arXiv subjects

Joachim Brod

Publications and source records attributed to Joachim Brod.

At least 37 records · Page 2Linked to original sources

arXiv2021

Generic One-Loop Matching Conditions for Rare Meson Decays

Leptonic and semileptonic meson decays that proceed via flavour-changing neutral currents provide excellent probes of physics of the standard model and beyond. We present explicit results for the Wilson coefficients of the weak effective Lagrangian for these decays in any perturbative model in which these processes proceed via one-loop contributions. We explicitly show that our results are finite and gauge independent, and provide Mathematica code that implements our results in an easily usable form.

hep-ph↗

arXiv2021

The Z Penguin in Generic Extensions of the Standard Model

Precision flavour observables play an important role in the interpretation of results at the LHC in terms of models of new physics. We present the result for the one-loop Z penguin in generic extensions of the standard model which exhibit exact perturbative unitarity. We use Slavnov-Taylor identities to study the implications of unitarity on the renormalisation of the Z penguin, and derive a manifestly finite result that depends on a reduced set of physical couplings.

hep-ph↗

arXiv2021

Radiative three-body D-meson decays in and beyond the standard model

We study radiative charm decays $D \to P_1 P_2 γ$, $P_{1,2}=π,K$ in QCD factorization at leading order and within heavy hadron chiral perturbation theory. Branching ratios including resonance contributions are around $\sim 10^{-3}$ for the Cabibbo-favored modes into $K πγ$ and $\sim 10^{-5}$ for the singly Cabibbo-suppressed modes into $π^+ π^- γ, K^+ K^- γ$, and thus in reach of the flavor factories BES III and Belle II. Dalitz plots and forward-backward asymmetries reveal significant differences between the two QCD frameworks; such observables are therefore ideally suited for a data-driven identification of relevant decay mechanisms in the standard-model dominated $D \to K πγ$ decays. This increases the potential to probe new physics with the $D \to π^+ π^- γ$ and $D \to K^+ K^- γ$ decays, which are sensitive to enhanced dipole operators. CP asymmetries are useful to test the SM and look for new physics in neutral $|ΔC|=1$ transitions. Cuts in the Dalitz plot enhance the sensitivity to new physics due to the presence of both $s$- and $t,u$-channel intermediate resonances.

hep-ph↗

arXiv2020

Two-loop Beta Function for Complex Scalar Electroweak Multiplets

We present the general form of the renormalizable four-point interactions of a complex scalar field furnishing an irreducible representation of SU(2), and derive a set of algebraic identities that facilitates the calculation of higher-order radiative corrections. As an application, we calculate the two-loop beta function for the SM extended by a scalar multiplet, and provide the result explicitly in terms of the group invariants. Our results include the evolution of the Higgs-portal couplings, as well as scalar "minimal dark matter". We present numerical results for the two-loop evolution of the various couplings.

hep-ph↗

arXiv2020

Renormalization Group Effects in Dark Matter Interactions

We present a renormalization-group (RG) analysis of dark matter interactions with the standard model, where dark matter is allowed to be a component of an electroweak multiplet, and has a mass at or below the electroweak scale. We consider, in addition to the gauge interactions, the complete set of effective operators for dark matter interactions with the standard model above the weak scale, up to and including mass dimension six. We calculate the RG evolution of these operators from the high scale Lambda down to the weak scale, and perform the matching to the tower of effective theories below the weak scale. We also summarize the RG evolution below the weak scale and the matching to the nonrelativistic nuclear interactions. We present several numerical examples and show that in certain cases the dark matter - nucleus scattering rate can change by orders of magnitude when the electroweak running is included.

hep-ph↗

arXiv2019

Standard-model prediction of $ε_K$ with manifest CKM unitarity

The parameter $ε_K$ describes CP violation in the neutral kaon system and is one of the most sensitive probes of new physics. The large uncertainties related to the charm-quark contribution to $ε_K$ have so far prevented a reliable standard-model prediction. We show that CKM unitarity enforces a unique form of the $|ΔS = 2|$ weak effective Lagrangian in which the short-distance theory uncertainty of the imaginary part is dramatically reduced. The uncertainty related to the charm-quark contribution is now at the percent level. We present the updated standard-model prediction $ε_K = 2.16(6)(8)(15) \times 10^{-3}$, where the errors in brackets correspond to QCD short-distance and long-distance, and parametric uncertainties, respectively.

