1. Seminar, QMULTracey Berry (RHUL) 1 1 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

2. Seminar, QMULTracey Berry (RHUL) 2 Overview Introduction: Beyond the Standard Model Fundamental Symmetries: Heavy Gauge Bosons Z’ (ee,  ) Extra Dimensions (ee,  ) Contact Interactions (ee,  ) ATLAS & LHC Searches for new physics Summary and Outlook

3. Seminar, QMULTracey Berry (RHUL) 3 3 The Standard Model Motivation for searching for something beyond the SM…. Gravity is not included! The SM : particles + forces

4. Seminar, QMULTracey Berry (RHUL) 4 4 Forces in Nature M EW (10 3 GeV) << M Planck (10 19 GeV)? Gravity is very weak! → Hierarchy Problem Table from Cigdem Issever, Oxford

5. Seminar, QMULTracey Berry (RHUL) 5 5 Extra Dimensions: Motivations  if compact space (R  ) is large M Pl 2 ~ R  M Pl(4+  ) (2+  ) Effective M Pl ~ 1TeV if warp factor kR c ~11-12 Planck TeV brane Arkani-Hamed, Dimopoulos, Dvali, Phys Lett B429 (98) Randall, Sundrum, Phys Rev Lett 83 (99) M EW (1 TeV) << M Planck (10 19 GeV)? Many (  ) large compactified EDs In which G can propagate 1 highly curved ED Gravity localised in the ED In the late 90’s Large Extra Dimensions (LED) were proposed as a solution to the hierarchy problem Some of these models can be/have been experimentally tested at high energy colliders Since then, new Extra Dimensional models have been developed and been used to solved other problems: Dark Matter, Dark Energy, SUSY Breaking, etc   = M pl e -kR c    ~ TeV

6. Seminar, QMULTracey Berry (RHUL) 6 6 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 If ED exist, why haven’t we observed them? But an ant can go around the tightrope as well … The “extra” dimensions may be too small to be detectable at energies less than ~ 10 19 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. Seminar, QMULTracey Berry (RHUL) 7 7 Or only some kinds of matter are able to move in the extra dimensions, and we are confined to our world. The “extra” dimensions may be too small to be detectable at energies less than ~ 1019 GeV like something that was forced to reside on the surface of a tabletop, being unaware of any such thing as up or down. G Our Brane/ World Extra Dimensions

8. Seminar, QMULTracey Berry (RHUL) 8 8 KK towers/particles When particles go into the extra dimensions…. M n = √(M 0 2 +n 2 /R 2 )) Like QM particles in a box Spacing & (summation of ) KK towers determines the search signature: narrow resonance (RS) or broad increase in cross-section (ADD)

9. Seminar, QMULTracey Berry (RHUL) 9 Questions remaining with the Standard Model  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?  Reasons to search for new physics…

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

11. Seminar, QMULTracey Berry (RHUL) 11 Questions remaining with the Standard Model  Fundamental symmetries: New Gauge Bosons?  Extra Dimensions?  Contact Interactions ?  Search for evidence of beyond the SM physics in dilepton + diphoton data...… schooljotter.comthegreatfiles.blogspot.com empireonline.com

12. Seminar, QMULTracey Berry (RHUL) 12 Signature for BSM in ATLAS  Resonance  Broad Increase in Cross-section G*: ADD CI G*: RS

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

15. Seminar, QMULTracey Berry (RHUL) 15 ATLAS Detector

16. Seminar, QMULTracey Berry (RHUL) 16 A Toroidal LHC AppartuS (ATLAS) DETECTOR Precision Muon Spectrometer,  /p T  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  ) Hadron Calorimeters,  /E  50% /  E(GeV)  3% Good jet and E T miss performance Inner Detector: Si Pixel and strips (SCT) & Transition radiation tracker (TRT)  /p T  5  10 -4 p T  0.001 Good impact parameter res.  (d 0 )=15  m@20GeV Magnets: solenoid (Inner Detector) 2T, air-core toroids (Muon Spectrometer) ~0.5T Full coverage for |  |<2.5

17. Seminar, QMULTracey Berry (RHUL) 17 ATLAS Detector

18. Seminar, QMULTracey Berry (RHUL) 18 Dilepton BSM Searches Resonance Searches  Z’  RS model Extra Dimensions G Non-Resonance Searches  Contact Interactions  ADD model Extra Dimensions (to come...)

19. Seminar, QMULTracey Berry (RHUL) 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. Seminar, QMULTracey Berry (RHUL) 20 Selection 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”) Muon channel ● Trigger requiring single Muon with p T > 22 GeV ● Primary vertex with |z| < 200 mm ● 2 muons with: — p T > 25 GeV — |  | < 2.4 — Hits in all 3 muon stations — Hit in nonbending plane — Veto on overlapping hits in barrel and endcaps — |d 0 | < 0.2 mm, |z 0 | < 1 mm — Isolation: within a cone of  R < 0.3 — Opposite charge Σp T trk <0.05 p T

21. Seminar, QMULTracey Berry (RHUL) 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 muon candidate 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 pT resolution at 1 TeV ranges from 15% to 44% (for |η| > 2).

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

24. Seminar, QMULTracey Berry (RHUL) 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. Seminar, QMULTracey Berry (RHUL) 25 Z' Muon Kinematics Good agreement with background expectations 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_T muons, only four belong to a dimuon pair with mass greater than 300 GeV.

