arXiv subjects

Lian-Tao Wang

Publications and source records attributed to Lian-Tao Wang.

At least 181 records · Page 10Linked to original sources

arXiv2007

Spin Measurements in Cascade Decays at the LHC

We systematically study the possibility of determining the spin of new particles after their discovery at the LHC. We concentrate on angular correlations in cascade decays. Motivated by constraints of electroweak precision tests and the potential of providing a Cold Dark Matter candidate, we focus on scenarios of new physics in which some discrete symmetry guarantees the existence of stable neutral particles which escape the detector. More specifically, we compare supersymmetry with another generic scenario in which new physics particles have the same spin as their Standard Model partners. A survey of possibilities of observing spin correlations in a broad range of decay channels is carried out, with interesting ones identified. Rather than confining ourselves to one "collider friendly" benchmark point (such as SPS1a), we describe the parameter region in which any particular decay channel is effective. We conduct a more detailed study of chargino's spin determination in the decay channel $\tilde{q}\to q + \tilde{C}^\pm \to q + W^\pm + LSP$. A scan over the chargino and neutralino masses is performed. We find that as long as the spectrum is not too degenerate the prospects for spin determination in this channel are rather good.

hep-ph↗

arXiv2007

MARMOSET: The Path from LHC Data to the New Standard Model via On-Shell Effective Theories

We describe a coherent strategy and set of tools for reconstructing the fundamental theory of the TeV scale from LHC data. We show that On-Shell Effective Theories (OSETs) effectively characterize hadron collider data in terms of masses, production cross sections, and decay modes of candidate new particles. An OSET description of the data strongly constrains the underlying new physics, and sharply motivates the construction of its Lagrangian. Simulating OSETs allows efficient analysis of new-physics signals, especially when they arise from complicated production and decay topologies. To this end, we present MARMOSET, a Monte Carlo tool for simulating the OSET version of essentially any new-physics model. MARMOSET enables rapid testing of theoretical hypotheses suggested by both data and model-building intuition, which together chart a path to the underlying theory. We illustrate this process by working through a number of data challenges, where the most important features of TeV-scale physics are reconstructed with as little as 5 inverse femtobarns of simulated LHC signals.

hep-ph↗

arXiv2007

The Bulk RS KK-gluon at the LHC

We study the possibility of discovering and measuring the properties of the lightest Kaluza-Klein excitation of the gluon in a Randall-Sundrum scenario where the Standard Model matter and gauge fields propagate in the bulk. The KK-gluon decays primarily into top quarks. We discuss how to use the $t \bar{t}$ final states to discover and probe the properties of the KK-gluon. Identification of highly energetic tops is crucial for this analysis. We show that conventional identification methods relying on well separated decay products will not work for heavy resonances but suggest alternative methods for top identification for energetic tops. We find, conservatively, that resonances with masses less than 5 TeV can be discovered if the algorithm to identify high $p_T$ tops can reject the QCD background by a factor of 10. We also find that for similar or lighter masses the spin can be determined and for lighter masses the chirality of the coupling to $t\bar t$ can be measured. Since the energetic top pair final state is a generic signature for a large class of new physics as the top quark presumably couples most strongly to the electroweak symmetry breaking sector, the methods we have outlined to study the properties of the KK-gluon should also be important in other scenarios.

hep-ph↗

arXiv2006

Little M-theory

Using the language of theory space, i.e. moose models, we develop a unified framework for studying composite Higgs models at the LHC. This framework--denoted little M-theory--is conveniently described by a theoretically consistent three-site moose diagram which implements minimal flavor and isospin violation. By taking different limits of the couplings, one can interpolate between simple group-like and minimal moose-like models with and without T-parity. In this way, little M-theory reveals a large model space for composite Higgs theories. We argue that this framework is suitable as a starting point for a comprehensive study of composite Higgs scenarios. The rich collider phenomenology of this framework is briefly discussed.

hep-ph↗

arXiv2006

Top Partners in Little Higgs Theories with T-parity

We consider a class of little Higgs theories with T-parity where ALL new particles responsible for canceling the standard model contributions to the one-loop quadratic divergences in the Higgs potential are odd under T-parity, including the heavy top partner which was previously taken to be T-even. The new construction significantly simplifies the spectrum in the top sector and completely changes the phenomenology of the top partner. At hadron colliders the signals of this class of T-invariant models appear to be even more similar to supersymmetry. We initiate a study on the collider phenomenology and discuss the challenge of distinguishing this class of models from supersymmetry at the Large Hadron Collider.

hep-ph↗

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.

