rare weak decays of heavy quark and lepton. The experiment is performed at the high luminosity SuperKEKB e^+ e^- collider with 80 x 10^34 cm^-2 s^-1 as an ultimate instantaneous luminosity. In order to develop and test an appropriate trigger algorithm under much higher luminosity and beam background environment than previous KEKB collider, a detail simulation study of the Belle II calorimeter trigger system is very crucial to operate Belle II Trigger and DAQ system in stable. We report preliminary results on various trigger logics and their efficiencies using physics and beam background Monte Carlo events with a Belle II Geant4-based analysis framework called Basf2.

Shape coexistence and multiparticle-multihole structures in Cd110,112

From detailed spectroscopy of Cd-110 and Cd-112 following the beta(+)/EC decay of In-110,In-112 and the beta(-) decay of Ag-112, the presence of very weak decay branches from nonyrast states is revealed. In Cd-112, 2(5)(+) -> 0(4)(+) and 4(6)(+) -> 2(5)(+) transitions are observed that yield B(E2; 2(5)(+) -> 0(4)(+)) = 34 +/- 15 W.u. and B(E2; 4(6)(+) -> 2(5)(+)) = 77 +/- 30 W.u., respectively, clearly indicating a collective structure. In 110Cd, a weak decay branch from the 4(6)(+) level to the 2(5)(+) level is observed, and from a lifetime measurement following the (n, n gamma' reaction, B(E2; 4(6)(+) -> 2(5)(+)) = 55 +/- 14 W.u. is determined. A new branch is also observed for the decay of the 6(4)(+) level to the 4(6)(+) state, indicating that the sequence 2(5)(+), 4(6)(+), and 6(4)(+) forms part of a collective structure. The presence of 3(3)(+) and 5(2)(+) levels spaced between the previous sequence is highly suggestive of a gamma band built on the 0(2)(+) shape-coexisting intruder state. The 0(4)(+) levels in Cd-110,Cd-112,Cd-114 have preferred decays to the lowest 2(+) members of the intruder bands, and for 114Cd a previous measurement had established an enhanced B(E2; 0(4)(+) -> 2(3)(+)). The energy systematics of the 0(2)(+), 0(3)(+), and 0(4)(+) levels all display the characteristic parabolic-shaped pattern, suggesting that they are built on multiparticle-multihole proton excitations. The results are compared with beyond-mean-field calculations that reproduce qualitatively the observed levels and their decays and suggest that the 0(1)(+), 0(2)(+), 0(3)(+), and 0(4)(+) levels and the excited states built on them possess different deformations.

Bootstrapping max statistics in high dimensions: Near-parametric rates under weak variance decay and application to functional and multinomial data

In recent years, bootstrap methods have drawn attention for their ability to approximate the laws of “max statistics” in high-dimensional problems. A leading example of such a statistic is the coordinatewise maximum of a sample average of $n$ random vectors in $\mathbb{R}^{p}$. Existing results for this statistic show that the bootstrap can work when $n\ll p$, and rates of approximation (in Kolmogorov distance) have been obtained with only logarithmic dependence in $p$. Nevertheless, one of the challenging aspects of this setting is that established rates tend to scale like $n^{-1/6}$ as a function of $n$. The main purpose of this paper is to demonstrate that improvement in rate is possible when extra model structure is available. Specifically, we show that if the coordinatewise variances of the observations exhibit decay, then a nearly $n^{-1/2}$ rate can be achieved, independent of $p$. Furthermore, a surprising aspect of this dimension-free rate is that it holds even when the decay is very weak. Lastly, we provide examples showing how these ideas can be applied to inference problems dealing with functional and multinomial data.

