arXiv subjects

D. M. Ghilencea

Publications and source records attributed to D. M. Ghilencea.

At least 55 records · Page 3Linked to original sources

arXiv2012

Higgs diphoton rate enhancement from supersymmetric physics beyond the MSSM

We show that supersymmetric "new physics" beyond the MSSM can naturally accommodate a Higgs mass near 126 GeV and enhance the signal rate in the Higgs to diphoton channel, while the signal rates in all the other Higgs decay channels coincide with Standard Model expectations, except possibly the Higgs to Z-photon channel. The "new physics" that corrects the relevant Higgs couplings can be captured by two supersymmetric effective operators. We provide a simple example of an underlying model in which these operators are simultaneously generated. The scale of "new physics" that generates these operators can be around 5 TeV or larger, and outside the reach of the LHC.

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arXiv2012

Tuning supersymmetric models at the LHC: A comparative analysis at two-loop level

We provide a comparative study of the fine tuning amount (Delta) at the two-loop leading log level in supersymmetric models commonly used in SUSY searches at the LHC. These are the constrained MSSM (CMSSM), non-universal Higgs masses models (NUHM1, NUHM2), non-universal gaugino masses model (NUGM) and GUT related gaugino masses models (NUGMd). Two definitions of the fine tuning are used, the first (Delta_{max}) measures maximal fine-tuning wrt individual parameters while the second (Delta_q) adds their contribution in "quadrature". As a direct result of two theoretical constraints (the EW minimum conditions), fine tuning (Delta_q) emerges as a suppressing factor (effective prior) of the averaged likelihood (under the priors), under the integral of the global probability of measuring the data (Bayesian evidence p(D)). For each model, there is little difference between Delta_q, Delta_{max} in the region allowed by the data, with similar behaviour as functions of the Higgs, gluino, stop mass or SUSY scale (m_{susy}=(m_{\tilde t_1} m_{\tilde t_2})^{1/2}) or dark matter and g-2 constraints. The analysis has the advantage that by replacing any of these mass scales or constraints by their latest bounds one easily infers for each model the value of Delta_q, Delta_{max} or vice versa. For all models, minimal fine tuning is achieved for M_{higgs} near 115 GeV with a Delta_q\approx Delta_{max}\approx 10 to 100 depending on the model, and in the CMSSM this is actually a global minimum. Due to a strong ($\approx$ exponential) dependence of Delta on M_{higgs}, for a Higgs mass near 125 GeV, the above values of Delta_q\approx Delta_{max} increase to between 500 and 1000. Possible corrections to these values are briefly discussed.

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arXiv2012

On non-universal Goldstino couplings to matter

Using the constrained superfields formalism to describe the interactions of a light goldstino to matter fields in supersymmetric models, we identify generalised, higher-order holomorphic superfield constraints that project out the superpartners and capture the non-universal couplings of the goldstino to matter fields. These arise from microscopic theories in which heavy superpartners masses are of the order of the supersymmetry breaking scale (\sqrt f). In the decoupling limit of infinite superpartners masses, these constraints reduce to the familiar, lower-order universal constraints discussed recently, that describe the universal goldstino-matter fields couplings, suppressed by inverse powers of \sqrt f. We initiate the study of the couplings of the Standard Model (SM) fields to goldstino in the constrained superfields formalism.

