gustavo conesa balbastre

4 Convegno sulla Fisica di ALICE

1/20

Review on direct photon measurement

and jet correlations with EMCal

Gustavo Conesa Balbastre


2/20

Outline

Introduction: Why direct photon measurement and correlation with jets.

Photon / 0 discrimination in EMCal: Shower shape analysis Summary of work done by Cynthia Hadjidakis

Direct prompt photon identification in EMCal: Isolation cut Summary of work done by Gustavo Conesa and Amaya

Casanova

Tagging jets with photons: Gamma in PHOS: Summary of work made by Gustavo Conesa Gamma in EMCal: Summary of work made by Guénolé

Bourdaud


3/20

Partons traversing the medium of high colored density are modified. Energy loss through gluon

emission High pT jets are our

tomographical probe of QGP. Constant energy loss E ~ 20

GeV. Jet reconstruction in AA is not

feasible for energies smaller than 50 GeV and in general energy measurement is not very precise.

Direct photon tagged-jets are an useful observable.: EEjet Back-to-back in azimuth

The measurement: Particle species spectra

• S ( pTh)

• RAA = SNN/(Norm x Spp) Fragmentation function

• FF ( z = pTh/Ejet )

• RFF = FFNN/(Norm x FFpp)

Jet

prompt

Nucleus A

Nucleus B

parton parton

QGP

} Medium coefficient transport

Introduction: Why jets and photonsJet-quenching


4/20

Introduction: Direct photon sources

Thermal radiation

PbPb

QGP

Direct EM probes convey unperturbed information and their production probe the medium Tag medium-modified jets:

Prompt photons from 2->2 hard process (E > 10 GeV)

Medium modified production: Fragmentation photons (E < Ejet)

Medium produced photon: Bremsstrahlung and jet conversion (E < Ejet) Thermal photons (E < 10 GeV)

γFragmentation

NLOHigh z isolated photonsHigh z isolated photons

γg g

q q

q

g

Jet

Prompt

LOIsolated photonsIsolated photons

γ γ

gg

q

q

q

q

Bremsstrahlung/jet coversion induced by medium


5/20

Photon sources

Pre-equilibrium: Prompt photons

Equilibrium:Thermal-Bremsstrahlung-jet conversion photons

Freeze-outDecay photons

Photons are produced during all stages of the collision.

Challenge: Disentangle the different sources. Neutral mesons decay.

But decay photons provide a first choice probe of medium effects

Identify real photons (EM calorimetry, trigger) and e+e- from virtual and converted photons (tracking and PID, trigger)


6/20

RHIC : PHENIX RAA

Hadron suppression

of factor 5 at high pT. Leading jet particle

suppression Run 2: No direct

suppression (PRL 94, 232301 (2005)).

Run 4 (QM06): High pT direct suppression Isospin (PDF)

effect Fragmentation

photon suppression? Something else?


7/20

RHIC : PHENIX RAA

PQM model

RAA is not a very good discriminator to calculate transport coefficient!Need to study what is really quenched, the parton-jet

~13

J. Nagle HP2008


8/20

Jet fragmentation function with quenching

L. Cunqueiro HP2008

Suppresion of leading particles at low (as observed with RAA)

Increase of soft particles at high


9/20

Introduction: How many direct photons?

10k/year

Large sample of direct LO -jet for pT < 30 GeV/c in PHOS and pT < 50 GeV/c in EMCal …

… but0 = 0,01-0,1 for pT > 10 GeV/cWe need a good 0 PID


10/20

-0 discrimination

Three regions of analysis

well separated clusters invariant mass analysis

< 15 GeV/c in EMCal< 30 GeV/c in PHOS

merged clustersnot spherical shower shape analysis

5 - 40 GeV/c in EMCal 30 - 100 GeV/c in PHOS

increasing pT

Opening angle << 1 cellall 0’s at this energy are in jets isolation cut

> 40 GeV/c only method in EMCal

R

PHOS/EMCal

TPC

candidate

IP

Isolated if:•no particle in cone with pT > pT thres

• or pT sum in cone, pT < pT

thres


11/20

/0 discrimination in EMCal Shower Shape Analysis

Nd

irec

t

/ N

from

pGeV/c pGeV/c

p+p 14 TeV

central Pb+Pb 5.5 TeVfactor 5 suppression

→ 0 suppression factor = 5-10 for pT =[10-30] GeV/c in p+p

→ 0 suppression factor > 5 for pT =[15-30] GeV/c in Pb+Pb

→ Ndirect / Nfrom ~ 1 in Pb+Pb for pT =[20-30] GeV/c

See Cynthia’s H. talk about EMCal PID during PWG4 meeting in 11 December 2007 for more details

Photons have0 < 0.25


12/20

Study presented for the CD2 Photons are correctly identified with a 90%

efficiency in pp collisions and more than 60% in PbPb collisions in the pT range from10 to 100 GeV/c

0 are misidentified as photons and with an efficiency that varies from 20 to 10 % going from 10 to 20 GeV and increases to 40 % at 40 GeV in pp collisions, similar in PbPb collisions

The PID helps to improve the ratio direct 0 in the region from 10 to 40 GeV, but it is still too small.

