arXiv subjects

Bruce Mellado

Publications and source records attributed to Bruce Mellado.

At least 55 records · Page 3Linked to original sources

arXiv2021

The anomalous production of multi-lepton and its impact on the measurement of $Wh$ production at the LHC

Anomalies in multi-lepton final states at the Large Hadron Collider (LHC) have been reported in Refs.~\cite{vonBuddenbrock:2017gvy,vonBuddenbrock:2019ajh}. These can be interpreted in terms of the production of a heavy boson, $H$, decaying into a Standard Model (SM) Higgs boson, $h$, and a singlet scalar, $S$, which is treated as a SM Higgs-like boson. This process would naturally affect the measurement of the $Wh$ signal strength at the LHC, where $h$ is produced in association with leptons and di-jets. Here, $h$ would be produced with lower transverse momentum, $p_{Th}$, compared to SM processes. Corners of the phase-space are fixed according to the model parameters derived in Refs.~\cite{vonBuddenbrock:2016rmr,vonBuddenbrock:2017gvy} without additional tuning, thus nullifying potential look-else-where effects or selection biases. Provided that no stringent requirements are made on $p_{Th}$ or related observables, the signal strength of $Wh$ is $μ(Wh)=2.41 \pm 0.37$. This corresponds to a deviation from the SM of $3.8σ$. This result further strengthens the need to measure with precision the SM Higgs boson couplings in $e^+e^-$, and $e^-p$ collisions, in addition to $pp$ collisions.

hep-ph↗

arXiv2020

Anatomy of inclusive $t\overline{t}W$ production at hadron colliders

In LHC searches for new and rare phenomena the top-associated channel $pp \to t\overline{t}W^\pm +X$ is a challenging background that multilepton analyses must overcome. Motivated by sustained measurements of enhanced rates of same-sign and multi-lepton final states, we reexamine the importance of higher jet multiplicities in $pp \to t\overline{t}W^\pm +X$ that enter at $\mathcal{O}(α_s^3α)$ and $\mathcal{O}(α_s^4α)$, i.e., that contribute at NLO and NNLO in QCD in inclusive $t\overline{t}W^\pm$ production. Using fixed-order computations, we estimate that a mixture of real and virtual corrections at $\mathcal{O}(α_s^4α)$ in well-defined regions of phase space can arguably increase the total $t\overline{t}W^\pm$ rate at NLO by at least $10\%-14\%$. However, by using non-unitary NLO multi-jet matching, we estimate that these same corrections are at most $10\%-12\%$, and at the same time exhibit the enhanced jet multiplicities that are slightly favored by data. This seeming incongruity suggests a need for the full NNLO result. We comment on implications for the $t\overline{t}Z$ process.

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arXiv2020

Connecting the Muon Anomalous Magnetic Moment and the Multi-lepton Anomalies at the LHC

A number of predictions were made in von Buddenbrock et al (2016 Eur. Phys. J. C 76, 10, 580) pertaining to the anomalous production of multiple leptons at the Large Hadron Collider. Discrepancies in multi-lepton final states have now become statistically compelling with the available Run~2 data. These could be connected with a heavy boson, $H$, decaying predominantly into a SM Higgs boson, $h$, and a singlet scalar, $S$, where $m_H\approx 270$\,GeV and $m_S\approx 150$\,GeV. These can be embedded into a scenario where a Two Higgs Doublet is considered with an additional singlet scalar, 2HDM+S. The long-standing discrepancy in the muon anomalous magnetic moment, $Δa_μ$, is interpreted in the context of the 2HDM+S type-II and type-X, along with additional fermionic degrees of freedom. The 2HDM+S model alone with the constraints from the LHC data does not seem to explain the $Δa_μ$ anomaly. However, adding fermions with mass of order $\mathcal{O}(100)$\,GeV can explain the discrepancy for low enough values of fermion-scalar couplings.

