68 research outputs found
Decoupling heavy sparticles in Effective SUSY scenarios: Unification, Higgs masses and tachyon bounds
Using two-loop renormalization group equations implementing the decoupling of
heavy scalars, Effective SUSY scenarios are studied in the limit in which there
is a single low energy Higgs field. Gauge coupling unification is shown to hold
with similar or better precision than in standard MSSM scenarios. b-tau
unification is examined, and Higgs masses are computed using the effective
potential, including two-loop contributions from scalars. A 125 GeV Higgs is
compatible with stops/sbottoms at around 300 GeV with non-universal boundary
conditions at the scale of the heavy sparticles if some of the trilinear
couplings at this scale take values of the order of 1-2 TeV; if more
constrained boundary conditions inspired by msugra or gauge mediation are set
at a higher scale, heavier colored sparticles are required in general. Finally,
since the decoupled RG flow for third-generation scalar masses departs very
significantly from the MSSM DR-bar one, tachyon bounds for light scalars are
revisited and shown to be relaxed by up to a TeV or more.Comment: 35 pages, 17 figures. v2: Updated some scans, allowing for changes in
sign of some parameters, minor improvements. v3: Typos corrected in formulae
in the appendices, added some clarifying remarks about flavor mixing being
ignore
Stop the Top Background of the Stop Search
The main background for the supersymmetric stop direct production search
comes from Standard Model ttbar events. For the single-lepton search channel,
we introduce a few kinematic variables to further suppress this background by
focusing on its dileptonic and semileptonic topologies. All are defined to have
end points in the background, but not signal distributions. They can
substantially improve the stop signal significance and mass reach when combined
with traditional kinematic variables such as the total missing transverse
energy. Among them, our variable M^W_T2 has the best overall performance
because it uses all available kinematic information, including the on-shell
mass of both W's. We see 20%-30% improvement on the discovery significance and
estimate that the 8 TeV LHC run with 20 fb-1 of data would be able to reach an
exclusion limit of 650-700 GeV for direct stop production, as long as the stop
decays dominantly to the top quark and a light stable neutralino. Most of the
mass range required for the supersymmetric solution of the naturalness problem
in the standard scenario can be covered.Comment: 16 pages, 5 figure
SUSY, the Third Generation and the LHC
We develop a bottom-up approach to studying SUSY with light stops and
sbottoms, but with other squarks and sleptons heavy and beyond reach of the
LHC. We discuss the range of squark, gaugino and Higgsino masses for which the
electroweak scale is radiatively stable over the "little hierarchy" below ~ 10
TeV. We review and expand on indirect constraints on this scenario, in
particular from flavor and CP tests. We emphasize that in this context,
R-parity violation is very well motivated. The phenomenological differences
between Majorana and Dirac gauginos are also discussed. Finally, we focus on
the light subsystem of stops, sbottom and neutralino with R-parity, in order to
probe the current collider bounds. We find that 1/fb LHC bounds are mild and
large parts of the motivated parameter space remain open, while the 10/fb data
can be much more decisive.Comment: 42 pages, 8 figures, 1 table. V2: minor corrections, references adde
SUSY Stops at a Bump
We discuss collider signatures of the "natural supersymmetry" scenario with
baryon-number violating R-parity violation. We argue that this is one of the
few remaining viable incarnations of weak scale supersymmetry consistent with
full electroweak naturalness. We show that this intriguing and challenging
scenario contains distinctive LHC signals, resonances of hard jets in
conjunction with relatively soft leptons and missing energy, which are easily
overlooked by existing LHC searches. We propose novel strategies for
distinguishing these signals above background, and estimate their potential
reach at the 8 TeV LHC. We show that other multi-lepton signals of this
scenario can be seen by currently existing searches with increased statistics,
but these opportunities are more spectrum-dependent.Comment: 23 pages, 7 figures, 3 tables. V2: spectrum discussion corrected,
most of the changes are in Sec. 2. Benchmarks, analysis and conclusions
unchanged. References adde
Hitting sbottom in natural SUSY
We compare the experimental prospects of direct stop and sbottom pair
production searches at the LHC. Such searches for stops are of great interest
as they directly probe for states that are motivated by the SUSY solution to
the hierarchy problem of the Higgs mass parameter - leading to a "Natural" SUSY
spectrum. Noting that sbottom searches are less experimentally challenging and
scale up in reach directly with the improvement on b-tagging algorithms, we
discuss the interplay of small TeV scale custodial symmetry violation with
sbottom direct pair production searches as a path to obtaining strong sub-TeV
constraints on stops in a natural SUSY scenario. We argue that if a weak scale
natural SUSY spectrum does not exist within the reach of LHC, then hopes for
such a spectrum for large regions of parameter space should sbottom out.
