Precise measurement of W mass at LHC with the CMS detector M. Bachtis*, M.R. d Alfonso, L. Perrozzi +GR+ CERN O. Cerri, N. Foppiani, E. Manca, A. Stacchiotti Students @ Scuola Normale - Pisa * now UCLA 1
arxiv:1608.01509 GeV
Measurement of the TOP quark mass Classical method 3
Top mass Alternative methods 4
Past (and present) W mass measurements New ATLAS Measurement december 2016 mw = 80.370 ± 19 MeV ATLAS Gigi s Breakdown 9-14 2-13 6-12 8-9 8-10 5-3 7 7ATLAS Numbers 11 14 5
Observable sensitive to Mw : muon transverse momentum Normalized entries / 0.5 GeV Mean 10.56 ± 0.01145 0.03 0.025 0.02 0.015 0.01 W Pt spectrum Std Dev 7.433 ± 0.008099 0.005 0 5 10 15 20 25 30 W Pt [GeV] Measure μ pt : you need the prediction of the W pt distribution to measure the W mass. 6
How to extract the W mass information from a distribution? 1- Compare one measured distribution (eg pt of the muon) with several simulated distributions generated with different mass assumptions and compute the likelihood ratio. 100 MeV This technique implies a perfect mastering of the simulation : W production and measurement of the event properties (muon, recoil) 7
Transverse mass: in principle a more robuste observable In these formulae p== pt PT ηcm sensitive to production and decay dynamics The jacobian edge preserves the mass information 8
Small distortions due to ISR and muon resolution Plot done with ideal measurement of the hadronic recoil 9
. however it is difficult to measure precisely the hadronic recoil! With a resolution of 7 GeV the edge is lost. Still very small dependence on the Wpt distribution Resolution in CDF ~6-8 GeV Resolution in ATLAS ~ 13 GeV 10
W mass @ CMS muon pt ~ 39 GeV CERN Sep. 2013 gigi.rolandi@cern.ch 11 /55
All silicon tracker ~ 1500 pixel modules and 15000 strip modules providing ~ 14 measurements ~ 10-50 μm per track in a magnetic field of 3.8 T 12
Tracker Resolution 13
W mass measurement strategy Calibration of the muon momentum scale J/Psi - Y samples Calibration of the hadronic recoil Z samples Precise definition of the production model Ancillary measurements 14
CMS Statistics on tape Lumi (1/)) Pileup J/Psi Y Z W+ W- Million Events 7 TeV 5 7 3.5 1 1.4 13 9 8 TeV 22 22 5.3 3 6.2 61 45 13 TeV 4+35 13+24 122 49 39 ~350 ~250 Using 8 TeV only the statistical error is <2 MeV 15
Calibration of the momentum scale Tracker in a very uniform magnetic field Map measured when CMS was the surface assembly hall. 2D approx. map used in the reconstruction Momentum biassed by Energy Loss How well do we know the tracker material? The 16500 modules of the tracker are aligned with collision data and cosmic rays. Local precision 2 μm thanks to overlaps of nearby modules. Alignment has weak modes - geometry is prone to global scale deformations K=curvature=1/p Kc=AK+ε sinθ K 2 + qm B field and radial length material effects transverse alignment 10 η bins 20 η bins 10 η bins 16
Fitting dimuon resonances: kalman filter with target mass mc = dimuon generator mass after FSR Michalis Bachtis Magnetic field in data about 0.05 % higher than in simulation expected because of more iron in the cavern x 500 Radial deformations < 0.1% level 17
Transverse alignment terms ~ few 10-5 GeV -1 Material effects < 10 MeV Calibration performed only with J/ψ and Υ Z used in the closure plots 18
Fit correlations 70% correlation between material terms and B-field terms 19
In situ measurement of the Material Budget Using ~ 1-2 GeV tracks compute local sagitta s using measurements in three consecutive layers Elisabetta Manca The variance measures the multiple scattering 2 out of the 39 measured tracker layers 100 s of data taking 20
Calibration of the recoil + Montecarlo study : which particles to use to measure the W recoil? At fix muon momentum plot the component of the recoil parallel to the muon direction 21
Oxford Jan 2017 gigi.rolandi@cern.ch 22
Hadronic recoil, definition and calibration Ideal detector, no pileup tracks + clusters Maria Rosaria d Alfonso Real detector, pileup tracks+clusters Ideal detector, no pileup only charged tracks Real detector, pileup only charged tracks from muon vertex 23
Hadronic recoil calibrated with Z μμ events Good closure of the recoil 24
Hadronic recoil calibrated with Z μμ events Response ~ 50% Resolution ~ 10 GeV Tracks Response CDF ~ 65%, ATLAS ~ 90% 25
March 2016 26
Measurement of the Z mass as W CMS-SMP-14-007 Production model addressed reweighing the Z MC to Z data: This measurement is just a test of the experimental calibrations 27
180 k Z, stat 36 MeV, muons scale 13 MeV, recoil 10 MeV CDF 620 k W, stat 16 MeV, muons scale 7 MeV, recoil 8 MeV 28
More on recoil Exploiting correlations Nicolo Foppiani Olmo Cerri Different muon pt bins 29
1st results on MC resolution from 10 GeV > 8 GeV 30
Modeling W production 31
Effect of the PDF uncertainties on W mass PT pt>30 GeV ~ ηcm <0.7 ηcm YW PT cut ηcm ηl At a given ηl you sample different Y corresponding to different pt changing average pt. And also W polarization effects 32
PDF Effects At LHC, differently from Tevatron, W + and W - have different cross sections and have ~ equal rate because at leading order the s and bar(s) content of the proton is the same. Subtracting W+ and W- distributions selects valence sea combinations and the PDF s of c and s quarks enter only at Cabibbo suppressed level. ATLAS 33
PDF Mitigation Fitting transverse mass in different η bins helps in reducing PDF systematic error. Fit the μ pt-η rate with fine binning together with the W mass 34
W production modeling 35
Angular coefficients ATLAS use fixed order NNLO prediction validating the model with Z measurement resulting in 5 MeV error. Perform the measurement at low pt? 36
V Transverse Momentum Distribution Fit prediction to Z data and apply to W ATLAS considered Powheg+Pytia8 and Pytia8 standalone After tuning 1% agreement primordial kt αisr ISR cut-off Exported to W 37
Collinear gluon emission, factorization scale and quark masses Gluon radiation has a cutoff depending on the quark mass Depending how you vary the scale (W/Z correlations), you get different results 38
Resummation codes predict an harder pt W spectrum for a given measured pt Z spectrum 39
Conclusions CMS has calibrated the muon momentum scale to 0.02 % and there is room for further improvements Measuring the W pt using charged tracks is possible and reasonably good resolution can be achieved. This allows to exploit the large luminosity collected at high pileup The (non) agreement of the Z (W) pt spectrum with more advanced calculations based on resummation must be understood We welcome very much the ATLAS measurement 40