BSM, Durham, Jan 2012Tracey Berry 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 Add UK contribut ion from

BSM, Durham, Jan 2012Tracey Berry 2 Overview Introduction: Motivation for Beyond the Standard Model Searches ATLAS & LHC Dilepton Search: Limits on Fundamental Symmetries: Heavy Gauge Bosons Z’ (ee,  ) Extra Dimensions RS (ee,  ) Contact Interactions (ee,  ) Diphoton Search Limits on Extra Dimensions RS (ee,  ) & ADD Summary and Outlook

BSM, Durham, Jan 2012Tracey Berry 3 3 The Standard Model Motivation for searching for something beyond the SM….  Standard Model describes data very well, but is only a low energy effective theory

BSM, Durham, Jan 2012Tracey Berry 4 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 (ED)? (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…

BSM, Durham, Jan 2012Tracey Berry 5 Questions remaining with the Standard Model  New Gauge Bosons?  Contact Interactions?  Extra Dimensions?  Search for evidence of beyond the SM physics in dilepton + diphoton data...… schooljotter.com thegreatfiles.blogspot.com empireonline.com G*  

BSM, Durham, Jan 2012Tracey Berry 6 Signatures for BSM  Resonance  Non-Resonance/Broad Increase in Cross-section G*: ADD ED CI G*: RS ED Resonance Searches  Z’  RS model ED Non-Resonance Searches  Contact Interactions  ADD model ED Z’

BSM, Durham, Jan 2012Tracey Berry 7 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  p T  Good impact parameter res.  (d 0 )=15  Magnets: solenoid (Inner Detector) 2T, air-core toroids (Muon Spectrometer) ~0.5T Full coverage for |  |<2.5

BSM, Durham, Jan 2012Tracey Berry 8 LHC data equivalent of 2010 dataset: collected in one day in 2011 running 2010 data: PLB700: , 2011 (~40 pb-1) 2011 data 200 pb-1 update: ATL-CONF data: ~1 fb-1 : 2010: 30 th March – 31 st October pb -1 data delivered 45pb -1 recorded Of pb −1 delivered: 40% in the last week over 60% in the last month (1 fb-1: 20 times more data than 33 pb-1 results) 2010: 45pb-1recorded 2011 (till 1/07): 1.23fb-1recorded Peak Lumiof 1.26x1033cm-2s-1 6 interactions per BC on average 2010: pb-1 data delivered to the experiments: 45pb-1recorded 30 th March – 31 st October Peak stable luminosity: 2.07×1032 cm−2s− (till 1/07): 1.23fb-1recorded Peak Lumiof 1.26x1033cm-2s-1 6 interactions per BC on average 2010: 30 th March – 31 st October pb -1 data delivered 45pb -1 recorded Peak stable luminosity: 2.07×10 32 cm −2 s −1. Of the pb−1 delivered to experiments in 2010, 40% was in the last week of data taking, with over 60% being delivered in the last month. Proton physics data taking in 2011 took place between the 12th of March and 22nd of November, delivering a total integrated luminosity of 5 fb−1 to the experiments th March - 22nd November

BSM, Durham, Jan 2012Tracey Berry 9 Dilepton Search

BSM, Durham, Jan 2012Tracey Berry 10 Analysis Procedure & Event Selection Electron channel Trigger on single Medium electron with E T > 20 GeV 2 electrons with: p T > 25 GeV |  | < 2.47, exclude crack region 1.37 < |  | < 1.52 Medium Electron ID Hit in first pixel layer (“Blayer”) No opposite charge requirement – to minimize impact of mis-ID Muon channel Trigger on 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 non-bending plane Veto 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 Select events with two leptons of same flavor (ee,  ) Search for excess above SM expectations in high invariant mass region

BSM, Durham, Jan 2012Tracey Berry 11 Highest mass ee event E T 257 GeV (  )=(-0.76, 1.14) E T 207 GeV (  )=(2.05, -2.05) Only tracks with p_T > 1 GeV are shown. M ee = 993 GeV

