1 physical program on alice experiment investigation of qcd predictions about deconfinement and high...
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Physical program on ALICE experiment
• Investigation of QCD predictions about deconfinement and high energy-
density phase of matter (so-called quark-gluon plasma).
• Examination of fundamental aspects of hard interaction: deconfinement, chiral symmetry restoration, non-perturbed QCD-vacuum…
• Determination of state characteristics and phase transitions in the Early Universe right after the Big Bang.
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Theoretical foundations for searching the new QCD-phase
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Highlights from the ultrarelativistic heavy ions experimental
results, that are different from hadron-hadron collisions Strangeness enhancement
J Suppression (1984)
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Hard gluon induced quarkonium breakup
quarkonium survival probabilities versus QGP lifetime
set(i)(ii) : lower (higher) bottomonium (charmonium) binding energy
• none of the J/ survives the LHC-(PbPb)QGP! (importance of pt & system size studies)
• probes the QGP lifetime & temperature
RHIC
RHIC
LHC LHC
J/
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PDF (Parton Distribution Function)• PDF is a distribution of x for a given parton type (e.g. gluon, valence quark, sea
quark) and gives a probability to pick up a parton with momentum fraction x from the proton.
• The LHC will allow to probe the PDF of a nucleon and, in the case of pN and NN collisions, also their modifications in the nucleon, down to unprecedented low x.
• Interesting is the following: the charm and beauty production cross sections at the LHC are significantly affected by parton dynamics in the small-x region.
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Nuclear PDF
• To extract separately the quark and gluon content in nuclear distribution, we can use these ratios (e – electric charge, q,g – momentum-space distributions):
at x<0.1 the ratio is smaller than 1 – gluon shadowing
At x~0.1 a small enhancement – “antishadowing”
Significant depletion of Rf2 – “EMC effect”
Strong rise caused by Fermi motion
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Shadowing
• Gluon shadowing (is a result of coherence, is a leading twist effect): at small values of x gluon clouds overlap in longitudinal direction and may fuse. As a result the gluon density is expected to be reduced compared to a free nucleon. Parton shadowing may leads to an additional nuclear suppression especially for exclusive vector meson production like charmonium (needs at least two gluon exchange)
The interpretation of the phenomenon of gluon shadowing depends very much on the reference frame. It looks like glue-glue fusion in the infinite momentum frame of the nucleus: although the nucleus is Lorentz contracted, the bound nucleons are still well separated since they contract too. However, the gluon clouds of the nucleons are contracted less since they have a smaller momentum fraction x. Therefore, they do overlap and interact at small x, and gluons originating from dierent nucleons can fuse leading to a reduction of the gluon density.
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Shadowing Pb+Pb vs p+Pb
1/ ppABR
Nonrelativistic QCD predictions for pPb and Pb+Pb collisions relative pp at =5.5 TeV/nucleons
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Energy loss
• Energy loss and shadowing are competing effects. When a proton enters a nucleus the first (soft) inelastic collision liberates a quark, which then loses energy via hadronisation and interaction. In the target rest frame proton-nucleus collision with lepton pair production is treated as “bremsstrahlung”.
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Antishadowing
• Nuclear modifications of the gluon distribution is poorly known.
• There is no reliable explanation for gluon shadowing until now (2003)
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pA interactions
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pA interactions
• pA collisions are useful as a reference in which no QGP is expected while there are some high density effects
• Saturation, shadowingSaturation effects are more pronounced for a large
projectileUsually included within collinear factorization by
using special parton distribution functions (e.g. EKS98, HKM)
• Rescattering effectsStrong color field produced in the collision
• Heavy quark production in pA collisions is also interesting per se as a means of studying the physics of saturation
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Glauber modelGlauber model for ion-ion collisions:
-nucleons in a nucleus are distributed according to NDF;
-Nucleons follow straight trajectories and do not deviate even in another nucleus;
-Nucleons interact according to inelastic cross-section for pp-interaction.
hardpp
b
Ahardppc
hardpA AbTAdbbb
c
0
2
, - функция ядерной плотности Вудса-Саксона.
szdzsT ii ,)(
i
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RapidityIn high energy physics we use the laws of the relativistic kinematics, but corresponding velocity conversion equation is not additive:
This is not convenient because the difference depends on the reference frame in this case. To save the additivity we introduce a new variable – rapidity (y), it is unambiguously determined by the velocity:
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1cvvvv
v
21 vv
vc
vcy
ln5.0
The applicable domain of the rapidity: (-c , c)
The range of values of the rapidity: (-~, ~)
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Collisions of A1 and A2 nuclei (in case of a proton A=1, Z=1):
and
with the longitudinal rapidity of pair
In case of asymmetric collisions, e.g. p-Pb and Pb-p, we have a rapidity
shift: corresponding to +0.47
)exp(1
11 QQ
pp
QQ ys
M
Z
Ax )exp(
2
22 QQ
pp
QQ ys
M
Z
Ax
Modelling
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21.. ln2
1
AZ
AZy mc
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21
2
1ln2
1ln2
1
AZ
AZ
x
x
pE
pEy
z
zQQ
(-0.47) for p-Pb (Pb-p) collisions
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Modelling
• In PYTHIA event generator, the process giving rise to contributions above leading order are calculating using a massless matrix element. As a consequence the cross-section for these processes diverge as vanishes. But the region of low is of prime interest for ALICE physics. So the approach was to tune the PYTHIA parameters in order to reproduce the NLO predictions.
• My task was to change the tuned PYTHIA’s code in order to allow to simulate asymmetrical collisions like p-Pb and Pb-p. The Lorenz shift was included in the program to provide asymmetrical distribution in c.m. The results are graphically presented below:
hard
tp
tp
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Modelling
• Statistics = 5000, =8.8ТэВ NNs
These are rapidity distributions for 3 type of mesons: Pi, K, Mu. The shift depends on the direction of the collision between p and PB and equals to +-0.47.
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Modelling
• The same distributions for Pi-mesons are plotted on the same axes to show the difference between two possible directions.
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Heavy flavors with ALICE
(di-)muons: J/, ’, , ’,’’, open charm, open bottom
(di-)electrons: J/, ’, , ’,’’, open charm, open bottom
hadrons: exclusive D0
electron-muon coincidences: open charm & bottom
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Heavy flavors with CMS
muon spectrometer & silicon tracker in central barrel & end-caps
large acceptance, excellent resolution
J/, ’, , ’,’’, open charm, open bottom, Z0
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Heavy flavors with ATLAS
muon spectrometer & silicon tracker in central barrel & end-caps
large acceptance
studies limited to , ’,’’ reconstruction & b-jet tagging so far
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Conclusions:
It is possible now to simulate asymmetric collisions with
PYTHIA.
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References
• Charm and beauty production al LHC, N.Carrer, Geneva,CERN
• High energy nuclear collisions, K.J.Eskola, Finland
• Nuclear quark and gluon distributions in coordinate space, K.J.Eskola, Finland
• Heavy flavor production off protons and in a nuclear environment, B.Z.Kopeliovich, J.Raufeisen, Germany-Russia
• Energy loss of fast quarks in nuclei, FNAL
• ROOT and PYTHIA guide books.