We investigate here the particle acceleration and collisions with extremely large center-of-mass energies in a perfectly regular spacetime containing neither singularity nor an event horizon. The ultrahigh energy collisions of particles near the event horizon of extremal Kerr black hole, and also in many other examples of extremal black holes, have been investigated and reported recently. We studied an analogous particle acceleration process in the Kerr and Reissner-Nordstrom spacetimes without horizon, containing naked singularities. Further to this, we show here that the particle acceleration and collision process is in fact independent of black holes and naked singularities, and can happen in a fully regular spacetime containing neither of these. We derive the conditions on the general static spherically symmetric metric for such a phenomena to happen. We show that in order to have ultrahigh energy collisions it is necessary for the norm of the timelike Killing vector to admit a maximum with a vanishingly small but a negative value. This is also a condition implying the presence of a surface with an extremely large but nevertheless finite value of the redshift or blueshift. Conditions to have ultrahigh energy collisions and a regular center imply the violation of the strong energy condition near the center, while the weak energy condition is respected in the region around the center. Thus the central region is surrounded by a dark energy fluid. Both the energy conditions are respected at the location where the high-energy collisions take place. As a concrete example, we then investigate the acceleration process in the spacetime geometry derived by Bardeen which is sourced by a nonlinear self-gravitating magnetic monopole.

We present and analyze exact solutions of the Einstein-Maxwell equations in higher dimensions which form a large subclass of the Kundt family of spacetimes. We assume that the cosmological constant may be nonvanishing, and the matter consists of a background aligned electromagnetic field and an additional pure radiation (gyratonic) field with a spin. We show that the field equations reduce to a set of linear equations on the transverse space which can be solved exactly and expressed in terms of the Green functions. We thus find explicit exact gyratonic gravitational and electromagnetic fields created by a radiation beam of null matter with arbitrary profiles of energy density and angular momenta. In the absence of the gyratonic matter we obtain pure nonexpanding higher-dimensional gravitational waves. In particular, we investigate gyratons and waves propagating on backgrounds which are a direct product of two-spaces of constant curvature. Such type D or 0 background spacetimes generalize four-dimensional Nariai, anti-Nariai, and Plebański-Hacyan universes, and conformally flat Bertotti-Robinson and Minkowski spaces. These spacetimes belong to a wider class of spaces which admit the Kähler structure related to the background magnetic field. The obtained wave and gyraton solutions are also members of the recently discussed class of spacetimes with constant scalar invariants of the curvature tensor.

We investigate the effect of a variation of fundamental constants on primordial element production in big bang nucleosynthesis (BBN). We focus on the effect of a possible change in the nucleon-nucleon interaction on nuclear reaction rates involving the $A=5$ (${}^5\mathrm{Li}$ and ${}^5\mathrm{He}$) and $A=8$ (${}^8\mathrm{Be}$) unstable nuclei. The reaction rates for ${}^3\mathrm{He}(\mathrm{d},\mathrm{p}){}^4\mathrm{He}$ and ${}^3\mathrm{H}(\mathrm{d},\mathrm{n}){}^4\mathrm{He}$ are dominated by the properties of broad analog resonances in ${}^5\mathrm{He}$ and ${}^5\mathrm{Li}$ compound nuclei, respectively. While the triple alpha process ${}^4\mathrm{He}(\alpha \alpha ,\gamma ){}^{12}\mathrm{C}$ is normally not effective in BBN, its rate is very sensitive to the position of the “Hoyle state” and could in principle be drastically affected if ${}^8\mathrm{Be}$ were stable during BBN. The nuclear properties (resonance energies in ${}^5\mathrm{He}$ and ${}^5\mathrm{Li}$ nuclei, and the binding energies of ${}^8\mathrm{Be}$ and D) are all computed in a consistent way using a microscopic cluster model. The $\mathrm{n}(\mathrm{p},\gamma )\mathrm{d}$, ${}^3\mathrm{He}(\mathrm{d},\mathrm{p}){}^4\mathrm{He}$, ${}^3\mathrm{H}(\mathrm{d},\mathrm{n}){}^4\mathrm{He}$, and ${}^4\mathrm{He}(\alpha \alpha ,\gamma ){}^{12}\mathrm{C}$ reaction rates are subsequently calculated as a function of the nucleon-nucleon interaction that can be related to the fundamental constants. We found that the effect of the variation of constants on the ${}^3\mathrm{He}(\mathrm{d},\mathrm{p}){}^4\mathrm{He}$, ${}^3\mathrm{H}(\mathrm{d},\mathrm{n}){}^4\mathrm{He}$, and ${}^4\mathrm{He}(\alpha \alpha ,\gamma ){}^{12}\mathrm{C}$ reaction rates is not sufficient to induce a significant effect on BBN, even if ${}^8\mathrm{Be}$ was stable. In particular, no significant production of carbon by the triple alpha reaction is found when compared to standard BBN. Finally, we complement our earlier work on the effect of coupled variation of fundamental constants inducing variations of the binding energy of deuterium, the Fermi constant, and the neutron–proton mass difference, that affect the weak rates and the $\mathrm{n}(\mathrm{p},\gamma )\mathrm{d}$, $A=2$ bottleneck reaction.

