Study of the pp→ Z → µ+µ− Process at ATLAS:Detector Performance and First Cross-Section

Measurement at 7 TeV

Candidate: Sara BorroniSupervisor: Cesare Bini

Co-supervisor: Stefano Rosati

Sapienza, Università di RomaXXIII Ciclo di Dottorato

November 3, 2010

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Theoretical IntroductionThe Standard Model of Particle Physics

The past century has seen a really big step forward for the elementary particle physics. The Standard Model (SM)has been developed which describes the behavior of matter and its interactions. This theoretical framework

managed to give a unified description of strong, electromagnetic and weak interactions but the gravity

The SM has been extensively tested in the last decades at LEP and Tevatron and it successfully explains most of theknown phenomena in elementary particle physics. Nevertheless a number of open problems are still left which needfor further studies to be done.

Experimental facts not explained by SMevidence that neutrinos have non-zero massesdark matter must existobserved matter-antimatter asymmetry...

Theoretical issues not solved in the SMthe Higgs or some new physics must exist to make the SM consistentunification of the gauge couplingsunification of gravity is still missing...

Sara Borroni November 3, 2010 2/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Theoretical IntroductionThe Standard Model of Particle Physics

The past century has seen a really big step forward for the elementary particle physics. The Standard Model (SM)has been developed which describes the behavior of matter and its interactions. This theoretical framework

managed to give a unified description of strong, electromagnetic and weak interactions but the gravity

The SM has been extensively tested in the last decades at LEP and Tevatron and it successfully explains most of theknown phenomena in elementary particle physics. Nevertheless a number of open problems are still left which needfor further studies to be done.

Experimental facts not explained by SMevidence that neutrinos have non-zero massesdark matter must existobserved matter-antimatter asymmetry...

Theoretical issues not solved in the SMthe Higgs or some new physics must exist to make the SM consistentunification of the gauge couplingsunification of gravity is still missing...

Sara Borroni November 3, 2010 2/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Theoretical IntroductionThe Standard Model of Particle Physics

The past century has seen a really big step forward for the elementary particle physics. The Standard Model (SM)has been developed which describes the behavior of matter and its interactions. This theoretical framework

managed to give a unified description of strong, electromagnetic and weak interactions but the gravity

The SM has been extensively tested in the last decades at LEP and Tevatron and it successfully explains most of theknown phenomena in elementary particle physics. Nevertheless a number of open problems are still left which needfor further studies to be done.

Experimental facts not explained by SMevidence that neutrinos have non-zero massesdark matter must existobserved matter-antimatter asymmetry...

Theoretical issues not solved in the SMthe Higgs or some new physics must exist to make the SM consistentunification of the gauge couplingsunification of gravity is still missing...

Sara Borroni November 3, 2010 2/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Theoretical IntroductionThe Standard Model of Particle Physics

The past century has seen a really big step forward for the elementary particle physics. The Standard Model (SM)has been developed which describes the behavior of matter and its interactions. This theoretical framework

managed to give a unified description of strong, electromagnetic and weak interactions but the gravity

The SM has been extensively tested in the last decades at LEP and Tevatron and it successfully explains most of theknown phenomena in elementary particle physics. Nevertheless a number of open problems are still left which needfor further studies to be done.

Experimental facts not explained by SMevidence that neutrinos have non-zero massesdark matter must existobserved matter-antimatter asymmetry...

Theoretical issues not solved in the SMthe Higgs or some new physics must exist to make the SM consistentunification of the gauge couplingsunification of gravity is still missing...

Sara Borroni November 3, 2010 2/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

The Large Hadron ColliderMotivations and Experiments

Answer these kind of questions is the reason why the Large Hadron Collider (LHC) has been built, with its fourexperiments: ATLAS, CMS, LHCb and ALICE.

LHC today

� proton - proton collider� 7 TeV center of mass energy

(proj. 14 TeV )� instantaneous peak

luminosity 2 · 1032cm−2s−1

(proj. 1034cm−2s−1 )� integrated luminosity

delivered ∼ 40 pb−1

� 2011 full year data taking -2012 technical stop (?)

Sara Borroni November 3, 2010 3/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

The ATLAS Experiment

ATLAS in (Very-)Short

� Usual collider multi-purposeexperiment: many concentrical layers todetect different particles

� Inner Tracker in a solenoidal magneticfield (2 T ) for charged particlesdetection→ resolution order 10 ÷ 100µm

� Electromagnetic and Hadroniccalorimeters for electrons, photons andhadrons

∆E

E(EM) =

11.0%p

E[GeV ]⊕ 0.4%

∆E

E(HAD) =

50%p

E[GeV ]⊕ 3%

� External Muon Spectrometer in anair-core toroidal magnetic field (0.6 T )for muons→ resolution ∼ 50 µm onthe sagitta

Sara Borroni November 3, 2010 4/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

The ATLAS Experiment

Variables used to describe theevent kinematics in an hadroncollider experiment

� only the transversecomponents of energies andmomenta (pT , missingtransverse energy..)

