ATLAS at the LHC

Aurelio Juste


Since 1993, the IFAE ATLAS group has made major contributions to the construction of the ATLAS detector and its trigger system, the reconstruction software, and preparatory physics studies. Using the data collected by the ATLAS experiment during Run 1 (2010-2012) and Run 2 (2015-2018) of the LHC, the IFAE group has carried out a strong physics program.


Introduction

During 2018, the last year of Run 2, the LHC continued the operation at a centre-of-mass energy of 13 TeV with exceptional performance. The machine’s peak luminosity regularly reached $2x10^{34}$ $cm^{-2}s^{-1}$, resulting in an integrated luminosity delivered to the ATLAS experiment of 65 $fb^{-1}$. The brings the total delivered integrated luminosity in Run 2 to 158 $fb^{-1}$, out of which 149 $fb^{-1}$ were recorded. Throughout the year the IFAE group has remained strongly involved in both detector operations and physics analyses. The group has also maintained its visibility within the ATLAS Collaboration through a number of important management positions.

PHYSICS ANALYSES

The year 2018 was characterized by the publication of many physics results based on the data recorded by ATLAS in 2015 and 2016 at √s=13 TeV, corresponding to an integrated luminosity of 36 $fb^{-1}$, as well as the completion of first results including also 2017 data, and thus using up to 80 $fb^{-1}$ of data. IFAE continued to play a leading role in several physics research lines, including Higgs boson studies, as well as monojet searches (probing e.g. extra spatial dimensions and dark matter), searches for Supersymmetry, searches for new phenomena in top-quark final states. A summary of the results obtained is provided below. Two PhD theses were completed in 2018.

Higgs Boson Physics

During Run 2 the IFAE team is strongly involved in the search for ttH production, the search for flavor-violating t→Hq decays, and searches for additional Higgs bosons, both charged and neutral.

Probing top-Higgs interactions

Direct tests of top-Higgs interactions are extremely interesting since the top quark is the SM particle most strongly coupled to the Higgs sector. The most sensitive direct probe of the top-Higgs Yukawa coupling is provided by the measurement of the ttH cross section. During Run 1 and throughout Run 2 the IFAE team has actively participated in the ttH(→bb) search, resulting in several public results. The most recent Run 2 result is based on 36 $fb^{-1}$ of Run 2 data, and was published in Phys. Rev. D 97 (2018) 072016. This result was combined with other recent ATLAS searches to establish first evidence [Phys. Rev. D 97 (2018) 072003], and then observation [Phys. Lett. B 784 (2018) 173] of ttH production (see Fig. 1), a milestone result that represents the first step towards a precise measurement of the top-Higgs Yukawa coupling. The IFAE team is currently involved in the ttH search with H→WW, ZZ, ττ, giving multilepton final states, using the full Run 2 dataset, which will be part of S. Kazakos’ PhD Thesis. The team is currently finalizing a publication based on 80 $fb^{-1}$ of data, with A. Juste acting as Corresponding Editor.

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Figure 1


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Figure 2


Another exciting topic is the search for flavor-changing top-Higgs interactions. Using Run 1 data, the IFAE team pioneered the first search for t→H(→bb)q decays in tt events, obtaining the most sensitive result at the time. Recently the IFAE team has also completed an improved search for t→h(→bb)q using 36 $fb^{-1}$ of Run 2 data, with N. Orlando acting as coordinator of the analysis effort. This result, which was also combined with other ATLAS searches, was submitted to publication to JHEP (arXiv:1812.11568), with A. Juste acting as Corresponding Editor. Since then, the IFAE team has focused on the analysis of the full Run 2 dataset, including further improvements to the search strategy, some of which were developed in the context of L. Pereira’s Master Thesis, defended in June 2018.

Searches for additional Higgs bosons

During 2018 the IFAE team (M. Bosman, M.P. Casado, J. Glatzer, Ll.-M. Mir, Adrià Salvador, I. Riu) continued its strong involvement in the search for a heavy charged Higgs boson (H+) decaying to a top quark and a bottom quark. Its main contribution has been the development of a likelihood-based discriminating variable that significantly improves the sensitivity to H+ with a mass of 200-300 GeV in the lepton+jets final state. The result (see Fig. 2) was published in JHEP 11 (2018) 085. J. Glatzer acted as corresponding editor for the paper and Ll.-M. Mir as analysis contact.

