Tests of the Standard Model by means of ¡ (3𝑆)
meson decays with the BABAR detector
Hossain Ahmed
St. Francis Xavier Universityhahmed@stfx.ca
On behalf of the BaBar Collaboration

Tests of the Standard Model by means of ¡ (3𝑆)
meson decays with the BABAR detector§ Lepton Universality
ü Precision Measurement of the Ratio ℬ(Υ(3𝑆) → 𝜏 !𝜏 ")/ℬ(Υ(3𝑆) → 𝜇!𝜇")
Phys. Rev. Lett. 125, 241801 (2020) by BABAR Collaboration§ Charged Lepton Flavor Violation
ü Search for Lepton Flavor Violation in 𝛶(3𝑆) → 𝑒 ±𝜇∓
A journal (PRL) paper will be submitted in the near future

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Motivation one
q Lepton Universality• In the SM, the branching fraction for the decay of Υ 𝑛𝑆 → 𝑙 !𝑙 "(𝑛 = 1,2,3 & 𝑙 =𝑒, 𝜇, 𝜏) is independent of the flavour of 𝑙 excluding a tiny lepton mass effect• Any deviation from the unity for the ratio of branching fractions would indicate thenew physics• Lepton Universality, therefore, is an excellent test of the SM prediction• Leptonic decays of the Υ 𝑛𝑆 mesons are also important in search for phenomena
beyond the SM

𝐴# → 𝑐𝑝 𝑜𝑑𝑑 − 𝐻𝑖𝑔𝑔𝑠
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Motivation twoq Charged Lepton Flavor Violation (CLFV)• Observation of mixing between the neutrino flavour eigenstate permits the CLFV
in higher-order• However, in the Standard Model (SM), CLFV is highly suppressed due to the small
neutrino masses, e.g.

∆%"!
&"#'• Observation of CLFV is, therefore, a clear sign of new physics (NP) beyond theSM

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BaBar Detector
Between Dec. 2007 – Apr. 2008, BaBar collected data below Υ(4𝑆)592 papers published,
June 2021 data!

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Analysis Modes
Lepton Universalityℬ(Υ(3𝑆) → 𝜏 "𝜏 !)/ℬ(Υ(3𝑆) → 𝜇"𝜇!)

CLFV
𝛶(3𝑆) → 𝑒 ±𝜇∓§ 27.96 fb-1 Υ 3𝑆 on-peak data
§ 27.9 fb-1 Υ 3𝑆 on-peak data
§ 78.3 fb-1 Υ 4𝑆 on-peak data and§ 78.31 fb-1 Υ 4𝑆 on-peak data for§ 7.75 fb-1 Υ 4𝑆 off-peak data (40 MeV
sytematic studies (data driven
below the on-peak) andcontinuum background)§ 2.62 fb -1 Υ 3𝑆 off-peak data (40§ 2.62 fb -1 Υ 3𝑆 off-peak data (40MeV below the on-peak) as control
MeV below the on-peak) andsamples§ 7.75 fb-1 Υ 4𝑆 off-peak data (40 MeV§ Control samples are used to evaluate
below the on-peak) to validate theproperties of background, to study
systematic studies
systematic sffects, and to calculate§ MC signal: 𝑒 "𝑒 ! → Υ 3𝑆 →corrections to MC based efficiencies!
𝑒 ±𝜇∓: 103000 events

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Lepton Universalityℬ(Υ(3𝑆) → 𝜏 ! 𝜏 " )/ℬ(Υ(3𝑆) → 𝜇! 𝜇" ) Phys. Rev. Lett. 125, 241801

v Monte Carlo (MC) samples:Ø Continuum: 𝜏 "𝜏 !, 𝜇"𝜇!, 𝐵ℎ𝑎𝑏ℎ𝑎𝑠, 𝑢𝑑𝑠, 𝑐 𝑐(̅ 𝑠 = 𝑚*(,-) 𝑎𝑛𝑑 𝑠 = 𝑚*(/-) )Ø 𝜏 "𝜏 !, 𝜇"𝜇! -- KKMC (with radiative effects)Ø Bhabhas –BHWIDEØ Hadronic continuum and generic Υ(3𝑆) – EvtGen (PHOTOS)Ø Υ 3𝑆 → 𝜏 "𝜏 !, 𝜇"𝜇! signal events – KKMC (ISR turned off) – Signal MC
sample is about three times the size of the data sample
Ø GEANT4 for detector acceptancev Event selections:
Ø two oppositeley charged tracks (each in one hemisphere)
𝝁! 𝝁"