� azimutal angle φ

� pseudo-rapidityη = ln tan θ/2

� Distance between tworeconstructed objects∆R =

p∆η2 + ∆φ2

Sara Borroni November 3, 2010 5/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

W/Z Bosons PhysicsA sketch

The hadronic nature of the proton-proton collisions opens a wide range of exploration possibilities, from the precisemeasurements of Standard Model parameters to the search for new physics phenomena up to the TeV scale

H1

H2

γ, Z,W±

p1

p2

x1p1

x1p2

k1

k2

�, ν

�̄, ν̄

Uncertainty sources:� During each bunch crossing are

NOT the proton to collide but thepartons inside the protons, eachcarrying a momentum fraction x

� Uncertainty on the PartonDistribution Functions (PDF)→describe the x distribution foreach parton type

Sara Borroni November 3, 2010 6/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

W/Z Bosons PhysicsA sketch

The hadronic nature of the proton-proton collisions opens a wide range of exploration possibilities, from the precisemeasurements of Standard Model parameters to the search for new physics phenomena up to the TeV scale

H1

H2

γ, Z,W±

p1

p2

x1p1

x1p2

k1

k2

�, ν

�̄, ν̄

Uncertainty sources:� During each bunch crossing are

NOT the proton to collide but thepartons inside the protons, eachcarrying a momentum fraction x

� Uncertainty on the PartonDistribution Functions (PDF)→describe the x distribution foreach parton type

Sara Borroni November 3, 2010 6/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

W/Z Bosons PhysicsA sketch

The measurement of the well known Standard Model processes is the first step towards all new physics searches

� theoretical predictions tuning at anew unexplored energy: whichtheoretical model correctlydescribes the data?

� standard candles both for thedetectors understanding andperformance assessment

Sara Borroni November 3, 2010 7/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

W/Z Bosons PhysicsA sketch

The W± and Z 0 bosons are suitable� theoretical predictions (NNLO) have a small uncertainty (about 5%) mainly due to the uncertainty on the PDF� measured with high experimental precision at LEP and Tevatron� copiously produced at LHC� leptonic decays are clear and “simple" to disentangle from the background

Expected W/Z events @ 7 TeV

σ × BR(Z → ll) ∼ 1 nb : →∼ 33000 evts in 100 pb−1 after selectionσ × BR(W → lν) ∼ 10 nb : → 370000 evts in 100 pb−1 after selection

Sara Borroni November 3, 2010 8/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

A cross-section measurementThe Standard Model Re-discovery

Let’s re-discover the Standard Model and assess the detector performance starting from the Z boson cross-sectionmeasurement!

The cross-section measurement is a counting experiment where one has to deduce how many events NS of acertain type have been produced when Nsel of them are observed.

NS = L · σ −→ σ =Nsel − Nb

L · A · �

Example: efficiency for Z → µ+ µ− process:� efficiency is correlated to the probability of the event with two muons in ζ1 and ζ2 kinematic bins to be

selected� which is a combination of the single muon efficiencies P(R, ζi) and P(T |R, ζi)

P(event, �ζ) =

2

4Y

i=1,n

P(R, ζi)

3

5 ·

2

41−Y

i=1,n

(1− P(T |R, ζi))

3

5

I want to measure the reconstruction efficiency P(R, ζi) and the trigger efficiency for a reconstructed muonP(T |R, ζi)

Sara Borroni November 3, 2010 9/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

A cross-section measurementThe Standard Model Re-discovery

Let’s re-discover the Standard Model and assess the detector performance starting from the Z boson cross-sectionmeasurement!

The cross-section measurement is a counting experiment where one has to deduce how many events NS of acertain type have been produced when Nsel of them are observed.

NS = L · σ −→ σ =Nsel − Nb

L · A · �

Example: efficiency for Z → µ+ µ− process:� efficiency is correlated to the probability of the event with two muons in ζ1 and ζ2 kinematic bins to be

selected� which is a combination of the single muon efficiencies P(R, ζi) and P(T |R, ζi)

P(event, �ζ) =

2

4Y

i=1,n

P(R, ζi)

3

5 ·

2

41−Y

i=1,n

(1− P(T |R, ζi))

3

5

I want to measure the reconstruction efficiency P(R, ζi) and the trigger efficiency for a reconstructed muonP(T |R, ζi)

Sara Borroni November 3, 2010 9/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

A cross-section measurementThe Standard Model Re-discovery

Let’s re-discover the Standard Model and assess the detector performance starting from the Z boson cross-sectionmeasurement!

The cross-section measurement is a counting experiment where one has to deduce how many events NS of acertain type have been produced when Nsel of them are observed.

NS = L · σ −→ σ =Nsel − Nb

L · A · �

Example: efficiency for Z → µ+ µ− process:� efficiency is correlated to the probability of the event with two muons in ζ1 and ζ2 kinematic bins to be

selected� which is a combination of the single muon efficiencies P(R, ζi) and P(T |R, ζi)

P(event, �ζ) =

2

4Y

i=1,n

P(R, ζi)

3

5 ·

2

41−Y

i=1,n

(1− P(T |R, ζi))

3

5

I want to measure the reconstruction efficiency P(R, ζi) and the trigger efficiency for a reconstructed muonP(T |R, ζi)

Sara Borroni November 3, 2010 9/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

A cross-section measurementThe Standard Model Re-discovery

Let’s re-discover the Standard Model and assess the detector performance starting from the Z boson cross-sectionmeasurement!

The cross-section measurement is a counting experiment where one has to deduce how many events NS of acertain type have been produced when Nsel of them are observed.

