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Minimum bias and underlying event studies at ATLAS

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Title: Minimum bias and underlying event studies at ATLAS


1
Minimum bias and underlying event studies at ATLAS
  • Arthur M. Moraes
  • Brookhaven National Laboratory

University of Florida HEP Seminar
Gainesville, March 21, 2006
2
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3
Outline
  • LHC and ATLAS
  • Hadron collisions how can we model them?
  • Describing soft hadronic interactions
  • Minimum-bias measurements
  • The underlying event in jet analysis
  • LHC predictions
  • New (improved?) models new measurements
  • Measuring minimum bias and the underlying event
    at ATLAS
  • Concluding remarks.

4
Outline
  • LHC and ATLAS
  • Hadron collisions how can we model them?
  • Describing soft hadronic interactions
  • Minimum-bias measurements
  • The underlying event in jet analysis
  • LHC predictions
  • New (improved?) models new measurements
  • Measuring minimum bias and the underlying event
    at ATLAS
  • Concluding remarks.

5
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6
LHC (Large Hadron Collider)
p-p collisions at vs 14TeV
bunch crossing every 25 ns (40 MHz)
  • low-luminosity L 2 x 1033cm-2s-1 (L 20
    fb-1/year)
  • high-luminosity L 1034cm-2s-1
  • (L 100 fb-1/year)

7
ATLAS A Toroidal LHC AparatuS
Muon Detectors
Liquid Argon Calorimeter
Tile Calorimeter
TRT Tracker
Toroid Magnets
Pixel Detector
Solenoid Magnet
SCT Tracker
8
ATLAS A Toroidal LHC AparatuS
7,000 tons
Multi-purpose detector coverage up to ?
5 design to operate at L 1034cm-2s-1
Inner Detector (tracker) Si pixel strip
detectors TRT 2 T magnetic field coverage up
to ?lt 2.5.
44m
  • Calorimetry
  • highly granular LAr EM calorimeter (?lt 3.2)
  • hadron calorimeter scintillator tile (?lt 4.9).

22m
  • Muon Spectrometer
  • air-core toroid system
  • (? lt 2.7).

9
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10
ATLAS installation progress
11
Physics goals and potential for the first year (a
few examples)
Expected event rates for ATLAS (or CMS) at L
1033 cm-2 s-1
  • Already in the first year, large statistics
    expected from
  • known SM processes (understand the detector and
    physics at the LHC)
  • several new physics scenarios can be tested

12
  • LHC and ATLAS
  • Hadron collisions how can we model them?
  • Describing soft hadronic interactions
  • Minimum-bias measurements
  • The underlying event in jet analysis
  • LHC predictions
  • New (improved?) models new measurements
  • Measuring minimum bias and the underlying event
    at ATLAS
  • Concluding remarks.

13
pp collisions at the LHC
  • Essentially all physics at LHC are connected to
    the interactions of quarks and gluons (small
    large transferred momentum).
  • Hard processes (high-pT) well described by
    perturbative QCD
  • Soft interactions (low-pT) require
    non-perturbative phenomenological models

14
Models for soft hadronic interactions
  • Questionable modelling for
  • Energy dependence
  • Minimum-bias and UE hard component
  • Hard/soft correlation
  • Uncorrelated soft scatter HERWIG/UA5 model
    (S.U.E.) (http//hepwww.rl.ac.uk/theory/seymour/he
    rwig/ )

15
Minimum-bias events
  • A minimum-bias event is what one would see with a
    totally inclusive trigger.
  • On average, it has low transverse energy, low
    multiplicity. Many can be diffractive (single and
    double).
  • Experimental definition depends on the
    experiments trigger!
  • Minimum bias is usually associated to
    non-single-diffractive events (NSD), e.g. ISR,
    UA5, E735, CDF,

stot 102 - 118 mb
sNSD 65 - 73mb
(PYTHIA)
(PHOJET)
(PYTHIA)
(PHOJET)
  • At the LHC, studies on minimum-bias should be
    done early on, at low luminosity to remove the
    effect of overlapping proton-proton collisions!

16
Minimum bias events Trigger simulation vs MC
direct selection
-
-
-
Switch NSD inelastic processes in the MC PYTHIA
ISUB 94 and 95 PHOJET IPRON 1, 4 and 7.
-
Selecting NSD inelastic processes in the MC event
generators, minimum bias events are reproduced
reasonably well.
17
Minimum bias data
Multiplicity information nch, dN/d?, KNO, FB,
etc.
Experiment
Colliding beams
CERN ISR
pp at vs 30.4, 44.5, 52.6 and 62.2 GeV
-
UA5 SPS
pp at vs 200, 546 and 900GeV
Set p0, K0s and ?0 stable
-
CDF - Tevatron
pp at vs 1.8TeV
E735 - Tevatron
  • Data samples are (usually) corrected for
    detector effects (pT cuts, limited ? range, etc.)

18
Tuning models to minimum-bias data
F(z) ltnchg gt P(nchg)
19
Underlying event in charged jet evolution(CDF
analysis Run I data)
Phys. Rev. D, 65 092002 (2002)
  • All particles from a single particle collision
    except the process of interest.
  • Sometimes, the underlying event can also be
    defined as everything in the collision except the
    hard process.
  • It is not only minimum bias event!

