arXiv subjects

Tao Han

Publications and source records attributed to Tao Han.

At least 289 records · Page 16Linked to original sources

arXiv2009

Collider Signatures for Heavy Lepton Triplet in Type I+III Seesaw

The minimal SU(5) theory augmented by the fermionic adjoint representation restores the coupling constant unification and gives realistic neutrino masses and mixing through the hybrid Type I and Type III seesaw. The crucial prediction of the theory is an SU(2) lepton triplet with the mass below TeV. We study the signature of these heavy leptons and propose the strategy to test this mechanism at the hadron and lepton colliders. The smoking gun evidence of the theory is Delta L=2 lepton number violation through events of a pair of like-sign leptons plus four jets without significant missing energy at hadron colliders. We find that via this unique channel, the heavy lepton can be searched for up to a mass of 200 GeV at the Tevatron with 8 fb^-1, and up to 450 (700) GeV at the LHC of 14 TeV C.M.energy with 10 (100) fb^-1. The signal rate at the 10 TeV LHC is reduced to 60-35% for a mass of 200-700 GeV. We also comment on how to distinguish this theory from other models with similar heavy leptons. Finally, we compare the production rates and angular distributions of heavy leptons in e+e- collisions for various models.

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arXiv2008

Leptoquarks and Neutrino Masses at the LHC

The properties of light leptoquarks predicted in the context of a simple grand unified theory and their observability at the LHC are investigated. The SU(5) symmetry of the theory implies that the leptoquark couplings to matter are related to the neutrino mass matrix. We study the resulting connection between neutrino masses and mixing parameters and the leptoquark decays, and show that different light neutrino hierarchies imply distinctive leptoquark decay signatures. We also discuss low-energy constraints implied by searches for charged lepton flavour violation, studies of meson decays, and electroweak precision data. We perform a detailed parton-level study of the leptoquark signals and the Standard Model backgrounds at the LHC. With the clean final states containing a di-lepton plus two jets, the QCD production of the leptoquark pair can be observed for a leptoquark mass of one TeV and beyond. By examining the lepton flavor structure of the observed events, one could further test the model predictions related to the neutrino mass spectrum. In particular, b-flavor tagging will be useful in distinguishing the neutrino mass pattern and possibly probing an unknown Majorana phase in the Inverted Hierarchy or the Quasi-Degenerate scenario. Electroweak associated production of the leptoquark doublet can also be useful in identifying the quantum numbers of the leptoquarks and distinguishing between the neutrino mass spectra, even though the corresponding event rates are smaller than for QCD production. We find that with only the clean channel of mu+ E_T jets, one could expect an observable signal for a leptoquark masses of about 600 GeV or higher.

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arXiv2008

Testing a Neutrino Mass Generation Mechanism at the Large Hadron Collider

The Large Hadron Collider could be a discovery machine for the neutrino mass pattern and its Majorana nature in the context of a well-motivated TeV scale Type II seesaw model. This is achieved by identifying the flavor structure of the lepton number violating decays of the charged Higgs bosons. The observation of either H^+ --> tau^+ nubar or H^+ --> e^+ nubar will be particularly robust to determine the neutrino spectra since they are independent of the unknown Majorana phases, which could be probed via the H^{++} --> e^+_i e^+_j decays. In a less favorable scenario when the leptonic channels are suppressed, one needs to observe the decays H^+ --> W^+ H_1, and H^+ --> t bbar to confirm the triplet-doublet mixing that implies the Type II relation. The associated production H^{++} H^{-} is crucial in order to test the triplet nature of the Higgs field.

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arXiv2008

LHC Signals for Warped Electroweak Charged Gauge Bosons

We study signals at the Large Hadron Collider (LHC) for the Kaluza-Klein (KK) excitations of electroweak charged gauge bosons in the framework of the Standard Model (SM) fields propagating in the bulk of a warped extra dimension. Such a scenario can solve both the Planck-weak and flavor hierarchy problems of the SM. There are two such charged states in this scenario with couplings to light quarks and leptons being suppressed relative to those in the SM, whereas the couplings to top/bottom quarks are enhanced, similar to the case of electroweak neutral gauge bosons previously studied. However, unlike the case of electroweak neutral gauge bosons, there is no irreducible QCD background (including pollution from possibly degenerate KK gluons) for decays to top + bottom final state so that this channel is useful for the discovery of the charged states. Moreover, decays of electroweak charged gauge bosons to longitudinal W, Z and Higgs are enhanced just as for the neutral bosons. However, unlike for the neutral gauge bosons, the purely leptonic (and hence clean) decay mode of the WZ are fully reconstructible so that the ratio of the signal to the SM (electroweak) background can potentially be enhanced by restricting to the resonance region more efficiently. We show that such final states can give sensitivity to 2 (3) TeV masses with an integrated luminosity of 100 (300) inverse fb. We emphasize that improvements in discriminating a QCD-jet from a highly boosted hadronically decaying W, and a highly boosted top-jet from a bottom-jet will enhance the reach for these KK particles, and that the signals we study for the warped extra dimensional model might actually be applicable also to a wider class of non-supersymmetric models of electroweak symmetry breaking.