NS = L · σ −→ σ =Nsel − Nb

L · A · �

Example: efficiency for Z → µ+ µ− process:� efficiency is correlated to the probability of the event with two muons in ζ1 and ζ2 kinematic bins to be

selected� which is a combination of the single muon efficiencies P(R, ζi) and P(T |R, ζi)

P(event, �ζ) =

2

4Y

i=1,n

P(R, ζi)

3

5 ·

2

41−Y

i=1,n

(1− P(T |R, ζi))

3

5

I want to measure the reconstruction efficiency P(R, ζi) and the trigger efficiency for a reconstructed muonP(T |R, ζi)

Sara Borroni November 3, 2010 9/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

A cross-section measurementThe Standard Model Re-discovery

Let’s re-discover the Standard Model and assess the detector performance starting from the Z boson cross-sectionmeasurement!

The cross-section measurement is a counting experiment where one has to deduce how many events NS of acertain type have been produced when Nsel of them are observed.

NS = L · σ −→ σ =Nsel − Nb

L · A · �

Example: efficiency for Z → µ+ µ− process:� efficiency is correlated to the probability of the event with two muons in ζ1 and ζ2 kinematic bins to be

selected� which is a combination of the single muon efficiencies P(R, ζi) and P(T |R, ζi)

P(event, �ζ) =

2

4Y

i=1,n

P(R, ζi)

3

5 ·

2

41−Y

i=1,n

(1− P(T |R, ζi))

3

5

I want to measure the reconstruction efficiency P(R, ζi) and the trigger efficiency for a reconstructed muonP(T |R, ζi)

Sara Borroni November 3, 2010 9/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

A cross-section measurementThe Standard Model Re-discovery

Let’s re-discover the Standard Model and assess the detector performance starting from the Z boson cross-sectionmeasurement!

The cross-section measurement is a counting experiment where one has to deduce how many events NS of acertain type have been produced when Nsel of them are observed.

NS = L · σ −→ σ =Nsel − Nb

L · A · �

Example: efficiency for Z → µ+ µ− process:� efficiency is correlated to the probability of the event with two muons in ζ1 and ζ2 kinematic bins to be

selected� which is a combination of the single muon efficiencies P(R, ζi) and P(T |R, ζi)

P(event, �ζ) =

2

4Y

i=1,n

P(R, ζi)

3

5 ·

2

41−Y

i=1,n

(1− P(T |R, ζi))

3

5

I want to measure the reconstruction efficiency P(R, ζi) and the trigger efficiency for a reconstructed muonP(T |R, ζi)

Sara Borroni November 3, 2010 9/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Reconstruction

� Muons in the Muon Spectrometer (MS) arecurved by the toroidal magnetic field

� The momentum of the muon is measuredreconstructing the track sagitta (stand-alonereconstruction)

� The stand-alone track can be combined withan track reconstructed in the inner detector tohave a more precise measurement (combinedreconstruction)

� the reconstruction efficiency is flat in pT starting from ∼ 10 GeV (muons with pT < 3 don’t arrive to the MS,due to the energy loss in the calorimeters)

� η and φ structure is defined by the geometry and the acceptance of the precision chambers� plateau value is around 94% due to the detector acceptance (central crack, lefts and feet)

Sara Borroni November 3, 2010 10/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Reconstruction

� Muons in the Muon Spectrometer (MS) arecurved by the toroidal magnetic field

� The momentum of the muon is measuredreconstructing the track sagitta (stand-alonereconstruction)

� The stand-alone track can be combined withan track reconstructed in the inner detector tohave a more precise measurement (combinedreconstruction)

� the reconstruction efficiency is flat in pT starting from ∼ 10 GeV (muons with pT < 3 don’t arrive to the MS,due to the energy loss in the calorimeters)

� η and φ structure is defined by the geometry and the acceptance of the precision chambers� plateau value is around 94% due to the detector acceptance (central crack, lefts and feet)

Sara Borroni November 3, 2010 10/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Reconstruction

� Muons in the Muon Spectrometer (MS) arecurved by the toroidal magnetic field

� The momentum of the muon is measuredreconstructing the track sagitta (stand-alonereconstruction)

� The stand-alone track can be combined withan track reconstructed in the inner detector tohave a more precise measurement (combinedreconstruction)

� the reconstruction efficiency is flat in pT starting from ∼ 10 GeV (muons with pT < 3 don’t arrive to the MS,due to the energy loss in the calorimeters)

� η and φ structure is defined by the geometry and the acceptance of the precision chambers� plateau value is around 94% due to the detector acceptance (central crack, lefts and feet)

Sara Borroni November 3, 2010 10/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Reconstruction

� Muons in the Muon Spectrometer (MS) arecurved by the toroidal magnetic field

� The momentum of the muon is measuredreconstructing the track sagitta (stand-alonereconstruction)

� The stand-alone track can be combined withan track reconstructed in the inner detector tohave a more precise measurement (combinedreconstruction)

� the reconstruction efficiency is flat in pT starting from ∼ 10 GeV (muons with pT < 3 don’t arrive to the MS,due to the energy loss in the calorimeters)

� η and φ structure is defined by the geometry and the acceptance of the precision chambers� plateau value is around 94% due to the detector acceptance (central crack, lefts and feet)

Sara Borroni November 3, 2010 10/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Reconstruction

� Muons in the Muon Spectrometer (MS) arecurved by the toroidal magnetic field

� The momentum of the muon is measuredreconstructing the track sagitta (stand-alonereconstruction)

� The stand-alone track can be combined withan track reconstructed in the inner detector tohave a more precise measurement (combinedreconstruction)

� the reconstruction efficiency is flat in pT starting from ∼ 10 GeV (muons with pT < 3 don’t arrive to the MS,due to the energy loss in the calorimeters)

