Low Transverse Momentum Research Articles

Measurement of double-parton scattering in inclusive production of four jets with low transverse momentum in proton-proton collisions at sqrt{s} = 13 TeV

A measurement of inclusive four-jet production in proton-proton collisions at a center-of-mass energy of 13 TeV is presented. The transverse momenta of jets within |η| < 4.7 are required to exceed 35, 30, 25, and 20 GeV for the first-, second-, third-, and fourth-leading jet, respectively. Differential cross sections are measured as functions of the jet transverse momentum, jet pseudorapidity, and several other observables that describe the angular correlations between the jets. The measured distributions show sensitivity to different aspects of the underlying event, parton shower modeling, and matrix element calculations. In particular, the interplay between angular correlations caused by parton shower and double-parton scattering contributions is shown to be important. The double-parton scattering contribution is extracted by means of a template fit to the data, using distributions for single-parton scattering obtained from Monte Carlo event generators and a double-parton scattering distribution constructed from inclusive single-jet events in data. The effective double-parton scattering cross section is calculated and discussed in view of previous measurements and of its dependence on the models used to describe the single- parton scattering background.

Recent milestones from STAR: new developments and open questions

In these proceedings, an overview of recent STAR results on selected topics is presented. These results utilize Au+Au collisions at various energies, and are aimed at understanding the properties of QED and QCD, characterizing the quark-gluon plasma, as well as searching for the possible critical point in the QCD phase diagram. Specifically, following measurements are discussed: global hyperon polarization, net-proton fluctuations, low transverse momentum dimuon pair production, hyperon-baryon correlations, and f0(980) elliptic flow.

Spin alignment measurement of vector mesons produced in high energy collisions

This paper covers the recent experimental development on spin alignment measurements of [Formula: see text] and [Formula: see text] vector mesons in heavy-ion and [Formula: see text] collisions at RHIC and LHC energies. Measurements in [Formula: see text] collisions at LEP energies are also discussed. Spin alignment of vector mesons is studied by measuring the second diagonal element [Formula: see text] of spin density matrix. The spin density matrix element [Formula: see text] is obtained by measuring the angular distribution of vector meson decay daughter with respect to the quantization axis in vector meson rest frame. Measured [Formula: see text] values for vector mesons are found to be larger than [Formula: see text] at high momentum in [Formula: see text] collisions at LEP energies, suggesting the preferential production of vector meson with helicity zero state from the fragmentation process. The [Formula: see text] values are found to be smaller than [Formula: see text] ([Formula: see text] implies no spin alignment) for [Formula: see text] and [Formula: see text] vector mesons at low transverse momentum in Pb–Pb collisions at [Formula: see text] TeV. These observations are qualitatively consistent with the expectation from models which attribute the spin alignment effect due to polarization of quarks in the presence of large initial angular momentum in noncentral heavy-ion collisions and its subsequent hadronization by the process of recombination. No significant spin alignment effect is observed for [Formula: see text] [Formula: see text] in mid-central Pb–Pb collisions and for vector mesons in [Formula: see text] collisions. However, the preliminary results of [Formula: see text] for [Formula: see text] mesons are larger than [Formula: see text] at intermediate [Formula: see text] in Au–Au collisions at RHIC energies and can be attributed to the presence of [Formula: see text] meson field. Although there is evidence of spin alignment effect of vector mesons in heavy-ion collisions but the measured effect is surprisingly larger in context of hyperon polarization. Therefore, these results will trigger further theoretical study.

Effect of thermal gluon absorption and medium fluctuations on heavy flavour nuclear modification factor at RHIC and LHC energies

The presence of thermal gluons reduces the stimulated gluon emission off a heavy quark propagating in the quark-gluon plasma (QGP) while absorption causes reduction of the radiative energy loss. On the other hand, the chromo-electromagnetic field fluctuations present in the QGP lead to collisional energy gain of the heavy quark. The net effect of the thermal gluon absorption and field fluctuations is a reduction of the total energy loss of the heavy quark, prominent at the lower momenta. We consider both kind of the energy gains along with the usual losses, and compute the nuclear modification factor ($R_{AA}$) of heavy mesons, viz., $D$ and $B$ mesons. The calculations have been compared with the experimental measurements in Au-Au collisions at $\sqrt{s_{NN}} = 200$ GeV from STAR and PHENIX experiments at the RHIC and Pb-Pb collisions at $\sqrt{s_{NN}} = 2.76$ TeV and $5.02$ TeV from CMS and ALICE experiments at the LHC. We find a significant effect of the total energy gain due to thermal gluon absorption and field fluctuations on heavy flavour suppression, especially at the lower transverse momenta.