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arXiv2011

Non-linear MSSM

Using the formalism of constrained superfields, we derive the most general effective action of a light goldstino coupled to the minimal supersymmetric standard model (MSSM) and study its phenomenological consequences. The goldstino-induced couplings become important when the (hidden sector) scale of spontaneous supersymmetry breaking, $\sqrt f$, is relatively low, of the order of few TeV. In particular, we compute the Higgs potential and show that the (tree level) mass of the lightest Higgs scalar can be increased to the LEP bound for $\sqrt f\sim 2$ TeV to 7 TeV. Moreover, the effective quartic Higgs coupling is increased due to additional tree-level contributions proportional to the ratio of visible to hidden sector supersymmetry breaking scales. This increase can alleviate the amount of fine tuning of the electroweak scale that exists in the MSSM. Among the new goldstino couplings, beyond those in MSSM, the most important ones generate an invisible decay of the Higgs boson into a goldstino and neutralino (if m_h>m_{χ_1^0}), with a partial decay rate that can be comparable to the SM channel h^0-> γγ. A similar decay of Z boson is possible if m_Z>m_{χ_1^0} and brings a lower bound on $\sqrt f$ that must be of about 700 GeV. Additional decay modes of the Higgs or Z bosons into a pair of light goldstinos, while possible, are suppressed by an extra 1/f factor and have no significant impact on the model.

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arXiv2011

A review of naturalness and dark matter prediction for the Higgs mass in MSSM and beyond

Within a two-loop leading-log approximation, we review the prediction for the lightest Higgs mass (m_h) in the framework of constrained MSSM (CMSSM), derived from the naturalness requirement of minimal fine-tuning (Delta) of the electroweak scale and dark matter consistency. As a result, the Higgs mass is predicted to be just above the LEP2 bound, m_h=115.9\pm 2 GeV, corresponding to a minimal Delta=17.8, value obtained from consistency with electroweak and WMAP (3σ) constraints, but without the LEP2 bound. Due to quantum corrections (largely QCD ones for m_h above LEP2 bound), Delta grows \approx exponentially on either side of the above value of m_h, which stresses the relevance of this prediction. A value m_h>121 (126) GeV cannot be accommodated within the CMSSM unless one accepts a fine-tuning cost worse than Delta>100 (1000), respectively. We review how the above prediction for m_h and Delta changes under the addition of new physics beyond the MSSM Higgs sector, parametrized by effective operators of dimensions d=5 and d=6. For d=5 operators, one can obtain values m_h\leq 130 GeV for Delta<10. The size of the supersymmetric correction that each individual operator of d=6 brings to the value of m_h for points with Delta<100, is found to be small, of few (<4) GeV for M=8 TeV, where M is the scale of new physics. This value decreases (increases) by approximately 1 GeV for a 1 TeV increase (decrease) of the scale M. The relation of these results to the Atlas/CMS supersymmetry exclusion limits is presented together with their impact for the CMSSM regions of lowest fine-tuning.

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arXiv2011

Beyond the MSSM Higgs with d=6 effective operators

We continue a previous study of the MSSM Higgs Lagrangian extended by all effective operators of dimension d=6 that can be present beyond the MSSM, consistent with its symmetries. By supersymmetry, such operators also extend the neutralino and chargino sectors, and the corresponding component fields Lagrangian is computed onshell. The corrections to the neutralino and chargino masses, due to these operators, are computed analytically in function of the MSSM corresponding values. For individual operators, the corrections are small, of few GeV for the constrained MSSM (CMSSM) viable parameter space. We investigate the correction to the lightest Higgs mass, which receives, from individual operators, a supersymmetric correction of up to 4 (6) GeV above the 2-loop leading-log CMSSM value, from those CMSSM phase space points with: EW fine tuning Delta<200, consistent with WMAP relic density (3$σ$), and for a scale of the operators of M=10 (8) TeV, respectively. Applied to the CMSSM point of minimal fine tuning (Delta=18), such increase gives an upper limit $m_h=120(122)\pm 2$ GeV, respectively. The increase of m_h from individual operators can be larger ($\sim$ 10-30 GeV) for those CMSSM phase space points with Delta>200; these can now be phenomenologically viable, with reduced Delta, and this includes those points that would have otherwise violated the LEP2 bound by this value. The neutralino/chargino Lagrangian extended by the effective operators can be used in studies of dark matter relic density within extensions of the MSSM, by implementing it in public codes like micrOMEGAs.

