arXiv subjects

P. Chang

Publications and source records attributed to P. Chang.

288 records · Page 16Linked to original sources

arXiv2008

Study of B-> phi phi K Decays

We report an observation of the decay B^\pm -> ϕϕK^\pm and evidence for B^0 -> ϕϕK^0. These results are based on a 414 fb^{-1} data sample collected with the Belle detector at the KEKB asymmetric-energy e^+e^- collider operating at the Υ(4S) resonance. The branching fractions for these decay modes are measured to be Br(B^{\pm} -> ϕϕK^\pm) = (3.2^{+0.6}_{-0.5} +- 0.3) * 10^{-6} and Br(B^{0} \to ϕϕK^{0}) = (2.3^{+1.0}_{-0.7} +- 0.2) * 10^{-6} for ϕϕinvariant mass below 2.85 GeV/c^2. The corresponding partial rate asymmetry for the charged B mode is measured to be A_{CP}(B^\pm -> ϕϕK^\pm) = 0.01^{+0.19}_{-0.16} +- 0.02. We also study the decays B^\pm -> J/ψK^\pm and B^\pm -> η_c K^\pm, where the J/ψand η_c decay to final states with four charged kaons. We find A_{CP}(B^\pm -> ϕϕK^\pm) with the ϕϕcandidates within the η_c mass region is 0.15^{+0.16}_{-0.17} +- 0.02, consistent with no asymmetry.

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arXiv2007

Long Term Evolution of Magnetic Turbulence in Relativistic Collisionless Shocks: Electron-Positron Plasmas

We study the long term evolution of magnetic fields generated by a collisionless relativistic $e^+e^-$ shock which is initially unmagnetized. Our 2D particle-in-cell numerical simulations show that downstream of such a Weibel-mediated shock, particle distributions are close to isotropic, relativistic Maxwellians, and the magnetic turbulence is highly intermittent spatially, with the non-propagating magnetic fields forming relatively isolated regions with transverse dimension $\sim 10-20$ skin depths. These structures decay in amplitude, with little sign of downstream merging. The fields start with magnetic energy density $\sim (0.1-0.2)$ of the upstream kinetic energy within the shock transition, but rapid downstream decay drives the fields to much smaller values, below $10^{-3}$ of equipartition after $10^3$ skin depths. In an attempt to construct a theory that follows field decay to these smaller values, we explore the hypothesis that the observed damping is a variant of Landau damping in an unmagnetized plasma. The model is based on the small value of the downstream magnetic energy density, which suggests that particle orbits are only weakly perturbed from straight line motion, if the turbulence is homogeneous. Using linear kinetic theory applied to electromagnetic fields in an isotropic, relativistic Maxwellian plasma, we find a simple analytic form for the damping rates, $γ_k$, in two and three dimensions for small amplitude, subluminous electromagnetic fields. We find that magnetic energy does damp due to phase mixing of current carrying particles as $(ω_p t)^{-q}$ with $q \sim 1$. (abridged)

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arXiv2007

The Origin of the Young Stars in the Nucleus of M31

The triple nucleus of M31 consists of a population of old red stars in an eccentric disk (P1 and P2) and another population of younger A stars in a circular disk (P3) around M31's central supermassive black hole (SMBH). We argue that P1 and P2 determine the maximal radial extent of the younger A star population and provide the gas that fueled the starburst that generated P3. The eccentric stellar disk creates an $m=1$ non-axisymmetric perturbation to the potential. This perturbed potential drives gas into the inner parsec around the SMBH, if the pattern speed of the eccentric stellar disk is $Ω_p \lesssim 3-10 {\rm km s^{-1} pc^{-1}}$. We show that stellar mass loss from P1 and P2 is sufficient to create a gravitationally unstable gaseous disk of $\sim 10^5\Msun$ every $0.1-1$ Gyrs, consistent with the 200 Myr age of P3. Similar processes may act in other systems to produce very compact nuclear starbursts.

