We present measurements of the differential cross section and Lambda recoil polarization for the gamma p to K+ Lambda reaction made using the CLAS detector at Jefferson Lab. These measurements cover the center-of-mass energy range from 1.62 to 2.84 GeV and a wide range of center-of-mass K+ production angles. Independent analyses were performed using the K+ p pi- and K+ p (missing pi -) final-state topologies/ results from these analyses were found to exhibit good agreement. These differential cross section measurements show excellent agreement with previous CLAS and LEPS results and offer increased precision and a 300 MeV increase in energy coverage. The recoil polarization data agree well with previous results and offer a large increase in precision and a 500 MeV extension in energy range. The increased center-of-mass energy range that these data represent will allow for independent study of non-resonant K+ Lambda photoproduction mechanisms at all production angles.
Differential cross section as a function of COS(THETA(K)) for the centre-of-mass range 1.62-1.63 GeV.
Differential cross section as a function of COS(THETA(K)) for the centre-of-mass range 1.63-1.64 GeV.
Differential cross section as a function of COS(THETA(K)) for the centre-of-mass range 1.64-1.65 GeV.
This Letter describes the current most precise measurement of the $W$ boson pair production cross section and most sensitive test of anomalous $WW\gamma$ and $WWZ$ couplings in $p \bar p$ collisions at a center-of-mass energy of 1.96 TeV. The $WW$ candidates are reconstructed from decays containing two charged leptons and two neutrinos, where the charged leptons are either electrons or muons. Using data collected by the CDF II detector from 3.6 fb$^{-1}$ of integrated luminosity, a total of 654 candidate events are observed with an expected background contribution of $320 \pm 47$ events. The measured total cross section is $\sigma (p \bar p \to W^+ W^- + X) = 12.1 \pm 0.9 \textrm{(stat)} ^{+1.6}_{-1.4} \textrm{(syst)}$ pb, which is in good agreement with the standard model prediction. The same data sample is used to place constraints on anomalous $WW\gamma$ and $WWZ$ couplings.
Measured cross section for inclusive W+ W- production.
The cross section for photon production in association with at least one jet containing a $b$-quark hadron has been measured in proton antiproton collisions at $\sqrt{s}=1.96$ TeV. The analysis uses a data sample corresponding to an integrated luminosity of 340 pb$^{-1}$ collected with the CDF II detector. Both the differential cross section as a function of photon transverse energy $E_T^{\gamma}$, $d \sigma$($p \overline{p} \to \gamma + \geq 1 b$-jet)/$d E_T^{\gamma}$ and the total cross section $\sigma$($p \overline{p} \to \gamma + \geq 1 b$-jet/ $E_T^{\gamma}> 20$ GeV) are measured. Comparisons to a next-to-leading order prediction of the process are presented.
b + photon cross section as a function of photon ET.
b + photon total cross section for photon ET > 20 GeV.
We report a measurement of high-p_T inclusive pi^0, eta, and direct photon production in p+p and d+Au collisions at sqrt(s_NN) = 200 GeV at midrapidity (0 < eta < 1). Photons from the decay pi^0 -> gamma gamma were detected in the Barrel Electromagnetic Calorimeter of the STAR experiment at the Relativistic Heavy Ion Collider. The eta -> gamma gamma decay was also observed and constituted the first eta measurement by STAR. The first direct photon cross section measurement by STAR is also presented, the signal was extracted statistically by subtracting the pi^0, eta, and omega(782) decay background from the inclusive photon distribution observed in the calorimeter. The analysis is described in detail, and the results are found to be in good agreement with earlier measurements and with next-to-leading order perturbative QCD calculations.
Cross sections for inclusive $\pi^0$ production in p + p and d + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The solid lines correspond to NLO pQCD calculations. The measured $\pi^0$ cross sections were not corrected for feed-down contributions $\eta$ -> 3$\pi^0$, $\eta$ -> $\pi^+\pi^-\pi^0$, and $K_S^0$ -> $\pi^0\pi^0$, which were expected to be negligible. Normalization uncertainties of 11.7% for p+p and 5.3% for d+Au are not shown.