An additional condition to separate direct photons from the rest is needed: Isolation Cut

/0 discrimination in EMCal Shower Shape Analysis, Summary and conclusions


13/20

Direct photon identification in EMCal:

Isolation Cut : Prompt photon / jet clusters Ratio isolated clusters in -jet / isolated clusters in jet-jet

Clusters selected with PID 20< 0.25

pp @ pp @ √14 TeV√14 TeV PbPb @ PbPb @ √5.5 TeV√5.5 TeV PbPb @ PbPb @ √5.5 TeV, qhat = 50√5.5 TeV, qhat = 50

Prompt photons signal larger than jet-jet Prompt photons signal larger than jet-jet clusters background for pclusters background for pTT larger than around larger than around

15 GeV/c for pp and quenched PbPb events15 GeV/c for pp and quenched PbPb events

Talk in Prague Physics week


14/20

pp @ 14 TeVpp @ 14 TeV

Direct photon identification in EMCal:Isolated Spectra

PbPb @ 5.5 TeV, qhat=0PbPb @ 5.5 TeV, qhat=0PbPb @ 5.5 TeV, qhat=50PbPb @ 5.5 TeV, qhat=50

With PIDWith PID

With PIDWith PID

With PIDWith PID

IC Parameters: R=0.4, pTth = 0.5 (pp), 2 (PbPb) GeV/c

Isolation seems to work in pp and PbPb quenched events for pT > 15-20 GeV/c.I would like to have larger amount of jet-jet simulations to quantify better the effect of other particles background at high pT

Talk in Prague Physics week


15/20

Direct photon identification in EMCal: Direct & Fragmentation photons Isolation efficiency.

Prompt and ISR are isolated, FSR at most 50% are not isolated, high z. Are isolated fragmentation photons similar to prompt photons?

Prompt photons and isolated fragmentation photons have a correlation with particles in the away side, not in the near side.

But correlation/fragmentation function of both isolated fragmentation and prompt photons seems not to follow exactly the same trend due to the fact that isolated fragmentation photons do not carry all the parton energy (more in back-up).

Pythia SimulationsNo detectors, no reconstruction

Talk in Prague, and April PWG4 meeting


16/20

Direct photon identification in EMCal :

Summary & conclusions

jet-jet and -jet events, pp and Pb-Pb collisions, quenched and not quenched, have been generated and fully reconstructed.

A simple PID and Isolation Cut have been applied and the measurements seem feasible for prompt energies larger than 10-20 GeV/c in pp and Pb-Pb quenched collisions.

Need a large production of jet-jet events to improve background estimation after isolation and use a more sophisticated PID.

PYTHIA predicts that fragmentation photons in jet-jet events are produced in a same amount as prompt photons. Most of them are isolated, carry a large value of the parton energy. Can we use them safely as normal prompt photons? Isolation studies with hadrons show similar results.


17/20

Tagging jet with photon

Search identified prompt photon (PHOS or EMCal) with largest pT (E > 20 GeV).

Strategy (event by event):

min max

LeadingJet core

Search leading particle or jet core :

-leading180ºR Reconstruct the jet in TPC and

EMCal (if available): With Standard jet algorithms, only high pT, not enough statistics.

Construct jet with particles around the leading or jet core inside a cone of size R

IP

EMCal

TPC

EMCal/PHOS

MonteCarlo studies: Pythia pp events to generate gamma and jet signal Hijing to generate PbPb background


18/20

-tagged Fragmentation FunctionF and RFF

photon in PHOS, jets in TPC+EMCal

Systematic errors due to jet(0)-jet background

Sensitive to medium modifications at low z if larger than ~5% in both configurations.

