arXiv subjects
Publications and source records attributed to William Shepherd.
This document summarises the proposal of the LHC Dark Matter Working Group on how to present LHC results on $s$-channel simplified dark matter models and to compare them to direct (indirect) detection experiments.
We discuss some consistency tests that must be passed for a successful explanation of a diphoton excess at larger mass scales, generated by a scalar or pseudoscalar state, possibly of a composite nature, decaying to two photons. Scalar states at mass scales above the electroweak scale decaying significantly into photon final states generically lead to modifications of Standard Model Higgs phenomenology. We characterise this effect using the formalism of Effective Field Theory (EFT) and study the modification of the effective couplings to photons and gluons of the Higgs. The modification of Higgs phenomenology comes about in a variety of ways. For scalar $0^+$states, the Higgs and the heavy boson can mix. Lower energy phenomenology gives a limit on the mixing angle, which gets generated at one loop in any such theory explaining the diphoton excess. Even if the mixing angle is set to zero, we demonstrate that a relation exists between lower energy Higgs data and a massive scaler decaying to diphoton final states. If the new boson is a pseudoscalar, we note that if it is composite, it is generic to have an excited scalar partner that can mix with the Higgs, which has a stronger coupling to photons. In the case of a pseudoscalar, we also characterize how lower energy Higgs phenomenology is directly modified using EFT, even without assuming a scalar partner of the pseudoscalar state. We find that naturalness concerns can be accommodated, and that pseudoscalar models are more protected from lower energy constraints.
We investigate the relation between the annihilation of dark matter (DM) particles into lepton pairs and into 2-body final states including one or two photons. We parametrize the DM interactions with leptons in terms of contact interactions, and calculate the loop-level annihilation into monochromatic gamma rays, specifically computing the ratio of the DM annihilation cross sections into two gamma rays versus lepton pairs. While the loop-level processes are generically suppressed in comparison with the tree-level annihilation into leptons, we find that some choices for the mediator spin and coupling structure lead to large branching fractions into gamma-ray lines. This result has implications for a dark matter contribution to the AMS-02 positron excess. We also explore the possibility of mediators which are charged under a dark symmetry and find that, for these loop-level processes, an effective field theory description is accurate for DM masses up to about half the mediator mass.
With the discovery of the Higgs boson the Standard Model has become a complete and comprehensive theory, which has been verified with unparalleled precision and in principle might be valid at all scales. However, several reasons remain why we firmly believe that there should be physics beyond the Standard Model. Experiments such as the LHC, new $B$ factories, and earth- and space-based astro-particle experiments provide us with unique opportunities to discover a coherent framework for many of the long-standing puzzles of our field. Here we explore several significant interconnections between the physics of the Higgs boson, the physics of flavour, and the experimental clues we have about dark matter.
In this report we summarize the many dark matter searches currently being pursued through four complementary approaches: direct detection, indirect detection, collider experiments, and astrophysical probes. The essential features of broad classes of experiments are described, each with their own strengths and weaknesses. The complementarity of the different dark matter searches is discussed qualitatively and illustrated quantitatively in two simple theoretical frameworks. Our primary conclusion is that the diversity of possible dark matter candidates requires a balanced program drawing from all four approaches.
Dark matter that was once in thermal equilibrium with the Standard Model is generally prohibited from obtaining all of its mass from the electroweak or QCD phase transitions. This implies a new scale of physics and mediator particles needed to facilitate dark matter annihilations. In this work, we consider scenarios where thermal dark matter annihilates via scalar mediators that are colored and/or electrically charged. We show how partial wave unitarity places upper bounds on the masses and couplings on both the dark matter and mediators. To do this, we employ effective field theories with dark matter as well as three flavors of sleptons or squarks with minimum flavor violation. For Dirac (Majorana) dark matter that annihilates via mediators charged as left-handed sleptons, we find an upper bound around 45 TeV (7 TeV) for both the mediator and dark matter masses, respectively. These bounds vary as the square root of the number of colors times the number of flavors involved. Therefore the bounds diminish by root two for right handed selectrons. The bounds increase by root three and root six for right and left handed squarks, respectively. Finally, because of the interest in natural models, we also focus on an effective field theory with only stops. We find an upper bound around 32 TeV (5 TeV) for both the Dirac (Majorana) dark matter and stop masses. In comparison to traditional naturalness arguments, the stop bound gives a firmer, alternative expectation on when new physics will appear. Similar to naturalness, all of the bounds quoted above are valid outside of a defined fine-tuned regions where the dark matter can co-annihilate. The bounds in this region of parameter space can exceed the well-known bounds from Griest and Kamionkowski. We briefly describe the impact on planned and existing direct detection experiments and colliders.
