Electroweak Measurements at LHCb! Simone Bifani University of Birmingham (UK) on behalf of the LHCb collaboration LHCb workshop on quantum interference effects, QCD measurements and generator tuning CERN, 2th - 22nd October 214
Outline Introduction Analyses» W µν» Z ll (l = µ, e, τ)#» Z µµ in pa Prospects Summary Simone Bifani 2
LHCb A Forward Spectrometer Designed to look at CP violation in B decays @ LHC Fully instrumented within 2. < η < 5. 25 mrad Beam mrad Beam JINST 3 (28) S85 Simone Bifani 3
LHCb A Forward Spectrometer Overlap with ATLAS & CMS 2. < η < 2.5 Complementary range 2.5 < η < 5. η# in LHCb 17%(16%) of W + (W - ) within LHCb 8% of Z within LHCb Simone Bifani 4
Datasets Analyses based on 211(213) pp(pa) data samples at 7(5) TeV 212 pp data at 8 TeV are currently being analysed» L 211 ~1 fb -1» L 213 ~1.6 nb -1 Due to luminosity levelling, same running conditions throughout fills EW trigger thresholds unchanged during years Simone Bifani 5
W & Z Production and PDFs LHCb s forward acceptance provides very interesting possibilities to study the proton Parton Density Functions Take large-x from one proton and a small-x from the other probe two distinct regions in the (x, Q2) space» W and Z (x of -4 and -1)» Low-mass Drell-Yan (x down to -6) Simone Bifani ATLAS/CMS LHCb CDF/D W&Z HERA Fixed Target 8 7 6 Q2 [GeV2] Can probe the low-x, high-q2 region inaccessible to other experiments LHC 7 TeV Kinematics 9 5 W&Z γ* 4 3 2 γ* 1 7 y =6 6 y =4 5 y =2 4 M x1,2 = p e±y s y = y =2 3 y =4 2 y =6 1 x Q2 = M 2 6
W & Z Production at LHCb arxiv:148.4354 LHCb-CONF-213-7 LHCb has measured the W and Z production cross-section at 7 TeV using several final states Pull Events per 1 GeV/c 3 2 5-5 LHCb µ + Purity ~77% 2. < η < 4.5 Data Fit W µν K/π µ ν Electroweak Heavy flavour 2 3 4 5 6 7 2 3 4 5 6 7 p [GeV/c] W µν# T - µ ) 2 Events / (1 GeV/c 7 6 5 4 3 2 Z µµ# LHCb preliminary Data Purity >99% 6 8 12 Dimuon invariant mass [GeV/c 2 ] 211 Z->µµ analysis with much reduced systematic uncertainty currently under review Simone Bifani 7
W & Z Production at LHCb JHEP 2 (213) 6 JHEP 1 (213) 111 LHCb has measured the W and Z production cross-section at 7 TeV using several final states Events / (2 GeV/c 2 ) 12 8 6 4 2 Z ee LHCb 4 6 8 12 M (e Data Signal Background Purity ~95% + e - ) [GeV/c 2 ] Events / (5 GeV/c 2 ) Events / (5 GeV/c 2 ) 8 Data 7 Z æ QCD 6 EWK t t æ µe 5 WW æ µe 4 3 2 LHCb Ô s =7TeV 2 4 6 8 12 M µe [GeV/c 2 ] 2 4 6 8 12 M µh [GeV/c 2 ] Events / (5 GeV/c 2 ) 4 Data 35 Z æ QCD 3 EWK t t æ eµ 25 WW æ eµ 2 Z ττ# Purity 6-7% 35 3 25 2 15 5 Data Z æ QCD EWK t t æ µh WW æ µh Z æ µµ LHCb Ô s =7TeV Events / (5 GeV/c 2 ) 15 5 LHCb Ô s =7TeV 2 4 6 8 12 M eµ [GeV/c 2 ] 3 25 2 15 5 Data Z æ QCD EWK t t æ eh WW æ eh Z æ ee LHCb Ô s =7TeV 2 4 6 8 12 M eh [GeV/c 2 ] Simone Bifani 8
