Zero Degree Calorimeters Research Articles

Two-particle azimuthal correlations in photonuclear ultraperipheral Pb+Pb collisions at 5.02 TeV with ATLAS

Two-particle long-range azimuthal correlations are measured in photonuclear collisions using 1.7 nb$^{-1}$ of 5.02 TeV Pb+Pb collision data collected by the ATLAS experiment at the LHC. Candidate events are selected using a dedicated high-multiplicity photonuclear event trigger, a combination of information from the zero-degree calorimeters and forward calorimeters, and from pseudorapidity gaps constructed using calorimeter energy clusters and charged-particle tracks. Distributions of event properties are compared between data and Monte Carlo simulations of photonuclear processes. Two-particle correlation functions are formed using charged-particle tracks in the selected events, and a template-fitting method is employed to subtract the non-flow contribution to the correlation. Significant nonzero values of the second- and third-order flow coefficients are observed and presented as a function of charged-particle multiplicity and transverse momentum. The results are compared with flow coefficients obtained in proton-proton and proton-lead collisions in similar multiplicity ranges, and with theoretical expectations. The unique initial conditions present in this measurement provide a new way to probe the origin of the collective signatures previously observed only in hadronic collisions.

Performance of the CMS Zero Degree Calorimeters in pPb collisions at the LHC

The two Zero Degree Calorimeters (ZDCs) of the CMS experiment are located at ± 140 m from the collision point and detect neutral particles in the |η| > 8.3 pseudorapidity region. This paper presents a study on the performance of the ZDC in the 2016 pPb run. The response of the detectors to ultrarelativistic neutrons is studied using in-depth Monte Carlo simulations. A method of signal extraction based on template fits is presented, along with a dedicated calibration procedure. A deconvolution technique for the correction of overlapping collision events is discussed.

Search for CME in U+U and Au+Au collisions in STAR with different approaches of handling backgrounds

The chiral magnetic effect (CME) refers to charge separation along a strong magnetic field between left- and right-handed quarks, caused by interactions with topological gluon fields from QCD vacuum fluctuations. We present two approaches to handle the dominant elliptic flow (v2) background in the three-particle correlator (Δγ112), sensitive to CME.In the first approach, we present the Δγ112 and Δγ123 measurements in U+U and Au+Au collisions. While hydrodynamic simulations including resonance decays and local charge conservation predict that Δγ112 scaled by Npart/ν2 will be similar in U+U and Au+Au collisions, the projected B-field exhibits a distinct difference between the two systems and with varying Npart. Therefore, U+U and Au+Au collisions provide configurations with different expectations for both CME signal and background. Moreover, the three-particle observable Δγ123 scaled by Npart/ν3 provide baseline measurement for only the background.In the second approach, we handle the ν2 background by measuring Δγ112 with respect to the planes of spectators measured by Zero Degree Calorimeters and participants measured by Time Projection Chamber. These measurements contain different amounts of contributions from CME signal (along B-field, due to spectators) and ν2 background (determined by the participant geometry). With the two Δγ112 measurements, the possible CME signal and the background contribution can be determined. We report such a measurement in Au+Au collisions at sNN=27GeV with the newly installed event plane detector, and report the new findings in U+U system where the spectator-participant plane correlations are expected to differ from those in Au+Au collisions.

Methods for centrality determination in nucleus-nucleus collisions with forward hadron calorimeters at the BM@N experiment

The BM@N (Baryonic Matter at Nuclotron) is the fixed target experiment at the Nuclotron (JINR, Dubna, Russia) accelerator complex. The main goal of the BM@N experiment is studying properties of dense nuclear matter produced in the ion-ion collisions at beam energies up to 4.5 AGeV. First experiments were done for carbon, argon and krypton ions beams with C, Al, Cu and Pb targets. The methods of central events selection on event-by-event base with ZDC (Zero Degree Calorimeter) for these reactions are presented. The new Forward Hadron Calorimeter (FHCal) with transverse and longitudinal segmentation will be used to measure the collision centrality in heavy ion experiments after the BM@N upgrade. The FHCal has the beam hole in the center due to expected high beam intensities. New forward quartz hodoscope is been developed to be placed in the beam hole to measure the charge of fragments. New methods of centrality determination with the FHCal and forward quartz hodoscope are discussed.

