Physics prospects of the ZZ 4 l production

Physics prospects of the ZZ 4 l production measurements with ATLAS experiment for the first year of RUN 2 Cong GENG The Univ. of Michigan Ann Arbor/USTC

Outline • Introduction • ZZ 4 l measurements at RUN 1 • Analysis prospects for 13 Te. V • ZZ 4 l candidates at the beginning of RUN 2 • Summary DPF 2015 2

The First SM Milestone in Run 2 Measurement of the σ(ZZ 4 l) H Z qq->4 l gg->4 l The most clean experimental signature DPF 2015 3

Test of SM EW sector at the LHC ü ZZ cross section measurements of RUN 1 match NNLO SM predictions (ar. Xiv: 1507: 06257). ü At RUN 2, Continue to measure ZZ diboson cross section at 13 Te. V to test SM at new energy frontier, compare to SM calculations with DPF 2015 higher-order corrections 4

Irreducible background to Higgs Detection and Searches for New Physics ü Continuum ZZ production is an irreducible background for Higgs boson re-discovery, and its property measurements. ü The SM ZZ production is also an important background for high mass Higgs search and other exotic physics searches with heavy particle decay DPF 2015 to ZZ 4 l. 5

Anomalous TGC and VBS Process ZZZ, or ZZγ ü Probing anomalous couplings for triple-gauge-boson couplings is a sensitive tool to search for new physics with a model-independent way. ü Observation and cross section measurement of VBS process is important to validate the SM Higgs mechanism. ü These measurements will need large integrated luminosity. DPF 2015 6

ZZ Production Cross-Section Measurement at 8 Te. V with 20. 3 fb-1 ATLAS-CONF-2013 -020 4 e, 4μ, 2 e 2 are used to measure the ZZ production cross section. Both Z’s are produced on shell. Consistent with the SM prediction calculated by MCFM (with CT 10 PDFs) : 7. 2 +0. 3/-0. 2 pb. DPF 2015 7

Measurement of 4 l from Z/H/ZZ productions at 8 Te. V New! ATLAS-CONF-2015 -031 Physics goal: measure the 4 l production cross section (integrated and differential) inclusively (Z/H/ZZ) from 80 Ge. V to 1 Te. V. Signal model: NLO qq. ZZ and higher-order correction; LO gg. ZZ Background: combined data-driven and MC estimation; contribute ~5% of the total event yield. DPF 2015 8

Measured s in Fiducial and Extended Phase Space DPF 2015 New! 9

Measured differential s of 4 l from Z/H/ZZ productions at 8 Te. V ATLAS-CONF-2015 -031 New! ü In M(4 l)>180 Ge. V region, the signal strength of non-resonant gg ZZ w. r. t. to LO calculation is estimated to be mgg=2. 4± 1. 0(stat)± 0. 5(syst)± 0. 8(th. ). This is done by fitting the m 4 l spectrum and constraining the qq 4 l with theoretical calculations at NNLO QCD and NLO EW DPF 2015 10

Theoretical Calculations for σ(qq ZZ) For on-shell qq ZZ productions ar. Xiv: 1405. 2219 [hep-ph] NNLO QCD calculation is available for on-shell ZZ production ü NLO/LO k-factor ~1. 45, NNLO/NLO 1. 12 -1. 17 ü LO gg ZZ contributes 60% of NNLO-NLO correction ü Higher order gg ZZ can bring sizable NNLO correction to ZZ production DPF 2015 11

Cross Section Comparison for 8 and 13 Te. V Process Cross-section At 8 Te. V (pb) Cross-section At 13 Te. V (pb) On-shell qq ZZ (NLO) 6. 8 13. 5 On-shell gg ZZ (LO) 0. 43 1. 18 v The cross sections are calculated with MCFM using QCD Scales= M(ZZ)/2 and CT 10 PDFs; for dilepton mass range 66 Ge. V < M(ll) < 116 Ge. V v More than a factor of two increase from 8 Te. V to 13 Te. V. Expecting gg ZZ increases faster than qq ZZ. v Higher order corrections on cross section calculations are expected to further increase the cross sections for both qq and gg production mechanisms v Measure the ZZ production cross section is the first milestone in diboson physics studies in Run 2 DPF 2015 12

Prospect of the ZZ Production Measurement in Run 2 Ø Event Selection for 4 l signal ü Triggers : single and di-leptons; with high efficiency. ü Four isolated leptons, two on-shell Z candidates ü Using MC to determine the acceptance and efficiencies. The higher-order corrections will be applied to cross sections and kinematic distributions. ü Higher luminosities have small impact on event selections Ø Background ü Main background Z+jets, tt, WZ, where two leptons from Z decays and one or two fake leptons from the jets. ü The backgrounds are estimated from data and also from MC simulations DPF 2015 13

