1 Gianluca Usai – University of Cagliari and INFN Electromagnetic Probes of Strongly interacting...
-
Upload
oscar-hyde -
Category
Documents
-
view
212 -
download
0
Transcript of 1 Gianluca Usai – University of Cagliari and INFN Electromagnetic Probes of Strongly interacting...
1
Gianluca Usai – University of Cagliari and INFN
Electromagnetic Probes of Strongly interacting Matter in ECT* Trento - 23/05/2013
The QCD phase diagram and search of critical point: dilepton measurements in the range 20-158 AGeV at the CERN-SPS
2
NA60: past precision di-muon measurements in HI at top-most SPS energy (158 AGeV)
Outline
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
dipole magnet
hadron absorber
targets
beam tracker Si-pixel trackermuon trigger and tracking
magnetic field
>10m<1m
Track matching in coordinate and momentum spaceImproved dimuon mass resolution Distinguish prompt from decay dimuons
Radiation-hard silicon pixel detectors (LHC developments) High luminosity of dimuon experiments maintained
High precision muon measurements in HI collisions
Concept working at high energy (NA60)
Does it work also at low energy (20-30 GeV)?
4
NA60: In-In at 158 AGeV Measurement of muon offsets :distance between interaction vertex and track impact point
Charm not enhanced Excess above 1 GeV prompt
Eur. Phys. J. C 59 (2009) 607
440000 events below 1 GeV (3 weeks run)
23 MeV mass resolution at the
Phys. Rev. Lett. 96 (2006) 162302
5
[Eur. Phys. J. C 59 (2009) 607-623]CERN Courier 11/ 2009, 31-35 Chiral 2010 , AIP Conf.Proc. 1322 (2010) 1-10
M<1 GeV
ρ dominates, ‘melts’ close to Tc
best described by H/R model
~ exponential fall-off ‘Planck-like’
M>1 GeV
)/exp(/ 2/3 TMMdMdN fit to
T>Tc: partons dominate
range 1.1-2.0 GeV: T=205±12 MeV 1.1-2.4 GeV: T=230±10 MeV
only described by R/R and D/Z models
All known sources subtracted
integrated over pT
Fully corrected for acceptance
absolutely normalized to dNch/dη
Inclusive excess mass spectrum
6
Dilepton measurements in the range 20-158 AGeV at the CERN-SPS
7
SPS
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?
The QCD phase diagram
8
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?
Experimental search of critical point at SPS:
Energy scan from ~ 20-30 GeV towards topmost SPS energy
Central Pb-Pb
Dileptons measurements: which energy interval to explore?
Phys. Rev. Lett. 91 (2003) 042301
Higher baryon density at 40 than at 158 AGeV
Enhancement factor:
5.9±1.5(stat.)±1.2(syst.)
(published ±1.8 (syst. cocktail) removed due to the new NA60 results on the η and ω FFs)
Larger enhancement and stronger broadening in support of the decisive role of baryon interactions
9
Measuring dileptons at lower SPS energies
10
NA60 precision measurement of excess yield (-clock):
provided the most precise constraint in the fireball lifetime (6.5±0.5 fm/c) in heavy ion collisions to date!
Crucial in corroborating extended lifetime due to soft mixed phase around CP:
if increased FB observed with identical final state hadron spectra (in terms of flow) → lifetime extension in a soft phase
Nice example of complementary measurements with NA61
Low mass dileptons: constraints in fireball lifetimeEur. Phys. J. C (2009) in press
1111
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)
Investigation of IMR at lower energies
12
partly complementary programs planned at CERN SPS BNL RHIC DUBNA NICA GSI SIS-CBM
NA60’ experiment at CERN
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
Competitiveness
Muon spectrometer
Toroid field (R=160 cm)
beam
5 m
13
Compress the spectrometer reducing the absorber and enlarge transverse dimensions
dimuons@160 GeV (NA60) rapidity coverage 2.9<y<4.5
beam
2.4 m
Longitudinally scalable setup for running at different energies
From the high energy to a low energy apparatus layout
dimuons@20 GeV rapidity coverage 1.9<y< 4
3m
9 m
Muon spectrometer
Toroid field (R=180 cm)
toroid
Vertex spectrometerDipole field
Pixel plane: - 400 m silicon + 1 mm carbon substrate - material budget ≈0.5% X0
Required rapidity coverage @20 GeV starting from <y>=1.9 (ϑ~0.3 rad)
3 T dipole field along
x40 cm
x
z
5 silicon pixel planes at 7<z<38 cm
The vertex spectrometer
15
540 cm
Fe 40 cm
BeO-Al2O3 50 cm
graphite
The hadron absorber and muon wall
240 cm
graphite
High energy setup absorber
BeO 110 cmLow 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 yLab~2 and pT≥0.5 GeV
7.3 I , 14.7 X0: potentially not containing fully the hadronic showerLow energy muon wall: 120-160 cm graphite
14 I , 50 X0: contains fully hadronic showers
High energy muon wall: 120 cm Fe
The muon spectrometerMuon Tracker4 tracking stations (z=295, 360, 550, 650 cm)
Trigger system2 trigger stations placed after muon wall (ALICE-like) at z = 840, 890 cmNo particular topological constraints introduced contrary to NA60 hodo system (muons required in different sextants)
z
x
R=290 cmMuon wall (120 or 160 cm)
Muon spectrometer fieldtoroid magnet B0/r – B0 = 0.16 Tm
380<z<530 cm; r<180 cm
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 and K event generator (built-in decayer for and K)
