Electroweak Bosons Research Articles

Overview of ATLAS results

The heavy-ion programme in the ATLAS experiment at the Large Hadron Collider aims to probe and characterise hot and dense matter created in relativistic lead-lead collisions. Moreover, smaller collision systems involving nuclei and hadrons are of interest to disentangle initial- from final-state effects. This report presents new results based on lead-lead and proton-proton data collected at sNN=5.02 TeV in 2015, including measurements of bulk collectivity, charged-particle production, electroweak bosons, photon-jet correlations, and quarkonium suppression. First attempts to measure electromagnetic processes in ultra-peripheral collisions are also discussed.

KMR kt-factorization procedure for the description of the LHCb forward hadron–hadron Z0 production at [formula omitted

Quite recently, two sets of new experimental data from the LHCb and the CMS Collaborations have been published, concerning the production of the Z0 vector boson in hadron–hadron collisions with the center-of-mass energy ECM=s=13TeV. On the other hand, in our recent work, we have conducted a set of semi-NLO calculations for the production of the electro-weak gauge vector bosons, utilizing the unintegrated parton distribution functions (UPDF) in the frameworks of Kimber–Martin–Ryskin (KMR) or Martin–Ryskin–Watt (MRW) and the kt-factorization formalism, concluding that the results of the KMR scheme are arguably better in describing the existing experimental data, coming from D0, CDF, CMS and ATLAS Collaborations. In the present work, we intend to follow the same semi-NLO formalism and calculate the rate of the production of the Z0 vector boson, utilizing the UPDF of KMR within the dynamics of the recent data. It will be shown that our results are in good agreement with the new measurements of the LHCb and the CMS Collaborations.

Associated heavy quarks pair production with Higgs as a tool for a search for non-perturbative effects of the electroweak interaction at the LHC

Assuming an existence of the anomalous triple electro-weak bosons interaction being defined by coupling constant λ we calculate its contribution to interactions of the Higgs with pairs of heavy particles. Bearing in mind experimental restrictions −0.011<λ<0.011 we present results for possible effects in processes pp→W+W−H,pp→W+ZH,pp→W−ZH,pp→t¯tH, pp→b¯bH. Effects could be significant with negative sign of λ in associated heavy quarks t,b pairs production with the Higgs. In calculations we rely on results of the non-perturbative approach to a spontaneous generation of effective interactions, which defines the form-factor of the three-boson anomalous interaction.

Direct Photon Production at Next-to-Next-to-Leading Order.

We present the first calculation of direct photon production at next-to-next-to-leading order (NNLO) accuracy in QCD. For this process, although the final-state cuts mandate only the presence of a single electroweak boson, the underlying kinematics resembles that of a generic vector boson plus jet topology. In order to regulate the infrared singularities present at this order, we use the N-jettiness slicing procedure, applied for the first time to a final state that at the Born level includes colored partons but no required jet. We compare our predictions to ATLAS 8TeV data and find that the inclusion of the NNLO terms in the perturbative expansion, supplemented by electroweak corrections, provides an excellent description of the data with greatly reduced theoretical uncertainties.

Open heavy-flavour and electroweak boson measurements via the (di-)muonic decay channel with ALICE at the LHC

Heavy flavours (charm and beauty) and electroweak bosons (W and Z) are produced in initial hard partonic scatterings. The former interact strongly with the medium formed in ultra-relativistic heavy-ion collisions throughout its evolution, thus making them well suited to investigate its properties. Furthermore, heavy-flavour measurements in proton-nucleus collisions can be used to investigate initial-state effects whereas in proton-proton (pp) collisions they are considered an important test for perturbative Quantum ChromoDynamics (pQCD) predictions. In addition, open heavy-flavour measurements in pp collisions are used as a reference for proton-lead (p–Pb) and lead-lead (Pb–Pb) collisions. On the other hand, electroweak bosons and their leptonic decay products only interact weakly with the QCD matter and thus are suitable probes to test the validity of binary-collision scaling of hard processes. Moreover, their measurements in p–Pb collisions could help to constrain nuclear parton distribution functions. The ALICE muon spectrometer allows the measurement of open heavy flavour, W- and Z-boson production at forward rapidity (−4.0 < η < −2.5) exploiting their (di)muonic decay channel. In this talk the results obtained with the LHC Run I data in pp, p–Pb and Pb–Pb collisions will be discussed and compared with theoretical predictions.