ALICE-INT-2005-014

G. Conesa et al., NIM A 585(2008) 28

No quenching in simulation

PbPb fragmentation functionUE background removed statistically

If signal is quenchedIf signal is quenched If signal is quenchedIf signal is quenched


19/20

Realistic spectrum simulated: E-jet > 30 GeV Jet core is the jet seed

1/10 of a year statistic. range usable is 0.5 < <

3.2

Tagging jets with photons in EMCal Final Fragmentation function and Nuclear Modification Factor

No quenching

Bkg not substracted

Bkg substracted

PbPb/pp

PbPb/pp With quenching

Deviation from 1 used to calculate systematical errors

G. BourdaudSee July PWG4 meeting


20/20

Conclusion of gamma tagging jets with gamma in EMCal

The modification of the fragmentation function is usable with 30 GeV for 0.5 < < 3.2.

HI Background is the main source of error. More studies of bkg subtraction are needed.

bkg area, min pT cut,

testing other kind of algorithm... Jet-Jet background still not studied.


21/20

Back-up


22/20

PHOS identified spectrum pp and PbPb annual statistics

Yaxian M. poster QM2008

IC: R =0.3, (pT)=2 GeV/c IC: R =0.2, pT>2 GeV/c

At least one count up to ~100 GeV, counting rates for correlations good up to 30 GeV.

ALICE-INT-2005-014G. Conesa et al., NIM A 580 (2007) 1446

2 PHOS modules


23/20pGeV/cpGeV/c

-0 discrimination in EMCal:

Shower Shape Analysis

identified as identified as

Uniform and 0 distributions, weighted to match INCNLO predictions

identified : 02 < 0.25

identified 0: 02 > 0.25

PID

eff

icie

ncy

as

as

See Cynthia’s H. talk about EMCal PID during PWG4 meeting in 11 December 2007 for more details

p+p

PID

eff

icie

ncy

Pb+Pb

identified as

identified as

as

as


24/20

PHOS identified spectrum pp and PbPb annual statistics

Yaxian M. poster QM2008

IC: R =0.3, (pT)=2 GeV/c IC: R =0.2, pT>2 GeV/c

Statistics limits to ~100 GeV

ALICE-INT-2005-014G. Conesa et al., NIM A 580 (2007) 1446


25/20

Direct photon identification in EMCal:Event generation

pp PYTHIA collisions : 3 simulation cases:

1) pp @ √14 TeV2) pp @ √5.5 TeV, merged with HIJING, no quenching in PYTHIA.3) pp @ √5.5 TeV, merged with HIJING, quenching: qhat = 50.

1 +jet in final state (MSEL=10) – jet Prompt is the signal under study.

2) 2 jets in final state (MSEL=1) jet – jet These events constitute the background: decay , fragmentation and

hadrons.1) Jet-Jet Event generation with jets containing at least one 0 with pT>5

GeV/c in the acceptance of EMCAL.: production of fragmentation photons suppressed.1) Also did a pp simulation without trigger in the same bins, similar

number of events, to correct for this bias effect. ~10 k events in different pT hard bins.

1) Pb-Pb collisions @ √s = 5.5A TeV: pp simulations 2) and 3) merged with HIJING central events (b < 3fm) @ √s = 5.5A TeV.

2) Simple PID with shower shape used to discriminate photons from other particles: if 0

2 < 0.25 cluster is a photon (Cynthia H., PWG4 meeting, 12/07)

3) Full ALICE simulation with AliRoot 4.07 Release.4) Fidutial cut applied to all events.

See my talk in Prague ALICE meeting.


26/20

Fidutial cuts

Structures in front of EMCAL increase the amount of conversions, more than the 50% expected.Apply fidutial cuts to get rid of extra particles in analysis of candidates to prompt photons (but extra clusters can be inside the isolation cone).About 25% of the calorimeter acceptance after the cuts is taken out.With fidutial cutsWith fidutial cuts With fidutial cutsWith fidutial cuts

With PIDWith PID With PIDWith PID


27/20

Direct photon identification in EMCal: Generated and reconstructed+PID spectra: -jet : pp @ √14 TeV

50 % of convert in the material before EMCal (5-10% in ITS-TPC). Cluster reconstruction efficiency is almost 95% but with shower shape selection the efficiency descends to 60-80% due to the non photon shape of some converted clusters.

PYTHIAPYTHIA


28/20

Other approach: photon conversionsStudy performed by Ana Marin (GSI)

Loss of efficiency at high pT under investigation

Needs to be improved !

Identify photons converting in the beampipe, ITS and TPC

Clean photon identificationProvide directional information Non vertex background (important source of systematic errors in measurement of direct photons) can be rejected.