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arXiv2011

Goldstino and sgoldstino in microscopic models and the constrained superfields formalism

We examine the exact relation between the superconformal symmetry breaking chiral superfield (X) and the goldstino superfield in microscopic models of an arbitrary Kahler potential (K) and in the presence of matter fields. We investigate the decoupling of the massive sgoldstino and scalar matter fields and the offshell/onshell-SUSY expressions of their superfields in terms of the fermions composites. For general K of two superfields, we study the properties of the superfield X after integrating out these scalar fields, to show that in the infrared it satisfies (offshell) the condition $X^3=0$ and $X^2\not=0$. We then compare our results to those of the well-known method of constrained superfields discussed in the literature, based on the conjecture $X^2=0$. Our results can be used in applications, to couple offshell the (s)goldstino fields to realistic models such as the MSSM.

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arXiv2011

Fine-tuning implications for complementary dark matter and LHC SUSY searches

The requirement that SUSY should solve the hierarchy problem without undue fine-tuning imposes severe constraints on the new supersymmetric states. With the MSSM spectrum and soft SUSY breaking originating from universal scalar and gaugino masses at the Grand Unification scale, we show that the low-fine-tuned regions fall into two classes that will require complementary collider and dark matter searches to explore in the near future. The first class has relatively light gluinos or squarks which should be found by the LHC in its first run. We identify the multijet plus E_T^miss signal as the optimal channel and determine the discovery potential in the first run. The second class has heavier gluinos and squarks but the LSP has a significant Higgsino component and should be seen by the next generation of direct dark matter detection experiments. The combined information from the 7 TeV LHC run and the next generation of direct detection experiments can test almost all of the CMSSM parameter space consistent with dark matter and EW constraints, corresponding to a fine-tuning not worse than 1:100. To cover the complete low-fine-tuned region by SUSY searches at the LHC will require running at the full 14 TeV CM energy; in addition it may be tested indirectly by Higgs searches covering the mass range below 120 GeV.

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arXiv2010

Testing SUSY

If SUSY provides a solution to the hierarchy problem then supersymmetric states should not be too heavy. This requirement is quantified by a fine tuning measure that provides a quantitative test of SUSY as a solution to the hierarchy problem. The measure is useful in correlating the impact of the various experimental measurements relevant to the search for supersymmetry and also in identifying the most sensitive measurements for testing SUSY. In this paper we apply the measure to the CMSSM, computing it to two-loop order and taking account of current experimental limits and the constraint on dark matter abundance. Using this we determine the present limits on the CMSSM parameter space and identify the measurements at the LHC that are most significant in covering the remaining parameter space. Without imposing the LEP Higgs mass bound we show that the smallest fine tuning (1:13) consistent with a relic density within the WMAP bound corresponds to a Higgs mass of 114$\pm$2 GeV. Fine tuning rises rapidly for heavier Higgs.

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arXiv2010

Fine tuning as an indication of physics beyond the MSSM

We investigate the amount of fine tuning of the electroweak scale in the presence of new physics beyond the MSSM, parametrized by higher dimensional operators. We show that these significantly reduce the MSSM fine tuning to Delta<10 for a Higgs mass between the LEPII bound and 130 GeV, and a corresponding scale M_* of new physics as high as 30 to 65 times the Higgsino mass. If the fine-tuning criterion is indeed of physical relevance, the findings indicate the presence of new physics in the form of new states of mass of O(M_*) that generated the effective operators in the first instance. At small $\tanβ$ these states can be a gauge singlet or a SU(2) triplet. We derive analytical results for the EW scale fine-tuning for the MSSM with higher dimensional operators, including the quantum corrections which are also applicable to the pure MSSM case in the limit the coefficients of the higher dimension operators vanish. A general expression for the fine-tuning is also obtained for an arbitrary two-Higgs doublet potential.