The $\eta/\pi^0$ ratio measured in p + p collisions at $\sqrt{s_{NN}}$ = 200 GeV, compared to the PHENIX measurements [27] and to the $m_T$ scaling predictions. Shaded bands in plot are $p_T$-correlated systematic uncertainties and the error bars are statistical uncertainties.
The $\eta/\pi^0$ ratio measured in d + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, compared to the PHENIX measurements [27] and to the $m_T$ scaling predictions. Shaded bands in plot are $p_T$-correlated systematic uncertainties and the error bars are statistical uncertainties.
We present a measurement of pi^+pi^-pi^+pi^- photonuclear production in ultra-peripheral Au-Au collisions at sqrt(s_{NN}) = 200 GeV from the STAR experiment. The pi^+pi^-pi^+pi^- final states are observed at low transverse momentum and are accompanied by mutual nuclear excitation of the beam particles. The strong enhancement of the production cross section at low transverse momentum is consistent with coherent photoproduction. The pi^+pi^-pi^+pi^- invariant mass spectrum of the coherent events exhibits a broad peak around 1540 pm 40 MeV/c^2 with a width of 570 pm 60 MeV/c^2, in agreement with the photoproduction data for the rho^0(1700). We do not observe a corresponding peak in the pi^+pi^- final state and measure an upper limit for the ratio of the branching fractions of the rho^0(1700) to pi^+pi^- and pi^+pi^-pi^+pi^- of 2.5 % at 90 % confidence level. The ratio of rho^0(1700) and rho^0(770) coherent production cross sections is measured to be 13.4 pm 0.8 (stat.) pm 4.4 (syst.) %.
Distribution of the $\pi^{+}\pi^{-}\pi^{+}\pi^{-}$ transverse momentum $p_{T} = |\sum_{i=1}^{4}\overrightarrow{p}_{T,i}|$: The filled circles are the measured points with the statistical errors. The hatched filled histogram shows the expected distribution from simulation of coherent photoproduction (cf. section III). The strong enhancement at low transverse momenta is due to coherently produced $\pi^{+}\pi^{-}\pi^{+}\pi^{-}$. This unique signature is used in the event selection which requires $p_{T}$ < 150 MeV/c (arrow). The remaining background is estimated from +2 or −2 charged four-prong combinations, by normalizing (factor = 1.186 $\pm$ 0.054) their $p_{T}$ distribution (gray filled histogram) to that of the neutral four-prongs in the region of $p_{T}$ > 250 MeV/c (vertical line) yielding the unfilled histogram (see section IV).
Invariant Mass distribution of two-pion subsystems: The filled circles show the measured $\pi^{+}\pi^{-}$ invariant mass spectrum for the selected four-prong sample (four entries per event) with statistical errors. The open circles represent the mass spectrum of the like-sign pion pairs (two entries per event). The unlike-sign mass distribution exhibits an enhancement with respect to the like-sign pairs in the $\rho^{0}$(770) region. The solid line histograms show the prediction from simulation assuming the relative S-wave decay $\rho`\rightarrow\rho^{0}$(770) $f_{0}$(600).
Invariant Mass distribution of two-pion subsystems: The open circles show the measured invariant mass spectrum of the lightest $\pi^{+}\pi^{-}$ pair in the event with the bars indicating the statistical errors. The filled circles represent the invariant mass distribution of the $\pi^{+}\pi^{-}$ that is recoiling against the lightest pair. The spectrum exhibits a clear peak in the $\rho^{0}$(770) region. The solid line histograms show the prediction from simulation assuming the relative S-wave decay $\rho`\rightarrow\rho^{0}$(770) $f_{0}$(600).
We report the first three-particle coincidence measurement in pseudorapidity ($\Delta\eta$) between a high transverse momentum ($p_{\perp}$) trigger particle and two lower $p_{\perp}$ associated particles within azimuth $\mid$$\Delta\phi$$\mid$$<$0.7 in $\sqrt{{\it s}_{NN}}$ = 200 GeV $d$+Au and Au+Au collisions. Charge ordering properties are exploited to separate the jet-like component and the ridge (long-range $\Delta\eta$ correlation). The results indicate that the particles from the ridge are uncorrelated in $\Delta\eta$ not only with the trigger particle but also between themselves event-by-event. In addition, the production of the ridge appears to be uncorrelated to the presence of the narrow jet-like component.
