Search Results (120 total)

The observation of the Egret experiment of an excess of diffuse gamma rays with energies above E_γ=1  GeV has previously been interpreted in the context of the minimal supergravity model (mSUGRA) as coming from neutralino annihilation into mainly b-quarks in the galactic halo, with neutralino mass in the vicinity of 50–70 GeV. We observe that, in order to obtain the correct relic abundance of neutralinos in accord with WMAP measurements, the corresponding neutralino-proton direct detection (DD) rates should be in excess of recent limits from the Xenon-10 collaboration. While it does not appear possible to satisfy the Egret, WMAP, and Xenon-10 constraints simultaneously within the mSUGRA model, we find that it is easily possible in models with nonuniversal Higgs soft masses (NUHM). In either case, gluino pair production from m_g˜∼400–500  GeV should occur at large rates at the CERN LHC, and a gluino pair production signal should be visible with just 0.1  fb^-1 of integrated luminosity. The NUHM interpretation predicts a rather light spectrum of heavy Higgs bosons with m_A∼140–200  GeV over the whole parameter space which would interpret Egret data. Spin-independent DD rates are predicted to be just above 10^-8  pb, within range of the next round of direct dark matter detection experiments.

We show that, in the minimal supersymmetric standard model, the possibility for the lightest CP-even Higgs boson to be lighter than Z boson (as low as about 60 GeV) is, contrary to the usual belief, not yet excluded by the CERN LEP2 Higgs search nor any direct searches for supersymmetric particles at high energy colliders. The characteristic of the light Higgs boson scenario (LHS) is that the ZZh coupling and the decay branching ratio Br(h/A→bb̅ ) are simultaneously suppressed as a result of generic supersymmetric loop corrections. Consequently, the W^±H^∓h coupling has to be large due to the sum rule of Higgs couplings to weak gauge bosons. We discuss the potential of the Fermilab Tevatron and B factories to test the LHS, and show that the associated neutral and charged Higgs boson production process, pp→H^±h(A), can completely probe the LHS at the CERN Large Hadron Collider.

Magnetotransport phenomena in the high-density two-dimensional electron gas of AlGaN∕GaN heterostrutures are investigated. Peculiarities of low- and high-field magnetotransport measurements in the temperature range from 0.3  to  10  K are discussed. The weak localization and antilocalization effects are observed and analyzed. The Rashba constant describing the spin-orbit interaction is extracted from the results of low-field magnetotransport for the structure with 33% aluminum content in the barrier and an electron sheet density of 1.1×10¹³  cm⁻² and appears to be of 1.01×10⁻¹⁰  eV  cm.

We study production of Kaluza-Klein (KK) gluons at the Large Hadron Collider (LHC) in the framework of a warped extra dimension with the standard model fields propagating in the bulk. We show that the detection of the KK gluon is challenging since its production is suppressed by small couplings to the proton’s constituents. Moreover, the KK gluon decays mostly to top pairs due to an enhanced coupling and hence is broad. Nevertheless, we demonstrate that for M_KKG≲4  TeV, 100  fb^-1 of data at the LHC can provide discovery of the KK gluon. We utilize a sizable left-right polarization asymmetry from the KK gluon resonance to maximize the signal significance, and we explore the novel feature of extremely highly energetic “top-jets.” We briefly discuss how the detection of electroweak gauge KK states (Z/W) faces a similar challenge since their leptonic decays (golden modes) are suppressed. Our analysis suggests that other frameworks, for example, little Higgs, which rely on UV completion via strong dynamics might face similar challenges, namely, (1) suppressed production rates for the new particles (such as Z^′), due to their “light-fermion-phobic” nature, and (2) difficulties in detection since the new particles are broad and decay predominantly to third generation quarks and longitudinal gauge bosons.

It has been claimed recently that half-lives of radioactive nuclei embedded in metals would be significantly affected by electron screening provided by the metal. The effect would further be strengthened at low temperatures. We have determined the half-life-of ²⁵³Es nuclei embedded in iron at temperatures between 4 K and 50 mK. Our results agree with the room temperature literature value within about 2% and show no dependence on temperature over a range of almost two orders of magnitude.

