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ALICE physics. Guy Pai ć Instituto de Ciencias Nucleares UNAM Mexico. 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 - PowerPoint PPT Presentation
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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