Radiation damage from energetic particles at GRad-level of SiO[formula omitted] fibers of the Large Hadron Collider ATLAS Zero-Degree Calorimeter (ZDC)

Core SiO2 quartz fibers of the Large Hadron Collider (LHC) ATLAS Zero-degree Calorimeter (ZDC) are expected to experience integrated doses of a few giga-Rad (Grad) at their closest position to the LHC beam. An array of fibers was irradiated with 200 MeV protons and spallation-generated mixed spectra (primarily fast neutrons) at the Brookhaven National Laboratory (BNL) Linac. Specifically, 1 mm- and 2 mm-diameter quartz (GE 124) rods of 50 mm length were exposed to direct 200 MeV protons leading to peak integrated dose of ∼28 Grad (∼0.28 GGy). Exposure of 1 mm-diameter SiO2 fibers to a neutron flux was also achieved in the spallation field generated by 128 MeV protons. In the post-irradiation analysis, the quartz fiber transmittance was evaluated as a function of the absorbed dose. Significant degradation of the transmittance and increased radiation damage of the material were observed. Microscopic evaluation of the fibers revealed extensive micro-structural damage and irradiation-induced defects. The measurements revealed that a threshold fluence (∼2.6 1016 p/cm2) or dose of ∼10 Grad (0.1 GGy) appears to exist beyond which light transmittance drops below 10%. Also observed is that fiber transmittance loss increased drastically with SiO2 fiber diameter (1 mm vs. 2 mm diameter). This is attributed, in part, to the earlier lateral leakage from the 1 mm fiber of knock-on electrons and primary protons implying that more damage-inducing protons travel within the bulk of the 50 mm long 2-mm fibers. While Monte Carlo simulations performed tend to support such assumption, future experiments and sensitivity studies are envisioned to address the fiber diameter influence on degradation.

Coherent photoproduction of \u03c10 vector mesons in ultra-peripheral Pb-Pb collisions at sqrt{{mathrm{s}}_{mathrm{NN}}} = 5.02 TeV

Cross sections for the coherent photoproduction of ρ0 vector mesons in ultra-peripheral Pb-Pb collisions at sqrt{{mathrm{s}}_{mathrm{NN}}} = 5.02 TeV are reported. The measurements, which rely on the π+π− decay channel, are presented in three regions of rapidity covering the range |y| < 0.8. For each rapidity interval, cross sections are shown for different nuclear-breakup classes defined according to the presence of neutrons measured in the zero-degree calorimeters. The results are compared with predictions based on different models of nuclear shadowing. Finally, the observation of a coherently produced resonance-like structure with a mass around 1.7 GeV/c2 and a width of about 140 MeV/c2 is reported and compared with similar observations from other experiments.

Study of Very Forward Neutrons with the CMS Zero Degree Calorimeter

Forward neutrons are studied in proton-lead collisions at the CMS experiment at the CERN LHC. They provide information on the centrality and event plane of collisions and provide an opportunity to study nuclear breakup. At the CMS experiment they are detected by the Zero Degree Calorimeters (ZDCs) in the | η | &gt; 8.5 pseudorapidity range. The ZDCs are quartz fiber Cherenkov calorimeters using tungsten as absorber. Test beam data and events with a single spectator neutron are used for the calibration of these detectors. A Fourier-based method is used correct for the effect of multiple pPb collisions. The corrected ZDC energy distribution is used to calculate centrality percentiles and unfold the neutron multiplicity distribution.

