Neutral Meson Production at High p T with the PHENIX Experiment at RHIC

Neutral Meson Production at High pT with the PHENIX Experiment at RHIC

Henner BüschingFIAS / University of Frankfurt

Jet Physics in Heavy Ion Collisions at the

LHCECT*, Trento

September 1, 2006

The other famous “workshop”…

Council of Trent 1545-1563

=18 Years !

Let’s hope we come to conclusions a lot faster!

response to the theological

challenges of the Protestant Reformation

3

The Physics

Introduction:A case study

The Analysis

The Challenges

0

4

A Good General Case Study: The 0 Analysis in PHENIX

• The observables:– Important part of one of the first PHENIX papers (paper

#3)

– Highest citations of all PHENIX papers:• Phys.Rev.Lett.88:022301,2002 : Spires 335 citations

• Phys.Rev.Lett.91:072301,2003 : Spires 258 citations

– PHENIX is analysing 0 for 6 years now

– 0 analysis one of the PHENIX working horses

– It still needs some time to publish

Fast first results

High impact

Long experience

Established analysis scheme

New challenges with every run

5

A Good General Case Study: The 0 Analysis in PHENIX

Fast first results

High impact

Long experience



• Successful analysis series• Running long enough for

critical review • What can we learn ?

Why?How?What?

6

The 0 Analysis in PHENIX

Fast first results

High impact

• In principle a simple analysis– Self calibrating– Self identifying

• Experience from previous experiments• PbGl detector re-used from WA98• Two independent detectors• Two analysis groups – cross check

• Strong discovery• Predictions available at start• Theory-friendly (easy to calculate)• Identified particle• High pT

7

The 0 Analysis in PHENIX• Calorimeter “veterans” from previous experiments• New expertise developed in PHENIX• Human resource development

– Young people start– Experienced people can move on– No gaps in analysis strategy

• High interest from non pure 0 analyses– Direct photon– Spin– Correlations

• Guarantees fresh ideas and critical perspective

• Different reaction systems / energies• New data sets – changing detectors

Long experience



8

0

The Physics

9

evt 2AB AB T

AB 2AB pp T

1 N d N /dydpR

T d /dydp

Phys. Rev. Lett. 91, 072301 (2003)

/coll NNN

First years : main discoveries

Phys. Rev. Lett. 91, 072303 (2003)

Phys. Rev. Lett. 94, 232301 (2005)

1 2 3

10

Are we done? – No• Improve p+p reference

• New Data:– Better Centrality dependence

– Higher pT reach

– System size dependence– Energy dependence– Particle species dependence

Better Understanding of :

Influence of initial state effectsInfluence of final state effects

11

p+p d+Au Au+Au Cu+Cu

200 GeV

130 GeV

62.4 GeV

22.4 GeV

Reference sQGP ? Comp.

Neutral mesons in PHENIX

0 Run 6

12

can be measured at high pT in PHENIX-pT (, 0) ≥ 1 GeV/c-bulk: GeV/c

Should hadronize in vacuum Neutral meson with 4 x mass of 0

Second largest source for -decay photons -decay e ±

Important for-direct photon-single electron + dielectron

55.0pT

Signal

Background

The Meson

13

p+p reference

Run3 Data

Run5 Data

PHENIX preliminary

14

Initial state effects

No strong initial-state effects

15

Run4 DataCu+Cu 200 GeV56 M min-bias events1.9 M high-pT events

2.2 B sampled

Au+Au 200 GeV Luminosity 241b-1

(sampled)1.5B events

Run5 Data

0 spectra

16

Au+Au 200 GeV34 M minimum-bias events

+ 30 M high-pT (LVL2 events) sampled

spectra

nucl-ex/0601037

Run2 Data

(New) PHENIXpaper

17

Run4 0 Data

RAA in AuAu at 200 GeV

Photons are not suppressed

and even at high pT suppressedSuppression is flat at high pT

18

Similar suppression pattern 0 and 0 ~ 0.4 - 0.5. in all systems and for all centralities

d+Au

0 and

p+p

Au+Au

Universal suppression for light mesonsSuppression at partonic level

before fragmentation !(?)

