arXiv subjects

Seung J. Lee

Publications and source records attributed to Seung J. Lee.

70 records · Page 4Linked to original sources

arXiv2011

Implications of the CDF t\bar{t} Forward-Backward Asymmetry for Boosted Top Physics

New physics at a high scale Lambda can affect top-related observables at O(1/Lambda^2) via the interference of effective four quark operators with the SM amplitude. The (\bar{u} gamma_mu gamma^5 T^a u)(\bar{t} gamma^mu gamma^5 T^a t) operator modifies the large M_{t\bar{t}} forward-backward asymmetry, and can account for the recent CDF measurement. The (\bar{u} gamma_mu T^a u)(\bar{t} gamma^mu T^a t) operator modifies the differential cross section, but cannot enhance the cross section of ultra-massive boosted jets by more than 60%. The hint for a larger enhancement from a recent CDF measurement may not persist future experimental improvements, or may be a QCD effect that is not accounted for by leading order and matched Monte Carlo tools or naive factorization. If it comes from new physics, it may stem from new light states or an O(1/Lambda^4) new physics effect.

hep-ph↗

arXiv2011

Ultra Visible Warped Model From Flavor Triviality & Improved Naturalness

A warped extra-dimensional model, where the Standard Model Yukawa hierarchy is set by UV physics, is shown to have a sweet spot of parameters with improved experimental visibility and possibly naturalness. Upon marginalizing over all the model parameters, a Kaluza-Klein scale of 2.1 TeV can be obtained at 2 sigma (95.4 CL) without conflicting with electroweak precision measurements. Fitting all relevant parameters simultaneously can relax this bound to 1.7 TeV. In this bulk version of the Rattazzi-Zaffaroni shining model, flavor violation is also highly suppressed, yielding a bound of 2.4 TeV. Non-trivial flavor physics at the LHC in the form of flavor gauge bosons is predicted. The model is also characterized by a depletion of the third generation couplings -- as predicted by the general minimal flavor violation framework -- which can be tested via flavor precision measurements. In particular, sizable CP violation in Delta B=2 transitions can be obtained, and there is a natural region where Bs mixing is predicted to be larger than Bd mixing, as favored by recent Tevatron data. Unlike other proposals, the new contributions are not linked to Higgs or any scalar exchange processes.

hep-ph↗

arXiv2011

Long-Distance Dominance of the CP Asymmetry in B->X_{s,d}+gamma Decays

We show that in the Standard Model the parametrically leading (by a factor 1/alpha_s) contribution to the inclusive CP asymmetry in B->X_{s,d}+gamma decays arises from a long-distance effect in the interference of the electromagnetic dipole amplitude with the amplitude for an up-quark penguin transition accompanied by soft gluon emission. This contribution is governed by a single hadronic parameter Lambda_{17}^u related to a matrix elements of a non-local operator. In view of current experimental data, a future precision measurement of the flavor-averaged CP asymmetry in B->X_s+gamma will signal the presence of new physics only if a value below -2% is found. A cleaner probe of new physics is offered by the difference of the CP asymmetries in charged versus neutral B-meson decays.

hep-ph↗

arXiv2010

Factorization at Subleading Power and Irreducible Uncertainties in $\bar B\to X_sγ$ Decay

Using methods from soft-collinear and heavy-quark effective theory, a systematic factorization analysis is performed for the $\bar B\to X_sγ$ photon spectrum in the endpoint region $m_b-2E_γ={\cal O}(Λ_{\rm QCD})$. It is proposed that, to all orders in $1/m_b$, the spectrum obeys a novel factorization formula, which besides terms with the structure $H\,J\otimes S$ familiar from inclusive $\bar B\to X_u l\,\barν$ decay distributions contains "resolved photon" contributions of the form $H\,J\otimes S\otimes\bar J$ and $H\,J\otimes S\otimes\bar J\otimes\bar J$. Here $S$ and $\bar J$ are new soft and jet functions, whose form is derived. These contributions arise whenever the photon couples to light partons instead of coupling directly to the effective weak interaction. The new contributions appear first at order $1/m_b$ and are related to operators other than $Q_{7γ}$ in the effective weak Hamiltonian. They give rise to non-vanishing $1/m_b$ corrections to the total decay rate, which cannot be described using a local operator product expansion. A systematic analysis of these effects is performed at tree level in hard and hard-collinear interactions. The resulting uncertainty on the decay rate defined with a cut $E_γ>1.6$ GeV is estimated to be approximately $\pm 5%$. It could be reduced by an improved measurement of the isospin asymmetry $Δ_{0-}$ to the level of $\pm 4%$. We see no possibility to reduce this uncertainty further using reliable theoretical methods.

