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. Over the last six years, with the arrival of the LHC Run 1 (2011-2012) and early Run 2 data (2015-2017), the IFAE group has carried out a strong physics program.


During 2017 the IFAE group has remained strongly involved in both detector operations and physics analyses. In particular, a large number of high-profile physics results were produced using Run 2 data. The group has also maintained its visibility within the ATLAS Collaboration through a number of important management positions.


The year 2107 was characterized by the analysis of data recorded by ATLAS in 2015 and 2016 at √s=13 TeV, corresponding to an integrated luminosity of 36 $fb^{-1}$. IFAE continued to play a leading role in several physics research lines, including monojet searches (probing e.g. extra spatial dimensions and dark matter), searches for Supersymmetry (SUSY), searches for new phenomena in top-quark final states, as well as Higgs boson studies. A summary of the results obtained is provided below. Most of these results were submitted to publication in 2017 and/or will published in early 2018. Two PhD theses were completed in 2017.

Higgs Boson Physics

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

Search for ttH production

Since 2011 IFAE has lead searches for ttH production with H→bb, in the lepton+jets channel. During 2017 members of the IFAE team (A. Juste and Y. Rodina) participated in this search by developing a novel likelihood-based discriminating variable to separate signal from the dominant background from tt+heavy-flavor jets. The results of this search were published in Phys. Rev. D 97 (2018) 072016, and were included in Y. Rodina’s PhD thesis, defended in November 2017. The combination of all available ATLAS searches (see Fig. 1) has recently established first evidence for ttH production (Phys. Rev. D 97 (2018) 072003).

Searches for additional Higgs bosons

During 2017 the IFAE team (M. Bosman, M.P. Casado, J. Glatzer, Ll.-M. Mir, and I. Riu) has been actively involved in the search for a heavy H+ boson decaying to a top quark and a bottom quark in the lepton+jets final state. Its main contribution has been the development of a likelihood-based discriminant that significantly improves the sensitivity to a H+ boson with a mass of 200-300 GeV. This analysis is already approved and will be published in Spring 2018. J. Glatzer is acting as Corresponding Editor for the paper and Ll.-M. Mir is the analysis contact.

Figure 1: Summary of the observed $μ_{ttH}$ signal strength measurements from the individual analyses and for their combination, assuming mH=125 GeV. The total (tot.), statistical (stat.), and systematic (syst.) uncertainties on $μ_{ttH}$are shown. The SM $μ_{ttH}$=1 (0) expectation is shown as the black (grey) vertical line. From Phys. Rev. D 97 (2018) 072016.

Searches for new phenomena in Jet+X

In 2017 the IFAE ATLAS team (C. Fischer, M. Martínez and R. Zaidan) 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 data were translated into significantly better exclusion limits for searches for large extra spatial dimensions, the production of squarks in the framework of SUSY, and the search for dark matter (WIMP) production at the LHC. For the latter, a variety of simplified models for dark matter pair production were considered (see Fig. 2), demonstrating the complementary between the LHC and direct detection experiments in searches for dark matter, and the unique sensitivity of the LHC experiments at low WIMP masses. The results were published in JHEP 01 (2018) 126, and were part of C. Fischer’s PhD thesis, which was defended in September 2017.

Figure 2: A comparison of the inferred limits (black line) to the constraints from direct detection experiments (purple line) on the spin-dependent WIMP–proton scattering cross section in the context of the simplified model with axial-vector couplings. Unlike in the $m_{ZA}$–$m_\chi$ parameter plane, the limits are shown at 90% CL. The results from this analysis, excluding the region to the left of the contour, are compared with limits from the PICO experiment. The comparison is model-dependent and solely valid in the context of this model, assuming minimal mediator width and the coupling values $g_q$=1/4 and $g_χ=1$. From JHEP 01 (2018) 126.

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 areas of strong involvement by the IFAE team.

Searches for direct sbottom and stop pair production and dark matter

In 2017, the IFAE team (M. Martínez, A. Rodríguez, M. Tripiana, and R. Zaidan) 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. Final results on the search for bottom squark pair production have been published in JHEP 11 (2017) 195, leading to the exclusion of bottom squarks with mass below 1 TeV for relatively light neutralinos. Similarly, during 2017 the search for the top squark pair production in the fully hadronic channel, using the full 2015+2016 dataset, came to conclusion. This analysis has demonstrated to have unique sensitivity in the scenario with a heavy top squark decaying into a top quark and a neutralino, excluding top squark masses below 1 TeV for light neutralinos (see Fig. 3).

The IFAE team has also participated in dedicated searches for dark matter (DM) produced in association with heavy flavor quarks. Models with scalar or pseudo-scalar color-neutral mediators are explored. Figure 4 presents the results in case of a scalar mediator for which DM+bb and DM+tt have been combined. Within the theoretical model considered, the data have sensitivity for exclusion in the case of mediator masses below 100 GeV. The results have been recently published in Eur. Phys. J. C 78 (2018) 18. Altogether, this will constitute the main subject of the PhD thesis by A. Rodríguez, to be defended in 2018.

