用羊八井ASγ实验数据寻找TeV能区的γ射线暴

γ射线暴的TeV能区辐射对研究其起源、辐射机制等是非常重要的.利用西藏羊八井ASγ实验三期阵列的重建数据,通过在给定的小天区和时间间隔内寻找较高显著性事例团的方法对TeV能区的γ射线暴进行了寻找,在计算过程中采用“等天顶角法”来估计背景.工作中采用了两种途径来寻找γ射线暴,一种是与卫星γ射线暴的符合寻找,另一种是全天区独立寻找.结果发现少量事例团对背景有明显超出,考虑试验次数后,其超出还不足以认定为γ射线暴.通过Monte Carlo模拟,给出了在95%置信水平下,到达大气顶部流强上限的估计值为3.32×10^-9—1.24×10^-7 cm^-2s^-1. 关键词： γ射线暴 TeV能区 ASγ实验 宇宙射线     

寻找10TeV能区的γ射线暴

利用西藏羊八井大气簇射实验数据,开展了对10TeV能区γ暴的全天区搜寻. 分析了4亿个簇射事例,找出在给定的时间间隔和给定的小天区内出现的簇射事例团. 采用等天顶角方法来估计背景. 有少量事例团显示了对背景的超出,但它们的显著性还不够高,尚不足以认定为γ暴. 讨论了羊八井二期阵列实验对寻找10TeVγ暴所具有的高灵敏度.     

用羊八井加密阵列数据寻找3TeV能区γ暴的初步结果

报道了用 1 996年 1 0月完成的羊八井 EAS加密阵列探测器 ( Tibet 和 HD)于 1 997- 1 998年进行的全天区观测数据 ,对 3Te Vγ暴进行寻找的结果 .发现了一批显著性明显超过 3σ、少数显著性超过 4σ的事例团. A search for γ ray bursts at 3 TeV energy region was done by using the data of Tibet HD air shower array, which was obtained in the period of Febuary 1997-June 1998. About 2.5×10 9 EAS events were analysed to search for the shower clusters appearing in a fixed time intervial and small sky bin. An equal zenith angle method is used to estimate the background and three different time intervals(Δ t =1, 10, 100 s) were adopted in the research. A part of clusters show excess over the...     

Prospects of Bc Mesons at the Compact Muon Solenoid

The results of two Monte Carlo generators for production of Bc mesons in pp collisions are compared at the large hadron collider energy √s = 14 TeV. In the study, the produced Bc mesons were forced to decay into the final states Bslv, Bsπ, J/ψlv, and J/ψπ. We have estimated that about 104 decays of Bc are expected to be recorded during the first year of the CMS running with a lepton trigger. Many Bc studies can be carried out by using these events with a statistical error at the level of 10^-2.     

ARGO实验探测γ暴的灵敏度研究

西藏羊八井ARGO实验是对广延大气簇射事例进行观测研究的"全覆盖式"地面宇宙线观测实验, 其主要目的之一就是探测E>10GeV的γ暴. 通过Monte Carlo模拟, 估算了ARGO实验探测10GeV γ暴所具有的灵敏度.     

An updated search of steady TeV γ-ray point sources in northern hemisphere using the Tibet air shower array

Using the data taken from Tibet II High Density (HD) Array (1997 February—1999 September) and Tibet-III array (1999 November—2005 November), our previous northern sky survey for TeV γ-ray point sources has now been updated by a factor of 2.8 improved statistics. From 0.0° to 60.0° in declination (Dec) range, no new TeV γ-ray point sources with sufficiently high significance were identified while the well-known Crab Nebula and Mrk421 remain to be the brightest TeV γ-ray sources within the field of view of the Tibet air shower array. Based on the currently available data and at the 90% confidence level (C.L.), the flux upper limits for different power law index assumption are re-derived, which are approximately improved by 1.7 times as compared with our previous reported limits.     

Study of neutron spectra in a water bath from a Pb target irradiated by 250 MeV protons

Spallation neutrons were produced by the irradiation of Pb with 250 MeV protons. The Pb target was surrounded by water which was used to slow down the emitted neutrons. The moderated neutrons in the water bath were measured by using the resonance detectors of Au, Mn and In with a cadmium (Cd) cover. According to the measured activities of the foils, the neutron flux at different resonance energies were deduced and the epithermal neutron spectra were proposed. Corresponding results calculated with the Monte Carlo code MCNPX were compared with the experimental data to check the validity of the code. The comparison showed that the simulation could give a good prediction for the neutron spectra above 50 eV, while the finite thickness of the foils greatly effected the experimental data in low energy. It was also found that the resonance detectors themselves had great impact on the simulated energy spectra.     