hep-ph↗

arXiv2010

Template Overlap Method for Massive Jets

We introduce a new class of infrared safe jet observables, which we refer to as template overlaps, designed to filter targeted highly boosted particle decays from QCD jets and other background. Template overlaps are functional measures that quantify how well the energy flow of a physical jet matches the flow of a boosted partonic decay. Any region of the partonic phase space for the boosted decays defines a template. We will refer to the maximum functional overlap found this way as the template overlap. To illustrate the method, we test lowest-order templates designed to distinguish highly-boosted top and Higgs decays from backgrounds produced by event generators. For the functional overlap, we find good results with a simple construction based on a Gaussian in energy differences within angular regions surrounding the template partons. Although different event generators give different averages for our template overlaps, we find in each case excellent rejection power, especially when combined with cuts based on jet shapes. The template overlaps are capable of systematic improvement by including higher order corrections in the template phase space.

hep-ph↗

arXiv2009

Astrophysical Implications of a Visible Dark Matter Sector from a Custodially Warped-GUT

We explore, within the warped extra dimensional framework, the possibility of finding anti-matter signals in cosmic rays (CRs) from dark matter (DM) annihilation. Exchange of order 100 GeV radion, an integral part of our setup, generically results in Sommerfeld enhancement of the annihilation rate for TeV DM mass. No dark sector is required to obtain boosted annihilation cross sections. A mild hierarchy between the radion and DM masses can be natural due to the pseudo-Goldstone boson nature of the radion. Implications of Sommerfeld enhancement in warped grand unified theory (GUT) models, where proton stability implies a DM candidate, are studied. We show, via partially unified Pati-Salam group, how to incorporate a custodial symmetry for Z->b\bar b into the GUT framework such that a few TeV Kaluza-Klein (KK) mass scale is allowed by precision tests. The model with smallest fully unified SO(10) representation allows us to decouple the DM from the electroweak sector. Thus, a correct DM relic density is obtained and direct detection bounds are satisfied. Looking at robust CR observables, a possible future signal in the \bar p / p flux ratio is found. We show how to embed a similar custodial symmetry for the right handed tau, allowing it to be strongly coupled to KK particles. Such a scenario might lead to observed signal in CR positrons; however, the DM candidate in this case can not constitute all of the DM in the universe. Independently of the above, the strong coupling between KK particles and tau's can lead to striking LHC signals.

hep-ph↗

arXiv2009

Spin Determination via Third Generation Cascade Decays

Once new particles are discovered at the LHC and their masses are measured, it will be of crucial importance to determine their spin, in order to identify the underlying new physics model. We investigate the method first suggested by Barr and later extended by others to distinguish between Supersymmetry and alternative models, e.g. Universal Extra Dimensions, in a certain cascade decay. This method uses invariant mass distributions of the outgoing Standard Model particles to measure the spin of intermediate particles, by exploiting the quark/anti-quark asymmetry of the LHC as a pp collider, which is limited for first generation quarks. In this work, we suggest instead to measure the charge of the outgoing quark, in case it is a third generation quark. The resulting asymmetry for a bottom quark is similar to the previous method, while it is independent of hadronic uncertainties. Furthermore, for a top quark, the asymmetry allows better distinction between the models, as demonstrated by a quantitative analysis of model discrimination. We also show that the top's decay products can be used instead of the top itself, when the reconstruction of the top momentum is difficult to accomplish, and still provide information about the spin.

hep-ph↗

arXiv2009

Dark matter and Higgs boson collider implications of fermions in an abelian-gauged hidden sector

