I
.-
SLAC-PUB-5616
March 1992
T/E
Rapidity Gaps and Jets as a New Physics Signature
in Very High Energy Hadron-Hadron Collisions*
J. D.
BJORKEN
Stanford Linear Accelerator Center
Stanford University, Stanford, California 94309
Submitted to Physica. Review II
* Work supported by the Department of Energy, contract DE-AC03-76SF00515.
I
:
1. Introduction
At SSC/LHC
energies there emerges a new class of processes which are of
importance in the attempt to push beyond the standard-model phenomenology.
These reactions are characterized by the presence of virtual electroweak bosons in
the hard subprocesses. The most familiar-and perha.ps important-of these [l] is
the two-body scattering of W’s and Z’s, with the W’s a,nd Z’s treated as partons of
the incoming proton bea.ms (Fig. la.). Closely rela.ted is the production of a Higgs
boson (or other new elect,rowea.k/Higgs-sector pa.rticle) via IV-W fusion (Fig. lb).
(b)
P
Figure 1. Basic mechanism for producing W-N’ int,eraction processes in high-energy pp col-
lisions, with the presence of a rapidit.y-gap in the final state.
At the naive, “fa.ctorized,” level depicted in Fig. 1, the event-structure is atyp-
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- ical. For example, in the W-W scattering example, let the W’s decay leptonically.
Then there will be a large “rapidity-gap,”
i.e.
a region of (pseudo-) rapidity in
which no hadrons are found, separating the bea.m-jets containing the fragments of
the left-moving and right-moving projectiles.
This is the event morphology characteristic of double-diffraction, which has a
large cross-section.
The presence of isolated leptons, however, largely suppresses
this. And if la.rge transverse momentum is excha.nged between left and right movers
in the process, this double-diffra.ction background will itself be highly suppressed.
As will be discussed further in Section 2, the signal event, as shown in Fig. 2, has
the characteristic feature of “tagging-jets” at the edge of the ra.pidity-gap [2]. These
are simply the ha.dronization products of the init,ia.l-sta.te quarks that emitted the
W’S.
“.‘A t-;-j I;T””
Figure 2. Even morphology in lego variables
for the processes depicted in Fig. 1. The
tagging jets are the hadronization products of the quarks, while for large Higgs masses, almost
a.11 of the W-decay product.s lie within the dashed circles.
The remaining region, marked gap,
contains on avera.ge no more than 2 or 3 hadrons.
The combination of ra.pidity-ga.ps, tagging-jets,
a.nd leptons within the gap
would seem to be a strong signature for this process. Indeed even if one allows
hadronic decays of the W’s, the signatures still look quite good. Therefore we
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believe that the possibility of using this “underlying-event” structure should be
studied seriously by theorists, phenomenologists, and experimentalists. The ba-
sic idea of utilizing the rapidity-gap signature is due to Dokshitzer, Troyan and
Khoze [3]. But up to now not much has been done in developing it [4]. There are
many difficult issues involved. They include the following:
1. How big must the ra,pidity-gaps be in order that multiplicity fluctuations do
not mimic their effect?
2. How big are strong-intera.ction (Pomeron-exchange) backgrounds and how
do they scale with energy a.nd
pi?
3. What fraction of a given electroweak-boson exchange process, as defined at
the parton level, rea.lly leads to a final state conta.ining the rapidity-gap. Most
of the time specta.tor intera.ctions will fill in the ga.p present at the naive level
considered above. We estimate in Section 3 tl1a.t the survival probability of
the rapidity gap is of order 5%, but there are serious theoretical issues here
which need further explora.tion.
To make a complete feasibility study of this stra.tegy requires a considerable
amount of serious hflonte-Carlo simulation work. It is not the purpose of this paper
to provide any of t,ha.t.
While such work is necessa.ry, it is not sufficient. There
are several fundamental theoretical issues, most ha.ving to do with the physics
of rapidity-ga.p crea.tion in strong processes (“Pomeron physics”), which need to
be a.ddressed before one can really a.ssess whet.her the
inputs
to a Monte-Carlo
simulation are realistic. It is the purpose of this paper to look at some of these
underlying issues, and discuss how they might be a.ddressed, both from the point
of view of funda.mental theory a.s well as from experiment.
In Section 2 we survey semi-quantita.tively some typical electroweak-boson ex-
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- change processes in order to get some feel for the pra.cticality of the strategy,
and how they are calculated. In Section 3 we look at the physics underlying the
“survival of the rapidity gap,” i.e.
what fraction of events retain the factorized
structure containing the rapidity-gap. Section 4 considers potential backgrounds
from “hard diffraction” processes, i.e.
high-pl double or multiple diffraction. A
conclusion from tha.t section is that it is arguable that these corrections will be
large. If so, these strong-interaction processes may be able to be utilized for new-
physics as well. Section 5 is devoted to concluding comments, and enumeration of
suggestions for further experimental and theoretica. work.
2. Hard Collisions with Electroweak-Boson Exchange
Processes involving electroweak-boson exchanges have by now been considered
at great length in connection with the high-energy ha.dron collider programs such
as SSC and LHC. It is not our purpose t,o repea.t any of that work here [5], but
only to describe the revisions needed if one is to utilize the ra.pidity-gap signa.ture.
The processes we consider here are as follows:
a) Exchange of a single 7, IV, or 2.
b) Two-boson nonresonant processes, in particu1a.r yy -+ pspL-, and yy + X
or w+w- + .x.
c) Resona,nt production of the Higgs boson aad elastic scattering of TV’s and/or
Z’S.
a,) Single-boson exchange:
We begin with a description of the photon-excha,nge process described in Fig.
3, with final-state interactions of specta.tor partons temporarily disregarded. If q”
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