Quest for in-medium modifications in p+A, +A, +A collisions

Goal of experiment Dielectron cocktail ( "elementary dielectron sources") Review of existing experimental results:• E325@KEK, • CLAS@JLAB • CBTAPS @ ELSA Plans: HADES @ GSI

Main goal: Studies of vector meson (//) spectral functions in nuclear

medium via e+, e- pair spectroscopy at N 0 and T=0

2sinppm ee

eeee

e+

e-

R(A=100) 5.5 fm Dielectron two-body decays of light Vector Mesons

High resolution spectroscopy of e+e- pairs, no final state interaction !

Meson Mass(MeV/c2)

(MeV/c2)

c (fm)

Main decay

e+e- BR

768 152 1.3 4.4 x 10-5

782 8.43 23.4 7.2 x 10-5

1019 4.43 44.4 K+ K- 3.1 x 10-4

Decay length LD= c R (decay in medium)

N(L)=N(0)exp(-L/LD)

Theoretical predictions (some examples)

quark-meson-exchange effective chiral lagrangian

15%

K. Saito et.al

Phys.Rev.C55(1997)2637

T. Renk et.al

Phys. Rev. C 66 (2002) 014902

meson masses scale with density as , order parameter of ChSB

• Brown-Rho scaling('92) : mV*= mV(1-/0)

qq

Hadronic scenario: meson spectral function

2222 )]M(Im[)]M(RemM[

)M(Im2)M(A

GeVm

mimM

77.0

)()(

Vacuum:Vacuum:

W. Peters et.al. NPA 632(1998)109:

meson spectral function A(q,m)

In mediumIn medium: :

+

N-1

N(1520)

+ ...

(1232)

N-1

• m, depends on meson momentum (q) A(m,p) spectral function

•Connection to chiral symmetry?

Mass (width) changes – "trivial" effects

Broad resonances () : modification of line shape due mass dependent widths (m) and due to available phase space : important for HI and NN collisions at low energy (see second lecture)

Collision broadening

decay in Pb nucleus

() =vac () + coll()

Ntot

Ncoll v )(

M. Effenberger at al PHYS. REV. C 60 044614

Experimental considerations: 1. cross sections2. "inside" vs "outside" decays

cross sections for pp and p

pA A2/3

Lund String Model

3.5 12

GeV GeV

KEK (4.6 mb)

exclusive

SAPHIR (Bonn)CBELSA (Bonn)DAPHNE, TAPS (Mainz)GRAAL (Grenoble)

+ o

+ -

+ o

o o

▲ K+ K+ o

Ko ’

p

"Tomography" of vector meson decay Transport calculations for p// + A collisions HSD -E. Bratkovskaya, V. Cassing Phys. Rep. 308(1999) 65)

• vacuum spectral functions

• full in-medium meson propagation (scattering in medium)

production decay production on 93Nb with p/

3.5 GeV

1.17 GeV

Decay length LD= c R (decay in medium)

N(L)=N(0)exp(-L/LD)

Beam energy dependence for /

• for / Ebeam ≈1.1-1.5 GeV is optimum (but ~100 times less for !)

• for p/ beams less decays inside

protonspions pionsprotons

for 12 GeV p+A:~ 6% decays ~55% decays

For : factor 2

less in-medium decays

as compared to beams

e+,e- spectrum components: "dielectron cocktail"

Dielectron sources: „free” hadron: dielectron cocktail

2102.10

eeBR

• Two body meson decays (peaks):

• 3-body Dalitz- decays (continuum):

Me+e [GeV/c2}-

• CB – e+ e- combinatorial background

• Signal/CB usually <1 !

V → e+e-

V → e+e- X

e+

e-

e+

e-

2sin

eeeeee ppM

Meson Dalitz (3 body:e+e-X ) decays

Dielectron spectrum

|F(q2)| - electromagnetic form factor : time-like q2>0

q2<0 : space like – probed via electron scattering experiments

Vector Meson Dominance (VMD)

or Vector Dominance Model (VDM)

BR(0e+e-) = 1.2*10-2

BR(e+e-) = 6*10-3

Electromagnetic transition form-factors

• known from di-muon G-experiment

600 events33±7 events

60±8 events

Dielectron decays : Dalitz decays of Baryons

Baryon decays : (1232), N*(1440), ...Ne+e-

N*( )->Ne+,e- - not measured in elementary reactions !

