Central Rapidity Region Research Articles

Development of the Baldin approach to analysis of hadronic and nuclear processes at high energies

This article presents the development of the A.M. Baldin approach using a description relativistic nuclear interactions in the four velocity space. This approach allows one to perform calculations of pion yields in hadron-hadron interactions in the central rapidity region depending on the transverse momentum. Results of our calculations have coincided with experimental data in a wide energy range with high precision. This approach gives a good description of the experimental results at high (LHC) and at lower energies (the Nuclotron).

Bose-Einstein correlations in perturbative QCD: vn dependence on multiplicity

In this paper we study the dependence of Bose-Einstein correlations on the multiplicity of an event. We found that events with large multiplicity, stem from the production of several parton showers, while the additional production of small multiplicity in the central rapidity region (central diffraction), gives a negligible contribution due to emission of soft gluons, that leads to the Sudakov suppression of the exclusive production of two gluon jets. Hence, the Bose-Einstein correlation is the main source of the azimuthal angle correlations which generates $v_n$ with odd and even $n$. We found, that without this suppression, the measurement of an event with given multipilicity, yields $v_{n,n} < 0$ for odd $n$. It appears that in hadron-nucleus and nucleus-nucleus collisions, the Bose-Einstein correlations do not depend on multiplicity, while for hadron-hadron scattering, such dependence can be considerable. We proposed a simple Kharzeev-Levin-Nardi (KLN) type model, to describe the dependence of azimuthal angle correlations on the centrality of the event, in ion-ion collisions.

J/Ψ photoproduction in peripheral AA collisions

The exclusive photoproduction of the heavy vector mesons $J/\Psi$ is investigated in the context of peripheral lead-lead collisions for the energies available at the LHC, $\sqrt{s}=2.76$ TeV and $\sqrt{s}=5.02$ TeV. Using the light-cone color dipole formalism, it was calculated the rapidity distribution in two centrality bins 50\%-70\% and 70\%-90\% in order to evaluate its robustness in extrapolating down to smaller impact parameter. It is introduced a modified photon flux, without change in the photonuclear cross section in relation to the ultraperipheral (UPC) case. Results were obtained for the two regions analized, which presented a maximum difference of 27\% in frontal rapidity for the two regions. Comparing the results for $\sqrt{s}=2.76$ TeV and $\sqrt{s}=5.02$ TeV, it was verified an increase of approximately half the one obtained in ultraperipheral regime in the central rapidity region.

Light neutral CP -even Higgs boson within the next-to-minimal supersymmetric standard model at the Large Hadron Electron Collider

We analyze the prospects of observing the light CP-even neutral Higgs bosons ($h_1$) in their decays into $b \bar b$ quarks, in the neutral and charged current production processes $e h_1 q$ and $\nu h_1 q$ at the upcoming LHeC, with $\sqrt s \approx 1.296$ TeV. Assuming that the intermediate Higgs boson ($h_2$) is Standard Model (SM)-like, we study the Higgs production within the framework of NMSSM. We consider the constraints from Dark-matter, Sparticle masses, and the Higgs boson data. The signal in our analysis can be classified as three jets, with electron (missing energy)coming from the neutral (charged) current interaction. We demand that the number of b-tagged jets in the central rapidity region be greater or equal to two. The remaining jet is tagged in the forward regions. With this forward jet and two $b$-tagged jets in the central region, we reconstructed three jets invariant masses. Applying some lower limits on these invariant masses turns out to be an essential criterion to enhance the signal--to--background rates, with slightly different sets of kinematical selections in the two different channels. We consider almost all reducible and irreducible SM background processes. We find that the non-SM like Higgs boson, $h_1$, would be accessible in some of the NMSSM benchmark points, at approximately 0.4$\sigma$ (2.5$\sigma$) level in the $e$+3j channel up to Higgs boson masses of 75 GeV and in the $E\!\!\!\!/_T$+3j channel could be discovered with 1.7$\sigma$ (2.4$\sigma$) level up to Higgs boson masses of 88 GeV with 100 fb$^{-1}$ of data in a simple cut-based (with optimization) selection. With ten times more data accumulation at the end of the LHeC run and using optimization, one can have 5$\sigma$ discovery in the electron (missing energy) channel up to 85 (more than 90) GeV.