Correlated hadron distribution in ∆φ(|η|<1 with a high-p⊥trigger particle in 0-12% Au+Au collisions for 3<p(t)⊥<10 GeV/cand 1<p(a)⊥<3GeV/c. The ZYA1-normalized flow background is shown by the curve.
Correlated hadron distribution ∆η(|∆φ|<0.7) with a high-p⊥ trigger particle in 0-12% Au+Au collisions for 3<p(t)⊥<10 GeV/c and 1<p(a)⊥<3GeV/c. The ∆η distributions are background subtracted and corrected for ∆η acceptance and are for like and unlike-sign pairs separately. The curves in are Gaussian fits. Errors are statistical.
Background-subtracted charge-independent (AAT ) correlated hadron pair density in minimum bias d+Au collisions for 3<p(t)⊥<10 GeV/cand 1<p(a)⊥<3 GeV/c. The results are for near-side correlated hadrons within |∆φ1,2|<0.7, and corrected for the 3-particle ∆η-∆η acceptance. Statistical errors at (∆η1,∆η2)∼(0,0)are approximately 0.033 for d+Au respectively.
Charged-particle spectra associated with direct photon ($\gamma_{dir} $) and $\pi^0$ are measured in $p$+$p$ and Au+Au collisions at center-of-mass energy $\sqrt{s_{_{NN}}}=200$ GeV with the STAR detector at RHIC. A hower-shape analysis is used to partially discriminate between $\gamma_{dir}$ and $\pi^0$. Assuming no associated charged particles in the $\gamma_{dir}$ direction (near side) and small contribution from fragmentation photons ($\gamma_{frag}$), the associated charged-particle yields opposite to $\gamma_{dir}$ (away side) are extracted. At mid-rapidity ($|\eta|<0.9$) in central Au+Au collisions, charged-particle yields associated with $\gamma_{dir}$ and $\pi^0$ at high transverse momentum ($8< p_{T}^{trig}<16$ GeV/$c$) are suppressed by a factor of 3-5 compared with $p$ + $p$ collisions. The observed suppression of the associated charged particles, in the kinematic range $|\eta|<1$ and $3< p_{T}^{assoc} < 16$ GeV/$c$, is similar for $\gamma_{dir}$ and $\pi^0$, and independent of the $\gamma_{dir}$ energy within uncertainties. These measurements indicate that the parton energy loss, in the covered kinematic range, is insensitive to the parton path length.
The $z_{T}$ dependence of $\pi^{0}-h^{\pm}$ near side and away-side associated particle yields. The errors denoted 'syst' are systematic errors correlated in $z_{T}$. The errors denoted 'syst uncorr' are point-to-point systematic errors.
The $z_{T}$ dependence of away-side associated-particle yields for $\pi^{0}-h^{\pm}$ triggers and $\gamma_{dir}$ triggers. The errors denoted 'syst' are systematic errors correlated in $z_{T}$. The errors denoted 'syst uncorr' are point-to-point systematic errors.
The $z_{T}$ dependence $I_{AA}$ for $\pi^{0}-h^{\pm}$ triggers and $\gamma_{dir}$ triggers. The errors denoted 'syst' are systematic errors correlated in $z_{T}$. The errors denoted 'syst uncorr' are point-to-point systematic errors.
We report the measurement of the transverse momentum dependence of inclusive J/psi polarization in p+p collisions at sqrt(s)=200 GeV performed by the PHENIX Experiment at RHIC. The polarization is studied in the helicity, Gottfried-Jackson, and Collins-Soper frames for p_T < 5 GeV/c and |y| < 0.35. The J/psi polarization in the helicity and Gottfried-Jackson frames is consistent with zero for all transverse momenta, with a slight (1.8 sigma) trend towards longitudinal polarization for transverse momenta above 2 GeV/c. No conclusion is allowed due to the limited acceptance in the Collins-Soper frame and the uncertainties of the current data. The results are compared to observations for other collision systems and center of mass energies and to different quarkonia production models.