Dark matter that was once in thermal equilibrium with the Standard Model is generally prohibited from obtaining all of its mass from the electroweak phase transition. This implies a new scale of physics and mediator particles to facilitate dark matter annihilation. In this work, we focus on dark matter that annihilates through a generic gauge boson portal. We show how partial wave unitarity places upper bounds on the dark gauge boson, dark Higgs and dark matter masses. Outside of well-defined fine-tuned regions, we find an upper bound of 9 TeV for the dark matter mass when the dark Higgs and dark gauge bosons both facilitate the dark matter annihilations. In this scenario, the upper bound on the dark Higgs and dark gauge boson masses are 10 TeV and 16 TeV, respectively. When only the dark gauge boson facilitates dark matter annihilations, we find an upper bound of 3 TeV and 6 TeV for the dark matter and dark gauge boson, respectively. Overall, using the gauge portal as a template, we describe a method to not only place upper bounds on the dark matter mass but also on the new particles with Standard Model quantum numbers. We briefly discuss the reach of future accelerator, direct and indirect detection experiments for this class of models.
We explore the possibility of explaining a gamma-ray excess in the Galactic Center, originally pointed out by Hooper, collaborators, and other groups, in an effective field theory framework. We assume that dark matter annihilation is mediated by particles heavy enough to be integrated out, and that such particles couple to all quark families. We calculate the effective coupling required to explain the annihilation signal in the Galactic Center, and compare with bounds from direct detection, collider searches, and the requirement that the dark matter particle make up the appropriate fraction of the universal energy budget. We find that only a very small set of operators can explain the gamma-ray excess while being consistent with other constraints. Specifically, for scalar dark matter the viable options are one scalar-type coupling to quarks and one interaction with gluons, while for fermionic (Dirac) dark matter the viable options are two scalar-type dimension-7 operators or a dimension-6 vector-type operator. In all cases, future searches with the Large Hadron Collider should probe the relevant operators' effective energy scale, while all viable interactions should escape direct detection experiments.
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.
Theoretical and experimental techniques employed in dedicated searches for dark matter at hadron colliders are reviewed. Bounds from the 7 and 8 TeV proton-proton collisions at the LHC on dark matter interactions have been collected and the results interpreted. We review the current status of the Effective Field Theory picture of dark matter interactions with the Standard Model. Currently, LHC experiments have stronger bounds on operators leading to spin-dependent scattering than direct detection experiments, while direct detection probes are more constraining for spin-independent scattering for WIMP masses above a few GeV.
We consider the contributions of individual new particles to the anomalous magnetic moment of the muon, utilizing the generic framework of simplified models. We also present analytic results for all possible one-loop contributions, allowing easy application of these results for more complete models which predict more than one particle capable of correcting the muon magnetic moment. Additionally, we provide a Mathematica code to allow the reader straightforwardly compute any 1-loop contribution. Furthermore, we derive bounds on each new particle considered, assuming either the absence of other significant contributions to $a_μ$ or that the anomaly has been resolved by some other mechanism. The simplified models we consider are constructed without the requirement of $SU(2)_L$ invariance, but appropriate chiral coupling choices are also considered. In summary, we found the following particles capable of explaining the current discrepancy, assuming unit couplings: $2$~TeV ($0.3$~TeV) neutral scalar with pure scalar (chiral) couplings, $4$~TeV doubly charged scalar with pure pseudoscalar coupling, $0.3-1$~TeV neutral vector boson depending on what couplings are used (vector, axial, or mixed), $0.5-1$~TeV singly-charged vector boson depending on which couplings are chosen, and $3$~TeV doubly-charged vector-coupled bosons. We also derive the following $1σ$ lower bounds on new particle masses assuming unit couplings and that the experimental anomaly has been otherwise resolved: a doubly charged pseudo-scalar must be heavier than $7$~TeV, a neutral scalar than $3$~TeV, a vector-coupled new neutral boson $600$~GeV, an axial-coupled neutral boson $1.5$~TeV, a singly-charged vector-coupled $W^\prime$ $1$~TeV, a doubly-charged vector-coupled boson $5$~TeV, scalar leptoquarks $10$~TeV, and vector leptoquarks $10$~TeV.
We present a new technique for distinguishing the hadronic decays of boosted heavy particles from QCD backgrounds based on wavelet transforms. As an initial exploration, we illustrate the technique in the particular case of hadronic $W$ boson decays, comparing it to the ``mass drop'' cut currently used by the LHC experiments. We apply wavelet cuts, which make use of complementary information, and in combination with the mass drop cut results in an improvement of $\sim$7% in discovery reach of hadronic $W$ boson final states over a wide range of transverse momenta.
