Central Rapidity Region Research Articles

LHC phenomenology of lowest massive Regge recurrences in the Randall-Sundrum orbifold

We consider string realizations of the Randall-Sundrum effective theory for electroweak symmetry breaking and explore the search for the lowest massive Regge excitation of the gluon and of the extra (color singlet) gauge boson inherent in D-brane constructions. In these curved backgrounds, the higher-spin Regge recurrences of standard model fields localized near the IR brane are warped down to close to the TeV range and hence can be produced at collider experiments. Assuming that the theory is weakly coupled, we make use of four gauge boson amplitudes evaluated near the first Regge pole to determine the discovery potential of LHC. We study the inclusive dijet mass spectrum in the central rapidity region $|{y}_{\mathrm{jet}}|<1.0$ for dijet masses $M\ensuremath{\ge}2.5\text{ }\text{ }\mathrm{TeV}$. We find that with an integrated luminosity of $100\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, the $5\ensuremath{\sigma}$ discovery reach can be as high as 4.7 TeV. Observations of resonant structures in $pp\ensuremath{\rightarrow}\mathrm{\text{direct}}\ensuremath{\gamma}+\mathrm{\text{jet}}$ can provide interesting corroboration for string physics up to 3.0 TeV. We also study the ratio of dijet mass spectra at small and large scattering angles. We show that with the first ${\mathrm{fb}}^{\ensuremath{-}1}$ such a ratio can probe lowest-lying Regge states for masses $\ensuremath{\sim}2.5\text{ }\text{ }\mathrm{TeV}$.

Single transverse spin asymmetry of dilepton production nearZ0pole

Using the latest quark Sivers functions extracted from the global analysis of available data on single transverse spin asymmetry (SSA), we calculate the SSA of Drell-Yan inclusive production of lepton pairs of invariant mass $Q$ at both $4<Q<9$ GeV and $Q\sim M_Z$ in $p^\uparrow p$ collisions at RHIC energies. We find that the features of the asymmetry $A_N$ for $Q\sim M_Z$ are significantly different from that when $4<Q<9$ GeV. The $A_N$ near $Z^0$ pole is positive and sizable in the central rapidity region while the $A_N$ at low $Q$ is negative and only sizable in the forward rapidity region. We show that the size of $A_N$ is sufficiently large for a wide range of $Q$ around the $Z^0$ pole, and even with the lepton pair's invariant mass integrated from Q=70 GeV to 110 GeV, the $A_N$ is still close to 10 percent for the central and near forward rapidity region. We argue that the SSAs of Drell-Yan inclusive dilepton production at low $Q$ and that near the $Z^0$ pole provide complementary information for testing the time-reversal modified universality (the sign change) of the Sivers functions.

Diphoton production at the Tevatron in the quasi-multi-Regge-kinematic approach

We study the production of prompt diphotons in the central region of rapidity within the framework of the quasi-multi-Regge-kinematics approach applying the hypothesis of quark and gluon Reggeization. We describe accurately and without free parameters the experimental data which were obtained by the CDF Collaboration at the Tevatron Collider. It is shown that the main contribution to studied process is given by the direct fusion of two Reggeized gluons into a photon pair, which is described by the effective Reggeon-Reggeon to particle-particle vertex. The contribution from the annihilation of Reggeized quark-antiquark pair into a diphoton is also considered. At the stage of numerical calculations we use the Kimber-Martin-Ryskin prescription for unintegrated quark and gluon distribution functions, with the Martin-Roberts-Stirling-Thorne collinear parton densities for a proton as input.

The feature study on the π and proton rapidity distributions at AGS, SPS and RHIC

The features of nuclear stopping power and multi-hadron production systematically are studied by making an analysis of rapidity distributions of pion and proton at AGS, SPS and RHIC in this work. It is found that nuclear stopping power increases linearly with project rapidity yp at AGS and SPS, but that is not liner at RHIC. It is argued that the average rapidity loss is saturated at central rapidity region at RHIC. For pion distribution, it is found that the phase space of pion distribution distributes uniformly in the longitudinal direction, and a linear relationship of 〈βγ〉L with log√s is given at AGS and SPS. Non-uniform flow model may explain the features of the distribution at AGS and SPS, but may not explain those of at RHIC.

