Soma Sanyal

Effect of pre-existing baryon inhomogeneities on the dynamics of quark-hadron transition

Baryon number inhomogeneities may be generated during the epoch when the baryon asymmetry of the universe is produced, e.g. at the electroweak phase transition. These lumps will have a lower temperature than the background. Also the value ofT c will be different in these regions. Since a first-order quark-hadron (Q–H) transition is susceptible to small changes in temperature, we investigate the effect of the presence of such baryonic lumps on the dynamics of the Q–H transition. We find that the phase transition is delayed in these lumps for significant overdensities. Consequently, we argue that baryon concentration in these regions grows by the end of the transition. We mention some models which may give rise to such high baryon overdensities before the Q–H transition.

Cosmic string induced sheetlike baryon inhomogeneities at the quark-hadron transition

Cosmic strings moving through matter produce wakes where density is higher than the background density. We investigate the effects of such wakes occurring at the time of a first order quark-hadron transition in the early universe and show that they can lead to separation of quark-gluon plasma phase in the wake region, while the region outside the wake converts to the hadronic phase. Moving interfaces then trap large baryon densities in sheet like regions which can extend across the entire horizon. Typical separation between such sheets, at formation, is of the order of a km. Regions of baryon inhomogeneity of this nature, i.e. having a planar geometry, and separated by such large distance scales, appear to be well suited for the recent models of inhomogeneous nucleosynthesis to reconcile with the large baryon to photon ratio implied by the recent measurements of the cosmic microwave background power spectrum.

EXCITED HADRONS AS A SIGNAL FOR QUARK–GLUON PLASMA FORMATION

At the quark–hadron transition, when quarks get confined to hadrons, certain orbitally excited states, namely those which have excitation energies above the respective L = 0 states of the same order as the transition temperature Tc, may form easily because of thermal velocities of quarks at the transition temperature. We propose that the ratio of multiplicities of such excited states to the respective L = 0 states can serve as an almost model independent signal for the quark–gluon plasma (QGP) formation in relativistic heavy-ion collisions. For example, the ratio R* of multiplicities of and when plotted with respect to the center-of-mass energy of the collision (or vs. centrality/number of participants), should show a jump at the value of beyond which the QGP formation occurs. This should happen irrespective of the shape of the overall plot of R* vs. . Recent data from RHIC on Λ*/Λ vs. Npart for large values of Npart may be indicative of such a behavior, though there are large error bars. We give a list of several other such candidate hadronic states.

Baryon inhomogeneity generation via cosmic strings at QCD scale and its effects on nucleosynthesis

We have earlier shown that cosmic strings moving through the plasma at the time of a first order quark-hadron transition in the early universe can generate large scale baryon inhomogeneities. In this paper, we calculate detailed structure of these inhomogeneities at the quark-hadron transition. Our calculations show that the inhomogeneities generated by cosmic string wakes can strongly affect nucleosynthesis calculations. A comparison with observational data suggests that such baryon inhomogeneities should not have existed at the nucleosynthesis epoch. If this disagreement holds with more accurate observations, then it will lead to the conclusions that cosmic string formation scales above

BARYOGENESIS VIA DENSITY FLUCTUATIONS WITH A SECOND ORDER ELECTROWEAK PHASE TRANSITION

{10}^{14}--{10}^{15}

Skyrmion formation in (1+1)-dimensions with chemical potential

GeV may not be consistent with nucleosynthesis and CMBR observations. Alternatively, some other input in our calculation should be constrained, for example, if the average string velocity remains sufficiently small so that significant density perturbations are never produced at the QCD scale, or if strings move ultrarelativistically so that string wakes are very thin, trapping negligible amount of baryons. Finally, if the quark-hadron transition is not of first order then our calculations do not apply.

Baryogenesis via density fluctuations with a second-order electroweak phase transition

We consider the presence of cosmic string induced density fluctuations in the universe at temperatures below the electroweak phase transition temperature. Resulting temperature fluctuations can restore the electroweak symmetry locally, depending on the amplitude of fluctuations and the background temperature. The symmetry will be spontaneously broken again in a given fluctuation region as the temperature drops there (for fluctuations with length scales smaller than the horizon), resulting in the production of baryon asymmetry. The time scale of the transition will be governed by the wavelength of fluctuation and, hence, can be much smaller than the Hubble time. This leads to strong enhancement in the production of baryon asymmetry for a second order electroweak phase transition as compared to the case when transition happens due to the cooling of the universe via expansion. For a two-Higgs extension of the Standard Model (with appropriate CP violation), we show that one can get the required baryon to entr...

SUSTAINING SUPERCOOLED MIXED PHASE VIA RESONANT OSCILLATIONS OF THE ORDER PARAMETER

Formation of topological objects during phase transitions has been discussed extensively in literature. In all these discussions, defects and antidefects form with equal probabilities. In contrast, many physical situations, such as formation of baryons in relativistic heavy-ion collisions at present energies, flux tube formation in superconductors in the presence of external magnetic field, and formation of superfluid vortices in a rotating vessel, require a mechanism which can bias (say) defects over antidefects. Such a bias can crucially affect defect–antidefect correlations, apart from its effects on defect density. In this paper we initiate an investigation for the basic mechanism of biased formation of defects. For Skyrmions in 1+1 dimensions, we show that incorporation of a chemical potential term in the effective potential leads to a domain structure where order parameter is spatially varying. We show that this leads to biased formation of Skyrmions.

Implications of cosmic string-induced density fluctuations at the quark-hadron transition

We consider the presence of cosmic string-induced density fluctuations in the early universe at temperatures below the electroweak phase transition temperature. Resulting temperature fluctuations can restore the electroweak symmetry locally, depending on the amplitude of fluctuations and the background temperature. The symmetry will be spontaneously broken again in a given region as the temperature drops there (for fluctuations with length scales smaller than the horizon), resulting in the production of baryon asymmetry. The time-scale of the transition will be governed by the wavelength of fluctuation and, hence, can be much smaller than the Hubble time. This leads to strong enhancement in the production of baryon asymmetry for a second-order electroweak phase transition as compared to the case when transition happens due to the cooling of the universe via expansion. For a two-Higgs doublet model (with appropriate CP violation), we show that one can get the required baryon asymmetry if fluctuations propagate without getting significantly damped. If fluctuations are damped rapidly, then a volume factor suppresses the baryon production, though it is still 3–4 orders of magnitude larger than the conventional case of second-order transition.

Baryon inhomogeneities due to cosmic string wakes at the quark-hadron transition

We investigate the dynamics of a first order transition when the order parameter field undergoes resonant oscillations, driven by a periodically varying parameter of the free energy. This parameter could be a background oscillating field as in models of pre-heating after inflation. In the context of condensed matter systems, it could be temperature T, or pressure, external electric/magnetic field etc. We show that with suitable driving frequency and amplitude, the system remains in a type of mixed phase, without ever completing transition to the stable phase, even when the oscillating parameter of the free energy remains below the corresponding critical value (for example, with oscillating temperature, T always remains below the critical temperature Tc). This phenomenon may have important implications. In cosmology, it will imply prolonged mixed phase in a first order transition due to coupling with background oscillating fields. In condensed matter systems, it will imply that using oscillating temperature (or, more appropriately, pressure waves) one may be able to sustain liquids in a mixed phase indefinitely at low temperatures, without making transition to the frozen phase.