Search Results (74 total)

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

We present a measurement of the W boson mass using data collected by the DØ experiment at the Fermilab Tevatron during 1994–1995. We identify W bosons by their decays to eν final states where the electron is detected in a forward calorimeter. We extract the W boson mass M_W by fitting the transverse mass and transverse electron and neutrino momentum spectra from a sample of 11 089 W→eν decay candidates. We use a sample of 1687 dielectron events, mostly due to Z→ee decays, to constrain our model of the detector response. Using the forward calorimeter data, we measure M_W=80.691±0.227 GeV. Combining the forward calorimeter measurements with our previously published central calorimeter results, we obtain M_W=80.482±0.091 GeV.

Limits on anomalous WWγ and WWZ couplings are presented from a study of WW/WZ→eνjj events produced in pp̅ collisions at sqrt[s]=1.8 TeV. Results from the analysis of data collected using the DØ detector during the 1993–1995 Tevatron collider run at Fermilab are combined with those of an earlier study from the 1992–1993 run. A fit to the transverse momentum spectrum of the W boson yields direct limits on anomalous WWγ and WWZ couplings. With the assumption that the WWγ and WWZ couplings are equal, we obtain -0.34<λ<0.36 (with Δκ=0) and -0.43<Δκ<0.59 (with λ=0) at the 95% confidence level for a form-factor scale Λ=2.0 TeV.

We report on a measurement of σ(pp̅ →W+X)B(W→τν) in pp̅ collisions at sqrt[s]  =  1.8 TeV at the Fermilab Tevatron. The measurement is based on an integrated luminosity (lum) of 18 pb^-1 of data collected with the D0 detector during 1994–1995. We find that σ(pp̅ →W+X)B(W→τν)  =  2.22±0.09 (stat)±0.10 (syst)±0.10 (lum) nb. Lepton universality predicts that the ratio of the tau and electron electroweak charged current couplings to the W boson, g_τ^W/g_e^W, be unity. We find g_τ^W/g_e^W  =  0.980±0.031, in agreement with lepton universality.

This Letter describes a measurement of the muon cross section originating from b-quark decay in the forward rapidity range 2.4<| y^μ|<3.2 in pp̅ collisions at sqrt[s]  =  1.8 TeV. The data used in this analysis were collected by the D0 experiment at the Fermilab Tevatron. We find that next-to-leading-order QCD calculations underestimate b-quark production by a factor of 4 in the forward rapidity region.

We present a measurement of the transverse momentum distribution of Z bosons produced in pp̅ collisions at sqrt[s]  =  1.8 TeV from data collected by the DØ experiment at the Fermilab Tevatron Collider. We find good agreement between our results and current resummation calculations, and also use our data to extract nonperturbative parameters for a particular version of the resummation formalism. The resulting values are significantly more precise than obtained in previous determinations.

We report a new measurement of the cross section for the production of isolated photons with transverse energies ( E_T^γ) above 10 GeV and pseudorapidities |η|<2.5 in pp̅ collisions at sqrt[s]  =  1.8 TeV. The results are based on a data sample of 107.6 pb^-1 recorded during 1992–1995 with the D0 detector at the Fermilab Tevatron collider. The background, predominantly from jets which fragment to neutral mesons, was estimated using the longitudinal shower shape of photon candidates in the calorimeter. The measured cross section is in good agreement with the next-to-leading order QCD calculation for E_T^γ≳36 GeV.

We have searched for second-generation leptoquark (LQ) pairs in the μμ+jets channel using 94±5 pb^-1 of p̅ p collider data collected by the D0 experiment at the Fermilab Tevatron during 1993–1996. No evidence for a signal is observed. These results are combined with those from the μν+jets and νν+jets channels to obtain 95% confidence level (C.L.) upper limits on the LQ pair production cross section as a function of mass and β, the branching fraction of a LQ decay into a charged lepton and a quark. Lower limits of 200(180) GeV/c² for β  =  1(1 / 2) are set at the 95% C.L. on the mass of scalar LQ. Mass limits are also set on vector leptoquarks as a function of β.

We report on measurements on inclusive cross sections times branching fractions into electrons for W and Z bosons produced in pp̅ collisions at sqrt[s]=1.8 TeV. From an integrated luminosity of 84.5 pb^-1 recorded in 1994–1995 using the DØ detector at the Fermilab Tevatron, we determine σ(pp̅ →W+X)×B(W→eν)=2310±10(stat)±50(syst)±100(lum)pb and σ(pp̅ →Z+X)×B(Z→ee)=221±3(stat)±4(syst)±10(lum)pb. From these, we derive σ(pp̅ →W+X)×B(W→eν)/σ(pp̅ →Z+X)×B(Z→ee)=10.43±0.15(stat)±0.20(syst)±0.10(NLO), B(W→eν)=0.1044±0.0015(stat)±0.0020(syst)±0.0017(theory)±0.0010(NLO), and Γ_W=2.169±0.031(stat)±0.042(syst)±0.041(theory)±0.022(NLO)GeV. We use the latter to set a 95% confidence level upper limit on the partial decay width of the W boson into nonstandard model final states, Γ_W^inv, of 0.213 GeV. Combining these results with those from the 1992–1993 data gives σ(pp̅ →W+X)×B(W→eν)/σ(pp̅ →Z+X)×B(Z→ee)=10.51±0.25, Γ_W=2.152±0.066 GeV, and a 95% C.L. upper limit on Γ_W^inv of 0.191 GeV. Using a sample with a luminosity of 505 nb^-1 taken at sqrt[s]=630 GeV, we measure σ(pp̅ →W+X)×B(W→eν)=658±67 pb.

