TeV Hadron Collider Research Articles

Testing the CKM unitarity at high energy via the W+W− production at the LHC and future colliders

We propose a novel test to assess the unitarity of the Cabibbo-Kobayashi-Maskawa matrix, VCKM, at present and future collider experiments. Our strategy makes use of the W+W− production cross section to directly probe the VCKM†VCKM product, which regulates the high-energy behavior of the observable. The violation of unitarity is signaled by an anomalous behavior of the cross section that grows quadratically with the W+W− invariant mass with respect to the Standard Model prediction. By using the recent ATLAS measurements of the W+W− cross section we are able to constrain the maximal unitarity violation allowed by current data, producing a bound complementary to the results of flavor physics experiments. Forecasts for the high luminosity phase of the LHC and for the future 100 TeV hadron collider are also discussed.

Exploration of heavy Higgs bosons at a 100 TeV hadron collider withinthe semi-constrained next-to-minimal supersymmetric standard model* *Supported by the National Natural Science Foundation of China (12275066, 11605123)

In this study, we explore the detectability of heavy Higgs bosons in the channel at a 100 TeV hadron collider within the semi-constrained next-to-minimal supersymmetric standard model. We calculate their production cross sections and decay branching ratios and compare them with simulation results from literature. We focus on the heavy doublet-dominated CP-even Higgs H and CP-odd Higgs A, with mass limits set below 10 TeV to ensure detectability. At a collider with an integrated luminosity of 3 ab , the potential for detecting heavy Higgs bosons varies significantly with their mass and . Heavy Higgs bosons with masses below 2 TeV are within the testable range, while those heavier than 7 TeV are below the exclusion and discovery thresholds, rendering them undetectable. For masses between 2 and 7 TeV, heavy Higgs bosons with smaller than 20 can be detected, whereas those with larger than 20 are beyond the current discovery or exclusion capabilities.

Investigating Higgs self-interaction through di-Higgs plus jet production at a 100 TeV hadron collider

The Higgs self coupling measurement is quite essential for determining the shape of the Higgs potential and nature of the Higgs boson. We propose the di-Higgs plus jet final states at hadron colliders to increase the discovery sensitivity of the Higgs self coupling at the low invariant mass region. Our simulation indicates that the allowed region of the Higgs self coupling would be further narrowed from $[-1.5,6.7]$ from the most recent ATLAS report down to $[0.5, 1.7]$. Furthermore, we find negative Higgs self couplings would be disfavored beyond $2\sigma$ confidence level at a future 100TeV collider with the help of this signal.

Detecting heavy Higgs bosons from natural SUSY at a 100 TeV hadron collider

Supersymmetric models with radiatively-driven naturalness (RNS) enjoy low electroweak fine-tuning whilst respecting LHC search limits on gluinos and top squarks and allowing for $m_h\simeq 125$ GeV. While the heavier Higgs bosons $H,\ A$ may have TeV-scale masses, the SUSY conserving $\mu$ parameter must lie in the few hundred GeV range. Thus, in natural SUSY models there should occur large heavy Higgs boson branching fractions to electroweakinos, with Higgs boson decays to higgsino plus gaugino dominating when they are kinematically accessible. These SUSY decays can open up new avenues for discovery. We investigate the prospects of discovering heavy neutral Higgs bosons $H$ and $A$ decaying into light plus heavy chargino pairs which can yield a four isolated lepton plus missing transverse energy signature at the LHC and at a future 100~TeV $pp$ collider. We find that discovery of heavy Higgs decay to electroweakinos via its $4\ell$ decay mode is very difficult at HL-LHC. For FCC-hh or SPPC, we study the $H,\ A \to $ SUSY reaction along with dominant physics backgrounds from the Standard Model and devise suitable selection requirements to extract a clean signal for FCC-hh or SPPC with $\sqrt{s}=100$ TeV, assuming an integrated luminosity of 15 $ab^{-1}$. We find that while a conventional cut-and-count analysis yields a signal statistical significance greater than $5\sigma$ for $m_{A,H}\sim 1.1-1.65$ TeV, a boosted-decision-tree analysis allows for heavy Higgs signal discovery at FCC-hh or SPPC for $m_{A,H}\sim 1-2$ TeV.