Dark energy perturbation affects the growth of matter perturbations even in scenarios with noninteracting dark energy. We investigate the integrated Sachs-Wolfe effect in various canonical scalar field models with perturbed dark energy. We do this analysis for models belonging to the thawing and freezing classes, which are classes based on the way the equation of state evolves with time. For thawing models, the dark energy equation of state remains the same as that of a cosmological constant and deviates from this value at late times. In freezing models, the equation of state evolves in the opposite manner, namely it “freezes” to cosmological constant-type behavior at late times. We show that between these classes there is no clear difference for the Sachs-Wolfe effect. We show that on taking perturbations into account, the contribution due to different models is closer to each other and to the cosmological constant model than it is to the case of an unperturbed scalar field, i.e., the dark energy component is homogeneous. Therefore, considering dark energy to be homogeneous gives an overestimate in distinction between different models. However, the difference between contribution to the angular power spectrum due to different models remains large, and future observations pertaining to growth of perturbations may be able to distinguish between these.

We revisit the annihilation of dark matter to neutrinos in the Sun near the $W^{+}{W}^{-}$ and $t\overline{t}$ kinematic thresholds. We investigate the potential importance of annihilation to $W{W}^{*}$ in a minimal dark matter model in which a Majorana singlet is mixed with a vectorlike electroweak doublet, but many results generalize to other models of weakly interacting dark matter. We reevaluate the indirect detection constraints on this model and find that, once all annihilation channels are properly taken into account, the most stringent constraints on spin-dependent scattering for dark matter mass $60\mathrm{GeV}\lesssim m_{\chi }\lesssim m_t$ are derived from the results of the Super-Kamiokande experiment. Moreover, we establish the model-independent statement that Majorana dark matter whose thermal relic abundance and neutrino signals are both controlled by annihilation via an $s$-channel $Z$ boson is excluded for $70\mathrm{GeV}\lesssim m_{\chi }\lesssim m_W$. In some models, annihilation to $t{t}^{*}$ can affect indirect detection, notably by competing with annihilation to gauge boson final states and thereby weakening neutrino signals. However, in the minimal model, this final state is largely negligible, only allowing dark matter with mass a few GeV below the top quark mass to evade exclusion.

We discuss the gravitational collapse of a spherically symmetric massive core of a star in which the fluid component is interacting with a growing vacuum energy density. The influence of the variable vacuum in the collapsing core is quantified by a phenomenological $\beta$ parameter as predicted by dimensional arguments and the renormalization group approach. For all reasonable values of this free parameter, we find that the vacuum energy density increases the collapsing time, but it cannot prevent the formation of a singular point. However, the nature of the singularity depends on the value of $\beta$. In the radiation case, a trapped surface is formed for $\beta \le 1/2$, whereas for $\beta \ge 1/2$, a naked singularity is developed. In general, the critical value is $\beta =1–2/3(1+\omega )$, where $\omega$ is the parameter describing the equation of state of the fluid component.

The downgoing atmospheric ${\nu }_{\mu }$ and ${\overline{\nu }}_{\mu }$ fluxes can be significantly altered due to the presence of ${\mathrm{eV}}^2$-scale active-sterile oscillations. We study the sensitivity of a large liquid argon detector and a large magnetized iron detector (like the proposed ICAL at INO) to these oscillations. Such oscillations are indicated by results from LSND, and more recently, from MiniBooNE and from reanalyses of reactor experiments following recent recalculations of reactor fluxes. There are other tentative indications of the presence of sterile states in both the $\nu$ and $\overline{\nu }$ sectors as well. Using the allowed sterile parameter ranges in a $3+1$ mixing framework in order to test these results, we perform a fit assuming active-sterile oscillations in both the muon neutrino and antineutrino sectors, and compute oscillation exclusion limits using atmospheric downgoing muon neutrino and antineutrino events. We find that (for both ${\nu }_{\mu }$ and ${\overline{\nu }}_{\mu }$) a liquid argon detector, an ICAL-like detector or a combined analysis of both detectors with an exposure of $1\mathrm{Mt}/\mathrm{yr}$ provides significant sensitivity to regions of parameter space in the range $0.1<\Delta m^2<5{\mathrm{eV}}^2$ for ${sin}^{2}2{\Theta }_{\mu \mu }\ge 0.08$. Thus atmospheric neutrino experiments can provide complementary coverage in these regions, improving sensitivity limits in combination with bounds from other experiments on these parameters. We also analyze the bounds using muon antineutrino events only and compare them with the results from MiniBooNE.

