Transverse Momentum Region Research Articles

TheaTdistribution of theZboson at hadron colliders

We provide the first theoretical study of a novel variable, $a_T$, proposed in Ref.[1] as a more accurate probe of the region of low transverse momentum $p_T$, for the $Z$ boson $p_T$ distribution at hadron colliders. The $a_T$ is the component of $p_T$ transverse to a suitably defined axis. Our study involves resummation of large logarithms in $a_T$ up to the next-to--leading logarithmic accuracy and we compare the results to those for the well-known $p_T$ distribution, identifying important physical differences between the two cases. We also test our resummed result at the two-loop level by comparing its expansion to order $\alpha_s^2$ with the corresponding fixed-order results and find agreement with our expectations.

Jet-Photon Production at RHIC and LHC

We calculate the production of prompt and thermal photons which includes the contribution of gluons in relativistic heavy ion collisions with the equilibrium and non-equilibrium quark-gluon plasma. We develop a new thermal jet-photon conversion mechanism which plays a vital role in the low transverse momentum region. The effect of the non-equilibrium quark-gluon plasma enhances the contribution of the thermal photons. The shadowing and iso-spin of the nucleus which can properly estimate the prompt photon production are also considered in our calculation.

Collins Function and the Single Transverse Spin Asymmetry

We study the Collins mechanism for the single transverse spin asymmetry in the collinear factorization approach. The corresponding twist-three fragmentation function is identified. We show that the Collins function calculated in this approach is universal. We further examine its contribution to the single transverse spin asymmetry of semi-inclusive hadron production in deep inelastic scattering and demonstrate that the transverse momentum dependent and twist-three collinear approaches are consistent in the intermediate transverse momentum region where both apply.

Drell–Yan lepton pair azimuthal asymmetry in hadronic processes

We study the azimuthal asymmetry (cos2ϕ) in the Drell–Yan lepton pair production in hadronic scattering processes at moderate transverse momentum region, taking into account the contributions from the twist-three quark–gluon correlations from the unpolarized hadrons. The contributions are found to dominate the asymmetry, and are not power suppressed by q⊥/Q at small q⊥ where q⊥ and Q are the transverse momentum and invariant mass of the lepton pair. Accordingly, the Lam–Tung relation will be violated at this momentum region, and its violation depends on the twist-three functions. However, at large transverse momentum q⊥∼Q, the Lam–Tung relation still holds because all corrections are power suppressed by Λ2/q⊥2∼Λ2/Q2 where Λ is the typical nonperturbative scale.

Experimental review of jet physics in hadronic collisions

In this contribution, a comprehensive review of the main aspects of high p T jet physics in Run II at the Tevatron is presented. Recent measurements on inclusive jet production are discussed using different jet algorithms and covering a wide region of jet transverse momentum and jet rapidity. Several measurements, sensitive to a proper description of soft gluon radiation and the underlying event in hadron collisions, are shown. Finally, high p T prompt photon measurements, and studies on the production of electroweak bosons in association with jets in the final state are discussed.

Next-to-leading-order QCD corrections to [formula omitted] production at the LHC

In this work, we calculate the next-to-leading-order (NLO) QCD corrections to the process p + p → J / ψ + γ via the color-singlet mechanism at the LHC. The results show that the partial cross section ( p t J / ψ > 3 GeV and | y J / ψ , γ | < 3 ) is enhanced by a factor of about 2.0, and the differential cross section can be enhanced by two orders of magnitude in the large transverse momentum region of J / ψ . Furthermore, the polarization of J / ψ changes from transverse polarization at leading-order to longitudinal polarization at NLO. For the inclusive J / ψ hadroproduction, it is known that the color-octet contributions are one order of magnitude larger than the color-singlet contribution, and the polarization distribution is dominated by the color-octet behavior at NLO. In contrast, for J / ψ + γ production the color-singlet contribution is of the same order as the color-octet contribution, and the polarization distribution arises from both the color-singlet and color-octet. Therefore, measurements of J / ψ production associated with a direct photon at the hadron collider could be an important supplement to testify the theoretical framework treating with the heavy quarkonium. In addition, an NLO QCD correction to ϒ + γ production at the LHC is also presented in this Letter.

