A strong-focusing momentum channel has been arranged to form a beam from antiprotons produced by 6.0-Bev protons striking an internal target of the Bevatron. The channel consists of five 4-inch-diameter magnetic quadrupole lenses and two deflecting magnets adjusted to give a ±5% momentum interval. The antiprotons were selected from a large background of mesons by a scintillation counter telescope with a time-of-flight coincidence circuit having a resolution of ±2×10−9 second. This system allowed detection of approximately 400 antiprotons per hour. With a liquid hydrogen attenuator, the total antiproton-proton cross section at four different energies, 190, 300, 500, and 700 Mev, has been observed to be 135, 104, 97, and 94 mb, respectively. Also, the total cross sections for antiprotons incident on Be and C have been measured at two energies. The inelastic cross sections for carbon have been measured by observing the pulse heights produced by the interactions in a target of liquid scintillator. To measure the inelastic cross section for a high-Z element, lead wafers were immersed in the liquid scintillator, and to select inelastic events the pulse heights were measured.
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Exclusive electroproduction of rho^0 mesons has been measured using the ZEUS detector at HERA in two Q^2 ranges, 0.25<Q^2<0.85 GeV^2 and 3<Q^2<30 GeV^2. The low-Q^2 data span the range 20<W<90 GeV; the high-Q^2 data cover the 40<W<120 GeV interval. Both samples extend up to four-momentum transfers of |t|=0.6 GeV^2. The distribution in the azimuthal angle between the positron scattering plane and the rho^0 production plane shows a small but significant violation of s-channel helicity conservation, corresponding to the production of longitudinally polarised (i.e. helicity zero) rho^0 mesons from transverse photons. Measurements of the 15 combinations of spin-density matrix elements which completely define the angular distributions are presented and discussed.
The spin-density matrix elements obtained from the BPC low Q**2 data set.
The spin-density matrix elements obtained from the DIS high Q**2 data set.
The spin-density matrix elements obtained from the low Q**2 BPC data set in two W intervals.
We report here additional positive results of a search for muonless neutrino- and anti-neutrino-induced events using an enriched antineutrino beam and a muon identifier of relatively high geometric detection efficiency. The ratio of muonless to muon event rates is observed to be R=0.20±0.05. We observe no background derived from ordinary neutrino or antineutrino interactions that is capable of explaining the muonless signal.
No description provided.
Diffractive dissociation of quasi-real photons at a photon-proton centre of mass energy of W 200 GeV is studied with the ZEUS detector at HERA. The process under consideration is gamma p -> X N, where X is the diffractively dissociated photon system of mass M_X and N is either a proton or a nucleonic system with mass M_N < 2GeV. The cross section for this process in the interval 3 < M_X < 24 GeV relative to the total photoproduction cross section was measured to be sigma~partial_D / sigma_tot = 6.2 +- 0.2(stat) +- 1.4(syst)%. After extrapolating this result to the mass interval of m_phi~2 < M_X~2 < 0.05 W~2 and correcting it for proton dissociation, the fraction of the total cross section attributed to single diffractive photon dissociation, gamma p -> X p, is found to be sigma_SD / sigma_tot = 13.3 +- 0.5(stat) +- 3.6(syst)%. The mass spectrum of the dissociated photon system in the interval 8 < M_X < 24 GeV can be described by the triple pomeron (PPP) diagram with an effective pomeron intercept of alpha_P(0) = 1.12 +- 0.04(stat) +- 0.08(syst). The cross section for photon dissociation in the range 3 < M_X < 8 GeV is significantly higher than that expected from the triple pomeron amplitude describing the region 8 < M_X < 24 GeV. Assuming that this discrepancy is due to a pomeron-pomeron-reggeon (PPR) term, its contribution to the diffractive cross section in the interval 3 < M_X < 24 GeV is estimated to be f_PPR = 26 +- 3(stat) +- 12(syst)%.
Fraction of the total photoproduction cross section attributed to the photon dissociation.
