Measurement of the W-boson mass at LHC

Transcription

1 Measurement of the W-boson mass at LHC Student: Marco Cipriani Supervisor: Prof. Shahram Rahatlou Università di Roma La Sapienza e INFN Roma Seminario dei dottorandi del XXXI ciclo - 14/02/2017 1

2 Motivation The Standard Model (SM) of particle physics describes matter in terms of fundamental particles and their interactions Z/W/γ mediate the electroweak (EWK) interaction Z/W experimentally massive spontaneous simmetry breaking to explain their mass m W depends on EWK parameters higher order corrections modify m W main contributions from heavy particles (top quark and Higgs boson) new particles would also contribute precise measurement of m W is a crucial test of the SM 2

3 The Large Hadron Collider (LHC) CMS LHCb ATLAS ALICE Center of mass energy [TeV] Period Data (CMS) [fb -1 ] pp WX (W lν) # events simultaneous pp collisions

4 The Compact Muon Solenoid (CMS) (ECAL) muon electron charged hadron (e.g. π + ) neutral hadron (e.g. neutron) photon we focus on W μν and W eν decay 4

5 Seeing the invisible colliding partons carry a fraction of proton momentum according to parton distribution functions (PDF) initial state longitudinal momentum unknown can still rely on momentum conservation in transverse plane (orthogonal to beam axis) jet neutrinos do not interact with the detector measure their momentum as missing transverse energy E T miss E T miss E T miss = visible particles p T (ECAL) p T : momentum in transverse plane event candidate for Z(νν)+jet 5

6 Ph.D road map 2) search for dark matter (DM) in monojet events (also master thesis) ) Master s degree, ) (ECAL) energy response intercalibration with π 0 /η γγ ongoing 4) Monitoring and optimization of the π 0 /η High Level Trigger (HLT) stream ongoing 5) Search for DM from decays of Higgslike particles produced through vector boson fusion (VBF) early ) Measurement of the W-boson mass 2017, Ph.D project 6

7 Search for dark matter 2) search for dark matter (DM) in monojet events (also master thesis) ) Master s degree, ) (ECAL) energy response intercalibration with π 0 /η γγ ongoing 4) Monitoring and optimization of the π 0 /η High Level Trigger (HLT) stream ongoing 5) Search for DM from decays of Higgslike particles produced through vector boson fusion (VBF) early ) Measurement of the W-boson mass 2017, Ph.D project 7

8 MonoJet and H inv analysis monojet: most sensitive DM search channel VBF topology: rarer but cleaner process Signal: missing transverse energy (E T miss ) due to DM one or more jets from initial state radiation (trigger) Main backgrounds: Z(νν)/W(lν) + jets (irreducible, 95%) performance of recoil measurement E T miss g recoil monojet VBF H inv My contributions: study of E T miss response and resolution development of strategy and data-driven techniques for estimation of background events 8

9 Public results monojet paper will be published soon based on 12.9 fb 13 TeV exclusion limits on DM and mediator masses for (axial-)vector, (pseudo-)scalar mediator limits compared to non-collider searches full dataset analysis (36.4 fb 13 TeV ) also expected to be published (summer timescale) VBF H inv analysis should be presented for publication by next summer based on 36.4 fb 13 TeV exclusion limits on H inv decay branching ratio 9

10 Detector activities 2) search for dark matter (DM) in monojet events (also master thesis) ) Master s degree, ) (ECAL) energy response intercalibration with π 0 /η γγ ongoing 4) Monitoring and optimization of the π 0 /η High Level Trigger (HLT) stream ongoing 5) Search for DM from decays of Higgslike particles produced through vector boson fusion (VBF) early ) Measurement of the W-boson mass 2017, Ph.D project 10

11 ECAL intercalibration with π 0 /η e/γ release most of their energy in ECAL homogeneous calorimeter PbWO 4 crystals accurate calibration fundamental to achieve excellent energy resolution crucial for analysis with e/γ in their final states * constant term dominant at high energy largest contribution from crystal-to-crystal response variation exploit diphoton invariant mass peak from π 0 /η γγ to equalize the energy response among ECAL crystals * as measured in electron test beam 11

12 Thesis project 2) search for dark matter (DM) in monojet events (also master thesis) ) Master s degree, ) (ECAL) energy response intercalibration with π 0 /η γγ ongoing 4) Monitoring and optimization of the π 0 /η High Level Trigger (HLT) stream ongoing 5) Search for DM from decays of Higgslike particles produced through vector boson fusion (VBF) early ) Measurement of the W-boson mass 2017, Ph.D project 12

13 Previous measurements of m W first measurement by UA1 collaboration at Sp ps synchotron (CERN) m W = 81 ± 5 GeV W and Z boson just discovered by UA1 in 1983 Rubbia and Van Der Meer won the Nobel Prize for this discovery (1984) LEP collider (CERN) in e e collisions m W = ± 33 MeV CDF + D0 experiments at Tevatron (Fermilab) in p p collisions (2014) m W = ± 16 MeV ATLAS collaboration at LHC (CERN) in pp collisions (2016) m W = ± 19 MeV current experimental world average (no ATLAS) is m W = ± 15 MeV theoretical indirect estimate from global EWK fit to SM parameters m W = ± 8 MeV experiments must target δm W < 10 MeV 13

