The 50 most-cited articles in Physics Letters B, 2016

Measurements of two- and multi-particle angular correlations in pp collisions at s = 5 , 7 , and 13 TeV are presented as a function of charged-particle multiplicity. The data, corresponding to integrated luminosities of 1.0 pb − 1 (5 TeV), 6.2 pb − 1 (7 TeV), and 0.7 pb − 1 (13 TeV), were collected using the CMS detector at the LHC. The second-order ( v 2 ) and third-order ( v 3 ) azimuthal anisotropy harmonics of unidentified charged particles, as well as v 2 of K S 0 and Λ / Λ ‾ particles, are extracted from long-range two-particle correlations as functions of particle multiplicity and transverse momentum. For high-multiplicity pp events, a mass ordering is observed for the v 2 values of charged hadrons (mostly pions), K S 0 , and Λ / Λ ‾ , with lighter particle species exhibiting a stronger azimuthal anisotropy signal below p T ≈ 2 GeV/ c . For 13 TeV data, the v 2 signals are also extracted from four- and six-particle correlations for the first time in pp collisions, with comparable magnitude to those from two-particle correlations. These observations are similar to those seen…

The recent determination of the local value of the Hubble constant by Riess et al., 2016 (hereafter R16) is now 3.3 sigma higher than the value derived from the most recent CMB anisotropy data provided by the Planck satellite in a ΛCDM model. Here we perform a combined analysis of the Planck and R16 results in an extended parameter space, varying simultaneously 12 cosmological parameters instead of the usual 6. We find that a phantom-like dark energy component, with effective equation of state w = − 1.29 − 0.12 + 0.15 at 68 % c.l. can solve the current tension between the Planck dataset and the R16 prior in an extended ΛCDM scenario. On the other hand, the neutrino effective number is fully compatible with standard expectations. This result is confirmed when including cosmic shear data from the CFHTLenS survey and CMB lensing constraints from Planck. However, when BAO measurements are included we find that some of the tension with R16 remains, as also is the case when we include the supernova type Ia luminosity distances from the JLA catalog.

We propose that three recent anomalies in B meson decays, RD(⁎), RK, and P5′, might be explained by only one vector leptoquark weak triplet state. The constraints on the parameter space are obtained by considering t→bτ+ν data, lepton flavor universality tests in the kaon sector, bounds on the lepton flavor violating decay B→Kμτ, and b→cμ−ν¯ decays. The presence of such vector leptoquark could be exposed in precise measurements of top semitauonic decays to b quark. The model predicts enhanced decay rate of B→Kν¯ν, approximate equality of lepton flavor universality ratios RK⁎, RK, and suppressed branching fraction of Bs→μ+μ−.

We confront the indications of lepton flavor universality (LFU) violation observed in semi-tauonic B meson decays with new physics (NP) searches using high pT tau leptons at the LHC. Using effective field theory arguments we correlate possible non-standard contributions to semi-tauonic charged currents with the τ+τ− signature at high energy hadron colliders. Several representative standard model extensions put forward to explain the anomaly are examined in detail: (i) weak triplet of color-neutral vector resonances, (ii) second Higgs doublet and (iii) scalar or (iv) vector leptoquark. We find that, in general, τ+τ− searches pose a serious challenge to NP explanations of the LFU anomaly. Recasting existing 8 TeV and 13 TeV LHC analyses, stringent limits are set on all considered simplified models. Future projections of the τ+τ− constraints as well as caveats in interpreting them within more elaborate models are also discussed.

With the advent of gravitational-wave astronomy marked by the aLIGO GW150914 and GW151226 observations, a measurement of the cosmological speed of gravity will likely soon be realised. We show that a confirmation of equality to the speed of light as indicated by indirect Galactic observations will have important consequences for a very large class of alternative explanations of the late-time accelerated expansion of our Universe. It will break the dark degeneracy of self-accelerated Horndeski scalar–tensor theories in the large-scale structure that currently limits a rigorous discrimination between acceleration from modified gravity and from a cosmological constant or dark energy. Signatures of a self-acceleration must then manifest in the linear, unscreened cosmological structure. We describe the minimal modification required for self-acceleration with standard gravitational-wave speed and show that its maximum likelihood yields a 3σ poorer fit to cosmological observations compared to a cosmological constant. Hence, equality between the speeds challenges the concept of cosmic…

We find that several thresholds can contribute to the enhancements of the newly observed heavy pentaquark candidates P c + ( 4380 ) and P c + ( 4450 ) via the anomalous triangle singularity (ATS) transitions in the specific kinematics of Λ b → J / ψ K − p . Apart from the observed two peaks we find that another peaks around 4.5 GeV can also be produced by the ATS. We also show that the ⁎ Σ c ( ⁎ ) can be produced at leading order in Λ b decay. This process is different from the triangle diagram and its threshold enhancement only appears as CUSP effects if there is no pole structure or the ATS involved. The threshold interaction associated with the presence of the ATS turns out to be a general phenomenon and plays a crucial role in the understanding of candidates for exotic states.

