arXiv subjects

Wan-Zhe Feng

Publications and source records attributed to Wan-Zhe Feng.

33 records · Page 2Linked to original sources

arXiv2017

Dirac dark matter and $b \to s \ell^+ \ell^-$ with $\mathrm{U(1)}$ gauge symmetry

We revisit the possibility of a Dirac fermion dark matter candidate in the light of current $b \to s \ell^+ \ell^-$ anomalies by investigating a minimal extension of the Standard Model with a horizontal $\mathrm{U(1)}^{\prime}$ local symmetry. Dark matter stability is protected by a remnant $\mathcal{Z}_2$ symmetry arising after spontaneous symmetry breaking of $\mathrm{U(1)}^{\prime}$. The associated $Z^{\prime}$ gauge boson can accommodate current hints of new physics in $b \to s \ell^+ \ell^-$ decays, and acts as a vector portal between dark matter and the visible sector. We find that the model is severely constrained by a combination of precision measurements at flavour factories, LHC searches for dilepton resonances, as well as direct and indirect dark matter searches. Despite this, viable regions of the parameter space accommodating the observed dark matter relic abundance and the $b \to s \ell^+ \ell^-$ anomalies still persist for dark matter and $Z^{\prime}$ masses in the TeV range.

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arXiv2016

Baryogenesis and Dark Matter in $U(1)$ Extensions

A brief review is given of some recent works where baryogenesis and dark matter have a common origin within the $U(1)$ extensions of the standard model and of the minimal supersymmetric standard model. The models considered generate the desired baryon asymmetry and the dark matter to baryon ratio. In one model all of the fundamental interactions do not violate lepton number, and the total $B-L$ in the Universe vanishes. In addition, one may also generate a normal hierarchy of neutrino masses and mixings in conformity with the current data. Specifically one can accommodate $θ_{13}\sim 9^{\circ}$ consistent with the data from Daya Bay reactor neutrino experiment.

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arXiv2016

ATLAS Diboson Excess from Stueckelberg Mechanism

We discuss the diboson excess seen by the ATLAS detector around 2 TeV in the LHC run I at $\sqrt{s}=8$ TeV. We explore the possibility that such an excess can arise from a $Z'$ boson which acquires mass through a $U(1)_X$ Stueckelberg extension. The corresponding $Z'$ gauge boson is leptophobic with a mass of around 2 TeV and has interactions with $SU(2)_L$ Yang-Mills fields and gauge fields of the hypercharge. The analysis predicts $Z'$ decays into $WW$ and $ZZ$ as well as into $Zγ$. Further three-body as well as four-body decays of the $Z'$ such as $WWZ, WWγ, WWZZ$ etc are predicted. In the analysis we use the helicity formalism which allows us to exhibit the helicity structure of the $Z'$ decay processes in an transparent manner. In particular, we are able to show the set of vanishing helicity amplitudes in the decay of the massive $Z'$ into two vector bosons due to angular momentum conservation with a special choice of the reference momenta. The residual set of non-vanishing helicity amplitudes are identified. The parameter space of the model compatible with the diboson excess seen by the ATLAS experiment at $\sqrt s=8$ TeV is exhibited. Estimate of the diboson excess expected at $\sqrt s= 13$ TeV with 20 fb$^{-1}$ of integrated luminosity at LHC run II is also given. It is shown that the $WW$, $ZZ$ and $Zγ$ modes are predicted to be in the approximate ratio $1:\cos^2θ_W (1+ α\tan^2θ_W)^2/2: (1-α)^2\sin^2θ_W/2$ where $α$ is the relative strength of the couplings of hypercharge gauge fields to the couplings of the Yang-Mills gauge fields. Thus observation of the $Zγ$ mode as well as three-body and four-body decay modes of the $Z'$ will provide a definite test of the model and of a possible new source of interaction beyond the standard model.

