Presentation on theme: "W mass and width at CDF Emily Nurse University of Manchester Seminar 30th March 2006."— Presentation transcript:
W mass and width at CDF Emily Nurse University of Manchester Seminar 30th March 2006
E. Nurse, U. of Manchester Seminar2 Outline EWK precision measurements Unstable particles: mass and width The W mass and width: –Motivation –Current status –CDF width measurement –CDF mass measurement (brief summary) Summary and projections
30th March 2006E. Nurse, U. of Manchester Seminar3 Introduction: EWK measurements The Standard Model contains free parameters that must be found from experiment. Relationships between these parameters are given in the theory. Measuring the parameters to a high precision is a stringent test of the theory and deviations from expected values indicate physics Beyond The Standard Model.
30th March 2006E. Nurse, U. of Manchester Seminar4 Introduction : unstable particles mass and width The properties of short lived particles can be measured by reconstructing the invariant mass of their decay products. This distribution peaks at the particle mass and has a finite intrinsic width. The width is due to the Heisenberg uncertainty principle: E t ≥ ћ The shorter the lifetime the larger the width.
30th March 2006E. Nurse, U. of Manchester Seminar5 Z bosons decaying to neutrinos cannot be detected. This decay mode will, however, contribute to the width. The LEP experiment measurements of the Z width gives the number of neutrino species.
30th March 2006E. Nurse, U. of Manchester Seminar6 W mass and width: motivation r~M t 2 r~ln M H Plot from LEP EWK working group web page Measured to 0.0009% Measured to 0.004% Measured to 0.014% “on-shell” mass scheme
30th March 2006E. Nurse, U. of Manchester Seminar7 W mass and width: current status Plots from LEP EWK working group web page direct indirect cf/ Z-Boson: mass = 91.1876 ± 0.0021 GeV width = 2.4952 ± 0.0023 GeV
30th March 2006E. Nurse, U. of Manchester Seminar8 The Tevatron The highest energy accelerator in the world, collides protons with antiprotons at a centre of mass energy of 1.96 TeV. Each experiment currently has ~1.2 fb -1 on tape. Aim to have ~8 fb -1 by 2009.
30th March 2006E. Nurse, U. of Manchester Seminar9 Collider Detector at Fermilab = 1.0 = 2.8 = 2.0 ■ Silicon tracking detectors ■ Central drift chambers (COT) ■ Solenoid Coil ■ EM calorimeter ■ Hadronic calorimeter ■ Muon scintillator counters ■ Muon drift chambers ■ Steel shielding
30th March 2006E. Nurse, U. of Manchester Seminar10 W and Z production W e, , , q e, , , q ’ P P q’ q’ q The large masses (~100 GeV ) of W and Z bosons means their decay products will have large p T. The electron and muon channels are used to measure W properties, due to their clean experimental signature.
30th March 2006E. Nurse, U. of Manchester Seminar11 CDF W width analysis note: this analysis is work in progress! current analysis based on 350 pb -1
30th March 2006E. Nurse, U. of Manchester Seminar12 Analysis strategy Ideally, the mass and width of the W would be found from the invariant mass distribution of its decay products (line- shape). We cannot reconstruct the W invariant mass distribution since the neutrino escapes detection. Instead we reconstruct the transverse mass: M T = 2 p T p T (1 - cos∆ ) p T found by summing total “transverse” energy in detector ( calorimeter towers) + lepton to give the “missing E T ”. l l
30th March 2006E. Nurse, U. of Manchester Seminar13 Analysis strategy Width and mass found by fitting M T in data to that in Monte Carlo. The line-shape of the W is described by a Breit-Wigner distribution with a scale dependent width: x1x2sx1x2s
30th March 2006E. Nurse, U. of Manchester Seminar14 Analysis strategy Same principle for W mass, with fit region 50 - 100 GeV Simulate M T distributions using Monte Carlo event generator with various “blinded” input width values. Normalise to data in the low M T region. Fit to data in the high M T region (exactly where is a trade off between systematics and statistics). A fast simulation is required (rather than full detector simulation). Z events used to tune the detector response to particles.
30th March 2006E. Nurse, U. of Manchester Seminar15 The data Select 4 data samples: Z and Z ee (control samples). W and W e Muons identified in COT, calorimeters (MIP signature) and muon detectors. Electrons identified in COT and EM calorimeter.
