arXiv subjects

Michael J. Ramsey-Musolf

Publications and source records attributed to Michael J. Ramsey-Musolf.

At least 73 records · Page 4Linked to original sources

arXiv2016

Right-Handed Neutrinos and T-Violating, P-Conserving Interactions

We show that experimental probes of the P-conserving, T-violating triple correlation in polarized neutron or nuclear $β$-decay provide a unique probe of possible T-violation at the TeV scale in the presence of right-handed neutrinos. In contrast to other possible sources of semileptonic T-violation involving only left-handed neutrinos, those involving right-handed neutrinos are relatively unconstrained by present limits on the permanent electric dipole moments of the electron, neutral atoms, and the neutron. On the other hand, LHC results for $pp\to e+$ missing transverse energy imply that an order of magnitude of improvement in $D$-coefficient sensitivity would be needed for discovery. Finally, we discuss the interplay with the scale of neutrino mass and naturalness considerations.

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arXiv2016

Singlet-Catalyzed Electroweak Phase Transitions in the 100 TeV Frontier

We study the prospects for probing a gauge singlet scalar-driven strong first order electroweak phase transition with a future proton-proton collider in the 100 TeV range. Singlet-Higgs mixing enables resonantly-enhanced di-Higgs production, potentially aiding discovery prospects. We perform Monte Carlo scans of the parameter space to identify regions associated with a strong first-order electroweak phase transition, analyze the corresponding di-Higgs signal, and select a set of benchmark points that span the range of di-Higgs signal strengths. For the $b\bar{b}γγ$ and $4τ$ final states, we investigate discovery prospects for each benchmark point for the high luminosity phase of the Large Hadron Collider and for a future $pp$ collider with $\sqrt{s}$ = 50, 100, or 200 TeV. We find that any of these future collider scenarios could significantly extend the reach beyond that of the high luminosity LHC, and that with $\sqrt{s}$ = 100 TeV (200 TeV) and 30 ab$^{-1}$, the full region of parameter space favorable to strong first order electroweak phase transitions is almost fully (fully) discoverable.

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arXiv2016

Indirect Detection Imprint of a CP Violating Dark Sector

We introduce a simple scenario involving fermionic dark matter ($χ$) and singlet scalar mediators that may account for the Galactic Center GeV $γ$-ray excess while satisfying present direct detection constraints. CP-violation in the scalar potential leads to mixing between the Standard Model Higgs boson and the scalar singlet, resulting in three scalars $h_{1,2,3}$ of indefinite CP-transformation properties. This mixing enables s-wave $χ{\barχ}$ annihilation into di-scalar states, followed by decays into four fermion final states. The observed $γ$-ray spectrum can be fitted while respecting present direct detection bounds and Higgs boson properties for $m_χ = 60 \sim 80 $ GeV, and $m_{h_3} \sim m_χ$. Searches for the Higgs exotic decay channel $h_1 \to h_3 h_3$ at the 14 TeV LHC should be able to further probe the parameter region favored by the $γ$-ray excess.

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arXiv2016

Stop-Catalyzed Baryogenesis Beyond the MSSM

Non-minimal supersymmetric models that predict a tree-level Higgs mass above the Minimal Supersymmetric Standard Model (MSSM) bound are well motivated by naturalness considerations. Indirect constraints on the stop sector parameters of such models are significantly relaxed compared to the MSSM; in particular, both stops can have weak-scale masses. We revisit the stop-catalyzed electroweak baryogenesis (EWB) scenario in this context. We find that the LHC measurements of the Higgs boson production and decay rates already rule out the possibility of stop-catalyzed EWB. We also introduce a gauge-invariant analysis framework that may generalize to other scenarios in which interactions outside the gauge sector drive the electroweak phase transition.