𝝉! 𝝉"

§ At least one 𝜇 hit IFR (suppressing Bhabha)§ 0.8 < 𝑀)) / 𝑠 <1.1
§ 99.9% purity

8/19/21§ One track is required to be identified as anelectron (based on PID) and the other doesn’t§ Angle between two track in the center-of-mass >110o§ 𝑐𝑜𝑠𝜃*+,, < 0.85 in the center-of-mass frame§ 98.9 % purity

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Lepton Universalityℬ(Υ(3𝑆) → 𝜏 ! 𝜏 " )/ℬ(Υ(3𝑆) → 𝜇! 𝜇" ) Phys. Rev. Lett. 125, 241801

q Data Analysis§ Off-resonance data (𝛶 4𝑆 , 𝛶(3𝑆)) are used to correct the differences
between MC and data (𝜏 "𝜏 !/ 𝜇"𝜇!) selection efficiency ratios§ Continuum modeling:

ISR effect ishighlysuppressed!
----- 23%; Υ 4𝑆 off-resonance___7%; Υ 3𝑆 decay in MC simulation§ For the 𝜏 "𝜏 ! events the total reconstructed event energy scaled to thecenter-of-mass energy 𝐸00 / 𝑠 is plotted§ Cascade decays are considered (via radiative and hadronic transitions)
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q Data Analysis: Fitting*(,-)§ To extract the ratio ℛ01 , a binned maximum likelihood fit is employedon 𝑀11 / 𝑠 and 𝐸00 / 𝑠§ Υ(3𝑆) → 𝜇"𝜇! and Υ(3𝑆) → 𝜏 "𝜏 ! are taken from KKMC (no ISR)
§ Υ(2𝑆) → 𝑙 "𝑙 !, Υ(1𝑆) → 𝑙 "𝑙 !, and Υ(𝑛𝑆) → ℎ𝑎𝑑𝑟𝑜𝑛𝑠 are from EvtGen
MC8/19/21§ Cascade decays are clearly separated in dimuon events and nearly
indistinguishable in 𝜏 "𝜏 ! events.
§ Continuum templates are then use data control samples: Υ(4𝑆) run6 data
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Lepton Universalityℬ(Υ(3𝑆) → 𝜏 ! 𝜏 " )/ℬ(Υ(3𝑆) → 𝜇! 𝜇" ) Phys. Rev. Lett. 125, 241801

q Data Analysis: Fit Result§ 𝑀11 / 𝑠 and 𝐸00 / 𝑠 are simultaneously fit using MC and data derived
templates§ The free parameters of the fit are the number of Υ(3𝑆) → 𝜇"𝜇! events
2
𝑁11 and the raw ratio 𝑅M01 = ..
2//

§ Systematics:

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Lepton Universalityℬ(Υ(3𝑆) → 𝜏 ! 𝜏 " )/ℬ(Υ(3𝑆) → 𝜇! 𝜇" ) Phys. Rev. Lett. 125, 241801

q Data Analysis: Fit Result§ 𝑅M01 is the fit result, CMC is the data/MC correction, 𝜀11 /𝜀00 is the MC
selection efficiency, and 𝛿334 is the correction from 𝐵𝐵Q events§ Using Υ(3𝑆) data with Υ 4𝑆 and off-resonance control samples BaBar
measures the ratio of the leptonic branching fractions of the Υ(3𝑆) mesonℬ(*(,-)→00 01 )*(,-)
is: ℛ01 == 0.966 ± 0.0087898 ± 0.0147:78ℬ(*(,-)→10 11 )= 0.966 ± 0.0168;8§ Six times more precise than the CLEO measurement [PRL 98, 052002(2007)]§ The final ratio is with 2𝜎 of the SM value 0.9948 [J. High Energy Phys.
06 (2017) 019]
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Charged Lepton Flavor Violation
q Theory:
Forbidden in the SM
Lepton Flavor Violation

