It’s A Wrap! Summary of ATLAS Updates from 2016

Article: Latest ATLAS results from Run 2
Authors: Claudia Gemme on behalf of the ATLAS Collaboration
Reference: arXiv:1612.01987 [hep-ex]

2016 certainly has been an…interesting year. There’s been a lot of division, so as the year winds down, this seems like a perfect time to reflect on something we can all get excited about: particle physics! This has been an incredible year for the ATLAS experiment and the LHC in general. Run 2 began in 2015, but really hit its stride this year. The LHC reached a peak luminosity of 1.37 x 1034 cm-2s-1, which exceeds the design luminosity by almost 40%! Additionally, the pile up (number of interactions per bunch crossing) nearly doubled with respect to 2015. From these delivered events, ATLAS was able to record 30 fb-1 of data at 13 TeV center-of-mass energy, with a DAQ (data acquisition) efficiency above 90%.

Before discussing the many analyses being done with this amazing new data set, I’d like to highlight some of the upgrades performed during the long shutdown (2013-2014) that allowed ATLAS to perform so well during Run 2. One of the most important improvements was the addition of the Insertable B-layer (IBL). The IBL is now the innermost part of the inner detector, located only 3.3 cm from the interaction point. The IBL was designed to combat radiation damage to the inner detector; due to its insertable nature, it can eventually be replaced and it shields the other 3 detector layers from the bulk of the radiation damage. Of course, it also allows more tracking points which further improves track reconstruction. There were also extensive improvements to the magnetic and cryogenic systems to provide more powerful cooling and repair damage from Run 1. To deal with the increased data load, the ATLAS DAQ and trigger systems were also upgraded (reminder, triggers are data collection elements that decide which events get stored for analysis and which get thrown out). The High Level Trigger, ATLAS’s only hardware level trigger was updated to output 100kHz of data (previously 75kHz), and the 2 software level triggers were merged into one. This allowed ATLAS DAQ to have an average physics data output of 1kHz, allowing more of the Run 2 events to be processed and stored.

Now, lets talk about ATLAS’s physics program in 2016. A precise understanding of the Standard Model is incredibly important for all particle physics programs. It allows physicists to test theoretical mass and coupling predictions, and to make accurate predictions for background processes in BSM searches. ATLAS improved the precision of many SM measurements in 2016, and a summary of some important cross section measurements is shown in Figure 1. In particular, the measurements of the Z+jets and diboson cross sections were compatible with new next-to-next-leading-order calculations from theorists, confirming theoretical predictions.

Overview of cross-section measurements for a variety of SM processes compared to theoretical expectations

Another major goal of the Run 2 ATLAS physics program was to confirm the discovery of the Higgs Boson and further study it. During Run 1, both ATLAS and CMS found evidence of a Higgs boson with a combined measured mass of 125.09 GeV using the H→gg and H→ZZ*→4l channels. Furthermore, nearly all measured couplings were consistent with SM predictions within 2σ. In Run 2, ATLAS ‘rediscovered’ the Higgs at a compatible mass with a local significance of 10 (compared to 8.6 expected) using the same channels. The Run 2 cross section is slightly higher than the SM prediction, but still compatible within uncertainty. A summary of the cross-section measurements at various energies can be seen in Figure 2. Additionally, ATLAS physicists sought to show conclusive evidence of the H→bb decay channel in Run 2. This decay mode of the Higgs has the largest predicted branching fraction (58%) according to the SM, but has been difficult to measure due to large multi-jet backgrounds. The increased luminosity in Run 2 made studying the similar X+H→bb decay a promising alternative, despite its cross-section being much lower. This decay process is easier to isolate as leptonic decays of the W or Z create a clean signature. However, the measured significance of this decay was only 0.42, compared to a SM expectation of 1.94. The analysis procedure was well validated by measuring SM (W/Z)Z yield, so this large discrepancy is certainly an area for more study in 2017.

Total pp->H cross-sections measured at different center-of-mass energies compared to SM predictions

The final component of the ATLAS physics program is, of course, the many searches for BSM physics. The discovery potential for many of these processes is increased in Run 2 due to the enhanced cross-sections at the larger energy. There are 3 important channels for general BSM searches: diboson, diphoton, and dilepton. Many extensions of the SM predict new heavy particles including heavy neutral Higgs, Heavy Vector Triplet W, and some Gravitons, that could decay into vector-boson pairs. General searches in this channel are done by looking for hadronic decays of the boosted W and Z bosons within a single large-radius jet and using jet substructure properties. Unfortunately, no significant excess is observed. The diphoton channel became the hot topic of 2016, due to a deviation of at least 3σ from the SM only hypothesis seen in both ATLAS and CMS early this year using the 2015 data set. However, this excess was not confirmed with the full 2016 data set, which has 4x more statistics. The largest excess seen by ATLAS in the diphoton channel is now 2.3σ for a mass near 710 GeV. The dilepton final states have excellent sensitivity to a variety of new phenomena and have high signal selection efficiencies. Again, however, the 2016 measurements are consistent with the SM prediction. Lower limits on a resonance mass have been set, enhancing exclusion up to 1 TeV more than in Run 1.

