arXiv subjects

G. F. Giudice

Publications and source records attributed to G. F. Giudice.

At least 55 records · Page 3Linked to original sources

arXiv1998

Nucleosynthesis Bounds in Gauge-Mediated Supersymmetry Breaking Theories

In gauge-mediated supersymmetry breaking theories the next-to-lightest supersymmetric particle can decay during or after the nucleosynthesis epoch. The decay products such as photons and hadrons can destroy the light element abundances. Restricting the damage that these decays can do leads to constraints on the abundance and lifetime of the NLSP. We compute the freezeout abundance of the NLSP by including all coannhilation thresholds which are particularly important in the case in which the NLSP is the lightest stau. We find that the upper bound on the messenger scale can be as stringent as 10^12 GeV when the NLSP is the lightest neutralino and 10^13 GeV when the NLSP is the lightest stau. Our findings disfavour models of gauge mediation where the messenger scale is close to the GUT scale or results from balancing renormalisable interactions with non-renormalisable operators at the Planck scale. When combined with the requirement of no gravitino overabundance, our bound implies that the reheating temperature after inflation must be less than 10^7 GeV.

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arXiv1998

QED Logarithms in the Electroweak Corrections to the Muon Anomalous Magnetic Moment

We employ an effective Lagrangian approach to derive the leading-logarithm two-loop electroweak contributions to the muon anomalous magnetic moment, a_mu. We show that these corrections can be obtained using known results on the anomalous dimensions of composite operators. We confirm the result of Czarnecki et al. for the bosonic part and present the complete sin^2 θ_W dependence of the fermionic contribution. The approach is then used to compute the leading-logarithm three-loop electroweak contribution to a_mu. Finally we derive, in a fairly model-independent way, the QED improvement of new-physics contributions to a_mu and to the electric dipole moment (EDM) of the electron. We find that the QED corrections reduce the effect of new physics at the electroweak scale by 6% (for a_mu) and by 11% (for the electron EDM).

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arXiv1998

Supersymmetry-Breaking Loops from Analytic Continuation into Superspace

We extend to all orders in perturbation theory a method to calculate supersymmetry-breaking effects by analytic continuation of the renormalization group into superspace. A central observation is that the renormalized gauge coupling can be extended to a real vector superfield, thereby including soft breaking effects in the gauge sector. We explain the relation between this vector superfield coupling and the "holomorphic" gauge coupling, which is a chiral superfield running only at 1 loop. We consider these issues for a number of regulators, including dimensional reduction. With this method, the renormalization group equations for soft supersymmetry breaking terms are directly related to supersymmetric beta functions and anomalous dimensions to all orders in perturbation theory. However, the real power of the formalism lies in computing finite soft breaking effects corresponding to high-loop component calculations. We prove that the gaugino mass in gauge-mediated supersymmetry breaking is ``screened'' from strong interactions in the messenger sector. We present the complete next-to-leading calculation of gaugino masses (2 loops) and sfermion masses (3 loops) in minimal gauge mediation, and several other calculations of phenomenological relevance.

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arXiv1998

Next-to-Leading QCD Corrections to B -> X_s gamma: Standard Model and Two-Higgs Doublet Model

We present the QCD corrections to the matching conditions of the Delta B =1 magnetic and chromo-magnetic operators in the Standard Model and in two-Higgs doublet models. We use an off-shell matching procedure which allows us to perform the computation using Taylor series in the external momenta, instead of asymptotic expansions. In the Standard Model case, we confirm previous results derived on-shell and we obtain BR(B -> X_s gamma)=(3.62\pm 0.33) 10^{-4}. In the case of the usual two-Higgs doublet model, we show that going from the leading to the next-to-leading order result improves the CLEO bound on the charged-Higgs mass from 260 GeV to 380 GeV. This limit is very sensitive to the definition of errors and we carefully discuss the theoretical uncertainties. Finally, in the case of the two-Higgs doublet model in which both up- and down-type quarks couple to the same Higgs field, the theoretical prediction for BR(B -> X_s gamma) can be reduced by at most 20% with respect to the Standard Model value.

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arXiv1997

Extracting Supersymmetry-Breaking Effects from Wave-Function Renormalization

We show that in theories in which supersymmetry breaking is communicated by renormalizable perturbative interactions, it is possible to extract the soft terms for the observable fields from wave-function renormalization. Therefore all the information about soft terms can be obtained from anomalous dimensions and beta functions, with no need to further compute any Feynman diagram. This method greatly simplifies calculations which are rather involved if performed in terms of component fields. For illustrative purposes we reproduce known results of theories with gauge-mediated supersymmetry breaking. We then use our method to obtain new results of phenomenological importance. We calculate the next-to-leading correction to the Higgs mass parameters, the two-loop soft terms induced by messenger-matter superpotential couplings, and the soft terms generated by messengers belonging to vector supermultiplets.

