arXiv subjects

Howard Baer

Publications and source records attributed to Howard Baer.

At least 181 records · Page 10Linked to original sources

arXiv2008

Cosmological consequences of Yukawa-unified SUSY with mixed axion/axino cold and warm dark matter

Supersymmetric models with t-b-τYukawa unification at M_{GUT} qualitatively predict a sparticle mass spectrum including first and second generation scalars at the 3--15 TeV scale, third generation scalars at the (few) TeV scale and gluinos in the sub-TeV range. The neutralino relic density in these models typically turns out to lie far above the measured dark matter abundance, prompting the suggestion that instead dark matter is composed of an axion/axino mixture. We explore the axion and thermal and non-thermal axino dark matter abundance in Yukawa-unified SUSY models. We find in this scenario that {\it i}). rather large values of Peccei-Quinn symmetry breaking scale f_a\sim 10^{12} GeV are favored and {\it ii}). rather large values of GUT scale scalar masses \sim 10-15 TeV allow for the re-heat temperature T_R of the universe to be T_R\agt 10^6 GeV. This allows in turn a solution to the gravitino/Big Bang Nucleosynthesis problem while also allowing for baryogenesis via non-thermal leptogenesis. The large scalar masses for Yukawa-unified models are also favored by data on b\to sγand B_s\to μ^+μ^- decay. Testable consequences from this scenario include a variety of robust LHC signatures, a possible axion detection at axion search experiments, but null results from direct and indirect WIMP search experiments.

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arXiv2008

Dark matter from SUGRA GUTs: mSUGRA, NUSUGRA and Yukawa-unified SUGRA

Gravity-mediated SUSY breaking models with R-parity conservation give rise to dark matter in the universe. I review neutralino dark matter in the minimal supergravity model (mSUGRA), models with non-universal soft SUSY breaking terms (NUSUGRA) which yield a well-tempered neutralino, and models with unified Yukawa couplings at the GUT scale (as may occur in an SO(10) SUSY GUT theory). These latter models have difficulty accommodating neutralino dark matter, but work very well if the dark matter particles are axions and axinos.

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arXiv2008

Prospects for Yukawa Unified SO(10) SUSY GUTs at the CERN LHC

The requirement of t-b-τYukawa coupling unification is common in simple grand unified models based on the gauge group SO(10), and it also places a severe constraint on the expected spectrum of superpartners. For Yukawa-unified models with μ>0, the spectrum is characterized by three mass scales: {\it i}). first and second generation scalars in the multi-TeV range, {\it ii}). third generation scalars, μand m_A in the few-TeV range and {\it iii}). gluinos in the \sim 350-500 GeV range with chargino masses around 100-160 GeV. In such a scenario, gluino pair production should occur at large rates at the CERN LHC, followed by gluino three-body decays into neutralinos or charginos. Discovery of Yukawa-unified SUSY at the LHC should hence be possible with only 1 fb^{-1} of integrated luminosity, by tagging multi-jet events with 2--3 isolated leptons, without relying on missing E_T. A characteristic dilepton mass edge should easily be apparent above Standard Model background. Combining dileptons with b-jets, along with the gluino pair production cross section information, should allow for gluino and neutralino mass reconstruction. A secondary corroborative signal should be visible at higher integrated luminosity in the W1Z2-> 3\ell channel, and should exhibit the same dilepton mass edge as in the gluino cascade decay signal.

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arXiv2008

SO(10) SUSY GUTs, the gravitino problem, non-thermal leptogenesis and axino dark matter

Simple SUSY GUT models based on the gauge group SO(10) require t-b-τYukawa coupling unification, in addition to gauge coupling and matter unification. The Yukawa coupling unification places strong constraints on the expected superparticle mass spectrum, with scalar masses \sim 10 TeV while gaugino masses are quite light. A problem generic to all supergravity models comes from overproduction of gravitinos in the early universe: if gravitinos are unstable, then their late decays may destroy the predictions of Big Bang nucleosynthesis. We present a Yukawa-unified SO(10) SUSY GUT scenario which avoids the gravitino problem, gives rise to the correct matter-antimatter asymmetry via non-thermal leptogenesis, and is consistent with the WMAP-measured abundance of cold dark matter due to the presence of an axino LSP. To maintain a consistent cosmology for Yukawa-unified SUSY models, we require a re-heat temperature T_R\sim 10^6-10^7 GeV, an axino mass around \sim 0.1-10 MeV, and a PQ breaking scale f_a\sim 10^{12} GeV.