(1232): 3 Form. factors (GE(q2), GM(q2), Gc(q2)) not known electromagnetic structure of nucleon

Extended Extended VectVector Meson Domince or Meson Domince

Contribution from several vector Contribution from several vector mesons – interferences !mesons – interferences !

Various treatments:

1.

2. Baryons are treated as point like particles (QED)

C. Fuchs: Phys. Rev. C67 025202(2003)M. Krivoruchenko et al. Ann.Phys296 (2002)299

B. Lautrup, J.Smith Phys.Rev D3(1971)1122, C. Ernst et al. PRC 58 (1998) 447, M. Thomere et al PRC 75(2007) 0604902,...

gives similar BR as 1

Ne+e-=5.02 KeV (BR=4*10-5)

Two body Vector Meson decays

Meson Mass(MeV/c2)

(MeV/c2)

c (fm) Main decay

e+e- BR

768 152 1.3 4.4 x 10-5

782 8.43 23.4 7.2 x 10-5

1019 4.43 44.4 K+ K- 3.1 x 10-4

I =1

qq

24

KK

Inverse process: e+e- * hadrons

I =0,1

+ 2 + 3+...

JP = 1-

Vector Mesons carry same quantum numbers

as photon

...quark em.current!

At meson pole !

Combinatorial Background

Combinatorial background (CB)

e+ and e- comming from different vertices Ne+e- unlike-sign pairs

Ne-Ne- and Ne+Ne+ like-sign pairs

absolute normalization CB= Signal

S+-= Ne+e- - CB+-

0 e-

e+

e-

Combinatorial Background

eeee NN2

Ị Multi 0 Dalitz decays small e+e-

• External Pair Conversion (EPC) of photons from 0 small e+e-

• e+e- from Dalitz and EPC small but their

combination can form large e+e- and large mass!!

0 e-

e+

dedicated talk in student session !

E325 ExperimentE325 Experiment

Measures Invariant Mass of e+e-, K+K- at KEK (1996-2002) in 12GeV p + A // + X reactions, s=5.1 GeVMass Resolution for e+e- : 8.0MeV/c2 for 10.7MeV/c2 for

Primary proton beam (~7*108/s)

Five targets; Carbon x 1 and Copper x 4 aligned in line Very thin targets to suppress conversion

Beam

Target

Target ConfigurationTarget Configuration

material beam intensity(p/spill)

Interaction length(%)

radiation length(%)

C ~1x109 0.2% 0.4%

CuX4 ~1x109 0.05%X4 0.5%X4

23mm

CCu

Very thin target with clean and high intensity beam

Vertex Distribution

Beam

Experiment KEK-PS E325

Forward LG Calorimeter

Rear LG Calorimeter

Side LG Calorimeter

Front Gas Cherenkov

Rear Gas Cherenkov

Barrel Drift Chamber

Cylindrical DC

Vertex Drift chamber

1m

Detector SetupDetector Setup

12GeV proto

n

beam

B

Hodoscope

Aerogel Cherenkov

Forward TOF

Start Timing Counter

Trigger: e+ and e- requested in opposite arms

E325 Model calculations of meson line shape

/ mesons are generated uniformly at surface of target nucleus ( ~ A2/3)

momentum distribution: as measured in xp pole mass downward shift : m*/m = 1 – k1 /0

k1 parameter decay width increase : */ = 1 + k2 /0

k2 parameter density distribution

Woods-Saxon : R: C:2.3fm/Cu:4.1fm e-

e+

e+

e-

•Blue histogram : Detector Simulation

•Red line : Breit-Wigner (gaussian convoluted) fitting result

• agrees with measured: Ks + -

• Detector response :

1

0

])/)exp[(1(

Rr

• CB subtracted

Data described (both nuclei) assumming / mass modification m*=m

0( 1 - .092/0)

• fit supports * consistent with the vacuum for /

/=0.7±0.2 /=0.9±0.2

• CB shape from mixed event but w.o absolute normalization (fit parameter)!

