arXiv subjects

D. Comelli

Publications and source records attributed to D. Comelli.

At least 37 records · Page 2Linked to original sources

arXiv2000

Bloch-Nordsieck violating electroweak corrections to inclusive TeV scale hard processes

We point out that, since the colliders initial states (e+ e-,p p, p pbar, ... ) carry a definite nonabelian flavor, electroweak radiative corrections to inclusive hard cross sections at the TeV scale are affected by peculiar Bloch-Nordsieck violating double logs. We recall the setup of soft cancellation theorems, and we analyze the magnitude of the noncancelling terms in the example of electron - positron annihilation into hadrons.

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arXiv1999

Logarithmic expansion of electroweak corrections to four-fermion processes in the TeV region

Starting from a theoretical representation of the electroweak component of four-fermion neutral current processes that uses as theoretical input the experimental measurements at the Z peak, we consider the asymptotic high energy behaviour in the Standard Model at one loop of those gauge-invariant combinations of self-energies, vertices and boxes that contribute all the different observables. We find that the logarithmic contribution due to the renormalization group running of the various couplings is numerically overwhelmed by single and double logarithmic terms of purely electroweak (Sudakov-type) origin, whose separate relative effects grow with energy, reaching the 10% size at about one TeV. We then propose a simple "effective" parametrization that aims at describing the various observables in the TeV region, and discuss its validity both beyond and below 1 TeV, in particular in the expected energy range of future linear electron-positron (LC) and muon-muon colliders.

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arXiv1999

Electroweak Sudakov form factors and nonfactorizable soft QED effects at NLC energies

We study the leading log infrared behavior of electroweak corrections at TeV scale energies, that will be reached by next generation of linear colliders (NLC). We show that, contrary to what happens at typical LEP energies, it is not anymore possible to disentangle ``pure electroweak'' from ``photonic'' corrections. This means that soft QED effects do not factorize and therefore cannot be treated in the usual ``naive'' way they were accounted for in the LEP-era. The nonfactorizable effects come up first at the two loop LL level, that we calculate explicitly for a fermion source that is neutral under the SU(2)$\otimes$U(1) gauge group (explicitly, a Z' decay into two fermions). The basic formalism we set up can be used to calculate LL effects at any order of perturbation theory. The results of this paper might be important for future calculations of electroweak corrections at NLC energies.

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arXiv1999

The role of the top mass in b-production at future lepton colliders

We compute the one loop contribution coming from vertex and box diagrams, where virtual top quarks are exchanged, to the asymptotic energy behaviour of $b\bar b$ pair production at future lepton colliders. We find that the effect of the top mass is an extra linear logarithmic term of Sudakov type that is not present in the case of (u,d,s,c) production. This appears to be particularly relevant in the case of the $b\bar{b}$ cross section.

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arXiv1999

Susy Virtual effects at LEP2 Boundary

We examine the possibility that SUSY particles are light, i.e. have a mass just beyond the final kinematical reach of LEP2. In this case, even if light particles are not directly detected, their virtual effects are enhanced by a ``close to threshold'' resonance in the s-channel. We find that this resonant effect is absent in the case of light sfermions, while it is enhanced in the case of light gauginos, since neutralinos and charginos add coherently in some regions of the allowed parameter space. We discuss this ``virtual-alliance'' in detail and try to examine the possibilities of its experimental verification.

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arXiv1999

Flatons and Peccei-Quinn Symmetry

We study in detail a supersymmetric Peccei-Quinn model, which has a DFSZ and a KSVZ version. The fields breaking the Peccei-Quinn (PQ) symmetry correspond to flat directions (flaton fields) and have unsuppressed couplings when PQ symmetry is unbroken. The models have interesting particle physics phenomenology. The PQ scale is naturally generated through radiative corrections; also, in the DFSZ case the $μ$ problem can be solved and neutrino masses can be generated. Cosmologically they lead to a short period of thermal inflation making the axion an excellent dark matter candidate if one of the flaton fields has a positive effective mass-squared at early times but with too low a density in the opposite case. A highly relativistic population of axions is produced by flaton decay during the subsequent reheating, whose density is constrained by nucleosynthesis. We evaluate all of the relevant reaction rates and evaluate the nucleosynthesis constraint. We find that the KSVZ model is practically ruled out, while the DFSZ model has a sizable allowed region of parameter space.