Quarkonium production at the LHC: A data-driven analysis of remarkably simple experimental patterns

The LHC quarkonium production data reveal a startling observation: the J/ψ, ψ(2S), χc1, χc2 and ϒ(nS)pT-differential cross sections in the central rapidity region are compatible with one universal momentum scaling pattern. Considering also the absence of strong polarizations of directly and indirectly produced S-wave mesons, we conclude that there is currently no evidence of a dependence of the partonic production mechanisms on the quantum numbers and mass of the final state. The experimental observations supporting this universal production scenario are remarkably significant, as shown by a new analysis approach, unbiased by specific theoretical calculations of partonic cross sections, which are only considered a posteriori, in comparisons with the data-driven results.

Charged particle nuclear modification factor in PbPb at 5.02 TeV with CMS

In the high-luminosity 5.02 TeV collision-energy per nucleon pair PbPb and pp data provided by LHC in 2015, CMS measured the nuclear modification factor of charged particles from a transverse momentum of 0.7 GeV/c to 400 GeV/c in the central rapidity region. The centrality dependence of the nuclear modification factor is explored in several bins of collision centrality, from the most central 0–5% to the peripheral 50–70% centrality range. Comparisons of the measured nuclear modification factor of charged particles at 5.02 TeV are made to theory calculations and to measurements at lower collision energies. The nuclear modification factors are also compared to the measurements at 2.76 TeV with charged particles and fully reconstructed jets.

Rapidity dependence of the average transverse momentum in p+p and p+Pb collisions revisited

We revisit the calculation of rapidity dependence of the average transverse momentum $⟨{p}_{T}⟩$ of single inclusive jet production or particle in a $p+p$ and $p\ensuremath{-}\mathrm{Pb}$ collision, using the high energy factorization. We update previous predictions for the $⟨{p}_{T}⟩$ both in central and forward rapidity region using parton densities following from extended Balitsky-Kovchegov and Balitsky-Fadin-Kuraev-Lipatov evolution equations to account for corrections of higher orders as well as one obtained by Sudakov resummation. Furthermore, we demonstrate that in the midrapidity region the saturation based formalisms predict increase of the transversal momentum while in the forward rapidity region the $⟨{p}_{T}(y)⟩$ decreases.

Measurement of the D+ -meson production cross section at low transverse momentum in pp¯ collisions at s=1.96 TeV

We report on a measurement of the $D^{+}$-meson production cross section as a function of transverse momentum ($p_T$) in proton-antiproton ($p\bar{p}$) collisions at 1.96 TeV center-of-mass energy, using the full data set collected by the Collider Detector at Fermilab in Tevatron Run II and corresponding to 10 fb$^{-1}$ of integrated luminosity. We use $D^{+} \to K^-\pi^+\pi^+$ decays fully reconstructed in the central rapidity region $|y|<1$ with transverse momentum down to 1.5 GeV/$c$, a range previously unexplored in $p\bar{p}$ collisions. Inelastic $p\bar{p}$-scattering events are selected online using minimally-biasing requirements followed by an optimized offline selection. The $K^-\pi^+\pi^+$ mass distribution is used to identify the $D^+$ signal, and the $D^+$ transverse impact-parameter distribution is used to separate prompt production, occurring directly in the hard scattering process, from secondary production from $b$-hadron decays. We obtain a prompt $D^+$ signal of 2950 candidates corresponding to a total cross section $\sigma(D^+, 1.5 < p_T < 14.5~\mbox{GeV/}c, |y|<1) = 71.9 \pm 6.8 (\mbox{stat}) \pm 9.3 (\mbox{syst})~\mu$b. While the measured cross sections are consistent with theoretical estimates in each $p_T$ bin, the shape of the observed $p_T$ spectrum is softer than the expectation from quantum chromodynamics. The results are unique in $p\bar{p}$ collisions and can improve the shape and uncertainties of future predictions.