� η and φ structure is defined by the geometry and the acceptance of the precision chambers� plateau value is around 94% due to the detector acceptance (central crack, lefts and feet)

Sara Borroni November 3, 2010 10/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Reconstruction

� Muons in the Muon Spectrometer (MS) arecurved by the toroidal magnetic field

� The momentum of the muon is measuredreconstructing the track sagitta (stand-alonereconstruction)

� The stand-alone track can be combined withan track reconstructed in the inner detector tohave a more precise measurement (combinedreconstruction)

� the reconstruction efficiency is flat in pT starting from ∼ 10 GeV (muons with pT < 3 don’t arrive to the MS,due to the energy loss in the calorimeters)

� η and φ structure is defined by the geometry and the acceptance of the precision chambers� plateau value is around 94% due to the detector acceptance (central crack, lefts and feet)

Sara Borroni November 3, 2010 10/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Trigger

Three trigger levels

L1 hardware based on geometrical coincidences,with 6 programmable pT thresholds (selectsmuons with pT > threshold). It defines theRegions of Interest (ROIs)

L2 refinement of the raw pT measurement doneat L1, accessing ony data of the trackingdetectors in the RoI

EF (Event Filter) refinement of trigger decisionusing the whole event and sub-detectors info

� pT structure of the trigger turn-on curves is defined by the threshold logic� η and φ structure is defined by the geometry and the acceptance of the trigger chambers� plateau value is around 85% due to the detector acceptance (mainly in the barrel)

Sara Borroni November 3, 2010 11/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Trigger

Three trigger levels

L1 hardware based on geometrical coincidences,with 6 programmable pT thresholds (selectsmuons with pT > threshold). It defines theRegions of Interest (ROIs)

L2 refinement of the raw pT measurement doneat L1, accessing ony data of the trackingdetectors in the RoI

EF (Event Filter) refinement of trigger decisionusing the whole event and sub-detectors info

� pT structure of the trigger turn-on curves is defined by the threshold logic� η and φ structure is defined by the geometry and the acceptance of the trigger chambers� plateau value is around 85% due to the detector acceptance (mainly in the barrel)

Sara Borroni November 3, 2010 11/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Trigger

Three trigger levels

L1 hardware based on geometrical coincidences,with 6 programmable pT thresholds (selectsmuons with pT > threshold). It defines theRegions of Interest (ROIs)

L2 refinement of the raw pT measurement doneat L1, accessing ony data of the trackingdetectors in the RoI

EF (Event Filter) refinement of trigger decisionusing the whole event and sub-detectors info

� pT structure of the trigger turn-on curves is defined by the threshold logic� η and φ structure is defined by the geometry and the acceptance of the trigger chambers� plateau value is around 85% due to the detector acceptance (mainly in the barrel)

Sara Borroni November 3, 2010 11/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Trigger

Three trigger levels

L1 hardware based on geometrical coincidences,with 6 programmable pT thresholds (selectsmuons with pT > threshold). It defines theRegions of Interest (ROIs)

L2 refinement of the raw pT measurement doneat L1, accessing ony data of the trackingdetectors in the RoI

EF (Event Filter) refinement of trigger decisionusing the whole event and sub-detectors info

� pT structure of the trigger turn-on curves is defined by the threshold logic� η and φ structure is defined by the geometry and the acceptance of the trigger chambers� plateau value is around 85% due to the detector acceptance (mainly in the barrel)

Sara Borroni November 3, 2010 11/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Trigger

Three trigger levels

L1 hardware based on geometrical coincidences,with 6 programmable pT thresholds (selectsmuons with pT > threshold). It defines theRegions of Interest (ROIs)

L2 refinement of the raw pT measurement doneat L1, accessing ony data of the trackingdetectors in the RoI

EF (Event Filter) refinement of trigger decisionusing the whole event and sub-detectors info

� pT structure of the trigger turn-on curves is defined by the threshold logic� η and φ structure is defined by the geometry and the acceptance of the trigger chambers� plateau value is around 85% due to the detector acceptance (mainly in the barrel)

Sara Borroni November 3, 2010 11/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Trigger

Three trigger levels

L1 hardware based on geometrical coincidences,with 6 programmable pT thresholds (selectsmuons with pT > threshold). It defines theRegions of Interest (ROIs)

L2 refinement of the raw pT measurement doneat L1, accessing ony data of the trackingdetectors in the RoI

EF (Event Filter) refinement of trigger decisionusing the whole event and sub-detectors info

� pT structure of the trigger turn-on curves is defined by the threshold logic� η and φ structure is defined by the geometry and the acceptance of the trigger chambers� plateau value is around 85% due to the detector acceptance (mainly in the barrel)

Sara Borroni November 3, 2010 11/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies MeasurementThe Concept

� to measure the efficiency of the Muon Spectrometer(MS) we need to select a muon without using theMS and test if the MS reconstructed/triggered it

� how to recognize a muon without using the MS?!?� take advantage of a known process:

Z → µ+ µ−

� the Tag&Probe method has been developed toprovide an in-situ determination of muon triggerand reconstruction efficiency taking advantage ofthe kinematic correlation between muons comingfrom Z boson decay

Sara Borroni November 3, 2010 12/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies MeasurementThe Concept

� to measure the efficiency of the Muon Spectrometer(MS) we need to select a muon without using theMS and test if the MS reconstructed/triggered it

