Rare Decays Research Articles

Luminescence of ZnWO4 crystals under X-ray excitation

Zinc tungstate crystal scintillator is an interesting detector material for double beta decay, dark matter, and rare alpha decay experiments. The luminescence under X-ray excitation in the temperature region from 85 K to 420 K was studied with five samples of ZnWO4 crystals developed by different methods: with a stoichiometric composition, and with excess of Zn or W. It is found that doping a ZnWO4 crystal with 0.2–0.3% Zn and W does not affect the luminescence properties of the material. A theoretical dependence of the sample luminescence intensity on the competing nonradiative recombination centers and traps is obtained. The main center of the recombination luminescence in a ZnWO4 crystal with a spectral maximum at 2.5 eV is the tungsten complex with a structural defect located next to it.

Study of B(s)0→ϕϕ→(K+K−)(K+K−) decays in the perturbative QCD approach

In this work, we make a detailed analysis on the penguin-dominant processes $B_{(s)}^0 \to \phi\phi \to (K^+K^-)(K^+K^-)$ in the perturbative QCD (PQCD) approach. In addition to the dominant $P$-wave resonance, the scalar background $f_0(980) \to K^+K^-$ is also accounted for. We improve the Gegenbauer moments in $KK$ two-meson distribution amplitudes by fitting the PQCD factorization formulas to measured branching ratios of three-body and four-body $B$ decays. We extract the branching ratios of two-body $B_{(s)}^0 \to \phi\phi$ decays from the corresponding four-body decay modes and calculate the relevant polarization fractions together with two relative phases $\phi_{\parallel,\perp}$, which are consistent with the previous theoretical predictions. The PQCD predictions for the "true" triple product asymmetries (TPAs) are zero which are expected in the standard model due to the vanishing weak phase difference, and support the current data reported by the CDF and LHCb Collaborations. A large "fake" TPA $\mathcal{A}_\text{T-fake}^1=30.4\%$ of the decay $B^0_s \to \phi\phi \to (K^+K^-)(K^+K^-)$ is predicted for the first time, which indicates the presence of the significant final-state interactions. The TPAs of the rare decay channel $B^0 \to \phi\phi\to (K^+K^-)(K^+K^-)$ are also predicted and can be tested in the near future.

Prompt acceleration of a μ + beam in a toroidal wakefield driven by a shaped steep-rising-front Laguerre–Gaussian laser pulse

Recent experimental data for anomalous magnetic moments strongly indicates the existence of new physics beyond the Standard Model. Energetic μ + bunches are relevant to μ + rare decay, spin rotation, resonance and relaxation (μSR) technology, future muon colliders, and neutrino factories. In this paper, we propose prompt μ + acceleration in a nonlinear toroidal wakefield driven by a shaped steep-rising-front Laguerre–Gaussian (LG) laser pulse. An analytical model is described, which shows that a μ + beam can be focused by an electron cylinder at the centerline of a toroidal bubble and accelerated by the front part of the longitudinal wakefield. A shaped LG laser with a short rise time can push plasma electrons, generating a higher-density electron sheath at the front of the bubble, which can enhance the acceleration field. The acceleration field driven by the shaped steep-rising-front LG laser pulse is about four times greater than that driven by a normal LG laser pulse. Our simulation results show that a 300 MeV μ + bunch can be accelerated to 2 GeV and its transverse size is focused from an initial value of w 0 = 5 μm to w = 2 μm in the toroidal bubble driven by the shaped steep-rising-front LG laser pulse with a normalized amplitude of a = 22.

Search for new physics with rare charm decays at LHCb

The LHCb experiment is playing a crucial role in the study of rare and forbidden decays of charm hadrons, which are unique probes for hints of physics beyond the Standard Model. These proceedings present results of the search for semileptonic three-body decays of the form $D_{(s)}^+$ \rightarrow $h^\pm$ \ellp $\ell^{(')\mp}$, where $h^\pm$ is a charged pion or kaon and $\ell^{(')\mp}$ is an electron or a muon. Furthermore, studies of the four-body neutral meson charm decays $D^0 \rightarrow \pi^+ \pi^- \mu^+ \mu^-$ and $D^0 \rightarrow K^+ K^- \mu^+ \mu^-$ are discussed. Prospects for leptonic and semileptonic rare charm decays at LHCb in the future are presented in the end.

