Coannihilation Region Research Articles

Determining the Dark Matter Relic Density in the Minimal Supergravity Stau-Neutralino Coannihilation Region at the Large Hadron Collider

We examine the stau-neutralino coannihilation (CA) mechanism of the early Universe. We use the minimal supergravity (mSUGRA) model and show that from measurements at the CERN Large Hadron Collider one can predict the dark matter relic density with an uncertainty of 6% with 30 fb(-1) of data, which is comparable to the direct measurement by the Wilkinson Microwave Anisotropy Probe. This is done by measuring four mSUGRA parameters m(0), m(1/2), A(0), and tanbeta without requiring direct measurements of the top squark and bottom squark masses. We also provide precision measurements of the gaugino, squark, and lighter stau masses in this CA region without assuming gaugino universality.

The fine-tuning price of neutralino dark matter in models with non-universal Higgs masses

We study the amounts of fine-tuning of the parameters of the MSSM with non-universal soft supersymmetry-breaking contributions to the Higgs masses (the NUHM) that would be required for the relic neutralino density to lie within the range favoured by WMAP and other astrophysical and cosmological observations. Such dark matter fine-tuning is analogous to the commonly studied electroweak fine-tuning associated with satisfying the electroweak symmetry breaking conditions, which we also study for completeness. We identify several distinct regions of the NUHM parameter space: a bulk region, a $\stau-\neut$ coannihilation region, a pseudoscalar Higgs funnel region, a focus-point bino/higgsino region and a $\sneut-\neut$ coannihilation region. Within each region, we analyse specific representative points for which we provide breakdowns of the contributions to the dark matter fine-tuning associated with the different NUHM parameters. In general, the NUHM offers points with both both smaller and larger amounts of dark matter fine-tuning than points in the corresponding regions of the CMSSM. Lower amounts of dark matter fine-tuning typically arise at points where several different (co)annihilation processes contribute, e.g., at junctions between regions with different dominant processes. We comment on the prospects for using collider measurements to estimate the likely dark matter density within the NUHM framework.

Long-lived charged sleptons in the minimal supergravity standard model

We performed a study on the probable existence of long-lived charged sleptons in the so-called coannihilation region of parameter space in the framework of the MSSM with supergravity-inspired soft SUSY breaking. We show the possibility to get long-lived charged sleptons in a tiny thin band of mSUGRA parameters space which is consistent with the WMAP restrictions. We show also the strong dependence of this region on the values of tanβ. In this context, we analyze the production cross section of long-lived charged sleptons at LHC in annihilation channels and found that, for light long-lived charged \( \tilde \tau \), the production cross section may reach a few percent of pb.

Full one-loop corrections to the relic density in the MSSM: A few examples

We show the impact of the electroweak, and in one instance the QCD, one-loop corrections on the relic density of dark matter in the MSSM which is provided by the lightest neutralino. We cover here some of the most important scenarios: annihilation into fermions for a bino-like neutralino, annihilation involving gauge bosons in the case of a mixed neutralino, the neutralino–stau co-annihilation region and annihilation into a bottom quark pair. The corrections can be large and should be taken into account in view of the present and forthcoming increasing precision on the relic density measurements. Our calculations are made possible thanks to a newly developed automatic tool for the calculation at one-loop of any process in the MSSM. We have implemented a complete on-shell gauge invariant renormalisation scheme, with the possibility of switching to other schemes. In particular we will report on the impact of different renormalisation schemes for tanβ.

SUSY-GUTs, SUSY-Seesaw and the neutralino dark matter

We will consider a SUSY-SU(5) with one right-handed neutrino with a large top like Yukawa coupling. Assuming universal soft masses at high scale we compute the low-energy spectrum and subsequently the neutralino LSP relic density taking also into consideration SU(5) as well as the see-saw running effects above the gauge coupling unification scale. We found that there exists no viable region in parameter space for tan β 35. The coannihilation process starts becoming efficient for tan β 35−40. However, this process is significantly constrained by the limited range in which the stau is lighter than the neutralino. In fact, for a given tan β we find that there exists an upper bound on the lightest neutralino mass (Mχ10) in this region. The A-pole funnel region appears at very large tan β 45−50, while the focus-point region does not make an appearance till large (m0, M1/2), namely a few TeV. Large A0 terms at high scale can lead to extended regions consistent with WMAP constraints and remove the upper bounds in the stau coannihilation regions.

