Browsing by Author "Kuday, Sinan"
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Item A search for tt ¯ resonances with the ATLAS detector in 2.05 fb−1 of proton-proton collisions at √s = 7 TeV(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Abbas Kenan; Çiftçi, Rena; Çakır, Orhan; Fen FakültesiA search for top quark pair resonances in final states containing at least one electron or muon has been performed with the ATLAS experiment at the CERN Large Hadron Collider. The search uses a data sample corresponding to an integrated luminosity of 2.05 fb−1, which was recorded in 2011 at a proton-proton centre-of-mass energy of 7 TeV. No evidence for a resonance is found and limits are set on the production cross-section times branching ratio to tt for narrow and wide resonances. For narrow Z′ bosons, the observed 95 % Bayesian credibility level limits range from 9.3 pb to 0.95 pb for masses in the range of mZ′ = 500 GeV to mZ ′= 1300 GeV. The corresponding excluded mass region for a leptophobic topcolour Z′ boson (Kaluza-Klein gluon excitation in the Randall-Sundrum model) is mZ < 880 GeV (mgKK < 1130 GeV). © CERN for the benefit of the ATLAS collaboration 2012.Item ATLAS search for a heavy gauge boson decaying to a charged lepton and a neutrino in pp collisions at √s = 7 TeV(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Rena; Çiftçi, Abbas Kenan; Çakır, Orhan; Fen FakültesiThe ATLAS detector at the LHC is used to search for high-mass states, such as heavy charged gauge bosons (Wʹ), decaying to a charged lepton (electron or muon) and a neutrino. Results are presented based on the analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 4.7 fb−1. No excess beyond Standard Model expectations is observed. AWʹ with Sequential Standard Model couplings is excluded at the 95% credibility level for masses up to 2.55 TeV. Excited chiral bosons (W*) with equivalent coupling strength are excluded for masses up to 2.42 TeV. © CERN for the benefit of the ATLAS collaboration 2012.Item ATLAS search for new phenomena in dijet mass and angular distributions using pp collisions at √s = 7 TeV(2013) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Rena; Çiftçi, Abbas Kenan; Çakır, Orhan; Fen FakültesiMass and angular distributions of dijets produced in LHC proton-proton collisions at a centre-of-mass energy √s = 7TeV have been studied with the ATLAS detector using the full 2011 data set with an integrated luminosity of 4.8 fb-1. Dijet masses up to ~ 4.0TeV have been probed. No resonance-like features have been observed in the dijet mass spectrum, and all angular distributions are consistent with the predictions of QCD. Exclusion limits on six hypotheses of new phenomena have been set at 95% CL in terms of mass or energy scale, as appropriate. These hypotheses include excited quarks below 2.83 TeV, colour octet scalars below 1.86TeV, heavy W bosons below 1.68 TeV, string resonances below 3.61 TeV, quantum black holes with six extra space-time dimensions for quantum gravity scales below 4.11 TeV, and quark contact interactions below a compositeness scale of 7.6 TeV in a destructive interference scenario. © CERN, for the benefit of the ATLAS collaboration.Item Büyük hadron çarpıştırıcısında üçüncü aile sfermiyonlarının üretimi(Fen Bilimleri Enstitüsü, 2012) Kuday, Sinan; Çakır, OrhanCERN’de halen aktif olarak çalışmaya devam eden Büyük Hadron Çarpıştırıcısı’nda, üçüncü aile sfermiyonlarının üretimi hem R-pariteyi koruduğu, hem de büyük Yukawa çiftlenimleri nedeniyle karışım etkilerine ve kütle farklılaşmalarına neden olduğu için önemlidir. LEP ve Tevatron’da yapılan araştırmalarda skaler üst kuark ve skaler alt kuark kütleleri için getirilen sınırlamalar ele alınan süpersimetrik senaryoya göre 90 – 200 GeV arasında değişebilmektedir. Bununla birlikte, Büyük Hadron Çarpıştırıcısı’nın duyarlılık bölgesini TeV ölçeğine çıkarması beklenmektedir. Büyük Hadron Çarpıştırıcısı’nda yapılan güncel araştırmalar ışığında bazı test noktaları ve bu test noktalarında üçüncü aile sfermiyonlarının üretimleri incelenmiştir. Kütle merkezi enerjisi 7 (14) TeV ve 1(100) fb-1 ışınlık için parametre uzayında ulaşılabilir bölgeler ve süpersimetrinin dördüncü aile genişletimi de sunulmuştur. Dördüncü aile sleptonlarının R-parite bozan süreçler yoluyla rezonans üretimleri ele alınmıştır. Özel olarak, dördüncü aile sleptonu ’ün W bozonu ve nötrinoya bozunumunun, son durumda dördüncü aile nötrinosu Majorana yapısındaysa, eş işaretli dimuon ve dijet sinyali olarak gözlenebileceği ele alınmıştır. R-parite bozan süreçlerle üretilecek parçacıkların ve katsayılarına gelecek sınırlamalar 7 ve 14 TeV kütle merkezi