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arXiv2019

The emergence of multi-lepton anomalies at the LHC and their compatibility with new physics at the EW scale

A recent study [1] has shown that a simplified model predicting a heavy scalar of mass 270 GeV ($H$) that decays to a Standard Model (SM) Higgs boson in association with a scalar singlet of mass 150 GeV ($S$) can accommodate several anomalous multi-lepton results in proton-proton collisions at the Large Hadron Collider (LHC). With this in mind, the goal of this article is to provide a more formal study of a wider set of LHC results pertaining to the production of multiple leptons. We find that a combination of such results lead to strong discrepancies between the data and SM Monte Carlo predictions. These discrepancies appear in corners of the phase-space where different SM processes dominate, indicating that the potential mismodeling of a single SM process is unlikely to explain them. Systematic uncertainties from the prediction of SM processes evaluated with currently available tools seem unable to explain away these discrepancies. A combination is able to constrain the simplified model's single degree of freedom $β_g^2$, related to the size of the Yukawa coupling of $H$ to the top quark, to a value of $2.92\pm 0.35$. This is in contrast to the absence of signal, where $β_g=0$. This result is discussed in the independent contexts of both potential for new physics in the existing LHC data as well as the limitations of our current understanding of the SM. That being said, QCD NNLO and EW NLO corrections in di-lepton final states are not expected to change the conclusions of this study. New results pertaining to the production of two opposite sign different flavour charged leptons with a full jet veto further confirm the presence of anomalies in similar corners of the leptonic phase-space.

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arXiv2019

Anomalies in the production of multiple leptons at the LHC

Based on a number of features from proton-proton collisions taken during Run 1 data taking period at the LHC, a boson with a mass around the Electro-Weak scale was postulated such that a significant fraction of its decays would comprise the Standard Model (SM) Higgs boson and an additional scalar, $S$. One of the phenomenological implications of a simplified model, where $S$ is treated a SM Higgs boson, is the anomalous production of high transverse momentum leptons. A combined study of Run 1 and Run 2 data is indicative of very significant discrepancies between the data and SM Monte Carlos in a variety of final states involving multiple leptons with and without $b$-quarks. These discrepancies appear in corners of the phase-space where different SM processes dominate, indicating that the potential mismodeling of a particular SM process is unlikely to explain them. Systematic uncertainties from the prediction of SM processes evaluated with currently available tools seem unable to explain away these discrepancies. The internal consistency of these anomalies and their interpretation in the framework of the original hypothesis is quantified.

hep-ph↗

arXiv2019

Constraints on a 2HDM with a singlet scalar and implications in the search for heavy bosons at the LHC

We study a two-Higgs doublet model extended with an additional singlet scalar (2HDM+S), and provide a brief introduction to the model and its parameters. Constraints are applied to the parameter space of this model in order to accommodate a number of features in the data that have been interpreted in von Buddenbrock et al (2018 J. Phys. G 45 115003) as the result of the $H\rightarrow Sh$ decay produced via gluon-gluon fusion and in association with top quarks. Implications on the phenomenology of the heavy pseudo-scalar ($A$) and charged scalar ($H^+$) are discussed. In particular, the decays $A\rightarrow ZH$ and $H^+\rightarrow W^+H$ become prominent. This leads to final states with multiple leptons and $b$-quarks. The decay $A\rightarrow ZH\rightarrow ZSh$ results in the production of a high transverse momentum $Z$ produced in association with a lepton and two $b$-quarks with little additional jet activity. These predictions are compared to the data with model's benchmark points. With the parameters obtained here the model is able to accommodate the features at the LHC reported in von Buddenbrock et al (2018 J. Phys. G 45 115003). Without varying these parameters additional excesses in the $Zb\overline{b}$ and $t\overline{t}$ invariant mass spectra, and the production of $3$ leptons plus two $b$-tagged jets can be explained assuming $m_A\approx 600$\,GeV.