Charged-particle pseudorapidity density at mid-rapidity in p\u2013Pb collisions at pmb {sqrt{s_{scriptscriptstyle {mathrm{NN}}}}} = 8.16 TeV

The pseudorapidity density of charged particles, mathrm {d}N_{mathrm{ch}}/mathrm {d}eta , in p–Pb collisions has been measured at a centre-of-mass energy per nucleon–nucleon pair of sqrt{s_{scriptscriptstyle {mathrm{NN}}}} = 8.16 TeV at mid-pseudorapidity for non-single-diffractive events. The results cover 3.6 units of pseudorapidity, |eta |<1.8. The mathrm {d}N_{mathrm{ch}}/mathrm {d}eta value is 19.1pm 0.7 at |eta |<0.5. This quantity divided by langle N_{mathrm{part}} rangle / 2 is 4.73pm 0.20, where langle N_{mathrm{part}} rangle is the average number of participating nucleons, is 9.5% higher than the corresponding value for p–Pb collisions at sqrt{s_{scriptscriptstyle {mathrm{NN}}}} = 5.02 TeV. Measurements are compared with models based on different mechanisms for particle production. All models agree within uncertainties with data in the Pb-going side, while HIJING overestimates, showing a symmetric behaviour, and EPOS underestimates the p-going side of the mathrm {d}N_{mathrm{ch}}/mathrm {d}eta distribution. Saturation-based models reproduce the distributions well for eta >-1.3. The mathrm {d}N_{mathrm{ch}}/mathrm {d}eta is also measured for different centrality estimators, based both on the charged-particle multiplicity and on the energy deposited in the Zero-Degree Calorimeters. A study of the implications of the large multiplicity fluctuations due to the small number of participants for systems like p–Pb in the centrality calculation for multiplicity-based estimators is discussed, demonstrating the advantages of determining the centrality with energy deposited near beam rapidity.

On the Detection of Multinucleon Events in Nucleus—Nucleus Collisions with Forward Calorimeters (ZDC)

The detection of spectator nucleons emitted forward in the beam direction in experiments on the study of nucleus—nucleus interactions using forward Zero Degree Calorimeters (ZDCs) allows the determination of the event centrality and the detection of the nucleons from electromagnetic dissociation (EMD). In this paper, we propose a simple combinatorial model for calculating the energy releases in the ZDC, taking into account the calorimeter acceptance and efficiency. Using this model, the effect of the limited acceptance and nucleon detection efficiency on the spectral shape in the ZDC and the possibility of measuring the yield of a certain number of nucleons from the EMD is studied at LHC energies.

Performance of the CMS Zero Degree Calorimeters in the 2016 pPb run

Two neutral particle detectors, Zero Degree Calorimeters (ZDCs) at the LHC-CMS experiment, cover the |η| > 8.5 region. The ZDCs are Cherenkov calorimeters that use tungsten as the absorber and quartz clad quartz fibers as the active medium. They have a five element electromagnetic section followed by a hadronic section divided into four depth segments. For the 2016 pPb run, the ZDCs were calibrated using test beam data and the single spectator neutron peak at 2.56 TeV. Peaks corresponding to 1, 2 and 3 neutrons are visible in the ZDC total signal distribution. The effect of pileup is corrected by a Fourier deconvolution method. Using this, the spectator neutron number distribution can be unfolded by a linear regularization method. This information serves as a strong constraint to models of pPb collisions and has the potential to produce an unbiased measure of centrality in pPb collisions.

Performance of the ALICE Zero Degree Calorimeters and upgrade strategy

The Zero Degree Calorimeters (ZDCs) of the ALICE experiment were designed with the twofold purpose of both estimating the centrality in heavy ion collisions by measuring the energy carried away by the spectator nucleons and of measuring the luminosity delivered to the experiment exploiting the high cross sections for neutron emission from electromagnetic dissociation processes. The measurement of centrality has been successfully extended to p–A collisions with the detection of nucleons ejected from the nucleus by the collisions with the projectile proton (“gray” nucleons) and those resulting from de-excitation processes (“black” nucleons). The applications of the detector in triggering and analysis have expanded during the years of operation in the LHC RUN1 and RUN2. These now include both the reaction plane and the longitudinal asymmetry measurements in heavy ion collisions. Moreover the ZDCs are used to reject the parasitic interactions of main bunches with satellite bunches in A–A and p–A collisions and to tag diffractive events in pp collisions. The foreseen operation in RUN3 with the tenfold increase in the luminosity delivered by LHC in heavy ion collisions, together with the continuous acquisition strategy that is being adopted by ALICE, will be challenging for the ZDC readout system. The readout upgrade will be based on FMC digitizers with trigger, timing and charge integration functionality performed through FPGA.

