arXiv subjects

Emmanuel Stamou

Publications and source records attributed to Emmanuel Stamou.

At least 37 records · Page 2Linked to original sources

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↗

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

Light Resonances and the Low-$q^2$ Bin of $R_{K^*}$

LHCb has reported hints of lepton-flavor universality violation in the rare decays $B \to K^{(*)} \ell^+\ell^-$, both in high- and low-$q^2$ bins. Although the high-$q^2$ hint may be explained by new short-ranged interactions, the low-$q^2$ one cannot. We thus explore the possibility that the latter is explained by a new light resonance. We find that LHCb's central value of $R_{K^*}$ in the low-$q^2$ bin is achievable in a restricted parameter space of new-physics scenarios in which the new, light resonance decays preferentially to electrons and has a mass within approximately $10$ MeV of the di-muon threshold. Interestingly, such an explanation can have a kinematic origin and does not require a source of lepton-flavor universality violation. A model-independent prediction is a narrow peak in the differential $B \to K^* e^+e^-$ rate close to the di-muon threshold. If such a peak is observed, other observables, such as the differential $B \to K e^+e^-$ rate and $R_K$, may be employed to distinguish between models. However, if a low-mass resonance is not observed and the low-$q^2$ anomaly increases in significance, then the case for an experimental origin of the lepton-flavor universality violating anomalies would be strengthened. To further explore this, we also point out that, in analogy to $J/ψ$ decays, $e^+e^-$ and $μ^+μ^-$ decays of $ϕ$ mesons can be used as a cross check of lepton-flavor universality by LHCb with $5$ fb$^{-1}$ of integrated luminosity.

hep-ph↗

arXiv2017

Scaling dimensions in QED$_3$ from the $ε$-expansion

We study the fixed point that controls the IR dynamics of QED in $d = 4 - 2ε$. We derive the scaling dimensions of four-fermion and bilinear operators beyond leading order in $ε$-expansion. For the four-fermion operators, this requires the computation of a two-loop mixing that was not known before. We then extrapolate these scaling dimensions to $d = 3$ to estimate their value at the IR fixed point of QED$_3$ as function of the number of fermions $N_f$. The next-to-leading order result for the four-fermion operators corrects significantly the leading one. Our best estimate at this order indicates that they do not cross marginality for any value of $N_f$, which would imply that they cannot trigger a departure from the conformal phase. For the scaling dimensions of bilinear operators, we observe better convergence as we increase the order. In particular, $ε$-expansion provides a convincing estimate for the dimension of the flavor-singlet scalar in the full range of $N_f$.

hep-th↗

arXiv2017

Operator mixing in $\boldsymbolε$-expansion: scheme and evanescent (in)dependence

We consider theories with fermionic degrees of freedom that have a fixed point of Wilson-Fisher type in non-integer dimension $d = 4-2ε$. Due to the presence of evanescent operators, i.e., operators that vanish in integer dimensions, these theories contain families of infinitely many operators that can mix with each other under renormalization. We clarify the dependence of the corresponding anomalous-dimension matrix on the choice of renormalization scheme beyond leading order in $ε$-expansion. In standard choices of scheme, we find that eigenvalues at the fixed point cannot be extracted from a finite-dimensional block. We illustrate in examples a truncation approach to compute the eigenvalues. These are observable scaling dimensions, and, indeed, we find that the dependence on the choice of scheme cancels. As an application, we obtain the IR scaling dimension of four-fermion operators in QED in $d=4-2ε$ at order $\mathcal{O}(ε^2)$.