hep-ph↗

arXiv2019

Precision Higgs Physics at CEPC

The discovery of the Higgs boson with its mass around 125 GeV by the ATLAS and CMS Collaborations marked the beginning of a new era in high energy physics. The Higgs boson will be the subject of extensive studies of the ongoing LHC program. At the same time, lepton collider based Higgs factories have been proposed as a possible next step beyond the LHC, with its main goal to precisely measure the properties of the Higgs boson and probe potential new physics associated with the Higgs boson. The Circular Electron Positron Collider~(CEPC) is one of such proposed Higgs factories. The CEPC is an $e^+e^-$ circular collider proposed by and to be hosted in China. Located in a tunnel of approximately 100~km in circumference, it will operate at a center-of-mass energy of 240~GeV as the Higgs factory. In this paper, we present the first estimates on the precision of the Higgs boson property measurements achievable at the CEPC and discuss implications of these measurements.

hep-ex↗

arXiv2018

Effects of neutron radiation on the optical and structural properties of blue and green emitting plastic scintillators

We report on the optical and structural properties of plastic scintillators irradiated with neutron beams produced by the IBR-2 reactor of the Frank Laboratory of Neutron Physics in JINR, Dubna. Blue UPS-923A and green plastic scintillators were irradiated with neutron fluence ranging from 10$^{13}$ to 10$^{17}$ n/cm$^2$. Discolouring in the plastic scintillators was observed after irradiation. The effects of radiation damage on the optical and structural properties of the samples were characterized by conducting light yield, light transmission, light fluorescence and Raman spectroscopy studies. The results showed that neutron radiation induced damage in the material. The disappearance of the Raman peak features in green scintillators at frequencies of 1165.8, 1574.7 and 1651.2 cm$^{-1}$ revealed significant structural alterations due to neutron bombardment. Losses in fluorescence intensity, light yield and light transmission in the plastic scintillators were observed.

physics.ins-det↗

arXiv2018

Multi-lepton signatures of additional scalar bosons beyond the Standard Model at the LHC

Following a prediction made in Refs.~\cite{vonBuddenbrock:2015ema,Kumar:2016vut,vonBuddenbrock:2016rmr}, this paper focuses on multi-lepton signatures arising from two new hypothetical scalar bosons, $H$ and $S$, at the Large Hadron Collider (LHC). These two new bosons are an extension to the Standard Model (SM) and interact with the SM Higgs boson, $h$. We consider two production modes for $H$, one being gluon fusion and the other being in association with top quarks. The $H \to S h$ decay mode is considered, where leptonic final states are studied. The CP properties of $S$ are characterised by considering effective couplings derived from dimension six operators through $SWW$ vertices. The nature of the $S$ boson is considered in two separate contexts. Firstly in a simplified model, it is considered to have Higgs-like couplings. Secondly, we consider a heavy neutrino model and its interactions with the $Z, W$ and $S$ bosons. The predictions of the models are compared both to ATLAS and CMS results at $\sqrt{s} = 8$ and $13$~TeV, where appropriate. The data is interpreted using a simplified model where all the signal comes from $H \to S h$, assuming $S$ to be Higgs-like, $m_H=270$~GeV and $m_S=150$~GeV. The combined result yields gives a best fit value for the parameter $β_g$ (the strength of the Yukawa coupling of $H$ to top quarks), $β_g^2=1.38\pm 0.22$. A number of regions of the phase space are suggested to the experiments for further exploration.

hep-ph↗

arXiv2017

The Madala hypothesis with Run 1 and 2 data at the LHC

The Madala hypothesis postulates a new heavy scalar, H, which explains several independent anomalous features seen in ATLAS and CMS data simultaneously. It has already been discussed and constrained in the literature by Run 1 results, and its underlying theory has been explored under the interpretation of a two Higgs doublet model coupled with a scalar singlet, $S$. When applying the hypothesis to Run 2 results, it can be shown that the constraints from the data are compatible with those obtained using Run 1 results.

hep-ph↗

arXiv2017

Extraction of Top Backgrounds in the Higgs Boson Search with the $H\to WW^\ast \to \ell\ell + E_{\rm T}^{\rm miss}$ Decay with a Full-Jet Veto at the LHC

