Search Results (29 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.

Results are presented on the exclusive production of four-prong final states in photon-photon collisions from the TPC/Two-Gamma detector at the SLAC e⁺e^- storage ring PEP. Measurement of dE/dx and momentum in the time-projection chamber (TPC) provides identification of the final states 2π⁺2π^-, K⁺K^-π⁺π^-, and 2K⁺2K^-. For two quasireal incident photons, both the 2π⁺2π^- and K⁺K^-π⁺π^- cross sections show a steep rise from threshold to a peak value, followed by a decrease at higher mass. Cross sections for the production of the final states ρ⁰ρ⁰, ρ⁰π⁺π^-, and φπ⁺π^- are presented, together with upper limits for φρ⁰, φφ, and K^*0K¯ ^*0. The ρ⁰ρ⁰ contribution dominates the four-pion cross section at low masses, but falls to nearly zero above 2 GeV. Such behavior is inconsistent with expectations from vector dominance but can be accommodated by four-quark resonance models or by t-channel factorization. Angular distributions for the part of the data dominated by ρ⁰ρ⁰ final states are consistent with the production of J^P=2⁺ or 0⁺ resonances but also with isotropic (nonresonant) production. When one of the virtual photons has mass (m_γ²=-Q²≠0), the four-pion cross section is still dominated by ρ⁰ρ⁰ at low final-state masses W_γγ and by 2π⁺2π^- at higher mass. Further, the dependence of the cross section on Q² becomes increasingly flat as W_γγ increases.

We report cross sections for the process γγ→pp¯ at center-of-mass energies W from 2.0 to 2.8 GeV. These results have been extracted from measurements of e⁺e^-→e⁺e^-pp¯ at an overall center-of-mass energy of 29 GeV, using the TPC/Two-Gamma facility at the SLAC storage ring PEP. Cross sections for the untagged mode [both photons nearly real] are shown to lie well above QCD predictions. Results are also presented for the single-tagged mode [one photon in the range 0.16<Q²<1.6 (GeV/c)²].

We set an upper limit on the branching fraction B(τ^-→ν_τK^-K⁰)<0.26% at the 95% confidence level. The data sample was obtained with the TPC/Two-Gamma detector facility at the SLAC e⁺e^- storage ring PEP. The process τ^-→ν_τK^-K⁰ is related via SU(3) to the second-class-current decay τ^-→ν_τπ^-η. Our limit is nearly 20 times smaller than the recently reported branching fraction B(τ^-→ν_τπ^-η) of (5.1±1.0±1.2)%, whereas SU(3) symmetry predicts the ratio of πη to KK¯ production to be at most 5:1. We also measure the branching fraction B(τ^-→ν_τK^*-)=(1.5±0.4*0.4%.

Two-photon formation of the η’ in the reaction e⁺e^-→e⁺e^-ηm’→e⁺e^-π⁺π^-γ has been studied for nearly real and virtual photons at an e⁺e^- center-of-mass energy of 29 GeV. The γγ width of the η’ is found to be 4.5±0.3(stat.)±0.7(syst.) keV. The measured dependence of this γγ width upon the photon four-momentum squared in the range 0–5 GeV² agrees with predictions both from QCD and from ρ dominance. A spin analysis results in a strong preference for a spin-zero, relative to a spin-two, assignment for the η’. No evidence for C-parity violation is found in this η’→π⁺π^-γ decay mode.

We have measured the branching ratios for several τ decay modes. We use e⁺e^-→τ⁺τ^- events accumulated with the TPC/Two-Gamma facility at the SLAC e⁺e^- storage ring PEP. The data correspond to an integrated luminosity of 77 pb^-1 at a center-of-mass energy of 29 GeV. The one- and three-charged-particle inclusive branching ratios of the τ decay are measured to be B₁=(84.7±1.0)% and B₃=(15.1±1.0)%, where B₁+B₃ is constrained to be 99.85%. The branching ratios of the two leptonic decay modes are B(τ^-→e^- ν¯_eν_τ)=(18.4±1.6)% and B(τ^-→μ^- ν¯_μν_τ)=(17.7±1.4)%. If we then assume lepton universality, we obtain B(τ^-→e^- ν¯_eν_τ)=(18.3±0.9)% and B(τ^-→μ^- ν¯_μν_τ)=(17.8±0.9)%. We measure the Cabibbo-allowed semihadronic decay mode B(τ^-→π^-+neutral particles)=(47.0±1.5)%, and the Cabibbo-suppressed τ decay mode B(τ^-→K^-+neutral particles)=(1.6±0.4)%. By looking for associated photons, we find B(τ^-→K^-π⁰+neutral particles) to be (1.2±0.6)%. Using the channel K*^-→K_S^Oπ⁺π^-, we find B(τ^-→K^*-(892)+neutral particles)=(1.4±0.9)%. The quoted errors are the combined statistical and systematic errors.

