We study the light-quark mass and spatial volume dependence of the matrix elements of $\mathrm{\Delta }B=0$ four-quark operators relevant for the determination of $V_{ub}$ and the lifetime ratios of single-$b$ hadrons. To this end, one-loop diagrams are computed in the framework of heavy hadron chiral perturbation theory with partially quenched formalism for three light-quark flavors in the isospin limit; flavor-connected and -disconnected diagrams are carefully analyzed. These calculations include the leading light-quark flavor and heavy-quark spin symmetry breaking effects in the heavy hadron spectrum. Our results can be used in the chiral extrapolation of lattice calculations of the matrix elements to the physical light-quark masses and to infinite volume. To provide insight on such chiral extrapolation, we evaluate the one-loop contributions to the matrix elements containing external $B_d$, $B_s$ mesons and ${\mathrm{\Lambda }}_b$ baryon in the QCD limit, where sea and valence quark masses become equal. In particular, we find that the matrix elements of the ${\lambda }_3$ flavor-octet operators with an external $B_d$ meson receive the contributions solely from connected diagrams in which current lattice techniques are capable of precise determination of the matrix elements. Finite volume effects are at most a few percent for typical lattice sizes and pion masses.

A key problem in making precise perturbative QCD (pQCD) predictions is how to set the renormalization scale of the running coupling unambiguously at each finite order. The elimination of the uncertainty in setting the renormalization scale in pQCD will greatly increase the precision of collider tests of the Standard Model and the sensitivity to new phenomena. Renormalization group invariance requires that predictions for observables must also be independent on the choice of the renormalization scheme. The well-known Brodsky-Lepage-Mackenzie (BLM) approach cannot be easily extended beyond next-to-next-to-leading order of pQCD. Several suggestions have been proposed to extend the BLM approach to all orders. In this paper we discuss two distinct methods. One is based on the “Principle of Maximum Conformality” (PMC), which provides a systematic all-orders method to eliminate the scale and scheme ambiguities of pQCD. The PMC extends the BLM procedure to all orders using renormalization group methods; as an outcome, it significantly improves the pQCD convergence by eliminating renormalon divergences. An alternative method is the “sequential extended BLM” (seBLM) approach, which has been primarily designed to improve the convergence of pQCD series. The seBLM, as originally proposed, introduces auxiliary fields and follows the pattern of the ${\beta }_0$-expansion to fix the renormalization scale. However, the seBLM requires a recomputation of pQCD amplitudes including the auxiliary fields; due to the limited availability of calculations using these auxiliary fields, the seBLM has only been applied to a few processes at low orders. In order to avoid the complications of adding extra fields, we propose a modified version of seBLM which allows us to apply this method to higher orders. We then perform detailed numerical comparisons of the two alternative scale-setting approaches by investigating their predictions for the annihilation cross section ratio $R_{e^{+}{e}^{-}}$ at four-loop order in pQCD.

Factorization in gauge theories holds at the amplitude or amplitude-squared level for states of given soft or collinear momenta. When performing phase-space integrals over such states, one would generally like to avoid putting in explicit cuts to separate soft from collinear momenta. Removing these cuts induces an overcounting of the soft-collinear region and adds new infrared-ultraviolet divergences in the collinear region. In this paper, we first present a regulator-independent subtraction algorithm for removing soft-collinear overlap at the amplitude level which may be useful in pertubative QCD. We then discuss how both the soft-collinear and infrared-ultraviolet overlaps can be undone for certain observables in a way which respects factorization. Our discussion clarifies some of the subtleties in phase-space subtractions and includes a proof of the infrared finiteness of a suitably subtracted jet function. These results complete the connection between factorized QCD and the soft-collinear effective theory.