enerjilerde elde edilmiştir.Abstract Production of third family sfermions is an important case at the Large Hadron Collider, which is actively running at CERN, due to both R-parity conservation and their Yukawa couplings leading to mixing effects and mass splitting. Searches at the LEP and the Tevatron put lower limits on the scalar top and scalar bottom quark masses in the range 90 – 200 GeV depending on supersymmetric scenarios. However, the LHC is expected to extend the range of sensitivity to TeV scale. Third family sfermion production at some benchmarks and their subsequent decays are studied at the LHC. We present an extension of supersmmetry into fourth family and analysis for the parameter space reachable at the LHC with center of mass energy of 7 (14) TeV and luminosity L = 1(100) fb-1. Specifically, the resonant production of the fourth family slepton via R-parity violating interactions of supersymmetry at the LHC has been investigated. We study the decay mode of into fourth family neutrino and W boson whose final state signal will be a like-sign dimuon and dijet if the fourth family neutrino has Majorana nature. We discuss the constraints on the R-parity violating couplings and of the fourth family charged slepton at the LHC with center of mass energies 7 and 14 TeV.Item Evidence for the associated production of a √ W boson and a top quark in ATLAS at s = 7 TeV ✩(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Rena; Çiftçi, Abbas Kenan; Çakır, Orhan; Fen FakültesiThis Letter presents evidence for the associated production of a W boson and a top quark using 2.05 fb−1 of pp collision data at s=7TeV accumulated with the ATLAS detector at the LHC. The analysis is based on the selection of the dileptonic final states with events featuring two isolated leptons, electron or muon, with significant transverse missing momentum and at least one jet. An approach based on boosted decision trees has been developed to improve the discrimination of single top-quark Wt events from background. A template fit to the final classifier distributions is performed to determine the cross-section. The result is incompatible with the background-only hypothesis at the 3.3σ level, the expected sensitivity assuming the Standard Model production rate being 3.4σ. The corresponding cross-section is determined and found to be σWt=16.8±2.9 (stat)±4.9 (syst) pb, in good agreement with the Standard Model expectation. From this result the CKM matrix element |Vtb|=1.03−0.19 +0.16 is derived assuming that the Wt production through |Vts| and |Vtd| is small. © 2012 CERNItem Measurement of event shapes at large momentum transfer with the ATLAS detector in pp collisions at √s = 7 TeV(2012) Yıldız (Duran), Hatice; Çiftçi, Abbas Kenan; Çiftçi, Rena; Çakır, Orhan; Kuday, Sinan; Fen FakültesiA measurement of event shape variables is presented for large momentum transfer proton-proton collisions using the ATLAS detector at the Large Hadron Collider. Six event shape variables calculated using hadronic jets are studied in inclusive multi-jet events in 35 pb-1 of integrated luminosity at a center-of-mass energy of √s = 7 TeV. These measurements are compared to predictions by three Monte Carlo event generators containing leading-logarithmic parton showers matched to leading order matrix elements for 2→2 and 2→n (n = 2, …, 6) scattering. Measurements of the third-jet resolution parameter, aplanarity, thrust, sphericity, and transverse sphericity are generally well described. The mean value of each event shape variable is evaluated as a function of the average momentum of the two leading jets pT,1 and pT,2, with a mean pT approaching 1 TeV. © 2012, CERN for the benefit of the ATLAS collaboration.Item Measurement of the azimuthal anisotropy for charged particle production in √sN N = 2.76 TeV lead-lead collisions with the ATLAS detector(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Abbas Kenan; Çiftçi, Rena; Çakır, Orhan; Fen FakültesiDifferential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at √sNN = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 μb− 1. This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle relative to the reaction plane, with the coefficients vn denoting the magnitude of the anisotropy. Significant v2–v6 values are obtained as a function of transverse momentum (0.5 < pT < 20 GeV), pseudorapidity (|η| < 2.5), and centrality using an event plane method. The vn values for n ≥ 3 are found to vary weakly with both η and centrality, and their pT dependencies are found to follow an approximate scaling relation, v1/n n (pT) ∝ v1/2 2 (pT), except in the top 5% most central