Leptonic scalars and collider signatures in a UV-complete model

We study the non-standard interactions of neutrinos with light leptonic scalars (ϕ) in a global (B − L)-conserved ultraviolet (UV)-complete model. The model utilizes Type-II seesaw motivated neutrino interactions with an SU(2)L-triplet scalar, along with an additional singlet in the scalar sector. This UV-completion leads to an enriched spectrum and consequently new observable signatures. We examine the low-energy lepton flavor violation constraints, as well as the perturbativity and unitarity constraints on the model parameters. Then we lay out a search strategy for the unique signature of the model resulting from the leptonic scalars at the hadron colliders via the processes H±±→ W±W±ϕ and H±→ W±ϕ for both small and large leptonic Yukawa coupling cases. We find that via these associated production processes at the HL-LHC, the prospects of doubly-charged scalar H±± can reach up to 800 (500) GeV and 1.1 (0.8) TeV at the 2σ (5σ) significance for small and large Yukawa couplings, respectively. A future 100 TeV hadron collider will further increase the mass reaches up to 3.8 (2.6) TeV and 4 (2.7) TeV, at the 2σ (5σ) significance, respectively. We also demonstrate that the mass of ϕ can be determined at about 10% accuracy at the LHC for the large Yukawa coupling case even though it escapes as missing energy from the detectors.

Non-leptonic decays of bileptons

We provide a detailed analysis of the decays of doubly-charged bilepton gauge bosons Y±±, as predicted in models based on a SU(3)C×SU(3)L×U(1)X symmetry. In addition to the decay modes into same-sign lepton pairs, which were already investigated in scenarios wherein each branching ratio was about one third, there are, depending on the mass spectrum, possible non-leptonic decays which reduce the leptonic rates. These non-leptonic decays are typically into a light quark (antiquark) and a heavy TeV-scale antiquark (quark), which carries double lepton number and decays via virtual bileptons. We choose two benchmark points to represent the particle spectrum and present a phenomenological analysis of such decays. We find that, while the LHC statistics are too low, even in the high-luminosity phase, they can lead to a visible signal at a future 100 TeV hadron collider (FCC-hh). A more exhaustive exploration with a rigorous inclusion of tagging efficiencies, detector simulations and higher-order corrections to the partonic cross section, aiming at assessing the reach of LHC and FCC-hh for bileptons as a function of the model parameters, as well as a recast of four-top searches in terms of the bilepton model, are deferred to future work.

Neutrino and doubly charged Higgs boson phenomenology in flavored-TNMSSM

We develop a neutrino mass model based on A4×Z3 flavor symmetry in the context of the NMSSM extended by supersymmetric type II seesaw mechanism (flavored-TNMSSM). We show that the NMSSM-singlet contributes significantly to the light neutrino masses, leading to the trimaximal neutrino mixing and providing a source for CP violation. The model favors normal hierarchy for neutrino masses, and offers testable predictions for upcoming experiments aimed at developing the measurements of the parameters mνe, |mee|. We also show that the decay of the doubly charged Higgs boson (DCHB) into dilepton of the same sign is dominant; and that the muon-tau (μτ) channel is the most relevant, thus giving rise to a collider signature. We prove that the model allows LFV processes, such as τ±→e±μ∓e± and τ±→μ±e∓μ±. We find further that when the triplet VEV exceeds 0.5 eV, a discovery of the DCHB would be possible at the current run of the LHC; whereas for a broad part of the parameter space when the triplet VEV is less than about 0.5 eV, the DCHB can acquire mass up to few TeVs, which can be probed at HL-LHC, HE-LHC, and future 100 TeV hadron colliders.

Probing the top-Higgs sector with composite Higgs models at present and future hadron colliders

We study the production of toverline{t}h and toverline{t} hh at hadron colliders, in the minimal Composite Higgs Models, based on the coset SO(5)/SO(4). We explore the fermionic representations 5 and 14. A detailed phenomenological analysis is performed, covering the energy range of the LHC and its High Luminosity upgrade, as well as that of a future 100 TeV hadron collider. Both resonant and non-resonant production are considered, stressing the interplay and complementary interest of these channels with each other and double Higgs production. We provide sets of representative points with detailed experimental outcomes in terms of modification of the cross sections as well as resonance masses and branching ratios. For non-resonant production, we gauge the relative importance of Yukawa, Higgs trilinear, and contact toverline{t} hh vertices to these processes, and consider the prospect for distinguishing the fermion representations from each other and from the Standard Model. In the production of top partners, we find that the three-body decay channel W+W–t becomes significant in certain regions of parameter space having a degenerate spectrum, and is further enhanced by increasing the top partner’s mass. This motivates both higher energy machines as well as the need to go beyond the current analysis performed for the searches for these resonances.

