Perturbative QCD Calculations Research Articles

Heavy-flavor correlations and multiplicity dependence in pp and p–Pb collisions with ALICE

The production of heavy quarks in pp collisions at the LHC energies provides a reliable test of perturbative QCD calculations. Comparisons of pp and p–Pb measurements of their hadronization products allow us to investigate how cold nuclear matter effects affect the heavy-quark production. We present ALICE measurements of azimuthal correlations of prompt D mesons with charged hadrons in pp collisions at s=7 TeV and p–Pb collisions at sNN=5.02 TeV. We also show the per-event D-meson yields as a function of the charged-particle multiplicity in pp collisions at s=7 TeV.

Jet-like correlations with neutral pion triggers in pp and central Pb–Pb collisions at 2.76 TeV

We present measurements of two-particle correlations with neutral pion trigger particles of transverse momenta $8 < p_{\mathrm{T}}^{\rm trig} < 16~\mathrm{GeV}/c$ and associated charged particles of $0.5 < p_{\mathrm{T}}^{\rm assoc} < 10~\mathrm{GeV}/c$ versus the azimuthal angle difference $\Delta\varphi$ at midrapidity in pp and central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV with ALICE. The new measurements exploit associated charged hadrons down to $0.5~\mathrm{GeV}/c$, which significantly extends our previous measurement that only used charged hadrons above $3~\mathrm{GeV}/c$. After subtracting the contributions of the flow background, $v_2$ to $v_5$, the per-trigger yields are extracted for $|\Delta\varphi|<0.7$ on the near and for $|\Delta\varphi-\pi| < 1.1$ on the away side. The ratio of per-trigger yields in Pb--Pb to those in pp collisions, $I_{\mathrm{AA}}$, is measured on the near and away side for the $0$--$10$\% most central Pb--Pb collisions. On the away side, the per-trigger yields in Pb--Pb are strongly suppressed to the level of $I_{\mathrm{AA}} \approx 0.6$ for $p_{\mathrm{T}}^{\rm assoc} > 3~\mathrm{GeV}/c$, while with decreasing momenta an enhancement develops reaching about $5$ at low $p_{\mathrm{T}}^{\rm assoc}$. On the near side, an enhancement of $I_{\mathrm{AA}}$ between $1.2$ at the highest to $1.8$ at the lowest $p_{\mathrm{T}}^{\rm assoc}$ is observed. The data are compared to parton-energy-loss predictions of the JEWEL and AMPT event generators, as well as to a perturbative QCD calculation with medium-modified fragmentation functions. All calculations qualitatively describe the away-side suppression at high $p_{\mathrm{T}}^{\rm assoc}$. Only AMPT captures the enhancement at low $p_{\mathrm{T}}^{\rm assoc}$, both on the near and away side. However, it also underpredicts $I_{\mathrm{AA}}$ above $5$ GeV/$c$, in particular on the near-side.

Photon and neutral meson measurements with the ALICE experiment at the LHC

The measurements of photons and neutral mesons, such as ϕ0 and ϕ, allow to explore the QCD matter created in heavy-ions collisions. In pp collisions, observables such as differential particle production cross sections can be used to test perturbative QCD calculations and constrain PDFs. In Pb-Pb collisions, neutral meson spectra address the medium induced suppression, whereas low transverse momentum direct photons allow to obtain information on the QCD medium temperature or anisotropic flow.We present an overview of photon and neutral meson measurements, employing three different methods which use both ALICE calorimeters, as well as the central tracking system via the photon conversion method. These experimental results are compared with theoretical predictions.

