A Precision Measurement of the Neutral Pion ? :=+2/#"1F,+/'2%21='>#1/'",%#+12/'",(?!+(4=/(%0"#%/3+%! G ='0+/'+? 8!9&:;%1/%HI%J+K

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1 A Precision Measurement of the Neutral Pion Lifetime: the PRIMEX Experiment!"#$%&'()'*+, -,'.+#('/$%"0%&1((1234(+//(6%5*3+#(/%5*3+#(/ 1,7%/3+ 8!9&:;%<"==1>"#1/'",? 0!""? A3+%8!9&:;%+BC+#'*+,/%1/%DE1>? :=+2/#"*1F,+/'2%21='>#1/'",%#+12/'",(?!+(4=/(%0"#%/3+%! G ='0+/'*+? 8!9&:;%1/%HI%J+K L'B/3%9,/+#,1/'",1=%<",0+#+,2+%",%8+#(C+2/'.+(%',%M17#",'2%83$('2( A#'+(/+%N 9/1=$6%&1$%HIOHP6%IGGQ

2 A short review of! 0!"" : discovery! HRSQT%U4)1@1%C"(/4=1/+(%1%,+4/#1=% *+(",%>1(+7%",%">(+#.1/'",(%"0% 231#F+%',7+C+,7+,2+%"0%/3+%VV% 0"#2+! HRWGT%L1)1/1%+(/'*1/+(%#! HG OHP (% 0"#%/3+%! G 0#"*%%%%%%=""C%7'1F#1*%% pp? HRWQT%%XCC+,3+'*+#%(4FF+(/(%/31/%! G% 7+21$(%1#+%#+(C",('>=+%0"#%F1**1% >12)F#"4,7(%',%3'F3%1=/'/47+%2"(*'2% #1$(? HRYGT%%! G 7'(2".+#+7%1/%Z+#)+=+$% 7% %Z % <$2="/#",%

3 A short review of! 0!"" : era of current algebra?a3+%("0/oc'",%='*'/%"0%8<5<%c#+7'2/(%5[!!""$%\%g? 57=+#6%Z+==6%D12)'@%1,7%Z1#7++, 7'(2".+#%/#'1,F=+%%%%%%%%%%%% 7'1F#1*(%/31/%1=/+#%8<5<%C#+7'2/'",(%0"#%! " 7+21$ # $ % $ &

4 A short review of! 0!"" : era of QCD and effective interactions? ]+((6%^4*',"%1,7%]'//+, 2",(/#42/%1,"*1="4(% X[C W _%=1F#1,F'1,%/31/%C+#*'/(%/#1,('/'",(%>+/@++,% +.+,%1,7%"77%,4*>+#(%"0%C(+47"O(21=1#%*+(",( (21=1#%*+(",(? A3+%23'#1=%1,"*1=$%31(%(C+2'1=%(/1/4(%',%`<aT% /3+#+%1#+%,"%="@%+,+#F$%2",(/1,/(%',%=1F#1,F'1, F F I ' V2 I *! S )! S I YbP! d! & G! $ 2! ( "" * * bcbiybiy +K

5 A short review of! 0!"" : era of chiral dynamics? <"##+2/'",(%74+%/"% ['_%4O 7O e41#)%*1((+( [''_%%'("(C',%>#+1)',F%[!6%+6%1,7%+%+ %1,7%+ *'B',F_%C#"C"#/'",1=% /"%e41#)%*1((%7'00+#+,2+(?!+2+,/%21=24=1/'",(%',%vex%<38a%f'.+( f & G ( "" $ * QcHG +K )!! Yg%3'F3+#%/31,%EX6%@'/3%4,2+#/1',/$%"0%=+((%/31,%Hg %J % %Z % 7%Z % = %83 % % % % 7% ' Dc%J"'/$6%5c%Z+#,(/+',6%1,7%Zc%M"=(/+',6%83$(c%!+.c%aPPTGbPGHW6%IGGI6%1,7% Zc%5,1,/31,1#1$1,%1,7%Zc%&"4((1==1*6%DM:8GY%[IGGI_%GYIc%

6 Experimental results used in PDG average V"%2"=='7+#%#+(4=/(% + h + O ("";; 1#+%',%/3'(% %/3' % 1.+#1F+ ' `<a%(4*%#4=+%21=24=1/'",t%zc%9"00+6%5c%xf1,+('1,6%83$(c%e+//c%zpwb6%sqr%[iggb_c

$L"=4/'",T%%%<#+1/+%+,+#F+/'2%! G i(6 E%\%.#! :j*! &+1(4#+%!$ $G 7+21$%=+,F/3% d"#%:\%hggg%j+k %HGGG%J+K6%E%E *+1,, HGG%!*! )[! G (""_%\%bcsw+k.$

