arXiv subjects
Publications and source records attributed to Sanghyeon Chang.
The renormalization evolution of all parameters in the neutrino mass matrix depends only on one variable, the energy scale. This fact, coupled with rephasing considerations, leads to a set of renormalization invariants, correlating the evolution of physical parameters. For the general three flavor case, we obtain these invariants explicitly and discuss their implications.
A simple axion model is proposed in the scenario of large extra dimensions where gravity scale is as low as 1 TeV. To obtain an intermediate-scale decay constant of the axion, the axion is assumed to live in a sub-spacetime (brane) of the whole bulk. In this model there appear Kaluza-Klein modes of the axion which have stronger interaction than those of the graviton. The axion brane plays a role of absorber of the graviton Kaluza-Klein modes. Because of these reasons phenomenology and cosmology of the axion become highly non-trivial. We discuss various cosmological constraints as well as astrophysical ones and show that the model is viable for certain choices of the dimensionality of the axion brane. The structure of the model proposed here provides a viable realization of the fat brane idea to relax otherwise very severe cosmological constraints.
We discuss issues in an attempt to put the Standard Model (SM) in five-dimensional anti-de Sitter spacetime compactified on $S^1/Z_2$. The recently-proposed approach to the gauge hierarchy problem by using this background geometry, with the SM confined on a boundary, is extended to a situation where (some of) the SM particles reside in the five dimensional bulk. In particular, we find a localization of zero modes of bulk fermions near the boundary with a negative tension. Unlike the compactification with the flat metric, these fermion zero modes couple to Kaluza-Klein (KK) excitations of the SM gauge bosons. Interestingly, only low-lying modes of such KK gauge bosons have non-negligible couplings. Current electroweak precision data give a constraint that the first KK mode be heavier than 9 TeV. We also argue that at least the Higgs field should be confined on the brane to utilize the Randall-Sundrum background as a solution to the gauge hierarchy.
Interaction of bulk gravitons to Standard Model particles is examined in the scenario of warped extra dimension. It is found that the length of the extra dimension is of physical importance as it determines the decay widths of graviton Kaluza-Klein modes. We show that cosmology as well as collider signals of the Kaluza-Klein modes are very different for different choices of the length of the fifth dimension.
We examine the axion model in the large extra dimensions scenario with TeV scale gravity. To obtain an intermediate-scale decay constant of the axion, the axion is assumed to live in a sub-spacetime (brane) of the whole bulk. In this model there appear Kaluza-Klein modes of the axion which have stronger interaction than those of the graviton. The axion brane plays a role of absorber of the graviton Kaluza-Klein modes. We discuss various cosmological constraints as well as astrophysical ones and show that the model is viable for certain choices of the dimensionality of the axion brane. The structure of the model proposed here provides a viable realization of the fat brane idea to relax otherwise very severe cosmological constraints.
We present predictions for the non-singlet Drell-Yan longitudinal spin cross sections and spin asymmetry, $A_{LL}$, in proton-proton collisions at large $p_T$ at the RHIC energy of $200\gev$ at next-to-leading order QCD. The higher order corrections to the non-singlet polarized cross section, $σ_{NS}^{LL}$, are sizeable and similar to those found for the unpolarized cross section. The non-singlet asymmetry parameter, $A^{NS}_{LL}$, is very stable against higher order corrections and is a direct measurement of the non-singlet (i.e. valence) polarized quark distributions within the proton.
A study of the order $α_s^2$ corrections to the Drell Yan (non-singlet) cross section for incoming states of arbitrary longitudinal helicities is presented. The study is performed in the case of the transverse momentum distributions, $(q_T)$, of the lepton pair and extends the calculation of Ellis Martinelli and Petronzio (EMP) to the polarized case. We use the $\bar{MS}$ scheme and the t'Hooft-Veltman regularization for the helicity projectors. From our results one can obtain the bulk of the totally inclusive NNLO cross section for the production of a Drell Yan pair in the non-singlet sector by a simple integration over the heavy photon momentum. We show that in the $\bar{MS}$ helicity is not conserved, unless a finite renormalization is done to reenter into a physical scheme ($\bar{MS}_p$). This aspect of the calculation follows the same trend as in polarized production of single and double photon to $O(α_s^2)$. In the unpolarized limit we reproduce all the results of EMP.
A study of prompt photon plus associated jet production is performed at next-to-leading order (O($αα_s^2$)) in QCD at $\sqrt{S}=200-500$ GeV, appropriate for the RHIC polarized $\vec{p}\vec{p}$ collider experiment. Momentum correlations between the jet and photon are examined and the utility of the process as a method for constraining the size and shape of the polarized gluon density of the proton $ΔG$ is examined.
We investigate the role of the transverse spin dependence in Drell Yan lepton pair production to NLO in QCD at parton level. In our analysis we deal with the large $p_T$ distributions. We give very compact expressions for the virtual $O(α_s^2)$ corrections to the cross section and show that the singularities factorize. The study is performed in the $\bar{MS}$ scheme in Dimensional Regularization, and with the t'Hooft-Veltman prescription for $γ_5$. A discussion of the structure of the real emissions is included, and detailed methods for the study of these contributions are formulated.
We present the complete $O(α_s^2)$ radiative corrections to the (non singlet) polarized Drell Yan cross section for the production of a lepton pair with a nonzero $q_T$. The helicity of the incoming states is arbitrary. In the case of double photon, results for the longitudinal asymmetries and on the $p_T$ behavior of the cross section are also given (to $O(α_{em}^2α_s )$) in the central rapidity region of the tagged photon.
