# Presentation: 170355

## The Top Mass Measurement in ATLASat the LHC“A Test of Systematics”

**James Proudfoot**
- Argonne National Laboratory
- Argonne, Illinois, USA
***On behalf of the ATLAS Collaboration***

## Overview

**The LHC parameters**
**A brief overview of the Atlas detector**** **
**unique capabilities**
**Construction and installation status**
**Tile calorimeter commissioning**
**LHC Status (from a talk by Lynn Evans in June 2004)**
**The top mass measurement:**
**A Personal Bias**
**Trigger**
**Jet measurement and calibration**
**SYTEMATICS**
**Modeling**
**Comparison to the TeVatron  **
**Summary**

## The Large Hadron Collider

**Luminosity**
- 1033 cm-2 s-1 to 1034 cm-2 s-1
**25 ns bunch crossing (40 MHz)**
**High luminosity and large ********inc ****at LHC****  ****implies:**
- ~ 23 minimum bias events per BC
- ~ 70 charged tracks/event with
-     pT > 1 GeV/c for || < 2.5
**Mass reach  up to ******** 5 TeV**

**Production cross-sections and dynamics are largely controlled by QCD**

## The ATLAS Detector

**Tracking in 2T solenoid:**
**Si pixel + strips (3+4 layers)**
**TRT ******** particle id.**
-  / pT ~ 5 10-4 pT  0.01 in 2 Tesla field
**EM calorimeter: Pb – liquid Argon**
**   ****presampler + segmented EM calo.**
- 	          / E ~ 10% / E(GeV)
**Had. Calorimeter:**
**Fe –scintillator (barrel)**
- 	    / E ~ 50% / E(GeV)  0.03
**Cu/W – liquid Argon(endcaps/Forwd)**
-        / E ~ 60% / E(GeV)  0.03
**Muons: “Air”;  instrumented large toroid magnet**
**            ** / pT ~ 10 % at 1 TeV/c
**Luminosity Uncertainty < 5%**

**A General Purpose Detector**

## ATLAS has a Unique Capability in the Muon Spectrometer

## Next 18 Months on the Schedule

- BT construction and installation
- Barrel calorimeter installation
- Barrel services installation
- Cabling campaigns
- Muon barrel installation

- Barrel installation will be done in October 04

- First toroid will be installed in  Oct 04 (Scheduled for Oct 26th.)

## The Atlas Cavern in June 2004

**The Atlas Cavern in June 2004**

**October 26th**

**October 28****th****=>**

## Concurrently: Install services and begin to commission the detector electronics

**Concurrently: Install services and begin to commission the detector electronics**

## Services Being Installed on Barrel (in Cavern)

## Tile Calorimeter Commissioning (an example of what lies before us)

**Drawer**

**Readout part**

**HV part**

**PMT blocks**

**Tile Calorimeter Commissioning ****(an example of what lies before us)**

**Insert LVPS, and close with Iron Plates**

## Phase1: Tile Calorimeter Commissioning  Work Packages

- Detector

- FE
- electronics

_**Cooling,**_
_**Sniffers**_

- Cables

- FE
- Electronics
- on the truck

- BE
- electronics

_**Racks,**_
_**DCS**_
_**DAQ**_
_**DB**_

- FE-BE
- integration

- Cosmics L1
- receivers

_**Offline**_

- LV
- system

- HV
- system

- Cesium
- system

- Laser
- system

- TILES
- DCS

_**Racks,DCS**_

_**Interlocks**_

- TTC-LTP
- system

_**Racks,**_
_**DCS**_

_**Racks**_

-  L1
- receivers

-  Laser
- calibration

***Drawers and LVPS are inaccessible when the detector is closed for data-taking***

## Status of the LHC Project

**Status of the LHC Project**
- Lyn Evans
- CERN, 24 June 2004

**The most critical single issue is the installation of the QRL**** Installation planning is being revised to maintain summer 2007 for first beams**