Photons from anisotropic quark-gluon plasma

We calculate medium photons due to Compton and annihilation processes in an anisotropic media. The effects of time-dependent momentum-space anisotropy of {\em Quark-Gluon-Plasma} (QGP) on the medium photon production are discussed. Such an anisotropy can results from the initial rapid longitudinal expansion of the matter, created in relativistic heavy ion collisions. A phenomenological model for the time-dependence of the parton hard momentum scale, $p_{hard}(\tau)$, and anisotropy parameter, $\xi(\tau)$, has been used to describe the plasma space-time evolution. We find significant dependency of photon yield on the isotropization time ($\tau_{iso}$). It is shown that the introduction of early time momentum-space anisotropy can enhance the photon production by a factor of $10 (1.5)$ (in the central rapidity region) for {\em free streaming} ({\em collisionally-broadened}) {\em interpolating} model if we assume fixed initial condition. On the other hand, enforcing the fixed final multiplicity significantly reduces the enhancement of medium photon production.

P t -Multiplicity correlations in a multi-pomeron-exchange model with string collective effects

The 〈pt〉Nch - Nch correlations experimentally observed in the central rapidity region in pp and p\( \bar p \) collisions, in a wide energy range from the ISR to Tevatron, are described in the framework of a multi-Pomeron exchange model in which string collectivity has been included in an effective way. Three parameters are obtained from the fit to data: the string tension, the average number of particles per string, and a parameter which effectively introduces string collective effects. The model successfully reproduces the rise of 〈pt〉 of charged particles, the flattening with growing rapidity density of charged particles and with the collision energy, and the negative pt-Nch correlation at low energies. The string tension and the average number of particles per string are energy independent, while the parameter that includes effectively string collective effects shows a smooth increasing behavior with energy.

Chemical freeze-out in ultrarelativistic heavy ion collisions atsNN=130and 200 GeV

A comprehensive and detailed analysis of hadronic abundances measured in $\mathrm{Au}\text{\ensuremath{-}}\mathrm{Au}$ collisions at RHIC at $\sqrt{{s}_{\mathit{NN}}}=130$ and 200 GeV is presented. The rapidity densities measured in the central rapidity region have been fitted to the statistical hadronization model and the chemical freeze-out parameters determined as a function of centrality, using data from experiments BRAHMS, PHENIX, and STAR. The chemical freeze-out temperature turns out to be independent of centrality to accuracy of a few percentages, whereas the strangeness undersaturation parameter ${\ensuremath{\gamma}}_{S}$ decreases from almost unity in central collisions to a significantly lower value in peripheral collisions. Our results are in essential agreement with previous analyses, with the exception that fit quality at $\sqrt{{s}_{\mathit{NN}}}=200$ GeV is not as good as previously found. From the comparison of the two different energies, we conclude that the difference in fit quality, as described by ${\ensuremath{\chi}}^{2}$ values, is owing to the improved resolution of measurements that has probably exceeded the intrinsic accuracy of the simplified theoretical formula used in the fits.

Heavy flavour physics in ALICE

With a centre-of-mass energy of 5.5 TeV per nucleon, Pb–Pb collisions at the LHC are expected to provide a copious yield of heavy quarks. In this paper, an overview of the ALICE perspectives for heavy flavour physics will be presented. The ALICE experiment will detect quarkonia both in the e+e− and in the μ+μ− decay channels. Open heavy flavoured hadrons will be studied in the semileptonic decay channels at both central and forward rapidities. In addition, exclusive reconstruction of selected hadronic decay modes of charmed hadrons will be possible in the central rapidity region (|η| < 0.9).

Measurement of the differential cross section for the production of an isolated photon with associated jet in [formula omitted] collisions at [formula omitted

The process pp¯→γ+jet+X is studied using 1.0 fb−1 of data collected by the D0 detector at the Fermilab Tevatron pp¯ collider at a center-of-mass energy s=1.96 TeV. Photons are reconstructed in the central rapidity region |yγ|<1.0 with transverse momenta in the range 30<pTγ<400 GeV while jets are reconstructed in either the central |yjet|<0.8 or forward 1.5<|yjet|<2.5 rapidity intervals with pTjet>15 GeV. The differential cross section d3σ/dpTγdyγdyjet is measured as a function of pTγ in four regions, differing by the relative orientations of the photon and the jet in rapidity. Ratios between the differential cross sections in each region are also presented. Next-to-leading order QCD predictions using different parameterizations of parton distribution functions and theoretical scale choices are compared to the data. The predictions do not simultaneously describe the measured normalization and pTγ dependence of the cross section in the four measured regions.

