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Abhilasha Saini Research scholar(Physics) Suresh gyan Vihar university Email: kashvini.abhi@gmail.com
Abstract The quantum chromo-dynamics (QCD) explains the state of the quarks and gluons with which this universe is composed of, in the form of hadrons. The QCD calculations performed on the lattice (LQCD) suggests that at very high temperature and density a phase transition from hadron matter into plasma of quarks and gluons takes place and famously called Quark-Gluon plasma (QGP), where quarks and gluons are deconfined [1]. These ultra-relativistic heavy ion collisions can be studied
experimentally on earth and the one way which provides the opportunity to create and study the QGP is Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL). Here the heavy nuclei are accelerated and collided at high energies for the purpose of exploration of such high energy physics and some of the most basic phenomena of universe.
The Relativistic Heavy-Ion Collider
The Relativistic Heavy-Ion Collider is set up at Brookhaven National Laboratory (BNL), New York. It is the first establishment which enables physicists to collide heavy ions primarily of gold, one of the heaviest elements and to look back in the time to see the behavior of the matter at the start of the universe. The collider is able to produce the ions ranging from A=1(protons) to A=197(gold) and has a facility to collide them with centre-of-mass energies per nucleon pair (√ ) up to 200 GeV for Au+Au and 500GeV for p+p collisions [2].
The acceleration and ionization in collision takes place a number of stages before the complete ionization of nuclei and the process achieves the maximum collision
energy. The figure given below ( Fig. 1) elaborates the relativistic heavy ion collider’s different parts and accelerators. For Au+Au collisions, firstly the pulses sputter ion source produces the ions in the charge state-1and then they are made to be accelerated through Tandem Van de Graaff accelerator by electrostatic method. Then the stripping foils allow the ions to go through them achieve the energy of 1MeV/u and charge state of+32 while leaving the Tandem. To get the higher acceleration of the order of 95 MeV the ions pass through the Booster synchrotron. As the ions come out of booster a foil strips the ions of all but two of their remaining electrons. In the next step the ions further get the entry to the Alternating Gradient Synchrotron
(AGS) where they speedup to be accelerated upto 10.8 GeV/u. A last foil to pass through is kept to strip of last two electrons to get the +79 charge state, after exit of ions from AGS. Now the ions are ready to travel down the AGS to RHIC (AtR) beam line from where they are sent to the RHIC accelerator. In RHIC there are two counter-circulating superconducting magnet rings, of circumference 3.8 km, which intersect in at 6 points for the purpose of collisions. The currents are allowed to their top values to ramp up the magnets for duration around two minutes, to accelerate the ions to their maximum collision energy. In the process of RHIC run, ions pass through in packets or bunches inside the rings and typically every ring contains 56 bunches. Approximately 109 ions are present in each bunch. For an average running time of 10 hours at 100GeV/u the expected luminosity is ∼ 2 × 1026−2−1. The production of collisions at RHIC started in 2000, after that a large number of collisions over a wide range of energy and various systems have taken place. The collisions with Au+Au for √ upto 200GeV have been logged through PHOBOS experiment, during 2000 to 2005. The other collisions for which data have been collected are deuteron-gold collisions (d+Au), copper-copper collisions (Cu+Cu), and polarized proton (p+p) collisions at different energies.
Figure 1: The RHIC accelerator complex at Brookhaven National Laboratory and the locations of the four experiments on the RHIC ring [2].
RHIC Experiments
There are four sites at RHIC facility which experiments and study the relativistic heavy ion collisions. Actually RHIC endows six beam intersection domains out of which only four employ heavy ion experiments. The two largest experiments are STAR and PHENIX and in addition to these other two are small experiments named as PHOBOS and BRAHMS. Every detector is specialized in measuring different aspects and features of the collisions, also able to provide complimentary measurements of different variables. There are four sites located at the RHIC ring. The ring is very similar and resembles
to a clock face, like the uppermost point indicating at the 12 o’clock position. In the same manner, Brahms at 2 o’clock, STAR at 6 o’clock, PHENIX at 8 o’clock, and PHOBOS at 10 o’clock are featured.
PHOBOS
PHOBOS detector is among one of the experiments at RHIC and is fabricated and designed to experiment and analyze the collision between large numbers of unselected gold ions [3]. The detector is able to provide a global picture and after effects for each Au+Au collision and even the detailed knowledge of the small fragments shower out from the hot and dense region. The interaction region is surrounded with many silicon detectors can be said as silicon multiplicity array. The advantage is that the produced particle number and their angular momentum distribution can be achieved which enables one to get good azimuthal and longitudinal coverage along the beam axis.
Figure 2: The PHOBOS detector [3]
This array of detectors is also a way to capture the fluctuation in the number of particles and in angular distribution. For the purpose of detailed information about these events and for the identification of particles and their momentum, a high quality magnetic spectrometer is also incorporated. This spectrometer is very helpful in studying and analyzing for about 1% of particles ejected. It can provide the accurate measurement of temperature, size and density of the fire ball created during collision process also the estimation of various particle ratios. It made the study and detection of transition phase between ordinary and QGP state easy. The PHOBOS is additionally consisting of a chain of plastic scintillator detectors for triggering, and two time-of-flight walls [4]. In the fast detectors the time and energy information is used by the collision triggers to access whether a particular event is beneficial and should be logged for consideration. After getting the signal in several detectors they are recorded and processed accordingly, further the vertex position and fractional cross-section are estimated. PHOBOS experiment is having a peerless feature of surveying the charged particles at very low transverse momenta (< 100MeV).The knowledge about the integrated event reconstru
ction assents further choice on the subservience of recorded collisions for multifold physics analyses.