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arXiv2010

MSSM Higgs with dimension-six operators

We investigate an extension of the MSSM Higgs sector by including the effects of all dimension-five and dimension-six effective operators and their associated supersymmetry breaking terms. The corrections to the masses of the neutral CP-even and CP-odd Higgs bosons due to the d=5 and d=6 operators are computed. When the d=5 and d=6 operators are generated by the same physics (i.e. when suppressed by powers of the same scale M), due to the relative $\tanβ$ enhancement of the latter, which compensates their extra scale suppression (1/M), the mass corrections from d=6 operators can be comparable to those of d=5 operators, even for conservative values of the scale M. We identify the effective operators with the largest individual corrections to the lightest Higgs mass and discuss whether at the microscopic level and in the simplest cases, these operators are generated by "new physics" with a sign consistent with an increase of m_h. Simple numerical estimates easily allow an increase of m_h due to d=6 operators alone in the region of 10-30 GeV, while for a much larger increase light new states beyond MSSM may be needed, in which case the effective description is unreliable. Special attention is paid to the treatment of the effective operators with higher derivatives. These can be removed by non-linear field redefinitions or by an "unfolding" technique, which effectively ensure that any ghost degrees of freedom (of mass $\geqsim M$) are integrated out and absent in the effective theory at scales much smaller than M. Considering general coefficients of the susy operators with a scale of new physics above the LHC reach, it is possible to increase the tree-level prediction for the Higgs mass to the LEPII bound, thus alleviating the MSSM fine-tuning.

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arXiv2010

Testing SUSY at the LHC: Electroweak and Dark matter fine tuning at two-loop order

In the framework of the Constrained Minimal Supersymmetric Standard Model (CMSSM) we evaluate the electroweak fine tuning measure that provides a quantitative test of supersymmetry as a solution to the hierarchy problem. Taking account of current experimental constraints we compute the fine tuning at two-loop order and determine the limits on the CMSSM parameter space and the measurements at the LHC most relevant in covering it. Without imposing the LEPII bound on the Higgs mass, it is shown that the fine tuning computed at two-loop has a minimum $Δ=8.8$ corresponding to a Higgs mass $m_h=114\pm 2$ GeV. Adding the constraint that the SUSY dark matter relic density should be within present bounds we find $Δ=15$ corresponding to $m_h=114.7\pm 2$ GeV and this rises to $Δ=17.8$ ($m_h=115.9\pm 2$ GeV) for SUSY dark matter abundance within 3$σ$ of the WMAP constraint. We extend the analysis to include the contribution of dark matter fine tuning. In this case the overall fine tuning and Higgs mass are only marginally larger for the case SUSY dark matter is subdominant and rises to $Δ=28.7$ ($m_h=116.98\pm 2$ GeV) for the case of SUSY dark matter saturates the WMAP bound. For a Higgs mass above these values, fine tuning rises exponentially fast. The CMSSM spectrum that corresponds to minimal fine tuning is computed and provides a benchmark for future searches. It is characterised by heavy squarks and sleptons and light neutralinos, charginos and gluinos.

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arXiv2009

Electroweak and Dark Matter Constraints on a Z' in Models with a Hidden Valley

We consider current precision electroweak data, Z' searches and dark matter constraints and analyse their implications for an extension of the SM that includes an extra U(1)' massive gauge boson and a particular hidden sector ("hidden valley") with a confining (QCD-like) gauge group. The constraints on the Z' with arbitrary Z-Z' kinetic mixing coming from direct searches and precision tests of the Standard Model are analysed and shown to lead to a lower limit of 800 GeV on its mass. Renormalisable interactions involving the Z' probe the physics of the hidden valley sector which contains a pseudoscalar dark matter candidate. We find that dark matter constraints place an upper bound on the mass of the Z' of O(10) TeV. A TeV mass scale is needed for the hidden valley states, and the Sommerfeld factor for p-wave dark matter annihilation is found significantly to suppress the allowed parameter space of the model.

