Run-2 Data Research Articles

Current status on pair-produced muon-philic vectorlike leptons in multilepton channels at the LHC

In this work, we obtain the current limits on the pair production of vectorlike leptons decaying to a Standard Model gauge boson and a lepton in the second generation using the Run-2 data at the LHC. Since there is no dedicated search out of Run-2 data, we recast the ATLAS analyses searching for the type-III seesaw heavy leptons in the multi-lepton channels. There is no limit for the SU(2)L singlet vectorlike lepton beyond about 100 GeV, while the limit is about 780 GeV for the doublet one. Thus, dedicated searches for the vectorlike leptons are necessary, especially for the singlet one. We also study the general cases of the vectorlike lepton decays and future sensitivities at the HL-LHC.

Searches for lepton-flavour violating decays of the Higgs boson into eτ and μτ in √(s) = 13 TeV pp collisions with the ATLAS detector

Direct searches for lepton-flavour-violating decays of the Higgs boson into eτ and μτ final states with the ATLAS detector at the LHC with Run-2 data are presented. Both leptonically and hadronically decaying τ-leptons are included and two different background estimation techniques are employed: a MC-template method, based on data-corrected simulation samples, and a data-driven method, based on exploiting the symmetry between electrons and muons in the Standard Model backgrounds. Observed (Expected) upper limits are set on the branching ratios at 95 % confidence level, ℬ(H → eτ)<0.20 % (0.12 %) and ℬ(H → μτ)<0.18% (0.09 %), and a best-fit branching ratio difference, ℬ(H → μτ) - ℬ(H → eτ), of 0.25 ± 0.10 % is found in the channel where the τ-lepton decays to leptons, compatible with a value of zero within 2.5 σ.

Prospects of gluino searches in multi-lepton channels in light of the ongoing LHC RUN-III

This study investigates the prospect of discovering strongly interacting gluinos in different multi-lepton channels with lepton multiplicities greater than or equal to 2 at LHC RUN-III, considering several pMSSM scenarios. The effectiveness of the multivariate analysis (MVA) method with the boosted decision tree (BDT) algorithm is explored to obtain a better significance for different models. Promising results are obtained for the 3-lepton channels, indicating that the use of MVA methods can improve the sensitivity of the search for gluinos at LHC RUN-III. The study probes the multi-lepton signatures arising from gluinos via intermediate eweakinos and sleptons at an early stage of the LHC RUN-III. The heavier eweakinos can give rise to three or four lepton signals, in which the squark hierarchy between the L and R types plays a crucial role. The study considers two sets of benchmark points that satisfy all the collider constraints obtained from the LHC RUN-II data. Moreover, these sets of benchmark points are mostly consistent with WMAP/PLANCK data and the muon (g-2) constraint. The corresponding results from the MVA technique demonstrate that, even for an integrated luminosity of 270 , the 5 σ discovery prospect of for TeV in the wino type model is promising. The study also presents various other models that may emerge at the early stage of LHC RUN-III. Wino type models in the scenario where left squarks are light and right squarks are heavy exhibit the best prospect of discovering gluinos in multi-lepton channels in the LHC RUN-III experiment. The findings of this study provide crucial insights into the potential discovery of gluinos in multi-lepton channels.

Drell-Yan tails beyond the Standard Model

We investigate the high-pT tails of the pp → ℓν and pp → ℓℓ Drell-Yan processes as probes of New Physics in semileptonic interactions with an arbitrary flavor structure. For this purpose, we provide a general decomposition of the 2 → 2 scattering amplitudes in terms of form-factors that we match to specific scenarios, such as the Standard Model Effective Field Theory (SMEFT), including all relevant operators up to dimension-8, as well as ultraviolet scenarios giving rise to tree-level exchange of new bosonic mediators with masses at the TeV scale. By using the latest LHC run-II data in the monolepton (eν, μν, τν) and dilepton (ee, μμ, ττ, eμ, eτ, μτ) production channels, we derive constraints on the SMEFT Wilson coefficients for semileptonic four-fermion and dipole operators with the most general flavor structure, as well as on all possible leptoquark models. For the SMEFT, we discuss the range of validity of the EFT description, the relevance of mathcal{O} (1/Λ2) and mathcal{O} (1/Λ4) truncations, the impact of d = 8 operators and the effects of different quark-flavor alignments. Finally, as a highlight, we extract for several New Physics scenarios the combined limits from high-pT processes, electroweak pole measurements and low-energy flavor data for the b → cτν transition, showing the complementarity between these different observables. Our results are compiled in HighPT (https://highpt.github.io), a package in Mathematica which provides a simple way for users to extract the Drell-Yan tails likelihoods for semileptonic effective operators and for leptoquark models.

