To guarantee the stability of the cosmological constant sector against radiative corrections coming from quantum matter fields, one of the most natural ingredients to invoke is the symmetry under scale transformations of the gravitational field. Previous attempts to follow this path have nevertheless failed in providing a consistent picture. Here, we point out that this failure is intimately tied up to an assumption that is typically embedded in modern studies of the gravitational interaction: invariance under the full group of diffeomorphisms. We base the discussion on the gravitational theory known as Weyl transverse gravity. While leading to the same classical solutions as general relativity, and so to the same classical phenomenology, we show that in the presence of quantum matter (i) the degeneracy between these theories is broken (general relativity exhibits the well-known cosmological constant problem, while in Weyl transverse gravity, the cosmological constant sector is protected due to gravitational scale invariance), and (ii) this is possible as the result of abandoning the assumption of full diffeomorphism invariance, which permits circumventing classic results on scale-invariance anomalies and guarantees that gravitational scale invariance survives quantum corrections. Both results signal new directions in the quest of finding an ultraviolet completion of gravity.

We perform the two-point diagnostic for the $Om(z)$ function proposed by Sahni et al. in 2014 for the Starobinsky and the Hu and Sawicki models in $f(R)$ gravity. We show that the observed values of the $Om{h}^2$ function can be explained in $f(R)$ models, while in $\mathrm{\Lambda }$CDM the $Om{h}^2$ function is expected to be a redshift independent number. We perform the analysis for some particular values of ${\mathrm{\Omega }}_m^0$, finding a cumulative probability ($P({\chi }^2\le {\chi }_{\text{model}}^2)$) $P\sim 0.16$ or $\sim 0.09$ for the better cases versus a cumulative probability of $P\sim 0.98$ in the $\mathrm{\Lambda }\mathrm{CDM}$ scenario. We also show that these models present a characteristic signature around the interval between $z\sim 2$ and $z\sim 4$ that could be confronted with future observations using the same test.

Dissipative dark matter, where dark matter particles interact with a massless (or very light) boson, is studied. Such dark matter can arise in simple hidden sector gauge models, including those featuring an unbroken $U(1{)}^{\prime }$ gauge symmetry, leading to a dark photon. Previous work has shown that such models can not only explain the large scale structure and cosmic microwave background, but potentially also dark matter phenomena on small scales, such as the inferred cored structure of dark matter halos. In this picture, dark matter halos of disk galaxies not only cool via dissipative interactions but are also heated via ordinary supernovae (facilitated by an assumed photon–dark photon kinetic mixing interaction). This interaction between the dark matter halo and ordinary baryons, a very special feature of these types of models, plays a critical role in governing the physical properties of the dark matter halo. Here, we further study the implications of this type of dissipative dark matter for disk galaxies. Building on earlier work, we develop a simple formalism which aims to describe the effects of dissipative dark matter in a fairly model independent way. This formalism is then applied to generic disk galaxies. We also consider specific examples, including NGC 1560 and a sample of dwarf galaxies from the LITTLE THINGS survey. We find that dissipative dark matter, as developed here, does a fairly good job accounting for the rotation curves of the galaxies considered. Not only does dissipative dark matter explain the linear rise of the rotational velocity of dwarf galaxies at small radii, but it can also explain the observed wiggles in rotation curves which are known to be correlated with corresponding features in the disk gas distribution.

We propose a renormalizable dark matter model in which a fermionic dark matter (DM) candidate communicates with the standard model particles through two distinct portals: Higgs and vector portals. The dark sector is charged under a $U(1{)}^{\prime }$ gauge symmetry while the standard model has a leptophobic interaction with the dark vector boson. The leading contribution of the DM-nucleon elastic scattering cross section begins at one-loop level. The model meets all the constraints imposed by direct detection experiments provided by LUX and XENON100, observed relic abundance according to WMAP and Planck, and the invisible Higgs decay width measured at the LHC. It turns out that the dark matter mass in the viable parameter space can take values from a few GeV up to 1 TeV. This is a new feature which is absent in the models with only one portal. In addition, we can find in the constrained regions of the parameter space a DM mass of $\sim 34\mathrm{GeV}$ annihilating into $b$ quark pair, which explains the Fermi-LAT gamma-ray excess.

