# Presentation: 279053

## Measurement of the open charm cross-section	

- Alexandre SHABETAI
- for the STAR collaboration

- in 200 GeV Cu+Cu collisions using STAR @ RHIC

## Physics motivations

- 1

-  _Charm:_
-  is primarily produced by **gluon fusion**
- little contribution from **gluon** **splitting** (see talk by A. Mischke and poster by X. Dong).
-  **is sensitive ****to early stage ****of the collision **
-    (quark produced at
-  has an undisturbed  **mass**

** ****l****arge** uncertainty in pQCD
******s** large
**gluon PDF** undetermined
- for **P****T** < **M****c**
- the cross-section is little constrained by theory

## Binary scaling

- _Binary scaling_

- Charm production is a hard process: expect binary scaling.
- Data so far supports this  d+Au to Au+Au.
- What about Cu+Cu?

- 2

## Open charm measurements via semileptonic decays

- p

- _Open charm measurements __via __semileptonic decays_

## Electrons and Muons....

** ****muon** from heavy quark semileptonic decay
-  **electron** from heavy quark semileptonic decay

******-**

**c ******** e****+**** + X **
**c ******** ********+**** + X **
**D****0**** ******** e****+**** + X (6.87 %)**
**D****0**** ******** ********+**** + X**

- _Electrons and Muons...._

- semileptonic decay

- Measured by STAR and PHENIX

- 3

-   Straightforward using detectors with **e**** ****–**** PID **
** ****	****  **(TPC, EMC  and TOF)
- Background is difficult to assess
- 	(photonic)
- Limited to higher **P****T**
**e****- **does not reflect full **D** kinematics

## Charm Cross-Section to Date

- _Charm Cross-Section to Date_

- STAR **PRL 98 (2007) 192301**

- 4

- Non photonic electrons

- Non photonic electrons,
-  Muons and D mesons

|  |  | p+p | Au+Au (MinBias) | d+Au |
| --- | --- | --- | --- | --- |
| STAR  | driven by D mesons |   | 1.26 ± 0.09 ± 0.23  | 1.4 ± 0.2 ± 0.4 |
| PHENIX | from electrons | 0.567 ± 0.057 ± 0.224  | 0.622 ± 0.057 ± 0.160  |   |

- Cross-section measurements so far:

- What
- about
- Cu+Cu?

## Open charm measurements via hadronic decays

- p

- _Open charm measurements __via hadronic decays_

## Direct reconstruction

-  **electron** from heavy quark semileptonic decay

******-**

- semileptonic decay

**c ******** e****+**** + anything **
**c ******** ********+**** + anything **
**D****0**** ******** e****+**** + anything **
**D****0**** ******** ********+**** + anything**

- Hadronic decay channels
**(D****0********Kp (B.R.: 3.8%)**

- _Direct reconstruction_

-  Direct measurement, covers **large fraction **of the **cross-section**
-  **Large **combinatorial background
-  High precision **vertexing **is needed

- 5

** ****hadronic **“Direct“ D0                               reconstruction
**(****event. Mixing technique)**

## Direct D0 reconstruction in STAR

- _Direct D__0 __reconstruction in STAR_

- Pions and Kaons are selected using the TPC
- Combine “same event” pairs  signal+background
- Combine pairs coming from different events  background 	       (“mixed events” or “track rotating ”)
- After subtraction   signal

- 6

## Cu+Cu collisions @ 200 GeV

- ~ 28 Million events used
- All the statistics available
-  Cu+Cu  « minimum bias » (RHIC run V)
-  After track rotating or mixed event subtraction: residual background
-  Low S/B ratio:

-  Measurement only possible because of large S (~ 150 000)
- Large STAR acceptance !

