arXiv subjects
Publications and source records attributed to C. -P. Yuan.
I will briefly review the physics of top quark at high energy colliders. A new discovery of single-top event at the Fermilab Tevatron is expected. At the CERN Large Hadron Collider, detailed top quark properties can be measured and new physics ideas in which top quark plays a special role can be tested. I will also discuss a few phenomenological methods for analyzing experimental data to study top quark interactions.
We evaluate the effect of the bottom-quark mass on resummed transverse momentum distributions of supersymmetric Higgs bosons at the Tevatron and LHC. The mass of the bottom quark acts as a non-negligible momentum scale at small transverse momenta and affects resummation of soft and collinear radiation in this region. The improved treatment of the b-quark mass and kinematical effects lead to observable modifications in the resummed predictions for both colliders.
We study the impact of initial-state multiple parton radiation on transverse momentum $(q_T)$ distribution of Higgs boson produced via bottom quark fusion at hadron colliders. The shape of the resulting $q_T$ distribution is affected by the bottom-quark mass corrections and by the strong kinematical behavior of the bottom-quark parton density. We account for both features in the full range of $q_T$. To do this, we formulate the resummation calculation in a general-mass factorization (S-ACOT) scheme and introduce a correction in the resummed-term to account for the effect from large-$q_T$ kinematics of Higgs boson. The results of this resummation are compared to fixed-order and PYTHIA predictions.
We calculate the production cross section of the ``CP-odd'' Higgs boson via gluon fusion in the minimal supersymmetric standard model with explicit CP violation in the stop sector. We show that there is a parameter region in which the cross section is enhanced by a factor of about 1000, as compared to the case without CP violation in the stop sector. In the parameter region where the ``CP-odd'' Higgs boson can decay into a stop pair, the stop pair events will be the important signature of the enhanced ``CP-odd'' Higgs boson. In the case where the ``CP-odd'' Higgs boson cannot decay into any superparticles, the gamma gamma and tau tau decay channels could become important for discovering the ``CP-odd'' Higgs boson. We also discuss the constraints from electric dipole moments of electron, neutron and mercury on the viable parameter space mentioned above.
Analysis of semi-inclusive DIS hadroproduction suggests broadening of transverse momentum distributions at small x below 1E-3 ~ 1E-2 which can be modeled in the Collins-Soper-Sterman formalism by a modification of impact parameter dependent parton densities. We investigate these consequences for the production of electroweak bosons at the Tevatron and the LHC. If substantial small-x broadening is observed in forward Z boson production in the Tevatron Run-2, it will strongly affect the predicted q_T distributions for W and Z boson production at the LHC.
We present the calculations of the complete next-to-leading order (NLO) QCD corrections (including supersymmetric QCD) to the inclusive total cross sections of the associated production processes $pp\to A^0Z^0+X$ in the Minimal Supersymmetric Standard Model at the CERN Large Hadron Collider. Both the dimensional regularization scheme and the dimensional reduction scheme are used to organize the calculations which yield the same NLO rates. The NLO correction can either enhance or reduce the total cross sections, but it generally efficiently reduces the dependence of the total cross sections on the renormalization/factorization scale. We also examine the uncertainty of the total cross sections due to the parton distribution function uncertainties.
Analysis of semi-inclusive DIS hadroproduction suggests broadening of transverse momentum distributions at $x$ below a few $10^{-3}$, which can be modeled in the Collins-Soper-Sterman formalism by a modification of impact-parameter-dependent parton densities. We discuss the consequences of such a modification for the production of electroweak bosons at hadron-hadron colliders. If substantial small-$x$ broadening is observed in forward Z boson production in the Tevatron Run-2, it will strongly affect predicted transverse momentum distributions for W, Z, and Higgs boson production at the Large Hadron Collider.
We present a study of the $t$-channel mode of single top quark production at the upgraded Tevatron $p\bar{p}$ collider, including the next-to-leading order (NLO) QCD corrections to the production and the decay of a single top quark. The narrow width approximation was adopted in order to preserve the spin of the top quark in its production and decay. We discuss the effects of different $O(α_s)$ contributions on the inclusive cross section as well as various kinematic distributions after imposing the relevant cuts to select $t$-channel single top signal events.
We present a calculation of the next-to-leading order QCD corrections, with one-scale phase space slicing method, to single top quark production and decay process $p\bar{p},pp\to t\bar{b}+X\to b\ellν\bar{b}+X$ at hadron colliders. Using the helicity amplitude method, the angular correlation of the final state partons and the spin correlation of the top quark are preserved. The effect of the top quark width is also examined.
We present a study of s-channel single top quark production at the upgraded Tevatron $p\bar{p}$ collider, including the next-to-leading order (NLO) QCD corrections to the production and the decay of the top quark. The "modified" narrow width approximation was adopted to preserve the spin of the top quark in its production and decay. We discuss the effect of the different $O(α_s)$ contributions on the inclusive cross section as well as various kinematic distributions after imposing the relevant cuts to select s-channel single top signal events. In particular the $O(α_s)$ decay contribution, while small in size, has a significant impact on several distributions. With the help of the best-jet algorithm to reconstruct the top quark we demonstrate that it is possible to study kinematical and spin correlations in s-channel single top events. We furthermore compare top quark spin measurements in two different basis and show how NLO corrections have to be taken into consideration in searches for the Higgs boson through $W^{\pm}H$ associated production at the Tevatron.
