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A. G. Akeroyd

Publications and source records attributed to A. G. Akeroyd.

At least 19 recordsLinked to original sources
arXiv2025

Optimizing $pp\to A\to Z^{*}h\to \ell^+\ell^- b\bar b$ Searches at the LHC in the 2HDM Type-I with Inverted Hierarchy

In this study, we investigate the Large Hadron Collider (LHC) search for the signal process $pp\to A \to Z^{*}h \to \ell^{+} \ell^{-} b \bar{b}$ ($\ell=e,μ$) within the framework of the 2-Higgs-Doublet Model (2HDM) Type-I, considering an Inverted Hierarchy (IH) scenario wherein the Standard Model (SM)-like Higgs boson $H$ is heavier than $h$ (i.e., $m_h < m_H=125$ GeV). We reproduce the dominant background distributions from a CMS analysis of $pp\to H \to Z A \to \ell^{+} \ell^{-} b \bar{b}$ (for a on-shell $Z$), which we do for validation purposes, so that we can explore different invariant mass selection criteria to enhance the signal significance of our target process. Specifically, we compare the CMS baseline cuts ($70$ GeV $ < m_{\ell^+\ell^-} < 110$ GeV, no $m_{b\bar b}$ restriction) with the alternative selections $20~{\rm GeV}~< m_{\ell^+\ell^-} < 50$ GeV with $m_{b\bar b} < 100$ GeV. The latter cuts are enforced to account for the off-shellness of the $Z^*$ boson and the low mass Higgs state in our case. We show that these modifications reduce drastically the dominant Drell-Yan (DY) and top-(anti)quark pair backgrounds, leading to a significant excess from the analysis of the reconstructed $m_{\ell^+\ell^-b\bar b}$ invariant mass in the illustrative ranges 145 GeV $<m_{A}<$ 150 GeV and 68 GeV $<m_{h}<$ 75 GeV. Our results demonstrate that such cuts enable effective signal discrimination, suggesting an optimized strategy in future searches for an established topology but in a new kinematical regime, which is of particular relevance to the 2HDM Type-I with IH in its mass spectrum.

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arXiv2025

Probing the decay $A^0\to h^0 Z^{(*)}$ in Two-Higgs-Doublet Models in the inverted hierarchy scenario at the Large Hadron Collider

Searches are being carried out at the Large Hadron Collider (LHC) for the decay of the CP-odd scalar ($A^0$) in Two-Higgs-Doublet Models (2HDMs) with Natural Flavour Conservation (NFC) in the channel $A^0\to h^0 Z$, where $h^0$ is either the discovered 125 GeV Higgs boson or is an undiscovered CP-even scalar with a mass below 125 GeV. The latter possibility is called the "inverted hierarchy scenario" (IH) and would provide the opportunity of simultaneous discovery of two scalars. In both searches the selection cuts are optimised for the case of an on-shell $Z$ boson. For the case of the $Z$ boson being off-shell (denoted by $Z^*$, for which $m_{A^0} - m_{h^0}<m_Z$) no limits are set on the relevant 2HDM parameters from this process. It is known that the decay $A^0\to h^0 Z^*$ can have a large branching ratio (BR) in 2HDMs (especially in the Type I structure). In the context of the IH scenario and developing our previous work, we calculate the signal cross section $σ(gg\to A^0)\times {\rm BR}(A^0\to h^0Z^{(*)})\times {\rm BR}(h^0\to b\overline b,τ\overline τ)$ in the four types of 2HDMs with NFC. We also suggest some selection cuts that could provide sensitivity to $A^0\to h^0 Z^*$ in the IH scenario.

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arXiv2024

The Process $gg\to h^0 Z^{*}$ in the Inverted Hierarchy Scenario of the 2HDM Type-I at the LHC

