arXiv subjects

Jernej F. Kamenik

Publications and source records attributed to Jernej F. Kamenik.

At least 73 records · Page 4Linked to original sources

arXiv2013

Light Higgs and Vector-like Quarks without Prejudice

Light vector-like quarks with non-renormalizable couplings to the Higgs are a common feature of models trying to address the electroweak (EW) hierarchy problem by treating the Higgs as a pseudo-goldstone boson of a global (approximate) symmetry. We systematically investigate the implications of the leading dimension five operators on Higgs phenomenology in presence of dynamical up- and down-type weak singlet as well as weak doublet vector-like quarks. After taking into account constraints from precision EW and flavour observables we show that contrary to the renormalizable models, significant modifications of Higgs properties are still possible and could shed light on the role of vector-like quarks in solutions to the EW hierarchy problem. We also briefly discuss implications of higher dimensional operators for direct vector-like quark searches at the LHC.

hep-ph↗

arXiv2013

Constraining the dipole moments of the top quark

We investigate the direct and indirect bounds on dipole operators involving the top quark. A careful analysis shows that the experimental upper limit on the neutron electric dipole moment strongly constrains the chromo-electric dipole of the top. We improve previous bounds by two orders of magnitude. This has significant implications for new physics models and it also means that CP violation in top pair production mediated by dipole operators will not be accessible at the LHC. The CP conserving chromo-magnetic dipole moments are constrained by recent measurements of the t\bar t spectrum by the ATLAS collaboration. We also update the indirect constraints on electric and magnetic dipole moments from radiative b -> s transitions, finding that they can be considerably larger than their colored counterparts.

hep-ph↗

arXiv2013

Direct CP violation in non-leptonic charm decays: Implications for new physics

I review the implications of recent measurements of CP violation in D meson decays in the context of standard model extensions. Using effective theory methods, one can derive significant constraints on the possible non-standard contributions from measurements of neutral D meson mixing and direct CP violation in kaon decays. Due to an approximate universality of CP violation in new physics scenarios which only break the SU(3)_Q flavor symmetry of the standard model kinetic Lagrangian, such contributions are particularly constrained by epsilon'/epsilon. Explanations of the observed effect within several explicit well-motivated new physics frameworks are briefly discussed. Finally, possible future experimental tests able to distinguish standard vs. non-standard explanations of the observed CP violation in the charm sector are mentioned.

hep-ph↗

arXiv2013

Implications of lepton flavor universality violations in B decays

Present measurements of b->c tau nu and b->u tau nu transitions differ from the standard model predictions of lepton flavor universality by almost 4sigma. We examine new physics interpretations of this anomaly. An effective field theory analysis shows that minimal flavor violating models are not preferred as an explanation, but are also not yet excluded. Allowing for general flavor violation, right-right vector and right-left scalar quark currents are identified as viable candidates. We discuss explicit examples of two Higgs doublet models, leptoquarks as well as quark and lepton compositeness. Finally, implications for LHC searches and future measurements at the (super)B- factories are presented.

hep-ph↗

arXiv2012

Higgs Uncovering Light Scalar Remnants of High Scale Matter Unification

We consider the impact of colored scalars that can couple directly to matter fields on the recently measured h to gamma gamma excess. Among all possible candidates only scalar states transforming as (8, 2,1/2) and (6,3,1/3) under the Standard Model gauge group can individually accommodate the excess and remain in agreement with all available data. Current experimental constraints require such colored states to have an order one coupling to the Standard Model Higgs and a mass below 300 GeV. We use the best fit values to predict the correlated effect in h to Z gamma and di-Higgs production. We furthermore discuss where and how these states appear in extensions of the Standard Model with primary focus on scenarios of matter unification. We revisit two simple SU(5) setups to show that these two full-fledged models not only accommodate a light color octet state but correlate its mass with observable partial proton decay lifetimes.

hep-ph↗

arXiv2012

Forward Tevatron Tops and Backward LHC Tops with Associates

The anomalous forward-backward asymmetry (A_FB) in t tbar production at the Tevatron is in apparent contradiction with the t tbar charge asymmetry (A_C) measurements at the LHC, which agree well with the standard model predictions. We argue that associated production of a state with [u tbar] flavor quantum numbers can lead to a sizable negative contribution to A_C. Exchange of such a resonance in the t-channel would lead to positive contributions to A_FB and A_C, as has been extensively discussed in the literature. Given the additional negative A_C contribution, both the Tevatron and LHC data can be naturally accommodated within this framework. A simple realization of this setup is the well known example of a Z' with flavor off-diagonal up-top couplings. We provide a detailed study of this model, demonstrating that it indeed reproduces the t tbar asymmetry data, and is compatible with other constraints, e.g. t+jet resonance searches, t tbar inclusive jet multiplicities, and atomic parity violation.