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arXiv2008

Phenomenology of Hidden Valleys at Hadron Colliders

We study the phenomenology of, and search techniques for, a class of "Hidden Valleys." These models are characterized by low mass (well below a TeV) bound states resulting from a confining gauge interaction in a hidden sector; the states include a spin-one resonance that can decay to lepton pairs. Assuming that the hidden sector communicates to the Standard Model (SM) through TeV suppressed operators, taking into account the constraint from the $Z$ pole physics at LEP, searches at Tevatron may be difficult in the particular class of Hidden Valleys we consider, so that we concentrate on the searches at the LHC. Hidden Valley events are characterized by high multiplicities of jets and leptons in the final state. Depending on the scale of confinement in the hidden sector, the events are typically more spherical, with lower thrust and higher incidences of isolated leptons, than those from the SM background processes. Most notably, high cluster invariant mass and very narrow, low mass resonances in lepton pairs are the key observables to identify the signal. We use these characteristics to develop a set of cuts to separate the Hidden Valley from SM, and show that with these cuts LHC has a significant reach in the parameter space. Our strategies are quite general and should apply well beyond the particular class of models studied here.

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arXiv2008

Neutrino Masses and the LHC: Testing Type II Seesaw

We demonstrate how to systematically test a well-motivated mechanism for neutrino mass generation (Type-II seesaw) at the LHC, in which a Higgs triplet is introduced. In the optimistic scenarios with a small Higgs triplet vacuum expectation value vd < 10^{-4} GeV, one can look for clean signals of lepton number violation in the decays of doubly charged and singly charged Higgs bosons to distinguish the Normal Hierarchy (NH), the Inverted Hierarchy (IH) and the Quasi-Degenerate (QD) spectrum for the light neutrino masses. The observation of either H+ --> tau+ nubar or H+ --> e+ nubar will be particularly robust for the spectrum test since they are independent of the unknown Majorana phases. The H++ decays moderately depend on a Majorana phase Phi2 in the NH, but sensitively depend on Phi1 in the IH. In a less favorable scenario vd > 2 10^{-4} GeV, when the leptonic channels are suppressed, one needs to observe the decays H+ --> W+ H_1 and H+ --> t bbar to confirm the triplet-doublet mixing which in turn implies the existence of the same gauge-invariant interaction between the lepton doublet and the Higgs triplet responsible for the neutrino mass generation. In the most optimistic situation, vd approx 10^{-4} GeV, both channels of the lepton pairs and gauge boson pairs may be available simultaneously. The determination of their relative branching fractions would give a measurement for the value of vd.

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arXiv2008

The "Top Priority" at the LHC

The LHC will be a top-quark factory. With 80 million pairs of top quarks and an additional 34 million single tops produced annually at the designed high luminosity, the properties of this particle will be studied to a great accuracy. The fact that the top quark is the heaviest elementary particle in the Standard Model with a mass right at the electroweak scale makes it tempting to contemplate its role in electroweak symmetry breaking, as well as its potential as a window to unknown new physics at the TeV scale. We summarize the expectations for top-quark physics at the LHC, and outline new physics scenarios in which the top quark is crucially involved.

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arXiv2008

Higgs Signal for h to aa at Hadron Colliders

We assess the prospect of observing a neutral Higgs boson at hadron colliders in its decay to two spin-zero states, a, for a Higgs mass of 90-130 GeV, when produced in association with a W or Z boson. Such a decay is allowed in extensions of the MSSM with CP-violating interactions and in the NMSSM, and can dominate Higgs boson final states, thereby evading the LEP constraints on standard Higgs boson production. The light spin-zero state decays primarily via a to bb and tau+tau-, so this signal channel retains features distinct from the main backgrounds. Our study shows that at the Tevatron, there may be potential to observe a few events in the bb tau+tau- or bbbb channels with relatively small background, although this observation would be statistically limited. At the LHC, the background problem is more severe, but with cross sections and integrated luminosities orders of magnitude larger than at the Tevatron, the observation of a Higgs boson in this decay mode would be possible. The channel h to aa to bbbb would provide a large statistical significance, with a signal-to-background ratio on the order of 1:2. In these searches, the main challenge would be to retain the adequate tagging efficiency of b's and tau's in the low p_T region.

