# Presentation: 361048

## PHENIX Opportunities with Direct Photonsand Heavy Flavor

- RHIC/AGS Users’ Meeting WS
- June 18, 2007
- Yuji Goto (RIKEN/RBRC)

**Notes:**
I will talk about this given title from organizers, PHENIX opportunities with direct photons and heavy flavor

## Direct photon, heavy flavor, ...

- For precision physics, measurements of multi-channels (and by multi-experiments) with different kinematic coverage (with overlap region) and systematic uncertainty are important
- Direct photon and heavy flavor production channels are golden channels for G measurement
- All these data must be consistently understood on the basis of common physics framework
- global QCD analysis
- test of experimental data
- test of QCD

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
We are measuring Delta-G, and we have many important channels for this purpose.
Those are direct photon, heavy-flavor, etc.
For precision physics, measurements of multi-channels and by multi-experiments with different kinematic coverage with overlap region
and different systematic uncertainty are important.
In those measurments, direct photon and heavy flavor production channels are two golden channels.
All these data must be consistently understood on the basis of common physics framework.
This is the global QCD analysis, and it will serve to test both experimental data and QCD.
I don’t talk much about the global QCD analysis today. I will show our current status and future measurement of direct photon
and heavy flaovr production.

## Direct photon

*A**LL*() provides G with sensitivity to its sign
*A**LL*(0) can give us two solutions on G
- Smaller statistics than 0
- but significant at high-*x*

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
First topic is direct photon. I will compare it with pi0 measurement to know its importance.
A_LL(photon) provides us Delta-G information with sensitivity to its sign, while A_LL(pi0) can give us two solution on Delta-G, as shown by this figure.
This is because the direct photon process is dominated by the gluon-quark reaction, so-called gluon Compton process, and the pi0 production is
dominated by the gluon-gluon process at low p_T region.
Disadvantage of direct photon seems to be small statistics than that of pi0, but this is not true.
When we have much enough statistics for high p_T region, direct photon statistics is significant.
Please compare these figures for A_LL(pi0) and A_LL(photon) at luminosity 65 inverse-picobarn.
This luminosity will be accumulated in Run-9 (or Run-8?) at PHENIX.

## Direct photon

- Status and issues – background
- experimentally subtracted and evaluated
- large background from 0 and 
- theoretically evaluated
- gluon Compton and annihilation
- higher order – bremsstrahlung and fragmentation
- Cross section measurement
- better understood than 0?
- scale and fragmentation function
- For *A**LL* measurement, the isolation cut is essential
- to increase the signal purity
- fragmentation issue to be investigated

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
So, I will show our current status and issues.
Issues are background.
Large background from pi0 and eta are experimentally subtracted and evaluated.
Theoretically, also contamination of gluon Compton, annihilation, and higher order diagrams, bremsstrahlung
and fragmentation, are evaluated.
We investigate these issues in the cross section measurement.
Our current published result of the cross section is that obtained by the so-called cocktail method and pi0-tagging method. -> go to next slide

In addition to scale uncertainty, we should compare uncertainty from the fragmentation function.

For A_LL measurement, the isolation cut is essential to increase the signal purity.
It also provide us information to understand the fragmentation component of photon.

## Direct photon

**200GeV p+p direct photon**

- Run3 published
- PRL 98 012002

- Run5 preliminary

- Cocktail method and 0-tagging method
- evaluate or subtract known background sources
- the NLO pQCD calculation describes our result well.

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
The cocktail method is a method to evaluate all inclusive photon including those from decay with known background source,
mainly pi0 decay and eta decay.
pi0-tagging method is a method to subtract decay photons from pi0 by invariant mass reconstruction, and evaluate remaining
photons with known background source.
This published run-3 result was made by using both methods.
On the other hand, this preliminary run-5 result was made by using only the pi0-tagging method.
Anyway, the NLO pQCD calculation describes our result well.

## Direct photon

- 0 vs direct photon
- high statistics at high pT
- better scale uncertainty?

- Run5 preliminary

**200GeV p+p direct photon**

**200GeV p+p ********0**** production**

- Run5 final
- arXiv:0704.3599
- [hep-ex]

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
Next, let’s compare the result with pi0 cross section result.
Both results are those of run-5 and high p_T coverages are similar and statistical significance in the high-pT region is almost same.
Scale uncertainties shown by these band is better for photon, though it is hard to see in these figures. It is like -> next page

## Direct photon

***Direct photon***

- Scale uncertainty of NLO pQCD calculation
- better for photon...

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
This is the comparison of the scale uncertainty of the NLO pQCD calculation.
That of direct photon is smaller than that of inclusive hadrons.

## Direct photon

- Isolated photon
- for asymmetry measurement, it is important to reduce background event-by-event, otherwise the asymmetry is diluted by that of the background
- though our detectors aren’t large enough, an isolation cut is applied
- also important to know the asymmetry of the background  isolated 0 measurement

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
For asymmetry measurement, it is important to reduce background event-by-event, otherwise the asymmetry is diluted by that of the background.
Though our detectors are not large enough, an isolation cut is applied shown by these figures.
Isolation cone was defined R = 0.5, anyway, though it is larger than the central arm acceptance, and all photon energy and charged particle momentum in the cone was summed up and this isolation cut condition was applied -> next slide

## Direct photon

- Isolated photon
- background reduction
- significant

- before isolation cut

- after isolation cut

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
These are comparison of signal fraction in all photon sample detected by the central arm before and after the isolation cut.
Background reduction is clear, and the signal purity is made much high by the isolation cut.