hep-ph↗

arXiv2012

Minimal lepton flavor violating realizations of minimal seesaw models

We study the implications of the global U(1)R symmetry present in minimal lepton flavor violating implementations of the seesaw mechanism for neutrino masses. In the context of minimal type I seesaw scenarios with a slightly broken U(1)R, we show that, depending on the R-charge assignments, two classes of generic models can be identified. Models where the right-handed neutrino masses and the lepton number breaking scale are decoupled, and models where the parameters that slightly break the U(1)R induce a suppression in the light neutrino mass matrix. We show that within the first class of models, contributions of right-handed neutrinos to charged lepton flavor violating processes are severely suppressed. Within the second class of models we study the charged lepton flavor violating phenomenology in detail, focusing on mu to e gamma, mu to 3e and mu to e conversion in nuclei. We show that sizable contributions to these processes are naturally obtained for right-handed neutrino masses at the TeV scale. We then discuss the interplay with the effects of the right-handed neutrino interactions on primordial B - L asymmetries, finding that sizable right-handed neutrino contributions to charged lepton flavor violating processes are incompatible with the requirement of generating (or even preserving preexisting) B - L asymmetries consistent with the observed baryon asymmetry of the Universe.

hep-ph↗

arXiv2012

Discerning New Physics in Top-Antitop Production using Top Spin Observables at Hadron Colliders

Copious production of top-antitop quark pairs at hadron colliders has enabled various probes into the properties and interactions of top quarks. Among the various presently measured observables, the forward-backward asymmetry (FBA) in t tbar production measured at the Tevatron significantly deviates from the standard model predictions, and many models of new physics have been invented to explain the puzzle. We consider the consistency of the simplified single-resonance models containing a color octet axial-vector ("axigluon"), color triplet or sextet weak singlet scalars, weak isodoublet scalar, flavor-changing neutral Z', or charged W' vector boson with existing t tbar production measurements. Among the considered models only an axigluon can reproduce all Tevatron observables, without being in severe tension with the recent LHC results on t tbar production cross section, charge asymmetry and top-spin correlations. The LHC charge asymmetry measurements exclude the W' and Z' explanations of the Tevatron FBA anomaly. On the other hand, all scalar models predict notable deviations in several top spin observables, and the recent top spin correlation measurement using the "helicity" spin quantization axis by ATLAS already provides a significant constraint on possible explanations of the Tevatron FBA anomaly. Future precise measurements of top spin correlations and especially top polarization could differentiate between scalar t(u)-channel models, while they are less sensitive to pure axigluon contributions.

hep-ph↗

arXiv2012

On the Universality of CP Violation in Delta F = 1 Processes

We show that new physics which breaks the left-handed SU(3)_Q quark flavor symmetry induces contributions to CP violation in Delta F = 1 couplings which are approximately universal, in that they are not affected by flavor rotations between the up and the down mass bases. (Only the short distance contributions are universal, while observables are also affected by hadronic matrix elements.) Therefore, such flavor violation cannot be aligned, and is constrained by the strongest bound from either the up or the down sectors. We use this result to show that the bound from eps'/eps prohibits an SU(3)_Q breaking explanation of the recent LHCb evidence for CP violation in D meson decays. Another consequence of this universality is that supersymmetric alignment models with a moderate mediation scale are consistent with the data, and are harder to probe via CP violating observables. With current constraints, therefore, squarks need not be degenerate. However, future improvements in the measurement of CP violation in D-Dbar mixing will start to probe alignment models.

hep-ph↗

arXiv2012

Charming CP Violation and Dipole Operators from RS Flavor Anarchy

Recently the LHCb collaboration reported evidence for direct CP violation in charm decays. The value is sufficiently large that either substantially enhanced Standard Model contributions or non-Standard Model physics is required to explain it. In the latter case only a limited number of possibilities would be consistent with other existing flavor-changing constraints. We show that warped extra dimensional models that explain the quark spectrum through flavor anarchy can naturally give rise to contributions of the size required to explain the the LHCb result. The D meson asymmetry arises through a sizable CP-violating contribution to a chromomagnetic dipole operator. This happens naturally without introducing inconsistencies with existing constraints in the up quark sector. We discuss some subtleties in the loop calculation that are similar to those in Higgs to γγ. Loop-induced dipole operators in warped scenarios and their composite analogs exhibit non-trivial dependence on the Higgs profile, with the contributions monotonically decreasing when the Higgs is pushed away from the IR brane. We show that the size of the dipole operator quickly saturates as the Higgs profile approaches the IR brane, implying small dependence on the precise details of the Higgs profile when it is quasi IR localized. We also explain why the calculation of the coefficient of the lowest dimension 5D operator is guaranteed to be finite. This is true not only in the charm sector but also with other radiative processes such as electric dipole moments, b to sγ, ε'/ε_K and μ to eγ. We furthermore discuss the interpretation of this contribution within the framework of partial compositeness in four dimensions and highlight some qualitative differences between the generic result of composite models and that obtained for dynamics that reproduces the warped scenario.

