Search

We discuss the future prospects of heavy neutrino searches at next generation lepton colliders. In particular, we focus on the planned electron-positron colliders, operating in two different beam modes, namely, e+e- and e-e-. In the e+e- beam mode, we consider various production and decay modes of the heavy neutrino (N) and find that the final state with e+ 2j + E, arising from the e+e- -> Nv production mode, is the most promising channel. However, since this mode is insensitive to the Majorana nature of the heavy neutrinos, we also study a new production channel e+e- -> Ne±WT±, which leads to a same-sign dilepton plus four jet final state, thus directly probing the lepton number violation in e+e- colliders. In the e-e- beam mode, we study the prospects of the lepton number violating process of e-e- → W-W-, mediated by a heavy Majorana neutrino. We use both cut-based and multivariate analysis techniques to make a realistic calculation of the relevant signal and background events, including detector effects for a generic linear collider detector. We find that with the cut-based analysis, the light-heavy neutrino mixing parameter |VeN|² can be probed down to ~10-4 at 95% C.L. for the heavy neutrino mass up to 400 GeV or so at √s = 500 GeV with 100 fb-1 of integrated luminosity. For smaller mixing values, we show that a multivariate analysis can improve the signal significance by up to an order of magnitude. These limits will be at least an order of magnitude better than the current best limits from electroweak precision data, as well as the projected limits from √s = 14 TeV LHC. [ABSTRACT FROM AUTHOR]

We study the discovery prospects of a heavy neutral scalar arising from a U(l)B-L extension of the Standard Model (SM) during the Large Hadron Collider's high luminosity runs (HL-LHC). This heavy neutral scalar mixes with the SM Higgs boson through a Higgs portal and interacts with the SM particles with an interaction strength proportional to the sine of the mixing angle. The mixing between the two Higgs bosons is constrained by direct and indirect measurements. We choose an experimentally viable mixing angle and explore in detail the ZZ and WW decay modes of the heavy Higgs boson. For the ZZ case, we focus on the cleanest 4ℓ and 2ℓ2j final states and find that a heavy Higgs boson of mass smaller than 500 GeV can be discovered at the HL-LHC. For the WW decay mode, we analyze the ℓ jj ... signature. We implement novel background reduction techniques in order to tackle the huge background by performing both cut-based and multivariate analyses. However, large backgrounds render this channel challenging. We briefly discuss the discovery prospects of the heavy Z'-boson arising in this model. [ABSTRACT FROM AUTHOR]

We investigate the effect of introducing a sequential generation of chiral fermions in the Higgs triplet model with nontdvial mixing between the doublet and triplet Higgs. We use the available Large Hadron Collider data for Higgs boson production and decay rates, the constraints on the fourth generation masses, and impose electroweak precision constraints from the S, T and U parameters. Our analysis shows that a standard model (SM)-like Higgs boson state at ~ 125 GeV can be accommodated in the Higgs triplet model with four generadons, and thus, that four generadons survive collider and electroweak precision constraints in models beyond SM. [ABSTRACT FROM AUTHOR]

We examine the relic density of the light mass dark matter region in the inert doublet model (IDM) when the dominant process is due to coannihilation between the lightest neutral scalars of the model. The full one-loop electroweak corrections are computed in an on shell scheme and are found to be well approximated as an effective cross section expressed in terms of Z observables. The electroweak corrections to the subdominant process, which consists of an annihilation into an on shell W and an off shell W, that is calculated as a annihilation into a three-body final state, is also performed. In the calculation of the latter in this case of coannihilation, although the SM Higgs exchange is absent, it is induced at one loop, but the cross section does not show a peak behavior despite the crossing of the Higgs resonance. This cross section reveals an important dependence on a parameter that describes the self-interaction of the new scalars (solely within the dark sector), a parameter which is not accessible in tree-level calculations of standard model-IDM interactions. [ABSTRACT FROM AUTHOR]