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arXiv2016

Stringy explanation of $b \to s \ell^+ \ell^-$ anomalies

We show that the recent anomalies in $b \to s \ell^+ \ell^-$ transitions observed by the LHCb collaboration can be accommodated within string motivated models with a low mass $Z^{\prime}$ gauge boson. Such $Z^{\prime}$ gauge boson can be obtained in compactifications with a low string scale. We consider a class of intersecting D-brane models, in which different families of quarks and leptons are simultaneously realized at different D-brane intersections. The explanation of $b \to s \ell^+ \ell^-$ anomalies via a stringy $Z^{\prime}$ sets important restrictions on these viable D-brane constructions.

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arXiv2014

String Resonances at Hadron Colliders

[Abridged] We consider extensions of the standard model based on open strings ending on D-branes. Assuming that the fundamental string mass scale M_s is in the TeV range and that the theory is weakly coupled, we discuss possible signals of string physics at the upcoming HL-LHC run (3000 fb^{-1}) with \sqrt{s} = 14 TeV, and at potential future pp colliders, HE-LHC and VLHC, operating at \sqrt{s} = 33 and 100 TeV, respectively. In such D-brane constructions, the dominant contributions to full-fledged string amplitudes for all the common QCD parton subprocesses leading to dijets and γ+ jet are completely independent of the details of compactification, and can be evaluated in a parameter-free manner. We make use of these amplitudes evaluated near the first (n=1) and second (n=2) resonant poles to determine the discovery potential for Regge excitations of the quark, the gluon, and the color singlet living on the QCD stack. We show that for string scales as large as 7.1 TeV (6.1 TeV), lowest massive Regge excitations are open to discovery at 5σin dijet (γ+ jet) HL-LHC data. We also show that for n=1, the dijet discovery potential at HE-LHC and VLHC exceedingly improves: up to 15 TeV and 41 TeV, respectively. To compute the signal-to-noise ratio for n=2 resonances, we first carry out a complete calculation of all relevant decay widths of the second massive level string states. We demonstrate that for string scales M_s <~ 10.5 TeV (M_s <~ 28 TeV), detection of n=2 Regge recurrences at HE-LHC (VLHC) would become the smoking gun for D-brane string compactifications. Our calculations have been performed using a semi-analytic parton model approach which is cross checked against an original software package. The string event generator interfaces with HERWIG and Pythia through BlackMax. The source code is publically available in the hepforge repository.

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arXiv2014

Probing Hidden Sectors with Stückelberg U(1) Gauge Fields

We propose a framework in which visible matter interacts with matter from a hidden sector through mass mixings of Stückelberg U(1) gauge fields. In contrast to other Z' mediation scenarios, our setup has the added appealing features that (i) the choice of Z's can be significantly broadened without necessarily introducing unwanted exotic matter and (ii) there can be sizable tree-level interactions between the visible and hidden sectors. String theory embeddings of this scenario and their phenomenological features are briefly discussed.

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arXiv2014

Cogenesis in a universe with vanishing $B-L$ within a gauged $U(1)_x$ extension

We consider a gauged $U(1)_x$ extension of the standard model and of the minimal supersymmetric standard model where the dark matter fields are charged under $U(1)_x$ and carry lepton number while the standard model fields and fields of the minimal supersymmetric standard model are neutral under $U(1)_x$. We consider leptogenesis in this class of models with all fundamental interactions having no violation of lepton number, and the total $B-L$ in the universe vanishes. Such leptogenesis leads to equal and opposite lepton numbers in the visible sector and in the dark sector, and thus also produces asymmetric dark matter. Part of the lepton numbers generated in the leptonic sector subsequently transfer to the baryonic sector via sphaleron interactions. The stability of the dark particles is protected by the $U(1)_x$ gauge symmetry. A kinetic mixing between the $U(1)_x$ and the $U(1)_Y$ gauge bosons allows for dissipation of the symmetric component of dark matter. The case when $U(1)_x$ is $U(1)_{B-L}$ is also discussed for the supersymmetric case. This case is particularly interesting in that we have a gauged $U(1)_{B-L}$ which ensures the conservation of $B-L$ with an initial condition of a vanishing $B-L$ in the universe. Phenomenological implications of the proposed extensions are discussed, which include implications for electroweak physics, neutrino masses and mixings, and lepton flavor changing processes such as $\ell_i \to \ell_j γ$. We also briefly discuss the direct detection of the dark matter in the model.

