arXiv subjects

Howard Baer

Publications and source records attributed to Howard Baer.

At least 19 recordsLinked to original sources

arXiv2026

Exothermic dark matter and the 248 keV nuclear recoil in LUX-ZEPLIN

The single 248-keV nuclear-recoil candidate reported by LZ is reproduced by dark matter that downscatters, with the relic in the excited state of a pseudo-Dirac pair and a splitting \delta\simeq-350\keV. Every interpretation so far advanced is endothermic and places the splitting within a few percent of the xenon kinematic ceiling, so that the predicted count changes from zero to 143 times its central value as the halo escape speed runs from 500 to 600 km/s. The downscattering branch has no threshold speed; the same variation changes its count by 0.3 %. The ratio of counts below 70 keV to those above is 0.011, so the null standard search follows from the kinematics. A weak-strength coupling overproduces the event by 8.9\times10^{5}, so the mediator is a dark-sector one, and longevity of the relic excited state requires |\delta|<2m_e. Solving the coupled Boltzmann system removes the one free normalization. Rotating to the eigenchannels of the off-diagonal potential gives \sigma_0=(\pi/k^2)\sin^2(\delta_+-\delta_-), whose phase difference is 0.03 to 1.3 rad, so the Born rate exceeds the s-wave unitarity bound where it is used. One event then requires \sigma_p\simeq2\times10^{-42} cm^2 and a kinetic mixing \epsilon\simeq1\times10^{-6} at m_{A'}=1\GeV, with beam-dump coverage below 0.6\GeV pushing the mediator above that value. The ground state upscatters endothermically through the same operator, which excludes |\delta|\lesssim300 keV and makes the published endothermic reading the small-splitting limit of one model. The decisive test is argon, whose first form-factor zero lies at 695 keV against 94.6 keV for xenon; argon yields 4 to 6 events per tonne-year near 250 keV, where the endothermic reading is forbidden at any exposure.

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arXiv2026

Cosmological moduli problem ameliorated by decaying WIMPs

We investigate the cosmological moduli problem (CMP) in the context of supersymmetric models where discrete R-symmetries are used to suppress the mu term, and where mu is regenerated via Kim--Nilles superpotential operators. In such theories, a global U(1)_{PQ} emerges as an accidental, approximate symmetry which solves the strong CP problem. R-parity is also generated as accidental but approximate, where RPV operators gain a (f_a/m_P)^n suppression. When n=1, the thermally-produced LSPs may decay before BBN in the early universe, leaving axion-only dark matter of the SUSY DFSZ type. Meanwhile, the dominant cosmological constraint on light stringy moduli previously came from the modulus-induced WIMP overproduction problem. WIMP overproduction is now ameliorated by the RPV WIMP decays, but the moduli-induced BBN bound may be exacerbated by convolving long-lived modulus decay with long-lived RPV WIMP decay: long-lived particle (LLP) cascade decays. In this context, moduli as light as m_\phi ~ 130 TeV may be allowed which can reconcile naturalness with the presence of stringy moduli in the early universe.

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arXiv2026

Serendipitous supersymmetric solution to the strong CP problem

The Minimal Supersymmetric Standard Model (MSSM) has several problems: 1. its $\mu$ term must be forbidden, then regenerated at the weak scale, 2. it allows for $R$-parity violating superpotential terms which lead to rapid proton decay, 3. it allows for dimension-5 proton decay operators. The usual imposition of $R$- or matter parity $P_M$ solves only the second of these, whereas anomaly-free discrete $\mathbb{Z}_n^R$ symmetries (consistent with grand unification) address all of them. Once the $\mu$-term is forbidden by the imposition of a discrete $\mathbb{Z}_n^R$ symmetry (which can emerge as a discrete remnant of string compactifications to 4-dimensions), the MSSM develops an accidental global $U(1)_{PQ}$ symmetry (thus providing a plausible origin for the global $U(1)_{PQ}$ needed for solving the strong CP problem). By coupling the Higgs fields to PQ-charged gauge singlet fields $X,\ Y$ (in the Kim-Nilles mechanism), and imposing SUSY breaking, one regenerates $\mu$ at the weak scale whilst breaking the discrete $\mathbb{Z}_n^R$ and the $U(1)_{PQ}$. The broken global $U(1)_{PQ}$ develops a pseudo-Goldstone boson, the DFSZ axion, thus (perhaps inadvertently) solving the strong CP problem. In this setting, SUSY develops a dark matter candidate, the SUSY DFSZ axion, and possibly, though not necessarily, a WIMP dark matter candidate as well, depending on the order of the induced $R$-parity violating operators.