Search for the decay D0→π0νν¯

We present the first experimental search for the rare charm decay $D^{0} \to \pi^{0} \nu \bar{\nu}$. It is based on an $e^+e^-$ collision sample consisting of $10.6\times10^{6}$ pairs of $D^0\bar{D}^0$ mesons collected by the BESIII detector at $\sqrt{s}$=3.773 GeV, corresponding to an integrated luminosity of 2.93~fb$^{-1}$. A data-driven method is used to ensure the reliability of the background modeling. No significant $D^{0} \to \pi^{0} \nu \bar{\nu}$ signal is observed in data and an upper limit of the branching fraction is set to be $2.1\times 10^{-4}$ at the 90$\%$ confidence level. This is the first experimental constraint on charmed-hadron decays into dineutrino final states.

Heavy neutrino searches via same-sign lepton pairs at a Higgs boson factory

This paper investigates the $e^-e^+\to Zh_1$ sensitivity for Higgs boson's rare decay into heavy neutrinos $h_1 \to NN$ at the proposed electron-positron collider, with the focus on multi-lepton final states that contain same-sign lepton pairs. $h_1 \to NN$ decay can derive from Higgs boson's mixing with new physics scalar(s) that is complementary to the contribution from active-sterile neutrino mixings. We consider the scenario with a singlet scalar which gives the heavy neutrino mass and has a small mixing with the SM Higgs boson. We analyze the semileptonic, fully leptonic and mixed $NN$ decay scenarios, and categorize the signal on the number of leptons in the final state: $ \ell^\pm \ell^\pm$ with at least 3 jets, $\ell^\pm \ell^\pm \ell$ with at least 2 jets, and $e^\pm e^\pm \mu^\mp \mu^\mp$ plus with at least 1 jet, each containing one or two same-sign dilepton system(s). Selection cuts are optimized for the presence of the associated $Z$-boson, which leads to additional backgrounds at the $e^-e^+$ collider. The Standard Model background channels are systematically analyzed. Sensitivity limits on $h_1 \rightarrow NN$ branching fractions are derived for signals with 2-4 final leptons assuming the heavy neutrino masses are between 10 and 60 GeV. With 240 GeV center-of-mass energy and 5.6 ab$^{-1}$ design luminosity, $h_1 \rightarrow NN$ branching fraction can be probed to $2\times 10^{-4}$ in $2\ell$ and $3\ell$ channels, and $6\times 10^{-4}$ in the $4\ell$ channel at $95\%$ confidence level. $3\ell, 4\ell$ channels expect one or fewer background event, and their sensitivities saturate the statistic limit at 5.6 ab$^{-1}$ luminosity. A same-sign trilepton ($\ell^\pm \ell^\pm \ell^\pm$) signal in the $3\ell$ channel is also discussed. Our search strategy provides an approach to discovering the singlet scalar and exploring the origin of neutrino masses at future $e^-e^+$ colliders.

Probing the [formula omitted] anomalous couplings via exclusive Z boson decay

We propose to utilize the exclusive Z-boson rare decays Z→ϒ(ns)+γ to constrain the Zbb¯ couplings at the HL-LHC and 100 TeV proton-proton collider. We demonstrate that the event yield of the proposed processes is sensitive to the axial-vector component of the Zbb¯ coupling and can provide complementary information to the jet-charge weighted single-spin asymmetry measurement at the EIC and the gg→Zh production rate measurement at the LHC. By applying the NRQCD factorization formalism, we calculate the partial decay width of Z→ϒ(ns)+γ to the NLO accuracy in strong interaction, which is found to agree with those obtained from the light-cone distribution amplitude approach. We show that the HL-LHC can break the degeneracy of the Zbb¯ couplings, as implied by the precision electroweak data at LEP and SLC, if the signal efficiency can be improved by a factor of 1.7 from the present ATLAS analysis at the 13 TeV LHC with an integrated luminosity of 36.1fb−1.

