Transcript Document

Heavy Ion Collisions at LHC with the
ATLAS Detector
Helio Takai
[email protected]
Brookhaven National Laboratory
(J. Nagle, B. Cole and S. White)
Hadron 2002
Bento Gonçalves, April 2002
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.
Helio Takai, Brookhaven National Laboratory
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April, 2002
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 can’t pass
the opportunity to mention that
everybody missed one good heavy
ion experiment:
Helio Takai, Brookhaven National Laboratory
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April, 2002
Plan for this presentation
i.
Flirting with heavy ions
ii. The ATLAS detector
iii. Heavy Ion Physics at LHC energies
iv. A nuclear physics program for ATLAS
v. What is going to happen next
vi. Summary and conclusions
Helio Takai, Brookhaven National Laboratory
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April, 2002
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
L=1034 cm-2s-1, or 40 MHz collision rate.
The detector is designed to cover a range of |h|<5.0 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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April, 2002
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.
2
This has sparked renewed
interest by a group of physicists
within ATLAS to revisit the heavy
ion physics program with the
detector.
Helio Takai, Brookhaven National Laboratory
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0
6
2
4
pT(GeV/c)
April, 2002
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…
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April, 2002
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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April, 2002
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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April, 2002
CERN
CERN
BNL
Where is CERN? It is in Switzerland, not Swaziland…
and about 6,000 km from New York.
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April, 2002
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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April, 2002
Inner Detectors
The same event, H  bb in full and low luminosity running conditions.
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April, 2002
Calorimeters
Calorimeters in ATLAS
cover a wide range of
pseudo-rapidity, |h|<5.
The electromagnetic
calorimeter is realized in
liquid argon technology
The hadronic calorimeter is
implemented as a ironscintillator device, except in
the forward direction.
The very forward region is
covered by axial drift liquid
argon calorimeter.
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April, 2002
Electromagnetic Calorimeter
Used by the
Inner tracker
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April, 2002
Electromagnetic Calorimeter Segmentation
The Electromagnetic
Calorimeter is segmented
longitudinally and transversely.
Will be used for p0
identification.
g
p0
g
Segmentation helps the
identification of g in the
background of p0s.
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April, 2002
Hadronic Tile Calorimeter
The hadronic tile calorimeter ‘hugs’
the liquid argon electromagnetic
calorimeter. It is built in ironscintillator technology and readout
by WLS optical fibers.
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Hadronic Tile Calorimeter
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Calorimeter Characteristics
Electromagnetic Cal.
Energy Resolution
E
E
EM Angular Resolution

 

EM Timing Resolution

Hadronic Calorimeter
Energy Resolution
 
E
E

Helio Takai, Brookhaven National Laboratory

10%
 0.7%
E
20
60mrad
E
4.15
ns GeV
E
(p ) 
47%
 2%
E
April, 2002
Jet Energy Resolution
hadronic
Large acceptance allows for
detection of back-to-back
jets.
Jet Energy Resolution :

E
E

50%
 2%
E
Electromagnetic

Ha! But this is for pp
collisions!!!
Di-jet event in ATLAS
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Muon Spectrometer
Toroid! Toroid! Toroid!
Muon detectors
Air Core Toroidal Magnet System
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Muon Spectrometer
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April, 2002
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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April, 2002
Heavy Ion collisions
…the picture says it all !!!!
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April, 2002
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 x~10-3 - 10-5.
Note that xg(x) is enhanced by A1/3
~ 6 in Pb over the proton.
LHC
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RHIC
April, 2002
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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April, 2002
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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April, 2002
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.
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Baier, Dokshitzer, Mueller, Schiff, hep-ph/9907267
Gyulassy, Levai, Vitev, hep-pl/9907461
Wang, nucl-th/9812021
and many more…..
April, 2002
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
Jet pT
Jet pT
Jet pT
> 50 GeV
> 100 GeV
> 150 GeV
> 200 GeV
30 million !
1.5 million
190,000
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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April, 2002
ATLAS Jet Measurements
ATLAS can measure jets with E > 70 GeV with reasonable resolution
and efficiency in the highest multiplicity central Pb-Pb events. More
detailed studies are currently underway.
Substantial 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
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April, 2002
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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April, 2002
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
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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April, 2002
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 pT~30 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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April, 2002
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).
D
n
B
b
b
ATLAS can tag B jets via a high pT
muon in the muon detectors.
Y.L.Dokshitzer and D.E. Kharzeev, hep-ph/0106202
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April, 2002
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 > 40 GeV.
Z0-jet reconstruction is possible,
but less than 500 total Z0 events
per month.
Helio Takai, Brookhaven National Laboratory
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April, 2002
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.
state
Mass [GeV}
B.E. [GeV]
Td/Tc
J/y
3.096
0.64
---
cc
3.415
0.2
0.74
y'
U(1s)
3.686
0.05
0.15
9.46
1.1
---
cb
9.859
0.67
---
U(2s)
10.023
0.54
0.93
cb'
10.232
0.31
0.83
U(3s)
10.355
0.2
0.74
ATLAS is currently investigating the mass resolution in the muon
system with alternate track matching algorithms.
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April, 2002
Correlated Global Measures
ATLAS will measure many global observables and have high statistics
for correlating them with high pT probes.
1)
2)
3)
4)
Transverse energy
Charged particle multiplicity
Zero degree energy
Reaction plane
z
y
x
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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April, 2002
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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April, 2002
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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April, 2002
p-A Physics in ATLAS
• Study of p-A collisions is essential @ 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 @ high gluon density.
Test factorization.
Multiple hard scattering – Measure parton correlations in nucleon
(and nucleus ?)
• ATLAS is ideal detector for p-A studies
– h coverage, calorimeter performance, b tagging, lepton
identification, inner tracking.
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April, 2002
RHIC and LHC, g+hadron colliders
gbeamRhic=100
S=2mA’
=>Nucleus at rest,effective Lorentz geff=2*gbeam2-1
Heavy Ions
e-Hadron collider
1 dN() 2aZ 2 0.681hcg ef f

ln(
)
2 d
p
Rnucl eus  
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dN( ) 2a me  g ef f
 ln(
)
d
p

April, 2002
Ultra Peripheral Collisions
Heavy Ion Physics=
Opportunities with a tool
that we are just learning
to exploit (c.f. e+ephysics)
LHC energy scale
Equivalent g flux
Up to 100’s of TeV
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April, 2002
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 : g+jet, g*+jet, Z0+jet
Heavy Quarks : beauty jets
Quarkonia :  suppression
p+A physics
Ultra Peripheral Collisions
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April, 2002
Conclusions and Outlook
ATLAS is a world class detector designed for high rate protonproton 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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April, 2002
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!!!
Helio Takai, Brookhaven National Laboratory
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April, 2002