# Presentation: 191494

## Direct Photons from RHIC to the LHC

**Stefan Bathe**
**RBRC**

**Nuclear Physics Seminar**
**BNL, August 19, 2008**

**Notes:**
Changed title 

## What I will do in this talk- a disclaimer -

- I’ll talk little about RHIC physics
- I’ll talk some about LHC physics
- I’ll talk much about LHC calorimeters

- BNL NuclPhysSeminar

- Stefan Bathe

## RHIC Key Physics Result

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**Nucl.Phys.A757:184-283,2005**

- High-pT hadron suppression
- E loss of partons in medium

**PRL91(2003)072304**

- 0

## Surface Bias

- Strong suppression (factor 5)
- Dominated by surface emission (black probe)
- Difficult to learn about suppression mechanism

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- Dainese et al., Eur. Phys. J. C 38, 461 (2005)

- Origin of partons that produce 5 GeV hadrons in central Au+Au

## How can we learn more?

**“****[Almost]** everything you want to know about jets can be found in 2-particle correlations.” Mike Tannenbaum
- “**Almost**”:  Shape of xE distribution not sensitive to FF
- Reason:  trigger bias
- Trigger bias can be overcome by
- -h correlations (also overcomes surface bias!)
- Full jet reconstruction (difficult, but higher rate)

- BNL NuclPhysSeminar

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****

- So what does -jet help vs. -h?
- Jet angle easy to measure
- Expect multiple scattering in medium even w/o E loss

## Jet Measurements in HI Collisions

- In cone of
- there is
-  *dE**T**/d*** in central Au+Au at RHIC is 600 GeV*
- →E= 300 GeV in cone with R=1
-            75 GeV in cone with R=0.5
- Compare to maximum jet energy:  100 GeV

**Jet Measurements in HI Collisions**

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*** Phys. Rev. C 71, 034908 (2005) **

**Jet measurement in HI at RHIC difficult!**

## Jet Measurements at LHC

-  *dE**T**/d*** in central Pb+Pb at LHC ≈ 1200 GeV*
- →E=  600 GeV in cone with R=1
-            150 GeV in cone with R=0.5
- Compare to: - maximum jet energy:  2750 GeV
-                      - typical jet energy:  100-200 GeV

**Jet Measurements at LHC**

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***Logarithmic extrapolation of √s dependence**
**of (*****dE******T******/d***********N****part****/2) from SIS to LHC**

**S. Milov, Jour.Phy.: Conf.Ser. 5 (2005) 17**

**Jet measurement in HI at LHC still challenging**
**eliminate b fluctuations (large acceptance)**

## RHIC to LHC

- √s:  0.2 TeV → 5.5 TeV (x 27.5)
- Relative rates (e.g. /0) ~ scale with xT
- e.g.  fraction = 10%:  3 GeV at RHIC
-                                       90 GeV at LHC
- hard increases by orders of magnitude
- Accessible pT range ~ 20-200 GeV (x10)
- New probes available
- Jets, direct -jet correlations

- BNL NuclPhysSeminar

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- Detailed study of hard scattering

- G. Roland, Hard Probes 2008

- Experimental limit at RHIC

**Pb+Pb design lumi:**
**One Jet with 150 GeV/s**

## Caveat

- “If energy loss is low-pT phenomenon, LHC will have nothing to measure.” (Mike Tannenbaum)

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**Direct ******** R****AA**** with measured **
**p+p reference data**

****** ****    ********0**

****

## Experiments at the LHC

**Experiments at the LHC**

- ATLAS

- CMS

- ALICE

- Stefan Bathe

## The ALICE Detector:  dedicated to HI

**The ALICE Detector:  dedicated to HI**

**Capabilities**
- Hadrons: 2 * 1.8 units  (dE/dx + ToF)
- Leptons (dE/dx, TOF, transition radiation)
- Photons (high resolution EM calorimetry, conversions)
- Tracking and PID pT range:  100 MeV/c -100 GeV/c

## The ATLAS Detector

**The ATLAS Detector**

- Inner tracking
- EM and Hadronic calorimeters
- External muon spectrometers
- Full azimuthal acceptance in all detectors
- Large pseudorapidity coverage

- 2

-  = 10 for Calorimetry

-  = 5 for tracking

**Capabilities**
- High-precision tracking < 2.5
- Muon identification < 2.5
- Highly segmented calorimetry < 5
- Forward coverage
- Large bandwidth: DAQ + Trigger

- Stefan Bathe

## The CMS Detector

- Large (mid-rapidity) acceptance (tracker and calorimetry)‏
- DAQ+HLT will inspect every single Pb+Pb event
- Large statistics for rare probes

- 2

-  = 5 for Si tracker

-  = 10 for Calorimetry

**Capabilities**
- High-precision tracking over < 2.5
- Muon identification over < 2.5
- High resolution calorimetry over < 5
- Forward coverage
- Large bandwidth: DAQ + Trigger

- J.Phys.G34:2307-2455,2007

- Very well suited for HI environment

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## Measuring -Jet Correlations

- q,g

**Hadrons**

**Jet**

- q,g

- q,g

**Photon**

**3) Jet axis ****provides parton direction**

**2) Photon energy ****tags parton energy E****T**

**4) Charged hadron tracks ****used to calculate **
**z = p****T(track)****/E****T******

**1) Multiplicity, <p****T****> and flow measurements ****characterize density, path length**

**Measuring ********-Jet Correlations**

- Collisional/non-perturbative
- vs. radiative E loss

- Lokhtin et al.

