arXiv subjects

Spencer Chang

Publications and source records attributed to Spencer Chang.

52 records · Page 3Linked to original sources

arXiv2009

Watching Worlds Collide: Effects on the CMB from Cosmological Bubble Collisions

We extend our previous work on the cosmology of Coleman-de Luccia bubble collisions. Within a set of approximations we calculate the effects on the cosmic microwave background (CMB) as seen from inside a bubble which has undergone such a collision. We find that the effects are always qualitatively similar--an anisotropy that depends only on the angle to the collision direction--but can produce a cold or hot spot of varying size, as well as power asymmetries along the axis determined by the collision. With other parameters held fixed the effects weaken as the amount of inflation which took place inside our bubble grows, but generically survive order 10 efolds past what is required to solve the horizon and flatness problems. In some regions of parameter space the effects can survive arbitrarily long inflation.

hep-th↗

arXiv2009

Discovering a Nonstandard Higgs in a Standard Way

In this note, we explore the Tevatron and LHC detection prospects for a nonstandard Higgs whose decays contain both missing and visible energy. In particular, we consider the decay $h \to χ_2 χ_1$ where $χ_1$ is stable and $χ_2$ decay back to $χ_1$ and additional visible particles. We focus on adapting standard searches for both Higgses and supersymmetry to these decays. In these channels, the Tevatron has little reach, but the LHC has good prospects for decays that involve leptons or neutrinos. In particular, the standard WW* search for a Higgs produced in vector boson fusion has the largest reach. Trilepton searches and invisible Higgs searches are less powerful, but can give corroborating evidence. However, such standard searches could become inefficient for these signals if the analyses adopt model dependent tools such as matrix elements and neural nets.

hep-ph↗

arXiv2009

Fake Dark Matter at Colliders

If the dark matter (DM) consists of a weakly interacting massive particle (WIMP), it can be produced and studied at future collider experiments like those at the LHC. The production of collider-stable WIMPs is characterized by hard scattering events with large missing transverse energy. Here we point out that the discovery of this well-characterized DM signal may turn out to be a red herring. We explore an alternative explanation -- fake dark matter -- where the only sources of missing transverse energy are standard model neutrinos. We present examples of such models, focusing on supersymmetric models with R-parity violation. We also briefly discuss means of differentiating fake dark matter from the production of new collider-stable particles.

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arXiv2008

Inelastic Dark Matter in Light of DAMA/LIBRA

Inelastic dark matter, in which WIMP-nucleus scatterings occur through a transition to an excited WIMP state ~ 100 keV above the ground state, provides a compelling explanation of the DAMA annual modulation signal. We demonstrate that the relative sensitivities of various dark matter direct detection experiments are modified such that the DAMA annual modulation signal can be reconciled with the absence of a reported signal at CDMS-Soudan, XENON10, ZEPLIN, CRESST, and KIMS for inelastic WIMPs with masses O(100 GeV). We review the status of these experiments, and make predictions for upcoming ones. In particular, we note that inelastic dark matter leads to highly suppressed signals at low energy, with most events typically occurring between 20 to 45 keV (unquenched) at xenon and iodine experiments, and generally no events at low (~ 10 keV) energies. Suppressing the background in this high energy region is essential to testing this scenario. The recent CRESST data suggest seven observed tungsten events, which is consistent with expectations from this model. If the tungsten signal persists at future CRESST runs, it would provide compelling evidence for inelastic dark matter, while its absence should exclude it.

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arXiv2008

Using the Energy Spectrum at DAMA/LIBRA to Probe Light Dark Matter

A weakly interacting massive particle (WIMP) weighing only a few GeV has been invoked as an explanation for the signal from the DAMA/LIBRA experiment. We show that the data from DAMA/LIBRA are now powerful enough to strongly constrain the properties of any putative WIMP. Accounting for the detailed recoil spectrum, a light WIMP with a Maxwellian velocity distribution and a spin-independent (SI) interaction cannot account for the data. Even neglecting the spectrum, much of the parameter space is excluded by limits from the DAMA unmodulated signal at low energies. Significant modifications to the astrophysics or particle physics can open light mass windows.