Two-, three-, and four-body decays in the constructive standard model

We further develop the massive constructive theory of the Standard Model and use it to calculate the amplitude and squared amplitude for all two-body decays, a collection of weak three-body decays, as well as Higgs decay to four neutrinos. We compare our results with those from Feynman diagrams and find complete agreement. We show that in all the cases considered here, the amplitudes of massive constructive theories are significantly simpler than those resulting from Feynman diagrams. In fact, a naive counting of the number of calculations required for a matrix-element generator to compute a phase-space point is orders-of-magnitude smaller for the result coming from the constructive method suggesting that these generators might benefit from this method in the future, even in the case of massive weak amplitudes. We also anticipate that our simpler expressions will produce numerically more stable expressions.

Core-Etched CC/SnO2 Nanotube Arrays as High-Performance Anodes for Lithium-Ion Batteries With Ionic Liquid Electrolyte

Despite the design of nano-structured SnO2 anodes has attracted much attention because of its high theoretical capacity, good electron mobility and low potential of lithium-ion intercalation, challenges remain due to their weak mechanical stability, complex processing and rapid capacity decay. The flexible one-dimensional binder-free porous CC/SnO2 nanotube arrays are synthesized with a well-suited core etching method to meet the needs of steady operation of flexible devices under mechanical deformation. This porous, binder-free nanostructure has large contact area with the electrolyte and excellent electron transport performance. The electrochemical measurements demonstrate that these nanotube arrays have high energy density and high-rate capability. After 500 cycles at a current density of 200 mA g−1, their stable capacity remains at 595.7 mA h g−1.

Charmed baryon weak decays with vector mesons

We give a systematic study of ${\bf B}_c\to {\bf B}_n V$ decays, where ${\bf B}_c$ and $ {\bf B}_n$ correspond to the anti-triplet charmed and octet baryons, respectively, while $V$ stand for the vector mesons. We calculate the color-symmetric contributions to the decays from the effective Hamiltonian with the factorization approach and extract the anti-symmetric ones based on the experimental measurements and $SU(3)_F$ flavor symmetry. We find that most of the existing experimental data for ${\bf B}_c\to {\bf B}_n V$ are consistent with our fitting results. We present all the branching ratios of the Cabbibo allowed, singly Cabbibo suppressed and doubly Cabbibo suppressed decays of ${\bf B}_c\to {\bf B}_n V$. The decay parameters for the daughter baryons and mesons in ${\bf B}_c\to {\bf B}_n V$ are also evaluated. In particular, we point out that the Cabbibo allowed decays of $\Lambda_c^+ \to \Lambda^0 \rho^+$ and $ \Xi_c^0 \to \Xi^- \rho^+$ as well as the singly Cabbibo suppressed ones of $\Lambda_c^+ \to \Lambda^0 K^{*+}$, $\Xi_c^+ \to \Sigma^+ \phi$ and $\Xi_c^0\to \Xi^- K^{*+}$ have large branching ratios and decay parameters with small uncertainties, which can be tested by the experimental searches at the charm facilities.

Effects of resonance weak decay and hadronic rescattering on the proton number fluctuations in Au+Au collisions at sNN=5GeV from a microscopic hadronic transport (JAM) model

Proton number fluctuation is sensitive observable to search for the QCD critical point in heavy-ion collisions. In this paper, we studied rapidity acceptance dependence of the proton cumulants and correlation functions in most central Au+Au collisions at $\sqrt{s_\mathrm{NN}} = 5$ GeV from a microscopic hadronic transport model (JAM). At mid-rapidity, we found the effects of resonance weak decays and hadronic re-scattering on the proton cumulants and correlation functions are small, but those effects get larger when further increasing the rapidity acceptance. On the other hand, we found the baryon number conservation is a dominant background effect on the rapidity acceptance dependence of proton number fluctuations. It leads to a strong suppression of cumulants and cumulant ratios, as well as the negative proton correlation functions. We also studied those two effects on the energy dependence of cumulant ratios of net-proton distributions in most central Au+Au collisions at $\sqrt{s_\mathrm{NN}} = 5-200$ GeV from JAM model. This work can serve as a non-critical baseline for future QCD critical point search in heavy-ion collisions at high baryon density region.