Coherent [formula omitted] production—A novel feature at LHC?

Energy dependence of heavy quarkonia production in hadron–nucleus collisions is studied in the framework of the Glauber–Gribov theory. We emphasize a change in the space–time picture of heavy-quark state production on nuclei with energy. Longitudinally ordered scattering of a heavy-quark system takes place at low energies, while with increasing energy it transforms to a coherent scattering of projectile partons on the nuclear target. The characteristic energy scale for this transition depends on masses and rapidities of produced particles. For J/ψ, produced in the central rapidity region, the transition happens at RHIC energies. The parameter-free calculation of J/ψ in dAu collisions is in good agreement with recent RHIC data. We use distributions of gluons in nuclei to predict suppression of heavy quarkonia at LHC.

Measurement of D0, D+, Ds + and D*+ production in fixed target 920 GeV proton–nucleus collisions

The inclusive production cross sections of the charmed mesons D0,D+,Ds + and D*+ have been measured in interactions of 920 GeV protons on C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of transverse momentum and Feynman’s x variable are given for the central rapidity region and for transverse momenta up to pT=3.5 GeV/c. The atomic mass number dependence and the leading to non-leading particle production asymmetries are presented as well.

Intermittency in 3.7 A GeV 16 O-emulsion interactions

The intermittency effect has been studied for an interaction of 3.7 A GeV 16O with emulsion using the distributions of both the pseudorapidity intervals and the azimuthal angle intervals of the shower particles emitted in a central rapidity region. The scaled factorial moments, reduced scaled factorial moments and multifractal moments as functions of the bin size in pseudorapidity and in azimuthal angle have been calculated and have revealed the presence of an intermittent behaviour which may be due to the random cascading property of the reaction. The anomalous fractal dimension has been found to increase with the increase of rank of the moment.

Measurement of (anti)deuteron and (anti)proton production in DIS at HERA

The first observation of (anti)deuterons in deep inelastic scattering at HERA has been made with the ZEUS detector at a centre-of-mass energy of 300–318 GeV using an integrated luminosity of 120 pb −1. The measurement was performed in the central rapidity region for transverse momentum per unit of mass in the range 0.3 < p T / M < 0.7 . The particle rates have been extracted and interpreted in terms of the coalescence model. The (anti)deuteron production yield is smaller than the (anti)proton yield by approximately three orders of magnitude, consistent with the world measurements.

Wounded quarks and diquarks in heavy ion collisions

A model in which the soft collisions of the nucleon are described in terms of interactions of its two constituents (a quark and a diquark) is proposed. When adjusted to describe precisely the elastic proton–proton scattering data and supplemented with the idea of wounded constituents, the model accounts rather well for the centrality dependence of particle production in the central rapidity region at RHIC energies.

Diffractive χ production at the Tevatron and the LHC

We present predictions for the diffractive production of χ mesons in the central rapidity region usually covered by collider detectors. The predicted cross sections are based on the Bialas–Landshoff formalism for both exclusive and inclusive productions and make use of the DPEMC Monte Carlo simulation adapted with kinematics appropriate for small-mass diffractive production. We compare generator-level results with a CDF measurement for exclusive χ production, and study background and other scenarios including the contribution of inclusive χ production. The results agree with the Tevatron data and are extrapolated, highlighting the exclusive χ c 0 production at LHC energies. A possible new measurement at the Tevatron using the DØ forward detectors is investigated, taking advantage of the dominance of exclusive production for high enough diffractive mass fraction.

K*0 and φ meson production in proton–nucleus interactions at $\sqrt{s}=41.6\text{GeV}$

The inclusive production cross sections of the strange vector mesons K*0, K*0bar, and phi have been measured in interactions of 920 GeV protons with C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of rapidity and transverse momentum have been measured in the central rapidity region and for transverse momenta up to pT=3.5 GeV/c. The atomic number dependence is parametrised as sigma(pA) = sigma(pN)*A**alpha, where sigma(pN) is the proton-nucleon cross section. Within the phase space accessible, alpha(K*0) = 0.86+/-0.03, alpha(K*0bar) = 0.87+/-0.03, and alpha(phi) = 0.96+/-0.02. The total proton-nucleon cross sections, determined by extrapolating the differential measurements to full phase space, are sigma(pN->K*0) = 5.06+/-0.54 mb, sigma(pN->K*0bar) = 4.02+/-0.45 mb, and sigma(pN->phi) = 1.17+/-0.11 mb. The Cronin effect is observed for the first time for vector mesons containing strange quarks; compared to the measurements of Cronin et al. for K+- mesons, the measured values of alpha for phi mesons coincide with those of K- mesons for all transverse momenta, while the enhancement for K*0 / K*0bar mesons is smaller.

