Napoli Doct. School 9 JULY 07 1 DRELL-YAN /Z/Z q q e,e, e+, +e+, +

Napoli Doct. School 9 JULY 07 1

DRELL-YANDRELL-YAN

/Zq

q

e,

e+, +


Story of DiscoveriesStory of Discoveries


Story of DiscoveriesStory of Discoveries

Lederman 1978Lederman 1978

19831983


Drell YanDrell Yan

LHC 1 fb-1

pT > 6 GeV

|| < 2.5

• Total cross section pdf parton lumi search for Z, extra dim. , ... Much higher mass reach ascompared to Tevatron

/Zq

q

e,

e+, +

Inversion of e+e qq at LEP

Z pole


Comparison with jet-jet cross sectionComparison with jet-jet cross section

At m = 1 TeV jet jet cross section is 1 pb/GeV (|At m = 1 TeV jet jet cross section is 1 pb/GeV (||<1) and Drell-Yang (||<1) and Drell-Yang (|||<2.5) is 10<2.5) is 10-5-5 pb/GeV. pb/GeV.

Do we understand this qualitatively ?Do we understand this qualitatively ?

First we pay a ratio (First we pay a ratio (_em/_em/_s)_s)22 for the coupling to the photon (Z) vs the for the coupling to the photon (Z) vs the coupling to gluon. coupling to gluon.

Secondly Drell Yang can happen only Secondly Drell Yang can happen only via qqvia qqbar bar of the same flavour, this of the same flavour, this gives another factor ~ 1/20.gives another factor ~ 1/20.

Thirdly Drell-Yang is a s channel only Thirdly Drell-Yang is a s channel only process, while qqprocess, while qqbar bar has also a very has also a very important t-channel contribution. important t-channel contribution.

Fourtly in s channel jet jet one has Fourtly in s channel jet jet one has 6x3 final states + gluon gluon 6x3 final states + gluon gluon


GeneraltiesGeneralties

Di-lepton final state usually easy to trigger and to identify. Trigger and Di-lepton final state usually easy to trigger and to identify. Trigger and identification efficiency depend on the lepton energy. Typically one can identification efficiency depend on the lepton energy. Typically one can trigger on single lepton > 20 GeV.trigger on single lepton > 20 GeV.


ElectronsElectronsElectron showers deposit their energy in several crystals in the ECAL. Approximately Electron showers deposit their energy in several crystals in the ECAL. Approximately 94% of the incident energy of a single electron or photon is contained in 3×3 crystals, 94% of the incident energy of a single electron or photon is contained in 3×3 crystals, and 97% in 5×5 crystals. Summing the energy measured in such fixed arrays gives the and 97% in 5×5 crystals. Summing the energy measured in such fixed arrays gives the best performance for unconverted photons, or for electrons in the test beam. The best performance for unconverted photons, or for electrons in the test beam. The presence in CMS of material in front of the calorimeter results in bremsstrahlung and presence in CMS of material in front of the calorimeter results in bremsstrahlung and photon conversions. Because of the strong magnetic field the energy reaching the photon conversions. Because of the strong magnetic field the energy reaching the calorimeter is spread in calorimeter is spread in . The spread energy is clustered by building a cluster of . The spread energy is clustered by building a cluster of clusters, a “supercluster,” which is extended in clusters, a “supercluster,” which is extended in ..

Energy and position are obtained from the PH and shape of the cluster. A number of Energy and position are obtained from the PH and shape of the cluster. A number of corrections have to be applied. Calibration is very important: uniformity and energy corrections have to be applied. Calibration is very important: uniformity and energy scale.scale.

Once crystals are intercalibrated the energy scale is set by the Z peak into Once crystals are intercalibrated the energy scale is set by the Z peak into e+e- e+e-


Electron Triggers - Level 1Electron Triggers - Level 1Leve1 Trigger: from calorimeter. Ingredients : Energy and Leve1 Trigger: from calorimeter. Ingredients : Energy and Isolation.Isolation.


Electron Triggers - Level 2Electron Triggers - Level 2•Re-clustering and re-definition of energy with finer grainRe-clustering and re-definition of energy with finer grain

• Search for a pixel hit in coincidence with the electron trajectory (typically Search for a pixel hit in coincidence with the electron trajectory (typically search region in r-phi very small due to constraint on the beam spot).search region in r-phi very small due to constraint on the beam spot).

Pixel MatchingPixel MatchingTrack matched with Track matched with pixelpixel


Muon Trigger - Level 1Muon Trigger - Level 1

Efficiency of Level 1 Muon Trigger as function of Efficiency of Level 1 Muon Trigger as function of ThresholdThreshold

Ingredients: Muon Reconstruction and IsolationIngredients: Muon Reconstruction and Isolation


Muon Trigger - Level2Muon Trigger - Level2


Randal Sundrum ModelsRandal Sundrum Models


Z’ ModelsZ’ Models


High Mass Electrons Pairs -leakegeHigh Mass Electrons Pairs -leakege

An important characteristic of the signal events is that the very energetic final state An important characteristic of the signal events is that the very energetic final state electrons may have a significant energy leakage beyond the ECAL. The energy deposit electrons may have a significant energy leakage beyond the ECAL. The energy deposit in the CMS hadronic calorimeter (HCAL) cell behind the ECAL cluster is included in the in the CMS hadronic calorimeter (HCAL) cell behind the ECAL cluster is included in the measurement, event by event. This procedure leads to an improvement of the energy measurement, event by event. This procedure leads to an improvement of the energy determination, as shown by the study of single electron and photon calibration files at determination, as shown by the study of single electron and photon calibration files at fixed energy.fixed energy.


