The polarized longitudinal-transverse structure function $\sigma_{LT^\prime}$ measures the interference between real and imaginary amplitudes in pion electroproduction and can be used to probe the coupling between resonant and non-resonant processes. We report new measurements of $\sigma_{LT^\prime}$ in the $N(1440){1/2}^+$ (Roper) resonance region at $Q^2=0.40$ and 0.65 GeV$^2$ for both the $\pi^0 p$ and $\pi^+ n$ channels. The experiment was performed at Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS) using longitudinally polarized electrons at a beam energy of 1.515 GeV. Complete angular distributions were obtained and are compared to recent phenomenological models. The $\sigma_{LT^\prime}(\pi^+ n)$ channel shows a large sensitivity to the Roper resonance multipoles $M_{1-}$ and $S_{1-}$ and provides new constraints on models of resonance formation.
Polarized structure function of the reaction E- P --> E- PI0 P for Q**2 = 0.40 and W = 1.1 GeV.
Polarized structure function of the reaction E- P --> E- PI0 P for Q**2 = 0.40 and W = 1.14 GeV.
Polarized structure function of the reaction E- P --> E- PI0 P for Q**2 = 0.40 and W = 1.18 GeV.
Exclusive electroproduction of $\phi$ mesons has been studied in $e^\pm p$ collisions at $\sqrt{s}=318 \gev$ with the ZEUS detector at HERA using an integrated luminosity of 65.1 pb$^{-1}$. The $\gamma^*p$ cross section is presented in the kinematic range $2<Q^2<70 \gev^2$, $35<W<145 \gev$ and ${|t|}<0.6 \gev^2$. The cross sections as functions of $Q^2$, $W$, $t$ and helicity angle $\theta_h$ are compared to cross sections for other vector mesons. The ratios $R$ of the cross sections for longitudinally and transversely polarized virtual photons are presented as functions of $Q^2$ and $W$. The data are also compared to predictions from theoretical models.
Exclusive GAMMA* P --> PHI P cross section in the Q**2 range 2 to 3 GeV**2.
Exclusive GAMMA* P --> PHI P cross section in the Q**2 range 3 to 5 GeV**2.
Exclusive GAMMA* P --> PHI P cross section in the Q**2 range 5 to 9 GeV**2.
The exclusive omega electroproduction off the proton was studied in a large kinematical domain above the nucleon resonance region and for the highest possible photon virtuality (Q2) with the 5.75 GeV beam at CEBAF and the CLAS spectrometer. Cross sections were measured up to large values of the four-momentum transfer (-t < 2.7 GeV2) to the proton. The contributions of the interference terms sigma_TT and sigma_TL to the cross sections, as well as an analysis of the omega spin density matrix, indicate that helicity is not conserved in this process. The t-channel pi0 exchange, or more generally the exchange of the associated Regge trajectory, seems to dominate the reaction gamma* p -> omega p, even for Q2 as large as 5 GeV2. Contributions of handbag diagrams, related to Generalized Parton Distributions in the nucleon, are therefore difficult to extract for this process. Remarkably, the high-t behaviour of the cross sections is nearly Q2-independent, which may be interpreted as a coupling of the photon to a point-like object in this kinematical limit.
Total cross sections and interference terms (TT and TL).
Differential cross sections DSIG/DT for Q**2 = 1.725 GeV**2 and W = 2.77 GeV.
Differential cross sections DSIG/DT for Q**2 = 1.752 GeV**2 and W = 2.48 GeV.
The first measurement of direct photons in Au+Au collisions at sqrt(s_NN) = 200 GeV is presented. The direct photon signal is extracted as a function of the Au+Au collision centrality and compared to NLO pQCD calculations. The direct photon yield is shown to scale with the number of nucleon-nucleon collisions for all centralities.
