US participation in Heavy Ion Physics with
Compact Muon Solenoid at LHC
M. Ballintijn,K Barish,R Betts, BE Bonner, W.Busza, D.Cebra, G.Eppley, E.Garcia,
F.Geurts, C.Halliwell, D.Hofman, P.Kulinich, W.Llope, M.Murray, G. van
Nieuwenhuizen, E.Norbeck, R.Nouicer, Y.Onel, C.Roland, G.Roland, R.Seto,
G.S.F.Stephans, B. Wyslouch, P.Yepes
MIT, Rice, TAMU, UC Davis, UC Riverside,
UI Chicago, U Iowa
CMS HI workshop
was held at MIT
February 2002
US Heavy Ion physics today
• RHIC has completed first long run of Au-Au at (sNN)
=200 GeV/c (Its highest energy)
– Expect to run for many years
• It has reached design luminosity
– Expect improvements
• The four detectors: BRAHMS, PHENIX, PHOBOS and
STAR
– BRAHMS and PHOBOS have short lifetime (<4 y)
• Much better detectors and running conditions than
ever before in the field of relativistic heavy ions
• Practically no involvement at LHC
Main RHIC physics and tools
• Equation of state of QCD: connection to lattice results predicting
phase transitions
• Chiral symmetry restoration
• Quark Gluon Plasma, How does a high-temperature quark-gluon
state emerge from the collision of nuclei?
• Photon and di-lepton probes ( J/), studying effects of the
plasma on particle production and survival
• Production of high pt particles is affected by the energy loss in
nuclear matter
Early results are accumulating. No large, obvious effects seen, but
some puzzles start to appear
Integrated Au-Au luminosity
PHENIX during last 10 days:
24 (mb)-1/week
Lave(week) = 0.4  1026 cm-2 s-1
Lave(week)/Lave(store) = 27 %
FY2001 – 02
100 GeV/amu
FY2000
(66 GeV/amu)
Multiplicity per nucleon-nucleon interaction as a function of collision energy for
heavy ions and for proton-antiproton collisions
Comparison of momentum spectra of particles AuAu vs pp
Too few high pt particles ?
Au  Au
RAA 
 N binary pp
STAR
Preliminary
Production of high pT particles, e.g. p0
Heavy Ion physics at the LHC
Increase energy (sNN) 200->5500 GeV
• Saturated gluon distributions in
colliding nuclei, initial state of
the system calculable in
perturbative QCD
• Plasma has higher temperature
and lives longer in partonic state
• Spectrum of quarkonia produced
with high statistics
• Jets clearly visible and
identifiable
• Z0 produced in large numbers
HI group in CMS has produced many excellent physics studies
Results need some update and extensions to take into account RHIC results
The experiments
ALICE: dedicated HI experiment
CMS: pp experiment with HI program
ATLAS: pp experiment, recent HI interest
Design of LHC experiments is affected by
the multiplicity of charged particles
Extrapolated to LHC:
dN/dh~1400
LHC?
LHC detectors
are being
designed for
dN/dy~8000
Peter Steinberg, 2002
Quarkonia, CMS is very good here
J/
 family
Detailed studies using full simulation,
reconstruction, background subtraction
dN/dy studied from 2500 to 8000
Very large event rate
Uses muon detector, outer tracker, pixels
Yield/month (kevents, 50% eff)
Pb+Pb Sn+Sn Kr+Kr
Ar+Ar
J/psi
28.7
210
470
2200
psi'
0.8
5.5
12
57
Upsilon
22.6
150
320
1400
Upsilon'
12.4
80
180
770
Upsilon''
7
45
100
440
’/  ratio
affected by
the initial
conditions:
a plasma
thermometer
(Ramona Vogt)
Can we use tracker ?
