arXiv subjects

Chuan-Ren Chen

Publications and source records attributed to Chuan-Ren Chen.

52 records · Page 3Linked to original sources

arXiv2010

Probing Variant Axion Models at LHC

We study collider implications of variant axion models which naturally avoid the cosmological domain wall problem. We find that in such models the branching ratio of $h \to γγ$ can be enhanced by a factor of 5 up to 30 as compared with the standard model prediction. The $h \to γγ$ process is therefore a promising channel to discover a light Higgs boson at the LHC and to probe the Peccei-Quinn charge assignment of the standard model fields from Yukawa interactions.

hep-ph↗

arXiv2009

Gamma-ray Constraints on Hadronic and Leptonic Activities of Decaying Dark Matter

While the excess in cosmic-ray electrons and positrons reported by PAMELA and Fermi may be explained by dark matter decaying primarily into charged leptons, this does not necessarily mean that dark matter should not have any hadronic decay modes. In order to quantify the allowed hadronic activities, we derive constraints on the decay rates of dark matter into WW, ZZ, hh, qqbar and gg using the Fermi and HESS gamma-ray data. We also derive gamma-ray constraints on the leptonic e+e-, mu+mu- and tau+tau- final states. We find that dark matter must decay primarily into mu+mu- or tau+tau- in order to simultaneously explain the reported excess and meet all gamma-ray constraints.

hep-ph↗

arXiv2009

Kaluza-Klein Dark Matter After Fermi

Kaluza-Klein photon in universal extra dimension models is one of the most attractive dark matter candidates as a weakly interacting massive particle. Having a characteristic split spectrum in split universal extra dimension the relic density of Kaluza-Klein photon with 900 GeV mass is in good agreement with the observed dark matter amount in our Universe. Interestingly Kaluza-Klein photon in the same mass range also provides excellent fits to the recently observed excesses in cosmic electron and positron fluxes. The amount of gamma-ray contributions, mostly from tau decays, can be significant around 300 GeV, thus can be observed or constrained by the forthcoming Fermi-LAT diffuse gamma-ray data.

hep-ph↗

arXiv2009

High-energy Cosmic-Ray Positrons from Hidden-Gauge-Boson Dark Matter

We provide a scenario in which a hidden U(1) gauge boson constitutes dark matter of the Universe and decays into the standard-model particles through a kinetic mixing with an $U(1)_{B-L}$ gauge boson. Interestingly, our model can naturally account for the steep rise in the positron fraction recently reported by PAMELA. Moreover, we find that due to the charge assignment of $U(1)_{B-L}$, only a small amount of antiprotons are produced in the decay, which is also consistent with the PAMELA and other observational data.

hep-ph↗

arXiv2009

Anomalous gtt couplings in the Littlest Higgs Model with T-parity

In this work we calculate the leading electroweak (EW) corrections to the anomalous $gt\bar{t}$ coupling in the Littlest Higgs model with T-parity (LHT), by applying the Goldstone boson equivalence theorem. In the LHT model, such electroweak corrections arise from the loop diagrams of heavy fermions and the ``would-be'' Goldstone bosons. We further examine the EW corrections in the top quark pair production via the quark annihilation process at the LHC. The negative EW corrections in the Standard Model are partially canceled by the positive EW corrections from the loops of the new heavy particles, and the latter dominates in the large invariant mass of the top quark pair.

hep-ph↗

arXiv2009

Decaying Hidden Gauge Boson and the PAMELA and ATIC/PPB-BETS Anomalies

We show that the PAMELA anomaly in the positron fraction as well as the ATIC/PPB-BETS excesses in the e^- + e^+ flux are simultaneously explained in our scenario that a hidden U(1)H gauge boson constitutes dark matter of the Universe and decays into the standard-model particles through a kinetic mixing with an U(1)B-L gauge boson. Interestingly, the B-L charge assignment suppresses an antiproton flux in consistent with the PAMELA and BESS experiments, while the hierarchy between the B-L symmetry breaking scale and the weak scale naturally leads to the right lifetime of O(10^26) seconds.

