Naturalness,supersymmetry and implications for LHC and dark matter

It is shown that the Hyperbolic Branch of the radiative electroweak symmetry breaking contains in it three regions: the Focal Point, Focal Curves, and Focal Surfaces. Further, the Focal Point is shown to lie on the boundary of a Focal Curve. These focal regions allow for a small μ while scalar masses can become large and may lie in the several TeV region. It is shown that for the mSUGRA model the current LHC-7 constraint depletes the Focal Point region while regions on Focal Curves and Focal Surfaces remain largely intact. The LHC implications for models which lie on Focal Curves are briefly discussed as well as the implications of dark matter constraints for the Focal Point, Focal Curves and Focal Surfaces are discussed.     

Exclusion of black hole disaster scenarios at the LHC

The upcoming high energy experiments at the LHC are one of the most outstanding efforts for a better understanding of nature. It is associated with great hopes in the physics community. But there is also some fear in the public, that the conjectured production of mini black holes might lead to a dangerous chain reaction. In this Letter we summarize the most straightforward arguments that are necessary to rule out such doomsday scenarios.     

Chargino-Pair Productions via Drell-Yan Process at the LHC

We present the analytical and numerical investigations of chargino-pair productions via Drell-Yan process a t proton-proton collider in the frame of minimal supersymmetric extension at tree level. For the χ_i⁺χ_j^-(i,j= 1,2) pair productions at the LHC, the cross section is in the order of 3～2.5 ×10^-3 pb. We find that the cross sections of all kinds of chargino-pair production processes are strongly related to the SUSY Lagrangian parameters M₂ and μ.     

Further investigation of the model-independent probe of heavy neutral Higgs bosons at LHC Run 2

In one of our previous papers,we provided general,effective Higgs interactions for the lightest Higgs boson h(SM-like) and a heavier neutral Higgs boson H based on the effective Lagrangian formulation up to the dim-6 interactions,and then proposed two sensitive processes for probing H.We showed in several examples that the resonance peak of H and its dim-6 effective coupling constants(ECC) can be detected at LHC Run 2 with reasonable integrated luminosity.In this paper,we further perform a more thorough study of the most sensitive process,pp →VH*→VVV,providing information about the relations between the 1σ,3σ,5σ statistical significance and the corresponding ranges of the Higgs ECC for an integrated luminosity of 100 fb~(-1).These results have two useful applications in LHC Run 2:(A) realizing the experimental determination of the ECC in the dim-6 interactions if H is found and,(B) obtaining the theoretical exclusion bounds if H is not found.Some alternative processes sensitive for certain ranges of the ECC are also analyzed.     

Heavy Higgs bosons at the LHC upgrade

We evaluate the discovery potential for the heavy Higgs bosons at the LHC energy upgrade with \begin{document}$\sqrt{s}=27$\end{document} TeV. We assume the degenerate mass spectrum and an approximate alignment limit in the Type-II Two Higgs Doublet Model for illustration. We explore the observability of the heavy neutral Higgs bosons by examining the clean signals from \begin{document}$H^0\to W^+W^-, ZZ$\end{document} via gluon-gluon fusion production. The associated production of a top quark and a charged Higgs boson via \begin{document}$gb\to t H^\pm$\end{document} is adopted to predict the discovery potential of heavy charged Higgses. We also emphasize the potential importance of the electroweak production of Higgs boson pairs, i.e. \begin{document}$pp\to W^\ast \to H^\pm A^0$\end{document} and \begin{document}$pp\to Z^\ast/\gamma^\ast \to H^+ H^-$\end{document}. These are only governed by pure electroweak gauge couplings and can provide complementary information to the conventional signals in the determination of the nature of the Higgs sector.     

LHC/ILC/cosmology interplay

There is a strong and growing interplay between particle physics and cosmology. In this talk, I discuss some aspects of this interplay concerning dark matter candidates put forth by theories beyond the standard model. In explaining the requirements for collider tests of such dark matter candidates, I focus in particular on the case of the lightest neutralino in the MSSM.     

On the nuclear modification factor at RHIC and LHC

We show that pQCD factorization incorporated with pre-hadronization energy-loss effect naturally leads to flatness of the nuclear modification factor RAA$R_{\mathit{AA}}$ for produced hadrons at high transverse momentum pT$p_T$. We consider two possible scenarios for the pre-hadronization: In scenario 1, the produced gluon propagates through dense QCD medium and loses energy. In scenario 2, all gluons first decay to quark–antiquark pairs and then each pair loses energy as propagating through the medium. We show that the estimates of the energy-loss in these two different models lead to very close values and is able to explain the suppression of high-pT$p_T$ hadrons in nucleus–nucleus collisions at RHIC. We show that the onset of the flatness of RAA$R_{\mathit{AA}}$ for the produced hadron in central collisions at midrapidity is about pT≈15$p_T\approx 15$ and 25 GeV at RHIC and the LHC energies, respectively. We show that the smallness (RAA<0.5$R_{\mathit{AA}}<0.5$ ) and the high-pT$p_T$ flatness of RAA$R_{\mathit{AA}}$ obtained from the kT$k_T$ factorization supplemented with the Balitsky–Kovchegov (BK) equation is rather generic and it does not strongly depend on the details of the BK solutions. We show that energy-loss effect reduces the nuclear modification factor obtained from the kT$k_T$ factorization about 30–50% at moderate pT$p_T$.     