� how to recognize a muon without using the MS?!?� take advantage of a known process:

Z → µ+ µ−

� the Tag&Probe method has been developed toprovide an in-situ determination of muon triggerand reconstruction efficiency taking advantage ofthe kinematic correlation between muons comingfrom Z boson decay

Sara Borroni November 3, 2010 12/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies MeasurementThe Concept

� to measure the efficiency of the Muon Spectrometer(MS) we need to select a muon without using theMS and test if the MS reconstructed/triggered it

� how to recognize a muon without using the MS?!?� take advantage of a known process:

Z → µ+ µ−

� the Tag&Probe method has been developed toprovide an in-situ determination of muon triggerand reconstruction efficiency taking advantage ofthe kinematic correlation between muons comingfrom Z boson decay

Sara Borroni November 3, 2010 12/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies MeasurementThe Concept

� to measure the efficiency of the Muon Spectrometer(MS) we need to select a muon without using theMS and test if the MS reconstructed/triggered it

� how to recognize a muon without using the MS?!?� take advantage of a known process:

Z → µ+ µ−

� the Tag&Probe method has been developed toprovide an in-situ determination of muon triggerand reconstruction efficiency taking advantage ofthe kinematic correlation between muons comingfrom Z boson decay

Sara Borroni November 3, 2010 12/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies MeasurementThe Concept

� to measure the efficiency of the Muon Spectrometer(MS) we need to select a muon without using theMS and test if the MS reconstructed/triggered it

� how to recognize a muon without using the MS?!?� take advantage of a known process:

Z → µ+ µ−

� the Tag&Probe method has been developed toprovide an in-situ determination of muon triggerand reconstruction efficiency taking advantage ofthe kinematic correlation between muons comingfrom Z boson decay

Sara Borroni November 3, 2010 12/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies Measurement: the Tag&Probe MethodThe Concept

Tag A TAG muon is selected requiring tight criteria to ensurea pure selectionA fundamental requirement to not to bias the analysis isto ask the tag to have fired the trigger

Probe Then for each tag, taking advantage from the kinematicalcorrelation:

� select the second muon to be used as a PROBE from thetracks in the inner tracker and never using the MS info−→ has this track been reconstructed byMS?

� select the second muon to be used as a PROBE fromreconstructed muons and never using the MuonTrigger info −→ has this track beentriggered?

Muons The efficiencies are measured wrt a certain muonreconstruction definition i.e. the same used in thephysics analysis the efficiencies are meant to be usedfor

Sara Borroni November 3, 2010 13/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies Measurement: the Tag&Probe MethodThe Concept

Tag A TAG muon is selected requiring tight criteria to ensurea pure selectionA fundamental requirement to not to bias the analysis isto ask the tag to have fired the trigger

Probe Then for each tag, taking advantage from the kinematicalcorrelation:

� select the second muon to be used as a PROBE from thetracks in the inner tracker and never using the MS info−→ has this track been reconstructed byMS?

� select the second muon to be used as a PROBE fromreconstructed muons and never using the MuonTrigger info −→ has this track beentriggered?

Muons The efficiencies are measured wrt a certain muonreconstruction definition i.e. the same used in thephysics analysis the efficiencies are meant to be usedfor

Sara Borroni November 3, 2010 13/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies Measurement: the Tag&Probe MethodThe Concept

Tag A TAG muon is selected requiring tight criteria to ensurea pure selectionA fundamental requirement to not to bias the analysis isto ask the tag to have fired the trigger

Probe Then for each tag, taking advantage from the kinematicalcorrelation:

� select the second muon to be used as a PROBE from thetracks in the inner tracker and never using the MS info−→ has this track been reconstructed byMS?

� select the second muon to be used as a PROBE fromreconstructed muons and never using the MuonTrigger info −→ has this track beentriggered?

Muons The efficiencies are measured wrt a certain muonreconstruction definition i.e. the same used in thephysics analysis the efficiencies are meant to be usedfor

Sara Borroni November 3, 2010 13/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies Measurement: the Tag&Probe MethodThe Concept

Tag A TAG muon is selected requiring tight criteria to ensurea pure selectionA fundamental requirement to not to bias the analysis isto ask the tag to have fired the trigger

Probe Then for each tag, taking advantage from the kinematicalcorrelation:

� select the second muon to be used as a PROBE from thetracks in the inner tracker and never using the MS info−→ has this track been reconstructed byMS?

� select the second muon to be used as a PROBE fromreconstructed muons and never using the MuonTrigger info −→ has this track beentriggered?

Muons The efficiencies are measured wrt a certain muonreconstruction definition i.e. the same used in thephysics analysis the efficiencies are meant to be usedfor

Sara Borroni November 3, 2010 13/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies Measurement: the Tag&Probe MethodThe Concept

Tag A TAG muon is selected requiring tight criteria to ensurea pure selectionA fundamental requirement to not to bias the analysis isto ask the tag to have fired the trigger

Probe Then for each tag, taking advantage from the kinematicalcorrelation:

� select the second muon to be used as a PROBE from thetracks in the inner tracker and never using the MS info−→ has this track been reconstructed byMS?

� select the second muon to be used as a PROBE fromreconstructed muons and never using the MuonTrigger info −→ has this track beentriggered?

