Heavy Ion Collisions at LHC with the ATLAS Detector

1
Heavy Ion Collisions at LHC with the ATLAS
Detector

• Helio Takai
• takai_at_bnl.gov
• Brookhaven National Laboratory

(J. Nagle, B. Cole and S. White)
Hadron 2002 Bento Gonçalves,
April 2002
2
Before we start

• First of all my sincere thanks to the organizers !

• After I accepted the invitation I got worried !

What can I talk about? I decided to talk about
heavy ion physics with the ATLAS detector that it
is still far in the future. However it has been
our experience in ATLAS that help from our
theoretical colleagues in planning the
experiments has been a big plus. So this is the
spirit of this presentation.
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Before we start
Another reason why I got worried is because I
have been away from for
a long time - almost 8 years!

So I will behave myself, and make no jokes about
Our Lady of Perpetual Motion, the Church of
Christ Geologist or the Jojoba Witnesses. However
one cant pass the opportunity to mention that
everybody missed one good heavy ion experiment
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Plan for this presentation

1. Flirting with heavy ions
2. The ATLAS detector
3. Heavy Ion Physics at LHC energies
4. A nuclear physics program for ATLAS
5. What is going to happen next
6. Summary and conclusions

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ATLAS and Heavy Ions

• ATLAS is an experiment designed to study the
  origins of the electroweak symmetry breaking in
  proton-proton collisions, search for SUSY, and
  exotica - e.g., extra dimensions.
• ATLAS is designed to acquire data at full LHC
  design luminosity of L1034 cm-2s-1, or 40 MHz
  collision rate.
• The detector is designed to cover a range of
  hlt5.0 in rapidity with inner tracking,
  calorimetric coverage and muon spectrometer.
• ATLAS is also a large collaboration with over
  2,000 physicists from 157 institutions worldwide,
  including Brazil.

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ATLAS and Heavy Ions

• Recent RHIC data suggests that jets may be
  quenched.
• CERN theorists have been sponsoring a one year
  long workshop on hard probes in heavy ion
  physics.
• This has sparked renewed interest by a group of
  physicists within ATLAS to revisit the heavy ion
  physics program with the detector.

RAA
2
1
0
2
4
pT(GeV/c)
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ATLAS and Heavy Ions

• Large calorimetric coverage detector are well
  suited for jet physics
• Jets at LHC heavy ion energies are similar to
  Tevatron jets.
• For about one year, since the workshop in BNL, we
  have been developing a nuclear physics program
  for the ATLAS detector.
• However there are many hurdles to overcome
  people, funding, etc

8
The people

• The institutions involved in ATLAS heavy ions
  working group are
• Brookhaven National Laboratory
• CERN
• Columbia University
• Prague
• Rio de Janeiro
• University of Geneva
• Helsinki

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LHC and heavy ions

• LHC, the Large Hadron Collider, is now under
  construction and will accelerate heavy ions at an
  energy of 2.75 TeV/nucleon
• The energy available at the center of mass of a
  Lead-Lead collision is well over a 1,000 TeV.
• The energy density is expected to be about 30
  times what is currently observed at RHIC
• New and exciting physics could be observed by
  probing the hot QCD matter with hard probes.
• But what is ATLAS exactly? Where is the
  experiment?

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CERN
CERN
BNL
Where is CERN? It is in Switzerland, not
Swaziland and about 6,000 km from New York.
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CERN
Mt. Blanc
CMS
Mt Blanc des Americaine
ATLAS
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The ATLAS detector
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Inner Detectors
The Semiconductor Tracker are also silicon
devices.
The ATLAS inner detector is composed of three
systems Pixel, SCT and TRT
The transition radiation tracker are gas
detectors and used to identify electrons.
The pixel detectors are silicon devices and are
located near to the collision point.
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Inner Detectors
The same event, in full and low
luminosity running conditions.
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Calorimeters

• Calorimeters in ATLAS cover a wide range of
  pseudo-rapidity, hlt5.
• The electromagnetic calorimeter is realized in
  liquid argon technology
• The hadronic calorimeter is implemented as a
  iron-scintillator device, except in the forward
  direction.
• The very forward region is covered by axial drift
  liquid argon calorimeter.