BSM, Durham, Jan 2012Tracey Berry 12 Highest Mass  event P T of 510 GeV (  ) = (0.37, 3.01) P T of 437 GeV (  ) =(0.72, -0.12). Only tracks with P T > 0.5 GeV are shown M mm =959 GeV

BSM, Durham, Jan 2012Tracey Berry 13 Main Backgrounds SM Z/  Drell-Yan (irreducible, primary background) Produced using Pythia with MRST2007 LO* Interference with heavy resonances is small and ignored NNLO K-factors generated using PHOZPR with MSTW2008 QCD (electron channel only) estimated using “reversed electron identification" and others Top quark pair production Produced using 3.41 Predicted to approximate-NNLO with 10% uncert. SM W+jets (electron channel only) Produced using Alpgen cross-section rescaled to inclusive NNLO calculation of FEWZ Dibosons (WW, WZ, ZZ) Produced using Herwig with MRST2007 LO* NLO cross-sections calculated using MCFM Cosmic Rays (negligible contribution to muon channel)

BSM, Durham, Jan 2012Tracey Berry 14 Dilepton Distributions Backgrounds are normalised to data in Z-peak region ( GeV)

BSM, Durham, Jan 2012Tracey Berry 15 Dilepton Kinematics Good agreement with background expectations The bin width is chosen to be constant in sqrt(E_T).

BSM, Durham, Jan 2012Tracey Berry 16 Dilepton Kinematics 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

BSM, Durham, Jan 2012Tracey Berry 17 New Physics? No evidence of New Physics... so we set limits! p-value = 0.54p-value = 0.24  Resonances: Z’ & RS model Extra Dimensions G  Non-Resonances: Contact Interactions

BSM, Durham, Jan 2012Tracey Berry 18 Limits Setting and Errors  Because normalize MC to data in Z peak region (70 < m ℓℓ < 110 GeV) luminosity and other mass independent systematics cancel between Z and Z’/G  Uncertainties treated as correlated across all bins

BSM, Durham, Jan 2012Tracey Berry 19 Z’  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 to fermions as SM Z  Z’ width assumed comparable to detector resolution  Also consider string theory –inspired E6 models 2010 data: PLB700: , data 200 pb-1 update: ATL-CONF

BSM, Durham, Jan 2012Tracey Berry 20 Z’ Limits Update table Observed (Expected) 95 % C.L. mass lower limit in TeV on Z’ SSM resonance arXiv: v1 (hep-ex) Limits set using template shape fit — Bayesian method Upper limit on signal cross-section set at 95% C.L. Bayesian technique using a template shape fit & a prior assumed to be flat in signal cross-section Mass-dependent syst. uncert. integrated out as nuisance parameters

BSM, Durham, Jan 2012Tracey Berry 21 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 ) m + m - (40 pb -1 ) e + e - + m + m - (~40 pb -1 ) Tevatron experiments exclude M (Z′ SSM) < TeV LHC experiments, using ~40 pb − data exclude M (Z′ SSM) < TeV (ATLAS) & TeV (CMS) Indirect constraints from LEP extend these limits to TeV

BSM, Durham, Jan 2012Tracey Berry 22 Other Z’ models  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:

BSM, Durham, Jan 2012Tracey Berry 23 Z’ Combined Limits Update table

BSM, Durham, Jan 2012Tracey Berry 24 Extra Dimensions  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   = M pl e -kR c    ~ TeV

BSM, Durham, Jan 2012Tracey Berry 25 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)

BSM, Durham, Jan 2012Tracey Berry 26 RS model Signature: Narrow, high-mass resonance states in dilepton/dijet/diboson channels 700 GeV KK Graviton at the Tevatron k/M Pl = 1,0.7,0.5,0.3,0.2,0.1 from top to bottom M ll (GeV) Davoudiasl, Hewett, Rizzo hep-ph KK excitations can be excited individually on resonance 1500 GeV G KK and subsequent tower states K/M Pl LHC W u Z g Branching Fraction Couplings of each individual KK excitation are determined by the scale,   = M pl e -kRc  ~ TeV masses m n = kx n e -krc  (J 1 (x n )=0) 5D curve space with AdS 5 slice: two 3(brane)+1(extra)+time! Signature for Model Coupling proportional to  p-1 for KK levels above the fundamental level (n>=1) for n=0 graviton couples with the gravitational strenght d  /dM (pb/GeV) Model parameters: Gravity Scale: 1 st graviton excitation mass: m 1   = m 1 M pl /kx 1, & m n =kx n e krc  (J 1 (x n )=0) Coupling constant: c= k/M Pl  1 =  m 1 x 1 2 (k/M pl ) 2  width  position Resonance   = M pl e -kR c  k = curvature, R = compactification radius 1 extra warped dimension

