arXiv subjects

Vernon Barger

Publications and source records attributed to Vernon Barger.

At least 73 records · Page 4Linked to original sources

arXiv2021

Oscillations of sterile neutrinos from dark matter decay eliminates the IceCube-Fermi tension

IceCube has observed a flux of cosmic neutrinos, with a "bump" in the energy range $10 \lesssim E/{\rm TeV} \lesssim 100$ that creates a $3σ$ tension with gamma-ray data from the Fermi satellite. This has been interpreted as evidence for a population of hidden cosmic-ray accelerators. We propose an alternative explanation of this conundrum on the basis of cold dark matter which decays into sterile neutrinos that after oscillations produce the bump in the cosmic neutrino spectrum.

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arXiv2021

Anomalous muon magnetic moment, supersymmetry, naturalness, LHC search limits and the landscape

The recent measurement of the muon anomalous magnetic moment a_μ\equiv (g-2)_μ/2 by the Fermilab Muon g-2 experiment sharpens an earlier discrepancy between theory and the BNL E821 experiment. We examine the predicted Δa_μ\equiv a_μ(exp)-a_μ(th) in the context of supersymmetry with low electroweak naturalness (restricting to models which give a plausible explanation for the magnitude of the weak scale). A global analysis including LHC Higgs mass and sparticle search limits points to interpretation within the normal scalar mass hierarchy (NSMH) SUSY model wherein first/second generation matter scalars are much lighter than third generation scalars. We present a benchmark model for a viable NSMH point which is natural, obeys LHC Higgs and sparticle mass constraints and explains the muon magnetic anomaly. Aside from NSMH models, then we find the (g-2)_μanomaly cannot be explained within the context of natural SUSY, where a variety of data point to decoupled first/second generation scalars. The situation is worse within the string landscape where first/second generation matter scalars are pulled to values in the 10-50 TeV range. An alternative interpretation for SUSY models with decoupled scalar masses is that perhaps the recent lattice evaluation of the hadronic vacuum polarization could be confirmed which leads to a Standard Model theory-experiment agreement in which case there is no anomaly.

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arXiv2021

Distribution of supersymmetry mu parameter and Peccei-Quinn scale f_a from the landscape

A scan of soft SUSY breaking parameters within the string theory landscape with the MSSM assumed as the low energy effective field theory -- using a power-law draw to large soft terms coupled with an anthropic selection of a derived weak scale to be within a factor four of our measured value -- predicts a peak probability of m_h~125 GeV with sparticles masses typically beyond the reach of LHC Run 2. Such multiverse simulations usually assume a fixed value of the SUSY conserving superpotential mu parameter to be within the assumed anthropic range, mu<~ 350 GeV. However, depending on the assumed solution to the SUSY mu problem, the expected mu term distribution can actually be derived. In this paper, we examine two solutions to the SUSY mu problem. The first is the gravity-safe-Peccei-Quinn (GSPQ) model based on an assumed Z_{24}^R discrete R-symmetry which allows a gravity-safe accidental, approximate Peccei-Quinn global symmetry to emerge which also solves the strong CP problem. The second case is the Giudice-Masiero solution wherein the mu term effectively acts as a soft term and has a linear draw to large values. For the first case, we also present the expected landscape distribution for the PQ scale f_a; in this case, weak scale anthropics limits its range to the cosmological sweet zone of around f_a~ 10^{11} GeV.

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arXiv2021

Sparticle and Higgs boson masses from the landscape: dynamical versus spontaneous supersymmetry breaking

Perturbative supersymmetry breaking on the landscape of string vacua is expected to favor large soft terms as a power-law or log distribution, but tempered by an anthropic veto of inappropriate vacua or vacua leading to too large a value for the derived weak scale -- a violation of the atomic principle. Indeed, scans of such vacua yield a statistical prediction for light Higgs boson mass m_h~ 125 GeV with sparticles (save possibly light higgsinos) typically beyond LHC reach. In contrast, models of dynamical SUSY breaking (DSB) -- with a hidden sector gauge coupling g^2 scanned uniformly -- lead to gaugino condensation and a uniform distribution of soft parameters on a log scale. Then soft terms are expected to be distributed as $m_{\rm soft}^{-1}$ favoring small values. A scan of DSB soft terms generally leads to $m_h\ll 125$ GeV and sparticle masses usually below LHC limits. Thus, the DSB landscape scenario seems excluded from LHC search results. An alternative is that the exponential suppression of the weak scale is set anthropically on the landscape via the atomic principle.