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arXiv2014

Building a Stückelberg Portal

We construct explicit string theory models realizing the recently proposed "Stückelberg Portal" scenario, a framework for building Z' mediation models without the need to introduce unwanted exotic matter charged under the Standard Model. This scenario can be viewed purely field-theoretically, although it is particularly well motivated from string theory. By analyzing carefully the Stückelberg couplings between the Abelian gauge bosons and the RR axions, we construct the first global intersecting brane models which extend the Standard Model with a genuine hidden sector, to which it is nonetheless connected via U(1) mass mixings. Utilizing the explicit models we construct, we discuss some broad phenomenological properties and experimental implications of this scenario such as Z-Z' mixings, dark matter stability and relic density, and supersymmetry mediation. With an appropriate confining hidden sector, our setup also provides a minimal realization of the hidden valley scenario. We further explore the possibility of obtaining small Z' masses from a large ensemble of U(1) bosons. Related to the Stückelberg portal are two mechanisms that connect the visible and the hidden sectors, namely mediation by non-perturbative operators and the hidden photon scenario, on which we briefly comment.

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arXiv2013

Baryogenesis from dark matter

We consider the possibility that some primordial fields decay purely into the dark sector creating asymmetric dark matter. This asymmetry is subsequently transmuted into leptons and baryons. Within this paradigm we compute the amount of asymmetric dark matter created from the out of equilibrium decays of the primordial fields with CP violating Yukawa couplings. The dark matter asymmetry is then transferred to the visible sector by the asymmetry transfer equation and generates an excess of $B-L$. Baryogenesis occurs via sphaleron processes which conserve $B-L$ but violate $B+L$. A mechanism for the annihilation of the symmetric component of dark matter is also discussed. The model leads to multi-component dark matter consisting of both bosonic and fermionic components.

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arXiv2013

Higgs diphoton rate and mass enhancement with vector-like leptons and the scale of supersymmetry

Analysis of contributions from vector-like leptonic supermultiplets to the Higgs diphoton decay rate and to the Higgs boson mass is given. Corrections arising from the exchange of the new leptons and their super-partners, as well as their mirrors are computed analytically and numerically. We also study the correlation between the enhanced Higgs diphoton rate and the Higgs mass corrections. Specifically, we find two branches in the numerical analysis: on the lower branch the diphoton rate enhancement is flat while on the upper branch it has a strong correlation with the Higgs mass enhancement. It is seen that a factor of 1.4-1.8 enhancement of the Higgs diphoton rate on the upper branch can be achieved, and a 4-10 GeV positive correction to the Higgs mass can also be obtained simultaneously. The effect of this extra contribution to the Higgs mass is to release the constraint on weak scale supersymmetry, allowing its scale to be lower than in the theory without extra contributions. The vector-like supermultiplets also have collider implications which can be tested at the LHC and at the ILC.

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arXiv2012

Cosmic Coincidence and Asymmetric Dark Matter in a Stueckelberg Extension

We discuss the possibility of cogenesis generating the ratio of baryon asymmetry to dark matter in a Stueckelberg U(1) extension of the standard model and of the minimal supersymmetric standard model. For the U(1) we choose $L_μ-L_τ$ which is anomaly free and can be gauged. The dark matter candidate arising from this extension is a singlet of the standard model gauge group but is charged under $L_μ-L_τ$. Solutions to the Boltzmann equations for relics in the presence of asymmetric dark matter are discussed. It is shown that the ratio of the baryon asymmetry to dark matter consistent with the current WMAP data, i.e., the cosmic coincidence, can be successfully explained in this model with the depletion of the symmetric component of dark matter from resonant annihilation via the Stueckelberg gauge boson. For the extended MSSM model it is shown that one has a two component dark matter picture with asymmetric dark matter being the dominant component and the neutralino being the subdominant component (i.e., with relic density a small fraction of the WMAP cold dark matter value). Remarkably, the subdominant component can be detected in direct detection experiments such as SuperCDMS and XENON-100. Further, it is shown that the class of Stueckelberg models with a gauged $L_μ-L_τ$ will produce a dramatic signature at a muon collider with the $σ(μ^+μ^-\to μ^+μ^-,τ^+τ^-)$ showing a detectable $Z'$ resonance while $σ(μ^+μ^-\to e^+e^-)$ is devoid of this resonance. Asymmetric dark matter arising from a $U(1)_{B-L}$ Stueckelberg extension is also briefly discussed. Finally, in the models we propose the asymmetric dark matter does not oscillate and there is no danger of it being washed out from oscillations.

