arXiv subjects

Leszek Roszkowski

Publications and source records attributed to Leszek Roszkowski.

At least 91 records · Page 5Linked to original sources

arXiv2004

Weakened Constraints from $b \to s γ$ on Supersymmetry Flavor Mixing Due to Next-To-Leading-Order Corrections

We examine the process $B \to X_s γ$ in minimal supersymmetry (SUSY) with general squark flavor mixings. We include all relevant next--to--leading order (NLO) QCD corrections and dominant NLO SUSY effects from the gluino. We find that gluino--squark corrections to down--type quark masses induce large NLO corrections to the dominant Wilson coefficients whose size is often similar to those at LO, especially at large $\tanβ$. For $μ>0$, destructive interference, and suppression by the renormalization group running lead to a ``focusing effect'' of reducing the size of gluino corrections to the branching ratio, and also of reducing the LO sensitivity to flavor mixings among squarks. Constraints from $B(B \to X_s γ)$ on the SUSY--breaking scale can become significantly weakened relative to the minimal flavor violation case, even, at large $\tanβ$, for small flavor mixings. The case of $μ<0$ can also becomes allowed.

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arXiv2004

Axino Dark Matter and the CMSSM

If the axino is the lightest superpartner and satisfies cosmological bounds, including a preferred range of the relic abundance of cold dark matter, then the usual stringent constraints on the parameter space of the CMSSM become greatly relaxed. The lightest superpartner of the usual CMSSM spectrum will appear to be stable in collider experiments but will not necessarily obey relic abundance constraints. It may be either neutral (lightest neutralino) or charged (typically a stau). With the axino as cold dark matter, large regions of the CMSSM, often corresponding to heavy superpartners, become allowed, depending on the axino mass and the reheating temperature.

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arXiv2004

Particle Dark Matter - A Theorist's Perspective

Dark matter is presumably made of some new, exotic particle that appears in extensions of the Standard Model. After giving a brief overview of some popular candidates, I discuss in more detail the most appealing case of the supersymmetric neutralino.

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arXiv2003

Large Beyond-Leading-Order Effects in b -> s gamma in Supersymmetry with General Flavor Mixing

We examine squark--gluino loop effects on the process $b \to s γ$ in minimal supersymmetry with general flavor mixing in the squark sector. In the regime of heavy squarks and gluino, we derive analytic expressions for the beyond--LO corrections to the Wilson coefficients and find them to be often large, especially at large $\tanβ$ and $μ>0$. The ensuing ranges of values of the Wilson coefficients are typically smaller than in the LO approximation, and sometimes even change sign. This has the effect of often reducing, relative to the LO, the magnitude of supersymmetric contributions to $BR(B \to X_s γ)$. This ``focusing effect'' is caused by contributions from: (i) an RG evolution of the Wilson coefficients; (ii) a correction to the LO chargino contribution to the Wilson coefficients, which can considerably reduce the LO gluino contribution. This partial cancellation of the two contributions takes place only in the case of general flavor mixing. As a result, stringent lower bounds on the mass scale of superpartners, which apply in the case of minimal flavor violation, can be substantially reduced for even small departures from the scenario. The often disfavored case of $μ<0$ can also become allowed for $M_{SUSY}$ as small as $\sim$ 200 GeV, compared to > $\sim$ 500 GeV at LO and over 2 TeV in the case of minimal flavor violation. Limits on the allowed amount of flavor mixing among the 2nd and 3rd generation down--type squarks are also typically considerably weakened. The input CKM matrix element $K^{(0)}_{cb}$ can be larger than the experimental value by a factor of ten, or can be as small as zero.

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arXiv2002

Upper and Lower Limits on Neutralino WIMP Mass and Spin--Independent Scattering Cross Section, and Impact of New (g-2)_{mu} Measurement

We derive the allowed ranges of the spin--independent interaction cross section $\sigsip$ for the elastic scattering of neutralinos on proton for wide ranges of parameters of the general Minimal Supersymmetric Standard Model. We investigate the effects of the lower limits on Higgs and superpartner masses from colliders, as well as the impact of constraints from $\bsgamma$ and the new measurement of $\gmtwo$ on the upper and lower limits on $\sigsip$. We further explore the impact of the neutralino relic density, including coannihilation, and of theoretical assumptions about the largest allowed values of the supersymmetric parameters. For $μ>0$, requiring the latter to lie below $1\tev$ leads to $\sigsip\gsim 10^{-11}\pb$ at $\mchi\sim100\gev$ and $\sigsip\gsim 10^{-8}\pb$ at $\mchi\sim1\tev$. When the supersymmetric parameters are allowed above $1\tev$, for $440\gev \lsim \mchi\lsim 1020 \gev$ we derive a {\em parameter--independent lower limit} of $\sigsip \gsim 2\times 10^{-12}\pb$. (No similar lower limits can be set for $μ<0$ nor for $1020\gev\lsim\mchi\lsim2.6\tev$.) Requiring $\abundchi<0.3$ implies a {\em parameter--independent upper limit} $\mchi\lsim2.6\tev$. The new $\epem$--based measurement of $(g-2)_μ$ restricts $\mchi\lsim 350\gev$ at $1 σ$ CL and $\mchi\lsim515\gev$ at $2 σ$ CL, and implies $μ>0$. The largest allowed values of $\sigsip$ have already become accessible to recent experimental searches.

