Electroweak precision physics at the LHC. Stefano Camarda

Transcription

1 Electroweak precision physics at the LHC Research Progress Meetings LBNL 21 Feb 2017

2 Electroweak precision physics at the LHC Introduction Global fit of electroweak observables W-boson mass measurement Weak-mixing angle and W-boson width Summary and prospects 2

3 Standard model at the LHC The SM of particle physics has been very successful in predicting the observed rates of particle production processes 3

4 Open questions Despite its success, the SM cannot answer several open questions in particle physics: Dark matter, dark energy? Baryon asymmetry? Strong CP problem? Is the Higgs mass fine tuned? Are these the right questions? 4

5 How to answer these questions at the LHC? arxiv: Direct searches: look for bumps in invariant mass distributions arxiv: Enhanced cross sections typically in high pt corners of the phase space Indirect searches: look for deviations from SM predictions due to quantum loop effects of new virtual particles 5

6 Precision measurements of EW parameters In the recent past, the global electroweak fit was able to predict the masses of the top quark and Higgs boson before their discovery After the measurement of the Higgs mass, all the free parameters of the Standard Model are known Relations between electroweak observables can be predicted at 2-loop level Precise measurements of the electroweak parameters allow Stringent test of the self consistency of the SM Looking for hints of physics beyond the SM Eur.Phys.J. C74 (2014)

7 Measurements of SM parameters at the LHC Various SM parameters can be measured at the LHC with a precision competitive with previous determinations mh ± 0.24 GeV (ATLAS+CMS) arxiv: mt ± 0.70 GeV (ATLAS) ± 0.49 GeV (CMS) arxiv: arxiv: Uniquely measured at the LHC Comparable with Tevatron precision as(mz) arxiv: Currently dominated by ± (CMS jets) ± (CMS tt) large theory uncertainty arxiv: ± (ATLAS TEEC) arxiv: (MHO, PDFs) mw ± GeV (ATLAS) sin2qw ± (ATLAS) ± (CMS) ± (LHCb) GW 2144 ± 62 MeV (CMS) arxiv: arxiv: arxiv: Higgs QCD Competing with Tevatron precision Not yet competitive with LEP and SLD Electroweak arxiv: arxiv: From W/Z cross section ratio Focus of this talk 7

8 Electroweak sector The electroweak gauge sector of the Standard Model is constrained by three precisely measured parameters At tree level, other EW parameters can be expressed as 8

9 Electroweak sector The electroweak gauge sector of the Standard Model is constrained by three precisely measured parameters At tree level, other EW parameters can be expressed as Higher order corrections modify these relations, and determine sensitivity to other particle masses and couplings 9

10 Relation between top, Higgs and W masses Radiative corrections Dr to mw are dominated by top-quark and Higgs loops The relation between mt, mh and mw provides a stringent test of the SM Chin. Phys. C, 40, (2016) The comparison between the measured mh and the predicted mh is sensitive to new physics arxiv:

11 Relation between top, Higgs and W masses (*) arxiv: The measurements of the Higgs and topquark masses are currently more precise than their indirect determination from the global fit of the electroweak observables Indirect determination of mw (±8 MeV) is more precise than the experimental measurement Improving precision will not increase sensitivity to new physics Call for dmwexp < 10 MeV The W mass is nowadays the crucial measurement to improve the sensitivity of the global EW fits to new physics 11

12 Measurements of top, Higgs, and W masses Many more mt and mh measurements in recent years than mw (*) SM predictions from arxiv:

13 W-boson mass history 1983 CERN SPS W discovery 1983 UA1 mw = 81 ± 5 GeV 1992 UA2 (with mz from LEP) mw = ± 0.37 GeV 2013 LEP combined mw = ± GeV 2013 Tevatron combined mw = ± GeV Only four W-boson mass measurements in the last 7 years Complex measurements which require O(5-7) years 2017 LHC (ATLAS) mw = ± GeV 13

14 Experimental measurements at colliders The W-boson mass can be measured from: Kinematic properties of decay leptons in the final state in pp W ln processes (hadron colliders) Direct reconstruction from the final state in ee WW qqqq/qqln (e+e- colliders) SPS, Tevatron, LHC LEP best measurements W-pair production at thresholds (e+e- colliders) Limited by statistics at LEP, but most precise prospect at future colliders 14