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arXiv2009

Threshold effects near the compactification scale for gauge couplings on orbifolds

In 6D orbifold compactifications of N=1 supersymmetric gauge theories, the one-loop behaviour of the 4D {\it effective} gauge coupling and of its beta function are carefully investigated for momentum scales k^2 near the compactification scale(s). It is showed that as k^2 is crossing the compactification scale(s), there exists a smooth transition ("crossover") to a power-like behaviour of the 4D effective coupling, with a continuous one-loop beta function. Contrary to the naive expectation, the power-like behaviour sets in at momentum scales {\it smaller} than the compactification scale, to suggest a {\it global} effect of the Kaluza-Klein modes even below this scale. We argue that the experimental observation of such behaviour is not necessarily a quantum effect of the compactification, as often thought, and is dominated by classical effects. Special attention is paid to convergence issues near compactification scale(s) and to the scheme dependence of the results.

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arXiv2008

MSSM with Dimension-five Operators (MSSM_5)

We perform a general analysis of the R-parity conserving dimension-five operators that can be present beyond the Minimal Supersymmetric Standard Model. Not all these operators are actually independent. We present a method which employs spurion-dependent field redefinitions that removes this "redundancy" and establishes the minimal, irreducible set of these dimension-five operators. Their potential effects on the MSSM Higgs sector are discussed to show that the tree level bound $m_h\leq m_Z$ cannot be easily lifted within the approximations used, and quantum corrections are still needed to satisfy the LEPII bound. An ansatz is provided for the structure of the remaining couplings in the irreducible set of D=5 operators, which avoids phenomenological constraints from flavor changing neutral currents. The minimal set of operators brings new couplings in the effective Lagrangian, notably "wrong"-Higgs Yukawa couplings and contact fermion-fermion-scalar-scalar interactions, whose effects are expected to be larger than those generated in the MSSM at loop or even tree level. This has implications in particular for LHC searches for supersymmetry by direct squark production.

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arXiv2008

Higher Dimensional Operators in the MSSM

The origin and the implications of higher dimensional effective operators in 4-dimensional theories are discussed in non-supersymmetric and supersymmetric cases. Particular attention is paid to the role of general, derivative-dependent field redefinitions which one can employ to obtain a simpler form of the effective Lagrangian. An application is provided for the Minimal Supersymmetric Standard Model extended with dimension-five R-parity conserving operators, to identify the minimal irreducible set of such operators after supersymmetry breaking. Among the physical consequences of this set of operators are the presence of corrections to the MSSM Higgs sector and the generation of "wrong"-Higgs Yukawa couplings and fermion-fermion-scalar-scalar interactions. These couplings have implications for supersymmetry searches at the LHC.

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arXiv2008

Supersymmetric Models with Higher Dimensional Operators

In 4D renormalisable theories, integrating out massive states generates in the low energy effective action higher dimensional operators (derivative or otherwise). Using a superfield language it is shown that a 4D N=1 supersymmetric theory with higher derivative operators in either the Kahler or the superpotential part of the Lagrangian and with an otherwise arbitrary superpotential, is equivalent to a 4D N=1 theory of second order (i.e. without higher derivatives) with additional superfields and renormalised interactions. We provide examples where a free theory with trivial supersymmetry breaking provided by a linear superpotential becomes, in the presence of higher derivatives terms and in the second order version, a non-trivial interactive one with spontaneous supersymmetry breaking. The couplings of the equivalent theory acquire a threshold correction through their dependence on the scale of the higher dimensional operator(s). The scalar potential in the second order theory is not necessarily positive definite, and one can in principle have a vanishing potential with broken supersymmetry. We provide an application to MSSM and argue that at tree-level and for a mass scale associated to a higher derivative term in the TeV range, the Higgs mass can be lifted above the current experimental limits.

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arXiv2007

Higher dimensional operators and their effects in (non)supersymmetric models

It is shown that a 4D N=1 softly broken supersymmetric theory with higher derivative operators in the Kahler or the superpotential part of the Lagrangian and with an otherwise arbitrary superpotential, can be re-formulated as a theory without higher derivatives but with additional (ghost) superfields and modified interactions. The importance of the analytical continuation Minkowski-Euclidean space-time for the UV behaviour of such theories is discussed in detail. In particular it is shown that power counting for divergences in Minkowski space-time does not always work in models with higher derivative operators.

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