Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://nuclphys.sinp.msu.ru/conf/lpp14/240809/Eckweiler.pdf Äàòà èçìåíåíèÿ: Wed Sep 16 16:32:41 2009 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 00:42:52 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: jet

QCD Studies at LHC with the Atlas detector
Sebastian Eckweiler - University of Mainz
(on behalf of the ATLAS Collaboration)

· Introduction · Examples of QCD studies · Minimum bias & underlying event · Jet-physics · W/Z + Jets · Summary
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Introduction
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The Large Hadron Collider
pp / pp cross sections

· · · ·

p-p collision up to s = 14 TeV (x7 wrt Tevatron) Luminosities up to 1034cm-2s-1 (x30 wrt Tevatron) ~100 fb-1 per year at design luminosity Huge QCD cross sections jet( ET > 700 GeV) ~ 0.1 nb
(nb)
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Current schedule

· · ·

Start-up by mid-November First physics run in 2010 at s = 7-10 TeV
and lower luminosity

Aim to collect ~100pb-1 in first physics run:

Determines possible first physics!
Sebastian Eckweiler - University of Mainz Lomonosov Conference - August '09


The Atlas experiment

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What can be done with early data?
Process Process !(nb) Minimum Minimum8 10 bias bias Inclusive Inclusive 100 jets ­ jets ­ pT>200GeV pT>200GeV W ! e"W ! e15 " 1.5 Z ! e+e- ! e+eZ 0.2 Dibosons Dibosons !Ns-1 (nb) 108 107
-1 L=10pbL=10pb-1 -1 =10fb Ns-1 L=10fb L -1

L~1032 to 1033 cm-2 s-1 -2 s L~1032 to 1033 cm

-1

·

Extensive test beam beam characterization of Extensive test characterization of prototypes and final modules. Also used for for prototypes and final modules. Also used validation of G4 of G4 simulations. Shorvalidation simulations. t term plans:

107 10

12

1012 ~10

15

~10

15

· ·

Minimum bias & underlying event:

100 100

100 10

6

106~10

9

~10

9

15 15 1.5 1.5 10-3 0.2

15 10 1.5 10

5

10

5 ~108

~10 ~10 10

8

4

104~10 10 10

7

7

-3 1010

4

4

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commissioning; relation: early benchmark of MC commissioning; Angular de-cor Calibration with physics processes; Calibration with physics processes; generators Procedure valid valid sub-detectors, ECAL, - InclusivProcedure for yallforeall sub-detectors, ECAL, - e Jets: Reach be ond T vatron energy regime HCAL,HCAL, trackers, Muon Muon Chambers. inner inner trackers, Chambers. already in first data

Jets: Single beam and beam gasduring LHC LHC Single beam and beam gas runs runs during

· constrain uncertainties in MC generators from extrapolations to LHC energies in the ver y beginning `In situ' detector calibration: `In situ' detector calibration: baseline or understanding ·Cosmics fruns; runs; of pile up corrections Cosmics · ·

(Numbers for s = 14 TeV)

claiming any discovery of new physics!" claiming any discovery of new physics!"

Studies presented used s = 14 TeV and full Need to simulation Need to "re-discover the the LHC before detector"re-discover the SM at SM at the LHC before

·

Medium & long term plans:

· · ·

Use W, Z and top for calibration of detector and trigger Study W, Z, top and QCD multi-jets for proper background-estimation Improve current SM measurements to provide consistency tests of the underlying theor y
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few pb-1

several fb-1

Sebastian Eckweiler - University of Mainz

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Minimum bias measurements
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Experimental definition: any inelastic collision of two protons
Usually also referred to as `non-single-diffractive` (nsd): tot = elas + sd + dd + nd nsd
EPJ C 50, 435 (2007)

· ·

,Removal` of sd very difficult Extrapolations to LHC energies suffer from large energy gap in data

· · · · ·

tot 102 - 118 mb nsd 65 - 73 mb

LHC

Proper modeling of min. bias pile-up and underlying event requirement for high pT physics! Minimum bias studies should be done at low luminosity to minimize effects from pile-up Statistics only limited by allocated trigger bandwidth!

