QCD physics at HERA

QCD physics at HERA

Takahiro MatsumotoKEK, ZEUS Group

KEKPH 2007 1st March, 2007

Recent and results from:

Structure function

EW tests using polarized lepton

Hadronic Final States

KEKPH 2007 Takahiro Matsumoto 2

• HERA; start 1992… end June 30, 2007• Only one ep collider in the world

HERA accelerator

Proton

Electron or positron

Ring ~ 6.3 km

DESY, Hamburg

27.6 GeV 920 GeV√s = 318 GeV

e p


Longitudinal polarized lepton

• Self polarization of electrons by Synchrotron radiation (Sokolov-Ternov effect)

• Use spin rotator for longitudinal polarization– Applied to H1 and ZEUS fro

m 2002-• Polarization:

– 30-40% (both for L&R)• Useful for

EW tests in DIS


The Detectors

Uranium-Scintillator calorimeter

Multi-purpose detectors with Calorimeter, Tracking system

Both detectors provide competitive measurements on QCD

Liquid-Argon calorimeter

(E)/E = 35% for hadrons

(E)/E = 18% for electrons (E)/E = 12% for electrons

(E)/E = 50% for hadrons


Luminosity (HERA-I/II)

• HERA-I (1992~2000)– e+p 110 pb-1

– e-p 15 pb-1 (on tape)– Unpolarized leptonMatured analyses,

being finalized• HERA-II (2002~2007)

– e+p 130 pb-1

– e-p 250 pb-1 (on tape)– Longitudinally

polarized lepton– Si vertex detector

for b,c tagging New results with

larger statistics, new capabilities are being produced

~70% is recorded on the tape

02/03

06/07

04-06

92-00

as of Feb. 20, 2007


Deep Inelastic Scattering at HERA

• HERA; Giant electron micro scope of the proton• Q2 up to 4 Ee Ep ~ 105 GeV2

Space resolution; 0.001 fm Proton structure can be measured for wide kinematic range• Various QCD studies using Hadronic final states

27.6 GeV 920 GeV


DIS (cont’d)

][2 2

3242

2

Lpe FyxFYFY

xQdxdQ

d

bscduf

fff qqxeF,,,,

22 )(

),( 2Qxq f

Related to

x P

Q2 = - (k-k’)2

x = Q2/2P•(k-k’)

Y± = 1 ±(1-y)2, inelasticity y = Q2/xs

Parton Distribution Function

F2, F3, FL Structure function of the proton

3xF

LF

~ x (qf – qf)

Small for Q2 < < MZ2

Neutral Current DIS:

Longitudinal component Small, 0 for LO QCD dumped with y2

Main source


Kinematic regions• HERA covers

wide span (~O(106) ) for Q2 and x region, compared to Fixed targets and Tevatron

• Structure function:– Evolution in Q2, mea

sured very well– Low x region is only

covered by HERA Important input for

LHC !

LHC x region


Structure function, F2

• Sea quark is dynamically produced for high Q2 in low x

F 2

• Predicted by DGLAP (Dokschitzer-Gribov-Lipatov-Altarelli-Parisi) evolution equation for wide kinematic region

Triumph of perturbative QCD

Scaling violation


High Q2 DIS

• Low Q2

– NC, mediated by – CC, suppressed

by W mass• High Q2 ( >> MZ

2, MW2 )

– NC and CC cross sections compatible

Electroweak unification !

For NC, Q2 >~ MZ2 is

also interesting to see effect of /Z interference

Neutral Current

Charged Current

MZ2


F2, xF3 at High Q2

][2

3242

2

xFYFYxQdxdQ

dpe

Y± = 1± (1-y)2

F2 = Af x (qf + qf) = F2 (ve + Pe ae) Z F2

Z + O(Z2)

xF3 = Bf x (qf qf) = (ae + Pe ve)Z xF3Z + O(Z

2)

WZMQ

Q

2sin

1222

2

F2Z = 2 ef vf x ( qf + qf )

EW coupling

vf = T3f – 2ef sin2W, af = T3

f

ve ~ 0 ( because 2sin2W ~ ½)

