# Presentation: 690177

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

- Kensuke Okada
- (RIKEN-BNL Research Center)
- For the PHENIX collaboration

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Motivations

**Physics meaning of prompt photon**
**Is a good probe of parton structure in nucleon.**
**One of simple processes at hadron collisions.**
**At RHIC (a polarized proton collider capable of ********s up to 500GeV)**
**The highest energy available for pp********gamma+”X”**
**Prompt photon production can probe the gluon polarization in proton.**

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## 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

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## 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) )

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## 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

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## 0 tag

- Reason to miss the partner

- Out of EMCal Arm
- EMCal bad area
- Less than the minimum E
- Photon conversions
- Photon merging

- Assign edges only for partner search
- (guard veto region)
- Careful definition
- Set Emin as low as possible (at 150MeV)
- Charged veto with DC tracks, not with
- a detector in front of the EMCal
- In our pT region, those are rejected by the
- EM shower shape cut.

- Our efforts to recover them

- From MC,
-  from missed 0 / from tagged 0
-   =  40% @ pT=5GeV/c
-   =  20% @ pT=10GeV/c

- _Now we are ready to obtain prompt photon yield._

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Signal to noise ratio

- 0 tagging (subtraction)

- raw

- With Isolation Cut

**Photon components**

**(subtraction)**

**(with isolation cut)**

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

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## 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

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## 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.

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## 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

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## 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.)

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Backup slides

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## EMCal (PbSc)

- Sandwich type calorimeter
- 	Lead plates 55.2x55.2x1.5mm
- 	Scintillator plates 110.4x110.4x4mm
- Shish-kebab geometry wave shifter fiber readout
- 	6x6 fibers  1 PMT = 1 tower
- 	2 x 2 towers =  1 module
- 	6 x 6 module = 1 super module
- 	6 x 3 super module = 1 sector

- PbSc sector 2.0m x 4.0m

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## EMCal Trigger

- 2x2

- 4x4a
- 4x4b
- 4x4c

- Threshold

- 2x2

****2x2

****2x2

- overlapped

- EMCal tower 5.25*5.25cm(PbSc)
-                        4*4cm (PbGl)

- PMT

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## EMCal Trigger

- 2x2

- 4x4a
- 4x4b
- 4x4c

- Threshold

- 2x2

****2x2

****2x2

- overlapped

- EMCal tower 5.25*5.25cm(PbSc)
-                        4*4cm (PbGl)

- PMT

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## EMCal Trigger performance

- Plateau above 3GeV

- Black points: Data
- Red line: MC expectation

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Strategy 1

- Pi0 tag and subtraction

- Guard veto (the acceptance is reduced to 67%)
- was applied.

- Pi0 tag

- Photon not
- tagged as Pi0

-  etc

- Prompt photon = all - (pi0 tag)/r *(1+non pi0)

- r

- Pi0 photons won’t be tagged, since
-     partner’s energy is below Emin
-     partner is in the warn map mask

- MC

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## 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.

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## EM shower cut (photon)

- Photons are selected from pi0 mass window

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

- ratio

- 98% (pT independent)

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Counts (PbSc west)

- ecore>3GeV
- PC3 dead area is masked off.

- -------------------------------
- Run88007-88824, 89212-92466
- #event   4315M
- 170922 168940 157748 (92%)
- 26880 26666 20221    (75%)
- 12695 12632 7002     (55%)
- 2972 2960 1858       (62%)
- 2488 2477 1678       (67%)
- pc3match ratio : 13%
- dcmatch ratio  :  4.4%
- electron ratio(n0>0) :  1.2%
- electron ratio(n0>1) :  1.1%
- electron/dcmatch(org) : 20%
- electron/dcmatch      : 24%
- Electron/1M events   : 0.39

- The difference of 3 converter runs is
- consistent to electron group’s study.

**9% conversion**

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Counts (energy dependence)

- -------------------------------
- Run88007-88824, 89212-92466
- #event   4315M
- 170922 168940 157748 (92%) photon 97%
- 26880 26666 20221    (75%)
- 12695 12632 7002     (55%)
- 2972 2960 1858       (62%)
- 2488 2477 1678       (67%)
- pc3match ratio : 13%
- dcmatch ratio  :  4.4%
- electron ratio(n0>0) :  1.2%
- electron ratio(n0>1) :  1.1%
- electron/dcmatch(org) : 20%
- electron/dcmatch      : 24%
- Electron/1M events   : 0.39