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arXiv2012

Massive Supermultiplets in Four-Dimensional Superstring Theory

We extend the discussion of arXiv:1007.5254 on massive Regge excitations on the first mass level of four-dimensional superstring theory. For the lightest massive modes of the open string sector, universal supermultiplets common to all four-dimensional compactifications with N = 1, 2 and N = 4 spacetime supersymmetry are constructed respectively -- both their vertex operators and their supersymmetry variations. Massive spinor helicity methods shed light on the interplay between individual polarization states.

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arXiv2011

Higher Level String Resonances in Four Dimensions

We study higher level Regge resonances of open superstrings, focusing on the universal part of the Neveu-Schwarz sector common to all D-brane realizations of the standard model. For Regge states with masses far above the fundamental string scale, we discuss the spin-dependence of their decay rates into massless gauge bosons. Extending our previous work on lowest level string excitations, we study the second mass level at which spins range from 0 to 3. We construct the respective vertex operators and compute the amplitudes involving one massive particle and two or three gauge bosons. To illustrate the use of BCFW recursion relations in superstring theory, we build the four gluon amplitude from on-shell amplitudes involving string resonances and gauge bosons.

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arXiv2011

Searching for string resonances in e^+e^- and γγcollisions

If the fundamental mass scale of superstring theory is as low as few TeVs, the massive modes of vibrating strings, Regge excitations, will be copiously produced at the Large Hadron Collider (LHC). We discuss the complementary signals of low mass superstrings at the proposed electron-positron facility (CLIC), in e^+e^- and γγcollisions. We examine all relevant four-particle amplitudes evaluated at the center of mass energies near the mass of lightest Regge excitations and extract the corresponding pole terms. The Regge poles of all four-point amplitudes, in particular the spin content of the resonances, are completely model independent, universal properties of the entire landscape of string compactifications. We show that γγ\to e^+ e^- scattering proceeds only through a spin-2 Regge state. We estimate that for this particular channel, string scales as high as 4 TeV can be discovered at the 11σlevel with the first fb^{-1} of data collected at a center-of-mass energy \approx 5 TeV. We also show that for e^+e^- annihilation into fermion-antifermion pairs, string theory predicts the precise value, equal 1/3, of the relative weight of spin 2 and spin 1 contributions. This yields a dimuon angular distribution with a pronounced forward-backward asymmetry, which will help distinguishing between low mass strings and other beyond the standard model scenarios.

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arXiv2010

Direct Production of Lightest Regge Resonances

We discuss direct production of Regge excitations in the collisions of massless four-dimensional superstring states, focusing on the first excited level of open strings ending on D-branes extending into higher dimensions. We construct covariant vertex operators and identify ``universal'' Regge states with the internal parts either trivial or determined by the world-sheet SCFT describing superstrings propagating on an arbitrary Calabi-Yau manifold. We evaluate the amplitudes involving one such massive state and up to three massless ones and express them in the helicity basis. The most important phenomenological applications of our results are in the context of low-mass string (and large extra dimensions) scenarios in which excited string states are expected to be produced at the LHC as soon as the string mass threshold is reached in the center-of-mass energies of the colliding partons. In order to facilitate the use of partonic cross sections, we evaluate them and tabulate for all production processes: gluon fusion, quark absorbing a gluon, quark-antiquark annihilation and quark-quark scattering.

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