Results W µν arxiv:148.4354 Fiducial volume: p T,µ > 2 GeV/c, 2. < η µ < 4.5 LHCb, Data stat Data tot s = 7 TeV MSTW8 NNPDF23 CT ABM12 HERA15 JR9 8 82 84 86 88 9 92 p > 2 GeV/c T 2. < η < 4.5 σ W + µ + ν [pb] [pb] ν - µ - σ W 8 75 7 LHCb, Data s = 7 TeV Data stat Data tot MSTW8 NNPDF23 CT ABM12 HERA15 JR9 syst p > 2 GeV/c T 2. < η < 4.5 L ~37 pb -1 68.3% CL ellipse area 62 64 66 68 7 72 σ - W µ - ν σ W + µ + ν 1.2 1.25 1.3 1.35 σ W - µ - ν [pb].8% precision! 65 85 9 95 σ W + µ + ν [pb] Results in general agreement with NNLO predictions Main systematics: template fit, reconstruction efficiencies Luminosity uncertainty down to 1.7% Simone Bifani 9
Results W µν arxiv:148.4354 dσ W µν /dη [pb] Theory/Data 8 LHCb, s = 7 TeV 6 4 2 1.1.9 1 1.1.9 1 p T > 2 GeV/c Datastat Data Data Data tot stat tot + (W ) (W + ) - (W ) - (W ) MSTW8 NNPDF23 CT ABM12 HERA15 JR9 2 2.5 3 3.5 4 4.5 η dσ W µν /dη [pb] 6 4 2 s = 7 TeV LHCb (211 extrapolated) W LHCb (211 extrapolated) W + ATLAS (2) W µ + ν - ATLAS (2) W µ - ν µ + ν µ - ν 1 2 3 4 + - p T M T E miss > 2 GeV/c > 4 GeV/c > 25 GeV 2 η Good agreement with NNLO Good agreement with ATLAS (after η adapting to ATLAS cuts) Simone Bifani
Results W µν arxiv:148.4354 Lepton charge asymmetry Aµ.6.4.2 -.2 LHCb, Data Data s = 7 TeV stat tot MSTW8 NNPDF23 CT ABM12 HERA15 JR9 A l.6.4.2 p T > 25 GeV/c Theory-Data -.4.5 -.5 p T > 2 GeV/c 2 2.5 3 3.5 4 4.5 η -.2 -.4 s = 7 TeV LHCb (211 extrapolated) W µν CMS (211) W µν -.6 1 2 3 4 η Good agreement with NNLO Good agreement with CMS (after adapting to CMS cuts) Simone Bifani 11
Results Z ll LHCb-CONF-213-7 Fiducial volume: p T,l > 2 GeV/c, 2. < η l < 4.5, 6 < M ll < 12 GeV/c 2 LHCb Preliminary Z µµ LHCb preliminary (75.4 ±.3 stat ± 1.9 syst ± 2.6 lumi ) pb Z ee (76. ±.8 stat ± 2. syst ± 2.6 lumi ) pb Z ττ NNLO(MSTW8) (71.4 ± 3.5 stat ± 2.8 syst ± 2.5 lumi ) pb (74.7 +1.6 1.4 PDF ±.4 T heory) pb 5 6 7 8 9 σ(z ll) [pb] Results agree with each other and NNLO predictions Measurements limited by statistics (ττ) or luminosity (ee, µµ) Main systematics: reconstruction (µµ), tracking efficiency (ee), backgrounds and efficiency (ττ) Simone Bifani 12
Results Z ll LHCb-CONF-213-7 Graph dσ/dy [pb] 7 LHCb preliminary 6 5 4 3 2 Data (stat) Data (tot) MSTW8 (NNLO) CT (NNLO) NNPDF23 (NNLO) ABM11 (NNLO) dσ/dy [pb] 7 LHCb preliminary 6 5 4 3 Data (stat) Data (tot) Z ee 2 2 3 4 Z boson rapidity 2 2.5 3 3.5 4 Z boson rapidity Good agreement with NNLO Good agreement between µµ and ee final states Good agreement with ATLAS (after adapting to ATLAS cuts) dσ/dy [pb] 14 12 8 6 4 2 LHCb 211 preliminary, Z µµ extrapolated LHCb 211, Z ee extrapolated ATLAS 2, Z µµ, Z ee.5 1 1.5 2 2.5 3 3.5 4 4.5 y(z) Simone Bifani 13
Results Z ll LHCb-CONF-213-7 dσ/dφ* [pb] 3 LHCb preliminary Data (stat) Data (tot) NNLO - MSTW8 ResBos Powheg + PS 2-2 -1 1 φ* prediction/data 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.9.8.7 LHCb preliminary Data (stat) Data (tot) NNLO - MSTW8 ResBos Powheg + PS -2-1 1 φ* = tan (( )/2) cosh ( /2) p T m Z NNLO fails to describe φ* Bett er agreement when compared to ResBos and POWHEG dσ/dφ* [pb] 3 LHCb preliminary 2 1-2 Graph -1 1 Data (stat) Data (tot) Z ee φ* Simone Bifani 14
