The measurement of R at low energies: status and perspectives Federico Nguyen - Università degli Studi di Roma TRE

Still measuring the R ratio: why? Hadron cross section measurement with the KLOE DA  NE Published results by BESII and CMD-2 Preliminary results by BABAR Future perspectives and two Italian workshops Federico Nguyen 24/04/2003 Outline

Federico Nguyen 24/04/2003 Hadronic contributions to  em (M Z 2 ) reducing the uncertainty of the effective  em (s) is essential for SM tests  had (s) includes contribution from u,d,s,c,b quarks not evaluable in pQCD, but one can use DATA by means of optical theorem + analyticity: Im[ ]  | hadrons | 2  had (5) (M Z 2 ) =

Federico Nguyen 24/04/2003 Hadronic contributions to (g-2)  a  had suffers for similar nonperturbative problems of  had (5) (M Z 2 ), but again it can be provided by  (e + e -  hadrons) by means of dispersion relations: a  SM = a  QED + a  had + a  EW a  had (e + e - ) K(s) ~ 1/s   hadrons  decays provide complementary information: CVC + SU iso (2)  a  had (  )

Federico Nguyen 24/04/2003 The present situation for a  had a  exp = (  8)  [0.7ppm] September ‘02

Federico Nguyen 24/04/2003 Hadronic regions: contributions and errors 12 -  (+  ) (+J/ ,  ) 4 3  (+ ,  )22 > 4  (+KK) a  had :  had (M Z ):  2 [a  had ]:  2 [  had ]: < 1.8 GeV with 

Federico Nguyen 24/04/2003 The radiative return standard way : measuring  e+ e-  hadrons (s) by varying e  beams energy alternative approach: given a fixed s 1/2, by studying Initial State Radiation events H = radiation function  min = emitted  min. ang. Positive: uncertainties related to beam energy and luminosity are the same for each M had 2 value it doesn’t require a dedicated set-up (measurement performed in parallel with typical flavour-factory programs) Negative (with respect to the scan method):  it needs higher luminosity (rate decreased by a factor  em )  it requires an accurate knowledge of rad. corr. (  )

DEAR DA  NE is a e + e - collider at a fixed  s ~ GeV ~ M  The DA  NE collider 2000 run: L  25 pb -1  7.5 x 10 7  2001 run: L ~ 190 pb -1  5.7 x 10 8   2002 run: L ~ 300 pb -1  9 x 10 8  L peak ~ 7.74E+31 cm -2 s -1 since January machine is “down” for the insertion of a new interaction region 2001 collected luminosity is sufficient for  statist  1% in  (e + e -      ) Federico Nguyen 24/04/2003

 E /E = 5.7% /  E(GeV)   t = 57 ps /  E(GeV)  50 ps  E /E = 5.7% /  E(GeV)   t = 57 ps /  E(GeV)  50 ps  p / p < 0.4%  z = 2 mm Electromagnetic Calorimeter Pb / Scintillating Fibres Endcap + Barrel = 98% (4  ) Drift Chamber 4 m , 3.3 m length 90% He, 10% i-C 4 H 10 Superconducting Coil B=0.5 T accurate detection of :  with E ~ 20 MeV tracks p ~ [ ] MeV/c accurate detection of :  with E ~ 20 MeV tracks p ~ [ ] MeV/c The KLOE detector Federico Nguyen 24/04/2003

The selection of      events 2 charged tracks in 40 o    140 o, with a vertex in r  8 cm, |z|  15 cm  acceptance: for enhancing ISR (wrt FSR and    +  -   ) small angle photons are required, but this region is highly inefficient !!! only 2  ’s are detected and a cut on the direction of the missing momentum is applied:    15 o (  165 o      180 o ),    dipion system polar angle     Federico Nguyen 24/04/2003

Pion identification e  e       log L the event is selected if at least one of the 2 tracks is identified to be a pion ( log L > 0 ) it leads to a rejection power ~ 94% for e + e   e + e  events while keeping a selection efficiency  96% for e + e    +    events  /e separation is performed using a Likelihood function based on: – TOF of charged clusters – Shape and quantity of energy deposition of the “charged” cluster along the EMC – TOF of charged clusters – Shape and quantity of energy deposition of the “charged” cluster along the EMC Federico Nguyen 24/04/2003   

m trk (MeV) before likelihood after likelihood            Background identification L effect is seen in “track mass” distr., m trk, defined by 4-mom. Conserv. under the hypothesis of 2 equal mass tracks and a  Federico Nguyen 24/04/2003       (MC)      (MC) DATA region selected by our analysis (2  invariant mass) 2

 Select  detection efficiency, L = luminosity MC = Phokhara (G. Rodrigo et al.) M 2  (GeV 2 ) d  /dM 2  (nb/0.01GeV 2 ) MC DATA eventi da 73 pb -1 del 2001 Federico Nguyen 24/04/2003 Cross section and F  extraction = CMD2 = KLOE |F  | 2 M 2  (GeV 2 ) KLOE PRELIMINARY bckgr not yet subtracted

The luminosity measurement in KLOE KLOE can exploit Bhabha events to measure the luminosity N Bckgr =     ,      events, etc...  Bhabha MC = cross section (MC with rad. corr.) Large angle Bhabha (  ~ 428 nb): 55° <  e  < 125° acollinearity < 9° E e   400 MeV ee ee  Federico Nguyen 24/04/2003 * Data - Babayaga (MC) C.M.C. Calame et al. - BK-DV (MC) track energy (MeV) track polar angle (degrees)

