HEP2005, Marta Ruspa Vector meson production at large t at HERA 1

Vector meson production at large t at HERA

Marta Ruspa, Univ. Piemonte Orientale & INFN-Torino, Italy

HEP2005 International Europhysics Conference on High Energy Physics

Lisboa, Portugal July 21-27, 2005

On behalf of

Vector meson production at large t at HERA 2HEP2005, Marta Ruspa

Diffractive vector meson production at HERAExclusive Proton dissociative

VM = vector meson ( ρ, ω, φ, J/ψ, ... )

Q2 = (4-momentum exchanged at lepton vertex)2

W = c.m.energy of photon-proton system

|t| = (4-momentum exchanged at proton vertex)2

Experimentally clean process in wide kinematic range

Simultaneous control of different scales: Q2, t, MVM

Large Q2, t, MVM pQCD applicable, transition soft-hard regime

Vector meson production at large t at HERA 3HEP2005, Marta Ruspa

VM = qqbar dipole, interacts with the proton through the exchange of 2 gluons or a gluon ladder

standard DGLAP or BFKL evolution equations

pQCD predictions:

cross section t-dependence ~ |t|-n

steep rise of the cross section with W

possible SCHC violations

High |t| domain little explored so far

Proton dissociative processes dominate at high |t|

Proton dissociative VM production at high |t|

Vector meson production at large t at HERA 4HEP2005, Marta Ruspa

Cross section t-dependence Spin density matrix elements

J/ψ in photoproduction (ZEUS)Submitted paper n. 334

50 < W < 150 GeV

|t| < 20 GeV2

Proton remnant mass MY < 30 GeV

ρ0 in photoproduction (H1)Submitted paper n. 640

75 < W < 95 GeV

1.5 < |t| < 10 GeV2

Proton remnant mass MY < 5 GeV

Data samples

Vector meson production at large t at HERA 5HEP2005, Marta Ruspa

Electron scattered at small angles (detected in 44 m tagger)

Two oppositely charged tracks (pion candidates)

Nothing else in LAr above 400 MeV

DATA DiffVM MC

ρ0 selection

t = transverse momentum squared of ρ0

Detailed description of p-diss

Tuned to describe data t-dependence

Reweighted to take into account s-channel helicity non conservation

Also used to simulate ω, φ, ρ’ backgrounds

0

0 Yp

Vector meson production at large t at HERA 6HEP2005, Marta Ruspa

t-dependence well described by BFKL model of Poludniowski et al. [R.Enberg et al., JHEP 0309 (2003); G.G.Poludniowski et al., JHEP 0312 (2003) Parameters tuned to ZEUS data EPJ C26 (2003)]

Fit to a power law distribution of the form |t|-n

t-dependence of γ p ρ0 Y cross section

.)(.)(06.041.4 07.005.0 syststatn

obtained with MY < 25 GeV, also well described by the same model t-dependence sensitive to MY cut

.)(015.0.)(04.021.3 syststatnZEUS

Vector meson production at large t at HERA 7HEP2005, Marta Ruspa

γ p ρ0 Y: helicity analysis3 angles defined in photon-proton c.m.frame:

θ* φ* Φ

Photoproduction: can only measure θ*, φ* access to 3 of the 15 SDMEs

s-channel helicity conservation: vector meson retains photon helicity

all 3 SDMEs are predicted to be zero

Decay angular distribution related to spin of γ* and VM through Spin Density Matrix Elements (SDME)

study of transition between γ* and VM helicity states

Helicity=spin component along direction of the particle’s motion

Vector meson production at large t at HERA 8HEP2005, Marta Ruspa

γ p ρ0 Y: dependence on cosθ*

Production of longitudinally polarized ρ(0) from transversely polarized γ(+) (helicity “single flip”)

SCHC: expected zero04

00r

small: production dominated by transversely polarised ρ mesons Good agreement with ZEUS data Data well described by BFKL model of Poludniowski et al.

0400r

Vector meson production at large t at HERA 9HEP2005, Marta Ruspa

SCHC: expected zero04

11r

γ p ρ0 Y: dependence on *

0411r significantly different from zero: s-channel helicity non-

conservation Good agreement with ZEUS data BFKL model of model of Poludniowski et al. qualitatively reproduces data ciabut too large at low values of |t|

Production of transversely polarized ρ(+/-) from oppositely polarized γ(-/+) (helicity “double flip”)

Vector meson production at large t at HERA 10HEP2005, Marta Ruspa

Cross section t-dependence Cross section W-dependence

Cross section t-dependence Spin density matrix elements

J/ψ in photoproduction (ZEUS)Submitted paper n. 334

50 < W < 150 GeV

|t| < 20 GeV2

Proton remnant mass MY < 30 GeV

ρ0 in photoproduction (H1)Submitted paper n. 640

75 < W < 95 GeV

1.5 < |t| < 10 GeV2

Proton remnant mass MY < 5 GeV

Data samples

Vector meson production at large t at HERA 11HEP2005, Marta Ruspa

Electron undetected in CAL

Two oppositely charged tracks (muon candidates)

Nothing else in detector above 300 MeV

Signal simulated with EPSOFT MC tuned to describe data t-dependence

Non resonant background from γγ fusion process ep e +- Y simulated with GRAPE-DILEPTON MC

DATA

J/ selection

t = transverse momentum squared of dimuon system

/

/

J

YJp

Vector meson production at large t at HERA 12HEP2005, Marta Ruspa

t-dependence of γ p J/ Y cross section

DGLAP and BFKL LL as well as BFKL nonL with fixed s describe general behaviour of data

BFKL nonL with running s

too steep

[BFKL: J. Bartels et al., PL B 375 (1996); J. R. Forshaw and G. Poludniowski, EPJ C26 (2003); R. Enberg et al., EPJ C26 (2002); hep-ph 0502045 DGLAP: E. Gotsman et al., PL B 532 (2002) Parameters tuned to ZEUS data EPJ C26 (2003)]

Vector meson production at large t at HERA 13HEP2005, Marta Ruspa

Rise of cross section with W

Fit to a power law distribution of the form W

Extracted value of increase with rising |t|

Regge fit to the 4 t-bins:

Consistent with previous H1 result [PL B568 (2003)]

W-dependence of γ p J/ Y cross section

.)(039.0.)(048.0153.1)0( syststatIP

.)(010.0.)(014.0020.0' syststatIP

Regge theory: )1)((4 tIPWdt

d tt IPIPIP ')0()(

Vector meson production at large t at HERA 14HEP2005, Marta Ruspa

DGLAP LL not able to describe the data while BFKL LL qualitatively reproduces the rise

of the cross section

W-dependence of γ p J/ Y cross section

Vector meson production at large t at HERA 15HEP2005, Marta Ruspa

VM production shows at large |t| features of a hard process:

- hard t distributions

- steep rise of the cross section with W

- SCHC breaking

Photoproduction of ρ mesons measured up to |t|= 10 GeV2

- cross section power-like t-dependence well described by BFKL model of Poludniowski et al.

- small measured single flip amplitude agrees well with BFKL model of Poludniowski; large measured double flip amplitude confirms s-channel helicity non-conservation

Photoproduction of J/ mesons measured up to |t|= 20 GeV2

- cross section rises like W, suggesting onset of BFKL dynamics;

- significant t-dependence of

Summary