Gino Isidori, Jernej F. Kamenik
We analyse a simple Standard Model (SM) extension with only two new light fields: a scalar partner of the top (stop) (with mass above m_t) and a light neutral fermion chi^0 (with mass of a few GeV), coupled to SM quarks via a Yukawa interaction. We show that such model can lead to a significant enhancement of the forward-backward asymmetry in t tbar production at the Tevatron via the additional t tbar pairs produced from (stop anti-stop) decays. The model satisfies existing constraints on new-physics searches both at low and high energies, and could even address the cosmological dark-matter abundance. The implications for future searches at the LHC are briefly outlined.
Gino Isidori, Jernej F. Kamenik, Zoltan Ligeti, Gilad Perez
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.
Gino Isidori, Jernej F. Kamenik
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.
Ezequiel Alvarez, Estefania Coluccio Leskow, Jure Drobnak, Jernej F. Kamenik
We study the possibility of detecting New Physics (NP) phenomena at the LHC through a new search strategy looking at the monotop (top plus missing energy) signature which is common to a variety of NP models. We focus on the leptonic top decay mode and study the discovery or exclusion reach of the 2012 LHC data for three example models. Contrary to the hadronic mode, in this case the problematic QCD multijet background can be safely neglected. We find that the key kinematic variable to suppress most of the remaining SM backgrounds is the transverse mass of the charged lepton and missing energy. In fact, one could expect that the single-top production measurements already address the monotop signature in this mode. This is however not the case because in the SM single-top production the transverse mass has an end point determined by the W mass, while the NP signals typically have an additional source of missing energy. We compare, under the same conditions, our monotop search strategy with existing single-top measurements and find a considerable improvement in the monotop signature reach.
Jernej F. Kamenik, Yotam Soreq, Jure Zupan
We show that new physics models without new flavor violating interactions can explain the recent anomalies in the $b\to s\ell^+\ell^-$ transitions. The $b\to s\ell^+\ell^-$ arises from a $Z'$ penguin which automatically predicts the $V-A$ structure for the quark currents in the effective operators. This framework can be realized either in a renormalizable $U(1)'$ setup or be due to new strongly interacting dynamics. The di-muon resonance searches at the LHC are becoming sensitive to this scenario since the $Z'$ is relatively light, and could well be discovered in future searches by ATLAS and CMS.
Roberto Franceschini, Gian F. Giudice, Jernej F. Kamenik, Matthew McCullough, Alex Pomarol, Riccardo Rattazzi, Michele Redi, Francesco Riva, Alessandro Strumia, Riccardo Torre
Run 2 LHC data show hints of a new resonance in the diphoton distribution at an invariant mass of 750 GeV. We analyse the data in terms of a new boson, extracting information on its properties and exploring theoretical interpretations. Scenarios covered include a narrow resonance and, as preliminary indications suggest, a wider resonance. If the width indications persist, the new particle is likely to belong to a strongly-interacting sector. We also show how compatibility between Run 1 and Run 2 data is improved by postulating the existence of an additional heavy particle, whose decays are possibly related to dark matter.
Jernej F. Kamenik, Arman Korajac, Manuel Szewc, Michele Tammaro, Jure Zupan
Recent advances in $b$, $c$, and $s$ quark tagging coupled with novel statistical analysis techniques will allow future high energy and high statistics electron-positron colliders, such as the FCC-ee, to place phenomenologically relevant bounds on flavor violating Higgs and $Z$ decays to quarks. We assess the FCC-ee reach for $Z/h\to bs, cu$ decays as a function of jet tagging performance. We also update the SM predictions for the corresponding branching ratios, as well as the indirect constraints on the flavor violating Higgs and $Z$ couplings to quarks. Using type III two Higgs doublet model as an example of beyond the standard model physics, we show that the searches for $h\to bs, cu$ decays at FCC-ee can probe new parameter space not excluded by indirect searches. We also reinterpret the FCC-ee reach for $Z\to bs , cu$ in terms of the constraints on models with vectorlike quarks.
Jan O. Eeg, Svjetlana Fajfer, Jernej F. Kamenik
We consider chiral loop corrections to the scalar form factor in B_q to D_q l nu decays. First we consider chiral corrections to the 1/m_Q suppressed operators and then we propose the procedure for the extraction of the relevant form factor using lattice QCD results. In the case of B_s to D_s l nu decay we find that effects of kinematics and chiral corrections tend to cancel for the scalar form factor contributions. In particular the 1/m_Q suppression of chiral corrections is compensated by the SU(3) flavor symmetry breaking corrections which can be as large as 30%. The calculated corrections are relevant for the precise determination of possible new physics effects in B_q to D_q l nu decays.
Patrick D. Bolton, Svjetlana Fajfer, Jernej F. Kamenik, Martín Novoa-Brunet
Motivated by a recent Belle~II measurement that suggests an excess in the rare decay $B \to K\, E_{\rm miss}$, and building upon our recent differential decay rate likelihood analysis of the existing experimental information, we investigate possible new physics (NP) scenarios in which light invisible states participate in flavour-changing $b \to s$ transitions. In particular, we consider the total and differential $B\to K^* E_{\rm miss}$ decay rates and $K^*$ polarisation effects in each NP scenario preferred by the $B\to K E_{\rm miss}$ measurement. We show that future measurements of these $B \to K^* E_{\rm miss}$ observables will offer decisive discrimination among the different NP explanations. Our results highlight the strong complementarity of the rare semi-invisible $b$-hadron decay observables, and underline the importance of analysing their momentum transfer spectra when probing extensions of the Standard Model that feature new light degrees of freedom.