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Minimum bias measurements
·
· · · · · ·
Reconstructing minimum bias events:
Need to recover all charged particles -> main challenge: tracking! Default reconstruction only for track pT > 500 MeV
(lower energy particles curl up in magnetic field) pT ~ 250 MeV

Dedicated tracking software pushes low pT limit to ~150 MeV Avoids large corrections in uncovered phase space Integrated luminosity << 1 pb-1 Study done for s = 14 TeV

pT > 500 MeV

CERN-OPEN-2008-020

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Minimum bias measurements
· · ·
Charged particle multiplicity, pT > 150 MeV Corrections involve:

· · ·

Track-to-particle correction Vertex reconstruction correction Trigger (in)efficiency

Systematic uncertainties dominate:

Track selection cuts Mis-estimate of secondaries Vertex reconstruction bias Mis-alignment Beam-gas & pile-up Particle composition Diffractive cross sections

2% 1.5% 0.1% 6% 1% 2% 4%

Total:
Sebastian Eckweiler - University of Mainz

8%
CERN-OPEN-2008-020

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· · ·

Underlying event
· ·
Important check of MC generators ,jetty` environment - understanding needed for jet energy corrections CDF-Data used to tune ATLAS monte carlo
currently relying on Jimmy4.3 and PYTHIA6.416 (appropriately tuned) to describe UE both generators describe CDF data reasonably well < PT,Sum > - transverse region

< Nchg > - transverse region

ATLAS

ATLAS

A. Moraes at 4th Hera-LHC Workshop, 2006

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·
· · ·
< Nchg > - transverse region

Underlying event
Extrapolations to LHC (@ 14 TeV) energies differ
Generators agree in in transverse region Disagreement in in transverse region Validation needed with early data - combined with minimum bias description
< PT,Sum > - transverse region

LHC prediction

LHC prediction

A. Moraes at 4th Hera-LHC Workshop, 2006

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Dijet angular de-correlation
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Very early possible check of generators

· · ·

Sensitivity mainly to ISR / parton showering Uncomplicated measurement of = |jet1 -
~ : clean dijet event, small deviations due to soft radiation < : increasingly harder radation Probes transition from soft to hard radiation

jet2

|

Jet2



Jet1

·

Analysis divided into regions of leading jet pT
ATL-PHYS-PUB-2006-013

D0 data

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Dijet angular de-correlation
·
· ·
Very early possible check of generators

· · ·

Sensitivity mainly to ISR / parton showering Uncomplicated measurement of = |jet1 -
~ : clean dijet event, small deviations due to soft radiation < : increasingly harder radation Probes transition from soft to hard radiation

jet2

|

Jet2



Jet1

· ·

Analysis divided into regions of leading jet pT Again, extrapolations to LHC energies differ between generators
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LHC prediction D0 data
ATL-PHYS-PUB-2006-013

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Dijet angular de-correlation
· · ·
· ·
Cone jet algorithm (R = 0.7), |yJet| < 0.5 Defined two analysis regions:
Simulated 20k events for both regions: 1.6 pb-1 / 56 pb-1 integrated luminosity -> expect ~50 fold / doubled statistics by end of 2010 run 300 GeV < ET,Jet1 < 600 GeV; 600 GeV < ET,Jet1 < 1200 GeV
T,Jet2

Only ver y few further selections applied: E

> 80 GeV

Ag reement of measured and true distribution. Practically no corrections necessary!

ATLAS jet sample (J5) 300 < ET,Max< 600 GeV Cone - R = 0.7

ATLAS jet sample (J6) 600 < ET,Max< 1200 GeV Cone - R = 0.7

ATL-PHYS-PUB-2006-013

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Jet physics
· · · · · ·
Jets above 1 TeV will be first ,new physics`: O(100) Jets in 100pb-1 Di-jets provide easy access to x values of scattered partons -> constrain on PDFs esp. at high x and Q2 possible Currently large uncertainties in high x gluon densities estimated impact of ATLAS jet data

·

Toy-study made using ATLAS pseudo jet data: incl. cross section in bins of ||

Gluon fractional uncertainty

Not an early data analysis - simulated 10 fb-1 with 5% syst. uncertainty Significant improvement in gluon PDF! Systematic uncertainties dominate:

· · · ·

Luminosity Jet energy scale & resolution Jet trigger efficiencies Underlying event
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K. Rabbertz, 4th HERA and LHC Workshop

Sebastian Eckweiler - University of Mainz

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W/Z + Jets
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Important background in SM analysis & searches for physics beyond the SM Current MC generator and calculations differ by 10-60% Given 5% jet energy scale uncertainty, we`ll be competitive with generator-differences!

