Search Results (24 total)

A model-independent partial-wave analysis of the S-wave component of the Kπ system from decays of D⁺ mesons to the three-body K^-π⁺π⁺ final state is described. Data come from the Fermilab E791 experiment. Amplitude measurements are made independently for ranges of K^-π⁺ invariant mass, and results are obtained below 825  MeV/c², where previous measurements exist only in two mass bins. This method of parametrizing a three-body decay amplitude represents a new approach to analyzing such decays. Though no model is required for the S-wave, a parametrization of the relatively well-known reference P- and D-waves, optimized to describe the data used, is required. In this paper, a Breit-Wigner model is adopted to describe the resonances in these waves. The observed phase variation for the S-, P-, and D-waves do not match existing measurements of I=1 / 2 K^-π⁺ scattering in the invariant mass range in which scattering is predominantly elastic. If the data are mostly I=1 / 2, this observation indicates that the Watson theorem, which requires these phases to have the same dependence on invariant mass, may not apply to these decays without allowing for some interaction with the other pion. The production rate of K^-π⁺ from these decays, if assumed to be predominantly I=1 / 2, is also found to have a significant dependence on invariant mass in the region above 1.25  GeV/c². These measurements can provide a relatively model-free basis for future attempts to determine which strange scalar amplitudes contribute to the decays.

We study the Dalitz plot of the decay D⁺→K^-π⁺π⁺ with a sample of 15090 events from Fermilab experiment E791. Modeling the decay amplitude as the coherent sum of known Kπ resonances and a uniform nonresonant term, we do not obtain an acceptable fit. If we allow the mass and width of the K₀^*(1430) to float, we obtain values consistent with those from PDG but the χ² per degree of freedom of the fit is still unsatisfactory. A good fit is found when we allow for the presence of an additional scalar resonance, with mass 797±19±43   MeV/c² and width 410±43±87   MeV/c². The mass and width of the K₀^*(1430) become 1459±7±5   MeV/c² and 175±12±12   MeV/c², respectively. Our results provide new information on the scalar sector in hadron spectroscopy.

Using data from the E791 fixed-target hadroproduction experiment at Fermilab, we have studied the Cabibbo-favored but phase-space-suppressed decay D⁰→K^-K^-K⁺π⁺. We find the decay rate for this mode to be (0.54±0.16±0.08)×10^-2 times that for the normalization mode D⁰→K^-π^-π⁺π⁺. We observe a clear signal for D⁰→φK^-π⁺ which is consistent with producing 0.7±0.3 of the D⁰→K^-K^-K⁺π⁺ signal. In the context of simple models, we use our measurements to estimate the importance of decay amplitudes that produce extra quark-antiquark pairs from the vacuum relative to those that do not.

We have studied the diffractive dissociation into dijets of 500 GeV/c pions scattering coherently from carbon and platinum targets. Extrapolating to asymptotically high energies (where t_min→0), we find that when the per-nucleus cross section for this process is parametrized as σ  =  σ₀A^α, α has values near 1.6, the exact result depending on jet transverse momentum. These values are in agreement with those predicted by theoretical calculations of color-transparency.

We present the first direct measurements of the pion valence-quark momentum distribution which is related to the square of the pion light-cone wave function. The measurements were carried out using data on diffractive dissociation of 500 GeV/c π^- into dijets from a platinum target at Fermilab experiment E791. The results show that the |qq̅ 〉 light-cone asymptotic wave function describes the data well for Q²∼10 (GeV/c)² or more. We also measured the transverse momentum distribution of the diffractive dijets.

We report results of a search for flavor-changing neutral current (FCNC), lepton flavor, and lepton-number violating decays of the D⁰ (and its antiparticle) into three and four bodies. Using data from Fermilab charm hadroproduction experiment E791, we examine modes with two leptons (muons or electrons) and a ρ⁰, K̅ ^*0, or φ vector meson or a nonresonant ππ, Kπ, or KK pair of pseudoscalar mesons. No evidence for any of these decays is found. Therefore, we present branching-fraction upper limits at 90% confidence level for the 27 decay modes examined (18 new).

From a sample of 1172±61 D⁺→π^-π⁺π⁺ decays, we find Γ(D⁺→π^-π⁺π⁺)/Γ(D⁺→K^-π⁺π⁺)  =  0.0311±0.0018_-0.0026^+0.0016. Using a coherent amplitude analysis to fit the Dalitz plot of these decays, we find strong evidence that a scalar resonance of mass 478_-23⁺²⁴±17 MeV/c² and width 324_-40⁺⁴²±21 MeV/c² accounts for approximately half of all decays.

