# Presentation: 544546

## Revealing the Details of QCD Energy Loss with Jets:Prospects of ATLAS Heavy-Ion Jet Measurements

- Nathan Grau
*For the ATLAS Collaboration*
- Columbia University, Nevis Laboratories

## Discovery of Jet Quenching at RHIC

- Hot Quarks 2008

- 8/22/2008

## Fundamental Question of Energy Loss

- Is energy loss dominated by perturbative effects?
- Is it dominated by radiative energy loss?
- What is dE/dx (L,E)
- What is the medium’s response?

- Hot Quarks 2008

- 8/22/2008

## Fundamental Question of Energy Loss

- What is the mechanism if not perturbative?
- Insights from AdS/CFT Emax(m,L) [Kharzeev]
- Chromo-magnetic effects [Shuryak]
- Do jets fragment outside of the medium?
- Strongly coupled liquid so 			scattering from a field?

- Hot Quarks 2008

- 8/22/2008

## First Experimental Test

- Measure Jet RAA using standard algorithms
- Sensitive to
- Perturbative effects
- Collisional energy loss
- Energy radiated outside the cone

- Lohktin, PYQUEN

- Hot Quarks 2008

- 8/22/2008

## First Experimental Test

- Measure Jet RAA using standard algorithms
- Sensitive to
- Perturbative effects
- Collisional energy loss
- Energy radiated outside the cone
- Non-perturbative effects
- Loss of jets because not reconstructed as a jet

- Lohktin, PYQUEN

- ?

- Hot Quarks 2008

- 8/22/2008

## The ATLAS Detector

- Hot Quarks 2008

- 8/22/2008

## The ATLAS Detector

- Full azimuthal
- acceptance

- Tracking in 2T solenoid

- Hot Quarks 2008

- 8/22/2008

## The ATLAS Detector

- Full azimuthal
- acceptance

- Photons

- Tracking in 2T solenoid

- Hot Quarks 2008

- 8/22/2008

## The ATLAS Detector

- Full azimuthal
- acceptance

- Jets

- Photons

- Tracking in 2T solenoid

- Hot Quarks 2008

- 8/22/2008

## The ATLAS Detector

- Full azimuthal
- acceptance

- 100 GeV jet depositing energy

- Photons

- Tracking in 2T solenoid

- Jets

- Hot Quarks 2008

- 8/22/2008

## Cone Jet Reconstruction: Embedding

- Pythia di-jets embedded in unquenched HIJING
- Lots of correlations: Mini-jets, c-cbar, b-bbar, longitudinal strings, etc.

- -0.5<<-1.5

- Calorimeter
- energy in
- 0.1x0.1 towers

- Hot Quarks 2008

- 8/22/2008

## Cone Jet Reconstruction: Subtraction

- Remove <ET> layer-by-layer and vs. 

- -0.5<<-1.5

- Calorimeter
- energy in
- 0.1x0.1 towers

- Hot Quarks 2008

- 8/22/2008

## Cone Jet Reconstruction: Jets

- Reliably reconstruct the input jets
- And some fake jets

- -0.5<<-1.5

- Calorimeter
- energy in
- 0.1x0.1 towers

- Hot Quarks 2008

- 8/22/2008

## Evaluating Fake Jets

- Run HIJING with a hard cut of 10 GeV
- No direct hard scattering above this scale
- Still could have jets above 10 GeV because
- initial and final state radiation
- longitudinal string fragmentation
- Remove from reconstructed jets matching to parton with ET cut
- No embedded PYTHIA jets

- Hot Quarks 2008

- 8/22/2008

## Distinguishing Real and Fake Jets

- Left: Reconstructed jet from embedded PYTHIA
- Asymmetric fragmentation
- Right: Reconstructed jet from a HIJING sample without jets > 10 GeV and nothing embedded
- Large angle fragments and no core
- Need a distinguishing variable.

- ET=57 GeV

- Hot Quarks 2008

- 8/22/2008

## Need a variable which enhances the large angle towers/cells in the jet
Define

   with                         the angle from the jet

- Define
-    with                         the angle from the jet

- Fake Jet Rejection: SumJt

- Hot Quarks 2008

- 8/22/2008

## Fake Jet Rejection: SumJt

- Fake jets SumJt>Pythia jets
- <R> for Pythia jets decreases with increasing jet energy

- Hot Quarks 2008

- 8/22/2008

## Fake Jet Rejection: SumJt

- Fit the fake jet data from HIJING without jets to remove trivial ET dependence (by definition centered at 0 with width of 1)
- Real jets will have a much smaller SumJt and a cut to reject fake jets is made.