Cross section Extraction C factor: detection efficiency correction A factor: fiducial acceptance σfid. ZZ is the fiducial cross section depending on analysis fiducial definition. Generally, this cross section is firstly extracted by only correcting detection efficiency. σtotal. ZZ is the full phase sapce (PS) cross section which taking into account the factor (A) between experimental fiducial to full PS. DPF 2015 14

Cross section measurements • RUN 1 Fiducial phase space definition: p. T>7 Ge. V, |η|<2. 7, d. R(ll)>0. 2 ATLAS-CONF-2013 -020 RUN 1 Powheg ZZ 4μ (error) ZZ 4 e (error) ZZ 2 e 2μ (error) Azz 0. 64 (1. 3%) Czz 0. 83 (3. 0%) 0. 55 (6. 6%) 0. 66 (3. 9%) N @8 Te. V 20 fb-1 85 62 158 Backgrounds 1. 1± 1. 4± 0. 5 10. 0± 1. 8± 1. 4 9. 3± 2. 1± 3. 1 Totally around 300 4 l on-shell ZZ events (with background). kinematic topologies are similar between 8 Te. V and 13 Te. V for the final four leptons. We have found similar A and C factors for cross section measurements in Run 2. DPF 2015 15

Cross Section Measurement Uncertainties vs. Luminosity üCross section of ZZ increases from 8 Te. V to 13 Te. V by a factor of 2. üAssuming similar systematic uncertainty at 13 Te. V, we will reach the same sensitivity as RUN 1 if we have 10 fb-1 at RUN 2 at new energy frontier. Based on 8 Te. V 4 l event selection criteria: (and estimated 5% 4 l background): §Expect to select ~50 4 l events per fb-1 at 13 Te. V; §Expect to select ~30 4 l events from on-shell ZZ per fb-1 at 13 Te. V DPF 2015 16

Sensitivity of a. TGC Measurement by ATLAS JHEP 03 (2013) 128 2008 JINST 3 S 08003 • “mock data” samples was used to study the sensitivity of a. TGC • Fit the p. T (Z) distribution to probe the a. TGC (at 14 Te. V) Will improve the a. TGC sensitivity with the first 10 fb-1 data at Run 2 DPF 2015 17

ATLAS status at the beginning of RUN 2 ü ~ 100 pb-1 has been recorded. ü To expect 5 - 8 fb-1 at the end of the first year (2015) DPF 2015 18

Z ee, mm from Run 2 Initial Data Z mm Z ee Muon-ID: combing inner tracker and outer muon spectrometer. Required: p. T>20 Ge. V ET>20 Ge. V, e-ID with likelihood. Require loose isolation DPF 2015 19

Electron and muon detections Data (13 Te. V) comparing with MC • Z ee candidates ET = 40 – 50 Ge. V Tag and probe method • Z μμ candidates Combined muons p. T>20 Ge. V DPF 2015 20

Three ZZ 4 l events have been recorded by the ATLAS Detector with 0. 1 fb-1 RUN, event type mass 1 p. T 1 eta 1 p. T 2 eta 2 mass 2 p. T 1 eta 2 p. T 2 eta 2 mass ZZ 271298, 78224729 eemm 94. 0 (ee) 59. 6 0. 62 28. 3 -0. 49 85. 9 (mm) 60. 0 -0. 18 35. 0 -0. 17 191. 05 271421, 287349506 eemm 92. 0 (mm) 50. 8 1. 25 45. 5 1. 3 90. 1 (ee) 55. 4 -0. 53 33. 5 -1. 2 305. 23 Unit: Ge. V DPF 2015 21

Summary • ZZ 4 l production measurement is the first milestone in Run 2 for diboson physics studies • Measurement of ZZ 4 l is sensitive to new physics searches at Te. V mass scale and for precision Higgs property measurement. • At 13 Te. V, we expect to select ~30 events per fb-1 for on shell ZZ 4 l and ~50 inclusive 4 l events. At the end of the first year of RUN 2, we expect to reach the same cross section measurement precision as the Run 1 measurement. • Using the recorded about 100 pb-1 data by ATLAS by July 2015, we have observed three gold-plated on-shell ZZ events! DPF 2015 22

Backup slides DPF 2015 23

Fiducial phase space for 4 l measurement DPF 2015 24

Uncertainty for 4 l s analysis DPF 2015 25

Uncertainty for 4 l s analysis DPF 2015 26

Study the ZZ polarizations in Run 2 new physics involves enhancement of ZLZL In Z rest frame DPF 2015 27