NLO pQCD Calculations Research Articles

Measuring the light meson nuclear modification factor in p-Pb collisions over an unprecedented $p_{\text{T}}$ range with ALICE

Differential invariant cross sections of light neutral mesons in p-Pb collisions at \sqrt{s_{NN}}sNN = 8.16 TeV and in pp collisions at \sqrt{s}s = 8 TeV have been measured up to very high transverse momentum (p_TpT). By combining independent reconstruction techniques available in ALICE using the EMCal and PHOS calorimeters as well as the central barrel tracking detectors, the combined spectra cover almost two orders of magnitude in p_TpT~for the \pi^0π0~meson. The nuclear modification factor R_{pPb}RpPb~has been measured for the \pi^0π0~and \etaη~mesons and is found to be consistent with NLO pQCD, CGC and energy loss calculations. Comparisons to the R_{pPb}RpPb~of \pi^0π0~measured in \sqrt{s_{NN}}sNN = 5.02 TeV hint at a stronger suppression at low p_TpT~with increasing collision energy.

Measurement of inclusive charged-particle b-jet production in pp and p-Pb collisions at sqrt{s_{mathrm{NN}}} = 5.02 TeV

A measurement of the inclusive b-jet production cross section is presented in pp and p-Pb collisions at sqrt{s_{mathrm{NN}}} = 5.02 TeV, using data collected with the ALICE detector at the LHC. The jets were reconstructed in the central rapidity region |η| < 0.5 from charged particles using the anti-kT algorithm with resolution parameter R = 0.4. Identification of b jets exploits the long lifetime of b hadrons, using the properties of secondary vertices and impact parameter distributions. The pT-differential inclusive production cross section of b jets, as well as the corresponding inclusive b-jet fraction, are reported for pp and p-Pb collisions in the jet transverse momentum range 10 ≤ pT,ch jet≤ 100 GeV/c, together with the nuclear modification factor, {R}_{mathrm{pPb}}^{mathrm{b}-mathrm{jet}} . The analysis thus extends the lower pT limit of b-jet measurements at the LHC. The nuclear modification factor is found to be consistent with unity, indicating that the production of b jets in p-Pb at sqrt{s_{mathrm{NN}}} = 5.02 TeV is not affected by cold nuclear matter effects within the current precision. The measurements are well reproduced by POWHEG NLO pQCD calculations with PYTHIA fragmentation.

Measurements of heavy-flavor jets with ALICE at the LHC

Heavy quarks created in ultra-relativistic heavy-ion collisions are mostly produced in hard QCD processes during the early stages of the reaction. They interact with the hot and cold nuclear matter throughout the evolution of the medium via semi-hard and soft processes such as energy loss via gluon radiations and collisions. Nuclear modification of heavy flavors in p-A systems provides insight into cold nuclear matter effects such as (anti)shadowing and kT- broadening, and serves as a baseline for A-A studies. In addition to that the fully reconstructed heavy-flavor jets provide additional information on the flavor (or mass) dependence of fragmentation, color charge effects as well as insight into the contribution of late gluon splitting. In this contribution, we present the measurements of b-jet production in p–Pb collisions at sNN=5.02 TeV and c-jet production in pp, p–Pb and Pb–Pb collisions measured by the ALICE experiment at the LHC. The measurements of the nuclear modification factors for c-jet in p–Pb and Pb–Pb collisions are also presented. The experimental measurements are compared with the results from Monte Carlo event generators (PYTHIA 6, PYTHIA 8 and Herwig 7) and the NLO pQCD calculations (POWHEG+PYTHIA6). We find good agreement of the measurements with the results from Monte Carlo event generators and from NLO pQCD calculations.

