arXiv subjects

J. Lopez

Publications and source records attributed to J. Lopez.

At least 73 records · Page 4Linked to original sources

arXiv1994

A survey of phenomenological constraints on supergravity models

We advocate the study of supergravity models as well motivated few-parameter low-energy supersymmetric models. In this context we survey a broad range of phenomenological constraints and future tests, including present and near-future Tevatron ($\tilde q,\tilde g,χ^\pm_1,\tilde t_1$) and LEP ($h,χ^\pm_1$) mass limits, collider indirect tests ($Γ_Z^{\rm inv},R_b,m_t$), rare processes ($b\to sγ,(g-2)_μ$), proton decay, and dark matter (cosmology, direct and indirect detection).

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arXiv1994

Supersymmetry dileptons and trileptons at the Tevatron

We consider the production of supersymmetry neutralinos and charginos in $p\bar p$ collisions at the Tevatron, and their subsequent decay via hadronically quiet dileptons and trileptons. We perform our computations in the context of a variety of supergravity models, including generic four-parameter supergravity models, the minimal $SU(5)$ supergravity model, and $SU(5)\times U(1)$ supergravity with string inspired two- and one-parameter moduli and dilaton scenarios. Our results are contrasted with estimated experimental sensitivities for dileptons and trileptons for integrated luminosities of $100\ipb$ and $1\ifb$, which should be available in the short and long terms. We find that the dilepton mode is a needed complement to the trilepton signal when the latter is suppressed by small neutralino leptonic branching ratios. The estimated reaches in chargino masses can be as large as $100\,(150)\GeV$ for $100\ipb\,(1\ifb)$. We also discuss the task left for LEPII once the Tevatron has completed its short-term search for dilepton and trilepton production.

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arXiv1994

Direct detection of dark matter in SU(5)xU(1) supergravity

We compute the scattering rates for the lightest neutralino $χ^0_1$ in the forthcoming germanium $(^{73}{\rm Ge}+^{76}{\rm Ge})$ detector and a proposed lead detector $(^{207}{\rm Pb})$, within the framework of $SU(5)\times U(1)$ supergravity. We find that in only a small portion ($\lsim10\%$) of the parameter spaces of this class of models, are the rates in the germanium detector above the expected initial experimental sensitivity of 0.1 events/kg/day. However, a much larger portion ($\lsim40\%$) of the parameter spaces could be probed with an improved background rejection capability (0.01 events/kg/day) and/or a more sensitive detector $(^{207}{\rm Pb})$.

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arXiv1994

New phenomena in the standard no-scale supergravity model

We revisit the no-scale mechanism in the context of the simplest no-scale supergravity extension of the Standard Model. This model has the usual five-dimensional parameter space plus an additional parameter $ξ_{3/2}\equiv m_{3/2}/m_{1/2}$. We show how predictions of the model may be extracted over the whole parameter space. A necessary condition for the potential to be stable is ${\rm Str}{\cal M}^4>0$, which is satisfied if $\bf m_{3/2}\lsim2 m_{\tilde q}$. Order of magnitude calculations reveal a no-lose theorem guaranteeing interesting and potentially observable new phenomena in the neutral scalar sector of the theory which would constitute a ``smoking gun'' of the no-scale mechanism. This new phenomenology is model-independent and divides into three scenarios, depending on the ratio of the weak scale to the vev at the minimum of the no-scale direction. We also calculate the residual vacuum energy at the unification scale ($C_0\, m^4_{3/2}$), and find that in typical models one must require $C_0>10$. Such constraints should be important in the search for the correct string no-scale supergravity model. We also show how specific classes of string models fit within this framework.

