arXiv subjects

V. Ravindran

Publications and source records attributed to V. Ravindran.

140 records · Page 8Linked to original sources

arXiv1998

The Unpolarized Gluon Anomalous Dimension at Small x

We discuss the quantitative consequences of the resummation of the small-x contributions to the anomalous dimensions beyond next-to-leading order in alpha_s and up to next order in ln(1/x) (NLx) in a framework based on the renormalization group equations. We find large and negative effects leading to negative values for the {\sf total} splitting function P_{gg}(x,alpha_s) already for x \lsim 0.01 at Q^2 \simeq 20 GeV^2. Terms less singular than those under consideration turn out to be quantitatively as important and need to be included. We derive the effects of the conformal part of the NLx contributions to the anomalous dimensions and discuss the exponent omega describing the s^omega behavior of inclusive cross sections.

hep-ph↗

arXiv1998

Relations among polarized and unpolarized splitting functions beyond leading order

The role of various symmetries in the evaluation of splitting functions and coefficient functions is discussed. The scale invariance in hard processes is known to be a guiding tool to understand the dynamics. We discuss the constraints on splitting functions coming from various symmetries such as scale, conformal and supersymmetry. We also discuss the Drell-Levy-Yan relation among splitting and coefficient functions in various schemes. The relations coming from conformal symmetry are also presented.

hep-ph↗

arXiv1998

On the Gluon Regge Trajectory in $O(α_s^2)$

We recalculate the gluon Regge trajectory in next-to-leading order to clarify a discrepancy between two results in the literature on the constant part. We confirm the result obtained by Fadin et al.~\cite{FFK}. The effects on the anomalous dimension and on the $s^ω$ behavior of inclusive cross sections are also discussed.

hep-ph↗

arXiv1997

On the charm-squark interpretation of the HERA events

The charm squark resonance in the $e^+ q$ channel provides one of the plausible interpretations of the reported anomaly at HERA. We show that the relevant $R$ violating coupling $λ'_{121}$ is required to be large, typically around 0.1 in a large class of supergravity based models including the minimal one with the universal boundary condition at the GUT scale. Existing constraints on this coupling are reanalysed in this light and it is argued that a large $λ'_{121}$ may be feasible but would require fine tuned cancelations.

hep-ph↗

arXiv1997

Neutrino Mass Constraints on R violation and HERA anomaly

R parity violating trilinear couplings $λ'_{1jk}$ of the minimal supersymmetric standard model (MSSM) are constrained from the limit on the electron neutrino mass. Strong limits on these couplings follow from the earlier neglected contribution due to sneutrino vacuum expectation values. The limits on most of the $λ'_{1jk}$ couplings derived here are stronger than the existing ones for a wide range in parameters of MSSM. These limits strongly constrain the interpretation of recent HERA results in $e^+ p$ scattering in terms of production of squarks through $R$ violating couplings. In particular, the interpretation in terms of $\tilde{t}_L$ production off strange quark as suggested recently is not viable for wide ranges in MSSM parameters.

hep-ph↗

arXiv1996

Collinear Divergences at One-loop Order for External Particles in a Heat-bath

In hard interactions between external particles incident on a heat-bath, we show that large logarithms are generated when a radiated or absorbed gauge boson is collinear with the initial fermion momentum. These logarithms can be absorbed into process independent splitting/absorption probabilities. Unlike the zero-temperature case, however, they depend explicitly on the temperature and the scale of the interaction.

hep-ph↗

arXiv1996

Transverse structure function in the factorisation method

Deep Inelastic scattering experiments using transversely polarised targets yield information on the structure function $g_2$. By means of a free-field analysis, we study the operator structure of $g_2$ and demonstrate the need for retaining the twist three mass terms in order to maintain current-conservation. We show that the structure function $g_T=g_1+g_2$ has a much simpler operator structure as compared to $g_2$, in spite of the fact that, like $g_2$, $g_T$ has a twist-three component. We demonstrate factorisation of the hadronic tensor into hard and soft parts for the case of $g_T$. We show that the first moment of the gluonic contribution to $g_T$ vanishes, and discuss possible physical applications.