The fraction of the total photoproduction cross section due to single dif fractive photon dissociation, in the mass range M_phi**2 < M_DD < X >**2 < 0.05 *W**2.
Identification of the diffractive processes was performed on the basis of the shape of reconstructed hadronic mass spectrum. No rapidity-gap was required.
We present measurements of the structure function \Ft\ in $e~+p$ scattering at HERA in the range $3.5\;\Gevsq < \qsd < 5000\;\Gevsq$. A new reconstruction method has allowed a significant improvement in the resolution of the kinematic variables and an extension of the kinematic region covered by the experiment. At $ \qsd < 35 \;\Gevsq$ the range in $x$ now spans $6.3\cdot 10~{-5} < x < 0.08$ providing overlap with measurements from fixed target experiments. At values of $Q~2$ above 1000 GeV$~2$ the $x$ range extends to 0.5. Systematic errors below 5\perc\ have been achieved for most of the kinematic region. The structure function rises as \x\ decreases; the rise becomes more pronounced as \qsd\ increases. The behaviour of the structure function data is well described by next-to-leading order perturbative QCD as implemented in the DGLAP evolution equations.
No description provided.
No description provided.
No description provided.
Inclusive jet differential cross sections for the reaction e+ p --> e+ + jet + X with quasi-real photons have been measured with the ZEUS detector at HERA. These cross sections are given for the photon-proton centre-of-mass energy interval 134 < W < 277 GeV and jet pseudorapidity in the range -1 < eta(jet) < 2 in the laboratory frame. The results are presented for three cone radii in the eta-phi plane, R=1.0, 0.7 and 0.5. Measurements of dsigma/deta(jet) above various jet-transverse-energy thresholds up to 25 GeV and in three ranges of W are presented and compared to next-to-leading order (NLO) QCD calculations. For jets defined with R=1.0 differences between data and NLO calculations are seen at high eta(jet) and low E_T(jet). The measured cross sections for jets defined with R=0.7 are well described by the calculations in the entire measured range of eta(jet) and E_T(jet). The inclusive jet cross section for E_T(jet) > 21 GeV is consistent with an approximately linear variation with the cone radius R in the range between 0.5 and 1.0, and with NLO calculations.
Jet defining cone radius R = 1.0.
Jet defining cone radius R = 1.0.
Jet defining cone radius R = 1.0.
Elastic and proton-dissociative rho0 photoproduction (gamma p-->rho0 p,gamma p -->rho0 N,with rho0-->pi+pi-) has been studied in ep interactions at HERA for gamma-p centre-of-mass energies in the range 50<W<100 GeV and for |t|<0.5 GeV2; the results on the p-dissociative reaction are presented for masses of the dissociated proton system in the range MN^2<0.1W^2.For the elastic process,the pi+pi- invariant mass spectrum has been investigated as a function of t. As in fixed target experiments, the rho0 resonance shape is asymmetric;this asymmetry decreases with increasing |t|.The cross section has been studied as a function of W; a fit to the resonant part with the form W^a gives a=0.16\pm0.06(stat.) +0.11-0.15(syst.). The resonant part of the gamma p-->pi+pi- p cross section is 11.2\pm 0.1(stat.)+1.1-1.2 (syst.) mub at <W>=71.7 GeV. The t dependence of the cross section can be described by a function of the type Ae^(-b|t|+ct^2) with b=10.9\pm0.3(stat.)+1.0-0.5(syst.)GeV-2 and c=2.7\pm0.9(stat.)+1.9-1.7(syst.) GeV-4. The t dependence has also been studied as a function of W and a value of the slope of the pomeron trajectory 0.23\pm0.15(stat.)+0.10-0.07(syst.)GeV-2 has been deduced. The rho spin density matrix elements r^04_00,r^04_1-1 and Re[r^04_10] have been measured and found to be consistent with expectations based on SCHC. For p-dissociative pi+pi- photoproduction in the rho0 mass range, the distributions of the two-pion invariant mass, W and the polar and azimuthal angles of the pions in the helicity frame are the same within errors as those for the elastic process. The t distribution has been fitted to an exponential function with a slope parameter 5.8\pm0.3(stat.)\pm0.5(syst.)GeV-2. The ratio of the elastic to p-dissociative rho0 photoproduction cross section is 2.0\pm0.2(stat.)\pm0.7(syst.).