Muons The efficiencies are measured wrt a certain muonreconstruction definition i.e. the same used in thephysics analysis the efficiencies are meant to be usedfor

Sara Borroni November 3, 2010 13/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies Measurement: the Tag&Probe MethodThe Concept

Tag A TAG muon is selected requiring tight criteria to ensurea pure selectionA fundamental requirement to not to bias the analysis isto ask the tag to have fired the trigger

Probe Then for each tag, taking advantage from the kinematicalcorrelation:

� select the second muon to be used as a PROBE from thetracks in the inner tracker and never using the MS info−→ has this track been reconstructed byMS?

� select the second muon to be used as a PROBE fromreconstructed muons and never using the MuonTrigger info −→ has this track beentriggered?

Muons The efficiencies are measured wrt a certain muonreconstruction definition i.e. the same used in thephysics analysis the efficiencies are meant to be usedfor

Sara Borroni November 3, 2010 13/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Efficiencies Measurement: the Tag&Probe MethodThe Concept

Tag A TAG muon is selected requiring tight criteria to ensurea pure selectionA fundamental requirement to not to bias the analysis isto ask the tag to have fired the trigger

Probe Then for each tag, taking advantage from the kinematicalcorrelation:

� select the second muon to be used as a PROBE from thetracks in the inner tracker and never using the MS info−→ has this track been reconstructed byMS?

� select the second muon to be used as a PROBE fromreconstructed muons and never using the MuonTrigger info −→ has this track beentriggered?

Muons The efficiencies are measured wrt a certain muonreconstruction definition i.e. the same used in thephysics analysis the efficiencies are meant to be usedfor

Sara Borroni November 3, 2010 13/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Tag&Probe on the first 1.3 pb−1

The method has been applied to a data integrated luminosity of 1.3 pb−1 (April-August 2010 data-taking periods)

Indet probes −→ forreconstruction efficiency

Muon probes −→ for triggerefficiency (L1_MU10 is taken as

example)

The disagreement between dataand MC is manly due to the

difference in the efficiencies (seenext slide). No correction factors

are applied in the plot

Probes Selected with 1.3 pb−1

Indet probes Muon probes

# selected probes 655 567Expected from MC 714.0 ± 1.4 686.98 ± 0.59Background estimation (MC) 1.8 ± 0.4% 1.0 ± 0.1%

Sara Borroni November 3, 2010 14/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Reconstruction Efficiency from DataMuon reconstruction efficiency vs pT (left) and η (right)

Reconstruction EfficienciesRegion η value Efficiency Scale Factor

Barrel 0.1 < |η| < 1.05 0.90 ± 0.02 0.94 ± 0.02End-Caps 1.05 < |η| < 2.4 0.85 ± 0.03 0.91 ± 0.01Crack |η| < 0.1 0.69 ± 0.09 1.36 ± 0.17Average −2.4 < η < 2.4 0.89 ± 0.01 0.95 ± 0.01

Sara Borroni November 3, 2010 15/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Trigger Efficiency from Data

Muon trigger efficiency vs pT (left) and η (right)

Relative Trigger Efficiencies L1_MU10Region η value Efficiency Scale Factor

Barrel |η| < 1.05 0.81 ± 0.02 1.02 ± 0.02End-Caps 1.05 < |η| < 2.4 0.85 ± 0.02 0.89 ± 0.02Average |η| < 2.4 0.83 ± 0.01 0.94 ± 0.02

Sara Borroni November 3, 2010 16/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Z → µ+ µ− + jets cross-section measurement

� With the same 1.3 pb−1 datasample a first cross-sectionmeasurement of theZ → µ+ µ− process hasbeen performed

� 430Z → µ+ µ− candidateshave been found in a masswindow of 71 < mµµ < 111GeV

� correcting for the measuredefficiencies and all the otherfactors the resulting inclusivecross-section is:

σ = 427.3 ± 10.0(stat)+53.5−11.4 (syst) ± 47.0(lumi) pb

Sara Borroni November 3, 2010 17/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Z → µ+ µ− + jets cross-section measurement

� It’s interesting to perform adifferential cross-sectionmeasurement

� The shape of the differentialdistributions stronglydepend on the MC models

� These measurements areimportant to be comparedto the theoretical predictionsand to tune the MC

� It necessary to know theefficiencies in # of jets bins!

Sara Borroni November 3, 2010 18/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Dependence on the # of Jets

Muon reconstruction (left) and trigger (right) efficiencies vs # of jets in the eventThe dependence studied in MC is not jet significative in data (due to the statistics)

Sara Borroni November 3, 2010 19/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Inclusive Z → µ+ µ− cross-section: a comparison

Published inclusive cross-section measurement with ∼ 300 nb−1

Sara Borroni November 3, 2010 20/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Summary and Conclusions

� Development and optimization of a Tag&Probe method to measure the muon trigger and reconstructionefficiencies from data

� Optimization using MC samples� Application to the first 1.3 pb−1 of data @ 7 TeV→ efficiencies measurement and comparison with MC

expectations� Application of the efficiencies to measure the inclusive pp→ Z → µ+µ− cross-section

ATLAS Collaboration, “Measurement of the W → lν and Z/γ∗ → ll production cross sections in proton-proton collisionsat√

s = 7 TeV with the ATLAS detector" , arXiv:1010.2130v1 , submitted to JHEP

� Study of the efficiencies dependence from the jets variables to estimate the production of a Z boson inassociation with jetsA. Ahmada et al., “Measurement of the cross-section for jets produced in association with a Z -boson in pp collisions at

√s =

7 TeV", in preparationA. Ahmada et al., “Measurement of the cross-section for jets produced in association with a W -boson in pp collisions at

√s =

7 TeV", ATL-COM-PHYS-2010

THANKS!