BRAHMS
Another detector at RHIC ring is BRAHMS (Broad RAnge Hadron Magnetic Spectrometers) [5]. The purpose of designing of this detector is the study and measurement of charged hadrons over a
wide rapidity and transverse momentum region for understanding the reaction mechanism in highly excited matter state. Two spectrometer arms are there, one of which is kept in the mid-rapidity region and the other one in the forward region. Among all the RHIC experiments, BRAHMS possesses the largest rapidity coverage for particle identification as the two arms can be revolved around the beam axis, along the polar angle. This is the special feature of BRAHMS that the spectrometer arms can estimate large ranged rapidity distributions of identified particles, mainly the net-proton content of the collision frame as a function of rapidity. The experiment provided its first data during 2000.
Figure 3: The BRAHMS experiment detector [5].
BRAHMS also includes beam-beam counters, a multiplicity array, and zerodegree calorimeters (ZDCs) the common triggering detectors as all other RHIC detectors are having. The ZDCs estimate the overall energy of spectator neutrons in a greatly limited cone round the beam axis so that they can be utilized for collision centrality measurements and a minimum bias trigger in peripheral events is procured. An estimation of the event vertex can be obtained by the time difference in approaching the spectator neutrons and reaching the two arrays. Fig. 3 shows the BRAHMS detector with the major subdetectors labeled. One of the exclusive aspect BRAHMS has that it can discern and cognize charged particles up to very forward rapidities (y~ 3.5) and high transverse momenta because of its Ring Imaging Cherenkov (RICH) detector [6].
STAR (Solenoidal tracker)
Unlike the BRAMAS and PHOBOS, the Solenoid Tracker i.e. STAR is one of the large experiments at RHIC [7]. The special characteristic is the tracking of large number of particles produced in each ion collision. It’s very heavy detector weighing 1200 tons capable of searching the signature of QGP form of matter. The important segment and the heart of the detector is a 4π timeprojection chamber (TPC) situated in a magnetic field. The TPC chamber identifies and tracks the particles emerging from heavy ion collisions in the mid-rapidity region. The forward rapidity region is also taken under tracking with the use of two end cap forward TPCs. It also has a silicon vertex tracker, electromagnetic calorimeters, and time-of-flight detectors. The STAR experimental system compliance distends successively in a large range around mid-rapidity (|η| ≤ 1.8 with TPC only) and
has full azimuthal extension and that’s why making it exceptionally well suited for event by- event characterization of heavy ion collisions. The multi-strange hyperons can also be gauged with its silicon pixel tracker (SVT).All these detectors work coherently to acquire advanced data and subsequent analysis in detecting certain types of particles and characterizing their motion to give final statement about the collision.
Figure 4: The STAR detector [7].
PHENIX
The abbreviation can be elaborated as Pioneering High Energy Nuclear Interaction experiment (PHENIX) which is another large experiment at RHIC [8]. This is able to record different particles ejected in RHIC like photons, electrons, muons and hadrons.
Fig. 5 shows the PHENIX detector and its major components. As it is known that photons and leptons do not interact via the strong nuclear force and can emerge unaffected from the interior of heavy ion collision and carry the unaltered information about the process. This detector is focused in estimating these direct probes of the collision. The PHENIX detector also comprises two arms in the mid-rapidity region and surrounded with a magnetic field. It is massive and weights 4000 tons with a dozen
Figure 5: The PHENIX detector [8].
detectors and subsystem. The magnetic field is provided by three large steel magnets which compel charged particles to move along the curved path. The main parts included are drift chamber, pad chambers, ring imaging Cherenkov detector, time expansion chamber, time-of-flight detector, and electromagnetic calorimeter. The forward muon arms with muon tracking chambers and muon identifiers also kept in the magnetic field. From the mentioned parts the tracking chambers helps in determining each particle momentum by recording hits along the flight path to trace the curvature. Other detectors find the particle species and measure the particle energy.
Summary: The nuclear collisions performed at RHIC and other colliders provide the exclusive opportunity of approaching experimentally towards the confined state of matter i.e. the QGP. Before the experimentation in the field of high energy physics at RHIC the only source to explore high energy physics was the detection of cosmic rays events. RHIC utilizes its four different types of detectors each of which is specialized in detecting various characters and features of heavy ion collision. Here the heavy ion collisions are performed under controlled environment to have the advantage of studying the hot and dense matter. These collider experiments are vastly beneficial and challenging in formulating the veritable theories of multi body systems of quark and gluon and QGP and the further exploration
in this field is going on to achieve new challenges.
References: [1] W. Weise, in Quarks and Nuclei, Int. Rev. of Nucl. Phys. 1 (World Scientific, 1984) ed. W. Weise, p. 57.
[2] http://www.bnl.gov/rhic/
[3] http://www.phenix.bnl.gov/, B.B. Back et al., PHOBOS Collaboration, Nucl.Instr.Meth. A499 (2003) 603
[4] R. Bindel, E. Garcia, A. C. Mignerey, and L. P. Remsberg. Array of scintillator counters for PHOBOS at RHIC. Nucl. Instrum. Meth., A474:38-45, 2001.
[5] http://www4.rcf.bnl.gov/brahms/WWW/, M.Adamczyk et al., BRAHMS Collaboration, Nucl.Instr.Meth. A499 (2003) 437
[6] R. Debbe, C.E. Jorgensen, J. Olness, Z. Yin, Nucl.Instr.Meth. A570 (2007) 216
[7] http://www.star.bnl.gov/, K.H. Ackermann et al., STAR Collaboration, Nucl.Instr.Meth. A499 (2003) 624
[8] http://www.phenix.bnl.gov/, K. Adcox et al., PHENIX Collaboration, Nucl.Instr.Meth. A499 (2003) 469