ZEUS results on the measurement and phenomenology of F(2) at low x and low Q**2

Measurements of the proton structure function F-2 for 0.6 < Q(2) < 17 GeV2 and 1.2 x 10(-5) < x < 1.9 x 10(-3) from ZEUS 1995 shifted vertex data are presented. From ZEUS F-2 data the slopes dF(2)/dln Q(2) at fixed x and dlnF(2)/dln(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) less than or equal to 0.65 GeV2 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 x 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) similar to 1 GeV2, it is found that the q (q) over bar sea distribution is still rising at small x and the lowest Q(2) values whereas the gluon distribution is strongly suppressed

High E(T) inclusive jet cross-sections in photoproduction at HERA

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.Comment: 31 pages including 8 figure

Measurement of the diffractive cross section in deep inelastic scattering using ZEUS 1994 data

The DIS diffractive cross section, dσdiffγ*p→XN/dMx, has been measured in the mass range Mx &lt; 15 GeV for γ*p c.m. energies 60 &lt; W &lt; 200 GeV and photon virtualities Q2 = 7 to 140 GeV2. For fixed Q2 and Mx, the diffractive cross section rises rapidly with W, dσdiffγ*p→X N (Mx, W, Q2)/dMx ∝ Wadiff with adiff = 0.507 ± 0.034 (stat) +0.155-0.046 (syst) corresponding to a t-averaged pomeron trajectory of ̄αℙ = 1.127 ± 0.009 (stat) +0.039-0.012 (syst) which is larger than ̄αℙ observed in hadron-hadron scattering. The W dependence of the diffractive cross section is found to be the same as that of the total cross section for scattering of virtual photons on protons. The data are consistent with the assumption that the diffractive structure function FD(3)2 factorizes according to cursive greek chiℙFD(3)2(cursive greek chiℙ, β, Q2) = (cursive greek chi0//cursive greek chiℙ)nFD(2)2(β, Q2). They are also consistent with QCD based models which incorporate factorization breaking. The rise of cursive greek chiℙFD(3)2 with decreasing cursive greek chiℙ and the weak dependence of FD(2)2 on Q2 suggest a substantial contribution from partonic interactions

Measurement of the diffractive cross-section in deep inelastic scattering using ZEUS 1994 data

The DIS diffractive cross section, d sigma(gamma*p-->XN)(diff)/dM(X), has been measured in the mass range M(X) XN)(diff) (M(X), W, Q(2))/dM(X) proportional to W(adiff) with a(diff) = 0.507 +/- 0.034(stat)(-0.046)(+0.155) (syst) corresponding to a t-averaged pomeron trajectory of (P) = 1.127 +/- 0.009(stat)(-0.012)(+0.039) (syst) which is larger than (P) observed in hadron-hadron scattering The W dependence of the diffractive cross section is found to be the same as that of the total cross section for scattering of virtual photons on protons. The data are consistent with the assumption that the diffractive structure function F(2)(D(3)) factorizes according to x(P)F(2)(D(3))(x(p), beta, Q(2)) = (x(0)/x(P))(n)F(2)(D(2)) (beta, Q(2)). They are also consistent with QCD based models which incorporate factorization breaking. The rise of x(P)F(2)(D(3)) with decreasing x(P) and the weak dependence of F(2)(D(2)) On Q(2) suggest a substantial contribution from partonic interactions

Measurement of jet shapes in photoproduction at HERA

The shape of jets produced in quasi-real photon-proton collisions at centre-of-mass energies in the range 134-277 GeV has been measured using the hadronic energy flow. The measurement was done with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the η - φ plane with a cone radius of one unit. Measured jet shapes both in inclusive jet and dijet production with transverse energies EjetT &gt; 14 GeV are presented. The jet shape broadens as the jet pseudorapidity (ηjet) increases and narrows as EjetT increases. In dijet photoproduction, the jet shapes have been measured separately for samples dominated by resolved and by direct processes. Leading-logarithm parton-shower Monte Carlo calculations of resolved and direct processes describe well the measured jet shapes except for the inclusive production of jets with high ηjet and low EjetT. The observed broadening of the jet shape as ηjet increases is consistent with the predicted increase in the fraction of final state gluon jets

Measurement of jet shapes in high Q**2 deep inelastic scattering at HERA

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 cane 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 (p) over bar p 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

Study of photon dissociation in diffractive photoproduction at HERA

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 γp → XN, where X is the diffractively dissociated photon system of mass MX and N is either a proton or a nucleonic system with mass MN &lt; 2 GeV. The cross section for this process in the interval 3 &lt; MX &lt; 24 GeV relative to the total photoproduction cross section was measured to be σDpartial/σtot = 6.2 ± 0.2(stat) ± 1.4(syst)%. After extrapolating this result to the mass interval of mφ2 &lt; MX2 &lt; 0.05W2 and correcting it for proton dissociation, the fraction of the total cross section attributed to single diffractive photon dissociation, γp → Xp, is found to be σSD/σtot = 13.3 ± 0.5(stat) ± 3.6(syst)%. The mass spectrum of the dissociated photon system in the interval 8 &lt; MX &lt; 24 GeV can be described by the triple pomeron (IP IP IP) diagram with an effective pomeron intercept of αIP(0) = 1.12 ± 0.04(stat) ± 0.08(syst). The cross section for photon dissociation in the range 3 &lt; MX &lt; 8 GeV is significantly higher than that expected from the triple pomeron amplitude describing the region 8 &lt; MX &lt; 24 GeV. Assuming that this discrepancy is due to a pomeron-pomeron-reggeon (IP IP IR) term, its contribution to the diffractive cross section in the interval 3 &lt; MX &lt; 24 GeV is estimated to be fIP IP IR = 26 ± 3(stat) ± 12(syst)%. © Springer-Verlag 1997