Measurement of differential tt¯ production cross sections in the full kinematic range using lepton+jets events from proton-proton collisions at s=13 TeV

Measurements of differential and double-differential cross sections of top quark pair ($\text{t}\overline{\text{t}}$) production are presented in the lepton+jets channels with a single electron or muon and jets in the final state. The analysis combines for the first time signatures of top quarks with low transverse momentum $p_\text{T}$, where the top quark decay products can be identified as separated jets and isolated leptons, and with high $p_\text{T}$, where the decay products are collimated and overlap. The measurements are based on proton-proton collision data at $\sqrt{s} = $ 13 TeV collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 137 fb$^{-1}$. The cross sections are presented at the parton and particle levels, where the latter minimizes extrapolations based on theoretical assumptions. Most of the measured differential cross sections are well described by standard model predictions with the exception of some double-differential distributions. The inclusive $\text{t}\overline{\text{t}}$ production cross section is measured to be $\sigma_{\text{t}\overline{\text{t}}} = $ 791 $\pm$ 25 pb, which constitutes the most precise measurement in the lepton+jets channel to date.

Average minijet rapidity ratios in Mueller\u2013Navelet jets

We investigate different final state features in Mueller–Navelet jets events at hadron colliders. The focus lies on the average rapidity ratio between subsequent minijet emissions which has been investigated in previous works but now is modified to also incorporate the transverse momenta together with the rapidities of the emitted jets. We study the dependence of this observable on a lower transverse momentum veto which does affect the typical minijet multiplicity of the events under scrutiny. We find that this observable is stable when including higher order quantum corrections, also when collinear terms are resummed to all orders.

On improving NLO merging for mathrm{t}overline{mathrm{t}}mathrm{W} production

We introduce an improvement to the FxFx matrix element merging procedure for pp → toverline{t}W production at NLO in QCD with one and/or two additional jets. The main modification is an improved treatment of jets that are not logarithmically enhanced in the low transverse-momentum regime. We provide predictions for the inclusive cross section and the toverline{t}W differential distributions including parton-shower effects. Taking also the NLO EW corrections into account, this results in the most-accurate predictions for this process to date. We further proceed to include the on-shell LO decays of the toverline{t}W including the tree-level spin correlations within the narrow-width approximation, focusing on the multi-lepton signatures studied at the LHC. We find a ∼30% increase over the NLO QCD prediction and large non-flat K-factors to differential distributions.

Measurement of the production cross section of pairs of isolated photons in pp collisions at 13 TeV with the ATLAS detector

A measurement of prompt photon-pair production in proton-proton collisions at sqrt{s} = 13 TeV is presented. The data were recorded by the ATLAS detector at the LHC with an integrated luminosity of 139 fb−1. Events with two photons in the well-instrumented region of the detector are selected. The photons are required to be isolated and have a transverse momentum of {p}_{mathrm{T}{,}_{gamma 1(2)}} > 40 (30) GeV for the leading (sub-leading) photon. The differential cross sections as functions of several observables for the diphoton system are measured and compared with theoretical predictions from state-of-the-art Monte Carlo and fixed-order calculations. The QCD predictions from next-to-next-to-leading-order calculations and multi-leg merged calculations are able to describe the measured integrated and differential cross sections within uncertainties, whereas lower-order calculations show significant deviations, demonstrating that higher-order perturbative QCD corrections are crucial for this process. The resummed predictions with parton showers additionally provide an excellent description of the low transverse-momentum regime of the diphoton system.

Bremsstrahlung photons from stopping in heavy-ion collisions

We examine the spectrum of bremsstrahlung photons that results from the stopping of the initial net charge distributions in ultrarelativistic nucleus-nucleus collisions at the CERN Large Hadron Collier (LHC). This effect has escaped detection so far since it becomes sizable only at very low transverse momentum and at sufficiently forward rapidity. We argue that it may be within reach of the next-generation LHC heavy-ion detector ALICE-3 that is currently under study, and we comment on the physics motivation for measuring it.