hep-ph↗

arXiv2019

Electric dipole moment constraints on CP-violating light-quark Yukawas

Nonstandard CP violation in the Higgs sector can play an essential role in electroweak baryogenesis. We calculate the full two-loop matching conditions of the standard model, with Higgs Yukawa couplings to light quarks modified to include arbitrary CP-violating phases, onto an effective Lagrangian comprising CP-odd electric and chromoelectric light-quark (up, down, and strange) dipole operators. We find large isospin-breaking contributions of the electroweak diagrams. Using these results, we obtain significant constraints on the phases of the light-quark Yukawas from experimental bounds on the neutron and mercury electric dipole moments.

hep-ph↗

arXiv2018

Weak mixing below the weak scale in dark-matter direct detection

If dark matter couples predominantly to the axial-vector currents with heavy quarks, the leading contribution to dark-matter scattering on nuclei is either due to one-loop weak corrections or due to the heavy-quark axial charges of the nucleons. We calculate the effects of Higgs and weak gauge-boson exchanges for dark matter coupling to heavy-quark axial-vector currents in an effective theory below the weak scale. By explicit computation, we show that the leading-logarithmic QCD corrections are important, and thus resum them to all orders using the renormalization group.

hep-ph↗

arXiv2017

From quarks to nucleons in dark matter direct detection

We provide expressions for the nonperturbative matching of the effective field theory describing dark matter interactions with quarks and gluons to the effective theory of nonrelativistic dark matter interacting with nonrelativistic nucleons. We give the leading and subleading order expressions in chiral counting. In general, a single partonic operator already matches onto several nonrelativistic operators at leading order in chiral counting. Thus, keeping only one operator at the time in the nonrelativistic effective theory does not properly describe the scattering in direct detection. Moreover, the matching of the axial--axial partonic level operator, as well as the matching of the operators coupling DM to the QCD anomaly term, naively include momentum suppressed terms. However, these are still of leading chiral order due to pion poles and can be numerically important. We illustrate the impact of these effects with several examples.

hep-ph↗

arXiv2017

Chiral Effective Theory of Dark Matter Direct Detection

We present the effective field theory for dark matter interactions with the visible sector that is valid at scales of O(1 GeV). Starting with an effective theory describing the interactions of fermionic and scalar dark matter with quarks, gluons and photons via higher dimension operators that would arise from dimension-five and dimension-six operators above electroweak scale, we perform a nonperturbative matching onto a heavy baryon chiral perturbation theory that describes dark matter interactions with light mesons and nucleons. This is then used to obtain the coefficients of the nuclear response functions using a chiral effective theory description of nuclear forces. Our results consistently keep the leading contributions in chiral counting for each of the initial Wilson coefficients.

hep-ph↗

arXiv2017

DirectDM: a tool for dark matter direct detection

We provide a Mathematica package, DirectDM, that takes as input the Wilson coefficients of the relativistic effective theory describing the interactions of dark matter with quarks, gluons and photons, and matches it onto an effective theory describing the interactions of dark matter with neutrons and protons. The nonperturbative matching is performed at leading order in a chiral expansion. The one-loop QCD and QED renormalization-group evolution from the electroweak scale down to the hadronic scale, as well as finite corrections at the heavy quark thresholds are taken into account. We also provide an interface with the package DMFormFactor so that, starting from the relativistic effective theory, one can directly obtain the event rates for direct detection experiments.

hep-ph↗

arXiv2016

Project X: Physics Opportunities

Part 2 of "Project X: Accelerator Reference Design, Physics Opportunities, Broader Impacts". In this Part, we outline the particle-physics program that can be achieved with Project X, a staged superconducting linac for intensity-frontier particle physics. Topics include neutrino physics, kaon physics, muon physics, electric dipole moments, neutron-antineutron oscillations, new light particles, hadron structure, hadron spectroscopy, and lattice-QCD calculations. Part 1 is available as arXiv:1306.5022 [physics.acc-ph] and Part 3 is available as arXiv:1306.5024 [physics.acc-ph].

hep-ex↗

arXiv2016

Theory News Higgs

I review new developments in Higgs physics, with a focus on Yukawa couplings in and beyond the standard model. In particular, I discuss different methods of measuring the light Yukawas, new sources of CP violation in the Higgs sector, and lepton flavor violation.