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arXiv2002

$B \to X_s γ$ in minimal supersymmetric standard model with general flavor mixing

We present the results of our analysis on $b \to s γ$ in the minimal supersymmetric standard model with a general flavor mixing in the squark sector. We identify the next--to--leading order focusing effect, which considerably reduces the gluino contribution to $b \to s γ$ relative to the leading order calculation. We illustrate that $BR(B \to X_s γ)$ is still very sensitive to the flavor mixing parameters, $δ^d_{ll}$ and $δ^d_{lr}$ so that the lower bound for $m_{1/2}$ obtained in the minimal flavor violation can be easily removed even at $μ<0$, while it is rather stable against $δ^d_{rr}$ and $δ^d_{rl}$.

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arXiv2002

Exact Cross Sections for the Neutralino-Slepton Coannihilation

Coannihilation processes provide an important additional mechanism for reducing the density of stable relics in the Universe. In the case of the stable lightest neutralino of the MSSM, and in particular the Constrained MSSM (CMSSM), the coannihilation with sleptons plays a major role in opening up otherwise cosmologically excluded ranges of supersymmetric parameters. In this paper, we derive a full set of exact, analytic expressions for the coannihilation of the lightest neutralino with the sleptons into all two--body tree--level final states in the framework of minimal supersymmetry. We make no simplifying assumptions about the neutralino nor about sfermion masses and mixings other than the absence of explicit CP--violating terms and inter--family mixings. The expressions should be particularly useful in computing the neutralino WIMP relic abundance without the approximation of partial wave expansion. We illustrate the effect of our analytic results with numerical examples and demonstrate a sizeable difference with approximate expressions available in the literature.

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arXiv2002

New Cosmological and Experimental Constraints on the CMSSM

We analyze the implications of several recent cosmological and experimental measurements for the mass spectra of the Constrained MSSM (CMSSM). We compute the relic abundance of the neutralino and compare the new cosmologically expected and excluded mass ranges with those ruled out by the final LEP bounds on the lightest chargino and Higgs masses, with those excluded by current experimental values of $\br(B\to X_s γ)$, and with those favored by the recent measurement of the anomalous magnetic moment of the muon. We find that for $tanβ\lsim 45$ there remains relatively little room for the mass spectra to be consistent with the interplay of the several constraints. On the other hand, at larger values of $tanβ\$ the decreasing mass of the pseudoscalar Higgs gives rise to a wide resonance in the neutralino WIMP pair-annihilation, whose position depends on the ratio of top and bottom quark masses. As a consequence, the cosmologically expected regions consistent with other constraints often grow significantly and generally shift towards superpartner masses in the $\tev$ range.

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arXiv2002

Supersymmetric Dark Matter -- A Physics' Godot?

Where is the long--awaited one? A supersymmetric neutralino has been a favored candidate for the WIMP dark matter but, so far, it has not been found. One way to locate it is to identify where it can be hiding in the vast supersymmetric parameter space. A combination of theoretical, experimental and cosmological constraints leads to remarkably well--defined allowed regions which favor lighter neutralino, and other superpartner, mass ranges. Nevertheless the neutralino can still be as heavy as roughly $800\gev$. The resulting scalar direct detection cross sections range from some $10^{-7}$ pb down to $10^{-10}$ pb, which is at least one order of magnitude below the sensitivity of today's experiments but which will be almost fully explored by a new generation of planned detectors.

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arXiv2002

Effects of Squark Processes on the Axino CDM Abundance

We investigate the role of an effective dimension-4 axino-quark-squark coupling in the thermal processes producing stable cold axino relics in the early Universe. We find that, while the induced squark and quark scattering processes are always negligible, squark decays become important in the case of low reheat temperature and large gluino mass. The effect can tighten the bounds on the scenario from the requirement that cold dark matter axinos do not overclose the Universe.