We propose a simple scenario that directly connects the dark matter (DM) and neutrino mass scales. Based on an interaction between the DM particle $\chi$ and the neutrino $\nu$ of the form $\chi \chi \nu \nu /{\mathrm{\Lambda }}^2$, the DM annihilation cross section into the neutrino is determined and a neutrino mass is radiatively induced. Using the observed neutrino mass scale and the DM relic density, the DM mass and the effective scale $\mathrm{\Lambda }$ are found to be of the order MeV and GeV, respectively. We construct an ultraviolet-complete toy model based on the inverse seesaw mechanism which realizes this potential connection between DM and neutrino physics.

We propose new searches that exploit the unique signatures of decaying sterile neutrinos with masses below $M_W$ at the LHC, where they can be produced in rare decays of Standard Model gauge bosons. We show that, for few-GeV-scale sterile neutrinos, the LHC experiments can probe mixing angles at the level of ${10}^{-4}–{10}^{-3}$ through powerful searches that look for a prompt lepton in association with a displaced lepton jet. For higher-mass sterile neutrinos, i.e., $M_N\gtrsim 15\mathrm{GeV}$, run II can explore similarly small mixing angles in prompt multilepton final states. This represents an improvement of up to 2 orders of magnitude in sensitivity to the sterile neutrino production rate.

A search for the process $e^{+}{e}^{-}\to {\eta }^{\prime }$ has been performed with the SND detector at the VEPP-2000 $e^{+}{e}^{-}$ collider. The data were accumulated at the center-of-mass energy of $957.78\pm 0.06\mathrm{MeV}$ with an integrated luminosity of about $2.9{\mathrm{pb}}^{-1}$. For reconstruction of the ${\eta }^{\prime }$ meson, five decay chains have been used: ${\eta }^{\prime }\to \eta {\pi }^{+}{\pi }^{-}$ followed by the $\eta$ decays to $\gamma \gamma$ and $3{\pi }^0$ and ${\eta }^{\prime }\to \eta {\pi }^0{\pi }^0$ followed by the $\eta$ decays to ${\pi }^{+}{\pi }^{-}{\pi }^0$, $\gamma \gamma$, and $3{\pi }^0$. As a result, the following upper limit has been set on the ${\eta }^{\prime }$ electronic width: ${\mathrm{\Gamma }}_{{\eta }^{\prime }\to e^{+}{e}^{-}}<0.0020\mathrm{eV}$ at the 90% confidence level.

We report a measurement of the branching fraction for $\psi (3770)\to \gamma {\chi }_{c1}$ and search for the transition $\psi (3770)\to \gamma {\chi }_{c2}$ based on $2.92{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ data accumulated at $\sqrt{s}=3.773\mathrm{GeV}$ with the BESIII detector at the BEPCII collider. We measure $\mathcal{B}(\psi (3770)\to \gamma {\chi }_{c1})=(2.48\pm 0.15\pm 0.23)\times 10^{-3}$, which is the most precise measurement to date. The upper limit on the branching fraction of $\psi (3770)\to \gamma {\chi }_{c2}$ at a 90% confidence level is $\mathcal{B}(\psi (3770)\to \gamma {\chi }_{c2})<0.64\times 10^{-3}$. The corresponding partial widths are $\mathrm{\Gamma }(\psi (3770)\to \gamma {\chi }_{c1})=(67.5\pm 4.1\pm 6.7)\mathrm{keV}$ and $\mathrm{\Gamma }(\psi (3770)\to \gamma {\chi }_{c2})<17.4\mathrm{keV}$.

