3,128 research outputs found
Eviction of a 125 GeV "heavy"-Higgs from the MSSM
We prove that the present experimental constraints are already enough to rule
out the possibility of the ~125 GeV Higgs found at LHC being the second
lightest Higgs in a general MSSM context, even with explicit CP violation in
the Higgs potential. Contrary to previous studies, we are able to eliminate
this possibility analytically, using simple expressions for a relatively small
number of observables. We show that the present LHC constraints on the diphoton
signal strength, tau-tau production through Higgs and BR(B -> X_s gamma) are
enough to preclude the possibility of H_2 being the observed Higgs with m_H~125
GeV within an MSSM context, without leaving room for finely tuned
cancellations. As a by-product, we also comment on the difficulties of an MSSM
interpretation of the excess in the gamma-gamma production cross section
recently found at CMS that could correspond to a second Higgs resonance at
m_H~136 GeV.Comment: 38 pages, 9 figures. Final version accepted at JHEP. Sections 2, 3
and appendices simplified. Experimental results updated, several references
added. Small typos corrected and a new comparison of approximate formulas
with full expressions include
METing SUSY on the Z peak
Recently the ATLAS experiment announced a 3 excess at the Z-peak
consisting of 29 pairs of leptons together with two or more jets, GeV and GeV, to be compared with
expected lepton pairs in the Standard Model. No excess outside the Z-peak was
observed. By trying to explain this signal with SUSY we find that only
relatively light gluinos, TeV, together with a
heavy neutralino NLSP of GeV decaying
predominantly to Z-boson plus a light gravitino, such that nearly every gluino
produces at least one Z-boson in its decay chain, could reproduce the excess.
We construct an explicit general gauge mediation model able to reproduce the
observed signal overcoming all the experimental limits. Needless to say, more
sophisticated models could also reproduce the signal, however, any model would
have to exhibit the following features, light gluinos, or heavy particles with
a strong production cross-section, producing at least one Z-boson in its decay
chain. The implications of our findings for the Run II at LHC with the scaling
on the Z peak, as well as for the direct search of gluinos and other SUSY
particles, are pointed out.Comment: 24 pages, 17 figures, simulation improved, Checkmate analysis added,
new benchmark point included. Typos corrected, conclusions unchange
Hysteresis and Avalanches in the Random Anisotropy Ising Model
The behaviour of the Random Anisotropy Ising model at T=0 under local
relaxation dynamics is studied. The model includes a dominant ferromagnetic
interaction and assumes an infinite anisotropy at each site along local
anisotropy axes which are randomly aligned. Two different random distributions
of anisotropy axes have been studied. Both are characterized by a parameter
that allows control of the degree of disorder in the system. By using numerical
simulations we analyze the hysteresis loop properties and characterize the
statistical distribution of avalanches occuring during the metastable evolution
of the system driven by an external field. A disorder-induced critical point is
found in which the hysteresis loop changes from displaying a typical
ferromagnetic magnetization jump to a rather smooth loop exhibiting only tiny
avalanches. The critical point is characterized by a set of critical exponents,
which are consistent with the universal values proposed from the study of other
simpler models.Comment: 40 pages, 21 figures, Accepted for publication in Phys. Rev.
Recommended from our members
Dynamic trading and asset prices: Keynes vs. Hayek
We investigate the dynamics of prices, information, and expectations in a competitive, noisy, dynamic asset pricing equilibrium model with long-term investors. We argue that the fact that prices can score worse or better than consensus opinion in predicting the fundamentals is a product of endogenous short-term speculation. For a given positive level of residual pay-off uncertainty, if liquidity trades display low persistence, rational investors act like market makers and accommodate the order flow and prices are farther away from fundamentals compared to consensus. This defines a "Keynesian" region; the complementary region is "Hayekian" in that rational investors chase the trend and prices are systematically closer to fundamentals than average expectations. The standard case of no residual uncertainty and liquidity trading following a random walk is on the frontier of the two regions and identifies the set of deep parameters for which rational investors abide by Keynes' dictum of concentrating on an asset "long-term prospects and those only". The analysis also explains momentum and reversal in stock returns and how accommodation and trend-chasing strategies differ from these phenomena
Analysis of enhanced tan(beta) corrections in MFV GUT scenarios
We analyse a minimal supersymmetric standard model (MSSM) taking a minimal
flavour violation (MFV) structure at the GUT scale. We evaluate the parameters
at the electroweak scale taking into account the full flavour structure in the
evolution of the renormalization group equations. We concentrate mainly on the
decay Bs -> mu mu and its correlations with other observables like b -> s
gamma, b -> s l l, Delta M_Bs and the anomalous magnetic moment of the muon. We
