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JHEP11(2016)011

Published for SISSA by Springer

Received: July 12, 2016

Revised: September 26, 2016

Accepted: October 25, 2016

Published: November 2, 2016

Measuring c-quark polarization in W +c samples at

ATLAS and CMS

Yevgeny Kats

Department of Particle Physics and Astrophysics, Weizmann Institute of Science,

Rehovot 7610001, Israel

E-mail: yevgeny.kats@weizmann.ac.il

Abstract: The process pp → W

−

c produces polarized charm quarks. The polarization is

expected to be partly retained in Λ

baryons when those form in the c-quark hadronization.

We argue that it will likely be possible for ATLAS and CMS to measure the Λ

polarization

in the W +c samples in Run 2 of the LHC. This can become the ﬁrst measurement ever

of a longitudinal polarization of charm quarks. Its results will provide a unique input to

the understanding of polarization transfer in fragmentation. They will also allow applying

the same measurement technique to other (e.g., new physics) samples of charm quarks in

which the polarization is a priori unknown. The proposed analysis is similar to the ATLAS

and CMS measurements of the W +c cross section in the 7 TeV run that used reconstructed

D-meson decays for charm tagging.

Keywords: Charm physics, Hadron-Hadron scattering (experiments), Heavy quark

production, Polarization, proton-proton scattering

ArXiv ePrint: 1512.00438

Present address: Theoretical Physics Department, CERN, CH-1211 Geneva 23, Switzerland. E-mail:

yevgeny.kats@cern.ch.

Open Access,

 The Authors.

Article funded by SCOAP

doi:10.1007/JHEP11(2016)011

JHEP11(2016)011

Contents

1 Introduction 1

2 Overview of the theoretical picture 3

3 Λ

reconstruction 5

4 Λ

polarization measurement 8

5 Sensitivity estimate for Run 2 8

6 Comments on other possible strategies 11

7 Discussion 12

1 Introduction

The process

pp → W

−

c (1.1)

proceeds at leading order via the diagrams in ﬁgure 1 (see also [1, 2]).

Since the W couples

only to the left-handed quark ﬁelds, the charm quarks in the ﬁnal state are polarized. In

this paper we argue that ATLAS and CMS can likely measure this polarization in Run 2

of the LHC.

While measurements of top-quark polarization are now standard [3–9], measuring the

polarization of all the other quarks is more challenging because they hadronize. It is,

however, possible. For a heavy quark, m

 Λ

QCD

, an O(1) fraction of the quark polar-

ization is expected to be retained when its hadronization produces a baryon, which is most

commonly a Λ

in the b-quark case or a Λ

in the c-quark case [10–13]. The polarization

can then be determined from kinematic distributions of the baryon decay products. Evi-

dence of Λ

polarization has been seen in e

−

→ Z → b

b events at LEP [14–16]. There

have been no analogous measurements for the Λ

. Even though the charm quark is not

as heavy as the bottom, it is reasonable to expect the Λ

to also carry polarization, since

the LEP experiments observed O(1) polarization retention even for the (strange-based) Λ

baryons [17–19].

Carrying out charm-quark polarization measurements in Standard Model samples at

the LHC is important for several reasons. First, establishing the measurement technique on

a Standard Model sample will allow applying it conﬁdently to any new-physics sample that

will contain charm quarks. The polarization can in turn provide key information about

the new-physics Lagrangian. Importantly, as alluded to above, the c-quark polarization

Throughout the paper, the conjugate process pp → W

c is included implicitly. Its cross section is

slightly lower than that of pp → W

−

c, primarily because the valence quarks of the proton include a d but

not a

– 1 –

JHEP11(2016)011



 















Figure 1. Leading-order diagrams for W +c production. The contribution with the s quark in the

initial state is an order-of-magnitude larger than the CKM-suppressed d-quark contribution, while

the b-quark contribution is entirely negligible.

fraction carried by the Λ

is currently unknown. Therefore, for a full interpretation of any

new-physics measurement, it would be useful to have this fraction extracted from a known

sample, such as W +c. Second, regardless of whether any new physics is discovered at

the LHC, the polarization measurements will advance our understanding of fragmentation.

There are various theoretical approaches that attempt to describe the polarization transfer

from a heavy quark to a baryon. Some parameterize the nonperturbative QCD physics in

terms of certain quantities that can be determined also by other types of measurements [12]

(see also [13]), as we will review, and others attempt to model it [20, 21]. These frameworks

will greatly beneﬁt from experimental inputs.

The possibility of using Λ

decays for measuring charm-quark polarization in ATLAS

and CMS was ﬁrst analyzed in [13]. It was shown there, that in Run 2 it would be possible

to do such measurements in pp → t

t samples, where polarized charm quarks are produced

in W

→ c¯s decays. The process in eq. (1.1) might be even more promising than t

t because

it provides an order-of-magnitude larger sample of charm quarks (although the background

is also larger).

A relatively clean sample of W+c events can be obtained by using the decays W → `ν

(where ` = e or µ) along with charm tagging. Such samples were used by ATLAS [22] and

CMS [23] in the 7 TeV run for measuring the W +c cross section. Similar measurements were

done by CDF [24–26] and D0 [27, 28] at the Tevatron and by LHCb [29]. The tagging of

charm quarks in these analyses was based on either a reconstructed multiprong D

or D

∗+

decay, or an inclusively deﬁned displaced decay, or a soft muon from a semileptonic decay.

In the polarization measurement, one would instead use reconstructed decays of charmed

baryons, most importantly the Λ

, for tagging the charm and measuring its polarization.

While ATLAS and CMS have not yet reported any W +c analyses from the 8 or 13 TeV

runs, measurements of inclusive W production at 13 TeV [30, 31] show that the ATLAS

and CMS triggers still allow collecting the W → `ν decays without a signiﬁcant decrease

in eﬃciency. It is therefore realistic to obtain large samples of W +c events in Run 2.

Measuring the Λ

polarization in W +c samples may also be possible at LHCb. While

LHCb’s luminosity and acceptance (and therefore statistics) are inferior relative to ATLAS

and CMS, its particle identiﬁcation, vertexing ability and momentum resolution oﬀer sig-

niﬁcant advantages from the point of view of purity. This makes it plausible for an LHCb

measurement to be competitive with ATLAS and CMS [32]. However, it will not be pos-

sible for us to analyze a potential LHCb measurement in this work, as this would require

dedicated simulation tools, unlike in the case of ATLAS/CMS where we will be able to

build upon an existing analysis.

– 2 –

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