[email protected] Oxford Jan 2017 1
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
arXi
v:16
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[email protected] Oxford Jan 2017
5
Past (and present) W mass measurements
mW = 80.370 ± 19 MeV
New ATLAS Measurement december 2016
8-98-105-3
9-14 2-13
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ATLAS
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ATLA
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Measure μ pT : you need the prediction of the W pt distribution to measure the W mass.
Observable sensitive to Mw : muon transverse momentum
Mean 0.01145± 10.56
Std Dev 0.008099± 7.433
W Pt [GeV]0 5 10 15 20 25 30
Nor
mal
ized
ent
ries
/ 0.5
GeV
0.005
0.01
0.015
0.02
0.025
0.03 Mean 0.01145± 10.56
Std Dev 0.008099± 7.433
W Pt spectrum
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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 MeVThis technique implies a perfect mastering of the simulation : W production and measurement of the event properties (muon, recoil)
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Transverse mass: in principle a more robuste observableIn these formulae p== pT
The jacobian edge preserves the mass information
sensitive to production and
decay dynamics
PT
ηcm
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Small distortions due to ISR and muon resolution
Plot done with ideal measurement of the hadronic recoil
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…. 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
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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
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Tracker Resolution
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W mass measurement strategy
Calibration of the muon momentum scale
Calibration of the hadronic recoil
Precise definition of the production model
J/Psi - Y samples
Z samples
Ancillary measurements
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CMS Statistics on tape
Lumi(1/)) Pileup J/Psi Y Z W+ W-
7TeV 5 7 3.5 1 1.4 13 9
8TeV 22 22 5.3 3 6.2 61 45
13TeV 4+35 13+24 122 49 39 ~350 ~250
Using 8 TeV only the statistical error is <2 MeV
Million Events
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Calibration of the momentum scale
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
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 ?
Kc=AK+ε sinθ K2 + qM
B field and radial length
K=curvature=1/p
material effects transverse alignment10 η bins 10 η bins20 η bins
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Fitting dimuon resonances: kalman filter with target mass mc = dimuon generator mass after FSR
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
Michalis Bachtis
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Material effects < 10 MeV
Transverse alignment terms ~ few 10-5 GeV-1
Calibration performed only with J/ψ and ΥZ used in the closure plots
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Fit correlations
70% correlation between material terms and B-field terms
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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
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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
+……
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Hadronic recoil, definition and calibration
Ideal detector, no pileup tracks + clusters
Real detector, pileup tracks+clusters
Ideal detector, no pileup only charged tracks
Real detector, pileup only charged tracks from muon vertex
Maria Rosaria d’ Alfonso
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Hadronic recoil calibrated with Z μμ events
Good closure of the recoil
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Hadronic recoil calibrated with Z μμ events
Response ~ 50% Resolution ~ 10 GeV
Response CDF ~ 65%, ATLAS ~ 90%
Tracks
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March 2016
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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
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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
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More on recoil
Nicolo Foppiani Olmo CerriExploiting correlations
Different muon pT bins
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1st results on MC resolution from 10 GeV —> 8 GeV
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Modeling W production
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Effect of the PDF uncertainties on W mass
PT
ηcm
ηcm
ηL
YWpT>30 GeV ~ |ηcm | <0.7
At a given ηL you sample different Y corresponding to different pT changing average pT . And also W polarization effects
PT cut
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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
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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
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W production modeling
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Angular coefficients
Perform the measurement at low pT ?ATLAS use fixed order NNLO prediction validating the model with Z measurement resulting in 5 MeV error.
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V Transverse Momentum Distribution
Fit prediction to Z data and apply to W
ATLAS considered Powheg+Pytia8 and Pytia8 standalone
After tuning 1% agreementprimordial kt αISR ISR cut-off
Exported to W
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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
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Resummation codes predict an harder pT W spectrum for a given measured pT Z spectrum
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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