Dichroic subjettiness ratios to distinguish colour flows in boosted boson tagging

N-subjettiness ratios are in wide use for tagging heavy boosted objects, in particular the ratio of 2-subjettiness to 1-subjettiness for tagging boosted electroweak bosons. In this article we introduce a new, dichroic ratio, which uses different regions of a jet to determine the two subjettiness measures, emphasising the hard substructure for the 1-subjettiness and the full colour radiation pattern for the 2-subjettiness. Relative to existing N -subjettiness ratios, the dichroic extension, combined with SoftDrop (pre-)grooming, makes it possible to increase the ultimate signal significance by about 25% (for 2 TeV jets), or to reduce non-perturbative effects by a factor of 2−3 at 50% signal efficiency while maintaining comparable background rejection. We motivate the dichroic approach through the study of Lund diagrams, supplemented with resummed analytical calculations.

W+W−+3 -jet production at the Large Hadron Collider in next-to-leading-order QCD

We present next-to-leading order (NLO) QCD predictions to $W^+W^-$ production in association with up to three jets at hadron colliders. We include contributions from couplings of the $W$ bosons to light quarks as well as trilinear vector couplings. These processes are used in vector-boson coupling measurements, are background to Higgs signals and are needed to constrain many new physics scenarios. For the first time NLO QCD predictions are shown for electroweak di-vector boson production with three jets at a hadron collider. We show total and differential cross sections for the LHC with proton center-of-mass energies of 8 and 13 TeV. To perform the calculation we employ on-shell and unitarity methods implemented in the BlackHat library along with the SHERPA package. We have produced event files that can be accessed for future dedicated studies.

Measurements of properties of strong and electroweak forces with the ATLAS detector at the LHC

These proceedings summarise ATLAS measurements of properties of the strong and electroweak forces in proton-proton collisions at 7 and 8 TeV during Run-1 at the LHC. The specific measurements presented here are concerned with the gauge symmetries in the Standard Model and related parameters, starting with the production cross section of a W boson in association with jets, the forward-backward asymmetry in the production of Z/γ∗ decaying to electrons or muons and extraction of the effective weak mixing angle. Then a number of measurements specifically probing electroweak processes at very high energies are presented, including the first measurement of triple gauge boson production pprightarrow W(lnu )gamma gamma , the production of a leptonically decaying W boson in association with W or Z boson decaying to jets, electroweak production of two jets in association with a Z boson, and same-electric-charge diboson production in association with two jets providing the first evidence of electroweak vector boson scattering.

Vector boson production in joint resummation

We study the transverse momentum (QT ) distribution of an electro-weak vector boson produced via the Drell-Yan mechanism, in the context of joint resummation. This formalism allows for the simultaneous resummation of logarithmic contributions that are enhanced at small QT and at partonic threshold. We extend joint resummation to next-to-next-to leading logarithmic accuracy and we present resummed and matched results for three different phenomenological setups. In particular, we study the production of a Z boson at the Tevatron and at the Large Hadron Collider (LHC), as well as the production of a heavier Z′ at the LHC. We compare our findings to standard QT resummation, as well as to fixed-order perturbation theory. We find that joint resummation provides a moderate (but not flat) correction with respect to QT resummation and it leads to a reduction of the scale dependence of the results. However, our study also shows some limitations of this formalism. While the use of joint resummation for Z production at the Tevatron and Z′ production at the LHC appears to be justified, our implementation suffers from a stronger dependence on power corrections for processes which are further away from threshold, such as Z production at the LHC, for which we cannot claim an improvement over standard QT resummation.

Anomalous triple gauge couplings in the effective field theory approach at the LHC

We discuss how to perform consistent extractions of anomalous triple gauge couplings (aTGC) from electroweak boson pair production at the LHC in the Standard Model Effective Field Theory (SMEFT). After recasting recent ATLAS and CMS searches in pp → W Z(W W ) → ℓ′νℓ+ℓ−(νℓ) channels, we find that: (a) working consistently at order Λ−2 in the SMEFT expansion the existing aTGC bounds from Higgs and LEP-2 data are not improved, (b) the strong limits quoted by the experimental collaborations are due to the partial Λ−4 corrections (dimension-6 squared contributions). Using helicity selection rule arguments we are able to explain the suppression in some of the interference terms, and discuss conditions on New Physics (NP) models that can benefit from such LHC analyses. Furthermore, standard analyses assume implicitly a quite large NP scale, an assumption that can be relaxed by imposing cuts on the underlying scale of the process ( sqrt{widehat{s}} ). In practice, we find almost no correlation between sqrt{widehat{s}} and the experimentally accessible quantities, which complicates the SMEFT interpretation. Nevertheless, we provide a method to set (conservative) aTGC bounds in this situation, and recast the present searches accordingly. Finally, we introduce a simple NP model for aTGC to compare the bounds obtained directly in the model with those from the SMEFT analysis.

