Scale Of New Physics Research Articles

Searching for an anomalous coupling via single top quark production at a γγ collider

We investigate the potential of a high-energy $\gamma\gamma$ collider to detect an anomalous $\bar t q \gamma$ coupling from observation of the reaction $\gamma \gamma \to t\bar q$, $\bar t q$, where $q=c$ or $u$. We find that with $b$-tagging and suitable kinematic cuts this process should be observable if the anomalous coupling $\kappa/\Lambda$ is no less than about 0.05/TeV, where $\Lambda$ is the scale of new physics associated with the anomalous interaction. This improves upon the bound possible from observation of top decays at the Tevatron.

Phase diagram of the U(2)×U(2) σ model and its implications for chiral hierarchies

Motivated by the issue of whether it is possible to construct phenomenologically viable models where the electroweak symmetry breaking is triggered by new physics at a scale $\Lambda \gg 4\pi v$, where $v$ is the order parameter of the transition ($v\sim 250$ GeV) and $\Lambda$ is the scale of new physics, we have studied the phase diagram of the $U(2) \times U(2)$ model. This is the relevant low energy effective theory for a class of models which will be discussed below. We find that the phase transition in these models is first order in most of parameter space. The order parameter can not be made much smaller than the cut-off and, consequently a large hierarchy does not appear sustainable. In the relatively small region in the space of parameters where the phase transition is very weakly first order or second order the model effectively reduces to the O(8) theory for which the triviality considerations should apply.

Unitarity constraints on anomalous top quark couplings to weak gauge bosons

If there is new physics associated with the top quark, it could show up as anomalous couplings of the top quark to weak gauge bosons, such as $Z\ttbar$ and $W\tbbar$ vector and axial-vector couplings. We use the processes $\ttbar\to Z^0Z^0$, $\ttbar\to W^+W^-$, and $\ttbar\to Z^0H$ to obtain the unitarity constraints on these anomalous couplings, and combine these constraints with those from precision electroweak data. The unitarity constraints can impose additional limits on the anomalous couplings when the scale of new physics is as low as 2 TeV. A nonzero measurement of such an anomalous coupling leads to an upper limit on the new physics scale from the unitarity condition.

Effective Lagrangian description of top quark production and decay.

We propose a rather general description of residual New Physics (NP) effects on the top quark couplings. These effects are described in terms of 20 gauge invariant $dim=6$ operators involving gauge and Higgs bosons as well as quarks of the third family. We compute their implications for the $\gamma t\bar t$, $Z t \bar t$ and $tbW$ vertices and study their observability in the process $e^-e^+ \to t\bar t$ with $t\to bW \to b\ell^+\nu_\ell$. We present results for the integrated cross section, the angular distribution and various decay distribution and polarization asymmetries for NLC energies of $0.5-2~TeV$. Observability limits are discussed and interpreted in terms of the NP scales associated to each operator through the unitarity constraints. The general landscape of the residual NP effects in the heavy quark and bosonic sectors is also presented.

Standard model stability bounds for new physics within LHC reach

We analyse the stability lower bounds on the Standard Model Higgs mass by carefully controlling the scale independence of the effective potential. We include resummed leading and next-to-leading-log corrections, and physical pole masses for the Higgs boson, M H , and the top-quark, M t . Particular attention is devoted to the cases where the scale of new physics Λ is within LHC reach, i.e. Λ ≤ 10 TeV, which have been the object of recent controversial results. We clarify the origin of discrepancies and confirm our earlier results within the error of our previous estimate. In particular for Λ = 1 TeV we find that M H[GeV]>52+0.64(M 1[GeV]−175)−0.50 α s(M Z−0.118 0.006 For fixed values of M t and α s ( M Z ), the error from higher effects, as the lack of exact scale invariance of the effective potential and higher-order radiative corrections, is conservatively estimated to be ≲ 5 GeV.

Anomalous couplings in single Higgs production through γγ collisions

We show that single Higgs production in $\gamma\gamma$ collisions through laser backscattering provides the best way to look for New Physics (NP) effects inducing anomalous Higgs couplings. Our analysis is based on the four $dim=6$ operators $\O_{UB}$, $\O_{UW}$, $\ol{\O}_{UB}$ and $\ol{\O}_{UW}$ which describe New Physics effects in this sector. Using the Higgs production rate we establish observability limits for the couplings of the aforementioned operators at a level of $10^{-3}$, which means establishing lower bounds on the scale of NP of the order 30 to 200 TeV. Higgs branching ratios, especially the ratio $\Gamma(H\to\gamma\gamma)/\Gamma (H\to\gamma Z)$, are shown to provide powerful ways to disentangle the effects of the various operators.