30th March 2006E. Nurse, U. of Manchester Seminar16 2 electrons, E T > 25 GeV 2 muons, p T > 25 GeV 1 electron, E T > 25 GeV E T miss > 25 GeV 1 muon, p T > 25 GeV E T miss > 25 GeV | e | < 1| | < 1
30th March 2006E. Nurse, U. of Manchester Seminar17 The simulation Toygen event generator produces Z , W , Z ee and W e samples. Purely electroweak (no gluon radiation, i.e. p T (boson) = 0 GeV). p T (Z) is added from a functional form (fit to p T (Z) data). A theoretical calculation converts p T (Z) p T (W). QED radiation - Toygen is interfaced with Berends and Kleiss program: one photon FSR correction. UA2 program
30th March 2006E. Nurse, U. of Manchester Seminar18 Simulation of detector: –Simple tracking: helix extrapolation through detector with calorimeter and muon geometry simulated. –The ionisation energy loss of muon tracks in central trackers taken from the Bethe-Bloch equation. –Electron bremsstrahlung and photon conversion within central trackers simulated. –Clustering of electrons/photons in calorimeter simulated. nIter = 0 12 3 … etc. Ze+e-Ze+e- initial
30th March 2006E. Nurse, U. of Manchester Seminar19 –Lepton id and trigger efficiencies input from data. –COT momentum scale and resolution found by fitting to M in Z events. –Calorimeter energy scale and resolution found by fitting to M ee in Z ee events. –Calorimeter energy scale and resolution also found from E/P in W e events. –Recoil ( non-lepton calorimeter towers) distribution modeled by tuning to Z and Z ee data. E: energy of electron measured in calorimeter. P: momentum of electron measured in central tracker.
30th March 2006E. Nurse, U. of Manchester Seminar20 Sources of systematic uncertainty (anything affecting the M T distribution) PDFs and W p T distribution QED corrections Lepton energy scales and resolutions E T miss distribution (recoil model) Backgrounds in W samples
30th March 2006E. Nurse, U. of Manchester Seminar21 Parton Distribution Functions PDFs are used as inputs to the MC to give the momentum distributions of partons within the incoming protons. The PDFs have an uncertainty associated with them due to the uncertainty in the many datasets used to fit them. This uncertainty gives an uncertainty on the M T distribution and hence the width determination. Use the CTEQ6 and MRST PDF error sets to obtain the uncertainty on the W width: –CTEQ6: +25 MeV, -25 MeV –MRST: +14 MeV, -10 MeV
30th March 2006E. Nurse, U. of Manchester Seminar22 Z and W p T The Z p T is parameterised by an ad-hoc functional form: The parameters are found by fitting to the Z data. A theoretical calculation converts this to a W p T distribution. The bosons (generated with 0 p T ) obtain a p T from this functional form. P1 = 0.564716 ± 0.0235054 P2 = 5.04421 ± 0.247411 P3 = 16.1793 ± 1.00640 P4 = 0.921681 ± 0.0616547
30th March 2006E. Nurse, U. of Manchester Seminar23 Lepton momentum/energy scales and resolutions Since we do not model the complete CDF detector and event reconstruction we must smear the “true” lepton momentum (as measured in the COT) and energy (as measured in calorimeter). The smearing parameters are found by tuning to the data.
30th March 2006E. Nurse, U. of Manchester Seminar24 Get M distribution from Z-> events in data in region: 82 < M < 100 GeV. Simulate 100 million Z-> events. Make many M templates with different values for momentum resolution constant. Plot 2 vs resolution. Fit to a polynomial and minimise! 2 - 2 min 1 resolution Tuning method (e.g. track resolution)
30th March 2006E. Nurse, U. of Manchester Seminar25 Track momentum scale: p T meas = 0.9986 p T resolution: (1/p T ) = 0.00055 0.006 / p T curvature term multiple scattering term
30th March 2006E. Nurse, U. of Manchester Seminar26 Calorimeter energy scale: E meas = 1.0030 E resolution: (E) / E = 13.5% / E T 2.2% stochastic term constant term scale = 1.0030 0.0006 kappa = 2.22 0.14 % 2 = 18.5 / 18
30th March 2006E. Nurse, U. of Manchester Seminar27 Calorimeter energy: E/P Momentum loss in trackers through bremsstrahlung Energy leakage from electron calorimeter tower (not yet simulated) Calorimeter energy and track momentum resolutions scales consistent; resolutions have ~4 different! scale = 1.0030 0.0006 kappa = (2.22 0.14) % e energy in calorimeter e mom in COT
30th March 2006E. Nurse, U. of Manchester Seminar28 Recoil (U) comes from multiple interactions, underlying event and initial state gluon radiation. Defined as the sum of all non- lepton calorimeter towers. Measured and modeled in Z events and applied to W events. Recoil Model
30th March 2006E. Nurse, U. of Manchester Seminar29 U in Z events resolved into 2 directions: –U 1 : anti-parallel to the Z p T –U 2 : perpendicular to Z p T U 1 and U 2 are gaussian in bins of Z p T. = a +bp T +cp T 2 = 0 (U 1 ) = d + ep T + f (E T ) g (U 2 ) =h (E T ) i Z ee events : The recoil is then added in this way to the simulation for both Z and W events. Note: this is ongoing work, the simulation currently uses a Run I model.
