arXiv subjects

Christopher Smith

Publications and source records attributed to Christopher Smith.

At least 37 records · Page 2Linked to original sources

arXiv2019

Axions are blind to anomalies

The axion couplings to SM gauge bosons are derived in various models, and shown to always arise entirely from non-anomalous fermion loops. They are thus independent of the anomaly structure of the model. This fact is without consequence for vector gauge interactions like QCD and QED, but has a major impact for chiral gauge theories. For example, in the DFSZ axion model, the couplings of axions to electroweak gauge bosons do not follow the pattern expected from chiral anomalies, as we prove by an explicit calculation. The reason for this mismatch is traced back to triangle Feynman diagrams sensitive to the anomalous breaking of the vector Ward identity, and is ultimately related to the conservation of baryon and lepton numbers. Though our analyses are entirely done for true axion models, this observation could have important consequences for axion-like particle searches.

hep-ph↗

arXiv2018

Quark-lepton flavored unification

The unification of the quark and charged lepton flavor structures is revisited using the tools of Minimal Flavor Violation (MFV). Under that framework, we first identify a unique point in parameter space where flavor universality is maximally violated. Though from the point of view of the usual polynomial MFV expansions, standing close to that point would require unacceptable fine-tunings, it is automatically attained with infinite geometric MFV series. Remarkably, this peculiar point is precisely where the charged lepton Yukawa coupling can be expressed naturally in terms of those of the quarks. The most striking consequence would be that the physical electron field is dominantly the gauge lepton field of the third generation.

hep-ph↗

arXiv2018

The effective action for gauge bosons

By treating the vacuum as a medium, H. Euler and W. Heisenberg estimated the non-linear interactions between photons well before the advent of Quantum Electrodynamics. In a modern language, their result is often presented as the archetype of an Effective Field Theory (EFT). In this work, we develop a similar EFT for the gauge bosons of some generic gauge symmetry, valid for example for $SU(2)$, $SU(3)$, various grand unified groups, or mixed $U(1)\otimes SU(N)$ and $SU(M)\otimes SU(N)$ gauge groups. Using the diagrammatic approach, we perform a detailed matching procedure which remains manifestly gauge invariant at all steps, but does not rely on the equations of motion hence is valid off-shell. We provide explicit analytic expressions for the Wilson coefficients of the dimension four, six, and eight operators as induced by massive scalar, fermion, and vector fields in generic representations of the gauge group. These expressions rely on a careful analysis of the quartic Casimir invariants, for which we provide a review using conventions adapted to Feynman diagram calculations. Finally, our computations show that at one loop, some operators are redundant whatever the representation or spin of the particle being integrated out, reducing the apparent complexity of the operator basis that can be constructed solely based on symmetry arguments.

hep-ph↗

arXiv2017

Arcs on spheres intersecting at most twice

Let p be a puncture of a punctured sphere, and let Q be the set of all other punctures. We prove that the maximal cardinality of a set of arcs pairwise intersecting at most once, which start at p and end in Q, is |X|(|X| + 1). We deduce that the maximal cardinality of a set of arcs with arbitrary endpoints pairwise intersecting at most twice is |X|(|X| + 1)(|X| + 2).

math.GT↗

arXiv2017

EDM with and beyond flavor invariants

In this paper, the flavor structure of quark and lepton EDMs in the SM and beyond is investigated using tools inspired from Minimal Flavor Violation. While Jarlskog-like flavor invariants are adequate for estimating CP-violation from closed fermion loops, non-invariant structures arise from rainbow-like processes. Our goal is to systematically construct these latter flavor structures in the quark and lepton sectors, assuming different mechanisms for generating neutrino masses. Numerically, they are found typically much larger, and not necessarily correlated with, Jarlskog-like invariants. Finally, the formalism is adapted to deal with a third class of flavor structures, sensitive to the flavored U(1) phases, and used to study the impact of the strong CP-violating interaction and the interplay between the neutrino Majorana phases and possible baryon and/or lepton number violating interactions.

hep-ph↗

arXiv2016

Realizing posets as prime spectra of Leavitt path algebras

We associate in a natural way to any partially ordered set $(P,\leq)$ a directed graph $E_P$ (where the vertices of $E_P$ correspond to the elements of $P$, and the edges of $E_P$ correspond to related pairs of elements of $P$), and then describe the prime spectrum of the resulting Leavitt path algebra $L_K(E_P)$. This construction allows us to realize a wide class of partially ordered sets as the prime spectra of rings. More specifically, any partially ordered set in which every downward directed subset has a greatest lower bound, and where these greatest lower bounds satisfy certain compatibility conditions, can be so realized. In particular, any partially ordered set satisfying the descending chain condition is in this class.