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Electromagnetic Calorimeter
Used by the Inner tracker
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Electromagnetic Calorimeter Segmentation
The Electromagnetic Calorimeter is segmented
longitudinally and transversely. Will be used for
p0 identification.
Segmentation helps the identification of g in the
background of p0s.
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Hadronic Tile Calorimeter
The hadronic tile calorimeter hugs the liquid
argon electromagnetic calorimeter. It is built in
iron-scintillator technology and readout by WLS
optical fibers.
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Hadronic Tile Calorimeter
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Calorimeter Characteristics
Electromagnetic Cal. Energy Resolution
EM Angular Resolution
EM Timing Resolution
Hadronic Calorimeter Energy Resolution
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Jet Energy Resolution
hadronic
Large acceptance allows for detection of
back-to-back jets.
Jet Energy Resolution
Electromagnetic
Ha! But this is for pp collisions!!!
Di-jet event in ATLAS
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Muon Spectrometer
Muon detectors
Air Core Toroidal Magnet System
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Muon Spectrometer
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Muon spectrometer
It takes 3 GeV to go through. The rest stops!
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Muon Spectrometer Performance
Spectrometer only
Spectrometer ID
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The case for Heavy Ions

• ATLAS is a detector appropriate for high pT
  physics.
• It has a finely segmented calorimeter which is
  appropriate for jet physics. The energy
  resolution for jets is superb.
• It has a stand alone muon spectrometer with good
  momentum resolution.
• It is designed for high data rates for
  proton-proton collisions
• But, is there interesting physics to be done in
  the high Q2 regime?

