Interpreting top-quark LHC measurements in the standard-model effective field theory
This note proposes common standards and prescriptions for the effective-field-theory interpretation of top-quark measurements at the LHC.
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arXiv subjects
M. Fabbrichesi
Publications and source records attributed to M. Fabbrichesi.
Constraints on top quark non-standard interactions from Higgs and $t \bar t$ production cross sections
We identify the differential cross sections for $t\bar t$ production and the total cross section for Higgs production through gluon fusion as the processes in which the two effective operators describing the leading non-standard interactions of the top quark with the gluon can be disentangled and studied in an independent fashion. Current data on the Higgs production and the $ {\rm d}σ/{\rm d} {p^t_T}$ differential cross section provide limits comparable, but not more stringent, than those from the total $t\bar t$ cross sections measurements at the LHC and Tevatron, where however the two operators enter on the same footing and can only be constrained together. Given the present uncertainties, we find that the most stringent bounds are provided by a combination of data on the $t \bar t$ total cross sections together with those from the Higgs production. We conclude by stating the (modest) reduction in the uncertainties necessary to provide more stringent limits by means of the Higgs production and $t\bar t$ differential cross section observables at the LHC with the future luminosity of 300 and 3000 fb$^{-1}$.
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Hunting down massless dark photons in kaon physics
If dark photons are massless, they couple to standard-model particles only via higher dimensional operators, while the kinetic mixing with photons, which motivates most of the current experimental searches, is absent. We consider the effect of possible flavor-changing magnetic-dipole couplings of massless dark photons in kaon physics. In particular, we study the branching ratio for the process $K^+\rightarrow π^+π^0 \bar γ$ with a simplified-model approach, assuming the chiral quark model to evaluate the hadronic matrix element. Possible effects in the $K^0$-$\bar K^0$ mixing are taken into account. We find that branching ratios up to $O(10^{-7})$ are allowed---depending on the dark-sector masses and couplings. Such large branching ratios for $K^+\rightarrow π^+π^0 \bar γ$ could be of interest for experiments dedicated to rare $K^+$ decays like NA62 at CERN, where $\bar γ$ can be detected as a massless invisible system.
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Limits on anomalous top quark gauge couplings from Tevatron and LHC data
We review and update current limits on possible anomalous couplings of the top quark to gauge bosons. We consider data from top quark decay (as encoded in the $W$-boson helicity fractions) and single-top production (in the $t$-, $s$- and $Wt$-channels). We find improved limits with respect to previous results (in most cases of almost one order of magnitude) and extend the analysis to include four-quark operators. We find that new electroweak physics is constrained to live above an energy scale between 430 GeV and 3.2 TeV (depending on the form of its contribution); strongly interacting new physics is bounded by scales higher than 1.3 or 1.5 TeV (again depending on its contribution).
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Stringent limits on top-quark compositeness from top anti-top production at the Tevatron and the LHC
If the top quark is a composite state made out of some constituents, its interaction with the gluon will be modified. We introduce the leading effective operators that contribute to the radius and anomalous magnetic moment of the top quark and study their effect on the cross section for top anti-top production at the Tevatron and the LHC. Current measurements of the cross sections set a stringent limit on the scale of compositeness. This limit is comparable to similar limits obtained for light quarks and those from electroweak precision measurements. It can be used to constrain the parameter space of some composite Higgs models.
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Low-scale seesaw and dark matter
We discuss how two birds---the little hierarchy problem of low-scale type-I seesaw models and the search for a viable dark matter candidate---are (proverbially) killed by one stone: a new inert scalar state
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The little hierarchy problem for new physics just beyond the LHC
We discuss two possible extensions to the standard model in which an inert singlet scalar state that only interacts with the Higgs boson is added together with some fermions. In one model the fermions provide for a see-saw mechanism for the neutrino masses, in the other model for grand unification of the gauge couplings. Masses and interaction strengths are fixed by the requirement of controlling the finite one-loop corrections to the Higgs boson mass thus addressing the little hierarchy problem. The inert scalar could provide a viable dark matter candidate. Direct detection of this scalar singlet in nuclear scattering experiments is possible with a cross section within reach of future experiments.
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A simple inert model solves the little hierarchy problem and provides a dark matter candidate
We discuss a minimal extension to the standard model in which two singlet scalar states that only interacts with the Higgs boson is added. Their masses and interaction strengths are fixed by the two requirements of canceling the one-loop quadratic corrections to the Higgs boson mass and providing a viable dark matter candidate. Direct detection of the lightest of these new states in nuclear scattering experiments is possible with a cross section within reach of future experiments.
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Quark contact interactions at the LHC
Quark contact interactions are an important signal of new physics. We introduce a model in which the presence of a symmetry protects these new interactions from giving large corrections in flavor changing processes at low energies. This minimal model provides the basic set of operators which must be considered to contribute to the high-energy processes. To discuss their experimental signature in jet pairs produced in proton-proton colllisions, we simplify the number of possible operators down to two. We show (for a representative integrated luminosity of 200 pb^-1 at \surd s = 7 TeV) how the presence of two operators significantly modifies the bound on the characteristic energy scale of the contact interactions which is obtained by keeping a single operator.
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Asymptotic safety and the gauged SU(N) nonlinear sigma-model
We study the beta functions of the leading, two-derivative terms of the left-gauged SU(N) nonlinear sigma-model in d dimensions. In d>2, we find the usual Gaussian ultraviolet fixed point for the gauge coupling and an attractive non-Gaussian fixed point for the Goldstone boson coupling. The position of the latter fixed point controls the chiral expansion, unitarity and the strength of the tree-level Goldstone boson scattering amplitudes. Attention is paid to the gauge- and scheme-dependence of the results.
