# Presentation: 125594

## Measurement of prompt photon in s=200GeV pp collisions

- Kensuke Okada
- For the PHENIX collaboration

- K.Okada

- 11/23/2004 spin discussion

## Prompt photon production

**Gluon Compton Dominates**
**At ****LO**** no fragmentation function**
**Small contamination from annihilation**

**A****1**

- K.Okada

- 11/23/2004 spin discussion

## Data

**Central Arm** (West)
-      (Rapidity |y|<0.35)**  **
**        **Electromagnetic Calorimeter (EMCal)
-                  Photon detection
-                  High granularity (~10*10mrad2)
**      **Drift chamber (DC)
-                  Charged hadron veto
**Beam forward / backward**
**    **(Rapidity 3.1<|y|<3.9)
-    Beam-beam counter (BBC)
-            Triggering and vertex determination
- BBC and EMCal Trigger for the data taking

- RHIC-PHENIX detector

- RHIC run3 p+p

- 2003 April-May
- s=200GeV Proton-proton collisions
- Luminosity= 266nb-1

- K.Okada

- 11/23/2004 spin discussion

## EMCal Trigger 

- Plateau above 3GeV

- Black points: Data
- Red line: MC expectation

- Rejection power ~ 120

- Geometrical limit

- K.Okada

- 11/23/2004 spin discussion

## Analysis procedure 

- Prompt photon signal = All EMCal clusters – known contributions
- Signal / Noise = 0.2~1 (pT 5~17GeV/c) without 0  tag

- Non photon contributions (hadronic shower)

- Electromagnetic shower shape requirement
- Charged hadron veto with drift chamber tracking

- Photon contributions

- 0 with both photons seen in detector
-     —Eliminate using a 0 tag.
- 0 with only on photon seen in detector
-     —Estimate based on photon tagged as 0
- Photonic decay of , etc.
-     —Estimate to be 235% of the total 0 contributions
-         (=Production * Br(h) )

- K.Okada

- 11/23/2004 spin discussion

## Analysis procedure (+isolation cut)

- Total prompt photons
-                    = photons (gluon Compton scattering) + photons (fragmentation)

- We expect that the isolation cut  around well-identified high-pT photons
- reduces the background from hadronic decays (0, ,  ...)

- In addition, the isolation cut  could separate photon production processes.

- Pass the isolation cut

- Mostly eliminated by the isolation cut

- Yes, it’s not so simple. (cancellation of infrared divergences, underlying parton picture)
- In this analysis, we compare with/without the isolation cut
- _without efficiency correction_.

- Obtained by
- the subtraction method

- K.Okada

- 11/23/2004 spin discussion

## Analysis procedure summary

- After rejecting non photon clusters

- Subtraction method

- Isolation method

- A: ratio of total hadron contribution to pi0 contribution (1.23±0.05)
- R: ratio of photons from missing pi0 to ones from tagged pi0 (from MC)
- N’iso-pi0: photons from pi0 passing the isolation cut if the partner is missed

- Prompt photon signal = all – (hadron decay)
-                                    = all – (all hadron/pi0)*(pi0 contribution)
-                                    = all – (all hadron/pi0)*(pi0 tagged + untagged)

- Estimated using MC

- K.Okada

- 11/23/2004 spin discussion

## Contents

-  Hadronic shower rejection
-  Pi0 reconstruction
-  Fast MC
-  Eta, omega, etc/ pi0 ratio
-  Result

- K.Okada

- 11/23/2004 spin discussion

## ---------------------------------Hadronic shower rejection----------------------------------

-  Electromagnetic shower probability cut
-  DC matching veto

- Keywords

- K.Okada

- 11/23/2004 spin discussion

## EMCal cluster probability cut (PbSc)

- Electron enriched

- Charged pion enriched

- As we expected, charged hadrons (red line) have low “prob” values.

- K.Okada

- 11/23/2004 spin discussion

## Electromagnetic shower shape cut

- EM shower probability

- With Prob>0.02 cut
- 40% of hadronic shower hits are rejected
- 98% of photons are remained

- For charged hadrons
- Integral from x to 1.

- K.Okada

- 11/23/2004 spin discussion

## EM shower cut (photon)

- Photons are selected from pi0 mass window

- Black : all photon
- Red: with prob>0.02 cut

- ratio

- 98% (pT independent)

- K.Okada

- 11/23/2004 spin discussion

## A view of contents

- The component of clusters with TOF,prob cut is roughly,

- PC3 (14%)

- DC (4.5%)

- DC with RICH (1%)

- Conversions before DC
- Pi0 Dalitz

- Conversions after DC

- K.Okada

- 11/23/2004 spin discussion

## Summary (Hadronic shower rejection)

- After tof, prob, and DC veto

- Of total photon clusters,

- Charged hadron contribution is ~0.2% due to DC dead area.
- Neutral hadron is thought to be negligible so far. (from MC).

- K.Okada

- 11/23/2004 spin discussion

## -------------------------------------Pi0 reconstruction-------------------------------------

-  Strategy
-  Combinatorial background correction
-  Partner inefficiency correction

- K.Okada

- 11/23/2004 spin discussion

## Strategy

- 3x3 towers of dead or hot channels are rejected.

