Lepton Charge Asymmetry Research Articles

W±-boson production in p–Pb collisions at sqrt{s_{textrm{NN}}} = 8.16 TeV and Pb–Pb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV

The production of the W± bosons measured in p–Pb collisions at a centre-of-mass energy per nucleon–nucleon collision sqrt{s_{textrm{NN}}} = 8.16 TeV and Pb–Pb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV with ALICE at the LHC is presented. The W± bosons are measured via their muonic decay channel, with the muon reconstructed in the pseudorapidity region −4 < {eta}_{textrm{lab}}^{mu } < −2.5 with transverse momentum {p}_{textrm{T}}^{mu } > 10 GeV/c. While in Pb–Pb collisions the measurements are performed in the forward (2.5 < {y}_{textrm{cms}}^{mu } < 4) rapidity region, in p–Pb collisions, where the centre-of-mass frame is boosted with respect to the laboratory frame, the measurements are performed in the backward (−4.46 < {y}_{textrm{cms}}^{mu } < −2.96) and forward (2.03 < {y}_{textrm{cms}}^{mu } < 3.53) rapidity regions. The W− and W+ production cross sections, lepton-charge asymmetry, and nuclear modification factors are evaluated as a function of the muon rapidity. In order to study the production as a function of the p–Pb collision centrality, the production cross sections of the W− and W+ bosons are combined and normalised to the average number of binary nucleon–nucleon collision 〈Ncoll〉. In Pb–Pb collisions, the same measurements are presented as a function of the collision centrality. Study of the binary scaling of the W±-boson cross sections in p–Pb and Pb–Pb collisions is also reported. The results are compared with perturbative QCD calculations, with and without nuclear modifications of the Parton Distribution Functions (PDFs), as well as with available data at the LHC. Significant deviations from the theory expectations are found in the two collision systems, indicating that the measurements can provide additional constraints for the determination of nuclear PDFs and in particular of the light-quark distributions.

Linear scaling of lepton charge asymmetry in W± production in ultra-relativistic nuclear collisions from PACIAE model

The lepton charge asymmetry in [Formula: see text] production in the nuclear collisions at [Formula: see text][Formula: see text]TeV is investigated with a parton and hadron cascade model PACIAE. Recently published ALICE and the ATLAS data of lepton charge asymmetry are well reproduced. An interesting linear scaling behavior is observed in the lepton charge asymmetry as a function of the collision system valence quark number asymmetry among different sizes of nuclear collision systems at [Formula: see text][Formula: see text]TeV. This linear scaling behavior may serve as an additional constraint on the PDF (nPDF) extractions.

Understanding PDF uncertainty in W boson mass measurements* *JG and DL is supported by the National Natural Science Foundation of China(11875189 and 11835005), as well as the Yangyang Development Fund. KX is supported by the U.S. Department of Energy(DE-SC0007914), U.S. National Science Foundation (PHY-2112829), and in part by the PITT PACC

We study the dependence of the transverse mass distribution of charged leptons and the missing energy on parton distributions (PDFs) adapted to W boson mass measurements at the CDF and ATLAS experiments. We compare the shape variations of the distribution induced by different PDFs and find that the spread of predictions from different PDF sets can be significantly larger than the PDF uncertainty predicted by a specific PDF set. We suggest analyzing the experimental data using up-to-date PDFs to gain a better understanding of the PDF uncertainties in W boson mass measurements. We also perform a series of Lagrange multiplier scans to identify the constraints on the transverse mass distribution imposed by individual data sets in the CT18 global analysis. In the case of the CDF measurement, the distribution is mostly sensitive to d-quark PDFs in the intermediate x region, which are largely constrained by DIS and Drell-Yan data on deuteron targets and Tevatron lepton charge asymmetry data.

