Searching for supersymmetric needles in the LHC haystack

20 November 2023 | By

Searching for supersymmetric particles at the LHC is no easy task. Supersymmetry (SUSY) predicts a vast zoo of new particles, with even the simplest version of the theory more than doubling the number of known particles. Each of these new particles may decay in multiple different ways, leading to many possibilities for experimental signatures. That creates a lot of different needles in a huge haystack – so where do scientists start?

It is always good to begin with the low-hanging fruit – the superpartners of the gluon and quarks, the gluino and squarks, as these particles should be abundantly produced in the LHC if they are light enough. Unfortunately, no evidence of squarks and gluinos has been found so far, with researchers setting the lower bounds on the possible masses at a whopping 2 TeV. The more difficult supersymmetric particles to search for are the rarer, electroweak-produced superpartners of leptons, and the W, Z, and Higgs bosons: the sleptons, charginos and neutralinos. The ATLAS Collaboration has published three new results in this challenging area, using Boosted Decision Trees (BDTs) to isolate potential signals and ultimately set stringent constraints on the masses of SUSY particles.

Pushing new limits in the search for staus

Figure 1: The exclusion contour for mass-degenerate left and right-handed stau pair production. The red line encloses the area excluded by this analysis. This result pushes the sensitivity up to higher masses than the previous result shown in grey and closes the previous gap left at 87-110 GeV between the LEP limit (brown) and LHC limits. (Image: ATLAS Collaboration/CERN)

The search for the stau, the superpartner of third-generation leptons, is challenging on two fronts: its production rate is very low and its experimental signature is difficult to isolate. If staus exist, they may have played a pivotal role in creating the observed dark matter content in the universe by co-annihilating with a SUSY dark-matter candidate particle—the lightest neutralino.

At the LHC, staus are expected to decay into a lightest neutralino and a Standard-Model tau lepton. The neutralino would pass through the ATLAS detector unseen, but their presence could be inferred through an imbalance in energy conservation (missing energy). To spot the tau leptons, researchers use sophisticated reconstruction algorithms to detect their decay into a narrow spray of pions.

The search for stau pair production at ATLAS hinges on identifying two high-momentum taus and significant missing energy. The ATLAS Collaboration's latest approach used four BDTs each tailored to a specific region of the stau-neutralino mass parameter space. These BDTs significantly improve the separation between signal and background compared to previous searches. Despite these improvements, no significant excess in the data was observed. ATLAS researchers have set world-leading constraints on the stau mass, excluding mass-degenerate left and right-handed staus from 80 GeV to 480 GeV, thus closing the previous gap between LEP and LHC limits (see Figure 1). They also set the first ATLAS limit for right-handed-only staus, excluding masses up to 330 GeV.

Together, these new analyses showcase the cutting-edge techniques being used to explore electroweakino production at the LHC.

Exploring electroweakino pair production

Figure 2: Post-fit distribution of the data (black points) and contributions from expected Standard Model processes (solid histograms) in the signal regions of Wh channel for the BDT signal output score. Two representative SUSY signal models are overlaid in purple dashed lines for illustration. The uncertainty bands plotted include all statistical and systematic uncertainties. (Image: ATLAS Collaboration/CERN)

ATLAS researchers also set out to investigate electroweakino pair production, specifically chargino pairs and their interactions with neutralinos. The lightest neutralino, in addition to being a dark matter candidate particle, may hold the key to understanding the discrepancy between the g-2 measurement and Standard Model predictions.

For their new study, researchers looked for charginos decaying in three ways – via two W bosons (WW), a W boson and a Z boson (WZ), or a W boson and a Higgs boson (WH). These decay channels can all result in similar experimental signatures with one lepton. Researchers looked for unique collision-event signatures with isolated leptons, missing momentum and large-radius jets (or b-jets in the WH case). They applied improved cut-and-count strategies in the WW/WZ cases, and revised the previous cut-and-count WH analysis with new machine-learning techniques (see Figure 2). Using a BDT, researchers were able to enhance signal identification in scenarios where the chargino and next-to-lightest neutralino decays were mediated by a Higgs boson, or when their mass difference closely aligns with the mass of the Higgs boson itself.

While researchers found no significant deviations from well-established Standard Model expectations, they set new constraints on the potential masses of charginos and neutralinos. Notably, they have excluded chargino masses from 260 to 520 GeV in the WW scenario, and from 260 to 420 GeV in the WZ scenario. Remarkably, the incorporation of the BDT-based approach resulted in more stringent constraints, improving mass limits by up to 40 GeV within the mass range of 200 - 260 GeV and 280 - 470 GeV for the chargino/next-to-lightest neutralino system.

Hunting for higgsinos

Figure 3: The exclusion contour for pair-produced higgsinos decaying to either Higgs or Z bosons. The x-axis displays the mass of the produced higgsinos, while the y-axis shows the probability that the higgsino decays to a gravitino and higgs boson. Values of higgsino mass and decay probability above the line are excluded. The exclusion from both low-mass and high-mass channels is joined at the optimal point of 250 GeV, covering a large portion of the available phase space up to masses of 940 GeV. (Image: ATLAS Collaboration/CERN)

The discovery of the Higgs boson opened the door to a whole new world of supersymmetric particles: higgsinos. In "natural SUSY" models, which solve the mass hierarchy problem of the Standard Model, higgsinos are predicted to be light enough to be produced at the LHC.

In models where SUSY is broken by mediation of gauge interactions, a very light gravitino (the superpartner of the graviton) arises. The lightest neutralino can then decay to this new gravitino plus a Standard Model boson, with a Higgs boson expected to be produced in cases where the higgsino component dominates.

A recent ATLAS search targeted the production of higgsino-dominated neutralino pairs, focusing on events with two Higgs bosons and missing transverse energy (the invisible gravitinos) (see event display). The topology of such events depends significantly on the neutralino mass, thus different search strategies are used in the low-mass and high-mass channels. For the low-mass channel, researchers took a data-driven approach to predict backgrounds in the analysis regions, using a powerful statistical fit to enhance their sensitivity. For the high-mass channel, they used a BDT for excellent signal-to-background discrimination, pushing the mass reach through boosted-Higgs reconstruction techniques. Together, the low-mass and high-mass searches exclude masses up to 940 GeV for higgsinos decaying to Higgs bosons (see Figure 3).

Together, these new analyses showcase the cutting-edge techniques being used to explore electroweakino production at the LHC. They provide an affirmation of Standard Model predictions, while also increasing the mass limits associated with these enigmatic particles.

About the banner image: Event display for a higgsino-like event in the low-mass channel of the multi-b search. Four jets (yellow cones) produced in the decay of the two Higgs boson candidates are observed, with low missing transverse momentum. (Image: ATLAS Collaboration/CERN)

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