Heavy Ion Collisions at LHC with the ATLAS Detector

Heavy Ion Collisions at LHC with the ATLAS Detector

Helio [email protected]

Brookhaven National Laboratory

Hadron 2002 Bento Gonçalves, April

2002

(J. Nagle, B. Cole and S. White)

mailto:[email protected]

April, 2002Helio Takai, Brookhaven National Laboratory 2

Before we start

First of all my sincere thanks to the organizers !

After I accepted the invitation I got worried !

What can I talk about? I decided to talk about heavy ion physics with the ATLAS detector that it is still far in the future. However it has been our experience in ATLAS that help from our theoretical colleagues in planning the experiments has been a big plus. So this is the spirit of this presentation.


Before we start

Another reason why I got worried is because I have been away from for a long time - almost 8 years!

So I will behave myself, and make no jokes about Our Lady of Perpetual Motion, the Church of Christ Geologist or the Jojoba Witnesses. However one can’t pass the opportunity to mention that everybody missed one good heavy ion experiment:


Plan for this presentation

i. Flirting with heavy ions

ii. The ATLAS detector

iii. Heavy Ion Physics at LHC energies

iv. A nuclear physics program for ATLAS

v. What is going to happen next

vi. Summary and conclusions


ATLAS and Heavy Ions

ATLAS is an experiment designed to study the origins of the electroweak symmetry breaking in proton-proton collisions, search for SUSY, and exotica - e.g., extra dimensions.

ATLAS is designed to acquire data at full LHC design luminosity of L=1034 cm-2s-1, or 40 MHz collision rate.

The detector is designed to cover a range of ||<5.0 in rapidity with inner tracking, calorimetric coverage and muon spectrometer.

ATLAS is also a large collaboration with over 2,000 physicists from 157 institutions worldwide, including Brazil.



Recent RHIC data suggests that jets may be quenched.

CERN theorists have been sponsoring a one year long workshop on hard probes in heavy ion physics.

This has sparked renewed interest by a group of physicists within ATLAS to revisit the heavy ion physics program with the detector.

0 2 4

1

2

RA

A

pT(GeV/c)



Large calorimetric coverage detector are well suited for jet physics

Jets at LHC heavy ion energies are similar to Tevatron jets.

For about one year, since the workshop in BNL, we have been developing a nuclear physics program for the ATLAS detector.

However there are many hurdles to overcome … people, funding, etc…


The people

The institutions involved in ATLAS heavy ions working group are:

Brookhaven National Laboratory

CERN

Columbia University

Prague

Rio de Janeiro

University of Geneva

Helsinki


LHC and heavy ions

LHC, the Large Hadron Collider, is now under construction and will accelerate heavy ions at an energy of 2.75 TeV/nucleon

The energy available at the center of mass of a Lead-Lead collision is well over a 1,000 TeV.

The energy density is expected to be about 30 times what is currently observed at RHIC

New and exciting physics could be observed by probing the hot QCD matter with hard probes.

But what is ATLAS exactly? Where is the experiment?


CERN

BNL

CERN

Where is CERN? It is in Switzerland, not Swaziland… and about 6,000 km from New York.


CERN

ATLAS

CMSMt. Blanc

Mt Blancdes Americaine


The ATLAS detector


Inner Detectors

The ATLAS inner detector is composed of three systems: Pixel, SCT and TRT

The pixel detectors are silicon devices and are located near to the collision point.

The Semiconductor Tracker are also silicon devices.

The transition radiation tracker are gas detectors and used to identify electrons.


Inner Detectors

The same event, in full and low luminosity running conditions.

€

H → bb


Calorimeters

Calorimeters in ATLAS cover a wide range of pseudo-rapidity, | |<5.

The electromagnetic calorimeter is realized in liquid argon technology

The hadronic calorimeter is implemented as a iron-scintillator device, except in the forward direction.

The very forward region is covered by axial drift liquid argon calorimeter.


Electromagnetic Calorimeter

Used by theInner tracker


Electromagnetic Calorimeter Segmentation

The Electromagnetic Calorimeter is segmented longitudinally and transversely.

Will be used for 0 identification.

Segmentation helps the identification of in the background of 0s.

0


Hadronic Tile Calorimeter

The hadronic tile calorimeter ‘hugs’ the liquid argon electromagnetic calorimeter. It is built in iron-scintillator technology and readout by WLS optical fibers.