Sphalerons in composite and nonstandard Higgs models

After the discovery of the Higgs boson and the rather precise measurement of all electroweak boson's masses the local structure of the electroweak symmetry breaking potential is already quite well established. However, despite being a key ingredient to a fundamental understanding of the underlying mechanism of electroweak symmetry breaking, the global structure of the electroweak potential remains entirely unknown. The existence of sphalerons, unstable solutions of the classical action of motion that are interpolating between topologically distinct vacua, is a direct consequence of the Standard Model's $\mathrm{SU}(2)_L$ gauge group. Nevertheless, the sphaleron energy depends on the shape of the Higgs potential away from the minimum and can therefore be a litmus test for its global structure. Focusing on two scenarios, the minimal composite Higgs model $\mathrm{SO}(5)/\mathrm{SO}(4)$ or an elementary Higgs with a deformed electroweak potential, we calculate the change of the sphaleron energy compared to the Standard Model prediction. We find that the sphaleron energy would have to be measured to $\mathcal{O}(10)\%$ accuracy to exclude sizeable global deviations from the Standard Model Higgs potential. We further find that because of the periodicity of the scalar potential in composite Higgs models a second sphaleron branch with larger energy arises.

Top physics at the LHC ?

The status of top physics at the Large Hadron Collider is discussed, with particular attention on measurements of top production cross sections, including associate production of top quarks and electroweak bosons, and on measurements of the top mass.

Non-perturbative effects of the electroweak interaction at the LHC

Assuming an existence of the anomalous triple electro-weak bosons interaction being defined by coupling constant λ we calculate its contribution to interactions of the Higgs with pairs of heavy particles. Bearing in mind experimental restrictions -0.011 t, b pairs production with the Higgs. In calculations we rely on results of the non-perturbative approach to a spontaneous generation of effective interactions, which defines the form-factor of the three-boson anomalous interaction.

Performance of the ATLAS Hadronic Tile Calorimeter in Run-2 and its Upgrade for the High Luminosity LHC

The Tile Calorimeter (TileCal) of the ATLAS experiment at the LHC is the central hadronic calorimeter designed for energy reconstruction of hadrons, jets, tauparticles and missing transverse energy. TileCal is a scintillator-steel sampling calorimeter and it covers the region of pseudo-rapidity up to 1.7, with almost 10000 channels measuring energies ranging from ∼30 MeV to ∼2 TeV. Each stage of the signal production, from scintillation light to the signal reconstruction, is monitored and calibrated. The performance of the Tile calorimeter has been studied in-situ employing cosmic ray muons and a large sample of proton-proton collisions, acquired during the operations of the LHC. Prompt isolated muons of high momentum from electroweak bosons decays are employed to study the energy response of the calorimeter at the electromagnetic scale. The calorimeter response to hadronic particles is evaluated with a sample of isolated hadrons. The modelling of the response by the Monte Carlo simulation is discussed. The calorimeter timing calibration and resolutions are studied with a sample of multijets events. Results on the calorimeter operation and performance are presented, including the calibration, stability, absolute energy scale, uniformity and time resolution. TileCal performance satisfies the design requirements and has provided an essential contribution to physics results in ATLAS.The Large Hadron Collider (LHC) has envisaged a series of upgrades towards a High Luminosity LHC (HL-LHC), delivering five times the LHC nominal instantaneous luminosity. The ATLAS Phase II upgrade, in 2024, will accommodate the detector and data acquisition system for the HL-LHC. In particular, the Tile Calorimeter will undergo a major replacement of its on- and off-detector electronics. All signals will be digitised and then transferred directly to the off-detector electronics, where the signals will be reconstructed, stored, and sent to the first level of trigger at a rate of 40 MHz. This will provide better precision for the calorimeter signals used by the trigger system and will allow the development of more complex trigger algorithms. Changes to the electronics will also contribute to the reliability and redundancy of the system. Three different front-end options are presently being investigated for the upgrade. Results of extensive laboratory tests and with beams of the three options will be presented, as well as the latest results on the development of the power distribution and the off-detector electronics.

Recent heavy ion results from the ATLAS experiment

The ATLAS experiment at the LHC has undertaken a broad physics program to probe and characterize the hot nuclear matter created in relativistic lead-lead collisions. This overview presents recent results on production of electroweak bosons, charmonia, charged-particles, jets, and bulk particle collectivity from Pb+Pb and p +Pb collisions at 2.76 TeV and 5.02 TeV, respectively, as well as on electromagnetic processes in ultra-peripheral Pb+Pb collisions at 5.02 TeV.

