1. Author: Javier Santaolalla Camino
Measurement of electroweak processes in muon decay channels, in pp collisions at sqrt(s)=7 TeV, in the CMS experiment at the LHC
Author: Javier Santaolalla Camino
Supervisors:
Dr. Juan Alcaraz Maestre
Dr. Begona de la Cruz Martinez
Dr. Isabel Josa Mutuberria

2. Outline LHC and CMS Drift Tube Chamber Calibration
Muon momentum scale and resolution
W inclusive production
W + charm production

3. 27 km long accelerator
100 m underground
Proton-proton collisions at 7 TeV cms
~1033 cm-2 s-1

4. CMS

5. 5

6. CMS Commissioning underground
Cosmic muons
In absence of proton beam, the detector is commissioned with cosmic muons
Several data taking campaigns to calibrate the detector
CMS Commissioning underground
2011
2010
CRAFT‘08
B off
B on
2008

7. CMS Commissioning underground
Cosmic muons
In absence of proton beam, the detector is commissioned with cosmic muons
Several data taking campaigns to calibrate the detector
CMS Commissioning underground
2011
2010
2008
Drift Velocity
Segment Efficiency

8. Early operation phase. First 3 pb-1 collected (Calibration)
p-p collisions at 7 TeV
Studied with MC before collisions
~ 400 Z bosons into muons per pb-1 collected
3.6 pb-1 delivered
3.3 pb-1 recorded
2011
2010
2008
Early operation phase. First 3 pb-1 collected (Calibration)

9. Early operation phase. First 3 pb-1 collected (Calibration)
p-p collisions at 7 TeV
Studied with MC before collisions
~ 400 Z bosons into muons per pb-1 collected
Muon momentum scale and resolution
2011
2010
2008
Early operation phase. First 3 pb-1 collected (Calibration)

10. 36 pb-1 p-p collisions at 7 TeV Physics results
47 pb-1 delivered
36 pb-1 recorded
2010
2009
2008
Precision measurements on EWK physics

11. W boson inc. production W+charm production 36 pb-1
p-p collisions at 7 TeV
Physics results
W boson inc. production
W+charm production
2010
2009
2008
Precision measurements on EWK physics

12. DT chamber calibration
Drift
chambers
DT chamber calibration
Drift velocity
DT segment efficiency

13. Drift tubes in CMS
MB4
MB3
MB2
MB1 S5 y Φ
X 5 θ x z

14. Drift tubes in CMS D = (th – ti) vd
If we consider vd constant, we can compute the passing point of the muon as:
D = (th – ti) vd

15. Drift tubes in CMS D = (th – ti) vd
We can reconstruct the trajectory of the muon with up to 44 “hits” in the drift cells
Good calibration is necessary for a good reconstruction:
time offset calibration
drift velocity calibration
Guarantee a high reconstruction efficiency in a chamber (segment efficiency)

16. Drift Velocity Calibration: Meantimer
Cosmic muons
Drift velocity determination
With the muon incident angle
Drift Velocity
Calibration
With the magnetic field in the cell
Drift velocity variation with different effects
With the signal propagation

17. Drift Velocity Calibration: Meantimer
Cosmic muons
Drift velocity determination
With the muon incident angle
Drift Velocity
Calibration
With the magnetic field in the cell
Drift velocity variation with different effects
With the signal propagation

18. Drift Velocity Variation
Apparent drift velocity veriation with the signal propagation
With the muon incident angle

19. Drift Velocity Variation
With the magnetic field in the cell

20. 4D Segment efficiency We consider events…
Cosmic Muon
We consider events…
with 3 4D segments in the same sector and wheel
triggered in the other half-hemisphere of CMS
with the extrapolation point to the test chamber within a fiducial volume
with enough “hits” to ensure a good extrapolation (> 12 in φ, > 4 in θ) and low extrapolation error (< 1.5 cm)
Selected ~1:100 of the intial sample:
 ~150 Kevents

21. 4D Segment efficiency Cosmic muons 2% lower efficiency
in θ super-layers

23. Scale and resolution in muon momentum
The SiDrA method
Scale and resolution in muon momentum

24. Muon momentum scale and res
The accuracy on the pT measured will depend on the intrinsic resolution in the form:
The CMS event simulation try to reproduce the detector conditions, including effects that could spoil the pT resolution: multiple scattering in the iron, the mapping of the magnetic field…
However the real conditions could differ from what we simulate due to effects not considered in the simulation or not accurate enough.
Our goal is to have a MC simulation as close to reality as we can
We make use of the Z resonance