astro-ph↗

arXiv2009

Dark matter and collider phenomenology of split-UED

We explicitly show that split-universal extra dimension (split-UED), a recently suggested extension of universal extra dimension (UED) model, can nicely explain recent anomalies in cosmic-ray positrons and electrons observed by PAMELA and ATIC/PPB-BETS. Kaluza-Klein (KK) dark matters mainly annihilate into leptons because the hadronic branching fraction is highly suppressed by large KK quark masses and the antiproton flux agrees very well with the observation where no excess is found . The flux of cosmic gamma-rays from pion decay is also highly suppressed and hardly detected in low energy region (E<20 GeV). Collider signatures of colored KK particles at the LHC, especially q_1 q_1 production, are studied in detail. Due to the large split in masses of KK quarks and other particles, hard p_T jets and missing E_T are generated, which make it possible to suppress the standard model background and discover the signals.

hep-ph↗

arXiv2009

Color Sextet Scalars at the CERN Large Hadron Collider

Taking a phenomenological approach, we study a color sextet scalar at the LHC. We focus on the QCD production of a color sextet pair Φ_6\barΦ_{6} through gg fusion and q\bar{q} annihilation. Its unique coupling to \bar{ψ^c}ψallows the color sextet scalar to decay into same-sign diquark states, such as Φ_6\to tt/tt^*. We propose a new reconstruction in the multijet plus same sign dilepton with missing transverse energy samples to search for on-shell tt\bar{t}\bar{t} final states from sextet scalar pair production. Thanks to the large QCD production, the search covers the sextet mass range up to 1 TeV for 100 fb^-1 integrated luminosity.

hep-ph↗

arXiv2008

Dark Matter Model Selection and the ATIC/PPB-BETS anomaly

We argue that we may be able to sort out dark matter models in which electrons are generated through the annihilation and/or decay of dark matter, by using a fact that the initial energy spectrum is reflected in the cosmic-ray electron flux observed at the Earth even after propagation through the galactic magnetic field. To illustrate our idea we focus on three representative initial spectra: (i)monochromatic (ii)flat and (iii)double-peak ones. We find that those three cases result in significantly different energy spectra, which may be probed by the Fermi satellite in operation or an up-coming cosmic-ray detector such as CALET.

astro-ph↗

arXiv2008

Gamma rays and positrons from a decaying hidden gauge boson

We study a scenario that a hidden gauge boson constitutes the dominant component of dark matter and decays into the standard model particles through a gauge kinetic mixing. Interestingly, gamma rays and positrons produced from the decay of hidden gauge boson can explain both the EGRET excess of diffuse gamma rays and the HEAT anomaly in the positron fraction. The spectra of the gamma rays and the positrons have distinctive features; the absence of line emission of the gamma ray and a sharp peak in the positron fraction. Such features may be observed by the GLAST and PAMELA satellites.

hep-ph↗

arXiv2008

Cosmic rays from Leptonic Dark Matter

If dark matter possesses a lepton number, it is natural to expect the dark-matter annihilation and/or decay mainly produces the standard model leptons, while negligible amount of the antiproton is produced. To illustrate such a simple idea, we consider a scenario that a right-handed sneutrino dark matter decays into the standard model particles through tiny R-parity violating interactions. Interestingly enough, charged leptons as well as neutrinos are directly produced, and they can lead to a sharp peak in the predicted positron fraction. Moreover, the decay of the right-handed sneutrino also generates diffuse continuum gamma rays which may account for the excess observed by EGRET, while the primary antiproton flux can be suppressed. Those predictions on the cosmic-ray fluxes of the positrons, gamma rays and antiprotons will be tested by the PAMELA and FGST observatories.

hep-ph↗

arXiv2008

General Analysis of Single Top Production and W Helicity in Top Decay

We provide a framework for the analysis of the W boson helicity in the decay of the top quark that is based on a general effective tbW coupling. Four independent coupling coefficients can be uniquely determined by the fractions of longitudinal and transverse W boson polarizations as well as the single top production rates for the t-channel and the s-channel processes. The knowledge of these coefficients can be used to discriminate models of electroweak symmetry breaking.