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Heavy Ion collisions
the picture says it all !!!!
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Gluon Densities
HERA experiments have observed a dramatic
increase in the gluon density at low x. This
increase must end at some point when the gluon
density saturates. Large Hadron Collider Pb-Pb
collisions probe the gluon structure below x10-3
- 10-5. Note that xg(x) is enhanced by A1/3
6 in Pb over the proton.
RHIC
LHC
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LHC and RHIC
The saturation scale is much larger at the LHC
than at RHIC. Thus, the initial partonic state
may be dominated by the saturation region
(described as a color glass condensate). Also,
the cross section for high pT processes is much
larger, thus yielding better pQCD calibrated
probes of the possible gluon plasma.
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Freeing the Gluons (the QLF)
In a future Electron-Ion Collider (EIC) one can
probe the low-x gluon structure one gluon at a
time. At the LHC, tens of thousands of gluons,
quark and antiquarks are made physical in the
laboratory in every collision !
Very complementary physics.
Then we can study the nature of this very hot
bath of partons (QGP) ! The plasma should be
hotter and live longer than at RHIC.
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Jet Probes of the Plasma
Partons are expected to lose energy via induced
gluon radiation in traversing a dense partonic
medium. Coherence among these radiated gluons
leads to DE a L2
q
q
We want to measure the modification of jet
properties as we change the gluon density and
path length.
Baier, Dokshitzer, Mueller, Schiff,
hep-ph/9907267 Gyulassy, Levai, Vitev,
hep-pl/9907461 Wang, nucl-th/9812021 and many
more..
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ATLAS Jet Rates
In one month of Pb-Pb running with three
experiments at LHC, ATLAS will measure an
enormous number of jets.
Vitev - extrapolated to Pb-Pb
ATLAS accepted jets for central Pb-Pb Jet pT gt
50 GeV 30 million ! Jet pT gt 100 GeV
1.5 million Jet pT gt 150 GeV 190,000 Jet pT gt
200 GeV 44,000
Note that every accepted jet event is really an
accepted jet-jet event since ATLAS has nearly
complete phase space coverage !
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ATLAS Jet Measurements
ATLAS can measure jets with E gt 70 GeV with
reasonable resolution and efficiency in the
highest multiplicity central Pb-Pb events. More
detailed studies are currently underway. Substant
ial background reduction can be achieved by
simultaneously finding back-to-back jets.
200 GeV jet overlay on central Pb-Pb event with
ATLAS segmentation
h
f
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Jet Profile Analysis
The induced gluon radiation may be measurable due
to the broader angular energy distribution than
from the jet.
proton-proton jet cone
Possible observation of reduced jet cross
section from this effect.
U.A. Wiedemann, hep-ph/0008241. BDMS,
hep-ph/0105062.
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Fragmentation Functions
ATLAS can measure identified p0 and h mesons via
photons. Excellent energy and timing resolution
will help to limit background.
2nd sample 3x5 cluster invariant mass calculation
for identification.
2nd sample cluster two g shower shape
identification.
Dh x Df 0.025 x 0.025
Photon opening angle in degrees
1st sample cluster two g separation and total
energy measure in 2nd sample.
Dh x Df 0.003 x 0.1
pT (GeV)
Background and resolution studies are underway.
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g-Jet Physics
Wang and Huang, hep-ph/9701227
ATLAS g-jet rate is very large, thus allowing for
detailed studies. In one month, over 1000 events
with g energy 60 GeV in a 1 GeV bin ! Above a
certain pT30 GeV, ATLAS can no longer cleanly
separate single g from two g resulting from a p0
decay.
We are investigating whether with isolation cuts
on a single high energy shower and an opposite
side jet, which process dominates (1) g-jet
events (2) jet-jet events with one jet with a
high z fragmentation
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Beauty Jets
m
Radiative quark energy loss is qualitatively
different for heavy and light quarks. Finite
velocity of heavy quarks at finite pT leads to
suppression of co-linear gluon emission
(dead-cone effect). ATLAS can tag B jets
via a high pT muon in the muon detectors.
D
n
B
b
b
Y.L.Dokshitzer and D.E. Kharzeev, hep-ph/0106202
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g-Jet Physics
One can also study virtual photon-jet events,
where g ? m m-. Rate is down two orders of
magnitude from g-jet. Good muon coverage makes
this possible. In one month in central Pb-Pb,
ATLAS would accept 10,000 events with pT gt 40
GeV. Z0-jet reconstruction is possible, but
less than 500 total Z0 events per month.
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Probes of Deconfinement
Upsilon states (1s,2s,3s) span a large range in
binding energy and thus their suppression pattern
may allow for a mapping on the onset in the
screening on the long range color confining
potential.
ATLAS is currently investigating the mass
resolution in the muon system with alternate
track matching algorithms.
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Correlated Global Measures
ATLAS will measure many global observables and
have high statistics for correlating them with
high pT probes. 1) Transverse energy 2)
Charged particle multiplicity 3) Zero degree
energy 4) Reaction plane
Jet observables as a function of reaction
plane Azimuthal distribution of high pT p0 and h
Coverage over a broad range of pseudorapidity
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Unique Opportunity at LHC
A parton propagating through the hot QCD media
will radiate gluons loosing energy. The overall
energy is, however, conserved therefore the
measured jet properties will be modified. The
modification of the jet fragmentation function is
the most sensitive physical quantity. Other
expected modifications are the cone radius,
etc. Whatever measurement is done, it is
worthwhile balancing the jet energy by the
opposite g, g or Z0. Therefore large coverage is
beneficial.
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Heavy Quarks
Dokshitzer and Kharzeev have suggested that heavy
quarks would not radiate as much as light quarks
when propagating through the media. Therefore
b-jets should not be quenched as much as light
quark jets. b-jets can be identified by the
dislocated vertex or by tagging jets with the
associated muon. t-quarks although interesting do
not live long enough to feel the media.
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p-A Physics in ATLAS

