| Nuclear collisions at ultrarelativistic energies,such as those studied at Relativistic Heavy Ion Collider(RHIC),create a hot and dense phase of matter containing approxi-mately equal numbers of quarks and antiquarks,namely the quark-gluon plasma(QGP).The QGP persists for only a few times 10-23s,then cools and transitions into a lower temperature phase comprised of mesons,baryons,nucleus and hypernucleus,as well as their corresponding antimatter.Thus these collisions offer an ideal laboratory to explore fundamental physics involving nuclei,hypernuclei,and their antimatter partners.In this thesis,the masses of hypertriton and antihypertriton are measured by re-constructing its invariant masses through its mesonic decay channels Λ3H→3H+π-(two-body decay)and Λ3H→d+p+π-(three-body decay),using the high statistic data of Au+Au collisions at(?)=200 GeV collected in 2014 and 2016 by the STAR detector with the Heavy Flavor Tracker(HFT)at RHIC.The measured masses of hyper-triton and antihypertriton are 2990.95±0.13(stat.)±0.11(syst.)MeV/c2 and 2990.60±0.28(stat.)±0.11(syst.)MeV/c2,respectively.The average mass of hypertriton and antihypertriton weighted by the reciprocal of squared statistical uncertainties is 2990.89±0.12(stat.)±0.11(syst.)MeV/c2.The CPT theorem predicts that all processes must exactly conserve the combined operation of C(charge conjugation),P(parity,which reverses the direction of all spatial axes),and T(time reversal),and thus the matter and antimatter should have identical mass.Although CPT symmetry has been tested to a precision of 10-19 in the strange quark sector for kaons,the qualitatively different tests of CPT symmetry with improved accuracy or new systems are a continuing priority for fundamental physics.This thesis extracts the relative mass difference between hypertriton and antihypertriton using the measured masses of hypertriton and antihypertriton in conjunction with the latest mass differences of 3He and deuteron,namely(0.1±2.0(stat.)±1.0(syst.))×10-4.In addition,by taking the difference between the mass differences measured in the 2-body and 3-body decay channels of hypertriton in conjunction with the latest deuteron mass difference,a new constraint on the relative mass difference between 3He and 3He is placed,namely(-1.5±2.6(stat.)±1.2(syst.))×10-4 in which the precision is an order of magnitude improved over the latest measurement.The mass difference between hypertriton and antihypertriton is in agreement with zero,and the precision is higher than the previous data with same mass number by an order of magnitude.This is the first test of CPT symmetry in hypernuclei sector with a precision of 10-4.The hyperon-nucleon(YN)interaction plays an important role in understanding the strong nuclear force,and since hyperons may exist in the core of neutron stars,the YN interaction is also of importance for the study of neutron star properties.Hypernuclei are a natural YN interaction system and thus their Λ binding energy,BΛ,have a direct connection to the strength of YN interaction.The early measurements of BA of light hypernuclei performed about 50 years ago are revisited in this thesis.The masses of particle and nuclei used in the previous mea-surements are different from the current best estimated values.In this thesis,the BΛof light hypernuclei measured in 1967,1968,and 1973 are recalibrated by comparing the current best estimated masses of particle and nuclei with old values.The recali-brated BΛ are systematically larger(except in the case of Λ6 He)than the original values by about 100 keV.The effect of 100 keV is important for lighter hypernuclei,espe-cially for the hypertriton.Although this thesis provides a better estimates of BΛ of light hypernuclei by recalibration,those early measurements still suffer from an unknown systematic uncertainty as some references argued.More precise measurements using modern experimental methods are highly needed.B A for hypertriton and antihypertriton is also calculated using the measured masses of hypertriton and antihypertriton,namely BA=0.41±0.12(stat.)±0.11(syst.)MeV.This result differs from zero with a statistical significance of 3.4σ,which is different from the fact that the previous measurements are in agreement with 0 in statistical error.The central value of our measurement is larger than the commonly used measurement from 1973.The result also differs from the predictions where hypertriton is treated as a weakly bound state,and thus our result may indicate a stronger YN interaction than scientist expected.This new measurement place stringent constraints on the YN interaction,and have implications for understanding neutron star interiors. |
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