arXiv subjects

Ben Allanach

Publications and source records attributed to Ben Allanach.

At least 19 recordsLinked to original sources

arXiv2026

Neutrino masses and mixing angles in the Plan B Model

The Plan B Model was proposed to improve the global fit quality of the Standard Model to observables involving the $b\rightarrow s \ell^+ \ell^-$ transition whilst simultaneously providing a qualitative explanation for the small magnitudes of some off-diagonal quark mixing matrix entries. The model is based on applying an additional spontaneously broken $U(1)_{3B_3-L_e-2L_\mu}$ gauge symmetry to the Standard Model with family-dependent fermion charges. Right-handed neutrino fields are present and cancel gauge anomalies. We investigate the implications of the Plan B Model for neutrino masses and mixing angles. The model naturally suggests a Froggatt-Nielsen mechanism for neutrino masses and mixing hierarchies. We find that an implementation of the Froggatt-Nielsen mechanism works when additional suppression is present from a clockwork mechanism. We show how current neutrino oscillation data can be fit with order unity fundamental dimensionless couplings.

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arXiv2026

The Plan B Model: $Z^{\prime}$ collider phenomenology and discovery prospects

The Plan B Model was proposed by Allanach et al. (2023) and explains some gross features of the fermion mass spectrum. It also affects the predictions of various observables involving the $b \rightarrow s$ quark-flavour transition. The model predicts a new TeV-scale $Z^{\prime}$ which decays into various different final states. We constrain the viable parameter space of the model by re-casting several LHC direct searches for the $Z^{\prime}$ in addition to using Contur to test the model against unfolded measurements of various differential cross sections that are predicted to be non-zero in the Standard Model. We delineate the regions of parameter space that are excluded by current LHC data, and estimate the projected reach of the high luminosity LHC.

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arXiv2026

The Fundaments of Unity: ${\mathcal O}(1)$ Couplings in Quantum Field Theories

We critically examine the expectation that in a fundamental quantum field theory, dimensionless couplings in the Lagrangian density should all be of order unity. We propose a measure to quantify the adherence of a theory to this: the spread (the ratio of the largest to the smallest of the magnitudes) of such dimensionless couplings, obtaining various closed-form results. If we take independent and identically distributed (IID) couplings to parameterise our uncertainty on the values of the order-unity couplings, the spread can be much larger than one might naively expect. For a theory with 20 IID unit normal couplings, the probability that the spread is greater than 100 is 0.29, for example. Even when the IID couplings have exponentially suppressed tails, the distribution of the spread has fat power-law tails whose amplitude grows with the number of independent couplings.

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arXiv2025

Reinterpretation and preservation of data and analyses in HEP

Data from particle physics experiments are unique and are often the result of a very large investment of resources. Given the potential scientific impact of these data, which goes far beyond the immediate priorities of the experimental collaborations that obtain them, it is imperative that the collaborations and the wider particle physics community publish and preserve sufficient information to ensure that this impact can be realised, now and into the future. The information to be published and preserved includes the algorithms, statistical information, simulations and the recorded data. This publication and preservation requires significant resources, and should be a strategic priority with commensurate planning and resource allocation from the earliest stages of future facilities and experiments.

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arXiv2025

A $B-$anomaly motivated $Z^\prime$ boson at the energy and precision frontiers

TeV-scale $Z^\prime$ bosons with family-dependent couplings can explain some anomalies inferred from $B-$meson measurements of processes involving the $b \rightarrow s \ell^+\ell^-$ transition. A $Z^\prime$ originating from kinetically-mixed spontaneously broken $U(1)_{B_3-L_2}$ gauge symmetry has been shown to greatly ameliorate global fits~\cite{Allanach:2024ozu} in a `flavour-preferred' region of parameter space. We provide an exploration of this region at the high luminosity (HL-)LHC with particular attention to which signals could be verified across different discovery modes. Even if the HL-LHC does not discover the $Z^\prime$ boson in a resonant di-lepton channel, a FCC-ee $Z$-pole run would detect oblique corrections to the electroweak precision observables (EWPOs). Changes due to $Z^\prime$-induced non-oblique corrections are unlikely to be detected, to within experimental precision. In any case, the extended discovery potential offered by a 100 TeV $pp-$collider would afford sensitivity to the entire flavour-preferred region and enable a fine-grained and forensic analysis of the~model.