Exploring hot QCD matter via direct photons at ALICE

We review ALICE results on direct photon production in pp, pA and AA collisions and compare them to predictions of NLO pQCD calculations, hydrodynamic, and transport models. In pp and pA collisions the measured direct photon yield is consistent with NLO pQCD calculations, scaled with the number of binary collisions. In Pb-Pb collisions an additional direct photon contribution is observed at low pT < 4 GeV/c which can be interpreted as a thermal direct photon contribution. The measured yield is larger than predictions of hydrodynamic models, but still consistent with them within uncertainties. The elliptic collective flow of direct photons was measured in central and mid-central Pb–Pb collisions and compared with hydrodynamic model predictions.

Neutral Meson and Direct Photon Measurements with the ALICE Experiment

The ALICE experiment is designed to study the properties of the matter created in proton-proton and heavy-ion collisions at the LHC. Neutral mesons can be reconstructed in ALICE in a wide range of transverse momenta via two-photon decays. Neutral meson measurements in pp collisions give an opportunity to validate the NLO or NNLO pQCD calculations and to constrain the parton distribution functions and the parton fragmentation functions. Neutral meson spectra measured in pA and AA collisions allow us to test a modification of the parton distribution functions in nuclei and the parton energy loss in the hot matter created in AA collisions. The recent results from ALICE on direct photon measurements in the Pb–Pb, neutral pion and n meson productions in pp, p-Pb, and Pb–Pb collisions are presented.

Direct photon production in pp, p–Pb and Pb–Pb collisions measured with the ALICE experiment

We review recent ALICE results on direct photon production in pp, p–Pb and Pb–Pb collisions at LHC energies. In light systems, pp and p–Pb, no signal of direct photons at low pT &lt; 3-5 GeV/c is observed within uncertainties, while at high pT our measurements are consistent with NLO pQCD calculations. In central and mid-central Pb–Pb collisions a thermal photon contribution is observed at low pT &lt; 3 - 4 GeV/c with slopes Teff = (304 ± 11stat ± 40syst) MeV and (407 ± 61stat ± 96syst) MeV respectively. A collective elliptic flow of direct photons is measured and appeared to be close to the decay photon flow.

Direct photon yield in pp and in Pb–Pb collisions measured with the ALICE experiment

The measurement of direct photon production in Pb-Pb collisions at sNN=2.76 TeV and in pp collisions at s=7 TeV with the ALICE experiment is presented. In Pb-Pb collisions a clear direct photon signal below 3 GeV/c is observed only for the 0–20 % most central collisions. No excess is observed for semi-central and peripheral Pb-Pb collisions and in pp collisions.Furthermore, in pp collision the measurement of direct photon production in the range 10≤pT≤60 GeV/c is reported. The analysis is performed on the EMCal-triggered data taken in 2011. The two main sources of background, namely photons from fragmentation processes and decay photons, have been subtracted from the inclusive photon spectrum by means of an isolation technique combined with the study of the transverse dispersion of electromagnetic showers. The measurement is in agreement both with NLO pQCD calculations and with those performed by the ATLAS and the CMS collaborations, extending the isolated photon spectrum investigated at LHC towards lower values of pT.

Measurements of the production of prompt photons, jets and vector bosons+jets in pp collisions with the ATLAS detector

The production of jets, photons and gauge bosons in association with jets at hadron colliders provides a stringent test of perturbative QCD and can be used to probe the gluon density function of the proton. In this letter we present recent measurements of the inclusive jet cross section in data collected at center-of-mass energies of 8 TeV and 13 TeV. The measurements have been performed differentially in various kinematic and angular observables and are compared with state-of-the-art predictions at NLO in pQCD. ATLAS has performed precise measurements of the inclusive production of isolated prompt photons in data collected at center-of-mass energies of 8 and 13 TeV, differential in both rapidity and the photon transverse momentum. The measurements are compared with NLO pQCD calculations and with several MC generators. The cross section of photons, W and Z bosons in association with one or more jets have been measured differentially in various observables at center-of-mass energies of 8 and 13 TeV and compared with state-of-the-art pQCD calculations up to NNLO QCD and NLO EWK as well as LO and NLO ME+PS generators. Dedicated phase spaces sensitive to emission of massive bosons collinear with jets or the electroweak production of vector bosons are tested.