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arXiv1999

The Sphaleron in a Magnetic Field and Electroweak Baryogenesis

The presence of a primordial magnetic field in the early universe affects the dynamic of the electroweak phase transition enhancing its strength. This effect may enlarge the window for electroweak baryogenesis in the minimal supersymmetric extension of the standard model or even resurrect the electroweak baryogenesis scenario in the standard model. We compute the sphaleron energy in the background of the magnetic field and show that, due to the sphaleron dipole moment, the barrier between topologically inequivalent vacua is lowered. Therefore, the preservation of the baryon asymmetry calls for a much stronger phase transition than required in the absence of a magnetic field. We show that this effect overwhelms the gain in the phase transition strength, and conclude that magnetic fields do not help electroweak baryogenesis.

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arXiv1999

Sudakov Effects in Electroweak Corrections

In perturbation theory the infrared structure of the electroweak interactions produces large corrections proportional to double logarithms log^2 (s/m^2), similar to Sudakov logarithms in QED, when the scale s is much larger than the typical mass m of the particles running in the loops. These energy growing corrections can be particularly relevant for the planned Next Linear Colliders. We study these effects in the Standard Model for the process e+ e^->f fbar and we compare them with similar corrections coming from SUSY loops.

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arXiv1998

Field Evolution leading to hybrid inflation

In general the onset of hybrid inflation requires an extremely homogeneous field configuration at the end of Planck era. The field ϕorthogonal to the inflaton σmust be zero with high accuracy over the range exceeding the initial horizon size \sim m_{Pl}^{-1} by about two orders of magnitude. We consider a supersymmetric model that permits the decay of the oscillating field ϕinto light particles. We study the field evolution and find that the requirement of the extreme homogeneity can be relaxed. However, the field ϕmust still be smaller than the inflaton by a factor of order 1 over a region far exceeding the initial horizon size.

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arXiv1996

Hard $m_t$ Corrections as a Probe of the Symmetry Breaking Sector

Non-decoupling effects related to a large $m_t$ affecting non-oblique radiative corrections in vertices ($Z\bar{b}b$) and boxes ($B$-$\bar{B}$ mixing and $ε_K$) are very sensitive to the particular mechanism of spontaneous symmetry breaking. We analyze these corrections in the framework of a chiral electroweak standard model and find that there is only one operator in the effective lagrangian which modifies the longitudinal part of the $W^+$ boson without touching the oblique corrections. The inclusion of this operator affects the $Z\bar{b}b$ vertex, the $B$-$\bar{B}$ mixing and the CP-violating parameter $ε_K$, generating interesting correlations among the hard $m_t^4 \log m_t^2$ corrections to these observables, for example, the maximum vertex $Z b\bar{b}$ correction allowed by low energy physics is about one percent.

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arXiv1996

New limits on the mass of neutral Higgses in General Models

In general electroweak models with weakly coupled (and otherwise arbitrary) Higgs sector there always exists in the spectrum a scalar state with mass controlled by the electroweak scale. A new and simple recipe to compute an analytical tree-level upper bound on the mass of this light scalar is given. We compare this new bound with similar ones existing in the literature and show how to extract extra information on heavier neutral scalars in the spectrum from the interplay of independent bounds. Production of these states at future colliders is addressed and the implications for the decoupling limit in which only one Higgs is expected to remain light are discussed.

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arXiv1996

Bosonic Thermal Masses in Supersymmetry

Effective thermal masses of bosonic particles in a plasma play an important role in many different phenomena. We compute them in general supersymmetric models at leading order. The origin of different corrections is explicitly shown for the formulas to be applicable when some particles decouple. The correct treatment of Boltzmann decoupling in the presence of trilinear couplings and mass mixing is also discussed. As a relevant example, we present results for the Minimal Supersymmetric Standard Model.

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arXiv1996

Decoupling or nondecoupling: is that the $R_b$ question?

The top quark is well known for the nondecoupling effects it implies in $ρ$ and $R_b$. The recent experimental $R_b$ data exhibits a disagreement with the SM prediction at more than the $3 σ$ level. It is tempting to explore whether this might be due to nondecoupling New Physics effects, opposite to those of the top. We investigate this issue in the context of models with an extra family of right or left handed, singlet or doublet quarks. It is shown that, contrary to what one might naively expect, the nondecoupling properties of a mirror $t^\prime$ do not have an impact on $R_b$, due to a conspiracy of the mixing angles, imposed by the requirement that there be no $b$ - $b^\prime$ mixing. Our analysis agrees with an analysis performed independently, which includes this model as a particular case.

