# Presentation: 853453

## Open charm measurements with an HFT in STAR

- Charm at RHIC

- The goals of RHIC are identification and study of the properties of matter with partonic degrees of freedom. Previous studies have identified partonic collectivity, but have not yet demonstrated thermalization of the created matter. The study of heavy quark collectivity may allow us to address this issue.

**Figure 1:** Illustration of non-central heavy ion collision. The spatial asymmetry of the interaction region creates an asymmetry in particle momentum distributions.

- Charm quarks:
- produced early in the collision
- derive their mass from the Higgs field 	and stay massive even during chiral 	symmetry restoration
- their collectivity tests medium 	thermalization
- charm quark energy loss directly  	probes QCD medium at high 	temperature

**Figure 3:** Extreme collective behavior scenarios (Pythia, hydro). Compared to difference in D and B meson spectra, electron spectrum provides less sensitivity to dynamics [2,3].

**Figure 2:** Nuclear modification factor of non-photonic electrons in central Au+Au collisions shows similar suppression as hadrons [1]. Knowledge of relative contribution of charm and bottom is crucial to interpret the results.

**Need for direct reconstruction of open charm hadrons.**

- To address these challenges, STAR needs to upgrade its tracking system.  A proposed micro vertex detector , the Heavy Flavor Tracker (HFT, [4]),  will reconstruct displaced decay vertices of open charm hadrons.

- Heavy Flavor Tracker design

- 2 layers of PIXEL detector + 1 layer of IST (Intermediate Silicon Tracker)‏ + 1 layer SSD (Silicon Strip Detector)
**PIXEL**: measurement of open charm hadrons down to pT = 0.5 GeV/c requires low material budget – MAPS 	(Monolithic Active Pixel Sensors)‏
**IST**: fast detector between PIXEL and the existing SSD detector to cope with high luminosity at RHIC-II and 	long integration time of PIXEL
- design was improved for better hit matching and lower mass budget, not fully simulated yet

**Figure 4:** Subsystems of the HFT (PIXEL, IST - current design), with the existing SSD detector.

**Table 1:** Hit position resolution of SSD + HFT layers for the two design versions. IST2 (simulated design) has two layers (A,B) with crossed strips.

**Figure 5:** Design of PIXEL detector support structure, with ladders and wire bundles.

- PIXEL detector development

- MAPS chip prototypes:
- 30 μm pixel pitch, ~ 15 μm thick active region
- modified diode design for improved radiation 	tolerance successfully tested (MimoSTAR2)‏
- large ( ~ 1 x 2 cm2 ) chip prototype MimoSTAR3 	currently under testing
- analog readout
- 2 ms integration time

- h

**Figure 6:** MimoSTAR2 chip, 128 x 128 pixels, 4 x 4 mm2.

**Figure 7:** MAPS principle of operation: electrons created in the epitaxial layer thermally diffuse toward low potential n-well region.

- Development and deployment plan
- Phase 1 sensors:
- large area chips (2 x 2 cm2)‏
- thinned to 50 μm, 0.28% X0 including support
- digital readout and on-chip CDS
- 640 μs integration time
- build detector patch (~ 30% of full acceptance)‏
- Ultimate sensors:
- on-chip cluster finding (data sparsification)‏
- < 200 μs integration time

**Figure 8:** Detail of PIXEL detector patch design.

**Figure 9:** Prototype telescope with 3 MimoSTAR2 sensors.

**Figure 10:** Tracks from the interaction diamond, registered by the telescope, increased width due to MCS in the beam pipe. Beam background observed.

- MAPS telescope prototype test [5]
- telescope with 3 MimoSTAR2 chips placed 	inside STAR during 2007 Au+Au 200 GeV RHIC run
- located 150 cm from the interaction point, 5 cm 	below the beam pipe
- chip readout and integration with STAR trigger 	successfully tested

- Tracking performance

- Hand calculations & simulation with full STAR geometry and tracking
- assuming RHIC-II luminosity: 80 kHz minimum bias collision rate
- D0 embedded into HIJING central Au+Au events
- investigation of pile-up effects in PIXEL due to 200 μs integration time 	and high luminosity – addition of pseudo-random hits to PIXEL layers
- no pile-up in IST, SSD (fast detectors)‏

**Figure 11:** Reconstruction probability of pion and kaon tracks in |η| < 1 with correct hit association in both PIXEL layers.

**Figure 12:** Pointing resolution in z-direction to event primary vertex for tracks with hits in both PIXEL layers.

**Figure 13:** probability of wrong hit association (ghosting) in PIXEL layers. Highest contribution comes from pointing to PIXEL2 layer.

- D0 reconstruction

- D0 → K- π+,   B.R. 3.8%;   cτ = 123 μm
- decay daughters reconstructed requiring hits in both 	PIXEL layers
- Assumptions:
- perfect K-π separation for tracks with 	pT < 1.6 GeV/c (Time Of Flight detector)‏
- no PID for tracks with pT > 1.6 GeV/c
- Nbin scaling for D0 yields
- p+p D0 yield: dN/dy = 0.002
- D0 pT spectrum for central Au+Au: power-law, 	<pT> = 1.0 GeV/c and n = 11

- The STAR HFT will perform topological
- 		reconstruction of open charm hadrons 	even in central Au+Au collisions at 	RHIC-II luminosities.
- RCP and v2 measurements using 500M 	minimum bias events (one month of data) 	will provide strong constraints on theory.
- MAPS telescope prototype has been  	successfully tested in STAR environment 	during 2007 Au+Au RHIC run.