15 W mass at the LHC A proton-proton collider is the most challenging enviroment to measure mw, worse compared to e+e- and proton-antiproton In pp collisions W bosons are mostly produced in the same helicity state Further QCD complications In pp collisions they are equally distributed between positive and negative helicity states Heavy-flavour-initiated processes W+, W- and Z are produced by different light flavour fractions Larger gluon-induced W production Large PDF-induced W-polarisation uncertainty affecting the pt lepton distribution Larger Z samples, available for detector calibration given the precisely known Z mass most of the measurement is then the transfer from Z to W 15

16 W mass at the LHC The measurement of mw at the LHC is extremely challenging and prone to biases due to QCD effects They affect all aspects of the measurement: detector calibration, transfer from Z to W, PDF uncertainties, W polarisation, modelling of pt W Need to design the measurement to be waterproof from the point of view of detector calibration and physics modelling At the same time, the challenge makes it very interesting, and provides a great occasion to understand QCD 16

17 ATLAS W mass arxiv: Main signature: final state prompt and isolated lepton (electron or muon) The neutrino escapes detection, and its momentum can be reconstructed from momentum imbalance in the transverse plane: ptmiss The transverse mass mt is defined from variables measured in the transverse plane Observables sensitive to mw are Lepton transverse momentum W transverse mass Neutrino transverse momentum (from hadronic recoil) used only as cross-check 17

18 ATLAS W mass Measurement strategy mw extracted from the pt lepton and transverse mass (mt) distributions pt lepton has a Jacobian edge at mw/2 mt has a Jacobian edge at mw Template-fit approach: Vary the W-boson mass values in the theory prediction, and predict the pt lepton and mt distributions Compare to data, and determine the W mass by c2 minimization 18

19 ATLAS W mass Measurement overview Physics modelling Calibration Z-boson cross checks Background Combination 19

20 ATLAS W mass Measurement overview Physics modelling Build the physics modelling by supplementing the MC samples with higher-order corrections and fits to DY ancillary measurements Calibration Use Z ll events to calibrate the detector response to the energy scales and resolutions of the leptons and of the recoil Z-boson cross checks Validate the physics modelling and the calibration by extracting mz from pt lepton and mt in the Z sample Background Estimate and subtract the backgrounds in the W sample Combination Extract mw in several categories and combine. The categorisation validates the detector calibration and physics modelling and improves the accuracy 20

21 Physics modeling Breit-Wigner NNLO pqcd Parton Shower 21

22 Physics modeling pt W The Pythia8 pt-ordered parton shower is used as model for the pt W The parameters of the model are fit to the pt Z measurement at 7 TeV (AZ tune) The Pythia8 AZ tune describe the pt Z data within 2% inclusively and in rapidity bins Pythia8 is used to transfer from the pt Z to the pt W distribution and to evaluate theory uncertainties on the W/Z pt ratio 22

23 Uncertainties in the pt W modeling Heavy-flavour-initiated (HFI) production introduce differences between Z and W production HFI production determines a harder boson pt spectrum, cc Z and bb Z are 6% and 3% of Z production, cs W is ~20% of W production pt W theory uncertainties are evaluated as the sum of experimental Z pt unc. and theory unc. on the W/Z pt ratio HFI addressed with charm-quark mass variations, and by decorrelating the PS mf between light and HFI processes Central prediction and uncertainty validated with the recoil distribution when using the data to constrain the model we end up with compatible central value and similar uncertainties 23

24 Alternative higher order models for p T W Since the pt Z distribution is very well measured, the relevant theoretical uncertainties are those which affect the W/Z pt distribution Only Herwig, Pythia, and Powheg predict a monotonic falling W/Z pt ratio MINLO and NNLL analytic resummed predictions as Resbos, Cute, and DyRes are strongly disfavoured by the recoil distribution in data 24

25 Rapidity distributions Rapidity distributions are modeled with NNLO predictions, and the CT10nnlo PDF set, which provides good agreement with data thanks to its milder strangeness suppresion. CT14 and MMHT considered as uncertainty, other PDF sets excluded by data Strong indication of unsuppressed strangeness from the W, Z inclusive cross section measurement s +s R s= u + d 25