New forward hadron calorimeter for centrality and reaction plane determination at BM@N heavy ion experiments

It is proposed to replace existing Zero Degree Calorimeter of the BM@N setup at the Nuclotron (JINR) by a new forward hadron calorimeter for the measurement of the collision centrality and reaction plane orientation in heavy ions experiments. This calorimeter with transverse and longitudinal segmentation will be assembled from modules presently constructed for FHCal MPD and PSD CBM. The proposed design of the new calorimeter, simulation results for centrality and reaction plane determination by the new calorimeter, as well as radiation doses in the calorimeter simulated by FLUKA are discussed. The performance of hadron calorimeter supermodule studied at CERN is shown.

On the performance of Zero Degree Calorimeters in detecting multinucleon events

The facilities designed to study collisions of relativistic nuclei, such as the MPD at NICA (JINR), STAR at RHIC (BNL), ALICE, ATLAS and CMS at the LHC (CERN), are equipped with pairs of hadronic Zero Degree Calorimeters (ZDC) to detect forward nucleons at the both sides of the interaction point and estimate the collision centrality. The energy deposited in a ZDC fluctuates from one event to another, but on average it is proportional to the number of absorbed nucleons. Forward nucleons are also emitted in electromagnetic dissociation (EMD) of nuclei in ultraperipheral collisions, and they are used to monitor the luminosity. As known, ZDC energy spectra are specific to each facility, because they are affected by the ZDC acceptance, and the ZDC energy resolution depends on the beam energy. In this work a simple probabilistic model leading to handy formulas has been proposed to connect the numbers of emitted and detected forward nucleons taking into account a limited ZDC acceptance. The ZDC energy spectra from the EMD with the emission of one, two, three and four forward neutrons and protons have been modeled for the collision energies of NICA and the LHC. The case of a rather small ZDC acceptance has been investigated and a possibility to measure the inclusive nucleon emission cross section has been demonstrated.

Effects of longitudinal asymmetry in heavy-ion collisions

In collisions of identical nuclei at a given impact parameter, the number of nucleons participating in the overlap region of each nucleus can be unequal due to nuclear density fluctuations. The asymmetry due to the unequal number of participating nucleons, referred to as longitudinal asymmetry, causes a shift in the center of mass rapidity of the participant zone. The information of the event asymmetry allows us to isolate and study the effect of longitudinal asymmetry on rapidity distribution of final state particles. In a Monte Carlo Glauber model the average rapidity-shift is found to be almost linearly related to the asymmetry. Using toy models, as well as Monte Carlo data for Pb-Pb collisions at 2.76 TeV generated with HIJING, two different versions of AMPT and DPMJET models, we demonstrate that the effect of asymmetry on final state rapidity distribution can be quantitatively related to the average rapidity shift via a third-order polynomial with a dominantly linear term. The coefficients of the polynomial are proportional to the rapidity shift with the dependence being sensitive to the details of the rapidity distribution.Experimental estimates of the spectator asymmetry through the measurement of spectator nucleons in a Zero Degree Calorimeter may hence be used to further constrain the initial conditions in ultra-relativistic heavy-ion collisions.

Radiation hardness of Silicon Photomultipliers for CBM@FAIR, NA61@CERN and BM@N experiments

Results of radiation hardness study of Silicon Photomultipliers (SiPMs) to be used at forward hadron calorimeters, including designed Projectile Spectator Detector (PSD) of CBM, PSD of NA61 beyond 2020 upgrade and Zero Degree Calorimeter (ZDC) at BM@N are presented. High intensity beams at future heavy-ion accelerator facilities up to 106∕107 interactions/second will lead to high neutron fluence around calorimeters up to 3×1012 neq/cm2 accumulated during a year of the experiment operation. SiPMs produced by Ketek, Zecotek, Hamamatsu and Sensl manufacturers were irradiated at cyclotron of NPI Rez in a wide range of neutron fluences from 4×1010 up to 6×1012 n/cm2. Change of SiPM’s main parameters after irradiation including dark current, breakdown voltage, noise and signal response to LED was studied in the lab. Furthermore, several packs of irradiated SiPMs were installed at the currently operating NA61 PSD to study the cumulative radiation effect on the calorimeter performance. Beam scan with 20–80 GeV/c protons was performed to determine the dependency of detector energy resolution on beam energy before and after irradiation.