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arXiv2008

Nonstandard Higgs Boson Decays

This review summarizes the motivations for and phenomenological consequences of nonstandard Higgs boson decays, with emphasis on final states containing a pair of non-Standard-Model particles that subsequently decay to Standard Model particles. Typically these non-Standard-Model particles are part of a ``hidden'' sector, for example a pair of neutral Higgs bosons or a pair of unstable neutralinos. We emphasize that such decays allow for a Higgs substantially below the Standard Model Higgs LEP limit of 114 GeV. This in turn means that the ``fine-tuning'' problems of many Beyond the Standard Model (BSM) theories, in particular supersymmetric models, can be eliminated while achieving excellent consistency with precision electroweak data which favor a Higgs boson with mass below $100\gev$ and standard $WW$, $ZZ$, and top couplings.

hep-ph↗

arXiv2007

When Worlds Collide

We analyze the cosmological signatures visible to an observer in a Coleman-de Luccia bubble when another such bubble collides with it. We use a gluing procedure to generalize the results of Freivogel, Horowitz, and Shenker to the case of a general cosmological constant in each bubble and study the resulting spacetimes. The collision breaks the isotropy and homogeneity of the bubble universe and provides a cosmological "axis of evil" which can affect the cosmic microwave background in several unique and potentially detectable ways. Unlike more conventional perturbations to the inflationary initial state, these signatures can survive even relatively long periods of inflation. In addition, we find that for a given collision the observers in the bubble with smaller cosmological constant are safest from collisions with domain walls, possibly providing another anthropic selection principle for small positive vacuum energy.

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arXiv2007

Nonstandard Higgs Decays with Visible and Missing Energy

Experimental and theoretical clues both suggest that the Higgs boson has a mass below the LEP2 lower limit of 114.4 GeV. If true, this suggests that the dominant Higgs decay is nonstandard while the production cross sections remain unmodified. We consider the possibility of nonstandard Higgs decays in the language of On-Shell Effective Theories (OSETs), and discuss a little considered class of Higgs decays, with a topology of both visible and missing energy. We study existing LEP constraints on such decays, and find that such decays would in general be allowed experimentally for ~ 100 GeV mass Higgses. Simple model realizations of these decays exist, which can occur in supersymmetric models and also in models with additional massive neutrinos. Some potential searches that can be performed at Tevatron and LHC, contained in standard supersymmetry topologies of leptons and missing energy, offer the possibility of discovering such Higgses.

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arXiv2006

Visible Cascade Higgs Decays to Four Photons at Hadron Colliders

The presence of a new singlet scalar particle a can open up new decay channels for the Higgs boson, through cascades of the form h -> 2a -> X, possibly making discovery through standard model channels impossible. If a is CP-odd, its decay products are particularly sensitive to physics beyond the standard model. Quantum effects from heavy fields can naturally make gluonic decay, a -> 2g, the dominant decay mode, resulting in a h -> 4 g decay which is difficult to observe at hadron colliders, and is allowed by LEP for m_h > 82 GeV. However, there are usually associated decays with photons, either h -> 2g 2gamma or h -> 4gamma, which are more promising. The decay h -> 2g 2gamma only allows discovery of the a particle and not the Higgs whereas h -> 4gamma is a clean channel that can discover both particles. We determine what branching ratios are required for discovery at LHC and find that with 300 fb^-1 of luminosity, a branching ratio of order 10^-4 is sufficient for a large region of Higgs masses. Due to a lower expected luminosity of ~ 8 fb^-1, discovery at the Tevatron requires more than 5 x 10^-3 in branching ratio.

hep-ph↗

arXiv2006

Naturalness and Higgs Decays in the MSSM with a Singlet

The simplest extension of the supersymmetric standard model - the addition of one singlet superfield - can have a profound impact on the Higgs and its decays. We perform a general operator analysis of this scenario, focusing on the phenomenologically distinct scenarios that can arise, and not restricting the scope to the narrow framework of the NMSSM. We reexamine decays to four b quarks and four tau's, finding that they are still generally viable, but at the edge of LEP limits. We find a broad set of Higgs decay modes, some new, including those with four gluon final states, as well as more general six and eight parton final states. We find the phenomenology of these scenarios is dramatically impacted by operators typically ignored, specifically those arising from D-terms in the hidden sector, and those arising from weak-scale colored fields. In addition to sensitivity of m_Z, there are potential tunings of other aspects of the spectrum. In spite of this, these models can be very natural, with light stops and a Higgs as light as 82 GeV. These scenarios motivate further analyses of LEP data as well as studies of the detection capabilities of future colliders to the new decay channels presented.

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arXiv2006

A Supersymmetric Twin Higgs

We present a supersymmetric realization of the twin Higgs mechanism, which cancels off all contributions to the Higgs mass generated above a scale f. Radiative corrections induced by the top quark sector lead to a breaking of the twin sector electroweak symmetry at a scale f ~ TeV. In our sector, below the scale f, these radiative corrections from the top quark are present but greatly weakened, naturally allowing a Z boson mass an order of magnitude below f, even with a top squark mass of order 1 TeV and a messenger scale near the Planck mass. A sufficient quartic interaction for our Higgs boson arises from the usual gauge contribution together with a radiative contribution from a heavy top squark. The mechanism requires the presence of an SU(2)-adjoint superfield, and can be simply unified. Naturalness in these theories is usually associated with light winos and sleptons, and is largely independent of the scale of the colored particles. The assumption of unification naturally predicts the existence of many exotic fields. The theory often has particles which may be stable on collider timescales, including an additional color octet superfield. In the limit that m_SUSY >> f, the mechanism yields a UV completion of the non-supersymmetric twin Higgs, but with the notable improvement of a tree-level quartic for the standard model Higgs. In this framework, a successful UV completion requires the existence of new charged fields well below the scale f.