While searching at the Large Hadron Collider (LHC) for the production and decay of the CP-odd scalar ($A^0$) in the 2-Higgs-Doublet Model (2HDM) with Natural Flavour Conservation (NFC) via the channels $gg\to A^0$ (through one-loop triangle diagrams) and $A^0\to h^0 Z^*$ (with $m_{h^0} =125$ GeV or $m_{h^0} < 125$ GeV, with $Z$ off-shell), respectively, a factorisation of the two processes is normally performed, with the $A^0$ state being on-shell. While this approach is gauge-invariant, it is not capturing the presence of either of the following two channels: $gg\to Z^*\to h^0Z^*$ (through one-loop triangle diagrams) or $gg\to h^0Z^*$ (through one-loop box diagrams). As the resolution of the $A^0$ mass cannot be infinitely precise, we affirm that all such contributions should be computed simultaneously, whichever the $h^0$($Z^{*}$) decay(splitting) products, thereby including all possible interferences amongst themselves. The cross section of the ensuing complete process is significantly different from that obtained in the factorisation case, being of the order up to ten percent in either direction at the integrated level and larger (including changes in the shape of kinematical observables) at the differential level. We thus suggest that the complete calculation ought to be performed while searching for $A^0$ in this channel. We illustrate this need for the case of a 2HDM of Type-I in the inverted hierarchy scenario with $m_{h^0}<125$ GeV.

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arXiv2023

The decay $A^0\to h^0 Z^{(*)}$ in the inverted hierarchy scenario and its detection prospects at the Large Hadron Collider

Searches are being carried out at the Large Hadron Collider (LHC) for the decay of the CP-odd scalar ($A^0$) in Two-Higgs-Doublet Models (2HDMs) with Natural Flavour Conservation (NFC) in the channel $A^0\to h^0 Z$ (with $m_{h^0}=125$ GeV and $Z$ on-shell). In the absence of any signal, limits on the parameter space of $[\tanβ, \cos(β-α), m_{A^0}]$ in each 2HDM are derived for $m_{A^0} > 225$ GeV. In this work we consider the scenario of inverted hierarchy with $m_{h^0}<125$ GeV and $m_{H^0}=125$ GeV in which the decay $A^0\to h^0 Z^{(*)}$ (i.e. including the case of an off-shell $Z$) can have a large branching ratio in the 2HDM (Type I) for $m_{A^0}<225$ GeV. We calculate the signal cross section $σ(gg\to A^0)\times {\rm BR}(A^0\to h^0Z^{(*)})\times {\rm BR}(h^0\to b\overline b)$ in the 2HDM (Type I) with NFC and compare its magnitude with the cross section for the case of normal hierarchy ($m_{h^0}=125$ GeV) that is currently being searched for at the LHC. For the experimentally unexplored region $m_{A^0} < 225$ GeV it is shown that the above cross section for signal events in the scenario of inverted hierarchy can be of the order of a few picobarns. Such sizeable cross sections are several orders of magnitude larger than the cross sections for the case of normal hierarchy, thus motivating an extension of the ongoing searches for $A^0\to h^0 Z^{(*)}$ to probe the scenario of inverted hierarchy.

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arXiv2022

Slight excess at 130 GeV in search for a charged Higgs boson decaying to a charm quark and a bottom quark at the Large Hadron Collider

Searches for a charged Higgs boson ($H^\pm$) decaying to a charm quark and a bottom quark ($H^\pm \to cb$) have been carried out at the Large Hadron Collider (LHC) in the decay of top quarks ($t\to H^\pm b$). In a recent search by the ATLAS collaboration (with all Run II data, 139 fb$^{-1}$) a local excess of around $3σ$ has been observed, which is best fitted by a charged Higgs boson with a mass ($m_{H^\pm}$) of around 130 GeV and a product of branching ratios (BRs) given by BR$(t\to H^\pm b)\times{\rm BR}(H^\pm\to cb)=0.16\%\pm 0.06\%$. In the context of Two-Higgs-Doublet Models (2HDM) with independent Yukawa couplings for $H^\pm$ we present the parameter space for which this excess (assuming it to be genuine) can be accommodated, taking into account the limits from LHC searches for $H^\pm\to cs$ and $H^\pm\to τν$ at $m_{H^\pm}$=130 GeV and the constraint from $b\to sγ$. It is then shown that such an excess cannot be explained in 2HDMs with natural flavour conservation, but can be accommodated in the flipped Three-Higgs-Doublet Model (3HDM) and in the aligned 2HDM (A2HDM). Upcoming searches with 139 fb$^{-1}$ in the channels $H^\pm\to cb$ (CMS), $H^\pm \to cs$ (ATLAS/CMS) and $H^\pm \to τν$ (ATLAS/CMS) will determine if the excess is the first sign of an $H^\pm$ with $m_{H^\pm}=130$ GeV.

