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Quarkonia Studies in PbPb Collisions by the ATLAS Experiment at LHC. Alexandre Lebedev, Iowa State University for the ATLAS Collaboration. A.Lebedev Quark Matter 2008 02/10/2008 1.

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Quarkonia Studies in PbPb Collisions by the ATLAS Experiment at LHC

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Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Quarkonia Studies

in PbPb Collisions

by the ATLAS Experiment at LHC

Alexandre Lebedev, Iowa State University

for the ATLAS Collaboration

A.Lebedev Quark Matter 2008 02/10/2008 1


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Why Quarkonia?

Quarkonium dissociation due to color screening is considered as

a promising signature of QGP formation.

- Different quarkonia states are expected to “melt” at

different temperatures.

Recent RHIC results point to importance of recombination of

quarkonia in the later stages of the collisions.

- Also need to consider feed-down from higher resonances

Complicated picture:

- It is important to measure simultaneously different

quarkonia states in order to understand heavy ion collisions

In this talk we study possibility to measure charmonium (J/y)

and bottonium (U) states via di-muon channel in PbPb collisions

by the ATLAS experiment at LHC.

A.Lebedev Quark Matter 2008 02/10/2008 2


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Measuring quarkonia in ATLAS

Quarkonia are measured in di-muon decay channel. ATLAS has

excellent muon detection capabilities for |h| < 2.6 and PT > ~2.5 GeV/c

MDT: Monitored drift tubes (barrel and endcaps)

CSC: Cathode strip chambers (endcaps)

RPC: Resistive Plates Chambers (barrel trigger)

TGP: Thin Gap Chambers (endcaps and barrel trigger)

A.Lebedev Quark Matter 2008 02/10/2008 3


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Algorithm

U with flat PT and h distributions, weighted with Pythia distributions.

generate single U

simulated PbPb Hijing events

simulate single U

merge single U to

simulated PbPb Hijing events

digitize single U

reconstruct single U

reconstruct merged events

analyze results and fill histograms

Analyse ESD/AOD and produce AAN

analyze results and fill histograms

compare

A.Lebedev Quark Matter 2008 02/10/2008 4


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

U acceptance (singles)

Projections are done with PT and

h distributions from pythia.

U was considered to be in acceptance

if both muons produced hits in

muon detector.

A.Lebedev Quark Matter 2008 02/10/2008 5


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Single U (baseline for PbPb)

Integrated (over PT and h) mass resolution is 177 MeV.

Integrated acceptance times efficiency is ~0.19

A.Lebedev Quark Matter 2008 02/10/2008 6


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Improving mass resolution

Restrict muon pseudorapidity - but loose statistics.

Best mass resolution ~111 MeV (~10% improvement)

Tighter reconstruction cuts can slightly improve this number

- still further loss in statistics

A.Lebedev Quark Matter 2008 02/10/2008 7


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Mass resolution (PbPb)

Integrated (over PT and h) mass resolution is 192 MeV

Weighted with PT distribution from pythia

Intergated over PT and h

centrality dependence

PbPb

singles

A.Lebedev Quark Matter 2008 02/10/2008 8


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Reconstruction efficiency vs centrality

- Integrated (over PT and h) acceptance times efficiency is 0.12

- Factor of ~2 loss in most central collisions

A.Lebedev Quark Matter 2008 02/10/2008 9


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Muon triggers

ATLAS Trigger system:

- Level 1 (L1) – configurable hardware

Higher Level Triggers (HLT):

- Level 2 (L2) – software, relies on input from L1

- Event Filter (EF) – off-line algorithms and data model

In PbPb interaction rate is expected to be ~3kHz

Probably no need for L1 trigger for data taking

But need HLT for analysis

We studied L2 trigger efficiency using merged

Upsilon/PbPb events, and fake rate using

pure PbPb Hijing events.

A.Lebedev Quark Matter 2008 02/10/2008 10


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Triggering on Upsilons

efficiency = (U reconstructed & muon trigger fired)/(U reconstructed)

No fakes in ~250 PbPb Min. Bias Hijing events (without U merging)

Any L2 muon trigger fired

Trigger threshold 6GeV

A.Lebedev Quark Matter 2008 02/10/2008 11


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Background and yields (1/3)

Total pp cross-section at 5.5 TeV: ~100mb

bbar cross-section at 5.5 TeV: ~100mb

U cross-section: ~100nb

- Upsilon cross-section in pp collisions was studied using pythia:

http://dprice.web.cern.ch/dprice/work/oniumvalidation-jun06.pdf

- 34nb with default trigger cuts, ~150nb with relaxed trigger cuts.

One Upsilon reconstructed in ~1M pp events

PbPb expected luminosity is 4 1026 cm-2s-1 (Letter of Intent)

- Interaction rate ~3000 Hz

from Glauber calculation (David d'Enterria, nucl-ex/0302016):

- Total PbPb cross-section: 7.7b

- Number of binary collisions: 400 (MB); 1670 (central)

Assume that both high pT muons and U scale with number of binary collisions. Then background will scale as a square of signal, and S/sqrt(B) ratio will be the same for pp and PbPb.

A.Lebedev Quark Matter 2008 02/10/2008 12


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Background and yields (2/3)

Sources of background:

a) charm

b) beauty

c) hadron decays (mostly pi & K)

d) hadron punch-through

Calculating background:

- use pythia to get muons from charm/beauty (a and b)

- generate single pions and kaons, run full simulation,

plot reconstructed muon spectrum (c and d)

This gives us muon spectra and multiplicities

Add Upsilons to this mix.

Scale everything with number of binary collisions

Run MC simulation and produce di-muon invariant mass spectra.

A.Lebedev Quark Matter 2008 02/10/2008 13


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Background and yields (3/3)

This plot corresponds to 24 days of running, at 3 kHz collision rate

Acceptance and efficiency corrected

Barrel only (|h|<1), 120 MeV U mass resolution

~15k reconstructed

U1S

A.Lebedev Quark Matter 2008 02/10/2008 14


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Charmonium (J/y)

Main problem: low acceptance due to minimum muon PT ~2.5-3.0 GeV

Two methods considered:

- both muons fully reconstructed

- “tagging method” for one muon (allows muon reconstruction down to 1.5GeV).

“tagging” method

Mass resolution 68 MeV

~100k J/y per month, tagging method

full reconstruction

A.Lebedev Quark Matter 2008 02/10/2008 15


Quarkonia studies in pbpb collisions by the atlas experiment at lhc

Conclusions and outlook

- Reconstruction efficiency is reasonably good even in most

central PbPb events

- Mass resolution is almost unaffected in PbPb collisions

- Mass resolution is good enough to separate different U states

at least in the barrel region

- We should be able to see U and J/y peaks in a few weeks of running

- Quarkonium study in e+e- channel is underway

- cC study is underway

A.Lebedev Quark Matter 2008 02/10/2008 16


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