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arXiv2008

Top Quark Pair plus Large Missing Energy at the LHC

We study methods of extracting new physics signals in final states with a top-quark pair plus large missing energy at the LHC. We consider two typical examples of such new physics: pair production of a fermionic top partner (a $T'$ in Little Higgs models for example) and of a scalar top partner (a $\tilde{t}$ in SUSY). With a commonly-adopted discrete symmetry under which non Standard Model particles are odd, the top partner is assumed to decay predominantly to a top quark plus a massive neutral stable particle $A^0$. We focus on the case in which one of the top quarks decays leptonically and the other decays hadronically, $pp \to {\tt} A^0A^0 X \to bj_1j_2 \bar b\ell^- \barνA^0A^0 X + c.c.$, where the $A^0$s escape detection. We identify a key parameter for the signal observation: the mass splitting between the top partner and the missing particle. We reconstruct a transverse mass for the lepton-missing transverse energy system to separate the real $W$ background from the signal and propose a definition for the reconstructed top quark mass that allows it to take unphysical values as an indication of new physics. We perform a scan over the two masses to map out the discovery reach at the LHC in this channel. We also comment on the possibility of distinguishing between scalar and fermionic top partners using collider signatures.

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arXiv2008

Charged Higgs Boson Effects in the Production and Decay of a Heavy Majorana Neutrino at the LHC

We consider a new interaction between a heavy Majorana neutrino (N) and a charged Higgs boson (H+) and show that it can have drastic implications on lepton number violating (LNV) signal of same-sign dileptons at the LHC. The LNV signal of heavy Majorana neutrinos previously considered at the LHC, pp -> l+ N -> l+ l+ W-, may be overwhelmed by pp -> l+ N -> l+ l+ H-. With the subsequent decays H- -> b t(bar) or H- -> W- H^0, the heavy Majorana neutrino production leads to the spectacular events of l+ l+ b b(bar) + 2 jets. We also explore the case m_N < m_{H+}, where the decay H+ -> l+ N can become the dominant N-production mechanism at the LHC. In particular, we show that the process g b -> t H- followed by t -> b W+ and H- -> l- N -> l- l- W+ could lead to another type of spectacular events of l- l- b + 4 jets.

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arXiv2007

Top Quark Pairs at High Invariant Mass - A Model-Independent Discriminator of New Physics at the LHC

We study top quark pair production to probe new physics at the LHC. We propose reconstruction methods for $t\bar{t}$ semileptonic events and use them to reconstruct the $t\bar{t}$ invariant mass. The angular distribution of top quarks in their c.m. frame can determine the spin and production subprocess for each new physics resonance. Forward-backward asymmetry and CP-odd variables can be constructed to further delineate the nature of new physics. We parametrize the new resonances with a few generic parameters and show high invariant mass top pair production may provide an early indicator for new physics beyond the Standard Model.

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arXiv2007

LHC Signals for Warped Electroweak Neutral Gauge Bosons

We study signals at the Large Hadron Collider (LHC) for Kaluza-Klein (KK) excitations of the electroweak gauge bosons in the framework with the Standard Model (SM) gauge and fermion fields propagating in a warped extra dimension. Such a framework addresses both the Planck-weak and flavor hierarchy problems of the SM. Unlike the often studied Z' cases, in this framework, there are three neutral gauge bosons due to the underlying SU(2)_L x SU(2)_R x U(1)_X gauge group in the bulk. Furthermore, couplings of these KK states to light quarks and leptons are suppressed, whereas those to top and bottom quarks are enhanced compared to the SM gauge couplings. Therefore, the production of light quark and lepton states is suppressed relative to other beyond the SM constructions, and the fermionic decays of these states are dominated by the top and bottom quarks, which are, though, overwhelmed by KK gluons dominantly decaying into them. However, as we emphasize in this paper, decays of these states to longitudinal W, Z and Higgs are also enhanced similarly to the case of top and bottom quarks. We show that the W, Z and Higgs final states can give significant sensitivity at the LHC to 2 (3) TeV KK scale with an integrated luminosity of 100 invfb (1 invab). Since current theoretical framework(s) favor KK masses greater than about 3 TeV, luminosity upgrade of LHC is likely to be crucial in observing these states.

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arXiv2007

Pair Production of Doubly-Charged Scalars: Neutrino Mass Constraints and Signals at the LHC

We study the pair production of doubly charged Higgs bosons at the Large Hadron Collider (LHC), assuming the doubly charged Higgs to be part of an SU(2)_L triplet which generates Majorana masses for left-handed neutrinos. Such pair-production has the advantage that it is not constrained by the triplet vacuum expectation value, which tends to make the single production rate rather small. We point out that, in addition to the Drell-Yan (DY) production mechanism, two-photon processes also contribute to H++H++ production at a level comparable to the QCD corrections to the DY channel. Decays of the doubly charged Higgs into both the l+l+ and W+W+ modes are studied in detail to optimize the signal observation over the backgrounds. Doubly charged scalars should be observable at the LHC with 300 fb^-1 integrated luminosity in the ll channel upto the mass range of 1 TeV even with a branching fraction of about 60 %, and in the WW channel upto a mass of 700 GeV. Such a doubly charged Higgs, if it is a member of a triplet generating neutrino masses,cannot be long-lived on the scale of collider detectors although it might lead to a displaced secondary vertex during its decay if it is lighter than about 250 GeV.