7 Direct Measurement of Lifetime (CERN 1984)!# OHP!,HBHG (+2%-/""%(*1==%/"%*+1(4#+!L"=4/'",T%%%<#+1/+%+,+#F+/'2%! G i(6 E%\%.#! :j*! &+1(4#+%! G 7+21$%=+,F/3% d"#%:\%hggg%j+k %HGGG%J+K6%E%E *+1,, HGG%!*! )[! G (""_%\%bcsw+k.schg[/"/1=_ a"*',1,/%($(/+*1/'2%+##"#t -,2+#/1',/$%',%8! [.HcYg_ π 0 γγ Decay Width (ev) QCD Sum Rule +/-1.5% NLO, +/-1% Leading Order 7 CERN (Direct) Experiments

8 Primakoff Reaction dσ/dθ, µbarn/rad E γ =5.2GeV HI <%/1#F+/ 15 8#'*1)"00 V42=c%<"3+#+,/ ,/+#0+#+,2+% V42=c%9,2"3c θ 0 π (degr.) d d Pr ' Z 0 E 2 2 * )"" F..( Q) sin / 3 4 e m! 2 m! Q <31==+,F+T%%:B/#12/%/3+%8#'*1)"00%1*C='/47+

9 Previous Primakoff Experiments 5==%C#+.'"4(%+BC+#'*+,/(%4(+7T! -,/1FF+7%>#+*((/#13=4,F%" " >+1*! <",.+,/'",1=%8>OF=1((%21="#'*+/#$ π 0 γγ Decay Width (ev) QCD Sum Rule +/-1.5% DESY (Primakoff) NLO, +/-1% Leading Order 7 Cornell (Primakoff) Tomsk (Primakoff)

10 PRIMEX Collaboration Arizona State University, Tempe, AZ, Catholic University of America, Washington, DC, Chinese Institute of Atomic Energy, Beijing, China, Eastern Kentucky University, Richmond, KY, George Washington University, Washington, DC, Hampton University, Hampton, VA, Institute for High Energy Physics, Chinese Academy of Sciences, Beijing, China, Institute for High Energy Physics, Protvino, Moscow region, Russia, Institute for Theoretical and Experimental Physics, Moscow, Russia, Kharkov Institute of Physics and Technology, Kharkov, Ukraine, Massachusetts Institute of Technology, Cambridge, MA, Norfolk State University, Norfolk, VA, North Carolina A&T State University, Greensboro, NC, North Carolina Central University, Durham, NC, Thomas Jefferson National Accelerator Facility, Newport News, VA, Tomsk Polytechnical University, Tomsk, Russia, Idaho State University, Pocatello, ID, University of Illinois, Urbana, IL, University of Kentucky, Lexington, KY, University of Massachusetts, Amherst, MA, University of North Carolina at Wilmington, i Wilmington, i NC, University i of Virginia, i i Charlottesville, VA, Yerevan Physics Institute, Yerevan, Armenia

11 " DE1>%M1==%Z%3'F3%#+("=4/'",%3'F3 ',/+,('/$%C3"/",%/1FF',F%012'='/$ " C3"/",%0=4B%2",/#"=%1/%3'F3 ',/+,('/'+( " F $ *4=/'O231,,+=%21="#'*+/+# [MU<5E_

12 Jefferson Lab Photon Tagger

13 PRIMEX Targets: 12 C and 208 Pb Lead target :00+2/'.+%,4*>+#%"0%1/"*(j2* % 0% I ',%/3+%/1#F+/(T% <1#>",%),"@,%/"%k GcGWg E+17%),"@,%/"%k GcWg 34jA\GcSg

14 Pair Spectrometer "!+=1/'.+%C3"/",%0=4B%*",'/"#T >$%7+/+2/',F%+ h + O C1'#(%0#"*% >+1*%74#',F%/3+%+BC+#'*+,/ " <"*>',1/'",%"0T%! HP%lJB&%7'C"=+%*1F,+/! I%/+=+(2"C+(%"0%IBQ% (2',/'==1/',F%7+/+2/"#(

15 Stability of relative tagging ratios 8Lh/1FF+#! &",'/"#+7%>$%8L%74#',F%C#"742/'",%71/1%/1)',Fc A1FF+# %(

16 8#'*:B%M$>#'7%<1="#'*+/+#%%O M$<1= 5%3'F3=$%(+F*+,/+7%3$>#'7% 21="#'*+/+# N YbP%=+17OF=1((%7+/+2/"#(% [WcGG)WcGG)WG%2* GG)WG%2* S _ N HHYI%=+17O/4,F(/1/+% 7+/+2/"#(% [IcHIY)IcHIY)IHcY%2* S _ N +,+#F$%#+("=4/'",%HcSg N C"('/'",%#+("=4/'",%HcS%**

17 QED calibration reaction I: pair production on

18 Differential cross sections for pair production [mb] dσ dx Run 5141 x min x max σ exp. σ theory = 0.4 = = mb = mb x = E e -/E γ " 9,/+F#1/+7%2#"((%(+2/'",(%',%1F#++*+,/%@'/3%/3+"#$%1/% " 9,/+F#1/+7%2#"((%(+2/'",(%',%1F#++*+,/%@'/3%/3+"#$%1/% /3+%(4>OC+#2+,/%=+.+=