math.RA↗

arXiv2016

Towards a new paradigm for quark-lepton unification

The quark and lepton mass patterns upset their naive unification. In this paper, a new approach to solve this problem is proposed. Model-independently, we find that a successful unification can be achieved. A mechanism is identified by which the large top quark mass renders its third-generation leptonic partner very light. This state is thus identified with the electron. We then provide a generic dynamical implementation of this mechanism, using tree-level exchanges of vector leptons to relate the quark and lepton flavor structures. In a supersymmetric context, this same mechanism splits the squark masses, and third generation squarks end up much lighter than the others. Finally, the implementation of this mechanism in SU(5) GUT permits to avoid introducing any flavor structure beyond the two minimal Yukawa couplings, ensuring the absence of unknown mixing matrices and their potentially large impact on FCNC.

hep-ph↗

arXiv2016

Could the width of the diphoton anomaly signal a three-body decay ?

The recently observed diphoton anomaly at the LHC appears to suggest the presence of a rather broad resonance. In this note, it is pointed out that this does not hold if the two photons are produced along with an extra state. Specifically, the diphoton invariant mass arising from various $A \to Bγγ$ processes, with $A,B$ being scalars, fermions, or vectors, though peaked at a rather large value, would naturally be broad and could fit rather well the observed deviations. This interpretation has a number of advantages over the two-photon resonance hypothesis, for example with respect to the compatibility with the 8 TeV diphoton, dilepton or dijet searches, and opens many new routes for New Physics model construction.

hep-ph↗

arXiv2014

Real-time earthquake warning for astronomical observatories

Early earthquake warning is a rapidly developing capability that has significant ramifications for many fields, including astronomical observatories. In this work, we describe the susceptibility of astronomical facilities to seismic events, including large telescopes as well as second-generation ground-based gravitational-wave interferometers. We describe the potential warning times for observatories from current seismic networks and propose locations for future seismometers to maximize warning times.

astro-ph.IM↗

arXiv2014

The Flavour of Natural SUSY

An inverted mass hierarchy in the squark sector, as in so-called "natural supersymmetry", requires non-universal boundary conditions at the mediation scale of supersymmetry breaking. We propose a formalism to define such boundary conditions in a basis-independent manner and apply it to generic scenarios where the third-generation squarks are light, while the first two generation squarks are heavy and near-degenerate. We show that not only is our formalism particularly well-suited to study such hierarchical squark mass patterns, but in addition the resulting soft terms at the TeV scale are manifestly compatible with the principle of minimal flavour violation, and thus automatically obey constraints from flavour physics.

hep-ph↗

arXiv2014

Rare K decays: Challenges and Perspectives

At this stage of the LHC program, the prospect for a new physics signal in the very rare K ---> pi nu nu bar decays may be dented, but remains well alive thanks to their intrinsic qualities. First, these decays are among the cleanest observables in the quark flavor sector. When combined with their terrible suppression in the Standard Model, they thus offer uniquely sensitive probes. Second, the LHC capabilities are not ideal for all kinds of new physics, even below the TeV scale. For example, rather elusive scenarios like natural-SUSY-like hierarchical spectrum, baryon number violation, or new very light but very weakly interacting particles may well induce deviations in rare K decays. Even though experimentalists should brace themselves for tiny deviations, these modes thus have a clear role to play in the LHC era.

hep-ph↗

arXiv2014

Baryonic R-parity violation and its running

Baryonic R-parity violation arises naturally once Minimal Flavor Violation (MFV) is imposed on the supersymmetric flavor sector at the low scale. At the same time, the yet unknown flavor dynamics behind MFV could take place at a very high scale. In this paper, we analyze the renormalization group (RG) evolution of this scenario. We find that low-scale MFV is systematically reinforced through the evolution, with the R-parity violating couplings exhibiting infrared fixed points. Intriguingly, we also find that if holomorphy is imposed on MFV at some scale, it is preserved by the RG evolution. Furthermore, low-scale holomorphy is a powerful infrared attractor for a large class of non-holomorphic scenarios. Therefore, supersymmetry with minimally flavor violating baryon number violation at the low scale, especially in the holomorphic case but not only, is viable and resilient under the RG evolution, and should constitute a leading contender for the physics beyond the Standard Model waiting to be discovered at the LHC.