30th March 2006E. Nurse, U. of Manchester Seminar30 Efficiencies …measured in the data and applied to MC as a function of
30th March 2006E. Nurse, U. of Manchester Seminar31 Z ll Potential backgrounds to W events One lepton lost E T miss due to missing lepton decays to e/ intrinsic E T miss We need to know M T distribution of backgrounds as well as overall normalisation!! Residual background: run full event selections on signal and background MC samples passed through simulation of CDF detector to get fractional background. muon channel: Z : 4.7% W : 1.9% Background suppression: veto on additional high p T, isolated track electroweak W
30th March 2006E. Nurse, U. of Manchester Seminar32 Jet contains /fakes a lepton E T miss from misconstruction Residual background: Found from difference in isolation distributions in W and Z events. muon channel: 0.5%, electron channel: 1.4% Background suppression: total recoil in event < 20 GeV Background suppression: Cosmic tagger takes a seed track and searches for additional track hits on the other side of the interaction point............. Residual background: muon channel: negligible electron channel: N/A di-jet cosmic muons
30th March 2006E. Nurse, U. of Manchester Seminar33 Residual background: Found by fitting the d0 distribution in W events to that in Z + W + kaon (with fraction of kaon varying). muon channel: 1.00 0.19 % electron channel: N/A Kaon decays to a pair when passing through COT. kink in track can give a fake high p T and E T miss. Background suppression: Cut on track 2 and d0 (impact parameter). kaon decay Tracks with no silicon hits
30th March 2006E. Nurse, U. of Manchester Seminar34 Muon channel backgrounds: 1% kaon background is probably too much for us! Muon channel transverse mass distributions:
30th March 2006E. Nurse, U. of Manchester Seminar35 Width measurement: summary Fast simulation modeling the detector reasonably well. Work still required to ensure a good understanding of some effects: –electron energy resolution –recoil model Need to determine systematic uncertainties for all the effects discussed. When this is done fits for the width will be “un-blinded”.
30th March 2006E. Nurse, U. of Manchester Seminar36 Width measurement: estimated uncertainties Run I analysis achieved 130 MeV with 110 pb -1 (one third of our luminosity). A naïve estimate of most systematics can be obtained by scaling Run I numbers by increased statistics (since they are controlled by Z statistics). Some systematics are not determined by Z statistics (e.g. QED corrections). Estimate ~80 MeV with this dataset with dominant errors from statistics, recoil and backgrounds. In reality may be higher…
30th March 2006E. Nurse, U. of Manchester Seminar37 W mass status Current analysis uses 200 pb -1. Very similar to the width analysis, but with 50 - 100 GeV fit region. Some effects are more important and must be better understood (e.g. track momentum scale determined using J/ and decays to muons, with Z events as a cross check). Therefore their simulation is more sophisticated than ours. Use Resbos Monte Carlo event generator interfaced with Wgrad for QED corrections.
30th March 2006E. Nurse, U. of Manchester Seminar38 W mass status current work Total uncertainty 76 MeV (Run 1b: 79 MeV) Fits blinded with additive offsets 110 pb -1
30th March 2006E. Nurse, U. of Manchester Seminar39 Projections W width to a similar level of precision
30th March 2006E. Nurse, U. of Manchester Seminar40 Outlook Hope to have these first round measurements out by the summer(?) After that the plan is join forces on the “W mass 1 fb -1 challenge”. Watch this space….
30th March 2006E. Nurse, U. of Manchester Seminar41 Back-up slides
30th March 2006E. Nurse, U. of Manchester Seminar42