EW radiative corrections to theory predictions of charge asymmetry for W-boson hadroproduction

The higher-order predictions of the lepton charge asymmetry (the W-boson charge asymmetry ) for the leptonic final state of the W-boson hadroproduction in proton–proton (pp) collisions are presented. The predictions from the state-of-the-art computations are reported for achieving adequate description of the by including next-to-leading order (NLO) electroweak (EW) radiative corrections in combination with next-to-NLO (NNLO) quantum chromodynamics (QCD) radiative corrections. The combined predictions NNLO QCD+NLO EW and NNLO QCD × NLO EW, based on standard additive and factorised combination prescriptions in turn, are provided in the fiducial phase space of the pseudorapidity of the decay lepton (of the rapidity of the W-boson), comprising both central and forward detector acceptance regions as η l ≤ 4.5 (y W ≤ 4.5). The inclusion of the NLO EW effects for the () constitutes additional input for the relative u- and d-quark densities in the proton, which is also of high importance in the domain of the high-precision studies. The predicted distributions are compared with the actual measurements by CERN Large Hadron Collider (LHC) experiments at 8 TeV pp collisions energies. The combined predictions for the () distributions are also provided in comparisons with the NNLO QCD predictions at both 13 TeV and 14 TeV energies. The impact of the NLO EW corrections for the () distributions is extensively assessed by means of relative correction factor analysis with respect to the NNLO QCD predictions, in addition to a detailed K-factor analysis with respect to the leading order (LO) accuracy. The predicted results show that the NLO EW effects have larger impact in the forward η l region of the contrary to the central η l region, and is sizable in some of the y W ranges of the . The paper suggests inclusion of the presented EW corrections at NLO to have explicit accounting for the EW effects for the () in phenomenological studies.

Enhancing the Large Hadron Collider sensitivity to charged and neutral broad resonances of new gauge sectors

In scenarios beyond the Standard Model (BSM) characterised by charged (W′) or neutral (Z′) massive gauge bosons with large width, resonant mass searches are not very effective, so that one has to exploit the tails of the mass distributions measured at the Large Hadron Collider (LHC). In this case, the LHC sensitivity to new physics signals is influenced significantly by systematic uncertainties associated with the Parton Distribution Functions (PDF), particularly in the valence quark sector relevant for the multi-TeV mass region. As a BSM framework featuring such conditions, we consider the 4-Dimensional Composite Higgs Model (4DCHM), in which multiple W′ and Z′ broad resonances are present, with strongly correlated properties. By using the QCD tool xFitter, we study the implications on W′ and Z′ searches in Drell-Yan (DY) lepton decay channels that follow from the reduction of PDF uncertainties obtained through combining high-statistics precision measurements of DY lepton-charge and forward-backward asymmetries. We find that the sensitivity to the BSM states is greatly increased with respect to the case of base PDF sets, thereby enabling one to set more stringent limits on (or indeed discover) such new particles, both independently and in correlated searches.

Impact of lepton p T threshold on charge asymmetry predictions for inclusive W boson production in pp collisions at 13 TeV

This paper presents the impact of the lepton transverse momentum threshold on the boson charge asymmetry predictions in perturbative QCD for the inclusive production in proton-proton ( ) collisions. The predictions are obtained at various low- thresholds 20, 25, 30, and 40 GeV in a fiducial region encompassing both the central and forward detector acceptances in terms of the lepton pseudorapidity . The predicted distributions for the lepton charge asymmetry, which is defined by ( ), at the next-to-next-to-leading order (NNLO) accuracy are compared with the CMS and LHCb data at 8 TeV center-of-mass collision energy. The 8 TeV predictions reproduce the data fairly well within the quoted uncertainties. The predictions from the CT14 parton distribution function (PDF) model are in a slightly better agreement with the data over the other PDF sets that are tested. The 13 TeV predictions using various thresholds are reported for and the charge asymmetries that are defined in terms of the differential cross sections in bins of the boson rapidity ( ) and transverse momentum ( ). The NNLO predictions for the , , and distributions are assessed to be in close correlation with the value. The and distributions are particularly shown to be more correlated at a higher threshold. The distributions are also reported from the merged predictions with improved accuracy by the inclusion of the next-to-next-to-next-to-leading logarithm (N3LL) corrections, i.e., at NNLO+N3LL. The predicted distributions from various thresholds represent a finer probe in terms of the capability to provide more constraints on the ratio of and quark distribution functions in the parton momentum fraction range .