We discuss B-meson signatures of a Supersymmetric U(2) flavor model, with relatively light (electroweak scale masses) third generation right-handed scalars. We impose current B and K meson experimental constraints on such a theory, and obtain expectations for B->X_s gamma, B->X_s glue, B->X_s l+ l-, B->phi K_s, B_s-B_sbar mixing, and the dilepton asymmetry in B_s. We show that such a theory is compatible with all current data, and furthermore, could reconcile the apparent deviations from Standard Model predictions that have been found in some experiments.
The gold-plated discovery mode of a Standard Model like Higgs boson at the CERN Large Hadron Collider (LHC) is the H -> Z^0 Z^0 decay mode. To find and then measure the properties of the Higgs, it is crucial to have the most precise theoretical prediction both for the signal and the QCD background in this mode. In this work we calculate the effects of the initial-state multiple soft-gluon emission on the kinematic distributions of photon and Z^0 pairs produced in hadron collisions. The Collins-Soper-Sterman formalism is extended to resum the large logarithmic terms due to soft-gluons. The resummed total rates, the invariant mass, transverse momentum, and rapidity distributions of the photon and Z^0 pairs, and the transverse momentum distributions of the individual vector bosons are presented and compared to the fixed order predictions in the whole kinematic range, for the LHC energies and for the upgraded Fermilab Tevatron. Our conclusion is that the resummed predictions should be used when extracting the Higgs signal at the LHC.
We consider the collider signature of right-handed neutrinos propagating in $δ$ (large) extra dimensions, and interacting with Standard Model fields only through a Yukawa coupling to the left-handed neutrino and the Higgs boson. These theories are attractive as they can explain the smallness of the neutrino mass, as has already been shown. We show that if $δ$ is bigger than two, it can result in an enhancement in the production rate of the Higgs boson, decaying either invisibly or to a $b$ anti-$b$ quark pair, associated with an isolated high $p_T$ charged lepton and missing transverse energy at future hadron colliders, such as the LHC. The enhancement is due to the large number of Kaluza-Klein neutrinos produced in the final state. The observation of the signal event would provide an opportunity to distinguish between the normal and inverted neutrino mass hierarchies, and to determine the absolute scale of neutrino masses by measuring the asymmetry of the observed event numbers in the electron and muon channels.
We study new physics effects on the couplings of weak gauge bosons with the lightest CP-even Higgs boson ($h$), $hZZ$, and the tri-linear coupling of the lightest Higgs boson, $hhh$, at the one loop order, as predicted by the two Higgs doublet model. Those renormalized coupling constants can deviate from the Standard Model (SM) predictions due to two distinct origins; the tree level mixing effect of Higgs bosons and the quantum effect of additional particles in loop diagrams. The latter can be enhanced in the renormalized $hhh$ coupling constant when the additional particles show the non-decoupling property. Therefore, even in the case where the $hZZ$ coupling is close to the SM value, deviation in the $hhh$ coupling from the SM value can become as large as plus 100 percent, while that in the $hZZ$ coupling is at most minus 1 percent level. Such large quantum effect on the Higgs tri-linear coupling is distinguishable from the tree level mixing effect, and is expected to be detectable at a future linear collider.
Theoretical progress in Higgs boson production and background processes is discussed with particular emphasis on QCD corrections at and beyond next-to-leading order as well as next-to-leading order electroweak corrections. The residual theoretical uncertainties of the investigated processes are estimated in detail. Moreover, recent investigations of the MSSM Higgs sector and other extensions of the SM Higgs sector are presented. The potential of the LHC and a high-energy linear e+e- collider for the measurement of Higgs couplings is analyzed.
We consider right-handed neutrinos propagating in $δ$ (large) extra dimensions, whose only coupling to Standard Model fields is the Yukawa coupling to the left-handed neutrino and the Higgs boson. These theories are attractive as they can explain the smallness of the neutrino mass, as has already been shown. We show that if $δ$ is bigger than two, there are strong constraints on the radius of the extra dimensions, resulting from the experimental limit on the probability of an active state to mix into the large number of sterile Kaluza-Klein states of the bulk neutrino. We also calculate the bounds on the radius resulting from requiring that perturbative unitarity be valid in the theory, in an imagined Higgs-Higgs scattering channel.
We discuss the B-physics signature of a supersymmetric U(2) flavor model in which the third generation scalars are relatively light (electroweak scale masses). We impose current experimental constraints on such a framework and obtain expectations for various B-physics processes. Here we present CP violation in (b->s gamma) and (b->phi K), and, Bs-Bsbar mixing. We show that if realized in nature, such a framework can be discovered in current and upcoming experiments.
We propose a sensitive way to test the anomalous HVV couplings (V=W^\pm, Z^0) of the Higgs boson (H), which can arise from either the dimension-3 effective operator in a nonlinearly realized Higgs sector or the dimension-6 effective operators in a linearly realized Higgs sector, via studying the VV scattering processes at the CERN Large Hadron Collider (LHC). We show that, with an integrated luminosity of 300 fb^{-1} and sufficient kinematical cuts for suppressing the backgrounds, studying the process pp \to W^+ W^+ j j \to ł^+ νł^+ νj j can probe the anomalous HWW couplings at a few tens of percent level for the nonlinearly realized Higgs sector, and at the level of 0.01-0.08 TeV^{-1} for the linearly realized effective Lagrangian.