- 10 towers edge surrounding the arm were used
- as pi0 veto. ( only for the partner search)
- It corresponds 90% of 5GeV pi0.

- Target photon : as clean as possible
-                         (warn map,tof, prob cut, DC veto)
- Pi0 partner : as many as possible
-                         (only warn map)

- K.Okada

- 11/23/2004 spin discussion

## Mdistribution

- a+bx+cx^2+const*exp(-0.5*((x-mean)/sigma)^2)

- Fit:

- Net :

- cons*sqrt(2pi)*sigma*1/bin_size (bin_size=0.5e-2)

- West arm
- Guard veto (10)
- ert4x4c match
- -2<|tof|<5ns
- Prob>0.02
- DC veto

- Partner
- Ecore>0.15GeV

- 5-5.5

- 7-7.5

- 8.5-9

- 9-9.5

- K.Okada

- 11/23/2004 spin discussion

## Combinatorial background correction

- Suppose there are N photons in a pT bin. There are two kinds of photons.

- N= x + y
-     x: photons from pi0 should be reconstructed.
-     y: single photons

- Pair based method
-             **x=S******** **

- B) photon based method
-     From the number of photon tagged,

- pi0 window region         :  **N****w****= x+yp**
- vicinity region                :  **N****v****= (x+y)p**
- Where **p** is combinatorial probability which
-    can be estimated by the ratio of 2tag/1tag
- Then
-    **x=(N****w****-N****v****)/(1-p) **

- K.Okada

- 11/23/2004 spin discussion

## Photon base method

- Photon in pi0 window
- Photon in pi0 vicinity
- Vicinity width (=2)
- Combinatorial probability

**~10% correction**

- K.Okada

- 11/23/2004 spin discussion

## Partner photon efficiency

- Total 3% from MC (in pi0 cross section analysis)

- Conversion before DC  Completely lost (due to magnetic field)
- Conversion after DC    Partially lost

- K.Okada

- 11/23/2004 spin discussion

## ----------------------------------fast MC-----------------------------------

- Missing pi0 ratio
-       (pi0 photon merge)
- Photon acceptance and smearing
-        (pi0 mass peak, width)

- Input: EMCal warn map, pT slope

- Purpose:

- K.Okada

- 11/23/2004 spin discussion

## Energy Scale, Resolution

- Pi0 peak position

- Pi0 width

- The fast MC is tuned using pi0 peak and width.

- K.Okada

- 11/23/2004 spin discussion

## Study of cluster merging in PbSc

**“****prob” cut discards all merged photons up to 15~20 GeV.**

- No “prob” cut

- With “prob” cut

- AN240

- RIKEN Review No28 (May2000)

- K.Okada

- 11/23/2004 spin discussion

## Pi0 photon merging (example)

**---1--- (0,0)**
-   0.0   0.1   0.3   0.1   0.0
-   0.1   1.6   6.1   1.6   0.1
-   0.3   6.1  **66.5   **6.1   0.3
-   0.1   1.6   6.1   1.6   0.1
-   0.0   0.1   0.3   0.1   0.0
**---2--- (0,1.27273)**
-   0.0   0.0   0.1   0.3   0.2
-   0.0   0.1   1.1   7.0   3.7
-   0.0   0.2   3.3 ** 53.8**  17.5
-   0.0   0.1   1.1   7.0   3.7
-   0.0   0.0   0.1   0.3   0.2
**---1--- (0,-0.5)**
-   0.1   0.3   0.3   0.1   0.0
-   0.7   5.8   5.8   0.7   0.1
-   1.7  **34.4  34.4**   1.7   0.1
-   0.7   5.8   5.8   0.7   0.1
-   0.1   0.3   0.3   0.1   0.0
**---2--- (0,0.772727)**
-   0.0   0.1   0.2   0.3   0.1
-   0.0   0.3   3.2   6.9   1.2
-   0.0   0.7  14.7  **56.2**   3.6
-   0.0   0.3   3.2   6.9   1.2
-   0.0   0.1   0.2   0.3   0.1

**---1--- (-0.5,0)**
-   0.1   0.7   1.7   0.7   0.1
-   0.3   5.8  **34.4 **  5.8   0.3
-   0.3   5.8  **34.4**   5.8   0.3
-   0.1   0.7   1.7   0.7   0.1
-   0.0   0.1   0.1   0.1   0.0
**---2--- (-0.5,1.27273)**
-   0.0   0.1   0.4   1.7   1.1
-   0.0   0.2   2.8  **30.8**  12.8
-   0.0   0.2   2.8  **30.8**  12.8
-   0.0   0.1   0.4   1.7   1.1
-   0.0   0.0   0.1   0.1   0.1
**---1--- (-0.5,-0.5)**
-   0.3   1.4   1.4   0.3   0.0
-   1.4  **21.7  21.7**   1.4   0.1
-   1.4  **21.7  21.7**   1.4   0.1
-   0.3   1.4   1.4   0.3   0.0
-   0.0   0.1   0.1   0.0   0.0
**---2--- (-0.5,0.772727)**
-   0.0   0.2   1.0   1.7   0.5
-   0.0   0.6  11.1 ** 31.6 **  3.1
-   0.0   0.6  11.1  **31.6**   3.1
-   0.0   0.2   1.0   1.7   0.5
-   0.0   0.0   0.1   0.1   0.1