Reduction of PDF uncertainty in the measurement of the weak mixing angle at the ATLAS experiment * *Supported by the National Natural Science Foundation of China (11721505, 11875245) and the U.S. National Science Foundation (PHY-2013791)

We investigate the parton distribution function (PDF) uncertainty in the measurement of the effective weak mixing angle at the CERN Large Hadron Collider (LHC). The PDF-induced uncertainty is large in proton-proton collisions at the LHC due to the dilution effect. The measurement of the Drell-Yan forward-backward asymmetry ( ) at the LHC can be used to reduce the PDF uncertainty in the measurement. However, when including the full mass range of lepton pairs in the data analysis, the correlation between the PDF updating procedure and the extraction leads to a sizable bias in the obtained value. From our studies, we find that the bias can be significantly reduced by removing Drell-Yan events with invariant mass around the Z-pole region, while most of the sensitivity in reducing the PDF uncertainty remains. Furthermore, the lepton charge asymmetry in the W boson events as a function of the rapidity of the charged leptons, , is known to be another observable which can be used to reduce the PDF uncertainty in the measurement. The constraint from is complementary to that from , and thus no bias affects the extraction. The studies are performed using the error PDF Updating Method Package (ePump), which is based on Hessian updating methods. In this article, the CT14HERA2 PDF set is used as an example.

CPT Symmetry Test at KLOE-2

The entanglement in the neutral kaon pairs produced at the DAΦNE ϕ-factory is a unique tool to test discrete symmetries and quantum coherence at the utmost sensitivity, in particular strongly motivating the experimental searches of possible CPT violating effects, which would unambiguously signal New Physics. KLOE and KLOE-2 data sets with about 2.4×1010 ϕ-meson produced represent the largest sample ever collected at the ϕ-meson peak. The lepton charge asymmetry measured in K S semileptonic decays with 1.7 fb−1 of KLOE data, improving the statistical uncertainty of present result by about a factor two, has been presented together with the test of CPT in transitions in ϕ → K S K L → π ± e ∓ ν, 3π 0 and K S K L → π + π −, π ± e ∓ ν decays.

Dark CP violation and gauged lepton or baryon number for electroweak baryogenesis

We explore the generation of the baryon asymmetry in an extension of the Standard Model where the lepton number is promoted to a $U(1)_\ell$ gauge symmetry with an associated $Z^\prime$ gauge boson. This is based on a novel electroweak baryogenesis mechanism first proposed by us in Ref. \cite{Carena:2018cjh}. Extra fermionic degrees of freedom - including a fermionic dark matter $\chi$ - are introduced in the dark sector for anomaly cancellation. Lepton number is spontaneously broken at high scale and the effective theory, containing the Standard Model, the $Z^\prime$, the fermionic dark matter, and an additional complex scalar field $S$, violates CP in the dark sector. The complex scalar field couples to the Higgs portal and is essential in enabling a strong first order phase transition. Dark CP violation is diffused in front of the bubble walls and creates a chiral asymmetry for $\chi$, which in turn creates a chemical potential for the Standard Model leptons. Weak sphalerons are then in charge of transforming the net lepton charge asymmetry into net baryon number. We explore the model phenomenology related to the leptophilic $Z^\prime$, the dark matter candidate, the Higgs boson and the additional scalar, as well as implications for electric dipole moments. We also discuss the case when baryon number $U(1)_B$ is promoted to a gauge symmetry, and discuss electroweak baryogenesis and its corresponding phenomenology.

Measurement of W^pm boson production in Pb+Pb collisions at sqrt{s_{mathrm{NN}}} = 5.02~text {Te}text {V} with the ATLAS detector

A measurement of W^pm boson production in Pb+Pb collisions at sqrt{s_mathrm {NN}} = 5.02~text {Te}text {V} is reported using data recorded by the ATLAS experiment at the LHC in 2015, corresponding to a total integrated luminosity of 0.49;mathrm {nb^{-1}}. The W^pm bosons are reconstructed in the electron or muon leptonic decay channels. Production yields of leptonically decaying W^pm bosons, normalised by the total number of minimum-bias events and the nuclear thickness function, are measured within a fiducial region defined by the detector acceptance and the main kinematic requirements. These normalised yields are measured separately for W^+ and W^- bosons, and are presented as a function of the absolute value of pseudorapidity of the charged lepton and of the collision centrality. The lepton charge asymmetry is also measured as a function of the absolute value of lepton pseudorapidity. In addition, nuclear modification factors are calculated using the W^pm boson production cross-sections measured in pp collisions. The results are compared with predictions based on next-to-leading-order calculations with CT14 parton distribution functions as well as with predictions obtained with the EPPS16 and nCTEQ15 nuclear parton distribution functions. No dependence of normalised production yields on centrality and a good agreement with predictions are observed for mid-central and central collisions. For peripheral collisions, the data agree with predictions within 1.7 (0.9) standard deviations for W^- (W^+) bosons.

