Valerio Ippolito. Dottorato di Ricerca in Fisica - XXVI ciclo Sapienza Università di Roma

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1 Measurement of the properties of the new particle observed within the search for the Standard Model Higgs boson in the H ZZ (*) 4l decay channel at ATLAS Valerio Ippolito Dottorato di Ricerca in Fisica - XXVI ciclo Sapienza Università di Roma

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3 Status in ~October 2011 O Higgs boson, where art thou?? 3

4 2011 pp collisions at 7 TeV 2012 pp collisions at 8 TeV y pt 2 =px 2 +py 2 η=-2lnθ z x

5 Phys. Lett. B 726 (Jul, 2013) Phys. Lett. B 716 (Aug, 2012) 1 29 The Golden Channel charged leptons give the cleanest signatures: pp H ZZ(*) 4ℓ[ℓ=e,μ] ℓ- [mh= GeV] * low cross section: 2-5 fb (~106 times less probable than pp Z ℓ+ℓ-) ℓ+ Z H * low background contamination Z ℓ+ from other processes faking signal S/B ~ 1.4 e µ * fully reconstructed final state unlike H WW(*) ℓνℓν ℓ- * if mh < 2mZ, only one Z boson is on-shell e let s call it Z1 (its mass is m1) we label final states like e.g. 2µ2e (Z1 µµ, Z2 ee) Valerio Ippolito - PhD Seminar (November 29th, 2013) µ 5

6 Backgrounds irreducible background l + * pp ZZ (*) 4l [MC] * same final state as signal l - p Z p * dominant at high mh Z l - l + reducible backgrounds l + l - * Z+bb, Z+jets, tt [data, MC] * relevant contribution at low mh * rejection: ask leptons to be isolated and compatible with the primary interaction p jet jet p * estimated from data [MC modeling is hard] Z l - l + 6

7 How to Improve Significance? signal evidence is a peak in the four-lepton invariant mass distribution * the narrower the peak, the better the sensitivity improve resolution with Z mass constraint fit lepton momenta are corrected to obtain m2l=mz true, where! mass resolution Low Mass intrinsic width 7

8 Z Mass Constraint Fit O(10%) improvement in resolution need 5% less luminosity to obtain the same significance channel (125 GeV) σ [GeV] σ constr [GeV] 4µ µ2e e2µ e H 4µ H 4e 8

9 How to Improve Significance? * the lower the backgrounds, the higher the sensitivity keep the reducible background under control in high pile-up regime efficiency isolation efficiency must be as pile-up independent as possible sig, topoetcone20/pt < 0.2 conv+had, topoetcone20/pt < 0.2 sig, topoetcone20/pt < 0.25 conv+had, topoetcone20/pt < 0.25 sig, topoetcone20/pt < 0.3 conv+had, topoetcone20/pt < 0.3 high instantaneous luminosity (up to cm -2 s -1 ) * is a resource (obtain results earlier) * is a challenge (up to 40 additional interactions) electron efficiency where s the Z μμ event? sig, topoetcone20/pt < 0.2 conv+had, topoetcone20/pt < 0.2 sig, topoetcone20/pt < 0.25 conv+had, topoetcone20/pt < 0.25 sig, topoetcone20/pt < 0.3 conv+had, topoetcone20/pt < electron n VX 9

10 What did we Find? is it the SM Higgs boson? 6.6σ excess mass measurement [David Moir/Reuters] q q Z l l l l combination with other channels µ = events observed events expected 10

11 Why Spin-Parity? Phys. Lett. B 726 (Jul, 2013) ATLAS-CONF the SM requires H to be a parity-even scalar (0 + ) * * * it could be a J=1 state (and H γγ would be a different particle) it could be a graviton-like J=2 state it could be a CP-even/odd admixture we can relate what we measure and what we want to know (p1,p2,p3,p4) = f [A(H ZZ)] l - 8 unambiguous well-measured DOF l + Z Z H l + expressed in terms of m4l, m1, m2 and angles defined in the final state l - 11

12 Strategy we write the most general Lorentz-invariant decay amplitude A(H ZZ) e.g.: for J=0 SM Higgs pseudoscalar we relate it to the differential mass and angular distribution phase space + propagator d J (m 1,m 2, ) dm 1 dm 2 d = P (m 1,m 2 ) X / i K i (m 1,m 2 )f i ( ) J=0: three helicity combinations (A++,A--,A00) Ki = A++ 2,Re(A++A00 * ), Im(A++A00 * )... (9 terms) 12

13 The J P -MELA Discriminant collapse all final state DOF on a single real number J P -MELA 0 for alternative hypothesis, 1 for SM Higgs J P -MELA(0 + vs J P ) = p(h):=p(data H) p(0 + ) p(0 + ) + p(j P ) red: 0+ blue: 0- p = THEORY RECONSTRUCTION EFFECTS obtain typical JP-MELA distributions computing the discriminant on H0, H1, bkg events 13

14 Hypothesis Testing Results use distribution of log[l(h0)/l(h1)] sampled on pseudo-events to build a test statistics J=2 production mechanism unknown fqq s+b b exclusion given w.r.t. 0 + with CLs=CLs+b/CLb method >95% excluded by 4l+γγ+WW 14

15 ATL-PHYS-PUB Can we Say More? let s take again the most general H ZZ decay amplitude yes, if we assume J=0 in the SM: 2i O(10-2 ) (one loop diagrams) (suppressed by scale 2 of NP) 0 (three loops diagrams) non-zero g2, g4 affect final state distributions * CP even/odd admixture present if g4 and g1 are both non-zero can hint to CP violation (e.g. mixing between multiple Higgs particles) which might explain matter/antimatter asymmetry (excluded) pure pseudoscalar state corresponds to the limit g4/g1 * new physics could contribute in loops giving g2 0 15

16 Prospects with High Luminosity determine values of complex couplings gi CRUCIAL! * developed an 8D fitting technique to measure gi/g1 * correct d 7 Γ/(dm1dm2dΩ) for reconstruction effects * parametrize m4l and backgrounds from simulation * results cross-checked with a 2D J P -MELA-like method sensitivity to CP-even/odd admixtures CMS exp. 25 fb -1 ~fg4 ATLAS exp. 300 fb -1 ATLAS exp fb -1 need high statistics to probe the interesting region <fcp> << 1 studied for High Luminosity LHC upgrade (>2020) 16

17 Conclusions discovery looking for a low mass Higgs boson how we improved sensitivity: resolution, isolation optimization a new particle has been found is it the Standard Model Higgs boson? J P -MELA discriminant: exploit final state kinematics spin-parity studies: excluded 0-, 1+, 1-, 2+ against SM enough for 2013 Nobel Prize in Physics! is it really the Standard Model Higgs boson? probing the HZZ vertex: 8D and 2D matrix-element techniques projections for high luminosity (300/3000 fb -1 ): sensitive to 6-20% CPV fraction 17

18 Relevant Publications 8 papers Phys. Lett. B 726 (Jul, 2013) Phys. Lett. B 726 (Jul, 2013) Science 338 (Jan, 2013) Phys. Lett. B 716 (Aug, 2012) 1 29 pre-discovery: Phys. Rev. D 86 (Jul, 2012) , Phys. Lett. B 717 (Jun, 2012) 7088, Phys. Lett. B 710 (Feb, 2012) , Phys. Lett. B 710 (Feb, 2012) conference notes HL-LHC perspectives: ATL-PHYS-PUB internal supporting notes 18