Probing the parameter space for an MSSM inflation and the neutralino dark matter

The flat directions $LLe$ and $udd$ within the minimal supersymmetric standard model provide all the necessary ingredients for a successful inflation with the right amplitude of the scalar density perturbations, negligible gravity waves and the spectral tilt within $2\ensuremath{\sigma}$ observed range $0.92\ensuremath{\le}{n}_{s}\ensuremath{\le}1.0$. In this paper we explore the available parameter space for inflation in conjunction with a thermal cold dark matter abundance within the minimal supergravity model. Remarkably for the inflaton, which is a combination of squarks and sleptons, there is a stau-neutralino coannihilation region below the inflaton mass 500 GeV for the observed density perturbations and the tilt of the spectrum. For such a low mass of the inflaton the LHC is capable of discovering the inflaton candidates within a short period of its operation. Inflation is also compatible with the focus point region which opens up for the inflaton masses above TeV. We show that embedding MSSM within $SO(10)$ can naturally favor this region.

Indirect measurements of the τ˜–χ˜10 mass difference and Mg˜ in the co-annihilation region of mSUGRA models at the LHC

Abstract We study the prospects for the measurement of the τ ˜ – χ ˜ 1 0 mass difference (ΔM) and the g ˜ mass ( M g ˜ ) in the supersymmetric co-annihilation region of mSUGRA at the LHC using tau leptons. Recent WMAP measurements of the amount of cold dark matter and previous accelerator experiments favor the co-annihilation region of mSUGRA, characterized by a small ΔM (5–15 GeV). Focusing on taus from χ ˜ 2 0 → τ τ ˜ → τ τ χ ˜ 1 0 decays in g ˜ and q ˜ production, we consider inclusive 3 τ + jet + E T production, with two τ's above a high E T threshold and a third τ above a lower threshold. Two observables, the number of opposite-signed τ pairs minus the number of like-signed τ pairs, and the peak of the di-tau invariant mass distribution, allow for the simultaneous determination of ΔM and M g ˜ for Δ M ≳ 6 GeV . For example, for Δ M = 9 GeV and M g ˜ = 850 GeV with 30 fb −1 of data, we can measure ΔM to 15% and M g ˜ to 6%.

Natural dark matter from type I string theory

We study neutralino dark matter within a semi-realistic type I string model, where supersymmetry breaking arises from F-terms of moduli fields parameterised in terms of Goldstino angles, which automatically gives rise to non-universal soft third sfamily and gaugino masses. We study the fine-tuning sensitivities for dark matter and electroweak symmetry breaking across the parameter space of the type I string model, and compare the results to a similar analysis in the non-universal MSSM. Within the type I string model we find that neutralino dark matter can be naturally implemented in the $\stau$ bulk region, the $Z^0$ resonance region and the maximally tempered Bino/Wino/Higgsino region, in agreement with the results of the non-universal MSSM analysis. We also find that in the type I string model the ``well-tempered'' Bino/Wino region is less fine-tuned than in the MSSM, whereas the $\stau$ co-annihilation region exhibits a significantly higher degree of fine-tuning than in the MSSM.

Novel collider signatures for Little Higgs dark matter models

Abstract Little Higgs models with T-parity provide a stable neutral particle A H , the lightest T-odd particle (LTP), which was recently proposed to explain the cold dark matter of the Universe. In the coannihilation region of the LTP with the Little Higgs partner of the light quark Q H , we propose novel signatures for its detection at hadronic colliders by searching for monojet and dijet plus missing energy events due to the associated production of Q H A H and direct production of Q H Q ¯ H , respectively. Using the most recent WMAP data, we show that the coupling of A H – Q H – q is tightly constrained in the coannihilation region. This implies that the monojet signal is too small to be discernible but the dijet plus missing energy signal is potentially measurable at the LHC.

Corrigendum to: “Detection of SUSY in the stau-neutralino coannihilation region at the LHC” [Phys. Lett. B 639 (2006) 46

Corrigendum to: “Detection of SUSY in the stau-neutralino coannihilation region at the LHC” [Phys. Lett. B 639 (2006) 46

What if supersymmetry breaking appears below the GUT scale?