Single production of vector-like top partners in trilepton channel at future 100 TeV Hadron colliders

The new vector-like quarks are predicted by several new physics beyond the Standard Model at the TeV scale. In this paper, we study single production of vector-like quark T with electric charge |Q|=2/3 in the framework of a simplified model. We investigate the discovery potential of heavy vector-like T-quark decaying into a top quark and a Z boson in trilepton final states, at the future 100 TeV pp colliders including the realistic detector effects. The 95% confidence level (CL) excluded regions and the 5σ discovery reach in the parameter plane between the mass of T-quark and the coupling parameter g⁎, are obtained at a 100 TeV pp collider with the integrated luminosity of 30 ab−1. The projected sensitivity in terms of the production cross section times branching fraction is also predicted for 5% systematic uncertainty.

Pair production of heavy neutrinos in next-to-leading order QCD at the hadron colliders in the inverse seesaw framework

The explanation of the small neutrino mass can be depicted using some handsome models like type-I and inverse seesaw where the Standard Model gauge singlet heavy right-handed neutrinos are deployed. The common thing in these two models is a lepton number violating parameter, however, its order of magnitude creates a striking difference between them making the nature of the right-handed heavy neutrinos a major play factor. In the type-I seesaw a large lepton number violating parameter involves the heavy right-handed neutrinos in the form of Majorana fermions while a small lepton number violating parameter being involved in the inverse seesaw demands the pseudo-Dirac nature of the heavy right-handed neutrinos. Such heavy neutrinos are accommodated in these models through the sizable mixings with the Standard Model light neutrinos. In this paper we consider the purely inverse seesaw scenario to study the pair production of the pseudo-Dirac heavy neutrinos followed by their various multilepton decay modes through the leading branching fraction at the leading order and next-to-leading order QCD at the LHC with a center-of-mass energy of 13 TeV and a luminosity of 3000 fb[Formula: see text]. We also consider a prospective 100 TeV hadron collider with luminosities of 3000 fb[Formula: see text] and 30,000 fb[Formula: see text], respectively to study the process. Using anomalous multilepton search performed by the CMS at the 8 TeV with 19.5 fb[Formula: see text] luminosity we show prospective search reaches of the mixing angles for the three lepton and four lepton events at the 13 TeV LHC and 100 TeV hadron collider.

Improving heavy dijet resonance searches using jet substructure at the LHC

The search for new physics at high energy accelerators has been at the crossroads with very little hint of signals suggesting otherwise. The challenges at a hadronic machine such as the LHC is compounded by the fact that final states are swamped with jets which one needs to understand and unravel. A positive step in this direction would be to separate the jets in terms of their gluonic and quark identities, much in a similar spirit of distinguishing heavy quark jets from light quark jets that has helped in improving searches for both neutral and charged Higgs bosons at the LHC. In this work, we utilise this information using the jet substructure techniques to comment on possible improvements in sensitivity as well as discrimination of new resonances in the all hadronic mode that would be crucial in pinning down new physics signals at HL-LHC, HE-LHC and any future 100 TeV hadron collider.

Corrections to Yukawa couplings from higher dimensional operators in a natural SUSY SO(10) and HL-LHC implications

We consider a class of unified models based on the gauge group SO(10) which with appropriate choice of Higgs representations generate in a natural way a pair of light Higgs doublets needed to accomplish electroweak symmetry breaking. In this class of models higher dimensional operators of the form matter-matter-Higgs-Higgs in the superpotential after spontaneous breaking of the GUT symmetry generate contributions to Yukawa couplings which are comparable to the ones from cubic interactions. Specifically we consider an SO(10) model with a sector consisting of 126 + overline{126} + 210 of heavy Higgs which breaks the GUT symmetry down to the standard model gauge group and a sector consisting of 2 × 10 + 120 of light Higgs fields. In this model we compute the corrections from the quartic interactions to the Yukawa couplings for the top and the bottom quarks and for the tau lepton. It is then shown that inclusion of these corrections to the GUT scale Yukawas allows for consistency of the top, bottom and tau masses with experiment for low tan β with a value as low as tan β of 5–10. We compute the sparticle spectrum for a set of benchmarks and find that satisfaction of the relic density is achieved via a compressed spectrum and coannihilation and three sets of coannihilations appear: chargino-neutralino, stop-neutralino and stau-neutralino. We investigate the chargino-neutralino coannihilation in detail for the possibility of observation of the light chargino at the high luminosity LHC (HL-LHC) and at the high energy LHC (HE-LHC) which is a possible future 27 TeV hadron collider. It is shown that all benchmark models but one can be discovered at HL-LHC and all would be discoverable at HE-LHC. The ones discoverable at both machines require a much shorter time scale and a lower integrated luminosity at HE-LHC.

