Testing the scale dependence of the scale factor σ eff in double dijet production at the LHC

The scale factor σ _eff is the effective cross section used to characterize the measured rate of inclusive double dijet production in high-energy hadron collisions. It is sensitive to the two-parton distributions in the hadronic projectile. In principle, the scale factor depends on the center of mass energy and on the minimal transverse energy E _T,min? of the jets contributing to the double dijet cross section. Here, we point out that proton–proton collisions at the LHC will provide for the first time experimental access to these scale dependences in a logarithmically wide, nominally perturbative kinematic range 10?E _T,min??100 GeV. This constrains the dependence of two-parton distribution functions on parton momentum fractions and parton localization in impact parameter space. Novel information is to be expected about the transverse growth of hadronic distribution functions in the range of semi-hard Bjorken x (0.001?x?0.1) and high resolution Q ². We discuss to what extent one can disentangle different pictures of the x-evolution of two-parton distributions in the transverse plane by measuring double-hard scattering events at the LHC.     

B-meson measurement via secondary J/ψ production in Pb-Pb collisions at ?{sNN } \sqrt {s_{NN} } = 5.5 TeV in the CMS

The production of heavy quarks in heavy-ion collisions is important because it is expected to give information about high-density QCD matter. One interesting channel of heavy quark measurements at LHC is the decay of B to J/ψ. The J/ψ mesons from this channel can be separated from the primary J/ψ (produced at collision vertex) by using the secondary vertex which is efficiently reconstructed with the CMS tracker. In this work, the generator level results for the primary and secondary J/ψ obtained through dimuon pairs are presented for pp and PbPb collisions at $ \sqrt {s_{NN} } $ \sqrt {s_{NN} } = 5.5 TeV.     

Cold nuclear matter effects on dijet productions in relativistic heavy-ion reactions at LHC

We investigate the cold nuclear matter (CNM) effects on dijet productions in high-energy nuclear collisions at LHC with the next-to-leading order perturbative QCD. The nuclear modifications for dijet angular distributions, dijet invariant mass spectra, dijet transverse momentum spectra and dijet momentum imbalance due to CNM effects are calculated by incorporating EPS, EKS, HKN and DS parametrization sets of parton distributions in nucleus. It is found that dijet angular distributions and dijet momentum imbalance are insensitive to the initial-state CNM effects and thus provide optimal tools to study the final-state hot QGP effects such as jet quenching. On the other hand, the invariant mass spectra and the transverse momentum spectra of dijet are generally enhanced in a wide region of the invariant mass or transverse momentum due to CNM effects with a feature opposite to the expected suppression because of the final-state parton energy loss effect in the QGP. The difference of EPS, EKS, HKN and DS parametrization sets of nuclear parton distribution functions is appreciable for dijet invariant mass spectra and transverse momentum spectra at p+Pb collisions, and becomes more pronounced for those at Pb+Pb reactions.     

Charmonium-state production in heavy-ion collisions

An overview of existing experimental data on the production of charmonium states (J/ψ and ψ′) at the superproton synchrotron (SPS, CERN) and the relativistic heavy ion collider (RHIC, Brookhaven National Laboratory, United States) is given. The production of J/ψ mesons shows an anomalous suppression discovered by the NA50 Collaboration (CERN) in collisions of lead nuclei at an energy of 158 GeV per nucleon and confirmed in the NA60 experiment (CERN) in collisions of indium nuclei at the same energy. The suppression of J/ψ production depends on interaction centrality and becomes anomalous at about 122 participant nucleons in PbPb collisions and at about 86 participant nucleons in InIn collisions. The experimental data in question are compared with the predictions of existing theoretical models. None of the models is able to simultaneously describe data on PbPb and InIn collisions. Data obtained in the PHENIX experiment at RHIC from measurements of J/ψ production in AuAu and CuCu collisions at an energy of 200 GeV (in the nucleon-nucleon c.m. frame) indicate that the suppression of J/ψ production at such energies approximately corresponds to the suppression of J/ψ production in PbPb collisions at SPS. Theoretical models that describe SPS data on PbPb collisions predict a stronger suppression of J/ψ production. Models that take into account J/ψ-meson regeneration better describe experimental data obtained at RHIC. Measurement of cross sections for charmonium and bottomonium production at the Large Hadron Collider (LHC, CERN) would make it possible to study the properties of matter and to explore the mechanism of quarkonium production at ultrahigh energy densities and temperature and high transverse momenta, as well as to investigate the effect of the regeneration and suppression of quarkonium production as the energy increases.     