Simulating V+jet processes in heavy ion collisions with JEWEL

Processes in which a jet recoils against an electroweak boson complement studies of jet quenching in heavy ion collisions at the LHC. As the boson does not interact strongly it escapes the dense medium unmodified and thus provides a more direct access to the hard scattering kinematics than can be obtained in di-jet events. First measurements of jet modification in these processes are now available from the LHC experiments and will improve greatly with better statistics in the future. We present an extension of JEWEL to boson-jet processes. JEWEL is a dynamical framework for jet evolution in a dense background based on perturbative QCD, that is in agreement with a large variety of jet observables. We also obtain a good description of the CMS and ATLAS data for y+jet and Z+jet processes at 2.76 TeV and 5.02 TeV.

Improved jet substructure methods: Y-splitter and variants with grooming

It has recently been demonstrated with Monte Carlo studies that combining the well-known Y-splitter and trimming techniques gives rise to important gains in the signal significance achievable for boosted electroweak boson tagging at high $p_t$. Here we carry out analytical calculations that explain these findings from first principles of QCD both for grooming via trimming and via the modified mass-drop tagger (mMDT). We also suggest modifications to Y-splitter itself, which result in great simplifications to the analytical results both for pure Y-splitter as well as its combination with general grooming methods. The modifications also lead to further performance gains, while making the results largely independent of choice of groomer. We discuss the implications of these findings in the broader context of optimal methods for boosted object studies at hadron colliders.

A brief review of measurements of electroweak bosons at the LHCb experiment in LHC Run 1

The LHCb experiment is one of four major experiments at the large hadron collider (LHC). Despite being designed for the study of beauty and charm particles, it has made important contributions in other areas, such as the production and decay of W and Z bosons. Such measurements can be used to study and constrain parton distribution functions (PDFs), as well as to test perturbative quantum chromodynamics (QCD) in hard scattering processes. The angular structure of Z boson decays to leptons can also be studied and used to measure the weak mixing angle. The phase–space probed by LHCb is particularly sensitive to this quantity, and the LHCb measurement using the dimuon final state is currently the most precise determination of [Formula: see text] at the LHC. LHCb measurements made using data collected during the first period of LHC operations (LHC Run 1) are discussed in this review. The paper also considers the potential impact of related future measurements.

Measurement of W- and Z-boson production in p—Pb collisions at √sNN = 5.02 TeV with ALICE at the LHC

In hadronic collisions, electroweak bosons (W and Z) are produced in initial hard scattering processes and they are not affected by the strong interaction. In proton-proton collisions, they have been suggested as standard candles for luminosity monitoring and their measurement can improve the evaluation of detector performances. In nucleus-nucleus and proton-nucleus collisions, electroweak bosons provide a check at first order of the validity of binary collision scaling, while small deviations allow studying the nuclear modifications of parton distribution functions. In ALICE, the production of W and Z bosons is measured via the contribution of W-boson decays to the inclusive pT-differential muon yield and the invariant mass of unlike-sign muon pairs from Z-boson decays, respectively, at large rapidities. This paper reports on the production cross sections of muons from W muonic decay and of Z boson in p- Pb collisions at √sNN = 5.02 TeV, which are compared to the theoretical calculations, and on the W-boson yields normalized to the average number of binary nucleon-nucleon collisions as a function of the event activity.

NLO production of W± and Z0 vector bosons via hadron collisions in the frameworks of Kimber-Martin-Ryskin and Martin-Ryskin-Watt unintegrated parton distribution functions

In a series of papers, we have investigated the compatibility of the $Kimber$-$Martin$-$Ryskin$ ($KMR$) and $Martin$-$Ryskin$-$Watt$ ($MRW$) $unintegrated$ parton distribution functions ($UPDF$) as well as the description of the experimental data on the proton structure functions. The present work is a sequel to that survey, via calculation of the transverse momentum distribution of the electro-weak gauge vector bosons in the $k_t$-factorization scheme, by the means of the $KMR$, the $LO\;MRW$ and the $NLO\;MRW$ $UPDF$, in the next-to leading order ($NLO$). To this end, we have calculated and aggregated the invariant amplitudes of the corresponding $involved$ diagrams in the $NLO$, and counted the individual contributions in different frameworks. The preparation process for the $UPDF$ utilizes the $PDF$ of $Martin$ et al, $MSTW2008-LO$, $MSTW2008-NLO$, $MMHT2014-LO$ and $MMHT2014-NLO$ as the inputs. Afterwards, the results have been analyzed against each other, as well as the existing experimental data. Our calculation show excellent agreement with the experiment data. It is however interesting to point-out that, the calculation using the $KMR$ framework illustrates a stronger agreement with the experimental data, despite the fact that the $LO\;MRW$ and the $NLO\;MRW$ formalisms employ a better theoretical description of the $DGLAP$ evolution equation. This is of course due to the use of the different implementation of the angular ordering constraint in the $KMR$ approach, in which automatically includes the re-summation of $ln({1/x})$, $BFKL$ logarithms, in the $LO$-$DGLAP$ evolution equation.