Physics at HERA - DESY · 2007. 8. 22. · Physics at HERA Katja Krüger KirchhoffInstitut für Physik H1 Collaboration email: [email protected] Summer Student Lectures 20 22

Physics at HERA

Katja Krüger KirchhoffInstitut für Physik

H1 Collaborationemail: [email protected]

Summer Student Lectures20 22 August 2007

Katja Krüger Physics @ HERA 2

Overview Part 2

● Reminder● Parton Density Fits ● High Q2 and Electroweak Physics

– Neutral Current

– Charged Current

● Exotics– Model Dependent Searches

– Model Independent Searches


Reminder: Deep Inelastic Scattering● fourmomentum transfer

● inelasticity

● parton momentum fraction

Q2=−q2=−k−k '2

y=q⋅Pk⋅P

kk'

P

xP

q

F 2 x ,Q2=x∑ eq2 q x q x

d 2dx dQ2

=42

Q41x [1−y y

2

2 F2 x ,Q

2]

x=Q2

2 q⋅P=

Q2

ys


DGLAP Evolution Equations

● Q2 dependence of quark densities q(x,Q2) and gluon density g(x,Q2) is predicted


Parton Density FitsDGLAP predicts only Q2 dependence➔ assume parametrisation of the parton density functions

(PDFs) as a function of x at a starting scale Q02 (typically

around 47 GeV2):

➔ evolve the PDFs to all measured Q2, calculate F2, and fit

the parameters to match the data

some freedom in the procedure!– how many parameters, which Q0

2 ?

– how to combine quark and antiquark densities?

x q x ,Q02=a x b1−x c [1d x ]


Parton Density Fitsquark and antiquark densities:● most general:

● distinguish valence and sea quarks: (some assumptions on decomposition of sea needed)

● distinguish uptype and downtype quarks:

u ,u , d , d ,s ,s ,c ,c ,b ,b

uv , d v , sea

U=uc , D=ds b U=uc , D=ds buv=U−U , d v=D−D


H1 & ZEUS Parton Densities

differences:● H1:

–

–

– 10 free parameters

● ZEUS:–

–

– 11 free parameters

U ,U , D , D , gx U = a xb 1−x c

[1d x f x3 ]

uv , d v , sea , g

x uv = a xb 1−x c [1d x ]


High Q2 & Electroweak Physics


More Structure Functions

d 2NC±

dx dQ2=

22

Q41x

Y [F 2 x ,Q2 − y2Y F L x ,Q2 ∓ Y−Y x F3 x ,Q2]

/Z 0

FL = F

2 – 2xF

1 = 0 in the QPM

F3 :−Z0−interference

● FL relevant only at large y

● F3 relevant only at large Q2,

different sign for e+ and e–

e e

p

Y± = 1±1−y2


High Q2 Neutral Current

● difference between e+p and e–p only at large

➔

Q2≈M Z2

−Z 0−interference

M Z2


High Q2 Neutral Current

=x Q4

221

Y

d 2NC±

dx dQ2

e– positive interference

e+ negative interference

x F 3∝ x∑ eq2 q−q

direct handle on valence quark distribution!


Charged Current Interactions

e

pq

q'

e

neutrino not visiblein detector

imbalance in transverse plane

W


Charged Current Cross Section

● W bosons couple differently to up and downtype quarks

● in the QPM:

➔

d2CC±

dx dQ2=

GF2

4 x M W2M W2 Q2 2

Y[W 2± − y2Y W L± ∓ Y−Y x W 3±]

W 2=xUD , xW 3

=x D−U

W 2−=x UD , x W 3

−=x U−D

W L±=0

CC− ∝ x [U 1− y2 D ]

CC ∝ x [U 1−y2 D ]

e+

e–


Comparison NC vs. CC● at low Q2: different

dependences because of photon in NC

● at high Q2: „electroweak unification“ but:

∝1

Q4

∝1

M W2 Q22

d2CC±

dx dQ2≈

GF2

4 x⋅ MW2M W2 Q2

2

⋅YW 2±

d2NC±

dx dQ2≈

22

x⋅

1Q4

⋅Y F 2

GF≈4

2 M W2similar because


Electroweak Parameters: W Mass

● G = GF determined by normalization of the CC cross section

● Mprop = MW determined by the Q2 dependence of the CC cross section

82.87±1.82exp 0.30−0.16model GeV


CC & Polarization

● CC cross section depends on longitudinal electron/positron polarization Pe

● reason: W boson couples only to lefthanded (LH) particles and righthanded (RH) antiparticles:

d 2CC±

dx dQ2Pe ≈ 1±Pe

GF2

4 x⋅ MW2M W2 Q2

2

⋅YW 2±


Polarization @ HERA

● transverse polarization builds up in ~40 minutes through synchrotron radiation (SokolovTernov effect)

● spin rotators flip transverse longitudinal before experiments and back after

Pe =N RH−N LHN RHN LH


Polarization @ HERA

spin rotator


eP1 0.5 0 0.5 1

(p

b)

CC

σ

0

20

40

60

80

100

120p Scattering±Charged Current e

Xν→p e

Xν→p +e

2 > 400 GeV2Q

y


Electroweak Parameters: Z0 Couplings

1 0.5 0 0.5 1

1

0.5

0

0.5

1PDF (prel.)uvu ZEUSpola

total uncert. uncorr. uncert.

