arXiv subjects

Matthew J. Strassler

Publications and source records attributed to Matthew J. Strassler.

At least 37 records · Page 2Linked to original sources

arXiv2010

On the Eikonal Approximation in AdS Space

We explore the eikonal approximation to graviton exchange in AdS_5 space, as relevant to scattering in gauge theories. We restrict ourselves to the regime where conformal invariance of the dual gauge theory holds, and to large 't Hooft coupling where the computation involves pure gravity. We give a heuristic argument, a direct loop computation, and a shock wave derivation. The scalar propagator in AdS_3 plays a key role, indicating that even at strong coupling, two-dimensional conformal invariance controls high-energy four-dimensional gauge-theory scattering.

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arXiv2010

On The Pomeron at Large 't Hooft Coupling

We begin the process of unitarizing the Pomeron at large 't Hooft coupling. We do so first in the conformal regime, which applies to good accuracy to a number of real and toy problems in QCD. We rewrite the conformal Pomeron in the $J$-plane and transverse position space, and then work out the eikonal approximation to multiple Pomeron exchange. This is done in the context of a more general treatment of the complex $J$-plane and the geometric consequences of conformal invariance. The methods required are direct generalizations of our previous work on single Pomeron exchange and on multiple graviton exchange in AdS space, and should form a starting point for other investigations. We consider unitarity and saturation in the conformal regime, noting elastic and absorptive effects, and exploring where different processes dominate. Our methods extend to confining theories and we briefly consider the Pomeron kernel in this context. Though there is important model dependence that requires detailed consideration, the eikonal approximation indicates that the Froissart bound is generically both satisfied and saturated.

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arXiv2009

A Simple-Minded Unitarity Constraint and an Application to Unparticles

Unitarity, a powerful constraint on new physics, has not always been properly accounted for in the context of hidden sectors. Feng, Rajaraman and Tu have suggested that large (pb to nb) multi-photon or multi-lepton sugnals could be generated at the LHC through the three-point functions of a conformally-invariant hidden sector (an "unparticle" sector.) Because of the conformal invariance, the kinematic distributions are calculable. However, the cross-sections for many such processes grow rapidly with energy, and at some high scale, to preserve unitarity, conformal invariance must break down. Requiring that conformal invariance not be broken, and that no signals be already observed at the Tevatron, we obtain a strong unitarity bound on multi-photon events at the (10 TeV) LHC. For the model of Feng et al., even with extremely conservative assumptions, cross-sections must be below 25 fb, and for operator dimension near 2, well below 1 fb. In more general models, four-photon signals could still reach cross-sections of a few pb, though bounds below 200 fb are more typical. Our methods apply to a wide variety of other processes and settings.

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arXiv2009

A Pure-Glue Hidden Valley I. States and Decays

It is possible that the standard model is coupled, through new massive charged or colored particles, to a hidden sector whose low energy dynamics is controlled by a pure Yang-Mills theory, with no light matter. Such a sector would have numerous metastable "hidden glueballs" built from the hidden gluons. These states would decay to particles of the standard model. We consider the phenomenology of this scenario, and find formulas for the lifetimes and branching ratios of the most important of these states. The dominant decays are to two standard model gauge bosons, or by radiative decays with photon emission, leading to jet- and photon-rich signals.

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arXiv2009

Metastable supersymmetry breaking and multitrace deformations of SQCD

Metastable vacua in supersymmetric QCD in the presence of single and multitrace deformations of the superpotential are explored, with the aim of obtaining an acceptable phenomenology. The metastable vacua appear at one loop, have a broken R-symmetry, and a magnetic gauge group that is completely Higgsed. With only a single trace deformation, the adjoint fermions from the meson superfield are approximately massless at one loop, even though they are massive at tree level and R-symmetry is broken. Consequently, if charged under the standard model, they are unacceptably light. A multitrace quadratic deformation generates fermion masses proportional to the deformation parameter. Phenomenologically viable models of direct gauge mediation can then be obtained, and some of their features are discussed.

