G. Velev - Academia.edu (original) (raw)
Papers by G. Velev
Physical Review D, 2008
We search for evidence of resonant top quark pair production in 955 pb −1 of pp collisions at √ s... more We search for evidence of resonant top quark pair production in 955 pb −1 of pp collisions at √ s = 1.96 TeV recorded with the CDF II detector at the Fermilab Tevatron. For fully reconstructed candidate tt events triggered on leptons with large transverse momentum and containing at least one identified b-quark jet, we compare the invariant mass spectrum of tt pairs to the expected superposition of standard model tt, non-tt backgrounds, and a simple resonance model based on a sequential Z ′ boson. We establish upper limits for σ(pp → Z ′) • Br(Z ′ → tt) in the Z ′ mass interval from 450 GeV/c 2 to 900 GeV/c 2. A topcolor leptophobic Z ′ is ruled out below 720 GeV/c 2 , and the cross section of any narrow Z ′-like state decaying to tt is found to be less than 0.64 pb at 95% C.L. for M Z ′ above 700 GeV/c 2 .
Physical Review D, 2014
We combine six measurements of the inclusive top-quark pair (tt) production cross section (σ tt) ... more We combine six measurements of the inclusive top-quark pair (tt) production cross section (σ tt) from data collected with the CDF and D0 detectors at the Fermilab Tevatron with proton anti-proton collisions at √ s = 1.96 TeV. The data correspond to integrated luminosities of up to 8.8 fb −1. We obtain a value of σ tt = 7.60 ± 0.41 pb for a top-quark mass of mt = 172.5 GeV. The contributions to the uncertainty are 0.20 pb from statistical sources, 0.29 pb from systematic sources, and 0.21 pb from the uncertainty on the integrated luminosity. The result is in good agreement with the standard model expectation of 7.35 +0.28 −0.33 pb at NNLO+NNLL in pertubative QCD.
Physical review letters, Jan 25, 2014
We measure the inclusive forward-backward asymmetry of the charged-lepton pseudorapidities from t... more We measure the inclusive forward-backward asymmetry of the charged-lepton pseudorapidities from top-quark pairs produced in proton-antiproton collisions and decaying to final states that contain two charged leptons (electrons or muons). The data are collected with the Collider Detector at Fermilab and correspond to an integrated luminosity of 9.1 fb(-1). We measure the leptonic forward-backward asymmetry, A(FB)(ℓ), to be 0.072 ± 0.060 and the leptonic pair forward-backward asymmetry, A(FB)(ℓℓ), to be 0.076 ± 0.082. The measured values can be compared with the standard model predictions of A(FB)(ℓ) = 0.038 ± 0.003 and A(FB)(ℓℓ) = 0.048 ± 0.004, respectively. Additionally, we combine the A(FB)(ℓ) result with a previous determination from a final state with a single lepton and hadronic jets and obtain A(FB)(ℓ) = 0.090(-0.026)(+0.028).
Physics Letters B, 2010
We report on a CDF measurement of the total cross section and rapidity distribution, dσ /dy, for ... more We report on a CDF measurement of the total cross section and rapidity distribution, dσ /dy, for γ * /Z → e + e − events in the Z boson mass region (66 < M ee < 116 GeV/c 2) produced in pp collisions at √ s = 1.96 TeV with 2.1 fb −1 of integrated luminosity. The measured cross section of 257 ± 16 pb and dσ /dy distribution are compared with Next-to-Leading-Order (NLO) and Next-to-Next-to-Leading-Order (NNLO) QCD theory predictions with CTEQ and MRST/MSTW parton distribution functions (PDFs).
Physical Review Letters, 2001
Physical Review Letters, 2000
We report the results of a search for a W ′ boson produced in pp collisions at a center-of-mass e... more We report the results of a search for a W ′ boson produced in pp collisions at a center-of-mass energy of 1.8 TeV using a 107 pb −1 data sample recorded by the Collider Detector at Fermilab. We consider the decay channel W ′ → µν µ and search for anomalous production of high transverse mass µν µ lepton pairs. We observe no excess of events above background and set limits on the rate of W ′ boson production and decay relative to Standard Model W boson production and decay using a fit of the transverse mass distribution observed. If we assume Standard Model strength couplings of the W ′ boson to quark and lepton pairs, we exclude a W ′ boson with invariant mass less than 660 GeV/c 2
Physical Review Letters, 2006
We searched for scalar bottom quarks in 156 pb ÿ1 of pp collisions at s p 1:96 TeV recorded by th... more We searched for scalar bottom quarks in 156 pb ÿ1 of pp collisions at s p 1:96 TeV recorded by the Collider Detector at Fermilab II experiment at the Tevatron. Scalar bottom quarks can be produced from gluino decays in R-parity conserving models of supersymmetry when the mass of the gluino exceeds that of the scalar bottom quark. Then, a scalar bottom quark can decay into a bottom quark and a neutralino. To search for this scenario, we investigated events with large missing transverse energy and at least three jets, two or more of which were identified as containing a secondary vertex from the hadronization of b quarks. We found four candidate events, where 2:6 0:7 are expected from standard model processes, and placed 95% confidence level lower limits on gluino and scalar bottom quark masses of up to 280 and 240 GeV=c 2 , respectively.