Low-Mass Dielectron Production in pp, p–Pb and Pb–Pb Collisions with ALICE

The ALICE Collaboration measures the production of low-mass dielectrons in pp, p–Pb and Pb–Pb collisions at the LHC. The main detectors used in the analyses are the Inner Tracking System, Time Projection Chamber and Time-Of-Flight detector, all located at mid-rapidity. The dielectron yield in p–Pb collisions shows an overall agreement with the hadronic cocktail. The pair transverse momentum distributions are sensitive to the contributions from open heavy-flavours. In Pb–Pb collisions, uncorrected background-subtracted yields have been extracted in two centrality classes. In pp collisions the production of virtual photons relative to the inclusive yield is determined by analyzing the dielectron excess with respect to the expected hadronic sources. The direct photon cross section is then calculated and found to be in agreement with NLO pQCD calculations. A feasibility study for LHC Run 3 after the ALICE upgrade indicates the possibility for a future measurement of the early effective temperature.

Dielectron measurements in pp, p-Pb, and Pb-Pb collisions with the ALICE detector

Dielectrons are excellent tools to investigate the properties of the medium created in heavy-ion collisions. Since dielectrons generated in the medium are not affected by final state interactions, they carry information about the space-time evolution of the collision. The dielectron invariant mass shape is expected to be sensitive to the in-medium modification of vector mesons and thermal radiation from the hot medium created in heavy-ion collisions. In order to understand the nuclear effects, dielectron measurements in pp collisions serve as reference, while p-A collisions are important for the evaluation of the cold nuclear matter effects on the modification of the invariant mass shape. In this proceedings, the dielectron measurements with the ALICE central barrel detectors are presented. The invariant mass spectra in pp at s=7 TeV and p-Pb collisions at sNN=5.02 TeV are extracted and in agreement with the estimated hadronic cocktail calculation. The cross section of the direct photons measured via virtual photons is consistent with NLO pQCD calculations within uncertainties. The uncorrected dielectron yields are also extracted in central and semi-central Pb-Pb collisions at sNN=2.76 TeV. The performance study of the dielectron excess measurement in Pb-Pb collisions by the ALICE detector upgrade is also discussed.

Transverse Single Spin Asymmetry of π0 and η Mesons at RHIC/PHENIX

We presented measurements of the transverse single spin asymmetries ([Formula: see text]) for neutral [Formula: see text] and [Formula: see text] meson at forward rapidities and central rapidity with the PHENIX detector at RHIC at 62.4 GeV and 200 GeV. At mid-rapidity, [Formula: see text] and [Formula: see text] are reconstructed from di-photon decay. At forward rapidities, [Formula: see text] and [Formula: see text] meson are measured using di-photons decays and electromagnetic clusters due to the photon merging effects are significant for energy [Formula: see text]. The neutral-pion measurement of [Formula: see text] at mid-rapidity is consistent with zero with uncertainties a factor of 20 smaller than previous publications, which will lead to improved constraints on the gluon Sivers function. At higher rapidities, both neutral [Formula: see text] and [Formula: see text] [Formula: see text] exhibit sizable asymmetries. The origin of the forward [Formula: see text] is presently not understood quantitatively. We also measured [Formula: see text] meson cross section for [Formula: see text] GeV/c and [Formula: see text]. It is well described by a NLO pQCD calculation.

Erratum to: Measurement of the inclusive jet cross-section in proton-proton collisions at s = 7 $$ \sqrt{s}=7 $$ TeV using 4.5 fb−1 of data with the ATLAS detector

It was found that the non-perturbative corrections calculated using Pythia with the Perugia 2011 tune did not include the effect of the underlying event. The affected correction factors were recomputed using the Pythia 6.427 generator. These corrections are applied as baseline to the NLO pQCD calculations and thus the central values of the theoretical predictions have changed by a few percent with the new corrections. This has a minor impact on the agreement between the data and the theoretical predictions. Figures 2 and 6 to 13, and all the tables have been updated with the new values. A few sentences in the discussion in sections 5.2 and 9 were altered or removed. The online version of the original article can be found at http://dx.doi.org/10.1007/JHEP02(2015)153.