Measurement of the inclusive isolated prompt photon cross section in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

A measurement of the cross section for the inclusive production of isolated prompt photons in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV is presented. The measurement covers the pseudo rapidity ranges $|\eta^{\gamma}| < 1.37$ and $1.56 \leq |\eta^{\gamma}| < 2.37$ in the transverse energy range $25 < E_{\rm T}^{\gamma} < 1500$ GeV. The results are based on an integrated luminosity of 20.2 fb$^{-1}$, recorded by the ATLAS detector at the LHC. Photon candidates are identified by combining information from the calorimeters and the inner tracker. The background is subtracted using a data-driven technique, based on the observed calorimeter shower-shape variables and the deposition of hadronic energy in a narrow cone around the photon candidate. The measured cross sections are compared with leading-order and next-to-leading order perturbative QCD calculations and are found to be in a good agreement over ten orders of magnitude.

Momentum distribution of charged particles in jets in dijet events and comparison to perturbative QCD predictions

Inclusive momentum distributions of charged particles are measured in dijet events. Events were produced at the AMY detector with a centre of mass energy of 60 GeV. Our results were compared, on the one hand to those obtained from other e+e−, ep as well as CDF data, and on the other hand to the perturbative QCD calculations carried out in the framework of the modified leading log approximation (MLLA) and assuming local parton–hadron duality (LPHD). A fit of the shape of the distributions yields Qeff = 263±13 MeV for the AMY data. In addition, a fit to the evolution of the peak position with dijet mass using all data from different experiments gives Qeff = 226±18 MeV. Next, αs was extracted using the shape of the distribution at the Z0 scale, with a value of 0.118 ± 0.013. This is consistent, within the statistical errors, with many accurate measurements. We conclude that it is the success of LPHD + MLLA that the extracted value of αs is correct. Possible explanations for all these features will be presented in this paper.

Measurement of inclusive jet production and nuclear modifications in pPb collisions at [Formula: see text

Inclusive jet production in pPb collisions at a nucleon–nucleon (NN) center-of-mass energy of sqrt{s_{_mathrm {NN}}} =5.02,mathrm{TeV} is studied with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 30.1 nb^{-1} is analyzed. The jet transverse momentum spectra are studied in seven pseudorapidity intervals covering the range -2.0<eta _mathrm {CM}< 1.5 in the NN center-of-mass frame. The jet production yields at forward and backward pseudorapidity are compared and no significant asymmetry about eta _mathrm {CM} = 0 is observed in the measured kinematic range. The measurements in the pPb system are compared to reference jet spectra obtained by extrapolation from previous measurements in pp collisions at sqrt{s}=7,mathrm{TeV} . In all pseudorapidity ranges, nuclear modifications in inclusive jet production are found to be small, as predicted by next-to-leading order perturbative QCD calculations that incorporate nuclear effects in the parton distribution functions.

Measurements of D-meson production with ALICE

Heavy quarks (charm and beauty) are produced in hard parton scattering processes in the initial stages of hadronic collisions. The study of their production can give us different information in each type of collisions. In pp collisions at the LHC it provides a test of perturbative QCD calculations at the highest collision energy. In p-Pb collisions it allows one to characterise the nuclear modification of the parton distribution functions and the kT-broadening effect due to soft scatterings of the partons in the incoming nuclei. Moreover, it provides insight into possible final state effects that could occur in p-Pb collisions, like energy loss of heavy quarks in the cold nuclear matter and their collective behaviour. In Pb-Pb collisions, heavy-flavour production can be used to study the properties of the high energy-density and strongly-interacting medium created in heavy-ion collisions. Passing through all the phases of the system evolution, heavy quarks lose energy via gluon radiation and elastic collisions in the medium. Therefore, the study of their production can be used to test parton energy-loss models.In these proceedings we will focus on the ALICE results of prompt D mesons measured via the full reconstruction of their hadronic decays. We will present results on open-charm production in pp collisions at s=7 TeV, p-Pb collisions at sNN=5.02 TeV and Pb-Pb collisions at sNN=2.76 TeV.