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arXiv2007

Search for B^0-> p p-bar, Lambda Lambda-bar and B^+ -> p Lambda-bar at Belle

We report on a new search for two-body baryonic decays of the B meson. Improved sensitivity compared to previous Belle results is obtained from 414 fb^-1 of data that corresponds to 449 million B B-bar pairs, which were taken on the Upsilon(4S) resonance and collected with the Belle detector at the KEKB e^+e^- collider. No significant signals are observed and we set the 90% confidence level upper limits: Br(B^0-> p pbar) <1.1X10^-7, Br(B^0-> Lambda Lambda-bar) < 3.2X10^-7 and Br(B^+-> p Lambda-bar) < 3.2X10^-7.

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arXiv2007

Improved measurements of branching fractions for B -> K pi and B -> pi pi decays

We report improved measurements of branching fractions for B -> K pi and B -> pi pi decays based on a data sample of 449 million B Bbar pairs collected at the Upsilon(4S) resonance with the Belle detector at the KEKB e^+e^- collider. We also calculate the ratios of partial widths for the decays B -> K pi and pi pi, namely R_c = 1.08 +- 0.06 +- 0.08 and R_n = 1.08 +-0.08 +0.09-0.08, where the first and second errors are statistical and systematical, respectively. These ratios are sensitive to enhanced electroweak penguin contributions from new physics; the new measurements are, however, consistent with Standard Model expectations.

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arXiv2007

Observation of B Decays to Two Kaons

Using 449 million BBbar pairs collected with the Belle detector at the KEKB asymmetric-energy e^+e^- collider, we observe clear signals for B^+-> \bar{K}^0 K^+ and B^0-> \bar{K}^0 K^0 decays with 5.3 sigma and 6.0 sigma significance, respectively. We measure the branching fractions Br(B^+-> \bar{K}^0 K^+ = (1.22^{+0.32+0.13}_{-0.28-0.16})x10^{-6} and Br(B^0-> \bar{K}^0 K^0) = (0.87^{+0.25}_{-0.20} +- 0.09)x10^{-6}, and partial-rate asymmetries A_{CP}(B^+-> \bar{K}^0 K^+) = 0.13^{+0.23}_{-0.24}+- 0.02 and A_{CP}(B^0-> \bar{K}^0 K^0) = -0.58^{+0.73}_{-0.66}+- 0.04. From a simultaneous fit we also obtain Br(B^+-> K^0π^+)= (22.8^{+0.8}_{-0.7} +- 1.3)x 10^{-6} and A_{CP}(B^+-> K^0 π^+) = 0.03 +- 0.03 +- 0.01. The first and second error in the branching fractions and the partial-rate asymmetries are statistical and systematic, respectively. No signal is observed for B^0-> K^+ K^- decays, and for this branching fraction we set an upper limit of 4.1x10^{-7} at the 90% confidence level.

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arXiv2007

From Thin to Thick: The Impact of X-ray Irradiation on Accretion Disks in AGN

We argue that the X-ray and UV flux illuminating the parsec-scale accretion disk around luminous active galactic nuclei (AGN) is super-Eddington with respect to the local far-infrared dust opacity. The far infrared opacity may be larger than in the interstellar medium of the Milky Way due to a combination of supersolar metallicity and the growth of dust grains in the dense accretion disk. Because of the irradiating flux, the outer accretion disk puffs up with a vertical thickness $h\sim R$. This provides a mechanism for generating a geometrically thick obscuring region from an intrinsically thin disk. We find obscuring columns $\sim 10^{22} - 10^{23} {\rm cm}^{-2}$, in reasonable agreement with observations.

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arXiv2007

Improved Measurements of Branching Fractions and CP Asymmetries in B-> eta h Decays

We report improved measurements of B decays with an eta meson in the final state using 492 fb^{-1} of data collected by the Belle detector at the KEKB e^+ e^- collider. We observe the decays B^{+-}-> eta pi^{+-} and B^{+-}-> eta K^{+-}; the measured branching fractions are Br(B^{+-}-> eta pi^{+-}) = (4.2+- 0.4(stat)+- 0.2(sys))x10^{-6} and Br(B^{+-}-> eta K^{+-}) = (1.9+-0.3(stat)^{+0.2}_{-0.1}(sys))x10^{-6}. The corresponding CP-violating asymmetries are measured to be -0.23+- 0.09(stat) +-0.02(sys) for eta pi^{+-} and -0.39+-0.16(stat)+-0.03(sys) for eta K^{+-}. We also search for B^0-> eta K^0 decays and set an upper limit of 1.9x10^{-6} at the 90% confidence level.