Direct photon production at mid-rapidity in Pb–Pb collisions at s NN = 2.76 TeV was studied in the transverse momentum range 0.9 < p T < 14 GeV / c . Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the ALICE detector material with the e + e − pair reconstructed in the central tracking system. The results of the two methods were combined and direct photon spectra were measured for the 0–20%, 20–40%, and 40–80% centrality classes. For all three classes, agreement was found with perturbative QCD calculations for p T ≳ 5 GeV / c . Direct photon spectra down to p T ≈ 1 GeV / c could be extracted for the 20–40% and 0–20% centrality classes. The significance of the direct photon signal for 0.9 < p T < 2.1 GeV / c is 2.6 σ for the 0–20% class. The spectrum in this p T range and centrality class can be described by an exponential with an inverse slope parameter of ( 297 ± 12 stat ± 41 syst ) MeV . State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties.

We study the generic features of minimal gauge extensions of the Standard Model in view of recent hints of lepton-flavor non-universality in semi-leptonic b→sℓ+ℓ− and b→cℓν decays. We classify the possible models according to the symmetry-breaking pattern and the source of flavor non-universality. We find that in viable models the SU(2)L factor is embedded non-trivially in the extended gauge group, and that gauge couplings should be universal, hinting to the presence of new degrees of freedom sourcing non-universality. Finally, we provide an explicit model that can explain the B-decay anomalies in a coherent way and confront it with the relevant phenomenological constraints.

In this work we revisit the construction of theories for a massive vector field with derivative self-interactions such that only the 3 desired polarizations corresponding to a Proca field propagate. We start from the decoupling limit by constructing healthy interactions containing second derivatives of the Stueckelberg field with itself and also with the transverse modes. The resulting interactions can then be straightforwardly generalized beyond the decoupling limit. We then proceed to a systematic construction of the interactions by using the Levi–Civita tensors. Both approaches lead to a finite family of allowed derivative self-interactions for the Proca field. This construction allows us to show that some higher order terms recently introduced as new interactions trivialize in 4 dimensions by virtue of the Cayley–Hamilton theorem. Moreover, we discuss how the resulting derivative interactions can be written in a compact determinantal form, which can also be regarded as a generalization of the Born-Infeld lagrangian for electromagnetism. Finally, we generalize our results for a…

The effects of local Lorentz violation on dispersion and birefringence of gravitational waves are investigated. The covariant dispersion relation for gravitational waves involving gauge-invariant Lorentz-violating operators of arbitrary mass dimension is constructed. The chirp signal from the gravitational-wave event GW150914 is used to place numerous first constraints on gravitational Lorentz violation.

Where does Hawking radiation originate? A common picture is that it arises from excitations very near or at the horizon, and this viewpoint has supported the “firewall” argument and arguments for a key role for the UV-dependent entanglement entropy in describing the quantum mechanics of black holes. However, closer investigation of both the total emission rate and the stress tensor of Hawking radiation supports the statement that its source is a near-horizon quantum region, or “atmosphere,” whose radial extent is set by the horizon radius scale. This is potentially important, since Hawking radiation needs to be modified to restore unitarity, and a natural assumption is that the scales relevant to such modifications are comparable to those governing the Hawking radiation. Moreover, related discussion suggests a resolution to questions regarding extra energy flux in “nonviolent” scenarios, that does not spoil black hole thermodynamics as governed by the Bekenstein–Hawking entropy.

Recently the LIGO and VIRGO Collaborations reported the observation of gravitational-wave signal corresponding to the inspiral and merger of two black holes, resulting into formation of the final black hole. It was shown that the observations are consistent with the Einstein theory of gravity with high accuracy, limited mainly by the statistical error. Angular momentum and mass of the final black hole were determined with rather large allowance of tens of percents. Here we shall show that this indeterminacy in the range of the black-hole parameters allows for some non-negligible deformations of the Kerr spacetime leading to the same frequencies of the black-hole ringing. This means that at the current precision of the experiment there remains some possibility for alternative theories of gravity.