Physical Review Letters, 2008
We present the results of a search for pair production of a new heavy top-like quark t ′ decaying... more We present the results of a search for pair production of a new heavy top-like quark t ′ decaying to a W boson and another quark using the CDF II detector in Run II of the Tevatron pp collider. Using a data sample corresponding to 760 pb −1 of integrated luminosity, we fit the observed spectrum of total transverse energy and reconstructed t ′ quark mass to a combination of standard model processes and t ′ pair production. We see no evidence for t ′t′ production, and we infer a lower limit of 256 GeV/c 2 on the mass of the t ′ at 95% CL assuming standard strong couplings for the t ′ .
Physical Review Letters, 2008
Physical Review Letters, 2002
We report a measurement of the strong coupling constant, αs(MZ), extracted from inclusive jet pro... more We report a measurement of the strong coupling constant, αs(MZ), extracted from inclusive jet production in pp collisions at √ s =1800 GeV. The QCD prediction for the evolution of αs with jet transverse energy ET is tested over the range 40 < ET < 450 GeV using ET for the renormalization scale. The data show good agreement with QCD in the region below 250 GeV. In the text we discuss the data-theory comparison in the region from 250 to 450 GeV. The value of αs at the mass of the Z 0 boson averaged over the range 40 < ET < 250 GeV is found to be αs(MZ) = 0.1178 ± 0.0001(stat) +0.0081 −0.0095 (exp. syst). The associated theoretical uncertainties are mainly due to the choice of renormalization scale (+6% −4%) and input parton distribution functions (5%).
Physical Review Letters, 2007
We present a measurement of the W boson mass using 200 pb −1 of data collected in pp collisions a... more We present a measurement of the W boson mass using 200 pb −1 of data collected in pp collisions at √ s = 1.96 TeV by the CDF II detector at Run II of the Fermilab Tevatron. With a sample of 63964 W → eν candidates and 51128 W → µν candidates, we measure MW = (80413 ± 34stat ± 34syst = 80413 ± 48) MeV/c 2. This is the most precise single measurement of the W boson mass to date.
Physical Review Letters, 2010
Physical Review Letters, 2010
Physical Review Letters, 1997
Physical Review Letters, 2011
We present a search for a new particle T ′ decaying to top quark via T ′ → t + X, where X is an i... more We present a search for a new particle T ′ decaying to top quark via T ′ → t + X, where X is an invisible particle. In a data sample with 4.8 fb −1 of integrated luminosity collected by the CDF II detector at Fermilab in pp collisions with √ s = 1.96 TeV, we search for pair production of T ′ in the lepton+jets channel, pp → tt + X + X → ℓνbqq ′ b + X + X. We interpret our results primarily in terms of a model where T ′ are exotic fourth generation quarks and X are dark matter particles. Current direct and indirect bounds on such exotic quarks restrict their masses to be between 300 and 600 GeV/c 2 , the dark matter particle mass being anywhere below m T ′. The data are consistent with standard model expectations, and we set 95% confidence level limits on the generic production of T ′T ′ → tt + X + X. We apply these limits to the dark matter model and exclude the fourth generation exotic quarks T ′ at 95% confidence level up to m T ′ = 360 GeV/c 2 for mX ≤ 100 GeV/c 2 .
Physical Review Letters, 2013
Physical Review Letters, 2011
We present the results of a search for a very light CP-odd Higgs boson a 0 1 originating from top... more We present the results of a search for a very light CP-odd Higgs boson a 0 1 originating from top quark decays t → H ± b → W ± (*) a 0 1 b, and subsequently decaying into τ + τ −. Using a data sample corresponding to an integrated luminosity of 2.7 fb −1 collected by the CDF II detector in pp collisions at 1.96 TeV, we perform a search for events containing a lepton, three or more jets, and an additional isolated track with transverse momentum in the range 3 to 20 GeV/c. Observed events are consistent with background sources, and 95% C.L. limits are set on the branching ratio of t → H ± b for various masses of H ± and a 0 1 .