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arXiv2026

Can blind spots save neutralino dark matter in natural supersymmetry models?

Natural supersymmetry (SUSY) models remain viable even in the face of LHC Run 2 sparticle search limits. However, the LZ experiment has placed strong limits on light higgsino dark matter even when the higgsinos carry only their thermally-produced abundance, with the bulk of the dark matter composed of axions. One way out is the possibility of WIMP direct detection blind spots where cancellations in direct detection (DD) couplings lead to tiny DD rates. We examine natural SUSY models with mu <0 and \mu >0 but find that the surviving blind spots all lie in the unnatural region where the superpotential |mu | parameter is much greater than the weak scale gaugino masses; the few natural candidates are excluded by LHC soft-dilepton searches and by the measured Higgs mass. Within NUHM2/NUHM3-type gravity-mediated models with positive gaugino masses and assuming a thermally produced neutralino fractional abundance, direct-detection blind spots do not rescue stable light higgsino dark matter in the electroweak-natural region. Thus, within this framework, stable light higgsino dark matter is disfavored, although special circumstances like large entropy dilution of all relics is still possible. This points to SUSY models with {\it unstable} light higgsinos as perhaps the preferred alternative.

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arXiv2026

Natural SUSY with mixed axion/axino dark matter

While supersymmetric models provide a solution to the big hierarchy problem, natural SUSY is also allowed by the little hierarchy problem. In supersymmetric models which include the Peccei-Quinn (PQ) solution to the strong CP problem, one expects the presence of an axion-axino-saxion supermultiplet with a micro-eV-scale axion and a saxion with mass of order the soft breaking scale. The axino mass is much more model-dependent, and may occur in the range of keV-TeV: over 9 orders of magnitude. This leads to the possibility of the axino as lightest SUSY particle (LSP) and the presence of mixed axion plus axino dark matter. The case of natural SUSY with higgsino-like WIMPs as LSP seems (nearly) excluded by multi-ton noble liquid WIMP detector limits, even in the case where the LSP has a depleted abundance compared to axions. We examine the case where the axino is LSP leading to mixed axion-axino dark matter in a natural SUSY context. We map out regions of PQ scale f_a vs. axino mass m_{\ta} parameter space where such a scenario remains viable in both the SUSY DFSZ and KSVZ axion models. For axino mass ~100 keV, we find solutions in accord with the measured dark matter abundance with mainly warm axino dark matter for f_a~ 10^{11} GeV and also solutions with mainly axion cold DM and a tiny axino contribution for higher f_a~ 3\times 10^{12} GeV.

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arXiv2026

pMSSM versus complete models and the excellent prospects for top-squark discovery at HL-LHC

LHC sparticle search limits are usually performed within the context of simplified models and subsequently interpreted within the 19 parameter phenomenological MSSM (pMSSM) as to how many models avoid search limits for a particular sparticle mass, often including WIMP dark matter constraints. We provide a critical discussion of this procedure and how it can go wrong due to the introduction of new prejudices. By ameliorating these conditions, one is pushed into the more plausible four extra parameter non-universal Higgs model (NUHM4). Implementing a decoupling/quasi-degeneracy solution to the SUSY flavor and CP problems leads to first/second generation sfermions in the tens-of-TeV range. In this case, the natural solutions typically contain top-squarks in the 1-2 TeV range which are accessible to high-lumi LHC (HL-LHC) searches. This search channel, along with higgsino and wino pair production, may allow a nearly complete scan of natural/plausible parameter space by HL-LHC.

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arXiv2025

Natural supersymmetry at a muon collider

There is great interest within the particle physics community for building a $\mu^+\mu^-$ collider with center-of-mass (CoM) energies ranging from $\sqrt{s}\sim$ 1-14 TeV. For Beyond-the-Standard-Model (BSM) physics, natural supersymmetry seems perhaps the most motivated, plausible extension of the Standard Model. Here, we examine what can be accomplished by a muon collider with regards to natural SUSY at various muon collider CoM energies. In natural SUSY -- especially in the guise that would emerge from the string landscape -- one expects sparticles to be spread over two orders of magnitude in mass values. A muon collider with highly variable beam energies would be most useful for targeting 2-body reaction thresholds and Higgs boson resonances.