CMS Tracker Occupancy
• Calculated for PbPb dNch/dy=5000
• For reference: STAR TPC
occupancy reaches 22%
Jet fragmentation
•
•
•
Find jets using calorimetry
Study charged particle momenta inside of a jet using the tracker
For this study use 4-5 outer layers of the tracker (use conservative
resolution obtained in pp studies: AA plausible with low occupancy in
outer layers)
Particles in jet
Background
Global observables e.g. Et
Field OFF
Field OFF
Field ON
Field ON
Our proposal to DoE/Nuclear
• Extend the physics reach of the US heavy ion
community beyond RHIC’s energy scale
• Concentrate US physics effort on the study of
phenomena most likely to be affected by the energy
increase, the “hard probes”:
– Quarkonia and heavy quark production
– Jet production, jet-jet, jet-gamma and jet-Z0 correlations
• Provide US/RHIC expertise and tools to study high pt
processes in AA collisions at the LHC
• Use detector designed for high pt physics: CMS
• LHC starts in mid-2007 with pp, AA to follow in
2008(?): Be ready with strong group in 2008
Our proposal to DoE/Nuclear
Specific plans of the existing groups
• Physics studies, software development
• High Level Trigger code development + request to
fund 2/8 slices of Event Filter Farm (Rice, MIT, UC
Davis, UCR)
• Zero degree calorimeter (U Iowa, UIC, TAMU)
• Total ~ 5 M$ from DoE/Nuclear
• review took place 2&3 of April @ DoE, no final report
yet. Closeout conclusion: “continue studies, come
back later”
• competing with ALICE and ATLAS(!)
Centrality: Participants vs. Spectators
The collision geometry (i.e. the impact parameter) determines
the number of nucleons that participate in the collision
“Spectators”
Only ZDCs measure Npart
Zero-degree
Calorimeter
“Spectators”
Many things scale with Npart:
• Transverse Energy
• Particle Multiplicity
• Particle Spectra
“Participants”
Detectors at 90o
N part  A  N spec
Zero Degree Calorimetry for CMS
PAIR OF PANTS
(POP)
Beam pipe
splits 140m
from IR.
BEAM
ZDC
LOCATION
ALIGNMENT PINS
BEAM
INNER WALL
Z-AXIS
RHIC ZDCs work very well
High Level Trigger (HLT)
• All event data available:
– Fine data for Calorimetry
and Muon Detectors
– Tracker
• L1 in AA has larger backgrounds than
in pp due to underlying event.
• Efficiency trigger requires more careful
analysis. HLT can do a better job than
L1.
• HLT to play a greater role in AA
•
•
•
•
Refine triggered object
Allows to go lower in pT
Processing time O(s)
Filtering Farms of
commodity processors
(Linux)
AA Event Size & Data Flow
Pixel
ECAL
Muon
3000
Pixel
Si Tracker
ECAL
HCAL
Muon Det.
Total
# Channels
(1000)
45,000
12,000
230
14
400
57,644
Event Size
2500
Event Size (Kbytes)
Detector
Si Tracker
HCAL
2000
1500
1000
500
0
pp
OO
ArAr SnSn PbPb
Data Flow and Rates
HLT better
trigger job
L1
HLT
CPU Estimate of HI Online Tracking
dN/dy
Acceptance, |h|<
Max. Number Layers
Clustering
Clustering Type
All Factors
June 16, 2000 First STAR event
tracked on line
• Use STAR (Rice)
experience
• Scale linearly with #
clusters from STAR
AuAu to CMS PbPb
• Assume Moore’s Law
STAR
600
1.5
45
1
1
MIP s/event
MIP s/ CPU
Event/CPU/s
Max Input L3/HLT (Hz)
Minimum Number CPUs
CMS
3000
2.5
13
2
2
STAR
386
460
1.2
100
84
Moore’s Law: double/18 months
for ~8 years
Scaling
5.0
1.7
0.3
2.0
2.0
9.6
CMS
3706
13800
3.7
7400
1987
Near term plan
• Work together with heavy ion group in CMS on
expanding the physics scope of the heavy ion
program
• Develop good physics case for tracking in CMS, at
HLT level and in offline analysis
• Develop and possibly prototype Zero degree
calorimeter.
• Fight for funding from DoE, expand collaboration
Summary
• CMS has a potential to be an excellent detector for
heavy ion physics
• RHIC results will really determine the scope of
interesting physics, the US community will have
direct access to the RHIC experience
• HI physicists could provide useful expertise,
manpower and source of funding
Z0 production
•
•
•
Z0-mm can be
reconstructed with high
efficiency
A probe to study nuclear
shadowing and parton
energy loss
Z0 also proposed as
reference to 
production.
– Nuclear effects may
depend on mass
MZ>M
– Different production
mechanisms:
• Z0: antiquark-quark,
quark-gluon and
antiquark-gluon.
• : gluon-gluon.
Open b: high mass m+m- : medium
modification, energy loss for b-quarks
Muon tracking
(muon+tracker)
Displaced vertices
(pixels)