Production of vector bosons and jets at CMS

The production cross section of highly boosted vector bosons (V = W, Z) recoiling against jets is studied, with CMS data, differentially as function of the transverse momentum and angular correlations of the final state particles. The mechanism of production of heavy-flavoured mesons, containing b or c quarks, in association with vector bosons, W or Z, in the Standard Model is only partially understood. The study of events with one or two well-identified and isolated leptons accompanied by b-jets or b-mesons is therefore crucial to refine the theoretical calculations in perturbative QCD, as well as validate associated Monte Carlo techniques. A measurement of the WZ and ZZ production cross sections in proton-proton collisions at 8 TeV in final states where one Z boson decays to b-tagged jets, while the other gauge boson, either W or Z, is detected through its leptonic decay is also presented.

Centrality, rapidity, and transverse momentum dependence of isolated prompt photon production in lead-lead collisions atsNN=2.76TeV measured with the ATLAS detector

Prompt photon production in $\sqrt{s_{\mathrm{NN}}} = 2.76$ TeV Pb+Pb collisions has been measured by the ATLAS experiment at the Large Hadron Collider using data collected in 2011 with an integrated luminosity of 0.14 nb$^{-1}$. Inclusive photon yields, scaled by the mean nuclear thickness function, are presented as a function of collision centrality and transverse momentum in two pseudorapidity intervals, $|\eta| < 1.37$ and $1.52 < |\eta| < 2.37$. The scaled yields in the two pseudorapidity intervals, as well as the ratios of the forward yields to those at midrapidity, are compared to the expectations from next-to-leading order perturbative QCD calculations from JETPHOX. The measured cross sections agree well with the predictions for proton-proton collisions within statistical and systematic uncertainties. Both the yields and ratios are also compared to two other pQCD calculations, one which uses the isospin content appropriate to colliding lead nuclei, and another which includes the EPS09 nuclear modifications to the nucleon parton distribution functions.

Direct photon production in Pb–Pb collisions atsNN=2.76 TeV

Direct photon production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 2.76$ TeV was studied in the transverse momentum range $0.9 < p_\mathrm{T} < 14$ GeV$/c$. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the ALICE detector material with the $e^+e^-$ pair reconstructed in the central tracking system. The results of the two methods were combined and direct photon spectra were measured for the 0-20%, 20-40%, and 40-80% centrality classes. For all three classes, agreement was found with perturbative QCD calculations for $p_\mathrm{T} \gtrsim 5$ GeV$/c$. Direct photon spectra down to $p_\mathrm{T} \approx 1$ GeV$/c$ could be extracted for the 20-40% and 0-20% centrality classes. The significance of the direct photon signal for $0.9 < p_\mathrm{T} < 2.1$ GeV$/c$ is $2.6\sigma$ for the 0-20% class. The spectrum in this $p_\mathrm{T}$ range and centrality class can be described by an exponential with an inverse slope parameter of $(297 \pm 12^\mathrm{stat}\pm 41^\mathrm{syst})$ MeV. State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties.

The Drell–Yan process as a testing ground for parton distributions up to LHC

The Drell–Yan massive dilepton production in hadron–hadron collisions provides a unique tool, complementary to Deep Inelastic Scattering, for improving our understanding of hadronic substructure and in particular for testing parton distributions. We will consider measurements of the differential and double-differential Drell–Yan cross sections from FNAL Tevatron up to CERN LHC energies and they will be compared to the predictions of perturbative QCD calculations using most recent sets (CT14 and MMHT14) of parton distribution functions, as well as those provided by the statistical approach.