collisions. A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Δϕ = ϕa − ϕb) is performed to extract the coefficients vn, n = (cos nΔϕ). For pairs of charged particles with a large pseudorapidity gap (|Δη = ηa − ηb| > 2) and one particle with pT < 3 GeV, the v2,2–v6,6 values are found to factorize as vn, n(pa T, pb T) ≈ vn(pa T)vn(pb T) in central and midcentral events. Such factorization suggests that these values of v2,2–v6,6 are primarily attributable to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the v1,1(pa T, pb T) data are consistent with the combined contributions from a rapidity-even v1 and global momentum conservation.A two-component fit is used to extract the v1 contribution. The extracted v1 isobserved to cross zero at pT ≈ 1.0 GeV, reaches a maximum at 4–5 GeV with a value comparable to that for v3, and decreases at higher pT. © 2012 CERN, for the ATLAS Collaboration.Item Measurement of the inclusive cross-section for the production of jets in association with a Z boson in proton–proton collisions at 8 TeV using the ATLAS detector(2019) Çakır, Orhan; Duran Yıldız, Hatice; Kuday, Sinan; Sultansoy, Saleh; Türk Çakır, İlkay; Fen FakültesiThe inclusive cross-section for jet production in association with a Z boson decaying into an electron–positron pair is measured as a function of the transverse momentum and the absolute rapidity of jets using 19.9 fb- 1 of s=8 TeV proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider. The measured Z+ jets cross-section is unfolded to the particle level. The cross-section is compared with state-of-the-art Standard Model calculations, including the next-to-leading-order and next-to-next-to-leading-order perturbative QCD calculations, corrected for non-perturbative and QED radiation effects. The results of the measurements cover final-state jets with transverse momenta up to 1 TeV, and show good agreement with fixed-order calculations.Item Measurement of the t-channel single top-quark production cross section in pp collisions at √s = 7 TeV with the ATLAS detector(2012) Çakır, Orhan; Çiftçi, Rena; Yıldız (Duran), Hatice; Kuday, Sinan; Çiftçi, Abbas Kenan; Fen FakültesiWe report a measurement of the cross section of single top-quark production in the t-channel using 1.04 fb−1 of pp collision data at s=7 TeV recorded with the ATLAS detector at the LHC. Selected events feature one electron or muon, missing transverse momentum, and two or three jets, exactly one of them identified as originating from a b quark. The cross section is measured by fitting the distribution of a multivariate discriminant constructed with a neural network, yielding σt=83±4(stat.)−19 +20(syst.) pb, which is in good agreement with the prediction of the Standard Model. Using the ratio of the measured to the theoretically predicted cross section and assuming that the top-quark-related CKM matrix elements obey the relation |Vtb|≫|Vts|,|Vtd|, the coupling strength at the W-t-b vertex is determined to be |Vtb|=1.13−0.13 +0.14. If it is assumed that |Vtb|⩽1 a lower limit of |Vtb|>0.75 is obtained at the 95% confidence level. © 2012 CERNItem Measurement of the top quark pair cross section with ATLAS in pp collisions at √s = 7 TeV using final states with an electron or a muon and a hadronically decaying τ lepton ✩(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Rena; Çiftçi, Abbas Kenan; Çakır, Orhan; Fen FakültesiA measurement of the cross section of top quark pair production in proton–proton collisions recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb−1. Events with an isolated electron or muon and a τ lepton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, σtt¯=186±13(stat)±20(syst.)±7(lumi.) pb, is in good agreement with the Standard Model prediction. © 2012 CERNItem Measurement of tt ¯ production with a veto on additional central jet activity in pp collisions at √s = 7 TeV using the ATLAS detector(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çakır, Orhan; Çiftçi, Rena; Çiftçi, Abbas Kenan; Fen FakültesiA measurement of the jet activity in tt events produced in proton–proton collisions at a centre-of-mass energy of 7 TeV is presented, using 2.05 fb-1 of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. The tt events are selected in the dilepton decay channel with two identified b-jets from the top quark decays. Events are vetoed if they contain an additional jet with transverse momentum above a threshold in a central rapidity interval. The fraction of events surviving the jet veto is presented as a function of this threshold for four different central rapidity interval definitions. An alternate measurement is also performed, in which