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arXiv2006

Improved measurements of branching fractions and CP partial rate asymmetries for $B\toωK$ and $B\toωπ$

We report improved measurements of $B$ to pseudoscalar-vector decays containing an $ω$ meson in the final states. Our results are obtained from a data sample that contains $388\times 10^{6}$ $B\bar{B}$ pairs accumulated at the $Υ(4S)$ resonance, with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We measure the following branching fractions: ${\mathcal B}(B^{+} \to ωK^{+}) = [8.1\pm0.6({\rm stat.})\pm{0.6}({\rm syst.})]\times 10^{-6}$, ${\mathcal B}(B^{+} \to ωπ^{+}) = [6.9\pm{0.6}({\rm stat.})\pm{0.5}({\rm syst.})]\times 10^{-6}$, and ${\mathcal B}(B^{0} \to ωK^{0}) = [4.4^{+0.8}_{-0.7}({\rm stat.})\pm{0.4}({\rm syst.})]\times 10^{-6}.$ The partial width ratio $\frac{Γ(B^{+}\to ωK^{+})}{Γ(B^{0}\to ωK^{0})}$ = $1.8\pm0.4({\rm stat.})\pm0.1({\rm sys.})$. We also set the 90% confidence level upper limit ${\mathcal B}(B^{0} \to ωπ^{0}) < 2.0\times 10^{-6}.$ In addition, we obtain the partial rate asymmetries ${\mathcal A}_{CP} = 0.05^{+0.08}_{-0.07}(\rm {stat.}) \pm 0.01(\rm {syst.})$ for $B^{+} \to ωK^{+}$, and ${\mathcal A}_{CP} = -0.02 \pm 0.09(\rm {stat.}) \pm 0.01(\rm {syst.})$ for $B^{+}\to ωπ^{+}$.

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arXiv2006

Averages of b-hadron Properties at the End of 2005

This article reports world averages for measurements on b-hadron properties obtained by the Heavy Flavor Averaging Group (HFAG) using the available results as of at the end of 2005. In the averaging, the input parameters used in the various analyses are adjusted (rescaled) to common values, and all known correlations are taken into account. The averages include lifetimes, neutral meson mixing parameters, parameters of semileptonic decays, branching fractions of B meson decays to final states with open charm, charmonium and no charm, and measurements related to CP asymmetries.

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arXiv2005

Rotational Broadening of Atomic Spectral Features from Neutron Stars

The discovery of the first gravitationally redshifted spectral line from a neutron star (NS) by Cottam, Paerels and Mendez has triggered theoretical studies of the physics of atomic line formation in NS atmospheres. Chang, Bildsten and Wasserman showed that the hydrogenic Fe H$α$ line formed above the photosphere of a bursting NS is intrinsically broad. We now include rotational broadening within general relativity and compare the resulting profile to that observed during Type I bursts from EXO 0748-676. We show that the fine structure splitting of the line precludes a meaningful constraint on the radius. Our fitting of the data show that the line forming Fe column is ${\rm log}_{10} (N_{\rm Fe, n=2}/{\rm cm^{-2}})=17.9_{-0.42}^{+0.27}$ and gravitational redshift $1+z =1.345_{-0.008}^{+0.005}$ with 95% confidence. We calculate the detectability of this spectral feature for a large range of spins and inclinations assuming that the emission comes from the entire surface. We find that at 300 (600) Hz only 10-20% (5-10%) of NSs would have spectral features as deep as that seen in EXO 0748-676.

