arXiv subjects

Vernon Barger

Publications and source records attributed to Vernon Barger.

At least 145 records · Page 8Linked to original sources

arXiv2013

Radiative natural supersymmetry: Reconciling electroweak fine-tuning and the Higgs boson mass

Models of natural supersymmetry seek to solve the little hierarchy problem by positing a spectrum of light higgsinos <~ 200-300 GeV and light top squarks <~ 600 GeV along with very heavy squarks and TeV-scale gluinos. Such models have low electroweak fine-tuning and satisfy the LHC constraints. However, in the context of the MSSM, they predict too low a value of m(h), are frequently in conflict with the measured b\to sγbranching fraction and the relic density of thermally produced higgsino-like WIMPs falls well below dark matter (DM) measurements. We propose "radiative natural SUSY" (RNS) which can be realized within the MSSM (avoiding the addition of extra exotic matter) and which maintains features such as gauge coupling unification and radiative electroweak symmetry breaking. The RNS model can be generated from SUSY GUT type models with non-universal Higgs masses (NUHM). Allowing for high scale soft SUSY breaking Higgs mass m_{H_u}> m_0 leads to automatic cancellations during renormalization group (RG) running, and to radiatively-induced low fine-tuning at the electroweak scale. Coupled with large mixing in the top squark sector, RNS allows for fine-tuning at the 3-10% level with TeV-scale top squarks and a 125 GeV light Higgs scalar h. The model allows for at least a partial solution to the SUSY flavor, CP and gravitino problems since first/second generation scalars (and the gravitino) may exist in the 10-30 TeV regime. We outline some possible signatures for RNS at the LHC and at a linear e^+e^- collider. If the strong CP problem is solved by the Peccei-Quinn mechanism, then RNS naturally accommodates mixed axion-higgsino cold dark matter, where the light higgsino-like WIMPS - which in this case make up only a fraction of the measured relic abundance - should be detectable at upcoming WIMP detectors.

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arXiv2013

Post-LHC7 fine-tuning in the mSUGRA/CMSSM model with a 125 GeV Higgs boson

The recent discovery of a 125 GeV Higgs-like resonance at LHC, coupled with the lack of evidence for weak scale supersymmetry (SUSY), have severely constrained SUSY models such as mSUGRA/CMSSM. As LHC probes deeper into SUSY model parameter space, the little hierarchy problem -- how to reconcile the Z and Higgs boson mass scale with the scale of SUSY breaking -- will become increasingly exacerbated unless a sparticle signal is found. We evaluate two different measures of fine-tuning in the mSUGRA/CMSSM model. The more stringent of these, Δ_{HS}, includes effects that arise from the high scale origin of the mSUGRA parameters while the second measure, Δ_{EW}, is determined only by weak scale parameters: hence, it is universal to any model with the same particle spectrum and couplings. Our results incorporate the latest constraints from LHC7 sparticle searches, LHCb limits from B_s ->μ^+μ^- and also require a light Higgs scalar with m_h ~123-127 GeV. We present fine-tuning contours in the m_0 vs. m_{1/2} plane for several sets of A_0 and tan(β) values. We also present results for Δ_{HS} and Δ_{EW} from a scan over the entire viable model parameter space. We find a Δ_{HS} > 10^3, or at best 0.1% fine-tuning. For the less stringent electroweak fine tuning, we find Δ_{EW} > 10^2, or at best 1% fine-tuning. Two benchmark points are presented that have the lowest values of Δ_{HS} and Δ_{EW}. Our results provide a quantitative measure for ascertaining whether or not the remaining mSUGRA/CMSSM model parameter space is excessively fine-tuned, and so could provide impetus for considering alternative SUSY models.

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arXiv2013

LHC and dark matter signals of Z' bosons

We customize the simulation code FEWZ (Fully Exclusive W, Z Production) to study Z' production at the LHC for both \sqrt{s}=8 TeV and 14 TeV. Using the results of our simulation for several standard benchmark Z' models, we derive a semi-empirical expression for the differential cross section, that permits the determination of Z' couplings in a model-independent manner. We evaluate cross sections and other observables for large classes of models, including the common E_6, left-right and B-L models, as a function of model parameters. We also consider a hidden sector Z' that couples to standard model fermions via kinetic and mass mixing and serves as a mediator of isospin-violating interactions with dark matter. We combine the results of LHC Z' searches and dark matter direct detection experiments with global electroweak data to obtain mass-dependent constraints on the model parameters.

