A Monte Carlo model for ‘jet quenching’

We have developed the Monte Carlo simulation program Jewel 1.0 (Jet Evolution With Energy Loss), which interfaces a perturbative final-state parton shower with medium effects occurring in ultra-relativistic heavy-ion collisions. This is done by comparing for each jet fragment the probability of further perturbative splitting with the density-dependent probability of scattering with the medium. A simple hadronisation mechanism is included. In the absence of medium effects, we validate Jewel against a set of benchmark jet measurements. For elastic interactions with the medium, we characterise not only the medium-induced modification of the jet, but also the jet-induced modification of the medium. Our main physical result is the observation that collisional and radiative medium modifications lead to characteristic differences in the jet fragmentation pattern, which persist above a soft background cut. We argue that this should allow one to disentangle collisional and radiative parton energy loss mechanisms by measuring the n-jet fraction or a class of jet shape observables.     

Out of medium fragmentation from long-lived jet showers

We study the time structure of vacuum jet evolution via a simple uncertainty principle estimate in the kinematic range explored by current heavy ion collisions at the LHC. We observe that a large fraction of the partonic splittings occur at large times, of the order of several fm. We compare the time distribution of vacuum splittings with the distribution of path lengths traversed by jets in a heavy ion collision. We find that if no medium induced modification of the jet dynamics were present, a very large fraction (larger than 80% for inclusive jets) of the jet splittings would occur outside of the medium. We confront this observation with current available data on jet properties in heavy ion collisions and discuss its implications for the dynamics of jet–medium interactions.     

Electromagnetic showers in crystals at high energies

The development of an electromagnetic shower in crystals at high energies has been studied. It is shown that coherence effects in radiation and e⁺e^--pair production can make the shower develop in a crystal over an appreciably shorter length than in an amorphous medium. The possibility of photon shower development in a crystal has been demonstrated.     

Mach cone induced by γ-triggered jets in high-energy heavy-ion collisions

Medium excitation by jet shower propagation inside a quark-gluon plasma is studied within a linear Boltzmann transport and a multiphase transport model. Contrary to the naive expectation, it is the deflection of both the jet shower and the Mach-cone-like excitation in an expanding medium that is found to give rise to a double-peak azimuthal particle distribution with respect to the initial jet direction. Such a deflection is the strongest for hadron-triggered jets which are often produced close to the surface of a dense medium due to trigger bias and travel against or tangential to the radial flow. Without such trigger bias, the effect of deflection on γ-jet showers and their medium excitation is weaker. Comparative study of hadron and γ-triggered particle correlations can therefore reveal the dynamics of jet-induced medium excitation in high-energy heavy-ion collisions.     

Multiple collisions and induced gluon bremsstrahlung in QCD

Induced soft gluon bremsstrahlung associated with multiple collisions is calculated via perturbative QCD. We derive the non-abelian analog of the Landau-Pomeranchuk effect that suppresses induced soft radiation with formation times exceeding the mean free path. The dependence of the suppression effect on the SU(N) representation of the jet parton as well as the kinematic variables is expressed through a radiation formation factor. The soft radiation with k < μ, where μ is the infrared screening scale in the medium, is shown to lead to an approximately constant radiative energy loss per unit length.     

Jet hadrochemistry as a characteristic of jet quenching

Jets produced in nucleus–nucleus collisions at the LHC are expected to be strongly modified due to the interaction of the parton shower with the dense QCD matter. Here, we point out that jet quenching can leave signatures not only in the longitudinal and transverse jet energy and multiplicity distributions, but also in the hadrochemical composition of the jet fragments. In particular, we show that even in the absence of medium-effects at or after hadronization, the medium-modification of the parton shower may result in significant changes in jet hadrochemistry. We discuss how jet hadrochemistry can be studied within the high-multiplicity environment of nucleus–nucleus collisions at the LHC.     

Jet charge in high-energy nuclear collisions

The averaged jet charge characterizes the electric charge of the initiating parton and provides a powerful tool to distinguish quark jets from gluon jets.We predict,for the first time,the medium modification of the averaged jet charge in the heavy-ion collisions at the LHC,where jet productions in p+p collisions are simulated by PYTHIA6,and the parton energy loss in QGP is calculated with two Monte Carlo models of jet quenching:PYQUEN and JEWEL.We found that the distribution of averaged jet charge is significantly suppressed by initial state isospin effects due to the participation of neutrons with zero electric charge during nuclear collisions.The considerable enhancement of the averaged jet charge in central Pb+Pb collisions is observed relative to peripheral collisions,since the jet quenching effect is more pronounced in central collisions.The distinct feature of the averaged jet charge between quark and gluon jets,along with the sensitivity of medium modifications on the jet charge to flavor dependence of the parton energy loss,could be very useful to discriminate the energy loss pattern between quark and gluon jets in heavy-ion collisions.     