Measurement of charged jet production cross sections and nuclear modification in p–Pb collisions at [formula omitted

Charged jet production cross sections in p–Pb collisions at sNN=5.02 TeV measured with the ALICE detector at the LHC are presented. Using the anti-kT algorithm, jets have been reconstructed in the central rapidity region from charged particles with resolution parameters R=0.2 and R=0.4. The reconstructed jets have been corrected for detector effects and the underlying event background. To calculate the nuclear modification factor, RpPb, of charged jets in p–Pb collisions, a pp reference was constructed by scaling previously measured charged jet spectra at s=7 TeV. In the transverse momentum range 20≤pT,chjet≤120 GeV/c, RpPb is found to be consistent with unity, indicating the absence of strong nuclear matter effects on jet production. Major modifications to the radial jet structure are probed via the ratio of jet production cross sections reconstructed with the two different resolution parameters. This ratio is found to be similar to the measurement in pp collisions at s=7 TeV and to the expectations from PYTHIA pp simulations and NLO pQCD calculations at sNN=5.02 TeV.

Low-Mass Dielectron Production in pp, p–Pb and Pb–Pb Collisions with ALICE

The ALICE Collaboration measures the production of low-mass dielectrons in pp, p-Pb and Pb-Pb collisions at the LHC. The main detectors used in the analyses are the Inner Tracking System, Time Projection Chamber and Time-Of-Flight detector, all located around mid-rapidity. The production of virtual photons relative to the inclusive yield in pp collisions is determined by analyzing the dielectron excess with respect to the expected hadronic sources. The direct photon cross section is then calculated and found to be in agreement with NLO pQCD calculations. Results from the invariant mass analysis in p-Pb collisions show an overall agreement between data and hadronic cocktail. In Pb-Pb collisions, uncorrected background- subtracted yields have been extracted in two centrality classes. A feasibility study for LHC run 3 after the ALICE upgrade indicates the possibility for a future measurement of the early effective temperature.

Study of Z boson production in pp, pPb and PbPb collisions in CMS

The Z boson production is studied in PbPb and pp collisions at = 2.76 TeV and in pPbcollisions at = 5.02 TeV using data collected by the CMS experiment at the LHC. The Z boson production is observed to be proportional to the number of elementary nucleon-nucleon collisions in both PbPb and pPb collisions. The measured differential cross sections as a function of transverse momentum and rapidity are in agreement with NLO pQCD calculations using different parton distribution functions with and without nuclear effects. Using the PbPb and pp data, the nuclear modification factor is presented as well as the forward- backward ratio measured in pPb collisions.

Dielectron measurements in pp, p–Pb and Pb–Pb collisions with ALICE at the LHC

Electromagnetic probes are excellent messengers from the hot and dense medium created in high-energy heavy-ion collisions. Since leptons do not interact strongly, their spectra reflect the entire space-time evolution of the collision. The surrounding medium can lead to modifications of the dielectron production with respect to the vacuum rate. To quantify modifications in heavy-ion collisions, measurements in pp collisions serve as a reference, while the analysis of p-A collisions allows for the disentanglement of cold nuclear matter effects from those of the hot and dense medium. In this proceedings, dielectron measurements with the ALICE central barrel detectors are presented. The invariant mass distributions in the range $ 0<m_{ee}<3 $ GeV/$ c^{2} $ are compared to the expected yields from hadronic sources for pp collisions at $ \sqrt{s}=7 $ TeV, and for p-Pb collisions at $ \sqrt{s_{\rm NN}}=5.02 $ TeV. The cross section of direct photons measured via virtual photons in pp collisions is compared to predictions from NLO pQCD calculations as a function of the transverse momentum. The status of the analysis of Pb-Pb collisions at $ \sqrt{s_{\rm NN}}=2.76 $ TeV is presented.

