arXiv subjects

A. Strumia

Publications and source records attributed to A. Strumia.

27 records · Page 2Linked to original sources

arXiv1998

Two-loop heavy top corrections to the b --> s gamma decay

We compute the two-loop corrections to the coefficient of the b --> s gamma magnetic penguin present in the limits of heavy top and/or heavy higgs. This kind of corrections affects in a significant way the observables measured at LEP. On the contrary we find that, due to a numerical accident, the correction to BR(B --> X_s gamma) is negligible (below the 1% level for any possible value of the higgs mass) when the leading order result is expressed in the usual way in terms of the semileptonic BR(B --> X_c \bar{e}ν).

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arXiv1997

Two-loop QCD corrections to charged-Higgs-mediated $b\to sγ$ decay

The charged-Higgs-mediated contribution to the Wilson coefficient of the $b\to sγ$ magnetic penguin is expected to be one of the more promising candidates for a supersymmetric effect in B physics, probably the only one in gauge-mediated models. We compute the two-loop QCD correction to it. With naive dimensional regularization and MSbar subtraction, for reasonable values of the charged Higgs mass and for mu-bar = m_top, we find a (10--20)% reduction of the corresponding one-loop effect.

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arXiv1996

On axion-mediated macroscopic forces again

In a supersymmetric unified theory or in a generic model where a large neutron electric dipole moment $d_N$ is expected, close to the present bound, we estimate the relation g_aNN = 10^{-21\pm1}(d_N/10^{-25} e cm)(10^10 GeV/f_a) between d_N itself, the scalar coupling g_{aNN} to nucleons of the axion, assumed to exist, and the breaking scale, f_a, of the Peccei-Quinn symmetry. Newly developing techniques to search for sub-cm macroscopic forces might reveal a signal due to axion exchange at least in a favorable range of f_a.

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arXiv1996

Charge and color breaking minima and constraints on the MSSM parameters

The MSSM potential can have unphysical minima deeper than the physically acceptable one. We point out that their presence is quite generic in SO(10) unification with supergravity mediated soft terms. However, at least for moderate values of tanβ, the physically acceptable vacuum has a life-time longer than the universe age. Furthermore, by discussing the evolution of the universe in an inflationary scenario, we show that the correct vacuum is the natural expectation. This is not the tendency in more general cases. Even in the SO(10) case, the weak assumptions necessary for this may prevent to use the \tildeνh^u direction for baryogenesis a la Affleck-Dine.

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arXiv1995

Electric dipole moments as signals of supersymmetric unification

If supersymmetric unification is true, we show how the combined effort of several experiments under way to try to measure an electric dipole moment of the electron or of the neutron has a significant chance not only of producing a positive signal but also of providing crucial information to understand the physical origin of the signal itself.

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arXiv1995

Light thresholds in Grand Unified Theories

In a generic Grand Unified Theory with a relatively small dispersion of the spectrum around the $Z$-boson and the unification masses, a connection is established, exact at one loop level, between $M_Z$, $G_{\rm F}$, $α(M_Z)$ and the strong coupling constant $α_3(M_Z)$. At this level of precision, this avoids the logical and phenomenological inconsistency of predicting $α_3(M_Z)$ by means of the electroweak couplings as extracted from the data in the Standard Model rather than in the complete theory. Attention is paid to the independence of the physical results from regularization and/or renormalization schemes. As a particularly relevant example, the analysis is specialized to the case of the Minimal Supersymmetric Standard Model, with emphasis on light charginos and neutralinos.

hep-ph↗

arXiv1995

Violations of lepton flavour and CP in supersymmetric unified theories

As a consequence of the large top quark Yukawa coupling, supersymmetric unified theories with soft supersymmetry breaking terms generated at the Planck scale predict lepton flavour and CP violating processes with significant rates. The flavour violating parameters of the low energy theory are derived in both SU(5) and SO(10) theories, and are used to calculate the rate for mu --> e gamma. The sensitivity of the search for mu --> e gamma is compared with that for mu --> e conversion in atoms, tau --> mu gamma and the electric dipole moment of the electron. The experimental search for these processes is shown to provide a very significant test of supersymmetric unification, especially in SO(10) but also in SU(5).

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arXiv1994

Flavour in supersymmetric Grand Unification: a democratic approach

We consider the flavour problem in a supersymmetric Grand Unified theory with gauged SU(6) group, where the Higgs doublets are understood as pseudo-Goldstone bosons of a larger $\SU(6)\otimes\SU(6)$ global symmetry of the Higgs superpotential. A key element of this work is that we never appeal to any flavour symmetry. One main interesting feature emerges: only one of the light fermions, an up-type quark, to be identified with the top, can get a Yukawa coupling at renormalizable level. This fact, together with bottom-tau Yukawa unification, also implied in our scheme, gives rise to a characteristic correlation between the top and the Higgs mass. By including a flavour-blind discrete symmetry and requiring that all higher dimensional operators be mediated by the exchange of appropriate heavy multiplets, it is possible to give an approximate description of all masses and mixing angles in term of a hierarchy of grand unified scales. A special ``texture'' arises, implying a relation between the top mass and the third generation mixing angles. Several other possible consequences of this approach are pointed out, concerning the $μ/s$ mass ratio, the Cabibbo angle and the proton decay.

hep-ph↗

arXiv1994

Strings versus supersymmetric GUTs: can they be reconciled?

We describe a class of supersymmetric unified models with the following properties: i) the full breaking of the gauge group is achieved by Higgs fields in the fundamental representation; ii) the correct unification of the strong and electroweak coupling constants is obtained without the need of any intermediate scale; iii) the problems of the doublet-triplet splitting and of the proton decay at dimension-5 level may receive a natural solution. The models, other than being interesting unified field theories per se, may constitute examples of string-derivable GUTs.

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