Muons

HCAL ECAL

4.7 Muons

Electron tracker isolation I_TRK^e , the scalar sum of the transverse momenta of tracks originating from the same primary vertex, withpT >0.7 GeV, within an annulus of 0.015<∆R <0.3 around the electron momentum direction, exclud- ing a rectangular region of 0.03×0.3 in ∆η×∆φ centered on the electron to protect the isolation sum from the inclusion of the electron track and conversion tracks of eventual bremsstrahlung photons [112, 113].

Electron ECAL isolation I_ECAL^e , the scalar sum of transverse energy of ECAL crystal deposits within a 3∆η0 <∆R <0.3 cone around the supercluster ECAL location, excluding a rectangular strip of 3∆η0×0.3 in ∆η×∆φto protect the isolation sum from the inclusion of energy deposits due bremsstrahlung showers and secondary conversions. Here, ∆η0 is the size of a single crystal in units of pseudorapidityη. In the ECAL barrel, only deposits with energies E >80 MeV are included in the sum. In the ECAL endcaps, only deposits with transverse energiesET >100 MeV are included [114, 115].

Electron HCAL isolation I_HCAL^e , the scalar sum of the transverse energy of HCAL tower deposits within a solid cone of ∆R < 0.3 around the ECAL supercluster location.

Electron combined relative isolation I_comb^{rel, e} = (I_TRK^e +I_ECAL^e +I_HCAL^e )/pT, the sum of the tracker, ECAL and HCAL isolations divided by the electron trans- verse momentum.

Similarly to photons, to minimize the impact of the pileup energy deposits on the isolation, we correct the combined isolation by subtracting the average energy deposited by the pileup using eq. (4.5). Here, the effective area is given byA^eff =πR² with R= 0.3.

isolated hits or a reconstructed track. There are two algorithms used. The tracker muon reconstruction starts with the inner tracks. It propagates them through the muon system using the known track parameters and the magnetic field map, and searches for consistent hits in the muon system. The global muon reconstruction requires that a global track can be fitted to the hits in both the tracker and the muon system simultaneously.

Figure 4.14 shows the reconstruction efficiency for muons in the inner tracker from Z → µ⁺µ⁻ decays, measured in both data and simulation with the tag-and- probe method. Similarly, Figures 4.15 and 4.16 show the efficiencies of the outer track reconstruction and inner-outer track matching of the global muon reconstruction algorithm, respectively.

The reconstruction is optimized to have a very high efficiency. However, it can happen that signals that do not originate from primary isolated muons are also re- constructed. These may come from a number of sources:

• hadronic punch-through,

• decays in flight,

• accidental track-to-segment matches, potentially involving pile-up tracks,

• cosmic muons.

A set of requirements serves to reject such undesired muon candidates and to identify signal muons with high efficiency.

We use the following muon identification and isolation variables:

Global muon - the candidate is reconstructed as a global muon.

Reduced χ squared χ²/ndof, the ratio of theχ² and the number of degrees of free- dom ndof of the global track fit. Requiring low values of χ²/ndof suppresses hadronic punch-through and muons from decays in flight.

η Muon

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

(GeV)

T

Muon p

20 40 60 80 100 120 140

0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Inner Track Reco. Efficiency CMS Preliminary 1

η Muon

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

(GeV)

T

Muon p

20 40 60 80 100 120 140

0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Inner Track Reco. Efficiency CMS Simulation 1

Figure 4.14: Muon inner track reconstruction efficiency map in the pT-η plane [107].

Top: data, bottom: simulation.

η Muon

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

(GeV)

T

Muon p

20 40 60 80 100 120 140

0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Outer Track Reco. Efficiency CMS Preliminary 1

η Muon

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

(GeV)

T

Muon p

20 40 60 80 100 120 140

0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Outer Track Reco. Efficiency CMS Simulation 1

Figure 4.15: Muon outer track reconstruction efficiency map in the pT-η plane [107].

Top: data, bottom: simulation.

η Muon

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

(GeV)

T

Muon p

20 40 60 80 100 120 140

0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Matching Efficiency CMS Preliminary 1

η Muon

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

(GeV)

T

Muon p

20 40 60 80 100 120 140

0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Matching Efficiency CMS Simulation 1

Figure 4.16: Muon inner-outer track matching efficiency map in the pT-η plane [107].

Top: data, bottom: simulation.

Number of muon chamber hits n_hits, the total number of valid muon chamber hits included in the global track fit, for which a high value further discriminates signal muons from hadronic punch-through and muons from decays in flight.

Number of matched stations nstations, the total number of muon stations with muon segments. Requiring higher values of nstations suppresses punch-through and accidental track-to-segment matches. This also guarantees the consistency between the offline selection and the muon trigger selection, since the same criterion is used in the trigger to establish that the muon transverse momentum estimate is reasonably accurate.

Transverse impact parameter dxy, the signed minimum distance in thexy plane of the extrapolated muon inner track and the primary vertex. Signal muons tend to have lower values of |dxy|than cosmic muons and those from decays in flight.

Longitudinal impact parameter dz, the signed minimum distance along the z coordinate of the extrapolated muon inner track and the primary vertex. As for dxy, requiring low values of |dz| contributes to rejecting cosmic muons and muons from decays in flight.

Number of pixel hits n_pixel, the total number of reconstructed hits in the pixel detector. The signal muons tend to record more hits in the pixel detector than the muons from decays in flight.

Number of tracker hits n_tracker, the total number of the hits in the inner tracker including the silicon pixel and strip detectors. Requiring greater values ofntracker

further suppresses muons from decays in flight and ensures a reasonable estimate of the inner track muon transverse momentum estimate.

In addition to the muon identification variables, this analysis uses the following muon isolation variables:

Muon Tracker Isolation I_TRK^µ , the scalar sum of the transverse momenta of tracks originating from the same primary vertex, with pT > 1.5 GeV, and within an annulus of 0.015<∆R <0.3 around the muon momentum direction.

Muon ECAL Isolation I_ECAL^µ , the scalar sum of transverse energy of ECAL rec hits within a cone of ∆R <0.3 around the muon direction.

Muon HCAL Isolation I_HCAL^µ , the scalar sum of the transverse energy of HCAL towers within a cone of ∆R <0.3 around the near muon direction.

Muon Combined Relative Isolation I_comb^rel,µ = (I_TRK^µ + I_ECAL^µ + I_HCAL^µ )/pT, the sum of the tracker, ECAL and HCAL isolations divided by the muon transverse momentum.

Hadronic punch through and decays in flight tend to have higher isolation values on average than the signal muons. The tracker isolation is very robust with respect to the pileup. The ECAL isolation is efficient in rejecting events with hard radiative photons that tend to be collinear. The combined relative isolation has greater signal-to- background rejection power due to reduced fluctuation coming from jet hadronization and detector noise.

Chapter 5