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arXiv2008

Flesh and Blood, or Merely Ghosts? Some Comments on the Multi-Muon Study at CDF

A recent paper by the CDF collaboration suggests (but does not claim) an anomalous event sample containing muons produced with large impact parameter, often with high multiplicity and at small angles from one another. This curious hint of a signal is potentially consistent with the hidden valley scenario, as well as with some other classes of models. Despite its tenuous nature, this hint highlights the experimental difficulties raised by such signals, and merits some consideration. Some of the simplest interpretations of the data, such as a light neutral particle decaying to muon and/or tau pairs, are largely disfavored; three-body decays to $ττν$ appear slightly better. An alternative speculative possibility -- a "micro-cascade decay" -- might be consistent with the data. It is suggested that the experimentalists involved provide additional plots showing invariant mass distributions of same- and opposite-sign dimuon pairs, invariant masses of various classes of displaced vertices, and spatial correlations among vertices within a cone.

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arXiv2008

Phenomenology of Hidden Valleys at Hadron Colliders

We study the phenomenology of, and search techniques for, a class of "Hidden Valleys." These models are characterized by low mass (well below a TeV) bound states resulting from a confining gauge interaction in a hidden sector; the states include a spin-one resonance that can decay to lepton pairs. Assuming that the hidden sector communicates to the Standard Model (SM) through TeV suppressed operators, taking into account the constraint from the $Z$ pole physics at LEP, searches at Tevatron may be difficult in the particular class of Hidden Valleys we consider, so that we concentrate on the searches at the LHC. Hidden Valley events are characterized by high multiplicities of jets and leptons in the final state. Depending on the scale of confinement in the hidden sector, the events are typically more spherical, with lower thrust and higher incidences of isolated leptons, than those from the SM background processes. Most notably, high cluster invariant mass and very narrow, low mass resonances in lepton pairs are the key observables to identify the signal. We use these characteristics to develop a set of cuts to separate the Hidden Valley from SM, and show that with these cuts LHC has a significant reach in the parameter space. Our strategies are quite general and should apply well beyond the particular class of models studied here.

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arXiv2008

On the Phenomenology of Hidden Valleys with Heavy Flavor

A preliminary investigation of a large class of Hidden Valley models is presented. These models are more challenging than those considered in arXiv:0712.2041 [hep-ph]; although they produce a new light resonance which decays to heavy standard model fermions, they exhibit no light dilepton resonance. A heavy $Z'$ decaying to v-hadrons, which in turn decay mainly to bottom quarks and tau leptons, is considered; six case studies are investigated, using a new Monte Carlo simulation package. It is found that the one-to-one correspondence of jets and partons is badly broken, and the high-multiplicity heavy-flavor signal probably cannot be isolated by counting jets, with or without heavy-flavor tags. Instead, other measures, such as counting and correlating vertices or displaced tracks, and possibly counting of (non-isolated) muons and use of event-shape variables, should be combined with scalar transverse energy and/or missing transverse energy to reduce backgrounds. Within the resulting sample, searches for the v-pion mass resonance in both di-jet and single-jet invariant mass can help confirm a signal. The best observable in a perfect calorimeter seems to be single-jet invariant mass for jets of larger radius ($R$=0.7), although this needs further study in a realistic setting. A more detailed signal-to-background study is needed as a next step, but will face the difficulty of estimating the various high-multiplicity backgrounds.

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arXiv2008

Holographic Double Diffractive Scattering

The holographic description of Pomeron exchange in a strongly-coupled gauge theory with an AdS dual is extended to the case of two to three scattering. We study the production event of a central particle via hadron-hadron scattering in the double Regge kinematic regime of large center-of-momentum energy and fixed momentum transfer. The computation reduces to the overlap of a holographic wave function for the central particle with a source function for the Pomerons. The formalism is applied to scalar glueball production and the resulting amplitude is studied in various kinematic limits.