$J/\psi$ yield times dielectron branching ratio ($B$) after detector acceptance and efficiency corrections for the real data with $A$ = 28.7 $\pm$ 1.0 nb/GeV/$c$, $b$ = 3.41 $\pm$ 0.21 GeV/$c$, and $n$ = 4.6 $\pm$ 0.4.
$J/\psi$ polarization parameter ($\lambda_{J/\psi}$) versus transverse momentum ($p_T$).
$J/\psi$ polarization parameter ($\lambda_{J/\psi}$) versus transverse momentum ($p_T$).
PHENIX has measured the e^+e^- pair continuum in sqrt(s_NN)=200 GeV Au+Au and p+p collisions over a wide range of mass and transverse momenta. The e^+e^- yield is compared to the expectations from hadronic sources, based on PHENIX measurements. In the intermediate mass region, between the masses of the phi and the J/psi meson, the yield is consistent with expectations from correlated c^bar-c production, though other mechanisms are not ruled out. In the low mass region (below the phi) the p+p inclusive mass spectrum is well described by known contributions from light meson decays. In contrast, the Au+Au minimum bias inclusive mass spectrum in this region shows an enhancement by a factor of 4.7+/-0.4(stat)+/-1.5(syst)+/-0.9(model) At low mass (m_ee<0.3 GeV/c^2) and high p_T (1<p_T<5 GeV/c) an enhanced e^+e^- pair yield is observed that is consistent with production of virtual direct photons. This excess is used to infer the yield of real direct photons. In central Au+Au collisions, the excess of the direct photon yield over the p+p is exponential in p_T, with inverse slope T=221+/-19(stat)+/-19(syst) MeV. Hydrodynamical models with initial temperatures ranging from T_init ~=300--600 MeV at times of 0.6--0.15 fm/c after the collision are in qualitative agreement with the direct photon data in Au+Au. For low p_T<1 GeV/c the low mass region shows a further significant enhancement that increases with centrality and has an inverse slope of T ~=100 MeV. Theoretical models under predict the low mass, low p_T enhancement.
(Color online) Inclusive mass spectrum of $e^+e^-$ pairs in the PHENIX acceptance in $p$+$p$ collisions compared to the expectations from the decays of light hadrons and correlated decays of charm, bottom, and Drell-Yan. The contribution from hadron decays is independently normalized based on meson measurements in PHENIX. The bottom panel shows the ratio of data to the cocktail of known sources. The systematic uncertainties of the data are shown as boxes, while the uncertainty on the cocktail is shown as band around 1.
(Color online) Inclusive mass spectrum of $e^+e^-$ pairs in the PHENIX acceptance in minimum-bias Au+Au compared to expectations from the decays of light hadrons and correlated decays of charm, bottom, and Drell-Yan. The charm contribution expected if the dynamic correlation of $c$ and $\bar{c}$ is removed is shown separately. Statistical (bars) and systematic (boxes) uncertainties are shown separately. The contribution from hadron decays is independently normalized based on meson measurements in PHENIX. The bottom panel shows the ratio of data to the cocktail of known sources. The systematic uncertainties of the data are shown as boxes, while the uncertainty on the cocktail is shown as band around 1.
(Color online) Inclusive mass spectrum of $e^+e^-$ pairs in the PHENIX acceptance in minimum-bias Au+Au compared to expectations from the decays of light hadrons and correlated decays of charm, bottom, and Drell-Yan. The charm contribution expected if the dynamic correlation of $c$ and $\bar{c}$ is removed is shown separately. Statistical (bars) and systematic (boxes) uncertainties are shown separately. The contribution from hadron decays is independently normalized based on meson measurements in PHENIX. The bottom panel shows the ratio of data to the cocktail of known sources. The systematic uncertainties of the data are shown as boxes, while the uncertainty on the cocktail is shown as band around 1.
We present a measurement of the ttbar cross section using high-multiplicity jet events produced in ppbar collisions at sqrt{s}=1.96 TeV. These data were recorded at the Fermilab Tevatron collider with the D0 detector. Events with at least six jets, two of them identified as b jets, were selected from a 1 fb-1 data set. The measured cross section, assuming a top quark mass of 175 GeV/c^2, is 6.9 \pm 2.0 pb, in agreement with theoretical expectations.
Measured top topbar cross section at two values of the top mass.
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