Marcela Carena

University of Chicago, Chicago, Illinois, United States

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Publications (70)134.03 Total impact

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    ABSTRACT: Warped extra-dimension models in which the Higgs boson is allowed to propagate in the bulk of a compact AdS$_5$ space are conjectured to be dual to models featuring a partially composite Higgs boson. They offer a framework with which to investigate the implications of changing the scaling dimension of the Higgs operator, which can be used to reduce the constraints from electroweak precision data. In the context of such models, we calculate the cross section for Higgs production in gluon fusion and the $H\to\gamma\gamma$ decay rate and show that they are finite (at one-loop order) as a consequence of gauge invariance. The extended scalar sector comprising the Kaluza-Klein excitations of the Standard Model scalars is constructed in detail. The largest effects are due to virtual KK fermions, whose contributions to the cross section and decay rate introduce a quadratic sensitivity to the maximum allowed value $y_\ast$ of the random complex entries of the 5D anarchic Yukawa matrices. We find an enhancement of the gluon-fusion cross section and a reduction of the $H\to\gamma\gamma$ rate as well as of the tree-level Higgs couplings to fermions and electroweak gauge bosons. We perform a detailed study of the correlated signal strengths for different production mechanisms and decay channels as functions of $y_\ast$, the mass scale of Kaluza-Klein resonances and the scaling dimension of the composite Higgs operator.
    08/2014;
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    ABSTRACT: We study classically scale invariant models in which the Standard Model Higgs mass term is replaced in the Lagrangian by a Higgs portal coupling to a complex scalar field of a dark sector. We focus on models that are weakly coupled with the quartic scalar couplings nearly vanishing at the Planck scale. The dark sector contains fermions and scalars charged under dark SU(2) x U(1) gauge interactions. Radiative breaking of the dark gauge group triggers electroweak symmetry breaking through the Higgs portal coupling. Requiring both a Higgs boson mass of 125.5 GeV and stability of the Higgs potential up to the Planck scale implies that the radiative breaking of the dark gauge group occurs at the TeV scale. We present a particular model which features a long-range abelian dark force. The dominant dark matter component is neutral dark fermions, with the correct thermal relic abundance, and in reach of future direct detection experiments. The model also has lighter stable dark fermions charged under the dark force, with observable effects on galactic-scale structure. Collider signatures include a dark sector scalar boson with mass < 250 GeV that decays through mixing with the Higgs boson, and can be detected at the LHC. The Higgs boson, as well as the new scalar, may have significant invisible decays into dark sector particles.
    08/2014;
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    Marcela Carena, Leandro Da Rold, Eduardo Ponton
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    ABSTRACT: We consider composite Higgs models where the Higgs is a pseudo-Nambu Goldstone boson arising from the spontaneous breaking of an approximate global symmetry by some underlying strong dynamics. We focus on the SO(5) -> SO(4) symmetry breaking pattern, assuming the partial compositeness paradigm. We study the consequences on Higgs physics of the fermionic representations produced by the strong dynamics, that mix with the Standard Model (SM) degrees of freedom. We consider models based on the lowest-dimensional representations of SO(5) that allow for the custodial protection of the Z -> b b coupling, i.e. the 5, 10 and 14. We find a generic suppression of the gluon fusion process, while the Higgs branching fractions can be enhanced or suppressed compared to the SM. Interestingly, a precise measurement of the Higgs boson couplings can distinguish between different realizations in the fermionic sector, thus providing crucial information about the nature of the UV dynamics.
    02/2014;
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    ABSTRACT: In models with an extended Higgs sector there exists an alignment limit, in which the lightest CP-even Higgs boson mimics the Standard Model Higgs. The alignment limit is commonly associated with the decoupling limit, where all non-standard scalars are significantly heavier than the $Z$ boson. However, alignment can occur irrespective of the mass scale of the rest of the Higgs sector. In this work we discuss the general conditions that lead to "alignment without decoupling", therefore allowing for the existence of additional non-standard Higgs bosons at the weak scale. The values of $\tan\beta$ for which this happens are derived in terms of the effective Higgs quartic couplings in general two-Higgs-doublet models as well as in supersymmetric theories, including the MSSM and the NMSSM. Moreover, we study the information encoded in the variations of the SM Higgs-fermion couplings to explore regions in the $m_A - \tan\beta$ parameter space.