- Hot Quarks 2008

- 8/22/2008

## Cone Jet Results: Efficiency

- Efficiency is independent of centrality
- Important for jet RCP, centrality-dependent effects
- Affected by
- 5 GeV seed selection
- Fake rejection

- Hot Quarks 2008

- 8/22/2008

## Cone Jet Results: Position Resolution

- Position resolution in  and  is very good

- Hot Quarks 2008

- 8/22/2008

## Cone Jet Results: Energy Resolution

- Jet scale good to 2% for ET>50 GeV using p+p calibrations
- Energy resolution < 25% for ET>70 GeV for extreme conditions of unquenched HIJING and dN/d = 2700

- Hot Quarks 2008

- 8/22/2008

## Cone Jet Results: Energy Resolution

- Energy resolution
- decrease to    ||~3
- FCAL(||>3.2) same as ||~0

- Hot Quarks 2008

- 8/22/2008

## Cone Jet Results: Spectrum

- Reconstructed spectrum uncorrected for ET res. and efficiency
- Fake spectrum (red squares) after rejection from dashed line.

- dN/d=2700

- Reco spectrum uncorrected for
- efficiency and energy resolution

- Hot Quarks 2008

- 8/22/2008

## Cone Jet Results: Spectrum

- Ratio ~20%
- Will be sensitive to this level of effects from perturbative and non-perturbative effects.

- Reco spectrum uncorrected for
- efficiency and energy resolution

- Hot Quarks 2008

- 8/22/2008

## Physics Conclusion

- Only after we establish that jets we measure lose energy in some perturbative way can we go to measure D(z), jT distributions, etc. to understand the details of the perturbative energy loss.
- Until then we must, at both RHIC and the LHC, to find measurements which are sensitive to perturbative and non-perturbative energy loss.

- Hot Quarks 2008

- 8/22/2008

## A Further Outlook

- Whole set of results not shown which will be necessary experimental tools for understanding jet energy loss:
- kT algorithm, -jets, heavy flavor tagging
- Background subtraction will be complicated by the medium response to jets
- Fake jet rejection will be complicated by the energy loss mechanism
- What other new physics is waiting to be discovered?

- Hot Quarks 2008

- 8/22/2008

## The ATLAS Heavy Ion Working Group

- Brookhaven National Laboratory, Upton, USA
- Charles University, Prague, Czech Republic
- Columbia University, New York, USA
- University of Geneva, Geneva, Switzerland
- IHEP, Moscow, Russia
- IFJ PAN, Krakow, Poland
- Iowa State University, Ames, USA
- JINR, Dubna, Russia
- MePHI, Moscow, Russia
- Universidad Catolica de Chile, Santiago, Chile
- Santa Maria University, Valparaiso, Chile
- Stony Brook University (Chemistry) Stony Brook, USA
- Weizmann Institute, Rehovot, Israel

- Hot Quarks 2008

- 8/22/2008

## Backup Slides

## Life at the LHC

- Plenty of high-ET jets visible above the background
- Use standard jet reconstruction algorithms to measure full jets instead of di-hadrons
- But there are two important issues: the underlying event and fake jets.

- 75 GeV

- 72 GeV

- dN/d=
- 2700

- Hot Quarks 2008

- 8/22/2008

## Subtraction of the Background

- Overall scheme: subtract the average background calorimetric ET (<ET>)
- Done for each layer of the calorimeter since background contribution (and noise) is different in each layer
- Done as a function of  because of physics and because of cracks, inner detector material differences as a function of .
- Do not include *jets* in the <ET> calculation of the *background*

- Hot Quarks 2008

- 8/22/2008

## Finding High Towers

- Use a 0.3x0.3 size sliding window

- Hot Quarks 2008

- 8/22/2008

## Finding High Towers

- Use a 0.3x0.3 size sliding window

- Hot Quarks 2008

- 8/22/2008

## Finding High Towers

- Use a 0.3x0.3 size sliding window
- Find <ET> and RMS of summed towers
- Select high tail

- High
- Towers

- Hot Quarks 2008

- 8/22/2008

## An Example Event : Towers

- PYTHIA event in dN/d=2700 unquenched HIJING
- 0.1x0.1 Towers

- Hot Quarks 2008

- 8/22/2008

## An Example Event : Truth Jets

- 75 GeV

- 72 GeV

- 75 GeV

- 72 GeV

- 25 GeV

- 12 GeV

- PYTHIA event in dN/d=2700 unquenched HIJING
- 0.1x0.1 Towers
- Truth jets from PYTHIA

- Hot Quarks 2008

- 8/22/2008

## An Example Event : High Towers

- PYTHIA event in dN/d=2700 unquenched HIJING
- 0.1x0.1 Towers
- Truth jets from PYTHIA
- High towers regions exclude jets from <ET>

