CMS-PAS-EXO-24-012

CMS-PAS-EXO-24-012
Search for a low mass resonance decaying to $ \tau\tau $ using data collected with a dedicated high-rate data stream
CMS Collaboration
7 May 2025

Abstract: An inclusive search for a low-mass resonance decaying to two $ \tau $ leptons is presented, probing the mass range from 20 to 60 GeV for the first time at a hadron collider. The search uses proton-proton collision data at $ \sqrt{s}= $ 13.6 TeV corresponding to an integrated luminosity of 61.9 fb$^{-1}$ collected by the CMS experiment in 2022 and 2023. The data is collected using a high-rate trigger stream that records limited event information which allows for lower thresholds. A novel algorithm is used to reconstruct hadronic $ \tau $ decays with very low momentum. No significant excess is observed, and upper limits are set on the production cross section times branching fraction to $ \tau $ lepton pairs.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Resonance shape consistent with $ \Upsilon $ simulation (left, center) and SM candle Z (right). The backgrounds for the $ \Upsilon $ region are estimated using a second order Chebychev polynomial, while the resonance is modeled by a Gaussian distribution. A fit to $ \Upsilon $ simulation is overlaid on the resonance shape extracted from data with arbitrary scaling. The Z backgrounds are determined by simulation scaled to expected cross section.
png pdf	Figure 1-a: Resonance shape consistent with $ \Upsilon $ simulation (left, center) and SM candle Z (right). The backgrounds for the $ \Upsilon $ region are estimated using a second order Chebychev polynomial, while the resonance is modeled by a Gaussian distribution. A fit to $ \Upsilon $ simulation is overlaid on the resonance shape extracted from data with arbitrary scaling. The Z backgrounds are determined by simulation scaled to expected cross section.
png pdf	Figure 1-b: Resonance shape consistent with $ \Upsilon $ simulation (left, center) and SM candle Z (right). The backgrounds for the $ \Upsilon $ region are estimated using a second order Chebychev polynomial, while the resonance is modeled by a Gaussian distribution. A fit to $ \Upsilon $ simulation is overlaid on the resonance shape extracted from data with arbitrary scaling. The Z backgrounds are determined by simulation scaled to expected cross section.
png pdf	Figure 1-c: Resonance shape consistent with $ \Upsilon $ simulation (left, center) and SM candle Z (right). The backgrounds for the $ \Upsilon $ region are estimated using a second order Chebychev polynomial, while the resonance is modeled by a Gaussian distribution. A fit to $ \Upsilon $ simulation is overlaid on the resonance shape extracted from data with arbitrary scaling. The Z backgrounds are determined by simulation scaled to expected cross section.
png pdf	Figure 2: Distributions of $ \tau_{\mu} \tau_\mathrm{h} $ final state mass (points) in each hadronic decay mode with regressions (lines). All plots contain a $ \mathrm{Z}/\gamma^{*}\to\tau\tau $ component determined by simulation overlaid on the continuum fit. The leftmost plot shows the single prong final state with pollution due to $ \mathrm{Z}\to\mu\mu $ events (orange) at 91 GeV. The center and right plots show the three prong and one prong plus one or two strips respectively. The $ m_{vis} $ range corresponding to $ \phi $ masses from 20 to 60 GeV is 10 to 40 GeV.
png pdf	Figure 2-a: Distributions of $ \tau_{\mu} \tau_\mathrm{h} $ final state mass (points) in each hadronic decay mode with regressions (lines). All plots contain a $ \mathrm{Z}/\gamma^{*}\to\tau\tau $ component determined by simulation overlaid on the continuum fit. The leftmost plot shows the single prong final state with pollution due to $ \mathrm{Z}\to\mu\mu $ events (orange) at 91 GeV. The center and right plots show the three prong and one prong plus one or two strips respectively. The $ m_{vis} $ range corresponding to $ \phi $ masses from 20 to 60 GeV is 10 to 40 GeV.
png pdf	Figure 2-b: Distributions of $ \tau_{\mu} \tau_\mathrm{h} $ final state mass (points) in each hadronic decay mode with regressions (lines). All plots contain a $ \mathrm{Z}/\gamma^{*}\to\tau\tau $ component determined by simulation overlaid on the continuum fit. The leftmost plot shows the single prong final state with pollution due to $ \mathrm{Z}\to\mu\mu $ events (orange) at 91 GeV. The center and right plots show the three prong and one prong plus one or two strips respectively. The $ m_{vis} $ range corresponding to $ \phi $ masses from 20 to 60 GeV is 10 to 40 GeV.
png pdf	Figure 2-c: Distributions of $ \tau_{\mu} \tau_\mathrm{h} $ final state mass (points) in each hadronic decay mode with regressions (lines). All plots contain a $ \mathrm{Z}/\gamma^{*}\to\tau\tau $ component determined by simulation overlaid on the continuum fit. The leftmost plot shows the single prong final state with pollution due to $ \mathrm{Z}\to\mu\mu $ events (orange) at 91 GeV. The center and right plots show the three prong and one prong plus one or two strips respectively. The $ m_{vis} $ range corresponding to $ \phi $ masses from 20 to 60 GeV is 10 to 40 GeV.
png pdf	Figure 3: 95% CL upper limits on production cross section for $ \mathrm{p}\mathrm{p}\to\phi\to\tau\tau $ in pb. The dotted line represents the median expected limits while the yellow and blue bands indicate the 68 and 95% expected bands, respectively. The solid line shows the observed limits.

Tables
png pdf	Table 1: Summary of analysis selections for the $ \phi $, Z, and $ \Upsilon $ regions. The offline trigger object cuts are not applied for the $ \Upsilon $ region.
png pdf	Table 2: Summary of the systematic uncertainties in the signal yield and shape. The ``lnN'' denotes lognormal distribution.

Summary

In summary, an inclusive search for new low mass scalar $ \phi $ bosons decaying to two $ \tau $ leptons has been performed in a never before searched mass regime at CMS or ATLAS. The search uses proton-proton collision data at $ \sqrt{s}= $ 13.6 TeV, collected with the CMS detector at the LHC in 2022 fb$^{-1}$2023. A high-rate data stream with low trigger thresholds and a compact data format, a strategy known as data scouting, is used to enhance sensitivity to low masses. We implement a new hadronic $ \tau $ reconstruction methodology demonstrating sensitivity to extremely low $ p_{\mathrm{T}} \tau_\mathrm{h} $ decays.

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