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arXiv2008

SUSY backgrounds to Standard Model calibration processes at the LHC

One of the first orders of business for LHC experiments after beam turn-on will be to calibrate the detectors using well understood Standard Model (SM) processes such as W and Z production and ttbar production. These familiar SM processes can be used to calibrate the electromagnetic and hadronic calorimeters, and also to calibrate the associated missing transverse energy signal. However, the presence of new physics may already affect the results coming from these standard benchmark processes. We show that the presence of relatively low mass supersymmetry (SUSY) particles may give rise to significant deviations from SM predictions of Z+jets and W+jets events for jet multiplicity $\ge 4$ or $\ge 5$, respectively. Furthermore, the presence of low mass SUSY may cause non-standard deviations to appear in top quark invariant and transverse mass distributions. Thus, effects that might be construed as detector mal-performance could in fact be the presence of new physics. We advocate several methods to check when new physics might be present within SM calibration data.

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arXiv2008

Dark matter and the LHC

An abundance of astrophysical evidence indicates that the bulk of matter in the universe is made up of massive, electrically neutral particles that form the dark matter (DM). While the density of DM has been precisely measured, the identity of the DM particle (or particles) is a complete mystery. In fact, within the laws of physics as we know them (the Standard Model, or SM), none of the particles have the right properties to make up DM. Remarkably, many new physics extensions of the SM -- designed to address theoretical issues with the electroweak symmetry breaking sector -- require the introduction of new particles, some of which are excellent DM candidates. As the LHC era begins, there are high hopes that DM particles, along with their associated new matter states, will be produced in pp collisions. We discuss how LHC experiments, along with other DM searches, may serve to determine the identity of DM particles and elucidate the associated physics. Most of our discussion centers around theories with weak-scale supersymmetry, and allows for several different DM candidate particles.

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arXiv2008

Collider signals and neutralino dark matter detection in relic-density-consistent models without universality

We present brief synopses of supersymmetric models where either the neutralino composition or its mass is adjusted so that thermal relic neutralinos from the Big Bang saturate the measured abundance of cold dark matter in the universe. We first review minimal supergravity (mSUGRA), and then examine its various one-parameter extensions where we relax the assumed universality of the soft supersymmetry breaking parameters. Our goal is to correlate relic-density-allowed parameter choices with expected phenomena in direct, indirect and collider dark matter search experiments. For every non-universal model, we first provide plots to facilitate the selection of ``dark-matter allowed'' parameter space points, and then present salient features of each model with respect to searches at Tevatron, LHC and ILC and also direct and indirect dark matter searches. We present benchmark scenarios that allow one to compare and contrast the non-universal models with one another and with the paradigm mSUGRA framework. We show that many implications about sparticle properties and collider signals drawn from the analysis of the relic density constraint within mSUGRA do not carry over to simple one-parameter extensions of the mSUGRA framework. We find that in many relic-density-consistent models, there is one (or more) detectable edge in the invariant mass distribution of same-flavour, opposite sign dileptons in SUSY cascade decay events at the LHC. Finally, we scan the parameter space of these various models, requiring consistency with the LEP2 constraint on the chargino mass, and with the observed relic density, and examine prospects for direct and indirect dark matter detection. We find that in a large number of cases the mechanism that causes the early universe neutralino annihilation rate to be large (so as to produce the measured relic density) also enhances the direct detection rate, and often also the rates for indirect detection of neutralino dark matter.

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arXiv2008

Dark matter allowed scenarios for Yukawa-unified SO(10) SUSY GUTs

Simple supersymmetric grand unified models based on the gauge group SO(10) require --in addition to gauge and matter unification-- the unification of t-b-τYukawa couplings. Yukawa unification, however, only occurs for very special values of the soft SUSY breaking parameters. We perform a search using a Markov Chain Monte Carlo (MCMC) technique to investigate model parameters and sparticle mass spectra which occur in Yukawa-unified SUSY models, where we also require the relic density of neutralino dark matter to saturate the WMAP-measured abundance. We find the spectrum is characterizd by three mass scales: first/second generation scalars in the multi-TeV range, third generation scalars in the TeV range, and gauginos in the \sim 100 GeV range. Most solutions give far too high a relic abundance of neutralino dark matter. The dark matter discrepancy can be rectified by 1. allowing for neutralino decay to axino plus photon, 2. imposing gaugino mass non-universality or 3. imposing generational non-universality. In addition, the MCMC approach finds 4. a compromise solution where scalar masses are not too heavy, and where neutralino annihilation occurs via the light Higgs h resonance. By imposing weak scale Higgs soft term boundary conditions, we are also able to generate 5. low μ, m_A solutions with neutralino annihilation via a light A resonance, though these solutions seem to be excluded by CDF/D0 measurements of the B_s\to μ^+μ^- branching fraction. Based on the dual requirements of Yukawa coupling unification and dark matter relic density, we predict new physics signals at the LHC from pair production of 350--450 GeV gluinos. The events are characterized by very high b-jet multiplicity and a dilepton mass edge around mz2-mz1 \sim 50-75 GeV.