Hadronic Tile Calorimeter


Calorimeter Characteristics

Electromagnetic Cal. Energy Resolution

EM Angular Resolution

EM Timing Resolution

Hadronic Calorimeter Energy Resolution

€

σ E

E=

10%E⊕0.7%

€

σ θ =60mrad

E

€

σ τ =4.15Ens •GeV

€

σ E

E(π ) =

47%E⊕2%


Jet Energy Resolution

Di-jet event in ATLAS

hadronic

Electromagnetic

Large acceptance allows for detection of back-to-back jets.

Jet Energy Resolution :

€

σE

E=

50%E⊕2%

Ha! But this is for pp collisions!!!


Muon Spectrometer

Air Core Toroidal Magnet System Muon detectors

Toroid! Toroid! Toroid!


Muon Spectrometer


Muon spectrometer

It takes 3 GeV to gothrough. Therest stops!


Muon Spectrometer Performance

Spectrometer only

Spectrometer + ID


The case for Heavy Ions

ATLAS is a detector appropriate for high pT physics.

It has a finely segmented calorimeter which is appropriate for jet physics. The energy resolution for jets is superb.

It has a stand alone muon spectrometer with good momentum resolution.

It is designed for high data rates for proton-proton collisions

But, is there interesting physics to be done in the high Q2 regime?


Heavy Ion collisions

…the picture says it all !!!!


Gluon Densities

HERA experiments have observed a dramatic increase in the gluon density at low x.

This increase must end at some point when the gluon density saturates.

Large Hadron Collider Pb-Pb collisions probe the gluon structure below x~10-3 - 10-5.

Note that xg(x) is enhanced by A1/3 ~ 6 in Pb over the proton.

RHICLHC


LHC and RHIC

The saturation scale is much larger at the LHC than at RHIC.

Thus, the initial partonic state may be dominated by the saturation region (described as a color glass condensate).

Also, the cross section for high pT processes is much larger, thus yielding better pQCD calibrated probes of the possible gluon plasma.


Freeing the Gluons (the QLF)

Then we can study the nature of this very hot bath of partons (QGP) !The plasma should be hotter and live longer than at RHIC.

In a future Electron-Ion Collider (EIC) one can probe the low-x gluon structure one gluon at a time.

At the LHC, tens of thousands of gluons, quark and antiquarks are made physical in the laboratory in every collision !

Very complementary physics.


Jet Probes of the Plasma

Baier, Dokshitzer, Mueller, Schiff, hep-ph/9907267Gyulassy, Levai, Vitev, hep-pl/9907461Wang, nucl-th/9812021and many more…..

Partons are expected to lose energy via induced gluon radiation in traversing a dense partonic medium.

Coherence among these radiated gluons leads to E L2 q

q

We want to measure the modification of jet properties as we change the gluon density and path length.


ATLAS Jet Rates

In one month of Pb-Pb running with three experiments at LHC,ATLAS will measure an enormous number of jets.

ATLAS accepted jets for central Pb-Pb

Jet pT > 50 GeV 30 million !Jet pT > 100 GeV 1.5 millionJet pT > 150 GeV 190,000Jet pT > 200 GeV 44,000

Vitev - extrapolated to Pb-Pb

Note that every accepted jet event is really an accepted jet-jet event since ATLAS has nearly complete phase space coverage !


ATLAS Jet Measurements

ATLAS can measure jets with E > 70 GeV with reasonable resolution and efficiency in the highest multiplicity central Pb-Pb events. More detailed studies are currently underway.

Substantial background reduction can be achieved by simultaneously finding back-to-back jets.

200 GeV jet overlay on central Pb-Pb event

with ATLAS segmentation


Jet Profile Analysis

The induced gluon radiation may be measurable due to the broader angular energy distribution than from the jet.

Possible observation of reduced “jet” cross section from this effect.

U.A. Wiedemann, hep-ph/0008241.BDMS, hep-ph/0105062.

proton-proton jet cone


Fragmentation Functions

pT (GeV)

Pho

ton

op

enin

g a n

gle

in d

e gre

es

2nd sample 3x5 cluster invariant mass calculation for identification.

ATLAS can measure identified 0 and mesons via photons. Excellent energy and timing resolution will help to limit background.

Background and resolution studies are underway.

x = 0.003 x 0.1

x = 0.025 x 0.025

2nd sample cluster two shower shape identification.

1st sample cluster two separation and total energy measure in 2nd sample.


-Jet Physics

ATLAS -jet rate is very large, thus allowing for detailed studies.

In one month, over 1000 events with energy = 60 GeV in a 1 GeV bin !

Above a certain pT~30 GeV, ATLAS can

no longer cleanly separate single from

two resulting from a 0 decay.

We are investigating whether with isolation cuts on a single high energy shower and an opposite side jet, which process dominates(1) -jet events(2) jet-jet events with one jet with a high z fragmentation

Wang and Huang, hep-ph/9701227


Beauty Jets

Radiative quark energy loss is qualitatively different for heavy and light quarks.