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arXiv2025

Reach of e^+e^- Higgs factory for light higgsinos via electroweak precision observables and comparison with other future facilities

Light higgsinos with mass ~100-400 GeV are well-motivated from naturalness considerations within supersymmetric models. However, at hadron colliders such as CERN LHC, they are rather difficult to search for due to the small visible energy release from heavy higgsino decay to the lightest higgsino, assumed here to be the lightest SUSY particle (LSP). An alternative way to search for the sparticles of supersymmetry is via their virtual effects on electroweak precision observables (EWPO) such as the W boson mass or the effective weak mixing angle \sin^2\theta_{\rm eff}. We quantify the ability of an e^+e^- Higgs factory operating at \sqrt{s}\sim 90-250 GeV to indirectly detect higgsinos via EWPO in the so-called higgsino discovery plane. The latter allows one to compare the relative reach of LHC and high-lumi LHC with an e^+e^- Higgs factory and with a linear e^+e^- collider operating at \sqrt{s}~ 0.5 TeV.

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arXiv2025

Aspects of the WIMP quality problem and R-parity violation in natural supersymmetry with all axion dark matter

In supersymmetric models where the mu problem is solved via discrete R-symmetries, then both the global U(1)_{PQ} (Peccei-Quinn, needed to solve the strong CP problem) and R-parity conservation (RPC, needed for proton stability) are expected to arise as accidental, approximate symmetries. Then in some cases, SUSY dark matter is expected to be all axions since the relic lightest SUSY particles (LSPs) can decay away via small R-parity violating (RPV) couplings. We examine several aspects of this {\it all axion} SUSY dark matter scenario. 1. We catalogue the operator suppression which is gained from discrete R-symmetry breaking via four two-extra-field base models. 2. We present exact tree-level LSP decay rates including mixing and phase space effects and compare to results from simple, approximate formulae. 3. Natural SUSY models are characterized by light higgsinos with mass ~100-350 GeV so that the dominant sparticle production cross sections at LHC14 are expected to be higgsino pair production which occurs at the 10^2-10^4 fb level. Assuming nature is natural, the lack of an RPV signal from higgsino pair production in LHC data translates into rather strong upper bounds on nearly all trilinear RPV couplings in order to render the SUSY signal (nearly) invisible. Thus, in natural SUSY models with light higgsinos, the RPV-couplings must be small enough that the LSP has a rather high quality of RPC.

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arXiv2025

Prospects for supersymmetry at high luminosity LHC

Weak scale supersymmetry (SUSY) is highly motivated in that it provides a 't Hooft technically natural solution to the gauge hierarchy problem. However, recent strong limits from superparticle searches at LHC Run 2 may exacerbate a so-called Little Hierarchy problem (LHP) which is a matter of practical naturalness: why is m_{weak}<< m_{soft}? We review recent LHC and WIMP dark matter search bounds as well as their impact on a variety of proposed SUSY models: gravity-, gauge-, anomaly-, mirage- and gaugino-mediation along with some dark matter proposals such as well-tempered neutralinos. We address the naturalness question. We also address the emergence of the string landscape at the beginning of the 21st century and its impact on expectations for SUSY. Rather generally, the string landscape statistically prefers large soft SUSY breaking terms but subject to the anthropic requirement that the derived value of the weak scale for each pocket universe (PU) within the greater multiverse lies with the ABDS window of values. This {\it stringy natural} (SN) approach implies m_h~ 125 GeV more often than not with sparticles beyond or well-beyond present LHC search limits. We review detailed reach calculations of the high-lumi LHC (HL-LHC) for non-universal Higgs mass models which present perhaps the most plausible realization of SUSY from the string landscape. In contrast to conventional wisdom, from a stringy naturalness point of view, the search for SUSY at LHC has only just begun to explore the interesting regimes of parameter space. We comment on how non-universal Higgs models could be differentiated from other expressions of natural SUSY such as natural anomaly-mediation and natural mirage mediation at HL-LHC.

