ALICE physics
Guy PaićInstituto de Ciencias Nucleares
UNAMMexico
05/04/2006 Guy Paic LISHEP- ALICE physics
The aim
• The energy density reached in heavy ion collisions at LHC is large – according to the predictions of QCD theory of
strong interactions, nuclear matter will go through a QGP (Quark Gluon Plasma) phase
• state of deconfined partons (in a large volume).
• the question at LHC is not actually to put in evidence the QGP but rather to study its properties and hadronisation.
05/04/2006 Guy Paic LISHEP- ALICE physics
The aim cont’d
• observe phenomena that are very difficult to explain from a hadronic perspective but have a simple qualitative explanation based on quarks and gluons.
• make quantitative predictions for the emission of various kinds of “hard” radiation from the quark gluon plasma.
05/04/2006 Guy Paic LISHEP- ALICE physics
QCD phase transition
in vacuum• linear increase with distance • strong attractive force• quark confinement in hadrons baryons (qqq) and mesons (qq)
QCD potential:
in dense and hot matter• screening of color charges• potential vanishes for large distance scales• deconfinement of quarks !
05/04/2006 Guy Paic LISHEP- ALICE physics
Dynamics of a collision
•
After S. Bass
before the collision – before the collision – coherent field coherent field configuration the clouds configuration the clouds of gluons and quarks of gluons and quarks represented by the QCD represented by the QCD approximation of A x approximation of A x structure functions of structure functions of nucleonsnucleonsThe QCD fields persist The QCD fields persist after the nuclear after the nuclear valence valence quarkquark pancakes collide – pancakes collide – the interaction lasts for ~ the interaction lasts for ~ 30 fm/c =1030 fm/c =10-22-22 s sHadronization! The fields Hadronization! The fields hadronize in a way that is hadronize in a way that is not well understoodnot well understoodThe dense final state The dense final state debris further interact as debris further interact as it expandsit expands
05/04/2006 Guy Paic LISHEP- ALICE physics
Change of perspective
• SPS - evidence for collective phenomena in Nucleus- nucleus collisions
• many interesting signals telltale of phase transition
• not much hard processes• Dynamics of a collision • first inkling of new processes (hard) at RHIC
more to be seen at LHC– dominance of minijets– dominance of gluon-gluon interactions– importance of parton shadowing– parton saturation phenomena– high initial temperatures– jet quenching
05/04/2006 Guy Paic LISHEP- ALICE physics
Two main classes
• Soft physics – low pt (<3 GeV/c)• Hard probes
05/04/2006 Guy Paic LISHEP- ALICE physics
Time evolution
e
distance
time jet
AuAu
E
xpan
sion
p K
QGP
e
05/04/2006 Guy Paic LISHEP- ALICE physics
Soft physics
05/04/2006 Guy Paic LISHEP- ALICE physics
Soft Physics in Pb-Pb and pp
Expansion dynamics Space-time structure Radial, anisotropic flow Momentum correlations
Chemical composition Hadronisation mechanisms
Event characterization Centrality selection Global observables
Event by event physics Fluctuations
Bulk properties: soft hadrons + interplay hard–soft Identified particle spectra (wide pT range)
1
05/04/2006 Guy Paic LISHEP- ALICE physics
Pseudorapidity distributions at ALICE and Atlas
05/04/2006 Guy Paic LISHEP- ALICE physics
Global event characterization in Pb-PbCentrality determination
Event by event determination of the centrality Zero degree hadronic calorimeters (ZDC) + electromagnetic calorimeters (ZEM)EZDC , EZEM Nspec Npart impact parameter (b)
Correlations between ZDC and ZEME
ve
nts
EZEM (GeV)
EZ
DC (
Te
V)
b ~ 1fm
bgen (fm)
bre
c(f
m)
reconstructed
Npart
3
Npart
generated
Npart ~15
05/04/2006 Guy Paic LISHEP- ALICE physics
Global event properties in Pb-Pb
Multiplicity distribution (dNch/d) in Pb-Pb
Silicon Pixel Detector (SPD) : -1.6 < < +1.6 + Forward Multiplicity Detector (FMD): -5, +3.5
Energy density
dN/d % centrality (Npart) Fraction of particles produced in hard processes
Npart
(dN
/d)
||<
0.5
(dN/d)||<0.5
Generated Tracklets
Generated Tracklets
1 central Hijing event
4
05/04/2006 Guy Paic LISHEP- ALICE physics
Identified particle spectra in Pb-Pb and pp Excitation functions of bulk observables for identified hadrons New regime at LHC: strong influence of hard processes
Chemical composition
Equilibrium vs non equilibrium stat. models ?
