Experimental Tests of the BFKL Mechanism Zarah Casilum State University of New York at Stony Brook for the D0 Collaboration 1997 Joint April APS/AAPT Meeting April 18-21, 1997 Washington, DC

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 x 1, k T1 x 2, k T2 x n, k Tn

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

Measuring azimuthal decorrelation (cartoon analogy) Qualitative: broadening 1/N dN/d  as  Quantitative: falling off  cos(  as  LO p t1     p t2     p t1     p t2     Higher Order

The DO Detector 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: E T sums in 0.7 cone in  x  space Single jet triggers: Inclusive: E T >12 Gev Forward: E T >12 GeV and |  Multiple pp interactions and fake jets removed Offline defines jets with E T >20 GeV, and |  Rapidity ordering and tagging jets at extreme rapidities: (     ),(     ) Require rapidity boost Analysis variables:  1  2 and     X(J i )    Tagging Jet2 Tagging Jet1 J1J1 J2J2

vs.  1/N dN/d  vs.  Data Results: 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 x 1, x 2, Q 2 and take the ratio of the cross sections at two different center of mass energies, s A and s B –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+12  s ln2/  (using  s (Q 2 )=0.16)

Q 2 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 Q 2 and x values: 630 GeV 1800 GeV 1 bin: GeV 2 several bins:

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

Q 2 Distribution 630 GeV 1800 GeV