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arXiv2025

All axion dark matter from supersymmetric models

Supersymmetric models accompanied by certain anomaly-free discrete R-symmetries Z_n^R are attractive in that 1. the R-symmetry (which can arise from compactified string theory as a remnant of the broken 10-d Lorentz symmetry) forbids unwanted superpotential terms while allowing for the generation of an accidental, approximate global U(1)_{PQ} symmetry needed to solve the strong CP problem and 2. they provide a raison d'etre for an otherwise ad-hoc R-parity conservation. We augment the minimal supersymmetric Standard Model (MSSM) by two additional Z_n^R- and PQ-charged fields X and Y wherein SUSY breaking at an intermediate scale m_{hidden} leads to PQ breaking at a scale f_a\sim 10^{11} GeV leading to a SUSY DFSZ axion. The same SUSY breaking can trigger R-parity breaking via higher-dimensional operators leading to tiny R-violating couplings of order (f_a/m_P)^N and a WIMP quality problem. For Z_4^R and Z_8^R, we find only an N=1 suppression. Then the lightest SUSY particle (LSP) of the MSSM becomes unstable with a lifetime of order ~ 10^{-3}-10 seconds so the LSPs all decay away before the present epoch. That leaves a universe with all axion cold dark matter and no WIMPs in accord with recent LZ-2024 WIMP search results.

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arXiv2024

Implications of Higgs mass for hidden sector SUSY breaking

Hidden sector SUSY breaking where charged hidden sector fields obtain SUSY breaking vevs once seemed common in dynamical SUSY breaking (DSB). In such a case, scalars can obtain large masses but gauginos and A-terms gain loop-suppressed anomaly-mediated contributions which may be smaller by factors of 1/16\pi^2 ~1/160. This situation leads to models such as PeV or mini-split supersymmetry with m(scalars)~ 160 m(gauginos). In order to generate a light Higgs mass m_h~ 125 GeV, the scalar mass terms are required in the 10-100 TeV range, leading to large, unnatural contributions to the weak scale. Alternatively, in gravity mediation with singlet hidden sector fields, then m(scalars)~ m(gauginos)~ A-terms and the large A-terms lift m_h ->125 GeV even for natural values of m(stop1)~ 1-3 TeV. Requiring naturalness, which is probabilistically preferred by the string landscape, then the measured Higgs mass seems to favor singlets in the hidden sector, which can be common in metastable and retrofitted DSB models.

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arXiv2024

Living dangerously with decoupled first/second generation scalars: SUSY prospects at the LHC

The string landscape statistical draw to large scalar soft masses leads to a mixed quasi-degeneracy/decoupling solution to the SUSY flavor and CP problems where first/second generation matter scalars lie in the 20-40 TeV range. With increasing first/second generation scalars, SUSY models actually become more natural due to two-loop RG effects which suppress the corresponding third generation soft masses. This can also lead to substantial parameter space regions which are forbidden by the presence of charge and/or color breaking (CCB) minima of the scalar potential. We outline the allowed SUSY parameter space for the gravity-mediated three extra-parameter-non-universal Higgs model NUHM3. The natural regions with m_h~ 125 GeV, \Delta_{EW}<~ 30 and decoupled first/second generation scalar are characterized by rather heavy gluinos and EW gauginos, but with rather small \mu and top-squarks not far beyond LHC Run 2 limits. This scenario also explains why SUSY has so far eluded discovery at LHC in that the parameter space with small scalar and gaugino masses is all excluded by the presence of CCB minima.

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arXiv2024

Minding the gap: testing natural anomaly-mediated SUSY breaking at high luminosity LHC

While the minimal anomaly-mediated SUSY breaking model (mAMSB) seems ruled out by constraints on Higgs mass, naturalness and wino dark matter, a slightly generalized version dubbed natural AMSB (nAMSB) remains both viable and compelling. Like mAMSB, nAMSB features winos as the lightest gauginos, but unlike mAMSB, nAMSB allows a small mu parameter so that higgsinos are the lightest of electroweakinos (EWinos). nAMSB spectra depend on the input value of gravitino mass m_{3/2}, where the lower range of m_{3/2} is excluded by LHC gluino pair searches while a higher m_{3/2} band is excluded by LHC limits on wino pair production followed by boosted hadronic wino decays. A remaining intermediate gap in m_{3/2} values remains allowed by present LHC searches, but appears to be completely explorable by high luminosity ugrades of LHC (HL-LHC). We explore a variety of compelling discovery channels that may allow one to close the intermediate gap in m_{3/2} values: 1. same-sign diboson +MET (SSdB) production arising from wino pair production, leading to same-sign dileptons plus MET, 2. trilepton production arising from wino pair production and 3. soft dilepton plus jet events from higgsino pair production, 4. top-squark pair production. From our signal-to-background analysis along a nAMSB model line, we expect HL-LHC to either discover or rule out the nAMSB model with 3000 fb^{-1} of integrated luminosity.