(EM) Z (EW Z order) Z (EW Z2 order)

E polarization

visible @High Q2

xF3Z = 2 ef af x ( qf – qf )

EW effect?Unpol data seen in xF3 ((e-p) – (e+p)) af of quarks

pol data also in F2 af, vf of quarks


xF3 (unpol)

xF3 ~ x (qf – qf), obtained from (e-p) – (e+p)

xF3 at High Q2 is useful to determine valence quark PDF

complementary to neutrino DIS at Low Q2


Polarization asymmetry at High Q2

e+p

e-p

A+

A-

)()(

)()(2

LNCRNC

LNCRNC

LR PP

PP

PPA

PR > 0

PL < 0

First observation of parity violation in e±p NC DIS


Light quark EW coupling

• ZEUS fit to HERA-I e±p DIS, jets + HERA-II pol e-p DIS– Consistent with SM– Better precision than H1 unpol results (especially for vu,d)– Better precision than Tevatron, LEP


Obtained PDF

• Inclusion of HERA-II data with polarization – Consistent results with HERA-I– Smaller uncertainties of PDF at High Q2 Good News for

LHC

HERA-I

+HERA-II

HERA-I

+ HERA-II


Hadronic final states @HERA

This talk covers recent results on:- fragmentation function

- hard process QCD (s, low x) - other topics (multi-parton interaction, antideuteron)

ss

),,())(,,,(),,(,,

sFpRsRFasFagqqa

xDxdxfdx

Parton distribution function of the proton

Cross section of the hard process

Fragmentation function

Cross section for HFS:

Jet production at HERA

Regarded as hadronic correction ~ 1 + had in jet studies


Breit frame

Advantages in Breit frame:

02

qpxBJ

parton from proton (for the interaction) is back scattered with the same momenta

Current region can be compared to e+e- experiments

Current quark and proton remnants are clearly separated

Target region[Proton remnants]

Current region


Scaled charged particle momentum distributions in DIS

• For tracks in current region of Breit frame, define:– xp = p/(Q/2)

• Measurements of dn/dxp for Q <~100 GeV and full xp range

H1prelim-05-133

L = 44 pb-1 (2000)

1/N

dn/

dxp

xp

Q increasing

Scaling violation is clearly observed

Agreement with e+e- experiments, and RAPGAP(PS) MC tuned to e+e- universality of quark fragmentation function


• theoretically favored, no overlapping

Jet analysis

dii = ET,i2 R2

dij = min[ET,i2,ET, ｊ

2][2 + 2]

Order all dii and dij

For each Energy Flow Object, calculate dii, dij under Breit frame

If dii is minimum i is regarded as jet

If dij is minimum i and j are merged,

Radius parameter [usually set to 1]

No gluon No PT components

Contributions from QCD radiation can be seen in high ET jet

Breit frame

Longitudinally invariant kT algorithm

further iteration


Inclusive jet in DIS

NLO describes inclusive jets well

for s

)()())]((/[ 221

2Zs

iZs

iiZs MCMCMdQd

Small uncertainty of s (main concern is theoretical part) is obtained from Q2>500GeV2 and R=1

Agreement also for smaller radius (R=0.7,0.5)

Interesting for studying jets from heavy particles

s extraction


s running is clearly seen

H1prelim-05-133

ZEUS (DESY-06-241)

Consistent with PDG’2006: s(MZ) = 0.1176 ±0.002


s measurements at HERAs running

many s from jets and scaling violation in the structure function All give consistent results


Low x QCD• We want to estimate

LHC PDF from Tevatron and HERA data

• DGLAP QCD evolution sufficiently works?