Hard And Soft QCD Physics In ATLAS

Hard and soft QCD results using proton-proton collisions recorded with the ATLAS detector at the LHC are reported. Charged-particle distributions and forward- backward correlations have been studied in low-luminosity minimum bias data taken at centre-of-mass energies of √ s = 0.9, 2.36 and 7 TeV. Recent measurements on under- lying event characteristics using charged-particle jets are also presented. The results are tested against various phenomenological soft QCD models implemented in Monte-Carlo generators. A summary of hard QCD measurements involving high transverse momen- tum jets is also given. Inclusive jet and dijet cross-sections have been measured at a centre-of-mass energy of 7 TeV and are compared to expectations based on NLO pQCD calculations corrected for non-perturbative effects as well as to NLO Monte Carlo predic- tions. Recent studies exploiting jet substructure techniques to identify hadronic decays of boosted massive particles are reported.

Energy dependence of the transverse momentum distributions of charged particles in pp collisions measured by ALICE.

Differential cross sections of charged particles in inelastic pp collisions as a function of p T have been measured at sqrt{s} = 0.9, 2.76 text{and} 7 text{TeV} at the LHC. The p T spectra are compared to NLO-pQCD calculations. Though the differential cross section for an individual sqrt{s} cannot be described by NLO-pQCD, the relative increase of cross section with sqrt{s} is in agreement with NLO-pQCD. Based on these measurements and observations, procedures are discussed to construct pp reference spectra at sqrt{s} = 2.76 text{and} 5.02~text{TeV} up to p T=50 GeV/c as required for the calculation of the nuclear modification factor in nucleus–nucleus and proton–nucleus collisions.

Elliptic flow of thermal photons from an event-by-event hydrodynamic model

Elliptic flow of direct photons in relativistic heavy ion collisions is believed to be dominated by contribution from thermal radiation of quark gluon plasma up to $p_T$ $\sim 5$ GeV/$c$, although other sources start outshining the thermal contribution at already smaller values of $p_T$ in the direct photon spectrum. The elliptic flow of thermal photons from ideal hydrodynamics considering a smooth initial density distribution under-predicts the PHENIX direct photon data from 200A GeV Au+Au collisions at RHIC by a large margin in the range $1 < p_T < 5$ GeV/$c$. However, a significant enhancement of thermal photon production due to fluctuations in the initial QCD matter density distributions is expected. We show that such fluctuations result in substantially larger photon elliptic flow for $p_T > 2.5 $ GeV/$c$ compared to a smooth initial-state-averaged density profile. The results from event-by-event hydrodynamics are found to be sensitive to the fluctuation size parameter. However, the effects of initial state fluctuations are insufficient to account for the discrepancy to the PHENIX data for direct photon elliptic flow. Furthermore, the photon $v_2$ is reduced even more when we include the NLO pQCD prompt photon component. We also calculate the spectra and elliptic flow of thermal photons for 2.76A TeV Pb+Pb collisions at LHC and for the 0--40% centrality bin. Thermal photons from event-by-event hydrodynamics along with prompt photons from NLO pQCD calculations explain the ALICE preliminary direct photon data well in the region $p_T \ge 2.5$ GeV/c. Similar to RHIC, the elliptic flow results at LHC are again found to be much smaller than the ALICE preliminary $v_2$ data.

Isolated prompt photon pair production at hadron colliders with k T -factorization

In the framework of the kt-factorization approach, the isolated prompt photon pair production in pp and p \bar p collisions at high energies is studied. The consideration is based on the quark-antiquark annihilation, quark-gluon scattering and gluon-gluon fusion subprocesses, where the non-zero transverse momenta of incoming partons are taken into account. The unintegrated quark and gluon densities in a proton are determined using the Kimber-Martin-Ryskin prescription. The numerical analysis covers the total and differential production cross sections and extends to specific angular correlations between the produced prompt photons. Theoretical uncertainties of our evaluations are studied and comparison with the NLO pQCD calculations is performed. The numerical predictions are compared with the recent experimental data taken by the D0, CDF, CMS and ATLAS collaborations at the Tevatron and LHC energies.