Search for neutrinoless decays τ→3ℓ

We have searched for neutrinoless τ lepton decays into three charged leptons using an 87.1fb-1 data sample collected with the Belle detector at the KEKB e+e- collider. Since the number of signal candidate events is compatible with that expected from the background, we set 90% confidence level upper limits on the branching fractions in the range (1.9-3.5)×10-7 for various decay modes τ -→ℓ-ℓ+ℓ- where ℓ represents e or μ. © 2004 Published by Elsevier B.V.

A search for heavy long-lived staus in the LHCb detector at √s = 7 and 8 TeV

Thi Viet Nga La

The Large Hadron Collider (LHC) has been producing pp collisions at 7 and 8 TeV since 2010 and promises a new era of discoveries in particle physics. One of its experiments, the Large Hadron Collider beauty (LHCb) experiment, was constructed to study CP violation in the B meson system. In addition to B physics, new Physics beyond the Standard Model can also be searched for at this single-arm forward spectrometer. With the different sub-detectors and the high resolution of the tracking system, the LHCb detector has the ability to search for heavy, long-lived and charged particles, which are predicted by extensions of the Standard Model. One of these extensions, the minimal Gauge Mediated Supersymmetry Breaking (mGMSB), proposes such a particle, named stau (τ~) - the SUSY bosonic counterpart of the heavy lepton tau (τ). The theory proposes that the staus may be pair-produced in pp collisions or in the decays of heavier particles, and have only electromagnetic interactions with the atoms of the medium like the muons. Therefore, we expect that at the energy of the LHC these particles can be produced if they do exist and that we have a chance to discover them at LHCb, as well as at the other experiments of the LHC. This thesis is dedicated to the search for stau pairs produced in pp collisions at the centre-of-mass energies √s = 7 and 8 TeV in the LHCb detector. For this purpose, we generated the stau pairs with seven different particle masses ranging from 124 to 309 GeV/c2 and simulated their path through the LHCb detector, as well as their muon background from the decays Z0, γ∗ → μ+μ−. Based on the results from the simulation, a set of cuts are then defined to select the stau pairs. Some muon pairs at high energies will also pass the selection cuts. Thus, to separate the stau pairs from the muon pairs, the Neural Network technique has been used. A first Neural Network has been used to distinguish the stau tracks from the muon tracks using their signals left in the sub-detectors: the VELO silicon detector, the electromagnetic calorimeter, the hadron calorimeter and the RICH detectors. Then, two methods to select the stau pairs have been developed: the first one is based on the product of the two responses from the first Neural Network (NN1) for the two tracks, the second one employs a second Neural Network to separate the stau pairs from the muon pairs by using the above product of the two NN1 responses and the invariant mass of pair. Finally, a favourable region for the staus finding has been defined and the expected numbers of stau and muon pairs in this region have been evaluated. The training of the Neural Network has been achieved with the Monte Carlo variables, then the trained Neural Network has been used to classify the data. The data used in our work were collected by the LHCb experiment in 2011 and 2012 and correspond to integrated luminosities of 1 fb−1 at √s = 7 TeV and of 2 fb−1 at √s = 8 TeV. No significant excess of signal has been observed. Upper limits at 95% CL on the cross section for stau pair production in pp collisions at √s = 7 and 8 TeV have been computed by using the profile likelihood method, which is derived from the well known Feldman and Cousins method.

EPFL2013

Observation of top quark pairs produced in association with a vector boson in pp collisions at $ \sqrt{s}=8 $ TeV

Muhammad Ahmad, Georgios Anagnostou, Konstantin Androsov, Alexandre Aubin, Roberto Castello, Juan Ramon Castiñeiras De Saa, Yixing Chen, Tian Cheng, Davide Cieri, Giuseppe Codispoti, Alessandro Degano, Charles Dietz, Milos Dordevic, Dipanwita Dutta, Matthias Finger, Francesco Fiori, Daniel Gonzalez, Ruchi Gupta, Csaba Hajdu, Peter Hansen, Alexis Kalogeropoulos, Viktor Khristenko, Ji Hyun Kim, Donghyun Kim, Vineet Kumar, Ajay Kumar, Sanjeev Kumar, Ekaterina Kuznetsova, Wei Li, Ho Ling Li, Shuai Liu, Hao Liu, Werner Lustermann, Bibhuprasad Mahakud, Matteo Marone, Thomas Muller, Ioannis Papadopoulos, Vladimir Petrov, Quentin Python, Andrea Rizzi, Paolo Ronchese, Sourav Sen, Varun Sharma, Ashish Sharma, Lesya Shchutska, Muhammad Shoaib, Michal Simon, Gurpreet Singh, Jan Steggemann, Xin Sun, Wei Sun, Marco Trovato, Andromachi Tsirou, Paul Turner, Joao Varela, Rui Wang, Qian Wang, Yi Wang, Siyuan Wang, Zheng Wang, Jian Wang, Matthias Weber, Matthias Wolf, Fan Xia, Meng Xiao, Zhirui Xu, Fan Yang, Yong Yang, Kai Yi