Fine structure in the azimuthal transverse momentum correlations at $\sqrt{s_{NN}}=200$ GeV using the event shape analysis

The experimental results on transverse momentum azimuthal hadron correlations at RHIC have opened a rich field for parton energy loss analysis in heavy-ion collisions. Recently, a considerable amount of work has been devoted to study the shapes of the “away-side” jet which exhibit an interesting and unexpected “double hump” structure not observed in the analogous treatment of the pp data. Driven by the possibility that the latter result might just mean that such a structure exists already in the case of pp collisions, but that its relative intensity could be small, here we use the Event Shape Analysis to show that it is possible to identify and select well defined event topologies in pp collisions, among which a double hump structure for the away-side jet emerges. Using two shape parameters, the sphericity in the transverse plane and the recoil to analyze a sample of PYTHIA generated pp collisions at  GeV, we show that this structure corresponds to two jets emitted in the backward hemisphere. Finally, we show that Q-PYTHIA qualitatively reproduces the decrease in the yield of dijet events and the increase of the double hump structure in the away side observed in heavy-ion collisions. The implications for the treatment of parton energy loss in heavy-ion collisions are discussed.     

Determination of the effect of final state energy loss on the acoplanarity and momentum imbalance in jets

The energy loss of partons leaving the hot media created in the heavy ion collisions has been at the center of the interest of the experimental and theoretical communities of the field. Hence an increase in the transverse momentum imbalance and acoplanarity of dijets may be a way to study the parton energy loss. In order to study the effect, we have made simulations of dijet events where the intrinsic k _T of the partons, the initial and final state radiation are taken into account. The results of these simulations for p + p and Pb + Pb collisions, as a function of the center of mass energy of collision will be presented.     

Physics with the ALICE experiment

ALICE experiment at LHC collects data in pp collisions at 1497-1 = 0.9, 2.76, and 7 TeV and in PbPb collisions at 2.76 TeV. Highlights of the detector performance and an overview of experimental results measured with ALICE in pp and AA collisions are presented in this paper. Physics with protonproton collisions is focused on hadron spectroscopy at low and moderate p _t . Measurements with lead-lead collisions are shown in comparison with those in pp collisions, and the properties of hot quark matter are discussed.     

Study of photon-tagged jet events in high-energy heavy ion collisions with CMS

The energy loss of fast partons traversing the strongly interacting matter produced in high energy nuclear collisions is one of the most interesting observables to probe the nature of the produced medium. The collisional and radiative energy loss of the partons will modify the fragmentation functions depending on the path length in the medium. Pb + Pb collisions at  GeV at the LHC will allow detailed measurements of the in-medium modifications of fragmentation functions of parton initiated jets, using the γ-jet channel. Since the photon does not strongly interact with the medium, the initial transverse energy of the fragmenting parton can be related to the photon transverse energy. This in turn allows for precision studies of the fragmentation function underlying these jets.     

Cold nuclear modifications at RHIC and LHC

We use recent nuclear parton distributions, among them the Hirai — Kumano — Nagai (HKN) and Eskola — Paukkunen — Salgado (EPS08) parameterizations, in our pQCD-improved parton model to calculate the nuclear modification factor, R _AA′ (p _T ), at RHIC and at the LHC. At RHIC, the deuteron-gold nuclear modification factor for pions, measured at p _T ≥ 10 GeV/c in central collisions, appears to deviate more from unity than the model results. The slopes of the calculated R _dAu (p _T ) are similar to the slopes of the PHENIX pion and photon data. At LHC, without final-state effects we see a small enhancement of R _dPb (p _T ) in the transverse momentum range 10 GeV/c ≥ p _T ≥ 100 GeV/c for most parameterizations. The inclusion of final-state energy loss will reduce the R _dPb (p _T ) values.     