Analysis of Drell-Yan lepton pair production in the p−p(p¯) colliders using different angular ordering constraints and the kt -factorization approach

In this work, the $p\ensuremath{-}p(\overline{p})$ Drell-Yan lepton pair production (DY) differential cross sections at hadron colliders, such as LHC and TEVATRON, are studied in the ${k}_{t}$-factorization framework. In order to take into account the transverse momenta of incoming partons, we use the unintegrated parton distribution functions of Kimber et al. (KMR) and Martin et al. (MRW) in the leading order (LO), and next-to-leading-order (NLO) levels with the input MMHT2014 PDF libraries. Based on the different off shell partonic matrix elements, we analyze the behaviors of DY differential cross sections with respect to the invariant mass, the transverse momentum and the rapidity as well as the specific angular correlation between the produced leptons. The numerical results are compared with the experimental data, in different energies, which are reported by various collaborations, such as CDF, CMS, ATLAS, and LHCb. It is shown that the NLO-MRW and KMR schemes predict closer results to the data compared to the LO-MRW, since we do not have fragmentation. It is demonstrated that while the ${q}^{*}+{\overline{q}}^{*}\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}/Z+g\ensuremath{\rightarrow}{l}^{+}+{l}^{\ensuremath{-}}+g$ subprocess has a negligible contribution, it has a sizable effect in the low dilepton transverse momentum. In agreement with the NNLO, perturbative quantum chromodynamics (pQCD), report (pythia, sherpa, etc.) by including the higher-order perturbative contributions the better results are archived. On the other hand as the scale of energy increases, for the LHC energies, the Compton subprocess, i.e., ${q}^{*}+{g}^{*}\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}/Z\ensuremath{\rightarrow}{l}^{+}+{l}^{\ensuremath{-}}+q$, has the largest contribution to the differential cross section in the most intervals of some observables, as is expected. The variation of the differential cross section with respect to the various variables such as the invariant mass, the center of mass energy, etc. are discussed. In order to validate our results, we also consider the strong ordering constraint and the KaTie parton-level event generator.

Study of transverse momentum and nuclear modification factors distribution of charged particles produced in pp and Pb–Pb collisions at s NN = 2.76TeV and 5.02TeV

We have studied transverse momentum distributions of charged particles produced in pp and Pb–Pb collisions at [Formula: see text] TeV and 5.02 TeV in the pseudorapidity interval [Formula: see text] and transverse momentum range [Formula: see text][Formula: see text]GeV/[Formula: see text]. We simulated data using EPOS-LHC, EPOS-1.99 and QGSJETII-04 models. The simulation data is compared with the ALICE experimental data values at [Formula: see text] TeV and 5.02 TeV for pp and most central Pb–Pb collisions. It has been observed that, EPOS-LHC and QGSJETII-04 models explain the experimental results for pp collision at [Formula: see text] TeV and 5.02 TeV. The behavior of nuclear modification factors has been studied. The simulation codes of all three models EPOS-LHC, EPOS-1.99 and QGSJETII-04 overestimate the experimental results at low transverse momentum interval: [Formula: see text] GeV/[Formula: see text], for Pb–Pb collisions at [Formula: see text] TeV and 5.02 TeV. However, only EPOS-LHC model can explain the experimental data at high transverse momentum in the range: [Formula: see text] GeV/[Formula: see text]. EPOS-1.99 and QGSJETII-04 underestimate in the region of Cronin effect and cannot give satisfactory estimates for the [Formula: see text] values for which [Formula: see text] demonstrates stronger suppression because of the collective parton effect. It can be inferred that these effects are not taken into account in EPOS-1.99 and QGSJETII-04 models. These models, however, satisfactorily explain the ALICE experimental data in the ranges of [Formula: see text] for which nuclear modification factor [Formula: see text] shows rising trend.

