# Presentation: 035237

## Particle multiplicity & ET measurements with ATLAS 

**Andrzej Olszewski**
**Institute of Nuclear Physics, Kraków, Poland**

- Andrzej Olszewski

- June-2003

## Outline of the Talk

- Andrzej Olszewski

- June-2003

## Heavy Ions at the LHC

** ****Hot/dense matter effects should dominate over**
**   ****initial and final state effects.**
** ****The effects may be seen already in global**
**   ****observables like particle multiplicity and E****T**

**Quantitative studies of a QGP properties:**

- Andrzej Olszewski

- June-2003

## The ATLAS Detector

**The ATLAS Detector**

- Andrzej Olszewski

- June-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**

**Global event characterization**
**Tracking particles with p****T**** ******** 1.0 GeV/c**
**High p****T**** probes, heavy quarks, quarkonia ...**

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

_**Physics:**_

- Andrzej Olszewski

- June-2003

**Notes:**
ATLAS is an excellent detector for high pT physics and jet studies

## Heavy Ion Interactions in Atlas

- Andrzej Olszewski

- June-2003

## Simulation Tools

-  **Event generator HIJING**
**  ****Based on PYTHIA  and Lund fragmentation scheme**
**  ****with nuclear effects: nuclear shadowing, jet quenching**
** ****GEANT3 detector simulation**
**  ****Full GEANT3 ATLAS detector simulations**
**  ****High Geant cuts: 1 MeV tracking/10 MeV production**
**  ****Only particles within |y| < 3.2**
** ****Sample generated**
**  ****5,000 events in each of 5 impact parameter bins:**
**  ****b = 0-1, 1-3, 3-6, 6-10, 10-15 fm**
** ****Detectors used in analysis**
**  ****Silicon Pixel, SCT. EM and HAD Calorimeters**

- Andrzej Olszewski

- June-2003

## Zoom on Atlas ID & Calorimeters

- Andrzej Olszewski

- June-2003

## Average Occupancies

**Average Occupancies**

** ****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**

- Andrzej Olszewski

- June-2003

## Global Measurements

- Andrzej Olszewski

- June-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**

- Andrzej Olszewski

- June-2003

## Calibration

**Linear calibration:**

**Quadratic calibration:**

- Andrzej Olszewski

- June-2003

## Measurements of Nch(|| < 3)

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

- Andrzej Olszewski

- June-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**

- Andrzej Olszewski

- June-2003

## dNhits/d vs. dNch/d

- Andrzej Olszewski

- June-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**

- Andrzej Olszewski

- June-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%**

- Andrzej Olszewski

- June-2003

## Reconstructed dNch/d

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

- Andrzej Olszewski

- June-2003

## 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**.

- Andrzej Olszewski

- June-2003

## Stability of the Centrality Estimate

**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.**

- Andrzej Olszewski

- June-2003

## Resolution of the Centrality Estimate

**High resolution available by using narrow cuts**
**on centrality correlated quantity**

- Andrzej Olszewski

- June-2003

## Track Reconstruction

**Track Reconstruction**

**Pixel and SCT detectors – ATLAS xKalman algorithm**

**p****T**** threshold for reconstructable **
**  ****tracks is 1 GeV (reduce CPU).**
**T****racking cuts are optimized to **
** **** ****get a decent efficiency  and **
** **** ****low rate of fake tracks.**
**V****ertex constraints applied**

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

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

- Andrzej Olszewski

- June-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!**

- Andrzej Olszewski

- June-2003

## Summary

**Global measurements are needed for detailed**
**  ****studies of heavy ion interaction properties**
**ATLAS detector is capable of providing **
**  ****measurements of the total charged multiplicity**
**  ****and transverse energy as well as their rapidity**
**  ****densities using very simple reconstruction**
**  ****procedures**
**Precision of the measurements seems to be high and**
**  ****reasonably independent of the true collision**
**  ****properties**
**These results, available already in the first days of**
**  ****LHC Heavy Ion run, may provide on their own crude**
**  ****verification of some ideas in heavy ion models**

- Andrzej Olszewski

- June-2003

## BACKUPS

- Andrzej Olszewski

- June-2003

## Zoom on Atlas ID & Calorimeters

- Andrzej Olszewski

- June-2003

## Detector Occupancies

**Detector Occupancies**

**Occ**

**Occ**

**z**

**z**

**N****ch**

**N****ch**

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

**Pix1**

**SCT1**

- Andrzej Olszewski

- June-2003

## Detector occupancies

**Pixel Detector**

**Silicon Tracker**

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

- Andrzej Olszewski

- June-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*

- Andrzej Olszewski

- June-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.**

- Andrzej Olszewski

- June-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)**

- Andrzej Olszewski

- June-2003