arXiv subjects

Xun-Jie Xu

Publications and source records attributed to Xun-Jie Xu.

At least 55 records · Page 3Linked to original sources

arXiv2022

Searching for new physics from SMEFT and leptoquarks at the P2 experiment

The P2 experiment aims at high-precision measurements of the parity-violating asymmetry in elastic electron-proton and electron-$^{12}$C scatterings with longitudinally polarized electrons. We discuss here the sensitivity of P2 to leptoquarks, which within the P2 energy range can be described in the language of Standard Model Effective Field Theory (SMEFT). We give the expected P2 limits on the SMEFT operators and on the leptoquark parameters, which will test energy scales up to 15 TeV. In many cases those limits exceed current constraints from LHC and atomic parity violation (APV) experiments. We also demonstrate that degeneracies of different SMEFT operators can partially be resolved by use of APV experiments and different targets (protons and $^{12}$C) at P2. Moreover, we show that P2 could confirm or resolve potential tensions between the theoretical and experimental determinations of the weak charge of $^{133}$Cs.

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arXiv2022

High-energy colliders as a probe of neutrino properties

The mediators of neutrino mass generation can provide a probe of neutrino properties at the next round of high-energy hadron (FCC-hh) and lepton colliders (FCC-ee/ILC/CEPC/CLIC). We show how the decays of the Higgs triplet scalars mediating the simplest seesaw mechanism can shed light on the neutrino mass scale and mass-ordering, as well as the atmospheric octant. Four-lepton signatures at the high-energy frontier may provide the discovery-site for charged lepton flavour non-conservation in nature, rather than low-energy intensity frontier experiments.

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arXiv2022

Towards deconstructing the simplest seesaw mechanism

The triplet or type-II seesaw mechanism is the simplest way to endow neutrinos with mass in the Standard Model (SM). Here we review its associated theory and phenomenology, including restrictions from $S$, $T$, $U$ parameters, neutrino experiments, charged lepton flavour violations as well as collider searches. We also examine restrictions coming from requiring consistency of electroweak symmetry breaking, i.e. perturbative unitarity and stability of the vacuum. Finally, we discuss novel effects associated to the scalar mediator of neutrino mass generation namely, (i) rare processes, e.g. $l_α\to l_βγ$ decays, at the intensity frontier, and also (ii) four-lepton signatures in colliders at the high-energy frontier. These can be used to probe neutrino properties in an important way, providing a test of the absolute neutrino mass and mass-ordering, as well as of the atmospheric octant. They may also provide the first evidence for charged lepton flavour violation in nature. In contrast, neutrino non-standard interaction strengths are found to lie below current detectability.

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arXiv2022

Neutrino forces and the Sommerfeld enhancement

The Sommerfeld enhancement plays an important role in dark matter (DM) physics, and can significantly enhance the annihilation cross section of non-relativistic DM particles. In this paper, we study the effect of neutrino forces, which are generated by the exchange of a pair of light neutrinos, on the Sommerfeld enhancement. We demonstrate that in certain cases, a neutrino force can cause a significant correction to the Sommerfeld enhancement. Models that can realise DM-neutrino interactions and sizeable Sommerfeld enhancement are also briefly discussed, together with the impacts on DM phenomenology of neutrino forces.

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arXiv2021

Dark matter produced from neutrinos

In the presence of interactions between neutrinos and dark matter (DM), DM can potentially be produced via freeze-in from the neutrino sector. We investigate the implications of such a scenario for the evolution of both DM and neutrinos in the early Universe, and show that the future cosmic neutrino detection experiment PTOLEMY might be sensitive to neutrino signals that originate from DM annihilation in this model.

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arXiv2021

Probing the muon $g-2$ with future beam dump experiments

We consider the light $Z'$ explanation of the muon $g-2$ anomaly. Even if such a $Z'$ has no tree-level coupling to electrons, in general one will be induced at loop-level. We show that future beam dump experiments are powerful enough to place stringent constraints on$-$or discover$-$a $Z'$ with loop-suppressed couplings to electrons. Such bounds are avoided only if the $Z'$ has a large interaction with neutrinos, in which case the scenario will be bounded by ongoing neutrino scattering experiments. The complementarity between beam dump and neutrino scattering experiments therefore indicates that there are good prospects of probing a large part of the $Z'$ parameter space in the near future.

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arXiv2021

Searching for $Z'$ bosons at the P2 experiment

The P2 experiment aims at high-precision measurements of the parity-violating asymmetry in elastic electron-proton and electron-$^{12}$C scatterings with longitudinally polarized electrons. We discuss here the sensitivity of P2 to new physics mediated by an additional neutral gauge boson $Z'$ of a new $U(1)'$ gauge symmetry. If the charge assignment of the $U(1)'$ is chiral, i.e., left- and right-handed fermions have different charges under $U(1)'$, additional parity-violation is induced directly. On the other hand, if the $U(1)'$ has a non-chiral charge assignment, additional parity-violation can be induced via mass or kinetic $Z$-$Z'$ mixing. By comparing the P2 sensitivity to existing constraints, we show that in both cases P2 has discovery potential over a wide range of $Z'$ mass. In particular, for chiral models, the P2 experiment can probe gauge couplings at the order of $10^{-5}$ when the $Z'$ boson is light, and heavy $Z'$ bosons up to 79 (90) TeV in the proton ($^{12}$C) mode. For non-chiral models with mass mixing, the P2 experiment is sensitive to mass mixing angles smaller than roughly $10^{-4}$, depending on model details and gauge coupling magnitude.