We obtain solutions to the coupled Schrödinger-Poisson equations. The solutions describe the evolution of cold dark matter density perturbations in an otherwise homogeneous expanding Friedmann universe. We discuss the relationships between descriptions of cold dark matter in terms of a pressureless fluid, in terms of a wave function, of a classical scalar field, and a quantum scalar field. We identify the regimes where the various descriptions coincide and where they differ.

It may well happen that the two main components of the dark sector of the Universe, dark matter and dark energy, do not evolve separately but interact nongravitationally with one another. However, given our current lack of knowledge of the microscopic nature of these two components, there is no clear theoretical path to determine their interaction. Yet, over the years, phenomenological interaction terms have been proposed on mathematical simplicity and heuristic arguments. In this paper, based on the likely evolution of the ratio between the energy densities of these dark components, we lay down reasonable criteria to obtain phenomenological, useful, expressions of the said term independent of any gravity theory. We illustrate this with different proposals which seem compatible with the known evolution of the Universe at the background level. Likewise, we show that two possible degeneracies with noninteracting models are only apparent as they can be readily broken at the background level. Further, we analyze some interaction terms that appear in the literature.

We construct the cosmological model to explain the cosmological constant problem. We built the extension of the standard cosmological model $\mathrm{\Lambda }\mathrm{CDM}$ by consideration of decaying vacuum energy represented by the running cosmological term. From the principles of quantum mechanics one can find that in the long-term behavior survival probability of unstable states is a decreasing function of the cosmological time and has the inverse powerlike form. This implies that cosmological constant ${\rho }_{\mathrm{vac}}=\mathrm{\Lambda }(t)={\mathrm{\Lambda }}_{\text{bare}}+\frac{\alpha }{t^2}$ where ${\mathrm{\Lambda }}_{\text{bare}}$ and $\alpha$ are constants. We investigate the dynamics of this model using dynamical system methods due to a link to the $\mathrm{\Lambda }(H)$ cosmologies. We have found the exact solution for the scale factor as well as the indicators of its variability like the deceleration parameter and the jerk. From the calculation of the jerk we obtain a simple test of the decaying vacuum in the Friedman-Robertson-Walker universe. Using astronomical data [SNIa, $H(z)$, CMB, BAO] we have estimated the model parameters and compared this model with the $\mathrm{\Lambda }\mathrm{CDM}$ model. Our statistical results indicate that the decaying vacuum model is a little worse than the $\mathrm{\Lambda }\mathrm{CDM}$ model. But the decaying vacuum cosmological model explains the small value of the cosmological constant today.

We present a measurement of the atmospheric ${\nu }_e$ spectrum at energies between 0.1 and 100 TeV using data from the first year of the complete IceCube detector. Atmospheric ${\nu }_e$ originate mainly from the decays of kaons produced in cosmic-ray air showers. This analysis selects 1078 fully contained events in 332 days of live time, and then identifies those consistent with particle showers. A likelihood analysis with improved event selection extends our previous measurement of the conventional ${\nu }_e$ fluxes to higher energies. The data constrain the conventional ${\nu }_e$ flux to be $1.3_{-0.3}^{+0.4}$ times a baseline prediction from a Honda’s calculation, including the knee of the cosmic-ray spectrum. A fit to the kaon contribution ($\xi$) to the neutrino flux finds a kaon component that is $\xi =1.3_{-0.4}^{+0.5}$ times the baseline value. The fitted/measured prompt neutrino flux from charmed hadron decays strongly depends on the assumed astrophysical flux and shape. If the astrophysical component follows a power law, the result for the prompt flux is $0.0_{-0.0}^{+3.0}$ times a calculated flux based on the work by Enberg, Reno, and Sarcevic.