hep-ph↗

arXiv2005

Supersymmetry and the LHC Inverse Problem

Given experimental evidence at the LHC for physics beyond the standard model, how can we determine the nature of the underlying theory? We initiate an approach to studying the "inverse map" from the space of LHC signatures to the parameter space of theoretical models within the context of low-energy supersymmetry, using 1808 LHC observables including essentially all those suggested in the literature and a 15 dimensional parametrization of the supersymmetric standard model. We show that the inverse map of a point in signature space consists of a number of isolated islands in parameter space, indicating the existence of "degeneracies"--qualitatively different models with the same LHC signatures. The degeneracies have simple physical characterizations, largely reflecting discrete ambiguities in electroweak-ino spectrum, accompanied by small adjustments for the remaining soft parameters. The number of degeneracies falls in the range 1<d<100, depending on whether or not sleptons are copiously produced in cascade decays. This number is large enough to represent a clear challenge but small enough to encourage looking for new observables that can further break the degeneracies and determine at the LHC most of the SUSY physics we care about. Degeneracies occur because signatures are not independent, and our approach allows testing of any new signature for its independence. Our methods can also be applied to any other theory of physics beyond the standard model, allowing one to study how model footprints differ in signature space and to test ways of distinguishing qualitatively different possibilities for new physics at the LHC.

hep-ph↗

arXiv2004

Model Building and Phenomenology of Flux-Induced Supersymmetry Breaking on D3-branes

We study supersymmetry breaking effects induced on D3-branes at singularities by the presence of NSNS and RR 3-form fluxes. First, we discuss some local constructions of chiral models from D3-branes at singularities, as well as their global embedding in flux compactifications. The low energy spectrum of these constructions contains features of the supersymmetric Standard Model. In these models, both the soft SUSY parameters and the mu-term are generated by turning on the 3-form NSNS and RR fluxes. We then explore some model-independent phenomenological features as, e.g., the fine-tuning problem of electroweak symmetry breaking in flux compactifications. We also comment on other phenomenological features of this scenario.

hep-th↗

arXiv2004

Possible Evidence For Axino Dark Matter In The Galactic Bulge

Recently, the SPI spectrometer on the INTEGRAL satellite observed strong 511 keV line emission from the galactic bulge. Although the angular distribution (spherically symmetric with width of \sim 9 degree) of this emission is difficult to account for with traditional astrophysical scenarios, light dark matter particles could account for the observation. In this letter, we consider the possibility that decaying axinos in an R-parity violating model of supersymmetry may be the source of this emission. We find that \sim 1-300 MeV axinos with R-parity violating couplings can naturally produce the observed emission.

hep-ph↗

arXiv2004

Loop induced decays of the Little Higgs: H --> gg, gamma gamma

We analyze the loop induced decays of the Higgs boson into pairs of gluons and photons in the Littlest Higgs model. We find that the deviation of the partial widths for these decays relative to their Standard Model values scales with 1/f^2, where f ~ TeV is the mass scale of the new heavy particles in the model. For f = 1 TeV, Gamma(H -> gg) is reduced by 6-10% and Gamma(H -> gamma gamma) is reduced by 5-7% compared to their Standard Model values. While the LHC and a linear e+e- collider would be sensitive to these deviations only for relatively low values of f <~ 650 GeV, a photon collider could probe the deviation in Gamma(H -> gamma gamma) up to f <~ 1.1 (0.7) TeV at the 2 (5) sigma level.

hep-ph↗

arXiv2004

High Scale Study of Possible B \to ϕK_S CP Physics

Some rare decay processes are particularly sensitive to physics beyond the Standard Model (SM) because they have no SM tree contributions. We focus on one of these, $B_{d}\to ϕK_{s}$. Our study is in terms of the high scale effective theory, and high scale models for the underlying theory, while previous studies have been focusing on the low scale effective Lagrangian. We examine phenomenologically the high scale parameter space with full calculations, but largely report the results in terms of mass insertion techniques since they are then easily pictured. We also determine the ranges of different mass insertions that could produce large non-SM CP effects. Then we exhibit classes of high scale models that can or cannot provide large non-SM CP effects, thus demonstrating that data on $B_{d}\to ϕK_{s}$ can probe both supersymmetry breaking and the underlying high scale theory and even make relatively direct contact with string-motivated models. We provide a novel and systematic technique to understand the relations between high and low scale parameters from RGE running. We include all constraints from other data, particularly $b\to sγ$ and EDMs.

hep-ph↗

arXiv2004

Theoretical Implications of the LEP Higgs Search

We study the implications for the minimal supersymmetric standard model (MSSM) of the absence of a direct discovery of a Higgs boson at LEP. First we exhibit 15 physically different ways in which one or more Higgs bosons lighter than the LEP limit could still exist. For each of these cases -- as well as the case that the lightest Higgs eigenstate is at, or slightly above, the current LEP limit -- we provide explicit sample configurations of the Higgs sector as well as the soft supersymmetry breaking Lagrangian parameters necessary to generate these outcomes. We argue that all of the cases seem fine-tuned, with the least fine-tuned outcome being that with Higgs mass near 115 GeV. Seeking to minimize this tuning we investigate ways in which the ``maximal-mixing'' scenario with large top-quark trilinear A-term can be obtained from simple string-inspired supergravity models. We find these obvious approaches lead to heavy gauginos and/or problematic low-energy phenomenology with minimal improvement in fine-tuning.