Rapidity dependence of charged pion production at relativistic energies using Tsallis statistics

Transverse momentum distributions of charged pions produced in Au+Au collisions at 62.4GeV, 130GeV, 200GeV, Cu+Cu and d+Au collisions at 200GeV, p+p collisions at 62.4 and 200GeV and Pb+Pb collisions at 17.3GeV are studied using the Tsallis distribution as a parametrization. The non-extensivity parameter and Tsallis volume increase with energy, while the Tsallis temperature shows a decrease at higher energies. Using BRAHMS $ p_T$ spectra obtained in Au+Au collisions at 62.4GeV and 200GeV, Tsallis fit parameters are obtained on a very wide rapidity range. Results are compared with p+p and Cu+Cu data and changes of Tsallis parameters with rapidity and energy are investigated. We found that non-extensivity parameter q shows a decrease from midrapidity to forward rapidities for all the studied systems. Tsallis volume, V , increases with the system size from p+p, Cu+Cu to Au+Au, both in central rapidity region and at forward rapidities. The values of temperatures increase with rapidity, but the $ T/\cosh(y)$ ratio is constant as a function of rapidity.

Stability of azimuthal-angle observables under higher order corrections in inclusive three-jet production

Recently, a new family of observables consisting of azimuthal-angle generalised ratios was proposed in a kinematical setup that resembles the usual Mueller-Navelet jets but with an additional tagged jet in the central region of rapidity. Non-tagged minijet activity between the three jets can affect significantly the azimuthal angle orientation of the jets and is accounted for by the introduction of two BFKL gluon Green functions. Here, we calculate the, presumably, most relevant higher order corrections to the observables by now convoluting the three leading-order jet vertices with two gluon Green functions at next-to-leading logarithmic approximation. The corrections appear to be mostly moderate giving us confidence that the recently proposed observables are actually an excellent way to probe the BFKL dynamics at the LHC. Furthermore, we allow for the jets to take values in different rapidity bins in various configurations such that a comparison between our predictions and the experimental data is a straightforward task.

Achieving high baryon densities in the fragmentation regions in heavy ion collisions at top RHIC energy

Heavy ion collisions at extremely high energy, such as the top energy at RHIC, exhibit the property of transparency where there is a clear separation between the almost net-baryon-free central rapidity region and the net-baryon-rich fragmentation region. We calculate the net-baryon rapidity loss and the nuclear excitation energy using the energy-momentum tensor obtained from the McLerran-Venugopalan model. Nuclear compression during the collision is further estimated using a simple space-time picture. The results show that extremely high baryon densities, about twenty times larger than the normal nuclear density, can be achieved in the fragmentation regions.

Excitation functions of parameters extracted from three-source (net-)proton rapidity distributions in Au-Au and Pb-Pb collisions over an energy range from AGS to RHIC

Experimental results of the rapidity spectra of protons and net-protons (protons minus antiprotons) emitted in gold-gold (Au-Au) and lead-lead (Pb-Pb) collisions, measured by a few collaborations at the alternating gradient synchrotron (AGS), super proton synchrotron (SPS), and relativistic heavy ion collider (RHIC), are described by a three-source distribution. The values of the distribution width $\sigma_C$ and contribution ratio (relative contribution) $k_C$ of the central rapidity region, and the distribution width $\sigma_F$ and rapidity shift $\Delta y$ of the forward/backward rapidity regions, are then obtained. The excitation function of $\sigma_C$ increases generally with increase of the center-of-mass energy per nucleon pair $\sqrt{s_{NN}}$. The excitation function of $\sigma_F$ shows a saturation at $\sqrt{s_{NN}}=8.8$ GeV. The excitation function of $k_C$ shows a minimum at $\sqrt{s_{NN}}=8.8$ GeV and a saturation at $\sqrt{s_{NN}}\approx 17$ GeV. The excitation function of $\Delta y$ increase monotonously with $\ln \sqrt{s_{NN}}$ in the considered energy range.

