The Case for a Low Extragalactic Gamma-ray Background

Measurements of the diffuse extragalactic gamma-ray background (EGRB) are complicated by a strong Galactic foreground. Estimates of the EGRB flux and spectrum, obtained by modeling the Galactic emission, have produced a variety of (sometimes conflicting) results. The latest analysis of the EGRET data found an isotropic flux I_x=1.45+-0.05 above 100 MeV, in units of 10^-5 s^-1 cm^-2 sr^-1. We analyze the EGRET data in search for robust constraints on the EGRB flux, finding the gamma-ray sky strongly dominated by Galactic foreground even at high latitudes, with no conclusive evidence for an additional isotropic component. The gamma-ray intensity measured towards the Galactic poles is similar to or lower than previous estimates of I_x. The high latitude profile of the gamma-ray data is disk-like for 40<|b[deg]|<70, and even steeper for |b|>70; overall it exhibits strong Galactic features and is well fit by a simple Galactic model. Based on the |b|>40 data we find that I_x<0.5 at a 99% confidence level, with evidence for a much lower flux. We show that correlations with Galactic tracers, previously used to identify the Galactic foreground and estimate I_x, are not satisfactory; the results depend on the tracers used and on the part of the sky examined, because the Galactic emission is not linear in the Galactic tracers and exhibits spectral variations across the sky. The low EGRB flux favored by our analysis places stringent limits on extragalactic scenarios involving gamma-ray emission, such as radiation from blazars, intergalactic shocks and production of ultra-high energy cosmic rays and neutrinos. We suggest methods by which future gamma-ray missions such as GLAST and AGILE could indirectly identify the EGRB.Comment: Accepted for publication in JCAP. Increased sizes of polar regions examined, and added discussion of spectral data. Results unchange

Semi-Inclusive Lambda and Kshort Production in p-Au Collisions at 17.5 GeV/c

The first detailed measurements of the centrality dependence of strangeness production in p-A collisions are presented. Lambda and Kshort dn/dy distributions from 17.5 GeV/c p-Au collisions are shown as a function of "grey" track multiplicity and the estimated number of collisions, nu, made by the proton. The nu dependence of the Lambda yield deviates from a scaling of p-p data by the number of participants, increasing faster than this scaling for nu<=5 and saturating for larger nu. A slower growth in Kshort multiplicity with nu is observed, consistent with a weaker nu dependence of K-Kbar production than Y-K production.Comment: 5 pages, 3 figures, formatted with RevTex, current version has enlarged figure catpion

Why do UK banks securitize?

Working paper seriesThe eight years from 2000 to 2008 saw a rapid growth in the use of securitization by UK banks. We aim to identify the reasons that contributed to this rapid growth. The time period (2000 to 2010) covered by our study is noteworthy as it covers the pre- nancial crisis credit- boom, the peak of the nancial crisis and its aftermath. In the wake of the nancial crisis, many governments, regulators and political commentators have pointed an accusing nger at the securitization market - even in the absence of a detailed statistical and economic analysis. We contribute to the extant literature by performing such an analysis on UK banks, fo- cussing principally on whether it is the need for liquidity (i.e. the funding of their balance sheets), or the desire to engage in regulatory capital arbitrage or the need for credit risk trans- fer that has led to UK banks securitizing their assets. We show that securitization has been signi cantly driven by liquidity reasons. In addition, we observe a positive link between securitization and banks credit risk. We interpret these latter ndings as evidence that UK banks which engaged in securitization did so, in part, to transfer credit risk and that, in comparison to UK banks which did not use securitization, they had more credit risk to transfer in the sense that they originated lower quality loans and held lower quality assets. We show that banks which issued more asset-backed securities before the nancial crisis su¤ered more defaults after the nancial crisis.The eight years from 2000 to 2008 saw a rapid growth in the use of securitization by UK banks. We aim to identify the reasons that contributed to this rapid growth. The time period (2000 to 2010) covered by our study is noteworthy as it covers the pre-financial crisis credit- boom, the peak of the financial crisis and its aftermath. In the wake of the financial crisis, many governments, regulators and political commentators have pointed an accusing finger at the securitization market - even in the absence of a detailed statistical and economic analysis. We contribute to the extant literature by performing such an analysis on UK banks, fo- cussing principally on whether it is the need for liquidity (i.e. the funding of their balance sheets), or the desire to engage in regulatory capital arbitrage or the need for credit risk trans- fer that has led to UK banks securitizing their assets. We show that securitization has been significantly driven by liquidity reasons. In addition, we observe a positive link between securitization and banks credit risk. We interpret these latter findings as evidence that UK banks which engaged in securitization did so, in part, to transfer credit risk and that, in comparison to UK banks which did not use securitization, they had more credit risk to transfer in the sense that they originated lower quality loans and held lower quality assets. We show that banks which issued more asset-backed securities before the financial crisis suffered more defaults after the financial crisis