arXiv subjects

Howard Baer

Publications and source records attributed to Howard Baer.

At least 235 records · Page 13Linked to original sources

arXiv2000

Calculable Sparticle Masses with Radiatively Driven Inverted Mass Hierarchy

Supersymmetric models with an inverted mass hierarchy (IMH: multi-TeV first and second generation matter scalars, and sub-TeV third generation scalars) can ameliorate problems arising from flavor changing neutral currents, $CP$ violating phases and electric dipole moments, while at the same time satisfying conditions on naturalness. It has recently been shown that such an IMH can be generated radiatively, making use of infra-red fixed point properties of renormalization group equations given Yukawa coupling unification and suitable $GUT$ scale boundary conditions on soft SUSY breaking masses. In these models, explicit spectra cannot be obtained due to problems implementing radiative electroweak symmetry breaking (REWSB). We show that use of SO(10) $D$-term contributions to scalar masses can allow REWSB to occur, while maintaining much of the desired IMH. A somewhat improved IMH is obtained if splittings are applied only to Higgs scalar masses.

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arXiv2000

Trilepton Signal for Supersymmetry at the Fermilab Tevatron Revisited

Within a wide class of models, the LEP2 lower limit of 95 GeV on the chargino mass implies gluinos are heavier than $\sim$300 GeV. In this case electroweak $\tw_1\twb_1$ and $\tw_1\tz_2$ production are the dominant SUSY processes at the Tevatron, and the extensively examined isolated trilepton signal from $\tw_1\tz_2$ production assumes an even greater importance. We update our previous calculations of the SUSY reach of luminosity upgrades of the Fermilab Tevatron in this channel incorporating ({\it i}) decay matrix elements in the computation of the momenta of leptons from chargino and neutralino decays, ({\it ii}) the trilepton background from $W^*Z^*$ and $W^*γ^*$ production which, though neglected in previous analyses, turns out to be the dominant background, and finally, ({\it iii}) modified sets of cuts designed to reduce these new backgrounds and increase the range of model parameters for which the signal is observable. We show our improved projections for the reach for SUSY of both the Fermilab Main Injector and the proposed TeV33 upgrade. We also present opposite sign same flavor dilepton invariant mass distributions as well as the $p_T$ distributions of leptons in SUSY trilepton events, and comment upon how the inclusion of decay matrix elements impacts upon the Tevatron reach, as well as upon the extraction of neutralino masses.

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arXiv2000

The Reach of the CERN Large Hadron Collider for Gauge-Mediated Supersymmetry Breaking Models

We examine signals for sparticle production at the CERN Large Hadron Collider (LHC) within the framework of gauge mediated supersymmetry breaking models with a low SUSY breaking scale for four different model lines, each of which leads to qualitatively different signatures. We first examine the reach of the LHC via the canonical E_T^miss and multilepton channels that have been advocated within the mSUGRA framework. Next, we examine special features of each of these model lines that could serve to further enhance the SUSY signal over Standard Model backgrounds. We use ISAJET to evaluate the SUSY reach of experiments at the LHC. We find that the SUSY reach, measured in terms of m(gluino), is at least as large, and sometimes larger, than in the mSUGRA framework. In the best case of the co-NLSP scenario, the reach extends to m(gluino) >~ 3 TeV, assuming 10 fb^-1 of integrated luminosity.

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arXiv2000

Sparticle Mass Spectra from SO(10) Grand Unified Models with Yukawa Coupling Unification

We examine the spectrum of superparticles obtained from the minimal SO(10) grand unified model, where it is assumed the gauge symmetry breaking yields the Minimal Supersymmetric Standard Model (MSSM) as the effective theory at $M_{GUT}\sim 2\times 10^{16}$ GeV. In this model, unification of Yukawa couplings implies a value of $\tanβ\sim 45-55$. At such high values of $\tanβ$, assuming universality of scalar masses, the usual mechanism of radiative electroweak symmetry breaking breaks down. We show that a set of weak scale sparticle masses consistent with radiative electroweak symmetry breaking can be generated by imposing non-universal GUT scale scalar masses consistent with universality within SO(10) plus extra $D$-term contributions associated with the reduction in rank of the gauge symmetry group when SO(10) spontaneously breaks to $SU(3)\times SU(2)\times U(1)$. We comment upon the consequences of the sparticle mass spectrum for collider searches for supersymmetry. One implication of SO(10) unification is that the light bottom squark can be by far the lightest of the squarks. This motivates a dedicated search for bottom squark pair production at $p\bar{p}$ and $e^+e^-$ colliders.

