Forward Protons from the SPS to the Tevatron

Andrew Brandt, University of Texas at Arlington

Physics SeminarMay 17, 2006DESY

Thanks for slides: Koji Terashi, Dino Goulianos, Mike Albrow,Rainer Wallny Michele Arneodo, and othersDOE, NSF, UTA, Texas ARP for support

Elastic “dip” Structure fromPhys. Rev. Lett. 54, 2180 (1985).

Examples of Soft Diffraction

Modeled by Regge Theory Analysis of poles in the complex angular momentum plane give rise to trajectories

that describe particle exchange P.D.B. Collins, An Introduction to Regge Theory and High Energy Physics, Cambridge Univ. Press,

Cambridge 1977

Non-perturbative QCD

Elastic Single Diffraction

Priorto 1985

all diffractionwassoft

diffraction

Ingelman-Schlein Propose Hard Diffraction possibility in 1985 Factorization allows us to look at the diffractive reaction as a

two step process. Hadron A emits a Pomeron (pomeron flux) then partons in the Pomeron interact with hadron B in a standard QCD gg hard scattering. (basis of POMPYT, POMWIG MC’s)

The Pomeron to leading order is proposed to have a minimal structure of two gluons in order to have quantum numbers of the vacuum

AA*

BJ1

J2

P

X

G. Ingelman and P. Schlein, Phys. Lett. B 152, 256 (1985)

My first trip toDESY was April 1987

to meet Gunnar, begin work onPYTHIA 4.8X, precursor to POMPYT

UA8

UA8 = UA2 + Roman-pot Spectrometer

UA8 Dijet Production in Diffraction

Hard Diffraction exists! Pomeron has a “super-hard” component.

A. Brandt et al., P.L. B 297(1992) 417 (196 citations!)

x(2-jet)

CDF Confirms UA8 Result

K. Hatakeyama’sthesis, Rockefeller

2003

Diffractive Deep Inelastic Scattering

e

p

HERA

Proton energy = 920 GeVElectron energy = 27.5 GeVs=318 GeV

Q2 = virtuality of photon = = (4-momentum exchanged at e vertex)2

t = (4-momentum exchanged at p vertex)2

typically: |t|<1 GeV2

W = invariant mass of photon-proton system

xIP = fraction of proton’s momentum

taken by Pomeron = in Fermilab jargon = Bjorken’s variable for the Pomeron = fraction of Pomeron’s momentum carried by struck quark

LRGIP

Q2

t

W X

e’

p’

*e

p

920 GeV27.5 GeV

s 320 GeV

ZEUS

pe

X

e

p’p

e

e’

IP dPDF

1) Diffractive PDFs: probability to find a parton of given x in the proton under condition that proton stays intact – sensitive to low-x partons in proton, complementary to standard PDFs (ingredient for all inclusive diffractive processes at Tevatron and LHC)

2) Generalised Parton Distributions (GPD) quantify correlations between parton momenta in the proton; t-dependence sensitive to parton distribution in transverse plane• When x’=x, GPDs are proportional to the square of the usual PDFs (ingredient for all exclusive diffractive processes)

VM, exclusive dijets…Higgs

x’ x

p p

GPD

Two fundamental physics quantities can be accessed in diffractive DIS: dPDFs and GPDs

Rather than IP exchange: probe diffractive PDFs of proton

Applying dPDFs to FNAL/LHC Requires Care

CDF data

Extrapolationfrom HERA

F2D

GPDs and diffractive PDFs measured at HERA cannot be used blindly in pp (or ) interactions.

In addition to the hard diffractive scattering, there are soft interactions among spectator partons. They fill the rapidity gap and reduce the rate of diffractive events.

Multi-Pomeron-exchange effects (a.k.a. “renormalization”, “screening”,“shadowing”, “damping”, “absorption”)

pp

CDF Run 1-0 (1988-89)

Elastic, single diffractive, and total cross sections @ 546 and 1800 GeV

Roman Pot Spectrometers

Roman Pot Detectors Scintillation trigger counters Wire chamber Double-sided silicon strip detector

Results Total cross section tot ~ s Elastic cross section d/dt ~ exp[2’ lns] shrinking forward peak Single diffraction Breakdown of Regge factorization

Additional DetectorsTrackers up to || = 7

SSC is a four letterword in Texas

1992 Small-x Workshop

DESY seminar Oct. 1997 on DØ Hard

Diffraction leads to collaboration with young

Brian Cox

E

DØ Run I GapsDØ Run I Gaps

•Pioneered central gaps between jets: Color-Singlet fractions at s = 630 & 1800 GeV; Color-Singlet Dependence on , ET, s (parton-x). PRL 72, 2332(1994); PRL 76, 734 (1996);