Physical Review D, 2008
We search for evidence of resonant top quark pair production in 955 pb −1 of pp collisions at √ s... more We search for evidence of resonant top quark pair production in 955 pb −1 of pp collisions at √ s = 1.96 TeV recorded with the CDF II detector at the Fermilab Tevatron. For fully reconstructed candidate tt events triggered on leptons with large transverse momentum and containing at least one identified b-quark jet, we compare the invariant mass spectrum of tt pairs to the expected superposition of standard model tt, non-tt backgrounds, and a simple resonance model based on a sequential Z ′ boson. We establish upper limits for σ(pp → Z ′) • Br(Z ′ → tt) in the Z ′ mass interval from 450 GeV/c 2 to 900 GeV/c 2. A topcolor leptophobic Z ′ is ruled out below 720 GeV/c 2 , and the cross section of any narrow Z ′-like state decaying to tt is found to be less than 0.64 pb at 95% C.L. for M Z ′ above 700 GeV/c 2 .
Physical Review D, 2014
We combine six measurements of the inclusive top-quark pair (tt) production cross section (σ tt) ... more We combine six measurements of the inclusive top-quark pair (tt) production cross section (σ tt) from data collected with the CDF and D0 detectors at the Fermilab Tevatron with proton anti-proton collisions at √ s = 1.96 TeV. The data correspond to integrated luminosities of up to 8.8 fb −1. We obtain a value of σ tt = 7.60 ± 0.41 pb for a top-quark mass of mt = 172.5 GeV. The contributions to the uncertainty are 0.20 pb from statistical sources, 0.29 pb from systematic sources, and 0.21 pb from the uncertainty on the integrated luminosity. The result is in good agreement with the standard model expectation of 7.35 +0.28 −0.33 pb at NNLO+NNLL in pertubative QCD.
Physical review letters, Jan 25, 2014
We measure the inclusive forward-backward asymmetry of the charged-lepton pseudorapidities from t... more We measure the inclusive forward-backward asymmetry of the charged-lepton pseudorapidities from top-quark pairs produced in proton-antiproton collisions and decaying to final states that contain two charged leptons (electrons or muons). The data are collected with the Collider Detector at Fermilab and correspond to an integrated luminosity of 9.1 fb(-1). We measure the leptonic forward-backward asymmetry, A(FB)(ℓ), to be 0.072 ± 0.060 and the leptonic pair forward-backward asymmetry, A(FB)(ℓℓ), to be 0.076 ± 0.082. The measured values can be compared with the standard model predictions of A(FB)(ℓ) = 0.038 ± 0.003 and A(FB)(ℓℓ) = 0.048 ± 0.004, respectively. Additionally, we combine the A(FB)(ℓ) result with a previous determination from a final state with a single lepton and hadronic jets and obtain A(FB)(ℓ) = 0.090(-0.026)(+0.028).
Physics Letters B, 2010
We report on a CDF measurement of the total cross section and rapidity distribution, dσ /dy, for ... more We report on a CDF measurement of the total cross section and rapidity distribution, dσ /dy, for γ * /Z → e + e − events in the Z boson mass region (66 < M ee < 116 GeV/c 2) produced in pp collisions at √ s = 1.96 TeV with 2.1 fb −1 of integrated luminosity. The measured cross section of 257 ± 16 pb and dσ /dy distribution are compared with Next-to-Leading-Order (NLO) and Next-to-Next-to-Leading-Order (NNLO) QCD theory predictions with CTEQ and MRST/MSTW parton distribution functions (PDFs).
Physical Review Letters, 2001
Physical Review Letters, 2000
We report the results of a search for a W ′ boson produced in pp collisions at a center-of-mass e... more We report the results of a search for a W ′ boson produced in pp collisions at a center-of-mass energy of 1.8 TeV using a 107 pb −1 data sample recorded by the Collider Detector at Fermilab. We consider the decay channel W ′ → µν µ and search for anomalous production of high transverse mass µν µ lepton pairs. We observe no excess of events above background and set limits on the rate of W ′ boson production and decay relative to Standard Model W boson production and decay using a fit of the transverse mass distribution observed. If we assume Standard Model strength couplings of the W ′ boson to quark and lepton pairs, we exclude a W ′ boson with invariant mass less than 660 GeV/c 2
Physical Review Letters, 2006
We searched for scalar bottom quarks in 156 pb ÿ1 of pp collisions at s p 1:96 TeV recorded by th... more We searched for scalar bottom quarks in 156 pb ÿ1 of pp collisions at s p 1:96 TeV recorded by the Collider Detector at Fermilab II experiment at the Tevatron. Scalar bottom quarks can be produced from gluino decays in R-parity conserving models of supersymmetry when the mass of the gluino exceeds that of the scalar bottom quark. Then, a scalar bottom quark can decay into a bottom quark and a neutralino. To search for this scenario, we investigated events with large missing transverse energy and at least three jets, two or more of which were identified as containing a secondary vertex from the hadronization of b quarks. We found four candidate events, where 2:6 0:7 are expected from standard model processes, and placed 95% confidence level lower limits on gluino and scalar bottom quark masses of up to 280 and 240 GeV=c 2 , respectively.
