W mass Oxford - University of Oxford · Oxford Jan 2017 gigi.rolandi@cern.ch 1 ... W Pt spectrum....

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gigi.rolandi@cern.ch 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

Measurement of the TOP quark mass

Classical method

Top mass

Alternative methods

gigi.rolandi@cern.ch Oxford Jan 2017

Past (and present) W mass measurements

mW = 80.370 ± 19 MeV

New ATLAS Measurement december 2016

8-98-105-3

9-14 2-13

i’s B

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

0.03 Mean 0.01145± 10.56

Std Dev 0.008099± 7.433

W Pt spectrum

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)

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

Small distortions due to ISR and muon resolution

Plot done with ideal measurement of the hadronic recoil

…. 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

/55CERNSep. 2013 gigi.rolandi@cern.ch

muon pt ~ 39 GeV

W mass @ CMS

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

Tracker Resolution

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

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

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

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

Radial deformations < 0.1% level

Michalis Bachtis

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

Fit correlations

70% correlation between material terms and B-field terms

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

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

+……

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

Hadronic recoil calibrated with Z μμ events

Good closure of the recoil

Hadronic recoil calibrated with Z μμ events

Response ~ 50% Resolution ~ 10 GeV

Response CDF ~ 65%, ATLAS ~ 90%

Tracks

March 2016

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

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

W mass Oxford - University of Oxford · Oxford Jan 2017 gigi.rolandi@cern.ch 1 ... W Pt spectrum....

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