Z boson studies at the ATLAS experiment at CERN Giacomo Artoni Ph.D Thesis Project June 6, 2011
Outline Introduction to the LHC and ATLAS ((Very) Brief) Z boson history Measurement of σ Backgrounds Acceptances Ongoing and future work: ( ) + + Conclusions 2
The Large Hadron Collider Proton-proton collider; 27 kilometers of circumference; Center of Mass Energy 7 TeV; Peak Luminosity 1.1 33 cm -2 s ( as of May 26 ); 3
Physics at a Hadron Collider The total energy and momenta of the protons are known......but interactions occur at parton level, not at proton level! Each quark interacts with an energy and a momentum that we cannot know a priori however, the projection of the quarks quadri-momenta on the transverse plane can be considered zero and we can apply kinematic rules. p x 1 p x 2 So which kinematic variables should we use to describe our events at the LHC? p p 4
Useful variables We can define a system of spherical coordinates, but then we need to make some adjustments... x φ ϑ Original variable What we really use y ϑ = η = z (beam axis) ϑ = η =. p (particle momentum) = + ϑ η = (ϑ/ ) ϑ = η =. ϑ = η = 5
M ll and isolation An important quantity that we use is the invariant mass of a pair of particles ( 1 and 2 ), which is simply: =( + ) ( + ) Another important variable at the LHC is the isolation of the leptons: e,µ = Σ <. = η + φ This variable is very useful when we need to discriminate between muons and electrons coming from W, Z and muons coming from jets! 6
The ATLAS Experiment 7
Particle ID at ATLAS From the interaction point: Tracking system; Electromagnetic calorimeter; Hadronic calorimeter; Muon chambers; Peculiarity: our magnetic field is solenoidal in the inner tracking system but toroidal in the muon chambers! 8
A great work for the LHC The LHC has achieved an instantaneous luminosity of 1.1 33 cm -2 s and the statistics at our disposal is increasing every day! ] Total Integrated Luminosity [pb 60 50 40 30 20 Data 20 Data 2011 ATLAS Online Luminosity LHC Delivered ATLAS Recorded Total Delivered: 48.1 pb Total Recorded: 45.0 pb s = 7 TeV ] Total Integrated Luminosity [pb 500 400 300 200 0 ATLAS Online Luminosity LHC Delivered ATLAS Recorded Total Delivered: 427.0 pb Total Recorded: 403.9 pb s = 7 TeV 0 24/03 19/05 14/07 08/09 03/11 Day in 20 0 24/02 26/03 25/04 26/05 Day in 2011 9
Data 20 analysis (~40 pb ) Z boson cross section measurement
A small reminder The electroweak unification made by Glashow, Weinberg and Salam postulated the existence of three vector bosons, one of which had to be neutral; m Z = απ G 2 2 sin(2θ W ) In 1973 Gargamelle showed the existence of neutral current interactions; We had to wait until May 1983 to see the Z boson, at UA1 and UA2. 11
Why the Z at ATLAS? Is the standard high-p T candle for detector performance: Its signature is very clean and should provide two high-p T leptons; They can be used for commissioning of the particle reconstruction at ATLAS; Its production cross-section can be ( and will be ) used as a luminosity monitor for ATLAS; A cross-section measurement can provide a good test of different PDF sets in NLO theoretical calculations; σ Z BR(Z l + l ) l = e, µ We then decided to focus on. 12
The measurement Number of candidates that pass our selections Estimated number of background events passing our selections σ tot = σ Z BR(Z ll) = N B A Z C Z L int Acceptance factors Trigger and reconstruction efficiencies Integrated luminosity used (in our case 35 pb ) 13
Electroweak backgrounds (µµ) [GeV] 1500 All these backgrounds have been 00 estimated using Monte Carlo generated 500 samples and theoretical cross-sections; 0 W µν : 0.006%; Z ττ : 0.07%; tt : 0.1%; Di-boson : 0.2%. Entries/GeV Entries/GeV 500 0 3500 3000 2500 2000 4000 3500 3000 2500 2000 1500 00 500 0 00 60 80 0 120 1 500 60 80 0 120 m µµ [GeV] m µµ [GeV] 0 60 80 0 120 (a) p T > 15 GeV, q 1 q 2 < 0, pt ID /p T <0.2 [GeV] Data 20 Z!µµ " L dt = 34 pb # 2 /NDF = 28/27 Entries/GeV Entries/GeV 4000 3500 3 3000 2500 2 2000 1500 m µµ (b) p T > 20 GeV, q 1 q 00 1 60 80 0 120 500 60 80 0 120 m µµ [GeV] m µµ [GeV] 0 60 80 0 120 (b) p T > 20 GeV, q 1 q 2 < 0, pt ID /p T <0.4 [GeV] Data 20 Z!µµ " L dt = 34 pb # 2 /NDF = 27/27 60 80 0 120 m µµ Normal scale Logarithmic scale [GeV] Entries/GeV 4 Data 20 Z!µµ L dt = 34 pb W!#" W!µ" WW Data 20 % Z!µµ " 2 /NDF = 28/27 ZZ L dt = 34 pb WZ tt Z!## # 2 /NDF = 27/27 QCD m µµ (c) p T > 20 GeV, q 1 q 2 WW % Figure 78: Fit results using data-driven 2 /NDF = 27/27 ZZ QCD temp WZ algorithm. 3 The tt cuts specific to each selection are Z!## results have been QCD found using the MuId chain. 4 2 1 Data 20 Z!µµ W!#" W!µ" 60 80 0 120 $ $ L dt = 34 pb m µµ [GeV] (c) p T > 20 GeV, q 1 q 2 < 0, pt ID /p T <0.2 14