Two-body weak decays of doubly charmed baryons

The hadronic two-body weak decays of the doubly charmed baryons ${\mathrm{\ensuremath{\Xi}}}_{cc}^{++},{\mathrm{\ensuremath{\Xi}}}_{cc}^{+}$, and ${\mathrm{\ensuremath{\Omega}}}_{cc}^{+}$ are studied in this work. To estimate the nonfactorizable contributions, we work in the pole model for the $P$-wave amplitudes and current algebra for $S$-wave ones. For the ${\mathrm{\ensuremath{\Xi}}}_{cc}^{++}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{+}{\ensuremath{\pi}}^{+}$ mode, we find a large destructive interference between factorizable and nonfactorizable contributions for both $S$- and $P$-wave amplitudes. Our prediction of $\ensuremath{\sim}0.70%$ for its branching fraction is smaller than the earlier estimates in which nonfactorizable effects were not considered, but agrees nicely with the result based on an entirely different approach, namely, the covariant confined quark model. On the contrary, a large constructive interference was found in the $P$-wave amplitude by Dhir and Sharma, leading to a branching fraction of order (7--16)%. Using the current results for the absolute branching fractions of $({\mathrm{\ensuremath{\Lambda}}}_{c}^{+},{\mathrm{\ensuremath{\Xi}}}_{c}^{+})\ensuremath{\rightarrow}p{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ and the LHCb measurement of ${\mathrm{\ensuremath{\Xi}}}_{cc}^{++}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{+}{\ensuremath{\pi}}^{+}$ relative to ${\mathrm{\ensuremath{\Xi}}}_{cc}^{++}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Lambda}}}_{c}^{+}{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{+}$, we obtain $\mathcal{B}({\mathrm{\ensuremath{\Xi}}}_{cc}^{++}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{+}{\ensuremath{\pi}}^{+}{)}_{\text{expt}}\ensuremath{\approx}(1.83\ifmmode\pm\else\textpm\fi{}1.01)%$ after employing the latest prediction of $\mathcal{B}({\mathrm{\ensuremath{\Xi}}}_{cc}^{++}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Sigma}}}_{c}^{++}{\overline{K}}^{*0})$. Our prediction of $\mathcal{B}({\mathrm{\ensuremath{\Xi}}}_{cc}^{++}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{+}{\ensuremath{\pi}}^{+})\ensuremath{\approx}0.7%$ is thus consistent with the experimental value but in the lower end. It is important to pin down the branching fraction of this mode in future study. Factorizable and nonfactorizable $S$-wave amplitudes interfere constructively in ${\mathrm{\ensuremath{\Xi}}}_{cc}^{+}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{0}{\ensuremath{\pi}}^{+}$. Its large branching fraction of order 4% may enable experimentalists to search for the ${\mathrm{\ensuremath{\Xi}}}_{cc}^{+}$ through this mode. That is, the ${\mathrm{\ensuremath{\Xi}}}_{cc}^{+}$ is reconstructed through the ${\mathrm{\ensuremath{\Xi}}}_{cc}^{+}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{0}{\ensuremath{\pi}}^{+}$ followed by the decay chain ${\mathrm{\ensuremath{\Xi}}}_{c}^{0}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}p{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$. Besides ${\mathrm{\ensuremath{\Xi}}}_{cc}^{+}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{0}{\ensuremath{\pi}}^{+}$, the ${\mathrm{\ensuremath{\Xi}}}_{cc}^{+}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{+}({\ensuremath{\pi}}^{0},\ensuremath{\eta})$ modes also receive large nonfactorizable contributions to their $S$-wave amplitudes. Hence, they have large branching fractions among ${\mathrm{\ensuremath{\Xi}}}_{cc}^{+}\ensuremath{\rightarrow}{\mathcal{B}}_{c}+P$ channels. Nonfactorizable amplitudes in ${\mathrm{\ensuremath{\Xi}}}_{cc}^{++}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{\ensuremath{'}+}{\ensuremath{\pi}}^{+}$ and ${\mathrm{\ensuremath{\Omega}}}_{cc}^{+}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{\ensuremath{'}+}{\overline{K}}^{0}$ are very small compared to the factorizable ones owing to the Pati-Woo theorem for the inner $W$-emission amplitude. Likewise, nonfactorizable $S$-wave amplitudes in ${\mathrm{\ensuremath{\Xi}}}_{cc}^{+}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{\ensuremath{'}+}({\ensuremath{\pi}}^{0},\ensuremath{\eta})$ decays are also suppressed by the same mechanism.

