We examine a model with multiple scalar fields to see whether it is possible to reduce the fine-tuning of the Standard Model Higgs mass without introducing low scale top partners. Our approach may be regarded as a generalization of the condition proposed by Veltman, who attempted to predict the Higgs mass using the criterion that the various low energy contributions to the quadratic divergence of the Higgs mass cancel. Although the Veltman condition predicts the wrong Higgs mass in the Standard Model, it can still be adapted to extended Higgs sectors. Furthermore, theories with additional Higgs bosons can lead to suppressed Yukawa couplings of the top quark to the 125 GeV Higgs, making the associated one-loop divergence smaller. Here, we review possible extensions of the Standard Model where the Veltman condition could be realized, and study in detail one minimal model with two extra scalar fields. For this model and for a cutoff of 5 TeV, we show that the overall fine-tuning can be considerably lowered without introducing low-scale Landau poles, albeit the Higgs sector will contain fairly large couplings at the cutoff. Models where the top Yukawa coupling is reduced, in particular, will be within the reach of the upcoming LHC searches.

The present paper incorporates the effects induced by massless spectator quarks in the renormalization group improved Wilson coefficients associated to the $\mathcal{O}(1/m^3)$ spin-dependent heavy quark effective theory Lagrangian operators. The computation is carried out in the Coulomb gauge with leading logarithmic precision. The result completes the Lagrangian to $\mathcal{O}(1/m^3)$ with that accuracy.

With fourth-order derivative theories leading to propagators of the generic ghostlike $1/(k^2-M_1^2)-1/(k^2-M_2^2)$ form, it would appear that such theories have negative norm ghost states and are not unitary. However on constructing the associated quantum Hilbert space for the free theory that would produce such a propagator, Bender and Mannheim found that the Hamiltonian of the free theory is not Hermitian but is instead $PT$ symmetric, and that there are in fact no negative norm ghost states, with all Hilbert space norms being both positive and preserved in time. Even though perturbative radiative corrections cannot change the signature of a Hilbert space inner product, nonetheless it is not immediately apparent how such a ghostlike propagator would not then lead to negative probability contributions in loop diagrams. Here we obtain the relevant Feynman rules and show that all states obtained in cutting intermediate lines in loop diagrams have positive norm. Also we show that due to the specific way that unitarity (conservation of probability) is implemented in the theory, negative signatured discontinuities across cuts in loop diagrams are canceled by a novel and unanticipated contribution of the states in which tree approximation (no loop) graphs are calculated, an effect that is foreign to standard Hermitian theories. Perturbatively then, the fourth-order derivative theory with propagator $1/(k^2-M_1^2)-1/(k^2-M_2^2)$ is viable. Implications of our results for the pure massless $1/k^4$ propagator are also discussed.

We present high statistics results for the isovector charges $g_A^{u-d}$, $g_S^{u-d}$ and $g_T^{u-d}$ of the nucleon. Calculations were carried out on eleven ensembles of gauge configurations generated by the MILC collaboration using highly improved staggered quarks action with $2+1+1$ dynamical flavors. These ensembles span four lattice spacings $a\approx 0.06$, 0.09, 0.12 and 0.15 fm and light-quark masses corresponding to $M_{\pi }\approx 135$, 225 and 315 MeV. Excited-state contamination in the nucleon three-point correlation functions is controlled by including up to three-states in the spectral decomposition. Remaining systematic uncertainties associated with lattice discretization, lattice volume and light-quark masses are controlled using a simultaneous fit in these three variables. Our final estimates of the isovector charges in the $\overline{\mathrm{MS}}$ scheme at 2 GeV are $g_A^{u-d}=1.218(25)(30)$, $g_S^{u-d}=1.022(80)(60)$ and $g_T^{u-d}=0.989(32)(10)$. The first error includes statistical and all systematic uncertainties except that due to the extrapolation ansatz, which is given by the second error estimate. We provide a detailed comparison with the recent result of $g_A^{u-d}=1.271(13)$ by the CalLat collaboration and argue that our error estimate is more realistic. Combining our estimate for $g_S^{u-d}$ with the difference of light quark masses $(m_d-m_u{)}^{\mathrm{QCD}}=2.572(66)\mathrm{MeV}$ given by the MILC/Fermilab/TUMQCD collaboration for $2+1+1$-flavor theory, we obtain $(M_N-M_P{)}^{\mathrm{QCD}}=2.63(27)\mathrm{MeV}$. We update the low-energy constraints on novel scalar and tensor interactions, ${\epsilon }_S$ and ${\epsilon }_T$, at the TeV scale by combining our new estimates for $g_S^{u-d}$ and $g_T^{u-d}$ with precision low-energy nuclear experiments, and find them comparable to those from the ATLAS and the CMS experiments at the LHC.