Measurements of the cross sections for top quark pairs produced in association with a W or Z boson are presented, using 8 TeV pp collision data corresponding to an integrated luminosity of 19.5 fb

^{−1}

, collected by the CMS experiment at the LHC. Final states are selected in which the associated W boson decays to a charged lepton and a neutrino or the Z boson decays to two charged leptons. Signal events are identified by matching reconstructed objects in the detector to specific final state particles from

\mathrm{t}\overline{\mathrm{t}}\mathrm{W}

\mathrm{t}\overline{\mathrm{t}}\mathrm{Z}

decays. The

\mathrm{t}\overline{\mathrm{t}}\mathrm{W}

cross section is measured to be 382

_{− 102}^{+ 117}

fb with a significance of 4.8 standard deviations from the background-only hypothesis. The

\mathrm{t}\overline{\mathrm{t}}\mathrm{Z}

cross section is measured to be 242

_{− 55}^{+ 65}

fb with a significance of 6.4 standard deviations from the background-only hypothesis. These measurements are used to set bounds on five anomalous dimension-six operators that would affect the

\mathrm{t}\overline{\mathrm{t}}\mathrm{W}

and

\mathrm{t}\overline{\mathrm{t}}\mathrm{Z}

cross sections.

2016

Search for vector-like quarks in events with two oppositely charged leptons and jets in proton-proton collisions at $\sqrt{s} =$ 13 TeV

Muhammad Ahmad, Benjamin William Allen, Georgios Anagnostou, Konstantin Androsov, Tagir Aushev, Michele Bianco, Xin Chen, Yixing Chen, Tian Cheng, Davide Cieri, Giuseppe Codispoti, Pratyush Das, Abhisek Datta, Milos Dordevic, Dipanwita Dutta, Matthias Finger, Francesco Fiori, Daniel Gonzalez, Ruchi Gupta, Seungkyu Ha, Csaba Hajdu, Peter Hansen, Ali Harb, Miao Hu, Alexis Kalogeropoulos, Viktor Khristenko, Ji Hyun Kim, Donghyun Kim, Sanjeev Kumar, Ajay Kumar, Vineet Kumar, Ekaterina Kuznetsova, Jing Li, Ho Ling Li, Wei Li, Lei Li, Zhen Liu, Hao Liu, Werner Lustermann, Bibhuprasad Mahakud, Maren Tabea Meinhard, Amr Mohamed, Thomas Muller, Ioannis Papadopoulos, Vladimir Petrov, Quentin Python, Andrea Rizzi, Paolo Ronchese, Sourav Sen, Varun Sharma, Ashish Sharma, Lesya Shchutska, Wei Shi, Kun Shi, Muhammad Shoaib, Jan Steggemann, Marco Trovato, Andromachi Tsirou, David Vannerom, Joao Varela, Zheng Wang, Siyuan Wang, Yi Wang, Qian Wang, Hui Wang, Xiao Wang, Jian Wang, Mingkui Wang, Muhammad Waqas, Matthias Weber, Matthias Wolf, Fan Xia, Meng Xiao, Yong Yang

A search for the pair production of heavy vector-like partners

\mathrm {T}

and

\mathrm {B}

of the top and bottom quarks has been performed by the CMS experiment at the CERN LHC using proton–proton collisions at

\sqrt{s} = 13\,\text {Te}\text {V}

. The data sample was collected in 2016 and corresponds to an integrated luminosity of 35.9