We present a measurement of the differential cross section as a function of transverse momentum of the Z boson in pp̅ collisions at sqrt[s]=1.8 TeV using data collected by the DØ experiment at the Fermilab Tevatron Collider during 1994–1996. We find good agreement between our data and the next-to-next-to-next-leading-logarithmic resummation prediction and extract values of the non-perturbative parameters for the resummed prediction from a fit to the differential cross section.

We report a measurement of the W boson mass based on an integrated luminosity of 82 pb^-1 from pp̅ collisions at sqrt[s]  =  1.8 TeV recorded in 1994–1995 by the D0 detector at the Fermilab Tevatron. We identify W bosons by their decays to eν, where the electron is detected in the forward calorimeters. We extract the mass by fitting the transverse mass and the electron and neutrino transverse momentum spectra of 11 089 W boson candidates. We measure M_W  =  80.691±0.227 GeV. By combining this measurement with our previously published central calorimeter results from data taken in 1992–1993 and 1994–1995, we obtain M_W  =  80.482±0.091 GeV.

Using data corresponding to an integrated luminosity of 79 pb^-1, D0 has searched for events containing multiple jets and large missing transverse energy in pp̅ collisions at sqrt[s]  =  1.8  TeV at the Fermilab Tevatron collider. Observing no significant excess beyond what is expected from the standard model, we set limits on the masses of squarks and gluinos and on the model parameters m₀ and m_1/2, in the framework of the minimal low-energy supergravity models of supersymmetry. For tanβ  =  2 and A₀  =  0, with μ<0, we exclude all models with m_q̃<250  GeV/c². For models with equal squark and gluino masses, we exclude m<260  GeV/c².

We report on a search for R-parity-violating supersymmetry in pp̅ collisions at sqrt[s]  =  1.8 TeV using the D0 detector at Fermilab. Events with at least two electrons and four or more jets were studied. We observe two events in 99±4.4 pb^-1 of data, consistent with the expected background of 1.8±0.4 events. This result is interpreted within the framework of minimal low-energy supergravity supersymmetry models. Squarks with mass below 243 GeV/c² and gluinos with mass below 227 GeV/c² are excluded at the 95% C.L. for A₀  =  0, μ<0, tanβ  =  2, and a finite value for any one of the six R-parity-violating couplings λ_1jk^′ ( j  =  1, 2 and k  =  1, 2, 3).

We report on a search for second generation leptoquarks (LQ) produced in pp̅ collisions at sqrt[s]  =  1.8 TeV using the D0 detector at Fermilab. Second generation leptoquarks are assumed to be produced in pairs and to decay to either μ or ν and either a strange or a charm quark (q). Limits are placed on σ(pp̅ →LQLQ̅ →μν+jets) as a function of the mass of the leptoquark. For equal branching ratios to μq and νq, second generation scalar leptoquarks with a mass below 160 GeV/c², vector leptoquarks with anomalous minimal vector couplings with a mass below 240 GeV/c², and vector leptoquarks with Yang-Mills couplings with a mass below 290 GeV/c², are excluded at the 95% confidence level.

Evidence of anomalous WW and WZ production was sought in pp̅ collisions at a center-of-mass energy of sqrt[s]=1.8 TeV. The final states WW(WZ)→μν jet jet+X, WZ→μνee+X and WZ→eνee+X were studied using a data sample corresponding to an integrated luminosity of approximately 90 pb^-1. No evidence of anomalous diboson production was found. Limits were set on anomalous WWγ and WWZ couplings and were combined with our previous results. The combined 95% confidence level anomalous coupling limits for Λ=2 TeV are -0.25<~Δκ<~0.39 (λ=0) and -0.18<~λ<~0.19 (Δκ=0), assuming the WWγ couplings are equal to the WWZ couplings.

We present a measurement of tt̅ production in pp̅ collisions at sqrt[s]  =  1.8 TeV from 110 pb^-1 of data collected in the all-jets decay channel with the D0 detector at Fermilab. A neural network analysis yields a cross section of 7.1±2.8(stat)±1.5(syst) pb at a top quark mass (m_t) of 172.1 GeV/c². Using previous D0 measurements from dilepton and single lepton channels, the combined D0 result for the tt̅ production cross section is 5.9±1.2(stat)±1.1(syst) pb for m_t  =  172.1  GeV/c².