The basic building block for Lorentz-invariant and ghost-free massive gravity is the square root of the combination $g^{-1}\eta$, where $g^{-1}$ is the inverse of the physical metric and $\eta$ is a reference metric. Since the square root of a matrix is not uniquely defined, it is possible to have physically inequivalent potentials corresponding to different branches. We show that around the Minkowski background, the only perturbatively well-defined branch is the potential proposed by de Rham, Gabadadze and Tolley. On the other hand, if Lorentz symmetry is broken spontaneously, other potentials exist with a standard perturbative expansion. We show this explicitly building new Lorentz-invariant, ghost-free massive gravity potentials for theories that in the background preserve rotational invariance but break Lorentz boosts.

I address the validity of results presented in [S. Khalil, S. Salem, and M. Allam, Phys. Rev. D 89, 095011 (2014)] with regard to unification of gauge couplings within a particular $SU(5)$ framework. The scalar sector of the proposed $SU(5)$ model contains one 5-dimensional, one 24-dimensional, and one 45-dimensional representation. The authors discuss one specific unification scenario that supports the case for the LHC accessible color octet scalar. I show that the unification analysis in question is based on (i) an erroneous assumption related to the issue of nucleon stability and (ii) an incorrect input for the applicable set of renormalization group equations. This, in my view, invalidates the aforementioned gauge coupling unification study. I also question a source of the fermion mass relations presented in that work.

In this paper we investigate the role of inelastic collisions in the kinetic evolution of a highly overpopulated gluon system starting from a glasma-type initial condition. Using the Gunion-Bertsch formula we derive the inelastic collision kernel under the collinear and small-angle approximations. With both numerics and analytic analysis, we show that the inelastic process has two effects: globally changing (mostly reducing) the total particle number, while locally in the small-momentum regime always filling up the infrared modes extremely quickly. This latter effect is found to significantly speed up the emergence of a local thermal distribution in the infrared regime with vanishing local “chemical potential” and thus catalyze the onset of dynamical Bose-Einstein condensation to occur faster (as compared with the purely elastic case) in the overpopulated glasma.

We consider the possibility that the heavier $CP$-even Higgs boson ($H^0$) in the minimal supersymmetric standard model (MSSM) decays invisibly into neutralinos in the light of the recent discovery of the 126 GeV resonance at the CERN Large Hadron Collider (LHC). For this purpose we consider the minimal supersymmetric standard model with universal, nonuniversal and arbitrary boundary conditions on the supersymmetry breaking gaugino mass parameters at the grand unified scale. Typically, scenarios with universal and nonuniversal gaugino masses do not allow invisible decays of the lightest Higgs boson ($h^0$), which is identified with the 126 GeV resonance, into the lightest neutralinos in the MSSM. With arbitrary gaugino masses at the grand unified scale, such an invisible decay is possible. The second lightest Higgs boson can decay into various invisible final states for a considerable region of the MSSM parameter space with arbitrary gaugino masses as well as with the gaugino masses restricted by universal and nonuniversal boundary conditions at the grand unified scale. The possibility of the second lightest Higgs boson of the MSSM decaying into invisible channels is more likely for arbitrary gaugino masses at the grand unified scale. The heavier Higgs boson decay into lighter particles leads to the intriguing possibility that the entire Higgs boson spectrum of the MSSM may be visible at the LHC even if it decays invisibly, during the searches for an extended Higgs boson sector at the LHC. In such a scenario the nonobservation of the extended Higgs sector of the MSSM may carefully be used to rule out regions of the MSSM parameter space at the LHC.

The discovery of a 125 GeV Higgs boson and rising lower bounds on the masses of superpartners have led to concerns that supersymmetric models are now fine-tuned. Large stop masses, required for a 125 GeV Higgs, feed into the electroweak symmetry breaking conditions through renormalization group equations forcing one to fine-tune these parameters to obtain the correct electroweak vacuum expectation value. Nonetheless, this fine-tuning depends crucially on our assumptions about the supersymmetry breaking scale. At the same time, $U(1)$ extensions provide the most compelling solution to the $\mu$ problem, which is also a naturalness issue, and allow the tree-level Higgs mass to be raised substantially above $M_Z$. These very well-motivated supersymmetric models predict a new $Z^{\prime }$ boson which could be discovered at the LHC, and the naturalness of the model requires that the $Z^{\prime }$ boson mass should not be too far above the TeV scale. Moreover, this fine-tuning appears at the tree level, making it less dependent on assumptions about the supersymmetry breaking mechanism. Here we study this fine-tuning for several $U(1)$ supersymmetric extensions of the Standard Model and compare it to the situation in the MSSM where the most direct tree-level fine-tuning can be probed through chargino mass limits. We show that future LHC $Z^{\prime }$ searches are extremely important for challenging the most natural scenarios in these models.