From a sample of 848±44 D_s⁺→π^-π⁺π⁺ decays, we find Γ(D_s⁺→π^-π⁺π⁺)/Γ(D_s⁺→φπ⁺)  =  0.245±0.028_-0.012^+0.019. Using a Dalitz plot analysis of this three body decay, we find significant contributions from the channels ρ⁰(770)π⁺, ρ⁰(1450)π⁺, f₀(980)π⁺, f₂(1270)π⁺, and f₀(1370)π⁺. We also present the values obtained for masses and widths of the resonances f₀(980) and f₀(1370).

Using the large hadroproduced charm sample collected in experiment E791 at Fermilab, we report the first directly measured constraint on the decay-width difference ΔΓ for the mass eigenstates of the D⁰- D̅ ⁰ system. We obtain our result from lifetime measurements of the decays D⁰→K^-π⁺ and D⁰→K^-K⁺, under the assumption of CP invariance, which implies that the CP eigenstates and the mass eigenstates are the same. The lifetime of D⁰→K^-K⁺ (the CP-even final state) is τ_KK  =  0.410±0.011±0.006 ps, and the lifetime of D⁰→K^-π⁺ (an equal mixture of CP-odd and CP-even final states) is τ_Kπ  =  0.413±0.003±0.004 ps. The decay-width difference is ΔΓ  =  2(Γ_KK-Γ_Kπ)  =  0.04±0.14±0.05 ps^-1. We relate these measurements to measurements of mixing in the neutral D-meson system.

We report results of the first search for the pentaquark P_{c̅ s} which is predicted to be a doublet of states: P_{c̅ s}⁰  =  |c̅ suud〉 and P_{c̅ s}^-  =  |c̅ sddu〉. A search was made for the decay P_{c̅ s}⁰→φπp in data from Fermilab experiment E791, in which a 500 GeV/c π^- beam interacted with nuclear targets. We present upper limits at 90% confidence level for the ratio of cross section times branching fraction of this decay to that for the decay D_s^±→φπ^±. The upper limits vary between 0.022 and 0.046 for M(P_{c̅ s}⁰) between 2.750 and 2.907 GeV/c², assuming a P_{c̅ s}⁰ lifetime of 0.4 ps.

We present a measurement of the form-factor ratios r_V  =  V(0)/A₁(0) and r₂  =  A₂(0)/A₁(0) for the decay D⁺→K̅ ^*0e⁺ν_e. The measurement is based on a signal of approximately 3000 D⁺→K̅ ^*0e⁺ν_e, K̅ ^*0→K^-π⁺ decays reconstructed in data from charm hadroproduction experiment E791 at Fermilab. The results are r_V  =  1.84±0.11±0.08 and r₂  =  0.71±0.08±0.09.

We present results of a search for D⁰-D⁰ mixing and doubly-Cabibbo-suppressed decays of the D⁰ in Fermilab experiment E791, a fixed-target charm hadroproduction experiment. We look for evidence of mixing in the decay chain D^*→πD→π(Kπ or Kπππ). If the charge of the pion from the D^* decay is the same as the charge of the kaon from the D decay (a “wrong-sign” event), mixing may have occurred. Mixing can be distinguished from other sources of wrong-sign events (such as doubly-Cabibbo-suppressed decays) by analyzing the distribution of decay times. We see no evidence of mixing. Allowing for CP violation in the interference between DCS and mixing amplitudes our fitted ratio for mixed to unmixed decay rates is r_mix=(0.39_-0.32^+0.36±0.16)%. This corresponds to a 90% C.L. upper limit of r_mix<0.85%. The sensitivity of this result is comparable to that of previous measurements, but the assumptions made in fitting the data are notably more general. We present results from many fits to our data under various assumptions. If we assume r_mix=0, we find a two-sigma wrong-sign enhancement in the Kπ mode which we ascribe to doubly Cabibbo-suppressed decays. The ratios of doubly Cabibbo-suppressed decays to Cabibbo-favored decays are r_dcs(Kπ)=(0.68_-0.33^+0.34±0.07)% and r_dcs(Kπππ)=(0.25_-0.34^+0.36±0.03)%.

We present a measurement of the atomic mass dependence of central Ξ^- and Ξ̅ ⁺ production. It is measured using a sample of 22 459 Ξ^-’s and Ξ̅ ⁺’s produced in collisions between a 250 GeV π^- beam and targets of beryllium, aluminum, copper, and tungsten. The relative cross sections are fit to the two parameter function σ₀A^α, where A is the atomic mass. We measure α=0.924±0.020±0.025, for Feynman x in the range -0.09<x_F<0.15.