The scenario of two families of compact stars

We present several arguments which favor the scenario of two coexisting families of compact stars: hadronic stars and quark stars. Besides the well known hyperon puzzle of the physics of compact stars, a similar puzzle exists also when considering delta resonances. We show that these particles appear at densities close to twice saturation density and must be therefore included in the calculations of the hadronic equation of state. Such an early appearance is strictly related to the value of the L parameter of the symmetry energy that has been found, in recent phenomenological studies, to lie in the range $40<L<62$ MeV. We discuss also the threshold for the formation of deltas and hyperons for hot and lepton rich hadronic matter. Similarly to the case of hyperons, also delta resonances cause a softening of the equation of state which makes it difficult to obtain massive hadronic stars. Quark stars, on the other hand, can reach masses up to $2.75 M_{\odot}$ as predicted by perturbative QCD calculations. We then discuss the observational constraints on the masses and the radii of compact stars. The tension between the precise measurements of high masses and the indications of the existence of very compact stellar objects (with radii of the order of $10$ km) is relieved when assuming that very massive compact stars are quark stars and very compact stars are hadronic stars. Finally, we discuss recent interesting measurements of the eccentricities of the orbits of millisecond pulsars in low mass X-ray binaries. The high values of the eccentricities found in some cases could be explained by assuming that the hadronic star, initially present in the binary system, converts to a quark star due to the increase of its central density.

Z 0 / γ ∗ +Jet via electron decay mode at s = $\boldsymbol {\sqrt {s}=}$ 7 TeVin CMS@LHC

The area-normalized angular distributions in events containing a Z 0 boson and a jet, using the electron decay mode, are presented. The data samples correspond to 5 fb −1 of proton–proton collisions at $\sqrt {s}=7$ TeV, collected by the CMS detector. Events in which there is a Z boson and at least one jet, with a jet transverse momentum threshold of 30 GeV /c and absolute jet rapidity less than 2.4, are selected for this analysis. We compare our measurements with a next-to-leading order perturbative QCD calculation and two generators that combine tree-level matrix element calculations with parton showers.

Study of B Meson Production in p+Pb Collisions at √[S(NN)]=5.02 TeV Using Exclusive Hadronic Decays.

The production cross sections of the B^{+}, B^{0}, and B_{s}^{0} mesons, and of their charge conjugates, are measured via exclusive hadronic decays in p+Pb collisions at the center-of-mass energy sqrt[s_{NN}]=5.02 TeV with the CMS detector at the CERN LHC. The data set used for this analysis corresponds to an integrated luminosity of 34.6 nb^{-1}. The production cross sections are measured in the transverse momentum range between 10 and 60 GeV/c. No significant modification is observed compared to proton-proton perturbative QCD calculations scaled by the number of incoherent nucleon-nucleon collisions. These results provide a baseline for the study of in-medium b quark energy loss in Pb+Pb collisions.

Early Run 2 Hard QCD Results from the ATLAS Collaboration

We provide an overview of hard QCD results based on data collected with the ATLAS detector in proton-proton collision at √ s = 13 TeV at the Large Hadron Collider. The production of high transverse momentum jets, photons and photon-pairs were studied; the inclusive jet cross section is found to agree well with the prediction of perturbative QCD calculations performed at next-to-leading accuracy. The production cross sections for W and Z bosons in their e and μ decays was measured; in general, agreement is found with the expectation of next-to-next-to leading order QCD calculations and interesting sensitivities to the proton structure functions are already observed. The top production cross sections were measured in different top decay channels and found to agree with the state of the art QCD predictions.

Regularization of the light-cone gauge gluon propagator singularities using sub-gauge conditions

Perturbative QCD calculations in the light-cone gauge have long suffered from the ambiguity associated with the regularization of the poles in the gluon propagator. In this work we study sub-gauge conditions within the light-cone gauge corresponding to several known ways of regulating the gluon propagator. Using the functional integral calculation of the gluon propagator, we rederive the known sub-gauge conditions for the theta-function gauges and identify the sub-gauge condition for the principal value (PV) regularization of the gluon propagator's light-cone poles. The obtained sub-gauge condition for the PV case is further verified by a sample calculation of the classical Yang-Mills field of two collinear ultrarelativistic point color charges. Our method does not allow one to construct a sub-gauge condition corresponding to the well-known Mandelstam-Leibbrandt prescription for regulating the gluon propagator poles.

