Gianluca Usai

1
The QCD phase diagram and search of critical
point dilepton measurements in the range 20-158
AGeV at the CERN-SPS
Gianluca Usai University of Cagliari and INFN
Electromagnetic Probes of Strongly interacting
Matter in ECT Trento - 23/05/2013
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Outline

• NA60 past precision di-muon measurements in HI
  at top-most
• SPS energy (158 AGeV)

• Search of the QCD critical point and onset of
  deconfinement
• di-muon measurements from top-most SPS energy
  to 20 AGeV

• Results for a new spectrometer for low energy
  dilepton (and
• quarkonia?) measurements at the CERN SPS

3
High precision muon measurements in HI collisions
gt10m
lt1m

• Concept working at high energy (NA60)

Does it work also at low energy (20-30 GeV)?
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NA60 In-In at 158 AGeV
Measurement of muon offsets Dmdistance between
interaction vertex and track impact point
Eur. Phys. J. C 59 (2009) 607
Phys. Rev. Lett. 96 (2006) 162302

• Charm not enhanced
• Excess above 1 GeV prompt

• 440000 events below 1 GeV (3 weeks run)
• 23 MeV mass resolution at the w

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Inclusive excess mass spectrum
Eur. Phys. J. C 59 (2009) 607-623CERN Courier
11/ 2009, 31-35 Chiral 2010 , AIP Conf.Proc.
1322 (2010) 1-10
only described by R/R and D/Z models
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Dilepton measurements in the range 20-158 AGeV at
the CERN-SPS
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The QCD phase diagram
QCD phase diagram poorly known in the region of
highest baryon densities and moderate
temperatures Fundamental question of strong
interaction theory is there a critical
point?
SPS
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Dileptons measurements which energy interval to
explore?
Steepening of the increase of the pion yield with
collision energy and sharp maximum in the energy
dependence of the strangeness to pion ratio.
Onset of deconfinement?
Central Pb-Pb
Experimental search of critical point at
SPS Energy scan from 20-30 GeV towards topmost
SPS energy
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Measuring dileptons at lower SPS energies
Phys. Rev. Lett. 91 (2003) 042301
Enhancement factor 5.91.5(stat.)1.2(syst.) (pu
blished 1.8 (syst. cocktail) removed due to the
new NA60 results on the ? and ? FFs)
Higher baryon density at 40 than at 158 AGeV
Larger enhancement and stronger r broadening in
support of the decisive role of baryon
interactions
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Low mass dileptons constraints in fireball
lifetime
Eur. Phys. J. C (2009) in press
NA60 precision measurement of excess yield
(r-clock) provided the most precise constraint
in the fireball lifetime (6.50.5 fm/c) in heavy
ion collisions to date!
Crucial in corroborating extended lifetime due to
soft mixed phase around CP if increased tFB
observed with identical final state hadron
spectra (in terms of flow) ? lifetime extension
in a soft phase Nice example of complementary
measurements with NA61
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Investigation of IMR at lower energies

• How will the drop decrease or disappear (partonic
  radiation significant at 158 AGeV)? Where?

• Direct extraction of source temperature
  measurement of T vs beam energy from mass
  distribution (Lorentz invariant)

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Competitiveness
NA60 experiment at CERN
partly complementary programs planned at CERN
SPS BNL RHIC
DUBNA NICA GSI SIS-CBM

• Availability of ion beams at CERN dilepton
  experiment covering a large energy interval -
  crucial for QCD phase diagram
• Energy interval not covered by any other
  experiment unique experiment
• Experiment focused on dileptons highly optimized
  for precision measurements
• Complementary to NA61

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From the high energy to a low energy apparatus
layout
Muon spectrometer Toroid field (R160 cm)
5 m
beam
Compress the spectrometer reducing the absorber
and enlarge transverse dimensions
dimuons_at_160 GeV (NA60) rapidity coverage 2.9ltylt4.5
Muon spectrometer Toroid field (R180 cm)
3m
toroid
2.4 m
9 m
Vertex spectrometer Dipole field
beam
dimuons_at_20 GeV rapidity coverage
1.9ltylt 4
Longitudinally scalable setup for running at
different energies
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The vertex spectrometer
3 T dipole field along x
40 cm

• Required rapidity coverage _at_20 GeV starting from
  ltygt1.9 (?0.3 rad)

x

• 5 silicon pixel planes at 7ltzlt38 cm

• Pixel plane
• - 400 mm silicon 1 mm carbon substrate
• - material budget 0.5 X0

z
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The hadron absorber and muon wall
High energy setup absorber
BeO-Al2O3 50 cm