Electroweak top couplings, partial compositeness, and top partner searches

Partial top quark compositeness is a crucial aspect of theories with strong electroweak symmetry breaking. Together with the heavy top partners that lift the top quark mass to its observed value, these theories predict correlated modifications of the top quark's electroweak couplings. Associated measurements therefore provide direct constraints on the ultraviolet structure of the underlying hypercolour dynamics. In this paper we employ a minimal version of top compositeness to discuss how measurements related to the top's electroweak gauge interactions can inform the potential composite nature of the TeV scale. In doing so, we identify the dominant factors that limit the BSM sensitivity. Extrapolating to a future 100 TeV hadron collider, we demonstrate that top quark measurements performed at highest precision can provide complementary information to resonance search by performing a representative resonant top partner search that specifically targets the correlated resonant electroweak top partner signatures.

Measuring the trilinear Higgs boson self-coupling at the 100 TeV hadron collider via multivariate analysis

We perform a multivariate analysis of Higgs-pair production via the decay channel $HH \to b\bar b \gamma\gamma$ at the future 100 TeV $pp$ collider to determine the trilinear Higgs self--coupling (THSC) $\lambda_{3H}$, which takes the value of 1 in the standard model. We consider all known background processes. For the signal we adopt the most recent event generator of {\tt POWHEG-BOX-V2} to exploit the NLO distributions for Toolkit for Multivariate Data Analysis (TMVA). Through the technique of Boosted Decision Tree (BDT) analysis trained for $\lambda_{3H}=1$, compared to the the conventional cut-and-count approach, the signal-to-background ratio improves tremendously from about $1/10$ to $1$ and the significance can reach up to $20.5$ with a luminosity of 3 ab$^{-1}$ without including systematic uncertainties. In addition, by implementing a likelihood fitting of the signal-plus-background $M_{\gamma\gamma b b}$ distribution with optimized bin sizes, it is possible to determine the THSC with the precision of 7.5\% at 68\% CL even at the early stage of 100 TeV hadron collider with 3 ab$^{-1}$.

Preliminary conceptual design of FCC-hh cryo-refrigerators: Air Liquide Study

In the framework of a world-wide international collaboration, the FCC-hh, a 100 TeV hadron collider in a 100-km long tunnel, is proposed as a future circular collider beyond LHC at CERN offering the broadest discovery potential at the energy frontier. For such high performance hadron collider, the cryogenic system has to distribute very large cooling capacities all along the 100-km tunnel for the superconducting magnets continuously cooled at 1.9 K and for the beam screens operated between 40 and 60 K. The required total cooling power will be produced in 10 refrigeration plants with a unit equivalent capacity of 100 kW at 4.5 K, up to 4 times larger than the present state-of-the-art. Half of the entropic refrigeration load is due to the synchrotron radiation produced by the high-energy proton beams and deposited on beam screens actively cooled around 50 K. This non-conventional thermal load distribution is an additional challenge for the FCC-hh cryogenic system. Furthermore, the cryogenic system has to cool down the cold mass of the FCC-hh machine in less than 20 days with controlled thermal gradients in the cryo-magnets and beam screens. Based on preliminary design works from research institutes, an engineering study was performed with world-leader industries to assess a preliminary conceptual design for the FCC-hh cryogenic plants with industrial solutions and innovative technologies. The present paper recalls the FCC-hh cryogenic requirements and presents the main results of Air Liquide study confirming the novel precooling refrigeration option down to 40 K.

Phenomenology of GUT-inspired gauge-Higgs unification

We perform a detailed investigation of a Grand Unified Theory (GUT)-inspired theory of gauge-Higgs unification. Scanning the model's parameter space with adapted numerical techniques, we contrast the scenario's low energy limit with existing SM and collider search constraints. We discuss potential modifications of di-Higgs phenomenology at hadron colliders as sensitive probes of the gauge-like character of the Higgs self-interactions and find that for phenomenologically viable parameter choices modifications of the order of 20% compared to the SM cross section can be expected. While these modifications are challenging to observe at the LHC, a future 100 TeV hadron collider might be able to constrain the scenario through more precise di-Higgs measurements. We point out alternative signatures that can be employed to constrain this model in the near future.