Independent measurement of the same quantities, with different systematics compared to PHOS/EMCAL. Increase level of confidence in the results Counting annual statistics for pT > 20 GeV/c (very very rough stimations)

pp @

√s=14 TeV

PbPb @ √s=5.5A TeV

EMCal 20000 20000

PHOS 3000 3000

Tracking 4000 4000


29/20

• pT threshold candidate isolated if:

•no particle in cone with pT > pT thres

•pT sum in cone, pT < pTthres

• p T min of all particles (charged and neutral) in cone is at least 0.5 GeV/c

• Consider in cone particles :•Charged in TPC acceptance: ||<0.7, 0º<<360º•Neutral in

•EMCal acceptance: ||<0.7, 80º<<190º•PHOS acceptance: ||<0.12, 220º<<320º

• Prompt are likely to be produced isolated.

• Two parameters define isolation:• Cone size

22R +φ=

Isolation cut method

R

PHOS/EMCal

TPC

candidate

IP

ALICE-INT-2005-014 - G. Conesa et al, NIM A 580 (2007) 1446

Reminder


30/20

Pb-Pb collisions, pT, part > 0.5 GeV/c

Tagging jets with photons in PHOSReconstructed jet selection 40 GeV jets

TPC alone

TPC+EMCAL

p-p collisions, pT, part > 0.5 GeV/c

ALICE-INT-2005-014



31/20

TPC alone

TPC+EMCAL

p-p collisions, pT, part > 0.5 GeV/c

TPC alone

TPC+EMCAL

Pb-Pb collisions, pT, part > 2 GeV/c

ALICE-INT-2005-014


Tagging jets with photons in PHOSReconstructed jet selection 40 GeV jets


32/20

E > 20 GeV/c; TPC+EMCal detect jet particles, PHOS

Pb-Pb collisions

Background

Signal

HIC background

Any neutral signal in PHOS

z = pT, jet particle /E

Prompt identified in PHOS

If signal is quenchedIf signal is quenched

ALICE-INT-2005-014


HI Background subtracted statistically, accumulate a fake fragmentation function with particle in cone at opposite and same as jet

Tagging jets with photons in PHOSFragmentation function


33/20


Prompt & Fragmentation photons: pT distribution

PYTHIA pp collisions √s=14 TeV

|| < 1

Pythia produces non hadronic decay photons: Prompt photons: Compton and

annihilation processes Jet-Jet events:

Initial state photons (ISR), radiated by hard parton before scattering. Few

Final state photons (FSR), scattered partons radiate / fragment into bremsstrahlung / fragmentation photons

Pure Pythia Generation, millions of events in several pT hard bins.

Spectrum of all photons in || < 1 in all plots.

Isolation done on the pure PYTHIA particles, R=0.4, pT

th = 1 GeV/c. Particles with status code 1


34/20


|| < 0.5

NLO calculation thanks to Lamia B.


Prompt & Fragmentation photons: pT

distribution


35/20

Correlation Function:

Plots for quark jets (similar for gluon jets)As expected, without isolation fragmentation have a correlation in the near and away sides, direct only in the away side. Isolated fragmentation only correlate in the away sideTo understand: why away side descends a bit in isolated FSR photons.

All charged hadrons with pT > 2 GeV/c



36/20

Fragmentation Function: zT

All charged hadrons with pT > 0.1 GeV/c inside cone of size R=1 around jet axis

Difference in FF for fragmentation and prompt due to z<1 for fragmentation photons?


Gluon Jet Quark JetpT FSR / pT jet > 0.5


37/20

100 GeV -jets, no bkg.

Fraction of reconstructed jet energy vs RReconstructed jet energy in HI /HI Background fluctuation

For Ejet = 30 GeV and R = 0.25 ! Ejet / fluctuations bkg = 2.

Tagging jets with photons in EMCalJet energy reconstruction


38/20

RHIC : PHENIX RAA

Hadron suppression

of factor 5 at high pT. Leading jet particle

suppression Run 2: No direct

suppression (PRL 94, 232301 (2005)).

Run 4 (QM06): High pT direct suppression Isospin (PDF)

effect Fragmentation

photon suppression? Something else?


39/20

RHIC : PHENIX RAA

PQM model

RAA is not a very good discriminator to calculate transport coefficient!Need to study what is really quenched, the parton-jet

~13

J. Nagle HP2008


40/20

How can we measure direct photons and jets in ALICE?

E > 10 GeV E/E < 1.5%, x =[0.5,2.5] mm

Tracking System resolution

p/p = 2%, =1.1º

E > 10 GeV E/E < 3 %

x =[3,50] mm

Currently under construction at LNF

gustavo conesa balbastre

Documents

direct photons

emcal4 convegno sulla

trigger4 convegno sulla

direct photon measurement

fragmentation photons

ejetthermal photons

ejet medium produced

photon p0 discrimination