Physical Review Letters, 2008
We present the results of a search for pair production of a new heavy top-like quark t ′ decaying... more We present the results of a search for pair production of a new heavy top-like quark t ′ decaying to a W boson and another quark using the CDF II detector in Run II of the Tevatron pp collider. Using a data sample corresponding to 760 pb −1 of integrated luminosity, we fit the observed spectrum of total transverse energy and reconstructed t ′ quark mass to a combination of standard model processes and t ′ pair production. We see no evidence for t ′t′ production, and we infer a lower limit of 256 GeV/c 2 on the mass of the t ′ at 95% CL assuming standard strong couplings for the t ′ .
Physical Review Letters, 2008
Physical Review Letters, 2002
We report a measurement of the strong coupling constant, αs(MZ), extracted from inclusive jet pro... more We report a measurement of the strong coupling constant, αs(MZ), extracted from inclusive jet production in pp collisions at √ s =1800 GeV. The QCD prediction for the evolution of αs with jet transverse energy ET is tested over the range 40 < ET < 450 GeV using ET for the renormalization scale. The data show good agreement with QCD in the region below 250 GeV. In the text we discuss the data-theory comparison in the region from 250 to 450 GeV. The value of αs at the mass of the Z 0 boson averaged over the range 40 < ET < 250 GeV is found to be αs(MZ) = 0.1178 ± 0.0001(stat) +0.0081 −0.0095 (exp. syst). The associated theoretical uncertainties are mainly due to the choice of renormalization scale (+6% −4%) and input parton distribution functions (5%).
Physical Review Letters, 2007
We present a measurement of the W boson mass using 200 pb −1 of data collected in pp collisions a... more We present a measurement of the W boson mass using 200 pb −1 of data collected in pp collisions at √ s = 1.96 TeV by the CDF II detector at Run II of the Fermilab Tevatron. With a sample of 63964 W → eν candidates and 51128 W → µν candidates, we measure MW = (80413 ± 34stat ± 34syst = 80413 ± 48) MeV/c 2. This is the most precise single measurement of the W boson mass to date.
Physical Review Letters, 2010
Physical Review Letters, 2010
Physical Review Letters, 1997
Physical Review Letters, 2011
We present a search for a new particle T ′ decaying to top quark via T ′ → t + X, where X is an i... more We present a search for a new particle T ′ decaying to top quark via T ′ → t + X, where X is an invisible particle. In a data sample with 4.8 fb −1 of integrated luminosity collected by the CDF II detector at Fermilab in pp collisions with √ s = 1.96 TeV, we search for pair production of T ′ in the lepton+jets channel, pp → tt + X + X → ℓνbqq ′ b + X + X. We interpret our results primarily in terms of a model where T ′ are exotic fourth generation quarks and X are dark matter particles. Current direct and indirect bounds on such exotic quarks restrict their masses to be between 300 and 600 GeV/c 2 , the dark matter particle mass being anywhere below m T ′. The data are consistent with standard model expectations, and we set 95% confidence level limits on the generic production of T ′T ′ → tt + X + X. We apply these limits to the dark matter model and exclude the fourth generation exotic quarks T ′ at 95% confidence level up to m T ′ = 360 GeV/c 2 for mX ≤ 100 GeV/c 2 .
Physical Review Letters, 2013
Physical Review Letters, 2011
We present the results of a search for a very light CP-odd Higgs boson a 0 1 originating from top... more We present the results of a search for a very light CP-odd Higgs boson a 0 1 originating from top quark decays t → H ± b → W ± (*) a 0 1 b, and subsequently decaying into τ + τ −. Using a data sample corresponding to an integrated luminosity of 2.7 fb −1 collected by the CDF II detector in pp collisions at 1.96 TeV, we perform a search for events containing a lepton, three or more jets, and an additional isolated track with transverse momentum in the range 3 to 20 GeV/c. Observed events are consistent with background sources, and 95% C.L. limits are set on the branching ratio of t → H ± b for various masses of H ± and a 0 1 .