We suggest and briefly review a new sort of superrenormalizable models of higher derivative quantum gravity. The higher derivative terms in the action can be introduced in such a way that all the unphysical massive states have complex poles. According to the literature on Lee–Wick quantization, in this case the theory can be formulated as unitary, since all massive ghosts-like degrees of freedom are unstable.

An evidence for a diphoton resonance at a mass of 750 GeV has been observed in the data collected at the LHC run at a center of mass energy of 13 TeV. We explore several interpretations of this signal in terms of Higgs-like resonances in a two-Higgs doublet model and its supersymmetric incarnation, in which the heavier CP-even and CP-odd states present in the model are produced in gluon fusion and decay into two photons through top quark loops. We show that one cannot accommodate the observed signal in the minimal versions of these models and that an additional particle content is necessary. We then consider the possibility that vector-like quarks or leptons strongly enhance the heavy Higgs couplings to photons and eventually gluons, without altering those of the already observed 125 GeV state.

We study the topological susceptibility in 2+1 flavor QCD above the chiral crossover transition temperature using Highly Improved Staggered Quark action and several lattice spacings corresponding to temporal extent of the lattice, Nτ=6,8,10 and 12. We observe very distinct temperature dependences of the topological susceptibility in the ranges above and below 250MeV. While for temperatures above 250MeV, the dependence is found to be consistent with dilute instanton gas approximation, at lower temperatures the fall-off of topological susceptibility is milder. We discuss the consequence of our results for cosmology wherein we estimate the bounds on the axion decay constant and the oscillation temperature if indeed the QCD axion is a possible dark matter candidate.

We present composite models explaining the diphoton excess of mass around 750 GeV recently reported by the LHC experiments.

The production of the hypertriton nuclei H Λ 3 and H ‾ Λ ¯ 3 has been measured for the first time in Pb–Pb collisions at s NN = 2.76 TeV with the ALICE experiment at LHC. The p T -integrated H Λ 3 yield in one unity of rapidity, d N / d y × B . R . ( H Λ 3 → He 3 , π − ) = ( 3.86 ± 0.77 ( stat. ) ± 0.68 ( syst. ) ) × 10 − 5 in the 0–10% most central collisions, is consistent with the predictions from a statistical thermal model using the same temperature as for the light hadrons. The coalescence parameter B 3 shows a dependence on the transverse momentum, similar to the B 2 of deuterons and the B 3 of 3 He nuclei. The ratio of yields S 3 = H Λ 3 / ( He 3 × Λ / p ) was measured to be S 3 = 0.60 ± 0.13 ( stat. ) ± 0.21 ( syst. ) in 0–10% centrality events; this value is compared to different theoretical models. The measured S 3 is compatible with thermal model predictions. The measured H Λ 3 lifetime, τ = 181 − 39 + 54 ( stat. ) ± 33 ( syst. ) ps is in agreement within 1 σ with the world average value.

The multi-strange baryon yields in Pb Pb collisions have been shown to exhibit an enhancement relative to pp reactions. In this work, Ξ and Ω production rates have been measured with the ALICE experiment as a function of transverse momentum, p T , in p Pb collisions at a centre-of-mass energy of s NN = 5.02 TeV . The results cover the kinematic ranges 0.6 GeV / c < p T < 7.2 GeV / c and 0.8 GeV / c < p T < 5 GeV / c , for Ξ and Ω respectively, in the common rapidity interval − 0.5 < y CMS < 0 . Multi-strange baryons have been identified by reconstructing their weak decays into charged particles. The p T spectra are analysed as a function of event charged-particle multiplicity, which in p Pb collisions ranges over one order of magnitude and lies between those observed in pp and Pb Pb collisions. The measured p T distributions are compared to the expectations from a Blast-Wave model. The parameters which describe the production of lighter hadron species also describe the hyperon spectra in high multiplicity p Pb collisions. The yield of hyperons relative to charged pions is studied…

This Letter describes a model-agnostic search for pairs of jets (dijets) produced by resonant and non-resonant phenomena beyond the Standard Model in 3.6 fb − 1 of proton–proton collisions with a centre-of-mass energy of s = 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. The distribution of the invariant mass of the two leading jets is examined for local excesses above a data-derived estimate of the smoothly falling prediction of the Standard Model. The data are also compared to a Monte Carlo simulation of Standard Model angular distributions derived from the rapidity of the two jets. No evidence of anomalous phenomena is observed in the data, which are used to exclude, at 95% CL, quantum black holes with threshold masses below 8.3 TeV, 8.1 TeV, or 5.1 TeV in three different benchmark scenarios; resonance masses below 5.2 TeV for excited quarks, 2.6 TeV in a W ′ model, a range of masses starting from m Z ′ = 1.5 TeV and couplings from g q = 0.2 in a Z ′ model; and contact interactions with a compositeness scale below 12.0 TeV and 17.5 TeV respectively for…