We have measured the scintillation and ionization yield of recoiling nuclei in liquid argon as a function of applied electric field by exposing a dual-phase liquid argon time projection chamber (LAr-TPC) to a low energy pulsed narrow band neutron beam produced at the Notre Dame Institute for Structure and Nuclear Astrophysics. Liquid scintillation counters were arranged to detect and identify neutrons scattered in the TPC and to select the energy of the recoiling nuclei. We report measurements of the scintillation yields for nuclear recoils with energies from 10.3 to 57.3 keV and for median applied electric fields from 0 to $970\mathrm{V}/\mathrm{cm}$. For the ionization yields, we report measurements from 16.9 to 57.3 keV and for electric fields from 96.4 to $486\mathrm{V}/\mathrm{cm}$. We also report the observation of an anticorrelation between scintillation and ionization from nuclear recoils, which is similar to the anticorrelation between scintillation and ionization from electron recoils. Assuming that the energy loss partitions into excitons and ion pairs from ${}^{83m}\mathrm{Kr}$ internal conversion electrons is comparable to that from ${}^{207}\mathrm{Bi}$ conversion electrons, we obtained the numbers of excitons ($N_{\text{ex}}$) and ion pairs ($N_{\mathrm{i}}$) and their ratio ($N_{\text{ex}}/N_{\mathrm{i}}$) produced by nuclear recoils from 16.9 to 57.3 keV. Motivated by arguments suggesting direction sensitivity in LAr-TPC signals due to columnar recombination, a comparison of the light and charge yield of recoils parallel and perpendicular to the applied electric field is presented for the first time.

We study the Casimir friction due to the relative, uniform, lateral motion of two parallel semitransparent mirrors coupled to a vacuum real scalar field $\varphi$. We follow a functional approach, whereby nonlocal terms in the action for $\varphi$, concentrated on the mirrors’ loci, appear after functional integration of the microscopic degrees of freedom. This action for $\varphi$, which incorporates the relevant properties of the mirrors, is then used as the starting point for two complementary evaluations: Firstly, we calculate the in-out effective action for the system, which develops an imaginary part, hence a nonvanishing probability for the decay (because of friction) of the initial vacuum state. Secondly, we evaluate another observable: the vacuum expectation value of the frictional force, using the in-in or closed time path formalism. Explicit results are presented for zero-width mirrors and half-spaces, in a model where the microscopic degrees of freedom at the mirrors are a set of identical quantum harmonic oscillators, linearly coupled to $\varphi$.

We calculate the Casimir interaction energy in $d=2$ spatial dimensions between two (zero-width) mirrors—one flat and the other slightly curved—upon which imperfect conductor boundary conditions are imposed for an electromagnetic (EM) field. Our main result is a second-order derivative expansion (DE) approximation for the Casimir energy, which is studied in different interesting limits. In particular, we focus on the emergence of a nonanalyticity beyond the leading-order term in the DE, when approaching the limit of perfectly conducting mirrors. We also show that the system considered is equivalent to a dual one, consisting of a massless real scalar field satisfying imperfect Neumann conditions (on the very same boundaries). Therefore, the results obtained for the EM field hold true also for the scalar field model.

The electron-positron pair production due to the dynamical Schwinger process in a slowly oscillating strong electric field is enhanced by the superposition of a rapidly oscillating weaker electric field. A systematic account of the enhancement by the resulting bifrequent field is provided for the residual phase space distribution. The enhancement is explained by a severe reduction of the suppression in both the tunneling and multiphoton regimes.

We revisit Starobinsky inflation in a quantum gravitational context, by means of the exact renormalization group (RG). We calculate the nonperturbative beta functions for Newton’s “constant” $G$ and the dimensionless $R^2$ coupling, and show that there exists an attractive UV fixed point where the latter one vanishes but not the former one, and we provide the corresponding beta functions. The smallness of the $R^2$ coupling, required for agreement with inflationary observables, is naturally ensured by its vanishing at the UV fixed point, ensuring the smallness of the primordial fluctuations, as well as providing a theoretical motivation for the initial conditions needed for successful inflation in this context. We discuss the corresponding RG dynamics, showing both how inflationary and classical observations define the renormalization conditions for the couplings, and also how the UV regime is connected with lower energies along the RG flow. Finally, we discuss the consistency of our results when higher-order curvature corrections are included, and show that they are robust to the inclusion of $R^3$ corrections.