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arXiv1997

What if Charged Current Events at Large Q2 are Observed at HERA ?

An excess of events at large Q2 with a positron in the final state has been observed at HERA which, if confirmed, would be a signal of new physics. It is not clear at present if a signal of comparable rate is also seen in the charged current channel (with an antineutrino in the final state). In this note we analyse the implications of the presence of such a signal in models of new physics based on contact terms, leptoquarks and squarks with R-violating decays. We find that in all cases the most likely possibility is that the charged current signal is absent. As a consequence if this signal is present the resulting indications are very selective. In particular for squarks only charged current events with multi-quark final states are possible with quite definite predictions on the spectrum of supersymmetric particles.

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arXiv1997

R-Parity Violation and Unification

The reported anomaly in deep-inelastic scattering at HERA has revived interest in the phenomenology of R-parity violation. From the theoretical point of view, the existence of R-violating interactions poses two considerable problems. The first one concerns the flavour structure of the interactions and the origin of an appropriate suppression of flavour-changing neutral-current processes and lepton-family transitions. The second one concerns the way of embedding R-violating interactions in a grand unified theory (GUT) without introducing unacceptable nucleon decay rates. We show that the second problem can be solved by a mechanism which is purely group theoretical and does not rely on details of the flavour theory. We construct explicit GUT models in which our mechanism can be realized.

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arXiv1997

Pursuing interpretations of the HERA large-Q2 data

We explore interpretations of the anomaly observed by H1 and ZEUS at HERA in deep-inelastic e^+ p scattering at very large Q^2. We discuss the possibilities of new effective interactions and the production of a narrow state of mass 200 GeV with leptoquark couplings. We compare these models with the measured Q^2 distributions: for the contact terms, constraints from LEP2 and the Tevatron allow only a few choices of helicity and flavour structure that could roughly fit the HERA data. The data are instead quite consistent with the Q^2 distribution expected from a leptoquark state. We study the production cross sections of such a particle at the Tevatron and at HERA. The absence of a signal at the Tevatron disfavours the likelihood that any such leptoquark decays only into e^+ q. We then focus on the possibility that the leptoquark is a squark with R-violating couplings. In view of the present experimental limits on such couplings, the most likely production channels are e^+d -> scharm_L or perhaps e^+d->stop, with e^+s->stop a more marginal possibility. Possible tests of our preferred model include the absence both of analogous events in e^- p collisions and of charged current events, and the presence of detectable cascade decays whose kinematical signatures we discuss. We also discuss the possible implications for K->pi nu nubar, neutrinoless double-beta decay, the Tevatron and for e^+ e^- ->q qbar and neutralinos at LEP2.

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arXiv1996

Four-jet Signal at LEP2 and Supersymmetry

ALEPH has reported a significant excess of four-jet events in the LEP runs above the Z^0 resonance, which however has not been confirmed by the other LEP collaborations. We assume here that this excess corresponds to a physics signal and try to interpret it in the context of supersymmetric models with R-parity violation. Associated production of a left and right selectron can explain all the distinctive features of the ALEPH data: the value of the cross section, the di-jet mass difference, the absence of bottom quarks in the final state, and the di-jet charge content. Our proposed scenario makes definite predictions which can be tested at future LEP runs at higher energies.

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arXiv1996

Constraints on Supersymmetric Models from the Muon Anomalous Magnetic Moment

We study the impact of present and future $(g-2)_μ$ measurements on supersymmetric models. The corrections to $(g-2)_μ$ become particularly relevant in the presence of light sleptons, charginos and neutralinos, especially in the large $\tanβ$ regime. For moderate or large values of $\tanβ$, it is possible to rule out scenarios in which charginos and sneutrinos are both light, but nevertheless escape detection at the LEP2 collider. Furthermore, models in which supersymmetry breaking is transferred to the observable sector through gauge interactions can be efficiently constrained by the $(g-2)_μ$ measurement.

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arXiv1996

Multi-Messenger Theories of Gauge-Mediated Supersymmetry Breaking

We study gauge-mediated theories containing several messengers with the most general SU(5)-invariant mass and supersymmetry-breaking parameters. We show that these theories are predictive, containing only two relevant parameters more than the minimal gauge-mediated model. Hypercharge D-terms can contribute significantly to the right-handed charged sleptons and bring them closer in mass to the left-handed sleptons. The messenger masses must be invariant under either SU(5) or a ``messenger parity" to avoid spontaneous breaking of charge conservation.

