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

Examples of Soft Diffraction

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

Elastic Single Diffraction

Priorto 1985

all diffractionwassoft

diffraction

Modeled by Regge TheoryAnalysis 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

Ingelman-SchleinPropose Hard Diffraction possibility in 1985Factorization allows us to look at the diffractive reaction as atwo 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 A

A*

BJ1

J2P

X

My first trip toDESY was April 1987

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

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

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 GeV√s=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 momentumtaken by Pomeron = ξ in Fermilab jargon

β = Bjorken’s variable for the Pomeron = fraction of Pomeron’s momentum carried by

struck quark

LRGIP

Q2

t

W X

e’

p’

γ*e

p

920 GeV27.5 GeV

√s ≈ 320 GeV

ZEUS

pe

X

e∆η

p’p

ee’

IPdPDF

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 diffractiveprocesses at Tevatron and LHC)

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

Rather than IP exchange: probe diffractive PDFs of proton

2) Generalised Parton Distributions (GPD)quantify correlations between parton momenta in the proton; t-dependencesensitive 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, γ, exclusivedijets…Higgs

x’ xp p

γ∗

GPD

Applying dPDFs to FNAL/LHC Requires Care

CDF data

Extrapolationfrom HERA

F D

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.

2

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 GeVRoman Pot Spectrometers

Roman Pot DetectorsScintillation trigger countersWire chamber Double-sided silicon strip detector

ResultsTotal cross section σtot ~ sε

Elastic cross section dσ/dt ~ exp[2α’ lns] shrinking forward peakSingle diffraction Breakdown of Regge factorization

Additional DetectorsTrackers up to |η| = 7

SSC is a four letterword in Texas 1992 Small-x

Workshop

DØ Run I GapsDØ Run I Gaps

DESY seminar Oct. 1997 on DØ Hard

Diffraction leads to collaboration with young

Brian Cox

φ η

E

η

∆ηφ

η

•Pioneered central gaps between jets: Color-Singlet fractions at √s = 630 & 1800 GeV; Color-Singlet Dependence on ∆η, 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 3.8σDIFFWsignal

DØ Observation of Diffractive W/Z

Observed clear Diffractively produced W and Z boson signalsEvents have typical W/Z characteristicsBackground from fake W/Zgives negligible change in gap fractions

Sample Diffractive Probability BackgroundAll Fluctuates to Data

Central W (1.08 + 0.19 - 0.17)% 7.7σForward W (0.64 + 0.18 - 0.16)% 5.3σAll W (0.89 + 0.19 – 0.17)% 7.5σAll Z (1.44 + 0.61 - 0.52)% 4.4σ

ncalnL0

Diffractive W and Z Boson Signals

Central electron W Forward electron W

All Z

ncalnL0

ncalnL0

•Phys. Lett. B 574, 169 (2003)

Soft Diffraction and Elastic Scattering:Inclusive Single Diffraction Elastic scattering (t dependence) Inclusive double pomeronSearch for glueballs/exotics

Hard Diffraction:Diffractive jetDiffractive b,c ,t Diffractive W/ZDiffractive photon Other hard diffractive topics Double Pomeron + jetsOther Hard Double Pomeron topics

Exclusive Production of Dijets

DØ Run II Diffractive TopicsDØ Run II Diffractive Topics

Topics in RED were studiedwith gaps only in Run I

Diffractive Z ProductionEvent Selection: Z→µ+µ- EventsTwo 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.

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

Mass (GeV)0 100 200 300

Eve

nts

/ G

eV

200

400

600

800

1000

Mass (GeV)0 100 200 300

Ev

en

ts /

2 G

eV

5

10

15

20

25

DØ PrelimDØ Prelim

Candidate Diffractive Z Events

Forward Proton DetectorNine independent spectrometers each consisting of two detectors

z [m]

QuadrupoleMagnets

Separator

DipoleMagnets

Separator

PDOWN SpectrometerDipoleSpectrometer ADOWN Spectrometer

AUP Spectrometer PUP Spectrometer

IP

Reconstruct particle tracks from detector (scintillating fiber) hits

Scattered antiprotons Scattered Protons

QuadrupoleMagnets

78 nsec109 nsec 78 nsec 109 nsec200 nsec

Dipole Spectrometer Quadrupole Spectrometers

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

ξ > 0.04 ξ > 0.0

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

TDC’S!

Brown U.Hardware

commissioned by Manchester Engineers

Elastics/Halo BackgroundA1U A2U

P2DP1D

P

Pbar

LMVCElastic

78 nsec

109nsec

78 nsec

109nsec

A1U A2U

P2DP1D

LMVC

Proton 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 β* Store

Physics Goals:1. Low-t

elastic scattering

2. Low-t single diffractive and double pomeronscattering

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

Estimatedt range accessible with injection tune

pot position

integrated luminosity

Hit Maps from 1x1 StoreLarge β∗ store (4647)(no low β squeeze)Typical Store

20 Million events; first results this summer/fall

potstypically

9-15σfrom beam

(no jet ET dependence either)

CDF Exclusive Dijets in Run IPRL 85 (2000) 4215

Expected shape of signal events

Dijet Mass fraction X

jjjj M

MR =

Exclusive dijet limit: σjj (excl.) < 3.7 nb (95% CL)

Theoretical expectation (KMR) ~1 nb

Hard Diffraction hascome a long way from UA8

days (from the SPS to Fermilab via HERA)

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