Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 48 (2015)

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,∗2and the PHENIX FVTX group
Protons and neutrons, which are the components offamiliar substances, are composed of quarks and gluons that bind quarks together. Immediately following
the big bang, under extremely high-density and hightemperature conditions, quarks and gluons are considered 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 Heavy
Ion Collider (RHIC). In particular, in Au+Au and
Pb+Pb collisions, the particle angular correlation exhibits a similar azimuthal pattern throughout the wide
range of the rapidity region, referred to as ʠ ridge. ʡThe ridge correlation is considered as a consequenceof the hydrodynamic ﬂow of products, and it is interpreted as a characteristic of QGP. Certain experiments
at the LHC have recently reported that the ridge wasobserved especially in high-multiplicity events in small
colliding systems, such as p+p.1) Similar ridge phenomena were observed in d+Au and He3+Au at RHIC.
However, they have not yet been observed in p+p.
Previous experiments demonstrated high-multiplicity
events are the key to observe the ridge in p+p collisions. In PHENIX, the Forward Silicon Vertex Detector (FVTX) is suitable to select such high-multiplicity
events 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 azimuthal
slice of the FVTX sensor, each FEM judges whether
there is any track, following which the FEM-IB counts

Fig. 1. FVTX trigger design based on the FPGA logic
implemented on the readout electronics
∗1
∗2
∗3

the number of track ﬂag sent from the FEM, and ﬁnally the FVTX trigger ﬁres when the number of track
is 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 tested
through evaluation of the process eﬃciency depending on the number of hits on the detector. The hits
were generated by using a calibration pulse that injects typically 13 strips per detector per event. Fig.
2 shows the correlation between the number of hits
per event measured from oﬄine data analysis and thetrigger state; the vertical axis shows the trigger ﬂag
at an FEM that is supposed to give 1 for the events
where the number of hits is greater than the threshold.
The eﬃciency, deﬁned as the number of trigger events
divided by the number of events with more hits than
the trigger threshold, was 94%, and a few fake triggers
were observed.
New serial line cables for FEM - FEM-IB communication were installed in the trigger system, and theirsignal transmission test was performed. Performance
was monitored by scanning the signal rate, and the cables exhibited good performance up to a trigger rate of
4.7 MHz. The timing of the trigger signal was tuned
with GL1. It was found that GL1 does not recognize
the trigger signal depending on timing. It is adjusted
using the BBC trigger signal, which is already used asa trigger.
References
1) V. Khachatryan et al.: CMS Collaboration, JHEP

1009 (2010) 091
2) arXiv:1311.3594v2 [physics.ins-det] 14 Feb. 2014

RIKEN Nishina Center
Department of Physics, Rikkyo University
J-Parc- 115 -完全版2014_本文.indd 115

15/10/16 17:46