- 75 GeV

- 72 GeV

- 75 GeV

- 72 GeV

- 25 GeV

- 12 GeV

- Hot Quarks 2008

- 8/22/2008

## An Example Event: Subtraction

- Hot Quarks 2008

- 8/22/2008

## An Example Event: Subtraction

- Note the
- scale

- Hot Quarks 2008

- 8/22/2008

## An Example Event: Before

- 75 GeV

- 72 GeV

- dN/d=
- 2700

- Hot Quarks 2008

- 8/22/2008

## An Example Event: After

- 75 GeV

- 72 GeV

- dN/d=
- 2700

- Hot Quarks 2008

- 8/22/2008

## An Example Event: Reconstructed Jets

- 75 GeV (truth)
-           67 GeV (reco)

- 72 GeV (truth)
-  54 GeV (reco)

- Applying an R=0.4 iterative cone algorithm with seed > 5 GeV tower we find jets

- dN/d=
- 2700

- Hot Quarks 2008

- 8/22/2008

## An Example Event: Fake Jets

- Algorithm also finds a stable cone for fluctuations in the background
- Need to reject fake jets

- 75 GeV (truth)
-           67 GeV (reco)

- 72 GeV (truth)
-  54 GeV (reco)

- 25 GeV (reco)

- dN/d=
- 2700

- Hot Quarks 2008

- 8/22/2008

## Some Physics: jT

- Match charged tracks to jets
- Reproduce input jT before jet corrections

- Salgado, Wiedemann Phys. Rev. Lett. 93 042301 (2004)

- Hot Quarks 2008

- 8/22/2008

## Some Physics: D(z)

- Reliable reproduction of D(z) before jet energy resolution correction.

- Gluon to Pion D(z)

- Armesto et al. JHEP02(2008)048

- Hot Quarks 2008

- 8/22/2008

## Some Physics: Di-jet Correlations

- Correlations of reconstructed jets as a function of  and pout = ETBsin
- Clear peak at  indicative of hard back-to-back jets.

- Hot Quarks 2008

- 8/22/2008

## Further Avenue: kT Algorithm

- Using the kT algorithm
- Clusters based on relative energy between nearby towers/cells/particles but not in a fixed cone
- Infrared and collinearly safe
- Because the kT algorithm is O(N log N) [FastJet by Cacciari, Gavin, and Salam] run directly on HI events before background subtraction

- Hot Quarks 2008

- 8/22/2008

## Example kT Event

- Example event with Pythia di-jet embedded in unquenched HIJING

- Hot Quarks 2008

- 8/22/2008

## Example kT Event

- Applying the kT algorithm directly on the HI event clusters all towers into a jet

- Pink circles indicate pythia di-jets
- All other jets are composed primarily of background

- Hot Quarks 2008

- 8/22/2008

## Example kT Event: Discrimination

- Define a (several) discriminating variable(s) to distinguish between real and fake jets

-  Example is max/avg tower ET jet-by-jet.
-  Pythia dijets are clearly visible.
-  Use fake jets to subtract background

- Hot Quarks 2008

- 8/22/2008

## Cone and kT Performance Comparison

- Performance differences due to differences
- Intrinsic to the jet algorithms
- In handling the background
- In rejection fake jets
- Multiple methods allow control of systematics

- Hot Quarks 2008

- 8/22/2008

## Photon ID: The Strip Layer

- Designed to measure
-     and rejecting di-jets
-  and 0 separation for wide range of ET
- Front layer: strips
- Typically 0.003x0.1 in x
- Over ||<2.5

- Hot Quarks 2008

- 8/22/2008

## Photon ID: The Strip Layer

**Single**
**Particles**

**dN/d********=**
**2700**

- Hot Quarks 2008

- 8/22/2008

## Photon ID: Strip Layer Rejection

- Rejection of background from strip layer
- Before isolation means measurement of fragmentation photons!

- Hot Quarks 2008

- 8/22/2008

## Photon ID: Isolation

- Require ETEMC<f(cent,ET) and no pT>g(cent,ET)

- Hot Quarks 2008

- 8/22/2008

## Combined Strip/Isolation Rejection

- Worst case scenario: RAAh=1
- If RAAh=0.2, S/B=1 at 30 GeV for dN/d=2700
- Expect 200k direct photons for ET>30 GeV

- Hot Quarks 2008

- 8/22/2008

## Photon Spectra

- Estimated spectra for different centrality from 1 LHC year (1 month, 0.5 nb-1)

- Hot Quarks 2008

- 8/22/2008

## -Jet Correlations

- Clear back-to-back correlations down to 40 GeV!
- Small  dominated by fake reconstructed jets
- Use correlations to study background rejection of jets

- Hot Quarks 2008

- 8/22/2008

## Muon Tagging: Heavy Quark E-loss

- Require muon in jet or in recoil jet
- Cut on muon pT

- Hot Quarks 2008

- 8/22/2008