We explore the potential of the European Spallation Source (ESS) in probing physics within and beyond the Standard Model (SM), based on future measurements of coherent elastic neutrino-nucleus scattering (CE$\textit{ν}$NS). We consider two SM physics cases, namely the weak mixing angle and the nuclear radius. Regarding physics beyond the SM, we focus on neutrino generalized interactions (NGIs) and on various aspects of sterile neutrino and sterile neutral lepton phenomenology. For this, we explore the violation of lepton unitarity, active-sterile oscillations as well as interesting upscattering channels such as the sterile dipole portal and the production of sterile neutral leptons via NGIs. The projected ESS sensitivities are estimated by performing a statistical analysis considering the various CE$\textit{ν}$NS detectors and expected backgrounds. We find that the enhanced statistics achievable in view of the highly intense ESS neutrino beam, will offer a drastic improvement in the current constraints obtained from existing CE$\textit{ν}$NS measurements. Finally, we discuss how the ESS has the potential to provide the leading CE$\textit{ν}$NS-based constraints, complementing also further experimental probes and astrophysical observations.

India

Ayan Chattaraj

Anirban Majumdar

Spain

Dimitrios Papoulias

Rahul Srivastava

Probing conventional and new physics at the ESS with coherent elastic neutrino-nucleus scattering

<p>Coherent elastic neutrino-nucleus (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>CE</mi><mi>ν</mi><mi>NS</mi></mrow></math>) and elastic neutrino-electron scattering (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi mathvariant="normal">E</mi><mi>ν</mi><mi>ES</mi></mrow></math>) data are exploited to constrain “chiral” <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><msub><mo stretchy="false">)</mo><mi>X</mi></msub></math> gauged models with light vector mediator mass. These models fall under a distinct class of new symmetries called dark hypercharge symmetries. A key feature is the fact that the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> boson can couple to all Standard Model fermions at tree level, with the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><msub><mo stretchy="false">)</mo><mi>X</mi></msub></math> charges determined by the requirement of anomaly cancellation. Notably, the charges of leptons and quarks can differ significantly depending on the specific anomaly cancellation solution. As a result, different models exhibit distinct phenomenological signatures and can be constrained through various experiments. In this work, we analyze the recent data from the COHERENT experiment, along with results from dark matter (DM) direct detection experiments such as XENONnT, LUX-ZEPLIN, and PandaX-4T, and place new constraints on three benchmark models. Additionally, we set constraints from a performed analysis of TEXONO data and discuss the prospects of improvement in view of the next-generation DM direct detection DARWIN experiment.</p>

Dimitrios K. Papoulias

Hemant Prajapati

Constraining low scale dark hypercharge symmetry at spallation, reactor, and dark matter direct detection experiments

The Dirac Scotogenic model provides an elegant mechanism for generating small Dirac neutrino masses at the one-loop level. A single abelian discrete $$ {\mathcal{Z}}_6 $$ symmetry simultaneously protects the “Diracness” of the neutrinos and the stability of the dark matter candidate. This symmetry originates as an unbroken subgroup of the so-called $\textit{445}$ U(1) symmetry. Here, we thoroughly explore the phenomenological implications of this construction, including an analysis of electroweak vacuum stability, charged lepton flavor violation, and the dark matter phenomenology. After considering all constraints, we also show that the model allows for the possibility of novel low-mass scalar and fermionic dark matter, a feature not shared by its canonical Majorana counterpart.

Germany

Salvador Chuliá

Sushant Yadav

Comprehensive phenomenology of the Dirac Scotogenic Model: Novel low-mass dark matter

We explore the potential of reactor antineutrino-induced Coherent Elastic Neutrino-Nucleus Scattering (CE <italic>&#957;</italic>NS) data from the CONUS+ experiment to investigate both the Standard Model (SM) and Beyond Standard Model (BSM) scenarios. Alongside CE <italic>&#957;</italic>NS, Elastic Neutrino-Electron Scattering (E <italic>&#957;</italic>ES) events are also included in our analysis, enabling more stringent constraints on new physics. Within the SM, we examine the weak mixing angle as a precision test of the electroweak sector. For BSM scenarios, we constrain the parameter space of light mediators arising from neutrino generalized interactions (NGI), while also setting limits on the electromagnetic properties of neutrinos, including their charge radius, millicharge, and magnetic moment.