Randall-Sundrum models provide a possible explanation of (gauge-gravity) hierarchy, whereas discrete symmetry flavor groups yield a possible description of the texture of Standard Model (SM) fermion masses. We use both these ingredients to propose a five-dimensional extension of the Standard Model where the mass hierarchy of the four-dimensional effective field theory is obtained only using localization parameters of order 1. We consider a bulk custodial gauge symmetry group together with an Abelian $Z_4$ group: the model turns out to yield a rather minimal extension of the SM as it only requires two brane Higgs fields to provide the desired Yukawa interactions and the required spontaneous symmetry breaking pattern. In fact, the presence of an extra dimension allows the use of the Scherk-Schwarz mechanism to contribute to the breaking of the bulk custodial group down to the SM gauge symmetry. Moreover, no right-handed neutrinos are present and neutrino masses are generated radiatively with the help of a bulk charged scalar field that provides the lepton number violation. Using experimental inputs from the Global Neutrino Analysis and recent Daya Bay results, a numerical analysis is performed and allowed parameter regions are displayed.

In an earlier work we analyzed the $CP$-even scalar sector of an ${\mathsf{S}}_3$ flavor model, where we identified some novel decay signatures of an exotic scalar. In this work we extend our analysis by including the complete set of scalars/pseudoscalars, revisiting the potential minimization conditions in a more general setup, setting the spectrum in conformity with the current LHC limits on the scalar mass, and identifying yet another spectacularly novel decay channel which might be revealed from an intense study of rare top decays at the LHC into modes containing multileptons of different flavors.

We derive a sum rule for the total quark angular momentum of a spin-one hadronic system within a gauge-invariant decomposition of the hadron’s spin. We show that the total angular momentum can be measured through deeply virtual Compton scattering experiments using transversely polarized deuterons.

We investigate the implications of models that achieve a Standard Model-like Higgs boson of mass near 125 GeV by introducing additional TeV-scale supermultiplets in the vectorlike $\mathbf{10}+\overline{\mathbf{10}}$ representation of $SU(5)$, within the context of gauge-mediated supersymmetry breaking. We study the resulting mass spectrum of superpartners, comparing and contrasting to the usual gauge-mediated and constrained minimal supersymmetric Standard Model scenarios, and discuss implications for LHC supersymmetry searches. This approach implies that exotic vectorlike fermions $t_{1,2}^{\prime }$, $b^{\prime }$, and ${\tau }^{\prime }$ should be within the reach of the LHC. We discuss the masses, the couplings to electroweak bosons, and the decay branching ratios of the exotic fermions, with and without various unification assumptions for the mass and mixing parameters. We comment on LHC prospects for discovery of the exotic fermion states, both for decays that are prompt and for those that are nonprompt on detector-crossing time scales.

We investigate the possibility of setting model independent limits for a nonstandard anomalous magnetic moment $a_{\tau }^{\mathrm{NP}}$ of the tau lepton, in future $\gamma \gamma$ colliders based on Compton backscattering. For a hypothetical collider we find that, at various levels of confidence, the limits for $a_{\tau }^{\mathrm{NP}}$ could be improved, compared to previous studies based on LEP1, LEP2 and SLD data. We show the results for a realistic range of the center of mass energy of the $e^{+}{e}^{-}$ collider. As a more direct application, we also present the results of the simulation for the photon collider at the TESLA project.