Machine learning predictor for ‘measurement-to-track’ association for the ATLAS Inner Detector trigger

The track finding algorithm adopted for the LHC Run-2 data taking period is based on combinatorial track following, where the number of seeds scales non-linearly with the number of hits. The corresponding CPU time increase, close to cubical, creates huge and ever-increasing demand for computing power. This is particularly problematic for the silicon tracking detectors, where the hit occupancy is the largest. Therefore, it is essential that resource use is reduced, whilst maintaining the ability to reconstruct tracks with minimal loss in efficiency. This paper briefly summarises the work that has been done to optimise the HLT ID track seeding software for ATLAS Run-3 and beyond, in order to reduce the number of fake seeds constructed. An ML-based algorithm has been developed to predict if a pair of hits belong to the same track given input hit features, focusing on cluster width and inverse track inclination. The implementation of the trained predictor in the form of Look-Up Tables is presented, along with performance results in terms of tracking efficiency and speed-up factor using simulated data.

Status of the Muon g − 2 experiment

The Muon g−2 Experiment at Fermi National Accelerator Laboratory was designed to measure the anomalous magnetic moment of the muon, aµ, with a precision of 140 parts-per-billion; a four-fold improvement over the former BNL measurement. The Fermilab experiment was motivated by the about 3.5 standard deviation between the experiment and the Standard Model calculation of aµ which could be a hint of new physics. The experiment at Fermilab relies on the well-established storage ring technique using magic momentum muons but employs new detector systems and a higher rate of muons per injection cycle to achieve the significant improvement in precision. A first result from the Run-1 data taking period has achieved an uncertainty of 0.46 parts-per-million and confirmed the BNL discrepancy, further increasing the tension with the Standard Model to 4.2 σ. The experimental technique, key aspects of the measurement, and the data analysis of Run-1 will be summarized.

CMS RPC data taking during the LHC Run-2 and activities during Long Shutdown 2

The CMS experiment collected around 150 fb−1 of proton-proton collision data at TeV during the Run-2 data taking period of LHC. The CMS RPC system provided redundant information for robust muon triggering, reconstruction and identification. To ensure stable data taking, the CMS RPC collaboration has performed detector operation, calibration and performance studies. After the end of Run-2, it was started the second LHC long shutdown period (LS2), an important opportunity for maintenance and preparation for the next data taking period (Run-3) and the installation of services in preparation for the Phase-II upgrade. The activities included maintenance of power, gas and online systems. In this presentation, the overall performance of the CMS RPC system during the Run-2 period is summarized as well as all the activities done in preparation for future data taking periods.

Leptonic cascade decays of a heavy Higgs boson through vectorlike leptons at the LHC

We demonstrate the potential of fully leptonic cascade decays of a heavy neutral Higgs boson through vectorlike leptons as a simultaneous probe for extended Higgs sectors and extra matter particles at the LHC. The processes we explore are unique in that their event topologies lead to di-boson-like leptonic final states with a lepton pair which does not reconstruct the mass of a gauge boson. By recasting existing 2ell +{E}_R^{textrm{miss}} and 3/4ℓ searches channels using run2 data from the LHC we obtain model independent bounds on the masses of heavy scalars and vectorlike leptons and use these results to explore future prospects at the HL-LHC. Our results can be directly applied to any kind of new physics scenarios sharing the final states and the event topology. For concreteness, we apply our results to a benchmark scenario: a two Higgs doublet model type-II augmented with vectorlike leptons. Remarkably, even with current data the sensitivity of our analysis shows a reach for masses of a heavy neutral Higgs and vectorlike leptons up to 2 TeV and 1.5 TeV, respectively. Even for low tan β ≳ 1, the analysis retains sensitivity to heavy Higgs masses slightly above 1 TeV. The future sensitivities at the HL-LHC extend the reach for heavy Higgses and new leptons to 2.7 TeV and 2 TeV, respectively.

Searches for supersymmetry in CMS

The results from the CMS search for supersymmetric particles based on Run-2 data recorded at a center-of-mass energy of 13[Formula: see text]TeV are summarized. Strong and weak production of SUSY scenarios are considered. Results presented include the searches for squarks and gluinos, direct production of charginos, neutralinos and sleptons. These searches involve final state objects comprising jets, missing transverse momentum, electrons or muons, taus or photons, as well as long-lived particles. The data in these searches are found to be consistent with Standard Model predictions and no significant excess is observed. Upper limits have been set on the masses of supersymmetric particles from a variety of search channels.