• Need to care low x– Saturation?– Breakdown of DGLAP? Study at HERA

LHCTevatron

HERA


Evolution, Resummation

• DGLAP (Dokshitzer-Gribov-Lipatov-Altarelli-Parisi) – kT ordering, describe evolution in Q2, but fails at low x

• BFKL (Balitsky-Fadir-Kuraev-Lipatov)– No kT ordering, applicable to low x

• CCFM (Ciafaloni-Catani-Fiorani-Marchesini)– angular ordering, describe evolution both in Q2 and x

( applicable to low x @ BFKL and High Q2 @ DGLAP)

BFKL: expect more energetic jets in forward

~(lnQ2)n

~(ln(1/x))n

Gluon ladder diagram


Forward jet

• LEPTO (DGLAP)– Disagree with data

• CASCADE (CCFM) – Set 2 ( with no singula

r term ) is better – OK for Q2 and ET

– But fail in xBj and • ARIADNE

– Color dipole model– unordered kT,

but not BFKL…

– Good agreement

L = 82pb-

1

Extended to forward rapidity 2 < jet < 3.5

0.0004 < xBj < 0.005

Comparison with LO+PS MC


• NLO, improved from LO calculation (k=(NLO)/(LO)~10)

• Data is larger than NLO prediction, but within large theoretical uncertainty

* except lowest xBj

Forward jet (cont’d) Comparison with NLO QCD calculationwith hadronization correction

* DISENT: Fixed order QCD partonic cross section, on mass shell ME + DGLAP


3-jet in DIS for low x

• Low x DIS– DGLAP (kT ordering)

may be insufficient to describe g radiation

• 3-jet in DIS– Low Q2: 5 < Q2 < 80 GeV2

– Low x: 10-4 < x < 10-2

– 2 jets in forward• Sensitive to g radiation

• Fixed order QCD prediction, O(s3) is greatly improved from O(s2) , but still discrepancy exists at small x need NNLO or unordered gluon radiation?

L= 44pb-

1

~ s3

(usually 3-jet: ~s

2)

jet < 20º

xjet = Ejet*/Ep,beam

> 0.035


Four jets in photoproduction

• Test of pQCD in higer orders of s

• Can adjust multi-parton interaction model to agree with data related to LHC (underly

ing event)

4jet direct (tree level)

Schematic view of MPI

O(s3)

Multi-jet at HERA

Q2 < 1 GeV2

quasi-photon


First observation of 4 jets in photoproduction

MC without MPI underestimates the data in low Mnj region

HERWIG with tuned MPI describes the data well

Fraction of the photon’s momentum exchanged in the interaction

ZEUS (prel.) 121 pb-1


Antideuteron production in DIS

• Antideuteron production:– Studied in nuclear experim

ents (PHENIX etc.), but not well known for elementary particle collisions

• Search in DIS first time – Mass was calculated fro

m dE/dx, p– Also use DCA to beam sp

ot, Zvtx

• First observation of antideuteron in DIS– N(d) = 61 ±8

ZEUS (prel.) 120 pb-1

dE/dx(mips)>2.5

Q2 > 1 GeV2


d/p, p/p ratios in DIS

• Ratios were measured after correction of efficiency for tracking and dE/dx cut

• d/p ~ 5 x 10-4

– Consistent with (1S)(ARGUS), pp

– But d production is suppressed in e+e- qq

• p/p ~ 1– Asymmetry is no

t observed

0.3 < pT/M < 0.7, |y|< 0.4

Q2 > 1 GeV2


Coalescence model

• Process dependent– Pp ~ pA ~ gp– Suppressed in e+

e- and also in AASuppression in AA

larger source size

B2 ~ 1 / V (freeze out volume)

Recent measurements in e+e- also show the suppression (mechanism, not well known yet…)

HERA Consistent with pp, pA


Summary• Structure function in wide Q2 and x range

– F2; Steep rise in F2 at low x (sea quark and gluon), well described by pQCD (DGLAP)

– xF3; Z interference at High Q2, sensitive to valence quark– Light quark EW coupling; benefit from polarized lepton– Proton PDF determination

• Hadronic final states– Various QCD studies; fragmentation, s, low x,

multiple parton interaction, coalescence• Many other interesting items were missed this time

– Diffraction, heavy flavor, new physics search etc.

• HERA, operating 15 years and end up with this year. Inclusion of HERA-II results with polarized lepton, finally will improve our knowledge on QCD

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QCD physics at HERA