26 Physics modelling angular coefficients A i The DY cross section can be reorganised by factorising the dynamic of the boson production, and the kinematic of the boson decay Pi (cos θ, φ) are spherical harmonics. In the assumption of spin 1 of the boson and spin ½ of the fermions, the 9 harmonics of order 0, 1, and 2 provide a complete decomposition Angular coefficients are modelled with fixed order perturbative QCD at NNLO Ai predictions are validated by comparisons to the Z measurement at 8 TeV 26

27 Physics modelling Summary of QCD uncertainties PDFs are the dominant uncertainty, followed by pt W uncertainty due to heavy-flavour-initiated production PDF uncertainties are partially anti-correlated between W+ and W-, and significantly reduced by the combination of these two categories. pt W uncertainties are similar for mw extracted from pt lepton and from mt 27

28 ATLAS detector calibration Object reconstruction: Muons (Inner Detector, Muon Spectrometer) Electrons (Inner Detector, EM calorimeter) Recoil (full calorimeter system) 28

29 Muon calibration Muon identification using combined ID+MS tracks Momentum measurement from ID only simplifies calibration, some loss in resolution Parameterisation of momentum corrections: a: radial bias (scale) d: sagitta bias b: resolution correction Charge dependent corrections Scale and resolution corrections derived form Z mm line shape, sagitta bias also from E/p in W en 29

30 Muon calibration Z mm line shape well modelled after corrections Momentum scale is the dominant uncertainty 30

31 Recoil calibration The recoil ut is the vector sum of the transverse energy of all the calorimeter clusters: ut is a measure of pt W Calibration steps: Correct pile-up multiplicity in MC to match the data Correct for residual differences in the SET distribution Derive scale and resolution corrections from the pt balance in Z events 31

32 Recoil calibration ut distribution well modelled after corrections SET correction is the dominant uncertainty 32

33 ATLAS W mass Results I am really in perfect shape

34 Compatibility of categories All categories give consistent extractions of mw Strong validation of physics modelling and detector calibration 34

35 W mass measurement result The ATLAS result equals in precision the previous single-experiment best measurement of CDF MW = ± 18.5 MeV MW = ± 6.8 (stat) ± 10.6 (exp.syst.) ± 13.6 (model.syst.) MeV The dominant uncertainty is due to the physics modelling and the largest contributions are from QCD/PDF 35

36 ATLAS W mass result The tension between measurement and SM prediction is reduced with respect to the previous Tevatron results arxiv: The consistency of the standard model is confirmed 36

37 Weak-mixing angle W width 37

38 2 Z forward-backward asymmetry and sin qw The Drell-Yan production cross section is a function of the scattering angle q Linear term in cos(q) due to Z/g* and V-A interference The linear term in cos(q) gives rise to a nonvanishing forward-backward asymmetry The V-A interference contribution is proportional to gv ga, and depends on the weak mixing angle qw The Z/g* interference contribution is proportional to (s - mz2) AFB changes sign at the Z pole 38

39 Z forward-backward asymmetry and qw The orientation of the incoming quark is unknown Asymmetry as a function of boson y CMS ATLAS CC In pp collisions, it is more likely to be in the same orientation as the Z boson, due to the u/u and d/d valence asymmetry ATLAS CC+CF LHCb Use q* scattering angle defined in the Collins-Soper frame, with z-axis orientation defined by the Z rapidity At the LHC the colliding proton beams are FB symmetric the asymmetry vanishes at central rapidity, and grows as a function of rapidity What we actually observe in the detector is a Forward-Central asymmetry, which is converted to a FB asymmetry by the choice of the z-axis 39

40 Measurement of weak-mixing angle qw ATLAS and LHCb: qw extracted from template fits to Z AFB as a function of dilepton invariant mass mll CMS: multivariate likelihood technique, qw extracted from mll, cos(qw), yll sin2(qweff) ATLAS 7 TeV 4.8 fb ± (stat) ± (syst) arxiv: CMS 7 TeV 1.1 fb ± (stat) ± (syst) Phys. Rev. D 84, (2011) LHCb 7/8 TeV 1/2 fb ± (stat) ± (syst) ± (theory) LEP+SLD ± arxiv: arxiv:hep-ex/ Still 10 times worse than LEP+SLD Measurements affected by significant PDF uncertainty 40