The Higgs boson of the Standard Model is being searched for at the LHC in the channel $H\to \gamma \gamma$. In the Higgs Triplet Model (HTM), there are contributions to this decay from loops of doubly charged scalars ($H^{\pm \pm }$) and singly charged scalars ($H^{\pm }$) which are not present in the Standard Model. These additional contributions are mediated by the trilinear couplings $H_1{H}^{++}{H}^{--}$ and $H_1{H}^{+}{H}^{-}$, where $H_1$ is the lightest $CP$-even Higgs boson. We point out the possibility of constructive interference of the $H^{\pm \pm }$ contribution with that of the $W^{\pm }$ contribution, which enables a substantial enhancement of the branching ratio of $H_1\to \gamma \gamma$ in the HTM. The magnitude of the contribution of $H^{\pm \pm }$ is essentially determined by the mass of $H^{\pm \pm }$ ($m_{H^{\pm \pm }}$) and a quartic scalar coupling (${\lambda }_1$), with constructive interference arising for ${\lambda }_1<0$. Consequently, the ongoing searches for $H\to \gamma \gamma$ restrict the parameter space of $[m_{H^{\pm \pm }},{\lambda }_1]$ more stringently for ${\lambda }_1<0$ than for the recently studied case of destructive interference with ${\lambda }_1>0$. Moreover, if the excess of $\gamma \gamma$ events around a mass of 125 GeV in the LHC searches for $H\to \gamma \gamma$ is substantiated with larger data samples, and the branching ratio is measured to be somewhat larger than that for the SM Higgs boson, then such an enhancement could be readily accommodated by $H_1$ in the HTM with ${\lambda }_1<0$.

The BABAR Collaboration has recently reported the measurement of the ratio of the branching fractions of $\overline{B}\to D(D^{*}){\tau }^{-}{\overline{\nu }}_{\tau }$ to $\overline{B}\to D(D^{*}){\ell }^{-}{\overline{\nu }}_{\ell }$ which deviates from the Standard Model prediction by $2\sigma$ ($2.7\sigma$). This deviation goes up to $3.4\sigma$ level when the two measurements in the $D$ and $D^{*}$ modes are taken together and could indicate new physics. Using an effective Lagrangian for the new physics, we study the implication of these results and calculate other observables that can shed light on the nature of the new physics. We show that the measurements of the forward-backward asymmetries and the $\tau$ and $D^{*}$ polarization fractions can be distinguished among the various couplings of the new physics operators.

We compute, for $t\overline{t}$ production at the LHC and at the Tevatron, several charge asymmetries to next-to-leading order QCD, including also the electromagnetic and weak-interaction corrections. We calculate these asymmetries both inclusively and with additional kinematic cuts and compare our results, where possible, with recent experimental results and with Standard Model (SM) predictions. The $t\overline{t}$ asymmetries induce also corresponding asymmetries for the charged leptons from semileptonic top-quark decay. Although these asymmetries are, in the SM, smaller than the corresponding ones for top quarks, they are expected to be measurable quite precisely. In fact, measurement of a lepton asymmetry in $\ell +\mathrm{\text{jets}}$ events was reported by the D0 [Phys. Rev. D 84, 112005 (2011)] and CDF [CDF Report No. 10807 (unpublished)] experiments. We analyze and compute to next-to-leading order in the gauge couplings leptonic charge asymmetries for dileptonic and semileptonic $t\overline{t}$ events, with and without acceptance cuts, at the Tevatron and the LHC.

The Nambu–Jona-Lasinio jet model is extended to accommodate hadronization of a transversely polarized quark in order to explore the Collins effect within a multihadron emission framework. This is accomplished by calculating the polarized quark spin flip probabilities after a pseudoscalar hadron emission and the elementary Collins functions. The model is then used to calculate the number densities of the hadrons produced in the polarized quark’s decay chain. The full Collins fragmentation function is extracted from the sine modulation of the polarized number densities with respect to the polar angle between the initial quark’s spin and hadron’s transverse momentum. Two cases are studied here. First, a toy model for elementary Collins function is used to study the features of the transversely polarized quark-jet model. Second, a full model calculation of transverse momentum dependent pion and kaon Collins functions is presented. The remarkable feature of our model is that the $1/2$ moments of the favored Collins fragmentation functions are positive and peak at large values of $z$ but decrease and oscillate at small values of $z$. The $1/2$ moments of the unfavored Collins functions have comparable magnitude and opposite sign to the favored functions, vanish at large $z$ and peak at small values of $z$. This feature is observed for both the toy model and full calculation and can therefore be attributed to the quark-jet picture of hadronization. Moreover, the transverse momentum dependencies of the model Collins functions differ significantly from the Gaussian form widely used in the empirical parametrizations. Finally, a naïve interpretation of the Schäfer-Teryaev sum rule is proven not to hold in our model, where the transverse momentum conservation is explicitly enforced. This is attributed to the sizable average transverse momentum of the remnant quark that needs to be accounted for to satisfy the transverse momentum sum-rule.

