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