41 W width The W width GW can be measured directly from the tail of the mt distribution, or indirectly from the W/Z inclusive cross section ratio S.C et al. arxiv: Assuming the SM predictions of the branching ratios and of the CKM matrix, and using the LEP measurement of GZ, GW can be extracted from the ratio A Tevatron+LHC combination yields In agreement with the combination of direct measurements at LEP and Tevatron Chin. Phys. C, 40, (2016) 41

42 Prospects Future measurements of W mass at the LHC W mass at future colliders Weak-mixing angle at the LHC 42

43 W mass at the LHC The statistical uncertainty is expected to be reduced by factors of 2 to 7 by analysing 8 and 13 TeV datasets sqrt(s) 7 TeV 8 TeV TeV -1 Lumi ~4.5 fb Events 15x x x10-6 Stat Unc.[MeV] Expected Expected Measured ~20 fb ~100 fb PDF uncertainties will be reduced by the inclusion of the latest HERA I+II and W asymmetry data in the global PDF fits (expected 30% reduction) EW uncertainties can be largely reduced by including available HO corrections PT W can be reduced by using analytic resummation at NNLL (if calculations will improve the agreement with the data) Muon calibration can be improved using J/psi 43

44 W mass at the LHC [MeV] Stat Muon Electron Recoil Background QCD EW PDF Total 7 TeV 6.8 (measured) TeV (expected) Realistic target for the next round with 8 TeV data: MeV 44

45 W mass at future colliders The ultimate precision on mw can be achieved at e+e- colliders through an energy scan of the WW production threshold Near threshold, the WW cross section is proportional to the non-relativistic W velocity arxiv: Phys.Rept. 532 (2013) ILC Giga-Z program Energy scan 160 to 170 GeV dmw = 6-7 MeV LC simulated data JHEP 1401 (2014) 164 TLEP OkuW program dmw = 0.5 MeV dominated by statistical uncertainty Dominant theory uncertainties Initial state QED corrections Parametrization of cross section near threshold Physics at LHC and beyond LC-PHSM

46 Weak mixing angle Current measurements are limited by statistical and PDF uncertainties The statistical uncertainty is expected to be reduced by factors of 2 to 6 by analysing 8 and 13 TeV datasets sqrt(s) 7 TeV 8 TeV 13 TeV Lumi ~4.5 fb-1 ~20 fb-1 ~100 fb-1 Events 2x106 10x106 80x106 Stat Unc.[10-5] PDF uncertainties will be reduced by the inclusion of the latest HERA I+II and W asymmetry data in the global PDF fits PDF can be further reduced with multi-differential measurements in m and y Realistic target: 30 [10-5] Physics at LHC and beyond 46

47 Weak mixing angle Asymmetry as a function of boson y The maximum sensitivity to the FB asymmetry is at y ~ 3-4 The measurement will benefit from improved tracking in the forward region Run muons HL-LHC upgrade For the HL-LHC (2026) ATLAS and CMS will replace the inner detectors and likely extend the coverage in rapidity Current coverage Improvements will depend on the balance between increased sensitivity and worse resolution in the forward region Physics at LHC and beyond 47

48 Summary Precise measurements of electroweak observables performed at the LHC are sensitive to new physics, and may provide guidance for the next big discovery The LHC has delivered the first measurement of the W mass, a milestone in its physics programme. The new measurement reaches the precision of CDF and is now the world leading measurement Electroweak measurements at the LHC provide an opportunity to understand QCD and EW physics at a very deep level. In these measurements the SM is not a background, but a complex and precise model which is used to interpret the data 48

49 BACKUP 49

50 W mass at the LHC A large fraction of W production at the LHC is inititiated by sea quarks The W polarisation at the LHC is more influenced by PDF uncertainties, implying larger uncertainties on the lepton pt distribution The valence-sea difference, as well as the amount of sea quarks with u and d flavour, must be known with better precision than needed at the Tevatron The effect can be isolated by switching off spin correlations O(10-20) MeV effect for mw extracted from the pt lepton distribution Large reduction of PDF uncertainties near the Jacobian peak ATL-PHYS-PUB

