Experimental Tests of the BFKL Mechanism

Zarah CasilumState University of New York at Stony

Brookfor the D0 Collaboration

1997 Joint April APS/AAPT MeetingApril 18-21, 1997Washington, DC

Title: d0logo.epsCreator: CLARIS EPSF Export Filter V1.0CreationDate: 2/24/97 5:03:44 p.m.

BFKL Dynamics at the Tevatron

• Inclusive dijet production at large rapidities

• Semi-hard region:

For large x:

• The BFKL formalism resums (s) to all orders in s (only leading logs) into the partonic cross section by using a multigluon amplitude

QCD Q s

ln

,s

Q2 1 2 1

p p J p J p X 1 1 1 2 2 2( , ) ( , )

dijet x P x Q x P x Q 1 1 12

2 2 22( , ) ( , )

p

J p1 1 1( ),

J p2 2 2( ),

p

X pi i i( ),

x1, kT1x2, kT2

xn, kTn

s x x s 1 2

• The BFKL resummation results in an exponential growth of the partonic cross section

• Contribution of real gluons leads to additional radiation between the two jets

Decorrelation of the dijets in the azimuth

Dijet decorrelation at large rapidities• Tevatron at a fixed cms energy (1.8 TeV)

-At large PDF’s control dijet cross section

-Difficult to see the growth of cross section

with -Look for evidence of additional radiation as

increases

,

BFKLsQ

e BFKL

1

2

1

BFKLS 1

12 2ln

• Measuring azimuthal decorrelation (cartoon analogy)

Qualitative: broadening 1/N dN/d as

Quantitative: falling off cos(as

Title: USR$ROOT1:[SYJUN.DPF96]BFKL_PREDICTION.EPS;1Creator: HIGZ Version 1.22/09CreationDate: 06/08/96 21.49

LO pt1

pt2

pt1

pt2

Higher Order

The DO Detector

Title: iplot9118.psCreator: I-DEASCreationDate:

• Hermetic coverage over large rapidity (|)• Fine transverse segmentation (0.1 x 0.1 in x )• Good jet energy response and resolution

Side view of the DO Detector

Event topology and Analysis variables

• Jets: ET sums in 0.7 cone in x space

• Single jet triggers: Inclusive: ET>12 Gev

Forward: ET>12 GeV and |

• Multiple pp interactions and fake jets removed

• Offline defines jets with ET>20 GeV , and |

• Rapidity ordering and tagging jets at extreme rapidities: (),()

• Require rapidity boost

• Analysis variables: 12 and

1 2 i

X(Ji)

Tagging Jet2

Tagging Jet1

J1

J2

1 2

20 5.

<cos(-)> vs.

1/N dN/d vs. Title: NU01_1:[TMP321.SYJUN.DPF]DELTA_PHI_DPF96_COLOR.EPS;Creator: HIGZ Version 1.22/09CreationDate: 04/08/96 01.28

Data Results:

Title: NU01_1:[TMP321.SYJUN.DPF]COS_GLB_DPF96_V2_COLOR.EPSCreator: HIGZ Version 1.22/09CreationDate: 09/08/96 02.52

LLA

1800/630 GeV Dijet Cross Section Ratio at large

• Mueller and Navelet suggested a method to explore BFKL dynamics by describing s dependence (Nucl. Phys. B282 (1987) 727)

• Fix x1, x2, Q2 and take the ratio of the cross sections at two different center of mass energies, sA and sB

– Cancellation of the parton distribution functions, hence removing large source of theoretical error

• By measuring R(1800,630) at the Tevatron, we can probe the perturbative BFKL dynamics and extract the effective coupling of the BFKL pomeron, BFKL=1+12sln2/ (using s(Q2)=0.16)

R s sx x Q s

x x Q s

eA B

A

B

s s

A B

BFKL A B

( , )( , , , )

( , , , ) /

( ) ln( / )

1 2

2

1 22

1

LO HERWIG BFKL

R 1.0 ~1.4 ~1.7

dijetS

x P x Q x P x QQ

BFKLe~ ( , ) ( , )( )

1 1 12

2 2 22

2

11

Q2 and x

• Data set– 1800 GeV: integrated Lum = 1.5 nb-1 – 630 GeV: integrated Lum = 34 nb-1

• Offline cuts: similar cuts as the decorrelation analysis, except that ||>2.0

• Q2 and x values:

Q E ET T2

1 2

xE

seT

1 21 22

,, cosh

Title: NU01_1:[TMP321.SYJUN.RESUM.RATIO]SMALLX_X1X2_NOYBAR.EPS;1Creator: HIGZ Version 1.22/09CreationDate: 07/08/96 14.48

630 GeV1800 GeV

1 bin: 400-1000GeV2

several bins: 0.06-0.25

Summary and Outlook

• Decorrelation study -Qualitatively, data show trends consistent with

BFKL

-However, quantitative results do not agree with

LLA BFKL but agree well with Herwig(V5.8,

parton shower model, including ISR,FSR,

interference terms)

-LLA BFKL with improved kinematics will be

available soon and NLL BFKL calculations are in

progress

• Dijet cross section ratio study -study under way

-Very promising measurement;directly sensitive

to BFKL dynamics

Title: smallx_q2.epsCreator: HIGZ Version 1.22/09CreationDate: 96/09/25 11.13

Q2 Distribution

630 GeV1800 GeV