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reasonable goal for 1 fb-1

Comparably small PDF uncertainty

CERN-OPEN-2008-020

CERN-OPEN-2008-020

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Jets in cosmic ray data
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· · · · · · ·
Today`s data - tomorrow`s background
Muons experiencing catastrophic energy loss are visible as ,jets` study properties for future background rejection Good agreement of cosmic monte carlo and data
Normalized Number of Jets ATLAS Cosmic 2008 Preliminary Cosmic Data L1Calo Cosmic Single Muon MC QCD Dijet MC

Examined jet cleaning cuts: Most muon showers either in em. or had. calorimeter Fraction of total energy in EM calorimeter : 0.2 < JetEMFraction < 0.97: Number of clusters: NClus > 6 Rejection by factor ~1000

9 1 8 7 10-1 6
2 10-5 -3

4 3

10

4 10-2 1

10 0

-5

-0.2

0

0.2

0.4

0.6

0.8

1 1.2 Jet EM Fraction

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Jets in cosmic ray data
·
· · · · · · ·
Today`s data - tomorrow`s background
Muons experiencing catastrophic energy loss are visible as ,jets` study properties for future background rejection Good agreement of cosmic monte carlo and data
Normalized Number of Jets
5 100 10

â10

3

Examined jet cleaning cuts: Most muon showers either in em. or had. calorimeter Fraction of total energy in EM calorimeter : 0.2 < JetEMFraction < 0.97: Number of clusters: NClus > 6 Rejection by factor ~1000

90 9 1 8 104 80 70 7 10-1 3 106 60
2 50 10-5 102 40 4

ATLAS Cosmic 2008 Preliminary ATLAS Cosmic 2008 Preliminary Cosmic Data L1Calo Cosmic Single Muon MC Cosmic Data L1Calo QCD Dijet MC Cosmic Single Muon MC Before Cleaning Cuts After EMF Cut After EMF + NClus Cut

10 30 3 10 20 4 10-2 10 1 1 -5 10 0 0 -0.2

-3

500 0

0.2 1000 0.4

0.6 1500 0.8 2000 1 1.2 2500 Jet Transverse Energy (GeV) Jet EM Fraction

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Summary
·
· · · ·
ATLAS studied a variety of QCD analysis
Minimum bias and underlying event studies in early data will help to tune MC generators Angular decorrelation will discriminate between ISR models Early jet physics will reach beyond Tevatron`s kinematic limit Ready for the re-discovery of the Standard Model

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Jet calibration at ATLAS
Jet pT /pT-1

0.04 0.02 0 -0.02 -0.04 -0.06 -0.08

!

· ·
!/E

ATLAS

Focus on in-situ calibration procedures in early data Use /Z+jet events to validate/set jet energy scale
0.24 0.22 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0
0.0 < Jet Eta < 0.7 EM H1 Samp

Default ! selection Tight ! selection Truth
100 200 300 400 500 600 700 800 ! pT (GeV)

-0.1 0

ATLAS
500 1000 1500 2000

· · · ·
Jet Energy (GeV)

Longer term plans: Validate monte carlo based calibrations Based on energy density cell weighting Supposed to double energy resolution at high jet energies

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Jet physics
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· · · · ·
Jets above 1 TeV will be first ,new physics`

·

O(100) Jets in 100pb-1

Will provide test of pQCD in new energy regime Inclusive and multi jet cross section allows s measurement Multi-jets are significant background for several physics studies Statistical uncertainties will be small Systematic uncertainties:
CERN-LHCC-99-015

· · · · ·

Luminosity (if applicable): 5 - 10% Jet energy scale - conser vative: 10-20% in first data Jet trigger efficiencies Underlying event Theory: PDFs
K. Rabbertz, 4th HERA and LHC Workshop

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