hep-th↗

arXiv2016

Interpreting a 750 GeV Diphoton Resonance

We discuss the implications of the significant excesses in the diphoton final state observed by the LHC experiments ATLAS and CMS around a diphoton invariant mass of 750 GeV. The interpretation of the excess as a spin-zero s-channel resonance implies model-independent lower bounds on both its branching ratio and its coupling to photons, which stringently constrain dynamical models. We consider both the case where the excess is described by a narrow and a broad resonance. We also obtain model-independent constraints on the allowed couplings and branching fractions to final states other than diphotons, by including the interplay with 8 TeV searches. These results can guide attempts to construct viable dynamical models of the resonance. Turning to specific models, our findings suggest that the anomaly cannot be accounted for by the presence of only an additional singlet or doublet spin-zero field and the Standard Model degrees of freedom; this includes all two-Higgs-doublet models. Likewise, heavy scalars in the MSSM cannot explain the excess if stability of the electroweak vacuum is required, at least in a leading-order analysis. If we assume that the resonance is broad we find that it is challenging to find a weakly coupled explanation. However, we provide an existence proof in the form of a model with vectorlike quarks with large electric charge that is perturbative up to the 100 TeV scale. For the narrow-resonance case a similar model can be perturbative up to high scales also with smaller charges. We also find that, in their simplest form, dilaton models cannot explain the size of the excess. Some implications for flavor physics are briefly discussed.

hep-ph↗

arXiv2016

Large BR(h->tau mu) in the MSSM?

We study how large the rate of the lepton-flavor violating Higgs decay h->tau mu can be in the (R-parity conserving) MSSM. We make no assumptions, such as universality or alignment, about the flavor structure of the MSSM. We only assume that all couplings and, in particular, the trilinear scalar ones, are perturbative. We take into account lower bounds on the bino and slepton masses from tau->mu gamma and h->gamma gamma as well as upper bounds on the trilinear scalar couplings from the requirement that the global minimum is not charge breaking. We find that in highly fine-tuned regions of parameter space, the ratio BR(h->tau mu)/BR(h->tau tau) can be enhanced by about three orders of magnitude above the estimate from naive dimensional analysis, but still about two orders of magnitude below the present bound. Thus, if h->tau mu is experimentally established to be close to present bounds, the MSSM will be excluded.

hep-ph↗

arXiv2016

Prospects for measuring the Higgs coupling to light quarks

We discuss the prospects to probe the light-quark Yukawa couplings to the Higgs boson. The Higgs coupling to the charm quark can be probed both via inclusive and exclusive approaches. On the inclusive frontier, we use our recently proposed method together with published experimental studies for the sensitivity of the Higgs coupling to bottom quarks to find that the high-luminosity LHC can be sensitive to modifications of the charm Yukawa of the order of a few times its standard model (SM) value. We also present a preliminary study of this mode for a 100 TeV hadronic machine (with similar luminosity) and find that the bound can be further improved, possibly within the reach of the expected signal in the SM. On the exclusive frontier, we use the recent ATLAS search for charmonia and photon final state. This study yields the first measurement of the background relevant to these modes. Using this background measurement we project that at the high-luminosity LHC, unless the analysis strategy is changed, the sensitivity of the exclusive final state to the charm Yukawa to the charm Yukawa will be rather poor, of the order of 50 times the SM coupling. We then use a Monte-Carlo study to rescale the above backgrounds to the h->phi gamma case and obtain a much weaker sensitivity to the strange Yukawa, of order of 3000 times the SM value. We briefly speculate what would be required to improve the prospects of the exclusive modes.

hep-ph↗

arXiv2015

Heavy baryons as polarimeters at colliders

In new-physics processes that produce b or c jets, a measurement of the initial b or c-quark polarization could provide crucial information about the structure of the new physics. In the heavy-quark limit, the b and c-quark polarizations are preserved in the lightest baryons they hadronize into, Lambda_b and Lambda_c, respectively. We revisit the prediction for the polarization retention after the hadronization process and extend it to the case of transverse polarization. We show how ATLAS and CMS can measure the b-quark polarization using semileptonic Lambda_b decays, and the c-quark polarization using Lambda_c+ -> p K- pi+ decays. For calibrating both measurements we suggest to use ttbar samples in which these polarizations can be measured with precision of order 10% using 100/fb of data in Run 2 of the LHC. Measurements of the transverse polarization in QCD events at ATLAS, CMS and LHCb are motivated as well. The proposed measurements give access to nonperturbative QCD parameters relevant to the dynamics of the hadronization process.