Jet propagation vs thermalization ?
Production mechanisms for different hadron species also in pp
Interplay between hard and soft processes at intermediate pT
Parton recombination + fragmentation ?
or soft (hydro -> flow) + quenching ? or … ?
Rcp: central over peripheral yields/<Nbin> Baryon/meson ratio Elliptic flow
RHIC
5
05/04/2006 Guy Paic LISHEP- ALICE physics
Particle reconstruction and identification capabilities: unique to ALICE Global tracking (ITS-TPC-TRD) + dE/dx (low pT + relativ. rise), TOF, HMPID, PHOS, …
Invariant mass, topological reconstruction Acceptance / efficiency / reconstruction rate () / contamination pT range (PID or stat. limits) for 107 central Pb-Pb and 109 min. bias pp
Identified particle spectra
Pb-Pb
Mid-rapidityPID in the relativistic rise
p
K
Pb-Pb
pT (GeV/c)
6
For ~ 20 particle species for -1 < y < +1 and -4 < y < +2.5
, K, p: 0.1- 0.15 50 GeV Weak or strong decaying particles: until 10-15 GeV
05/04/2006 Guy Paic LISHEP- ALICE physics
Topological identification of strange particles
Secondary vertex and cascade finding
Identification of K+, K- via their kink topology K
Pb-Pb central
13 recons./event
pT dependent cuts -> optimizeefficiency over the whole pT range
Statistical limit : pT ~11 - 13 GeV for K+, K-, K0s, , 7 - 10 GeV for
6x104 pp collisions
Reconst. rates: : 0.1/event : 0.01/eventpT: 1 7-10 GeV
About the samepT limit for 109 pp
pp collisions
300 Hijingevents
11-12 GeV
Limit of combined PID
7
05/04/2006 Guy Paic LISHEP- ALICE physics
Resonances ( K*, …) Time difference between chemical and kinetic freeze-out In medium modifications of mass, width, comparison between hadronic and leptonic channels partial chiral symmetry restoration
Invariant mass reconstruction, background subtracted (like-sign method) mass resolutions ~ 1.5 - 3 MeV and pT stat. limits from 8 () to 15 GeV (,K*)
central Pb-Pb
Mass resolution ~ 2-3 MeV
K*(892)0 K 15000 central Pb-Pb
K+K- Mass resolution ~ 1.2 MeV
Generated & reconstructed for 107 central Pb-Pb
Invariant mass (GeV/c2)
8
05/04/2006 Guy Paic LISHEP- ALICE physics
Anisotropic FlowHydro limit (full local thermalization) at RHIC ? More likely at LHC ?
Initial conditions CGC + hydro (until T ~ 170 MeV)i.e., contribution of the QGP + hadronic cascade At LHC, contribution from QGP much larger than at RHIC
Relation between V2 and higher harmonics(V4, V6, …) to test perfect liquid % viscous fluid
2 2
2 2 2cos(2 )x y
x y
p pv
p p
y
x
py
px
2
2 21 2 cos( )
2 nnT T
d N dNv n
dp d dp
V2, V4, ...
At LHC: more sensitivity to the QGP
Flow of identified hadrons-> partonic d’s of freedom ?
RHIC
9
Hard Probes
05/04/2006 Guy Paic LISHEP- ALICE physics
• The yields of hard probes give rather direct information about the initial state of the collision– PDFs – the environment they have to traverse on there way out(QGP).
