# Presentation: 572488

## Heavy Ion Physics with the ATLAS Detector

**Barbara Wosiek**
- _[Barbara.Wosiek@ifj.edu.pl](mailto:Barbara.Wosiek@ifj.edu.pl)_
**Institute of Nuclear Physics, Krak****ów, Poland**

**For the ATLAS Heavy Ion Group:**

**S. Aronson, K. Assamagan, B. Cole, M. Dobbs, J. Dolej****s****i,**
**H. Gordon, F. Gianotti, S. Kabana, M. Levine, F. Marroquin,**
**J. Nagle, P. Nevski, A. Olszewski, L. Rosselet, P. Sawicki,**
**H. Takai, S. Tapprogge, A. Trzupek, M.A.B. Vale,**
**S. White, R. Witt, B. Wosiek, K. Wo****ź****niak and ......**

- Barbara Wosiek

- May-2003

## Outline of the Talk

-  **INTRODUCTION**

** ****Why Heavy Ions at the LHC?**
** ****Why ATLAS as a Detector for Heavy Ions?**

-  **ATLAS PERFORMANCE FOR HEAVY ION PHYSICS**

** ****Monte Carlo Simulations**
** ****Detector Occupancies**
** ****Global Measurements**
** ****Event Characterization**
** ****Tracking with ATLAS ID(Si)**
** ****B-tagging**
** ****Quarkonia Studies**
** ****JET PHYSICS    ********  Ketevi’s talk**

- Barbara Wosiek

- May-2003

## Heavy Ions at the LHC

**Study of QCD matter at extremely ****high energy **
**densities and ~vanishing baryon chemical potential****.**

** ****Initial energy density about 5 times higher than at RHIC.**
** ****Lifetime of a ****hot & dense matter**** much longer **
**    ****10-15 fm/c at LHC as compared to 1.5-4 fm/c at RHIC**
** ****Access to truly hard probes with sufficiently high rates       **** **
**             ****p****T**** > 100 GeV/c  (at RHIC p****T**** ******** 20 GeV/c)**
**           ****copious production of b  and c quarks**

**  ****deconfinement**
**  ****restoration of the chiral symmetry,**
**  ****physics of parton densities close to saturation**

**RHIC          LHC**
** ****200          5500 GeV**

- Barbara Wosiek

- May-2003

## Heavy Ions at the LHC

**Quantitative studies of a QGP properties:**
** ****Hot/dense matter**** effects should dominate over**
**  **** ****initial and final**** ****state effects.**
** ****Studies facilitated by many hard probes.**

_**LHC – HI  Phase I**_

**Pb + Pb     **
**E=2.75 TeV/beam    L****max**** = 1****·10****27**** cm****-2**** s****-1****   Interaction Rate = 8 kHz**
**Exploratory run of a few days in 2007**
**Extended run in 2008 (1 nb****-1****)**

_**LHC – HI  Phase I**__**I and later**_

** ****p + A collisions (benchmarking nuclear effects)**
** ****Possible lighter ion species: ****1****15**** ****I****n, ****84****Kr, ****40****Ar, ****16****O**

- Barbara Wosiek

- May-2003

## Heavy Ion Physics with ATLAS Detector

**ATLAS interest in heavy ion physics ****was ****activated **
**by highlights from**** ****RHIC ****experiments!**

**Hot/dense Nuclear Matter Diagnostics**

** ****Suppression of ****high p****T**** particles**
** ****Disappearance of back-to-back ****high p****T**** jet correlations**
** ****Huge azimuthal asymmetry at ****high p****T**

**ATLAS is an excellent detector **
**for high p****T**** physics and jet studies**

- Barbara Wosiek

- May-2003

## ATLAS as a Heavy Ion Detector

**High Resolution Calorimeters**
**Hermetic coverage up to |********| < 4.9**
**Fine granularity (with longitudinal segmentation)**

**Large Acceptance Muon Spectrometer**
**Coverage up to |********| < 2.7**

**Si Tracker**
**Large coverage up to |********| < 2.5**
**Finely segmented pixel and strip detectors**
**Good momentum resolution**