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Figure 3


Searches for new phenomena in Jet+X

In 2018 the IFAE ATLAS team (C. Fischer, J.L. Muñoz, M. Martínez, R. Rosten, M. Praderio, S. González) continued to be a driving force in the search for a jet plus large missing transverse momentum (ETmiss) in ATLAS at √s=13 TeV. The results of the analysis of the full 2015+2016 were finally published in JHEP 01 (2018) 126 (see Fig. 3). Since then the group is focused on the analysis of the 2017+2018 data, which increases the dataset by a factor of four. Improvements in the analysis are being implemented in terms of additional control regions to calibrate the SM backgrounds, the implementation of a τ-lepton veto, and extended coverage in missing transverse momentum, and new models including axion-like particles as candidates for dark matter. Moreover, a significant effort has been put in evaluating the feasibility of implementing deep learning algorithms in the analysis leading to better sensitivity. The latter led to M. Praderio’s Master Thesis, defended in September 2018.

In addition, the group has opened a new venue focused in searches for mono-V (vector boson), as a natural complement of the mono-jet analysis, accessing new interpretations related to dark matter Higgs portal models and the measurement of the invisibly decaying Higgs branching fraction. In this case, the reconstruction of the boosted vector bosons at very high transverse momentum is required, for which both QCD-inspired sub-jet quantities in large-R jets and a deep learning approach will be employed in separating vector boson decay products from gluon radiation. First results are expected in 2019.

Supersymmetry searches

One possible solution to the gauge hierarchy problem is provided by weak-scale SUSY, which extends the SM by introducing supersymmetric partners for all Standard Model (SM) particles. Searches for gluinos, top/bottom squarks, and higgsinos are a high priority for the LHC Run 2 and beyond, and are areas of strong involvement by the IFAE team.

Searches for direct sbottom and stop pair production and dark matter

In 2018, the IFAE team (M. Martínez, A. Rodríguez, R. Rosten) continued the program for searches for 3rd generation squarks and the associated production of dark matter and heavy flavor quarks using the 2015+2016 dataset. The IFAE team participated in dedicated searches for dark matter (DM) produced in association with heavy flavor quarks (see Fig. 4), published in Eur. Phys. J. C 78 (2018) 18, where different interpretations in terms of dark matter simplified models are considered. As expected the LHC results are complementary to direct-detection experiments providing a unique sensitivity at low dark matter mass. Altogether, this constituted the main subject of the PhD thesis by A. Rodríguez, defended in October 2018.

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Figure 4


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Figure 5


Searches for gluino-mediated top/bottom squark production

Since 2015 the IFAE team (A. Juste, C. Moreno, Ch. Rizzi) is playing a leading role in the search for gluino-mediated top/bottom squark production. The result of the analysis of the 2015+2016 Run 2 dataset was published in JHEP 1806 (2018) 107. Following this publication, the analysis team produced an updated result using the full 2015-2017 dataset (~80 $fb^{-1}$), with Ch. Rizzi acting as coordinator of this effort. This result (see Fig. 5), also summarized in Ch. Rizzi‘s PhD thesis (defended in October 2018), was released as a preliminary result in ATLAS-CONF-2018-067 and was presented at the SUSY2018 conference in July 2018.

Searches for Higgsino production in multi-b-jets final states

The IFAE team (A. Juste and Ch. Rizzi) has also lead a search for pair production of higgsinos in gauge-mediated SUSY-breaking scenarios. The search is based on the 2015+2016 Run 2 dataset and places some of the most restrictive bounds to date on mass-degenerate higgsinos that decay dominantly to the Higgs boson and the gravitino (see Fig. 6). An interpretation of the limits in terms of the branching ratio of the higgsino to Z boson or Higgs boson decays is also provided. This result, also part of Ch. Rizzi‘s PhD thesis, was published in Phys. Rev. D 98 (2018) 092002, with Ch. Rizzi acting as coordinator of the analysis team. The IFAE team is currently analyzing the full Run 2 dataset, which will be part of C. Moreno’s PhD Thesis.

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Figure 6


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Figure 7


Searches for compositeness

Models of partial compositeness represent another solution to the gauge hierarchy problem, predicting heavy vector-like quarks (VLQ) and new strong interactions resulting in a significant increase of the four top-quark production rate. During 2018 the IFAE team (A. Juste, N. Orlando, T. Van Daalen) continued to lead searches for pair production of vector-like top quarks (T), with at least one of the T-quarks decaying into a top quark and a Higgs boson or a Z boson, leading to a busy environment with high jet and b-tagged jet multiplicities. The most recent search is based on the 2015+2016 dataset and constitutes the single most-sensitive search for TT production at the LHC. These results were published in JHEP 1807 (2018) 089 with A. Juste acting as Corresponding Editor. This constitutes the single most-sensitive search for TT production at the LHC to date and was included in the recent VLQ ATLAS combination, which was published in Phys. Rev. Lett. 121 (2018) 211801 (see Fig. 7). This search will be extended to the full Run 2 dataset, and a brand-new search for single T(→ht/Zt) production will be performed. Both searches will be part of T. Van Daalen's PhD thesis.