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arXiv2021

Landscape Higgs and sparticle mass predictions from a logarithmic soft term distribution

Recent work on calculating string theory landscape statistical predictions for the Higgs and sparticle mass spectrum from an assumed power-law soft term distribution yields an expectation for m(h)~ 125 GeV with sparticles (save light higgsinos) somewhat beyond reach of high-luminosity LHC. A recent examination of statistics of SUSY breaking in IIB string models with stabilized moduli suggests a power-law for models based on KKLT stabilization and uplifting while models based on large-volume scenario (LVS) instead yield an expected logarithmic soft term distribution. We evaluate statistical distributions for Higgs and sparticle masses from the landscape with a log soft term distribution and find the Higgs mass still peaks around ~125 GeV with sparticles beyond LHC reach, albeit with somewhat softer distributions than those arising from a power-law.

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arXiv2020

Midi-review: Status of weak scale supersymmetry after LHC Run 2 and ton-scale noble liquid WIMP searches

While LHC has discovered a very Standard Model-like Higgs boson of mass m_h~ 125 GeV, no solid signal for physics beyond the Standard Model has emerged so far at LHC or at WIMP seach experiments. For the case of weak scale supersymmetry (SUSY), LHC has found rather generally that gluinos are beyond about 2.2 TeV whilst top squark must lie beyond 1.1 TeV. These limits contradict older simplistic notions of naturalness that emerged in the 1980s-1990s, leading to the rather pessimistic view that SUSY is now excluded except for perhaps some remaining narrow corners of parameter space. Yet, this picture ignores several important developments in SUSY/string theory that emerged in the 21st century: 1. the emergence of the string theory landscape and its solution to the cosmological constant problem, 2. a more nuanced view of naturalness including the notion of "stringy naturalness", 3. the emergence of anomaly-free discrete R-symmetries and their connection to R-parity, Peccei-Quinn symmetry, the SUSY mu problem and proton decay and 4. the importance of including a solution to the strong CP problem. Rather general considerations from the string theory landscape favor large values of soft terms, subject to the vacuum selection criteria that electroweak symmetry is properly broken (no CCB minima) and the resulting magnitude of the weak scale is not too far from our measured value. Then stringy naturalness predicts a Higgs mass m_h~ 125 GeV whilst sparticle masses are typically lifted beyond present LHC bounds. In light of these refinements in theory perspective confronted by LHC and dark matter search results, we review the most likely LHC, ILC and dark matter signatures that are expected to arise from weak scale SUSY as we understand it today.

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arXiv2020

The LHC higgsino discovery plane for present and future SUSY searches

Considerations from electroweak naturalness and stringy naturalness imply a little hierarchy in supersymmetric models where the superpotential higgsino mass parameter mu is of order the weak scale whilst the soft SUSY breaking terms may be in the (multi-) TeV range. In such a case, discovery of SUSY at LHC may be most likely in the higgsino pair production channel. Indeed, ATLAS and CMS are performing searches in the higgsino mass discovery plane of m(chi_2^0) vs. Δm^0 = m(chi_2^0)-m(chi_1^0). We examine several theoretical aspects of this discovery plane in both the gravity-mediation NUHM2 model and the general mirage-mediation (GMM') models. These include: the associated chargino mass m(chi_1^+), the expected regions of the bottom-up notion of electroweak naturalness Δ_{EW}, and the expected regions of stringy naturalness. While compatibility with electroweak naturalness allows for mass gaps Δm^0~ 4-20 GeV, stringy naturalness exhibits a clear preference for yet smaller mass gaps of 4-10 GeV. For still smaller mass gaps, the plane becomes sharply unnatural since very large gaugino masses are required. This study informs the most promising SUSY search channels and parameter space regions for the upcoming HL-LHC runs and possible HE-LHC option.

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arXiv2020

Naturalness versus stringy naturalness (with implications for collider and dark matter searches)

The notion of stringy naturalness-- that an observable O_2 is more natural than O_1 if more (phenomenologically acceptable) vacua solutions lead to O_2 rather than O_1-- is examined within the context of the Standard Model (SM) and various SUSY extensions: CMSSM/mSUGRA, high-scale SUSY and radiatively-driven natural SUSY (RNS). Rather general arguments from string theory suggest a (possibly mild) statistical draw towards vacua with large soft SUSY breaking terms. These vacua must be tempered by an anthropic veto of non-standard vacua or vacua with too large a value of the weak scale m(weak). We argue that the SM, the CMSSM and the various high-scale SUSY models are all expected to be relatively rare occurances within the string theory landscape of vacua. In contrast, models with TeV-scale soft terms but with m(weak)~100 GeV and consequent light higgsinos (SUSY with radiatively-driven naturalness) should be much more common on the landscape. These latter models have a statistical preference for m_h~ 125 GeV and strongly interacting sparticles beyond current LHC reach. Thus, while conventional naturalness favors sparticles close to the weak scale, stringy naturalness favors sparticles so heavy that electroweak symmetry is barely broken and one is living dangerously close to vacua with charge-or-color breaking minima, no electroweak breaking or pocket universe weak scale values too far from our measured value. Expectations for how landscape SUSY would manifest itself at collider and dark matter search experiments are then modified compared to usual notions.