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arXiv2025

Computation of FCC-ee Sensitivity to Heavy New Physics with Interactions of Any Flavor Structure

We present a tool to compute the sensitivity of the Future Circular Electron--Positron Collider (FCC-ee) to the interactions of new, heavy particles via publicly available extensions to the smelli and flavio computer programs. We parameterize new particles' effects without any flavor assumptions and take into account the projected experimental and correlated theoretical uncertainties of various electroweak and Higgs observables at the proposed collider. We illustrate a use of the tool by estimating the sensitivity of the FCC-ee to a $Z^\prime$ model with flavor-specific couplings which explains anomalies inferred from present-day measurements and Standard Model predictions of observables that involve the $b \rightarrow s \ell^+ \ell^-$ transition.

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arXiv2024

Constraints on the malaphoric $B_3-L_2$ model from di-lepton resonance searches at the LHC

We confront the malaphoric $B_3-L_2$ model with bounds coming from a search for resonances in the di-lepton channels at the 13~TeV LHC. In contrast to the original $B_3-L_2$ model, the $Z^\prime$ of the malaphoric $B_3-L_2$ model has sizeable couplings to the lighter two families; these originate from order unity kinetic mixing with the hypercharge gauge boson and ameliorate the fit to lepton flavour universality measurements in $B-$meson decays. The $Z^\prime$ coupling to the first two families of quark means that the resulting constraints from resonant di-lepton searches are stronger. Nevertheless, we find that for $M_{Z^\prime}>2.8$ TeV there remains a non-negligible region of allowed parameter space where the model significantly improves upon several Standard Model predictions for observables involving the $b \rightarrow s l^+ l^-$ transition. We estimate that the 3000 fb$^{-1}$ HL-LHC will extend this sensitivity to $M_{Z^\prime}= 4.2$ TeV.

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arXiv2024

Malaphoric $Z'$ models for $b \rightarrow s \ell^+ \ell^-$ anomalies

We study some phenomenological effects of kinetic mixing between the hypercharge field and a new $U(1)$ gauge field in specific $Z^\prime$ models that ameliorate the tensions between measurements and Standard Model predictions in $b \rightarrow s\ell^+ \ell^-$ decays. To this end, we rederive the dimension-6 SMEFT coefficients resulting from integrating out the kinetically-mixed (`malaphoric') $Z^\prime$ field. The kinetic mixing provides a family-universal component to the couplings of the $Z^\prime$ field, which can improve fits to lepton flavour universality observables. We show how kinetic mixing improves the best fit of the $B_3-L_2$ model to $b\rightarrow s$ data by 7.0 units of $\chi^2$ while remaining compatible with other relevant data sets such as electroweak precision observables and measurements of $e^+ e^- \rightarrow \ell^+ \ell^-$ at LEP2.

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arXiv2024

LHC di-lepton searches for $Z^\prime$ bosons which explain measurements of $b \rightarrow s l^+l^-$ transitions

Several current measurements of $b \rightarrow s l^+ l^-$ processes are in tension with Standard Model predictions, whereas others (e.g.\ $R_K$ and $R_{K^\ast}$) are in reasonable agreement with them. We examine some recent $Z^\prime$ models that fit the data as a whole appreciably better than does the Standard Model, confronting the models with ATLAS resonance searches in the $e^+e^-$ and $\mu^+\mu^-$ channels. Both channels constrain the models; we find that the searches rule out less than one fifth of the allowed range of $M_{Z^\prime}$. We estimate that the HL-LHC can roughly double the current sensitivity in $M_{Z^\prime}$, but covering the whole parameter space would take a more powerful machine, such as a 100 TeV $pp$ collider.

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arXiv2024

Supersymmetry, Part I (Theory)

This is a review of the theoretical aspects of the supersymmetric extension of the Standard Model of particle physics, extracted from Chapter 87 of the 2026 Review of Particle Physics, F. Takahashi et al. (Particle Data Group), Int. J. Mod. Phys. A 41, 2630011 (2026). The companion review, co-authored by M. D'Onofrio and F. Moortgat, "Supersymmetry, Part II (Experiment)," can be found in Chapter 88 of the 2026 Review of Particle Physics (op. cit.).

