arXiv subjects

Adam Martin

Publications and source records attributed to Adam Martin.

At least 73 records · Page 4Linked to original sources

arXiv2016

Minimal Models of Loop-Induced Higgs Lepton Flavor Violation

The LHC has recently reported a slight excess in the $h\rightarrow τμ$ channel. If this lepton flavor violating (LFV) decay is confirmed, an extension of the Standard Model (SM) will be required to explain it. In this paper we investigate two different possibilities to accommodate such a LFV process: the first scenario is based on flavor off-diagonal $A$-terms in the Minimal Supersymmetric Standard Model (MSSM), and the second is a model where the Higgs couples to new vectorlike fermions that couple to the SM leptons through a LFV four fermion interaction. In the supersymmetric model, we find that the sizes of the $A$-terms needed to accommodate the $h\rightarrow τμ$ excess are in conflict with charge- and color-breaking vacuum constraints. In the second model, the excess can be successfully explained while satisfying all other flavor constrains, with order one couplings, vectorlike fermion masses as low as 15 TeV, and a UV scale higher than 35 TeV.

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arXiv2016

Constraints on $L_μ- L_τ$ interactions at the LHC and beyond

In this paper we examine the constraints dedicated LHC multi lepton searches can place on $Z'$ bosons coming from gauged muon number minus tau number, $L_μ-L_τ$. As the $L_μ-L_τ$ gauge boson does not couple to proton constituents or electrons at tree level, the current bounds are fairly loose, especially for $M_{Z'} \gtrsim 1\, \rm GeV$. For $2m_μ < M_{Z'} < M_Z/2$ we develop search strategies using the $pp \rightarrow Z \to 4\, μ$ channel. The cleanliness of the final state, combined with the fact that $pp \to Z \to 4 e$, $Z \to 2e\,2μ$ can be used as background control samples, allow us to spot $L_μ-L_τ$ $Z'$ with couplings $\mathcal O(10^{-3})$ times the Standard Model couplings. For lighter $Z'$, we propose the mode $pp \rightarrow 2μ+ \displaystyle{\not} E_T$. The presence of missing energy means there is a wider set of backgrounds to consider in this final state, such as Drell-Yan production of leptonically decaying $τ$ pairs, however we find these can be controlled with careful cuts. Combining the $4\, μ$ and $2\,μ+ \displaystyle{\not} E_T$ modes, we find that with $\sim3 \, \text{ab}^{-1}$ of integrated luminosity we are sensitive to couplings $g_{Z'} \gtrsim 0.005\, g_1$ and $0.5\, \rm GeV \le M_{Z'} \le 40\, \rm GeV$ and $g_{Z'} \gtrsim 0.001\, g_1$ for $M_{Z'} < 2\, m_μ$. This region includes the parameter space where $L_μ-L_τ$ models can ameliorate the muon $g-2$ anomaly. We repeat these analyses at a future $e^+e^-$ Z-factory, where we find improved sensitivity. Specifically, given $2.6\, \rm ab^{-1}$ of luminosity, we can exclude $g_{Z'} \gtrsim 0.001\, g_1$ for $2\, m_μ \le M_{Z'} \le M_Z/2$.

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arXiv2015

The Diboson Excess: Experimental Situation and Classification of Explanations; A Les Houches Pre-Proceeding

We examine the `diboson' excess at $\sim 2$ TeV seen by the LHC experiments in various channels. We provide a comparison of the excess significances as a function of the mass of the tentative resonance and give the signal cross sections needed to explain the excesses. We also present a survey of available theoretical explanations of the resonance, classified in three main approaches. Beyond that, we discuss methods to verify the anomaly, determining the major properties of the various surpluses and exploring how different models can be discriminated. Finally, we give a tabular summary of the numerous explanations, presenting their main phenomenological features.