We explore the potential of the CERN Large Hadron Collider (LHC) to test the dynamical torsion parameters. The form of the torsion action can be established from the requirements of consistency of effective quantum field theory. The most phenomenologically relevant part of the torsion tensor is dual to a massive axial vector field. This axial vector has geometric nature, that means it does not belong to any representation of the gauge group of the SM extension or GUT theory. At the same time, torsion should interact with all fermions, that opens the way for the phenomenological applications. We demonstrate that LHC collider can establish unique constraints on the interactions between fermions and torsion field considerably exceeding present experimental lower bounds on the torsion couplings and its mass. It is also shown how possible nonuniversal nature of torsion couplings due to the renormalization group running between the Planck and TeV energy scales can be tested via the combined analysis of Drell-Yan and tt̅ production processes.

We study the collider phenomenology of a littlest Higgs model with T parity. We first stress the important role of the T-odd SU(2)-doublet fermions (introduced to make the model T-parity invariant) in high energy scattering processes, such as qq̅ →W_H⁺W_H^- where W_H^± are the T-odd partners of W bosons. Because the mass of the T-odd SU(2)-doublet fermions cannot be too heavy to be consistent with low energy data, they can be copiously produced at the CERN Large Hadron Collider (LHC). Therefore, we study the collider phenomenology of the model with emphasis on the contributions of the T-odd fermion to the production of the heavy T-parity partners (either bosons or fermions) of the usual particles at the LHC. The production cross sections and the decay branching ratios of the new heavy particles are classified and various experimental signatures are discussed.

In this paper we discuss how to extract information about physics beyond the standard model (SM) from searches for a light SM Higgs boson at Tevatron Run II and CERN LHC. We demonstrate that new (pseudo)scalar states predicted in both supersymmetric and dynamical models can have enhanced visibility in standard Higgs search channels, making them potentially discoverable at Tevatron Run II and CERN LHC. We discuss the likely sizes of the enhancements in the various search channels for each model and identify the model features having the largest influence on the degree of enhancement. We compare the key signals for the nonstandard scalars across models and also with expectations in the SM, to show how one could start to identify which state has actually been found. In particular, we suggest the likely mass reach of the Higgs search in pp̅ /pp→H→τ⁺τ^- for each kind of nonstandard scalar state and we demonstrate that pp̅ /pp→H→γγ may cleanly distinguish the scalars of supersymmetric models from those of dynamical models.

Within the minimal supergravity model (mSUGRA) framework, the expectation for the relic density of neutralinos exceeds the WMAP determination, unless neutralinos (a) have a significant Higgsino component, (b) have a mass close to half that of a heavy Higgs boson, or (c) can efficiently coannihilate with a charged or colored particle. Within a one-parameter extension of the mSUGRA model which includes nonuniversal Higgs masses, we show that agreement with the WMAP data can be obtained over a wide range of mSUGRA parameters for scenarios (a) and (b), so that the phenomenological implications may be much more diverse than in mSUGRA. We show that direct and/or indirect detection of neutralino dark matter should be possible at various current and planned facilities.

The anisotropy in the angular distribution of α particles from oriented ^253,254Es and ²⁵⁵Fm nuclei, which are among the strongest deformed α emitters, was measured. Large α anisotropies have been observed for all three nuclei. The results are compared with calculations based on α-particle tunneling through a deformed Coulomb barrier.

The results based on 1992–95 data (Run 1) from the CDF and D0 experiments on the measurements of the W boson mass and width are presented, along with the combined results. We report a Tevatron collider average M_W=80.456±0.059  GeV. We also report the Tevatron collider average of the directly measured W boson width Γ_W=2.115±0.105  GeV. We describe a new joint analysis of the direct W mass and width measurements. Assuming the validity of the standard model, we combine the directly measured W boson width with the width extracted from the ratio of W and Z boson leptonic partial cross sections. This combined result for the Tevatron is Γ_W=2.135±0.050  GeV. Finally, we use the measurements of the direct total W width and the leptonic branching ratio to extract the leptonic partial width Γ(W→eν)=224±13  MeV.