We add fermions to an abelian-gauged hidden sector. We show that the lightest can be the dark matter with the right thermal relic abundance, and discovery is within reach of upcoming dark matter detectors. We also show that these fermions change Higgs boson phenomenology at the Large Hadron Collider (LHC), and in particular could induce a large invisible width to the lightest Higgs boson state. Such an invisibly decaying Higgs boson can be discovered with good significance in the vector boson fusion channel at the LHC.

hep-ph↗

arXiv2008

Top Jets at the LHC

We study the reconstruction of high p_T hadronically-decaying top quarks at the LHC. The main challenge in identifying energetic top quarks is that the decay products become increasingly collimated. This reduces the efficacy of conventional methods that exploit the topology of the top decay chain. We focus on the cases where the decay products of the top quark are reconstructed as a single jet, a "top-jet". The most basic "top-tag" method based on jet mass measurement is considered in detail. To analyze the top-tagging method, theoretical and experimental aspects of the QCD jet background are examined. Based on QCD factorization, we derive a simple analytic approximation for the shape of the QCD jet mass spectrum. We observe a good agreement with the Monte Carlo simulation. We consider high-p_T t\bar{t} production in the Standard Model as an example, and show that our theoretical QCD jet mass distributions can efficiently characterize the background via sideband analyses. We show that with 25 fb^{-1} of data, our approach allows us to resolve top-jets with p_T $\ge$ 1 TeV from the QCD background, and about 1.5 TeV top-jets with 100 fb^{-1}, without relying on b-tagging. To further improve the significance we consider jet shapes (analyzed in 0807.0234 [hep-ph]), which resolve the substructure of energy flow inside cone jets. A method of measuring top quark polarization by using the b-quark p_T is also presented. The main advantages of our approach are: (i) the mass distributions are driven by first principle calculations, instead of relying solely on Monte Carlo simulation; (ii) for high p_T jets (p_T $\ge$1 TeV), IR-safe jet shape variables are robust against detector resolution effects. Our analysis is applicable to other boosted massive particles such as the electroweak gauge bosons and the Higgs.

hep-ph↗

arXiv2008

Substructure of high-p_T Jets at the LHC

We study high-pt jets from QCD and from highly-boosted massive particles such as tops, W, Z and Higgs, and argue that infrared-safe observables can help reduce QCD backgrounds. Jets from QCD are characterized by different patterns of energy flow compared to the products of highly-boosted heavy particle decays, and we employ a variety of jet shapes, observables restricted to energy flow within a jet, to explore this difference. Results from Monte Carlo generators and arguments based on perturbation theory support the discriminating power of the shapes we refer to as planar flow and angularities. We emphasize that for massive jets, these and other observables can be analyzed perturbatively.

hep-ph↗

arXiv2007

Enhanced Non-local Power Corrections to the B->X_s+gamma Decay Rate

A new class of enhanced non-perturbative corrections to the inclusive B->X_s+gamma decay rate is identified, which contribute first at order Lambda/m_b in the heavy-quark expansion and cannot be described using a local operator product expansion. Instead, these effects are described in terms of hadronic matrix elements of non-local operators with component fields separated by light-like distances. They contribute to the high-energy part of the photon-energy spectrum but do not reduce to local operators when an integral over energy is taken to obtain the total inclusive decay rate. The dominant corrections depend on the flavor of the B-meson spectator quark and are described by tri-local four-quark operators. Their contribution is estimated using the vacuum insertion approximation. The corresponding uncertainty in the total decay rate is found to be at the few percent level. This new effect accounts for the leading contribution to the rate difference between B^- and B^0 mesons.