- 7

-  Challenging  measurement

## 		            		pT bins

- D0  Mass  in **agreement with the PDG **
***(M******D******0****** =1864.84 ± 0.17 MeV/c******2***)

- 8

| PT  (GeV/c) | Mass (GeV/c2) | Width (MeV) |
| --- | --- | --- |
| 0.5-1.2 | 1.864 ± 0.005 | 8.4 ± 8.7 |
| 1.2-2.0 | 1.864 ± 0.005 | 8.3 ± 13 |
| 2.0-3.3 | 1.850 ± 0.015 | 12 ± 18 |

## Spectra

- _Spectra_

_**After corrections:**_

- Fit using an Exponential or a Boltzmann  function
-  **same results **(within stat. errors).

- 9

## Extraction of the cross-section

- _Extraction of the cross-section_

** ****Inelastic cross-section  in p+p **
**(UA5)**

**Conversion to full rapidity**
**(Pythia)**

**Ratio obtained from e****+****e****-**** collisions**

** ****Number of binary collisions (Glauber)**

_**STAR Preliminary:**_

- 10

## d/dy in STAR...

- _d____/dy in STAR..._

-  Scaling with the number of binary collisions
-  from d+Au to Au+Au **confirmed in Cu+Cu.**

- Accurate background
- subtraction is crucial
- Systematic study  is ongoing

- 11

- R. Vogt  [hep/ph 0709.2531]

## Summary

- _Today: _
- The charm cross-section was measured in Cu+Cu @ 200 GeV ;
- A direct measurement  in Cu-Cu is **consistent with a scaling of the cross-section with N****bin**** (****at low ****pT**).
- Theory: l**arge uncertainty in pQCD calculations **and **data points are needed.**
- _In the Future:_
- STAR **low material runs**
** **use of **SSD/SVT** and eventually the **HFT **upgrade (2010-2012)
-  will allow:
**precise ****measurements** of the charm cross-section
- direct **topological **measurements of charm** **and of its **anisotropy parameter V****2****, **RAA, RCP
- isolation of the **bottom contributions**

- 12

## Outlook

- « Upgrade » for RHIC2
- and especially
-        The future STAR ***Heavy Flavor Tracker***

** ****Direct and ****topological **** measurements of charm and precise V****2**
**See the HFT poster**
- by Jan Kapitan
- (for the STAR HFT collaboration)

- 13

- _Outlook_

-  Low material run (without the SVT/SSD)
-         low radiation length in run VIII
-  reduce the photonic background
-  Reconstructing the  secondary vertex with SVT/SSD in Au+Au (run VII)

## Backup

- p

- _Backup_

## PID

- p

- _PID_

## Electron ID - TOF

-  TOF measures particle velocity

-  TPC measures particle energy loss

-  The cut |1/-1|<0.03 with TOF excludes kaons and protons

-  TPC dE/dx further separates the electron and pion bands

- _Electron ID - TOF_

## Electron ID - EMC

- electrons

-  Charged tracks selected by TPC

-  EMC Tower hits association with TPC tracks required

-  Shower size measured by Shower Max Detector (SMD)

-  Small shower size for hadrons

-  Large shower size for electrons

-  Momentum/Energy ratio is cut to be around one for electron candidates

-  Both inclusive electron yield and hadron contamination obtained from Gaussian fit

- _Electron ID - EMC_

## Muon ID – TPC + TOF

- 

- 

- 0.17<pT<0.21 GeV/c

- Muon and pion bands slightly separated at low momentum in TPC

- TOF can further help to identify muons in mass2 distribution

- Backgrounds are mainly from , K+ decays, can be subtracted from DCA
- distributions  charm decayed muons!!

- m2 (GeV2/c4)

- STAR Preliminary

- 0-12% Au+Au

- _Muon ID – TPC + TOF_

## Photonic Background

- γ conversion
- π0 Dalitz decay
- η Dalitz decay
-  Kaon decay
-  vector meson decays

-  For each tagged e+(e-), we select a partner e-(e+) identified only with the TPC and calculate the invariant mass of the pair.

-  Combinatorial background reconstructed by track rotating or like-sign technique.