Tests of anomalous Higgs boson couplings through −e+ → ZH and γH

We show how the processes − e + → ZH, γH which will be studied at LEP2 and at high energy colliders, could be used to search for types of New Physics (NP) characterized by an effective NP scale in the few TeV range and affecting the scalar sector only. In particular, for e − e + → ZH, we propose the measurement of suitable azimuthal asymmetries determined from the angular distribution of the Z-decay plane with respect to the Z-production one. Such asymmetries, together with the ZH angular distribution may allow a complete disentangling of the five dimension-6 operators O UB, O UW, O UB, O UW, O φ2, describing the residual NP effects to the Higgs couplings. We note that the first four of these operators contribute to processes relevant to LEPI precision measurements only at the 1-loop level while the last operator remains completely unconstrained at this level. The process e − e + -+ γH is also very sensitive to NP and should bring important independent information.

Signatures of anomalous couplings in boson pair production through γγ collisions

We discuss possible New Physics (NP) effects on the processes $\gamma\gamma \to W^+W^-$, $ZZ$, $Z\gamma$, $\gamma\gamma$, $HH$ which are observable in $\gamma\gamma$ collisions. Such collisions may be achieved through laser backscattering at a high energy $e^+e^-$ linear collider. To the extent that no new particles will be directly produced in the future colliders, it has already been emphasized that the new physics possibly hidden in the bosonic interactions, may be represented by the seven $dim=6$ operators $\O_W$, $\O_{B\Phi}$, $\O_{W\Phi}$, $\O_{UB}$, $\O_{UW}$, $\ol{\O}_{UB}$ and $\ol{\O}_{UW}$ (the last two ones being CP-violating). In this paper, we show that the above processes are sensitive to NP scales at the several TeV range, and we subsequently discuss the possibility to disentangle the effects of the various operators.

Higgs boson production at e+e− colliders: a model independent approach

We consider non-Standard Model physics effects using an effective lagrangian parameterization. We determine the operators whose effects are most significant and extract the sensitivity to the scale of new physics generated by the existing data. We then consider processes containing the Higgs particle in e + e − colliders as a probe for new physics effects, and demonstrate their usefulness in this area.

Anomalous couplings in single Higgs production throughγγ collisions

We show that single Higgs production inγγ collisions through laser backscattering provides the best way to look for New Physics (NP) effects inducing anomalous Higgs couplings. Our analysis is based on the fourdim=6 operators\({\mathcal{O}}_{UB} , {\mathcal{O}}_{UW, } \bar {\mathcal{O}}_{UB} \), and\(\bar {\mathcal{O}}_{UW} \) which describe New Physics effects in this sector. Using the Higgs production rate we establish observability limits for the couplings of the aforementioned operators at a level of 10−3, which means establishing lower bounds on the scale of NP of the order 30 to 200 TeV. Higgs branching ratios, especially the ratio Γ(H→γγ)/Γ(H→γZ), are shown to provide powerful ways to disentangle the effects of the various operators.

Signatures of anomalous couplings in boson pair production throughγγ collisions

We discuss possible New Physics (NP) effects on the processesγγ →W+W−,ZZ, Zγ, γγ, HH which are observable inγγ collisions. Such collisions may be achieved through laser backscattering at a high energye+e− linear collider. To the extent that no new particles will be directly produced in the future colliders, it has already been emphasized that the new physics possibly hidden in the bosonic interactions, may be represented by the sevendim=6 operatorsOW,OBΦ,OWΦ,OUB,OUW\(\bar {\mathcal{O}}_{UB} \) and\(\bar {\mathcal{O}}_{UW} \) (the last two ones being CP-violating). In this paper, we show that the above processes are sensitive to NP scales at the several TeV range, and we subsequently discuss the possibility to disentangle the effects of the various operators.

On quadratic divergences and the Higgs mass

The quadratic divergences in the scalar sector of the standard model are considered. Since the divergences are present also in the unbroken theory, a natural scale for the divergence formula is proposed to be at the scale of new physics. The implications of top quark mass on the Higgs mass are investigated by means of the renormalization group equations. The Coleman-Weinberg mechanism for spontaneous symmetry breaking is also considered.

Relation between the fermion pole mass and MS-bar Yukawa coupling in the standard model.

