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K. Sridhar

Publications and source records attributed to K. Sridhar.

At least 73 records · Page 4Linked to original sources
arXiv1999

Issues in Quarkonium Production

In this talk, I start with a brief introduction to Non-Relativistic QCD (NRQCD) and its applications to quarkonium physics. This theory has provided a consistent framework for the physics of quarkonia, in particular, the colour-octet Fock components predicted by NRQCD have important implications for the phenomenology of charmonium production in experiments. The applications of NRQCD to $J/ψ$ production at Tevatron and the tests of the theory in other experiments is discussed. In particular, the apparent disagreement of NRQCD with results from HERA on inelastic photoproduction of $J/ψ$ is discussed and it is shown that the results are rather susceptible to intrinsic transverse momentum smearing. The photoproduction data, therefore, do not provide a good test of NRQCD. It is argued that NRQCD may be tested stringently by looking for the production of other charmonium resonances at the Tevatron, because the production rates for these resonances can be predicted within the NRQCD framework.

hep-ph↗
arXiv1998

eta_c Production at the Tevatron: A test of NRQCD

We use NRQCD to predict the cross section for $η_c$ production at the Tevatron. The non-perturbative matrix elements required are obtained, using heavy-quark symmetry, from the matrix elements determined from the CDF $J/ψ$ data. Our numbers are, therefore, predictions of NRQCD and provide a very good test of it. Even after taking into account the small branching ratio of the $η_c$ into two photons, a substantial rate for the production of this resonance at the Tevatron is expected.

hep-ph↗
arXiv1998

J/psi production at the Tevatron and HERA: the effect of k_T smearing

We study the effects of intrinsic transverse momentum smearing on J/psi production both at the Tevatron and at HERA. For the case of large-p_T J/psi production at the Tevatron, the effects due to k_T smearing are mild. On the other hand, inelastic J/psi photoproduction at HERA is very sensitive to the k_T smearing and, in fact, with a reasonable value of it is possible to resolve the large-z discrepancy seen by comparing non-relativistic QCD (NRQCD) predictions with the HERA data. We conclude that, with the present kinematic cuts, photoproduction at HERA is not a good test of NRQCD.

hep-ph↗
arXiv1997

Aspects of hadronic $J/ψ$ production

In this talk, I start with a brief introduction to Non-Relativistic QCD (NRQCD) and its applications to quarkonium physics. This theory has provided a consistent framework for the physics of quarkonia, in particular, the colour-octet Fock components predicted by NRQCD have important implications for the phenomenology of charmonium production in experiments. We discuss the applications of NRQCD to $J/ψ$ production at Tevatron and the tests of the theory in other experiments. Finally we discuss the production of the ${}^1P_1$ charmonium state, $h_c$, at large-$p_T$ at the Tevatron. The observation of $h_c$ is interesting, since its existence has yet to be experimentally confirmed. Moreover, this rate is a $prediction$ of NRQCD, and the observation of the $h_c$ at the Tevatron can, therefore, be used as a test of NRQCD.

hep-ph↗
arXiv1997

W + 1 jet to W + 0 jet Ratio at the Tevatron: A Hint of New Physics?

We interpret the reported disagreement between the measured ratio, R_{10}, of the W + 1 jet cross-section to the W + 0 jet cross-section at the Tevatron and the Standard Model (SM) prediction, as the effect of interactions mediated by a colour-octet analogue of the W-boson, the W_8. The presence of a W_8 with mass O(300) GeV, and with couplings to quarks and gluons of the order of electroweak strength, allows the observations of the D0 collaboration to be reproduced quite accurately. Such an interaction is not in contradiction with the present CDF data on W+ W- or dijet production, though higher luminosities may reveal measurable effects.

hep-ph↗
arXiv1997

h_c production at the Tevatron

Non-Relativistic QCD (NRQCD) has provided a consistent framework for the physics of quarkonia. The colour-octet Fock components predicted by NRQCD have important implications for the phenomenology of quarkonium production in experiments. We have considered the consequences of the colour-octet channels for the production of the $^1P_1$ charmonium state, $h_c$, at large-p_T at the Tevatron. The colour-octet contributions to this state are found to be dominant and give a reasonably large rate for the production of $h_c$. This should make it feasible to look for this resonance in the $J/ψ+π$ decay channel. The observation of $h_c$ is interesting in its own right, since its existence has yet to be experimentally confirmed. Moreover, this rate is a prediction of NRQCD, and the observation of the $h_c$ at the Tevatron can, therefore, be used as a test of NRQCD.

hep-ph↗
arXiv1997

Pursuing the Strange Stop Interpretation of the HERA Large Q^2 data

We explore the possible interpretation of the large-$Q^2$ anomaly reported by the H1 and ZEUS collaborations in terms of stop squark production off a strange quark in the proton via an R-violating interaction. This "strange stop" interpretation is constrained by LEP measurements of the $Z \to e^+ e^-$ decay rate in addition to constraints from the electroweak $ρ$ parameter and CDF and D0 searches for first-generation leptoquarks. We investigate the interplay between these constraints, taking full account of stop mixing effects. We find that if $m_{\tilde t} \le 200$ GeV only relatively small domains of the chargino and neutralino parameters are consistent with these constraints, and explore the extent to which this scenario may be probed further by searches for contact interactions at LEP~2 and experiments with $e^-$ and polarized beams at HERA.