19 `:a%21='>#1/'",%#+12/'",%99t%<"*c/",%(21//+#',f% " 6 e ( " 5 6 e5

21 Latest results for the! 0 lifetime

22 Extracting Elastic Pion Yields versus /! : : H : H 6 I /1FF+77" & "" A3#++%F#"4C(%1,1=$m+7%/3+%71/1%',7+C+,7+,/=$T% C $ :c<=',/",%[-&1((_6%ac%&2v4=/$%[&9ajde1>_%1,7%9c%e1#',%[9a:8_

23 π o yield per 0.02 o per incident γ s a'00+#+,/'1=%<#"((%(+2/'", π 0 Photoproduction Yield (crystal only) Elastic π 0 Yield 12 C target dσ/dθ per 0.02 o, µbarn/rad C target θ π o, (degr.) θ π o, (degr.) :BC+#'*+,/1=%U'+=7% C+#%38! GEANT:! 122+C/1,2+(n! +00'2'+,2'+(n%! #+("=4/'",(n a'00c%2#"((%(+2/'",

24 d'/%/"%:b/#12/%"[! G (""_%a+21$%]'7/3 " Theoretical angular distributions smeared with experimental resolutions are fit to the data HI < IGQ 8>

25 IGQ8> dn/dθ, events/0.02 deg. HI< dn/dθ, events/0.02 deg. <"*>',+7%L4*4=/1,+"4(%d'/%/"%Z"/3%A1#F+/( Γ CS-C φ-c CI-C CS-Pb φ-pb CI-Pb / E E E E E θ(π ), degrees Γ CS-C φ-c CI-C CS-Pb φ-pb CI-Pb 0 / E E E E E θ(π0), degrees 2

26 5,1=$('(%5.+#1F+(%0"#%)[! G!""_ Target )&! 9!""$ HI < bcqp%k GcIH%+K IGQ 8> <"*>',+7%21#>",h=+17 bcrr%k GcHQ%+K bcrs%k GcHQ%+K

27 :(/'*1/+7%L$(/+*1/'2%:##"#( Contributions 83"/",%0=4B A1#F+/%,4*>+# Z12)F#"4,7%(4>/#12/'", Errors HcGg GcHg GcRg :.+,/%(+=+2/'", GcYg MU<5E%#+(C",(+% 04,2/'", Z+1*%C1#1*+/+#( 522+C/1,2+ &"7+=%+##"#(%[/3+"#$_ 83$('2(%>12)F#"4,7 Z#1,23',F%#1/'"%[8aJ_ GcYg GcWg GcSg GcIYg GcIWg GcGSg Total 1.6%

28 Current PrimEx Result )[!(""_%\%bcrs+k.icsg.hcpg &*

29 d4/4#+%de1>%!4,%

30 PrimEx Program at 12 GeV JLAB PrimEx Program at 12 GeV JLAB ]+%C#"C"(+%/"%*+1(4#+T N A@"O83"/",%a+21$%]'7/3(T% "[#$o!!_ o!!_6%"[# %"[#"$o!!_% HH%J+K%+ C3"/"C#"742/'",% /' ",%C#"/", N A#1,('/'",%d"#*%d12/"#%d ""p "0%% G 6%#%1,7%# %#%1,7%#" 1/%="@%`I [GcGGHOOGcY%J+K I j2 I _.'1%/3+%%8#'*1)"00%+00+2/c

31 Fundamental input to physics: # a+/+#*',1/'",%"0%e41#)%*1((%#1/'" 2! 2 m m 1 Q 7 2 * s, where mˆ * ( m 2 2 u m m 7 m 6 2 d d u # +-+ *'B',F # 9,/+#12/'",%#17'4(%"0%! 0 6%+ 1,7%+ # 9(%/3+ + 1,%1CC#"B'*1/+%J"=7(/",+% >"(",q )

32 8!9&:;%1/%HI%J+KT%J=4+;%7+/+2/"#%',% DE1>%M1==%a

33 Summary and Conclusions " ]+%1#+%',%/3+%C#"2+((%"0%0',1='m',F%/3+%% " ='0+/'*+ 1,1=$('(c%%X4#%#+(4=/%'(T )! G * bcrs. GcHQ. ("" GcHS+K " X4#%#+(4=/%'(%',%1F#++*+,/%@'/3%EX%1,7%VEX%<38A6%1,7% /3+%`<a%(4*%#4=+%21=24=1/'",c%% " 577'/'",1=%#4,,',F%21,%#+742+%"4#%2"*>',+7%(/1/'(/'21=% 1,7%($(/+*1/'2%+##"#(%>$%1>"4/%rc%]+%1#+%1CC#".+7% 0"#%1,"/3+#%#4,%1/%D+00+#(",%E1>c% " 5%8#'*1)"00%C#"F#1*%1/%HI%J+K%3"=7(%F#+1/%C#"*'(+% C F F C 0"#%(/47'+(%"0%/3+%C(+47"O(21=1#%*+(",(

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