hep-ph↗

arXiv1996

The Polarised $Λ$ production in QCD

The $Q^2$ evolution of polarised parton fragmentation functions is discussed using Altarelli-Parisi evolution equations. The first moments of both polarised quark and gluon fragmentation functions are shown to behave in a similar fashion at very high energies. This analysis is applicable to any hard processes involving production of polarised hadrons. The polarised $Λ$ hyperon production in $e^+~e^-$ annihilation where this can be realised is considered. We present a complete $α_s(Q^2)$ corrections to the asymmetries discussed in the paper of Burkardt and Jaffe which demonstrates the extraction of various polarised fragmentation functions. To this order, these corrections are found to be scheme dependent similar to that of structure functions.

hep-ph↗

arXiv1996

Photon Structure Functions: Target Photon Mass Effects and QCD Corrections

We present a systematic analysis of the polarised and unpolarised processes $e^+ ~e^- \rightarrow e^+ ~e^- X$ in the deep inelastic limit and study the effects of target photon mass (virtuality) on the photon structure functions. The effect of target photon virtuality manifests as new singly polarised structure functions and also alters the physical interpretation of the unpolarised structure functions. The physical interpretation of these structure functions in terms of hadronic components is studied using the free field analysis. We also retrieve the real photon results in the limit the virtuality goes to zero. Assuming factorisation of the photon structure tensor, the relevant QCD corrections to the various twist two structure functions are evaluated.

hep-ph↗

arXiv1996

Gluon fragmentation function in polarised $Λ$ hyperon production: The Method of Factorisation

We discuss the polarised fragmentation functions of quarks and gluons in Perturbative Quantum Chromodynamics. The Altarelli-Parisi evolution equations governing these fragmentation functions are presented. We find that the first moment of the polarised gluon fragmentation function to order $α_s$ is as important as that of the quark and anti-quark fragmentation functions. The $Λ$ production in polarised $e^+ e^-$ annihilation where this can be realised is discussed. We propose the factorisation method to compute the gluonic contribution to $Λ$ production to order $α_s$. With appropriate operator definitions for the quark and the gluon fragmentation functions, we find that the hard part of the factorisation formula is free of any infrared singularities confirming the factorisation.

hep-ph↗

arXiv1996

Order $α_s(Q^2)$ QCD corrections to the polarised $e^+~e^- \to Λ~X$

The importance of polarised gluons fragmenting into $Λ$ in the polarised $e^+ e^-$ annihilation is discussed using the Altarelli-Parisi evolution equations satisfied by the quark and gluon fragmentation functions. In this context, the polarised fragmentaion function $\hat g_1^Λ(x,Q^2)$ appearing in the cross section is discussed within the parton model. We relate this fragmentation function to the quark, anti-quark and gluon fragmentation functions and also find the QCD corrections to order $α_s$.

hep-ph↗

arXiv1995

Bloch-Nordsieck Thermometers: One-loop Exponentiation in Finite Temperature QED

We study the scattering of hard external particles in a heat bath in a real-time formalism for finite temperature QED. We investigate the distribution of the 4-momentum difference of initial and final hard particles in a fully covariant manner when the scale of the process, $Q$, is much larger than the temperature, $T$. Our computations are valid for all $T$ subject to this constraint. We exponentiate the leading infra-red term at one-loop order through a resummation of soft (thermal) photon emissions and absorptions. For $T>0$, we find that tensor structures arise which are not present at $T=0$. These carry thermal signatures. As a result, external particles can serve as thermometers introduced into the heat bath. We investigate the phase space origin of $\log(Q/m)$ and $\log(Q/T)$ terms.

hep-ph↗

arXiv1994

Photon Structure Functions: Target Photon Mass Effects and QCD Corrections

We present a systematic analysis of the processes $e^+ ~e^- \rightarrow e^+ ~e^- X$ to study the polarised and unpolarised photon structure functions. The effect of target photon mass, which manifests itself as new singly polarised structure functions is studied. The physical interpretation of these structure functions in terms of hadronic components is given using the free field analysis. Assuming factorisation of the photon structure tensor, the relevant QCD corrections to the various structure functions are evaluated. The effect of the target photon mass and QCD corrections on the unpolarised and polarised cross sections are also studied.

hep-ph↗

arXiv1992

Can Polarised Drell-Yan Shed More Light On The Proton Spin?

We analyse polarised Drell-Yan process using the factorisation method and derive operator definitions for polarised parton distribution functions. We demonstrate that a factorisation analogous to that in the unpolarised Drell-Yan case holds in this process. We study the leading order gluonic contribution to the first moment of polarised Drell-Yan function and show that it is consistent with results obtained from polarised deep inelastic scattering.

hep-ph↗