Searches for in the dilepton (and diphoton ) final states with the ATLAS detector

1. New Physics Searches for in the dilepton(and diphoton) final states with the ATLAS detector Dr Tracey Berry Royal Holloway University of London And Prospects for the LHC Extra Dimensions

2. Overview • Introduction: • Beyond the Standard Model • Fundamental Symmetries: Heavy Gauge Bosons Z’ (ee, mm) • Extra Dimensions (ee, mm + gg) • Contact Interactions (ee, mm) • ATLAS & LHC • Searches for new physics • Summary and Outlook

3. Standard Model The Standard Model The SM : particles + forces Gravity is not included! Motivation for searching for something beyond the SM….

4. Forces in Nature Gravity is very weak! → Hierarchy Problem MEW (103 GeV) << MPlanck (1019 GeV)? Table from Cigdem Issever, Oxford

5. Extra Dimensions: Motivations In the late 90’s Large Extra Dimensions (LED) were proposed as a solution to the hierarchy problem MEW (1 TeV) << MPlanck (1019 GeV)? Randall, Sundrum,Phys Rev Lett 83 (99) Arkani-Hamed, Dimopoulos, Dvali,Phys Lett B429 (98) RS ADD 1 highly curved ED Gravity localised in the ED Many (d) large compactified EDs In which G can propagate Planck TeV brane MPl2 ~ RdMPl(4+d)(2+d) = Mple-kRc ~ TeV  ifcompact space (Rd) is large Effective MPl ~ 1TeV if warp factorkRc ~11-12 Since then, new Extra Dimensional models have been developed and been used to solved other problems: Dark Matter, Dark Energy, SUSY Breaking, etc Some of these models can be/have been experimentally tested at high energy colliders

6. Extra Dimensions? If ED exist, why haven’t we observed them? The “extra” dimensions could be hidden to us: • E.g. To a tightrope walker, the tightrope is one-dimensional: he/she can only move forward or backward • But an ant can go around the tightrope as well … • The “extra” dimensions may be too small to be detectable at energies less than ~ 1019 GeV (E.g. they are small that only extremely energetic particles could fit into them (so we need high energies to probe them)) • But to an ant, the rope has an extra dimension: the ant can travel around the rope as well

7. G Extra Dimensions? • Or only some kinds of matter are able to move in the extra dimensions, and we are confined to our world. Extra Dimensions like something that was forced to reside on the surface of a tabletop, being unaware of any such thing as up or down. Our Brane/ World • The “extra” dimensions may be too small to be detectable at energies less than ~ 1019 GeV

8. KK towers/particles When particles go into the extra dimensions…. Like QM particles in a box Mn = √(M02+n2/R2)) • Spacing & (summation of ) KK towers determines the search signature: • narrow resonance (RS) or • broad increase in cross-section (ADD)

9. Questions remaining with the Standard Model  Reasons to search for new physics… • Fundamental symmetries: • Are there more symmetries beyond SU(3)C SU(2)L U(1)Y? GUTs with larger symmetry group? Left-right symmetry? • How can we address the hierarchy problem? / Include Gravity into the SM? • Are there warped extra dimensions? (RS model) • Are there large extra dimensions? (ADD model) • Quark and lepton generations: • Why are there 3 generations?  Fermions composite? • Is there a lepto(n)-quark symmetry? • More than 3 generations of quarks & leptons?

10. Introduction • Standard Model describes data very well, but is only a low energy effective theory New Physics is needed at the weak scale ~ 1 TeV • Physics Processes at LHC: • Total cross-section • SM Processes: W->ev • New Physics: hard lepton/photon (Z’->ee, etc)

11. Questions remaining with the Standard Model • Search for evidence of beyond the SM physics • in dilepton+ diphoton data...… • Fundamental symmetries: New Gauge Bosons? • Extra Dimensions? • Contact Interactions ? & Beyond! empireonline.com thegreatfiles.blogspot.com schooljotter.com

12. Signature for BSM in ATLAS • Resonance • Broad Increase in Cross-section G*: RS G*: ADD CI

13. Introduction • Standard Model describes data very well, but is only a low energy effective theory • New Physics is needed at the weak scale ~ 1 TeV • Physics Processes at LHC: • Total cross-section • SM Processes: W->ev • New Physics: hard lepton/photon (Z’->ee, etc) • Need large luminosity and efficient background rejection to see interesting signal events: & effective lepton/photon identification