𝛶(3𝑆) → 𝑒 ± 𝜇∓

S. Nussinov, et. al.
Phys.Rev. D63 (2001) 016003

Re-ordering
incoming/outgoing particles§ Compare the re-ordered diagram with the ordinary Muon decay𝜞 𝝁 → 𝟑𝒆 𝑽"𝒆𝒙𝒄𝒉𝒂𝒏𝒈𝒆≈ 𝑩𝑹 𝝁 → 𝟑𝒆𝜞(𝝁 → 𝒆𝝂(𝝂)𝜞(𝑽 → 𝒆+ 𝒆" )𝜞(𝑽 → 𝒆± 𝝁∓ ) 𝑴𝑾 𝟔𝑽"𝒆𝒙𝒄𝒉. ≈𝜞𝟐 (𝑾 → 𝒆𝝂)𝑴𝑽Existing Measurements Results

CL (%)

BF(Υ(3𝑆) → 𝑒 ± 𝜏 ∓ )< 4.2 × 10-690

BF(Υ(3𝑆) → 𝜇 ± 𝜏 ∓ )< 3.1 × 10-690

BF(Υ(3𝑆) → 𝜇 ± 𝜏 ∓ )< 20.3 × 10-695

Collaboration
J.P. Lees et al. PR D89 111102[BaBar Collaboration]
Love et al. PRL 101, 201601[CLEO Collaboration]§ No published experimental measurement of the decay on 𝜰(𝟑𝑺)→ 𝑒 ±𝜇∓yet!
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Charged Lepton Flavor Violation
𝛶(3𝑆) → 𝑒 ± 𝜇∓q Data Analysis: selection and reconstruction§§§§§§

8/19/21𝑝)𝑠 ∗ 0.5−13< 0.01

two oppositely charged tracks (directlyfrom 𝛶(3𝑆) à signal candidates!)
momentum close to beam energy(𝐸% =§3𝑝2−1 +𝑠 ∗ 0.5&'))*
angle between two tracks: 𝜃('>179+
energy deposit by muons on EMC >50 MeVEMC acceptance 24+ < 𝜃,-. < 130+
main sources of backgrounds: 𝑒 ! 𝑒 " →𝜏 ! 𝜏 " , 𝜇! 𝜇" 𝛾 , 𝑒 ! 𝑒 " (𝛾)in second stage tighter and optimized
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Charged Lepton Flavor Violation
𝛶(3𝑆) → 𝑒 ± 𝜇∓q Data Analysis: results

q Branching Fraction:

𝑵𝐂𝐚𝐧𝐝𝐢𝐝𝐚𝐭𝐞 "𝑵𝑩𝑮
=𝜺𝒔𝒊𝒈× 𝑵𝚼

𝟏. 𝟎 ± 𝟏. 𝟒𝒔𝒕𝒂𝒕(𝑵𝑪𝒂𝒏𝒅𝒊𝒅𝒂𝒕𝒆) ± 𝟎. 𝟖𝒔𝒚𝒔𝒕 ×𝟏𝟎"𝟕

q Set an upper limit at 90% confidence level:[J.Phys.G 28 (2002) 2693-2704]
q Paper is now in final preparation for PRL
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Summary• BABAR has made a significant contribution to Lepton Universality search byℬ(Υ(3𝑆) → 𝜏 "𝜏 !)/ℬ(Υ(3𝑆) → 𝜇"𝜇!) and verify the SM prediction-- the result is six times precise than the only previous measurement byCLEO.
-- result published in Phys. Rev. Lett. 125, 241801• No significant evidence for Charged Lepton Flavor Violation in 𝛶(3𝑆) → 𝑒 ±𝜇∓decay and an upper limit has been set.

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Backup Slides (LU)
§ Extended van Royen–Weisskopf formalism for lepton–antilepton meson decaywidths:

q Muon selection: more

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Backup Slides (LU)q Tau selection: more

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Backup Slides (CLFV)v Monte Carlo (MC) samples:Ø Continuum: 𝜏 "𝜏 !, 𝜇"𝜇!, 𝐵ℎ𝑎𝑏ℎ𝑎𝑠, 𝑢𝑑𝑠, 𝑐 𝑐(̅ 𝑠 = 𝑚*(,-) 𝑎𝑛𝑑 𝑠 = 𝑚*(/-) )Ø 𝜏 "𝜏 !, 𝜇"𝜇! -- KKMC (with radiative effects)Ø Bhabhas –BHWIDEØ Hadronic condinuum and generic Υ(3𝑆) – EvtGen (PHOTOS)v Limit of New Physics (NP):Ø This result is the first reported experimental upper limit on its kindØ It can be interpreted as a limit on NP using the relationship:@90% CL

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Backup Slides (CLFV)v Systematics

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