51 W mass measurement definitions The recoil is the vector sum of the transverse energy of all the calorimeter clusters: ut is a measure of pt W u and uperp are the parallel and perpendicular projections of the recoil on the charged lepton (W events) or on the dilepton pt (Z events) pt nu is inferred from the momentum imbalance in the transverse plane The transverse mt is defined from variables measured in the transverse plane 51

52 W mass electroweak corrections QED FSR: dominant correction, included in the MC with PHOTOS, uncertainty from comparison with YFS. QED ISR also included Running widths (and running of a for Z) included in the BW parametrisation NLO electroweak: pure weak corrections and ISR-FSR interference, estimated with WINHAC. QCD ISR included to predict a realistic pt W distribution (at Tevatron it was evaluated at pt W = 0). Estimated and added as uncertainty FSR lepton pair production g* ll : formally higher order (NNLO), but significant correction. Estimated and added as uncertainty 52

53 pt W uncertainties on pt lepton and mt pt W uncertainties are similar for mw extracted from pt lepton and from mt mt is less sensitive to pt W, but pt W variations on mt are less distinguishable from mw variations 53

54 Electron calibration Electron measurement: energy from the EM calorimeter, h, f from the ID Scale and resolution corrections derived from the Z ee line shape f-dependent corrections are important for the Z to W extrapolation The ptmiss requirement, which is only used for W events, induces a f asymmetry in the selected W events distribution 54

55 Electron calibration Z ee line shape well modelled after corrections Energy scale is the dominant uncertainty 55

56 Z-boson mass fits The physics modelling and the detector calibration are validated by performing an extraction of mz The extraction is a closure test, and not a measurement of m Z, because the LEP measurement is used as input for detector calibration 56

57 W mass multijet background Novel technique for the multijet background estimation The multijet background is determined with template fits, and by extrapolation of the lepton isolation to the signal region Both normalisation and shape are extrapolated 57

58 Measurement strategy categories A crucial aspect of the measurement design is the categorisation. Events are categorised according to their type and kinematic range. The importance of categories is twofold: validate detector calibration and physics modelling and improve accuracy The various set of categories are sensitive to different experimental and theoretical biases, the consistency of mw across categories validates our knowledge of the detector and of QCD The measurement was considered ready for unblinding only when all the categories yield consistent values of mw The experimental and theoretical uncertainties have different correlation or anticorrelation patterns, the categorisation allows to constrain them, and increase the sensitivity to mw Categories used for the combination (28 in total): 58

59 W mass categories pt lepton is very sensitive to ptw modelling, polarisation, PDFs, mt is less sensitive to these effects Biases in the QCD modelling would produce discrepancies between pt lepton and mt determinations of mw W+ and W- have different helicity states, and are produced by different quark flavours in the initial state. Charm-initiated production is relatively larger for W h lepton bins are sensitive to PDFs Biases in the modelling of the W polarisation or HFI production would produce discrepancies between W+ and W- determinations of mw Some of the PDF uncertainties are anticorrelated between W+ and W-, and in h bins. The combination reduce PDF uncertainties 59

60 W mass categories for cross check Recoil bins (ut) Validate pt W modelling and recoil calibration Upar bins Validate pt W modelling 60

61 Control plots - electrons 61

62 Control plots - muons 62

63 Measurement categories 63

64 Post fit plots - electrons 64

65 Post fit plots - muons 65

66 Overall modelling quality The physics modelling and the detector calibration are validated with data/theory comparison plots for 6 observables and 14 categories The distribution of the c2 probabilities for the 84 control and post-fit distributions considered in the measurement is flat 66

67 Differences between PS and resummation F. Tackman, Mainz Feb

68 MSSM constraints from the W mass measurement 68

69 Z forward-backward asymmetry and qw Z/g* interference V-A interference 69

70 Tevatron weak-mixing angle Tevatron combination of sin2qw reached a precision comparable to that of LEP+SLD Tevatron analyses correct for differences between leptonic and hadronic effective mixing angles with the EW form factors k Allows a direct comparison with sin2qweff leptonic measured at LEP and SLD These corrections are of the order of 10-4 and will be needed for the precision of future measurements at the LHC The results are also interpreted in terms of the on-shell sin2qw = 1 m2w/m2z SM-dependent interpretation, assuming the measured values of mt and mh 70

71 LHCb weak-mixing angle 71

72 W width from direct measurements Chin. Phys. C, 40, (2016) 72