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arXiv2015

Baryon Washout, Electroweak Phase Transition, and Perturbation Theory

We analyze the conventional perturbative treatment of sphaleron-induced baryon number washout relevant for electroweak baryogenesis and show that it is not gauge-independent due to the failure of consistently implementing the Nielsen identities order-by-order in perturbation theory. We provide a gauge-independent criterion for baryon number preservation in place of the conventional (gauge-dependent) criterion needed for successful electroweak baryogenesis. We also review the arguments leading to the preservation criterion and analyze several sources of theoretical uncertainties in obtaining a numerical bound. In various beyond the standard model scenarios, a realistic perturbative treatment will likely require knowledge of the complete two-loop finite temperature effective potential and the one-loop sphaleron rate.

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arXiv2015

Two-Step Electroweak Baryogenesis

We analyze electroweak baryogenesis during a two-step electroweak symmetry breaking transition, wherein the baryon asymmetry is generated during the first step and preserved during the second. Focusing on the dynamics of CP-violation required for asymmetry generation, we discuss general considerations for successful two-step baryogenesis. Using a concrete model realization, we illustrate in detail the viability of this scenario and the implications for present and future electric dipole moment (EDM) searches. We find that CP-violation associated with a partially excluded sector may yield the observed baryon asymmetry while evading present and future EDM constraints.

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arXiv2015

TeV Lepton Number Violation: From Neutrinoless Double Beta Decay to the LHC

We analyze the sensitivity of next-generation tonne-scale neutrinoless double $β$-decay ($0νββ$) experiments and searches for same-sign di-electrons plus jets at the Large Hadron Collider to TeV scale lepton number violating interactions. Taking into account previously unaccounted for physics and detector backgrounds at the LHC, renormalization group evolution, and long-range contributions to $0νββ$ nuclear matrix elements, we find that the reach of tonne-scale $0νββ$ generally exceeds that of the LHC. However, for a range of heavy particle masses near the TeV scale, the high luminosity LHC and tonne-scale $0νββ$ may provide complementary probes.

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arXiv2015

Catalysis of Electroweak Baryogenesis via Fermionic Higgs Portal Dark Matter

We investigate catalysis of electroweak baryogenesis by fermionic Higgs portal dark matter using a two Higgs doublet model augmented by vector-like fermions. The lightest neutral fermion mass eigenstate provides a viable dark matter candidate in the presence of a stabilizing symmetry Z_2 or gauged U(1)_D symmetry. Allowing for a non-vanishing CP-violating phase in the lowest-dimension Higgs portal dark matter interactions allows generation of the observed dark matter relic density while evading direct detection bounds. The same phase provides a source for electroweak baryogenesis. We show that it is possible to obtain the observed abundances of visible and dark matter while satisfying present bounds from electric dipole moment (EDM) searches and direct detection experiments. Improving the present electron (neutron) EDM sensitivity by one (two) orders of magnitude would provide a conclusive test of this scenario.

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arXiv2014

Differences between Axions and Generic Light Scalars in Laboratory Experiments

It is well-known that electric dipole moment (EDM) constraints provide the most stringent bounds on axion-mediated macroscopic spin-dependent (SD) and time reversal and parity violating (TVPV) forces. These bounds are several orders of magnitude stronger than those arising from direct searches in fifth-force experiments and combining astrophysical bounds on stellar energy loss with Eotvos tests of the weak equivalence principle (WEP). This is a consequence of the specific properties of the axion, invoked to solve the Strong CP problem. However, the situation is quite different for generic light scalars that are unrelated to the strong CP problem. In this case, bounds from fifth-force experiments and astrophysical processes are far more stringent than the EDM bounds, for the mass range explored in direct searches.