- At 5m away, distance of 2 photons is

- 140cm / E_pi0[GeV]
- = 25.4 towers / E_pi0[GeV]

- For example, 20GeV pi0 makes
- 10GeV photons with 1.27towers distance.
- Numbers show the deposit energy [%]
- of these two photon in 5x5 towers.
- These are merged and will be an
- 20GeV cluster, but it can’t pass the
- EMCal shower shape criteria.

- K.Okada

- 11/23/2004 spin discussion

## pi0 photon missing ratio (with MC)

- Input: pi0 spectra, Energy resolution, Shower size from measurements
- The same MC used in pi0 cross section measurement.
- For the case both photon weren’t lost before EMCal.

- Miss/ tag (=B/A)
- 40% at 5GeV/c
- 20% at 10GeV/c

- Photon from pi0

- A

- B

- This figure shows components of
- photon from pi0.

- (to be discarded by
- the shower shape cut
- And those have almost
- double energy)

- (B) 1photon (the partner missing)

- (A) 2 photons tagged as pi0

- Merged photons

- K.Okada

- 11/23/2004 spin discussion

## ---------------------------------------eta, omega, etc /pi0 ratio---------------------------------------

- World data, PHENIX preliminary data
- Self veto effect in the isolation cut method

- K.Okada

- 11/23/2004 spin discussion

## Eta/pi0 ratio

- Eta/pi0 = 0.45*0.394/0.988=0.18 (assuming no pT dependence)
-                 + omega, eta’,... = 0.23 ± 0.05

- K.Okada

- 11/23/2004 spin discussion

## -----------------------------------result-----------------------------------

- Photon components (S/N)
- Systematic error
- Spectra with Werner’s calculation
- With/without isolation cut

- K.Okada

- 11/23/2004 spin discussion

## Prompt photon signal and photon without pi0 tag

- signal

- Tagged as pi0

- untagged

- (subtraction)

- raw

- (With Isolation Cut)

**(with isolation cut)**

- With the isolation cut,
- The signal is enhanced.

- signal

- Tagged as pi0

-  untagged

**(subtraction)**

**Photon components**

- Signal to untagged ratio

- K.Okada

- 11/23/2004 spin discussion

## Cross section calculation

- Factors
-    1/Luminosity: 1/266nb-1  (=5.450e9 events/20.5mb)
-    1/bbc_bias: 1/0.785
-    1/(acceptance+smearing): 1/0.0982
-    1/(shower shape cut efficiency): 1/0.98
-    1/(Conversion probability): 1/0.97

- K.Okada

- 11/23/2004 spin discussion

## Systematic error sources

***Pi0 photon estimation:***** **
-      combinatorial bg correction, partner inefficiency correction
-      missing pi0 estimation
***Non pi0 hadron contribution:***
-      production ratio to pi0
***Photon acceptance & smearing:***
-      energy scale ambiguity
***Photon conversion effect:***
***     ***material, revival electron

***Luminosity measurement:***
-       BBC cross section measurement by a vernier scan.
***BBC trigger bias:***
-       pi0 analysis

- The followings are considered for now.

- K.Okada

- 11/23/2004 spin discussion

## Systematic error

-  Errors on the backgrounds result in enlarged errors on the signal,
- especially at low-pT region.

-  With the isolation cut, those are less thanks to high S/N ratio.

- For 0 tagging (subtraction) method

- K.Okada

- 11/23/2004 spin discussion

## Systematic error

- For isolation cut method

- K.Okada

- 11/23/2004 spin discussion

## Result with NLO pQCD calculation

- NLO-pQCD calculation
- CTEQ6M PDF.
- Gluon Compton scattering
-      + fragmentation photon
- Set Renormalization scale
-     and factorization scale pT/2,pT,2pT

- The theory calculation shows a good
- agreement with our result.

- (Subtraction)

- Bands represent systematic errors.

- Errors on the backgrounds result in
- enlarged errors on the signal,
- especially at low-pT region.

- K.Okada

- 11/23/2004 spin discussion

## Isolation cut

- It suggests
-  — low rejection power for the fragmentation
-      photon contributions
- or/and
-  — a large contribution from gluon Compton
-       scattering

_**Without isolation cut efficiency **_
_**correction**_

- The result from isolation cut method is
- almost identical with the result from
- subtraction method

- K.Okada

- 11/23/2004 spin discussion

## Summary

-  The prompt photon cross section for 200GeV p+p collisions
-     has been measured at PHENIX.
- In the analysis, it is important to tag 0.
- NLO pQCD calculation agrees well with our measurement.
- The isolation cut improves the signal-to-noise ratio
-     without significantly reducing the signal.
-    (It is important for future spin asymmetry measurement.)

- K.Okada

- 11/23/2004 spin discussion