    10/2013;
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    ABSTRACT: We study extensions of the standard model by one generation of vector-like leptons with non-standard hypercharges, which allow for a sizable modification of the h -> gamma gamma decay rate for new lepton masses in the 300 GeV - 1 TeV range. We analyze vaccum stability implications for different hypercharges. Effects in h -> Z gamma are typically much smaller than in h -> gamma gamma, but distinct among the considered hypercharge assignments. Non-standard hypercharges constrain or entirely forbid possible mixing operators with standard model leptons. As a consequence, the leading contributions to the experimentally strongly constrained electric dipole moments of standard model fermions are only generated at the two loop level by the new CP violating sources of the considered setups. We derive the bounds from dipole moments, electro-weak precision observables and lepton flavor violating processes, and discuss their implications. Finally, we examine the production and decay channels of the vector-like leptons at the LHC, and find that signatures with multiple light leptons or taus are already probing interesting regions of parameter space.
    08/2013;
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    ABSTRACT: The ATLAS and CMS experiments have recently announced the discovery of a Higgs-like resonance with mass close to 125 GeV. Overall, the data is consistent with a Standard Model (SM)-like Higgs boson. Such a particle may arise in the minimal super-symmetric extension of the SM with average stop masses of the order of the TeV scale and a sizable stop mixing parameter. In this article we discuss properties of the SM-like Higgs production and decay rates induced by the possible presence of light staus and light stops. Light staus can affect the decay rate of the Higgs into di-photons and, in the case of sizable left-right mixing, induce an enhancement in this production channel up to ~ 50% of the Standard Model rate. Light stops may induce sizable modifications of the Higgs gluon fusion production rate and correlated modifications to the Higgs diphoton decay. Departures from SM values of the bottom-quark and tau-lepton couplings to the Higgs can be obtained due to Higgs mixing effects triggered by light third generation scalar superpartners. We describe the phenomenological implications of light staus on searches for light stops and non-standard Higgs bosons. Finally, we discuss the current status of the search for light staus produced in association with sneutrinos, in final states containing a W gauge boson and a pair of τ s.
    Journal of High Energy Physics 08/2013; · 5.62 Impact Factor
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    ABSTRACT: Part 2 of "Project X: Accelerator Reference Design, Physics Opportunities, Broader Impacts". In this Part, we outline the particle-physics program that can be achieved with Project X, a staged superconducting linac for intensity-frontier particle physics. Topics include neutrino physics, kaon physics, muon physics, electric dipole moments, neutron-antineutron oscillations, new light particles, hadron structure, hadron spectroscopy, and lattice-QCD calculations. Part 1 is available as arXiv:1306.5022 [physics.acc-ph] and Part 3 is available as arXiv:1306.5024 [physics.acc-ph].
    06/2013;
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    ABSTRACT: Current Higgs data at the Large Hadron Collider is compatible with a SM signal at the 2$\sigma$ level, but the central value of the signal strength in the diphoton channel is enhanced with respect to the SM expectation. If the enhancement resides in the diphoton partial decay width, the data could be accommodated in the Minimally Supersymmetric Standard Model (MSSM) with highly mixed light staus. We revisit the issue of vacuum instability induced by large mixing in the stau sector, including effects of a radiatively-corrected tau Yukawa coupling. Further, we emphasize the importance of taking into account the $\tan\beta$ dependence in the stability bound. While the metastability of the Universe constrains the possible enhancement in the Higgs to diphoton decay width in the light stau scenario, an increase of the order of 50% can be achieved in the region of large $\tan\beta$. Larger enhancements may be obtained, but would require values of $\tan\beta$ associated with non-perturbative values of the tau Yukawa coupling at scales below the GUT scale, thereby implying the presence of new physics beyond the MSSM.
    Journal of High Energy Physics 11/2012; 2013(2). · 5.62 Impact Factor
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    ABSTRACT: We study the minimal supersymmetric standard model (MSSM) with minimal flavor violation (MFV), imposing constraints from flavor physics observables and MSSM Higgs searches, in light of the recent discovery of a 125 GeV Higgs boson by ATLAS and CMS. We analyze the electroweak vacuum stability conditions to further restrict the MSSM parameter space. In addition, a connection to ultraviolet physics is shown via an implementation of renormalization group running, which determines the TeV-scale spectrum from a small set of minimal supergravity parameters. Finally, we investigate the impact from dark matter direct detection searches. Our work highlights the complementarity of collider, flavor and dark matter probes in exploring the MSSM, and shows that even in a MFV framework, flavor observables constrain the MSSM parameter space well beyond the current reach of direct SUSY particle searches.