Measurements of Higgs properties from CMS

<p indent=0mm>The Large Hadron Collider (LHC) exhibits the highest colliding energy in the world. The compact muon solenoid (CMS) is one of the main experiments conducted on the LHC, and its objective is to discover the Higgs boson and new particles. In this report, the discovery of the Higgs Boson and the measurements of Higgs properties using CMS RUN2 data were briefly presented. Higgs production modes—ggH, VBF, VH, and ttH—were observed using the CMS, and decay modes— ZZ to four leptons, diphotons, WW, ττ, and bb—were discovered using the CMS. Higgs coupling with vector bosons and third-generation fermions t, b, and τ were measured. Moreover, the Higgs mass, width, decay branching ratio, spin and parity, cross-section, and differential cross-section were studied. The results are consistent with those obtained using the standard model within an error range. The results are mainly obtained using 2016 data, and the errors are large. Nevertheless, the results will be improved when all RUN2 data analyses are published.

Towards constraining triple gluon operators through tops

Effective field theory techniques provide us important tools to probe for physics beyond the Standard Model in a relatively model-independent way. In this work, we revisit the CP-even dimension-6 purely gluonic operator to investigate the possible constraints on it by studying its effect on top-pair production at the LHC, in particular the high $p_T$ and $m_{t\bar{t}}$ tails of the distribution. Cut-based analysis reveals that the scale of New Physics when this operator alone contributes to the production process is greater than 3.4 TeV at 95% C.L., which is a much stronger bound compared to the bound of 850 GeV obtained from Run-I data using the same channel. This is reinforced by an analysis using Machine Learning techniques. Our study complements similar studies that have focussed on other collider channels to study this operator.

Beam dynamics corrections to the Run-1 measurement of the muon anomalous magnetic moment at Fermilab

This paper presents the beam dynamics systematic corrections and their uncertainties for the Run-1 data set of the Fermilab Muon g-2 Experiment. Two corrections to the measured muon precession frequency $\omega_a^m$ are associated with well-known effects owing to the use of electrostatic quadrupole (ESQ) vertical focusing in the storage ring. An average vertically oriented motional magnetic field is felt by relativistic muons passing transversely through the radial electric field components created by the ESQ system. The correction depends on the stored momentum distribution and the tunes of the ring, which has relatively weak vertical focusing. Vertical betatron motions imply that the muons do not orbit the ring in a plane exactly orthogonal to the vertical magnetic field direction. A correction is necessary to account for an average pitch angle associated with their trajectories. A third small correction is necessary because muons that escape the ring during the storage time are slightly biased in initial spin phase compared to the parent distribution. Finally, because two high-voltage resistors in the ESQ network had longer than designed RC time constants, the vertical and horizontal centroids and envelopes of the stored muon beam drifted slightly, but coherently, during each storage ring fill. This led to the discovery of an important phase-acceptance relationship that requires a correction. The sum of the corrections to $\omega_a^m$ is 0.50 $\pm$ 0.09 ppm; the uncertainty is small compared to the 0.43 ppm statistical precision of $\omega_a^m$.

Magnetic-field measurement and analysis for the Muon g−2 Experiment at Fermilab

The Fermi National Accelerator Laboratory has measured the anomalous precession frequency $a^{}_\mu = (g^{}_\mu-2)/2$ of the muon to a combined precision of 0.46 parts per million with data collected during its first physics run in 2018. This paper documents the measurement of the magnetic field in the muon storage ring. The magnetic field is monitored by nuclear magnetic resonance systems and calibrated in terms of the equivalent proton spin precession frequency in a spherical water sample at 34.7$^\circ$C. The measured field is weighted by the muon distribution resulting in $\tilde{\omega}'^{}_p$, the denominator in the ratio $\omega^{}_a$/$\tilde{\omega}'^{}_p$ that together with known fundamental constants yields $a^{}_\mu$. The reported uncertainty on $\tilde{\omega}'^{}_p$ for the Run-1 data set is 114 ppb consisting of uncertainty contributions from frequency extraction, calibration, mapping, tracking, and averaging of 56 ppb, and contributions from fast transient fields of 99 ppb.

Studies of excited states of beauty hadrons at CMS

Studies of excited states of beauty mesons and baryons, performed at the CMS Experiment, are presented. The first observation of and states in the invariant mass spectrum is reported as well as the study of excited Λb(5912)0, Λb(5920)0, Λb(6146)0 and Λb(6152)0 baryons decaying to . The analyses use Run-2 data, collected in pp collisions at .