events are vetoed if the scalar transverse momentum sum of the additional jets in each rapidity interval is above a threshold. In both measurements, the data are corrected for detector effects and compared to the theoretical models implemented in MC@NLO, POWHEG, ALPGEN and SHERPA. The experimental uncertainties are often smaller than the spread of theoretical predictions, allowing deviations between data and theory to be observed in some regions of phase space. © 2012, CERN for the benefit of the ATLAS collaboration.Item Measurement of W γ and Zγ production cross sections in pp collisions at √s = 7 TeV and limits on anomalous triple gauge couplings with the ATLAS detector ✩(2012) Çakır, Orhan; Çiftçi, Abbas Kenan; Çiftçi, Rena; Yıldız (Duran), Hatice; Kuday, Sinan; Fen FakültesiThis Letter presents measurements of l±νγ and l+l−γ (l=e,μ) production in 1.02 fb−1 of pp collision data recorded at s=7 TeV with the ATLAS detector at the LHC in the first half of 2011. Events dominated by Wγ and Zγ production with leptonic decays of the W and Z bosons are selected, and their production cross sections and kinematic properties are measured in several ranges of the photon transverse energy. The results are compared to Standard Model predictions and are used to determine limits on anomalous WWγ and ZZγ/Zγγ couplings. © 2012 CERNItem Measurement of W±Z production in proton-proton collisions at √s = 7 TeV with the ATLAS detector(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Rena; Çiftçi, Abbas Kenan; Çakır, Orhan; Fen FakültesiA study of W ± Z production in proton-proton collisions at √s = 7 TeV is presented using data corresponding to an integrated luminosity of 4.6 fb−1 collected with the ATLAS detector at the Large Hadron Collider in 2011. In total, 317 candidates, with a background expectation of 68 ± 10 events, are observed in double-leptonic decay final states with electrons, muons and missing transverse momentum. The total cross-section is determined to be be σtot WZ = 19.0+1.4 −1.3 (stat.) ± 0.9(syst.) ± 0.4(lumi.) pb, consistent with the Standard Model expectation of 17.6+1.1 −1.0 pb. Limits on anomalous triple gauge boson couplings are derived using the transverse momentum spectrum of Z bosons in the selected events. The cross-section is also presented as a function of Z boson transverse momentum and diboson invariant mass. © 2012, CERN for the benefit of the ATLAS collaboration.Item Measurement of Z Boson Production in Pb-Pb Collisions at ffiffiffiffiffiffiffiffiffi sNNp ¼2:76 TeVwith the ATLAS Detector(2013) Çakır, Orhan; Çiftçi, Abbas Kenan; Çiftçi, Rena; Yıldız (Duran), Hatice; Kuday, Sinan; Fen FakültesiThe ATLAS experiment has observed 1995 Z boson candidates in data corresponding to 0:15 nb-1 of integrated luminosity obtained in the 2011 LHC Pb + Pb run at √SNN = 2.76 TeV. The Z bosons are reconstructed via dielectron and dimuon decay channels, with a background contamination of less than 3%. Results from the two channels are consistent and are combined. Within the statistical and systematic uncertainties, the per-event Z boson yield is proportional to the number of binary collisions estimated by the Glauber model. The elliptic anisotropy of the azimuthal distribution of the Z boson with respect to the event plane is found to be consistent with zero. © 2013 CERN.Item Measurement of τ polarization in W → τν decays with the ATLAS detector in pp collisions at √s = 7 TeV(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Rena; Çiftçi, Abbas Kenan; Çakır, Orhan; Fen FakültesiIn this paper, a measurement of τ polarization in W → τν decays is presented. It is measured from the energies of the decay products in hadronic τ decays with a single final state charged particle. The data, corresponding to an integrated luminosity of 24 pb−1, were collected by the ATLAS experiment at the Large Hadron Collider in 2010. The measured value of the τ polarization is Pτ = −1.06±0.04 (stat) +0.05−0.07 (syst), in agreement with the Standard Model prediction, and is consistent with a physically allowed 95 % CL interval [−1,−0.91]. Measurements of τ polarization have not previously been made at hadron colliders. © 2012, CERN for the benefit of the ATLAS collaboration.Item Search for a fermiophobic Higgs boson in the diphoton decay channel with the ATLAS detector(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Rena; Çiftçi, Abbas Kenan; Çakır, Orhan; Fen FakültesiA search for a fermiophobic Higgs boson using diphoton events produced in proton-proton collisions at a centre-of-mass energy of √s = 7 TeV is performed using data corresponding to an integrated luminosity of 4.9 fb−1collected