• Fit, excluding excess region, gives / <0.15 (Cu) and <0.32(C) with CL of 95% ! In contrast to other pp data /~1

2/dof =2.3

2/dof =1.8

p+A @ 12 GeV KEK-PS E325M. Naruki Phys.Rev.Lett 96 (2006) 092301

e+e- modification in KEK-PS E325

• only small fraction of decays is inside medium selection on

distribution

• Fit again excluding the region where the excess was seen; 0.95~1.01GeV/c2

excluded from the fitting

Mass modification of for Cu targets

Best fit (both nuclei) achieved for k1=0.035 and k2=2.6 (* ) . Mass shift of

Yokkaichi Meson2006

• Integrate the amount of the excess in the above region(0.95~1.01 GeV/c2) Nexcess

Photon experiment G7 @ JLaB

Photon beam: E =0.6-2.85 GeV

E>1.1 GeV needed for / production

Toroid with 6 coils – 6 sectors

Electron identification in CLAS

C Fe C Pb C Ti C

D2

~1.1%

dedicated talk in student session !

Detector acceptance

no acceptance !

• e-,e+ ID only for 8-450 : EmCal. coverage

• electrons in same sector forbidden!

Pair acceptance

Pair spectra

• Combinatorial background: CB from mixed events with normalization from like-sign pairs :

Signal pairs (CB subtracted)

Final result for the meson spectral function

cocktail of free // meson decays subtracted spectral function

k1=0.02±0.02 consistent with 0!

contradiction to KEK PS E325

Why CLAS result contradicts KEK-E325?• KEK-E325 fits CB (no absolute determination!) G7 spectra with CB fitted (no absolute normalization!) different results CB normalization essential !

Crystal Barrel & TAPS (CBTAPS) @ ELSA

Detector (photon calorimeter)

Strategy of the experiment: 0A A (0) A with photons from 3 GeV e- beam

Reference experiment: + N

CB/TAPS @ ELSAD. Trinka Phys. Rev, Lett (2005) 192303

m = m0 (1 - k /0); k = 0.14 mass shift of

m =722 at averaged =0.60

G7

p dependence

g7 is not sensitive to CBTAPS effect !

In-medium spectral function

in_medium 90 MeV !

HADES experiment @ GSIGSI

• 2007: FW hodoscope added

< 70

• proton/pion/HI beams

•e+e-/p//K id

• mass resolution

M/M~2% at /

more details in second lecture !

/ momentum distributions

• HADES is sensitive to both / : M ()~1.5% at low p (<1.2)

3.5 GeV

1.17 GeV

+A: smaller <p/ > momentum +A: low beam intensity, broad focus

• proton beams:

higher intensity

excellent beam focus (1-2 mm)

reference reaction p +p @ 3.5 GeV done

1.2

Expectations for in-medium effect

similar effect for pA

Background for p+A - conversionbeam

4x Nb segments : 4 x 0.5% I0 1x Be segment (2%)106 p/sec

p+Nb @ 3.5 GeV p+ Pb @ 3.5 GeV

S/B>1 for M>0.5 !

• segmented target :

Larger CB!

Pairs from pp @ 3.5 GeV

• ~ 5.5*109 LVL1 events collected (Apr07 -12 days running time)

• ~70% "online" analyzed (with reasonable calibration and tracking alignment)

• signal pairs: 54 k (all)

visible i on-line spectrum ~35 MeV/c2 !

S/BS/B

1010

11

0.5 1.0 0.5 1.0

MMeeee

"HADES online pair spectrum from April 2007"

to be continued with pA to be continued with pA and and A with HADES..A with HADES..

Summary

Meson line shape modifications seen in p+A/+A reactions:1. E325/KEK : downward mass shift (~ 9%) 2. E325/KEK: downward mass shift (~3%) and broadening 3. CBTAPS : downward mass shift (~14%) and 10-fold! broadening.

Strong momentum dependece m(p). No sensitivity to

.. but G7/CLAS: no mass shift of but broadening possible

explanation of contradiction to E325/KEK CB normalization in E325

G7/CLAS: no effect on mass shift ... but CLAS acceptance is not sensitive to CBTAPS no contradiction to CPTABS

New HADES experiment @ GSI : sensitive to both /

Line shape modification of in pp? : intermediate resonaces

D. Schumacher , S. Vogel et.al (UrQMD)

• produced through Baryonic resonance

N*(1520), N*(1720) and

(1700), (1905) involved

to be continued with pA to be continued with pA and and A with HADES..A with HADES..

e+ e-

N

N*()

m, pt, y distributions• HADES acceptance and reconstruction efficiency filter:

• HADES acceptance is flat for M> 0.5 GeV/c2

3.5 GeV

1.17 GeV

HADES acceptance

e+,e- pairs with e+e-> 90