Isolation QCD background (µµ) In the case of QCD background, we can exploit the fact that there is no correlation between M µµ and isolation and avoid using our Monte Carlo samples;? 0.1 D B C A We can thus use data from sideband regions to evaluate the number of background events in our fiducial window: N A = N B N C N D Also this background is very small, about 0.2%. Isolation 20? 66 116 6 5 4 3 2 1 QCD MC 0! 0 40 60 80 0 120 140 160 180 200 3 2.5 2 1.5 1 0.5 M µµ (GeV) M µµ (GeV) 15
W eν : p T,e > 20 GeV, η e < 2.47 excluding 1.37 < η e < 1.52, p T,ν > 25 GeV, m T > 40 GeV W µν : p T,µ > 20 GeV, η µ < 2.4, p T,ν > 25 GeV, m T > 40 GeV Z ee : Forward Z ee : Acceptances 16 p T,e > 20 GeV, both η e < 2.47 excluding 1.37 < η e < 1.52, 66 < m ee < 116 GeV p T,e > 20 GeV, one η e < 2.47 excluding 1.37 < η e < 1.52, other 2.5 < η e < 4.9, 66 < m ee < 116 GeV Z µµ : p T,µ > 20 GeV, both η µ < 2.4, 66 < m µµ < 116 GeV Acceptance factors are needed to pass from the fiducial measurement to a measurement that can be compared with theoretical predictions and other experiments; We need to combine three different measurements and for each of these A Z must be calculated using Monte Carlo samples.
Final result σ [ ] =. ±. ( ) ±. ( ) ±. ( ) Z ee Z ee fwd ±. ( ) Z µµ 17
Sensitivity to NNLO PDFs Our experimental precision is soon going to allow us to discriminate between different sets of PDFs at NNLO;! W [nb] 11 ATLAS Preliminary This really good result has been achieved thanks to the great work made in ATLAS for the integrated luminosity determination uncertainty, that passed from 11% to 3.4%. 9 Data 20 ( MSTW08 HERA ABKM09 JR09 s = 7 TeV) total uncertainty uncorr. exp. + stat. uncertainty L dt = 33-36 pb 0.8 0.9 1 1.1 "! Z [nb] 18
Data 2011-2012 analysis (???? pb ) Higgs boson searches 19
Pros: We have deeply studied the Z boson at ATLAS, so we already know how to treat most of the detector issues; Electrons and muons are the cleanest signature at a hadron collider; We have the best resolution on M H achievable by ATLAS; Cons: H ZZ( * ) 4l The last missing piece of the Standard Model is the Higgs Boson and ATLAS (along with CMS) has been designed especially for discovering this particle; Our goal is to study the Higgs decaying in two Zs, which then should decay in muon or electron pairs each; 0 * E123F A ZZ pair goes to 4µ/2µ2e/4e only 4 times over about one thousand! 20
Current ATLAS results So far we have had 40 pb of 20 data to work with and the result is that we do not have yet the statistics we need for this measurement... 21
Closer than ever... We already have 400 pb of data, here are our projections: The 4 leptons channel represents the best channel for a Higgs of mass between 200 GeV and ~260 GeV; In every other (high mass) scenario it is the best channel in terms of resolution on the Higgs invariant mass; H ZZ 4l NNLO 95% CL Upper Bound on / SM 1 H ZZ 4l ± 1 ± 2 s=7 TeV Projection L dt=1 fb ATLAS Preliminary (Simulation) 0 150 200 250 300 350 400 450 500 550 600 m H [GeV] Here are the expected background and signal events for different Higgs masses with 1 fb at 7 TeV: ( ) ( ) All channels NNLO 95% CL Upper Bound on / SM 1 s=7 TeV ATLAS Preliminary (Simulation) H WW H 4l H ZZ llbb H ZZ ll H VH, H bb H Combined! 1! 2 L dt=1 fb -2 0 200 300 400 500 600 m H [GeV] 22
Conclusions My thesis project started with a complete study of the Z boson (since in 20 we gathered only 40 pb ) that ended up in a Conference Note (ATLAS-CONF-2011-041, see http://cdsweb.cern.ch/record/1338570) and soon will be published as a paper; The real thesis project will be focused on the Higgs boson, trying to use my knowledge of the Z to study H ZZ 4l; Two scenarios are possible: There is no standard Higgs boson: we should be able to exclude its presence in an unexplored mass range; There is the standard Higgs boson: we will contribute directly to the discovery and we will be able to measure its mass (if above 180 GeV) with the best precision possible at our experiment. 23