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arXiv2020

CP asymmetries of ${\overline B}\to X_s/X_dγ$ in models with three Higgs doublets

Direct CP asymmetries (${\cal A}_{CP}$) in the inclusive decays of ${\overline B}\to X_sγ$ and ${\overline B}\to X_{s+d}γ$ of the order of $1\%$ will be probed at the BELLE II experiment. In this work, three such asymmetries are studied in the context of a three-Higgs-doublet model (3HDM), and it is shown that all three ${\cal A}_{CP}$ can be as large as the current experimental limits. Of particular interest is ${\cal A}_{CP}$ for ${\overline B}\to X_{s+d}γ$, which is predicted to be effectively zero in the Standard Model (SM). A measurement of $2.5\%$ or more for this observable with the full BELLE II data would give $5σ$ evidence for physics beyond the SM. We display parameter space in the 3HDM for which such a clear signal is possible.

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arXiv2019

Light charged Higgs boson with dominant decay to a charm quark and a bottom quark and its search at LEP2 and future $e^+e^-$ colliders

The possibility of a light charged Higgs boson $H^\pm$ that decays predominantly to $cb$ and with a mass in the range 80 GeV $\le M_{H^\pm} \le 90$ GeV is studied in the context of a 3-Higgs Doublet Model (3HDM). Searches for this decay at the Large Hadron Collider (LHC) do not have sensitivity to this mass region at present. It is shown that the searches for $H^\pm$ at LEP2 could be supplemented by either one or two $b$-tags, which would enable such large branching ratios for $H^\pm\to cb$ to be probed in the above mass region. We comment on the possibility of this 3HDM scenario to explain a slight excess in the searches for $H^\pm$ at LEP2, which is best fit by $M_{H^\pm}$ of around 90 GeV, and discuss the prospects for detecting $H^\pm \to cb$ decays at future $e^+e^-$ colliders.

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arXiv2018

Light charged Higgs boson with dominant decay to quarks and its search at LHC and future colliders

The possibility of a light charged Higgs boson $H^\pm$ that decays predominantly to quarks ($cs$ and/or $cb$) and with a mass in the range 80 GeV $\le m_{H^\pm} \le 90$ GeV is studied in the context of Three-Higgs-Doublet Models (3HDMs). At present the Large Hadron Collider (LHC) has little sensitivity to this scenario, and currently the best constraints are from LEP2 and Tevatron searches. The branching ratio of $H^\pm\to cb$ can be dominant in two of the five types of 3HDM, and we determine the parameter space where this occurs. The decay $H^\pm\to cb$ has recently been searched for at the LHC for the first time, and with increased integrated luminosity one would expect sensitivity to the region 80 GeV $\le m_{H^\pm} \le 90$ GeV due to the smaller backgrounds with respect to $H^\pm\to cs$ decays.

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arXiv2017

Constraint on the branching ratio of B_c \to tau nu from LEP1 and consequences for R(D(*)) anomaly

Recently there has been interest in the correlation between R(D*) and the branching ratio (BR) of $B_c \to τν$ in models with a charged scalar H^\pm. Any enhancement of R(D*) by $H^\pm$ alone (in order to agree with current data) also enhances $BR(B_c \to τν$), for which there has been no direct search at hadron colliders. We show that LEP data taken at the Z peak requires BR($B_c \to τν$) < 10%, and this constraint is significantly stronger than the recent constraint BR($B_c \to τν$) < 30% from considering the lifetime of B_c. In order to respect this new constraint, any explanation of the R(D) and R(D*) anomaly in terms of $H^\pm$ alone would require the future measurements of R(D*) to be even closer to the Standard Model prediction. A stronger limit on BR($B_c \to τν$) (or its first measurement) would be obtained if the L3 collaboration used all its data taken at the Z peak.

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arXiv2017

Prospects for charged Higgs searches at the LHC

The goal of this report is to summarize the current situation and discuss possible search strategies for charged scalars, in non-supersymmetric extensions of the Standard Model at the LHC. Such scalars appear in Multi-Higgs-Doublet models (MHDM), in particular in the popular Two-Higgs-Doublet model (2HDM), allowing for charged and additional neutral Higgs bosons. These models have the attractive property that electroweak precision observables are automatically in agreement with the Standard Model at the tree level. For the most popular version of this framework, Model~II, a discovery of a charged Higgs boson remains challenging, since the parameter space is becoming very constrained, and the QCD background is very high. We also briefly comment on models with dark matter which constrain the corresponding charged scalars that occur in these models. The stakes of a possible discovery of an extended scalar sector are very high, and these searches should be pursued in all conceivable channels, at the LHC and at future colliders.