Measurements of the W ± → ℓ ± ν and Z → ℓ + ℓ − production cross sections (where ℓ ± = e ± , μ ± ) in proton–proton collisions at s = 13 TeV are presented using data recorded by the ATLAS experiment at the Large Hadron Collider, corresponding to a total integrated luminosity of 81 pb −1 . The total inclusive W ± -boson production cross sections times the single-lepton-flavour branching ratios are σ W + tot = 11.83 ± 0.02 (stat) ± 0.32 (sys) ± 0.25 (lumi) nb and σ W − tot = 8.79 ± 0.02 (stat) ± 0.24 (sys) ± 0.18 (lumi) nb for W + and W − , respectively. The total inclusive Z -boson production cross section times leptonic branching ratio, within the invariant mass window 66 < m ℓ ℓ < 116 GeV , is σ Z tot = 1.981 ± 0.007 (stat) ± 0.038 (sys) ± 0.042 (lumi) nb . The W + , W − , and Z -boson production cross sections and cross-section ratios within a fiducial region defined by the detector acceptance are also measured. The cross-section ratios benefit from significant cancellation of experimental uncertainties, resulting in σ W + fid / σ W − fid = 1.295 ± 0.003 (stat) ± 0.010 (sys) and σ…

We report on the first computation of the next-to-next-to-leading order (NNLO) QCD corrections to W ± Z production in proton collisions. We consider both the inclusive production of on-shell W ± Z pairs at LHC energies and the total W ± Z rates including off-shell effects of the W and Z bosons. In the off-shell computation, the invariant mass of the lepton pairs from the Z boson decay is required to be in a given mass window, and the results are compared with the corresponding measurements obtained by the ATLAS and CMS collaborations. The NNLO corrections range from 8 % at s = 7 TeV to 11 % at s = 14 TeV and significantly improve the agreement with the LHC data at s = 7 and 8 TeV .

The production of charged pions, kaons and (anti)protons has been measured at mid-rapidity ( − 0.5 < y < 0 ) in p–Pb collisions at s NN = 5.02 TeV using the ALICE detector at the LHC. Exploiting particle identification capabilities at high transverse momentum ( p T ), the previously published p T spectra have been extended to include measurements up to 20 GeV/ c for seven event multiplicity classes. The p T spectra for pp collisions at s = 7 TeV , needed to interpolate a pp reference spectrum, have also been extended up to 20 GeV/ c to measure the nuclear modification factor ( R pPb ) in non-single diffractive p–Pb collisions. At intermediate transverse momentum ( 2 < p T < 10 GeV / c ) the proton-to-pion ratio increases with multiplicity in p–Pb collisions, a similar effect is not present in the kaon-to-pion ratio. The p T dependent structure of such increase is qualitatively similar to those observed in pp and heavy-ion collisions. At high p T ( > 10 GeV / c ), the particle ratios are consistent with those reported for pp and Pb–Pb collisions at the LHC energies. At intermediate p…

We argue that the QCD axion can arise from many aligned axions with decay constants much smaller than the conventional axion window. If the typical decay constant is of O(100) GeV to 1 TeV, one or more of the axions or saxions may account for the recently found diphoton excess at ∼750 GeV. Our scenario predicts many axions and saxions coupled to gluons with decay constants of order the weak scale, and therefore many collider signatures by heavy axions and saxions will show up at different energy scales. In particular, if the inferred broad decay width is due to multiple axions or saxions, a non-trivial peak structure may become evident when more data is collected. We also discuss cosmological implications of the aligned QCD axion scenario. In the Appendix we give a possible UV completion and argue that the high quality of the Peccei–Quinn symmetry is naturally explained in our scenario.

A resonance with a mass of approximately 750 GeV has recently been “observed” at the LHC in its diphoton decay. If this state is not simply a statistical fluctuation which will disappear with more data, it could have important implications not only for particle physics but also for cosmology. In this note, we analyze the implications of such a resonance for the dark matter (DM). Assuming a spin- 1 2 DM particle, we first verify that indeed the correct relic density can be obtained for a wide range of the particle mass and weak scale coupling, that are compatible with present data. We then show that the combination of near future direct and indirect detection experiments will allow to probe the CP-nature of the mediator resonance, i.e. discriminate whether it is a scalar or a pseudoscalar like particle.