    Journal of High Energy Physics 11/2012; · 5.62 Impact Factor
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    ABSTRACT: Electroweak baryogenesis is an attractive scenario for the generation of the baryon asymmetry of the universe as its realization depends on the presence at the weak scale of new particles which may be searched for at high energy colliders. In the MSSM it may only be realized in the presence of light stops, and with moderate or small mixing between the left- and right-handed components. Consistency with the observed Higgs mass around 125 GeV demands the heavier stop mass to be much larger than the weak scale. Moreover the lighter stop leads to an increase of the gluon-gluon fusion Higgs production cross section which seems to be in contradiction with indications from current LHC data. We show that this tension may be considerably relaxed in the presence of a light neutralino with a mass lower than about 60 GeV, satisfying all present experimental constraints. In such a case the Higgs may have a significant invisible decay width and the stop decays through a three or four body decay channel, including a bottom quark and the lightest neutralino in the final state. All these properties make this scenario testable at a high luminosity LHC.
    Journal of High Energy Physics 07/2012; 2013(2). · 5.62 Impact Factor
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    Marcela Carena, Ian Low, Carlos E. M. Wagner
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    ABSTRACT: Motivated by recent results from Higgs searches at the Large Hadron Collider, we consider possibilities to enhance the diphoton decay width of the Higgs boson over the Standard Model expectation, without modifying either its production rate or the partial widths in the WW and ZZ channels. Studying effects of new charged scalars, fermions and vector bosons, we find that significant variations in the diphoton width may be possible if the new particles have light masses of the order of a few hundred GeV and sizeable couplings to the Higgs boson. Such couplings could arise naturally if there is large mass mixing between two charged particles that is induced by the Higgs vacuum expectation value. In addition, there is generically also a shift in the Z + Gamma partial width, which in the case of new vector bosons tends to be of similar magnitude as the shift in the diphoton partial width, but smaller in other cases. Therefore simultaneous measurements in these two channels could reveal properties of new charged particles at the electroweak scale.
    Journal of High Energy Physics 06/2012; 2012(8). · 5.62 Impact Factor
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    ABSTRACT: The LHC has started to constrain supersymmetry-breaking parameters by setting bounds on possible colored particles at the weak scale. Moreover, constraints from Higgs physics, flavor physics, the anomalous magnetic moment of the muon, as well as from searches at LEP and the Tevatron have set additional bounds on these parameters. Renormalization Group Invariants (RGIs) provide a very useful way of representing the allowed parameter space by making direct connection with the values of these parameters at the messenger scale. Using a general approach, based on the pMSSM parametrization of the soft supersymmetry-breaking parameters, we analyze the current experimental constraints to determine the probability distributions for the RGIs. As examples of their application, we use these distributions to analyze the question of Gaugino Mass Unification and to probabilistically determine the parameters of General and Minimal Gauge Mediation with arbitrary Higgs mass parameters at the Messenger Scale.
    Physical review D: Particles and fields 05/2012; 86(7).
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    ABSTRACT: Recently, an excess of events consistent with a Higgs boson with mass of about 125 GeV was reported by the CMS and ATLAS experiments. This Higgs boson mass is consistent with the values that may be obtained in minimal supersymmetric extensions of the Standard Model (SM), with both stop masses less than a TeV and large mixing. The apparently enhanced photon production rate associated with this potential Higgs signal may be the result of light staus with large mixing. Large stau mixing and large coupling of the staus to the SM-like Higgs boson may be obtained for large values of tan \beta, and moderate to large values of the Higgsino mass parameter, \mu. We study the phenomenological properties of this scenario, including precision electroweak data, the muon anomalous magnetic moment, Dark Matter, and the evolution of the soft supersymmetry-breaking parameters to high energies. We also analyze the possible collider signatures of light third generation sleptons and demonstrate that it is possible to find evidence of their production at the 8 TeV and the 14 TeV LHC. The most promising channel is stau and tau sneutrino associated production, with the sneutrino decaying into a W boson plus a light stau.
    Journal of High Energy Physics. 05/2012; 2012(7).