We present the first measurements of the longitudinal double-spin asymmetry $A_{LL}$ for dijets with at least one jet reconstructed within the pseudorapidity range $0.8<\eta <1.8$. The dijets were measured in polarized $pp$ collisions at a center-of-mass energy $\sqrt{s}=200\mathrm{GeV}$. Values for $A_{LL}$ are determined for several distinct event topologies, defined by the jet pseudorapidities, and span a range of parton momentum fraction $x$ down to $x\sim 0.01$. The measured asymmetries are found to be consistent with the predictions of global analyses that incorporate the results of previous RHIC measurements. They will provide new constraints on $\mathrm{\Delta }g(x)$ in this poorly constrained region when included in future global analyses.

The Review summarizes much of particle physics and cosmology. Using data from previous editions, plus 2,873 new measurements from 758 papers, we list, evaluate, and average measured properties of gauge bosons and the recently discovered Higgs boson, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as supersymmetric particles, heavy bosons, axions, dark photons, etc. Particle properties and search limits are listed in Summary Tables. We give numerous tables, figures, formulae, and reviews of topics such as Higgs Boson Physics, Supersymmetry, Grand Unified Theories, Neutrino Mixing, Dark Energy, Dark Matter, Cosmology, Particle Detectors, Colliders, Probability and Statistics. Among the 118 reviews are many that are new or heavily revised, including a new review on Neutrinos in Cosmology.

Starting with this edition, the Review is divided into two volumes. Volume 1 includes the Summary Tables and all review articles. Volume 2 consists of the Particle Listings. Review articles that were previously part of the Listings are now included in volume 1.

The complete Review (both volumes) is published online on the website of the Particle Data Group (http://pdg.lbl.gov) and in a journal. Volume 1 is available in print as the PDG Book. A Particle Physics Booklet with the Summary Tables and essential tables, figures, and equations from selected review articles is also available.

The 2018 edition of the Review of Particle Physics should be cited as: M. Tanabashi et al. (Particle Data Group), Phys. Rev. D 98, 030001 (2018).

Canonical quantization is often used to suggest new effects in quantum gravity, in the dynamics as well as the structure of space-time. Usually, possible phenomena are first seen in a modified version of the classical dynamics, for instance in an effective Friedmann equation, but there should also be implications for a modified space-time structure. Quantum space-time effects, however, are often ignored in this setting because they are not obvious: they require a careful analysis of gauge transformations and the anomaly problem. It is shown here how modified space-time structures and effective line elements can be derived unambiguously, provided an off-shell anomaly-free system of modified constraints exists. The resulting effective line elements reveal signature change as an inescapable consequence of nonclassical gauge transformations in the presence of holonomy modifications. The general framework is then specialized to black-hole models in loop quantum gravity. In contrast to previous studies, a self-consistent space-time structure is taken into account, leading to a new picture of black-hole interiors.

In this paper, we investigate the sensitivity to additional gravitational wave polarization modes of future detectors. We first look at the upcoming Einstein Telescope and its combination with existing or planned Earth-based detectors in the case of a stochastic gravitational wave background. We then study its correlation with a possible future space-borne detector sensitive to high frequencies, like DECIGO. Finally, we adapt those results for a single gravitational wave source and establish the sensitivity of the modes as well as the localization on the sky.

