Ultralight scalars in leptonic observables

Abstract Many new physics scenarios contain ultralight scalars, states which are either exactly massless or much lighter than any other massive particle in the model. Axions and majorons constitute well-motivated examples of this type of particle. In this work, we explore the phenomenology of these states in low-energy leptonic observables. After adopting a model independent approach that includes both scalar and pseudoscalar interactions, we briefly discuss the current limits on the diagonal couplings to charged leptons and consider processes in which the ultralight scalar ϕ is directly produced, such as μ → eϕ, or acts as a mediator, as in τ → μμμ. Contributions to the charged leptons magnetic and electric moments are studied as well.

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Z lepton flavour violation as a probe for new physics at future $$e^+e^-$$ colliders

The European Physical Journal C ◽

10.1140/epjc/s10052-021-09777-3 ◽

2021 ◽

Vol 81 (12) ◽

Author(s):

Lorenzo Calibbi ◽

Xabier Marcano ◽

Joydeep Roy

Keyword(s):

Field Theory ◽

Effective Field Theory ◽

New Physics ◽

Effective Field ◽

Low Energy ◽

The Standard Model ◽

Discovery Potential ◽

Flavour Violation ◽

Model Independent ◽

Lepton Flavour

AbstractIn this work we assess the potential of discovering new physics by searching for lepton-flavour-violating (LFV) decays of the Z boson, $$Z\rightarrow \ell _i \ell _j$$ Z → ℓ i ℓ j , at the proposed circular $$e^+e^-$$ e + e - colliders CEPC and FCC-ee. Both projects plan to run at the Z-pole as a “Tera Z factory”, i.e., collecting $${\mathcal {O}}\left( 10^{12} \right) $$ O 10 12 Z decays. In order to discuss the discovery potential in a model-independent way, we revisit the LFV Z decays in the context of the Standard Model effective field theory and study the indirect constraints from LFV $$\mu $$ μ and $$\tau $$ τ decays on the operators that can induce $$Z\rightarrow \ell _i \ell _j$$ Z → ℓ i ℓ j . We find that, while the $$Z\rightarrow \mu e$$ Z → μ e rates are beyond the expected sensitivities, a Tera Z factory is promising for $$Z\rightarrow \tau \ell $$ Z → τ ℓ decays, probing New Physics at the same level of future low-energy LFV observables.

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Charming ALPs

Journal of High Energy Physics ◽

10.1007/jhep08(2021)121 ◽

2021 ◽

Vol 2021 (8) ◽

Author(s):

Adrian Carmona ◽

Christiane Scherb ◽

Pedro Schwaller

Keyword(s):

Ultra Violet ◽

New Physics ◽

Low Energy ◽

Effective Theory ◽

Tree Level ◽

Meson Decay ◽

Fixed Target ◽

The Standard Model ◽

Model Independent ◽

Key Features

Abstract Axion-like particles (ALPs) are ubiquitous in models of new physics explaining some of the most pressing puzzles of the Standard Model. However, until relatively recently, little attention has been paid to its interplay with flavour. In this work, we study in detail the phenomenology of ALPs that exclusively interact with up-type quarks at the tree-level, which arise in some well-motivated ultra-violet completions such as QCD-like dark sectors or Froggatt-Nielsen type models of flavour. Our study is performed in the low-energy effective theory to highlight the key features of these scenarios in a model independent way. We derive all the existing constraints on these models and demonstrate how upcoming experiments at fixed-target facilities and the LHC can probe regions of the parameter space which are currently not excluded by cosmological and astrophysical bounds. We also emphasize how a future measurement of the currently unavailable meson decay D → π + invisible could complement these upcoming searches.

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Disentangling QCD and new physics in D → πℓ+ℓ−

Journal of High Energy Physics ◽

10.1007/jhep04(2021)158 ◽

2021 ◽

Vol 2021 (4) ◽

Author(s):

Aoife Bharucha ◽

Diogo Boito ◽

Cédric Méaux

Keyword(s):

Standard Model ◽

Branching Ratio ◽

Branching Ratios ◽

New Physics ◽

Operator Product ◽

Beyond The Standard Model ◽

Spectral Functions ◽

The Standard Model ◽

Model Independent ◽

Weak Annihilation

Abstract In this paper we consider the decay D+ → π+ℓ+ℓ−, addressing in particular the resonance contributions as well as the relatively large contributions from the weak annihilation diagrams. For the weak annihilation diagrams we include known results from QCD factorisation at low q2 and at high q2, adapting the existing calculation for B decays in the Operator Product Expansion. The hadronic resonance contributions are obtained through a dispersion relation, modelling the spectral functions as towers of Regge-like resonances in each channel, as suggested by Shifman, imposing the partonic behaviour in the deep Euclidean. The parameters of the model are extracted using e+e− → (hadrons) and τ → (hadrons) + ντ data as well as the branching ratios for the resonant decays D+ → π+R(R → ℓ+ℓ−), with R = ρ, ω, and ϕ. We perform a thorough error analysis, and present our results for the Standard Model differential branching ratio as a function of q2. Focusing then on the observables FH and AFB, we consider the sensitivity of this channel to effects of physics beyond the Standard Model, both in a model independent way and for the case of leptoquarks.