A period of inflation in the early Universe produces a nearly scale-invariant spectrum of gravitational waves over a huge range in wavelength. If the amplitude of this gravitational wave background is large enough to be detectable with microwave background polarization measurements, it will also be detectable directly with a space-based laser interferometer. Using a Monte Carlo sampling of inflation models, we demonstrate that the combination of these two measurements will strongly constrain the expansion history during inflation and the physical mechanism driving it.

We revisit the phenomenology of $n\text{−}\overline{n}$ oscillations in the presence of external magnetic fields, highlighting the role of spin. We show, contrary to long-held belief, that the $n\text{−}\overline{n}$ transition rate need not be suppressed, opening new opportunities for its empirical study.

We point out that if neutron-antineutron oscillation is observed in a free neutron oscillation experiment, it will put an upper limit on the strengths of Lorentz invariance violating (LIV) mass operators for neutrons at the level of ${10}^{-23}\mathrm{GeV}$ or so, which would be the most stringent limit for neutrons. We also study constraints on $\mathrm{\Delta }B=2$ LIV operators and find that for one particular operator degaussing is not necessary to obtain a visible signal. We also note that observation of the $n-\overline{n}$ oscillation signal in the nucleon decay search experiment involving nuclei does not lead to any limit on LIV operators since the nuclear potential difference between neutrons and antineutrons will mask any Lorentz violating effect.

We investigate a simplified model of dark matter where a Majorana fermion $\chi$ coannihilates with a colored scalar top partner $\tilde{t}$. We explore the cosmological history, with particular emphasis on the most relevant low-energy parameters: the mass splitting between the dark matter and the coannihilator, and the Yukawa coupling $y_{\chi }$ that connects these fields to the Standard Model top quarks. We also allow a free quartic coupling ${\lambda }_h$ between a pair of Higgs bosons and $\tilde{t}$ pairs. We pay special attention to the case where the values take on those expected where $\tilde{t}$ corresponds to the superpartner of the right-handed top, and $\chi$ is a bino. Direct detection, indirect detection, and colliders are complementary probes of this simple model.

The experimental limit on the neutron electric dipole moment is used as a possible probe of new physics beyond the standard model. Within the minimal supersymmetric standard model (MSSM), we use the current experimental limit on the neutron EDM and possible future improvement as a probe of high-scale supersymmetry (SUSY). Quantitative analyses show that scalar masses as large as a PeV and larger could be probed in an improved experiment far above the scales accessible at future colliders. We also discuss the neutron EDM as a probe of new physics models beyond MSSM. Specifically, we consider an MSSM extension with a particle content including a vectorlike multiplet. Such an extension brings in new sources of charge conjugation and parity ($CP$) violation beyond those in the MSSM. These $CP$ phases contribute to the EDM of the quarks and to the neutron EDM. These contributions are analyzed in this work where we include the supersymmetric loop diagrams involving the neutralinos, charginos, and the gluino, squark and mirror squark exchange diagrams at the one-loop level. We also take into account the contributions from the $W$, $Z$, quark and mirror quark exchanges arising from the mixings of the vectorlike generation with the three generations. It is shown that the experimental limit on the neutron EDM can be used to probe such new physics models. In the future, one expects the neutron EDM to improve an order of magnitude or more allowing one to extend the probe of high-scale SUSY and of new physics models. For the MSSM, the probe of high scales could go up to and beyond PeV scale masses.

We describe an approach to detect dark matter and other invisible particles with mass below a GeV, exploiting missing energy-momentum measurements and other kinematic features of fixed-target production. In the case of an invisibly decaying MeV–GeV-scale dark photon, this approach can improve on present constraints by 2–6 orders of magnitude over the entire mass range, reaching sensitivity as low as ${\epsilon }^2\sim 10^{-14}$. Moreover, the approach can explore essentially all of the viable parameter space for thermal or asymmetric dark matter annihilating through the vector portal.