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arXiv2005

The New Fat Higgs: Slimmer and More Attractive

In this paper we increase the MSSM tree level higgs mass bound to a value that is naturally larger than the LEP-II search constraint by adding to the superpotential a $λS H_{u}H_{d}$ term, as in the NMSSM, and UV completing with new strong dynamics {\it before} $λ$ becomes non-perturbative. Unlike other models of this type the higgs fields remain elementary, alleviating the supersymmetric fine-tuning problem while maintaining unification in a natural way.

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arXiv2004

Unitarity of Little Higgs Models Signals New Physics of UV Completion

The ``Little Higgs'' opens up a new avenue for natural electroweak symmetry breaking in which the standard model Higgs particle is realized as a pseudo-Goldstone boson and thus is generically light. The symmetry breaking structure of the Little Higgs models predicts a large multiplet of (pseudo-)Goldstone bosons and their low energy interactions below the ultraviolet (UV) completion scale $Λ\sim 4πf \sim O(10)$ TeV, where $f$ is the Goldstone decay constant. We study unitarity of the Little Higgs models by systematically analyzing the high energy scatterings of these (pseudo-)Goldstone bosons. We reveal that the collective effect of the Goldstone scatterings via coupled channel analysis tends to push the unitarity violation scale $Λ_U$ significantly below the conventional UV scale $Λ\sim 4πf$ as estimated by naive dimensional analysis (NDA). Specifically, $Λ_U \sim (3-4)f$, lying in the multi-TeV range for $f\sim 1$ TeV. We interpret this as an encouraging sign that the upcoming LHC may explore aspects of Little Higgs UV completions, and we discuss some potential signatures. The meanings of the two estimated UV scales $Λ_U$ (from unitarity violation) and $Λ$ (from NDA) together with their implications for an effective field theory analysis of the Little Higgs models are also discussed.

hep-ph↗

arXiv2004

A "Littlest Higgs" Model with Custodial SU(2) Symmetry

In this note, a ``littlest higgs'' model is presented which has an approximate custodial SU(2) symmetry. The model is based on the coset space $SO(9)/(SO(5)\times SO(4))$. The light pseudo-goldstone bosons of the theory include a {\it single} higgs doublet below a TeV and a set of three $SU(2)_W$ triplets and an electroweak singlet in the TeV range. All of these scalars obtain approximately custodial SU(2) preserving vacuum expectation values. This model addresses a defect in the earlier $SO(5)\times SU(2)\times U(1)$ moose model, with the only extra complication being an extended top sector. Some of the precision electroweak observables are computed and do not deviate appreciably from Standard Model predictions. In an S-T oblique analysis, the dominant non-Standard Model contributions are the extended top sector and higgs doublet contributions. In conclusion, a wide range of higgs masses is allowed in a large region of parameter space consistent with naturalness, where large higgs masses requires some mild custodial SU(2) violation from the extended top sector.

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arXiv2004

Little Higgs and Custodial SU(2)

In this note we present a little Higgs model that has custodial SU(2) as an approximate symmetry. This theory is a simple modification of the ``Minimal Moose'' with SO(5) global symmetries protecting the Higgs mass. This allows for a simple limit where TeV physics makes small contributions to precision electroweak observables. The spectrum of particles and their couplings to Standard Model fields are studied in detail. At low energies this model has two Higgs doublets and it favours a light Higgs from precision electroweak bounds, though for different reasons than in the Standard Model. The limit on the breaking scale, f, is roughly 700 GeV, with a top partner of 2 TeV, W' and B' of 2.5 TeV, and heavy Higgs partners of 2 TeV. These particles are easily accessible at hadron colliders.

hep-ph↗

arXiv2003

Quantum Modified Mooses

We summarize our findings on the quantum moduli constraints and superpotentials of an infinite family of moose extensions of $n_f = n_c$ SUSY QCD. For $n_c=2$, we perform concrete calculations using traditional integrating out techniques as well as Intriligator's ``integrating in'' technique. Checking the constraints and superpotentials in the limits of setting $Λ$'s to zero or integrating out mass terms, we find that the quantum moduli constraints are local in theory space and are equivalent to a consistent structure of ``splitting relations'' amongst the different theories. Extending the results to arbitrary $n_c$, we show that the splitting relations, along with a set of rules for flowing from a high energy theory to a low energy theory, incorporate much of the physics of the moose chain. The relations can be used both to simplify perturbative calculations of symmetry breaking and to incorporate nonperturbative effects.

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