hep-ph↗

arXiv2007

Radion Phenomenology in Realistic Warped Space Models

We investigate the phenomenology of the Randall-Sundrum radion in realistic models of electroweak symmetry breaking with bulk gauge and fermion fields, since the radion may turn out to be the lightest particle in such models. We calculate the coupling of the radion in such scenarios to bulk fermion and gauge modes. Special attention needs to be devoted to the coupling to massless gauge fields (photon, gluon), since it is well known that loop effects may be important for these fields. We also present a detailed explanation of these couplings from the CFT interpretation. We then use these couplings to determine the radion branching fractions and discuss some of the discovery potential of the LHC for the radion. We find that the gamma-gamma signal is enhanced over most of the range of the radion mass over the gamma-gamma signal of a SM Higgs, as long as the RS scale is sufficiently low. However, the signal significance depends strongly on free parameters that characterize the magnitude of bare brane-localized kinetic terms for the massless gauge fields. In the absence of such terms, the signal can be be enhanced over the traditional RS1 models (where all standard model fields are localized on the IR brane), but the signal can also be reduced compared to RS1 if the brane localized terms are sizeable. We also show that for larger radion masses, where the gamma-gamma signal is no longer significant, one can use the usual 4 lepton signal to discover the radion.

hep-ph↗

arXiv2005

The flavor of a little Higgs with T-parity

We analyze flavor constraints in the littlest Higgs model with T-parity. In particular, we focus on neutral meson mixing in the K, B, and D systems due to one loop contributions from T-parity odd fermions and gauge bosons. We calculate the short distance contributions to mixing for a general choice of T-odd fermion Yukawa couplings. We find that for a generic choice of textures, a TeV scale GIM suppression is necessary to avoid large contributions. If order one mixing angles are allowed in the extended flavor structure, the mass spectrum is severely constrained, and must be degenerate at the 1-5% level. However, there are still regions of parameter space where only a loose degeneracy is necessary to avoid constraints. We also consider the B(s) system, and identify a scenario in which the mixing can be significantly enhanced beyond the standard model prediction while still satisfying bounds on the other mixing observables. We present both analytical and numerical results as functions of the T-odd fermion mass eigenvalues.

hep-ph↗

arXiv2005

Model-Independent Properties of the B-Meson Distribution Amplitude

The operator product expansion is used to obtain model-independent predictions for the first two moments of the renormalized B-meson light-cone distribution amplitude phi_+(omega,mu), defined with a cutoff omega >mu. By solving the evolution equation for the distribution amplitude, an integral representation for phi_+(omega,mu) is obtained in terms an initial function phi_+(omega,mu_0) defined at a lower renormalization scale. A realistic model of the B-meson light-cone distribution amplitude is proposed, which satisfies the moment relations and has the correct asymptotic behavior. This model provides an estimate for the first inverse moment and the associated parameter lambda_B.

hep-ph↗

arXiv2004

Sudakov Resummation for Subleading SCET Currents and Heavy-to-Light Form Factors

The hard-scattering contributions to heavy-to-light form factors at large recoil are studied systematically in soft-collinear effective theory (SCET). Large logarithms arising from multiple energy scales are resummed by matching QCD onto SCET in two stages via an intermediate effective theory. Anomalous dimensions in the intermediate theory are computed, and their form is shown to be constrained by conformal symmetry. Renormalization-group evolution equations are solved to give a complete leading-order analysis of the hard-scattering contributions, in which all single and double logarithms are resummed. In two cases, spin-symmetry relations for the soft-overlap contributions to form factors are shown not to be broken at any order in perturbation theory by hard-scattering corrections. One-loop matching calculations in the two effective theories are performed in sample cases, for which the relative importance of renormalization-group evolution and matching corrections is investigated. The asymptotic behavior of Sudakov logarithms appearing in the coefficient functions of the soft-overlap and hard-scattering contributions to form factors is analyzed.

hep-ph↗

arXiv2002

Active cooling control of the CLEO detector using a hydrocarbon coolant farm

We describe a novel approach to particle-detector cooling in which a modular farm of active coolant-control platforms provides independent and regulated heat removal from four recently upgraded subsystems of the CLEO detector: the ring-imaging Cherenkov detector, the drift chamber, the silicon vertex detector, and the beryllium beam pipe. We report on several aspects of the system: the suitability of using the aliphatic-hydrocarbon solvent PF(TM)-200IG as a heat-transfer fluid, the sensor elements and the mechanical design of the farm platforms, a control system that is founded upon a commercial programmable logic controller employed in industrial process-control applications, and a diagnostic system based on virtual instrumentation. We summarize the system's performance and point out the potential application of the design to future high-energy physics apparatus.

hep-ex↗