**High p****T**** probes**

**Muons from ********, Z****0**** decays**

**Tracking particles with p****T**** ******** 1.0 GeV/c**

**2.****+**** 3.**

**Heavy quarks(b), quarkonium suppression(********, ********’)**

**1.& 3.**

**Global event characterization**

- Barbara Wosiek

- May-2003

## Simulation Tools

_**Event Generator HIJING**_** : **
**Based on PYTHIA **** and Lund fragmentation scheme**
**with ****nuclear effects****:**** **nuclear shadowing, jet quenching

_**Simulated **__**event samples**_

**HIJING + full GEANT3 ATLAS detector simulations**
**Only particles within |y| < 3.2**

** ****High Geant cuts: 1 MeV tracking/10 MeV production**

** ****5,000 events in each of 5 impact parameter bins:**
**    ****b = 0-1, 1-3, 3-6, 6-10, 10-15 fm**

** ****Standard ATLAS cuts:100 keV tracking/1 MeV production**

-  **1,000 central events, b = 0-1fm**

** ****Initial layout – 2 pixel barrel layers**

-  **1,000 central events, b = 0-1fm**

- Barbara Wosiek

- May-2003

## Central Pb+Pb Collision 

** ****About**** 75,000 ****stable particles**
** ****~**** 40,000 ****particles in ****|********| ******** 3**
** ****CPU – 6 ****h per central event**** (800MHz)**
** ****Event size**** 50MB (****without**** TRT)**

**N****ch****(****|y|********0.5)**

- Barbara Wosiek

- May-2003

## Average Occupancies

**Average Occupancies**

**Central Collision Events b=0-1 fm**

** ****Occupancies still reasonable in all Si Detectors:**
**    ****below 2% in Pixels and below 20% in Strips **
**    ****(after accounting for local fluctuations in the data with low GEANT cuts)**
** ****TRT unusable – too high occupancy**

- Barbara Wosiek

- May-2003

## Global Measurements

**DAY-ONE MEASUREMENTS!**
**N****ch****,  dN****ch****/****d********,  ********E****T****,  dE****T****/d********, b**

** ****Constrain model prediction**
** ****Indispensable for all physics analyses**

_**Predictions for Pb+Pb central collisions at LHC**_

-  (**dN****ch****/****d********)********0**

**Model/data**

** ****~ 6500          HIJING:with quenching, with shadowing**
** ****~ 3200          HIJING:no quenching, ****with**** shadowing**
** ****~ 2300          Saturation Model (Kharzeev & Nardi)**
** ****~ 1500          Extrapolation from lower energy data**

- Barbara Wosiek

- May-2003

## Measurements of Nch(|| < 3)

**                 **

**Based on the correlation **
**between measurable quantity**
**Q and the true number of **
**charged primary particles:**
**Q = f(N****ch****)**

**Q:**** N****sig**** **
** ****(all Si detectors,except PixB-B)**
**    ********E****tot****EM****, ********E****tot****HAD**
**    ********E****T****EM ****, ********E****T****HAD**** **

**Caution:**
**Consistency between the measured signals**
** ****and the simulated ones**
**Monte Carlo dependency**

- Barbara Wosiek

- May-2003

## Measurements of Nch(|| < 3)

**Reconstructed multiplicity**
**distribution (N****sig****)**

**Relative reconstruction**
**errors: |N****rec****-N****ch****|/N****ch**

**Histogram – true N****ch**

**Points – reconstructed N****ch**

**Uncertainty up to 10% at low N****ch****, less than 3% at high N****ch**

- Barbara Wosiek

- May-2003

## Reconstruction of dNch/d

_**Motivation:**_** shape of the dN****ch****/d******** distribution is **
**             ****sensitive to dynamical effects like **
**             ****e.g. quenching and shadowing.**

** ****Analysis is based on signals only from Pixel barrel layers **
**  ****(done separately for each layer).**

** ****Clusterization procedure i.e. merging of hits in neighbor **
**   ****pixels is applied (particle traverses more than one pixel **
**   ****when ******** ******** 0).**

