Presentation on theme: "The charmonium mass spectrum"— Presentation transcript:
1The charmonium mass spectrum Presented by:Wander BaldiniFerrara University and INFNInformal workshop on charmonium spectroscopy Genova June 7th-8th 2001
2Outline A little bit of history: the November revolution. Main experimental techniques for the study of charmonium:The charmonium spectrum: present status
3The November revolution From ”The Rise of the Standard Model:”"As I look back to the first three years at SPEAR, I consider this one of the most revolutionary, or perhaps the most revolutionary, experiment in the history of particle physics....."G.Goldhaber
4The discovery of the J/y On November 10-th 1974, at SLAC and BNL an extremely narrow resonance was discovered at an energy of about 3100 MeVThe resonance was immediately confirmed at FrascatiIts small width couldn't be explained in terms of the known quarks u,d or sThis resonance was called J at BNL and y at SLAC (for a reason that I will explain soon)
5The discovery of the J/y at SLAC……and at the BrookhavenNational Laboratory
6What is this resonance made of? A few years before, Glashow, Iliopoulos and Maiani proposed a model to explain the absence of the S=1 neutral weak currents:This model predicted the existence of a new quark “charm” with charge +2/3The discovery of the J/y confirmed this prediction and was actually the definitive confirmation of the existence of quarks
7Why y?You may wonder why this resonance was called y look at the picture of this event….The name was clearly right!
8Experimental techniques Three main methods are used to study the charmonium resonances:electron-positron annihilations:proton- antiproton annihilations:two-photon collisions:
9Electron-Positron annihilations This method is one of the first exploitedIt allows the direct formation of the charmonium states with the same quantum number of the photonAll the other states are studied through radiative decay of andIt provides a low background method for the identification of charmonium statesMarkI,II,III and Crystal Ball at SLAC are someof the experiments that exploited this technique.
10Crystal Ball Crystal ball is a non magnetic detector designed to study the charmoniumstates mainly through the detection ofphotons emitted in radiative transitions:energy resolution:Angular coverage: 98% 4pMain detector made of 672pyramidal NaI(Tl) blocksEach block is 15.7 radiationlengths thick
11Proton-antiproton annihilations This method allows the directformation of all the charmoniumresonancesThe mass and width of the resonanceare obtained from beam parametersand do not depend on the detectorenergy resolutionThe charmonium signal can be clearly selected over the largehadronic background by studying the electromagnetic decaysThis technique has been pioneered by R704 at the IntersectingStorage Ring at CERN and extensively used by E760/E835 at theFermilab Antiproton Accumulator
12E760/E835 at Fermilab Non magnetic spectrometer designed to study the charmoniumresonances through their e.m.decays:Angular coverage: 33% 4pAngular and energy resolutions:from 1.5 to 5 mrad
13Two photon collisions With this technique C-even charmonium states can beproduced through the fusion oftwo quasi-real photons emitted bye+ and e- :The e+ and e- usually goundetected along the beam pipe(untagged events)CLEO-II and LEPexperiments are presentlyusing this techniqueThe andresonances can be produced,forbidden by Young theorem
14Resonance scan Each charmonium resonance is studied by changing the c.m.energy in small stepsThe charmonium is detectedthrough its e.m. decaysThe measured excitation curve isthe convolution of the resonancecross section (Breit-Wigner) andof the beam energy distributionThe mass and the width of theresonance are extracted from theexcitation curve with a maximumlikelihood fit
15Beam energy measurement The beam energy is calculated fromthe orbit length (Lorb.) andfrom the revolution frequency (f)The uncertainty on the energymeasurement is dominated byThe reference orbit length Lref is calculated at the energywith a precision ofThe orbit length at all the other energies is calculated thanksto 48 Beam Position Monitors, which provide the orbit lengthdifference :
17The fundamental state The resonance observed by Crystal Ball… preliminary results…and by E835 in thedecay channel:
18The fundamental state Mass measurements Total width measurements preliminary results
19The state Crystal ball is the only evidence of this resonance experiment which saw anevidence of this resonanceE760/E835 searched for thisresonance in the energy region:Ecm=( ) MeV, in thedecay channel: but noevidence of a signal was foundCrystal BallMass:Total width:
20The state Search of the resonance in the decay channel: …and in the channel
29The D wave states The charmonium “D states” are above the open charm threshold (3730 MeV ) butthe widths of the J= 2 statesand are expectedto be small:forbidden by parity conservationforbidden by energy conservationOnly the , considered to be largely state, hasbeen clearly observed
30The D wave states The only evidence of another D state has been observed at Fermilabby experiment E705 at an energy of3836 MeV, in the reaction:This evidence was not confirmedby the same experiment in thereactionand more recently by BES
31Conclusions After almost 30 years since its discovery we have learned a lot about charmonium. But, still, manyquestions, like the non observation of the , theconfirmation of the resonance and the poorknowledge of the D states are still open questionsand efforts should be put to solve them.