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![Page 1: Neutral Meson Production at High p T with the PHENIX Experiment at RHIC Henner Büsching FIAS / University of Frankfurt Jet Physics in Heavy Ion Collisions.](https://reader034.fdocuments.in/reader034/viewer/2022051401/56649ec95503460f94bd6a9e/html5/thumbnails/1.jpg)
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
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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
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The Physics
Introduction:A case study
The Analysis
The Challenges
0
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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
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A Good General Case Study: The 0 Analysis in PHENIX
Fast first results
High impact
Long experience
Established analysis scheme
New challenges with every run
• Successful analysis series• Running long enough for
critical review • What can we learn ?
Why?How?What?
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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
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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
New challenges with every run
Established analysis scheme
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0
The Physics
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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
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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
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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
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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
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p+p reference
Run3 Data
Run5 Data
PHENIX preliminary
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Initial state effects
No strong initial-state effects
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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
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Au+Au 200 GeV34 M minimum-bias events
+ 30 M high-pT (LVL2 events) sampled
spectra
nucl-ex/0601037
Run2 Data
(New) PHENIXpaper
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Run4 0 Data
RAA in AuAu at 200 GeV
Photons are not suppressed
and even at high pT suppressedSuppression is flat at high pT
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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 !(?)
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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
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RAA – Reaction systems
Steeper slope at low Npart
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• 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
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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
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Reaction Plane dependence
RAA(pT
)RAA(pT)
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Reaction Plane dependence
Multiplied by inclusive RAA
0
/2
Au+Au – 200 GeV
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The Analysis
0
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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
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• 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
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PbGl Sector
PbSc Super Module
The EMCal Detector
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PbSc towers: 5.52 x 5.52 x 33 cm3
(18 X0)
6 sectors with15552 blocks total
%.E
%.
EE 91
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PbSc tower
66 sampling cells1.5 mm Pb, 4 mm Sc
penetrating wavelength shifting fibers
for light collection
The Lead Scintillator
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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
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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
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• 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
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• 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
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• 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
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• 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
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• Photon PID Cuts
• Asymmetry cut on pairs of photons
• Invariant Mass Distribution
• Mixed Event Background Subtraction
• Acceptance + Efficiency Corrections
Analysis Outline
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• 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
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• 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
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• 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
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• 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
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• 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
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• 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
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• 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
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Invariant Mass [GeV/c2]
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
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Invariant Mass [GeV/c2]
Invariant Mass [GeV/c2]
Invariant Mass [GeV/c2]pT =4-4.5 GeV/c
Example : p+p
Real/Mix
Real normalized Mix
Real -normalized Mix
0
Yield Extraction
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pT =4-4.5 GeV/c
Example : p+p
Real/Mix
Real normalized Mix
Real -normalized Mix
Invariant Mass [GeV/c2]
Invariant Mass [GeV/c2]
Invariant Mass [GeV/c2]
S/B: 0.21 – 2.0
0
Yield Extraction
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pT =3-4 GeV/c
Real/Mix
Real -normalized Mix
Real normalized Mix
Invariant Mass [GeV/c2]
min bias Au+Au
S/B: 0.002 – 1.5
0
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• 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?
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• 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
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• 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
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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
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The Challenges
0
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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!
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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
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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
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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
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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
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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
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BackupPhysics
0
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RAA : Centrality Dependence
Run4 DataAu+Au – 200 GeV
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nucl-ex/0601037
RAA : Centrality Dependence
Run2 Data
Au+Au – 200 GeV
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RAA – Reaction systems
Central Cu+Cu also suppressedConsistent with energy-loss calculation dNg/dy = 370
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RAA – Reaction systems
• Geometrical model with “corona” effect– More jets from surface – Correlated with ellipticity
Au+Au30-40%
Npart
= 114
Cu+Cu0-10%
Npart
= 98.2
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0 RAA 40-50%
Ncoll
= 22.9 4.4 Npart
= 23.1 3.3
PbSc PbGl
Uncertainty in Ncoll
and p+p param. (20%)
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0 RAA 20-40%PbSc PbGl
Ncoll
= 48.4 6.5 Npart
= 41 3.6
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0 RAA10-20%PbSc PbGl
Ncoll
= 93.6 9.4 Npart
= 67.8 3.1
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0 RAA 0-10%PbSc PbGl
Ncoll
= 140.7 14.8 Npart
= 92.2 2.2
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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
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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
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Comparison to theory Vitev, Trento 2005
Energy loss in cold nuclear matter to explain forward rapiditiesPower corrections( high twist shadowing)
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The world data I
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The world data II
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p+p Reference @ 62 GeV
J.Phys.G31:S491-S512,2005
David d’Enterria
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p+p Reference @ 62 GeV
J.Phys.G31:S491-S512,2005David
d’Enterria
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p+p Reference @ 22 GeV
David d’Enterria
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p+p Reference @ 22 GeV
David d’Enterria
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BackupAnalysis
0
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– 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
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• Photons massless– travel at speed of light
• Hadrons have mass– slower than photons
• TOF cut – rejects hadrons– keeps photons
PID: TOF
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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
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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
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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
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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