Abstract We consider the possibility that the soft supersymmetry-breaking parameters m 1 / 2 and m 0 of the MSSM are universal at some scale M in below the supersymmetric grand unification scale M GUT , as might occur in scenarios where either the primordial supersymmetry-breaking mechanism or its communication to the observable sector involve a dynamical scale below M GUT . We analyze the ( m 1 / 2 , m 0 ) planes of such sub-GUT CMSSM models, noting the dependences of phenomenological, experimental and cosmological constraints on M in . In particular, we find that the coannihilation, focus-point and rapid-annihilation funnel regions of the GUT-scale CMSSM approach and merge when M in ∼ 10 12 GeV . We discuss sparticle spectra and the possible sensitivity of LHC measurements to the value of M in .

Detection of SUSY in the stau–neutralino coannihilation region at the LHC

Abstract We study the feasibility of detecting the stau neutralino ( τ ˜ 1 – χ ˜ 1 0 ) coannihilation region at the LHC using tau (τ) leptons. The signal is characterized by multiple low energy τ leptons from χ ˜ 2 0 → τ τ ˜ 1 → τ τ χ ˜ 1 0 decays, where the τ ˜ 1 and χ ˜ 1 0 mass difference (ΔM) is constrained to be 5–15 GeV by current experimental bounds including the bound on the amount of neutralino cold dark matter. Within the framework of minimal supergravity models, we show that if hadronically decaying τ's can be identified with 50% efficiency for visible p T > 20 GeV the observation of such signals is possible in the final state of two τ leptons plus large missing energy and two jets. With a gluino mass of 830 GeV the signal can be observed with as few as 3 – 10 fb −1 of data (depending on the size of ΔM). Using a mass measurement of the τ pairs with 10 fb −1 we can determine ΔM with a statistical uncertainty of 12% for Δ M = 10 GeV and an additional systematic uncertainty of 14% if the gluino mass has an uncertainty of 5%.

PRODUCTION OF LONG-LIVED CHARGED SLEPTONS AT LHC

We analyze the MSSM parameter space and discuss the narrow band in the so-called co-annihilation region where sleptons may be long-lived particles. This region is consistent with the WMAP restrictions on the Dark matter and depends on the value of tan β. In this region staus are long-lived and may go through the detector. Due to a relatively small mass (150–850 GeV) their production cross-section at LHC may reach a few % pb.

Comparison of supersymmetric spectrum calculations and impact on the relic density constraints from WMAP

We compare results of four public supersymmetric (SUSY) spectrum codes, Isajet, Softsusy, Spheno and Suspect to estimate the present-day uncertainty in the calculation of the relic density of dark matter in mSUGRA models. We find that even for mass differences of about 1% the spread in the obtained relic densities can be 10%. In difficult regions of the parameter space, such as large tan(beta) or large m_0, discrepancies in the relic density are much larger. We also find important differences in the stau co-annihilation region. We show the impact of these uncertainties on the bounds from WMAP for several scenarios, concentrating on the regions of parameter space most relevant for collider phenomenology. We also discuss the case of non-zero A_0 and the stop co-annihilation region. Moreover, we present a web application for the online comparison of the spectrum codes.

Tau flavour violation in sparticle decays at the LHC

We consider sparticle decays that violate τ lepton number, motivated by neutrino oscillation data. We work in the context of the constrained minimal supersymmetric extension of the Standard Model (CMSSM), in which the different sleptons have identical masses at the GUT scale, and neutrino Dirac Yukawa couplings mix them. We find that the branching ratio for decay of the heavier neutralino χ2→χ+τ±μ∓ is enhanced when the LSP mass mχ∼mτ˜1, including the region of CMSSM parameter space where coannihilation keeps the relic χ density within the range preferred by cosmology. Thus χ2→χ+τ±μ∓ decay may provide a physics opportunity for observing the violation of τ lepton number at the LHC that is complementary to τ→μ+γ decay. Likewise, χ2→χ+e±μ∓ decay is also enhanced in the coannihilation region, providing a complement to μ→e+γ decay.

The stau neutralino co-annihilation region at an International Linear Collider

We probe the stau neutralino co-annihilation domain of the parameter space allowed by the current experimental bounds on the light Higgs mass, the b→sγ decay, and the amount of neutralino cold dark matter within the framework of minimal SUGRA models at a 500 GeV e+e− linear collider. The most favorable signals of SUSY are stau pair production and neutralino pair production where the small mass difference between the lighter stau and the lightest neutralino in the co-annihilation region is ∼5–15 GeV and hence generates low-energy tau leptons in the final state. This small mass difference would be a striking signal of many SUGRA models. We find that a calorimeter covering down to 1° from the beams is crucial to reduce the two-photon background and the mass difference could be measured at a level of 10% with 500 fb−1 of data where an invariant mass of two-tau jets and missing energy is used as a discriminator.