-  Photonic background is subtracted in a statistical manner: Nphotonic = (un_like – rotating)/bkgrd_eff

- STAR Preliminary

- _Photonic Background_

## NPE

- p

- _NPE_

## Non Photonic Electron Spectra

-  Non-photonic electron spectra measured by TOF and EMC are consistent with each other by proper Nbin scaling

- STAR Preliminary

- _Non Photonic Electron Spectra_

## Combined Fit

- Power-law function with parameters
- dN/dy, <pT> and n to describe the D0
- spectrum

- D0, e ,  combined fit

- Generate D0e decay kinematics
- according to the above parameters

- Vary (dN/dy, <pT>, n) to get the min.
- 2 by comparing power-law to D0
- data and the decayed e shape to e
- and  data

- Advantage: D0 and  constrain low pT
- e constrains higher pT

- Spectra difference between e and  ~5% (included into sys. error)

- _Combined Fit_

## Others

- p

- _Others_

## The charm cross-section

- _The charm cross-section_

- _Experiment :_

- _Calculation:_

**Fragmentation Functions**

**pQCD + PDF**

- 2

**Simulation (MC)**** **
**of decay**

***Measured (differential)***
***cross section***

## Cross-section – How well is the calculation constrained?

- _Cross-section – __How well is the calculation constrained?_

- -

**The pQCD error band is large and data points are needed...**

## How to compare measurements to calculations ?

- Using QCD (and pQCD) :
-  Heavy flavor cross-section can be correctly predicted
-  Differential cross-section (as function of momentum, rapidity or energy...), requires « **adding a **_**minimal**_**, **_**self-consistent and universal **_**set of  non perturbative input parameters »**

- To make an accurate comparison,  one should:
-  Use **dedicated theoretical tools **(FONLL and now NNLO)
-  Use **adequate parameters** (mass, renormalization and factorization scales, coupling),  Partons Distribution  Functions  (PDF) and Fragmentation Functions (FF).
-  _**Minimize extrapolations and deconvolutions **__between measurements and theory_
** ****If and only if** all those conditions are satisfied, **a good agreement between measurements and calculations can be reached**

**Matteo Cacciari     ****ISMD 2007**

**In real life, the error band is large and data points are needed...**

## PHENIX vs. STAR

- The value of the cross-section
- is not the same (factor 2 to 3)
-  STAR and  PHENIX are both above
-  FONLL predictions...

- - Spectra shapes are the same.
- STAR and PHENIX are seeing the same scaling with Nbin.

- Disagreement but
- let’s look at the RAA ...

- A. Suaide QM2006 (nucl-ex/0702035)

- STAR **PRL 98 (2007) 192301**

## RAA: (e-) from d+Au to Au+Au central

- Dead cone effect
- not observed ...
- (non photonic e-)

- RAA in agreement between STAR and PHENIX
- Is there a normalization issue?

- d+Au: no suppression expected but
-  small enhancement (cronin effect)

## D0 Analysys

- p

- _D__0__ Analysys_

## Cuts

- 0-60%  selected

- Primary vertex                          -30 cm  < Vz < 30 cm

## PID

- Kaon Tracks
- Pion Tracks
- Unused Tracks

## MC study / Efficiency 

## Background subtractionComparison between event mixing and track rotating

- Only one 180° Rotation
- larger stat error
- (more background fluctuations)

- Flow effects?

- Flow effects?

## Comparison

- obtained using
- 5 degree Rotations

## Normalisation

- as

## Systematicsunder progress

-  Background subtraction method
- 1 versus several rotations / event mixing  (and normalization)
-  Flow effects?
-  Range of the Invariant mass fit
-  Function used to subtract the residual background
- (1st order polynomial versus 3 order polynomial)
-  Bin width
-  Cuts (&  PID)

## Data correction

- Efficiency
- Number of events
- PID
- Branching ratio
- MC Temperature correction