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arXiv2008

Why Unparticle Models with Mass Gaps are Examples of Hidden Valleys

Hidden valleys, hidden sectors with multi-particle dynamics and a mass gap, can produce striking and unusual final states at the LHC. Unparticle models, hidden-sectors with conformal dynamics and no (or a very small) mass gap, can result in unusual kinematic features that indirectly reflect the conformal dynamics. When sufficiently large mass gaps are added to unparticle models, they become hidden valley models. Predictions using unparticle propagators alone overlook the most striking signals, which are typically of hidden-valley type. Inclusive signatures often cannot be predicted from unparticle dimensions, and exclusive signatures are often visible and can be spectacular. Among possible signatures are: Higgs decays to pairs of particles that in turn decay to two quarks, leptons or gauge bosons, possibly with displaced vertices; Higgs, top, and neutralino decays to more than six particles; resonances below an ``unparticle'' continuum which produce multi-body final states; etc. The Stephanov model is deconstructed, reconstructed, and shown to be a hidden valley model. Some effects of strong dynamics on hidden valley observables, not predictable using unparticle methods, are discussed, including resonances, reduced flavor symmetry breaking, reduced supersymmetry breaking, and a strongly enhanced hidden parton shower.

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arXiv2007

The Pomeron and Gauge/String Duality

The traditional description of high-energy small-angle scattering in QCD has two components -- a soft Pomeron Regge pole for the tensor glueball, and a hard BFKL Pomeron in leading order at weak coupling. On the basis of gauge/string duality, we present a coherent treatment of the Pomeron. In large-N QCD-like theories, we use curved-space string-theory to describe simultaneously both the BFKL regime and the classic Regge regime. The problem reduces to finding the spectrum of a single j-plane Schrodinger operator. For ultraviolet-conformal theories, the spectrum exhibits a set of Regge trajectories at positive t, and a leading j-plane cut for negative t, the cross-over point being model-dependent. For theories with logarithmically-running couplings, one instead finds a discrete spectrum of poles at all t, where the Regge trajectories at positive t continuously become a set of slowly-varying and closely-spaced poles at negative t. Our results agree with expectations for the BFKL Pomeron at negative t, and with the expected glueball spectrum at positive t, but provide a framework in which they are unified. Effects beyond the single Pomeron exchange are briefly discussed.

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arXiv2006

Possible Effects of a Hidden Valley on Supersymmetric Phenomenology

A hidden valley sector may havea profound impact on the classic phenomenology of supersymmetry. This occurs if the LSP lies in the valley sector. In addition to reducing the standard missing energy signals and possibly providing displaced vertices (phenomena familiar from gauge-mediated and R-parity-violating models) it may lead to a variable multiplicity of new neutral particles, whose decays produce soft jets and/or leptons, and perhaps additional displaced vertices. Combined, these issues might obscure supersymmetric particle production from search strategies used on current Tevatron data and planned for the LHC. The same concerns arise more generally for any model that has a symmetry (such as T-parity or KK-parity) realized nontrivially in both the standard-model and the hidden-valley sectors. Possible strategies for experimental detection are discussed, and the potential importance of the LHCb detector is noted.

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arXiv2006

Discovering the Higgs Through Highly-Displaced Vertices

We suggest that the Higgs could be discovered at the Tevatron or the LHC (perhaps at the LHCb detector) through decays with one or more substantially displaced vertices from the decay of new neutral particles. This signal may occur with a small but measurable branching fraction in the recently-described ``hidden valley'' models, hep-ph/0604261; weakly-coupled models with multiple scalars, including those of hep-ph/0511250, can also provide such signals, potentially with a much larger branching fraction. This decay channel may extend the Higgs mass reach for the Tevatron. Unusual combinations of b jets, lepton pairs and/or missing energy may accompany this signal.

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arXiv2006

Echoes of a Hidden Valley at Hadron Colliders

We consider examples of ``hidden-valley'' models, in which a new confining gauge group is added to the standard model. Such models often arise in string constructions, and elsewhere. The resulting (electrically-neutral) bound states can have low masses and long lifetimes, and could be observed at the LHC and Tevatron. Production multiplicities are often large. Final states with heavy flavor are common; lepton pairs, displaced vertices and/or missing energy are possible. Accounting for LEP constraints, we find LHC production cross-sections typically in the 1-100 fb range, though they can be larger. It is possible the Higgs boson could be discovered at the Tevatron through rare decays to the new particles.