Classifiers for centrality determination in proton-nucleus and nucleus-nucleus collisions

Centrality, as a geometrical property of the collision, is crucial for the physical interpretation of nucleus-nucleus and proton-nucleus experimental data. However, it cannot be directly accessed in event-by-event data analysis. Common methods for centrality estimation in A-A and p-A collisions usually rely on a single detector (either on the signal in zero-degree calorimeters or on the multiplicity in some semi-central rapidity range). In the present work, we made an attempt to develop an approach for centrality determination that is based on machine-learning techniques and utilizes information from several detector subsystems simultaneously. Different event classifiers are suggested and evaluated for their selectivity power in terms of the number of nucleons-participants and the impact parameter of the collision. Finer centrality resolution may allow to reduce impact from so-called volume fluctuations on physical observables being studied in heavy-ion experiments like ALICE at the LHC and fixed target experiment NA61/SHINE on SPS.

Study of nuclear fragmentation at MPD/NICA

Due to the much lower beam energy of NICA compared to the RHIC and LHC hadron colliders and the fixed target experiments at SPS the role and performance of the forward detectors of NICA are quite different. The Neutron Zero Degree Calorimeter could be used for the measurement and monitoring of luminosity, however with lower efficiency of neutron detection produced in ultra-peripheral collisions. The use of Forward Hadron Calorimeter for the determination of centrality is impossible by simply counting the number of spectators because of the ambiguity of the impact parameter dependence. This ambiguity could be removed if the angular distribution of the spectators will be taken into account. It is shown by the simulation with LAQGSM model that the forward multiplicity detector like V0 of ALICE could not be used for the determination of centrality. However it could provide the valuable information on the nuclear fragmentation of heavy ions.

Longitudinal Asymmetry and its Measurable Effects in Pb–Pb Collisions at 2.76 TeV

Collisions of identical nuclei at finite impact parameter have an unequal number of participating nucleons from each nucleus due to fluctuations. The event-by-event fluctuations have been estimated by measuring the difference of energy in the zero-degree calorimeters on either side of interaction vertex. The fluctuations affect the global variables such as the rapidity distributions, and the effect has been correlated with a measure of these fluctuations.

Physics performance of the STAR zero degree calorimeter at relativistic heavy ion collider

The zero degree calorimeter (ZDC) at RHIC-STAR was installed in the year 2000. After running for more than 10 years, the performance of the STAR-ZDC cannot maintain a proper status because of the radiation damage. The ZDC on RHIC-BRAHMS had been moved to STAR in 2011 after some tests. We present here the result of the tests as well as the physical performance of those ZDC modules between the 2011 and 2015 RHIC runs. The excellent energy resolution of the ZDC in heavy ion collision provides a good candidate for future detector development, such as the CSR experiment at CAS-Lanzhou facility.

The First Transverse Single Spin Measurement in High Energy Polarized Proton-Nucleus Collision at the PHENIX experiment at RHIC

Large single spin asymmetries in very forward neutron production seen using the PHENIX zero-degree calorimeters are a long established feature of transversely polarized proton-proton collisions at RHIC. Neutron production near zero degrees is well described by the one-pion exchange framework. The absorptive correction to the OPE generates the asymmetry as a consequence of a phase shift between the spin flip and non-spin flip amplitudes. However, the amplitude predicted by the OPE is too small to explain the large observed asymmetries. A model introducing interference of pion and a1-Reggeon exchanges has been successful in reproducing the experimental data. During the RHIC experiment in year 2015, RHIC delivered polarized proton collisions with Au and Al nuclei for the first time, enabling the exploration of the mechanism of transverse single-spin asymmetries with nuclear collisions. The observed asymmetries showed surprisingly strong A-dependence in the inclusive forward neutron production, while the existing framework which was successfull in p+p only predicts moderate A- dependence. Thus the observed data are absolutely unexpected and unpredicted. In this report, experimental and theoretical efforts are discussed to disentangle the observed A-dependence using somewhat semi-inclusive type measurements and Monte-Carlo study, respectively.