An updated search of steady TeV γ-ray point sources in northern hemisphere using the Tibet air shower array

Using the data taken from Tibet Ⅱ High Density (HD) Array (1997 February--1999 September) and Tibet-Ⅲ array (1999 November--2005 November), our previous northern sky survey for TeV γ-ray point sources has now been updated by a factor of 2.8 improved statistics. From 0.0° to 60.0° in declination (Dec) range, no new TeV γ-ray point sources with sufficiently high significance were identified while the well-known Crab Nebula and Mrk421 remain to be the brightest TeV T-ray sources within the field of view of the Tibet air shower array. Based on the currently available data and at the 90% confidence level (C.L.), the flux upper limits for different power law index assumption are re-derived, which are approximately improved by 1.7 times as compared with our previous reported limits.     

TeV γ 暴搜寻中最优窗口角半径的探讨

作为搜寻TeVγ暴最主要的观测手段—广延大气簇射阵列,其阵列角分辨值常用于确定搜寻窗口的大小.为了在一个合理大小的窗口内对TeVγ暴进行搜寻,本文从理论上推导出一个统计意义上的最优窗口角半径,然后用Monte Carlo方法进行了模拟验证.结合入射信号强度、背景强度、计算显著性的不同方法等因素,进一步对窗口角半径进行了讨论.并在小统计情况下,给出了计算显著性的经验公式.     

Study of neutron activation yields in spallation reaction of 400 MeV/u carbon on a thick lead target

The spallation-neutron yield was studied experimentally by bombarding a thick lead target with 400 MeV/u carbon beam. The data were obtained with the activation analysis method using foils of Au, Mn, Al, Fe and In. The yields of produced isotopes were deduced by analyzing the measured γ spectra of the irradiated foils. According to the isotopes yields, the spatial and energy distributions of the neutron field were discussed. The experimental results were compared with Monte Carlo simulations performed by the GEANT4+FLUKA code.     

Spectral properties of Fermi/GBM gamma-ray bursts and the GeV emission detection rate with Fermi/LAT

With a sample of 58 Fermi/GBM GRBs detected before 2009 May, we compare the spectral properties of GBM GRBs with those detected by CGRO/BTASE and HETE-2. Our results show that the spectral index distributions are very consistent with those observed by BATSE. However, the E _p distribution is quite different from that observed with BATSE and HETE-2. The GBM GRBs tend to be softer than the BATSE sample, but harder than the HETE-2 sample. This may be due to the instrumental selection effects and artificial sample effect on the BATSE sample. The distribution of the pseudo redshifts derived from the luminosity indicator based on the Amati-relation shows rough consistency with the spectroscopic redshifts of Swift GRBs. We estimate the detection rate of GBM GRBs with LAT based on the observed spectrum in the GBM band, and the inferred burst ratio of LAT detection with over 5 photons to GBM detection is 6%, yielding a detection rate pf 12 GRBs/yr with over 5 photons in the 1–300 GeV band. This is roughly consistent with the results in the first half year of Fermi operation. The low detection rate compared with theoretical predictions is a key for revealing the radiation mechanisms and particle acceleration of the prompt gamma-rays.     

Properties of intermediate GRBs subset,according to the GBM,BAT, and BATSE data

The duration of gamma ray bursts (GRBs) is usually characterized by time interval t ₉₀, in which the total number of registered counts grows from 5 to 95%. Classes of short and long GRBs were first detected in analyzing the BATSE experiment data from the Compton Gamma Ray Observatory (CGRO); burst duration separation point was found to be t _90lim ~2 s. A group of bursts of intermediate duration was first detected in analyzing the data of the same experiment in 1999 in the interval of ~1 to ~40 s with an average event duration of 〈t ₉₀〉 ~ 3.5 s. The results from analyzing the catalog of gamma-ray burst data selected while ground processing BATSE data (i.e., the catalog of nontriggered events) showed that the intensity of intermediate bursts is lower than that of short and long bursts. Preliminary results from investigating the GBM catalog (onboard the Fermi Space Observatory) and the BAT catalog (onboard the Swift satellite) confirm the detection of events with similar properties.     