ATLAS physicists also study the infamous SUSY model by looking for signatures of gluinos and squarks (predicted to be the lightest SUSY particles). These particles could be observed in events with high jet multiplicity and lots of missing energy. Once again, no significant excess was found. These results were interpreted within 2 SUSY models, and gluinos with mass up to 1600 GeV were excluded.

So, to conclude, no evidence of BSM physics has been found at ATLAS in Run 2, but the  new limits on some SUSY models and various other BSM phenomena have been improved and can be seen in Figures 3 and 4. Precision measurements of the SM were improved, the detector equipment was updated, and the Higgs was rediscovered, but ATLAS physicists and particle enthusiasts are still hoping for something more exciting. Lest we leave 2016 on a low note, its important to remember that only 50% of ATLAS searches have been updated to the Run 2 energy, so there are still many more channels to be explored. Plus, there are already some modest excesses, as well as the discrepancy in the H→bb measurement. There is certainly more to be discovered as Run 2 continues, so here’s to 2017!

Mass reach of selected ATLAS SUSY searches
Mass reaches of selected non-SUSY BSM searches at ATLAS

References and further reading:


Saving SUSY: Interpreting the 3.3σ ATLAS Stop Excess

Article: Surviving scenario of stop decays for ATLAS l+jets+E_T^miss search
Authors: Chengcheng Han, Mihoko M. Nojiri, Michihisa Takeuchi, and Tsutomu T. Yanagida
Reference: arXiv:1609.09303v1 [hep-ph]

If you’ve been following the ongoing SUSY searches, you know that much of the phase space accessible at colliders like the LHC has been ruled out. Nevertheless, many phenomenologists are working diligently to develop alternative frameworks that maintain the compatibility of the Minimal Supersymmetric Standard Model (MSSM, one of the most compelling SUSY models) with recent experimental results. If evidence of SUSY is found at the LHC, it could help us start answering questions about naturalness, dark matter, gauge coupling unification, and other BSM questions, so it’s no wonder so many researchers are invested in keeping the SUSY search alive.

This particular paper discusses recent 13 TeV ATLAS results in the l+jets+ETmiss channel where an excess of up to 3.3σ above Standard Model expectations were seen in the initial 13TeV Run 2 dataset. Although CMS hasn’t reported any significant excess in this channel, both experiments see a moderate excess in the 0 lepton channel, so there’s some strong motivation for phenomenologists to explain these preliminary results as the presence of a new particle, namely a SUSY particle.

This ATLAS search is aimed at stop (the SUSY partner of the top quark) production where the stop then decays into a top and neutralino (a mixed state of the higgsino and gaugino, ie the SUSY partners of the Higgs and the gauge boson), and the top then decays to leptons. The stop is a particularly interesting SUSY particle, because it plays a critical role in the naturalness of SUSY models; most natural SUSY models predict a light stop and higgsino. The analysis group defined 7 (non exclusive) potential signal regions for this search, and excesses above 2σ were seen in 3 of them: DM_low, bC2x_diag, and SR1. The selection cuts for these regions are shown in Table 1. This paper discussed models to explain the DM_low excess of 3.3σ, but similar models could be used to explain the other excesses as well. The authors sought to create models which are compatible both with these results and the previous stop parameter limits set by ATLAS and CMS.

Table 1: Summary of the selection cuts for the 7 signal regions considered in this search
Table 1: Summary of the selection cuts for the 7 signal regions considered in this search

They first explored the two simplest one-step stop decays. Depending on what the Lightest Supersymmetric Particle (LSP) is, these decays can have different constraints, so they conducted a scan over the entire parameter space. There are two cases for this type of decay: (1) the LSP is a bino (SUSY partner of weak hypercharge boson) and the Next Lightest Super Symmetric Particle (NLSP) is the stop,leading to the simple decay shown in Figure 1a, or (2) the LSP is a higgsino which can be charged or neutral, with each possibility having different masses, which leads to the split decay shown in Figure 1b.

Figure 1: Decay diagrams for the Bino LSP scenario (left) and Higgsino LSP scenario (right)
Figure 1: Decay diagrams for the Bino LSP scenario (left) and Higgsino LSP scenario (right)

We can see in Figure 2 that most (or all in the case of the higgsino) of the preferred 2σ phase-space for these models are ruled out by existing constraints on stop production, so unfortunately these models aren’t particularly promising. Consequently, the authors designed an additional model essentially combing these two processes, where the LSP is a bino and the NLSP is a higgsino. This allows for both one step and two step decays, as shown in Figure 3. The results for this model are much more exciting; almost all of the preferred 2σ phase space exists outside of the existing constraints, as shown in Figure 4!

