We present the first search for the lepton-flavor-violating decay modes <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mrow><mi>B</mi></mrow><mrow><mn>0</mn></mrow></msup><mo stretchy="false">→</mo><msubsup><mrow><mi>K</mi></mrow><mrow><mi>S</mi></mrow><mrow><mn>0</mn></mrow></msubsup><msup><mrow><mi>τ</mi></mrow><mrow><mo>±</mo></mrow></msup><msup><mrow><mo>ℓ</mo></mrow><mrow><mo>∓</mo></mrow></msup></mrow></math> (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mo>ℓ</mo><mo>=</mo><mi>μ</mi><mo>,</mo><mi>e</mi></mrow></math>) using the 711 and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>365</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> data samples recorded by the Belle and Belle II detectors, respectively. We use a hadronic <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>B</mi></math>-tagging technique to fully reconstruct a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>B</mi></math> meson and search for signal decays in the system recoiling against the tagged meson, considering <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>τ</mi></math> decays to either light leptons, one charged hadron, or one charged hadron and a neutral pion. We find no evidence for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mrow><mi>B</mi></mrow><mrow><mn>0</mn></mrow></msup><mo stretchy="false">→</mo><msubsup><mrow><mi>K</mi></mrow><mrow><mi>S</mi></mrow><mrow><mn>0</mn></mrow></msubsup><msup><mrow><mi>τ</mi></mrow><mrow><mo>±</mo></mrow></msup><msup><mrow><mo>ℓ</mo></mrow><mrow><mo>∓</mo></mrow></msup></mrow></math> decays and set 90% confidence level upper limits on the branching fractions in the range of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">[</mo><mn>0.8</mn><mo>,</mo><mn>3.6</mn><mo stretchy="false">]</mo><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>5</mn></mrow></msup></math>.

I. Adachi

K. Adamczyk

L. Aggarwal

H. Ahmed

H. Aihara

N. Akopov

M. Alhakami

A. Aloisio

N. Althubiti

N. Anh Ky

D.M. Asner

H. Atmacan

V. Aushev

M. Aversano

R. Ayad

V. Babu

H. Bae

N.K. Baghel

S. Bahinipati

P. Bambade

Sw. Banerjee

S. Bansal

M. Barrett

M. Bartl

J. Baudot

A. Baur

A. Beaubien

F. Becherer

J. Becker

J.V. Bennett

F.U. Bernlochner

V. Bertacchi

M. Bertemes

E. Bertholet

M. Bessner

S. Bettarini

V. Bhardwaj

B. Bhuyan

F. Bianchi

D. Biswas

A. Bobrov

D. Bodrov

A. Bolz

A. Boschetti

A. Bozek

M. Bračko

P. Branchini

R.A. Briere

T.E. Browder

A. Budano

S. Bussino

Q. Campagna

M. Campajola

L. Cao

G. Casarosa

C. Cecchi

J. Cerasoli

M.C. Chang

P. Chang

R. Cheaib

P. Cheema

B.G. Cheon

K. Chilikin

K. Chirapatpimol

H.E. Cho

K. Cho

S.J. Cho

S.K. Choi

S. Choudhury

J. Cochran

L. Corona

J.X. Cui

E. De La Cruz-Burelo

S.A. De La Motte

G. de Marino

G. De Nardo

G. De Pietro

R. de Sangro

M. Destefanis

S. Dey

R. Dhamija

A. Di Canto

F. Di Capua

J. Dingfelder

Z. Doležal

I. Domínguez Jiménez

T.V. Dong

X. Dong

M. Dorigo

D. Dossett

S. Dubey

K. Dugic

G. Dujany

P. Ecker

P. Feichtinger

T. Ferber

T. Fillinger

C. Finck

G. Finocchiaro

A. Fodor

Search for Lepton-Flavor-Violating Decay Modes <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>B</mi><mn>0</mn></msup><mo stretchy="false">→</mo><msubsup><mi>K</mi><mi>S</mi><mn>0</mn></msubsup><msup><mi>τ</mi><mo>±</mo></msup><msup><mo>ℓ</mo><mo>∓</mo></msup></math> with Hadronic <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>B</mi></math> Tagging at Belle and Belle II