19

RAA – Reaction systems

Similar suppression at similar NPart

Systematic comparison possible

Au+Au Cu+Cu

Au+Au 30-40 %, NPart = 114.2

Cu+Cu 0-10 %, NPart = 98.2

20


Steeper slope at low Npart

• Close to SPS Energies– p+p data at 21.7 – 23

GeV– Use of

parameterization as reference

• 3 days of RHIC Run5– 6.8M Events after

quality cuts– Centrality via PC1

multiplicity

Cu+Cu @ 22.4 GeV

22

Vitev nucl-th/0404052

RAA – Energy dependence

Now we can study influence of collision energy on scaling behavior

62 GeV 22.4 GeV

Vitev nucl-th/0404052

dNg/dy=650-800Au+Au Cu+Cupp ref:

D. d’Enterria

23

Reaction Plane dependence

RAA(pT

)RAA(pT)

24

Reaction Plane dependence

Multiplied by inclusive RAA

0

/2

Au+Au – 200 GeV

25

The Analysis

0

26

0

• Mass– 135.0 MeV

• Decay Modes– 2 98.8%– e+e- 1.2%

• Mean life – 8.4*10 -17 sec

• cτ = 25 nm• 40 times nuclear radius

– leaves collision zone before it decays

• 1/250,000 BBC resolution– decays at measured vertex position

The Basics

0

27

• 0via 0 • Lead-scintillator

calorimeter

• Lead-glass calorimeter

• Centrality, vertex

– Beam-Beam Counter (BBC) 3.0 < || < 3.9

– Zero-Degree Calorimeter

(pseudorapidity || < 0.35)

PHENIX Central Arm

28

PbGl Sector

PbSc Super Module

The EMCal Detector

29

PbSc towers: 5.52 x 5.52 x 33 cm3

(18 X0)

6 sectors with15552 blocks total

%.E

%.

EE 91

28

PbSc tower

66 sampling cells1.5 mm Pb, 4 mm Sc

penetrating wavelength shifting fibers

for light collection

The Lead Scintillator

30

Lead glass blocks 4 x 4 x 40 cm3 (14.4 X0)

2 sectors with9216 blocks total

%.E

%.

EE 03

55

The Lead Glass Calorimeter

31

Pb + Scintillator

generateshower

generatelight

collectlight

Fiber

• Charged shower particles generate Cherenkov photons in the PbGl

• The Ch. Photons propagate with a wavelength dependent attenuation to the PMT

homogeneous lead-glass

Cherenkov radiator

PMT

• Electrons and Photons: • Bremsstrahlung, pair production• Electromagnetic shower

• Strongly interacting particles: • Hadronic shower, MIP

• Calorimeter measures energy, position, and TOF

PbSc

PbGl

Measuring Photons

32

• 0 -> 2• E2 = p2 + minv

2 (c=1)

• Conservation of energy and momentum– Valid for both 0 and 2

system

– E2(2) – p2(2) = minv2 (0)

– E(2) = E(1) + E(2)

– (2) = (1) + (2)

• Take any two photons in event

• Calculate minv

• If minv = 135 MeV -> 0

p

p

p

)( p

)( 2p

)( 0p

)( 0inv m

)2( E

Principle of Measurements

33

• Alternative formula: minv2 = 2E1E2(1-cosψ)

– Not good to calculate minv • cos needs more CPU than vector addition!

– But illustrates• the higher the 0 pT the smaller the opening angle

– natural limit of 0 measurement

Limits of Measurements

34

• High pT

– From 10 (15) GeV on• clusters start to

merge

– Beyond 25 (30) GeV • photons overlap

completely• look like single photon

• Low pT:– nonlinearity of EMCal response– corresponding uncertainty– 0 spectra so far only starting from 1 GeV

– Going to lower pT might be possible but is challenging

Limits of Measurements II

35

• There are less eta mesons compared to 0

– ratio ~ 0.5

• The branching ratio into photons is smaller (40%)

• Mass is higher compared to pions:– For given pT, opening angle is bigger– At low pT - harder to hit the detectors– At high pT – easier to measure as merging starts later

Measuring eta

36

• Photon PID Cuts

• Asymmetry cut on pairs of photons

• Invariant Mass Distribution

• Mixed Event Background Subtraction

• Acceptance + Efficiency Corrections

Analysis Outline

37

• 0 peak is the best Particle Identification criterion one can ask for

• Photon PID not mandatory

• So why bother with additional PID cuts?

• hadrons contribute to background

• Getting rid of hadrons – increases

signal/background ratio– decreases statistical error

of 0 yield

• Comparison of different PID’s to estimate systematic uncertainty

PID Cuts on Photons

38

• At low energy calorimeter response is nonlinear

• Shower maximum close to detector surface

• in case of PbGl– Cherenkov photons have to

travel all the way through lead glass

– absorption

• Nonlinearity not known well enough (simulations)

• large uncertainty on energy

PID: Energy Cut

We can use PID cuts to eliminate detector disadvantages – and optimize

39

• Hadron shower: λint > X0

• Spread of hadronic shower larger– longitudinally – laterally

• Lateral shower spread used to reject hadronic showers

PID: Shower Shape

Example for PID cuts to use detector design

40

• Asymmetry cut α– Energy asymmetry of photon

pair– pairs from 0 decays

• Flat asymmetry distribution • random orientation of decay

axis relative to 0 momentum

– Random pairs • asymmetric energies favored

– Reason• steeply falling photon energy spectrum • many low-energy photons available to

form random pairs

E2E1

E2E1α

Asymmetry cut

Asymmetry cut increases signal/background

41

• Identified by minv of decay-photon pair• Which two photons in event originate

from 0 ?