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arXiv2020

A string landscape guide to soft SUSY breaking terms

We examine several issues pertaining to statistical predictivity of the string theory landscape for weak scale supersymmetry (SUSY). We work within a predictive landscape wherein super-renormalizable terms scan while renormalizable terms do not. We require stringy naturalness wherein the likelihood of values for observables is proportional to their frequency within a fertile patch of landscape including the MSSM as low energy effective theory with a pocket-universe value for the weak scale nearby to its measured value in our universe. In the string theory landscape, it is reasonable that the soft terms enjoy a statistical power-law draw to large values, subject to the existence of atoms as we know them (atomic principle). We argue that gaugino masses, scalar masses and trilinear soft terms should each scan independently. In addition, the various scalars should scan independently of each other unless protected by some symmetry. The expected non-universality of scalar masses-- once regarded as an undesirable feature-- emerges as an asset within the context of the string landscape picture. In models such as heterotic compactifications on Calabi-Yau manifolds, where the tree-level gauge kinetic function depends only on the dilaton, then gaugino masses may scale mildly, while scalar masses and A-terms, which depend on all the moduli, may scale much more strongly leading to a landscape solution to the SUSY flavor and CP problems in spite of non-diagonal Kähler metrics. We present numerical results for Higgs and sparticle mass predictions from the landscape within the generalized mirage mediation SUSY model and discuss resulting consequences for LHC SUSY and WIMP dark matter searches.

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arXiv2020

Probing the top Yukawa coupling at the LHC via associated production of single top and Higgs

We study Higgs boson production associated with single top or anti-top via $t$-channel weak boson exchange at the LHC. The process is an ideal probe of the top quark Yukawa coupling, because we can measure the relative phase of $htt$ and $hWW$ couplings, thanks to the significant interference between the two amplitudes. By choosing the emitted $W$ momentum along the polar axis in the $th\,(\bar{t}h)$ rest frame, we obtain the helicity amplitudes for all the contributing subprocesses analytically, with possible CP phase of the Yukawa coupling. We study the azimuthal asymmetry between the $W$ emission and the $Wb\,(\bar{b})\to t(\bar{t})\,h$ scattering planes, as well as several $t$ and $\bar{t}$ polarization asymmetries as a signal of CP violating phase in the $htt$ coupling. Both the azimuthal asymmetry and the polarization perpendicular to the scattering plane are found to have the opposite sign between the top and anti-top events. We identify the origin of the sign of asymmetries, and propose the possibility of direct CP violation test in $pp$ collisions by comparing the top and anti-top polarization at the LHC.

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arXiv2019

Mirage mediation from the landscape

Rather general considerations from the string theory landscape suggest a statistical preference within the multiverse for soft SUSY breaking terms as large as possible subject to a pocket universe value for the weak scale not greater than a factor of 2-5 from our measured value. Within the gravity/moduli-mediated SUSY breaking framework, the Higgs mass is pulled to m_h~ 125 GeV while first/second generation scalars are pulled to tens of TeV scale and gauginos and third generation scalars remain at the few TeV range. In this case, one then expects comparable moduli- and anomaly-mediated contributions to soft terms, leading to mirage mediation. For an assumed stringy natural value of the SUSY mu parameter, we evaluate predicted sparticle mass spectra for mirage mediation from a statistical scan of the string landscape. We then expect a compressed spectrum of gauginos along with a higgsino-like LSP. For a linear (quadratic) statistical draw with gravitino mass m_{3/2}~ 20 TeV, then the most probable mirage scale is predicted to be around μ_{mir}~10^{13} (10^{14}) GeV. SUSY should appear at high-luminosity LHC via higgsino pair production into soft dilepton pairs. Distinguishing mirage mediation from models with unified gaugino masses may have to await construction of an ILC with \sqrt{s}>2m(higgsino).