Apparatus geometry defined in consistent way with fast simulation tool
Hits in detector planes recorded in external file for reconstruction
Track reconstruction
Setup parameters: - x, y resolutions of active layers - detector efficiencies: 90% for pixel, 90% muon stations
Background hits sampled from multiplicity distributions ( 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
Single muon acceptances
5x103 + generated with uniform 1.5<y<4.5 and 0<pT<3 GeV and tracked through spectrometer
+ -
fast sim + rec (correct match)85% global eff
Fluka sim + rec (correct match)81% global eff
20 GeV hadron cocktail: pT vs y coveragemid y – 20 GeV = 1.9 mid y – 20 GeV = 1.9
Optimized setup with dipole+toroid
Wall = 120 cm
y vs pT : comparison with NA60 @ 158 GeV
Low energy setupmid y – 20 GeV = 1.9
NA60 high energy setupmid y – 158 GeV = 2.9
p T [G
eV]
y y
Signal reconstruction efficiency vs transverse momentum
pixel resolution 15 m – 2<1.5
Light absorber: broader y-pT coverage No topological constraints imposed by trigger
Thick absober: narrower y-pT coverage Trigger: muons required in different
hodo sextants Dead zones in toroid magnet
Low energy setup: rec eff x Acc better by more than one order of magnitude
NA60 high energy setupLow energy reduced setup (dipole+toroid field, wall 120 cm )
Very good agreement between fast and full simulation
NA60 high energy setup – full simulationNA60 high energy setup – fast 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
Cross check: high energy setup rec x Acc vs pT
NA60 high energy setup: Mass resolution 23 MeV
NA60 high energy setup
New setup: mass resolution can be improved at least by a factor 2-3 …..with respect to the old high energy setup
M [GeV]
Mass resolution
NA60 low energy setup: mass resolution 8 MeV
M [GeV]
Low energy setup (dipole+ toroid field)
25
Sources of combinatorial background
Keep this distance as small as possible ~40 cm
, K µoffset
vertex
Beam Tracker
µdipole field
Vertex Detector
prompt muon
primary hadron punch-
through
decay muons from primary hadrons
Seconday hadron punch-through
decay muons from secondary hadrons
Muon wall (not to scale)
Hadron absorber (not to scale)
Fluka: - Full hadronic shower development in absorber - Punch-through of primary and secondary hadrons (, K, p) - Muons from secondary hadrons
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: dN+K/dy(NA49) ≈ 180- 30 GeV: dN+K/dy(NA49) ≈ 210
Protons:
- 20 GeV: dNp/dy(NA49) ≈ 80
- 30 GeV: dNp/dy(NA49) ≈ 60
choose hadron cocktail in mass window 0.5-0.6 GeV for S
- free from prejudices on any excess; no ‘bootstrap’; most sensitive region - unambiguous scaling between experiments; B/S dNch/dy 27
B
S
B/S=
35
Assessment of B/S: choice of S
B/S @ 600 MeV = 90Pix res 15 m
B/S @ 600 MeV =140Pix res 15 m
20 GeV 30 GeV
- Correct signal matches- Signal fakes
- Correct signal matches- Signal fakes
Hadronic cocktail and bkg in central Pb-Pb collisions at 20 and 30 GeV (wall 120 cm)
, K, p all included in bkg estimation
B/S @ 600 MeV = 75Pix res 15 m
B/S @ 600 MeV = 110Pix res 15 m
20 GeV 30 GeV
- Correct signal matches- Signal fakes
- Correct signal matches- Signal fakes
Hadronic cocktail and bkg in central Pb-Pb collisions at 20 and 30 GeV (wall 160 cm)
, K, p all included in bkg estimation
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
30
Tracking with 2 fields vs 1 field
Qualitative argument: concept succesfully exploited by ATLAS/CMS. Caveat: - works well for hard muons (quarkonia)
- low masses (CMS): pT cut at 7 GeV
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 demonstrated by NA60 in heavy ions at 158 GeV/c
×
CBM 25 GeV Au-Au central
B/S=600
New Much setup and selection cuts
CBM 35 GeV Au-Au central
B/S=200
30 GeV Pb-Pb
NA60’-B/S = 110
Comparison to CBM
B/S: factor ≈ 2 difference
Reconstruction efficiency: factor ≈ 10 difference tracking problem in active absorber?
NA60’ setup: tracking switching off toroid field
B/S @ 600 MeV = 300Pix res 15 m
20 GeV
- Correct signal matches- Signal fakes
Strong increase of signal fakes and, in particular, combinatorial background (factor ≈ 3)
The tracking after the absorber requires a second field
1.5 m
0.75 m
Rapidity coverage:
Sitting at 380<z<455 covers down to ≈1.8
Issues:
- Maximum quoted B=0.12 T bending power 1/2 what considered in simulations
- Magnet operated in pulsed mode with a neutrino beam: cooling aspect to be considered
Search for and existing toroid magnet
Magnet: critical element to determine the cost looking for existing magnets
Chorus magnet at CERN
34
Experimental aspects
High precision: - very good B/S (<100 in central collisions) - pT acceptance (a factor >10 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 m cell, ≈0.5% material budget/plane
Muon tracking chambers: large area but conventional detectors
Muon spectrometer magnet: - toroid magnet (R≈1.5-2 m) required with relatively low field strength
Further work for optimizing the setup required
35
Summary
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
Study of QCD phase diagram and critical point search: fundamental physics aspects addressed