The diquark–triquark model is used to explain charmonium-pentaquark states, i.e., Pc(4380) and Pc(4450), which were observed recently by the LHCb Collaboration. For the first time, we investigate the properties of the color attractive configuration of a triquark and we define a nonlocal light cone distribution amplitude for pentaquark states, where both diquark and triquark are not pointlike, but they have nonzero size. We establish an effective diquark–triquark Hamiltonian based on spin–orbital interaction. According to the Hamiltonian, we show that the minimum mass splitting between 52+ and 32− is around 100 MeV, which may naturally solve the challenging problem of small mass splitting between Pc(4450) and Pc(4380). This helps to understand the peculiarities of Pc(4380) with a broad decay width whereas Pc(4450) has a narrow decay width. Based on the diquark–triquark model, we predict more pentaquark states, which will hopefully be measured in future experiments.

We discuss a minimal solution to the long-standing (g−2)μ anomaly in a simple extension of the Standard Model with an extra Z′ vector boson that has only flavor off-diagonal couplings to the second and third generation of leptons, i.e. μ,τ,νμ,ντ and their antiparticles. A simplified model realization, as well as various collider and low-energy constraints on this model, are discussed. We find that the (g−2)μ-favored region for a Z′ lighter than the tau lepton is totally excluded, while a heavier Z′ solution is still allowed. Some testable implications of this scenario in future experiments, such as lepton-flavor universality-violating tau decays at Belle 2, and a new four-lepton signature involving same-sign di-muons and di-taus at HL-LHC and FCC-ee, are pointed out. A characteristic resonant absorption feature in the high-energy neutrino spectrum might also be observed by neutrino telescopes like IceCube and KM3NeT.

We study the heavy quarkonium within the basis light-front quantization approach. We implement the one-gluon exchange interaction and a confining potential inspired by light-front holography. We adopt the holographic light-front wavefunction (LFWF) as our basis function and solve the non-perturbative dynamics by diagonalizing the Hamiltonian matrix. We obtain the mass spectrum for charmonium and bottomonium. With the obtained LFWFs, we also compute the decay constants and the charge form factors for selected eigenstates. The results are compared with the experimental measurements and with other established methods.

We show that the new precise measurements of Cosmic Microwave Background (CMB) temperature and polarization anisotropies made by the Planck satellite significantly improves previous constraints on the cosmic gravitational waves background (CGWB) at frequencies f>10−15 Hz. On scales smaller than the horizon at the time of decoupling, primordial gravitational waves contribute to the total radiation content of the Universe. Considering adiabatic perturbations, CGWB affects temperature and polarization CMB power spectra and matter power spectrum in a manner identical to relativistic particles. Considering the latest Planck results we constrain the CGWB energy density to Ωgwh2<1.7×10−6 at 95% CL. Combining CMB power spectra with lensing, BAO and primordial Deuterium abundance observations, we obtain Ωgwh2<1.2×10−6 at 95% CL, improving previous Planck bounds by a factor 3 and the recent direct upper limit from the LIGO and VIRGO experiments a factor 2. A combined analysis of future satellite missions as COrE and EUCLID could improve current bound by more than an order of magnitude.

In this letter, we consider a class of scenarios in which the dark matter is part of a heavy hidden sector that is thermally decoupled from the Standard Model in the early universe. The dark matter freezes-out by annihilating to a lighter, metastable state, whose subsequent abundance can naturally come to dominate the energy density of the universe. When this state decays, it reheats the visible sector and dilutes all relic abundances, thereby allowing the dark matter to be orders of magnitude heavier than the weak scale. For concreteness, we consider a simple realization with a Dirac fermion dark matter candidate coupled to a massive gauge boson that decays to the Standard Model through its kinetic mixing with hypercharge. We identify viable parameter space in which the dark matter can be as heavy as ∼1–100 PeV without being overproduced in the early universe.

A geometric formulation of Higgs Effective Field Theory (HEFT) is presented. Experimental observables are given in terms of geometric invariants of the scalar sigma model sector such as the curvature of the scalar field manifold M. We show how the curvature can be measured experimentally via Higgs cross-sections, WL scattering, and the S parameter. The one-loop action of HEFT is given in terms of geometric invariants of M. The distinction between the Standard Model (SM) and HEFT is whether M is flat or curved, and the curvature is a signal of the scale of new physics.

ATLAS and CMS recently released the first results of searches for diphoton resonances in 13 TeV data, revealing a modest excess at an invariant mass of approximately 750 GeV. We find that it is generically possible that a singlet scalar resonance is the origin of the excess while avoiding all other constraints. We highlight some of the implications of this model and how compatible it is with certain features of the experimental results. In particular, we find that the very large total width of the excess is difficult to explain with loop-level decays alone, pointing to other interesting bounds and signals if this feature of the data persists. Finally we comment on the robust Zγ signature that will always accompany the model we investigate.

