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Register a new userUnitarity Constraints on Semi-hard Jet Production in Impact Parameter Space
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The perturbative QCD formula for minijet production consitutes an important ingredient in models describing the total cross section and multiparticle production in hadron-hadron scattering at high energies. Using arguments based on s-channel unitarity we set bounds on the minimum value of p_T for which the leading twist minijet formula can be used. For large impact parameters where correlations between partons appear to be small we find that the minimum value of p_T should be greater than 2.5 GeV for LHC energies and greater than 3.5 GeV for cosmic ray energies of about 50 TeV. We also argue that for collisions with values of impact parameters typical for heavy particle production the values of minimum p_T are likely to be considerably larger. We also analyze and quantify the potential role of saturation effects in the gluon density. We find that although saturation effects alone are not sufficient to restore unitarity, they are likely to play an important role at LHC energies.
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In a suitably chosen back-to-back kinematics, four-jet production in hadronic collisions is known to be dominated by contributions from two independent partonic scattering processes, thus giving experimental access to the structure of generalized two-parton distributions 2GPDs. Here, we show that a combined measurement of the double hard four-jet cross section in proton-proton and proton-nucleus collisions will allow one to disentangle different sources of two-parton correlations in the proton, that cannot be disentangled with 4-jet measurements in proton-proton collisions alone. To this end, we analyze in detail the structure of 2GPDs in the nucleus (A), we calculate in the independent nucleon approximation all contributions to the double hard four-jet cross section in pA, and we determine corrections arising from the nuclear dependence of single parton distribution functions. We then outline an experimental strategy for determining the longitudinal two-parton correlations in the proton.
In mSUGRA models the lightest supersymmetric particle (assumed to be the lightest neutralino) provides an excellent cold dark matter (CDM) candidate. The supersymmetric parameter space is significantly reduced, if the limits on the CDM relic density, obtained from WMAP data, are used. Assuming a vanishing trilinear scalar coupling A0 and fixed values of tan(beta), these limits result in narrow lines of allowed regions in the m0-m12 plane, the so called WMAP strips. In this analysis the trilinear coupling A0 has been varied within +/-4TeV resulting in largely extended areas in the m0-m12 plane which are no longer excluded.
We derive partial-wave unitarity constraints on gauge-invariant interactions of an Axion-Like Particle (ALP) up to dimension-6 from all allowed $2to2$ scattering processes in the limit of large center-of-mass energy. We find that the strongest bounds stem from scattering amplitudes with one external ALP and only apply to the coupling to a pair of $SU(2)_L$ gauge bosons. Couplings to $U(1)_Y$ and $SU(3)_C$ gauge bosons and to fermions are more loosely constrained.
At large virtuality $Q^2$, the coupling to the vector meson production channels provides us with a natural explanation of the surprisingly large cross section of the neutral pion electroproduction recently measured at Jefferson Laboratory, without destroying the good agreement between the Regge pole model and the data at the real photon point. Elastic rescattering of the $pi^0$ provides us with a way to explain why the node, that appears at $tsim -0.5$ GeV$^2$ at the real photon point, disappears as soon as $Q^2$ differs from zero.
We obtain the partial-wave unitarity constraints on dimension-six operators stemming from the analyses of vector boson and Higgs scattering processes as well as the inelastic scattering of standard model fermions into electroweak gauge bosons. We take into account all coupled channels, all possible helicity amplitudes, and explore a six-dimensional parameter space of anomalous couplings. Our analysis shows that for those operators affecting the Higgs couplings, present 90% confidence level constraints from global data analysis of Higgs and electroweak data are such that unitarity is not violated if $sqrt{s}leq 3.2;{rm TeV}$. For the purely gauge-boson operator $O_{WWW}$, the present bounds from triple-gauge boson analysis indicate that within its presently allowed 90% confidence level range unitarity can be violated in $fbar f to V V$ at center-of-mass energy $sqrt{s}geq 2.4;{rm TeV}$.
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