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arXiv1994

Experimental consequences of one-parameter no-scale supergravity models

We consider the experimental predictions of two {\em one-parameter} no-scale $SU(5)\times U(1)$ supergravity models with string-inspired moduli and dilaton seeds of supersymmetry breaking. These predictions have been considerably sharpened with the new information on the top-quark mass from the Tevatron, and the actual measurement of the $B(b\to sγ)$ branching ratio from CLEO. In particular, the sign of the Higgs mixing parameter $μ$ is fixed. A more precise measurement of the top-quark mass above (below) $\approx160\GeV$ would disfavor the dilaton (moduli) scenario. Similarly a measurement of the lightest Higgs-boson mass above 90 GeV (below 100 GeV) would disfavor the dilaton (moduli) scenario. At the Tevatron with $100\ipb$, the reach into parameter space is significant only in the dilaton scenario ($m_{χ^\pm_1}\lsim80\GeV$) via the trilepton and top-squark signals. At LEPII the dilaton scenario could be probed up the kinematical limit via chargino and top-squark pair production, and the discovery of the lightest Higgs boson is guaranteed. In the moduli scenario only selectron pair production looks promising. We also calculate the supersymmetric contribution to the anomalous magnetic moment of the muon.

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arXiv1994

Tevatron energy and luminosity upgrades beyond the Main Injector

The Fermilab Tevatron will be the world's highest energy hadron collider until the LHC is commissioned, it has the world's highest energy fixed target beams, and Fermilab will be the leading high energy physics laboratory in the US for the foreseeable future. Following the demise of the SSC, a number of possible upgrades to the Tevatron complex, beyond construction of the Main Injector, are being discussed. Using existing technology, it appears possible to increase the luminosity of the $\bar{p}p$ Collider to at least $10^{33}$cm$^{-2}$sec$^{-1}$ (Tevatron-Star) and to increase the beam energy to 2 TeV (DiTevatron). Fixed target beam of energy about 1.5 TeV could also be delivered. Leaving the existing Tevatron in the tunnel and constructing bypasses around the collider halls would allow simultaneous 800 GeV fixed target and $\sqrt{s}$ = 4 TeV collider operation. These upgrades would give Fermilab an exciting physics program which would be complementary to the LHC, and they would lay the groundwork for the construction of a possible post-LHC ultra-high energy hadron collider. (Presented at the Eighth Meeting of the Division of Particles and Fields Albuquerque, New Mexico, August 2-6, 1994.)

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arXiv1994

Theoretical expectations for the top-quark mass

I review the theoretical expectactions for the top-quark mass in a variety of models: the Standard Model, unified models (GUTs), low-energy supersymmetric models (SUSY), unified supersymmetric models (SUSY GUTs), supergravity models, and superstring models. In all instances I consider the constraints on the top-quark mass which arise by demanding that these theories be weakly interacting. This assumption is quantified by the use of partial-wave unitarity or triviality. The resulting upper bounds on the top-quark mass are most stringent in SUSY GUTs models ($m^{\rm pole}_t\lsim200\sinβ\GeV$). I also discuss a class of $SU(5)\times U(1)$ superstring models where $m^{\rm pole}_t\sim(170-195)\GeV$ is predicted. I conclude that experimental determinations of the top-quark mass can be {\em naturally} understood in SUSY GUTs and superstring models. (Lecture presented at the International School of Subnuclear Physics, 32nd Course: From Superstring to Present-Day Physics, Erice--Sicily: July 3--11, 1994.)

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arXiv1994

New constraints on supergravity models from $b\to sγ$

We perform a detailed study of the constraints from $b\to sγ$ on a large class of supergravity models, including generic four-parameter supergravity models, the minimal $SU(5)$ supergravity model, and $SU(5)\times U(1)$ supergravity. For each point in the parameter spaces of these models we obtain a range of $B(b\to sγ)$ values which should conservatively account for the unknown next-to-leading-order QCD corrections. We then classify these points into three categories: ``excluded" points have ranges of $B(b\to sγ)$ which do not overlap with the experimentally allowed range, ``preferred" points have $B(b\to sγ)$ ranges which overlap with the Standard Model prediction, and ``Ok" points are neither ``excluded" nor ``preferred" but may become ``excluded" should new CLEO data be consistent with the Standard Model prediction. In {\em all} cases we observe a strong tendency for the ``preferred" points towards one sign of the Higgs mixing parameter $μ$. For the opposite sign of $μ$ there is an upper bound on $\tanβ$: $\tanβ\lsim25$ in general, and $\tanβ\lsim6$ for the ``preferred" points. We conclude that new CLEO data will provide a decisive test of supergravity models.