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arXiv2005

Formation of Resonant Atomic Lines during Thermonuclear Flashes on Neutron Stars

Motivated by the measurement of redshifted Fe H$α$ lines during type I X-ray bursts on EXO 0748-676 (Cottam, Paerels & Mendez), we study the formation of atomic Fe lines above the photosphere of a bursting neutron star ($k_BT_{\rm eff} \approx 1-2 {\rm keV}$). We discuss the effects of Stark broadening, resonant scattering and NLTE (level population) on the formation of hydrogenic Fe H$α$, Ly$α$ and P$α$ lines. From the observed equivalent width of the Fe H$α$ line, we find an implied Fe column of $1-3 \times 10^{20} {\rm cm}^{-2}$, which is 3-10 times larger than the Fe column calculated from the accretion/spallation model of Bildsten, Chang & Paerels. We also estimate that the implied Fe column is about a factor of 2-3 larger than a uniform solar metallicity atmosphere. We discuss the effects of rotational broadening and find that the rotation rate of \EXO must be slow, as confirmed by the recent measurement of a 45 Hz burst oscillation by Villarreal & Strohmayer. We also show that the Fe Ly$α$ EW $\approx$ 15-20 eV (redshifted 11-15 eV) and the P$α$ EW $\approx$ 4-7 eV (redshifted 3-5 eV) when the H$α$ EW is 10 eV (redshifted 8 eV). The Ly$α$ line is rotationally broadened to a depth of $\approx 10%$, making it difficult to observe with {\it Chandra}. We also show that radiative levitation can likely support the Fe column needed to explain the line.

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arXiv2004

Hydrogen Burning on Magnetar Surfaces

We compute the rate of diffusive nuclear burning for hydrogen on the surface of a "magnetar" (Soft Gamma-Ray Repeater or Anomalous X-Ray Pulsar). We find that hydrogen at the photosphere will be burned on an extremely rapid timescale of hours to years, depending on composition of the underlying material. Improving on our previous studies, we explore the effect of a maximally thick "inert" helium layer, previously thought to slow down the burning rate. Since hydrogen diffuses faster in helium than through heavier elements, we find this helium buffer actually increases the burning rate for magnetars. We compute simple analytic scalings of the burning rate with temperature and magnetic field for a range of core temperature. We conclude that magnetar photospheres are very unlikely to contain hydrogen. This motivates theoretical work on heavy element atmospheres that are needed to measure effective temperature from the observed thermal emission and constrains models of AXPs that rely on magnetar cooling through thick light element envelopes.

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arXiv2004

Improved Measurements of Partial Rate Asymmetry in B -> h h Decays

We report improved measurements of the partial rate asymmetry (Acp) in B -> h h decays with 140fb^-1 of data collected with the Belle detector at the KEKB e+e- collider. Here h stands for a charged or neutral pion or kaon and in total five decay modes are included: K-+ pi+-, K0s pi-+, K-+ pi0, pi-+ pi0 and K0s pi0. The flavor of the last decay mode is determined from the accompanying B meson. Using a data sample 4.7 times larger than that of our previous measurement, we find Acp(K-+ pi+-) -0.088+-0.035+-0.013, 2.4 sigma from zero. Results for other decay modes are also presented.

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arXiv2004

Observation of the Decays B0->K+pi-pi0 and B0->rho-K+

We report the observation of B^0 decays to the K^+pi^-pi^0 final state using a data sample of 78 fb^-1 collected by the Belle detector at the KEKB e^+e^- collider. With no assumptions about intermediate states in the decay, the branching fraction is measured to be (36.6^{+4.2}_{-4.3}+- 3.0)*10^-6.We also search for B decays to intermediate two-body states with the same K^+pi^-pi^0 final state. Significant B signals are observed in the rho(770)^- K^+ and K^*(892)^+pi^- channels, with branching fractions of (15.1^{+3.4+1.4+2.0}_{-3.3-1.5-2.1})* 10^-6 and (14.8^{+4.6+1.5+2.4}_{-4.4-1.0-0.9})* 10^-6, respectively. The first error is statistical, the second is systematic and the third is due to the largest possible interference. Contributions from other possible two-body states will be discussed. No CP asymmetry is found in the inclusive K^+pi^-pi^0 or rho^-K^+ modes, and we set 90% confidence level bounds on the asymmetry of -0.12<A_{CP}<0.26 and -0.18<A_{CP}<0.64, respectively.