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arXiv2019

A landscape solution to the SUSY flavor and CP problems

In a fertile patch of the string landscape which includes the Minimal Supersymmetric Standard Model (MSSM) as the low energy effective theory, rather general arguments from Douglas suggest a power-law statistical selection of soft breaking terms (m(soft)^n where n=2n_F+n_D-1 with n_F the number of hidden sector F-SUSY breaking fields and n_D the number of D-term SUSY breaking fields). The statistical draw towards large soft terms must be tempered by requiring an appropriate breakdown of electroweak (EW) symmetry with no contributions to the weak scale larger than a factor 2-5 of its measured value, lest one violates the (anthropic) atomic principle. Such a simple picture of stringy naturalness generates a light Higgs boson with mass m_h~ 125 GeV with sparticles (other than higgsinos) typically beyond LHC reach. Then we expect first and second generation matter scalars to be drawn independently to the tens of TeV regime where the upper cutoff arises from two-loop RGE terms which drive third generation soft masses towards tachyonic values. Since the upper bounds on m_0(1,2) are the same for each generation, and flavor independent, then these will be drawn toward quasi-degenerate values. This mechanism leads to a natural mixed decoupling/quasi-degeneracy solution to the SUSY flavor problem and a decoupling solution to the SUSY CP problem.

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arXiv2019

Lepton flavor violation from SUSY with non-universal scalars

Right-handed neutrinos in supersymmetric models can act as the source of lepton flavor violation (LFV). We present experimental implications of lepton flavor-violating processes within a supersymmetric type-I seesaw framework in the three-extra-parameter non-universal Higgs model (NUHM3) for large (PMNS-like) and small (CKM-like) Yukawa mixing scenarios. We highlight LFV predictions for the natural (low $Δ_{\rm EW}$) portion of parameter space. Our numerical analysis includes full 2-loop renormalization group running effects for the three neutrino masses and mass matrices. We show the projected discovery reach of various LFV experiments ($\textit{i.e.}$ Mu2e, Mu3e, MEG-II, Belle-II), and specify regions that have already been excluded by the LHC searches. Our results depend strongly on whether one has a normal sneutrino hierarchy (NSH) or an inverted sneutrino hierarchy (ISH). Natural SUSY with a NSH is already excluded by MEG-2013 results while large portions of ISH have been or will soon be tested. However, LFV processes from natural SUSY with small Yukawa mixing and an ISH seem below any projected sensitivities. A substantial amount of the remaining parameter space of models with large PMNS-like sneutrino mixing will be probed by Mu2e and MEG-II experiments whereas small, CKM-like Yukawa mixing predicts LFV decays which can hide from LFV experiments.

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arXiv2019

Is the magnitude of the Peccei-Quinn scale set by the landscape?

Rather general considerations of the string theory landscape imply a mild statistical draw towards large soft SUSY breaking terms tempered by the requirement of proper electroweak symmetry breaking where SUSY contributions to the weak scale are not too far from m(weak)~ 100 GeV. Such a picture leads to the prediction that m_h~ 125 GeV while most sparticles are beyond current LHC reach. Here we explore the possibility that the magnitude of the Peccei-Quinn (PQ) scale f_a is also set by string landscape considerations within the framework of a compelling SUSY axion model. First, we examine the case where the PQ symmetry arises as an accidental approximate global symmetry from a more fundamental gravity-safe Z(24)^R symmetry and where the SUSY mu parameter arises from a Kim-Nilles operator. The pull towards large soft terms then also pulls the PQ scale as large as possible. Unless this is tempered by rather severe (unknown) cosmological or anthropic bounds on the density of dark matter, then we would expect a far greater abundance of dark matter than is observed. This conclusion cannot be negated by adopting a tiny axion misalignment angle theta_i because WIMPs are also overproduced at large f_a. Hence, we conclude that setting the PQ scale via anthropics is highly unlikely. Instead, requiring soft SUSY breaking terms of order the gravity-mediation scale m_{3/2}~ 10-100 TeV places the mixed axion-neutralino dark matter abundance into the intermediate scale sweet zone where f_a~ 10^{11}-10^{12} GeV. We compare our analysis to the more general case of a generic SUSY DFSZ axion model with uniform selection on theta_i but leading to the measured dark matter abundance: this approach leads to a preference for f_a~ 10^{12} GeV.