** ****Correction factors need to be applied to account for the**
**   ****excess of clusters at large |********|:**
** ****double hits from overlapping sensors**
**  ****magnetic field effects (low p****T**** particles bending back)**
**  ****production of secondary particles**

- Barbara Wosiek

- May-2003

## Reconstructed dNch/d

**Reconstructed dN****ch****/d******

**Single Pb+Pb event, b =0-1fm**

**5 peripheral collision, b =10-15fm**

***Correction factors are ~ centrality independent!***

**Comparison of the reconstructed ****dN****ch****/d******** distributions with the true one of charged primary particles**.
**One single correction function C(********) calculated from**
**  ****a sample of central events is used.**

**Reconstruction errors ~5%**

**Reconstruction errors ~13%**

- Barbara Wosiek

- May-2003

## Reconstructed dNch/d

**Single Pb+Pb HIJING event**
**with jet quenching, b =0-1fm**

**100 p+p events at ********s=200 GeV**

***Correction factors are ~insensitive to the detailed***
***properties of generated particles!***

**Different shape and higher density **
**are correctly reproduced!!**

- Barbara Wosiek

- May-2003

## Estimate of the Collision Centrality

**Estimate of the Collision Centrality**

**Based on the correlation between the measurable **
**quantity Q and the centrality parameter: b, (N****part****, N****coll****)**

**Monotonic relation between Q and b allows for assigning **
**to a certain fraction of events selected by cuts on Q, **
**a well defined average**** impact parameter**.

**N****sig**

**E****T ****- EM**

**E****T ****- HAD**

- Barbara Wosiek

- May-2003

## Estimate of the Collision Centrality

**Resolution of the estimated impact parameter**

**Remark: A better approach would be to use a quantity measured **
**          ****outside the mid-rapidity region,e.g. energy in forward **
**          ****calorimeters, which is less sensitive to dynamical effects.**

- Barbara Wosiek

- May-2003

## Track Reconstruction

**Track Reconstruction**

**Pixel and SCT detectors**** – ATLAS xKalman algorithm**
**Starting with software release 6.1.0 all modifications specific to Heavy Ion track reconstruction are included in the official xKalman code.**

-  pT threshold for reconstructable
-     tracks is 1 GeV (reduce CPU).
- Tracking cuts are optimized to
-    get a decent efficiency  and
-    low rate of fake tracks.

**At least 10 measurements per track**
**Maximum two shared measurements**
******2****/ndf ******** 4**

** ****Tracking in the |********| < 2.5**
** ****For p****T****: ****1 ****-**** 15 GeV/c: efficiency ~ 70; fake rate<10%**
** ****Fake rate at high pT can be reduced by matching with Calo data**

- Barbara Wosiek

- May-2003

## Track Reconstruction 

**Track Reconstruction **

**Momentum resolution**

**Efficiency versus rapidity **

**Flat dependency for |y| < 2**
**Higher in EC (more layers)**

**~3% for p****T**** up to 20 GeV/c**
**(for |********| < 2.5)**

**Tracking in HI events looks promising, still can be optimized!**

- Barbara Wosiek

- May-2003

## Heavy-Quark Production

** ****Heavy quarks live through the thermalization of QGP**
**   ********can be affected by the presence of QGP**
** ****Their radiative energy loss is qualitatively different **
**   ****than for light quarks**.

**Open Beauty via semi-leptonic decays**

**Tagging of the B-jets:**
** ****the high p****T**** ******** in the  MS **
** ****displaced vertex in the ID**

- Barbara Wosiek

- May-2003

## B-jet Tagging

**Preliminary study: **
**Standard ATLAS algorithm for pp**
**Higgs events embedded into pp or Pb-Pb event**
**Cuts on the vertex impact parameter in the Pixel and SCT**

**Promising, should be improved when combined with muon tagging!**

**Rejection factors against light quarks versus b-tagging efficiency**

**p-p**

**Pb-Pb**

- Barbara Wosiek

- May-2003

## Quarkonium Suppression

**Upsilon mass reconstruction using the Muon Spectrometer, Silicon Tracker and the Pixel Detector (barrel sections only). **