hep-ph↗

arXiv2016

The Coannihilation Codex

We present a general classification of simplified models that lead to dark matter (DM) coannihilation processes of the form DM + X $\rightarrow$ SM$_1$ + SM$_2$, where X is a coannihilation partner for the DM particle and SM$_1$, SM$_2$ are Standard Model fields. Our classification also encompasses regular DM pair annihilation scenarios if DM and X are identical. Each coannhilation scenario motivates the introduction of a mediating particle M that can either belong to the Standard Model or be a new field, whereby the resulting interactions between the dark sector and the Standard Model are realized as tree-level and dimension-four couplings. We construct a basis of coannihilation models, classified by the $SU(3)_C\times SU(2)_L\times U(1)_Y$ quantum numbers of DM, X and M. Our main assumptions are that dark matter is an electrically neutral color singlet and that all new particles are either scalars, Dirac or Majorana fermions, or vectors. We illustrate how new scenarios arising from electroweak symmetry breaking effects can be connected to our electroweak symmetric simplified models. We offer a comprehensive discussion of the phenomenological features of our models, encompassing the physics of thermal freeze-out, direct and indirect detection constraints, and in particular searches at the Large Hadron Collider (LHC). Many novel signatures that are not covered in current LHC searches are emphasized, and new and improved LHC analyses tackling these signatures are proposed. We discuss how the coannihilation simplified models can be used to connect results from all classes of experiments in a straightforward and transparent way. This point is illustrated with a detailed discussion of the phenomenology of a particular simplified model featuring leptoquark-mediated dark matter coannihilation.

hep-ph↗

arXiv2015

Size of direct CP violation in singly Cabibbo-suppressed D decays

The first experimental evidence for direct CP violation in charm-quark decays has recently been presented by the LHCb collaboration in the difference between the D -> K+ K- and D -> pi+ pi- time-integrated CP asymmetries. We estimate the size of the effects that can be expected within the Standard Model and find that at leading order in 1/mc they are an order of magnitude smaller. However, tree-level annihilation type amplitudes are known to be large experimentally. This implies that certain formally 1/mc-suppressed penguin amplitudes could plausibly account for the LHCb measurement. Simultaneously, the flavor-breaking parts of these amplitudes could explain the large difference between the D -> K+ K- and D -> pi+ pi- decay rates.

hep-ph↗

arXiv2015

Nonstandard Yukawa Couplings and Higgs Portal Dark Matter

We study the implications of non-standard Higgs Yukawa couplings to light quarks on Higgs-portal dark matter phenomenology. Saturating the present experimental bounds on up-quark, down-quark, or strange-quark Yukawa couplings, the predicted direct dark matter detection scattering rate can increase by up to four orders of magnitude. The effect on the dark matter annihilation cross section, on the other hand, is subleading unless the dark matter is very light -- a scenario that is already excluded by measurements of the Higgs invisible decay width. We investigate the expected size of corrections in multi-Higgs-doublet models with natural flavor conservation, the type-II two-Higgs-doublet model, the Giudice-Lebedev model of light quark masses, minimal flavor violation new physics models, Randall-Sundrum, and composite Higgs models. We find that an enhancement in the dark matter scattering rate of an order of magnitude is possible. Finally, we point out that a discovery of Higgs-portal dark matter could lead to interesting bounds on the light-quark Yukawa couplings.

hep-ph↗

arXiv2015

Experimental constraints on the coupling of the Higgs boson to electrons

In the standard model (SM), the coupling of the Higgs boson to electrons is real and very small, proportional to the electron mass. New physics could significantly modify both real and imaginary parts of this coupling. We discuss experiments which are sensitive to the Higgs-electron coupling and derive the current bounds on new physics contributing to this coupling. The strongest constraint follows from the ACME bound on the electron electric dipole moment (EDM). We calculate the full analytic two-loop result for the electron EDM and show that it bounds the imaginary part of the Higgs-electron coupling to be less than 1.7 x 10^-2 times the SM electron Yukawa coupling. Deviations of the real part are much less constrained. We discuss bounds from Higgs decays, resonant Higgs production at electron colliders, Higgs mediated B -> e^+ e^- decays, and the anomalous magnetic moment of the electron. Currently, the strongest constraint comes from h -> e^+ e^- at the LHC, bounding the coupling to be less than ~600 times the SM Yukawa coupling. Important improvements can be expected from future EDM measurements as well as from resonant Higgs production at a next-generation high-luminosity e^+ e^- collider.

hep-ph↗

arXiv2015

Probing anomalous $t\bar t Z$ interactions with rare meson decays

Anomalous couplings of the $Z$ boson to top quarks are only marginally constrained by direct searches and are still sensitive to new particle dynamics at the TeV scale. Employing an effective field theory approach we consider the dimension-six operators which generate deviations from the standard-model vector and axial-vector interactions. We show that rare $B$ and $K$ meson decays together with electroweak precision observables provide strong constraints on these couplings. We also consider constraints from t-channel single-top production.

hep-ph↗