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arXiv2019

Measuring the Black Hole Mass Spectrum from Redshifts of aLIGO Binary Merger Events

The binary black hole merger events observed by the Advanced LIGO (aLIGO) and VIRGO collaboration can shed light on the origins of black holes. Many studies based on black hole stellar origins have shown a maximum mass for stellar black holes, which can be measured or constrained from the observed black hole mass distribution. In this paper, we point out that the redshift distribution of the observed merger events can provide complementary information for studying the black hole mass distribution, because the detectability correlates the event redshift to the black hole masses. Based on the five observed events and using the Kolmogorov-Smirnov test, we have found that the maximum of the stellar black hole masses are constrained to be below $76\,M_\odot$ at 90\% confidence level, for a negative power-law index of 2.3 for the heavier black hole. With the improved sensitivity of aLIGO, a few dozen merger events may be obtained, for which we estimate that the maximum mass will be constrained to $10\,M_\odot$ accuracy.

astro-ph.HE↗

arXiv2019

Revisiting the SUSY mu problem and its solutions in the LHC era

The supersymmetry preserving mu parameter in SUSY theories is naively expected to be of order the Planck scale while phenomenology requires it to be of order the weak scale. This is the famous SUSY mu problem. Its solution involves two steps: 1. first forbid mu, perhaps via some symmetry, and then 2. re-generate it of order the scale of soft SUSY breaking terms. However, present LHC limits suggest the soft breaking scale m_{soft} lies in the multi-TeV regime whilst naturalness requires mu~ m_{W,Z,h}~ 100 GeV so that a Little Hierarchy (LH) appears with mu << m_{soft}. We review twenty previously devised solutions to the SUSY mu problem and re-evaluate them in light of whether they are apt to support the LH. We organize the twenty solutions according to: 1. solutions from supergravity/superstring constructions, 2. extended MSSM solutions, 3. solutions from an extra local U(1)' and 4. solutions involving Peccei-Quinn (PQ) symmetry and axions. Early solutions would invoke a global Peccei-Quinn symmetry to forbid the mu term while relating the mu solution to solving the strong CP problem via the axion. We discuss the gravity-safety issue pertaining to global symmetries and the movement instead toward local gauge symmetries or R-symmetries, either continuous or discrete. At present, discrete R-symmetries of order M (Z_M^R) which emerge as remnants of Lorentz symmetry of compact dimensions seem favored. Even so, a wide variety of regenerative mechanisms are possible, some of which relate to other issues such as the strong CP problem or the generation of neutrino masses. We also discuss the issue of experimental verification or falsifiability of various solutions to the mu problem. Almost all solutions seem able to accommodate the LH.

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arXiv2019

LHC SUSY and WIMP dark matter searches confront the string theory landscape

The string theory landscape of vacua solutions provides physicists with some understanding as to the magnitude of the cosmological constant. Similar reasoning can be applied to the magnitude of the soft SUSY breaking terms in supersymmetric models of particle physics: there appears to be a statistical draw towards large soft terms which is tempered by the anthropic requirement of the weak scale lying not too far from ~100 GeV. For a mild statistical draw of m_{soft}^n with n=1 (as expected from SUSY breaking due to a single F term) then the light Higgs mass is preferred at ~125 GeV while sparticles are all pulled beyond LHC bounds. We confront a variety of LHC and WIMP dark matter search limits with the statistical expectations from a fertile patch of string theory landscape. The end result is that LHC and WIMP dark matter detectors see exactly that which is expected from the string theory landscape: a Standard Model-like Higgs boson of mass 125 GeV but as yet no sign of sparticles or WIMP dark matter. SUSY from the n=1 landscape is most likely to emerge at LHC in the soft opposite-sign dilepton plus jet plus MET channel. Multi-ton noble liquid WIMP detectors should be able to completely explore the n=1 landscape parameter space.

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arXiv2018

Gravity safe, electroweak natural axionic solution to strong CP and SUSY mu problems

Particle physics models with Peccei-Quinn (PQ) symmetry breaking as a consequence of supersymmetry (SUSY) breaking are attractive in that they solve the strong CP problem with a SUSY DFSZ-like axion, link the SUSY breaking and PQ breaking intermediate mass scales and can resolve the SUSY mu problem with a naturalness-required weak scale mu term whilst soft SUSY breaking terms inhabit the multi-TeV regime as required by LHC sparticle mass limits and the Higgs mass measurement. On the negative ledger, models based on global symmetries suffer a generic gravity spoliation problem. We present a model based on the discrete R-symmetry Z_{24}^R-- which may emerge from compactification of 10-d Lorentzian spacetime in string theory-- where the mu term and dangerous proton decay and R-parity violating operators are either suppressed or forbidden while a gravity-safe PQ symmetry emerges as an accidental approximate global symmetry leading to a solution to the strong CP problem and a weak-scale/natural value for the mu term.

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