• Rescattering• Energy loss• Color screening
• A, pA and pp necessary and compulsory to be able to interpret the results
• Open flavor • Heavy quarks produced copiously at LHC
– 120 ccbar et 5 bbbar per central Pb-Pb, event– produced at (~1/2 mQ ~0.1 fm/c compared to τQGP ~10 fm/c)
• Should test: – pQCD – Test the medium thru energy loss of partons (jet quenching)– test the color screening of quarkonia.
•
05/04/2006 Guy Paic LISHEP- ALICE physics
gluon radiation
Parton energy loss
• High energy partons, resulting from a initial hard scattering, will create a high energy collimated spray of particles → jets
• Partons traveling through a dense colour medium are expected to lose energy via medium induced gluon radiation, “jet quenching”, and the magnitude of the energy loss depends on the gluon density of the medium
• Total jet energy is conserved, but “quenching” changes the jet structure and fragmentation function
Measurement of the parton fragmentation products reveals information about the QCD medium
2ˆLqCE Rs
05/04/2006 Guy Paic LISHEP- ALICE physics
Jet rates at LHC
4 108 central PbPb collisions/month
6 105 events
|y| < 0.5
ET threshol
d
Njets
50 GeV 2 107
100 GeV 6 105
150 GeV 1.2 105
200 GeV 2.0 104
Copious production:
Several jets per central PbPb collisions for ET > 20 GeV
However, for measuring the jet fragmentation function close to z = 1, >104 jets are needed. In addition you want to bin, i.e. perform studies relative to reaction plane to map out L dependence.
05/04/2006 Guy Paic LISHEP- ALICE physics
nucl-ex/0406012
x5
● PHENIX (π0)
High-pT suppression in central AuAu collisions
High-pT hadrons of recoiling jet suppressed in AuAu but not in dAu
gluon radiation
Evidence for partonic energy loss in heavy ion collisions
ddpNd
ddpNd
bNR
Tpp
TAA
collAA /
/
)(
12
2
1/Ntriggerd
N/d
()
PRL91, 072304 (2003)
Results from RHIC
05/04/2006 Guy Paic LISHEP- ALICE physics
Full jet reconstructionEskola et al., hep-ph/0406319
Leading Particle
Reconstructed Jet
Ideally, the analysis of reconstructed jets will allow us to measure the original parton 4-momentum and the jet structure. → Study the properties of the medium through modifications of the jet structure:
– Decrease of particles with high z, increase of particles with low z– Broadening of the momentum distribution perpendicular to jet axis
Leading particle becomes fragile as a probe• Surface emission:
–Small sensitivity of RAA to medium properties.
• For increasing in medium path length L, the momentum of the leading particle is less and less correlated with the original parton 4-momentum.
jetT
T
E
pz
05/04/2006 Guy Paic LISHEP- ALICE physics
Jet rates at the LHC
Huge jet statistics from ET ~10 GeV to ET~100 GeV
• Jets with ET > 50 GeV will allow full reconstruction of hadronic jets, even in the underlying heavy-ion environment.•Multijet production per event extends to ~ 20 GeV
100
coneR
pt (GeV)2 20 100 200
100/event 1/event 100K/year
05/04/2006 Guy Paic LISHEP- ALICE physics
50 GeV jet
50 – 100 GeV jets in Pb–Pb
η–φ lego plot with Δη 0.08 Δφ 0.25
At large enough jet energy – jet clearly visibleBut still large fluctuation in underlying energy
Central Pb–Pb event (HIJING simulation) with 100 GeV di-jet (PYTHIA simulation)
C. Loizides
100 GeV
05/04/2006 Guy Paic LISHEP- ALICE physics
Q
Heavy Quarks – dead cone
• Heavy quarks with momenta < 20–30 GeV/c v << c
• Gluon radiation is suppressed at angles < mQ/EQ “dead-cone” effect– Due to destructive interference– Contributes to the harder fragmentation of heavy
quarks
• Yu.L.Dokshitzer and D.E.Kharzeev: dead cone implies lower energy loss
Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett. B519 (2001) 199 [arXiv:hep-ph/0106202].