Hyperon polarization in unpolarized scattering processes

Transverse polarization in the hyperon ($\ensuremath{\Lambda}$) production in the unpolarized deep inelastic scattering and $pp$ collisions is studied in the twist-three approach, considering the contribution from the quark-gluon-antiquark correlation distribution in nucleon. We further compare our results for deep inelastic scattering to a transverse momentum dependent factorization approach, and find consistency between the two approaches in the intermediate transverse momentum region. We also find that in $pp$ collisions, there are only derivative terms contributions, and the nonderivative terms vanish.

Dilepton spectra in p+p and Au+Au collisions at RHIC

This work presents measurements of electron-positron pairs from p+p collisions at sqrt(s) = 200 GeV collected during the 2005 RHIC run and compares them to results from Au+Au collisions at sqrt(s_NN) = 200 GeV taken in 2004 with the PHENIX detector. The invariant mass distribution of e+e- pairs in p+p is consistent with the expected contributions from Dalitz decays of light hadrons, dielectron decays of vector mesons and correlated charm production, which have been measured in the same experiment. The charm and bottom cross section extracted from the measured dielectron yield are sigma_cc = 544 +/- 39(stat.) +/- 142(syst.) +/- 200(model) microbarns and sigma_bb = 3.9 +/- 2.4(stat.) +3/-2(syst.) microbarns, respectively. In min. bias Au+Au collisions the yield of dielectrons in the low mass region is enhanced by a factor of 4.0 +/- 0.3(stat.) +/- 1.5(syst.) +/- 0.8(model) compared to the known hadronic sources. The excess dominates the yield in the transverse momentum region below 1 GeV/c and shows significantly lower average p_T than the expected sources. The low p_T enhancement is currently not understood by any theoretical model of heavy ion collisions. The enhancement extends to larger transverse momenta where it is also observed in p+p and explained by virtual direct photons. The p+p measurement serves as an important test to pQCD calculations of direct photon production from hard scattering processes in this momentum range. An excess with an inverse slope of T_eff = 221 +/- 23(stat.) +/- 18(syst.) MeV is observed in central Au+Au collisions above the binary scaled direct photon yield in p+p. This can be qualitatively explained by hydrodynamical models including thermal photon radiation with initial temperatures of 300 - 600 MeV and formation times of 0.12 - 0.6 fm/c.

Matches and mismatches in the descriptions of semi-inclusive processes at low and high transverse momentum

We investigate the transverse-momentum-dependence in semi-inclusive deep inelastic leptoproduction of hadrons. There are two different theoretical approaches to study this dependence, one for low and one for high transverse momentum of the observed hadron. We systematically investigate their connection, paying special attention to azimuthal distributions and to polarization dependence. In the region of intermediate transverse momentum, where both approaches are applicable, we find that their results match for certain observables but not for others. Interpolating expressions are discussed for the case where one has no matching. We then use power counting to determine which mechanism is dominant in various azimuthal and spin asymmetries that are integrated over the transverse momentum. Our findings have consequences for the extension of transverse-momentum-dependent factorization beyond leading twist. They also shed light on the problem of resumming logarithms of transverse momentum for azimuthal distributions. Our results can be carried over to the Drell-Yan process and to two-hadron production in e+e? annihilation.

Top production from black holes at the LHC

In theories with large extra dimension and with low quantum gravity scale near a TeV, it is expected that TeV-scale black holes to be produced in proton-proton collisions at the LHC with the center of mass energy of 14 TeV. Since the black holes temperature can be around 1 TeV, top quark production is expected from them via Hawking radiation. Within the Standard Model of particle physics top quarks are produced via strong interaction in $t\bar{t}$ pairs or via electroweak interaction singly. Therefore, black holes can be the new source of top quark production. In this article we present the total cross sections and transverse momentum distributions of top quark production from black holes at the LHC. We find that the top quarks from black holes tend to reside at very high transverse momentum region so it can be a very useful signature for the black holes at the LHC.