Z µµ in pa! LHCb-PAPER-214-22 Sensitive to npdf in a region where there is no experimental input Candidates / (2 GeV/c 2 ) 4 3 2 LHCb ppb s pn forward = 5 TeV LHCb ppb s pn = 5 TeV [nb] σ Z µ + µ 4 3 2 LHCb stat. LHCb syst. + lumi. LHCb ppb s pn = 5 TeV FEWZ NNLO + MSTW8 FEWZ NNLO + MSTW8 + EPS9 (NLO) 1 6 8 12 m [GeV/c 2 µ + µ ] forward backward Forward (pa): σ(z µµ) = (13.5 +5.4-4. stat ± 1.1 syst ) pb Backward (Ap): σ(z µµ) = (.7 +8.4-5.1 stat ± 1. syst ) pb Simone Bifani 15
Impact on PDFs M.Ubiali @ LHCb Impication Workshop Simone Bifani 16
Prospects Updates with 8 TeV data for all analyses Perform inclusive, differential and associated measurements with Run-II data @ 13(14) TeV Measurement of event properties, multiplicities and multiplicity ratios in Z events with identified particles (p, pbar, K ±, Lambda->pπ, K S ->ππ, maybe ρ and φ) What should we measure with higher priority that would be an important input for MC tuning? Simone Bifani 17
Summary LHCb probes a unique region in η and low p T reach at LHC and can provide important inputs to PDFs in the forward region W and Z production cross-sections measured by using a variety of final states Results consistent with theoretical predictions calculated at NNLO Many other measurements expected soon! Simone Bifani 18
Backup
W & Z Production and PDFs Theoretical predictions» Partonic cross-sections known at NNLO to few %» PDF uncertainty dominates at large rapidities (1% for y<2, 6-8% at y~5) Experimental measurements» Muon modes provide clean signature and easily reconstructible final states» Electron and tau modes provide complementary measurements and test lepton universality Cross-section measurements @ LHCb can constrain PDFs Simone Bifani 2
W & Z Production and PDFs Cancel or highlight PDF uncertainties with ratios» A µ = (dσ W+ - dσ W- ) / (dσ W+ + dσ W- ) tests u V and d V difference A µ# R W#» R W = dσ W+ / dσ W- tests d V /u V ratio» R WZ = dσ W / dσ Z almost insensitive to PDFs precise test of SM Many systematic errors cancel Simone Bifani 21
W µν! Single muon trigger: p T > GeV/c 1 reconstructed & isolated muon» p T > 2 GeV/c» 2. < η < 4.5» Cone p T (R=.5) < 2 GeV/c» Extra muon veto (p T > 2 GeV/c)» Impact parameter < 4 µm» E / pc < 4% Backgrounds» Z/γ* µµ (MC)» W τν and Z ττ (MC)» Heavy flavour (Data)» K/π decay in flight (Data) N Candidates = 147k + 116k - Purity ~ 79% + 78% - Events per 1 GeV/c Pull 3 2 5-5 LHCb L ~37 pb -1 (2) µ + 2. < η < 4.5 Data Fit W µν K/π µ ν Electroweak Heavy flavour 2 3 4 5 6 7 2 3 4 5 6 7 p [GeV/c] Template fit in 5 η bins W and Z muon p T spectrum at NNLO Simone Bifani 22 T - µ