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

Flipping t tbar asymmetries at the Tevatron and the LHC

We show that the charge asymmetry in t tbar production at the LHC, A_C, and the forward-backward asymmetry at the Tevatron, A_FB, are in general not tightly correlated. They can even have opposite signs, if the underlying new physics (NP) model is general enough. We demonstrate this using two examples of NP: a light axigluon, and a vector that is a color octet and electroweak triplet. The small value of A_C measured at the LHC is thus shown not to exclude a NP interpretation of the anomalously large A_FB at the Tevatron. We identify two observables where significant NP effects are still expected at the Tevatron and the LHC, the b bbar production forward-backward asymmetry and spin polarizations of the pair-produced tops and anti-tops.

hep-ph↗

arXiv2012

Shedding light on CP violation in the charm system via D to V gamma decays

Recent evidence for direct CP violation in non-leptonic charm decays cannot be easily accommodated within the Standard Model (SM). On the other hand, it fits well in new physics models generating CP violating Delta C=1 chromomagnetic dipole operators. We show that in these frameworks sizable direct CP asymmetries in radiative D to P+ P- gamma decays (P=pi,K), with M_PP close to the rho or the phi peak, can be expected. Enhanced matrix elements of the electromagnetic dipole operators can partly compensate the long distance dominance in these decays, leading to CP asymmetries of the order of several percent. If observed at this level, these would provide a clean signal of physics beyond the SM and of new dynamics associated to dipole operators. We briefly comment on related CP violating observables accessible via time dependent D(Dbar) to P+ P- gamma studies and angular decay product distributions in rare semileptonic D decays.

hep-ph↗

arXiv2012

On the B to D* tau nu Sensitivity to New Physics

B physics has played a prominent role in investigations of new physics effects at low-energies. Presently, the largest discrepancy between a standard model prediction and experimental measurements appears in the branching ratio of the charged current mediated B to tau nu decay, where the large tau mass lifts the helicity suppression arising in leptonic B decays. Less significant systematic deviations are also observed in the semileptonic B to D(*) tau nu rates. Due to the rich spin structure of the final state, the decay mode B to D* tau nu offers a number of tests of such possible standard model deviations. We investigate the most general set of lowest dimensional effective operators leading to helicity suppressed modifications of b to c (semi)leptonic transitions. We explore such contributions to the B to D* tau nu decay amplitudes by determining the differential decay rate, longitudinal D* polarization fraction, D* - tau opening angle asymmetry and the tau helicity asymmetry. We identify the size of possible new physics contributions constrained by the present B to D(*) tau nu rate measurements and find significant modifications are still possible in all of them. In particular, the opening angle asymmetry can be shifted by almost 30%, relative to the standard model prediction, while the tau helicity asymmetry can still deviate by as much as 80%.

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

Implications of the LHCb Evidence for Charm CP Violation

The LHCb collaboration recently announced preliminary evidence for CP violation in D meson decays. We discuss this result in the context of the standard model (SM), as well as its extensions. In the absence of reliable methods to evaluate the hadronic matrix elements involved, we can only estimate qualitatively the magnitude of the non-SM tree level operators required to generate the observed central value. In the context of an effective theory, we list the operators that can give rise to the measured CP violation and investigate constraints on them from other processes.

hep-ph↗

arXiv2012

ElectroWeak Precision Observables at One-Loop in Higgsless models

We study the viability of generic Higgsless models at low energies when compliance with EWPO and unitarity constraints up to the TeV scale are imposed. Our analysis shows that consistency with S and T can be achieved at the one-loop level even with a single light vector state, m_V <= 500 GeV. However, this scenario turns out to be strongly disfavored when direct resonance searches at the Tevatron are also taken into account. We show that a fully consistent picture can be obtained if an axial state is introduced. Interestingly, m_V is still predicted to be light (below 1 TeV) while typical values of m_A span over the window 1.2 m_V <= m_A <= 1.4 m_V. Our results for the vector channel are rather robust and well within the reach of present-day colliders, while the axial channel is more loosely constrained.

hep-ph↗

arXiv2011

Limits on scalar leptoquark interactions and consequences for GUTs

A colored weak singlet scalar state with hypercharge 4/3 is one of the possible candidates for the explanation of the unexpectedly large forward-backward asymmetry in t tbar production as measured by the CDF and D0 experiments. We investigate the role of this state in a plethora of flavor changing neutral current processes and precision observables of down-quarks and charged leptons. Our analysis includes tree- and loop-level mediated observables in the K and B systems, the charged lepton sector, as well as the Z to b bbar decay width. We perform a global fit of the relevant scalar couplings. This approach can explain the (g-2)_mu anomaly while tensions among the CP violating observables in the quark sector, most notably the nonstandard CP phase (and width difference) in the Bs system cannot be fully relaxed. The results are interpreted in a class of grand unified models which allow for a light colored scalar with a mass below 1TeV. We find that the renormalizable SU(5) scenario is not compatible with our global fit, while in the SO(10) case the viability requires the presence of both the 126- and 120-dimensional representations.

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