Pdf studies with early data at Pdf studies with early data at ATLAS ATLAS

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SM group meeting – 01 July 2009

Pdf studies with early data at Pdf studies with early data at

ATLAS ATLAS

• Motivation

• MC study

• Conclusions Elisa Piccaro

Eram Rizvi

Queen Mary, University of London

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SM group meeting – 01 July 2009 2

Motivation Motivation

Measure the Drell-Yan cross section at low masses Why measure it? Because we can!

Constrain PDFs at low x Stringent test of QCD

At low masses : - expected large cross section - sensitive to low x pdf

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Overview Overview

 ATLAS pseudo-rapidity range -2.5 to 2.5 (LHCb overlap between 1.9 and 2.5)

 Overlap with HERA and LHCb

 Need knowledge of low x Pdf when calculating total cross-sections

 Good test of QCD

R.Thorne

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4

Total cross sections Total cross sections

High masses: cross section dominated by Z productionHigh masses

d quark contribution only slightly enhanced

Low masses: cross section dominated by Drell-Yan production of Low masses dimuon pairs from virtual photons

more sensitive to u quark

This difference as function on Q ²will allow us to differentiate up type from down type quarks

~ 0.29 ~ 0.37

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 Uncertainty of Drell-Yan dimuon production cross section at low mass increases as y increases ( and as the mass increases )

 In the region that we will be able to

study the uncertainty is maximum 10%

 So by measuring in this region we will be able to constrain this uncertainty!

Uncertainty Uncertainty

R.Thorne

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Invariant mass Invariant mass

Lepton filter:

pT > 4GeV

|eta| < 2.5

Truth study

• Use one lepton filter to be unbiased in the trigger study, and ideally to be able to use mu6 at the early stages of run

No filter – 100000 events 1 filter – 42905 events 2 filters – 20903 events

At low M:

- Big cross section even with filters

- More background and lower trigger efficiency

- Cross-section = 7.13 nb for a mass range from 8 to 60 GeV (PYTHIA). This corresponds to 400000 events with one filter in 100 pb^-1

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x measurement x measurement

 At Q² = 100 we can measure down to x= 10^-4

8GeV < M < 60 GeV 2 lepton filters

(pT> 4GeV)

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Request MC for studies Request MC for studies

• Want to measure the mu channel because:

- possibly cleaner

- the electron channel has higher trigger threshold - 2mu4 is unprescaled

• Request MC sample with: - 500000 events

- 8 < M < 60 GeV, cme = 10 TeV

- 1 lepton filter pT > 3GeV (to measure trigger threshold at 4GeV)

• Test sample generated by Pavel Staroba and validated in 14.2.20. Reference to the validation talk @ http://indico.cern.ch/conferenceDisplay.py?confId=59906

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Conclusions Conclusions

• Accurate knowledge of PDFs is required in precision phenomenology studies at the LHC

• Excellent cross check when measuring in regions of direct overlap between 2 experiments

 Low-mass Drell-Yan production needed in order to investigate new region of phase space not yet studied

• Signal MC needed

• One lepton filter ideal for an unbiased study (useful if using mu6 instead of 2mu4)