Probing standard model and beyond with reactor CE <italic>&#957;</italic>NS data of CONUS+ experiment

Small neutrino masses can be sourced by a tiny vacuum expectation value of a leptophilic Higgs doublet, and mediated by Quasi-Dirac heavy neutrinos. In such simplest linear seesaw picture the neutrino mass mediators can be accessible to colliders. We describe novel charged Higgs and heavy neutrino production mechanisms that can be sizeable at <math><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup></math>, <math><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup><mi>γ</mi></math>, pp, or muon colliders and discuss some of the associated signatures. The oscillation length of the heavy neutrino mediators is directly related to the light neutrino mass ordering. Moreover, lepton number violation can be large despite the smallness of neutrino masses, and may shed light on the Majorana nature of neutrinos and the significance of basic symmetries in weak interaction.

Portugal

Aditya Batra

Praveen Bharadwaj

South Korea

Sanjoy Mandal

José W.F. Valle

Large lepton number violation at colliders: Predictions from the minimal linear seesaw mechanism

We introduce a framework for hybrid neutrino mass generation, wherein scotogenic dark sector particles, including dark matter, are charged non-trivially under the A 4 flavor symmetry. The spontaneous breaking of the A 4 group to residual   <math> <msub> <mi>Z</mi> <mn>2</mn> </msub> </math>  $$ {\mathcal{Z}}_2 $$  subgroup results in the “cutting” of the radiative loop. As a consequence the neutrinos acquire mass through the hybrid “scoto-seesaw” mass mechanism, combining aspects of both the tree-level seesaw and one-loop scotogenic mechanisms, with the residual   <math> <msub> <mi>Z</mi> <mn>2</mn> </msub> </math>  $$ {\mathcal{Z}}_2 $$  subgroup ensuring the stability of the dark matter. The flavor symmetry also leads to several predictions including the normal ordering of neutrino masses and “generalized μ − τ reflection symmetry” in leptonic mixing. Additionally, it gives testable predictions for neutrinoless double beta decay and a lower limit on the lightest neutrino mass. Finally, A 4 →   <math> <msub> <mi>Z</mi> <mn>2</mn> </msub> </math>  $$ {\mathcal{Z}}_2 $$  breaking also leaves its imprint on the dark sector and ties it with the neutrino masses and mixing. The model allows only scalar dark matter, whose mass has a theoretical upper limit of ≲ 600 GeV, with viable parameter space satisfying all dark matter constraints, available only up to about 80 GeV. Conversely, fermionic dark matter is excluded due to constraints from the neutrino sector. Various aspects of this highly predictive framework can be tested in both current and upcoming neutrino and dark matter experiments.

