Papers by Karolos Potamianos
The European Physical Journal C
A muon collider would enable the big jump ahead in energy reach that is needed for a fruitful exp... more A muon collider would enable the big jump ahead in energy reach that is needed for a fruitful exploration of fundamental interactions. The challenges of producing muon collisions at high luminosity and 10 TeV centre of mass energy are being investigated by the recently-formed International Muon Collider Collaboration. This Review summarises the status and the recent advances on muon colliders design, physics and detector studies. The aim is to provide a global perspective of the field and to outline directions for future work.
The semiconductor tracker is a silicon microstrip detector forming part of the inner tracking sys... more The semiconductor tracker is a silicon microstrip detector forming part of the inner tracking system of the ATLAS experiment at the LHC. The operation and performance of the semiconductor tracker during the first years of LHC running are described. More than 99% of the detector modules were operational during this period, with an average intrinsic hit efficiency of (99.74±0.04)%. The evolution of the noise occupancy is discussed, and measurements of the Lorentz angle, δ-ray production and energy loss presented. The alignment of the detector is found to be stable at the few-micron level over long periods of time. Radiation damage measurements, which include the evolution of detector leakage currents, are found to be consistent with predictions and are used in the verification of radiation background simulations. KEYWORDS: Solid state detectors; Charge transport and multiplication in solid media; Particle tracking detectors (Solid-state detectors); Detector modelling and simulations I (interaction of radiation with matter, interaction of photons with matter, interaction of hadrons with matter, etc)
A search is reported for Higgs boson decay to µ + µ − using data with an integrated luminosity of... more A search is reported for Higgs boson decay to µ + µ − using data with an integrated luminosity of 24.8 fb −1 collected with the ATLAS detector in pp collisions at √ s =7 and 8 TeV at the CERN Large Hadron Collider. The observed dimuon invariant mass distribution is consistent with the Standard Model background-only hypothesis in the 120-150 GeV search range. For a Higgs boson with a mass of 125.5 GeV, the observed (expected) upper limit at the 95% confidence level is 7.0 (7.2) times the Standard Model expectation. This corresponds to an upper limit on the branching ratio BR(H → µ + µ −) of 1.5 × 10 −3 .
This letter presents a search for new resonances decaying to final states with a vector boson pro... more This letter presents a search for new resonances decaying to final states with a vector boson produced in association with a high transverse momentum photon, V γ, with V = W (→ ν) or Z(→ + −), where = e or µ. The measurements use 20.3 fb −1 of proton-proton collision data at a center-of-mass energy of √ s = 8 TeV recorded with the ATLAS detector. No deviations from the Standard Model expectations are found, and production cross section limits are set at 95% confidence level. Masses of the hypothetical a T and ω T states of a benchmark Low Scale Technicolor model are excluded in the ranges [275, 960] GeV and [200, 700] ∪ [750, 890] GeV, respectively. Limits at 95% confidence level on the production cross section of a singlet scalar resonance decaying to Zγ final states have also been obtained for masses below 1180 GeV.
The ratio of the production cross sections for W and Z bosons in association with jets has been m... more The ratio of the production cross sections for W and Z bosons in association with jets has been measured in proton-proton collisions at √ s = 7 TeV with the ATLAS experiment at the Large Hadron Collider. The measurement is based on the entire 2011 dataset, corresponding to an integrated luminosity of 4.6 fb −1. Inclusive and differential cross-section ratios for massive vector bosons decaying to electrons and muons are measured in association with jets with transverse momentum p T > 30 GeV and jet rapidity |y| < 4.4. The measurements are compared to nextto-leading-order perturbative QCD calculations and to predictions from different Monte Carlo generators implementing leading-order matrix elements supplemented by parton showers.
Physical Review D - Particles, Fields, Gravitation and Cosmology, 2013
A search for nonresonant new phenomena, originating from either contact interactions or large ext... more A search for nonresonant new phenomena, originating from either contact interactions or large extra spatial dimensions, has been carried out using events with two isolated electrons or muons. These events, produced at the LHC in proton-proton collisions at ffiffi ffi s p ¼ 7 TeV, were recorded by the ATLAS detector. The data sample, collected throughout 2011, corresponds to an integrated luminosity of 4.9 and 5:0 fb À1 in the e þ e À and þ À channels, respectively. No significant deviations from the Standard Model expectation are observed. Using a Bayesian approach, 95% confidence level lower limits ranging from 9.0 to 13.9 TeV are placed on the energy scale of ''qq contact interactions in the left-left isoscalar model. Lower limits ranging from 2.4 to 3.9 TeV are also set on the string scale in large extra dimension models. After combining these limits with results from a similar search in the diphoton channel, slightly more stringent limits are obtained.