• All possible combinations of photon pairs

• Background of pairs that randomly have right invariant mass

0 Reconstruction

0

42

• Random background estimated by mixed events technique

• Pair combinations of photons from different events

• By construction, photons cannot originate from same 0

• Random minv distribution

• In p+p: Fit of random background good enough

0

Mixed Events

43

• minv distribution for real and mixed events• Mixed event distribution has to be normalized to real event

distribution• In principle one knows the normalization• In practice much too complicated to know

– # pairs real: n(n-1)/2– # pairs mixed: n*m– n, m vary event by event

• Would need to keep track of all n, m– Correlated pairs in peak in real events become uncorrelated pairs in

mixed events• Would require iterative procedure to calculate normalization (and peak

content)– Other correlations in real events whose size is not known (η, other

resonances, back-to-back correlations, non-vertex 0’s, HBT)

0

Mixed Events II

Normalization simply from real/mix ratio outside peak region, but close to it

44

Invariant Mass [GeV/c2]


Invariant Mass [GeV/c2]pT =1-1.5 GeV/c

Example : 0 d+Au

Real/Mix

Real normalized Mix

Real -normalized Mix

First order polynomial

Constant for syst. error

0

Yield Extraction

45



Invariant Mass [GeV/c2]pT =4-4.5 GeV/c

Example : p+p

Real/Mix

Real normalized Mix


0

Yield Extraction

46

pT =4-4.5 GeV/c

Example : p+p

Real/Mix

Real normalized Mix





S/B: 0.21 – 2.0

0

Yield Extraction

47

pT =3-4 GeV/c

Real/Mix


Real normalized Mix


min bias Au+Au

S/B: 0.002 – 1.5

0

48

• It’s not at the mass !

• Natural with of 0 peak is 7.7 eV -> negligible• Measured width comes from limited energy resolution of

detector• Random up and down fluctuations of energy along with

steeply falling spectrum increase the average observed energy in a given bin

• This shifts the 0 peak up

Where is the peak?

49

• A fast Monte Carlo– Generate photons and 0’s randomly according to assumed pT and y distribution

– Smear energy and position of photons – Shower overlap:

• Decide randomly for each photon whether overlap takes place

– model PID cut losses by energy dependent photon survival functions

• Tuning– tune smearing and overlap probability by comparing 0

peak position and width from Fast MC to real data

– estimate PID cut losses by comparing raw 0 spectra for different PID’s

Simulation I

Fast MC: fast, good description if occupancy lowlimitations in central Au+Au

50

• A full simulation– detector response of single photons or ’s simulated

with GEANT– single particle response embedded into measured

events before reconstruction• Assumption: no significant change of event properties

– reconstruct event– compare pT of embedded particle after

reconstruction to input pT

• Tuning– Adjust energy and position resolution to match 0

peak position and width in real data

Simulation II

Full simulation difficult to maintainLearn from full simulation,

use fast implementation as soon as possible

51

0 p+p2 GeV 3 GeV 10 GeV 15 GeV8.5 GeV

totalE- scale

7 %5 %

12 %9.4 %

17 %11 %

Au+Au

totalyield extr.

14 %10 %

38 %37 %

0 Au+Au

totalefficienc

y

19 %11 %

E- scale 11 %

19 %11 %11 %

Systematic errors

52

The Challenges

0

53

CThe Big C…

entrality - Bias

ross section

hoice of events - Trigger

alibration

heck of data quality C

C

CC

C

It’s an established procedure…The same procedure as every year?(Un-)Fortunately not!