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arXiv2015

Low-derivative operators of the Standard Model effective field theory via Hilbert series methods

In this work, we explore an extension of Hilbert series techniques to count operators that include derivatives. For sufficiently low-derivative operators, we find an algorithm that gives the number of invariant operators, properly accounting for redundancies due to the equations of motion and integration by parts. Specifically, the technique can be applied whenever there is only one Lorentz invariant for a given partitioning of derivatives among the fields. At higher numbers of derivatives, equation of motion redundancies can be removed, but the increased number of Lorentz contractions spoils the subtraction of integration by parts redundancies. While restricted, this technique is sufficient to automatically generate the complete set of invariant operators of the Standard Model effective field theory for dimensions 6 and 7 (for arbitrary numbers of flavors). At dimension 8, the algorithm does not automatically generate the complete operator set; however, it suffices for all but five classes of operators. For these remaining classes, there is a well defined procedure to manually determine the number of invariants. Using these methods, we thereby derive the set of 535 dimension-8 $N_f = 1$ operators.

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arXiv2015

Towards the Final Word on Neutralino Dark Matter

We present a complete phenomenological prospectus for thermal relic neutralinos. Including Sommerfeld enhancements to relic abundance and halo annihilation calculations, we obtain direct, indirect, and collider discovery prospects for all neutralinos with mass parameters $M_1,M_2,|μ| < 4$ TeV, that freeze out to the observed dark matter abundance, with scalar superpartners decoupled. Much of the relic neutralino sector will be uncovered by the direct detection experiments Xenon1T and LZ, as well as indirect detection with CTA. We emphasize that thermal relic higgsinos will be found by next-generation direct detection experiments, so long as $M_{1,2} < 4$ TeV. Charged tracks at a 100 TeV hadron collider complement indirect searches for relic winos. Thermal relic bino-winos still evade all planned experiments, including disappearing charged-track searches. However, they can be discovered by compressed electroweakino searches at a 100 TeV collider, completing the full coverage of the relic neutralino surface.

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arXiv2015

The Relic Neutralino Surface at a 100 TeV collider

We map the parameter space for MSSM neutralino dark matter which freezes out to the observed relic abundance, in the limit that all superpartners except the neutralinos and charginos are decoupled. In this space of relic neutralinos, we show the dominant dark matter annihilation modes, the mass splittings among the electroweakinos, direct detection rates, and collider cross-sections. The mass difference between the dark matter and the next-to-lightest neutral and charged states is typically much less than electroweak gauge boson masses. With these small mass differences, the relic neutralino surface is accessible to a future 100 TeV hadron collider, which can discover inter-neutralino mass splittings down to 1 GeV and thermal relic dark matter neutralino masses up to 1.5 TeV with a few inverse attobarns of luminosity. This coverage is a direct consequence of the increased collider energy: the Standard Model events with missing transverse momentum in the TeV range have mostly hard electroweak radiation, distinct from the soft radiation shed in compressed electroweakino decays. We exploit this kinematic feature in final states including photons and leptons, tailored to the 100 TeV collider environment.

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arXiv2015

Hilbert Series for Constructing Lagrangians: expanding the phenomenologist's toolbox

This note presents the Hilbert series technique to a wider audience in the context of constructing group-invariant Lagrangians. This technique provides a fast way to calculate the number of operators of a specified mass dimension for a given field content, and is a useful cross check on more well-known group theoretical methods. In addition, at least when restricted to invariants without derivatives, the Hilbert series technique supplies a robust way of counting invariants in scenarios which, due to the large number of fields involved or to high dimensional group representations, are intractable by traditional methods. We work out several practical examples.

