Transcript Present status of charmonium spectroscopy in proton

Present status of charmonium
spectroscopy inpp annihilations
Marco Pallavicini
University and I.N.F.N. Genova
on behalf of Fermilab E835 Collaboration:
Fermilab
University and I.N.F.N. Ferrara
University and I.N.F.N. Genova
University of California at Irvine
Northwestern University
University and I.N.F.N. Torino
Talk overview
 Physical motivations forpp annihilations in
charmonium spectroscopy and open problems
 Experimental technique
 Antiproton Accumulator and E835 Detector Layout
 Results from 1996/1997 fixed target run
 Comparison with previous measurements
 Future perspectives and conclusions
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Physics motivations
 Charmonium states have been studied for 25 years at e+e–
colliders, in production at fixed target spectrometers and
using pp annihilations.
 Whypp ?
 All charmonium states can be formed.
 e+ e– : Only 1- - states are
directly accessible. cc and
hc only from cascade
decays of y’; h’c and 1P1
unobserved or very poorly
studied
 Production at fixed target:
all states are produced but
mass and width resolution
depends on detetector
 Resonance mass and width are determined from
beam parameters and do not depend on detector
energy-momentum resolution.
 Thep beam quality is crucial
 There is a clean electromagnetic signal over the large
hadronic background even ifpp coupling tocc is
small.
  p p  hadrons   70 mb
  p p  cc   5 b
 eff  p p  J /y  e  e    25 nb
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Charmonium spectrum
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Previous experiments and E835
 Charmonium spectroscopy inpp annihilations was
pioneered by CERN experiment R704 at ISR in mid-80s.
 R704 proved that the experiment was feasible with
an internal hydrogen jet target.
 It produced the first measurements of cc1 and cc2
 Fermilab experiment E760 has collected 35 pb-1 of
integrated luminosity in 1990 and 1991.
 First evidence of 1P1 state.
 Precise measurement of cc1 and cc2 masses and
widths.
 First direct observation of hc inpp annihilations
 Experiment E835 has continued E760 program collecting
150 pb-1 and achieving:
 The first observation of cc0 in pp annihilations.
 Precise measurement of hc parameters
 Extensive search for h’c
 Improved cc1 and cc2 angular distributions
 Improved y’ branching ratios measurement
 Proton form factor in time like region
 p0p0 cross section
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Experimental Technique
The Source
 Proton anti-proton annihilations are obtained by
intersecting the cooledp beam of the Fermilab
Antiproton Accumulator with a molecular hydrogen jet
target.
 Small beam energy spread : Dp/p ~ 2 10–4
 Minimum center of mass energy step ~ 250 KeV, to
be compared to charmonium widths ranging from
~100 KeV to ~ 10 MeV
 High intensity. Beam current up to 60-80 mA,
corresponding to 6-8 1011 p circulating
 Good compensation for dE/dx in the target by
stochastic cooling
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Experimental technique
Beam energy measurement
 Beam energy is measured from beam revolution
frequency and reference orbit length:
m pc2
c  LREF f REV
Ep 
2
f REV LREF 

1 
c 


 DL

2 3 2  Df 
DE p  m p c  p  p     ORB 
 f   LORB 
2
2



1
2
 Beam revolution frequency is well measured : Df/f ~ 10-7
 DLORB is the dominant source of error
 LREF measured from y’ peak position (known up to 100
KeV).
BEAM ENERGY CALIBRATION
LREF  c
f REV
Dmy '  100KeV  DLREF  0.67mm
 At each energy point we measure:
c  LREF  DL  f
48 Beam Position Monitors to
measure the distance between
reference orbit and real orbit
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Experimental technique
The Target
 The proton target is done with a molecular hydrogen jet
made up of microdroplets (clusters) formed inside a
trumpet shaped nozzle at temperature (T~35°K) and
pressure (P~1 bar) near saturation.
 Main features
 Well localized target : ~ 0.5 x 0.5 x 0.5 cm3
 Large target density. Instantaneous luminosity has
been limited by DAQ to 2.5 1031 cm–2 s–1
 Density control and beam current feedback to be able
to run at constant luminosity
 Motorized nozzle for allignment
 Very efficient use of p
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Experimental technique
Resonance Scan
 Each charmonium state is studied by changing Ebeam (Ecm)
in small steps (~ 250 KeV in c.m.)
 Detection of charmonium formation through
electromagnetic final states
 All beam parameters are known: orbit length and
frequency spectrum is recorded

