Natural and Mathematical Sciences

Permanent URI for this collectionhttps://hdl.handle.net/10679/313

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Now showing 1 - 20 of 1083

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Theoretical determination of K(1)(1270,1400) mixing angle in QCD
(IOP Publishing, 2012) Dağ, Hüseyin; Ozpineci, A.; Çağıl, A.; Erkol, Güray; Natural and Mathematical Sciences; ERKOL, Güray; DAĞ, Hüseyin
In quark model, the strange axial vector mesons K1(1270) and K1(1400) are defined as the mixtures of orbital angular momentum states K1A and K1B. In this work, by using the orthogonality of the mass eigenstates, we have estimated the K1(1270, 1400) mixing angle θK1, where we have found that θK1 sime −(39 ± 4)°.
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Study of W boson production in PbPb and pp collisions at root s(NN)=2.76 TeV
(Elsevier, 2012) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
A measurement is presented of W-boson production in PbPb collisions carried out at a nucleon-nucleon (NN) centre-of-mass energy root S-NN of 2.76 TeV at the LHC using the CMS detector. In data corresponding to an integrated luminosity of 7.3 mu b(-1), the number of W -> mu v(mu) decays is extracted in the region of muon pseudorapidity vertical bar eta mu vertical bar < 2.1 and transverse momentum p(T)(mu) > 25 GeV/c. Yields of muons found per unit of pseudorapidity correspond to (159 +/- 10(stat.) +/- 12(syst.)) x 10(-8) W and (154 +/- 10(stat.) +/- 12(syst.)) x 10(-8) W- bosons per minimum-bias PbPb collision. The dependence of W production on the centrality of PbPb collisions is consistent with a scaling of the yield by the number of incoherent NN collisions. The yield of W bosons is also studied in a sample of pp interactions at root S = 2.76 TeV corresponding to an integrated luminosity of 231 nb(-1). The individual W+ and W- yields in PbPb and pp collisions are found to agree, once the neutron and proton content in Pb nuclei is taken into account. Likewise, the difference observed in the dependence of the positive and negative muon production on pseudorapidity is consistent with next-to-leading-order perturbative QCD calculations.
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Search for three-jet resonances in pp collisions at root s=7 TeV
(Elsevier, 2012) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
Results are reported from a search for the production of three-jet resonances in pp collisions at a center-of-mass energy root s = 7 TeV. The study uses the data sample collected by the CMS experiment at the LHC in 2011, corresponding to an integrated luminosity of 5.0 fb(-1). Events with high jet multiplicity and a large scalar sum of jet transverse momenta are analyzed for the presence of resonances in the three-jet invariant mass spectrum. No evidence for a narrow resonance is found in the data, and limits are set on the cross section for gluino pair production in an R-parity-violating supersymmetry model, for gluino masses greater than 280 GeV. Assuming a branching fraction for gluino decay into three jets of 100%, gluino masses below 460 GeV are excluded at 95% confidence level. These results significantly extend the range of previous limits.
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Observation of a new Xi(b) baryon
(American Physical Society, 2012-06-22) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
The observation of a new b baryon via its strong decay into Xi(-)(b)pi(+) (plus charge conjugates) is reported. The measurement uses a data sample of pp collisions at root s = 7 TeV collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 5.3 fb(-1). The known Xi(-)(b) baryon is reconstructed via the decay chain Xi(-)(b) -> J/psi Xi(-) -> mu(+)mu(-)Lambda(0)pi(-), with Lambda(0) -> p pi(-). A peak is observed in the distribution of the difference between the mass of the Xi(-)(b)pi(+) system and the sum of the masses of the Xi(-)(b) and pi(+), with a significance exceeding 5 standard deviations. The mass difference of the peak is 14.84 +/- 0.74(stat) +/- 0.28(syst) MeV. The new state most likely corresponds to the J(P) = 3/2(+) companion of the Xi(b).
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Search for a light charged Higgs boson in top quark decays in pp collisions at root s=7 TeV
(Springer Science+Business Media, 2012-07) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
Results are presented on a search for a light charged Higgs boson that can be produced in the decay of the top quark t -> H(+)b and which, in turn, decays into tau(+)nu(tau). The analysed data correspond to an integrated luminosity of about 2 fb(-1) recorded in proton-proton collisions at root s = 7 TeV by the CMS experiment at the LHC. The search is sensitive to the decays of the top quark pairs t (t) over bar -> H(+/-)W(-/+)b (b) over bar and t (t) over bar -> H +/- H -/+ b (b) over bar. Various final states have been studied separately, all requiring presence of a tau lepton from H+ decays, missing transverse energy, and multiple jets. Upper limits on the branching fraction B(t -> H(+)b) in the range of 2-4% are established for charged Higgs boson masses between 80 and 160 GeV, under the assumption that B(H+ -> tau(+)nu(tau)) = 1.
Open Access
Search for stopped long-lived particles produced in pp collisions at root s=7 TeV
(Springer Science+Business Media, 2012-08) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
A search has been performed for long-lived particles that have stopped in the CMS detector, during 7TeV proton-proton operations of the CERN LHC. The existence of such particles could be inferred from observation of their decays when there were no proton-proton collisions in the CMS detector, namely during gaps between LHC beam crossings. Using a data set in which CMS recorded an integrated luminosity of 4.0 fb(-1), and a search interval corresponding to 246 hours of trigger live time, 12 events are observed, with a mean background prediction of 8.6 +/- 2.4 events. Limits are presented at 95% confidence level on long-lived gluino and stop production, over 13 orders of magnitude of particle lifetime. Assuming the "cloud model" of R-hadron interactions, a gluino with mass below 640 GeV and a stop with mass below 340 GeV are excluded, for lifetimes between 10 mu s and 1000s.
Open Access
Search for new physics in events with same-sign dileptons and b-tagged jets in pp collisions at root s=7 TeV
(Springer Science+Business Media, 2012-08) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
A search for new physics is performed using isolated same-sign dileptons with at least two b-quark jets in the final state. Results are based on a 4.98 fb(-1) sample of proton-proton collisions at a centre-of-mass energy of 7TeV collected by the CMS detector. No excess above the standard model background is observed. Upper limits at 95% confidence level are set on the number of events from non-standard-model sources. These limits are used to set constraints on a number of new physics models. Information on acceptance and efficiencies are also provided so that the results can be used to confront additional models in an approximate way.
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Search for narrow resonances in dilepton mass spectra in pp collisions at root s=7 TeV
(Elsevier, 2012-08-14) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
A search for narrow, high-mass resonances decaying to electron or muon pairs has been performed using pp collision data collected at root s = 7 TeV by the CMS experiment in 2011. The data sample corresponds to an integrated luminosity of approximately 5 fb(-1). The event yields observed in the signal regions are consistent with predictions of the standard model backgrounds, and upper limits on the cross section times branching fraction for a resonance decaying to dileptons are extracted from a shape analysis of the dilepton invariant mass distribution. The resulting mass limits at 95% confidence level are 2330 GeV for the Z' in the Sequential Standard Model, 2000 GeV for the superstring-inspired Z'(psi) resonance, 890 (540) GeV for the Stueckelberg extension Z'(St) the mass parameter epsilon = 0.06 (0.04), and 2140 (1810) GeV for Kaluza-Klein gravitons with the coupling parameter k/(M) over bar (Pl) of 0.10 (0.05). These limits are the most stringent to date.
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Measurement of the Λb cross section and the (Lambda)over-bar(b) to Λb ratio with J/ψΛ decays in pp collisions at root s=7 TeV
(Elsevier, 2012-08-14) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
The Lambda b differential production cross section and the cross section ratio sigma((Lambda) over bar (b))/sigma(Lambda(b)) are measured as functions of transverse momentum p(T)(Lambda b) rapidity vertical bar y(Lambda b)vertical bar in pp collisions at root s = 7 TeV using data collected by the CMS experiment at the LHC. The measurements are based on Lambda(b) decays reconstructed in the exclusive final state J/psi Lambda, with the subsequent decays J/psi -> mu(+)mu(-) and Lambda -> p pi, using a data sample corresponding to an integrated luminosity of 1.9 fb(-1). The product sigma(Lambda(b)) x B(Lambda(b) -> J/psi Lambda) versus p(T)(Lambda b) falls faster than that of b mesons. The measured value of sigma(Lambda(b)) x B(Lambda(b) -> J/psi Lambda) for p(T)(Lambda b) > 10 GeV and vertical bar y(Lambda b)vertical bar < 2.0 is 1.16 +/- 0.06 +/- 0.12 nb, and the integrated sigma(<(Lambda)over bar>(b))/sigma(Lambda(b)) ratio is 1.02 +/- 0.07 +/- 0.09, where the uncertainties are statistical and systematic, respectively.
Open Access
Search for new physics with same-sign isolated dilepton events with jets and missing transverse energy
(American Physical Society, 2012-08-17) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
A search for new physics is performed in events with two same-sign isolated leptons, hadronic jets, and missing transverse energy in the final state. The analysis is based on a data sample corresponding to an integrated luminosity of 4: 98 fb(-1) produced in pp collisions at a center-of-mass energy of 7 TeV collected by the CMS experiment at the LHC. This constitutes a factor of 140 increase in integrated luminosity over previously published results. The observed yields agree with the standard model predictions and thus no evidence for new physics is found. The observations are used to set upper limits on possible new physics contributions and to constrain supersymmetric models. To facilitate the interpretation of the data in a broader range of new physics scenarios, information on the event selection, detector response, and efficiencies is provided.
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Search for dark matter and large extra dimensions in monojet events in pp collisions at root s=7 TeV
(Springer Science+Business Media, 2012-09) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
A search has been made for events containing an energetic jet and an imbalance in transverse momentum using a data sample of pp collisions at a center-of-mass energy of 7 TeV. This signature is common to both dark matter and extra dimensions models. The data were collected by the CMS detector at the LHC and correspond to an integrated luminosity of 5.0 fb(-1). The number of observed events is consistent with the standard model expectation. Constraints on the dark matter-nucleon scattering cross sections are determined for both spin-independent and spin-dependent interaction models. For the spin-independent model, these are the most constraining limits for a dark matter particle with mass below 3.5 GeV/c(2), a region unexplored by direct detection experiments. For the spin-dependent model, these are the most stringent constraints over the 0.1-200 GeV/c(2) mass range. The constraints on the Arkani-Hamed, Dimopoulos, and Dvali model parameter M D determined as a function of the number of extra dimensions are also an improvement over the previous results.
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Search for a fermiophobic Higgs boson in pp collisions at root s=7 TeV
(Springer Science+Business Media, 2012-09) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
Combined results are reported from searches for a fermiophobic Higgs boson in the gamma gamma, WW, and ZZ decay modes in proton-proton collisions at root s = 7 TeV. The explored Higgs boson mass range is 110-300 GeV. The data sample corresponds to an integrated luminosity of 4.9-5.1 fb(-1). A fermiophobic Higgs boson is excluded at 95% confidence level in the mass range 110-194 GeV, and at 99% confidence level in the mass ranges 110-124.5 GeV, 127-147.5 GeV, and 155-180 GeV.
Open Access
Search for pair production of first- and second-generation scalar leptoquarks in pp collisions at root s=7 TeV
(American Physical Society, 2012-09-01) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
Results are presented from a search for the pair production of first- and second-generation scalar leptoquarks in proton-proton collisions at root s = 7 TeV. The data sample corresponds to an integrated luminosity of 5.0 fb(-1), collected by the CMS detector at the LHC. The search signatures involve either two charged leptons of the same flavor (electrons or muons) and at least two jets or a single charged lepton (electron or muon), missing transverse energy, and at least two jets. If the branching fraction of the leptoquark decay into a charged lepton and a quark is assumed to be beta = 1, leptoquark pair production is excluded at the 95% confidence level for masses below 830 GeV and 840 GeV for the first and second generations, respectively. For beta = 0.5, masses below 640 GeV and 650 GeV are excluded. These limits are the most stringent to date.
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Measurement of the electron charge asymmetry in inclusive w production in pp collisions at root s=7 TeV
(American Physical Society, 2012-09-14) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
A measurement of the electron charge asymmetry in inclusive pp -> W + X -> ev + X production at root s = 7 TeV is presented based on data recorded by the CMS detector at the LHC and corresponding to an integrated luminosity of 840 pb(-1). The electron charge asymmetry reflects the unequal production of W+ and W- bosons in pp collisions. The electron charge asymmetry is measured in bins of the absolute value of electron pseudorapidity in the range of vertical bar eta vertical bar < 2.4. The asymmetry rises from about 0.1 to 0.2 as a function of the pseudorapidity and is measured with a relative precision better than 7%. This measurement provides new stringent constraints for parton distribution functions.
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Search for high-mass resonances decaying into tau-lepton pairs in pp collisions at root s=7 TeV
(Elsevier, 2012-09-17) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
A search for high-mass resonances decaying into tau(+)tau(-) is performed using a data sample of pp collisions at root s = 7 TeV. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of 4.9 fb(-1). The number of observed events is in agreement with the standard model prediction. An upper limit on the product of the resonance cross section and branching fraction into tau-lepton pairs is calculated as a function of the resonance mass. Using the sequential standard model resonance Z'(SSM) and the superstring-inspired E-6 model with resonance Z'(psi) as benchmarks, resonances with standard model couplings with masses below 1.4 and 1.1 TeV, respectively, are excluded at 95% confidence level.
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Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC
(Elsevier, 2012-09-17) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
Results are presented from searches for the standard model Higgs boson in proton-proton collisions at root s = 7 and 8 TeV in the Compact Muon Solenoid experiment at the LHC, using data samples corresponding to integrated luminosities of up to 5.1 fb(-1) at 7 TeV and 5.3 fb(-1) at 8 TeV. The search is performed in five decay modes: gamma gamma, ZZ, W+W-, tau(+)tau(-), and b (b) over bar. An excess of events is observed above the expected background, with a local significance of 5.0 standard deviations, at a mass near 125 GeV, signalling the production of a new particle. The expected significance for a standard model Higgs boson of that mass is 5.8 standard deviations. The excess is most significant in the two decay modes with the best mass resolution, gamma gamma and ZZ; a fit to these signals gives a mass of 125.3 +/- 0.4(stat.) +/- 0.5(syst.) GeV. The decay to two photons indicates that the new particle is a boson with spin different from one.
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Search for a light pseudoscalar higgs boson in the dimuon decay channel in pp collisions at root s=7 TeV
(American Physical Society, 2012-09-21) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
The dimuon invariant mass spectrum is searched in the range between 5.5 and 14 GeV for a light pseudoscalar Higgs boson a, predicted in a number of new physics models, including the next-to-minimal supersymmetric standard model. The data sample used in the search corresponds to an integrated luminosity of 1: 3 fb(-1) collected in pp collisions at root s = 7 TeV with the CMS detector at the LHC. No excess is observed above the background predictions and upper limits are set on the cross section times branching fraction sigma x B(pp -> a -> mu(+) mu(-)) in the range of 1.5-7.5 pb. These results improve on existing bounds on the ab (b) over bar coupling for m(a) < m(gamma(1s)) and are the first significant limits for m(a) > m(gamma(3S)). Constraints on the supersymmetric parameter space are presented in the context of the next-to-minimal model.
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Search for supersymmetry in hadronic final states using M-T2 in pp collisions at root s=7 TeV
(Springer Science+Business Media, 2012-10) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
A search for supersymmetry or other new physics resulting in similar final states is presented using a data sample of 4.73 fb(-1) of pp collisions collected at root s = 7 TeV with the CMS detector at the LHC. Fully hadronic final states are selected based on the variable M-T2, an extension of the transverse mass in events with two invisible particles. Two complementary studies are performed. The first targets the region of parameter space with medium to high squark and gluino masses, in which the signal can be separated from the standard model backgrounds by a tight requirement on M-T2. The second is optimized to be sensitive to events with a light gluino and heavy squarks. In this case, the M-T2 requirement is relaxed, but a higher jet multiplicity and at least one b-tagged jet are required. No significant excess of events over the standard model expectations is observed. Exclusion limits are derived for the parameter space of the constrained minimal supersymmetric extension of the standard model, as well as on a variety of simplified model spectra.
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Measurement of jet fragmentation into charged particles in pp and PbPb collisions at root s(NN)=2.76 TeV
(Springer Science+Business Media, 2012-10) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
Jet fragmentation in pp and PbPb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair was studied using data collected with the CMS detector at the LHC. Fragmentation functions are constructed using charged-particle tracks with transverse momenta p(T) > 4 GeV/c for dijet events with a leading jet of p(T) > 100 GeV/c. The fragmentation functions in PbPb events are compared to those in pp data as a function of collision centrality, as well as dijet-p(T) imbalance. Special emphasis is placed on the most central PbPb events including dijets with unbalanced momentum, indicative of energy loss of the hard scattered parent partons. The fragmentation patterns for both the leading and subleading jets in PbPb collisions agree with those seen in pp data at 2.76 TeV. The results provide evidence that, despite the large parton energy loss observed in PbPb collisions, the partition of the remaining momentum within the jet cone into high-p(T) particles is not strongly modified in comparison to that observed for jets in vacuum.
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Measurement of the top-quark mass in t(t)over-bar events with dilepton final states in pp collisions at root s=7 TeV
(Springer Science+Business Media, 2012-10) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
The top-quark mass is measured in proton-proton collisions at root s = 7 TeV using a data sample corresponding to an integrated luminosity of 5.0 fb(-1) collected by the CMS experiment at the LHC. The measurement is performed in the dilepton decay channel t (t) over bar -> (l(+)nu(l)b) (l-(nu) over bar (l)(b) over bar), where l = e, mu. Candidate top-quark decays are selected by requiring two leptons, at least two jets, and imbalance in transverse momentum. The mass is reconstructed with an analytical matrix weighting technique using distributions derived from simulated samples. Using a maximum-likelihood fit, the top-quark mass is determined to be 172.5 +/- 0.4 (stat.) +/- 1.5 (syst.) GeV.

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