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arXiv2008

SUSY interpretation of the Egret GeV anomaly, Xenon-10 dark matter search limits and the LHC

The observation of the Egret experiment of an excess of diffuse gamma rays with energies above E_γ=1 GeV has previously been interpreted in the context of the minimal supergravity model (mSUGRA) as coming from neutralino annihilation into mainly b-quarks in the galactic halo, with neutralino mass in the vicinity of 50-70 GeV. We observe that in order to obtain the correct relic abundance of neutralinos in accord with WMAP measurements, the corresponding neutralino-proton direct detection (DD) rates should be in excess of recent limits from the Xenon-10 collaboration. While it does not appear possible to satisfy the Egret, WMAP and Xenon-10 constraints simultaneously within the mSUGRA model, we find that it is easily possible in models with non-universal Higgs soft masses (NUHM). In either case, gluino pair production from m(gluino)\sim 400-500 GeV should occur at large rates at the CERN LHC, and a gluino pair production signal should be visible with just 0.1 fb^{-1} of integrated luminosity. The NUHM interpretation predicts a rather light spectrum of heavy Higgs bosons with m_A\sim 140-200 GeV over the whole parameter space which would interpret Egret data. Spin-independent DD rates are predicted to be just above 10^{-8} pb, within range of the next round of direct dark matter detection experiments.

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arXiv2008

Early SUSY discovery at LHC without missing E_T: the role of multi-leptons

Traditional searches for SUSY at hadron colliders rely heavily on the presence of large missing transverse energy (ME_T) to reject background compared to signal. On the other hand, initial searches for new physics at the LHC may not be able to rely on ME_T due to a variety of detector calibration issues. We show that much of SUSY parameter space is accessible to discovery even {\it without} using ME_T, and with rather low integrated luminosities 0.1-1 fb^{-1}. A key role is played by isolated lepton multiplicity which arises from gluino and squark cascade decays. Requiring \ge 3 isolated leptons plus jets yields a high rate of background rejection compared to signal. We find an LHC reach in m(gluino) of about 700-750 GeV for just 0.1 fb^{-1} of integrated luminosity by requiring events with \ge 4 jets plus \ge 3 isolated leptons but {\it without} using ME_T. If a large enough event sample is assembled, then kinematic reconstruction of sparticle mass properties should be possible just as in the case where large ME_T is required. SUSY without ME_T can also be seen in opposite-sign/same flavor {\it dilepton plus jets} events when a characteristic invariant mass edge stands out against background.

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arXiv2007

Mixed Higgsino Dark Matter from a Large SU(2) Gaugino Mass

We observe that in SUSY models with non-universal GUT scale gaugino mass parameters, raising the GUT scale SU(2) gaugino mass |M_2| from its unified value results in a smaller value of -m_{H_u}^2 at the weak scale. By the electroweak symmetry breaking conditions, this implies a reduced value of μ^2 {\it vis à vis} models with gaugino mass unification. The lightest neutralino can then be mixed Higgsino dark matter with a relic density in agreement with the measured abundance of cold dark matter (DM). We explore the phenomenology of this high |M_2| DM model. The spectrum is characterized by a very large wino mass and a concomitantly large splitting between left- and right- sfermion masses. In addition, the lighter chargino and three light neutralinos are relatively light with substantial higgsino components. The higgsino content of the LSP implies large rates for direct detection of neutralino dark matter, and enhanced rates for its indirect detection relative to mSUGRA. We find that experiments at the LHC should be able to discover SUSY over the portion of parameter space where m_{\tg} \alt 2350-2750 ~GeV, depending on the squark mass, while a 1 TeV electron-positron collider has a reach comparable to that of the LHC. The dilepton mass spectrum in multi-jet + \ell^+\ell^- + \eslt events at the LHC will likely show more than one mass edge, while its shape should provide indirect evidence for the large higgsino content of the decaying neutralinos.