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arXiv2014

Catching sparks from well-forged neutralinos

In this paper we present a new search technique for electroweakinos, the superpartners of electroweak gauge and Higgs bosons, based on final states with missing transverse energy, a photon, and a dilepton pair, $\ell^+\,\ell^- + γ+ \displaystyle{\not} E_T$. Unlike traditional electroweakino searches, which perform best when $m_{\widetildeχ^0_{2,3}} - m_{\widetildeχ^0_1}, m_{\widetildeχ^{\pm}} - m_{\widetildeχ^0_1} > m_Z$, our search favors nearly degenerate spectra; degenerate electroweakinos typically have a larger branching ratio to photons, and the cut $m_{\ell\ell} \ll m_Z$ effectively removes on-shell Z boson backgrounds while retaining the signal. This feature makes our technique optimal for `well-tempered' scenarios, where the dark matter relic abundance is achieved with inter-electroweakino splittings of $\sim 20 - 70\,\text{GeV}$. Additionally, our strategy applies to a wider range of scenarios where the lightest neutralinos are almost degenerate, but only make up a subdominant component of the dark matter -- a spectrum we dub `well-forged'. Focusing on bino-Higgsino admixtures, we present optimal cuts and expected efficiencies for several benchmark scenarios. We find bino-Higgsino mixtures with $m_{\widetildeχ^0_{2,3}} \lesssim 190\,\text{GeV}$ and $m_{\widetildeχ^0_{2,3}} - m_{\widetildeχ^0_1} \cong 30\,\text{GeV}$ can be uncovered after roughly $600\,\text{fb}^{-1}$ of luminosity at the 14 TeV LHC. Scenarios with lighter states require less data for discovery, while scenarios with heavier states or larger mass splittings are harder to discriminate from the background and require more data. Unlike many searches for supersymmetry, electroweakino searches are one area where the high luminosity of the next LHC run, rather than the increased energy, is crucial for discovery.

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arXiv2014

Fitting the Galactic Center Gamma-Ray Excess with Cascade Annihilations

The apparent excess of gamma rays in an extended region in the direction of the galactic center has a spatial distribution and amplitude that are suggestive of dark matter annihilations. If this excess is indeed due to dark matter annihilations, it would indicate the presence of both dark matter and an additional particle beyond the Standard Model that mediates the interactions between the dark matter and Standard Model states. We introduce reference models describing dark matter annihilation to pairs of these new mediators, which decouples the SM-mediator coupling from the thermal annihilation cross section and easily explains the lack of direct detection signals. We determine the parameter regions that give good descriptions of the gamma ray excess for several motivated choices of mediator couplings to the SM. We find fermion dark matter with mass 7-26 GeV and a dark vector mediator, or scalar dark matter in the 10-50 GeV range (Higgs portal mediator) or 10-65 GeV range (gluophilic mediator) can provide a comparable or improved fit, compared to the case of direct annihilation. We demonstrate that these models can easily satisfy all constraints from collider experiments, direct detection, and cosmology.

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arXiv2014

Boosted Higgses from chromomagnetic $b$'s: $b\bar{b}h$ at high luminosity

This paper examines detection prospects and constraints on the chromomagnetic dipole operator for the bottom quark. This operator has a flavor, chirality and Lorentz structure that is distinct from other dimension six operators considered in Higgs coupling studies. Its non-standard Lorentz structure bolsters boosted $b \bar{b} h$ events, providing a rate independent signal of new physics. To date, we find this operator is unconstrained by $p p \rightarrow h + {\rm jets}$ and $pp \rightarrow \bar b b $ searches: for order-one couplings the permitted cutoff $Λ$ for this operator can be as low as $Λ\sim 1~{\rm TeV}$. We show how to improve this bound with collider cuts that allow a $b$-tagged Higgs plus dijet search in the Higgs to diphoton decay channel to exclude cutoffs as high as $\sim 6~{\rm TeV}$ at $2 σ$ with 3 $\text{ab}^{-1}$ of luminosity at the 14 TeV LHC. Cuts on the $p_T$ of the Higgs are key to this search, because the chromomagnetic dipole yields a non-standard fraction of boosted Higgses.

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arXiv2014

Interpretations of anomalous LHC events with electrons and jets

The CMS Collaboration has recently reported some excess events in final states with electrons and jets, in searches for leptoquarks and $W'$ bosons. Although these excesses may be due to some yet-to-be-understood background mismodeling, it is useful to seek realistic interpretations involving new particles that could generate such events. We show that resonant pair production of vector-like leptons that decay to an electron and two jets leads to kinematic distributions consistent with the CMS data.

