Development of the FVTX high multiplicity trigger system for thePHENIX experiment

T. Nagashima,∗1,∗2 Y. Fukushima,∗1,∗2 S. Hasegawa,∗3 I. Nakagawa,∗1 W. Saito,∗1,∗2

and the PHENIX FVTX group

Protons and neutrons, which are the components offamiliar substances, are composed of quarks and glu-ons that bind quarks together. Immediately followingthe big bang, under extremely high-density and high-temperature conditions, quarks and gluons are consid-ered to escape from the boundary of nucleons. Thisliquid-like state is called Quark Gluon Plasma (QGP).

The PHENIX group investigates the behavior ofmatter in such a high-density and high-temperaturestate produced by collision using Relativistic HeavyIon Collider (RHIC). In particular, in Au+Au andPb+Pb collisions, the particle angular correlation ex-hibits a similar azimuthal pattern throughout the widerange of the rapidity region, referred to as ridge.The ridge correlation is considered as a consequenceof the hydrodynamic flow of products, and it is inter-preted as a characteristic of QGP. Certain experimentsat the LHC have recently reported that the ridge wasobserved especially in high-multiplicity events in smallcolliding systems, such as p+p.1) Similar ridge phe-nomena were observed in d+Au and He3+Au at RHIC.However, they have not yet been observed in p+p.

Previous experiments demonstrated high-multiplicityevents are the key to observe the ridge in p+p colli-sions. In PHENIX, the Forward Silicon Vertex Detec-tor (FVTX) is suitable to select such high-multiplicityevents since it is the tracker closest to the vertex point.The FVTX trigger design is shown in Fig. 1. Thetrigger signal is generated on FPGAs implementedon FVTX readout electronics,2) Front End Module(FEM) and FEM Interface Board (FEM-IB). Using thefeature that each FEM corresponds to an azimuthalslice of the FVTX sensor, each FEM judges whetherthere is any track, following which the FEM-IB counts

Fig. 1. FVTX trigger design based on the FPGA logicimplemented on the readout electronics

∗1 RIKEN Nishina Center∗2 Department of Physics, Rikkyo University∗3 J-Parc

the number of track flag sent from the FEM, and fi-nally the FVTX trigger fires when the number of trackis greater than that of our interest.

Fig. 2. Correlation between the number of hits and thetrigger state

The trigger algorithm in the FEM FPGA was testedthrough evaluation of the process efficiency depend-ing on the number of hits on the detector. The hitswere generated by using a calibration pulse that in-jects typically 13 strips per detector per event. Fig.2 shows the correlation between the number of hitsper event measured from offline data analysis and thetrigger state; the vertical axis shows the trigger flagat an FEM that is supposed to give 1 for the eventswhere the number of hits is greater than the threshold.The efficiency, defined as the number of trigger eventsdivided by the number of events with more hits thanthe trigger threshold, was 94%, and a few fake triggerswere observed.

New serial line cables for FEM - FEM-IB communi-cation were installed in the trigger system, and theirsignal transmission test was performed. Performancewas monitored by scanning the signal rate, and the ca-bles exhibited good performance up to a trigger rate of4.7 MHz. The timing of the trigger signal was tunedwith GL1. It was found that GL1 does not recognizethe trigger signal depending on timing. It is adjustedusing the BBC trigger signal, which is already used asa trigger.

References1) V. Khachatryan et al.: CMS Collaboration, JHEP

1009 (2010) 0912) arXiv:1311.3594v2 [physics.ins-det] 14 Feb. 2014

Current status of bottom and charm measurements using VTX atRHIC-PHENIX

T. Hachiya,∗1 A. Adare,∗2 Y. Akiba,∗1 H. Asano,∗1,∗3 S. Bathe,∗1,∗4 J. Bryslawskeyj,∗4 T. Koblesky,∗2

M. Kurosawa,∗1 D. Mcglinchey,∗1 T. Moon,∗1,∗5 H. Nakagomi,∗1,∗6 R. Nouicer,∗7 T. Rinn,∗8 Z. Rowan,∗4

T. Sumita,∗1 and A. Taketani∗1

One of the most noteworthy results at RHIC was thestrong suppression of inclusive open heavy quarks (bot-toms and charms) at high pT in central Au+Au colli-sions1). It was expected that heavy quarks would beless suppressed due to their large mass. However, theresults showed that the suppression was comparablystrong with pions. In order to study the suppression indetail, we developed a silicon vertex tracker (VTX) atPHENIX. VTX can measure bottoms and charms sep-arately using the distance of closest approach (DCA)to the primary vertex.We reported the preliminary result of the fraction

of (b → e)/(b → e + c → e) using VTX in Au+Aucollisions2). Numerous improvements of the analysisfor the final result are underway. Here, we have listeda few of these improvements:

(1) Rejecting photon conversions and Dalitz decays:Photon conversions and Dalitz decays of lightneutral mesons are the main background sourcesin single electron measurement. Electron pairsfrom the backgrounds are generated with smallopening angles and the pair makes correlatedhits in VTX. Therefore, VTX can significantlyreject the backgrounds by requiring pair-wisehits in VTX. On the other hand, heavy flavorelectrons (b → e and c → e) rarely make thecorrelated hits. Figure 1 shows the fraction ofheavy flavor electrons to inclusive electrons. Theopen and closed circles correspond to the resultbefore and after VTX installation, respectively.This plot indicates that VTX could reject thebackground effectively and provide a good sig-nal to noise ratio.

(2) Unfolding of DCA distribution:We developed an unfolding method to decom-pose bottom and charm contributions3). Themethod is a Bayesian approach using the Markovchain Monte Carlo sampler4). Using thismethod, the bottom and charm yields are ob-tained by fitting both the DCA and the pT distri-bution of heavy flavor electrons simultaneously.The DCA shape is correlated with the pT distri-

∗1 RIKEN Nishina Center∗2 Department of Physics, University of Colorado, Boulder∗3 Department of Physics, Kyoto University∗4 Department of Natural Science, Baruch College, CUNY∗5 Department of Physics, Yonsei University∗6 Department of Physics, University of Tsukuba∗7 Brookhaven National Laboratory∗8 Department of Physics, Iowa State University

bution of the parent B and D mesons becausethe DCA is determined by the convolution oftwo effects: the decay length of the parent par-ticle and the decay pT kick relative to the parentmomentum. As a consistency test, we verified ifthe known input can be reproduced using a sim-ulation3).

(3) Detector efficiency:The detector efficiency was determined by fullGEANT simulation with the actual dead map.The raw pT spectrum of heavy flavor electronswas corrected using the efficiency, and the cor-rected spectrum is consistent with the publisheddata5)

(4) DCA smearing by random associationThe DCA shape is smeared and modified by therandom association of electron tracks with VTX.The smearing effect was studied by embeddingsimulated electron tracks into real events. Then,we found that the unphysically large DCA tailin the data was explained by the smearing effect.

The current analysis of Au+Au data will be com-pleted for publication soon. We took 20 billion Au+Audata in year 2014, which is 20 times larger statistics.This new dataset will be analyzed. The alignment cal-ibration of the dataset was completed6) and the dataproduction was started.

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Fig. 1. Fraction of heavy flavor electrons to inclusive elec-

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References1) A. Adare et al: Phy. Rev. Lett. 98, 172301 (2007).2) R. Akimoto et al.: RIKEN Accel. Prog. Rep. 46, 60

(2013).3) A. Adare et al.: Quark Matter 2014 poster.4) G. Choudalakis.: arXiv:1201.4612v45) H. Asano at al.: In this report.6) T. Moon at al.: In this report.

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