Using 85.2±3.6   pb^-1 of pp̅ collisions collected at sqrt[s]=1.8  TeV with the D0 detector at Fermilab’s Tevatron Collider, we present the results of a search for direct pair production of scalar top quarks (t˜), the supersymmetric partners of the top quark. We examined events containing two or more jets and missing transverse energy, the signature of light scalar top quark decays to charm quarks and neutralinos. After selections, we observe 27 events while expecting 31.1±6.4 events from known standard model processes. Comparing these results to next-to-leading-order production cross sections, we exclude a significant region of t˜ and neutralino phase space. In particular, we exclude the t˜ mass m_t˜<122  GeV/c² for a neutralino mass of 45  GeV/c².

A search for narrow resonances that decay into tt̅ pairs has been performed using 130   pb^-1 of data in the lepton + jets channel collected by the DØ detector in pp̅ collisions at sqrt[s]=1.8   TeV. There is no significant deviation observed from the standard-model predictions at a top-quark mass of 175   GeV/c². We therefore present upper limits at the 95% confidence level on the product of the production cross section and branching fraction to tt̅ for narrow resonances as a function of the resonance mass M_X. These limits are used to exclude the existence of a leptophobic top-color particle with mass M_X<560   GeV/c², using a theoretical cross section for a width Γ_X=0.012M_X.

We present the results of a search for the production of new particles decaying into two jets in p̅ p collisions at sqrt[s]=1.8 TeV, using the DØ 1992–1995 data set corresponding to 109 pb^-1. We exclude at the 95% confidence level the production of excited quarks (q^*) with masses below 775 GeV/c², the most restrictive limit to date. We also exclude standard-model-like W^′ (Z^′) bosons with masses between 300 and 800 GeV/c² (400 and 640 GeV/c²). A W^′ boson with mass <786 GeV/c² has been excluded by previous measurements, and our lower limit is therefore the most stringent to date.

We report calculations for the low-energy near-threshold inelastic collision cross sections between the Li(2s,2p,3s,3p)+H(1s) states. Results are obtained by solving the coupled-channel equations. Order-of-magnitude estimates for higher states have been made with the multichannel Landau-Zener model. Potentials and couplings from H. Croft et al [J. Phys. B 32, 81 (1999)] are employed. The calculated cross sections are much smaller than ones predicted by the classical Thomsom atom formula currently employed in astrophysics. This result is important for the interpretation of stellar spectra.

The current through a p-i-n diode containing a layer of self-assembled InAs quantum dots in the intrinsic region is investigated. A series of peaks is observed in the differential conductance below the flat band regime which we attribute to electron tunneling in the quantum dot and wetting layer states, combining the carrier recombination and the carrier relaxation effects. This phenomenon is investigated numerically on the basis of a master equation model. Criteria for the observability of the charging of individual quantum dots are discussed. A key point is the presence of Coulomb screening as otherwise the long-range interdot interactions smear out the energy level spectrum.

We present a search for large extra dimensions (ED) in pp̅ collisions at a center-of-mass energy of 1.8 TeV using data collected by the DØ detector at the Fermilab Tevatron in 1994–1996. Data corresponding to 78.8±3.9   pb^-1 are examined for events with large missing transverse energy, one high-p_T jet, and no isolated muons. There is no excess observed beyond expectation from the standard model, and we place lower limits on the fundamental Planck scale of 1.0 and 0.6 TeV for 2 and 7 ED, respectively.

We present data on multiple production of jets with transverse energies near 20 GeV in pp̅ collisions at sqrt[s]=1.8 TeV. QCD calculations in the parton-shower approximation of PYTHIA and HERWIG and the next-to-leading order approximation of JETRAD are compared to the data for one, two, three, and four jet inclusive production. Transverse energy spectra and multiple jet angular and summed transverse-energy distributions are adequately described by the shower approximation while next-to-leading order calculations describe the data poorly.