## The ATLAS Physics Capability – Some Background

**Rates are for an instantaneous luminosity of **
**2x 10****33**** cm****-2**** s****-1**

_***I. Fleck, Eur Phys J C 34, 201, s 185***_

**30Hz of W****→e******
**3 Hz of Z****→****ee**
**2Hz of ttbar**

## Atlas Trigger: Primarily an Inclusive Trigger at Level 1 & 2

**Level 1:**
**Uses coarse grained calorimeter and muon information**
**Level 2:**
**Uses full granularity and full resolution but in Regions of Interest**
**Level 3:**
**On passing Level 2, the full event is built and passed to the Event Filter**

**Keep trigger ****inclusive**** since there might also be physics that we have not modeled !**

## The Top Mass Measurement  A Personal Bias

**Compositeness ****→ di-photons, di-jets, Etmiss**
**→ ****Calorimeter Performance**
**Higgs ****→ Photons, Etmiss, Forward Jet Tagging**
**→ ****Calorimeter Performance**
**SUSY ****→ Jets, Etmiss, Leptons**
**→ ****Calorimeter Performance**
**W, Z, W’, Z’ ****→ Isolated Leptons**
**→ ****Calorimeter Performance**
**Top ****→ Isolated Leptons, Jet Et, Etmiss**
**→ ****Calorimeter Performance**

## Why Pick the the Top Mass?

**To establish the performance of the detector, we need to MEASURE  a process which has a CLEAR signature**
**A PRECISION measurement of the top mass is an appropriate process:**
***The production rate for t-tbar is PREDICTED to be enormous***
***The measurement of the top mass touches the performance of the ENTIRE  ATLAS Detector***
***The top mass is an important measurement in its own right***

## The top quark mass, when combined with precision electroweak data, constrains the mass of the elusive Higgs Boson

- The top mass: a precision SM measurement to be made at the LHC

***(Chris Hill, VthRencontres du Vietnam)***

**For an equal error on the indirect determination of the Higgs mass, the uncertainty on the W mass has to be 150 time smaller than that of the Top quark Mass**

## Atlas Event Selection for Top: Isolated lepton + Jets

-  **p****T**** (lepton) > 20 GeV**
** ****p****T****miss****  > 20 GeV**
** ****≥ ****4 jets with p****T****  > 40 GeV**
**|********| < 2.4**
**Cone ********R = 0.4**
** ≥ ****2 b-tagged jets**

**L**** =  10****33**** cm****-2****s****-1**

| Period | evts | dMtop(stat) |
| --- | --- | --- |
| 1 year | 3x105 | 0.1 GeV |
| 1 month | 7x104 | 0.2 GeV |
| 1 week | 2x103 | 0.4 GeV |

## Jet Reconstruction Algorithms

**Cone Algorithm**
**Highest E****T**** tower for jet seed + cone**
**Iteration of cone direction, jet overlap, energy sharing, merging**
**K****T**** Clustering Algorithm**
**Pairs closest calorimeter towers, merging all particles into jets**
**Cone size influence on reconstructed jet energy and resolution**

- ******R=0.4**
******R=0.7**
******R=1.5**

|  | a (%GeV1/2) | b (%) |
| --- | --- | --- |
| Full Calo | 48.2 ± 0.9 | 1.8 ± 0.1 |
| R=0.7 | 52.3 ± 1.1 | 1.7 ± 1.1 |
| R=0.4 | 62.4 ± 1.4 | 1.7 ± 0.2 |

**Cone Algorithm**

***B. Caron, LHC2003***

## Atlantis top events

## Signal and Background Rates

**Based on Pythia Generator, with Fast Simulation**

***P. Roy,  Thesis 2002, Aubière : Clermont-Ferrand 2. Lab. Phys. Corpusc. Cosmol***_  _

**Assumes b-jet efficiency = 60%**

**C5: >=2 b-jets**
**C4: >= 1b-jet**

## Jet Reconstruction Issues

**Physics effects**
**Fragmentation**
**Initial and final state radiation**
**Underlying event**
**Minimum bias events**
**Detector performance effects**
**Non-linear response**
**Magnetic field**
**Dead material and cracks between calorimeters**
**Longitudinal leakage**
**Lateral shower size and granularity**
**Finite cone size (out-of-cone loss)**
**Electronic noise**

