Search for light pseudoscalar boson pairs produced from Higgs boson decays using the 4$\tau$ and 2$\mu$2$\tau$ final states in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Using 138 fb⁻¹ of CMS data from 13 TeV proton-proton collisions, this study searched for light pseudoscalar boson pairs produced from Higgs boson decays in the 4$\tau$ and 2$\mu$2$\tau$ channels, finding no excess above Standard Model expectations and setting upper limits on the branching fraction, particularly within the context of 2HD+S models.

The Gist

The Cosmic Hide-and-Seek: Hunting for "Ghostly Twins"

Imagine you are at a massive, high-stakes masquerade ball. The star of the show is the Higgs Boson—a famous, heavy, and very important guest. Everyone knows the Higgs is there, and everyone knows how he usually behaves.

However, some scientists suspect that the Higgs has a secret hobby: sometimes, instead of staying as himself, he performs a magic trick and splits into two identical, much smaller, and much faster "ghostly twins" called pseudoscalar bosons (or $a_1$ for short).

These twins are incredibly hard to find. They are small, they move incredibly fast, and they don't stay in one piece for long—they immediately decay into even smaller particles, like tau leptons or muons. It’s like watching a magician throw two handfuls of glitter into a windstorm; you don't see the glitter itself, you only see the tiny, shimmering patterns left in the air.

This paper is a report from the CMS Collaboration at CERN, describing their latest attempt to catch these "glitter patterns" using the world's most powerful microscope: the Large Hadron Collider (LHC).

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The Challenge: Finding Needles in a Haystack of Hay

The researchers were looking for a very specific pattern: a Higgs boson turning into two $a_1$ twins, which then turn into four specific particles (either four "taus" or two "muons" and two "taus").

The problem? The LHC is a chaotic place. It’s like trying to hear a specific person whisper "hello" in the middle of a heavy metal concert. The "noise" (known as QCD background) is deafening. Most of the particles flying around the detector are just random debris from the collisions, not the magical twins we are looking for.

To solve this, the scientists used a few clever tricks:

1. The "Same-Sign" Filter: They looked for two muons with the same electrical charge. In the "normal" world, this is quite rare, so if they see it, it’s a signal that something unusual—perhaps our ghostly twins—might be happening.
2. The "Boosted" Strategy: Because these twins are so light and the Higgs is so heavy, the twins are "boosted"—they fly away from each other with incredible speed. This causes their decay products to be squeezed very close together, like a tightly packed bundle of sticks. The scientists had to develop special "eyes" (algorithms) to recognize these tightly packed bundles.

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The Result: A "No" for Now

After combing through a massive amount of data (equivalent to 138 femtobarns of collisions), the scientists had a result: They didn't find the twins.

They didn't see any "excess" glitter that couldn't be explained by the usual background noise. In science, a "no" is still a huge "yes." By not finding them, they were able to draw a line in the sand. They said, "If these twins exist, they must be even smaller or rarer than we thought."

They set "upper limits." Think of this like a detective saying, "I didn't find the thief, but I can tell you for certain that the thief isn't taller than 5 feet and doesn't weigh more than 150 pounds." They have narrowed down the "hiding spots" where these particles could exist.

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Why Does This Matter?

You might ask, "Who cares about tiny, ghostly twins?"

The reason is that our current "Rulebook of the Universe" (the Standard Model) is incomplete. It explains almost everything, but it fails to explain big mysteries like Dark Matter or why the universe is made of matter instead of antimatter.

The "twins" described in this paper are part of "Extended Higgs" theories. These theories are like "expanded rulebooks" that might finally explain those big mysteries. By searching for these particles, scientists are testing the very foundations of reality, trying to see if the universe is even more complex and magical than we currently believe.

In short: The hunt continues. The twins are still hiding, but the scientists just made the haystack a lot smaller.