The structure function F₂^γ for a quasireal photon has been measured in the reaction ee→eeX for Q² in the range 0.2<Q²<7 GeV², by use of 9200 multihadron events obtained with the TPC/Two-Gamma detector at the SLAC storage ring PEP. The data have been corrected for detector effects by a regularized unfolding procedure and are presented as F₂^γ(x,Q²). The structure function shows scaling in the region 0.3<Q²<1.6 GeV², x<0.3, and rises for higher Q² and x>0.1. Below Q²=0.3 GeV², scaling breaks down in accordance with the finite cross-section bound for real photons.

We report on the first search with virtual photon-photon collisions for narrow, neutral resonances with even C parity in the mass range 4.5<W<19 GeV. The data were obtained via the process e⁺e^-→e⁺e^-γ^*γ^*→e ⁺e^-R with both the scattered e⁺ and e^- detected. We find upper limits (95% confidence level) for the partial decay width of a resonance into two photons, ranging from 50 keV at W=4.5 GeV to 10 MeV at W=19 GeV. These limits constrain theoretical models involving neutral composite bosons.

We report measurements of two-particle correlations in rapidity space between a p¯ or Λ¯ and an additional p, p¯, Λ, or Λ¯. We find evidence for local conservation of baryon number, and for the first time observe a pronounced anticorrelation between baryons with the same value of baryon number. Such an anticorrelation is expected in fragmentation models where the rapidity order of particles closely reflects their ‘‘color order,’’ as is the case, for example, in recent versions of the Lund string model.

Two-photon production of K_S⁰K^±π^∓ states has been studied by the TPC/Two-Gamma experiment at the SLAC storage ring PEP. A resonance of mass 1.42 GeV was seen when one of the photons was quite virtual but not when both photons were nearly real. Production of a spin-1 meson, which cannot be made by two real photons, would fit these observations. The Q² dependence of the data in the resonance region agrees with this spin assignment and is incompatible with a spin-0 hypothesis. The mass and width of the resonance are similar to those of the E meson, which has been assigned J^P=0^- and J^P=1⁺ by different experiments.

We measure an inclusive branching fraction of (13.9 ± 2.0_-2.2^+1.9)% for the decay τ^-→ν_τπ^-π⁰+nh⁰(n>~1), where h⁰ is a π⁰ or an η. The data sample, obtained with the time-projection-chamber detector facility at the SLAC e⁺e^- storage ring PEP, corresponds to an integrated luminosity of 72 pb^-1 at 29 GeV center-of-mass energy. The measured branching fraction is somewhat greater than the theoretical prediction and, with errors taken into account, could resolve the present difference between the inclusive and the sum of the exclusive τ^± branching fractions into one charged prong.

We compare the particle flow in the event plane of three-jet qq̅ g (quark-antiquark-gluon) events with the particle flow in radiative annihilation events qq̅ γ (quark-antiquark-photon) for similar kinematic configurations. In the angular region between quark and antiquark jet, we find a significant decrease in particle density for qq̅ g as compared to qq̅ γ. This effect is predicted in QCD as a result of destructive interference between soft-gluon radiation from quark, antiquark, and hard gluon.

We report measurements of the two-photon processes e⁺e^-→e⁺e^-π⁺π^- and e⁺e^-→e⁺e^-K⁺K^-, at an e⁺e^- center-of-mass energy of 29 GeV. In the π⁺π^- data a high-statistics analysis of the f(1270) results in a γγ width Γ(γγ→f)=3.2±0.4 keV. The π⁺π^- continuum below the f mass is well described by a QED Born approximation, whereas above the f mass it is consistent with a QCD-model calculation if a large contribution from the f is assumed. For the K⁺K^- data we find agreement of the high-mass continuum with the QCD prediction; limits on f^′(1520) and θ(1720) formation are presented.

By observing the reaction γγ→K_s⁰K^±π^∓, the TPC/Two-Gamma experiment at the SLAC e⁺e^- storage ring PEP has obtained a 95%-confidence-level limit of Γ_ι→γγB(ι→KK¯π)<1.6 keV for the ι(1450) meson. If, as is likely, the ι decays predominantly into KK¯π, the resulting Γ_ι→γγ limit appears to conflict with previous assignments of an observed ργ decay to ι and also with many analyses of η-η’-ι mixing. The contrast of this small γγ width with the large rate for J/ψ→γι is evidence that the ι is a glueball with little admixture of qq¯ states.