Higgs-boson-pair production H(→bb¯)H(→γγ) from gluon fusion at the HL-LHC and HL-100 TeV hadron collider

We perform the most up-to-date comprehensive signal-background analysis for Higgs-pair production in $HH \to b\bar b \gamma\gamma$ channel at the HL-LHC and HL-100 TeV hadron collider, with the goal of probing the self-coupling $\lambda_{3H}$ of the Higgs boson. We simulate all the standard-model signal and background processes with the simulation tools almost as sophisticated as what experimentalists are using. We find that even for the most promising channel $HH \to b\bar{b}\gamma\gamma$ at the HL-LHC with a luminosity of 3000 fb$^{-1}$, the significance is still not high enough to establish the Higgs self-coupling at the standard model (SM) value. Instead, we can only constrain the self-coupling to $-1.0 < \lambda_{3H} < 7.6 $ at $ 95\% $ confidence level after considering the uncertainties associated with the top-Yukawa coupling and the estimation of backgrounds. Here we also extend the study to the HL-100 TeV hadron collider. With a luminosity of 3 ab$^{-1}$, we find there exists a bulk region of $2.6 \lsim \lambda_{3H} \lsim 4.8 $ in which one cannot pin down the trilinear coupling. Otherwise one can measure the coupling with a high precision. At the SM value, for example, we show that the coupling can be measured with about 20 \% accuracy. While assuming 30 ab$^{-1}$, the bulk region reduces to $3.1 \lsim \lambda_{3H} \lsim 4.3$ and the trilinear coupling can be measured with about 7 \% accuracy at the SM value. With all the simulated background events and results, our study can be useful to probe the Higgs potential of various models with extended Higgs sector, such as two-Higgs-doublet model, MSSM, etc.

CP violating effects in heavy neutrino oscillations: implications for colliders and leptogenesis

Two of the important implications of the seesaw mechanism are: (i) a simple way to understand the small neutrino masses, and (ii) the origin of matter-anti-matter asymmetry in the universe via the leptogenesis mechanism. For TeV-scale seesaw models, successful leptogenesis requires that the right-handed neutrinos (RHNs) must be quasi-degenerate and if they have CP violating phases, they also contribute to the CP asymmetry. We investigate this in the TeV-scale left-right models for seesaw and point out a way to probe the quasi-degeneracy possibility with CP violating mixings for RHNs in hadron colliders using simple observables constructed out of same-sign dilepton charge asymmetry (SSCA). In particular, we isolate the parameter regions of the model, where the viability of leptogenesis can be tested using the SSCA at the Large Hadron Collider, as well as future 27 TeV and 100 TeV hadron colliders. We also independently confirm an earlier result that there is a generic lower bound on the WR mass of about 10 TeV for leptogenesis to work.

Resolving the degeneracy in top quark Yukawa coupling with Higgs pair production

The top quark Yukawa coupling (yt) can be modified by two dimension-six operators OH and Oy with the corresponding Wilson coefficients cH and cy, whose individual contribution cannot be distinguished by measuring yt alone. However, such a degeneracy can be resolved with Higgs boson pair production. In this work we explore the potential of resolving the degeneracy of the unknown Wilson coefficients cH and cy at the 14 TeV LHC and the 100 TeV hadron colliders. Combining the information of the single Higgs production, tt¯h associated production and Higgs pair production, the individual contribution of cH and cy to yt can be separated. Regardless of the value of cH, the Higgs pair production can give a strong constraint on cy at the 100 TeV hadron collider. We further show that it is possible to differentiate various cy and ct values predicted in several benchmark models.

Indirect studies of electroweakly interacting particles at 100 TeV hadron colliders

There are many extensions of the standard model that predict the existence of electroweakly interacting massive particles (EWIMPs), in particular in the context of the dark matter. In this paper, we provide a way for indirectly studying EWIMPs through the precise study of the pair production processes of charged leptons or that of a charged lepton and a neutrino at future 100 TeV collider experiments. It is revealed that this search method is suitable in particular for Higgsino, providing us the $5\sigma$ discovery reach of Higgsino in supersymmetric model with mass up to 850 GeV. We also discuss how accurately one can extract the mass, gauge charge, and spin of EWIMPs in our method.