25. Muon momentum scale and res
Z mass peak highly unaffected by theoretical uncertainties (PDF, ISR…)
Comparison Pythia-ResBos
Z boson selection
Selection of events with 2 high pT, isolated muons.
Quality criteria applied to the reconstructed muons.
Invariant mass of the dimuon system in the range [70,110] GeV.
~ 400 Zµµ per pb-1

26. The SIDRA method Z boson NLO POWHEG sample Take muon pT from MC
Distort it with terms
Compute Z resonance and compare with data one
Change δ and σ
We get a prediction that
reproduces better the data
Fit converged? NO
YES
Binned log-likelihood fit

27. The SIDRA method In general σ and δ may depend on the muon η and φ
Tests on this dependence
suggests
A sinusoidal form in φ and a parabolic form in η for δ
A parabolic form in η for σ

28. The SIDRA method: results
Syst. unc. of the order of
100 MeV (δ) and 400 MeV (σ)

30. Cross section measurement
W inclusive
production
W selection
Signal extraction
Cross section measurement

31. W inclusive production
W are mainly produced in p-p collisions via:
Measurement of the production cross section is interesting:
To test the SM in a new energy regime
It gives constraints to proton PDF
Benchmark for W and lepton reconstruction
Background of many analysis

32. W inclusive cross section at 7 TeV in the muon channel
Measurement
W inclusive cross section at 7 TeV in the muon channel
Signal extraction
Selection
Acceptance
Luminosity
Efficiency

33. W signature High pT, isolated lepton Large MET in the event (neutrino)
Isolation = Amount of energy in a cone centered in the µ direction
High pT, isolated lepton
Large MET in the event (neutrino)
Lepton and MET back-to-back in the transverse plane (low acoplanarity)
MET
MET
Missing Transverse Energy =
-ΣET
Acoplanarity = Angle between the µ and the ν in the transverse plane

34. Cosmic muons, decays in flight
Backgrounds
Several processes with similar signature to that of the signal:
Highest cross section
Partially removed exploiting the differences with signal
QCD multijet events
Zµµ, reduced with 2nd muon veto
Wτν, Zττ
Dibosons, low cross section
EWK processes
2 W bosons in the final state
Low cross section
ttbar
“Fake muons” that can contaminate the sample
Cosmic muons, decays in flight

35. Cosmic muons, decays in flight
Backgrounds
Several processes with similar signature to that of the signal:
Highest cross section
Partially removed exploiting the differences with signal
QCD multijet events
Zµµ, reduced with 2nd muon veto
Wτν, Zττ
Dibosons, low cross section
EWK processes
2 W bosons in the final state
Low cross section
ttbar
“Fake muons” that can contaminate the sample
Cosmic muons, decays in flight

36. 1 2 3 4 Low pT muons QCD multijet events Flat isolation distribution
Low MT of the W candidate
Flat acop. 4

37. Reject cosmic background
Wµν selection
Good pT measurement
Selection of a good muon:
Good χ2 of the fit
Minimum number the “hits” in the tracker
Detection by at least 2 muon chambers
Transverse impact parameter < 2mm
Muon in the range -2.1 < η < 2.1
High muon pT (>~25 GeV)
Low isolation variable (<~0.1)
The MT/MET is used for the signal extraction
Reject punch-through
Reject cosmic background
Optimization process
QCD multijet events
0.6%
With MET>20 GeV
EWK processes 6%
ttbar
0.4%

38. A pT cut at ~25 GeV seems to be optimal
Optimization process
Goal: minimize the uncertainty
pT cut
TOTAL
A pT cut at ~25 GeV seems to be optimal
ISR effect
PDF unc.
QCD modelling
The uncertainty on the ISR (Initial State Radiation) affects the muon pT.
ISR effect
The W yield is extracted from a fit to the MET components. A missmodeling of the QCD bkg could affect the yield measured
QCD Mod.

39. A pT cut at ~25 GeV seems to be optimal
Optimization process
Goal: minimize the uncertainty
pT cut
TOTAL
A pT cut at ~25 GeV seems to be optimal
ISR effect
PDF unc.
QCD modelling
As we increase the pT cut, we enter the signal domain. Any uncertainty on the muon pT affects directly the yield
ISR effect
As we increase the pT cut, we reject more QCD background, so we reduce the uncertainty in its modelling
QCD Mod.
PDF unc
No effect expected/observed

40. An iso cut at ~0.1 seems to be optimal
Optimization process
Goal: minimize the uncertainty
Iso cut
TOTAL
ISR effect
PDF unc.
QCD
modelling
An iso cut at ~0.1 seems to be optimal
Iso = 0.15
Iso = 0.1

41. Signal extraction
We fit the MT/MET distribution to the different components (templates):
Wµν
MC template corrected with data
QCD multijet events
Template from data
EWK, ttbar and others
Template from MC
Fixed in the fit