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arXiv2003

Evolution of Young Neutron Star Envelopes

We extend our initial study of diffusive nuclear burning (DNB) for neutron stars (NSs) with Hydrogen atmospheres and an underlying layer of proton capturing nuclei. Our initial study showed that DNB can alter the photospheric abundance of Hydrogen on surprisingly short timescales ($10^{2-4}\yrs$). Significant composition evolution impacts the radiated thermal spectrum from the NS as well as its overall cooling rate. In this paper, we consider the case when the rate limiting step for the H consumption is diffusion to the burning layer, rather than the local nuclear timescale. This is relevant for NSs with surface temperatures in excess of $10^6 {\rm K}$, such as young ($<10^5$ yr) radio pulsars and accreting NSs in quiescence. When downward diffusion is the limiting rate in DNB, the rate of H consumption is suppressed by 1-2 orders of magnitude compared to a DNB estimate that assumes diffusive equilibrium. In order to apply our ongoing study to young neutron stars, we also include the important effects of strong magnetic fields ($B \sim 10^{12} {\rm G}$). In this initial study of magnetic modifications to DNB, we find that the H burning time is lengthened by 2-3 orders of magnitude for a $10^{12} {\rm G}$ field. However, even for NSs with dipole field strengths of $10^{12}$ G, we find that all of the H can be burned before the pulsar reaches an age of $\sim 10^5 \ {\rm yr}$, thus potentially revealing the underlying proton-capturing elements. Finally, we conclude by providing an overview of what can be learned about fallback and pulsar winds from measuring the surface composition of a young NS.

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arXiv2003

Atomic Spectral Features During Thermonuclear Flashes on Neutron Stars

The gravitational redshift measured by Cottam, Paerels and Mendez for the neutron star (NS) in the low-mass X-ray binary EXO 0748-676 depends on the identification of an absorption line during a type I burst as the H$α$ line from hydrogenic Fe. We show that Fe is present above the photosphere as long as $\dot M>4\times 10^{-13}M_\odot {\rm yr^{-1}}$ during the burst. In this limit, the total Fe column is $N_{\rm Fe}\approx 3\times 10^{19}{\rm cm^{-2}}$ for incident material of solar abundances and only depends on the nuclear physics of the proton spallation. The Fe destruction creates many heavy elements with $Z<26$ which may imprint photo-ionization edges on the NS spectra during a radius expansion event or in a burst cooling tail. Detecting these features in concert with those from Fe would confirm a redshift measurement. We also begin to address the radiative transfer problem, and find that a concentrated Fe layer with $kT=1.2-1.4 {\rm keV}$ and column $N_{\rm Fe}= 7-20 \times 10^{20} {\rm cm}^{-2}$ (depending on the line depth) above the hotter continuum photosphere is required to create the H$α$ line of the observed strength. This estimate must be refined by considerations of non-LTE effects as well as resonant line transport. Until these are carried out, we cannot say whether the Fe column from accretion and spallation is in conflict with the observations. We also show that hydrogenic Fe might remain in the photosphere due to radiative levitation from the high burst flux.

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arXiv2002

Diffusive Nuclear Burning on Neutron Star Envelopes

We calculate the rate of hydrogen burning for neutron stars (NSs) with hydrogen atmospheres and an underlying reservoir of nuclei capable of proton capture. This burning occurs in the exponentially suppressed diffusive tail of H that extends to the hotter depths of the envelope where protons are rapidly captured. This process, which we call diffusive nuclear burning (DNB), can change the H abundance at the NS photosphere on timescales as short as $10^{2-4}$ years. In the absence of diffusion, the hydrogen at the photosphere (where $T\approx 10^6 {\rm K}$ and $ρ\sim 0.1 {\rm g cm^{-2}}$) would last for far longer than a Hubble time. Our work impacts the understanding of the evolution of surface abundances of isolated NSs, which is important to their thermal spectrum and their effective temperature-core temperature relation. In this paper, we calculate the rate of H burning when the overall consumption rate is controlled by the nuclear timescales, rather than diffusion timescales. The immediate application is for H burning on millisecond radio pulsars and in quiescence for the accreting NS Cen X-4. We will apply this work to young radio pulsars and magnetars once we have incorporated the effects of strong $B>10^{12} {\rm G}$ magnetic fields.

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