We present the full one-loop correction to the relation between the running MSbar Yukawa coupling, $\overline h_f(\mu)$, at some renormalization scale, $\mu$, and the pole mass, $m_f$, of a fermion, $f$, in the standard model. Our result complements previous analyses that included just the QCD correction to this relation in the case of quarks. This allows us to convert, without loss of information, the threshold value, $\overline h_f(m_f)$, of $\overline h_f(\mu)$, which satisfies a renormalization-group equation and may be driven up to some high-energy scale of new physics, to physical observables at low energy. We investigate the limit of the Higgs boson and/or the top quark being much heavier than all the other particles. We also list simple and yet rather precise approximation formulae, which are convenient for applications.

Vacuum stability in the singlet Majoron model

We study the vacuum stability of the singlet Majoron model using full renormalization group improved scalar potential and Monte Carlo techniques. We show in the perturbative regime of the various free parameters, the vacuum stability requirement together with LEP limits is passed by 18% of the parameter space if the scale of new physics is 10 TeV and 6% if the scale is 10 14 GeV. Moreover, if the baryogenesis condition for scalar couplings is required, no portion of the parameter space survives.

Mass matrix models: the sequel

The smallness of the quark sector parameters and the hierarchy between them could be the result of a horizontal symmetry broken by a small parameter. Such an explicitly broken symmetry can arise from an exact symmetry which is spontaneously broken. Constraints on the scales of new physics arise from new flavor-changing interactions and from Landau poles, but do not exclude the possibility of observable signatures at the TeV scale. Such a horizontal symmetry could also lead to many interesting results: (i) quark—squark alignment that would suppress, without squark degeneracy, flavor-changing neutral currents induced by supersymmetric particles, (ii) exact relations between mass ratios and mixing angles, (iii) a solution of the μ-problem and (iv) a natural mechanism for obtaining hierarchy among various symmetry-breaking scales.

Top quark production: Sensitivity to new physics.

The production cross section and distributions of the top quark are sensitive to new physics; e.g., the $t\overline{t}$ system can be a probe of new resonances or gauge bosons that are strongly coupled to the top quark, in analogy with Drell-Yan production. The existence of such new physics is expected in dynamical electroweak symmetry-breaking schemes, and associated with the large mass of the top quark. The total top quark production cross section can be more than doubled, and distributions significantly distorted with a chosen scale of new physics of \ensuremath{\sim} 1 TeV in the vector color singlet or octet $s$ channel. New resonance physics is most readily discernible in the high-${p}_{T}$ distributions of the single top quark and of the $W$ boson, and the mass distribution of the $t\overline{t}$ pair.

A global fit to extended oblique parameters

The STU formalism of Peskin and Takeuchi is an elegant method for encoding the measurable effects of new physics which couples to light fermions dominantly through its effects on electroweak boson propagation. However, this formalism cannot handle the case where the scale of new physics is not much larger than the weak scale. In this case three new parameters ( V, W and X) are required. A global fit to precision electroweak data for these six parameters is performed. Our results differ from what is found for just STU. In particular we find that the preference for S<0 is no longer statistically significant.

Anomalous neutrino reactions at DESY HERA

We study the sensitivity of HERA to new physics using the helicity suppressed reaction $e_R p \rightarrow \nu X $, where the final neutrino can be a standard model one or a heavy neutrino. The approach is model independent and is based on an effective lagrangian parametrization. It is shown that HERA will put significant bounds on the scale of new physics, though, in general, these are more modest than previously thought. If deviations from the standard model are observed in the above processes, future colliders such as the SSC and LHC will be able to directly probe the physics responsible for these discrepancies}

Implications of naturalness in effective field theory on the masses of resonances.

Many years ago Weinberg formulated a definition of ``naturalness'' for effective theories: if an effective theory is to make sense, coefficients must not change too much when the cutoff scale is changed by a factor of order 1. As an example we consider simple field theories in which an O(N) symmetry spontaneously breaks to O(N-1). We show that in these theories Weinberg's criterion for a natural effective theory may be applied directly to the S matrix; it implies that the scale of new physics, beyond the Goldstone bosons, may not be too large: there is always a particle or a cut of mass below or about 4\ensuremath{\pi}f/ \ensuremath{\surd}N . We discuss the range of convergence of the expansion of the chiral Lagrangian. It appears to be impossible to construct an underlying theory of the type considered here that fails to satisfy Weinberg's criterion.

Top quark condensation from broken family symmetry

We investigate the vacuum alignment in a fully dynamical “moose” model which breaks electroweak symmetry with a top quark condensate and show how the prefered vacuum depends on the explicit masses of the fermions involved. Our results are also applicable to the cases in which electroweak symmetry is broken by a fourth family of quarks, or by a technicolour mechanism. Such moose models allow the scale of new physics, Λ, to be in the TeV range with the absence of flavour changing neutral currents ensured by a GIM mechanism.