High baryon densities in heavy ion collisions at energies attainable at the BNL Relativistic Heavy-Ion Collider and the CERN Large Hadron Collider

In very high energy collisions nuclei are practically transparent to each other but produce very hot, nearly baryon-free, matter in the so-called central rapidity region. The energy in the central rapidity region comes from the kinetic energy of the colliding nuclei. We calculate the energy and rapidity loss of the nuclei using the color glass condensate model. This model also predicts the excitation energy of the nuclear fragments. Using a space-time picture of the collision we calculate the baryon and energy densities of the receding baryonic fireballs. For central collisions of gold nuclei at the highest energy attainable at the Relativistic Heavy Ion Collider, for example, we find baryon densities more than ten times that of atomic nuclei over a large volume.

Transverse Momentum Distribution in Ψ(1S,2S) Photoproduction in PP and AA Collisions at the LHC

The exclusive photoproduction of the heavy vector mesons [Formula: see text] is investigated in the context of ultra-peripheral collisions proton-proton and nucleus-nucleus for the energies available at the LHC run 2. Using the light-cone color dipole formalism, it was calculated the transverse momentum distribution in the central rapidity region, in which it is expected major contribution for the process.

Measurement of D0-meson production in Pb-Pb and p-Pb collisions with the ALICE experiment at the LHC

In this article, a review of the measurements of the production of prompt charmed D0 mesons relative to the reaction plane in Pb-Pb collisions at the centre-of-mass energy per nucleon-nucleon collision of $\sqrt{s_{NN}} = 2.76$ TeV, and of the D0 production in p-Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV with the ALICE experiment at the CERN Large Hadron Collider is presented. The azimuthal anisotropy is quantified via the second coefficient v2 in a Fourier expansion of the D0 azimuthal distribution, and the nuclear modification factor $R_{AA}$ , measured with respect to the reaction plane. The D0 production is measured in both systems by reconstructing the two-prong hadronic decay $D^{0} \rightarrow K^{-}\pi^{+}$ in the central rapidity region. A positive elliptic flow is observed in Pb-Pb collisions in the centrality class 30-50%, with a mean value of $0.204^{+0.099}_{-0.036}$ in the interval $2< p_{T} < 6$ GeV/c, which decreases towards more central collisions. Consequently, the nuclear modification factor shows a stronger suppression in the direction orthogonal to the reaction plane. The D0 nuclear modification factor $ R_{pPb}$ in p-Pb collisions is compatible with unity within uncertainties. The multiplicity dependence of the D0 production is also studied by comparing yields in p-Pb collisions in four event classes with those in pp collisions, scaled by the number of binary nucleon-nucleon collisions, as well as by evaluating the yields per event in multiplicity intervals normalised to the multiplicity-integrated ones. The nuclear modification factor is consistent with unity in the four considered event classes, within uncertainties. The relative D0-meson yields increase as a function of the charged-particle multiplicity. The Pb-Pb and p-Pb results are compared to theoretical calculations of charm quark transport and energy loss in high-density strongly interacting matter at high temperature and including initial-state effects induced by the nuclear environment, respectively.

Flavor violating signatures of lighter and heavier Higgs bosons within the two Higgs doublet model type III at the LHeC

We analyze the prospects for observing the CP-even neutral Higgs bosons (h and H) in their decays into flavor violating $b \bar s+ c.c.$ channels at the proposed LHeC, with $\sqrt s \approx 1.3$ TeV, in the 2HDM Type-III, assuming a four-zero texture in the Yukawa matrices and a general Higgs potential. We consider theoretically consistent scenarios in agreement with current experimental data from flavor and Higgs physics. We investigate the charged current production process $\nu_e \phi q$ in presence of flavor violating decays of the Higgs bosons, that lead to a 3-jets + $E\!\!\!\!/_T$ signature. We demand exactly two jets, one tagged $b$-jet and one light-flavor jet, all in the central rapidity region. The remaining jet (originated by the remanent quark $q$) is tagged in the forward or backward regions and this together with a central jet veto (not more than one light-flavor jet) are essential criterions to enhance the signal--to--background rates. We consider the most relevant Standard Model (SM) backgrounds, treating $c$-jets separately from light-flavor and gluon ones, while allowing for mis-tagging. We find that the SM-like Higgs boson, $h$, would be accessible within several parameter configurations of our model at approximately the 1-2$\sigma$ level with 100 fb$^{-1}$ of data. We also find that the heaviest neutral Higgs boson, $H$, with mass up to 150 GeV, would have a 1$\sigma$ significance for the same data sample. At the end of the LHeC running, one would have ten times data accumulation and for all the Higgs masses the significances are enhanced so as to allow for detection of both the $h$ and $H$ state. Hence, one of the most viable extensions of 2HDMs with FCNC's generated at tree level but controlled by a four-zero texture approach in the Yukawa matrices, as opposed to the adoption of ad hoc discrete symmetries, could be put under scrutiny at a future LHeC.