The search for a Standard Model Higgs boson with the $H\to WW^\ast\to \ell\ell + E_{\rm T}^{\rm miss}$ decay and the application of a full-jet veto yields a strong sensitivity in the mass range $130<m_H<200$ GeV. One of the residual backgrounds is related to processes involving top quarks. A method for the extraction of top backgrounds is evaluated at the parton and hadronic levels based on the data-driven extraction of the jet veto survival probability. The uncertainty of the proposed method is about 15%.

hep-ph↗

arXiv2017

Exploring CP-even scalars of a Two Higgs-doublet model in future $e^-\,p$ colliders

In this proceeding we shall explore the potential of a future $e^- \,p$ collider to probe the CP-even scalars in a two Higgs doublet model. We consider Type-I in this study. The mass of the lighter scalar particle is considered to be the Higgs-boson, $m_h = 125$ GeV, and a heavy scalar $m_H = 270$ GeV. The centre of mass energy for the $e^-\,p$ collision is considered as in the Large Hadron Electron Collider and the Future Circular Hadron Electron Collider configurations, by fixing the proton beam energy to be $E_p = 7$ and 50 TeV, respectively, and an electron beam energy of $E_e = 60$ GeV. Production cross sections of these scalars are also shown at higher electron beam energies. Future prospects of these studies are also discussed.

hep-ph↗

arXiv2017

Impact of additional bosons on the exploration of the Higgs boson at the LHC

The LHC is making strides in the exploration of the properties of the newly discoverd Higgs boson, $h$. In Refs.~\cite{vonBuddenbrock:2015ema,Kumar:2016vut,vonBuddenbrock:2016rmr} the compatibility of the proton-proton data reported in the Run I period with the presence of a heavy scalar, $H$, with a mass around 270\,GeV and its implications were explored. This boson would decay predominantly to $H\rightarrow Sh$, where $S$, is a lighter scalar boson. The production cross-section of $pp\rightarrow H(\rightarrow Sh) + X$ is considerable and it would significantly affect the inclusive rate of $h$. The contamination from this new production mechanism would depend strongly on the final state used to measure the rate of $h$. The contamination in the rate measurement of $Vh(\rightarrow b\overline{b}), V=Z,W$ is estimated to be small. This statement does not depend strongly on assumptions made on the decay of $S$.

hep-ph↗

arXiv2017

Measuring CP nature of top-Higgs couplings at the future Large Hadron electron collider

We investigate the sensitivity of top-Higgs coupling by considering the associated vertex as CP phase ($ζ_t$) dependent through the process $p\, e^- \to \bar t \,h \,ν_e$ in the future Large Hadron electron collider. In particular the decay modes are taken to be $h \to b\bar b$ and $\bar t \to$ leptonic mode. Several distinct $ζ_t$ dependent features are demonstrated by considering observables like cross sections, top-quark polarisation, rapidity difference between $h$ and $\bar t$ and different angular asymmetries. Luminosity ($L$) dependent exclusion limits are obtained for $ζ_t$ by considering significance based on fiducial cross sections at different $σ$-levels. For electron and proton beam-energies of 60 GeV and 7 TeV respectively, at $L = 100$~fb$^{-1}$, the regions above $π/5 < ζ_t \leq π$ are excluded at 2$σ$ confidence level, which reflects better sensitivity expected at the Large Hadron Collider. With appropriate error fitting methodology we find that the accuracy of SM top-Higgs coupling could be measured to be $κ= 1.00 \pm 0.17 (0.08)$ at $\sqrt{s} = 1.3 (1.8)$ TeV for an ultimate $L = 1 \,\rm{ab}^{-1}$.

hep-ph↗

arXiv2017

Data driven method for predicting the shape of the dijet mass in $Z\,h + {E_{\rm T}}^{\rm {miss}}$ analysis