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arXiv2014

Singlet-Catalyzed Electroweak Phase Transitions and Precision Higgs Studies

We update the phenomenology of gauge singlet extensions of the Standard Model scalar sector and their implications for the electroweak phase transition. Considering the introduction of one real scalar singlet to the scalar potential, we analyze present constraints on the potential parameters from Higgs coupling measurements at the Large Hadron Collider (LHC) and electroweak precision observables for the kinematic regime in which no new scalar decay modes arise. We then show how future precision measurements of Higgs boson signal strengths and Higgs self-coupling could probe the scalar potential parameter space associated with a strong first-order electroweak phase transition. We illustrate using benchmark precision for several future collider options, including the High Luminosity LHC (HL-LHC), the International Linear Collider (ILC), TLEP, China Electron Positron Collider (CEPC), and a 100 TeV proton-proton collider, such as the Very High Energy LHC (VHE-LHC) or the Super proton-proton Collider (SPPC). For the regions of parameter space leading to a strong first order electroweak phase transition, we find that there exists considerable potential for observable deviations from purely Standard Model Higgs properties at these prospective future colliders.

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arXiv2014

Distinguishing axions from generic light scalars using EDM and fifth-force experiments

We derive electric dipole moment (EDM) constraints on possible new macroscopic time reversal and parity violating (TVPV) spin-dependent forces. These constraints are compared to those derived from direct searches in fifth-force experiments and from combining laboratory searches with astrophysical bounds on stellar energy loss. For axion-mediated TVPV spin-dependent forces, EDM constraints dominate over fifth-force limits by several orders of magnitude. However, we show that for a generic light scalar, unrelated to the strong CP problem, present bounds from direct fifth- force searches are more stringent than those inferred from EDM limits, for the interaction ranges explored by fifth-force experiments. Thus, correlating observations in EDM and fifth-force experiments could help distinguish axions from more generic light scalar scenarios.

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arXiv2014

Impact of LSP Character on Slepton Reach at the LHC

Searches for supersymmetry at the Large Hadron Collider (LHC) have significantly constrained the parameter space associated with colored superpartners, whereas the constraints on color-singlet superpartners are considerably less severe. In this study, we investigate the dependence of slepton decay branching fractions on the nature of the lightest supersymmetric particle (LSP). In particular, in the Higgsino-like LSP scenarios, both decay branching fractions of $\tilde\ell_L$ and $\tildeν_\ell$ depend strongly on the sign and value of $M_1/M_2$, which has strong implications for the reach of dilepton plus MET searches for slepton pair production. We extend the experimental results for same flavor, opposite sign dilepton plus MET searches at the 8 TeV LHC to various LSP scenarios. We find that the LHC bounds on sleptons are strongly enhanced for a non-Bino-like LSP: the 95% C.L. limit for $m_{\tilde\ell_L}$ extends from 300 GeV for a Bino-like LSP to about 370 GeV for a Wino-like LSP. The bound for $\tilde\ell_L$ with a Higgsino-like LSP is the strongest (~ 490 GeV) for $M_1/M_2$ ~ $-\tan^2θ_W$ and is the weakest (~ 220 GeV) for $M_1/M_2$ ~ $\tan^2θ_W$. We also calculate prospective slepton search reaches at the 14 TeV LHC. With 100 fb$^{-1}$ integrated luminosity, the projected 95% C.L. mass reach for the left-handed slepton varies from 550 (670) GeV for a Bino-like (Wino-like) LSP to 900 (390) GeV for a Higgsino-like LSP under the most optimistic (pessimistic) scenario. The reach for the right-handed slepton is about 440 GeV. The corresponding 5$σ$ discovery sensitivity is about 100 GeV smaller. For 300 fb$^{-1}$ integrated luminosity, the reach is about 50 - 100 GeV higher.

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arXiv2014

CPV Phenomenology of Flavor Conserving Two Higgs Doublet Models

We analyze the constraints on CP-violating, flavor conserving Two Higgs Doublet Models (2HDMs) implied by measurements of Higgs boson properties at the Large Hadron Collider (LHC) and by the non-observation of permanent electric dipole moments (EDMs) of molecules, atoms and the neutron. We find that the LHC and EDM constraints are largely complementary, with the LHC studies constraining the mixing between the neutral CP-even states and EDMs probing the effect of mixing between the CP-even and CP-odd scalars. The presently most stringent constraints are implied by the non-observation of the ThO molecule EDM signal. Future improvements in the sensitivity of neutron and diamagnetic atom EDM searches could yield competitive or even more severe constraints. We analyze the quantitative impact of hadronic and nuclear theory uncertainties on the interpretation of the latter systems and conclude that these uncertainties cloud the impact of projected improvements in the corresponding experimental sensitivities.