hep-ph↗

arXiv2015

Quantum Electrodynamics in d=3 from the epsilon-expansion

We study Quantum Electrodynamics in d=3 (QED_3) coupled to N_f flavors of fermions. The theory flows to an IR fixed point for N_f larger than some critical number N_f^c. For N_f<= N_f^c, chiral-symmetry breaking is believed to take place. In analogy with the Wilson-Fisher description of the critical O(N) models in d=3, we make use of the existence of a perturbative fixed point in d=4-2epsilon to study the three-dimensional conformal theory. We compute in perturbation theory the IR dimensions of fermion bilinear and quadrilinear operators. For small N_f, a quadrilinear operator can become relevant in the IR and destabilize the fixed point. Therefore, the epsilon-expansion can be used to estimate N_f^c. An interesting novelty compared to the O(N) models is that the theory in d=3 has an enhanced symmetry due to the structure of 3d spinors. We identify the operators in d=4-2epsilon that correspond to the additional conserved currents at d=3 and compute their infrared dimensions.

hep-th↗

arXiv2015

Constraining the Charm Yukawa and Higgs-quark Coupling Universality

We introduce four different types of data-driven analyses with different level of robustness that constrain the size of the Higgs-charm Yukawa coupling: (i) recasting the vector-boson associated, Vh, analyses that search for bottom-pair final state. We use this mode to directly and model independently constrain the Higgs to charm coupling, y_c/y_c^{SM} < 234; (ii) the direct measurement of the total width, y_c/y_c^{SM} < 120-140; (iii) the search to h -> J/ψγ, y_c/y_c^{SM} < 220; (iv) a global fit to the Higgs signal strengths, y_c/y_c^{SM} < 6.2. A comparison with t\bar{t}h data allows us to show that current data eliminates the possibility that the Higgs couples to quarks in a universal way, as is consistent with the Standard Model (SM) prediction. Finally, we demonstrate how the experimental collaborations can further improve our direct bound by roughly an order of magnitude by charm-tagging, as already used in new physics searches.

hep-ph↗

arXiv2015

Dark Matter and Gauged Flavor Symmetries

We investigate the phenomenology of flavored dark matter (DM). DM stability is guaranteed by an accidental ${\mathcal Z}_3$ symmetry, a subgroup of the standard model (SM) flavor group that is not broken by the SM Yukawa interactions. We consider an explicit realization where the quark part of the SM flavor group is fully gauged. If the dominant interactions between DM and visible sector are through flavor gauge bosons, as we show for Dirac fermion flavored DM, then the DM mass is bounded between roughly $0.5$ TeV and $5$ TeV if the DM multiplet mass is split only radiatively. In general, however, no such relation exists. We demonstrate this using scalar flavored DM where the main interaction with the SM is through the Higgs portal. For both cases we derive constraints from flavor, cosmology, direct and indirect DM detection, and collider searches.

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↗

arXiv2014

Stealth QCD-like Strong Interactions and the t-tbar Asymmetry

We show that a new strongly interacting sector can produce large enhancements of the t-tbar asymmetries at the Tevatron. The Standard Model is extended by a new vector-like flavor triplet of fermions and one heavy scalar, all charged under a hypercolor gauge group SU(3) HC. This simple extension results in a number of new resonances. The predictions of our model are rather rigid once a small number of UV parameters are fixed, since all the strong dynamics can be directly taken over from our understanding of QCD dynamics. Despite the rather low hypercolor confinement scale of ~100 GeV, the new strongly interacting sector is stealth. It is shielded from present direct and indirect NP searches since the light resonances are QCD singlets, whereas the production of the heavier QCD colored resonances leads predominantly to high multiplicity final states. Improved searches can potentially be devised using top tagged final states or decays into a small number of hypercolor pions.