We present an analysis of new physics searches in $Z\,h$ with missing energy final states at the Large Hadron Collider considering $Z \to l^+ l^-$ (where $l^\pm = e^\pm, μ^\pm$) and $h \to b\bar b$ decay modes. For this analysis we consider production of a $CP$-odd scalar $A$ through gluon-fusion which decay into a heavy $CP$-even neutral scalar $H$ with $Z$-boson. Further $H$ decays into a lighter $CP$-even Higgs boson $h$ in association with dark matter candidate $χ$ - a source of missing energy. The masses of these scalars are considered as $m_h = 125$ GeV, $m_χ= 60$ GeV, $2 m_h < m_H < 2 m_t$ and $m_A > 2 m_t$. A data-driven method have been applied to reduce the considerable backgrounds from electroweak processes $W$+ jets and $Z$+ jets, in addition with top-pair and single-top production. The di-jet mass distributions have been studied with same- and opposite-flavour lepton selections.

hep-ph↗

arXiv2016

Probing anomalous couplings using di-Higgs production in electron-proton collisions

A proposed high energy Future Circular Hadron-Electron Collider would provide sufficient energy in a clean environment to probe di-Higgs production. Using this channel we show that the azimuthal angle correlation between the missing transverse energy and the forward jet is a very good probe for the non-standard $hhh$ and $hhWW$ couplings. We give the exclusion limits on these couplings as a function of integrated luminosity at a $95$\% C.L. using the fiducial cross sections. With appropriate error fitting methodology we find that the Higgs boson self coupling could be measured to be $g^{(1)}_{hhh} = 1.00^{+0.24(0.14)}_{-0.17(0.12)}$ of its expected Standard Model value at $\sqrt s = 3.5(5.0)$ TeV for an ultimate 10 ab$^{-1}$ of integrated luminosity.

hep-ph↗

arXiv2016

Phenomenological signatures of additional scalar bosons at the LHC

We investigate the search prospects for new scalars beyond the Standard Model (SM) at the Large Hadron Collider (LHC). In these studies two real scalars $S$ and $χ$ have been introduced in a two Higgs doublet model (2HDM), where $S$ is a portal to dark matter (DM) through its interaction with $χ$, a DM candidate and a possible source of missing transverse energy (\MET). Previous studies focused on a heavy scalar $H$ decay mode $H \to hχχ$, which was studied using an effective theory in order to explain a distortion in the Higgs ($h$) transverse momentum spectrum [16]. In this work, the effective decay is understood more deeply by including a mediator $S$, and the focus is changed to $H \to h S,~SS$ with $S \to χχ$. Phenomenological signatures of all the new scalars in the proposed 2HDM are discussed in the energy regime of the LHC, and their mass bounds have been set accordingly. Additionally, we have performed several analyses with final states including leptons and \MET, with $H \to 4 W$, $t\bar t H \to 6 W$ and $A \to ZH$ channels, in order to understand the impact these scalars have on current searches.

hep-ph↗

arXiv2016

Phenomenology of additional scalar bosons at the LHC

The confirmation of the Higgs boson in Run I data at the Large Hadron Collider (LHC) and the excesses in recent Run II data suggest scenarios beyond the Standard Model (SM). We pursue a study in a minimal model which is an extension of a scalar doublet in the SM known as two-Higgs doublet model (THDM). Following earlier suggestions two real scalars $χ$ and $S$ have been introduced in the THDM where $χ$ is treated as a candidate for dark matter. $χ$ does not receive any vacuum expectation value ($vev$) in the model whereas the Higgs-like scalar $S$ acquires $vev$. This allows small mixing between the $CP$-even scalars of the THDM, $h$, $H$ and $S$. In this study the mass spectrum of new scalars is taken to be $2 m_h < m_H < 2 m_t$, $m_χ< m_h/2$, $m_h \lesssim m_S \lesssim m_H - m_h$, $m_A > 2 m_t$ and $m_H^\pm < m_A$, where $m_h$ and $m_t$ is masses of the SM Higgs and top-quark respectively, $m_H, m_A$ and $m_{H^\pm}$ are the masses of the heavy $CP$-even scalar $H$, $CP$-odd scalar $A$, and charged Higgs $H^\pm$, respectively. A partial list of potential search channels at the LHC has been provided with possible phenomenological consequences. The expected phenomenology and constraints on parameters are also discussed in a model-independent approach .

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