The present paper is the first in a series that addresses the calculation of the full one-loop corrections of dark matter (DM) annihilation cross sections in the low mass region of the inert doublet model (IDM). This series is a sequel to our recent publication concerning these corrections in the high mass region. We first review the renormalization of the model both in a fully on shell (OS) scheme as well as in a mixed scheme that combines on shell (for the masses) and a MS approach when the partial invisible width is closed and does not allow the use of a full OS scheme. The scale dependence introduced by the mixed scheme is shown to be tracked through an analysis of a parametrization of the tree-level cross section and the β constant of a specific coupling; this analysis could be followed in theories. We discuss how to minimize the scale dependence. The theoretical uncertainty brought by the scale dependence leads us to introduce a new criterion on the perturbativity of the IDM. This criterion further delimits the allowed parameter space, which we investigate carefully by including a host of constraints, both theoretical and experimental, including in particular, new data from the LHC. We come up with a set of benchmark points that cover three different mechanisms for a viable relic density of DM: (i) a dominance of coannihilation into a fermion pair, (ii) annihilation into two vector bosons of which one is off shell that requires the calculation of a 2 → 3 process, (iii) annihilation that proceeds through the very narrow standard model Higgs resonance. Since the 2 → 3 vector boson channel features in all three channels and is essentially a buildup on the simpler annihilation to OS vector bosons, we study the latter in detail in the present paper. We confirm again that the corrected cross sections involve a parameter that can be considered as rescattering in the dark sector, which a tree-level computation is not sensitive to. The setup of the renormalization detailed in the present paper will be the backbone of the accompanying papers where each mechanism requires, calculationally, a specific treatment. One-loop corrections to 2 → 3 processes for DM annihilation are technically challenging and have not been attempted before. We dedicate one of the accompanying papers to such a computation. The one-loop correction in the presence of a resonance will be presented separately since we need to supplement our general schemes with a complex scheme. For the coannihilation into fermions, our study will show that the annihilation cross sections can be excellently parametrized through simple effective couplings. [ABSTRACT FROM AUTHOR]

One-loop electroweak corrections to the annihilation cross sections of dark matter in the Higgs resonance region of the inert doublet model are investigated. The procedure of how to implement the width of the Higgs in order to regularize the amplitude both at tree level and at one loop together with the renormalization of a key parameter of the model, are thoroughly scrutinized. The discussions go beyond the application to the relic density calculation and also beyond the inert doublet model so that addressing these technical issues can help in a wider context. We look in particular at the dominant channels with the bb̈ final state and the more involved three-body final state, W ff̈', where both a resonance and an antiresonance, due to interference effects, are present. We also discuss how to integrate over such configurations when converting the cross sections into a calculation of the relic density. [ABSTRACT FROM AUTHOR]

We perform the first one-loop electroweak corrections for 2 → 3 processes for dark matter annihilation. These are the dominant processes that enter the computation of the relic density for the low mass region of the inert doublet model (IDM) when annihilations to two on shell vector bosons are closed. The impact of the one-loop corrections are important as they involve, through rescattering effects, not only a dependence on the parameter controlling the dark sector, not present if a calculation at tree level is conducted, but also on the renormalization scale. These combined effects should be taken into account in analyses based on tree-level cross sections of the relic density calculations, as a theoretical uncertainty, which we find to be much larger than the cursory 10% uncertainty that is routinely assumed, independently of the model parameters. [ABSTRACT FROM AUTHOR]

We propose differential observables for pp→Z(ℓ+ℓ-)h(bb̅) that can be used to completely determine the tensor structure of the hZZ*/hZf̅f couplings relevant to this process in the dimension-six Standard Model (SM) effective field theory. In particular, we propose a strategy to probe the anomalous hZμνZμν and hZμν˜Z~μν vertices at the percent level. We show that this can be achieved by resurrecting the interference term between the transverse Zh amplitude, which receives contributions from the above couplings, and the dominant SM longitudinal amplitude. These contributions are hard to isolate without knowledge of the analytical amplitude, as they vanish unless the process is studied differentially in three different angular variables at the level of the Z-decay products. By also including the differential distributions with respect to energy variables, we obtain projected bounds for the two other tensor structures of the Higgs coupling to Z bosons. [ABSTRACT FROM AUTHOR]

A full renormalization of the inert doublet model is presented and exploited for a precise calculation of the relic density of dark matter (DM) at one-loop. In this first paper, we study the case of a DM candidate with mDM~500 GeV. In this regime, the coannihilation channels are important. We therefore compute, for a wide range of relative velocities, the full next-to-leading order electroweak corrections to seven annihilation/coannihilation processes that contribute ~70% to the relic density of DM. These corrected cross sections are interfaced with micromegas to obtain the one-loop correction to the freeze-out relic density. Due to the accurate measurement of this observable, the one-loop corrections are relevant. We discuss the one-loop renormalization scheme dependence and point out the influence, at one-loop, of a parameter that solely describes the scattering in the dark sector. A tree-level computation of the relic density is not sensitive to this parameter. [ABSTRACT FROM AUTHOR]

The exploration of the scalar sector of the Standard Model is at the core of current and future science programs at collider experiments, with increasing focus on the self-interaction of the Higgs boson. This important parameter of the Higgs sector can be measured in various channels, among the production of a Higgs boson associated with a top-quark pair, tt̄hh. In this paper we study this channel and its potential to measure or constrain the self-coupling and possible new physics contributions at a future 100 TeV proton-proton collider. Analyzing this highly complex final state adds to the sensitivity for enhanced self-coupling interactions, and we argue that a measurement of this process is a necessity to constrain blind directions in the multidimensional parameter space of well-motivated new physics scenarios. [ABSTRACT FROM AUTHOR]