AZIMUTHAL ANISOTROPY MEASUREMENT OF DIRECT PHOTON IN $\sqrt{^SNN} = 200\ {\rm GeV}$Au + Au COLLISIONS AT RHIC-PHENIX

Azimuthal anisotropy of direct photons is measured in [Formula: see text] Au + Au collisions by the PHENIX experiment at RHIC. Direct photons are one of the most effective probes to study properties of hot dense medium at initial state (also QGP state) of heavy ion collisions because photons do not interact strongly with medium once produced. They keep their conditions when they are created. Within statistical and systematic errors, the elliptic flow parameter (v2) of direct photons is consistent with zero. Direct photons v2 is measured by the hadron decay photon background v2 from inclusive photon v2 in intermediate to high transverse momentum (pT) region (0 to 10 GeV/c) for 3 centrality selections (20% steps) and minimum bias.

MEASUREMENTS OF MULTI-STRANGE (ANTI-)BARYON PRODUCTION IN Cu+Cu COLLISIONS WITH THE STAR EXPERIMENT AT RHIC

Strange particle production has always been an important probe to study various aspects of the high density, strong interacting matter created in ultra-relativistic heavy-ion collisions. Yields and spectra of strange hadrons have been measured by the STAR experiment for center of mass energies of 62.4 GeV, 130 GeV and 200 GeV per nucleon pair for Au + Au , p + p and d + Au collisions. We have now measured Cu + Cu collisions at 62.4 GeV and 200 GeV, complementing the study of the system size dependence initiated with the centrality dependence study of Au + Au data. We will present preliminary results on Ξ, Ω and their anti-particles produced in Cu + Cu collisions. Yields and ratios are compared to results from Au + Au collisions normalized by the equivalent number of participants. The nuclear modification factor of the measured particles from Cu + Cu data also shows the same suppression effects in the high transverse momentum region and a baryon/meson difference in the intermediate momentum region.

Generalized z-scaling and pp collisions at RHIC

New generalization of the z-scaling in inclusive particle production is proposed. The scaling variable z is expressed in terms of the momentum fractions x1 and x2 of the incoming protons. Explicit dependence of z on the momentum fractions ya and yb of the scattered and recoil constituents carried by the inclusive particle and recoil object is included. The scaling function y (z) for charged and identified hadrons produced in proton-proton collisions is constructed. The scheme allows unique description of data on inclusive cross sections of charged hadrons, pions, kaons, antiprotons and lambdas produced at RHIC energies. The obtained results suggest that the z-scaling may be used as a tool for searching for new physics phenomena of particle production in high transverse momentum and high multiplicity region at proton-proton colliders RHIC and LHC.

Global properties of relativistic heavy-ion collisions proved by PHENIX

We report recent experimental results of elliptic flows ( v 2 ), event-by-event fluctuations and Bose-Einstein correlations (HBT) of identified hadrons measured by PHENIX detector at low transverse momentum ( p T < ∼ 2 GeV/c ) and mid-rapidity ( | η | < 0.35 ) region. We focus on results observed in Au+Au collision at S N N = 200 GeV , but results from Au+Au at 63 GeV and Cu+Cu at 63 and 200 GeV are also reported. These observables give us detailed insights into the properties of the space-time evolution such as the order of the phase transition and the collective dynamics of the hot-and-dense matter.

Counting valence quarks at RHIC and LHC

Abstract We consider the nuclear modification ratios in heavy ion collisions, R C P and R A A , in the region of intermediate transverse momentum, and study the dependency upon the constituent quark composition of the observed hadron. Adopting a two component recombination/fragmentation model, validated by experimental information from STAR and PHENIX, we show that a clear distinction is predicted for the f 0 ( 980 ) between the assumptions of s s ¯ or diquark–antidiquark content.