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arXiv2002

Exact Cross Sections for the Neutralino WIMP Pair-Annihilation

We derive a full set of exact, analytic expressions for the annihilation of the lightest neutralino pairs into all two-body tree-level final states in the framework of minimal supersymmetry. We make no simplifying assumptions about the neutralino nor about sfermion masses and mixings other than the absence of explicit CP--violating terms. The expressions should be particularly useful in computing the neutralino WIMP relic abundance without the usual approximation of partial wave expansion.

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arXiv2001

Axinos as Dark Matter

Supersymmetric extensions of the Standard Model that incorporate the axion solution to the strong CP problem necessarily contain also the axino, the fermionic partner of the axion. In contrast to the neutralino and the gravitino, the axino mass is generically not of the order of the supersymmetry-breaking scale and can be much smaller. The axino is therefore an intriguing candidate for a stable superpartner. In a previous Letter [1] it was shown that axinos are a natural candidate for cold dark matter in the Universe when they are generated non-thermally through out-of-equilibrium neutralino decays. Here, we extend the study of non-thermal production and include a competing thermal production mechanism through scatterings and decays of particles in the plasma. We identify axino masses in the range of tens of MeV to several GeV (depending on the scenario) as corresponding to cold axino relics if the reheating temperature $\treh$ is less than about $5\times10^4\gev$. At higher $\treh$ and lower mass, axinos could constitute warm dark matter. In the scenario with axinos as relics, the gravitino problem finds a natural solution. The lightest superpartner of the Standard Model spectrum remains effectively stable in high-energy detectors but may be either neutral or charged. The usual highly restrictive constraint $\abundchi\lsim1$ on the relic abundance of the lightest neutralino becomes void.

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arXiv2001

Axino - New Candidate for Cold Dark Matter

Supersymmetric extensions of the Standard Model when combined with the Peccei-Quinn solution to the strong CP problem necessarily contain also the axino, the fermionic partner of the axion. In contrast to the neutralino and the gravitino, the axino mass is generically not of the order of the supersymmetry breaking scale and can be much smaller. The axino is therefore an intriguing candidate for a stable superpartner. The axinos are a natural candidate for cold dark matter in the Universe when they are generated non-thermally through out-of-equilibrium neutralino decays or via a competing thermal production mechanism through scatterings and decays of particles in the plasma. We identify axino masses in the range of tens of MeV to several GeV (depending on the scenario) as corresponding to cold axino relics if the reheating temperature T_R is less than about 5 \times 10^4 GeV. At higher T_R and lower mass, axinos could constitute warm dark matter. In the scenario with axinos as stable relics the gravitino problem finds a natural solution. The lightest superpartner of the Standard Model spectrum will remain stable in high-energy detectors but may be either neutral or charged. The usual constraint Omega*h^2<1 on the relic abundance of the lightest neutralino does not hold.

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arXiv2001

Hide and Seek with Neutralino Dark Matter WIMP

As experimental sensitivity increases, one is approaching the range of WIMP-nucleon interaction strengths characteristic of neutralinos. But this continuing progress also unearths new experimental challenges and uncertainties.

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arXiv2001

Non-Baryonic Dark Matter

There exist several well-motivated candidates for non-baryonic cold dark matter, including neutralinos, axions, axinos, gravitinos, Wimpzillas. I review the dark matter properties of the neutralino and the current status of its detection. I also discuss the axino as a new interesting alternative.

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arXiv2001

Towards An Accurate Calculation of the Neutralino Relic Density

We compute the neutralino relic density in the minimal supersymmetric standard model by using exact expressions for the neutralino annihilation cross section into all tree-level final states, including all contributions and interference terms. We find that several final states may give comparable contributions to the relic density, which illustrates the importance of performing a complete calculation. We compare the exact results with those of the usual expansion method and demonstrate a sizeable discrepancy (of more than 10%) over a significant range of the neutralino mass of up to several tens of GeV which is caused by the presence of resonances and new final-state thresholds. We perform several related checks and comparisons. In particular, we find that the often employed approximate iterative procedure of computing the neutralino freeze-out temperature gives generally very accurate results, except when the expansion method is used near resonances and thresholds.

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arXiv1999

Axinos as Cold Dark Matter

We show that axinos produced in the early Universe in the decay of the lightest neutralinos are a natural candidate for cold dark matter. We argue that axinos may well provide the main component of the missing mass in the Universe because their relic density is often comparable with the critical density.

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arXiv1999

WIMP Velocity Impact on Direct Dark Matter Searches

We examine the effect of some uncertainties in the input astrophysical parameters on direct detection searches for WIMPs in the Galactic halo. We concentrate on the possible WIMP annual modulation signal recently reported by the DAMA Collaboration. We find that allowing for a reasonable uncertainty in a WIMP Maxwellian velocity distribution leads to significantly relaxed constraints on the WIMP mass as compared to the original DAMA analysis.

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