In the standard cosmological framework of the zeroth-order FLRW metric and the use of perfect fluids in the stress-energy tensor, dark energy with an equation-of-state parameter $w<-1$ (known as phantom dark energy) implies negative kinetic energy and vacuum instability when modeled as a scalar field. However, the accepted values for present-day $w$ from Planck and WMAP9 include a significant range of values less than $-1$. A flip of the sign in front of the kinetic energy term in the Lagrangian remedies the negative kinetic energy but introduces ghostlike instabilities, which perhaps may be rendered unobservable, but certainly not without great cost to the theory. Staying within the confines of observational constraints and general relativity, for which there is good experimental validation, we consider a reasonable departure from the standard zeroth-order framework in an attempt to see if negative kinetic energy can be avoided despite an apparent $w<-1$, all without flipping the sign of the kinetic energy term. We consider a more accurate description of the Universe through the perturbing of the isotropic and homogeneous FLRW metric and the components of the stress-energy tensor to first order. We treat dark energy as a quantum scalar field in the background of this first-order FLRW spacetime, find an approximation for the Green’s function, and calculate the expectation value of the field’s kinetic energy for $w<-1$ using adiabatic expansion to renormalize and obtain a finite value. We find that the kinetic energy is positive for values of $w$ less than $-1$ in zeroth- or first-order FLRW space, thus giving more theoretical credence to observational values of $w<-1$ and demonstrating that phantom dark energy does not categorically have negative kinetic energy. For a nonminimal coupling parameter $\xi =0$, kinetic energy is positive for $w\gtrsim -1.22$, which includes virtually all values of constant $w$ allowed by cosmological data constraints, and more negative values of $w$ give positive kinetic energy for nonzero values of $\xi$. Also, our results are generally applicable for a massive free field or a field with a small potential in a zeroth- or first-order FLRW background dominated by a fluid with a constant $w$.

Features during inflation and reionization leave corresponding features in the temperature and polarization power spectra that could potentially explain anomalies in the Planck 2015 data but require a joint analysis to disentangle. We study the interplay between these two effects using a model-independent parametrization of the inflationary power spectrum and the ionization history. Preference for a sharp suppression of large-scale power is driven by a feature in the temperature power spectrum at multipoles $\ell \sim 20$, whereas preference for a component of high redshift ionization is driven by a sharp excess of polarization power at $\ell \sim 10$ when compared with the lowest multipoles. Marginalizing inflationary freedom does not weaken the preference for $z\gtrsim 10$ ionization, whereas marginalizing reionization freedom slightly enhances the preference for an inflationary feature but can also mask its direct signature in polarization. The inflation and reionization interpretation of these features makes predictions for the polarization spectrum which can be tested in future precision measurements especially at $10\lesssim \ell \lesssim 40$.

We present fully general relativistic simulations of binary neutron star mergers, employing a new zero-temperature chiral effective field theory equation of state (EOS), the BL EOS. We offer a comparison with respect to the older GM3 EOS, which is based on standard relativistic mean-field theory, and separately determine the impact of the mass. We provide a detailed analysis of the dynamics, with focus on the postmerger phase. For all models, we extract the gravitational wave strain and the postmerger frequency spectrum. Further, we determine the amount, velocity, and polar distribution of ejected matter and provide estimates for the resulting kilonova signals. We also study the evolution of the disk while it is interacting with the hypermassive remnant and discuss the merits of different disk mass definitions applicable before collapse, with regard to the mass remaining after black hole formation. Finally, we investigate the radial mass distribution and rotation profile of the remnants, which validate previous results and also corroborate a recently proposed stability criterion.