We use the time-projection chamber detector to study mechanisms of baryon production in the central rapidity region of jets from e⁺e^- annihilation. On the basis of angular correlations between protons and antiprotons, we exclude the isotropic decay of heavy mesonic clusters as a dominant source of baryons at 95% confidence level. The diquark mechanisms used in string fragmentation models reproduce the data, provided that diquarks are formed in a two-step process which allows mesons to be produced in between the baryon and antibaryon.

We report a measurement of the reaction γγ→K⁺K^-π⁺π^- in both tagged and untagged events at PEP. The cross section rises with invariant γγ mass to about 15 nb at 2 GeV and falls slowly at higher masses. We find clear evidence for the processes γγ→φπ⁺π^- and γγ→K^*0(892)Kπ. Upper limits (95% C.L.) of 1.5 and 5.7 nb in the mass range from 1.7 to 3.7 GeV are obtained for φρ⁰ and K^*0K̅ ^*0 production, respectively.

We have studied the production of prompt muons in hadronic events from e⁺e^- annihilation at a center-of-mass energy of 29 GeV with the PEP4-TPC (Time Projection Chamber) detector. The muon p and p_t distributions are well described by a combination of bottom- and charm-quark decays, with fitted semimuonic branching fractions of (15.2±1.9±1.2)% and (6.9±1.1±1.1)%, respectively. The muon spectra imply hard fragmentation functions for both b and c quarks, with 〈z(b quark)〉=0.80±0.05±0.05 and 〈z(c quark)〉=0.60±0.06±0.04. We derive neutral-current axial-vector couplings of a(b quark)=-0.9±1.1±0.3 and a(c quark)=1.5±1.5±0.5 from the forward-backward asymmetries.

Bose-Einstein correlations between like-sign pions have been investigated in e⁺e^- annihilation at √s =29 GeV using the Time Projection Chamber detector at the SLAC e⁺e^- storage ring PEP. The production rate of like-sign pion pairs with small relative momentum is found to be increased by more than 50% over the rate expected for uncorrelated production of pions. From the correlation length, a typical source radius of 0.65 fm is derived. Data are consistent with a spherical shape of the pion source. No dependence of radius or correlation strength on the event multiplicity is observed.

The inclusive production cross section of Λ, Λ̅ in e⁺e^- annihilation at a c.m. energy of 29 GeV has been measured with the time-projection-chamber detector at PEP. The average Λ, Λ̅ multiplicity has been measured to be 0.197 ± 0.012(stat.) ±0.017(syst.). Λ-Λ̅ pairs have been observed in jets for the first time, and the average number of Λ-Λ̅ pairs per event has been measured to be 0.042 ± 0.017 ± 0.014.

The distribution of particles in three-jet events is compared with the predictions of three fragmentation models currently in use: the Lund string model, the Webber cluster model, and an independent fragmentation model. The Lund model and, to a certain extent, the Webber model provide reasonable descriptions of the data. The independent fragmentation model does not describe the distribution of particles at large angles with respect to the jet axes. The results provide evidence that the sources of hadrons are Lorentz boosted with respect to the overall c.m.

Evidence for a narrow state decaying into an F meson and a photon has been obtained in e⁺e^- annihilation events at 29-GeV c.m. energy. This state lies 139.5 ± 8.3(stat.) ± 9.7(syst.) MeV above the F-meson mass and is consistent with the expected F^* meson. The F mesons are identified by a peak in the K⁺K^-Kπ^± mass at 1.948±0.028±0.010 GeV.

The inclusive production cross sections and transverse momentum distributions of K^*0 and K_S⁰ mesons in e⁺e^- annihilation at a center-of-mass energy of 29 GeV have been measured by means of the time projection chamber detector in the PEP-4 experiment. The mean multiplicites are found to be 0.49 ± 0.04(stat.) ± 0.07(syst.) (K^*0+K―^*0) and 1.22 ± 0.03(stat.) ±0.15(syst.) (K⁰+K―⁰) per event.

Correlations in rapidity space are presented for identified π^± and K^± in e⁺e^- annihilation at 29-GeV c.m. energy. Short-range KK correlations indicate local flavor compensation in the hadronization process. Long-range KK and ππ correlations prove that the initial partons carry flavor. In addition, we observe significant Kπ correlations as a result of heavy-quark decays.

From a study of τ decays using the Time Projection Chamber (TPC) detector at the SLAC e⁺e^- storage ring PEP, we have measured the one-, three-, and five-charged-particle inclusive branching fractions of the τ to be (85.2±1.7)%, (14.8±1.7)%, and 0.3%, respectively. Using particle identification by dE / dx in the TPC we have searched for charged K mesons in three-prong τ decays, and have set an upper limit on B (τ^±→K^±+2 charged+neutrals) of 0.6%.