H1 prel. (HERA I+II 9505)

SM CDF LEP

1 0.5 0 0.5 1

1

0.5

0

0.5

1

ua

uv

68% CL

ZEUS

1 0.5 0 0.5 1

1

0.5

0

0.5

1PDF (prel.)dvd ZEUSpola

total uncert. uncorr. uncert.

H1 prel. (HERA I+II 9505)

SM CDF LEP

1 0.5 0 0.5 1

1

0.5

0

0.5

1

da

dv

68% CL

ZEUS

polarization also allows better sensitivity to vector and axialvector couplings of up and downtype quarks to the Z0


Exotics orBeyond the Standard Modell


New Particles

many theories predict more particles than the SM:● SUSY:

– every Standard Model particles has a supersymmetric partner

– fermion partners are bosons, boson partners fermions

● leptoquarks– particle with lepton and quark properties

– can be produced resonantly in ep collisions

● ... exited fermions, contact interactions, large extradimensions ...

but experimentally search also modelindependent!


Leptoquarks

● MLQ2 = (xP + k)2 = xs

● compare measured cross section with SM expectation

● derive limits on coupling

/ GeVLQM50 100 150 200 250 300

Eve

nts

/ 20

GeV

1

10

210

310

410

/ GeVLQM50 100 150 200 250 300

Eve

nts

/ 20

GeV

1

10

210

310

410

NC, P=27%H1 data (prelim.)SMSM uncertainty

/ GeVLQM50 100 150 200 250 300

Eve

nts

/ 20

GeV

1

10

210

310

/ GeVLQM50 100 150 200 250 300

Eve

nts

/ 20

GeV

1

10

210

310

CC, P=27%H1 data (prelim.)SMSM uncertainty

e e

q q

LQk

xP


Limits on Leptoquarks

100 150 200 250 300 350 400

210

110

1

/ GeVLQM

λ

d)νu, (e0,LS

u) (e0,RS

d) (e0,RS~

d)νu, d, e (e1,LS

d)νu, (e0,LS

u) (e0,RS

d) (e0,RS~

d)νu, d, e (e1,LS

d)νu, (e0,LS

u) (e0,RS

d) (e0,RS~

d)νu, d, e (e1,LS

d)νu, (e0,LS

u) (e0,RS

d) (e0,RS~

d)νu, d, e (e1,LS

Exclu

ded

at 95

% C

.L.

H1 preliminary


SUSY● R parity violation:

single SUSY particle can be produced

● limits depend on many parameters (masses, couplings)

● example: stop

ZEUS

102

101

1

100 120 140 160 180 200 220 240 260 280 300Mstop (GeV)

λ'13

1

Exclud

ed at 9

5% CL

ZEUS (prel.) 9900 e+p

100 GeV Λ M2 Λ 300 GeV

300 GeV Λµ Λ 300 GeV

tanβ=6

Excluded in part of SUSY parameters

λ` 131 (APV)


General Searches

● idea: new particles have typically large mass

➔ final state should contain particles with large transverse momentum from the decay– jets

– electrons

– muons

– photons

– neutrinos (missing transverse momentum)


General Searches

every channel in reasonable agreement with the standard model


MultiLeptonsin HERA1 a small excess of di and trielectron events at high transverse momenta observed by H1

[GeV]12M0 20 40 60 80 100 120 140 160 180

Eve

nts

110

1

10

210

310

[GeV]12M0 20 40 60 80 100 120 140 160 180

Eve

nts

110

1

10

210

310SM

SM Signal

Data (prelim.)

)1p, 0.94 fb±Dielectrons, HERA I+II (e

HE

RA

Exo

tics

Wo

rkin

g G

roup

2e H1+ZEUS (common phase space)

[GeV]12M0 20 40 60 80 100 120 140 160 180

Eve

nts

110

1

10

210

310

[GeV]12M0 20 40 60 80 100 120 140 160 180

Eve

nts

110

1

10

210

310

SM

SM Signal

Data (prelim.)

)1p, 0.94 fb±Trielectrons, HERA I+II (e

HE

RA

Exo

tics

Wo

rkin

g G

roup

3e H1+ZEUS (common phase space)

HERA2 data show no significant excess


Isolated Leptons and Missing PT

● spectacular events

● excess in HERA1 data at large transverse momenta of the hadronic system (PTX) seen by H1



● no excess in e– data● what about e+?



● H1+ZEUS combined: 1.8 excess● H1 alone: 2.9 excess

?

Documents

Physics at HERA - DESY · 2007. 8. 22. · Physics at HERA Katja Krüger KirchhoffInstitut für Physik H1 Collaboration email: [email protected] Summer Student Lectures 20 22