In this work we investigate the muon anomalous magnetic dipole moment <math altimg='si1.svg'><msub><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow></msub></math> in a model that extends the Standard Model with a scalar triplet and a lepton triplet. Different from previous studies, we find that there is still viable parameter space in this model to explain the discrepancy <math altimg='si2.svg'><mi mathvariant='normal'>&#916;</mi><msub><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><msub><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow></msub><mo stretchy='false'>(</mo><mrow><mi mathvariant='normal'>Exp</mi></mrow><mo stretchy='false'>)</mo><mo linebreak='goodbreak' linebreakstyle='after'>&#8722;</mo><msub><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow></msub><mo stretchy='false'>(</mo><mrow><mi mathvariant='normal'>SM</mi></mrow><mo stretchy='false'>)</mo></math>. While being consistent with the current data of neutrino mass, electroweak precision measurements and the perturbativity of couplings, our model can provide new physics contribution <math altimg='si128.svg'><msubsup><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow><mrow><mtext mathvariant='normal'>NP</mtext></mrow></msubsup></math> to cover the central region of <math altimg='si18.svg'><mi mathvariant='normal'>&#916;</mi><msub><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow></msub></math> with new scalar and lepton mass as low as around TeV. This mass scale is allowed by the current collider searches for doubly charged scalars and the lepton triplet, and they can be tested at future high energy and/or high luminosity colliders.

China

Lei Cai

South Korea

Chengcheng Han

Shi-Ping He

Peiwen Wu

Hybrid Type-II and Type-III seesaw model for the muon <italic>g</italic>&#8239;&#8722;&#8239;2 anomaly

Among the three known types of static solutions proposed within the Hamiltonian constraint approach to effective quantum gravity (EQG), the first two have been extensively investigated, whereas the third type-which preserves general covariance, is free of Cauchy horizons, and was only recently obtained-remains relatively unexplored. This solution can describe a black hole with an event horizon for certain parameter ranges, or a horizonless compact object beyond those ranges. In this paper, we focus on the third type and show that its light rings feature both stable and unstable branches, and that the black hole shadow size grows with the quantum parameter-unlike in the first two types. However, when we account for both the shadow and the lensing ring, the overall behavior closely resembles that of the second type, in which an increasing quantum parameter leads to a larger portion of the lensing ring being occupied by the shadow. This feature can serve as a hallmark of black holes in EQG, offering a potential way to distinguish them from their GR counterparts. Remarkably, the parameter ranges under which the solution remains a black hole are highly consistent with the current observational constraints on black hole shadows, lending strong support to the classification of the third type of compact object in EQG as a black hole endowed with an event horizon.

Wentao Liu

Di Wu

Jieci Wang

Light rings and shadows of static black holes in effective quantum gravity II: A new solution without Cauchy horizons

We measure the time-integrated <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><mi>P</mi></math> asymmetry in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mrow><mi>D</mi></mrow><mrow><mn>0</mn></mrow></msup><mo stretchy="false">→</mo><msubsup><mrow><mi>K</mi></mrow><mrow><mi mathvariant="normal">S</mi></mrow><mrow><mn>0</mn></mrow></msubsup><msubsup><mrow><mi>K</mi></mrow><mrow><mi mathvariant="normal">S</mi></mrow><mrow><mn>0</mn></mrow></msubsup></mrow></math> decays reconstructed in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup><mo stretchy="false">→</mo><mrow><mi>c</mi><mover accent="true"><mi>c</mi><mo stretchy="false">¯</mo></mover></mrow></math> events collected by the Belle and Belle II experiments. The corresponding data samples have integrated luminosities of 980 and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>428</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></math>, respectively. To infer the flavor of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>D</mi><mn>0</mn></msup></math> meson, we exploit the correlation between the flavor of the reconstructed decay and the electric charges of particles reconstructed in the rest of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></mrow><mo stretchy="false">→</mo><mrow><mi>c</mi><mover accent="true"><mi>c</mi><mo stretchy="false">¯</mo></mover></mrow></math> event. This results in a sample which is independent from any other previously used at Belle or Belle II. The result, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>A</mi></mrow><mrow><mi>C</mi><mi>P</mi></mrow></msub><mo stretchy="false">(</mo><msup><mrow><mi>D</mi></mrow><mrow><mn>0</mn></mrow></msup><mo stretchy="false">→</mo><msubsup><mrow><mi>K</mi></mrow><mrow><mi mathvariant="normal">S</mi></mrow><mrow><mn>0</mn></mrow></msubsup><msubsup><mrow><mi>K</mi></mrow><mrow><mi mathvariant="normal">S</mi></mrow><mrow><mn>0</mn></mrow></msubsup><mo stretchy="false">)</mo><mo>=</mo><mo stretchy="false">(</mo><mn>1.3</mn><mo>±</mo><mn>2.0</mn><mo>±</mo><mn>0.2</mn><mo stretchy="false">)</mo><mo>%</mo></mrow></math>, where the first uncertainty is statistical and the second systematic, is consistent with previous determinations and with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><mi>P</mi></math> symmetry.