The Sommerfeld enhancement for scalar particles and application to sfermion co-annihilation regions

We study the impact of the Sommerfeld enhancement on the thermal relic density of the lightest neutralino in the case of large co-annihilation effects with a scalar particle. The proper way of including the Sommerfeld effect in this case is discussed, and the appropriate formulas for a general scenario with a set of particles with arbitrary masses and (off-)diagonal interactions are provided. We implement these results to compute the relic density in the neutralino sfermion co-annihilation regions in the mSUGRA framework. We find non-negligible effects in whole sfermion co-annihilation regimes. For stau co-annihilations the correction to the relic density is of the order of several per cent, while for stop co-annihilations is much larger, reaching a factor of 5 in some regions of the parameter space. A numerical package for computing the neutralino relic density including the Sommerfeld effect in a general MSSM setup is made public available.     

Search for heavy sterile neutrinos in trileptons at the LHC

We present a search strategy for both Dirac and Majorana sterile neutrinos from the purely leptonic decays of W~±→e~±e~±μ~?ν and μ~±μ~± e~?ν at the 14 TeV LHC. The discovery and exclusion limits for sterile neutrinos are shown using both the Cut-and-Count(CC) and Multi-Variate Analysis(MVA) methods. We also discriminate between Dirac and Majorana sterile neutrinos by exploiting a set of kinematic observables which differ between the Dirac and Majorana cases. We find that the MVA method, compared to the more common CC method, can greatly enhance the discovery and discrimination limits. Two benchmark points with sterile neutrino mass m N =20 GeV and 50 GeV are tested. For an integrated luminosity of 3000 fb~(-1), sterile neutrinos can be found with 5σ significance if heavy-to-light neutrino mixings |U_(Ne)|~2~|U_(Nμ)|~2~10~(-6), while Majorana vs. Dirac discrimination can be reached if at least one of the mixings is of order 10~(-5).     

Pair production of the Elementary Goldstone Higgs boson at the LHC

The Elementary Goldstone Higgs(EGH) model is a perturbative extension of the standard model(SM),which identifies the EGH boson as the observed Higgs boson. In this paper, we study pair production of the EGH boson via gluon fusion at the LHC and find that the resonant contribution of the heavy scalar is very small and the SM-like triangle diagram contribution is strongly suppressed. The total production cross section mainly comes from the box diagram contribution and its value can be significantly enhanced with respect to the SM prediction.     

Search for W' signal in single top quark production at the LHC

Heavy charged gauge bosons are proposed in some theories beyond the standard model. We explore the discovery potential for W →tb with top quark semi-leptonic decay at the LHC. We concentrate on the new physics signal search with the deviation from the standard model prediction if the resonance peak of W cannot be observed directly. Signal events with two jets plus one charged lepton and missing energy are simulated, together with the dominant standard model backgrounds. In this paper, it is found that suitable cuts on the kinematic observables can effectively suppress the standard model backgrounds, so that it is possible to search for a W signal at the LHC if its mass is less than 6.6 Te V.     

Higgs production cross-section in a Standard Model with four generations at the LHC

We present theoretical predictions for the Higgs boson production cross-section via gluon fusion at the LHC in a Standard Model with four generations. We include QCD corrections through NLO retaining the full dependence on the quark masses, and the NNLO corrections in the heavy quark effective theory approximation. We also include electroweak corrections through three loops. Electroweak and bottom-quark contributions are suppressed in comparison to the Standard Model with three generations.     

Associated production of Higgs boson and tt at LHC

One of the future goals of the LHC is to precisely measure the properties of the Higgs boson. The associated production of a Higgs boson and top quark pair is a promising process to investigate the related Yukawa interaction and the properties of the Higgs. Compared with the pure scalar sector in the Standard Model, the Higgs sector contains both scalars and pseudoscalars in many new physics models, which makes the ttH interaction more complex and provides a variety of phenomena. To investigate the ttH interaction and the properties of the Higgs, we study the top quark spin correlation observables at the LHC.     

Critical comment on the recent microscopic black hole search at the LHC

Recently the CMS Collaboration at the LHC reported “the first direct limit on black hole production at a particle accelerator” using a data sample corresponding to an integrated luminosity of 35 pb−1 of pp collision at a center of mass energy of 7 TeV (Khachatryan et al., 2011) [1]. Even though the result has a strong impact on future searches, the interpretation lacks enough theoretical support. In this Letter, we show that the parameter range which was considered by the CMS Collaboration is actually out of the validity range of semi-classical black hole picture so that the Monte Carlo simulation result which was crucially used in the analysis still needs further solid scientific basis.