Physical Review D, 2012
We present a search for new phenomena in events with two reconstructed Z bosons and large missing... more We present a search for new phenomena in events with two reconstructed Z bosons and large missing transverse momentum, sensitive to processes pp → X2X2 → ZZX1X1, where X2 is an unstable particle decaying as X2 → ZX1 and X1 is undetected. The particles X1 and X2 may be, among other possibilities, fourth generation neutrinos or supersymmetric particles. We study the final state in which one Z boson decays to two charged leptons and the second decays hadronically. In data corresponding to an integrated luminosity of 4.2 fb −1 from proton-antiproton collisions recorded by the CDF II detector at the Tevatron, with center-of-mass energy of 1.96 TeV, we find agreement between data and standard-model backgrounds. We calculate 95% confidence level upper limits on the cross section of the process pp → X2X2 → ZZX1X1 ranging from 50 fb to 1 pb, depending on the masses of X1 and X2.
Physical Review D, 2011
The Çð1SÞ, Çð2SÞ, and Çð3SÞ production cross sections in proton-proton collisions at ffiffi ffi s... more The Çð1SÞ, Çð2SÞ, and Çð3SÞ production cross sections in proton-proton collisions at ffiffi ffi s p ¼ 7 TeV are measured using a data sample collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 3:1 AE 0:3 pb À1. Integrated over the rapidity range jyj < 2, we find the product of the Çð1SÞ production cross section and branching fraction to dimuons to be ðpp ! Çð1SÞXÞ Á BðÇð1SÞ ! þ À Þ ¼ 7:37 AE 0:13 þ0:61 À0:42 AE 0:81 nb, where the first uncertainty is statistical, the second is systematic, and the third is associated with the estimation of the integrated luminosity of the data sample. This cross section is obtained assuming unpolarized Çð1SÞ production. With the assumption of fully transverse or fully longitudinal production polarization, the measured cross section changes by about 20%. We also report the measurement of the Çð1SÞ, Çð2SÞ, and Çð3SÞ differential cross sections as a function of transverse momentum and rapidity.
Journal of High Energy Physics, 2014
The prompt and non-prompt production cross-sections for ψ(2S) mesons are measured using 2.1 fb −1... more The prompt and non-prompt production cross-sections for ψ(2S) mesons are measured using 2.1 fb −1 of pp collision data at a centre-of-mass energy of 7 TeV recorded by the ATLAS experiment at the LHC. The measurement exploits the ψ(2S) → J/ψ(→ µ + µ −)π + π − decay mode, and probes ψ(2S) mesons with transverse momenta in the range 10 ≤ p T < 100 GeV and rapidity |y| < 2.0. The results are compared to other measurements of ψ(2S) production at the LHC and to various theoretical models for prompt and non-prompt quarkonium production.
The European Physical Journal C, 2014
Additional jet activity in dijet events is measured using pp collisions at ATLAS at a centre-of-m... more Additional jet activity in dijet events is measured using pp collisions at ATLAS at a centre-of-mass energy of 7 TeV, for jets reconstructed using the anti-k t algorithm with radius parameter R = 0.6. This is done using variables such as the fraction of dijet events without an additional jet in the rapidity interval bounded by the dijet subsystem and correlations between the azimuthal angles of the dijets. They are presented, both with and without a veto on additional jet activity in the rapidity interval, as a function of the scalar average of the transverse momenta of the dijets and of the rapidity interval size. The double differential dijet cross section is also measured as a function of the interval size and the azimuthal angle between the dijets. These variables probe differences in the approach to resummation of large logarithms when performing QCD calculations. The data are compared to powheg, interfaced to the pythia 8 and herwig parton shower generators, as well as to hej with and without interfacing it to the ariadne parton shower generator. None of the theoretical predictions agree with the data across the full phase-space considered; however, powheg+pythia 8 and hej+ariadne are found to provide the best agreement with the data. These measurements use the full data sample collected with the ATLAS detector in 7 TeV pp collisions at the LHC and correspond to integrated luminosities of 36.1 pb −1 and 4.5 fb −1 for data collected during 2010 and 2011, respectively.
The European Physical Journal C, 2014
ATLAS measurements of the azimuthal anisotropy in lead-lead collisions at √ s NN = 2.76 TeV are s... more ATLAS measurements of the azimuthal anisotropy in lead-lead collisions at √ s NN = 2.76 TeV are shown using a dataset of approximately 7 µb −1 collected at the LHC in 2010. The measurements are performed for charged particles with transverse momenta 0.5 < p T < 20 GeV and in the pseudorapidity range |η| < 2.5. The anisotropy is characterized by the Fourier coefficients, v n , of the charged-particle azimuthal angle distribution for n = 2-4. The Fourier coefficients are evaluated using multi-particle cumulants calculated with the generating function method. Results on the transverse momentum, pseudorapidity and centrality dependence of the v n coefficients are presented. The elliptic flow, v 2 , is obtained from the two-, four-, six-and eight-particle cumulants while higher-order coefficients, v 3 and v 4 , are determined with two-and four-particle cumulants. Flow harmonics v n measured with four-particle cumulants are significantly reduced compared to the measurement involving two-particle cumulants. A comparison to v n measurements obtained using different analysis methods and previously reported by the LHC experiments is also shown. Results of measurements of flow fluctuations evaluated with multiparticle cumulants are shown as a function of transverse momentum and the collision centrality. Models of the initial spatial geometry and its fluctuations fail to describe the flow fluctuations measurements. ation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the
Journal of High Energy Physics, 2014
The prompt and non-prompt production cross-sections for the χ c1 and χ c2 charmonium states are m... more The prompt and non-prompt production cross-sections for the χ c1 and χ c2 charmonium states are measured in pp collisions at √ s = 7 TeV with the ATLAS detector at the LHC using 4.5 fb −1 of integrated luminosity. The χ c states are reconstructed through the radiative decay χ c → J/ψγ (with J/ψ → µ + µ −) where photons are reconstructed from γ → e + e − conversions. The production rate of the χ c2 state relative to the χ c1 state is measured for prompt and non-prompt χ c as a function of J/ψ transverse momentum. The prompt χ c cross-sections are combined with existing measurements of prompt J/ψ production to derive the fraction of prompt J/ψ produced in feed-down from χ c decays. The fractions of χ c1 and χ c2 produced in b-hadron decays are also measured.
Journal of High Energy Physics, 2014
Search for direct production of charginos, neutralinos and sleptons in final states with two lept... more Search for direct production of charginos, neutralinos and sleptons in final states with two leptons and missing transverse momentum in pp collisions at √ s = 8 TeV with the ATLAS detector The ATLAS collaboration
Physics Letters B, 2013
A search for highly ionising, penetrating particles with electric charges from |q| = 2e to 6e is ... more A search for highly ionising, penetrating particles with electric charges from |q| = 2e to 6e is performed using the ATLAS detector at the CERN Large Hadron Collider. Proton-proton collision data taken at √ s = 7 TeV during the 2011 running period, corresponding to an integrated luminosity of 4.4 fb −1 , are analysed. No signal candidates are observed, and 95% confidence level cross-section upper limits are interpreted as mass-exclusion lower limits for a simplified Drell-Yan production model. In this model, masses are excluded from 50 GeV up to 430, 480, 490, 470 and 420 GeV for charges 2e, 3e, 4e, 5e and 6e, respectively.
Physics Letters B, 2012
A measurement of the top quark mass (Mtop) in the all-hadronic decay channel is presented. It use... more A measurement of the top quark mass (Mtop) in the all-hadronic decay channel is presented. It uses 5.8 fb −1 of pp data collected with the CDF II detector at the Fermilab Tevatron Collider. Events with six to eight jets are selected by a neural network algorithm and by the requirement that at least one of the jets is tagged as a b-quark jet. The measurement is performed with a likelihood fit technique, which simultaneously determines Mtop and the jet energy scale (JES) calibration. The fit yields a value of Mtop = 172.5 ± 1.4 (stat) ± 1.0 (JES) ± 1.1 (syst) GeV/c 2 .
Physical Review Letters, 2012
The angular distributions of muons from Υ(1S, 2S, 3S) → µ + µ − decays are measured using data fr... more The angular distributions of muons from Υ(1S, 2S, 3S) → µ + µ − decays are measured using data from pp collisions at √ s = 1.96 TeV corresponding to an integrated luminosity of 6.7 fb −1 and collected with the CDF II detector at the Fermilab Tevatron. This analysis is the first to report the full angular distributions as functions of transverse momentum pT for Υ mesons in both the Collins-Soper and s-channel helicity frames. This is also the first measurement of spin alignment of Υ(3S) mesons. Within the kinematic range of Υ rapidity |y| < 0.6 and pT up to 40 GeV/c, the angular distributions are found to be nearly isotropic.
Physical Review Letters, 2011
A search for new dielectron mass resonances using data recorded by the CDF II detector and corres... more A search for new dielectron mass resonances using data recorded by the CDF II detector and corresponding to an integrated luminosity of 5.7 fb −1 is presented. No significant excess over the expected standard model prediction is observed. In this dataset, an event with the highest dielectron mass ever observed (960 GeV/c 2) was recorded. The results are interpreted in the Randall-Sundrum (RS) model. Combined with the 5.4 fb −1 diphoton analysis, the RS-graviton lower mass limit for the coupling k/M P l = 0.1 is 1058 GeV/c 2 , making it the strongest limit to date.
Physical Review Letters, 2011
Physical Review Letters, 2011
We report the first observation of the baryonic flavor-changing neutral current decay Λ 0 b → Λµ ... more We report the first observation of the baryonic flavor-changing neutral current decay Λ 0 b → Λµ + µ − with 24 signal events and a statistical significance of 5.8 Gaussian standard deviations. This measurement uses a pp collisions data sample corresponding to 6.8 fb −1 at √ s = 1.96 TeV collected by the CDF II detector at the Tevatron collider. The total and differential branching ratios for Λ 0 b → Λµ + µ − are measured. We find B(Λ 0 b → Λµ + µ −) = [1.73 ± 0.42(stat) ± 0.55(syst)] × 10 −6. We also report the first measurement of the differential branching ratio of B 0 s → φµ + µ − using 49 signal events. In addition, we report branching ratios for B + → K + µ + µ − , B 0 → K 0 µ + µ − and B → K * (892)µ + µ − decays.
Uploads
Papers by Karolos Potamianos