Manifestation of Jet Quenching in the Total Transverse

Energy Distributions in the Nucleus-Nucleus Collisions


M.V. Savina, S.V. Shmatov, N.V. Slavin, P.I. Zarubin

Joint Institute for Nuclear Reseach,



20 December, 1997




Abstract



In the framework of the HIJING model global characteristics of nucleus-

nucleus collisions are studied for a Large Hadron Collider energy scale. An inter-

esting model prediction is the presence of a central bump over a pseudorapidity

plateau of a total transverse energy distribution. The bump is induced by a jet

quenching effect in a dense nuclear matter. It is shown that on a wide acceptance

calorimeter with a pseudorapidity coverage -5 < < 5 allow to obtain experimental

arXiv: v2 13 Jul 2000 confirmation of such an effect.

The investigation has been performed at the Laboratory of High Energies,

JINR.





1


A dense nuclear matter formation (or quark-gluon plasma) in central heavy ion

collisions at high energies is predicted by many models [1]. One of the predicted

properties of such nuclear matter is parton jet energy losses as a result of final state

interactions with a dense nuclear matter called jet quenching [2]. One may expect a

manifestation of this effect in differential distributions of a transverse energy flow,

Et in a wide pseudorapidity region. In this paper we concentrated our attention on

distributions dEt/d generated in the framework of the HIJING model [3]. A HIJING

program steering makes it possible to switch on or to switch off a jet quenching in an

easy way.

In the HIJING generator [3] soft hadron-hadron processes are simulated as

classical strings with kinks and valence quark ends following the FRITIOF model

[4]. Hard parton processes and the jet production are based on the well known

PYTHIA program package [5]. The number of jets in inelastic pp-collisions is

calculated by the eikonal approximation. The HIJING model provides a dependence

of the parton structure function on a collision impact parameter.

One of the most important features of the HIJING model is the inclusion of

energy losses dE/dx mechanism of a parton traversing dense nuclear matter. High

energy quark and gluon losses in a hot chromodynamic matter are estimated in [6]. It

was shown that a dominant mechanism of energy losses is radiative ones induced by

a gluon emission in soft final state interactions (bremsstrahlung). In the HIJING

model this mechanism is fulfilled as consecutive transmissions of a part of the quark

and gluon energy ldE/dx from one string configuration to another. Energy losses per

unit of path for quark and gluon jets are considered to be the same. It is supposed

that the energy losses are proportional to a distance l traversed by a jet after last

interaction and occur only in a transverse direction within a nucleus radius RA.. The

average free path is denoted as << 2
s E/d , where E is the hard parton energy d is

the Debye colour screening length. Parton interaction points are defined by a

probability function:

dl
dP e l S
= -
s



2


An interaction proceeds until a parton jet stays inside the considered volume or

the jet energy exceeds the jet production energy limit. Let's note that a space-time

picture of parton shower evolution is not considered in the HIJING, and the effect

of dense matter formation is introduced by a phenomenological way with

parameters s =1 fm and dE/dx = 2 GeV/fm.

The HIJING generator was used to produce 10000 events of minimal bias PbPb,

NbNb, CaCa, OO interactions at a 5 TeV/nucleon collision energy. Fig. 1 a and b

shows a differential distribution dEt/d of the total transverse energy (i.e. hadron and

electromagnetic components) over pseudorapidity for PbPb-collisions with and

without jet quenching, respectively. A distinctive feature is the presence of the bump

in central pseudorapidity (-2<<2) region when jet quenching is switched on and

absence of the bump in case when quenching is switched off. Being normalized to

the transverse energy flow integral, pp distributions reproduce a AA case well

enough in fragmentation regions.

We explored the collision energy dependence of an enhanced transverse energy

flow. In fig.2 differential transverse energy distributions are presented for 5, 2, 1, 0.5

TeV/nucleon lead-lead collisions. One may conclude that the bump becomes distin-

guishable over a plateau starting just from the energy value larger than 5

TeV/nucleon. For lower energy values such an effect is not so profound due to

smaller rapidity difference of fragmentation regions.