BSM, Durham, Jan 2012Tracey Berry 27 ADD Collider Signatures Signature: deviations in  and asymmetries of SM processes e.g. q q  l + l -,   & new processes e.g. gg  l + l -  Virtual Graviton exchange Run I CDF Run I =+1 Broad increase in  due to closely spaced summed over KK towers M ll  independent of the number of ED* in Hewett convention Excess above di-lepton continuum  Virtual Graviton Emission  Parameterise  in terms of  Treating  as a free parameter, and extract limits on M S

BSM, Durham, Jan 2012Tracey Berry 28 RS Search: Analysis Strategy  Use same dataset as for Z’ search  Set limits in a similar bayesian way

BSM, Durham, Jan 2012Tracey Berry 29 RS G* limits Observed 95 % C.L. mass lower limit in TeV on RS Gravitons See later for combination with γγ channel!

BSM, Durham, Jan 2012Tracey Berry 30 Contact Interactions  Four-fermion contact interactions (CI) at low energy limit describe phenomena as: Quark-lepton compositeness  Benchmark composite model is left-left isoscalar model arXiv: ,CERN-PH-EP Submitted to PRX. (Dec 2011)

BSM, Durham, Jan 2012Tracey Berry 31 CI Analysis Strategy  Use same dataset as for Z’/G* search  Search for a broad increase: excess above 200 GeV  No excess observed  Set limits in a similar bayesian way to Z’/G analysis

BSM, Durham, Jan 2012Tracey Berry 32 Contact Interactions Using a Bayesian approach with a prior flat in 1/  2 : 95 % CL limits: ee:  - > 10.1 (9.4) TeV for constructive (destructive) interference  + > 8.0 (7.0) TeV for destructive interference  in the left-left isoscalar CI model arxiv: v1, PRX Mm 34 pb-1: arxiv: , PRD

BSM, Durham, Jan 2012Tracey Berry 33 Diphoton Search!

BSM, Durham, Jan 2012Tracey Berry 34 Analysis Procedure & Event Selection Diphoton channel Trigger on 2  (or e) with E T > 20 GeV Good Runs >= 1 primary vertex with >=3 tracks 2  with E T > 25 GeV Passing Tight  ID criteria |  | < 1.37 or 1.52 < |  | < 2.37    Isolation (0.4)< 5 GeV Energy correction to reduce pile-up & underlying event effects ee Overlap removal (so can combine results with G->ee result) Select events with two diphotons Search for excess above SM expectations in high invariant mass region  cluster E cell (Q>4000)  cluster E cell > 0.8. where  is measured in the second layer of the EM calorimeter Photon cleaning selection

BSM, Durham, Jan 2012Tracey Berry 35 Main Backgrounds  Irreducible Background SM  production  Reducible Background   + (misidentified) jet  jet + jet Shape determined using data-driven background enriched control samples & extrapolated to high mass  Total Background: normalised to data 140 Gev < m  < 400 GeV Born process box process bremsstrahlung process Sample Feynman diagrams of photon pair production, at the lowest order in terms of  S. 3 main processes contribute at tree level simulated with pythia (v6.424) and MRST2007LOMOD PDFs pythia events reweighted as a function of m  to the differential cross section predicted by the NLO calculation of diphox (v 1.3.2). (The reweighting factor varied from 1.6 for a diphoton mass of 140 GeV, decreasing smoothly to 1 for large masses. )