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arXiv1999

Measuring the Top Quark Yukawa Coupling at a Linear e^+e^- Collider

The cross section for the reaction $e^+e^- \to t\bar{t} H$ depends sensitively on the top quark Yukwawa coupling $λ_t$. We calculate the rate for $t\bar{t}H$ production, followed by the decay $H\to b\bar{b}$, for a Standard Model Higgs boson with 100 < m_H <130 GeV. We interface with ISAJET to generate QCD radiation, hadronization and particle decays. We also calculate the dominant $t\bar{t}b\bar{b}$ backgrounds from electroweak and QCD processes. We consider both semileptonic and fully hadronic decays of the $t\bar{t}$ system. In our analysis, we attempt full reconstruction of the top quark and W boson masses in the generated events. The invariant mass of the remaining b-jets should show evidence of Higgs boson production. We estimate the accuracy with which $λ_t$ can be measured at a linear e^+e^- collider. Our results, including statistical but not systematic errors, show that the top quark Yukawa coupling can be measured to 6-8 % accuracy with 1000 fb^{-1} at $E_{CM}=1 TeV$, assuming 100 % efficiency for b-jet tagging. The accuracy of the measurement drops to 17-22 % if only a 60 % efficiency for b-tagging is achieved.

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arXiv1999

The Reach of Fermilab Tevatron Upgrades for SU(5) Supergravity Models with Non-universal Gaugino Masses

We explore the reach of luminosity upgrades of the Fermilab Tevatron collider for SU(5) supergravity models in which non-universal GUT-scale gaugino masses arise via a vacuum expectation value for the auxiliary component of a superfield that transforms as a 24, 75 or 200 dimensional representation of SU(5). This results in a different pattern of sparticle masses and mixing angles from what is expected in the minimal supergravity model (mSUGRA) with universal GUT scale gaugino masses. We find that the resulting signal cross sections, and hence the reach of the Tevatron, are sensitive to the gaugino masses at the GUT scale. In the 24 model, the large splitting amongst the two lightest neutralinos leads to SUSY events containing many isolated leptons, including events with a real leptonic Z boson plus jets plus missing energy signal which is visible over much of parameter space. In contrast, in the 75 and 200 models, the reach via leptonic SUSY signals is greatly reduced relative to mSUGRA, and the signal is usually visible only via the canonical $\eslt +$jets channel.

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arXiv1999

The Reach of Tevatron Upgrades in Gauge-Mediated Supersymmetry Breaking Models

We examine signals for sparticle production at the Tevatron within the framework of gauge mediated supersymmetry breaking models for four different model lines, each of which leads to qualitatively different signatures. We identify cuts to enhance the signal above Standard Model backgrounds, and use ISAJET to evaluate the SUSY reach of experiments at the Fermilab Main Injector and at the proposed TeV33. For the model lines that we have examined, we find that the reach is at least as large, and frequently larger, than in the mSUGRA framework. For two of these model lines, we find that the ability to identify $b$-quarks and $τ$-leptons with high efficiency and purity is essential for the detection of the signal.

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arXiv1998

The Reach of CERN LEP2 and Fermilab Tevatron Upgrades for Higgs Bosons in Supersymmetric Models

Luminosity upgrades of the Fermilab Tevatron pbar-p collider have been shown to allow experimental detection of a Standard Model (SM) Higgs boson up to $m_{H_{SM}}\sim 120$ GeV via $WH_{SM} \to \ellνb\bar{b}$ events. This limit nearly saturates the parameter space for many models of weak scale supersymmetry (SUSY) with a minimal particle content. It is therefore interesting to examine the SUSY Higgs reach of future Tevatron experiments. Contours are presented of Higgs boson reach for CERN LEP2 and Tevatron luminosity upgrades for three models of weak scale SUSY: the Minimal Supersymmetric Standard Model (MSSM), the minimal Supergravity model (mSUGRA) and a simple Gauge Mediated SUSY Breaking Model (GMSB). In each case we find a substantial gain in reach at the Tevatron with integrated luminosity increasing from 10 fb^{-1} to 25-30 fb^{-1}. With the larger integrated luminosity, a Higgs search at the Tevatron should be able to probe essentially the entire parameter space of these models. While a discovery would be very exciting, a negative result would severely constrain our ideas about how weak scale supersymmetry is realized.