hep-ph↗

arXiv2013

Constraining Higgs mediated dark matter interactions

We perform an analysis of Higgs portal models of dark matter (DM), where DM is light enough to contribute to invisible Higgs decays. Using effective field theory we show that DM can be a thermal relic only if there are additional light particles present with masses below a few 100 GeV. We give three concrete examples of viable Higgs portal models of light DM: (i) the SM extended by DM scalar along with an electroweak triplet and a singlet, (ii) a Two Higgs Doublet Model of type II with additional scalar DM, (iii) SM with DM and an extra scalar singlet that is lighter than DM. In all three examples the Br(h to invisible) constraint is not too restrictive, because it is governed by different parameters than the relic abundance. Additional light particles can have implications for flavor violation and collider searches.

hep-ph↗

arXiv2013

The same-sign top signature of R-parity violation

Baryonic R-parity violation could explain why low-scale supersymmetry has not yet been discovered at colliders: sparticles would be hidden in the intense hadronic activity. However, if the known flavor structures are any guide, the largest baryon number violating couplings are those involving the top/stop, so a copious production of same-sign top-quark pairs is in principle possible. Such a signal, with its low irreducible background and efficient identification through same-sign dileptons, provides us with tell-tale signs of baryon number violating supersymmetry. Interestingly, this statement is mostly independent of the details of the supersymmetric mass spectrum. So, in this paper, after analyzing the sparticle decay chains and lifetimes, we formulate a simplified benchmark strategy that covers most supersymmetric scenarios. We then use this information to interpret the same-sign dilepton searches of CMS, draw approximate bounds on the gluino and squark masses, and extrapolate the reach of the future 14 TeV runs.

hep-ph↗

arXiv2013

Three-generation baryon and lepton number violation at the LHC

One of the most puzzling questions in particle physics concerns the status of the baryon (B) and lepton (L) quantum numbers. On the theoretical side, most new physics scenarios naturally lead to their non-conservation and some amount of violation is actually needed to explain the baryon asymmetry of the Universe. On the experimental side, low-energy constraints such as those on proton decay are so stringent that it is generally believed that no B and L violation will ever be seen in laboratories. We observe that this apparent contradiction, however, disappears when the flavor symmetries involving all three generations are taken into account. We then identify model-independent classes of B and/or L violating six-fermion-based processes that indeed simultaneously satisfy low-energy constraints and produce clearly identifiable signals at the LHC. Finally, through simplified models, we study two classes characterized by (ΔB;ΔL) = (\pm 1;\pm 3) and (\pm 2; 0), that lead to particularly striking signatures (t μ^+ e^+ and \bar t \bar t + jets, respectively).

hep-ph↗

arXiv2012

Could a light Higgs boson illuminate the dark sector?

The impact a new neutral light particle of spin 0, 1/2, 1, or 3/2 could have on the tiny width of a light Higgs boson is systematically analyzed. To this end, we include all the relevant effective interactions, whether renormalizable or not, and review the possible signatures in the Higgs decay modes with missing energy. This includes the fully invisible Higgs boson decay, as well as modes with SM gauge boson or fermions in the final state. In many cases, simply preventing these modes from being dominant suffices to set tight model-independent constraints on the masses and couplings of the new light states.

hep-ph↗

arXiv2012

FCNC portals to the dark sector

The most general basis of operators parametrizing a low-scale departure from the SM particle content is constructed. The SM gauge invariance is enforced, and operators of lowest dimensions are retained separately for a new light neutral particle of spin 0, 1/2, 1, and 3/2. The basis is further decomposed into couplings to the SM Higgs/gauge fields, to pairs of quark/lepton fields, and to baryon/lepton number violating combinations of fermion fields. This basis is then used to systematically investigate the discovery potential of the rare FCNC decays of the K and B mesons with missing energy in the final state. The most sensitive decay modes in the s to d, b to d, and b to s sectors are identified and compared for each type of couplings to the new invisible state.

hep-ph↗

arXiv2012

Proton stability from a fourth family

The possibility to violate baryon or lepton number without introducing any new flavor structures, beyond those needed to account for the known fermion masses and mixings, is analyzed. With four generations, but only three colors, this minimality requirement is shown to lead to baryon number conservation, up to negligible dimension-18 operators. In a supersymmetric context, this same minimality principle allows only superpotential terms with an even number of flavored superfields, hence effectively enforces R-parity both within the MSSM and in a GUT context.

hep-ph↗