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arXiv2024

Decoding the gaugino code, naturally, at high-lumi LHC

Natural supersymmetry with light higgsinos is most likely to emerge from the string landscape since the volume of scan parameter space shrinks to tiny volumes for electroweak unnatural models. Rather general arguments favor a landscape selection of soft SUSY breaking terms tilted to large values, but tempered by the atomic principle: that the derived value of the weak scale in each pocket universe lie not too far from its measured value in our universe. But that leaves (at least) three different paradigms for gaugino masses in natural SUSY models: unified (as in nonuniversal Higgs models), anomaly-mediation form (as in natural AMSB) and mirage mediation form (with comparable moduli- and anomaly-mediated contributions). We perform landscape scans for each of these, and show they populate different, but overlapping, positions in m(\ell\bar{\ell}) and m(wino) space. The first of these may be directly measurable at high-lumi LHC via the soft opposite-sign dilepton plus jets plus MET signature arising from higgsino pair production while the second of these could be extracted from direct wino pair production leading to same-sign diboson production.

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arXiv2024

Beyond the Standard Model: An overview

At present, the Standard Model (SM) agrees with almost all collider data. Yet, three finetuning issues -- the Higgs mass problem, the strong CP problem and the cosmological constant problem -- all call for new physics. The most plausible solutions at present are weak scale SUSY, the PQWW axion and the string landscape. A re-evaluation of EW finetuning in SUSY allows for a higgsino-like LSP and naturalness upper bounds well beyond LHC limits. Rather general arguments from string theory allow for statistical predictions that m_h~ 125 GeV with sparticles beyond present LHC limits. The most lucrative LHC search channel may be for light higgsino pair production. Dark matter turns out to be a SUSY DFSZ axion along with a diminished abundance of higgsino-like WIMPs.

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arXiv2024

Stau pairs from natural SUSY at high luminosity LHC

Natural supersymmetry (SUSY) with light higgsinos is perhaps the most plausible of all weak scale SUSY models while a variety of motivations point to (right) tau sleptons as the lightest of all the sleptons. We examine a SUSY model line with rather light right-staus embedded within natural SUSY. For light stau_1 of a few hundred GeV, then the decays stau_1 -> \tau\tchi_{1,2}^0 and \nu_\tau\tchi_1^- occur at comparable rates where the (higgsino-like) \tchi_1^\pm and \tchi_2^0 release only small visible energy: in this case, the expected \tau^+\tau^- +\eslt signature is diminished from usual expectations due to the presence of the nearly invisible decay mode \ttau_1 -> \nu_\tau\tchi_1^-. However, once m_{\ttau_1}> ~m(bino), then decays to binos such as \ttau_1 -> \tau\tchi_3^0 open up where \tchi_3^0 decays to higgsinos plus W^\pm, Z^0 and h at comparable rates. For these heavier staus, then stau pair production gives rise to diboson+\eslt events which may contain 0, 1 or 2 additional hard \tau leptons. From these considerations, we examine the potential for future discovery of tau-slepton pair production at high-luminosity LHC. While we do not find a 5\sigma HL-LHC discovery reach for 3000 fb^{-1}, we do find a 95\% CL exclusion reach, ranging between m_{\ttau_1}:100-450 GeV for m_{\tchi_1^0}~ 100 GeV. This latter reach disappears for m_{\tchi_1^0}>~ 200 GeV.

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arXiv2024

Weak scale supersymmetry emergent from the string landscape

Superstring flux compactifications can stabilize all moduli while leading to an enormous number of vacua solutions, each leading to different $4-d$ laws of physics. While the string landscape provides at present the only plausible explanation for the size of the cosmological constant, it may also predict the form of weak scale supersymmetry which is expected to emerge. Rather general arguments suggest a power-law draw to large soft terms, but these are subject to an anthropic selection of not-too-large a value for the weak scale. The combined selection allows one to compute relative probabilities for the emergence of supersymmetric models from the landscape. Models with weak scale naturalness appear most likely to emerge since they have the largest parameter space on the landscape. For finetuned models such as high scale SUSY or split SUSY, the required weak scale finetuning shrinks their parameter space to tiny volumes, making them much less likely to appear compared to natural models. Probability distributions for sparticle and Higgs masses from natural models show a preference for Higgs mass $m_h\sim 125$ GeV with sparticles typically beyond present LHC limits, in accord with data. From these considerations, we briefly describe how natural SUSY is expected to be revealed at future LHC upgrades. This article is a contribution to the Special Edition of the journal {\it Entropy} honoring Paul Frampton on his 80th birthday.

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