42. 1 2 3 W and Z bosons Have similar recoil models
Are produced at similar Q2
Production and decay are similar
MC template corrected with data
QCD multijet events
EWK, ttbar and others
Extract information from the
Z boson and apply it to the W boson
Goal: improve the MET description
in the MC W sample 1
We take the MC W boson pT
We extract the MET
corresponding to that W pT for the Z in data
We apply this MET to the W boson 2
Data
MET = all energy in the event except the 2 muons 3

43. 1 2 Wµν Isolation inversion QCD multijet events Template from data
EWK, ttbar and others 2
Inversion correction
Final template

44. Measurement Nsignal  Selection procedure
Nbackground  Signal extraction (MT/MET fit)
Acceptance  MC simulation, # events in the detector phase space
Efficiency  Computed with the Tag&Probe method (Z resonance)
Luminosity  36 pb-1 ±4%
Efficiency
Acceptance

45. Systematic uncertainties
Momentum scale
PDF uncertainty
SIDRA
Difference between PDF sets
MET scale
Background modelling
Difference between templates (MC and data)
Difference between templates (MC and data)

46. Results W+ W- W Yield Cross section (nb) 84315±290 56911±239
141226±376
5.93±0.26
4.17±0.19
10.1±0.43

47. Results

49. W + charm
prodcution
W+charm selection
Rc± and R± measurement

50. Direct access to s quark PDF
W+charm production
In proton-proton collisions the W+c proceeds predominantly via:
Direct access to s quark PDF
Other channels: d
Are Cabbibo supressed

51. W+charm measurement We measure the ratios
that are sensitive to the s quark PDF
With this definition:
We avoid the computation of efficiencies
Many systematic uncertainties cancel out.
Theoretical predictions

52. Event selection We select events with a W… …and at least one jet…
Same requirements than in the W inclusive prod. + MT>50 GeV
…and at least one jet…
In the region -2.1 < η < 2.1
ET > 20 GeV
Less than 3 jets with ET > 40 GeV (ttbar rejection)
Main backgrounds
Top and single-top
W + light jets
Other bckg
High jet multiplicity
High decay length
Biggest contribution
Low decay length
W+b(b), EWK

53. Event selection We select events with a W… …and at least one jet…
Signal:
12.6% of final sample
We select events with a W…
Same requirements than in the W inclusive prod. + MT>50 GeV
…and at least one jet…
In the region -2.1 < η < 2.1
eT > 20 GeV
Less than 3 jets with eT > 40 GeV (ttbar rejection)
Main backgrounds
Top and single-top
W + light jets
Other bckg
0.8% of final sample
82% of final sample
4.6% of final sample

54. W+c extraction
SSVHE
Simple Secondary Vertex High Efficiency
Defined as log(1+S) with S the decay length significance
Separates heavy and light jets from charm
HE  at least two tracks per vertex
POSITIVE
NEGATIVE

55. W+c extraction
SSVHE
Simple Secondary Vertex High Efficiency
Defined as log(1+S) with S the decay length significance
Separates heavy and light jets from charm
HE  at least two tracks per vertex
Different shapes  We are
sensitive to the amount of
W+charm
Clear signal observed

56. Event selection We select events with a W… …and at least one jet…
12.6% % signal
We select events with a W…
Same requirements than in the W inclusive prod. + MT>50 GeV
…and at least one jet…
In the region -2.1 < η < 2.1
ET > 20 GeV
Less than 3 jets with ET > 40 GeV (ttbar rejection)
…and b-tagging discriminator
Main backgrounds
Top and single-top
W + light jets
Other bckg
0.8% 20%
82% %
4.6% %

57. W+c extraction

58. Systematic uncertainties
Affecting the tagging
efficiency

59. Results

61. Conclusions
Pre-collision era: the detector needs to be calibrated and ready for the data taking (using cosmic rays)
The drift velocity is computed for all DT chambers in CMS and variations of the apparent drift velocity are observed with the magnetic field, the muon incident angle and the signal propagation.
Early collision data: we present the SIDRA method to compute the muon scale and resolution
Small shifts observed with θ and φ dependence (~15 MeV for a 30 GeV muon)
2 sc. papers
1 sc. paper

62. Conclusions W inclusive production W + charm production:
A measurement of the W boson production cross section is presented in this theses.
W + charm production:
The production of a W boson with a charm jet in the final state is studied in this thesis, and prospects for the improvement with 1 fb-1 data are suggested.
Cross section (nb) W+ W- W
5.93±0.26
4.17±0.19
10.1±0.43
2 sc. papers
1 public anal.