We consider higher-order derivative interactions beyond second-order generalized Proca theories that propagate only the three desired polarizations of a massive vector field besides the two tensor polarizations from gravity. These new interactions follow the similar construction criteria to those arising in the extension of scalar–tensor Horndeski theories to Gleyzes–Langlois–Piazza–Vernizzi (GLPV) theories. On the isotropic cosmological background, we show the existence of a constraint with a vanishing Hamiltonian that removes the would-be Ostrogradski ghost. We study the behavior of linear perturbations on top of the isotropic cosmological background in the presence of a matter perfect fluid and find the same number of propagating degrees of freedom as in generalized Proca theories (two tensor polarizations, two transverse vector modes, and two scalar modes). Moreover, we obtain the conditions for the avoidance of ghosts and Laplacian instabilities of tensor, vector, and scalar perturbations. We observe key differences in the scalar sound speed, which is mixed with the matter…

We use phase-equivalent transformations to adjust off-shell properties of similarity renormalization group evolved chiral effective field theory NN interaction (Idaho N3LO) to fit selected binding energies and spectra of light nuclei in an ab exitu approach. We then test the transformed interaction on a set of additional observables in light nuclei to verify that it provides reasonable descriptions of these observables with an apparent reduced need for three- and many-nucleon interactions.

We consider the possibility of interpreting the recently reported diphoton excess at 750 GeV as a spin-two massive particle (such as a Kaluza–Klein graviton in warped extra-dimensions) which serves as a mediator to Dark Matter via its gravitational couplings to the dark sector and to the Standard Model (SM). We model non-universal couplings of the resonance to gauge bosons in the SM and to Dark Matter as a function on their localization in the extra dimension. We find that scalar, fermion or vector dark matter can saturate the dark matter relic density by the annihilation of dark matter into a pair of the SM particles or heavy resonances, in agreement with the diphoton resonance signal strength. We check the compatibility of our hypothesis with other searches for the KK graviton. We show that the invisible decay rate of the resonance into a pair of dark matter is subdominant in the region of the correct relic density, hence leading to no constraints from the mono-jet bound at 8 TeV via the gluon coupling. We also discuss the kinematic features of the decay products of a KK graviton…

We recently showed how it is possible to use a cubic Galileon action to construct classical cosmological solutions that enter a contracting null energy condition (NEC) violating phase, bounce at finite values of the scale factor and exit into an expanding NEC-satisfying phase without encountering any singularities or pathologies. A drawback of these examples is that singular behavior is encountered at some time either just before or just after the NEC-violating phase. In this Letter, we show that it is possible to circumvent this problem by extending our method to actions that include the next order L 4 Galileon interaction. Using this approach, we construct non-singular classical bouncing cosmological solutions that are non-pathological for all times.

A measurement of the total pp cross section at the LHC at s = 8 TeV is presented. An integrated luminosity of 500 μ b − 1 was accumulated in a special run with high- β ⋆ beam optics to measure the differential elastic cross section as a function of the Mandelstam momentum transfer variable t . The measurement is performed with the ALFA sub-detector of ATLAS. Using a fit to the differential elastic cross section in the − t range from 0.014 GeV 2 to 0.1 GeV 2 to extrapolate t → 0 , the total cross section, σ tot ( p p → X ) , is measured via the optical theorem to be σ tot ( p p → X ) = 96.07 ± 0.18 ( stat. ) ± 0.85 ( exp. ) ± 0.31 ( extr. ) mb , where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation t → 0 . In addition, the slope of the exponential function describing the elastic cross section at small t is determined to be B = 19.74 ± 0.05 ( stat. ) ± 0.23 ( syst. ) GeV − 2 .

The basic features of quark and lepton mass matrices can be successfully explained by natural minima of a generic potential with dynamical Yukawa fields invariant under the [ S U ( 3 ) ] 5 × O ( 3 ) flavor symmetry. If this symmetry is gauged, in order to avoid potentially dangerous Goldstone bosons, and small perturbations are added to exactly fit the observed pattern of fermion masses, the spectrum of massive flavor gauge bosons can naturally explain the hints for new physics in b → s ℓ + ℓ − transitions, including R K . In particular, the desired pattern of the Standard Model Yukawa couplings is compatible with a gauged U ( 1 ) q in the quark sector, and U ( 1 ) μ − τ in the lepton sector spontaneously broken around the TeV scale. In order to explain the aforementioned experimental hints, the corresponding neutral gauge bosons are required to mix, yielding to potentially observable signals in dimuon resonance searches at the LHC.