Jet energy density in hadron-hadron collisions at high energies

The average particle multiplicity density dN _ch/dη is the dynamical quantity that reflects some regularities of particle production in the low-p _T range. The quantity is an important ingredient of z scaling. Experimental results on charged particle density are available for pp, pA, and AA collisions, while experimental properties of the jet density are still an open question. The goal of this work is to find the variable that will reflect the main features of the jet production in a low transverse energy range and play the role of the scale factor for the scaling function ψ(z) and variable z in data z-presentation. The appropriate candidate is the variable we called “scaled jet energy density.” The scaled jet energy density is the probability of having a jet with defined E _T in defined x _T and pseudorapidity regions. The PYTHIA6.2 Monte Carlo generator is used for calculation of the scaled jet energy density in proton-proton collisions over a high energy range and at η = 0. The properties of the new variable are discussed and sensitivity to “physical scenarios” applied in the standard Monte Carlo generator is noted. The results of the calculations of scaled jet energy density at LHC energies are presented and compared with predictions based on z scaling. The text was submitted by the authors in English.     

Bulk matter physics and its future at the Large Hadron Collider

Measurements at low transverse momentum will be performed at the LHC for studying particle production mechanisms in pp and heavy-ion collisions. Some of the experimental capabilities for bulk matter physics are presented, focusing on tracking elements and particle identification. In order to anticipate the study of baryon production for both colliding systems at multi-TeV energies, measurements for identified species and recent model extrapolations are discussed. Several mechanisms are expected to compete for hadro-production in the low momentum region. For this reason, experimental observables that could be used for investigating multi-parton interactions and help understanding the “underlying event” content in the first pp collisions at the LHC are also mentioned.     

Heavy-ion physics with the ATLAS detector

The CERN LHC collider will operate with lead ions at √s of 5.5 TeV/nucleon. The ATLAS detector, designed to study high-pT physics in the pp mode of the LHC, has the potential to study ultrarelativistic heavy-ion collisions in a full range of observables characterized extremely dense matter and the formation of a quark-gluon plasma. The ATLAS physics program includes global event measurements (particle multiplicities, transverse momentum), suppression of heavy-quarkonia production, jet quenching, and a study of ultraperipheral collisions. (on behalf of the ATLAS Collaboration) The text was submitted by the author in English.     

Angular pattern of minijet transverse energy flow in hadron and nuclear collisions

The azimuthal asymmetry of a minijet system produced at the early stage of nucleon–nucleon and nuclear collisions in a central rapidity window is studied. We show that in pp collisions the minijet transverse energy production in a central rapidity window is essentially unbalanced in the azimuth due to asymmetric contributions in which only one minijet hits the acceptance window. We further study the angular pattern of the transverse energy flow generated by the semihard degrees of freedom at the early stage of high energy nuclear collisions and its dependence on the number of semihard collisions in the models both including and neglecting soft contributions to the inelastic cross section at RHIC and LHC energies as well as on the choice of the infrared cutoff. Received: 19 January 1999 / Revised version: 18 February 2000 / Published online: 26 July 2000     

High-pT spectra of charged hadrons in Au + Au collisions at √sNN = 9.2 GeV in STAR

The production of hadrons in heavy-ion collisions at high-p _T provides an important information on mechanism of particle formation and constituent energy loss in medium. Such information is needed for search of a Critical Point and signatures of phase transition. Measurements by the STAR Collaboration of charged hadron production in Au + Au collisions at √s _NN = 9.2 GeV over a wide transverse momentum p _T = 0.2−4 GeV/c and at mid-rapidity range are reported. It allows for a first measurement of the spectra for charged hadrons at high p _T at this energy. The spectra demonstrate the dependence on centrality which enhances with p _T . The constituent energy loss and its dependence on transverse momentum of particle, and centrality of collisions are estimated in the z-scaling approach.     

Dark matter production at the LHC from black hole remnants

We study dark matter production at CERN LHC from black hole remnants (BHR). We find that the typical mass of these BHR at the LHC is ∼5–10 TeV which is heavier than other dark matter candidates, such as axion, axino, neutralino, etc. We propose the detection of this dark matter via single jet production in the process pp → jet + BHR (dark matter) at CERN LHC. We find that for zero impact parameter partonic collisions, the monojet cross section is not negligible in comparison to the standard model background and is much higher than the other dark matter scenarios studied so far. We also find that dσ/dp _T of jet production in this process increases as p _T increases, whereas in all other dark matter scenarios the dσ/dp _T decreases at CERN LHC. This may provide a useful signature for dark matter detection at the LHC. However, we find that when the impact parameter dependent effect of inelasticity is included, the monojet cross section from the above process becomes much smaller than the standard model background and may not be detectable at the LHC.     

<Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> suppression and <Emphasis Type="Italic">p</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript> spectra in RHIC and LHC energy collisions

In a hydrodynamic model, we have studied J/ψ production in Au+Au/Cu+Cu collisions at RHIC energy, GeV. At the initial time, J/ψ’s are randomly distributed in the fluid. As the fluid evolves in time, the free streaming J/ψ’s are dissolved if the local fluid temperature exceeds a threshold temperature T _J/ψ . Sequential melting of charmonium states (χ _c , ψ ^′ and J/ψ), with melting temperatures , T _J/ψ ≈2T _c and feed-down fraction F≈0.3, explains the PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions. J/ψ p _T spectra and the nuclear modification factor in Au+Au collisions are also well explained in the model. The model however overpredicts the centrality dependence of J/ψ suppression in Cu+Cu collisions by 20–30%. The J/ψ p _T spectra are underpredicted by 20–30%. The model predicts that in central Pb+Pb collisions at LHC energy,  GeV, J/ψ’s are suppressed by a factor of ∼10. The model predicted a J/ψ p _T distribution in Pb+Pb collisions at LHC is similar to that in Au+Au collisions at RHIC.     

Charmonium production in fixed-target experiments with SPS and LHC beams at CERN

The possibility of measuring cross sections for the production of J/ψ mesons in fixed-target experiments with the proton and ion beams of the Large Hadron Collider (LHC) at CERN (Switzerland) is considered. At the present time, measurements of charmonium production in proton-proton collisions at an energy of 7 TeV have begun at LHC. Previously, the production of J/ψ and ψ′ mesons was studied in the NA38, NA50, and NA60 fixed-target experiments with beams of the CERN synchrotron (SPS) and in the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC, Brookhaven National Laboratory, USA). A normal nuclear absorption of J/ψ mesons in proton-nucleus collisions and an enhanced, anomalous, suppression of the production of charmonium states in central collisions of relativistic nuclei were observed. At the present time, there are no theoretical models that could describe the entire body of experimental data. Measurements over a broad interval of proton and ion energies are required. Measurements of charmonium production using LHC beams with fixed targets in the energy range between the SPS and RHIC energies-a beam of 7-TeV protons (√s = 114.6 GeV) and a beam of 2.75-TeV/nucleon lead ions (√s = 71.8 GeV)-will provide an additional possibility for studying the charmonium-production mechanism. Estimates of the geometric acceptance, luminosity, and counting rate for the production of J/ψ mesons are presented.     

Study of QGP signatures with the ? → K+K– signal in Pb-Pb ALICE events

The ϕ → K ⁺ K ^– decay channel in Pb-Pb collisions at LHC is studied through a full simulation of the ALICE detector. The study focuses on possible signatures in this channel of quark-gluon plasma (QGP) formation. On a basis of 10⁶ collisions at high centrality some proposed QGP signatures are clearly visible both in K ⁺ K ^– invariant mass and transverse mass distributions. The high significance of this observation appears to reside heavily on the use of the TOF (Time Of Flight) system of ALICE in addition to its central tracking detectors. An erratum to this article is available at .     

Charmonium suppression by gluon bremsstrahlung in p-A and A-B collisions

Prompt gluons are an additional source for charmonium suppression in nuclear collisions, in particular for nucleus-nucleus collisions. These gluons are radiated as bremsstrahlung in N-N collisions and interact inelastically with the charmonium states while the nuclei still overlap. The spectra and mean number <n _g> of the prompt gluons are calculated perturbatively and the inelastic cross section σ_abs ^Ψg is estimated. The integrated cross sections σ(A B →J/ψX) for p-A and A-B collisions and the dependence on transverse energy for S-U and Pb-Pb can be described quantitatively with some adjustment of one parameter <n _g>σ_abs ^Ψg. Received: 20 August 1999