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**Radiation outside jet cone**

**Radiation inside jet cone**

## Photon Energy Tag

**Photon Energy Tag**

** **Use photon to tag parton energy
-  Goal: Best correlation of photon and parton energy
-  Ideal: Use leading order photons
-  In practice: **Isolated** photons to select events with
- good correlation of photon and parton energy

- Background photons usually associated with jets
- Remove using isolation cuts in data analysis **and **calculations

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## RHIC Calorimeters

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## STAR Calorimeter

- Similar to LHC calorimeters
- large coverage
- high resolution in Shower Max for neutral hadron rejection
- lower resolution in Towers for 0 measurement

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## PHENIX Calorimeter

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- Two calorimeters:  PbSc (sampling); PbGl (homogenous)
- Spatial resolution:  x = 0.01x0.01
- Energy resolution:  (E)/E = [1-2]%  [6-8]%/√E
- Virtually complete background rejection for pT< 25 GeV through reconstruction of neutral hadrons

## PHENIX Isolation Cut in p+p

- Why does isolation cut only suppresses background by factor 5 at 10 GeV?
- Small acceptance?---Small effect.
- More likely due to underlying event

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- Signal fraction

- pT

- Uncorrected for cluster merging

- bckgrd fraction

- w/ iso

- w/o iso

## The Higgs:  driving the design of LHC calorimetry

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- Higgs capabilities depend on
- Rejection (ATLAS:  lateral, longitudinal segmentation)
- E Resolution (CMS:  crystal)

## Coverage and Other Differences

- ALICE:  1000 scientists, 1000 HI
- ATLAS:  2500 scientists, ~35+ HI (11 inst.), photon experts
- CMS:     2600 scientists, ~100 HI (25 inst.)

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**From 2010**

## Photon Detectors at LHC

**Photon Detectors at LHC**

| Exp. | ATLAS |  | CMS |  | ALICE |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Name | LAr Barrel | LArEndCap | ECAL(EB) | ECAL(EE) | PHOS | EMCal | Barrel |
| Structure
 | Liquid Argon
 |  | PWO+APD
 |  | PWO+APD
 | Pb+APD
 | TPC+ITS+
TRD+TOF |
| Coverage
 | 0<||<1.4
2
 | 1.4<||<3.2
2
 | 0<||<1.5
2
 | 1.5<||<3.
2
 | 0<||<0.12
0.6
 | 0<||<0.7
0.6 | (0<||<0.9
2
7% X/X0 |
| Granularity
x
 | 0.003x0.100
0.025x0.025
0.025x0.050 | 0.003x0.100
0.025x0.025
0.025x0.050 | 0.017x0.017
 | 0.017x0.017
to
0.05x0.05 | 0.004x0.004

 | 0.014x0.014
 | 3.10-4x 2.10-4
(resolution)
 |
| Resolution
 | 10%/√E 
0.5% | 10%/√E 
0.5% | 2.7%/√E  0.55% | 5.7%/√E  0.55% | 3.3%/√E  1.1% | 7%/√E 
1.5% | 2% low p T
5% high p T |

- A. Marin, Hard Probes 2008

## ALICE

## Identifying prompt in ALICE (PHOS)

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**Identifying prompt ********in ALICE (PHOS)**

- p+p
*R* = 0.3, *p*T < 2 GeV/*c*
- Efficiency: 69%
- Background rejection: 1/170
- First year (10 pb-1)
- 3000  (*E* > 20 GeV)
- Pb+Pb
*R* = 0.2, *p*Tthresh = 2 GeV/*c*
- Efficiency: 50%
- Background rejection: 1/14
- One month of running
- 2000  (*E* > 20 GeV)

- Y. Mao, Poster QM2008,
- ALICE-INT-2007-021

- G. Conesa et al., NIM A 580(2007) 1446,
- NIM A 585(2008) 28

- Stefan Bathe

- Why S/B so good (100)?