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arXiv2017

Light charged Higgs boson scenario in 3-Higgs doublet models

The constraints from the measurements of the $B\to X_sγ$ decay rate on the parameter space of 3-Higgs Doublet Models (3HDMs), where all the doublets have non-zero vacuum expectation values, are studied at the next-to-leading order in QCD. In order to naturally avoid the presence of flavour changing neutral currents at the tree level, we impose two softly-broken discrete $Z_2$ symmetries. This gives rise to five independent types of 3HDMs that differ in their Yukawa couplings. We show that in all these 3HDMs (including the case of type-II-like Yukawa interactions) both masses of the two charged Higgs bosons $m_{H_1^\pm}$ and $m_{H_2^\pm}$ can be smaller than the top mass $m_t$ while complying with the constraints from $B\to X_sγ$. As an interesting phenomenological consequence, the branching ratios of the charged Higgs bosons decay into the $cb$ final states can be as large as $80\%$ when their masses are taken to be below $m_t$ in two of the five 3HDMs (named as Type-Y and Type-Z). This light charged Higgs boson scenario provides a hallmark 3HDM signature that cannot be realised in $Z_2$ symmetric 2-Higgs doublet models. We find that in the Type-Y and Type-Z 3HDMs the scenario with $90\,{\rm GeV}< m_{H^\pm_1}^{}\,, m_{H^\pm_2}^{}<m_t$ is ruled out by the direct searches at the LHC, but in the Type-Y 3HDM $80~\text{GeV}<m_{H_1^\pm}^{}<90~\text{GeV}$ and $90~\text{GeV}<m_{H_2^\pm}^{}<m_t$ is allowed by $B\to X_sγ$ and direct searches at LEP2, Tevatron and LHC due to the reduced sensitivity of these searches to the degenerate case $m_{H_1^\pm}\approx m_{W^\pm}$. The cases where only one or both charged Higgs bosons are above the top quark mass are also naturally allowed in the both Type-Y and Type-Z 3HDMs.

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arXiv2016

Light charged Higgs boson scenario in 3HDMs

The measurement of the $B\to X_sγ$ process gives important constraints on physics related to charged Higgs bosons ($H^\pm$). In 2-Higgs Doublet Models (2HDMs) with a softly-broken $Z_2$ symmetry, a light $H^\pm$ scenario, in which $H^\pm$ can be produced via the top decay, is possible in two of four types of Yukawa interactions (the so-called Type-I and Type-X). In these types of 2HDMs, the $H^\pm \to τ^\pm ν$ decay mode is dominant in wide regions of the parameter space. In this report, we discuss the other possibility of a light charged Higgs boson scenario in 3-Higgs-Doublet Models (3HDMs) based on the results obtained in Ref. [1]. We show that charged Higgs bosons can mainly decay into $cb$ without contradiction with the $B\to X_sγ$ data and the direct searches for charged Higgs bosons at the LHC, and this scenario cannot be realized in the 2HDMs.

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arXiv2014

$H^\pm\to cb$ in models with two or more Higgs doublets

Searches for light $H^\pm$s via $t\to H^\pm b$ are being carried out at the LHC. Herein, it is normally assumed that the dominant decay channels are $H^\pm\to τν$ and $H^\pm\to cs$ and separate data analyses are performed with comparable sensitivity to the underlying model assumptions. However, the $H^\pm\to cb$ decay rate can be as large as $80\%$ in models with two or more Higgs doublets with natural flavour conservation, while satisfying the constraint from $b\to sγ$ for $m_{H^\pm}< m_t$. Despite the current search strategy for $H^\pm\to cs$ is also sensitive to $H^\pm\to cb$, a significant gain in sensitivity could be obtained by tagging the $b$ quark from the decay $H^\pm\to cb$.

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arXiv2013

Dependence of the leptonic decays of H^- on the neutrino mixing angles theta_{13} and theta_{23} in models with neutrinophilic charged scalars

In the Higgs Triplet Model and the neutrinophilic Two-Higgs-Doublet Model the observed neutrinos obtain mass from a vacuum expectation value which is much smaller than the vacuum expectation value of the Higgs boson in the Standard Model. Both models contain a singly charged Higgs boson (H^-) whose Yukawa coupling is directly related to the neutrino mass (i.e. a "neutrinophilic charged Higgs"). The partial decay widths of H^- into a charged lepton and a neutrino (H^- to l^- nu) depend identically on the neutrino masses and mixings in the two models. We quantify the impact of the recent measurement of sin^2(2theta_{13}), which plays a crucial role in determining the magnitude of the branching ratio of H^- to e^- nu for the case of a normal neutrino mass ordering if the lightest neutrino mass m_0 < 10^{-3} eV. We also discuss the sizeable dependence of H^- to mu^- nu and H^- to tau^- nu on sin^2(theta_{23}), which would enable information to be obtained on sin^2(theta_{23}) and the sign of Δm^2_{31} if these decays are measured. Such information would help neutrino oscillation experiments to determine the CP-violating phase δ.