The strong decays of charm-strange baryons up to $N=2$ shell are studied in a chiral quark model. The theoretical predictions for the well-determined charm-strange baryons, ${\Xi }_c^{*}(2645)$, ${\Xi }_c(2790)$, and ${\Xi }_c(2815)$, are in good agreement with the experimental data. This model is also extended to analyze the strong decays of the other newly observed charm-strange baryons ${\Xi }_c(2930)$, ${\Xi }_c(2980)$, ${\Xi }_c(3055)$, ${\Xi }_c(3080)$, and ${\Xi }_c(3123)$. Our predictions are given as follows: (i) ${\Xi }_c(2930)$ might be the first orbital ($1P$) excitation of ${\Xi }_c^{\prime }$ with $J^P=1/2^{-}$ and favors the $|{\Xi }_c^{\prime }{}^{2}P_{\lambda }1/2^{-}⟩$ or $|{\Xi }_c^{\prime }{}^{4}P_{\lambda }1/2^{-}⟩$ state. (ii) ${\Xi }_c(2980)$ might correspond to two overlapping $1P$ excitations of ${\Xi }_c^{\prime }$. The broader resonance with a mass $m\simeq 2.98\mathrm{GeV}$ observed in the ${\Lambda }_c^{+}\overline{K}\pi$ final state is most likely to be the $|{\Xi }_c^{\prime }{}^{2}P_{\rho }1/2^{-}⟩$ state, while the narrower resonance with a mass $m\simeq 2.97\mathrm{GeV}$ observed in the ${\Xi }_c^{*}(2645)\pi$ channel is most likely to be the $|{\Xi }_c^{\prime }{}^{2}P_{\rho }3/2^{-}⟩$ state. (iii) ${\Xi }_c(3080)$ could be classified as the $|{\Xi }_c{S}_{\rho \rho }1/2^{+}⟩$, i.e., the first radial ($2S$) excitation of ${\Xi }_c$. (iv) ${\Xi }_c(3055)$ is most likely to be the second orbital ($1D$) excitation of ${\Xi }_c$ with $J^P=3/2^{+}$ and favors the $|{\Xi }_c{}^{2}D_{\lambda \lambda }3/2^{+}⟩$ state. (v) ${\Xi }_c(3123)$ might be assigned to the $|{\Xi }_c^{\prime }{}^{4}D_{\lambda \lambda }3/2^{+}⟩$, $|{\Xi }_c^{\prime }{}^{4}D_{\lambda \lambda }5/2^{+}⟩$, or $|{\Xi }_c{}^{2}D_{\rho \rho }5/2^{+}⟩$ state. As a byproduct, we calculate the strong decays of the bottom baryons ${\Sigma }_b^{\pm }$, ${\Sigma }_b^{*\pm }$, and ${\Xi }_b^{*}$, which are in good agreement with the recent observations as well.

We compute next-to-leading order (NLO) perturbative QCD corrections to the correlators of interpolating pentaquark currents and their absorptive parts. We employ modular techniques in configuration space which saves us from the onus of having to do loop calculations. The modular technique is explained in some detail. We present explicit NLO results for several interpolating pentaquark currents that have been written down in the literature. Our modular approach is easily adapted to the case of NLO corrections to multiquark correlators with an arbitrary number of quarks/antiquarks.

We use sampling techniques to find robust constraints on the masses of a possible fourth sequential fermion generation from electroweak oblique variables. We find that in the case of a light (115 GeV) Higgs from a single electroweak symmetry breaking doublet, inverted mass hierarchies are possible for both quarks and leptons, but a mass splitting more than $M_W$ in the quark sector is unlikely. We also find constraints in the case of a heavy (600 GeV) Higgs in a single doublet model. As recent data from the Large Hadron Collider hints at the existence of a resonance at 124.5 GeV and a single Higgs doublet at that mass is inconsistent with a fourth fermion generation, we examine a Type II two Higgs doublet model. In this model, there are ranges of parameter space where the Higgs sector can potentially counteract the effects of the fourth generation. Even so, we find that such scenarios produce qualitatively similar fermion mass distributions.

We provide predictions for the production of a $W$ boson in association with up to two jets including at least one $b$ jet at next-to-leading order (NLO) in QCD at the CERN Large Hadron Collider with 7 TeV center-of-mass energy. Both exclusive and inclusive event cross section and $b$-jet cross sections are presented. The calculation is performed consistently in the five-flavor-number scheme where both $q{\overline{q}}^{\prime }$ and $bq$ ($q\ne b$) initiated parton level processes are included at NLO QCD. We study the residual theoretical uncertainties of the NLO predictions due to the renormalization and factorization scale dependence, to the uncertainty from the parton distribution functions, and to the values of ${\alpha }_s$ and the bottom-quark mass.