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arXiv2014

Electroweak Baryogenesis, Electric Dipole Moments, and Higgs Diphoton Decays

We study the viability of electroweak baryogenesis in a two Higgs doublet model scenario augmented by vector-like, electroweakly interacting fermions. Considering a limited, but illustrative region of the model parameter space, we obtain the observed cosmic baryon asymmetry while satisfying present constraints from the non-observation of the permanent electric dipole moment (EDM) of the electron and the combined ATLAS and CMS result for the Higgs boson diphoton decay rate. The observation of a non-zero electron EDM in a next generation experiment and/or the observation of an excess (over the Standard Model) of Higgs to diphoton events with the 14 TeV LHC run or a future $e^+e^-$ collider would be consistent with generation of the observed baryon asymmetry in this scenario.

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arXiv2014

Stepping Into Electroweak Symmetry Breaking: Phase Transitions and Higgs Phenomenology

We study the dynamics of electroweak symmetry-breaking in an extension of the Standard Model where the Higgs sector is augmented by the addition of a real (Y = 0) isospin triplet. We show that this scenario exhibits a novel, two-step electroweak phase transition, wherein the first step provides the strongly first order transition as required for electroweak baryogenesis followed by a second step to the Standard Model Higgs phase that also admits a cold dark matter candidate. We analyze the constraints on this scenario from recent results obtained at the Large Hadron Collider for the Higgs diphoton decay channel. We argue that this two-step scenario can be generalized to extensions of the Standard Model with additional higher-dimensional scalar multiplets that may yield realistic baryogenesis dynamics.

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arXiv2014

Stop on Top

The most natural supersymmetric solution to the hierarchy problem prefers the scalar top partner to be close in mass to the top quark. Experimental searches exclude top squarks across a wide range of masses, but a gap remains when the difference between the masses of the stop and the lightest supersymmetric particle is close to the top mass. We propose to search for stops in this regime by exploiting the azimuthal angular correlation of forward tagging jets in (s)top pair production. As shown in earlier work, this correlation is sensitive to the spin of the heavy states, allowing one to distinguish between top and stop pair production. Here, we demonstrate that this angular information can give a statistically significant stop pair production signal in the upcoming LHC run. While the appropriate simulation including parton showering and detector simulation requires some care, we find stable predictions for the angular correlation using multi-jet merging.

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arXiv2013

Fundamental Symmetry Tests with Nucleons, Nuclei, and Atoms: A Snowmass Report

Present and prospective fundamental symmetry tests with nucleons, nuclei and atoms are probing for possible new physics at the TeV scale and beyond. These ongoing and proposed table-top as well as accelerator-based experiments are thus a vital component of the Intensity Frontier. At the same time, these tests provide increasingly sophisticated probes of long-distance strong interactions that are responsible for the structure of nucleons and nuclei. In this community report, some of the most compelling opportunities with nucleons, nuclei and atoms are summarized, drawing largely on input received from the nuclear and atomic physics communities. In particular, this report includes many contributions submitted to two recent Intensity Frontier Workshops.

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arXiv2013

Theory Uncertainty in Extracting the Proton's Weak Charge: White Paper

We review the state-of-the-art and address open questions relative to the theory uncertainty of the $γ-Z$ box contribution to the $A_{PV}$ measurement within the QWEAK experiment at Jefferson Lab. This white paper summarizes the contributions by participants and discussion sessions on this topic within the MITP Workshop on Precision Electroweak Physics held in Mainz, Germany, September 23 - October 11, 2013 \url{http://indico.cern.ch/conferenceDisplay.py?confId=248072

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