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Fermions and Goldstone bosons in an asymptotically safe model
We consider a model in which Goldstone bosons, described by a SU(N) chiral nonlinear sigma model, are coupled to an N-plet of colored fermions by means of a Yukawa interaction. We study the one-loop renormalization group flow and show that the non-Gaussian UV fixed point, which is present in the purely bosonic model, is lost because of fermion loop effects unless N is sufficiently large. We then add four-fermion contact interactions to the lagrangian and show that in this case there exist several non-Gaussian fixed points. The strength of the contact interactions, predicted by the requirement that the theory flows towards a fixed point in the UV, is compared to the current experimental bounds. This toy model could provide an important building block of an asymptotically safe model of the weak interactions.
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Telling the spin of the "Higgs boson" at the LHC
We assume that the Higgs boson or a possible resonance---playing its role in strongly interacting models of electroweak symmetry breaking---has been discovered at the LHC and propose a search strategy to determine its spin based on two simple asymmetries in the ZZ, W+W- and t t-bar decays channels. We consider some benchmark values for its mass (in the interval from 182 GeV/c^2 to 1 TeV/c^2) and discuss the relative advantages of the different decay processes. A full analysis, including the background, is given. For a center-of-mass energy of 14 TeV, we find that the lowest integrated luminosity required to discriminate between the different spins is, depending on the process and the resonance mass, between 40 fb^{-1} and 250 fb^{-1}.
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The electroweak S and T parameters from a fixed point condition
We consider the standard model without the Higgs boson, where the Goldstone modes are described by a nonlinear sigma model. We study the renormalization group flow of the sigma model coupling f and of the electroweak parameters S and T. The condition that the couplings reach a fixed point at high energy leaves the low energy values of f and T arbitrary (to be determined experimentally) and fixes S to a value compatible with electroweak precision data.
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Proceedings of the Workshop on Monte Carlo's, Physics and Simulations at the LHC PART II
These proceedings collect the presentations given at the first three meetings of the INFN "Workshop on Monte Carlo's, Physics and Simulations at the LHC", held at the Frascati National Laboratories in 2006. The first part of these proceedings contains pedagogical introductions to several basic topics of both theoretical and experimental high pT LHC physics. The second part collects more specialised presentations.
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Proceedings of the Workshop on Monte Carlo's, Physics and Simulations at the LHC PART I
These proceedings collect the presentations given at the first three meetings of the INFN "Workshop on Monte Carlo's, Physics and Simulations at the LHC", held at the Frascati National Laboratories in 2006. The first part of these proceedings contains pedagogical introductions to several basic topics of both theoretical and experimental high pT LHC physics. The second part collects more specialised presentations.
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Possible experimental signatures at the LHC of strongly interacting electro-weak symmetry breaking
If electro-weak symmetry is broken by a new strongly interacting sector, new physics will probably manifest itself in gauge boson scattering at the LHC. The relevant dynamics is well described in terms of an effective lagrangian. We discuss the probable size of the coefficients of the relevant operators under a combination of model-independent constraints and reasonable assumptions based on two models of the strongly interacting sector. We compare these values with LHC sensitivity and argue that they will be too small to be seen. Therefore, the presence of vector and scalar resonances required by unitarity will be the only characteristic signature. We analyze the most likely masses and widths of these resonances.
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Just so Higgs boson
We discuss a minimal extension to the standard model in which there are two Higgs bosons and, in addition to the usual fermion content, two fermion doublets and one fermion singlet. The little hierachy problem is solved by the vanishing of the one-loop corrections to the quadratic terms of the scalar potential. The electro-weak ground state is therefore stable for values of the cut off up to 10 TeV. The Higgs boson mass can take values significantly larger than the current LEP bounds and still be consistent with electro-weak precision measurements.
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The littlest Higgs is a cruiserweight
We study the exact (one-loop) effective potential of the littlest Higgs model and determine the dependence of physical quantities, such as the vacuum expectation value v_W and mass m_h of the Higgs boson, on the fundamental parameters of the Lagrangian--masses, couplings of new states, the fundamental scale f of the sigma model, and the coefficients of operators quadratically sensitive to the cutoff of the theory. On the one hand, we show that it is possible to have the electroweak ground state and a relatively large cutoff Λ= 4πf with f in the 2 TeV range without requiring unnaturally small coefficients for quadratically divergent quantities, and with only moderate cancellations between the contribution of different sectors to the effective potential of the Higgs. On the other hand, this cannot be achieved while at the same time keeping m_h close to its (bantamweight) current lower bound of 114.4 GeV. The natural expectation for m_h is O(f), mainly because of large logarithmically divergent contributions to the effective potential of the top-quark sector. Even a fine-tuning at the level of O(10^{-2}) in the coefficients of the quadratic divergences is not enough to produce small physical Higgs masses, and the natural expectation is in the 800 GeV range (cruiserweight) for f \sim 2 TeV. We conclude that the littlest Higgs model is a solution of the little hierarchy problem, in the sense that it stabilizes the electroweak symmetry breaking scale to be a factor of 100 less than the cutoff of the theory, but this requires a quite large physical mass for the Higgs, and hence precision electroweak studies should be redone accordingly. We also study finite temperature corrections.
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