Integrated elastic rho0 photoproduction cross section.
Integrated elastic pi+ pi- photoproduction cross section.
Differential T distribution. Statistical errors only.
The shapes of jets with transverse energies, E_T(jet), up to 45 GeV produced in neutral- and charged-current deep inelastic e+p scattering (DIS) at Q**2 > 100 GeV**2 have been measured with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the eta-phi plane with a cone radius of one unit. The jets become narrower as E_T(jet) increases. The jet shapes in neutral- and charged-current DIS are found to be very similar. The jets in neutral-current DIS are narrower than those in resolved processes in photoproduction and closer to those in direct-photon processes for the same ranges in E_T(jet) and jet pseudorapidity. The jet shapes in DIS are observed to be similar to those in e+e- interactions and narrower than those in pbarp collisions for comparable E_T(jet). Since the jets in e+e- interactions and e+p DIS are predominantly quark initiated in both cases, the similarity in the jet shapes indicates that the pattern of QCD radiation within a quark jet is to a large extent independent of the hard scattering process in these reactions.
Measured differential jet shapes, corrected to the hadron level, in neutral-current DIS for jets with ET greater than 14 GeV in different etarap regions.
Measured differential jet shapes, corrected to the hadron level, in neutral-current DIS for jets with ET greater than 14 GeV in different etarap regions.
Measured differential jet shapes, corrected to the hadron level, in neutral-current DIS for jets with ET greater than 14 GeV in different etarap regions.
Photon diffractive dissociation, $\gamma p \to Xp$, has been studied at HERA with the ZEUS detector using $ep$ interactions where the virtuality $Q^2$ of the exchanged photon is smaller than 0.02 GeV$^2$. The squared four-momentum $t$ exchanged at the proton vertex was determined in the range $0.073<|t|<0.40$ GeV$^2$ by measuring the scattered proton in the ZEUS Leading Proton Spectrometer. In the photon-proton centre-of-mass energy interval $176<W<225$ GeV and for masses of the dissociated photon system $4<M_X<32$ GeV, the $t$ distribution has an exponential shape, $dN/d|t| \propto \exp{(-b|t|)}$, with a slope parameter $b=6.8 \pm 0.9$~(stat.)~$ ^{+1.2}_{-1.1}$~(syst.)~GeV$^{-2}$.
T is the squared four momentum transfer at the proton vertex.
SLOPE of the DN/DT distribution.
Measurements of the proton structure function $F_2$ for $0.6 < Q^2 < 17 {GeV}^2$ and $1.2 \times 10^{-5} < x <1.9 \times 10^{-3}$ from ZEUS 1995 shifted vertex data are presented. From ZEUS $F_2$ data the slopes $dF_2/d\ln Q^2$ at fixed $x$ and $d\ln F_2/d\ln(1/x)$ for $x < 0.01$ at fixed $Q^2$ are derived. For the latter E665 data are also used. The transition region in $Q^2$ is explored using the simplest non-perturbative models and NLO QCD. The data at very low $Q^2$ $\leq 0.65 {GeV}^2$ are described successfully by a combination of generalised vector meson dominance and Regge theory. From a NLO QCD fit to ZEUS data the gluon density in the proton is extracted in the range $3\times 10^{-5} < x < 0.7$. Data from NMC and BCDMS constrain the fit at large $x$. Assuming the NLO QCD description to be valid down to $Q^2\sim 1 {GeV}^2$, it is found that the $q\bar{q}$ sea distribution is still rising at small $x$ and the lowest $Q^2$ values whereas the gluon distribution is strongly suppressed.
F2.
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Forum