High Mass electron pairs- SaturationHigh Mass electron pairs- Saturation

For very energetic electrons and photons, saturation occurs in the ECAL single For very energetic electrons and photons, saturation occurs in the ECAL single crystal electronics because of the limited dynamical range of the Multi-Gain-crystal electronics because of the limited dynamical range of the Multi-Gain-Pre-Amplifier. From 2004 test beam data analysis, the saturation threshold Pre-Amplifier. From 2004 test beam data analysis, the saturation threshold has been established to be at 1.7 TeV in the barrel crystals and 3.0 TeV for the has been established to be at 1.7 TeV in the barrel crystals and 3.0 TeV for the endcaps.endcaps.

The Method uses the crystal near the saturated one , sampling a fraction of the The Method uses the crystal near the saturated one , sampling a fraction of the shower. The correct estimate of energy deposit with a resolution of around 7%. In shower. The correct estimate of energy deposit with a resolution of around 7%. In addition, dedicated corrections, depending both on the electron energy and %, addition, dedicated corrections, depending both on the electron energy and %, are performed for remaining losses.are performed for remaining losses.


High mass electron pairs - SelectionHigh mass electron pairs - Selection

Events with electrons trigger. At Events with electrons trigger. At least 2 Super Clusters energy > least 2 Super Clusters energy > 100GeV.100GeV.

H/E<0.1 against isolated H/E<0.1 against isolated hadronshadrons

EEconecone /E /Escsc < 0.02 (Cone < 0.02 (Cone R 0.5)R 0.5)


Hign mass electron pairs- resultsHign mass electron pairs- results


High energy electrons Discovery luminosityHigh energy electrons Discovery luminosity

Z’Z’RS GravitonRS Graviton


High Mass Muon Pairs - SelectionHigh Mass Muon Pairs - Selection

• Requires 2 muon tracks of opposite sign.Requires 2 muon tracks of opposite sign.

• For each track examine the variation of the fit including/excluding For each track examine the variation of the fit including/excluding all muon chambers but from 1st station and all muon chambers hits all muon chambers but from 1st station and all muon chambers hits that looks contaminated by em showers. Take fit with best that looks contaminated by em showers. Take fit with best 22


Distinguishing among different Z’ modelsDistinguishing among different Z’ models

The forward-backward asymmetry, AFB, of the leptonic decay products The forward-backward asymmetry, AFB, of the leptonic decay products provides information on parity-violating couplings, on and off resonanceprovides information on parity-violating couplings, on and off resonance

For CM energies well above the ZFor CM energies well above the Z0 0 peak, the Drell-Yan background has a peak, the Drell-Yan background has a characteristic AFB of about 0.6 and provides a useful starting point.characteristic AFB of about 0.6 and provides a useful starting point.


The sign of cosThe sign of cos**

• In proton-proton interactions, the quark direction is ambiguous experimentally since a quark can originate with equal probability from either proton, and the sign of cos * is not directly measurable. We follow infer the sign of cos * by assuming that the longitudinal motion of the dimuon system is in the direction of the proton contributing the annihilating quark, since a quark in a proton typically carries a larger momentum fraction x than does an anti-quark. We refer to the inference of the wrong sign of cos * as “mistagging” the sign. If not accounted for, the mistagged events, particularly at low y, reduce (“dilute”) the apparent value of AFB. Some authors deal with this problem by removing events below a chosen y threshold, or by examining AFB in bins of y. An approached is described which assigns the probability of a mistag on an event-by-event basis, thus using all events in a given sample. The knowledge of the mistagging probability depends on the Parton Distribution .


How good is this approximization ?How good is this approximization ?

MC TruthMC Truth

Sign InferredSign Inferred

All events, no acceptanceAll events, no acceptanceWatch the y scaleWatch the y scale


Miss tag probabilityMiss tag probability

All eventsAll events Miss tagged eventsMiss tagged events RatioRatio

DEPEND ON PDFsDEPEND ON PDFs


Effect of the acceptanceEffect of the acceptanceBefore Acceptance Before Acceptance

AFTER Acceptance AFTER Acceptance


Evolution of the AsymmetryEvolution of the Asymmetry


First 500 Events (4 fb-1)First 500 Events (4 fb-1)


Asymmetry for various modelsAsymmetry for various models


Capability to distinguish among Capability to distinguish among ModelsModels


Comparison among mu and e Comparison among mu and e channelschannels


RS Graviton reachRS Graviton reach

Napoli Doct. School 9 JULY 07 1 DRELL-YAN /Z/Z q q e,e, e+, +e+, +

Documents

Transcript of Napoli Doct. School 9 JULY 07 1 DRELL-YAN /Z/Z q q e,e, e+, +e+, +