arXiv subjects

Shadman Salam

Publications and source records attributed to Shadman Salam.

14 recordsLinked to original sources

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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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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arXiv2023

On practical naturalness and its implications for weak scale supersymmetry

We revisit the various measures of naturalness for models of weak scale supersymmetry including 1. electroweak (EW) naturalness, 2. naturalness via sensitivity to high scale parameters (EENZ/BG), 3. sensitivity of Higgs soft term due to high scale (HS) radiative corrections and 4. stringy naturalness (SN) from the landscape. The EW measure is most conservative and seems unavoidable; it is also model independent in that its value is fixed only by the weak scale spectra which ensues, no matter which model is used to generate it. The EENZ/BG measure is ambiguous depending on which ``parameters of ignorance'' one includes in the low energy effective field theory (LE-EFT). For models with calculable soft breaking terms, then the EENZ/BG measure reduces to the tree-level EW measure. The HS measure began life as a figurative expression and probably shouldn't be taken more seriously than that. SN is closely related to EW naturalness via the atomic principle, although it is also sensitive to the distribution of soft terms on the landscape. If the landscape favors large soft terms, as in a power law distribution, then it favors m(h) ~ 125 GeV along with sparticles beyond present LHC reach. In this context, SN appears as a probability measure where more natural models are expected to be more prevalent on the landscape than finetuned models. We evaluate by how much the different measures vary against one another with an eye to determining by how much they may overestimate finetuning; we find overestimates can range up to a factor of over 1000. In contrast to much of the literature, we expect the string landscape to favor EW natural SUSY models over finetuned models so that the landscape is not an alternative to naturalness.

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arXiv2022

TF08 Snowmass Report: BSM Model Building

We summarize the state of Beyond the Standard Model (BSM) model building in particle physics for Snowmass 2021, focusing mainly on several whitepaper contributions to BSM model building (TF08) and closely related areas.

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arXiv2022

Fine-tuned vs. natural supersymmetry: what does the string landscape predict?

A vast array of (metastable) vacuum solutions arise from string compactifications, each leading to different 4-d laws of physics. The space of these solutions, known as the string landscape, allows for an environmental solution to the cosmological constant problem. We examine the possibility of an environmental solution to the gauge hierarchy problem. We argue that the landscape favors softly broken supersymmetric models over particle physics models containing quadratic divergences, such as the Standard Model. We present a scheme for computing relative probabilities for supersymmetric models to emerge from the landscape. The probabilities are related to the likelihood that the derived value of the weak scale lies within the Agrawal et al. (ABDS) allowed window of values leading to atoms as we know them. This then favors natural SUSY models over unnatural (SUSY and other) models via a computable probability measure.

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arXiv2022

Radiative natural supersymmetry emergent from the string landscape

In string theory with flux compactifications, anthropic selection for structure formation from a discretuum of vacuum energy values provides at present our only understanding of the tiny yet positive value of the cosmological constant. We apply similar reasoning to a toy model of the multiverse restricted to vacua with the MSSM as the low energy effective theory. Here, one expects a statistical selection favoring large soft SUSY breaking terms leading to a derived value of the weak scale in each pocket universe (with appropriate electroweak symmetry breaking) which differs from the weak scale as measured in our universe. In contrast, the SUSY preserving μparameter is selected uniformly on a log scale as is consistent with the distribution of SM fermion masses: this favors smaller values of μ. An anthropic selection of the weak scale to within a factor of a few of our measured value -- in order to produce complex nuclei as we know them (atomic principle) -- provides statistical predictions for Higgs and sparticle masses in accord with LHC measurements. The statistical selection then more often leads to (radiatively-driven) {\it natural} SUSY models over the Standard Model or finely-tuned SUSY models such as mSUGRA/CMSSM, split, mini-split, spread, high scale or PeV SUSY. The predicted Higgs and superparticle spectra might be testable at HL-LHC via higgsino pair production but is certainly testable at higher energy hadron colliders with \sqrt{s}~ 30-100 TeV.

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arXiv2022

Mini-review: Expectations for supersymmetry from the string landscape

In this mini-review, we summarize a variety of findings pertaining to consequences of the landscape of string theory for supersymmetry (SUSY) phenomenology. The idea is to adopt the MSSM as the most parsimonious 4-d low energy EFT after string compactification but where the scale of SUSY breaking is as yet undetermined. A power-law landscape draw to large soft terms is tempered by the requirement that the derived value of the weak scale lie within the anthropic window of Agrawal {\it et al.} (ABDS). Such a set-up predicts a light Higgs mass m_h~ 125 GeV with sparticles generally beyond LHC bounds. We discuss consequences for LHC searches: light higgsinos, highly mixed TeV-scale top squarks, same-sign diboson events and m_{\tg}~ 2-5 TeV. We expect dark matter to consist of an axion/higgsino-like WIMP admixture.

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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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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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