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arXiv2012

Scalar-Top Masses from SUSY Loops with 125 GeV mh and precise Mw, mt

We constrain the masses of scalar-tops (stop) by analyzing the new precision Tevatron measurement of the W-boson mass and the LHC/Tevatron indications of a Higgs boson of mass 125.5+-1 GeV. Our study adopts Natural SUSY with low fine-tuning, which has multi-TeV first and second generation squarks and a light Higgsino mixing parameter mu=150 GeV. An effective Lagrangian calculation is made of mh to 3-loops using the H3m program with weak scale SUSY parameters obtained from RGE evolution from the GUT scale in the Natural SUSY scenario. The SUSY radiative corrections to the Higgs mass imply maximal off-diagonal elements of the stop mass-matrix and a mass splitting of the two stops larger than 400 GeV.

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arXiv2012

Flavor-Tuned 125 GeV SUSY Higgs Boson at the LHC: MSSM and NATURAL SUSY TESTS

We show that an enhanced two-photon signal of the Higgs boson, h, observed with 125 GeV mass by the ATLAS and CMS collaborations, can be obtained if it is identified principally with the neutral $H_u^0$ of the two Higgs doublets of minimal Supersymmetry. We focus on sparticles and the pseudoscalar Higgs $A$ at the TeV scale. The off-diagonal element of the (Hu0,Hd0) mass matrix in the flavor basis must be suppressed, and this requires both a large Higgsino mass parameter, mu \sim TeV, and large tan beta. A MSSM sum rule is derived that relates gamma gamma and bb rates, and the gamma gamma enhancement predicts the bb reduction. On the contrary, Natural SUSY requires | mu | < 0.5 TeV, for which gamma gamma is reduced and $b\bar b$ is enhanced. This conclusion is independent of the mA-value and the SUSY quantum correction Deltab. Relative tau tau to bb rates are sensitive to Deltab.

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arXiv2012

2HDM Portal Dark Matter: LHC data and the Fermi-LAT 135 GeV Line

We study a two Higgs doublet model augmented by a scalar dark matter particle that provides an excellent fit to the LHC Higgs data and the Fermi-LAT 135 GeV line. The heavy CP-even Higgs boson, which predominantly mediates annihilation and scattering, must have a coupling to weak gauge bosons at or below percent level to suppress the continuum gamma-ray spectrum below the limit from the Fermi-LAT data and the anti-proton spectrum constrained by the PAMELA data. Discovering or excluding this CP-even Higgs boson at the LHC with a mass between 265 and 280 GeV and an enhanced diphoton branching ratio is crucial to test this scenario.

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arXiv2012

The 2HDM-X and Large Hadron Collider Data

We study the consistency of two Higgs doublet models in light of the new bosonic particle discovery at the LHC. We work within a general setup that we call the 2HDM-X, in which the quarks couple to both scalar doublets with aligned couplings such that flavor-changing neutral currents are absent at tree level. The 2HDM-X encompasses the traditional Type I, Type II, lepton specific, and flipped models, but also provides for more general possibilities. The best fit to the current data with a suppressed tau tau signal and a b bbar signal of Standard Model strength is given by the 2HDM-X with specific parameter choices; however, very good fits are also obtained within the lepton-specific model and a democratic model, the 2HDM-D, in which both the up-type and down-type quarks couple to each doublet with equal strengths. The approach provides a general framework in which to interpret future LHC Higgs data within extensions of the Standard Model with two Higgs doublets.

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arXiv2012

Radiative natural SUSY with a 125 GeV Higgs boson

It has been argued that requiring low electroweak fine-tuning (EWFT) along with a (partial) decoupling solution to the SUSY flavor and CP problems leads to a sparticle mass spectra characterized by light Higgsinos at 100-300 GeV, sub-TeV third generation scalars, gluinos at a few TeV and multi-TeV first/second generation scalars (natural SUSY). We show that by starting with multi-TeV first/second and third generation scalars and trilinear soft breaking terms, the natural SUSY spectrum can be generated radiatively via renormalization group running effects. Using the complete 1-loop effective potential to calculate EWFT, significantly heavier third generation squarks can be allowed even with low EWFT. The large negative trilinear term and heavier top squarks allow for a light Higgs scalar in the ~125 GeV regime.