hep-ph↗
arXiv1996

Quarkonium production via fragmentation: A review

We review the physics of quarkonium production at high energy colliders, with a specific emphasis on production $via$ fragmentation. We discuss the fragmentation picture for the production of quarkonia at large-$p_T$ at the Tevatron and emphasise the importance of the colour-octet components of the quarkonium wave function which are included in the fragmentation functions. Applications of these theoretical ideas to other processes, both at the Tevatron and at other experiments like LEP, HERA and the LHC are important as they will provide stringent tests of these ideas.

hep-ph↗
arXiv1996

Colour-octet Contributions to $x_F$ Distributions in $J/ψ$ Hadro-production at Fixed Target Energies

We study J/ψproduction at fixed-target energies, including the colour-octet production mechanisms predicted by NRQCD. In an earlier paper, we found that the octet components were crucial in understanding p_T-integrated forward (x_F > 0) cross-sections. Here we make a detailed comparison of the theoretical predictions with measured x_F distributions from fixed-target experiments. We find that the model predictions agree well with data. Taking into account higher orders in NRQCD, we argue that sigma(chi_1)/sigma(chi_2)=0.6, in excellent agreement with data. Similiar arguments also predict that sigma(chi)/sigma(J/psi) is roughly equal in photo- and hadro-production.

hep-ph↗
arXiv1996

1P1 charmonium production at the Tevatron

The production of the ${}^1P_1$ charmonium state, $h_c$, at large-$p_T$ at the Tevatron is considered. The colour-octet contributions to this state are found to be dominant and give a reasonably large rate for the production of $h_c$. This should make it feasible to look for this resonance in the $J/ψ+π$ decay channel. This production rate is a prediction of NRQCD, and the observation of the $h_c$ at the Tevatron can, therefore, be used as a test of the colour-octet predictions of NRQCD.

hep-ph↗
arXiv1996

Bounds from $t \bar t$ production on R-parity violating models of supersymmetry

We study $t \bar t$ production in R-parity violating supersymmetry. The annihilation channel $q \bar q \rightarrow t \bar t$ gets new contributions from $t$-channel exchange of squarks or sleptons. With the data from Tevatron on $t \bar t$ production, we find that the squark- or slepton-exchange processes constrain the $B$-violating $λ''$ couplings or the $L$-violating $λ'$ couplings, respectively. Our bounds are already comparable to the few existing constraints on third-generation $R$-parity violating couplings, and will improve when more precise measurements of the $t \bar t$ production cross-section become available. We also discuss the effects of these couplings for top production at the LHC.

hep-ph↗
arXiv1996

Charmonium Production

Theoretical analyses of the results on quarkonium production at large transverse momentum ($p_T$) in $p\bar p$ collisions, at the Tevatron have revealed two novel features of the physics of quarkonium production : 1) the contribution of fragmentation of gluons and charm quarks to the cross-section at large $p_T$, and 2) the importance of colour octet components of the quarkonium wave-function. I discuss the theoretical developments which have contributed to a reasonably consistent picture of quarkonium production at the Tevatron, and discuss large-$p_T$ $J/ψ$ production at HERA and at LHC as important tests of the fragmentation picture. I also discuss our recent analysis of $J/ψ$ production cross-sections at fixed-target energies, where we find that the energy dependence of the $p_T$-integrated cross-sections for both $pp$ and $π^- - p$ collisions is reasonably well reproduced, when the colour-octet components are included.

hep-ph↗
arXiv1996

Charmonium Production at the LHC

The analyses of large transverse momentum charmonium production at the Tevatron have shown that fragmentation of gluons is an important production mechanism. We study large-$p_T$ charmonium production in $pp$ collisions at the LHC, and find that due to the copious gluon production at this energy, the gluon fragmentation contribution completely overwhelms the fusion contribution and the charm quark fragmentation contribution. Our analysis shows that for $J/ψ$ production at the LHC, there is a significant event rate even for $p_T \sim$~100~GeV. The measurement of the cross-section at such large values of $p_T$ will provide a very important test of the fragmentation mechanism.

hep-ph↗
arXiv1996

Colour-octet Contributions to J/psi Hadroproduction at Fixed Target Energies

We investigate the integrated cross section for forward $\jpsi$ production in proton-proton and pion-proton collisions at centre of mass energies upto 60 GeV. We find that colour-octet contributions to the cross section are crucially important, and their inclusion produces a very good description of the data. Since the values of non-perturbative matrix elements are fixed by other experiments, these predictions are parameter-free.

hep-ph↗
arXiv1995

J/psi production via fragmentation at HERA

We compute the contributions to large-$p_T$ $J/ψ$ production at HERA coming from fragmentation of gluons and charm quarks. We find that the charm quark fragmentation contribution dominates over the direct production of $J/ψ$ via photon-gluon fusion at large-$p_T$, while the gluon fragmentation is negligibly small over the whole range of $p_T$. An experimental study of $p_T$ distributions of $J/ψ$ at HERA will provide a direct probe of the charm quark fragmentation functions.

hep-ph↗
arXiv1995

New LEP constraints on some supersymmetric Yukawa interactions that violate R-parity

We consider one-loop corrections to partial widths of the $Z$ induced by supersymmetric Yukawa interactions that violate $R$-parity. The precise experimental values of the leptonic $Z$ partial widths bound these Yukawa couplings, with the most interesting constraints being those on couplings involving the $τ$, since previous constraints on them were very mild.

hep-ph↗
arXiv1995

Quarkonium production in hadronic collisions

We summarize the theoretical description of charmonium and bottonium production in hadronic collisions and compare it to the available data from hadron-nucleon interactions. With the parameters of the theory established by these data, we obtain predictions for quarkonium production at RHIC and LHC energies.

hep-ph↗