restrict our analysis to the regions in parameter space consistent with the
dark matter constraints. We find that the BR(Bs -> mu mu) can exceed the
current experimental limit in the regions of parameter space which are allowed
by all other constraints thus providing an additional bound on supersymmetric
parameters. This holds even in the constrained MSSM. Assuming an hypothetical
measurement of BR(Bs -> mu mu) ~ 10^-7 we analyse the predicted MSSM spectrum
and flavour violating decay modes of supersymmetric particles which are found
to be small.Comment: 47 pages, 16 figures (best viewed printed or in pdf format), updated
lattice inputs used, version submitted to PR
Cold Positrons from Decaying Dark Matter
Many models of dark matter contain more than one new particle beyond those in
the Standard Model. Often heavier particles decay into the lightest dark matter
particle as the Universe evolves. Here we explore the possibilities that arise
if one of the products in a (Heavy Particle) (Dark Matter) decay
is a positron, and the lifetime is shorter than the age of the Universe. The
positrons cool down by scattering off the cosmic microwave background and
eventually annihilate when they fall into Galactic potential wells. The
resulting 511 keV flux not only places constraints on this class of models but
might even be consistent with that observed by the INTEGRAL satellite.Comment: 20 pages, 7 figure
Tree-level flavor-changing neutral currents in the B system: From CP asymmetries to rare decays
Tree-level flavor-changing neutral currents (FCNC) are characteristic of models with extra vectorlike quarks. These new couplings can strongly modify the B/sup 0/ CP asymmetries without conflicting with low-energy constraints. In the light of low CP asymmetry in B to J/ psi K/sub S/, we discuss the implications of these contributions. We find that even these low values can be easily accommodated in these models. Furthermore, we show that the new data from B factories tend to favor an O(20) enhancement of the b to dll transition over the SM expectation. (25 refs)
Influence of the driving mechanism on the response of systems with athermal dynamics: the example of the random-field Ising model
We investigate the influence of the driving mechanism on the hysteretic
response of systems with athermal dynamics. In the framework of local-mean
field theory at finite temperature (but neglecting thermallly activated
processes), we compare the rate-independent hysteresis loops obtained in the
random field Ising model (RFIM) when controlling either the external magnetic
field or the extensive magnetization . Two distinct behaviors are
observed, depending on disorder strength. At large disorder, the -driven and
-driven protocols yield identical hysteresis loops in the thermodynamic
limit. At low disorder, when the -driven magnetization curve is
discontinuous (due to the presence of a macroscopic avalanche), the -driven
loop is re-entrant while the induced field exhibits strong intermittent
fluctuations and is only weakly self-averaging. The relevance of these results
to the experimental observations in ferromagnetic materials, shape memory
alloys, and other disordered systems is discussed.Comment: 11 pages, 11 figure
The effect of progressive muscle relaxation on daily cortisol secretion
Abbreviated progressive muscle relaxation (APMR) is a much used stress-management technique. Its efficacy relevant to placebo control is already established in the literature and the primary aim of the present study was to ascertain whether its proven impact on psychological stress measures is matched by a decrease in prevailing levels of the stress-associated hormone cortisol, using accurate and robust measurement based on multiple sampling of full diurnal cortisol secretion profiles. First-year university students can face significant stress in adjustment to academic demands and immersion in a novel social network and provided a convenient study population. One hundred and one first-year students completed APMR with prevailing stress levels assessed a week before and after intervention. Both cortisol and self-report measures were significantly reduced post-intervention by 8% and 10%, respectively. The efficacy of the intervention was independent of, and not modulated by neuroticism, gender, age and smoking status. We also demonstrated that cortisol reduction was unlikely to have been a consequence of adaptation to any initial cortisol elevation prompted by the challenge of the demanding saliva collection protocol. We conclude that the efficacy of APMR in this population extends to reduction in biologically expressed stress levels as well as levels based solely on self-report
How sensitive to FCNC can CP asymmetries be?
We show that the study of CP asymmetries in neutral B-meson decays provides a
very sensitive probe of flavour-changing neutral currents (FCNC). We introduce
two new angles, and , whose main feature is that they
can be readily obtained from the measurement of the CP asymmetries , and the ratio , providing a quantitative test of the
presence of new physics in a model-independent way.
Assuming that new physics is due to the presence of an isosinglet down-type
quark, we indicate how to reconstruct the unitarity quadrangles and point out
that the measurements of the above asymmetries, within the expected
experimental errors, may detect FCNC effects, even for values of at the level of a few times .Comment: 19 pages including 9 figure
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