High-energy behavior of photon, neutrino, and proton cross sections

By combining the color dipole model of the nucleon with the assumption that cross sections behave asymptotically as $\ln^2(s)$, we are able to describe the data for photon, neutrino and hadron interactions with protons at all energies, $s$ is the center-of-mass energy of the interacting particles. Specifically, we extrapolate the perturbative QCD calculations into the regime of small fractional parton momenta $x$ using a color dipole description of the proton target that guarantees an asymptotic $\ln^2(s)$ behavior of all cross sections. The ambiguity of introducing a parametrization associated with the dipole approximation is mitigated by the requirement that the saturation of the small-$x$ structure functions produces $\ln^2(s)$-behaved asymptotic cross sections, in agreement with the data. The same formalism allows us to calculate the cross section for the hadronic pair production of charm particles. The results, in particular those for the high-energy neutrino and charm cross sections, are relevant for evaluating the sensitivity as well as the background in neutrino telescopes.

Single-inclusive production of hadrons and jets in lepton-nucleon scattering at NLO

We present next-to-leading order (NLO) perturbative-QCD calculations of the cross sections for $\ensuremath{\ell}N\ensuremath{\rightarrow}hX$ and $\ensuremath{\ell}N\ensuremath{\rightarrow}\mathrm{jet}X$. The main feature of these processes is that the scattered lepton is not observed, so that the hard scale that makes them perturbative is set by the transverse momentum of the hadron or jet. Kinematically, the two processes thus become direct analogs of single-inclusive production in hadronic collisions which, as has been pointed out in the literature, makes them promising tools for exploring transverse spin phenomena in QCD when the incident nucleon is transversely polarized. We find that the NLO corrections are sizable for the spin-averaged cross section. We also investigate in how far the scattering is dominated by the exchange of almost real (Weizs\"acker-Williams) photons. We present numerical estimates of the cross sections for present-day fixed target experiments and for a possible future electron-ion collider.

Top-quark production in proton–nucleus and nucleus–nucleus collisions at LHC energies and beyond

Single and pair top-quark production in proton–lead (p–Pb) and lead–lead (Pb–Pb) collisions at the CERN Large Hadron Collider (LHC) and Future Circular Collider (FCC) energies, are studied with next-to-leading-order perturbative QCD calculations including nuclear parton distribution functions. At the LHC, the pair-production cross sections amount to σtt¯=3.4 μb in Pb–Pb at sNN=5.5 TeV, and σtt¯=60 nb in p–Pb at sNN=8.8 TeV. At the FCC energies of sNN=39 and 63 TeV, the same cross sections are factors of 90 and 55 times larger respectively. In the leptonic final-state tt¯→W+bW−b¯→bb¯ℓℓνν with ℓ=e±,μ±, after typical acceptance and efficiency cuts, one expects about 90 and 300 top-quarks per nominal LHC-year and 4.7⋅104 and 105 per FCC-year in Pb–Pb and p–Pb collisions respectively. The total tt¯ cross sections, dominated by gluon fusion processes, are enhanced by 3–8% in nuclear compared to p–p collisions due to an overall net gluon antishadowing, although different regions of their differential distributions are depleted due to shadowing or EMC-effect corrections. The rapidity distributions of the decay leptons in tt¯ processes can be used to reduce the uncertainty on the Pb gluon density at high virtualities by up to 30% at the LHC (full heavy-ion programme), and by 70% per FCC-year. The cross sections for single-top production in electroweak processes are also computed, yielding about a factor of 30 smaller number of measurable top-quarks after cuts, per system and per year.

Measurement of Hadronic Cross Sections with ISR/Two-Photon Processes at Belle

We present cross section measurements in two-photon and ISR processes at the Belle experiment. Measured differential and integrated cross sections in two-photon processes are compared with perturbative and non-perturbative QCD calculations. We summarize ISR measurements at Belle and present recent updated analysis of e+e−→γISRπ+π−ψ(2S).