PDF profiling using the forward-backward asymmetry in Neutral Current Drell-Yan production

Non-perturbative QCD effects from Parton Distribution Functions (PDFs) may be constrained by using high-statistics Large Hadron Collider (LHC) data. Drell-Yan (DY) measurements in the Charged Current (CC) case provide one of the primary means to do this, in the form of the lepton charge asymmetry. We investigate here the impact of measurements in Neutral Current (NC) DY data mapped onto the Forward-Backward Asymmetry (AFB) on PDF determinations, by using the open source fit platform xFitter. We demonstrate the potential impact of AFB data on PDF determinations and perform a thorough analysis of related uncertainties.

Discrete symmetries studies at KLOE-2

The KLOE and KLOE-2 experiments at the Laboratori Nazionali di Frascati (Italy) collected almost 8 fb−1 of integrated luminosity at the energy equal to the mass of ϕ meson. The excellent time resolution of the electromagnetic calorimeter and the very good accuracy on both momentum and vertex reconstruction of the tracking system allow to study discrete symmetries to the utmost precision as well as light meson spectroscopy, dark forces searches, hadronic cross-section measurements and studies of γγ-physics. CPT symmetry test with the lepton charge asymmetry measured in KS semileptonic decays with 1.7 fb−1 of KLOE data, tests of time reversal and CPT in transitions in $\phi \to {K_S}{K_L} \to \pi ev,\,3{\pi ^0}(2{\pi ^0})$ decays with newly acquired data with the KLOE-2 detector will be presented and discussed.

Inclusive W boson QCD predictions and lepton charge asymmetry in proton–proton collisions at = 14 TeV

Inclusive W ([Formula: see text]) boson QCD predictions and lepton charge asymmetry in proton–proton collisions at [Formula: see text] = 14 TeV is performed in this study. Total and fiducial cross section predictions are obtained up to next to next to leading order (NNLO) QCD corrections using Monte Carlo for FeMtobarn processes (MCFM) MC generator. To validate the predictions, a detailed comparison of NNLO QCD calculations with 8 TeV CMS results is performed. To discuss the advantage of the higher order QCD predictions on the scale uncertainty, a scale dependence study is presented based on the choice of renormalization (μR) and factorization (μF) scale variations. W boson – lepton charge asymmetry and differential cross section as a function of lepton pseudorapidity at [Formula: see text] = 14 TeV are further performed in 11 |η| regions.

Single lepton charge asymmetries in t {bar{t}} and t {bar{t}} gamma production at the LHC

We discuss lepton charge asymmetries in t {bar{t}} and t {bar{t}} gamma production at the LHC, which can be measured in the semileptonic decay channel t {bar{t}} rightarrow W^+ b , W^- {bar{b}} rightarrow ell ^+ nu b , q {bar{q}}' {bar{b}} (or the charge conjugate). Considering several variants of a new physics scenario with a light colour octet, it is seen that for t {bar{t}} these asymmetries may have a sensitivity competitive with the dilepton asymmetry already measured. For t {bar{t}} gamma the new leptonic asymmetries, as well as the t {bar{t}} charge asymmetry, will reach their full potential with the high luminosity LHC upgrade. These asymmetries can pinpoint deviations at the 3sigma -4sigma level for new physics scenarios where the charge asymmetries already measured in t {bar{t}} production agree within 1sigma .