NLO inclusive jet production inkT-factorization

The inclusive production of jets in the central region of rapidity is studied in $k_T$-factorization at next-to-leading order (NLO) in QCD perturbation theory. Calculations are performed in the Regge limit making use of the NLO BFKL results. A jet cone definition is introduced and a proper phase--space separation into multi-Regge and quasi-multi-Regge kinematic regions is carried out. Two situations are discussed: scattering of highly virtual photons, which requires a symmetric energy scale to separate the impact factors from the gluon Green's function, and hadron-hadron collisions, where a non--symmetric scale choice is needed.

Recent results from the BRAHMS experiment

We present recent results obtained by the BRAHMS experiment at the Relativistic Heavy Ion Collider (RHIC) for the systems of Au+Au and Cu+Cu at s NN = 200 GeV and at 62.4 GeV, and p+p at s NN = 200 GeV . BRAHMS explores reaction dynamics and properties of the hot and high energy density matter produced in ultra-relativistic heavy-ion collisions versus its longitudinal expansion. Overall charged-particle production and particle spectra over a large rapidity interval and p T range are presented. Nuclear modification factors for Au+Au and Cu+Cu collisions are discussed. The observed number of charged particles produced per unit of rapidity in the central rapidity region indicates that a high energy density system is produced at the initial stage of the Au+Au reaction. Analysis of anti-particle to particle ratios as a function of rapidity and collision energy reveal that particle populations at the chemical freeze-out stage for heavy-ion reactions at and above SPS energies are controlled by the baryon chemical potential. From the particle spectra we deduce significant radial expansion ( β ≈ 0.75 ), as expected for systems created with a large initial energy density. We also measure the elliptic flow parameter v 2 versus rapidity and p T . A weak dependence on rapidity of the p T differential v 2 is observed. We present rapidity dependent p / π ratios within 0 y 3 for Au+Au and Cu+Cu at s NN = 200 GeV . The ratios are enhanced in nucleus-nucleus collisions as compared to p+p collisions. The particle ratios are discussed in terms of their system size and rapidity dependence. We compare R AA for Au+Au at s NN = 200 GeV and at 62.4 GeV, and for Au+Au and Cu+Cu at s NN = 200 GeV . R AA is found to increase with decreasing collision energy, decreasing system size, and when going towards more peripheral collisions. However, R AA shows only a very weak dependence on rapidity (for 0 y 3.2 ), both for pions and protons.

Geometric scaling violations in the central rapidity region of [formula omitted] collisions at RHIC

We show that geometric scaling is satisfied to good accuracy in the forward region of d + Au collisions at RHIC. Scaling violations do show up, however, at mid-rapidity, and the anomalous dimension of the small- x gluon distribution evolves to near its DGLAP limit for transverse momenta of a few GeV. This represents a first consistency check of RHIC deuteron–nucleus and HERA DIS phenomenology, and of the universality of the underlying color glass condensate (CGC) theory, which describes both phenomena. It also reconciles successful leading-twist LO and NLO perturbative QCD computations of mid-rapidity particle production with small- x evolution. Finally, we introduce a new parameterization for the anomalous dimension of the small- x gluon distribution which properly reproduces known theoretical limits at large rapidity, at large virtuality, and on the saturation boundary, and still fits the available data from d + Au collisions at RHIC. We find indications that sub-asymptotic terms in the rapidity-evolution of the anomalous dimension are large.

Single Electrons from Heavy-Flavor Decays inp+pCollisions ats=200 GeV

The invariant differential cross section for inclusive electron production in p+p collisions at [FORMULA: SEE TEXT] has been measured by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider over the transverse momentum range 0.4<or=pT<OR=5.0 GeV/c in the central rapidity region ([FORMULA: SEE TEXT]). The contribution to the inclusive electron spectrum from semileptonic decays of hadrons carrying heavy flavor, i.e., charm quarks or, at high , bottom quarks, is determined via three independent methods. The resulting electron spectrum from heavy-flavor decays is compared to recent leading and next-to-leading order perturbative QCD calculations. The total cross section of charm quark-antiquark pair production is determined to be [FORMULA: SEE TEXT].