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arXiv1994

A light stop and its consequences at the Tevatron and LEPII

An interesting prediction of a string-inspired {\em one-parameter} $SU(5)\times U(1)$ supergravity model, is the fact that the lightest member ($\tilde t_1$) of the top-squark doublet $(\tilde t_1,\tilde t_2)$, may be substantially lighter than the top quark. This sparticle ($\tilde t_1$) may be readily pair-produced at the Tevatron and, if $m_{\tilde t_1}\lsim130\GeV$, even be observed at the end of Run IB. Top-squark production may also be an important source of sought-for top-quark signatures in the dilepton and $\ell$+jets channels. Therefore, a re-analysis of the top data sample in the presence of a possibly light top-squark appears necessary before definitive statements concerning the discovery of the top quark can be made. Such a light top-squark is linked with a light supersymmetric spectrum, which can certainly be searched for at the Tevatron through trilepton and squark-gluino searches, and at LEPII through direct $\tilde t_1$ pair-production (for $m_{\tilde t_1}\lsim100\GeV$) and via chargino and Higgs-boson searches.

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arXiv1994

Supersymmetry at the DiTevatron

We study the signals for supersymmetry at the Tevatron and DiTevatron ($\sqrt{s}=4\TeV$) in various well-motivated supersymmetric models. We consider the trilepton signature in the decay of pair-produced charginos and neutralinos, the missing energy signature in gluino and squark production, and the $b\bar b$ signal in the decay of the lightest supersymmetric Higgs boson produced in association with a $W$ or $Z$ boson. In each case we perform signal and background studies, using Monte Carlo and/or real data to estimate the sensitivity to these signals at the Tevatron and DiTevatron with the Main Injector, for short- and long-term integrated luminosities of ${\cal L}=10$ and $25\ifb$, and $5σ$ statistical significance. We conclude that one could probe chargino masses as high as $m_{χ^\pm_1}\sim180\,(200)\GeV$, gluino masses as high as $m_{\tilde g}\sim450\,(750)\GeV$, and lightest Higgs boson masses as high as $m_h\sim110\,(120)\GeV$ at the Tevatron (DiTevatron). A high-luminosity option at the Tevatron ($10^{33}\cm^{-2}\s^{-1}$) may compensate somewhat for the higher reach of the DiTevatron, but only in the trilepton and Higgs signals. However, these gains may be severely compromised once the multiple-interaction environment of the high-luminosity Tevatron is accounted for.

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arXiv1994

String and string-inspired phenomenology

In these lectures I review the progress made over the last few years in the subject of string and string-inspired phenomenology. I take a practical approach, thereby concentrating more on explicit examples rather than on formal developments. Topics covered include: introduction to string theory, the free-fermionic formulation and its general features, generic conformal field theory properties, $SU(5)\times U(1)$ GUT and string model-building, supersymmetry breaking, the bottom-up approach to string-inspired models, radiative electroweak symmetry breaking, the determination of the allowed parameter space of supergravity models and the experimental constraints on this class of models, and prospects for direct and indirect tests of string-inspired models. (Lectures delivered at the XXII ITEP International Winter School of Physics, Moscow, Russia, February 22 -- March 2, 1994)