by the ATLAS experiment at the Large Hadron Collider. A specific benchmark model is considered where all the fermion couplings to the Higgs boson are set to zero and the bosonic couplings are kept at the Standard Model values (fermiophobic Higgs model). The largest excess with respect to the background-only hypothesis is found at 125.5 GeV, with a local significance of 2.9 standard deviations, which reduces to 1.6 standard deviations when taking into account the look-elsewhere effect. The data exclude the fermiophobic Higgs model in the ranges 110.0–118.0 GeV and 119.5–121.0 GeV at 95 % confidence level. © 2012, CERN for the benefit of the ATLAS collaboration.Item Search for a heavy top-quark partner in final states with two leptons with the ATLAS detector at the LHC(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Rena; Çiftçi, Abbas Kenan; Çakır, Orhan; Fen FakültesiThe results of a search for direct pair production of heavy top-quark partners in 4.7 fb–1 of integrated luminosity from pp collisions at √s = 7 TeV collected by the ATLAS detector at the LHC are reported. Heavy top-quark partners decaying into a top quark and a neutral non-interacting particle are searched for in events with two leptons in the final state. No excess above the Standard Model expectation is observed. Limits are placed on the mass of a supersymmetric scalar top and of a spin-1/2 top-quark partner. A spin-1/2 top-quark partner with a mass between 300 GeV and 480 GeV, decaying to a top quark and a neutral non-interacting particle lighter than 100 GeV, is excluded at 95% confidence level. © CERN.Item Search for a standard model Higgs boson in the H → ZZ → +−νν¯ decay channel using 4.7 fb−1 of √s = 7 TeV data with the ATLAS detector ✩(2012) Çakır, Orhan; Yıldız (Duran), Hatice; Kuday, Sinan; Çiftçi, Rena; Çiftçi, Abbas Kenan; Fen FakültesiA search for a Standard Model Higgs boson decaying via H→ZZ→ℓ+ℓ−νν¯ where ℓ represents electrons or muons, is presented. It is based on proton-proton collision data at s=7 TeV, collected by the ATLAS experiment at the LHC during 2011 and corresponding to an integrated luminosity of 4.7 fb−1. The data agree with the expected Standard Model backgrounds. Upper limits on the Higgs boson production cross section are derived for Higgs boson masses between 200 GeV and 600 GeV and the production of a Standard Model Higgs boson with a mass in the range 319–558 GeV is excluded at the 95% confidence level. © 2012 CERNItem Search for a Standard Model Higgs boson in the mass range 200–600 GeV in the H → Z Z → +−qq¯ decay channel with the ATLAS detector(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Rena; Çiftçi, Abbas Kenan; Çakır, Orhan; Fen FakültesiA search for a heavy Standard Model Higgs boson decaying via H→ZZ→ℓ+ℓ−qq¯, where ℓ=e or μ, is presented. The search uses a data set of pp collisions at s=7 TeV, corresponding to an integrated luminosity of 4.7 fb−1 collected in 2011 by the ATLAS detector at the CERN LHC. No significant excess of events above the estimated background is found. Upper limits at 95% confidence level on the production cross section of a Higgs boson with a mass in the range between 200 and 600 GeV are derived. A Standard Model Higgs boson with a mass in the range 300 GeV⩽mH⩽322 GeV or 353 GeV⩽mH⩽410 GeV is excluded at 95% CL. The corresponding expected exclusion range is 351 GeV⩽mH⩽404 GeV at 95% CL. © 2012 CERNItem Search for doubly charged Higgs bosons in like-sign dilepton final states at √s = 7 TeV with the ATLAS detector(2012) Kuday, Sinan; Yıldız (Duran), Hatice; Çiftçi, Rena; Çiftçi, Abbas Kenan; Çakır, Orhan; Fen FakültesiA search for doubly charged Higgs bosons decaying to pairs of electrons and/or muons is presented. The search is performed using a data sample corresponding to an integrated luminosity of 4.7 fb−1of pp collisions at √s = 7 TeV collected by the ATLAS detector at the LHC. Pairs of prompt, isolated, high-pT leptons with the same electric charge (e±e±, e±μ±, μ±μ±) are selected, and their invariant mass distribution is searched for a narrow resonance. No significant excess over Standard Model background expectations is observed, and limits are placed on the cross section times branching ratio for pair production of doubly charged Higgs bosons. The masses of doubly charged Higgs bosons are constrained depending on the branching ratio into these leptonic final states. Assuming pair production, coupling to left-handed fermions, and a branching ratio of 100% for each final state, masses below 409 GeV, 375 GeV, and 398 GeV are excluded for e±e±, e±μ±, and μ±μ±, respectively. © 2013, CERN for the benefit of the ATLAS collaboration.