14. LHC data 2010 data: PLB700: 163-180, 2011 (~40 pb-1) 2011 data 200 pb-1 update: ATL-CONF-2011-083 2011 data: ~1 fb-1 : 2010 data: collected in one day in 2011 running (Dan’s chapter)

15. ATLAS Detector

16. AToroidal LHC AppartuS (ATLAS) DETECTOR Precision Muon Spectrometer, s/pT 10% at 1 TeV/c Fast response for trigger Good p resolution (e.g., Z’  ) EM Calorimeters, /E  10%/E(GeV)  0.7% excellent electron/photon identification Good E resolution (e.g., Ggg) Full coverage for ||<2.5 Hadron Calorimeters, /E  50% / E(GeV)  3% Good jet and ET miss performance Inner Detector: Si Pixel and strips (SCT) & Transition radiation tracker (TRT) s/pT 5 10-4pT 0.001 Good impact parameter res. (d0)=15m@20GeV Magnets: solenoid (Inner Detector) 2T, air-core toroids (Muon Spectrometer) ~0.5T

17. ATLAS Detector

18. DileptonBSM Searches Resonance Searches • Z’ • RS model Extra Dimensions G Non-Resonance Searches • Contact Interactions • ADD model Extra Dimensions (to come...)

19. Signal and Backgrounds Select events with two leptons of same flavor (ee, ) Search for excess above Standard Model expectations in high invariant mass region ● Main backgrounds: — SM Z (irreducible, primary background) — QCD (electron channel only) — Top quark pair production — SM W+jets (electron channel only) — Dibosons (WW, WZ, ZZ) ● Negligible backgrounds for muon channel: — QCD, SM W+jets — Cosmic rays

20. Selection Muon channel ● Trigger requiring single Muon with pT> 22 GeV ● Primary vertex with |z| < 200 mm ● 2 muons with: — pT> 25 GeV — || < 2.4 — Hits in all 3 muon stations — Hit in nonbendingplane — Veto on overlapping hits in barrel and endcaps — |d0| < 0.2 mm, |z0| < 1 mm — Isolation: within a cone of R < 0.3 — Opposite charge Electron channel ● Trigger requiring single Medium electron with ET > 20 GeV ● 2 electrons with: — pT> 25 GeV — || < 2.47, exclude crack region 1.37 < || < 1.52 — Medium Electron ID — Hit in first pixel layer (“Blayer”) ΣpTtrk<0.05 pT

21. Muons Muon tracks are reconstructed independently in both the inner detector and muon spectrometer, and their momenta are determined from a combined fit to these two measurements. To optimize the momentum resolution, each muoncandidate is required to pass quality cuts in the inner detector and to have at least three hits in each of the inner, middle, and outer layers of the muon system. Muons with hits in both the barrel and the endcap regions are discarded because of residual misalignment between these two parts of the muon spectrometer. The effects of misalignments and intrinsic position resolution are included in the simulation. The pTresolution at 1 TeV ranges from 15% to 44% (for |η| > 2).

22. Electron QCD Background • Estimated in data using two techniques, extrapolated to high mass search region • Inverted identification method reverses Medium ID cuts, uses template fit in invariant mass • Isolation fit method requires standard Medium ID, reverses other ID cuts, uses template fit in calorimeter isolation

23. Z' Electron Kinematics Good agreement with background expectations The bin width is chosen to be constant in sqrt(E_T).

24. Calorimeter Isolation Calorimeter isolation spectra for the leading electron after event selection, but before the isolation cut is applied on the leading electron. The points represent ATLAS data and the filled histograms show the Monte Carlo stacked backgrounds. The isolation is the sum of the calorimeter energy in a cone delta(R) < 0.2, corrected for leakage and pileup effects. The QCD is estimated from data, using the reverse identification method, and is less isolated than the Drell-Yan.