PLB 440, 189 (1998)

•Observed forward gaps in jet events at s = 630 & 1800 GeV. Rates much smaller than expected from naïve Ingelman-Schlein model. Require a different normalization and significant soft component to describe data. Large fraction of proton momentum frequently involved in collision.PLB 531, 52 (2002)

•Observed W and Z boson events with gaps: measured fractions, properties first observation of diffractive Z. PLB 574, 169 (2003)

• Observed jet events with forward/backward gaps at s = 630 and 1800 GeV

Diffractive W Boson

Predicts15-20%

of W’s arediffractively

produced

CDF {PRL 78 2698 (1997)} measured RW = 1.15 ± 0.55%where RW = Ratio of diffractive/non-diffractive W

a significance of 3.8DIFFWsignal

DØ Observation of Diffractive W/Z

Observed clear Diffractively produced W and Z boson signals

Events have typical W/Z characteristics Background from fake W/Z gives negligible change in gap

fractions

Sample Diffractive Probability Background All Fluctuates to Data Central W (1.08 + 0.19 - 0.17)% 7.7Forward W (0.64 + 0.18 - 0.16)% 5.3All W (0.89 + 0.19 – 0.17)% 7.5All Z (1.44 + 0.61 - 0.52)% 4.4

ncalnL0

Diffractive W and Z Boson Signals

Central electron W Forward electron W

All Z

ncalnL0

ncal

nL0

•Phys. Lett. B 574, 169 (2003)

Soft Diffraction and Elastic Scattering: Inclusive Single Diffraction

Elastic scattering (t dependence)

Inclusive double pomeron

Search for glueballs/exotics

Hard Diffraction: Diffractive jet

Diffractive b,c ,t

Diffractive W/Z

Diffractive photon

Other hard diffractive topics

Double Pomeron + jets

Other Hard Double Pomeron topics

Exclusive Production of Dijets

DØ Run II Diffractive TopicsDØ Run II Diffractive Topics

Topics in RED were studied

with gaps only in Run I

Event Selection: Z→μ+μ- Events Two Good (PT > 15GeV) Oppositely Charged TracksBoth Identified as muonsBKGD Rejection: Min one muon Isolated in Tracker and Calorimeter (suppress Heavy Flavour BKGD), Cosmic Ray Rejection.

Diffractive Z Production

Demand Activity North and South Forward Gap (North or South)

Candidate Diffractive Z Events

DØ PrelimDØ Prelim

Forward Proton Detector

z [m]

Dipole Spectrometer Quadrupole Spectrometers

|t| ~ 0.0 GeV2 |t| > 0.8 GeV2

> 0.04> 0.0

18 Pots integrated into DØ readout and inserted every store since Jan 2004 Simultaneously tag/reconstruct protons and antiprotons

QuadrupoleMagnets

Separator

DipoleMagnets

Separator

PDOWN Spectrometer

DipoleSpectrometer

AUP Spectrometer

ADOWN Spectrometer

PUP Spectrometer

IP

Nine independent spectrometers each consisting of two detectors

Reconstruct particle tracks from detector (scintillating fiber) hits

Scattered antiprotons Scattered Protons

QuadrupoleMagnets

78 nsec109 nsec 78 nsec 109 nsec200 nsec

TDC’S!

Brown U.Hardware

commissioned by Manchester Engineers

Elastics/Halo BackgroundA1U A2U

P2DP1D

P

Pbar

LM

VCElastic

78 nsec

109nsec

78 nsec

109nsec

A1U A2U

P2DP1D

LM

VCProton Halo

-78 nsec

-109nsec

In-time Bit set if pulse detected (above threshold) in in-time windowHalo Timing Bit set if pulse detected in early time window

double halo could be backgroundto elastics

p

Large * Store

Physics Goals:1. Low-t

elastic scattering

2. Low-t single diffractive and double pomeronscattering

Two day run of accelerator at injection tune *=1.6 m1x1 bunchLum=0.5E30

Estimatedt range accessible with injection tune

pot position

integrated luminosity

Hit Maps from 1x1 Store

Typical StoreLarge store (4647)(no low squeeze)

20 Million events; first results this summer/fall

potstypically

9-15from beam

(no jet ET dependence either)

CDF Exclusive Dijets in Run I

Exclusive dijet limit:

jj (excl.) < 3.7 nb (95% CL)

Expected shape of

signal events

Theoretical expectation (KMR) ~1 nb

PRL 85 (2000) 4215

Dijet Mass fraction X

jjjj M

MR

Hard Diffraction has come a long way from UA8

days (from the SPS to Fermilab via HERA)

SPS: Jets, FNAL: W/Z, at LHC: Higgs?