Soft-Dielectron Excess in Proton-Proton Collisions at s=13 TeV

A measurement of dielectron production in proton-proton (pp) collisions at $\sqrt{s} = 13$ TeV, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron invariant mass $m_{\rm ee}$ and pair transverse momentum $p_{\rm T,ee}$ that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of $m_{\rm ee}$, $p_{\rm T,ee}$, and event multiplicity ${\rm d} N_{\rm ch}/{\rm d} \eta$. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured $\eta/\pi^0$ ratio in pp and proton-nucleus (p-A) collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at $0.15<m_{\rm ee}<0.6$ GeV/$c^2$ and for $p_{\rm T,ee} < 0.4$ GeV/$c$ indicates an enhancement of soft dielectrons, reminiscent of the 'anomalous' soft-photon and -dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to $1.61\pm 0.13\,(\rm{stat.})\pm 0.17\,(\rm{syst., data}) \pm 0.34\,(\rm{syst., cocktail})$ in the ALICE acceptance. Acceptance-corrected excess spectra in $m_{\rm ee}$ and $p_{\rm T,ee}$ are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach.

Saturation effects in SIDIS at very forward rapidities

Using the dipole picture for electron-nucleus deep inelastic scattering at small Bjorken x, we study the effects of gluon saturation in the nuclear target on the cross-section for SIDIS (single inclusive hadron, or jet, production). We argue that the sensitivity of this process to gluon saturation can be enhanced by tagging on a hadron (or jet) which carries a large fraction z ≃ 1 of the longitudinal momentum of the virtual photon. This opens the possibility to study gluon saturation in relatively hard processes, where the virtuality Q2 is (much) larger than the target saturation momentum {Q}_s^2 , but such that z(1 − z)Q2 ≲ {Q}_s^2 . Working in the limit z(1 − z)Q2 ≪ {Q}_s^2 , we predict new phenomena which would signal saturation in the SIDIS cross-section. For sufficiently low transverse momenta k⊥ ≪ Qs of the produced particle, the dominant contribution comes from elastic scattering in the black disk limit, which exposes the unintegrated quark distribution in the virtual photon. For larger momenta k⊥ ≳ Qs, inelastic collisions take the leading role. They explore gluon saturation via multiple scattering, leading to a Gaussian distribution in k⊥ centred around Qs. When z(1 − z)Q2 ≪ Q2, this results in a Cronin peak in the nuclear modification factor (the RpA ratio) at moderate values of x. With decreasing x, this peak is washed out by the high-energy evolution and replaced by nuclear suppression (RpA< 1) up to large momenta k⊥ ≫ Qs. Still for z(1 − z)Q2 ≪ {Q}_s^2 , we also compute SIDIS cross-sections integrated over k⊥. We find that both elastic and inelastic scattering are controlled by the black disk limit, so they yield similar contributions, of zeroth order in the QCD coupling.

Exploring the non-perturbative Sudakov factor via Z0-boson production in pp collisions

Z0-boson production at low transverse momentum offers a unique opportunity to explore the non-perturbative Sudakov factor. In this paper, we employ three parameterizations of the non-perturbative Sudakov factor to calculate the ϕ⁎-distribution and compare our results with the ultra-precise experimental data. We extract the free parameters in each parameterization with a χ2 analysis. The parton-momentum-fraction dependence of these free parameters is also studied by comparing the values extracted at different collision energies and different rapidity ranges.

W-boson production in TMD factorization

At hadron colliders, the differential cross section for W production can be factorized and it is sensitive transverse momentum dependent distributions (TMD) for low boson transverse momentum. While, often, the corresponding non-perturbative QCD contributions are extrapolated from Z boson production, here we use an existing extraction (based on the code Artemide) of TMD which includes data coming from Drell–Yan and semi-inclusive deep inelastic scattering, to provide checks and predictions for the W case. Including fiducial cuts with different configurations and kinematical power corrections, we consider transverse momentum dependent cross sections within several intervals of the vector boson transverse mass. We perform the same study for the p_T^{W^-}/p_T^{W^+} and p_T^Z/p_T^W distributions. We compare our predictions with recent extractions of these quantities at ATLAS and CMS and results from TeVatron. The results encourage a broader experimental and phenomenological work, and a deeper study of TMD for the W case.

Coulomb nuclear interference effect in dipion production in ultraperipheral heavy ion collisions

We study exclusive $\pi^+ \pi^-$ pair production near $\rho^0$ resonance peak in ultraperipheral heavy ion collisions. Pion pair can either be produced via two photon fusion process or from the decay of $\rho^0$ in photon-nuclear reaction. At very low pair transverse momentum, the electromagnetic and nuclear amplitudes become comparable. We show that the Coulomb nuclear interference amplitude gives rise to sizable $\cos \phi$ and $\cos 3\phi$ azimuthal asymmetries, which can be used to constrain the phase of the dipole-nucleus scattering amplitude.