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    ABSTRACT: Measurements of the Higgs-boson production cross section at the LHC are an important tool for studying electroweak symmetry breaking at the quantum level, since the main production mechanism gg-->h is loop-suppressed in the Standard Model (SM). Higgs production in extra-dimensional extensions of the SM is sensitive to the Kaluza-Klein (KK) excitations of the quarks, which can be exchanged as virtual particles in the loop. In the context of the minimal Randall-Sundrum (RS) model with bulk fields and a brane-localized Higgs sector, we derive closed analytical expressions for the gluon-gluon fusion process, finding that the effect of the infinite tower of virtual KK states can be described in terms of a simple function of the fundamental (5D) Yukawa matrices. Given a specific RS model, this will allow one to easily constrain the parameter space, once a Higgs signal has been established. We explain that discrepancies between existing calculations of Higgs production in RS models are related to the non-commutativity of two limits: taking the number of KK states to infinity and removing the regulator on the Higgs-boson profile, which is required in an intermediate step to make the relevant overlap integrals well defined. Even though the one-loop gg-->h amplitude is finite in RS scenarios with a brane-localized Higgs sector, it is important to introduce a consistent ultraviolet regulator in order to obtain the correct result.
    Journal of High Energy Physics 03/2012; 2012(8). · 5.62 Impact Factor
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    ABSTRACT: In a variety of well motivated models, such as two Higgs Doublet Models (2HDMs) and the Minimal Supersymmetric Standard Model (MSSM), there are neutral Higgs bosons that have significantly enhanced couplings to b-quarks and tau leptons in comparison to those of the SM Higgs. These so called non-standard Higgs bosons could be copiously produced at the LHC in association with b quarks, and subsequently decay into b-quark pairs. However, this production channel suffers from large irreducible QCD backgrounds. We propose a new search strategy for non-standard neutral Higgs bosons at the 7 TeV LHC in the 3b's final state topology. We perform a simulation of the signal and backgrounds, using state of the art tools and methods for different sets of selection cuts, and conclude that neutral Higgs bosons with couplings to b-quarks of about 0.3 or larger, and masses up to 400 GeV, could be seen with a luminosity of 30 fb^{-1}. In the case of the MSSM we also discuss the complementarity between the 3b channel and the inclusive tau pair channel in exploring the supersymmetric parameter space.
    Journal of High Energy Physics 03/2012; 2012(7). · 5.62 Impact Factor
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    Marcela Carena, Eduardo Pontón, José Zurita
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    ABSTRACT: We consider the Higgs sector in extensions of the minimal supersymmetric standard model by higher-dimension operators in the superpotential and the Kähler potential, in the context of Higgs searches at the LHC 7 TeV run. Such an effective field theory approach, also referred to as BMSSM, allows for a model-independent description that may correspond to the combined effects of additional supersymmetric sectors, such as heavy singlets, triplets or gauge bosons, in which the supersymmetry-breaking mass splittings can be treated as a perturbation. We consider the current LHC dataset, based on about 1–2 fb-1 of data to set exclusion limits on a large class of BMSSM models. We also present projections for integrated luminosities of 5 and 15 fb-1, assuming that the ATLAS and CMS collaborations will combine their results in each channel. Our study shows that the majority of the parameter space will be probed at the 2σ level with 15 fb-1 of data. A nonobservation of a Higgs boson with about 10 fb-1 of data will point towards a Higgs SUSY spectrum with intermediate tan⁡β (≈a few to10) and a light SM-like Higgs with somewhat enhanced couplings to bottom and tau pairs. We define a number of BMSSM benchmark scenarios and analyze the possible exclusion/discovery channels and the projected required luminosity to probe them. We also discuss the results of the effective field theory framework for two specific models, one with a singlet superfield and one with SU(2)L triplets.
    Physical review D: Particles and fields 02/2012; 85(3).
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    ABSTRACT: We consider the possibility of a Standard Model (SM)-like Higgs in the context of the Minimal Supersymmetric Standard Model (MSSM), with a mass of about 125 GeV and with a production times decay rate into two photons which is similar or somewhat larger than the SM one. The relatively large value of the SM-like Higgs mass demands stops in the several hundred GeV mass range with somewhat large mixing, or a large hierarchy between the two stop masses in the case that one of the two stops is light. We find that, in general, if the heaviest stop mass is smaller than a few TeV, the rate of gluon fusion production of Higgs bosons decaying into two photons tends to be somewhat suppressed with respect to the SM one in this region of parameters. However, we show that an enhancement of the photon decay rate may be obtained for light third generation sleptons with large mixing, which can be naturally obtained for large values of $\tan\beta$ and sizable values of the Higgsino mass parameter.