## Direct photon

- Isolated photon
- in contrast to 0 decay photons, direct photons are mostly isolated
- theory calculations agree well to the measurement  at high *p**T* region (*p**T* > ~7GeV/*c*)
- to be studied by higher statistics
- effect of underlying event larger at lower *p**T*
- larger fragmentation component at lower *p**T*
- isolated 0 asymmetry to be used for background asymmetry

- Run3 published

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
In our run-3 analysis of direct photon cross section, we showed isolated direct photon and photon from pi0 as shown by this figure.
In contract to pi0 decay photon, direct photons are mostly isolated.
Theory calculations agree well to the measurement at high p_T region more than about 7 GeV/c.
This result should be studied by higher statistics.
At low p_T region, the difference from the theory is larger. One reason is that the underlying event effect is larger at lower p_T.
Fragmentation component is also larger at low p_T, and we have to understand this with higher statistics data in the future.
This isolated pi0 sample is important sample to know the background asymmetry for the isolated photon asymmetry result in the future.

## Direct photon

***Direct photon***

- photon + jet with VTX
- jet axis obtained with VTX

- jet  : reconstructed by charged particles
-         using Cone algorithm.

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
In the future, for the direct photon analysis at PHENIX, we will have VTX silicon vertex tracker, and will become possible to detect jet axis for photon + jet measurement.
Expected jet axis resolution is .13 for pseudorapidity direction.
By using this information, we’ll be able to calculate the x_1 and x_2 of partons and x_gluon uncertainty is evaluated to become much better by showing these figures.
The figure shows generated x_gluon by the PYTHIA generator, and reconstructed x_gluon without VTX and with VTX. We can see a significant improvement and avoid misidentification.

## Heavy flavor

- J/
- preliminary *A**LL* result
- more (and more) statistics necessary as an input for G

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
Next topic is our heavy-flavor physics.
We already have a preliminary A_LL result of J/psi, as shown by this figure, but more and more statistics is necessary to be an input for Delta-G, and comparison with theory is made possible.

## Heavy flavor

- (charm) PRL97 252002: Run5

***Heavy flavor***

**c quark**
-   D  e (+anything),  K
-   J/ e+e-

**b quark**
-   B  e (+anything)
-   B  J/+ X

**In single electron analysis**

-  background.
- 0,  Dalitz decay
-      conversion (  e+e-)

-  can not distinguish electron
-   from D and B (and J/)

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
Another high statisics channel is single-electron measurement which is a decay object from c-quark and b-quark.
Cross section is already measured and published shown in this figure.
In our current single electron analysis, we cannot distinguish electrons from D meson, B meson, and background from pi0/eta Dalitz decay and conversion.

## Heavy flavor

**Black : charged particles **
**             ****from collision point**
**Blue  : electron from D or B**

**Azimuthal angle**

**DCA**

***Heavy flavor***

- single electron with VTX

- DCA (**D**istance of **C**losest **A**pproach) of D and B meson are different.
**can distinguish each other, and from background**

| meson | Life-time(m) |
| --- | --- |
| D± | 315 |
| D0 | 124 |
| B± | 496 |
| B0 | 464 |

**DCA (********m)**

**studied by Vladimir L. Rykov with tuned PYTHIA**

- RHIC/AGS Users' Meeting WS

- June 18, 2007

**Notes:**
These will be separated by the VTX detector.
The VTX detector measure DCA (distance of closest approach) of tracks and tracks from D and B meson can be distinguished

## Heavy flavor

- single electron with VTX

**Red     : Dalitz+Conversion**
**Green : Charm**
**Blue    : Beauty**

**No DCA cut **
**     ****e****all****(no DCA cut) = ****e****Dalitz**** ****+ ****e****D**** + ****e****B **
**With DCA cut **
**     ****e****all****(DCA cut)       = R****dalitz****(p****T****) ****e****Dalitz ****+ R****D ****(p****T****) ****e****D**** + R****B ****(p****T****) ****e****B**
**R****XXX**** is electron ratio of “no DCA cut” to “DCA cut” for xxx. **
**(can be estimated by simulation)****             **

** ****Fraction of electron source            **

**p****T**

**no DCA cut**

**With DCA cut**

- RHIC/AGS Users' Meeting WS

- June 18, 2007

## Heavy flavor

- single electron with VTX

**Red     : Dalitz+Conversion**
**Green : Charm**
**Blue    : Beauty**

** ****Fraction of electron source            **

**p****T**

**no DCA cut**

**With DCA cut**

- RHIC/AGS Users' Meeting WS

- June 18, 2007

## Summary

- Direct photon and heavy flavor production channels are golden channels for G measurement
- VTX will extend PHENIX capability for both measurements significantly
- VTX will be installed at PHENIX in 2008 and will be available from 2008-2009 run (Run-9)
- All these data must be consistently understood on the basis of common physics framework by global QCD analysis, finally

- RHIC/AGS Users' Meeting WS

- June 18, 2007