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arXiv2012

Dipole Moment Dark Matter at the LHC

Monojet and monophoton final states with large missing transverse energy (${\not E}_T$) are important for dark matter (DM) searches at colliders. We present analytic expressions for the differential cross sections for the parton-level processes, $q\bar{q}(qg)\to g(q)χ\barχ$ and $q\bar{q}\to γχ\barχ$, for a neutral DM particle with a magnetic dipole moment (MDM) or an electric dipole moment (EDM). We collectively call such DM candidates dipole moment dark matter (DMDM). We also provide monojet cross sections for scalar, vector and axial-vector interactions. We then use ATLAS/CMS monojet${+\not E}_T$ data and CMS monophoton$+{\not E}_T$ data to constrain DMDM. We find that 7 TeV LHC bounds on the MDM DM-proton scattering cross section are about six orders of magnitude weaker than on the conventional spin-independent cross section.

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arXiv2012

Neutrino mass hierarchy and octant determination with atmospheric neutrinos

The recent discovery by the Daya-Bay and RENO experiments, that θ_{13} is nonzero and relatively large, significantly impacts existing experiments and the planning of future facilities. In many scenarios, the nonzero value of θ_{13} implies that θ_{23} is likely to be different from π/4. Additionally, large detectors will be sensitive to matter effects on the oscillations of atmospheric neutrinos, making it possible to determine the neutrino mass hierarchy and the octant of θ_{23}. We show that a 50 kT magnetized liquid argon neutrino detector can ascertain the mass hierarchy with a significance larger than 4 sigma with moderate exposure times, and the octant at the level of 2-3 sigma with greater exposure.

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arXiv2012

Discovery potential for SUSY at a high luminosity upgrade of LHC14

After completion of the LHC8 run in 2012, the plan is to upgrade the LHC for operation close to its design energy sqrt{s}=14 TeV, with a goal of collecting hundreds of fb^{-1} of integrated luminosity. The time is propitious to begin thinking of what is gained by even further LHC upgrades. In this report, we compute an LHC14 reach for SUSY in the mSUGRA/CMSSM model with an anticipated high luminosity upgrade. We find that LHC14 with 300 (3000) fb^{-1} has a reach for SUSY via gluino/squark searches of m(gluino)\sim3.2 TeV (3.6 TeV) for m(squark)\sim m)gluino), and a reach of m(gluino)\sim1.8 TeV (2.3 TeV) for m(squark)>> m(gluino). In the case where m(squark)>> m(gluino), then the LHC14 reach for chargino-neutralino production with decay into the Wh+MET final state reaches to m(gluino)\sim2.6 TeV for 3000 fb^{-1}.

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arXiv2012

Total Width of 125 GeV Higgs Boson

By using the LHC and Tevatron measurements of the cross sections to various decay channels relative to the standard model Higgs boson, the total width of the putative 125 GeV Higgs boson is determined as 6.1 +7.7-2.9 MeV. We describe a way to estimate the branching fraction for Higgs decay to dark matter. We also discuss a No-Go theorem for the gammagamma signal of the Higgs boson at the LHC.

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arXiv2012

Implications of a 125 GeV Higgs scalar for LHC SUSY and neutralino dark matter searches

The ATLAS and CMS collaborations have reported an excess of events in the γγ, ZZ^*\to 4\ell and WW^* search channels at an invariant mass m \simeq 125 GeV, which could be the first evidence for the long-awaited Higgs boson. We investigate the consequences of requiring m_h\simeq 125 GeV in both the mSUGRA and NUHM2 SUSY models. In mSUGRA, large values of trilinear soft breaking parameter |A_0| are required, and universal scalar m_0\agt 0.8 TeV is favored so that we expect squark and slepton masses typically in the multi-TeV range. This typically gives rise to an "effective SUSY" type of sparticle mass spectrum. In this case, we expect gluino pair production as the dominant sparticle creation reaction at LHC. For m_0< 5 TeV, the superpotential parameter μ> 2 TeV and m_A> 0.8 TeV, greatly restricting neutralino annihilation mechanisms. These latter conclusions are softened if m_0\sim 10-20 TeV or if one proceeds to the NUHM2 model. The standard neutralino abundance tends to be far above WMAP-measured values unless the neutralino is higgsino-like. We remark upon possible non-standard (but perhaps more attractive) cosmological scenarios which can bring the predicted dark matter abundance into accord with the measured value, and discuss the implications for direct and indirect detection of neutralino cold dark matter.