We study the effect of dimension 7 and 8 operators on inclusive semileptonic B decays and the extraction of |Vcb|. Using moments of semileptonic B decay spectra and information based on the Lowest-Lying State saturation Approximation (LLSA) we perform a global fit of the non-perturbative parameters of the heavy quark expansion including for the first time the O(1/mb4,5) contributions. Higher power corrections appear to have a very small effect on the extraction of |Vcb|, independently of the weight we attribute to the LLSA.

A search for muon neutrinos originating from dark matter annihilations in the Sun is performed using the data recorded by the ANTARES neutrino telescope from 2007 to 2012. In order to obtain the best possible sensitivities to dark matter signals, an optimisation of the event selection criteria is performed taking into account the background of atmospheric muons, atmospheric neutrinos and the energy spectra of the expected neutrino signals. No significant excess over the background is observed and 90 % C.L. upper limits on the neutrino flux, the spin-dependent and spin-independent WIMP-nucleon cross-sections are derived for WIMP masses ranging from 50 GeV to 5 TeV for the annihilation channels WIMP + WIMP → b b ¯ , W + W − and τ + τ − .

If neutrinos are truly Dirac fermions, the smallness of their masses may still be natural if certain symmetries exist beyond those of the standard model of quarks and leptons. We perform a systematic study of how this may occur at tree level and in one loop. We also propose a scotogenic version of the left–right gauge model with naturally small Dirac neutrino masses in one loop.

We present a determination of nucleon-nucleon scattering phase shifts for ℓ ≥ 0 . The S , P , D and F phase shifts for both the spin-triplet and spin-singlet channels are computed with lattice Quantum ChromoDynamics. For ℓ > 0 , this is the first lattice QCD calculation using the Lüscher finite-volume formalism. This required the design and implementation of novel lattice methods involving displaced sources and momentum-space cubic sinks. To demonstrate the utility of our approach, the calculations were performed in the S U ( 3 ) -flavor limit where the light quark masses have been tuned to the physical strange quark mass, corresponding to m π = m K ≈ 800 MeV . In this work, we have assumed that only the lowest partial waves contribute to each channel, ignoring the unphysical partial wave mixing that arises within the finite-volume formalism. This assumption is only valid for sufficiently low energies; we present evidence that it holds for our study using two different channels. Two spatial volumes of V ≈ ( 3.5 fm ) 3 and V ≈ ( 4.6 fm ) 3 were used. The finite-volume spectrum is…

We report a first measurement for ultrahigh energy cosmic rays of the correlation between the depth of shower maximum and the signal in the water Cherenkov stations of air-showers registered simultaneously by the fluorescence and the surface detectors of the Pierre Auger Observatory. Such a correlation measurement is a unique feature of a hybrid air-shower observatory with sensitivity to both the electromagnetic and muonic components. It allows an accurate determination of the spread of primary masses in the cosmic-ray flux. Up till now, constraints on the spread of primary masses have been dominated by systematic uncertainties. The present correlation measurement is not affected by systematics in the measurement of the depth of shower maximum or the signal in the water Cherenkov stations. The analysis relies on general characteristics of air showers and is thus robust also with respect to uncertainties in hadronic event generators. The observed correlation in the energy range around the ‘ankle’ at lg ⁡ ( E / eV ) = 18.5 – 19.0 differs significantly from expectations for pure…

Two high-accuracy goniometers equipped with two bent silicon crystals were installed in the betatron cleaning insertion of the CERN Large Hadron Collider (LHC) during its long shutdown. First beam tests were recently performed at the LHC with 450 GeV/ c and 6500 GeV/ c stored proton beams to investigate the feasibility of beam halo collimation assisted by bent crystals. For the first time channeling of 6500 GeV/ c protons was observed in a particle accelerator . A strong reduction of beam losses due to nuclear inelastic interactions in the aligned crystal in comparison with its amorphous orientation was detected. The loss reduction value was about 24. Thus, the results show that deflection of particles by a bent crystal due to channeling is effective for this record particle energy.