Sara Borroni November 3, 2010 21/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Summary and Conclusions

� Development and optimization of a Tag&Probe method to measure the muon trigger and reconstructionefficiencies from data

� Optimization using MC samples� Application to the first 1.3 pb−1 of data @ 7 TeV→ efficiencies measurement and comparison with MC

expectations� Application of the efficiencies to measure the inclusive pp→ Z → µ+µ− cross-section

ATLAS Collaboration, “Measurement of the W → lν and Z/γ∗ → ll production cross sections in proton-proton collisionsat√

s = 7 TeV with the ATLAS detector" , arXiv:1010.2130v1 , submitted to JHEP

� Study of the efficiencies dependence from the jets variables to estimate the production of a Z boson inassociation with jetsA. Ahmada et al., “Measurement of the cross-section for jets produced in association with a Z -boson in pp collisions at

√s =

7 TeV", in preparationA. Ahmada et al., “Measurement of the cross-section for jets produced in association with a W -boson in pp collisions at

√s =

7 TeV", ATL-COM-PHYS-2010

THANKS!

Sara Borroni November 3, 2010 21/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

PublicationsContributions to Conferences:

"XVIII International Workshop on Deep-Inelastic Scattering and Related Subjects", Convitto della Calza, Firenze,19 - 23 April 2010; talk given "W/Z Production at ATLAS", ATL-COM-PHYS-2010-179

"Incontri di Fisica delle Alte Energie", Sapienza Univeristá di Roma, Italy April 7 - 9 2010; poster presented"Inclusive Muon Production in 900 GeV p-p Collisions with ATLAS Detector"

”The 2009 Europhysics Conference on High Energy Physics", Krakow, Poland July 16 - 22 2009; posterpresented ”ATLAS Electroweak Measurements With Early Data", ATL-PHYS-SLIDE-2009-196

”Signaling the Arrival of the LHC Era", ICTP Trieste, Italy December 8 - 13 2008; talk given ”Studies on ATLASmuon efficiency measurement with Z→ µµ", ATL-MUON-SLIDE-2009-151, ATL-COM-MUON-2008-017

Publications with major contribution:

A. Ahmada et al., “Measurement of the cross-section for jets produced in association with a Z -boson in ppcollisions at

√s = 7 TeV", in preparation

A. Ahmada et al., “Measurement of the cross-section for jets produced in association with a W -boson in ppcollisions at

√s = 7 TeV", ATL-COM-PHYS-2010

J. Barreiro Guimaraes da Costa et al., “W → µν and Z → µµ cross-sections measurements inproton-proton collisions at

√s = 7 TeV with the ATLAS Detector", ATL-COM-PHYS-2010-685

ATLAS Collaboration, “Measurement of the W → lν and Z/γ∗ → ll production cross sections inproton-proton collisions at

√s = 7 TeV with the ATLAS detector" , arXiv:1010.2130v1 , submitted to JHEP

P.Bagnaia et al.,“Calibration model for the MDT chambers of the ATLAS Muon Spectrometer", ATLAS groupnote, ATL-MUON-PUB-2008-004, ATL-COM-MUON-2008-006

Sara Borroni November 3, 2010 22/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Publications

Other publications:

ATLAS Collaboration, "Measurement of inclusive jet and di-jet cross sections in proton-proton collisions at 7TeV centre-of-mass energy with the ATLAS detector", arXiv:1009.5908v2, submitted to EPJC

ATLAS Collaboration, "Search for Quark Contact Interactions in Di-jet Angular Distributions in pp Collisions atsqrt(s) = 7 TeV Measured with the ATLAS Detector", arXiv:1009.5069v1, submitted to PLB

ATLAS Collaboration, "Search for New Particles in Two-Jet Final States in 7 TeV Proton-Proton Collisions with theATLAS Detector at the LHC", Phys. Rev. Lett. 105, 161801 (2010), arXiv:1008.2461v2

ATLAS Collaboration, "Charged-particle multiplicities in pp interactions at√

s = 900 GeV measured with theATLAS detector at the LHC", CERN-PH-EP-2010-004, arXiv:1003.3124, Phys Lett B 688, 2010, Issue 1, 21-42

ATLAS Collaboration, "Readiness of the ATLAS Liquid Argon Calorimeter for LHC Collisions", arXiv:0912.2642,submitted to EPJC

ATLAS Collaboration, "Drift Time Measurement in the ATLAS Liquid Argon Electromagnetic Calorimeter usingCosmic Muons", arXiv:1002.4189, submitted to EPJC

ATLAS Collaboration, "ATLAS Inner Detector commissioning and calibration", arXiv:1004.5293, submitted to EPJC

Sara Borroni November 3, 2010 23/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Backup

Sara Borroni November 3, 2010 24/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

W/Z Bosons PhysicsA sketch

The hadronic nature of the proton-proton collisions opens a wide range of exploration possibilities, from the precisemeasurements of Standard Model parameters to the search for new physics phenomena up to the TeV scale

H1

H2

γ, Z,W±

p1

p2

x1p1

x1p2

k1

k2

�, ν

�̄, ν̄

Uncertainty sources:� extrapolation to a new kinematical

x region→ x = momentumfraction carried by the collidingparton

� uncertainty on the PartonDistribution Functions (PDF)→describe the x distribution foreach parton type

Sara Borroni November 3, 2010 25/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Tag&Probe Selection DetailsMuon Pre-selection Cuts

Muon Pre-Selection Cuts

Type Combined MuonpT > 15.0 GeV|η| < 2.5Track Isolation (0.2) < 1.8 GeV# PIX hits > 1# SCT hits > 5# TRT hits > 0 (only for |η| < 2.0)|pID

T − pMST |/pID

T < 0.5pMST > 10 GeV

d0 < 0.1 mmz0 < 10 mm

Table: Muon pre-selection cuts. This is chosen to be the same used for the inclusive Z and Z+jets 7 TeV analysis. The variables usedin the selection are defined in the text.