We followed a jet quenching sensitivity to a mass number for lighter colliding

nuclei. Energy losses of hard parton jets depend on a parton path in a dense matter

like E(l)=ldE/dx. Therefore reduction of the colliding nucleus radii might lead to a

bump reduction in the central region with the respect to the fragmentation part. This

provides an additional test of jet quenching phenomena. Fig.3 shows that lead-lead

collisions demonstrate maximum quenching dependence while for CaCa and lighter

ion cases the bump can be seen hardly.

Initially the observation of jet quenching was proposed as a modification of in-

clusive particle spectra in the central rapidity region. Fig.4 shows this effect in trans-

verse momentum spectra. Parton energy losses in dense nuclear matter lead to an

increase of the particle production at small and moderate pt and to a decrease of it at

large pt simultaneously.

3


We note that the CMS [7] experiment with a wide calorimeter acceptance -5 <

< 5 is sufficient to observe the described modification in dEt/d () distributions.

These distributions can be obtained with a practically unlimited accuracy for various

colliding nuclei by a direct summation of signals from each CMS calorimeter tower.

One of the CMS calorimeter practical problems is energy resolution depletion

on boundaries of different calorimeter sections, i.e. barrel, forward, and very forward

ones. We propose to smooth transverse energy spectra by normalizing nucleus-

nucleus distributions to appropriate distributions for pp collisions (fig.5) with the

same dependence of the energy resolution in calorimeter border regions.

It's interesting to note that jet quenching manifestation happens just in the cen-

tral rapidity part leaving fragmentation regions 3<||<5 unmodified. The latter cir-

cumstance is particularly useful for definition of nuclear collision geometry (i.e. im-

pact parameter) independently of collision dynamics details. Besides it is useful for

luminosity monitoring of various colliding ion species.





Conclusion



Using the generator HIJING we have shown that jet quenching effect can be

observed in the CMS experiment calorimeter by measuring the transverse energy

differential distribution in the whole CMS pseudorapidity coverage.

In a more general sense the study of the global energy flow makes it possible to

define quite common rules of nucleus-nucleus collisions dynamic in ultrarelativistic

energy range as well as to verify some important predictions of dense nuclear matter

formation model in a sufficiently simple way.

Besides, the performed analysis allows one to conclude that the jet quenching

effect is small for lighter nuclei like O, Ca enabling to investigate parton distribu-

tions in nuclei with minor destortion by final state interactions.

We would like to express our thanks to Profs. I.A. Golutvin, A.I. Malakhov, and

V.N. Penev for encouraging support and stimulating discussions.





4


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5


Fig.1 The differential distribution of the total transverse energy over pseudorapidity
for proton-proton collisions and lead-lead ones with (a) and without (b) jet quenching

effect ( s =
= 5 T
eV nucl). The distribution are normalized on an integral transverse
energy flow.


Fig.2 The differential distribution of the total transverse energy over pseudorapidity

for PbPb-collisions at energy values s =
= 5 2
0 5 TeV nucl

Fig.3 The differential distribution of the total transverse energy over pseudorapidity
for collisions of nuclei of various mass number (Pb, Nb, Ca, O) with and without jet

quenching ( s =
= 5 T
eV nucl).

Fig.4 The proton transverse momentum distributions for central AuAu collisions at

(a) RHIC ( s =
= 200 G
eV nucl) and PbPb collisions at (b) LHC ( s == 5 TeV nucl)
energies (-1<
<1). The solid and empty circles correspond to the distributions with
and without jet quenching effect, respectively.


Fig.5 The differential distribution of the total transverse energy over pseudorapidity
for collisions of nuclei of various mass number (Pb, Nb, Ca, O) with and without jet

quenching ( s =
= 5 T
eV nucl) normalized to proton-proton case.





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