Measurement of jet activity in top quark events using the eμ final state with two b-tagged jets in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

Measurements of the jet activity in $t\bar{t}$ events produced in proton--proton collisions at $\sqrt{s}=8$ TeV are presented, using $20.3\,$fb$^{-1}$ of data collected by the ATLAS experiment at the Large Hadron Collider. The events were selected in the dilepton $e\mu$ decay channel with two identified $b$-jets. The numbers of additional jets for various jet transverse momentum ($p_T$) thresholds, and the normalised differential cross-sections as a function of $p_T$ for the five highest-$p_T$ additional jets, were measured in the jet pseudorapidity range $|\eta|<4.5$. The gap fraction, the fraction of events which do not contain an additional jet in a central rapidity region, was measured for several rapidity intervals as a function of the minimum $p_T$ of a single jet or the scalar sum of $p_T$ of all additional jets. These fractions were also measured in different regions of the invariant mass of the $e\mu b\bar{b}$ system. All measurements were corrected for detector effects, and found to be mostly well-described by predictions from next-to-leading-order and leading-order $t\bar{t}$ event generators with appropriate parameter choices. The results can be used to further optimise the parameters used in such generators.

Detailed rapidity dependence ofJ/ψproduction at energies available at the Large Hadron Collider

$J/\psi$ suppression with rapidity is studied with both (3+1) dimensional transport equation and hydrodynamics in details at $\sqrt{s_{NN}}=2.76$ TeV Pb+Pb collisions. In central rapidity region of the hot medium, $J/\psi$ nuclear modification factor shows weak rapidity dependence due to the boost invariance of the medium. But in forward rapidity region for entire transverse momentum bin, $J/\psi$ regeneration is strongly suppressed due to the decreasing charm pair cross section. This makes $J/\psi$ $R_{AA}(y)$ decrease with rapidity. The ratio $r_{AA}$ of mean transverse momentum square in AA collisions to that in pp collisions increases strongly in forward rapidity region and is more sensitive to the $J/\psi$ production mechanisms.

Centrality dependence of charged jet production in p-Pb collisions at [Formula: see text] = 5.02 TeV.

Measurements of charged jet production as a function of centrality are presented for p–Pb collisions recorded at sqrt{s_mathrm {NN}}= 5.02 TeV with the ALICE detector. Centrality classes are determined via the energy deposit in neutron calorimeters at zero degree, close to the beam direction, to minimise dynamical biases of the selection. The corresponding number of participants or binary nucleon–nucleon collisions is determined based on the particle production in the Pb-going rapidity region. Jets have been reconstructed in the central rapidity region from charged particles with the anti-k_mathrm {T} algorithm for resolution parameters R = 0.2 and R = 0.4 in the transverse momentum range 20 to 120 GeV/c. The reconstructed jet momentum and yields have been corrected for detector effects and underlying-event background. In the five centrality bins considered, the charged jet production in p–Pb collisions is consistent with the production expected from binary scaling from pp collisions. The ratio of jet yields reconstructed with the two different resolution parameters is also independent of the centrality selection, demonstrating the absence of major modifications of the radial jet structure in the reported centrality classes.

Muon momentum scale and resolution in pp collisions at [formula omitted] in ATLAS

This work shows the performance in the muon momentum scale and resolution achieved by ATLAS during the 2012 run at s=8. The momentum scale is known with an uncertainty ranging from 0.05% (in the central rapidity region) to 0.2% (at large rapidities). The muon momentum resolution is measured to be 1.7% for central muons with low momentum (pT≃10 GeV) and reaches 4% for high-pT muons in the forward regions of the ATLAS detector.