The recent discovery of a Higgs boson with mass of about 125 GeV, along with its striking similarity to the prediction from the Standard Model, informs and constrains many models of new physics. The Higgs mass exhausts one out of three input parameters of the minimal, five-dimensional version of universal extra dimension models, the other two parameters being the Kaluza-Klein (KK) scale and the cut-off scale of the theory. The presence of KK fermions with large coupling to the Higgs implies a short-lived electro-weak vacuum, unless the cut-off scale is at most a few times higher than the KK mass scale, providing an additional tight constraint to the theory parameter space. Here, we focus on the lightest KK particle as a dark matter candidate, and investigate the regions of parameter space where such particle has a thermal relic density in accord with the cosmological dark matter density. We find the paradoxical result that, for low enough cutoff scales consistent with vacuum stability, larger than previously thought KK mass scales become preferred to explain the dark matter abundance in the universe. We explain this phenomenon by pinpointing the additional particles which, at such low cutoffs, become close enough in mass to the dark matter candidate to coannihilate with it. We make predictions for both collider and direct dark matter searches that might soon close in on all viable theory parameter space.
It is sometimes argued that a virtue of pushing the supersymmetry breaking scale above 1 PeV is that no particular flavor structure is required in the soft sector in order to evade bounds on flavor-changing neutral currents. However, without flavor structure, suppressing generic Planck-suppressed contributions to proton decay requires even higher SUSY scales, of order $10^{11}$ ($10^9$) GeV for degenerate (mini-split) gauginos and scalars. With flavor structure, the question of whether proton decay or flavor symmetries are more constraining is model-dependent, but it straightforward to find simple models where both constraints are satisfied for much lower SUSY scales.
We study the muon anomalous magnetic moment $(g-2)_μ$ in the context of the reduced minimal 3-3-1 model recently proposed in the literature. In particular, its spectrum contains a doubly charged scalar ($H^{\pm \pm}$) and gauge boson ($U^{\pm \pm}$), new singly charged vectors ($V^{\pm}$) and a $Z^{\prime}$ boson, each of which might give a sizeable contribution to the $(g-2)_μ$. We compute the 1-loop contributions from all these new particles to the $(g-2)_μ$. We conclude that the doubly charged vector boson provides the dominant contribution, and by comparing our results with the experimental constraints we derive an expected value for the scale of $SU(3)_L\otimes U(1)_N$ symmetry breaking $v_χ \sim 2$ TeV. We also note that, if the discrepancy in the anomalous moment is resolved in the future without this model then the constraints will tighten to requiring $v_χ> 2.7$ TeV with current precision, and will entirely rule out the model if the expected precision is achieved by the future experiment at Fermilab.
We consider LHC searches for dark matter in the mono-Higgs channel using the tools of effective field theory. This channel takes unique advantage of the presence of $SU(2)_L$ breaking in those operators to avoid the need for any initial-state radiation, usually necessary to tag the production of invisible particles. We find that sensitivities to parameters describing dark matter interactions with standard model particles are comparable to those from monojet searches for a subset of the usually-considered operators, and we present for the first time bounds from collider searches on operators which couple DM to only the Higgs field or its covariant derivatives.
In this Report we discuss the four complementary searches for the identity of dark matter: direct detection experiments that look for dark matter interacting in the lab, indirect detection experiments that connect lab signals to dark matter in our own and other galaxies, collider experiments that elucidate the particle properties of dark matter, and astrophysical probes sensitive to non-gravitational interactions of dark matter. The complementarity among the different dark matter searches is discussed qualitatively and illustrated quantitatively in several theoretical scenarios. Our primary conclusion is that the diversity of possible dark matter candidates requires a balanced program based on all four of those approaches.
We explore the connection between pair production of dark matter particles at collider experiments and annihilation of dark matter in the early and late universe, with a focus on the correlation between the two time-reversed processes. We consider both a model-independent effective theory framework, where the initial and final states are assumed to not change under time-reversal, and concrete UV-complete models within the framework of supersymmetric extensions to the Standard Model. Even within the effective theory framework (where crossing symmetry is in some sense assumed), we find that the predictions of that symmetry can vary by orders of magnitude depending on the details of the selected effective interaction. Within the supersymmetric models we consider, we find that there is an even wilder variation in the expectations one can derive for collider observables based on cross-symmetric processes such as having a thermal relic or given indirect dark matter detection rates. We also explore additional "pitfalls" where naïve crossing symmetry badly fails, including models with very light mediators leading to Sommerfeld enhancements and/or dark matter bound states.