We compute the next-to-leading order (NLO) QCD corrections to the g g → W + W − → l 1 + ν 1 l 2 − ν ¯ 2 process, mediated by a massless quark loop, at the LHC. This process first contributes to the hadroproduction of W + W − at O ( α s 2 ) , but, nevertheless, has a sizable impact on the total production rate. We find that the NLO QCD corrections to the g g → W + W − process amount to O ( 50 ) % , and increase the NNLO QCD cross sections of p p → W + W − by approximately two percent, at both the 8 TeV and 13 TeV LHC. We also compute the NLO corrections to gluonic W + W − production within a fiducial volume used by the ATLAS Collaboration in their 8 TeV measurement of the W + W − production rate and find that the QCD corrections are significantly smaller than in the inclusive case. While the current experimental uncertainties are still too large to make these differences relevant, the observed strong dependence of perturbative corrections on kinematic cuts underscores that extrapolation from a fiducial measurement to the total cross section is an extremely delicate matter, and calls…

We consider a solution of the effective four-dimensional Einstein equations, obtained from the general relativistic Schwarzschild metric through the principle of Minimal Geometric Deformation (MGD). Since the brane tension can, in general, introduce new singularities on a relativistic Eötvös brane model in the MGD framework, we require the absence of observed singularities, in order to constrain the brane tension. We then study the corresponding Bose–Einstein condensate (BEC) gravitational system and determine the critical stability region of BEC MGD stellar configurations. Finally, the critical stellar densities are shown to be related with critical points of the information entropy.

General considerations in general relativity and quantum mechanics are known to potentially rule out continuous global symmetries in the context of any consistent theory of quantum gravity. Assuming the validity of such considerations, we derive stringent bounds from gamma-ray, X-ray, cosmic-ray, neutrino, and CMB data on models that invoke global symmetries to stabilize the dark matter particle. We compute up-to-date, robust model-independent limits on the dark matter lifetime for a variety of Planck-scale suppressed dimension-five effective operators. We then specialize our analysis and apply our bounds to specific models including the Two-Higgs-Doublet, Left–Right, Singlet Fermionic, Zee–Babu, 3-3-1 and Radiative See-Saw models. Assuming that (i) global symmetries are broken at the Planck scale, that (ii) the non-renormalizable operators mediating dark matter decay have O(1) couplings, that (iii) the dark matter is a singlet field, and that (iv) the dark matter density distribution is well described by a NFW profile, we are able to rule out fermionic, vector, and scalar dark…

We report on new results on the infrared behavior of the three-gluon vertex in quenched Quantum Chromodynamics , obtained from large-volume lattice simulations. The main focus of our study is the appearance of the characteristic infrared feature known as ‘zero crossing’, the origin of which is intimately connected with the nonperturbative masslessness of the Faddeev–Popov ghost. The appearance of this effect is clearly visible in one of the two kinematic configurations analyzed, and its theoretical origin is discussed in the framework of Schwinger–Dyson equations. The effective coupling in the momentum subtraction scheme that corresponds to the three-gluon vertex is constructed, revealing the vanishing of the effective interaction at the exact location of the zero crossing .

The frequency-dependent time delays in fast radio bursts (FRBs) can be used to constrain the photon mass, if the FRB redshifts are known, but the similarity between the frequency dependences of dispersion due to plasma effects and a photon mass complicates the derivation of a limit on mγ. The dispersion measure (DM) of FRB 150418 is known to ∼0.1%, and there is a claim to have measured its redshift with an accuracy of ∼2%, but the strength of the constraint on mγ is limited by uncertainties in the modelling of the host galaxy and the Milky Way, as well as possible inhomogeneities in the intergalactic medium (IGM). Allowing for these uncertainties, the recent data on FRB 150418 indicate that mγ≲1.8×10−14 eVc−2 (3.2×10−50 kg), if FRB 150418 indeed has a redshift z=0.492 as initially reported. In the future, the different redshift dependences of the plasma and photon mass contributions to DM can be used to improve the sensitivity to mγ if more FRB redshifts are measured. For a fixed fractional uncertainty in the extra-galactic contribution to the DM of an FRB, one with a lower redshift…

We prove that for a given operator in the Standard Model (SM) with baryon number ΔB and lepton number ΔL, that the operator's dimension is even (odd) if (ΔB−ΔL)/2 is even (odd). Consequently, this establishes the veracity of statements that were long observed or expected to be true, but not proven, e.g., operators with ΔB−ΔL=0 are of even dimension, ΔB−ΔL must be an even number, etc. These results remain true even if the SM is augmented by any number of right-handed neutrinos with ΔL=1.

We discuss the importance of boundary effects on fermionic matter in a rotating frame. By explicit calculations at zero temperature we show that the scalar condensate of fermion and anti-fermion cannot be modified by the rotation once the boundary condition is properly implemented. The situation is qualitatively changed at finite temperature and/or in the presence of a sufficiently strong magnetic field that supersedes the boundary effects. Therefore, to establish an interpretation of the rotation as an effective chemical potential, it is crucial to consider further environmental effects such as the finite temperature and magnetic field .