    12/2011;
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    ABSTRACT: We analyze the stability of the vacuum and the electroweak phase transition in the NMSSM close to the Peccei-Quinn symmetry limit. This limit contains light Dark Matter (DM) particles with a mass significantly smaller than the weak scale and also light CP-even and CP-odd Higgs bosons. Such light particles lead to a consistent relic density and facilitate a large spin-independent direct DM detection cross section, that may accommodate the recently reported signatures at the DAMA and CoGeNT experiments. Studying the one-loop effective potential at finite temperature, we show that when the lightest CP-even Higgs mass is of the order of a few GeV, the electroweak phase transition tends to become first order and strong. The inverse relationship between the direct-detection cross-section and the lightest CP-even Higgs mass implies that a cross-section of the order of 10$^{-41}$ cm$^2$ is correlated with a strong first order phase transition.
    Physical review D: Particles and fields 10/2011;
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    Wolfgang Altmannshofer, Marcela Carena
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    ABSTRACT: We study a minimal flavor violating extension of the MSSM, where higher dimensional operators in the Kahler potential induce tree level non-holomorphic Higgs couplings that are controlled by the scale of the physics beyond the MSSM, and analyze their possible impact on CP violation in Bs and Bd mixing. We consider results on the time dependent CP asymmetries in Bs --> psi phi and Bs --> psi f_0 from LHCb, in Bs --> psi phi from CDF and D0 and in B --> psi K_S from the B factories as well as the measurement of an anomalous like-sign dimuon charge asymmetry at D0. Taking into account the stringent bounds on the branching ratio of the rare Bs --> mu+mu- decay, we investigate to which extent the framework allows to address the observed (2-3)sigma discrepancies in fits of the unitarity triangle. We find that a non-standard Bd mixing phase, that is in agreement with the current bounds on CP violation in Bs mixing, requires the presence of higher dimensional operators both in the Kahler potential and the superpotential. The corresponding region of parameter space is characterized by small tan(beta)~5, a light Higgs spectrum with masses below ~400 GeV and will be probed by future measurements at LHCb.
    Physical review D: Particles and fields 10/2011;
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    ABSTRACT: The search for the Higgs boson is entering a decisive phase. The Large Hadron Collider experiments have collected more than 1 fb$^{-1}$ of data and are now capable of efficiently probing the high Higgs mass region, $m_H > 140$ GeV. The low mass region is more challenging at the LHC, but if the Higgs has Standard Model (SM)-like properties, the LHC should find evidence for it by the end of next year. In low energy supersymmetric extensions of the SM, the situation is similar for large values of the CP-odd Higgs mass $m_A$, but more interesting for lower values of $m_A$. The ($\sqrt{s}$ =7 TeV) LHC searches for a low-mass Standard Model Higgs boson predominantly in the $h\rightarrow \gamma\gamma,WW$ decay modes, which may be suppressed by an increase in the $h\rightarrow b\bar{b}, \tau^+ \tau^-$ partial widths (and thus the total $h$ width) for $m_A\lesssim 500$ GeV. Although $h\rightarrow b\bar{b}, \tau^+ \tau^-$ are sought at the LHC, these channels are not powerful enough to fully counter this suppression in the first year of running. We consider two alternative possibilities for probing the low $m_A$ region: nonstandard Higgs boson searches at the LHC, and a statistical combination with the Tevatron, where $Vh\rightarrow b\bar{b}$ is the primary search channel for light $h$. We also study an MSSM scenario in which the $h\rightarrow\gamma\gamma$ rate is enhanced at low $m_A$ to the point where discovery is possible in the near future.
    Physical review D: Particles and fields 07/2011; 84.

Publication Stats

3k Citations
134.03 Total Impact Points

Institutions

  • 2011–2014
    • University of Chicago
      • Enrico Fermi Institute
      Chicago, Illinois, United States
  • 2002–2012
    • Fermi National Accelerator Laboratory (Fermilab)
      • Theoretical Physics Department
      Batavia, Illinois, United States
  • 2000
    • CERN
      Genève, Geneva, Switzerland
  • 1987
    • Centro Atómico Bariloche
      San Carlos de Bariloche, Río Negro, Argentina