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arXiv2023

Hide and Seek with the Third Family Hypercharge Model's $Z^\prime$ at the Large Hadron Collider

The Third Family Hypercharge Model predicts a $Z^\prime$ gauge boson with flavour dependent couplings which has been used to explain anomalies in meson decay processes which involve the $b\rightarrow sμ^+μ^-$ transition. The model predicts that a TeV-scale $Z^\prime$ should decay to particle-antiparticle pairs of muons, taus, top quarks and bottom quarks with appreciable branching ratios. We reinterpret various ATLAS and CMS search limits for $Z^\prime$ production followed by such decays at the LHC over a parameter space of the model that results from a successful combined fit to the $b\rightarrow s l^+ l^-$ data and precision electroweak observables. Current exclusions in parameter space and expected sensitivities of the various different channels in the high-luminosity run are compared. We find that the high-luminosity run of the LHC will significantly increase the sensitivity to the model beyond the existing observational limits, which we find to be surprisingly weak.

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arXiv2023

Fits to measurements of rare heavy flavour decays

This write-up is intended to form part of the proceedings for Lepton-Photon 2023. We review the decays $b\rightarrow c \ell \bar ν_\ell$ (where $\ell \in \{e,μ,τ\}$) as well as $b\rightarrow s μ^+ μ^-$ and $b\rightarrow s e^+ e^-$, giving the current state-of-the-art in terms of measurements. We review fits to such data of new physics weak effective field theory operators, before closing with interpretations in terms of simplified TeV-scale field theories.

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arXiv2023

Plan B: New ${Z^\prime}$ models for $b\rightarrow sl^+l^-$ anomalies

Measurements of $b \rightarrow s \mu^+ \mu^-$ transitions indicate that there may be a new physics field coupling to di-muon pairs associated with the $b$ to $s$ flavour transition. Including the 2022 LHCb reanalysis of $R_K$ and $R_{K^\ast}$, one infers that there may also be associated new physics in $b\rightarrow e^+ e^-$ transitions. Here, we examine the extent of the statistical preference for $Z^\prime$ models coupling to di-electron pairs taking into account the relevant constraints, in particular from experiments at LEP-2. We identify an anomaly-free set of models which interpolates between the $Z^\prime$ not coupling to electrons at all, to one in which there is an equal $Z^\prime$ coupling to muons and electrons (but where in all models in the set, the $Z^\prime$ boson can mediate $b\rightarrow \mu^+ \mu^-$ transitions). A $3B_3-L_e-2L_\mu$ model provides a close-to-optimal fit to the pertinent measurements along the line of interpolation. We have (re-)calculated predictions for the relevant LEP-2 observables in terms of dimension-6 SMEFT operators and put them into the ${\tt flavio}$ computer program, so that they are available for global fits.

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arXiv2023

The Rumble in the Meson: a leptoquark versus a $Z^\prime$ to fit $b \rightarrow s μ^+ μ^-$ anomalies including 2022 LHCb $R_{K^{(\ast)}}$ measurements

We juxtapose global fits of two bottom-up models (an $S_3$ scalar leptoquark model and a ${B_3-L_2}$ $Z^\prime$ model) of \bsll\ anomalies to flavour data in order to quantify statistical preference or lack thereof. The leptoquark model couples directly to left-handed di-muon pairs, whereas the $Z^\prime$ model couples to di-muon pairs with a vector-like coupling. $B_s-\overline{B_s}$ mixing is a focus because it is typically expected to disfavour $Z^\prime$ explanations. In two-parameter fits to 247 flavour observables, including $B_{s/d} \to μ^+ μ^-$ branching ratios for which we provide an updated combination and LHCb $R_{K^{(\ast)}}$ measurements from December 2022, we show that each model provides a similar improvement in quality-of-fit of $\sqrt{Δχ^2}=3.6$ with respect to the Standard Model. The main effect of the $B_s-\overline{B_s}$ mixing constraint in the $Z^\prime$ model is to disfavour values of the $s_L-b_L$ mixing angle greater than about $5|V_{cb}|$. This limit is rather loose, meaning that a good fit to data does not require `alignment' in either quark Yukawa matrix. No curtailment of the $s_L-b_L$ mixing angle is evident in the $S_3$ model.