• Study of p-A collisions is essential _at_ LHC
• To provide baseline for heavy ion measurements.
• Physics intrinsically compelling
• Mini-jet production, multiple semi-hard
  scattering.
• Shadowing test of Eikonal QCD.
• Gluon saturation probe QCD _at_ high gluon
  density.
• Test factorization.
• Multiple hard scattering Measure parton
  correlations in nucleon (and nucleus ?)
• ATLAS is ideal detector for p-A studies
• ? coverage, calorimeter performance, b tagging,
  lepton identification, inner tracking.

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Test Perturbative QCD Factorization

• ATLAS ( CMS) will provide the most comprehensive
  detectors ever built for study of hard physics in
  p-A.
• Simultaneously study di/?/Z-jet, Z, W, b-bbar,
  DY,
• Check that same nuclear PDFs can reproduce all
  results.
• Ask questions like
• Does jet fragmentation change in bath of dense
  gluons from nucleus ?
• Are hard radiative processes the same in p-A
  (A-A) collisions ?
• Study double parton scattering events
• CDF ?3 jet data (twice the expected rate)
  interpretation
• Stronger than expected parton correlations in
  nucleon.
• Correlation between valence quark separation
  gluonsea distribution ??
• Study _at_ LHC using WW/W-W-, double b-bbar, ?3
  jet, W2 jet
• Different processes involve different valence-sea
  combinations.
• In both p-p and p-A (p-A relaxes correlations in
  one nucleon).
• Estimated cross-section for double b-bbar, 10
  ?Barn in p-p (10-3/p-Pb ev)

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Summary of pA physics

• ATLAS detector provides an ideal tool for the
  study of perturbative high gluon density QCD in
  p-A collisions.
• Measurements of mini-jet production and gluon
  shadowing are critical for heavy ion collisions _at_
  LHC.
• Measurements will complement RHIC EIC
• Can match RHIC Qs (2 GeV ?) in cleaner events.
• Use azimuthal rapidity coverage to test
  saturation model predictions.
• Comparing to EIC data can test
• Equal only at leading twist but saturation ? all
  twist terms important.
• McLerran Not valid for Q2 ltlt ?s2Qs2.
• Make detailed study of pQCD in nuclear
  environment.
• Study parton correlations in proton/nucleus w/
  double parton scattering events.

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RHIC and LHC, ghadron colliders
gbeamRhic100
S2mA?
gtNucleus at rest,effective Lorentz
geff2gbeam2-1
Heavy Ions
e-Hadron collider
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Ultra Peripheral Collisions
Heavy Ion Physics Opportunities with a tool that
we are just learning to exploit (c.f. ee-
physics)
LHC energy scale Equivalent g flux Up to 100s
of TeV
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Ultra Peripheral Collisions
The photon spectrum
The photon spectrum depends on the rigorous
treatment of the form factors.
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Summary of the physics program
The ATLAS Nuclear Physics program includes
Global variable measurements measurement of ET,
dET/dh, charged particle multiplicity
N, dN/dh, and elliptical flow. Jet Physics
gjet, gjet, Z0jet Heavy Quarks
beauty jets Quarkonia ? suppression
pA physics Ultra Peripheral Collisions
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Conclusions and Outlook
ATLAS is a world class detector designed for high
rate proton-proton collisions and will provide
great opportunity to study high Q2 physics in
nucleus nucleus collisions.
The ATLAS heavy ion working group is being formed
and simulation is under way. Input from Theorist
and Experimentalists are very welcome
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Conclusions and Outlook
Strong input from phenomenologists is the ticket
for a successful program. We have experience in
the proton-proton program and has made a huge
difference. In early april the proposed program
was presented to DOE as a letter of intent and
will formalize a proposal to LHCC late fall (in
the northern hemisphere) - so join in!!!