Results are presented on a measurement of the tt̅ pair production cross section in pp̅ collisions at sqrt[s]=1.8 TeV from nine independent decay channels. The data were collected by the DØ experiment during the 1992–1996 run of the Fermilab Tevatron Collider. A total of 80 candidate events is observed with an expected background of 38.8±3.3 events. For a top quark mass of 172.1 GeV/c², the measured cross section is 5.69±1.21(stat)±1.04(syst) pb.

We describe a search for evidence of minimal supergravity (MSUGRA) in 92.7 pb^-1 of data collected with the DØ detector at the Fermilab Tevatron pp̅ collider at sqrt[s]=1.8 TeV. Events with a single electron, four or more jets, and large missing transverse energy were used in this search. The major backgrounds are from W+jets, misidentified multijet, tt̅ , and WW production. We observe no excess above the expected number of background events in our data. A new limit in terms of MSUGRA model parameters is obtained.

We report the first search for supersymmetric particles via s-channel production and decay of smuons or muon sneutrinos at hadronic colliders. The data for the two-muon and two-jets final states were collected by the D0 experiment and correspond to an integrated luminosity of 94±5   pb^-1. Assuming that R parity is violated via the single coupling λ₂₁₁^′, the number of candidate events is in agreement with expectation from the standard model. Exclusion contours are given in the (m₀,m_1/2) and (m_{χ ˜},m_{ν ˜}) planes for λ₂₁₁^′=0.09, 0.08, and 0.07.

We present results of a search for R-parity-violating decay of the neutralino χ ˜₁⁰, taken as the lightest supersymmetric particle, to a muon and two jets. The decay proceeds through a lepton-number violating coupling λ_2jk^′ (j=1,2; k=1,2,3), with R-parity conservation in all other production and decay processes. In the absence of candidate events from 77.5±3.9   pb^-1 of data collected by the D0 experiment at the Fermilab Tevatron in pp̅ collisions at sqrt[s]=1.8  TeV, and with an expected background of 0.18±0.03±0.02 events, we set limits on squark and gluino masses within the framework of the minimal low-energy supergravity-supersymmetry model.

Based on 85 pb^-1 data of pp̅ collisions at sqrt[s]=1.8 TeV collected using the DØ detector at Fermilab during the 1994–1995 run of the Tevatron, we present a direct measurement of the total decay width of the W boson Γ_W. The width is determined from the transverse mass spectrum in the W→e+ν_e decay channel and found to be Γ_W=2.23_-0.14^+0.15(stat)±0.10(syst) GeV, consistent with the expectation from the standard model.

We have measured the W boson mass using the DØ detector and a data sample of 82 pb^-1 from the Fermilab Tevatron collider. This measurement uses W→eν decays, where the electron is close to a boundary of a central electromagnetic calorimeter module. Such “edge” electrons have not been used in any previous DØ analysis, and represent a 14% increase in the W boson sample size. For these electrons, new response and resolution parameters are determined, and revised backgrounds and underlying event energy flow measurements are made. When the current measurement is combined with previous DØ W boson mass measurements, we obtain M_W=80.483±0.084 GeV. The 8% improvement from the previous DØ measurement is primarily due to the improved determination of the response parameters for non-edge electrons using the sample of Z bosons with non-edge and edge electrons.

We present the results of a search for leptoquark (LQ) pairs in (85.2±3.7)  pb^-1 of pp̅ collider data collected by the D0 experiment at the Fermilab Tevatron. We observe no evidence for leptoquark production and set a limit on σ(pp̅ →LQLQ̅ →νν+jets) as a function of the mass of the leptoquark (m_LQ). Assuming the decay LQ→νq, we exclude scalar leptoquarks for m_LQ<98 GeV/c², and vector leptoquarks for m_LQ<200 GeV/c² and coupling which produces the minimum cross section, at a 95% confidence level.