On the interpretation of the results of the experimental test of the Askar’yan effect

The effect of the environment on the results of measuring the radio radiation spectrum is considered in the experimental simulation of the cascade shower by a high-energy γ-ray beam in a dense medium. The calculation shows that the character of the measured energy spectrum depends on the location of the receiving antenna with respect to the shower axis.     

Photon radiation and dilepton production induced by rescattering in strong interacting medium

Using the opacity expansion technique we investigate the photon radiation and dilepton production induced by multiple rescattering as an energetic parton jet passing through the strong interacting medium. The real photon radiation and dilepton invariant-mass spectra of the bremsstrahlung contribution from an energetic quark jet are presented. The leading contribution of total energy loss by photon emission in the medium of a high energetic quark jet is found to be proportional to the jet energy and has a linear dependence on the thickness of the nuclear target. The rescattering contribution to the dilepton production is important only when the pair has a small invariant mass and the jet has relatively low energy. The contribution fraction of the dilepton induced by rescattering in medium is found to be nearly a constant when the ratio of the jet energy to Debye screening mass E/μ is large.     

Q-PYTHIA: a medium-modified implementation of final state radiation

We present a Monte Carlo implementation of medium-induced gluon radiation in the final-state branching process. Medium effects are introduced through an additive term in the splitting functions. We have implemented such modification within PYTHIA. We show the medium effects on the hump-backed plateau, and the transverse-momentum and angular distributions with respect to the parent parton. As expected, with increasing medium densities there is an increase (decrease) of partons with small (large) momentum fraction, and angular broadening is observed. The effects on the transverse-momentum distributions are more involved, with an enhancement of low- and intermediate-p _T partons and a decrease at large p _T, which is related to energy conservation, and to the lack of momentum exchange with the medium in our approach.     

Beam-quality measurement of a focused high-order harmonic beam

Using a multilayer spherical mirror, we focus the high-order harmonic radiation produced near 55 nm by the nonlinear interaction of an intense femtosecond laser pulse and a xenon gas jet. The focused XUV beam is characterized by a knife-edge technique in the focal region and by far-field imaging. We show that good-quality beams, nearly two times diffraction limited, can be generated, a conclusion that is at variance with recent predictions of harmonic phase-front distortion. Spot sizes close to 10 microm are obtained, resulting in a high XUV intensity. Increasing the gas density and the length of the generating medium results in a large increase in the divergence in and degradation of the beam quality.     

Measuring the collective flow with jets

In nucleus-nucleus collisions, high-p(T) partons interact with a dense medium, which possesses strong collective flow components. Here, we demonstrate that the resulting medium-induced gluon radiation does not depend solely on the energy density of the medium, but also on the collective flow. Both components can be disentangled by the measurement of particle production associated with high-p(T) trigger particles, jetlike correlations, and jets. In particular, we show that flow effects lead to a characteristic breaking of the rotational symmetry of the average jet energy and jet multiplicity distribution in the eta x phi plane. We argue that data on the medium-induced broadening of jetlike particle correlations in Au + Au collisions at the Relativistic Heavy-Ion Collider may provide evidence for a significant distortion of parton fragmentation due to the longitudinal collective flow.     

Implementation of a medium-modified parton shower algorithm

We present a Monte Carlo implementation of medium-induced gluon radiation in the final-state branching process. Medium effects are introduced through an additive term in the splitting functions. We have implemented such modification within PYTHIA. We show the medium effects on the hump-backed plateau, and the transverse-momentum and angular distributions with respect to the parent parton. As expected, with increasing medium densities there is an increase (decrease) of partons with small (large) momentum fraction, and angular broadening is observed. The effects on the transverse-momentum distributions are more involved, with an enhancement of low- and intermediate-p _T partons and a decrease at large p _T, which is related to energy conservation, and to the lack of momentum exchange with the medium in our approach.     