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arXiv1994

A Vision for High Energy Physics

Following the termination of the Superconducting Super Collider, there is an urgent need to develop a strategic plan for the future of high energy physics and an accompanying vision to guide the priorities of the U.S. program. This document proposes such a strategic plan and presents a singular opportunity for the U.S. program. The existing hadron collider at Fermilab could be upgraded to create a major discovery potential for supersymmetry, one of the most profound concepts in the world of elementary particles. Using a single ring of SSC magnets in the existing tunnel, the recently improved understanding of SUSY phenomenology, and the upgraded detectors in place at the Tevatron, the DiTevatron could be doing physics within five years and reach most of the range of parameters permitted in SUSY models. We propose that it be funded as a worthy component of the SSC termination, bringing to fruition both the technology and the science of the SSC at very modest cost.

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arXiv1994

Status of the superworld: from theory to experiment

Review to appear in Progress in Particle and Nuclear Physics. Contents: {1}Introduction}{1} {2}High precision LEP data and convergence of couplings: physics is not Euclidean geometry}{2} {3}Interconnections between the measured quantities due to Unification}{7} {4}The origin of $M_{SUSY}$ and why it should be abandoned: masses and spectra are needed}{13} {5}The new step forward: Supergravity}{21} {6}The SU(5) Supergravity Model}{22} {7}SU(5)xU(1) Supergravity}{32} {8}Detailed calculations for the Tevatron}{47} {9}Detailed calculations for LEP}{48} {10}Detailed calculations for HERA}{53} {11}Detailed calculations for Underground Labs and Underwater facilities}{55} {12}Detailed calculations for indirect experimental detection}{65} {13}The problem of mass and $m_t$}{73} {14}Conclusions}{77}

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arXiv1994

Closing the Universe by relaxing the cosmological constant

We propose a string-inspired model which correlates several aspects of particle physics and cosmology. Inspired by the flat directions and the absence of adjoint Higgs representations found in typical string models, we consider a no-scale $SU(5)\times U(1)$ supergravity model. This model entails well determined low-energy phenomenology, such as the value of the neutralino dark matter relic abundance and a negative contribution to the vacuum energy. A positive contribution to the vacuum energy is also typically present in string theory as a consequence of the running of the fundamental constants towards their fixed point values. If these two contributions cancel appropriately, one may end up with a vacuum energy which brings many cosmological observations into better agreement with theoretical expectations. The present abundance of neutralinos would then be fixed. We delineate the regions of parameter space allowed in this scenario, and study the ensuing predictions for the sparticle and Higgs-boson masses in this model.

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arXiv1994

The top-quark mass in SU(5)xU(1) supergravity

We show that the currently experimentally preferred values of the top-quark mass (\ie, $130\lsim m_t\lsim180\GeV$) are naturally understood in the context of string models, where the top-quark Yukawa coupling at the string scale is generically given by $λ_t={\cal O}(g)$, with $g$ the unified gauge coupling. A detailed study of the Yukawa sector of $SU(5)\times U(1)$ supergravity shows that the ratio of the bottom-quark to tau-lepton Yukawa couplings at the string scale is required to be in the range $0.7\lsimλ_b/λ_τ\lsim1$, depending on the values of $m_t$ and $m_b$. This result is consistent with $SU(5)\times U(1)$ symmetry, which does {\em not} require the equality of these Yukawa couplings in the unbroken symmetry phase of the theory. As a means of possibly predicting the value of $m_t$, we propose a procedure whereby the size of the allowed parameter space is determined as a function of $m_t$, since all sparticle and Higgs-boson masses and couplings depend non-trivially on $m_t$. At present, no significant preference for particular values of $m_t$ in $SU(5)\times U(1)$ supergravity is observed, except that high-precision LEP data requires $m_t\lsim180\GeV$.