Y. Ahn

S. Alghamdi

K. Amos

M. Angelsmark

C. Antonioli

T. Aushev

T. Bilka

A. Bondar

J. Borah

J. Chin

I. Consigny

D. Epifanov

R. Farkas

F. Forti

B.G. Fulsom

A. Gabrielli

A. Gale

E. Ganiev

M. Garcia-Hernandez

R. Garg

G. Gaudino

V. Gaur

Measurement of the time-integrated <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><mi>P</mi></math> asymmetry in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mrow><mi>D</mi></mrow><mrow><mn>0</mn></mrow></msup><mo stretchy="false">→</mo><msubsup><mrow><mi>K</mi></mrow><mrow><mi mathvariant="normal">S</mi></mrow><mrow><mn>0</mn></mrow></msubsup><msubsup><mrow><mi>K</mi></mrow><mrow><mi mathvariant="normal">S</mi></mrow><mrow><mn>0</mn></mrow></msubsup></mrow></math> decays using opposite-side flavor tagging at Belle and Belle II

The charge-velocity-dependent one-scale model is an extension of the canonical velocity-dependent one-scale model that explicitly incorporates additional degrees of freedom on the string world sheet, such as arbitrary currents and charges, expected in physically realistic models. A previous paper [<a href="http://dx.doi.org/10.1103/PhysRevD.110.023540">Phys. Rev. D 110, 023540 (2024)</a>] started an in-depth classification of its possible asymptotic scaling solutions, with the goal of identifying distinguishing features which may be tested against numerical simulations or future observations. This earlier analysis restricted itself to unbiased solutions; in the present work we relax this assumption, allowing for the possibility of energy loss biases between the bare string and the charge/current or between the charge and the current themselves. We find additional scaling solutions, some of which are parametric extensions of the unbiased ones, while others are new scaling branches that do not exist in the unbiased case. Overall, it remains the case that there are three broad classes of solutions, mainly depending on the balance between the expansion rate and the available energy loss mechanisms. Our results enable a quantitative calibration of the model, using forthcoming high-resolution numerical simulations.

Portugal

F.C.N.Q. Pimenta

C.J.A.P. Martins

Scaling solutions for current-carrying cosmic string networks. II. Biased solutions

In this study, we implement the type-II seesaw mechanism for Dirac neutrino masses within the framework of a 3-3-1 model. To this end, we introduce a scalar sextet and impose both lepton number conservation and invariance under a discrete $\textit{Z}$ symmetry in the Lagrangian. This mechanism naturally generates small Dirac neutrino masses by allowing the soft breaking of the $\textit{Z}$ symmetry through a unique term in the scalar potential, while preserving lepton number. Specifically, we explore the realization of this model at low-energy scales. Regarding flavor implications, we analyze its contributions to the rare decay processes $\textit{μ}$ → $\textit{eγ}$ and $$ \mu \to \overline{e} ee $$ . In the cosmological context, we analyze the influence of right-handed neutrinos on the effective number of neutrino species, $\textit{N}$ , through interactions mediated by the $\textit{Z}$ boson. Our findings establish a lower bound of $$ {m}_{Z^{\prime }} $$ > 4.4 TeV, which slightly exceeds the current lower limit set by the Large Hadron Collider (LHC).