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arXiv1996

Dark Matter in Theories of Gauge-Mediated Supersymmetry Breaking

In gauge-mediated theories supersymmetry breaking originates in a strongly interacting sector and is communicated to the ordinary sparticles via SU(3)$\times$SU(2)$\times$U(1) carrying ``messenger'' particles. Stable baryons of the strongly interacting supersymmetry breaking sector naturally weigh $\sim$ 100 TeV and are viable cold dark matter candidates. They interact too weakly to be observed in dark matter detectors. The lightest messenger particle is a viable cold dark matter candidate under particular assumptions. It weighs less than 5 TeV, has zero spin and is easily observable in dark matter detectors.

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arXiv1996

The Chromomagnetic Dipole Operator and the B Semileptonic Branching Ratio

We consider the possibility of having a large branching ratio for the decay $b\to s g$ coming from an enhanced Wilson coefficient of the chromomagnetic dipole operator. We show that values of $BR(b\to s g)$ up to $\sim 10\%$ or more are compatible with the constraints coming from the CLEO experimental results on $BR(B\to X_sγ)$ and $BR(B\to X_sϕ)$. Such large values can reconcile the predictions of both the semileptonic branching ratio and the charm counting with the present experimental results. We also discuss a supersymmetric model with gluino-mediated flavour violations, which can account for such large values of $BR(b\to s g)$.

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arXiv1996

The $μ$-Problem in Theories with Gauge-Mediated Supersymmetry Breaking

We point out that the $μ$-problem in theories in which supersymmetry breaking is communicated to the observable sector by gauge interactions is more severe than the one encountered in the conventional gravity-mediated scenarios. The difficulty is that once $μ$ is generated by a one-loop diagram, then usually $\bmu$ is also generated at the same loop order. This leads to the problematic relation $\bmu \sim μΛ$, where $Λ\sim$ 10--100 TeV is the effective supersymmetry-breaking scale. We present a class of theories for which this problem is naturally solved. Here, without any fine tuning among parameters, $μ$ is generated at one loop, while $\bmu$ arises only at the two-loop level. This mechanism can naturally lead to an interpretation of the Higgs doublets as pseudo-Goldstone bosons of an approximate global symmetry.

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arXiv1996

Macroscopic Forces from Supersymmetry

We argue that theories in which supersymmetry breaking originates at low energies often contain scalar particles that mediate coherent gravitational strength forces at distances less than a cm. We estimate the strength and range of these forces in several cases. Present limits on such forces are inadequate. However new techniques, such as those based on small cryogenic mechanical oscillators, may improve the present limits by ten orders of magnitude or discover new forces as weak as 1 \% of gravity at distances down to 40 microns.

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arXiv1996

Searches For New Physics

Review of prospects for discovery of new physics signals at LEP2. The areas covered include SUSY, exotic fermions, BESS models, leptoquarks, virtual effects and CP violating observables.

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arXiv1995

Naturalness Constraints in Supersymmetric Theories with Non-Universal Soft Terms

In the absence of universality the naturalness upper limits on supersymmetric particle masses increase significantly. The superpartners of the two light generations can be much heavier than the weak scale without extreme fine-tunings; they can weigh up to about 900 GeV --- or even up to 5 TeV, if SU(5) universality is invoked. This supresses sparticle-mediated rare processes and consequently ameliorates the problem of supersymmetric flavor violations. On the other hand, even without universality, the gluino and stop remain below about 400 GeV while the charginos and neutralinos are likely to be accessible at LEP2.

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arXiv1995

Disoriented and Plastic Soft Terms: A Dynamical Solution to the Problem of Supersymmetric Flavor Violations

We postulate that the orientation of the soft supersymmetry-breaking terms in flavor space is not fixed by tree level physics at the Planck scale; it is a dynamical variable which depends on fields that have no tree level potential. These fields can be thought of as either moduli or as the Nambu-Goldstone bosons of the spontaneously broken flavor symmetry which is non-linearly realized by the soft terms. We show that the soft terms align with the quark and lepton Yukawa couplings, just as spins align with an external magnetic field. As a result, the soft terms conserve individual lepton numbers and do not cause large flavor or CP violations. The vacuum adjusts so as to allow large sparticle splittings to naturally coexist with flavor conservation. Consequently, the resulting phenomenology is different from that of minimal supersymmetric theories. We also propose theories in which the shape of the soft terms in flavor space is a dynamical variable which depends on fields that have no tree level potential. This dynamically leads to partial degeneracy among sparticles and further supression of flavor violations. The ideas of this paper suggest a connection between the space of moduli and the spontaneously broken flavor group.

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