25. Z' Muon Kinematics The small excess in the tails of the distributions may be explained by the imperfect modelling of highly energetic jets in PYTHIA, since the agreement is much better with ALPGEN which generates more high energy jets which can boost the dilepton system in the Z+jets events. Of the ten highest p_Tmuons, only four belong to a dimuon pair with mass greater than 300 GeV. Good agreement with background expectations

26. Pseudorapidity The dips in the pseudorapidity distribution are caused by the requirement that hits are measured in all three layers of the muon chambers. Most chambers in the middle station near |eta|=1.2 have not been installed yet, which causes the two gaps. The central dip is due to the passage of inner detectors' (tracker and calorimeter) services. |eta|=1.45 transition region between the barrel and endcap calorimeters

27. Rapidity

28. Resonant Searches • Z’ • RS model Extra Dimensions G

29. 2010 data: PLB700: 163-180, 2011 2011 data 200 pb-1 update: ATL-CONF-2011-083

30. Z’ Theoretical Motivations • New heavy gauge bosons are predicted in several extensions of the Standard Model • Benchmark model for these searches is the Sequential Standard Model (SSM) • Z’ has the same couplings as SM Z • Z’ width assumed comparable to detector resolution • Also consider string theory –inspired E6 models

31. Invariant Mass Distributions

32. Data & SM Background

33. Highest mass ee event ET257 GeV,(eta, phi) (-0.76, 1.14). ET207 GeV(eta, phi) of (2.05, -2.05). Mee= 993 GeV. Only tracks with p_T > 1 GeV are shown.

34. Highest Mass mm event PTof 510 GeV, (\eta, phi) =(0.37, 3.01). PTof 437 GeV, (eta, phi) =(0.72, -0.12). Mmm=959 GeV. . Only tracks with P_T > 0.5 GeV are shown

35. Signal? The significance of a signal is summarized by a p-value, the probability of observing an excess at least as signal like as the one observed in data, in the absence of signal. The outcome of the search is ranked using a likelihood ratio, which is scanned as a function of Z′ cross section and mZ′ over the full considered mass range. The data are consistent with the SM hypothesis, with p-values of 54% for e+e− and 24% for the μ+μ− channels.

36. Limits Setting and Errors • Normalize MC to data in Z peak region (70 < mℓℓ < 110 GeV) • Luminosity and other mass independent systematicscancel • Uncertainties treated as correlated across all bins

37. Z' Individual Channel Limits Limits set using template shape fit — Bayesian method Observed (Expected) 95 % C.L. mass lower limit in TeV on Z’SSM resonance an upper limit on the signal cross section is determined at the 95% confidence level (C.L.) using a Bayesian approach [41] with a flat prior on the signal cross section.

38. Mass Limits Z' (GeV/c2) Z’ Combined Limits Observed (Expected) 95 % C.L. mass lower limit in TeV on Z’SSM resonance arXiv:1108.1582v1 (hep-ex) Update table

39. Comparison of Z’SSM Limits The Tevatron experiments exclude a Z′ SSM with a mass lower than 1.071 TeV. Recent measurements from the LHC experiments, based on 40 pb−1 of data recorded in 2010, exclude a Z′ SSM with a mass lower than 1.042 TeV (ATLAS) and 1.140 TeV (CMS) Indirect constraints from LEP extend these limits to 1.787 TeV.

40. Other Z’ models • In the Z′ are the Sequential Standard Model (SSM) the Z’ has the same couplings to fermions as the Z boson • Also when the E6 grand unified symmetry group is broken into SU(5) and two additional U(1) groups leads to new neutral gauge fields ψ and χ. • The particles associated with the additional fields can mix in a linear combination to form the Z′ candidate: Z′(θE6 ) = Z′ψcosθE6 + Z′χsin θE6 , where θE6 is the mixing angle between the two gauge bosons. • The pattern of spontaneous symmetry breaking and the value of θE6 determine the Z′ couplings to fermions; • six well motivated choices of θE6lead to the specific Z′states named:

41. Mass Limits Z' (GeV/c2) Combined Limits Update table

42. Z’ limits vs Other Experiments

43. G*

44. Good agreement with background expectations Z’ compared to G* Muon/Electron Kinematics

45. G* limits

46. G* limits Observed 95 % C.L. mass lower limit in TeV on RS Gravitons

47. G* limits • Mass Limits RS Graviton (GeV/c2)

48. Combination of ee +mm with updated ggresult Coming Soon! Under Approval

49. Update Coming Soon! Tetiana Berger-Hryn’ova, on behalf of the ATLAS collaboration EPS, Grenoble, France, 21 July 2011

50. Non-Resonant Searches • Contact Interactions • ADD model Extra Dimensions