Double ratio of measured $(\gamma/\pi^0)_{Measured}$ invariant yield ratio to the background decay $(\gamma/\pi^0)_{Background}$ ratio as a function of $p_T$ for minimum bias and for five centralities of Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The bin range is not an uncertainty in the x-axis because the actual uncertainty by having the finite bin width is corrected for by the bin-shift correction. These bins were constructed using the corrected finite values as centers.
Double ratio of measured $(\gamma/\pi^0)_{Measured}$ invariant yield ratio to the background decay $(\gamma/\pi^0)_{Background}$ ratio as a function of $p_T$ for minimum bias and for five centralities of Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The bin range is not an uncertainty in the x-axis because the actual uncertainty by having the finite bin width is corrected for by the bin-shift correction. These bins were constructed using the corrected finite values as centers.
Direct $\gamma$ invariant yields as a function of transverse momentum for 9 centrality selections and minimum bias Au+AU collisions at $\sqrt{s_{NN}}$ = 200 GeV. Data with no errors represents 90% confidence level upper limit. The bin range is not an uncertainty in the x-axis because the actual uncertainty by having the finite bin width is corrected for by the bin-shift correction. These bins were constructed using the corrected finite values as centers.
The transverse momentum dependence of the azimuthal anisotropy parameter v_2, the second harmonic of the azimuthal distribution, for electrons at mid-rapidity (|eta| < 0.35) has been measured with the PHENIX detector in Au+Au collisions at sqrt(s_NN) = 200 GeV. The measurement was made with respect to the reaction plane defined at high rapidities (|eta| = 3.1 -- 3.9). From the result we have measured the v_2 of electrons from heavy flavor decay after subtraction of the v_2 of electrons from other sources such as photon conversions and Dalitz decay from light neutral mesons. We observe a non-zero single electron v_2 with a 90% confidence level in the intermediate p_T region.
Transverse momentum dependence of inclusive electron $v_2$ and heavy quark electron $v_2$.
A measurement of direct photons in p+p collisions at sqrt(s)=200 GeV is presented. A photon excess above background from pi^0 --> gamma+gamma, eta --> gamma+gamma, and other decays is observed in the transverse momentum range 5.5 < p_T < 7 GeV/c. The result is compared to a next-to-leading-order perturbative QCD calculation. Within errors, good agreement is found between the QCD calculation and the measured result.
<p>Measured cross section and NLO pQCD calculations for direct-photon production in p+p collisions at $\sqrt{s}=$200 GeV. The normalization error of 9.6% is not shown. The two data points plotted with an arrow indicate the beginning of the low- and high-$p_\mathrm{T}$ ranges where the direct photon signal is consistent with zero. The upper edges of the arrows indicate an upper limit (90% confidence level) for the direct photon cross section calculated from the statistical and systematic uncertainty.</p>
Multijet production rates in neutral current deep inelastic scattering have been measured in the range of exchanged boson virtualities 10 < Q2 < 5000 GeV2. The data were taken at the ep collider HERA with centre-of-mass energy sqrt(s) = 318 GeV using the ZEUS detector and correspond to an integrated luminosity of 82.2 pb-1. Jets were identified in the Breit frame using the k_T cluster algorithm in the longitudinally invariant inclusive mode. Measurements of differential dijet and trijet cross sections are presented as functions of jet transverse energy E_{T,B}{jet}, pseudorapidity eta_{LAB}{jet} and Q2 with E_{T,B}{jet} > 5 GeV and -1 < eta_{LAB}{jet} < 2.5. Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant alpha_s(M_Z), determined from the ratio of the trijet to dijet cross sections, is alpha_s(M_Z) = 0.1179 pm 0.0013(stat.) {+0.0028}_{-0.0046}(exp.) {+0.0064}_{-0.0046}(th.)
Inclusive trijet cross section as a function of the jet transverse energy in the Breit frame for the jet with the highest transverse energy.
Inclusive trijet cross section as a function of the jet transverse energy in the Breit frame for the jet with the second highest transverse energy.