Hadrophilic dark sectors at the Forward Physics Facility

Models with light dark sector and dark matter particles motivate qualitatively new collider searches. Here we carry out a comprehensive study of hadrophilic models with U(1)$_B$ and U(1)$_{B-3L_{\tau}}$ gauge bosons coupled to light dark matter. The new mediator particles in these models couple to quarks, but have suppressed couplings to leptons, providing a useful foil to the well-studied dark photon models. We consider current bounds from accelerator and collider searches, rare anomaly-induced decays, neutrino non-standard interactions, and dark matter direct detection. Despite the many existing constraints, these models predict a range of new signatures that can be seen in current and near future experiments, including dark gauge boson decays to the hadronic final states $\pi^+ \pi^- \pi^0$, $\pi^0 \gamma$, $K^+ K^-$, and $K_S K_L$ in FASER at LHC Run 3, enhancements of $\nu_{\tau}$ scattering rates in far-forward neutrino detectors, and thermal dark matter scattering in FLArE in the HL-LHC era. These models therefore motivate an array of different experiments in the far-forward region at the LHC, as could be accommodated in the proposed Forward Physics Facility.

One-loop matching of the type-II seesaw model onto the Standard Model effective field theory

In this paper, we continue to construct the low-energy effective field theories (EFTs) of the canonical seesaw models, which are natural extensions of the Standard Model (SM) to accommodate tiny but nonzero neutrino masses. Different from three right-handed neutrino singlets in the type-I seesaw model, the Higgs triplet in the type-II seesaw model participates directly in the electroweak gauge interactions, rendering the EFT construction more challenging. By integrating out the heavy Higgs triplet in the functional-integral formalism, we carry out a complete one-loop matching of the type-II seesaw model onto the so-called Standard Model Effective Field Theory (SMEFT). It turns out that 41 dimension-six operators (barring flavor structures and Hermitian conjugates) in the Warsaw basis of the SMEFT can be obtained, covering all those 31 dimension-six operators in the case of type-I seesaw model. The Wilson coefficients for 41 dimension-six operators are computed up to mathcal{O}left({M}_{Delta }^{-2}right) with M∆ being the mass scale of the Higgs triplet. Moreover, the branching ratios of rare radiative decays of charged leptons {l}_{alpha}^{-}to {l}_{beta}^{-}+gamma are calculated in the EFT and compared with that in the full theory in order to demonstrate the practical application and the correctness of our EFT construction.

Optical, luminescence, and scintillation properties of advanced ZnWO4 crystal scintillators

Zinc tungstate (ZnWO4) crystal scintillators are promising detection material for the experiments searching for double beta decay, dark matter, and investigating rare alpha decays. An extended R&D was performed to develop advanced quality ZnWO4 crystal scintillators. The R&D programme included the selection of the initial materials, the variation of the compound stoichiometry, the application of single and double crystallization, and the annealing of the crystal boules. The optical transmittance of the produced boules was measured, and the luminescence under X-ray excitation in the temperature region from 85 K to room temperature was studied (thermally stimulated luminescence was measured till 350 K). The energy resolution and the relative scintillation pulse amplitude were measured with gamma-sources demonstrating high scintillation properties of the samples produced by single crystallization from deeply purified zinc and tungsten oxides, with stoichiometric composition, annealed in air atmosphere.

CP violation in the rare Higgs decays via exchange of on-shell almost degenerate Majorana neutrinos, Hto {v}_k{N}_jto {v}_k{mathrm{ell}}^{-}Uoverline{D} and Hto {v}_k{N}_jto {v}_k{mathrm{ell}}^{+}overline{U}D

We investigate rare decays of Higgs via exchange of two almost degenerate heavy on-shell Majorana neutrinos Nj (j = 1, 2): Γ± = Γ(h → νkNj→ νkℓ±π∓), and into the open quark channels Γ± = Γ(h → νkNj→ νkℓ±UD), where UD are two jets of open quarks ( overline{U}D , or Uoverline{D} , where U = u, c and D = d, s). The related CP violation asymmetry ACP = (Γ−− Γ+)/(Γ− + Γ+) is studied in detail. We take into account the N1-N2 overlap and oscillation effects. We can see that for certain, presently acceptable, range of input parameters, such decays with open quark channels, and their asymmetries, could be detected in the International Linear Collider (ILC).

Analysis of Neutral B-Meson Decays into Two Muons.