Search for strong gravitational lensing effect in the current GRB data of BATSE

Because gamma-ray bursts(GRBs)trace the high-z universe,there is an appreciable probability for a GRB to be gravitational lensed by galaxies in the universe.Herein we consider the gravitational lensing effect of GRBs contributed by the dark matter halos in galaxies.Assuming that all halos have the singular isothermal sphere(SIS)mass profile in the mass range 1010h?1M?M2×1013h?1M?and all GRB samples follow the intrinsic redshift distribution and luminosity function derived from the Swift LGRBs sample,we calculated the gravitational lensing probability in BATSE,Swift/BAT and Fermi/GBM GRBs,respectively.With an derived probability result in BATSE GRBs,we searched for lensed GRB pairs in the BATSE5B GRB Spectral catalog.The search did not find any convincing gravitationally lensed events.We discuss our result and future observations for GRB lensing observation.     

Search for dijet resonances in 7 TeV pp collisions at CMS

A search for narrow resonances in the dijet mass spectrum is performed using data corresponding to an integrated luminosity of 2.9 pb?1 collected by the CMS experiment at the Large Hadron Collider. Upper limits at the 95% confidence level are presented on the product of the resonance cross section, branching fraction into dijets, and acceptance, separately for decays into quark-quark, quark-gluon, or gluon-gluon pairs. The data exclude new particles predicted in the following models at the 95% confidence level: string resonances, with mass less than 2.50 TeV, excited quarks, with mass less than 1.58 TeV, and axigluons, colorons, and E6 diquarks, in specific mass intervals. This extends previously published limits on these models.     

Velocity variation of internal shock waves in gamma ray bursts: Observational properties

The work uses the data in the TTS mode of BATSE to analyze the time lags and pulse widths of GRB960113 and GRB960722 in high as well as low energy bands. The results show that their time lags increase monotonously. This phenomenon can reasonably be interpreted with the model of internal shock waves of γ-ray bursts (GRB). Perhaps we obtain the direct observational evidence for the fireball model of GRBs for the first time.     

Ultra-high energy cosmic rays and prompt TeV gamma rays from gamma ray bursts

Gamma ray bursts (GRBs) have been proposed as one possible class of sources of the ultrahigh energy cosmic ray (UHECR) events observed up to energies ≳ 10²⁰ eV. The synchrotron radiation of the highest energy protons accelerated within the GRB source should produce gamma rays up to TeV energies. Here we briefly discuss the implications on the energetics of the GRB from the point of view of the detectability of the prompt TeV γ-rays of proton-synchrotron origin in GRBs in the up-coming ICECUBE muon detector in the south pole.     

Search for quark compositeness with the dijet centrality ratio in pp collisions at √s=7 TeV

A search for quark compositeness in the form of quark contact interactions, based on hadronic jet pairs (dijets) produced in proton-proton collisions at √s=7 TeV, is described. The data sample of the study corresponds to an integrated luminosity of 2.9 pb(-1) collected with the CMS detector at the LHC. The dijet centrality ratio, which quantifies the angular distribution of the dijets, is measured as a function of the invariant mass of the dijet system and is found to agree with the predictions of the standard model. A statistical analysis of the data provides a lower limit on the energy scale of quark contact interactions. The sensitivity of the analysis is such that the expected limit is 2.9 TeV; because the observed value of the centrality ratio at high invariant mass is below the expectation, the observed limit is 4.0 TeV at the 95% confidence level.     

Search for Kaluza-Klein graviton emission in pp collisions at square root[s] = 1.8 TeV using the missing energy signature

We report on a search for direct Kaluza-Klein graviton production in a data sample of 84 pb(-1) of ppmacr; collisions at sqrt[s]=1.8 TeV, recorded by the Collider Detector at Fermilab. We investigate the final state of large missing transverse energy and one or two high energy jets. We compare the data with the predictions from a (3+1+n)-dimensional Kaluza-Klein scenario in which gravity becomes strong at the TeV scale. At 95% confidence level (C.L.) for n=2, 4, and 6 we exclude an effective Planck scale below 1.0, 0.77, and 0.71 TeV, respectively.