1 ultraperipheral heavy ion collisions timoshenko s., emelyanov v. moscow engineering physics...
TRANSCRIPT
1
Ultraperipheral heavy ion collisions
Timoshenko S., Emelyanov V.
Moscow Engineering Physics Institute (State University)
(for the STAR Collaboration)
XXXIII International Conference on High Energy Physics (ICHEP'06), Moscow 2006.
2
The STAR Collaboration~400 collaborators, 49 institutions, 8 countries
3
Overwiev
Introduction in UPC
Vector meson production
STAR
AuAu
dAu
4 prong analysis
interference
Conclusion
4
Introduction in ultraperipheral collisions
The two nuclei geometrically “miss” each other b > 2RA
Ions are source of fields photons s ~ Z4
pomerons sp ~ Z2A2 – for ‘heavy’ states
sp ~ Z2A5/3 - for lighter mesons
Photon and pomeron can couple coherently to the nuclei if its have:
• Small transverse momentum:
pT < h/RA~ 30 MeV
• Maximum longitudinal componentpL < h/RA ~ 3 GeV
A
nuclear form factor
, P
b
A
A
Pomeron carry the strong interaction but is colorless and it has the quantum number of the vacuum JP = 0++
E ~ 3 (80) GeV at RHIC (LHC)W ~ 6 (160) GeV at RHIC (LHC)
5
Vector meson production Vector mesons production (photon-pomeron interaction)
Exclusive vector meson production can be described in terms of elastic scattering. Photon emitted by one nucleus fluctuates to virtual ‘qq’ pairs, this state elastically scatters from the other nucleus (absorb some of the photon wave function) , then ‘qq’ pairs can be emerge as a vector meson. The “pomeron” represents the absorptive part of nuclear cross section.
, J/Y ,Y
Klein S. and Nystrand J.,hep-ph/0311164
00 V q u,d,s,c,b q l e, , lV , , ,J / ,| c | c | V c | qq c | l l
~ 8 % of (had.) for gold at 200 GeV/nucleon• 120 events/sec at RHIC design luminosity
6
STAR
7
Types of trigger Topology trigger (no break up d)
CTB (central trigger barrel) 240 scintillator slats CTB divided into 4 azimuthal
quadrants Hits in the North and South
sectors Top and bottom reject cosmic
ray Low multiplicity (2 or 4 events) Vertex position
typical event
8
Types of trigger Topology trigger + ZDCs (0 in TPC + signals in forward (zero
degree calorimeters) Topology trigger + West ZDC: Au+d->Au+pn
required break up d Topology trigger + both ZDC: Au+Au->AuAu+Xn
Backgrounds peripheral hadronic events cosmic rays, beam gas interactions, pile-up
TPCAu d(Au)
ZDC-West ZDC-East
CTB-topology
9
AuAu -> 0Au*Au* 200 GeV
Signal region:
pT<0.15 GeV
0 Rapidity
After detector simulation
1.7 million ZDC coincidence triggers in 2002 Require a 2 track vertex and - model background
scaled up to 2 single (1n) and multiple (Xn) neutron production
1n mostly from Giant Dipole Resonance Cross section and rapidity distribution match soft Pomeron model
STAR Preliminary
10
d -> 0pn Mass Spectra
2
2 2
M MdA B
dM M M iM
Topology trigger + ZDC for d breakup
Data shows clear 1n signal Large, clean 0 sample
~13400 events Well fit by 0 + direct
0 mass, width consistent with particle data book
0:direct pp ratio slightly lower than AuAu data
= 2.63±0.32±0.73 mb
preliminary
no cut pT
STAR Preliminary
11
d --> 0pn t spectra Fit to t ~ pT
2
Sensitive to nucleon form factor
ZEUS used F(t)=e-bt+ct*t
for proton form factor slope b ~ 9 GeV-2
Same as ZEUS R=√4b=(1.2±0.3) fm
Good fit to ZEUS at large t Drop off at small t,
Too little energy to dissociate the deuteron
Seen by fixed target experiments
Deuteron does not dissociate
t (GeV/c)2
ds/d
t, m
b/G
eV2
12
4 prong analysis Very preliminary ‘Model’ reaction
A->*(1450/1700) ->++-
Expect ~ 100 events Follows 2-prong analysis
pT < 100 MeV/c Excess for ++-
• Over ++-
• Only at low pT
Analysis on a fraction of data Background subtracted mass
spectrum peaks at ~1.5 GeV
mass (GeV)
En
trie
s
Neutral 4 pion combos
Charged 4 pion combos
En
trie
s
Net Signal
STAR Preliminary
13
Interference 2 indistinguishable possibilities
Interference!!
2-source interferometerwith separation b
is negative parity For pp, AA parity transform ->
~ |A1 - A2eip·b|2
At y=0=0[1 - cos(pb)] For pbar p: CP transform ->
~ |A1 + A2eip·b|2
b is unknown Reduction for pT <<1/<b>
0 w/ mutual Coulomb dissoc. 0.1< |y| < 0.6
t (GeV/c)2
dN/d
t
int
noint
14
Interference
Efficiency corrected t 1764 events total R(t) = Int(t)/Noint(t)
Fit with polynomial dN/dt =A*exp(-bt)[1+c(R(t)-1)]
A is overall normalization b is slope of nuclear form factor
b = 301 +/- 14 GeV-2 304 +/- 15 GeV-2
syst. uncertainties: ±8(syst)±15%(theory) c=0 -- > no interference c=1 -- > “full” interference
Data and interference model match c = 1.01 +/- 0.08
0.78 +/- 0.13
dN/d
tdN
/dt
STAR Preliminary
STAR Preliminary
Data (w/ fit) Noint Int
Data (w/ fit) Noint Int
t (GeV2)
t (GeV2)
0.1 < |y| < 0.5
0.5 < |y| < 1.0
AuAu -> r0Au*Au* 200 GeV
15
Conclusion
STAR has observed incoherent photonuclear 0 meson production in dAu collisions. The 0 cross sections and kinematic distributions
agree with theoretical models. dN/dt reflects d/Au size/form factor
STAR has observed (likely the 0*) production in AuAu collisions
We observe 2-source interference in 0 production. The interference occurs even though the 0 decay
before the wave functions of the two sources can overlap.