Conversely, the same arguments make clear that a discovery of such a spectrum
is likely to proceed in a sbottom up manner.Comment: 18 pages, 8 figures,v2 refs added, JHEP versio
Flavor Mediation Delivers Natural SUSY
If supersymmetry (SUSY) solves the hierarchy problem, then naturalness
considerations coupled with recent LHC bounds require non-trivial superpartner
flavor structures. Such "Natural SUSY" models exhibit a large mass hierarchy
between scalars of the third and first two generations as well as degeneracy
(or alignment) among the first two generations. In this work, we show how this
specific beyond the standard model (SM) flavor structure can be tied directly
to SM flavor via "Flavor Mediation". The SM contains an anomaly-free SU(3)
flavor symmetry, broken only by Yukawa couplings. By gauging this flavor
symmetry in addition to SM gauge symmetries, we can mediate SUSY breaking via
(Higgsed) gauge mediation. This automatically delivers a natural SUSY spectrum.
Third-generation scalar masses are suppressed due to the dominant breaking of
the flavor gauge symmetry in the top direction. More subtly, the
first-two-generation scalars remain highly degenerate due to a custodial U(2)
symmetry, where the SU(2) factor arises because SU(3) is rank two. This
custodial symmetry is broken only at order (m_c/m_t)^2. SUSY gauge coupling
unification predictions are preserved, since no new charged matter is
introduced, the SM gauge structure is unaltered, and the flavor symmetry treats
all matter multiplets equally. Moreover, the uniqueness of the anomaly-free
SU(3) flavor group makes possible a number of concrete predictions for the
superpartner spectrum.Comment: 17 pages, 7 figures, 2 tables. v2 references added, minor changes to
flavor constraints and a little discussion adde
Supersymmetry with Light Stops
Recent LHC data, together with the electroweak naturalness argument, suggest
that the top squarks may be significantly lighter than the other sfermions. We
present supersymmetric models in which such a split spectrum is obtained
through "geometries": being "close to" electroweak symmetry breaking implies
being "away from" supersymmetry breaking, and vice versa. In particular, we
present models in 5D warped spacetime, in which supersymmetry breaking and
Higgs fields are located on the ultraviolet and infrared branes, respectively,
and the top multiplets are localized to the infrared brane. The hierarchy of
the Yukawa matrices can be obtained while keeping near flavor degeneracy
between the first two generation sfermions, avoiding stringent constraints from
flavor and CP violation. Through the AdS/CFT correspondence, the models can be
interpreted as purely 4D theories in which the top and Higgs multiplets are
composites of some strongly interacting sector exhibiting nontrivial dynamics
at a low energy. Because of the compositeness of the Higgs and top multiplets,
Landau pole constraints for the Higgs and top couplings apply only up to the
dynamical scale, allowing for a relatively heavy Higgs boson, including m_h =
125 GeV as suggested by the recent LHC data. We analyze electroweak symmetry
breaking for a well-motivated subset of these models, and find that fine-tuning
in electroweak symmetry breaking is indeed ameliorated. We also discuss a flat
space realization of the scenario in which supersymmetry is broken by boundary
conditions, with the top multiplets localized to a brane while other matter
multiplets delocalized in the bulk.Comment: 27 pages, 7 figure
Excess Higgs Production in Neutralino Decays
The ATLAS and CMS experiments have recently claimed discovery of a Higgs
boson-like particle at ~5 sigma confidence and are beginning to test the
Standard Model predictions for its production and decay. In a variety of
supersymmetric models, a neutralino NLSP can decay dominantly to the Higgs and
the LSP. In natural SUSY models, a light third generation squark decaying
through this chain can lead to large excess Higgs production while evading
existing BSM searches. Such models can be observed at the 8 TeV LHC in channels
exploiting the rare diphoton decays of the Higgs produced in the cascade decay.
Identifying a diphoton resonance in association with missing energy, a lepton,
or b-tagged jets is a promising search strategy for discovery of these models,
and would immediately signal new physics involving production of a Higgs boson.
We also discuss the possibility that excess Higgs production in these SUSY
decays can be responsible for enhancements of up to 50% over the SM prediction
for the observed rate in the existing inclusive diphoton searches, a scenario
which would likely by the end of the 8 TeV run be accompanied by excesses in
the diphoton + lepton/MET and SUSY multi-lepton/b searches and a potential
discovery in a diphoton + 2b search.Comment: 42 pages, 19 figure
Probing natural SUSY from stop pair production at the LHC
We consider the natural supersymmetry scenario in the framework of the
R-parity conserving minimal supersymmetric standard model (called natural MSSM)
and examine the observability of stop pair production at the LHC. We first scan
the parameters of this scenario under various experimental constraints,
including the SM-like Higgs boson mass, the indirect limits from precision
electroweak data and B-decays. Then in the allowed parameter space we study the
stop pair production at the LHC followed by the stop decay into a top quark
plus a lightest neutralino or into a bottom quark plus a chargino. From
detailed Monte Carlo simulations of the signals and backgrounds, we find the
two decay modes are complementary to each other in probing the stop pair
production, and the LHC with TeV and 100 luminosity is
capable of discovering the stop predicted in natural MSSM up to 450 GeV. If no
excess events were observed at the LHC, the 95% C.L. exclusion limits of the
stop masses can reach around 537 GeV.Comment: 19 pages, 10 figures, version accepted by JHE
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