hep-ph↗

arXiv2007

Resummation Effects in the Search of SM Higgs Boson at Hadron Colliders

We examine the soft-gluon resummation effects, including the exact spin correlations among the final state particles, in the search of the Standard Model Higgs boson, via the process $gg\to H\to WW/ZZ \to 4 leptons, at the Tevatron and the LHC. A comparison between the resummation and the Next-to-Leading order (NLO) calculation is performed after imposing various kinematics cuts suggested in the literature for the Higgs boson search. For the $H\to ZZ$ mode, the resummation effects increase the acceptance of the signal events by about 25%, as compared to the NLO prediction, and dramatically alter various kinematics distributions of the final state leptons. For the $H\to WW$ mode, the acceptance rates of the signal events predicted by the resummation and NLO calculations are almost the same, but some of the predicted kinematical distributions are quite different. Thus, to precisely determine the properties of the Higgs boson at hadron colliders, the soft-gluon resummation effects have to be taken into account.

hep-ph↗

arXiv2007

Signatures of Extra Gauge Bosons in the Littlest Higgs Model with T-parity at Future Colliders

We study the collider signatures of a T-odd gauge boson $W_{H}$ pair production in the Littlest Higgs Model with T-parity (LHT) at Large Hadron Collider (LHC) and Linear Collider (LC). At the LHC, we search for the $W_{H}$ boson using its leptonic decay, i.e. $pp\to W_{H}^{+}W_{H}^{-}\to A_{H}A_{H}\ell^{+}ν_{\ell}\ell^{\prime-}\barν_{\ell^{\prime}}$, which gives rise to a collider signature of $\ell^{+}\ell^{\prime-}+\met$. We demonstrate that the LHC not only has a great potential of discovering the $W_{H}$ boson in this channel, but also can probe enormous parameter space of the LHT. Due to four missing particles in the final state, one cannot reconstruct the mass of $W_{H}$ at the LHC. But such a mass measurement can be easily achieved at the LC in the process of $e^{+}e^{-}\to W_{H}^{+}W_{H}^{-}\to A_{H}A_{H}W^{+}W^{-}\to A_{H}A_{H}jjjj$. We present an algorithm of measuring the mass and spin of the $W_{H}$ boson at the LC. Furthermore, we illustrate that the spin correlation between the $W$ boson and its mother particle ($W_{H}$) can be used to distinguish the LHT from other new physics models.

hep-ph↗

arXiv2007

Phenomenology of Littlest Higgs Model with T-parity: including effects of T-odd fermions

We study the collider phenomenology of a Littlest Higgs model with T-parity. We first stress the important role of the T-odd SU(2) doublet fermions (introduced to make the model T-parity invariant) in high energy scattering processes, such as $q\bar{q}\to W_H^+ W_H^-$ where $W_H^\pm$ are the T-odd partners of $W$-bosons. Because the mass of the T-odd SU(2) doublet fermions cannot be too heavy to be consistent with low energy data, they can be copiously produced at the CERN Large Hadron Collider (LHC). Therefore, we study the collider phenomenology of the model with emphasis on the contributions of the T-odd fermion to the production of the heavy T-parity partners (either bosons or fermions) of the usual particles at the LHC. The production cross sections and the decay branching ratios of the new heavy particles are classified and various experimental signatures are discussed.

hep-ph↗

arXiv2006

Higgs Boson Production and Decay in Little Higgs Models with T-parity

We study Higgs boson production and decay in a certain class of Little Higgs models with T-parity in which some T-parity partners of the Standard Model (SM) fermions gain their masses through Yukawa-type couplings. We find that the Higgs boson production cross section of a 120 GeV Higgs boson at the CERN LHC via gg fusion process at one-loop level could be reduced by about 45%, 35% and 20%, as compared to its SM prediction, for a relatively low new particle mass scale f = 600, 700 and 1000 GeV, respectively. On the other hand, the weak boson fusion cross section is close to the SM value. Furthermore, the Higgs boson decay branching ratio into di-photon mode can be enhanced by about 35% in small Higgs mass region in certain case, for the total decay width of Higgs boson in the Little Higgs model is always smaller than that in the SM.

hep-ph↗