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arXiv2007

Implications of Compressed Supersymmetry for Collider and Dark Matter Searches

Martin has proposed a scenario dubbed ``compressed supersymmetry'' (SUSY) where the MSSM is the effective field theory between energy scales M_{\rm weak} and M_{\rm GUT}, but with the GUT scale SU(3) gaugino mass M_3<< M_1 or M_2. As a result, squark and gluino masses are suppressed relative to slepton, chargino and neutralino masses, leading to a compressed sparticle mass spectrum, and where the dark matter relic density in the early universe may be dominantly governed by neutralino annihilation into ttbar pairs via exchange of a light top squark. We explore the dark matter and collider signals expected from compressed SUSY for two distinct model lines with differing assumptions about GUT scale gaugino mass parameters. For dark matter signals, the compressed squark spectrum leads to an enhancement in direct detection rates compared to models with unified gaugino masses. Meanwhile, neutralino halo annihilation rates to gamma rays and anti-matter are also enhanced relative to related scenarios with unified gaugino masses but, depending on the halo dark matter distribution, may yet be below the sensitivity of indirect searches underway. In the case of collider signals, we compare the rates for the potentially dominant decay modes of the stop_1 which may be expected to be produced in cascade decay chains at the LHC: \tst_1\to c\tz_1 and \tst_1\to bW\tz_1. We examine the extent to which multilepton signal rates are reduced when the two-body decay mode dominates. For the model lines that we examine here, the multi-lepton signals, though reduced, still remain observable at the LHC.

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arXiv2007

Collider and Dark Matter Phenomenology of Models with Mirage Unification

We examine supersymmetric models with mixed modulus-anomaly mediated SUSY breaking (MM-AMSB) soft terms which get comparable contributions to SUSY breaking from moduli-mediation and anomaly-mediation. The apparent (mirage) unification of soft SUSY breaking terms at Q=mu_mir not associated with any physical threshold is the hallmark of this scenario. The MM-AMSB structure of soft terms arises in models of string compactification with fluxes, where the addition of an anti-brane leads to an uplifting potential and a de Sitter universe, as first constructed by Kachru {\it et al.}. The phenomenology mainly depends on the relative strength of moduli- and anomaly-mediated SUSY breaking contributions, and on the Higgs and matter field modular weights, which are determined by the location of these fields in the extra dimensions. We delineate the allowed parameter space for a low and high value of tan(beta), for a wide range of modular weight choices. We calculate the neutralino relic density and display the WMAP-allowed regions. We show the reach of the CERN LHC and of the International Linear Collider. We discuss aspects of MM-AMSB models for Tevatron, LHC and ILC searches, muon g-2 and b->s γbranching fraction. We also calculate direct and indirect dark matter detection rates, and show that almost all WMAP-allowed models should be accessible to a ton-scale noble gas detector. Finally, we comment on the potential of colliders to measure the mirage unification scale and modular weights in the difficult case where mu_mir>>M_GUT.

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arXiv2007

Precision gluino mass at the LHC in SUSY models with decoupled scalars

One way to ameliorate the SUSY flavor and CP problems is to postulate that scalar masses lie in the TeV or beyond regime. For example, the focus point (FP) region of the minimal supergravity (mSUGRA) model is especially compelling in that heavy scalar masses can co-exist with low fine-tuning while yielding the required relic abundance of cold dark matter (via a mixed higgsino-bino neutralino). We examine many of the characteristics of collider events expected to arise at the CERN LHC in models with multi-TeV scalars, taking the mSUGRA FP region as a case study. The collider events are characterized by a hard component arising from gluino pair production, plus a soft component arising from direct chargino and neutralino production. Gluino decays in the FP region are characterized by lengthy cascades yielding very large jet and lepton multiplicities, and a large b-jet multiplicity. Thus, as one steps to higher jet, b-jet or lepton multiplicity, signal-over-background rates should steadily improve. The lengthy cascade decays make mass reconstruction via kinematic edges difficult; however, since the hard component is nearly pure gluino pair production, the gluino mass can be extracted to +- 8% via total rate for \eslt +\ge 7-jet +\ge 2 b-jet events, assuming 100 fb^{-1} of integrated luminosity. The distribution of invariant mass of opposite-sign/same-flavor dileptons in the hard component exhibits two dilepton mass edges: m_{\tz_2}-m_{\tz_1} and m_{\tz_3}-m_{\tz_1}. As a consistency check, the same mass edges should be seen in isolated opposite-sign dileptons occurring in the soft component trilepton signal which originates mainly from chargino-neutralino production.