• 14 lI , 50 X0 contains fully hadronic showers
• High energy muon wall 120 cm Fe

540 cm
graphite
Fe 40 cm
BeO 110 cm

• Low energy setup absorber
• E loss main factor together with detector
  transverse size which determines pT-y
  differential acceptance
• Compromise with signal muon energy loss 1 GeV
  at most to get muons into the spectrometer with
  yLab2 and pT0.5 GeV
• 7.3 lI , 14.7 X0 potentially not containing
  fully the hadronic shower
• Low energy muon wall 120-160 cm graphite

240 cm
graphite
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The muon spectrometer
Muon wall (120 or 160 cm)

• Muon Tracker
• 4 tracking stations (z295, 360, 550, 650 cm)

R290 cm
x
z

• Muon spectrometer field
• toroid magnet B0/r B0 0.16 Tm
• 380ltzlt530 cm rlt180 cm

• Trigger system
• 2 trigger stations placed after muon wall
  (ALICE-like) at z 840, 890 cm
• No particular topological constraints introduced
  contrary to NA60 hodo system (muons required in
  different sextants)

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Simulation tools

• Fast simulation (signal)
• Hadron cocktail generator derived from
  NA60GenGenesis
• Apparatus defined in setup file describing layer
  properties
• - geometric dimensions of active and passive
  layers
• - material properties
• Multiple scattering generated in gaussian
  approximation (Geant code)
• Energy loss imposed and corrected for
  deterministically according to Bethe-Bloch

• Fluka (background)
• parametric p and K event generator (built-in
  decayer for p and K)
• Apparatus geometry defined in consistent way with
  fast simulation tool
• Hits in detector planes recorded in external file
  for reconstruction

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Track reconstruction

• Setup parameters
• - x, y resolutions of active layers
• - detector efficiencies 90 for pixel, 90
  muon stations
• Background hits sampled from multiplicity
  distributions (p and K) to populate vertex
  detector (signal embedded in background)
• Track reconstruction started from hits in trigger
  stations
• Kalman fit adding hits in muon stations and
  vertex detector
• Matched tracks
• - Correct match only correct hits
  associated to track
  
• - Fake match one or more wrong hits
  associated to track

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Single muon acceptances
5x103 m generated with uniform 1.5ltylt4.5 and
0ltpTlt3 GeV and tracked through spectrometer
Fluka sim rec (correct match) 81 global eff
fast sim rec (correct match) 85 global eff
m
m-
20
20 GeV hadron cocktail pT vs y coverage
mid y 20 GeV 1.9
mid y 20 GeV 1.9
??mmg
w?mmg
Optimized setup with dipoletoroid Wall 120 cm
w?mm
r?mm
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y vs pT r comparison with NA60 _at_ 158 GeV
Low energy setup mid y 20 GeV 1.9
NA60 high energy setup mid y 158 GeV 2.9
r?mm
r?mm
pT GeV
y
y
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Signal reconstruction efficiency vs transverse
momentum
Low energy reduced setup
(dipoletoroid field, wall 120 cm )
NA60 high energy setup
pixel resolution 15 mm c2lt1.5

• Thick absober narrower y-pT coverage
• Trigger muons required in different hodo
  sextants
• Dead zones in toroid magnet

• Light absorber broader y-pT coverage
• No topological constraints imposed by trigger

? Low energy setup rec eff x Acc better by more
than one order of magnitude
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Cross check high energy setup rec e x Acc vs pT
NA60 high energy setup fast simulation
NA60 high energy setup full simulation

• Rec eff in different mass bins average of rec
  eff for single cocktail processes (simple average
  in fast simulation)
• Hodo trigger condition and dead zones taken into
  account in fast simulation

• Very good agreement between fast and full
  simulation

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Mass resolution
NA60 high energy setup
Low energy setup (dipole toroid field)
M GeV
M GeV
NA60 low energy setup mass resolution 8 MeV
NA60 high energy setup
Mass resolution 23 MeV
? New setup mass resolution can be improved at
least by a factor 2-3 ..with respect to the
old high energy setup
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Sources of combinatorial background
Keep this distance as small as possible 40 cm
Muon wall (not to scale)
dipole field
µ
prompt muon
primary hadron punch-through
Seconday hadron punch-through
?, K ? µ
decay muons from primary hadrons
Beam Tracker
decay muons from secondary hadrons
Vertex Detector
Hadron absorber (not to scale)