14 Measurement of the W-boson mass W μν and W eν decay channels undetected neutrino can t compute m W from the Lorentz 4-vector lepton mass extracted from the Jacobian edge in two kinematic distributions: p T l : clean observable but large theoretical uncertainties (p T W PDF, QCD corrections ) m T W : large experimental uncertainties (neutrino E T miss resolution) E T miss m T W = 2 p T l E T miss 1 cos Δϕ lν ϕ : angle in transverse plane p T l [GeV] m T W [GeV] 14

15 There is more to it than meets the eye Experimental systematic uncertainty: lepton momentum scale muon chambers alignment, ECAL calibration hadronic recoil resolution (p T W 0 in lab frame W recoils against jets) lepton Theoretical systematic uncertainty : incomplete knowledge of PDF dominant uncertainty QCD modelling higher order corrections EWK corrections QED initial and final state radiation of photons from leptons E T miss recoil p T l [GeV] m T W [GeV] 15

16 Cross-check with Z boson Crucial to get precise templates deep understanding of detector performance fine tuning of the Monte Carlo simulations method validation with Z sample measure m Z in a W-like manner by neglecting one lepton (done by 7 TeV) m Z well known tune physics modelling constrain theoretical uncertainty on p T W distribution take Z as standard candle and extrapolate to W (rely on theory) many uncertainties cancel out in the W/Z ratio q l + q l + q Z l q W + ν l treat as neutrino 16

17 Plans and prospects I will work on the completion of all the aspects of the analysis for both W μν and W eν decay channels background modelling W mass fitting PDF modelling and uncertainty I will focus on the introduction and development of the electron channel efficiency, energy scale with Z ee events Expected publication on the measurement by the end of the Ph.D likely more than one many related and important measurement from CMS still missing 17

18 BACKUP 18

19 COMPACT MUON SOLENOID 14/02/2017 Marco Cipriani

20 CMS longitudinal view transverse direction beam direction η = ln tg θ 2 η differences are Lorentz invariant for high energy particles 20

21 Parton distributions functions (PDF) Collisions occur between proton constituents (quarks or gluons) energy fraction carried by each parton distributed according to PDF final state longitudinal momentum unknown can still rely on momentum conservation in transverse plane (orthogonal to beam axis) 21

22 Searching for dark matter (DM) 3 types of searches, with high degree of complementarity indirect DM annihilation to SM particles constraints from cosmological parameters measurements direct DM scattering on nuclei sensitive to m DM 10 GeV colliders DM production in pp collisions rich phenomenology 22

23 Dark Matter at colliders MET+X searches: DM seen as missing transverse energy (MET) X is a Standard Model (SM) particle(s) from initial state radiation (ISR) trigger on the event signal: excess of events in the high MET region monojet E T miss recoil monotop one topology, many final states monophoton monoz Z 23

24 Dark Matter at colliders Other interesting channels: multijet + MET from Susy searches gluinos/squarks decay to quarks and lightest supersymmetric particle t t + MET, b b + MET Higgs + MET Higgs ISR suppressed probe Higgs coupling to DM dijet MET-less, but provides DM interpretation MET + (lots of) jets q Z q q dijet q MET + heavy quarks monohiggs 24

25 Theoretical overview Interpretation with simplified models Dark Matter Forum prescriptions arxiv: benchmark of Run2 interpretation new mediator connecting SM and DM g q Z g DM free parameters: m DM, M med, g DM, g q Assumptions: DM is a Dirac fermion DM produced on-shell in pairs minimal decay width for mediator minimal flavour violation g DM = 1 and g q = 0.25 limits strongly depends on the couplings choice and model change in couplings affects mediator s width more details: arxiv: v1 25

26 Comparison of some channels (g l = 0) dijet search dominates the picture (no DM production but limits on mediator mass) plot from Tristan du Pree, EXO Workshop

27 Comparison of some channels (g l = g q ) Z (ee) most sensitive probe, if allowed. Exclusion (or discovery) potential driven by couplings choice and M med plot from Tristan du Pree, EXO Workshop

28 Comparison of some channels (smaller g l ) Exclusion (or discovery) potential driven by couplings choice and M med plot from Tristan du Pree, EXO Workshop

29 Comparison of some channels (smaller g q and g l ) Monojet dominates when coupling to DM bigger than that to quaks or leptons plot from Tristan du Pree, EXO Workshop

30 Theoretical overview Limits 95% CL in the m DM, M med plane switch to limits on cross section as a function of m DM 90% CL used to compare with direct searches g q Z g DM collider searches limits flat with respect to m DM and sensitive also to low m DM 30

31 ICHEP summary plots monojet most sensitive channel for vector mediator direct searches more sensitive than collider searches for m DM > few GeV CERN-CMS-DP