Sweden

Vinícius Oliveira

Brazil

Patricio Escalona

Chile

Lucia Angel

C. Pires

Farinaldo Queiroz

Type-II seesaw mechanism for Dirac neutrinos and its implications on $\textit{N}$

Dark matter (DM) genesis via Ultraviolet (UV) freeze-in embeds the seed of reheating temperature and dynamics in its relic density. Thus, discovery of such a DM candidate can possibly open the window for post-inflationary dynamics. However, there are several challenges in this exercise, as freezing-in DM possesses feeble interaction with the visible sector and therefore very low production cross-section at the collider. We show that mono-photon (and dilepton) signal at the ILC, arising from DM effective operators connected to the SM field strength tensors, can still warrant a signal discovery. We study both the scalar and fermionic DM production during reheating via UV freeze-in, when the inflaton oscillates at the bottom of a general monomial potential. Interestingly, we see, right DM abundance can be achieved only in the case of bosonic reheating scenario, satisfying bounds from big bang nucleosynthesis (BBN). This provides a unique correlation between collider signal and the post-inflationary dynamics of the Universe within single-field inflationary models.

India

Basabendu Barman

Subhaditya Bhattacharya

Sahabub Jahedi

Dipankar Pradhan

Abhik Sarkar

Lepton collider as a window to reheating via freezing in dark matter detection. Part II

The observation of a resonance structure in the opening angle of the electron-positron pairs in the $$^{7}$$ Li(p,e $$^+$$ e $$^-$$ ) $$^{8}$$ Be reaction was claimed and interpreted as the production and subsequent decay of a hypothetical particle (X17). Similar excesses, consistent with this particle, were later observed in processes involving $$^{4}$$ He and $$^{12}$$ C nuclei with the same experimental technique. The MEG II apparatus at PSI, designed to search for the $$\mu ^+ \rightarrow \textrm{e}^+ \gamma $$ decay, can be exploited to investigate the existence of this particle and study its nature. Protons from a Cockroft–Walton accelerator, with an energy up to 1.1 MeV, were delivered on a dedicated Li-based target. The $$\gamma $$ and the e $$^{+}$$ e $$^{-}$$ pair emerging from the $$^8\textrm{Be}^*$$ transitions were studied with calorimeters and a spectrometer, featuring a broader angular acceptance than previous experiments. We present in this paper the analysis of a four-week data-taking in 2023 with a beam energy of 1080 keV, resulting in the excitation of two different resonances with Q-value 17.6 and 18.1 MeV. No significant signal was found, and limits at 90% C.L. on the branching ratios (relative to the $$\gamma $$ emission) of the two resonances to X17 were set, $$R_{17.6} <{1.8}\,\times \,10^{-6}$$ and $$R_{18.1} < {1.2}\,\times \,10^{-5}$$ in the mass range between $${16.5}\hbox { MeV}/\hbox {c}^{2}$$ and $${17.1}\hbox { MeV}/\hbox {c}^{2}$$ .

Russia

K. Afanaciev

Italy

A. Baldini

Japan

S. Ban

H. Benmansour

G. Boca

P. Cattaneo

G. Cavoto

F. Cei

M. Chiappini

A. Corvaglia

Switzerland

G. Maso

A. Bari

M. Gerone

L. Barusso

M. Francesconi

L. Galli

G. Gallucci

F. Gatti

L. Gerritzen

F. Grancagnolo

E. Grandoni

M. Grassi

D. Grigoriev

M. Hildebrandt

United Kingdom

F. Ignatov

F. Ikeda

T. Iwamoto

S. Karpov

P. Kettle

N. Khomutov

A. Kolesnikov

N. Kravchuk

V. Krylov

Search for the X17 particle in $$^{7}\textrm{Li}(\textrm{p},\textrm{e}^+ \textrm{e}^{-}) ^{8}\textrm{Be}$$ processes with the MEG II detector