Towards a two-dimensional readout of the improved CMS Resistive Plate Chamber with a new front-end electronics

As part of the Compact Muon Solenoid experiment Phase-II upgrade program, new resistive plate chambers will be installed in the region at low angle with respect to the beam collision axis, in order to improve the detection of muons with a low transverse momentum. High background conditions are expected in this region during the high-luminosity phase of the Large Hadron Collider, therefore an improved-RPC design has been proposed with a new front-end electronics to sustain a higher particle rate capability and better time resolution. A new technology is used in the front-end electronics resulting in low achievable signal detection of 1–20 fC. Crucial in the design of the improved-RPC is the capability of a two-dimensional readout in order to improve the spatial resolution, mainly motivated by trigger requirements. In this work, the first performance results towards this two-dimensional readout are presented, based on data taken on a real-size prototype chamber with two embedded readout planes with orthogonal strips.

The anomalous production of multi-leptons and its impact on the measurement of Wh production at the LHC

Anomalies in multi-lepton final states at the Large Hadron Collider (LHC) have been reported in Refs. (von Buddenbrock et al., J Phys G 45(11):115003, arXiv:1711.07874 [hep-ph], 2018; Buddenbrock et al., JHEP 1910:157, arXiv:1901.05300 [hep-ph], 2019). These can be interpreted in terms of the production of a heavy boson, H, decaying into a standard model (SM) Higgs boson, h, and a singlet scalar, S, which is treated as a SM Higgs-like boson. This process would naturally affect the measurement of the Wh signal strength at the LHC, where h is produced in association with leptons and di-jets. Here, h would be produced with lower transverse momentum, p_{Th}, compared to SM processes. Corners of the phase-space are fixed according to the model parameters derived in Refs. (von Buddenbrock et al., J Phys G 45(11):115003, arXiv:1711.07874 [hep-ph], 2018; von Buddenbrock et al., Eur Phys J C 76(10):580, arXiv:1606.01674 [hep-ph], 2016) without additional tuning, thus nullifying potential look-else-where effects or selection biases. Provided that no stringent requirements are made on p_{Th} or related observables, the signal strength of Wh is mu (Wh)=2.41 pm 0.37. This corresponds to a deviation from the SM of 3.8sigma . This result further strengthens the need to measure with precision the SM Higgs boson couplings in e^+e^-, and e^-p collisions, in addition to pp collisions.

Elliptic Flow of Particles under the Influence of Electromagnetic Field Evolution in Relativistic Heavy Ion Collision

The bending of flow of identified particles from the relativistic heavy-ion collision is investigated using the iEBE-VISHNU framework. The Maxwell's equations are applied to compute the incremental drift velocity and the change in the elliptic flow of particles from the four sources of the electric force, which are of coulomb (EC), Lorentz (EL), Faraday (EF) and Plasma-based. We find out that the field evolution arouses flow at lower transverse momentum and suppresses it at higher. Heavier particles get higher initial push, and particles and their anti-particles get crudely the same elliptic flow changes. Moreover, elliptic flow is found to show different percentage increase for different collision energies. To conclude, the present study shows that besides the inclusion of electromagnetic fields, the increase in collision energy affects the elliptic flow of particles in a non-uniform fashion throughout the evolution. Further study by softening many of the crude assumptions we made and keeping the functionality of parameters is needed to establish a better understanding of the electromagnetic field evolution and its effects on the created system.

Associate production J/ψ + γ at the Large Hadron Collider and Electron–Ion Collider

AbstractCharmonium production associated with a photon in coherent hadron–hadron interactions at high‐energy colliders is a suitable observable to test the partonic density function in the proton and in nuclei. We employ nonrelativistic quantum chromodynamics to investigate the transverse momentum distribution and total cross sections of J/ψ + γ production at the Large Hadron Collider (LHC) and the electron‐ion collider (EIC). Our results show that discrimination can be made to test the parameters for the matrix elements operators, which varies by a factor of 3. A higher transverse momentum region is more sensitive for pPb collisions at the LHC, although the EIC kinematic region is also favorable at low transverse momentum.