Federico Nguyen 24/04/2003 The Novosibirsk experiments Beam energy 2  ( ) MeV Peak luminosity3  cm -2 s -1 Interaction regions 2 Years of activity Data collected by CMD-2 : 2  10 7  decays 5  10 6  +  - events 3  10 6  decays 2  10 7 multi-hadrons VEPP-2M parameters: the Cryogenic Magnetic Detector (CMD-2): drift chamber + proportional Z-chamber inside a superconduct. solenoid ~ 1T and calorimeters (CsI-barrel + BGO-endcaps) provide simultaneous measurement of particle momentum and energy deposition the Spherical Neutral Detector (SND): 3-layer spherical electromagnetic calorimeter made of NaI(Tl) crystals makes it optimized for multiphoton final states and provides good discrimination  /K L

Federico Nguyen 24/04/2003 the selection of collinear events inside the fiducial volume e/  /  separation by energy depositions rejection of cosmic background by means of the spatial distribution of the vertex The  (e + e   +   ) analysis from CMD-2 Source of systematic error Value beam energy 0.1 % fiducial volume 0.2 % registration efficiency 0.2 % fitting procedure 0.2 % correction on pion losses 0.2 % radiative corrections 0.4 % Total systematic error 0.6 %  e 

Federico Nguyen 24/04/2003 The pion form factor from CMD-2  i = rad. corr.,  i = efficiencies  N,D = pion losses,   =  term  -  interference

Federico Nguyen 24/04/2003 VEPP-2M impact on a  Channel Accuracy % Error in a  ppm ++ + 0+  K+KK+K KLKSKLKS + 00+ 0 + ++ +  0 ,  Total0.29 contributions to a  above VEPP-2m: 8.6  0.6 ppm

Federico Nguyen 24/04/2003 The BEPC machine and the BESII detector the only e + e  machine in 2-5 GeV since 1989 L ~ 5  cm -2 s -1 at the J/  peak the BEijing Spectrometer the Beijing Electron Positron Collider vertex chamber + drift chamber inside a solenoid coil (~ 0.4 T) TOF + shower counters muon counters luminosity monitors TOF and drift chamber provide good  /K/p separation for momentum  GeV/c

Federico Nguyen 24/04/2003 Data analysis for the R scan Source BESII (%) Luminosity 2 – 3 Detection eff. 3 – 4 Trigger eff. 0.5 Rad. corr Hadron decay model Statistical 2.5 Total 6 – 7 N had =obs. had. events, N beam =beam-gas interacts., N ll =lept. pairs, N  =  events, L =integrated luminosity,  had = detection efficiency for hadronic events

Federico Nguyen 24/04/2003 R scan from BESII March-May 1998: 6 points with E  [2.6,5] GeV February-June 1999: 85 points with E  [2,4.8] GeV

Federico Nguyen 24/04/2003 BESII impact on M Higgs Without BES data With BES data (95% C.L.)

DesignPerformance Peak Lumi3*10 33 cm -2 s *10 33 cm -2 s -1  Lumi/day 135 pb pb -1 The PEPII machine and the BABAR detector Federico Nguyen 24/04/2003 PEPII is an asymmetric e + (E beam = 9 GeV) e   beam = 3.1GeV  collider   s ~ M Y(4S) = GeV Si vertex tracker + drift chamber + EMC(CsI) inside a solenoid magnet (1.5 T) Cherenkov light based particle ID vertexing for B mesons lepton (from J/  ) ident.  /K separation

Federico Nguyen 24/04/2003 The pion form factor at BABAR  = detect. effs.  rad = rad. corr. s’=“effective c.m. energy scale” tagged photon (kinematic fit)

Federico Nguyen 24/04/2003 Comparison VEPP-2M vs. BABAR for 2    2   BaBar 22fb -1 Very Preliminary! Statistical errors only! 4 charged tracks + hard photon + Kaon/Muon veto BABAR only covers the whole mass range very competitive with only 22 fb -1 dominated by 4  M 2  2  (MeV)  (e + e   2   2   )

Federico Nguyen 24/04/2003 New machines and detectors Region below 2 GeV VEPP-2000 –Novosibirsk (2004?) 2  (0.4-2) GeV e + e - collider Luminosity  cm -2 sec  s = 2 GeV Upgrade of SND and CMD-3 Several hadronic channels and NNbar Region between 2 GeV and 5 GeV BEPC and BES upgrade (BEPCII and BESIII) (2006?) Peak luminosity (3.8-10)  cm -2 sec -1  R/R~ 1% in this region CLEO-C: Modify CESR for High L in 3<Ecm<5 GeV (2005?) L~ 2  3.6  cm -2 sec -1

Federico Nguyen 24/04/2003 Conclusions and perspectives So far CMD2 and SND experiments at VEPP-2M, Novosibirsk and BESII at BEPC, Beijing, have improved quite a lot theaccuracy on R in the region E<1.4 GeV and 2<E<5 GeV Other improvements are expected from KLOE in the region E<1 GeV and BABAR in the region GeV, both using ISR   had MethodRef  data<12 GeV (new data CMD2 & BES)FJ ’01  ( )  up to J/  up to  )  a m had ( ) MethodRef  10.8 data<12 GeV (new data CMD2 & BES)FJ ’01  6.0 (  2.0)  up to1.4 GeV (  to J/   4.0 BNL a  exp final goal

R related workshops in Italy Federico Nguyen 24/04/2003