## ALICE: Photon-Tag Jets Fragmentation Function

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**ALICE: Photon-Tag Jets Fragmentation Function**

- pp

- PbPb

- Y. Mao, Poster QM2008, ALICE-INT-2007-021

- Isolated photons and
- background subtracted

- Imbalance distribution

- G. Conesa et al., NIM A 580(2007) 1446,
- NIM A 585(2008) 28

- Pb+Pb

- p+p

- Stefan Bathe

##  ALICE: -tagged FF RAA

- Charged + EM

** ****ALICE: ********-tagged FF R****AA**

- Systematic errors due
- to jet(0)-jet background

**Sensitive to medium modifications at low z **
**if larger than ~5%****.**** **

- G. Conesa et al., NIM A 580(2007) 1446,
- NIM A 585(2008) 28

- Stefan Bathe

## ATLAS

## Lateral Segmentation

**Lateral Segmentation**

- First EMCal layer has high separation in 
- Low occupancy even in central Pb+Pb
- rejection for ET  70 GeV

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**Central Pb+Pb**

**Central Pb+Pb**

**Single particle**

**Single particle**

## Photon ID:  Loose Cuts

**Photon ID:  Loose Cuts**

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- Relative rejection of 2 at 85% efficiency with loose photon ID cuts below ~ 50 GeV

- unquenched

## Photon ID:  Tight Cuts

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- Relative rejection of 1.5-2.2 at ~50% efficiency with tight photon ID cuts

- unquenched

## Isolation Cut

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- Relative rejection of 5 at ~60% efficiency in unquenched Pb+Pb

## Photon Spectrum

- S/B > 1 for ET > 70 GeV in unquenched central Pb+Pb
- Relative rejection factor ~ 15

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## S/B

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- S/B = 1 at ET = 70 GeV for unquenched Pb+Pb
- S/B ~ 4 at ET = 70 GeV for quenched Pb+Pb

## Medium Photons

**Medium Photons**

- Photons are abundantly produced at LHC
- Jet-photon conversion in the plasma dominates 8<pT<14 GeV
- Prompt hard NN scattering dominant for p T>20GeV at LHC
- ATLAS better suited than CMS

- Turbide, Gale, Jeon, and Moore
- PRC (2004) 014906

- Stefan Bathe

## CMS

## ECAL Response in p+p and Pb+Pb

**ECAL Response in p+p and Pb+Pb**

**p+p** (selective readout)

**p+p** after seed threshold (0.5/0.18 GeV)

**Pb+Pb**
- dN/d ~ 2400

**Pb+Pb** after seed threshold (0.5/0.18 GeV)

- ECAL clusters

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## Photon Selection

**Photon Selection**

- Selection variables
- Cluster shape in ECAL
- HCAL/ECAL energy ratios
- Track isolation
- 21 variables total
- Linear discriminant analysis to select optimal cuts
- Large acceptance essential for effective cuts w/o loss of signal

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## Photon identification performance

**Photon identification performance**

- Set working point to **60%** signal efficiency
- Leads to **96.5%**  background rejection
- Training is done on unquenched samples only

- WP

- p+p

- Pb+Pb

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**Notes:**
So here is the obtained photon id performance, given as signal efficiency vs background rejection. Obviously one would like to have 100% signal and 100% background rejection. For pp as you can see we get close to this. For Pb+Pb we set our working point to 60% sig eff, which corresponds to about 96.5% background rejection.

## Photon Selection Performance

- Before cuts:

- After cuts:

**S/B=4.5**

**S/B=0.3**

**Photon Selection Performance**

** **
**Photon isolation/shape cuts improve S/B by factor ~15**
**(at 60% signal acceptance, 97% background rejection)**

- PYQUEN/quenched Pb+Pb

- C. Loizides nucl-ex/0804.3679v1

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## Photon Selection Performance

- Before cuts:

**S/B=0.3**

**Photon Selection Performance**

## Photon Selection Performance

- Before cuts:

**S/B=0.3**

**Photon Selection Performance**

******0**

- Main background:  high-z neutral hadrons

- Why /0 flat (and not rising)?

## Fragmentation function ratio

**Fragmentation function ratio**

- Medium modification of fragmentation functions can be measured
- High significance for 0.2 < ξ < 5 for both, ET0GeV and ET>100GeV

- ET70GeV

- ET0GeV

- Reco quenched Pb+Pb / MC unquenched p+p

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**Notes:**
Still the overall systematic errors are small compared to the effect we are after. Here you see the ratio of the reconstructed fragmentation function in Pb+Pb quenched case to the MC truth unquenched case for gamma et cut of 70 GeV left and 100 GeV right.

In comparison with the horizontal line indicating no modification, it is obvious that for xi between 0.2 and 5 modified fragmentation functions will be measurable.  

## Discrepancy in p+p

- CMS:  factor 200 background rejection
- S/B = 10 after cuts
- ATLAS:  factor 15 rejection
- S/B = 1 after cuts     at (70 GeV)

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## Summary

- LHC gives access to new regime of HI physics
- High rates for hard scattering
- New observables:  jets, direct photon-jet
- One dedicated HI experiment (ALICE)
- Two large-acceptance HEP experiments (ATLAS, CMS) with excellent HI capabilities
- ATLAS and CMS calorimeters ideally suited for jet measurement

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## ATLAS Calorimeter I

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## ATLAS Calorimeter II

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