Technical Summary

Technical Summary: Search for Light Pseudoscalar Boson Pairs in Higgs Decays

1. Problem Statement

While the Standard Model (SM) has successfully described the Higgs boson discovered at 125 GeV, it remains an incomplete theory, failing to account for dark matter, neutrino masses, and the matter-antimatter asymmetry. Many theoretical extensions, such as the Two-Higgs-Doublet Model plus a Singlet (2HD+S), predict an extended Higgs sector. A key signature of these models is the decay of the SM-like Higgs boson ($H$) into a pair of light pseudoscalar bosons ($a_1$), denoted as $H \to a_1a_1$.

The experimental challenge lies in the kinematic regime where the $a_1$ boson is light (4–15 GeV). In this range, the $a_1$ bosons are highly Lorentz-boosted, causing their decay products (tau leptons or muons) to be highly collimated (overlapping). Furthermore, the low transverse momentum ($p_T$) of the resulting leptons makes them difficult to distinguish from the massive background of QCD multijet production.

2. Methodology

The CMS Collaboration performed a search using proton-proton collision data at $\sqrt{s} = 13$ TeV with an integrated luminosity of $138\text{ fb}^{-1}$.

• Final States: The analysis combines two decay channels:
  1. $4\tau$ channel: $H \to a_1a_1 \to (\tau\tau)(\tau\tau)$.
  2. $2\mu2\tau$ channel: $H \to a_1a_1 \to (\mu\mu)(\tau\tau)$.
• Signal Selection Strategy: To handle the collimated decay products, the study targets a specific topology: nonisolated same-sign muon pairs. Each muon is required to be accompanied by a nearby oppositely charged track (a "1-prong" decay product). This "muon + track" system is used to reconstruct the $a_1$ candidates.
• Discriminant: A two-dimensional (2D) distribution of the invariant masses of the two muon+track systems $(m_1, m_2)$ is used as the final discriminant to separate signal from background.
• Background Modeling: Since QCD multijets are the dominant background, the researchers used a binned template method. They constructed a 1D invariant mass distribution ($f_{1D}$) from a dedicated Control Region (CR) and used a symmetric correlation matrix ($C_{i,j}$) to model the 2D distribution.
• Statistical Inference: A binned maximum-likelihood fit was performed using the COMBINE tool to extract the signal strength modifier, relative to the SM Higgs production cross-section.

3. Key Contributions

• Enhanced Sensitivity: This study significantly improves upon previous CMS searches by using approximately four times more data and employing optimized selection criteria, such as a b-jet veto to suppress heavy-flavor backgrounds.
• Channel Combination: By combining the $4\tau$ and $2\mu2\tau$ channels, the analysis leverages the resonant structure of the muon channel to improve the overall sensitivity, resulting in a 25–35% improvement in expected limits.
• Specialized Reconstruction: The development of a strategy to identify "boosted" leptons (where decay products overlap) addresses a major bottleneck in searching for light exotic Higgs decays.

4. Results

• No Discovery: No significant excess of events over the SM background was observed. The observed data is compatible with the background-only hypothesis (p-value = 0.45).
• Upper Limits: The study sets 95% Confidence Level (CL) upper limits on the product of the Higgs production cross-section and the branching fraction to the $4\tau$ final state.
  • Observed limits: Range from 0.007 (at $m_{a1} = 11$ GeV) to 0.079 (at $m_{a1} = 4$ GeV).
  • Expected limits: Range from 0.011 to 0.066.
• Model Interpretation: The results were applied to 2HD+S models. The Type III 2HD+S model provided the tightest constraints. For $m_{a1}$ between 5 and 15 GeV and $\tan\beta > 2$, the branching ratio $B(H \to a_1a_1)$ exceeding 16% is excluded at 95% CL.

5. Significance

This paper provides some of the most stringent constraints to date on light pseudoscalar bosons produced in Higgs decays. By pushing the boundaries of low-$p_T$ lepton reconstruction and collimated object identification, the CMS Collaboration has significantly narrowed the parameter space for extended Higgs sectors (like the NMSSM or 2HD+S). This work is vital for guiding future searches for New Physics and understanding the potential complexity of the Higgs sector.