Branching fraction and effective lifetime measurements of the rare decay B_{s}^{0}→μ^{+}μ^{-} and searches for the decays B^{0}→μ^{+}μ^{-} and B_{s}^{0}→μ^{+}μ^{-}γ are reported using proton-proton collision data collected with the LHCb detector at center-of-mass energies of 7, 8, and 13TeV, corresponding to a luminosity of 9 fb^{-1}. The branching fraction B(B_{s}^{0}→μ^{+}μ^{-})=(3.09_{-0.43-0.11}^{+0.46+0.15})×10^{-9} and the effective lifetime τ(B_{s}^{0}→μ^{+}μ^{-})=2.07±0.29±0.03 ps are measured, where the first uncertainty is statistical and the second systematic. No significant signal for B^{0}→μ^{+}μ^{-} and B_{s}^{0}→μ^{+}μ^{-}γ decays is found and upper limits B(B^{0}→μ^{+}μ^{-})<2.6×10^{-10} and B(B_{s}^{0}→μ^{+}μ^{-}γ)<2.0×10^{-9} at the 95%C.L. are determined, where the latter is limited to the range m_{μμ}>4.9 GeV/c^{2}. The results are in agreement with the standard model expectations.

REST-for-Physics, a ROOT-based framework for event oriented data analysis and combined Monte Carlo response

The REST-for-Physics (Rare Event Searches Toolkit for Physics) framework is a ROOT-based solution providing the means to process and analyze experimental or Monte Carlo event data. Special care has been taken to the traceability of the code and the validation of the results produced within the framework, together with the connectivity between code and stored data, registered through specific version metadata members.The framework development was originally motivated to cover the needs of Rare Event Searches experiments (experiments looking for phenomena having extremely low occurrence probability, like dark matter or neutrino interactions or rare nuclear decays). The framework components naturally implement tools to address the challenges in these kinds of experiments. The integration of a detector physics response, the implementation of signal processing routines, or topological algorithms for physical event identification are some examples. Despite this specialization, the framework was conceived thinking in scalability. Other event-oriented applications could benefit from the data processing routines and/or metadata description implemented in REST, being the generic framework tools completely decoupled from dedicated libraries.REST-for-Physics is a consolidated piece of software already serving the needs of different physics experiments - using gaseous Time Projection Chambers (TPCs) as detection technology - for detector data analysis and characterization, as well as generic R&D. Even though REST has been exploited mainly with gaseous TPCs, the code could be easily applied or adapted to other detector technologies. We present in this work an overview of REST-for-Physics, providing a broad perspective to the infrastructure and organization of the project as a whole. The framework and its different components will be described in the text.

Singlet fermion dark matter and Dirac neutrinos from Peccei-Quinn symmetry

The Peccei-Quinn (PQ) mechanism not only acts as an explanation for the absence of strong CP violation but also can play a main role in the solution to other open questions in particle physics and cosmology. Here we consider a model that identifies the PQ symmetry as a common thread in the solution to the strong CP problem, the generation of radiative Dirac neutrino masses and the origin of a multicomponent dark sector. Specifically, scotogenic neutrino masses arise at one loop level with the lightest fermionic mediator field acting as the second dark matter (DM) candidate thanks to the residual $Z_2$ symmetry resulting from the PQ symmetry breaking. We perform a phenomenological analysis addressing the constraints coming from the direct searches of DM, neutrino oscillation data and charged lepton flavor violating (LFV) processes. We find that the model can be partially probed in future facilities searching for WIMPs and axions, and accommodates rates for rare leptonic decays that are within the expected sensitivity of upcoming LFV experiments.

Present status of radiative and rare kaon decays

I will review recent developments in rare and radiative kaon decays from the theory side, with emphasis on those modes that are actively analyzed by the experimental collaborations.

Search for the radiative {Xi}_b^{-} \u2192 \u039e\u2212\u03b3 decay

The first search for the rare radiative decay {Xi}_b^{-} → Ξ−γ is performed using data collected by the LHCb experiment in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb−1. The {Xi}_b^{-} → Ξ−J/ψ channel is used as normalization. No {Xi}_b^{-} → Ξ−γ signal is found and an upper limit of mathcal{B} ( {Xi}_b^{-} → Ξ−γ) < 1.3 × 10−4 at 95% confidence level is obtained.