In addition, the IFAE team is participating in the search for four top-quark (tttt) production using the full Run 2 dataset. This analysis will be used to measure the SM tttt production cross-section, and to set bounds on BSM production via a contact interaction. In addition, a dedicated search for resonant tttt production will be performed, which will be part of A. Sonay’s PhD Thesis.

Searches for leptoquarks

Over the last few years, results from the B-factories and the LHCb experiment show intriguing deviations of ~2–3σ in the ratios RK() and RD(), where accurate tests of Lepton Flavor Universality can be performed. Currently, the favored BSM explanation is a leptoquark (LQ) with a mass in the TeV scale, and preferentially coupled to 3rd-generation quarks and 2nd and 3rd generation leptons. During 2018 the IFAE team has started participating in a search for pair production of a scalar leptoquark decaying into LQ→tτ, resulting in multilepton final states, including τ-leptons. This search will use the full Run 2 dataset and will be part of S. Kazakos’ PhD Thesis.

TILECAL OPERATIONS AND UPGRADE

In 2018, members of the IFAE group contributed strongly to the ATLAS Tile calorimeter (TileCal) operation, to the calorimeter calibration, to the measurement of the ATLAS luminosity, and to the preparation of the detector upgrades for the high-luminosity LHC (HL-LHC) operation.

IFAE’s TileCal team (J. Glatzer, I. Korolkov, R. Rosten, T. Van Daalen, and S. González) continues its commitment to fully support the TileCal “Minimum Bias” data calibration system. The system is based on the components developed and maintained exclusively by IFAE and is used to monitor on a daily basis the stability of the TileCal response in time (see Fig. 8) and, together with other luminosity monitors of ATLAS, to measure the luminosity delivered to the ATLAS detector by the LHC.

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Figure 8


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Figure 9


During 2018, the group coordinated the beam tests dedicated to the validation of the new TileCal readout electronics for the HL-LHC upgrade. Spare TileCal modules were instrumented with prototype readout electronics and were exposed to different particle beams of muons, positrons and hadrons of different energies from the SPS (see Fig. 9). The above prototype will be introduced for further tests in ATLAS in 2019.

TRIGGER OPERATIONS, PERFORMANCE AND UPGRADE

Since the Long Shutdown 1 and during the whole Run 2, the IFAE ATLAS group has been involved in the Level-1 (L1) topological trigger system (L1Topo), consisting of two electronic boards with FPGA processors programmed to perform real-time event selection based on event topological variables. The group wrote the simulation of the topological trigger algorithms, was responsible for its evolution and provided diagnostic tools to identify sources of discrepancies or hardware malfunctioning.

In 2018, I. Riu co-coordinated the L1Topo commissioning group while PhD student C. Moreno qualified as ATLAS author by maintaining the simulation software of the L1Topo algorithms, providing additional monitoring tools, and analyzing differences between hardware and software (see Fig. 10).

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Figure 10


Since the Long Shutdown 1 and during the whole Run 2, the IFAE ATLAS group has been involved in the Level-1 (L1) topological trigger system (L1Topo), consisting of two electronic boards with FPGA processors programmed to perform real-time event selection based on event topological variables. The group wrote the simulation of the topological trigger algorithms, was responsible for its evolution and provided diagnostic tools to identify sources of discrepancies or hardware malfunctioning.

In 2018, I. Riu co-coordinated the L1Topo commissioning group while PhD student C. Moreno qualified as ATLAS author by maintaining the simulation software of the L1Topo algorithms, providing additional monitoring tools, and analyzing differences between hardware and software (see Fig. 10). Figure 10 presents the observed online mismatch rates between the L1Topo hardware and simulation for a selected list of triggers. Overall, discrepancies are smaller than O(10-3). Also, together with a master student, D. Cintas, we studied the trigger efficiency turn-on of various muon-related topological triggers aiming at triggering J/ψ or B-physics events. They showed good agreement with Monte Carlo simulation.

Throughout 2017 and early 2018, I. Riu co-edited the TDAQ Phase-II Upgrade Technical Design Report (TDR), in preparation for the HL-LHC data-taking period foreseen to start in 2026. Following a positive review of the TDR by the LHC Experiments Committee, the project was approved by the CERN Research Board in April 2018 and published in June 2018 (ATL-TDR-029, CERN-LHCC-2017-020).

The IFAE group also contributed to the trigger operations by taking online triggers shifts in ATLAS, supporting the online software release process and validating trigger performance during various shift periods in 2018.