Figure 3: Summary of the dedicated ATLAS searches for top squark (stop) pair production based on 3.2 - 36 $fb^{-1}$ of pp collision data taken at √s=13 TeV. Exclusion limits at 95% CL are shown in the stop1-chi0 mass plane. The dashed and solid lines show the expected and observed limits, respectively, including all uncertainties except the theoretical signal cross section uncertainty (PDF and scale). Four decay modes are considered separately with 100% branching ratio: stop1→t+chi0 (where the stop1 is mostly right), stop1→W+b+chi0 (3-body decay for m(stop1)<m(top)+m(chi0)), stop1→c+chi0 and stop1→f+f’+b+chi0 (4-body decay). The latter two decay modes are superimposed. Note that these plots overlay contours belonging to different stop decay channels, different sparticle mass hierarchies, and simplified decay scenarios.
Figure 4: Exclusion limits for color-neutral tt/bb+φ scalar model as a function of the mediator mass for a DM mass of 1 GeV. The limits are calculated at 95% CL and are expressed in terms of the ratio of the excluded cross-section to the nominal cross-section for a coupling assumption of $g = g_q = g_χ = 1$. From Eur. Phys. J. C 78 (2018) 18.
Figure 5: 95% CL exclusion limits in the chi0 vs gluino mass for the Gtt model. The dashed and solid bold lines show the 95% CL expected and observed limits respectively. From arXiv:1711.01901.

Searches for gluino-mediated top/bottom squark production

Since 2015 the IFAE team (A. Juste, Ch. Rizzi, and L. Valéry) is playing a leading role in the search for gluino-mediated top/bottom s-quark production. The most recent result, based on the 2015+2016 Run 2 dataset, was presented at the SUSY 2017 conference and was submitted for publication to JHEP (arXiv:1711.01901), with L. Valéry acting as Corresponding Editor. This publication provides the most restrictive available direct bounds on the mass of the gluino in these scenarios (see e.g. Fig. 5), thanks to an optimized multi-channel analysis strategy developed by the IFAE team. This search will be part of Ch. Rizzi’s PhD thesis, to be defended in 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. An interpretation of the limits in terms of the branching ratio of the higgsino to Z boson or Higgs boson decays is also provided (see Fig. 6). The results were presented at the SUSY 2017 conference (ATLAS-CONF-2017-081), and will be submitted to publication in Spring 2018. They will also be part of Ch. Rizzi’s PhD thesis.

Searches for new phenomena in Top+X

During 2017 the IFAE team (D. Gerbaudo, A. Juste, L. Valéry, and T. van Daalen) continued to lead searches for pair production of vector-like top quarks, 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 (see Fig. 7). These results were recently submitted to publication in JHEP (arXiv:1803.09678), with L. Valéry acting as analysis contact and A. Juste acting as Corresponding Editor.

Figure 6: Exclusion limits at the 95% CL on the branching ratio for higgsino decay into the Higgs boson and the gravitino vs higgsino mass. The dashed and solid bold lines show the 95% CL expected and observed limits respectively. From ATLAS-CONF-2017-081.
Figure 7: Observed limit (95% CL) on the mass of the T quark in the plane of BR(T → Ht) versus BR(T → Wb) for the combination of the 1-lepton and 0-lepton TT→Ht+X searches. Contour lines are provided to guide the eye. From arXiv:1803.09678.


In 2017, the IFAE group members 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 G. Volpi) 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 and, together with other luminosity monitors of ATLAS, to measure the luminosity delivered to the ATLAS detector by the LHC. 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.

During 2017, the group (J. Glatzer, I. Korolkov, A. Rodríguez, R. Rosten, and G. Volpi) 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). Beam tests of more components will follow in 2018.

Figure 8: Beam-induced drift of the gain of a given Tile readout channel type measured by various calibration systems in 2017.
Figure 9: Tile detector response to positrons in the 2017 test beam. The electronics readout used is the final candidate for the HL-LHC upgrade.


Since the Long Shutdown 1 after Run 1, the IFAE ATLAS group has been involved in the Level-1 (L1) topological trigger processor (L1Topo), a system designed to perform real-time event selection based on event topological variables. The group took the responsibility of writing the simulation of the topological trigger algorithms in addition to designing and estimating trigger acceptance for various physics signals.

In 2017, D. Gerbaudo co-coordinated the L1Topo commissioning group at CERN and P. Casado investigated the discrepancies of the di-tau topological triggers observed between the simulation and hardware decisions. Figure 10 shows the significant rate reduction, at no significant signal efficiency loss, provided by the di-muon topological selection consisting on both invariant mass and angular distance requirements.

The IFAE group also contributed to the trigger operations by providing support on the releases to be installed for online usage, the trigger menu, on-call trigger operations and trigger performance validation through various shift periods during the year.

I. Riu co-edited the TDAQ Phase-II Upgrade Technical Design Report (TDR), in preparation for the HL-LHC data-taking period foreseen starting 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.

Figure 10: Event rate as a function of time of the L1 trigger that requires two muons with pT > 6 GeV without (red) or in addition to (blue) a L1Topo requirement. L1Topo requires the two selected L1 muons to have an invariant mass between 2 and 9 GeV and an angular separation ΔR between 0.2 and 1.5.