Figure 2: 2 sigma preferred region and exclusion limits from experiments for Bino LSP (left) and Higgsino LSP (right)
Figure 2: 2 sigma preferred region and exclusion limits from experiments for Bino LSP (left) and Higgsino LSP (right)

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2 additional references are included in the Figure 4 plot. 3 benchmark models for different combinations of higgsino and stop masses are indicated with red crosses, and all of them fall in this allowed phase space. Furthermore, direct dark matter detection limits from the LUX experiment are shown as a dashed black line. The left side of this line has been excluded by LUX, so one of the considered benchmark models is not allowed. The second benchmark model falls near the exclusion line, so upcoming dark matter results will play an important role in telling us if this SUSY model can actually explain the excess!

Figure 3: Decay diagram for the Bino LSP, Higgsino NLSP scenario
Figure 3: Decay diagram for the Bino LSP, Higgsino NLSP scenario
Figure 4: 2 sigma preferred region and exclusion limits for the Bino LSP and Higgsino NLSP model with benchmark points and LUX exclusion limit
Figure 4: 2 sigma preferred region and exclusion limits for the Bino LSP and Higgsino NLSP model with benchmark points and LUX exclusion limit

So, SUSY hasn’t been found at the LHC, but its not dead yet! There are promising excesses in the current ATLAS dataset which are consistent with benchmark MSSM models with expected LSP candidates. We look forward to new data from the LHC and other experiments to tell us more!

References and further reading:

  • Stephen P. Martin, “Supersymmetry primer” (arXiv:hep-ph/9709356)
  • Sven Krippendorf, Fernando Quevedo, Oliver Schlotterer, “Cambridge Lectures on Supersymmetry and Extra Dimensions” (arXiv:1011.1491)
  • John Ellis, “Supersymmetry, Dark Matter, and the LHC” (slides)


Gravity in the Next Dimension: Micro Black Holes at ATLAS

Article: Search for TeV-scale gravity signatures in high-mass final states with leptons and jets with the ATLAS detector at sqrt(s)=13 TeV
Authors: The ATLAS Collaboration
Reference: arXiv:1606.02265 [hep-ex]

What would gravity look like if we lived in a 6-dimensional space-time? Models of TeV-scale gravity theorize that the fundamental scale of gravity, MD, is much lower than what’s measured here in our normal, 4-dimensional space-time. If true, this could explain the large difference between the scale of electroweak interactions (order of 100 GeV) and gravity (order of 1016 GeV), an important open question in particle physics. There are several theoretical models to describe these extra dimensions, and they all predict interesting new signatures in the form of non-perturbative gravitational states. One of the coolest examples of such a state is microscopic black holes. Conveniently, this particular signature could be produced and measured at the LHC!

Sounds cool, but how do you actually look for microscopic black holes with a proton-proton collider? Because we don’t have a full theory of quantum gravity (yet), ATLAS researchers made predictions for the production cross-sections of these black holes using semi-classical approximations that are valid when the black hole mass is above MD. This production cross-section is also expected to dramatically larger when the energy scale of the interactions (pp collisions) surpasses MD. We can’t directly detect black holes with ATLAS, but many of the decay channels of these black holes include leptons in the final state, which IS something that can be measured at ATLAS! This particular ATLAS search looked for final states with at least 3 high transverse momentum (pt) jets, at least one of which must be a leptonic (electron or muon) jet (the others can be hadronic or leptonic). The sum of the transverse momenta, is used as a discriminating variable since the signal is expected to appear only at high pt.

This search used the full 3.2 fb-1 of 13 TeV data collected by ATLAS in 2015 to search for this signal above relevant Standard Model backgrounds (Z+jets, W+jets, and ttbar, all of which produce similar jet final states). The results are shown in Figure 1 (electron and muon channels are presented separately).  The various backgrounds are shown in various colored histograms, the data in black points, and two microscopic black hole models in green and blue lines. There is a slight excess in the 3 TeV region in the electron channel, which corresponds to a p-value of only 1% when tested against the background only hypothesis. Unfortunately, this isn’t enough evidence to indicate new physics yet, but it’s an exciting result nonetheless! This analysis was also used to improve exclusion limits on individual extra-dimensional gravity models, as shown in Figure 2. All limits were much stronger than those set in Run 1.

Figure 1: momentum distributions in the electron (a) and muon (b) channels


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Figure 2: Exclusion limits in the Mth, MD plane for models with various numbers of extra dimensions

So: no evidence of microscopic black holes or extra-dimensional gravity at the LHC yet, but there is a promising excess and Run 2 has only just begun. Since publication, ATLAS has collected another 10 fb-1 of sqrt(13) TeV data that has yet to be analyzed. These results could also be used to constrain other Beyond the Standard Model searches at the TeV scale that have similar high pt leptonic jet final states, which would give us more information about what can and can’t exist outside of the Standard Model. There is certainly more to be learned from this search!



References and further reading: