Design da identidade visual: website e aplicação móvel para a empresa Manuel Pataco

O trabalho que se apresenta foi realizado na unidade curricular (UC) de Investigação em Design orientada pela professora Jacinta Costa e parte da análise da proposta realizada na unidade curricular de Multimédia, no curso de Arte e Design, minor em design da Escola Superior de Educação, sob a docência do professor Miguel Gata. Teve como objetivo o redesign da imagem visual e website da empresa Manuel Pataco, situada em Macedo de Cavaleiros, Bragança. Esta possuía uma identidade visual e ferramentas online que poderiam ser melhoradas através da aplicação do método de design.N/

Exclusive photon-photon production of muon pairs in proton-proton collisions at sqrt(s) = 7 TeV

Abstract A measurement of the exclusive two-photon production of muon pairs in protonproton collisions at √ s = 7 TeV, pp → pµ + µ − p, is reported using data corresponding to an integrated luminosity of 40 pb −1 . For muon pairs with invariant mass greater than 11.5 GeV, transverse momentum p T (µ) > 4 GeV and pseudorapidity |η(µ)| < 2.1, a fit to the dimuon p T (µ + µ − ) distribution results in a measured cross section of σ(p → pµ + µ − p) = 3.38 +0.58 −0.55 (stat.) ± 0.16 (syst.) ± 0.14 (lumi.) pb, consistent with the theoretical prediction evaluated with the event generator LPAIR. The ratio to the predicted cross section is 0.83 +0.14 −0.13 (stat.) ± 0.04 (syst.) ± 0.03 (lumi.). The characteristic distributions of the muon pairs produced via γγ fusion, such as the muon acoplanarity, the muon pair invariant mass and transverse momentum agree with those from the theory. Submitted to the Journal of High Energy Physics Introduction The exclusive two-photon production of lepton pairs may be reliably calculated within the framework of quantum electrodynamics (QED) At the Tevatron, the exclusive two-photon production of electron [4, 5] and muon [5, 6] pairs in pp collisions has been measured with the CDF detector. Observations have been made of QED signals, leading to measurements of exclusive charmonium photoproduction [6] and searches for anomalous high-mass exclusive dilepton production [5]. However, all such measurements have very limited numbers of selected events because the data samples were restricted to single interaction bunch crossings. The higher energies and increased luminosity available at the Large Hadron Collider (LHC) will allow significant improvements in these measurements, if this limitation can be avoided. As a result of the small theoretical uncertainties and characteristic kinematic distributions in γγ → µ + µ − , this process has been proposed as a candidate for a complementary absolute calibration of the luminosity of pp collisions Unless both outgoing protons are detected, the semi-exclusive two-photon production, involving single or double proton dissociation 3 Simulated Samples The paper is organized as follows. In Section 2, a brief description of the CMS detector is provided. Section 3 describes the data and samples of simulated events used in the analysis. Section 4 documents the criteria used to select events, and Section 5 the method used to extract the signal yield from the data. The systematic uncertainties and cross-checks performed are discussed in Section 6, while Section 7 contains plots comparing the selected events in data and simulation. Finally, the results of the measurement are given in Section 8 and summarized in Section 9. The CMS detector A detailed description of the CMS experiment can be found elsewhere Simulated Samples The LPAIR 4.0 event generator [9, 10] is used to produce simulated samples of two-photon production of muon pairs. The generator uses full leading-order QED matrix elements, and the cross sections for the exclusive events depend on the proton electromagnetic form-factors to account for the distribution of charge within the proton. For proton dissociation, the cross sections depend on the proton structure function. In order to simulate the fragmentation of the dissociated proton into a low-mass system N, the LUND model shower routine Event selection The analysis uses a sample of pp collisions at √ s = 7 TeV, collected during 2010 at the LHC and corresponding to an integrated luminosity of 40 pb −1 . The sample includes 36 pb −1 of data passing the standard CMS quality criteria for all detector subsystems, and 4 pb −1 in which the quality criteria are satisfied for the tracking and muon systems used in the analysis. From the sample of triggered events, the presence of two reconstructed muons is required. Then the exclusivity selection is performed to keep only events with a vertex having no tracks other than those from the two muons. Finally, the signal muons are required to satisfy identification criteria, and kinematic constraints are imposed using their four-momentum. All selection steps are described in the following sections. Trigger and muon reconstruction Events are selected online by triggers requiring the presence of two muons with a minimum p T of 3 GeV. No requirement on the charge of the muons is applied at the trigger level. Muons are reconstructed offline by combining information from the muon chambers with that on chargedparticle tracks reconstructed in the silicon tracker Vertex and track exclusivity selection With single interactions, the exclusive signal is characterized by the presence of two muons, no additional tracks, and no activity above the noise threshold in the calorimeters. The presence of additional interactions in the same bunch crossing will spoil this signature by producing additional tracks and energy deposits in the calorimeters. In the 2010 data, less than 20% of the total luminosity was estimated to have been collected from bunch crossings where only a single interaction look place, leading to a significant decrease in signal efficiency if the conditions of no extra tracks or calorimeter energy are required. The selection of exclusive events is therefore applied using the pixel and silicon tracker only, since the primary vertex reconstruction In order to reduce the background from inclusive DY and QCD dimuon production, which typically have many tracks originating from the same vertex as a prompt muon pair, the dimuon vertex is required to be separated in three dimensions by more than 2 mm from any additional tracks in the event. This value is selected to optimize the signal efficiency and background rejection found in events triggered only by the presence of colliding bunches ("zero-bias" events), and in DY Monte Carlo simulation. For the zero-bias data, this is accomplished by introducing an artificial additional dimuon vertex into each event as a proxy for an exclusive dimuon interaction. Thus, in this study, beam crossings with no real vertex present are counted as "single vertex" events, and crossings with one real vertex are counted as having an additional pileup event. Event selection The effects of the track veto on the signal efficiency and on the efficiency for misidentifying background as signal are studied as a function of the distance to the closest track for the zerobias sample and DY background Muon identification Each muon of the pair is required to pass a "tight" muon selection Kinematic selection In order to minimize the systematic uncertainties related to the knowledge of the low-p T and large-η muon efficiencies, only muons with p T > 4 GeV and |η| < 2.1 are selected. The p T and |η| requirements retain muon pairs from exclusive photoproduction of upsilon mesons, γp → Υp → µ + µ − p. This process occurs when a photon emitted from one proton fluctuates into a qq pair, which interacts with the second proton via a color-singlet exchange. This contribution is removed by requiring that the muons have an invariant mass m(µ + µ − ) > 11.5 GeV. In order to suppress further the proton dissociation background, the muon pair is required to be back-to-back in azimuthal angle (1 − |∆φ(µ + µ − )/π| < 0.1) and balanced in the scalar difference in the p T of the two muons (|∆p T (µ + µ − )| < 1.0 GeV). A possible contamination could arise from cosmic-ray muons, which would produce a signature similar to the exclusive γγ → µ + µ − signal. The three-dimensional opening angle of the pair, defined as the arccosine of the normalized scalar product of the muon momentum vectors, is therefore required to be smaller than 0.95 π, to reduce any contribution from cosmic-ray muons. The effect of each step of the selection on the data and simulated signal and background samples is shown in Signal extraction Efficiency corrections A correction is applied to account for the presence of extra proton-proton interactions in the same bunch crossing as a signal event. These pileup interactions will result in an inefficiency if they produce a track with a position within the nominal 2 mm veto distance around the dimuon vertex. This effect is studied in zero-bias data using the method described in Section 4.2. The nominal 2 mm veto is then applied around the dimuon vertex, and the event is accepted if no tracks fall within the veto distance. The efficiency is measured as a function of the instantaneous luminosity per colliding bunch. The average efficiency is calculated based on 6 5 Signal extraction The trigger, tracking, and offline muon selection efficiencies are each obtained from the tagand-probe The effect of the vertexing efficiency is studied both in inclusive dimuon data and signal simulation, by performing an independent selection of all muon pairs with a longitudinal separation of less than 0.5 mm. A Kalman filter Maximum likelihood fit The elastic pp → pµ + µ − p contribution is extracted by performing a binned maximum-likelihood fit to the measured p T (µ + µ − ) distribution. Shapes from Monte Carlo simulation are used for the signal, single-proton dissociation, double-proton dissociation, and DY contributions, with all corrections described in Section 4.4 applied. Three parameters are determined from the fit: the elastic signal yield relative to the LPAIR prediction for an integrated luminosity of 40 pb −1 (R El−El ), the single-proton dissociation yield relative to the LPAIR single-proton dissociation prediction for 40 pb −1 (R diss−El ), and an exponential modification factor for the shape of the p T distribution, characterized by the parameter a. The modification parameter is included to account for possible rescattering effects not included in the simulation, as described in Section 3. Given the small number of events expected in 40 pb −1 , the double-proton dissociation and DY contributions cannot be treated as free parameters and are fixed from simulation to their predicted values. The contribution from exclusive γγ → τ + τ − production is estimated to be 0.1 events from the simulation, and is neglected. The p T (µ + µ − ) distribution in data is shown overlaid with the result of the fit to the shapes from Monte Carlo simulation in data-theory signal ratio: R El−El = 0.83 +0.14 −0.13 ; single-proton dissociation yield ratio: R diss−El = 0.73 +0.16 −0.14 ; modification parameter: a = 0.04 with asymmetric statistical uncertainties computed using MINOS As a cross-check, a fit to the 1 − |∆φ(µ + µ − )/π| distribution is performed, with the signal and single-proton dissociation yields as free parameters, and the shape of the single-proton dissociation component fixed from the simulation. The resulting value of the data-theory signal ratio is 0.81 +0.14 −0.13 , consistent with the nominal fit result. The central values of the signal and single-proton dissociation yields from the fit are both below the mean number expected for 40 pb −1 , consistent with the deficit shown in The fits to the data with these looser selection requirements are shown in Control plots The dimuon invariant mass and acoplanarity distributions for events passing all selection criteria listed in In Systematic uncertainties and cross-checks Systematic uncertainties related to the pileup efficiency correction, muon trigger and reconstruction efficiency corrections, momentum scale, LHC crossing angle, and description of the backgrounds in the fit are considered. The systematic uncertainties related to the muon identification, trigger, and tracking efficiencies are determined from the statistical uncertainties of the J/ψ and Z control samples used to derive the corrections. The remaining systematic uncertainties are evaluated by varying each contribution as described in the following sections, and repeating the fit with the same three free parameters R El−El , R diss−El , and the shape correction a. The relative difference of the data-theory signal ratio between the modified and the nominal fit result is taken as a systematic uncertainty. Pileup correction systematic uncertainties Charged tracks from pileup interactions more than 2.0 mm from the dimuon vertex may induce a signal inefficiency, if they are misreconstructed to originate from within the 2.0 mm veto window. The η-dependent single-track impact parameter resolution in CMS has been measured to be less than 0.2 mm in the transverse direction, and less than 1.0 mm in the longitudinal direction As a further check, the same variations are applied to the selected sample of dimuon events, removing the Υ mass cut m < 11.5 GeV to increase the statistics with photo-produced exclusive upsilon events. The change in the number of events selected in the dimuon sample is found to be consistent with the expectation from the zero-bias sample. Muon efficiencies and momentum scale The statistical uncertainty on the muon efficiency correction is evaluated by performing a fast Monte Carlo study in which each single-muon correction evaluated from the tag-andprobe study is varied independently using a Gaussian distribution having a width equal to the measured uncertainty. The r.m.s. of the distribution of the resulting variations in the overall dimuon efficiency correction is taken as the systematic uncertainty. From 1000 pseudoexperiments, this results in an uncertainty of 0.8%. In addition, we study the effect of correlations in the dimuon efficiency. The tag-and-probe study is only sensitive to single-muon efficiencies. Since we take the dimuon efficiency as the product of the single-muon efficiencies, the effect of correlations in the efficiency are not modeled. To evaluate the size of this effect, the efficiency corrections are computed after removing events in the J/ψ control sample in which the two muons bend towards each other in the r-φ plane, potentially becoming very close or overlapping. Such events may introduce larger correlations in the efficiency of the dimuon pair than would be present in the well separated signal muons. Repeating the signal extraction with this change results in a relative difference of 0.7% from the nominal efficiency, which is taken as a systematic uncertainty. Using studies of the muon momentum scale derived from Z → µ + µ − [23], the muon p T is shifted by the observed p T -dependent bias, and the nominal fit is performed again. The resulting relative change in the signal yield is 0.1%, which is taken as a systematic uncertainty. As a cross-check using a sample kinematically closer to the signal, we apply all the selections except for the veto on the Υ mass region, and perform a fit to the Υ(1S) resonance. The resulting mass is consistent with the PDG value Vertexing and tracking efficiencies Since the study described in Section 4.4 shows no significant difference in the vertexing efficiency between data and simulation, the 0.1% statistical uncertainty of the measurement in data is taken as a systematic uncertainty. For the tracking efficiency, the difference between data and simulation is applied as a single correction without binning in p T or η. The statistical uncertainty of 0.1% on the correction for the dimuon is taken as a systematic uncertainty. Crossing angle The non-zero crossing angle of the LHC beams leads to a boost of the dimuon system in the x direction. Consequently, the p T of the pair is over-estimated by a few MeV, especially for high-mass dimuon events. This effect is estimated by applying a correction for the Lorentz boost, using a half-angle of 100 µrad in the x-z plane. This results in a 1.0% variation from the nominal fit value, and is taken as an additional systematic uncertainty. Fit stability Checks of the fit stability are performed by testing different bin widths and fit ranges. Starting from the nominal number of 20 bins in the range 0-3 GeV, variations in the bin width from 0.1 to 0.2 GeV and fit range [0, 2] to [0, 4] GeV show deviations by at most 3.3% with respect to the nominal yield. The fit bias is studied by performing a series of Monte Carlo pseudoexperiments for different input values of the signal and proton-dissociation yields, using events drawn from the fully simulated samples. The means of the pull distributions are found to be 7.6 Backgrounds 13 consistent with zero. Since the pseudo-experiments with the nominal binning and fit range show no significant bias, no additional systematics are assigned in this case. Backgrounds The yields of the double-proton dissociation and DY contributions are fixed in the nominal fit. To estimate the systematic uncertainty from this constraint, the fit is repeated with each of these varied independently by a factor of 2. The resulting changes in the fitted signal yield are 0.9% and 0.4%, respectively, where because of the similar shapes of the single and double proton dissociation components, this variation is partly absorbed into the fitted single-proton dissociation yield. As a cross-check of this procedure, the |∆p T (µ + µ − )| and 1 − |∆φ(µ + µ − )/π| requirements are inverted to select samples of events expected to be dominated by doubleproton dissociation and DY backgrounds. The agreement between data and simulation in these regions is found to be within the factor of 2 used as a systematic variation. The possibility of a large contamination from cosmic-ray muons, which may fake a signal since they will not be correlated with other tracks in the event, is studied by comparing the vertex position and three-dimensional opening angle in data and simulations of collision backgrounds. A total of three events fail the vertex position selection in data, after all other selection criteria are applied. All three also fail the opening angle selection, which is consistent with the expected signature from cosmic muons. We conclude that the opening angle requirement effectively rejects cosmic muons, and do not assign a systematic uncertainty for this possible contamination. A similar check for contamination from beam-halo muons is performed by applying the nominal analysis selection to non-collision events triggered by the presence of a single beam. Within the limited statistics, zero events pass all the analysis selections, and therefore no additional systematic uncertainty is assigned in this case. Summary of systematic uncertainties The individual variations in the definition of the track-veto are taken as correlated uncertainties, with the largest variation taken as a contribution to the systematic uncertainty. The largest variation related to the track quality, obtained when requiring high-purity tracks with > 10 hits instead of the nominal value of > 3 hits, is also taken as a contribution. The larger variation resulting from increasing or decreasing the double-proton dissociation background normalization by a factor of 2, and the larger variation resulting from increasing or decreasing the DY background normalization by a factor of 2, are each taken as contributions to the systematic uncertainty. The variation in the crossing angle, muon identification and trigger efficiencies, tracking efficiency, bias due to correlations in the J/ψ control sample, and vertexing efficiency are treated as uncorrelated uncertainties. Summing quadratically all uncorrelated contributions gives an overall relative systematic uncertainty of 4.8% on the signal yield Results For muon pairs with invariant mass greater than 11.5 GeV, single-muon transverse momentum p T (µ) > 4 GeV, and single-muon pseudorapidity in the range |η(µ)| < 2.1, 148 events pass all selections. Approximately half of these are ascribed to fully exclusive (elastic) production. The number of events expected from Monte Carlo simulation of signal, proton dissociation, and DY backgrounds for an integrated luminosity of 40 pb −1 is 184. +0.58 −0.55 (stat.) ± 0.16 (syst.) ± 0.14 (lumi.) pb, and the corresponding data-theory signal ratio is 0.83 +0.14 −0.13 (stat.) ± 0.04 (syst.) ± 0.03 (lumi.), where the statistical uncertainties are strongly correlated with the single-proton dissociation background. Summary A measurement is reported of the exclusive two-photon production of muon pairs, pp → pµ + µ − p, in a 40 pb −1 sample of proton-proton collisions collected at √ s = 7 TeV during 2010 at the LHC. The measured cross section +0.58 −0.55 (stat.) ± 0.16 (syst.) ± 0.14 (lumi.) pb, is consistent with the predicted value, and the characteristic distributions of the muon pairs produced via γγ fusion, such as the pair acoplanarity and transverse momentum, are well described by the full simulation using the matrix-element event generator LPAIR. The detection efficiencies are determined from control samples in data, including corrections for the significant event pileup. The signal yield is correlated with the dominant background from two-photon production with proton dissociation, for which the current estimate from a fit to the p T (µ + µ − ) distribution can be improved with additional data. The efficiency for the exclusivity selection is above 90% in the full data sample collected by CMS during the 2010 LHC run. With increasing instantaneous luminosity this efficiency will decrease, but without possible improvements to the selection remains above 60% with up to 8 additional pileup vertices. Since the process may be calculated reliably in the framework of QED, within uncertainties associated with the proton form factor, this represents a first step towards a complementary luminosity measurement, and a reference for other exclusive production measurements to be performed with pileup. Acknowledgments We wish to congratulate our colleagues in the CERN accelerator departments for the excellent performance of the LHC machine. [10] S. P. Baranov et al., "LPAIR -A generator