\,\text {fb}^{-1}

. Final states studied for

\mathrm {T} \overline{\mathrm {T}}

production include those where one of the

\mathrm {T}

quarks decays via

\mathrm {T} \rightarrow \mathrm {t}\mathrm {Z}

and the other via

\mathrm {T} \rightarrow \mathrm {b}\mathrm {W}

\mathrm {t}\mathrm {Z}

, or

\mathrm {t}\mathrm {H}

, where

\mathrm {H}

is a Higgs boson. For the

\mathrm {B} \overline{\mathrm {B}}

case, final states include those where one of the

\mathrm {B}

quarks decays via

\mathrm {B} \rightarrow \mathrm {b}\mathrm {Z}

and the other

\mathrm {B} \rightarrow \mathrm {t}\mathrm {W}

\mathrm {b}\mathrm {Z}

, or

\mathrm {b}\mathrm {H}

. Events with two oppositely charged electrons or muons, consistent with coming from the decay of a

\mathrm {Z}

boson, and jets are investigated. The number of observed events is consistent with standard model background estimations. Lower limits at 95% confidence level are placed on the masses of the

\mathrm {T}

and

\mathrm {B}

quarks for a range of branching fractions. Assuming 100% branching fractions for

\mathrm {T} \rightarrow \mathrm {t}\mathrm {Z}

, and

\mathrm {B} \rightarrow \mathrm {b}\mathrm {Z}

\mathrm {T}

and

\mathrm {B}

quark mass values below 1280 and 1130

\,\text {Ge}\text {V}

, respectively, are excluded.

Springer2019

A search for heavy long-lived staus in the LHCb detector at √s = 7 and 8 TeV

Thi Viet Nga La

EPFL2013

Search for vector-like quarks in events with two oppositely charged leptons and jets in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A search for the pair production of heavy vector-like partners

\mathrm {T}

and

\mathrm {B}

of the top and bottom quarks has been performed by the CMS experiment at the CERN LHC using proton–proton collisions at

\sqrt{s} = 13\,\text {Te}\text {V}

. The data sample was collected in 2016 and corresponds to an integrated luminosity of 35.9

\,\text {fb}^{-1}

. Final states studied for

\mathrm {T} \overline{\mathrm {T}}

production include those where one of the

\mathrm {T}

quarks decays via

\mathrm {T} \rightarrow \mathrm {t}\mathrm {Z}

and the other via

\mathrm {T} \rightarrow \mathrm {b}\mathrm {W}

\mathrm {t}\mathrm {Z}

, or

\mathrm {t}\mathrm {H}

, where

\mathrm {H}

is a Higgs boson. For the

\mathrm {B} \overline{\mathrm {B}}

case, final states include those where one of the

\mathrm {B}

quarks decays via

\mathrm {B} \rightarrow \mathrm {b}\mathrm {Z}

and the other

\mathrm {B} \rightarrow \mathrm {t}\mathrm {W}

\mathrm {b}\mathrm {Z}

, or

\mathrm {b}\mathrm {H}

. Events with two oppositely charged electrons or muons, consistent with coming from the decay of a

\mathrm {Z}

boson, and jets are investigated. The number of observed events is consistent with standard model background estimations. Lower limits at 95% confidence level are placed on the masses of the

\mathrm {T}

and

\mathrm {B}

quarks for a range of branching fractions. Assuming 100% branching fractions for

\mathrm {T} \rightarrow \mathrm {t}\mathrm {Z}

, and

\mathrm {B} \rightarrow \mathrm {b}\mathrm {Z}

\mathrm {T}

and

\mathrm {B}

quark mass values below 1280 and 1130

\,\text {Ge}\text {V}

, respectively, are excluded.

Springer2019

Observation of top quark pairs produced in association with a vector boson in pp collisions at $ \sqrt{s}=8 $ TeV

Measurements of the cross sections for top quark pairs produced in association with a W or Z boson are presented, using 8 TeV pp collision data corresponding to an integrated luminosity of 19.5 fb

^{−1}

\mathrm{t}\overline{\mathrm{t}}\mathrm{W}

\mathrm{t}\overline{\mathrm{t}}\mathrm{Z}

decays. The

\mathrm{t}\overline{\mathrm{t}}\mathrm{W}

cross section is measured to be 382

_{− 102}^{+ 117}

fb with a significance of 4.8 standard deviations from the background-only hypothesis. The

\mathrm{t}\overline{\mathrm{t}}\mathrm{Z}

cross section is measured to be 242

_{− 55}^{+ 65}

fb with a significance of 6.4 standard deviations from the background-only hypothesis. These measurements are used to set bounds on five anomalous dimension-six operators that would affect the

\mathrm{t}\overline{\mathrm{t}}\mathrm{W}

and

\mathrm{t}\overline{\mathrm{t}}\mathrm{Z}

cross sections.

2016