# Presentation: 707114

## D0 Measurement in Cu+Cu Collisions at √s=200GeV at STAR using the Silicon Inner Tracker (SVT+SSD)

- Sarah LaPointe
- Wayne State University
- For the STAR Collaboration
- DNP Meeting, October 24, 2008

## Motivation

- Heavy quarks are also produced at RHIC
- The most likely production mechanism for charm is gluon fusion in        parton-parton hard scattering.
- Thus, charm production occurs in the early stage of the collision, making charm an attractive probe of the initial conditions.
- Questions:
- What is the energy loss (RAA) of the heavy quarks in the partonic medium?
- Does the heavy quark experience a collective motion (v2), similar to what is observed for the light quarks?

- Sarah LaPointe

- DNP - Oakland, October 24, 2008

## Detection Methods	

-  D0  K 			3.8%
-  D+/-  K  		            9.2%

**Semileptonic Channels**

-  D0    e+ + anything	6.9%
-  D+/-  e+/- + anything 	17.2%
-  B0     e+ + anything	            10.9%

**Techniques used to measure open charm**
**Single electrons**
**Reconstruction from hadronic decay **

**Hadronic Channels****	**Branching Ratio

- Sarah LaPointe

- DNP - Oakland, October 24, 2008

## Single Electrons	

- STAR measures single electrons in the **TPC, TOF and EMC**.  The combination of all three detectors gives us a pT spectrum from              200 MeV – 10 GeV

**Disadvantages:**
-  Missing decay products
-  Need to reliably subtract photonic electrons and non-photonic background electrons
-  Challenging to disentangle D and B decays

**      ****In principle, single electrons are sensitive to charm and beauty, but relative fraction of b/c uncertain.**

- STAR: B. I. Abelev *et al.*, *Phys. Rev. Lett.* **98** 192301 (2007)
- PHENIX: A. Adare *et al.*, *Phys. Rev. Lett.* **98** 172301 (2007)

- M. Cacciari *et al.*, *Phys. Rev. Lett.* **95** 122001 (2005)

## Direct Hadron Reconstruction

- D0 → K-π+
- Invariant mass analysis
- Visualization of decay vertex not possible because the decay occurs before the tracking detectors
- Reconstruction via combinatorial methods

-  Oppositely charged tracks are paired and projected toward the primary vertex
-  The two trajectories are compared to see if they cross at some point
**If so, the tracks are considered candidates for D****0 ****decay.**

- Sarah LaPointe

- DNP - Oakland, October 24, 2008

## Secondary Vertexing Technique using Silicon

-           **c******** for D****±**** is 318 ********m **
**             ****c******** for D****0**** is 126 ********m **
- Most of the D decay vertices are within a millimeter of the primary vertex.

**What is necessary in order to perform direct reconstruction of the D mesons?**
-  Impact parameter resolution need to be      comparable to the decay length
-  TPC alone  ~ 1.8mm
-  TPC+SVT+SSD ~ 200μm (@ 1GeV/c)
-  Event vertex resolution
-  Currently the TPC yields  ~ 2mm
-  SVT+SSD yields ~ 90m

- Sarah LaPointe

- DNP - Oakland, October 24, 2008

## Cu+Cu 200GeV

- Require TPC hits ≥ 7
- Initial background (B) estimated to be 6.5x109
- Initial signal (S) estimated to be 23,000, take from
**D****0****+D****0**** **= 0.45 in y ±1, 3.8% branching ratio, 80% primary tracking efficiency, 40% Si hit matching efficiency in CuCu.
- Significance can be defined as
- The initial S and B give a **significance of  0.2**

- In order to obtain a reasonable significance one must suppress the background while retaining the signal.  Look at:
- Geometrical Cuts
- PID cuts, dE/dx

- Sarah LaPointe

- DNP - Oakland, October 24, 2008

## Geometrical Distributions

## Cu+Cu 200GeV, with Geometrical Cuts

- Applying the following cuts:

- TPC hits ≥ 15
- SVT hits ≥ 1
- If SVT hits =1, SSD hit = 1
- pK,π ≥ 200MeV/c
**Decay Length < 500****μ**
**D0 dca PV ≤ 300****μ**
**DCA daughters  ≤ 400****μ**
**Daugs DCA PV ≤ 400**
- |nSigmaPion| < 2.5
- |nSigmaKaon| < 2.5

- Cu+Cu background is reduced by a factor of **270**.
- By looking at a Pythia sample of D0, one finds that the signal is reduced by a factor** 6**.
- Significance = **0.5**

- Sarah LaPointe

- DNP - Oakland, October 24, 2008

## PID using dE/dx

- Kaons and pions can be chosen via PID from energy loss in the TPC.
- dE/dx is momentum dependent

- |nSigmaPion| < 1.5
- |nSigmaKaon| < 1.5
- For daughter track with p < 900MeV/c
- |nSigmaPion| < 0.75
- |nSigmaKaon| < 0.75

- Background is reduced by a factor **340**.
- Signal reduction can not be estimated because Pythia contains no dE/dx information
- At best the significance = **0.7**

- Cut

## Cu+Cu 200GeV results 

- PID Cuts
- |nSigmaPion| < 1.5
- |nSigmaKaon| < 1.5
- For p < 900Mev/c
- |nSigmaPion| < 0.75
- |nSigmaKaon| < 0.75

- CuCu background is reduced by a factor of **6225**.
- By looking at a Pythia sample of D0, one finds that the signal is reduced by a factor** 77**.
- Significance = **2.0 **

- TPC hits **≥ 25**
**SVT hits ≥ 1**
**If SVT = 1, SSD hit =1**
- pK,π ≥ 200MeV/c
- Decay Length < 500μ, **> 100****μ**
- D0 dca PV ≤ 300μ
- Daugs DCA PV ≤ 400μ, **≥ 100****μ**
- DCA daughters **≤ 50****μ**

- After a some further optimization:

- Sarah LaPointe

- DNP - Oakland, October 24, 2008

## Background Suppression and Future Plans

- Sarah LaPointe

- DNP - Oakland, October 24, 2008

## Backup Slides

- Sarah LaPointe

- DNP - Oakland, October 24, 2008

## Signals and Background (all candidates)

- To claim a D0 signal (S) one looks at the Kπ invariant mass spectrum for excess candidates in a window around the D0 mass (1.864 GeV/c2)
- There is also background in that region.  I currently estimate the background by fitting a polynomial function outside the window that sits around the D0 peak.
- It is reasonable, for the CuCu data set to require the statistical significance, σ to be greater than 4 (currently the mixed event technique gives σ=4.3).

- Sarah LaPointe

- DNP - Oakland, October 24, 2008