The CMS experiment recently reported an excess consistent with an invariant mass edge in opposite-sign same flavor leptons, when produced in conjunction with at least two jets and missing transverse momentum. We provide an interpretation of the edge in terms of (anti)squark pair production followed by the “golden cascade” decay for one of the squarks: $\tilde{q}\to {\tilde{\chi }}_2^{0}q\to \tilde{l}lq\to {\tilde{\chi }}_1^{0}qll$ in the minimal supersymmetric standard model. A simplified model involving binos, winos, an on-shell slepton, and the first two generations of squarks fits the event rate and the invariant mass edge. We check consistency with a recent ATLAS search in a similar region, finding that much of the good-fit parameter space is still allowed at the 95% confidence level (C.L.). However, a combination of other LHC searches, notably two-lepton stop pair searches and jets plus ${\cancel{p}}_T$, rule out all of the remaining parameter space at the 95% C.L.

Recently, a new symmetry of mesons has been found upon truncation of the quasizero modes of the overlap Dirac operator in lattice simulations. Namely, the $\rho$, ${\rho }^{\prime }$, $\omega$, ${\omega }^{\prime }$, $a_1$, $b_1$, $h_1$, and possibly $f_1$ $J=1$ mesons get degenerate after removal of the quasizero modes. This emergent symmetry has been established to be $SU(4)\supset SU(2{)}_{\mathrm{L}}\times SU(2{)}_{\mathrm{R}}\times U(1{)}_{\mathrm{A}}$. It is higher than the symmetry of the QCD Lagrangian and provides not only a mixing of quarks of given chirality in the isospin space, but also the mixing of left-handed and right-handed components. Here we study, with the overlap Dirac operator, the isovector $J=2$ mesons upon the quasizero mode reduction and observe a similar degeneracy. This result further supports the $SU(4)$ symmetry in mesons of given spin $J\ge 1$.

We consider the process of Higgs boson pair production at a future proton collider with a center-of-mass energy of 100 TeV, focusing on rare final states that include a bottom-antibottom quark pair and multiple isolated leptons: $hh\to (b\overline{b})+n\ell +X$, $n=\{2,4\}$, $X=\{{\cancel{E}}_T,\gamma ,-\}$. We construct experimental search strategies for observing the process through these channels and make suggestions on the desired requirements for the detector design of the future collider.

Results of a search for decays of massive particles to fully hadronic final states are presented. This search uses $20.3{\mathrm{fb}}^{-1}$ of data collected by the ATLAS detector in $\sqrt{s}=8\mathrm{TeV}$ proton-proton collisions at the LHC. Signatures based on high jet multiplicities without requirements on the missing transverse momentum are used to search for $R$-parity-violating supersymmetric gluino pair production with subsequent decays to quarks. The analysis is performed using a requirement on the number of jets, in combination with separate requirements on the number of $b$-tagged jets, as well as a topological observable formed from the scalar sum of the mass values of large-radius jets in the event. Results are interpreted in the context of all possible branching ratios of direct gluino decays to various quark flavors. No significant deviation is observed from the expected Standard Model backgrounds estimated using jet counting as well as data-driven templates of the total-jet-mass spectra. Gluino pair decays to ten or more quarks via intermediate neutralinos are excluded for a gluino with mass $m_{\tilde{g}}<1\mathrm{TeV}$ for a neutralino mass $m_{{\tilde{\chi }}_1^0}=500\mathrm{GeV}$. Direct gluino decays to six quarks are excluded for $m_{\tilde{g}}<917\mathrm{GeV}$ for light-flavor final states, and results for various flavor hypotheses are presented.