26. Seminar, QMULTracey Berry (RHUL) 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. Seminar, QMULTracey Berry (RHUL) 27 Rapidity

28. Seminar, QMULTracey Berry (RHUL) 28 Resonant Searches  Z’  RS model Extra Dimensions G

29. Seminar, QMULTracey Berry (RHUL) 29 2010 data: PLB700: 163-180, 2011 2011 data 200 pb-1 update: ATL-CONF-2011-083

30. Seminar, QMULTracey Berry (RHUL) 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. Seminar, QMULTracey Berry (RHUL) 31 Invariant Mass Distributions

32. Seminar, QMULTracey Berry (RHUL) 32 Data & SM Background

33. Seminar, QMULTracey Berry (RHUL) 33 Highest mass ee event E T 257 GeV,(eta, phi) (-0.76, 1.14). E T 207 GeV (eta, phi) of (2.05, -2.05). M ee = 993 GeV. Only tracks with p_T > 1 GeV are shown.

34. Seminar, QMULTracey Berry (RHUL) 34 Highest Mass  event P T of 510 GeV, (\eta, phi) = (0.37, 3.01). P T of 437 GeV, (eta, phi) =(0.72, -0.12). M mm =959 GeV.. Only tracks with P_T > 0.5 GeV are shown

35. Seminar, QMULTracey Berry (RHUL) 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. Seminar, QMULTracey Berry (RHUL) 36 Limits Setting and Errors  Normalize MC to data in Z peak region (70 < mℓℓ < 110 GeV)  Luminosity and other mass independent systematics cancel  Uncertainties treated as correlated across all bins

37. Seminar, QMULTracey Berry (RHUL) 37 Z' Individual Channel Limits 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. Limits set using template shape fit — Bayesian method Observed (Expected) 95 % C.L. mass lower limit in TeV on Z’ SSM resonance

38. Seminar, QMULTracey Berry (RHUL) 38 Z’ Combined Limits Update table Mass Limits Z' (GeV/c 2 ) Observed (Expected) 95 % C.L. mass lower limit in TeV on Z’ SSM resonance arXiv:1108.1582v1 (hep-ex)

39. Seminar, QMULTracey Berry (RHUL) 39 Comparison of Z’ SSM Limits Z' ModelZ' SM Z' &eta Z' &chi Z' &psi Z' &iota Z' sec Z' N e + e - (39 pb -1 ) 957633829604779807626 m + m - (40 pb -1 ) 834645702720736662659 e + e - + m + m - (~40 pb -1 ) 1048771900738842871763 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. Seminar, QMULTracey Berry (RHUL) 40 Other Z’ models  I n 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′(θ E6 ) = Z′ ψ cos θ E6 + Z′ χ sin θ E6, where θ E6 is the mixing angle between the two gauge bosons.  The pattern of spontaneous symmetry breaking and the value of θ E6 determine the Z′ couplings to fermions;  six well motivated choices of θ E6 lead to the specific Z′states named:

41. Seminar, QMULTracey Berry (RHUL) 41 Combined Limits Update table Mass Limits Z' (GeV/c 2 )

42. Seminar, QMULTracey Berry (RHUL) 42 Z’ limits vs Other Experiments

43. Seminar, QMULTracey Berry (RHUL) 43

44. Seminar, QMULTracey Berry (RHUL) 44 Z’ compared to G* Muon/Electron Kinematics Good agreement with background expectations

45. Seminar, QMULTracey Berry (RHUL) 45 G* limits

46. Seminar, QMULTracey Berry (RHUL) 46 G* limits Observed 95 % C.L. mass lower limit in TeV on RS Gravitons

47. Seminar, QMULTracey Berry (RHUL) 47 G* limits k/M Pl 0.0100.0200.0250.0300.0500.0700.100 e+e-e+e- 385-595740701745993 m+m-m+m- 333-4937807468241000 gg230-500-694782850 e + e - + m + m - 425--850729-1080 e + e - + gg 560750580-9358201050 ATLASCMSCDFD0 Mass Limits RS Graviton (GeV/c 2 )

48. Seminar, QMULTracey Berry (RHUL) 48 Combination of ee +  with updated  result

49. Seminar, QMULTracey Berry (RHUL) 49 Tetiana Berger-Hryn’ova, on behalf of the ATLAS collaboration EPS, Grenoble, France, 21 July 2011

50. Seminar, QMULTracey Berry (RHUL) 50 Non-Resonant Searches  Contact Interactions  ADD model Extra Dimensions

51. Seminar, QMULTracey Berry (RHUL) 51

52. Seminar, QMULTracey Berry (RHUL) 52 Contact Interactions  Four-fermion contact interactions (CI) at low energy limit describe phenomena as:  Large Extra Dimension ADD model  Quark-lepton compositeness  Benchmark composite model is left-left isoscalar model

53. Seminar, QMULTracey Berry (RHUL) 53 Contact Interactions (  ) Taking the integral No excess, limits at 95 % CL:  - > 4.9 TeV for constructive interference  + > 4.5 TeV for destructive interference arxiv:1104.4398, PRD

54. Seminar, QMULTracey Berry (RHUL) 54 Contact Interaction Search Combination of ee +  with ~ 1fb -1 for mid Nov HCP conference

55. Seminar, QMULTracey Berry (RHUL) 55 Conclusion  LHC is working very well  ATLAS detector is efficiently collecting data  We are searching for New Physics  many limits are reaching ~1TeV range  distributions so far consistent with SM expectations  previous limits substantially improved  We already have more than 5fb -1 of data, more exciting results from ATLAS to come… using full 2011 dataset

56. Seminar, QMULTracey Berry (RHUL) 56

57. Seminar, QMULTracey Berry (RHUL) 57 Thank you for inviting me!

58. Seminar, QMULTracey Berry (RHUL) 58 Sensitivity: 10 Signal Events  D. Olivito’s PLHC 2011 talk, June 10 2011

59. Seminar, QMULTracey Berry (RHUL) 59 Sensitivity: Limit Setting