**Direct probe of the QGP:**
**Color screening of the binding potential leads to the**
** ****dissociation of the quarkonium states.**

**Upsilon family**                                     ******(1s)      ********(2s)     ********(3s) **
**Binding energies (GeV)                 1.1           0.54        0.2**
**Dissociation at the temperature     ~2.5T****c****       ~0.9T****c****    ~0.7T****c**

**Important to separate ********(1s) and ********(2s) **

****** ******** ********+********–**

- Barbara Wosiek

- May-2003

## Quarkonium Suppression

** ****GEANT3 simulations of pure ********(1s) and ********(2s) states  ******** ********+********–**
** ****Muons with p****T ****> 3GeV are tracked backwards to the ID **
** ****Invariant mass is calculated from the overall fit.**

** ****Background estimate (HIJING+G3) ******** S/B ~ 0.6**
** ****Acceptance 10-15% providing 100% efficient dimuon trigger **
** ****Overlay with HIJING Event is under study!**

****** ****= 130 MeV**

- Barbara Wosiek

- May-2003

## Summary

** ****ATLAS detector will be capable of measuring **
**   ****many aspects of ****relatively ****low p****T**** heavy-ion **
**   ****physics**
** ****Let’s see the detector performance for **
**   ****studying the ****truly ****high p****T**** phenomena ...**

- Barbara Wosiek

- May-2003

## BACKUPS

- Barbara Wosiek

- May-2003

## Detector Occupancies

**Detector Occupancies**

**Occ**

**Occ**

**z**

**z**

**N****ch**

**N****ch**

**Examples of occupancy versus z and N****ch**
**(high GEANT thresholds)**

**Pix****1**

**SCT1**

- Barbara Wosiek

- May-2003

## Detector occupancies

**Pixel Detector**

**Silicon Tracker**

**Central collisions b=0-1 fm, low GEANT  thresholds**

- Barbara Wosiek

- May-2003

## Trigger DAQ

**For Pb+Pb collisions the interaction rate is 8kHz, a factor of 10 smaller than LVL 1 bandwidth.**
**We expect further reduction to 1kHz by requiring central collisions and pre-scaled minimum bias events  (or high p****T**** jets or muons).**
**The event size for a central collision is ~ 5 Mbytes.**
**Similar bandwidth to storage as pp at design L implies that we can afford ~ 50 Hz data recording.**

*~200 Hz*

- Barbara Wosiek

- May-2003

## Correction Factors

**Correction Factors**

**Correction factors are defined as:**

**C(********) calculated from the **
**sample of 50 central**
**(b=0-1fm) Pb+Pb events,**
**and then parameterized.**

**Correction function for**
**the inner most barrel **
**layer.**

- Barbara Wosiek

- May-2003

## Maximal Cluster Size

** ****Define the expected maximal size of the cluster:**

** ****In ****Z-direction**** the number of pixels to be merged **
**    ****depends on the Z-coordinate of the hit:**

**e.g. for R=5cm, Z****hit****=40cm  N****pixel ******** 6 -7**

** ****In ********-direction**** the number of traversed pixels depends on p****T.**
**   ****For a track with a curvature r, an angle at which particle **
**   ****enters the sensor is cos(********)=R/2r (assuming that sensors form **
**   ****an ideal tube). **
**   ****Taking r = 15cm (corresponding to p****T****=90 MeV/c):**

**N****pixels**** = 4 – 6  for R=12cm**
**N****pixels**** = 3 – 4  for R= 5cm**

- Barbara Wosiek

- May-2003

## Cluster Formation

** ****Choose seeds ******** large signals > 10,000 electrons**
** ****Start with the seed with the largest signal**
** ****Attached to it a signal in the adjacent pixel as long as:**

** ****There is a signal in a pixel**
** ****One of the closest neighbor pixels already belongs to the cluster**
** ****The distance from the seed to the pixel is not larger than the **
**  ****expected maximal size of the cluster (in both Z and ******** directions)**
**up to**** ********6**** ****pixels**** ****in**** Z (****depending on ****Z****hit****)**** and ********3 ****pixels in ******** (depending on R)**

- Barbara Wosiek

- May-2003