The stau-neutralino co-annihilation region and the collider experiments

We examine the cold dark matter relic density allowed region of SUGRA parameter space in collider experiments. All current experimental bounds on the mSUGRA parameter space are imposed. We study the SUSY signals that would be observable for the dominant stau-neutralino co-annihilation region in a 500/800 GeV linear collider. The detection of the signals in this region requires a complicated analysis due to the fact that the dark matter co-annihilation constraint requires that the τ ˜ 1 and χ ˜ 1 0 mass difference be ⩽15 GeV. The signal contains soft taus. We analyze the SM background and develop necessary cuts to extract the signal. We then utilize these event acceptances to probe the SUGRA models which explain the the recent Belle and BABAR measurements of the B → ϕK CP violating parameters and branching ratios.

Dark matter, light top squarks, and electroweak baryogenesis

We examine the neutralino relic density in the presence of a light top squark, such as the one required for the realization of the electroweak baryogenesis mechanism, within the minimal supersymmetric standard model. We show that there are three clearly distinguishable regions of parameter space, where the relic density is consistent with WMAP and other cosmological data. These regions are characterized by annihilation cross sections mediated by either light Higgs bosons, Z bosons, or by the co-annihilation with the lightest top squark. Tevatron collider experiments can test the presence of the light top squark in most of the parameter space. In the co-annihilation region, however, the mass difference between the light top squark and the lightest neutralino varies between 15 and 30 GeV, presenting an interesting challenge for top squark searches at hadron colliders. We present the prospects for direct detection of dark matter, which provides a complementary way of testing this scenario. We also derive the required structure of the high energy soft supersymmetry breaking mass parameters where the neutralino is a dark matter candidate and the top squark spectrum is consistent with electroweak baryogenesis and the present bounds on the lightest Higgs boson mass.

Two Photon Background and the Reach of a Linear Collider for Supersymmetry in WMAP Favoured Coannihilation Regions

A neutralino relic density in accord with WMAP measurements can be found in the minimal supergravity (mSUGRA) model in several regions of parameter space: the stau co-annihilation corridor at low m_0 and the hyperbolic branch/focus point (HB/FP) region at large m_0 at the edges of parameter space, and the bulk and Higgs boson funnel regions within. In the regions at the edge, the mass gap between the next-to-lightest SUSY particle (NLSP) and the lightest SUSY particle (LSP) becomes small, and backgrounds from $\gamma\gamma\to f\bar{f}$ ($f$ is a SM fermion) become important for NLSP detection at an $e^+e^-$ linear collider. We evaluate these backgrounds from bremsstrahlung and beamstrahlung photons, and demonstrate that these do not preclude the observability of the signal for the cases where either the stau or the chargino is the NLSP. We also delineate the additional portion of the stau coannihilation region, beyond what can be accessed via a search for selectrons and smuons, that can be probed by a search for di-tau-jet events plus missing energy. The reach of a LC for SUSY in the HB/FP region is shown for an updated value of $m_t\simeq 180$ GeV as recently measured by the $D\O$ experiment.

Likelihood analysis of the constrained minimal supersymmetric standard model parameter space

We present a likelihood analysis of the parameter space of the constrained minimal supersymmetric extension of the Standard Model (CMSSM), in which the input scalar masses m_0 and fermion masses m_{1/2} are each assumed to be universal. We include the full experimental likelihood function from the LEP Higgs search as well as the likelihood from a global precision electroweak fit. We also include the likelihoods for b to s gamma decay and (optionally) g_mu - 2. For each of these inputs, both the experimental and theoretical errors are treated. We include the systematic errors stemming from the uncertainties in m_t and m_b, which are important for delineating the allowed CMSSM parameter space as well as calculating the relic density of supersymmetric particles. We assume that these dominate the cold dark matter density, with a density in the range favoured by WMAP. We display the global likelihood function along cuts in the (m_{1/2}, m_0) planes for tan beta = 10 and both signs of mu, tan beta = 35, mu < 0 and tan beta = 50, mu > 0, which illustrate the relevance of g_mu - 2 and the uncertainty in m_t. We also display likelihood contours in the (m_{1/2}, m_0) planes for these values of tan beta. The likelihood function is generally larger for mu > 0 than for mu < 0, and smaller in the focus-point region than in the bulk and coannihilation regions, but none of these possibilities can yet be excluded.