54

CChoice of events

• PHENIX uses various high pT triggers

– In/without coincidence with main trigger (BBC)

• Advantage for fast analyses: – Filtering of data– Splitting of data in smaller nDSTs, pDSTS…– Easier to handle– No overhead of “useless” events

• Important: Synchronise smaller DST units

Create small subsets of the DST with sharp event selection for distinct analyses

55

CTriggered events:

Mixing • Always one high-pT photon in each event• Event mixing: high-pt photon in each of the two

events• Pair of two high-energy photons—a very unlikely

case for real events• minv distribution is biased, does not match the

random background in real events

• Solution: Triggered events are mixed with MB events

• In the age of filtered triggered events– Minimum bias events are often not readily available– Pseudo minimum bias events– In one of the triggered events the photon that triggered

the ERT is not used for mixing

56

CCalibration

• Often the calibration in the DST is not good enough

• 0 peak position can be used to get both– relative (tower-by-tower)– absolute calibration

• Relative calibration– Fill minv disribution for each tower

– Balance tower gain factors so that all peaks are at the same position

Make sure in the DST (+derivatives) you have allthe information to correct the calibration

57

CCalibration II

– Predict expected peak position (not at 135 MeV!) with simulation

• Tune energy resolution in simulation to match peak width vs pT in data

• Match peak position vs. E in data to prediction from tuned simulation

• To get 0 peak position vs E:– replace 0 pT by average photon energy and apply tight asymmetry

cut

58

CCalibration III

Make sure to organize afterburner on the collaboration level

• Information will change• Changes have to be communicated• It’s impossible to wait for the “final”

correction• Hide it from the user• Centralize it for the experts• Make it impossible to get the wrong data

59

60

BackupPhysics

0

61

RAA : Centrality Dependence

Run4 DataAu+Au – 200 GeV

62

nucl-ex/0601037

RAA : Centrality Dependence

Run2 Data

Au+Au – 200 GeV

63


Central Cu+Cu also suppressedConsistent with energy-loss calculation dNg/dy = 370

64


• Geometrical model with “corona” effect– More jets from surface – Correlated with ellipticity

Au+Au30-40%

Npart

= 114

Cu+Cu0-10%

Npart

= 98.2

0 RAA 40-50%

Ncoll

= 22.9 4.4 Npart

= 23.1 3.3

PbSc PbGl

Uncertainty in Ncoll

and p+p param. (20%)

0 RAA 20-40%PbSc PbGl

Ncoll

= 48.4 6.5 Npart

= 41 3.6

0 RAA10-20%PbSc PbGl

Ncoll

= 93.6 9.4 Npart

= 67.8 3.1

0 RAA 0-10%PbSc PbGl

Ncoll

= 140.7 14.8 Npart

= 92.2 2.2

69

The /0 Ratio

constant fitpT > 2 GeV/c

Au+Au 0-20 %0.40 ± 0.04

d+Au min bias0.47 ± 0.03

p+p0.48 ± 0.03

0s in d+Au

<kT>= 0.52 GeV2

Accardi, Gyulassy. Partonic Glauber-Eikonal approach: sequential multiple partonic collisions. Phys. Lett. B 586 (2004) 244.

PHENIX preliminary

71

Comparison to theory Vitev, Trento 2005

Energy loss in cold nuclear matter to explain forward rapiditiesPower corrections( high twist shadowing)

The world data I

The world data II

p+p Reference @ 62 GeV

J.Phys.G31:S491-S512,2005

David d’Enterria


J.Phys.G31:S491-S512,2005David

d’Enterria


David d’Enterria


David d’Enterria

78

BackupAnalysis

0

79

– Dispersion cut • rejects ~ 50% of hadronic showers • Keep 98% of photons (PbGl)

– PbSc• shower shape is compared to typical shape of electromagnetic

shower• Similarity to electromagnetic shower calculated

PID: Shower Shape II

Dcorr

Photons

Pions

80

• Photons massless– travel at speed of light

• Hadrons have mass– slower than photons

• TOF cut – rejects hadrons– keeps photons

PID: TOF

81

More Pitfalls

• Make sure you also exclude η peak• Make sure you apply cut on minimum

distance between two photons– Two photons cannot come closer than spatial

resolution of detector allows in real events– In mixed events any distance is possible– If you don’t apply minimum distance cut in mixed

events the minv distribution won’t match the one in real events

82

Corrections

• Geometrical Acceptance– Not all 0’s hit EMCal

• Limited η and φ coverage• Towers not used in analysis• Even if one of the decay photon hits, the other might

still miss

• Opening angle of photon pair is pT dependent

• Acceptance is pT dependent

83

Calculate Acceptance• MC simulation:

generate 0’s and photons

• || < 0.45 (account for vertex variation)

• Gaussian Rapidity distribution

• calculate kinematics of 0 decay photons

• take inactive detector areas into account

84

Efficiency

• Corrects for detector effects like– limited resolution– shower overlaps– photon losses due to PID cuts

• Two approaches: Fast MC, full simulation

truepN

pNp

|d/d

|d/d)(

T

measuredTT

• Definition:

p+p, Fast MC

Documents

Neutral Meson Production at High p T with the PHENIX Experiment at RHIC