Nonleptonic beauty baryon decays and CP asymmetries based on an SU(3) -flavor analysis

We consider hadronic weak decays of beauty-baryons into charmless baryons and pseudoscalar mesons in a general framework based on $SU(3)$ decomposition of the decay amplitudes. The advantage of the approach lies in the ability to perform an $SU(3)$ analysis of these decays without any particular set of dynamical assumptions while accounting for the effects of an arbitrarily broken $SU(3)$ flavor symmetry. Dictated by the symmetries of the effective Hamiltonian that allow us to relate or neglect reduced $SU(3)$ amplitudes, we derive several sum rule relations between amplitudes and relations between $CP$ asymmetries in these decays and identify those that hold even if $SU(3)$ is broken.

Nonleptonic Decays of Doubly Charmed Baryons

In this lecture, we provide a basic introduction into the topic of charmed baryons and their nonleptonic two-body decays. Some features of the baryon weak decays on the quark level are discussed in detail in the framework of effective field theory. The calculation of the matrix elements of the four-quark operators arising in the effective theory proceeds by using the covariant constituent quark model. The model allows one to evaluate not only the factorizing tree-level diagrams but also more complicated diagrams with the internal W–exchange. The technique required for such calculation is discussed in some detail. Finally, the numerical results are presented, and comparison of the contributions coming from the W–exchange diagrams with those from the tree-level are carefully performed.

Status of the HOLMES Experiment

The absolute neutrino mass is still an unknown parameter in the modern landscape of particle physics. The HOLMES experiment aims at exploiting the calorimetric approach to directly measure the neutrino mass through the kinematic measurement of the decay products of the weak process decay of $$^{163}$$Ho. This low energy decaying isotope, in fact, undergoes electron capture emitting a neutrino and leaving the daughter atom, $$^{163}$$Dy$$^*$$, in an atomic excited state. This, in turn, relaxes by emitting electrons and, to a considerably lesser extent, photons. The high-energy portion of the calorimetric spectrum of this decay is affected by the non-vanishing neutrino mass value. Given the small fraction of events falling within the region of interest, to achieve a high experimental sensitivity on the neutrino mass, it is important to have a high activity combined with a very small undetected pileup contribution. To achieve these targets, the final configuration of HOLMES foresees the deployment of a large number of $$^{163}$$Ho ion-implanted TESs characterized by an ambitiously high activity of 300 Hz each. In this paper, we outline the status of the major tasks that will bring HOLMES to achieve a statistical sensitivity on the neutrino mass as low as 2 eV/c$$^2$$.

Weak decays of magnetized charged pions in the symmetric gauge

We consider the decay $\pi^- \to l \, \bar{\nu}_l$ ($l=e^-,\,\mu^-$) in the presence of an arbitrary large uniform magnetic field, using the symmetric gauge. The consequences of the axial symmetry of the problem and the issue of angular momentum conservation are discussed in detail. In particular, we analyze the projection of both the canonical and the mechanical total angular momenta along the direction of the magnetic field. It is found that while the former is conserved in the symmetric gauge, the latter is not conserved in both the symmetric and Landau gauges. We derive an expression for the integrated $\pi^- \to l \, \bar{\nu}_l$ width that coincides exactly with the one we previously found using the Landau gauge, providing an explicit test of the gauge independence of that result. Such an expression implies that for nonzero magnetic fields the decay width does not vanish in the limit in which the outgoing charged leptons can be considered as massless, i.e. it does not exhibit the helicity suppression found in the case of no external field.