hep-ph↗

arXiv2004

Phenomenology of the Little Higgs Model

We study the low energy phenomenology of the little Higgs model. We first discuss the linearized effective theory of the "littlest Higgs model" and study the low energy constraints on the model parameters. We identify sources of the corrections to low energy observables, discuss model-dependent arbitrariness, and outline some possible directions of extensions of the model in order to evade the precision electroweak constraints. We then explore the characteristic signatures to test the model in the current and future collider experiments. We find that the LHC has great potential to discover the new SU(2) gauge bosons and the possible new U(1) gauge boson to the multi-TeV mass scale. Other states such as the colored vector-like quark T and doubly-charged Higgs boson Phi^{++} may also provide interesting signals. At a linear collider, precision measurements on the triple gauge boson couplings could be sensitive to the new physics scale of a few TeV. We provide a comprehensive list of the linearized interactions and vertices for the littlest Higgs model in the appendices.

hep-ph↗

arXiv2003

The Soft Supersymmetry-Breaking Lagrangian: Theory and Applications

After an introduction recalling the theoretical motivation for low energy (100 GeV to TeV scale) supersymmetry, this review describes the theory and experimental implications of the soft supersymmetry-breaking Lagrangian of the general minimal supersymmetric standard model (MSSM). Extensions to include neutrino masses and nonminimal theories are also discussed. Topics covered include models of supersymmetry breaking, phenomenological constraints from electroweak symmetry breaking, flavor/CP violation, collider searches, and cosmological constraints including dark matter and implications for baryogenesis and inflation.

hep-ph↗

arXiv2003

Relating Incomplete Data and Incomplete Theory

Assuming string theorists will not soon provide a compelling case for the primary theory underlying particle physics, the field will proceed as it has historically: with data stimulating and testing ideas. Ideally the soft supersymmetry breaking Lagrangian will be measured and its patterns will point to the underlying theory. But there are two new problems. First a matter of principle: the theory may be simplest at distance scales and in numbers of dimensions where direct experiments are not possible. Second a practical problem: in the foreseeable future (with mainly hadron collider data) too few observables can be measured to lead to direct connections between experiment and theory. In this paper we discuss and study these issues and consider ways to circumvent the problems, studying models to test methods. We propose a semi-quantitative method for focusing and sharpening thinking when trying to relate incomplete data to incomplete theory, as will probably be necessary.

hep-ph↗

arXiv2003

D-Matter

We study the properties and phenomenology of particle-like states originating from D-branes whose spatial dimensions are all compactified. They are non-perturbative states in string theory and we refer to them as D-matter. In contrast to other non-perturbative objects such as 't Hooft-Polyakov monopoles, D-matter states could have perturbative couplings among themselves and with ordinary matter. The lightest D-particle (LDP) could be stable because it is the lightest state carrying certain (integer or discrete) quantum numbers. Depending on the string scale, they could be cold dark matter candidates with properties similar to that of wimps or wimpzillas. The spectrum of excited states of D-matter exhibits an interesting pattern which could be distinguished from that of Kaluza-Klein modes, winding states, and string resonances. We speculate about possible signatures of D-matter from ultra-high energy cosmic rays and colliders.

hep-ph↗

arXiv2003

B --> Phi K_S and Supersymmetry

The rare decay B --> Phi K_S is a well-known probe of physics beyond the Standard Model because it arises only through loop effects yet has the same time-dependent CP asymmetry as B --> Psi K_S. Motivated by recent data suggesting new physics in B --> Phi K_S, we look to supersymmetry for possible explanations, including contributions mediated by gluino loops and by Higgs bosons. Chirality-preserving LL and RR gluino contributions are generically small, unless gluinos and squarks masses are close to the current lower bounds. Higgs contributions are also too small to explain a large asymmetry if we impose the current upper limit on B(B_s --> mu mu). On the other hand, chirality-flipping LR and RL gluino contributions can provide sizable effects and while remaining consistent with related results in B --> Psi K_S, Delta M_s, B --> X_s gamma and other processes. We discuss how the LR and RL insertions can be distinguished using other observables, and we provide a string-based model and other estimates to show that the needed sizes of mass insertions are reasonable.

hep-ph↗

arXiv2003

Direct and Indirect Detection of Neutralino Dark Matter In Selected Supersymmetry Breaking Scenarios

Various methods of searching for supersymmetric dark matter are sensitive to WIMPs with different properties. One consequence of this is that the phenomenology of dark matter detection can vary dramatically in different supersymmetric breaking scenarios. In this paper, we consider the sensitivities to supersymmetric dark matter of different detection methods and techniques in a wide variety of supersymmetric breaking scenarios. We discuss the ability of various astrophysical experiments, such as direct experiments, gamma-rays satellites, neutrino telescopes and positron and anti-proton cosmic ray experiments, to test various supersymmetry breaking scenarios. We also discuss what information can be revealed about supersymmetry breaking by combining results from complementary experiments. We place an emphasis on the differences between various experimental techniques.

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