Ranjeet Kumar

Italy

Newton Nath

Cutting the scotogenic loop: adding flavor to dark matter

The relativistic degrees of freedom (<math><msub><mi>N</mi><mi>eff</mi></msub></math>) is one of the crucial cosmological parameters. The precise measurement of <math><msub><mi>N</mi><mi>eff</mi></msub></math> at the time of cosmic microwave background formation, by Planck 2018 can be used to understand the new fundamental interactions, in particular involving light mediators. Presence of any new particle with sufficient energy density and sizeable interactions with Standard Model particles at the temperature around <math><mo>∼</mo><mi>MeV</mi></math> can significantly alter the neutrino decoupling and hence <math><msub><mi>N</mi><mi>eff</mi></msub></math>. Thus the bound on <math><msub><mi>N</mi><mi>eff</mi></msub></math> can place stringent constraints on various beyond Standard Model paradigms involving light particles. <math><mrow><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>X</mi></msub></mrow></math> models are among such scenarios and are widely studied in several aspects. In this work, we consider several popular <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>X</mi></msub></math> models with light <math><msup><mi>Z</mi><mo>′</mo></msup></math> boson like <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub></math>, <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><mi>B</mi><mo>−</mo><mn>3</mn><msub><mi>L</mi><mi>i</mi></msub></mrow></msub></math>, <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><msub><mi>B</mi><mi>i</mi></msub><mo>−</mo><mn>3</mn><msub><mi>L</mi><mi>j</mi></msub></mrow></msub></math>, <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><msub><mi>L</mi><mi>i</mi></msub><mo>−</mo><msub><mi>L</mi><mi>j</mi></msub></mrow></msub></math>; <math><mrow><mrow><mi>i</mi></mrow><mo>,</mo><mrow><mi>j</mi><mo>=</mo><mn>1</mn></mrow><mo>,</mo><mn>2</mn><mo>,</mo><mn>3</mn></mrow></math> being the flavor indices and study their impact on <math><msub><mi>N</mi><mi>eff</mi></msub></math>. We also examine the constraints from ground based experiments like Xenon1T, Borexino, trident, etc. Our analysis shows that for light mass <math><msub><mi>M</mi><msup><mi>Z</mi><mo>′</mo></msup></msub><mo>≲</mo><mi>O</mi><mo>(</mo><mi>MeV</mi><mo>)</mo></math> the <math><msub><mi>N</mi><mi>eff</mi></msub></math> provides the most stringent constraints on the <math><msup><mi>Z</mi><mo>′</mo></msup></math> mass and coupling, far exceeding the existing constraints from other experiments.

Dilip Kumar Ghosh

Purusottam Ghosh

Sk Jeesun

<math><mrow><msub><mrow><mi>N</mi></mrow><mrow><mi>eff</mi></mrow></msub></mrow></math> at CMB challenges <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>X</mi></msub></math> light gauge boson scenarios

The current upper limit on  $$N_\textrm{eff}$$  <math> <msub> <mi>N</mi> <mtext>eff</mtext> </msub> </math>   at the time of CMB by Planck 2018 can place stringent constraints in the parameter space of BSM paradigms where their additional interactions may affect neutrino decoupling. Motivated by this fact in this paper we explore the consequences of light gauge boson ( $$Z'$$  <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math>  ) emerging from local  $$U(1)_X$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>X</mi> </msub> </mrow> </math>   symmetry in  $$N_\textrm{eff}$$  <math> <msub> <mi>N</mi> <mtext>eff</mtext> </msub> </math>   at the time of CMB. First, we analyze the generic  $$U(1)_X$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>X</mi> </msub> </mrow> </math>   models with arbitrary charge assignments for the SM fermions and show that, in the context of  $$N_\textrm{eff}$$  <math> <msub> <mi>N</mi> <mtext>eff</mtext> </msub> </math>   the generic  $$U(1)_X$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>X</mi> </msub> </mrow> </math>   gauged models can be broadly classified into two categories, depending on the charge assignments of first generation leptons. We then perform a detailed analysis with two specific  $$U(1)_X$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>X</mi> </msub> </mrow> </math>   models:  $$U(1)_{B_3-3L_e}$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <msub> <mi>B</mi> <mn>3</mn> </msub> <mo>-</mo> <mn>3</mn> <msub> <mi>L</mi> <mi>e</mi> </msub> </mrow> </msub> </mrow> </math>   and  $$U(1)_{B_3-3L_\mu }$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <msub> <mi>B</mi> <mn>3</mn> </msub> <mo>-</mo> <mn>3</mn> <msub> <mi>L</mi> <mi>μ</mi> </msub> </mrow> </msub> </mrow> </math>   and explore the contribution in  $$N_\textrm{eff}$$  <math> <msub> <mi>N</mi> <mtext>eff</mtext> </msub> </math>   due to the presence of  $$Z'$$  <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math>   realized in those models. For comparison, we also showcase the constraints from low energy experiments like: Borexino, Xenon 1T, neutrino trident, etc. We show that in a specific parameter space, particularly in the low mass region of  $$Z'$$  <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math>  , the bound from  $$N_\textrm{eff}$$  <math> <msub> <mi>N</mi> <mtext>eff</mtext> </msub> </math>   (Planck 2018) is more stringent than the experimental constraints. Additionally, a part of the regions of the same parameter space may also relax the  $$H_0$$  <math> <msub> <mi>H</mi> <mn>0</mn> </msub> </math>   tension.