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arXiv1998

Probing Minimal Supergravity at the CERN LHC for Large $\tanβ$

For large values of the minimal supergravity model parameter $\tanβ$, the tau lepton and the bottom quark Yukawa couplings become large, leading to reduced masses of $τ$-sleptons and $b$-squarks relative to their first and second generation counterparts, and to enhanced decays of charginos and neutralinos to $τ$-leptons and $b$-quarks. We evaluate the reach of the CERN LHC $pp$ collider for supersymmetry in the mSUGRA model parameter space. We find that values of $m_{\tg}\sim 1500-2000$ GeV can be probed with just 10 fb$^{-1}$ of integrated luminosity for $\tanβ$ values as high as 45, so that mSUGRA cannot escape the scrutiny of LHC experiments by virtue of having a large value of $\tanβ$. We also perform a case study of an mSUGRA model at $\tanβ=45$ where $\tz_2\to τ\ttau_1$ and $\tw_1\to \ttau_1ν_τ$ with $\sim 100%$ branching fraction. In this case, at least within our simplistic study, we show that a di-tau mass edge, which determines the value of $m_{\tz_2}-m_{\tz_1}$, can still be reconstructed. This information can be used as a starting point for reconstructing SUSY cascade decays on an event-by-event basis, and can provide a strong constraint in determining the underlying model parameters. Finally, we show that for large $\tanβ$ there can be an observable excess of $τ$ leptons, and argue that $τ$ signals might serve to provide new information about the underlying model framework.

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arXiv1998

Searching for Bottom Squarks at Luminosity Upgrades of the Fermilab Tevatron

Because of their Yukawa interactions, third generation squarks may be substantially lighter than those of the first two generations. Assuming that $\tb_1 \to b\tz_1$ and $\tz_1$ escapes experimental detection, we show that experiments at the Main Injector upgrade (integrated luminosity of $2 fb^{-1}$) of the Tevatron should be sensitive to $b$-squark masses up to 210 GeV for $m_{\tz_1} \leq 120$ GeV. For integrated luminosities of $10 fb^{-1}$ (25 $fb^{-1}$) the sbottom mass reach increases by 20 GeV (35 GeV). If the channel $\tb_1 \to b\tz_2$ is also accessible, the reach becomes model-dependent and may be degraded relative to the case where only the decay to $\tz_1$ is allowed. In models with gaugino mass unification and $μ$ much larger than gaugino masses, we argue that this degradation is unlikely to be larger than 30-40 GeV.

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arXiv1998

LHC Reach for Gauge Mediated Supersymmetry Breaking Models Via Prompt Photon Channels

We evaluate the supersymmetry reach of the Large Hadron Collider within the gauge-mediated low energy supersymmetry breaking framework, assuming that a neutralino is the second lightest sparticle and that it decays promptly into a gravitino which escapes detection. We find that the maximum reach is obtained via a search for inclusive $γγ+\eslt$ events coming dominantly from chargino and neutralino production. Assuming an integrated luminosity of 10~$fb^{-1}$, we find that LHC experiments will be able to probe values of the model parameter $Λ\alt 400$ TeV, corresponding to $m_{\tg} \leq 2.8$ TeV. A measure of the model parameter $Λ$ may be possible from the photon $p_T$ spectrum.

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arXiv1998

Supersymmetry Reach of Tevatron Upgrades: The Large $\tanβ$ Case

The Yukawa couplings of the tau lepton and the bottom quark become comparable to, or even exceed, electroweak gauge couplings for large values of the SUSY parameter $\tanβ$. As a result, the lightest tau slepton $\ttau_1$ and bottom squark $\tb_1$ can be significantly lighter than corresponding sleptons and squarks of the first two generations. Gluino, chargino and neutralino decays to third generation particles are significantly enhanced when $\tanβ$ is large. This affects projections for collider experiment reach for supersymmetric particles. In this paper, we evaluate the reach of the Fermilab Tevatron $p\bar p$ collider for supersymmetric signals in the framework of the mSUGRA model. We find that the reach via signatures with multiple isolated leptons ($e$ and $μ$) is considerably reduced. For very large $\tanβ$, the greatest reach is attained in the multi-jet$+\eslt$ signature. Some significant extra regions may be probed by requiring the presence of an identified $b$-jet in jets$+\eslt$ events, or by requiring one of the identified leptons in clean trilepton events to actually be a hadronic 1 or 3 charged prong tau. In an appendix, we present formulae for chargino, neutralino and gluino three body decays which are valid at large $\tanβ$.

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arXiv1997

Next-to-leading order slepton pair production at hadron colliders

We compute total cross sections for various slepton pair production reactions in next-to-leading order QCD. For pbarp collisions at root S=2TeV, we find leading order cross sections to be enhanced by typically 35% to 40%. For pp collisions at root S=14TeV, the enhancement ranges from 25% to 35% depending on the mass of the sleptons. We comment upon the phenomenological implications of these results.