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arXiv2021

Dark matter electromagnetic dipoles: the WIMP expectation

We perform a systematic study of the electric and magnetic dipole moments of dark matter (DM) that are induced at the one-loop level when DM experiences four-fermion interactions with Standard Model (SM) charged fermions. Related to their loop nature these moments can largely depend on the UV completion at the origin of the four-fermion operators. We illustrate this property by considering explicitly two simple ways to generate these operators, from $t$- or $s$-channel tree-level exchange. Fixing the strength of these interactions from the DM relic density constraint, we obtain in particular a magnetic moment that, depending on the interaction considered, lies typically between $10^{-20}$ to $10^{-23}$ ecm or identically vanishes. These non-vanishing values induce, via photon exchange, DM-nucleus scattering cross sections that could be probed by current or near future direct detection experiments.

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arXiv2021

Dirac neutrinos and $N_{\rm eff}$ II: the freeze-in case

We discuss Dirac neutrinos whose right-handed component $ν_R$ has new interactions that may lead to a measurable contribution to the effective number of relativistic neutrino species $N_{\rm eff}$. We aim at a model-independent and comprehensive study on a variety of possibilities. Processes for $ν_R$-genesis from decay or scattering of thermal species, with spin-0, spin-1/2, or spin-1 initial or final states are all covered. We calculate numerically and analytically the contribution of $ν_R$ to $N_{\rm eff}$ primarily in the freeze-in regime, since the freeze-out regime has been studied before. While our approximate analytical results apply only to freeze-in, our numerical calculations work for freeze-out as well, including the transition between the two regimes. Using current and future constraints on $N_{\rm eff}$, we obtain limits and sensitivities of CMB experiments on masses and couplings of the new interactions. As a by-product, we obtain the contribution of Higgs-neutrino interactions, $ΔN_{\rm eff}^{\rm SM} \approx 7.5\times10^{-12}$, assuming the neutrino mass is 0.1 eV and generated by the standard Higgs mechanism.

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arXiv2021

How dark is the $ν_R$-philic dark photon?

We consider a generic dark photon that arises from a hidden $U(1)$ gauge symmetry imposed on right-handed neutrinos ($ν_{R}$). Such a $ν_{R}$-philic dark photon is naturally dark due to the absence of tree-level couplings to normal matter. However, loop-induced couplings to charged leptons and quarks are inevitable, provided that $ν_{R}$ mix with left-handed neutrinos via Dirac mass terms. We investigate the loop-induced couplings and find that the $ν_{R}$-philic dark photon is not inaccessibly dark, which could be of potential importance to future dark photon searches at SHiP, FASER, Belle-II, LHC 14 TeV, etc.

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arXiv2021

Wolfenstein potentials for neutrinos induced by ultra-light mediators

New physics can emerge at low energy scales, involving very light and very weakly interacting new particles. These particles can mediate interactions between neutrinos and usual matter and contribute to the Wolfenstein potential relevant for neutrino oscillations. We compute the Wolfenstein potential in the presence of ultra-light scalar and vector mediators and study the dependence of the potential on the mediator mass $m_A$, taking the finite size of matter distribution (Earth, Sun, supernovae) into consideration. For ultra-light mediators with $m_{A}^{-1}$ comparable to the size of the medium ($R$), the usual $m_{A}^{-2}$ dependence of the potential is modified. In particular, when $m_{A}^{-1}\gg R$, the potential does not depend on $m_{A}$. Taking into account existing bounds on light mediators, we find that for the scalar case significant effects on neutrino propagation are not possible, while for the vector case large matter effects are allowed for $m_{A} \in [2\times10^{-17}$, $4\times10^{-14}]$ eV and the gauge coupling $g\sim 10^{-25}$.

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arXiv2020

Flavoured Neutrinoless Double Beta Decay

We discuss a mechanism of neutrinoless double beta decay, where neutrinos of different flavours come into play. This is realized by effective flavour-violating scalar interactions. As one consequence, we find that within the normal mass ordering the neutrino effective mass may no longer vanish due to contributions from other flavours. We evaluate the necessary nuclear matrix elements, consider the interference between the standard diagram and the new scalar one, and analyze a UV-complete model that realizes the scalar interaction. Tests of the complete model are possible at colliders and future neutrino experiments. Our scenario represents an alternative mechanism for neutrinoless double beta decay, where nevertheless lepton number violation resides only in Majorana mass terms of light neutrinos.