We measure the differential cross sections with respect to Feynman x ( x_F) and transverse momentum ( p_T) for π, K, and p-induced charm meson production using fully reconstructed D⁺, D⁰, and D_s decays. The shapes of these cross sections are compared to the theoretical predictions for charm quark production of next-to-leading order perturbative QCD using modern parametrizations of the pion and nucleon parton distributions. We observe the differences expected in production induced by projectiles with different gluon distributions, harder distributions being indicated for mesons than for protons.

We measure forward cross sections for production of D⁺, D⁰, D_s, D^*+, and Λ_c in collisions of π^±, K^±, and p on a nuclear target. Production induced by different beam particles is found to be the same within statistics. Strange and baryonic final states are seen to contribute appreciably to the total charm cross section, which our measurements indicate is larger than but consistent with QCD predictions. The energy dependence mapped out by these and previous measurements is consistent with theory. Leading-particle asymmetry measurements for K and p-induced charm production are also presented.

We report the result of a search for D⁰D̅ ⁰ mixing in the data from hadroproduction experiment E791 at Fermilab. We use the charge of the pion from the strong decay D^*+→D⁰π⁺ (and charge conjugate) to identify the charm quantum number of the neutral D at production, and the charge of the lepton and the kaon in the semileptonic decays D⁰→Keν and Kμν to identify the charm at the time of decay. No evidence of mixing is seen. We set a 90% confidence level upper limit on mixing of r<0.50%, where r  =  γ(D⁰→D̅ ⁰→K⁺l^-ν̅ _l)/γ(D⁰→K^-l⁺ν_l).

We report the results of a search for the flavor-changing neutral-current decays D⁺→π⁺μ⁺μ^- and D⁺→π⁺e⁺e^- in data from Fermilab charm hadroproduction experiment E791. No signal above background is found, and we obtain upper limits on branching fractions, B(D⁺→π⁺μ⁺μ^-)<1.8×10^-5 and B(D⁺→π⁺e⁺e^-)<6.6×10^-5, at the 90% confidence level.

We report results from Fermilab experiment E769 on the differential cross sections of D^*± charm vector mesons with respect to Feynman-x (x_F) and transverse momentum (P_T), and on the atomic mass dependence of the production. The D^* mesons were produced by a 250 GeV π beam on a target of Be, Al, Cu, and W foils. The dσ / dx_F distribution is fit by the form ((1-x_F)ⁿ) with n=3.5±0.3±0.1, the dσ / dP_T² distribution by exp(-b×P_T²) with b=0.70±0.07±0.04 GeV^-2, and the cross section A dependence by A^α with α=1.00±0.07±0.02. These results are compared to the equivalent parameters for the production of pseudoscalar D⁰ and D^± charm mesons.

A leading charm meson is one with longitudinal momentum fraction, x_F>0, whose light quark (or antiquark) is of the same type as one of the quarks in the beam particles. We report on the production asymmetry, A=[σ(leading-σ(nonleading)]/[σ(leading)+σ(nonleading)] as a function of x_F. The data consist of 1500 fully reconstructed D^± and D^*± decays in Fermilab experiment E 769. We find a significant asymmetry for the production of charm quarks is not expected in perturbative quantum chromodynamics.

We measure the relative cross sections for D mesons produced in interactions of π^- and π⁺ beams with targets of Be, Cu, Al, and W. The measurement is based on 1400 fully reconstructed decays of the types D⁰→K^-π⁺, D⁺→K^-π⁺π⁺, and charge conjugates. We find that the cross section for the production of both neutral and charged D’s by either π^- or π⁺ is well fitted by the form A^α where A is the atomic mass and α=1.00±0.05±0.02, where the errors are statistical and systematic, respectively. There is no significant dependence of α on the transverse or longitudinal momentum of the D meson or on the charge of either the incident pion or the produced D mesons.

We measure the differential cross section with respect to Feynman-x (x_F) and transverse momentum (P_T) for charm meson production using targets of Be, Al, Cu, and W. In the range 0.1<x_F<0.7, dσ/dx_F is well fit by the form (1-x_F)ⁿ with n=3.9±0.3. The difference between n values for D^- and D⁺ is 1.1±0.7. However, we find an asymmetry of 0.18±0.06 favoring the production of D^- compared to D⁺. In the lower P_T range, <2 GeV, dσ/dP_T² is well fit by the form exp(-b×P_T²) with b=1.03±0.06 GeV^-2, while in the higher P_T range, 0.8 to 3.6 GeV, it is well fit by the form exp(-b’×P_T) with b’=2.76±0.08 GeV^-1. The shape of the differential cross section has no significant dependence on atomic mass of the target material.