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arXiv1997

$b\to sγ$ Constraints on the Minimal Supergravity Model with Large $\tanβ$

In the minimal supergravity model (mSUGRA), as the parameter $\tanβ$ increases, the charged Higgs boson and light bottom squark masses decrease, which can potentially increase contributions from $tH^\pm$, $\tg\tb_j$ and $\tz_i\tb_j$ loops in the decay $b\to sγ$. We update a previous QCD improved $b\to sγ$ decay calculation to include in addition the effects of gluino and neutralino loops. We find that in the mSUGRA model, loops involving charginos also increase, and dominate over $tW$, $tH^\pm$, $\tg\tq$ and $\tz_i\tq$ contributions for $\tanβ\agt 5-10$. We find for large values of $\tanβ\sim 35$ that most of the parameter space of the mSUGRA model for $μ<0$ is ruled out due to too large a value of branching ratio $B(b\to sγ)$. For $μ>0$ and large $\tanβ$, most of parameter space is allowed, although the regions with the least fine-tuning (low $m_0$ and $m_{1/2}$) are ruled out due to too low a value of $B(b\to sγ)$. We compare the constraints from $b\to sγ$ to constraints from the neutralino relic density, and to expectations for sparticle discovery at LEP2 and the Fermilab Tevatron $p\bar p$ colliders. Finally, we show that non-universal GUT scale soft breaking squark mass terms can enhance gluino loop contributions to $b\to sγ$ decay rate even if these are diagonal.

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arXiv1997

Trilepton Higgs signal at hadron colliders

Determining the origin of electroweak symmetry breaking will be one of the primary functions of high energy colliders. We point out that most Higgs boson searches pursued at hadron colliders require Yukawa interactions either in the production or the decay of a Higgs boson. We propose a trilepton Higgs boson search based only upon the gauge interactions of the Higgs. This strategy can be utilized successfully for the standard model (SM) Higgs boson as well as non-standard Higgs bosons which break electroweak symmetry but have little to do with fermion mass generation. The trileptons come from Wh production followed by Wh -> WWW^(*) -> 3l decays. A SM Higgs trilepton signal would be difficult to detect at the Tevatron collider: with 100 fb^{-1} of data, only a $3σ$ signal above background can be achieved after cuts if 140 GeV < mh < 175 GeV. Some discrimination of signal over background can be gained by analyzing the opposite sign dilepton pT distributions. At the LHC with 30 (100) fb^{-1} a clear discovery above the $5σ$ level is possible for a Higgs mass in the range 140-185 (125-200) GeV. Prospects for a trilepton Higgs discovery are greatly improved for models with non-standard Higgs sectors where a Higgs boson couples preferentially to vector bosons rather than to fermions.

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arXiv1997

Neutralino Dark Matter in Minimal Supergravity: Direct Detection vs. Collider Searches

We calculate expected event rates for direct detection of relic neutralinos as a function of parameter space of the minimal supergravity model. Numerical results are presented for the specific case of a $^{73}$Ge detector. We find significant detection rates ($R> 0.01$ events/kg/day) in regions of parameter space most favored by constraints from $B\to X_sγ$ and the cosmological relic density of neutralinos. The detection rates are especially large in regions of large $\tanβ$, where many conventional signals for supersymmetry at collider experiments are difficult to detect. If the parameter $\tanβ$ is large, then there is a significant probability that the first direct evidence for supersymmetry could come from direct detection experiments, rather than from collider searches for sparticles.

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arXiv1997

Collider Phenomenology for Supersymmetry at Large $tanβ$

If the parameter $tan β$ of the minimal supersymmetric model is large, then $b$ and $τ$ Yukawa interactions are important. These can significantly modify the masses and decays of sparticles. We describe new calculations which allow a reliable exploration of large $tan β$ values, and discuss implications for collider experiments. For large values of $tan β$, charginos and neutralinos may dominantly decay to $τ$-leptons or $b$-quarks. The usual cross sections for multilepton signatures may be greatly reduced, but SUSY may be detectable via new signals involving $τ$'s or $b$'s in the final state.

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arXiv1996

QCD Improved $b\to sγ$ Constraints on the Minimal Supergravity Model

Recent advances in the QCD corrections to $b\to sγ$ decay in the MSSM include i.) evaluation of the relevant operators, Wilson coefficients and anomalous dimension matrix elements for the various MSSM effective theories valid at scales beyond $Q =M_W$, ii.) calculations of most of the needed anomalous dimension matrix elements to next-to-leading order for scales $m_b\alt Q <M_W$, and iii.) calculations of ${\cal O}(α_s)$ virtual and bremsstrahlung corrections to the $b\to sγ$ decay operators at scale $Q\sim m_b$. We assemble all these known results to gain an estimate of $B(b\to sγ)$ for the parameter space of the minimal supergravity model (mSUGRA). We find a much reduced scale dependence of our result compared to usual leading-log evaluations. Comparison with the latest CLEO results yields stringent constraints on parameter space. Much of mSUGRA parameter space is ruled out for $μ<0$, especially for large $tanβ$. We compare these results with other constraints from cosmology and non-standard vacua. Also, we compare with expectations for discovering mSUGRA at LEP2, the Tevatron and the CERN LHC.

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