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arXiv2012

Enhancement of H to gamma gamma from doubly charged scalars in the Higgs Triplet Model

The Higgs boson of the Standard Model is being searched for at the LHC in the channel H -> gamma gamma. In the Higgs Triplet Model (HTM) there are contributions to this decay from loops of doubly charged scalars (H^{\pm\pm}) and singly charged scalars (H^\pm) that are not present in the Standard Model. These additional contributions are mediated by the trilinear couplings H_1 H^{++}H^{--} and H_1 H^{+}H^{-}, where H_1 is the lightest CP-even Higgs boson. We point out the possibility of constructive interference of the H^{\pm\pm} contribution with that of the W^\pm contribution, which enables a substantial enhancement of the branching ratio of H_1 -> gamma gamma in the HTM. The magnitude of the contribution of $H^{\pm\pm}$ is essentially determined by the mass of H^{\pm\pm} (m_{H^{\pm\pm}}) and a quartic scalar coupling (λ_1), with constructive interference arising for λ_1 < 0. Consequently, the ongoing searches for H -> gamma gamma restrict the parameter space of [m_{H^{\pm\pm}},λ_1] more stringently for λ_1 < 0 than for the recently-studied case of destructive interference with λ_1 > 0. Moreover, if the excess of gamma gamma events around a mass of 125 GeV in the LHC searches for H -> gamma gamma is substantiated with larger data samples, and the branching ratio is measured to be somewhat larger than that for the SM Higgs boson, then such an enhancement could be readily accommodated by H_1 in the HTM with λ_1 <0.

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arXiv2012

Light charged Higgs bosons decaying to charm and bottom quarks in models with two or more Higgs doublets

Searches for light charged Higgs bosons (H^\pm) in the decay of top quarks, t -> H^\pm b, are being carried out at the LHC and at the Tevatron. It is assumed that the dominant decay channels for such an H^\pm state are either H^\pm -> τνor H^\pm -> cs, and separate searches are performed with comparable sensitivity to the parameters m_{H^\pm} and tanbeta of the scalar potential. The branching ratio for the decay H^\pm -> cb can be as large as 80% in the Aligned Two Higgs Doublet Model and in models with three or more Higgs doublets with natural flavour conservation, while satisfying the constraint from b -> s gamma for m_{H^\pm} < m_t. Although the current search strategy for H^\pm -> cs is also sensitive to H^\pm -> cb, a considerable gain in sensitivity could be obtained by tagging the b quark from the decay H^\pm -> cb. Such an analysis, which could be readily performed at the Tevatron and in the 7 TeV and 8 TeV runs of the LHC, would probe a parameter space of the fermionic couplings of H^\pm in the above models which at present cannot be probed by experimental observables in flavour physics.

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arXiv2012

Five-lepton and six-lepton signatures from production of neutral triplet scalars in the Higgs Triplet Model

The neutral triplet scalars (H^0, A^0) in the Higgs Triplet Model (HTM) are difficult to detect at the LHC for the case of mass degeneracy with the singly charged (H^+) and doubly charged (H^++) scalars. A non-zero value of a specific quartic coupling in the scalar potential removes this degeneracy, and positive values of this coupling would give rise to the mass hierarchy m_{A^0,H^0} > m_{H^+} > m_{H^++}. In this scenario the decays H^0-> H^+ W^* and A^0-> H^+ W^*, followed by H^+ -> H^{++} W^*, can proceed with large branching ratios, even for small mass splittings among the triplet scalars. Such a cascade process would lead to production of H^{++}H^{--} accompanied by several virtual W bosons, and would provide a way of observing H^0 and A^0 with a signature which is different from that of the conventional production process for doubly charged scalars, q qbar -> γ^*, Z^* -> H^{++}H^{--}. Assuming the decay H^{++} to l^+ l^+, we quantify the magnitude of the five-lepton and six-lepton signals in these channels, which have essentially negligible backgrounds at the LHC.

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