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arXiv2006

Target dark matter detection rates in models with a well-tempered neutralino

In the post-LEP2 era, and in light of recent measurements of the cosmic abundance of cold dark matter (CDM) in the universe from WMAP, many supersymmetric models tend to predict 1. an overabundance of CDM and 2. pessimistically low rates for direct detection of neutralino dark matter. However, in models with a ``well-tempered neutralino'', where the neutralino composition is adjusted to give the measured abundance of CDM, the neutralino is typically of the mixed bino-wino or mixed bino-higgsino state. Along with the necessary enhancement to neutralino annihilation rates, these models tend to give elevated direct detection scattering rates compared to predictions from SUSY models with universal soft breaking terms. We present neutralino direct detection cross sections from a variety of models containing a well-tempered neutralino, and find cross section asymptotes with detectable scattering rates. These asymptotic rates provide targets that various direct CDM detection experiments should aim for. In contrast, in models where the neutralino mass rather than its composition is varied to give the WMAP relic density via either resonance annihilation or co-annihilation, the neutralino remains essentially bino-like, and direct detection rates may be below the projected reaches of all proposed experiments.

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arXiv2006

Probing SUSY beyond the reach of LEP2 at the Fermilab Tevatron: low |M_3| dark matter models

In supersymmetric models where the magnitude of the GUT scale gaugino mass parameter M_3 is suppressed relative to M_1 and M_2, the lightest neutralino can be a mixed higgsino-bino state with a thermal relic abundance in agreement with the WMAP central value for Ω_{\rm CDM} h^2 and consistent with all other phenomenological constraints. In these models, the gluino can be as light as 200 GeV without conflicting with the LEP2 bounds on the chargino mass. Thus, gluino pair production can be accessible at the Fermilab Tevatron at high rates. In this framework, gluinos decay radiatively with a large branching fraction to a gluon plus a neutralino. We find that experiments at the Fermilab Tevatron, with 5 fb^{-1} of integrated luminosity, will be sensitive to gluino pair production in the m_{\tg}\sim 200-350 GeV range via the multi-jet +etmissing and multi-jet +\ell^+\ell^- + etmissing channels at the 5σlevel, while trilepton signatures are expected to be below this level of detectability. Dilepton mass edges from both \tz_2 and \tz_3 decays may be measurable in the dilepton + multi-jet +etmissing channel.

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arXiv2006

Collider and Dark Matter Searches in Models with Mixed Modulus-Anomaly Mediated SUSY Breaking

We investigate the phenomenology of supersymmetric models where moduli fields and the Weyl anomaly make comparable contributions to SUSY breaking effects in the observable sector of fields. This mixed modulus-anomaly mediated supersymmetry breaking (MM-AMSB) scenario is inspired by models of string compactification with fluxes, which have been shown to yield a de Sitter vacuum (as in the recent construction by Kachru {\it et al}). The phenomenology depends on the so-called modular weights which, in turn, depend on the location of various fields in the extra dimensions. We find that the model with zero modular weights gives mass spectra characterized by very light top squarks and/or tau sleptons, or where M_1\sim -M_2 so that the bino and wino are approximately degenerate. The top squark mass can be in the range required by successful electroweak baryogenesis. The measured relic density of cold dark matter can be obtained via top squark co-annihilation at low \tanβ, tau slepton co-annihilation at large \tanβor via bino-wino coannihilation. Then, we typically find low rates for direct and indirect detection of neutralino dark matter. However, essentially all the WMAP-allowed parameter space can be probed by experiments at the CERN LHC, while significant portions may also be explored at an e^+e^- collider with \sqrt{s}=0.5--1 TeV. We also investigate a case with non-zero modular weights. In this case, co-annihilation, A-funnel annihilation and bulk annihilation of neutralinos are all allowed. Results for future colliders are qualitatively similar, but prospects for indirect dark matter searches via gamma rays and anti-particles are somewhat better.

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arXiv2006

Measuring Modular Weights in Mirage Unification Models at the LHC and ILC

String compactification with fluxes yields MSSM soft SUSY breaking terms that receive comparable contributions from modulus and anomaly mediation whose relative strength is governed by a phenomenological parameter $α$. Gaugino and first/second generation (and sometimes also Higgs and third generation) scalar mass parameters unify at a mirage unification scale $Q \not= M_{\rm GUT}$, determined by the value of $α$. The ratio of scalar to gaugino masses at this mirage unification scale depends directly on the scalar field modular weights, which are fixed in turn by the brane or brane intersections on which the MSSM fields are localized. We outline a program of measurements which can in principle be made at the CERN LHC and the International Linear $e^+e^-$ collider (ILC) which can lead to a determination of the modular weights.

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