Measurement of W and Z Boson Production in 5 TeV pp, p+Pb and Pb+Pb Collisions with the ATLAS Detector

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Quark shapes of most modern parton distribution functions

Parton distribution functions (PDFs) describe the internal structure of the proton and are necessary inputs to almost all theory predictions of hadron colliders. One way to do a precise measurement of a PDF is to measure the lepton charge asymmetry of W boson where u$\bar{d}\to W^{+}$ and $d\bar{u}\to W^{-}$. Therefore, u, d, $\bar{u}$, and $\bar{d}$ quark shapes of the most modern PDF models (NNPDF3.1, NNPDF3.0, CT14, MMHT2014, and HERAPDF2.0) are inspected using the APFEL online cluster in this paper. The ratio of $d$ to $u$ is further investigated since the charge asymmetry is sensitive to the value of $d$ over $u$ momentum distributions in the proton. $Q$ scale dependence of PDFs are further studied in a range from 1 to 100 GeV.

Vector-boson production in p + Pb collisions measured with ATLAS at the LHC

Electroweak–boson production processes (W, Z and photon) provide access to the earliest moments of heavy–ion collisions. Furthermore, because they do not undergo strong interactions, they are sensitive to the initial–state geometry of the collision and potentially the details of the nuclear parton distribution functions. ATLAS results on vector–boson yields have demonstrated binary collision scaling in Pb+Pb collisions. In p + Pb collisions, the measurement of vector bosons provides possible constraints on the nuclear parton distribution functions and insights into the details of the initial collision geometry. We report on the latest results of vector–boson production in p + Pb collisions at sNN=5.02 TeV. Production yields of Z and W bosons and lepton charge asymmetry of W bosons are presented as a function of pseudo-rapidity and centrality. The vector–boson yields are compared to calculations incorporating different parton distribution functions, as well as different centrality calculations.

Scaling properties of inclusive W[Formula: see text] production at hadron colliders.

We consider the hadroproduction of W gauge bosons in their leptonic decay mode. Starting from the leading-order expressions, we show that by defining a suitable scaling variable the centre-of-mass dependence of the cross sections at the LHC energies can be essentially described by a simple power law. The scaling exponent is directly linked to the small-x behaviour of parton distribution functions (PDF) which, at the high virtualities involved in W production, is largely dictated by QCD evolution equations. This entails a particularly simple scaling law for the lepton charge asymmetry and also predicts that measurements in different collision systems (p–p, p–{overline{mathrm{p}}}, p–Pb Pb–Pb) are straightforwardly related. The expectations are compared with the existing data and a very good overall agreement is observed. It is shown that the PDF uncertainty in certain cross-section ratios between nearby centre-of-mass energies can be significantly reduced by taking the ratios at fixed value of the scaling variable instead of fixed rapidity.

Signatures of top flavour-changing dark matter

We develop the phenomenology of scenarios in which a dark matter candidate interacts with a top quark through flavour-changing couplings, employing a simplified dark matter model with an s-channel vector-like mediator. We study in detail the top-charm flavour-changing interaction, by investigating the single top plus large missing energy signature at the LHC as well as constraints from the relic density and direct and indirect dark matter detection experiments. We present strategies to distinguish between the top-charm and top-up flavour-changing models by taking advantage of the lepton charge asymmetry as well as by using charm-tagging techniques on an extra jet. We also show the complementarity between the LHC and canonical dark matter experiments in exploring the viable parameter space of the models.

High Order QCD Predictions for Inclusive Production ofWBosons inppCollisions ats=13 TeV

Predictions of fiducial cross sections, differential cross sections, and lepton charge asymmetry are presented for the production ofW±bosons with leptonic decay up to next-to-next-to-leading order (NNLO) in perturbative QCD. Differential cross sections ofW±bosons andWboson lepton charge asymmetry are computed as a function of lepton pseudorapidity for a defined fiducial region inppcollisions ats=13 TeV. Numerical results of fiducialW±cross section predictions are presented with the latest modern PDF models at next-to-leading order (NLO) and NNLO. It is found that the CT14 and NNPDF 3.0 predictions with NNLO QCD corrections are about 4% higher than the NLO CT14 and NNPDF 3.0 predictions while MMHT 2014 predictions with NLO QCD corrections are smaller than its NNLO QCD predictions by approximately 6%. In addition, the NNLO QCD corrections reduce the scale variation uncertainty on the cross section by a factor of 3.5. The prediction of central values and considered uncertainties are obtained using FEWZ 3.1 program.