32 ICHEP summary plots monojet most sensitive channel for axial-vector mediator collider searches more sensitive than direct searches everywhere CERN-CMS-DP

33 MonoJet analysis Signal: missing transverse energy (E T miss ) due to DM one or more jets from initial state radiation trigger on the event Main backgrounds: Z(νν)/W(lν) + jets (irreducible, 95%) Analysis strategy: data driven estimate of main backgrounds five control regions (CR) Z(ll) / W(lν) / γ + jets (l = e,μ) signal from fit to E T miss distribution g E T miss recoil My contributions: study of E T miss response and resolution development of leptons CR photon purity studies for γ+jets CR 33 33

34 Background estimate Z(μμ)+jets Z(νν)+jets Z(ee)+jets γ+jets W(lν)+jets W(μν)+jets W(eν)+jets 34

35 MonoJet/V limits

36 VBF H inv analysis VBF topology as a 2-jets category in the monojet analysis monojet focus on DM mediator coupling only to fermions VBF channel probes coupling to vector bosons Benchmark: VBF Higgs (125) invisible distinctive topology: two high pt forward jets and E T miss signal discriminating variables: Δη j1,j2 * and dijet invariant mass M j1,j2 VBF H(125) Z(νν) (QCD) My contributions: set up the analysis selection optimization control regions fit to extract signal improve analysis strategy (shape analysis of Δη j1,j2, M j1,j2 ) Z(νν) (EWK) * η = ln tg θ, θ is the polar angle wrt the beam axis 2 36

37 VBF H inv properties Discriminating variables: MET does not separate signal and background Δη j1,j2 and M j1,j2 are good variables (correlated) other variables are Δφ(jets,MET) and Δφ(j1,j2) (embed information on boson s spin) VBF H(125) Z(νν) (QCD) Z(νν) (EWK) VBF H(125) Z(νν) (QCD) Z(νν) (EWK) VBF H(125) Z(νν) (QCD) Z(νν) (EWK) VBF H(125) Z(νν) (QCD) Z(νν) (EWK) 37

38 ECAL PbO 4 crystals in the barrel (EB) 7324 for each endcap (EE) EB: 36 supermodules with 4 modules for each 360-fold granularity in φ 85-fold granularity in η > 0 EE: crystals arranged in 5x5 towers ES (preshower): enhance γ identification capabilities 38

39 ECAL intercalibration with π 0 /η e/γ release most of their energy in ECAL homogeneous calorimeter PbWO 4 crystals accurate calibration fundamental to achieve excellent energy resolution crucial for analysis with e/γ in their final states * constant term dominant at high energy largest contribution from crystal-to-crystal response variation ECAL crystal response variation e, γ electromagnetic shower 39 energy * as measured in electron test beam

40 ECAL intercalibration with π 0 /η exploit diphoton invariant mass peak from π 0 /η γγ to equalize the energy response among ECAL crystals CMS γ reconstruction optimized for high p T physics ( 60 GeV), while typical p T (π 0 /η γγ ) is only few GeV select calibration sample with dedicated HLT stream ad hoc clustering algorithm (3x3 crystal matrix) significantly reduce event size allow for higher trigger rate (15 khz) larger calibration sample with respect to Z bosons allow for calibration in early stage of data taking need at least 3000 γ/crystal to achieve statistical uncertainty < 0.5% in the barrel 40

41 Cross-check with Z boson Z ll events used for detector calibration method validation with Z sample measure m Z in a W-like manner by neglecting one lepton (done by 7 TeV) m Z well known tune physics modelling constrain theoretical uncertainty on p T W distribution take Z as standard candle and extrapolate to W (rely on theory) many uncertainties cancel out in the W/Z ratio CAVEAT: different initial state (PDF), background, Z W extrapolation uncertainty differential cross section W/Z cross section ratio 41

42 There is more to it than meets the eye Strategy: 1. build simulated templates for several m W values (1-10 MeV steps) 2. perform compatibility test with observed distribution 3. extract m W minimizing the test statistics Crucial to get precise templates deep understanding of detector performance fine tuning of the Monte Carlo simulations Systematic uncertainties Experimental: lepton momentum scale and resolution muon chambers alignment, ECAL calibration hadronic recoil resolution (p T W 0 in lab frame W recoils against jets) Theoretical: incomplete knowledge of PDF dominant uncertainty QCD modelling parton shower, missing higher orders in the perturbative expansion of the strong coupling constant α S (m Z ) EWK corrections QED initial and final state radiation of photons from leptons 42

43 Systematic uncertainties Experimental: lepton momentum scale and resolution muon chambers alignment, ECAL calibration hadronic recoil resolution (p T W 0 in lab frame W recoils against jets) Theoretical: incomplete knowledge of PDF dominant uncertainty QCD modelling parton shower, missing higher orders in the perturbative expansion of the strong coupling constant α S (m Z ) EWK corrections QED initial and final state radiation of photons from leptons Example: tracker alignment affect muon momentum scale p T [GeV] = 0.3 R[m] B[T] B: magnetic field R: curvature radius 43

44 W production asymmetry 44