CDF The Collider Detector at Fermilab
20
ltNchggt distributions (particles from different
angular regions)
Phys. Rev. D, 65 092002 (2002)
ltNchggt - event
21
ltNchggt and ltpTsumgt distributions (particles from
the underlying event)
Average multiplicity of charged particles in the
underlying event associated to a leading jet with
Ptljet (GeV).
22
Tuning models to the underlying event
23
pT spectrum charged particles in the underlying
event
Phys. Rev. D, 65 092002 (2002)
24
PARP(67) Q2 scale of the hard scattering is
multiplied by PARP(67) to define the maximum
parton virtuality allowed in showers
pTljet gt 30 GeV
PARP(67)4 harder pT spectrum
PARP(67)1
25
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26
Tuned models (a few examples)
27
LHC predictions pp collisions at vs 14 TeV
28
LHC predictions pp collisions at vs 14 TeV
dNchg/d? at ?0
LHC
vs (GeV)
  • PYTHIA models favour ln2(s)
  • PHOJET suggests a ln(s) dependence.

29
How are we going to improve our model predictions?
  • New models

30
  • Look at more measurements

31
  • Look at more measurements

32
  • LHC and ATLAS
  • Hadron collisions how can we model them?
  • Describing soft hadronic interactions
  • Minimum-bias measurements
  • The underlying event in jet analysis
  • LHC predictions
  • New (improved?) models new measurements
  • Measuring minimum bias and the underlying event
    at ATLAS
  • Concluding remarks.

33
Early measurements with minimum bias data
  • Min-bias events are associated with inelastic pp
    collisions
  • Atlass first data!
  • Dominated by soft interactions
  • Obvious first measurements with min-bias data
    are
  • dNch/d?
  • dNch/dpT

(Only require several thousand events)
  • Weve made many min-bias studies usually
    comparing MC predictions with data from ISR, SPS
    and Tevatron experiments and extrapolating to
    LHC energies.
  • Now time to start using the full ATHENA software
    chain, and ask how well can we make these
    measurements when ATLAS data comes along?

Started looking at dNch/d? at ? 0
34
Charged particle density at ? 0
(Only need central inner tracker and a few
thousand pp events)
35
Measuring the minimum bias events at ATLAS
  • Minimum-bias trigger (commissioning
    low-luminosity runs)

Minimum bias trigger scintilator
36
  • Simulated events

Track pT gt 400 MeV
37
Measuring the minimum bias events at ATLAS
  • Reconstructing tracks

Black Generated (Pythia6.2) Blue
TrkTrack iPatRec Red TrkTrack xKalman
dNch/d?
  • Only a fraction of tracks reconstructed, because
  • limited rapidity coverage
  • can only reconstruct track pT with good
    efficiency down to 500MeV, and most particles in
    min-bias events have pT lt 500MeV.

h
dNch/dpT
Reconstruct tracks with 1) pTgt500MeV 2)
d0 lt 1mm 3) B-layer hits gt 1 4)
precision hits gt 8
  • Previous dNch/d? measurements published for pT gt
    0, so need to apply correction factor. Biggest
    systematic uncertainty?

pT (MeV)
38
Measuring the underlying event at ATLAS
ET jet 1 1.63TeV
ET jet 2 1.16TeV
39
Njets gt 1, ?jet lt 2.5, ETjet gt10 GeV,
?track lt 2.5, pTtrack gt 1.0 GeV/c
Selecting the underlying event
charged particles (generator level)
40
Njets gt 1, ?jet lt 2.5, ETjet gt10 GeV,
?track lt 2.5, pTtrack gt 1.0 GeV/c
Selecting the underlying event
41
Concluding Remarks
  • Current minimum bias and underlying event data
    can be described with appropriate tunings for
    PYTHIA and PHOJET (JIMMY4.1 can be tuned to
    describe the UE data at the Tevatron!).
  • There are sizeable uncertainties in LHC
    predictions generated by different models.
  • We need to understand better how to tune the
    energy dependence of the event activity multiple
    parton scattering rate?
  • New models offer new insights on soft-hadronic
    physics.
  • New measurements at different colliding energies
    will certainly improve our understanding of the
    phenomenology.
  • At the LHC, the best chance to measure
    minimum-bias and the UE is at the low (very low?)
    luminosity runs. We are currently developing
    systematic studies on strategies to perform these
    measurements at ATLAS.

42
Backup
43
Dijet azimuthal decorrelation
hep-ex/0409040 Sep. 2004
Jets are defined in the central region using
seed-based cone algorithm (R0.7)
leading jet pTmax gt 75 GeV
second leading jet pTmax gt 40 GeV
both leading pT jets yjet lt 0.5
44
hep-ex/0409040 Sep. 2004
45
Dijet azimuthal decorrelation ISR
PARP(67) defines the maximum parton virtuality
allowed in ISR showers (PARP(67) x hard scale Q2)
PARP(67)1 (default) distributions underestimate
the data! Need to increase the decorrelation
effect, i.e. increase radiative and multiple
interaction effects.
Increasing PARP(67) (from 1 to 4) the azimuthal
decorrelation is increased.
Best value is somewhere between PARP(67) 1 and
4!
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