Fiducial cross sections for the lepton-pair-plus-photon decay mode in Higgs production up to NNLO QCD

The rare three-body decay of a Higgs boson to a lepton-antilepton pair and a photon is starting to become experimentally accessible at the LHC. We investigate how higher-order QCD corrections to the dominant gluon-fusion production process impact on the fiducial cross sections in this specific Higgs decay mode for electrons and muons. Corrections up to NNLO QCD are found to be sizeable. They are generally uniform in kinematical variables related to the Higgs boson, but display several distinctive features in the kinematics of its individual decay products.

Likelihood analysis of the flavour anomalies and g \u2013 2 in the general two Higgs doublet model

We present a likelihood analysis of the general two Higgs doublet model, using the most important currently measured flavour observables, in view of the anomalies in charged current tree-level and neutral current one-loop rare decays of B mesons in b → cl overline{nu} and b → sμ+μ− transitions, respectively. We corroborate that the model explains the latter and it is able to simultaneously fit the experimental values of the R(D) charged current ratio at 1σ, but it can not accommodate the D* charmed meson observables R(D*) and FL(D*). We find that the fitted values for the angular observables in b → sμ+μ− transitions exhibit better agreement with the general two Higgs double model in comparison to the SM. We also make predictions for future collider observables BR(t → ch), BR(h → bs), BR(h → τμ), BR(Bs → τ+τ−), BR(B+ → K+τ+τ−) and the flavour violating decays of the τ lepton, BR(τ → 3μ) and BR(τ → μγ). The model predicts values of BR(t → ch), BR(Bs → τ+τ−) and BR(B+ → K+τ+τ−) that are out of reach of future experiments, but its predictions for BR(h → bs) and BR(h → τμ) are within the future sensitivity of the HL-LHC or the ILC. We also find that the predictions for the τ → 3μ and τ → μγ decays are well within the projected limits of the Belle II experiment. Finally, using the latest measurement of the Fermilab Muon g − 2 Collaboration, we performed a simultaneous fit to ∆aμ constrained by the charged anomalies, finding solutions at the 1σ level. Once the neutral anomalies are included, however, a simultaneous explanation is unfeasible.

Weak dissipation for high-fidelity qubit-state preparation and measurement

Highly state-selective, weakly dissipative population transfer is used to irreversibly move the population of one ground state qubit level of an atomic ion to an effectively stable excited manifold with high fidelity. Subsequent laser interrogation accurately distinguishes these electronic manifolds, and we demonstrate a total qubit state preparation and measurement (SPAM) inaccuracy $\epsilon_\mathrm{SPAM} < 1.7 \times 10^{-4}$ ($-38 \mbox{ dB}$), limited by imperfect population transfer between qubit eigenstates. We show experimentally that full transfer would yield an inaccuracy less than $8.0 \times 10^{-5}$ ($-41 \mbox{ dB}$). The high precision of this method revealed a rare ($\approx 10^{-4}$) magnetic dipole decay induced error that we demonstrate can be corrected by driving an additional transition. Since this technique allows fluorescence collection for effectively unlimited periods, high fidelity qubit SPAM is achievable even with limited optical access and low quantum efficiency.

CP violation in top quark decay via heavy Majorana neutrinos at the LHC

The tiny neutrino masses can be explained naturally by extending the standard model with right-handed Majorana neutrinos in a low-scale seesaw mechanism, and the $CP$ violation effect induced by the Majorana phase is interesting and worth studying at colliders. In this paper, we investigate the prospects for measuring $CP$ violation in top quark pair production and rare decay via Majorana neutrinos at the LHC. The $CP$ asymmetry stems from the significant interference of contributions from two different Majorana neutrinos and can be appreciable when the two neutrino masses are nearly degenerate. It is found that in the Majorana neutrino mass range of $10\text{ }\text{ }\mathrm{GeV}&lt;{m}_{N}&lt;80\text{ }\text{ }\mathrm{GeV}$, the $CP$ asymmetry is independent of the Majorana neutrino mass at the LHC. Any possible new observation of $CP$ violation will be the clear evidence of new physics beyond the standard model.