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3–4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from a heavy-Z target and proceeding through the phase rotation and decay (π→μν_μ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96 Workshop [R. B. Palmer, A. Sessler, and A. Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics (Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].

DØ has measured the inclusive production cross section of W and Z bosons in a sample of 13 pb^-1 of data collected at the Fermilab Tevatron. The cross sections, multiplied by their leptonic branching fractions, for production in pp̅  collisions at sqrt[s]=1.8 TeV are σ_WB(W→eν)=2.36±0.02±0.08±0.13 nb, σ_WB(W→μν)=2.09±0.06±0.22±0.11 nb, σ_ZB(Z→e⁺e^-)=0.218±0.008±0.008±0.012 nb, and σ_ZB(Z→μ⁺μ^-)=0.178±0.022±0.021±0.009 nb, where the first uncertainty is statistical and the second systematic; the third reflects the uncertainty in the integrated luminosity. For the combined electron and muon analyses, we find σ_WB(W→lν)/σ_ZB(Z→l⁺l^-)=10.90±0.52. Assuming standard model couplings, we use this result to determine the width of the W boson, and obtain Γ(W)=2.044±0.097 GeV.

We report a measurement of the top quark mass using six candidate events for the process pp̅ →tt̅ +X→l⁺νbl^-ν̅ b̅ +X, observed in the D0 experiment at the Fermilab pp̅ collider. Using maximum likelihood fits to the dynamics of the decays, we measure a mass for the top quark of m_t=168.4±12.3(stat)±3.6(syst) Gev. We combine this result with our previous measurement in the tt̅ →l+jets channel to obtain m_t=172.1±7.1 GeV as the best value of the mass of the top quark measured by D0.

We report on a search for bottom squarks (b̃) produced in pp̅ collisions at sqrt[s]=1.8 TeV using the DØ detector at Fermilab. Bottom squarks are assumed to be produced in pairs and to decay to the lightest supersymmetric particle (LSP) and a b quark with a branching fraction of 100%. The LSP is assumed to be the lightest neutralino and stable. We set limits on the production cross section as a function of b̃ mass and LSP mass.

We present a search for charged Higgs bosons in decays of pair-produced top quarks using 109.2±5.8 pb^-1 of data recorded from pp̅ collisions at sqrt[s]  =  1.8 TeV by the D0 detector during 1992–1996 at the Fermilab Tevatron. No evidence is found for charged Higgs production, and most parts of the [M_H⁺,tanβ] parameter space where the decay t→H⁺b has a branching fraction close to or larger than that for t→W⁺b are excluded at 95% confidence level. Assuming m_t  =  175 GeV and σ(pp̅ →tt̅ )  =  5.5 pb, for M_H⁺  =  60 GeV, we exclude tanβ<0.97 and tanβ>40.9.

We present a measurement of the Drell-Yan cross section at high dielectron invariant mass using 120 pb ^-1 of data collected in pp̅ collisions at sqrt[s]  =  1.8 TeV by the D0 Collaboration during 1992–1996. No deviation from standard model expectations is observed. We use the data to set limits on the quark-electron compositeness scale. The 95% confidence level lower limits on the compositeness scale vary between 3.3 and 6.1 TeV depending on the assumed form of the effective contact interaction.

We have studied tt̅ production using multijet final states in pp̅ collisions at a center-of-mass energy of 1.8 TeV, with an integrated luminosity of 110.3 pb^-1. Each of the top quarks with these final states decays exclusively to a bottom quark and a W boson, with the W bosons decaying into quark-antiquark pairs. The analysis has been optimized using neural networks to achieve the smallest expected fractional uncertainty on the tt̅ production cross section, and yields a cross section of 7.1±2.8 (stat)±1.5 (syst) pb, assuming a top quark mass of 172.1 GeV/c². Combining this result with previous DØ measurements, where one or both of the W bosons decay leptonically, gives a tt̅ production cross section of 5.9±1.2 (stat)±1.1 (syst) pb.

Using the D0 detector at the 1.8 TeV p̅ p Fermilab Tevatron collider, we have measured the inclusive dijet mass spectrum in the central pseudorapidity region |η_jet|<1.0 for dijet masses greater than 200 GeV/c². We have also measured the ratio of spectra σ(|η_jet|<0.5)/σ(0.5<|η_jet|<1.0). The order α_s³ quantum chromodynamics predictions are in good agreement with the data and we rule out models of quark compositeness with a contact interaction scale <2.4 TeV at the 95% confidence level.

We have made a precise measurement of the central inclusive jet cross section at sqrt[s]  =  1.8 TeV. The measurement is based on an integrated luminosity of 92 pb^-1 collected at the Fermilab Tevatron p̅ p Collider with the D0 detector. The cross section, reported as a function of jet transverse energy (E_T≥60 GeV) in the pseudorapidity interval |η|≤0.5, is in good agreement with predictions from next-to-leading order quantum chromodynamics.