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arXiv2014

Clearing the Brush: The Last Stand of Solo Small Field Inflation

By incorporating both the tensor-to-scalar ratio and the measured value of the spectral index, we set a bound on solo small field inflation of $Δϕ/ m_{Pl} \geq 1.00 \sqrt{r/0.1}$. Unlike previous bounds which require monotonic $ε_V,$ $|η_V| <1$, and 60 e-folds of inflation, the bound remains valid for non-monotonic $ε_V$, $|η_V| \gtrsim 1$, and for inflation which occurs only over the 8 e-folds which have been observed on the cosmic microwave background. The negative value of the spectral index over the observed 8 e-folds is what makes the bound strong; we illustrate this by surveying single field models and finding that for $r \gtrsim 0.1$ and 8 e-folds of inflation, there is no simple potential which reproduces observed CMB perturbations and remains sub-Planckian. Models that are sub-Planckian after 8 e-folds must be patched together with a second epoch of inflation that fills out the remaining $\sim 50$ e-folds. This second, post-CMB epoch is characterized by extremely small $ε_V$ and therefore an increasing scalar power spectrum. Using the fact that large power can overabundantly produce primordial black holes, we bound the maximum energy level of the second phase of inflation.

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arXiv2014

Split Dirac Supersymmetry: An Ultraviolet Completion of Higgsino Dark Matter

Motivated by the observation that the Higgs quartic coupling runs to zero at an intermediate scale, we propose a new framework for models of split supersymmetry, in which gauginos acquire intermediate scale Dirac masses of $\sim 10^{8-11}$ GeV. Scalar masses arise from one-loop finite contributions as well as direct gravity-mediated contributions. Like split supersymmetry, one Higgs doublet is fine-tuned to be light. The scale at which the Dirac gauginos are introduced to make the Higgs quartic zero is the same as is necessary for gauge coupling unification. Thus, gauge coupling unification persists (nontrivially, due to adjoint multiplets), though with a somewhat higher unification scale $\gtrsim 10^{17}$ GeV. The $μ$-term is naturally at the weak scale, and provides an opportunity for experimental verification. We present two manifestations of Split Dirac Supersymmetry. In the "Pure Dirac" model, the lightest Higgsino must decay through R-parity violating couplings, leading to an array of interesting signals in colliders. In the "Hypercharge Impure" model, the bino acquires a Majorana mass that is one-loop suppressed compared with the Dirac gluino and wino. This leads to weak scale Higgsino dark matter whose overall mass scale, as well as the mass splitting between the neutral components, is naturally generated from the same UV dynamics. We outline the challenges to discovering pseudo-Dirac Higgsino dark matter in collider and dark matter detection experiments.

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arXiv2014

Cornering a Hyper Higgs: Angular Kinematics for Boosted Higgs Bosons with Top Pairs

In the wake of the Higgs discovery and over the long haul of the LHC run, one should keep a lookout for kinematic anomalies in the most massive known trio of coupled particles, $t \bar{t} h$. After surveying the scope of prior constraints on chromomagnetic dipole and Higgs-gluon kinetic couplings, we focus on surpluses of boosted-$p_T$ Higgs bosons fomented by these momentum dependent dimension-six operators in $t \bar{t} h$ final states. We uncover a number of simple, $p_T$ weighted angular variables useful for discriminating Standard Model from dimension-6 boosted Higgs distributions, and make headway arguing that one of these variables may improve the reach of existing Standard Model top-Higgs searches. The approach we take is model independent, because we just consider a set of effective operators that contribute to the same three-body final state.