Dilip Ghosh

Hubble tension and cosmological imprints of  $$U(1)_X$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mi>X</mi> </msub> </mrow> </math>   gauge symmetry:  $$U(1)_{B_3-3 L_i}$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <msub> <mi>B</mi> <mn>3</mn> </msub> <mo>-</mo> <mn>3</mn> <msub> <mi>L</mi> <mi>i</mi> </msub> </mrow> </msub> </mrow> </math>   as a case study

The simplest linear seesaw mechanism can accommodate the new CDF-II <italic>W</italic> mass measurement. In addition to Standard Model particles, the model includes quasi-Dirac leptons, and a second, leptophilic, scalar doublet seeding small neutrino masses. Our proposal is consistent with electroweak precision tests, neutrino physics, rare decays and collider restrictions, requiring a new charged scalar below a few TeV, split in mass from the new degenerate scalar and pseudoscalar neutral Higgs bosons.

W-mass anomaly in the simplest linear seesaw mechanism

This paper's novelty lies in introducing a hybrid scoto-seesaw model rooted in <math><msub><mrow><mi>A</mi></mrow><mrow><mn>4</mn></mrow></msub></math> discrete modular symmetry leading to several interesting phenomenological implications. The scoto-seesaw framework leads to generation of one mass square difference <math><mo>(</mo><mi>Δ</mi><msubsup><mrow><mi>m</mi></mrow><mrow><mi>atm</mi></mrow><mrow><mn>2</mn></mrow></msubsup></math>) using the type-I seesaw mechanism at the tree level. Additionally, the scotogenic contribution is vital in obtaining the other mass square difference (<math><mi>Δ</mi><msubsup><mrow><mi>m</mi></mrow><mrow><mi>sol</mi></mrow><mrow><mn>2</mn></mrow></msubsup></math>) at the loop level, thus providing a clear interpretation of the two different mass square differences. The non-trivial transformation of Yukawa couplings under the <math><msub><mrow><mi>A</mi></mrow><mrow><mn>4</mn></mrow></msub></math> modular symmetry helps to explore neutrino phenomenology with a specific flavor structure of the mass matrix. In addition to predictions for neutrino mass ordering, mixing angles and CP phases, this setup leads to precise predictions for <math><mo>∑</mo><msub><mrow><mi>m</mi></mrow><mrow><mi>i</mi></mrow></msub></math> as well as <math><mo>|</mo><msub><mrow><mi>m</mi></mrow><mrow><mi>e</mi><mi>e</mi></mrow></msub><mo>|</mo></math>. In particular, the model predicts <math><mo>∑</mo><msub><mrow><mi>m</mi></mrow><mrow><mi>i</mi></mrow></msub><mo>∈</mo><mo>(</mo><mn>0.073</mn><mo>,</mo><mn>0.097</mn><mo>)</mo></math> eV and <math><mrow><mo>|</mo><msub><mrow><mi>m</mi></mrow><mrow><mi>e</mi><mi>e</mi></mrow></msub><mo>|</mo></mrow><mo>∈</mo><mo>(</mo><mn>3.15</mn><mo>,</mo><mn>6.66</mn><mo>)</mo><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup></math> eV range; within reach of upcoming experiments. Furthermore, our model is also promising for addressing lepton flavor violations, i.e., <math><msub><mrow><mi>ℓ</mi></mrow><mrow><mi>α</mi></mrow></msub><mo>→</mo><msub><mrow><mi>ℓ</mi></mrow><mrow><mi>β</mi></mrow></msub><mi>γ</mi></math>, <math><msub><mrow><mi>ℓ</mi></mrow><mrow><mi>α</mi></mrow></msub><mo>→</mo><mn>3</mn><msub><mrow><mi>ℓ</mi></mrow><mrow><mi>β</mi></mrow></msub></math> and <math><mi>μ</mi><mo>−</mo><mi>e</mi></math> conversion rates while staying within the realm of current experimental limits.