The exploration of the scalar sector of the Standard Model is at the core of current and future science programs at collider experiments, with increasing focus on the self-interaction of the Higgs boson. This important parameter of the Higgs sector can be measured in various channels, among the production of a Higgs boson associated with a top-quark pair, tt̄hh. In this paper we study this channel and its potential to measure or constrain the self-coupling and possible new physics contributions at a future 100 TeV proton-proton collider. Analyzing this highly complex final state adds to the sensitivity for enhanced self-coupling interactions, and we argue that a measurement of this process is a necessity to constrain blind directions in the multidimensional parameter space of well-motivated new physics scenarios. [ABSTRACT FROM AUTHOR]

In contrast to the decay products ensuing from a fast moving particle which are collimated along the original direction of the parent, those from a slow moving particle are distributed over a wide region. In the context of searches for heavy long-lived particles (LLP) at the Large Hadron Collider (LHC), we quantitatively demonstrate, using a few benchmark models, that objects which emerge from a secondary vertex due to the decay of a LLP at the TeV scale can be at large angular separations with respect to the direction of the parent LLP. A fraction of the decay products, the backward moving objects (BMOs), can even go in the backward direction. These give rise to striking signatures in the detectors at the LHC as these particles traverse different layers of the detector outside-in towards the direction of the beam pipe. Based on a simple geometrical modeling of the detector, we give examples of how this effect translates into the fraction of energy deposited in the tracker, from particles coming as far as from the hadron calorimeter, as well as those that could be entering from outside the detector into the muon chamber. The largest effect is from LLP candidates that come to rest inside the detector, such as the stopped R-hadrons. But the results are promising even in the case of not so heavy LLPs and/or when some of the available energy is carried by a massive invisible daughter. This urges us to look more in detail at these unusual signatures, taking into account the particularities of each layer that constitutes the detector. From the BMO perspective, we review how each layer of the detector could be exploited and what improvements can be made to enhance the shower shapes and the timing information, for instance. We also argue that the cosmic ray events, the most important background, can be easily dealt with. [ABSTRACT FROM AUTHOR]

Motivated by grand unification considerations, we analyse a simple extension of the minimal supersymmetric standard model with additional pairs of vector-like chiral supermultiplets. We focus on the so-called LND setup, which enlarges the particle content of the minimal model by two vector-like pairs of weak doublets (one pair of leptons and one pair of down-type quarks) and one vector-like pair of neutrino singlets. Imposing collider and low-energy constraints, sneutrinos and neutralinos both emerge as possible lightest supersymmetric particles and thus dark matter candidates. We perform a complete analysis of the dark sector and study the viability of these neutralino and sneutrino dark matter options. We show that cosmological considerations (the dark matter relic abundance and its direct and indirect detection signals) restrict neutralino dark matter to exhibit similar properties as in the minimal supersymmetric standard model, and impose the sneutrino dark matter candidate to be singlet-like, rather than doublet-like. Allowing the mixing of the fermionic component of the new supermultiplets with the Standard Model third generation fermions, we moreover demonstrate the existence of collider signals that are distinguishable from other, more minimal, supersymmetric scenarios by virtue of an enhanced production of events enriched in tau leptons. We furthermore show that this signature yields robust LHC signals, that could potentially be differentiated from the background in future data. [ABSTRACT FROM AUTHOR]

We study the process pp→Z(ℓ+ℓ-)h(b&b) in the standard model effective field theory (SMEFT) at high energies using subjet techniques to reconstruct the Higgs boson. We show that at high energies this process probes four directions in the dimension-6 EFT space, namely the operators that contribute to the four contact interactions, hZμfγμf, where f=uL,uR,dL and dR. These four directions are, however, already constrained by the Z-pole and diboson measurements at LEP. We show that by utilizing the energy growth of this process in the SMEFT and the accuracy that can be achieved by using subjet techniques at the High Luminosity LHC, one can obtain bounds on these operators that are an order of magnitude better than existing LEP bounds. [ABSTRACT FROM AUTHOR]

Observation of Higgs pair production is an important long term objective of the LHC physics program as it will shed light on the scalar potential of the Higgs field and the nature of electroweak symmetry breaking. While numerous studies have examined the impact of new physics on di-Higgs production, little attention has been given to the well-motivated possibility of exotic Higgs decays in this channel. Here we investigate the consequences of exotic invisible Higgs decays in di-Higgs production. We outline a search sensitive to such invisible decays in the bb + ET channel. We demonstrate that probing invisible branching ratios of order 10% during the LHC's high-luminosity run will be challenging, but in resonance enhanced di-Higgs production, this final state can become crucial to establish the existence of physics beyond the Standard Model at collider energies. We also briefly discuss the outlook for other exotic Higgs decay modes and the potential to observe such exotic decays in the di-Higgs channel. [ABSTRACT FROM AUTHOR]