hep-ph↗

arXiv1994

Experimental aspects of SU(5)xU(1) supergravity

We study various aspects of $SU(5)\times U(1)$ supergravity as they relate to the experimental verification or falsification of this model. We consider two string-inspired, universal, one-parameter, no-scale soft-supersymmetry-breaking scenarios, driven by the $F$-terms of the moduli and dilaton fields. The model is described in terms of the supersymmetry mass scale (\ie, the chargino mass $m_{χ^\pm_1}$), $\tanβ$, and the top-quark mass. We first determine the combined effect on the parameter space of all presently available direct and indirect experimental constraints, including the LEP lower bounds on sparticle and Higgs-boson masses, the $b\to sγ$ rate, the anomalous magnetic moment of the muon, the high-precision electroweak parameters $ε_1,ε_b$ (which imply $m_t\lsim180\GeV$), and the muon fluxes in underground detectors (neutrino telescopes). For the still-allowed points in $(m_{χ^\pm_1},\tanβ)$ parameter space, we re-evaluate the experimental situation at the Tevatron, LEPII, and HERA. In the 1994 run, the Tevatron could probe chargino masses as high as 100 GeV. At LEPII the parameter space could be explored with probes of different resolutions: Higgs boson searches, selectron searches, and chargino searches. Moreover, for $m_t\lsim150\GeV$, these Higgs-boson searches could explore all of the allowed parameter space with $\sqrt{s}\lsim210\GeV$.

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arXiv1993

First Constraints on SU(5)xU(1) Supergravity from Trilepton Searches at the Tevatron

We present the first constraints on the parameter space of $SU(5)\times U(1)$ supergravity (in both no-scale and dilaton scenarios) which arise from the recently announced limits on trilepton searches at the Tevatron. The trilepton rate has been calculated for those points in parameter space which satisfy not only the minimal theoretical and experimental LEP constraints, but also the {\em combined} effect of the following indirect experimental constraints: (i) the CLEO limits on the $b\to sγ$ rate, (ii) the long-standing limit on the anomalous magnetic moment of the muon, (iii) the non-observation of anomalous muon fluxes in underground detectors (``neutrino telescopes"), and (iv) the electroweak LEP high-precision measurements in the form of the $ε_{1},ε_b$ parameters. For $m_t=150\GeV$, the trilepton constraint rules out some regions of parameter space with chargino masses as high as $m_{χ^\pm_1}\approx105\GeV$, although it is not possible to establish a new absolute lower bound on the chargino mass. For $m_t=170\GeV$, the simultaneous imposition of {\em all} of the above constraints excludes the dilaton scenario completely, and leaves only a few allowed points in parameter space in the no-scale scenario (with $m_{\tilde q}\approx m_{\tilde g}\lsim285\GeV$). The five-fold increase in integrated luminosity expected in the upcoming Tevatron run should probe some regions of parameter space with chargino masses much beyond the reach of LEPII.

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arXiv1993

A Layman's guide to SUSY GUTs

The determination of the most straightforward evidence for the existence of the Superworld requires a guide for non-experts (especially experimental physicists) for them to make their own judgement on the value of such predictions. For this purpose we review the most basic results of Super-Grand unification in a simple and clear way. We focus the attention on two specific models and their predictions. These two models represent an example of a direct comparison between a traditional unified-theory and a string-inspired approach to the solution of the many open problems of the Standard Model. We emphasize that viable models must satisfy {\em all} available experimental constraints and be as simple as theoretically possible. The two well defined supergravity models, $SU(5)$ and $SU(5)\times U(1)$, can be described in terms of only a few parameters (five and three respectively) instead of the more than twenty needed in the MSSM model, \ie, the Minimal Supersymmetric extension of the Standard Model. A case of special interest is the strict no-scale $SU(5)\times U(1)$ supergravity where all predictions depend on only one parameter (plus the top-quark mass). A general consequence of these analyses is that supersymmetric particles can be at the verge of discovery, lurking around the corner at present and near future facilities. This review should help anyone distinguish between well motivated predictions and predictions based on arbitrary choices of parameters in undefined models.

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