A positive-weight next-to-leading-order Monte Carlo forZpair hadroproduction

We present a first application of a previously published method for the computation of QCD processes that is accurate at next-to-leading order, and that can be interfaced consistently to standard shower Monte Carlo programs. We have considered Z pair production in hadron-hadron collisions, a process whose complexity is sufficient to test the general applicability of the method. We have interfaced our result to the HERWIG and PYTHIA shower Monte Carlo programs. Previous work on next-to-leading order corrections in a shower Monte Carlo (the MC@NLO program) may involve the generation of events with negative weights, that are avoided with the present method. We have compared our results with those obtained with MC@NLO, and found remarkable consistency. Our method can also be used as a standalone, alternative implementation of QCD corrections, with the advantage of positivity, improved convergence, and next-to-leading logarithmic accuracy in the region of small transverse momentum of the radiated parton.

PROBING POLARIZED GLUON DISTRIBUTIONS THROUGH CHARMED HADRON PRODUCTION IN POLARIZED pp COLLISIONS AT BNL-RHIC ENERGY

In order to extract information about behavior of polarized gluons in the nucleon, charmed hadron productions, i.e. D*+ meson and [Formula: see text] baryon productions, are studied in polarized pp reactions at BNL-RHIC energy. For these processes, the spin correlation asymmetry DLL between the target proton and the produced charmed hadron, and its statistical sensitivity δDLL are calculated. From analyses on these processes, we found that the pseudorapidity distribution of DLL in the limited transverse momentum region is quite effective for distinguishing the model of polarized gluons as well as the model of spin-dependent fragmentation functions.

Resonance production in heavy-ion collisions

Recent results of resonance production from RHIC at and from SPS at are presented and discussed in terms of the evolution and freeze-out conditions of a hot and dense fireball medium. Yields and spectra are compared with thermal model predictions at chemical freeze-out. Deviations in the low transverse momentum region of the resonance spectrum of the hadronic decay channel suggest a strongly interaction hadronic phase between chemical and kinetic freeze-out. Microscopic models including resonance rescattering and regeneration are able to describe the trend of the data. The magnitude of the regeneration cross sections for different inverse decay channels is discussed. Model calculations which include elastic hadronic interactions between chemical freeze-out and thermal freeze-out based on the K(892)/K and Λ(1520)/Λ ratios suggest a time between two freeze-outs surfaces of Δτ > 4 fm/c. The difference in momentum distributions and yields for the ϕ(1020) resonance reconstructed from the leptonic and hadronic decay channels at SPS energy are discussed taking into account the impact of a hadronic phase and possible medium modifications.

Color-octet contributions toP-waveBcmeson hadroproduction

The contributions from the color-octet components $|(c\overline{b}{)}_{\mathbf{8}}(^{1}S_{0})g⟩$ and $|(c\overline{b}{)}_{\mathbf{8}}(^{3}S_{1})g⟩$ to the ${h}_{{B}_{c}}$ or ${\ensuremath{\chi}}_{{B}_{c}}^{J}$ (the $P$-wave ${B}_{c}$ meson) hadroproduction are estimated in terms of the complete $\mathcal{O}({\ensuremath{\alpha}}_{s}^{4})$ calculation. As necessary inputs in the estimate, we take the values of the octet matrix elements according to the nonrelativistic quantum chromodynamics scaling rules, and as a result, we have found that the contributions to the $P$-wave production may be the same in order of magnitude as those from the color-singlet ones, $|(c\overline{b}{)}_{\mathbf{1}}(^{1}P_{1})⟩$ and $|(c\overline{b}{)}_{\mathbf{1}}(^{3}P_{J})⟩$ ($J=1,2,3$). Especially, our result indicates that the observation of the color-octet contributions to the $P$-wave production in the low transverse momentum region is not very far from the present experimental capability at Tevatron and LHC.

Testing models of hadron production with strange hadrons in the intermediate pT region using the STAR detector

We report on the transverse momentum dependence of strangeness production measured with the STAR detector. Identified particle spectra are used to provide a test of models of hadron production in the intermediate transverse momentum region, 2 < pT < 6 GeV/c. Further insight into hadron production is obtained from the nuclear modification factor, Rcp, in Au+Au collisions, which shows that there is a different pT dependence of hadron suppression in central collisions for baryons and mesons, consistent with quark coalescence. Evidence for a similar baryon–meson difference is also found in d+Au collisions.