**Figure 14:** D0 → K- π+ decay topology.

**Figure 15:** Estimated Minv distributions from 100M central Au+Au events. Entries per 5 MeV/c2.

**Figure 17:** Two extreme scenarios for D0 elliptic flow. Statistical errors correspond to 1 month of data taking.

**Figure 16:** Efficiency to reconstruct D0 decay daughters and D0 (after applying cuts) for D0 in |η| < 1.

**dca****πK**

**Key measurements from 1 month of data taking at RHIC-II**

**Table 2:** Estimated hit densities (cm-2) at detector center (|η| = 0). Pile-up for RHIC-II luminosity is calculated assuming primary vertex diamond σPV = 15 cm.

- HFT performance - high luminosity environment

- Furthermore, we have studied D0 reconstruction for different levels of pile-up hit densities in PIXEL detector.

**Figure 19:** The performance of the intermediate tracking depends on the integration time of the PXL chip
- P(good association) = 1/(1+S)   if  S =  2 x y 

- Conclusions

**With thanks to:**
- E. Anderssen7, J. Baudot5, D. Beavis1, M. Bystersky4, R. Cendejas2, C. Chasman1, R. Debbe1, X. Dong7, L. Greiner7, A. Hirsch8, H. Huang2, C. Hu-Guo5, J. Joseph3, J. Kapitan4, D. Keane3, J. Kelsey6, V. Kushpil4, J.H. Lee1, M.J. Levine1, S. Margetis3, H.S. Matis7, R. Milner6, S. Morgan7, M. Plesko6, R. Redwine6, H.G. Ritter7, A. Rose7, V. Rykov3, S. Sakai2, A. Shabetai5, E. Sichtermann7, F. Simon6, R.P. Singh7, B. Srivastava8, T. Stezelberger7, M. Sumbera4, X. Sun7, B. Surrow6, M. Szelezniak5, J.H. Thomas7, V. Tram7, G. Van Nieuwenhuizen6, F. Videbaek1, C. Vu7, F. Wang8, C. Whitten2, H.H. Wieman7, M. Winter5,  W. Xie8, N. Xu7, Z. Xu1, W.M. Zhang3
- 1Brookhaven National Laboratory, Upton, NY; 2University of California, Los Angeles, CA; 3Kent State University, Kent, OH; 4Nuclear Physics Institute AS CR, Rez/Prague, Czech Republic; 5Institut Pluridisciplinaire Hubert Curien, Strasbourg, France; 6Massachusetts Institute of Technology, Cambridge, MA; 7Lawrence Berkeley National Laboratory, Berkeley, CA; 8Purdue University, West Lafayette, IN

- J. Kapitán*(NPI ASCR Prague) and J.H. Thomas* (LBL)  for the STAR Collaboration

**D****0**** reconstruction cuts:**
- dcaπK < 100 μm
- dca_π, dca_K > 50 μm
- cos(θ) > 0.98
- 1.83 < Minv(GeV/c2) < 1.90

**References:**

**(flow)****‏**

**(flow)****‏**

- [1]   B.I. Abelev *et. al.*, Phys. Rev. Lett. **98** 192301 (2007).
- [2]   S. Batsouli, S. Kelly, M. Gyulassy and J.L. Nagle, Phys. Lett. B **557**, 26 (2003).
- [3]   K. Adcox *et. al.*, Phys. Rev. Lett. **88 **192303 (2002).
- [4]   E. Anderssen *et. al.*, A Heavy Flavor Tracker for STAR (available at http://rnc.lbl.gov/hft/docs/hft_final_submission_version.pdf).
- [5]   M. A. Szelezniak *et al.*, Small-scale readout system prototype for the STAR PIXEL detector (IEEE NSS 2007 Conference Record Vol. 2 1474-1481 (2007), 10.1109/NSSMIC.2007.4437278).

- S/√(S+B) = 38

- S/√(S+B) = 69

- S/√(S+B) = 22

**stack of 3 MimoSTAR2 pixel chips**

- ladders: 2 x 20 cm2 active area

- wire bundles

- Support structure of the PIXEL detector:
- low mass budget
- repeatable positioning with ~ 10 μm accuracy

**IST at r=14cm**

**PIXEL at r=2.5cm and r=8cm**** **

**SSD at r=23cm**

**Figure 18:** Estimated statistical errors of RCP measurement (corresponding to 1 month of data taking). Expected D0 yield in peripheral collisions at high pT is scaled by 1/RCP with respect to Nbin scaling.

- “Characterization of the QGP with Heavy Quarks”, Bad Honnef, 2008

- * E-mail address: kapitan@rcf.rhic.bnl.gov ,  jhthomas@lbl.gov