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arXiv2006

Adjoint Trapping: A New Phenomenon at Strong 't Hooft Coupling

Adding matter of mass m, in the fundamental representation of SU(N), to N=4 supersymmetric Yang-Mills theory, we study ``generalized quarkonium'' containing a (s)quark, an anti(s)quark, and J massless (or very light) adjoint particles. At large 't Hooft coupling $λ$ >> 1, the states of spin <= 1 are surprisingly light (Kruczenski et al., hep-th/0304032) and small (hep-th/0312071) with a J-independent size of order $\sqrtλ/m$. This ``trapping'' of adjoint matter in a region small compared with its Compton wavelength and compared to any confinement scale in the theory is an unfamiliar phenomenon, as it does not occur at small $λ$. We explore adjoint trapping further by considering the limit of large J. In particular, for J >> $\sqrtλ$ >> 1, we expect the trapping phenomenon to become unstable. Using Wilson loop methods, we show that a sharp transition, in which the generalized quarkonium states become unbound (for massless adjoints) occurs at $J \simeq 0.22 \sqrtλ$. If the adjoint scalars of N=4 are massive and the theory is confining (as, for instance, in N=1* theories) then the transition becomes a cross-over, across which the size of the states changes rapidly from ~$\sqrtλ/m$ to something of order the confinement scale ~ $Λ^{-1}$.

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arXiv2005

On the Couplings of Vector Mesons in AdS/QCD

We address, in the AdS/CFT context, the issue of the universality of the couplings of the rho meson to other hadrons. Exploring some models, we find that generically the rho-dominance prediction f_ρg_{ρH H}=m_ρ^2 does not hold, and that g_{ρH H} is not independent of the hadron H. However, we prove that, in any model within the AdS/QCD context, there are two limiting regimes where the g_{ρH H}, along with the couplings of all excited vector mesons as well, become H-independent: (1) when H is created by an operator of large dimension, and (2) when H is a highly-excited hadron. We also find a sector of a particular model where universality for the rho coupling is exact. Still, in none of these cases need it be true that f_ρg_ρ=m_ρ^2, although we find empirically that the relation does hold approximately (up to a factor of order two) within the models we have studied.

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arXiv2005

The Duality Cascade

The duality cascade, and its dual description as string theory on the warped deformed conifold, brings together several sophisticated topics, some of which are not widely known. These lectures, which contain a number of previously unpublished results, and are intended for experts as well as students, seek to explain the physics of duality cascades. Seiberg duality is carefully introduced, with detailed attention to the physical implications of duality away from the far infrared. The conifold is briefly introduced and strings on the conifold (the Klebanov-Witten model) are discussed. Next, fractional branes are introduced. The duality cascade is then constructed in field theory and in its dual supergravity description. Among the newly published results: it is shown why supergravity sees the cascade as smooth; how the two holomorphic couplings (dilaton and integrated two-form in supergravity) are related to the three physical couplings in the gauge theory; that there are actually twice as many approximate fixed points in the cascade as might be naively expected. These notes are based on lectures given at TASI 2003 and at the 2003 PIMS Summer School on Strings, Gravity & Cosmology.

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arXiv2005

In Search of Lonely Top Quarks at the Tevatron

Single top-quark production, via weak-interaction processes, is an important test of the standard model, potentially sensitive to new physics. However, it is becoming known that this measurement is much more challenging at the Tevatron than originally expected. We reexamine this process and suggest new methods, using shape variables, that can supplement the methods that have been discussed previously. In particular, by focusing on correlations and asymmetries, we can reduce backgrounds substantially without low acceptance for the signal. Our method also allows for a self-consistency check on the modeling of the backgrounds. However, at the present time, serious systematic problems remain, especially concerning the background from W-plus-jets; these must be studied further by experimentalists and theorists alike.

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