**GOAL**→

- Dead Material Map

## Energy Calibration “Tools”

**electronics calibration and/or response of the calorimeters to a known calibration “source”**
**Testbeam data plus simulation to refine the energy scale determination**
**Di-jet balance to take out zeroth order non-uniformity (in ********)**
**In situ jet calibration from  Z/******** + Jets (where jets may be either tagged b-jets or non-tagged jets**
**Z ****→ e****+**** e****-**** **** mass calibration to determine the electromagnetic energy scale**
**Z ****→ ********+**** ********-**** **** mass calibration to determine the momentum scale**
**Top specific jet calibration using constrained fit to W mass**
**Eventually calibrate b-jet energy scale using Z ****→ b bbar**

## ATLAS Test Beam Results(one example)

- Combined Test: EM LAr  and Hadronic Tile Calorimeter
-  Energy Resolution

|  | a (%GeV1/2) | b (%) | c (GeV) |
| --- | --- | --- | --- |
| Data | 69.8 ± 0.2 | 3.3 ± 0.2 | 1.8± 0.1 |
| G-CALOR | 61.7 ± 0.1 | 2.9 ± 0.3 | 1.5 fixed |

**Expect to set jet scale to 5-10% from testbeam + simulation**

## Offline Jet Energy Determinationfrom Monte Carlo “Truth”

- Sampling Method
- Weights applied to different calorimeter compartments
- Enlarged cone size yields increased electronic noise
- H1 Method
- Weights applied directly                   to cell energies
- Better resolution and               residual nonlinearities

**Resolution for**
**Back-to-Back dijet events**

- ||=0.3

| Parameter | Sampling Method |  | H1 Method |  |
| --- | --- | --- | --- | --- |
|  | R=0.4 | R=0.7 | R=0.4 | R=0.7 |
| a (%GeV1/2) | 66.0 ± 1.5 | 61.2 ± 1.3 | 53.9 ± 1.3 | 51.5 ± 1.1 |
| b (%) | 1.2 ± 0.3 | 1.4 ± 0.2 | 1.3 ± 0.2 | 2.5 ± 0.2 |
| 2 prob. (%) | 1.6 | 0.8 | 27.3 | 66.7 |

## Z + Jets: Pt Balance

**Use p****T**** balance between highest p****T**** jet and leptonic Z decay**
**Apply tight 2****nd**** jet veto and ********>3.06 to reach ±1% sensitivity level**

- GeV

**Full Simulation**
**Atlas Reconstruction code 8.4.0**

## W → Jet Jet in situ Calibration

- W → Jet Jet *in situ* Calibration

***Compare reconstructed Jet to Monte Carlo “truth”***

**1% mismeasurement of jet energies induces a top mass shift of 1.6GeV**
**Mismeasurement of cos(********bW****) induces a top mass shift of 1.2GeV**
**Residual effects:**
**FSR effects large for Pt~50GeV due to out-off-cone energy**
**Jet overlap for Pt>200GeV**

## M (W) = 80.3 ± 0.4

***Before calibration***

***After calibration***

**M (W) = 80.3 ****± 0.4**
** ******** ****= 6.9 ± 0.5**

**Fit ********1****, ********2 ****to minimize:**

**Where ********W**** = 2.1GeV, and ********1**** and ********2**** are the jet resolutions – taken from Monte Carlo**

## Mtop reconstruction

-  W from jet pair with the closest invariant mass to MW
- cut on |mjj-mW| < 20 GeV
- Associate W with a b-tagged-jet
**Cut on |mjjb-<mjjb>| < 20 GeV**
_**Kinematic fit**_
- Using remaining l+b-jet, the leptonic part is reconstructed
-  |mlb-<mjjb>| < 35 GeV
-  Kinematic fit to the ttbar hypothesis,  with Mtop and MW mass constraints

**M****top ****reconstruction**

- P. Grenier

**Predominately combinatorial background from real ttbar events**

## Systematics

**Light and B-Jet jet energy scales**
**Apply miscalibration to reconstructed jet energy and evaluate effect on reconstructed top mass**
**For light jet energy scale, the in situ calibration greatly reduces the impact of the scale uncertainty**
**Initial State Radiation**** **
**Compare difference in mass for ISR on/off. Use 20% of the as systematic**
**Final State Radiation**
**Compare difference in mass for ISR on/off. Use 20% of the as systematic**
**B quark fragmentation**
**Vary Person fragmentation parameter within 1******** of it default value**
**Combinatorial background**
**Estimate by varying the assumptions for the background shape**