# ev.
   f B E  s  BW E dE   B
Ldt

0

 BW 
2 J  1p
4k 2

B  p p  R B R  f GR2
2
GR2
s  MR 
4


 MR, GR and BRinxBRout are extracted from the excitation
curve using maximum likelihood fit. MR depends on
beam energy calibration only. GR on energy spectrum
width. BRinxBRout needs efficiency and acceptance
estimate.
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E835 detector
p p  cc  ee
p p  c c  J /y X  e  e  X
p p  c c  
p p  multi 
p p    K  K  K  K 
pp  pp
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Total Integrated Luminosity
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Event selection for e+e– exclusive and
inclusive events
 Hardware 1st level trigger: candidate events are selected
requiring two electrons (defined by scintillators and
Cerenkov counter concidences) and two high invariant
mass CCAL clusters (> 2 GeV).
 Online software filter selects “golden” candidates by
requiring an inclusive invariant mass above 2.4 GeV
 Event selection is based on electron id, event topology,
timing and kinematic fit.
• Electron i.d. is done using 8
variables
from
hodoscope
counters
dE/dx,
Cerenkov
photoelectrons, CCAL cluster
shape variables. Background can
be reduced to a few pb while
keeping efficiency around 85%
• CCAL cluster timing (TDC) is
used to reject pile-up clusters due
to high intensity
•Event topology and kinematic
fit are used to classify final states
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Event selection at the y’ energy
T itolo:
/usr/us ers/pallas/OFFE835/NT UPLE/cut_pl ot.eps
Autore:
HIGZ Versi on 1.23/07
Anteprima:
L'immagine EPS non è s tata salvata
c on l'anteprima inclus a i n ess a.
Commento:
L'immagine EPS potrà es s ere s tampata c on una s tampante
Pos tScri pt e non con
altri tipi di stampante.
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First observation of c0 inpp annihilations
 We have gathered ~ 3.5 pb-1 on resonance (Ecm = 34063430 MeV) and ~ 1.8 pb-1 off resonance (Ecm = 3270,
3320 and 3495 MeV)
 We performed two different analysis to search evidence
for the resonant process:
p p  c 0  J /y  e e
 A inclusive J/y analysis ignoring 
 B exclusive J/y  analysis
Event Selection
 2 good electrons 15°<q<60° and 2.75<Mee<3.35 GeV
 at most one CCAL cluster “on time” E > 50 MeV and
3°<q<68°
 if Eg<100 MeV, OP(e±, )<10 are discarded to
remove background from bremmstrahlung
 inclusive or exclusive kinematic fit probability
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First observation of c0 inpp annihilations (2)
Preliminary Fit Results
 1.7
Mass 3417.9
MeV
 1.8
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 4.9
Width 15.0
MeV
 3.6
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Form factor
 We have performed the only measurement of the proton
form factor in the time-like region above 6 GeV2 by
measuring the non-resonant cross section
p p  e e
 First order QED predicts
4m 2p
d
p 2 2c 2 
2
2
2 *
2 *

G
1

cos
q

G
1

cos
q

M
E
2 xs 
s
d cosq *







 Background from p0p0, p0,  and pp- has been
carefully evaluated and is negligible
 We could not measure the angular distribution beacuse of
lack of statistics and geometrical acceptance. The form
factors are extracted from the data assuming the
following two different hypothesis:
a)
b)
|GE|=|GM|/p
|GE| is assumed negligible
GM
p
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C

2
s 2 ln  s 2 
  
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C and  free
parameters
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Form factor (preliminary)
T itolo:
/homej/s tanc ari /ff/pl ots /evlum.eps
Autore:
HIGZ Versi on 1.22/09
Anteprima:
L'immagine EPS non è s tata salvata
c on l'anteprima inclus a i n ess a.
Commento:
L'immagine EPS potrà es s ere s tampata c on una s tampante
Pos tScri pt e non con
altri tipi di stampante.
T itolo:
/homej/s tanc ari /ff/ana/../plots /ff.eps
Autore:
HIGZ Versi on 1.22/09
Anteprima:
L'immagine EPS non è s tata salvata
c on l'anteprima inclus a i n ess a.
Commento:
L'immagine EPS potrà es s ere s tampata c on una s tampante
Pos tScri pt e non con
altri tipi di stampante.
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hc
 BACK
 preliminary
 s0 

 A
 s 
B
 Ldt  25 pb
1
Event selection
 2 On Time clusters. No on time extra clusters
 Reject events with internal p0 and h
 Invariant mass > 3 GeV and cos(q*)<0.2
 Kinematic fit probability > 10 %
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hc
 results (preliminary)
 We have collected 18 pb-1 in the hc region ( from 2900 to
3100 MeV ) and searched for  events in the angular
region cos(qcm) < 0.2
 The preliminary results are:
1.7
 Mass : 2985 1.8
MeV
 Total width: 17.8 57..92
 G :
3.711..35 1.2
 BRpp x BR :
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c2
MeV
assuming BRpp=12±4 10-4
24.633..89 108
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h’ c
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 and c2
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
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s calculation
 Perturbative QCD with first order radiative corrections
predicts:
s

8 2  1  3.4 p
hc   2 
Ggg
9 s  1  4.8 s
p

G





s 

8 2  1  16 3p 
c 2   2 
Ggg
9 s  1  2.2 s 
p

G
 From our preliminary measurement we get:
 hc : s(mc) = 0.32 ± 0.05 assuming Ggg ~ GTOT
 c2 : s(mc) = 0.33 ± 0.02 assuming Ggg ~ GTOT - GJ/y
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h’c search (preliminary)
 We fit the data (maximum likelihood) with hypothesis
of a spin 0 resonance plus a power law background
 We find with G=5 Mev and 3576<Ecm<3660 MeV :
BR( R  p p) BR( R   )  9 108
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95% c.l.
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hc’ search in other channels
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p0 p0 at 90°
 We have measured the cross section:
pp
p0p0

with p0s scattered at 90° in the c.m. frame
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CONCLUSIONS
 We have gathered 145 pb-1 integrated luminosity with ~5 109
events to be processed and analized.
 e+e— and  events are already on disk and preliminary
analysis are completed in many channels
 I have shown preliminary results on hc (mass, width and
branching ratios), c0, hc’ search (negative, so far), y’ branching
ratios, p0p0 cross section, proton form factor
 We are working on:
 Multi  analysis (2-3 neutral mesons)
 pp elastic at 90º
 FF-->4K
FUTURE PROGRAMS
 Run extension in 1999 has been approved by Fermilab
and INFN
 New antiproton accumulator!
 To be done:
 Complete hc’ search
 Confirm 1P1 measured by E-760
 Search for 3D2, improve c0 measurement
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