The flavor conversion of a neutrino usually occurs at densities $\lesssim G_{\mathrm{F}}^{-1}\omega$, whether in ordinary matter or a dense neutrino medium, and on time/distance scales of order ${\omega }^{-1}$, where $G_{\mathrm{F}}$ is the Fermi weak coupling constant and $\omega$ is the typical vacuum oscillation frequency of the neutrino. In contrast, fast neutrino flavor conversions or fast neutrino oscillations can occur on scales much shorter than ${\omega }^{-1}$ in a very dense, anisotropic neutrino gas such as that in a core-collapse supernova or a binary neutron star merger. The origin of fast neutrino oscillations still seems elusive except that it is a mathematical solution to the equation of motion. It has been suggested that the fast oscillations in stationary neutrino gases require a crossing in the electron lepton number angular distribution of the neutrino and that they are suppressed at large matter densities as normal oscillations are. By inspecting a simple four-beam neutrino model we illustrate how the multi-angle effects that were once found to suppress collective neutrino oscillations now give rise to fast flavor conversions. As a result, a large matter density can induce fast oscillations in certain astrophysical scenarios such as at the early epoch of a core-collapse supernova. We also provide an explicit proof of the necessity of a crossed neutrino angular distribution for fast oscillations to occur in an outward flowing, axially symmetric neutrino flux such as in the multi-bulb supernova model. However, fast oscillations can occur without a crossed angular distribution when both inward and outward flowing neutrino fluxes are present in a stationary neutrino gas.

In relativistic quantum field theory particles of half-integer spin must obey Fermi-Dirac statistics. Their quantum operators must anticommute at spacelike separation in contrast to commuting physical observables. We show that Fermi-Dirac spin $1/2$ operators can be emergent in a fully commuting field theory forming directed strings and loops of spin 0 and 1 constituents, reproducing massive Dirac dynamics with background fields. Such underlying description may violate relativistic invariance but there are no manifest interactions at a distance and rotation symmetry remains preserved. We show that under some constraints on the model there exists a well-defined ground state—Fermi sea that it is stable—fermions cannot convert to bosons.

We study the radiation produced by an accelerated time delay acting on the left moving modes. Through analysis via the Schrödinger picture, we find that the final state is a two-mode squeezed state of the left moving Unruh modes, implying particle production. We analyze the system from an operational point of view via the use of self-homodyne detection with broadband inertial detectors. We obtain semianalytical solutions that show that the radiation appears decohered when such an inertial observer analyzes the information of the radiation from the accelerated time-delay source. We make a connection with the case of the accelerated mirror. We investigate the operational conditions under which the signal observed by the inertial observer can be purified.

We apply the perturbative grand unification due to renormalization to distinguish TeV-scale relics of supersymmetric SO(10) scenarios. With rational theoretical constraints taken into account, we find that for the breaking pattern of either SU(5) or Pati-Salam, only extra matter 16 supermultiplet of SO(10) can appear at the TeV scale, apart from the MSSM spectrum.

In a truly model-independent approach, we reexamine a minimal extension of the Standard Model (SM) through the introduction of an additional $U(1)$ symmetry leading to a new neutral gauge boson ($Z^{\prime }$), allowing its kinetic mixing with the hypercharge gauge boson. An SM neutral scalar is used to spontaneously break this extra symmetry, leading to the mass of the $Z^{\prime }$. Except for three right-handed neutrinos, no other fermions are added. We use the current LHC Drell-Yan data to put model-independent constraints in the parameter space of three quantities, namely, $M_{Z^{\prime }}$, the $Z\text{−}{Z}^{\prime }$ mixing angle (${\alpha }_z$), and the extra $U(1)$ effective gauge coupling ($g_x^{\prime }$), which absorb all model dependence. We impose additional constraints from unitarity and low-energy neutrino-electron scattering. However, limits extracted from direct searches turn out to be most stringent. We obtain $M_{Z^{\prime }}>4.4\mathrm{TeV}$ and $|{\alpha }_z|<0.001$ at 95% C.L., when the strength of the additional $U(1)$ gauge coupling is the same as that of the SM $SU(2{)}_L$.