## Initial performance : uncertainty on b-jet scale expected to dominate

- Systematic error on Mtop (TDR performance, 10 fb-1)

- _Initial performance_ : uncertainty on b-jet scale expected to dominate

- c.f.: 10% on q-jet scale  3 GeV on Mtop

## Top Mass Systematics at the TeVatron

- _D0:- Template Analysis_

| Systematic Uncertainties | 
Mtop(GeV/c2)   |
| --- | --- |
| Jet Energy Scale | 5.3 |
| ISR | 0.5 |
| FSR | 0.5 |
| PDF | 2.0  |
| Generator | 0.6  |
| Spin correlation | 0.4  |
| NLO effect | 0.4 |
| Bkg fraction(±5%) | 0.5 |
| Bkg Modeling | 0.5 |
| MC Modeling(jet,UE) | 0.5 |
| Transfer function | 2.0  |
| Total | 6.2  |

- _CDF:- DLM _

**There are several systematics at the ~0.5-1GeV level that are missing in the Atlas enumeration**
**This is good as it means that there is still work to do while we wait for the accelerator !**

- (Inc. PDFs)

## Comparison to present day Measurements from TeVatron and LEP

**New world average**
***m******top***** = 178.0 ± 4.3 GeV/c****2**
- Changes Higgs mass value favored by electroweak fits
***m******H*****   ******** 113 GeV/c****2**

**Recent, more precise D0 measurement**
*m**top* = 180.1 ± 5.4 GeV/c2
***Nature***** **429**, 638-642** (2004)

***(Chris Hill, VthRencontres du Vietnam)***

**In first year of data-taking, Atlas can reduce the uncertainty on the top mass by a factor of ~2**

## Other Modeling Issues(I Don’t Know but I’ve Been Told.......)

**Top Pt spectrum**
**Monte Carlo simulation of Z + jets:**
**Jet Pt spectrum**
**Jet multiplicity**
** ********s**
**Full Detector simulation**
**Signal AND Background**
**Improve understanding of b-jet energy scale**

**Comparison to analytic calculation (Weber & Frixione) suggests that Pythia is not a good representation - another area for further study**

## Some Other Analysis Options Possibilities

**Rate has its advantages !**
**Different jet reconstruction algorithms (e.g. continuous jet algorithm)**
**Use only large transverse momentum ttbar events (has different systematics)**
**Measure top mass in events with** a **J/******** ****in the final state**** **

## Backgrounds ?One Instrumental Effect in the Tile Calorimeter

**The Atlas Trigger and event selection will preferentially select background events from events with poorly measured Etmiss**

## More Ways to Generate Fake Etmiss & Thereby Backgrounds

**Gap between barrel and endcap calorimeters**
**e/h = 1**
**Mis-calibration**
**Barrel EM calorimeter insensitive region at ********~0**
**Detector “average center” vs beam center**
**+...**

## The Impact of Staging

**Staged detector components include:**
**The middle layer of the pixel detector**
**A large part of the high level trigger and data acquisition hardware**
**The impact include:**
***A 30% degradation in the b-jet identification efficiency ***
***Reduced capability for triggering on high rate QCD processes, which may be needed to understand backgrounds and detector efficiencies***

## Summary

**Many members of the Atlas collaboration have been intensively** **studying detector performance – I have shown some of these results and want to acknowledge in particular input I received from the Atlas Top Conveners Dominique Pallin and Marina Cobal**
**I hope I have convinced you that these studies give confidence that Atlas will make a precision measurement of the top quark mass and that in combination with the W mass and (hoped for) Higgs mass we can explore the limits of the Standard Model**
**Along the way, it is clear that there is much yet to be learned about the detector performance and basic W & Z production .. Both for energy scale calibration and as a “calibration” of the QCD predictions for jet spectra and multiplicity distributions and Etmiss**