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arXiv2023

Flavonstrahlung in the $B_3-L_2$ $Z'$ Model at Current and Future Colliders

The $B_3-L_2$ $Z^\prime$ model may explain some gross features of the fermion mass spectrum as well as $b\rightarrow s \ell \ell$ anomalies. A TeV-scale physical scalar field associated with gauged $U(1)_{B_3-L_2}$ spontaneous symmetry breaking, the flavon field $\vartheta$, affects Higgs phenomenology via mixing. In this paper, we investigate the collider phenomenology of the flavon field. Higgs and $W$ boson mass data are used to place bounds upon parameter space. We then examine flavonstrahlung (${Z^\prime} \rightarrow Z^\prime \vartheta$ production) at colliders as a means to directly produce and discover flavon particles, providing direct empirical evidence tying it to $U(1)_{B_3-L_2}$ symmetry breaking. A 100 TeV FCC-hh or a 10 TeV muon collider would have high sensitivity to flavonstrahlung, whereas the HL-LHC can observe it only in extreme corners of parameter space.

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arXiv2022

$M_W$ helps select $Z^\prime$ models for $b \rightarrow s \ell \ell$ anomalies

As shown in Ref. \cite{Allanach:2021kzj}, the Third Family Hypercharge ($Y_3$) Model changes the Standard Model prediction for $M_W$ whilst simultaneously explaining anomalies in $b\to s\ell\ell$ transitions via a heavy $Z^\prime$ gauge boson which is spawned by a spontaneously broken gauged $U(1)_{Y_3}$ symmetry. The 2022 CDF II measurement of $M_W$, which is far from the Standard Model prediction, somewhat disfavours the $Y_3$ model. Here, we generalise the gauge charge assignments to the anomaly-free combination $s Y_3 + t (B_3-L_3)$ and show that incorporating the 2022 CDF II measurement of $M_W$ selects a viable domain of integers $s$ and $t$. For example, $s=1, t=-3$ yields a $p-$value of .12 in a two-parameter global fit to 277 electroweak and flavour changing $b$ data, much improving a SM $p-$value of $5\times 10^{-6}$.

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arXiv2017

Z' models for the LHCb and g-2 muon anomalies

We revisit a class of Z' explanations of the anomalies found by the LHCb collaboration in $B$ decays, and show that the scenario is tightly constrained by a combination of constraints: (i) LHC searches for di-muon resonances, (ii) pertubativity of the Z' couplings; (iii) the $B_s$ mass difference, and (iv) and electro-weak precision data. Solutions are found by suppressing the Z' coupling to electrons and to light quarks and/or by allowing for a Z' decay width into dark matter. We also present a simplified framework where a TeV-scale Z' gauge boson that couples to standard leptons as well as to new heavy vector-like leptons, can simultaneously accommodate the LHCb anomalies and the muon g-2 anomaly.

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arXiv2015

Interpreting a CMS $lljjp_T^{\rm miss}$ Excess With the Golden Cascade of the MSSM

The CMS experiment recently reported an excess consistent with an invariant mass edge in opposite-sign same flavor (OSSF) leptons, when produced in conjunction with at least two jets and missing transverse momentum. We provide an interpretation of the edge in terms of (anti-)squark pair production followed by the `golden cascade' decay for one of the squarks: $\tilde q \rightarrow \tildeχ_2^0 q \to \tilde l l q \to \tildeχ_1^0 q l l$ in the minimal supersymmetric standard model (MSSM). A simplified model involving binos, winos, an on-shell slepton, and the first two generations of squarks fits the event rate and the invariant mass edge. We check consistency with a recent ATLAS search in a similar region, finding that much of the good-fit parameter space is still allowed at the 95% confidence level (CL). However, a combination of other LHC searches, notably two-lepton stop pair searches and jets plus $p_T^{\rm miss}$, rule out all of the remaining parameter space at the 95% CL.

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