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arXiv2014

Hunting Quasi-Degenerate Higgsinos

We present a new strategy to uncover light, quasi-degenerate Higgsinos, a likely ingredient in a natural supersymmetric model. Our strategy focuses on Higgsinos with inter-state splittings of O(5-50) GeV that are produced in association with a hard, initial state jet and decay via off-shell gauge bosons to two or more leptons and missing energy, $pp \to j + \text{MET}\, + 2^+\, \ell$. The additional jet is used for triggering, allowing us to significantly loosen the lepton requirements and gain sensitivity to small inter-Higgsino splittings. Focusing on the two-lepton signal, we find the seemingly large backgrounds from diboson plus jet, $\bar tt$ and $Z/γ^* + j$ can be reduced with careful cuts, and that fake backgrounds appear minor. For Higgsino masses $m_χ$ just above the current LEP II bound ($μ\simeq 110\,$) GeV we find the significance can be as high as 3 sigma at the LHC using the existing 20 fb$^{-1}$ of 8 TeV data. Extrapolating to LHC at 14 TeV with 100 fb$^{-1}$ data, and as one example $M_1 = M_2 = 500$ GeV, we find 5 sigma evidence for $m_χ \lesssim\, 140\,$ GeV and 2 sigma evidence for $m_χ \lesssim\, 200\,$ GeV . We also present a reinterpretation of ATLAS/CMS monojet bounds in terms of degenerate Higgsino ($δm_χ \ll 5\,$) GeV plus jet production. We find the current monojet bounds on $m_χ$ are no better than the chargino bounds from LEP II.

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arXiv2013

Probing top-partners in Higgs + jets

Fermionic top-partners arise in models such as Composite Higgs and Little Higgs. They modify Higgs properties, in particular how the Higgs couples to top quarks. Alas, there is a low-energy cancellation acting in the coupling of the Higgs boson to gluons and photons. As a result of this cancellation, no information about the spectrum and couplings of the top-partners can be obtained in gluon fusion to Higgs, just the overall new physics scale. In this paper we show that this is not the case when hard radiation is taken into account. Indeed, differential distributions in Higgs plus jets are sensitive to the top-partner mass and coupling to the Higgs. We exploit the transverse momentum distribution of the hard jet to obtain limits on the top-partners in the 14 TeV LHC run, finding that 300 ifb of data of 14 TeV LHC are sufficient to rule out top-sector mixing angles $\sin^2 (θ_R)$ > 0.05 for top-partners with masses from 300 GeV to above 2 TeV.

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arXiv2013

Dirac Gauginos in Supersymmetry -- Suppressed Jets + MET Signals: A Snowmass Whitepaper

We consider the modifications to squark production in the presence of a naturally heavier Dirac gluino. First generation squark production is highly suppressed, providing an interesting but challenging signal find or rule out. No dedicated searches for supersymmetry with a Dirac gluino have been performed, however a reinterpretation of a "decoupled gluino" simplified model suggests the bounds on a common first and second generation squark mass is much smaller than in the MSSM: $\lsim 850$ GeV for a massless LSP, and no bound for an LSP heavier than about 300 GeV. We compare and contrast the squark production cross sections between a model with a Dirac gluino and one with a Majorana gluino, updating earlier results in the literature to a $pp$ collider operating at $\sqrt{s} = 14$ and 33 TeV. Associated production of squark+gluino is likely very small at $\sqrt{s} = 14$ TeV, while is a challenging but important signal at even higher energy $pp$ colliders. Several other salient implications of Dirac gauginos are mentioned, with some thought-provoking discussion as it regards the importance of the various experiments planned or proposed.

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arXiv2013

Measuring CP Violation in $h \to τ^+ τ^-$ at Colliders

We investigate the LHC and Higgs Factory prospects for measuring the CP phase in the Higgs-tau-tau coupling. Currently this phase can be anywhere between 0 degrees (CP even) and 90 degrees (CP odd). A new, ideal observable is identified from an analytic calculation for the $τ^\pm \to ρ^\pmν\to π^\pm π^0 ν$ channel. It is demonstrated to have promising sensitivity at the LHC and superior sensitivity at the ILC compared to previous proposals. Our observable requires the reconstruction of the internal substructure of decaying taus but does not rely on measuring the impact parameter of tau decays. It is the first proposal for such a measurement at the LHC. For the 14 TeV LHC, we estimate that about 1 ab$^{-1}$ data can discriminate CP-even versus CP-odd at the $5σ$ level. With 3 ab$^{-1}$, the CP phase should be measurable to an accuracy of $\sim 11$ degrees. At an $e^+e^-$ Higgs Factory, we project that a 250 GeV run with 1 ab$^{-1}$ luminosity can measure the phase to $\sim 4.4$ degrees accuracy.

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