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arXiv2007

QCD corrections to single slepton production at hadron colliders

We evaluate the cross section for single slepton production at hadron colliders in supersymmetric theories with R-parity violating interactions to the next-to-leading order in QCD. We obtain fully differential cross section by using the phase space slicing method. We also perform soft-gluon resummation to all order in $α_s$ of leading logarithm to obtain a complete transverse momentum spectrum of the slepton. We find that the full transverse momentum spectrum is peaked at a few GeV, consistent with the early results for Drell-Yan production of lepton pairs. We also consider the contribution from gluon fusion via quark-triangle loop diagrams dominated by the $b$-quark loop. The cross section of this process is significantly smaller than that of the tree-level process induced by the initial $b\bar{b}$ annihilation.

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arXiv2006

Signatures for Majorana neutrinos at hadron colliders

The Majorana nature of neutrinos may only be experimentally verified via lepton-number violating processes involving charged leptons. We explore the $ΔL=2$ like-sign dilepton production at hadron colliders to search for signals of Majorana neutrinos. We find significant sensitivity for resonant production of a Majorana neutrino in the mass range of 10-80 GeV at the current run of the Tevatron with 2 fb$^{-1}$ integrated luminosity, and in the range of 10-400 GeV at the LHC with 100 fb$^{-1}$.

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arXiv2006

Smoking-gun signatures of little Higgs models

Little Higgs models predict new gauge bosons, fermions and scalars at the TeV scale that stabilize the Higgs mass against quadratically divergent one-loop radiative corrections. We categorize the many little Higgs models into two classes based on the structure of the extended electroweak gauge group and examine the experimental signatures that identify the little Higgs mechanism in addition to those that identify the particular little Higgs model. We find that by examining the properties of the new heavy fermion(s) at the LHC, one can distinguish the structure of the top quark mass generation mechanism and test the little Higgs mechanism in the top sector. Similarly, by studying the couplings of the new gauge bosons to the light Higgs boson and to the Standard Model fermions, one can confirm the little Higgs mechanism and determine the structure of the extended electroweak gauge group.

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arXiv2005

Anomalous gauge couplings of the Higgs boson at high energy photon colliders

We study the sensitivity of testing the anomalous gauge couplings $g_{HVV}$'s of the Higgs boson in the formulation of linearly realized gauge symmetry via the processes $γγ\to ZZ$ and $γγ\to WWWW$ at polarized and unpolarized photon colliders based on $e^+e^-$ linear colliders of c.m.~energies 500 GeV, 1 TeV, and 3 TeV. Signals beyond the standard model (SM) and SM backgrounds are carefully studied. We propose certain kinematic cuts to suppress the standard model backgrounds. For an integrated luminosity of 1 ab$^{-1}$, we show that (a) $γγ\to ZZ$ can provide a test of $g_{Hγγ}$ to the $3σ$ sensitivity of $O(10^{-3}-10^{-2})$ TeV$^{-1}$ at a 500 GeV ILC, and $O(10^{-3})$ TeV$^{-1}$ at a 1 TeV ILC and a 3 TeV CLIC, and (b) $γγ\to WWWW$ at a 3 TeV CLIC can test all the anomalous couplings $g_{HVV}$'s to the $3σ$ sensitivity of $O(10^{-3}-10^{-2})$ TeV$^{-1}$.

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arXiv2005

Exotic Neutrino Interactions at the Pierre Auger Observatory

The Pierre Auger Observatory for cosmic rays provides a laboratory for studying fundamental interactions at energies well beyond those available at colliders. In addition to hadrons or photons, Auger is sensitive to ultra-high energy neutrinos in the cosmic radiation and models for new physics can be explored by observing neutrino interactions at center-of-mass energies beyond the TeV scale. By comparing the rate for quasi-horizontal, deeply penetrating air showers triggered by all types of neutrinos with the rate for slightly upgoing showers generated by Earth-skimming tau neutrinos, any deviation of the neutrino-nucleon cross-section from the Standard Model expectation can be constrained. We show that this can test models of low-scale quantum gravity (including processes such as Kaluza-Klein graviton exchange, microscopic black hole production and string resonances), as well as non-perturbative electroweak instanton mediated processes. Moreover, the observed ratios of neutrino flavors would severely constrain the possibility of neutrino decay.

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