Finite velocity of heavy quarks at finite pT leads to suppression of co-linear gluon emission (“dead-cone” effect).

ATLAS can tag B jets via a high pT

muon in the muon detectors.

Y.L.Dokshitzer and D.E. Kharzeev, hep-ph/0106202

b

b

B

D


-Jet Physics

One can also study virtual photon-jet events, where * + -.

Rate is down two orders of magnitude from -jet.Good muon coverage makes this possible.

In one month in central Pb-Pb, ATLAS would accept ~ 10,000 events withpT > 40 GeV.

Z0-jet reconstruction is possible, but less than 500 total Z0 events per month.


Probes of Deconfinement

state J/ψ χcψ' Υ(1 )s χb

Υ(2 )s χb' Υ(3 )s

[ }Mass GeV 3.096 3.415 3.686 9.46 9.859 10.023 10.232 10.355. . [ ]B E GeV 0.64 0.2 0.05 1.1 0.67 0.54 0.31 0.2

Td/Tc --- 0.74 0.15 --- --- 0.93 0.83 0.74

Upsilon states (1s,2s,3s) span a large range in binding energy and thus their suppression pattern may allow for a mapping on the onset in the screening on the long range color confining potential.

ATLAS is currently investigating the mass resolution in the muon system with alternate track matching algorithms.


Correlated Global Measures

ATLAS will measure many global observables and have high statistics for correlating them with high pT probes.

1) Transverse energy2) Charged particle multiplicity 3) Zero degree energy4) Reaction plane

x

z

y

Jet observables as a function of reaction planeAzimuthal distribution of high pT 0 and Coverage over a broad range of pseudorapidity


Unique Opportunity at LHC

A parton propagating through the hot QCD media will radiate gluons loosing energy. The overall energy is, however, conserved therefore the measured jet properties will be modified.

The modification of the jet fragmentation function is the most sensitive physical quantity. Other expected modifications are the cone radius, etc.

Whatever measurement is done, it is worthwhile balancing the jet energy by the opposite , * or Z0. Therefore large coverage is beneficial.


Heavy Quarks

Dokshitzer and Kharzeev have suggested that heavy quarks would not radiate as much as light quarks when propagating through the media.

Therefore b-jets should not be quenched as much as light quark jets.

b-jets can be identified by the dislocated vertex or by tagging jets with the associated muon.

t-quarks although interesting do not live long enough to ‘feel’ the media.


p-A Physics in ATLAS

• Study of p-A collisions is essential @ LHC– To provide baseline for heavy ion measurements.– Physics intrinsically compelling

• Mini-jet production, multiple semi-hard scattering.• Shadowing – test of “Eikonal” QCD.• Gluon saturation – probe QCD @ high gluon density.• Test factorization.• Multiple hard scattering – Measure parton correlations in

nucleon (and nucleus ?)

• ATLAS is ideal detector for p-A studies coverage, calorimeter performance, b tagging, lepton

identification, inner tracking.


=>Nucleus at rest,effective Lorentz eff=2*beam2-1

€

dN(ω)dω

=2απ

ln(me •γeff

ω)

€

12

dN(ω)dω

=2αZ2

πln(

0.681hcγeff

Rnucleus•ω)

Heavy Ions e-Hadron collider

beamRhic=100

S=2mA’

RHIC and LHC, +hadron colliders


Heavy Ion Physics= Opportunities with a tool that we are just learning to exploit (c.f. e+e- physics)

LHC energy scale

Equivalent fluxUp to 100’s of TeV

Ultra Peripheral Collisions


Summary of the physics program

The ATLAS Nuclear Physics program includes

Global variable measurements: measurement of ET, dET/d, charged particle multiplicity N, dN/d, and elliptical flow.

Jet Physics : +jet, +jet, Z0+jet

Heavy Quarks : beauty jets

Quarkonia : suppression

p+A physics

Ultra Peripheral Collisions


Conclusions and Outlook

ATLAS is a world class detector designed for high rate proton-proton collisions and will provide great opportunity to study high Q2 physics in nucleus nucleus collisions.

The ATLAS heavy ion working group is being formed and simulation is under way. Input from Theorist and Experimentalists are very welcome


Conclusions and Outlook

Strong input from phenomenologists is the ticket for a successful program. We have experience in the proton-proton program and has made a huge difference.

In early april the proposed program was presented to DOE as a letter of intent and will formalize a proposal to LHCC late fall (in the northern hemisphere) - so join in!!!

Documents

Heavy Ion Collisions at LHC with the ATLAS Detector