• Fluka
• - Full hadronic shower development in
  absorber
• - Punch-through of primary and secondary
  hadrons (p, K, p)
• - Muons from secondary hadrons

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Input parameters for background simulation
pions, kaons and protons

• Pions, kaons and protons generated according to
  NA49 measurements for 0-5 Pb-Pb central
  collisions at 20, 30 GeV
• Pions, kaons

- 20 GeV dNpK/dy(NA49) 180
- 30 GeV dNpK/dy(NA49) 210

• Protons

- 20 GeV dNp/dy(NA49) 80
- 30 GeV dNp/dy(NA49) 60
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Assessment of B/S choice of S
choose hadron cocktail in mass window 0.5-0.6 GeV
for S
- free from prejudices on any excess no
bootstrap most sensitive region
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Hadronic cocktail and bkg in central Pb-Pb
collisions at 20 and 30 GeV (wall 120 cm)
B/S _at_ 600 MeV 140 Pix res 15 mm
B/S _at_ 600 MeV 90 Pix res 15 mm
20 GeV
30 GeV
- Correct signal matches - Signal fakes
- Correct signal matches - Signal fakes

• p, K, p all included in bkg estimation

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Hadronic cocktail and bkg in central Pb-Pb
collisions at 20 and 30 GeV (wall 160 cm)
B/S _at_ 600 MeV 110 Pix res 15 mm
B/S _at_ 600 MeV 75 Pix res 15 mm
20 GeV
30 GeV
- Correct signal matches - Signal fakes
- Correct signal matches - Signal fakes

• p, K, p all included in bkg estimation

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Tracking with 2 fields vs 1 field

• CBM vertex spectrometer active absorber
• Advantages
• - very compact
• Potential disadvantages
• - Fe absorber not optimized for MS
• - matching not exploiting momentum
• - tracking in very high multiplicity

• NA60 setup (vertex muon spectrometers)
• Advantages
• - tracking in low multiplicity environment
• - matching exploits momentum info
• - optimized for MS
• Potential disadvantages
• - larger apparatus

• Qualitative argument concept succesfully
  exploited by ATLAS/CMS. Caveat
  - works well for hard muons
  (quarkonia)
• - low masses (CMS) pT cut at 7 GeV

• Qualitative argument concept succesfully
  demonstrated by NA60 in heavy ions at 158 GeV/c

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Comparison to CBM
CBM 25 GeV Au-Au central B/S600
NA60-B/S 110
30 GeV Pb-Pb

New Much setup and selection cuts
CBM 35 GeV Au-Au central B/S200

• B/S factor 2 difference
• Reconstruction efficiency factor 10 difference
  ? tracking problem in active absorber?

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NA60 setup tracking switching off toroid field

• Strong increase of signal fakes and, in
  particular, combinatorial background (factor 3)
• ? The tracking after the absorber requires a
  second field

B/S _at_ 600 MeV 300 Pix res 15 mm
20 GeV
- Correct signal matches - Signal fakes
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Search for and existing toroid magnet
Magnet critical element to determine the cost ?
looking for existing magnets
Chorus magnet at CERN
0.75 m

• Rapidity coverage
• Sitting at 380ltzlt455 covers down to h1.8
• Issues
• - Maximum quoted B0.12 T bending power 1/2 what
  considered in simulations
• - Magnet operated in pulsed mode with a neutrino
  beam cooling aspect to be considered

1.5 m
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Experimental aspects

• High precision
• - very good B/S (lt100 in central collisions)
• - pT acceptance (a factor gt10 better than
  high energy setup)
• - 8 MeV mass resolution (a factor 2-3 better
  than high energy setup)
• Silicon detector use of existing hybrid
  technologies possible
• - 50 mm cell, 0.5 material budget/plane
• Muon tracking chambers large area but
  conventional detectors
• Muon spectrometer magnet
• - toroid magnet (R1.5-2 m) required with
  relatively low field strength
• Further work for optimizing the setup required

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Summary

• Study of QCD phase diagram and critical point
  search
• fundamental physics aspects addressed

• Ion beams exists and will be available for
  experiments at the SPS in the incoming years
  (presently scehduled up to 2021)

• A Dilepton experiment at CERN can provide crucial
  information on the QCD phase diagram
• Covering a large energy interval crucial for QCD
  phase diagram
• Energy interval together with high luminosity not
  covered by any other experiment unique
  experiment
• High precision very good B/S, pT acceptance,
  mass resolution