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arXiv1995

Weak magnetic dipole moments in two-Higgs-doublet models

We investigate the effects of the new scalars in a two-Higgs-doublet model on the weak magnetic dipole moments of the fermions at the $Z$ peak. Proportionality of the Yukawa couplings to the fermion masses, and to $\tanβ$, makes such effects more important for the third family, and potentially relevant. For the $τ$ lepton, the new diagrams are suppressed by $v_τ= 2 \sin^2 θ_W - 1/2$, or by powers of $m_τ/M_Z$, but may still be comparable to the SM electroweak contributions. In contrast, we find that the new contributions for the bottom quark may be much larger than the SM electroweak contributions. These new effects may even compete with the gluonic contribution, if the extra scalars are light and $\tan β$ is large. We also comment on the problem of the gauge dependence of the vertex, arising when the $Z$ is off mass shell. We compute the contributions from the new scalars to the magnetic dipole moments for top-quark production at the NLC, and for bottom and $ τ$ production at LEP2. In the case of the top, we find that the SM electroweak and gluonic contributions to the $Z t {\bar t}$ vertex are comparable. The new contributions may be of the same order of magnitude as the standard-model ones, but not much larger.

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arXiv1995

Particle Currents in a Space-Time dependent and CP-violating Higgs Background: a Field Theory Approach

Motivated by cosmological applications like electroweak baryogenesis, we develop a field theoretic approach to the computation of particle currents on a space-time dependent and CP-violating Higgs background. We consider the Standard Model model with two Higgs doublets and CP violation in the scalar sector, and compute both fermionic and Higgs currents by means of an expansion in the background fields. We discuss the gauge dependence of the results and the renormalization of the current operators, showing that in the limit of local equilibrium, no extra renormalization conditions are needed in order to specify the system completely.

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arXiv1995

Particle Currents on a CP Violating Higgs Background and the Spontaneous Baryogenesis Mechanism

We compute the particle currents induced on a bubble wall background at finite temperature in a model with CP violation in the Higgs sector. Using a field theory approach we show that fermionic currents arise at one loop, so that a suppression factor ${\cal O}(h_t ϕ/πT)^2$ with respect to previous computations is found. The contributions to the Higgs currents are also derived and their relevancy for the spontaneous baryogenesis mechanism is discussed.

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arXiv1994

Implications of a light stop for the spontaneous CP breaking at finite temperature in a nonminimal supersymmetric standard model

We study the implications of a light stop for the spontaneous CP breaking at finite temperature in the Higgs sector of the Minimal Supersymmetric Standard Model with a gauge singlet. Assuming CP conservation at zero temperature, we show that the presence of a large mixing between the left- and the right-handed stops can trigger easily the spontaneous breaking of CP inside the bubbles nucleated during the electroweak phase transition. This allows to avoid the fine-tuning among the vacuum expectation values in the region of interest for the generation of the baryon asymmetry, namely the bubble walls, which has been recently analized.

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arXiv1994

On Spontaneous Baryogenesis and Transport Phenomena

The spontaneous baryogenesis mechanism by Cohen, Kaplan and Nelson is reconsidered taking into account the transport of particles inside the electroweak bubble walls. Using linear response theory, we calculate the modifications on the thermal averages of the charges of the system due to the presence of a space time dependent `charge potential' for a quantum number not orthogonal to baryon number. The local equilibrium configuration is discussed, showing that, as a consequence of non zero densities for conserved charges, the $B+L$ density is driven to non zero values by sphaleronic processes. Solving a rate equation for the baryon number generation, we obtain an expression for the final baryon asymmetry of the Universe containing the relevant parameters of the bubble wall, {\it i.e.} its velocity, its width, and the width of the region in which sphalerons are active. Compared to previous estimates in which transport effects were not taken into account, we find an enhancement of nearly three orders of magnitude in the baryon asymmetry. Finally, the role of QCD sphalerons in cooperation with transport effects is analyzed, showing that the net result depends crucially on the particle species which enter into the charge potential.

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