We present parton-level analytical results for the next-to-leading order non-singlet virtual and real corrections to the Drell-Yan differential cross-section. The dependence of the differential cross section on the helicity of the initial state partons is shown explicitly (the spins of the final state partons are summed). The calculation is implemented in dimensional regularization within the $\bar{MS}$ scheme and with the t'Hooft Veltman prescriptions for the n-dimensional $γ_5$. Both the polarized initial state and the unpolarized cross sections can be obtained from our result. Our unpolarized cross section agrees with the previous result of Ellis, Martinelli and Petronzio in the non-singlet sector.
Perturbative gauge coupling unification in realistic superstring models suggests the existence of additional heavy down-type quarks, beyond the minimal supersymmetric standard model. The mass scale of the heavy down-type quarks is constrained by requiring agreement between the measured low energy gauge parameters and the string-scale gauge coupling unification. These additional quarks arise and may be stable due to the gauge symmetry breaking by ``Wilson lines'' in the superstring models. We argue that there is a window in the parameter space within which this down-type quark is a good candidate for the dark matter.
We investigate the cosmological constraints on exotic stable matter states which arise in realistic free fermionic superstring models. These states appear in the superstring models due to a ``Wilson--line'' breaking of the unifying non--Abelian gauge symmetry. In the models that we consider the unifying $SO(10)$ gauge symmetry is broken at the string level to $SO(6)\times SO(4)$, $SU(5)\times U(1)$ or $SU(3)\times SU(2)\times U(1)^2$. The exotic matter states are classified according to the patterns of the $SO(10)$ symmetry breaking. In $SO(6)\times SO(4)$ and $SU(5)\times U(1)$ type models one obtains fractionally charged states with $Q_{\rm e.m.}=\pm1/2$. In $SU(3)\times SU(2)\times U(1)^2$ type models one also obtains states with the regular charges under the Standard Model gauge group but with ``fractional'' charges under the $U(1)_{Z^\prime}$ symmetry. These states include down--like color triplets and electroweak doublets, as well as states which are Standard Model singlets. By analyzing the renormalizable and nonrenormalizable terms of the superpotential in a specific superstring model, we show that these exotic states can be stable. We investigate the cosmological constraints on the masses and relic density of the exotic states. We propose that, while the abundance and the masses of the fractionally charged states are highly constrained, the Standard Model -- like states, and in particular the Standard Model singlet, are good dark matter candidates.
We study the effect of the late decaying saxino (the scalar superpartner of the axion) and find out that there is a possible dark matter solution from a class of supersymmetric extensions of the invisible axion model. In this class of models, the saxino which decays into two axions acts as the late decaying particle which reconciles the cold dark matter model with high values of the Hubble constant. Recent observations of the Hubble constant are converging to $H_0=70\!-\!80\,{\rm km}\,{\rm sec}^{-1}\,{\rm Mpc}^{-1}$, which would be inconsistent with the standard mixed dark matter model. This class of models provides a plausible framework for the alternative cold dark matter plus late decaying particle model, with the interesting possibility that both cold dark matter and the extra radiation consist of axion.
We suggest the fermion doubling for all quarks and leptons. It is a generalization of the neutrino doubling of the seesaw mechanism. The new quarks and leptons are $SU(2)$ singlets and carry the electromagnetic charges of their lighter counterparts. An $SU(3)$ {\it anomaly free global symmetry} or a discrete symmetry can be introduced to restrict the Yukawa couplings. The form of mass matrix is belonging to that of Nelson and Barr even though our model does not belong to Barr's criterion. The weak CP violation of the Kobayashi-Maskawa form is obtained through the spontaneous breaking of CP symmetry at high energy scale. The strong CP solution is through a specific form of the mass matrix. At low energy, the particle content is the same as in the standard model. For a model with a global symmetry, in addition there exists a massless majoron.
We show that heavy quark axion is not necessarily a hadronic axion, which manifests in the quark and lepton seesaw mechanism. We introduce a heavy $SU(2)$ singlet fermion for each known fermion in order to unify the axion scale and the seesaw scale. The light quarks and leptons gain their masses by the seesaw mechanism. Even though our axion model gives a kind of heavy quark axion, the axion has tree level lepton--axion coupling suppressed by $F_a$, contrary to a widely known belief that heavy quark axions are hadronic.
Hadronic axions with the decay constant $f_a\simeq 10^{6}$ GeV may fulfill all astrophysical and laboratory constraints discussed so far. In this paper, we reexamine the possibility of the hadronic axion window while taking into account the uncertainties of some parameters describing low energy axion dynamics. It is found that $f_a$ in the range from $3\times 10^5$ GeV to $3\times 10^6$ can not be excluded by existing arguments. We then examine the implication of this hadronic axion window for the big-bang nucleosynthesis (NS) by evaluating the energy density of thermal axions at the nucleosynthesis epoch. Our analysis yields $(ρ_a/ρ_ν)_{NS}=0.4\sim 0.5$ which exceeds slightly the current best bound $(ρ_a/ρ_ν)_{NS}\leq 0.3$.
We examine whether observable majoron emission in double beta decay can be compatible with the big-bang nucleosynthesis (NS) and the observed neutrino flux from SN1987A. It is found that the NS upper bound on $^4$He abundance implies that the majoron-neutrino Yukawa coupling constant $g\leq 9\times 10^{-6}$ and its maximal value is allowed only when the scalar quartic coupling constant $λ$ is extremely small, $λ\leq 100g^2$. It is also observed that, although quite less restrictive, SN1987A also provides independent constraints on coupling constants.