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arXiv2012

Four-Lepton Resonance at the Large Hadron Collider

A spin-1 weakly interacting vector boson, Z', is predicted by many new physics theories. Searches at colliders for such a Z' resonance typically focus on lepton-antilepton or top-antitop events. Here we present a novel channel with a Z' resonance that decays to 4 leptons, but not to 2 leptons, and discuss its possible discovery at the Large Hadron Collider. This baryonic gauge boson is well motivated in a supersymmetry framework.

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arXiv2012

Natural Supersymmetry: LHC, dark matter and ILC searches

Particle physics models with Natural Supersymmetry are characterized by a superpotential parameter μ\sim m_h \sim125$ GeV, while third generation squarks have mass <0.5-1.5 TeV. Gluinos should be lighter than several TeV so as not to destabilize the lighter squarks. First and second generation sfermions can be at the tens-of-TeV level which yields a decoupling solution to the SUSY flavor and CP problems. Adopting a top-down approach, we delineate the range of GUT scale SUSY model parameters which leads to a Natural SUSY mass spectrum. We find natural SUSY models to be tightly constrained by the b-> sγbranching fraction measurement while it is also difficult but not impossible to accommodate a light Higgs scalar of mass ~125 GeV. We present several benchmark points which are expandable to slopes and planes. Natural SUSY is difficult to see at LHC unless some third generation squarks are very light. The top- and bottom- squarks cascade decay mainly to higgsino-like charginos and neutralinos via numerous possibilities, leading to a rather complex set of signatures. Meanwhile, a linear e^+e^- collider operating at \sqrt{s}\sim 0.25-0.5 TeV would be a higgsino factory and is essentially guaranteed a SUSY discovery of the low-lying charged and neutral higgsino states. Since thermal neutralino cold dark matter is underproduced, we conjecture that the incorporation of a Peccei-Quinn sector or light moduli into the theory will augment higgsino dark matter production, possibly together with an admixture of axions. We present rates for direct and indirect higgsino dark matter detection for the case where light higgsinos dominate the dark matter abundance.

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arXiv2012

Azimuthal Correlations in Top Pair Decays and The Effects of New Heavy Scalars

We present the azimuthal correlations in the decay products of the top quark pair created in the process $gg \to t \tbar \to b \bbar \wplus \wminus$ calculated in leading-order QCD. We then discuss deviations in this distribution that could arise from New Physics contributions, focusing, in particular, on the case of a new heavy scalar coupling to the top quark and produced via the gluon fusion mechanism. Within this scenario we consider two cases: one in which the top quark is the only particle with a significant coupling to the scalar and another in which there are additional heavy fermions that modify the effective coupling of the scalar to gluons. We discuss the effects on the overall cross section and azimuthal correlations of the top quark decay products, with particular emphasis on the effect of interference between the QCD and New Physics contributions and on the existence of CP-nonconserving couplings.

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arXiv2012

Randall-Sundrum Reality at the LHC

The radion is expected to be the first signal of the Randall-Sundrum (RS) model. We explore the possibility of finding it in the ongoing Higgs searches at the LHC. The little RS model (LRS), which has a fundamental scale at about 1000 TeV, is excluded over wide ranges of the radion mass from the latest $WW$ and gamma gamma data by ATLAS and CMS.

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arXiv2012

Differentiating the Higgs boson from the dilaton and the radion at hadron colliders

A number of candidate theories beyond the standard model (SM) predict new scalar bosons below the TeV region. Among these, the radion, which is predicted in the Randall-Sundrum model, and the dilaton, which is predicted in spontaneous scale symmetry breaking, have very similar couplings to those of the SM Higgs boson, and it is very difficult to differentiate these three spin-0 particles in the expected signals of the Higgs boson at the LHC and Tevatron. We demonstrate that the observation of the ratio sigma(gamma gamma)/sigma(WW) gives a simple and decisive way, independently of the values of model parameters: the VEVs of the radion and dilaton fields.

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