Sara Borroni November 3, 2010 26/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Tag&Probe Selection DetailsProbes Selection Cuts

Tag&Probe Selection @ 7 TeVCut Tag Indet Probes Muon Probes

Type Combined Muon InDet Track Pre-selected MuonCharge - OS OSTrigger L1_MU10 - -pT > 10.0 GeV > 5.0 GeV -|η| < 2.5 < 2.5 -d0 - 0.1 mm -∆z0 - 0.5 mm 0.5 mmTrack Isolation - < 0.1 -∆Φ - > 2.14 rad > 2.14 radCaloMuon matching - True False∆M - MZ ± 20 GeV MZ ± 20 GeV

Table: Tag&Probe selection for 7 TeV analysis. Jet selection: pT > 20 GeV in |y| < 2.8

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Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Tag&Probe Selection DetailsSelection Variables Distributions

Sara Borroni November 3, 2010 28/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Tag&Probe Cut-FlowEvent Selection

Cut Data MC

Initial Events 24105444 –GRL 21793697 –

Indet Probes SelectionCut Data MC

pT 1727031 15972.56|η| 1718121 15856.2Charge 862812 8655.86d0 152147 3055.87∆z0 118744 2911.73Track Isolation 21933 1499.05∆Φ 12567 1178.82CaloMuon 1675 953.79∆M 655 713.99 ± 1.40

Muon Probes SelectionCut Data MC

Type 5883 7059.27Charge 880 890.23∆z0 825 878.68∆Φ 715 796.73∆M 567 686.98 ± 0.59

Table: Probes selection cut-flow, data and MC expectations. MC is normalized to the data integrated luminosity (1.3 pb−1 )Sara Borroni November 3, 2010 29/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Tag&Probe Systematic Uncertainties

Systematic UncertaintiesCut Reconstruction Absolute Trigger Relative Trigger

Efficiency Efficiency Efficiency

d0 0.3% 0.2% –Isolation 1.2% 0.4% –∆z0 0.6% 0.6% 0.3%∆Φ 0.5% 0.8% 0.6%∆M 0.6% 0.1% 0.6%

1.6% 1.1% 0.9%

Table: Systematics uncertainties on efficiencies measurement. The unceratinties are evaluated varying the selection cuts in the rangeexplained in the text and estimating the corresponding maximal average efficiency variation.

Sara Borroni November 3, 2010 30/36

Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Dependence on # of Jets

Efficiencies vs Number of JetsCut Reconstruction Relative Trigger Absolute Trigger

Efficiency Efficiency EfficiencyData

No cut 0.889 ± 0.013% 0.782 ± 0.016% 0.825 ± 0.016> 0 0.902 ± 0.023% 0.762 ± 0.030% 0.823 ± 0.030> 1 0.942 ± 0.035% 0.811 ± 0.046% 0.808 ± 0.054> 2 0.909 ± 0.097% 0.800 ± 0.090% 0.818 ± 0.113

Monte Carlo

No cut 0.924 ± 0.001% 0.903 ± 0.001% 0.859 ± 0.001> 0 0.920 ± 0.003% 0.898 ± 0.003% 0.857 ± 0.001> 1 0.915 ± 0.007% 0.895 ± 0.007% 0.857 ± 0.004> 2 0.913 ± 0.017% 0.875 ± 0.008% 0.883 ± 0.020

Systematicsl Uncertainty

> 0 0.4% 0.5% 0.2%> 1 0.9% 0.8% 0.2%> 2 1.1% 2.7% 2.4%

Table: Dependence on the muon efficiencies from the number of jets in the event from data and Monte Carlo. Within the statisticalerrors data don’t show a clear dependence from the jets so the average value can be considered instead of the binned one. Thesystematic error introduced integrating over this variable can be estimated from the Monte Carlo samples.

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Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Details of Z+Jets Analysis

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Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Details of Z+Jets Analysis

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Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Resolution

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Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Muon Resolution

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Introduction and Motivations Tag&Probe Method for Efficiencies Z+jets measurement Summary and Conclusions Backup

Example: Efficiency for 2 Muons ProcessSingle object and event efficiencies

Suppose to measure the cross-section of a 2 muon process (e.g Z → µ+ µ− ):

� Efficiency is correlated to the probability of the event with two muons in ζ1 and ζ2 kinematic bins to beselected

� =

Zdζ1dζ2

df

dζ1dζ2P(event, ζ1, ζ2)

� which is a combination of the single muon efficiencies P(R, ζi) and P(T |R, ζi)

P(event, �ζ) =

2

4Y

i=1,n

P(R, ζi)

3

5 ·

2

41−Y

i=1,n

(1− P(T |R, ζi))

3

5

I want to measure the reconstruction efficiency P(R, ζi) and the trigger efficiency for a reconstructed muonP(T |R, ζi)

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