Experiences from the RPC data taking during the CMS RUN-2

The CMS experiment recorded 177.75 fb−1 of proton-proton collision data during the RUN-1 and RUN-2 data taking period. Successful data taking at increasing instantaneous luminosities with the evolving detector configuration was a big achievement of the collaboration. The CMS RPC system provided redundant information for the robust muon triggering, reconstruction, and identification. To ensure stable data taking, the CMS RPC collaboration has performed detector operation, calibration, and performance studies. Various software and related tools are developed and maintained accordingly. In this paper, the overall performance of the CMS RPC system and experiences of the data taking during the RUN-2 period are summarised.

Performance of the missing transverse momentum triggers for the ATLAS detector during Run-2 data taking

The factor of four increase in the LHC luminosity, from 0.5 × 1034 cm−2s−1 to 2.0 × 1034cm−2s−1, and the corresponding increase in pile-up collisions during the 2015–2018 data-taking period, presented a challenge for the ATLAS trigger, particularly for those algorithms that select events with missing transverse momentum. The output data rate at fixed threshold typically increases exponentially with the number of pile-up collisions, so the legacy algorithms from previous LHC data-taking periods had to be tuned and new approaches developed to maintain the high trigger efficiency achieved in earlier operations. A study of the trigger performance and comparisons with simulations show that these changes resulted in event selection efficiencies of > 98% for this period, meeting and in some cases exceeding the performance of similar triggers in earlier run periods, while at the same time keeping the necessary bandwidth within acceptable limits.

Top Quark Pair-Production in Noncommutative Standard Model

The differential cross-section of the top quark pair production via the quark-antiquark annihilation subprocess in hadron collision is calculated within the noncommutative standard model. A pure NC analytical expression for the forward-backward asymmetry at the tree level is obtained. Moreover, using recent Tevatron results from the full RUN2 data, a new lower bound on the noncommutative geometry parameter is deduced.

Particle tracking and radiation field characterization with Timepix3 in ATLAS

Four hybrid pixel detectors of Timepix3 technology, installed in the ATLAS experiment, were continuously taking data from April 2018 until the end of the Run-2 data taking period (December 2019). These detectors are synchronized with each other and the LHC orbit clock. They are capable of resolving the bunch structure of the LHC beams due to their time resolution of ∼2ns. Thus, they allow the characterization of the radiation field inside and outside bunch-crossing periods. This is shown for Timepix3 detectors at the extended barrel (x=-3.58 m, y=0.97 m, z=2.83 m). We apply pattern recognition methods to decompose the radiation field and determine the directionality of the minimum ionizing particles (MIP) component of the radiation field.

Calibration and performance of the CMS Electromagnetic Calorimeter in LHC Run-II

Many physics analyses using the Compact Muon Solenoid (CMS) detector at the LHC require accurate, high resolution electron and photon energy measurements. Excellent energy resolution is crucial for studies of Higgs boson decays with electromagnetic particles in the final state, as well as searches for very high mass resonances decaying to energetic photons or electrons. The CMS electromagnetic calorimeter (ECAL) is a fundamental instrument for these analyses and its energy resolution is crucial for the Higgs boson mass measurement. Recently the energy response of the calorimeter has been precisely calibrated exploiting the full Run-II data, aiming at a legacy reprocessing of the data. A dedicated calibration of each detector channel has been performed with physics events such as electrons from W and Z boson decays, and photons from π0/η decays. This paper presents the calibration strategies that have been implemented and the excellent performance achieved by the CMS ECAL with the ultimate calibration of Run-II data, in terms of energy scale stability and energy resolution.

ATLAS LAr Calorimeter performance in LHC Run 2

Liquid argon (LAr) sampling calorimeters are employed by ATLAS for all electromagnetic calorimetry and for hadronic and forward calorimetry in the region 1.5 < |η| < 4.9. In the first LHC run a total luminosity of 27 fb−1 has been collected at center-of-mass energies of 7–8 TeV . After detector consolidation during a long shutdown, Run-2 started in 2015 and 139 fb−1 of data at a center-of-mass energy of 13 TeV have been recorded. In order to accomplish the Level-1 acceptance rate of 100 kHz in Run-2 data taking, the number of read-out samples recorded and used for the energy and the time measurement has been modified from five to four while keeping the expected performance. The well calibrated and highly granular LAr Calorimeter reached its design goals, both in terms of energy and direction measurement. This contribution gives an overview of the detector operation, hardware improvements, monitoring and data quality procedures, as well as the achieved performance, including the calibration and stability of the electromagnetic scale, noise level, response uniformity and time resolution.