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Annual report

This annual report summarizes the various physics analyses conducted by the USTC ATLAS Group, including the search for heavy neutral particle decays, measurements of cross sections, and searches for resonances in various channels.

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Annual report

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  1. Annual report USTC ATLAS GROUP 22.Dec

  2. Physics analyses we are involved in • 1 Search for a heavy neutral particle decaying into an electron and a muon • 2 Measurement of WZ cross section • 3 Measurement of W+jet/Z+jet cross section ratio • 4 Search for Higgs-> γγ • 5 Search for graviton-> γγ • 6 Measurement of the W+W- Cross Section • 7 Measurement of W γand Z γcross section • 8 Search for resonant WZ production in the WZ->lvll channel • 9 Search for high mass dilepton resonance

  3. Search for a heavy neutral particle decaying into an electron and a muon • predicted by a number of extensions to SM:R-parity violating(RPV) SUSY,and Z’ with lepton flavor violating(LFV) interactions. • the e muon final state channel is chosen due to clean detector signature and low SM bkg in high mass region. • background: 1.Z/γ->tautau,ttbar,single top,dibosons 2 .W/Z+γ,W/Z+jet,and multijets

  4. Number of data and estimated background

  5. The observed and expected 95% C.L.limits on sigmma*BR

  6. Measurement of W+jet/Z+jet cross section ratio • an important test of SM description of the strong interaction in perturbative QCD • individual measurements of kinematics observables in W+jet and Z+jet are limited by systematic uncertainties commom to both . • to exploit the cancellation of theoretical and experiment uncertainties,building the foundation of a high precision test of SM.

  7. Measurement of W+jet/Z+jet cross section ratio

  8. Measurement of W+jet/Z+jet cross section ratio • the number of events after selection and bkg subtraction are corrected to the particle level, using trigger efficiency, lepton identification efficiency,and boson reconstruction and resolution

  9. Measurement of W+jet/Z+jet cross section ratio • Rjet in the electron and muon channel

  10. Measurement of W+jet/Z+jet cross section ratio • combined results in a common fiducial region and full phase space

  11. Search for Higgs-> γγ • below114.4GeV and above 141GeV has been excluded • background: 1.diphoton process 2.photon candidate from misidentified jet 3. misidentified electron (Drell-Yan)

  12. To optimize the sensitivety to a potential Higgs signal,introduce a new observable to split the events into categories.

  13. signal & background modelling

  14. the background is estimated from the data by fitting the diphoton mass spectrum in the whole range of 100-160GeV with a single exponential function.

  15. Results The maximum deviation from the background only expectation is observed at 126GeV with a local significance of 2.8 standard deviations. A SM higgs boson is excluded at 95% CL in the mass range of 114-115 and 135-136 GeV

  16. Search for graviton-> γγ • To address the hiercrchy problem(the enormous difference between the Planck scale and the electroweak scale) through the existence of extra spatial dimention. • the KK gravitions can be investigate due to their coupling to SM particle-antiparticle pairs. • Models: 1. ADD model postulate the existence of n flat additional spatial dimentions conpactified with radius R,in which only gravitons propagate. 2 .RS model posits the existence of a fifth dimension with warped geometry . channel : γγ clean experimental signature,excellent diphoton mass resolution,modest background,and a branch ratio that is twice of that to charged individual lepton pairs.

  17. Background : 1.irreducible SM γγproduction 2.reducible,misidentified photon: γ+jet,jet+jet the total bkg is normalized to the number of events in low mass control region in which signal has been excluded.

  18. Given the absence of evidence for a signal, set limits on ADD and RS model.

  19. Measurement of the W+W- Cross Section • The production rate and kinematic distribution of w+w- are sensitive to the triple gauge couplings of the w bosons. w+w- production is an important background to Standard Model higgs boson search • Search channel :w+wllnunu • Background: 1. W+jets with one jet misidentified as a lepton 2.Drell-Yan production 3.top production (ttbar and Wt) 4.other diboson process

  20. Measurement of the W+W- Cross Section

  21. Kinematic distribution of the observed events are consistent with the standard model expection.

  22. Measurement of the W+W- Cross Section conclusions 1.use a maxmimum likelihood fitting method to combine the three channels. . In good agreement with the Standard Model prediction of

  23. Measurement of W γand Z γcross section • Important test of SM: the Wγprocess is directly sensitive to the triple gauge boson couplings.

  24. Measurement of W γand Z γcross section • Background :1.mainly W(Z)+jets events where photons from the decay products of mesons produced by the jet fragmentation 2.EW+ttbar bkg:For Wγ: For Zγ

  25. Fiducial cross section & production cross section

  26. Results:

  27. Search for resonant WZ production in the WZ->lvll channel • Many extensions predicts: Extended Gauge Model(EGM), through W’ Low Scale Technicolor Model(LSTC), through techni-rho The transverse mass of the selected candidates is examined for the appearance of a new resonance. Background: 1.mainly SM WZ production, and ZZ, Zgamma, estimated using MC simulation 2. ll+jets estiamated by data-driven approach ,fake factor

  28. Search for resonant WZ production in the WZ->lvll channel • The number of observed candidates is consistent with the bkg only hypothesis and no statistical significance excess is observed, limits are set.

  29. 95% C.L. expected and observed excluded mass region

  30. Search for high mass dilepton resonance • Narrow high mass resonance decaying to e+e-,mu+mu- pairs. • Models:1. Z’: SSM,E6 2. RS graviton, 3. G* 4. Z* Background : mainly Drell-Yan process and ttbar ,dibosons,W+jets,QCD

  31. Expected and observed number of events in ee and mumu channel

  32. Dielectron and dimuon invariant mass distribution after final selection.

  33. Expected and observed 95% C.L. upper limit on sigma*B for Z’ and G*; and mass limit

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