W µν! Specific cuts implemented to reduce each background component Z/γ* µµ» Veto on 2 nd muon with p T > 2 GeV/c W τν, Z ττ and Heavy flavour» Impact parameter < 4 µm K/π punchthrough» E / pc <.4 Largest residual backgrounds» Decays in flight of K/π» Z µµ events with one muon outside the acceptance.5 1 1.5 IP [mm] Simone Bifani 23 Events per.2 mm Events per.2 6 5 4 3 2 6 5 4 3 2 LHCb Data Fit Pseudo- W (data) W τν (simulation) bb+cc Xµ (simulation) LHCb Data Fit Pseudo-W (data) Hadrons (data).5 1 1.5 / pc E calo
Z µµ! Single muon trigger: p T > GeV/c 2 reconstructed muons» p T > 2 GeV/c» 2. < η < 4.5» 6 < M µµ < 12 GeV/c 2 Backgrounds» Heavy flavour (Data)» K/π mis-id (Data)» Z ττ (MC)» tt - (MC)» WW (MC) ) 2 Events / (1 GeV/c 7 6 5 4 3 2 LHCb preliminary Data L ~1 fb -1 (211) N Candidates = 52626 Purity > 99% 6 8 12 Dimuon invariant mass [GeV/c 2 ] Simone Bifani 24
Z ee! Single electron trigger: p T > 15 GeV/c 2 reconstructed electrons» p T > 2 GeV/c» 2. < η < 4.5» E ECal / pc >.1» E HCal / pc <.5» M ee > 4 GeV/c 2 Backgrounds» K/π mis-id (Data)» Heavy flavour (Data)» Z ττ (MC)» tt - (MC) Events / (2 GeV/c 2 ) 12 8 6 4 2 LHCb L ~945 pb -1 (211) Data Signal Background N Candidates = 2142 Purity ~95% 4 6 8 12 M (e + e - ) [GeV/c 2 ] Mass peak distorted by bremsstrahlung Simone Bifani 25
Z ττ! Single lepton trigger: p T,µ(e) > (15) GeV/c 5 final states: µµ, µe, eµ, µh, eh» p T,1 > 2 GeV/c, p T,2 > GeV/c» 2. < η l < 4.5, 2.25 < η h < 3.75» M 12 > 2 GeV/c 2 Backgrounds» QCD (Data)» EW (Data)» tt (MC)» WW (MC)» Z ll (MC) 2 4 6 8 12 - M µµ [GeV/c 2 ] N Candidates = 99 Purity ~6-7% Simone Bifani 26 Events / (5 GeV/c 2 ) Events / (5 GeV/c 2 ) L ~1 fb -1 (211) 4 Data 35 3 25 2 15 5 4 3 2 LHCb Ô s =7TeV Z æ QCD EWK t t æ µµ WW æ µµ Z æ µµ 8 Data 7 Z æ QCD 6 EWK t t æ µe 5 WW æ µe LHCb Ô s =7TeV 2 4 6 8 12 M µe [GeV/c 2 ]
Cross-Section The cross-section for boson production can be expressed as BR= 1 R L N Candidates A " T rigger " Tracking " P articleid " Selection f FSR Measurements performed in the forward region (2.<η<4.5) for leptons with p T >2 GeV/c: Acceptance = 1 for Z µµ and W µν, but obtained from MC for Z ee and Z ττ Efficiencies determined mostly from data and cross checked with simulation FSR correction evaluated using HERWIG++ and PHOTOS+PYTHIA Simone Bifani 27
Reconstruction Efficiencies Trigger» Tag: triggered muon#» Probe: offline identified muon Tracking Tracking» Tag: identified muon track» Probe: trajectory from muon stub and minimal tracking information Particle ID Particle ID» Tag: identified muon#» Probe: reconstructed track Simone Bifani 28
W µν Simone Bifani 29
Z ll Simone Bifani 3
Z ττ! Simone Bifani 31
Z µµ in pa Simone Bifani 32
Z µµ in pa Candidates / (2 GeV/c 2 ) 4 3 2 1 LHCb ppb s pn forward = 5 TeV LHCb Preliminary ppb = 5 TeV s pn Candidates / (2 GeV/c 2 ) 3 2 1 LHCb ppb s pn backward = 5 TeV LHCb Preliminary ppb = 5 TeV s pn 6 8 12 m [GeV/c 2 µ + µ ] 6 8 12 m [GeV/c 2 µ + µ ] Candidates / (.2) 4 3 2 LHCb Preliminary ppb = 5 TeV s pn forward Candidates / (.2) 4 3 2 LHCb Preliminary ppb = 5 TeV s pn backward 1 1 1 2 3 4 5 6 y Z,lab 1 2 3 4 5 6 y Z,lab 1 2 3 4 5 y' 2 3 4 5 6 y' Simone Bifani 33
References Simone Bifani 34