A Precision Measurement of the Mass of the Top Quark Abazov, V. M. et al. (D0 Collaboration). Nature 429, (2004) Presented by: Helen Coyle

Introduction: Top Quark, t Discovered in 1995 at Fermilab Predicted by the Standard Model Its mass is close to that of a Gold nucleus but its size is smaller than a proton

Introduction: Some Elementary Particles of the Standard Model

Introduction:

Introduction: Top Quark Is produced during high energy proton and antiproton head on collisions in particle accelerators Jets: products of the fragmentation of a quark into a group of particles emitted in the original direction of the quark during quark production by proton- antiproton collisions

Introduction: Fermilab Tevatron Particle Accelerator 4 mile circumference World’s second largest particle accelerator Shut down in

Introduction:

D0 (DZero) Detector DZero Detector has 3 layers: Inner Layer: Silicon detectors to detect map the flight paths Argon filled calorimeter to measure energy Outer Layer: Muon detectors Silicon detector Introduction:

Luminosity: number of collisions/area/sec Integrated luminosity: collisions/area Unit of Integrated luminosity: pb -1 1b (barn) = cm 2 1pb = barn Introduction:

Significance of a Precise Measurement of the Top Quark Mass The top quark is thought to acquire it mass from interactions with the Higgs field (a field made up of Higgs bosons that occupies space) Knowing its mass narrows the range for the search of the Higgs boson Introduction:

Review of Literature Observation of the Top Quark: Abe, F. et al. (CDF Collaboration). Observation of top quark production in p¯p collisions with the Collider Detector at Fermilab. Phys. Rev. Lett. 74, 2626–2631 (1995). Abachi, S. et al. (DØ Collaboration). Observation of the top quark. Phys. Rev. Lett. 74, 2632–2637 (1995).

Review of Literature Previous work on Mass of Top Quark: Abbot, B. et al. (D0 Collaboration). Direct measurement of the top quark mass by the D0 collaboration. Phys. Rev. D 58, (1998) Affolder, T. et al. (CDF Collaboration). Measurement of the top quark mass with the Collider Detector at Fermilab. Phys. Rev. D 63, (2001), pp. 1–43 Abbott, B et al. (DØ Collaboration). Measurement of the top quark mass in the dilepton channel. Phys. Rev. D 60, (1999), pp. 1–21. The CDF Collaboration, the DØ Collaboration, and the TEVATRON Electro-Weak Working Group. Combination of CDF and DØ Results on the Top-Quark Mass. Preprint hep-ex/ , 1–7, (2004).

Review of Literature Information about the D0 detector : Abachi, S. et al. (D0 Collaboration). The D0 detector. Nucl. Instrum. Methods Phys. Res. A 338, (1994)

Review of Literature Previous work on Higgs boson Hagiwara, K. et al.. Review of particle physics. Phys. Rev. D 66, (2002), pp. 271–433. Barate, R. et al. (ALEPH Collaboration, DELPHI Collaboration, L3 Collaboration, OPAL Collaboration, and LEP Working Group for Higgs boson searches). Search for the standard model Higgs boson at LEP. Phys. Lett. B565, 61–75 (2003).

Hypothesis/Problem Statement The mass of the top quark can be measured with an improved precision by a new method developed during a D0 experiment at Fermilab.

Methods D0 Detector at Fermilab Tevatron Total Energy 1.8 TeV Head on collisions of 900GeV proton and 900 GeV antiproton Integrated Luminosity: 125 events/pb Cross Section: 5.7pb Produced 700 top and antitop quark pairs

Methods A top and antitop quark is produced by head on collisions of protons and antiprotons Each top quark decays to a bottom quark and a W + boson almost immediately. Each antitop quark decays to a antibottom quark and a W - boson almost immediately.

Methods Difference with previous methods is the data analysis in the lepton +jets decay: 1.Assignment of more weight to well measured or more likely to occur events 2.Only events with four jets are kept in the analysis Fewer events are used in the analysis (Narrowed down to 71 events and then following differential probability calculations and Monte Carlo methods to 22 events)

Results Previously reported result: M t = ± 5.6 (stat) ± 5.5 (syst) GeV/c 2 New Result M t = ± 3.6 (stat) ± 3.9 (syst) GeV/c 2 Both the statistical and the systematic error was reduced

Results D0 run new average top quark mass: GeV/c GeV/c 2 Average including previous D0 runs: GeV/c GeV/c 2 Combining with previous CDF run gives the new world average top quark mass: GeV/c 2 The new Higgs mass research target is now shifted from the previous 96 GeV/c 2 to the new 117 GeV/c 2 with a maximum of 219 GeV/c 2 to 251 GeV/c 2

Discussion Figure 1 Feynman Diagrams for top quark and antitop quark production in proton antiproton collisions. (a) is dominant, but gluon fusion (b) contributes 10% to the cross-section. This particular final state (electron, antineutrino, antibottom, bottom, up, antidown) is one of the channels used in the analysis.

Figure 2 Relative importance of top and antitop decay modes. The 'lepton + jets' channel used in this analysis corresponds to the two offset slices of the pie-chart and amounts to 30% of all the t decays. Discussion

FIGURE 4. Determination of the mass of the top quark using the maximum-likelihood method. The points represent the likelihood of the fit used to extract M t divided by its maximum value, as a function of M t (after a correction for a -0.5 GeV/c 2 mass bias, see text). The solid line shows a gaussian fit to the points. The maximum likelihood corresponds to a mass of GeV/c 2, which is the new DØ measurement of M t in the lepton + jets channel. The shaded band corresponds to the range of 1 standard deviation, and indicates the 3.6 GeV/c 2 statistical uncertainty of the fit. Discussion

Figure 3. Current experimental constraints on the mass of the Higgs boson. The  2 for a global fit to electroweak data is shown as a function of the Higgs mass. -Solid line: previous world-averaged M t = GeV/c 2 -Blue band: impact of theoretical uncertainty. -Dotted line: the new world-averaged M t of GeV/c 2 -Dashed line: using only the new DØ average of GeV/c 2. -Yellow-shaded area: the region of Higgs masses excluded by experiment (>114.4 GeV/c 2 at the 95% confidence level 5 ). The improved M t measurement shifts the most likely value of the Higgs mass above the experimentally excluded range. Discussion

Conclusion The hypothesis was supported in that the new world average top quark mass was measured with an improved precision as: GeV/c 2 The previous measurement was GeV/c 2 The new Higgs mass research target is now shifted from the previous 96 GeV/c 2 to the new 117 GeV/c 2 with a maximum of 219 GeV/c 2 to 251 GeV/c 2