D mesons quenching reducedRatio D/hadrons (or D/p0) enhanced and sensitive to medium properties
05/04/2006 Guy Paic LISHEP- ALICE physics
Detection strategy for D0 K- +
• Weak decay with mean proper length c = 124 m• Impact Parameter (distance of closest approach of a track to the primary vertex) of the decay products d0 ~ 100 m
• STRATEGY: invariant mass analysis of fully-reconstructed topologies originating from (displaced) secondary vertices– Measurement of Impact Parameters– Measurement of Momenta– Particle identification to tag the two decay products
05/04/2006 Guy Paic LISHEP- ALICE physics
Hadronic charmCombine ALICE tracking + secondary vertex finding capabilities (d0~60m@1GeV/c pT) + large acceptance PID to detect processes as D0K-+
~1 in acceptance / central event ~0.001/central event accepted after rec. and all cuts
S/B+S ~ 37
S/B+S ~ 8for 1<pT<2 GeV/c(~12 if K ID required)
significance vs pTResults for 107 PbPb ev. (~ 1/2 a run)
05/04/2006 Guy Paic LISHEP- ALICE physics
Sensitivity on RAA for D0 mesons
‘High’ pt (6–15 GeV/c)here energy loss can be studied(it’s the only expected effect)
Low pt (< 6–7 GeV/c)Nuclear shadowing+ kt broadening+ ? thermal charm ?
A.Dainese nucl-ex/0311004
05/04/2006 Guy Paic LISHEP- ALICE physics
Jet quenching
• Excellent jet reconstruction… but challenging to measure medium modification of its shape…
• Et=100 GeV (reduced average jet energy fraction inside R):– Radiated energy ~20% – R=0.3 E/E=3%– Et
UE ~ 100 GeV
RMedium induced redistribution of jet energy occurs inside cone
C.A. Salgado, U.A. Wiedemann hep-ph/0310079
vacuum
medium
Et = 50 GeV
Et = 100 GeV
0.200
0.4 0.6 0.8 1
0.2
R=√(2+2)
0.4
0.6
0.810
0.20.4
0.6
0.8
1
(R
)
05/04/2006 Guy Paic LISHEP- ALICE physics
Fragmentation functions
0 0.5 1z
10-4
10-2
1vacuummedium
pjet
z
kt
z=pt/pjet
05/04/2006 Guy Paic LISHEP- ALICE physics
The quarkonia physics
05/04/2006 Guy Paic LISHEP- ALICE physics
Acceptance for quarkonia measurements
• ALICE can measure J/ down to pt = 0 (unique @ the LHC)
• ALICE-muon can measure J/ & at large y
05/04/2006 Guy Paic LISHEP- ALICE physics
mass resolution( 100 MeV @ M ~ 10 GeV is needed to separate the sub-states)
• ALICE (& CMS) can measure the sub-states
• warning: ≠ simulation frameworks & inputs
ALICE dielectrons
background level 1 = 2 HIJING evts with dNch/d = 6000 @ = 0 each
ALICE dimuons
ATLAS
> 120 MeV
CMS
~ 80 MeV
ATLAS CERN/LHCC/2004-009, CMS NOTE 2000-060 (updated)
05/04/2006 Guy Paic LISHEP- ALICE physics
Extract signals
1. get invariant mass cocktail for all centrality & pt bins
2. subtract non-correlated dimuons (assuming a perfect event-mixing subtraction)
3. fit invariant mass spectra with 3 modified Landau convoluted with Gaussian & exponential for background
1. 2. 3.
0 < b < 3 fm 0 < b < 3 fm0 < b < 3 fm
05/04/2006 Guy Paic LISHEP- ALICE physics
Centrality dependence of ’/
• statistics : one month PbPb• nuclear absorption not in
• interest to combine pt
dependence of the ratio• systematic errors underway
05/04/2006 Guy Paic LISHEP- ALICE physics
1 month of dielectrons in the central barrel
05/04/2006 Guy Paic LISHEP- ALICE physics
• W LO production process is: • NLO processes contribute just ~ 13% to the total cross section
• LO dominant contribution (~ 80%) comes from udbar for W+,dubar for W-
• detection?– Via their leptonic decay:– Where?