Inclusive trijet cross section as a function of the jet transverse energy in the Breit frame for the jet with the third highest transverse energy.
Deep inelastic scattering and its diffractive component, ep -> e'gamma*p ->e'XN, have been studied at HERA with the ZEUS detector using an integrated luminosity of 4.2 pb-1. The measurement covers a wide range in the gamma*p c.m. energy W (37 - 245 GeV), photon virtuality Q2 (2.2 - 80 GeV2) and mass Mx. The diffractive cross section for Mx > 2 GeV rises strongly with W: the rise is steeper with increasing Q2. The latter observation excludes the description of diffractive deep inelastic scattering in terms of the exchange of a single Pomeron. The ratio of diffractive to total cross section is constant as a function of W, in contradiction to the expectation of Regge phenomenology combined with a naive extension of the optical theorem to gamma*p scattering. Above Mx of 8 GeV, the ratio is flat with Q2, indicating a leading-twist behaviour of the diffractive cross section. The data are also presented in terms of the diffractive structure function, F2D(3)(beta,xpom,Q2), of the proton. For fixed beta, the Q2 dependence of xpom F2D(3) changes with xpom in violation of Regge factorisation. For fixed xpom, xpom F2D(3) rises as beta -> 0, the rise accelerating with increasing Q2. These positive scaling violations suggest substantial contributions of perturbative effects in the diffractive DIS cross section.
Measurement of the proton structure function F2 at Q**2 = 2.7 GeV**2.
Measurement of the proton structure function F2 at Q**2 = 4.0 GeV**2.
Measurement of the proton structure function F2 at Q**2 = 6.0 GeV**2.
Differential inclusive jet cross sections in neutral current deep inelastic ep scattering have been measured with the ZEUS detector. Three phase-space regions have been selected in order to study parton dynamics where the effects of BFKL evolution might be present. The measurements have been compared to the predictions of leading-logarithm parton shower Monte Carlo models and fixed-order perturbative QCD calculations. In the forward region, QCD calculations at order alpha_s^1 underestimate the data up to an order of magnitude at low x. An improved description of the data in this region is obtained by including QCD corrections at order alpha_s^2, which account for the lowest-order t-channel gluon-exchange diagrams, highlighting the importance of such terms in parton dynamics at low x.
Inclusive jet cross section DSIG/DETARAP for jets of hadrons in the global phase space.
Inclusive jet cross section DSIG/DET for jets of hadrons in the global phase space.
Inclusive jet cross section DSIG/DQ**2 for jets of hadrons in the global phase space.
New measurements are presented for charged hadron azimuthal correlations at mid-rapidity in Au+Au collisions at sqrt(s_NN) = 62.4 and 200 GeV. They are compared to earlier measurements obtained at sqrt(s_NN) = 130 GeV and in Pb+Pb collisions at sqrt(s_NN) = 17.2 GeV. Sizeable anisotropies are observed with centrality and transverse momentum (p_T) dependence characteristic of elliptic flow (v_2). For a broad range of centralities, the observed magnitudes and trends of the differential anisotropy, v_2(p_T), change very little over the collision energy range sqrt(s_NN) = 62-200 GeV, indicating saturation of the excitation function for v_2 at these energies. Such a saturation may be indicative of the dominance of a very soft equation of state for sqrt(s_NN) = 62-200 GeV.
Assorted-$p_T$ correlation functions (0.65 < $p_{T,ref}$ < 2.5 GeV/$c$) for charged hadrons of 0.5 < $p_T$ < 0.7 GeV/$c$ and 1.0 < $p_T$ < 1.5 obtained in Au+Au collisions at $\sqrt{S_{NN}}$ = 62.4 GeV.
Differential anisotropy $v_2$($p_T$) for charged hadrons in Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV obtained via cumulants method
Differential anisotropy $v_2$($p_T$) for charged hadrons in Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV obtained via correlation function method
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