- -------------------------------
- Run88007-88824, 89212-92466
- #event   4315M
- 8660 8372 7868  (91%)   photon 97%
- 1358 1330 1048  (77%)
- 577 572 341       (59%)
- 267 265 153       (57%)
- pc3match ratio : 13%
- dcmatch ratio  :  4.3%
- electron ratio(n0>0) :
- electron ratio(n0>1) : 1.9%
- electron/dcmatch(org) :  46%
- electron/dcmatch      :  45%
- Electron/1M events   : 0.035

- -------------------------------
- Run88007-88824, 89212-92466
- #event   4315M
- 1279 1177 1080  (84%)     photon 95%
- 188 181 141       (75%)
- 81 79 45            (56%)
- 54 53 34            (63%)
- pc3match ratio :  13%
- dcmatch ratio  :    4.2%
- electron ratio(n0>0) :
- electron ratio(n0>1) :  3.1%
- electron/dcmatch(org) : 67%
- electron/dcmatch      :  76%
- Electron/1M events   :  0.0079

- Ecore>3GeV

- Ecore>5GeV

- Ecore>7GeV

- High pT charged pion contamination can be seen in electron/dcmatch ratio.
- Matching behavior looks the same.

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Isolation cut cone size

- 0.38rad

- /4

- 0.5rad

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Invariant mass of two photons

- 0

- 0.5GeV

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Invariant mass of two photons 

- With the isolation cut

- 0

- 0.5GeV

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## pi0 photon merging

- (second tower) / (first tower)

- 0.2/2 = 10% (tower center)

- 0.5/1.25=40% (center + 0.3unit)

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Pi0 photon merging

**---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 is 20GeV cluster,
- but it can’t pass the EMCal shower shape
- criteria.

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## pi0 photon missing ratio (with MC)

- Input: pi0 spectra, Energy resolution, Shower size from measurements
- The same MC used in pi0 cross section measurement.

- 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

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Eta/pi0 ratio

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Yields (West arm)

-  black : all photons
-    red : photons tagged as pi0
-    green : applied a combinatorial bg correction to (red)
-    blue : all pi0 contributions (green *(1+missing ratio))
-    magenta : (black)-(blue)*1.23

-  bg ratios to the total
-    red :
-    green :
-    blue :
-    magenta : all hadron contributions (blue)*1.23

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Yields (west arm) (with an isolation cut)

-  black : all isolated photons
-    red : isolated photon tagged as pi0
-    green : applied a combinatorial bg correction to (red)
-    blue : isolated photon estimated from isolated pi0s
-    magenta : (black)-(green+blue)*1.23

-    bg ratios to the total
-    red:
-    green:
-    blue:
-    magenta: all hadron contribution (green+blue)*1.23

- Yields (west arm) (with an isolation cut)

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Systematic error sources

-  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

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Systematic error sources

- For isolation cut method

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Comparison to NLO pQCD calculation

- NLO-pQCD calculation
- Private communication with W.Vogelsang
- CTEQ6M PDF.
- direct photon + fragmentation photon
- Set Renormalization scale
-     and factorization scale pT/2,pT,2pT

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

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Results (gamma/pi0 ratio)

- Pi0 fit function
- y=20.3/pow(x,8.285)

- BBC related systematical
- error is	canceled out.

- PHENIX Preliminary

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Comparison

- Comparison with existing data from p+p and p+pbar collisions
- The cross section increases systematically as the energy increases.

- Preliminary

- Systematic errors are not shown

- Preliminary

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## QCD理論によると、以下の仮定
PDFとFFのQ2スケーリング
Coupling constant(s)がQ2に非依存。

n=定数。xT=2pT/s
相互作用項と構造を表す項に分離
xTスケーリング
定数nに対する予想
Leading order n=4
Next-to-leading order: n=4+

- PDFとFFのQ2スケーリング
- Coupling constant(s)がQ2に非依存。
- n=定数。xT=2pT/s
- 相互作用項と構造を表す項に分離
- xTスケーリング
- 定数nに対する予想
- Leading order n=4
- Next-to-leading order: n=4+

- xT scaling

- xTスケーリング則n=5付近で成立

- 今回得られたデータを過去のデータ(s>20GeV)と比較して、xTスケーリング則がn=5付近で成り立つ
- X-T scaling shows a point like interaction of the measurement.

- Preliminary

- Kensuke Okada (RBRC)

- Oct..2004 Italy

## Bias on Physics Process

- Two processes
- Direct
- Fragmentation
- Direct photon measurement at pT>5GeV/c is dominated by direct process.
- The difference between two methods is small.

- Direct

- Fragmentation

- Private communication with W.Vogelsang

- Kensuke Okada (RBRC)

- Oct..2004 Italy