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The low-energy effective theory of axions and ALPs

Journal of High Energy Physics ◽

10.1007/jhep04(2021)063 ◽

2021 ◽

Vol 2021 (4) ◽

Author(s):

Martin Bauer ◽

Matthias Neubert ◽

Sophie Renner ◽

Marvin Schnubel ◽

Andrea Thamm

Keyword(s):

High Energy ◽

New Physics ◽

Mass Scale ◽

Low Energy ◽

Effective Theory ◽

Loop Order ◽

Flavor Changing ◽

Effective Lagrangian ◽

Anomalous Dimensions ◽

Pseudoscalar Mesons

Abstract Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model, which interact with the known particles through higher-dimensional operators suppressed by the mass scale Λ of the new-physics sector. Starting from the most general dimension-5 interactions, we discuss in detail the evolution of the ALP couplings from the new-physics scale to energies at and below the scale of electroweak symmetry breaking. We derive the relevant anomalous dimensions at two-loop order in gauge couplings and one-loop order in Yukawa interactions, carefully considering the treatment of a redundant operator involving an ALP coupling to the Higgs current. We account for one-loop (and partially two-loop) matching contributions at the weak scale, including in particular flavor-changing effects. The relations between different equivalent forms of the effective Lagrangian are discussed in detail. We also construct the effective chiral Lagrangian for an ALP interacting with photons and light pseudoscalar mesons, pointing out important differences with the corresponding Lagrangian for the QCD axion.

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Flavour Anomalies in a U(1) SUSY Extension of the SM

Symmetry ◽

10.3390/sym13020191 ◽

2021 ◽

Vol 13 (2) ◽

pp. 191

Author(s):

Alexander Bednyakov ◽

Alfiia Mukhaeva

Keyword(s):

Standard Model ◽

Parameter Space ◽

Gauge Boson ◽

New Physics ◽

Low Energy ◽

Effective Theory ◽

The Standard Model ◽

Meson Mixing ◽

Additional Anomaly ◽

Lepton Flavour

Flavour anomalies have attracted a lot of attention over recent years as they provide unique hints for possible New Physics. Here, we consider a supersymmetric (SUSY) extension of the Standard Model (SM) with an additional anomaly-free gauge U(1) group. The key feature of our model is the particular choice of non-universal charges to the gauge boson Z′, which not only allows a relaxation of the flavour discrepancies but, contrary to previous studies, can reproduce the SM mixing matrices both in the quark and lepton sectors. We pay special attention to the latter and explicitly enumerate all parameters relevant for our calculation in the low-energy effective theory. We find regions in the parameter space that satisfy experimental constraints on meson mixing and LHC Z′ searches and can alleviate the flavour anomalies. In addition, we also discuss the predictions for lepton-flavour violating decays B+→K+μτ and B+→K+eτ.

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SEARCH FOR NEW PHYSICS IN eμX DATA AT THE TEVATRON USING SLEUTH: A QUASI-MODEL-INDEPENDENT SEARCH STRATEGY FOR NEW PHYSICS

International Journal of Modern Physics A ◽

10.1142/s0217751x01008400 ◽

2001 ◽

Vol 16 (supp01b) ◽

pp. 888-890

Author(s):

◽

BRUCE KNUTESON

Keyword(s):

Standard Model ◽

Search Strategy ◽

New Physics ◽

Beyond The Standard Model ◽

Final States ◽

Final State ◽

The Standard Model ◽

Model Independent

We present a quasi-model-independent search for physics beyond the standard model. We define final states to be studied, and construct a rule that identifies a set of variables appropriate for any particular final state. A new algorithm ("Sleuth") searches for regions of excess in the space of those variables and quantifies the significance of any detected excess. After demonstrating the sensititvity of the method, we apply it to the semi-inclusive channel eμX collected in ≈108 pb -1 of [Formula: see text] collisions at [Formula: see text] at the DØ experiment at the Fermilab Tevatron. We find no evidence of new high pT physics in this sample.

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Non-standard interactions in SMEFT confronted with terrestrial neutrino experiments

Journal of High Energy Physics ◽

10.1007/jhep03(2021)019 ◽

2021 ◽

Vol 2021 (3) ◽

Author(s):

Yong Du ◽

Hao-Lin Li ◽

Jian Tang ◽

Sampsa Vihonen ◽

Jiang-Hao Yu

Keyword(s):

Neutrino Oscillation ◽

New Physics ◽

Effective Field ◽

Scalar Leptoquark ◽

Double Chooz ◽

The Standard Model ◽

Reactor Neutrino ◽

Model Independent ◽

Neutrino Experiments ◽

Individual Dimension

Abstract The Standard Model Effective Field Theory (SMEFT) provides a systematic and model-independent framework to study neutrino non-standard interactions (NSIs). We study the constraining power of the on-going neutrino oscillation experiments T2K, NOνA, Daya Bay, Double Chooz and RENO in the SMEFT framework. A full consideration of matching is provided between different effective field theories and the renormalization group running at different scales, filling the gap between the low-energy neutrino oscillation experiments and SMEFT at the UV scale. We first illustrate our method with a top- down approach in a simplified scalar leptoquark model, showing more stringent constraints from the neutrino oscillation experiments compared to collider studies. We then provide a bottom-up study on individual dimension-6 SMEFT operators and find NSIs in neutrino experiments already sensitive to new physics at ∼20 TeV when the Wilson coefficients are fixed at unity. We also investigate the correlation among multiple operators at the UV scale and find it could change the constraints on SMEFT operators by several orders of magnitude compared with when only one operator is considered. Furthermore, we find that accelerator and reactor neutrino experiments are sensitive to different SMEFT operators, which highlights the complementarity of the two experiment types.

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