Ilja Dorsner, Jure Drobnak, Svjetlana Fajfer, Jernej F. Kamenik, Nejc Kosnik
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.
Oram Gedalia, Jernej F. Kamenik, Zoltan Ligeti, Gilad Perez
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.
Jure Drobnak, Jernej F. Kamenik, Jure Zupan
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.
Svjetlana Fajfer, Jernej F. Kamenik, Blazenka Melic
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.
Sebastien Descotes-Genon, Jernej F. Kamenik
We reconsider the recent observation by the D0 experiment of a sizable like-sign dimuon charge asymmetry, highlighting that it could be affected by CP-violating New Physics contributions not only in Bd- and Bs-meson mixings, but also in semileptonic decays of b and c quarks producing muons. The D0, measurement could be reconciled with the Standard Model expectations for neutral-meson mixings, provided that the CP asymmetry in semileptonic b (c) decays reaches 0.3 % (1%). Such effects, which lie within the available (rather loose) experimental bounds, would be clear indications of New Physics and should be investigated experimentally.
Jan O. Eeg, Svjetlana Fajfer, Jernej F. Kamenik
We determine chiral loop corrections to the B meson decay amplitudes to positive and negative parity charmed mesons within a framework which combines heavy quark and chiral symmetries. We find that corrections due to states of opposite parity are competitive with the contributions arising from K and eta meson loops. Since lattice studies rely on the chiral behavior of the amplitudes we discuss the chiral limit of our results. We also comment on the extraction of 1/m_B, 1/m_D subleading form factors relevant for the studies of B_q to D_q tau nu decays, which are sensitive to possible helicity-suppressed new physics contributions.
Tobias Hurth, Gino Isidori, Jernej F. Kamenik, Federico Mescia
We analyse the constraints on dimension-six Delta F=1 effective operators in models respecting the MFV hypothesis, both in the one-Higgs doublet case and in the two-Higgs doublet scenario with large tan beta. The constraints are derived mainly from the b -> s inclusive observables measured at the B factories. The implications of these bounds in view of improved measurements in exclusive and inclusive observables in b -> s l^+ l^- and s -> d nu nubar transitions are discussed.
Jernej F. Kamenik, Federico Mescia
In the Standard Model, scalar contributions to leptonic and semileptonic decays are helicity suppressed. The hypothesis of additional physical neutral/charged Higgses can enhance such scalar contributions and give detectable effects especially in B physics. For the charged Higgs, experimental information on both Br(B -> D tau nu) and Br(B -> tau nu) has already become available and in particular the B -> D tau nu branching ratio measurements will be further improved in the coming years. Hadronic uncertainties of scalar contributions in semileptonic decays are already in much better shape than the ones plaguing the helicity suppressed leptonic decays B -> tau nu. Combining existing experimental information form the B factories, we explore which existing and future lattice estimates will be useful to directly address new physics effects from measurements of Br(B_{u,d,s} -> D_{u,d,s} tau nu), which can be performed also at hadron colliders.
Adam Falkowski, Jernej F. Kamenik
The diphoton excess around 750 GeV observed by ATLAS and CMS can be interpreted as coming from a massive spin-2 excitation. We explore this possibility in the context of warped five-dimensional models with the Standard Model (SM) fields propagating in the bulk of the extra dimension. The 750 GeV resonance is identified with the first Kaluza-Klein (KK) excitation of the five-dimensional graviton that is parametrically lighter than KK resonances of SM fields. Our set-up makes it possible to realize non-universal couplings of the spin-2 resonance to matter, and thus to explain non-observation of the 750 GeV resonance in leptonic channels. Phenomenological predictions of the model depend on the localization of fields in the extra dimension. If, as required by naturalness arguments, the zero modes of the Higgs and top fields are localized near the IR brane, one expects large branching fractions to ttbar, hh, WW, and ZZ final states. Decays to a Z boson and a photon can also be observable when the KK graviton couplings to the SM gauge fields are non-universal.
Jernej F. Kamenik, Benjamin R. Safdi, Yotam Soreq, Jure Zupan
We consider the diphoton excess observed by ATLAS and CMS using the most up-to-date data and estimate the preferred enhancement in the production rate between 8 TeV and 13 TeV. Within the framework of effective field theory (EFT), we then show that for both spin-0 and spin-2 Standard Model (SM) gauge-singlet resonances, two of the three processes S to ZZ, S to Z gamma, and S to WW must occur with a non-zero rate. Moreover, we demonstrate that these branching ratios are highly correlated in the EFT. Couplings of S to additional SM states may be constrained and differentiated by comparing the S production rates with and without the vector-boson fusion (VBF) cuts. We find that for a given VBF to inclusive production ratio there is maximum rate of S to gauge bosons, b b-bar, and lighter quark anti-quark pairs. Simultaneous measurements of the width and the VBF ratio may be able to point towards the existence of hidden decays.
Ulrich Haisch, Jernej F. Kamenik
We study the phenomenology of light spin-0 particles and stress that they can be efficiently searched for at the LHCb experiment in the form of dimuon resonances. Given the large production cross sections in the forward rapidity region together with the efficient triggering and excellent mass resolution, it is argued that LHCb can provide unique sensitivity to such states. We illustrate our proposal using the recent measurement of Upsilon production by LHCb, emphasising the importance of mixing effects in the bottomonium resonance region. The implications for dimuon decays of spin-0 bottomonium states are also briefly discussed.