Jet physics at the LHC with ALICE

In central Pb-Pb collisions at the LHC, jet rates are expected to be high at energies at which ALICE can reconstruct jets over the background of the underlying event. This will open the possibility to quantify the effect of partonic energy loss through medium induced gluon radiation, jet quenching, by detailed measurement of the modification of the longitudinal and transverse structure of identified jets. In order to obtain probes sensitive to the properties of the QCD medium, it is mandatory to measure the high- parton fragments together with the low- particles from the radiated gluons. Hence, the excellent charged particle tracking capabilities of ALICE combined with the proposed electromagnetic calorimeter for ALICE, EMCAL, represent an ideal tool for jet quenching studies at the LHC. Electronic Supplementary Material Supplementary material is available for this article at PACS: 25.75.Nq, 24.85. + p, 13.87.-a     

强作用介质诱导的光子辐射和双轻子产生

考察了相对论性高能重离子碰撞中产生的硬部分子喷注穿过强作用介质时,喷注与介质中的部分子多次散射诱导的光子辐射与双轻子产生,得到了对应于opacity展开第一阶的光子横动量谱,辐射光子导致的喷注的能量损失以及双轻子的不变质量谱,结果表明,光子的产生率随横动量的增加而降低,双轻子的产生率随其不变质量的增加而减小,辐射光子导致的能量损失线性依赖强作用介质靶的厚度。     

Salt neutrino detector for ultrahigh-energy neutrinos

Rock salt and limestone are studied to determine their suitability for use as a radio-wave transmission medium in an ultrahigh energy (UHE) cosmic neutrino detector. A sensible radio wave would be emitted by the coherent Cherenkov radiation from negative excess charges inside an electromagnetic shower upon interaction of a UHE neutrino in a high-density medium (Askar’yan effect). If the attenuation length for the radio wave in the material is large, a relatively small number of radio-wave sensors could detect the interaction occurring in the massive material. We measured the complex permittivity of the rock salt and limestone by the perturbed cavity resonator method at 9.4 and 1 GHz to good precision. We obtained new results of measurements at the frequency at 1.0 GHz. The measured value of the radio-wave attenuation length of synthetic rock salt samples is 1080 m. The samples from the Hockley salt mine in the United States show attenuation length of 180 m at 1 GHz, and then we estimate it by extrapolation to be as long as 900 m at 200 MHz. The results show that there is a possibility of utilizing natural massive deposits of rock salt for a UHE neutrino detector. A salt neutrino detector with a size of 2×2×2 km would detect 10 UHE neutrino/yr generated through the GZK process.     

常压射流等离子体发射光谱研究

使用改进介质阻挡放电装置生成常压射流等离子体,采用光纤光栅光谱仪在300～1 000 nm范围记录了不同放电电压的氩气发射光谱,并比较了空气和氩气常压介质阻挡放电等离子体发射光谱,分析发现氩气发射光谱中的谱线都是氩原子的发射谱线,表明常压射流装置产生的等离子体全部为氩等离子体,而无其他空气成分参与放电。为测量电子激发温度,选用相距较近的763.51和772.42 nm两条光谱线对电子温度进行分析,结果表明电子激发温度的范围在0.1～0.3 eV,而且它还随着放电电压的增加而增加。初步使用“红外测温仪”测量被处理材料表面温度,结果发现材料表面的温度也随着放电电压的增加而增加,范围在50～100 ℃,材料表面温度的变化趋势可以近似表征等离子体宏观温度变化趋势。通过分析常压射流等离子体的温度特性,探讨了常压射流等离子体温度对材料改性研究的意义。     

Formation of a reflected wave under propagation of a pulse with high peak intensity through a Kerr medium

Propagation through a Kerr medium of short pulses depending only on the longitudinal coordinate is investigated. High values of the peak intensity are considered for which the nonlinear part of the relative permittivity is on the order of unity. When a short pulse propagates through such a medium, the leading edge of the pulse is extended, while the trailing edge runs into the slowly propagating central part of the pulse; shock waves are generated at the trailing edge, giving rise to high spatial frequencies and backward reflected radiation. The duration of the pulse increases due to the high-frequency jet that forms at the trailing edge, and the peak intensity decreases. The spectrum of the backward reflected radiation is investigated as a function of the peak intensity of the pulse and the characteristics of the time dispersion of the medium.     

Cold Nuclear Matter Effects on Isolated Prompt Photon and Isolated Prompt Photon+Jet Productions in Relativistic Heavy-Ion Collisions

We investigate the cold nuclear matter(CNM) effects on isolated prompt photon and isolated prompt photon associated jet productions in nuclear collisions at the NLO accuracy by using the EPS09 NLO nuclear parton distribution functions and their error sets.Nuclear modification factors of isolated prompt photon and isolated prompt photon+jet productions due to CNM effects in p+A and A+A reactions at the RHIC and the LHC are provided with varying rapidity and transverse momentum of the final state photon.It is shown that the CNM effects on isolated prompt photon and photon+jet are modest,which give a small enhancement at low pT region and a more obvious suppression at large pT at central rapidity.At forward rapidity a pronounced suppression of γ as well as γ+jet is always observed.