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arXiv2020

The $ν_{R}$-philic scalar: its loop-induced interactions and Yukawa forces in LIGO observations

Right-handed neutrinos ($ν_{R}$) are often considered as a portal to new hidden physics. It is tempting to consider a gauge singlet scalar $(ϕ)$ that exclusively couples to $ν_{R}$ via a $ν_{R}ν_{R}ϕ$ term. Such a $ν_{R}$-philic scalar does not interact with charged fermions at tree level but loop-induced effective interactions are inevitable, which are systematically investigated in this work. The magnitude of the loop-induced couplings coincidentally meets the current sensitivity of fifth-force searches. In particular, the loop-induced coupling to muons could be tested in the recent LIGO observations of neutron star mergers as there might be a sizable Yukawa force in the binary system mediated by the $ν_{R}$-philic scalar.

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arXiv2020

Revisiting Neutrino Self-Interaction Constraints from $Z$ and $τ$ decays

Given the elusive nature of neutrinos, their self-interaction is particularly difficult to probe. Nevertheless, upper limits on the strength of such an interaction can be set by using data from terrestrial experiments. In this work we focus on additional contributions to the invisible decay width of $Z$ boson as well as the leptonic $τ$ decay width in the presence of a neutrino coupling to a relatively light scalar. For invisible $Z$ decays we derive a complete set of constraints by considering both three-body bremsstrahlung as well as the loop correction to two-body decays. While the latter is usually regarded to give rather weak limits we find that through the interference with the Standard Model diagram it actually yields a competitive constraint. As far as leptonic decays of $τ$ are concerned, we derive a first limit on neutrino self-interactions that is valid across the whole mass range of a light scalar mediator. Our bounds on the neutrino self-interaction are leading for $m_ϕ\gtrsim 300$ MeV and interactions that prefer $ν_τ$. Bounds on such $ν$-philic scalar are particularly relevant in light of the recently proposed alleviation of the Hubble tension in the presence of such couplings.

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arXiv2020

MUonE sensitivity to new physics explanations of the muon anomalous magnetic moment

The MUonE experiment aims at a precision measurement of the hadronic vacuum polarization contribution to the muon $g-2$, via elastic muon-electron scattering. Since the current muon $g-2$ anomaly hints at the potential existence of new physics (NP) related to the muon, the question then arises as to whether the measurement of hadronic vacuum polarization in MUonE could be affected by the same NP as well. In this work, we address this question by investigating a variety of NP explanations of the muon $g-2$ anomaly via either vector or scalar mediators with either flavor-universal, non-universal or even flavor-violating couplings to electrons and muons. We derive the corresponding MUonE sensitivity in each case and find that the measurement of hadronic vacuum polarization at the MUonE is not vulnerable to any of these NP scenarios.

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arXiv2020

Dirac neutrinos and $N_{\rm eff}$

If neutrinos are Dirac particles the existence of light right-handed neutrinos $ν_{R}$ is implied. Those would contribute to the effective number of relativistic neutrino species $N_{\rm eff}$ in the early Universe. With pure standard model interactions, the contribution is negligibly small. In the presence of new interactions, however, the contribution could be significantly enhanced. We consider the most general effective four-fermion interactions for neutrinos (scalar, pseudo-scalar, vector, axial-vector and tensor), and compute the contribution of right-handed neutrinos to $N_{\rm eff}$. Taking the Planck 2018 measurement of $N_{\rm eff}$, strong constraints on the effective four-fermion coupling are obtained, corresponding to interaction strengths of $10^{-5}\sim10^{-3}$ in units of the Fermi constant. This translates in new physics scales of up to 43 TeV and higher. Future experiments such as CMB-S4 can probe or exclude the existence of effective 4-neutrino operators for Dirac neutrinos. Ways to avoid this conclusion are discussed.

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arXiv2020

Neutrino Self-Interactions and Double Beta Decay

Neutrino Self-Interactions ($ν$SI) beyond the Standard Model are an attractive possibility to soften cosmological constraints on neutrino properties and also to explain the tension in late and early time measurements of the Hubble expansion rate. The required strength of $ν$SI to explain the $4σ$ Hubble tension is in terms of a point-like effective four-fermion coupling that can be as high as $10^9\, G_F$, where $G_F$ is the Fermi constant. In this work, we show that such strong $ν$SI can cause significant effects in two-neutrino double beta decay, leading to an observable enhancement of decay rates and to spectrum distortions. We analyze self-interactions via an effective operator as well as when mediated by a light scalar. Data from observed two-neutrino double beta decay is used to constrain $ν$SI, which rules out the regime around $10^9\, G_F$.

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arXiv2020

Borexino and General Neutrino Interactions

We derive constraints on all possible general neutrino-electron interactions (scalar, vector, pseudoscalar, axialvector and tensor) using the recent real time Borexino event rate measurements of $pp$, $pep$ and $^{7}Be$ solar neutrinos. The limits improve several previous ones from TEXONO and CHARM-II for incoming electron and muon neutrinos, and are the first ones for the tau flavor. Future improvements by next-generation solar neutrino experiments are also studied. The limits extend the physics reach of solar neutrino measurements to TeV-scale physics. Finally, the different properties of the new interactions for Dirac and Majorana neutrinos are discussed.

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