ATLAS strategy is to measure at < 2.4 and e at< 2.5
CMS will be able to measure spectra for < 2.4
ALICE can measure e for < 0.9 and for – 4.0 << – 2.5
for – 4.02.5 ALICE is the only LHC Experiment able to measure W boson production
W detection in ALICE (Z.Conesa del Vale)
Frixione & Mangano, hep-ph/0405130
Martin, et al, hep-ph/9907231
u d d u
Wq' q
)( )()W(Wq' q llll
05/04/2006 Guy Paic LISHEP- ALICE physics
Single Muons at LHC• Some estimations for pp and PbPb nominal
runs at LHC Point 2...– pp @ 14 TeV
627.000 ’s generated from W decay in the ALICE IP 337.000 at Pt (30,50) GeV/c
88.800 ’s generated from W decay in the Muon Spectrometer Acceptance
51.000 at Pt (30,50) GeV/c
– PbPb @ 5.5 TeV, Min Bias 142.000 ’s generated from W decay in the ALICE IP
77.000 at Pt (30,50) GeV/c 15.500 ’s generated from W decay in the Muon
Spectrometer Acceptance 7.800 at Pt (30,50) GeV/c
W at LHC
05/04/2006 Guy Paic LISHEP- ALICE physics
The first three minutes….
05/04/2006 Guy Paic LISHEP- ALICE physics
Beam characteristics (LHC-OP-BCP-0001 rev 1.)• The highest possible beam energy will be achieved soon, however, with a
small number of bunches, and low intensity • Beam conditions will be ideal for ALICE pp physics – TPC
drift time ~80s – no or small pile-up – L 1x1029cm-2s-1 corresponds to 1 inel event in 160s
Beam Energy (TeV) 6 to 7 6 to 7 6 to 7
Number of bunches 43 43 156
* [m] 10 10 10Crossing Angle [rad] 0 0 0Transverse emittance [m]
3.75 3.75 3.75
Bunch spacing [ns] 2025 2025 525Bunch Intensity 1x101
0
4x101
0
4x1010
Luminosity [cm–2 s–1] 6x102
8
1x103
0
3.5x103
0
Inelastic Rate [Hz] 3600 60000 210000
936
75
Only 3 minutes to collect sample of 104 events…
1.3x1032
later
05/04/2006 Guy Paic LISHEP- ALICE physics
Motivation for pp study
• First insight in pp collisions in new energy domain (s 14 TeV), study of evolution of soft hadronic physics
– Cosmic ray interactions show `knee’ in 10151016 eV region and `ankle’ in 10181019 eV region
s 14 TeV corresponds to 1017 eV in lab frame• Contribution to knowledge of underlying
minimum bias (background) pp events for other LHC physics programmes (Higgs search, B-physics, etc.)
• Provide pp data as a reference for study of other collision systems (p-A, A-A)
• Low multiplicity data to commission and calibrate various components of ALICE
05/04/2006 Guy Paic LISHEP- ALICE physics
• It only takes a handful of events to measure a few
important global event properties (dN/d, d/dpT, etc.) – after LHC start-up, with few tens of thousand events we will do: Claus Jorgensen
Mean pT vs multiplicity
Multiplicity distribution
pT spectrumof chargedparticles
Pseudorapidity density dN/dη
CDF:Phys. Rev. D41, 2330 (1990)30000 events at √s=1.8TeV9400 events at √s=640TeV
UA5:Z. Phys43, 357 (1989)6839 events at √s=900GeV4256 events at √s=200GeV
CDF:Phys. Rev. Lett.61, 1819 (1988)55700 events at √s=1.8TeV
CDF:Phys. Rev. D65,72005(2002)3.3M events at 1.8TeV2.6M events at 630GeV