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Title: Search for lepton flavor violating decay τ - →ℓ -ℓ +ℓ -ℓ = e, μ at BaBar

Abstract

The Standard Model (SM) is one of the most tested and verified physical theories of all time, present experimental observations are consistent with SM expectations. On the other hand SM can not explain many physical observations: the cosmological observations possibly infer the presence of dark matter which is clearly beyond the SM expectations; the SM Higgs model, while explaining the generation of fermion masses, can not explain the hierarchy problem and a non natural fine tuning of SM is needed to cancel out quadratic divergences in the Higgs boson mass. New physics (NP) beyond SM should hence be investigated: rising the energy above NP processes thresholds, and detecting new particles or new effects not predicted by the standard model directly, is one of the possible approaches; another approach is to make precision measurements of well known processes or looking for rare processes which involve higher order contribution from NP processes, this approach need higher luminosities with respect to the previous approach but lower beam energies. Search for Lepton Flavor Violation (LFV) in charged lepton decays is promising: neutrino physics provides indeed a clear and unambiguous evidence of LFV in the neutral lepton sector via mixing processes, which have been observedmore » for the first time by the Homestake collaboration. We expect LFV in the charged sector as well, both in {mu} and {tau} sector, but current experimental searches for LFV processes did not find any evidence for those processes, and more results are expected to come from new experiments in the coming years.« less

Authors:
 [1]
+ Show Author Affiliations
  1. Univ. of Pisa (Italy)
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
982087
Report Number(s):
SLAC-R-943
TRN: US1004373
DOE Contract Number:
AC02-76SF00515
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ACCURACY; DECAY; FERMIONS; HIGGS BOSONS; HIGGS MODEL; LEPTONS; NEUTRINOS; NONLUMINOUS MATTER; PHYSICS; STANDARD MODEL; TUNING; Experiment-HEP; HEPEX

Citation Formats

Cervelli, Alberto. Search for lepton flavor violating decay τ- →ℓ-ℓ+ℓ-ℓ = e, μ at BaBar. United States: N. p., 2010. Web. doi:10.2172/982087.
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Cervelli, Alberto. Search for lepton flavor violating decay τ- →ℓ-ℓ+ℓ-ℓ = e, μ at BaBar. United States. doi:10.2172/982087.
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Cervelli, Alberto. Wed . "Search for lepton flavor violating decay τ- →ℓ-ℓ+ℓ-ℓ = e, μ at BaBar". United States. doi:10.2172/982087. https://www.osti.gov/servlets/purl/982087.
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@article{osti_982087,
title = {Search for lepton flavor violating decay τ- →ℓ-ℓ+ℓ-ℓ = e, μ at BaBar},
author = {Cervelli, Alberto},
abstractNote = {The Standard Model (SM) is one of the most tested and verified physical theories of all time, present experimental observations are consistent with SM expectations. On the other hand SM can not explain many physical observations: the cosmological observations possibly infer the presence of dark matter which is clearly beyond the SM expectations; the SM Higgs model, while explaining the generation of fermion masses, can not explain the hierarchy problem and a non natural fine tuning of SM is needed to cancel out quadratic divergences in the Higgs boson mass. New physics (NP) beyond SM should hence be investigated: rising the energy above NP processes thresholds, and detecting new particles or new effects not predicted by the standard model directly, is one of the possible approaches; another approach is to make precision measurements of well known processes or looking for rare processes which involve higher order contribution from NP processes, this approach need higher luminosities with respect to the previous approach but lower beam energies. Search for Lepton Flavor Violation (LFV) in charged lepton decays is promising: neutrino physics provides indeed a clear and unambiguous evidence of LFV in the neutral lepton sector via mixing processes, which have been observed for the first time by the Homestake collaboration. We expect LFV in the charged sector as well, both in {mu} and {tau} sector, but current experimental searches for LFV processes did not find any evidence for those processes, and more results are expected to come from new experiments in the coming years.},
doi = {10.2172/982087},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed May 26 00:00:00 EDT 2010},
month = {Wed May 26 00:00:00 EDT 2010}
}
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Thesis/Dissertation:
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DOI:  10.2172/982087
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  • This thesis presents results on the search for the lepton-flavor-violating decay K{sub L}{sup 0} {yields} {mu}e. This decay is forbidden in the Standard Model while many alternate models allow it at various rates. These data were collected using a new kaon spectrometer at the Brookhaven National Laboratory Alternating Gradient Synchrotron facility during May, 1987. The sensitivity of the data set is calculated from approximately one million effective K{sub L}{sup 0} {yields} {pi}{sup +}{pi}{sup {minus}} decays. Two K{sub L}{sup 0} {yields} {mu}{mu} events were identified, a quantity consistent with the known branching ratio for this decay. No candidate K{sub L}{sup 0}more » {yields} {mu}e events were detected and a 90% confidence level limit on the branching ratio of B(K{sub L}{sup 0} {yields} {mu}e) < 0.98 {times} 10{sup {minus}8} was established. These results represent a significant improvement over the previous limits and serve to constrain some models that allow for lepton flavor violation. The sensitivity of this result probes, through virtual effects, a mass scale on the order of 20 TeV.« less

  • We present the search for the lepton flavour violating decay τ→ lK 0 s with the BaBar experiment data. This process and many other lepton flavour violating τ decays, like τ→ μγ and τ→ lll, are one of the most promising channel to search for evidence of new physics. According to the Standard Model and the neutrino mixing parameters, branching fractions are estimated well below 10 -14, but many models of new physics allow for branching fractions values close to the present experimental sensitivity. This analysis is based on a data sample of 469fb -1 collected by BABAR detector atmore » the PEP-II storage ring from 1999 to 2007, equivalent to 431 millions of τ pairs. the BABAR experiment, initially designed for studying CP violation in B mesons, has demonstrated to be one of the most suitable environments for studying τ decays. The tracking system, the calorimeter and the particle identification of BABAR, together with the knowledge of the τ initial energy, allow an extremely powerful rejection of background events that, for this analysis, is better than 10 -9. Being τ→lK 0 s a decay mode without neutrinos, the signal τ decay can be fully reconstructed. Kinematical constraints are used in a fit that provides a decay tree reconstruction with a high resolution. For this analysis MC simulated events play a decisive role for estimating the signal efficiency and study the residual background. High statistics MC sample are produced simulating detector conditions for different periods of data collection, in order to reduce any discrepancies with the data. When discrepancies can not be removed, we perform studies to compute a correction factor or an estimation of systematic errors that need to be included in the final measurement. A significant improvement of the current result can be reached only with a higher statistics and, therefore, with a new collider providing a luminosity from 10 to 100 times more than PEP-II. A new detector, with improved performance and able to collect data in a high background environment, is also requested to fully exploit the capability of such amount of data. In fact, only keeping the efficiency and the background as similar as possible to present ones, we will be able to scale almost linearly the estimated upper limit according to the luminosity. The strong potential of improvement for the search of lepton flavour violation τ decays makes the building of such a machine highly desirable.« less

  • In this thesis, work undertaken on the D0 Silicon Microstrip Tracker is described before a study of signed impact parameters in the channel Z 0 → τ +τ - → e ± v e μ ±more » $$\bar{v}$$ μ v τ $$\bar{v}$$ τ is presented.« less

  • The rare decay B → πℓℓ - is the simplest manifestation of a b → dℓ +ℓ - flavor-changing neutral current (FCNC) process. This type of process only proceeds through penguin loop or box diagrams and is sensitive to physics at the electroweak scale. It can be used to constrain parameters of the Standard Model and its extensions. B → πℓ +ℓ - events have not yet been observed; the branching fraction is expected to be an order of magnitude smaller than the measured branching fraction for the similar B → Kℓ +ℓ - decay. Using 230 million Bmore » $$\bar{B}$$ meson pairs collected with the BABAR detector, we have done a search for the rare decay B →πℓ +ℓ - . The data was produced in e +e - collision at the Y(4S) resonance in the PEP-II collider between 1999 and 2004. Four exclusive B-meson decay modes have been reconstructed: B + → π +ℓ +ℓ - and B 0 → π 0ℓ +ℓ - , where ℓ + ℓ - is either an electron pair (e +e -) or a muon pair (μ +μ -). We find no evidence for a signal, and we obtain upper limits on the branching fractions β. Assuming the isospin relation β(B + → π +ℓ +ℓ - ) 2 x τB +/ τB 0 β(B 0 → π 0ℓ +ℓ -), we obtain an upper limit at 90% confidence level on the lepton-flavor-averaged branching fraction of B → πℓ +ℓ - to be β(B → πeμ) < 9.2 x 10 -8 at 90% C.L. We have also reconstructed two control modes B +→ π +e ±μ ± and B 0 → π 0e ±μ ± and we also obtain an upper limit at 90% confidence level on the lepton-flavor-violating decay B → πeμ of β(B → πeμ) < 9.2 x 10 -8 at 90% C.L. This is the first search for these rare decays at the current B-Factory experiments. This limit is an improvement by four orders of magnitude with respect to the previous experimental limit, and about a factor three above Standard Model predictions.« less