In a large class of scenarios, dark matter (DM) particles that belong to a multiplet of the standard model (SM) weak interactions are challenging to probe in direct detection experiments due to loop-suppressed cross sections. Direct production at colliders is thus crucial to look for such DM candidates, and under current estimates, future runs of the 14-TeV LHC are projected to probe masses of around 300 GeV for DM belonging to an SU(2) doublet (Higgsino-like), and 900 GeV for SU(2) triplet (winolike). We examine how far this mass reach can be extended at the proposed 27-TeV high-energy upgrade of the LHC (HE-LHC), and compare the results to the case for a 100-TeV hadron collider. Following a detector setup similar to that of the ATLAS tracking system for the run-2 LHC upgrade, with a new insertable b-layer (IBL), a disappearing charged track analysis at the HE-LHC can probe Higgsino-like (winolike) DM mass of up to 600 GeV (2.1 TeV) at the 95% C.L. The monojet and missing transverse momentum search, on the otherhand, has a weaker reach of 490 GeV (700 GeV) at 95% C.L. for the Higgsino-like (winolike) states. The mass range accessible in the collider searches can be complementary to the indirect detection probes using gamma rays from dwarf-spheroidal galaxies.

The recent high energy electron and positron flux observed by the DAMPE experiment indicates possible excess events near 1.4 TeV. Such an excess may be evidence of dark matter annihilations or decays in a dark matter subhalo that is located close to the solar system. We give here an analysis of this excess from annihilations of Dirac fermion dark matter which is charged under a new $U(1{)}_X$ gauge symmetry. The interactions between dark matter and the standard model particles are mediated by the $U(1{)}_X$ gauge boson. We show that dark matter annihilations from a local subhalo can explain the excess with the canonical thermal annihilation cross section. We further discuss the constraints from the relic density, from the dark matter direct detection, from the dark matter indirect detection, from the cosmic microwave background, and from the particle colliders.

We examine the type-II two-Higgs-doublet model with a light scalar dark matter ($S$) after imposing the constraints from the Higgs searches at the LHC and dark matter experiments. We first assume that both $CP$-even Higgses ($h$ and $H$) are portals between the dark matter and standard model (SM) sectors, and the $CP$-odd Higgs ($A$) and $H$ are heavier than 130 GeV. We find that the dark matter with a mass of 10–50 GeV is disfavored by the joint constraints of the 125 GeV Higgs signal data, the relic density, XENON1T (2018), and Fermi-LAT. Next, we consider a special scenario in which the heavy $CP$-even Higgs is taken as the 125 GeV Higgs. The light $CP$-even Higgs is the only portal between the dark matter and SM sectors, and the dark matter mass is slightly below the Higgs resonance. We find that the signal data of the 125 GeV Higgs restrict $\tan \beta$ to be in the range of 1–1.5 for $m_h<62\mathrm{GeV}$. The $gg\to A\to hZ$ and $b\overline{b}\to h\to {\tau }^{+}{\tau }^{-}$ channels at the LHC can impose lower limits and upper limits on $\tan \beta$, respectively. For appropriate values of $\tan \beta$, ${\lambda }_h$, and $m_h$, the dark matter with a mass of 10–50 GeV is allowed by the constraints of the Higgs searches at the LHC and dark matter experiments. For example, $\tan \beta$ is restricted to be in the range of 1.0–1.5 for $10\mathrm{GeV}<m_s<28\mathrm{GeV}$, and $\frac{m_h}{2m_S}>1.12$ is excluded for $30\mathrm{GeV}<m_S<50\mathrm{GeV}$.

A certain class of new physics models includes long-lived particles which are singlet under the standard model (SM) gauge group. A displaced vertex is a spectacular signature to probe such particles productions at the high energy colliders, with a negligible SM background. In the context of the minimal gauged $B-L$ extended SM, we consider a pair creation of Majorana right-handed neutrinos (RHNs) at the high energy colliders through the production of the SM and the $B-L$ Higgs bosons and their subsequent decays into RHNs. With parameters reproducing the neutrino oscillation data, we show that the RHNs are long-lived and their displaced vertex signature can be observed at the next generation displaced vertex search experiments, such as the HL-LHC, the MATHUSLA, the LHeC, and the FCC-eh. We find that the lifetime of the RHNs is controlled by the lightest light neutrino mass, which leads to a correlation between the displaced vertex search and the search limit of the future neutrinoless double beta-decay experiments.