BSM, Durham, Jan 2012Tracey Berry 36 Diphoton Distributions The bin width is constant in log(m γγ ). The bin-by- bin significance of the difference between data and background is shown in the lower panel. Good agreement with data and expected background P=0.28

BSM, Durham, Jan 2012Tracey Berry 37 Uncertainties Source of Uncertainty Signal Uncertainty (%) Integrated Luminosity 3.7 MC Statistics 1.0 Bunch Crossing Identification 1.0 Photon Trigger 2.0 Pileup 2.5 Photon Efficiency and ID 4.3 Total Signal Uncertainty 6.7 arXiv: ,CERN-PH-EP , submitted to PRL  Limits obtained using a Bayesian approach, with a flat prior on the signal cross-section.  Systematic uncertainties incorporated as Gaussian nuisance parameters and integrated over

BSM, Durham, Jan 2012Tracey Berry 38 RS Limits The theory curves are drawn assuming a k-factor of The thickness of the theory curve for \coupling=0.1 illust rates the theoretical uncertainties. LO NLO  m  > 500 GeV  Limits obtained using same method, as for dilepton search  BR for G is twice that of G → γ γ

BSM, Durham, Jan 2012Tracey Berry 39 ADD Limits  Limits  Search Region m  > 1100 GeV  Parameterise  in terms of  Fit the differential  to this, treating  as a free parameter, and extract limits on M S 1.18±0.24  Parameterise  in terms of  Search Region m  > 1100 GeV  Treating  as a free parameter, and extract limits on M S Limits  Observed (expected) 95 % CL upper limit on  = 2.53 (1.95) fb  Translated into 95 % CL limits on the parameter on  and M S :

BSM, Durham, Jan 2012Tracey Berry 40 Future...  Analysis improvements/ development  add more data (5 fb -1 )!  aim to increase acceptance of dilepton search  ee channel  mm channel (3+2 station muons)  add angular information

BSM, Durham, Jan 2012Tracey Berry 41 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…

BSM, Durham, Jan 2012Tracey Berry 42 ADD Model

BSM, Durham, Jan 2012Tracey Berry 43 Non-Search Constraints on the Model Run I M Pl 2 ~ R  M Pl(4+  ) (2+  ) For M Pl ~ GeV and M Pl(4+  ) ~M EW  R ~10 32/  x cm   =1  R ~10 13 cm, ruled out because deviations from Newtonian gravity over solar distances have not been observed   =2  R ~1 mm, not likely because of cosmological arguments: In particular graviton emission from Supernova 1987a * implies M D >50 TeV Closest allowed M Pl(4+n) value for  =2 is ~30 TeV, out of reach at LHC *Cullen, Perelstein Phys. Rev. Lett 83,268 (1999) LEP & Tevatron limits is MPl(4+d) ~> 1TeV d>6 difficult to probe at LHC since cross-sections are very low G Arkani-Hamed, Dimopoulos, Dvali, Phys Lett B429 (98) (Many) Large flat Extra-Dimensions (LED) could be as large as a few  m In which G can propagate, SM particles restricted to 3D brane

BSM, Durham, Jan 2012Tracey Berry 44 Sensitivity: 10 Signal Events

BSM, Durham, Jan 2012Tracey Berry 45 Sensitivity: Limit Setting

BSM, Durham, Jan 2012Tracey Berry 46 CI Limits

BSM, Durham, Jan 2012Tracey Berry 47 Data & SM Background

BSM, Durham, Jan 2012Tracey Berry 48 Rapidity

BSM, Durham, Jan 2012Tracey Berry 49 Z’ compared to G* Muon/Electron Kinematics Good agreement with background expectations

BSM, Durham, Jan 2012Tracey Berry 50 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

BSM, Durham, Jan 2012Tracey Berry 51 G* limits k/M Pl e+e-e+e m+m-m+m gg e + e - + m + m e + e - + gg ATLASCMSCDFD0 Mass Limits RS Graviton (GeV/c 2 )

BSM, Durham, Jan 2012Tracey Berry 52 G* limits

BSM, Durham, Jan 2012Tracey Berry 53 

BSM, Durham, Jan 2012Tracey Berry 54 The End!