hep-ph↗

arXiv2014

B_{s,d} -> l+ l- in the Standard Model with Reduced Theoretical Uncertainty

We combine our new results for the O(alpha_em) and O(alpha_s^2) corrections to B_{s,d} -> l^+ l^-, and present updated branching ratio predictions for these decays in the standard model. Inclusion of the new corrections removes major theoretical uncertainties of perturbative origin that have just begun to dominate over the parametric ones. For the recently observed muonic decay of the B_s meson, our calculation gives BR(B_s -> mu^+ mu^-) = (3.65 +_ 0.23) * 10^(-9).

hep-ph↗

arXiv2013

Electroweak Corrections to $B_{s,d} \to \ell^+ \ell^-$

We calculate the full two-loop electroweak matching corrections to the operator governing the decay B_q --> l^+ l^- in the Standard Model. Their inclusion removes an electroweak scheme and scale uncertainty of about 7% of the branching ratio. Using different renormalization schemes of the involved electroweak parameters, we estimate residual perturbative electroweak and QED uncertainties to be less than 1% at the level of the branching ratio.

hep-ph↗

arXiv2013

Discrete Flavour Groups, θ_13 and Lepton Flavour Violation

Discrete flavour groups have been studied in connection with special patterns of neutrino mixing suggested by the data, such as Tri-Bimaximal mixing (groups A4, S4...) or Bi-Maximal mixing (group S4...) etc. We review the predictions for sin(θ_13) in a number of these models and confront them with the experimental measurements. We compare the performances of the different classes of models in this respect. We then consider, in a supersymmetric framework, the important implications of these flavour symmetries on lepton flavour violating processes, like μ-> e gamma and similar processes. We discuss how the existing limits constrain these models, once their parameters are adjusted so as to optimize the agreement with the measured values of the mixing angles. In the simplified CMSSM context, adopted here just for indicative purposes, the small tan(beta) range and heavy SUSY mass scales are favoured by lepton flavour violating processes, which makes it even more difficult to reproduce the reported muon g-2 discrepancy.

hep-ph↗

arXiv2012

Phenomenology of a Gauged SU(3)^3 Flavour Model

We present an extensive analysis of Delta F=2 observables and of B -> X_s gamma in the framework of a specific Maximally Gauged Flavour (MGF) model of Grinstein et al. including all relevant contributions, in particular tree-level heavy gauge boson exchanges that are considered in the present paper for the first time. The model allows in principle for significant deviations from the Standard Model predictions for epsilon_K, Delta M_{B_{d,s}}, mixing induced CP-asymmetries S_{psi K_S} and S_{psi phi} and B -> X_s gamma decay. The tension between epsilon_K and S_{psi K_S} present in the SM can be removed by enhancing |epsilon_K| without modifying S_{psi K_S}. In this case, we find that in this model i) the results for S_{psi phi} and B -> X_s gamma turn out to be SM-like, ii) the exclusive determination of |Vub| is favoured and most importantly iii) the values of Delta M_{B_d} and Delta M_{B_s} being strongly correlated in this model with epsilon_K turn out to be much larger than the data for the central values of input parameters: Delta M_{B_d}~0.75/ps and Delta M_{B_s}~27/ps. Therefore, from the present perspective, the model suffers from a serious epsilon_K-Delta M_{B_{d,s}} tension. However, this tension can be softened considering theoretical and parametric uncertainties and in particular the decrease of the weak decay constants. On the other side, the model can be strongly constrained considering the theoretically cleaner ratios Delta M_{B_d}/Delta M_{B_s} and BR(B^+ -> tau^+ nu)/Delta M_{B_d} and we find that it is unable to remove simultaneously all the SM tensions on the data. Finally, we compare the pattern of flavour violation in MGF with selected extensions of the SM.

hep-ph↗